Steel plates, components, and methods for manufacturing them.

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

Technical Field

[0001] The present invention relates to steel plates, members, and methods for manufacturing them.

Background Art

[0002] In automotive steel plates, excellent stretch flangeability in addition to strength is required. In addition, high toughness is necessary so that automotive parts do not break during a collision. In Patent Document 1, a high-strength steel plate excellent in workability and a method for manufacturing the same are disclosed. In Patent Document 2, a high-strength cold-rolled steel plate and a method for manufacturing the same are disclosed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the steel plates described in Patent Document 1 and Patent Document 2, the tensile strength TS is less than 1180 MPa in both cases, and stretch flangeability and toughness are not taken into consideration.

[0005] The present invention has been made in view of such circumstances, and an object thereof is to provide a steel plate, a member, and a method for manufacturing them, which have a tensile strength TS of 1180 MPa or more, have high strength, and are excellent in stretch flangeability and toughness.

[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 flange properties mean that the hole expansion ratio, as measured in accordance with JIS Z 2256 (2010), is 30% or more.

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

[0009] In order to achieve the above-mentioned objectives, the inventors conducted extensive research and obtained the following findings.

[0010] To obtain a tensile strength TS: 1180 MPa or higher, it is preferable to use martensite or bainite as the main phase. In this case, if the area ratio is 1% or less of ferrite, 95% or more of the total of martensite and bainite, and 5% or less of retained austenite, void formation at the interface between the soft ferrite and the hard martensite or bainite will be suppressed, and the elongation flangeability will be improved.

[0011] Furthermore, the addition of Nb refines the prior austenite grain size to 10 μm or less, improving toughness. In addition, the addition of B causes B to segregate at the prior austenite grain boundaries, strengthening the grain boundaries and thus improving toughness. On the other hand, in steel with added B, the toughness sometimes improved slightly and sometimes significantly, so we investigated in more detail the conditions for significantly improving toughness.

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

[0013] Furthermore, we investigated methods for uniformly segregating B on the prior austenite grain boundaries. We found that uniform segregation of B occurs by going through the segregation process on the grain boundaries twice. When a cold-rolled sheet is annealed once in the austenite region, B segregates at the austenite grain boundaries, but at this point, the diffusion of B is insufficient, and the segregated B is non-uniform. If this steel is cooled to form a martensite and bainite structure, 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 will act as a nucleus for the formation of an austenite structure with the same crystal orientation as during the first annealing. Furthermore, since martensite and bainite contain a large amount of dislocations, the carbon (B) that was dissolved in them diffuses rapidly into the austenite grain boundaries through the dislocations during the second annealing, causing the B to segregate uniformly. In order to form retained austenite as a nucleus before the second annealing, it is preferable to stabilize the austenite by distributing carbon to the untransformed structure through a partial quenching-distribution treatment after the first annealing.

[0014] This invention is based on the above findings. Specifically, the gist of this invention is as follows: [1] 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, B: 0.0015% or more and 0.0040% or less It contains and satisfies the following formula (1), and has a component composition in which the remainder consists of Fe and unavoidable impurities, The combined 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 grain size of prior austenite 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 at the prior austenite grain boundary is 0.05% or more by mass%, a steel sheet in which the variation of the B 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 ··· Formula (1) In Formula (1), [%N] and [%Ti] respectively indicate the contents (mass%) of N and Ti in the steel. [2] As the component composition, further by 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 [1] above, containing at least one selected from the above. [3] A steel sheet according to [1] or [2] above, 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] above. [5] A hot rolling step of subjecting a steel slab having the component composition according to [1] or [2] above to hot rolling to obtain a hot rolled sheet, The pickling process involves pickling the hot-rolled sheet, A cold rolling step is performed on the hot-rolled sheet after the pickling step to obtain a cold-rolled sheet. The cold-rolled sheet is heated to a first heating temperature of Ac3 or higher in a first annealing step, A cooling step is performed on the cold-rolled sheet after the first annealing step, in which cooling is started from the first heating temperature and the sheet is cooled at an average cooling rate of 50°C / s or more until it reaches a cooling stop temperature of 100°C or higher but below the Ms point. After the cooling step, the first reheating step involves heating to a first reheating temperature of 300°C to 400°C, holding at the first reheating temperature for 60 seconds or more, and then cooling to room temperature. After the first reheating step, a second annealing step is performed in which the heating is carried out to a second heating temperature of Ac3 or higher. A method for manufacturing a steel sheet, comprising: a second annealing step, a second reheating step, a second annealing step, a second reheating step, a second reheating step, a second reheating step, a second reheating step, a second reheating step, and a second reheating step, a second reheating step, a second reheating step, a second reheating step, a second reheating step, a second reheating step, a second reheating step, a second anneal reheating [6] The method for manufacturing a steel sheet according to [5], further comprising a plating step of plating the steel sheet after the second annealing step and before the second reheating step. [7] A method for manufacturing a steel sheet according to [6], comprising an alloying step of applying an alloying treatment to the steel sheet after the plating step. [8] A method for manufacturing a component, comprising the step of forming and joining a steel plate according to any one of [1] to [3] above to obtain a component. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide steel plates, members, and methods for manufacturing the same, which have a tensile strength TS of 1180 MPa or more, high strength, and excellent tensile flangeability and toughness. [Modes for carrying out the invention]

[0016] Embodiments of the present invention will be described below. However, the present invention is not limited to the embodiments described below.

