Classifications

• • 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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  • 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
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0447—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing characterised by the heat treatment
  • C21D8/0473—Final recrystallisation annealing
• • 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
• • 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
• • 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
• • 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
• • 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
• • 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
• • 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
• • 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
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
• • 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
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • 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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • 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
  • C21D2211/001—Austenite
• • 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
  • C21D2211/002—Bainite
• • 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
• • 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 steel sheet, particularly a high-strength steel sheet that is excellent in all of the following: component strength, stretch flangeability, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state, and a method for manufacturing the same.
• the steel sheet of the present invention can be suitably used as a structural member for automobile parts, etc.
• High strength steel sheets used in automotive reinforcing parts and frame structure parts are often required to have high part strength.
• the problem with high-strength steel plates with a tensile strength of 1,180 MPa or more is that hydrogen that penetrates into the steel in the corrosive atmospheric environment in which the car is traveling can cause delayed fracture, in which the component suddenly breaks.
• automotive steel sheets are subjected to stress during press working and part assembly, and there is a risk that hydrogen from the environment will subsequently penetrate into the steel sheet, so there is a demand for improved delayed fracture resistance in high-strength steel sheets.
• delayed fracture resistance in a painted state under atmospheric corrosion there is a demand for improved delayed fracture resistance in bent sections with sheared ends in a painted state in an environment where dry and wet cycles occur (hereinafter referred to as "delayed fracture resistance in a painted state under atmospheric corrosion").
• Patent Document 1 discloses an ultra-high strength cold-rolled steel sheet that has excellent hydrogen embrittlement resistance and a tensile strength of 1,300 MPa or more, and a method for manufacturing the same.
• the high-strength steel plate described in the above-mentioned Patent Document 1 has excellent delayed fracture properties at the punched end surface in a hydrochloric acid solution with a pH of 1. However, no consideration is given to the delayed fracture properties of the coating state in an environment where repeated dry and wet cycles occur during the day and night, which is a characteristic of atmospheric corrosion environments.
• the present invention was developed in consideration of these circumstances, and aims to obtain a high-strength steel plate that is excellent in all of the following: component strength, stretch flangeability, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state, and to provide an advantageous method for manufacturing the high-strength steel plate. It also aims to provide a component made of such a high-strength steel plate, and an automobile frame structural part or automobile reinforcing part made of such a component.
• high strength steel plate refers to a steel plate having a tensile strength (TS) of 1180 MPa or more as determined by a tensile test described below.
• Excellent member strength means that the yield ratio (YR) determined by the tensile test described below is more than 75%.
• excellent stretch flangeability means that the hole expansion ratio ( ⁇ ) determined by the hole expansion test described below is 30% or more.
• excellent bendability means that the limit bending radius (R/t) determined by the bending test described below is 5.0 or less.
• excellent delayed fracture resistance under atmospheric corrosion and in a painted state means that a delayed fracture test piece having a sheared end face described below is bent, stressed, and then electrochemically coated with a chemical electrodeposition coating, and then subjected to a corrosion cycle test in which dry and wet cycles are repeated, and no cracks are observed after 30 days.
• the gist of the present invention is as follows. 1. A component composition containing, by mass%, C: 0.090% or more and 0.390% or less, Si: 0.01% or more and 2.00% or less, Mn: 2.00% or more and 4.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities, At 1/4 of the thickness of the steel plate, The microstructure is Area ratio of tempered martensite: 75% or more, Area ratio of fresh martensite: 15% or less, Sum of area ratio of ferrite and bainite: 15% or less, And the area ratio of retained austenite is 15% or less, Furthermore, the average value [ ⁇ q] of the plastic deformation initiation stress ⁇ q measured by
• a steel slab having the composition according to 1 or 2 above is A method for producing a steel sheet, comprising the steps of performing rough rolling under conditions of 4 or more passes in a temperature range of 1000°C or higher, a rolling reduction of 15% or more in each pass, and an average strain rate in the range of 9 x 10-4 /s to 1 x 10-2 /s, followed by finish rolling and coiling to obtain a hot-rolled sheet, and then performing pickling and cold rolling to obtain a cold-rolled sheet by pickling and cold rolling, and then annealing the cold-rolled sheet, cooling it to 150°C or lower, and further reheating, The annealing is performed under conditions in which the heating temperature is 800° C. or higher, the dew point is ⁇ 25° C.
• [%C] is the C content in the steel plate
• t 0 (s)
• T t (° C.) is the average temperature of the cold-rolled sheet at time t: t-1 to t (s)
• the reheating step is also The reheating is carried out under conditions where the temperature X, which is the maximum temperature reached, and the holding time Y at a temperature of X-10°C or higher satisfy the following formula 2. 8000 ⁇ (273+X) ⁇ (20+Log(Y/3600)) ⁇ 12000...Formula 2 However, the unit of the temperature X is °C, and the unit of the holding time Y is s.
• a method for manufacturing a component comprising the step of subjecting the steel plate described in any one of items 1 to 3 to at least one of forming and joining to form the component.
• the present invention it is possible to provide a high-strength steel sheet which is excellent in all of member strength, stretch flangeability, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state. It is also possible to provide a member made of the above steel plate. Furthermore, according to the present invention, it is possible to provide a method for manufacturing the above-mentioned steel plate and member. In addition, according to the present invention, it is possible to provide an automobile frame structural part or an automobile reinforcing part that is made using the above-mentioned member.
• C 0.090% or more and 0.390% or less
• C is one of the important basic components of steel, and in this high strength steel plate, it affects the area ratio of tempered martensite at the 1/4 position of the plate thickness and the delayed fracture resistance. If the C content is too low, the area ratio of tempered martensite at the 1/4 position of the sheet thickness decreases, making it difficult to achieve a TS of 1180 MPa or more. For this reason, the C content is set to 0.090%.
• the C content is preferably 0.115% or more, and more preferably 0.140% or more.
• the C content is set to 0.390% or less.
