High-strength steel sheet, high-strength plated steel sheet, their manufacturing method, and component

Abstract

The objective of the present invention is to provide a high-strength steel sheet of 1,180 MPa or more, which has excellent bendability and inter-steel cracking properties in spot weld HAZ, and which enables the manufacture of parts with high dimensional accuracy, a high-strength plated steel sheet, their manufacturing methods, and components. Having a predetermined composition, x Ti,eff =x Ti -x N -x S (where each element symbol is the molar fraction of each element in steel) Ti,eff ) is 0.001 or more, and at the 1 / 4 position of the sheet thickness, the area ratio of martensite is 80% to 99%, and the area ratio of ferrite and / or the volume ratio of retained austenite is more than 0% and 20% or less in total; the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is 0.10% to 0.60%, and the number density of MnS present in the Mn segregated part on the steel sheet surface is 5.0 particles / mm 2 The following refers to a high-strength steel plate having a standard deviation of Vickers hardness on the steel plate surface of 15 or less, and a hardness fluctuation frequency of 7 or less per 1,100 μm in the plate width direction on the steel plate surface.

Description

[Technical field]

[0001] The present disclosure relates to a high-strength steel sheet, a high-strength plated steel sheet, and manufacturing methods thereof, and members that are optimal for reinforcement parts and frame structural parts of automobiles. [Background technology]

[0002] CO reduction due to vehicle weight reduction 2 In order to achieve both reduced emissions and improved crashworthiness through vehicle weight reduction, efforts are underway to increase the strength of steel sheets for automobiles. New legal regulations are also being introduced one after another. As a result, in order to increase the strength of the vehicle body, there has been an increase in the number of cases where high-strength steel sheets are used for the main structural parts and reinforcing parts that form the framework of the automobile cabin (hereinafter also referred to as automotive framework structural parts, etc.). In particular, there has been an increase in the number of cases where high-strength steel sheets with a tensile strength (hereinafter also referred to simply as TS) of 1180 MPa or more are used.

[0003] High-strength steel sheets used in automotive reinforcement parts and frame structural parts are required to have excellent formability. Furthermore, the parts after forming are required to have excellent dimensional accuracy. For example, parts such as crash boxes have punched end faces and bent parts, so from the viewpoint of formability, steel sheets with high stretch flangeability and bendability are suitable. From the viewpoint of part performance, increasing the yield ratio of the steel sheet (YR = yield strength YS / tensile strength TS) can increase the impact absorption energy during a collision. Furthermore, from the viewpoint of dimensional accuracy of parts, controlling the yield ratio (YR) of the steel sheet within a certain range can suppress springback after steel sheet forming and control the dimensional accuracy of parts. In order to increase the application ratio of high-strength steel sheets to automotive parts, it is required to comprehensively satisfy these characteristics.

[0004] Furthermore, among the structural components of automobiles, for example crash boxes have bent parts. Furthermore, in order to increase the ratio of high-strength steel sheets applied to parts having a relatively small bending radius, there is a demand for high-strength steel sheets having high bendability that does not cause large cracks in bent parts where the value R / t obtained by dividing the bending radius (R) by the plate thickness (t) is less than 5.0.

[0005] More recently, it has been confirmed that when high-strength galvanized steel sheets are spot-welded, zinc from the coating layer diffuses into the grain boundaries of the steel sheet surface, causing liquid metal embrittlement (LME). As a result, LME cracks occur in the HAZ of spot welds. Since LME cracks can occur in the HAZ of spot welds even in high-strength steel sheets without a galvanized layer, as long as the welding partner is a galvanized steel sheet, they are becoming a problem for all high-strength steel sheets. In mass production, steel sheets with various thicknesses are spot-welded, and the hold time required to suppress LME cracks in the HAZ of spot welds increases as the plate thickness increases. However, the increase in hold time leads to a decrease in productivity, so there is a demand for high-strength steel sheets that can suppress LME cracks in the HAZ of spot welds even with a short hold time.

[0006] In response to these demands, for example, Patent Document 1 provides a high-strength steel sheet having a tensile strength of 980 MPa or more, which is excellent in bendability and LME resistance and enables the manufacture of parts with high dimensional accuracy. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6787535 Summary of the Invention [Problem to be solved by the invention]

[0008] The high-strength steel plate described in Patent Document 1 satisfies overall bendability and LME resistance properties, and allows the manufacture of parts with high dimensional accuracy. However, the high-strength steel plate described in Patent Document 1 has a TS of 980 MPa, and there is room for further improvement in strength.

[0009] The present disclosure has been developed in consideration of the above circumstances, and aims to obtain a high-strength steel plate of 1,180 MPa or more that has excellent bendability and inter-steel sheet cracking resistance in spot weld HAZ and is capable of manufacturing parts with high dimensional accuracy, and to provide an advantageous method for manufacturing the high-strength steel plate.

[0010] In this disclosure, being able to manufacture parts with high dimensional accuracy (high dimensional accuracy during molding) means that the yield ratio (YR) is 65% or more and 90% or less. YR can be calculated using the following formula (2). YR = YS / TS × 100 (2) Regarding bendability, a bending test is performed using the V-block method with a bending angle of 90 degrees, and five samples are subjected to bending tests at an R where the value R / t obtained by dividing the bending radius (R) by the plate thickness (t) is about 4.5, i.e., 4.3 to 4.7. Next, the length of the crack at the ridgeline of the bend apex of all five samples is evaluated, and if the crack length is 200 μm or less, it is determined that the bendability is excellent.

[0011] For the inter-steel-plate cracking resistance of the spot weld HAZ, the cross section of the weld described in the examples was observed with an optical microscope (200x) and the inter-steel-plate cracking resistance of the spot weld HAZ was evaluated according to the following criteria. If it was A or B, it was determined that the spot weld HAZ had excellent inter-steel-plate cracking resistance. If it was C, it was determined that the spot weld HAZ had poor inter-steel-plate cracking resistance. A: No cracks longer than 0.1 mm were observed with a hold time of 0.16 seconds. B: A crack longer than 0.1 mm is observed at a hold time of 0.16 seconds, but no crack longer than 0.1 mm is observed at a hold time of 0.20 seconds. C: A crack longer than 0.1 mm is observed at a hold time of 0.20 seconds.

[0012] Here, the hold time refers to the time from when the welding current is finished flowing to when the electrodes start to be released. [Means for solving the problem]

[0013] Means for Solving the Problems The present inventors have conducted extensive research to achieve the above object, and as a result have obtained the following findings. (1) By using a microstructure that is mainly composed of martensite (quenched martensite and tempered martensite) and further contains ferrite and / or retained austenite, it is possible to achieve a YR, which is an index of the dimensional accuracy of a part, of 65% to 90%. (2) The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is 0.10% to 0.60%, and the number density of MnS present in the Mn segregated part of the steel sheet surface is 5.0 particles / mm 2 Hereinafter, the standard deviation of the Vickers hardness of the steel sheet surface is set to 15 or less. Furthermore, by setting the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface to 7 or less, good bendability can be realized. (3) By setting the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction to 0.10% or more and 0.60% or less, good resistance to inter-steel sheet cracking in the spot weld HAZ can be achieved.

[0014] The present disclosure has been made based on the above findings. That is, the gist of the present disclosure is as follows. [1] In mass%, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, and Ti: 0.002% or more and 0.200% or less, and the effective Ti mole fraction (x Ti,eff) is 0.001 or more, with the balance consisting of Fe and unavoidable impurities; the steel structure has an area ratio of martensite of 80% to 99%, and an area ratio of ferrite and / or a volume ratio of retained austenite of more than 0% and 20% or less in total at the 1 / 4 position in the sheet thickness; the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is 0.10% to 0.60%, and the number density of MnS present in the Mn segregated part on the steel sheet surface is 5.0 particles / mm 2 The following refers to a high-strength steel plate having a standard deviation of Vickers hardness on the steel plate surface of 15 or less, and a hardness fluctuation frequency of 7 or less per 1,100 μm in the plate width direction on the steel plate surface. Note x Ti,eff =x Ti -x N -x S (1) In addition, x in the formula Ti , x N , x S indicates the content (molar fraction) of each element in the steel sheet. [2] The high-strength steel plate according to [1], wherein the chemical composition further contains, by mass%, at least one element selected from 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] A high-strength plated steel sheet having a plating layer on at least one side of the high-strength steel sheet according to [1] or [2]. [4] A steel slab having the composition described in [1] or [2] is heated at an average heating rate of 25°C / min or less in a temperature range of 900°C to 1150°C, a slab heating temperature of 1150°C or more, and a residence time from 1100°C to the slab heating temperature of 20 minutes or more. Then, the steel slab is subjected to a finish rolling process in which the reduction ratio of the final pass is 9% to 15%, the reduction ratio of the pass before the final pass is 15% to 21%, and the reduction ratio of the pass two passes before the final pass is 20% to 25%. A method for producing a high strength steel sheet, comprising the steps of: hot rolling a steel sheet having a rolling reduction of 1% to 27% to obtain a hot rolled sheet; pickling the hot rolled sheet to obtain a pickled sheet; cold rolling the pickled sheet with a cumulative rolling reduction of 20% to 75% to obtain a cold rolled sheet; and annealing the cold rolled sheet to a heating temperature of 780°C or higher, wherein the average heating rate in a temperature range of 250°C to 700°C is 100°C / s or less and the residence time from 750°C to the heating temperature is 10 s or more. [5] The method for producing a high-strength steel plate according to [4], wherein in the annealing step, the oxygen concentration of the atmosphere at a temperature of 750°C or higher and the heating temperature or lower is 0.5 volume% or higher and 5.0 volume% or lower, and the dew point of the atmosphere is -35°C or higher. [6] The method for producing a high strength steel plate according to [4] or [5], wherein a cooling step is further carried out following the annealing step, and in that step, the average cooling rate in the temperature range of 250°C or more and 400°C or less is 1.0°C / s or more. [7] The method for producing a high-strength steel plate according to [4] or [5], further comprising a cooling step following the annealing step, in which the steel sheet is kept at a temperature of 100°C or higher and 450°C or lower for 5 s or more. [8] The method for producing a high strength steel sheet according to [4] or [5], further comprising the steps of: performing a cooling step following the annealing step; setting the cooling stop temperature to 250°C or lower; and then reheating the cold-rolled sheet to a reheating temperature of (the cooling stop temperature + 50°C) or higher and 450°C or lower, and maintaining the temperature at the reheating temperature for 5 s or more. [9] A method for producing a high-strength plated steel sheet, comprising the steps of: after the annealing step according to any one of [4] to [8], performing a plating step of plating at least one surface of the cold-rolled sheet.

