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

Abstract

The objective of the present invention is to provide a high-strength steel sheet of 1,180 MPa or more that has excellent stretch flangeability, bendability, and inter-steel sheet cracking resistance in the HAZ of spot welds, and that enables the manufacture of parts with high dimensional accuracy, a high-strength plated steel sheet, their manufacturing methods, and components. Having a predetermined composition, x Ti,eff =x Ti -x N -x S (However, x Ti , x N , x S is the effective Ti mole fraction (x Ti,eff ) is 0.001 or more, and at the 1 / 4 position of the plate thickness, the area ratio of martensite is 60% to 99%, the area ratio of ferrite and / or the volume ratio of retained austenite is more than 0% and 40% in total. The boron atom concentration at the prior austenite grain boundary with an orientation difference of 15 degrees or more is 0.3 at% to 6.0 at%, and the number density of precipitates with a size of 2 μm or more is 150 / mm 2 The following is a high-strength steel plate.

Description

[Technical field]

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

[0002] Steel sheets for automobiles are being made stronger in order to reduce CO2 emissions by making the vehicle lighter and to improve crashworthiness by making the vehicle lighter. New legal regulations are also being introduced one after another. Therefore, in order to increase the strength of the vehicle body, the number of cases where high-strength steel sheets are used for the main structural parts and reinforcing parts that form the framework of the automobile cabin (hereinafter also referred to as automotive framework structural parts) is increasing. In particular, the number of cases where high-strength steel sheets with a tensile strength (hereinafter also referred to as TS) of 1180 MPa or more is being used is increasing.

[0003] High-strength steel sheets used in automotive reinforcement parts and frame structural parts are required to have excellent formability. Furthermore, the parts after forming are required to have excellent dimensional accuracy. For example, parts such as crash boxes have punched end faces and bent parts, so from the viewpoint of formability, steel sheets with high stretch flangeability and bendability are suitable. From the viewpoint of part performance, increasing the yield ratio of the steel sheet (YR = yield strength YS / tensile strength TS × 100) increases the impact absorption energy during a collision. Furthermore, from the viewpoint of dimensional accuracy of parts, controlling the yield ratio (YR) of the steel sheet within a certain range makes it possible to suppress springback after steel sheet forming and control the dimensional accuracy of parts. In order to increase the application ratio of high-strength steel sheets to automotive parts, it is required to comprehensively satisfy these characteristics.

[0004] Furthermore, among the structural components of automobiles, for example, crash boxes have punched end faces and bent parts. Therefore, from the viewpoint of formability, it is preferable to apply steel sheets having high stretch flangeability and bendability to such parts. Furthermore, for bent parts, in order to increase the application ratio of high-strength steel sheets to parts having a relatively small bending radius, there is a demand for high-strength steel sheets having high bendability that does not cause large cracks in bent parts where the value R / t obtained by dividing the bending radius (R) by the plate thickness (t) is less than 5.0.

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

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

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

[0008] The high-strength steel sheet described in Patent Document 1 comprehensively satisfies stretch flangeability, bendability, and LME resistance, and can be used to manufacture parts with high dimensional accuracy. However, the high-strength steel sheet described in Patent Document 1 has a TS of 980 MPa, leaving room for further improvement in strength.

[0009] The present disclosure has been developed in view of the above circumstances. It is an object of the present disclosure to provide a high-strength steel sheet of 1180 MPa or more that is excellent in stretch flangeability, bendability, and inter-steel sheet cracking resistance in spot weld HAZ and that can be used to manufacture parts with high dimensional accuracy, and to provide an advantageous method for manufacturing the high-strength steel sheet.

[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 stretch flangeability, when the hole expansion ratio (hereinafter also simply referred to as λ) measured in accordance with JIS Z 2256 is 30% or more, it is determined that the stretch flangeability is excellent.

[0011] Regarding bendability, a bending test is performed using the V-block method with a bending angle of 90 degrees, and five samples are bent at an R where the value R / t obtained by dividing the bending radius (R) by the plate thickness (t) is about 4.5, i.e., 4.3 to 4.7. Next, the crack length at the ridgeline of the bend apex of all five samples is evaluated, and if the crack length is 200 μm or less, it is determined that the bendability is excellent.

[0012] For the inter-steel-plate cracking resistance of the spot weld HAZ, the cross section of the weld described in the examples was observed with an optical microscope (200x) and the inter-steel-plate cracking resistance of the spot weld HAZ was evaluated according to the following criteria. If it was A or B, it was determined that the spot weld HAZ had excellent inter-steel-plate cracking resistance. If it was C, it was determined that the spot weld HAZ had poor inter-steel-plate cracking resistance. A: No cracks longer than 0.1 mm were observed with a hold time of 0.16 seconds. B: A crack longer than 0.1 mm is observed with a hold time of 0.16 seconds, but no crack longer than 0.1 mm is observed with a hold time of 0.20 seconds. C: A crack longer than 0.1 mm is observed at a hold time of 0.20 seconds.

[0013] Here, the hold time refers to the time from when the welding current is finished flowing to when the electrodes start to be released. [Means for solving the problem]

[0014] Means for Solving the Problems The present inventors have conducted extensive research to achieve the above object, and as a result have obtained the following findings. (1) By using a steel structure that is mainly composed of martensite (quenched martensite and tempered martensite) and further contains ferrite and / or retained austenite, it is possible to achieve a YR, which is an index of the dimensional accuracy of parts, of 65% to 90%. (2) By setting the boron atom concentration in the prior austenite grain boundaries with a misorientation of 15 degrees or more to 0.3 at% or more and 6.0 at% or less, good inter-steel sheet cracking resistance properties of the spot weld HAZ can be realized. (3) The number density of precipitates with a size of 2 μm or more is 150 / mm 2 By setting the following, good stretch flangeability and bendability can be achieved.

[0015] The present disclosure has been made based on the above findings. That is, the gist of the present disclosure is as follows. [1] In mass%, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0.002% or more and 0.200% or less, and B: 0.0002% or more and 0.0100% or less, and the effective Ti mole fraction (x Ti,eff ) is 0.001 or more, with the balance being Fe and unavoidable impurities; At the 1 / 4 position of the plate thickness, the area ratio of martensite is 60% to 99%, the area ratio of ferrite and / or the volume ratio of retained austenite is more than 0% and less than 40% in total, and the boron atom concentration at the prior austenite grain boundary with an orientation difference of 15 degrees or more is 0.3 at% to 6.0 at%, and the number density of precipitates with a size of 2 μm or more is 150 / mm 2 The following is a high-strength steel plate. Note x Ti,eff =x Ti -x N -x S (1) In addition, x in the formula Ti , x N , x S 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, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 1.00% 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 [1], containing at least one element selected from the following: [3] A high-strength plated steel sheet having a plating layer on at least one side of the high-strength steel sheet according to [1] or [2]. [4] A steel slab having the composition according to [1] or [2] is prepared, and the steel slab is cooled at an average cooling rate of 50°C / hr or more and 500°C / hr or less in a temperature range of 700°C or more and 1000°C or less. The steel slab is then heated at an average heating rate of 25°C / min or less in a temperature range of 900°C or more and 1150°C or less to a slab heating temperature of 1150°C or more, with a residence time from 1100°C to the slab heating temperature being 20 minutes or more. The steel slab is then subjected to a rough rolling delivery temperature of 1100°C or more and a finish rolling delivery temperature of 1100°C or more. A method for producing a high strength steel sheet, comprising the steps of: hot rolling at an entry temperature of 1050°C or more to obtain a hot rolled sheet; pickling the hot rolled sheet to obtain a pickled sheet; cold rolling the pickled sheet at a cumulative reduction of 20% to 95% and at a sheet passing speed of 50 mpm or more in a final pass of cold rolling to obtain a cold rolled sheet; annealing the cold rolled sheet to a heating temperature of 780°C or more; and first cooling the cold rolled sheet at an average cooling rate of 0.5°C / s to 50°C / s in a temperature range from the heating temperature to 650°C. [5] The method for producing a high-strength steel plate according to [4], further comprising: performing second cooling after the first cooling, with the average cooling rate in the temperature range of 250°C or more and 400°C or less being 1.0°C / s or more. [6] The method for producing a high-strength steel plate according to [4], further comprising: after the first cooling, performing a second cooling by maintaining the temperature at 100°C or higher and 450°C or lower for 5 s or more. [7] The method for producing a high strength steel sheet according to [4], further comprising cooling the sheet to 250°C or lower after the first cooling, 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 for 5 s or more to perform a second cooling. [8] A method for producing a high-strength plated steel sheet, comprising the steps of: after the annealing step according to [4], performing a plating step of plating at least one surface of the cold-rolled sheet. [9] The method for producing a high-strength plated steel sheet according to [8], further comprising, after the first cooling, performing a second cooling at an average cooling rate of 1.0°C / s or more in a temperature range of 250°C or more and 400°C or less.