• the C content is preferably set to 0.375% or less, and more preferably set to 0.360% or less.
• Silicon increases the strength of the steel sheet by suppressing the precipitation of cementite in tempered martensite and fresh martensite and by solid solution strengthening.
• the silicon content is set to 0.01% or more.
• the content is preferably 0.05% or more, and more preferably 0.10% or more.
• the Si content is too high, the precipitation of carbides during bainite transformation and martensite transformation is significantly suppressed, the amount of retained austenite at the 1/4 position of the sheet thickness increases excessively, and the carbide particles generated from the retained austenite during shearing are reduced. The hardness of the induced martensite increases significantly.
• the Si content is set to 2.00%.
• the Si content is preferably 1.75% or less, and more preferably 1.50% or less.
• Mn is one of the important basic components of steel, and in the present invention in particular, it greatly affects the area ratio of tempered martensite, delayed fracture resistance, and stretch flangeability. If the Mn content is too low, the area ratio of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. For this reason, the Mn content is set to 2.00% or more. is preferably 2.20% or more, and more preferably 2.40% or more.
• the Mn content is set to 4.00% or less. It is preferably 70% or less, more preferably 3.50% or less, and even more preferably 3.30% or less.
• the P content must be 0.100% or less.
• the P content is 0.070% or less.
• the P content is preferably 0.001% or more.
• S 0.0200% or less
• S exists as sulfide and reduces the ultimate deformability of the steel sheet, which reduces the bendability. Therefore, the S content must be 0.0200% or less.
• the lower limit of the S content is not particularly specified, but due to restrictions on production technology, the S content is preferably 0.0001% or more.
• Al 1.000% or less
• Al provides sufficient deoxidization and reduces inclusions in the steel. If the Al content is too high, a large amount of ferrite is generated, and delayed fracture cracks tend to propagate at the interface between ferrite and tempered martensite or between ferrite and fresh martensite, and the delayed fracture resistance in a painted state under atmospheric corrosion is reduced. The properties are deteriorated. Therefore, the Al content is set to 1.000% or less, preferably 0.500% or less, and more preferably 0.100% or less. On the other hand, in order to perform stable deoxidation, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more.
• N 0.0100% or less
• N exists as a nitride and reduces the ultimate deformability of the steel sheet, thereby reducing the bendability. Therefore, the N content is set to 0.0100% or less, preferably 0.0050% or less. Although there is no particular lower limit for the N content, due to restrictions in production technology, the N content is preferably 0.0001% or more.
• O exists as an oxide and reduces the ultimate deformability of the steel sheet, which reduces the bendability. Therefore, the O content is set to 0.0100% or less. Preferably, the O content is set to 0.0050% or less. Although there is no particular lower limit for the O content, due to restrictions in production technology, the O content is preferably 0.0001% or more.
• the high-strength steel plate according to the present invention has a composition containing the above-mentioned components, with the remainder being Fe and unavoidable impurities.
• unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. It is permissible for these impurities to be contained in a total amount of 0.100% or less.
• the steel plate according to the present invention further contains Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn At least one element selected from the group consisting of: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less may be contained alone or in combination.
• Ti, Nb and V are each 0.200% or less, large amounts of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced, so that bendability is not reduced. Therefore, when these elements are contained, the Ti, Nb and V contents are preferably 0.200% or less, and more preferably 0.100% or less. On the other hand, there is no particular lower limit for the Ti, Nb and V contents. Note that Ti, Nb and V increase the strength of the steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing. Therefore, the Ti, Nb and V contents are preferably 0.001% or more.
• the Ta and W contents are each 0.10% or less, large amounts of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced, so that bendability is not reduced. Therefore, the Ta and W contents are preferably 0.10% or less, and more preferably 0.08% or less. On the other hand, there is no particular lower limit for the Ta and W contents. Note that Ta and W increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, the Ta and W contents are preferably 0.01% or more, respectively.
• the B content is preferably 0.0100% or less, and more preferably 0.0080% or less.
• the B content is preferably 0.0003% or more.
• the Cr, Mo and Ni contents are preferably 1.00% or less, and more preferably 0.80% or less.
• the Cr, Mo and Ni contents are preferably 0.01% or more.
• the Co content is 0.010% or less, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not decrease, so bendability will not decrease. Therefore, the Co content is preferably 0.010% or less, and more preferably 0.008% or less. On the other hand, there is no particular lower limit for the Co content. Note that, since Co is an element that improves hardenability, the Co content is preferably 0.001% or more.
• the Cu content is preferably 1.00% or less, and more preferably 0.80% or less. On the other hand, there is no particular lower limit for the Cu content. Note that, since Cu is an element that improves hardenability, the Cu content is preferably 0.01% or more.
• the Sn content is 0.200% or less, cracks will not form inside the steel plate during casting or hot rolling, and the ultimate deformability of the steel plate will not decrease, so bendability will not decrease. Therefore, the Sn content is preferably 0.200% or less, and more preferably 0.100% or less. On the other hand, there is no particular lower limit for the Sn content. Since Sn is an element that improves hardenability, the Sn content is preferably 0.001% or more.
• the Sb content is preferably 0.200% or less, and more preferably 0.100% or less. On the other hand, there is no particular lower limit for the Sb content. Since Sb is an element that controls the surface softening thickness and enables strength adjustment, the Sb content is preferably 0.001% or more.
• the Ca, Mg and REM if each is 0.0100% or less, do not increase coarse precipitates or inclusions, do not reduce the ultimate deformability of the steel sheet, and therefore do not reduce bendability. Therefore, the Ca, Mg and REM contents are preferably 0.0100% or less, and more preferably 0.0050% or less. On the other hand, there is no particular lower limit for the Ca, Mg and REM contents. Note that, since these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, it is preferable that the Ca, Mg and REM contents are 0.0005% or more.
• the Zr and Te contents are preferably 0.100% or less, and more preferably 0.080% or less.
• the Zr and Te contents are 0.001% or more, respectively.
