Steel plates and members, and their manufacturing methods

Abstract

The present invention provides a steel sheet having a TS of 1180 MPa or more, a high EL, and excellent toughness and impact properties after paint baking. The steel sheet has a specified composition, an area fraction of tempered martensite of 83% or more, an area fraction of retained austenite of less than 3%, a total area fraction of ferrite and bainitic ferrite of 5% or more and less than 15%, and a grain boundary density of 20° or more in the tempered martensite of 1.0 μm / μm 2 As described above, the organization satisfies the following formula (1). KAM(S) / KAM(C) > 1.00 ·····(1)

Description

[Technical field]

[0001] The present invention relates to a steel plate, a member made of the steel plate, and a method for manufacturing the same. [Background technology]

[0002] In order to achieve both reduced CO2 emissions by reducing the weight of automobile bodies and improved crashworthiness, efforts are being made to increase the strength of steel sheets used in automobile parts. New legal regulations are also being introduced one after another. As a result, the number of cases in which steel sheets with a tensile strength (hereinafter referred to as TS): of 1180 MPa or more are being used in major automobile structural parts is increasing.

[0003] Moreover, steel sheets used as 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) from the viewpoint of press formability.

[0004] For example, Patent Document 1 describes a steel sheet used as a material for automobile parts as follows: "By mass%, C: 0.12% or more and 0.40% or less, Si: 0.01% or more and 1.5% or less, Mn: over 1.7% and up to 3.5% P: 0.05% or less, S: 0.010% or less, Sol.Al: 1.00% or less, N: 0.010% or less, B: 0.0002% or more and 0.0050% or less, and A composition containing one or two of Nb and Ti in a total amount of 0.010% or more and 0.080% or less, with the balance being Fe and unavoidable impurities; A steel structure having an area ratio of martensite of 70% or more, an area ratio of bainite of 30% or less, and a total area ratio of ferrite and retained austenite of 10% or less, The density of carbides with a major axis of 0.5 μm or more at the 1 / 4 position of the steel plate thickness is 60,000 pieces / mm 2is as follows: The number density of inclusion particles with a circular equivalent diameter of 4.0 μm or more in the range of 1 / 4 to 3 / 4 of the plate thickness of the steel plate is 10 particles / mm 2 More than 30 pieces / mm 2 is as follows: The density of inclusion particles with a circular equivalent diameter of 4.0 μm or more in the area from the surface of the steel plate to 1 / 4 of the plate thickness was 27 particles / mm 2 is as follows: Steel plate with a tensile strength of 1310 MPa or more. has been disclosed.

[0005] In Patent Document 2, "By mass%, C: 0.05% to 0.40%, Si: 0.05% to 3.0%, Mn: 1.5%~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%~0.005%, Ca: 0.000%~0.005%, Ce: 0.000%~0.005%, and La: 0.000%~0.005%, and further comprising Nb: 0.0002%~0.04%, Ti: 0.0002% to 0.08%, and V and Ta: 0.01% to 0.3% in total, Contains one or more selected from the group consisting of The balance: Fe and impurities, The chemical composition is represented by In area%, First martensite having two or more iron carbides with a circle equivalent diameter of 2 nm to 500 nm in the lath: 20% to 95%; Ferrite: 15% or less, Retained austenite: 15% or less, and The balance: bainite or second martensite having less than two iron carbides with a circle equivalent diameter of 2 nm to 500 nm in the lath, or both of these. The steel structure is represented by ND / / <111> Grain orientation and ND / / <100> The total area fraction of oriented grains is 40% or less; The amount of solute C is 0.44 ppm or more, The above ND / / <111> Oriented grains have a crystal orientation parallel to the normal direction of the plate surface. <111> The grains are oriented with a deviation of 10° or less from the normal direction. The above ND / / <100> Oriented grains have a crystal orientation parallel to the normal direction of the plate surface. <100> A steel sheet characterized in that the crystal grains have a crystal orientation that deviates from the direction by 10° or less. has been disclosed.

[0006] In Patent Document 3, "By mass%, C: 0.09% or more and 0.37% or less, Si: more than 0.70% and less than 2.00%, Mn: 2.60% or more and 3.60% or less, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% to 1.000% and N: 0.0100% or less, the balance being Fe and unavoidable impurities; The area fraction of martensite having a carbon concentration greater than 0.7×[%C] and less than 1.5×[%C] is 55% or more, The area ratio of tempered martensite having a carbon concentration of 0.7 × [%C] or less is 5% or more and 40% or less, The ratio of the carbon concentration in the retained austenite to the volume fraction of the retained austenite is 0.05 or more and 0.40 or less, The martensite and the tempered martensite have an average grain size of 5.3 μm or less, The steel structure further has a softened surface thickness of 10 μm or more and 100 μm or less, High-strength steel plate with a tensile strength of 1180MPa or more. In addition, [%C] indicates the content (mass%) of the component element C in the steel. has been disclosed. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 7001197 [Patent Document 2] Patent No. 6497443 [Patent Document 3] Patent No. 6747612 Summary of the Invention [Problem to be solved by the invention]

[0008] Meanwhile, paint baking is often performed on automobile parts. Here, the toughness and impact properties of the steel sheet may change significantly before and after the paint baking. Therefore, in recent years, the steel sheet used as the material for automobile parts is also required to have excellent toughness and impact properties after the paint baking in order to further improve the safety of automobiles.

[0009] However, no consideration is given to the toughness and impact properties after baking of the paint in any of the steel sheets disclosed in Patent Documents 1 to 3. Therefore, there is currently a demand for the development of a steel sheet having a TS of 1180 MPa or more, a high EL, and excellent toughness and impact properties after baking of the paint.

[0010] The present invention has been developed in view of the above-mentioned current situation, and has an object to provide a steel plate having a TS of 1180 MPa or more, a high EL, and further having excellent toughness and impact properties after baking paint, together with an advantageous method for producing the same. Another object of the present invention is to provide a member made of the above-mentioned steel plate and a method for manufacturing the same.

[0011] Here, TS is measured by a tensile test in accordance with JIS Z 2241:2022. High EL means an EL of 10% or more, and is measured by a tensile test in accordance with JIS Z 2241:2022. "Excellent toughness after paint baking" means that the brittle-ductile transition temperature after aging treatment is -40°C or lower. Here, the aging treatment conditions are a treatment temperature of 170°C and a treatment time of 20 minutes. The brittle-ductile transition temperature is measured by a Charpy impact test in accordance with JIS Z 2242:2018. "Excellent impact properties after baking paint" means that the YR after aging treatment is 0.85 or more and the fracture stress ratio after aging treatment is 0.90 or less. Here, the aging treatment conditions are a treatment temperature of 170°C and a treatment time of 20 minutes. The YR after aging treatment and the fracture stress ratio after aging treatment are calculated by the following formulas from the TS, YS (yield stress) and fracture stress after aging treatment measured by a tensile test conforming to JIS Z 2241:2022. [YR after aging treatment] = [YS after aging treatment] / [TS after aging treatment] [Fracture stress ratio after aging treatment] = [Fracture stress after aging treatment] / [TS after aging treatment] Details of the measurement methods are as described in the Examples below. [Means for solving the problem]

[0012] In order to achieve the above object, the inventors have conducted extensive research and have obtained the following findings. (A) In order to obtain a TS of 1180 MPa or more, it is important that the area fraction of tempered martensite is 83% or more and the total area fraction of ferrite and bainitic ferrite is less than 15%. This makes it possible to obtain a TS of 1180 MPa or more while ensuring the required properties. (B) In order to obtain a high EL, it is important that the total area fraction of ferrite and bainitic ferrite is 5% or more. This makes it possible to obtain a high EL while ensuring the required properties. (C) To obtain excellent toughness after paint baking, the area fraction of retained austenite should be less than 3% and the density of grain boundaries of 20° or more in tempered martensite should be 1.0 μm / μm 2 It is important to satisfy the above requirements. This makes it possible to obtain excellent toughness after painting and baking while ensuring the specified required properties. (D) To obtain excellent impact properties after paint baking, the grain boundary density of 20° or more in tempered martensite should be 1.0 μm / μm 2 It is important to satisfy the above and the following formula (1). This makes it possible to obtain excellent crashworthiness after painting and baking while ensuring the specified required characteristics. KAM(S) / KAM(C) > 1.00 ·····(1) During the ceremony, KAM(S): KAM value at a depth of 100 μm from the steel plate surface, KAM(C): KAM value at the center of thickness of steel plate It is. The present invention was completed based on the above findings and through further investigation.

