WO2022202716A1 - Galvanized steel sheet and member, and method for manufacturing same - Google Patents
Galvanized steel sheet and member, and method for manufacturing same Download PDFInfo
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- WO2022202716A1 WO2022202716A1 PCT/JP2022/012855 JP2022012855W WO2022202716A1 WO 2022202716 A1 WO2022202716 A1 WO 2022202716A1 JP 2022012855 W JP2022012855 W JP 2022012855W WO 2022202716 A1 WO2022202716 A1 WO 2022202716A1
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- steel sheet
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- galvanized steel
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Classifications
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
- C21D1/76—Adjusting the composition of the atmosphere
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
- C21D3/02—Extraction of non-metals
- C21D3/04—Decarburising
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0236—Cold rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0273—Final recrystallisation annealing
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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Definitions
- the present invention relates to a galvanized steel sheet, a member made of the galvanized steel sheet, and a method for manufacturing the same.
- Patent Document 1 describes steel sheets that are used as materials for such automotive members. "Expressed in mass %, C 0.04 to 0.22%, Si 1.0% or less, Mn 3.0% or less, P 0.05% or less, S 0.01% or less, It contains 0.01 to 0.1% of Al and 0.001 to 0.005% of N, and has a component composition consisting of the balance Fe and inevitable impurities, and has a ferrite phase as the main phase and a second phase.
- a high-strength steel sheet having excellent stretch-flange formability and collision resistance, characterized by being composed of a certain martensite phase and having a maximum grain size of 2 ⁇ m or less and an area ratio of the martensite phase of 5% or more. is disclosed.
- Patent Document 2 "A hot-dip galvanized layer on the surface of a cold-rolled steel sheet obtained by pre-plating 0.2 g/m 2 or more and 2.0 g/m 2 or less of Ni on a cold-rolled steel sheet from which the surface layer has been ground and removed to a thickness of 0.1 ⁇ m or more.
- a galvanized steel sheet in % by mass, C: 0.05% or more, 0.4% or less, Si: 0.01% or more, 3.0% or less, Mn: 0.1% or more, 3.0% or less, P: 0.04% or less, S: 0.05% or less, N: 0.01% or less, Al: 0.01% or more, 2.0% or less, Si+Al>0.5%, containing, the remainder consisting of Fe and unavoidable impurities,
- the microstructure contains 40% or more ferrite as the main phase in volume fraction, 8% or more retained austenite, and the following three types of martensite [1] [2] [3] martensite [3] and 1% or more of bainite and 0 to 10% of pearlite, and the three types of martensite [1] [2] [3] each have a volume fraction of Martensite [1]: 0% or more and 50% or less, Martensite [2]: 0% or more and less than 20%, Martensite [3]: 1% or more and
- Patent Document 3 "In mass%, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% Below, S: 0.02% or less, Al: 0.01% or more and 2.5% or less, and the balance having a component composition consisting of Fe and unavoidable impurities, In terms of area ratio, tempered martensite phase: 30% to 73%, ferrite phase: 25% to 68%, retained austenite phase: 2% to 20%, other phases: 10% or less (including 0%) And, as the other phases, martensite phase: 3% or less (including 0%), bainitic ferrite phase: less than 5% (including 0%), and the average of the tempered martensite phase A high-strength hot-dip galvanized steel sheet having a steel sheet structure with a grain size of 8 ⁇ m or less and a C content in the retained austenite phase of less than 0.7% by mass. ” is disclosed.
- YS yield stress
- impact absorbed energy energy absorbed at the time of impact
- TS and YS of a steel sheet generally reduces formability, particularly properties such as ductility, work hardening ability, and hole expansibility. These properties correlate with the resistance to cracking of members in bending crush tests and axial crush tests that simulate collision tests. Therefore, if such a steel sheet with increased TS and YS is applied to the impact energy absorption member of the above-mentioned automobile, not only is it difficult to form, but the member will crack in a test that simulates a crash test.
- the impact energy absorbing member is limited to a steel plate with a TS of 590 MPa class. Work hardenability and hole expansibility correlate with stretchability and stretch flangeability, respectively.
- the steel sheets disclosed in Patent Documents 1 to 3 cannot be said to have TS: 980 MPa or more, high YS, and excellent ductility, work hardening ability and hole expansibility.
- the present invention has been developed in view of the above-mentioned current situation, and provides a galvanized steel sheet having a TS of 980 MPa or more, a high YS, and excellent ductility, work hardening ability and hole expansibility. It is an object to provide, together with an advantageous manufacturing method. Another object of the present invention is to provide a member made of the above galvanized steel sheet, and a method for manufacturing the same.
- ⁇ YS measured in a tensile test in accordance with JIS Z 2241 satisfies the following formula according to TS measured in the tensile test
- TS measured in the tensile test 980 MPa ⁇ TS ⁇ 1180 MPa, 550 MPa ⁇ YS
- 1180 MPa ⁇ TS ⁇ 1310 MPa 700 MPa ⁇ YS 800 MPa ⁇ YS when 1310 MPa ⁇ TS
- the total elongation (El) measured by a tensile test in accordance with JIS Z 2241 satisfies the following formula according to the TS measured by the tensile test
- 980 MPa ⁇ TS ⁇ 1180 MPa 13.0% ⁇ El
- 1180 MPa ⁇ TS ⁇ 1310 MPa 12.0% ⁇ El 10.0% ⁇ El when 1310 MPa ⁇ TS ⁇
- the chemical composition of the base steel sheet of the galvanized steel sheet is appropriately adjusted, and the steel structure of the base steel sheet of the galvanized steel sheet is Ferrite area ratio: 65.0% or less (including 0%), Area ratio of bainitic ferrite: 5.0% or more and 40.0% or less, Area ratio of tempered martensite: 0.5% or more and 80.0% or less, Area ratio of retained austenite: 3.0% or more, Area ratio of fresh martensite: 20.0% or less (including 0%), S BF + STM + 2 x SMA : 65.0% or more, SMA1 / SMA : 0.80 or less and SMA2 / SMA : 0.20 or more, so that TS: 980 MPa or more, high YS, excellent ductility, work hardening ability and hole It was found that a galvanized steel sheet having spreadability can be obtained. The present invention has been completed based on the above findings and further studies.
- the gist and configuration of the present invention are as follows. 1.
- a galvanized steel sheet having a base steel sheet and a galvanized layer on the surface of the base steel sheet,
- the base steel plate is in % by mass, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, a carbon equivalent Ceq of 0.540% or more, and the balance being Fe and unavoidable impurities.
- the base steel plate is Ferrite area ratio: 65.0% or less (including 0%), Area ratio of bainitic ferrite: 5.0% or more and 40.0% or less, Area ratio of tempered martensite: 0.5% or more and 80.0% or less, Area ratio of retained austenite: 3.0% or more, Area ratio of fresh martensite: 20.0% or less (including 0%), S BF + STM + 2 x SMA : 65.0% or more, having a steel structure in which SMA1/SMA: 0.80 or less and SMA2 / SMA : 0.20 or more; A galvanized steel sheet having a tensile strength of 980 MPa or more.
- S BF Area ratio of the bainitic ferrite
- STM Area ratio of the tempered martensite
- SMA Area ratio of the hard second phase composed of the retained austenite and the fresh martensite
- SMA1 The hard second phase
- S MA2 the hard second phase Of the constituent island regions, 1% or more of the circumference is the total area ratio of the island regions in contact with the bainitic ferrite.
- the chemical composition of the base steel plate is further, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.500% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, 2.
- a plating step of galvanizing the cold-rolled steel sheet to form a galvanized steel sheet a second cooling step of cooling the galvanized steel sheet to a second cooling stop temperature of 100° C. or more and less than 300° C.;
- the galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower, and the galvanized steel sheet is heated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower.
- a reheating step of holding in the temperature range for 10 seconds or more and 2000 seconds or less; has A method for producing a galvanized steel sheet, wherein the first cooling stop temperature and the temperature of the galvanizing bath in the galvanizing treatment satisfy the relationship of the following formula (1). ⁇ 150° C. ⁇ T 0 ⁇ T 1 ⁇ 50° C. (1)
- T0 is the first cooling stop temperature (°C)
- T1 is the temperature (°C) of the zinc plating bath in the zinc plating treatment.
- a method for producing a member comprising the step of subjecting the galvanized steel sheet according to any one of the above 1 to 8 to at least one of forming and joining to form a member.
- a galvanized steel sheet having a TS of 980 MPa or more, a high YS, and excellent ductility, work hardening ability and hole expansibility can be obtained.
- members made of the galvanized steel sheet of the present invention have high strength and excellent impact resistance, they can be very advantageously applied to impact energy absorbing members of automobiles.
- (A) is an example of a tissue image obtained by SEM used for tissue identification
- (B) is the tissue image of (A) color-coded using Adobe Photoshop of Adobe Systems.
- (A) is an example of a tissue image obtained by SEM used for determining the island-shaped regions of the hard second phase, in particular, an example of a tissue image including the island-shaped regions determined by MA1
- (B) is an example of (A).
- the tissue image of is color-coded using Adobe Photoshop of Adobe Systems.
- (A) is an example of a tissue image obtained by SEM used for determining the island-shaped regions of the hard second phase, in particular, an example of a tissue image including the island-shaped regions determined by MA2;
- the tissue image of is color-coded using Adobe Photoshop of Adobe Systems.
- (A) is an example of a tissue image obtained by SEM used for determining the island-shaped regions of the hard second phase, particularly an example of a tissue image including the island-shaped regions determined by MA3, and (B) is an example of (A)
- the tissue image of is color-coded using Adobe Photoshop of Adobe Systems.
- (A) is a schematic diagram for explaining a method for evaluating resistance weld cracking resistance in a weld zone, and the upper diagram of (B) is a top view of a plate assembly after resistance spot welding used in the same evaluation, and (B ) is a sectional view taken along the line AA of the upper figure.
- C 0.050% or more and 0.400% or less C generates appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite to ensure a TS of 980 MPa or more and a high YS. It is an effective element.
- the C content is less than 0.050%, the ferrite area ratio increases, making it difficult to increase the TS to 980 MPa or more. In addition, a decrease in YS is also caused.
- the C content exceeds 0.400%, the carbon concentration in retained austenite increases excessively. Therefore, when a steel plate is punched, the hardness of fresh martensite generated from retained austenite increases significantly.
- the C content should be 0.050% or more and 0.400% or less.
- the C content is preferably 0.100% or more. Also, the C content is preferably 0.300% or less.
- Si 0.20% to 3.00% Si suppresses the formation of carbides during annealing and promotes the formation of retained austenite. That is, Si is an element that affects the area ratio of retained austenite and the carbon concentration in retained austenite.
- Si content if the Si content is less than 0.20%, the area ratio of retained austenite decreases and the ductility decreases.
- the Si content exceeds 3.00%, the ferrite area ratio increases excessively, making it difficult to increase the TS to 980 MPa or more. In addition, a decrease in YS is also caused.
- the carbon concentration in retained austenite increases excessively.
- the Si content should be 0.20% or more and 3.00% or less.
- the Si content is preferably 0.40% or more.
- the Si content exceeds 2.00%, the resistance weld cracking resistance may be lowered, so the Si content is preferably 2.00% or less.
- Mn 1.00% or more and less than 3.50%
- Mn is an element that adjusts the area ratio of bainitic ferrite, tempered martensite, and the like.
- the Mn content is less than 1.00%, the ferrite area ratio increases excessively, making it difficult to achieve a TS of 980 MPa or more. In addition, a decrease in YS is also caused.
- the Mn content is 3.50% or more, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite excessively increases. As a result, the desired ductility is not obtained. Therefore, the Mn content should be 1.00% or more and less than 3.50%.
- the Mn content is preferably 1.80% or more. Also, the Mn content is preferably less than 3.20%.
- P 0.001% or more and 0.100% or less
- P is an element that has a solid-solution strengthening action and increases the strength of the steel sheet.
- the P content is made 0.001% or more.
- P segregates at the prior austenite grain boundaries and embrittles the grain boundaries. Therefore, when the steel sheet is punched, the amount of voids generated increases, leading to a decrease in hole expansibility. Therefore, the P content should be 0.001% or more and 0.100% or less.
- the P content is preferably 0.030% or less.
- S 0.0200% or less S exists as a sulfide in steel.
- the S content exceeds 0.0200%, the ultimate deformability of the steel sheet is lowered. Therefore, when the steel sheet is punched, the amount of voids generated increases, leading to a decrease in hole expansibility. Therefore, the S content should be 0.0200% or less.
- the S content is preferably 0.0080% or less. Although the lower limit of the S content is not specified, it is preferable that the S content is 0.0001% or more due to production technology restrictions.
- Al 0.010% to 2.000%
- Al suppresses the formation of carbide during annealing and promotes the formation of retained austenite. That is, Al is an element that affects the area ratio of retained austenite and the carbon concentration in retained austenite. In order to obtain such effects, the Al content is set to 0.010% or more. On the other hand, if the Al content exceeds 2.000%, the ferrite area ratio increases excessively, making it difficult to increase the TS to 980 MPa or more. In addition, a decrease in YS is also caused. Therefore, the content of Al is set to 0.010% or more and 2.000% or less. The Al content is preferably 0.015% or more. Also, the Al content is preferably 1.000% or less.
- N 0.0100% or less N exists as a nitride in steel.
- the N content exceeds 0.0100%, the ultimate deformability of the steel sheet is lowered. Therefore, when the steel sheet is punched, the amount of voids generated increases, leading to a decrease in hole expansibility. Therefore, the N content should be 0.0100% or less.
- the N content is preferably 0.0050% or less.
- the lower limit of the N content is not particularly specified, it is preferable that the N content is 0.0005% or more due to production technology restrictions.
- Carbon Equivalent Ceq 0.540% or More Carbon equivalent Ceq affects TS. In particular, when the carbon equivalent Ceq is less than 0.540%, it becomes difficult to increase the TS to 980 MPa or more. Therefore, the carbon equivalent Ceq is set to 0.540% or more.
- the basic components of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention have been described above.
- the balance has a component composition containing Fe (iron) and unavoidable impurities.
- the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention preferably has a chemical composition containing the above-mentioned basic ingredients and the balance being Fe and unavoidable impurities.
- the substrate steel sheet of the galvanized steel sheet according to one embodiment of the present invention may contain at least one selected from the following optional ingredients in addition to the basic ingredients described above. In addition, since the effect of this invention is acquired if it contains the optional component shown below below the upper limit shown below, a lower limit in particular is not set.
- the optional components are included as unavoidable impurities.
- Ti 0.200% or less Ti increases TS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. In order to obtain such effects, it is preferable to set the Ti content to 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, when the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.
- Nb 0.200% or less
- the Nb content is preferably 0.001% or more.
- the Nb content is more preferably 0.005% or more.
- the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less.
- the Nb content is more preferably 0.060% or less.
- V 0.100% or less
- V raises TS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing.
- the V content is preferably 0.001% or more.
- the V content is more preferably 0.005% or more.
- the V content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when V is contained, the V content is preferably 0.100% or less.
- the V content is more preferably 0.060% or less.
- B 0.0100% or less
- B is an element that increases the hardenability by segregating at the austenite grain boundary.
- B is an element that suppresses the formation of ferrite and grain growth during cooling after annealing.
- the B content is preferably 0.0001% or more.
- the B content is more preferably 0.0002% or more.
- the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling, which may reduce the ultimate deformability of the steel sheet.
- the B content is preferably 0.0100% or less.
- the B content is more preferably 0.0050% or less.
- Cu 1.000% or less
- Cu is an element that enhances hardenability.
- Cu is an element effective for adjusting the area ratio of hard fresh martensite and the like to a more suitable range, thereby adjusting TS to a more suitable range.
- the Cu content is preferably 0.005% or more.
- Cu content is more preferably 0.020% or more.
- the area ratio of fresh martensite increases excessively, resulting in excessively high TS. Also, a large amount of coarse precipitates and inclusions may be generated.
- the Cu content is preferably 1.000% or less.
- the Cu content is more preferably 0.200% or less.
- Cr 1.000% or less
- Cr is an element that enhances hardenability
- Cr is an element that is effective for generating retained austenite and fresh martensite.
- the Cr content is preferably 0.0005% or more.
- the Cr content is more preferably 0.010% or more from the viewpoint of making TS in a more suitable range.
- the Cr content exceeds 1.000%, the area ratio of hard fresh martensite increases excessively, which may lead to a decrease in hole expansibility. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less.
- the Cr content is more preferably 0.250% or less, and still more preferably 0.100% or less.
- Ni 1.000% or less
- Ni is an element that enhances hardenability. Further, Ni is an element effective for adjusting the area ratio of retained austenite and fresh martensite to a more suitable range, thereby adjusting TS to a more suitable range.
- the Ni content is preferably 0.005% or more.
- the Ni content is more preferably 0.020% or more.
- the Ni content exceeds 1.000%, the area ratio of fresh martensite may excessively increase, and ductility and dimensional accuracy during forming may deteriorate. Also, a large amount of coarse precipitates and inclusions may be generated.
- the Ni content is preferably 1.000% or less.
- the Ni content is more preferably 0.800% or less.
- Mo 0.500% or less
- Mo is an element that enhances hardenability. Moreover, Mo is an effective element for generating hard fresh martensite and the like. In order to obtain such effects, the Mo content is preferably 0.010% or more. Mo content is more preferably 0.030% or more. On the other hand, when the Mo content exceeds 0.500%, the area ratio of fresh martensite increases excessively, which may lead to a decrease in hole expansibility. Therefore, when Mo is contained, the Mo content is preferably 0.500% or less. The Mo content is more preferably 0.450% or less, still more preferably 0.400% or less.
- Sb 0.200% or less
- Sb is an element effective for suppressing the diffusion of C in the vicinity of the steel sheet surface during annealing and controlling the formation of a soft layer in the vicinity of the steel sheet surface.
- the Sb content is preferably 0.002% or more.
- the Sb content is more preferably 0.005% or more.
- the Sb content is preferably 0.200% or less.
- the Sb content is more preferably 0.020% or less.
- Sn 0.200% or less
- Sn is an element effective in suppressing the diffusion of C in the vicinity of the steel sheet surface during annealing and controlling the formation of a soft layer in the vicinity of the steel sheet surface.
- the Sn content is preferably 0.002% or more.
- the Sn content is more preferably 0.005% or more.
- the Sn content is preferably 0.200% or less.
- the Sn content is more preferably 0.020% or less.
- Ta 0.100% or less Ta, like Ti, Nb and V, raises TS by forming fine carbides, nitrides or carbonitrides during hot rolling and annealing.
- Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta) (C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This improves TS and also YS.
- the Ta content is preferably 0.100% or less.
- W 0.500% or less W is an effective element for improving hardenability and adjusting TS to a more suitable range.
- the W content is preferably 0.001% or more.
- the W content is more preferably 0.030% or more.
- the W content is preferably 0.500% or less.
- the W content is more preferably 0.450% or less, still more preferably 0.400% or less.
- Mg 0.0200% or less
- Mg is an element effective for making inclusions such as sulfides and oxides spherical and improving the ultimate deformability and hole expandability of the steel sheet.
- the Mg content is preferably 0.0001% or more.
- the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen exists in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less.
- Zn 0.0200% or less
- Zn is an element effective for making the shape of inclusions spherical and improving the ultimate deformability and hole expandability of the steel sheet.
- the Zn content is preferably 0.0010% or more.
- the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen exists in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less.
- Co 0.0200% or less
- Co is an element effective in making inclusions spherical and improving the ultimate deformability and hole expandability of the steel sheet.
- the Co content is preferably 0.0010% or more.
- the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Co is contained, the Co content is preferably 0.0200% or less.
- Zr 0.0200% or less
- Zr is an element effective in making inclusions spherical and improving the ultimate deformability and hole expandability of the steel sheet.
- the Zr content is preferably 0.0010% or more.
- the Zr content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Zr is contained, the Zr content is preferably 0.0200% or less.
- Ca 0.0200% or less
- Ca exists as inclusions in steel.
- the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be generated. In such a case, if diffusible hydrogen exists in the steel sheet, the coarse inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less.
- the Ca content is preferably 0.0020% or less.
- the lower limit of the Ca content is not particularly limited, the Ca content is preferably 0.0005% or more. Moreover, from the restrictions on production technology, 0.0010% or more of Ca content is more preferable.
- the contents of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are each preferably 0.0001% or more.
- the contents of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM each exceed 0.0200%, a large amount of coarse precipitates and inclusions may be generated. be.
- coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when at least one of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM is included, the content thereof is preferably 0.0200% or less.
- the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention is in % by mass, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, and a carbon equivalent Ceq of 0.540% or more, optionally, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.500% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Mg
- the steel structure of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention is Ferrite area ratio: 65.0% or less (including 0%), Area ratio of bainitic ferrite: 5.0% or more and 40.0% or less, Area ratio of tempered martensite: 0.5% or more and 80.0% or less, Area ratio of retained austenite: 3.0% or more, Area ratio of fresh martensite: 20.0% or less (including 0%), S BF + STM + 2 x SMA : 65.0% or more, A steel structure in which SMA1/SMA: 0.80 or less and SMA2 / SMA : 0.20 or more.
- S BF Area ratio of the bainitic ferrite
- STM Area ratio of the tempered martensite
- SMA Area ratio of the hard second phase composed of the retained austenite and the fresh martensite
- the hard second phase Among the constituent island regions, the total area ratio of the island regions having an equivalent circle diameter of 2.0 ⁇ m or more and having 20% or less of the circumference contacting the tempered martensite SMA2 : The hard second phase Of the constituent island regions, 1% or more of the circumference is the total area ratio of the island regions in contact with the bainitic ferrite.
- the area ratio of ferrite is set to 65.0% or less.
- the area ratio of ferrite is preferably 35.0% or less, more preferably 25.0% or less.
- the lower limit of the area ratio of ferrite is not particularly limited, and may be 0%.
- the area ratio of ferrite is preferably 5.0% or more.
- Area ratio of bainitic ferrite 5.0% or more and 40.0% or less
- Bainitic ferrite has intermediate hardness between soft ferrite and hard fresh martensite, etc., and ensures good hole expandability. is an important phase for Bainitic ferrite is also a useful phase for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite. Therefore, the area ratio of bainitic ferrite is set to 5.0% or more. Also, the area ratio of bainitic ferrite is preferably 10.0% or more. On the other hand, if the area ratio of bainitic ferrite is excessively increased, the hole expansibility is rather lowered. Therefore, the area ratio of bainitic ferrite is set to 40.0% or less. Also, the area ratio of bainitic ferrite is preferably 35.0% or less.
- the area ratio of tempered martensite 0.5% or more and 80.0% or less important phase. Therefore, the area ratio of tempered martensite is set to 0.5% or more.
- the area ratio of tempered martensite is preferably 40.0% or more.
- the area ratio of tempered martensite is set to 80.0% or less. Further, the area ratio of tempered martensite is preferably 75.0% or less.
- the area ratio of retained austenite is made 3.0% or more.
- the area ratio of retained austenite is preferably 5.0% or more.
- the upper limit of the area ratio of retained austenite is not particularly limited, the area ratio of retained austenite is preferably 20.0% or less.
- Area ratio of fresh martensite 20.0% or less (including 0%)
- the area ratio of fresh martensite is set to 20.0% or less from the viewpoint of ensuring good hole expandability.
- the lower limit of the area ratio of fresh martensite is not particularly limited, and may be 0%.
- the area ratio of fresh martensite is preferably 3.0% or more.
- Fresh martensite is martensite as quenched (not tempered).
- the area ratio of the residual structure other than the above is preferably 10.0% or less.
- the area ratio of the residual tissue is more preferably 5.0% or less.
- the area ratio of the residual tissue may be 0%.
- the residual structure is not particularly limited, and examples thereof include carbides such as lower bainite, pearlite, and cementite. The type of residual tissue can be confirmed, for example, by observation using a SEM (Scanning Electron Microscope).
- the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard second phase are measured at the 1/4 thickness position of the base steel sheet as follows. That is, a sample is cut out from the base steel plate so that the plate thickness cross-section parallel to the rolling direction of the base steel plate becomes the observation surface. Next, the observation surface of the sample is mirror-polished using diamond paste. Then, the observation surface of the sample was subjected to final polishing using colloidal silica, and then subjected to 3 vol. Etch with % nital to reveal the tissue.
- Ferrite A black region with a blocky shape. In addition, it contains almost no iron-based carbides. However, when iron-based carbide is included, the area of ferrite is also included in the area of ferrite. The same applies to bainitic ferrite and tempered martensite, which will be described later.
- Bainitic ferrite A black to dark gray region with a massive or irregular shape. In addition, it does not contain iron-based carbides or contains a relatively small number of iron-based carbides.
- Tempered martensite A gray area with an amorphous shape. In addition, it contains a relatively large number of iron-based carbides.
- Hard second phase restored austenite + fresh martensite
- the austenite structure in the annealing process can be reproduced from these structures and confirmed. Such points serve as judgment materials for tissue identification.
- the fact that the C concentration and Mn concentration differ depending on the tissue serves as a criterion for identifying the tissue.
- the C concentration of ferrite and bainitic ferrite is lower than the C concentration of a region mainly composed of tempered martensite (including fine hard secondary phases, carbides, etc.).
- the Mn concentration may be lower than in other tissues.
- the difference in hardness depending on the structure is a criterion for identifying the structure.
- ferrite has the lowest hardness and hard secondary phase has the highest hardness.
- Bainitic ferrite and tempered martensite also exhibit a hardness between that of ferrite and that of hard secondary phases.
- FIG. 1(A) is a part extracted from one visual field of the sample observation area (25.6 ⁇ m ⁇ 17.6 ⁇ m) for the above explanation.
- the area ratio of retained austenite is measured as follows. That is, after mechanically grinding the substrate steel plate to the 1/4 position of the plate thickness in the plate thickness direction (depth direction), chemical polishing with oxalic acid is performed to obtain an observation surface. Then, the observation surface is observed by the X-ray diffraction method. CoK ⁇ rays were used as the incident X-rays, and the diffraction intensity of the (200), (211) and (220) planes of bcc iron (200), (220) and (311) planes of fcc iron (austenite) were compared.
- the ratio of the diffraction intensity of each surface is obtained, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensity of each surface. Then, assuming that the retained austenite is three-dimensionally homogeneous, the volume ratio of the retained austenite is defined as the area ratio of the retained austenite.
- the area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite from the area ratio of the hard second phase obtained as described above.
- [Area ratio of fresh martensite (%)] [Area ratio of hard second phase (%)] - [Area ratio of retained austenite (%)]
- the area ratio of the residual structure is obtained by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of the hard second phase obtained as described above from 100%.
- Ask. [Area ratio of residual structure (%)] 100 - [Area ratio of ferrite (%)] - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Hard Second phase area ratio (%)]
- S BF + STM + 2 x SMA 65.0% or more From the viewpoint of ensuring a TS of 980 MPa or more, S BF + STM + 2 x SMA is set to 65.0% or more.
- the upper limit of S BF +S TM +2 ⁇ S MA is not particularly limited, but is preferably 130.0% or less.
- SMA area ratio of a hard second phase composed of retained austenite and fresh martensite.
- a hard second phase (hereinafter also referred to as MA ) composed of retained austenite and fresh martensite is composed of a plurality of island regions.
- an island region (hereinafter also referred to as MA1) having an equivalent circle diameter of 2.0 ⁇ m or more and having a circumference of 20% or less in contact with tempered martensite (hereinafter also referred to as MA1) has a solid solution C Low concentration.
- the stability of retained austenite contained in MA1 is low. Therefore, MA1 does not contribute to ensuring good ductility.
- MA1 since MA1 has a high ratio of fresh martensite, MA1 reduces the hole expandability.
- S MA1 /S MA which is the ratio of the area ratio of MA1 to the area ratio of the hard second phase, is set to 0.80 or less.
- S MA1 /S MA is preferably 0.75 or less, more preferably 0.40 or less.