• the Hf content is preferably 0.10% or less, and more preferably 0.08% or less.
• the Hf content is preferably 0.01% or more.
• the Bi content is preferably 0.200% or less, and more preferably 0.100% or less. On the other hand, there is no particular lower limit for the Bi content. Since Bi is an element that reduces segregation, the Bi content is preferably 0.001% or more.
• the microstructure at the 1/4 position of the sheet thickness of the steel sheet of the present invention will be described.
• Area ratio of tempered martensite 75% or more
• a TS of 1180 MPa or more can be realized.
• the YR can be increased.
• the area ratio of tempered martensite is set to 75% or more.
• the area ratio of such tempered martensite is preferably 80% or more, and more preferably 85% or more.
• the upper limit of the area ratio of tempered martensite is not particularly limited, and the above-mentioned effects can be obtained even if the area ratio of tempered martensite is 100%.
• Tempered martensite is defined as martensite in which carbides are observed by SEM observation, which will be described later. Such carbides include cementite ( ⁇ ), epsilon ( ⁇ ), eta ( ⁇ ), and chi ( ⁇ ). Tempered martensite also includes lower bainite that is formed at or below the Ms point. The observation position of such tempered martensite is, as will be described later, a quarter position in the sheet thickness of the steel sheet.
• Fresh martensite is defined as martensite in which no carbide is observed by SEM observation, which will be described later. Note that the observation position of fresh martensite is a quarter position in the sheet thickness of the steel sheet, as will be described later.
• Total area ratio of ferrite and bainite 15% or less
• the total area ratio of ferrite and bainite is set to 15% or less.
• the total area ratio of ferrite and bainite is preferably 13% or less, more preferably 10% or less. Note that the effect of the present invention can be obtained even if the total area ratio of ferrite and bainite is 0%.
• Ferrite is a soft BCC iron formed at high temperatures, and includes allotriomorph ferrite and idiomorph ferrite.
• Bainite is an angular BCC iron containing fine carbides that forms at temperatures above Ms. The observation position for ferrite and bainite was a quarter of the thickness of the steel plate.
• the method for measuring the area ratio of retained austenite is as follows. First, the steel plate to be measured is ground so that the 1/4 position of the plate thickness (the position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate) becomes the measurement surface, and then further polished by 0.1 mm by chemical polishing to obtain a sample.
• the measurement surface of the sample is measured by an X-ray diffraction device using a Co K ⁇ source to measure the integrated reflection intensities of the (200), (220) and (311) faces of fcc iron (austenite), and the (200), (211) and (220) faces of bcc iron.
• the intensity ratios of the integrated reflection intensities of each face of fcc iron to the integrated reflection intensities of each face of bcc iron thus obtained are obtained, and a total of nine intensity ratios are obtained.
• the average value of these nine intensity ratios is regarded as the volume fraction of retained austenite.
• the volume fraction of retained austenite is regarded as the area fraction of retained austenite.
• the structure at the 1/4 position of the sheet thickness of the steel sheet of the present invention may have a structure (remaining structure) other than the above-mentioned tempered martensite, fresh martensite, ferrite, bainite and retained austenite.
• the area ratio of the remaining structure is preferably 5% or less in terms of area ratio, because the effect of the present invention is not impaired. Examples of the remaining structure include pearlite, alloy carbonitrides precipitated in ferrite, and other structures known as structures of steel sheets.
• the average value [ ⁇ q] of the plastic deformation initiation stress ⁇ q is 2.50 GPa or more and 4.10 GPa or less]
• the stress at which plastic deformation begins at the 1/4 position of the plate thickness of the steel plate is an important constituent element in the present invention.
• the plastic deformation initiation stress is a value obtained at an early stage of a load-displacement curve obtained by nanoindentation as described below, and means a stress at which a transition occurs from an elastic region to a plastic region in a local region.
• the plastic deformation initiation stress of the present invention is a stress corresponding to the generation and emission of dislocations in a local region, and is completely different from the nanohardness obtained by the conventional nanoindentation test or the yield strength obtained by the conventional tensile test.
• the inventors have conducted extensive research into the relationship between the plastic deformation initiation stress obtained by nanoindentation and the delayed fracture resistance properties under atmospheric corrosion and in a coated state.
• the average value [ ⁇ q] of the plastic deformation initiation stress ⁇ q at the 1/4 position of the plate thickness of the steel plate to 2.50 GPa or more and 4.10 GPa or less.
• the delayed fracture resistance properties under atmospheric corrosion and in a painted state are improved.
• the average value [ ⁇ q] of the plastic deformation initiation stress ⁇ q to an appropriate value as described above, the generation and emission of dislocations at the tip of the delayed fracture crack is optimized, and the propagation of the delayed fracture crack is suppressed.
• the delayed fracture resistance under atmospheric corrosion and in a painted state is improved.
• the average value [ ⁇ q] of the plastic deformation initiation stress ⁇ q is set to 2.50 GPa or more.
• the average value [ ⁇ q] of the plastic deformation initiation stress ⁇ q is 2.65 GPa or more. More preferably, the average value [ ⁇ q] of the plastic deformation initiation stress ⁇ q is 2.80 GPa or more.
• the average value [ ⁇ q] of the plastic deformation initiation stress ⁇ q exceeds 4.10 GPa, the generation and emission of dislocations at the crack tip is suppressed, and the delayed fracture crack propagates brittlely along the grain boundary. As a result, the delayed fracture resistance under atmospheric corrosion and in a painted state is reduced, particularly in the case of a steel plate having a high yield ratio. Therefore, the average value [ ⁇ q] of the stress at which plastic deformation begins needs to be 4.10 GPa or less.
• the average value [ ⁇ q] of the stress at which plastic deformation begins is 3.90 GPa or less. More preferably, the average value [ ⁇ q] of the stress at which plastic deformation begins is 3.80 GPa or less.
• Standard deviation ⁇ q of plastic deformation initiation stress is 0.30 GPa or less
• the standard deviation ⁇ q of the plastic deformation initiation stress at the 1/4 position of the steel plate thickness is an important constituent element in the present invention.
