High strength steel sheet, high strength plated steel sheet, methods for producing same, and member

Abstract

The purpose of the present invention is to provide: a high strength steel sheet and a high strength plated steel sheet which have a strength of 1180 MPa or more, which have excellent bendability and inter-steel-sheet cracking characteristics at spot-welding resistant parts HAZ, and from which it is possible to manufacture components with high dimensional accuracy; methods for producing the high strength steel sheet and the high strength plated steel sheet; and a member.　This high strength steel sheet has a prescribed component composition, has an effective Ti molar fraction (xTi, eff) of 0.001 or more as determined from xTi, eff = xTi-xN-xS (wherein each element symbol represents the molar fraction of a corresponding element in steel), and has a steel structure in which the area percentage of martensite is 80-99% and the total of the area percentage of ferrite and / or the volume percentage retained austenite is more than 0% but not more than 20% at a 1 / 4-position in the sheet thickness, the Si concentration at a 5-µm positon in the sheet thickness direction from the steel sheet surface is 0.10-0.60%, the number density of MnS present in Mn-segregated portions in the surface of the steel sheet is 5.0 pieces / mm2 or less, the standard deviation of Vickers hardness in the surface of the steel sheet is 15 or less, and the hardness fluctuation frequency per 1100 µm in the sheet width direction in the surface of the steel sheet is seven times or less.

Description

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

[0001] The present disclosure relates to high-strength steel sheets, high-strength plated steel sheets, methods for manufacturing the same, and components that are optimal for automotive reinforcing parts and frame structural parts.

[0002] CO2 emissions from lighter vehicles ２ With the aim of achieving both reduced emissions and improved crashworthiness through vehicle weight reduction, efforts are underway to increase the strength of automotive steel sheets. Furthermore, new legal regulations are being introduced one after another. Therefore, 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 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 simply referred to 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 formed parts are required to have excellent dimensional accuracy. For example, parts such as crash boxes have punched edges and bent sections, so steel sheets with high stretch flangeability and bendability are suitable from the standpoint of formability. Furthermore, from the standpoint of part performance, increasing the yield ratio (YR = yield strength YS / tensile strength TS) of the steel sheet can increase the impact absorption energy during a collision. Furthermore, from the standpoint of part dimensional accuracy, 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 the part. To increase the application rate of high-strength steel sheets to automotive parts, it is necessary to comprehensively satisfy these properties.

[0004] Furthermore, among the structural components of automobiles, for example, crash boxes have bent portions. Furthermore, in order to increase the application rate of high-strength steel sheets to components 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 portions where R / t, the value 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 spot welding high-strength galvanized steel sheets, zinc from the coating layer diffuses into the grain boundaries of the steel sheet surface, causing liquid metal embrittlement (LME). As a result, inter-steel sheet cracking (LME cracking) occurs in the HAZ of the spot weld. Inter-steel sheet cracking in the HAZ of the spot weld can occur even in high-strength steel sheets without a galvanized layer, as long as the welding partner is a galvanized steel sheet, and is therefore becoming a problem for all high-strength steel sheets. During mass production, steel sheets of various thicknesses are spot welded, and as the sheet thickness increases, the hold time required to suppress inter-steel sheet cracking in the HAZ of the spot weld typically increases. However, since an increase in hold time leads to a decrease in productivity, there is a demand for high-strength steel sheets that can suppress inter-steel sheet cracking in the HAZ of the spot weld 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 which allows parts to be manufactured with high dimensional accuracy.

[0007] Patent No. 6787535

[0008] The high-strength steel sheet described in Patent Document 1 comprehensively satisfies the bendability and LME resistance characteristics, and enables the production of parts with high dimensional accuracy. However, the high-strength steel sheet described in Patent Document 1 has a TS of 980 MPa, and there is room for further improvement in strength.

[0009] The present disclosure was developed in consideration of the above circumstances, and aims to obtain a high-strength steel plate of 1180 MPa or more that has excellent bendability and inter-steel plate cracking resistance in the HAZ of spot welds and that can be used to manufacture parts with high dimensional accuracy, as well as to provide an advantageous method for manufacturing such a 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. Five samples are tested at an R where R / t, the value obtained by dividing the bending radius (R) by the plate thickness (t), is approximately 4.5, i.e., 4.3 to 4.7. Next, the crack lengths at the ridges of the bend apexes of all five samples are evaluated, and samples with crack lengths of 200 μm or less are judged to have excellent bendability.

[0011] Furthermore, the resistance to inter-steel plate cracking in the HAZ of the spot weld was evaluated by observing the cross section of the weld described in the examples with an optical microscope (200x magnification) and using the following criteria. A rating of A or B indicates that the resistance to inter-steel plate cracking in the HAZ of the spot weld is excellent. A rating of C indicates that the resistance to inter-steel plate cracking in the HAZ of the spot weld is poor. A: No cracks of 0.1 mm or more in length were observed with a hold time of 0.16 seconds. B: Cracks of 0.1 mm or more in length were observed with a hold time of 0.16 seconds, but no cracks of 0.1 mm or more in length were observed with a hold time of 0.20 seconds. C: Cracks of 0.1 mm or more in length were observed with a hold time of 0.20 seconds.

[0012] Here, the hold time refers to the time from when the welding current has finished flowing until when the electrodes start to be released.

[0013] The present inventors have conducted extensive research to achieve the above-mentioned objectives, and as a result have made the following discoveries: (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 of 65% or more and 90% or less, which is an index of dimensional accuracy of parts; (2) 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, and the number density of MnS present in Mn segregated parts on the steel sheet surface is 5.0 particles / mm ２Hereinafter, the standard deviation of the Vickers hardness of the steel sheet surface is set to 15 or less. Also, good bendability can be achieved by setting the hardness fluctuation frequency per 1,100 μm in the sheet width direction on the steel sheet surface to 7 or less. (3) Good inter-steel sheet cracking resistance in the spot weld HAZ can be achieved 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.

