Steel plates, members, and parts, and their manufacturing methods

Abstract

Provided is a high-strength steel plate that is excellent in all of strain dispersion ability, ductility, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state. The steel plate has a specified composition and microstructure, and has an average value [τq] of the stress at which plastic deformation begins, τq, measured by nanoindentation at a position 1 / 4 of the plate thickness of the steel plate, of 1.70 GPa to 3.30 GPa, a standard deviation σq of said τq of 0.42 GPa or less, and further, when the average value [τs] of the stress at which plastic deformation begins, τs, measured by nanoindentation at a position 10 μm from the surface of the steel plate is taken as 0.85 × [τs], the proportion of measurement points that are less than 30.0% is 1.

Description

[Technical field]

[0001] The present invention relates to a steel sheet, in particular to a high-strength steel sheet excellent in all of strain dispersion ability, ductility, bendability, and delayed fracture resistance at a bent portion having a sheared end surface in a painted state, and to a method for producing the same. The steel sheet of the present invention can be suitably used as a structural member for automobile parts and the like. [Background technology]

[0002] CO reduction due to vehicle weight reduction 2 In order to achieve both reduced emissions and improved crashworthiness, progress is being made in increasing the strength of thin steel sheets for automobiles, and new legal regulations are being introduced one after another. As a result, the use of high-strength steel plates with a tensile strength (TS) of 1,180 MPa or higher is increasing in the main structural components that form the framework of automobile cabins.

[0003] Stretch forming and bending are used for high-strength steel sheets used in automotive reinforcing parts and frame structural parts. Stretch forming requires high strain dispersion, so it is required to have a low yield ratio (YR = yield strength YS / tensile strength TS x 100) and good ductility. On the other hand, bending forming requires good bendability.

[0004] Here, in the case of high-strength steel plates with a tensile strength of 1180 MPa or more, a delayed fracture phenomenon occurs in which the component suddenly breaks due to hydrogen that penetrates into the steel in the atmospheric corrosive environment in which the automobile is traveling.

[0005] That is, in the case of steel sheets for automobiles, stress is applied during press working and component assembly, and there is a risk that hydrogen will subsequently enter the steel sheet from the environment, so there is a demand for improving the delayed fracture resistance of high-strength steel sheets.

[0006] Furthermore, in automobile parts, there is a high risk of delayed fracture occurring at bent portions and sheared end surfaces, so there is a demand for improved delayed fracture resistance in bent portions having sheared end surfaces. In particular, automotive parts are exposed to atmospheric corrosive environments in a painted state. Unlike an unpainted state, a painted state inevitably produces paint defects. In an environment where dry and wet conditions occur repeatedly, corrosion may progress locally on the surface layer of the steel sheet starting from the paint defects, and hydrogen may penetrate through the corroded areas, leading to delayed fracture.

[0007] Therefore, there is a demand for improved delayed fracture resistance in bent sections having sheared ends in a painted state in an environment where dry and wet cycles occur (hereinafter, also referred to as "delayed fracture resistance in a painted state under atmospheric corrosion"). Thus, in order to increase the ratio of high strength steel sheets used in automobile parts, it is required to comprehensively satisfy the various characteristics described above.

[0008] In response to the above demands, for example, Patent Document 1 discloses an ultra-high strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and a tensile strength of 1300 MPa or more, and a method for producing the same. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2018 / 011978 Summary of the Invention [Problem to be solved by the invention]

[0010] The high-strength steel sheet described in Patent Document 1 has excellent delayed fracture resistance at a punched end surface in a hydrochloric acid aqueous solution of pH 1. However, no consideration is given to delayed fracture resistance under atmospheric corrosion and in a painted state.

[0011] Therefore, in order to widely apply high-strength steel sheets to automotive parts, it is necessary to improve resistance to delayed fracture under atmospheric corrosion and in a painted state, in addition to excellent strain dispersion ability, ductility, and bendability.

[0012] The present invention has been developed in view of the above circumstances, and aims to provide a high-strength steel plate excellent in all of strain dispersion ability, ductility, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state, and to provide an advantageous method for producing the high-strength steel plate. Another aim is to provide a member made of the high-strength steel plate, and an automobile frame structural part or automobile reinforcing part made of the member.

[0013] In the present invention, the term "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 strain dispersion ability" means that the yield ratio (YR) determined by the tensile test described below is 80% or less. "Excellent ductility" means that the total elongation (El) determined by the tensile test described below is 7.0% or more. The term "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 test piece having a sheared end face, as described below, is bent, stress-loaded, and then a chemical electrodeposition coating is applied to the delayed fracture test piece, and then a corrosion cycle test in which dry and wet cycles are repeated is performed, and no cracks are observed after 30 days. [Means for solving the problem]

[0014] As a result of extensive investigations, the present inventors have found that the above object can be achieved by employing the configuration described below, and have completed the present invention. That is, the gist of the present invention is as follows. 1. A 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 martensite: 70% or more, Total area ratio of ferrite and bainite: 15% or less, And the area ratio of retained austenite is 15% or less, Furthermore, the standard deviation of the carbon concentration σ C but, 0.10[%C]≦σ C ≦0.28[%C] Where, [%C] is the carbon content in the steel (% by mass) Satisfied, The average value [τq] of the plastic deformation start stress τq measured by the nanoindentation method at the 1 / 4 position of the plate thickness is 1.70 GPa or more and 3.30 GPa or less, and the standard deviation σq of the plastic deformation start stress τq is 0.42 GPa or less; At a position 10 μm from the surface of the steel plate, The microstructure is Area ratio of martensite: 40% or less, Perlite area ratio: 15% or less, The sum of the area ratios of ferrite and bainite is 60% or more, When the average value of the plastic deformation initiation stress τs measured by the nanoindentation method at the 10 μm position is [τs], the percentage of measurement points that are less than 0.85 × [τs] is 30.0% or less. steel plate.

[0015] 2. The steel sheet according to 1 above, wherein the chemical composition further contains, by mass%, at least one element selected from the group consisting of 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: 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.

[0016] 3. The steel sheet according to 1 or 2 above, which has a plating layer on the surface of the steel sheet.

[0017] 4. A member made of the steel plate according to any one of 1 to 3 above.

[0018] 5. An automobile frame structural part or an automobile reinforcing part made of the member described in 4 above.

[0019] 6. A steel slab having the composition described in 1 or 2 above is The number of passes in the temperature range of 1000°C or higher is 4 passes or more, the rolling reduction rate in each pass is 15% or more, and the average strain rate is 9×10 -4 / s or more 1×10 -2 A method for producing a steel sheet, comprising the steps of: rough rolling under conditions in the range of 0.15 to 1.5 mm / s; finish rolling; and coiling to obtain a hot-rolled sheet; pickling and cold rolling to obtain a cold-rolled sheet by pickling and cold rolling the hot-rolled sheet; annealing the cold-rolled sheet; and cooling the cold-rolled sheet to 50° C. or less. The annealing is performed under conditions in which the heating temperature is 800° C. or higher, the dew point is −25° C. or higher in a heating temperature region T1 of 800° C. or higher, and the following formula 1 is satisfied: 2.0≦K≦60.0...Equation 1 In addition,

number

[0020] 7. The method for producing a steel sheet as described in 6 above, further comprising carrying out a plating treatment during the cooling process.