[10] The method for producing a high-strength plated steel sheet according to [8], further comprising: after the first cooling, performing a second cooling by maintaining the temperature at 100°C or higher and 450°C or lower for 5 s or more.

[11] The method for producing a high-strength plated steel sheet according to [8], further comprising cooling to 250°C or lower after the first cooling, 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 for 5 s or more at the reheating temperature, followed by second cooling.

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

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

[0016] According to the present disclosure, it is possible to provide a high-strength steel plate and member of 1180 MPa or more which has excellent stretch flangeability, bendability, and inter-steel sheet cracking resistance in spot weld HAZ, and which enables the manufacture of parts with high dimensional accuracy. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, embodiments of the present disclosure will be described. Note that the present disclosure is not limited to the following embodiments.

[0018] First, the appropriate range of the composition of the steel sheet and the reason for limiting it will be described. In the following description, "%" representing the content of the component elements of the steel sheet means "mass%" unless otherwise specified. In addition, in this specification, a numerical range expressed with "~" means a range including the numerical values ​​written before and after "~" as the lower and upper limits.

[0019] [C: 0.030% or more and 0.500% or less] C is one of the important basic components of steel, and in the present disclosure, in particular, it is an important element that affects the area ratio of martensite, ferrite, and the volume ratio of retained austenite. If the C content is less than 0.030%, the area ratio of martensite decreases, and the area ratio of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. It also makes it difficult to achieve a desired YR. On the other hand, if the C content exceeds 0.500%, the volume ratio of retained austenite increases, so that λ and bendability decrease. 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.

[0020] [Si:0.01% or more and 2.50% or less] Si is one of the important basic components of steel, and in the present disclosure, Si suppresses the formation of carbides during annealing and promotes the formation of retained austenite, and is therefore an element that affects the volume fraction of retained austenite. If the Si content is less than 0.01%, the area fraction of ferrite and / or the volume fraction of retained austenite decreases, making it difficult to achieve a desired YR. On the other hand, if the Si content exceeds 2.50%, the volume fraction of retained austenite increases, making it difficult to achieve a desired YR. 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.

[0021] [Mn: 0.10% or more and 5.00% or less] Mn is one of the important basic components of steel, and in the present disclosure, Mn is an important element that affects the area ratio of martensite, ferrite, and the volume ratio of retained austenite. If the Mn content is less than 0.10%, the area ratio of martensite decreases, and the area ratio of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. It also makes it difficult to achieve a desired YR. On the other hand, if the Mn content exceeds 5.00%, the area ratio of martensite increases, and the desired area ratio of ferrite and / or volume ratio of retained austenite cannot be obtained, making it difficult to achieve a desired YR. In addition, λ and bendability decrease. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably set to 1.00% or more. The Mn content is preferably set to 4.00% or less. The Mn content is more preferably set to 2.00% or more. The Mn content is more preferably set to 3.50% or less.

[0022] [P:0.100% or less] If P is excessive, it segregates at the prior austenite grain boundaries, embrittling the grain boundaries and reducing the ultimate deformability of the steel sheet, resulting in reduced λ and bendability. Therefore, the P content must be 0.100% or less. Although there is no particular lower limit for the P content, it is preferable that it be 0.001% or more since P is a solid solution strengthening element and can increase the strength of the steel sheet. Therefore, the P content is 0.100% or less. The P content is preferably 0.001% or more. The P content is preferably 0.070% or less.

[0023] [S:0.0200% or less] S exists as sulfides and increases the number density of precipitates with a size of 2 μm or more, which reduces λ and bendability. Therefore, the S content must be 0.0200% or less. Although there is no particular lower limit for the S content, due to constraints on production technology, the S content is preferably 0.0001% or more. Therefore, the S content is 0.0200% or less. The S content is preferably 0.0001% or more. The S content is preferably 0.0050% or less.

[0024] [Al:0.100% or less] If Al is excessive, the A3 transformation point rises and a large amount of ferrite is included in the steel structure, making it difficult to achieve the desired YR. Therefore, the Al content must 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 0.100% or less. The Al content is preferably 0.001% or more. The Al content is preferably 0.050% or less.

[0025] [N:0.0100% or less] N exists as nitrides and increases the number density of precipitates with a size of 2 μm or more, which reduces λ and bendability. Therefore, the N content must be 0.0100% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0005% or more. Therefore, the N content is 0.0100% or less. The N content is preferably 0.0005% or more. The N content is preferably 0.0050% or less.

[0026] [O:0.0100% or less] O exists as an oxide and increases the number density of precipitates with a size of 2 μm or more, which reduces λ and bendability. Therefore, the O content must be 0.0100% or less. Although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is 0.0100% or less. The O content is preferably 0.0001% or more. The O content is preferably 0.0050% or less.

[0027] [Ti: 0.002% or more and 0.200% or less] Ti increases the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. In addition, the addition of Ti can reduce the number density of precipitates with a size of 2 μm or more. Furthermore, Ti can increase the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more by forming precipitates with N and S. In order to obtain such effects, 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.

[0028] [Effective Ti mole fraction (x) calculated from equation (1) Ti,eff ) is 0.001 or more] Effective Ti mole fraction (x Ti,eff )=x Ti -x N -x S (1) In addition, x in the formula Ti , x N , x S indicates the content (molar fraction) of each element in the steel sheet. By setting the effective Ti molar fraction calculated from the above formula (1) to a certain level or more, the number density of precipitates with a size of 2 μm or more can be reduced, and good λ and bendability can be achieved. Furthermore, Ti can increase the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more by forming precipitates with N and S. To achieve this effect, the effective Ti molar fraction is set to 0.001% or more. Although there is no particular upper limit for the effective Ti molar fraction, it is preferable that the effective Ti molar fraction is set to 0.040 or less, since the amount of carbides, nitrides, or carbonitrides increases and it becomes difficult to achieve the desired YR. Therefore, the effective Ti molar fraction is set to 0.001% or more. The effective Ti molar fraction is preferably set to 0.002% or more. The effective Ti molar fraction is preferably set to 0.040% or less.

[0029] [B: 0.0002% or more and 0.0100% or less] B increases the boron atom concentration in the prior austenite grain boundary with an orientation difference of 15 degrees or more, and can realize good inter-steel sheet cracking resistance properties of the spot weld HAZ. To obtain such an effect, the B content needs to be 0.0002% or more. On the other hand, if the B content exceeds 0.0100%, the number density of precipitates with a size of 2 μm or more increases, so that λ and bendability decrease. Therefore, the B content is set to 0.0002% or more and 0.0100% or less. The B content is preferably set to 0.0004% or more. The B content is preferably set to 0.0080% or less. The B content is more preferably set to 0.0005% or more. The B content is more preferably set to 0.0050% or less.

[0030] A high-strength steel sheet according to an embodiment of the present invention has a composition containing the above elements with the balance including Fe and unavoidable impurities. Preferably, a high-strength steel sheet according to an embodiment of the present invention has a composition containing the above elements with the balance including Fe and unavoidable impurities. Here, examples of the unavoidable impurities include Zn, Pb, and As. These impurities are allowed to be contained as long as their total amount is 0.100% or less.

[0031] In addition to the above essential components, the composition of the high-strength steel plate of the present disclosure may further contain, by mass%, at least one element selected from Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination.

[0032] [Nb:0.200% or less] Nb generates a large amount of coarse precipitates and inclusions, which reduces the ultimate deformability of the steel sheet, so that when the Nb content exceeds 0.200%, λ and bendability are reduced. Therefore, the Nb content is set to 0.200% or less. Although the lower limit of the Nb content is not particularly specified, by setting the Nb content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet and controlling the YR to a desired range. For this reason, the Nb content is preferably set to 0.001% or more. Therefore, when 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.

[0033] [V:0.200% or less] V generates a large amount of coarse precipitates and inclusions, which reduces the ultimate deformability of the steel sheet, so that if the V content exceeds 0.200%, λ and bendability are reduced. Therefore, the V content is set to 0.200% or less. Although the lower limit of the V content is not particularly specified, by setting the V content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet and controlling the YR to a desired range. For this reason, the V content is preferably set to 0.001% or more. Therefore, when added, the V content is set to 0.200% or less. The V content is preferably set to 0.001% or more. The V content is preferably set to 0.100% or less.