[0014] The present disclosure has been made based on the above findings. That is, the gist of the present disclosure is as follows: [1] A steel sheet containing, by 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 an effective Ti mole fraction (x Ｔｉ，ｅｆｆ ) is 0.001 or more, with the balance consisting of Fe and unavoidable impurities; a steel structure in which, at a quarter-thickness position, the area fraction of martensite is 80% or more and 99% or less, and the area fraction of ferrite and / or the volume fraction of retained austenite is more than 0% and 20% or less in total; a 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, and the number density of MnS present in Mn segregated parts on the steel sheet surface is 5.0 particles / mm ２ Hereinafter, 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 per 1100 μm in the plate width direction on the steel plate surface of 7 or less. Ｔｉ，ｅｆｆ = x Ｔｉ -x Ｎ -x Ｓ ... (1) Note that x in the formula Ｔｉ , x Ｎ , x Ｓrepresents 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, in 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 component composition according to [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, and then the steel slab is subjected to finish rolling with a final pass reduction of 9% to 15%, a pass one pass before the final pass reduction of 15% to 21%, and a pass two passes before the final pass reduction of 20% to 21%. [5] A method for producing a high-strength steel sheet according to [4], wherein the atmosphere at a temperature of 750°C or higher and lower than the heating temperature has an oxygen concentration of 0.5% by volume or higher and 5.0% by volume or lower, and a 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 the average cooling rate in the temperature range of 250°C or higher and 400°C or lower is 1.0°C / s or higher.[7] A method for producing a high-strength steel sheet according to [4] or [5], further comprising: performing a cooling step after the annealing step, wherein the cooling step is performed at a temperature of 100°C to 450°C for 5 seconds or more. [8] A method for producing a high-strength steel sheet according to [4] or [5], further comprising: performing a cooling step after the annealing step, wherein the cooling stop temperature is set to 250°C or less, and then reheating the cold-rolled sheet to a reheating temperature of (the cooling stop temperature + 50°C) to 450°C or less, and maintaining the reheating temperature for 5 seconds or more. [9] A method for producing a high-strength plated steel sheet, comprising: performing a plating step of plating at least one surface of the cold-rolled sheet after the annealing step according to any one of [4] to [8].

[10] A member at least partially made using the high-strength steel sheet according to [1] or [2].

[11] A member at least partially made using the high-strength plated steel sheet according to [3].

[0015] According to the present disclosure, it is possible to provide high-strength steel plates and components of 1180 MPa or more that have excellent bendability and inter-steel sheet cracking resistance in the HAZ of spot welds, and that can be used to manufacture parts with high dimensional accuracy.

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

[0017] First, the appropriate range of the chemical composition of the steel sheet and the reasons for limiting it will be explained. In the following explanation, "%" representing the content of the chemical elements of the steel sheet means "mass%" unless otherwise specified. Furthermore, in this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" 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, C is an important element that affects the area fractions of martensite and ferrite, the volume fraction of retained austenite, the standard deviation of Vickers hardness on the steel sheet surface, and the frequency of hardness fluctuations per 1,100 μm in the sheet width direction on the steel sheet surface. If the C content is less than 0.030%, the area fraction of martensite decreases and the area fraction of ferrite increases, making it difficult to achieve a TS of 1,180 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 Vickers hardness on the steel sheet surface exceeds 15, and the frequency of hardness fluctuations per 1,100 μm in the sheet width direction on the steel sheet surface exceeds 7, resulting in reduced 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 set to 0.400% or less. The C content is more preferably set to 0.110% or more. The C content is more preferably set to 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. In particular, in the present disclosure, Si suppresses carbide formation during annealing and promotes the formation of retained austenite, thereby affecting the volume fraction of retained austenite. Furthermore, Si exhibits high tempering softening resistance at temperatures below 400°C, making it an important element that affects the standard deviation of Vickers hardness on the steel sheet surface and the frequency of hardness fluctuations per 1,100 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 Vickers hardness on the steel sheet surface exceeds 15, and the frequency of hardness fluctuations per 1,100 μm in the sheet width direction on the steel sheet surface exceeds 7, resulting in reduced 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, resulting in a decrease in the inter-steel sheet cracking resistance of the spot weld HAZ. 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 fractions of martensite and ferrite, the volume fraction of retained austenite, the standard deviation of Vickers hardness on the steel sheet surface, and the frequency of hardness fluctuations per 1100 μm in the sheet width direction on the steel sheet surface. If the Mn content is less than 0.10%, the area fraction of martensite decreases and the area fraction 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 Vickers hardness on 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, resulting in reduced bendability. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably 1.00% or more. The Mn content is preferably 4.00% or less. The Mn content is more preferably 2.00% or more. The Mn content is more preferably 3.50% or less.

[0021] [P: 0.100% or less] Excessive P segregates at prior austenite grain boundaries, embrittling the grain boundaries and reducing the ultimate deformability of the steel sheet, resulting in reduced bendability. Therefore, the P content must be 0.100% or less. 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 preferably 0.001% or more. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.001% or more, and more preferably 0.070% or less.

[0022] [S: 0.0200% or less] S exists as sulfides and reduces the ultimate deformability of steel, thereby reducing bendability. Therefore, the S content needs to 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 set to 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 is excessive, A ３ The transformation point rises and a large amount of ferrite is contained in the microstructure, making it difficult to achieve the desired YR. Therefore, the Al content needs to be 0.100% or less. Although there is no particular lower limit for the Al content, 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 set to 0.100% or less. The Al content is preferably set to 0.001% or more. The Al content is preferably set to 0.050% or less.

[0024] [N: 0.0100% or less] N exists as a nitride and reduces the ultimate deformability of the steel sheet, thereby reducing bendability. Therefore, the N content needs to 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, thereby reducing bendability. Therefore, the O content needs to 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 set to 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. Furthermore, adding Ti can reduce the number density of MnS present in Mn segregated areas on the steel sheet surface. To achieve this 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 the 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) Note that x in the formula Ｔｉ , x Ｎ , x Ｓ represents 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 higher, the number density of MnS present in the Mn segregated portion on the steel sheet surface can be reduced, thereby achieving good bendability. To achieve this effect, the effective Ti molar fraction is set to 0.001% or higher. Note that there is no particular upper limit for the effective Ti molar fraction, but since the amount of carbides, nitrides, or carbonitrides increases and it becomes difficult to achieve the desired YR, it is preferable that the effective Ti molar fraction be 0.040% or lower. Therefore, the effective Ti molar fraction is set to 0.001% or higher. The effective Ti molar fraction is preferably set to 0.002% or higher. The effective Ti molar fraction is preferably set to 0.040% or lower. Note that the molar fraction is the molar fraction obtained by converting the components contained in mass % into molar fraction.

[0028] A high-strength steel sheet according to one embodiment of the present invention has a composition containing the above elements with the balance being Fe and unavoidable impurities. Preferably, a high-strength steel sheet according to one embodiment of the present invention has a composition containing the above elements with the balance being Fe and unavoidable impurities. Examples of the unavoidable impurities include Zn, Pb, and As. The inclusion of these impurities is permitted as long as the total content is 0.100% or less.

[0029] In addition to the above-described essential components, the component composition of the high-strength steel plate of the present disclosure further contains, 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, and Bi: 0.200% or less. At least one element selected from these may be contained alone or in combination.

[0030] [Nb: 0.200% or less] Nb generates large amounts of coarse precipitates and inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the Nb content exceeds 0.200%, the bendability decreases. Therefore, the Nb content is set to 0.200% or less. Although there is no particular lower limit for the Nb content, 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 within a desired range. For this reason, the Nb content is preferably set to 0.001% or more. Therefore, when Nb is 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 large amounts of coarse precipitates and inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the V content exceeds 0.200%, bendability decreases. Therefore, the V content is set to 0.200% or less. Although there is no particular lower limit for the V content, 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 within 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%, large amounts of coarse precipitates and inclusions are formed, reducing the ultimate deformability of the steel sheet and therefore reducing bendability. Therefore, the Ta and W contents are each set to 0.10% or less. Although there are no particular lower limits for the Ta and W contents, the Ta and W contents are preferably set to 0.01% or more because 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.