[0021] 8. A method for producing a component, comprising the step of subjecting the steel plate according to any one of 1 to 3 above to at least one of forming and joining to form a component. Effect of the Invention

[0022] According to the present invention, it is possible to provide a high-strength steel sheet which is excellent in all of strain dispersion ability, ductility, 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. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Hereinafter, the high strength steel plate and the manufacturing method thereof according to the present invention will be described in detail with respect to its component composition, microstructure, and manufacturing method. First, the appropriate range of the composition and the reason for limiting it will be explained. In the following explanation, "%" representing the content of the component elements of the steel means "mass %" unless otherwise specified.

[0024] [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 martensite at the 1 / 4 position of the plate thickness and the amount of retained austenite. If the C content is too low, the area ratio of 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% or more. The C content is preferably 0.115% or more, and more preferably 0.140% or more. On the other hand, if the C content is too high, the strength of the martensite at the 1 / 4 plate thickness position increases significantly, promoting crack propagation during delayed fracture testing and reducing delayed fracture resistance under atmospheric corrosion and in a painted state. For this reason, the C content is set to 0.390% or less. The C content is preferably 0.375% or less, and more preferably 0.360% or less.

[0025] [Si: 0.01% or more and 2.00% or less] Silicon increases the strength of steel sheet by suppressing the precipitation of cementite in martensite and by solid solution strengthening. To obtain this effect, the silicon content is set to 0.01% or more. The silicon content is preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, if the Si content is too high, carbide precipitation during bainite transformation and martensite transformation is significantly suppressed, the amount of retained austenite at the 1 / 4 plate thickness position increases excessively, and the hardness of martensite generated from the retained austenite during shearing increases significantly. As a result, the delayed fracture resistance under atmospheric corrosion and in a painted state decreases. For this reason, the Si content is set to 2.00% or less. The Si content is preferably 1.75% or less, and more preferably 1.50% or less.

[0026] [Mn:2.00% or more and 4.00% or less] Mn is one of the important basic components of steel, and particularly in the present invention, it affects the area ratio of martensite and the delayed fracture resistance. If the Mn content is too low, the area ratio of martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the Mn content is set to 2.00% or more. The Mn content is preferably 2.20% or more, and more preferably 2.40% or more. On the other hand, if the Mn content is too high, the austenite is stabilized, the amount of retained austenite at the 1 / 4 plate thickness position increases excessively, and the hardness of the martensite generated from the retained austenite during shearing increases significantly. As a result, the delayed fracture resistance in atmospheric corrosion and in a painted state decreases. For this reason, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less, more preferably 3.50% or less, and even more preferably 3.30% or less.

[0027] [P:0.100% or less] P segregates at prior austenite grain boundaries, embrittling the grain boundaries and reducing the ultimate deformability of the steel sheet, resulting in reduced bendability. Therefore, the P content must be 0.100% or less. The P content is preferably 0.070% or less. There is no particular lower limit for the P content, but since P is a solid solution strengthening element and increases the strength of the steel sheet, the P content is preferably 0.001% or more.

[0028] [S:0.0200% or less] S exists as sulfides and reduces the ultimate deformability of the steel sheet, which reduces its bendability. Therefore, the S content must be 0.0200% or less. The S content is preferably 0.0050% or less. There is no particular lower limit for the S content, but due to production technology constraints, the S content is preferably 0.0001% or more.

[0029] [Al: 1.000% or less] Al provides sufficient deoxidation and reduces inclusions in steel. If the Al content is too high, a large amount of ferrite is generated, which makes delayed fracture cracks more likely to grow at the interface between ferrite and martensite, and reduces delayed fracture resistance under atmospheric corrosion and in a painted state. For this reason, the Al content is set to 1.000% or less. The Al content is 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.

[0030] [N:0.0100% or less] N exists as a nitride and reduces the ultimate deformability of the steel sheet, which reduces the bendability. Therefore, the N content is set to 0.0100% or less. Preferably, the N content is set to 0.0050% or less. There is no particular lower limit for the N content, but due to production technology constraints, the N content is preferably set to 0.0001% or more.

[0031] [O:0.0100% or less] 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. There is no particular lower limit for the O content, but due to production technology constraints, the O content is preferably set to 0.0001% or more.

[0032] The high-strength steel plate according to the present invention has a composition containing the above-mentioned components, with the balance being Fe and unavoidable impurities. The unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. The total content of these impurities is permitted to be 0.100% or less.

[0033] In addition to the above-mentioned chemical composition, the steel sheet according to the present invention may further contain at least one element selected from the group consisting of 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: 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, either alone or in combination.

[0034] If 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 the bendability is not reduced. Therefore, when Ti, Nb and V are contained, the contents of Ti, Nb and V 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 contents of Ti, Nb and V. 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 contents of Ti, Nb and V are preferably 0.001% or more.

[0035] If the contents of Ta and W 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 the bendability is not reduced. Therefore, the contents of Ta and W are preferably 0.10% or less, and more preferably 0.08% or less. On the other hand, the lower limits of the contents of Ta and W are not particularly specified. 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 contents of Ta and W are preferably 0.01% or more, respectively.

[0036] If the B content is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not decrease, so that bendability will not decrease. Therefore, the B content is preferably 0.0100% or less, and more preferably 0.0080% or less. On the other hand, there is no particular lower limit for the B content. Note that, since B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, the B content is preferably 0.0003% or more.

[0037] If the content of Cr, Mo, and Ni is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not decrease, so that the bendability will not decrease. Therefore, the content of Cr, Mo, and Ni 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 content of Cr, Mo, and Ni. Note that, since Cr, Mo, and Ni are elements that improve hardenability, the content of Cr, Mo, and Ni is preferably 0.01% or more.

[0038] If 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 that the 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.

[0039] If Cu is 1.00% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so that bendability does not decrease. Therefore, 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.

[0040] If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not decrease, so that 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.

[0041] If Sb is 0.200% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so that bendability does not decrease. Therefore, 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. Note that, since Sb is an element that controls the surface softening thickness and enables strength adjustment, the Sb content is preferably 0.001% or more.

[0042] If the content of Ca, Mg and REM is 0.0100% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so that the bendability does not decrease. Therefore, the content of Ca, Mg and REM is preferably 0.0100% or less, and more preferably 0.0050% or less. On the other hand, the lower limit of the content of Ca, Mg and REM is not particularly specified. Note that, since Ca, Mg and REM are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, the content of Ca, Mg and REM is preferably 0.0005% or more.

[0043] If the content of Zr and Te is 0.100% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so that the bendability does not decrease. Therefore, the content of Zr and Te is preferably 0.100% or less, and more preferably 0.080% or less. On the other hand, the lower limit of the content of Zr and Te is not particularly specified. Note that Zr and Te are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, so that the content of Zr and Te is preferably 0.001% or more.

[0044] If the Hf content is 0.10% or less, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not decrease, so that the bendability will not decrease. Therefore, the Hf content is preferably 0.10% or less, and more preferably 0.08% or less. On the other hand, there is no particular lower limit for the Hf content. Note that, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, the Hf content is preferably 0.01% or more.

[0045] If the Bi content is 0.200% or less, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not decrease, so that the bendability will not decrease. Therefore, 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.

