High-strength steel plate, high-strength plated steel plate, methods for manufacturing same, and member

Abstract

The purpose of the present invention is to provide: a high-strength steel sheet and a high-strength plated steel sheet of 1180 MPa or more, both having excellent elongation flange properties, bendability, and resistance to interplate cracking of the heat affected zones of spot weld sections, and allowing the production of components with high dimensional accuracy; methods for manufacturing the same; and a member.　This high-strength steel plate has a prescribed component composition, has a steel structure in which: the effective Ti molar fraction (xTi, eff), as determined from XTi, eff = XTi - XN - XS (where, XTi, XN, and XS are the molar fractions of respective elements in steel) is 0.001 or more; at a position 1 / 4 the plate thickness, the area ratio of martensite is 60% to 99% inclusive; and the area ratio of ferrite and / or the volume ratio of retained austenite in total is more than 0% but no more than 40%. At the grain boundaries of former austenite with a directional difference of 15 degrees or more, the concentration of boron atoms is 0.3 at% to 6.0 at% inclusive, and the number density of precipitates with a size of 2 μm or more is 150 or less per mm2.

Description

High-strength steel sheets, high-strength plated steel sheets, and manufacturing methods and components thereof

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

[0002] CO2 emissions from lighter vehicles ２ With the aim of achieving both reduced emissions and improved crashworthiness through vehicle weight reduction, efforts are underway to increase the strength of automotive steel sheets. Furthermore, new legal regulations are being introduced one after another. Therefore, in order to increase the strength of the vehicle body, there has been an increase in the number of cases where high-strength steel sheets are used for the main structural and reinforcing parts that form the framework of the automobile cabin (hereinafter also referred to as automotive framework structural parts, etc.). In particular, there has been an increase in the number of cases where high-strength steel sheets with a tensile strength (hereinafter also simply referred to as TS) of 1180 MPa or more are used.

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

[0004] Furthermore, among automotive structural components, crash boxes, for example, have punched end faces and bent portions. 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 portions, 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 do not cause large cracks in bent portions where R / t, the value obtained by dividing the bending radius (R) by the plate thickness (t), is less than 5.0.

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

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

[0007] Patent No. 6787535

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

[0009] The present disclosure has been developed in view of the above circumstances. An object of the present disclosure is to obtain a high-strength steel sheet of 1180 MPa or more that is excellent in stretch flangeability, bendability, and inter-steel sheet cracking resistance in the HAZ of a spot weld 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 the present disclosure, being able to manufacture parts with high dimensional accuracy (high dimensional accuracy during forming) 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, a material is considered to have excellent stretch flangeability when the hole expansion ratio (hereinafter simply referred to as λ) measured in accordance with JIS Z 2256 is 30% or more.

[0011] Regarding bendability, a bending test is performed using the V-block method with a bending angle of 90 degrees, and five samples are subjected to a bending test at an R where the value R / t obtained by dividing the bending radius (R) by the plate thickness (t) is approximately 4.5, i.e., 4.3 to 4.7. Next, the length of the crack at the ridge line 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] Furthermore, the resistance to inter-steel plate cracking of the spot weld HAZ was evaluated by observing the cross section of the weld described in the examples with an optical microscope (200x magnification) and using the following criteria: A or B indicates that the resistance to inter-steel plate cracking of the spot weld HAZ is excellent; C indicates that the resistance to inter-steel plate cracking of the spot weld HAZ is poor. A: No cracks of 0.1 mm or more in length were observed with a hold time of 0.16 seconds; B: Cracks of 0.1 mm or more in length were observed with a hold time of 0.16 seconds, but no cracks of 0.1 mm or more in length were observed with a hold time of 0.20 seconds; C: Cracks of 0.1 mm or more in length were observed with a hold time of 0.20 seconds.

[0013] Here, the hold time refers to the time from when the welding current has finished flowing until when the electrodes start to be released.

[0014] The inventors of the present invention have conducted extensive research to achieve the above-mentioned objectives, and as a result have made the following discoveries. (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 of 65% or more and 90% or less, which is an index of dimensional accuracy of parts. (2) By setting the boron atom concentration in the prior austenite grain boundaries with an orientation difference of 15 degrees or more to 0.3 at% or more and 6.0 at% or less, it is possible to achieve good inter-steel sheet cracking resistance in the HAZ of spot welds. (3) By setting the number density of precipitates with a size of 2 μm or more to 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] A steel sheet containing, by mass%, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0.002% or more and 0.200% or less, and B: 0.0002% or more and 0.0100% or less, and an effective Ti mole fraction (x Ｔｉ，ｅｆｆ ) is 0.001 or more, with the balance consisting of Fe and unavoidable impurities, and a steel structure in which the area fraction of martensite is 60% or more and 99% or less, and the area fraction of ferrite and / or the volume fraction of retained austenite is more than 0% and 40% or less in total at the 1 / 4 position of the plate thickness, and the boron atom concentration at the prior austenite grain boundary with a misorientation of 15 degrees or more is 0.3 at% or more and 6.0 at% or less, and the number density of precipitates with a size of 2 μm or more is 150 / mm ２ High strength steel plate as follows: Ｔｉ，ｅｆｆ = x Ｔｉ -x Ｎ -x Ｓ ... (1) Note that x in the formula Ｔｉ , x Ｎ , x Ｓ[2] The high-strength steel sheet according to [1], wherein the chemical composition further contains, in mass %, at least one element selected from Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, 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, and Bi: 0.200% or less. [3] A high-strength plated steel sheet having a plating layer on at least one side of the high-strength steel sheet according to [1] or [2]. [4] A steel slab having the component composition according to [1] or [2] is cooled at an average cooling rate of 50°C / hr to 500°C / hr in a temperature range of 700°C to 1000°C, and then the steel slab is heated at an average heating rate of 25°C / min or less in a temperature range of 900°C to 1150°C to a slab heating temperature of 1150°C or higher, with a residence time from 1100°C to the slab heating temperature being 20 minutes or longer. Then, the steel slab is subjected to a rough rolling process at a delivery temperature of 1100°C or higher and a finish rolling process at a delivery temperature of 1100°C or higher. [5] A method for producing a high-strength steel sheet according to [4], wherein after the first cooling, a second cooling is performed at an average cooling rate of 1.0°C / s or more in a temperature range from the heating temperature to 650°C. [6] The method for producing a high-strength steel plate according to [4], wherein after the first cooling, a second cooling is further performed in which the temperature is maintained at a temperature of 100°C or higher and 450°C or lower for 5 seconds or more.[7] A method for producing a high-strength steel sheet according to [4], further comprising: after the first cooling, further cooling to 250°C or less, 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 maintaining the reheating temperature for 5 seconds or more. [8] A method for producing a high-strength plated steel sheet, further comprising: after the annealing step according to [4], performing a plating step of plating at least one surface of the cold-rolled sheet. [9] A method for producing a high-strength plated steel sheet according to [8], further comprising: after the first cooling, further performing 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] A method for producing a high-strength plated steel sheet according to [8], further performing second cooling at a retention temperature of 100°C or more and 450°C or less for 5 seconds or more.