[0033] [B: 0.0100% or less] If B is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not be reduced, so 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 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 even more preferably 0.0080% or less.

[0034] [Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less] If the Cr, Mo, and Ni contents each exceed 1.00%, the amount of coarse precipitates and inclusions increases, reducing the ultimate deformability of the steel sheet and therefore reducing bendability. Therefore, the Cr, Mo, and Ni contents are each set to 1.00% or less. Although there are no particular lower limits for the Cr, Mo, and Ni contents, these elements improve hardenability, so the Cr, Mo, and Ni contents are preferably set to 0.01% or more. Therefore, when added, the Cr, Mo, and Ni contents are each set to 1.00% or less. The Cr, Mo, and Ni contents are preferably set to 0.01% or more. The Cr, Mo, and Ni contents 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, reducing the ultimate deformability of the steel sheet and therefore reducing bendability. 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, the Co content is preferably set to 0.001% or more. Therefore, when Co is added, the Co content is set to 0.010% or less. The Co content is preferably set to 0.001% or more. The Co content is preferably set to 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, reducing the ultimate deformability of the steel sheet and therefore reducing 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, the Cu content is preferably set to 0.01% or more. Therefore, when added, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.01% or more. The Cu content is preferably set to 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, reducing the ultimate deformability of the steel sheet and therefore reducing 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, reducing the ultimate deformability of the steel sheet and therefore reducing bendability. 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 softened surface 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 Ca, Mg, and REM contents exceed 0.0100%, the amount of coarse precipitates and inclusions increases, reducing the ultimate deformability of the steel sheet and therefore reducing bendability. Therefore, the Ca, Mg, and REM contents are each set to 0.0100% or less. Although there are no particular lower limits for the Ca, Mg, and REM contents, these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, when added, the Ca, Mg, and REM contents are each set to 0.0100% or less. The Ca, Mg, and REM contents are preferably set to 0.0005% or more. The contents of Ca, Mg, and REM are preferably 0.0050% or less. Note that REM (rare earth elements) is a collective term for Sc, Y, and 15 elements ranging from 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 exceed 0.100%, the amount of coarse precipitates and inclusions increases, reducing the ultimate deformability of the steel sheet and therefore reducing bendability. Therefore, the Zr and Te contents must each be 0.100% or less. Although there are no particular lower limits for the Zr and Te contents, since these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, it is preferable that the Zr and Te contents be 0.001% or more. Therefore, when added, the Zr and Te contents are 0.100% or less. The Zr and Te contents are preferably 0.001% or more. The Zr and Te contents are preferably 0.080% or less.

[0041] [Hf: 0.10% or less] If the Hf content exceeds 0.10%, the amount of coarse precipitates and inclusions increases, reducing the ultimate deformability of the steel sheet and therefore reducing bendability. 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, reducing the ultimate deformability of the steel sheet and therefore reducing bendability. 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 content of each of the above-mentioned Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferred lower limit, the effect of the present invention is not impaired, and therefore, they are included as unavoidable impurities.

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

[0045] [Area Fraction of Martensite: 80% or More and 99% or Less] If the area fraction of martensite is less than 80%, the area fraction 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 becomes non-uniform, causing the standard deviation of the Vickers hardness on the steel sheet surface to exceed 15, and the frequency of hardness fluctuations per 1100 μm in the sheet width direction on the steel sheet surface to exceed 7, resulting in reduced bendability. On the other hand, if the area fraction of martensite exceeds 99%, ferrite and / or retained austenite, which are effective for controlling the YR, are not present in the steel structure, making it difficult to achieve a desired YR. Therefore, the area fraction of martensite is set to 80% or more and 99% or less. The area fraction of martensite is preferably 85% or more. The area fraction of martensite is preferably 98% or less. The area fraction of martensite is more preferably 87% or more. The area fraction 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 a quarter position in the thickness of the steel plate, as described later.

[0046] [Total of Ferrite Area Fraction and / or Retained Austenite Volume Fraction: More than 0% and Not More than 20%] When the total of the ferrite area fraction and / or the retained austenite volume fraction is 0%, the steel structure becomes a martensite single-phase structure, making it difficult to achieve the desired YR. On the other hand, when the total of the ferrite area fraction and / or the retained austenite volume fraction exceeds 20%, the martensite area fraction decreases, making it difficult to achieve a TS of 1180 MPa or more. It also makes it difficult to achieve the desired YR. Furthermore, the hardness distribution in the sheet width direction becomes non-uniform, causing the standard deviation of the Vickers hardness on the steel sheet surface to exceed 15, and the frequency of hardness fluctuations per 1100 μm in the sheet width direction on the steel sheet surface to exceed 7, resulting in poor bendability. Therefore, the total of the ferrite area fraction and / or the retained austenite volume fraction is set to be more than 0% and not more than 20%. The total of the ferrite area fraction and / or the retained austenite volume fraction is preferably 1% or more. The area fraction of martensite is preferably 18% or less. The area fraction of martensite is more preferably 2% or more. The area fraction of martensite is preferably 15% or less. Note that ferrite here includes bainitic ferrite. Note that the observation position for the area fraction of ferrite and the volume fraction of retained austenite is set at a position ¼ of the thickness of the steel plate, as described below.

[0047] Here, the area ratios of martensite (quenched martensite, tempered martensite, and bainite) and ferrite (bainitic ferrite) are measured as follows.

[0048] After cutting out a 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. Under the condition of an acceleration voltage of 15 kV, using an SEM (Scanning Electron Microscope), the observation position is set to 1 / 4 of the plate thickness of the steel plate, and at a magnification of 5000 times, three fields of view are observed in a field range of 17 μm × 23 μm. The obtained structure image is used Adobe Photoshop from Adobe Systems 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), martensite (tempered martensite, bainite, and quenched martensite). These values ​​are averaged to determine the area ratio of each structure. In the above structural image, ferrite (bainitic ferrite) is a flat structure containing no carbides in the recessed portions, while tempered martensite and bainite are structures containing fine carbides in the recessed portions. Furthermore, quenched martensite is a structure with protruding portions and fine irregularities inside the structure, 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 was measured as follows.

