Steel sheet, member, and methods for producing same

Abstract

Provided are: a high-strength steel sheet having excellent stretch flange properties and toughness; a member; and methods for manufacturing the steel sheet and the member.　The steel sheet has a component composition in which, in mass%, C, Si, Mn, P, Al, N, Ti, Nb, and B have specific contents, the total area ratio of martensite and bainite is 95% or more, the area ratio of retained austenite is 5% or less, the area ratio of ferrite is 1% or less, the prior austenite grain size is 10 μm or less, the C concentration in the prior austenite grain boundary is at least 1.5 times the C content in the steel, the B concentration in the prior austenite grain boundary is, in mass%, 0.05% or more, and the variation of the B concentration in the prior austenite grain boundary in the same grain boundary is, in mass%, less than 0.010%.

Description

Steel plates, components, and their manufacturing methods

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

[0002] Automotive steel sheets require not only strength but also excellent stretch flangeability. High toughness is also required so that automotive parts do not break during a collision. Patent Document 1 discloses a high-strength steel sheet with excellent formability and a manufacturing method thereof. Patent Document 2 discloses a high-strength cold-rolled steel sheet and a manufacturing method thereof.

[0003] JP 2013-147736 A JP 2008-106351 A

[0004] However, the steel sheets described in Patent Documents 1 and 2 both have a tensile strength TS of less than 1180 MPa, and stretch flangeability and toughness are not taken into consideration.

[0005] The present invention has been made in view of the above circumstances, and aims to provide a steel plate and a member having a tensile strength TS of 1180 MPa or more, high strength, and excellent stretch flangeability and toughness, as well as a method for manufacturing the same.

[0006] In the present invention, high strength means that the tensile strength TS measured in accordance with JIS Z2241 (2011) is 1180 MPa or more.

[0007] Furthermore, excellent stretch flangeability means that the hole expansion ratio measured in accordance with JIS Z 2256 (2010) is 30% or more.

[0008] Furthermore, excellent toughness means that the brittle-ductile transition temperature is −40° C. or lower in a Charpy impact test performed in accordance with JIS Z2242 (2018).

[0009] The present inventors have conducted extensive research to achieve the above-mentioned object, and as a result have obtained the following findings.

[0010] To obtain a tensile strength TS of 1,180 MPa or more, it is advisable to use martensite or bainite as the main phase. In this case, if the area ratio of ferrite is 1% or less, the total of martensite and bainite is 95% or more, and further the area ratio of retained austenite is 5% or less, void generation at the interface between the soft ferrite and the hard martensite or bainite is suppressed, and stretch flangeability is improved.

[0011] Furthermore, the addition of Nb refines the prior austenite grain size to 10 μm or less, improving toughness. Furthermore, the addition of B segregates at the prior austenite grain boundaries, strengthening the grain boundaries and improving toughness. Meanwhile, even in steels to which B is added, toughness may improve slightly or greatly, and the conditions for greatly improving toughness were investigated in more detail.

[0012] It was found that B can segregate non-uniformly or uniformly on prior austenite grain boundaries, and that toughness is significantly improved when it segregates at a uniform concentration. As an index of the uniformity of B segregation, the variation in the B concentration within the same prior austenite grain boundary was focused on, and toughness was significantly improved when this variation was less than 0.010% by mass.

[0013] Furthermore, the inventors investigated means for uniformly segregating B on prior austenite grain boundaries. They found that B segregates uniformly by performing the process of segregating B on grain boundaries twice. When a cold-rolled sheet is annealed once in the austenite region, B segregates on the austenite grain boundaries, but at this point, the diffusion of B is insufficient, and the segregated B is non-uniform. When this steel is cooled once to form martensite and bainite structures, and then annealed a second time, an austenite reverse transformation structure is formed. In this regard, if retained austenite is present after the first annealing and cooling, it serves as a nucleus to form an austenite structure with the same crystal orientation as in the first annealing. Furthermore, martensite and bainite contain a large number of dislocations, and B dissolved in them diffuses rapidly to the austenite grain boundaries through the dislocations during the second annealing, resulting in uniform segregation of B. In order to form retained austenite as nuclei before the second annealing, it is advisable to distribute carbon to the untransformed structure by partial quenching and distribution treatment after the first annealing, thereby stabilizing the austenite.

[0014] The present invention has been made based on the above findings. That is, the gist and configuration of the present invention are as follows. [1] A steel sheet containing, in mass%, C: 0.10% or more and 0.30% or less, Si: 0.20% or more and 1.20% or less, Mn: 2.5% or more and 4.0% or less, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.01% or less, Ti: 0.100% or less, Nb: 0.002% or more and 0.050% or less, and B: 0.0015% or more and 0.0040% or less, and satisfying the following formula (1) with the balance consisting of Fe and unavoidable impurities, wherein the sum of the area fractions of martensite and bainite is 95% or more, the area fraction of retained austenite is 5% or less, the area fraction of ferrite is 1% or less, and the prior austenite grain size is 10 μm or less, A steel sheet in which the carbon concentration at the prior austenite grain boundaries is 1.5 times or more the carbon content in the steel, the boron concentration at the prior austenite grain boundaries is 0.05% or more by mass, and the variation in the boron concentration at the prior austenite grain boundaries within the same grain boundary is less than 0.010% by mass: ([%N] / 14) / ([%Ti] / 47.9)<1.0...Equation (1) In equation (1), [%N] and [%Ti] represent the nitrogen and titanium contents (mass%) in the steel, respectively. [2] The steel sheet according to [1], wherein the chemical composition further contains, by mass%, at least one selected from V: 0.100% or less, Mo: 0.500% or less, Cr: 1.00% or less, Cu: 1.00% or less, Ni: 0.50% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.200% or less, W: 0.400% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Co: 0.020% or less, REM: 0.0200% or less, Te: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less. [3] The steel sheet according to [1] or [2], having a plating layer on at least one side. [4] A member made using the steel sheet according to any one of [1] to [3].[5] A hot rolling process in which a steel slab having the chemical composition according to [1] or [2] is hot rolled to obtain a hot rolled sheet; a pickling process in which the hot rolled sheet is pickled; a cold rolling process in which the hot rolled sheet after the pickling process is cold rolled to obtain a cold rolled sheet; a first annealing process in which the cold rolled sheet is heated to a first heating temperature of Ac3 point or higher; a cooling process in which cooling of the cold rolled sheet after the first annealing process is started from the first heating temperature and cooled to a cooling stop temperature of 100°C or higher but lower than Ms point at an average cooling rate of 50°C / s or higher; a first reheating process in which, after the cooling process, the cold rolled sheet is heated to a first reheating temperature of 300°C or higher and 400°C or lower, and held at the first reheating temperature for 60 seconds or longer to cool to room temperature; and a second annealing process in which, after the first reheating process, the cold rolled sheet is heated to a second heating temperature of Ac3 point or higher. and a second reheating step of cooling the steel sheet after the second annealing step, heating it to a second reheating temperature of 70°C or more and 200°C or less, and holding it at the second reheating temperature for 600 seconds or more to obtain the steel sheet. [6] A method for producing the steel sheet according to [5] above, comprising a plating step of plating the steel sheet after the second annealing step and before the second reheating step. [7] A method for producing the steel sheet according to [6] above, comprising an alloying step of alloying the steel sheet after the plating step. [8] A method for producing a member, comprising a step of subjecting the steel sheet according to any one of [1] to [3] above to at least one of forming and joining to form a member.

