Steel sheet, member, and production methods for these

Abstract

Provided are a steel sheet and a member that have high strength as well as excellent bending workability and excellent resistance to delayed fractures, and production methods for these.　The steel sheet contains, in mass%, 0.15% to 0.45% of C, 0.3% to 2.0% of Si, 1.7% to 4.0% of Mn, 0.10% or less of P, 0.01% or less of S, 0.50% or less of sol. Al, 0.010% or less of N, and 0.0008% to 0.0100% of B, with the remainder consisting of Fe and unavoidable impurities. The steel sheet has a steel structure in which the area ratio of martensite to the whole structure is at least 85% but less than 95%, and the area ratio of retained austenite to the whole structure is 5% to 15%. The number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the formula: A (precipitates / mm2)≤8.5×105×[B].

Description

Steel plates, components, and their manufacturing methods

[0001] The present invention relates to a steel sheet such as a high-strength steel sheet for cold press forming that is used after cold press forming in automobiles and the like, a member using the steel sheet, and a method for manufacturing the same.

[0002] In recent years, steel sheets with a tensile strength TS of 1310 MPa or higher have been increasingly used in automotive frame parts for the purposes of reducing the weight of automobiles and improving collision safety. Also, steel sheets with a tensile strength TS of 1470 MPa or higher have been increasingly used in bumpers, impact beams, and other parts.

[0003] Steel sheets used in such automotive frame parts are required to have excellent formability, particularly excellent bend formability, since press forming becomes more difficult as the strength of the steel sheet increases.

[0004] Furthermore, when a high-strength steel sheet having a tensile strength TS of 1470 MPa or more is formed into a part by cold pressing, delayed fracture may occur due to an increase in residual stress within the part or a deterioration in the delayed fracture resistance of the steel sheet itself.

[0005] Here, delayed fracture refers to a phenomenon in which, when a component is placed in a hydrogen penetration environment while high stress is applied to the component, hydrogen penetrates into the steel sheet that makes up the component, reducing the interatomic bonding strength and causing localized deformation, resulting in microcracks, which then propagate and lead to fracture.

[0006] As a technique for improving such delayed fracture resistance, for example, Patent Document 1 discloses a steel sheet containing, in mass %, C: 0.13% or more and 0.40% or less, Si: 0.02% or more and 1.5% or less, Mn: 0.4% or more and 1.7% or less, P: 0.030% or less, S: 0.0002% or more and less than 0.0010%, sol. The alloy contains Al: 0.01% or more and 0.20% or less, N: 0.0055% or less, O: 0.0025% or less, Nb: 0.002% or more and 0.035% or less, and Ti: 0.002% or more and 0.040% or less so as to satisfy the formulas (1) and (2), with the balance being Fe and unavoidable impurities, and the area ratio of martensite and bainite to the entire structure being 95% or more and 100% or less in total, with the balance being one or both of ferrite and retained austenite, the average grain size of prior austenite grains exceeding 5 μm, and the alloy satisfies the following conditions, and the number of inclusions having a major axis length of 20 to 80 μm is 5 / mm ２ The present invention discloses a high-strength steel plate having excellent delayed fracture resistance, characterized by a tensile strength of 1,320 MPa or more, and a steel structure having the following: [%Ti] + [%Nb] > 0.007 (1) [%Ti] × [%Nb] ２ ≦7.5×10 －６ (2) Here, [% Nb] and [% Ti] represent the contents (%) of Nb and Ti.

[0007] Patent Document 2 also describes a steel containing, in mass%, C: 0.05 to 0.30%, Si: 2.0% or less (including 0%), Mn: more than 0.1% and 2.8% or less, P: 0.1% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.01 to 0.50% or less, and one or more of Nb, Ti, and Zr, which are 0.01% or more in total, and [%C] - [%Nb] / 92.9 × 12 - [% The alloy has a composition in which the ratio of [%Ti] / 47.9×12 - [%Zr] / 91.2×12 > 0.03 is satisfied, with the balance being iron and unavoidable impurities, and has a structure in which tempered martensite accounts for 50% or more (including 100%) in area ratio, with the balance being ferrite, and the distribution state of precipitates in the tempered martensite is such that precipitates having a circle equivalent diameter of 1 to 10 nm are present in the tempered martensite at 1 μm or less. ２ The precipitates are 20 or more per 1000 micrometers and have an equivalent circle diameter of 20 nm or more, and contain one or more of Nb, Ti and Zr. ２ The document discloses a high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and workability, characterized in that the average grain size of ferrite surrounded by high-angle grain boundaries with a crystal orientation difference of 15° or more is 5 μm or less, and the number of grains per grain is 10 or less.

[0008] Patent No. 6388085 Patent No. 4712882

[0009] However, the conventional technology is not sufficient to ensure a tensile strength TS of 1470 MPa or more while also achieving excellent bending formability and excellent resistance to delayed fracture, and there has been a demand for the establishment of a new technology.

[0010] The present invention has been made to solve such problems, and an object of the present invention is to provide a steel plate, a member, and a method for manufacturing the same, which have a tensile strength of 1470 MPa or more (TS≧1470 MPa) and which combine excellent bend formability and excellent delayed fracture resistance.

[0011] "Excellent bend formability" refers to a judgment that the steel sheet has excellent bend formability based on the following evaluation. (1) JIS No. 3 test pieces are taken from each steel sheet, with the longitudinal direction perpendicular to the rolling direction (coil width direction), and a 90° V-bend test is performed using the V-block method in accordance with JIS Z 2248 (2022) at various bending radii. (2) Next, bendability is evaluated by the value (R / t) obtained by dividing the minimum bending radius R at which cracks of 0.3 mm or more do not occur on the surface of the test piece by the sheet thickness t. The direction of the bend ridge is parallel to the rolling direction. (3) A steel sheet with an R / t of 3.0 or less is judged to have excellent bendability.

[0012] "Excellent delayed fracture resistance" refers to a judgment that the steel sheet (coil) has excellent delayed fracture resistance based on the following evaluation. (1) First, a strip test piece measuring 100 mm in the direction perpendicular to the rolling direction and 30 mm in the rolling direction is taken from the end of the width direction of the obtained steel sheet (coil) at a position 1 / 4 of the coil width. (2) The end face of the long side with a length of 100 mm is sheared, and the test piece is bent in the as-sheared state (without machining to remove burrs) so that the burrs are on the outer periphery of the bend. The test piece is fixed with bolts while maintaining the shape of the bent piece. The shearing clearance is 13% and the rake angle is 1°. The bending is performed with a tip bending radius of 10 mm and an inner angle of the bend apex of 90 degrees (V-bend). The punch used has a tip radius R equal to the tip bending radius R and is U-shaped (the tip R portion is semicircular and the punch body thickness is 2R), and the die has a corner R of 30 mm. The depth to which the punch pushes the steel plate is then adjusted to form a 90-degree bend angle (the angle at the inner side of the bend apex) (V-shape). The test specimen is clamped and tightened with a hydraulic jack so that the distance between the flange ends of the straight pieces during bending is the same as when the specimen was bent (to cancel out the opening of the straight pieces due to springback), and then bolted in this state. The bolt is passed through an elliptical hole (minor axis 10 mm, major axis 15 mm) pre-installed 10 mm inward from the short edge of the strip specimen to secure it in place. (3) The bolted specimen is then immersed in a solution prepared by mixing 0.1% by mass of ammonium thiocyanate aqueous solution and McIlvaine buffer solution in a 1:1 mass ratio and adjusting the pH to 8.0 to perform a delayed fracture resistance evaluation test. The solution temperature is 20°C, and the specimen is measured for a surface area of ​​1 cm. ３ (4) After 24 hours, the presence or absence of visually observable cracks (length of 1 mm or more) is checked, and those in which no cracks are observed are judged to have excellent delayed fracture resistance.