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

[0051] [Si concentration at a position 5 μm from the steel sheet surface in the thickness direction: 0.10% or more and 0.60% or less] This is an extremely important invention requirement in the present disclosure. To control the inter-steel-sheet cracking resistance of the spot weld HAZ, it is important to control the element concentration at a position 5 μm from the steel sheet surface in the thickness direction. In particular, reducing the Si concentration at a position 5 μm from the steel sheet surface in the thickness direction can improve the inter-steel-sheet cracking resistance of the spot weld HAZ. To achieve this effect, the Si concentration at a position 5 μm from the steel sheet surface in the thickness direction must be 0.60% or less. On the other hand, if the Si concentration at a position 5 μm from the steel sheet surface in the thickness direction is less than 0.10%, the hardness distribution of martensite in the width direction becomes non-uniform. As a result, the standard deviation of the Vickers hardness at the steel sheet surface exceeds 15, and the hardness fluctuation frequency per 1,100 μm in the width direction at the steel sheet surface exceeds 7, resulting in reduced bendability. Therefore, the Si concentration at a position 5 μm from the steel sheet surface in the 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 thickness direction is preferably set to 0.15% or more. The Si concentration at a position 5 μm from the steel sheet surface in the thickness direction is preferably set to 0.55% or less. The Si concentration at a position 5 μm from the steel sheet surface in the thickness direction is more preferably set to 0.20% or more. The Si concentration at a position 5 μm from the steel sheet surface in the thickness direction is preferably set to 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 surface of the steel sheet 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 sheet. The surface of the high-strength steel sheet is used as the measurement surface, and measurement is performed by glow discharge optical emission spectrometry (hereinafter referred to as GDS). At this time, the Si concentration is analyzed along the sheet 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 concentrations at a position 5 μm from the steel sheet surface are averaged and calculated as the Si concentration at a position 5 μm from the sheet thickness surface. The measurement data is converted to Si concentration using a calibration curve method.

[0054] [Number density of MnS present in Mn segregated parts on the steel sheet surface: 5.0 pieces / mm ２ [The following paragraphs refer to the present disclosure] This is an extremely important invention constituent feature in the present disclosure. In order to control bendability, it is important to control the number density of inclusions present in the Mn segregated portions on the steel sheet surface, that is, in the region 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 portions on the steel sheet surface. In order to obtain this effect, it is necessary to reduce the number density of MnS present in the Mn segregated portions on the steel sheet surface to 5.0 particles / 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 number density of MnS present in the Mn segregated portion on the steel sheet surface is 5.0 particles / mm ２ The number density of MnS present in the Mn segregated portion on 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 was performed using the V-block method with a bending angle of 90 degrees, with an R of approximately 4.5 (i.e., 4.3 to 4.7) where R / t was 4.5. A sample of 20 mm in the rolling direction and 5 mm in the width direction was cut to include the crack at the ridge of the bend apex. The outer surface of the bend was used as the observation surface, and the observation surface was mirror-polished using diamond paste. Next, measurements were performed using an electron probe microanalyzer (EPMA) (JXA-8230: manufactured by JEOL Ltd.). At this time, the acceleration voltage was 15 kV, the measurement area was 1.2 mm in the rolling direction x 1.0 mm in the width direction, and the irradiation current was 1.0 x 10 －７ Under the condition of A, Mn and S are measured in three visual fields. The measurement data is converted to C concentration using the calibration curve method. Then, when the obtained three visual fields are mapped with Mn element, the location where a large amount of Mn is detected is identified as Mn segregation area. Furthermore, by mapping with S element, the S-enriched area in the Mn segregation area, that is, MnS, is identified and its number is evaluated. The number of MnS present in the obtained Mn segregation area is counted in a measurement area of ​​1.2 mm. ２ The number density of MnS present in the Mn segregated portion on the steel sheet surface is calculated by dividing by the number density.

[0057] [Standard deviation of Vickers hardness of steel sheet surface: 15 or less] This is an extremely important invention requirement in the present disclosure. Uniform hardness distribution on the steel sheet surface is important for controlling bendability, and reducing the standard deviation of Vickers hardness on the steel sheet surface can improve bendability. To achieve this effect, the standard deviation of Vickers hardness on the steel sheet surface needs to be 15 or less. While there is no particular lower limit for the standard deviation of Vickers hardness on the steel sheet surface, the lower the standard deviation of Vickers hardness on the steel sheet surface, the more preferable it is. Even if the standard deviation of Vickers hardness on the steel sheet surface is 0, the effects of the present disclosure can be achieved. Therefore, the standard deviation of Vickers hardness on the steel sheet surface is 15 or less. The standard deviation of Vickers hardness on the steel sheet surface is preferably 0 or more. The standard deviation of Vickers hardness on the steel sheet surface is preferably 13 or less.

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

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

[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 constituent feature of the present disclosure. By reducing the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface, desired bendability can be achieved. To achieve this effect, the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface needs to be 7 times or less. Note that there is no particular lower limit for the hardness fluctuation frequency per 1100 μm in the sheet width direction on the steel sheet surface, but a lower hardness fluctuation frequency is preferable, and even if it is 0 times, the effects 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 fluctuation per 1100 μm in the sheet width direction on the surface of the steel sheet is as follows.

[0062] A bending test is performed using the V-block method with a bending angle of 90 degrees, with an R where R / t is approximately 4.5, i.e., 4.3 to 4.7. A sample 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 bent specimen is used as the observation surface, and the observation surface is mirror-polished using diamond paste. Next, the Vickers hardness of the mirror-polished observation surface is measured at 11 points along the plate width direction at 100 μm intervals using a Vickers hardness tester under a load of 100 gf. The measurement position is 500 μm away from the end of the crack in the rolling direction, with the sixth point being parallel to the crack. From the obtained results, a hardness distribution is created by measuring the steel plate surface in the plate width direction using a Vickers hardness tester. In this hardness distribution, first, {(maximum hardness Hv ｍａｘ ) - (minimum hardness Hv ｍｉｎ )} / 2 is calculated. ｍａｘ ) - (minimum hardness Hv ｍｉｎ )} / 2 is used as the reference fluctuation amount, and each time the hardness fluctuates above or below the reference 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, when the hardness fluctuates above or below the reference fluctuation amount, it is counted as one occurrence, and when the hardness fluctuates below the reference fluctuation amount, it is counted as one occurrence. Therefore, when the hardness fluctuates above and below once, the hardness fluctuation frequency is two.

[0063] Furthermore, the steel structure according to the present disclosure may contain a residual structure other than the above-mentioned martensite (quenched martensite, tempered martensite, and bainite), ferrite (including bainitic ferrite), and retained austenite. The effects of the present disclosure are not impaired even if the residual structure contains carbides such as pearlite, cementite, and metastable carbides, as well as other known structures of steel sheets, as long as the area ratio is within a range of 5% or less. Here, metastable carbides include, for example, epsilon (ε) carbide, eta (η) carbide, and chi (χ) carbide.