[0034] [Ta: 0.10% or less, W: 0.10% or less] If the Ta and W contents exceed 0.10%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel sheet, and therefore the lambda and bendability are reduced. Therefore, the Ta and W contents are each set to 0.10% or less. Although the lower limits of the Ta and W contents are not particularly specified, the Ta and W contents are preferably set to 0.01% or more, respectively, 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, respectively. The Ta and W contents are preferably set to 0.08% or less, respectively.

[0035] [Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less] If the contents of Cr, Mo, and Ni exceed 1.00%, coarse precipitates and inclusions increase, lowering the ultimate deformability of the steel sheet, and therefore λ and bendability decrease. Therefore, the contents of Cr, Mo, and Ni are each set to 1.00% or less. Although there is no particular lower limit for the contents of Cr, Mo, and Ni, these elements improve hardenability, so the contents of Cr, Mo, and Ni are preferably set to 0.01% or more. Therefore, when added, the contents of Cr, Mo, and Ni are each set to 1.00% or less. The contents of Cr, Mo, and Ni are preferably set to 0.01% or more. The contents of Cr, Mo, and Ni are preferably set to 0.80% or less.

[0036] [Co:0.010% or less] If the Co content exceeds 0.010%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the lambda and bendability decrease. Therefore, the Co content is set to 0.010% or less. Although there is no particular lower limit for the Co content, since Co is an element that improves hardenability, it is preferable that the Co content be 0.001% or more. Therefore, when Co is added, the Co content is set to 0.010% or less. The Co content is preferably 0.001% or more. The Co content is preferably 0.008% or less.

[0037] [Cu:1.00% or less] If the Cu content exceeds 1.00%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the lambda and bendability decrease. 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 Cu is 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.

[0038] [Sn:0.200% or less] If the Sn content exceeds 0.200%, cracks are generated inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet is reduced, resulting in reduced λ and 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 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.

[0039] [Sb:0.200% or less] If the Sb content exceeds 0.200%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the lambda and bendability decrease. 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.

[0040] [Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less] If the contents of Ca, Mg and REM exceed 0.0100%, coarse precipitates and inclusions increase, lowering the ultimate deformability of the steel sheet, and therefore λ and bendability decrease. Therefore, the contents of Ca, Mg and REM are each set to 0.0100% or less. Although there is no particular lower limit for the contents of Ca, Mg and REM, these elements are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, so it is preferable that the contents of Ca, Mg and REM are each set to 0.0005% or more. Therefore, when added, the contents of Ca, Mg and REM are each set to 0.0100% or less. The contents of Ca, Mg and REM are preferably set to 0.0005% or more. The contents of Ca, Mg and REM are preferably set to 0.0050% or less. Note that REM (rare earth elements) is a general term for 15 elements ranging from Sc, Y, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here refers to the total content of these elements.

[0041] [Zr: 0.100% or less, Te: 0.100% or less] If Zr and Te exceed 0.100%, coarse precipitates and inclusions increase, lowering the ultimate deformability of the steel sheet, and therefore λ and bendability decrease. Therefore, the contents of Zr and Te must be 0.100% or less. Although there is no particular lower limit for the contents of Zr and Te, the contents of Zr and Te are preferably 0.001% or more, since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, when added, the contents of Zr and Te are 0.100% or less. The contents of Zr and Te are preferably 0.001% or more, respectively. The contents of Zr and Te are preferably 0.080% or less, respectively.

[0042] [Hf:0.10% or less] If the Hf content exceeds 0.10%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the lambda and bendability decrease. 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.

[0043] [Bi:0.200% or less] If the Bi content exceeds 0.200%, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and therefore the λ and bendability decrease. 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 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.

[0044] In addition, when the contents of the above-mentioned Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi are less than the preferable lower limit, the effect of the present invention is not impaired. Therefore, they are included as unavoidable impurities.

[0045] Next, the steel structure of the steel plate will be described.

[0046] [Area ratio of martensite: 60% to 99%] If the area ratio of martensite is less than 60%, the area ratio of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. It also makes it difficult to achieve a desired YR. On the other hand, if the area ratio of martensite is more than 99%, ferrite and / or retained austenite, which are effective for controlling the YR, do not exist in the steel structure, making it difficult to achieve a desired YR. Therefore, the area ratio of martensite is set to 60% or more and 99% or less. The area ratio of martensite is preferably 65% ​​or more. The area ratio of martensite is preferably 98% or less. The area ratio of martensite is more preferably 67% or more. The area ratio of martensite is preferably 97% or less. In addition to quenched martensite (fresh martensite), the martensite includes tempered martensite and bainite. The observation position for the area ratio of martensite is set to 1 / 4 of the sheet thickness of the steel sheet, as described later.

[0047] [Total area ratio of ferrite and / or volume ratio of retained austenite: more than 0% and less than 40%] If the total of the area ratio of ferrite and / or the volume ratio of retained austenite is 0%, the steel structure will be a martensite single phase structure, making it difficult to achieve the desired YR. On the other hand, if the total of the area ratio of ferrite and / or the volume ratio of retained austenite exceeds 40%, the area ratio of martensite will decrease, making it difficult to achieve a TS of 1180 MPa or more. Also, it will be difficult to achieve the desired YR. Therefore, the total of the area ratio of ferrite and / or the volume ratio of retained austenite is more than 0% and 40% or less. The total of the area ratio of ferrite and / or the volume ratio of retained austenite is preferably 1% or more. The total of the area ratio of ferrite and / or the volume ratio of retained austenite is preferably 38% or less. The total of the area ratio of ferrite and / or the volume ratio of retained austenite is more preferably 2% or more. The total area ratio of the area ratio of ferrite and / or the volume ratio of retained austenite is preferably 35% or less. The ferrite referred to here includes bainitic ferrite. The observation position for the area ratio of ferrite and the volume ratio of retained austenite is set at a 1 / 4 position in the sheet thickness of the steel sheet, as described later.

[0048] Here, the method for measuring the area ratios of martensite (quenched martensite, tempered martensite, and bainite) and ferrite (including bainitic ferrite) is as follows.

[0049] After cutting out the sample so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface, the observation surface is mirror-polished using diamond paste, and then etched with 3 vol.% nital to reveal the structure. Using a scanning electron microscope (SEM) under the condition of an acceleration voltage of 15 kV, the observation position is set to 1 / 4 of the plate thickness of the steel plate, and three fields of view are observed at a magnification of 5000 times and a field of view of 17 μm × 23 μm. The area ratio of each constituent structure is calculated by dividing the area of ​​the obtained structure image by the measured area using Adobe Photoshop of Adobe Systems Inc. for three fields of view. The area ratio of each structure is calculated by averaging these values. In addition, in the above structure image, ferrite (including bainitic ferrite) is a flat structure that does not contain carbides in the structure of the recesses, and tempered martensite and bainite are structures that contain fine carbides in the structure of the recesses. In addition, the quenched martensite has a convex portion and a fine unevenness inside the structure, and can be distinguished from the tempered martensite, the tempered martensite, and the bainite. Note that the tempered martensite, the tempered martensite, and the bainite do not need to be distinguishable from each other because the total area ratio is calculated as the area ratio of martensite.

[0050] The volume fraction of retained austenite is measured by the following method.

[0051] After grinding, the steel plate is polished by chemical polishing for an additional 0.1 mm so that the observation surface is located at 1 / 4 of the plate thickness from the surface layer (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface). For this surface, an X-ray diffractometer is used to measure the integrated reflection intensities of the (200), (220), and (311) faces of fcc iron (austenite) and the (200), (211), and (220) faces of bcc iron using a Co Kα radiation source. Next, the volume fraction of austenite is calculated from the intensity ratio of the integrated reflection intensity from each face of fcc iron (austenite) to the integrated reflection intensity from each face of bcc iron, and this is taken as the volume fraction of retained austenite.

[0052] [Boron atom concentration in prior austenite grain boundaries with misorientation of 15 degrees or more: 0.3 at% to 6.0 at%] This is an extremely important invention constituent element in the present disclosure. In order to control the inter-steel plate crack resistance of the spot weld HAZ, it is important to control element segregation in the prior austenite grain boundaries with an orientation difference of 15 degrees or more. In particular, by increasing the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more, the inter-steel plate crack resistance of the spot weld HAZ can be improved. In order to obtain such an effect, the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more must be 0.3 at% or more. On the other hand, if the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more exceeds 6.0 at%, the number density of precipitates with a size of 2 μm or more increases, and λ and bendability decrease. Therefore, the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more is set to 0.3 at% or more and 6.0 at% or less. The boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more is preferably set to 0.4 at% or more. The boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more is preferably 5.0 at% or less. The boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more is more preferably 0.5 at% or more. The boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more is preferably 4.0 at% or less. The boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more is further preferably 0.6 at% or more. The boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more is further preferably 3.0 at% or less.

