Steel sheet and component and method for manufacturing same

Abstract

Provided is a steel sheet having a TS of 1320 MPa or more and excellent stretch flangeability, toughness after coating and baking, and impact characteristics. The steel sheet has a prescribed component composition and a structure in which the area fraction of tempered martensite is 95% or more, the area fraction of retained austenite is less than 3%, the total area fraction of ferrite and bainitic ferrite is less than 5%, the grain boundary density at 20 DEG C or more in the tempered martensite is 1.0 [mu] m / [mu] m2 or more, and the following formula (1) is satisfied. (1) KAM (S) / KAM (C) > 1.00... (1).

Description

Technical Field

[0001] The present invention relates to a steel plate, a component using the steel plate as a blank, and a method for manufacturing the same. Background Art

[0002] To balance reducing CO2 emissions and improving crashworthiness through lightweighting of automobiles, the strength of steel sheets used as base materials for automotive parts is being continuously increased. Furthermore, new laws and regulations are being introduced. Consequently, the use of steel sheets with a tensile strength (TS) of 1320 MPa or higher in key automotive structural components is increasing.

[0003] Many automotive parts require excellent stretch-flange formability in steel sheets. For example, automotive parts like crash boxes have stamped end faces. Therefore, from a formability perspective, excellent stretch-flange formability is required in steel sheets used as the base material for these automotive parts.

[0004] Patent Document 1 discloses, for example, a steel sheet for use as a base material for automobile parts, “a steel sheet having a composition and a steel structure comprising, in mass%, 0.12% to 0.40% C, 0.01% to 1.5% Si, more than 1.7% and 3.5% or less Mn, 0.05% or less P, 0.010% or less S, 1.00% or less sol. Al, 0.010% or less N, 0.0002% to 0.0050% B, and 0.010% to 0.080% in total of one or both of Nb and Ti, with the remainder being Fe and unavoidable impurities, wherein the steel structure comprises an area fraction of martensite of 70% or more, an area fraction of bainite of 30% or less, and a total area fraction of ferrite and retained austenite of 10% or less;”

[0005] The number density of carbides with a major diameter of 0.5 μm or more at the 1 / 4 thickness position of the steel plate is 60,000 / mm 2 the following,

[0006] The number density of inclusion particles with an equivalent circle diameter of 4.0 μm or more in the range of 1 / 4 to 3 / 4 of the thickness of the steel plate is 10 particles / mm 2 ~30 pieces / mm 2 ,

[0007] The number density of inclusion particles with an equivalent circle diameter of 4.0 μm or more in the range from the surface of the steel plate to 1 / 4 of the plate thickness is 27 particles / mm 2 the following,

[0008] The tensile strength is over 1310 MPa.”

[0009] Patent Document 2 discloses a steel plate having the following chemical composition and steel structure, wherein the chemical composition contains, in mass%, C: 0.05% to 0.40%, Si: 0.05% to 3.0%, Mn: 1.5% to 3.5%, Al: 1.5% or less, N: 0.010% or less, P: 0.10% or less, S: 0.005% or less, Cr, Cu, Ni, Sn and Mo: 0.0% to 1.0% in total, B: 0.000% to 0.005%, Ca: 0.000% to 0.005%, Ce: 0.000% to 0.005%, and La: 0.000% to 0.005%. 0.005%, further containing one or more selected from Nb: 0.0002% to 0.04%, Ti: 0.0002% to 0.08%, and V and Ta: 0.01% to 0.3% in total, the remainder being Fe and impurities, wherein the steel structure comprises, in terms of area%, 20% to 95% of first martensite having two or more iron carbides with an equivalent circle diameter of 2 nm to 500 nm in the lath, 15% or less of ferrite, 15% or less of retained austenite, and the remainder being bainite or second martensite having less than two iron carbides with an equivalent circle diameter of 2 nm to 500 nm in the lath, or both;

[0010] The total area fraction of ND / / <111> oriented grains and ND / / <100> oriented grains is 40% or less.

[0011] The amount of solid solution C is 0.44ppm or more,

[0012] The ND / / <111> oriented grains are grains whose crystal orientation parallel to the normal direction of the plate surface deviates from the <111> direction by 10° or less.

[0013] The above-mentioned ND / / <100> oriented grains refer to grains whose crystal orientation parallel to the normal direction of the plate surface deviates from the <100> direction by 10° or less.

[0014] Patent Document 3 discloses a high-strength steel sheet comprising, in mass%, 0.09% to 0.37% C, more than 0.70% and less than 2.00% Si, 2.60% to 3.60% Mn, 0.001% to 0.100% P, less than 0.0200% S, 0.010% to 1.000% Al, and less than 0.0100% N, with the remainder being Fe and unavoidable impurities.

[0015] The steel structure comprises: martensite having a carbon concentration greater than 0.7×[%C] and less than 1.5×[%C] accounting for 55% or more by area ratio, tempered martensite having a carbon concentration of 0.7×[%C] or less accounting for 5% to 40% by area ratio, a ratio of the carbon concentration in retained austenite to the volume fraction of retained austenite being 0.05 to 0.40, and an average crystal grain size of each of the martensite and the tempered martensite being 5.3 μm or less.

[0016] In addition, in the above steel structure, the surface softening thickness is 10μm to 100μm.

[0017] The tensile strength is 1180 MPa or more.

[0018] It should be noted that [%C] represents the content (mass %) of the component element C in steel.

[0019] Prior art literature

[0020] Patent Literature

[0021] Patent Document 1: Japanese Patent No. 7001197

[0022] Patent Document 2: Japanese Patent No. 6497443

[0023] Patent Document 3: Japanese Patent No. 6747612 Summary of the Invention

[0024] However, many automotive parts undergo a paint bake process. The toughness and collision resistance of steel sheets can vary significantly before and after the paint bake process. Therefore, in recent years, to further improve automotive safety, there has been a growing demand for steel sheets used as base materials for automotive parts to exhibit superior toughness and collision resistance after the paint bake process.

[0025] However, none of the steel sheets disclosed in Patent Documents 1 to 3 consider toughness and collision properties after paint baking. Therefore, there is a current demand for the development of a steel sheet with a TS of 1320 MPa or higher and excellent stretch flangeability, toughness after paint baking, and collision properties.

[0026] The present invention has been developed in view of the above-mentioned current situation, and an object of the present invention is to provide a steel sheet having a TS of 1320 MPa or more, excellent in stretch flange formability, toughness after paint baking, and collision characteristics, and to provide an advantageous production method thereof.

[0027] Another object of the present invention is to provide a component using the above-mentioned steel plate as a base material and a method for manufacturing the component.

[0028] Here, TS is measured by a tensile test in accordance with JIS Z 2241:2022.

[0029] Excellent stretch flange formability means that the limiting hole expansion ratio λ is 30% or more. The limiting hole expansion ratio λ is measured by a hole expansion test in accordance with JIS Z2256:2020.

[0030] Excellent toughness after baking paint means a brittle-ductile transition temperature of -40°C or lower after aging treatment. Aging treatment conditions are 170°C and 20 minutes. The brittle-ductile transition temperature is measured using a Charpy impact test in accordance with JIS Z 2242:2018.

[0031] Excellent post-baked paint crash properties mean a YR after aging treatment of 0.85 or higher and a fracture stress ratio after aging treatment of 0.90 or lower. Aging treatment conditions are a treatment temperature of 170°C and a treatment time of 20 minutes. Furthermore, the YR after aging treatment and the fracture stress ratio after aging treatment are calculated using the following formulas, respectively, from the TS, YS (yield stress), and fracture stress after aging treatment, measured by a tensile test according to JIS Z 2241:2022.

[0032] [YR after aging treatment] = [YS after aging treatment] / [TS after aging treatment]

[0033] [Ratio of fracture stress after aging treatment] = [Rupture stress after aging treatment] / [TS after aging treatment]

[0034] In addition, the details of the measurement method are described in the Examples described later.

[0035] Therefore, the inventors have conducted intensive studies to achieve the above-mentioned object and have obtained the following findings.

[0036] (A) To achieve TS: 1320 MPa or more, it is important to make the area fraction of tempered martensite 95% or more. This ensures the required properties while achieving TS: 1320 MPa or more.

[0037] (B) To achieve excellent stretch-flange formability, it is important to reduce the combined area fraction of ferrite and bainitic ferrite to less than 5%. This ensures the required properties while achieving excellent stretch-flange formability.

