WO2015102048A1 - 熱間成形部材およびその製造方法 - Google Patents
熱間成形部材およびその製造方法 Download PDFInfo
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- WO2015102048A1 WO2015102048A1 PCT/JP2014/050019 JP2014050019W WO2015102048A1 WO 2015102048 A1 WO2015102048 A1 WO 2015102048A1 JP 2014050019 W JP2014050019 W JP 2014050019W WO 2015102048 A1 WO2015102048 A1 WO 2015102048A1
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D37/00—Tools as parts of machines covered by this subclass
- B21D37/16—Heating or cooling
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
- B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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- 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
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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
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- 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
- C21D8/0273—Final recrystallisation annealing
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- 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
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- 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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- 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
- C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- 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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- 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
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- 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
- 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
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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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
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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- 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
- C23C2/06—Zinc or cadmium or alloys based thereon
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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
- C23C2/26—After-treatment
- C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
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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
- C23C2/26—After-treatment
- C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
- C23C2/29—Cooling or 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
- C21D1/62—Quenching devices
- C21D1/673—Quenching devices for die 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present invention relates to a hot-formed member used for, for example, a machine structural part such as a body structural part of an automobile, and a manufacturing method thereof. Specifically, the present invention relates to a hot-formed member having excellent ductility and impact characteristics while having a tensile strength of 900 MPa or more, and a method for producing the same.
- Patent Document 1 in a method called hot press in which a heated steel plate is press-formed, a member having a complicated shape can be formed from a high-strength steel plate with high dimensional accuracy.
- the hot pressing process the steel sheet is processed in a state of being heated to a high temperature, so that the steel sheet at the time of processing is soft and has high ductility.
- the steel sheet is heated to an austenite single-phase region before pressing, and the steel sheet is rapidly cooled (quenched) in the mold after pressing to increase the strength of the member by martensitic transformation. Can also be achieved. Therefore, the hot pressing method is an excellent forming method that can simultaneously ensure the strength of the member and the formability of the steel sheet.
- Patent Document 2 a steel plate is formed into a predetermined shape in advance at room temperature, the member obtained thereby is heated to an austenite region, and further quenched in a mold to increase the strength of the member.
- a pre-press quench method to achieve is disclosed.
- the pre-press quench method which is an aspect of hot pressing, can restrain a member from being deformed due to thermal strain by restraining the member with a mold.
- the pre-press quench method is an excellent molding method capable of increasing the strength of a member and obtaining high dimensional accuracy.
- Patent Document 3 a steel sheet in which the content of C is limited to 0.1% or less is heated to an austenite single-phase region, and hot pressing is performed to obtain a multiphase structure containing ferrite and martensite. By doing so, the member considered to be excellent in ductility is disclosed. By heating the steel plate to the austenite single phase region in this way, the metal structure of the member becomes uniform.
- the member described in Patent Document 3 limits the C content to 0.1% or less, so the tensile strength of the member is 700 MPa at most. And does not have sufficient strength to contribute to weight reduction of automobiles.
- Patent Document 4 discloses that a steel sheet to which a large amount of Cr is added is heated to an austenite single-phase region, and a part of austenite is transformed into ferrite before and after pressing to form a multiphase structure, specifically, ferrite and martensite. A member having a tensile strength of 980 MPa or more and excellent ductility by using two phases of the site is disclosed.
- steel containing a large amount of Cr as disclosed in Patent Document 4 is used, cementite formed in the steel and carbides such as M 23 C 6 are not easily dissolved during heating, so that the stable To ensure mechanical properties, heating for a long time is required.
- Patent Documents 5 to 7 describe a cold-rolled steel sheet having an average particle size (the average particle size of the ferrite phase or, if further including the second phase, the average particle size of the ferrite phase and the second phase) of 15 ⁇ m or less. Is heated to a two-phase structure of ferrite and austenite, pressed while maintaining the structure, and rapidly cooled in the mold, so that the structure is a two-phase structure of ferrite and martensite. A member having a particle size of 7 ⁇ m or less and high strength and excellent ductility is disclosed.
- the metal structure of the hot-formed member is affected by the metal structure of the steel sheet used for hot pressing.
- miniaturization of the metal structure is an important structure control method that contributes to improving the ductility of the member, as shown in Patent Documents 5 to 7.
- the present inventors have newly found that it is possible to improve the impact characteristics of a member by refining and homogenizing the structure of a steel sheet used for hot forming. And in order to refine
- the annealing temperature is controlled in the vicinity of the Ac 1 point in order to refine the microstructure of the steel sheet used for hot pressing.
- a large amount of non-recrystallized ferrite remains in the steel sheet to be subjected to hot pressing under such manufacturing conditions.
- Such unrecrystallized ferrite does not recrystallize even when heated to a two-phase temperature range in which ferrite and austenite coexist, and therefore the structure after hot pressing becomes extremely non-uniform.
- the steel sheet contains a large amount of Ti.
- a specific problem of the present invention is to provide a hot-formed member having a tensile strength of 900 MPa or more, excellent in ductility and impact properties, and a method for producing the same, which have been impossible to mass-produce as described above. It is.
- the present inventors have (1) a limited range of Ti content in the hot-formed member.
- the hot-formed member having such a metal structure uses a steel sheet having the above-described chemical composition and a fine and uniform metal structure as a steel sheet for hot forming, and has the heat treatment conditions during the hot forming. New knowledge that it can be achieved by optimizing.
- the hot-formed member according to one aspect of the present invention has a chemical composition of mass%, C: 0.10% to 0.40%, Si: 0% to 2.0%, Mn: 1.%. 0% to 3.0%, P: 0.05% or less, S: 0.01% or less, sol.
- Al 0.001% to 1.0%, Ti: 0.050% to 0.30%, N: 0.01% or less, Nb: 0% to 0.4%, V: 0% to 0.4% %, Cr: 0% to 1.0%, Mo: 0% to 1.0%, Cu: 0% to 1.0%, Ni: 0% to 1.0%, Ca: 0% to 0.01% %, Mg: 0% to 0.01%, REM: 0% to 0.01%, Zr: 0% to 0.01%, B: 0% to 0.01%, Bi: 0% to 0.01% %, And the balance: Fe and impurities, in area%, ferrite: 10% to 90%, unrecrystallized ferrite: 0% to 2.0%, and martensite: 10% to 90%, the ferrite And the total area ratio of the martensite: 90% to 100%, the ferrite has an average grain size of 0.5 ⁇ m to 5.0 ⁇ m, and a tensile strength of 9 It is 00 MPa to 1800 MPa.
- the chemical composition is mass%, Nb: 0.003% to 0.4%, V: 0.003% to 0.4%, Cr: 0.005% to 1.0%, Mo: 0.005% to 1.0%, Cu: 0.005% to 1.0%, and Ni: 0.005% to 1.0% You may contain 1 type, or 2 or more types chosen from the group.
- the chemical composition is, by mass%, Ca: 0.0003% to 0.01%, Mg: 0.0003% to 0
- One or more selected from the group consisting of 0.01%, REM: 0.0003% to 0.01%, and Zr: 0.0003% to 0.01% may be contained.
- a method for producing a hot-formed member according to another aspect of the present invention has the same chemical composition as the chemical composition of the hot-formed member according to any one of (1) to (5) above.
- a material steel plate having a metal structure in which the content of non-recrystallized ferrite is 0 area% to 2.0 area% and the average grain diameter of ferrite is 0.5 ⁇ m to 7.0 ⁇ m is 720 ° C. or higher, Ac 3 points
- a hot forming step in which hot forming is performed on the raw steel plate, and, after the hot forming step, the raw steel plate has an average cooling rate of 20 ° C./second to 500 ° C. in a temperature range of 600 ° C. to 150 ° C. And a cooling step of cooling at a condition of ° C./second.
- a method for producing a hot-formed member according to another aspect of the present invention has the same chemical composition as the chemical composition of the hot-formed member according to any one of (1) to (5).
- a material steel plate having a metal structure in which unrecrystallized ferrite exceeds 2.0 area% and the average grain size of ferrite is 0.5 ⁇ m to 7.0 ⁇ m or less is a temperature of Ac 3 points to Ac 3 points + 100 ° C.