[0053] The method for measuring the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more is as follows.

[0054] After cutting out the specimen so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate is the observation surface, the observation surface is mirror-polished using diamond paste, and then etched with 3 vol.% nital to reveal the structure. Using an SEM with an acceleration voltage of 15 kV, the observation position is set to 1 / 4 of the plate thickness of the steel plate, and the prior γ grain boundaries are observed in multiple fields of view at a magnification of 3000 to 5000 times. The crystal orientation of the fields of view is measured using FE-SEM / EBSD (JSM7100F: manufactured by JEOL Ltd., OIM: manufactured by TSL Co., Ltd.). The EBSD measurement is performed under sufficient measurement conditions (measurement field of view: 28 μm (rolling direction) × 30 μm (plate thickness direction), measurement point interval: 40 nm) to evaluate the martensite substructure by crystal orientation. The parent phase is reconstructed from the EBSD observation results using the Nishiyama-Wasserman relationship in the Parent Grain Reconstruction function of the OIM Analysis v8.6 software. In other words, the orientation of martensite is converted to the orientation of austenite, and prior austenite grain boundaries with an orientation difference of 15 degrees or more are identified by drawing boundaries with an orientation difference of 15 degrees or more on the obtained IPF (Inverse Pole Figure) image of the austenite. Next, needle-shaped samples are prepared using a forced ion beam (FIB) for prior austenite grain boundaries that have been identified as having an orientation difference of 15 degrees or more. After that, the boron atom concentration is analyzed using a 3D atom probe (EIKOS-UV: CAMECA). The 3D atom probe is used in laser mode, and the measurement conditions are 7 nJ for the purse energy. From the obtained results, the maximum value of the boron atom concentration at the prior austenite grain boundaries is calculated, and this is taken as the boron atom concentration at the prior austenite grain boundaries with an orientation difference of 15 degrees or more.

[0055] [Number density of precipitates with size of 2 μm or more: 150 / mm 2 below] This is an extremely important invention constituent element in the present disclosure. In order to control λ and bendability, it is important to control the number density of precipitates, particularly the number density of precipitates with a size of 2 μm or more, and λ and bendability can be improved by reducing the number density of precipitates with a size of 2 μm or more. In order to obtain this effect, it is necessary to keep the number density of precipitates with a size of 2 μm or more below 150 precipitates / mm 2 Although there is no particular lower limit for the number density of precipitates having a size of 2 μm or more, the lower the number density of precipitates having a size of 2 μm or more, the more preferable it is, and the lower the number density of precipitates having a size of 2 μm or more, the more preferable it is. 2 Therefore, the effect of the present disclosure can be obtained even if the number density of precipitates having a size of 2 μm or more is 150 pieces / mm 2 The number density of precipitates with a size of 2 μm or more is preferably 0 pieces / mm 2 The number density of precipitates having a size of 2 μm or more is preferably 100 pieces / mm 2 Here, the precipitates having a size of 2 μm or more refer to coarse inclusions that are generated as a result of the composite precipitation of sulfides, nitrides, oxides, borides, and the like.

[0056] The method for measuring the number density of precipitates having a size of 2 μm or more is as follows.

[0057] A sample is cut out so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate is the observation surface, and then the observation surface is mirror-polished using diamond paste. The entire thickness of the steel plate is observed in two rows in the rolling direction at a magnification of 3000 times using a backscattered electron image of an SEM under conditions of an accelerating voltage of 15 kV. The number of precipitates with a size of 2 μm or more in that field of view is counted and divided by the measured area to calculate the number density of precipitates with a size of 2 μm or more. Note that "size" here means the maximum length of the precipitates.

[0058] Furthermore, the steel structure according to the present disclosure may contain the following structures in addition to the above-mentioned martensite (quenched martensite, tempered martensite, bainite), ferrite (including bainitic ferrite), and retained austenite. In other words, the effects of the present disclosure are not impaired even if pearlite, cementite, metastable carbides, and other carbides known as structures of steel sheets are contained within a range of 5% or less in terms of area ratio. Note that metastable carbides include epsilon (ε) carbides, eta (η) carbides, chi (χ) carbides, and the like.

[0059] [High strength steel plate] The chemical composition and steel structure of the high strength steel plate are as described above. The plate thickness of the high strength steel plate is not particularly limited, but is usually 0.3 mm or more and 2.8 mm or less.

[0060] [High-strength plated steel sheet] The high-strength plated steel sheet of the present disclosure is a high-strength plated steel sheet having a plating layer on at least one side of the high-strength steel sheet of the present disclosure. The type of plating layer is not particularly limited, and may be, for example, either a hot-dip plating layer or an electroplating layer. The plating layer may also be an alloyed plating layer. The plating layer is preferably a zinc plating layer. The zinc plating layer may contain Al and Mg. Also, hot-dip zinc-aluminum-magnesium alloy plating (Zn-Al-Mg plating layer) is also preferred. In this case, it is preferable that the Al content is 1 mass% or more and 22 mass% or less, the Mg content is 0.1 mass% or more and 10 mass% or less, and the balance is Zn. In addition, in the case of a Zn-Al-Mg plating layer, in addition to Zn, Al, and Mg, one or more selected from Si, Ni, Ce, and La may be contained in a total of 1 mass% or less. In addition, since the plating metal is not particularly limited, in addition to the above-mentioned Zn plating, Al plating, etc. may be used.

[0061] The composition of the plating layer is not particularly limited, and may be any common one. For example, in the case of a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, it generally contains Fe: 20 mass% or less, and Al: 0.001 mass% or more and 1.0 mass% or less. Furthermore, it contains one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0 mass% or more and 3.5 mass% or less, with the balance being Zn and unavoidable impurities. In the present disclosure, the plating coating weight per side is 20 to 80 g / m 2 It is preferable that the coating layer has a hot-dip galvanized layer, which is further alloyed with the hot-dip galvanized layer. When the coating layer is a hot-dip galvanized layer, the Fe content in the coating layer may be less than 7 mass%, and when the coating layer is an alloyed hot-dip galvanized layer, the Fe content in the coating layer may be 7 to 20 mass%.

[0062] Next, a method for producing the high strength steel plate according to the present disclosure will be described.

[0063] First, a steel material having the above-mentioned composition is melted to produce a steel slab. In the present invention, the method of melting the steel material is not particularly limited, and any known melting method such as a converter or an electric furnace is suitable. In addition, the steel slab (slab) is preferably produced by a continuous casting method to prevent macrosegregation, but it can also be produced by an ingot casting method or a thin slab casting method.

[0064] Next, after producing the steel slab, in the cooling step, the steel slab is cooled at an average cooling rate of 50°C / hr to 500°C / hr in the temperature range of 700°C to 1000°C.

[0065] [Average cooling rate in the temperature range from 700℃ to 1000℃: 50℃ / hr to 500℃ / hr] The average cooling rate in the temperature range of 700°C to 1000°C is an extremely important invention constituent element. By increasing the average cooling rate in the temperature range of 700°C to 1000°C, the number density of precipitates with a size of 2 μm or more can be reduced. In order to obtain such an effect, the average cooling rate in the temperature range of 700°C to 1000°C is set to 50°C / hr or more. On the other hand, if the average cooling rate in the temperature range of 700°C to 1000°C exceeds 500°C / hr, the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more decreases, so that good inter-steel plate cracking resistance properties of the spot weld HAZ cannot be realized. Therefore, the average cooling rate in the temperature range of 700°C to 1000°C is set to 50°C / hr or more and 500°C / hr or less. The average cooling rate in the temperature range of 700°C to 1000°C is preferably set to 100°C / hr or more. The average cooling rate in the temperature range of 700°C or more and 1000°C or less is preferably 450°C / hr or less. The average cooling rate in the temperature range of 700°C or more and 1000°C or less is more preferably 150°C / hr or more. The average cooling rate in the temperature range of 700°C or more and 1000°C or less is more preferably 400°C / hr or less. Note that the temperature of the steel slab here refers to the surface temperature of the steel slab.

[0066] After the above cooling, the steel slab may be cooled to room temperature and then reheated as in the conventional method, but an energy-saving process such as direct rolling may also be applied. Direct rolling is a process in which the steel slab is charged into a heating furnace as a hot slab without being cooled to room temperature.

[0067] 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 minutes or more.

