Steel sheet, member, and method for producing same

Abstract

Provided are: a steel sheet having high strength and excellent stretch flangeability and toughness; a member; and methods for producing the steel sheet and the member. A steel sheet having a component composition in which C, Si, Mn, P, Al, N, Ti, Nb, and B are contained in specific amounts in mass%, the total area ratio of martensite and bainite being 95% or more, the area ratio of retained austenite being 5% or less, the area ratio of ferrite being 1% or less, the prior austenite particle size being 10 [mu] m or less, the C concentration at the prior austenite grain boundary being 1.5 times or more of the C content in the steel, and the steel sheet having a steel structure in which the total area ratio of martensite and bainite is 1% or less. The B concentration at the prior austenite grain boundary is 0.05% by mass or more, and the deviation of the B concentration at the prior austenite grain boundary within the same grain boundary is less than 0.010% by mass.

Description

Technical Field

[0001] The present invention relates to steel plates, components and methods for manufacturing the same. Background Art

[0002] Automotive steel sheets are required to have not only strength but also excellent stretch-flangeability. Furthermore, they are also required to have high toughness to prevent automotive parts from breaking during collisions.

[0003] Patent Document 1 discloses a high-strength steel sheet having excellent workability and a method for producing the same. Patent Document 2 discloses a high-strength cold-rolled steel sheet and a method for producing the same.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2013-147736

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2008-106351 Summary of the Invention

[0008] Problems to be solved by the invention

[0009] However, the steel plates described in Patent Documents 1 and 2 have a tensile strength TS of less than 1180 MPa, and neither of them takes stretch flange formability and toughness into consideration.

[0010] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel plate and a member having a tensile strength TS of 1180 MPa or more, high strength, excellent stretch flange formability and toughness, and a method for producing the same.

[0011] In addition, in the present invention, high strength means that the tensile strength TS measured in accordance with JIS Z2241 (2011) is 1180 MPa or more.

[0012] Furthermore, excellent stretch flangeability means that the hole expansion ratio measured in accordance with JIS Z 2256 (2010) is 30% or more.

[0013] In addition, excellent toughness means that the brittle-ductile transition temperature in the Charpy impact test conducted in accordance with JIS Z2242 (2018) is -40°C or lower.

[0014] Methods used to solve problems

[0015] The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have obtained the following findings.

[0016] To achieve a tensile strength TS of 1180 MPa or higher, martensite or bainite may be used as the main phase. In this case, if ferrite is controlled to be 1% or less, the total of martensite and bainite is controlled to be 95% or more, and retained austenite is controlled to be 5% or less, the formation of voids at the interface between the soft ferrite and the hard martensite or bainite is suppressed, thereby improving stretch flangeability.

[0017] Furthermore, the addition of Nb refines the prior austenite grain size to 10 μm or less, thereby improving toughness. Furthermore, the addition of B causes B to segregate at prior austenite grain boundaries, strengthening the grain boundaries and thereby improving toughness.

[0018] On the other hand, in steels to which B is added, the toughness may be slightly improved or greatly improved. The conditions for greatly improving the toughness have been studied in more detail.

[0019] It was found that B can segregate unevenly or at uniform concentrations at prior austenite grain boundaries, with the toughness significantly improved when B segregates at uniform concentrations. As an indicator of the uniformity of B segregation, the variation in B concentration at prior austenite grain boundaries within the same grain boundary was considered. When this variation was less than 0.010% by mass, toughness was significantly improved.

[0020] Furthermore, researchers studied methods for uniformly segregating B at prior austenite grain boundaries. They discovered that by performing two B segregation steps at grain boundaries, B segregates uniformly. While a single annealing of a cold-rolled sheet in the austenite region segregates B at the austenite grain boundaries, the diffusion of B at this point is insufficient, resulting in uneven segregation.

[0021] After the steel is temporarily cooled to form martensite and bainite structures, a second annealing is performed to form an austenite reverse transformation structure. In this regard, if retained austenite exists after the first annealing and cooling, it serves as the nucleus for the formation of an austenite structure with the same crystal orientation as during the first annealing. Furthermore, martensite and bainite contain a large number of dislocations, and the boron dissolved therein diffuses rapidly to the austenite grain boundaries via dislocations during the second annealing, resulting in uniform boron segregation. In order to form retained austenite as a nucleus before the second annealing, after the first annealing, a local quenching-partitioning treatment can be performed to distribute carbon in the untransformed structure, thereby stabilizing the austenite.

[0022] The present invention has been completed based on the above findings. Specifically, the gist of the present invention is as follows.

[0023] [1] A steel plate having a composition comprising, in mass%, C: 0.10% to 0.30%, Si: 0.20% to 1.20%, Mn: 2.5% to 4.0%, P: 0.050% to 0.020%, S: 0.10% to 0.10%, N: 0.01% to 0.01%, Ti: 0.100% to 0.002% to 0.050%, and B: 0.0015% to 0.0040%, satisfying the following formula (1), with the balance consisting of Fe and unavoidable impurities.

[0024] The total area ratio of martensite and bainite is 95% or more.

[0025] The area ratio of retained austenite is less than 5%,

[0026] The area ratio of ferrite is less than 1%,

[0027] The original austenite grain size is less than 10 μm.

[0028] The C concentration at the original austenite grain boundary is more than 1.5 times the C content in the steel.

[0029] The B concentration in the prior austenite grain boundaries is 0.05% or more by mass.

[0030] The deviation of the B concentration in the prior austenite grain boundaries within the same grain boundary is less than 0.010% in terms of mass %.

[0031] ([%N] / 14) / ([%Ti] / 47.9)<1.0…Equation (1)

[0032] In formula (1), [%N] and [%Ti] represent the contents of N and Ti in steel (mass %), respectively.

[0033] [2] The steel sheet according to [1], further comprising, as the above-mentioned chemical composition, at least one selected from the group consisting of 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.

[0034] [3] The steel sheet according to [1] or [2] above, having a plating layer on at least one surface.

[0035] [4] A member formed using the steel plate according to any one of [1] to [3] above.