• the standard deviation ⁇ q of the plastic deformation initiation stress 0.30 GPa or less
• the delayed fracture resistance under atmospheric corrosion and in a painted state is improved, particularly in a steel plate having a high yield ratio.
• the standard deviation ⁇ q of 0.30 GPa or less suppresses the variation in the microscopic plastic deformation initiation stress.
• the variation in the generation and emission behavior of dislocations at the crack tip is suppressed, and the delayed fracture crack is suppressed from selectively propagating through weak parts in the structure.
• it is estimated that the delayed fracture resistance under atmospheric corrosion and in a painted state is improved.
• the standard deviation ⁇ q of the plastic deformation initiation stress exceeds 0.30 GPa, the microscopic variation in the plastic deformation initiation stress is large, and delayed fracture cracks will selectively propagate through weak parts in the structure. As a result, particularly in the case of a steel plate having a high yield ratio, the delayed fracture resistance under atmospheric corrosion and in a painted state will decrease. Therefore, the standard deviation ⁇ q of the plastic deformation initiation stress must be 0.30 GPa or less. Preferably, the standard deviation ⁇ q of the plastic deformation initiation stress is 0.26 GPa or less. On the other hand, the smaller the standard deviation ⁇ q of the plastic deformation initiation stress, the better, and it may be 0 GPa.
• the area ratio of the tempered martensite in the microstructure at a position 10 ⁇ m from the steel sheet surface is set to 40% or less.
• the area ratio of the tempered martensite is preferably 35% or less, and more preferably 30% or less. Note that the effect of the present invention can be obtained even if the area ratio of the tempered martensite is 0%.
• Tempered martensite is defined as martensite in which carbides are observed by SEM observation, which will be described later.
• carbides include cementite ( ⁇ ), epsilon ( ⁇ ), eta ( ⁇ ), and chi ( ⁇ ).
• the observation position of the tempered martensite is 10 ⁇ m from the surface of the steel sheet, as described below.
• the area ratio of pearlite in the microstructure at a position 10 ⁇ m from the steel sheet surface is set to 15% or less.
• the area ratio of pearlite is preferably 10% or less. However, the effect can be obtained even if the area ratio of pearlite is 0%.
• the observation position for such pearlite is 10 ⁇ m from the surface of the steel sheet, as described below.
• Total area ratio of ferrite and bainite 60% or more
• the bendability and the resistance to delayed fracture under atmospheric corrosion and in a painted state are improved. That is, by increasing the total area ratio of the soft phases ferrite and bainite, the number of starting points for cracks during bending is reduced, improving bendability.
• ferrite and bainite have fewer lattice defects and fewer hydrogen trapping sites than tempered martensite and fresh martensite, which suppresses hydrogen penetration due to corrosion that occurs in the steel sheet surface layer directly below the paint, making delayed fracture cracks less likely to occur. As a result, delayed fracture resistance is improved under atmospheric corrosion and in a painted state.
• the total area ratio of ferrite and bainite is 60% or more, preferably 65% or more, and more preferably 70% or more.
• the above-mentioned effects can be obtained even if the total area ratio of ferrite and bainite is 100%.
• Ferrite is a soft BCC iron formed at high temperatures, and includes allotriomorph ferrite and idiomorph ferrite.
• Bainite is an angular BCC iron containing fine carbides that forms at temperatures above Ms. The observation position for the ferrite and bainite was 10 ⁇ m from the surface of the steel sheet.
• the steel structure at a position 10 ⁇ m from the steel sheet surface may have a structure (remaining structure) other than the above-mentioned tempered martensite, pearlite, ferrite and bainite.
• the area ratio of the remaining structure is preferably 5% or less in terms of area ratio because the effect of the present invention is not impaired.
• Examples of the remaining structure include pearlite, alloy carbonitrides precipitated in ferrite, and other structures known as the structure of steel sheets.
• the method for measuring the area ratios of tempered martensite, fresh martensite, pearlite, ferrite and bainite at a position 1/4 of the sheet thickness of the steel sheet or at a position 10 ⁇ m from the surface of the steel sheet is as follows. First, a sample is cut out from a steel sheet so that the plate thickness cross section (L cross section) parallel to the rolling direction becomes the observation surface. The observation surface of the sample is mirror-polished with diamond paste, then finish-polished with colloidal silica, and further etched with 1 volume % nital to reveal the structure.
• a scanning electron microscope (SEM) with an acceleration voltage of 10 kV is used to observe three fields of view at a position 1/4 of the sheet thickness of the steel sheet or a position 10 ⁇ m from the surface of the steel sheet on the observation surface of the sample at a magnification of 3000 times to obtain SEM images of the three fields of view.
• the area ratio of each structure is calculated using Adobe Photoshop (Adobe Systems, Inc.). Specifically, the value obtained by dividing the area of each structure by the measured area is regarded as the area ratio of each structure. The area ratio of each structure is calculated for three fields of view, and the average of these is regarded as the area ratio of each structure.
• tempered martensite has a hierarchical structure with fine internal irregularities, and is a structure region containing fine carbides with white contrast.
• Fresh martensite has a hierarchical structure with fine internal irregularities, and is a structure region not containing fine carbides with white contrast.
• Pearlite is a layered structure region consisting of gray ferrite and white cementite. Ferrite is gray and is a flat structure region not containing carbides. Bainite is gray and is a structure region containing fine carbides.
• the percentage of measurement points that are less than 0.85 ⁇ [ ⁇ s] is 25.0% or less]
• the proportion of measurement points defined by the average value of the plastic deformation initiation stress ⁇ s at a position 10 ⁇ m from the steel sheet surface measured by the nanoindentation method is an important constituent element in the present invention. That is, when the average value is [ ⁇ s], by making the proportion of measurement points where the stress is less than 0.85 ⁇ [ ⁇ s] 25.0% or less, the delayed fracture resistance property is improved under atmospheric corrosion and in a painted state, particularly in a steel sheet having a high yield ratio. This is because the structure region where the stress is less than 0.85 ⁇ [ ⁇ s] is a region where local dislocation generation and emission are likely to occur, and becomes the starting point of delayed fracture cracks on the steel sheet surface.