[10] A member, at least in part, using the high-strength steel plate according to [1] or [2].

[11] A member, at least in part, using the high-strength plated steel sheet according to [3]. Effect of the Invention

[0015] According to the present disclosure, it is possible to provide a high-strength steel plate and member of 1180 MPa or more which has excellent bendability and inter-steel sheet cracking resistance in spot weld HAZ, and which enables the manufacture of parts with high dimensional accuracy. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, embodiments of the present disclosure will be described. Note that the present disclosure is not limited to the following embodiments.

[0017] First, the appropriate range of the composition of the steel sheet and the reason for limiting it will be described. In the following description, "%" representing the content of the component elements of the steel sheet means "mass%" unless otherwise specified. In addition, in this specification, a numerical range expressed with "~" means a range including the numerical values ​​written before and after "~" as the lower and upper limits.

[0018] [C: 0.030% or more and 0.500% or less] C is one of the important basic components of steel. In particular, in the present disclosure, it is an important element that affects the area ratio of martensite and ferrite, the volume ratio of retained austenite, the standard deviation of the Vickers hardness of the steel sheet surface, and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. If the C content is less than 0.030%, the area ratio of martensite decreases, and the area ratio of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. It also makes it difficult to achieve a desired YR. On the other hand, if the C content exceeds 0.500%, the hardness distribution of martensite in the sheet width direction becomes non-uniform. As a result, the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15, and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface exceeds 7 times, resulting in a decrease in bendability. Therefore, the C content is set to 0.030% or more and 0.500% or less. The C content is preferably set to 0.080% or more. The C content is preferably 0.400% or less. The C content is more preferably 0.110% or more. The C content is more preferably 0.350% or less.

[0019] [Si:0.01% or more and 2.50% or less] Si is one of the important basic components of steel, and in particular, in the present disclosure, Si suppresses the formation of carbides during annealing and promotes the formation of retained austenite, and is therefore an element that affects the volume fraction of retained austenite. In addition, Si exhibits high tempering softening resistance at 400°C or less, and is therefore an important element that affects the standard deviation of the Vickers hardness of the steel sheet surface and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. If the Si content is less than 0.01%, the hardness distribution of martensite in the sheet width direction becomes non-uniform. As a result, the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15, and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface exceeds 7 times, resulting in a decrease in bendability. On the other hand, if the Si content exceeds 2.50%, the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction increases, and the inter-sheet cracking properties of the spot weld HAZ are reduced. Therefore, the Si content is set to 0.01% or more and 2.50% or less. The Si content is preferably set to 0.20% or more. The Si content is preferably set to 2.00% or less. The Si content is more preferably set to 0.25% or more. The Si content is more preferably set to 1.50% or less.

[0020] [Mn:0.10% or more and 5.00% or less] Mn is one of the important basic components of steel. In particular, in the present disclosure, Mn is an important element that affects the area ratio of martensite and ferrite, the volume ratio of retained austenite, the standard deviation of the Vickers hardness of the steel sheet surface, and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. If the Mn content is less than 0.10%, the area ratio of martensite decreases, and the area ratio of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. It also makes it difficult to achieve a desired YR. On the other hand, if the Mn content exceeds 5.00%, the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15, and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface exceeds 7 times, resulting in a decrease in bendability. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably set to 1.00% or more. The Mn content is preferably set to 4.00% or less. The Mn content is more preferably 2.00% or more, and more preferably 3.50% or less.

[0021] [P:0.100% or less] If P is excessive, it segregates at the prior austenite grain boundaries, embrittling the grain boundaries and reducing the ultimate deformability of the steel sheet, resulting in reduced bendability. Therefore, the P content must be 0.100% or less. Although there is no particular lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable to set it to 0.001% or more. Therefore, the P content is set to 0.100% or less. The P content is preferably set to 0.001% or more, and more preferably set to 0.070% or less.

[0022] [S:0.0200% or less] S exists as sulfides and reduces the ultimate deformability of steel, which reduces bendability. Therefore, the S content must be 0.0200% or less. Although there is no particular lower limit for the S content, due to constraints on production technology, the S content is preferably 0.0001% or more. Therefore, the S content is 0.0200% or less. The S content is preferably 0.0001% or more. The S content is preferably 0.0050% or less.

[0023] [Al:0.100% or less] When Al becomes excessive, A 3 The transformation point rises and a large amount of ferrite is included in the microstructure, making it difficult to achieve the desired YR. Therefore, the Al content must be 0.100% or less. Although the lower limit of the Al content is not particularly specified, the Al content is preferably 0.001% or more because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite. Therefore, the Al content is 0.100% or less. The Al content is preferably 0.001% or more. The Al content is preferably 0.050% or less.

[0024] [N:0.0100% or less] N exists as a nitride and reduces the ultimate deformability of the steel sheet, which reduces the bendability. Therefore, the N content must be 0.0100% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0005% or more. Therefore, the N content is 0.0100% or less. The N content is preferably 0.0005% or more. The N content is preferably 0.0050% or less.

[0025] [O:0.0100% or less] O exists as an oxide and reduces the ultimate deformability of the steel sheet, which reduces the bendability. Therefore, the O content must be 0.0100% or less. Although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is 0.0100% or less. The O content is preferably 0.0001% or more. The O content is preferably 0.0050% or less.

[0026] [Ti: 0.002% or more and 0.200% or less] Ti increases the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. In addition, the number density of MnS present in the Mn segregation part on the steel sheet surface can be reduced by adding Ti. In order to obtain such an effect, the Ti content needs to be 0.002% or more. On the other hand, if the Ti content exceeds 0.200%, the amount of carbides, nitrides, or carbonitrides increases, making it difficult to achieve a desired YR. Therefore, the Ti content is set to 0.002% or more and 0.200% or less. The Ti content is preferably set to 0.006% or more. The Ti content is preferably set to 0.100% or less. The Ti content is more preferably set to 0.010% or more. The Ti content is more preferably set to 0.050% or less.

[0027] [Effective Ti mole fraction (x) calculated from equation (1) Ti,eff ) is 0.001 or more] Effective Ti mole fraction (x Ti,eff )=x Ti -x N -x S (1) In addition, x in the formula Ti , x N , x S indicates the content (molar fraction) of each element in the steel sheet. By setting the effective Ti molar fraction calculated from the above formula (1) to a certain level or more, the number density of MnS present in the Mn segregation part on the steel sheet surface can be reduced, and good bendability can be achieved. In order to achieve this effect, the effective Ti molar fraction is set to 0.001% or more. Although the upper limit of the effective Ti molar fraction is not particularly specified, the effective Ti molar fraction is preferably set to 0.040 or less because the amount of carbides, nitrides, or carbonitrides increases and it becomes difficult to achieve the desired YR. Therefore, the effective Ti molar fraction is set to 0.001% or more. The effective Ti molar fraction is preferably set to 0.002% or more. The effective Ti molar fraction is preferably set to 0.040% or less. The molar fraction is a molar fraction obtained by converting the components contained as mass% into molar fraction.

[0028] A high-strength steel sheet according to an embodiment of the present invention has a composition containing the above elements with the balance including Fe and unavoidable impurities. Preferably, a high-strength steel sheet according to an embodiment of the present invention has a composition containing the above elements with the balance including Fe and unavoidable impurities. Here, examples of the unavoidable impurities include Zn, Pb, and As. These impurities are allowed to be contained as long as their total amount is 0.100% or less.

[0029] In addition to the above essential components, the composition of the high-strength steel plate of the present disclosure may further contain, by mass%, at least one element selected from Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination.

[0030] [Nb:0.200% or less] Nb generates a large amount of coarse precipitates and inclusions, which reduces the ultimate deformability of the steel sheet, so if the Nb content exceeds 0.200%, the bendability decreases. Therefore, the Nb content is set to 0.200% or less. Although the lower limit of the Nb content is not particularly specified, by setting the Nb content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet and controlling the YR to a desired range. For this reason, the Nb content is preferably set to 0.001% or more. Therefore, when added, the Nb content is set to 0.200% or less. The Nb content is preferably set to 0.001% or more. The Nb content is preferably set to 0.100% or less.

[0031] [V:0.200% or less] V generates a large amount of coarse precipitates and inclusions, which reduces the ultimate deformability of the steel sheet, so that if the V content exceeds 0.200%, the bendability decreases. Therefore, the V content is set to 0.200% or less. Although the lower limit of the V content is not particularly specified, by setting the V content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet and controlling the YR to a desired range. For this reason, the V content is preferably set to 0.001% or more. Therefore, when added, the V content is set to 0.200% or less. The V content is preferably set to 0.001% or more. The V content is preferably set to 0.100% or less.

[0032] [Ta: 0.10% or less, W: 0.10% or less] If the Ta and W contents exceed 0.10%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel sheet, and therefore the bendability is reduced. Therefore, the Ta and W contents are each set to 0.10% or less. Although the lower limits of the Ta and W contents are not particularly specified, the Ta and W contents are preferably set to 0.01% or more, since they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, when added, the Ta and W contents are each set to 0.10% or less. The Ta and W contents are preferably set to 0.01% or more. The Ta and W contents are preferably set to 0.08% or less, respectively.

[0033] [B:0.0100% or less] If B is 0.0100% or less, cracks will not be generated inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not be reduced, so that bendability will not be reduced. Therefore, the B content is preferably 0.0100% or less. Although there is no particular lower limit for the B content, since B is an element that segregates to the austenite grain boundaries during annealing and improves hardenability, the B content is more preferably 0.0003% or more. Therefore, when B is contained, its content is 0.0100% or less. The B content is more preferably 0.0003% or more. The B content is further preferably 0.0080% or less.