[11] The method for producing a high-strength plated steel sheet according to [8], wherein after the first cooling, the temperature is further reduced to 250°C or less, and then the cold-rolled sheet is reheated to a reheating temperature of (the cooling stop temperature + 50°C) or more and 450°C or less, and maintained at the reheating temperature for 5 seconds or more, thereby performing a second cooling.

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

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

[0016] According to the present disclosure, it is possible to provide high-strength steel plates and components of 1180 MPa or more that have excellent stretch flangeability, bendability, and inter-steel sheet cracking resistance in the HAZ of spot welds, and that can be used to manufacture parts with high dimensional accuracy.

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

[0018] First, the appropriate range of the chemical composition of the steel sheet and the reasons for limiting it will be explained. In the following explanation, "%" representing the content of the chemical elements of the steel sheet means "mass%" unless otherwise specified. Furthermore, in this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits.

[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, C is an important element that affects the area fractions of martensite, ferrite, and retained austenite, particularly. If the C content is less than 0.030%, the area fraction of martensite decreases, and the area fraction of ferrite increases, making it difficult to achieve a TS of 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 fraction of retained austenite increases, resulting in decreased λ and bendability. Therefore, the C content is set to 0.030% or more and 0.500% or less. The C content is preferably set to 0.080% or more. The C content is preferably set to 0.400% or less. The C content is more preferably set to 0.110% or more. The C content is more preferably set to 0.350% or less.

[0020] [Si: 0.01% or more and 2.50% or less] Si is one of the important basic components of steel. In particular, in the present disclosure, Si suppresses 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 the 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 the 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, it is an important element that affects the area fractions of martensite, ferrite, and retained austenite, particularly. If the Mn content is less than 0.10%, the area fraction of martensite decreases, and the area fraction of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. It also makes it difficult to achieve the desired YR. On the other hand, if the Mn content exceeds 5.00%, the area fraction of martensite increases, making it difficult to obtain the desired area fraction of ferrite and / or the volume fraction of retained austenite, making it difficult to achieve the desired YR. It also reduces λ and bendability. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably set to 1.00% or more. The Mn content is preferably set to 4.00% or less. The Mn content is more preferably set to 2.00% or more. The Mn content is more preferably 3.50% or less.

[0022] [P: 0.100% or less] Excessive P segregates at prior austenite grain boundaries, embrittling the grain boundaries and reducing the ultimate deformability of the steel sheet, resulting in reduced λ and bendability. Therefore, the P content must be 0.100% or less. Although there is no particular lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.001% or more. 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, thereby reducing λ and bendability. Therefore, the S content needs to be 0.0200% or less. Although there is no particular lower limit for the S content, due to constraints on production technology, the S content is preferably 0.0001% or more. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0001% or more. The S content is preferably 0.0050% or less.

[0024] [Al: 0.100% or less] When Al is excessive, A ３ The transformation point rises and a large amount of ferrite is contained in the steel structure, making it difficult to achieve the desired YR. Therefore, the Al content needs to be 0.100% or less. Although there is no particular lower limit for the Al content, the Al content is preferably 0.001% or more because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite. Therefore, the Al content is set to 0.100% or less. The Al content is preferably set to 0.001% or more. The Al content is preferably set to 0.050% or less.

[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, thereby reducing 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, thereby reducing 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 set to 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 steel sheets by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Furthermore, 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 at prior austenite grain boundaries with a misorientation of 15 degrees or more by forming precipitates with N and S. To achieve these effects, the Ti content must 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 0.010% or more, and more preferably 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) Note that x in the formula Ｔｉ , x Ｎ , x Ｓrepresents the content (molar fraction) of each element in the steel sheet. By setting the effective Ti molar fraction calculated from the above formula (1) to a certain level or higher, the number density of precipitates with a size of 2 μm or larger can be reduced, thereby achieving good λ and bendability. Furthermore, Ti can form precipitates with N and S, thereby increasing the boron atom concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more. To achieve these effects, the effective Ti molar fraction is set to 0.001% or higher. While there is no particular upper limit for the effective Ti molar fraction, it is preferable to set the effective Ti molar fraction to 0.040% or lower because the amount of carbides, nitrides, or carbonitrides increases, making it difficult to achieve the desired YR. Therefore, the effective Ti molar fraction is set to 0.001% or higher. The effective Ti molar fraction is preferably set to 0.002% or higher. The effective Ti molar fraction is preferably set to 0.040% or lower.

[0029] [B: 0.0002% or more and 0.0100% or less] B increases the boron atom concentration in prior austenite grain boundaries with an orientation difference of 15 degrees or more, thereby achieving good inter-steel sheet cracking resistance in the spot weld HAZ. To achieve this 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, thereby reducing λ and bendability. 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 one embodiment of the present invention has a composition containing the above elements with the balance being Fe and unavoidable impurities. Preferably, a high-strength steel sheet according to one embodiment of the present invention has a composition containing the above elements with the balance being Fe and unavoidable impurities. Examples of the unavoidable impurities include Zn, Pb, and As. The inclusion of these impurities is permitted as long as the total content is 0.100% or less.

[0031] In addition to the above-described essential components, the component composition of the high-strength steel plate of the present disclosure may further contain, in mass%, at least one element selected from Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, 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. These elements may be contained alone or in combination.

[0032] [Nb: 0.200% or less] Nb generates large amounts of coarse precipitates and inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the Nb content exceeds 0.200%, λ and bendability decrease. Therefore, the Nb content is set to 0.200% or less. Although there is no particular lower limit for the Nb content, by setting the Nb content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet and controlling the YR within a desired range. For this reason, the Nb content is preferably set to 0.001% or more. Therefore, when Nb is added, the Nb content is set to 0.200% or less. The Nb content is preferably set to 0.001% or more. The Nb content is preferably set to 0.100% or less.

[0033] [V: 0.200% or less] V generates large amounts of coarse precipitates and inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the V content exceeds 0.200%, λ and bendability decrease. Therefore, the V content is set to 0.200% or less. Although there is no particular lower limit for the V content, by setting the V content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet and controlling the YR within a desired range. For this reason, the V content is preferably set to 0.001% or more. Therefore, when added, the V content is set to 0.200% or less. The V content is preferably set to 0.001% or more. The V content is preferably set to 0.100% or less.