- a hot forming step in which hot forming is performed on the raw steel plate, and, after the hot forming step, the raw steel plate is subjected to an average cooling rate of 3 ° C./second in a temperature range of Ac 3 to 600 ° C. And a cooling step of cooling under the condition of 20 ° C./second.
- the material steel plate is selected from the group consisting of a cold-rolled steel plate, a hot-dip galvanized steel plate, and an alloyed hot-dip galvanized steel plate.
- a cold-rolled steel plate a hot-dip galvanized steel plate
- an alloyed hot-dip galvanized steel plate One kind may be sufficient.
- a hot-formed member having excellent ductility and impact properties and a tensile strength of 900 MPa or more can be put into practical use and mass production of such a hot-formed member can be performed for the first time. Achieved.
- hot forming will be described by taking a hot press as a specific embodiment as an example. However, if substantially the same manufacturing conditions as the manufacturing conditions disclosed in the following description are achieved, a molding method other than hot pressing, such as roll molding, may be adopted as the hot molding method. .
- the tensile strength of the hot-formed member according to this embodiment is 900 MPa to 1800 MPa. In order to reduce the weight of mechanical structural parts such as automobiles, the tensile strength of the material needs to be 900 MPa or more. Further, in order to prevent the occurrence of brittle fracture of the steel plate, the tensile strength needs to be 1800 MPa or less. In this embodiment, such tensile strength is achieved by appropriately controlling the content of various alloy elements such as C and the manufacturing method.
- C 0.10% to 0.40%
- C is a very important element that enhances the hardenability of steel and most strongly affects the strength after quenching.
- the C content is 0.10% or more.
- the C content is preferably 0.11% or more.
- the C content is set to 0.40% or less. From the viewpoint of weldability, the C content is preferably 0.28% or less.
- the Si content is an element having an effect of increasing the strength after quenching without deteriorating the ductility or improving the ductility. If the Si content is less than 0.001%, it is difficult to obtain the above effect. Therefore, to obtain the above effect, the Si content may be 0.001% or more. Note that when the Si content is 0.05% or more, the ductility is further improved. Therefore, the Si content is preferably 0.05% or more. On the other hand, if the Si content exceeds 2.0%, the effects of the above action are saturated and disadvantageous economically, and the surface properties are significantly deteriorated. Therefore, the Si content is 2.0% or less. Preferably it is 1.5% or less.
- Mn is a very effective element in order to improve the hardenability of steel and to secure the strength after quenching stably.
- the Mn content is 1.0% or more. If the Mn content is 1.6% or more, it is possible to ensure a tensile strength of 980 MPa or more after quenching. For this reason, it is preferable that Mn content shall be 1.6% or more.
- the Mn content is 3.0% or less.
- the Mn content is preferably 2.4% or less.
- P 0.05% or less
- P is an impurity inevitably contained in steel.
- P has an effect of increasing the strength of the hot-formed member by solid solution strengthening, and therefore P may be positively included.
- the P content is 0.05% or less.
- the P content is preferably set to 0.02% or less.
- the P content is 0%, it is not necessary to limit the lower limit value of the P content because the characteristics necessary to solve the problem can be obtained. That is, the lower limit value of the P content is 0%.
- S is an impurity contained in the steel.
- the S content In order to improve weldability, the lower the S content, the better. If the S content is more than 0.01%, the weldability deteriorates to an unacceptable extent. Therefore, the S content is 0.01% or less.
- the S content In order to further prevent deterioration of weldability, the S content is preferably 0.003% or less, and more preferably 0.0015% or less. The smaller the S content, the better. Therefore, it is not necessary to define the lower limit of the S content. That is, the lower limit of the S content is 0%.
- sol.Al 0.001% to 1.0%) sol.
- Al refers to solid solution Al present in steel in a solid solution state.
- Al is an element having a function of deoxidizing steel, and is also an element having a function of preventing carbonitride-forming elements such as Ti from being oxidized and promoting the formation of carbonitride. By these actions, generation of surface flaws in the steel material can be suppressed, and the yield of the steel material can be improved.
- sol. If the Al content is less than 0.001%, it is difficult to obtain the above effect. Therefore, sol.
- the Al content is 0.001% or more. In order to obtain the above action more reliably, sol.
- the Al content is preferably 0.015% or more. On the other hand, sol.
- sol. Al content shall be 1.0% or less. In order to more reliably avoid the above phenomenon, sol.
- the Al content is preferably 0.080% or less.
- Ti 0.050% to 0.30%)
- Ti is an important element in the present embodiment.
- By containing Ti it is possible to form fine precipitates that are Ti carbide, Ti nitride, and / or Ti carbonitride in the hot-formed member, and to refine the metal structure after quenching.
- the ductility of the hot-formed member is remarkably improved.
- the Ti content is less than 0.050%, the metal structure after quenching does not become fine and ductility cannot be improved. Therefore, the Ti content is 0.050% or more.
- the Ti content is 0.070% or more.
- the Ti content is set to 0.30% or less.
- the Ti content is preferably 0.25% or less, more preferably 0.20% or less.
- N is an impurity contained in the steel.
- the N content is low.
- the N content exceeds 0.01%, the decrease in weldability of the hot-formed member becomes significant to an unacceptable level. Therefore, the N content is 0.01% or less.
- the N content is preferably 0.006% or less. The smaller the N content, the better. Therefore, it is not necessary to define the lower limit of the N content. That is, the lower limit of the N content is 0%.
- the balance is Fe and impurities.
- Impurities are components mixed in due to various factors of raw materials such as ore or scrap, or manufacturing processes when industrially manufacturing steel materials, and are allowed within a range that does not adversely affect this embodiment. Means something.
- the hot forming member according to the embodiment may further contain an element as described below as an optional component.
- limit the lower limit of arbitrary element content that is, the lower limit of the content of each arbitrary element is 0%.
- Ni one or more selected from the group consisting of 0% to 1.0%) All of these elements are effective elements for enhancing the hardenability of steel and stably securing the strength of the hot-formed member after quenching. Accordingly, one or more of these elements may be included in the hot-formed member. However, if Nb and V are contained in amounts exceeding 0.4%, it becomes difficult to perform hot rolling and cold rolling in the manufacturing process.
- Nb and V are each contained in excess of 0.4%, the structure of the hot formed member after quenching tends to be non-uniform, and the impact characteristics of the hot formed member are significantly deteriorated.
- Cr, Mo, Cu and Ni are contained in an amount exceeding 1.0%, the effects of the above action are saturated and economically disadvantageous, and hot rolling and cold rolling are performed in the manufacturing process. It becomes difficult.
- Nb 0.003% or more
- V 0.003% or more
- Cr 0.005% or more
- any of these elements contributes to inclusion control, in particular, fine dispersion of inclusions, and has an effect of increasing the low temperature toughness of the hot formed member. Therefore, you may contain 1 type, or 2 or more types among these elements. However, if any element is contained in excess of 0.01%, the surface properties of the hot-formed member may be deteriorated. Therefore, when each element is contained, the content of each element is as described above.
- REM refers to a total of 17 elements composed of Sc, Y, and a lanthanoid
- REM content means the total content of these 17 elements.
- B is an element having an effect of increasing the low temperature toughness of the hot formed member. Therefore, B may be contained in the hot-formed member. However, when B is contained exceeding 0.01%, the hot workability of the raw steel plate is deteriorated, and it is difficult to perform hot rolling. Therefore, when B is contained in the hot-formed member, the B content is 0.01% or less. In addition, in order to acquire the effect by the said action more reliably, it is preferable to make B content 0.0003% or more.
- Bi 0% to 0.01%
- Bi is an element having an effect of making the metal structure of the hot-formed member uniform and improving the impact characteristics of the hot-formed member. Therefore, Bi may be included in the hot-formed member.
- the Bi content is 0.01% or less.
- the structure of the metal structure described below is a structure at a position from about 1/2 t to about 1/4 t of the plate thickness and not the center segregation portion.
- the center segregation part may have a metal structure different from a typical metal structure of a steel material.
- the center segregation portion is a minute region with respect to the entire plate thickness, and hardly affects the characteristics of the steel material. That is, it cannot be said that the metal structure of the central segregation part represents the metal structure of the steel material.