[0013] The present inventors have conducted extensive research to solve the above problems and have found that delayed fracture resistance can be significantly improved by satisfying all of the following conditions: i) The area ratio of martensite is 85% or more and less than 95%; ii) The area ratio of retained austenite to the entire structure is 5% or more and 15% or less; and iii) The number density A of precipitates with a circle equivalent diameter of 500 nm or more satisfies the following conditions: A (numbers / mm ２ ) ≦ 8.5 × 10 ５ ×[B] Here, [B] represents the content of B (% by mass).

[0014] The present invention was completed through further studies based on the above findings, and the gist of the present invention is as follows: [1] A steel sheet having a chemical composition containing, by mass%, C: 0.15% to 0.45%, Si: 0.3% to 2.0%, Mn: 1.7% to 4.0%, P: 0.10% or less, S: 0.01% or less, sol. Al: 0.50% or less, N: 0.010% or less, B: 0.0008% to 0.0100% or less, with the balance being Fe and unavoidable impurities, having a steel structure in which the area ratio of martensite to the entire structure is 85% to less than 95%, and the area ratio of retained austenite to the entire structure is 5% to 15%, and in which the number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the following formula (1): A (numbers / mm ２ ) ≦ 8.5 × 10 ５×[B] Formula (1) Here, [B] represents the content of B (% by mass). [2] The steel sheet according to [1], wherein the chemical composition further contains, in mass%, one or more elements selected from Cu: 1.00% or less, Cr: 1.00% or less, Nb: 0.10% or less, Ti: 0.10% or less, V: 0.50% or less, Mo: 0.50% or less, Ni: 1.00% or less, Sb: 0.10% or less, Sn: 0.10% or less, As: 0.10% or less, Ta: 0.10% or less, Ca: 0.020% or less, Mg: 0.020% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, W: 0.020% or less, and REM: 0.020% or less. [3] The steel sheet according to [1] or [2], having a plating layer on the surface of the steel sheet. [4] A member made using the steel sheet according to any one of [1] to [3]. [5] A steel slab having the chemical composition according to [1] or [2] is held at a heating holding temperature of 1100°C or higher at the slab surface temperature for 30 minutes or more, and then hot finish rolling is performed under conditions of a residence time at 900 to 1000°C of 20 seconds or more and 150 seconds or less, and a finish rolling temperature of 850°C or higher, cooling is performed at an average cooling rate of 40°C / second or more in the range from the finish rolling temperature to 650°C, and then coiled at a coiling temperature of 650°C or lower to obtain a hot rolled steel sheet, and the hot rolled steel sheet is cold rolled at a rolling reduction of 40% or more to obtain a cold rolled steel sheet, and an annealing temperature is set to 830 to 950°C, and the cold rolled steel sheet is heated from 400°C to the annealing temperature at an average heating rate of 1.0°C / second or more, and held at the annealing temperature for 600 seconds or less,

[0013] A method for producing a steel sheet, comprising: cooling from the annealing temperature to a cooling stop temperature of 150 to 250°C at an average cooling rate of 10°C / second or more; heating from the cooling stop temperature to a reheating holding temperature of 250 to 450°C; and then performing continuous annealing by holding at the reheating holding temperature for 20 to 1,500 seconds. [6] A method for producing a steel sheet according to [5], wherein a plating treatment is performed on the surface of the steel sheet after the continuous annealing. [7] A method for producing a member, comprising a step of performing at least one of forming and joining on the steel sheet according to any of [1] to [3] to form a member.

[0015] According to the present invention, there are provided a steel plate and a member having high strength and excellent bending formability and delayed fracture resistance, and methods for producing the same.

[0016] Hereinafter, an embodiment of the present invention will be described.

[0017] The steel sheet of the present invention has a composition containing, by mass%, C: 0.15% to 0.45%, Si: 0.3% to 2.0%, Mn: 1.7% to 4.0%, P: 0.10% or less, S: 0.01% or less, sol. Al: 0.50% or less, N: 0.010% or less, B: 0.0008% to 0.0100% or less, with the balance being Fe and unavoidable impurities, and has a steel structure in which the area ratio of martensite to the entire structure is 85% to less than 95%, and the area ratio of retained austenite to the entire structure is 5% to 15%, and the number density A of precipitates having a circle equivalent diameter of 500 nm or more satisfies the following formula (1): A (numbers / mm ２ ) ≦ 8.5 × 10 ５ ×[B] Formula (1) Here, [B] represents the content of B (% by mass).

[0018] The reasons for limiting the range of the chemical composition of the steel sheet of the present invention will be explained below. Note that % regarding the content of each element is "mass %".

[0019] C: 0.15% or more and 0.45% or less C is contained to increase the strength of martensite and obtain a tensile strength of 1470 MPa or more (hereinafter also referred to as TS≧1470 MPa). Therefore, in order to obtain a desired TS, the C content is set to 0.15% or more. From the viewpoint of reducing the weight of automotive frame parts by increasing strength, the C content is preferably 0.20% or more, more preferably 0.25% or more. On the other hand, if excessive C is added, C concentrates and stabilizes austenite, which leads to the excessive formation of retained austenite and deteriorates delayed fracture resistance. Therefore, the C content is set to 0.45% or less. The C content is preferably 0.40% or less, more preferably 0.35% or less.

[0020] Si: 0.3% or more and 2.0% or less Si suppresses the formation of cementite, thereby suppressing a decrease in strength and deterioration of delayed fracture resistance. Si also suppresses the formation of cementite and promotes the formation of retained austenite, thereby improving bendability. Therefore, the Si content is set to 0.3% or more. The Si content is preferably 0.5% or more. On the other hand, excessive addition of Si leads to deterioration of delayed fracture resistance due to segregation of Si. Therefore, the Si content is set to 2.0% or less. The Si content is preferably 1.8% or less, and more preferably 1.5% or less.

[0021] Mn: 1.7% or more and 4.0% or less Mn is an element effective in improving the hardenability of steel. In order to suppress the formation of ferrite and stably obtain the desired area ratio of martensite, the Mn content is set to 1.7% or more. The Mn content is preferably 2.3% or more. On the other hand, if excessive Mn is added, martensitic transformation is excessively suppressed, and the desired martensite and retained austenite cannot be obtained. Therefore, the Mn content is set to 4.0% or less. The Mn content is preferably 3.2% or less.

[0022] P: 0.10% or less P segregates at grain boundaries and reduces grain boundary strength, thereby deteriorating delayed fracture resistance. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.05% or less, more preferably 0.02% or less, and even more preferably 0.01% or less. There is no specified lower limit for the P content, but the lower limit currently industrially feasible is 0.002%. Therefore, the P content is preferably set to 0.002% or more.