[0046] In addition, when the contents of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi are less than the above-mentioned preferable lower limit, the effect of the present invention is not impaired. Therefore, in such a case, they are treated as unavoidable impurities.

[0047] Next, 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 martensite: 70% or more] In the microstructure, by containing a certain amount or more of martensite, a TS of 1180 MPa or more can be realized. Therefore, the area ratio of martensite is set to 70% or more. The area ratio of martensite is preferably 80% or more, and more preferably 90% or more. On the other hand, there is no particular upper limit to the area ratio of martensite, and the above-mentioned effects can be obtained even if the area ratio of martensite is 100%. Martensite is a transformation phase that forms below the Ms point, regardless of whether it is tempered or not. Martensite also includes lower bainite that forms below the Ms point. The observation position for such martensite is at 1 / 4 of the plate thickness of the steel plate, as described later.

[0048] [Total area ratio of ferrite and bainite: 15% or less] If the microstructure contains too much ferrite and bainite, the strength of the steel sheet decreases. In addition, the difference in hardness between the structures increases, which promotes the propagation of delayed fracture cracks and reduces the delayed fracture resistance under atmospheric corrosion and in a painted state. For this reason, 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, and more preferably 10% or less. The effects 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 1 / 4 of the thickness of the steel plate.

[0049] [Area ratio of retained austenite: 15% or less] If there is too much retained austenite, a large amount of hard martensite is generated from the retained austenite during shearing. As a result, the difference in hardness between the structures becomes large, the propagation of delayed fracture cracks is promoted, and the delayed fracture resistance under atmospheric corrosion and in a painted state is reduced. For this reason, the area ratio of the retained austenite is set to 15% or less. The area ratio of the retained austenite is preferably 10% or less. On the other hand, the lower limit is not particularly limited, and the effects of the present invention can be obtained even if the area ratio of retained austenite is 0%.

[0050] In the present invention, 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 diffractometer using a Co Kα 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. Furthermore, 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. In the present invention, the average value of the nine intensity ratios is regarded as the volume fraction of retained austenite. In the present invention, the volume fraction of retained austenite is regarded as the area fraction of retained austenite.

[0051] [Standard deviation of carbon concentration in steel sheet structure σ C is 0.10[%C]≦σ C ≦0.28[%C] where [%C] is the carbon content in the steel (% by mass). Standard deviation σ of carbon concentration in the steel plate structure at 1 / 4 of the plate thickness C is 0.10[%C]≦σ CBy satisfying the relationship of ≦0.28 [%C], high strain dispersion and high ductility are obtained. Since the elongation and strength of martensite depend on the amount of carbon contained in the martensite, if the standard deviation of the carbon concentration in the steel plate is set to an appropriate value, the structure will be a mixture of martensite that is responsible for elongation and martensite that is responsible for strength. As a result, it is presumed that the work hardening ability increases, resulting in a low yield ratio and high elongation.

[0052] σ C If C is less than 0.10%, the steel plate structure at the 1 / 4 position of the plate thickness will not be a mixture of martensite, which is responsible for elongation, and martensite, which is responsible for strength. C If the carbon concentration is too low, i.e., if the carbon concentration is too uniform, the work hardening ability decreases, the yield ratio increases, and the elongation decreases.

[0053] On the other hand, σ C If the C content exceeds 0.28%, the steel sheet structure is a mixture of martensite, which has high elongation but extremely low strength, and martensite, which has extremely high strength. Therefore, delayed fracture cracks tend to propagate inside the martensite, which has extremely high strength, and the delayed fracture resistance under atmospheric corrosion and in a painted state is reduced. Therefore, the standard deviation of the carbon concentration in the steel plate structure at the 1 / 4 position of the plate thickness σ C is 0.10[%C]≦σ C It is necessary to satisfy ≦0.28[%C]. Preferably, σ C is 0.12[%C] or more. On the other hand, σ C is 0.26[%C] or less. More preferably, σ C is 0.14[%C] or more. On the other hand, more preferably, σ C is less than 0.24[%C].

[0054] In the present invention, the standard deviation σ Cis calculated as follows. First, a sample is cut out so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface, and the observation surface is polished with diamond paste, and then finish polished with alumina. Next, the polished observation surface is measured in three fields of view using an electron probe micro analyzer (EPMA) at an acceleration voltage of 7 kV and a measurement area of ​​22.5 μm × 22.5 μm. The data after this measurement is converted into carbon concentration using the calibration curve method. The obtained carbon concentration data for the three fields of view is summed to create a histogram, and the standard deviation of the carbon concentration σ C Request.

[0055] [Remaining tissue] 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 martensite, ferrite, bainite and retained austenite. However, the area ratio of the remaining structure is preferably 5% or less in terms of area ratio, so that the effect of the present invention is not impaired. Examples of the remaining structure include pearlite, alloy carbonitride precipitated in ferrite, and other structures known as structures of steel sheets.

[0056] [The average value of the plastic deformation initial stress τq [τq] is 1.70 GPa or more and 3.30 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. Here, 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.

[0057] Here, 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. As a result, it was found that the delayed fracture resistance properties under atmospheric corrosion and in a painted state can be improved by setting the average plastic deformation initiation stress τq [τq] at the 1 / 4 position of the steel plate thickness to be between 1.70 GPa and 3.30 GPa. This is because, by setting the average value [τq] of the plastic deformation initiation stress τq 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. As a result, it is presumed that the delayed fracture resistance properties in a painted state under atmospheric corrosion are improved.

[0058] That is, when the average value [τq] of the plastic deformation initiation stress τq is less than 1.70 GPa, the generation and emission of dislocations at the crack tip becomes significant, promoting the generation of holes and voids, and as a result, the delayed fracture crack propagates. In particular, in steel plates having high strain dispersion ability, the delayed fracture resistance in atmospheric corrosion and in a painted state decreases. Therefore, the average value [τq] of the plastic deformation initiation stress τq is set to 1.70 GPa or more. Preferably, the average value [τq] of the plastic deformation initiation stress τq is 1.85 GPa or more. More preferably, the average value [τq] of the plastic deformation initiation stress τq is 2.00 GPa or more.

[0059] On the other hand, when the average value of the plastic deformation initiation stress τq [τq] exceeds 3.30 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, especially in the case of steel plates with high strain dispersion ability. Therefore, the average value [τq] of the stress at which plastic deformation begins must be 3.30 GPa or less. Preferably, the average value [τq] of the stress at which plastic deformation begins is 3.20 GPa or less. More preferably, the average value [τq] of the stress at which plastic deformation begins is 3.10 GPa or less.

[0060] [Standard deviation of plastic deformation starting stress σq is 0.42 GPa or less] The standard deviation σq 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. By setting the standard deviation σq of the plastic deformation initiation stress to 0.42 GPa or less, the delayed fracture resistance properties are improved under atmospheric corrosion and in a painted state, particularly in steel plates having high strain dispersion ability. This is because the standard deviation σq is set to 0.42 GPa or less, and the microscopic variation in the plastic deformation initiation stress is suppressed. In other words, 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. As a result, it is estimated that the delayed fracture resistance properties are improved under atmospheric corrosion and in a painted state.