[0064] [High-strength steel plate] The chemical composition and steel structure of the high-strength steel plate are as described above. The 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 comprising a plating layer on at least one surface 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 electroplated 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. A 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% by mass or more and 22% by mass or less, the Mg content is 0.1% by mass or more and 10% by mass or less, and the remainder is Zn. Furthermore, the Zn-Al-Mg plating layer may contain, in addition to Zn, Al, and Mg, one or more elements selected from Si, Ni, Ce, and La in a total amount of 1% by mass or less. The plating metal is not particularly limited, and in addition to the Zn plating described above, an Al plating or the like may also 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 20 mass% or less of Fe and 0.001 mass% to 1.0 mass% of Al. Furthermore, it contains one or more elements 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% to 3.5 mass%, with the balance consisting of Zn and unavoidable impurities. In the present disclosure, the plating coating weight per side is 20 to 80 g / m. ２ Preferably, the coating layer has a hot-dip galvanized layer, which is further alloyed to form a galvannealed 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 manufacturing a high-strength steel sheet according to the present disclosure will be described.

[0068] First, a steel material having the above-described composition is melted to produce a steel slab. In the present disclosure, the method for melting the steel material is not particularly limited, and any known melting method, such as a converter or an electric furnace, is suitable. Furthermore, 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, a thin slab casting method, or the like. Furthermore, in addition to the conventional method in which the steel slab is cooled to room temperature after production and then reheated, energy-saving processes such as direct rolling, in which the slab is charged into a heating furnace as a hot slab without cooling, 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 the slab in the temperature range of 900°C to 1150°C: 25°C / min or less] This is an extremely important invention requirement in the present disclosure. By slowing 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 Si desorption 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. Furthermore, by slowing the average heating rate of the slab in the temperature range of 900°C to 1150°C, it is possible to reduce Mn segregation formed during casting and reduce the number density of MnS present in the Mn segregated portion 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 fluctuations per 1100 μm in the sheet width direction on the steel sheet surface. To achieve 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 there is no particular lower limit for the average heating rate of the slab in the temperature range of 900°C or higher and 1150°C or lower, it is preferably 5°C / min or higher in order to suitably prevent an increase in the softened thickness of the surface layer after annealing and to keep TS within a more suitable range. Therefore, the average heating rate of the slab in the temperature range of 900°C or higher and 1150°C or lower is 25°C / min or lower. The average heating rate of the slab in the temperature range of 900°C or higher and 1150°C or lower is preferably 5°C / min or higher. The average heating rate of the slab in the temperature range of 900°C or higher and 1150°C or lower is preferably 15°C / min or lower. The slab heating temperature is the surface temperature of the steel slab during slab heating.

[0071] [Slab heating temperature: 1150°C or higher] This is an extremely important invention constituent element in the present disclosure. Increasing the slab heating temperature to 1150°C or higher promotes Si desorption from the steel sheet surface and reduces the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. Furthermore, increasing the slab heating temperature reduces Mn segregation formed during casting and reduces the number density of MnS present in the Mn segregated portion on the steel sheet surface. Furthermore, it is possible to reduce the standard deviation of the Vickers hardness on the steel sheet surface and the frequency of hardness fluctuations per 1100 μm in the sheet width direction on the steel sheet surface. To achieve these effects, the slab heating temperature is set to 1150°C or higher. While there is no particular upper limit for the slab reheating temperature, it is preferable to set it to 1300°C or lower in order to appropriately prevent an increase in the surface softening thickness after annealing and to keep the TS within a more suitable range. Therefore, the slab heating temperature is set to 1150°C or higher. The slab heating temperature is preferably set to 1180°C or higher. The slab heating temperature is preferably 1300° C. or less. The slab heating temperature refers to the temperature of the surface of the steel slab during heating.

[0072] [Residence time from 1100°C to the slab heating temperature: 20 min or more] This is an extremely important invention requirement in the present disclosure. By extending the residence time from 1100°C to the slab heating temperature to 20 min or more, Si desorption from the steel sheet surface can be promoted, and the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction can be reduced. Furthermore, extending the residence time from 1100°C to the slab heating temperature can reduce Mn segregation formed during casting and the number density of MnS present in the Mn segregated portion on the steel sheet surface. By extending 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 the frequency of hardness fluctuations per 1100 μm in the sheet width direction on the steel sheet surface. To achieve these 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 to the residence time from 1100°C to the slab heating temperature, it is preferably 100 min or less in order to suitably prevent an increase in the softened surface thickness after annealing and to keep 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 made into a sheet bar by rough rolling under normal conditions, but if the slab heating temperature is set 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 ratio of the final pass of finish rolling: 9% or more and 15% or less] [Reduction ratio of the pass two passes before the final pass: 15% or more and 21% or less] [Reduction ratio of the pass two passes before the final pass: 21% or more and 27% or less] These are extremely important constituent elements of the present disclosure. 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 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 two passes 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 at the steel sheet surface, and the frequency of hardness fluctuation per 1,100 μm in the sheet width direction at the steel sheet surface.

[0075] If the reduction ratio in the final pass of 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, thereby suppressing the diffusion of Si to the steel sheet surface. 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. Furthermore, since Ti precipitation, i.e., sulfide precipitation, is suppressed, it is not possible to reduce the number density of MnS present in the Mn-segregated portion on the steel sheet surface. On the other hand, if the reduction ratio in 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 on the steel sheet surface or the frequency of hardness fluctuations per 1,100 μm in the sheet width direction on the steel sheet surface. Therefore, the reduction ratio in the final pass of finish rolling is set to be 9% or more and 15% or less.

[0076] If the reduction ratio before the final pass is less than 15%, 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, thereby suppressing the diffusion of Si to the steel sheet surface. 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. Furthermore, Ti precipitation, i.e., sulfide precipitation, is suppressed, so the number density of MnS present in the Mn-segregated portion on the steel sheet surface cannot be reduced. On the other hand, if the reduction ratio 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 on the steel sheet surface or the frequency of hardness fluctuations per 1,100 μm in the sheet width direction on the steel sheet surface. Therefore, the reduction ratio before the final pass of finish rolling is set to 15% or more and 21% or less.

[0077] If the reduction ratio in 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, i.e., the crystal grain size in the annealed sheet becomes coarse, thereby suppressing Si diffusion to the steel sheet surface. 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. Furthermore, Ti precipitation, i.e., sulfide precipitation, is suppressed, so the number density of MnS present in the Mn-segregated portion on the steel sheet surface cannot be reduced. On the other hand, if the reduction ratio in 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 on the steel sheet surface or the frequency of hardness fluctuations per 1,100 μm in the sheet width direction on the steel sheet surface. Therefore, the reduction ratio in 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 the reduction ratio in the unrecrystallized austenite state, 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 ３Transformation point (°C) = 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] represents the content (mass%) of the corresponding element in the above composition, and is set to 0 if the element is not contained.

[0079] During hot rolling, the rough-rolled sheets may be joined together and continuously finished. The rough-rolled sheets may also be wound up once. To reduce the rolling load during hot rolling, some or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniforming the shape and material properties of the steel sheet. The coefficient of friction during lubricated rolling is preferably 0.10 or more and 0.25 or less.