- the definition of the metal structure of the hot-formed member according to the present embodiment is assumed to be at a position from about 1/2 t to about 1/4 t of the plate thickness and not at the center segregation portion.
- the “1/2 t position” indicates a position that is 1/2 the thickness of the member thickness t from the surface of the hot formed member, and the “1/4 t position” indicates the hot formed member surface. To a position that is a quarter of the member thickness t.
- ferrite that has undergone plastic deformation due to rolling and is stretched in the rolling direction and remains without being recrystallized thereafter is referred to as “unrecrystallized ferrite”.
- ferrite other than non-recrystallized ferrite is referred to as “ferrite” or “normal ferrite”.
- the term “unrecrystallized ferrite” is a term well known to those skilled in the art.
- the ferrite includes recrystallized ferrite generated by recrystallization and transformed ferrite generated by phase transformation.
- the crystal orientation continuously changes due to plastic deformation due to rolling.
- the crystal orientation in the grains of normal ferrite is almost uniform, and the crystal orientations of adjacent normal ferrite crystal grains are greatly different. Due to such differences, unrecrystallized ferrite usually has a higher hardness than ferrite.
- non-recrystallized ferrite and normal ferrite can be distinguished by observing the metal structure with a microscope.
- the crystal orientation measurement data of the electron back scattering analysis image (EBSP) of the metal structure is obtained by the Kernel Average Misoration method (KAM method).
- KAM method Kernel Average Misoration method
- the ratio of ferrite to martensite is not particularly limited as long as the area ratio of ferrite falls within the above range, but is preferably ferrite: 25 to 85% and martensite: 15 to 75%.
- the non-recrystallized ferrite remains in the metal structure of the hot-formed member, so that the strength of the hot-formed member after quenching is increased, but the metal structure becomes extremely non-uniform. Impact characteristics are significantly degraded. Specifically, when the area ratio of non-recrystallized ferrite exceeds 2.0%, desired ductility and impact characteristics cannot be obtained. Therefore, the area ratio of the non-recrystallized ferrite of the hot-formed member is 2.0% or less (including the case of 0%).
- the hot-formed member according to the present embodiment has a metal structure mainly composed of ferrite and martensite, but as a phase or structure other than ferrite and martensite, bainite, retained austenite, cementite, and pearlite depending on manufacturing conditions. 1 type or 2 types or more may mix in a metal structure.
- the area ratio of the phase or metal structure other than ferrite and martensite exceeds 10%, the intended characteristics may not be obtained due to the influence of these phases or metal structure. Therefore, the area ratio of the phase or structure other than ferrite and martensite is less than 10%. That is, the total area ratio of ferrite and martensite is 90% or more. Since it is not necessary to define the upper limit of the total area ratio of ferrite and martensite, the upper limit of the total area ratio of ferrite and martensite is 100%.
- test piece is taken from the hot-formed member along the rolling direction of the raw steel plate that is the raw material of the hot-formed member and the direction perpendicular to the rolling direction. Collect. Next, the metal structure of the cross section of the test piece along the rolling direction and the cross section perpendicular to the rolling direction is photographed with an electron microscope.
- the area ratio of unrecrystallized ferrite, ferrite, and martensite is calculated by image analysis of the electron micrograph of the 800 ⁇ m ⁇ 800 ⁇ m region (800 ⁇ m square region) obtained in this way. It is easy to distinguish the ferrite particles and martensite particles from the surrounding structure using an electron microscope. Further, to distinguish between ferrite particles and non-recrystallized ferrite particles, the aspect ratio of the particles is calculated from the shape of the particles, the ferrite particles having an aspect ratio of 4 or more are judged as non-recrystallized ferrite particles, and the aspect ratio This can be done by judging that ferrite particles having a particle size of less than 4 are ferrite particles.
- the average particle diameter of ferrite is set to 5.0 ⁇ m or less. Since it is preferable that the average particle diameter of the ferrite is small, it is not necessary to define the lower limit of the average particle diameter of the ferrite. However, considering the capacity of the manufacturing facility, etc., about 0.5 ⁇ m is the practical lower limit of the average grain size of ferrite.
- the hot-formed member means a member hot-formed from a raw steel plate, and includes, for example, a hot-pressed steel member.
- Typical hot-formed members include parts of automobile body structures such as door guard bars and bumper reinforcement, and mechanical structural parts such as hot-formed steel pipes for building structures.
- the structure of the metal structure described below is a structure at a position from about 1/2 t to about 1/4 t of the plate thickness and not the center segregation portion.
- the center segregation part may have a metal structure different from a typical metal structure of a steel material.
- the center segregation portion is a minute region with respect to the entire plate thickness, and hardly affects the characteristics of the steel material. That is, it cannot be said that the metal structure of the central segregation part represents the metal structure of the steel material. Therefore, the definition of the metal structure of the hot-formed member according to the present embodiment is assumed to be at a position from about 1/2 t to about 1/4 t of the plate thickness and not at the center segregation portion.
- the metal structure (final metal structure) of the hot-formed member after quenching is Area%, including ferrite 10% to 90%, non-recrystallized ferrite 0% to 2.0%, and martensite 10% to 90%, the total area ratio of ferrite and martensite being 90% or more, It is necessary that the average particle diameter of the ferrite is 5.0 ⁇ m or less.
- the metal structure of the raw steel plate (also referred to as “starting steel plate”) before being subjected to hot press forming is adjusted in advance to a predetermined state.
- hot pressing is performed under predetermined hot press molding conditions.
- a steel plate having a composition and having a metal structure in which non-recrystallized ferrite is 0 area% to 2.0 area% and the average grain size of ferrite is 0.5 ⁇ m to 7.0 ⁇ m is prepared as a raw steel sheet.
- a raw steel plate in which the amount of amorphous ferrite is 2.0 area% or less can be obtained, for example, by subjecting the steel plate to a recrystallization annealing process for a sufficient time while cold rolling.
- the steel sheet can be produced, for example, by annealing a cold-rolled steel sheet in a temperature range of (Ac 3 points ⁇ 20 ° C.) or higher.
- the thus-prepared hot-formed member has the same chemical composition as the above-mentioned hot-formed member, non-recrystallized ferrite is 2.0 area% or less, and the average grain size of ferrite is 0.5 ⁇ m to
- a hot press steel plate which is a raw steel plate having a metal structure of 7.0 ⁇ m, is hot press formed according to the conditions shown below. Moreover, since the area ratio of the non-recrystallized ferrite of the raw steel plate is limited to 2.0 area% or less, the metal structure of the hot-formed member does not become a non-uniform structure.
- the metal structure of the raw steel plate is a refined structure, the ductility and impact characteristics of the hot-formed member can be greatly improved by the manufacturing method according to this embodiment.
- the lower limit value of non-recrystallized ferrite the lower the non-recrystallized ferrite, the better. Therefore, the lower limit value of non-recrystallized ferrite is practically 0%.
- the area ratio of each metal structure of the raw steel plate described above can be obtained by the same method as the method of obtaining the area ratio of each metal structure of the hot-formed member.
- the material steel plate prepared as described above is heated to a temperature range of 720 ° C. or more and less than Ac 3 points in the heating step, and then the temperature of the material steel plate is set to 720 ° C. or more and less than Ac 3 points in the holding step. Hold in the temperature range for 1 to 20 minutes, then hot press in the hot forming step, and in the cooling step, the average cooling rate in the temperature range of 600 ° C to 150 ° C is 20 ° C / second Cooling is performed under the condition of 500 ° C./second. According to the manufacturing method which concerns on this embodiment, a raw steel plate is not heated to an austenite single phase area, and a process for a short time is attained.
- the raw steel plate used for hot pressing has the same chemical composition as that of the hot-formed steel plate, non-recrystallized ferrite is 2.0 area% or less, and the average grain size of ferrite is 0.5 ⁇ m.
- a cold-rolled steel sheet or a hot-dip galvanized cold-rolled steel sheet having a metal structure of ⁇ 7.0 ⁇ m can be used.
- the chemical composition of the raw steel plate is specified as described above, and in particular, C, Mn, and Ti are specified within a specific range, so that sufficient recrystallization annealing is performed under normal conditions. By performing this, the above-mentioned raw steel plate can be easily obtained.