- S MA1 /S MA is preferably 0.50 or less, more preferably 0.30 or less.
- the lower limit of S MA1 /S MA is not particularly limited, and may be zero.
- Each island-shaped region is separated from other island-shaped regions of the second hard phase by a phase other than the hard second phase (individual island-shaped regions are phase). Further, the specific shape of each island-shaped region is not particularly limited, and may be, for example, circular, elliptical, polygonal, or amoeba-shaped (a shape extending in a plurality of irregular directions).
- SMA2 / SMA 0.20 or more Of the island-shaped regions constituting the hard second phase composed of retained austenite and fresh martensite, 1% or more of the island-shaped region in contact with the bainitic ferrite of the circumference (hereinafter referred to as Also called MA2.) has a high dissolved C concentration. In other words, the stability of retained austenite contained in MA2 is high. Therefore, MA2 plays a very important role in ensuring good work hardenability and ductility. That is, if bainitic ferrite is formed under appropriate conditions during cooling after annealing, solute C that diffuses from the bainitic ferrite into the surrounding untransformed austenite does not sufficiently diffuse into the untransformed austenite.
- S MA2 /S MA which is the ratio of the area ratio of MA2 to the area ratio of the hard second phase, should be 0.20 or more.
- SMA2 / SMA is preferably 0.25 or more, more preferably 0.30 or more.
- the upper limit of S MA2 /S MA is not particularly limited, and may be one. Moreover, from the viewpoint of ensuring high YS and excellent hole expansibility , SMA2 /SMA is preferably 0.98 or less when 980 MPa ⁇ TS ⁇ 1180 MPa is required. Moreover, when 1180 MPa ⁇ TS is required, S MA2 /S MA is preferably 0.70 or less.
- the steel structure of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention further has SMA3 / SMA of 0.05 or more.
- SMA3 Of the island-shaped regions constituting the hard second phase, 1% or more of the peripheral length is in contact with bainitic ferrite, and more than 20% of the peripheral length is in contact with tempered martensite. is the area ratio of
- SMA3 / SMA 0.05 or more Of the island-shaped regions that constitute the hard second phase composed of retained austenite and fresh martensite, 1% or more of the peripheral length is in contact with the bainitic ferrite, and 1% or more of the peripheral length
- An island-shaped region (hereinafter also referred to as MA3) in which more than 20% is in contact with tempered martensite has a particularly high dissolved C concentration among MA2. That is, in MA3, in addition to bainitic ferrite, solid solution C diffuses from tempered martensite, so the solid solution C concentration is particularly high. Therefore, MA3 particularly effectively contributes to ensuring good work hardening ability and ductility.
- S MA3 /S MA which is the ratio of the area ratio of MA3 to the area ratio of the hard second phase, is preferably 0.05 or more.
- SMA3 / SMA is preferably 0.07 or more, more preferably 0.10 or more.
- the upper limit of S MA3 /S MA is not particularly limited, and may be one. Also, S MA3 /S MA is preferably 0.70 or less.
- S MA1 , S MA2 and S MA3 are each measured as follows. That is, in the manner described above, ferrite, bainitic ferrite, tempered martensite and hard second phase ( (retained austenite + fresh martensite). Next, after color-coding (quaternary imaging) using Adobe Photoshop from Adobe Systems, the island-shaped regions of the hard second phase were extracted, and the equivalent circle diameter of each island-shaped region was measured using the open source ImageJ. , the perimeter of each island region, and the length of contact of each island region with bainitic ferrite and tempered martensite. Note that the pixel density of the tissue image when determining the circumference is set to 30 pixels/ ⁇ m or more and 100 pixels/ ⁇ m or less.
- each island-shaped area corresponds to MA1, MA2, or MA3, respectively, and color-coded using Adobe Photoshop of Adobe Systems (for example, FIG. 2 (B), FIG. 3 (B) , and FIG. 4(B)), each area is calculated. Then, the total area of each of the island-shaped regions identified as MA1, MA2, and MA3 was divided by the area of the observed region (25.6 ⁇ m ⁇ 17.6 ⁇ m) and multiplied by 100 to calculate the value (area ratio) for 5 fields of view. do. Then, the average values of the values (area ratios) for five fields of view for each of MA1, MA2 and MA3 are defined as S MA1 , S MA2 and S MA3 .
- the amount of diffusible hydrogen is 0.50 ppm by mass or less.
- Amount of diffusible hydrogen in substrate steel sheet 0.50 ppm by mass or less
- the amount of diffusible hydrogen in the substrate steel sheet is preferably 0.50 ppm by mass or less.
- the amount of diffusible hydrogen in the base steel sheet is more preferably 0.35 ppm by mass or less.
- the lower limit of the amount of diffusible hydrogen in the base steel sheet is not particularly specified, and may be 0 ppm by mass.
- the amount of diffusible hydrogen in the base steel sheet is more preferably 0.01 ppm by mass or more.
- the amount of diffusible hydrogen in the base steel sheet is measured as follows. Specifically, a test piece having a length of 30 mm and a width of 5 mm is taken from a galvanized steel sheet, and the galvanized layer is removed with an alkali. Then, the amount of hydrogen released from the test piece is measured by thermal desorption spectroscopy. Specifically, the test piece is continuously heated from room temperature to 300° C. at a heating rate of 200° C./h, and then cooled to room temperature. At this time, the amount of hydrogen released from the test piece (accumulated amount of hydrogen) is measured in the temperature range from room temperature to 210° C. during the continuous heating. Then, the measured amount of hydrogen is divided by the mass of the test piece (the test piece after removing the galvanized layer and before continuous heating), and the value converted to mass ppm is taken as the diffusible hydrogen amount of the base steel sheet.
- test pieces are cut out from the products placed in a general usage environment, and diffusible hydrogen is removed from the base steel sheet in the same manner as described above. If the amount is 0.50 mass ppm or less, the amount of diffusible hydrogen in the base steel sheet of the galvanized steel sheet at the raw material stage before forming or joining is also 0.50 mass ppm or less. can be regarded as
- the galvanized steel sheet according to one embodiment of the present invention preferably has a decarburized layer.
- the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention has a decarburized layer.
- Steel sheets containing Si, particularly plated steel sheets using a steel sheet with a high Si content as a base steel sheet may have a problem of cracking due to liquid metal embrittlement (LME) during resistance spot welding.
- LME liquid metal embrittlement
- the galvanized steel sheet has a decarburized layer particularly on the surface layer of the base steel sheet, resistance weld cracking resistance can be improved even when the Si content of the base steel sheet is high.
- the thickness of the decarburized layer in other words, the depth in the sheet thickness direction from the surface of the base steel sheet is preferably 30 ⁇ m or more, more preferably 40 ⁇ m or more.
- the thickness of the decarburized layer is preferably 130 ⁇ m or less in order to keep the tensile strength within a good range.
- the decarburized layer is defined as a region where the C concentration of the base steel sheet is analyzed from the surface of the base steel sheet in the plate thickness direction, and the C concentration is 80% or less of the C content in the chemical composition of the base steel sheet. is defined as the thickness of the region.
- the thickness of the decarburized layer is determined by surface analysis or line analysis of the elemental distribution near the surface layer of the base steel sheet using an electron probe micro analyzer (EPMA) for the cross-sectionally processed sample.
- EPMA electron probe micro analyzer
- a resin-embedded galvanized steel sheet is polished to finish a cross section perpendicular to the rolling direction for observation, and then removed from the resin to obtain a sample for measurement.
- the acceleration voltage is 7 kV
- the irradiation current is 50 nA
- the C strength is measured by performing area analysis or line analysis of the sample cross section in 1 ⁇ m steps in a range of 300 ⁇ 300 ⁇ m including the outermost layer (surface) of the base steel plate.
- plasma cleaners are used to remove hydrocarbons from the surface and surroundings of the sample in two places, the measurement room and the sample preparation room, before starting the measurement.
- the measurement is performed while the sample temperature is maintained at a maximum of 100° C. on the stage.
- the C intensity is converted to the C concentration (% by mass) using a calibration curve separately prepared by measuring a standard sample. Confirm that the C detection limit is sufficiently lower than 0.10% by mass due to the effect of suppressing contamination.
- the details of the equipment used and the method of contamination control are as described in reference 1 below.
- a line profile in the plate thickness direction is extracted from the surface of the base steel plate, and it is averaged for 300 points in the direction parallel to the base steel plate surface to obtain a profile of the C concentration in the plate thickness direction.
- the obtained C concentration profile in the plate thickness direction is smoothed by a simple moving average method. At this time, it is preferable to set the number of smoothing points to about 21 points.
- the thickness of the decarburized layer is determined by specifying the range in the plate thickness direction where the C concentration is 80% or less of the C content in the chemical composition of the base steel plate.
- the tensile strength of the galvanized steel sheet according to one embodiment of the present invention is 980 MPa or more.
- the tensile strength of the galvanized steel sheet according to one embodiment of the present invention is preferably 1180 MPa or more.
- TS tensile strength
- Yield stress Yield stress
- El total elongation
- n value work hardening index/yield ratio
- ⁇ limit hole expansion rate
- the galvanized layer of the galvanized steel sheet according to one embodiment of the present invention may be provided only on one surface of the base steel sheet, or may be provided on both surfaces.
- the galvanized layer refers to a galvanized layer containing Zn as a main component (Zn content is 50% or more), and examples thereof include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer.
- the hot-dip galvanized layer is preferably composed of, for example, Zn, 20% by mass or less of Fe, and 0.001% by mass or more and 1.0% by mass or less of Al.
- the hot-dip galvanized layer optionally includes one selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM.
- a total of 0 mass % or more and 3.5 mass % or less of the seed or two or more elements may be contained.
- the Fe content of the hot-dip galvanized layer is more preferably less than 7% by mass. The remainder other than the above elements is unavoidable impurities.
- the alloyed hot-dip galvanized layer is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al.
- the alloyed hot-dip galvanized layer is optionally selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM. 0% by mass or more and 3.5% by mass or less in total of one or two or more elements of
- the Fe content of the alloyed hot-dip galvanized layer is more preferably 7% by mass or more, and still more preferably 8% by mass or more. Further, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15% by mass or less, still more preferably 12% by mass or less. The remainder other than the above elements is unavoidable impurities.
- the coating weight per side of the galvanized layer is not particularly limited, it is preferably 20 to 80 g/m 2 .
- the plating adhesion amount of the galvanized layer is measured as follows. That is, a treatment liquid is prepared by adding 0.6 g of a corrosion inhibitor against Fe ("Ibit 700BK" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10% by mass hydrochloric acid aqueous solution. Then, a galvanized steel sheet as a test material is immersed in the treatment liquid to dissolve the galvanized layer. Then, the mass reduction amount of the test material before and after melting was measured, and the value was divided by the surface area of the base steel plate (the surface area of the part coated with plating) to obtain the coating amount (g / m 2 ) is calculated.
- a corrosion inhibitor against Fe (Ibit 700BK" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.)
- the galvanized steel sheet according to one embodiment of the present invention may have a metal plating layer other than the galvanized layer in at least one of the base steel sheet and the galvanized layer.
- the metal plating layer contributes to improvement of resistance weld crack resistance. Further, the formation of the metal plating layer can suppress resistance weld cracking even when the base steel sheet has a large Si content.
- the mechanism by which the resistance weld cracking resistance is improved by the metal plating layer is not necessarily clear, but the inventors have found that when the metal plating layer is provided between the base steel sheet and the galvanized layer, in other words, on the surface of the base steel sheet, It is thought that the metal plating layer acts as a barrier layer that prevents the zinc in the galvanizing layer from melting and penetrating into the base steel plate during resistance spot welding, making it difficult for resistance weld cracks to occur (zinc intrusion suppression effect).
- the galvanized layer is provided on both sides of the base steel sheet, only one of the layers between the base steel sheet and the galvanized layer may have a metal plating layer. may have a metal plating layer on both.
- the adhesion amount of the metal plating layer is preferably more than 0 g/m 2 , more preferably 2.0 g/m 2 or more.
- the upper limit of the amount of the metal plating layer deposited on one side is not particularly limited, from the viewpoint of cost, the amount of the metal plating layer deposited is preferably 60 g/m 2 or less.
- the adhesion amount of the metal plating layer is more preferably 50 g/m 2 or less, still more preferably 40 g/m 2 or less, and even more preferably 30 g/m 2 or less. It should be noted that the adhesion amount of the metal plating layer referred to here is per one side.
- the adhesion amount of the metal plating layer is measured as follows. That is, a 10 ⁇ 15 mm size sample is taken from a galvanized steel sheet and embedded in resin to obtain a cross-sectional embedded sample. Any three points of the cross section of the same sample are observed using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV at a magnification of 2000 to 10000 times depending on the thickness of the metal plating layer. The thickness of the metal plating layer is measured at various points, and the average value is calculated. Then, the calculated average value is multiplied by the specific gravity of the metal forming the metal plating layer to convert to the adhesion amount per one side of the metal plating layer.
- SEM scanning electron microscope
- a metal with a higher melting point than Zn is desirable, and for example, metals such as Fe and Ni can be used. Further, in addition to the effect of suppressing zinc penetration described above, the following effect of suppressing a decrease in toughness can be expected, so the Fe-based plating layer is desirable.
- the Fe-based plating layer when the amount of Si in the vicinity of the surface of the base steel sheet is large, it is thought that the toughness of the weld zone is lowered and the resistance weld cracking resistance of the weld zone is deteriorated.
- the Fe-based plating layer when the Fe-based plating layer is provided between the base steel sheet and the galvanized layer, that is, on the surface of the base steel sheet, the Fe-based plating layer acts as a solid solution Si depleted layer, and the amount of Si dissolved in the weld zone is reduced. do. It is believed that this suppresses a decrease in the toughness of the weld zone and improves the resistance weld cracking resistance of the weld zone (effect of suppressing a decrease in toughness).
- the Fe-based plating layer functions as a soft layer and relieves the stress applied to the steel sheet surface during resistance spot welding. As a result, it is believed that the residual stress in the weld zone can be reduced and the resistance weld crack resistance can be improved (stress relaxation effect).
- Fe-based plating layer in addition to a pure Fe plating layer, for example, Fe--B alloy, Fe--C alloy, Fe--P alloy, Fe--N alloy, Fe--O alloy, Fe--Ni alloy, Fe--Mn Alloy plating layers such as alloys, Fe--Mo alloys, and Fe--W alloys can be used.
- the component composition of the Fe-based plating layer is not particularly limited as long as the Fe content is 50% by mass or more. Contains 10% by mass or less in total of one or more elements selected from the group consisting of Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the balance is Fe and unavoidable impurities Component composition is preferred.
- the total content of these elements is 10% by mass or less to prevent a decrease in electrolysis efficiency and to form an Fe-based plating layer, particularly an Fe-based electroplating, at a low cost. Layers can be formed.
- the C content is preferably 0.08% by mass or less.
- the galvanized steel sheet according to one embodiment of the present invention may have a metal plating layer and a decarburized layer at the same time (that is, from the surface of the galvanized steel sheet, the galvanized layer, the metal plating layer, ( It becomes the decarburized layer) of the surface layer of the base steel plate).
- a metal plating layer the C concentration is analyzed in the sheet thickness direction from the surface of the metal plating layer or the interface between the galvanized layer and the cold-rolled steel sheet by the above method, and the thickness of the decarburized layer (underlying thickness direction depth from the surface of the steel sheet) may be evaluated.
- the plate thickness of the galvanized steel sheet according to one embodiment of the present invention is not particularly limited, it is preferably 0.5 mm or more and 3.0 mm or less.
- a member according to one embodiment of the present invention is a member (as a raw material) using the galvanized steel sheet described above.
- a galvanized steel sheet which is a raw material, is subjected to at least one of molding and joining to form a member.
- the galvanized steel sheet has a TS of 980 MPa or more, a high YS, and excellent ductility, work hardening ability and hole expandability. Therefore, the member according to one embodiment of the present invention has high strength and excellent impact resistance. Therefore, the member according to one embodiment of the present invention is particularly suitable for application as an impact energy absorbing member used in the automobile field.
- a method for manufacturing a galvanized steel sheet comprises: A hot-rolling step of hot-rolling a steel slab having the above chemical composition to form a hot-rolled steel sheet; A cold rolling step of cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet; An annealing step of annealing the cold-rolled steel sheet at an annealing temperature of 760° C. or more and 900° C. or less and an annealing time of 20 seconds or more; a first cooling step of cooling the cold-rolled steel sheet to a first cooling stop temperature of 300° C. or higher and 550° C. or lower; a holding step of holding the cold-rolled steel sheet in a temperature range of 300° C.
- the galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower, and the galvanized steel sheet is heated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower.
- a reheating step of holding in the temperature range for 10 seconds or more and 2000 seconds or less; has The first cooling stop temperature and the temperature of the zinc plating bath in the zinc plating process satisfy the relationship of the following formula (1). ⁇ 150° C. ⁇ T 0 ⁇ T 1 ⁇ 50° C. (1)
- T0 is the first cooling stop temperature (°C)
- T1 is the temperature (°C) of the zinc plating bath in the zinc plating treatment. Note that each of the above temperatures means the surface temperature of the steel slab and steel plate, unless otherwise specified.
- a steel slab having the above chemical composition For example, a steel material is melted to obtain molten steel having the above chemical composition.
- the smelting method is not particularly limited, and known smelting methods such as converter smelting and electric furnace smelting can be used.
- the resulting molten steel is then solidified into a steel slab.
- the method of obtaining a steel slab from molten steel is not particularly limited, and for example, a continuous casting method, an ingot casting method, a thin slab casting method, or the like can be used.
- a continuous casting method is preferable from the viewpoint of preventing macro segregation.
- Hot rolling process Then, the steel slab is hot rolled to obtain a hot rolled steel sheet.
- Hot rolling may be performed by applying an energy saving process.
- Energy-saving processes include direct rolling (a method in which steel slabs are not cooled to room temperature, but are charged into a heating furnace and hot rolled) or direct rolling (a steel slab is slightly heat-retained). a method of rolling immediately afterwards).
- the hot rolling conditions are not particularly limited, and the hot rolling can be performed under the following conditions, for example. That is, the steel slab is once cooled to room temperature, then reheated and then rolled.
- the slab heating temperature (reheating temperature) is preferably 1100° C. or higher from the viewpoint of dissolving carbides and reducing the rolling load.
- the slab heating temperature is preferably 1300° C. or less.
- the slab heating temperature is based on the surface temperature of the steel slab.
- the steel slab is subjected to rough rolling according to a conventional method to obtain a rough rolled plate (hereinafter also referred to as sheet bar).
- the sheet bar is subjected to finish rolling to obtain a hot-rolled steel sheet.
- the slab heating temperature is lowered, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling from the viewpoint of preventing troubles during finish rolling.
- the finish rolling temperature is preferably the Ar 3 transformation point or higher.
- Ar 3 transformation point is obtained by the following formula.
- the sheet bars may be joined together during hot rolling, and finish rolling may be performed continuously. Also, the sheet bar may be wound once before finishing rolling. In order to reduce the rolling load during hot rolling, part or all of the finish rolling may be lubricated rolling. Performing lubricating rolling is also effective from the viewpoint of homogenizing the shape of the steel sheet and homogenizing the quality of the steel sheet.
- the coefficient of friction during lubricating rolling is preferably in the range of 0.10 or more and 0.25 or less.
- hot rolling processes including rough rolling and finish rolling generally steel slabs are rough rolled into sheet bars and finish rolled into hot rolled steel sheets. However, depending on the mill capacity, there is no problem as long as the predetermined size is achieved regardless of such classification.
- the finish rolling temperature is preferably in the range of 800°C or higher and 950°C or higher.
- the winding temperature is preferably 450° C. or higher and 750° C. or lower.
- the hot-rolled steel sheet after the hot-rolling process is pickled.
- pickling oxides on the surface of the steel sheet can be removed, and good chemical conversion treatability and plating quality are ensured.
- pickling may be performed only once, and may be divided into several times and may be performed.
- the pickling conditions are not particularly limited, and conventional methods may be followed.
- Cold rolling process Then, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
- Cold rolling is performed, for example, by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.
- the rolling reduction in cold rolling is not particularly limited, it is preferably 20% or more and 80% or less. If the rolling reduction in cold rolling is less than 20%, the steel structure tends to become coarse and non-uniform in the annealing process, and the strength and workability of the final product may deteriorate. On the other hand, if the rolling reduction in cold rolling exceeds 80%, the shape of the steel sheet tends to be defective, and the amount of galvanized coating may become non-uniform.
- the cold-rolled steel sheet obtained after cold rolling may also be pickled.
- At least one of the cold-rolled steel sheets obtained as described above is optionally added after the cold rolling step and before the annealing step described later.
- a metal plating treatment may be applied to form a metal plating layer on the surface of the .
- a cold-rolled steel sheet having a metal-plated layer on at least one surface before undergoing an annealing step, which will be described later may be hereinafter referred to as a metal-plated steel sheet.
- the metal plating method is not particularly limited, but electroplating is preferable from the viewpoint of manufacturability.
- the metal plating bath a sulfuric acid bath, a hydrochloric acid bath, or a mixed solution of both can be used.
- the adhesion amount of the metal plating layer can be adjusted by the energization time or the like.
- the metal-plated steel sheet means a steel sheet having a metal-plated layer on at least one surface of the cold-rolled steel sheet before undergoing the annealing process described later. It does not exclude the aspect in which the steel sheet is pre-annealed.
- a metal used for metal plating a metal with a higher melting point than Zn is desirable, and for example, metals such as Fe and Ni can be used.
- metals such as Fe and Ni can be used.
- the plating bath for forming the Fe-based plating layer contains B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and One or two or more elements selected from the group consisting of Co can be contained.
- the total content of these elements in the plating bath is preferably 10% by mass or less in the chemical composition of the metal plating layer of the metal plated steel sheet.
- Metal elements may be contained as metal ions, and non-metal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acids, and the like.
- the iron sulfate plating solution may contain conductivity aids such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.
- degreasing treatment and water washing for cleaning the surface of the cold-rolled steel sheet As a pretreatment before the metal plating treatment, optionally, degreasing treatment and water washing for cleaning the surface of the cold-rolled steel sheet, further pickling treatment for activating the surface of the cold-rolled steel sheet and It may be washed with water. Following these pretreatments, the metal plating treatment described above is performed.
- Methods of degreasing treatment and washing with water are not particularly limited, and ordinary methods can be used.
- Various acids such as sulfuric acid, hydrochloric acid, nitric acid and mixtures thereof can be used in the pickling process. Among them, sulfuric acid, hydrochloric acid or mixtures thereof are preferred.
- the concentration of the acid is not particularly specified, it is preferably about 1 to 20% by mass in consideration of the ability to remove the oxide film and the prevention of surface roughness (surface defects) due to over-acid washing.
- the pickling treatment liquid may contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, and the like.
- the cold-rolled steel sheets (including metal plated steel sheets) obtained as described above are annealed at an annealing temperature of 760° C. or more and 900° C. or less for an annealing time of 20 seconds or more.
- the number of annealing times may be two or more, but one time is preferable from the viewpoint of energy efficiency.
- Annealing temperature 760° C. or higher and 900° C. or lower If the annealing temperature is lower than 760° C., the rate of austenite formation during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite excessively increases after annealing, and YS decreases. In addition, the hole expansibility is also lowered. Furthermore, it becomes difficult to make TS 980 MPa or more. On the other hand, if the annealing temperature exceeds 900° C., the grain growth of austenite occurs excessively, and the formation rate of bainitic ferrite in the post-process slows down. This makes it impossible to obtain appropriate amounts of bainitic ferrite and retained austenite area ratios.
- the annealing temperature should be 760° C. or higher and 900° C. or lower.
- the annealing temperature is preferably 780°C or higher, more preferably above 790°C.
- the annealing temperature is preferably 880° C. or lower.
- the annealing temperature is the highest temperature reached in the annealing process.
- Annealing time 20 seconds or more If the annealing time is less than 20 seconds, the rate of austenite formation during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite excessively increases after annealing, and YS decreases. In addition, the hole expansibility is also lowered. Furthermore, it becomes difficult to set TS to 980 MPa or more. Therefore, the annealing time should be 20 seconds or more. Although the upper limit of the annealing time is not particularly limited, it is preferably 900 seconds or less. The annealing time is the holding time in the temperature range of (annealing temperature -40°C) to the annealing temperature.
- the annealing time includes, in addition to the holding time at the annealing temperature, the holding time in the temperature range above (annealing temperature -40°C) and below the annealing temperature in heating and cooling before and after reaching the annealing temperature.
- the dew point of the annealing atmosphere in the annealing step is preferably over -30°C.
- the dew point is preferably -20°C or higher, more preferably -5°C or higher.
- the upper limit of the dew point is not particularly limited, but from the viewpoint of suitably preventing oxidation of the surface of the cold-rolled steel sheet or metal plating layer and improving the plating adhesion when providing the zinc plating layer, the dew point is preferably 30 ° C. or less. preferable.
- First cooling stop temperature 300° C. or higher and 550° C. or lower
- the area ratio of tempered martensite increases excessively, and the area ratio of appropriate amounts of bainitic ferrite and retained austenite decreases. will not be obtained.
- untransformed austenite may decompose into pearlite and carbides in the subsequent galvanizing process. Therefore, SMA2 / SMA and SMA3 / SMA are decreased, and ductility and work hardening ability are decreased.
- the first cooling stop temperature exceeds 550°C, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite excessively increases.
- the first cooling stop temperature is set to 300°C or higher and 550°C or lower.
- the first cooling stop temperature is preferably 350° C. or higher.
- the first cooling stop temperature is preferably 510° C. or lower.
- the cold-rolled steel sheet is held in a temperature range of 300° C. or higher and 550° C. or lower (hereinafter also referred to as a holding temperature range) for 3 seconds or longer and 600 seconds or shorter.
- Holding time in the holding temperature range 3 seconds to 600 seconds
- bainitic ferrite is generated, and C diffuses from the generated bainitic ferrite to untransformed austenite adjacent to the bainitic ferrite. occurs.
- a predetermined amount of area ratio of retained austenite is secured, and SMA2 / SMA and further SMA3 / SMA are increased.
- the holding time in the holding temperature range is less than 3 seconds
- the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite excessively increases.
- SMA2 / SMA and further SMA3 / SMA decrease, and ductility and work hardening ability decrease.
- the holding time in the holding temperature range exceeds 600 seconds, the area ratio of bainitic ferrite may excessively increase, resulting in a decrease in YS.
- C may excessively diffuse from bainitic ferrite to untransformed austenite, resulting in an increase in SMA1 / SMA and a decrease in hole expansibility.
- C diffuses excessively inside the untransformed austenite, and it becomes impossible to make only the untransformed austenite around the bainitic ferrite locally high in solid solution C amount.
- S MA2 /S MA and further S MA3 /S MA may decrease and ductility may decrease. Therefore, the holding time in the holding temperature range is 3 seconds or more and 600 seconds or less.
- the holding time in the holding temperature range is preferably 5 seconds or longer, more preferably 10 seconds or longer. Also, the holding time in the holding temperature range is preferably less than 200 seconds, more preferably less than 80 seconds.
- the retention time in the retention temperature range includes the retention time in the temperature range until the first cooling stop temperature is reached in the first cooling step, and the cooling time until the start of zinc plating treatment in the plating step described later. It includes the residence time of the rolled steel sheet in the relevant temperature range (for example, the residence time in the relevant temperature range until the cold-rolled steel sheet is immersed in the galvanizing bath). However, the holding time in the holding temperature range does not include the residence time of the galvanized steel sheet after hot-dip galvanizing in the plating process.
- the cold-rolled steel sheet is subjected to galvanizing treatment to obtain a galvanized steel sheet.
- galvanizing include hot dip galvanizing and alloyed galvanizing.