[0080] The hot-rolled steel sheet thus produced is then subjected to pickling. Pickling is important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet, as it can remove oxides from the steel sheet surface. 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) that has been optionally heat-treated after pickling is cold-rolled to obtain a cold-rolled sheet. Since strain is introduced uniformly and efficiently and a uniform structure is obtained, cold rolling is preferably performed by multi-pass rolling that requires two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0082] In addition, it is preferable to perform bending and bending back processing at least once before cold rolling 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. 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 or more. 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] [Cold Rolling Cumulative Reduction: 20% or More and 75% or Less] Increasing the cumulative reduction in cold rolling reduces the ferrite area ratio, i.e., the total of the ferrite area ratio and / or the retained austenite volume ratio can be set to 20% or less. Furthermore, increasing the cumulative reduction in cold rolling introduces a large amount of shear bands into the steel sheet surface, thereby reducing the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. To achieve this effect, the cumulative reduction in cold rolling is set to 20% or more. On the other hand, if the cumulative reduction in cold rolling exceeds 75%, the grain size of austenite generated during annealing becomes finer, and the amount of retained austenite in the annealed sheet increases. In other words, the total of the ferrite area ratio and / or the retained austenite volume ratio increases, making it impossible to achieve the desired YR. Furthermore, since the hardness distribution in the sheet width direction becomes non-uniform, the standard deviation of the Vickers hardness on the steel sheet surface exceeds 15, and furthermore, the frequency of hardness fluctuations per 1,100 μm in the sheet width direction on the steel sheet surface exceeds 7 times, resulting in reduced bendability. Therefore, the cumulative reduction rate of cold rolling is set to 20% or more and 75% or less. The cumulative reduction rate of cold rolling is preferably set to 25% or more. The cumulative reduction rate of cold rolling is preferably set to 70% or less. The cumulative reduction rate of cold rolling is more preferably set to 27% or more. The cumulative reduction rate 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 of 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, thereby reducing the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction. To achieve this effect, the average heating rate in the temperature range of 250°C to 700°C needs to be 100°C / s or less. While there is no particular lower limit for the average heating rate in the temperature range of 250°C to 700°C, from the viewpoint of suppressing coarsening of austenite grain size during heating and optimizing the YR, the average heating rate is preferably 5°C / s or more, and more preferably 10°C / s or more. Therefore, the average heating rate in the temperature range of 250°C to 700°C is set to 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 or higher and 700°C or lower is preferably 75°C / s or lower. The average heating rate in the temperature range of 250°C or higher and 700°C or lower is more preferably 10°C / s or higher. The average heating rate in the temperature range of 250°C or higher and 700°C or lower is more preferably 50°C / s or lower. The average heating rate is measured based on the temperature of the steel sheet surface.

[0086] [Heating temperature: 780°C or higher] If the heating temperature (annealing temperature) is lower than 780°C, the annealing process will be performed in the two-phase region of ferrite and austenite, resulting in a large amount of ferrite after annealing, making it difficult to achieve a TS of 1180 MPa or higher and thus a desired YR. Furthermore, the hardness distribution in the sheet width direction will be non-uniform, resulting in a standard deviation of the Vickers hardness on the steel sheet surface exceeding 15, and the frequency of hardness fluctuations per 1100 μm in the sheet width direction on the steel sheet surface exceeding 7, resulting in reduced bendability. Furthermore, Si cannot diffuse to the steel sheet surface, making it impossible to reduce the Si concentration at a position 5 μm from the steel sheet surface in the sheet thickness direction, making it impossible to improve the inter-steel sheet cracking resistance of the spot weld HAZ. While the upper limit of the heating temperature is not particularly specified, a temperature of 1000°C or lower is preferred because an increase in the heating temperature increases the thickness of the softened surface layer after annealing, reduces the TS, coarsens the prior austenite grain size, and reduces the YR. Therefore, the heating temperature is set to 780°C or higher. The heating temperature is more preferably set to 820°C or higher. The heating temperature is further preferably set to 830°C or higher. The upper limit of the heating temperature is preferably set to 1000°C or lower. The upper limit of the heating temperature is more preferably set to 980°C or lower. The heating temperature is measured based on the temperature of the steel sheet surface.

[0087] [Residence time between 750°C and the heating temperature: 10 seconds or more] This is an extremely important inventive feature of the present disclosure. Increasing the residence time between 750°C and the heating temperature can promote Si desorption from the steel sheet surface and reduce the Si concentration at a position 5 μm from the steel sheet surface in the thickness direction. Furthermore, it is possible to reduce Mn segregation formed during casting, reduce the standard deviation of Vickers hardness on the steel sheet surface, and reduce the frequency of hardness fluctuations per 1,100 μm in the width direction on the steel sheet surface. To achieve these effects, the residence time between 750°C and the heating temperature is set to 10 seconds or more. There is no particular upper limit to the residence time between 750°C and the heating temperature. However, increasing the residence time between 750°C and the heating temperature can increase the thickness of the softened surface layer after annealing, which may result in a decrease in TS, or coarsen the prior austenite grain size, which may result in a decrease in YR. Therefore, the residence time between 750°C and the heating temperature is set to 10 seconds or more. The residence time from 750°C to the heating temperature is preferably 20 seconds or more. The residence time from 750°C to the heating temperature is preferably 400 seconds or less. The residence time from 750°C to the heating temperature is more preferably 25 seconds or more. The residence time from 750°C to the heating temperature is more preferably 300 seconds or less.

[0088] [Oxygen concentration of the atmosphere at a heating temperature of 750°C or higher: 0.5% by volume or higher and 5.0% by volume or lower (preferred conditions)] By increasing the oxygen concentration at a heating temperature of 750°C or higher during annealing, oxygen in the air can be used to promote Si desorption from the steel sheet surface, thereby reducing the Si concentration at a position 5 μm in the thickness direction from the steel sheet surface. Furthermore, Mn desorption from the steel sheet surface can be promoted, reducing the standard deviation of the Vickers hardness of the steel sheet surface and reducing the frequency of hardness fluctuations per 1,100 μm in the width direction at the steel sheet surface. To achieve these effects, the oxygen concentration at a heating temperature of 750°C or higher 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. Meanwhile, as the oxygen concentration at a heating temperature of 750°C or higher increases, 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 temperature of 750°C or higher and the heating temperature or lower is based on the steel sheet surface temperature. That is, when the steel sheet surface temperature is 750°C or higher and the heating temperature or lower, the oxygen concentration is adjusted to be within the above range.