[0017] The steel sheet according to this embodiment contains, by mass%, C: 0.10% to 0.30%, Si: 0.20% to 1.20%, Mn: 2.5% to 4.0%, 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% to 0.050%, and B: 0.0015% to 0.0040%, satisfying the following formula (1), with the remainder being Fe and unavoidable impurities. The material has the following characteristics: 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 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 boron concentration within the same grain boundary at the prior austenite grain boundaries is less than 0.010% by mass. ([%N] / 14) / ([%Ti] / 47.9)<1.0...Equation (1) In equation (1), [%N] and [%Ti] represent the mass %) content of N and Ti in the steel, respectively.

[0018] [Component composition] First, we will explain the appropriate range of the steel sheet's composition and the reasons for its limitation. In the following explanation, "%" representing the content of the steel sheet's constituent elements means "mass percent" unless otherwise specified. "ppm" means "mass ppm" unless otherwise specified. Furthermore, in this specification, numerical ranges expressed using "~" mean a range that includes the values ​​written before and after "~" as the lower and upper limits, respectively.

[0019] C: 0.10% or more and 0.30% or less Carbon (C) has the effect of strengthening the martensite-bainite structure. Furthermore, reheating after the second annealing process causes segregation 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 higher cannot be obtained. Therefore, the C content should be 0.10% or more. Preferably, the C content should be 0.11% or more. On the other hand, if the carbon content exceeds 0.30%, carbonoborides are formed between boron and iron during annealing, making it impossible to segregate a sufficient amount of boron on the grain boundaries. Therefore, the carbon content should be 0.30% or less. The C content is preferably 0.28% or less.

[0020] Si: 0.20% or more and 1.20% or less Si is an effective element for solid solution strengthening and requires a content of 0.20% or more. Therefore, the Si content should be 0.20% or more. Preferably, the Si content should be 0.50% or more. On the other hand, Si is a ferrite-stabilizing element, and if its content exceeds 1.20%, ferrite is formed, reducing strength, ductility, and toughness. Therefore, the Si content should be 1.20% or less. Preferably, the Si content should be 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 should be 2.5% or more. Preferably, the Mn content should be 2.8% or more. On the other hand, if the Mn content exceeds 4.0%, the segregated areas become excessively hardened, reducing toughness. Therefore, the Mn content should be 4.0% or less. Preferably, the Mn content should be 3.5% or less.

[0022] P:0.050% or less Since phosphorus (P) segregates at the prior austenite grain boundaries and reduces toughness, the P content should be 0.050% or less. Preferably, the P content should be 0.025% or less. There is no lower limit for the P content, and it may be 0%, but since a content of less than 0.001% increases manufacturing costs, a content of 0.001% or more is preferred.

[0023] S: 0.020% or less Since sulfur (S) segregates at the prior austenite grain boundaries and reduces toughness, the S content should be 0.020% or less. Preferably, the S content should be 0.018% or less. More preferably, the S content should be 0.0040% or less, and even more preferably, 0.0020% or less. There is no specific lower limit for the sulfur content, but since setting it below 0.0001% increases manufacturing costs, it is preferable to set it at 0.0001% or higher.

[0024] Al: 0.10% or less Al acts as a deoxidizing agent, and to obtain such an 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 form, and the strength decreases. Therefore, the Al content should be 0.10% or less. Preferably, the Al content is 0.05% or less.

[0025] N: 0.01% or less N forms nitrides with Nb and B, reducing the added effects of Nb and B. Therefore, the N content should be 0.01% or less. Preferably, the N content should be 0.006% or less. There is no particular lower limit, but from the viewpoint of manufacturing costs, it is preferable that the N content be 0.0001% or more.

[0026] Ti:0.100% or less Ti fixes N in the steel as TiN, suppresses the formation of BN and NbN, improves the effects of Nb and B addition, and enhances toughness and tensile 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 form on the grain boundaries, reducing toughness. Therefore, the Ti content should be 0.100% or less. Preferably, the Ti content should be 0.050% or less.

[0027] Nb: 0.002% or more and 0.050% or less Nb precipitates as a solid solution or fine carbides, suppressing the growth of austenite grains during annealing. This refines the grain size, complicates the fracture pathway, and improves toughness. To obtain these effects, the Nb content should be 0.002% or higher. Preferably, the Nb content should be 0.005% or higher. 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 should be 0.050% or less. Preferably, the Nb content is 0.040% or less.

[0028] B: 0.0015% or more and 0.0040% or less B has the effect of segregating at the prior austenite grain boundaries, increasing grain boundary strength and improving toughness. To obtain such an effect, the B content should be 0.0015% or more. Preferably, the B content should be 0.0016% or more. On the other hand, if the B content exceeds 0.0040%, carbon borides are formed, reducing toughness. Therefore, the B content should be 0.0040% or less. Preferably, the B content should be 0.0030% or less.

[0029] ([%N] / 14) / ([%Ti] / 47.9)<1.0...Equation (1) To obtain the aforementioned effects of adding B and Nb, N, which readily bonds with these elements, needs to be fixed by Ti. Therefore, the mole fraction of N is made smaller than the mole fraction of Ti. That is, the N and Ti content in the steel is adjusted to satisfy equation (1) above. ([%N] / 14) / ([%Ti] / 47.9) is preferably 0.6 or less. In formula (1), [%N] and [%Ti] represent the mass percentage of N and Ti in the steel, respectively.

[0030] The remainder of the components other than those mentioned above consists of Fe and unavoidable impurities. Regarding the optional components described later, if their content is below the lower limit, they do not impair the effects of the present invention; therefore, if these optional elements are present below the lower limit, they are treated as unavoidable impurities.

