Steel sheet and component and method for manufacturing same

Abstract

Provided is a steel sheet having a TS of 1180 MPa or more, a high EL, and excellent toughness and impact characteristics after coating and baking. The steel sheet has a prescribed component composition and a structure in which the area fraction of tempered martensite is 83% or more, the area fraction of retained austenite is less than 3%, the total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%, 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 increased. Furthermore, new laws and regulations are being introduced. Consequently, the use of steel sheets with a tensile strength (TS) of 1180 MPa or higher in key automotive structural components is increasing.

[0003] Furthermore, from the viewpoint of press formability, steel sheets used as blank materials for automobile parts, for example, automobile frame parts such as bumpers, are often required to have high elongation (hereinafter also referred to as EL).

[0004] As a steel sheet for a base material of automobile parts, for example, Patent Document 1 discloses “a steel sheet having the following composition and steel structure, wherein the composition contains, in mass %, C: 0.12% to 0.40%, Si: 0.01% to 1.5%, Mn: greater than 1.7% and 3.5% or less, P: 0.05% or less, S: 0.010% or less, sol. Al: 1.00% or less, N: 0.010% or less, B: 0.0002% to 0.0050%, and a total of one or both of Nb and Ti: 0.010% to 0.080%, with the remainder consisting of Fe and inevitable impurities; wherein the steel structure has 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 per 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: 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 less than 40%.

[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% to 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 having a TS of 1180 MPa or higher, high EL, and excellent toughness and collision properties after paint baking.

[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 1180 MPa or higher, a high EL, and excellent toughness and collision characteristics after paint baking, 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] High EL means an EL of 10% or more. EL is measured by a tensile test in accordance with JIS Z 2241:2022.

[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 by 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: 170°C, 20 minutes. YR after aging treatment and 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 tensile testing 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 a TS of 1180 MPa or higher, it is important to maintain an area fraction of tempered martensite of 83% or higher and a combined area fraction of ferrite and bainitic ferrite of less than 15%. This ensures the required properties while achieving a TS of 1180 MPa or higher.

[0037] (B) To obtain high EL, it is important to set the combined area fraction of ferrite and bainitic ferrite to 5% or more. This ensures the required properties while achieving high EL.

[0038] (C) To obtain excellent toughness after baking, it is important to reduce the area fraction of retained austenite to less than 3% and to reduce the grain boundary density of 20° or greater in tempered martensite to 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] In the above composition, in terms of mass %, C: 0.030% to 0.500%, Si: 0.010% to 2.500%, Mn: 0.10% to 5.00%, P: 0.100% or less, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, and Al: 1.000% or less, with the remainder being Fe and unavoidable impurities.

[0048] In the above structure, the area fraction of tempered martensite is 83% or more, the area fraction of retained austenite is less than 3%, the total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%, 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): The 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 its surface.

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

[0056] 5. A method for manufacturing 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 750° C. to 850° C. and an annealing time t1 of 10 seconds to 1000 seconds.

[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 0.5°C / s or more and less than 10.0°C / s 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 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.

[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 producing 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 sheet having a TS of 1180 MPa or higher, high EL, and excellent toughness and crash resistance after paint baking can be obtained. Application of the steel sheet of the present invention in, for example, automotive parts can improve fuel efficiency through lightweighting of vehicle bodies, significantly contributing to reductions in CO₂ emissions. Therefore, it has significant industrial value. 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 "%".