[0023] S: 0.01% or less S forms coarse inclusions with Mn, which become the starting points for delayed fracture, thereby deteriorating delayed fracture resistance. Therefore, the S content is set to 0.01% or less. The S content is preferably 0.003% or less, more preferably 0.0015% or less, and even more preferably 0.0008% or less. Although no lower limit is specified, the lower limit currently industrially feasible is 0.0002%. Therefore, the S content is preferably set to 0.0002% or more.

[0024] Sol. Al: 0.50% or less Al is contained to perform sufficient deoxidation and reduce inclusions in steel. Although the lower limit of sol. Al is not particularly specified, in order to perform stable deoxidation, it is desirable that the sol. Al content be 0.005% or more. The sol. Al content is more preferably 0.01% or more, and even more preferably 0.02% or more. On the other hand, if the sol. Al content exceeds 0.50%, cementite formed during coiling is less likely to dissolve in the annealing process, the number density A of precipitates cannot be set within the desired range, and delayed fracture resistance is significantly deteriorated. Therefore, the sol. Al content is set to 0.50% or less. The sol. Al content is preferably 0.20% or less, and more preferably 0.05% or less.

[0025] N: 0.010% or less N forms precipitates such as AlN, which act as initiation sites for delayed fracture, thereby deteriorating delayed fracture resistance. In particular, if the N content exceeds 0.010%, the number density A of precipitates cannot be set within the desired range, resulting in significant deterioration of delayed fracture resistance. Therefore, the N content is set to 0.010% or less. The N content is preferably set to 0.005% or less. Although no lower limit is specified, the lowest limit currently industrially feasible is 0.0006%. Therefore, the N content is preferably set to 0.0006% or more.

[0026] B: 0.0008% or more and 0.0100% or less B is an element that improves the hardenability of steel, and has the effect of generating a predetermined area ratio of martensite even with a small Mn content. In addition, B segregates at grain boundaries to increase the bonding strength of the grain boundaries and suppresses the segregation of P, which reduces grain boundary strength. In order to obtain the desired delayed fracture resistance due to these effects, the B content is set to 0.0008% or more. The B content is preferably 0.0015% or more, and more preferably 0.0020% or more. On the other hand, excessive addition of B increases the Fe content. ２３ (C, B) ６It has been found that the addition of B results in the formation of BN and BN, which become the starting point of delayed fracture, thereby deteriorating the delayed fracture resistance. Therefore, in order to obtain the effect of improving the delayed fracture resistance by adding B, it is necessary to add B while keeping the number density A of precipitates within a desired range. If the B content exceeds 0.0100%, it is difficult to reduce the B-based precipitates even by controlling the hot rolling conditions and annealing conditions, and the number density A of precipitates cannot be kept within a desired range. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, and more preferably 0.0060% or less.

[0027] The steel sheet of the present invention preferably has a composition containing the above-mentioned basic elements with the balance being iron (Fe) and unavoidable impurities.

[0028] In the present invention, the composition may contain the following: one or more elements selected from, by mass%, Cu: 1.00% or less, Cr: 1.00% or less, Nb: 0.10% or less, Ti: 0.10% or less, V: 0.50% or less, Mo: 0.50% or less, Ni: 1.00% or less, Sb: 0.10% or less, Sn: 0.10% or less, As: 0.10% or less, Ta: 0.10% or less, Ca: 0.020% or less, Mg: 0.020% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, W: 0.020% or less, and REM: 0.020% or less.

[0029] Cu: 1.00% or less Cu has the effects of improving the corrosion resistance of the steel sheet, reducing hydrogen penetration into the steel sheet, and improving delayed fracture resistance. Although there is no lower limit for the Cu content, to achieve these effects, the Cu content is desirably 0.01% or more. The Cu content is preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, excessive addition of Cu increases the number of coarse precipitates, making it impossible to achieve a desired number density A of the precipitates, and deteriorating delayed fracture resistance. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. The Cu content is preferably 0.50% or less, and more preferably 0.30% or less.

[0030] Cr: 1.00% or less Cr is an element effective in improving the hardenability of steel. Cr can be added to stably obtain a desired structure. While the lower limit of the Cr content is not particularly specified, to obtain this effect, the Cr content is desirably 0.01% or more. The Cr content is more preferably 0.05% or more, and even more preferably 0.10% or more. On the other hand, excessive addition of Cr delays the solid solution of cementite during annealing, leaving a large amount of undissolved cementite. This prevents the number density A of precipitates from being within the desired range, resulting in a deterioration of delayed fracture resistance. Therefore, when Cr is contained, the Cr content is set to 1.00% or less. The Cr content is preferably 0.50% or less, and more preferably 0.30% or less.

[0031] Nb: 0.10% or less Nb forms fine precipitates such as NbC in steel, thereby refining the prior austenite grain size through a pinning effect and improving delayed fracture resistance. While there is no lower limit for the Nb content, in order to obtain this effect of improving delayed fracture resistance, the Nb content is desirably 0.005% or more. The Nb content is preferably 0.01% or more. On the other hand, excessive addition of Nb increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, thereby deteriorating delayed fracture resistance. Therefore, when Nb is contained, the Nb content is set to 0.10% or less. The Nb content is preferably 0.08% or less, and more preferably 0.06% or less.

[0032] Ti: 0.10% or less Ti forms fine precipitates such as TiC in steel, thereby refining the prior austenite grain size through a pinning effect and improving delayed fracture resistance. While there is no lower limit for the Ti content, to achieve this effect, the Ti content is desirably 0.005% or more. The Ti content is preferably 0.01% or more. On the other hand, excessive addition of Ti increases the number of coarse precipitates, making it impossible to achieve a desired precipitate number density A, thereby deteriorating delayed fracture resistance. Therefore, when Ti is contained, the Ti content is set to 0.10% or less. The Ti content is preferably 0.08% or less, more preferably 0.06% or less.

[0033] V: 0.50% or less V has the effect of generating fine V-containing carbides that act as hydrogen trapping sites, thereby improving delayed fracture resistance. Furthermore, by forming fine precipitates, the prior austenite grain size is refined through a pinning effect, thereby improving delayed fracture resistance. While there is no specified lower limit for the V content, to achieve this effect, the V content is desirably 0.003% or more. The V content is preferably 0.01% or more, and more preferably 0.03% or more. On the other hand, excessive V addition increases the number of coarse precipitates, making it impossible to achieve a desired precipitate number density A, thereby deteriorating delayed fracture resistance. Therefore, when V is contained, the V content is set to 0.50% or less. The V content is preferably 0.20% or less, more preferably 0.10% or less, and even more preferably 0.06% or less.

[0034] Mo: 0.50% or less Mo has the effect of forming fine carbides containing Mo that act as hydrogen trapping sites, thereby improving delayed fracture resistance. Furthermore, by forming fine precipitates, the prior austenite grain size is refined through a pinning effect, thereby improving delayed fracture resistance. While there is no specified lower limit for the Mo content, in order to achieve this effect of improving delayed fracture resistance, the Mo content is desirably 0.003% or more. The Mo content is preferably 0.01% or more, more preferably 0.03% or more. On the other hand, excessive addition of Mo increases the number of coarse precipitates, making it impossible to achieve a desired precipitate number density A, thereby deteriorating delayed fracture resistance. Therefore, when Mo is contained, the Mo content is set to 0.50% or less. The Mo content is preferably 0.20% or less, more preferably 0.10% or less.