[0034] [Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less] If the contents of Cr, Mo, and Ni exceed 1.00% each, coarse precipitates and inclusions increase, lowering the ultimate deformability of the steel sheet, and thus reducing the bendability. Therefore, the contents of Cr, Mo, and Ni are each set to 1.00% or less. Although there is no particular lower limit for the contents of Cr, Mo, and Ni, these elements improve the hardenability, so the contents of Cr, Mo, and Ni are preferably set to 0.01% or more each. Therefore, when added, the contents of Cr, Mo, and Ni are each set to 1.00% or less. The contents of Cr, Mo, and Ni are preferably set to 0.01% or more. The contents of Cr, Mo, and Ni are preferably set to 0.80% or less.

[0035] [Co:0.010% or less] If the Co content exceeds 0.010%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the bendability decreases. Therefore, the Co content is set to 0.010% or less. Although there is no particular lower limit for the Co content, since Co is an element that improves hardenability, it is preferable that the Co content be 0.001% or more. Therefore, when Co is added, the Co content is set to 0.010% or less. The Co content is preferably 0.001% or more. The Co content is preferably 0.008% or less.

[0036] [Cu:1.00% or less] If the Cu content exceeds 1.00%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore reduces the bendability. Therefore, the Cu content is set to 1.00% or less. Although there is no particular lower limit for the Cu content, since Cu is an element that improves hardenability, it is preferable that the Cu content be 0.01% or more. Therefore, when added, the Cu content is set to 1.00% or less. The Cu content is preferably 0.01% or more. The Cu content is preferably 0.80% or less.

[0037] [Sn:0.200% or less] If the Sn content exceeds 0.200%, cracks are generated inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet is reduced, resulting in reduced bendability. Therefore, the Sn content is set to 0.200% or less. Although there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability, the Sn content is preferably set to 0.001% or more. Therefore, when Sn is added, the Sn content is set to 0.200% or less. The Sn content is preferably set to 0.001% or more. The Sn content is preferably set to 0.100% or less.

[0038] [Sb:0.200% or less] If the Sb content exceeds 0.200%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the bendability decreases. Therefore, the Sb content is set to 0.200% or less. Although there is no particular lower limit for the Sb content, since Sb is an element that controls the surface softening thickness and enables strength adjustment, the Sb content is preferably set to 0.001% or more. Therefore, when Sb is added, the Sb content is set to 0.200% or less. The Sb content is preferably set to 0.001% or more. The Sb content is preferably set to 0.100% or less.

[0039] [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 more than 0.0100%, coarse precipitates and inclusions increase, lowering the ultimate deformability of the steel sheet, and thus reducing the bendability. Therefore, the contents of Ca, Mg and REM are each set to 0.0100% or less. Although there is no particular lower limit for the contents of Ca, Mg and REM, these elements are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, so it is preferable that the contents of Ca, Mg and REM are each set to 0.0005% or more. Therefore, when added, the contents of Ca, Mg and REM are each set to 0.0100% or less. The contents of Ca, Mg and REM are preferably set to 0.0005% or more. The contents of Ca, Mg and REM are preferably set to 0.0050% or less. Note that REM (rare earth elements) is a general term for 15 elements ranging from Sc, Y, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here refers to the total content of these elements.

[0040] [Zr: 0.100% or less, Te: 0.100% or less] If the Zr and Te contents are each more than 0.100%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the bendability decreases. Therefore, the Zr and Te contents must each be 0.100% or less. Although there is no particular lower limit for the Zr and Te contents, the Zr and Te contents are preferably 0.001% or more, since they are elements that make the shape of nitrides and sulfides spheroidal and improve the ultimate deformability of the steel sheet. Therefore, when added, the Zr and Te contents are 0.100% or less. The Zr and Te contents are preferably 0.001% or more, respectively. The Zr and Te contents are preferably 0.080% or less, respectively.

[0041] [Hf:0.10% or less] If the Hf content exceeds 0.10%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the bendability decreases. Therefore, the Hf content is set to 0.10% or less. Although there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, the Hf content is preferably set to 0.01% or more. Therefore, when Hf is added, the Hf content is set to 0.10% or less. The Hf content is preferably set to 0.01% or more. The Hf content is preferably set to 0.08% or less.

[0042] [Bi:0.200% or less] If the Bi content exceeds 0.200%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the bendability decreases. Therefore, the Bi content is set to 0.200% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is preferably set to 0.001% or more. Therefore, when Bi is added, the Bi content is set to 0.200% or less. The Bi content is preferably set to 0.001% or more. The Bi content is preferably set to 0.100% or less.

[0043] In addition, when the contents of the above-mentioned Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi are less than the preferable lower limit, the effect of the present invention is not impaired. Therefore, they are included as unavoidable impurities.

[0044] Next, the steel structure of the steel plate will be described.

[0045] [Area ratio of martensite: 80% to 99%] If the area ratio of martensite is less than 80%, the area ratio of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. It also makes it difficult to achieve a desired YR. Furthermore, the hardness distribution in the sheet width direction is made non-uniform, so the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15, and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface exceeds 7 times, resulting in a decrease in bendability. On the other hand, if the area ratio of martensite exceeds 99%, ferrite and / or retained austenite effective for controlling the YR does not exist in the steel structure, making it difficult to achieve a desired YR. Therefore, the area ratio of martensite is set to 80% or more and 99% or less. The area ratio of martensite is preferably 85% or more. The area ratio of martensite is preferably 98% or less. The area ratio of martensite is more preferably 87% or more. The area ratio of martensite is preferably 97% or less. The martensite referred to here includes tempered martensite and bainite in addition to quenched martensite (fresh martensite). The observation position for the area ratio of martensite is set at 1 / 4 of the thickness of the steel plate, as described later.

[0046] [Total area fraction of ferrite and / or volume fraction of retained austenite: more than 0% and up to 20%] If the total of the area ratio of ferrite and / or the volume ratio of retained austenite is 0%, the steel structure will be a martensite single phase structure, making it difficult to achieve the desired YR. On the other hand, if the total of the area ratio of ferrite and / or the volume ratio of retained austenite exceeds 20%, the area ratio of martensite will decrease, making it difficult to achieve a TS of 1180 MPa or more. It will also be difficult to achieve the desired YR. Furthermore, the hardness distribution in the sheet width direction will be made non-uniform, so the standard deviation of the Vickers hardness of the steel sheet surface will exceed 15, and the frequency of hardness fluctuations per 1100 μm in the sheet width direction on the steel sheet surface will exceed 7 times, resulting in reduced bendability. Therefore, the total of the area ratio of ferrite and / or the volume ratio of retained austenite is set to more than 0% and 20% or less. The total of the area ratio of ferrite and / or the volume ratio of retained austenite is preferably 1% or more. The area ratio of martensite is preferably 18% or less. The area ratio of martensite is more preferably 2% or more. The area ratio of martensite is preferably 15% or less. Note that the ferrite referred to here includes bainitic ferrite. Note that the observation position for the area ratio of ferrite and the volume ratio of retained austenite is set at a 1 / 4 position of the sheet thickness of the steel sheet, as described later.

[0047] Here, the method for measuring the area ratios of martensite (quenched martensite, tempered martensite, and bainite) and ferrite (bainitic ferrite) is as follows.

[0048] After cutting out the sample so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface, the observation surface is mirror-polished using diamond paste, and then etched with 3 vol.% nital to reveal the structure. Using a scanning electron microscope (SEM) under the condition of an acceleration voltage of 15 kV, the observation position is set to 1 / 4 of the plate thickness of the steel plate, and three fields of view are observed at a magnification of 5000 times and a field of view of 17 μm × 23 μm. The obtained structure image is used Adobe Photoshop of Adobe Systems Inc. to calculate the area ratio of each constituent structure divided by the measured area for three fields of view. Here, each constituent structure means ferrite (bainitic ferrite) and martensite (tempered martensite, bainite, and quenched martensite). The area ratio of each structure is calculated by averaging these values. In the above structural image, ferrite (bainitic ferrite) is a flat structure that does not contain carbides in the recessed portion, and tempered martensite and bainite are structures that contain fine carbides in the recessed portion. Hardened martensite is a structure that has fine irregularities in the interior of the structure in the protruding portion, and they are distinguishable from each other. Note that tempered martensite, tempered martensite, and bainite do not need to be distinguishable from each other because the total area ratio is calculated as the area ratio of martensite.

[0049] The volume fraction of retained austenite is measured by the following method.

[0050] After grinding, the steel plate is polished by chemical polishing for an additional 0.1 mm so that the observation surface is located 1 / 4 of the plate thickness from the surface (a position corresponding to 1 / 4 of the plate thickness from the surface of the steel plate in the depth direction). For this surface, an X-ray diffraction device is used with a Co Kα source to measure the integrated reflection intensities of the (200), (220), and (311) surfaces of fcc iron (austenite) and the (200), (211), and (220) surfaces of bcc iron. The volume fraction of austenite is calculated from the intensity ratio of the integrated reflection intensity from each surface of fcc iron (austenite) to the integrated reflection intensity from each surface of bcc iron, and this is taken as the volume fraction of retained austenite.

[0051] [Si concentration at 5 μm from the steel plate surface in the plate thickness direction: 0.10% to 0.60%] This is an extremely important invention constituent element in the present disclosure. In order to control the inter-steel plate crack resistance of the spot weld HAZ, it is important to control the concentration of elements at a position 5 μm from the steel plate surface in the plate thickness direction. In particular, by reducing the Si concentration at a position 5 μm from the steel plate surface in the plate thickness direction, the inter-steel plate crack resistance of the spot weld HAZ can be improved. In order to obtain such an effect, it is necessary to make the Si concentration at a position 5 μm from the steel plate surface in the plate thickness direction 0.60% or less. On the other hand, if the Si concentration at a position 5 μm from the steel plate surface in the plate thickness direction is less than 0.10%, the hardness distribution of martensite in the plate width direction becomes non-uniform. Therefore, the standard deviation of the Vickers hardness of the steel plate surface exceeds 15, and further, the frequency of hardness fluctuation per 1100 μm in the plate width direction on the steel plate surface exceeds 7 times, and the bendability is reduced. Therefore, the Si concentration at a position 5 μm from the steel plate surface in the plate thickness direction is set to 0.10% or more and 0.60% or less. The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is preferably 0.15% or more. The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is preferably 0.55% or less. The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is more preferably 0.20% or more. The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is preferably 0.50% or less. The unit of this Si concentration is mass%.