[0031] [Optional ingredients] In addition to the above-mentioned component composition, the steel sheet according to this embodiment may further contain, in mass%, at least one element selected from among 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 increasing strength by forming fine carbides. If the V content exceeds 0.100%, coarse V carbides may precipitate, which may reduce toughness. Therefore, when V is included, the V content should be 0.100% or less. Preferably, the V content is 0.080% or less, and more preferably 0.060% or less. The lower limit of the V content is not particularly limited and may be 0.000%, but it is preferable to have a V content of 0.001% or more because it has the effect of forming fine carbides and increasing strength. 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 improves hardenability and increases the area ratio of bainite and martensite. The effect saturates when the Mo content exceeds 0.500%. Therefore, when Mo is included, the Mo content should be 0.500% or less. Preferably, the Mo content is 0.200% or less, and more preferably 0.150% or less. The lower limit of the Mo content is not particularly limited and may be 0.000%, but it is preferable to have a Mo content of 0.010% or more, as this has the effect of improving hardenability and increasing the area ratio of bainite and martensite. 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 improves hardenability and increases the area ratio of bainite and martensite. The effect saturates when the Cr content exceeds 1.00%. Therefore, when Cr is included, the Cr content should be 1.00% or less. The Cr content is preferably 0.300% or less, and more preferably 0.250% or less. The lower limit of the Cr content is not particularly limited and may be 0.000%, but it is preferable to have a Cr content of 0.01% or more, as this has the effect of improving hardenability and increasing the area ratio of bainite and martensite. 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. Furthermore, Cu 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 included, the Cu content should be 1.00% or less. Preferably, the Cu content is 0.60% or less, and more preferably 0.30% or less. The lower limit of the Cu content is not particularly limited and may be 0.000%, but it is preferable to have a Cu content of 0.01% or more because it has the effect of increasing strength through solid solution. 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 the effect saturates when the Ni content exceeds 0.50%. Therefore, when Ni is included, the Ni content should be 0.50% or less. Preferably, the Ni content is 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 it is preferable to have a Ni content of 0.01% or more, as this has the effect of improving hardenability. 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 steel sheets, but the effect saturates when the Sb content exceeds 0.200%. Therefore, when Sb is included, the Sb content should be 0.200% or less. Preferably, the Sb content is 0.050% or less, and more preferably 0.020% or less. The lower limit of the Sb content is not particularly limited and may be 0.000%, but it is preferable to have a content of 0.001% or more because it has the effect of suppressing surface oxidation, nitriding, and decarburization of the steel sheet. The Sb content is more preferably 0.002% or more, and even more preferably 0.005% or more.

[0038] Sn: 0.200% or less Similar to Sb, Sn has the effect of suppressing surface oxidation, nitriding, and decarburization of steel sheets. The effect saturates when the Sn content exceeds 0.200%. Therefore, when Sn is included, the Sn content should be 0.200% or less. Preferably, the Sn content is 0.050% or less, and more preferably 0.020% or less. The lower limit of the Sn content is not particularly limited and may be 0.000%, but it is preferable to have a Sn content of 0.001% or more, as this has the effect of suppressing surface oxidation, nitriding, and decarburization of the steel sheet. 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 increasing strength by forming fine carbides. If the Ta content exceeds 0.200%, coarse Ta carbides may precipitate, reducing toughness. Therefore, when Ta is included, the Ta content should be 0.200% or less. Preferably, the Ta content is 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 it is preferable to have a content of 0.001% or more because it has the effect of forming fine carbides and increasing strength. 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 increasing strength by forming fine carbides. If the W content exceeds 0.400%, coarse W carbides may precipitate, reducing toughness. Therefore, when W is included, the W content should be 0.400% or less. Preferably, the W content is 0.300% or less, and more preferably 0.250% or less. The lower limit of the W content is not particularly limited and may be 0.000%, but it is preferable to have a W content of 0.001% or more, as this has the effect of forming fine carbides and increasing strength. 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 improving toughness by spheroidizing the shape of inclusions and suppressing stress concentration. If the Zr content exceeds 0.0200%, a large amount of inclusions may form, which may reduce toughness. Therefore, when Zr is included, the Zr content should be 0.0200% or less. Preferably, the Zr content is 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 as low as 0.0000%, but it is preferable to have a Zr content of 0.0001% or more, as this has the effect of spheroidizing the shape of the inclusions, suppressing stress concentration, and improving toughness. 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 deoxidizing agent. If the Ca content exceeds 0.0200%, a large amount of Ca-based inclusions may be formed, which may reduce toughness. Therefore, if Ca is included, the Ca content should be 0.0200% or less. Preferably, the Ca content is 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 it is preferable to have a Ca content of 0.0001% or more, as it can be used as a deoxidizing agent. 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 deoxidizing agent. If the Mg content exceeds 0.0200%, a large amount of Mg-based inclusions may be formed, which may reduce toughness. Therefore, if Mg is included, the Mg content should be 0.0200% or less. Preferably, the Mg content is 0.0100% or less, and more preferably 0.0080% or less. The lower limit of the Mg content is not particularly limited and may be 0.0000%, but it is preferable to have a content of 0.0001% or more, as it can be used as a deoxidizing agent. 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. The effect saturates when the Co content exceeds 0.020%. Therefore, when Co is included, the Co content should be 0.020% or less. Preferably, the Co content is 0.015% or less, and more preferably 0.010% or less. The lower limit of the Co content is not particularly limited and may be 0.000%, but it is preferable to have a Co content of 0.001% or more, as this has the effect of increasing strength through solid solution strengthening. 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 improving toughness by spheroidizing the shape of inclusions and suppressing stress concentration. If the REM content exceeds 0.0200%, a large amount of inclusions may form, which may reduce toughness. Therefore, when REM is included, the REM content should be 0.0200% or less. Preferably, the REM content is 0.0100% or less, and more preferably 0.0050% or less. The lower limit of the REM content is not particularly limited and may be as low as 0.0000%, but it is preferable to have a REM content of 0.0001% or more, as this has the effect of spheroidizing the shape of the inclusions, suppressing stress concentration, and improving toughness. 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 lanthanide elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. REM content refers to the total content of one or more elements selected from the above-mentioned REM. Preferably, the REM contains La, Ce, and Nd.