[0036] [5] A method for manufacturing a steel plate, comprising:

[0037] A hot rolling step, wherein a steel slab having the composition described in [1] or [2] is hot rolled to produce a hot-rolled sheet;

[0038] A pickling process, wherein the hot-rolled plate is pickled;

[0039] a cold rolling step, wherein the hot-rolled sheet after the pickling step is cold-rolled to produce a cold-rolled sheet;

[0040] a first annealing step, wherein the cold-rolled sheet is heated to a first heating temperature above the Ac3 point;

[0041] a cooling step, wherein the cold-rolled sheet after the first annealing step is cooled from the first heating temperature at an average cooling rate of 50°C / s or more to a cooling stop temperature of 100°C or more and lower than the Ms point;

[0042] a first reheating step, wherein, after the cooling step, the steel is heated to a first reheating temperature of 300° C. to 400° C., maintained at the first reheating temperature for 60 seconds or more, and then cooled to room temperature;

[0043] A second annealing step, wherein, after the first reheating step, heating is performed to a second heating temperature above the Ac3 point; and

[0044] The second reheating step comprises cooling the steel sheet after the second annealing step, heating the steel sheet to a second reheating temperature of 70° C. to 200° C., and maintaining the temperature at the second reheating temperature for 600 seconds or more to obtain the steel sheet.

[0045] [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.

[0046] [7] The method for manufacturing a steel sheet according to [6] above, further comprising an alloying step of alloying the steel sheet after the plating step.

[0047] [8] A method for manufacturing a component, comprising the step of performing at least one of forming and joining on the steel plate described in any one of [1] to [3] above to produce the component.

[0048] Effects of the Invention

[0049] According to the present invention, it is possible to provide a steel plate and a member having a tensile strength TS of 1180 MPa or more, which has high strength, excellent stretch flange formability and toughness, and a method for producing the same. DETAILED DESCRIPTION

[0050] Hereinafter, embodiments of the present invention will be described. However, it should be noted that the present invention is not limited to the following embodiments.

[0051] The steel sheet of the present embodiment contains, in mass%, C: 0.10% to 0.30%, Si: 0.20% to 1.20%, Mn: 2.5% to 4.0%, P: 0.050% to 0.020%, S: 0.010% to 0.10%, N: 0.01% to 0.01%, Ti: 0.100% to 0.002% to 0.050%, and B: 0.0015% to 0.0040%, satisfying The following formula (1) is used, the balance is composed of components consisting of Fe and inevitable impurities, 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 at the prior austenite grain boundaries is 1.5 times or more of the C content in the steel, the B concentration at the prior austenite grain boundaries is 0.05% or more in mass%, and the deviation of the B concentration at the prior austenite grain boundaries within the same grain boundary is less than 0.010% in mass%.

[0052] ([%N] / 14) / ([%Ti] / 47.9)<1.0…Equation (1)

[0053] In formula (1), [%N] and [%Ti] represent the contents of N and Ti in steel (mass %), respectively.

[0054] [Ingredients]

[0055] First, the appropriate range of chemical compositions for steel sheets and the reasons for their limitations will be explained. It should be noted that, in the following description, "%" indicating the content of a steel sheet's constituent elements refers to "mass %" unless otherwise specified. "ppm" refers to "mass ppm" unless otherwise specified. Furthermore, in this specification, numerical ranges expressed using "to" refer to ranges that include the numerical values ​​before and after "to" as the lower and upper limits.

[0056] C: 0.10% or more and 0.30% or less

[0057] C has the effect of strengthening martensite and bainite structures. In addition, it has the effect of segregating at the original austenite grain boundaries and improving toughness through the reheating treatment after the second annealing step. When the C content is less than 0.10%, the area ratio of martensite and bainite is reduced, and a tensile strength TS (hereinafter sometimes abbreviated 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.

[0058] On the other hand, if the C content exceeds 0.30%, carborides with B and iron are formed during annealing, and a sufficient amount of B cannot be segregated at the grain boundaries. Therefore, the C content is set to 0.30% or less.

[0059] The C content is preferably set to 0.28% or less.

[0060] Si: 0.20% or more and 1.20% or less

[0061] Si is an element effective for solid solution strengthening and needs to be contained at least 0.20%. Therefore, the Si content is set to at least 0.20%. The Si content is preferably set to at least 0.50%.

[0062] On the other hand, Si is a ferrite-stabilizing element. If its content exceeds 1.20%, ferrite is formed, which reduces strength, stretch-flange formability, and toughness. Therefore, the Si content is set to 1.20% or less. The Si content is preferably set to 1.10% or less.

[0063] Mn: 2.5% or more and 4.0% or less

[0064] Mn is effective in improving hardenability. When the Mn content is less than 2.5%, the area ratio of martensite and bainite decreases, and the strength decreases. Therefore, the Mn content is set to 2.5% or more. The Mn content is preferably set to 2.8% or more.

[0065] On the other hand, if the Mn content exceeds 4.0%, the segregation portion becomes excessively hardened, and the toughness decreases. Therefore, the Mn content is set to 4.0% or less. The Mn content is preferably set to 3.5% or less.

[0066] P: 0.050% or less

[0067] P segregates at the prior austenite grain boundaries, reducing 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 content less than 0.001% increases manufacturing costs, so it is preferably 0.001% or more.

[0068] S: 0.020% or less

[0069] Since S segregates at prior austenite grain boundaries and reduces toughness, 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 even more preferably 0.0020% or less.

[0070] The lower limit of the S content is not particularly set, but a content less than 0.0001% increases production costs, so it is preferably set to 0.0001% or more.

[0071] Al: 0.10% or less

[0072] Al is an element that acts as a deoxidizing agent. To achieve this effect, the Al content is preferably set to 0.005% or more. On the other hand, if the Al content exceeds 0.10%, ferrite is easily formed, which reduces the strength. Therefore, the Al content is set to 0.10% or less. The Al content is preferably set to 0.05% or less.

[0073] N: 0.01% or less

[0074] N forms nitrides with Nb and B, reducing the effects of Nb and B addition. Therefore, the N content is set to 0.01% or less. The N content is preferably set to 0.006% or less. There is no particular lower limit, but from the perspective of manufacturing cost, the N content is preferably set to 0.0001% or more.

[0075] Ti: 0.100% or less

[0076] Ti has the effects of fixing N in steel as TiN, suppressing the formation of BN and NbN, thereby enhancing the effect of adding Nb and B, and improving toughness and stretch flangeability. To achieve these effects, the Ti content is preferably set to 0.005% or more.