[0034] [Ta: 0.10% or less, W: 0.10% or less] If the Ta and W contents exceed 0.10%, large amounts of coarse precipitates and inclusions are formed, reducing the ultimate deformability of the steel sheet and thereby reducing λ and bendability. Therefore, the Ta and W contents are each set to 0.10% or less. Although there are no particular lower limits for the Ta and W contents, the Ta and W contents are preferably set to 0.01% or more because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, when added, the Ta and W contents are each set to 0.10% or less. The Ta and W contents are preferably set to 0.01% or more. The Ta and W contents are preferably set to 0.08% or less.

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

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

[0037] [Cu: 1.00% or less] If the Cu content exceeds 1.00%, coarse precipitates and inclusions increase, reducing the ultimate deformability of the steel sheet, thereby reducing λ and bendability. Therefore, the Cu content is set to 1.00% or less. Although there is no particular lower limit for the Cu content, since Cu is an element that improves hardenability, the Cu content is preferably set to 0.01% or more. Therefore, when 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, reducing the ultimate deformability of the steel sheet, 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 Sn is added, the Sn content is set to 0.200% or less. The Sn content is preferably set to 0.001% or more. The Sn content is preferably set to 0.100% or less.

[0039] [Sb: 0.200% or less] If the Sb content exceeds 0.200%, coarse precipitates and inclusions increase, reducing the ultimate deformability of the steel sheet, thereby reducing λ and bendability. Therefore, the Sb content is set to 0.200% or less. Although there is no particular lower limit for the Sb content, since Sb is an element that controls the softened surface thickness and enables strength adjustment, the Sb content is preferably set to 0.001% or more. Therefore, when Sb is added, the Sb content is set to 0.200% or less. The Sb content is preferably set to 0.001% or more. The Sb content is preferably set to 0.100% or less.

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

[0041] [Zr: 0.100% or less, Te: 0.100% or less] If the Zr and Te contents exceed 0.100%, coarse precipitates and inclusions increase, reducing the ultimate deformability of the steel sheet, resulting in reduced λ and bendability. Therefore, the Zr and Te contents must each be 0.100% or less. Although there are no particular lower limits for the Zr and Te contents, the Zr and Te contents are preferably 0.001% or more because they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, when added, the Zr and Te contents are set to 0.100% or less. The Zr and Te contents are preferably 0.001% or more. The Zr and Te contents are preferably 0.080% or less.

[0042] [Hf: 0.10% or less] If the Hf content exceeds 0.10%, coarse precipitates and inclusions increase, reducing the ultimate deformability of the steel sheet, and therefore reducing λ and bendability. Therefore, the Hf content is set to 0.10% or less. Although there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, the Hf content is preferably set to 0.01% or more. Therefore, when Hf is added, the Hf content is set to 0.10% or less. The Hf content is preferably set to 0.01% or more. The Hf content is preferably set to 0.08% or less.

[0043] [Bi: 0.200% or less] If the Bi content exceeds 0.200%, the amount of coarse precipitates and inclusions increases, reducing the ultimate deformability of the steel sheet, and therefore reducing λ and bendability. Therefore, the Bi content is set to 0.200% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is preferably set to 0.001% or more. Therefore, when Bi is added, the Bi content is set to 0.200% or less. The Bi content is preferably set to 0.001% or more. The Bi content is preferably set to 0.100% or less.

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

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

[0046] [Area Fraction of Martensite: 60% or More and 99% or Less] If the area fraction of martensite is less than 60%, the area fraction of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. It also makes it difficult to achieve the desired YR. On the other hand, if the area fraction of martensite exceeds 99%, ferrite and / or retained austenite, which are effective for controlling the YR, are not present in the steel structure, making it difficult to achieve the desired YR. Therefore, the area fraction of martensite is set to 60% or more and 99% or less. The area fraction of martensite is preferably set to 65% or more. The area fraction of martensite is preferably set to 98% or less. The area fraction of martensite is more preferably set to 67% or more. The area fraction of martensite is preferably set to 97% or less. Here, martensite includes tempered martensite and bainite in addition to quenched martensite (fresh martensite). The observation position for the area fraction of martensite is set to ¼ of the steel plate thickness, as described below.

[0047] [Total of Area Fraction of Ferrite and / or Volume Fraction of Retained Austenite: More than 0% and Not More than 40%] When the total of the area fraction of ferrite and / or the volume fraction of retained austenite is 0%, the steel structure becomes a martensite single phase structure, making it difficult to achieve the desired YR. On the other hand, when the total of the area fraction of ferrite and / or the volume fraction of retained austenite exceeds 40%, the area fraction of martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. It also makes it difficult to achieve the desired YR. Therefore, the total of the area fraction of ferrite and / or the volume fraction of retained austenite is set to be more than 0% and not more than 40%. The total of the area fraction of ferrite and / or the volume fraction of retained austenite is preferably 1% or more. The total of the area fraction of ferrite and / or the volume fraction of retained austenite is preferably 38% or less. The total of the area fraction of ferrite and / or the volume fraction of retained austenite is more preferably 2% or more. The total area ratio of the ferrite area ratio and / or the volume ratio of the retained austenite is preferably 35% or less. Note that ferrite here includes bainitic ferrite. Note that the observation position for the ferrite area ratio and the volume ratio of the retained austenite is set at a position ¼ of the thickness of the steel plate, as described below.

[0048] Here, the area ratios of martensite (quenched martensite, tempered martensite, and bainite) and ferrite (including bainitic ferrite) are measured as follows.

[0049] After cutting out a sample so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface, the observation surface is mirror-polished using diamond paste, and then etched with 3 vol.% nital to reveal the structure. Under the condition of an acceleration voltage of 15 kV, using an SEM (Scanning Electron Microscope), the observation position is set to 1 / 4 of the plate thickness of the steel plate, and at a magnification of 5000 times, three fields of view are observed in a field range of 17 μm × 23 μm. The obtained structure image is used Adobe Photoshop from Adobe Systems to calculate the area ratio of each constituent structure divided by the measured area for three fields of view. These values ​​are averaged to obtain the area ratio of each structure. In the above structural images, ferrite (including bainitic ferrite) is a flat structure containing no carbides in the recessed portions, while tempered martensite and bainite are structures containing fine carbides in the recessed portions. Furthermore, quenched martensite is a structure with protrusions and fine irregularities inside the structure, and they are distinguishable from each other. Note that tempered martensite, tempered martensite, and bainite do not need to be distinguishable from each other because the total area ratio is calculated as the area ratio of martensite.

[0050] The volume fraction of retained austenite was measured as follows.