- the material steel sheet having the metal structure is hot-pressed under the heat treatment conditions described below, thereby having a desired metal structure, a tensile strength of 900 MPa or more, and excellent ductility and impact characteristics. An intermediate formed member is obtained.
- cold-rolled steel sheet and hot-dip galvanized cold-rolled steel sheet having the above metal structure can be produced by annealing at a temperature range of not lower than (Ac 3 point -20 ° C.) .
- Heating temperature of raw steel plate temperature range of 720 ° C or more and less than Ac 3 points
- Heating temperature and holding time of the steel plate Hold for 1 to 20 minutes in a temperature range of 720 ° C or more and less than Ac 3 points
- the raw steel plate in the hot forming step the raw steel plate is heated to a temperature range of 720 ° C. or higher and less than Ac 3 points (° C.).
- the temperature of the raw steel plate is held in the above temperature range, that is, in the temperature range of 720 ° C. or higher and less than Ac 3 points for 1 minute to 20 minutes.
- Ac 3 point is a temperature defined by the following formula (i) obtained by experiment, and when the steel is heated to a temperature range of Ac 3 point or higher, the metal structure of the steel becomes an austenite single phase.
- the heating temperature in the heating step and the holding temperature in the holding step are less than 720 ° C.
- the metal structure of the material steel sheet becomes a structure close to a ferrite single phase, and it becomes difficult to ensure a tensile strength of 900 MPa or more after quenching. Therefore, the heating temperature and the holding temperature are set to 720 ° C. or higher.
- the heating temperature in the heating step and the holding temperature in the holding step are Ac 3 points or more
- the metal structure of the hot-formed member after quenching becomes a martensite single phase, and the deterioration of the ductility of the hot-formed member is significant. Become. Therefore, the heating temperature and the holding temperature are less than Ac 3 points.
- the holding time in the holding process is less than 1 minute, undissolved carbides such as cementite remain in the hot-formed member, and the impact characteristics of the hot-formed member deteriorate. Accordingly, the holding time is 1 minute or longer.
- the holding time exceeds 20 minutes, the productivity is lowered, and the surface properties of the hot-formed member are deteriorated due to the generation of scale and zinc-based oxide. Accordingly, the holding time is 20 minutes or less.
- the average heating rate from the temperature range of 720 ° C. to Ac 3 points in the heating step is not particularly limited, but is preferably 0.2 ° C./second to 100 ° C./second.
- the average heating rate is 100 ° C./second or less, the heating temperature can be easily controlled in the case of heating using a normal furnace.
- the heating temperature can be accurately controlled even if heating is performed at a heating rate exceeding 100 ° C./second.
- Cooling in the temperature range of 600 ° C. to 150 ° C. is performed so that diffusional transformation does not occur. If the average cooling rate in the above temperature range is less than 20 ° C./second, the bainite transformation proceeds excessively, and it becomes impossible to secure the area ratio of martensite, which is a phase that strengthens the strength of the hot-formed member (strengthening phase). It becomes difficult to ensure a tensile strength of 900 MPa or more after quenching. Therefore, the average cooling rate in the temperature range is set to 20 ° C./second or more.
- the average cooling rate in the temperature range is set to 500 ° C./second or less.
- the average cooling rate in the temperature range is preferably 200 ° C./second or less.
- cooling is usually achieved by a mold having a normal temperature or a temperature of several tens of degrees Celsius immediately before hot pressing, taking heat away from the hot forming member. Therefore, in order to change the cooling rate, the heat capacity of the mold may be changed by changing the dimensions of the mold.
- the cooling rate can also be changed by changing the mold material to a different metal (for example, copper). If the mold dimensions cannot be changed, the cooling rate can also be changed by using a fluid cooling mold and changing the flow rate of the cooling medium.
- the cooling rate can also be changed by using a mold having grooves cut in advance and passing a cooling medium (water or gas) through the grooves during pressing.
- the cooling rate can also be changed by operating the press machine during pressing to separate the mold and the hot forming member and flowing a cooling medium between them. Furthermore, the cooling rate can also be changed by changing the mold clearance and changing the contact area between the mold and the steel plate (hot forming member). In view of the above matters, the following means can be considered as means for changing the cooling rate around 600 ° C.
- the hot forming member is moved to a mold having a different heat capacity or a mold at room temperature to change the cooling rate;
- the cooling rate is changed by changing the flow rate of the cooling medium in the mold immediately after reaching 600 ° C .;
- a cooling medium is passed between the mold and the member, and the cooling rate is changed by changing the flow rate.
- a hot-formed member having the above-described metal structure In order to obtain a hot-formed member having the above-described metal structure, it has the same chemical composition as that of the above-mentioned hot-formed member, the average grain size of ferrite is 7.0 ⁇ m or less, and is not recrystallized.
- a steel plate having a metal structure with a ferrite content exceeding 2.0 area% is prepared as a raw steel plate.
- it can be produced by annealing a cold-rolled steel sheet in a temperature range lower than (Ac 3 points ⁇ 20 ° C.).
- the raw steel plate prepared as described above is hot pressed in the temperature range of Ac 3 points to Ac 3 points + 100 ° C for 30 seconds to less than 20 minutes, and then hot pressed to reach the temperature range from Ac 3 points to 600 ° C. Cool at an average cooling rate of 3 ° C / second to 20 ° C / second.
- the steel sheet used for hot pressing has the same chemical composition as that of the hot-formed member, the average grain size of ferrite is 7.0 ⁇ m or less, and the amount of unrecrystallized ferrite exceeds 2.0 area%.
- a cold-rolled steel sheet or a hot-dip galvanized cold-rolled steel sheet having a metal structure as described above can be used.
- the steel sheet By hot-pressing the material steel sheet having the above metal structure under the heat treatment conditions described later, the steel sheet has a desired metal structure, a tensile strength of 900 MPa or more, and excellent in ductility and impact properties. A molded member is obtained.
- Heating temperature of the steel plate Ac 3 points to Ac 3 points + 100 ° C temperature range
- Heating temperature and holding time of the steel plate Ac 3 points to Ac 3 points + 100 ° C. for 30 seconds or more and less than 20 minutes
- Heating of the steel sheet to be subjected to hot pressing is carried out by maintaining at a temperature range of Ac 3 points (° C.) to Ac 3 points + 100 ° C. defined by the above empirical formula (i) for 30 seconds or more and less than 20 minutes.
- the holding temperature is set to Ac 3 points or more.
- the holding temperature is Ac 3 point + 100 ° C. or higher, grain boundary oxides are generated in the metal structure, and the impact characteristics of the hot-formed member are significantly lowered. Accordingly, the holding temperature is set to Ac 3 points + 100 ° C. or lower.
- the holding time is less than 30 seconds, the strength fluctuation of the steel material becomes large. Conditions under which such a phenomenon occurs are not suitable for mass production technology, so the holding time is 30 seconds or more. On the other hand, if the holding time is 20 minutes or more, austenite grains grow excessively, the metal structure becomes non-uniform, and the impact characteristics of the hot-formed member are remarkably deteriorated. Accordingly, the holding time is less than 20 minutes.
- the heating rate up to the temperature range of Ac 3 point to Ac 3 point + 100 ° C. is preferably 0.2 ° C./second to 100 ° C./second.
- the average heating rate is 100 ° C./second or less, the heating temperature can be easily controlled in the case of heating using a normal furnace.
- the heating temperature can be accurately controlled even if heating is performed at a heating rate exceeding 100 ° C./second.
- the cooling in the temperature range of Ac 3 points to 600 ° C . is performed so that the average cooling rate is 3 ° C./second to 20 ° C./second. If the average cooling rate in the said temperature range is less than 3 degree-C / sec, a grain-boundary oxide will produce
- the average cooling rate in the temperature range is set to 20 ° C./second or less.
- the average cooling rate in the temperature range below 600 ° C. is 20 ° C./second to 500 ° C./second.
- the form of molding in the hot press method in this embodiment is not particularly limited.
- Exemplified molding forms are bending, drawing, stretch molding, hole expansion molding, and flange molding. What is necessary is just to select a preferable thing among the above-mentioned shaping
- molding forms suitably according to the kind and shape of the target hot forming member.