[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 more than 1180 MPa. 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 combined area fraction of ferrite and bainitic ferrite is less than 5%, and it is difficult to achieve a high EL. 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 1180 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 1180 MPa or more. On the other hand, if the Mn content exceeds 5.00%, the combined area fraction of ferrite and bainitic ferrite is less than 5%, and it is difficult to achieve a high EL. 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 original austenite grain boundaries, embrittles the grain boundaries, and reduces the ultimate deformation capacity of the steel plate. Therefore, if the P content is excessive, it is difficult to achieve excellent toughness after coating 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 particular lower limit for the P content. However, P is a solid solution strengthening element that can increase the strength of the steel plate. 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 deformation capacity of the steel sheet. Therefore, if the S content is excessive, it is difficult to achieve excellent toughness after baking and coating. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0050% or less. It should be noted that 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 deformation capacity of the steel sheet. Therefore, if the N content is excessive, it is difficult to achieve excellent toughness after painting and baking. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0050% or less. It should be noted that 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 deformation capacity of the steel plate. Therefore, if the O content is excessive, it is difficult to achieve excellent toughness after coating and baking. 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 deformation capacity of the steel sheet. Therefore, if the Al content is excessive, it is difficult to achieve excellent toughness after painting and baking. 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 Ti, Nb and V are respectively less than 0.200%, 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 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 improving the strength of the steel plate. Therefore, the contents of Ti, Nb and V are preferably more than 0.001%.

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

[0094] If Ta and W are respectively 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, it will not lead to a decrease in toughness after coating and baking. 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 special 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 preferably more than 0.01%.

[0095] [B: 0.0100% or less]

[0096] If B is 0.0100% or less, no cracks will form inside the steel plate during casting or hot rolling, and the ultimate deformation capacity of the steel plate will not be reduced. Therefore, it will not lead to a decrease in toughness after coating and baking. Therefore, when B is contained, its content is preferably 0.0100% or less. The content of B is more preferably 0.0080% or less. It should be noted that there is no particular lower limit for the content of B. However, B segregates at the austenite grain boundaries during annealing and is an element that improves hardenability. Therefore, the content of B 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 content of Cr, Mo and Ni is 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, it will not lead to a decrease in toughness after coating and baking. Therefore, when containing Cr, Mo and Ni, their content is preferably less than 1.00%. The content of Cr, Mo and Ni is more preferably less than 0.80%. It should be noted that there is no special lower limit for the content of Cr, Mo and Ni. However, Cr, Mo and Ni are elements that improve hardenability. Therefore, the content of Cr, Mo and Ni is preferably more than 0.01%.

[0099] [Co: 0.010% or less]

[0100] If Co is below 0.010%, 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 containing Co, its content is preferably below 0.010%. The content of Co is more preferably below 0.008%. It should be noted that the lower limit of the content of Co is not particularly specified. However, Co is an element that improves hardenability. Therefore, the content of Co is preferably above 0.001%.

[0101] [Cu: 1.00% or less]

[0102] If Cu is below 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 containing Cu, its content is preferably below 1.00%. The content of Cu is more preferably below 0.80%. It should be noted that the lower limit of the content of Cu is not particularly specified. However, Cu is an element that improves hardenability. Therefore, the content of Cu is preferably above 0.01%.

[0103] [Sn: 0.200% or less]

[0104] If Sn is below 0.200%, no cracks will occur 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 containing Sn, its content is preferably below 0.200%. The content of Sn is more preferably below 0.100%. It should be noted that the lower limit of the content of Sn is not particularly specified. However, Sn is an element that improves hardenability and is also an element that improves corrosion resistance. Therefore, the content of Sn is preferably more than 0.001%.

[0105] [Sb: 0.200% or less]

[0106] If Sb is 0.200% 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, it will not lead to a decrease in toughness after coating and baking. 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 particular lower limit for the Sb content. However, Sb is an element that can control the softening thickness of the surface layer and 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 respectively 0.0100% or less, 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 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 deformation capacity of the steel sheet. Therefore, the contents of Ca, Mg, and REM are preferably 0.0005% or more, respectively.

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

[0110] If the Zr and Te contents are each less than 0.100%, 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 Zr and Te are contained, their contents are preferably less than 0.100%. The Zr and Te contents are more preferably less than 0.080%, respectively. It should be noted that there is no particular 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 deformation capacity of the steel plate. Therefore, the Zr and Te contents are each preferably greater than 0.001%.

[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 coating and baking 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 particular 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 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 Bi is contained, 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 particular 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 83%.