[0013] That is, the gist and configuration of the present invention are as follows. 1. In mass percent, C: 0.030% or more and 0.500% or less, Si: 0.010% or more and 2.500% or less, Mn: 0.10% or more and 5.00% or less, 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 and the remainder being Fe and unavoidable impurities; Area fraction of tempered martensite: 83% or more, Area fraction of retained austenite: less than 3% Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%; and The tempered martensite has a grain boundary density of 1.0 μm / μm or more at 20°. 2 That's all. A steel sheet having a structure that satisfies the following formula (1): KAM(S) / KAM(C) > 1.00 ·····(1) During the ceremony, KAM(S): Average KAM value at a depth of 100 μm from the steel plate surface. KAM(C): Average KAM value at the center of thickness of steel plate It is.

[0014] 2. The composition further comprises, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less and Bi:0.200% or less The steel sheet according to 1 above, comprising at least one selected from the following:

[0015] 3. The steel sheet according to 1 or 2 above, having a plating layer on the surface.

[0016] 4. A member made using the steel plate according to any one of 1 to 3 above.

[0017] 5. A method for producing a steel sheet according to any one of 1 to 3 above, A preparation step of preparing a base steel sheet having the chemical composition according to 1 or 2 above; Next, the base steel sheet is Average heating rate in the temperature range of 700℃~750℃: 5.0℃ / s or less A heating process of heating to an annealing temperature T1 under the above conditions; Next, the base steel sheet is The annealing temperature T1: 750°C or more and 850°C or less; Annealing time t1: 10 seconds or more and 1000 seconds or less An annealing process, Next, the base steel plate is A bending process in which bending is performed at least once using a roll with a radius of 800 mm or less in a temperature range of the annealing temperature T1 to 700 ° C.; Next, the base steel sheet is Average cooling rate in the temperature range of 700℃~550℃: 0.5℃ / s or more and less than 10.0℃ / s A first cooling step of cooling the mixture to a first cooling end temperature under the above condition; Next, the base steel sheet is Average cooling rate in the temperature range of 300℃~100℃: 300℃ / s or more, Tension applied to the base steel plate in the temperature range of 300℃ to 100℃: 5MPa or more A second cooling step of cooling the mixture to a second cooling end temperature under the above condition. Next, the base steel sheet is Tempering temperature T2: 100℃ or more and 400℃ or less, Tempering time t2: 10 seconds or more and 10000 seconds or less A tempering process, Next, the base steel plate is Straightening start temperature: 100℃ or less, Inlet intermesh push-in amount: 4.0mm to 10.0mm Output intermesh push-in amount: 1.0mm to 10.0mm Inlet tension: 20MPa or more and 500MPa or less, Output tension: 25MPa or more and 550MPa or less A straightening process in which straightening is performed by leveling under the above conditions; A manufacturing method for steel plates that:

[0018] 6. The method for producing steel plate as described in 5 above, further comprising a plating process for plating the base steel plate between the first cooling process and the second cooling process, or between the tempering process and the straightening process.

[0019] 7. A method for producing a component, comprising the step of subjecting the steel plate according to any one of 1 to 3 above to at least one of forming and joining to form a component. Effect of the Invention

[0020] According to the present invention, a steel sheet having a TS of 1180 MPa or more, a high EL, and excellent toughness and crash properties after baking paint can be obtained. By applying the steel sheet of the present invention to, for example, a material for automobile parts, it is possible to improve fuel efficiency by reducing the weight of the automobile body, which can greatly contribute to reducing CO2 emissions. Therefore, the industrial utility value is extremely high. [Brief description of the drawings]

[0021] [Figure 1] FIG. 2 is a schematic diagram for explaining the definitions of an inlet intermesh push-in amount and an outlet intermesh push-in amount. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The present invention will be described based on the following embodiments. [1] Steel plate First, the composition of the steel sheet according to one embodiment of the present invention will be described. Note that the unit of the composition is always "mass%", but hereinafter, unless otherwise specified, it will be simply represented as "%".

[0023] [C: 0.030% or more and 0.500% or less] C is one of the important basic components of steel. In particular, in a steel sheet according to an embodiment of the present invention, C is an important element that affects the area fraction of tempered martensite and the impact properties after paint baking. If the C content is less than 0.030%, the area fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. It is also difficult to achieve excellent impact properties after paint baking. On the other hand, if the C content exceeds 0.500%, the total area fraction of ferrite and bainitic ferrite is less than 5%, making it difficult to achieve a high EL. Therefore, the C content is set to 0.030% or more and 0.500% or less. The C content is preferably set to 0.050% or more, more preferably 0.100% or more. The C content is preferably set to 0.400% or less, more preferably 0.350% or less.

[0024] [Si:0.010% or more and 2.500% or less] Si is one of the important basic components of steel. In particular, in the steel sheet according to one embodiment of the present invention, Si suppresses the formation of carbides during annealing and promotes the formation of retained austenite. That is, Si is an important element that affects the area fraction of retained austenite. If 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 paint baking. Therefore, the Si content is set to 0.010% or more and 2.500% or less. 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.

[0025] [Mn: 0.10% or more and 5.00% or less] Mn is one of the important basic components of steel. In particular, in the steel sheet 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 paint baking. If the Mn content is less than 0.10%, the area fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. On the other hand, if the Mn content exceeds 5.00%, the total area fraction of ferrite and bainitic ferrite becomes less than 5%, making it difficult to achieve a high EL. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably 0.50% or more, more preferably 0.80% or more. The Mn content is preferably 4.50% or less, more preferably 4.00% or less.

[0026] [P:0.100% or less] P segregates at prior austenite grain boundaries, embrittling the grain boundaries and reducing the ultimate deformability of the steel sheet. Therefore, if the P content is excessive, it becomes difficult to achieve excellent toughness after paint baking. Therefore, the P content is set to 0.100% or less. The P content is preferably set to 0.070% or less. There is no particular lower limit for the P content. However, P is a solid solution strengthening element and can increase the strength of the steel sheet. Therefore, the P content is preferably set to 0.001% or more.

[0027] [S:0.0200% or less] S exists as sulfides and reduces the ultimate deformability of steel sheet. Therefore, if the S content is excessive, it becomes difficult to achieve excellent toughness after paint baking. Therefore, the S content is set to 0.0200% or less. The S content is preferably set to 0.0050% or less. There is no particular lower limit for the S content. However, due to production technology constraints, the S content is preferably set to 0.0001% or more.