[0038] (C) To obtain excellent toughness after baking, it is important to make the area fraction of retained austenite less than 3% and to make the density of grain boundaries of 20° or more in tempered martensite 1.0 μm / μm. 2 As a result, it is possible to ensure the required properties and obtain excellent toughness after baking the coating.

[0039] (D) In ​​order to obtain excellent collision properties after baking, it is important to make the density of grain boundaries of 20° or more in tempered martensite 1.0 μm / μm 2 The above conditions are satisfied, and the following formula (1) is satisfied. Thus, it is possible to obtain excellent post-baking collision characteristics while ensuring predetermined required characteristics.

[0040] KAM(S) / KAM(C)>1.00·····(1)

[0041] Where,

[0042] KAM(S): KAM value at a depth of 100 μm from the steel plate surface.

[0043] KAM(C): KAM value at the center of the steel plate thickness

[0044] The present invention has been completed through further research based on the above findings.

[0045] That is, the main configuration of the present invention is as follows.

[0046] 1. A steel plate having the following composition and structure:

[0047] The above composition comprises, by mass%, 0.030% to 0.500% C, 0.010% to 2.500% Si, 0.10% to 5.00% Mn, 0.100% or less P, 0.0200% or less S, 0.0100% or less N, 0.0100% or less O, and 1.000% or less Al, with the remainder being Fe and unavoidable impurities.

[0048] In the above structure, the area fraction of tempered martensite is 95% or more, the area fraction of retained austenite is less than 3%, the total area fraction of ferrite and bainitic ferrite is less than 5%, and the density of grain boundaries above 20° in the above tempered martensite is 1.0 μm / μm 2 The above satisfies the following formula (1).

[0049] KAM(S) / KAM(C)>1.00·····(1)

[0050] Where,

[0051] KAM(S): The average KAM value at a depth of 100 μm from the steel plate surface.

[0052] KAM(C): Average KAM value at the center of the steel plate thickness

[0053] 2. The steel sheet according to item 1 above, wherein the chemical composition further contains, in mass %, at least one 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.

[0054] 3. The steel sheet according to 1 or 2 above, having a plated layer on the surface.

[0055] 4. A component formed using the steel plate according to any one of 1 to 3 above.

[0056] 5. A method for producing a steel plate, comprising the following steps:

[0057] A preparation step of preparing a steel plate having the composition described in 1 or 2 above;

[0058] The heating step is to heat the steel plate to an annealing temperature T1 in a temperature range of 700°C to 750°C at an average heating rate of 5.0°C / s or less.

[0059] Annealing step: annealing the steel sheet at an annealing temperature T1 of 800° C. or higher and an annealing time t1 of 10 seconds or longer.

[0060] A bending step, wherein the steel sheet is bent at least once in a temperature range of the annealing temperature T1 to 700° C. using a roller having a radius of 800 mm or less.

[0061] A first cooling step is then performed, wherein the steel plate is cooled to a first cooling end temperature at an average cooling rate of 10°C / s or higher in a temperature range of 700°C to 550°C.

[0062] In a second cooling step, the steel sheet is cooled to a second cooling completion temperature under the conditions that the average cooling rate in the temperature range of 300°C to 100°C is 300°C / s or higher and the tension applied to the steel sheet in the temperature range of 300°C to 100°C is 5 MPa or higher.

[0063] Tempering step: Next, the steel plate is tempered at a tempering temperature T2 of 100°C to 400°C and a tempering time t2 of 10 seconds to 10,000 seconds.

[0064] The correction process then uses a straightening machine to correct the above-mentioned billet steel plate under the conditions of a correction starting temperature of less than 100°C, an inlet side meshing press-in amount of 4.0mm~10.0mm, an outlet side meshing press-in amount of 1.0mm~10.0mm, an inlet side tension of 20MPa~500MPa, and an outlet side tension of 25MPa~550MPa.

[0065] 6. The method for manufacturing a steel plate according to 5 above, further comprising a plating step of plating the steel sheet between the first cooling step and the second cooling step or between the tempering step and the straightening step.

[0066] 7. A method for manufacturing a component, comprising the step of subjecting the steel sheet according to any one of 1 to 3 above to at least one of forming and joining to produce the component.

[0067] According to the present invention, a steel plate having a TS of 1320 MPa or higher, excellent stretch flangeability, toughness after paint baking, and crash characteristics can be obtained. By using the steel plate of the present invention in, for example, automotive parts, fuel efficiency can be improved by reducing vehicle weight, significantly contributing to the reduction of CO2 emissions. Therefore, its industrial application value is extremely high. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 This is a schematic diagram for explaining the definitions of the inlet-side meshing press-in amount and the outlet-side meshing press-in amount. DETAILED DESCRIPTION

[0069] The present invention will be described based on the following embodiments.

[0070] [1] Steel plate

[0071] First, the chemical composition of the steel sheet according to one embodiment of the present invention will be described. It should be noted that all units in the chemical composition are "mass %" and, unless otherwise specified, are expressed in "%" only.

[0072] [C: 0.030% to 0.500%]

[0073] C is one of the important basic components of steel. In particular, in the steel plate according to one embodiment of the present invention, C is an important element that affects the area fraction of tempered martensite and the collision characteristics after coating and baking. When the C content is less than 0.030%, the area fraction of tempered martensite decreases, and it is difficult to achieve a TS of 1320 MPa or more. In addition, it is also difficult to achieve excellent collision characteristics after coating and baking. On the other hand, if the C content exceeds 0.500%, the tempered martensite becomes brittle, and it is difficult to achieve excellent toughness after coating and baking. Therefore, the C content is set to 0.030% to 0.500%. The C content is preferably 0.050% or more, and more preferably 0.100% or more. The C content is preferably 0.400% or less, and more preferably 0.350% or less.

[0074] [Si: 0.010% to 2.500%]

[0075] Si is one of the important basic components of steel. In particular, in the steel plate according to one embodiment of the present invention, Si suppresses the formation of carbides during annealing and promotes the formation of retained austenite. In other words, Si is an important element that affects the area fraction of retained austenite. When the Si content is less than 0.010%, it is difficult to achieve a TS of 1320 MPa or more. On the other hand, if the Si content exceeds 2.500%, the retained austenite increases excessively, making it difficult to achieve excellent toughness after coating and baking. Therefore, the Si content is set to 0.010% to 2.500%. The Si content is preferably 0.050% or more, more preferably 0.100% or more. The Si content is preferably 2.000% or less, more preferably 1.200% or less.

[0076] [Mn: 0.10% to 5.00%]

[0077] Mn is one of the important basic components of steel. In particular, in the steel plate according to one embodiment of the present invention, Mn is an important element that affects the area fraction of tempered martensite and the toughness after coating and baking. When the Mn content is less than 0.10%, the area fraction of tempered martensite decreases, and it is difficult to achieve a TS of 1320 MPa or more. On the other hand, if the Mn content exceeds 5.00%, the tempered martensite becomes brittle, and it is difficult to achieve excellent toughness after coating and baking. Therefore, the Mn content is set to 0.10% to 5.00%. The Mn content is preferably 0.50% or more, and more preferably 0.80% or more. The Mn content is preferably 4.50% or less, and more preferably 4.00% or less.

[0078] [P: 0.100% or less]

[0079] P segregates at the prior austenite grain boundaries, embrittles them, and reduces the ultimate deformation capacity of the steel sheet. Therefore, if the P content is excessive, it is difficult to achieve excellent toughness after painting and baking. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.070% or less. It should be noted that there is no specific lower limit for the P content. However, P is a solid solution strengthening element that can increase the strength of the steel sheet. Therefore, the P content is preferably 0.001% or more.

[0080] [S: 0.0200% or less]

[0081] Sulfur exists as a sulfide, reducing the ultimate deformability of the steel sheet. Therefore, excessive S content makes it difficult to achieve excellent toughness after baking. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0050% or less. There is no specific lower limit for the S content. However, due to production technology constraints, the S content is preferably 0.0001% or more.

[0082] [N: 0.0100% or less]

[0083] Nitrogen exists as nitrides, reducing the ultimate deformability of steel sheets. Therefore, excessive N content makes it difficult to achieve excellent toughness after baking. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0050% or less. There is no specific lower limit for the N content. However, due to production technology constraints, the N content is preferably 0.0001% or more.