[0046] Te: 0.020% or less Te has the effect of improving toughness by spheroidizing the shape of inclusions and suppressing stress concentration. If the Te content exceeds 0.020%, a large amount of inclusions may form, which may reduce toughness. Therefore, when Te is included, the Te content should be 0.020% or less. Preferably, the Te content is 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 as low as 0.000%, but it is preferable to have a Te content of 0.001% or more, as this has the effect of spheroidizing the shape of the inclusions, suppressing stress concentration, and improving toughness. 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 improving toughness by spheroidizing the shape of inclusions and suppressing stress concentration. If the Hf content exceeds 0.10%, a large amount of inclusions are formed and toughness decreases. Therefore, when Hf is included, the Hf content should be 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 it is preferable to set it to 0.01% or more, as this has the effect of spheroidizing the shape of the inclusions, suppressing stress concentration, and improving toughness.

[0048] Bi:0.200% or less Bi has the effect of reducing segregation and improving flexibility. If the Bi content exceeds 0.200%, a large amount of inclusions may form, which may reduce flexibility. Therefore, when Bi is included, the Bi content should be 0.200% or less. Preferably, the Bi content is 0.100% or less, more preferably 0.050% or less. Even more preferably, the Bi content is 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 it is preferable to have a Bi content of 0.001% or more, as this has the effect of reducing segregation and improving flexibility. The Bi content is more preferably 0.002% or more, and even more preferably 0.003% or more.

[0049] [Steel structure] Next, we will explain the structural microstructure of steel plates.

[0050] Total area percentage of martensite and bainite: 95% or more Both martensite and bainite are hard phases and are necessary to achieve a total tensile strength (TS) of 1180 MPa or higher. Therefore, the combined area ratio of martensite and bainite should be 95% or more. Preferably, the combined area ratio of martensite and bainite should be 96% or more. There is no particular upper limit to the combined area ratio of martensite and bainite, and it may be 100%.

[0051] Area percentage of retained austenite: 5% or less Retained austenite may be included as the remaining structure other than martensite and bainite. Therefore, the area percentage of retained austenite should be 5% or less. Preferably, the area percentage of retained austenite is 4% or less. The area percentage of retained austenite may be 0% or greater than 0%.

[0052] Ferrite area ratio: 1% or less The presence of soft ferrite generates voids at the interface with the matrix phases of martensite and bainite, reducing the stretchable flange properties; therefore, the area ratio of ferrite should be 1% or less. The area ratio of ferrite may also be 0%.

[0053] Here, the area fraction of each structure is measured as follows. For the area fraction of retained austenite, the rolled surface of a specimen taken from each steel sheet is chemically polished up to the position of the sheet thickness t / 4, and the X-ray diffraction intensity and diffraction peak position of the polished surface are measured using an X-ray diffractometer (XRD) to calculate the volume fraction, and this value is taken as the area fraction of retained austenite. Next, the thickness cross section parallel to the rolling direction of each steel sheet is polished and then etched with 3 vol% nital, and the position of the sheet thickness t / 4 is used as the observation surface. For the observation surface, SEM images of three fields are taken at a magnification of 2000x with a field of view of 57.1 μm × 42.9 μm. From the obtained SEM images, the area fraction of martensite, bainite and retained austenite combined, as well as the area fraction of structures other than martensite, bainite and retained austenite (ferrite), are determined by image analysis. The area ratios of martensite, bainite, and retained austenite obtained by image analysis are used to determine the area ratios of martensite and bainite obtained by XRD by subtracting the area ratio of retained austenite obtained by XRD. The average of three fields of view is used as the area ratio of the tissue.

[0054] Previous austenite particle size: 10 μm or less Toughness can be improved by complicating the crack propagation path. To obtain this effect, the prior austenite grain size must be 10 μm or less. Therefore, the prior austenite grain size is set to 10 μm or less. Preferably, the prior austenite is 9 μm or less. There is no particular lower limit to the grain size of the prior austenite grains, but from a production technology viewpoint, it is preferable that the prior austenite grain size is 1 μm or more. Furthermore, it is more preferable that the prior austenite grain size is 2 μm or more, and even more preferable that it is 3 μm or more.

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

[0056] C concentration at the former austenite grain boundary: 1.5 times or more the C content in the steel. Like B, C strengthens the grain boundaries and improves toughness by segregating into the prior austenite grain boundaries. The above effect is obtained when the C concentration in the prior austenite grain boundaries is 1.5 times or more the C content in the steel. Therefore, the C concentration in the prior austenite grain boundaries should be 1.5 times or more the C content in the steel. That is, the C concentration in the prior austenite grain boundaries satisfies the following equation. C concentration (mass%) of the former austenite grain boundary / C content (mass%) in the steel ≥ 1.5 The carbon concentration at the prior austenite grain boundaries is preferably 2.0 times or more the carbon content in the steel, and more preferably 2.5 times or more. While there is no upper limit on the carbon concentration at the prior austenite grain boundaries, it is preferably less than 7% by mass to effectively prevent the precipitation of hard carbides or carbonoborides on the grain boundaries and further improve toughness. More preferably, it is 2% or less by mass.