[0035] Ni: 1.00% or less Ni has the effects of improving the corrosion resistance of steel sheets, suppressing hydrogen penetration into steel sheets, and improving delayed fracture resistance. Ni is also an element effective in improving the hardenability of steel and can be added to stably obtain a desired structure. While there is no specified lower limit for the Ni content, to obtain these effects, the Ni content is desirably 0.01% or more. The Ni content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, excessive Ni addition increases the number of coarse precipitates, making it impossible to achieve the desired precipitate number density A within the desired range, resulting in a deterioration in delayed fracture resistance. Therefore, when Ni is contained, the Ni content is set to 1.00% or less. The Ni content is preferably 0.50% or less, more preferably 0.30% or less.

[0036] Sb: 0.10% or less Sb suppresses oxidation and nitriding of the surface layer of the steel sheet, contributing to an increase in strength and an improvement in delayed fracture resistance. Although the lower limit of the Sb content is not specified, in order to obtain such effects as an increase in strength and an improvement in delayed fracture resistance, the Sb content is desirably 0.002% or more. The Sb content is preferably 0.004% or more, and more preferably 0.006% or more. On the other hand, excessive addition of Sb increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating the delayed fracture resistance. Therefore, when Sb is contained, the Sb content is set to 0.10% or less. The Sb content is preferably 0.05% or less, and more preferably 0.02% or less.

[0037] Sn: 0.10% or less Sn suppresses oxidation and nitriding of the surface layer of the steel sheet, contributing to increased strength and improved delayed fracture resistance. While there is no lower limit for the Sn content, in order to achieve such effects as increased strength and improved delayed fracture resistance, the Sn content is desirably 0.002% or more. The Sn content is preferably 0.004% or more, and more preferably 0.006% or more. On the other hand, excessive addition of Sn increases the number of coarse precipitates, making it impossible to achieve a desired number density A of the precipitates, and deteriorating delayed fracture resistance. Therefore, when Sn is contained, the Sn content is set to 0.10% or less. The Sn content is preferably 0.05% or less, and more preferably 0.02% or less.

[0038] As: 0.10% or less As has the effect of increasing the strength of steel. Although there is no lower limit for the As content, in order to obtain this effect of increasing the strength of steel, the As content is desirably 0.002% or more. The As content is preferably 0.004% or more, more preferably 0.006% or more. On the other hand, excessive addition of As increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating the delayed fracture resistance. Therefore, when As is contained, the As content is set to 0.10% or less. The As content is preferably 0.05% or less, more preferably 0.02% or less.

[0039] Ta: 0.10% or less Ta has the effect of increasing the strength of steel. Although there is no lower limit for the Ta content, in order to obtain this effect of increasing the strength of steel, the Ta content is desirably 0.002% or more. The Ta content is preferably 0.004% or more, more preferably 0.006% or more. On the other hand, if Ta is added in excess, the number density A of the precipitates increases, making it impossible to achieve the desired range, and the delayed fracture resistance deteriorates. Therefore, when Ta is contained, the Ta content is set to 0.10% or less. The Ta content is preferably 0.05% or less, more preferably 0.02% or less.

[0040] Ca: 0.020% or less Ca reduces the initiation points of delayed fracture by spheroidizing the shape of sulfides, thereby improving delayed fracture resistance. To achieve this effect of improving delayed fracture resistance, the Ca content is desirably 0.0002% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, excessive addition of Ca increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating delayed fracture resistance. Therefore, when Ca is contained, the Ca content is set to 0.020% or less. The Ca content is preferably 0.010% or less, and more preferably 0.008% or less.

[0041] Mg: 0.020% or less Mg reduces the initiation points of delayed fracture by spheroidizing the shape of sulfides, thereby improving delayed fracture resistance. To achieve this effect of improving delayed fracture resistance, the Mg content is desirably 0.0002% or more. The Mg content is preferably 0.001% or more, and more preferably 0.003% or more. On the other hand, excessive addition of Mg increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating delayed fracture resistance. Therefore, when Mg is contained, the Mg content is set to 0.020% or less. The Mg content is preferably 0.015% or less, and more preferably 0.010% or less.

[0042] Zn: 0.020% or less Zn improves delayed fracture resistance by refining the prior austenite grain size and spheroidizing the shape of inclusions. To achieve this effect of improving delayed fracture resistance, the Zn content is desirably 0.001% or more. The Zn content is preferably 0.003% or more. On the other hand, excessive addition of Zn increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating delayed fracture resistance. Therefore, when Zn is contained, the Zn content is set to 0.020% or less. The Zn content is preferably 0.015% or less, and more preferably 0.010% or less.

[0043] Co: 0.020% or less Co improves delayed fracture resistance by refining the prior austenite grain size and spheroidizing the shape of inclusions. To achieve this effect of improving delayed fracture resistance, the Co content is desirably 0.001% or more. The Co content is preferably 0.003% or more. On the other hand, excessive addition of Co increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating delayed fracture resistance. 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.

[0044] Zr: 0.020% or less Zr improves delayed fracture resistance by refining the prior austenite grain size and spheroidizing the shape of inclusions. To achieve this effect of improving delayed fracture resistance, the Zr content is desirably 0.001% or more. The Zr content is preferably 0.003% or more. On the other hand, excessive addition of Zr increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, thereby deteriorating delayed fracture resistance. Therefore, when Zr is contained, the Zr content is set to 0.020% or less. The Zr content is preferably 0.015% or less, and more preferably 0.010% or less.

[0045] W: 0.020% or less W refines the prior austenite grain size through the formation of precipitates, thereby improving delayed fracture resistance. To achieve this effect of improving delayed fracture resistance, the W content is desirably 0.001% or more. The W content is preferably 0.003% or more. On the other hand, excessive addition of W increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, thereby deteriorating delayed fracture resistance. Therefore, when W is contained, the W content is set to 0.020% or less. The W content is preferably 0.015% or less, and more preferably 0.010% or less.

[0046] REM: 0.020% or less. REM also contributes to improving delayed fracture resistance by spheroidizing inclusions. To achieve this effect of improving delayed fracture resistance, the REM content is desirably 0.0002% or more. The REM content is preferably 0.001% or more, more preferably 0.003% or more. On the other hand, excessive addition of REM increases the number of coarse precipitates, making it impossible to achieve the desired number density A of the precipitates, and deteriorating delayed fracture resistance. Therefore, when REM is contained, the REM content is set to 0.020% or less. The REM content is preferably 0.015% or less, more preferably 0.010% or less. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REMs. The REMs are not particularly limited, but are preferably La and / or Ce.

[0047] When the optional element is contained in an amount less than the preferable lower limit, the optional element is considered to be contained as an unavoidable impurity.

[0048] The steel structure of the steel plate of the present invention has the following features: (Feature 1) The area ratio of martensite to the entire structure is 85% or more and less than 95%, and the area ratio of retained austenite to the entire structure is 5% or more and 15% or less. (Feature 2) The number density A of precipitates having a circle equivalent diameter of 500 nm or more satisfies the following formula (1): A (numbers / mm ２ ) ≦ 8.5 × 10 ５ ×[B] Formula (1) Here, [B] represents the content of B (% by mass).