[0084] [O: 0.0100% or less]

[0085] O exists as an oxide, reducing the ultimate deformability of the steel sheet. Therefore, if the O content is excessive, achieving excellent toughness after baking and coating becomes difficult. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0050% or less. It should be noted that there is no specific lower limit for the O content. However, due to production technology constraints, the O content is preferably 0.0001% or more.

[0086] [Al: 1.000% or less]

[0087] Al exists as an oxide, reducing the ultimate deformability of the steel sheet. Therefore, excessive Al content makes it difficult to achieve excellent toughness after baking and painting. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 0.500% or less. It should be noted that there is no specific lower limit for the Al content. However, due to production technology constraints, the Al content is preferably 0.001% or more.

[0088] While the basic composition of the steel sheet according to one embodiment of the present invention has been described above, the steel sheet according to one embodiment of the present invention has a composition comprising the above-mentioned basic components, with the remainder consisting of Fe (iron) and inevitable impurities. The steel sheet according to one embodiment of the present invention preferably has a composition comprising the above-mentioned basic components, with the remainder consisting of Fe and inevitable impurities. In addition to the above-mentioned basic components, the steel sheet according to one embodiment of the present invention may contain at least one element selected from the following as an optional additive element, either alone or in combination.

[0089] 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

[0090] It should be noted that, as long as the above-mentioned arbitrarily added element contains below the above-mentioned upper limit amount, the effect of the present invention can be obtained, so the lower limit is not particularly provided. In addition, when containing the above-mentioned arbitrarily added element with less than the preferred lower limit value described later, the element contains as inevitable impurities.

[0091] [Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less]

[0092] If the contents of Ti, Nb, and V are each less than 0.200%, coarse precipitates and inclusions will not be generated in large quantities, and the ultimate deformation capacity of the steel sheet will not be reduced. Therefore, the toughness after coating and baking will not be reduced. Therefore, when Ti, Nb, and V are contained, their contents are preferably less than 0.200%. The contents of Ti, Nb, and V are more preferably less than 0.100%. It should be noted that there is no particular lower limit for the contents of Ti, Nb, and V. However, Ti, Nb, and V form fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing the strength of the steel sheet. Therefore, the contents of Ti, Nb, and V are each preferably greater than 0.001%.

[0093] [Ta: 0.10% or less, W: 0.10% or less]

[0094] If Ta and W are each less than 0.10%, coarse precipitates and inclusions will not be generated in large quantities, and the ultimate deformation capacity of the steel plate will not be reduced. Therefore, the toughness after coating and baking will not be reduced. Therefore, when Ta and W are contained, their contents are preferably less than 0.10%. The contents of Ta and W are more preferably less than 0.08%. There is no particular lower limit for the contents of Ta and W. However, Ta and W form fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby improving the strength of the steel plate. Therefore, the contents of Ta and W are each preferably greater than 0.01%.

[0095] [B: 0.0100% or less]

[0096] 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 deformation capacity of the steel sheet will not be reduced. Therefore, the toughness after coating and baking will not be reduced. Therefore, when B is contained, its content is preferably 0.0100% or less. The B content is more preferably 0.0080% or less. It should be noted that there is no particular lower limit for the B content. However, B segregates at austenite grain boundaries during annealing and is an element that improves hardenability. Therefore, the B content is preferably 0.0003% or more.

[0097] [Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less]

[0098] If the contents of Cr, Mo, and Ni are respectively less than 1.00%, coarse precipitates and inclusions will not be generated in large quantities, and the ultimate deformation capacity of the steel plate will not be reduced. Therefore, the toughness after coating and baking will not be reduced. Therefore, when Cr, Mo, and Ni are contained, their contents are preferably less than 1.00%. The contents of Cr, Mo, and Ni are more preferably less than 0.80%. It should be noted that there is no particular lower limit for the contents of Cr, Mo, and Ni. However, Cr, Mo, and Ni are elements that improve hardenability. Therefore, the contents of Cr, Mo, and Ni are preferably more than 0.01%.

[0099] [Co: 0.010% or less]

[0100] If Co is 0.010% or less, coarse precipitates and inclusions will not form in large quantities, and the ultimate deformation capacity of the steel plate will not be reduced. Therefore, the toughness after coating and baking will not be reduced. Therefore, when Co is contained, its content is preferably 0.010% or less. The Co content is more preferably 0.008% or less. It should be noted that there is no particular lower limit for the Co content. However, Co is an element that improves hardenability. Therefore, the Co content is preferably 0.001% or more.

[0101] [Cu: 1.00% or less]

[0102] If Cu is 1.00% or less, coarse precipitates and inclusions will not be generated in large quantities, and the ultimate deformation capacity of the steel plate will not be reduced. Therefore, the toughness after coating and baking will not be reduced. Therefore, when Cu is contained, its content is preferably 1.00% or less. The Cu content is more preferably 0.80% or less. It should be noted that there is no special lower limit for the Cu content. However, Cu is an element that improves hardenability. Therefore, the Cu content is preferably 0.01% or more.

[0103] [Sn: 0.200% or less]

[0104] If Sn is 0.200% or less, cracks will not form inside the steel plate during casting or hot rolling, and the ultimate deformation capacity of the steel plate will not be reduced. Therefore, the toughness after coating and baking will not be reduced. Therefore, when Sn is contained, its content is preferably 0.200% or less. The Sn content is more preferably 0.100% or less. It should be noted that there is no particular lower limit for the Sn content. However, Sn is an element that improves hardenability and is generally also an element that improves corrosion resistance. Therefore, the Sn content is preferably 0.001% or more.

[0105] [Sb: 0.200% or less]

[0106] If the Sb content is 0.200% or less, coarse precipitates and inclusions will not form in large quantities, and the ultimate deformation capacity of the steel plate will not be reduced. Therefore, the toughness after coating and baking will not be reduced. Therefore, when Sb is contained, its content is preferably 0.200% or less. The Sb content is more preferably 0.100% or less. It should be noted that there is no specific lower limit for the Sb content. However, Sb is an element that controls the softening thickness of the surface layer and can adjust the strength. Therefore, the Sb content is preferably 0.001% or more.

[0107] [Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less]

[0108] If the contents of Ca, Mg, and REM are each 0.0100% or less, coarse precipitates and inclusions will not form in large quantities, and the ultimate deformability of the steel sheet will not be reduced. Therefore, the toughness after baking and coating will not be reduced. Therefore, when Ca, Mg, and REM are contained, their contents are preferably 0.0100% or less. The contents of Ca, Mg, and REM are more preferably 0.0050% or less, respectively. It should be noted that there is no particular lower limit for the contents of Ca, Mg, and REM. However, Ca, Mg, and REM are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the contents of Ca, Mg, and REM are each preferably 0.0005% or more.

[0109] [Zr: 0.100% or less, Te: 0.100% or less]

[0110] If the Zr and Te contents are each 0.100% or less, coarse precipitates and inclusions will not form in large quantities, and the ultimate deformability of the steel sheet will not be reduced. Consequently, the toughness after baking and coating will not be reduced. Therefore, when Zr and Te are contained, their contents are preferably 0.100% or less, respectively. The Zr and Te contents are more preferably 0.080% or less, respectively. It should be noted that there is no specific lower limit for the Zr and Te contents. However, Zr and Te are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the Zr and Te contents are each preferably 0.001% or more.

[0111] [Hf: 0.10% or less]

[0112] If the Hf content is 0.10% or less, coarse precipitates and inclusions will not form in large quantities, nor will the ultimate deformability of the steel sheet be reduced. Consequently, the toughness after baking and coating will not be reduced. Therefore, when Hf is present, its content is preferably 0.10% or less. The Hf content is more preferably 0.08% or less. It should be noted that there is no specific lower limit for the Hf content. However, Hf is an element that spheroidizes the shape of nitrides and sulfides, thereby improving the ultimate deformability of the steel sheet. Therefore, the Hf content is preferably 0.01% or more.

[0113] [Bi: 0.200% or less]

[0114] If Bi is 0.200% or less, coarse precipitates and inclusions will not form in large quantities, nor will the ultimate deformability of the steel sheet be reduced. Consequently, toughness after baking and coating will not be reduced. Therefore, when Bi is present, its content is preferably 0.200% or less. The Bi content is more preferably 0.100% or less. It should be noted that there is no specific lower limit for the Bi content. However, Bi is an element that reduces segregation. Therefore, the Bi content is preferably 0.001% or more.