- the first cooling stop temperature in the first cooling step described above and the temperature of the zinc plating bath in the zinc plating treatment (hereinafter also referred to as the plating bath temperature) have the following relationship: It is necessary to satisfy ⁇ 150° C. ⁇ T 0 ⁇ T 1 ⁇ 50° C. (1)
- T0 is the first cooling stop temperature (°C)
- T1 is the temperature (°C) of the zinc plating bath in the zinc plating treatment.
- T 0 ⁇ T 1 exceeds 50° C. or is less than ⁇ 150° C.
- SMA2 /S MA and SMA3 /S MA decrease, and work hardening ability and ductility decrease.
- T 0 ⁇ T 1 is preferably ⁇ 120° C. or higher, more preferably ⁇ 100° C. or higher.
- T 0 ⁇ T 1 is preferably 45° C. or less, more preferably 40° C. or less.
- Conditions other than the above are not particularly limited, and may be performed in accordance with conventional methods.
- the plating bath temperature is 440° C. or higher and 500° C. or lower.
- the zinc plating bath is not particularly limited as long as it has the composition of the zinc plating layer described above, but for example, the Al content is 0.10% by mass or more and 0.23% by mass or less, and the balance is It is preferable to use a plating bath with a composition consisting of Zn and unavoidable impurities.
- the galvanized steel sheet it is preferable to heat the galvanized steel sheet to an alloying temperature of 450° C. or higher and 600° C. or lower after hot-dip galvanizing as described above. If the alloying temperature is lower than 450° C., the Zn—Fe alloying speed becomes slow and alloying may become difficult. On the other hand, if the alloying temperature exceeds 600° C., untransformed austenite may transform into pearlite, resulting in a decrease in TS and ductility.
- the alloying temperature is more preferably 470° C. or higher. Also, the alloying temperature is more preferably 570° C. or lower.
- the coating weight of both hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA) is 20 to 80 g/m 2 per side.
- the amount of plating deposited can be adjusted by gas wiping or the like.
- the galvanized steel sheet is held in a temperature range of 300 ° C. or higher and 550 ° C. or lower (hereinafter also referred to as an additional holding temperature range) for 3 seconds or more.
- An additional holding step of holding for 600 seconds or less may be performed.
- the additional holding step is a step for obtaining the same effect as the holding step.
- the additional holding step may be performed after or during the plating step as long as it is before the second cooling step described later.
- the additional holding process may be performed during the plating process. That is, the plating process may serve as the additional holding process.
- the total holding time of the holding step and the additional holding step is preferably 3 seconds or more and 600 seconds or less. More preferably, the total holding time of the holding step and the additional holding step is less than 200 seconds.
- Second cooling stop temperature 100 ° C. or more and less than 300 ° C.
- the second cooling step is performed in order to control the area ratio of tempered martensite and the area ratio of retained austenite generated in the reheating step, which is a subsequent step, within a predetermined range. This is a necessary process.
- the second cooling stop temperature is less than 100°C, substantially all of the untransformed austenite present in the steel is transformed into martensite in the second cooling step. This excessively increases the area ratio of tempered martensite and decreases the area ratio of retained austenite. As a result, ductility and work hardenability are reduced.
- the second cooling stop temperature is 300° C.
- the second cooling stop temperature is set at 100°C or higher and lower than 300°C.
- the second cooling stop temperature is preferably 120°C or higher.
- the second cooling stop temperature is preferably 280° C. or less.
- the galvanized steel sheet is reheated to a reheating temperature of (said second cooling stop temperature + 50°C) or more and 500°C or less, and the galvanized steel sheet is heated to (said second cooling stop temperature + 50°C) or more and 500°C or less. (hereinafter also referred to as reheating temperature range) for 10 seconds or more and 2000 seconds or less.
- reheating temperature range 10 seconds or more and 2000 seconds or less.
- Reheating temperature (second cooling stop temperature + 50 ° C) or more and 500 ° C or less
- the reheating temperature is less than (cooling stop temperature + 50 ° C)
- the martensite present in the steel at the end of the second cooling process is not transformed. Diffusion of C into austenite does not proceed sufficiently, and a predetermined amount of area ratio of retained austenite cannot be obtained. This reduces ductility. Moreover, fresh martensite increases. Furthermore, the release of hydrogen contained in the base steel plate to the outside becomes insufficient, and the amount of diffusible hydrogen in the base steel plate increases. As a result, the hole expansibility is lowered.
- the reheating temperature should be (cooling stop temperature + 50°C) or more and 500°C or less.
- the reheating temperature is preferably (cooling stop temperature + 70°C) or higher.
- the reheating temperature is preferably 450° C. or lower. Note that the reheating temperature is the highest temperature reached in the reheating process.
- Holding time in the reheating temperature range 10 seconds or more and 2000 seconds or less
- the holding time in the reheating temperature range is less than 10 seconds
- the martensite present in the steel at the end of the second cooling process transforms into untransformed austenite. Diffusion of C does not proceed sufficiently, and a predetermined amount of area ratio of retained austenite cannot be obtained. This reduces ductility.
- the release of hydrogen contained in the base steel sheet to the outside becomes insufficient, and the amount of diffusible hydrogen in the base steel sheet increases. As a result, there is a possibility that the hole expansibility may be deteriorated.
- the holding time in the reheating temperature range is set to 10 seconds or more and 2000 seconds or less.
- the holding time in the reheating temperature range is preferably 15 seconds or longer. Further, the holding time in the reheating temperature range is preferably 1200 seconds or less.
- the retention time in the reheating temperature range includes not only the retention time at the reheating temperature but also the retention time in the temperature range during heating and cooling before and after reaching the reheating temperature.
- the cooling conditions after holding in the reheating temperature range are not particularly limited, and may be in accordance with the usual method.
- Examples of cooling methods that can be applied include gas jet cooling, mist cooling, roll cooling, water cooling, and air cooling.
- the average cooling rate in cooling after holding in the reheating temperature range is preferably 1° C./second or more and 50° C./second or less, for example.
- the galvanized steel sheet obtained as described above may be further subjected to temper rolling. If the rolling reduction of the temper rolling exceeds 2.00%, the yield stress increases, and there is a risk that the dimensional accuracy when forming the galvanized steel sheet into a member decreases. Therefore, the rolling reduction of temper rolling is preferably 2.00% or less. Although the lower limit of the rolling reduction in temper rolling is not particularly limited, it is preferably 0.05% or more from the viewpoint of productivity.
- the temper rolling may be performed on an apparatus continuous with the annealing apparatus for performing each process described above (online), or on an apparatus discontinuous from the annealing apparatus for performing each process (offline). you can go Also, the number of times of temper rolling may be one or two or more. Note that rolling by a leveler or the like may be used as long as the same elongation rate as that of temper rolling can be imparted.
- a method for manufacturing a member according to an embodiment of the present invention includes subjecting the galvanized steel sheet (for example, a galvanized steel sheet manufactured by the method for manufacturing a galvanized steel sheet) to at least one of forming and joining. It has a step of forming a member.
- the molding method is not particularly limited, and for example, a general processing method such as press working can be used.
- the joining method is not particularly limited, and for example, general welding such as spot welding, laser welding, arc welding, riveting, caulking, or the like can be used.
- the molding conditions and bonding conditions are not particularly limited, and conventional methods may be followed.
- Example 1 A steel material having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting. The obtained steel slab was heated to 1250° C. After heating, the steel slab was subjected to hot rolling including rough rolling and finish rolling to obtain a hot rolled steel sheet. Then, the obtained hot-rolled steel sheets were pickled and cold-rolled (rolling reduction: 50%) to obtain cold-rolled steel sheets having thicknesses shown in Table 3.
- the obtained cold-rolled steel sheets were subjected to an annealing process, a first cooling process, a holding process, a plating process, a second cooling process and a reheating process under the conditions shown in Table 2 to obtain galvanized steel sheets.
- the dew point in the annealing process was -35°C to -30°C.
- hot-dip galvanizing treatment or alloying galvanizing treatment was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or an alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA).
- GI hot-dip galvanized steel sheet
- GA alloyed hot-dip galvanized steel sheet
- Table 2 the types of plating processes are also indicated as "GI” and "GA”. It should be noted that when the alloyed zinc plating treatment is performed, No. Except for Nos. 20, 27 and 28, the holding time in the holding temperature range and the holding time in the temperature range of 300° C. to 550° C. in the alloying treatment were set to be 3 seconds to 600 seconds in total.
- GI zinc plating bath
- GA a plating bath having a composition containing 0.20% by mass of Al and the balance being Zn and unavoidable impurities
- GA a plating bath containing 0.14% by mass of Al with the balance being Zn and unavoidable impurities was used.
- the amount of plating deposited was 45 to 72 g/m 2 per side when manufacturing GI, and 45 g/m 2 per side when manufacturing GA.
- the composition of the galvanized layer of the galvanized steel sheet finally obtained is, in GI, Fe: 0.1 to 1.0% by mass, Al: 0.2 to 1.0% by mass, and the balance was Zn and unavoidable impurities.
- GA contained 7 to 15% by mass of Fe, 0.1 to 1.0% by mass of Al, and the balance was Zn and unavoidable impurities. All galvanized layers were formed on both sides of the base steel plate.
- a tensile test and a hole expansion test are performed according to the following procedures, and according to the following criteria, tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR) and critical hole expansion ratio ( ⁇ ) were evaluated.
- the hole expanding test was performed according to JIS Z 2256. That is, from the obtained galvanized steel sheet, a test piece of 100 mm x 100 mm was cut by shearing. A 10 mm diameter hole was punched in the specimen with a clearance of 12.5%. Then, using a die with an inner diameter of 75 mm, a wrinkle holding force of 9 tons (88.26 kN) is applied around the hole, and a conical punch with an apex angle of 60° is pushed into the hole to reach the crack initiation limit (crack initiation The diameter of the hole in the test piece was measured. Then, the limit hole expansion rate: ⁇ (%) was obtained from the following equation. ⁇ is an index for evaluating stretch flangeability.
- the member obtained by molding or the member obtained by bonding has tensile strength (TS), yield stress (YS), total elongation (El), working It was found that both the hardening index (n value)/yield ratio (YR) and the critical hole expansion ratio ( ⁇ ) have the excellent properties characteristic of the present invention.
- Example 2 A steel material having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting. The obtained steel slab was heated to 1250° C. After heating, the steel slab was subjected to hot rolling including rough rolling and finish rolling to obtain a hot rolled steel sheet. Then, the obtained hot-rolled steel sheet was pickled and cold-rolled (rolling reduction: 50%) to obtain a cold-rolled steel sheet having a thickness of 1.6 mm.
- Table 1 the balance being Fe and unavoidable impurities
- Fe-based electroplating was performed as metal plating treatment to form a metal plating layer (Fe-based plating layer) on the surface of the cold-rolled steel sheet.
- the cold-rolled steel sheet was degreased with an alkali. Then, under the conditions shown below, the cold-rolled steel sheet was used as a cathode and electrolytic treatment was performed to form a metal plating layer on the surface of the cold-rolled steel sheet.
- the obtained cold-rolled steel sheets (including metal-plated steel sheets in which a metal plating layer is formed on the surface of the cold-rolled steel sheet) are subjected to an annealing step, a first cooling step, a holding step, a plating step, A second cooling step and a reheating step were performed to obtain a galvanized steel sheet.
- an alloyed galvanized steel sheet (GA) was obtained by performing an alloyed galvanized steel sheet.
- the treatment conditions other than those described in Table 5 are the same as in Example 1. All galvanized layers were formed on both sides of the base steel plate.
- the steel structure of the base steel sheet was identified, and the thickness of the decarburized layer, the amount of adhesion of the metal plating layer, and the amount of diffusible hydrogen were measured according to the procedures described above.
- Table 6 shows the results.
- F is ferrite
- BF bainitic ferrite
- TM tempered martensite
- RA retained austenite
- FM is fresh martensite
- LB lower bainite
- P pearlite
- ⁇ cementite.
- "-" for the thickness of the decarburized layer and the amount of the metal plating layer respectively means that the decarburized layer and the metal plating layer are not provided.
- Example 7 tensile strength/yield ratio (YR) and critical hole expansion ratio ( ⁇ ) were evaluated. The results are also shown in Table 7.
- the hot-dip alloyed galvannealed steel sheet 1 for testing had a coating amount of 50 g/m 2 per side of the galvannealed layer, and was cut into the same size as the test piece 2 .
- the plate assembly consists of the evaluation target surface of the test piece 2 (when the galvanized layer and the metal coated layer are only on one side, the galvanized layer on that side) and the zinc of the alloyed hot-dip galvanized steel sheet 1 for test It was assembled so that the plated layers faced each other.
- the plate assembly was fixed to a fixing table 4 via a spacer 3 having a thickness of 2.0 mm.
- the spacer 3 is a pair of steel plates measuring 50 mm in the longitudinal direction, 45 mm in the transverse direction, and 2.0 mm in thickness. As shown in FIG. It was arranged so as to be aligned with both end faces. Therefore, the distance between the pair of steel plates is 60 mm.
- the fixed base 8 is a single plate with a hole in the center.
- the plate assembly is pressed with a pair of electrodes 5 (tip diameter: 6 mm) while bending the plate assembly. : 3.5 kN, hold time: 0.12 seconds, 0.18 seconds or 0.24 seconds, and welding time: 0.36 seconds, with a welding current that makes the nugget diameter r 5.9 mm Resistance spot welding was performed to form a plate assembly with a welded portion.
- the pair of electrodes 5 pressurized the plate assembly from above and below in the vertical direction, and the lower electrode pressurized the test piece 2 through the hole of the fixing table 4 .
- the lower electrode and the fixing table 4 are fixed so that the lower electrode of the pair of electrodes 5 is in contact with a plane extending the surface where the spacer 3 and the fixing table 4 are in contact, and the upper electrode is fixed.
- the electrodes are movable.
- the upper electrode was brought into contact with the central portion of the test alloyed hot-dip galvanized steel sheet 1 . Welding was performed while the plate assembly was tilted 5° to the longitudinal direction of the plate assembly with respect to the horizontal direction.
- the hold time refers to the time from the end of the welding current to the start of opening the electrode.
- the nugget diameter r means the distance between the ends of the nugget 6 in the longitudinal direction of the plate assembly.
- the plate set with the welded portion is cut along the line AA in the upper diagram of FIG.
- Observation was made with a microscope (200x magnification), and resistance weld crack resistance in the weld zone was evaluated according to the following criteria. If it is A+, A or B, it is judged that the resistance weld crack resistance in the weld is excellent. If it is C, it is judged that the resistance weld crack resistance in the weld is inferior.
- the results are also shown in Table 7.
- A+ No cracks having a length of 0.1 mm or more were observed at any of the hold times of 0.12 seconds, 0.18 seconds and 0.24 seconds.
- A Cracks with a length of 0.1 mm or longer were observed at a hold time of 0.12 seconds, but cracks with a length of 0.1 mm or longer were observed at hold times of 0.18 seconds and 0.24 seconds. I didn't.
- B Cracks with a length of 0.1 mm or longer were observed at hold times of 0.12 seconds and 0.18 seconds, but cracks with a length of 0.1 mm or longer were observed at a hold time of 0.24 seconds. I didn't.
- C Cracks with a length of 0.1 mm or longer were observed at all hold times of 0.12 seconds, 0.18 seconds and 0.24 seconds.
- the crack generated in the test piece 2 is schematically indicated by reference numeral 7.
- the stress on the evaluation target steel sheet the steel sheets of each invention example and comparative example
- an appropriate evaluation cannot be obtained. For this reason, data in which no cracks occurred in the mating steel plate were used as examples.
- the resistance weld crack resistance in the weld zone was also excellent.
- the member obtained by molding or the member obtained by bonding has tensile strength (TS), yield stress (YS), total elongation (El), working It was found that the hardening index (n value)/yield ratio (YR), limit hole expansion ratio ( ⁇ ), and resistance weld cracking resistance in the weld all have excellent properties that are characteristic of the present invention.
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Abstract
Description
「質量%で表して、Cを0.04~0.22%、Siを1.0%以下、Mnを3.0%以下、Pを0.05%以下、Sを0.01%以下、Alを0.01~0.1%及びNを0.001~0.005%含有し、残部Fe及び不可避的不純物からなる成分組成を有するとともに、主相であるフェライト相と、第二相であるマルテンサイト相から構成され、かつマルテンサイト相の最大粒径が2μm以下で、その面積率が5%以上であることを特徴とする伸びフランジ性と耐衝突特性に優れた高強度鋼板。」
が開示されている。 For example,
"Expressed in mass %, C 0.04 to 0.22%, Si 1.0% or less, Mn 3.0% or less, P 0.05% or less, S 0.01% or less, It contains 0.01 to 0.1% of Al and 0.001 to 0.005% of N, and has a component composition consisting of the balance Fe and inevitable impurities, and has a ferrite phase as the main phase and a second phase. A high-strength steel sheet having excellent stretch-flange formability and collision resistance, characterized by being composed of a certain martensite phase and having a maximum grain size of 2 μm or less and an area ratio of the martensite phase of 5% or more.
is disclosed.
「表面層を厚さ0.1μm以上研削除去された冷延鋼板上にNiを0.2g/m2以上2.0g/m2以下プレめっきされた冷延鋼板の表面に溶融亜鉛めっき層を有する溶融亜鉛めっき鋼板であって、
質量%で、
C:0.05%以上、0.4%以下、
Si:0.01%以上、3.0%以下、
Mn:0.1%以上、3.0%以下、
P:0.04%以下、
S:0.05%以下、
N:0.01%以下、
Al:0.01%以上、2.0%以下、
Si+Al>0.5%、
を含有し、残部Fe及び不可避的不純物からなり、
ミクロ組織が、体積分率で主相としてフェライトを40%以上含有し、残留オーステナイトを8%以上、下記に規定する3種類のマルテンサイト[1][2][3]のマルテンサイト[3]を含む2種以上と1%以上のベイナイト及び0~10%のパーライトを含有し、且つ、前記3種類のマルテンサイト[1][2][3]がそれぞれ、体積分率で、
マルテンサイト[1]:0%以上、50%以下、
マルテンサイト[2]:0%以上、20%未満、
マルテンサイト[3]:1%以上、30%以下、
である鋼板の表面に、Feを7%未満含有し、残部がZn、Alおよび不可避的不純物からなる溶融亜鉛めっき層を有し、
引張強度TS(MPa)、全伸び率EL(%)、穴拡げ率λ(%)としてTS×ELが18000MPa・%以上、TS×λが35000MPa・%以上であり、引張強度980MPa以上有することを特徴とするめっき密着性と成形性に優れた高強度溶融亜鉛めっき鋼板。
マルテンサイト[1]:C濃度(CM1)が0.8%未満で、硬さHv1が、
Hv1/(-982.1×CM12+1676×CM1+189)≦0.60
マルテンサイト[2]:C濃度(CM2)が0.8%以上で、硬さHv2が、
Hv2/(-982.1×CM22+1676×CM2+189)≦0.60
マルテンサイト[3]:C濃度(CM3)が0.8%以上で、硬さHv3が、
Hv3/(-982.1×CM32+1676×CM3+189)≧0.80」
が開示されている。 In
"A hot-dip galvanized layer on the surface of a cold-rolled steel sheet obtained by pre-plating 0.2 g/m 2 or more and 2.0 g/m 2 or less of Ni on a cold-rolled steel sheet from which the surface layer has been ground and removed to a thickness of 0.1 μm or more. A galvanized steel sheet,
in % by mass,
C: 0.05% or more, 0.4% or less,
Si: 0.01% or more, 3.0% or less,
Mn: 0.1% or more, 3.0% or less,
P: 0.04% or less,
S: 0.05% or less,
N: 0.01% or less,
Al: 0.01% or more, 2.0% or less,
Si+Al>0.5%,
containing, the remainder consisting of Fe and unavoidable impurities,
The microstructure contains 40% or more ferrite as the main phase in volume fraction, 8% or more retained austenite, and the following three types of martensite [1] [2] [3] martensite [3] and 1% or more of bainite and 0 to 10% of pearlite, and the three types of martensite [1] [2] [3] each have a volume fraction of
Martensite [1]: 0% or more and 50% or less,
Martensite [2]: 0% or more and less than 20%,
Martensite [3]: 1% or more and 30% or less,
A hot-dip galvanized layer containing less than 7% Fe, the balance being Zn, Al and unavoidable impurities, on the surface of the steel sheet,
Tensile strength TS (MPa), total elongation EL (%), hole expansion rate λ (%), TS x EL is 18000 MPa · % or more, TS x λ is 35000 MPa · % or more, and tensile strength is 980 MPa or more High-strength hot-dip galvanized steel sheet with excellent plating adhesion and formability.
Martensite [1]: C concentration (CM1) is less than 0.8%, hardness Hv1 is
Hv1/(-982.1 x CM12 + 1676 x CM1 + 189) ≤ 0.60
Martensite [2]: C concentration (CM2) is 0.8% or more, hardness Hv2 is
Hv2/(-982.1 x CM22 + 1676 x CM2 + 189) ≤ 0.60
Martensite [3]: C concentration (CM3) is 0.8% or more, hardness Hv3 is
Hv3/(-982.1 x CM32 + 1676 x CM3 + 189) ≥ 0.80"
is disclosed.
「質量%で、C:0.15%以上0.25%以下、Si:0.50%以上2.5%以下、Mn:2.3%以上4.0%以下、P:0.100%以下、S:0.02%以下、Al:0.01%以上2.5%以下、残部がFeおよび不可避的不純物からなる成分組成を有し、
面積率で、焼戻しマルテンサイト相:30%以上73%以下、フェライト相:25%以上68%以下、残留オーステナイト相:2%以上20%以下、他の相:10%以下(0%を含む)であり、かつ、該他の相としてマルテンサイト相:3%以下(0%を含む)、ベイニティックフェライト相:5%未満(0%を含む)を有し、前記焼戻しマルテンサイト相の平均結晶粒径が8μm以下、前記残留オーステナイト相中のC量が0.7質量%未満である鋼板組織を有する高強度溶融亜鉛めっき鋼板。」
が開示されている。 In
"In mass%, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% Below, S: 0.02% or less, Al: 0.01% or more and 2.5% or less, and the balance having a component composition consisting of Fe and unavoidable impurities,
In terms of area ratio, tempered martensite phase: 30% to 73%, ferrite phase: 25% to 68%, retained austenite phase: 2% to 20%, other phases: 10% or less (including 0%) And, as the other phases, martensite phase: 3% or less (including 0%), bainitic ferrite phase: less than 5% (including 0%), and the average of the tempered martensite phase A high-strength hot-dip galvanized steel sheet having a steel sheet structure with a grain size of 8 μm or less and a C content in the retained austenite phase of less than 0.7% by mass. ”
is disclosed.
また、本発明は、上記の亜鉛めっき鋼板を素材とする部材、ならびに、その製造方法を提供することを目的とする。 The present invention has been developed in view of the above-mentioned current situation, and provides a galvanized steel sheet having a TS of 980 MPa or more, a high YS, and excellent ductility, work hardening ability and hole expansibility. It is an object to provide, together with an advantageous manufacturing method.
Another object of the present invention is to provide a member made of the above galvanized steel sheet, and a method for manufacturing the same.
・JIS Z 2241に準拠する引張試験で測定されるYSが、当該引張試験で測定されるTSに応じて、以下の式を満足し、
980MPa≦TS<1180MPaの場合、550MPa≦YS
1180MPa≦TS<1310MPaの場合、700MPa≦YS
1310MPa≦TSの場合、800MPa≦YS
・JIS Z 2241に準拠する引張試験で測定される全伸び(El)が、当該引張試験で測定されるTSに応じて、以下の式を満足し、
980MPa≦TS<1180MPaの場合、13.0%≦El
1180MPa≦TS<1310MPaの場合、12.0%≦El
1310MPa≦TSの場合、10.0%≦El
・JIS Z 2241に準拠する引張試験で測定されるn値/YRが、以下の式を満足し、
n値/YR≧0.070
・さらに、JIS Z 2256に準拠する穴広げ試験で測定される限界穴広げ率(λ)が20%以上である、
ことを意味する。 Here, having high YS and excellent ductility, work hardening ability and hole expansibility means
・YS measured in a tensile test in accordance with JIS Z 2241 satisfies the following formula according to TS measured in the tensile test,
When 980 MPa ≤ TS < 1180 MPa, 550 MPa ≤ YS
When 1180 MPa ≤ TS < 1310 MPa, 700 MPa ≤ YS
800 MPa ≤ YS when 1310 MPa ≤ TS
・The total elongation (El) measured by a tensile test in accordance with JIS Z 2241 satisfies the following formula according to the TS measured by the tensile test,
When 980 MPa ≤ TS < 1180 MPa, 13.0% ≤ El
When 1180 MPa ≤ TS < 1310 MPa, 12.0% ≤ El
10.0% ≤ El when 1310 MPa ≤ TS
・ The n value / YR measured by a tensile test in accordance with JIS Z 2241 satisfies the following formula,
n value/YR≧0.070
・Furthermore, the limit hole expansion rate (λ) measured in a hole expansion test in accordance with JIS Z 2256 is 20% or more.
means that
その結果、亜鉛めっき鋼板の下地鋼板の成分組成を適正に調整し、かつ、亜鉛めっき鋼板の下地鋼板の鋼組織を、
フェライトの面積率:65.0%以下(0%を含む)、
ベイニティックフェライトの面積率:5.0%以上40.0%以下、
焼戻しマルテンサイトの面積率:0.5%以上80.0%以下、
残留オーステナイトの面積率:3.0%以上、
フレッシュマルテンサイトの面積率:20.0%以下(0%を含む)、
SBF+STM+2×SMA:65.0%以上、
SMA1/SMA:0.80以下、および
SMA2/SMA:0.20以上
とする、ことにより、TS:980MPa以上であり、かつ、高いYSと、優れた延性、加工硬化能および穴広げ性とを有する亜鉛めっき鋼板が得られることを知見した。
本発明は、上記の知見に基づき、さらに検討を加えて完成されたものである。 The inventors have made extensive studies in order to achieve the above object.
As a result, the chemical composition of the base steel sheet of the galvanized steel sheet is appropriately adjusted, and the steel structure of the base steel sheet of the galvanized steel sheet is
Ferrite area ratio: 65.0% or less (including 0%),
Area ratio of bainitic ferrite: 5.0% or more and 40.0% or less,
Area ratio of tempered martensite: 0.5% or more and 80.0% or less,
Area ratio of retained austenite: 3.0% or more,
Area ratio of fresh martensite: 20.0% or less (including 0%),
S BF + STM + 2 x SMA : 65.0% or more,
SMA1 / SMA : 0.80 or less and SMA2 / SMA : 0.20 or more, so that TS: 980 MPa or more, high YS, excellent ductility, work hardening ability and hole It was found that a galvanized steel sheet having spreadability can be obtained.
The present invention has been completed based on the above findings and further studies.
1.下地鋼板と、該下地鋼板の表面に亜鉛めっき層と、を有する亜鉛めっき鋼板であって、
該下地鋼板は、
質量%で、
C:0.050%以上0.400%以下、
Si:0.20%以上3.00%以下、
Mn:1.00%以上3.50%未満、
P:0.001%以上0.100%以下、
S:0.0200%以下、
Al:0.010%以上2.000%以下および
N:0.0100%以下
であり、炭素当量Ceqが0.540%以上であり、残部がFe及び不可避的不純物である、成分組成を有し、
また、該下地鋼板は、
フェライトの面積率:65.0%以下(0%を含む)、
ベイニティックフェライトの面積率:5.0%以上40.0%以下、
焼戻しマルテンサイトの面積率:0.5%以上80.0%以下、
残留オーステナイトの面積率:3.0%以上、
フレッシュマルテンサイトの面積率:20.0%以下(0%を含む)、
SBF+STM+2×SMA:65.0%以上、
SMA1/SMA:0.80以下、および
SMA2/SMA:0.20以上
である、鋼組織を有し、
引張強さが980MPa以上である、亜鉛めっき鋼板。
ここで、
SBF:前記ベイニティックフェライトの面積率
STM:前記焼戻しマルテンサイトの面積率
SMA:前記残留オーステナイトおよび前記フレッシュマルテンサイトからなる硬質第二相の面積率
SMA1:前記硬質第二相を構成する島状領域のうち、円相当直径が2.0μm以上であり、かつ、周長の20%以下が焼戻しマルテンサイトと接する島状領域の合計の面積率
SMA2:前記硬質第二相を構成する島状領域のうち、周長の1%以上がベイニティックフェライトと接する島状領域の合計の面積率
である。 That is, the gist and configuration of the present invention are as follows.