[0015] According to the present invention, it is possible to provide a steel plate and a member having a high strength with a tensile strength TS of 1180 MPa or more and excellent stretch flangeability and toughness, and a method for manufacturing the same.

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

[0017] The steel sheet according to this embodiment contains, in mass%, C: 0.10% or more and 0.30% or less, Si: 0.20% or more and 1.20% or less, Mn: 2.5% or more and 4.0% or less, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.01% or less, Ti: 0.100% or less, Nb: 0.002% or more and 0.050% or less, and B: 0.0015% or more and 0.0040% or less, and satisfies the following formula (1), with the balance being Fe and unavoidable impurities. The steel has a composition in which the sum of the area fractions of martensite and bainite is 95% or more, the area fraction of retained austenite is 5% or less, the area fraction of ferrite is 1% or less, the prior austenite grain size is 10 μm or less, the carbon concentration at the prior austenite grain boundary is 1.5 times or more the carbon content in the steel, the boron concentration at the prior austenite grain boundary is 0.05% or more by mass, and the variation in the boron concentration at the prior austenite grain boundary within the same grain boundary is less than 0.010% by mass. ([%N] / 14) / ([%Ti] / 47.9)<1.0...Equation (1) In equation (1), [%N] and [%Ti] represent the contents (% by mass) of N and Ti in the steel, respectively.

[0018] [Component Composition] First, the appropriate range of the component composition of the steel sheet and the reasons for its limitation will be described. In the following description, "%" representing the content of component elements in the steel sheet means "% by mass" unless otherwise specified. "ppm" means "ppm by 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: 0.10% or more and 0.30% or less C has the effect of strengthening the martensite-bainite structure. Furthermore, by reheating after the second annealing step, C segregates at the prior austenite grain boundaries, improving toughness. If the C content is less than 0.10%, the area ratio of martensite and bainite decreases, and a tensile strength TS (hereinafter sometimes simply referred to as TS) of 1180 MPa or more cannot be obtained. Therefore, the C content is set to 0.10% or more. The C content is preferably set to 0.11% or more. On the other hand, if the C content exceeds 0.30%, boron and iron carboborides are formed during annealing, making it impossible to segregate a sufficient amount of boron on the grain boundaries. Therefore, the C content is set to 0.30% or less. The C content is preferably set to 0.28% or less.

[0020] Si: 0.20% or more and 1.20% or less Si is an element effective for solid solution strengthening, and a content of 0.20% or more is required. Therefore, the Si content is set to 0.20% or more. The Si content is preferably set to 0.50% or more. On the other hand, Si is a ferrite stabilizing element, and if the content exceeds 1.20%, ferrite is formed, resulting in a decrease in strength, stretch flangeability, and toughness. Therefore, the Si content is set to 1.20% or less. The Si content is preferably set to 1.10% or less.

[0021] Mn: 2.5% or more and 4.0% or less Mn is effective in improving hardenability. If the Mn content is less than 2.5%, the area ratio of martensite and bainite decreases, resulting in a decrease in strength. Therefore, the Mn content is set to 2.5% or more. The Mn content is preferably set to 2.8% or more. On the other hand, if the Mn content exceeds 4.0%, the segregated portions become excessively hardened, resulting in a decrease in toughness. Therefore, the Mn content is set to 4.0% or less. The Mn content is preferably set to 3.5% or less.

[0022] P: 0.050% or less P segregates at prior austenite grain boundaries and reduces toughness, so the P content is set to 0.050% or less. The P content is preferably set to 0.025% or less. There is no particular lower limit for the P content, and it may be 0%, but a P content of less than 0.001% increases production costs, so a P content of 0.001% or more is preferred.

[0023] S: 0.020% or less S segregates at prior austenite grain boundaries and reduces toughness, so the S content is set to 0.020% or less. The S content is preferably set to 0.018% or less. The S content is more preferably set to 0.0040% or less, and further preferably set to 0.0020% or less. There is no particular lower limit for the S content, but since a content of less than 0.0001% increases production costs, it is preferably set to 0.0001% or more.

[0024] Al: 0.10% or less Al is an element that acts as a deoxidizer, and to obtain this effect, the Al content is preferably 0.005% or more. On the other hand, if the Al content exceeds 0.10%, ferrite is more likely to be formed, resulting in a decrease in strength. Therefore, the Al content is set to 0.10% or less. The Al content is preferably set to 0.05% or less.

[0025] N: 0.01% or less N forms nitrides with Nb and B, reducing the effects of adding Nb and B. Therefore, the N content is set to 0.01% or less. The N content is preferably set to 0.006% or less. Although there is no particular lower limit, from the viewpoint of production costs, the N content is preferably set to 0.0001% or more.

[0026] Ti: 0.100% or less Ti fixes N in steel as TiN, suppresses the formation of BN and NbN, improves the effects of adding Nb and B, and has the effect of improving toughness and stretch flangeability. To obtain these effects, the Ti content is preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.100%, coarse Ti carbides are formed on the grain boundaries, reducing toughness. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably set to 0.050% or less.

[0027] Nb: 0.002% or more and 0.050% or less Nb precipitates as a solid solution or as fine carbides, suppressing the growth of austenite grains during annealing. It also refines the grain size, complicating the fracture path and improving toughness. To achieve this effect, the Nb content is set to 0.002% or more. The Nb content is preferably set to 0.005% or more. On the other hand, if the Nb content exceeds 0.050%, not only does the effect saturate, but coarse Nb carbides precipitate, reducing toughness. Therefore, the Nb content is set to 0.050% or less. The Nb content is preferably set to 0.040% or less.

[0028] B: 0.0015% or more and 0.0040% or less B has the effect of segregating at prior austenite grain boundaries to increase grain boundary strength and improve toughness. To obtain such an effect, the B content is set to 0.0015% or more. The B content is preferably set to 0.0016% or more. On the other hand, if the B content exceeds 0.0040%, carboborides are formed, reducing toughness. Therefore, the B content is set to 0.0040% or less. The B content is preferably set to 0.0030% or less.