- Examples of the material of the material steel plate used for the hot pressing method in the present embodiment include cold-rolled steel plates, hot-dip galvanized steel plates, and alloyed hot-dip galvanized steel plates.
- hot-formed members include door guard bars and bumper reinforcements that are reinforcing parts for automobiles.
- the hot-formed member is a bumper reinforcement
- the above-mentioned material steel plate that is an alloy hot-dip galvanized steel plate of a predetermined length is prepared, and it is bent into the mold under the above-mentioned conditions. These processes may be performed sequentially.
- the hot-formed member according to this embodiment is characterized by excellent ductility and impact characteristics.
- the hot-formed member according to this embodiment preferably has ductility such that the total elongation in the tensile test is 10% or more.
- the hot-forming member according to the present embodiment has an impact characteristic that an impact value of a Charpy test at 0 ° C. is 20 J / cm 2 or more.
- a hot-formed member having such characteristics is realized by satisfying the above-mentioned regulations concerning chemical composition and metal structure.
- shot blasting is usually applied to the hot formed member for scale removal.
- This shot blasting treatment has the effect of introducing compressive stress into the surface of the material to be treated. Therefore, subjecting the hot-formed member to the shot blasting treatment has the advantages of suppressing delayed fracture in the hot-formed member and improving the fatigue strength of the hot-formed member.
- the material steel plate is as soft as possible and has high ductility.
- the tensile strength of the material steel plate is desirably 800 MPa or less.
- hot forming has been described by taking an example of hot pressing, which is a specific aspect, but the manufacturing method according to the present embodiment is not limited to hot pressing.
- the manufacturing method according to the present embodiment can be applied to any hot forming including a means for cooling a steel sheet at the same time as forming or immediately after forming, similarly to hot pressing.
- An example of such hot forming is roll forming.
- a material steel plate (plate thickness t: 1.2 mm) having the chemical composition shown in Table 1 and the metal structure and tensile strength shown in Table 2 was subjected to hot pressing.
- These raw steel plates are steel plates manufactured by hot rolling, cold rolling, and recrystallization annealing of slabs melted in the laboratory (referred to as cold rolled steel plates in Table 2).
- some steel sheets were subjected to hot dip galvanizing treatment (plating adhesion amount per side was 60 g / m 2 ), alloyed hot dip galvanizing treatment (plating adhesion amount per side was 60 g / m). 2 and Fe content in the plating film was 15% by mass).
- each is described as a hot dip galvanized steel sheet and an alloyed hot dip galvanized steel sheet.
- the steel plate of the cold rolling (it describes with full hard in Table 2) which does not give recrystallization annealing was also used as a raw material steel plate.
- thermocouple was attached to the steel plate, and the cooling rate was also measured.
- Average heating rate in Table 3 represents an average value of heating rates in a temperature range from room temperature to 720 ° C.
- Heat indicates the time during which the steel material was held in a temperature range of 720 ° C. or higher.
- Cooling rate * 1 in Table 3 indicates the average cooling rate from Ac 3 point to 600 ° C. when the heating temperature is 3 points or more of Ac, and from the heating temperature when the heating temperature is less than 3 points of Ac.
- the average cooling rate up to 600 ° C is shown.
- “Cooling rate * 2” is an average cooling rate in a temperature range from 600 ° C. to 150 ° C.
- Tensile tests, Charpy tests, and metallographic observations were performed on steel sheets obtained under various production conditions.
- the steel plate member produced in this example is not hot pressed by a mold, but receives the same thermal history as the hot-formed member, so the mechanical properties of the steel plate are hot with the same thermal history. It is substantially the same as the molded member.
- Test material From each steel plate, a JIS No. 5 tensile test piece having a longitudinal direction perpendicular to the rolling direction was taken, and TS (tensile strength) and EL (total elongation) were measured. A test material having a TS of 900 MPa or more and an EL of 10% or more was determined to be acceptable.