1. A galvanized steel sheet having a base steel sheet and a galvanized layer on the surface of the base steel sheet,
The base steel plate is
in % by mass,
C: 0.050% or more and 0.400% or less,
Si: 0.20% or more and 3.00% or less,
Mn: 1.00% or more and less than 3.50%,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, a carbon equivalent Ceq of 0.540% or more, and the balance being Fe and unavoidable impurities. ,
In addition, the base steel plate is
Ferrite area ratio: 65.0% or less (including 0%),
Area ratio of bainitic ferrite: 5.0% or more and 40.0% or less,
Area ratio of tempered martensite: 0.5% or more and 80.0% or less,
Area ratio of retained austenite: 3.0% or more,
Area ratio of fresh martensite: 20.0% or less (including 0%),
S BF + STM + 2 x SMA : 65.0% or more,
having a steel structure in which SMA1/SMA: 0.80 or less and SMA2 / SMA : 0.20 or more;
A galvanized steel sheet having a tensile strength of 980 MPa or more.
here,
S BF : Area ratio of the bainitic ferrite STM : Area ratio of the tempered martensite SMA : Area ratio of the hard second phase composed of the retained austenite and the fresh martensite SMA1 : The hard second phase Among the constituent island regions, the total area ratio of the island regions having an equivalent circle diameter of 2.0 μm or more and having 20% or less of the circumference contacting the tempered martensite S MA2 : the hard second phase Of the constituent island regions, 1% or more of the circumference is the total area ratio of the island regions in contact with the bainitic ferrite.
Ti:0.200%以下、
Nb:0.200%以下、
V:0.100%以下、
B:0.0100%以下、
Cu:1.000%以下、
Cr:1.000%以下、
Ni:1.000%以下、
Mo:0.500%以下、
Sb:0.200%以下、
Sn:0.200%以下、
Ta:0.100%以下、
W:0.500%以下、
Mg:0.0200%以下、
Zn:0.0200%以下、
Co:0.0200%以下、
Zr:0.0200%以下、
Ca:0.0200%以下、
Ce:0.0200%以下、
Se:0.0200%以下、
Te:0.0200%以下、
Ge:0.0200%以下、
As:0.0200%以下、
Sr:0.0200%以下、
Cs:0.0200%以下、
Hf:0.0200%以下、
Pb:0.0200%以下、
Bi:0.0200%以下および
REM:0.0200%以下
のうちから選ばれる少なくとも1種を含有する、前記1に記載の亜鉛めっき鋼板。 2. The chemical composition of the base steel plate is further, in mass%,
Ti: 0.200% or less,
Nb: 0.200% or less,
V: 0.100% or less,
B: 0.0100% or less,
Cu: 1.000% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 0.500% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
Zr: 0.0200% or less,
Ca: 0.0200% or less,
Ce: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0200% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
2. The galvanized steel sheet according to 1 above, containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less.
ここで、
SMA3:前記硬質第二相を構成する島状領域のうち、周長の1%以上がベイニティックフェライトと接し、かつ、周長の20%超が焼戻しマルテンサイトと接する島状領域の合計の面積率
である。 3. 3. The galvanized steel sheet according to 1 or 2 above, wherein SMA3 / SMA is 0.05 or more in the steel structure of the base steel sheet.
here,
SMA3 : Of the island-shaped regions constituting the hard second phase, 1% or more of the peripheral length is in contact with bainitic ferrite, and more than 20% of the peripheral length is in contact with tempered martensite. is the area ratio of
前記熱延鋼板を冷間圧延して冷延鋼板とする、冷延工程と、
前記冷延鋼板を、焼鈍温度:760℃以上900℃以下および焼鈍時間:20秒以上で焼鈍する、焼鈍工程と、
前記冷延鋼板を300℃以上550℃以下の第一冷却停止温度まで冷却する、第一冷却工程と、
前記冷延鋼板を300℃以上550℃以下の温度域で3秒以上600秒以下保持する、保持工程と、
前記冷延鋼板に亜鉛めっき処理を施して亜鉛めっき鋼板とする、めっき工程と、
前記亜鉛めっき鋼板を、100℃以上300℃未満の第二冷却停止温度まで冷却する、第二冷却工程と、
前記亜鉛めっき鋼板を、(前記第二冷却停止温度+50℃)以上500℃以下の再加熱温度に再加熱し、前記亜鉛めっき鋼板を、(前記第二冷却停止温度+50℃)以上500℃以下の温度域で10秒以上2000秒以下保持する、再加熱工程と、
を有し、
前記第一冷却停止温度と、前記亜鉛めっき処理での亜鉛めっき浴の温度とが、次式(1)の関係を満足する、亜鉛めっき鋼板の製造方法。
-150℃≦T0-T1≦50℃ ・・・(1)
ここで、T0は第一冷却停止温度(℃)、T1は亜鉛めっき処理での亜鉛めっき浴の温度(℃)である。 10. A hot rolling step of subjecting a steel slab having the chemical composition according to 1 or 2 to hot rolling to obtain a hot rolled steel sheet;
A cold rolling step of cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet;
An annealing step of annealing the cold-rolled steel sheet at an annealing temperature of 760° C. or more and 900° C. or less and an annealing time of 20 seconds or more;
a first cooling step of cooling the cold-rolled steel sheet to a first cooling stop temperature of 300° C. or higher and 550° C. or lower;
a holding step of holding the cold-rolled steel sheet in a temperature range of 300° C. or higher and 550° C. or lower for 3 seconds or more and 600 seconds or less;
a plating step of galvanizing the cold-rolled steel sheet to form a galvanized steel sheet;
a second cooling step of cooling the galvanized steel sheet to a second cooling stop temperature of 100° C. or more and less than 300° C.;
The galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower, and the galvanized steel sheet is heated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower. A reheating step of holding in the temperature range for 10 seconds or more and 2000 seconds or less;
has
A method for producing a galvanized steel sheet, wherein the first cooling stop temperature and the temperature of the galvanizing bath in the galvanizing treatment satisfy the relationship of the following formula (1).
−150° C.≦T 0 −T 1 ≦50° C. (1)
Here, T0 is the first cooling stop temperature (°C), and T1 is the temperature (°C) of the zinc plating bath in the zinc plating treatment.
[1]亜鉛めっき鋼板
まず、本発明の一実施形態に従う亜鉛めっき鋼板の下地鋼板の成分組成について説明する。なお、成分組成における単位はいずれも「質量%」であるが、以下、特に断らない限り、単に「%」で示す。 The present invention will be described based on the following embodiments.
[1] Galvanized Steel Sheet First, the chemical composition of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention will be described. Incidentally, although the units in the component compositions are all "% by mass", they are indicated simply by "%" unless otherwise specified.
Cは、フレッシュマルテンサイト、焼戻しマルテンサイト、ベイニティックフェライトおよび残留オーステナイトを適正量生成させて、980MPa以上のTSと、高いYSを確保するために有効な元素である。ここで、C含有量が0.050%未満では、フェライトの面積率が増加して、TSを980MPa以上とすることが困難になる。また、YSの低下も招く。一方、C含有量が0.400%を超えると、残留オーステナイト中の炭素濃度が過度に増加する。そのため、鋼板に打抜き加工を施すと、残留オーステナイトから生成するフレッシュマルテンサイトの硬度が大幅に増加する。その結果、打抜き加工後の鋼板では、穴広げ時の亀裂進展が促進される(すなわち、穴広げ性の低下を招く)。
したがって、C含有量は、0.050%以上0.400%以下とする。C含有量は、好ましくは0.100%以上である。また、C含有量は、好ましくは0.300%以下である。 C: 0.050% or more and 0.400% or less C generates appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite to ensure a TS of 980 MPa or more and a high YS. It is an effective element. Here, if the C content is less than 0.050%, the ferrite area ratio increases, making it difficult to increase the TS to 980 MPa or more. In addition, a decrease in YS is also caused. On the other hand, when the C content exceeds 0.400%, the carbon concentration in retained austenite increases excessively. Therefore, when a steel plate is punched, the hardness of fresh martensite generated from retained austenite increases significantly. As a result, in the steel plate after punching, crack growth during hole expansion is accelerated (that is, hole expandability is reduced).
Therefore, the C content should be 0.050% or more and 0.400% or less. The C content is preferably 0.100% or more. Also, the C content is preferably 0.300% or less.
Siは、焼鈍中の炭化物生成を抑制し、残留オーステナイトの生成を促進する。すなわち、Siは、残留オーステナイトの面積率および残留オーステナイト中の炭素濃度に影響する元素である。ここで、Si含有量が0.20%未満では、残留オーステナイトの面積率が減少し、延性が低下する。一方、Si含有量が3.00%を超えると、フェライトの面積率が過度に増加し、TSを980MPa以上とすることが困難になる。また、YSの低下も招く。加えて、残留オーステナイト中の炭素濃度が過度に増加する。そのため、鋼板に打抜き加工を施すと、残留オーステナイトから生成するフレッシュマルテンサイトの硬度が大幅に増加する。その結果、打抜き加工後の鋼板では、穴広げ時の亀裂進展が促進される(すなわち、穴広げ性の低下を招く)。
したがって、Si含有量は、0.20%以上3.00%以下とする。Si含有量は、好ましくは0.40%以上である。また、Si含有量が2.00%を超えると耐抵抗溶接割れ特性の低下が懸念されるので、Si含有量は、好ましくは2.00%以下である。 Si: 0.20% to 3.00% Si suppresses the formation of carbides during annealing and promotes the formation of retained austenite. That is, Si is an element that affects the area ratio of retained austenite and the carbon concentration in retained austenite. Here, if the Si content is less than 0.20%, the area ratio of retained austenite decreases and the ductility decreases. On the other hand, if the Si content exceeds 3.00%, the ferrite area ratio increases excessively, making it difficult to increase the TS to 980 MPa or more. In addition, a decrease in YS is also caused. In addition, the carbon concentration in retained austenite increases excessively. Therefore, when a steel plate is punched, the hardness of fresh martensite generated from retained austenite significantly increases. As a result, in the steel plate after punching, crack growth during hole expansion is accelerated (that is, hole expandability is reduced).
Therefore, the Si content should be 0.20% or more and 3.00% or less. The Si content is preferably 0.40% or more. Moreover, if the Si content exceeds 2.00%, the resistance weld cracking resistance may be lowered, so the Si content is preferably 2.00% or less.
Mnは、ベイニティックフェライトや焼戻しマルテンサイトなどの面積率を調整する元素である。ここで、Mn含有量が1.00%未満では、フェライトの面積率が過度に増加して、TSを980MPa以上とすることが困難になる。また、YSの低下も招く。一方、Mn含有量が3.50%以上となると、ベイニティックフェライトの面積率が減少し、焼戻しマルテンサイトの面積率が過度に増加する。その結果、所望の延性が得られない。
したがって、Mn含有量は、1.00%以上3.50%未満とする。Mn含有量は、好ましくは、1.80%以上である。また、Mn含有量は、好ましくは3.20%未満である。 Mn: 1.00% or more and less than 3.50% Mn is an element that adjusts the area ratio of bainitic ferrite, tempered martensite, and the like. Here, if the Mn content is less than 1.00%, the ferrite area ratio increases excessively, making it difficult to achieve a TS of 980 MPa or more. In addition, a decrease in YS is also caused. On the other hand, when the Mn content is 3.50% or more, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite excessively increases. As a result, the desired ductility is not obtained.
Therefore, the Mn content should be 1.00% or more and less than 3.50%. The Mn content is preferably 1.80% or more. Also, the Mn content is preferably less than 3.20%.
Pは、固溶強化の作用を有し、鋼板の強度を上昇させる元素である。このような効果を得るため、P含有量を0.001%以上にする。一方、P含有量が0.100%を超えると、Pが旧オーステナイト粒界に偏析して粒界を脆化させる。そのため、鋼板に打抜き加工を施すと、ボイドの生成量が増加し、穴広げ性の低下を招く。
したがって、P含有量は、0.001%以上0.100%以下とする。P含有量は、好ましくは0.030%以下である。 P: 0.001% or more and 0.100% or less P is an element that has a solid-solution strengthening action and increases the strength of the steel sheet. In order to obtain such effects, the P content is made 0.001% or more. On the other hand, when the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries and embrittles the grain boundaries. Therefore, when the steel sheet is punched, the amount of voids generated increases, leading to a decrease in hole expansibility.
Therefore, the P content should be 0.001% or more and 0.100% or less. The P content is preferably 0.030% or less.
Sは、鋼中で硫化物として存在する。特に、S含有量が0.0200%を超えると、鋼板の極限変形能が低下する。そのため、鋼板に打抜き加工を施すと、ボイドの生成量が増加し、穴広げ性の低下を招く。
したがって、S含有量は0.0200%以下とする。S含有量は、好ましくは0.0080%以下である。なお、S含有量の下限は特に規定しないが、生産技術上の制約から、S含有量は0.0001%以上とすることが好ましい。 S: 0.0200% or less S exists as a sulfide in steel. In particular, when the S content exceeds 0.0200%, the ultimate deformability of the steel sheet is lowered. Therefore, when the steel sheet is punched, the amount of voids generated increases, leading to a decrease in hole expansibility.
Therefore, the S content should be 0.0200% or less. The S content is preferably 0.0080% or less. Although the lower limit of the S content is not specified, it is preferable that the S content is 0.0001% or more due to production technology restrictions.
Alは、焼鈍中の炭化物生成を抑制するとともに、残留オーステナイトの生成を促進する。すなわち、Alは、残留オーステナイトの面積率および残留オーステナイト中の炭素濃度に影響を及ぼす元素である。このような効果を得るために、Al含有量を0.010%以上とする。一方、Al含有量が2.000%を超えると、フェライトの面積率が過度に増加して、TSを980MPa以上とすることが困難になる。また、YSの低下も招く。
したがって、Alの含有量は、0.010%以上2.000%以下とする。Al含有量は、好ましくは、0.015%以上である。また、Al含有量は、好ましくは1.000%以下である。 Al: 0.010% to 2.000% Al suppresses the formation of carbide during annealing and promotes the formation of retained austenite. That is, Al is an element that affects the area ratio of retained austenite and the carbon concentration in retained austenite. In order to obtain such effects, the Al content is set to 0.010% or more. On the other hand, if the Al content exceeds 2.000%, the ferrite area ratio increases excessively, making it difficult to increase the TS to 980 MPa or more. In addition, a decrease in YS is also caused.
Therefore, the content of Al is set to 0.010% or more and 2.000% or less. The Al content is preferably 0.015% or more. Also, the Al content is preferably 1.000% or less.
Nは、鋼中で窒化物として存在する。特に、N含有量が0.0100%を超えると、鋼板の極限変形能が低下する。そのため、鋼板に打抜き加工を施すと、ボイドの生成量が増加し、穴広げ性の低下を招く。
したがって、N含有量は0.0100%以下とする。また、N含有量は、好ましくは0.0050%以下である。なお、N含有量の下限は特に規定しないが、生産技術上の制約から、N含有量は0.0005%以上が好ましい。 N: 0.0100% or less N exists as a nitride in steel. In particular, when the N content exceeds 0.0100%, the ultimate deformability of the steel sheet is lowered. Therefore, when the steel sheet is punched, the amount of voids generated increases, leading to a decrease in hole expansibility.
Therefore, the N content should be 0.0100% or less. Also, the N content is preferably 0.0050% or less. Although the lower limit of the N content is not particularly specified, it is preferable that the N content is 0.0005% or more due to production technology restrictions.
炭素当量CeqはTSに影響を与える。特に、炭素当量Ceqが0.540%未満になると、TSを980MPa以上とすることが困難となる。したがって、炭素当量Ceqは0.540%以上とする。
ここで、炭素当量Ceqは、以下の式により定義される。
炭素当量Ceq=[C%]+([Si%]/24)+([Mn%]/6)+([Ni%]/40)+([Cr%]/5)+([Mo%]/4)+([V%]/14)
なお、上記した式中の[元素記号%]は、下地鋼板の成分組成における当該元素の含有量(質量%)を表す。また、下地鋼板の成分組成に含有されない元素は0として計算する。 Carbon Equivalent Ceq: 0.540% or More Carbon equivalent Ceq affects TS. In particular, when the carbon equivalent Ceq is less than 0.540%, it becomes difficult to increase the TS to 980 MPa or more. Therefore, the carbon equivalent Ceq is set to 0.540% or more.
Here, the carbon equivalent Ceq is defined by the following formula.
Carbon equivalent Ceq = [C%] + ([Si%] / 24) + ([Mn%] / 6) + ([Ni%] / 40) + ([Cr%] / 5) + ([Mo%] /4) + ([V%]/14)
[Element symbol %] in the above formula represents the content (% by mass) of the element in the chemical composition of the base steel sheet. Elements not contained in the chemical composition of the base steel sheet are calculated as 0.
Ti:0.200%以下、
Nb:0.200%以下、
V:0.100%以下、
B:0.0100%以下、
Cu:1.000%以下、
Cr:1.000%以下、
Ni:1.000%以下、
Mo:0.500%以下、
Sb:0.200%以下、
Sn:0.200%以下、
Ta:0.100%以下、
W:0.500%以下、
Mg:0.0200%以下、
Zn:0.0200%以下、
Co:0.0200%以下、
Zr:0.0200%以下、
Ca:0.0200%以下、
Ce:0.0200%以下、
Se:0.0200%以下、
Te:0.0200%以下、
Ge:0.0200%以下、
As:0.0200%以下、
Sr:0.0200%以下、
Cs:0.0200%以下、
Hf:0.0200%以下、
Pb:0.0200%以下、
Bi:0.0200%以下および
REM:0.0200%以下 The basic components of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention have been described above. The balance has a component composition containing Fe (iron) and unavoidable impurities. Here, the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention preferably has a chemical composition containing the above-mentioned basic ingredients and the balance being Fe and unavoidable impurities. The substrate steel sheet of the galvanized steel sheet according to one embodiment of the present invention may contain at least one selected from the following optional ingredients in addition to the basic ingredients described above. In addition, since the effect of this invention is acquired if it contains the optional component shown below below the upper limit shown below, a lower limit in particular is not set. In addition, when the following optional components are included in less than the preferable lower limit values described later, the optional components are included as unavoidable impurities.
Ti: 0.200% or less,
Nb: 0.200% or less,
V: 0.100% or less,
B: 0.0100% or less,
Cu: 1.000% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 0.500% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
Zr: 0.0200% or less,
Ca: 0.0200% or less,
Ce: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0200% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
Bi: 0.0200% or less and REM: 0.0200% or less
Tiは、熱間圧延時や焼鈍時に、微細な炭化物、窒化物または炭窒化物を形成することによって、TSを上昇させる。このような効果を得るためには、Ti含有量を0.001%以上とすることが好ましい。Ti含有量は、より好ましくは0.005%以上である。一方、Ti含有量が0.200%を超えると、粗大な析出物や介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な析出物や介在物が穴広げ試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Tiを含有させる場合、Ti含有量は0.200%以下が好ましい。Ti含有量は、より好ましくは0.060%以下である。 Ti: 0.200% or less Ti increases TS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. In order to obtain such effects, it is preferable to set the Ti content to 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, when the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.
Nbは、Tiと同様、熱間圧延時や焼鈍時に、微細な炭化物、窒化物または炭窒化物を形成することによって、TSを上昇させる。このような効果を得るためには、Nb含有量を0.001%以上とすることが好ましい。Nb含有量は、より好ましくは0.005%以上である。一方、Nb含有量が0.200%を超えると、粗大な析出物や介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な析出物や介在物が穴広げ試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Nbを含有させる場合、Nb含有量は0.200%以下が好ましい。Nb含有量は、より好ましくは0.060%以下である。 Nb: 0.200% or less Like Ti, Nb increases TS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. In order to obtain such effects, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.
Vは、TiやNbと同様、熱間圧延時や焼鈍時に、微細な炭化物、窒化物または炭窒化物を形成することによって、TSを上昇させる。このような効果を得るためには、V含有量を0.001%以上とすることが好ましい。V含有量は、より好ましくは0.005%以上である。一方、V含有量が0.100%を超えると、粗大な析出物や介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な析出物や介在物が穴広げ試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Vを含有させる場合、V含有量は0.100%以下が好ましい。V含有量は、より好ましくは0.060%以下である。 V: 0.100% or less Like Ti and Nb, V raises TS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. In order to obtain such effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. On the other hand, when the V content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when V is contained, the V content is preferably 0.100% or less. The V content is more preferably 0.060% or less.
Bは、オーステナイト粒界に偏析することにより、焼入れ性を高める元素である。また、Bは、焼鈍後の冷却時に、フェライトの生成および粒成長を抑制する元素である。このような効果を得るためには、B含有量を0.0001%以上にすることが好ましい。B含有量は、より好ましくは0.0002%以上である。一方、B含有量が0.0100%を超えると、熱間圧延時に鋼板内部に割れが生じ、鋼板の極限変形能を低下させるおそれがある。また、鋼板の極限変形能の低下に伴い、鋼板に打抜き加工を施した際のボイドの生成量が増加し、穴広げ性の低下を招く。したがって、Bを含有させる場合、B含有量は0.0100%以下とすることが好ましい。B含有量は、より好ましくは0.0050%以下である。 B: 0.0100% or less B is an element that increases the hardenability by segregating at the austenite grain boundary. B is an element that suppresses the formation of ferrite and grain growth during cooling after annealing. In order to obtain such effects, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling, which may reduce the ultimate deformability of the steel sheet. In addition, as the ultimate deformability of the steel sheet decreases, the amount of voids generated when the steel sheet is punched increases, leading to a decrease in hole expansibility. Therefore, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.
Cuは、焼入れ性を高める元素である。特に、Cuは、硬質なフレッシュマルテンサイトなどの面積率をより好適な範囲に調整し、これにより、TSをより好適な範囲に調整するために有効な元素である。このような効果を得るためには、Cu含有量を0.005%以上にすることが好ましい。Cu含有量は、より好ましくは0.020%以上である。一方、Cu含有量が1.000%を超えると、フレッシュマルテンサイトの面積率が過度に増加し、TSが過剰に高くなる。また、粗大な析出物や介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な析出物や介在物が引張試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Cuを含有させる場合、Cu含有量は1.000%以下とすることが好ましい。Cuの含有量は、より好ましくは0.200%以下である。 Cu: 1.000% or less Cu is an element that enhances hardenability. In particular, Cu is an element effective for adjusting the area ratio of hard fresh martensite and the like to a more suitable range, thereby adjusting TS to a more suitable range. In order to obtain such effects, the Cu content is preferably 0.005% or more. Cu content is more preferably 0.020% or more. On the other hand, when the Cu content exceeds 1.000%, the area ratio of fresh martensite increases excessively, resulting in excessively high TS. Also, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the tensile test, that is, there is a risk of deterioration in hole expandability. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.
Crは、焼入れ性を高める元素である、また、Crは、残留オーステナイトやフレッシュマルテンサイトを生成させるために有効な元素である。このような効果を得るためには、Cr含有量は0.0005%以上にすることが好ましい。特に、TSをより好適な範囲とする観点から、Cr含有量は0.010%以上がより好ましい。一方、Cr含有量が1.000%を超えると、硬質なフレッシュマルテンサイトの面積率が過度に増加し、穴広げ性の低下を招くおそれがある。したがって、Crを含有させる場合、Cr含有量は1.000%以下にすることが好ましい。また、Cr含有量は、より好ましくは0.250%以下、さらに好ましくは0.100%以下である。 Cr: 1.000% or less Cr is an element that enhances hardenability, and Cr is an element that is effective for generating retained austenite and fresh martensite. In order to obtain such effects, the Cr content is preferably 0.0005% or more. In particular, the Cr content is more preferably 0.010% or more from the viewpoint of making TS in a more suitable range. On the other hand, when the Cr content exceeds 1.000%, the area ratio of hard fresh martensite increases excessively, which may lead to a decrease in hole expansibility. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less. Also, the Cr content is more preferably 0.250% or less, and still more preferably 0.100% or less.
Niは、焼入れ性を高める元素である。また、Niは、残留オーステナイトやフレッシュマルテンサイトの面積率をより好適な範囲に調整し、これにより、TSをより好適な範囲に調整するために有効な元素である。このような効果を得るためには、Ni含有量を0.005%以上にすることが好ましい。Ni含有量は、より好ましくは、0.020%以上である。一方、Niの含有量が1.000%を超えると、フレッシュマルテンサイトの面積率が過度に増加し、延性や成形時の寸法精度が低下するおそれがある。また、粗大な析出物や介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な析出物や介在物が穴広げ試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Niを含有させる場合、Ni含有量は1.000%以下とすることが好ましい。Ni含有量は、より好ましくは0.800%以下である。 Ni: 1.000% or less Ni is an element that enhances hardenability. Further, Ni is an element effective for adjusting the area ratio of retained austenite and fresh martensite to a more suitable range, thereby adjusting TS to a more suitable range. In order to obtain such effects, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. On the other hand, if the Ni content exceeds 1.000%, the area ratio of fresh martensite may excessively increase, and ductility and dimensional accuracy during forming may deteriorate. Also, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less.
Moは、焼入れ性を高める元素である。また、Moは、硬質なフレッシュマルテンサイトなどを生成させるために有効な元素である。このような効果を得るためには、Mo含有量を0.010%以上にすることが好ましい。Mo含有量は、より好ましくは、0.030%以上である。一方、Mo含有量が0.500%を超えると、フレッシュマルテンサイトの面積率が過度に増加し、穴広げ性の低下を招くおそれがある。したがって、Moを含有させる場合、Mo含有量は0.500%以下にすることが好ましい。Mo含有量は、より好ましくは0.450%以下、さらに好ましくは0.400%以下である。 Mo: 0.500% or less Mo is an element that enhances hardenability. Moreover, Mo is an effective element for generating hard fresh martensite and the like. In order to obtain such effects, the Mo content is preferably 0.010% or more. Mo content is more preferably 0.030% or more. On the other hand, when the Mo content exceeds 0.500%, the area ratio of fresh martensite increases excessively, which may lead to a decrease in hole expansibility. Therefore, when Mo is contained, the Mo content is preferably 0.500% or less. The Mo content is more preferably 0.450% or less, still more preferably 0.400% or less.
Sbは、焼鈍中の鋼板表面近傍でのCの拡散を抑制し、鋼板表面近傍における軟質層の形成を制御するために有効な元素である。このような効果を得るためには、Sb含有量を0.002%以上とすることが好ましい。Sb含有量は、より好ましくは0.005%以上である。一方、Sb含有量が0.200%を超えると、鋼板表面近傍に軟質層が形成されず、穴広げ性の低下を招くおそれがある。したがって、Sbを含有させる場合、Sb含有量は0.200%以下にすることが好ましい。Sb含有量は、より好ましくは0.020%以下である。 Sb: 0.200% or less Sb is an element effective for suppressing the diffusion of C in the vicinity of the steel sheet surface during annealing and controlling the formation of a soft layer in the vicinity of the steel sheet surface. In order to obtain such effects, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, when the Sb content exceeds 0.200%, a soft layer is not formed in the vicinity of the steel sheet surface, which may lead to deterioration in hole expansibility. Therefore, when Sb is contained, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less.