[0068] [Average heating rate of slab in the temperature range of 900℃ to 1150℃: 25℃ / min or less] This is an extremely important invention constituent element in the present disclosure. By slowing down the average heating rate of the slab to 25°C / min or less in the temperature range of 900°C to 1150°C, it is possible to increase the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more and to reduce the number density of precipitates with a size of 2 μm or more. In order to obtain such effects, the average heating rate of the slab is set to 25°C / min or less in the temperature range of 900°C to 1150°C. The lower limit of the average heating rate of the slab in the temperature range of 900°C to 1150°C is not particularly specified, but it is preferable to set it to 5°C / min or more in order to suitably prevent an increase in the softened thickness of the surface layer after annealing and to keep the TS in a more suitable range. Therefore, 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. The average heating rate of the slab in the temperature range of 900°C to 1150°C is preferably set to 5°C / min or more. The average heating rate of the slab is preferably 15° C. / min or less in the temperature range of 900° C. to 1150° C. The slab heating temperature refers to the temperature of the surface of the steel slab during heating.

[0069] [Slab heating temperature: 1150℃ or higher] This is an extremely important invention constituent element in the present disclosure. By increasing the slab heating temperature to 1150°C or more, it is possible to increase the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15° or more, and to reduce the number density of precipitates with a size of 2 μm or more. In order to obtain such effects, the slab heating temperature is set to 1150°C or more. Note that the upper limit of the slab reheating temperature is not particularly specified, but it is preferable to set it to 1300°C or less in order to suitably prevent an increase in the softened thickness of the surface layer after annealing and to bring TS within a more suitable range. Therefore, the slab heating temperature is set to 1150°C or more. The slab heating temperature is preferably set to 1180°C or more. The slab heating temperature is preferably set to 1300°C or less. Note that the slab heating temperature is the temperature of the surface of the steel slab during slab heating.

[0070] [Dwell time from 1100℃ to the slab heating temperature: 20 minutes or more] This is an extremely important invention constituent element in the present disclosure. By lengthening the residence time from 1100°C to the slab heating temperature to 20 minutes or more, it is possible to increase the boron atom concentration in the prior austenite grain boundary with an orientation difference of 15 degrees or more and to reduce the number density of precipitates with a size of 2 μm or more. In order to obtain such effects, the residence time from 1100°C to the slab heating temperature is set to 20 minutes or more. Although there is no particular upper limit to the residence time from 1100°C to the slab heating temperature, it is preferable to set it to 100 minutes or less in order to suitably prevent an increase in the softened thickness of the surface layer after annealing and to set the TS within a more suitable range. Therefore, the residence time from 1100°C to the slab heating temperature is set to 20 minutes or more. The residence time from 1100°C to the slab heating temperature is preferably set to 30 minutes or more. The residence time from 1100°C to the slab heating temperature is preferably set to 100 minutes or less. The slab heating temperature is the surface temperature of the steel slab during slab heating.

[0071] The slab is rough-rolled under normal conditions to produce a sheet bar. When the slab heating temperature is low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.

[0072] [Rough rolling exit temperature: 1100℃ or more] This is an extremely important invention constituent element in the present disclosure. By increasing the rough rolling exit temperature to 1100°C or more, it is possible to increase the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15° or more, and to reduce the number density of precipitates with a size of 2 μm or more. In order to obtain such effects, the rough rolling exit temperature is set to 1100°C or more. The upper limit of the rough rolling exit temperature is not particularly specified, but it is preferable to set it to 1200°C or less in order to suitably prevent an increase in the surface softening thickness after annealing and to set TS within a more suitable range. Therefore, the rough rolling exit temperature is set to 1100°C or more. The rough rolling exit temperature is preferably set to 1110°C or more. The rough rolling exit temperature is preferably set to 1200°C or less.

[0073] [Finish rolling entry temperature: 1050℃ or higher] This is an extremely important invention constituent element in the present disclosure. By increasing the finish rolling entry temperature to 1050°C or more, it is possible to increase the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15° or more, and to reduce the number density of precipitates with a size of 2 μm or more. In order to obtain such effects, the finish rolling entry temperature is set to 1050°C or more. The upper limit of the finish rolling entry temperature is not particularly specified, but it is preferable to set it to 1150°C or less in order to suitably prevent an increase in the surface softening thickness after annealing and to set TS within a more suitable range. Therefore, the finish rolling entry temperature is set to 1050°C or more. The finish rolling entry temperature is preferably set to 1100°C or more. The finish rolling entry temperature is preferably set to 1150°C or less.

[0074] Finish rolling is preferably performed at a finish rolling delivery temperature of the Ar3 transformation point or higher, since the rolling load increases, the reduction rate increases in the unrecrystallized state of austenite, and abnormal structures elongated in the rolling direction develop, which may result in a decrease in the workability of the annealed sheet. 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. The Ar3 transformation point temperature is calculated by the following formula. Ar3 transformation point (℃) = 868-396 x [%C] + 24.6 x [%Si] - 68.1 x [%Mn] - 36.1 x [%Ni] - 20.7 x [%Cu] - 24.8 x [%Cr] In the above formula, the [% element symbol] indicates the content (mass%) of the corresponding element in the above composition, and is set to 0 when the corresponding element is not contained.

[0075] In addition, the rough rolled sheets may be joined together during hot rolling and continuously finished rolling may be performed. The rough rolled sheets may be wound once. In order to reduce the rolling load during hot rolling, a part or all of the finish rolling may be lubricated rolling. Lubricated rolling is also effective from the viewpoint of uniforming the shape of the steel sheet and the material. The friction coefficient during lubricated rolling is preferably 0.10 or more and 0.25 or less.

[0076] The hot-rolled steel sheet thus produced is subjected to pickling. Pickling can remove oxides from the steel sheet surface, and is therefore important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet product. Pickling may be performed once or multiple times.

[0077] Next, the hot-rolled sheet after pickling or the hot-rolled sheet (hot-rolled annealed sheet) optionally subjected to heat treatment after pickling is cold-rolled to obtain a cold-rolled sheet. Since strain is uniformly and efficiently introduced and a uniform structure is obtained, it is preferable to perform cold rolling by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0078] In this case, it is extremely important in the present embodiment that the cumulative rolling reduction in cold rolling is 20% to 95%, and the sheet threading speed in the final pass of cold rolling is 50 mpm or more.

[0079] [Cumulative reduction rate of cold rolling: 20% to 95%] By increasing the cumulative reduction ratio of cold rolling, the area ratio of ferrite can be reduced, that is, the total of the area ratio of ferrite and / or the volume ratio of retained austenite can be set to 40% or less. In order to obtain such an effect, the cumulative reduction ratio of cold rolling is set to 20% or more. On the other hand, if the cumulative reduction ratio of cold rolling exceeds 95%, the grain size of austenite generated during annealing becomes fine, and the amount of retained austenite in the annealed sheet increases. In other words, the total of the area ratio of ferrite and / or the volume ratio of retained austenite increases, and therefore the desired YR cannot be realized. Therefore, the cumulative reduction ratio of cold rolling is set to 20% or more and 95% or less. The cumulative reduction ratio of cold rolling is preferably set to 25% or more. The cumulative reduction ratio of cold rolling is preferably set to 90%. The cumulative reduction ratio of cold rolling is more preferably set to 27% or more. The cumulative reduction ratio of cold rolling is more preferably set to 70% or less.

[0080] [Threading speed of the final pass of cold rolling: 50mpm or more] In the present disclosure, this is an extremely important invention constituent element. By increasing the sheet passing speed in the final pass of cold rolling, a large amount of strain can be introduced into the steel sheet, and the number of prior austenite grain boundaries with an orientation difference of 15 degrees or more can be increased. As a result, the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more can be increased, and as a result, good inter-steel crack resistance properties of the spot weld HAZ can be realized. In order to obtain such effects, the sheet passing speed in the final pass of cold rolling is 50 mpm or more. Note that the upper limit of the sheet passing speed in the final pass of cold rolling is not particularly specified, but due to constraints on production technology, it is preferable that it is 300 mpm or less. Therefore, the sheet passing speed in the final pass of cold rolling is 50 mpm or more. The sheet passing speed in the final pass of cold rolling is preferably 70 mpm or more. The sheet passing speed in the final pass of cold rolling is preferably 300 mpm or less.

[0081] The cold-rolled sheet obtained as described above is subjected to an annealing process under the following annealing conditions.