• the ratio of measurement points that are less than 0.85 ⁇ [ ⁇ s] must be 25.0% or less.
• the ratio of measurement points that are less than 0.85 ⁇ [ ⁇ s] is 20.0% or less.
• the lower limit is not particularly limited, and the above-mentioned effect can be obtained even if the ratio of measurement points that are less than 0.85 ⁇ [ ⁇ s] is 0%.
• a position 10 ⁇ m from the surface of the steel plate means a position 10 ⁇ m deep in the plate thickness direction from the surface of the steel plate (a surface perpendicular to the plate thickness direction).
• the above regulations must be satisfied at least on one side of the steel plate for both the 1/4 plate thickness position and the 10 ⁇ m position from the steel plate surface.
• the measurement sample is prepared by cutting out a sample so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the measurement surface, and then mirror-polishing the measurement surface using diamond paste, and then finish-polishing using colloidal silica.
• a nanoindentation device equipped with a Berkovich indenter is used to measure the stress at which plastic deformation begins.
• the measurement position is set at 1/4 the thickness of the steel plate or 10 ⁇ m from the surface of the steel plate, and a nanoindentation test is performed under load control with a loading rate and unloading rate of 50 ⁇ N/s, a maximum load of 500 ⁇ N, and a data collection time interval of 5 ms to obtain the load P (N) and the displacement h (nm) at that load.
• a nanoindentation test is performed at 40 points at each measurement position. The measurement is performed with a distance of 2 ⁇ m or more between indentations.
• the Hertz contact displacement hc (nm) is calculated at each load P (N) using the Hertz contact equation shown as Equation 3 below.
• hC (nm) is the displacement assuming elastic deformation obtained by the Hertz contact equation
• P (N) is the load
• Er (Pa) is the composite Young's modulus
• R (m) is the radius of curvature of the indenter tip.
• Er is the average value (for 40 points) of the composite Young's modulus obtained from the unloading curve in each measurement.
• R(m) changes depending on the wear state of the Berkovich indenter, so it is determined by fitting a load-displacement curve in the elastic region using a standard sample such as fused silica.
• plastic deformation initiation stress ⁇ (GPa) is calculated from the plastic deformation initiation load using the following formula 5.
• the plastic deformation initial stress ⁇ (GPa) is calculated for 40 points, and the average value thereof is calculated.
• the average value of the stress at which plastic deformation begins at the 1/4 position in the sheet thickness is [ ⁇ q]
• the average value of the stress at which plastic deformation begins at the 10 ⁇ m position from the steel sheet surface is [ ⁇ s].
• a histogram is also created from the values of the stress at which plastic deformation begins at 40 points in the 1/4 position in the sheet thickness, and the standard deviation ⁇ q is calculated.
• the ratio of measurement points that are less than 0.85 ⁇ [ ⁇ s] is calculated from the values of the stress at which plastic deformation begins at 40 points in the 10 ⁇ m position from the steel sheet surface.
• the thickness of the high-strength steel plate of the present invention is not particularly limited, but is usually preferably 0.3 mm or more and 2.8 mm or less.
• the steel sheet according to the present invention may have a plating layer on its surface.
• the plating layer is formed by a plating process described later.
• the type of plating is not particularly limited, and examples thereof include hot-dip plating and electroplating.
• Examples of the plating layer include a zinc plating layer (Zn plating layer) and an Al plating layer.
• the zinc plating layer is preferable.
• the zinc plating layer may contain elements such as Al and Mg.
• the plating layer may be an alloyed plating layer (alloyed plating layer).
• the composition of the plating layer is not particularly limited, and may be a general composition.
• the plating layer is a hot-dip galvanized layer or an alloyed hot-dip galvanized layer
• the following composition is typical: Fe: 20 mass% or less, Al: 0.001-1.0 mass%, and further containing at least one selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0 mass% to 3.5 mass%, with the remainder consisting of Zn and unavoidable impurities.
• the coating weight of the plating layer per side is preferably 20 g/m 2 or more.
• the coating weight of the plating layer per side is preferably 80 g/m 2 or less.
• a galvannealed hot-dip galvanized layer obtained by alloying a hot-dip galvanized layer having such a coating weight can also be used.
• the Fe content in the plating layer is preferably less than 7 mass%.
• the Fe content in the plating layer is preferably 7 mass% or more.
• the Fe content in the plating layer is preferably 20 mass% or less, and more preferably 15 mass% or less.
• the member according to the present invention is formed by using the steel plate according to the embodiment of the present invention described above. Such a member is formed, for example, by forming or joining the steel plate according to the embodiment of the present invention into a desired shape.
• the member according to one embodiment of the present invention is preferably a member for an automobile frame structural part or a member for an automobile reinforcement part. That is, the steel plate according to the present invention is a high-strength steel plate excellent in all of member strength, stretch flangeability, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state. Therefore, the member according to the present invention can be suitably used in general as a member for an automobile frame structural part or a member for an automobile reinforcement part.
• the part according to the present invention is made by using the member of the present invention described above.
• the part according to one embodiment of the present invention is preferably an automobile frame structural part or an automobile reinforcement part.
• the member of the present invention described above is excellent in all of member strength, stretch flangeability, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state. Therefore, the part according to one embodiment of the present invention made by using such a member can be particularly preferably used for automobile frame structural parts or automobile reinforcement parts in general.
• a method for producing a steel sheet according to the present invention will be described.
• a steel material having the above-mentioned composition is melted to produce a steel slab.
• the method for melting the molten steel to become the steel slab is not particularly limited, and a known melting method using a converter, an electric furnace, or the like can be adopted.