Snは、Sbと同様、焼鈍中の鋼板表面近傍でのCの拡散を抑制し、鋼板表面近傍における軟質層の形成を制御するために有効な元素である。このような効果を得るためには、Sn含有量を0.002%以上とすることが好ましい。Sn含有量は、より好ましくは0.005%以上である。一方、Sn含有量が0.200%を超えると、鋼板表面近傍に軟質層が形成されず、穴広げ性の低下を招くおそれがある。したがって、Snを含有させる場合、Sn含有量は0.200%以下にすることが好ましい。Sn含有量は、より好ましくは0.020%以下である。 Sn: 0.200% or less Sn, like Sb, is an element effective in suppressing the diffusion of C in the vicinity of the steel sheet surface during annealing and controlling the formation of a soft layer in the vicinity of the steel sheet surface. In order to obtain such effects, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more. On the other hand, when the Sn content exceeds 0.200%, a soft layer is not formed in the vicinity of the surface of the steel sheet, which may lead to deterioration of hole expansibility. Therefore, when Sn is contained, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.020% or less.
Taは、Ti、NbおよびVと同様に、熱間圧延時や焼鈍時に、微細な炭化物、窒化物または炭窒化物を形成することによって、TSを上昇させる。加えて、Taは、Nb炭化物やNb炭窒化物に一部固溶し、(Nb,Ta)(C,N)のような複合析出物を生成する。これにより、析出物の粗大化を抑制し、析出強化を安定化させる。これにより、TS、さらにはYSを向上させる。このような効果を得るためには、Ta含有量を0.001%以上とすることが好ましい。一方、Ta含有量が0.100%を超えると、粗大な析出物や介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な析出物や介在物が穴広げ試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Taを含有させる場合、Ta含有量は0.100%以下が好ましい。 Ta: 0.100% or less Ta, like Ti, Nb and V, raises TS by forming fine carbides, nitrides or carbonitrides during hot rolling and annealing. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta) (C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This improves TS and also YS. In order to obtain such effects, it is preferable to set the Ta content to 0.001% or more. On the other hand, when the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen exists in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less.
Wは、焼入れ性を高め、TSをより好適な範囲に調整するために有効な元素である。このような効果を得るためには、W含有量を0.001%以上とすることが好ましい。W含有量は、より好ましくは0.030%以上である。一方、W含有量が0.500%を超えると、硬質なフレッシュマルテンサイトの面積率が過度に増加して、穴広げ性の低下を招くおそれがある。したがって、Wを含有させる場合、W含有量は0.500%以下にすることが好ましい。W含有量は、より好ましくは0.450%以下、さらに好ましくは0.400%以下である。 W: 0.500% or less W is an effective element for improving hardenability and adjusting TS to a more suitable range. In order to obtain such effects, the W content is preferably 0.001% or more. The W content is more preferably 0.030% or more. On the other hand, if the W content exceeds 0.500%, the area ratio of hard fresh martensite may excessively increase, leading to a decrease in hole expansibility. Therefore, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, still more preferably 0.400% or less.
Mgは、硫化物や酸化物などの介在物の形状を球状化して、鋼板の極限変形能、さらには穴広げ性を向上させるために有効な元素である。このような効果を得るためには、Mg含有量を0.0001%以上とすることが好ましい。一方、Mg含有量が0.0200%を超えると、粗大な析出物や介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な析出物や介在物が穴広げ試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Mgを含有させる場合、Mg含有量は0.0200%以下とすることが好ましい。 Mg: 0.0200% or less Mg is an element effective for making inclusions such as sulfides and oxides spherical and improving the ultimate deformability and hole expandability of the steel sheet. In order to obtain such effects, the Mg content is preferably 0.0001% or more. On the other hand, when the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen exists in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less.
Znは、介在物の形状を球状化して、鋼板の極限変形能、さらには穴広げ性を向上させるために有効な元素である。このような効果を得るためには、Zn含有量は、0.0010%以上にすることが好ましい。一方、Zn含有量が0.0200%を超えると、粗大な析出物や介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な析出物や介在物が穴広げ試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Znを含有させる場合、Zn含有量は0.0200%以下とすることが好ましい。 Zn: 0.0200% or less Zn is an element effective for making the shape of inclusions spherical and improving the ultimate deformability and hole expandability of the steel sheet. In order to obtain such effects, the Zn content is preferably 0.0010% or more. On the other hand, when the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen exists in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less.
Coは、Znと同様、介在物の形状を球状化して、鋼板の極限変形能、さらには穴広げ性を向上させるために有効な元素である。このような効果を得るためには、Co含有量は、0.0010%以上にすることが好ましい。一方、Co含有量が0.0200%を超えると、粗大な析出物や介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な析出物や介在物が穴広げ試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Coを含有させる場合、Co含有量は0.0200%以下とすることが好ましい。 Co: 0.0200% or less Like Zn, Co is an element effective in making inclusions spherical and improving the ultimate deformability and hole expandability of the steel sheet. In order to obtain such effects, the Co content is preferably 0.0010% or more. On the other hand, when the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Co is contained, the Co content is preferably 0.0200% or less.
Zrは、ZnおよびCoと同様、介在物の形状を球状化して、鋼板の極限変形能、さらには穴広げ性を向上させるために有効な元素である。このような効果を得るためには、Zr含有量は、0.0010%以上にすることが好ましい。一方、Zr含有量が0.0200%を超えると、粗大な析出物や介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な析出物や介在物が穴広げ試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Zrを含有させる場合、Zr含有量は0.0200%以下とすることが好ましい。 Zr: 0.0200% or less Zr, like Zn and Co, is an element effective in making inclusions spherical and improving the ultimate deformability and hole expandability of the steel sheet. In order to obtain such effects, the Zr content is preferably 0.0010% or more. On the other hand, when the Zr content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Zr is contained, the Zr content is preferably 0.0200% or less.
Caは、鋼中で介在物として存在する。ここで、Ca含有量が0.0200%を超えると、粗大な介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な介在物が穴広げ試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Caを含有させる場合、Ca含有量は0.0200%以下にすることが好ましい。Ca含有量は、好ましくは0.0020%以下である。なお、Ca含有量の下限は特に限定されるものではないが、Ca含有量は0.0005%以上が好ましい。また、生産技術上の制約から、Ca含有量は0.0010%以上がより好ましい。 Ca: 0.0200% or less,
Ca exists as inclusions in steel. Here, when the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be generated. In such a case, if diffusible hydrogen exists in the steel sheet, the coarse inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. Although the lower limit of the Ca content is not particularly limited, the Ca content is preferably 0.0005% or more. Moreover, from the restrictions on production technology, 0.0010% or more of Ca content is more preferable.
Ce、Se、Te、Ge、As、Sr、Cs、Hf、Pb、BiおよびREMはいずれも、鋼板の極限変形能、さらには穴広げ性を向上させるために有効な元素である。このような効果を得るためには、Ce、Se、Te、Ge、As、Sr、Cs、Hf、Pb、BiおよびREMの含有量はそれぞれ0.0001%以上にすることが好ましい。一方、Ce、Se、Te、Ge、As、Sr、Cs、Hf、Pb、BiおよびREMの含有量がそれぞれ0.0200%を超えると、粗大な析出物や介在物が多量に生成する場合がある。このような場合に、鋼板中に拡散性水素が存在すると、粗大な析出物や介在物が穴広げ試験時に亀裂の起点となる、すなわち、穴広げ性の低下を招くおそれがある。したがって、Ce、Se、Te、Ge、As、Sr、Cs、Hf、Pb、BiおよびREMのうちの少なくとも1種を含有させる場合、その含有量はそれぞれ0.0200%以下とすることが好ましい。 Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less and REM: 0.0200% or less Ce, Se, Te, Ge, As, Sr, Cs , Hf, Pb, Bi, and REM are all effective elements for improving the ultimate deformability and hole expandability of the steel sheet. In order to obtain such effects, the contents of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are each preferably 0.0001% or more. On the other hand, when the contents of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM each exceed 0.0200%, a large amount of coarse precipitates and inclusions may be generated. be. In such a case, if diffusible hydrogen exists in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when at least one of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM is included, the content thereof is preferably 0.0200% or less.
質量%で、
C:0.050%以上0.400%以下、
Si:0.20%以上3.00%以下、
Mn:1.00%以上3.50%未満、
P:0.001%以上0.100%以下、
S:0.0200%以下、
Al:0.010%以上2.000%以下および
N:0.0100%以下
であり、炭素当量Ceqが0.540%以上であり、
任意に、
Ti:0.200%以下、
Nb:0.200%以下、
V:0.100%以下、
B:0.0100%以下、
Cu:1.000%以下、
Cr:1.000%以下、
Ni:1.000%以下、
Mo:0.500%以下、
Sb:0.200%以下、
Sn:0.200%以下、
Ta:0.100%以下、
W:0.500%以下、
Mg:0.0200%以下、
Zn:0.0200%以下、
Co:0.0200%以下、
Zr:0.0200%以下、
Ca:0.0200%以下、
Ce:0.0200%以下、
Se:0.0200%以下、
Te:0.0200%以下、
Ge:0.0200%以下、
As:0.0200%以下、
Sr:0.0200%以下、
Cs:0.0200%以下、
Hf:0.0200%以下、
Pb:0.0200%以下、
Bi:0.0200%以下および
REM:0.0200%以下
のうちから選ばれる少なくとも1種を含有し、残部がFe及び不可避的不純物である、成分組成を有する。 That is, the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention is
in % by mass,
C: 0.050% or more and 0.400% or less,
Si: 0.20% or more and 3.00% or less,
Mn: 1.00% or more and less than 3.50%,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, and a carbon equivalent Ceq of 0.540% or more,
optionally,
Ti: 0.200% or less,
Nb: 0.200% or less,
V: 0.100% or less,
B: 0.0100% or less,
Cu: 1.000% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 0.500% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
Zr: 0.0200% or less,
Ca: 0.0200% or less,
Ce: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0200% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
It has a component composition containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less, and the balance being Fe and unavoidable impurities.
本発明の一実施形態に従う亜鉛めっき鋼板の下地鋼板の鋼組織は、
フェライトの面積率:65.0%以下(0%を含む)、
ベイニティックフェライトの面積率:5.0%以上40.0%以下、
焼戻しマルテンサイトの面積率:0.5%以上80.0%以下、
残留オーステナイトの面積率:3.0%以上、
フレッシュマルテンサイトの面積率:20.0%以下(0%を含む)、
SBF+STM+2×SMA:65.0%以上、
SMA1/SMA:0.80以下、および
SMA2/SMA:0.20以上
である、鋼組織である。
ここで、
SBF:前記ベイニティックフェライトの面積率
STM:前記焼戻しマルテンサイトの面積率
SMA:前記残留オーステナイトおよび前記フレッシュマルテンサイトからなる硬質第二相の面積率
SMA1:前記硬質第二相を構成する島状領域のうち、円相当直径が2.0μm以上であり、かつ、周長の20%以下が焼戻しマルテンサイトと接する島状領域の合計の面積率
SMA2:前記硬質第二相を構成する島状領域のうち、周長の1%以上がベイニティックフェライトと接する島状領域の合計の面積率
である。
以下、それぞれの限定理由について説明する。 Next, the steel structure of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention will be described.
The steel structure of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention is
Ferrite area ratio: 65.0% or less (including 0%),
Area ratio of bainitic ferrite: 5.0% or more and 40.0% or less,
Area ratio of tempered martensite: 0.5% or more and 80.0% or less,
Area ratio of retained austenite: 3.0% or more,
Area ratio of fresh martensite: 20.0% or less (including 0%),
S BF + STM + 2 x SMA : 65.0% or more,
A steel structure in which SMA1/SMA: 0.80 or less and SMA2 / SMA : 0.20 or more.
here,
S BF : Area ratio of the bainitic ferrite STM : Area ratio of the tempered martensite SMA : Area ratio of the hard second phase composed of the retained austenite and the fresh martensite SMA1 : The hard second phase Among the constituent island regions, the total area ratio of the island regions having an equivalent circle diameter of 2.0 μm or more and having 20% or less of the circumference contacting the tempered martensite SMA2 : The hard second phase Of the constituent island regions, 1% or more of the circumference is the total area ratio of the island regions in contact with the bainitic ferrite.
The reasons for each limitation will be described below.
軟質なフェライトは、延性および加工硬化能を向上させる相である。しかし、980MPa以上のTS、高いYS、および良好な穴広げ性を確保する観点から、フェライトの面積率は65.0%以下とする。フェライトの面積率は、好ましくは35.0%以下、より好ましくは25.0%以下である。フェライトの面積率の下限については特に限定されず、0%であってもよい。特に、980MPa≦TS<1180MPaが求められる場合、フェライトの面積率は、好ましくは5.0%以上である。 Ferrite area ratio: 65.0% or less (including 0%)
Soft ferrite is a phase that improves ductility and work hardenability. However, from the viewpoint of ensuring a TS of 980 MPa or more, a high YS, and good hole expandability, the area ratio of ferrite is set to 65.0% or less. The area ratio of ferrite is preferably 35.0% or less, more preferably 25.0% or less. The lower limit of the area ratio of ferrite is not particularly limited, and may be 0%. In particular, when 980 MPa≦TS<1180 MPa is required, the area ratio of ferrite is preferably 5.0% or more.
ベイニティックフェライトは、軟質なフェライトと硬質なフレッシュマルテンサイトなどとの中間の硬度を持ち、良好な穴広げ性を確保するために重要な相である。また、ベイニティックフェライトは、ベイニティックフェライトから未変態オーステナイトへのCの拡散を活用して、適正量の残留オーステナイトを得るためにも有用な相である。そのため、ベイニティックフェライトの面積率は5.0%以上とする。また、ベイニティックフェライトの面積率は、好ましくは10.0%以上である。一方、ベイニティックフェライトの面積率が過度に増加すると、却って穴広げ性が低下する。そのため、ベイニティックフェライトの面積率を40.0%以下とする。また、ベイニティックフェライトの面積率は、好ましくは35.0%以下である。 Area ratio of bainitic ferrite: 5.0% or more and 40.0% or less Bainitic ferrite has intermediate hardness between soft ferrite and hard fresh martensite, etc., and ensures good hole expandability. is an important phase for Bainitic ferrite is also a useful phase for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite. Therefore, the area ratio of bainitic ferrite is set to 5.0% or more. Also, the area ratio of bainitic ferrite is preferably 10.0% or more. On the other hand, if the area ratio of bainitic ferrite is excessively increased, the hole expansibility is rather lowered. Therefore, the area ratio of bainitic ferrite is set to 40.0% or less. Also, the area ratio of bainitic ferrite is preferably 35.0% or less.
焼戻しマルテンサイトは、軟質なフェライトと硬質なフレッシュマルテンサイトなどとの中間の硬度を持ち、良好な穴広げ性を確保するために重要な相である。そのため、焼戻しマルテンサイトの面積率は0.5%以上とする。焼戻しマルテンサイトの面積率は、好ましくは40.0%以上である。一方、良好な延性を確保する観点から、焼戻しマルテンサイトの面積率は80.0%以下とする。また、焼戻しマルテンサイトの面積率は、好ましくは75.0%以下である。 Area ratio of tempered martensite: 0.5% or more and 80.0% or less important phase. Therefore, the area ratio of tempered martensite is set to 0.5% or more. The area ratio of tempered martensite is preferably 40.0% or more. On the other hand, from the viewpoint of ensuring good ductility, the area ratio of tempered martensite is set to 80.0% or less. Further, the area ratio of tempered martensite is preferably 75.0% or less.
良好な延性を得る観点から、残留オーステナイトの面積率は3.0%以上とする。残留オーステナイトの面積率は、好ましくは5.0%以上である。なお、残留オーステナイトの面積率の上限については特に限定されないが、残留オーステナイトの面積率は20.0%以下が好ましい。 Area Ratio of Retained Austenite: 3.0% or More From the viewpoint of obtaining good ductility, the area ratio of retained austenite is made 3.0% or more. The area ratio of retained austenite is preferably 5.0% or more. Although the upper limit of the area ratio of retained austenite is not particularly limited, the area ratio of retained austenite is preferably 20.0% or less.
良好な穴広げ性を確保する観点から、フレッシュマルテンサイトの面積率は20.0%以下とする。なお、フレッシュマルテンサイトの面積率の下限については特に限定されず、0%であってもよい。また、980MPa以上のTSを確保する観点から、フレッシュマルテンサイトの面積率は3.0%以上が好ましい。
なお、フレッシュマルテンサイトとは、焼入れままの(焼戻しを受けていない)マルテンサイトである。 Area ratio of fresh martensite: 20.0% or less (including 0%)
The area ratio of fresh martensite is set to 20.0% or less from the viewpoint of ensuring good hole expandability. The lower limit of the area ratio of fresh martensite is not particularly limited, and may be 0%. Moreover, from the viewpoint of ensuring a TS of 980 MPa or more, the area ratio of fresh martensite is preferably 3.0% or more.
Fresh martensite is martensite as quenched (not tempered).
なお、残部組織としては、特に限定されず、例えば、下部ベイナイトやパーライト、セメンタイトなどの炭化物が挙げられる。なお、残部組織の種類は、例えば、SEM(Scanning Electron Microscope;走査電子顕微鏡)による観察で確認することができる。 In addition, the area ratio of the residual structure other than the above is preferably 10.0% or less. The area ratio of the residual tissue is more preferably 5.0% or less. Also, the area ratio of the residual tissue may be 0%.
The residual structure is not particularly limited, and examples thereof include carbides such as lower bainite, pearlite, and cementite. The type of residual tissue can be confirmed, for example, by observation using a SEM (Scanning Electron Microscope).
すなわち、下地鋼板の圧延方向に平行な板厚断面が観察面となるように、下地鋼板から試料を切り出す。ついで、ダイヤモンドペーストを用いて試料の観察面を鏡面研磨する。ついで、試料の観察面にコロイダルシリカを用いて仕上げ研磨を施したのち、3vol.%ナイタールでエッチングして組織を現出させる。
そして、SEM(Scanning Electron Microscope;走査電子顕微鏡)により、加速電圧:15kV、倍率:5000倍の条件で、試料の観察面の25.6μm×17.6μmの視野を5視野観察する。
得られた組織画像(例えば、図1(A)参照)から、以下のようにして、フェライト、ベイニティックフェライト、焼戻しマルテンサイトおよび硬質第二相(残留オーステナイト+フレッシュマルテンサイト)を同定する。 Here, the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard second phase (retained austenite + fresh martensite) are measured at the 1/4 thickness position of the base steel sheet as follows.
That is, a sample is cut out from the base steel plate so that the plate thickness cross-section parallel to the rolling direction of the base steel plate becomes the observation surface. Next, the observation surface of the sample is mirror-polished using diamond paste. Then, the observation surface of the sample was subjected to final polishing using colloidal silica, and then subjected to 3 vol. Etch with % nital to reveal the tissue.
Then, five fields of 25.6 μm×17.6 μm of the observation surface of the sample are observed under the conditions of an acceleration voltage of 15 kV and a magnification of 5000 using a SEM (Scanning Electron Microscope).
Ferrite, bainitic ferrite, tempered martensite, and hard second phase (retained austenite + fresh martensite) are identified as follows from the obtained structural image (see, for example, FIG. 1(A)).
ベイニティックフェライト:黒色から濃い灰色を呈した領域であり、形態は塊状や不定形などである。また、鉄系炭化物を内包しないか、比較的少数内包する。
焼戻しマルテンサイト:灰色を呈した領域であり、形態は不定形である。また、鉄系炭化物を比較的多数内包する。
硬質第二相(残留オーステナイト+フレッシュマルテンサイト):白色から薄い灰色を呈する領域であり、形態は不定形である。また、鉄系炭化物を内包しない。なお、サイズが比較的大きい場合には、他組織との界面から離れるにつれて次第に色が濃くなり、内部は濃い灰色を呈する場合がある。
残部組織:上述した下部ベイナイトやパーライト、セメンタイトなどの炭化物が挙げられ、これらの形態等は公知のとおりである。 Ferrite: A black region with a blocky shape. In addition, it contains almost no iron-based carbides. However, when iron-based carbide is included, the area of ferrite is also included in the area of ferrite. The same applies to bainitic ferrite and tempered martensite, which will be described later.
Bainitic ferrite: A black to dark gray region with a massive or irregular shape. In addition, it does not contain iron-based carbides or contains a relatively small number of iron-based carbides.
Tempered martensite: A gray area with an amorphous shape. In addition, it contains a relatively large number of iron-based carbides.
Hard second phase (retained austenite + fresh martensite): A white to pale gray region with an amorphous shape. Also, it does not include iron-based carbides. In addition, when the size is relatively large, the color gradually becomes darker away from the interface with the other tissue, and the inside may be dark gray.
Residual structure: carbides such as the above-mentioned lower bainite, pearlite, and cementite can be mentioned, and their forms and the like are known.
例えば、EBSDによる解析では、フェライトは下部組織を持たない(観察されない)。一方、ベイニティックフェライト、焼戻しマルテンサイトおよびフレッシュマルテンサイトは、下部組織を持ち、残留オーステナイトと特定の結晶方位関係を有する。また、これらの組織から焼鈍工程でのオーステナイト組織を再現し、これを確認することなどができる。このような点が、組織同定の判断材料となる。
また、EPMAによる成分分析では、組織によってC濃度やMn濃度が異なる点が、組織同定の判断材料となる。例えば、フェライトやベイニティックフェライトのC濃度は、焼戻しマルテンサイトを主とする領域(微細な硬質第二相や炭化物などを含む)のC濃度と比べて低い。また、フェライトでは、Mn濃度が他の組織と比べて低くなる場合がある。
微小硬度計による硬度測定では、組織によって硬度が異なる点が組織同定の判断材料となる。例えば、フェライト、ベイニティックフェライト、焼戻しマルテンサイトおよび硬質第二相のうち、フェライトが最も硬度が低く、硬質第二相が最も硬度が高い。また、ベイニティックフェライトおよび焼戻しマルテンサイトは、フェライトの硬度と硬質第二相の硬度と間の硬度を示す。 In addition to the above observation by SEM, observation of carbide at a higher magnification, detailed structural analysis by EBSD (electron beam backscatter diffraction method) in the same field of view, component analysis by EPMA (electron probe microanalyzer), Local hardness measurement using a microhardness meter may be added as appropriate. For example, when it is difficult to identify the tissue by the above-described SEM observation, it is effective to add these measurements as appropriate.
For example, in analysis by EBSD, ferrite has no substructure (not observed). On the other hand, bainitic ferrite, tempered martensite and fresh martensite have a substructure and a specific crystal orientation relationship with retained austenite. In addition, the austenite structure in the annealing process can be reproduced from these structures and confirmed. Such points serve as judgment materials for tissue identification.
Further, in the component analysis by EPMA, the fact that the C concentration and Mn concentration differ depending on the tissue serves as a criterion for identifying the tissue. For example, the C concentration of ferrite and bainitic ferrite is lower than the C concentration of a region mainly composed of tempered martensite (including fine hard secondary phases, carbides, etc.). Also, in ferrite, the Mn concentration may be lower than in other tissues.
In hardness measurement using a microhardness meter, the difference in hardness depending on the structure is a criterion for identifying the structure. For example, among ferrite, bainitic ferrite, tempered martensite and hard secondary phase, ferrite has the lowest hardness and hard secondary phase has the highest hardness. Bainitic ferrite and tempered martensite also exhibit a hardness between that of ferrite and that of hard secondary phases.
すなわち、下地鋼板を板厚方向(深さ方向)に板厚の1/4位置まで機械研削した後、シュウ酸による化学研磨を行い、観察面とする。ついで、観察面を、X線回折法により観察する。入射X線にはCoKα線を使用し、bcc鉄の(200)、(211)および(220)各面の回折強度に対するfcc鉄(オーステナイト)の(200)、(220)および(311)各面の回折強度の比を求め、各面の回折強度の比から、残留オーステナイトの体積率を算出する。そして、残留オーステナイトが三次元的に均質であるとみなして、残留オーステナイトの体積率を、残留オーステナイトの面積率とする。 Moreover, the area ratio of retained austenite is measured as follows.
That is, after mechanically grinding the substrate steel plate to the 1/4 position of the plate thickness in the plate thickness direction (depth direction), chemical polishing with oxalic acid is performed to obtain an observation surface. Then, the observation surface is observed by the X-ray diffraction method. CoKα rays were used as the incident X-rays, and the diffraction intensity of the (200), (211) and (220) planes of bcc iron (200), (220) and (311) planes of fcc iron (austenite) were compared. The ratio of the diffraction intensity of each surface is obtained, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensity of each surface. Then, assuming that the retained austenite is three-dimensionally homogeneous, the volume ratio of the retained austenite is defined as the area ratio of the retained austenite.
[フレッシュマルテンサイトの面積率(%)]=[硬質第二相の面積率(%)]-[残留オーステナイトの面積率(%)] The area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite from the area ratio of the hard second phase obtained as described above.
[Area ratio of fresh martensite (%)] = [Area ratio of hard second phase (%)] - [Area ratio of retained austenite (%)]
[残部組織の面積率(%)]=100-[フェライトの面積率(%)]-[ベイニティックフェライトの面積率(%)]-[焼戻しマルテンサイトの面積率(%)]-[硬質第二相の面積率(%)] Further, the area ratio of the residual structure is obtained by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of the hard second phase obtained as described above from 100%. Ask.
[Area ratio of residual structure (%)] = 100 - [Area ratio of ferrite (%)] - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Hard Second phase area ratio (%)]
980MPa以上のTSを確保する観点から、SBF+STM+2×SMAは、65.0%以上とする。SBF+STM+2×SMAの上限については特に限定されないが、130.0%以下であることが好ましい。
ここで、
SBF:ベイニティックフェライトの面積率
STM:焼戻しマルテンサイトの面積率
SMA:残留オーステナイトおよび前記フレッシュマルテンサイトからなる硬質第二相の面積率
である。 S BF + STM + 2 x SMA : 65.0% or more From the viewpoint of ensuring a TS of 980 MPa or more, S BF + STM + 2 x SMA is set to 65.0% or more. The upper limit of S BF +S TM +2×S MA is not particularly limited, but is preferably 130.0% or less.
here,
S BF : area ratio of bainitic ferrite STM : area ratio of tempered martensite SMA : area ratio of a hard second phase composed of retained austenite and fresh martensite.
残留オーステナイトおよびフレッシュマルテンサイトからなる硬質第二相(以下、MAともいう。)は、複数の島状領域から構成される。このような島状領域のうち、円相当直径が2.0μm以上であり、かつ、周長の20%以下が焼戻しマルテンサイトと接する島状領域(以下、MA1ともいう。)は、固溶C濃度が低い。換言すれば、MA1中に含まれる残留オーステナイトの安定性が低い。そのため、MA1は、良好な延性の確保に寄与しない。また、MA1ではフレッシュマルテンサイトの比率が高いため、MA1は穴広げ性を低下させる。そのため、硬質第二相の面積率に対するMA1の面積率の比であるSMA1/SMAは、0.80以下とする。特に、980MPa≦TS<1180MPaが求められる場合、SMA1/SMAは、好ましくは0.75以下、より好ましくは0.40以下である。また、1180MPa≦TSが求められる場合、SMA1/SMAは、好ましくは0.50以下、より好ましくは0.30以下である。なお、SMA1/SMAの下限は特に限定されず、0であってもよい。
なお、個々の島状領域は、硬質第二相以外の相により、他の硬質第二相の島状領域と分離される(個々の島状領域は、その全周が硬質第二相以外の相と接する)。また、個々の島状領域の具体的な形状については特に限定されず、例えば円形、楕円形、多角形、アメーバ形(複数の不規則方向に延伸した形状)などのいずれであってもよい。 SMA1 /SMA: 0.80 or less A hard second phase (hereinafter also referred to as MA ) composed of retained austenite and fresh martensite is composed of a plurality of island regions. Among such island regions, an island region (hereinafter also referred to as MA1) having an equivalent circle diameter of 2.0 μm or more and having a circumference of 20% or less in contact with tempered martensite (hereinafter also referred to as MA1) has a solid solution C Low concentration. In other words, the stability of retained austenite contained in MA1 is low. Therefore, MA1 does not contribute to ensuring good ductility. In addition, since MA1 has a high ratio of fresh martensite, MA1 reduces the hole expandability. Therefore, S MA1 /S MA , which is the ratio of the area ratio of MA1 to the area ratio of the hard second phase, is set to 0.80 or less. In particular, when 980 MPa≦TS<1180 MPa is required, S MA1 /S MA is preferably 0.75 or less, more preferably 0.40 or less. Further, when 1180 MPa≦TS is required, S MA1 /S MA is preferably 0.50 or less, more preferably 0.30 or less. The lower limit of S MA1 /S MA is not particularly limited, and may be zero.