[0061] If the standard deviation σq of the stress at which plastic deformation begins exceeds 0.42 GPa, the microscopic variation in the stress at which plastic deformation begins is large, and delayed fracture cracks selectively propagate through weak parts in the structure. As a result, the delayed fracture resistance in a painted state under atmospheric corrosion is reduced, particularly in steel plates with high strain dispersion ability. Therefore, the standard deviation σq of the stress at which plastic deformation begins in the present invention must be 0.42 GPa or less. Preferably, the standard deviation σq of the stress at which plastic deformation begins is 0.36 GPa or less. More preferably, the standard deviation σq of the stress at which plastic deformation begins is 0.30 GPa or less. On the other hand, the smaller the standard deviation σq of the plastic deformation initiation stress, the better, and it may be 0 GPa.

[0062] Next, the microstructure at a position 10 μm from the surface of the steel sheet will be described. [Area ratio of martensite: 40% or less] If there is too much martensite in the microstructure 10 μm from the surface of the steel sheet, the number of bending crack initiation sites increases, and bendability deteriorates. In addition, in the martensite present in the surface layer of the steel sheet, hydrogen penetration due to corrosion occurring in the surface layer of the steel sheet directly below the paint is promoted, and delayed fracture cracks are more likely to occur, so that the delayed fracture resistance under atmospheric corrosion and in a painted state is reduced. Therefore, in the present invention, the area ratio of martensite in the microstructure at a position 10 μm from the steel sheet surface is set to 40% or less. The area ratio of martensite is preferably 35% or less, and more preferably 30% or less. However, the effects of the present invention can be obtained even if the area ratio of martensite is 0%.

[0063] Such martensite is a transformation phase that forms below the Ms point, and does not matter whether it is tempered or not. Martensite also includes lower bainite that forms below the Ms point. The martensite was observed at a position 10 μm from the steel sheet surface, as described below.

[0064] [Perlite area ratio: 15% or less] If there is too much pearlite in the microstructure 10 μm from the steel sheet surface, the number of bending crack initiation sites increases, and bendability deteriorates. In addition, stress during bending is concentrated in the pearlite present in the surface layer, making delayed fracture cracks more likely to occur, and therefore the delayed fracture resistance in atmospheric corrosion and painted condition is reduced. Therefore, in the present invention, the area ratio of pearlite in the microstructure at a position 10 μm 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%. As will be described later, the observation position of pearlite was 10 μm from the steel sheet surface.

[0065] [Total area ratio of ferrite and bainite: 60% or more] By making the total area ratio of ferrite and bainite at a position 10 μm from the steel sheet surface 60% or more, the bendability and 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. In addition, ferrite and bainite have fewer lattice defects and fewer hydrogen trapping sites than martensite, which suppresses hydrogen penetration due to corrosion that occurs in the surface layer of the steel sheet 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.

[0066] Therefore, it is essential that the total area ratio of ferrite and bainite is 60% or more, preferably 65% ​​or more, and more preferably 70% or more. On the other hand, there is no particular upper limit, and the above-mentioned effects can be obtained even if the total area ratio of ferrite and bainite is 100%.

[0067] 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. Moreover, the observation position for the ferrite and bainite was 10 μm from the steel sheet surface.

[0068] [Remaining tissue] The steel structure at a position 10 μm from the steel sheet surface may have a structure (remaining structure) other than the above-mentioned martensite, pearlite, ferrite and bainite. However, the area ratio of the remaining structure is preferably 5% or less in terms of area ratio so as not to impair the effects of the present invention. Examples of the remaining structure include pearlite, alloy carbonitrides precipitated in ferrite, and other structures known as the structure of steel sheets.

[0069] The method for measuring the area ratios of martensite, pearlite, ferrite and bainite at the 1 / 4 position of the sheet thickness of the steel sheet or at a position 10 μm 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% by volume of nital to reveal the structure. Next, a scanning electron microscope (SEM) with an accelerating 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 μm from the surface of the steel sheet, and a magnification of 3000 times is used to obtain SEM images of the three fields of view.

[0070] From the SEM images thus obtained, the area ratio of each structure is calculated using Adobe Photoshop (manufactured by Adobe Systems). 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 value of these is regarded as the area ratio of each structure.

[0071] In the SEM image, martensite is a structural region with a hierarchical structure with fine internal irregularities, pearlite is a layered structural region consisting of gray ferrite and white cementite, ferrite is a gray, flat structural region that does not contain carbides, and bainite is a gray structural region that contains fine carbides. Thus, martensite, pearlite, ferrite and bainite can be distinguished from one another.

[0072] [When the average value of the plastic deformation start stress τs measured by the nanoindentation method is [τs], the percentage of measurement points that are less than 0.85 × [τs] is 30.0% or less] The proportion of measurement points, which is defined by the average value of the plastic deformation initiation stress τs at a position 10 μm 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 [τs], by making the proportion of measurement points less than 0.85 × [τs] 30.0% or less, the delayed fracture resistance under atmospheric corrosion and in a painted state is improved, especially in steel sheets with high strain dispersion ability. This is because the structure region where the stress is less than 0.85 × [τs] 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.

[0073] As described above, by setting the ratio of measurement points with a value less than 0.85 × [τs] to 30.0% or less, the number of locations where dislocations are generated or emitted locally on the surface of the bent portion is reduced, and the starting points of delayed fracture are reduced. As a result, the delayed fracture resistance in a painted state under atmospheric corrosion is improved. Therefore, in the present invention, the ratio of measurement points that are less than 0.85 × [τs] must be 30.0% or less. Preferably, the ratio of measurement points that are less than 0.85 × [τs] is 20.0% or less. More preferably, the ratio of measurement points that are less than 0.85 × [τs] is 15.0% or less. On the other hand, there is no particular lower limit, and the above-mentioned effects can be obtained even if the ratio of measurement points that are less than 0.85 × [τs] is 0%.

[0074] In the present invention, the position 10 μm from the surface of the steel sheet means a position 10 μm deep in the sheet thickness direction from the steel sheet surface (plane perpendicular to the sheet thickness direction). In addition, both the 1 / 4 sheet thickness position and the position 10 μm from the steel sheet surface must satisfy the above-mentioned requirement at least on one side of the steel sheet.

[0075] A method for measuring the plastic deformation initiation stress according to the nanoindentation method will be described below. To prepare the measurement sample, the sample is cut out 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 the measurement surface is mirror-polished using diamond paste, and then finish-polished 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 of the plate thickness or 10 μm from the plate surface, and the nanoindentation test is performed under load control with a loading and unloading speed of 50 μN / s, a maximum load of 500 μN, and a data collection time interval of 5 ms to obtain the load P (N) and the displacement h (nm) at that load. Nanoindentation tests are performed at 40 points at each measurement position. Measurements are performed with a distance of 2 μm or more between indentations. Using the obtained load-displacement curve, the Hertzian contact displacement hc (nm) is calculated at each load P (N) using the Hertzian contact equation shown below as Equation 2.

[0076]

number

[0077] In the above formula 2, h C (nm) is the displacement assuming elastic deformation obtained by the Hertz contact equation, P(N) is the load, E r (Pa) is the composite Young's modulus, and R (m) is the radius of curvature of the indenter tip. r is the average value (40 points) of the composite Young's modulus obtained from the unloading curve in each measurement. The above R changes depending on the wear state of the Berkovich indenter, so it is determined by fitting the load-displacement curve in the elastic region using a standard sample such as fused silica.