[0052] The method for measuring the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is as follows.

[0053] A sample of 20 mm in the rolling direction and 20 mm in the width direction is cut from the steel plate. The surface of the high-strength steel plate is used as the measurement surface, and measurements are made using glow discharge optical emission spectrometry (GDS). The Si concentration is analyzed along the plate thickness direction under conditions of a high-frequency discharge pressure of 300 Pa, a high-frequency output of 35 W, and a pulse frequency of 100 Hz. The Si concentration at a position 5 μm from the steel plate surface is averaged, and calculated as the Si concentration at a position 5 μm from the plate surface. The measurement data is converted to Si concentration using the calibration curve method.

[0054] [MnS density in the Mn segregated area on the steel sheet surface: 5.0 particles / mm 2 below] This is an extremely important invention constituent element in the present disclosure. In order to control bendability, it is important to control the number density of inclusions present in the Mn segregated parts of the steel sheet surface, that is, in the regions containing martensite which is harder than the surrounding area. Bendability can be improved by reducing the number density of MnS present in the Mn segregated parts of the steel sheet surface. To obtain this effect, it is necessary to reduce the number density of MnS present in the Mn segregated parts of the steel sheet surface to 5.0 pieces / mm 2 Although there is no particular lower limit for the number density of MnS present in the Mn segregated portion on the steel sheet surface, the lower the number density of MnS, the better, and the lower the number density of MnS, the better. 2 Therefore, the effect of the present disclosure can be obtained even if the number density of MnS present in the Mn segregated part on the steel sheet surface is 5.0 pieces / mm 2 The number density of MnS present in the Mn segregated portion of the steel sheet surface is preferably 0.0 particles / mm 2 The number density of MnS present in the Mn segregated portion on the steel sheet surface is preferably 4.0 particles / mm 2 The following applies.

[0055] Here, the method for measuring the number density of MnS present in the Mn segregated portion on the steel sheet surface is as follows.

[0056] A bending test is performed using the V-block method with a bending angle of 90 degrees, with R being such that R / t is approximately 4.5, i.e., 4.3 to 4.7. A sample of 20 mm in the rolling direction and 5 mm in the width direction is cut to include the crack at the ridgeline of the bent apex. The outer surface of the bent specimen is used as the observation surface, and the observation surface is mirror-polished using diamond paste. Then, measurements are performed using an electron probe micro analyzer (EPMA) (JXA-8230: manufactured by JEOL Ltd.). At this time, the accelerating voltage is 15 kV, the measurement area is 1.2 mm in the rolling direction × 1.0 mm in the width direction, and the irradiation current is 1.0 × 10 -7 Under condition A, Mn and S are measured in three visual fields. The measurement data is converted to C concentration using the calibration curve method. The three visual fields obtained are then mapped with Mn element, and the locations where large amounts of Mn are detected are identified as Mn segregation areas. Furthermore, by mapping with S element, the S-enriched areas in the Mn segregation areas, that is, MnS, are identified and their number is evaluated. The number of MnS particles present in the obtained Mn segregation areas is counted on a measurement area of ​​1.2 mm2. 2 The number density of MnS present in the Mn segregated area on the steel sheet surface is calculated by dividing the number density by the number of MnS particles.

[0057] [Standard deviation of Vickers hardness of steel plate surface: 15 or less] This is an extremely important invention constituent element in the present disclosure. In order to control the bendability, it is important to make the hardness distribution of the steel sheet surface uniform, and the bendability can be improved by reducing the standard deviation of the Vickers hardness of the steel sheet surface. In order to obtain such an effect, it is necessary to make the standard deviation of the Vickers hardness of the steel sheet surface 15 or less. Although the lower limit of the standard deviation of the Vickers hardness of the steel sheet surface is not particularly limited, the lower the standard deviation of the Vickers hardness of the steel sheet surface, the more preferable it is, and even if it is 0, the effect of the present disclosure can be obtained. Therefore, the standard deviation of the Vickers hardness of the steel sheet surface is 15 or less. The standard deviation of the Vickers hardness of the steel sheet surface is preferably 0 or more. The standard deviation of the Vickers hardness of the steel sheet surface is preferably 13 or less.

[0058] The method for measuring the standard deviation of the Vickers hardness of the steel sheet surface is as follows.

[0059] A bending test is performed by the V-block method with a bending angle of 90 degrees, and a bending test is performed at R where R / t is about 4.5, i.e., 4.3 to 4.7. A sample of 20 mm in the rolling direction and 5 mm in the width direction is cut to include the crack at the ridge of the bend apex. The surface on the outside of the bend is used as the observation surface, and the observation surface is mirror-polished using diamond paste. Next, the Vickers hardness of the observation surface after mirror polishing is measured at 11 points along the plate width direction at 100 μm intervals under a load of 100 gf using a Vickers hardness tester. The measurement position is a position 500 μm away from the end of the crack in the rolling direction, and the sixth point is measured parallel to the crack. The standard deviation is calculated from the obtained results to calculate the standard deviation of the Vickers hardness of the steel plate surface.

[0060] [Hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface: 7 times or less] This is an extremely important invention constituent element in the present disclosure. By reducing the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface, the desired bendability can be achieved. In order to obtain such an effect, it is necessary to make the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface 7 times or less. The lower limit of the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface is not particularly limited, but the lower the hardness fluctuation frequency, the more preferable it is, and even if it is 0 times, the effect of the present disclosure can be obtained. Therefore, the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface is 7 times or less. The hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface is preferably 0 times or more. The hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface is preferably 6 times or less.

[0061] Here, the frequency of hardness variation per 1100 μm in the sheet width direction on the surface of the steel sheet is as follows.

[0062] A bending test is performed by the V-block method with a bending angle of 90 degrees, and the bending test is performed at an R where R / t is about 4.5, i.e., 4.3 to 4.7. A sample of 20 mm in the rolling direction and 5 mm in the width direction is cut to include the crack at the ridge of the bend apex. The outer surface of the bend is used as the observation surface, and the observation surface is mirror-polished using diamond paste. Next, the Vickers hardness of the observation surface after mirror polishing is measured at 11 points along the plate width direction at intervals of 100 μm under a load of 100 gf using a Vickers hardness tester. The measurement position is a position 500 μm away from the end of the crack in the rolling direction, and the sixth point is parallel to the crack. From the obtained results, a hardness distribution is created by measuring the surface of the steel plate in the plate width direction using a Vickers hardness tester. In the hardness distribution, first, {(maximum hardness Hv max )-(Minimum hardness value Hv min )} / 2. {(Maximum hardness Hv max )-(Minimum hardness value Hv min The value of {0.01} / 2 is used as the standard fluctuation amount, and each time the hardness fluctuates above or below the standard fluctuation amount, it is counted as one occurrence, and the number of times the hardness fluctuates in the area where the hardness was measured (length 1100 μm) is measured. In other words, a time when the hardness fluctuates above or below the standard fluctuation amount is counted as one occurrence, and a time when the hardness fluctuates below the standard fluctuation amount is counted as one occurrence. Therefore, when the hardness fluctuates above and below once, the hardness fluctuation frequency is two occurrences.

[0063] Furthermore, the steel structure according to the present disclosure may contain a remaining structure other than the above-mentioned martensite (quenched martensite, tempered martensite, bainite), ferrite (including bainitic ferrite), and retained austenite. The remaining structure may contain carbides such as pearlite, cementite, and metastable carbides, as well as other known structures of steel sheets, so long as the area ratio is within a range of 5% or less, without impairing the effects of the present disclosure. Here, metastable carbides include, for example, epsilon (ε) carbide, eta (η) carbide, chi (χ) carbide, and the like.

[0064] [High strength steel plate] The chemical composition and steel structure of the high strength steel plate are as described above. The plate thickness of the high strength steel plate is not particularly limited, but is usually 0.3 mm or more and 2.8 mm or less.

[0065] [High-strength plated steel sheet] The high-strength plated steel sheet of the present disclosure is a high-strength plated steel sheet having a plating layer on at least one side of the high-strength steel sheet of the present disclosure. The type of plating layer is not particularly limited, and may be, for example, either a hot-dip plating layer or an electroplating layer. The plating layer may also be an alloyed plating layer. The plating layer is preferably a zinc plating layer. The zinc plating layer may contain Al and Mg. Also, hot-dip zinc-aluminum-magnesium alloy plating (Zn-Al-Mg plating layer) is also preferred. In this case, it is preferable that the Al content is 1 mass% or more and 22 mass% or less, the Mg content is 0.1 mass% or more and 10 mass% or less, and the balance is Zn. In addition, in the case of a Zn-Al-Mg plating layer, in addition to Zn, Al, and Mg, one or more selected from Si, Ni, Ce, and La may be contained in a total of 1 mass% or less. In addition, since the plating metal is not particularly limited, in addition to the above-mentioned Zn plating, Al plating, etc. may be used.

[0066] The composition of the plating layer is not particularly limited, and may be any common one. For example, in the case of a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, it generally contains Fe: 20 mass% or less, and Al: 0.001 mass% or more and 1.0 mass% or less. Furthermore, it contains one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0 mass% or more and 3.5 mass% or less, with the balance being Zn and unavoidable impurities. In the present disclosure, the plating coating weight per side is 20 to 80 g / m 2 It is preferable that the coating layer has a hot-dip galvanized layer, which is further alloyed with the hot-dip galvanized layer. When the coating layer is a hot-dip galvanized layer, the Fe content in the coating layer may be less than 7 mass%, and when the coating layer is an alloyed hot-dip galvanized layer, the Fe content in the coating layer may be 7 to 20 mass%.

[0067] Next, a method for producing the high strength steel plate according to the present disclosure will be described.

[0068] First, a steel material having the above-mentioned composition is melted to produce a steel slab. In the present disclosure, the method of melting the steel material is not particularly limited, and any known melting method such as a converter or an electric furnace is suitable. In addition, the steel slab is preferably produced by a continuous casting method to prevent macrosegregation, but it can also be produced by an ingot casting method or a thin slab casting method. In addition to the conventional method of cooling the steel slab to room temperature after production and then reheating it, energy-saving processes such as direct rolling and direct rolling, in which the steel slab is charged into a heating furnace without being cooled, or is immediately rolled after a short period of heat retention, can also be applied without any problems.