[0115] Elements other than the above are Fe and inevitable impurities. Examples of inevitable impurities include Zn, Pb, As, Ge, Sr, and Cs. These inevitable impurities are permitted if their total content is 0.100% or less.

[0116] Next, the structure of the steel sheet according to one embodiment of the present invention will be described.

[0117] In the structure of the steel plate according to one embodiment of the present invention,

[0118] The area fraction of tempered martensite is more than 95%,

[0119] The area fraction of retained austenite is less than 3%,

[0120] The combined area fraction of ferrite and bainitic ferrite is less than 5%,

[0121] The density of grain boundaries above 20° in tempered martensite is 1.0 μm / μm 2 above,

[0122] Satisfies the above formula (1).

[0123] The reasons for each limitation are described below. It should be noted that the area fraction of each phase is the area ratio occupied by each phase in the entire structure.

[0124] [Area fraction of tempered martensite: 95% or more]

[0125] In the steel sheet according to one embodiment of the present invention, it is extremely important to achieve an area fraction of tempered martensite of 95% or greater. Specifically, by making tempered martensite the primary phase, and in particular by achieving an area fraction of 95% or greater, a TS of 1320 MPa or greater can be achieved. Therefore, the area fraction of tempered martensite is set to 95% or greater. The area fraction of tempered martensite is preferably 96% or greater, and more preferably 97% or greater. There is no particular upper limit on the area fraction of tempered martensite. The area fraction of tempered martensite may also be 100%.

[0126] [Area fraction of retained austenite: less than 3%]

[0127] In a steel plate according to one embodiment of the present invention, it is extremely important to keep the area fraction of retained austenite less than 3%. That is, when the area fraction of retained austenite is 3% or more, it is difficult to achieve excellent toughness after paint baking. One of the reasons for the reduction in toughness after paint baking is that the retained austenite undergoes a work-induced martensitic transformation during processing to become high-hardness martensite, which becomes the starting point of fracture. Therefore, the area fraction of retained austenite is set to less than 3%. The area fraction of retained austenite is preferably 1% or less. It should be noted that there is no particular lower limit for the area fraction of retained austenite. The area fraction of retained austenite may also be 0%.

[0128] [Total area fraction of ferrite and bainitic ferrite: less than 5%]

[0129] In a steel sheet according to one embodiment of the present invention, it is extremely important to keep the combined area fraction of ferrite and bainitic ferrite below 5%. Specifically, if the combined area fraction of ferrite and bainitic ferrite is 5% or greater, it is difficult to achieve a TS of 1320 MPa or greater. Furthermore, it is difficult to achieve excellent stretch flangeability. Therefore, the combined area fraction of ferrite and bainitic ferrite is set to less than 5%. The combined area fraction of ferrite and bainitic ferrite is preferably 3% or less, more preferably 2% or less. There is no particular lower limit for the combined area fraction of ferrite and bainitic ferrite. The combined area fraction of ferrite and bainitic ferrite may also be 0%. Ferrite and bainitic ferrite may be present alone or in combination.

[0130] The area fraction of the remaining structures other than those described above is preferably 5% or less. Examples of the remaining structures include pearlite, fresh martensite, and acicular ferrite. These remaining structures have no effect on the properties if they are 5% or less, so they may be included. The area fraction of the remaining structures may also be 0%.

[0131] Here, the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite are measured, for example, as follows.

[0132] That is, a sample is cut from the steel plate with the plate thickness section (L section) parallel to the rolling direction of the steel plate as the observation surface. Then, the observation surface of the sample is polished. Then, the observation surface of the sample is etched with 3 vol.% nitric alcohol to reveal the structure. Then, 10 fields of view at a position of 1 / 4 of the plate thickness of the steel plate (a position equivalent to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate) are observed using an SEM at a magnification of 2000 times. It should be noted that in the observation image, tempered martensite is a structure with fine concave and convex parts and carbides inside the structure. In addition, ferrite and bainitic ferrite are structures with flat structures in the concave parts and no carbides inside. Then, the area of ​​the area occupied by tempered martensite, ferrite and bainitic ferrite in each field of view is calculated. Then, the area of ​​the area occupied by tempered martensite, ferrite and bainitic ferrite is divided by the area of ​​the entire observation field and multiplied by 100. Then, the average values ​​thereof were respectively defined as the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite.

[0133] It should be noted that the structure of a steel plate is generally symmetrical in the thickness direction. Therefore, any one of the surfaces of the steel plate (the light-receiving side and the back side) can be used as the starting point of the thickness position (the thickness zero position), such as the 1 / 4 position and the position 100 μm deep from the steel plate surface.

[0134] In addition, the area fraction of retained austenite is measured, for example, as follows.

[0135] That is, the steel plate is mechanically ground to a depth of 1 / 4 of the plate thickness - 0.1 mm with the 1 / 4 position of the steel plate as the observation position, and then further ground by 0.1 mm by chemical grinding. The ground surface is used as the observation surface, and an X-ray diffraction device is used to use CoKα radiation to obtain the ratio of the integrated intensity of the diffraction peaks of {200}, {220} and {311} of fcc iron (austenite) to the integrated intensity of the diffraction peaks of {200}, {211} and {220} of bcc iron. Then, the volume fraction of retained austenite is calculated based on the ratio of the integrated intensity of each surface. Then, the retained austenite is regarded as three-dimensionally homogeneous, and the volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0136] Furthermore, the area fraction of the remaining structure is determined by subtracting the area fraction of tempered martensite, the total area fraction of ferrite and bainitic ferrite, and the area fraction of retained austenite determined as described above from 100%.

[0137] [Area fraction of the remaining structure (%)] = 100 - [Area fraction of tempered martensite (%)] - [Area fraction of the total of ferrite and bainitic ferrite (%)] - [Area fraction of retained austenite (%)]

[0138] [Grain boundary density of 20° or more in tempered martensite: 1.0 μm / μm 2 above]

[0139] In the steel sheet according to one embodiment of the present invention, the density of grain boundaries with an angle of 20° or greater in tempered martensite is set to 1.0 μm / μm. 2 The above is extremely important. The high-angle grain boundaries in tempered martensite, especially the grain boundaries with an angle of 20° or more in tempered martensite, become carbon segregation sites during coating and baking, thereby suppressing the fracture of the steel sheet. As a result, the fracture stress during tensile deformation is reduced. Therefore, if the density of grain boundaries with an angle of 20° or more in tempered martensite (hereinafter also referred to as the high-angle grain boundary density of tempered martensite) is less than 1.0 μm / μm 2 Therefore, the high-angle grain boundary density of tempered martensite is set to 1.0μm / μm. 2 The high-angle grain boundary density of tempered martensite is preferably 1.2 μm / μm 2 More preferably, 1.3 μm / μm 2 It should be noted that the upper limit of the high-angle grain boundary density of tempered martensite is not particularly specified. For example, the high-angle grain boundary density of tempered martensite is preferably 3.0 μm / μm 2 the following.

[0140] Here, the high-angle grain boundary density of tempered martensite is obtained, for example, as follows.

[0141] A test piece for microstructure observation was taken from the steel plate. The sampled test piece was then polished using colloidal silica vibration polishing with the rolling direction cross section (L cross section) as the observation surface. The observation surface was mirror-finished. Electron beam backscatter diffraction (EBSD) was then performed at a position 1 / 4 of the thickness of the steel plate (a position corresponding to 1 / 4 of the thickness in the depth direction from the steel plate surface) to obtain local crystal orientation data. The EBSD measurement used a step size of 0.10 μm and a measurement area of ​​50 μm square (50 μm × 50 μm). The obtained local crystal orientation data was then analyzed using the analysis software: OIM Analysis 7. The local crystal orientation data was analyzed for 10 fields of view at a position 1 / 4 of the thickness of the steel plate, and the average value was used. Prior to the analysis of the local crystal orientation data, a cleaning process was performed using the GrainDilation function (GrainTolerance Angle: 5, Minimum GrainSize: 2, Single Iteration: ON) of the analysis software. Next, the grain boundaries with an angle of 20° or greater in the tempered martensite are displayed, and the total length of the grain boundaries with an angle of 20° or greater in the tempered martensite is determined. The high-angle grain boundary density in the tempered martensite is then determined by dividing the total length of the grain boundaries with an angle of 20° or greater in the tempered martensite by the area of ​​the measurement region.