[0077] On the other hand, when the Ti content exceeds 0.100%, coarse Ti carbides are formed at 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.

[0078] Nb: 0.002% or more and 0.050% or less

[0079] Nb dissolves or precipitates as fine carbides, inhibiting the growth of austenite grains during annealing. Furthermore, it can refine 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.

[0080] On the other hand, if the Nb content exceeds 0.050%, not only will the effect be saturated, but coarse Nb carbides will precipitate, reducing toughness. Therefore, the Nb content is set to 0.050% or less. The Nb content is preferably 0.040% or less.

[0081] B: 0.0015% or more and 0.0040% or less

[0082] Boron has the effect of segregating at the prior austenite grain boundaries, increasing grain boundary strength and toughness. To achieve these effects, the B content is set to 0.0015% or more. The B content is preferably set to 0.0016% or more.

[0083] On the other hand, if the B content exceeds 0.0040%, carboride is formed, which reduces toughness. Therefore, the B content is set to 0.0040% or less. The B content is preferably set to 0.0030% or less.

[0084] ([%N] / 14) / ([%Ti] / 47.9)<1.0…Equation (1)

[0085] To achieve the aforementioned effects of adding B and Nb, N, which readily bonds with these elements, needs to be fixed with Ti. To achieve this, the molar fraction of N is made smaller than the molar fraction of Ti. In other words, the N and Ti contents in the steel are adjusted to satisfy the aforementioned formula (1).

[0086] ([%N] / 14) / ([%Ti] / 47.9) is preferably 0.6 or less.

[0087] It should be noted that in formula (1), [%N] and [%Ti] represent the contents (mass %) of N and Ti in steel, respectively.

[0088] The balance other than the above components is Fe and inevitable impurities. It should be noted that, regarding the optional components described below, if the content is less than the lower limit, since the effect of the present invention is not impaired, the optional elements contained in an amount less than the lower limit are treated as inevitable impurities.

[0089] [Optional Ingredients]

[0090] The steel sheet of the present embodiment may further contain, in addition to the above-described component composition, at least one element selected from the group consisting of 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, in mass %.

[0091] V: 0.100% or less

[0092] V has the effect of forming fine carbides and improving strength. When the V content exceeds 0.100%, coarse V carbides may precipitate, reducing 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.

[0093] The lower limit of the V content is not particularly limited and may be 0.000%, but is preferably 0.001% or more to form fine carbides and improve strength. The V content is more preferably 0.005% or more, and even more preferably 0.010% or more.

[0094] Mo: 0.500% or less

[0095] Mo has the effect of improving hardenability and increasing the area ratio of bainite and martensite. When the Mo content exceeds 0.500%, the effect is saturated. 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.

[0096] The lower limit of the Mo content is not particularly limited and may be 0.000%, but is preferably set to 0.010% or more due to the effects 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.

[0097] Cr: less than 1.00%

[0098] Cr improves hardenability and increases the area ratio of bainite and martensite. This effect saturates when the Cr content exceeds 1.00%. Therefore, when Cr is included, 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.

[0099] The lower limit of the Cr content is not particularly limited and may be 0.000%, but is preferably set to 0.01% or more due to its effects 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.

[0100] Cu: 1.00% or less

[0101] Cu has the effect of increasing strength by solid solution. In addition, Cu has the effect of improving delayed fracture resistance. When the Cu content exceeds 1.00%, intergranular cracks are easily generated. 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.

[0102] The lower limit of the Cu content is not particularly limited and may be 0.000%, but is preferably 0.01% or more due to its 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.

[0103] Ni: 0.50% or less

[0104] Ni has the effect of improving hardenability, but the effect is saturated when the Ni content exceeds 0.50%. 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.

[0105] The lower limit of the Ni content is not particularly limited and may be 0.00%, but is preferably 0.01% or more due to its effect of improving hardenability. The Ni content is more preferably 0.02% or more, and even more preferably 0.03% or more.

[0106] Sb: 0.200% or less

[0107] Sb has the effect of inhibiting surface oxidation, nitridation, and decarburization of steel sheets, but this effect saturates when the Sb content exceeds 0.200%. 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.

[0108] The lower limit of the Sb content is not particularly limited and may be 0.000%, but is preferably set to 0.001% or more to suppress surface oxidation, nitridation, and decarburization of the steel sheet. The Sb content is more preferably 0.002% or more, and even more preferably 0.005% or more.

[0109] Sn: 0.200% or less

[0110] Sn, like Sb, has the effect of inhibiting surface oxidation, nitridation, and decarburization of steel sheets. This effect saturates when the Sn content exceeds 0.200%. Therefore, when Sn is included, 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.

[0111] The lower limit of the Sn content is not particularly limited and may be 0.000%, but is preferably set to 0.001% or more to suppress surface oxidation, nitridation, and decarburization of the steel sheet. The Sn content is more preferably 0.002% or more, and even more preferably 0.005% or more.

[0112] Ta: 0.200% or less

[0113] Ta has the effect of forming fine carbides and increasing strength. When the Ta content exceeds 0.200%, coarse Ta carbides may precipitate, reducing 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.

[0114] The lower limit of the Ta content is not particularly limited and may be 0.000%, but is preferably set to 0.001% or more because it forms fine carbides and increases strength. The Ta content is more preferably 0.005% or more, and even more preferably 0.010% or more.

[0115] W: 0.400% or less

[0116] W has the effect of forming fine carbides and improving strength. When the W content exceeds 0.400%, coarse W carbides may precipitate, reducing 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.

[0117] The lower limit of the W content is not particularly limited and may be 0.000%, but is preferably 0.001% or more because it forms fine carbides and improves strength. The W content is more preferably 0.005% or more, and even more preferably 0.010% or more.

[0118] Zr: 0.0200% or less

[0119] Zr spheroidizes inclusions, suppresses stress concentration, and improves toughness. A Zr content exceeding 0.0200% may result in the formation of numerous inclusions, reducing toughness. Therefore, when Zr is present, 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.

[0120] The lower limit of the Zr content is not particularly limited and may be 0.0000%, but is preferably set to 0.0001% or more because it has the effects of spheroidizing the shape of 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.

[0121] Ca: 0.0200% or less

[0122] Ca can be used as a deoxidizing agent. When the Ca content exceeds 0.0200%, a large amount of Ca-based inclusions may be generated, which may reduce 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.