[0078] h calculated from Equation 2 for each load P (N) C Using the displacement h (nm) measured in the nanoindentation test, the minimum load P that satisfies the condition of the following equation 3 is calculated. min (N) is calculated and Pmin(N) is defined as the load at which plastic deformation begins.

[0079]

number

[0080] Furthermore, the plastic deformation starting stress τ (GPa) is calculated from the plastic deformation starting load using the following formula 4.

[0081]

number

[0082] Using the above formulas 2, 3, and 4, the plastic deformation starting stress τ is calculated for 40 points, and the average value is calculated. The average value of the stress at which plastic deformation begins at the 1 / 4 position in the plate thickness direction is [τq], and the average value of the stress at which plastic deformation begins at the 10 μm position from the plate surface is [τs]. 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 plate thickness direction, and the standard deviation σq is calculated. Furthermore, the percentage of measurement points where the value of the stress at which plastic deformation begins at 40 points in the 10 μm position from the plate surface is less than 0.85 × [τs] is calculated.

[0083] The plate thickness of the high strength steel plate of the present invention is not particularly limited, and is usually preferably 0.3 mm or more and 2.8 mm or less.

[0084] [Plating layer] 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.

[0085] For example, when the coating 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 to 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% or more and 3.5 mass% or less, with the balance being Zn and unavoidable impurities.

[0086] In addition, when the coating layer is a hot-dip galvanized layer, the coating weight of the coating layer on one side is 20 g / m 2 On the other hand, the coating weight of the plating layer on one side is preferably 80 g / m or more. 2 Furthermore, a galvannealed layer obtained by alloying a galvanized layer having such a coating amount can also be used.

[0087] In addition, when the plating layer is a hot-dip galvanized layer, the Fe content in the plating layer is preferably less than 7 mass%. When the plating layer is a galvannealed hot-dip galvanized layer, the Fe content in the plating layer is preferably 7 mass% or more. On the other hand, the Fe content in the plating layer is preferably 20 mass% or less, more preferably 15 mass% or less.

[0088] [Part] Hereinafter, a member according to one embodiment of the present invention will be described. 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. Moreover, 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 strain dispersion ability, ductility, 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.

[0089] [parts] Hereinafter, a part according to one embodiment of the present invention will be described. 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. Here, the member of the present invention described above is excellent in all of strain dispersion ability, ductility, 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 preferably used in particular as an automobile frame structural part or an automobile reinforcement part in general.

[0090] [Steel plate manufacturing method] Next, a method for producing a steel sheet according to the present invention will be described. First, 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.

[0091] Next, the steel slab is hot-rolled to obtain a hot-rolled sheet. In one example, the steel slab is once cooled to room temperature, and then heated again and hot-rolled (rough rolling and finish rolling). The produced steel slab may be charged into a heating furnace as a hot piece without being cooled to room temperature, or may be briefly kept at room temperature and then immediately rough-rolled.

[0092] <Rough rolling> 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. On the other hand, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is based on the surface temperature of the steel slab. Next, the steel slab heated to the slab heating temperature is subjected to rough rolling under the following conditions.

[0093] [Number of passes in the temperature range above 1000℃ is 4 or more] By making the number of passes in the temperature range of 1000°C or higher 4 passes or more, the ratio of measurement points with a value less than 0.85 × [τs] can be reduced. By increasing the number of passes in the temperature range of 1000°C or higher, the number of times strain is introduced near the surface increases, promoting the diffusion of substitutional solid solution elements such as Si and Mn and making them uniform. If the number of passes in the temperature range of 1000°C or higher is less than 4 passes, areas with low Si and Mn will appear on the steel sheet surface, the frictional force within the crystal will decrease, and the number of measurement points where the starting stress for plastic deformation is significantly reduced will increase. Therefore, it is necessary to make the number of passes in the temperature range of 1000°C or higher 4 passes or more.

[0094] [Reduction rate for each pass is 15% or more] By setting the reduction rate in each pass to 15% or more, the standard deviation σq of the stress at which plastic deformation begins can be reduced. By setting the reduction rate in each pass to 15% or more, dynamic recrystallization of γ grains is promoted inside the steel sheet, and the grain size and element concentration distribution become uniform. If the reduction rate in each pass is less than 15%, dynamic recrystallization of γ grains is not promoted in some places, the grain size and element concentration distribution become non-uniform, the variation in the stress at which plastic deformation begins in the final structure increases, and the standard deviation σq of the stress at which plastic deformation begins increases. Therefore, it is necessary to set the reduction rate in each pass to 15% or more.

[0095] [The average strain rate is 9×10 -4 / s or more 1×10 -2 / s or less] The average strain rate during rough rolling is 9×10 -4 / s or more 1×10 -2By setting the rolling speed in the range of 0.1 / s or less, the standard deviation σq of the stress at which plastic deformation begins can be reduced. The average strain rate during rough rolling is calculated by multiplying the rolling rate ε(-) from the first mill to the last mill in rough rolling by the time t required from the start of rolling at the first mill to the completion of rolling at the last mill. R (ε / t R ) is defined as The average strain rate during rough rolling is 1×10 -2 When the stress gradient exceeds 1 / s, the diffusion of solute atoms such as Si and Mn during the plastic deformation and dynamic recrystallization of austenite grains becomes insufficient, resulting in the appearance of regions with low and high concentrations of Si and Mn inside the steel sheet.Then, differences in the frictional forces within the crystals occur between these regions, resulting in a large variation in the stress at which plastic deformation begins, and an increase in the standard deviation σq of the stress at which plastic deformation begins.

[0096] On the other hand, the average strain rate during rough rolling is 9 × 10 -4 When the strain rate is less than / s, the dynamic recovery of dislocations in austenite grains is promoted. As a result, the dislocation density is reduced, and the dislocation pipe diffusion of solute atoms such as Si and Mn is suppressed. The diffusion of the solute atoms becomes insufficient, and regions where Si and Mn are scarce and regions where they are enriched appear inside the steel sheet. Then, a difference occurs in the frictional force within the crystal between the two regions, so the variation in the plastic deformation threshold stress increases, and the standard deviation σq of the plastic deformation threshold stress increases. Therefore, the average strain rate is 9×10 -4 / s or more 1×10 -2 The average strain rate is preferably in the range of 1×10 -3 On the other hand, the average strain rate is preferably 9×10 -3 / s or less.

[0097] <Finish rolling> Next, 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. When the slab heating temperature is set low, it is preferable to heat the rough rolled sheet using a bar heater or the like before the finish rolling in order to prevent troubles during hot rolling. The temperature when performing the finish rolling (finish rolling temperature) is preferably 700°C or higher. This reduces the rolling load. Furthermore, the rolling reduction in the non-recrystallized state of austenite is reduced, suppressing the development of abnormal structures elongated in the rolling direction, and 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.

[0098] In order to reduce the rolling load, a part or all of the finish rolling may be performed as lubricating rolling. Lubricating rolling is also preferable from the viewpoint of making the steel sheet shape and material uniform. The friction coefficient during lubricating 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.