• the steel slab is preferably produced by a continuous casting method in order to prevent macrosegregation, but it can also be produced by other methods such as an ingot casting method and a thin slab casting method.
• the steel sheet of the present invention includes a cold-rolled steel sheet, which is produced by hot rolling, pickling, cold rolling and annealing, and a steel sheet obtained by plating a cold-rolled steel sheet.
• the steel slab is hot rolled to produce a hot-rolled sheet.
• the steel slab is cooled to room temperature, and then heated again and hot rolled (rough rolling and finish rolling).
• the produced steel slab may be loaded into the heating furnace as a hot piece without being cooled to room temperature, or may be briefly heated and then immediately rough rolled.
• the steel slab is rough rolled under the following conditions to obtain a rough rolled plate.
• the temperature at which the steel slab is heated (slab heating 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 lower.
• the slab heating temperature is based on the surface temperature of the steel slab.
• the steel slab heated to the slab heating temperature is subjected to rough rolling under the following conditions.
• the standard deviation ⁇ q of the plastic deformation initiation stress can be reduced by setting the average strain rate during rough rolling to the range of 9 ⁇ 10 ⁇ 4 /s or more and 1 ⁇ 10 ⁇ 2 /s or less.
• the average strain rate during rough rolling is defined as the rolling ratio ⁇ (-) from the first mill to the last mill in rough rolling divided by the time t R (s) required from the start of rolling at the first mill to the completion of rolling at the last mill in rough rolling ( ⁇ /t R ).
• the average strain rate during rough rolling is less than 9 ⁇ 10 ⁇ 4 /s
• the dynamic recovery of dislocations in the austenite grains is promoted.
• the dislocation density is reduced, dislocation pipe diffusion of solute atoms such as Si and Mn is suppressed, and the diffusion of such solute atoms becomes insufficient, resulting in the appearance of regions in which Si and Mn are scarce and regions in which they are enriched inside the steel sheet.
• a difference occurs in the frictional force within the crystal between the two regions, so the variation in the plastic deformation initiation stress increases, and the standard deviation ⁇ q of the plastic deformation initiation stress increases.
• the average strain rate is in the range of 9 ⁇ 10 ⁇ 4 /s or more and 1 ⁇ 10 ⁇ 2 /s or less.
• the average strain rate is preferably 1 ⁇ 10 ⁇ 3 /s or more.
• the average strain rate is preferably 9 ⁇ 10 ⁇ 3 /s or less.
• the rough rolled sheet is subjected to finish rolling to obtain a hot rolled sheet (hot rolling process).
• the hot rolled sheet is appropriately wound.
• the temperature when performing the finish rolling is preferably 700° C. or higher. This reduces the rolling load. Furthermore, the rolling reduction in the non-recrystallized state of austenite is reduced, and the development of abnormal structures elongated in the rolling direction is suppressed, making it possible to obtain a steel sheet with excellent workability.
• the finish rolling may be performed continuously by joining the rough rolled sheets together. Also, the rough rolled sheet may be temporarily wound before the finish rolling.
• a part or all of the finish rolling may be lubricated rolling.
• Lubricated rolling is also preferred from the viewpoint of uniforming the steel sheet shape and material properties.
• the friction coefficient during lubricated rolling is preferably 0.10 or more, and 0.25 or less.
• the coiling temperature after hot rolling is preferably 300° C. or higher from the viewpoint of improving the sheet passing properties during cold rolling and annealing described below, but is preferably 700° C. or lower.
• the hot-rolled sheet obtained by hot rolling is appropriately pickled.
• Pickling removes oxides from the surface of the hot-rolled sheet, and the final product, a high-strength steel sheet, has excellent chemical conversion properties and plating layer quality. Pickling may be performed once or multiple times.
• the hot-rolled sheet after pickling is optionally subjected to softening heat treatment and then cold-rolled to obtain a cold-rolled sheet.
• the conditions of the cold rolling are not particularly limited and may be in accordance with a conventional method, but the cumulative reduction ratio of the cold rolling is preferably in the range of 20 to 75%.
• the number of rolling passes and the reduction ratio of each pass of the cold rolling are not particularly limited and may be in accordance with a conventional method.
• the cold-rolled sheet thus obtained is then annealed as described below, cooled to 150° C. or lower, and then reheated.
• Heating temperature is 800°C or higher. If the heating temperature in the annealing process is too low, the reverse transformation to austenite does not proceed sufficiently, and the area ratio of ferrite at the 1/4 position of the steel sheet increases, so that the area ratio of tempered martensite at the 1/4 position of the steel sheet decreases. Therefore, the heating temperature is set to 800°C or higher. The heating temperature is preferably 830°C or higher. On the other hand, the upper limit of the heating temperature is not particularly limited, but from the viewpoint of operability and damage to the furnace body, the heating temperature is preferably 1000°C or lower. The heating temperature is based on the surface of the steel sheet.
• the dew point is -25°C or higher. If the dew point of the atmosphere in the heating temperature range T1 of 800°C or more in the annealing process is too low, decarburization does not proceed on the surface, and the total area ratio of ferrite and bainite at a position 10 ⁇ m from the steel sheet surface becomes too low. In addition, if the dew point is too low, the decarburization distribution at a position 10 ⁇ m from the steel sheet surface becomes non-uniform, and a portion with a low plastic deformation initiation stress appears, and the proportion of measurement points at a position 10 ⁇ m from the steel sheet surface that are less than 0.85 ⁇ [ ⁇ s] increases. Therefore, the dew point is set to -25°C or higher. Such a dew point is preferably -20°C or higher. On the other hand, the upper limit of the dew point is not particularly limited, but from the viewpoint of operability and damage to the furnace body, the dew point is preferably +15°C or lower.
• T t (° C.) is the average temperature of the cold-rolled sheet during time t: t-1 to t (s). Additionally, [%C] refers to the C content in the steel plate.
• parameter K is based on the diffusion phenomenon of C in the austenite region. As described above, the parameter K is calculated from the amount of C in the steel sheet, the temperature during the annealing process, and time.