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

[0052] [Boron atomic concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more: 0.3 at% to 6.0 at%] This is an extremely important invention requirement in the present disclosure. To control the inter-steel-sheet cracking resistance of the HAZ of a spot weld, it is important to control element segregation at prior austenite grain boundaries with a misorientation of 15 degrees or more. In particular, increasing the boron atomic concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more can improve the inter-steel-sheet cracking resistance of the HAZ of a spot weld. To achieve this effect, the boron atomic concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more must be 0.3 at% or more. On the other hand, if the boron atomic concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more exceeds 6.0 at%, the number density of precipitates with a size of 2 μm or more increases, resulting in a decrease in λ and bendability. Therefore, the boron atomic concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more is set to 0.3 at% to 6.0 at%. The boron atom concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more is preferably 0.4 at% or more. The boron atom concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more is preferably 5.0 at% or less. The boron atom concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more is more preferably 0.5 at% or more. The boron atom concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more is preferably 4.0 at% or less. The boron atom concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more is even more preferably 0.6 at% or more. The boron atom concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more is even more preferably 3.0 at% or less.

[0053] Here, the method for measuring the boron atom concentration in the prior austenite grain boundaries with a misorientation of 15 degrees or more is as follows.

[0054] After cutting out a sample so that the thickness cross section (L cross section) parallel to the rolling direction of the steel sheet serves as 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 under conditions of an acceleration voltage of 15 kV, the observation position is set at 1 / 4 of the steel sheet thickness, and the prior γ grain boundaries are observed in multiple fields at a magnification of 3000 to 5000 times. The crystal orientation of each field is measured using an FE-SEM / EBSD (JSM7100F: manufactured by JEOL Ltd., OIM: manufactured by TSL Co., Ltd.). EBSD measurement is performed under measurement conditions sufficient to evaluate the martensite substructure by crystal orientation (measurement field: 28 μm (rolling direction) × 30 μm (sheet thickness direction), measurement point spacing: 40 nm). The obtained EBSD observation results are reconstructed using the Nishiyama-Wasserman relationship in the Parent Grain Reconstruction function of the OIM Analysis v8.6 software. In other words, the martensite orientation is converted to the austenite orientation, and prior austenite grain boundaries with an orientation difference of 15 degrees or more are identified by depicting boundaries with an orientation difference of 15 degrees or more on the resulting inverse pole figure (IPF) image of the austenite. Next, needle-shaped samples are prepared using a focused ion beam (FIB) for prior austenite grain boundaries identified as having an orientation difference of 15 degrees or more. The boron atomic concentration is then analyzed using a three-dimensional atom probe (EIKOS-UV: manufactured by CAMECA). The three-dimensional atom probe was used in laser mode, with a pulse energy of 7 nJ. From the results obtained, the maximum boron atom concentration at the prior austenite grain boundary was calculated, and this was taken as the boron atom concentration at the prior austenite grain boundary with a misorientation of 15 degrees or more.

[0055] [Number density of precipitates with a size of 2 μm or more: 150 pieces / mm ２

[0033] 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. By reducing the number density of precipitates with a size of 2 μm or more, λ and bendability can be improved. 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 ...0 particles / mm ２ Therefore, the number density of precipitates with 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 are coarse inclusions formed as a result of the composite precipitation of sulfides, nitrides, oxides, borides, etc.

[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 serves as the observation surface, and then the observation surface is mirror-polished using diamond paste. Using a backscattered electron image of an SEM, the entire thickness of the steel plate is observed in two rows in the rolling direction at a magnification of 3000 times under the condition of an acceleration voltage of 15 kV. The number of precipitates with a size of 2 μm or more in the 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 refers to 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 carbides such as pearlite, cementite, and metastable carbides, as well as other structures known as steel sheet structures, are contained within an area ratio of 5% or less. Metastable carbides include epsilon (ε) carbide, eta (η) carbide, chi (χ) carbide, etc.

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

[0060] [High-Strength Plated Steel Sheet] The high-strength plated steel sheet of the present disclosure is a high-strength plated steel sheet comprising a plating layer on at least one surface of the high-strength steel sheet of the present disclosure. The type of plating layer is not particularly limited, and may be, for example, either a hot-dip plating layer or an electroplated layer. The plating layer may also be an alloyed plating layer. The plating layer is preferably a zinc plating layer. The zinc plating layer may contain Al and Mg. A hot-dip zinc-aluminum-magnesium alloy plating (Zn-Al-Mg plating layer) is also preferred. In this case, it is preferable that the Al content is 1% by mass or more and 22% by mass or less, the Mg content is 0.1% by mass or more and 10% by mass or less, and the remainder is Zn. Furthermore, the Zn-Al-Mg plating layer may contain, in addition to Zn, Al, and Mg, one or more elements selected from Si, Ni, Ce, and La in a total amount of 1% by mass or less. The plating metal is not particularly limited, and in addition to the Zn plating described above, an Al plating or the like may also be used.

[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 20 mass% or less of Fe and 0.001 mass% to 1.0 mass% of Al. Furthermore, it contains one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0 mass% to 3.5 mass%, with the balance consisting of Zn and unavoidable impurities. In the present disclosure, the plating coating weight per side is 20 to 80 g / m. ２ Preferably, the coating layer has a hot-dip galvanized layer, which is further alloyed to form a galvannealed layer. When the coating layer is a hot-dip galvanized layer, the Fe content in the coating layer may be less than 7 mass %, and when the coating layer is an alloyed hot-dip galvanized layer, the Fe content in the coating layer may be 7 to 20 mass %.

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

[0063] First, a steel material having the above-described composition is melted to produce a steel slab. In the present invention, the method for melting the steel material is not particularly limited, and any known melting method such as a converter or an electric furnace is suitable. Furthermore, the steel slab is preferably produced by a continuous casting method to prevent macrosegregation, but it can also be produced by an ingot casting method 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 of 700°C to 1000°C: 50°C / hr to 500°C / hr] The average cooling rate in the temperature range of 700°C to 1000°C is an extremely important constituent element of the present invention. Increasing the average cooling rate in the temperature range of 700°C to 1000°C can reduce the number density of precipitates with a size of 2 μm or more. To achieve this 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 prior austenite grain boundaries with a misorientation of 15° or more decreases, making it impossible to achieve good inter-steel-plate cracking resistance in the spot weld HAZ. Therefore, the average cooling rate in the temperature range of 700°C to 1000°C is set to 50°C / hr to 500°C / hr. The average cooling rate in the temperature range of 700°C or higher and 1000°C or lower is preferably 100°C / hr or higher. The average cooling rate in the temperature range of 700°C or higher and 1000°C or lower is preferably 450°C / hr or lower. The average cooling rate in the temperature range of 700°C or higher and 1000°C or lower is more preferably 150°C / hr or higher. The average cooling rate in the temperature range of 700°C or higher and 1000°C or lower is more preferably 400°C / hr or lower. Note that the temperature of the steel slab here refers to the temperature of the surface of the steel slab.