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

[0120] The combined area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%,

[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: 83% or more]

[0125] In the steel sheet according to one embodiment of the present invention, it is extremely important to have an area fraction of tempered martensite of 83% or greater. Specifically, by making tempered martensite the primary phase, and in particular by having an area fraction of 83% or greater, a TS of 1180 MPa or greater can be achieved. Therefore, the area fraction of tempered martensite is set to 83% or greater. The area fraction of tempered martensite is preferably 85% or greater, more preferably 87% or greater. There is no particular upper limit on the area fraction of tempered martensite. For example, the area fraction of tempered martensite is preferably less than 95%, more preferably 94% or less, and even more preferably 93% or less.

[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 make 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 coating and baking. One of the reasons for the reduction in toughness after coating and baking is that the retained austenite undergoes a processing-induced martensitic transformation during processing and becomes 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 less than 1%. It should be noted that there is no special lower limit for the area fraction of retained austenite. The area fraction of retained austenite can also be 0%.

[0128] [Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%]

[0129] In a steel sheet according to one embodiment of the present invention, it is extremely important to have a combined area fraction of ferrite and bainitic ferrite of 5% or more and less than 15%. That is, if the combined area fraction of ferrite and bainitic ferrite is 15% or more, it is difficult to achieve a TS of 1180 MPa or more. On the other hand, if the combined area fraction of ferrite and bainitic ferrite is less than 5%, it is difficult to achieve a high EL. Therefore, the combined area fraction of ferrite and bainitic ferrite is set to 5% or more and less than 15%. The combined area fraction of ferrite and bainitic ferrite is preferably 6% or more, more preferably 7% or more. Furthermore, the combined area fraction of ferrite and bainitic ferrite is preferably 14% or less, more preferably 13% or less. It should be noted that ferrite and bainitic ferrite may be contained 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. If these remaining structures are 5% or less, they have no effect on the properties and may be contained. 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] Specifically, a sample was cut from the steel plate, with the cross section through the thickness (L-section) parallel to the rolling direction of the steel plate serving as the observation surface. The observation surface of the sample was then polished. The observation surface was then etched with 3 vol.% Nital to reveal the structure. Ten fields of view were then observed using a SEM at 2000x magnification, located at a position ¼ the thickness of the steel plate (equivalent to a position ¼ the thickness in the depth direction from the steel plate surface). In the observed images, tempered martensite is a structure with fine irregularities and carbides within the structure. Ferrite and bainitic ferrite are structures with flat interiors within the concave portions and no carbides. The area occupied by tempered martensite, ferrite, and bainitic ferrite was then calculated for each field of view. The area occupied by tempered martensite, ferrite, and bainitic ferrite was then divided by the total area of ​​each 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] Note that the structure of a steel plate is generally vertically symmetrical across the thickness. Therefore, any one of the steel plate's surfaces (the light-receiving side and the back side) can be used as the starting point for thickness positions (thickness zero position), such as the 1 / 4 thickness position or 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] Specifically, the steel plate was mechanically ground to a depth of 1 / 4 of the plate thickness minus 0.1 mm, with the observation position set at 1 / 4 of the plate thickness. The steel plate was then further ground by chemical grinding by 0.1 mm. Using the ground surface as the observation surface, an X-ray diffractometer using CoKα radiation was used to determine the ratio of the integrated intensity of the diffraction peaks at the {200}, {220}, and {311} planes of fcc iron (austenite) to the integrated intensity of the diffraction peaks at the {200}, {211}, and {220} planes of bcc iron. The volume fraction of retained austenite was then calculated based on the ratio of the integrated intensities for each plane. The retained austenite was then considered three-dimensionally homogeneous, and the volume fraction of retained austenite was used 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 ferrite and bainitic ferrite (%)] - [Area fraction of retained austenite (%)]