[0082] [Heating temperature: 780℃ or higher] If the heating temperature (annealing temperature) is less than 780°C, the annealing process is performed in the two-phase region of ferrite and austenite, and since a large amount of ferrite is contained after annealing, a TS of 1180 MPa or more cannot be achieved, and it becomes difficult to achieve the desired YR. In addition, the boron atom concentration in the prior austenite grain boundary with an orientation difference of 15 degrees or more decreases, so good inter-steel sheet cracking resistance properties of the spot weld HAZ cannot be achieved. Furthermore, the number density of precipitates with a size of 2 μm or more increases, so λ and bendability cannot be improved. Therefore, the heating temperature is set to 780°C or more. Although there is no particular upper limit for the heating temperature, if the heating temperature increases, the surface softening thickness after annealing increases, TS decreases, and prior austenite grain size becomes coarse, so that the heating temperature is preferably set to 1050°C or less. Therefore, the heating temperature is set to 780°C or more. Preferably, it is set to 800°C or more. Preferably, it is set to 1050°C or less. More preferably, it is set to 820°C or more. More preferably, it is set to 1000°C or less. The heating temperature is measured based on the temperature of the steel sheet surface.

[0083] In addition, since it is possible to increase the boron atom concentration in prior austenite grain boundaries with an orientation difference of 15 degrees or more and to reduce the number density of precipitates with a size of 2 μm or more, the residence time at 780°C or more is preferably 10 s or more. The residence time between 780°C or more and the heating temperature is more preferably 20 s or more. The residence time at 780°C or more is more preferably 600 s or less. The residence time at 780°C or more is even more preferably 25 s or more. The residence time at 780°C or more is even more preferably 300 s or less.

[0084] [Cooling process] After the annealing process, the cold-rolled sheet is cooled (referred to as a cooling process of the cold-rolled sheet). The cooling conditions are as follows.

[0085] [Average cooling rate in the temperature range from heating temperature to 650℃: 0.5℃ / s to 50℃ / s (first cooling)] This is an extremely important invention constituent element in the present disclosure. By increasing the average cooling rate in the temperature range from the heating temperature to 650°C, the area ratio of ferrite can be reduced and the desired TS and YR can be achieved. In addition, the number density of precipitates with a size of 2 μm or more can be reduced. In order to obtain such effects, the average cooling rate in the temperature range from the heating temperature to 650°C is set to 0.5°C / s or more. On the other hand, if the average cooling rate in the temperature range from the heating temperature to 650°C exceeds 50°C / s, the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more cannot be increased. Therefore, the average cooling rate in the temperature range from the heating temperature to 650°C is set to 0.5°C / s or more and 50°C / s or less. The average cooling rate in the temperature range from the heating temperature to 650°C is preferably 1°C / s or more. The average cooling rate in the temperature range from the heating temperature to 650°C is preferably 40°C / s or less. The average cooling rate in the temperature range from the heating temperature to 650° C. is more preferably 3° C. / s or more. The average cooling rate in the temperature range from the heating temperature to 650° C. is more preferably 35° C. / s or less.

[0086] [Cooling of cold-rolled sheet (secondary cooling)] After the first cooling, the cold-rolled sheet is preferably further cooled (second cooling). In this step, after the first cooling from the heating temperature to 650°C, the cold-rolled sheet is further cooled. The cooling stop temperature is not particularly limited and may be room temperature. The average cooling rate at 650°C or less is preferably 5°C / s or more and 30°C / s or less, unless otherwise specified below. In addition, in the temperature range of 650°C or less, the high-strength steel sheet may be cooled once and the steel sheet temperature may be increased again.

[0087] [Average cooling rate in the temperature range of 250°C to 400°C: 1.0°C / s or more (optimal conditions)] In the second cooling step, 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 YR, λ, and bendability can be further improved. The average cooling rate in the temperature range of 250°C to 400°C is preferably 1.0°C / s or more, more preferably 2.0°C / s or more, and even more preferably 3.0°C / s or more. The upper limit of the average cooling rate in the temperature range of 250°C to 400°C is not particularly specified, but due to constraints on production technology, it is preferably 100.0°C / s or less, and more preferably 80.0°C / s or less. When the cooling stop temperature exceeds 250°C, the average cooling rate is the value in the temperature range of the cooling stop temperature to 400°C. The average cooling rate is measured based on the temperature of the steel sheet surface.

[0088] As a cooling method in the temperature range of 250° C. or more and 400° C. or less, gas jet cooling, mist cooling, water cooling, air cooling, etc. can be applied.

[0089] [Heat retention temperature during cooling process: 100℃ to 450℃ (optimal conditions)] In the second cooling step, it is preferable to keep the heat at a temperature in the range of 100°C or more and 450°C or less. By keeping the temperature within the above range, the high-strength steel plate is kept at a temperature so that YR, λ, and bendability are kept within a more suitable range. In addition, the area ratio of bainitic ferrite can be further reduced, and TS can be further improved. The heat keeping temperature in the second cooling step is more preferably 150°C or more, and more preferably 200°C or more. In addition, the heat keeping temperature in the second cooling step is more preferably 400°C or less, and more preferably 350°C or less. The heat keeping temperature is based on the surface temperature of the steel plate.

[0090] [Heat retention time during cooling process: 5 seconds or more (optimal conditions)] By keeping the heat at the heat retention temperature, the YR, λ and bendability can be kept within a more suitable range. To obtain such an effect, the heat retention time at the heat retention temperature is preferably 5 s or more, more preferably 10 s or more, and even more preferably 15 s or more. There is no particular upper limit to the heat retention time, but to keep TS within a more suitable range, it is preferably 500 s or less, and more preferably 250 s or less.

[0091] [Cooling stop temperature: 250℃ or less (optimal conditions)] In the above-mentioned second cooling step, the cooling stop temperature is preferably 250°C or less, more preferably 200°C or less. If the cooling stop temperature is 250°C or less, it is possible to prevent a large amount of retained austenite from being generated after annealing, and to further improve the YR, λ, and bendability. Although the lower limit of the cooling stop temperature is not particularly specified, it is preferably room temperature or higher from the viewpoint of productivity. The cooling stop temperature is measured based on the temperature of the steel sheet surface.

[0092] Although the average cooling rate to the cooling stop temperature below 250° C. is not particularly specified, in order to further improve TS, the average cooling rate to the cooling stop temperature below 250° C. 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 the cooling stop temperature below 250° C. is preferably 1000° C. / s or less, and more preferably 150° C. / s or less.

[0093] The cold-rolled sheet may be cooled from the cooling stop temperature to room temperature. The average cooling rate from the cooling stop temperature to room temperature is not particularly limited, and the sheet may be cooled to room temperature by any method. Examples of the cooling method include gas jet cooling, mist cooling, water cooling, and air cooling.

[0094] The high-strength steel sheet annealed as described above may be cooled to the cooling stop temperature and then rolled. The elongation rate of the rolling is preferably 0.05% or more, more preferably 0.10% or more. By setting the elongation rate of the rolling performed after cooling to the cooling stop temperature to 0.05% or more, the YR can be controlled to a desired range. In addition, the elongation rate of the rolling is preferably 2.00% or less, more preferably 1.00% or less. By setting the elongation rate of the rolling performed after cooling to the cooling stop temperature to 2.00% or less, the volume fraction of the retained austenite can be set to a more suitable range, and the bendability and the damage degree of the sheared end surface in a corrosive environment can be set to a more suitable range.

[0095] The rolling after cooling to the cooling stop temperature may be performed on an apparatus continuous with the above-mentioned continuous annealing apparatus (online), or on an apparatus not continuous with the above-mentioned continuous annealing apparatus (offline). The target elongation may be achieved by one rolling, or a total of 0.05% to 2.00% elongation may be achieved by multiple rolling. The rolling described here generally refers to temper rolling, but it may be a processing method using repeated bending with a tension leveler or rolls, etc., as long as it can impart an elongation equivalent to that of temper rolling.

[0096] [Reheating temperature: (cooling stop temperature + 50℃) or more and 450℃ or less (optimal conditions)] After cooling to the cooling stop temperature, or after further rolling after cooling to the cooling stop temperature, the high-strength steel sheet may be reheated (reheating step). By reheating the high-strength steel sheet, the YR, λ, and bendability can be set within a more suitable range. In order to obtain such an effect, the reheating temperature is preferably (cooling stop temperature + 50°C) or higher, more preferably (cooling stop temperature + 100°C) or higher, and even more preferably (cooling stop temperature + 150°C) or higher. On the other hand, as the reheating temperature increases, the tempering of martensite progresses and the TS decreases, so the reheating temperature is preferably 450°C or lower, more preferably 400°C or lower, and even more preferably 380°C or lower. The reheating temperature is based on the surface temperature of the steel sheet.

