Steel sheet, member, and methods for manufacturing the same

Abstract

Provided are: a steel sheet and a member having a tensile strength of 1310 MPa or more and exhibiting excellent bend formability, delayed fracture resistance at a sheared edge surface, and sheared edge properties; and methods for manufacturing the steel sheet and the member.　The steel sheet has a chemical composition comprising specific amounts of C, Si, Mn, P, S, sol. Al, N, and Ti, with the balance being Fe and unavoidable impurities, and has a steel structure comprising, in terms of area ratio with respect to the entire structure, martensite at 85% inclusive to 95% exclusive, retained austenite at 5% to 15% inclusive, and the remainder composed of one or more of bainite and ferrite at less than 5%. The steel sheet has: Ti-based precipitates having an equivalent circle diameter of 5 nm to 30 nm inclusive in which the average spacing is 1.0 μm or less; Ti-based precipitates having an equivalent circle diameter of 80 nm to 300 nm inclusive in which the the number density is 80 / mm2 or more; and a tensile strength of 1310 MPa or more.

Description

Steel plates, components, and methods for manufacturing them.

[0001] The present invention relates to steel sheets such as high-strength steel sheets for cold press forming used in automobiles and the like, components using said steel sheets, and methods for manufacturing them.

[0002] In recent years, the use of steel sheets with a tensile strength (TS) of 1310 MPa or higher has been increasing in automotive structural components for the purpose of reducing vehicle weight and improving collision safety. Furthermore, the use of steel sheets with a tensile strength (TS) of 1470 MPa or higher has been increasing in components such as bumpers and impact beams.

[0003] When high-strength steel sheets with a tensile strength (TS) of 1310 MPa or higher are formed into parts by cold pressing, there is a risk of delayed fracture due to hydrogen penetration into the steel sheet during the manufacturing process or in corrosive environments. Delayed fracture is particularly likely to occur at shear end faces damaged by blanking, piercing, or trimming processes, and improving the properties of the shear end faces is important to suppress delayed fracture.

[0004] As a technique to suppress such delayed fracture, based on the finding that reducing coarse precipitates that serve as the starting point for delayed fracture improves the delayed fracture resistance of steel sheets, Patent Document 1 states, 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 composition contains Al: 0.01% to 0.20%, N: 0.0055% or less, O: 0.0025% or less, Nb: 0.002% to 0.035%, and Ti: 0.002% to 0.040% in a manner that satisfies formulas (1) and (2), with the remainder being Fe and unavoidable impurities, and the total area ratio of martensite and bainite to the overall structure being 95% to 100%, with the remainder being one or two types of ferrite and retained austenite, the average particle size of prior austenite grains exceeding 5 μm, and the inclusion group having 5 inclusions / mm that satisfy the following conditions and have a long axis length of 20 to 80 μm. ２A high-strength steel sheet with excellent delayed fracture resistance is disclosed, characterized by having the following steel structure and a tensile strength of 1320 MPa or more. [%Ti] + [%Nb] > 0.007 (1) [%Ti] × [%Nb] ２ ≤ 7.5 × 10 －６ (2) Here, [%Nb] and [%Ti] represent the percentage of Nb and Ti content.

[0005] Furthermore, Patent Document 2 states that, by mass%, C: 0.05 to 0.30%, Si: 2.0% or less (including 0%), Mn: greater 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 in total amount of 0.01% or more, and [%C] - [%Nb] / 92.9 × 12 - [% The material has a composition that satisfies the condition Ti] / 47.9×12-[%Zr] / 91.2×12 > 0.03, with the remainder being iron and unavoidable impurities, and has a structure in which tempered martensite accounts for 50% or more (including 100%) by area, with the remainder being ferrite, and the distribution of precipitates in the tempered martensite is such that precipitates with an equivalent circle diameter of 1 to 10 nm are tempered martensite 1 μm ２ Precipitates with 20 or more particles per unit, with a circular diameter of 20 nm or more, containing one or more of Nb, Ti, and Zr, are tempered martensite with a diameter of 1 μm. ２ A high-strength cold-rolled steel sheet is disclosed, characterized by having 10 or fewer particles per sheet and an average grain size of ferrite surrounded by large-angle grain boundaries with a crystal orientation difference of 15° or more, which is 5 μm or less, and exhibiting excellent hydrogen embrittlement resistance and workability.

[0006] Patent No. 6388085 Patent No. 4712882

[0007] However, conventional technology was not sufficient to achieve both a tensile strength (TS) of 1310 MPa or higher and excellent bendability and delayed fracture of the shear end face.

[0008] Furthermore, in recent years, CO2 has been used from the perspective of carbon neutrality. ２It is desirable to be able to manufacture steel sheets using electric arc furnace processes that utilize iron scrap, rather than blast furnace processes which have high emissions. However, electric arc furnace processes increase the amount of trump elements and nitrogen (N) mixed in as impurities in the steel. However, conventional technologies have not been sufficient to suppress delayed fracture of the shear end face when the amount of nitrogen in the steel increases.

[0009] The present invention has been made to solve these problems, and aims to provide steel plates, members, and methods for manufacturing them, which have a tensile strength of 1310 MPa or more, and excellent bend formability, delayed fracture resistance of the shear end face, and shear end face properties.

[0010] Here, the tensile strength (TS) is determined by a measurement method in accordance with JIS Z2241 (2011).

[0011] Furthermore, excellent bend formability means that a JIS No. 3 test specimen is taken from the steel sheet with the longitudinal direction perpendicular to the rolling direction (coil width direction), and a 90° V-bending test is performed using the V-block method in accordance with the provisions of JIS Z 2248 (2022) with varying bending radii, and the value R / t, which is the minimum bending radius R at which no cracks of 0.3 mm or more are produced on the surface of the test specimen divided by the sheet thickness t, is 3.0 or less.