[0066] After the above cooling process, the steel slab may be cooled to room temperature and then reheated as in the conventional method, or an energy-saving process such as direct rolling may 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°C to 1150°C: 25°C / min or less] This is an extremely important invention requirement in the present disclosure. By slowing the average heating rate of the slab 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 at prior austenite grain boundaries with a misorientation of 15° or more and reduce the number density of precipitates with a size of 2 μm or more. To achieve these effects, the average heating rate of the slab in the temperature range of 900°C to 1150°C is set to 25°C / min or less. Note that there is no particular lower limit for the average heating rate of the slab in the temperature range of 900°C to 1150°C, but it is preferably set to 5°C / min or more to effectively prevent an increase in the surface softening thickness after annealing and to keep the TS within a more suitable range. Therefore, the average heating rate of the slab in the temperature range of 900°C to 1150°C is set to 25°C / min or less. The average heating rate of the slab in the temperature range of 900° C. or higher and 1150° C. or lower is preferably 5° C. / min or higher. The average heating rate of the slab in the temperature range of 900° C. or higher and 1150° C. or lower is preferably 15° C. / min or lower. The slab heating temperature refers to the temperature of the surface of the steel slab during slab heating.

[0069] [Slab heating temperature: 1150°C or higher] This is an extremely important invention constituent element in the present disclosure. By increasing the slab heating temperature to 1150°C or higher, it is possible to increase the boron atom concentration at prior austenite grain boundaries with a misorientation of 15° or more and reduce the number density of precipitates with a size of 2 μm or more. To achieve these effects, the slab heating temperature is set to 1150°C or higher. Note that there is no particular upper limit for the slab reheating temperature, but it is preferable to set it to 1300°C or lower in order to effectively prevent an increase in the softened surface thickness after annealing and to keep the TS within a more suitable range. Therefore, the slab heating temperature is set to 1150°C or higher. The slab heating temperature is preferably set to 1180°C or higher. The slab heating temperature is preferably set to 1300°C or lower. Note that the slab heating temperature refers to the surface temperature of the steel slab during slab heating.

[0070] [Dwell Time from 1100°C to the Slab Heating Temperature: 20 Minutes or More] This is an extremely important inventive constituent element of the present disclosure. By increasing the dwell time from 1100°C to the slab heating temperature to 20 minutes or more, it is possible to increase the boron atom concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more and reduce the number density of precipitates with a size of 2 μm or more. To achieve these effects, the dwell time from 1100°C to the slab heating temperature is set to 20 minutes or more. While there is no particular upper limit to the dwell time from 1100°C to the slab heating temperature, it is preferable to set it to 100 minutes or less in order to effectively prevent an increase in the surface softening thickness after annealing and to keep the TS within a more suitable range. Therefore, the dwell time from 1100°C to the slab heating temperature is set to 20 minutes or more. The dwell time from 1100°C to the slab heating temperature is preferably set to 30 minutes or more. The dwell time from 1100°C to the slab heating temperature is preferably set to 100 minutes or less. The slab heating temperature is the temperature of the surface of the steel slab during heating.

[0071] The slab is made into a sheet bar by rough rolling under normal conditions, but if the slab heating temperature is set low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.

[0072] [Rough rolling outlet temperature: 1100°C or higher] This is an extremely important constituent element of the present disclosure. By increasing the rough rolling outlet temperature to 1100°C or higher, it is possible to increase the boron atom concentration at prior austenite grain boundaries with a misorientation of 15° or more and reduce the number density of precipitates with a size of 2 μm or more. To achieve these effects, the rough rolling outlet temperature is set to 1100°C or higher. Note that there is no particular upper limit for the rough rolling outlet temperature, but it is preferable to set it to 1200°C or lower in order to suitably prevent an increase in the surface softening thickness after annealing and to keep the TS within a more suitable range. Therefore, the rough rolling outlet temperature is set to 1100°C or higher. The rough rolling outlet temperature is preferably set to 1110°C or higher. The rough rolling outlet temperature is preferably set to 1200°C or lower.

[0073] [Finish rolling entry temperature: 1050°C or higher] This is an extremely important constituent element of the present disclosure. By increasing the finish rolling entry temperature to 1050°C or higher, it is possible to increase the boron atom concentration at prior austenite grain boundaries with a misorientation of 15° or more and reduce the number density of precipitates with a size of 2 μm or more. To achieve these effects, the finish rolling entry temperature is set to 1050°C or higher. Note that there is no particular upper limit for the finish rolling entry temperature, but it is preferable to set it to 1150°C or lower in order to suitably prevent an increase in the surface softening thickness after annealing and to keep TS within a more suitable range. Therefore, the finish rolling entry temperature is set to 1050°C or higher. The finish rolling entry temperature is preferably set to 1100°C or higher. The finish rolling entry temperature is preferably set to 1150°C or lower.

[0074] Finish rolling increases the rolling load and the reduction ratio in the unrecrystallized austenite state, which can lead to the development of abnormal structures elongated in the rolling direction, which can reduce the workability of the annealed sheet. ３ It is preferable to carry out the finish rolling at a temperature on the delivery side that is equal to or higher than the transformation point. In addition, the coiling temperature after hot rolling is preferably 300°C or higher and 700°C or lower in order to improve the workability after annealing. ３ The transformation temperature is calculated using the following formula: Ar ３ Transformation point (°C) = 868 - 396 x [%C] + 24.6 x [%Si] - 68.1 x [%Mn] - 36.1 x [%Ni] - 20.7 x [%Cu] - 24.8 x [%Cr]. Note that the [% element symbol] in the above formula represents the content (mass%) of the corresponding element in the above composition, and is set to 0 if the corresponding element is not contained.

[0075] During hot rolling, the rough-rolled sheets may be joined together and continuously finished. The rough-rolled sheets may also be wound up once. To reduce the rolling load during hot rolling, some or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniforming the shape and material properties of the steel sheet. The coefficient of friction during lubricated rolling is preferably 0.10 or more and 0.25 or less.

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

[0077] Next, the hot-rolled sheet after pickling or the hot-rolled sheet (hot-rolled annealed sheet) that has been optionally heat-treated after pickling is cold-rolled to obtain a cold-rolled sheet. Since strain is introduced uniformly and efficiently and a uniform structure is obtained, cold rolling is preferably performed by multi-pass rolling that requires two or more passes, such as tandem multi-stand rolling or reverse rolling.

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

[0079] [Cold Rolling Cumulative Reduction: 20% or More and 95% or Less] Increasing the cold rolling cumulative reduction reduces the ferrite area ratio, i.e., the total of the ferrite area ratio and / or the retained austenite volume ratio can be set to 40% or less. To achieve this effect, the cold rolling cumulative reduction is set to 20% or more. On the other hand, if the cold rolling cumulative reduction exceeds 95%, the grain size of austenite generated during annealing becomes finer, and the amount of retained austenite in the annealed sheet increases. In other words, the total of the ferrite area ratio and / or the retained austenite volume ratio increases, making it impossible to achieve the desired YR. Therefore, the cold rolling cumulative reduction is set to 20% or more and 95% or less. The cold rolling cumulative reduction is preferably set to 25% or more. The cold rolling cumulative reduction is preferably set to 90% or more. The cold rolling cumulative reduction is more preferably set to 27% or more. The cumulative reduction rate of the cold rolling is more preferably 70% or less.