[0138] [Grain boundary density above 20° 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. High-angle grain boundaries in tempered martensite, especially those with an angle of 20° or greater, serve as carbon segregation sites during coating and baking, thereby suppressing 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 greater in tempered martensite (hereinafter 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 than, 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 collected from the steel plate. The sampled test piece was then polished using colloidal silica vibration milling, with the rolling direction cross section (L-section) serving as the observation surface. The observation surface was mirror-finished. Electron beam backscatter diffraction (EBSD) was then measured at the 1 / 4 thickness position (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 at the 1 / 4 thickness position, and the average value was used. Prior to the analysis of the local crystal orientation data, a cleaning process was performed using the analysis software's grain dilation function (grain tolerance angle: 5, minimum grain size: 2, single iteration: ON). 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 have a KAM(S) / KAM(C) ratio greater than 1.00. Here, KAM(S) refers to the KAM value at a depth of 100 μm from the steel sheet surface, and KAM(C) refers to the KAM value at the center of the steel sheet thickness. The inventors conducted extensive research and discovered that changing the dislocation distribution from the surface to the interior of the steel sheet is effective in improving post-baking paint impact properties. Furthermore, achieving a KAM(S) / KAM(C) ratio greater than 1.00 can achieve excellent post-baking paint impact properties. Therefore, KAM(S) / KAM(C) is set to greater than 1.00. KAM(S) / KAM(C) is preferably 1.03 or greater. There is no specific upper limit for KAM(S) / KAM(C). For example, KAM(S) / KAM(C) is preferably 1.110 or less.

[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 plate surface and at the center of the plate 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 plate surface and at the center of the plate thickness. 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 the 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 a steel sheet having a 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 one 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] Components

[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 1180 MPa or higher, high EL, and excellent toughness and crash resistance after baking. 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 reducing 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 750° C. to 850° C. and an annealing time t1 of 10 seconds to 1000 seconds.

[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 0.5°C / s or more and less than 10.0°C / s 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 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.

[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, the smelting method of the steel slab (steel billet) is suitable for any known melting method such as a converter or an electric furnace. 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 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 during cold rolling is preferably 30% or more. The total reduction ratio during cold rolling is preferably 80% or less. It should be noted that the number of rolling passes and the reduction ratio of 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 later.

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

[0178] The inventors conducted repeated and 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 density of high-angle grain boundaries in tempered martensite. In other words, by setting the average heating rate to 5.0°C / s or less, the dissolution of carbides is promoted. As a result, the original austenite grain boundaries are refined, and the number of grain boundaries with an angle of 20° or greater after the martensitic transformation increases. Therefore, the density of high-angle grain boundaries in the tempered martensite in the steel sheet of the final product also increases, and the collision characteristics after coating and baking are improved. Therefore, the average heating rate is set to 5.0°C / s or less. The average heating rate is preferably 3.0°C / s or less. There is no particular upper limit on the average heating rate. For example, the average heating rate is preferably 0.1°C / s or more.

[0179] Annealing process

[0180] Next, the steel sheet is annealed at an annealing temperature T1 of 750° C. to 850° C. and an annealing time t1 of 10 seconds to 1000 seconds.

[0181] [Annealing temperature T1: 750℃~850℃]

[0182] When the annealing temperature T1 is less than 750°C, the combined area fraction of ferrite and bainitic ferrite is 15% or more, making it difficult to achieve a TS of 1180 MPa or more. On the other hand, when the annealing temperature T1 is greater than 850°C, the combined area fraction of ferrite and bainitic ferrite is less than 5%, making it difficult to achieve a high El. Therefore, the annealing temperature T1 is set to 750°C to 850°C. The annealing temperature T1 is preferably greater than 760°C, more preferably greater than 770°C. The annealing temperature T1 is preferably less than 840°C, more preferably less than 830°C. The annealing temperature referred to here is the holding temperature during 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 within the temperature range of 750°C to 850°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.