[0012] Furthermore, the delayed fracture resistance characteristics of the shear end face are determined as follows: (1) First, a strip test piece is taken from the widthwise end of the obtained steel plate (coil) at a position 1 / 4 of the coil width, with a length of 100 mm perpendicular to the rolling direction and 30 mm in the rolling direction. (2) The end face on the longer side, which is 100 mm in length, is cut out by shearing, and while still in the sheared state (without machining to remove burrs), it is bent so that the burrs are on the outer circumference of the bend, and the test piece is fixed with bolts while maintaining the shape of the test piece at the time of bending. The clearance for shearing is set to 15%, and the rake angle is set to 0°. The bending is performed with a tip bending radius of 10 mm, and the angle on the inside of the bend apex is 90 degrees (V-bend). A punch with a tip radius the same as the tip bending radius R above and a V-shape is used, and a die with a corner R of 30 mm is used. Then, the depth to which the punch presses into the steel plate is adjusted, and the tip is shaped so that the bending angle (angle on the inside of the bending apex) is 90 degrees (V-shape). The test piece is clamped and tightened with a hydraulic jack so that the distance between the flange ends of the straight section during bending is the same as the distance when bending (to cancel out the opening of the straight section due to springback), and then bolted in that state. The bolts are fixed by passing them through elliptical holes (minor axis 10 mm, major axis 15 mm) that have been made 10 mm inward from the short edge of the strip test piece. (3) The obtained bolted test piece is immersed in a solution prepared by mixing 0.1 mass% ammonium thiocyanate aqueous solution and McIlvaine buffer in a 1:1 mass ratio and adjusting the pH to 8.0, and a delayed fracture resistance evaluation test is performed. At this time, the temperature of the solution is 20°C, and the surface area of ​​the test piece is 1 cm ２ The liquid volume per sample shall be 20 ml. (4) After 100 hours, check for any cracks that can be visually confirmed (length of 1 mm or more). If no cracks are observed, the sample shall be judged to have excellent delayed fracture resistance.

[0013] Furthermore, the shear end surface properties are evaluated as follows: (1) A strip of test material is taken from the widthwise end of the obtained steel plate (coil) at a position 1 / 4 of the coil width, with a length of 100 mm perpendicular to the rolling direction and 30 mm in the rolling direction. At this time, the end surface on the longer side, which has a length of 100 mm, is cut by shearing, with a shearing clearance of 15% and a rake angle of 0°. (2) The shear end surface is observed using a stereomicroscope, and the average shear surface ratio in each of the three fields of view is determined. (3) For steel plates with a TS of 1310 MPa or more and less than 1600 MPa, a shear surface ratio of 25% or more is judged to be excellent in terms of shear end surface properties. For steel plates with a TS of 1600 MPa or more and less than 1800 MPa, a shear surface ratio of 15% or more is judged to be excellent in terms of shear end surface properties. For steel plates with a TS of 1800 MPa or higher, those with a shear surface ratio of 5% or more are judged to have excellent shear surface properties.

[0014] The inventors have diligently studied to solve the above problems and have found that the delayed fracture resistance and shear end surface properties can be significantly improved by satisfying all of the following conditions: i) 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. ii) The average spacing of Ti precipitates with an equivalent circle diameter of 5 nm or more and 30 nm or less is 1.0 μm or less. iii) The number density of Ti precipitates with an equivalent circle diameter of 80 nm or more and 300 nm or less is 80 particles / mm ２ It must be the above.

[0015] This invention was completed based on the above findings, and its gist is as follows.

[0016] [1] In mass%, C: 0.10% or more and 0.45% or less, Si: 1.5% or less, Mn: more than 1.7% and 4.0% or less, P: 0.10% or less, S: 0.010% or less, sol.Al: 0.50% or less, N: 0.0200% or less, the content of Ti satisfies the following formula (1), the balance consists of Fe and inevitable impurities, having a component composition, the area ratio of the entire martensite structure is 85% or more and less than 95%, the area ratio of the entire retained austenite structure is 5% or more and 15% or less, and the area ratio of the balance consisting of one or more of bainite and ferrite to the entire structure is less than 5%, having a steel structure, the average interval of Ti-based precipitates having a circle equivalent diameter of 5 nm or more and 30 nm or less is 1.0 μm or less, and the number density of Ti-based precipitates having a circle equivalent diameter of 80 nm or more and 300 nm or less is 80 pieces / mm ２The steel plate has the above characteristics and a tensile strength of 1310 MPa or more. [Ti] ≥ 0.01 + 3.4 × [N] + 1.5 × [S] ... Equation (1) Here, [Ti], [N], and [S] represent the content (mass %) of each element. [2] The steel sheet according to [1], further comprising, in mass%, one or more selected from the following as the component composition: B: 0.0100% or less, Cu: 1.00% or less, Cr: 1.00% or less, Nb: 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: 0.50% or less, Zr: 0.020% or less, W: 0.020% or less, 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 component composition described in [1] or [2] above is heated and held at a slab surface temperature of 1100°C or higher for 10 to 60 minutes, then hot finish rolling is performed under conditions where the residence time at 1000 to 1100°C is 20 to 120 seconds and the finish rolling temperature is 850°C or higher, and the slab is cooled at an average cooling rate of 40°C / second or more in the range from the finish rolling temperature to 650°C, then wound at a winding temperature of 650°C or lower, and after winding is held at a temperature of 500 to 650°C for 40 minutes or more to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is cold-rolled with a reduction ratio of 30% or more, and the cold-rolled steel sheet is heated at an average heating rate of 1.0°C / second or more from 400°C to the annealing temperature with an annealing temperature of 800 to 950°C, and held at the annealing temperature for 10 to 600 seconds, A method for manufacturing 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 and holding temperature of 250 to 450°C; and then performing continuous annealing, holding at the reheating and holding temperature for 20 to 1500 seconds. [6] The method for manufacturing 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 manufacturing a member, comprising the step of forming and joining a steel plate according to any one of [1] to [3] above to make a member.

[0017] According to the present invention, steel plates, members, and methods for manufacturing the same are provided, which have high strength, excellent bendability, delayed fracture resistance of the shear end face, and shear end face properties.

[0018] Embodiments of the present invention will be described below.

[0019] The steel sheet of the present invention has a structural composition in which, by mass%, C: 0.10% to 0.45%, Si: 1.5% or less, Mn: greater than 1.7% and 4.0% or less, P: 0.10% or less, S: 0.010% or less, sol. Al: 0.50% or less, and N: 0.0200% or less, the Ti content satisfies the following formula (1), and the remainder consists of Fe and unavoidable impurities, the area ratio of martensite to the total structure is 85% or more and less than 95%, the area ratio of retained austenite to the total structure is 5% or more and 15% or less, and the area ratio of the remainder consisting of one or more of bainite and ferrite to the total structure is less than 5%, the average spacing of Ti precipitates with an equivalent circle diameter of 5 nm to 30 nm is 1.0 μm or less, and the number density of Ti precipitates with an equivalent circle diameter of 80 nm to 300 nm is 80 particles / mm ２ The above results indicate a tensile strength of 1310 MPa or more. [Ti] ≥ 0.01 + 3.4 × [N] + 1.5 × [S] ... Equation (1) Here, [Ti], [N], and [S] represent the content (mass %) of each element.