[0080] [Threading speed in the final pass of cold rolling: 50 mpm or more] This is an extremely important inventive element in the present disclosure. Increasing the threading speed in the final pass of cold rolling introduces a large amount of strain into the steel sheet, increasing the number of prior austenite grain boundaries with a misorientation of 15 degrees or more. As a result, the boron atom concentration in the prior austenite grain boundaries with a misorientation of 15 degrees or more can be increased, resulting in the realization of good inter-steel sheet cracking resistance in the HAZ of spot welds. To achieve this effect, the threading speed in the final pass of cold rolling is set to 50 mpm or more. While there is no particular upper limit for the threading speed in the final pass of cold rolling, it is preferably 300 mpm or less due to production technology constraints. Therefore, the threading speed in the final pass of cold rolling is set to 50 mpm or more. The threading speed in the final pass of cold rolling is preferably 70 mpm or more. The threading 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°C or Higher] If the heating temperature (annealing temperature) is lower than 780°C, the annealing process will be performed in the two-phase region of ferrite and austenite. Since a large amount of ferrite will be present after annealing, a TS of 1180 MPa or higher will not be achieved, making it difficult to achieve the desired YR. Furthermore, the boron atom concentration at prior austenite grain boundaries with a misorientation of 15° or higher will decrease, making it impossible to achieve good inter-steel sheet cracking resistance in the spot weld HAZ. Furthermore, the number density of precipitates with a size of 2 μm or higher will increase, making it difficult to improve λ and bendability. Therefore, the heating temperature is set to 780°C or higher. While there is no particular upper limit for the heating temperature, an increase in the heating temperature may increase the thickness of the softened surface layer after annealing, resulting in a decrease in TS and coarsening of the prior austenite grain size, resulting in a decrease in YR. Therefore, the heating temperature is set to 780°C or higher. It is preferably set to 800°C or higher. It is preferably set to 1050°C or lower. More preferably, the heating temperature is 820° C. or higher, and more preferably, 1000° C. or lower. The heating temperature is measured based on the temperature of the steel sheet surface.

[0083] In addition, since this allows increasing the boron atom concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more and reducing the number density of precipitates with a size of 2 μm or more, the residence time at 780°C or more is preferably 10 seconds or more. The residence time at 780°C or more and the heating temperature or lower is more preferably 20 seconds or more. The residence time at 780°C or more is more preferably 600 seconds or less. The residence time at 780°C or more is even more preferably 25 seconds or more. The residence time at 780°C or more is even more preferably 300 seconds or less.

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

[0085] [Average cooling rate in the temperature range from the heating temperature to 650°C: 0.5°C / s or more and 50°C / s or less (first cooling)] This is an extremely important inventive feature of the present disclosure. By increasing the average cooling rate in the temperature range from the heating temperature to 650°C, the ferrite area fraction can be reduced, thereby achieving the desired TS and YR. Furthermore, the number density of precipitates with a size of 2 μm or more can be reduced. To achieve these 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, it is not possible to increase the boron atom concentration in prior austenite grain boundaries with a misorientation of 15 degrees or more. 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 set to 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 (second cooling)] After the first cooling, the obtained cold-rolled sheet is preferably further cooled (second cooling). In this process, 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. Note that, unless otherwise specified below, the average cooling rate at 650°C or less is preferably 5°C / s or more and 30°C / s or less. Furthermore, 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 (preferred condition)] 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 the 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. Note that there is no particular upper limit for the average cooling rate in the temperature range of 250°C to 400°C, but due to production technology constraints, it is preferably 100.0°C / s or less, and more preferably 80.0°C / s or less. When the cooling stop temperature exceeds 250°C, the average cooling rate is defined as the value in the temperature range of the cooling stop temperature to 400°C. Note that the average cooling rate is measured based on the temperature of the steel sheet surface.

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

[0089] [Heat retention temperature in cooling step: 100°C or higher and 450°C or lower (preferred conditions)] In the second cooling step, it is preferable to retain heat at a temperature in the range of 100°C or higher and 450°C or lower. By retaining the heat within the above range, it is possible to retain the YR, λ, and bendability within more preferable ranges for the high-strength steel sheet. In addition, it is possible to further reduce the area ratio of bainitic ferrite and further improve TS. The heat retention temperature in the second cooling step is more preferably 150°C or higher, and even more preferably 200°C or higher. In addition, the heat retention temperature in the second cooling step is more preferably 400°C or lower, and even more preferably 350°C or lower. The heat retention temperature is based on the surface temperature of the steel sheet.

[0090] [Heat retention time in cooling step: 5 seconds or more (preferred condition)] By retaining the heat at the heat retention temperature, the YR, λ, and bendability can be kept within a more preferable range. To achieve this effect, the heat retention time at the heat retention temperature is preferably 5 seconds or more, more preferably 10 seconds or more, and even more preferably 15 seconds or more. There is no particular upper limit to the heat retention time, but in order to keep TS within a more preferable range, it is preferably 500 seconds or less, more preferably 250 seconds or less.

[0091] [Cooling stop temperature: 250°C or less (preferred conditions)] In the second cooling step described above, the cooling stop temperature is preferably 250°C or less, more preferably 200°C or less. If the cooling stop temperature is 250°C or less, it is possible to prevent a large amount of retained austenite from being generated after annealing, and to further improve the YR, λ, and bendability. There is no particular restriction on the lower limit of the cooling stop temperature, but from the viewpoint of productivity, it is preferably room temperature or higher. 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 that can be used 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 within a desired range. Furthermore, the elongation rate of the rolling is preferably 2.00% or less, more preferably 1.00% or less. By setting the elongation rate of the rolling performed after cooling to the cooling stop temperature to 2.00% or less, the volume fraction of retained austenite can be set within a more suitable range, and the bendability and the degree of damage at the sheared edge in a corrosive environment can be set within more suitable ranges.

[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 discontinuous with the above-mentioned continuous annealing apparatus (offline). The target elongation may be achieved in a single rolling pass, or a total elongation of 0.05% to 2.00% may be achieved by performing multiple rolling passes. The rolling described here generally refers to temper rolling, but any other processing method, such as repeated bending using a tension leveler or rolls, may also be used as long as it can impart an elongation equivalent to that achieved by temper rolling.