[0057] B concentration at the former austenite grain boundaries: 0.05% or more by mass. B can strengthen the grain boundaries and improve toughness by segregating at the prior austenite grain boundaries. The above effect can be obtained if the B concentration at the prior austenite grain boundaries is 0.05% or more by mass. Therefore, the B concentration at the prior austenite grain boundaries is set to 0.05% or more by mass. Preferably, the B concentration at the prior austenite grain boundaries is 0.07% or more by mass, and more preferably 0.10% or more. There is no upper limit set for the B concentration at the prior austenite grain boundaries, but preferably it is less than 6% by mass to suitably prevent the precipitation of hard carbonobides on the grain boundaries and further improve toughness. More preferably, it is 2% or less by mass.

[0058] Variation in B concentration within the same grain boundary of the former austenite grain boundary: less than 0.010% by mass% To improve toughness, in addition to the B concentration at the grain boundaries mentioned above, it is also important that these concentrations are uniform across the grain boundaries. If the variation is 0.010% or more by mass, strength differences will occur within the grain boundaries, making it easier for cracks to form in areas with locally low B concentration, thus reducing the toughness improvement effect. Therefore, the variation in B concentration within the same grain boundary of the prior austenite grain boundary should be less than 0.010% by mass. Preferably, the variation is 0.009% or less by mass, and more preferably 0.008% or less by mass. While less variation is preferable, from a production technology standpoint, a variation of 0.001% or more is acceptable.

[0059] Here, the C concentration, B concentration, and variability of the prior austenite grain boundary are measured as follows: Needle-shaped samples are prepared from the region containing the prior austenite grain boundary using the SEM-FIB (Focused Ion Beam) method. 3DAP analysis is performed on the obtained needle-shaped samples using a 3DAP instrument (LEAP4000XSi, AMETEK). Measurements are performed in laser mode. The sample temperature is kept below 80K. The C and B concentrations of the prior austenite grain boundary are determined from the number of C and B ions detected from the prior austenite grain boundary and the number of other ions. The C and B concentrations are the average values ​​of two samples. In addition, five non-overlapping circular regions with a diameter of 5 nm to 10 nm are set on the surface of the measured prior austenite grain boundary, and a cylindrical volume extending in the direction normal to the grain interface is set for each circle. The B concentration on the grain boundary is determined for each cylinder, and its standard deviation is taken as the variability of the B concentration. In the above measurement, the prior austenite grain size is very large compared to the area sampled by the SEM-FIB method. Therefore, the grain boundary targeted by each sampled sample is always the same. For example, while the prior austenite grain size is about 9 μm, the area of ​​the sampled sample has a diameter of about 0.1 μm. Consequently, the variation in the obtained B concentration, etc., is due to variation within the same grain boundary.

[0060] According to the present invention, a steel sheet having a tensile strength TS of 1180 MPa or more can be provided. Preferably, the tensile strength TS of the steel sheet is 1250 MPa or more.

[0061] The steel sheet described above may have a plating layer on at least one side. The plating layer is preferably a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electroplated galvanized layer. The composition of the plating layer is not particularly limited and can be a known composition.

[0062] The composition of the hot-dip galvanized layer is not particularly limited and can be any general one. For example, the plating layer contains Fe: 20% by mass or less, Al: 0.001% by mass or more and 1.0% by mass or less, and further contains 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 in a total of 0% by mass or more and 3.5% by mass or less, with the remainder being Zn and unavoidable impurities. In the case of a hot-dip galvanized layer, in one example the Fe content in the plating layer is less than 7% by mass, and in the case of an alloyed hot-dip galvanized layer, in one example the Fe content in the plating layer is 7% by mass or more and 15% by mass or less, more preferably 8% by mass or more and 13% by mass or less.

[0063] The amount of plating applied is not particularly limited, but the amount of plating applied per side of the steel sheet is 20-80 g / m². 2 This is preferable. In one example, the plating layer is formed on both the front and back surfaces of the steel sheet (high-strength cold-rolled steel sheet).

[0064] Next, we will explain the manufacturing method of steel plates. The steel sheet manufacturing method according to this embodiment includes a hot rolling step of hot rolling a steel slab having the aforementioned component composition to form 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 form a cold-rolled sheet, a first annealing step of heating the cold-rolled sheet to a first heating temperature of Ac3 or higher, and a first cooling step of starting cooling from the first heating temperature to a cooling stop temperature of 100°C or higher but below the Ms point at a rate of 50°C / s or less for the cold-rolled sheet after the first annealing step. The process includes: a cooling step of cooling at the average cooling rate described above; a first reheating step of heating to a first reheating temperature of 300°C to 400°C after the cooling step, holding at the first reheating temperature for 60 seconds or more, and cooling to room temperature; a second annealing step of heating to a second heating temperature of Ac3 or higher after the first reheating step; and a second reheating step of cooling the steel plate after the second annealing step, heating to a second reheating temperature of 70°C to 200°C, and holding at the second reheating temperature for 600 seconds or more to obtain the steel plate.

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

[0066] [Hot rolling process] The steel slabs described above are hot-rolled into hot-rolled sheets. There are no particular restrictions on the hot-rolling process; it can be carried out according to conventional methods. There are also no particular restrictions on cooling after hot-rolling; the sheets are cooled to the winding temperature. Next, the hot-rolled sheets are wound into coils. The winding temperature is preferably 400°C or higher. This is because a winding temperature of 400°C or higher makes winding easier without increasing the strength of the hot-rolled sheets. A winding temperature of 550°C or higher is more preferable. Furthermore, to effectively prevent the formation of thick scale and further improve yield, it is preferable to set the winding temperature to 750°C or lower. Note that the hot-rolled sheets may be heat-treated before pickling for the purpose of softening.