[0020] The following explains the reasons for limiting the range of the component composition of the steel sheet of the present invention. Note that the percentages for component content are in "mass percent".

[0021] C: 0.10% to 0.45% C is included to increase the strength of martensite and obtain a tensile strength of 1310 MPa or more (hereinafter also referred to as TS ≥ 1310 MPa). Therefore, in order to obtain the desired TS, the C content should be 0.10% or more. From the viewpoint of reducing the weight of automotive frame parts by increasing strength, the C content is preferably 0.15% or more, more preferably 0.20% or more, and even more preferably 0.25% or more. On the other hand, if C is added in excess, the delayed fracture resistance deteriorates due to an excessive increase in strength. Therefore, the C content should be 0.45% or less. The C content is preferably 0.40% or less, and more preferably 0.35% or less.

[0022] Si: 1.5% or less. Si suppresses the formation of film-like carbides when tempering at temperatures above 200°C, thereby suppressing a decrease in strength and deterioration of delayed fracture resistance. There is no lower limit specified for the Si content, but it is preferable that the Si content be 0.02% or more. The Si content is more preferably 0.10% or more, and even more preferably 0.20% or more. On the other hand, adding too much Si leads to deterioration of delayed fracture resistance due to Si segregation. Therefore, the Si content should be 1.5% or less. The Si content is preferably 1.0% or less, and more preferably 0.6% or less.

[0023] Mn: greater than 1.7%, 4.0% or less. Mn is an effective element for improving the hardenability of steel. In cooling equipment such as gas cooling, the Mn content should be greater than 1.7% in order to stably obtain the desired martensite area ratio. The Mn content is preferably 1.8% or more, and more preferably 2.0% or more. On the other hand, if Mn is added in excess, the delayed fracture resistance deteriorates through the formation of inclusions such as MnS. Therefore, the Mn content should be 4.0% or less. The Mn content is preferably 3.5% or less, and more preferably 3.0% or less.

[0024] P: 0.10% or less. P segregates at grain boundaries, reducing grain boundary strength and leading to deterioration of delayed fracture resistance. Therefore, the P content should be 0.10% or less. Preferably, the P content is 0.05% or less, more preferably 0.02% or less, and even more preferably 0.01% or less. While there is no lower limit for the P content, the industrially feasible lower limit is 0.002%. Therefore, it is preferable that the P content be 0.002% or more.

[0025] S: 0.010% or less. S forms coarse inclusions with Mn and acts as an initiation point for delayed fracture, leading to a deterioration of delayed fracture resistance. Therefore, the S content should be 0.010% or less. Preferably, the S content is 0.003% or less, more preferably 0.0015% or less, and even more preferably 0.0008% or less. No lower limit is specified, but the industrially feasible lower limit is 0.0002%. Therefore, it is preferable that the S content be 0.0002% or more.

[0026] sol. Al: 0.50% or less. Al is included to ensure sufficient deoxidation and reduce inclusions in the steel. There is no specific lower limit for sol. Al, but to ensure stable deoxidation, it is preferable to have a sol. Al content of 0.005% or more. More preferably, the sol. Al content is 0.01% or more, and even more preferably 0.02% or more. On the other hand, if the sol. Al content exceeds 0.50%, the precipitation of AlN increases, the desired number density of TiN cannot be obtained, and the delayed fracture resistance deteriorates. Therefore, the sol. Al content should be 0.50% or less. Preferably, the sol. Al content is 0.20% or less, and more preferably 0.05% or less.

[0027] N: 0.0200% or less. N is an important constituent element of the present invention. N improves the properties of the shear end face and enhances the delayed fracture resistance of the shear end face by forming TiN controlled to be between 80 nm and 300 nm. The lower limit of the N content is not limited, but in order to obtain this effect, the N content is preferably 0.0030% or more. The N content is more preferably 0.0040% or more, even more preferably 0.0050% or more, and even more preferably 0.0060% or more. On the other hand, an excessive increase in the N content can lead to Fe becoming the starting point for delayed fracture. ３ This can lead to the formation of iron nitrides such as N, potentially degrading the delayed fracture resistance. Therefore, the N content should be 0.0200% or less. Preferably, the N content is 0.0180% or less, and more preferably 0.0150% or less.

[0028] Ti: [Ti] ≥ 0.01 + 3.4 × [N] + 1.5 × [S] ... Equation (1) Here, [Ti], [N], and [S] represent the mass %) content of each element in the steel sheet. Ti is an important constituent element of the present invention. Ti improves the properties of the shear end face through the formation of Ti-based precipitates controlled to have an equivalent circular diameter of 80 nm to 300 nm. Although the mechanism is not clear, it is thought that Ti-based precipitates controlled to have an equivalent circular diameter of 80 nm to 300 nm act as crack initiation points during shearing and stabilize crack propagation. Furthermore, the formation of Ti-based precipitates controlled to have an equivalent circular diameter of 5 nm to 30 nm refines the prior austenite grain size and functions as a hydrogen trapping site, improving delayed fracture resistance. From the viewpoint of obtaining Ti-based precipitates having a desired equivalent circle diameter, the Ti content must satisfy [Ti] ≥ 0.01 + 3.4 × [N] + 1.5 × [S]. The upper limit of the Ti content is not particularly limited, but from the viewpoint of stably obtaining Ti-based precipitates having a desired equivalent circle diameter, it is preferable to set [Ti] ≤ 0.10 + 3.4 × [N] + 1.5 × [S], and more preferably [Ti] ≤ 0.06 + 3.4 × [N] + 1.5 × [S]. Here, an example of a Ti-based precipitate controlled to have an equivalent circle diameter of 80 nm to 300 nm is TiN. An example of a Ti-based precipitate controlled to have an equivalent circle diameter of 5 nm to 30 nm is TiC.

[0029] The component composition of the steel sheet in the present invention contains the above-mentioned component elements as basic components, with the remainder being iron (Fe) and unavoidable impurities. Here, it is preferable that the steel sheet of the present invention has a component composition consisting of the above-mentioned basic components, with the remainder being iron (Fe) and unavoidable impurities.