[0029] ([%N] / 14) / ([%Ti] / 47.9)<1.0...Equation (1) In order to obtain the above-mentioned effects of adding B and Nb, N, which easily bonds with these elements, needs to be fixed by Ti. For this purpose, the molar fraction of N is made smaller than the molar fraction of Ti. In other words, the contents of N and Ti in the steel are adjusted so as to satisfy the above equation (1). ([%N] / 14) / ([%Ti] / 47.9) is preferably 0.6 or less. In addition, in equation (1), [%N] and [%Ti] represent the contents (mass%) of N and Ti in the steel, respectively.

[0030] The balance other than the above-mentioned components is Fe and inevitable impurities. Note that, for the optional components described below, if the content is less than the lower limit, the effect of the present invention is not impaired, so if these optional elements are contained in an amount less than the lower limit, they are treated as inevitable impurities.

[0031] [Optional Components] In addition to the above-described component composition, the steel sheet according to this embodiment may further contain, in mass %, at least one element selected from V: 0.100% or less, Mo: 0.500% or less, Cr: 1.00% or less, Cu: 1.00% or less, Ni: 0.50% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.200% or less, W: 0.400% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Co: 0.020% or less, REM: 0.0200% or less, Te: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

[0032] V: 0.100% or less V has the effect of forming fine carbides and increasing strength. If the V content exceeds 0.100%, coarse V carbides may precipitate, resulting in a decrease in toughness. Therefore, when V is contained, the V content is set to 0.100% or less. The V content is preferably 0.080% or less, and more preferably 0.060% or less. There is no particular restriction on the lower limit of the V content, and it may be 0.000%. However, since V has the effect of forming fine carbides and increasing strength, it is preferably set to 0.001% or more. The V content is more preferably 0.005% or more, and even more preferably 0.010% or more.

[0033] Mo: 0.500% or less Mo has the effect of improving hardenability and increasing the area fraction of bainite and martensite. If the Mo content exceeds 0.500%, the effect saturates. Therefore, when Mo is contained, the Mo content is set to 0.500% or less. The Mo content is preferably 0.200% or less, and more preferably 0.150% or less. There is no particular limitation on the lower limit of the Mo content, and it may be 0.000%, but since Mo has the effect of improving hardenability and increasing the area fraction of bainite and martensite, it is preferably 0.010% or more. The Mo content is more preferably 0.020% or more, and even more preferably 0.030% or more.

[0034] Cr: 1.00% or less Cr has the effect of improving hardenability and increasing the area ratio of bainite and martensite. If the Cr content exceeds 1.00%, the effect saturates. Therefore, when Cr is contained, the Cr content is set to 1.00% or less. The Cr content is preferably 0.300% or less, and more preferably 0.250% or less. There is no particular limitation on the lower limit of the Cr content, and it may be 0.000%, but since Cr has the effect of improving hardenability and increasing the area ratio of bainite and martensite, it is preferably 0.01% or more. The Cr content is more preferably 0.015% or more, and even more preferably 0.030% or more.

[0035] Cu: 1.00% or less Cu has the effect of increasing strength through solid solution. Cu also has the effect of improving delayed fracture resistance. If the Cu content exceeds 1.00%, grain boundary cracking is more likely to occur. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. The Cu content is preferably 0.60% or less, and more preferably 0.30% or less. There is no particular limitation on the lower limit of the Cu content, and it may be 0.000%, but since Cu has the effect of increasing strength through solid solution, it is preferably set to 0.01% or more. The Cu content is more preferably 0.02% or more, and even more preferably 0.05% or more.

[0036] Ni: 0.50% or less Ni has the effect of improving hardenability, but if the Ni content exceeds 0.50%, the effect saturates. Therefore, when Ni is contained, the Ni content is set to 0.50% or less. The Ni content is preferably 0.20% or less, and more preferably 0.15% or less. The lower limit of the Ni content is not particularly limited and may be 0.00%, but since Ni has the effect of improving hardenability, it is preferably 0.01% or more. The Ni content is more preferably 0.02% or more, and even more preferably 0.03% or more.

[0037] Sb: 0.200% or less Sb has the effect of suppressing surface oxidation, nitriding, and decarburization of the steel sheet, but if the Sb content exceeds 0.200%, the effect saturates. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. The Sb content is preferably 0.050% or less, and more preferably 0.020% or less. There is no particular restriction on the lower limit of the Sb content, and it may be 0.000%. However, since Sb has the effect of suppressing surface oxidation, nitriding, and decarburization of the steel sheet, it is preferably set to 0.001% or more. The Sb content is more preferably 0.002% or more, and even more preferably 0.005% or more.

[0038] Sn: 0.200% or less Like Sb, Sn has the effect of suppressing surface oxidation, nitriding, and decarburization of the steel sheet. If the Sn content exceeds 0.200%, the effect saturates. Therefore, when Sn is contained, the Sn content is set to 0.200% or less. The Sn content is preferably 0.050% or less, and more preferably 0.020% or less. There is no particular limitation on the lower limit of the Sn content, and it may be 0.000%, but since Sn has the effect of suppressing surface oxidation, nitriding, and decarburization of the steel sheet, it is preferably 0.001% or more. The Sn content is more preferably 0.002% or more, and even more preferably 0.005% or more.

[0039] Ta: 0.200% or less Ta has the effect of forming fine carbides and increasing strength. If the Ta content exceeds 0.200%, coarse Ta carbides may precipitate, resulting in a decrease in toughness. Therefore, when Ta is contained, the Ta content is set to 0.200% or less. The Ta content is preferably 0.100% or less, and more preferably 0.070% or less. The lower limit of the Ta content is not particularly limited and may be 0.000%, but since Ta has the effect of forming fine carbides and increasing strength, it is preferably set to 0.001% or more. The Ta content is more preferably 0.005% or more, and even more preferably 0.010% or more.

[0040] W: 0.400% or less W has the effect of forming fine carbides and increasing strength. If the W content exceeds 0.400%, coarse W carbides may precipitate, resulting in a decrease in toughness. Therefore, when W is contained, the W content is set to 0.400% or less. The W content is preferably 0.300% or less, and more preferably 0.250% or less. There is no particular limitation on the lower limit of the W content, and it may be 0.000%, but since W has the effect of forming fine carbides and increasing strength, it is preferably set to 0.001% or more. The W content is more preferably 0.005% or more, and even more preferably 0.010% or more.

[0041] Zr: 0.0200% or less Zr has the effect of spheroidizing the shape of inclusions and suppressing stress concentration, thereby improving toughness. If the Zr content exceeds 0.0200%, a large amount of inclusions may be formed, resulting in a decrease in toughness. Therefore, when Zr is contained, the Zr content is set to 0.0200% or less. The Zr content is preferably 0.0150% or less, and more preferably 0.0100% or less. The lower limit of the Zr content is not particularly limited and may be 0.0000%, but since Zr has the effect of spheroidizing the shape of inclusions, suppressing stress concentration, and improving toughness, it is preferably 0.0001% or more. The Zr content is more preferably 0.0010% or more, and even more preferably 0.0020% or more.