[0067] [Pickling process] After the hot rolling process, the hot-rolled sheet is pickled in the pickling process. The pickling process can remove scale from the hot-rolled sheet metal wound into a coil. While the method of scale removal is not particularly limited, it is preferable to perform the pickling while unwinding the hot-rolled coil to completely remove the scale. The pickling method is not particularly limited and can be any conventional method.

[0068] [Cold rolling process] After the pickling process, the hot-rolled sheet is subjected to cold rolling in the cold rolling process to produce a cold-rolled sheet. For example, a hot-rolled sheet from which scale has been removed is washed as appropriate, and then cold-rolled to produce a cold-rolled sheet. The method of cold rolling is not particularly limited and can be carried out according to conventional methods.

[0069] [First annealing step: Heat to the first heating temperature of Ac3 or higher] Next, in the first annealing step, the cold-rolled sheet is heated to a first heating temperature of Ac3 or higher and annealed in the austenite single-phase region. If the first heating temperature is below Ac3, ferrite will be formed. In this ferrite, the dislocations are reduced 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. Also, if the temperature is raised excessively, the austenite grain size will increase to more than 10 μm. Therefore, the first heating temperature should be Ac3 or higher. The first heating temperature is preferably Ac3 + 10°C or higher, and more preferably Ac3 + 20°C or higher. Since the austenite structure formed in the second annealing is the same as the austenite structure formed in the first annealing, it is preferable to set the first heating temperature to 980°C or lower so that the austenite grain size is 10 μm or less even in the first annealing. More preferably, the first heating temperature is 950°C or lower. The Ac3 points are calculated using the following formula. Ac3(℃)=881-206×[%C]+53×[%Si]-15×[%Mn]-27×[%Cu]-20×[%Ni]-1×[%Cr]+41×[%Mo] (In the above formula, [%M] represents the mass %) of element M contained in the steel sheet, and the value for elements that are not present is 0 (zero).)

[0070] [Cooling process: Cooling begins from the initial heating temperature and continues at an average cooling rate of 50°C / s or higher until the cooling stop temperature is 100°C or higher but below the Ms point.] [First reheating step: Heat to a first reheating temperature of 300°C to 400°C, hold at the first reheating temperature for 60 seconds or more, and then cool to room temperature.] After the first annealing step, in order to create a structure in which retained austenite exists at the prior austenite grain boundaries, the cold-rolled sheet is subjected to a partial quenching-distribution treatment during the cooling step. If the cooling stop temperature of the partial quenching is below 100°C, martensitic transformation occurs before C distribution occurs, and a sufficient amount of retained austenite cannot be obtained before the second annealing. If the amount of retained austenite is insufficient, austenite with a different orientation than that of the first annealing is generated, and it is difficult to uniformly segregate B (boron) at the grain boundaries of such austenite. Therefore, the cooling stop temperature in the cooling step should be 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, so carbon distribution does not occur during subsequent reheating, and retained austenite is not formed before the second annealing. If the amount of retained austenite is insufficient, austenite with a different orientation than that of the first annealing is formed, and it is difficult to uniformly segregate boron (B) at the grain boundaries of such austenite. Therefore, the cooling stop temperature in the cooling process should be below the Ms point. Preferably, the cooling stop temperature is Ms point - 20°C or lower, and more preferably Ms point - 30°C or lower. The Ms point is calculated using the following formula. Ms(℃)=499-308×[C]-10.8×[Si]-32.4×[Mn]-16.2×[Ni]-27×[Cr]-10.8×[Mo] (In the above formula, [M] represents the mass %) content of element M in the steel sheet, and the value for elements that are not present 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 process generates austenite with a different orientation than the first, resulting in non-uniformity of grain boundary B. Therefore, the average cooling rate should be 50°C / s or higher. Preferably, the average cooling rate should be 60°C / s or higher, and more preferably 70°C / s or higher. The upper limit of the average cooling rate is not particularly limited, 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 process is calculated as follows: "(First heating temperature (°C)) - (Cooling stop temperature (°C)) / (Cooling time from the first heating temperature (°C) to the cooling stop temperature (°C) (seconds))".

[0072] If the first reheating temperature is below 300°C, sufficient carbon distribution does not occur, 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 of the first annealing will be formed, making it difficult to uniformly segregate boron (B) at the grain boundaries of such austenite. Therefore, the first reheating temperature should be 300°C or higher. Preferably, the first reheating temperature is 310°C or higher, and more preferably 320°C or higher. On the other hand, if the first reheating temperature exceeds 400°C, the untransformed austenite decomposes into cementite, and retained austenite is not generated before the second annealing. If the amount of retained austenite is insufficient, austenite with a different orientation than that of the first annealing is generated, and it is difficult to uniformly segregate boron (B) at the grain boundaries of such austenite. Therefore, the first reheating temperature should be 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, the carbon distribution is insufficient and retained austenite will not be formed before the second annealing. If the amount of retained austenite is insufficient, austenite will be formed in a different orientation than that of the first annealing, making it difficult to uniformly segregate boron (B) at the grain boundaries of such austenite. Therefore, the holding time at the first reheating temperature should be 60 seconds or more. Preferably, the holding time should be 80 seconds or more, and more preferably 100 seconds or more. On the other hand, there is no particular upper limit to the holding time, but if the holding time is 900 s or longer, the untransformed austenite may decompose into cementite, and austenite may not be formed before the second annealing. Therefore, it is preferable that the holding time at the first reheating temperature be less than 900 s, and more preferably 600 s or less.

[0074] After being held at the first reheating temperature, the mixture is cooled to room temperature. While the room temperature at which cooling stops during the first reheating step is not particularly limited, it can be between 5 and 50°C.