[0049] Each component will be described below.

[0050] (Configuration 1) The area fraction of martensite relative to the entire structure is 85% or more but less than 95%, and the area fraction of retained austenite relative to the entire structure is 5% or more but less than 15%. In the present invention, by using martensite as the main phase, it is possible to achieve high strength of TS≧1470 MPa. To achieve this effect of achieving high strength, the area fraction of martensite needs to be 85% or more. The area fraction of martensite is preferably 88% or more. It has also been found that by using martensite as the main phase and generating an area fraction of retained austenite of 5% or more but less than 15%, excellent delayed fracture resistance and excellent bend formability can be obtained. To achieve both such excellent delayed fracture resistance and excellent bend formability, the area fraction of martensite is less than 95%, and the area fraction of retained austenite is 5% or more. The area fraction of martensite is preferably less than 93%, and the area fraction of retained austenite is preferably 7% or more. On the other hand, the generation of excessive retained austenite reduces strength and deteriorates delayed fracture resistance. Therefore, in order to stably obtain excellent delayed fracture resistance and excellent bend formability, the area fraction of retained austenite is set to 15% or less, and preferably 12% or less.

[0051] In addition, when the steel contains structures other than martensite and retained austenite, the remainder is preferably composed of ferrite, pearlite, and bainite. Other than these structures, trace amounts of carbides, sulfides, nitrides, and oxides may be present. The remainder structure is 10% or less in area ratio, preferably 5% or less, and more preferably 3% or less. The remainder structure may be 0%. In other words, the steel structure may be composed of martensite and retained austenite. Furthermore, martensite includes martensite that has undergone self-tempering during continuous cooling, as well as martensite that has not undergone tempering by staying at approximately 150°C or higher for a certain period of time.

[0052] (Configuration 2) The number density A of precipitates having a circle-equivalent diameter of 500 nm or more satisfies the following formula: A (number / mm ２ ) ≦ 8.5 × 10 ５ × [B] Formula (1) Here, [B] represents the content of B (mass%). In order to suppress delayed fracture in steel having a high strength of TS≧1470 MPa, it is necessary to strengthen the grain boundaries with B and suppress intergranular fracture, in addition to controlling the area ratio of martensite and the area ratio of retained austenite. However, simply increasing the B content increases not only the grain boundary segregation B but also the Fe, which is the starting point of delayed fracture. ２３ (C, B) ６ It was found that this actually reduces the delayed fracture resistance because it also increases B-based precipitates mainly composed of B. The inventors have found that by reducing the number density A of precipitates with a circle equivalent diameter of 500 nm or more by controlling the hot rolling conditions, etc., and satisfying the following conditions, it is possible to simultaneously improve the delayed fracture resistance due to grain boundary strengthening by B and suppress fracture originating from precipitates. A (number / mm ２ ) ≦ 8.5 × 10 ５ × [B] The number density A of precipitates is preferably A (pieces / mm ２ ) ≦ 6.5 × 10 ５ × [B], and more preferably, A (pieces / mm ２ ) ≦ 5.0 × 10 ５ In the present invention, the lower limit of A is not particularly limited, and A may be 0, and A (pieces / mm ２ ) ≧0.5×10 ５×[B] may also be used.

[0053] The method for measuring each component in the above steel structure will be described. The area ratios of martensite, bainite, and ferrite are measured by polishing an L-section of a steel sheet (a section parallel to the rolling direction and perpendicular to the steel sheet surface (hereinafter also referred to as a perpendicular section parallel to the rolling direction)) and then etching it with nital. Four fields of view within a range of 50 μm × 65 μm are observed with an SEM at a magnification of 2000 times at a position ¼ of the thickness from the steel sheet surface, and the photographed structure is analyzed by image analysis. Here, martensite and bainite refer to structures that appear gray or white in SEM. On the other hand, ferrite is a region that appears in black contrast in SEM. Note that martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides inside, but since it is difficult to exclude these, the area ratio of the region including these is defined as the area ratio.

[0054] Bainite has the following characteristics: It has an aspect ratio of 2.5 or more, exhibits a plate-like form, and is a slightly black structure compared to martensite. The width of the plates is 0.3 to 1.7 μm. The distribution density of carbides with a diameter of 10 to 200 nm inside the bainite is 0 to 3 particles / μm. ２ is.

[0055] The retained austenite (retained gamma) is measured by chemically polishing the surface of the steel sheet with oxalic acid for 200 μm, and then measuring the surface using the X-ray diffraction intensity method. It is calculated from the integrated intensity of the (200) α, (211) α, (220) α, (200) γ, (220) γ, and (311) γ diffraction peaks measured using Mo-Kα radiation.

[0056] The number density A of precipitates having a circle equivalent diameter of 500 nm or more is measured after polishing the L cross section (vertical cross section parallel to the rolling direction) of the steel sheet, in the region from the 1 / 5 position to the 4 / 5 position of the sheet thickness of the steel sheet, that is, in the region from the 1 / 5 position of the sheet thickness from the surface of the steel sheet to the 4 / 5 position, sandwiching the center of the sheet thickness. ２The area is photographed continuously with an SEM, and the number of such precipitates is counted from the photographed SEM photographs. The magnification for photographing is 2000 times. When analyzing the components of individual inclusion particles, the individual inclusion particles are magnified 10000 times and the above precipitates are analyzed. Here, precipitates with a circle equivalent diameter of 500 nm or more are considered to be Fe ２３ (C, B) ６ The presence or absence of a peak of B is examined by elemental analysis using energy dispersive X-ray spectroscopy (EDS) at an acceleration voltage of 3 kV, and if a peak of B is present, it is evaluated that the above-mentioned precipitates are present. The circle equivalent diameter refers to the diameter of a perfect circle having the area of ​​each precipitate calculated from an SEM photograph.

[0057] Tensile strength (TS): 1470 MPa or more When the tensile strength of a steel plate is 1470 MPa or more, the delayed fracture resistance is significantly deteriorated. In this regard, one of the features of the present invention is that the delayed fracture resistance is good even when the tensile strength is 1470 MPa or more. From the viewpoint of reducing the weight of automotive frame parts, the tensile strength is preferably 1700 MPa or more. The tensile strength of the steel plate of the present invention may be 2100 MPa or less.

[0058] The tensile strength can be measured by cutting a JIS No. 5 tensile test piece at a position 1 / 4 of the coil width so that the direction perpendicular to the rolling direction is the longitudinal direction, and performing a tensile test in accordance with JIS Z2241 (2022).

[0059] The steel sheet of the present invention may have a plating layer on its surface. The plating layer may be Zn plating or other metal plating. The plating layer may be either a hot-dip plating layer or an electroplated layer.