[0142] [KAM(S) / KAM(C)>1.00]

[0143] In the steel sheet according to one embodiment of the present invention, it is extremely important to make KAM(S) / KAM(C) greater than 1.00. Here, KAM(S) is the KAM value at a depth of 100 μm from the surface of the steel sheet, and KAM(C) is the KAM value at the center of the thickness of the steel sheet. The inventors have repeatedly conducted various studies and found that in order to improve the collision characteristics after paint baking, it is effective to change the dislocation distribution state from the surface to the inside of the steel sheet. Furthermore, in particular, by making KAM(S) / KAM(C) greater than 1.00, excellent collision characteristics after paint baking can be obtained. Therefore, KAM(S) / KAM(C) is set to greater than 1.00. KAM(S) / KAM(C) is preferably greater than 1.03. It should be noted that there is no particular upper limit for KAM(S) / KAM(C). For example, KAM(S) / KAM(C) is preferably less than 1.110.

[0144] Here, KAM(S) and KAM(C) are obtained, for example, as follows.

[0145] EBSD measurements were performed using the same procedures as for measuring the high-angle grain boundary density of tempered martensite, and the resulting local crystal orientation data were analyzed. The local crystal orientation data were analyzed for 10 fields at a depth of 100 μm from the steel sheet surface and at the center of the steel sheet thickness, and the average values ​​were used. The KAM values ​​of the bcc phase at each location were plotted based on the analysis results of the local crystal orientation data at a depth of 100 μm from the steel sheet surface and at the center of the steel sheet thickness, and the average values ​​were designated as KAM(S) and KAM(C), respectively.

[0146] In addition, the mechanical properties of the steel sheet according to one embodiment of the present invention are as described above.

[0147] In addition, the steel sheet according to one embodiment of the present invention may also have a coating on the surface. The coating may be provided on only one surface of the steel sheet or on both surfaces. The coating is not particularly limited. As the coating, a galvanized layer having Zn as a main component (Zn content is 50.0% by mass or more) can be exemplified. In addition, as the galvanized layer, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electro-galvanized layer can be exemplified. It should be noted that the steel sheet having the galvanized layer may also be referred to as a galvanized steel sheet. In addition, the steel sheets having the above-mentioned hot-dip galvanized layer, alloyed hot-dip galvanized layer, and electro-galvanized layer may also be referred to as hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA), and electro-galvanized steel sheet (EG), respectively.

[0148] Examples of the plating layer other than the zinc plating layer include an aluminum plating layer and an alloy plating layer. Examples of the alloy plating layer include a hot-dip zinc-aluminum-magnesium alloy plating layer and an electroplated Zn-Ni alloy plating layer.

[0149] The coating weight per single side of the plated layer is not particularly limited, but is preferably 20 g / m 2 ~80g / m 2 .

[0150] The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.50 mm to 2.50 mm.

[0151] [2] Parts

[0152] Next, components according to one embodiment of the present invention will be described.

[0153] A component according to one embodiment of the present invention is manufactured using the above-mentioned steel plate (as a blank material). For example, the component is manufactured by subjecting the steel plate as a blank material to at least one of forming and joining.

[0154] The steel plate has a TS of 1320 MPa or higher and exhibits excellent tensile flange formability, toughness after painting and baking, and crash resistance. Therefore, the component according to one embodiment of the present invention is particularly suitable for use as a base material for automotive parts, for example. This can improve fuel efficiency by reducing vehicle weight, significantly contributing to reduced CO2 emissions.

[0155] [3]Method for manufacturing steel plates

[0156] Next, a method for producing a steel sheet according to one embodiment of the present invention will be described.

[0157] A method for manufacturing a steel plate according to one embodiment of the present invention includes the following steps:

[0158] A preparation step of preparing a steel plate having the above-mentioned composition;

[0159] The heating step is to heat the steel plate to an annealing temperature T1 in a temperature range of 700°C to 750°C at an average heating rate of 5.0°C / s or less.

[0160] Annealing step: annealing the steel sheet at an annealing temperature T1 of 800° C. or higher and an annealing time t1 of 10 seconds or longer.

[0161] A bending step, wherein the steel sheet is bent at least once in a temperature range of the annealing temperature T1 to 700° C. using a roller having a radius of 800 mm or less.

[0162] A first cooling step is then performed, wherein the steel plate is cooled to a first cooling end temperature at an average cooling rate of 10°C / s or higher in a temperature range of 700°C to 550°C.

[0163] In a second cooling step, the steel sheet is cooled to a second cooling completion temperature under the conditions that the average cooling rate in the temperature range of 300°C to 100°C is 300°C / s or higher and the tension applied to the steel sheet in the temperature range of 300°C to 100°C is 5 MPa or higher.

[0164] Tempering step: Next, the steel plate is tempered at a tempering temperature T2 of 100°C to 400°C and a tempering time t2 of 10 seconds to 10,000 seconds.

[0165] The correction process then uses a straightening machine to correct the above-mentioned billet steel plate under the conditions of a correction starting temperature of less than 100°C, an inlet side meshing press-in amount of 4.0mm~10.0mm, an outlet side meshing press-in amount of 1.0mm~10.0mm, an inlet side tension of 20MPa~500MPa, and an outlet side tension of 25MPa~550MPa.

[0166] It should be noted that, unless otherwise specified, the above temperatures refer to the surface temperature of the steel plate. In addition, unless otherwise specified, the average heating rate and average cooling rate are also based on the surface temperature of the steel plate.

[0167] Preparation process

[0168] First, a steel plate having the above-mentioned composition is prepared. For example, the steel plate can be prepared by hot-rolling a steel slab to form a hot-rolled steel plate, optionally subjecting the hot-rolled steel plate to pickling and heat treatment, and then cold-rolling the steel plate. The conditions of these steps are not particularly limited and can be prepared using conventional methods.

[0169] For example, as for the melting method of the steel slab (steel billet), any known melting method such as a converter or an electric furnace is suitable. In order to prevent macrosegregation, the steel slab is preferably melted by a continuous casting method.

[0170] Hot rolling includes a method of heating the steel billet and then rolling it, a method of directly rolling the steel billet after continuous casting without heating it, a method of subjecting the steel billet after continuous casting to a short-time heating treatment and then rolling it, etc. In addition, the slab heating temperature, slab soaking time and coiling temperature in hot rolling are not particularly limited. The slab heating temperature is preferably above 1100°C. The slab heating temperature is preferably below 1300°C. The slab soaking time is preferably above 30 minutes. The slab soaking time is preferably below 250 minutes. The finishing temperature is preferably above the Ar3 transformation point. The coiling temperature is preferably above 350°C. The coiling temperature is preferably below 650°C. It should be noted that the Ar3 transformation point is obtained by the following formula.

[0171] Ar3 phase transition point (°C) = 868 - 396 × [% C] + 24.6 × [% Si] - 68.1 × [% Mn] - 36.1 × [% Ni] - 20.7 × [% Cu] - 24.8 × [% Cr]

[0172] In addition, the [% element symbol] in the above formula represents the content (mass %) of the element in the above component composition.

[0173] Pickling removes oxides from the surface of the hot-rolled steel sheet and is preferably performed to ensure good chemical conversion and coating quality in the final product. Pickling can be performed once or multiple times. Furthermore, the hot-rolled steel sheet can be heat treated after pickling.

[0174] The total reduction ratio in cold rolling is preferably 30% or more. The total reduction ratio in cold rolling is preferably 80% or less. It should be noted that the number of rolling passes and the reduction ratio in each pass are not particularly limited, and the specified effect can be achieved.

[0175] Heating process

[0176] Next, the steel plate prepared in the preparation step is heated to an annealing temperature T1 at an average heating rate of 5.0°C / s or less in a temperature range of 700°C to 750°C. The annealing temperature T1 will be described in the annealing step described below.

[0177] [Average heating rate in the temperature range of 700°C to 750°C: 5.0°C / s or less]

[0178] The inventors and others have repeatedly conducted in-depth research and found that the average heating rate in the temperature range of 700°C to 750°C (hereinafter referred to as the average heating rate) affects the high-angle grain boundary density of tempered martensite. That is, by setting the average heating rate to less than 5.0°C / s, the dissolution of carbides is promoted. As a result, the original austenite grain boundaries are refined, and the grain boundaries above 20° after the martensite transformation increase. Therefore, the high-angle grain boundary density of the tempered martensite in the steel plate of the final product is also increased, and the collision characteristics after coating and baking are improved. Therefore, the average heating rate is set to less than 5.0°C / s. The average heating rate is preferably less than 3.0°C / s. There is no special upper limit for the average heating rate. For example, the average heating rate is preferably greater than 0.1°C / s.