[0123] The lower limit of the Ca content is not particularly limited and may be 0.0000%, but is preferably 0.0001% or more because it can be used as a deoxidizing material. The Ca content is more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0124] Mg: 0.0200% or less

[0125] Mg can be used as a deoxidizing agent. When the Mg content exceeds 0.0200%, a large amount of Mg-based inclusions may form, reducing 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.

[0126] The lower limit of the Mg content is not particularly limited and may be 0.0000%, but is preferably 0.0001% or more because it can be used as a deoxidizing material. The Mg content is more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0127] Co: 0.020% or less

[0128] Co has the effect of increasing strength through solid solution strengthening. When the Co content exceeds 0.020%, the effect is saturated. 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.

[0129] The lower limit of the Co content is not particularly limited and may be 0.000%, but is preferably 0.001% or more due to the effect of improving strength through solid solution strengthening. The Co content is more preferably 0.002% or more, and even more preferably 0.005% or more.

[0130] REM: 0.0200% or less

[0131] REM has the effect of spheroidizing inclusions, suppressing stress concentration, and improving toughness. If the REM content exceeds 0.0200%, a large number of inclusions may form, reducing 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.

[0132] The lower limit of the REM content is not particularly limited and may be 0.0000%, but is preferably set to 0.0001% or more to spheroidize inclusions, suppress stress concentration, and improve toughness. The REM content is more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0133] Here, REM refers to lanthanoid elements, including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content refers to the total content of one or more elements selected from the above REMs. La, Ce, and Nd are preferably included as REMs.

[0134] Te: 0.020% or less

[0135] Te has the effect of spheroidizing the shape of inclusions, suppressing stress concentration, and thus improving toughness. When the Te content exceeds 0.020%, a large number of inclusions may form, reducing 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.

[0136] The lower limit of the Te content is not particularly limited and may be 0.000%, but is preferably set to 0.001% or more because it has the effects of spheroidizing the shape of 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.

[0137] Hf: 0.10% or less

[0138] Hf spheroidizes inclusions, suppresses stress concentration, and improves toughness. When the Hf content exceeds 0.10%, a large number of inclusions form, reducing toughness. Therefore, when Hf is present, 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.

[0139] The lower limit of the Hf content is not particularly limited and may be 0.000%. However, since Hf has the effects of spheroidizing the shape of inclusions, suppressing stress concentration, and improving toughness, it is preferably set to 0.01% or more.

[0140] Bi: 0.200% or less

[0141] Bi reduces segregation and improves bendability. A Bi content exceeding 0.200% may result in the formation of numerous inclusions, which can reduce bendability. Therefore, when Bi is present, it should be kept at 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.

[0142] The lower limit of the Bi content is not particularly limited and may be 0.000%, but is preferably set to 0.001% or more to reduce segregation and improve bendability. The Bi content is more preferably 0.002% or more, and even more preferably 0.003% or more.

[0143] [Steel Structure]

[0144] Next, the steel structure of the steel plate will be described.

[0145] Total area ratio of martensite and bainite: 95% or more

[0146] Both martensite and bainite are hard phases and are necessary to achieve a TS of 1180 MPa or higher. Therefore, the combined area ratio of martensite and bainite is set to 95% or higher. The combined area ratio of martensite and bainite is preferably 96% or higher. The upper limit of the combined area ratio of martensite and bainite is not particularly limited and may be 100%.

[0147] Area ratio of retained austenite: less than 5%

[0148] Retained austenite may be included 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 greater.

[0149] Ferrite area ratio: less than 1%

[0150] If soft ferrite is present, voids are generated at the interface with the matrix phases of martensite and bainite, which reduces stretch flangeability. Therefore, the area ratio of ferrite is set to 1% or less. The area ratio of ferrite may be 0%.

[0151] Here, the area ratio of each structure is measured as follows. Regarding the area ratio of retained austenite, in the test piece cut from each steel plate, the rolled surface is chemically ground to the plate thickness t / 4 position of the steel plate, and the X-ray diffraction intensity and diffraction peak position of the ground surface are measured using an X-ray diffraction device (X-raydiffraction: XRD), and the volume ratio is calculated, and this value is used as the area ratio of retained austenite. Next, after grinding the plate thickness section parallel to the rolling direction of each steel plate, it is corroded with a 3 volume % nitric acid ethanol solution, and the plate thickness t / 4 position is used as the observation surface. Regarding the observation surface, at a magnification of 2000 times, the field of view range is set to 57.1μm×42.9μm, and SEM images of three fields of view are taken. For the obtained SEM images, the total area ratio of martensite, bainite and retained austenite, and the area ratio of structures (ferrite) other than martensite, bainite and retained austenite are calculated by image analysis. The area ratio of martensite and bainite was calculated by subtracting the area ratio of retained austenite obtained by XRD from the area ratios of martensite, bainite, and retained austenite obtained by image analysis. The average value of the three fields of view was used as the area ratio of the structure.

[0152] Prior austenite grain size: less than 10 μm

[0153] By complicating the crack progression path, toughness can be improved. To achieve this effect, the prior austenite grain size needs to be set to 10 μm or less. Therefore, the prior austenite grain size is set to 10 μm or less. The prior austenite is preferably 9 μm or less. There is no particular lower limit on the grain size of the prior austenite grains; from a production technology perspective, the prior austenite grain size is preferably set to 1 μm or more. In addition, the prior austenite grain size is more preferably set to 2 μm or more, and even more preferably to 3 μm or more.

[0154] Here, the particle size of the original austenite grains is measured as follows. After grinding the plate thickness section parallel to the rolling direction of each steel plate, it is corroded with a picric acid aqueous solution to form an observation surface. In the observation surface, for the microstructure at the plate thickness t / 4 position, the field of view is set to 57.1μm×42.9μm by SEM at a magnification of 2000 times, and three fields of view are taken to obtain SEM images. The particle size of the original austenite grains is calculated from the obtained tissue image by image analysis, and the average value of the three fields of view is used as the particle size of the original austenite grains (average crystal grain size).

[0155] C concentration at the original austenite grain boundary: more than 1.5 times the C content in the steel

[0156] Like B, C segregates at prior austenite grain boundaries, strengthening them and improving toughness. This effect is achieved when the C concentration at the prior austenite grain boundaries is at least 1.5 times the C content in the steel. Therefore, the C concentration at the prior austenite grain boundaries is set to at least 1.5 times the C content in the steel. That is, the C concentration at the prior austenite grain boundaries satisfies the following equation.