[0183] [Annealing time t1: 10 seconds to 1000 seconds]

[0184] When annealing time t1 is less than 10 seconds, the combined area fraction of ferrite and bainitic ferrite exceeds 15%, making it difficult to achieve a TS of 1180 MPa or higher. On the other hand, when annealing time t1 exceeds 1000 seconds, the combined area fraction of ferrite and bainitic ferrite falls below 5%, making it difficult to achieve a high EL. Therefore, annealing time t1 is set to 10 to 1000 seconds. Annealing time t1 is preferably 30 seconds or longer. Annealing time t1 is preferably 800 seconds or less. Note that annealing time t1 here refers to the holding time at 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 conducted extensive research and discovered that bending within the temperature range from annealing temperature T1 to 700°C (hereinafter referred to as the bending temperature range) affects the density of high-angle grain boundaries in tempered martensite. In particular, bending within the bending temperature range using rollers with a radius of 800 mm or less promotes nucleation of martensitic transformation. This refines martensite and increases the number of grain boundaries with an angle of 20° or greater after martensitic transformation. Consequently, the density of high-angle grain boundaries in tempered martensite in the final steel sheet increases, improving collision resistance after coating and baking. Therefore, the number of bending operations (hereinafter referred to as the number of bends) using rollers with a radius of 800 mm or less within the bending temperature range is set to one or more.

[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 on 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°. This has a greater effect on promoting the nucleation of martensitic transformation. 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, as long as 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 0.5° C. / s or more and less than 10.0° C. / s.

[0194] [Average cooling rate in the temperature range of 700°C to 550°C: 0.5°C / s or higher and less than 10.0°C / s]

[0195] When the average cooling rate in the temperature range of 700°C to 550°C (hereinafter referred to as the first average cooling rate) is 10.0°C / s or higher, the combined area fraction of ferrite and bainitic ferrite is less than 5%, making it difficult to achieve a high EL. On the other hand, when the first average cooling rate is less than 0.5°C / s, the combined area fraction of ferrite and bainitic ferrite is 15% or higher, making it difficult to achieve a TS of 1180 MPa or higher. Therefore, the first average cooling rate is set to be greater than 0.5°C / s and less than 10.0°C / s. The first average cooling rate is preferably greater than 1.0°C / s. The first average cooling rate is preferably less than 8.0°C / s.

[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] If 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 exceeds 3%, making it difficult to achieve excellent toughness after baking. Therefore, the second average cooling rate is set to 300°C / s or higher. The second average cooling rate is preferably 800°C / s or higher. 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 lower.

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

[0202] The present inventors conducted extensive research and discovered that the tension applied to the steel sheet during cooling to a temperature range of 300°C to 100°C affects the density of high-angle grain boundaries in tempered martensite. In particular, applying a tension of 5 MPa or greater to the steel sheet in the 300°C to 100°C temperature range (hereinafter referred to as the applied tension) promotes martensitic transformation. This refines the martensite and increases the number of grain boundaries with an angle of 20° or greater after martensitic transformation. Consequently, the density of high-angle grain boundaries in the tempered martensite in the final product increases, improving the impact resistance after baking and coating. Therefore, the applied tension is set to 5 MPa or greater. Preferably, the applied tension is 10 MPa or greater. There is no specific 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 1180 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 1180 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. Consequently, the amount of strain introduced into the steel sheet's surface and center during straightening processing changes, causing KAM(S) / KAM(C) to fall below 1.00. Consequently, the impact resistance after paint baking deteriorates. Therefore, the straightening start temperature is set to 100°C or below. It is preferably 80°C or below. There is no specific lower limit for the straightening start temperature. For example, it is preferably -10°C or above.

[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 post-paint bake crash characteristics deteriorate. However, 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 1.0 mm, 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 10.0 mm or less. Therefore, the outlet-side meshing press-in amount is set to 1.0 mm to 10.0 mm.

[0225] Here, as Figure 1 As shown in the figure, the entry side engagement press-in amount refers to the second roller from the entry side in the straightening machine process ( 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 1The amount of penetration of the middle roll 8 into the steel plate plane (the steel plate surface on the side where the roll 8 is located). The number of rolls in the leveling process (leveler) is not particularly limited, but for example, five or more rolls are preferred.

[0226] [Inlet 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-baking impact properties deteriorate. However, due to production 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] [Exit side tension: 25MPa~550MPa]

[0229] Due to equipment limitations in the straightening machine process, the exit tension becomes higher than the entry tension. Here, if the exit meshing press-in amount is less than 25 MPa, the machining volume is insufficient. Consequently, KAM(S) / KAM(C) falls below 1.00, and post-paint bake impact properties degrade. Note 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 perform the series of processes, including the heating, annealing, and plating steps, on a continuous galvanizing line (CGL) from the perspective of productivity. Wiping after hot-dip galvanizing can be performed to adjust the coating weight.