[0179] Annealing process

[0180] Next, the steel sheet is annealed at an annealing temperature T1 of 800° C. or higher and an annealing time t1 of 10 seconds or longer.

[0181] [Annealing temperature T1: 800°C or above]

[0182] When the annealing temperature T1 is less than 800°C, the combined area fraction of ferrite and bainitic ferrite is 5% or more, making it difficult to achieve a TS of 1320 MPa or more. In addition, it is also difficult to achieve excellent stretch flangeability. Therefore, the annealing temperature T1 is set to 800°C or more. The annealing temperature T1 is preferably 820°C or more. There is no particular upper limit for the annealing temperature T1. For example, the annealing temperature T1 is preferably 1000°C or less. The annealing temperature referred to here is the holding temperature in the annealing process. It should be noted that the annealing temperature can also be constant during the holding process. In addition, if the annealing temperature is in a temperature range of 800°C or more and the temperature fluctuation is within ±10°C of the set temperature, it may not always be constant during the holding process.

[0183] [Annealing time t1: 10 seconds or more]

[0184] If the annealing time t1 is less than 10 seconds, the combined area fraction of ferrite and bainitic ferrite exceeds 5%, making it difficult to achieve a TS of 1320 MPa or higher. Furthermore, achieving excellent stretch flange formability is difficult. Therefore, the annealing time t1 is set to 10 seconds or longer. The annealing time t1 is preferably 30 seconds or longer. There is no particular upper limit for the annealing time t1. For example, the annealing time t1 is preferably 1000 seconds or less. Note that the annealing time t1 referred to herein refers to the holding time at the annealing temperature T1.

[0185] Bending process

[0186] Next, the steel sheet is bent at least once in a temperature range from the annealing temperature T1 to 700° C. using a roll having a radius of 800 mm or less.

[0187] [Number of bending processes using a roller with a radius of 800 mm or less in the temperature range of annealing temperature T1 to 700°C: one or more]

[0188] The inventors and others have repeatedly conducted in-depth research and found that bending in the temperature range of annealing temperature T1 to 700°C (hereinafter also referred to as the bending temperature range) affects the density of high-angle grain boundaries of tempered martensite. In particular, if bending is performed using a roller with a radius of 800 mm or less in the bending temperature range, the nucleation of martensitic transformation is promoted. As a result, the martensite is refined, and the number of grain boundaries above 20° after the martensitic transformation increases. Therefore, the density of high-angle grain boundaries of tempered martensite in the steel plate of the final product is also increased, and the collision characteristics after coating and baking are improved. Therefore, the number of bending processes using a roller with a radius of 800 mm or less in the bending temperature range (hereinafter also referred to as the number of bending times) is set to more than one.

[0189] It should be noted that the radius of the roller used in the bending process is preferably 600 mm or less. The lower limit of the radius of the roller used in the bending process is not particularly limited. For example, the radius of the roller used in the bending process is preferably 100 mm or more.

[0190] In addition, the number of bending times may be more than one. The number of bending times is preferably more than two times. There is no particular upper limit to the number of bending times. For example, the number of bending times is preferably 15 times or less. It should be noted that the bending process can also be performed by bending in one direction using a roller and then bending back in the opposite direction by the same amount. In this case, the number of bending times is counted as two times (bending once, bending back once). In addition, the bending angle is preferably 80 to 110°. As a result, the effect of promoting the nucleation of martensitic transformation becomes greater. It should be noted that the bending angle is set to the angle (acute angle) formed by the through-plate direction of the steel plate on the roller inlet side and the through-plate direction of the steel plate on the roller outlet side.

[0191] In addition, if bending using a roller with a radius of 800 mm or less in the bending temperature range is performed once or more, further bending that does not satisfy this condition may be performed.

[0192] ·1st cooling process

[0193] Next, the steel plate is cooled to a first cooling completion temperature under the condition that the average cooling rate in the temperature range of 700° C. to 550° C. is 10° C. / s or higher.

[0194] [Average cooling rate in the temperature range of 700°C to 550°C: 10°C / s or more]

[0195] When the average cooling rate in the temperature range of 700°C to 550°C (hereinafter also referred to as the first average cooling rate) is less than 10°C / s, the combined area fraction of ferrite and bainitic ferrite is 5% or more, making it difficult to achieve a TS of 1320 MPa or more. In addition, it is also difficult to achieve excellent stretch flangeability. Therefore, the first average cooling rate is set to 10°C / s or more. The first average cooling rate is preferably 30°C / s or more. There is no particular upper limit on the first average cooling rate. For example, the first average cooling rate is preferably 2000°C / s or less.

[0196] The first cooling end temperature may be, for example, 550° C. to 300° C. The base steel sheet may be plated between the first cooling step and the second cooling step described below. The plated treatment will be described below.

[0197] · Second cooling process

[0198] Next, the steel plate is cooled to a second cooling completion temperature under the conditions that the average cooling rate in the temperature range of 300°C to 100°C is 300°C / s or higher and the tension applied to the steel plate in the temperature range of 300°C to 100°C is 5 MPa or higher.

[0199] [Average cooling rate in the temperature range of 300°C to 100°C: 300°C / s or more]

[0200] When the average cooling rate in the temperature range of 300°C to 100°C (hereinafter referred to as the second average cooling rate) is less than 300°C / s, the area fraction of retained austenite is 3% or more, making it difficult to achieve excellent toughness after baking. Therefore, the second average cooling rate is set to 300°C / s or more. The second average cooling rate is preferably 800°C / s or more. There is no particular upper limit for the second average cooling rate. For example, the second average cooling rate is preferably 2000°C / s or less.

[0201] [Tension applied to the steel plate in the temperature range of 300°C to 100°C: 5 MPa or more]

[0202] The inventors of the present invention have repeatedly conducted in-depth research and found that the tension applied to the steel sheet when cooled to a temperature range of 300°C to 100°C affects the density of high-angle grain boundaries of tempered martensite. In particular, if the tension applied to the steel sheet in the temperature range of 300°C to 100°C (hereinafter referred to as the applied tension) is 5 MPa or more, martensitic transformation is promoted. As a result, the martensite is refined and the number of grain boundaries with an angle of 20° or more after the martensitic transformation increases. Therefore, the density of high-angle grain boundaries of tempered martensite in the steel sheet of the final product is also increased, and the collision characteristics after coating and baking are improved. Therefore, the applied tension is set to 5 MPa or more. The applied tension is preferably 10 MPa or more. There is no particular upper limit for the applied tension. For example, the applied tension is preferably 100 MPa or less.

[0203] In addition, the second cooling completion temperature may be, for example, lower than 100° C. The second cooling completion temperature may be, for example, approximately room temperature.

[0204] It should be noted that the bending process in the bending step increases the number of nucleation sites that serve as the starting points for martensitic transformation. Meanwhile, the application of tension in the second cooling step promotes the martensitic transformation itself. In other words, the effects achieved by the two processes are different.

[0205] Tempering process

[0206] Next, the steel plate is tempered under the conditions of a tempering temperature T2 of 100° C. to 400° C. and a tempering time t2 of 10 seconds to 10,000 seconds.

[0207] [Tempering temperature T2: 100℃~400℃]

[0208] Tempered martensite is produced by tempering martensite through tempering. Here, if the tempering temperature T2 is less than 100°C, the martensite is not fully tempered, and the structure becomes mainly quenched martensite. This structure mainly quenched martensite cannot obtain excellent toughness after coating and baking. On the other hand, if the tempering temperature T2 exceeds 400°C, the tempering of martensite proceeds excessively, making it difficult to achieve a TS of 1320 MPa or more. Therefore, the tempering temperature T2 is set to 100°C to 400°C. The tempering temperature T2 is preferably above 150°C. The tempering temperature T2 is preferably below 350°C. The tempering temperature mentioned here is the holding temperature in the tempering process. The tempering temperature can also be constant during the holding process. In addition, if the tempering temperature is in the temperature range of 100°C to 400°C and the temperature fluctuation is within ±10°C of the set temperature, it does not always need to be constant during the holding process.