[0099] Next, the hot-rolled sheet obtained by hot rolling is appropriately pickled. By pickling, oxides on the surface of the hot-rolled sheet are removed, and the final product, a high-strength steel sheet, has excellent chemical conversion treatability and coating layer quality. Pickling may be performed once or multiple times.

[0100] 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 subjected to annealing as described below, and cooled to 50° C. or lower.

[0101] <Annealing> [Heating temperature is 800℃ 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, resulting in a decrease in the area ratio of martensite at the 1 / 4 position of the steel sheet. 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.

[0102] [Dew point is -25℃ 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 μm from the steel sheet surface becomes too low. In addition, if the dew point is too low, the decarburization distribution at a position 10 μm from the steel sheet surface becomes uneven, and a portion with a low plastic deformation initiation stress appears, and the proportion of measurement points at a position 10 μm from the steel sheet surface that are less than 0.85 × [τs] 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.

[0103] [Annealing is performed under conditions that satisfy formula 1] Equation 1: 2.0≦K≦60.0 In addition, K (mm 2 ) is defined by the following formula:

number

[0104] In a steel sheet that has reached 800°C during the annealing process, reverse transformation to austenite is completed at the 1 / 4 position of the steel sheet, and diffusion reduces microsegregation and causes grain growth. In addition, decarburization occurs on the steel sheet surface, with C being released from the steel sheet surface. These phenomena are defined as parameter K, which 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. By appropriately controlling the value of the parameter K as described above, it is possible to control, within predetermined ranges, the standard deviation σq of the plastic deformation initiation stress at the 1 / 4 position on the steel sheet, the sum of the area fractions of ferrite and bainite at a position 10 μm from the steel sheet surface, and the proportion of measurement points where the sum is less than 0.85 × [τs].

[0105] That is, the K (mm 2 ) is too small, the diffusion of solute atoms in austenite at the 1 / 4 position on the steel plate is insufficient, microsegregation remains, and the standard deviation σq of the plastic deformation initiation stress at the 1 / 4 position on the steel plate becomes too high. Also, decarburization at a position 10 μm from the steel plate surface becomes insufficient, and the sum of the area ratios of ferrite and bainite at a position 10 μm from the steel plate surface becomes too low. Furthermore, the diffusion of solute atoms in austenite at the steel plate surface becomes insufficient, microsegregation remains, and the proportion of measurement points at a position 10 μm from the steel plate surface that are less than 0.85 × [τs] becomes too high. Therefore, K(mm 2 ) must be 2.0 or more. K(mm 2 ) is preferably 3.0 or more, and more preferably 4.0 or more.

[0106] On the other hand, K(mm 2 ) is too large, the grain growth in some of the austenite at the 1 / 4 position of the steel plate becomes excessive, and coarse prior γ grains are generated. When the prior γ grains become larger, the microscopic Ms point of each prior γ grain becomes higher, and there are places where martensitic transformation occurs locally at a higher temperature. This leads to a non-uniform structure, and the standard deviation σq of the starting stress for plastic deformation becomes too high. Therefore, 2 ) shall be 60.0 or less. K(mm 2) is preferably 45.0 or less, 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.

[0107] [The average cooling rate v2 in the temperature range T2 of 600°C to 750°C is 1.0°C / s to 15.0°C / s] The temperature range T2 of 600°C or more and 750°C or less is a temperature range in which ferrite transformation occurs at the 1 / 4 position of the steel sheet and at a position 10 μm from the steel sheet surface. If the average cooling rate v2 in this temperature range T2 is too low, ferrite transformation occurs excessively 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.

[0108] On the other hand, if the average cooling rate v2 is too high, ferrite transformation is difficult to occur at a position 10 μm from the steel sheet surface, and the area ratio of ferrite at a position 10 μm from the steel sheet surface decreases. Therefore, 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.

[0109] [Average cooling rate in the temperature range of 500℃ to 600℃ exceeds v2] 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 μm from the steel sheet surface. In other words, 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 μm 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 it is about 1000°C / s or less due to equipment considerations.

[0110] [Dwell time in the temperature range T3 of 400°C or more and less than 500°C is 10s or more and 150s or less] The temperature region T3 of 400°C or more and less than 500°C is a temperature region in which bainite transformation occurs at the 1 / 4 position of the steel sheet and at a position 10 μm from the steel sheet surface. If the residence time in this temperature region T3 is too short, bainite transformation becomes difficult to occur at a position 10 μm from the steel sheet surface, and the area ratio of bainite at a position 10 μm from the steel sheet surface decreases. Therefore, the residence time in the temperature region T3 is set to 10 s or more. The residence time in this temperature region T3 is preferably set to 15 s or more.

[0111] On the other hand, if the residence time in the temperature region T3 is too long, excessive bainite transformation occurs at the 1 / 4 position of the steel sheet, and the area ratio of bainite at the 1 / 4 position of the steel sheet becomes high. Therefore, the residence time in the temperature region T3 is set to 150 seconds or less. The residence time in the temperature region T3 is preferably set to 130 seconds or less.

[0112] [The average cooling rate v4 in the temperature range T4 between Ms-100℃ and Ms℃ is 2.0℃ / s or more and 25.0℃ / s or less] The temperature range T4 of Ms-100°C or more and Ms°C or less is a temperature range in which martensitic transformation and self-tempering of the formed 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 formed martensite and the distribution of C from the martensite to the untransformed austenite are significantly promoted, and the standard deviation σ of the carbon concentration in the steel sheet structure at the 1 / 4 position of the steel sheet C becomes too large. In addition, the structure becomes non-uniform, and the standard deviation σq of the stress at which plastic deformation begins becomes too large. Therefore, the average cooling rate v4 is set to 2.0° C. / s or more. The average cooling rate v4 is preferably 3.0° C. / s or more.

[0113] On the other hand, if the average cooling rate v4 is too fast, the self-tempering of the formed martensite and the distribution of C from the martensite to the untransformed austenite are significantly suppressed, so the standard deviation σ of the carbon concentration in the steel sheet structure at the 1 / 4 position of the steel sheet Cbecomes too small. In addition, C segregation to mobile dislocations accompanying martensitic transformation in the surface layer is suppressed, and the ratio of measurement points at 10 μm from the steel sheet surface where the average cooling rate is less than 0.85 × [τs] becomes too high. Therefore, the average cooling rate v4 is 25.0 ° C. / s or less. The average cooling rate v4 is preferably 20.0 ° C. / s or less.

[0114] [The average cooling rate v5 in the temperature range T5 of 100°C or more and less than Ms-100°C is 0.5°C / s or more and less than the average cooling rate v4] The temperature range T5 of 100°C or more and less than Ms-100°C is a temperature range where ε carbide precipitation and C fixation occur at dislocations in the formed martensite. If the average cooling rate v5 in this temperature range T5 is too slow, ε carbide precipitation and C fixation at dislocations in the formed martensite are significantly promoted, and dislocations are pinned. As a result, the average value [τq] of the plastic deformation initiation stress τq becomes too high. Therefore, the average cooling rate v5 is 0.5°C / s or more. The average cooling rate v5 is preferably 1.0°C / s or more.

[0115] On the other hand, if the average cooling rate v5 is too fast, the ε carbide precipitation and C fixation at dislocation sites in the generated martensite are significantly suppressed, the mobility of dislocations increases, and the average value [τq] of the plastic deformation initiation stress τq becomes too low. Therefore, the average cooling rate v5 is less than the v4. The average cooling rate v5 is preferably less than (v4-3°C / s).