[0157] C concentration at the original austenite grain boundary (mass %) / C content in steel (mass %) ≥ 1.5

[0158] The C concentration in 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.

[0159] There is no upper limit for the C concentration in the prior austenite grain boundaries; however, in order to appropriately prevent precipitation of hard carbides or carborides at the grain boundaries and further improve toughness, the C concentration is preferably less than 7% by mass, and more preferably 2% or less by mass.

[0160] B concentration in prior austenite grain boundaries: 0.05% or more by mass

[0161] B can strengthen the grain boundaries and improve toughness by segregating at the original austenite grain boundaries. When the B concentration of the original austenite grain boundaries is 0.05% or more by mass%, the above-mentioned effect can be obtained. Therefore, the B concentration of the original austenite grain boundaries is set to 0.05% or more by mass%. The B concentration of the original austenite grain boundaries is preferably 0.07% or more by mass%, more preferably 0.10% or more. The upper limit of the B concentration of the original austenite grain boundaries is not set. In order to properly prevent the precipitation of hard carboride 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.

[0162] The deviation of B concentration in the prior austenite grain boundary within the same grain boundary is less than 0.010% in mass %.

[0163] In order to improve toughness, in addition to the B concentration at the grain boundaries mentioned above, it is also important that these are uniform and independent of the grain boundaries. If the deviation is 0.010% or more by mass%, strength differences will occur within the grain boundaries, cracks will easily form in areas where the B concentration is locally low, and the toughness improvement effect will be reduced. Therefore, the deviation of the B concentration at the prior austenite grain boundaries within the same grain boundary is set to less than 0.010% by mass. The deviation is preferably 0.009% or less by mass, and more preferably 0.008% or less by mass.

[0164] The smaller the deviation, the better. However, from the viewpoint of production technology, the deviation can be set to 0.001% or more.

[0165] Here, the C concentration, B concentration and deviation of the original austenite grain boundary are measured as follows. A needle-shaped sample is made from an area containing the original austenite grain boundary by the SEM-FIB (Focused Ion Beam: Focused Ion Beam) method. For the obtained needle-shaped sample, 3DAP analysis is performed using a 3DAP device (LEAP4000XSi, manufactured by AMETEK). The measurement is carried out in laser mode. The sample temperature is set to below 80K. Based on the number of C ions, B ions and other ions detected from the original austenite grain boundary, the C and B concentrations of the original austenite grain boundary are calculated. The C and B concentrations are set to the average value of the two samples. In addition, 5 non-overlapping circular areas with a diameter of more than 5nm and less than 10nm are set on the surface of the original austenite grain boundary where the measurement is performed, and a cylindrical volume extending in the normal direction of the crystal interface is set for each circle. The B concentration on the grain boundary is calculated for each cylinder, and its standard deviation is used as the deviation of the B concentration.

[0166] It should be noted that in the above measurement, the prior austenite grain size is significantly larger than the area sampled by the SEM-FIB method, so the grain boundaries within a single sample are all identical. For example, the prior austenite grain size is approximately 9 μm, while the sampled area is approximately 0.1 μm in diameter. Therefore, the resulting variations in B concentration, etc., are variations within the same grain boundary.

[0167] According to the present invention, a steel plate having a tensile strength TS of 1180 MPa or more can be provided. The tensile strength TS of the steel plate is preferably 1250 MPa or more.

[0168] The steel sheet may have a coating on at least one side. The coating is preferably any one of a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrogalvanized layer. The composition of the coating is not particularly limited and may be a known composition.

[0169] The composition of the hot-dip galvanized layer is not particularly limited and may be a general composition. In one example, the coating layer has a composition comprising Fe: 20% by mass or less, Al: 0.001% by mass or more and 1.0% by mass or less, and a total of 0% by mass or more and 3.5% by mass or less of one or more 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.

[0170] When the coating layer is a hot-dip galvanized layer, in one example, the Fe content in the coating layer is less than 7 mass %. In the case of an alloyed hot-dip galvannealed layer, in one example, the Fe content in the coating layer is 7 mass % or more and 15 mass % or less, more preferably 8 mass % or more and 13 mass % or less.

[0171] The coating weight is not particularly limited, but is preferably set to 20 to 80 g / m2 per side of the steel sheet. 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).

[0172] Next, a method for producing a steel plate will be described.

[0173] The method for manufacturing a steel plate according to the present embodiment includes: a hot rolling step, wherein a steel slab having the above-mentioned composition is hot rolled to produce a hot-rolled plate; a pickling step, wherein the hot-rolled plate is pickled; a cold rolling step, wherein the hot-rolled plate after the pickling step is cold rolled to produce a cold-rolled plate; a first annealing step, wherein the cold-rolled plate is heated to a first heating temperature of at least Ac3 point; a cooling step, wherein the cold-rolled plate after the first annealing step is cooled from the first heating temperature at an average cooling rate of at least 50°C / s to a temperature of at least 100°C and below Ms point. a cooling stop temperature; a first reheating step, wherein, after the cooling step, the steel plate is heated to a first reheating temperature of 300° C. to 400° C., maintained at the first reheating temperature for more than 60 seconds, and cooled to room temperature; a second annealing step, wherein, after the first reheating step, the steel plate is heated to a second heating temperature of more than Ac3 point; and a second reheating step, wherein, after the second annealing step, the steel plate is cooled, heated to a second reheating temperature of 70° C. to 200° C., and maintained at the second reheating temperature for more than 600 seconds, to obtain a steel plate.

[0174] First, a steel billet having the above-mentioned composition is produced.

[0175] Here, an example of the manufacturing conditions of the steel slab before the hot rolling step will be described.

[0176] First, the steel raw material is melted to produce molten steel having the above-mentioned composition. The smelting method is not particularly limited, and any of the well-known smelting methods such as converter smelting and electric furnace smelting is suitable. The obtained molten steel is solidified to produce a slab. The method for producing a slab from molten steel is not particularly limited, and a continuous casting method, an ingot casting method or a thin slab casting method can be used. The slab can be hot-rolled after being temporarily cooled and then heated again, or it can be continuously hot-rolled without cooling the cast slab to room temperature. Taking into account the rolling load and the generation of oxide scale, the slab heating temperature is preferably set to 1100°C or more, and is preferably set to 1300°C or less. The slab heating method is not particularly limited, and for example, heating can be performed using a heating furnace according to a conventional method.