[0209] [Tempering time t2: 10 seconds to 10,000 seconds]

[0210] As mentioned above, tempered martensite is produced by tempering martensite through a tempering treatment. Here, if the tempering time t2 is less than 10 seconds, the martensite is not fully tempered, and the structure becomes mainly quenched martensite. This structure mainly quenched martensite cannot achieve excellent toughness after coating and baking. On the other hand, if the tempering time t2 exceeds 10,000 seconds, the tempering of the martensite proceeds excessively, making it difficult to achieve a TS of 1320 MPa or more. Therefore, the tempering time t2 is set to 10 seconds to 10,000 seconds. The tempering time t2 is preferably 50 seconds or more. The tempering time t2 is preferably 5000 seconds or less. It should be noted that the tempering time t2 mentioned here refers to the holding time at the tempering temperature T2.

[0211] It should be noted that there are no particular regulations for cooling after tempering. For example, cooling can be performed by any conventional method. The cooling completion temperature after tempering can be, for example, around room temperature. Furthermore, the steel sheet may be plated between the tempering step and the straightening step described below. The plating treatment will be described below.

[0212] Correction process

[0213] Next, the steel plate is straightened by leveling. In this case, in the method for producing a steel plate according to one embodiment of the present invention, it is extremely important to satisfy the following conditions.

[0214] Correction start temperature: below 100℃

[0215] Inlet side engagement press-in amount: 4.0mm~10.0mm

[0216] Outlet side engagement press-in amount: 1.0mm~10.0mm

[0217] Inlet side tension: 20MPa~500MPa

[0218] Outlet side tension: 25MPa~550MPa

[0219] [Correction start temperature: 100°C or below]

[0220] If the straightening start temperature exceeds 100°C, the steel sheet softens. As a result, the amount of strain introduced into the surface and center of the steel sheet by the straightening machine changes, and KAM(S) / KAM(C) becomes less than 1.00. As a result, the collision characteristics after the paint baking are reduced. Therefore, the straightening start temperature is set to 100°C or less. The straightening start temperature is preferably 80°C or less. There is no specific lower limit for the straightening start temperature. For example, the straightening start temperature is preferably above -10°C.

[0221] [Entry side engagement press-in amount: 4.0mm to 10.0mm]

[0222] If the entry-side meshing press-in amount is less than 4.0 mm, the machining amount is insufficient. Consequently, KAM(S) / KAM(C) falls below 1.00, and the post-paint bake crash characteristics deteriorate. It should be noted that due to production technology constraints, the upper limit of the entry-side meshing press-in amount is set to 10.0 mm or less. Therefore, the entry-side meshing press-in amount is set to 4.0 mm to 10.0 mm. The entry-side meshing press-in amount is preferably at least 5.0 mm.

[0223] [Outlet side engagement press-in amount: 1.0mm to 10.0mm]

[0224] If the outlet-side meshing press-in amount is less than 1.0mm, the machining amount is insufficient. Consequently, KAM(S) / KAM(C) falls below 1.00, and post-paint bake crash characteristics deteriorate. Note that due to production technology constraints, the upper limit of the outlet-side meshing press-in amount is set to 10.0mm or less. Therefore, the outlet-side meshing press-in amount is set to 1.0mm to 10.0mm.

[0225] Here, as Figure 1 As shown in the figure, the entry side meshing press-in amount refers to the second roller ( Figure 1 The amount of pressure that the roller 2 in the straightening machine presses into the steel plate plane (the steel plate surface on the side where the roller 2 is located). In addition, the outlet side engagement pressure is the amount of pressure that the second roller ( Figure 1 The amount of penetration of the middle roller 8) into the steel plate plane (the steel plate surface on the side where the roller 8 is located). It should be noted that the number of rollers in the straightening machine process (straightening machine) is not particularly limited, for example, the number of rollers is preferably 5 or more.

[0226] [Entry side tension: 20MPa~500MPa]

[0227] If the inlet tension is less than 20 MPa, the processing yield is insufficient. Consequently, KAM(S) / KAM(C) falls below 1.00, and post-paint baking crash characteristics deteriorate. Due to production technology constraints, the upper limit of the inlet tension is set at 500 MPa or less. Therefore, the inlet tension is set between 20 MPa and 500 MPa. The inlet tension is preferably at least 100 MPa.

[0228] [Export side tension: 25MPa~550MPa]

[0229] Due to equipment limitations in the straightening machine, the exit tension becomes higher than the entry tension. Here, if the exit meshing press is less than 25 MPa, the machining volume is insufficient. Consequently, KAM(S) / KAM(C) falls below 1.00, and the post-paint bake impact properties degrade. It should be noted that due to production technology limitations, the upper limit of the exit tension is set at 550 MPa. Therefore, the exit tension is set between 25 MPa and 550 MPa. The exit tension is preferably at least 100 MPa.

[0230] ·Plating process

[0231] In addition, the steel sheet may be subjected to any plating treatment. The plating treatment is not particularly limited. Examples of plating treatments include hot-dip galvanizing, alloyed hot-dip galvanizing, and electrogalvanizing. In addition, examples of plating treatments other than galvanizing include aluminum plating and alloy plating. Examples of alloy plating include hot-dip zinc-aluminum-magnesium alloy plating and electroplating Zn-Ni alloy plating. The treatment conditions may be in accordance with conventional methods. It should be noted that, as described above, the plating treatment is preferably performed between the first cooling step and the second cooling step or between the tempering step and the straightening step. For example, hot-dip galvanizing and alloyed hot-dip galvanizing are preferably performed between the first cooling step and the second cooling step. In addition, electrogalvanizing and electroplating Zn-Ni alloy plating are preferably performed between the tempering step and the straightening step.

[0232] In the case of hot-dip galvanizing and alloyed hot-dip galvanizing, it is preferable to carry out the series of treatments, including the heating step, annealing step, and plating step, on a hot-dip galvanizing line (CGL) from the perspective of productivity. Wiping can be performed after hot-dip galvanizing to adjust the coating amount.

[0233] Conditions other than those described above are not particularly limited and may be performed according to conventional methods. The method for producing a steel plate according to one embodiment of the present invention described above can produce a steel plate having a TS of 1320 MPa or higher, excellent stretch flangeability, toughness after paint baking, and excellent crash properties. The resulting steel plate is suitable, for example, for use as a base material for automotive parts.

[0234] [4] Component manufacturing method

[0235] Next, a method for manufacturing a component according to one embodiment of the present invention will be described.

[0236] A component manufacturing method according to one embodiment of the present invention includes a step of subjecting the steel sheet to at least one of forming and joining to produce the component.

[0237] The forming method is not particularly limited, and for example, conventional forming methods such as stamping can be used. The joining method is also not particularly limited, and for example, conventional welding methods such as spot welding, laser welding, and arc welding, as well as riveting and clinching can be used. The forming and joining conditions are not particularly limited, and conventional methods can be used.

[0238] Example

[0239] Steel having the composition shown in Table 1 (the remainder being Fe and inevitable impurities) was melted in a converter and formed into steel slabs by continuous casting. The slabs were then heated. The slabs were then hot-rolled to form hot-rolled steel sheets. The hot-rolled steel sheets were then pickled. The hot-rolled steel sheets were then cold-rolled to form cold-rolled steel sheets. This produced steel slabs. The prepared steel slabs were then subjected to a heating process, annealing, bending, first cooling, second cooling, tempering, and straightening process under the conditions shown in Table 2 to obtain steel slabs (thickness: 0.6-2.2 mm) as final products. The first cooling finish temperature was set to 550°C to 300°C. The second cooling finish temperature and the cooling finish temperature after tempering were both set to room temperature. The bending angle during bending was set to 80-110°. Some of the steel slabs (the GI, GA, and EG steel slabs listed in the Type column of Table 2) were plated. The GI and GA steel sheets listed in the Type column of Table 2 were plated between the first and second cooling steps. Furthermore, the EG steel sheets listed in the Type column of Table 2 were plated between the tempering and straightening steps. Conditions not explicitly stated were followed by conventional methods.

[0240] The steel sheets thus obtained were used to determine the area fraction of tempered martensite, the area fraction of retained austenite, the combined area fraction of ferrite and bainitic ferrite, the high-angle grain boundary density of tempered martensite, and KAM(S) / KAM(C) according to the above-described procedures. The results are shown in Table 3.