[0116] The Ms point is calculated using the following formula 5. Ms=499-308[%C]-10.8[%Si]-32.4[%Mn]-27[%Cr]-10.8[%Mo]...Formula 5 Here, [%M] indicates the M content in the steel (mass%).

[0117] [Cooling to below 50℃] Furthermore, the cold-rolled sheet cooled to the temperature region T5 is then cooled to 50° C. or lower. In this manner, a high-strength steel sheet (cold-rolled steel sheet) according to the present invention is obtained.

[0118] In the production method of the present invention, when a plating treatment described below is carried out, the obtained high-strength steel sheet is a plated steel sheet having a plating layer. In addition, as long as the series of heat treatments in the production method of the present invention satisfies the thermal history conditions described above, other conditions are not particularly limited, and there are also no particular limitations on the equipment in which the heat treatments are carried out.

[0119] <Plating process> In the manufacturing method of the present invention, the cold rolled sheet may be subjected to a plating treatment. Examples of such 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. Also, an electroplating layer may be formed by an electroplating treatment.

[0120] For example, when hot-dip galvanizing is performed, it is preferable to immerse the cold-rolled sheet in a galvanizing bath and then adjust the coating weight of the coating layer by gas wiping or the like. 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.

[0121] Here, such a galvanizing treatment is preferably carried out after the steel sheet is held in the temperature region T3 of 400° C. or higher and lower than 500° C. during the cooling process after the above-mentioned annealing. Next, the treatment temperature when performing alloying treatment is preferably 470°C or higher in order to more favorably improve the Zn-Fe alloying rate and productivity. On the other hand, in order to more favorably prevent the transformation of untransformed austenite into pearlite and to more favorably improve TS, 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.

[0122] The steel sheet after the cooling process 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. On the other hand, 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.

[0123] From the viewpoint of productivity, the series of treatments such as the above-mentioned annealing and plating treatments are preferably carried out in a CAL (Continuous Annealing Line) or a CGL (Continuous Galvanizing Line).

[0124] The production conditions other than those mentioned above can be the same as those in the ordinary methods.

[0125] [Material manufacturing method] 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.

[0126] In the method for producing a steel plate, a member, and a part according to the present invention, any item not described in this specification can be produced by a conventional method. EXAMPLES

[0127] The present invention will be specifically described below with reference to examples, although the present invention is not limited to the examples described below. <Steel plate manufacturing> Molten steel having the composition shown in Table 1 below (the balance being Fe and unavoidable impurities) was produced in a converter, and a steel slab was obtained by continuous casting.

[0128] [Table 1]

[0129] The thus obtained steel slab 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 rolling reduction in each pass, and the average strain rate shown in Table 2 below, and then finish rolling was performed at a finish rolling temperature of 900 ° C., and then coiled under the condition of 500 ° C. After being coiled in this manner, the hot rolled sheet was obtained by cooling to room temperature. The obtained hot rolled sheet was subjected to pickling, and then softening heat treatment was performed at a condition of 500 ° C., and then cold rolling was performed at a rolling reduction of 50%. In this way, a cold rolled sheet having a sheet thickness of 1.4 mm was obtained. In Table 2, those in which the rolling reduction in each pass in the rough rolling was 15% or more were marked with ◯, and those in which the rolling reduction in even one pass was less than 15% were marked with ×. The cold-rolled sheet was subjected to annealing and cooling treatments under the conditions shown in Table 2 below to obtain a high-strength steel sheet (cold-rolled steel sheet) of the present invention.

[0130] Plating For some of the cold-rolled sheets, after retention in the temperature region T3 of 400°C or higher and 500°C or lower, a hot-dip galvanizing process was performed to form a coating layer (hot-dip galvanized layer) on both sides, thereby obtaining a hot-dip galvanized steel sheet (GI). For the hot-dip galvanizing treatment, a hot-dip galvanizing bath (bath temperature: 470°C) containing 0.20 mass% Al, with the remainder being Zn and unavoidable impurities, was used. The coating weight of the hot-dip galvanizing layer per side was 45 to 72 g / m 2 That was the extent of it. The composition of the formed hot-dip galvanized layer contained 0.1 to 1.0 mass % Fe, 0.2 to 1.0 mass % Al, and the remainder being Zn and unavoidable impurities.

[0131] Another part of the cold-rolled sheet was subjected to a galvannealing treatment after retention in a temperature region T3 of 400° C. or more and 500° C. or less to form a coating layer (galvannealed layer) on both sides. That is, a galvannealed steel sheet (GA) was obtained. For the hot-dip galvanizing treatment, a hot-dip galvanizing bath (bath temperature: 470°C) containing 0.14 mass% Al with the remainder being Zn and unavoidable impurities was used. The alloying treatment was carried out at 550°C. The coating weight of the alloyed hot-dip galvanizing layer per side was 45 g / m 2 That was the extent of it. The composition of the formed galvannealed layer contained 7 to 15 mass % Fe, 0.1 to 1.0 mass % Al, and the remainder Zn and unavoidable impurities.

[0132] In the "Plating type" column of Table 2 below, the following is indicated: "GI" if a hot-dip galvanized layer was formed; "GA" if an alloyed hot-dip galvanized layer was formed; and "CR" if no plating layer was formed.

[0133] [Table 2] TIFF0007652346000008.tif223128

[0134] Observation of steel structure For the obtained steel plate at the 1 / 4 position of the plate thickness, the area ratios of martensite, ferrite, bainite, retained austenite and the remaining structure, as well as the standard deviation σ of the carbon concentration in the steel plate structure were measured according to the method described above. C In addition, the area ratios of martensite, pearlite, ferrite, bainite and the remaining structure were measured at a position 10 μm from the surface of the obtained steel sheet according to the above-mentioned method. The measurement results are shown in Table 3 below.

[0135] Nanoindentation Test The average value [τq] of the stress at which plastic deformation begins τq and the standard deviation σq of the stress at which plastic deformation begins were measured at the 1 / 4 position of the plate thickness of the obtained steel plate according to the method described above. In addition, the ratio of measurement points at a position 10 μm from the surface of the obtained steel plate that was less than 0.85 × [τs] was calculated, where [τs] is the average value of the stress at which plastic deformation begins τs measured by the nanoindentation method. The measurement results are shown in Table 3 below.

[0136] <evaluation> The obtained steel sheets were subjected to the tests described below to evaluate various properties, and the results are shown in Table 3 below.

[0137] Tensile test The tensile test was carried out in accordance with JIS Z 2241:2021. Specifically, JIS No. 5 test pieces were cut from the obtained steel plate so that the direction perpendicular to the rolling direction of the steel plate was the longitudinal direction. -1 Tensile tests were carried out under the condition of 1000 mm / s, and the yield strength (YS) [MPa], tensile strength (TS) [MPa] and total elongation (El) [%] were measured. In addition, the yield ratio (YR) (=100×YS / TS) [%] was calculated from the yield strength and tensile strength. In this example, a tensile strength (TS) of 1180 MPa or more was determined to be high strength. A yield ratio (YR) of 80% or less was determined to be excellent in strain dispersibility. A total elongation (El) of 7.0% or more was determined to be excellent in ductility.