• K (mm 2 ) is set to 2.0 or more.
• K (mm 2 ) is preferably 3.0 or more, and more preferably 4.0 or more.
• K ( mm2 ) is set to 60.0 or less.
• K ( mm2 ) is preferably 45.0 or less, and more preferably 30.0 or less.
• the temperature history in the annealing step is not particularly limited as long as the parameter K is within the above range.
• the temperature range T2 of 600° C. or more and 750° C. or less is 1.0° C./s or more and 15.0° C./s or less]
• the temperature range T2 of 600°C to 750°C is a temperature range in which ferrite transformation occurs at the 1/4 position of the steel sheet and at a position 10 ⁇ m from the steel sheet surface. If the average cooling rate v2 in this temperature range T2 is too low, excessive ferrite transformation occurs at the 1/4 position of the steel sheet, and the area ratio of ferrite at the 1/4 position of the steel sheet becomes high. Therefore, the average cooling rate v2 is set to 1.0°C/s or more.
• the average cooling rate v2 is preferably set to 2.0°C/s or more.
• the average cooling rate v2 is set to 15.0°C/s or less.
• the average cooling rate v2 is preferably set to 13.0°C/s or less.
• the temperature range of 500°C or more and less than 600°C is a temperature range in which pearlite transformation can occur at a position 10 ⁇ m from the steel sheet surface. That is, if the average cooling rate in the temperature range of 500°C or more and less than 600°C is equal to or less than v2, pearlite transformation occurs with the interface between ferrite and austenite as a nucleus, and the area ratio of pearlite at a position 10 ⁇ m from the steel sheet surface increases excessively. Therefore, the average cooling rate in the temperature range of 500°C or more and less than 600°C is made to exceed v2. Preferably, it is made to exceed (v2 + 2°C/s). The upper limit of the average cooling rate in this temperature range is not particularly specified, but is about 1000°C/s or less due to equipment.
• the temperature range T3 of 400° C. or more and less than 500° C. is 10 s or more and 150 s or less]
• the temperature range T3 of 400°C or more and less than 500°C is a temperature range in which bainite transformation occurs at the 1/4 position of the steel sheet and at a position 10 ⁇ m from the steel sheet surface. If the residence time in this temperature range T3 is too short, bainite transformation becomes difficult to occur at a position 10 ⁇ m from the steel sheet surface, and the area ratio of bainite at a position 10 ⁇ m from the steel sheet surface decreases. Therefore, the residence time in the temperature range T3 is set to 10 s or more. The residence time in this temperature range T3 is preferably set to 15 s or more.
• the residence time in the temperature range T3 is set to 150 seconds or less.
• the residence time in the temperature range T3 is preferably set to 130 seconds or less.
• the temperature range T4 from Ms-100°C to Ms°C is a temperature range in which martensitic transformation and self-tempering of the generated martensite occur, and C is distributed from the martensite to the untransformed austenite. If the average cooling rate v4 in the temperature range T4 is too slow, the self-tempering of the generated martensite and the distribution of C from the martensite to the untransformed austenite are significantly promoted, resulting in non-uniformity of the structure and an excessively high standard deviation ⁇ q of the plastic deformation initiation stress.
• the average cooling rate v4 is set to 3.0°C/s or more.
• the average cooling rate v4 is preferably 4.0°C/s or more.
• the upper limit of the average cooling rate v4 is not particularly limited, but from the viewpoint of reducing the capital investment burden, it is preferably about 1000° C./s or less.
• Ms 499-308[%C]-10.8[%Si]-32.4[%Mn]-27[%Cr]-10.8[%Mo]...Formula 6
• [%M] indicates the content of M in the steel (mass%).
• the martensitic transformation progresses sufficiently by cooling the steel sheet in the temperature range T4 to 150°C or less. If the cooling completion temperature is higher than 150°C, the martensitic transformation is not completed, and the subsequent reheating does not cause tempering, resulting in too much fresh martensite. Therefore, the cooling completion temperature is 150°C or less. Preferably, the cooling completion temperature is 100°C or less.
• variable part [(273+X) ⁇ (20+Log(Y/3600))] of the formula 2, which is represented by the temperature X, which is the maximum temperature reached in the reheating, and the holding time Y in the temperature range below this temperature X and above temperature X-10°C, is too small, the precipitation of carbides in the tempered martensite and the segregation of C on dislocations become insufficient, and YS decreases, resulting in a decrease in part strength.
• the mobility of dislocations in the tempered martensite increases, and the average value [ ⁇ q] of the plastic deformation initiation stress ⁇ q decreases. Therefore, the variable part is set to 8000 or more. Preferably, the variable part is 8500 or more.
• variable portion is set to 12,000 or less.
• the variable portion is set to 11,500 or less.
• the cold-rolled sheet that has been subjected to such heat treatment is then cooled to room temperature.
• a high-strength steel sheet (cold-rolled steel sheet) according to the present invention is obtained.
• the obtained high-strength steel sheet is a plated steel sheet having a plating layer.
• other conditions are not particularly limited, and there are also no particular limitations on the equipment in which the heat treatments are carried out.
• the cold rolled sheet may be subjected to a plating treatment.
• plating treatments include hot-dip galvanizing treatment (treatment for forming a hot-dip galvanized layer), alloyed hot-dip galvanizing treatment (treatment for forming an alloyed hot-dip galvanized layer by performing an alloying treatment after hot-dip galvanizing treatment), etc.
• an electroplating layer may be formed by an electroplating treatment.
• the bath temperature of the galvanizing bath is not particularly limited, but is preferably 440°C or higher and 500°C or lower.
• the Al content of the galvanizing bath is preferably 0.10 mass% or higher and 0.23 mass% or lower.
• such a galvanizing treatment is preferably carried out after the steel sheet is held in the temperature range T3 of 400° C. or more and less than 500° C. during the cooling process after the above-mentioned annealing.