[0069] Next, the steel slab is heated at an average heating rate of 25°C / min or less in the temperature range of 900°C to 1150°C, a slab heating temperature of 1150°C or more, and a residence time from 1100°C to the slab heating temperature of 20 min or more.

[0070] [Average heating rate of slab in the temperature range of 900℃ to 1150℃: 25℃ / min or less] This is an extremely important invention constituent element in the present disclosure. By slowing down the average heating rate of the slab in the temperature range of 900°C to 1150°C to 25°C / min or less, it is possible to promote the desorption of Si from the steel sheet surface and reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, by slowing down the average heating rate of the slab in the temperature range of 900°C to 1150°C, it is possible to reduce the Mn segregation formed during casting and reduce the number density of MnS present in the Mn segregation part on the steel sheet surface. Furthermore, it is possible to reduce the standard deviation of the Vickers hardness of the steel sheet surface and reduce the frequency of hardness fluctuation per 1100 μm in the sheet width direction on the steel sheet surface. In order to obtain these effects, the average heating rate of the slab in the temperature range of 900°C to 1150°C is set to 25°C / min or less. Although the lower limit of the average heating rate of the slab in the temperature range of 900°C to 1150°C is not particularly specified, it is preferably 5°C / min or more in order to suitably prevent an increase in the softened thickness of the surface layer after annealing and to keep the TS within a more suitable range. Therefore, the average heating rate of the slab in the temperature range of 900°C to 1150°C is 25°C / min or less. The average heating rate of the slab in the temperature range of 900°C to 1150°C is preferably 5°C / min or more. The average heating rate of the slab in the temperature range of 900°C to 1150°C is preferably 15°C / min or less. The slab heating temperature is the surface temperature of the steel slab during slab heating.

[0071] [Slab heating temperature: 1150℃ or higher] In the present disclosure, this is an extremely important invention constituent element. By increasing the slab heating temperature to 1150°C or more, it is possible to promote the desorption of Si from the steel sheet surface and reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, by increasing the slab heating temperature, Mn segregation formed during casting is reduced, and the number density of MnS present in the Mn segregated part on the steel sheet surface is reduced. In addition, it is possible to reduce the standard deviation of the Vickers hardness of the steel sheet surface and reduce the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. In order to obtain such effects, the slab heating temperature is set to 1150°C or more. Note that the upper limit of the slab reheating temperature is not particularly specified, but it is preferable to set it to 1300°C or less in order to suitably prevent an increase in the surface softening thickness after annealing and to set the TS within a more suitable range. Therefore, the slab heating temperature is set to 1150°C or more. The slab heating temperature is preferably set to 1180°C or more. The slab heating temperature is preferably set to 1300°C or less. The slab heating temperature refers to the temperature of the surface of the steel slab during heating.

[0072] [Dwell time from 1100℃ to the slab heating temperature: 20min or more] This is an extremely important invention constituent element in the present disclosure. By lengthening the residence time from 1100°C to the slab heating temperature to 20 min or more, it is possible to promote the desorption of Si from the steel sheet surface and reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, by lengthening the residence time from 1100°C to the slab heating temperature, it is possible to reduce Mn segregation formed during casting and reduce the number density of MnS present in the Mn segregation part on the steel sheet surface. By lengthening the residence time from 1100°C to the slab heating temperature, it is possible to further reduce the standard deviation of the Vickers hardness of the steel sheet surface and reduce the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. In order to obtain such effects, the residence time from 1100°C to the slab heating temperature is set to 20 min or more. Although there is no particular upper limit for the residence time from 1100°C to the slab heating temperature, it is preferable to set it to 100 min or less in order to suitably prevent an increase in the softened thickness of the surface layer after annealing and to set TS within a more suitable range. Therefore, the residence time from 1100°C to the slab heating temperature is 20 min or more. The residence time from 1100°C to the slab heating temperature is preferably 30 min or more. The residence time from 1100°C to the slab heating temperature is preferably 100 min or less. The slab heating temperature is the surface temperature of the steel slab during slab heating.

[0073] The slab is rough-rolled under normal conditions to produce a sheet bar. When the slab heating temperature is low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.

[0074] [Reduction rate of the final pass of finishing rolling: 9% to 15%] [Reduction rate of the pass before the final pass: 15% to 21%] [Reduction rate of the pass two passes before the final pass: 21% to 27%] This is an extremely important invention constituent element in the present disclosure. The reduction ratio of the pass one pass before the final pass is set to be equal to or greater than the reduction ratio of the final pass, and the reduction ratio of the pass two passes before the final pass is set to be equal to or greater than the reduction ratio of the pass one pass before the final pass. This makes it possible to appropriately control the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction, the standard deviation of the Vickers hardness of the steel sheet surface, and the frequency of hardness fluctuation per 1100 μm in the sheet width direction on the steel sheet surface.

[0075] If the reduction rate of the final pass of the finish rolling is less than 9%, the austenite grain size on the steel sheet surface during hot rolling becomes coarse, i.e., the crystal grain size in the annealed sheet becomes coarse, and the diffusion of Si to the steel sheet surface is suppressed. As a result, it is not possible to reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, since Ti precipitation, i.e., sulfide precipitation, is suppressed, it is not possible to reduce the number density of MnS present in the Mn segregation part on the steel sheet surface. On the other hand, if the reduction rate of the final pass exceeds 15%, it is not possible to reduce the Mn segregation formed during casting. Therefore, it is not possible to reduce the standard deviation of the Vickers hardness of the steel sheet surface or the frequency of hardness fluctuation per 1100 μm in the sheet width direction on the steel sheet surface. Therefore, the reduction rate of the final pass of the finish rolling is set to 9% or more and 15% or less.

[0076] If the reduction rate before the final pass is less than 15%, the austenite grain size on the steel sheet surface during hot rolling becomes coarse, that is, the crystal grain size in the annealed sheet becomes coarse, and the diffusion of Si to the steel sheet surface is suppressed. As a result, it is not possible to reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, since Ti precipitation, that is, precipitation of sulfides, is suppressed, it is not possible to reduce the number density of MnS present in the Mn segregation part on the steel sheet surface. On the other hand, if the reduction rate before the final pass exceeds 21%, it is not possible to reduce the Mn segregation formed during casting. Therefore, it is not possible to reduce the standard deviation of the Vickers hardness of the steel sheet surface or the frequency of hardness fluctuation per 1100 μm in the sheet width direction on the steel sheet surface. Therefore, the reduction rate before the final pass of finish rolling is set to 15% or more and 21% or less.

[0077] If the reduction rate of the second pass before the final pass is less than 21%, the austenite grain size on the steel sheet surface during hot rolling becomes coarse, that is, the crystal grain size in the annealed sheet becomes coarse, and the diffusion of Si to the steel sheet surface is suppressed. As a result, it is not possible to reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, since Ti precipitation, that is, precipitation of sulfides, is suppressed, it is not possible to reduce the number density of MnS present in the Mn segregation part on the steel sheet surface. On the other hand, if the reduction rate of the second pass before the final pass exceeds 27%, it is not possible to reduce the Mn segregation formed during casting. Therefore, it is not possible to reduce the standard deviation of the Vickers hardness of the steel sheet surface or the frequency of hardness fluctuation per 1100 μm in the sheet width direction on the steel sheet surface. Therefore, the reduction rate of the second pass before the final pass of finish rolling is set to 21% or more and 27% or less.

[0078] Finish rolling increases the rolling load and increases the reduction ratio in the unrecrystallized state of austenite, which can lead to the development of abnormal structures elongated in the rolling direction, which can reduce the workability of the annealed sheet. 3 It is preferable to carry out the finish rolling at a temperature equal to or higher than the transformation point. In addition, the coiling temperature after hot rolling is preferably 300°C or higher and 700°C or lower in order to improve the workability after annealing. 3 The transformation temperature is calculated using the following formula. Ar 3 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 [% element symbol] indicates the content (mass%) of the corresponding element in the above composition, and is set to 0 when the corresponding element is not contained.

[0079] In addition, the rough rolled sheets may be joined together during hot rolling and continuously finished rolling may be performed. The rough rolled sheets may be wound once. In order to reduce the rolling load during hot rolling, a part or all of the finish rolling may be lubricated rolling. Lubricated rolling is also effective from the viewpoint of uniforming the shape of the steel sheet and the material. The friction coefficient during lubricated rolling is preferably 0.10 or more and 0.25 or less.

[0080] The hot-rolled steel sheet thus produced is subjected to pickling. Pickling can remove oxides from the steel sheet surface, and is therefore important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet product. Pickling may be performed once or multiple times.

[0081] Next, the hot-rolled sheet after pickling or the hot-rolled sheet (hot-rolled annealed sheet) optionally subjected to heat treatment after pickling is cold-rolled to obtain a cold-rolled sheet. Since strain is uniformly and efficiently introduced and a uniform structure is obtained, it is preferable to perform cold rolling by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0082] In order to introduce processing strain into the steel sheet surface and reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction during annealing, it is preferable to perform bending and bending back processing at least once before cold rolling. The number of times of bending and bending back processing before cold rolling is not particularly specified, but it is preferable to perform it at least twice, and more preferably three times. In addition, the bending and bending back processing before cold rolling is generally performed using rolls with a roll diameter of 300 to 1500 mm.

[0083] [Cumulative reduction rate of cold rolling: 20% to 75%] By increasing the cumulative reduction rate of cold rolling, the area ratio of ferrite can be reduced, that is, the total of the area ratio of ferrite and / or the volume ratio of retained austenite can be set to 20% or less. In addition, by increasing the cumulative reduction rate of cold rolling, a large amount of shear bands can be introduced into the steel sheet surface, and the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction can be reduced. In order to obtain such effects, the cumulative reduction rate of cold rolling is set to 20% or more. On the other hand, if the cumulative reduction rate of cold rolling exceeds 75%, the grain size of austenite generated during annealing becomes fine, and the amount of retained austenite in the annealed sheet increases. In other words, the total area ratio of ferrite and / or the volume ratio of retained austenite increases, and therefore the desired YR cannot be realized. In addition, the hardness distribution in the sheet width direction becomes non-uniform, so that the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15, and the frequency of hardness fluctuations per 1100 μm in the sheet width direction on the steel sheet surface exceeds 7 times, resulting in a decrease in bendability. Therefore, the cumulative reduction ratio of cold rolling is set to 20% or more and 75% or less. The cumulative reduction ratio of cold rolling is preferably set to 25% or more. The cumulative reduction ratio of cold rolling is preferably set to 70% or less. The cumulative reduction ratio of cold rolling is more preferably set to 27% or more. The cumulative reduction ratio of cold rolling is more preferably set to 60% or less.