[0028] [N:0.0100% or less] N exists as nitrides and reduces the ultimate deformability of steel sheets. Therefore, if the N content is excessive, it becomes difficult to achieve excellent toughness after paint baking. Therefore, the N content is set to 0.0100% or less. The N content is preferably set to 0.0050% or less. There is no particular lower limit for the N content. However, due to production technology constraints, the N content is preferably set to 0.0001% or more.

[0029] [O:0.0100% or less] O exists as an oxide and reduces the ultimate deformability of the steel sheet. Therefore, if the O content is excessive, it becomes difficult to achieve excellent toughness after paint baking. Therefore, the O content is set to 0.0100% or less. The O content is preferably set to 0.0050% or less. There is no particular lower limit for the O content. However, due to constraints in production technology, the O content is preferably set to 0.0001% or more.

[0030] [Al: 1.000% or less] Al exists as an oxide and reduces the ultimate deformability of the steel sheet. Therefore, if the Al content is excessive, it becomes difficult to achieve excellent toughness after paint baking. Therefore, the Al content is set to 1.000% or less. The Al content is preferably set to 0.500% or less. There is no particular lower limit for the Al content. However, due to constraints in production technology, the Al content is preferably set to 0.001% or more.

[0031] The basic composition of the steel sheet according to one embodiment of the present invention has been described above, but the steel sheet according to one embodiment of the present invention has a composition containing the above basic components, with the balance other than the above basic components including Fe (iron) and unavoidable impurities. Here, it is preferable that the steel sheet according to one embodiment of the present invention has a composition containing the above basic components, with the balance consisting of Fe and unavoidable impurities. In addition to the above basic components, the steel sheet according to one embodiment of the present invention may contain at least one element selected from the following, either alone or in combination, as an optional added element. 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 In addition, the above-mentioned optional elements are not particularly limited in terms of lower limit because the effects of the present invention can be obtained as long as they are contained in amounts not exceeding the above-mentioned upper limits. In addition, when the above-mentioned optional elements are contained in amounts less than the preferred lower limits described below, the elements are considered to be contained as unavoidable impurities.

[0032] [Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less] If Ti, Nb and V are each 0.200% or less, large amounts of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after paint baking is not reduced. Therefore, when Ti, Nb and V are contained, the contents are preferably 0.200% or less. The contents of Ti, Nb and V are more preferably 0.100% or less. The lower limits of the contents of Ti, Nb and V are not particularly specified. However, Ti, Nb and V increase the strength of the steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. Therefore, the contents of Ti, Nb and V are preferably 0.001% or more.

[0033] [Ta: 0.10% or less, W: 0.10% or less] If the Ta and W contents are each 0.10% or less, large amounts of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after paint baking is not reduced. Therefore, when Ta and W are contained, the contents are preferably 0.10% or less. The contents of Ta and W are more preferably 0.08% or less. There is no particular lower limit for the contents of Ta and W. However, Ta and W increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Therefore, the contents of Ta and W are preferably 0.01% or more.

[0034] [B:0.0100% or less] If the B content is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not decrease. Therefore, the toughness after paint baking will not decrease. Therefore, when B is contained, its content is preferably 0.0100% or less. The B content is more preferably 0.0080% or less. There is no particular lower limit for the B content. However, B is an element that segregates to the austenite grain boundaries during annealing and improves hardenability. Therefore, the B content is preferably 0.0003% or more.

[0035] [Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less] If the contents of Cr, Mo and Ni are each 1.00% or less, large amounts of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after paint baking is not reduced. Therefore, when Cr, Mo and Ni are contained, the contents are preferably 1.00% or less. The contents of Cr, Mo and Ni are more preferably 0.80% or less. The lower limits of the contents of Cr, Mo and Ni are not particularly specified. However, Cr, Mo and Ni are elements that improve hardenability. Therefore, the contents of Cr, Mo and Ni are preferably 0.01% or more.

[0036] [Co:0.010% or less] If the Co content is 0.010% or less, large coarse precipitates and inclusions are not generated in large quantities, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after paint baking is not reduced. Therefore, when Co is contained, the content is preferably 0.010% or less. The Co content is more preferably 0.008% or less. 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.

[0037] [Cu:1.00% or less] If the Cu content is 1.00% or less, large amounts of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after paint baking is not reduced. Therefore, when Cu is contained, the content is preferably 1.00% or less. The Cu content is more preferably 0.80% or less. There is no particular lower limit for the Cu content. However, Cu is an element that improves hardenability. Therefore, the Cu content is preferably 0.01% or more.

[0038] [Sn:0.200% or less] If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not decrease. Therefore, the toughness after paint baking will not decrease. Therefore, when Sn is contained, the content is preferably 0.200% or less. The Sn content is more preferably 0.100% or less. There is no particular lower limit for the Sn content. However, Sn is an element that improves hardenability and generally also improves corrosion resistance. Therefore, the Sn content is preferably 0.001% or more.

[0039] [Sb:0.200% or less] If the Sb content is 0.200% or less, large coarse precipitates and inclusions are not generated in large quantities, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after paint baking is not reduced. Therefore, when Sb is contained, the content is preferably 0.200% or less. The Sb content is more preferably 0.100% or less. There is no particular lower limit for the Sb content. However, Sb is an element that controls the surface softening thickness and enables strength adjustment. Therefore, the Sb content is preferably 0.001% or more.

[0040] [Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less] If the contents of Ca, Mg and REM are each 0.0100% or less, large amounts of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after paint baking is not reduced. Therefore, when Ca, Mg and REM are contained, the contents are preferably 0.0100% or less. The contents of Ca, Mg and REM are more preferably 0.0050% or less. The lower limits of the contents of Ca, Mg and REM are not particularly specified. 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 preferably 0.0005% or more.

[0041] [Zr: 0.100% or less, Te: 0.100% or less] If the content of Zr and Te is 0.100% or less, a large amount of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after paint baking is not reduced. Therefore, when Zr and Te are contained, the content is preferably 0.100% or less. The content of Zr and Te is more preferably 0.080% or less. The lower limit of the content of Zr and Te is not particularly specified. 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 content of Zr and Te is preferably 0.001% or more.

[0042] [Hf:0.10% or less] If the Hf content is 0.10% or less, large coarse precipitates and inclusions are not generated in large quantities, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after paint baking is not reduced. Therefore, when Hf is contained, its content is preferably 0.10% or less. The Hf content is more preferably 0.08% or less. There is no particular lower limit for the Hf content. However, Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet. Therefore, the Hf content is preferably 0.01% or more.

[0043] [Bi:0.200% or less] If the Bi content is 0.200% or less, large amounts of coarse precipitates and inclusions are not generated, and the ultimate deformability of the steel sheet is not reduced. Therefore, the toughness after paint baking is not reduced. Therefore, when Bi is contained, the content is preferably 0.200% or less. The Bi content is more preferably 0.100% or less. 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.

[0044] The elements other than those mentioned above are Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. These unavoidable impurities are permitted to be contained in a total amount of 0.100% or less.

[0045] Next, the structure of a steel sheet according to one embodiment of the present invention will be described. The structure of the steel plate according to one embodiment of the present invention is Area fraction of tempered martensite: 83% or more, Area fraction of retained austenite: less than 3% Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%; and The density of grain boundaries of 20° or more in tempered martensite is 1.0 μm / μm 2 That's all. An organization that satisfies the above formula (1). The reasons for each of the limitations will be explained below. The area fraction of each phase is the area ratio that each phase occupies with respect to the entire structure.