[0060] Next, a method for producing a steel sheet according to the present invention will be described. In the method for producing a steel sheet according to the present invention, a steel slab having the above-described chemical composition is held at a heating holding temperature of 1100°C or higher at the surface of the slab for 30 minutes or more, and then subjected to hot finish rolling under conditions of a residence time of 20 to 150 seconds at 900 to 1000°C and a finish rolling temperature of 850°C or higher, followed by cooling at an average cooling rate of 40°C / sec or higher in the range from the finish rolling temperature to 650°C, and then coiled at a coiling temperature of 650°C or lower to produce a hot rolled steel sheet, and the hot rolled steel sheet is then cooled by 40% or more. This steel sheet manufacturing method includes cold rolling the steel sheet at a rolling reduction to obtain a cold-rolled steel sheet, annealing the steel sheet at a temperature of 830 to 950°C, heating the cold-rolled steel sheet from 400°C to the annealing temperature at an average heating rate of 1.0°C / second or more, holding the cold-rolled steel sheet at the annealing temperature for 600 seconds or less, cooling the cold-rolled steel sheet from the annealing temperature to a cooling stop temperature of 150 to 250°C at an average cooling rate of 10°C / second or more, heating the cold-rolled steel sheet from the cooling stop temperature to a reheating holding temperature of 250 to 450°C, and holding the reheating holding temperature for 20 to 1500 seconds. Note that the temperatures specified in each step in the present invention refer to the surface temperatures of the slab (steel slab) or steel sheet.

[0061] Hot rolling Heating holding temperature: slab surface temperature of 1100°C or higher Heating holding time: 30 minutes or longer In slab heating before hot rolling, the steel slab is held at a heating holding temperature (slab heating holding temperature) of 1100°C or higher (slab surface temperature) for 30 minutes or longer, which promotes solid solution of precipitates such as B-based precipitates and reduces the size and number of precipitates. The heating holding temperature is preferably 1200°C or higher. The heating holding temperature is preferably 1250°C or lower. Furthermore, the time held at the heating holding temperature (holding time (slab heating holding time)) is preferably 40 minutes or longer. The holding time is preferably 50 minutes or shorter.

[0062] Residence time at 900 to 1000°C: 20 seconds or more and 150 seconds or less During hot rolling, the slab is retained at 900 to 1000°C for 20 seconds or more and 150 seconds or less. Increasing the residence time in the 900 to 1000°C temperature range generates BN-based precipitates and coarsens these precipitates. Precipitates formed in these temperature ranges are difficult to dissolve during annealing, reducing the amount of dissolved B after annealing. A residence time of more than 150 seconds fails to obtain an amount of dissolved B effective in suppressing delayed fracture. Therefore, the residence time is 150 seconds or less, preferably 120 seconds or less, and more preferably 100 seconds or less. On the other hand, a residence time of less than 20 seconds may result in a non-uniform structure. Therefore, the residence time is 20 seconds or more. The residence time is preferably 30 seconds or more.

[0063] Finish rolling temperature: 850°C or higher In hot finish rolling, the finish rolling temperature (FT) is set to 850°C or higher to suppress non-uniformity of the hot rolled texture. The finish rolling temperature is preferably 870°C or higher. In addition, the finish rolling temperature is preferably 930°C or lower.

[0064] Average cooling rate in the range from the finish rolling temperature to 650°C (first average cooling rate): 40°C / sec or more In the cooling after hot finish rolling, cooling is performed at an average cooling rate of 40°C / sec or more in the range from the finish rolling temperature to 650°C. In the temperature range from the finish rolling temperature to 650°C, B segregates at the grain boundaries with the recrystallization of austenite, and Fe ２３ (C, B) ６ is precipitated. ２３ (C, B) ６ In order to minimize the precipitation of , the average cooling rate (first average cooling rate) is set to 40°C / second or more. The average cooling rate is preferably 60°C / second or more. The average cooling rate is preferably 500°C / second or less, and more preferably 300°C / second or less. The average cooling rate (first average cooling rate) in the hot rolling process is "(temperature at the start of cooling (finish rolling temperature) (°C) - temperature at the end of cooling (°C) (650°C)) / cooling time (seconds) from the start of cooling to the end of cooling."

[0065] Coiling temperature: 650°C or less After cooling to 650°C as described above, the wire is further cooled as needed before coiling. At this time, if the coiling temperature exceeds 650°C, the Fe ２３ (C, B) ６ Therefore, the coiling temperature is set to 650°C or lower. Preferably, the coiling temperature is 600°C or lower. Also, the coiling temperature is preferably 500°C or higher.

[0066] Cold rolling reduction: 40% or more If the reduction (cumulative reduction (cold rolling reduction)) in cold rolling is 40% or more, the recrystallization behavior and texture orientation in the subsequent continuous annealing can be stabilized. If the reduction is less than 40%, some austenite grains may become coarse during annealing, which may result in a decrease in strength. In addition, the reduction is preferably 80% or less.

[0067] Continuous annealing Average heating rate from 400°C to the annealing temperature: 1.0°C / sec or more After cold rolling, the steel sheet is annealed and tempered in a continuous annealing line (CAL), and further subjected to temper rolling as required. ２３ (C, B) ６ is generated in the ferrite region during annealing and coarsens, so Fe ２３ (C, B) ６ In order to reduce the amount of B and fully obtain the effect of grain boundary strengthening by B, it is necessary to increase the average heating rate at 400°C or higher. From this perspective, the average heating rate from 400°C to the annealing temperature is 1.0°C / second or higher. The average heating rate from 400°C to the annealing temperature is preferably 1.5°C / second or higher, and more preferably 3.0°C / second or higher. Furthermore, the average heating rate is preferably 10°C / second or lower. The average heating rate here is "annealing temperature (°C) - 400 (°C) (described below) / heating time (minutes) from 400°C to the annealing temperature".

[0068] Annealing temperature: 830 to 950°C Soaking time (holding time at annealing temperature): 600 seconds or less Undissolved Fe remaining after annealing ２３ (C, B) ６In order to sufficiently reduce precipitates such as Fe, annealing is performed at a high temperature for a long period of time. Specifically, the annealing temperature must be 830°C or higher. The annealing temperature is preferably 840°C or higher, and more preferably 850°C or higher. On the other hand, annealing at a temperature higher than 950°C coarsens the prior austenite grain size, and although the cause is not clear, the final retained austenite is reduced. If the annealing temperature exceeds 950°C, the desired amount of retained austenite cannot be obtained, so the annealing temperature is set to 950°C or lower. The annealing temperature is preferably 900°C or lower. In addition, an excessively long soaking time (holding time) also leads to coarsening of the prior austenite grain size and reduces the retained austenite. Therefore, the soaking time is set to 600 seconds or shorter. The soaking time is preferably 540 seconds or shorter, and more preferably 480 seconds or shorter. On the other hand, Fe remaining in a solid solution ２３ (C, B) ６ In order to sufficiently reduce the amount of precipitates such as Zn, the soaking time is preferably 10 seconds or more, more preferably 30 seconds or more, and even more preferably 60 seconds or more.

[0069] Average cooling rate from the annealing temperature to a cooling stop temperature of 150 to 250°C (second average cooling rate): 10°C / second or more In order to obtain a structure in which the area fraction of martensite is 85% or more and less than 95% and the area fraction of retained austenite is 5% or more and 15% or less, the steel is cooled from the annealing temperature to a cooling stop temperature of 150 to 250°C at an average cooling rate of 10°C / second or more, heated from the cooling stop temperature to a reheating holding temperature of 250 to 450°C, and then held at the reheating holding temperature for 20 to 1500 seconds.

[0070] Here, the average cooling rate (second average cooling rate) is "(annealing temperature (°C) - cooling stop temperature (°C)) / cooling time (seconds) from the annealing temperature to the cooling stop temperature".