[0042] Ca: 0.0200% or less Ca can be used as a deoxidizer. If the Ca content exceeds 0.0200%, a large amount of Ca-based inclusions may be formed, resulting in a decrease in toughness. Therefore, when Ca is contained, the Ca content is set to 0.0200% or less. The Ca content is preferably 0.0100% or less, and more preferably 0.0080% or less. The lower limit of the Ca content is not particularly limited and may be 0.0000%, but since Ca can be used as a deoxidizer, it is preferably 0.0001% or more. The Ca content is more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0043] Mg: 0.0200% or less Mg can be used as a deoxidizer. If the Mg content exceeds 0.0200%, a large amount of Mg-based inclusions may be formed, which may reduce toughness. Therefore, when Mg is contained, the Mg content is set to 0.0200% or less. The Mg content is preferably 0.0100% or less, and more preferably 0.0080% or less. There is no particular restriction on the lower limit of the Mg content, and it may be 0.0000%, but since Mg can be used as a deoxidizer, it is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0044] Co: 0.020% or less Co has the effect of increasing strength through solid solution strengthening. If the Co content exceeds 0.020%, the effect saturates. Therefore, when Co is contained, the Co content is set to 0.020% or less. The Co content is preferably 0.015% or less, and more preferably 0.010% or less. There is no particular limitation on the lower limit of the Co content, and it may be 0.000%, but since Co has the effect of increasing strength through solid solution strengthening, it is preferably 0.001% or more. The Co content is more preferably 0.002% or more, and even more preferably 0.005% or more.

[0045] REM: 0.0200% or less REM has the effect of spheroidizing the shape of inclusions and suppressing stress concentration, thereby improving toughness. If the REM content exceeds 0.0200%, a large amount of inclusions may be formed, resulting in a decrease in toughness. Therefore, when REM is contained, the REM content is set to 0.0200% or less. The REM content is preferably 0.0100% or less, and more preferably 0.0050% or less. There is no particular restriction on the lower limit of the REM content, and it may be 0.0000%, but since REM has the effect of spheroidizing the shape of inclusions and suppressing stress concentration, thereby improving toughness, it is preferably set to 0.0001% or more. The REM content is more preferably 0.0005% or more, and even more preferably 0.0010% or more. Here, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoid elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of one or more elements selected from the above REM. The REM preferably contains La, Ce, and Nd.

[0046] Te: 0.020% or less Te has the effect of spheroidizing the shape of inclusions and suppressing stress concentration, thereby improving toughness. If the Te content exceeds 0.020%, a large amount of inclusions may be formed, resulting in a decrease in toughness. Therefore, when Te is contained, the Te content is set to 0.020% or less. The Te content is preferably 0.015% or less, and more preferably 0.010% or less. There is no particular lower limit for the Te content, and it may be 0.000%, but since Te has the effect of spheroidizing the shape of inclusions and suppressing stress concentration, thereby improving toughness, it is preferably set to 0.001% or more. The Te content is more preferably 0.002% or more, and even more preferably 0.004% or more.

[0047] Hf: 0.10% or less Hf has the effect of spheroidizing the shape of inclusions and suppressing stress concentration, thereby improving toughness. If the Hf content exceeds 0.10%, a large amount of inclusions is formed, resulting in a decrease in toughness. Therefore, when Hf is contained, the Hf content is set to 0.10% or less. The Hf content is preferably 0.08% or less, and more preferably 0.05% or less. There is no particular lower limit for the Hf content, and it may be 0.000%, but since Hf has the effect of spheroidizing the shape of inclusions, suppressing stress concentration, and improving toughness, it is preferably set to 0.01% or more.

[0048] Bi: 0.200% or less Bi has the effect of reducing segregation and improving bendability. If the Bi content exceeds 0.200%, a large amount of inclusions may form, resulting in reduced bendability. Therefore, when Bi is contained, the Bi content is set to 0.200% or less. The Bi content is preferably 0.100% or less, more preferably 0.050% or less. The Bi content is further preferably 0.010% or less, and even more preferably 0.005% or less. There is no particular lower limit for the Bi content, and it may be 0.000%, but because Bi has the effect of reducing segregation and improving bendability, it is preferably 0.001% or more. The Bi content is more preferably 0.002% or more, and even more preferably 0.003% or more.

[0049] [Steel Structure] Next, the steel structure of the steel plate will be described.

[0050] Sum of area fractions of martensite and bainite: 95% or more Both martensite and bainite are hard phases and are necessary to achieve a TS of 1180 MPa or more. Therefore, the sum of the area fractions of martensite and bainite is set to 95% or more. The sum of the area fractions of martensite and bainite is preferably 96% or more. There is no particular upper limit for the sum of the area fractions of martensite and bainite, and it may be 100%.

[0051] Area fraction of retained austenite: 5% or less Retained austenite may be contained as a residual structure other than martensite and bainite. Therefore, the area fraction of retained austenite is set to 5% or less. The area fraction of retained austenite is preferably 4% or less. The area fraction of retained austenite may be 0% or more than 0%.

[0052] Ferrite area fraction: 1% or less If soft ferrite is present, voids are generated at the interface with the martensite and bainite parent phase, resulting in a decrease in stretch flangeability, so the ferrite area fraction is set to 1% or less. The ferrite area fraction may be 0%.

[0053] Here, the area ratio of each structure is measured as follows. The area ratio of retained austenite is determined by chemically polishing the rolled surface of a test piece taken from each steel plate up to the t / 4 position in the plate thickness of the steel plate, measuring the X-ray diffraction intensity and diffraction peak position of the polished surface using an X-ray diffraction (XRD) device, calculating the volume ratio, and using this value as the area ratio of retained austenite. Next, a cross section of the plate thickness parallel to the rolling direction of each steel plate is polished and then corroded with 3 vol% nital, and the t / 4 position in the plate thickness is used as the observation surface. SEM images of the observation surface are taken at a magnification of 2000 times, with a field of view of 57.1 μm × 42.9 μm, and from three fields of view. The obtained SEM images are analyzed to determine the total area ratio of martensite, bainite, and retained austenite, as well as the area ratio of the structure other than martensite, bainite, and retained austenite (ferrite). The area fractions of martensite and bainite are calculated by subtracting the area fraction of retained austenite obtained by XRD from the area fractions of martensite, bainite, and retained austenite obtained by image analysis. The average value of the three fields of view is taken as the area fraction of the structure.

[0054] Prior austenite grain size: 10 μm or less Complicating the crack propagation path makes it possible to improve toughness. To achieve this effect, the prior austenite grain size needs to be 10 μm or less. Therefore, the prior austenite grain size is set to 10 μm or less. The prior austenite grain size is preferably 9 μm or less. There is no particular lower limit on the grain size of the prior austenite grains, but from the viewpoint of production technology, the prior austenite grain size is preferably 1 μm or more. Furthermore, the prior austenite grain size is more preferably 2 μm or more, and even more preferably 3 μm or more.