[0030] In the present invention, as a component composition, one or more of the following may be contained as optional elements (selected elements). In mass%, B: 0.0100% or less, Cu: 1.00% or less, Cr: 1.00% or less, Nb: 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: 0.50% or less, Zr: 0.020% or less, W: 0.020% or less, REM: 0.020% or less

[0031] B: 0.0100% or less. B is an element that improves the hardenability of steel and has the effect of generating martensite with a predetermined area ratio even with a low Mn content. Also, B improves the stress corrosion cracking resistance by segregating at grain boundaries to increase the grain boundary binding force and suppressing the segregation of P that reduces the grain boundary strength. To obtain such effects, the B content is preferably 0.0003% or more. The B content is more preferably 0.0008% or more, and still more preferably 0.0013% or more. On the other hand, when B is added in excess, Fe ２３ (C, B) ６ or BN is formed, which rather deteriorates the stress corrosion cracking resistance by serving as a starting point for stress corrosion cracking. Therefore, when B is contained, the B content is 0.0100% or less. The B content is preferably 0.0080% or less, and more preferably 0.0050% or less.

[0032] Cu: 1.00% or less. Cu has the effect of improving the corrosion resistance of the steel sheet, reducing hydrogen ingress into the steel sheet, and improving the stress corrosion cracking resistance. The lower limit value of the Cu content is not specified, but to obtain such 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, when Cu is added in excess, coarse precipitates increase and the stress corrosion cracking resistance deteriorates. Therefore, when Cu is contained, the Cu content is 1.00% or less. The Cu content is preferably 0.50% or less, and more preferably 0.20% or less.

[0033] 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. Although the lower limit value of the Cr content is not particularly specified, in order to obtain such an effect, the Cr content is preferably 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, when Cr is added excessively, the solution of cementite during annealing is delayed, and a large amount of undissolved cementite remains, resulting in deterioration of the delayed fracture resistance characteristics. Therefore, when Cr is contained, the Cr content is 1.00% or less. The Cr content is preferably 0.50% or less, and more preferably 0.20% or less.

[0034] Nb: 0.10% or less. Nb forms fine precipitates such as NbC in steel, and has the effect of refining the prior austenite grain size by the pinning effect and improving the delayed fracture resistance characteristics. Although the lower limit value of the Nb content is not specified, in order to obtain such an effect, the Nb content is desirably 0.005% or more. The Nb content is preferably 0.01% or more. On the other hand, when Nb is added excessively, coarse precipitates increase and the delayed fracture resistance characteristics deteriorate. Therefore, when Nb is contained, the Nb content is 0.10% or less. The Nb content is preferably 0.05% or less, and more preferably 0.03% or less.

[0035] V: 0.50% or less. V generates fine carbides containing V which becomes a hydrogen trap site and has the effect of improving the delayed fracture resistance characteristics. Also, by forming fine precipitates, it has the effect of refining the prior austenite grain size by the pinning effect and improving the delayed fracture resistance characteristics. Although the lower limit value of the V content is not specified, in order to obtain such an 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, when V is added excessively, coarse precipitates increase and the delayed fracture resistance characteristics deteriorate. Therefore, when V is contained, the V content is 0.50% or less. The V content is preferably 0.20% or less, and even more preferably 0.10% or less.

[0036] Mo: 0.50% or less Mo has the effect of improving delayed fracture resistance by generating fine carbides containing Mo, which act as hydrogen trapping sites. In addition, by forming fine precipitates, it has the effect of refining the prior austenite grain size through a pinning effect, thereby improving delayed fracture resistance. There is no lower limit for the Mo content, but in order to obtain these effects, the Mo content is preferably 0.003% or more. The Mo content is more preferably 0.01% or more, and even more preferably 0.03% or more. On the other hand, if Mo is added in excess, the amount of coarse precipitates increases and the delayed fracture resistance deteriorates. Therefore, when Mo is included, the Mo content should be 0.50% or less. The Mo content is preferably 0.20% or less, and more preferably 0.10% or less.

[0037] Ni: 1.00% or less. Ni has the effect of improving the corrosion resistance of steel sheets, suppressing hydrogen penetration into the steel sheets, and improving delayed fracture resistance. Ni is also an effective element for improving the hardenability of steel and can be added to stably obtain the desired microstructure. There is no lower limit for the Ni content, but in order to obtain these effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.05% or more, and even more preferably 0.10% or more. On the other hand, if Ni is added in excess, coarse precipitates increase and the delayed fracture resistance deteriorates. Therefore, when Ni is included, the Ni content should be 1.00% or less. The Ni content is preferably 0.50% or less, and more preferably 0.30% or less.

[0038] Sb: 0.10% or less. Sb suppresses oxidation and nitriding of the surface layer, contributing to increased strength and improved resistance to delayed fracture. While there is no lower limit specified for Sb content, a Sb content of 0.002% is preferable to obtain these effects. More preferably, the Sb content is 0.004% or more, and even more preferably 0.006% or more. On the other hand, adding too much Sb deteriorates the resistance to delayed fracture. Therefore, if Sb is included, the Sb content should be 0.10% or less. Preferably, the Sb content is 0.05% or less, and more preferably 0.02% or less.

[0039] Sn: 0.10% or less. Sn suppresses oxidation and nitriding of the surface layer, contributing to increased strength and improved resistance to delayed fracture. While there is no lower limit specified for the Sn content, a Sn content of 0.002% is preferable to obtain these effects. More preferably, the Sn content is 0.004% or more, and even more preferably 0.006% or more. On the other hand, adding too much Sn increases coarse precipitates and deteriorates the resistance to delayed fracture. Therefore, when Sn is included, the Sn content should be 0.10% or less. Preferably, the Sn content is 0.05% or less, and more preferably 0.02% or less.

[0040] As: 0.10% or less. As has the effect of increasing the strength of steel. Although there is no lower limit specified for the As content, it is preferable that the As content be 0.002% or more in order to obtain this effect. More preferably, the As content is 0.004% or more, and even more preferably 0.006% or more. On the other hand, if As is added in excess, the amount of coarse precipitates increases and the delayed fracture resistance deteriorates. Therefore, if As is included, the As content should be 0.10% or less. The As content is preferably 0.05% or less, and more preferably 0.02% or less.

[0041] Ta: 0.10% or less. Ta has the effect of increasing the strength of steel. Although there is no lower limit specified for the Ta content, it is preferable that the Ta content be 0.002% or more in order to obtain this effect. More preferably, the Ta content is 0.004% or more, and even more preferably 0.006% or more. On the other hand, if Ta is added in excess, the amount of coarse precipitates increases and the delayed fracture resistance deteriorates. Therefore, if Ta is included, the Ta content should be 0.10% or less. The Ta content is preferably 0.05% or less, and more preferably 0.02% or less.