[0046] [Area fraction of tempered martensite: 83% or more] In a steel sheet according to an embodiment of the present invention, it is extremely important that the area fraction of tempered martensite is 83% or more. That is, by making tempered martensite the main phase, and in particular by making its area fraction 83% or more, it becomes possible to realize TS: 1180 MPa or more. Therefore, the area fraction of tempered martensite is 83% or more. The area fraction of tempered martensite is preferably 85% or more, more preferably 87% or more. There is no particular upper limit for the area fraction of tempered martensite. The area fraction of tempered martensite is, for example, preferably less than 95%, more preferably 94% or less, and further preferably 93% or less.

[0047] [Area fraction of retained austenite: less than 3%] In a steel sheet according to an embodiment of the present invention, it is extremely important that the area fraction of the retained austenite is less than 3%. That is, if the area fraction of the retained austenite is 3% or more, it becomes difficult to achieve excellent toughness after baking paint. One of the causes of the decrease in toughness after baking paint is that the retained austenite is transformed into high-hardness martensite during processing, which becomes the starting point of fracture. Therefore, the area fraction of the retained austenite is less than 3%. The area fraction of the retained austenite is preferably 1% or less. The lower limit of the area fraction of the retained austenite is not particularly specified. The area fraction of the retained austenite may be 0%.

[0048] [Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%] In the steel sheet according to one embodiment of the present invention, it is extremely important that the total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%. That is, when the total area fraction of ferrite and bainitic ferrite is 15% or more, it is difficult to realize a TS of 1180 MPa or more. On the other hand, when the total area fraction of ferrite and bainitic ferrite is less than 5%, it is difficult to realize a high EL. Therefore, the total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%. The total area fraction of ferrite and bainitic ferrite is preferably 6% or more, more preferably 7% or more. In addition, the total area fraction of ferrite and bainitic ferrite is preferably 14% or less, more preferably 13% or less. In addition, ferrite and bainitic ferrite may be contained alone, or both of them may be contained.

[0049] The area fraction of the remaining structure other than the above is preferably 5% or less. Examples of the remaining structure include pearlite, fresh martensite, and acicular ferrite. These remaining structures may be contained as long as they are 5% or less, since they do not affect the properties. The area fraction of the remaining structure may be 0%.

[0050] Here, the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite are measured, for example, as follows. That is, a sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished. Next, the observation surface of the sample is corroded with 3 vol.% nital to reveal the structure. Next, the 1 / 4 plate thickness position of the steel plate (the position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate) is observed by SEM at a magnification of 2000 times in 10 fields of view. In the observed images, the tempered martensite has fine irregularities in the structure and has carbides in the structure. In addition, the ferrite and bainitic ferrite are structures in which the structure is flat at the recesses and does not have carbides in the structure. Then, the areas of the regions occupied by the tempered martensite, ferrite, and bainitic ferrite are calculated for each field of view. Next, the areas of the regions occupied by tempered martensite, and ferrite and bainitic ferrite are each divided by the area of ​​the entire observation field, and multiplied by 100. The average values ​​are then taken as the area fraction of tempered martensite, and the total area fraction of ferrite and bainitic ferrite, respectively. In addition, the structure of a steel sheet is generally symmetrical from top to bottom in the sheet thickness direction. Therefore, any one of the surfaces (front and back) of the steel sheet may be set as the starting point of the sheet thickness position (sheet thickness 0 position), such as the 1 / 4 sheet thickness position or a position 100 μm deep from the steel sheet surface.

[0051] The area fraction of retained austenite is measured, for example, as follows. That is, the steel plate is mechanically ground to a depth of 1 / 4-0.1 mm of the plate thickness so that the observation position is 1 / 4 of the plate thickness, and then chemically polished for an additional 0.1 mm. Using the polished surface as the observation surface, the ratio of the integrated intensity of the diffraction peaks of the {200}, {211}, and {220} planes of fcc iron (austenite) to the integrated intensity of the diffraction peaks of the {200}, {220}, and {311} planes of bcc iron is calculated using CoKα radiation in an X-ray diffractometer. The volume fraction of the retained austenite is then calculated from the ratio of the integrated intensities of the diffraction peaks of the {200}, {211}, and {220} planes of bcc iron. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.

[0052] 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%. [Area fraction of residual structure (%)] = 100 - [Area fraction of tempered martensite (%)] - [Total area fraction of ferrite and bainitic ferrite (%)] - [Area fraction of retained austenite (%)]

[0053] [Grain boundary density of 20° or more in tempered martensite: 1.0 μm / μm 2 End] In the steel sheet according to one embodiment of the present invention, the density of grain boundaries of 20° or more in the tempered martensite is 1.0 μm / μm 2 It is extremely important that the density of high-angle grain boundaries in tempered martensite, particularly those with angles of 20° or more, is 1.0 μm / μm. High-angle grain boundaries in tempered martensite, especially those with angles of 20° or more, become carbon segregation sites during paint baking, preventing fracture of the steel sheet. As a result, the fracture stress during tensile deformation decreases. For this reason, it is important that the density of grain boundaries with angles of 20° or more in tempered martensite (hereinafter also referred to as the high-angle grain boundary density of tempered martensite) is 1.0 μm / μm. 2 If the density is less than 1.0 μm / μm, it becomes difficult to achieve excellent impact properties after baking. Therefore, the high angle grain boundary density of tempered martensite is 1.0 μm / μm 2The high angle grain boundary density of tempered martensite is preferably 1.2 μm / μm 2 More preferably, 1.3 μm / μm 2 The upper limit of the high angle grain boundary density of tempered martensite is not specified. For example, the high angle grain boundary density of tempered martensite is 3.0 μm / μm 2 The following is preferred:

[0054] Here, the high angle grain boundary density of the tempered martensite is determined, for example, as follows. A test piece for microstructural observation is taken from the steel plate. The taken test piece is then polished by colloidal silica vibration polishing so that the rolling direction cross section (L cross section) becomes the observation surface. The observation surface is mirror-finished. Next, electron backscatter diffraction (EBSD) measurement is performed at the 1 / 4 position of the steel plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface) to obtain local crystal orientation data. In the EBSD measurement, the step size is 0.10 μm and the measurement area is 50 μm square (50 μm × 50 μm). Next, the obtained local crystal orientation data is analyzed using analysis software: OIM Analysis 7. Analysis of the local crystal orientation data is performed for 10 fields of view at the 1 / 4 position of the plate thickness of the steel plate, and the average value is used. In addition, prior to the analysis of the local crystal orientation data, a cleanup process is performed once each using the Grain Dilation function of the analysis software (Grain Tolerance Angle: 5, Minimum Grain Size: 2, Single Iteration: ON). Next, the grain boundaries of 20° or more in the tempered martensite are displayed, and the total length of the grain boundaries of 20° or more in the tempered martensite is calculated. Then, the total length of the grain boundaries of 20° or more in the tempered martensite is divided by the area of ​​the measurement region to calculate the high angle grain boundary density of the tempered martensite.

[0055] [KAM(S) / KAM(C)>1.00] In the steel sheet according to one embodiment of the present invention, it is extremely important that KAM(S) / KAM(C) is more 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. As a result of various studies, the inventors have found that it is effective to change the dislocation distribution state from the surface layer to the inside of the steel sheet in order to improve the impact properties after baking of the paint. In particular, by making KAM(S) / KAM(C) more than 1.00, excellent impact properties after baking of the paint can be obtained. Therefore, KAM(S) / KAM(C) is made to be more than 1.00. KAM(S) / KAM(C) is preferably 1.03 or more. The upper limit of KAM(S) / KAM(C) is not particularly specified. For example, KAM(S) / KAM(C) is preferably 1.110 or less.