[0089] [Dew point of atmosphere at a heating temperature of 750°C or higher: -35°C or higher (preferred conditions)] During annealing, increasing the dew point of the atmosphere at a heating temperature of 750°C or higher promotes Si desorption from the steel sheet surface via moisture in the air, thereby reducing the Si concentration at a position 5 μm from the steel sheet surface in the thickness direction. Furthermore, 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 frequency of hardness fluctuations per 1,100 μm in the width direction on the steel sheet surface. To achieve these effects, the dew point at a heating temperature of 750°C or higher is preferably -35°C or higher, more preferably -30°C or higher, and even more preferably -25°C or higher. There is no particular upper limit to the dew point at a heating temperature of 750°C or higher. However, because the softened surface thickness after annealing increases and TS decreases, the dew point at a heating temperature of 750°C or higher is preferably 15°C or lower, more preferably 5°C or lower. The temperature of 750°C or higher and the heating temperature or lower is based on the surface temperature of the steel sheet. 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 step] After the annealing step, the cold-rolled steel sheet is optionally cooled. The average cooling rate from the heating temperature to 400°C or higher is not particularly limited, but is preferably 5°C / s or higher and 30°C / s or lower. In the temperature range from the heating temperature to 400°C or higher, the high-strength steel sheet may be cooled once and then 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 (preferred condition)] 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. Note that there is no particular upper limit for the average cooling rate in the temperature range of 250°C to 400°C, but due to production technology constraints, 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 defined as the value in the temperature range of the cooling stop temperature to 400°C. Note that 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 higher and 400° C. or lower, gas jet cooling, mist cooling, water cooling, air cooling, etc. can be applied.

[0093] [Heat retention temperature in cooling step: 100°C or higher and 450°C or lower (preferred conditions)] In the cooling step, it is preferable to retain heat for 5 seconds or longer at a temperature in the range of 100°C or higher and 450°C or lower. By retaining the heat within the above range, it is possible to retain the YR and bendability within more preferable ranges for the high-strength steel sheet. In addition, it is possible to further reduce the area ratio of bainitic ferrite and further improve TS. The heat retention temperature in the cooling step is more preferably 150°C or higher, and even more preferably 200°C or higher. In addition, the heat retention temperature in the cooling step is more preferably 400°C or lower, and even more preferably 350°C or lower. The temperature in the cooling step is based on the surface temperature of the steel sheet.

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

[0095] [Cooling stop temperature: 250°C or less (preferred conditions)] In the cooling step described above, 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. There is no particular lower limit for the cooling stop temperature, but from the viewpoint of productivity, it is preferably room temperature or higher. The cooling stop rate 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 that can be used 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 within a desired range. Furthermore, 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 retained austenite can be set within a more suitable range. Furthermore, the bendability and the damage degree of the sheared edge in a corrosive environment can be set within 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 discontinuous with the above-mentioned continuous annealing apparatus (offline). The target elongation may be achieved in a single rolling pass, or a total elongation of 0.05% to 2.00% may be achieved by performing multiple rolling passes. The rolling described here generally refers to temper rolling, but any other processing method, such as repeated bending using a tension leveler or rolls, may also be used as long as it can impart an elongation equivalent to that achieved by temper rolling.

[0100] [Reheating temperature: (cooling stop temperature + 50°C) or higher and 450°C or lower (preferred 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). Reheating the high-strength steel sheet can bring the YR and bendability within more preferred ranges. To achieve this 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, tempering of martensite progresses and 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. Note that the reheating temperature is based on the surface temperature of the steel sheet.

[0101] [Heat retention time at reheating temperature: 5 seconds or more (preferred condition)] By retaining the heat at the reheating temperature, the YR and bendability can be kept within a more preferable range. To achieve this effect, the heat retention time at the reheating temperature is preferably 5 seconds or more, more preferably 10 seconds or more, and even more preferably 15 seconds or more. There is no particular upper limit to the heat retention time at the reheating temperature, but in order to keep the TS within a more preferable range, the heat retention time at the reheating temperature is preferably 500 seconds or less, more preferably 250 seconds or less.

[0102] The material may be cooled from the reheating temperature 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, or air cooling.

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

[0104] [Method for manufacturing high-strength plated steel sheet] A high-strength plated steel sheet can be obtained by subjecting at least one surface of the high-strength steel sheet manufactured as described above to a plating treatment. For example, examples of the plating treatment include hot-dip galvanizing and hot-dip galvanizing followed by alloying. Annealing and galvanizing may also be performed continuously in a single line. Alternatively, the plating layer may be formed by electroplating such as Zn—Ni electrolytic alloy plating, or hot-dip zinc-aluminum-magnesium alloy plating. While the above description has focused on 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 immerse the high-strength steel sheet in a galvanizing bath at 440°C to 500°C, and then adjust the coating weight by gas wiping or the like. For hot-dip galvanizing, a galvanizing bath containing 0.10% by mass to 0.23% by mass of Al is preferably used. Furthermore, the temperature range for performing the galvanizing alloying treatment after hot-dip galvanizing is preferably 470°C to 600°C, more preferably 470°C to 560°C. By performing the alloying treatment at 470°C or higher, the Zn-Fe alloying rate is more favorable, and productivity is more favorable. Furthermore, by performing the alloying treatment at 600°C or lower, the transformation of untransformed austenite to pearlite is prevented, and TS is more favorable. Furthermore, electrogalvanizing may be performed. Furthermore, the coating weight is 20 to 80 g / m per side. ２ (Double-sided plating) is preferred, and the alloyed hot-dip galvanized 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 annealing step, the high-strength steel sheet may be plated in a temperature range of 400°C or higher but not higher than the heating temperature without being cooled, or the cold-rolled steel sheet may be once cooled to below 400°C, and the steel sheet temperature may be increased again to 400°C or higher before being plated.

[0107] The high-strength plated steel sheet that has been subjected to the above-described 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 within a desired range. Furthermore, 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 retained austenite can be set within a more suitable range, and the bendability and the degree of damage at the sheared edge in a corrosive environment can be set within more suitable ranges.

[0108] The rolling after the plating treatment 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 in a single rolling pass, or multiple rolling passes may be performed to achieve a total elongation of 0.05% to 2.00%. The rolling described here generally refers to temper rolling, but any processing method involving repeated bending using a tension leveler or rolls may also be used as long as it can impart an elongation equivalent to that achieved by temper rolling. Furthermore, a reheating treatment may be performed after the rolling after the plating treatment.

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

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

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

[0112] [Members] Next, members according to one embodiment of the present invention will be described.

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

[0114] 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 the HAZ of spot welds, and that 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 for automotive frame structural parts or automotive reinforcing parts.

[0115] Steel having the chemical composition shown in Table 1, with the balance being Fe and unavoidable impurities, was melted in a converter and formed into a slab by continuous casting. After cooling, the resulting 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. Pre-annealed cold-rolled steel sheets with a thickness of 1.4 mm were produced. In some examples, sheets with different thicknesses were also produced.