[0055] Here, the grain size of the prior austenite grains is measured as follows. A cross section of each steel sheet parallel to the rolling direction is polished and then etched with picral to obtain an observation surface. On the observation surface, the microstructure at a position t / 4 of the sheet thickness is photographed with an SEM at a magnification of 2000 times, with a field of view of 57.1 μm × 42.9 μm, and three fields of view are obtained to obtain SEM images. The grain size of each prior austenite grain is determined from the obtained structural image by image analysis, and the average value of the three fields of view is taken as the grain size (average crystal grain size) of the prior austenite grains.

[0056] C concentration at prior austenite grain boundaries: 1.5 times or more the C content in steel Like B, C segregates at prior austenite grain boundaries to strengthen the grain boundaries and improve toughness. The above effect can be achieved if the C concentration at the prior austenite grain boundaries is 1.5 times or more the C content in the steel. Therefore, the C concentration at the prior austenite grain boundaries is set to 1.5 times or more the C content in the steel. That is, the C concentration at the prior austenite grain boundaries satisfies the following formula: C concentration at prior austenite grain boundaries (mass%) / C content in steel (mass%)≧1.5 The C concentration at the prior austenite grain boundaries is preferably 2.0 times or more, more preferably 2.5 times or more, the C content in the steel. While there is no upper limit for the C concentration at the prior austenite grain boundaries, it is preferably less than 7% by mass to effectively prevent the precipitation of hard carbides or carboborides on the grain boundaries and further improve toughness. More preferably, it is 2% or less by mass.

[0057] B concentration at prior austenite grain boundaries: 0.05% or more by mass B segregates at the prior austenite grain boundaries, strengthening the grain boundaries and improving toughness. If the B concentration at the prior austenite grain boundaries is 0.05% or more by mass, the above effects can be obtained. Therefore, the B concentration at the prior austenite grain boundaries is set to 0.05% or more by mass. The B concentration at the prior austenite grain boundaries is preferably 0.07% or more by mass, and more preferably 0.10% or more by mass. There is no upper limit to the B concentration at the prior austenite grain boundaries, but in order to suitably prevent the precipitation of hard carboborides on the grain boundaries and further improve toughness, it is preferably less than 6% by mass. More preferably, it is 2% or less by mass.

[0058] Variation in B concentration within the same grain boundary at prior austenite grain boundaries: less than 0.010% by mass To improve toughness, in addition to the B concentration on the grain boundaries, it is also important that the B concentration be uniform regardless of the grain boundary. If the variation is 0.010% by mass or more, strength differences occur within the grain boundaries, making cracks more likely to occur in areas where the B concentration is locally low, thereby reducing the toughness improvement effect. Therefore, the variation in B concentration within the same grain boundary at prior austenite grain boundaries is set to less than 0.010% by mass. The variation is preferably 0.009% by mass or less, and more preferably 0.008% by mass or less. While smaller variations are preferable, from the viewpoint of production technology, the variation should be 0.001% or more.

[0059] Here, the C concentration, B concentration, and variation at the prior austenite grain boundary are measured as follows. A needle-shaped sample is prepared from a region including the prior austenite grain boundary by the SEM-FIB (Focused Ion Beam) method. The obtained needle-shaped sample is subjected to 3DAP analysis using a 3DAP device (LEAP4000XSi, manufactured by AMETEK). The measurement is performed in laser mode. The sample temperature is set to 80 K or less. The C and B concentrations at the prior austenite grain boundary are determined from the number of C ions, the number of B ions, and the number of other ions detected at the prior austenite grain boundary. The C and B concentrations are taken as the average values ​​of two samples. Furthermore, five non-overlapping circular regions with a diameter of 5 nm to 10 nm were set on the plane of the measured prior austenite grain boundary, and a cylindrical volume extending in the normal direction of the grain boundary was set for each circle. The B concentration on the grain boundary for each cylinder was determined, and the standard deviation was taken as the variation in B concentration. In the above measurement, since the prior austenite grain size is much larger than the region sampled by the SEM-FIB method, all grain boundaries targeted by a single sampled sample are the same grain boundary. For example, while the prior austenite grain size is about 9 μm, the diameter of the sampled region is about 0.1 μm. Therefore, the variation in the obtained B concentration is the variation within the same grain boundary.

[0060] According to the present invention, it is possible to provide a steel sheet having a tensile strength TS of 1180 MPa or more. The tensile strength TS of the steel sheet is preferably 1250 MPa or more.

[0061] The steel sheet may have a plating layer on at least one surface. The plating layer is preferably any one of a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, and an electrogalvanized layer. The composition of the plating layer is not particularly limited and may be a known composition.

[0062] The composition of the hot-dip galvanized layer is not particularly limited, and may be any common composition. In one example, the coating layer contains 20 mass% or less of Fe, 0.001 mass% to 1.0 mass% of Al, and further contains 0 mass% to 3.5 mass% in total of one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with the balance being Zn and unavoidable impurities. When the coating layer is a hot-dip galvanized layer, the Fe content in the coating layer is, in one example, less than 7 mass%, and when the coating layer is an alloyed hot-dip galvanized layer, the Fe content in the coating layer is, in one example, 7 mass% to 15 mass%, more preferably 8 mass% to 13 mass%.

[0063] The coating weight is not particularly limited, but the coating weight per one side of the steel sheet is preferably 20 to 80 g / m ２ In one example, the plating layer is formed on both the front and back surfaces of a steel sheet (high-strength cold-rolled steel sheet).

[0064] The method for producing a steel sheet according to this embodiment includes a hot rolling step of hot rolling a steel slab having the above-described chemical composition to obtain a hot-rolled sheet, a pickling step of pickling the hot-rolled sheet, a cold rolling step of cold-rolling the hot-rolled sheet after the pickling step to obtain a cold-rolled sheet, a first annealing step of heating the cold-rolled sheet to a first heating temperature of Ac3 point or higher, and a cooling step of starting cooling the cold-rolled sheet after the first annealing step from the first heating temperature to a cooling stop temperature of 100°C or higher but lower than Ms point at a rate of 50°C / s or faster. a first reheating step of, after the cooling step, heating to a first reheating temperature of 300°C or more and 400°C or less, holding at the first reheating temperature for 60 seconds or more, and cooling to room temperature; a second annealing step of, after the first reheating step, heating to a second heating temperature of Ac3 point or more; and a second reheating step of, after the second annealing step, cooling the steel sheet, heating to a second reheating temperature of 70°C or more and 200°C or less, and holding at the second reheating temperature for 600 seconds or more to obtain a steel sheet.