[0241] In addition, various evaluations were performed according to the following procedures. The evaluation results are also recorded in Table 3.

[0242] (TS's evaluation)

[0243] From the obtained steel plate, a JIS No. 5 test piece (mark distance: 50 mm, parallel portion width: 25 mm) was taken with the longitudinal direction of the test piece perpendicular to the rolling direction of the steel plate. Then, a tensile test was performed using the taken test piece in accordance with JIS Z2241:2022 to measure TS. The conditions were set at a crosshead speed of 1.67 × 10 －1 mm / sec. Then, evaluation was performed according to the following criteria.

[0244] Good (qualified, excellent): TS is 1320MPa or above

[0245] Bad (unqualified): TS less than 1320MPa

[0246] (Evaluation of Stretch Flange Formability)

[0247] The evaluation of tensile flangeability is carried out by a hole expansion test in accordance with JIS Z 2256:2020. That is, the obtained steel plate is cut into 100mm×100mm, and a test piece is taken. Then, a hole with a diameter of 10mm is punched out of the test piece with a gap of 12.5%. Then, a die with an inner diameter of 75mm is used to press the test piece with a pressing force of 9ton (88.26kN). Then, in this state, a conical punch with a top angle of 60° is pressed into the hole of the test piece, and the diameter of the hole of the test piece at the crack generation limit (when the crack occurs) is measured. Then, the limit hole expansion rate λ is calculated by the following formula.

[0248] λ(%)={(D f -D0) / D0}×100

[0249] here,

[0250] D f : Diameter of the hole of the test piece at the crack initiation limit (when cracks occur) (mm)

[0251] D0: initial diameter of the hole of the test piece (mm).

[0252] Then, stretch flangeability was evaluated according to the following criteria.

[0253] Good (qualified, excellent): λ is 30% or more

[0254] Defective (unqualified): λ less than 30%

[0255] (Evaluation of toughness after coating baking)

[0256] Multiple resulting steel plates were stacked and bolted together. After confirming that there were no gaps between the steel plates, a V-notch with a depth of 2 mm was applied to the stacked steel plates to produce stacked Charpy test specimens (hereinafter referred to as test specimens). The number of stacked steel plates was set to the number closest to 10 mm in thickness (if there were two closest to 10 mm, the smaller number was used). For example, if the steel plate thickness was 1.2 mm, eight plates were stacked. This resulted in a test specimen thickness of 9.6 mm. Furthermore, the test specimens were prepared with the width of the steel plates oriented along the longitudinal direction of the test specimens. The prepared test specimens were then aged at 170°C for 20 minutes. Charpy impact tests were then performed on the aged test specimens within a test temperature range of -120°C to +120°C. A transition curve was then calculated based on the resulting brittle fracture ratio, with the temperature at which the brittle fracture ratio reached 50% being the brittle-ductile transition temperature. The toughness after the paint baking was evaluated according to the following criteria. It should be noted that the conditions other than those described above were in accordance with JIS Z2242:2018.

[0257] Excellent (qualified, particularly excellent): The brittle-ductile transition temperature after aging treatment is below -60°C

[0258] Good (qualified, excellent): The brittle-ductile transition temperature after aging treatment is below -40°C (except excellent)

[0259] Bad (unqualified): The brittle-ductile transition temperature after aging treatment is greater than -40°C

[0260] (Crash characteristics after paint baking)

[0261] The obtained steel plate was subjected to an aging treatment at a temperature of 170°C and a treatment time of 20 minutes. Next, a JIS No. 5 test piece (mark distance: 50 mm, parallel portion width: 25 mm) was taken from the aged steel plate with the direction perpendicular to the rolling direction of the steel plate as the long side direction of the test piece. Next, using the taken test piece, a tensile test in accordance with JIS Z 2241:2022 was performed in the same manner as the above-mentioned TS evaluation to measure the TS, YS (yield stress) and fracture stress after aging treatment. Then, the impact characteristics after paint baking were evaluated according to the following criteria.

[0262] Good (qualified, excellent): YR after aging treatment is 0.85 or higher and the fracture stress ratio after aging treatment is 0.90 or lower

[0263] Bad (unqualified): does not meet at least one of the following conditions: YR after aging treatment is 0.85 or higher and the fracture stress ratio after aging treatment is 0.90 or lower.

[0264] In addition, YR and fracture stress ratio after aging treatment were respectively obtained by the following formulae.

[0265] [YR after aging treatment] = [YS after aging treatment] / [TS after aging treatment]

[0266] [Ratio of fracture stress after aging treatment] = [Rupture stress after aging treatment] / [TS after aging treatment]

[0267] The breaking stress refers to the stress at the breaking point in the above-mentioned tensile test (the stress applied when the test piece breaks).

[0268] [Table 1

[0269]

[0270] Underlined areas: indicate areas outside the scope of the present invention.

[0271]

[0272]

[0273]

[0274]

[0275]

[0276] As shown in Table 3, all inventive examples passed the TS, stretch-flange formability, toughness after paint bake, and crash characteristics standards. Furthermore, parts formed or joined using the inventive example steel sheets exhibited no cracking, maintained the target shape, and met acceptable standards for TS, stretch-flange formability, toughness after paint bake, and crash characteristics.

[0277] On the other hand, the comparative example failed to meet the standards for at least one of TS, stretch flange formability, toughness after paint baking, and collision characteristics.

Claims

1. A steel plate having the following composition and structure: The composition comprises, by mass%, 0.030% to 0.500% C, 0.010% to 2.500% Si, 0.10% to 5.00% Mn, 0.100% or less P, 0.0200% or less S, 0.0100% or less N, 0.0100% or less O, and 1.000% or less Al, with the remainder being Fe and unavoidable impurities. In the structure, the area fraction of tempered martensite is 95% or more, the area fraction of retained austenite is less than 3%, the combined area fraction of ferrite and bainitic ferrite is less than 5%, and the density of grain boundaries at 20° or greater in the tempered martensite is 1.0 μm / μm 2 Above, and satisfying the following formula (1), KAM(S) / KAM(C)>1.00·····(1) Where, KAM(S): The average KAM value at a depth of 100 μm from the steel plate surface. KAM(C): The average KAM value at the center of the steel plate thickness.

2. The steel plate according to claim 1, wherein The above-mentioned component composition further contains, in terms of mass%, at least one 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.

3. The steel plate according to claim 1, wherein The surface is plated.

4. The steel plate according to claim 2, wherein The surface is plated. 5 . A component formed using the steel plate according to claim 1 .

6. A method for manufacturing a steel plate, comprising the following steps: a preparation step of preparing a steel plate having the composition of claim 1 or 2; The heating step then heats the steel plate to an annealing temperature T1 in a temperature range of 700° C. to 750° C. at an average heating rate of 5.0° C. / s or less; Annealing step: annealing the steel plate at an annealing temperature T1 of 800° C. or higher and an annealing time t1 of 10 seconds or longer. A bending process, wherein the steel plate is bent at least once in a temperature range of the annealing temperature T1 to 700° C. using a roller having a radius of 800 mm or less. a first cooling step, wherein the steel plate is cooled to a first cooling end temperature at an average cooling rate of 10°C / s or higher in a temperature range of 700°C to 550°C; In a second cooling step, the steel sheet is cooled to a second cooling end temperature under the conditions that the average cooling rate in the temperature range of 300°C to 100°C is 300°C / s or higher and the tension applied to the steel sheet in the temperature range of 300°C to 100°C is 5 MPa or higher. Tempering step: Next, the steel plate is tempered at a tempering temperature T2 of 100° C. to 400° C. and a tempering time t2 of 10 seconds to 10,000 seconds. The straightening process then involves straightening the steel plate using a straightening machine under the following conditions: a straightening start temperature of 100°C or less, an inlet side meshing press-in amount of 4.0mm to 10.0mm, an outlet side meshing press-in amount of 1.0mm to 10.0mm, an inlet side tension of 20MPa to 500MPa, and an outlet side tension of 25MPa to 550MPa.

7. The method for manufacturing a steel plate according to claim 6, wherein: A plating step of plating the base steel plate is further provided between the first cooling step and the second cooling step or between the tempering step and the straightening step. 8 . A method for manufacturing a component, comprising the step of subjecting the steel sheet according to claim 1 to at least one of forming and joining to produce the component.

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