[0075] [Second annealing step: Heat to a second heating temperature of Ac3 or higher] In the second annealing step, the steel sheet (cold-rolled sheet) obtained as described above is heated again to a second heating temperature of Ac3 or higher and annealed. During this process, austenite with the same orientation as the austenite produced in the first annealing is formed using retained austenite as a nucleus. During the formation process, boron (B) dissolved in the solid solution through the dislocation of martensite before the austenite transformation rapidly diffuses into the austenite grain boundaries, resulting in uniform boron segregation. If the second heating temperature is below the Ac3 point, ferrite is formed, and the strength decreases. Therefore, the second heating temperature should be above the Ac3 point. The second heating temperature is preferably above the Ac3 point + 10°C, and more preferably above the Ac3 point + 20°C. On the other hand, if the temperature is raised excessively, the austenite grain size will increase to more than 10 μm and the toughness will decrease, so it is preferable that the second heating temperature be 980°C or lower. More preferably, the second heating temperature is 950°C or lower. After heating to the second heating temperature, the material may be cooled to room temperature in order to generate a sufficient amount of martensite in the second reheating process described later, and then tempered, or it may be overaged using the equipment of the annealing line.

[0076] [Second reheating process: After the second annealing process, the steel plate is cooled and heated to a second reheating temperature of 70°C to 200°C, and held at the second reheating temperature for 600 seconds or more.] In the second reheating process, after the second annealing process, the steel plate is cooled and heated to a second reheating temperature of 70°C to 200°C, and held at the second reheating temperature for 600 seconds or more. The cooling conditions before heating (reheating) are not particularly limited, but it is preferable to cool at an average cooling rate of 30°C / s or higher in order to suppress ferrite transformation and reduce strength. Furthermore, it is preferable to cool to room temperature. Furthermore, the average cooling rate (°C / s) in this process is "(second heating temperature (°C)) - (cooling stop temperature (°C)) / (cooling time from the second heating temperature (°C) to the cooling stop temperature (°C) (seconds))". After cooling, the material is heated (reheated) to a second reheating temperature. In addition to B, reheating causes C to segregate at the prior γ grain boundaries, improving toughness. If the second reheating temperature is below 70°C, the diffusion of C is slow, and C segregation is insufficient. Therefore, the second reheating temperature should be 70°C or higher. Preferably, the second reheating temperature is 90°C or higher. On the other hand, if the second reheating temperature exceeds 200°C, excessive tempering occurs, causing precipitation of carbonoborides, making it impossible to achieve a B concentration of 0.05% by mass or more at the prior austenite grain boundaries, resulting in a decrease in toughness. Therefore, the second reheating temperature should be 200°C or lower. Preferably, the second reheating temperature should be 190°C or lower. If the holding time at the second reheating temperature (reheating holding time) is less than 600 s, the diffusion of carbon (C) is slow, and carbon segregation is insufficient. Therefore, the holding time at the second reheating temperature should be 600 s or more. Preferably, the holding time at the second reheating temperature is 800 s or more. There is no particular upper limit to the holding time at the second reheating temperature, but in order to prevent the precipitation of carbonite, the second reheating temperature is preferably 43,200 seconds or less (0.5 days) or less.

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

[0078] Furthermore, manufacturing conditions other than those mentioned above can be met by conventional methods.

[0079] The steel plate obtained according to this embodiment as described above preferably has a thickness of 0.5 mm or more. Furthermore, it is preferable that the thickness be 2.0 mm or less.

[0080] [Components] In this embodiment, a member can be provided that uses at least a portion of the steel plate described above. The steel plate described above can be formed into a desired shape by press working, for example, to make an automobile part. Note that the automobile part may include steel plates other than the steel plate according to this embodiment as a material. According to this embodiment, a high-strength steel plate with a TS of 1180 MPa or higher and excellent ductility and toughness can be provided, and a member with a TS of 1180 MPa or higher and excellent ductility and toughness can be provided. The steel plate according to this embodiment can be suitably used as an automobile part that contributes to the weight reduction of the vehicle body. The steel plate according to this embodiment can be suitably used in all types of automobile parts, particularly as structural components or reinforcing components.

[0081] The above-described method for manufacturing the component includes a step of forming the steel plate described above and applying at least one of the following processes: forming and joining to form the component. Forming processes can utilize general processing methods such as press working without restriction. Joining processes can utilize general welding methods such as spot welding and arc welding, as well as riveting and crimping without restriction. [Examples]

[0082] Steel having the component composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a converter to form steel slabs. The obtained slabs were reheated and hot-rolled, then wound up to obtain hot-rolled coils (hot-rolled sheets). Next, the hot-rolled coils were unwound and pickled, then cold-rolled to obtain cold-rolled sheets. The thickness of the hot-rolled sheets was 3.0 mm, and the thickness of the cold-rolled sheets was 1.2 mm. Annealing (first annealing step, cooling step, first reheating step, second annealing step) was carried out on a continuous hot-dip galvanizing line under the conditions shown in Table 2 to obtain steel sheets (cold-rolled steel sheets (CR), hot-dip galvanized steel sheets (GI), alloyed hot-dip galvanized steel sheets (GA)). The hot-dip galvanized steel sheets were immersed in a 460°C plating bath at 35 g / m² per side. 2 The plating adhesion amount was set to 45 g / m² per side for alloyed hot-dip galvanized steel sheet. 2 The steel sheets were manufactured by adjusting the plating adhesion amount and then performing an alloying treatment held at 520°C for 40 seconds. Except for steel sheet No. 9, the obtained steel sheets were subjected to a reheating treatment (second reheating step) under the conditions shown in Table 2.