- Specimen No. which is an example of the present invention. 1, 2, 4-7, 11, 15, 16, 19, 21, 23, 25, 27, 29, 31, 33, 36, 37, and 39 have excellent ductility and impact properties.
- specimen No. 3 had poor ductility and impact characteristics because the average grain size of ferrite in the material steel plate was outside the range defined in the present invention. Since the manufacturing method prescribed
- Specimen No. 18 was poor in ductility because the manufacturing conditions were outside the range specified in the present invention and the desired structure could not be obtained.
- Specimen No. No. 34 was poor in ductility because the chemical composition was outside the range defined in the present invention, and a desired structure could not be obtained.
- Specimen No. No. 8 had poor ductility because the heating temperature was higher than Ac 3 point. Specimen No. No. 12, since the heating temperature was lower than 720 ° C., the target tensile strength could not be obtained. Specimen No. No. 20 was produced to such an extent that the surface scale was unacceptable because the retention time was outside the range defined in the present invention. Specimen No. No. 30, because the retention time at 750 ° C. or higher was outside the range specified in the present invention, the target impact characteristics could not be obtained. Specimen No. No. 24, because the cooling rate of less than 600 ° C.
- Specimen No. 35 was produced to such an extent that the surface scale was unacceptable because the Si content was outside the range defined in the present invention.
- Specimen No. 38 is Sol. Since the Al content was outside the range defined in the present invention, the surface scale was generated to an unacceptable level.
- Specimen No. In No. 40 the Ti content was outside the range defined by the present invention, so the impact characteristics were poor.
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Abstract
Description
(1)本発明の一態様に係る熱間成形部材は、化学組成が、質量%で、C:0.10%~0.40%、Si:0%~2.0%、Mn:1.0%~3.0%、P:0.05%以下、S:0.01%以下、sol.Al:0.001%~1.0%、Ti:0.050%~0.30%、N:0.01%以下、Nb:0%~0.4%、V:0%~0.4%、Cr:0%~1.0%、Mo:0%~1.0%、Cu:0%~1.0%、Ni:0%~1.0%、Ca:0%~0.01%、Mg:0%~0.01%、REM:0%~0.01%、Zr:0%~0.01%、B:0%~0.01%、Bi:0%~0.01%、および残部:Feおよび不純物であり、面積%で、フェライト:10%~90%、未再結晶フェライト:0%~2.0%、およびマルテンサイト:10%~90%であり、前記フェライトおよび前記マルテンサイトの合計面積率:90%~100%であり、前記フェライトの平均粒径が0.5μm~5.0μmである金属組織を有し、引張強度が900MPa~1800MPaである。
はじめに、本発明の一実施形態に係る熱間成形部材の化学組成を上述のように規定した理由を説明する。以下の説明において、各合金元素の含有量を表す「%」は、特に断りがない限り「質量%」を意味する。なお、鋼の化学組成は熱間成形が行われても変化しないので、熱間成形を受ける前の素材鋼板中の各元素の含有量と、熱間成形後の熱間成形部材中の各元素の含有量とはそれぞれ等しい。
Cは、鋼の焼入れ性を高め、かつ焼入れ後の強度に最も強く影響する、非常に重要な元素である。C含有量が0.10%未満では、焼入れ後に900MPa以上の引張強度を確保することが困難となる。したがって、C含有量は0.10%以上とする。上述の効果をさらに確実に得るために、好ましくは、C含有量は0.11%以上である。一方、C含有量が0.40%超では、熱間成形部材の衝撃特性の劣化が顕著となり、さらに熱間成形部材の溶接性が低下する場合もある。したがって、C含有量は0.40%以下とする。溶接性の観点からは、C含有量を0.28%以下とすることが好ましい。
本実施形態に係る熱間成形部材において、Siを含有することは必須ではない。従って、Si含有量の下限値は0%である。しかし、Siは、延性を劣化させることなく、あるいは、延性を向上させながら、焼入れ後の強度を高める作用を有する元素である。Si含有量が0.001%未満では上記作用を得ることが困難である。したがって、上記効果を得るために、Si含有量を0.001%以上としてもよい。なお、Si含有量を0.05%以上にすると、延性がさらに向上する。したがって、Si含有量は0.05%以上とすることが好ましい。一方、Si含有量が2.0%超では、上記作用による効果は飽和して経済的に不利となる上、表面性状の劣化が著しくなる。したがって、Si含有量は2.0%以下とする。好ましくは1.5%以下である。
Mnは、鋼の焼入れ性を高め、焼入れ後の強度を安定して確保するために、非常に効果的な元素である。しかし、Mn含有量が1.0%未満では、その効果が十分に得られず、焼入れ後に900MPa以上の引張強度を確保することが非常に困難となる。したがって、Mn含有量は1.0%以上とする。なお、Mn含有量を1.6%以上にすると、焼入れ後に980MPa以上の引張強度を確保することが可能となる。このため、Mn含有量は1.6%以上とすることが好ましい。一方、Mn含有量が3.0%超では、熱間成形部材の金属組織が不均一となり、衝撃特性の劣化が顕著となる。したがって、Mn含有量は3.0%以下とする。なお、熱間成形を適用する前の素材鋼板の引張強度を低くすると、後の熱間成形工程における生産性が向上する。この効果を得るためには、Mn含有量を2.4%以下とすることが好ましい。
Pは、一般的には鋼に不可避的に含有される不純物である。しかし本実施形態において、Pは固溶強化により熱間成形部材の強度を高める作用を有するので、Pを積極的に含有させてもよい。しかし、P含有量が0.05%超では、熱間成形部材の溶接性の劣化が著しくなる。したがって、P含有量は0.05%以下とする。熱間成形部材の溶接性の劣化をさらに確実に防止するためには、P含有量を0.02%以下とすることが好ましい。また、上記作用をより確実に得るには、P含有量を0.003%以上とすることが好ましい。しかしながら、P含有量が0%であったとしても、課題を解決するために必要な特性を得ることができるので、P含有量の下限値を制限する必要はない。即ち、P含有量の下限値は0%である。
Sは、鋼に含有される不純物である。溶接性を向上させるためには、S含有量が低いほど好ましい。S含有量が0.01%超では、溶接性の低下が、許容できない程度に著しくなる。したがって、S含有量は0.01%以下とする。溶接性の低下をさらに確実に防ぐためには、S含有量は、0.003%以下にすることが好ましく、0.0015%以下にすることがさらに好ましい。S含有量は少なければ少ないほど好ましいので、S含有量の下限値を規定する必要はない。即ち、S含有量の下限値は0%である。
sol.Alとは、固溶状態で鋼中に存在する固溶Alのことを示す。Alは、鋼を脱酸する作用を有する元素であり、また、Ti等の炭窒化物形成元素が酸化することを防ぎ、炭窒化物の形成を促進する作用を有する元素でもある。これらの作用によって、表面疵が鋼材に発生することを抑制し、鋼材の歩留りを向上させることができる。sol.Al含有量が0.001%未満では、上記作用を得ることが困難となる。したがって、sol.Al含有量は0.001%以上とする。上記作用をさらに確実に得るためには、sol.Al含有量が0.015%以上であることが好ましい。一方、sol.Al含有量が1.0%超では、熱間成形部材の溶接性が著しく低下するとともに、酸化物系介在物が熱間成形部材中に増加して、熱間成形部材の表面性状が著しく劣化する。したがって、sol.Al含有量は1.0%以下とする。上記の現象をさらに確実に回避するためには、sol.Al含有量が0.080%以下であることが好ましい。