• the treatment temperature when performing the alloying treatment is preferably 470°C or higher in order to more favorably improve the Zn-Fe alloying rate and productivity.
• the alloying treatment temperature is preferably 600°C or lower, more preferably 560°C or lower.
• the alloying treatment temperature is based on 530°C.
• the steel sheet after cooling to room temperature may be subjected to skin pass rolling.
• the reduction ratio of the skin pass rolling is preferably 0.01% or more from the viewpoint of stabilizing the shape.
• the upper limit of the reduction ratio is not particularly limited, but is preferably 1.50% or less from the viewpoint of productivity.
• the skin pass rolling may be performed either online or offline.
• the skin pass may be performed at a single time with a desired rolling reduction, or may be performed in several steps.
• Production conditions other than those mentioned above can be carried out according to conventional methods.
• the member according to the present invention can be manufactured by subjecting the above-mentioned high strength steel plate to at least one of forming and joining.
• the forming and joining can be performed by conventional methods.
• any items not described in this specification can be manufactured using conventional methods.
• the steel slab thus obtained was subjected to hot rolling to obtain a hot-rolled sheet. Specifically, the steel slab was heated to 1250 ° C., and rough-rolled at the number of passes in the temperature range of 1000 ° C. or more, the reduction rate in each pass, and the average strain rate shown in Table 2 below. Then, finish rolling was performed at a finish rolling temperature of 900 ° C., and then coiled under the condition of 500 ° C. After coiling in this manner, the hot-rolled sheet was obtained by cooling to room temperature. The obtained hot-rolled sheet was subjected to pickling, followed by softening heat treatment at a condition of 500 ° C., and then cold rolling at a rolling rate of 50%.
• a hot-dip galvanizing process was performed to form a plating layer (hot-dip galvanized layer) on both sides, thereby obtaining a hot-dip galvanized steel sheet (GI).
• a hot-dip galvanizing bath bath temperature: 470° C.
• the coating weight of the hot-dip galvanized layer per side was set to about 45 to 72 g/m2.
• the composition of the formed hot-dip galvanized layer contained 0.1 to 1.0 mass % of Fe, 0.2 to 1.0 mass % of Al, and the balance being Zn and unavoidable impurities.
• Another part of the cold-rolled sheets was subjected to a galvannealing treatment after retention in a temperature region T3 of 400° C. or higher and lower than 500° C. to form a plating layer (galvannealed layer) on both sides. That is, a galvannealed steel sheet (GA) was obtained.
• a hot-dip galvanizing bath bath temperature: 470° C.
• the alloying treatment was performed at 550° C.
• the coating weight of the alloyed hot-dip galvanized layer per side was about 45 g/m 2 .
• the composition of the formed alloyed hot-dip galvanized layer contained 7 to 15 mass % Fe, 0.1 to 1.0 mass % Al, and the balance being Zn and unavoidable impurities.
• GI hot-dip galvanized layer
• GA alloyed hot-dip galvanized layer
• CR CR in the "Plating type” column in Table 2 below.
• the bending test was carried out in accordance with JIS Z 2248:2022. Specifically, a rectangular test piece having a width of 30 mm and a length of 100 mm was taken from the obtained steel sheet so that the axial direction of the bending test was parallel to the rolling direction of the steel sheet. The end face in the longitudinal direction of the test piece was the ground end face. Using the collected test pieces, a 90° V-bend test was carried out under the conditions of an indentation load of 100 kN and a holding time of 5 seconds. That is, the 90° V-bend test was carried out on five test pieces with an appropriate bending radius R. Next, the presence or absence of cracks at the ridgeline of the bent apex was confirmed.
• a delayed fracture test to confirm the delayed fracture resistance properties under atmospheric corrosion and in a painted state was conducted by bending a test piece having a shear end surface, applying a chemical electrocoating to the delayed fracture test piece under stress, and then conducting a repeated wet and dry corrosion cycle test. Specifically, a rectangular test piece having a width of 30 mm and a length of 100 mm was cut from the obtained steel sheet so that the axial direction of the bending test was parallel to the rolling direction of the steel sheet. The longitudinal end face of the test piece was a shear end face (clearance: 15%, shear angle: 0°).
• the test piece was subjected to a 90° V-bend process so that R/t was 5.0, and then it was tightened with a bolt so that the load stress at the apex outside the bend was 1000 MPa.
• the test pieces thus loaded with stress were subjected to a chemical conversion treatment by immersion under standard conditions (35°C, 120 seconds) using "Palbond” manufactured by Nippon Parkerizing Co., Ltd., and then subjected to electrodeposition coating and baking treatment using electrodeposition paint "GT-100" manufactured by Kansai Paint Co., Ltd. to form a coating film.
• the coating film thickness of the electrodeposition coating was set to 15 ⁇ m, and was confirmed by measuring the film thickness using a commercially available electromagnetic coating thickness meter.
• the thus-prepared delayed fracture test pieces of the bent portion having a sheared end surface in a painted state were subjected to a corrosion cycle test.
• the corrosion cycle test was performed in a constant temperature and humidity chamber at 50°C, with one cycle consisting of drying (30% RH, 2h), humidity transition (30% ⁇ 90% RH, 2h), wetting (90% RH, 2h), humidity transition (90% ⁇ 30% RH, 2h).
• salt water was sprayed onto the surface of the delayed fracture test specimen twice a week so that the amount of NaCl attached was 3g/ m2 .
• the examples according to the present invention have high strength and are excellent in all of component strength, stretch flangeability, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state.
• the comparative examples are inferior in one or more of strength, component strength, stretch flangeability, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state.
• the present invention makes it possible to manufacture high-strength steel plates that are excellent in all of the following: component strength, stretch flangeability, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state. Furthermore, by applying the steel plates obtained according to the method of the present invention to, for example, automobile structural components, it is possible to improve fuel efficiency by reducing the weight of the vehicle body, making the steel plates extremely valuable in industry.

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• 2024
  • 2024-06-03 CN CN202480046032.7A patent/CN121464236A/zh active Pending
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