[0097] [Heat retention time at reheating temperature: 5 seconds or more (optimal conditions)] By maintaining the heat at the reheating temperature, the YR, λ, and bendability can be kept within a more suitable range. To achieve this effect, the heat-maintenance time at the reheating temperature is preferably 5 s or more, more preferably 10 s or more, and even more preferably 15 s or more. There is no particular upper limit to the heat-maintenance time at the reheating temperature, but to keep the TS within a more suitable range, the heat-maintenance time at the reheating temperature is preferably 500 s or less, and more preferably 250 s or less.

[0098] The reheating temperature may be cooled to room temperature, but the cooling rate from the reheating temperature to room temperature is not particularly limited, and the material may be cooled to room temperature by any method, such as gas jet cooling, mist cooling, water cooling, and air cooling.

[0099] When high strength steel sheets are traded, they are usually cooled to room temperature before being traded.

[0100] [Manufacturing method for high-strength plated steel sheets] A high-strength plated steel sheet can be obtained by subjecting at least one side of the high-strength steel sheet produced as described above to a plating process. For example, examples of the plating process include a hot-dip galvanizing process and a process of alloying after hot-dip galvanizing. Annealing and galvanizing may be performed continuously in one line. Alternatively, a plating layer may be formed by electroplating such as Zn-Ni electric alloy plating, or hot-dip zinc-aluminum-magnesium alloy plating may be applied. Note that, although the above description has focused on the case of galvanizing, the type of plating metal such as Zn plating or Al plating is not particularly limited.

[0101] The high-strength steel sheet manufactured as described above can be subjected to a plating process to obtain a plated steel sheet. For example, examples of the plating process include a hot-dip galvanizing process and a process of alloying after hot-dip galvanizing. Annealing and galvanizing may be performed continuously in one line. Alternatively, a plating layer may be formed by electroplating such as Zn-Ni electric alloy plating, or hot-dip zinc-aluminum-magnesium alloy plating may be applied. Note that, although the above description has focused on the case of galvanizing, the type of plating metal such as Zn plating or Al plating is not particularly limited.

[0102] When hot-dip galvanizing is performed, it is preferable to adjust the coating weight by gas wiping or the like after immersing the high-strength steel sheet in a galvanizing bath at 440°C to 500°C to perform hot-dip galvanizing. It is preferable to use a galvanizing bath containing 0.10% by mass to 0.23% by mass of Al for hot-dip galvanizing. In addition, the temperature range when performing alloying treatment of galvanizing after hot-dip galvanizing is preferably 470°C to 600°C, more preferably 470°C to 560°C. By performing alloying treatment at 470°C or higher, the Zn-Fe alloying rate is more suitable and the productivity is more suitable. In addition, by performing alloying treatment at 600°C or lower, the transformation of untransformed austenite to pearlite is prevented, and TS is more suitable. In addition, electrogalvanizing may be performed. In addition, the coating weight is 20 to 80 g / m per side. 2(Double-sided plating) is preferred, and the galvannealed steel sheet (GA) is preferably subjected to the following alloying treatment to make the Fe concentration in the plating layer 7 to 15 mass %.

[0103] Regarding the plating treatment, after the above-mentioned annealing step, the high-strength steel sheet may be plated at a temperature range of 400°C or more below the heating temperature without being cooled, or the cold-rolled steel sheet may be once cooled to less than 400°C and then the steel sheet temperature may be raised again to 400°C or more before the plating treatment is performed.

[0104] The high-strength steel sheet that has been subjected to the above-mentioned plating treatment may be subjected to rolling. The elongation rate of the rolling is preferably 0.05% or more, more preferably 0.10% or more. By setting the elongation rate of the rolling performed after the plating treatment to 0.05% or more, the YR can be controlled to a desired range. Moreover, the elongation rate of the rolling is preferably 2.00% or less, more preferably 1.00% or less. By setting the elongation rate of the rolling in the plating treatment to 2.00% or less, the volume fraction of the retained austenite can be set to a more suitable range, and the bendability and the damage degree of the sheared end surface in a corrosive environment can be set to a more suitable range.

[0105] The rolling after the plating process may be performed on an apparatus continuous with the above-mentioned continuous annealing apparatus (online) or on an apparatus discontinuous with the above-mentioned continuous annealing apparatus (offline). The target elongation may be achieved by one rolling, or a total of 0.05% to 2.00% elongation may be achieved by multiple rolling. The rolling described here generally refers to temper rolling, but may be a processing method such as repeated bending with a tension leveler or rolls, as long as it can impart an elongation equivalent to that of temper rolling. Reheating may be performed after rolling after the plating process.

[0106] Other conditions of the manufacturing method are not particularly limited, but from the viewpoint of productivity, it is preferable to carry out a series of processes such as the above-mentioned annealing, hot-dip galvanizing, and alloying treatment of galvanizing in a CGL (Continuous Galvanizing Line), which is a hot-dip galvanizing line. After hot-dip galvanizing, wiping is possible to adjust the coating weight of the plating. Note that the plating conditions other than the above-mentioned conditions can be based on the usual hot-dip galvanizing method.

[0107] When high-strength plated steel sheets are traded, they are usually cooled to room temperature before being traded.

[0108] The production conditions other than those mentioned above can be the same as those in the ordinary methods.

[0109] [Part] Next, a member according to one embodiment of the present invention will be described.

[0110] A member according to an embodiment of the present invention is a member made using the high-strength steel sheet or high-strength plated steel sheet according to the embodiment of the present invention described above. The member according to an embodiment of the present invention is, for example, a member obtained by forming the high-strength steel sheet or high-strength plated steel sheet according to the embodiment of the present invention described above into a target shape by cold press working or the like. Therefore, even after being formed into a member, it has the steel structure and various properties of the high-strength steel sheet and the high-strength plated steel sheet. The member according to an embodiment of the present invention is preferably used for automobile frame structural parts or automobile reinforcing parts.

[0111] Here, the high-strength steel plate according to one embodiment of the present invention is a high-strength steel plate of 1180 MPa or more that is excellent in bendability and inter-steel-sheet cracking resistance in spot weld HAZ, and can be used to manufacture parts with high dimensional accuracy. Therefore, the member according to one embodiment of the present invention can contribute to weight reduction of the vehicle body, and can be suitably used in general members for automobile frame structural parts or automobile reinforcing parts. EXAMPLES

[0112] Steel having the composition shown in Table 1, with the balance being Fe and unavoidable impurities, was melted in a converter and formed into a slab by a continuous casting method. After cooling, the obtained slab was heated, hot rolled, pickled, and then cold rolled. Table 2 shows the cooling rate after casting, hot rolling conditions, pickling conditions, and cold rolling conditions. A pre-annealed cold-rolled steel sheet with a thickness of 1.4 mm was produced. In some examples, sheets with different thicknesses were also produced.

[0113] [Table 1]

[0114] [Table 2]

[0115] [Table 3]

[0116] Next, the steel sheets were annealed, cooled, and reheated under the conditions shown in Tables 2 and 3 to obtain high-strength cold-rolled steel sheets (CR). Some of the steel sheets were then plated 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 are plated) and for GA, 45g / m per side 2 The thickness of the plating layer was set to about 1 / 2 (double-sided plating). The Fe concentration in the plating layer of the GA was set to 9% by mass or more and 12% by mass or less. The Ni content in the plating layer of the EG, which has a Zn-Ni plating layer, was set to 9% by mass or more and 25% by mass or less.

[0117] 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, stretch flangeability (hole expandability), bendability, and inter-steel cracking resistance of the spot weld HAZ were evaluated according to the following test methods. The results are shown in Table 4.

[0118] [Table 4]

[0119] [Tensile test] The tensile test was performed in accordance with JIS Z 2241:2011. JIS No. 5 test pieces were taken from the obtained steel plate perpendicular to the rolling direction of the steel plate, and the crosshead speed was 1.67 × 10 -1 A tensile test was performed under the condition of mm / s to measure YS and TS. In the present invention, a yield ratio (YR) of 65% or more and 90% or less was determined to have high dimensional accuracy. YR was calculated using the calculation method described in the above formula (2).

[0120] [Hole expansion test] The hole expansion test was carried out in accordance with JIS Z 2256. The obtained steel plate was sheared to 100 mm x 100 mm, and then a hole with a diameter of 10 mm was punched into the sheared steel plate with a clearance of 12.5%. Next, the steel plate was held down with a blank holding force of 9 tons (88.26 kN) using a die with an inner diameter of 75 mm, and in this state, a conical punch with an apex angle of 60° was pressed into the hole to measure the hole diameter at the crack initiation limit. Then, the (limit) hole expansion ratio: λ (%) was calculated using the following formula. Limit hole expansion ratio: λ(%)={(D f -D0) / D0}×100 Here, D f is the hole diameter (mm) when a crack occurs, and D0 is the initial hole diameter (mm). When the limiting hole expanding ratio λ is 30% or more, it was determined that the stretch flange formability immediately after production was excellent.