[0071] If the average cooling rate from the annealing temperature to the cooling stop temperature (second average cooling rate) is less than 10°C / sec, ferrite and bainite are excessively formed, and the desired strength cannot be obtained. Therefore, the average cooling rate from the annealing temperature to the cooling stop temperature (second average cooling rate) is set to 10°C / sec or more. The average cooling rate from the annealing temperature to the cooling stop temperature is preferably 20°C / sec or more. The second average cooling rate is preferably 300°C / sec or less, and more preferably 100°C / sec or less. If the cooling stop temperature is less than 150°C, the area fraction of martensite becomes 95% or more, and bend formability deteriorates. Therefore, the cooling stop temperature is set to 150°C or more. The cooling stop temperature is preferably 200°C or more. On the other hand, if the cooling stop temperature exceeds 250°C, the area fraction of martensite becomes less than 85%, and strength decreases. Therefore, the cooling stop temperature is set to 250°C or less. The cooling stop temperature is preferably 240°C or less, and more preferably 230°C or less.

[0072] Reheating temperature: 250 to 450°C If the reheating temperature is less than 250°C, the concentration of elements from martensite to austenite does not occur sufficiently, the amount of retained austenite decreases, and the bend formability deteriorates. Therefore, the reheating temperature is set to 250°C or higher. The reheating temperature is preferably 300°C or higher. On the other hand, if the reheating temperature exceeds 450°C, the transformation of austenite during reheating is promoted, the amount of retained austenite decreases, and the bend formability deteriorates. Therefore, the reheating temperature is set to 450°C or lower. The reheating temperature is preferably 400°C or lower, and more preferably 340°C or lower.

[0073] Holding time at reheating temperature: 20 to 1500 seconds If the holding time at the reheating temperature (reheating holding time) is less than 20 seconds, the concentration of elements from martensite to austenite is insufficient, the amount of retained austenite is reduced, and the bend formability is deteriorated. Therefore, the reheating holding time is set to 20 seconds or more. The reheating holding time is preferably 100 seconds or more. On the other hand, if the reheating holding time exceeds 1500 seconds, the transformation of austenite during reheating is promoted, the amount of retained austenite is reduced, and the bend formability is deteriorated. In addition, the tempering of martensite proceeds excessively, resulting in a decrease in strength. Therefore, the reheating holding time is set to 1500 seconds or less. The reheating holding time is preferably 1200 seconds or less.

[0074] The steel sheet obtained in this manner can be subjected to skin-pass rolling from the viewpoint of stabilizing press formability, such as adjusting the surface roughness and flattening the sheet shape. In this case, the skin-pass elongation is preferably 0.1% or more. Also, the skin-pass elongation is preferably 1.0% or less. In this case, it is preferable to use a dull roll as the skin-pass roll and adjust the roughness Ra of the steel sheet to 0.8 μm or more from the viewpoint of flattening the shape. Also, it is preferable to adjust the roughness Ra of the steel sheet to 1.8 μm or less.

[0075] The obtained steel sheet may be subjected to a plating treatment. That is, after continuous annealing, the surface of the steel sheet may be subjected to a plating treatment. By performing the plating treatment, a steel sheet having a plating layer on the surface can be obtained.

[0076] As described above, according to the present invention, the delayed fracture resistance of high-strength cold-rolled steel sheets is significantly improved, and the application of high-strength steel sheets contributes to improving the strength of parts and reducing their weight. The steel sheets of the present invention preferably have a thickness of 0.5 mm or more. Also, the thickness is preferably 2.0 mm or less.

[0077] Next, the member of the present invention and the method for manufacturing the same will be described.

[0078] The member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining. Also, the method for manufacturing the member of the present invention includes a step of subjecting the steel plate of the present invention to at least one of forming and joining to form the member.

[0079] The steel sheet of the present invention has a tensile strength of 1470 MPa or more, and has excellent bending workability and delayed fracture resistance. Therefore, members obtained using the steel sheet of the present invention also have high strength and are superior in delayed fracture resistance compared to conventional high-strength members. Furthermore, the use of the member of the present invention enables weight reduction. Therefore, the member of the present invention can be suitably used, for example, in vehicle body frame parts.

[0080] The forming process can be performed using a general processing method such as press working without any restrictions, and the joining process can be performed using general welding methods such as spot welding and arc welding, riveting, crimping, etc. without any restrictions.

[0081] Examples of the present invention will be described below. Steels having the chemical compositions shown in Table 1 were melted and then cast into slabs. These slabs were subjected to the heat treatment and rolling shown in Table 2 to obtain steel plates having a thickness of 1.4 mm.

[0082] Specifically, a slab having each component composition was held at the heating holding temperature shown in Table 2 for the heating holding time shown in Table 2, then subjected to hot finish rolling at the finish rolling temperature shown in Table 2 for the residence time at 900 to 1000°C shown in Table 2, cooled at the first average cooling rate shown in Table 2, and coiled at the coiling temperature shown in Table 2 to obtain a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was cold-rolled at the rolling reduction (cold rolling reduction) shown in Table 2 to obtain a cold-rolled steel sheet. Thereafter, the cold-rolled steel sheet was heated to the annealing temperature shown in Table 2 at the average heating rate shown in Table 2, held for the soaking time shown in Table 2, cooled to the cooling stop temperature at the second average cooling rate shown in Table 2, reheated to the reheating holding temperature shown in Table 2, held for the holding time shown in Table 2, and subjected to continuous annealing.

[0083] For No. 2, the obtained steel sheet was subjected to electroplating to obtain a steel sheet on which a Zn plating layer was formed.

[0084]

[0085]

[0086] The metallographic structure of the obtained steel sheets was quantified using the method described above, and further, tensile tests, bend formability evaluation tests, and delayed fracture resistance evaluation tests were conducted. Specifically, the microstructure measurement methods were as follows. The L-section of the steel sheet (a vertical section parallel to the rolling direction) was polished and then corroded with nital, and four fields of view within a 50 μm × 65 μm area were observed with an SEM at a magnification of 2000 times at a position ¼ of the thickness from the steel sheet surface. The photographed microstructure was then analyzed and measured. Here, martensite and bainite refer to structures that appear gray or white under an SEM. Bainite has the following characteristics. That is, it has an aspect ratio of 2.5 or more, exhibits a plate-like morphology, and is a slightly black structure compared to martensite. The width of the above plates is 0.3 to 1.7 μm. The distribution density of carbides with a diameter of 10 to 200 nm inside the bainite is 0 to 3 / μm. ２ On the other hand, ferrite is the region that shows a black contrast in the SEM. Note that martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides, but since it is difficult to exclude these, the area ratio of the region including these was used as the area ratio.

[0087] The retained austenite (retained γ) was measured by chemically polishing the surface of the steel sheet (200 μm) with oxalic acid, and the X-ray diffraction intensity was measured on the sheet surface. It was calculated from the integrated intensity of the (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ diffraction peaks measured with Mo-Kα radiation.