[0084] The cold-rolled sheet obtained as described above is subjected to an annealing process under the following annealing conditions.

[0085] [Average heating rate in the temperature range from 250°C to 700°C: 100°C / s or less] By reducing the average heating rate in the temperature range of 250°C to 700°C, Si diffuses to the steel sheet surface, and the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction can be reduced. In order to obtain such an effect, the average heating rate in the temperature range of 250°C to 700°C must be 100°C / s or less. Although the lower limit of the average heating rate in the temperature range of 250°C to 700°C is not particularly specified, it is preferably 5°C / s or more, more preferably 10°C / s or more, from the viewpoint of suppressing coarsening of austenite grain size during heating and optimizing YR. Therefore, the average heating rate in the temperature range of 250°C to 700°C is 100°C / s or less. The average heating rate in the temperature range of 250°C to 700°C is preferably 5°C / s or more. The average heating rate in the temperature range of 250°C to 700°C is preferably 75°C / s or less. The average heating rate in the temperature range of 250° C. or more and 700° C. or less is more preferably 10° C. / s or more. The average heating rate in the temperature range of 250° C. or more and 700° C. or less is more preferably 50° C. / s or less. The average heating rate is measured based on the temperature of the steel sheet surface.

[0086] [Heating temperature: 780℃ or higher] If the heating temperature (annealing temperature) is less than 780°C, the annealing process is performed in the two-phase region of ferrite and austenite, and since a large amount of ferrite is contained after annealing, a TS of 1180 MPa or more cannot be achieved, and it is difficult to achieve the desired YR. In addition, the hardness distribution in the sheet width direction is made non-uniform, so the standard deviation of the Vickers hardness of the steel sheet surface exceeds 15, and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface exceeds 7 times, resulting in a decrease in bendability. Furthermore, since Si cannot diffuse to the steel sheet surface and the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction cannot be reduced, the inter-steel sheet cracking resistance property of the spot welded HAZ cannot be improved. Note that the upper limit of the heating temperature is not particularly specified, but since an increase in the heating temperature increases the surface softening thickness after annealing, reduces the TS, and coarsens the prior austenite grain size, reducing the YR, it is preferable that the heating temperature is 1000°C or less. Therefore, the heating temperature is set to 780°C or more. The heating temperature is more preferably 820° C. or higher. The heating temperature is further preferably 830° C. or higher. The upper limit of the heating temperature is preferably 1000° C. or lower. The upper limit of the heating temperature is more preferably 980° C. or lower. The heating temperature is measured based on the temperature of the steel sheet surface.

[0087] [Dwell time between 750℃ and the heating temperature: 10s or more] In the present disclosure, this is an extremely important invention constituent element. By increasing the residence time between 750°C and the heating temperature, it is possible to promote the desorption of Si from the plate thickness surface and reduce the Si concentration at a position 5 μm from the plate surface in the plate thickness direction. In addition, it is possible to reduce Mn segregation formed during casting, reduce the standard deviation of the Vickers hardness of the steel plate surface, and reduce the hardness fluctuation frequency per 1100 μm in the plate width direction on the steel plate surface. In order to obtain such effects, the residence time between 750°C and the heating temperature is set to 10 s or more. Note that the upper limit of the residence time between 750°C and the heating temperature is not particularly specified. However, if the residence time between 750°C and the heating temperature is increased, the surface softening thickness after annealing may increase and TS may decrease, or the prior austenite grain size may become coarse and YR may decrease, so that 400 s or less is preferable. Therefore, the residence time between 750°C and the heating temperature is set to 10 s or more. The residence time between 750°C and the heating temperature is preferably set to 20 s or more. The residence time from 750°C to the heating temperature is preferably 400s or less. The residence time from 750°C to the heating temperature is more preferably 25s or more. The residence time up to 750°C to the heating temperature is more preferably 300s or less.

[0088] [Oxygen concentration in the atmosphere at temperatures above 750°C and below the heating temperature: 0.5% by volume to 5.0% by volume (optimal conditions)] During annealing, by increasing the oxygen concentration at 750°C or higher and the heating temperature or lower, it is possible to promote Si desorption from the sheet thickness surface via oxygen in the air, and reduce the Si concentration at a position of 5 μm from the steel sheet surface in the sheet thickness direction. In addition, it is possible to promote Mn desorption from the steel sheet surface, reduce the standard deviation of the Vickers hardness of the steel sheet surface, and reduce the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. In order to obtain such effects, the oxygen concentration at 750°C or higher and the heating temperature or lower is preferably 0.5% by volume or higher, more preferably 1.0% by volume or higher, and even more preferably 1.5% by volume or higher. On the other hand, with an increase in the oxygen concentration at 750°C or higher and the heating temperature or lower, the softened surface thickness after annealing increases and the TS decreases. Therefore, the oxygen concentration at 750°C or higher and the heating temperature or lower is preferably 5.0% by volume or lower, more preferably 4.5% by volume or lower, and even more preferably 4.0% by volume or lower. The above temperature at 750°C or higher and the heating temperature or lower is based on the steel sheet surface temperature. That is, when the surface temperature of the steel sheet is in the range of 750° C. or higher and the heating temperature or lower, the oxygen concentration is adjusted to fall within the above range.

[0089] [Dew point of atmosphere below heating temperature of 750℃ or higher: -35℃ or higher (optimal conditions)] During annealing, by increasing the dew point of the atmosphere at 750°C or higher and the heating temperature or lower, it is possible to promote Si desorption from the sheet thickness surface through moisture in the air, and reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. In addition, it is possible to promote Mn desorption from the steel sheet surface, reduce the standard deviation of the Vickers hardness of the steel sheet surface, and reduce the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface. In order to obtain such effects, the dew point at 750°C or higher and the heating temperature or lower is preferably -35°C or higher, more preferably -30°C or higher, and even more preferably -25°C or higher. Note that the upper limit of the dew point at 750°C or higher and the heating temperature or lower is not particularly specified. However, since the surface softening thickness after annealing increases and TS decreases, the dew point at 750°C or higher and the heating temperature or lower is preferably 15°C or lower, more preferably 5°C or lower. Note that the temperature at 750°C or higher and the heating temperature or lower is based on the steel sheet surface temperature. That is, when the surface temperature of the steel sheet is 750° C. or higher and the heating temperature or lower, the dew point is adjusted to fall within the above range.

[0090] [Cooling process] After the annealing step, the cold-rolled sheet is optionally cooled. The average cooling rate at a temperature range of 400°C or more below the heating temperature is not particularly limited, but is preferably 5°C / s or more and 30°C / s or less. In addition, in the temperature range of 400°C or more below the heating temperature, the high-strength steel sheet may be cooled once and the steel sheet temperature may be increased again.

[0091] [Average cooling rate in the temperature range of 250°C to 400°C: 1.0°C / s or more (optimal conditions)] If the average cooling rate in the temperature range of 250°C to 400°C is 1.0°C / s or more, the amount of bainitic ferrite contained after annealing can be further reduced, and the YR and bendability can be further improved. The average cooling rate in the temperature range of 250°C to 400°C is preferably 1.0°C / s or more, more preferably 2.0°C / s or more, and even more preferably 3.0°C / s or more. The upper limit of the average cooling rate in the temperature range of 250°C to 400°C is not particularly specified, but due to constraints on production technology, it is preferably 100.0°C / s or less, and more preferably 80.0°C / s or less. When the cooling stop temperature exceeds 250°C, the average cooling rate is the value in the temperature range of the cooling stop temperature to 400°C. The average cooling rate is measured based on the temperature of the steel sheet surface.

[0092] As a cooling method in the temperature range of 250° C. or more and 400° C. or less, gas jet cooling, mist cooling, water cooling, air cooling, etc. can be applied.

[0093] [Heat retention temperature during cooling process: 100℃ to 450℃ (optimal conditions)] In the cooling process, it is preferable to keep the heat at a temperature range of 100°C or more and 450°C or less for 5s or more. By keeping the temperature within the above range, the high-strength steel plate can be kept in a more suitable range for YR and bendability. In addition, the area ratio of bainitic ferrite can be further reduced, and TS can be further improved. The heat retention temperature in the cooling process is more preferably 150°C or more, and more preferably 200°C or more. In addition, the heat retention temperature in the cooling process is more preferably 400°C or less, and more preferably 350°C or less. The temperature in the cooling process is based on the surface temperature of the steel plate.

[0094] [Heat retention time during cooling process: 5 seconds or more (optimal conditions)] By maintaining the heat retention temperature in the cooling step, the YR and bendability can be kept within a more suitable range. To achieve this effect, the heat retention time at the heat retention temperature in the cooling step is preferably 5 s or more, more preferably 10 s or more, and even more preferably 15 s or more. There is no particular upper limit to the heat retention time at the heat retention temperature in the cooling step, but to keep the TS within a more suitable range, the heat retention time at the heat retention temperature in the cooling step is preferably 500 s or less, and more preferably 250 s or less.

[0095] [Cooling stop temperature: 250℃ or less (optimal conditions)] In the above-mentioned cooling step, the cooling stop temperature is preferably 250°C or less, more preferably 200°C or less. If the cooling stop temperature is 250°C or less, it is possible to prevent a large amount of retained austenite from being generated after annealing, and to further improve the YR and bendability. Although the lower limit of the cooling stop temperature is not particularly specified, it is preferably room temperature or higher from the viewpoint of productivity. The cooling stop speed is measured based on the temperature of the steel sheet surface.

[0096] Although the average cooling rate to 250° C. or less is not particularly specified, in order to further improve TS, the average cooling rate to 250° C. or less is preferably 1° C. / s or more, and more preferably 2° C. / s or more. On the other hand, due to constraints on production technology, the average cooling rate to 250° C. or less is preferably 1000° C. / s or less, and more preferably 150° C. / s or less.