[0177] [Hot rolling process]

[0178] The above-mentioned steel billet is hot rolled to form a hot-rolled plate. There is no particular restriction on hot rolling, and it can be carried out according to conventional methods. There is no particular restriction on cooling after hot rolling, and it is cooled to the coiling temperature. Then, the hot-rolled plate is coiled into a coil. The coiling temperature is preferably set to above 400°C. This is because when the coiling temperature is above 400°C, the strength of the hot-rolled plate does not increase, and coiling becomes easy. The coiling temperature is more preferably set to above 550°C. In addition, in order to appropriately prevent the formation of thicker oxide scales and further improve the yield, the coiling temperature is preferably set to below 750°C. It should be noted that before pickling, the hot-rolled plate can also be heat-treated for the purpose of softening.

[0179] [Pickling process]

[0180] After the hot rolling process, the hot rolled sheet is pickled in the pickling process.

[0181] The pickling process removes scale from the hot-rolled coil. While the method for removing scale is not particularly limited, it is preferable to perform pickling while the hot-rolled coil is being unwound to completely remove the scale. The pickling method is not particularly limited and can be performed using conventional methods.

[0182] [Cold rolling process]

[0183] After the pickling step, the hot-rolled sheet is cold-rolled in a cold rolling step to produce cold-rolled sheet. For example, the hot-rolled sheet, after descaling, is appropriately cleaned and then cold-rolled to produce cold-rolled sheet. The cold rolling method is not particularly limited and can be performed using conventional methods.

[0184] [First annealing step: heating to a first heating temperature above the Ac3 point]

[0185] Next, in the first annealing step, the cold-rolled sheet is heated to a first heating temperature above the Ac3 point and annealed in the austenite single phase region. If the first heating temperature is lower than the Ac3 point, ferrite is generated. The dislocations in the ferrite are reduced during annealing, and there are no dislocations that become diffusion paths for B (boron) during the second annealing, making it difficult to segregate boron uniformly. In addition, if the temperature is excessively increased, the austenite grain size increases to more than 10 μm. Therefore, the first heating temperature is set to above the Ac3 point. The first heating temperature is preferably above the Ac3 point + 10°C, and more preferably above the Ac3 point + 20°C.

[0186] The austenite structure formed in the second annealing has the same crystal structure as that formed in the first annealing. Therefore, the first heating temperature is preferably set to 980°C or lower so that the austenite grain size is also 10 μm or smaller in the first annealing. The first heating temperature is more preferably set to 950°C or lower.

[0187] Point Ac3 is obtained by the following formula.

[0188] Ac3(℃)=881-206×[%C]+53×[%Si]-15×[%Mn]-27×[%Cu]-20×[%Ni]-1×[%Cr]+41×[%Mo]

[0189] (In the above formula, [%M] is the content (mass %) of the element M in the steel plate, and the value of the element not contained is set to 0 (zero).)

[0190] [Cooling step: Cooling starts from the first heating temperature at an average cooling rate of 50°C / s or higher to a cooling stop temperature of 100°C or higher and lower than the Ms point]

[0191] [First reheating step: heating to a first reheating temperature of 300°C to 400°C, maintaining at the first reheating temperature for 60 seconds or longer, and cooling to room temperature]

[0192] After the first annealing step, in order to form a structure in which retained austenite exists at the original austenite grain boundaries, the cold-rolled sheet is subjected to a local quenching-partitioning treatment in the cooling step. When the cooling stop temperature of the local quenching is lower than 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 an orientation different from 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 is set to above 100°C. The cooling stop temperature is preferably above 120°C, and more preferably above 150°C.

[0193] On the other hand, when the cooling stop temperature is above the Ms point, martensitic transformation does not occur, so carbon distribution does not occur during the subsequent reheating, and retained austenite is not generated before the second annealing. If the amount of retained austenite is insufficient, austenite with an orientation different from that in the first annealing is generated, making it difficult to uniformly segregate B (boron) at the grain boundaries of such austenite.

[0194] Therefore, the cooling stop temperature in the cooling step is set to be lower than the Ms point. The cooling stop temperature is preferably Ms point - 20°C or lower, more preferably Ms point - 30°C or lower.

[0195] The Ms point is obtained by the following formula.

[0196] Ms(℃)=499-308×[C]-10.8×[Si]-32.4×[Mn]-16.2×[Ni]-27×[Cr]-10.8×[Mo]

[0197] (In the above formula, [M] is the content (mass %) of the element M in the steel plate, and the value of the element not contained is set to 0 (zero).)

[0198] If the average cooling rate is less than 50°C / s, ferrite transformation occurs. This ferrite does not have a specific orientation relationship with austenite, so during the second annealing, austenite with a different orientation than the first annealing is formed, and the boron content at the grain boundaries becomes uneven. Therefore, the average cooling rate is set to 50°C / s or higher. The average cooling rate is preferably set to 60°C / s or higher, and more preferably 70°C / s or higher.

[0199] The upper limit of the average cooling rate is not particularly limited. If the cooling rate is too high, it becomes difficult to control the cooling stop temperature. Therefore, the average cooling rate is preferably set to 1000°C / s or less, more preferably 200°C / s or less.

[0200] The average cooling rate (°C / s) in the cooling process is "(first heating temperature (°C)) - (cooling stop temperature (°C)) / (cooling time (seconds) from the first heating temperature (°C) to the cooling stop temperature (°C))".

[0201] If the first reheating temperature is lower than 300°C, 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, making it difficult to uniformly segregate boron at the grain boundaries of this austenite.

[0202] Therefore, the first reheating temperature is set to be higher than 300° C. The first reheating temperature is preferably higher than 310° C., and more preferably higher than 320° C.

[0203] On the other hand, if the first reheating temperature exceeds 400°C, untransformed austenite decomposes into cementite, 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, making it difficult to uniformly segregate boron (B) at the grain boundaries of this austenite. Therefore, the first reheating temperature is set to 400°C or less. The first reheating temperature is preferably 390°C or less, and more preferably 380°C or less.