[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 sheet according to one embodiment of the present invention described above can produce a steel sheet having a TS of 1180 MPa or greater, high EL, and excellent toughness and crash resistance after baking. The resulting steel sheet can be suitably used, for example, 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 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 between 550°C and 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 between 80° and 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 procedure. 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. The sampled test piece was then subjected to a tensile test in accordance with JIS Z 2241:2022 to measure TS and EL. The conditions were 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 above 1180MPa

[0245] Bad (unqualified): TS less than 1180MPa

[0246] (EL's evaluation)

[0247] Evaluation was performed based on the EL measured in the above-mentioned tensile test according to the following criteria.

[0248] Good (qualified, excellent): EL is more than 10%

[0249] Defective (unqualified): EL less than 10%

[0250] (Evaluation of toughness after paint baking)

[0251] 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 create stacked Charpy test specimens (hereinafter referred to as test specimens). The number of stacked steel plates was determined based on the number of sheets closest to 10 mm in thickness (if two sheets were closest to 10 mm, the smaller number was used). For example, if the steel plate thickness was 1.2 mm, eight sheets were stacked. This resulted in a test specimen thickness of 9.6 mm. The test specimens were then prepared with the width of the steel plates oriented along the longitudinal direction of the test specimens. These 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 Z 2242:2018.

[0252] Excellent (qualified, particularly excellent): the brittle-ductile transition temperature after aging treatment is below -60°C

[0253] Good (qualified, excellent): The brittle-ductile transition temperature after aging treatment is below -40°C (except excellent)

[0254] Bad (unqualified): The brittle-ductile transition temperature after aging treatment is greater than -40°C

[0255] (Crash characteristics after paint baking)

[0256] The resulting steel sheets were aged at 170°C for 20 minutes. JIS No. 5 test pieces (50 mm spacing, 25 mm parallel width) were then collected from the aged sheets, with the longitudinal direction perpendicular to the rolling direction. These specimens were then subjected to tensile testing in accordance with JIS Z 2241:2022, following the same procedures as for the TS evaluation described above. TS, YS (yield stress), and fracture stress after aging were measured. The impact properties after the paint bake were then evaluated according to the following criteria.

[0257] 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

[0258] Defective (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.

[0259] In addition, YR and fracture stress ratio after aging treatment were respectively obtained by the following formulae.

[0260] [YR after aging treatment] = [YS after aging treatment] / [TS after aging treatment]

[0261] [Ratio of fracture stress after aging treatment] = [Rupture stress after aging treatment] / [TS after aging treatment]

[0262] 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).

[0263]

[0264]

[0265]

[0266]

[0267]

[0268]

[0269]

[0270] As shown in Table 3, all inventive examples met acceptable standards for TS, EL, toughness after paint bake, and crash properties. Furthermore, parts formed or joined using the inventive example steel sheets exhibited no cracking, maintained the desired shape, and met acceptable standards for TS, EL, toughness after paint bake, and crash properties.

[0271] On the other hand, the comparative example failed to meet the standards for at least one of TS, EL, toughness after baking, and collision properties.

Claims

1. A steel plate having the following composition and structure: 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. In the structure, the area fraction of tempered martensite is 83% or more, the area fraction of retained austenite is less than 3%, the combined area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%, and the density of grain boundaries of 20° or more in the tempered martensite is 1.0 μm / μm 2 The above satisfies 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 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. Annealing step: annealing the steel plate at an annealing temperature T1 of 750° C. to 850° C. and an annealing time t1 of 10 seconds to 1000 seconds. 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, then cooling the steel plate to a first cooling end temperature at an average cooling rate of 0.5°C / s or more and less than 10.0°C / s in a temperature range of 700°C to 550°C; a second cooling step, wherein 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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