[0065] First, a steel slab having the above-described chemical composition is manufactured. Here, an example of the manufacturing conditions for the steel slab before the hot rolling process is described. First, a steel material is melted to produce molten steel having the above-described chemical composition. The melting method is not particularly limited, and any known melting method, such as converter melting or electric furnace melting, is suitable. The resulting molten steel is solidified to produce a steel slab. The method for producing a steel slab from molten steel is not particularly limited, and a continuous casting method, an ingot casting method, a thin slab casting method, or the like can be used. The steel slab may be cooled and then reheated before hot rolling, or the cast steel slab may be continuously hot rolled without being cooled to room temperature. The slab heating temperature is preferably 1100°C or higher and 1300°C or lower, taking into account the rolling load and scale generation. The slab heating method is not particularly limited, and for example, it can be heated in a heating furnace according to a conventional method.

[0066] [Hot Rolling Process] The above-described steel slab is hot rolled to obtain a hot-rolled sheet. There are no particular limitations on the hot rolling, and it may be performed according to a conventional method. There are also no particular limitations on the cooling after hot rolling, and it is cooled to the coiling temperature. Next, the hot-rolled sheet is wound into a coil. The coiling temperature is preferably 400°C or higher. This is because a coiling temperature of 400°C or higher facilitates coiling without increasing the strength of the hot-rolled sheet. The coiling temperature is more preferably 550°C or higher. Furthermore, in order to suitably prevent the formation of thick scale and further improve yield, the coiling temperature is preferably 750°C or lower. Note that the hot-rolled sheet may be heat-treated for the purpose of softening before pickling.

[0067] [Pickling Process] After the hot rolling process, the hot-rolled sheet is pickled in the pickling process. In the pickling process, scale can be removed from the hot-rolled sheet wound around a coil. There are no particular restrictions on the method for removing the scale, but in order to completely remove the scale, it is preferable to perform pickling while uncoiling the hot-rolled coil. There are no particular restrictions on the pickling method, and a conventional method may be used.

[0068] [Cold Rolling Step] After the pickling step, in the cold rolling step, the hot rolled sheet is cold rolled to obtain a cold rolled sheet. For example, the hot rolled sheet from which the scale has been removed is appropriately cleaned and then cold rolled to obtain a cold rolled sheet. The cold rolling method is not particularly limited and may be a conventional method.

[0069] [First Annealing Step: Heating to a First Heating Temperature of Ac3 Point or Higher] Next, in the first annealing step, the cold-rolled sheet is heated to a first heating temperature of Ac3 point or higher and annealed in the austenite single-phase region. If the first heating temperature is lower than Ac3 point, ferrite is generated. This ferrite has fewer dislocations during annealing, and there are no dislocations that serve as diffusion paths for B (boron) during the second annealing, making it difficult to uniformly segregate boron. Furthermore, excessive heating increases the austenite grain size to more than 10 μm. Therefore, the first heating temperature is set to Ac3 point or higher. The first heating temperature is preferably Ac3 point + 10°C or higher, more preferably Ac3 point + 20°C or higher. Since the austenite structure generated in the second annealing has the same crystalline structure as the austenite structure generated in the first annealing, the first heating temperature is preferably set to 980°C or lower so that the austenite grain size is 10 μm or less even during the first annealing. The first heating temperature is more preferably 950°C or lower. The Ac3 point is calculated using the following formula: Ac3 (°C) = 881 - 206 x [%C] + 53 x [%Si] - 15 x [%Mn] - 27 x [%Cu] - 20 x [%Ni] - 1 x [%Cr] + 41 x [%Mo] (In the above formula, [%M] is the content (mass%) of element M in the steel sheet, and the value of an element that is not contained is 0 (zero).)

[0070] [Cooling step: Starting from the first heating temperature, cooling is performed at an average cooling rate of 50°C / s or more to a cooling stop temperature of 100°C or more but less than the Ms point.] [First reheating step: Heating to a first reheating temperature of 300°C or more but less than 400°C, holding at the first reheating temperature for 60 seconds or more, and then cooling to room temperature.] After the first annealing step, the cold-rolled sheet is subjected to partial quenching and partitioning in the cooling step to create a structure in which retained austenite exists at the prior austenite grain boundaries. If the cooling stop temperature for partial quenching is less than 100°C, martensitic transformation occurs before C partitioning occurs, making it impossible to obtain a sufficient amount of retained austenite before the second annealing. If the amount of retained austenite is insufficient, austenite with a different orientation from that in the first annealing is generated, making it difficult to uniformly segregate B (boron) at the grain boundaries of such austenite. Therefore, the cooling stop temperature in the cooling step is set to 100°C or higher. The cooling stop temperature is preferably 120°C or higher, and more preferably 150°C or higher. On the other hand, if the cooling stop temperature is above the Ms point, martensitic transformation does not occur, carbon distribution does not occur during subsequent reheating, and retained austenite does not form before the second annealing. If the amount of retained austenite is insufficient, austenite with a different orientation than that formed during the first annealing will form, making it difficult to uniformly segregate B (boron) at the grain boundaries of such austenite. Therefore, the cooling stop temperature in the cooling process is set to below the Ms point. The cooling stop temperature is preferably the Ms point minus 20°C or lower, and more preferably the Ms point minus 30°C or lower. The Ms point is calculated using the following formula: Ms (°C) = 499 - 308 x [C] - 10.8 x [Si] - 32.4 x [Mn] - 16.2 x [Ni] - 27 x [Cr] - 10.8 x [Mo] (In the above formula, [M] is the content (mass%) of element M in the steel sheet, and the value of an element that is not contained is 0 (zero).)

[0071] If the average cooling rate is less than 50°C / s, ferrite transformation occurs. Since this ferrite does not have a specific orientation relationship with austenite, the second annealing generates austenite with a different orientation from that in the first annealing, resulting in non-uniform B on the grain boundaries. Therefore, the average cooling rate is set to 50°C / s or more. The average cooling rate is preferably 60°C / s or more, and more preferably 70°C / s or more. There is no particular limit to the upper limit of the average cooling rate, but if the cooling rate is too high, it becomes difficult to control the cooling stop temperature. Therefore, the average cooling rate is preferably 1000°C / s or less, and more preferably 200°C / s or less. The average cooling rate (°C / s) in the cooling step is "(first heating temperature (°C)) - (cooling stop temperature (°C)) / (cooling time (seconds) from the first heating temperature (°C) to the cooling stop temperature (°C))".

[0072] If the first reheating temperature is less than 300°C, carbon distribution will be insufficient, and retained austenite will not be formed before the second annealing. If the amount of retained austenite is insufficient, austenite with a different orientation from that in the first annealing will be formed, and it will be difficult to uniformly segregate B (boron) at the grain boundaries of such austenite. Therefore, the first reheating temperature is set to 300°C or higher. The first reheating temperature is preferably 310°C or higher, and more preferably 320°C or higher. On the other hand, if the first reheating temperature exceeds 400°C, untransformed austenite will decompose into cementite, and retained austenite will not be formed before the second annealing. If the amount of retained austenite is insufficient, austenite with a different orientation from that in the first annealing will be formed, and it will be difficult to uniformly segregate B (boron) at the grain boundaries of such austenite. Therefore, the first reheating temperature is set to 400°C or lower. The first reheating temperature is preferably 390°C or lower, and more preferably 380°C or lower.