[0042] Ca: 0.020% or less. Ca reduces the starting point of delayed fracture by spheroidizing the shape of sulfides, thereby improving delayed fracture resistance. To obtain this effect, the Ca content is preferably 0.0002% or more. More preferably, the Ca content is 0.0005% or more, and even more preferably 0.0010% or more. On the other hand, if Ca is added in excess, the amount of coarse precipitates increases, and the delayed fracture resistance deteriorates. Therefore, if Ca is included, the Ca content should be 0.020% or less. The Ca content is preferably 0.01% or less, and more preferably 0.005% or less.

[0043] Mg: 0.020% or less. Mg improves delayed fracture resistance by reducing the starting point of delayed fracture through spheroidization of the sulfide shape. To obtain this effect, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.001% or more, and even more preferably 0.003% or more. On the other hand, if Mg is added in excess, the amount of coarse precipitates increases and the delayed fracture characteristics deteriorate. Therefore, if Mg is included, the Mg content should be 0.020% or less. The Mg content is preferably 0.015% or less, and more preferably 0.010% or less.

[0044] Zn: 0.020% or less. Zn improves delayed fracture resistance by refining the prior austenite grain size and spheroidizing the shape of inclusions. To obtain these effects, the Zn content is preferably 0.001% or more. Preferably, the Zn content is 0.003% or more. On the other hand, adding too much Zn increases the amount of coarse precipitates and deteriorates the delayed fracture resistance. Therefore, when Zn is included, the Zn content should be 0.020% or less. Preferably, the Zn content is 0.015% or less, and more preferably 0.010% or less.

[0045] Co: 0.50% or less. Co improves delayed fracture resistance by refining the prior austenite grain size and spheroidizing the shape of inclusions. To obtain these effects, the Co content is preferably 0.001% or more. More preferably 0.003% or more. On the other hand, if Co is added in excess, the amount of coarse precipitates increases and the delayed fracture resistance deteriorates. Therefore, when Co is included, the Co content should be 0.50% or less. The Co content is preferably 0.10% or less, more preferably 0.030% or less. The Co content is even more preferably 0.020% or less, even more preferably 0.015% or less, and even more preferably 0.010% or less.

[0046] Zr: 0.020% or less. Zr improves delayed fracture resistance by refining the prior austenite grain size and spheroidizing the shape of inclusions. To obtain these effects, the Zr content is preferably 0.001% or more. More preferably 0.003% or more. On the other hand, if Zr is added in excess, the amount of coarse precipitates increases and the delayed fracture resistance deteriorates. Therefore, if Zr is included, the Zr content should be 0.020% or less. The Zr content is preferably 0.015% or less, and more preferably 0.010% or less.

[0047] W: 0.020% or less. W improves delayed fracture resistance by refining the prior austenite grain size through the formation of precipitates. To obtain this effect, the W content is preferably 0.001% or more. More preferably, the W content is 0.003% or more. On the other hand, if W is added in excess, the amount of coarse precipitates increases, and the delayed fracture resistance deteriorates. Therefore, if W is included, the W content should be 0.020% or less. The W content is preferably 0.015% or less, and more preferably 0.010% or less.

[0048] REM: 0.020% or less REM also contributes to the improvement of delayed fracture resistance by spheroidizing inclusions. To obtain this effect, the REM content is preferably 0.0002% or more. The REM content is more preferably 0.001% or more, and even more preferably 0.003% or more. On the other hand, if REM is added in excess, the amount of coarse precipitates increases and the delayed fracture resistance deteriorates. Therefore, when REM is included, the REM content should be 0.020% or less. The REM content is preferably 0.015% or less, and more preferably 0.010% or less. In this 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. In this invention, the REM content is the total content of one or more elements selected from the above-mentioned REM.

[0049] Furthermore, if the above-mentioned arbitrary element is present in a quantity below the preferred lower limit, it shall be considered to be present as an unavoidable impurity.

[0050] The steel structure of the steel sheet of the present invention has the following configuration: (Configuration 1) The area ratio of martensite to the total structure is 85% or more and less than 95%, the area ratio of retained austenite to the total structure is 5% or more and 15% or less, and the area ratio of the remainder consisting of one or more types of bainite and ferrite to the total structure is less than 5%. (Configuration 2) The average spacing of Ti precipitates with an equivalent circle diameter of 5 nm or more and 30 nm or less is 1.0 μm or less, and the number density of Ti precipitates with an equivalent circle diameter of 80 nm or more and 300 nm or less is 80 particles / mm ２ That's all.

[0051] The following describes each component.

[0052] (Structure 1) The structure has a martensite area fraction of 85% or more and less than 95% of the total structure, a retained austenite area fraction of 5% or more and less than 15% of the total structure, and the remainder consisting of one or more types of bainite and ferrite has an area fraction of less than 5% of the total structure. This is an important constituent element of the present invention. By using martensite as the main phase, it is possible to achieve a high strength of TS ≥ 1310 MPa. In order to obtain this effect, the area fraction of martensite must be 85% or more. The area fraction of martensite is preferably 87% or more. Furthermore, it has been found that by using martensite as the main phase and generating retained austenite in an area fraction of 5% or more and less than 15%, excellent delayed fracture resistance as well as excellent bend formability can be obtained. In order to obtain these effects, 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 formation of excessive retained austenite reduces strength and degrades delayed fracture resistance. Therefore, in order to stably obtain excellent delayed fracture resistance and excellent bendability, the area fraction of retained austenite should be 15% or less. Preferably, the area fraction of retained austenite is 12% or less. When materials other than martensite and retained austenite are included, the remainder consists of ferrite and bainite. The area fraction of the remaining structure is less than 5%, preferably 3% or less, and may be 0%. Furthermore, the martensite includes martensite that has not undergone tempering due to staying at approximately 250°C or higher for a certain period of time, including self-tempering during continuous cooling.

[0053] (Configuration 2) The average spacing of Ti precipitates with an equivalent circle diameter of 5 nm to 30 nm is 1.0 μm or less, and the number density of Ti precipitates with an equivalent circle diameter of 80 nm to 300 nm is 80 particles / mm². ２The above is the conclusion. In order to achieve excellent delayed fracture resistance of the shear end face in steel having high strength with TS ≥ 1310 MPa, it is important to improve the delayed fracture resistance of the base material and improve the properties of the shear end face. To improve the delayed fracture resistance of the base material, it is effective to reduce the prior austenite grain boundaries that serve as the initiation point of delayed fracture by segregation of impurity elements such as P, and to disperse fine precipitates such as TiC that serve as hydrogen trapping sites. Specifically, if the average spacing of Ti-based precipitates with an equivalent circle diameter of 5 nm to 30 nm is controlled to 1.0 μm or less, the delayed fracture resistance of the base material can be significantly improved due to the hydrogen trapping effect of the precipitates and the refinement effect of the prior austenite grain size due to pinning. Ti-based precipitates with an equivalent circle diameter exceeding 30 nm do not have a sufficient hydrogen trapping effect and contribute little to the improvement of delayed fracture resistance. Furthermore, according to the method of the present invention, the equivalent circle diameter of the main Ti-based precipitates is 5 nm or more. Therefore, in the present invention, the equivalent circular diameter of the Ti precipitates, which is effective in improving the delayed fracture resistance of the base material, is 5 nm to 30 nm. From the viewpoint of obtaining the hydrogen trapping effect and the effect of refining the prior austenite grain size by pinning, the average spacing of the Ti precipitates with the above equivalent circular diameter of 5 nm to 30 nm is set to 1.0 μm or less. This average spacing is preferably 0.8 μm or less, and more preferably 0.5 μm or less.