[0056] Here, KAM(S) and KAM(C) are calculated, for example, as follows. EBSD measurements were performed in the same manner as for measuring the high angle boundary density of tempered martensite, and the obtained local crystal orientation data was analyzed. Analysis of the local crystal orientation data was performed for 10 fields of view at a position 100μm deep from the steel plate surface and at the center of the steel plate thickness, and the average value was used. Then, from the analysis results of the local crystal orientation data at a position 100μm deep from the steel plate surface and at the center of the steel plate thickness, a chart of the KAM value of the bcc phase was created for each position, and the average values ​​were designated as KAM(S) and KAM(C), respectively.

[0057] The mechanical properties of the steel plate according to one embodiment of the present invention are as described above.

[0058] Moreover, the steel sheet according to an embodiment of the present invention may have a plating layer on the surface. The plating layer may be provided on only one surface of the steel sheet, or on both surfaces. The plating layer is not particularly limited. An example of the plating layer is a zinc plating layer containing Zn as a main component (Zn content is 50.0 mass% or more). Also, an example of the zinc plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer. Note that a steel sheet having a zinc plating layer can also be called a zinc-plated steel sheet. Also, the above-mentioned steel sheets having a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrogalvanized layer can also be called a hot-dip galvanized steel sheet (GI), an alloyed hot-dip galvanized steel sheet (GA), and an electrogalvanized steel sheet (EG), respectively.

[0059] 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 a Zn-Ni electric alloy plating layer.

[0060] In addition, the plating weight per side of the plating layer is not particularly limited, but is preferably 20 g / m 2 More than 80g / m 2 It is preferable that:

[0061] The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.50 mm or more and 2.50 mm or less.

[0062] [2] Materials Next, a member according to one embodiment of the present invention will be described. A member according to an embodiment of the present invention is a member made using the above-mentioned steel plate (as a raw material). For example, the raw material steel plate is subjected to at least one of forming and joining to form a member. Here, the above steel sheet has a TS of 1180 MPa or more, a high EL, and further has excellent toughness and crashworthiness after baking paint. Therefore, the member according to one embodiment of the present invention is particularly suitable for use as a material for automobile parts, for example. This allows for the improvement of fuel efficiency by reducing the weight of the automobile body, which contributes greatly to the reduction of CO2 emissions.

[0063] [3] Manufacturing method of steel sheets Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described.

[0064] A method for producing a steel sheet according to one embodiment of the present invention includes the steps of: A preparation step of preparing a base steel sheet having the above-mentioned composition; Next, the base steel sheet is Average heating rate in the temperature range of 700℃~750℃: 5.0℃ / s or less A heating process of heating to an annealing temperature T1 under the above conditions; Next, the base steel sheet is The annealing temperature T1: 750°C or more and 850°C or less; Annealing time t1: 10 seconds or more and 1000 seconds or less An annealing process, Next, the base steel plate is A bending process in which bending is performed at least once using a roll with a radius of 800 mm or less in a temperature range of the annealing temperature T1 to 700 ° C.; Next, the base steel sheet is Average cooling rate in the temperature range of 700℃~550℃: 0.5℃ / s or more and less than 10.0℃ / s A first cooling step of cooling the mixture to a first cooling end temperature under the above condition; Next, the base steel sheet is Average cooling rate in the temperature range of 300℃~100℃: 300℃ / s or more, Tension applied to the base steel plate in the temperature range of 300℃ to 100℃: 5MPa or more A second cooling step of cooling the mixture to a second cooling end temperature under the above condition. Next, the base steel sheet is Tempering temperature T2: 100℃ or more and 400℃ or less, Tempering time t2: 10 seconds or more and 10000 seconds or less A tempering process, Next, the base steel plate is Straightening start temperature: 100℃ or less, Inlet intermesh push-in amount: 4.0mm to 10.0mm Output intermesh push-in amount: 1.0mm to 10.0mm Inlet tension: 20MPa or more and 500MPa or less, Output tension: 25MPa or more and 550MPa or less A straightening process in which straightening is performed by leveling under the above conditions; It has the following. Unless otherwise specified, the above temperatures are the surface temperatures of the steel sheet. Furthermore, unless otherwise specified, the average heating rate and the average cooling rate are also based on the surface temperatures of the steel sheet.

[0065] ·Preparation process First, a base steel sheet having the above-mentioned composition is prepared. For example, the base steel sheet can be prepared by hot rolling a steel slab to obtain a hot-rolled steel sheet, optionally pickling and heat treating the hot-rolled steel sheet, and then cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet. The conditions for these steps are not particularly limited, and may be in accordance with conventional methods.

[0066] For example, the method of smelting steel slabs (steel material) may be any known smelting method such as a converter or an electric furnace, etc. In order to prevent macrosegregation, it is preferable to smelt steel slabs by a continuous casting method.

[0067] Examples of hot rolling include a method of rolling a steel slab after heating, a method of directly rolling a steel slab after continuous casting without heating it, and a method of rolling a steel slab after continuous casting by subjecting it to a short-term heat treatment. The slab heating temperature, slab soaking time, and coiling temperature in hot rolling are also not particularly limited. The slab heating temperature is preferably 1100°C or higher. The slab heating temperature is preferably 1300°C or lower. The slab soaking time is preferably 30 minutes or more. The slab soaking time is preferably 250 minutes or less. The finish rolling temperature is preferably the Ar3 transformation point or higher. The coiling temperature is preferably 350°C or higher. The coiling temperature is preferably 650°C or lower. The Ar3 transformation point is calculated by the following formula. Ar3 transformation point (℃) = 868-396 x [%C] + 24.6 x [%Si] - 68.1 x [%Mn] - 36.1 x [%Ni] - 20.7 x [%Cu] - 24.8 x [%Cr] In the above formula, the symbol [% element symbol] represents the content (mass %) of the corresponding element in the above composition.

[0068] Pickling can remove oxides from the surface of the hot-rolled steel sheet, and is preferably performed to ensure good chemical conversion treatability and plating quality in the final steel sheet product. Pickling may be performed once or multiple times. The hot-rolled steel sheet after pickling may be subjected to a heat treatment.

[0069] The total reduction in cold rolling is preferably 30% or more. The total reduction in cold rolling is preferably 80% or less. The number of rolling passes and the reduction in each pass are not particularly limited, and a desired effect can be obtained.

[0070] ·Heating process Next, the base steel sheet prepared in the preparation step is heated to an annealing temperature T1 under conditions of 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 explained in the annealing step described later.

[0071] [Average heating rate in the temperature range of 700℃~750℃: 5.0℃ / s or less] The inventors have conducted extensive research and found that the average heating rate in the temperature range of 700°C to 750°C (hereinafter, simply 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 5.0°C / s or less, the dissolution of carbides is promoted. This refines the prior austenite grain boundaries, and increases the number of grain boundaries with angles of 20° or more after martensite transformation. Therefore, the high-angle grain boundary density of tempered martensite in the final steel sheet product also increases, improving the impact properties after paint baking. 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 to the average heating rate. For example, the average heating rate is preferably 0.1°C / s or more.

[0072] Annealing process Next, the base steel sheet is annealed under the conditions of an annealing temperature T1 of 750° C. or more and 850° C. or less, and an annealing time t1 of 10 seconds or more and 1000 seconds or less.