[0204] 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 an orientation different from that in the first annealing is formed, making it difficult to uniformly segregate B (boron) at the grain boundaries of such austenite.

[0205] Therefore, the holding time at the first reheating temperature is set to 60 seconds or longer, preferably 80 seconds or longer, and more preferably 100 seconds or longer.

[0206] On the other hand, the upper limit of the holding time is not particularly limited. However, if the holding time is 900 s or longer, 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 set to less than 900 s, more preferably 600 s or less.

[0207] After being held at the first reheating temperature, the mixture is cooled to room temperature. Here, the room temperature as the cooling stop temperature in the first reheating step is not particularly limited, and can be set to 5 to 50°C.

[0208] [Second annealing step: heating to a second heating temperature higher than the Ac3 point]

[0209] In the second annealing step, the steel sheet (cold-rolled sheet) obtained as described above is heated again to a second heating temperature of the Ac3 point or higher and annealed.

[0210] In this process, austenite with the same orientation as the austenite generated in the first annealing is generated with retained austenite as the nucleus. During the generation process, the solid-solution B (boron) diffuses rapidly to the austenite grain boundaries through the dislocations of the martensite before the austenite transformation, and the boron segregation becomes uniform. When the second heating temperature is lower than the Ac3 point, ferrite is generated and the strength is reduced. Therefore, the second heating temperature is set to above the Ac3 point. The second heating temperature is preferably above Ac3 point + 10 ° C, more preferably above Ac3 point + 20 ° C.

[0211] On the other hand, if the temperature is excessively increased, the austenite grain size increases to more than 10 μm, and the toughness decreases. Therefore, the second heating temperature is preferably set to 980° C. or lower. More preferably, the second heating temperature is 950° C. or lower.

[0212] After heating to the second heating temperature, the steel may be cooled to room temperature or over-aged using equipment in an annealing line in order to generate a sufficient amount of martensite in this state and then tempered in the second reheating step described later.

[0213] [Second reheating step: After the second annealing step, the steel sheet is cooled, then heated to a second reheating temperature of 70°C to 200°C, and maintained at the second reheating temperature for 600 seconds or more]

[0214] In the second reheating step, after the second annealing step, the steel sheet is cooled, heated to a second reheating temperature of 70° C. to 200° C., and maintained at the second reheating temperature for 600 seconds or more.

[0215] The cooling conditions before heating (reheating) are not particularly limited, but from the viewpoint of suppressing ferrite transformation and reducing strength loss, cooling is preferably performed at an average cooling rate of 30°C / s or higher. Cooling is preferably performed to room temperature.

[0216] In addition, the average cooling rate (°C / s) in this process is "(second heating temperature (°C))-(cooling stop temperature (°C)) / (cooling time (seconds) from the second heating temperature (°C) to the cooling stop temperature (°C))".

[0217] After cooling, the steel is heated (reheated) to the second reheating temperature. Reheating not only segregates B but also C at the original γ grain boundaries, improving toughness. If the second reheating temperature is below 70°C, C diffusion is slow, resulting in insufficient C segregation. Therefore, the second reheating temperature is set to 70°C or higher. The second reheating temperature is preferably 90°C or higher.

[0218] On the other hand, when the second reheating temperature exceeds 200° C., tempering proceeds excessively, carborides precipitate, and the B concentration in the prior austenite grain boundaries cannot be made 0.05 mass % or more, resulting in a decrease in toughness.

[0219] Therefore, the second reheating temperature is set to be 200° C. or lower. The second reheating temperature is preferably 190° C. or lower.

[0220] If the holding time at the second reheating temperature (reheating holding time) is less than 600 seconds, the diffusion of C is slow and the segregation of C is insufficient. Therefore, the holding time at the second reheating temperature is set to 600 seconds or longer. The holding time at the second reheating temperature is preferably 800 seconds or longer.

[0221] The upper limit of the holding time at the second reheating temperature is not particularly limited. However, in order to prevent precipitation of carboride, the second reheating temperature is preferably 43200 s or less (0.5 day) or less.

[0222] [Plating process, alloying process]

[0223] After the second annealing step and before the second reheating step, at least one side of the steel sheet may be plated 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 heat-treated to alloy the plated layer of the steel sheet to obtain an alloyed plated steel sheet.

[0224] It should be noted that the production conditions other than the above conditions may be in accordance with conventional methods.

[0225] The thickness of the steel sheet of the present embodiment obtained as described above is preferably set to 0.5 mm or more, and preferably set to 2.0 mm or less.

[0226] [member]

[0227] In the present embodiment, a component can be provided that is at least partially made of the above-mentioned steel plate. In one example, the above-mentioned steel plate can be formed into a target shape by stamping to make an automobile part. It should be noted that the automobile part can include steel plates other than the steel plate of the present embodiment as raw materials. According to the present embodiment, a high-strength steel plate with a TS of 1180 MPa or more and excellent stretch flangeability and toughness can be provided, and therefore a component with a TS of 1180 MPa or more and excellent stretch flangeability and toughness can be provided. The steel plate of the present embodiment can be suitably used as an automobile part that helps to reduce the weight of the automobile body. The steel plate of the present embodiment can be appropriately used in automobile parts, especially in all components used as skeleton structural parts or reinforcement parts.

[0228] The method for manufacturing the member includes the step of performing at least one of forming and joining on the steel plate to produce the member.

[0229] The forming process can use a general processing method such as press working without limitation. In addition, the joining process can use a general welding method such as spot welding, arc welding, rivet joining, caulking joining, etc. without limitation.

[0230] Example

[0231] Steel having the composition shown in Table 1 and the balance consisting of Fe and inevitable impurities is melted in a converter to produce a steel billet. The obtained steel billet is reheated and hot-rolled, and coiled to obtain a hot-rolled coil (hot-rolled plate). Next, while uncoiling the hot-rolled coil, it is pickled and cold-rolled to obtain a cold-rolled plate. The plate thickness of the hot-rolled plate is set to 3.0 mm, and the plate thickness of the cold-rolled plate is set to 1.2 mm. Annealing (first annealing step, cooling step, first reheating step, second annealing step) is carried out using a continuous hot-dip galvanizing line under the conditions shown in Table 2 to obtain steel plates (cold-rolled steel plate (CR), hot-dip galvanized steel plate (GI), alloyed hot-dip galvanized steel plate (GA)). The hot-dip galvanized steel plate is immersed in a plating bath at 460°C, set to 35 g / m per single side. 2 The coating adhesion of alloyed hot-dip galvanized steel sheet is adjusted to 45g / m per single side. 2 The obtained steel sheets, except for Steel Sheet No. 9, were reheated under the conditions shown in Table 2 (second reheating step).