[0054] Furthermore, the number density of Ti-based precipitates with an equivalent circular diameter of 80 nm to 300 nm is 80 particles / mm². ２ It was found that the properties of the shear end face of the steel plate can be significantly improved by the above method. Ti precipitates with an equivalent circle diameter exceeding 300 nm can act as delayed fracture initiation points and may actually degrade the delayed fracture resistance. Furthermore, Ti precipitates with an equivalent circle diameter of less than 80 nm have insufficient effect on improving the properties of the shear end face. Therefore, in this invention, it is important to control the number density of Ti precipitates with an equivalent circle diameter of 80 nm to 300 nm. To obtain the above effect, the number density of Ti precipitates with an equivalent circle diameter of 80 nm to 300 nm should be 80 particles / mm². ２ The above is correct. The number density of Ti-based precipitates with an equivalent circular diameter of 80 nm to 300 nm is preferably 100 particles / mm². ２The above is more preferable: 120 pieces / mm ２ That concludes the explanation. Furthermore, the number density of Ti-based precipitates with an equivalent circular diameter of 80 nm to 300 nm is preferably 1000 particles / mm². ２ The following is more preferable: 500 pieces / mm ２ The following applies:

[0055] The measurement methods for each component in the above steel microstructure are described below. The area ratios of martensite, bainite, and ferrite are measured by polishing an L-shaped section of the 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)), etching it with nital, and observing four fields of view with a SEM at a magnification of 2000x at a position 1 / 4 of the thickness from the steel sheet surface, with a field of view of 48 μm × 60 μm. The microstructure images taken are then analyzed for measurement. Here, martensite and bainite refer to structures that appear gray or white in the SEM. On the other hand, ferrite is a region that appears black in contrast in the SEM. Note that martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides, but it is difficult to exclude these, so the area ratio of the region including these is taken as the area ratio.

[0056] Here, bainite has the following characteristics: it has an aspect ratio of 2.5 or greater, exhibits a plate-like morphology, and has a slightly darker texture compared to martensite. The width of the plate is 0.3 to 1.7 μm. The distribution density of carbides with a diameter of 10 to 200 nm inside bainite is 0 to 3 particles / μm ２ That is the case.

[0057] The residual austenite (residual γ) is measured by chemically polishing the top 200 μm of the steel plate with oxalic acid, and then determining it by X-ray diffraction intensity spectroscopy on the plate surface. It is calculated from the integrated intensities of the (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ diffraction plane peaks measured by Mo-Kα radiation.

[0058] The average spacing of Ti precipitates with an equivalent circle diameter of 5 nm to 30 nm was calculated by taking a thin film sample from the region at 1 / 4 of the steel plate thickness and observing it at TEM at a magnification of 20,000x with a field of view of 830 nm × 830 nm. A total of 10 consecutive fields of view were used for observation, and component analysis was performed on individual precipitate particles. The average spacing was calculated by measuring the spacing between the nearest Ti precipitates for each observed Ti precipitate with an equivalent circle diameter of 5 nm to 30 nm, and dividing the sum of the measured precipitate spacings (μm) by the number of measured spacings. The number density of precipitates with an equivalent circle diameter of 80 nm to 300 nm was calculated by polishing the L-section (perpendicular section parallel to the rolling direction) of the steel plate and observing the region from 1 / 5 to 4 / 5 of the steel plate thickness, i.e., from 1 / 5 of the thickness from the surface to 4 / 5 of the thickness, encompassing the center of the thickness, within a 0.5 mm area. ２ The region was continuously imaged using FE-SEM, and the number of such precipitates was determined by measuring the number of precipitates from the captured SEM images. The magnification used for imaging was 10,000x. Component analysis was also performed on individual precipitate particles. The number density was calculated by measuring the total number of Ti-based precipitates (particles) with an equivalent circle diameter of 80 nm to 300 nm within the measurement region (mm²). ２ The result was calculated by dividing by ). Note that the equivalent circle diameter refers to the diameter of a perfect circle with the area of ​​each precipitate calculated from the SEM image. For a single precipitate, including cases where TiN, TiC, etc., are deposited in combination, the region in the SEM image that is surrounded by other materials on its outer periphery and is formed as a single, uninterrupted entity is measured as one unit.

[0059] Tensile strength (TS): 1310 MPa or more. A feature of the present invention is that even with a tensile strength of 1310 MPa or more, the delayed fracture resistance of the shear end face is good. From the viewpoint of reducing the weight of automotive frame parts, a TS of 1470 MPa or more is preferable. From the viewpoint of weight reduction, a TS of 1700 MPa or more is preferable, and 1900 MPa or more is more preferable. The tensile strength of the steel sheet of the present invention may be 2100 MPa or less.

[0060] The tensile strength can be measured by cutting out a JIS No. 5 tensile test specimen so that the direction perpendicular to the rolling direction is the longitudinal direction at a point 1 / 4 of the coil width, and performing a tensile test in accordance with JIS Z2241 (2011).

[0061] The steel sheet of the present invention described above may be a steel sheet having a plating layer on its surface. The plating layer may be Zn plating or plating of other metals. Furthermore, the plating layer may be a hot-dip galvanized layer or an electroplated layer such as an electro-galvanized layer.

[0062] Next, the steel plate manufacturing method of the present invention will be described. In the steel plate manufacturing method of the present invention, a steel slab having the above-mentioned component composition is held at a heating and holding temperature of 1100°C or higher on the slab surface temperature for 10 to 60 minutes, then hot finish rolling is performed under conditions where the residence time at 1000 to 1100°C is 20 to 120 seconds and the finish rolling temperature is 850°C or higher, cooling is performed with an average cooling rate of 40°C / second or higher in the range from the above finish rolling temperature to 650°C, then winding is performed at a winding temperature of 650°C or lower, and after winding is held at a temperature of 500 to 650°C for 40 minutes or more. The material is then hot-rolled to produce a cold-rolled steel sheet, which is then cold-rolled with a reduction ratio of 30% or more to produce a cold-rolled steel sheet, with an annealing temperature of 800 to 950°C. 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, held at the annealing temperature for 10 to 600 seconds, then 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 and holding temperature of 250 to 450°C, and then continuously annealed by holding at the reheating and holding temperature for 20 to 1500 seconds.