Each island-shaped region is separated from other island-shaped regions of the second hard phase by a phase other than the hard second phase (individual island-shaped regions are phase). Further, the specific shape of each island-shaped region is not particularly limited, and may be, for example, circular, elliptical, polygonal, or amoeba-shaped (a shape extending in a plurality of irregular directions).
残留オーステナイトおよびフレッシュマルテンサイトからなる硬質第二相を構成する島状領域のうち、周長の1%以上がベイニティックフェライトと接する島状領域(以下、MA2ともいう。)は、固溶C濃度が高い。換言すれば、MA2中に含まれる残留オーステナイトの安定性が高い。そのため、MA2は、良好な加工硬化能および延性の確保に極めて重要な役割を果たす。
すなわち、ベイニティックフェライトを焼鈍後の冷却時に適切な条件で生成させると、ベイニティックフェライトから周囲の未変態オーステナイトに拡散する固溶Cは、未変態オーステナイトの内部へは十分に拡散しない。すなわち、未変態オーステナイトのうちベイニティックフェライトの周囲のみを局所的に固溶C量が高い状態にすることができる。その後、その状態で再加熱処理を適切な条件で行うことにより、ベイニティックフェライトの周囲に固溶C濃度が高い硬質第二相、すなわち、MA2が生成する。そのため、MA2に含まれる残留オーステナイトの安定性が高くなり、MA2が良好な加工硬化能および延性の確保に極めて重要な役割を果たすのである。
以上のことから、硬質第二相の面積率に対するMA2の面積率の比であるSMA2/SMAは、0.20以上とする。SMA2/SMAは、好ましくは0.25以上、より好ましくは 0.30以上である。なお、SMA2/SMAの上限は特に限定されず、1であってもよい。また、SMA2/SMAは、高いYSおよび優れた穴広げ性を確保する観点から、980MPa≦TS<1180MPaが求められる場合、好ましくは0.98以下である。また、1180MPa≦TSが求められる場合、SMA2/SMAは好ましくは0.70以下である。 SMA2 / SMA : 0.20 or more Of the island-shaped regions constituting the hard second phase composed of retained austenite and fresh martensite, 1% or more of the island-shaped region in contact with the bainitic ferrite of the circumference (hereinafter referred to as Also called MA2.) has a high dissolved C concentration. In other words, the stability of retained austenite contained in MA2 is high. Therefore, MA2 plays a very important role in ensuring good work hardenability and ductility.
That is, if bainitic ferrite is formed under appropriate conditions during cooling after annealing, solute C that diffuses from the bainitic ferrite into the surrounding untransformed austenite does not sufficiently diffuse into the untransformed austenite. That is, only the surroundings of the bainitic ferrite in the untransformed austenite can be locally made to have a high solid-solution C amount. After that, by performing reheating treatment under appropriate conditions in that state, a hard second phase having a high solid-solution C concentration, that is, MA2 is generated around the bainitic ferrite. Therefore, the stability of retained austenite contained in MA2 is increased, and MA2 plays an extremely important role in ensuring good work hardening ability and ductility.
Based on the above, S MA2 /S MA , which is the ratio of the area ratio of MA2 to the area ratio of the hard second phase, should be 0.20 or more. SMA2 / SMA is preferably 0.25 or more, more preferably 0.30 or more. The upper limit of S MA2 /S MA is not particularly limited, and may be one. Moreover, from the viewpoint of ensuring high YS and excellent hole expansibility , SMA2 /SMA is preferably 0.98 or less when 980 MPa≦TS<1180 MPa is required. Moreover, when 1180 MPa≦TS is required, S MA2 /S MA is preferably 0.70 or less.
ここで、
SMA3:前記硬質第二相を構成する島状領域のうち、周長の1%以上がベイニティックフェライトと接し、かつ、周長の20%超が焼戻しマルテンサイトと接する島状領域の合計の面積率
である。 Further, it is preferable that the steel structure of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention further has SMA3 / SMA of 0.05 or more.
here,
SMA3 : Of the island-shaped regions constituting the hard second phase, 1% or more of the peripheral length is in contact with bainitic ferrite, and more than 20% of the peripheral length is in contact with tempered martensite. is the area ratio of
残留オーステナイトおよびフレッシュマルテンサイトからなる硬質第二相を構成する島状領域のうち、周長の1%以上がベイニティックフェライトと接し、かつ、周長の20%超が焼戻しマルテンサイトと接する島状領域(以下、MA3ともいう。)は、MA2の中でも、特に固溶C濃度が高い。
すなわち、MA3では、ベイニティックフェライトに加え、焼戻しマルテンサイトからも固溶Cが拡散するため、特に固溶C濃度が高い。そのため、MA3は、良好な加工硬化能および延性の確保に特に有効に寄与する。
したがって、硬質第二相の面積率に対するMA3の面積率の比であるSMA3/SMAは、0.05以上とすることが好ましい。SMA3/SMAは、好ましくは0.07以上、より好ましくは0.10以上である。なお、SMA3/SMAの上限は特に限定されず、1であってもよい。また、SMA3/SMAは、好ましくは0.70以下である。 SMA3 / SMA : 0.05 or more Of the island-shaped regions that constitute the hard second phase composed of retained austenite and fresh martensite, 1% or more of the peripheral length is in contact with the bainitic ferrite, and 1% or more of the peripheral length An island-shaped region (hereinafter also referred to as MA3) in which more than 20% is in contact with tempered martensite has a particularly high dissolved C concentration among MA2.
That is, in MA3, in addition to bainitic ferrite, solid solution C diffuses from tempered martensite, so the solid solution C concentration is particularly high. Therefore, MA3 particularly effectively contributes to ensuring good work hardening ability and ductility.
Therefore, S MA3 /S MA , which is the ratio of the area ratio of MA3 to the area ratio of the hard second phase, is preferably 0.05 or more. SMA3 / SMA is preferably 0.07 or more, more preferably 0.10 or more. The upper limit of S MA3 /S MA is not particularly limited, and may be one. Also, S MA3 /S MA is preferably 0.70 or less.
すなわち、前述の要領で、組織画像(例えば、図2(A)、図3(A)、および図4(A)参照)において、フェライト、ベイニティックフェライト、焼戻しマルテンサイトおよび硬質第二相(残留オーステナイト+フレッシュマルテンサイト)を同定する。ついで、Adobe Systems社のAdobe Photoshopを用いて色分け(4値化画像化)した後、硬質第二相の島状領域を抽出し、オープンソースのImageJを用いて、各島状領域の円相当径、各島状領域の周長、ならびに、各島状領域がベイニティックフェライトおよび焼戻しマルテンサイトと接する長さを求める。なお、周長を求める際の組織画像のピクセル密度は、30ピクセル/μm以上100ピクセル/μm以下とする。そして、求めた値から、各島状領域をMA1、MA2およびMA3に該当するかそれぞれ判別し、Adobe Systems社のAdobe Photoshopを用いて色分けし(例えば、図2(B)、図3(B)、および図4(B)参照)、それぞれの面積を算出する。ついで、MA1、MA2およびMA3と判別した島状領域それぞれの合計の面積を観察領域の面積(25.6μm×17.6μm)で除し、100を乗じた値(面積率)を5視野分算出する。そして、MA1、MA2およびMA3ごとの5視野分の値(面積率)の平均値を、SMA1、SMA2およびSMA3とする。なお、MA1とMA2の双方に該当する島状領域については、MA1とMA2の双方で面積をカウントする。MA1とMA3、MA2とMA3についても同様である。また、図2(A)、図3(A)、および図4(A)はそれぞれ、試料の観察領域(25.6μm×17.6μm)の1視野から、上記の説明のためにその一部を抽出したものである。 Here, S MA1 , S MA2 and S MA3 are each measured as follows.
That is, in the manner described above, ferrite, bainitic ferrite, tempered martensite and hard second phase ( (retained austenite + fresh martensite). Next, after color-coding (quaternary imaging) using Adobe Photoshop from Adobe Systems, the island-shaped regions of the hard second phase were extracted, and the equivalent circle diameter of each island-shaped region was measured using the open source ImageJ. , the perimeter of each island region, and the length of contact of each island region with bainitic ferrite and tempered martensite. Note that the pixel density of the tissue image when determining the circumference is set to 30 pixels/μm or more and 100 pixels/μm or less. Then, from the obtained values, it is determined whether each island-shaped area corresponds to MA1, MA2, or MA3, respectively, and color-coded using Adobe Photoshop of Adobe Systems (for example, FIG. 2 (B), FIG. 3 (B) , and FIG. 4(B)), each area is calculated. Then, the total area of each of the island-shaped regions identified as MA1, MA2, and MA3 was divided by the area of the observed region (25.6 μm×17.6 μm) and multiplied by 100 to calculate the value (area ratio) for 5 fields of view. do. Then, the average values of the values (area ratios) for five fields of view for each of MA1, MA2 and MA3 are defined as S MA1 , S MA2 and S MA3 . Note that for an island-shaped region corresponding to both MA1 and MA2, the area is counted for both MA1 and MA2. The same is true for MA1 and MA3, and MA2 and MA3. 2(A), 3(A), and 4(A) are each from one field of view of the observation area (25.6 μm×17.6 μm) of the sample, and a part thereof for the above explanation. is extracted.
より優れた穴広げ性を得る観点から、下地鋼板の拡散性水素量は0.50質量ppm以下とすることが好ましい。また、下地鋼板の拡散性水素量は、より好ましくは0.35質量ppm以下である。なお、下地鋼板の拡散性水素量の下限は特に規定されず、0質量ppmであってもよい。また、生産技術上の制約から、下地鋼板の拡散性水素量は0.01質量ppm以上がより好ましい。 Amount of diffusible hydrogen in substrate steel sheet: 0.50 ppm by mass or less From the viewpoint of obtaining more excellent hole expandability, the amount of diffusible hydrogen in the substrate steel sheet is preferably 0.50 ppm by mass or less. Further, the amount of diffusible hydrogen in the base steel sheet is more preferably 0.35 ppm by mass or less. The lower limit of the amount of diffusible hydrogen in the base steel sheet is not particularly specified, and may be 0 ppm by mass. Moreover, due to production technology restrictions, the amount of diffusible hydrogen in the base steel sheet is more preferably 0.01 ppm by mass or more.
すなわち、亜鉛めっき鋼板から長さが30mm、幅が5mmの試験片を採取し、亜鉛めっき層をアルカリ除去する。ついで、昇温脱離分析法により、試験片から放出される水素量を測定する。具体的には、試験片を、室温から300℃までを昇温速度200℃/hで連続加熱した後、室温まで冷却する。この際、当該連続加熱における室温から210℃までの温度域で、試験片から放出される水素量(積算水素量)を測定する。そして、測定した水素量を、試験片(亜鉛めっき層除去後で、連続加熱前の試験片)の質量で除し、質量ppm単位に換算した値を、下地鋼板の拡散性水素量とする。 Here, the amount of diffusible hydrogen in the base steel sheet is measured as follows.
Specifically, a test piece having a length of 30 mm and a width of 5 mm is taken from a galvanized steel sheet, and the galvanized layer is removed with an alkali. Then, the amount of hydrogen released from the test piece is measured by thermal desorption spectroscopy. Specifically, the test piece is continuously heated from room temperature to 300° C. at a heating rate of 200° C./h, and then cooled to room temperature. At this time, the amount of hydrogen released from the test piece (accumulated amount of hydrogen) is measured in the temperature range from room temperature to 210° C. during the continuous heating. Then, the measured amount of hydrogen is divided by the mass of the test piece (the test piece after removing the galvanized layer and before continuous heating), and the value converted to mass ppm is taken as the diffusible hydrogen amount of the base steel sheet.
ただし、測定時のコンタミネーション対策の必要性は、使用する機種やコンディションによるため、必ずしも上記構成は必須ではない。すなわち、測定条件は十分な精度が得られていることが確認できていればよく、測定条件は本発明の効果に本質的に関わるものではない。 Reference 1: Yamashita et al., “Carbon distribution at the initial stage of proeutectoid ferritic transformation of low carbon steel by high-precision FE-EPMA,” Tetsu to Hagane, Vol. 103 (2017) No. 11. p14-20
However, the need for measures against contamination during measurement depends on the model and conditions used, so the above configuration is not necessarily essential. In other words, the measurement conditions only need to confirm that sufficient accuracy is obtained, and the measurement conditions are not essentially related to the effects of the present invention.
本発明の一実施形態に従う亜鉛系めっき鋼板の引張強さは、980MPa以上である。本発明の一実施形態に従う亜鉛系めっき鋼板の引張強さは、好ましくは1180MPa以上である。 Tensile strength (TS): 980 MPa or more The tensile strength of the galvanized steel sheet according to one embodiment of the present invention is 980 MPa or more. The tensile strength of the galvanized steel sheet according to one embodiment of the present invention is preferably 1180 MPa or more.
なお、ここでいう亜鉛めっき層は、Znを主成分(Zn含有量が50%以上)とするめっき層を指し、例えば、溶融亜鉛めっき層や合金化溶融亜鉛めっき層が挙げられる。
ここで、溶融亜鉛めっき層は、例えば、Znと、20質量%以下のFe、0.001質量%以上1.0質量%以下のAlにより構成することが好適である。また、溶融亜鉛めっき層には、任意に、Pb、Sb、Si、Sn、Mg、Mn、Ni、Cr、Co、Ca、Cu、Li、Ti、Be、BiおよびREMからなる群から選ばれる1種または2種以上の元素を合計で0質量%以上3.5質量%以下含有させてもよい。また、溶融亜鉛めっき層のFe含有量は、より好ましくは7質量%未満である。なお、上記の元素以外の残部は、不可避的不純物である。
また、合金化溶融亜鉛めっき層は、例えば、20質量%以下のFe、0.001質量%以上1.0質量%以下のAlにより構成することが好適である。また、合金化溶融亜鉛めっき層には、任意に、Pb、Sb、Si、Sn、Mg、Mn、Ni、Cr、Co、Ca、Cu、Li、Ti、Be、BiおよびREMからなる群から選ばれる1種または2種以上の元素を合計で0質量%以上3.5質量%以下含有させてもよい。合金化溶融亜鉛めっき層のFe含有量は、より好ましくは7質量%以上、さらに好ましくは8質量%以上である。また、合金化溶融亜鉛めっき層のFe含有量は、より好ましくは15質量%以下、さらに好ましくは12質量%以下である。なお、上記の元素以外の残部は、不可避的不純物である。 Moreover, the galvanized layer of the galvanized steel sheet according to one embodiment of the present invention may be provided only on one surface of the base steel sheet, or may be provided on both surfaces.
Here, the galvanized layer refers to a galvanized layer containing Zn as a main component (Zn content is 50% or more), and examples thereof include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer.
Here, the hot-dip galvanized layer is preferably composed of, for example, Zn, 20% by mass or less of Fe, and 0.001% by mass or more and 1.0% by mass or less of Al. In addition, the hot-dip galvanized layer optionally includes one selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM. A total of 0 mass % or more and 3.5 mass % or less of the seed or two or more elements may be contained. Further, the Fe content of the hot-dip galvanized layer is more preferably less than 7% by mass. The remainder other than the above elements is unavoidable impurities.
Also, the alloyed hot-dip galvanized layer is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al. Also, the alloyed hot-dip galvanized layer is optionally selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM. 0% by mass or more and 3.5% by mass or less in total of one or two or more elements of The Fe content of the alloyed hot-dip galvanized layer is more preferably 7% by mass or more, and still more preferably 8% by mass or more. Further, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15% by mass or less, still more preferably 12% by mass or less. The remainder other than the above elements is unavoidable impurities.
すなわち、10質量%塩酸水溶液1Lに対し、Feに対する腐食抑制剤(朝日化学工業(株)製「イビット700BK」(登録商標))を0.6g添加した処理液を調整する。ついで、該処理液に、供試材となる亜鉛めっき鋼板を浸漬し、亜鉛めっき層を溶解させる。そして、溶解前後での供試材の質量減少量を測定し、その値を、下地鋼板の表面積(めっきで被覆されていた部分の表面積)で除することにより、めっき付着量(g/m2)を算出する。 In addition, the plating adhesion amount of the galvanized layer is measured as follows.
That is, a treatment liquid is prepared by adding 0.6 g of a corrosion inhibitor against Fe ("Ibit 700BK" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10% by mass hydrochloric acid aqueous solution. Then, a galvanized steel sheet as a test material is immersed in the treatment liquid to dissolve the galvanized layer. Then, the mass reduction amount of the test material before and after melting was measured, and the value was divided by the surface area of the base steel plate (the surface area of the part coated with plating) to obtain the coating amount (g / m 2 ) is calculated.
つぎに、本発明の一実施形態に従う部材について、説明する。
本発明の一実施形態に従う部材は、上記の亜鉛めっき鋼板を用いてなる(素材とする)部材である。例えば、素材である亜鉛めっき鋼板に、成形加工または接合加工の少なくとも一方を施して部材とする。
ここで、上記の亜鉛めっき鋼板は、TS:980MPa以上であり、かつ、高いYSと、優れた延性、加工硬化能および穴広げ性とを有する。そのため、本発明の一実施形態に従う部材は、高強度であり、かつ、耐衝撃特性にも優れている。したがって、本発明の一実施形態に従う部材は、自動車分野で使用される衝撃エネルギー吸収部材に適用して特に好適である。 [2] Member Next, a member according to one embodiment of the present invention will be described.
A member according to one embodiment of the present invention is a member (as a raw material) using the galvanized steel sheet described above. For example, a galvanized steel sheet, which is a raw material, is subjected to at least one of molding and joining to form a member.
Here, the galvanized steel sheet has a TS of 980 MPa or more, a high YS, and excellent ductility, work hardening ability and hole expandability. Therefore, the member according to one embodiment of the present invention has high strength and excellent impact resistance. Therefore, the member according to one embodiment of the present invention is particularly suitable for application as an impact energy absorbing member used in the automobile field.
つぎに、本発明の一実施形態に従う亜鉛めっき鋼板の製造方法について、説明する。 [3] Method for producing galvanized steel sheet Next, a method for producing a galvanized steel sheet according to one embodiment of the present invention will be described.
上記の成分組成を有する鋼スラブに熱間圧延を施して熱延鋼板とする、熱延工程と、
前記熱延鋼板を冷間圧延して冷延鋼板とする、冷延工程と、
前記冷延鋼板を、焼鈍温度:760℃以上900℃以下および焼鈍時間:20秒以上で焼鈍する、焼鈍工程と、
前記冷延鋼板を300℃以上550℃以下の第一冷却停止温度まで冷却する、第一冷却工程と、
前記冷延鋼板を300℃以上550℃以下の温度域で3秒以上600秒以下保持する、保持工程と、
前記冷延鋼板に亜鉛めっき処理を施して亜鉛めっき鋼板とする、めっき工程と、
前記亜鉛めっき鋼板を、100℃以上300℃未満の第二冷却停止温度まで冷却する第二冷却工程と、
前記亜鉛めっき鋼板を、(前記第二冷却停止温度+50℃)以上500℃以下の再加熱温度に再加熱し、前記亜鉛めっき鋼板を、(前記第二冷却停止温度+50℃)以上500℃以下の温度域で10秒以上2000秒以下保持する、再加熱工程と、
を有し、
前記第一冷却停止温度と、前記亜鉛めっき処理での亜鉛めっき浴の温度とが、次式(1)の関係を満足する、というものである。
-150℃≦T0-T1≦50℃ ・・・(1)
ここで、T0は第一冷却停止温度(℃)、T1は亜鉛めっき処理での亜鉛めっき浴の温度(℃)である。
なお、上記の各温度は、特に説明がない限り、鋼スラブおよび鋼板の表面温度を意味する。 A method for manufacturing a galvanized steel sheet according to one embodiment of the present invention comprises:
A hot-rolling step of hot-rolling a steel slab having the above chemical composition to form a hot-rolled steel sheet;
A cold rolling step of cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet;
An annealing step of annealing the cold-rolled steel sheet at an annealing temperature of 760° C. or more and 900° C. or less and an annealing time of 20 seconds or more;
a first cooling step of cooling the cold-rolled steel sheet to a first cooling stop temperature of 300° C. or higher and 550° C. or lower;
a holding step of holding the cold-rolled steel sheet in a temperature range of 300° C. or higher and 550° C. or lower for 3 seconds or more and 600 seconds or less;
a plating step of galvanizing the cold-rolled steel sheet to form a galvanized steel sheet;
A second cooling step of cooling the galvanized steel sheet to a second cooling stop temperature of 100° C. or more and less than 300° C.;
The galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower, and the galvanized steel sheet is heated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower. A reheating step of holding in the temperature range for 10 seconds or more and 2000 seconds or less;
has
The first cooling stop temperature and the temperature of the zinc plating bath in the zinc plating process satisfy the relationship of the following formula (1).
−150° C.≦T 0 −T 1 ≦50° C. (1)
Here, T0 is the first cooling stop temperature (°C), and T1 is the temperature (°C) of the zinc plating bath in the zinc plating treatment.
Note that each of the above temperatures means the surface temperature of the steel slab and steel plate, unless otherwise specified.
ついで、鋼スラブに熱間圧延を施して熱延鋼板とする。
熱間圧延は、省エネルギープロセスを適用して行ってもよい。省エネルギープロセスとしては、直送圧延(鋼スラブを室温まで冷却せずに、温片のままで加熱炉に装入し、熱間圧延する方法)または直接圧延(鋼スラブにわずかの保熱を行った後に直ちに圧延する方法)などが挙げられる。
熱間圧延条件については特に限定されず、例えば、以下の条件で行うことができる。
すなわち、鋼スラブを、一旦室温まで冷却し、その後、再加熱してから圧延する。スラブ加熱温度(再加熱温度)は、炭化物の溶解や圧延荷重の低減といった観点から、1100℃以上とすることが好ましい。また、スケールロスの増大を防止するため、スラブ加熱温度は1300℃以下とすることが好ましい。なお、スラブ加熱温度は、鋼スラブ表面の温度を基準とする。
ついで、鋼スラブに、常法に従い粗圧延を施し、粗圧延板(以下、シートバーともいう)とする。ついで、シートバーに仕上げ圧延を施して、熱延鋼板とする。なお、スラブ加熱温度を低めにした場合は、仕上げ圧延時のトラブルを防止する観点から、仕上げ圧延前にバーヒーターなどを用いてシートバーを加熱することが好ましい。仕上げ圧延温度は、圧延負荷を低減するため、Ar3変態点以上とすることが好ましい。また、オーステナイトの未再結晶状態での圧下率が高くなると、圧延方向に伸長した異常な組織が発達し、焼鈍板の加工性を低下させるおそれがあることからも、仕上げ圧延温度はAr3変態点以上とすることが好ましい。なお、Ar3変態点は次式により求める。
Ar3(℃)=868-396×[C%]+25×[Si%]-68[Mn%]
なお、上記の式中の[元素記号%]は、下地鋼板の成分組成における当該元素の含有量(質量%)を表す。 [Hot rolling process]
Then, the steel slab is hot rolled to obtain a hot rolled steel sheet.
Hot rolling may be performed by applying an energy saving process. Energy-saving processes include direct rolling (a method in which steel slabs are not cooled to room temperature, but are charged into a heating furnace and hot rolled) or direct rolling (a steel slab is slightly heat-retained). a method of rolling immediately afterwards).
The hot rolling conditions are not particularly limited, and the hot rolling can be performed under the following conditions, for example.
That is, the steel slab is once cooled to room temperature, then reheated and then rolled. The slab heating temperature (reheating temperature) is preferably 1100° C. or higher from the viewpoint of dissolving carbides and reducing the rolling load. Moreover, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300° C. or less. The slab heating temperature is based on the surface temperature of the steel slab.
Next, the steel slab is subjected to rough rolling according to a conventional method to obtain a rough rolled plate (hereinafter also referred to as sheet bar). Then, the sheet bar is subjected to finish rolling to obtain a hot-rolled steel sheet. When the slab heating temperature is lowered, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling from the viewpoint of preventing troubles during finish rolling. In order to reduce the rolling load, the finish rolling temperature is preferably the Ar 3 transformation point or higher. In addition, if the rolling reduction in the non-recrystallized state of austenite increases , an abnormal structure elongated in the rolling direction may develop, which may reduce the workability of the annealed sheet. A point or more is preferable. The Ar 3 transformation point is obtained by the following formula.
Ar 3 (° C.)=868−396×[C%]+25×[Si%]−68[Mn%]
[Element symbol %] in the above formula represents the content (% by mass) of the element in the chemical composition of the base steel sheet.
粗圧延および仕上げ圧延を含む熱間圧延工程では、一般的に鋼スラブは粗圧延でシートバーとなり、仕上げ圧延によって熱延鋼板となる。ただし、ミル能力等によってはそのような区分けにこだわらず、所定のサイズになれば問題ない。
仕上げ圧延温度は、800℃以上950℃以上の範囲とすることが好ましい。仕上げ圧延温度を800℃以上にすることにより、熱延鋼板段階の鋼組織、ひいては、最終製品の鋼組織も均一になり易い。なお、鋼組織が不均一になると、曲げ性が低下する傾向がある。一方、仕上げ圧延温度が950℃を超えると、酸化物(スケール)生成量が多くなる。その結果、地鉄と酸化物の界面が荒れて、酸洗および冷間圧延後の鋼板の表面品質が劣化するおそれがある。また、結晶粒が粗大になることで、鋼板の強度や曲げ性を低下させる原因となるおそれもある。
仕上げ圧延後、熱延鋼板を巻き取る。巻取温度は、450℃以上750℃以下とすることが好ましい。 Note that the sheet bars may be joined together during hot rolling, and finish rolling may be performed continuously. Also, the sheet bar may be wound once before finishing rolling. In order to reduce the rolling load during hot rolling, part or all of the finish rolling may be lubricated rolling. Performing lubricating rolling is also effective from the viewpoint of homogenizing the shape of the steel sheet and homogenizing the quality of the steel sheet. The coefficient of friction during lubricating rolling is preferably in the range of 0.10 or more and 0.25 or less.
In hot rolling processes including rough rolling and finish rolling, generally steel slabs are rough rolled into sheet bars and finish rolled into hot rolled steel sheets. However, depending on the mill capacity, there is no problem as long as the predetermined size is achieved regardless of such classification.
The finish rolling temperature is preferably in the range of 800°C or higher and 950°C or higher. By setting the finish rolling temperature to 800° C. or higher, the steel structure at the stage of the hot-rolled steel sheet and, by extension, the steel structure of the final product tend to be uniform. In addition, when the steel structure becomes non-uniform, the bendability tends to decrease. On the other hand, if the finish rolling temperature exceeds 950°C, the amount of oxide (scale) produced increases. As a result, the interface between the base iron and the oxide may become rough, and the surface quality of the steel sheet after pickling and cold rolling may deteriorate. In addition, coarsening of the crystal grains may cause deterioration in the strength and bendability of the steel sheet.
After finishing rolling, the hot-rolled steel sheet is wound up. The winding temperature is preferably 450° C. or higher and 750° C. or lower.