[0097] The cold-rolled sheet may be further cooled from the cooling stop temperature to room temperature. The average cooling rate from the cooling stop temperature to room temperature is not particularly limited, and the sheet may be cooled to room temperature by any method. Examples of the cooling method include gas jet cooling, mist cooling, water cooling, and air cooling.

[0098] The cold-rolled sheet annealed as described above may be cooled to the cooling stop temperature and then rolled. The elongation rate of the rolling is preferably 0.05% or more, more preferably 0.10% or more. By setting the elongation rate of the rolling performed after cooling to the cooling stop temperature to 0.05% or more, the YR can be controlled to a desired range. In addition, the elongation rate of the rolling is preferably 2.00% or less, more preferably 1.00% or less. By setting the elongation rate of the rolling after cooling to the cooling stop temperature to 2.00% or less, the volume fraction of the retained austenite can be set to a more suitable range. In addition, the bendability and the damage degree of the sheared end surface in a corrosive environment can be set to a more suitable range.

[0099] The rolling after cooling to the cooling stop temperature may be performed on an apparatus continuous with the above-mentioned continuous annealing apparatus (online), or on an apparatus not continuous with the above-mentioned continuous annealing apparatus (offline). The target elongation may be achieved by one rolling, or a total of 0.05% to 2.00% elongation may be achieved by multiple rolling. The rolling described here generally refers to temper rolling, but it may be a processing method using repeated bending with a tension leveler or rolls, etc., as long as it can impart an elongation equivalent to that of temper rolling.

[0100] [Reheating temperature: (cooling stop temperature + 50℃) or more and 450℃ or less (optimal conditions)] After cooling to the cooling stop temperature, or after further rolling after cooling to the cooling stop temperature, the high-strength steel sheet may be reheated (reheating step). By reheating the high-strength steel sheet, the YR and bendability can be set within a more suitable range. In order to obtain such an effect, the reheating temperature is preferably (cooling stop temperature + 50°C) or higher, more preferably (cooling stop temperature + 100°C) or higher, and even more preferably (cooling stop temperature + 150°C) or higher. On the other hand, as the reheating temperature increases, the tempering of martensite progresses and the TS decreases, so the reheating temperature is preferably 450°C or lower, more preferably 400°C or lower, and even more preferably 380°C or lower. The reheating temperature is based on the surface temperature of the steel sheet.

[0101] [Heat retention time at reheating temperature: 5 seconds or more (optimal conditions)] By maintaining the heat at the reheating temperature, the YR and bendability can be kept within a more suitable range. To achieve this effect, the heat-maintenance time at the reheating temperature is preferably 5 s or more, more preferably 10 s or more, and even more preferably 15 s or more. There is no particular upper limit to the heat-maintenance time at the reheating temperature, but to keep the TS within a more suitable range, the heat-maintenance time at the reheating temperature is preferably 500 s or less, and more preferably 250 s or less.

[0102] The reheating temperature may be cooled to room temperature, but the cooling rate from the reheating temperature to room temperature is not particularly limited, and the material may be cooled to room temperature by any method, such as gas jet cooling, mist cooling, water cooling, and air cooling.

[0103] When high strength steel sheets are traded, they are usually cooled to room temperature before being traded.

[0104] [Manufacturing method for high-strength plated steel sheets] A high-strength plated steel sheet can be obtained by subjecting at least one side of the high-strength steel sheet produced as described above to a plating process. For example, examples of the plating process include a hot-dip galvanizing process and a process of alloying after hot-dip galvanizing. Annealing and galvanizing may be performed continuously in one line. Alternatively, a plating layer may be formed by electroplating such as Zn-Ni electric alloy plating, or hot-dip zinc-aluminum-magnesium alloy plating may be applied. Note that, although the above description has focused on the case of galvanizing, the type of plating metal such as Zn plating or Al plating is not particularly limited.

[0105] When hot-dip galvanizing is performed, it is preferable to adjust the coating weight by gas wiping or the like after immersing the high-strength steel sheet in a galvanizing bath at 440°C to 500°C to perform hot-dip galvanizing. It is preferable to use a galvanizing bath containing 0.10% by mass to 0.23% by mass of Al for hot-dip galvanizing. In addition, the temperature range when performing alloying treatment of galvanizing after hot-dip galvanizing is preferably 470°C to 600°C, more preferably 470°C to 560°C. By performing alloying treatment at 470°C or higher, the Zn-Fe alloying rate is more suitable and the productivity is more suitable. In addition, by performing alloying treatment at 600°C or lower, the transformation of untransformed austenite to pearlite is prevented, and TS is more suitable. In addition, electrogalvanizing may be performed. In addition, the coating weight is 20 to 80 g / m per side. 2 (Double-sided plating) is preferred, and the galvannealed steel sheet (GA) is preferably subjected to the following alloying treatment to make the Fe concentration in the plating layer 7 to 15 mass %.

[0106] Regarding the plating treatment, after the above-mentioned annealing step, the high-strength steel sheet may be plated at a temperature range of 400°C or more below the heating temperature without being cooled, or the cold-rolled steel sheet may be once cooled to less than 400°C and the steel sheet temperature may be raised again to 400°C or more before the plating treatment is performed.

[0107] The high-strength plated steel sheet that has been subjected to the above-mentioned plating treatment may be subjected to rolling. The elongation rate of the rolling is preferably 0.05% or more, more preferably 0.10% or more. By setting the elongation rate of the rolling performed after the plating treatment to 0.05% or more, the YR can be controlled to a desired range. Moreover, the elongation rate of the rolling is preferably 2.00% or less, more preferably 1.00% or less. By setting the elongation rate of the rolling in the plating treatment to 2.00% or less, the volume fraction of the retained austenite can be set to a more suitable range, and the bendability and the damage degree of the sheared end surface in a corrosive environment can be set to a more suitable range.

[0108] The rolling after the plating process may be performed on an apparatus continuous with the above-mentioned continuous annealing apparatus (online) or on an apparatus discontinuous with the above-mentioned continuous annealing apparatus (offline). The target elongation may be achieved by one rolling, or a total of 0.05% to 2.00% elongation may be achieved by multiple rolling. The rolling described here generally refers to temper rolling, but may be a processing method such as repeated bending with a tension leveler or rolls, as long as it can impart an elongation equivalent to that of temper rolling. Reheating may be performed after rolling after the plating process.

[0109] Other conditions of the manufacturing method are not particularly limited. However, from the viewpoint of productivity, it is preferable to carry out a series of processes such as the above-mentioned annealing, hot-dip galvanizing, and alloying treatment of galvanizing in a CGL (Continuous Galvanizing Line), which is a hot-dip galvanizing line. After hot-dip galvanizing, wiping is possible to adjust the coating weight of the coating. Note that the plating conditions other than the above-mentioned conditions can be based on the usual hot-dip galvanizing method.

[0110] When high-strength plated steel sheets are traded, they are usually cooled to room temperature before being traded.

[0111] The production conditions other than those mentioned above can be the same as those in the ordinary methods.

[0112] [Part] Next, a member according to one embodiment of the present invention will be described.

[0113] A member according to an embodiment of the present invention is a member made using the high-strength steel sheet or high-strength plated steel sheet according to the embodiment of the present invention described above. The member according to an embodiment of the present invention is, for example, a member obtained by forming the high-strength steel sheet or high-strength plated steel sheet according to the embodiment of the present invention described above into a target shape by cold press working or the like. Therefore, even after being formed into a member, it has the steel structure and various properties of the high-strength steel sheet and the high-strength plated steel sheet. The member according to an embodiment of the present invention is preferably used for automobile frame structural parts or automobile reinforcing parts.

[0114] Here, the high-strength steel plate according to one embodiment of the present invention is a high-strength steel plate of 1180 MPa or more that is excellent in bendability and inter-steel-sheet cracking resistance in spot weld HAZ, and can be used to manufacture parts with high dimensional accuracy. Therefore, the member according to one embodiment of the present invention can contribute to weight reduction of the vehicle body, and can be suitably used in general members for automobile frame structural parts or automobile reinforcing parts. EXAMPLES

[0115] Steel having the composition shown in Table 1, with the balance being Fe and unavoidable impurities, was melted in a converter and formed into a slab by a continuous casting method. After cooling, the obtained slab was heated, hot rolled, pickled, and then cold rolled. Table 2 shows the cooling rate after casting, hot rolling conditions, pickling conditions, and cold rolling conditions. A pre-annealed cold-rolled steel sheet with a thickness of 1.4 mm was produced. In some examples, sheets with different thicknesses were also produced.

[0116] [Table 1]

[0117] [Table 2]

[0118] [Table 3]

[0119] Next, annealing, cooling and reheating were performed under the conditions shown in Tables 2 and 3 to obtain high-strength cold-rolled steel sheets (CR). Furthermore, some of the thin steel sheets were subjected to a plating process to obtain hot-dip galvanized steel sheets (GI), alloyed hot-dip galvanized steel sheets (GA) and electrogalvanized steel sheets (EG). For the hot-dip galvanizing bath, a zinc bath containing 0.14-0.19 mass% Al was used for GI, and a zinc bath containing 0.14 mass% Al was used for GA, with the bath temperature set to 470°C. The coating weight for GI was 45-72 g / m per side. 2 (Both sides are plated) and for GA, 45g / m per side 2 The thickness of the plating layer was set to about 1 / 2 (double-sided plating). For GA, the Fe concentration in the plating layer was set to 9% by mass or more and 12% by mass or less. For EG, which has a Zn-Ni plating layer, the Ni content in the plating layer was set to 9% by mass or more and 25% by mass or less.

[0120] The high-strength cold-rolled steel sheets and high-strength plated steel sheets obtained as described above were used as test steels to evaluate the tensile properties, bendability, and inter-sheet cracking properties of the spot weld HAZ according to the following test methods. The results are shown in Table 4.

[0121] [Table 4]

[0122] [Tensile test] The tensile test was performed in accordance with JIS Z 2241:2022. JIS No. 5 test pieces were taken from the obtained steel plate perpendicular to the rolling direction of the steel plate, and the crosshead speed was 1.67 × 10 -1 A tensile test was performed under the condition of mm / s to measure YS and TS. In the present invention, a yield ratio (YR) of 65% or more and 90% or less was determined to have high dimensional accuracy. YR was calculated using the calculation method described in the above formula (2).