[0138] <Bending test> 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 a pressing load of 100 kN and a pressing hold time of 5 seconds. That is, a 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. The occurrence of cracks was confirmed by observing the ridgeline at the apex of bending with a digital microscope (RH-2000, manufactured by Hirox Corporation) at a magnification of 40 times. The minimum bending radius R at which no cracks occurred was determined for any of the five test pieces, and the value obtained by dividing the radius by the plate thickness t (R / t) was taken as the critical bending radius. A critical bending radius (R / t) of 5.0 or less was determined to have excellent bendability.

[0139] <Delayed fracture resistance under atmospheric corrosion and in a painted state> 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 with a width of 30 mm and a length of 100 mm was cut from the obtained steel plate so that the axial direction of the bending test was parallel to the rolling direction of the steel plate. The longitudinal end face of the test piece was a shear end face (clearance: 15%, shear angle: 0°).

[0140] 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 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 "GT-100" electrodeposition paint manufactured by Kansai Paint Co., Ltd. to form a coating film. The coating film thickness of the electrodeposition coating was set to 15 μm, and was confirmed by measuring the film thickness using a commercially available electromagnetic coating thickness meter.

[0141] 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% to 90% RH, 2h), wetting (90% RH, 2h), and humidity transition (90% to 30% RH, 2h). The test was performed twice a week with a NaCl adhesion of 3g / m 2 The surface of the delayed fracture test piece was sprayed with salt water so that the salt water concentration was 100%. Such a corrosion cycle test was carried out for 30 days, and if no cracks were observed after 30 days, the delayed fracture resistance under atmospheric corrosion and in a painted state was judged to be good, and this was indicated by a circle in Table 3. Furthermore, if no cracks were observed after 35 days, the delayed fracture resistance under atmospheric corrosion and in a painted state was judged to be particularly good, and this was indicated by a double circle in Table 3. On the other hand, if cracks were observed before 30 days had passed, this was indicated by an x ​​in Table 3.

[0142] [Table 3] TIFF0007652346000010.tif223120

[0143] As shown in Table 3, the examples according to the present invention have high strength and are excellent in all of strain dispersion ability, ductility, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state, whereas the comparative examples are inferior in one or more of strength, strain dispersion ability, ductility, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state.

[0144] Although the embodiment of the present invention has been described above, the present invention is not limited by the description of the present embodiment, which is a part of the disclosure of the present invention. In other words, other embodiments, examples, and operation techniques made by those skilled in the art based on the present embodiment are all included in the scope of the present invention. For example, in the series of heat treatments in the above-mentioned manufacturing method, the equipment for subjecting the steel sheet to heat treatment is not particularly limited as long as the heat history conditions are satisfied. [Industrial Applicability]

[0145] According to the present invention, it is possible to manufacture high-strength steel sheets excellent in all of strain dispersion ability, ductility, bendability, and delayed fracture resistance under atmospheric corrosion and in a painted state. Furthermore, by applying the steel sheets obtained according to the method of the present invention to, for example, automobile structural members, it is possible to improve fuel efficiency by reducing the weight of the automobile body, and the industrial value of the steel sheets is extremely great.

Claims

1. In mass percent, 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; O: 0.0100% or less The composition is composed of Fe and unavoidable impurities. At 1 / 4 of the thickness of the steel plate, The microstructure is Area ratio of martensite: 70% or more, Sum of area ratio of ferrite and bainite: 15% or less, And the area ratio of retained austenite is 15% or less, Furthermore, the standard deviation of the carbon concentration σ Ｃ but, 0.10[%C]≦σ Ｃ ≦0.28[%C] where [%C] is the carbon content in the steel (mass%) Satisfied, The average value [τq] of the plastic deformation initiation stress τq measured by a nanoindentation method at the sheet thickness 1 / 4 position is 1.70 GPa or more and 3.30 GPa or less, and the standard deviation σq of the plastic deformation initiation stress τq is 0.42 GPa or less; At a position 10 μm from the surface of the steel sheet, The microstructure is Area ratio of martensite: 40% or less, Perlite area ratio: 15% or less, The sum of the area ratios of ferrite and bainite is 60% or more, When the average value of the plastic deformation initiation stress τs measured by the nanoindentation method at the 10 μm position is [τs], the ratio of measurement points that are less than 0.85 × [τs] is 30.0% or less. steel plate.

2. The composition further includes, in mass%, 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: 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; Bi: 0.200% or less The steel sheet according to claim 1, further comprising at least one element selected from the group consisting of:

3. The steel sheet according to claim 1 or 2, which has a plating layer on the surface of the steel sheet.

4. A member made of the steel sheet according to claim 1 or 2.

5. A member made of the steel plate described in claim 3.

6. 5. An automobile frame structural part or an automobile reinforcing part, comprising the member according to claim 4.

7. An automobile skeletal structural part or an automobile reinforcing part comprising the member described in claim 5.

8. A steel slab having the composition according to claim 1 or 2, The number of passes in the temperature range of 1000°C or higher is 4 passes or more, the rolling reduction rate in each pass is 15% or more, and the average strain rate is 9 x 10 －４ / s or more 1×10 －２ A method for producing a steel sheet, comprising the steps of: rough rolling under conditions in the range of 0.1 to 1.5 mm / s; finish rolling; coiling; and a hot rolling process to obtain a hot-rolled sheet; pickling and cold rolling to obtain a cold-rolled sheet by pickling and cold rolling the hot-rolled sheet; annealing the cold-rolled sheet; and cooling the cold-rolled sheet to 50 ° C. or less. The annealing is performed under conditions in which the heating temperature is 800° C. or higher, the dew point is −25° C. or higher in a heating temperature region T1 of 800° C. or higher, and the following formula 1 is satisfied: 2.0≦K≦60.0...Formula 1 In addition, [0010] Furthermore, in the above, [%C] is the C content in the steel plate, The time when the temperature of the cold-rolled sheet first reaches 800° C. during the annealing is defined as t=0 (s), and t=tE (s) is the time when the annealing is completed and the temperature of the cold-rolled sheet again reaches 800° C., and T t (°C) is the average temperature of the cold-rolled sheet at time t: t-1 to t(s), In addition, in the cooling, The average cooling rate v2 in the temperature range T2 of 600° C. or more and 750° C. or less is set to 1.0° C. / s or more and 15.0° C. / s or less, The average cooling rate in the temperature range of 500 ° C. or more and less than 600 ° C. is greater than v2, The residence time in 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 average cooling rate v4 in the temperature range T4 from Ms-100 ° C. to Ms ° C. is 2.0 ° C. / s or more and 25.0 ° C. / s or less, The average cooling rate v5 in the temperature range T5 of 100° C. or more and less than Ms-100° C. is set to 0.5° C. / s or more and less than v4. Manufacturing method of steel plate.

9. The method for producing a steel sheet according to claim 8, further comprising the step of carrying out a plating treatment during the cooling process.

10. A method for manufacturing a component, comprising a step of subjecting the steel plate according to claim 1 or 2 to at least one of forming and joining to form a component.

11. A method for manufacturing a component, comprising the step of subjecting the steel plate described in claim 3 to at least one of forming processing or joining processing to form a component.

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