Tiは、本実施形態において重要な元素である。Tiを含有させることにより、熱間成形部材中に、Ti炭化物、Ti窒化物、および/またはTi炭窒化物である微細な析出物を形成し、焼入れ後の金属組織を微細化することが可能となり、これにより熱間成形部材の延性を著しく向上させる。Ti含有量が0.050%未満では、焼入れ後の金属組織が微細にならず、延性を向上させることができない。したがって、Ti含有量は0.050%以上とする。好ましくは、Ti含有量は0.070%以上である。一方、Ti含有量が0.30%超では、鋳造時および熱間圧延時に粗大な炭窒化物が形成され、熱間成形部材の衝撃特性の劣化が顕著となる。したがって、Ti含有量は0.30%以下とする。Ti含有量は、好ましくは0.25%以下、さらに好ましくは0.20%以下である。
Nは、鋼に含有される不純物である。溶接性を向上させるためには、N含有量が低い方が好ましい。N含有量が0.01%超では、熱間成形部材の溶接性の低下が、許容できない程度に著しくなる。したがって、N含有量は0.01%以下とする。溶接性の低下をさらに確実に回避するために、N含有量は好ましくは0.006%以下である。N含有量は少なければ少ないほど好ましいので、N含有量の下限値を規定する必要はない。即ち、N含有量の下限値は0%である。
これらの元素は、いずれも鋼の焼入れ性を高め、かつ焼入れ後の熱間成形部材の強度を安定して確保するために効果的な元素である。したがって、これらの元素のうち1種または2種以上を熱間成形部材に含有させてもよい。しかし、NbおよびVについては、それぞれ0.4%を超えて含有させると、製造工程において熱間圧延および冷間圧延の実施が困難になる。さらに、NbおよびVをそれぞれ0.4%を超えて含有させると、焼入れ後の熱間成形部材の組織が不均一になりやすくなり、熱間成形部材の衝撃特性が顕著に劣化する。また、Cr、Mo、CuおよびNiについては、1.0%を超えて含有させると、上記作用による効果が飽和して経済的に不利となるうえに、製造工程において熱間圧延および冷間圧延が困難となる。なお、上記作用による効果をより確実に得るには、Nb:0.003%以上、V:0.003%以上、Cr:0.005%以上、Mo:0.005%以上、Cu:0.005%以上およびNi:0.005%以上との数値範囲のうち少なくとも一つを満足させることが好ましい。
これらの元素は、いずれも介在物制御、特に介在物の微細分散化に寄与し、熱間成形部材の低温靭性を高める作用を有する元素である。したがって、これらの元素のうち1種または2種以上を含有させてもよい。しかし、いずれの元素も0.01%を超えて含有させると、熱間成形部材の表面性状を劣化させる場合がある。したがって、各元素を含有させる場合、各元素の含有量はそれぞれ上記のとおりとする。なお、上記作用による効果をより確実に得るためには、添加する上記各元素の含有量をそれぞれ0.0003%以上とすることが好ましい。
ここで、「REM」との用語は、Sc、Yおよびランタノイドからなる合計17元素を指し、「REMの含有量」とは、これら17元素の合計含有量を意味する。ランタノイドをREMとして用いる場合、工業的には、REMはミッシュメタルの形で添加される。
Bは、熱間成形部材の低温靭性を高める作用を有する元素である。したがって、熱間成形部材にBを含有させてもよい。しかし、0.01%を超えてBを含有させると、素材鋼板の熱間加工性が劣化して、熱間圧延の実施が困難になる。したがって、Bを熱間成形部材中に含有させる場合、B含有量は0.01%以下とする。なお、上記作用による効果をより確実に得るためには、B含有量を0.0003%以上とすることが好ましい。
Biは、熱間成形部材の金属組織を均一にし、熱間成形部材の衝撃特性を高める作用を有する元素である。したがって、Biを熱間成形部材に含有させてもよい。しかし、0.01%を超える量のBiを含有させると、素材鋼板の熱間加工性が劣化して、熱間圧延の実施が困難になる。したがって、Biを熱間成形部材中に含有させる場合、Bi含有量は0.01%以下とする。なお、上記作用による効果をより確実に得るためには、Bi含有量を0.0003%以上とすることが好ましい。
次に、本実施形態に係る熱間成形部材の金属組織について説明する。以下の説明において、各金属組織の含有量を表す「%」は、特に断りがない限り「面積%」を意味する。
以下で説明する金属組織の構成は、板厚の略1/2tの位置~略1/4tの位置であって、且つ中心偏析部ではない位置における構成である。中心偏析部は、鋼材の代表的な金属組織とは異なる金属組織を有する場合がある。しかしながら、中心偏析部は、板厚全体に対して微小な領域であり、鋼材の特性にほとんど影響を及ぼさない。すなわち、中心偏析部の金属組織は、鋼材の金属組織を代表していると言えない。従って、本実施形態に係る熱間成形部材の金属組織の規定は、板厚の略1/2tの位置~略1/4tの位置であって、且つ中心偏析部ではない位置におけるものとする。なお、「1/2tの位置」とは、熱間成形部材表面から部材厚さtの1/2の深さである位置を示し、「1/4tの位置」とは、熱間成形部材表面から部材厚さtの1/4の深さである位置を示す。
本実施形態では、圧延加工による塑性変形を受けて圧延方向に延伸され、その後に再結晶することなく残存したフェライトを「未再結晶フェライト」と称する。また、本実施形態では、未再結晶フェライト以外のフェライトを「フェライト」または「通常フェライト」と称する。なお、「未再結晶フェライト」との用語は、当業者にとって周知の用語である。通常フェライトは、再結晶によって生じた再結晶フェライト、および相変態によって生じた変態フェライトなどを含む。
未再結晶フェライトの粒内では、圧延による塑性変形のために、結晶方位が連続的に変化している。これに対して、通常フェライトの粒内の結晶方位はほぼ均一であり、隣接する通常フェライト結晶粒同士の結晶方位は大きく異なっている。このような相違に起因して、未再結晶フェライトは、通常フェライトよりも高い硬度を有する。
未再結晶フェライトは、圧延方向に延伸された形状を有するので、金属組織を顕微鏡で観察することにより、未再結晶フェライトと通常フェライトとを区別することができる。また、未再結晶フェライトと通常フェライトとは結晶方位の状態が異なるので、金属組織の電子後方散乱解析像(EBSP:Electron back scattering pattern)の結晶方位測定データをKernel Average Misorientation法(KAM法)によって解析することにより、未再結晶フェライトと通常フェライトとを区別することができる。本実施形態においては、アスペクト比が4以上であるフェライトを未再結晶フェライトとし、アスペクト比が4未満であるフェライトを通常フェライトとする。
フェライトの面積率が10%未満では、フェライトの結晶粒同士が隣接しなくなる。即ち、フェライトの殆どが孤立し、熱間成形部材の延性を向上させることができない。したがって、フェライトの面積率は10%以上とする。一方、フェライトの面積率が90%超では、マルテンサイトの面積率が10%未満となり、後述するように、焼入れ後に900MPa以上の引張強度を確保することが困難となる。したがって、フェライトの面積率は90%以下とする。フェライトとマルテンサイトとの割合は、フェライトの面積率が上記範囲に入る限り、特に制限されないが、好ましくは、フェライト:25~85%、マルテンサイト:15~75%である。
未再結晶フェライトが熱間成形部材の金属組織中に残存することにより、焼入れ後の熱間成形部材の強度は高くなるが、金属組織が極めて不均一となるので、熱間成形部材の延性と衝撃特性とは著しく劣化する。具体的には、未再結晶フェライトの面積率が2.0%超である場合、所望の延性と衝撃特性とが得られなくなる。したがって、熱間成形部材の未再結晶フェライトの面積率は2.0%以下とする(0%の場合も含む)。
マルテンサイトを熱間成形部材の金属組織中に形成させることにより、焼入れ後の熱間成形部材の強度を高めることができる。マルテンサイトの面積率が10%未満では、焼入れ後に900MPa以上の引張強度を確保することが困難となる。したがって、マルテンサイトの面積率は10%以上とする。一方、マルテンサイトの面積率が90%超では、フェライト(再結晶フェライト)の面積率が10%未満となり、上述したように、延性を向上させることができない。したがって、マルテンサイトの面積率は90%以下とする。
本実施形態に係る熱間成形部材は、主にフェライトおよびマルテンサイトからなる金属組織を有するが、製造条件に応じて、フェライトおよびマルテンサイト以外の相または組織として、ベイナイト、残留オーステナイト、セメンタイトおよびパーライトのうちの1種または2種以上が金属組織中に混入する場合がある。この場合、フェライトおよびマルテンサイト以外の相または金属組織の面積率が10%を超えると、これらの相または金属組織の影響により、目的とする特性が得られない場合がある。したがって、フェライトおよびマルテンサイト以外の相または組織の面積率は10%未満とする。すなわち、フェライトおよびマルテンサイトの合計面積率は90%以上とする。フェライトおよびマルテンサイトの合計面積率の上限を規定する必要はないので、フェライトおよびマルテンサイトの合計面積率の上限は100%となる。
焼入れ後の金属組織を微細化することにより、焼入れ後の強度、延性および衝撃特性を高めることができる。引張強度を900MPa以上に保ちながら、良好な延性および衝撃特性を確保するために、フェライトの平均粒径は5.0μm以下とする。フェライトの平均粒径は小さい方が好ましいので、フェライトの平均粒径の下限値を規定する必要はない。しかしながら、製造設備の能力などを考慮すると、0.5μm程度がフェライトの平均粒径の事実上の下限値となる。
次に、上記の特徴を有する本実施形態に係る熱間成形部材の好ましい製造方法について説明する。以下の説明において、各金属組織の含有量を表す「%」は、特に断りがない限り「面積%」を意味する。
以下で説明する金属組織の構成は、板厚の略1/2tの位置~略1/4tの位置であって、且つ中心偏析部ではない位置における構成である。中心偏析部は、鋼材の代表的な金属組織とは異なる金属組織を有する場合がある。しかしながら、中心偏析部は、板厚全体に対して微小な領域であり、鋼材の特性にほとんど影響を及ぼさない。すなわち、中心偏析部の金属組織は、鋼材の金属組織を代表していると言えない。従って、本実施形態に係る熱間成形部材の金属組織の規定は、板厚の略1/2tの位置~略1/4tの位置であって、且つ中心偏析部ではない位置におけるものとする。