熱延工程後の熱延鋼板を、任意に、酸洗する。酸洗によって、鋼板表面の酸化物を除去することができ、良好な化成処理性やめっき品質が確保される。なお、酸洗は、1回のみ行ってもよく、複数回に分けて行ってもよい。酸洗条件については特に限定されず、常法に従えばよい。 [Pickling process]
Optionally, the hot-rolled steel sheet after the hot-rolling process is pickled. By pickling, oxides on the surface of the steel sheet can be removed, and good chemical conversion treatability and plating quality are ensured. In addition, pickling may be performed only once, and may be divided into several times and may be performed. The pickling conditions are not particularly limited, and conventional methods may be followed.
ついで、熱延鋼板に冷間圧延を施して冷延鋼板とする。冷間圧延は、例えば、タンデム式の多スタンド圧延やリバース圧延等の、2パス以上のパス数を要する多パス圧延により行う。
冷間圧延の圧下率は特に限定されないが、20%以上80%以下とすることが好ましい。冷間圧延の圧下率が20%未満では、焼鈍工程において鋼組織の粗大化や不均一化が生じやすくなり、最終製品において強度や加工性が低下するおそれがある。一方、冷間圧延の圧下率が80%を超えると、鋼板の形状不良が生じやすくなり、亜鉛めっきの付着量が不均一になるおそれがある。
また、任意に、冷間圧延後に得られた冷延鋼板に酸洗を施してもよい。 [Cold rolling process]
Then, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. Cold rolling is performed, for example, by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.
Although the rolling reduction in cold rolling is not particularly limited, it is preferably 20% or more and 80% or less. If the rolling reduction in cold rolling is less than 20%, the steel structure tends to become coarse and non-uniform in the annealing process, and the strength and workability of the final product may deteriorate. On the other hand, if the rolling reduction in cold rolling exceeds 80%, the shape of the steel sheet tends to be defective, and the amount of galvanized coating may become non-uniform.
Optionally, the cold-rolled steel sheet obtained after cold rolling may also be pickled.
また、本発明の一実施形態に従う亜鉛めっき鋼板の製造方法では、任意に、冷延工程後で、かつ、後述する焼鈍工程の前に、上記のようにして得られた冷延鋼板の少なくとも一方の表面に金属めっき層を形成する金属めっき処理を施してもよい。ここで、後述する焼鈍工程を経る前の状態で、少なくとも一方の表面に金属めっき層を有する冷延鋼板を、以下、金属めっき鋼板という場合がある。金属めっき処理方法は特に限定されないが、製造性の観点から電気めっきが好ましい。金属めっき浴としては硫酸浴、塩酸浴または両者の混合溶液などを使用できる。金属めっき層の付着量は、電気めっきの場合、通電時間等によって調整することができる。なお、金属めっき鋼板とは、上述したように、後述する焼鈍工程を経る前の状態で、冷延鋼板の少なくとも一方の表面に金属めっき層を有する鋼板を意味し、金属めっき処理前の冷延鋼板について予め焼鈍された態様を除外するものではない。 [Metal plating process]
In addition, in the method for producing a galvanized steel sheet according to one embodiment of the present invention, at least one of the cold-rolled steel sheets obtained as described above is optionally added after the cold rolling step and before the annealing step described later. A metal plating treatment may be applied to form a metal plating layer on the surface of the . Here, a cold-rolled steel sheet having a metal-plated layer on at least one surface before undergoing an annealing step, which will be described later, may be hereinafter referred to as a metal-plated steel sheet. The metal plating method is not particularly limited, but electroplating is preferable from the viewpoint of manufacturability. As the metal plating bath, a sulfuric acid bath, a hydrochloric acid bath, or a mixed solution of both can be used. In the case of electroplating, the adhesion amount of the metal plating layer can be adjusted by the energization time or the like. In addition, as described above, the metal-plated steel sheet means a steel sheet having a metal-plated layer on at least one surface of the cold-rolled steel sheet before undergoing the annealing process described later. It does not exclude the aspect in which the steel sheet is pre-annealed.
ついで、上記のようにして得られた冷延鋼板(金属めっき鋼板の場合も含む)を、焼鈍温度:760℃以上900℃以下および焼鈍時間:20秒以上で焼鈍する。なお、焼鈍回数は2回以上でもよいが、エネルギー効率の観点から1回が好ましい。 [Annealing process]
Then, the cold-rolled steel sheets (including metal plated steel sheets) obtained as described above are annealed at an annealing temperature of 760° C. or more and 900° C. or less for an annealing time of 20 seconds or more. The number of annealing times may be two or more, but one time is preferable from the viewpoint of energy efficiency.
焼鈍温度が760℃未満の場合、フェライトとオーステナイトの二相域での加熱中におけるオーステナイトの生成割合が不十分になる。そのため、焼鈍後にフェライトの面積率が過度に増加して、YSが低下する。また、穴広げ性も低下する。さらに、TSを980MPa以上とすることが困難になる。一方、焼鈍温度が900℃を超えると、オーステナイトの粒成長が過度に生じ、後工程でのベイニティックフェライトの生成速度が遅くなる。これにより、適正量のベイニティックフェライトおよび残留オーステナイトの面積率が得られなくなる。そのため、SMA2/SMAが低下し、延性および加工硬化能が低下する。従って、焼鈍温度は760℃以上900℃以下とする。焼鈍温度は、好ましくは780℃以上、より好ましく790℃超である。また、焼鈍温度は、好ましくは880℃以下である。なお、焼鈍温度は、焼鈍工程での最高到達温度である。 Annealing temperature: 760° C. or higher and 900° C. or lower If the annealing temperature is lower than 760° C., the rate of austenite formation during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite excessively increases after annealing, and YS decreases. In addition, the hole expansibility is also lowered. Furthermore, it becomes difficult to make TS 980 MPa or more. On the other hand, if the annealing temperature exceeds 900° C., the grain growth of austenite occurs excessively, and the formation rate of bainitic ferrite in the post-process slows down. This makes it impossible to obtain appropriate amounts of bainitic ferrite and retained austenite area ratios. Therefore, SMA2 / SMA decreases, and ductility and work hardening ability decrease. Therefore, the annealing temperature should be 760° C. or higher and 900° C. or lower. The annealing temperature is preferably 780°C or higher, more preferably above 790°C. Also, the annealing temperature is preferably 880° C. or lower. The annealing temperature is the highest temperature reached in the annealing process.
焼鈍時間が20秒未満になると、フェライトとオーステナイトの二相域での加熱中におけるオーステナイトの生成割合が不十分になる。そのため、焼鈍後にフェライトの面積率が過度に増加して、YSが低下する。また、穴広げ性も低下する。さらに、TSを980MPa以上とすることが困難になる。そのため、焼鈍時間は20秒以上とする。なお、焼鈍時間の上限は特に限定されないが、900秒以下とすることが好ましい。なお、焼鈍時間とは、(焼鈍温度-40℃)以上焼鈍温度以下の温度域での保持時間である。すなわち、焼鈍時間には、焼鈍温度での保持時間に加え、焼鈍温度に到達する前後の加熱および冷却における(焼鈍温度-40℃)以上焼鈍温度以下の温度域での滞留時間も含まれる。 Annealing time: 20 seconds or more If the annealing time is less than 20 seconds, the rate of austenite formation during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite excessively increases after annealing, and YS decreases. In addition, the hole expansibility is also lowered. Furthermore, it becomes difficult to set TS to 980 MPa or more. Therefore, the annealing time should be 20 seconds or more. Although the upper limit of the annealing time is not particularly limited, it is preferably 900 seconds or less. The annealing time is the holding time in the temperature range of (annealing temperature -40°C) to the annealing temperature. That is, the annealing time includes, in addition to the holding time at the annealing temperature, the holding time in the temperature range above (annealing temperature -40°C) and below the annealing temperature in heating and cooling before and after reaching the annealing temperature.
また、本発明の一実施形態に従う亜鉛めっき鋼板の製造方法では、焼鈍工程における焼鈍雰囲気の露点を-30℃超とすることが好ましい。露点を-30℃超とすることにより、脱炭反応が促進され、冷延鋼板(下地鋼板)の表層のC濃度を低減して、脱炭層を形成することが可能となる。露点は、好ましくは-20℃以上、より好ましくは-5℃以上である。露点を-5℃以上とすることにより、溶接部における耐抵抗溶接割れ特性を一層高めることが可能となる。露点の上限は特に限定されないが、冷延鋼板または金属めっき層表面の酸化を好適に防ぎ、亜鉛めっき層を設ける際のめっき密着性を良好にする観点から、露点は30℃以下とすることが好ましい。 Dew Point: Over -30°C In the method for manufacturing a galvanized steel sheet according to an embodiment of the present invention, the dew point of the annealing atmosphere in the annealing step is preferably over -30°C. By setting the dew point above −30° C., the decarburization reaction is promoted, the C concentration in the surface layer of the cold-rolled steel sheet (base steel sheet) is reduced, and a decarburized layer can be formed. The dew point is preferably -20°C or higher, more preferably -5°C or higher. By setting the dew point to −5° C. or more, it is possible to further improve the resistance weld crack resistance of the weld zone. The upper limit of the dew point is not particularly limited, but from the viewpoint of suitably preventing oxidation of the surface of the cold-rolled steel sheet or metal plating layer and improving the plating adhesion when providing the zinc plating layer, the dew point is preferably 30 ° C. or less. preferable.
ついで、上記のようにして焼鈍を施した冷延鋼板を、300℃以上550℃以下の第一冷却停止温度まで冷却する。 [First cooling step]
Next, the cold-rolled steel sheet annealed as described above is cooled to a first cooling stop temperature of 300°C or higher and 550°C or lower.
第一冷却停止温度が300℃未満になると、焼戻しマルテンサイトの面積率が過度に増加し、適正量のベイニティックフェライトおよび残留オーステナイトの面積率が得られなくなる。また、後工程である亜鉛めっき処理においいて、未変態オーステナイトがパーライトや炭化物に分解する場合がある。そのため、SMA2/SMAおよびSMA3/SMAが低下し、延性および加工硬化能が低下する。一方、第一冷却停止温度が550℃を超えると、ベイニティックフェライトの面積率が減少し、焼戻しマルテンサイトの面積率が過度に増加する。また、SMA2/SMA、さらにはSMA3/SMAが低下し、延性および加工硬化能が低下する。したがって、第一冷却停止温度は300℃以上550℃以下とする。第一冷却停止温度は、好ましくは350℃以上である。また、第一冷却停止温度は、好ましくは510℃以下とする。 First cooling stop temperature: 300° C. or higher and 550° C. or lower When the first cooling stop temperature is lower than 300° C., the area ratio of tempered martensite increases excessively, and the area ratio of appropriate amounts of bainitic ferrite and retained austenite decreases. will not be obtained. In addition, untransformed austenite may decompose into pearlite and carbides in the subsequent galvanizing process. Therefore, SMA2 / SMA and SMA3 / SMA are decreased, and ductility and work hardening ability are decreased. On the other hand, when the first cooling stop temperature exceeds 550°C, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite excessively increases. In addition, SMA2 / SMA and further SMA3 / SMA decrease, and ductility and work hardening ability decrease. Therefore, the first cooling stop temperature is set to 300°C or higher and 550°C or lower. The first cooling stop temperature is preferably 350° C. or higher. Also, the first cooling stop temperature is preferably 510° C. or lower.
ついで、冷延鋼板を300℃以上550℃以下の温度域(以下、保持温度域ともいう)で3秒以上600秒以下保持する。 [Holding step]
Then, the cold-rolled steel sheet is held in a temperature range of 300° C. or higher and 550° C. or lower (hereinafter also referred to as a holding temperature range) for 3 seconds or longer and 600 seconds or shorter.
保持工程では、ベイニティックフェライトが生成するとともに、生成したベイニティックフェライトから該ベイニティックフェライトに隣接する未変態のオーステナイトへのCの拡散が生じる。その結果、所定量の残留オーステナイトの面積率が確保され、SMA2/SMA、さらにはSMA3/SMAが増加する。
ここで、保持温度域での保持時間が3秒未満になると、ベイニティックフェライトの面積率が減少し、焼戻しマルテンサイトの面積率が過度に増加する。また、SMA2/SMA、さらにはSMA3/SMAが低下し、延性および加工硬化能が低下する。一方、保持温度域での保持時間が600秒を超えると、ベイニティックフェライトの面積率が過度に増加し、YSが低下するおそれがある。また、ベイニティックフェライトから未変態オーステナイトへのCの拡散が過度に生じ、SMA1/SMAが増加し、穴広げ性が低下するおそれがある。さらに、未変態オーステナイト内部でのCの拡散が過度に生じ、ベイニティックフェライトの周囲の未変態オーステナイトのみを局所的に固溶C量が高い状態にすることができなくなる。その結果、SMA2/SMA、さらにはSMA3/SMAが低下し、延性が低下するおそれがある。したがって、保持温度域での保持時間は、3秒以上600秒以下とする。保持温度域での保持時間は、好ましくは5秒以上、より好ましくは10秒以上である。また、保持温度域での保持時間は、好ましくは200秒未満、より好ましくは80秒未満である。なお、保持温度域での保持時間には、第一冷却工程において第一冷却停止温度に到達するまでの当該温度域での滞留時間、および、後述するめっき工程における亜鉛めっき処理開始時点までの冷延鋼板の当該温度域での滞留時間(例えば、冷延鋼板を亜鉛めっき浴に浸漬させるまでの当該温度域での滞留時間)が含まれる。ただし、保持温度域での保持時間には、当該めっき工程において溶融亜鉛めっき処理を施した後の亜鉛めっき鋼板の当該温度域での滞留時間は含まない。 Holding time in the holding temperature range: 3 seconds to 600 seconds In the holding step, bainitic ferrite is generated, and C diffuses from the generated bainitic ferrite to untransformed austenite adjacent to the bainitic ferrite. occurs. As a result, a predetermined amount of area ratio of retained austenite is secured, and SMA2 / SMA and further SMA3 / SMA are increased.
Here, when the holding time in the holding temperature range is less than 3 seconds, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite excessively increases. In addition, SMA2 / SMA and further SMA3 / SMA decrease, and ductility and work hardening ability decrease. On the other hand, if the holding time in the holding temperature range exceeds 600 seconds, the area ratio of bainitic ferrite may excessively increase, resulting in a decrease in YS. In addition, C may excessively diffuse from bainitic ferrite to untransformed austenite, resulting in an increase in SMA1 / SMA and a decrease in hole expansibility. Furthermore, C diffuses excessively inside the untransformed austenite, and it becomes impossible to make only the untransformed austenite around the bainitic ferrite locally high in solid solution C amount. As a result, S MA2 /S MA and further S MA3 /S MA may decrease and ductility may decrease. Therefore, the holding time in the holding temperature range is 3 seconds or more and 600 seconds or less. The holding time in the holding temperature range is preferably 5 seconds or longer, more preferably 10 seconds or longer. Also, the holding time in the holding temperature range is preferably less than 200 seconds, more preferably less than 80 seconds. In addition, the retention time in the retention temperature range includes the retention time in the temperature range until the first cooling stop temperature is reached in the first cooling step, and the cooling time until the start of zinc plating treatment in the plating step described later. It includes the residence time of the rolled steel sheet in the relevant temperature range (for example, the residence time in the relevant temperature range until the cold-rolled steel sheet is immersed in the galvanizing bath). However, the holding time in the holding temperature range does not include the residence time of the galvanized steel sheet after hot-dip galvanizing in the plating process.
ついで、冷延鋼板に亜鉛めっき処理を施して亜鉛めっき鋼板とする。亜鉛めっき処理としては、例えば、溶融亜鉛めっき処理や合金化亜鉛めっき処理が挙げられる。そして、このめっき工程では、上述した第一冷却工程における第一冷却停止温度と、亜鉛めっき処理での亜鉛めっき浴の温度(以下、めっき浴温ともいう)とについて、次式(1)の関係を満足させることが必要である。
-150℃≦T0-T1≦50℃ ・・・(1)
ここで、T0は第一冷却停止温度(℃)、T1は亜鉛めっき処理での亜鉛めっき浴の温度(℃)である。 [Plating process]
Then, the cold-rolled steel sheet is subjected to galvanizing treatment to obtain a galvanized steel sheet. Examples of galvanizing include hot dip galvanizing and alloyed galvanizing. Then, in this plating step, the first cooling stop temperature in the first cooling step described above and the temperature of the zinc plating bath in the zinc plating treatment (hereinafter also referred to as the plating bath temperature) have the following relationship: It is necessary to satisfy
−150° C.≦T 0 −T 1 ≦50° C. (1)
Here, T0 is the first cooling stop temperature (°C), and T1 is the temperature (°C) of the zinc plating bath in the zinc plating treatment.
例えば、溶融亜鉛めっき処理の場合、冷延鋼板を、亜鉛めっき浴中に浸漬させた後、ガスワイピング等によって、めっき付着量を調整することが好ましい。めっき浴温としては、440℃以上500℃以下である。また、亜鉛めっき浴としては、上記した亜鉛めっき層の組成となれば特に限定されるものではないが、例えば、Al含有量が0.10質量%以上0.23質量%以下であり、残部がZnおよび不可避的不純物からなる組成のめっき浴を用いることが好ましい。
また、合金化亜鉛めっき処理の場合、上記の要領で溶融亜鉛めっき処理を施した後、亜鉛めっき鋼板を450℃以上600℃以下の合金化温度に加熱して合金化処理を施すことが好ましい。合金化温度が450℃未満では、Zn-Fe合金化速度が遅くなり、合金化が困難となる場合がある。一方、合金化温度が600℃を超えると、未変態オーステナイトがパーライトへ変態し、TSおよび延性が低下する場合がある。なお、合金化温度は、より好ましくは470℃以上である。また、合金化温度は、より好ましくは570℃以下である。 Conditions other than the above are not particularly limited, and may be performed in accordance with conventional methods.
For example, in the case of hot-dip galvanizing, it is preferable to adjust the coating weight by gas wiping or the like after the cold-rolled steel sheet is immersed in a galvanizing bath. The plating bath temperature is 440° C. or higher and 500° C. or lower. The zinc plating bath is not particularly limited as long as it has the composition of the zinc plating layer described above, but for example, the Al content is 0.10% by mass or more and 0.23% by mass or less, and the balance is It is preferable to use a plating bath with a composition consisting of Zn and unavoidable impurities.
In the case of galvannealing, it is preferable to heat the galvanized steel sheet to an alloying temperature of 450° C. or higher and 600° C. or lower after hot-dip galvanizing as described above. If the alloying temperature is lower than 450° C., the Zn—Fe alloying speed becomes slow and alloying may become difficult. On the other hand, if the alloying temperature exceeds 600° C., untransformed austenite may transform into pearlite, resulting in a decrease in TS and ductility. The alloying temperature is more preferably 470° C. or higher. Also, the alloying temperature is more preferably 570° C. or lower.
ついで、亜鉛めっき鋼板を、100℃以上300℃未満の第二冷却停止温度まで冷却する。 [Second cooling step]
Next, the galvanized steel sheet is cooled to a second cooling stop temperature of 100°C or more and less than 300°C.
第二冷却工程は、後工程である再加熱工程で生成する焼戻しマルテンサイトの面積率および残留オーステナイトの面積率を所定の範囲に制御とするために必要な工程である。ここで、第二冷却停止温度が100℃未満では、当該第二冷却工程において鋼中に存在する未変態オーステナイトが、ほぼ全量マルテンサイトに変態する。これにより、焼戻しマルテンサイトの面積率が過度に増加し、残留オーステナイトの面積率が減少する。その結果、延性および加工硬化能が低下する。一方、第二冷却停止温度が300℃以上では、焼戻しマルテンサイトの面積率が減少し、フレッシュマルテンサイトの面積率が増加する。このフレッシュマルテンサイトの面積率の増加に伴い、鋼板中の拡散性水素量が増加し、穴広げ性が低下する。また、SMA1/SMAが増加することによっても、穴広げ性が低下する。したがって、第二冷却停止温度は100℃以上300℃未満とする。第二冷却停止温度は、好ましくは120℃以上である。また、第二冷却停止温度は、好ましくは280℃以下である。 Second cooling stop temperature: 100 ° C. or more and less than 300 ° C. The second cooling step is performed in order to control the area ratio of tempered martensite and the area ratio of retained austenite generated in the reheating step, which is a subsequent step, within a predetermined range. This is a necessary process. Here, when the second cooling stop temperature is less than 100°C, substantially all of the untransformed austenite present in the steel is transformed into martensite in the second cooling step. This excessively increases the area ratio of tempered martensite and decreases the area ratio of retained austenite. As a result, ductility and work hardenability are reduced. On the other hand, when the second cooling stop temperature is 300° C. or higher, the area ratio of tempered martensite decreases and the area ratio of fresh martensite increases. As the area ratio of fresh martensite increases, the amount of diffusible hydrogen in the steel sheet increases, and the hole expandability decreases. Further, an increase in S MA1 /S MA also reduces the hole expansibility. Therefore, the second cooling stop temperature is set at 100°C or higher and lower than 300°C. The second cooling stop temperature is preferably 120°C or higher. Also, the second cooling stop temperature is preferably 280° C. or less.
ついで、亜鉛めっき鋼板を、(前記第二冷却停止温度+50℃)以上500℃以下の再加熱温度に再加熱し、前記亜鉛めっき鋼板を、(前記第二冷却停止温度+50℃)以上500℃以下の温度域(以下、再加熱温度域ともいう)で10秒以上2000秒以下保持する。
これにより、第二冷却工程終了時点で鋼中に存在するマルテンサイトを焼戻す。また、マルテンサイト中に過飽和に固溶したCを未変態オーステナイトへと拡散させることにより、室温で安定なオーステナイト、すなわち、残留オーステナイトを生成させる。 [Reheating process]
Next, the galvanized steel sheet is reheated to a reheating temperature of (said second cooling stop temperature + 50°C) or more and 500°C or less, and the galvanized steel sheet is heated to (said second cooling stop temperature + 50°C) or more and 500°C or less. (hereinafter also referred to as reheating temperature range) for 10 seconds or more and 2000 seconds or less.
This tempers the martensite present in the steel at the end of the second cooling step. In addition, by diffusing C supersaturated as a solid solution in martensite into untransformed austenite, austenite stable at room temperature, that is, retained austenite is generated.
再加熱温度が(冷却停止温度+50℃)未満になると、第二冷却工程終了時点で鋼中に存在するマルテンサイトから未変態オーステナイトへのCの拡散が十分には進行せず、所定量の残留オーステナイトの面積率が得られない。これにより、延性が低下する。また、フレッシュマルテンサイトが増加する。さらに、下地鋼板に含まれる水素の外部放出が不十分となり、下地鋼板の拡散性水素量が増加する。これにより、穴広げ性が低下する。一方、再加熱温度が500℃を超えると、第二冷却工程終了時点で鋼中に存在するマルテンサイトの焼戻しが過度に進行するため、TSを980MPa以上とすることが困難になる。また、第二冷却工程終了時点で鋼中に存在する未変態オーステナイトが、炭化物(パーライト)として分解してしまうため、延性が低下する。さらに、下地鋼板に含まれる水素の外部放出が不十分となり、下地鋼板の拡散性水素量が増加する。これにより、穴広げ性が低下する。したがって、再加熱温度は(冷却停止温度+50℃)以上500℃以下とする。再加熱温度は、好ましくは(冷却停止温度+70℃)以上である。また、再加熱温度は、好ましくは450℃以下である。なお、再加熱温度は、再加熱工程での最高到達温度である。 Reheating temperature: (second cooling stop temperature + 50 ° C) or more and 500 ° C or less When the reheating temperature is less than (cooling stop temperature + 50 ° C), the martensite present in the steel at the end of the second cooling process is not transformed. Diffusion of C into austenite does not proceed sufficiently, and a predetermined amount of area ratio of retained austenite cannot be obtained. This reduces ductility. Moreover, fresh martensite increases. Furthermore, the release of hydrogen contained in the base steel plate to the outside becomes insufficient, and the amount of diffusible hydrogen in the base steel plate increases. As a result, the hole expansibility is lowered. On the other hand, if the reheating temperature exceeds 500°C, tempering of martensite present in the steel proceeds excessively at the end of the second cooling step, making it difficult to increase the TS to 980 MPa or more. In addition, untransformed austenite present in the steel at the end of the second cooling step decomposes as carbide (pearlite), resulting in a decrease in ductility. Furthermore, the release of hydrogen contained in the base steel plate to the outside becomes insufficient, and the amount of diffusible hydrogen in the base steel plate increases. As a result, the hole expansibility is lowered. Therefore, the reheating temperature should be (cooling stop temperature + 50°C) or more and 500°C or less. The reheating temperature is preferably (cooling stop temperature + 70°C) or higher. Also, the reheating temperature is preferably 450° C. or lower. Note that the reheating temperature is the highest temperature reached in the reheating process.
再加熱温度域での保持時間が10秒未満になると、第二冷却工程終了時点で鋼中に存在するマルテンサイトから未変態オーステナイトへのCの拡散が十分には進行せず、所定量の残留オーステナイトの面積率が得られない。これにより、延性が低下する。また、フレッシュマルテンサイトが増加することに加えて、下地鋼板に含まれる水素の外部放出が不十分となり、下地鋼板の拡散性水素量が増加する。これにより、穴広げ性が低下するおそれもある。一方、再加熱温度域での保持時間が2000秒を超えると、第二冷却工程終了時点で鋼中に存在するマルテンサイトの焼戻しが過度に進行するため、TSを980MPa以上とすることが困難になる。また、第二冷却工程終了時点で鋼中に存在する未変態オーステナイトが、炭化物(パーライト)として分解してしまうため、延性が低下する。したがって、再加熱温度域での保持時間は10秒以上2000秒以下とする。再加熱温度域での保持時間は、好ましくは15秒以上である。また、再加熱温度域での保持時間は、好ましくは1200秒以下である。なお、再加熱温度域での保持時間には、再加熱温度での保持時間に加え、再加熱温度に到達する前後の加熱および冷却における当該温度域での滞留時間も含まれる。 Holding time in the reheating temperature range: 10 seconds or more and 2000 seconds or less When the holding time in the reheating temperature range is less than 10 seconds, the martensite present in the steel at the end of the second cooling process transforms into untransformed austenite. Diffusion of C does not proceed sufficiently, and a predetermined amount of area ratio of retained austenite cannot be obtained. This reduces ductility. In addition to the increase in fresh martensite, the release of hydrogen contained in the base steel sheet to the outside becomes insufficient, and the amount of diffusible hydrogen in the base steel sheet increases. As a result, there is a possibility that the hole expansibility may be deteriorated. On the other hand, if the holding time in the reheating temperature range exceeds 2000 seconds, the martensite present in the steel is excessively tempered at the end of the second cooling process, making it difficult to increase the TS to 980 MPa or more. Become. In addition, untransformed austenite present in the steel at the end of the second cooling step decomposes as carbide (pearlite), resulting in a decrease in ductility. Therefore, the holding time in the reheating temperature range is set to 10 seconds or more and 2000 seconds or less. The holding time in the reheating temperature range is preferably 15 seconds or longer. Further, the holding time in the reheating temperature range is preferably 1200 seconds or less. The retention time in the reheating temperature range includes not only the retention time at the reheating temperature but also the retention time in the temperature range during heating and cooling before and after reaching the reheating temperature.
つぎに、本発明の一実施形態に従う部材の製造方法について、説明する。
本発明の一実施形態に従う部材の製造方法は、上記の亜鉛めっき鋼板(例えば、上記の亜鉛めっき鋼板の製造方法により製造された亜鉛めっき鋼板)に、成形加工または接合加工の少なくとも一方を施して部材とする、工程を有する。
ここで、成形加工方法は、特に限定されず、例えば、プレス加工等の一般的な加工方法を用いることができる。また、接合加工方法も、特に限定されず、例えば、スポット溶接、レーザー溶接、アーク溶接等の一般的な溶接や、リベット接合、かしめ接合等を用いることができる。なお、成形条件および接合条件については特に限定されず、常法に従えばよい。 [4] Member Manufacturing Method Next, a member manufacturing method according to an embodiment of the present invention will be described.