[0073] Furthermore, if the holding time at the first reheating temperature (reheating holding time) is less than 60 seconds, carbon distribution is insufficient and retained austenite is not formed before the second annealing. If the amount of retained austenite is insufficient, austenite with a different orientation from that in the first annealing is formed, and it is difficult to uniformly segregate B (boron) at the grain boundaries of such austenite. Therefore, the holding time at the first reheating temperature is set to 60 seconds or more. The holding time is preferably 80 seconds or more, and more preferably 100 seconds or more. On the other hand, although there is no particular upper limit to the holding time, if the holding time is 900 seconds or more, untransformed austenite may decompose into cementite, and austenite may not be formed before the second annealing. Therefore, the holding time at the first reheating temperature is preferably less than 900 seconds, and more preferably 600 seconds or less.

[0074] After being held at the first reheating temperature, the mixture is cooled to room temperature. The room temperature at which the cooling is stopped in the first reheating step is not particularly limited, but may be 5 to 50°C.

[0075] [Second Annealing Step: Heating to a Second Heating Temperature of Ac3 Point or Higher] In the second annealing step, the steel sheet (cold-rolled sheet) obtained as described above is again heated to a second heating temperature of Ac3 point or higher and annealed. During this process, austenite having the same orientation as the austenite formed in the first annealing is formed using retained austenite as nuclei. During this formation process, B (boron) dissolved through dislocations in martensite before austenite transformation rapidly diffuses to the austenite grain boundaries, resulting in uniform boron segregation. If the second heating temperature is lower than the Ac3 point, ferrite is formed and strength is reduced. Therefore, the second heating temperature is set to the Ac3 point or higher. The second heating temperature is preferably the Ac3 point + 10°C or higher, more preferably the Ac3 point + 20°C or higher. On the other hand, excessive heating increases the austenite grain size to more than 10 μm, reducing toughness, so the second heating temperature is preferably 980°C or lower. The second heating temperature is more preferably 950°C or lower. After heating to the second heating temperature, the steel sheet may be cooled to room temperature in order to generate a sufficient amount of martensite and then temper it in the second reheating step described below, or may be subjected to overaging treatment using the facilities of an annealing line.

[0076] [Second Reheating Step: After the Second Annealing Step, the Steel Sheet is Cooled, Heated to a Second Reheating Temperature of 70°C or More and 200°C or Less, and Maintained at the Second Reheating Temperature for 600 Seconds or More] In the second reheating step, after the second annealing step, the steel sheet is cooled, heated to a second reheating temperature of 70°C or More and 200°C or Less, and maintained at the second reheating temperature for 600 seconds or more. The cooling conditions before heating (reheating) here are not particularly limited, but from the viewpoint of suppressing ferrite transformation and suppressing strength reduction, it is preferable to cool at an average cooling rate of 30°C / s or more. It is also preferable to cool to room temperature. The average cooling rate (°C / s) in this step is "(Second Heating Temperature (°C)) - (Cooling Stop Temperature (°C)) / (Cooling Time (Second Heating Temperature (°C)) from Cooling Stop Temperature (°C))". After cooling, the steel sheet is heated (reheated) to the second reheating temperature. In addition to B, C segregates to prior γ grain boundaries by reheating, improving toughness. If the second reheating temperature is less than 70°C, the diffusion of C is slow and C segregation is insufficient. Therefore, the second reheating temperature is set to 70°C or higher. The second reheating temperature is preferably 90°C or higher. On the other hand, if the second reheating temperature exceeds 200°C, tempering proceeds excessively, carboborides precipitate, the B concentration at the prior austenite grain boundaries cannot be made 0.05 mass% or higher, and toughness decreases. Therefore, the second reheating temperature is set to 200°C or lower. The second reheating temperature is preferably 190°C or lower. If the holding time at the second reheating temperature (reheating holding time) is less than 600 seconds, the diffusion of C is slow and C segregation is insufficient. Therefore, the holding time at the second reheating temperature is set to 600 seconds or higher. The holding time at the second reheating temperature is preferably 800 seconds or higher. There is no particular upper limit to the holding time at the second reheating temperature, but in order to prevent precipitation of carboborides, the second reheating temperature is preferably 43,200 s or less (0.5 days) or less.

[0077] [Plating step, alloying step] After the second annealing step and before the second reheating step, a plating treatment may be performed on at least one surface of the steel sheet in a plating step to obtain a steel sheet (high-strength plated steel sheet). Alternatively, after the plating step, the steel sheet (high-strength plated steel sheet) may be subjected to a heat treatment to alloy the plating layer of the steel sheet to obtain an alloyed-plated steel sheet.

[0078] The manufacturing conditions other than those mentioned above can be the same as those in the ordinary method.

[0079] The steel sheet according to this embodiment obtained as described above preferably has a thickness of 0.5 mm or more, and more preferably 2.0 mm or less.

[0080] [Component] In this embodiment, a component can be provided that is at least partially made of the above-described steel sheet. In one example, the above-described steel sheet can be formed into a desired shape by press working to form an automotive component. Note that the automotive component may contain a steel sheet other than the steel sheet according to this embodiment as a raw material. According to this embodiment, a high-strength steel sheet having a TS of 1180 MPa or more and having both excellent stretch-flange formability and toughness can be provided, and thus a component having a TS of 1180 MPa or more and having both excellent stretch-flange formability and toughness can be provided. The steel sheet according to this embodiment can be suitably used as an automotive component that contributes to reducing the weight of the vehicle body. The steel sheet according to this embodiment can be suitably used in general components used as automotive components, particularly as frame structural components or reinforcing components.

[0081] The manufacturing method of the above-mentioned member includes a step of subjecting the above-mentioned steel plate to at least one of forming and joining to form the member. The forming can be performed by a general processing method such as press working, without any restrictions. The joining can be performed by general welding, such as spot welding and arc welding, or by riveting, caulking, or the like, without any restrictions.

[0082] A steel having the chemical composition shown in Table 1, with the balance being Fe and unavoidable impurities, was melted in a converter to obtain a steel slab. The obtained slab was reheated and hot-rolled, and then coiled to obtain a hot-rolled coil (hot-rolled sheet). Next, the hot-rolled coil was pickled while uncoiling, and cold-rolled to obtain a cold-rolled sheet. The thickness of the hot-rolled sheet was 3.0 mm, and the thickness of the cold-rolled sheet was 1.2 mm. Annealing (first annealing step, cooling step, first reheating step, second annealing step) was performed in a continuous hot-dip galvanizing line under the conditions shown in Table 2 to obtain steel sheets (cold-rolled steel sheet (CR), hot-dip galvanized steel sheet (GI), and galvannealed steel sheet (GA)). The hot-dip galvanized steel sheet was immersed in a galvanizing bath at 460°C, and a galvannealing rate of 35 g / m per side was applied. ２ The coating weight of the galvannealed steel sheet was 45 g / m per side. ２ After adjusting the coating weight to 1000 ppm, the steel sheets were subjected to an alloying treatment by holding the steel sheets at 520°C for 40 seconds. The steel sheets obtained, except for Steel Sheet No. 9, were subjected to a reheating treatment (second reheating step) under the conditions shown in Table 2.