[0063] In the present invention, the method for manufacturing steel slabs, such as the melting method and casting method, is not particularly limited. For example, converter melting and electric furnace melting can be used as melting methods, and continuous casting and ingot forming methods can be used as casting methods. In the slab heating before hot rolling, holding the slab surface at a heating and holding temperature of 1100°C or higher for 10 to 60 minutes promotes the solid solution of precipitates such as Ti precipitates, thereby reducing the size and number of precipitates. Therefore, in the present invention, a steel slab having the above-mentioned component composition is held at a heating and holding temperature of 1100°C or higher for 10 to 60 minutes. The heating and holding temperature is preferably 1200°C or higher. The upper limit of the heating and holding temperature is not particularly limited, but the heating and holding temperature is preferably 1350°C or lower. The holding time is preferably 30 minutes or more.

[0064] Subsequently, hot rolling is performed, during which the slab is allowed to remain at 1000-1100°C for 20-120 seconds. Remaining at a temperature of 20 seconds or more in the 1000-1100°C range makes it possible to precipitate a large amount of relatively fine Ti-based precipitates, such as TiN, with a diameter of 80 nm to 300 nm. On the other hand, if the residence time exceeds 120 seconds, the solid-solution Ti is excessively reduced, and insufficient Ti-based precipitates with an equivalent circle diameter of 5 nm to 30 nm are obtained in the subsequent hot-rolling process. Therefore, the residence time is 120 seconds or less, preferably 100 seconds or less, and more preferably 80 seconds or less. Furthermore, the residence time is 20 seconds or more, preferably 50 seconds or more.

[0065] In hot finish rolling, the finish rolling temperature (FT) is set to 850°C or higher in order to suppress non-uniformity of the hot-rolled structure. Preferably, the finish rolling temperature is 950°C or lower, and more preferably 900°C or lower.

[0066] In the cooling process after hot finish rolling, cooling is performed so that the average cooling rate in the range from the finish rolling temperature to 650°C is 40°C / second or higher. In the temperature range from the finish rolling temperature to 650°C, Ti precipitates become excessively coarse, and Ti precipitates with an equivalent circle diameter of 80 nm to 300 nm and Ti precipitates with an equivalent circle diameter of 5 nm to 30 nm decrease. As a result, the desired precipitate state cannot be obtained. Therefore, the average cooling rate should be 40°C / second or higher. Preferably, the average cooling rate is 60°C / second or higher. Preferably, the average cooling rate is 200°C / second or lower. Note that the average cooling rate in the hot rolling process is "(temperature at the start of cooling (finish rolling temperature) (°C) - temperature at the completion of cooling (°C) (650°C)) / cooling time from the start of cooling to the completion of cooling (seconds)".

[0067] After cooling to 650°C as described above, further cooling is performed as needed before winding. At this time, if the winding temperature exceeds 650°C, the Ti-based precipitates become excessively coarse, and the amount of Ti-based precipitates with an equivalent circle diameter of 5 nm to 30 nm decreases. As a result, the desired precipitate state cannot be obtained, and the delayed fracture resistance deteriorates. Therefore, the winding temperature should be 650°C or lower. Preferably, the winding temperature is 600°C or lower. Also, preferably, the winding temperature is 500°C or higher.

[0068] The coil after winding is held at a temperature of 500 to 650°C for 40 minutes or more. Holding at a temperature of 500 to 650°C for 40 minutes or more promotes the precipitation of Ti-based precipitates having an equivalent circle diameter of 5 nm to 30 nm. The holding time at a temperature of 500 to 650°C is preferably 50 minutes or more, and more preferably 60 minutes or more. Furthermore, the holding time at a temperature of 500 to 650°C is preferably 1000 minutes or less, and more preferably 500 minutes or less.

[0069] In cold rolling, if the reduction ratio (cold rolling ratio, cumulative reduction ratio) is 30% or more, the recrystallization behavior and texture orientation during subsequent continuous annealing can be stabilized. If the reduction ratio (cold rolling ratio) is less than 30%, some austenite grains may become coarse during annealing, potentially reducing strength. Furthermore, it is preferable that the reduction ratio (cold rolling ratio) be 70% or less.

[0070] After continuous annealing and cold rolling, the steel sheet undergoes annealing and, if necessary, tempering and temper rolling on a continuous annealing line (CAL). Since TiC rapidly coarses in the ferrite region, it is important to increase the average heating rate above 400°C in order to obtain sufficient pinning effect of TiC during annealing. From this viewpoint, the average heating rate from 400°C to the annealing temperature described later is 1.0°C / second or more. Preferably, the average heating rate from 400°C to the annealing temperature described later is 1.5°C / second or more, and more preferably 3.0°C / second or more. Also, the average heating rate is preferably 10°C / second or less. Here, the average heating rate is "annealing temperature (°C) described later - 400 (°C) / heating time from 400°C to the annealing temperature (minutes)".

[0071] If annealing is performed at a temperature exceeding 950°C, the Ti precipitates become excessively coarse, and the amount of Ti precipitates with an equivalent circle diameter of 5 nm to 30 nm decreases, resulting in an undesirable precipitate state and degraded resistance to delayed fracture. Therefore, the annealing temperature should be 950°C or lower. Preferably, the annealing temperature is 900°C or lower. On the other hand, if the annealing temperature is below 800°C, martensite with the desired area ratio cannot be obtained. Therefore, the annealing temperature should be 800°C or higher. Preferably, the annealing temperature is 830°C or higher. Furthermore, excessively long soaking times (holding times) also cause the Ti precipitates to become excessively coarse, and the amount of Ti precipitates with an equivalent circle diameter of 5 nm to 30 nm decreases, resulting in an undesirable precipitate state and degraded resistance to delayed fracture. Therefore, the soaking time should be 600 seconds or less. Preferably, the soaking time is 480 seconds or less. Furthermore, if the soaking time is less than 10 seconds, martensite with the desired area ratio cannot be obtained. Therefore, the soaking time should be 10 seconds or more. Preferably, the soaking time should be 20 seconds or more.