[0083] [Table 1]

[0084] [Table 2]

[0085] For the obtained steel sheets, the total area ratio of martensite and bainite, the area ratio of retained austenite, the area ratio of ferrite, the grain size of prior austenite, the carbon concentration at the prior austenite grain boundaries, the boron concentration at the prior austenite grain boundaries, and the variation in boron concentration at the same grain boundary were evaluated according to the method described above. Furthermore, the tensile strength TS, elongation flangeability, and toughness were evaluated according to the method described later. The results are shown in Table 3.

[0086] [Tensile test] Tensile tests were performed on the obtained steel plates in accordance with JIS Z 2241 (2011). JIS No. 5 tensile test specimens were taken with the longitudinal direction perpendicular to the rolling direction, and the tensile strength (TS) was measured. A tensile strength TS of 1180 MPa or higher was considered to indicate good tensile strength.

[0087] [Charpy test] The Charpy impact test was performed in accordance with JIS Z 2242 (2018). From the obtained steel plate, a test specimen was taken with a width of 10 mm, a length of 55 mm, and a 90° V-notch with a notch depth of 2 mm in the middle of the length, so that the direction perpendicular to the rolling direction of the steel plate was the direction of V-notch application. Subsequently, the Charpy impact test was performed in the test temperature range of -120 to +120°C. The transition curve was determined from the obtained brittle fracture surface ratio, and the temperature at which the brittle fracture surface ratio became 50% was determined as the brittle-ductile transition temperature. In addition, if the brittle-ductile transition temperature obtained from the Charpy test was -40°C or lower, the toughness was judged to be good. In the table, if the brittle-ductile transition temperature is -40°C or lower, the toughness is indicated as "good," and if the brittle-ductile transition temperature is above -40°C, the toughness is indicated as "poor."

[0088] A 100mmW × 100mL test specimen was taken from a steel sheet (cold-rolled steel sheet or plated steel sheet), and a hole expansion test was performed in accordance with JIS Z 2256 (2010). A 10mmφ hole was punched into the test specimen under a clearance of 12±1%, and the hole was expanded by raising a conical punch with a 60° apex angle. The punch was stopped when a crack occurred in the thickness direction of the sheet, and the hole expansion ratio λ was calculated from the hole diameter after crack occurrence and the hole diameter before the test according to the following formula: Limit hole expansion ratio: λ(%) = {(Df-D0) / D0} × 100 (wherein Df is the hole diameter at the time of crack occurrence (mm), and D0 is the initial hole diameter (mm)). Regardless of the strength of the steel sheet, if the value of λ was 30% or more, it was judged that the tensile flange properties were good.

[0089] [Table 3]

[0090] Table 3 shows that the present invention example exhibits a tensile strength TS of 1180 MPa or higher, and superior elongation flangeability and toughness. In contrast, the comparative example is inferior in one or more of the following: tensile strength TS, elongation flangeability, and toughness.

[0091] Furthermore, it was found that members obtained by forming, members obtained by joining, and members obtained by forming and joining using the steel plate of the present invention example also possess high strength, excellent stretch flange formability, and toughness, similar to the steel plate of the present invention example, as the steel plate of the present invention example has high strength and excellent stretch flange formability and toughness.

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, B: 0.0015% or more and 0.0040% or less It contains and satisfies the following formula (1), and has a component composition in which the remainder consists of Fe and unavoidable impurities, The combined 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 carbon concentration at the prior austenite grain boundaries is 1.5 times or more the carbon content in the steel. The B concentration at the prior austenite grain boundary is 0.05% or more and 0.18% or less by mass. A steel sheet in which the variation in B concentration within the same grain boundary of the prior austenite grain boundary is less than 0.010% by mass. ([%N] / 14) / ([%Ti] / 47.9)<1.0...Formula (1) In formula (1), [%N] and [%Ti] represent the mass %) content of N and Ti in the steel, respectively.

2. The aforementioned component composition is further expressed 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, Bi: 0.200% or less The steel plate according to claim 1, comprising at least one selected from among the following.

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

4. A member made using the steel plate described in claim 1 or 2.

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

6. A hot rolling step of hot rolling a steel slab having the component composition described in claim 1 or 2 to obtain a hot-rolled plate, A pickling step for pickling the hot-rolled sheet, A cold rolling step is performed on the hot-rolled sheet after the pickling step to obtain a cold-rolled sheet. The cold-rolled sheet is heated to a first heating temperature of Ac3 or higher in a first annealing step, A cooling step is performed on the cold-rolled sheet after the first annealing step, in which cooling is started from the first heating temperature and the sheet is cooled at an average cooling rate of 50°C / s or more until it reaches a cooling stop temperature of 100°C or higher but below the Ms point. After the cooling step, the first reheating step involves heating to a first reheating temperature of 300°C to 400°C, holding at the first reheating temperature for 60 seconds or more, and then cooling to room temperature. After the first reheating step, a second annealing step is performed in which the heating is carried out to a second heating temperature of Ac3 or higher. The process includes a second reheating step in which, after the second annealing step, the steel sheet is cooled, heated to a second reheating temperature of 70°C to 200°C, and held at the second reheating temperature for 600 seconds or more to obtain the steel sheet. The combined 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 carbon concentration at the prior austenite grain boundaries is 1.5 times or more the carbon content in the steel. The B concentration at the prior austenite grain boundary is 0.05% or more and 0.18% or less by mass. A method for manufacturing steel sheets, wherein the variation in the concentration of B within the same grain boundary of the prior austenite grain boundary is less than 0.010% by mass.

7. A method for manufacturing a steel sheet according to claim 6, comprising a plating step of applying a plating treatment to the steel sheet after the second annealing step and before the second reheating step.

8. A method for manufacturing a steel sheet according to claim 7, further comprising an alloying step of applying an alloying treatment to the steel sheet after the plating step.

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

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

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