[0123] [Bending test] The bending test was performed in accordance with JIS Z 2248:2022. From the obtained steel plate, a rectangular test piece with a width of 30 mm and a length of 100 mm was taken so that the axial direction of the bending test was parallel to the rolling direction of the steel plate. Then, a 90° V bending test was performed under the conditions of a pressing load of 100 kN and a pressing holding time of 5 seconds. In this disclosure, bending tests were performed on five samples at R, where the value R / t obtained by dividing the bending radius (R) by the plate thickness (t) was about 4.5, that is, 4.3 to 4.7. Next, the crack length at the ridgeline of the bending apex of all five samples was evaluated, and when the crack length was 200 μm or less, it was determined that the bending property was excellent. Here, the crack length was evaluated by measuring the ridgeline of the bending apex at a magnification of 40 to 160 times using a digital microscope (RH-2000: manufactured by Hirox Co., Ltd.).

[0124] [Spot weld HAZ interplate cracking test] Test pieces were cut to a thickness of 1.4 mm, length of 30 mm, and width of 100 mm with the rolling direction as the longitudinal direction. The coating weight of the hot-dip galvanized layer on each side was 50 g / m 2 The plate assembly was then stacked with a test hot-dip galvanized steel sheet of the same material to form a plate assembly. Next, a servo motor pressurized single-phase AC (50 Hz) resistance welding machine and an electrode with a tip diameter of 6 mm were used to incline the plate assembly by 5° with respect to the electrode of the resistance welding machine, and resistance welding was performed with a clearance of 1.5 mm between the lower electrode and the lower steel sheet. Specifically, the plate assembly was subjected to resistance welding under the conditions of a pressure of 3.5 kN, a hold time of 0.16 seconds or 0.20 seconds, and a welding current and welding time that resulted in a nugget diameter of 5.9 mm to form a plate assembly with a welded portion. Next, the plate assembly with the welded portion was cut in half to include the welded portion, and the cross section of the welded portion was observed with an optical microscope (200 times magnification), and the inter-steel sheet cracking resistance property of the spot welded portion HAZ was evaluated according to the above-mentioned criteria. In addition, a similar evaluation was performed on some samples with plate thicknesses of 0.8 mm and 2.3 mm.

[0125] According to the above-mentioned method, the area ratio of martensite and ferrite, the volume ratio of retained austenite, the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction, and the number density of MnS present in the Mn segregation part of the steel sheet surface were obtained. Furthermore, the standard deviation of the Vickers hardness of the steel sheet surface and the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface were obtained. In addition, the remaining structure was observed by the method described below. After cutting out a sample so that the sheet thickness cross section (L cross section) parallel to the rolling direction of the steel sheet was the observation surface, the observation surface was mirror-polished using diamond paste, and then etched with 3 vol.% nital to reveal the structure. Under the condition of an acceleration voltage of 15 kV, the observation position was set to 1 / 4 of the sheet thickness of the steel sheet, and three fields of view were observed at a magnification of 5000 times and a field of view of 17 μm × 23 μm. Carbides were identified as the remaining structure from the obtained structure image.

[0126] As shown in Table 4, the examples of the present invention are excellent in TS, YR, bendability, and inter-steel cracking resistance of the spot weld HAZ, while the comparative examples are inferior in at least one of TS, YR, bendability, and inter-steel cracking resistance of the spot weld HAZ.

[0127] Although the embodiment of the present invention has been described above, the present invention is not limited by the description of the present embodiment, which is a part of the disclosure of the present invention. In other words, other embodiments, examples, and operation techniques made by those skilled in the art based on the present embodiment are all included in the scope of the present invention. For example, in the series of heat treatments in the above-mentioned manufacturing method, the equipment for subjecting the steel sheet to heat treatment is not particularly limited as long as the heat history conditions are satisfied. [Industrial Applicability]

[0128] According to the present invention, a high strength steel plate of 1180 MPa or more is obtained which has excellent bendability and inter-steel sheet cracking resistance in spot weld HAZ and is capable of manufacturing parts with high dimensional accuracy.

[0129] In particular, the high-strength steel sheet of the present invention has excellent inter-steel sheet cracking resistance in the HAZ of spot welds, and can be applied to automotive structural components of various sizes and shapes while obtaining high component strength, which can improve fuel efficiency by reducing the weight of the vehicle body, making the steel sheet of the present invention extremely valuable in industry.

Claims

1. In mass percent, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, and Ti: 0.002% or more and 0.200% or less; The effective Ti mole fraction (x Ｔｉ，ｅｆｆ ) is 0.001 or more, with the balance being Fe and unavoidable impurities; At the 1 / 4 plate thickness position, The area ratio of martensite is 80% or more and 99% or less, A steel structure having an area ratio of ferrite and / or a volume ratio of retained austenite in total of more than 0% and not more than 20%; The Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction is 0.10% or more and 0.60% or less; The number density of MnS present in the Mn segregated part of the steel sheet surface is 5.0 pieces / mm ２ below, The standard deviation of the Vickers hardness of the steel plate surface is 15 or less. The hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface is 7 times or less; High strength steel plate. Note x Ｔｉ，ｅｆｆ =x Ｔｉ -x Ｎ -x Ｓ ・・・(1) In addition, x in the formula Ｔｉ , x Ｎ , x Ｓ represents the content (molar fraction) of each element in the steel sheet.

2. The composition further includes, in mass%, 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 high strength steel plate according to claim 1, further comprising at least one element selected from the group consisting of:

3. A high-strength plated steel sheet comprising the high-strength steel sheet according to claim 1 or 2, and a plating layer on at least one surface of the high-strength steel sheet.

4. A steel slab having the composition according to claim 1, The slab is heated at an average heating rate of 25° C. / min or less in a temperature range of 900° C. or more and 1150° C. or less, a slab heating temperature of 1150° C. or more, and a residence time from 1100° C. to the slab heating temperature of 20 min or more; Next, the steel slab is subjected to hot rolling in which the reduction rate of the final pass of the finish rolling is 9% or more and 15% or less, the reduction rate of the pass before the final pass is 15% or more and 21% or less, and the reduction rate of the pass two passes before the final pass is 21% or more and 27% or less to obtain a hot-rolled sheet, Next, the hot-rolled sheet is subjected to pickling to obtain a pickled sheet. Next, the pickled sheet is subjected to cold rolling with a cumulative rolling reduction of 20% to 75% to obtain a cold-rolled sheet, Next, an annealing step is performed in which the cold-rolled sheet is heated to a heating temperature of 780°C or more, and in this case, the average heating rate in the temperature range of 250°C to 700°C is 100°C / s or less, and the residence time from 750°C to the heating temperature is 10s or more. A method for producing a high-strength steel sheet.

5. A steel slab having the composition according to claim 2, The slab is heated at an average heating rate of 25° C. / min or less in a temperature range of 900° C. or more and 1150° C. or less, a slab heating temperature of 1150° C. or more, and a residence time from 1100° C. to the slab heating temperature of 20 min or more; Next, the steel slab is subjected to hot rolling in which the reduction rate of the final pass of the finish rolling is 9% or more and 15% or less, the reduction rate of the pass before the final pass is 15% or more and 21% or less, and the reduction rate of the pass two passes before the final pass is 21% or more and 27% or less to obtain a hot-rolled sheet, Next, the hot-rolled sheet is subjected to pickling to obtain a pickled sheet. Next, the pickled sheet is subjected to cold rolling with a cumulative rolling reduction of 20% to 75% to obtain a cold-rolled sheet, Next, an annealing step is performed in which the cold-rolled sheet is heated to a heating temperature of 780°C or more, and in this case, the average heating rate in the temperature range of 250°C to 700°C is 100°C / s or less, and the residence time from 750°C to the heating temperature is 10s or more. A method for producing a high-strength steel sheet.

6. The method for producing a high-strength steel plate according to claim 4, wherein in the annealing step, the oxygen concentration of the atmosphere at a temperature of 750 ° C. or higher and the heating temperature or lower is 0.5 vol.% or higher and 5.0 vol.% or lower, and the dew point of the atmosphere is −35° C. or higher.

7. A method for manufacturing a high-strength steel plate as described in claim 5, wherein in the annealing process, the oxygen concentration of the atmosphere at a temperature of 750°C or higher and lower than the heating temperature is 0.5 volume% or higher and 5.0 volume% or lower, and the dew point of the atmosphere is -35°C or higher.

8. The method for producing a high strength steel plate according to any one of claims 4 to 7, wherein a cooling step is further performed following the annealing step, and an average cooling rate in a temperature range of 250 ° C. or more and 400 ° C. or less is 1.0 ° C. / s or more.

9. The method for producing a high strength steel plate according to any one of claims 4 to 7, further comprising a cooling step following the annealing step, in which the steel sheet is kept at a heat retention temperature of 100°C or higher and 450°C or lower for 5s or more.

10. The annealing step is followed by a cooling step, in which the cooling stop temperature is 250 ° C. or less, and the cold-rolled sheet is then reheated to a reheating temperature of (the cooling stop temperature + 50 ° C.) or more and 450 ° C. or less, and the reheating temperature is kept at that temperature for 5 s or more. The method for producing a high-strength steel sheet according to any one of claims 4 to 7.

11. A method for producing a high-strength plated steel sheet, comprising: performing a plating process on at least one surface of the cold-rolled sheet after the annealing process according to any one of claims 4 to 7.

12. A method for manufacturing a high-strength plated steel sheet, comprising, after the annealing process described in claim 8, a plating process for applying a plating treatment to at least one side of the cold-rolled sheet.

13. A method for manufacturing a high-strength plated steel sheet, comprising, after the annealing process described in claim 9, a plating process for applying a plating treatment to at least one side of the cold-rolled sheet.

14. A method for manufacturing a high-strength plated steel sheet, comprising, after the annealing process described in claim 10, a plating process for applying a plating treatment to at least one side of the cold-rolled sheet.

15. A member, at least in part, using the high strength steel plate according to claim 1 or 2.

16. A member, at least in part, comprising the high-strength plated steel sheet according to claim 3.