A method for manufacturing a member according to an embodiment of the present invention includes subjecting the galvanized steel sheet (for example, a galvanized steel sheet manufactured by the method for manufacturing a galvanized steel sheet) to at least one of forming and joining. It has a step of forming a member.
Here, the molding method is not particularly limited, and for example, a general processing method such as press working can be used. Also, the joining method is not particularly limited, and for example, general welding such as spot welding, laser welding, arc welding, riveting, caulking, or the like can be used. The molding conditions and bonding conditions are not particularly limited, and conventional methods may be followed.
表1に示す成分組成(残部はFe及び不可避的不純物)を有する鋼素材を転炉にて溶製し、連続鋳造法にて鋼スラブとした。得られた鋼スラブを1250℃に加熱し、加熱後、鋼スラブに粗圧延と仕上げ圧延からなる熱間圧延を施し、熱延鋼板とした。ついで、得られた熱延鋼板に、酸洗および冷間圧延(圧下率:50%)を施し、表3に示す板厚の冷延鋼板とした。ついで、得られた冷延鋼板に、表2に示す条件で、焼鈍工程、第一冷却工程、保持工程、めっき工程、第二冷却工程および再加熱工程を行い、亜鉛めっき鋼板を得た。なお、焼鈍工程での露点は、-35℃~-30℃とした。 ・Example 1
A steel material having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting. The obtained steel slab was heated to 1250° C. After heating, the steel slab was subjected to hot rolling including rough rolling and finish rolling to obtain a hot rolled steel sheet. Then, the obtained hot-rolled steel sheets were pickled and cold-rolled (rolling reduction: 50%) to obtain cold-rolled steel sheets having thicknesses shown in Table 3. Then, the obtained cold-rolled steel sheets were subjected to an annealing process, a first cooling process, a holding process, a plating process, a second cooling process and a reheating process under the conditions shown in Table 2 to obtain galvanized steel sheets. The dew point in the annealing process was -35°C to -30°C.
めっき付着量は、GIを製造する場合は、片面あたり45~72g/m2とし、GAを製造する場合は、片面あたり45g/m2とした。
なお、最終的に得られた亜鉛めっき鋼板の亜鉛めっき層の組成は、GIでは、Fe:0.1~1.0質量%、Al:0.2~1.0質量%を含有し、残部がZnおよび不可避的不純物であった。また、GAでは、Fe:7~15質量%、Al:0.1~1.0質量%を含有し、残部がZnおよび不可避的不純物であった。
また、亜鉛めっき層はいずれも、下地鋼板の両面に形成した。 As the zinc plating bath, when manufacturing GI, a plating bath having a composition containing 0.20% by mass of Al and the balance being Zn and unavoidable impurities was used. When manufacturing GA, a plating bath containing 0.14% by mass of Al with the balance being Zn and unavoidable impurities was used.
The amount of plating deposited was 45 to 72 g/m 2 per side when manufacturing GI, and 45 g/m 2 per side when manufacturing GA.
The composition of the galvanized layer of the galvanized steel sheet finally obtained is, in GI, Fe: 0.1 to 1.0% by mass, Al: 0.2 to 1.0% by mass, and the balance was Zn and unavoidable impurities. GA contained 7 to 15% by mass of Fe, 0.1 to 1.0% by mass of Al, and the balance was Zn and unavoidable impurities.
All galvanized layers were formed on both sides of the base steel plate.
・TS
〇(合格):980MPa以上
×(不合格):980MPa未満
・YS
〇(合格):
980MPa≦TS<1180MPaの場合、550MPa≦YS
1180MPa≦TS<1310MPaの場合、700MPa≦YS
1310MPa≦TSの場合、800MPa≦YS
×(不合格):
980MPa≦TS<1180MPaの場合、550MPa>YS
1180MPa≦TS<1310MPaの場合、700MPa>YS
1310MPa≦TSの場合、800MPa>YS
・El
〇(合格):
980MPa≦TS<1180MPaの場合、13.0%≦El
1180MPa≦TS<1310MPaの場合、12.0%≦El
1310MPa≦TSの場合、10.0%≦El
×(不合格):
980MPa≦TS<1180MPaの場合、13.0%>El
1180MPa≦TS<1310MPaの場合、12.0%>El
1310MPa≦TSの場合、10.0%>El
・n値/YR
〇(合格):n値/YR≧0.070
×(不合格):n値/YR<0.070
・λ
〇(合格):20%以上
×(不合格):20%未満 In addition, a tensile test and a hole expansion test are performed according to the following procedures, and according to the following criteria, tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR) and critical hole expansion ratio (λ) were evaluated.
・TS
○ (passed): 980 MPa or more × (failed): less than 980 MPa YS
〇 (Passed):
When 980 MPa ≤ TS < 1180 MPa, 550 MPa ≤ YS
When 1180 MPa ≤ TS < 1310 MPa, 700 MPa ≤ YS
800 MPa ≤ YS when 1310 MPa ≤ TS
× (failed):
When 980 MPa ≤ TS < 1180 MPa, 550 MPa > YS
When 1180 MPa ≤ TS < 1310 MPa, 700 MPa > YS
When 1310 MPa ≤ TS, 800 MPa > YS
・El
〇 (Passed):
When 980 MPa ≤ TS < 1180 MPa, 13.0% ≤ El
When 1180 MPa ≤ TS < 1310 MPa, 12.0% ≤ El
10.0% ≤ El when 1310 MPa ≤ TS
× (failed):
When 980 MPa ≤ TS < 1180 MPa, 13.0% > El
When 1180 MPa ≤ TS < 1310 MPa, 12.0% > El
10.0%> El when 1310 MPa ≤ TS
・n value/YR
○ (passed): n value / YR ≥ 0.070
× (failed): n value / YR < 0.070
・λ
○ (passed): 20% or more × (failed): less than 20%
引張試験は、JIS Z 2241に準拠して行った。すなわち、得られた亜鉛めっき鋼板から、長手方向が下地鋼板の圧延方向に対して直角となるようにJIS5号試験片を採取した。採取した試験片を用いて、クロスヘッド速度が10mm/minの条件で引張試験を行い、TS、YS、Elおよびn値を測定した。ここで、n値は、均一伸び(U-El)の0.4倍および0.8倍の時の伸びと強度から算出した。また、測定したYS、TSおよびn値から、降伏比YR(=YS/TS)およびn値/YRの値を算出した。なお、n値/YRの値は加工硬化能を表し、鋼板の成形性と耐衝撃特性とを総合的に評価する指標となるものである。結果を表3に併記する。 (1) Tensile test A tensile test was performed according to JIS Z 2241. That is, a JIS No. 5 test piece was taken from the obtained galvanized steel sheet so that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. Using the sampled test piece, a tensile test was performed at a crosshead speed of 10 mm/min, and TS, YS, El and n values were measured. Here, the n value was calculated from the elongation and strength at 0.4 and 0.8 times the uniform elongation (U-El). Also, the yield ratio YR (=YS/TS) and n value/YR were calculated from the measured YS, TS and n values. The value of n value/YR represents the work hardening ability and serves as an index for comprehensively evaluating the formability and impact resistance of the steel sheet. The results are also shown in Table 3.
穴広げ試験は、JIS Z 2256に準拠して行った。すなわち、得られた亜鉛めっき鋼板から、100mm×100mmの試験片を剪断加工により採取した。該試験片に、クリアランスを12.5%として直径10mmの穴を打ち抜いた。ついで、内径:75mmのダイスを用いて穴の周囲にしわ押さえ力:9ton(88.26kN)を加え、そのた状態で頂角:60°の円錐ポンチを穴に押し込み、亀裂発生限界(亀裂発生時)における試験片の穴の直径を測定した。そして、次式により、限界穴広げ率:λ(%)を求めた。なお、λは、伸びフランジ性を評価する指標となるものである。結果を表3に併記する。
λ(%)={(Df-D0)/D0}×100
ここで、
Df:亀裂発生時の試験片の穴の直径(mm)
D0:初期の試験片の穴の直径(mm)
である。 (2) Hole expanding test The hole expanding test was performed according to JIS Z 2256. That is, from the obtained galvanized steel sheet, a test piece of 100 mm x 100 mm was cut by shearing. A 10 mm diameter hole was punched in the specimen with a clearance of 12.5%. Then, using a die with an inner diameter of 75 mm, a wrinkle holding force of 9 tons (88.26 kN) is applied around the hole, and a conical punch with an apex angle of 60° is pushed into the hole to reach the crack initiation limit (crack initiation The diameter of the hole in the test piece was measured. Then, the limit hole expansion rate: λ (%) was obtained from the following equation. λ is an index for evaluating stretch flangeability. The results are also shown in Table 3.
λ (%) = {(D f −D 0 )/D 0 }×100
here,
D f : Diameter of hole in test piece when crack occurs (mm)
D 0 : diameter of hole in initial specimen (mm)
is.
一方、比較例では、引張強さ(TS)、降伏応力(YS)、全伸び(El)、加工硬化指数(n値)/降伏比(YR)、限界穴広げ率(λ)の少なくとも1つが十分ではなかった。
また、本発明例の鋼板を用いて、成形加工を施して得た部材または接合加工を施して得た部材は、引張強さ(TS)、降伏応力(YS)、全伸び(El)、加工硬化指数(n値)/降伏比(YR)、限界穴広げ率(λ)ともに、本発明で特徴とする優れた特性を有することがわかった。 As shown in Table 3, all the examples of the present invention have tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR), limit hole All spread ratios (λ) were acceptable.
On the other hand, in the comparative example, at least one of tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR), limit hole expansion ratio (λ) it wasn't enough.
Further, using the steel plate of the present invention, the member obtained by molding or the member obtained by bonding has tensile strength (TS), yield stress (YS), total elongation (El), working It was found that both the hardening index (n value)/yield ratio (YR) and the critical hole expansion ratio (λ) have the excellent properties characteristic of the present invention.
表1に示す成分組成(残部はFe及び不可避的不純物)を有する鋼素材を転炉にて溶製し、連続鋳造法にて鋼スラブとした。得られた鋼スラブを1250℃に加熱し、加熱後、鋼スラブに粗圧延と仕上げ圧延からなる熱間圧延を施し、熱延鋼板とした。ついで、得られた熱延鋼板に、酸洗および冷間圧延(圧下率:50%)を施し、板厚1.6mmの冷延鋼板とした。 ・Example 2
A steel material having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting. The obtained steel slab was heated to 1250° C. After heating, the steel slab was subjected to hot rolling including rough rolling and finish rolling to obtain a hot rolled steel sheet. Then, the obtained hot-rolled steel sheet was pickled and cold-rolled (rolling reduction: 50%) to obtain a cold-rolled steel sheet having a thickness of 1.6 mm.
[電解条件]
浴温:50℃
pH:2.0
電流密度:45A/dm2
めっき浴:Fe2+イオンを1.5mol/L含む硫酸浴
陽極:酸化イリジウム電極
なお、金属めっき層の付着量は通電時間によって制御した。 Next, among the obtained cold-rolled steel sheets, No. For Nos. 8 to 10, Fe-based electroplating was performed as metal plating treatment to form a metal plating layer (Fe-based plating layer) on the surface of the cold-rolled steel sheet. Specifically, first, the cold-rolled steel sheet was degreased with an alkali. Then, under the conditions shown below, the cold-rolled steel sheet was used as a cathode and electrolytic treatment was performed to form a metal plating layer on the surface of the cold-rolled steel sheet.
[Electrolysis conditions]
Bath temperature: 50°C
pH: 2.0
Current density: 45A/ dm2
Plating bath: Sulfuric acid bath containing 1.5 mol/L of Fe 2+ ions Anode: Iridium oxide electrode The adhesion amount of the metal plating layer was controlled by the current application time.
得られた亜鉛めっき鋼板から圧延直角方向(TD)を長手、圧延方向を短手として、長手方向150mm×短手方向50mmに切り出した試験片2を、試験用合金化溶融亜鉛めっき鋼板1(板厚:1.6mm、TS:980MPa級)と重ねて板組とした。なお、試験用合金化溶融亜鉛めっき鋼板1は、合金化溶融亜鉛めっき層の片面あたりの付着量が50g/m2であり、試験片2と同サイズに切り出したものである。板組は、試験片2の評価対象面(亜鉛めっき層および金属めっき層を一方の側のみに有する場合には、その側の亜鉛めっき層)と、試験用合金化溶融亜鉛めっき鋼板1の亜鉛めっき層とが向かい合うように組み立てた。当該板組を、厚さ2.0mmのスペーサー3を介して、固定台4に固定した。スペーサー3は、長手方向50mm×短手方向45mm×厚さ2.0mmの一対の鋼板であり、図5(A)に示すように、一対の鋼板各々の長手方向端面が、板組短手方向両端面とそろうように配置した。よって、一対の鋼板間の距離は60mmとなる。固定台8は、中央部に穴が開いた一枚の板である。 <Evaluation of Resistance Weld Cracking Resistance in Welds>
A
A+:ホールドタイム0.12秒、0.18秒および0.24秒のいずれの場合にも、0.1mm以上の長さのき裂が認められなかった。
A:ホールドタイム0.12秒で0.1mm以上の長さのき裂が認められたが、ホールドタイム0.18秒および0.24秒では0.1mm以上の長さのき裂が認められなかった。
B:ホールドタイム0.12秒および0.18秒で0.1mm以上の長さのき裂が認められたが、ホールドタイム0.24秒では0.1mm以上の長さのき裂が認められなかった。
C:ホールドタイム0.12秒、0.18秒および0.24秒のいずれの場合にも、0.1mm以上の長さのき裂が認められた。 Next, the plate set with the welded portion is cut along the line AA in the upper diagram of FIG. Observation was made with a microscope (200x magnification), and resistance weld crack resistance in the weld zone was evaluated according to the following criteria. If it is A+, A or B, it is judged that the resistance weld crack resistance in the weld is excellent. If it is C, it is judged that the resistance weld crack resistance in the weld is inferior. The results are also shown in Table 7.
A+: No cracks having a length of 0.1 mm or more were observed at any of the hold times of 0.12 seconds, 0.18 seconds and 0.24 seconds.
A: Cracks with a length of 0.1 mm or longer were observed at a hold time of 0.12 seconds, but cracks with a length of 0.1 mm or longer were observed at hold times of 0.18 seconds and 0.24 seconds. I didn't.
B: Cracks with a length of 0.1 mm or longer were observed at hold times of 0.12 seconds and 0.18 seconds, but cracks with a length of 0.1 mm or longer were observed at a hold time of 0.24 seconds. I didn't.
C: Cracks with a length of 0.1 mm or longer were observed at all hold times of 0.12 seconds, 0.18 seconds and 0.24 seconds.
加えて、No.1、6~10の発明例、なかでもNo.8および9の発明例では、溶接部における耐抵抗溶接割れ特性が非常に優れていた。
また、本発明例の鋼板を用いて、成形加工を施して得た部材または接合加工を施して得た部材は、引張強さ(TS)、降伏応力(YS)、全伸び(El)、加工硬化指数(n値)/降伏比(YR)、限界穴広げ率(λ)、溶接部における耐抵抗溶接割れ特性ともに、本発明で特徴とする優れた特性を有することがわかった。 As shown in Table 7, in all invention examples, tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR), limit hole expansion All of the ratios (λ) passed. In addition, the resistance weld crack resistance in the weld zone was also excellent.
In addition, no. 1, 6 to 10 invention examples, especially No. In invention examples 8 and 9, the resistance weld crack resistance at the weld zone was very excellent.
Further, using the steel plate of the present invention, the member obtained by molding or the member obtained by bonding has tensile strength (TS), yield stress (YS), total elongation (El), working It was found that the hardening index (n value)/yield ratio (YR), limit hole expansion ratio (λ), and resistance weld cracking resistance in the weld all have excellent properties that are characteristic of the present invention.
2 試験片
3 スペーサー
4 固定台
5 電極
6 ナゲット
7 き裂
REFERENCE SIGNS
Claims (15)
- 下地鋼板と、該下地鋼板の表面に亜鉛めっき層と、を有する亜鉛めっき鋼板であって、
該下地鋼板は、
質量%で、
C:0.050%以上0.400%以下、
Si:0.20%以上3.00%以下、
Mn:1.00%以上3.50%未満、
P:0.001%以上0.100%以下、
S:0.0200%以下、
Al:0.010%以上2.000%以下および
N:0.0100%以下
であり、炭素当量Ceqが0.540%以上であり、残部がFe及び不可避的不純物である、成分組成を有し、
また、該下地鋼板は、
フェライトの面積率:65.0%以下(0%を含む)、
ベイニティックフェライトの面積率:5.0%以上40.0%以下、
焼戻しマルテンサイトの面積率:0.5%以上80.0%以下、
残留オーステナイトの面積率:3.0%以上、
フレッシュマルテンサイトの面積率:20.0%以下(0%を含む)、
SBF+STM+2×SMA:65.0%以上、
SMA1/SMA:0.80以下、および
SMA2/SMA:0.20以上
である、鋼組織を有し、
引張強さが980MPa以上である、亜鉛めっき鋼板。
ここで、
SBF:前記ベイニティックフェライトの面積率
STM:前記焼戻しマルテンサイトの面積率
SMA:前記残留オーステナイトおよび前記フレッシュマルテンサイトからなる硬質第二相の面積率
SMA1:前記硬質第二相を構成する島状領域のうち、円相当直径が2.0μm以上であり、かつ、周長の20%以下が焼戻しマルテンサイトと接する島状領域の合計の面積率
SMA2:前記硬質第二相を構成する島状領域のうち、周長の1%以上がベイニティックフェライトと接する島状領域の合計の面積率
である。 A galvanized steel sheet having a base steel sheet and a galvanized layer on the surface of the base steel sheet,
The base steel plate is
in % by mass,
C: 0.050% or more and 0.400% or less,
Si: 0.20% or more and 3.00% or less,
Mn: 1.00% or more and less than 3.50%,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, a carbon equivalent Ceq of 0.540% or more, and the balance being Fe and unavoidable impurities. ,
In addition, the base steel plate is
Ferrite area ratio: 65.0% or less (including 0%),
Area ratio of bainitic ferrite: 5.0% or more and 40.0% or less,
Area ratio of tempered martensite: 0.5% or more and 80.0% or less,
Area ratio of retained austenite: 3.0% or more,
Area ratio of fresh martensite: 20.0% or less (including 0%),
S BF + STM + 2 x SMA : 65.0% or more,
having a steel structure in which SMA1/SMA: 0.80 or less and SMA2 / SMA : 0.20 or more;
A galvanized steel sheet having a tensile strength of 980 MPa or more.
here,
S BF : Area ratio of the bainitic ferrite STM : Area ratio of the tempered martensite SMA : Area ratio of the hard second phase composed of the retained austenite and the fresh martensite SMA1 : The hard second phase Among the constituent island regions, the total area ratio of the island regions having an equivalent circle diameter of 2.0 μm or more and having 20% or less of the circumference contacting the tempered martensite S MA2 : the hard second phase Of the constituent island regions, 1% or more of the circumference is the total area ratio of the island regions in contact with the bainitic ferrite. - 前記下地鋼板の成分組成が、さらに、質量%で、
Ti:0.200%以下、
Nb:0.200%以下、
V:0.100%以下、
B:0.0100%以下、
Cu:1.000%以下、
Cr:1.000%以下、
Ni:1.000%以下、
Mo:0.500%以下、
Sb:0.200%以下、
Sn:0.200%以下、
Ta:0.100%以下、
W:0.500%以下、
Mg:0.0200%以下、
Zn:0.0200%以下、
Co:0.0200%以下、
Zr:0.0200%以下、
Ca:0.0200%以下、
Ce:0.0200%以下、
Se:0.0200%以下、
Te:0.0200%以下、
Ge:0.0200%以下、
As:0.0200%以下、
Sr:0.0200%以下、
Cs:0.0200%以下、
Hf:0.0200%以下、
Pb:0.0200%以下、
Bi:0.0200%以下および
REM:0.0200%以下
のうちから選ばれる少なくとも1種を含有する、請求項1に記載の亜鉛めっき鋼板。 The chemical composition of the base steel plate is further, in mass%,
Ti: 0.200% or less,
Nb: 0.200% or less,
V: 0.100% or less,
B: 0.0100% or less,
Cu: 1.000% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 0.500% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
Zr: 0.0200% or less,
Ca: 0.0200% or less,
Ce: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0200% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
The galvanized steel sheet according to claim 1, containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less. - 前記下地鋼板の鋼組織においてSMA3/SMAが0.05以上である、請求項1または2に記載の亜鉛めっき鋼板。
ここで、
SMA3:前記硬質第二相を構成する島状領域のうち、周長の1%以上がベイニティックフェライトと接し、かつ、周長の20%超が焼戻しマルテンサイトと接する島状領域の合計の面積率
である。 The galvanized steel sheet according to claim 1 or 2, wherein SMA3 / SMA is 0.05 or more in the steel structure of the base steel sheet.
here,
SMA3 : Of the island-shaped regions constituting the hard second phase, 1% or more of the peripheral length is in contact with bainitic ferrite, and more than 20% of the peripheral length is in contact with tempered martensite. is the area ratio of - 前記下地鋼板の拡散性水素量が0.50質量ppm以下である、請求項1~3のいずれか一項に記載の亜鉛めっき鋼板。 The galvanized steel sheet according to any one of claims 1 to 3, wherein the base steel sheet has a diffusible hydrogen content of 0.50 mass ppm or less.
- 脱炭層を有する、請求項1~4のいずれか一項に記載の亜鉛めっき鋼板。 The galvanized steel sheet according to any one of claims 1 to 4, which has a decarburized layer.
- 前記下地鋼板と前記亜鉛めっき層の間の少なくとも一方において金属めっき層を有する、請求項1~5のいずれか一項に記載の亜鉛めっき鋼板。 The galvanized steel sheet according to any one of claims 1 to 5, which has a metal plating layer in at least one between the base steel sheet and the galvanization layer.
- 前記金属めっき層がFe系めっき層である、請求項6に記載の亜鉛めっき鋼板。 The galvanized steel sheet according to claim 6, wherein the metal plating layer is an Fe-based plating layer.
- 前記亜鉛めっき層が、溶融亜鉛めっき層または合金化溶融亜鉛めっき層である、請求項1~7のいずれか一項に記載の亜鉛めっき鋼板。 The galvanized steel sheet according to any one of claims 1 to 7, wherein the galvanized layer is a hot-dip galvanized layer or an alloyed hot-dip galvanized layer.
- 請求項1~8のいずれか一項に記載の亜鉛めっき鋼板を用いてなる、部材。 A member using the galvanized steel sheet according to any one of claims 1 to 8.
- 請求項1または2に記載の成分組成を有する鋼スラブに熱間圧延を施して熱延鋼板とする、熱延工程と、
前記熱延鋼板を冷間圧延して冷延鋼板とする、冷延工程と、
前記冷延鋼板を、焼鈍温度:760℃以上900℃以下および焼鈍時間:20秒以上で焼鈍する、焼鈍工程と、
前記冷延鋼板を300℃以上550℃以下の第一冷却停止温度まで冷却する、第一冷却工程と、
前記冷延鋼板を300℃以上550℃以下の温度域で3秒以上600秒以下保持する、保持工程と、
前記冷延鋼板に亜鉛めっき処理を施して亜鉛めっき鋼板とする、めっき工程と、
前記亜鉛めっき鋼板を、100℃以上300℃未満の第二冷却停止温度まで冷却する、第二冷却工程と、
前記亜鉛めっき鋼板を、(前記第二冷却停止温度+50℃)以上500℃以下の再加熱温度に再加熱し、前記亜鉛めっき鋼板を、(前記第二冷却停止温度+50℃)以上500℃以下の温度域で10秒以上2000秒以下保持する、再加熱工程と、
を有し、
前記第一冷却停止温度と、前記亜鉛めっき処理での亜鉛めっき浴の温度とが、次式(1)の関係を満足する、亜鉛めっき鋼板の製造方法。
-150℃≦T0-T1≦50℃ ・・・(1)
ここで、T0は第一冷却停止温度(℃)、T1は亜鉛めっき処理での亜鉛めっき浴の温度(℃)である。 A hot rolling step of hot rolling a steel slab having the chemical composition according to claim 1 or 2 to form a hot rolled steel sheet;
A cold rolling step of cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet;
An annealing step of annealing the cold-rolled steel sheet at an annealing temperature of 760° C. or more and 900° C. or less and an annealing time of 20 seconds or more;
a first cooling step of cooling the cold-rolled steel sheet to a first cooling stop temperature of 300° C. or higher and 550° C. or lower;
a holding step of holding the cold-rolled steel sheet in a temperature range of 300° C. or higher and 550° C. or lower for 3 seconds or more and 600 seconds or less;
a plating step of galvanizing the cold-rolled steel sheet to form a galvanized steel sheet;
a second cooling step of cooling the galvanized steel sheet to a second cooling stop temperature of 100° C. or more and less than 300° C.;
The galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower, and the galvanized steel sheet is heated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower. A reheating step of holding in the temperature range for 10 seconds or more and 2000 seconds or less;
has
A method for producing a galvanized steel sheet, wherein the first cooling stop temperature and the temperature of the galvanizing bath in the galvanizing treatment satisfy the relationship of the following formula (1).
−150° C.≦T 0 −T 1 ≦50° C. (1)
Here, T0 is the first cooling stop temperature (°C), and T1 is the temperature (°C) of the zinc plating bath in the zinc plating treatment. - 前記焼鈍工程の露点が-30℃超である、請求項10に記載の亜鉛めっき鋼板の製造方法。 The method for producing a galvanized steel sheet according to claim 10, wherein the dew point in the annealing step is over -30°C.
- 前記冷延工程後で、かつ、前記焼鈍工程の前に、前記冷延鋼板の少なくとも一方の表面に金属めっき層を形成する金属めっき処理を施す、金属めっき処理工程をさらに有する、請求項10または11に記載の亜鉛めっき鋼板の製造方法。 11. or, further comprising a metal plating process for forming a metal plating layer on at least one surface of the cold rolled steel sheet after the cold rolling process and before the annealing process. 12. A method for producing a galvanized steel sheet according to 11.
- 前記金属めっき層がFe系めっき層である、請求項12に記載の亜鉛めっき鋼板の製造方法。 The method for manufacturing a galvanized steel sheet according to claim 12, wherein the metal plating layer is an Fe-based plating layer.
- 前記亜鉛めっき処理が、溶融亜鉛めっき処理または合金化溶融亜鉛めっき処理である、請求項10~13のいずれか一項に記載の亜鉛めっき鋼板の製造方法。 The method for producing a galvanized steel sheet according to any one of claims 10 to 13, wherein the galvanizing treatment is hot-dip galvanizing treatment or alloying hot-dip galvanizing treatment.
- 請求項1~8のいずれか一項に記載の亜鉛めっき鋼板に、成形加工または接合加工の少なくとも一方を施して部材とする、工程を有する、部材の製造方法。 A method for manufacturing a member, comprising the step of subjecting the galvanized steel sheet according to any one of claims 1 to 8 to at least one of molding and joining to form a member.
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JP2022542316A JP7197063B1 (en) | 2021-03-23 | 2022-03-18 | Galvanized steel sheet and member, and manufacturing method thereof |
KR1020237028572A KR20230135631A (en) | 2021-03-23 | 2022-03-18 | Galvanized steel sheets and members, and their manufacturing methods |
EP22775503.0A EP4306672A1 (en) | 2021-03-23 | 2022-03-18 | Galvanized steel sheet and member, and method for manufacturing same |
MX2023011170A MX2023011170A (en) | 2021-03-23 | 2022-03-18 | Galvanized steel sheet and member, and method for manufacturing same. |
CN202280022963.4A CN117062928A (en) | 2021-03-23 | 2022-03-18 | Galvanized steel sheet, component, and method for producing same |
US18/546,428 US20240124964A1 (en) | 2021-03-23 | 2022-03-18 | Galvanized steel sheet and member, and method for manufacturing same |
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MX2023011170A (en) | 2023-09-29 |
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