[0083]

[0084]

[0085] The steel sheets thus obtained were evaluated for the total area fraction of martensite and bainite, the area fraction of retained austenite, the area fraction of ferrite, the prior austenite grain size, the carbon concentration at the prior austenite grain boundaries, the boron concentration at the prior austenite grain boundaries, and the variation in the boron concentration at the same grain boundary, according to the methods described above. Furthermore, the tensile strength (TS), stretch flangeability, and toughness were evaluated according to the methods described below. The results are shown in Table 3.

[0086] [Tensile test] The obtained steel sheets were subjected to a tensile test in accordance with JIS Z 2241 (2011). JIS No. 5 tensile test pieces were taken with the longitudinal direction perpendicular to the rolling direction, and the tensile test was performed to measure the tensile strength (TS). A tensile strength TS of 1180 MPa or more was determined to be good.

[0087] [Charpy impact test] Charpy impact tests were performed in accordance with JIS Z 2242 (2018). Test specimens were taken from the obtained steel plates. Each specimen had a width of 10 mm, a length of 55 mm, and a 90° V-notch at the center of the length with a notch depth of 2 mm, with the V-notch direction perpendicular to the rolling direction of the steel plate. Charpy impact tests were then performed in a test temperature range of -120 to +120°C. A transition curve was calculated from the obtained brittle fracture surface area ratio, and the temperature at which the brittle fracture surface area ratio reached 50% was determined as the brittle-ductile transition temperature. A brittle-ductile transition temperature of -40°C or lower obtained from the Charpy test was deemed to have good toughness. In the table, a brittle-ductile transition temperature of -40°C or lower was deemed to have excellent toughness, and a brittle-ductile transition temperature of above -40°C was deemed to have poor toughness.

[0088] A 100 mm wide x 100 mL test piece was taken from a steel sheet (cold-rolled steel sheet or plated steel sheet) and subjected to a hole expansion test in accordance with JIS Z 2256 (2010). A 10 mm diameter hole was punched into the test piece with a clearance of 12 ± 1%, and a conical punch with an apex angle of 60° was raised to expand the hole. The punch was stopped from rising when a crack occurred in the thickness direction. The hole expansion ratio λ was calculated from the hole diameter after crack initiation and the hole diameter before the test according to the following formula: Critical hole expansion ratio: λ (%) = {(Df - D0) / D0} × 100 (where Df is the hole diameter (mm) at the time of crack initiation, and D0 is the initial hole diameter (mm)). Regardless of the strength of the steel sheet, a λ value of 30% or more was determined to have good stretch flangeability.

[0089]

[0090] As can be seen from Table 3, the inventive examples have a tensile strength TS of 1180 MPa or more, and are excellent in stretch flangeability and toughness. On the other hand, the comparative examples are inferior in at least one of tensile strength TS, stretch flangeability and toughness.

[0091] Furthermore, it was found that the steel sheets of the present invention were used to produce components obtained by forming, joining, and further formed and joined, and that the steel sheets of the present invention have high strength and excellent stretch flange formability and toughness, similar to the steel sheets of the present invention.

Claims

1. In mass percent, C: 0.10% or more and 0.30% or less, Si: 0.20% or more and 1.20% or less, Mn: 2.5% or more and 4.0% or less, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.01% or less, Ti: 0.100% or less, Nb: 0.002% or more and 0.050% or less, and B: 0.0015% or more and 0.0040% or less and has a composition satisfying the following formula (1), with the balance being Fe and unavoidable impurities: The sum of the area ratios of martensite and bainite is 95% or more, The area ratio of retained austenite is 5% or less, The area ratio of ferrite is 1% or less, The prior austenite grain size is 10 μm or less, The C concentration at the prior austenite grain boundary is 1.5 times or more the C content in the steel, The B concentration in the prior austenite grain boundary is 0.05% or more by mass%, A steel sheet in which the variation in B concentration in prior austenite grain boundaries within the same grain boundary is less than 0.010% in mass percent. ([%N] / 14) / ([%Ti] / 47.9)<1.0...Formula (1) In formula (1), [%N] and [%Ti] respectively represent the N and Ti contents (mass%) in the steel.

2. The composition further includes, in mass%, V: 0.100% or less, Mo: 0.500% or less, Cr: 1.00% or less, Cu: 1.00% or less, Ni: 0.50% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.200% or less, W: 0.400% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Co: 0.020% or less, REM: 0.0200% or less, Te: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less The steel sheet according to claim 1, comprising at least one selected from the following:

3. The steel sheet according to claim 1 or 2, having a plating layer on at least one surface.

4. A member made using the steel sheet according to claim 1 or 2.

5. A component made using the steel plate described in claim 3.

6. A hot rolling process in which a steel slab having the composition according to claim 1 or 2 is hot rolled to obtain a hot rolled sheet; A pickling process for pickling the hot-rolled sheet; A cold rolling process in which the hot-rolled sheet after the pickling process is cold-rolled to obtain a cold-rolled sheet; A first annealing step of heating the cold-rolled sheet to a first heating temperature of Ac3 point or higher; A cooling step in which cooling is started from the first heating temperature to a cooling stop temperature of 100 ° C. or higher and lower than the Ms point at an average cooling rate of 50 ° C. / s or higher for the cold-rolled sheet after the first annealing step; After the cooling step, a first reheating step of heating to a first reheating temperature of 300 ° C. or more and 400 ° C. or less, holding the first reheating temperature for 60 s or more, and cooling to room temperature; After the first reheating step, a second annealing step of heating to a second heating temperature of Ac3 point or higher; After the second annealing step, the steel sheet is cooled, heated to a second reheating temperature of 70 ° C. or more and 200 ° C. or less, and held at the second reheating temperature for 600 s or more to obtain a steel sheet. A method for producing a steel sheet.

7. The method for producing a steel sheet according to claim 6, further comprising a plating step of plating the steel sheet after the second annealing step and before the second reheating step.

8. The method for producing a steel sheet according to claim 7 , further comprising, after the plating step, an alloying step of subjecting the steel sheet to an alloying treatment.

9. A method for manufacturing a component, comprising the step of subjecting the steel plate according to claim 1 or 2 to at least one of forming and joining to form a component.

10. A method for manufacturing a component, comprising the step of subjecting the steel plate described in claim 3 to at least one of forming processing and joining processing to form a component.