[0232]

[0233]

[0234] The resulting steel sheets were evaluated using the aforementioned methods for the combined area ratio of martensite and bainite, the area ratio of retained austenite, the area ratio of ferrite, the prior austenite grain size, the C concentration at prior austenite grain boundaries, the B concentration at prior austenite grain boundaries, and the variation in B concentration within the same grain boundary within prior austenite grain boundaries. Furthermore, tensile strength TS, stretch-flange formability, and toughness were evaluated using the methods described below. The results are shown in Table 3.

[0235] [Tensile test]

[0236] The resulting steel plates were subjected to a tensile test in accordance with JIS Z 2241 (2011). JIS No. 5 tensile test pieces were cut, with the longitudinal direction perpendicular to the rolling direction, and subjected to a tensile test to measure the tensile strength (TS). A tensile strength TS of 1180 MPa or greater was considered good.

[0237] [Charpy test]

[0238] The Charpy impact test was conducted in accordance with JIS Z 2242 (2018). A test piece with a width of 10 mm and a length of 55 mm was cut from the obtained steel plate in such a way that the V-notch was given in the direction perpendicular to the rolling direction of the steel plate, and a 90° V-notch was given in the center of the length with a notch depth of 2 mm. Then, the Charpy impact test was carried out in a test temperature range of -120 to +120°C. The transformation curve was calculated from the obtained brittle fracture rate, and the temperature at which the brittle fracture rate reached 50% was determined as the brittle-ductile transition temperature. It should be noted that the case where the brittle-ductile transition temperature obtained by the Charpy test is below -40°C is judged to be good toughness. In the table, the case where the brittle-ductile transition temperature is below -40°C is indicated as "excellent" toughness, and the case where the brittle-ductile transition temperature exceeds -40°C is indicated as "poor" toughness.

[0239] A 100 mmW x 100 mL test piece was cut from a steel plate (cold-rolled or plated) and subjected to a hole expansion test in accordance with JIS Z 2256 (2010). A 10 mm diameter hole was punched out of the test piece at a clearance of 12 ± 1%. A conical punch with a 60° apex angle was raised to expand the hole. The punch was stopped when cracking occurred in the plate thickness direction. The hole expansion ratio λ was calculated using the following formula based on the hole diameter after cracking and the hole diameter before the test. Limiting hole expansion ratio: λ (%) = {(Df - D0) / D0} × 100 (where Df is the hole diameter (mm) at the time of cracking, and D0 is the initial hole diameter (mm)). Regardless of the strength of the steel plate, a λ value of 30% or greater indicates good stretch flangeability.

[0240]

[0241] As can be seen from Table 3, the examples of the present invention have a tensile strength TS of 1180 MPa or more, and are excellent in stretch-flange formability and toughness. On the other hand, the comparative examples have poor tensile strength TS, stretch-flange formability, and toughness.

[0242] In addition, since the steel plate of the present invention example is high strength and has excellent stretch flange formability and toughness, the components obtained by forming the steel plate of the present invention example, the components obtained by performing a joining process, and the components obtained by further performing a forming process and a joining process are also high strength and have excellent stretch flange formability and toughness like the steel plate of the present invention example.

Claims

1. A steel sheet having a composition comprising, in mass %, 0.10% to 0.30% C, 0.20% to 1.20% Si, 2.5% to 4.0% Mn, 0.050% to 0.020% S, 0.10% to 0.10% N, 0.100% to 0.10% Ti, 0.002% to 0.050% Nb, and 0.0015% to 0.0040% B, satisfying the following formula (1), with the balance consisting of Fe and unavoidable impurities. The total area ratio of martensite and bainite is 95% or more. The area ratio of retained austenite is less than 5%, The area ratio of ferrite is less than 1%, The original austenite grain size is less than 10 μm. The C concentration at the original austenite grain boundary is more than 1.5 times the C content in the steel. The B concentration in the prior austenite grain boundaries is 0.05% or more by mass. The deviation of the B concentration in 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 formula (1), [%N] and [%Ti] represent the contents of N and Ti in steel (mass %), respectively.

2. The steel plate according to claim 1, wherein The component composition further contains, in mass%, at least one selected from the group consisting of 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 plate according to claim 1 or 2, wherein: The coating is provided on at least one side. 4 . A member formed using the steel plate according to claim 1 .

5. A method for manufacturing a steel plate, comprising: a hot rolling step, wherein a steel slab having the composition described in claim 1 or 2 is hot rolled to produce a hot-rolled sheet; A pickling process, wherein the hot-rolled plate is pickled; a cold rolling step, wherein the hot-rolled sheet after the pickling step is cold-rolled to produce a cold-rolled sheet; a first annealing step, wherein the cold-rolled sheet is heated to a first heating temperature above the Ac3 point; a cooling step, wherein the cold-rolled sheet after the first annealing step is cooled from the first heating temperature at an average cooling rate of 50° C. / s or higher to a cooling stop temperature of 100° C. or higher and lower than the Ms point; a first reheating step, wherein, after the cooling step, the material is heated to a first reheating temperature of 300° C. to 400° C., maintained at the first reheating temperature for more than 60 seconds, and then cooled to room temperature; A second annealing step, wherein, after the first reheating step, heating is performed to a second heating temperature above the Ac3 point; and The second reheating step comprises cooling the steel sheet after the second annealing step, heating the steel sheet to a second reheating temperature of 70° C. to 200° C., and maintaining the temperature at the second reheating temperature for 600 seconds or more to obtain the steel sheet.

6. The method for manufacturing a steel plate according to claim 5, wherein: The method includes a plating step of plating the steel sheet after the second annealing step and before the second reheating step.

7. The method for manufacturing a steel plate according to claim 6, wherein: The method includes an alloying step of performing an alloying treatment on the steel sheet after the plating step. 8 . A method for manufacturing a member, comprising the step of subjecting the steel sheet according to claim 1 to at least one of forming and joining to produce the member.

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