[0073] [Annealing temperature T1: 750℃ to 850℃] When the annealing temperature T1 is less than 750°C, the total area fraction of ferrite and bainitic ferrite is 15% or more, and it is difficult to achieve a TS of 1180MPa or more. On the other hand, when the annealing temperature T1 exceeds 850°C, the total area fraction of ferrite and bainitic ferrite is less than 5%, and it is difficult to achieve a high El. Therefore, the annealing temperature T1 is 750°C or more and 850°C or less. The annealing temperature T1 is preferably 760°C or more, more preferably 770°C or more. The annealing temperature T1 is preferably 840°C or less, more preferably 830°C or less. The annealing temperature here is the holding temperature in the annealing process. The annealing temperature may be constant during holding. In addition, the annealing temperature does not have to be constant during holding as long as it is in the temperature range of 750°C or more and 850°C or less, and the temperature fluctuation is within the set temperature ±10°C.

[0074] [Annealing time t1: 10 seconds or more and 1000 seconds or less] When the annealing time t1 is less than 10 seconds, the total area fraction of ferrite and bainitic ferrite is 15% or more, and it is difficult to achieve a TS of 1180 MPa or more. On the other hand, when the annealing time t1 exceeds 1000 seconds, the total area fraction of ferrite and bainitic ferrite is less than 5%, and it is difficult to achieve a high El. Therefore, the annealing time t1 is 10 seconds or more and 1000 seconds or less. The annealing time t1 is preferably 30 seconds or more. The annealing time t1 is preferably 800 seconds or less. The annealing time t1 here is the holding time at the annealing temperature T1.

[0075] · Bending process Next, the base steel sheet is subjected to bending at least once using rolls with a radius of 800 mm or less in a temperature range of annealing temperatures T1 to 700°C.

[0076] [Number of times bending with rolls of 800 mm radius or less in the annealing temperature range T1 to 700°C: 1 or more] As a result of intensive research, the inventors have found that bending in the temperature range from annealing temperature T1 to 700°C (hereinafter also referred to as bending temperature range) affects the high angle grain boundary density of tempered martensite. In particular, when bending is performed using a roll with a radius of 800 mm or less in the bending temperature range, nucleation of martensitic transformation is promoted. This makes martensite finer, and grain boundaries with angles of 20° or more increase after martensitic transformation. Therefore, the high angle grain boundary density of tempered martensite in the final steel sheet product also increases, improving the impact properties after paint baking. Therefore, the number of times bending using a roll with a radius of 800 mm or less in the bending temperature range (hereinafter also referred to as bending number) is set to one or more times.

[0077] The radius of the roll used in the bending process is preferably 600 mm or less. There is no particular lower limit to the radius of the roll used in the bending process. For example, the radius of the roll used in the bending process is preferably 100 mm or more.

[0078] The number of times of bending may be one or more. The number of times of bending is preferably two or more. There is no particular upper limit to the number of times of bending. For example, the number of times of bending is preferably 15 or less. The bending process may be performed by bending in one direction with a roll, and then bending back the same amount as the amount of bending in the opposite direction. In this case, the number of times of bending is counted as two (one bending and one bending back). The bending angle is preferably 80 to 110°. This further enhances the effect of promoting nucleation of martensitic transformation. The bending angle is the angle (acute angle) formed by the passing direction of the steel sheet on the roll entry side and the passing direction of the steel sheet on the roll exit side.

[0079] Note that, as long as bending using a roll with a radius of 800 mm or less is performed at least once in the bending temperature range, bending that does not satisfy the above condition may be performed further.

[0080] ·First cooling process Next, the base steel sheet is cooled to a first cooling end temperature under the condition of 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.

[0081] [Average cooling rate in the temperature range of 700℃~550℃: 0.5℃ / s or more and less than 10.0℃ / s] 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 10.0°C / s or more, the total area fraction of ferrite and bainitic ferrite becomes less than 5%, making it difficult to realize a high El. On the other hand, when the first average cooling rate is less than 0.5°C / s, the total area fraction of ferrite and bainitic ferrite becomes 15% or more, making it difficult to realize a TS of 1180 MPa or more. Therefore, the first average cooling rate is set to 0.5°C / s or more and less than 10.0°C / s. The first average cooling rate is preferably 1.0°C / s or more. The first average cooling rate is preferably 8.0°C / s or less.

[0082] The first cooling end temperature may be, for example, 550° C. to 300° C. Furthermore, between the first cooling step and the second cooling step described below, the base steel sheet may be subjected to a plating treatment. The plating treatment is as described below.

[0083] ·Second cooling process Next, the base steel sheet is cooled to a second cooling end temperature under conditions of an average cooling rate of 300°C / s or more in the temperature range of 300°C to 100°C and a tension applied to the base steel sheet in the temperature range of 300°C to 100°C: 5 MPa or more.

[0084] [Average cooling rate in the temperature range from 300℃ to 100℃: 300℃ / s or more] If the average cooling rate in the temperature range of 300°C to 100°C (hereinafter also referred to as the second average cooling rate) is less than 300°C / s, the area fraction of retained austenite will be 3% or more, making it difficult to achieve excellent toughness after paint 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.

[0085] [Applied tension to steel plate in the temperature range of 300℃~100℃: 5MPa or more] As a result of intensive research by the present inventors, it was found that the tension applied to the material steel sheet during cooling in the temperature range of 300°C to 100°C affects the high angle grain boundary density of tempered martensite. In particular, when the tension applied to the material steel sheet in the temperature range of 300°C to 100°C (hereinafter also simply referred to as the applied tension) is 5 MPa or more, martensitic transformation is promoted. This makes the martensite finer, and the grain boundaries of 20° or more increase after martensitic transformation. Therefore, the high angle grain boundary density of tempered martensite in the final steel sheet product also increases, and the impact properties after paint baking are improved. Therefore, the applied tension is 5 MPa or more. The applied tension is preferably 10 MPa or more. The upper limit of the applied tension is not particularly specified. For example, the applied tension is preferably 100 MPa or less.

[0086] Moreover, the second cooling end temperature may be, for example, less than 100° C. The second cooling end temperature may be, for example, about room temperature.

[0087] The bending in the bending process increases the number of nucleation sites where martensitic transformation starts. On the other hand, the tension applied in the second cooling process promotes the martensitic transformation itself. In other words, the effects obtained by the two processes are different.

[0088] ·Tempering process Next, the base steel sheet is tempered under the conditions of a tempering temperature T2: 100° C. or more and 400° C. or less, and a tempering time t2: 10 seconds or more and 10,000 seconds or less.

[0089] [Tempering temperature T2: 100℃ or higher and 400℃ or lower] Tempered martensite is generated by tempering martensite through a tempering process. Here, if the tempering temperature T2 is less than 100°C, the martensite is not sufficiently tempered, and the structure is mainly composed of as-quenched martensite. With such a structure mainly composed of as-quenched martensite, excellent toughness after paint baking cannot be obtained. 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 or more and 400°C or less. The tempering temperature T2 is preferably set to 150°C or more. The tempering temperature T2 is preferably set to 350°C or less. The tempering temperature here is the holding temperature in the tempering process. The tempering temperature may be constant during holding. In addition, the tempering temperature does not have to be constant during holding as long as it is in a temperature range of 100°C or more and 400°C or less, and the temperature fluctuation is within ±10°C of the set temperature.