[0088] The number density A of precipitates having a circle equivalent diameter of 500 nm or more is measured after polishing the L cross section (vertical cross section parallel to the rolling direction) of the steel sheet, in the region from the 1 / 5 position to the 4 / 5 position of the sheet thickness of the steel sheet, that is, in the region from the 1 / 5 position of the sheet thickness from the surface of the steel sheet to the 4 / 5 position, sandwiching the center of the sheet thickness. ２The area was photographed continuously with an SEM, and the number of such precipitates was counted from the photographed SEM photographs. The magnification for photographing was 2000 times. When analyzing the components of individual inclusion particles, the individual inclusion particles were magnified 10000 times and the above precipitates were analyzed. Here, precipitates with a circle equivalent diameter of 500 nm or more were considered to be Fe ２３ (C, B) ６ The presence or absence of a B peak was examined by elemental analysis using energy dispersive X-ray spectroscopy (EDS) at an acceleration voltage of 3 kV, and when a B peak was present, it was evaluated that the above-mentioned precipitates were present.

[0089] For the tensile test, a JIS No. 5 tensile test piece was cut out at a quarter position of the coil width so that the direction perpendicular to the rolling direction was the longitudinal direction, and a tensile test (in accordance with JIS Z2241 (2022)) was carried out to evaluate the tensile strength TS. A tensile strength TS of 1470 MPa or more was deemed to have passed.

[0090] The bend formability was evaluated as follows. JIS No. 3 test pieces were taken from each steel sheet, with the longitudinal direction perpendicular to the rolling direction (coil width direction). A 90° V-bend test was performed using the V-block method in accordance with JIS Z 2248 (2022) at various bending radii. The bendability was evaluated based on the value (R / t) obtained by dividing the minimum bending radius R at which cracks of 0.3 mm or more did not occur on the surface of the test piece by the sheet thickness t. The bending ridge direction was parallel to the rolling direction. In the present invention, steel sheets with an R / t of 3.0 or less were evaluated as having excellent bendability, and are indicated by "◯ (pass)" in Table 3. Steel sheets with an R / t of more than 3.0 were evaluated as having poor bendability, and are indicated by "× (fail)" in Table 3.

[0091] The delayed fracture resistance was evaluated as follows. Strip test pieces measuring 100 mm in the direction perpendicular to the rolling direction and 30 mm in the rolling direction were taken from the width direction of the obtained steel sheet (coil) at a position ¼ of the coil width. The end face of the long side with a length of 100 mm was sheared, and the sheared specimen was bent in its as-sheared state (without machining to remove burrs) so that the burrs were on the outer periphery of the bend. The specimen was fixed with bolts while maintaining the shape of the bent specimen. The shear clearance was 13% and the rake angle was 1°. The bending was performed with a tip bend radius of 10 mm and a 90° angle inside the bend apex (V-bend). The punch used had a tip radius R equal to the tip bend radius R and was U-shaped (semicircular tip R portion and punch body thickness of 2R), and the die had a corner R of 30 mm. The depth to which the punch pressed the steel plate was adjusted, and the bending angle at the tip (the angle at the inner side of the bend apex) was 90 degrees (V-shaped). The test specimen was clamped and tightened with a hydraulic jack so that the distance between the flange ends of the straight piece portions during bending was the same as when the specimen was bent (to cancel out the opening of the straight piece portions due to springback), and then bolted in this state. The bolt was fixed by passing it through an elliptical hole (minor axis 10 mm, major axis 15 mm) pre-installed 10 mm inside the short edge of the strip test specimen. The obtained bolt-tightened test specimen was immersed in a solution prepared by mixing 0.1 mass% ammonium thiocyanate aqueous solution and McIlvaine buffer solution in a mass ratio of 1:1 and adjusting the pH to 8.0 to perform a delayed fracture resistance evaluation test. The temperature of the solution was 20°C, and the test specimen surface area was 1 cm. ３ The amount of liquid per test piece was 20 ml. After 24 hours, the test pieces were checked for the presence or absence of visually observable cracks (length of 1 mm or more), and those in which no cracks were observed were judged to have excellent delayed fracture resistance. In Table 3, those marked "no cracks" passed the test, and those marked "cracks" failed the test.

[0092] The structure and properties of the obtained steel sheets are shown in Table 3.

[0093]

[0094] The steel sheets within the scope of the present invention had high strength and excellent bend formability and delayed fracture resistance, whereas the comparative examples were insufficient in at least one of tensile strength, bend formability, and delayed fracture resistance.

[0095] Furthermore, it was found that the members obtained by forming and joining the steel plates of the present invention have high strength, excellent bending formability, and excellent delayed fracture resistance, similar to the steel plates of the present invention, because the steel plates of the present invention have high strength, excellent bending formability, and excellent delayed fracture resistance.

Claims

1. In mass percent, C: 0.15% or more and 0.45% or less, Si: 0.3% or more and 2.0% or less, Mn: 1.7% or more and 4.0% or less, P: 0.10% or less, S: 0.01% or less, sol. Al: 0.50% or less, N: 0.010% or less, B: Contains 0.0008% or more and 0.0100% or less, The balance is Fe and unavoidable impurities, The area ratio of martensite to the entire structure is 85% or more and less than 95%, The steel has a steel structure in which the area ratio of retained austenite to the entire structure is 5% to 15%; A steel sheet in which the number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the following formula (1). A (pieces / mm ２ )≦ 8.5×10 ５ ×[B] ……Formula (1) Here, [B] represents the B content (mass %).

2. The composition further includes, in mass%, Cu: 1.00% or less, Cr: 1.00% or less, Nb: 0.10% or less, Ti: 0.10% or less, V: 0.50% or less, Mo: 0.50% or less, Ni: 1.00% or less, Sb: 0.10% or less, Sn: 0.10% or less, As: 0.10% or less, Ta: 0.10% or less, Ca: 0.020% or less, Mg: 0.020% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, W: 0.020% or less, The steel plate according to claim 1, further comprising one or more selected from the following: REM: 0.020% or less.

3. The steel sheet according to claim 1 , which has a plating layer on a surface of the steel sheet.

4. The steel plate according to claim 2, having a plating layer on the surface of the steel plate.

5. A member made using the steel plate according to any one of claims 1 to 4.

6. A steel slab having the composition according to claim 1 or 2 is heated and held at a slab surface temperature of 1100°C or higher for 30 minutes or more, and then Hot finish rolling is performed under conditions of a residence time at 900 to 1000°C of 20 seconds or more and 150 seconds or less and a finish rolling temperature of 850°C or more; Cooling is performed at an average cooling rate of 40° C. / sec or more in the range from the finish rolling temperature to 650° C., Thereafter, the hot-rolled steel sheet is obtained by coiling the steel sheet at a coiling temperature of 650°C or less. The hot-rolled steel sheet is cold-rolled at a rolling reduction of 40% or more to obtain a cold-rolled steel sheet; The annealing temperature is set to 830 to 950 ° C., and the cold-rolled steel sheet is heated from 400 ° C. to the annealing temperature at an average heating rate of 1.0 ° C. / sec or more, After holding at the annealing temperature for 600 seconds or less, Cooling from the annealing temperature to a cooling stop temperature of 150 to 250 ° C. at an average cooling rate of 10 ° C. / sec or more; Heat from the cooling stop temperature to a reheating holding temperature of 250 to 450 ° C. Then, continuous annealing is performed by holding the reheating holding temperature for 20 to 1500 seconds.

7. The method for producing a steel sheet according to claim 6, further comprising the step of: plating a surface of the steel sheet after the continuous annealing.

8. A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining to form a component.