上述した金属組織を有する熱間成形部材を得るためには、上述した熱間成形部材の化学組成と同一の化学組成を有し、且つ未再結晶フェライトが0面積%~2.0面積%であり、フェライトの平均粒径が0.5μm~7.0μmである金属組織を有する鋼板を素材鋼板として用意する。未結晶フェライトの量が2.0面積%以下である素材鋼板は、例えば、冷間圧延まま鋼板に十分な時間の再結晶焼鈍処理を行うことで得られる。未再結晶フェライトが2.0面積%以下であり、フェライトの平均粒径が0.5μm~7.0μmである金属組織を有する冷延鋼板、溶融亜鉛めっき冷延鋼板、および合金化溶融亜鉛めっき鋼板は、例えば、(Ac3点-20℃)以上の温度域で冷延鋼板を焼鈍することにより製造することができる。
なお、上述した素材鋼板の各金属組織の面積率は、熱間成形部材の各金属組織の面積率を求める方法と同じ方法によって求めることができる。
熱間プレスに供する素材鋼板には、熱間成形鋼板の化学組成と同じ化学組成を有し、かつ、未再結晶フェライトが2.0面積%以下であり、フェライトの平均粒径が0.5μm~7.0μmである金属組織を有する冷延鋼板または溶融亜鉛めっき冷延鋼板を用いることができる。
(素材鋼板の保持温度および保持時間:720℃以上Ac3点未満の温度域に1分間~20分間保持)
熱間成形工程における素材鋼板の加熱工程では、720℃以上、かつ、Ac3点(℃)未満の温度域まで素材鋼板を加熱する。素材鋼板の保持工程では、素材鋼板の温度を上記温度域、即ち720℃以上Ac3点未満の温度域に1分間~20分間保持する。Ac3点は、実験により求められた下記式(i)により規定される温度であり、Ac3点以上の温度域に鋼を加熱した場合、鋼の金属組織はオーステナイト単相になる。
ここで、上記式中における元素記号は、前記鋼板の化学組成における各元素の含有量(単位:質量%)を示す。「sol.Al」は、固溶Alの濃度(単位:質量%)を示す。
600℃~150℃の温度域における冷却は、拡散型変態が起きないように行う。上記温度域における平均冷却速度が20℃/秒未満では、ベイナイト変態が過度に進行してしまい、熱間成形部材の強度を強化する相(強化相)であるマルテンサイトの面積率を確保できなくなり、焼入れ後に900MPa以上の引張強度を確保することが困難となる。したがって、上記温度域における平均冷却速度は20℃/秒以上とする。一方、上記温度域における平均冷却速度を500℃/秒超とすることは、通常の設備においては困難である。そこで、上記温度域における平均冷却速度は500℃/秒以下とする。上記温度域における平均冷却速度は、好ましくは200℃/秒以下である。
(2)流体冷却方式の金型の場合、600℃到達直後に金型中の冷却媒体の流量を変化させて、冷却速度を変える;
(3)600℃到達直後に、金型と部材との間に冷却媒体を流し、その流量を変化させることで、冷却速度を変える。
素材鋼板の未再結晶フェライト量が2.0面積%以下である場合、上述した方法によって、所定の金属組織を有する熱間成形部材を得ることができる。しかし、素材鋼板の未再結晶フェライト量が2.0面積%超である場合も、以下の方法によって、所定の金属組織を有する熱間成形部材を得ることができる。
上述した金属組織を有する熱間成形部材を得るためには、上述した熱間成形部材の化学組成と同一の化学組成を有し、フェライトの平均粒径が7.0μm以下であり、未再結晶フェライト量が2.0面積%超である金属組織を有する鋼板を素材鋼板として用意する。フェライトの平均粒径が7.0μm以下であり、未再結晶フェライト量が2.0面積%超である金属組織を有する冷延鋼板、溶融亜鉛めっき冷延鋼板、および合金化溶融亜鉛めっき鋼板は、例えば、(Ac3点-20℃)未満の温度域で冷延鋼板を焼鈍することにより製造することができる。上述のようにして用意した素材鋼板を、Ac3点~Ac3点+100℃の温度域に30秒間以上20分間未満保持したのちに熱間プレスし、Ac3点から600℃の温度範囲までを3℃/秒~20℃/秒の平均冷却速度で冷却する。
熱間プレスに供する素材鋼板には、熱間成形部材の化学組成と同じ化学組成を有し、フェライトの平均粒径が7.0μm以下であり、未再結晶フェライト量が2.0面積%超である金属組織を有する冷延鋼板または溶融亜鉛めっき冷延鋼板を用いることができる。
(素材鋼板の保持温度および保持時間:Ac3点~Ac3点+100℃の温度域に30秒間以上20分間未満保持)
熱間プレスに供する鋼板の加熱は、上述の実験式(i)により規定されるAc3点(℃)~Ac3点+100℃の温度域に30秒間以上20分間未満保持することにより行う。
Ac3点~600℃の温度範囲における冷却は、平均冷却速度が3℃/秒~20℃/秒となるように行う。上記温度域における平均冷却速度が3℃/秒未満では、粒界酸化物が金属組織中に生成し、熱間成形部材の衝撃特性が著しく低下する。したがって、上記温度域における平均冷却速度は3℃/秒以上とする。一方、上記温度域における平均冷却速度を20℃/秒超とすると、熱間成形部材中のフェライトの量が不足する。そこで、上記温度域における平均冷却速度は20℃/秒以下とする。なお、600℃未満の温度範囲での平均冷却速度は20℃/秒~500℃/秒とする。
表1に示す化学組成と、表2に示す金属組織および引張強度とを有する素材鋼板(板厚t:1.2mm)を熱間プレスに供した。
各鋼板から、圧延方向に対して直角方向が長手方向となるJIS5号引張試験片を採取し、TS(引張強度)およびEL(全伸び)を測定した。TSが900MPa以上かつELが10%以上である供試材を合格とした。
1.2mm厚の鋼板を4枚積層してねじ止めした後、Vノッチ試験片を作製し、シャルピー衝撃試験に供した。衝撃特性は、0℃での衝撃値が20J/cm2以上となる場合を「良好」とした。それに達しない場合を「不良」とした。
素材鋼板および熱処理後の鋼板の圧延方向と、圧延方向に対して垂直な方向とに沿って、素材鋼板および熱処理後の鋼板から試験片を採取した。次いで、試験片の、圧延方向に沿った断面と圧延方向に対して垂直な断面との金属組織を電子顕微鏡で撮影した。これにより得られた、800μm×800μmの領域の電子顕微鏡写真を画像解析することによって、未再結晶フェライト、フェライト、およびマルテンサイトの面積率を算出した。
これらの試験の結果を表4および表5に示す。
Claims (8)
- 化学組成が、質量%で、
C:0.10%~0.40%、
Si:0%~2.0%、
Mn:1.0%~3.0%、
P:0.05%以下、
S:0.01%以下、
sol.Al:0.001%~1.0%、
Ti:0.050%~0.30%、
N:0.01%以下、
Nb:0%~0.4%、
V:0%~0.4%、
Cr:0%~1.0%、
Mo:0%~1.0%、
Cu:0%~1.0%、
Ni:0%~1.0%、
Ca:0%~0.01%、
Mg:0%~0.01%、
REM:0%~0.01%、
Zr:0%~0.01%、
B:0%~0.01%、
Bi:0%~0.01%、および
残部:Feおよび不純物であり、
面積%で、フェライト:10%~90%、未再結晶フェライト:0%~2.0%、およびマルテンサイト:10%~90%であり、前記フェライトおよび前記マルテンサイトの合計面積率:90%~100%であり、前記フェライトの平均粒径が0.5μm~5.0μmである金属組織を有し、
引張強度が900MPa~1800MPaであることを特徴とする熱間成形部材。 - 前記化学組成が、質量%で、
Nb:0.003%~0.4%、
V:0.003%~0.4%、
Cr:0.005%~1.0%、
Mo:0.005%~1.0%、
Cu:0.005%~1.0%、および
Ni:0.005%~1.0%からなる群から選ばれた1種または2種以上を含有することを特徴とする請求項1に記載の熱間成形部材。 - 前記化学組成が、質量%で、
Ca:0.0003%~0.01%、
Mg:0.0003%~0.01%、
REM:0.0003%~0.01%、および
Zr:0.0003%~0.01%からなる群から選ばれた1種または2種以上を含有することを特徴とする請求項1または2に記載の熱間成形部材。 - 前記化学組成が、質量%で、
B:0.0003%~0.01%を含有することを特徴とする請求項1~3のいずれか1項に記載の熱間成形部材。 - 前記化学組成が、質量%で、
Bi:0.0003%~0.01%以下を含有することを特徴とする請求項1~4のいずれか1項に記載の熱間成形部材。 - 請求項1~5のいずれか1項に記載の熱間成形部材の前記化学組成と同じ化学組成を有し、未再結晶フェライトの含有量が0面積%~2.0面積%であり、フェライトの平均粒径が0.5μm~7.0μmである金属組織を有する素材鋼板を720℃以上Ac3点未満の温度域に加熱する加熱工程と、
前記加熱工程に次いで、前記素材鋼板の温度を720℃以上Ac3点未満の前記温度域に1分間~20分間保持する保持工程と、
前記保持工程に次いで、前記素材鋼板に熱間成形を行う熱間成形工程と、
前記熱間成形工程に次いで、前記素材鋼板を、600℃~150℃の温度域にて平均冷却速度が20℃/秒~500℃/秒である条件で冷却する冷却工程と、
を含むことを特徴とする熱間成形部材の製造方法。 - 請求項1~5のいずれか1項に記載の熱間成形部材の前記化学組成と同じ化学組成を有し、未再結晶フェライトが2.0面積%超であり、フェライトの平均粒径が0.5μm~7.0μm以下である金属組織を有する素材鋼板をAc3点~Ac3点+100℃の温度域に加熱する加熱工程と、
前記加熱工程に次いで、前記素材鋼板の温度をAc3点~Ac3点+100℃の前記温度域に30秒間以上20分間未満保持する保持工程と、
前記保持工程に次いで、前記素材鋼板に熱間成形を行う熱間成形工程と、
前記熱間成形工程に次いで、前記素材鋼板を、Ac3点~600℃の温度域にて平均冷却速度が3℃/秒~20℃/秒である条件で冷却する冷却工程と、
を含むことを特徴とする熱間成形部材の製造方法。 - 前記素材鋼板が、冷延鋼板、溶融亜鉛めっき鋼板、および合金化溶融亜鉛めっき鋼板からなる群から選ばれた1種である、請求項6又は7に記載の熱間成形部材の製造方法。
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CN110423953A (zh) * | 2019-08-21 | 2019-11-08 | 马鞍山钢铁股份有限公司 | 一种抗拉强度1800MPa级以上的冷弯性能优良的热成形构件及其制备方法 |
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