[0096] [Reheating temperature: (cooling stop temperature + 50°C) or higher and 450°C or lower (preferred conditions)] After cooling to the cooling stop temperature, or after further rolling after cooling to the cooling stop temperature, the high-strength steel sheet may be reheated (reheating step). Reheating the high-strength steel sheet can bring the YR, λ, and bendability within more preferred ranges. To achieve this effect, the reheating temperature is preferably (cooling stop temperature + 50°C) or higher, more preferably (cooling stop temperature + 100°C) or higher, and even more preferably (cooling stop temperature + 150°C) or higher. On the other hand, as the reheating temperature increases, tempering of martensite progresses and TS decreases, so the reheating temperature is preferably 450°C or lower, more preferably 400°C or lower, and even more preferably 380°C or lower. Note that the reheating temperature is based on the surface temperature of the steel sheet.

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

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

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

[0100] [Method for manufacturing high-strength plated steel sheet] A high-strength plated steel sheet can be obtained by subjecting at least one surface of the high-strength steel sheet manufactured as described above to a plating treatment. For example, examples of the plating treatment include hot-dip galvanizing and hot-dip galvanizing followed by alloying. Annealing and galvanizing may also be performed continuously in a single line. Alternatively, the plating layer may be formed by electroplating such as Zn—Ni electrolytic alloy plating, or hot-dip zinc-aluminum-magnesium alloy plating. While the above description has focused on galvanizing, the type of plating metal, such as Zn plating or Al plating, is not particularly limited.

[0101] The high-strength steel sheet produced as described above can be subjected to a plating process to obtain a plated steel sheet. For example, examples of the plating process include hot-dip galvanizing and hot-dip galvanizing followed by alloying. Annealing and galvanizing may be performed continuously in one line. Alternatively, the plating layer may be formed by electroplating such as Zn—Ni electrolytic alloy plating, or hot-dip zinc-aluminum-magnesium alloy plating may be applied. While the above description focuses 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 immerse the high-strength steel sheet in a galvanizing bath at 440°C to 500°C, and then adjust the coating weight by gas wiping or the like. For hot-dip galvanizing, a galvanizing bath containing 0.10% by mass to 0.23% by mass of Al is preferably used. Furthermore, the temperature range for performing the galvanizing alloying treatment after hot-dip galvanizing is preferably 470°C to 600°C, more preferably 470°C to 560°C. By performing the alloying treatment at 470°C or higher, the Zn-Fe alloying rate is more favorable, and productivity is more favorable. Furthermore, by performing the alloying treatment at 600°C or lower, the transformation of untransformed austenite to pearlite is prevented, and TS is more favorable. Furthermore, electrogalvanizing may be performed. Furthermore, the coating weight is 20 to 80 g / m per side. ２(Double-sided plating) is preferred, and the alloyed hot-dip galvanized steel sheet (GA) is preferably subjected to the following alloying treatment to make the Fe concentration in the plating layer 7 to 15 mass %.

[0103] Regarding the plating treatment, after the annealing step, the high-strength steel sheet may be plated in 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 below 400°C and then the steel sheet temperature may be raised again to 400°C or more before being plated.

[0104] The high-strength steel sheet that has been subjected to the above-described plating treatment may be subjected to rolling. The elongation rate of the rolling is preferably 0.05% or more, more preferably 0.10% or more. By setting the elongation rate of the rolling performed after the plating treatment to 0.05% or more, it is possible to control the YR within a desired range. Furthermore, the elongation rate of the rolling is preferably 2.00% or less, more preferably 1.00% or less. By setting the elongation rate of the rolling in the plating treatment to 2.00% or less, it is possible to set the volume fraction of retained austenite within a more preferable range, and to set the bendability and the degree of damage at the sheared edge in a corrosive environment within more preferable ranges.

[0105] The rolling after the plating treatment may be performed on an apparatus continuous with the above-mentioned continuous annealing apparatus (online), or on an apparatus discontinuous with the above-mentioned continuous annealing apparatus (offline). The target elongation may be achieved in a single rolling pass, or multiple rolling passes may be performed to achieve a total elongation of 0.05% to 2.00%. The rolling described here generally refers to temper rolling, but any processing method involving repeated bending using a tension leveler or rolls may also be used as long as it can impart an elongation equivalent to that achieved by temper rolling. Furthermore, a reheating treatment may be performed after the rolling after the plating treatment.

[0106] Other conditions of the manufacturing method are not particularly limited, but from the viewpoint of productivity, it is preferable that the series of processes such as the annealing, hot-dip galvanizing, and alloying treatment of the galvanizing be carried out in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping can be performed to adjust the coating weight of the coating. Note that plating conditions other than those described above can be based on conventional hot-dip galvanizing methods.

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

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

[0109] [Members] Next, members according to one embodiment of the present invention will be described.

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

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

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

[0113]

[0114]

[0115]

[0116] Next, annealing, cooling, and reheating were performed under the conditions shown in Tables 2 and 3 to obtain high-strength cold-rolled steel sheets (CR). Furthermore, some of the steel sheets were subjected to a plating process to obtain hot-dip galvanized steel sheets (GI), galvannealed steel sheets (GA), and electrogalvanized steel sheets (EG). For the hot-dip galvanizing bath, a zinc bath containing 0.14 to 0.19 mass% Al was used for GI, and a zinc bath containing 0.14 mass% Al was used for GA, with the bath temperature set to 470°C. The coating weight for GI was 45 to 72 g / m per side. ２ (Both sides plating) and for GA, 45 g / m per side ２ The GA had an Fe concentration of 9% by mass or more and 12% by mass or less in the plating layer. The EG, which had a Zn—Ni plating layer, had an Ni content of 9% by mass or more and 25% by mass or less in the plating layer.

[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 sheet cracking resistance of the spot weld HAZ were evaluated according to the following test methods. The results are shown in Table 4.

[0118]

[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 sheets in a direction perpendicular to the rolling direction of the steel sheets, and the test pieces were rolled at a crosshead speed of 1.67 × 10 －１ Tensile tests were conducted at a speed of 1000 kJ / s, and the YS and TS were measured. In the present invention, a yield ratio (YR) of 65% or more and 90% or less was determined to be high dimensional accuracy. The YR was calculated using the formula (2) above.

[0120] [Hole Expanding Test] The hole expanding 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 using a die with an inner diameter of 75 mm with a blank holding force of 9 ton (88.26 kN), 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 expanding ratio: λ (%) was calculated using the following formula. Limit hole expanding ratio: λ (%) = {(D ｆ -D ０ ) / D ０}×100 where D ｆ is the hole diameter (mm) when the crack occurs, D ０ is the initial hole diameter (mm). When the limiting hole expanding ratio λ is 30% or more, it was determined that the stretch flangeability immediately after production was excellent.