[0121] [Bending test] The bending test was performed in accordance with JIS Z 2248:2022. From the obtained steel plate, a rectangular test piece with a width of 30 mm and a length of 100 mm was taken so that the axial direction of the bending test was parallel to the rolling direction of the steel plate. Then, a 90° V bending test was performed under the conditions of a pressing load of 100 kN and a pressing holding time of 5 seconds. In this disclosure, bending tests were performed on five samples at R, where the value R / t obtained by dividing the bending radius (R) by the plate thickness (t) was about 4.5, that is, 4.3 to 4.7. Next, the crack length at the ridgeline of the bending apex of all five samples was evaluated, and when the crack length was 200 μm or less, it was determined that the bending property was excellent. Here, the crack length was evaluated by measuring the ridgeline of the bending apex at a magnification of 40 to 160 times using a digital microscope (RH-2000: manufactured by Hirox Co., Ltd.).

[0122] [Spot weld HAZ interplate cracking test] Test pieces were cut to a thickness of 1.4 mm, length of 30 mm, and width of 100 mm with the rolling direction as the longitudinal direction. The coating weight of the hot-dip galvanized layer on each side was 50 g / m 2 The plate assembly was then stacked with a test hot-dip galvanized steel sheet of the same material to form a plate assembly. Next, a servo motor pressurized single-phase AC (50 Hz) resistance welding machine and an electrode with a tip diameter of 6 mm were used to incline the plate assembly by 5° with respect to the electrode of the resistance welding machine, and resistance welding was performed with a clearance of 1.5 mm between the lower electrode and the lower steel sheet. Specifically, the plate assembly was subjected to resistance welding under the conditions of a pressure of 3.5 kN, a hold time of 0.16 seconds or 0.20 seconds, and a welding current and welding time that resulted in a nugget diameter of 5.9 mm to form a plate assembly with a welded portion. Next, the plate assembly with the welded portion was cut in half to include the welded portion, and the cross section of the welded portion was observed with an optical microscope (200 times magnification), and the inter-steel sheet cracking resistance property of the spot welded portion HAZ was evaluated according to the above-mentioned criteria. In addition, a similar evaluation was performed on some samples with plate thicknesses of 0.8 mm and 2.3 mm.

[0123] According to the above-mentioned method, the area ratio of martensite and ferrite, the volume ratio of retained austenite, the boron atom concentration in the prior austenite grain boundary with an orientation difference of 15 degrees or more, and the number density of precipitates with a size of 2 μm or more were obtained. In addition, the remaining structure was observed by the method described below. After cutting out a sample so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate was the observation surface, the observation surface was mirror-polished using diamond paste, and then etched with 3 vol.% nital to reveal the structure. Under the condition of an acceleration voltage of 15 kV, using SEM, the observation position was set to 1 / 4 of the plate thickness of the steel plate, and observation was performed at a magnification of 5000 times and three fields of view with a field of view of 17 μm × 23 μm. Carbide was identified as the remaining structure from the obtained structure image.

[0124] As shown in Table 4, the examples of the present invention are excellent in TS, YR, λ, bendability, and inter-steel-plate cracking resistance of the spot weld HAZ, while the comparative examples are inferior in at least one of TS, YR, λ, bendability, and inter-steel-plate cracking resistance of the spot weld HAZ.

[0125] Although the embodiment of the present invention has been described above, the present invention is not limited by the description of the present embodiment, which is a part of the disclosure of the present invention. In other words, other embodiments, examples, and operation techniques made by those skilled in the art based on the present embodiment are all included in the scope of the present invention. For example, in the series of heat treatments in the above-mentioned manufacturing method, the equipment for subjecting the steel sheet to heat treatment is not particularly limited as long as the heat history conditions are satisfied. [Industrial Applicability]

[0126] According to the present invention, a high strength steel sheet of 1180 MPa or more is obtained which is excellent in stretch flangeability, bendability and inter-steel sheet cracking resistance in spot weld HAZ and which enables the production of parts with high dimensional accuracy.

[0127] In particular, the high-strength steel sheet of the present invention has excellent inter-steel sheet cracking resistance in the HAZ of spot welds, and can be applied to automotive structural components of various sizes and shapes while obtaining high component strength, which can improve fuel efficiency by reducing the weight of the vehicle body, making the steel sheet of the present invention extremely valuable in industry.

Claims

1. In mass percent, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0.002% or more and 0.200% or less, and B: Contains 0.0002% or more and 0.0100% or less, The effective Ti mole fraction (x Ｔｉ，ｅｆｆ ) is 0.001 or more, The balance is composed of Fe and unavoidable impurities; At the 1 / 4 plate thickness position, The area ratio of martensite, including quenched martensite, tempered martensite, and bainite, is 60% or more and 99% or less; A steel structure having an area ratio of ferrite including bainitic ferrite and / or a volume ratio of retained austenite that is more than 0% and not more than 40% in total; The boron atom concentration in the prior austenite grain boundary having an orientation difference of 15 degrees or more is 0.3 at% or more and 6.0 at% or less; The number density of precipitates with a size of 2 μm or more is 150 / mm ２ Here, the term "precipitates having a size of 2 μm or more" refers to the maximum length of precipitates observed in a field of view in which a specimen is cut out so that a cross section of the plate thickness parallel to the rolling direction of the steel plate serves as the observation surface, the observation surface is mirror-polished using diamond paste, and the entire thickness of the steel plate is observed in two rows in the rolling direction at 3000 times magnification using a backscattered electron image of an SEM under conditions of an accelerating voltage of 15 kV. Note x Ｔｉ，ｅｆｆ =x Ｔｉ -x Ｎ -x Ｓ ・・・(1) In addition, x in the formula Ｔｉ , x Ｎ , x Ｓ indicates the content (molar fraction) of each element in the steel sheet.

2. The 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, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, The high strength steel plate according to claim 1, further comprising at least one element selected from the group consisting of:

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

4. A method for producing a high strength steel plate according to claim 1 or 2, comprising the steps of: providing a steel slab having the above-mentioned composition; Cooling the steel slab at an average cooling rate of 50°C / hr or more and 500°C / hr or less in a temperature range of 700°C or more and 1000°C or less; Next, the steel slab is heated at an average heating rate of 25°C / min or less in a temperature range of 900°C or more and 1150°C or less to a slab heating temperature of 1150°C or more, with a residence time from 1100°C to the slab heating temperature of 20 minutes or more; Next, the steel slab is subjected to hot rolling with a rough rolling exit temperature of 1100°C or more and a finish rolling entry temperature of 1050°C or more to obtain a hot-rolled sheet, Next, the hot-rolled sheet is subjected to pickling to obtain a pickled sheet. Next, the pickled sheet is cold-rolled at a cumulative rolling reduction of 20% to 95% and a final pass speed of 50 mpm or more 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, Next, the cold-rolled sheet is first-cooled under conditions in which the average cooling rate in the temperature range from the heating temperature to 650°C is 0.5°C / s or more and 50°C / s or less.

5. The method for producing a high strength steel plate according to claim 4, further comprising: performing second cooling after the first cooling, the average cooling rate in a temperature range of 250°C or more and 400°C or less being 1.0°C / s or more.

6. The method for producing a high strength steel plate according to claim 4, further comprising the step of: after the first cooling, performing a second cooling in which the steel sheet is kept at a heat retention temperature of 100°C or more and 450°C or less for 5 seconds or more.

7. 5. The method for producing a high strength steel sheet according to claim 4, further comprising: setting a cooling stop temperature to 250°C or less after the first cooling; and then reheating the cold rolled sheet to a reheating temperature of (the cooling stop temperature + 50°C) or more and 450°C or less and retaining the reheating temperature for 5s or more.

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

9. The method for producing a high-strength plated steel sheet according to claim 8, further comprising: performing second cooling after the first cooling, the average cooling rate in a temperature range of 250°C or more and 400°C or less being 1.0°C / s or more.

10. The method for producing a high-strength plated steel sheet according to claim 8, further comprising: performing second cooling after the first cooling, in which the steel sheet is kept at a heat retention temperature of 100°C or more and 450°C or less for 5 s or more.

11. 9. The method for producing a high-strength plated steel sheet according to claim 8, further comprising the steps of: setting a cooling stop temperature to 250°C or lower after the first cooling; and then performing second cooling in which the cold-rolled sheet is reheated to a reheating temperature of (the cooling stop temperature + 50°C) or higher and 450°C or lower and kept at the reheating temperature for 5s or more.

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

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

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