[0116]

[0117]

[0118]

[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 steel sheets were subjected to a plating process to obtain hot-dip galvanized steel sheets (GI), galvannealed steel sheets (GA), and electrogalvanized steel sheets (EG). For the hot-dip galvanizing bath, a zinc bath containing 0.14 to 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 to 72 g / m per side. ２ (Both sides plating) and for GA, 45 g / m per side ２ The GA had an Fe concentration in the plating layer of 9% by mass or more and 12% by mass or less. The EG, which had a Zn—Ni plating layer, had an Ni content in the plating layer of 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, and the tensile properties, bendability, and inter-steel sheet cracking properties of the spot weld HAZ were evaluated according to the following test methods. The results are shown in Table 4.

[0121]

[0122] [Tensile Test] The tensile test was carried out in accordance with JIS Z 2241:2022. JIS No. 5 test pieces were taken from the obtained steel sheets in a direction perpendicular to the rolling direction of the steel sheets, and the test pieces were rolled at a crosshead speed of 1.67 × 10 －１ Tensile tests were conducted at a speed of 1000 kJ / s, and the YS and TS were measured. In the present invention, a yield ratio (YR) of 65% or more and 90% or less was determined to be high dimensional accuracy. The YR was calculated using the formula (2) above.

[0123] [Bending Test] The bending test was conducted in accordance with JIS Z 2248:2022. From the obtained steel plate, strip-shaped test pieces with a width of 30 mm and a length of 100 mm were taken, with the axial direction of the bending test parallel to the rolling direction of the steel plate. Then, a 90° V-bending test was conducted under conditions of an indentation load of 100 kN and a holding time of 5 seconds. In this disclosure, bending properties were evaluated by conducting bending tests on five samples at an R where R / t, the value obtained by dividing the bending radius (R) by the plate thickness (t), was approximately 4.5, i.e., 4.3 to 4.7. Next, the crack length at the ridgeline of the bending apex of all five samples was evaluated, and samples with a crack length of 200 μm or less were judged to have excellent bendability. 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 inter-plate cracking test] A test piece was cut out to a thickness of 1.4 mm, a length of 30 mm and a width of 100 mm with the rolling direction as the longitudinal direction. A test piece was cut out to the same size, and the coating weight of the hot dip galvanized layer per side was 50 g / m ２The sheet assembly was then overlapped with a test hot-dip galvanized steel sheet (100 mm diameter) to form a sheet assembly. Next, resistance welding was performed using a servomotor-pressurized, single-phase AC (50 Hz) resistance welding machine with an electrode tip diameter of 6 mm. The sheet assembly was tilted 5° relative to the electrode of the resistance welding machine, with a 1.5 mm clearance between the lower electrode and the lower steel sheet. Specifically, resistance welding was performed on the sheet assembly under the conditions of a pressure of 3.5 kN, a hold time of 0.16 or 0.20 seconds, and a welding current and welding time that resulted in a nugget diameter of 5.9 mm to form a sheet assembly with a weld. Next, the sheet assembly with the weld was cut in half to include the weld, and the cross section of the weld was observed with an optical microscope (200x magnification). The inter-steel sheet cracking resistance of the HAZ of the spot weld was evaluated according to the above criteria. In addition, similar evaluations were performed on some samples with sheet 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 segregated portion of the steel sheet surface were determined. Furthermore, 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 were determined. In addition, the remaining structure was observed using the method described below. After cutting out a sample so that the 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. Using an SEM under the condition of an acceleration voltage of 15 kV, the observation position was set at 1 / 4 of the steel sheet thickness, and observation was performed at 5000x magnification in a field of view range of 17 μm x 23 μm. Carbides were identified as the remaining structure from the obtained structure image.

[0126] As shown in Table 4, the inventive examples are excellent in TS, YR, bendability, and inter-steel plate cracking resistance in the HAZ of the spot welds. On the other hand, the comparative examples are inferior in one or more of TS, YR, bendability, and inter-steel plate cracking resistance in the HAZ of the spot welds.

[0127] Although the embodiments of the present invention have been described above, the present invention is not limited to the descriptions of the present embodiments, which form part of the disclosure of the present invention. In other words, other embodiments, examples, and operational techniques made by those skilled in the art based on the present embodiments are all included in the scope of the present invention. For example, in the series of heat treatments in the above-described manufacturing method, there are no particular limitations on the equipment used to perform the heat treatment on the steel sheet as long as the thermal history conditions are met.

[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 plate cracking resistance in the HAZ of spot welds and which can be used to manufacture parts with high dimensional accuracy.

[0129] In particular, the high-strength steel sheet of the present invention has excellent resistance to inter-steel sheet cracking in the HAZ of spot welds, and therefore can be applied to automotive structural parts of various sizes and shapes while achieving high part strength, which can lead to improved fuel efficiency through reduced vehicle weight and has extremely high industrial value.

Claims

1. Containing, by 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 an effective Ti mole fraction (x Ｔｉ，ｅｆｆ ) is 0.001 or more, with the balance consisting of Fe and unavoidable impurities; a steel structure in which, at a 1 / 4 position in the sheet thickness, the area fraction of martensite is 80% or more and 99% or less, and the area fraction of ferrite and / or the volume fraction of retained austenite is more than 0% and 20% or less in total; a 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, and a number density of MnS present in Mn segregated parts on the steel sheet surface is 5.0 pieces / mm ２ A high-strength steel plate having a standard deviation of Vickers hardness on the surface of the steel plate of 15 or less, and a hardness fluctuation frequency of 7 or less per 1100 μm in the sheet width direction on the surface of the steel plate. Ｔｉ，ｅｆｆ = x Ｔｉ -x Ｎ -x Ｓ ... (1) Note that x in the formula Ｔｉ , x Ｎ , x Ｓ indicates 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 having a plating layer on at least one side of the high-strength steel sheet according to claim 1 or 2.

4. A steel slab having the chemical composition according to claim 1 or 2 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 of 20 min or more from 1100°C to said slab heating temperature; then, hot rolling is performed on said steel slab with a final pass reduction of 9% to 15%, a pass before said final pass reduction of 15% to 21%, and a pass before said final pass reduction of 21% to 27% to obtain a hot-rolled sheet; then, pickling is performed on said hot-rolled sheet to obtain a pickled sheet; then, cold rolling is performed on said pickled sheet with a cumulative 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. The method for producing high-strength steel plate according to claim 4, wherein the oxygen concentration of the atmosphere at a temperature of 750°C or higher and lower than the heating temperature in the annealing step 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. A method for producing a high strength steel plate according to claim 4 or 5, further comprising a cooling step following the annealing step, in which the average cooling rate in the temperature range of 250°C or higher and 400°C or lower is 1.0°C / s or higher.

7. The method for producing a high strength steel plate according to claim 4 or 5, further comprising a cooling step following the annealing step, in which the plate is kept at a temperature of 100°C or higher and 450°C or lower for 5 seconds or longer.

8. A method for producing a high strength steel sheet according to claim 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 seconds 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 claims 4 to 8, 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 claim 1 or 2.

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

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