[0121] [Bending Test] The bending test was conducted in accordance with JIS Z 2248:2022. From the obtained steel plate, strip-shaped test pieces with a width of 30 mm and a length of 100 mm were taken, with the axial direction of the bending test parallel to the rolling direction of the steel plate. Then, a 90° V-bending test was conducted under conditions of an indentation load of 100 kN and a holding time of 5 seconds. In this disclosure, bending properties were evaluated by conducting bending tests on five samples at an R where R / t, the value obtained by dividing the bending radius (R) by the plate thickness (t), was approximately 4.5, i.e., 4.3 to 4.7. Next, the crack length at the ridgeline of the bending apex of all five samples was evaluated, and samples with a crack length of 200 μm or less were judged to have excellent bendability. Here, the crack length was evaluated by measuring the ridgeline of the bending apex at a magnification of 40 to 160 times using a digital microscope (RH-2000: manufactured by Hirox Co., Ltd.).

[0122] [Spot weld HAZ inter-plate cracking test] A test piece was cut out to a thickness of 1.4 mm, a length of 30 mm and a width of 100 mm with the rolling direction as the longitudinal direction. A test piece was cut out to the same size, and the coating weight of the hot dip galvanized layer per side was 50 g / m ２ The sheet assembly was then overlapped with a test hot-dip galvanized steel sheet (100 mm diameter) to form a sheet assembly. Next, resistance welding was performed using a servomotor-pressurized, single-phase AC (50 Hz) resistance welding machine with an electrode tip diameter of 6 mm. The sheet assembly was tilted 5° relative to the electrode of the resistance welding machine, with a 1.5 mm clearance between the lower electrode and the lower steel sheet. Specifically, resistance welding was performed on the sheet assembly under the conditions of a pressure of 3.5 kN, a hold time of 0.16 or 0.20 seconds, and a welding current and welding time that resulted in a nugget diameter of 5.9 mm to form a sheet assembly with a weld. Next, the sheet assembly with the weld was cut in half to include the weld, and the cross section of the weld was observed with an optical microscope (200x magnification). The inter-steel sheet cracking resistance of the HAZ of the spot weld was evaluated according to the above criteria. In addition, similar evaluations were performed on some samples with sheet thicknesses of 0.8 mm and 2.3 mm.

[0123] According to the above-mentioned method, the area ratio of martensite and ferrite, the volume ratio of retained austenite, the boron atom concentration at prior austenite grain boundaries with a misorientation of 15 degrees or more, and the number density of precipitates with a size of 2 μm or more were determined. Furthermore, the remaining structure was observed by the method described below. A sample was cut out so that the observation surface was a thickness cross section (L cross section) parallel to the rolling direction of the steel sheet, and the observation surface was mirror-polished using diamond paste. Then, the sample was etched with 3 vol.% nital to reveal the structure. Using an SEM under conditions of an acceleration voltage of 15 kV, the observation position was set at 1 / 4 of the steel sheet thickness, and observation was performed at a magnification of 5000 times, with a field of view of 17 μm × 23 μm. Carbides were identified as the remaining structure from the obtained structure image.

[0124] As shown in Table 4, the inventive examples are excellent in TS, YR, λ, bendability, and inter-steel plate cracking resistance in the HAZ of the spot welds. On the other hand, the comparative examples are inferior in one or more of TS, YR, λ, bendability, and inter-steel plate cracking resistance in the HAZ of the spot welds.

[0125] Although the embodiments of the present invention have been described above, the present invention is not limited to the descriptions of the present embodiments, which form part of the disclosure of the present invention. In other words, other embodiments, examples, and operational techniques made by those skilled in the art based on the present embodiments are all included in the scope of the present invention. For example, in the series of heat treatments in the above-described manufacturing method, there are no particular limitations on the equipment used to perform the heat treatment on the steel sheet as long as the thermal history conditions are met.

[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 the HAZ of spot welds and which can be used to manufacture parts with high dimensional precision.

[0127] In particular, the high-strength steel sheet of the present invention has excellent resistance to inter-steel sheet cracking in the HAZ of spot welds, and therefore can be applied to automotive structural parts of various sizes and shapes while achieving high part strength, which can lead to improved fuel efficiency through reduced vehicle weight and has extremely high industrial value.

Claims

1. The steel sheet contains, by mass%, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0.002% or more and 0.200% or less, and B: 0.0002% or more and 0.0100% or less, and has an effective Ti mole fraction (x Ｔｉ，ｅｆｆ ) is 0.001 or more, with the balance consisting of Fe and unavoidable impurities; and at the 1 / 4 position of the sheet thickness, the steel structure has an area fraction of martensite of 60% to 99%, and an area fraction of ferrite and / or a volume fraction of retained austenite of more than 0% and 40% or less in total; and the boron atom concentration at prior austenite grain boundaries 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 high strength steel plate. Ｔｉ，ｅｆｆ = x Ｔｉ -x Ｎ -x Ｓ ... (1) In the formula, x Ｔｉ , x Ｎ , x Ｓ indicates the content (molar fraction) of each element in the steel sheet.

2. A high-strength steel plate as described in claim 1, wherein the chemical composition further contains, by mass%, at least one element selected from the following: 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.

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

4. A steel slab having the composition according to claim 1 or 2 is prepared, 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, the steel slab is then heated at an average heating rate of 25°C / min or less in the temperature range of 900°C to 1150°C to a slab heating temperature of 1150°C or more, with a residence time of 20 minutes or more from 1100°C to the slab heating temperature, the steel slab is then hot-rolled at 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, and the hot-rolled sheet is then pickled to obtain a pickled sheet, Next, the pickled sheet is cold-rolled at a cumulative reduction rate of 20% or more and 95% or less and at a sheet passing speed of 50 mpm or more in the final pass of cold rolling to obtain a cold-rolled sheet, then an annealing step is performed in which the cold-rolled sheet is heated to a heating temperature of 780°C or more, and then the cold-rolled sheet is first-cooled under conditions where the average cooling rate in a temperature range from the heating temperature to 650°C is 0.5°C / s or more and 50°C / s or less.

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

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

7. A method for producing a high strength steel sheet according to claim 4, further comprising the steps of: after the first cooling, setting the cooling stop temperature to 250°C or lower; and then performing a 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 maintained at the reheating temperature for 5 seconds or longer.

8. A method for producing a high-strength plated steel sheet, comprising the steps of: after the annealing step as recited in claim 4; carrying out a plating step of plating at least one surface of the cold-rolled sheet.

9. A method for producing a high-strength plated steel sheet according to claim 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. A method for producing a high-strength plated steel sheet according to claim 8, further comprising, after the first cooling, performing a second cooling in which the sheet is kept at a temperature of 100°C or higher and 450°C or lower for 5 seconds or more.

11. A method for producing a high-strength plated steel sheet according to claim 8, further comprising the steps of: after the first cooling, setting the cooling stop temperature to 250°C or lower; 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 is kept at the reheating temperature for 5 seconds or longer.

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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