[0090] [Tempering time t2: 10 seconds or more and 10000 seconds or less] As described above, tempered martensite is generated by tempering martensite through a tempering process. Here, if the tempering time t2 is less than 10 seconds, the martensite is not sufficiently tempered, and the structure is mainly composed of as-quenched martensite. With such a structure mainly composed of as-quenched martensite, excellent toughness after paint baking cannot be obtained. On the other hand, if the tempering time t2 exceeds 10,000 seconds, the tempering of martensite progresses excessively, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the tempering time t2 is set to 10 seconds or more and 10,000 seconds or less. The tempering time t2 is preferably set to 50 seconds or more. The tempering time t2 is preferably set to 5,000 seconds or less. The tempering time t2 here is the holding time at the tempering temperature T2.

[0091] The cooling after tempering is not particularly specified. For example, cooling may be performed by any method according to a conventional method. The cooling end temperature after tempering may be, for example, about room temperature. In addition, the base steel sheet may be plated between the tempering process and the straightening process described later. The plating process is described later.

[0092] ·Correcting process Next, the raw steel sheet is subjected to straightening by leveling. At this time, it is extremely important in the manufacturing method of the steel sheet according to the embodiment of the present invention that the following conditions are satisfied. Straightening start temperature: 100℃ or less, Inlet intermesh push-in amount: 4.0mm to 10.0mm Output intermesh push-in amount: 1.0mm to 10.0mm Inlet tension: 20MPa or more and 500MPa or less, Output tension: 25MPa or more and 550MPa or less

[0093] [straightening start temperature: 100℃ or less] If the straightening start temperature exceeds 100°C, the steel sheet becomes soft. This changes the amount of strain introduced into the surface layer and center of the steel sheet by the leveling process, and KAM(S) / KAM(C) becomes 1.00 or less. As a result, the impact properties after paint baking deteriorate. 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 particular lower limit for the straightening start temperature. For example, the straightening start temperature is preferably -10°C or more.

[0094] [Inlet intermesh push-in amount: 4.0mm to 10.0mm] If the inlet intermesh push-in amount is less than 4.0 mm, the processing amount will be insufficient. As a result, KAM(S) / KAM(C) will be 1.00 or less, and the impact properties after paint baking will deteriorate. The upper limit of the inlet intermesh push-in amount is set to 10.0 mm or less due to production technology constraints. Therefore, the inlet intermesh push-in amount is set to 4.0 mm or more and 10.0 mm or less. The inlet intermesh push-in amount is preferably 5.0 mm or more.

[0095] [Outlet intermesh push-in amount: 1.0mm to 10.0mm] If the amount of push-in of the exit intermesh is less than 1.0 mm, the amount of processing will be insufficient. As a result, KAM(S) / KAM(C) will be 1.00 or less, and the impact properties after paint baking will deteriorate. Note that the upper limit of the amount of push-in of the exit intermesh is set to 10.0 mm or less due to production technology constraints. Therefore, the amount of push-in of the exit intermesh should be 1.0 mm or more and 10.0 mm or less.

[0096] Here, the entry intermesh push-in amount is the push-in amount of the second roll from the entry side in the leveller process (roll 2 in Fig. 1) against the steel sheet plane (the steel sheet surface on the side where roll 2 is located), as shown in Fig. 1. The exit intermesh push-in amount is the push-in amount of the second roll from the exit side in the leveller process (roll 8 in Fig. 1) against the steel sheet plane (the steel sheet surface on the side where roll 8 is located). The number of rolls in the leveller process (leveller) is not particularly limited, but for example, the number of rolls is preferably 5 or more.

[0097] [Inlet tension: 20MPa or more and 500MPa or less] If the entry tension is less than 20 MPa, the amount of processing is insufficient. As a result, KAM(S) / KAM(C) becomes 1.00 or less, and the impact properties after paint baking deteriorate. The upper limit of the entry tension is set to 500 MPa or less due to constraints on production technology. Therefore, the entry tension is set to 20 MPa or more and 500 MPa or less. The entry tension is preferably 100 MPa or more.

[0098] [Outside tension: 25MPa or more and 550MPa or less] Due to equipment constraints in the leveller processing, the exit tension is higher than the entry tension. Here, if the exit intermesh pushing amount is less than 25 MPa, the processing amount will be insufficient. As a result, KAM(S) / KAM(C) will be 1.00 or less, and the impact properties after paint baking will deteriorate. Note that the upper limit of the exit tension is set to 550 MPa due to production technology constraints. Therefore, the exit tension is set to 25 MPa or more and 550 MPa or less. The exit tension is preferably 100 MPa or more.

[0099] Plating process In addition, the steel sheet may be optionally plated. The plating is not particularly limited. Examples of the plating include galvanizing, hot-dip galvanizing, and electrogalvanizing. Examples of plating other than galvanizing include aluminum plating and alloy plating. Examples of alloy plating include hot-dip zinc-aluminum-magnesium alloy plating and Zn-Ni electric alloy plating. All of the treatment conditions may be in accordance with conventional methods. As described above, the plating is preferably performed between the first and second cooling steps, or between the tempering and straightening steps. For example, the hot-dip galvanizing and galvannealed hot-dip galvanizing are preferably performed between the first and second cooling steps. Also, the electrogalvanizing and Zn-Ni electric alloy plating are preferably performed between the tempering and straightening steps.

[0100] In the case of hot-dip galvanizing and galvannealing, a series of processes such as the heating process, annealing process, and plating process are preferably carried out in a continuous galvanizing line (CGL) from the viewpoint of productivity. After hot-dip galvanizing, wiping is possible to adjust the coating weight of the plating.

[0101] The other conditions are not particularly limited and may be the same as those described above. According to the method for producing a steel sheet according to one embodiment of the present invention, a steel sheet having a TS of 1180 MPa or more, a high EL, and excellent toughness and impact properties after baking paint can be obtained. The obtained steel sheet can be suitably used as a material for automobile parts, for example.

[0102] [4] Manufacturing methods for components Next, a method for manufacturing a member according to one embodiment of the present invention will be described. A method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate to at least one of forming and joining to form a component. Here, the molding method is not particularly limited, and for example, a general processing method such as press molding can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, crimp joining, etc. The molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods. EXAMPLES

[0103] Steel having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by a continuous casting method. The steel slab was then heated. The steel slab was then hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was then pickled. The hot-rolled steel sheet was then cold-rolled to obtain a cold-rolled steel sheet. In this manner, a base steel sheet was prepared. The prepared base steel sheet was then subjected to a heating process, an annealing process, a bending process, a first cooling process, a second cooling process, a tempering process, and a straightening process under the conditions shown in Table 2 to obtain a steel sheet (sheet thickness: 0.6 to 2.2 mm) as a final product. The first cooling end temperature was 550°C to 300°C in all cases. The second cooling end temperature and the cooling end temperature after tempering were both room temperature. The bending angle in the bending process was 80 to 110° in all cases. In addition, some of the steel sheets (those in the type column of Table 2 as GI, GA, and EG) were plated. Of these, for GI and GA steel sheets in the type column of Table 2, the plating was performed between the first and second cooling processes. For those in the type column of Table 2 as EG, the plating was performed between the tempering process and the straightening process. Conditions not specified were in accordance with standard methods.

[0104] Using the steel sheets thus obtained, the area fraction of tempered martensite, the area fraction of retained austenite, the total area fraction of ferrite and bainitic ferrite, the high angle grain boundary density of tempered martensite, and KAM(S) / KAM(C) were determined in the manner described above. The results are shown in Table 3.

[0105] In addition, each evaluation was performed according to the following procedures. The evaluation results are shown in Table 3.

[0106] (TS rating) From the obtained steel plate, a JIS No. 5 test piece (gauge length: 50 mm, parallel part width: 25 mm) was taken so that the direction perpendicular to the rolling direction of the steel plate was the longitudinal direction of the test piece. Next, a tensile test according to JIS Z 2241:2022 was performed using the taken test piece, and TS and EL were measured. The crosshead speed was 1.67 × 10 -1 The condition was set to mm / sec. The evaluation was based on the following criteria. Good (passed, excellent): TS is 1180MPa or more Bad (failed): TS is less than 1180MPa

[0107] (EL rating) The EL measured in the above tensile test was evaluated according to the following criteria. Good (pass, excellent): EL is 10% or more Bad (failed): EL is less than 10%

[0108] (Evaluation of toughness after painting and baking) The obtained steel plates were stacked and fastened with bolts. Then, after confirming that there was no gap between the steel plates, a V-notch with a depth of 2 mm was given to the stacked steel plates to prepare a laminated Charpy test piece (hereinafter, also simply referred to as a test piece). The number of steel plates to be stacked was set to the number that would make the thickness of the test piece closest to 10 mm (if there were two numbers that would make the thickness of the test piece closest to 10 mm, the smaller number would be used). For example, when the thickness of the steel plate was 1.2 mm, eight steel plates were stacked. That is, the thickness of the test piece was 9.6 mm. The test piece was prepared so that the width direction of the steel plate was the longitudinal direction of the test piece. Then, the prepared test piece was subjected to aging treatment at a treatment temperature of 170°C and a treatment time of 20 minutes. Then, a Charpy impact test was performed using the aged test piece in a test temperature range of -120°C to +120°C. A transition curve was then obtained from the brittle fracture surface ratio, and the temperature at which the brittle fracture surface ratio was 50% was defined as the brittle-ductile transition temperature. The toughness after paint baking was evaluated according to the following criteria. Note that the other conditions were in accordance with JIS Z 2242:2018. Excellent (passed, especially excellent): Brittle-ductile transition temperature after aging is -60°C or less Good (pass, excellent): Brittle-ductile transition temperature after aging is -40°C or lower (except excellent) Bad (failed): Brittle-ductile transition temperature after aging is greater than -40°C

[0109] (Impact characteristics after paint baking) The obtained steel plate was subjected to aging treatment at a treatment temperature of 170°C for 20 minutes. Then, a JIS No. 5 test piece (gauge length: 50 mm, parallel part width: 25 mm) was taken from the aging-treated steel plate so that the direction perpendicular to the rolling direction of the steel plate was the longitudinal direction of the test piece. Next, using the taken test piece, a tensile test according to JIS Z 2241:2022 was performed in the same manner as the above TS evaluation, and the TS, YS (yield stress) and fracture stress after aging treatment were measured. Then, the impact properties after baking the paint were evaluated according to the following criteria. Good (pass, excellent): YR after aging is 0.85 or more and the fracture stress ratio after aging is 0.90 or less Poor (rejected): Does not satisfy at least one of the following: YR after aging is 0.85 or more and fracture stress ratio after aging is 0.90 or less The YR and fracture stress ratio after aging treatment are calculated by the following formulas. [YR after aging treatment] = [YS after aging treatment] / [TS after aging treatment] [Fracture stress ratio after aging treatment] = [Fracture stress after aging treatment] / [TS after aging treatment] The breaking stress is the stress at the breaking point in the above tensile test (the stress applied when the test piece breaks).

[0110] [Table 1]

[0111] [Table 2] TIFF0007597272000003.tif230170TIFF0007597272000004.tif237170

[0112] [Table 3] TIFF0007597272000006.tif223170TIFF0007597272000007.tif228170

[0113] As shown in Table 3, all of the inventive examples passed the TS, EL, and toughness and crash properties after paint baking. In addition, all of the components obtained by forming or joining using the steel plates of the inventive examples had the desired shape without any cracks, and all of the TS, EL, toughness and crash properties after paint baking met the pass criteria. On the other hand, the comparative examples failed in at least one of TS, EL, and toughness and impact properties after paint baking.

Claims

1. In mass percent, C: 0.030% or more and 0.500% or less, Si: 0.010% or more and 2.500% or less, Mn: 0.10% or more and 5.00% or less, 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 and the balance being Fe and unavoidable impurities; Area fraction of tempered martensite: 83% or more, Area fraction of retained austenite: less than 3%; Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%; and The tempered martensite has a grain boundary density of 1.0 μm / μm or more at 20°. ２ That's all. A structure that satisfies the following formula (1), The density of grain boundaries of 20° or more in the tempered martensite is obtained by dividing the total length of grain boundaries of 20° or more in the tempered martensite by the area of ​​the structure. KAM(S) / KAM(C) > 1.00 (1) During the ceremony, KAM (S): average KAM value at a depth of 100 μm from the steel sheet surface, KAM (C): Average KAM value at the center position of the thickness of the steel plate It is.

2. The composition further comprises, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less; Bi: 0.200% or less The steel sheet according to claim 1, comprising at least one selected from the following:

3. The steel sheet according to claim 1 , which has a plating layer on a surface thereof.

4. The steel sheet according to claim 2 , which has a plating layer on a surface thereof.

5. A member made using the steel plate according to any one of claims 1 to 4.

6. A method for producing a steel sheet according to any one of claims 1 to 4, comprising the steps of: A preparation step of preparing a base steel sheet having the component composition according to claim 1 or 2; Next, the base steel sheet is Average heating rate in the temperature range of 700°C to 750°C: 5.0°C / s or less A heating step of heating to an annealing temperature T1 under the above conditions; Next, the base steel sheet is The annealing temperature T1: 750° C. or higher and 850° C. or lower, Annealing time t1: 10 seconds or more and 1000 seconds or less An annealing process, Next, the base steel plate is A bending process in which bending is performed at least once using a roll with a radius of 800 mm or less in a temperature range of the annealing temperature T1 to 700 ° C.; Next, the base steel sheet is Average cooling rate in the temperature range of 700°C to 550°C: 0.5°C / s or more and less than 10.0°C / s A first cooling step of cooling the mixture to a first cooling end temperature under the conditions of Next, the base steel sheet is Average cooling rate in the temperature range of 300°C to 100°C: 300°C / s or more; Tension applied to the base steel plate in the temperature range of 300°C to 100°C: 5 MPa or more A second cooling step of cooling the mixture to a second cooling end temperature under the conditions of Next, the base steel sheet is Tempering temperature T2: 100°C or more and 400°C or less, Tempering time t2: 10 seconds or more and 10,000 seconds or less A tempering process, Next, the base steel plate is Straightening start temperature: 100℃ or less, Inlet intermesh push-in amount: 4.0 mm or more and 10.0 mm or less, Amount of intermesh pushed in at the outlet: 1.0 mm or more and 10.0 mm or less, Inlet tension: 20 MPa or more and 500 MPa or less, Output tension: 25MPa or more and 550MPa or less A straightening process in which straightening is performed by leveling under the above conditions; A manufacturing method for steel plates that provides the following:

7. 7. The method for producing a steel sheet according to claim 6, further comprising a plating process for plating the base steel sheet between the first cooling process and the second cooling process, or between the tempering process and the straightening process.

8. A method for producing a component, comprising the step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining to form a component.

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