[0072] Subsequently, in order to obtain a microstructure in which the area ratio of martensite is 85% or more and less than 95%, and the area ratio of retained austenite is 5% or more and less than 15%, the material 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.

[0073] The average cooling rate here is defined as "(annealing temperature (°C) - cooling stop temperature (°C)) / cooling time from annealing temperature to cooling stop temperature (seconds)".

[0074] If the average cooling rate from the annealing temperature to the cooling stop temperature is less than 10°C / second, excessive ferrite and bainite will be formed, and the desired strength cannot be obtained. Therefore, the average cooling rate from the annealing temperature to the cooling stop temperature should be 10°C / second or more. Preferably, the average cooling rate from the annealing temperature to the cooling stop temperature is 20°C / second or more. Preferably, the average cooling rate from the annealing temperature to the cooling stop temperature is 200°C / second or less, and more preferably 100°C / second or less.

[0075] If the cooling stop temperature is below 150°C, the area fraction of martensite becomes 95% or more, and the flexibility deteriorates. Therefore, the cooling stop temperature should be 150°C or higher. Preferably, the cooling stop temperature is 200°C or higher. On the other hand, if the cooling stop temperature exceeds 250°C, the area fraction of martensite becomes less than 85%, and the strength decreases. Therefore, the cooling stop temperature should be 250°C or lower. Preferably, the cooling stop temperature is 230°C or lower.

[0076] If the reheating and holding temperature is below 250°C, the elemental enrichment from martensite to austenite does not occur sufficiently, resulting in a reduction in retained austenite and deterioration of bendability. Therefore, the reheating and holding temperature should be 250°C or higher. Preferably, the reheating and holding temperature is 300°C or higher. On the other hand, if the reheating and holding temperature exceeds 450°C, the transformation of austenite during reheating and holding is accelerated, resulting in a reduction in retained austenite and deterioration of bendability. Therefore, the reheating and holding temperature should be 450°C or lower. Preferably, the reheating and holding temperature is 400°C or lower, and more preferably 340°C or lower.

[0077] If the reheating and holding time is less than 20 seconds, the elemental concentration from martensite to austenite will not occur sufficiently, resulting in a reduction in retained austenite and deterioration of bendability. Therefore, the reheating and holding time should be 20 seconds or more. Preferably, the reheating and holding time is 100 seconds or more. More preferably, the reheating and holding time is 240 seconds or more. On the other hand, if the reheating and holding time exceeds 1500 seconds, the transformation of austenite during reheating and holding is accelerated, resulting in a reduction in retained austenite and deterioration of bendability. In addition, excessive tempering of the martensite may occur, leading to a decrease in strength. Therefore, the reheating and holding time should be 1500 seconds or less. Preferably, the reheating and holding time is 1200 seconds or less. More preferably, the reheating and holding time is 900 seconds or less.

[0078] 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, it is preferable that the skin pass elongation rate be 0.1% or more. It is also preferable that the skin pass elongation rate be 1.0% or less. In this case, it is preferable to use a dull roll for the skin pass rolling and adjust the roughness Ra of the steel sheet to 0.8 μm or more from the viewpoint of flattening the shape. It is also preferable that the roughness Ra of the steel sheet be adjusted to 1.8 μm or less.

[0079] Furthermore, the resulting steel sheet may be subjected to plating. That is, after continuous annealing, the surface of the steel sheet may be plated. By performing the plating treatment, a steel sheet having a plating layer on its surface can be obtained.

[0080] As described above, the present invention significantly improves the delayed fracture resistance of high-strength cold-rolled steel sheets, contributing to improved component strength and weight reduction through the application of high-strength steel sheets. The steel sheet of the present invention preferably has a thickness of 0.5 mm or more. Furthermore, it is preferable that the thickness be 2.0 mm or less.

[0081] Next, the component and method for manufacturing the component of the present invention will be described. The component of the present invention is obtained by subjecting a steel sheet of the present invention to at least one of forming and joining processes. Furthermore, the method for manufacturing the component of the present invention includes the step of subjecting a steel sheet of the present invention to at least one of forming and joining processes to form a component.

[0082] The component of the present invention has the same component composition as the steel sheet of the present invention, and at least except for the corner portion after processing, it has the same steel structure and properties as the steel sheet of the present invention described above. Furthermore, when the component of the present invention is welded in a joining process, at least except for the heat-affected zone, it has the same steel structure and properties as the steel sheet of the present invention described above. The steel sheet of the present invention has a tensile strength of 1310 MPa or more, and has excellent bend formability, excellent delayed fracture resistance of the shear end face, and excellent shear end face properties. Therefore, the component obtained using the steel sheet of the present invention is also high strength and is superior to conventional high-strength components in terms of bend formability, delayed fracture resistance of the shear end face, and shear end face properties. Furthermore, weight reduction is possible when using the component of the present invention. Accordingly, the component of the present invention can be suitably used, for example, in vehicle body frame components.

[0083] Forming processes can utilize general processing methods such as press working without restriction. Joining processes can utilize general welding methods such as spot welding and arc welding, as well as riveting and crimping without restriction.

[0084] The following describes embodiments of the present invention. Steel with the component composition shown in Table 1 was melted and cast into a slab. This slab was subjected to the heat treatment and rolling shown in Table 2 to obtain a steel plate with a thickness of 1.4 mm.

[0085] Specifically, slabs having each component composition were heated at the slab heating temperature and held for the heating time shown in Table 2, then hot finish rolling was performed at the residence time of 1000-1100°C and finish rolling temperature shown in Table 2, followed by cooling at the average cooling rate shown in Table 2, winding at the winding temperature shown in Table 2, and holding the coil at 500°C for 60 minutes to obtain a hot-rolled steel sheet. Subsequently, the hot-rolled steel sheet was cold-rolled at the reduction ratio (cold rolling ratio) shown in Table 2 to obtain a cold-rolled steel sheet. Then, the cold-rolled steel sheet was heated to the annealing temperature shown in Table 2 at the heating rate shown in Table 2, held for the soaking time shown in Table 2, cooled to the cooling stop temperature under the cooling conditions shown in Table 2, reheated, and continuously annealed by holding at the reheating holding temperature shown in Table 2 for the holding time shown in Table 2.

[0086] Furthermore, for No. 2, the obtained steel sheet was subjected to electroplating to obtain a steel sheet with a Zn plating layer formed on it.

[0087]

[0088]

[0089] The obtained steel plates were subjected to quantification of their microstructure using the method described above, followed by tensile tests and delayed fracture resistance evaluation tests. Specifically, the microstructure was measured as follows: The area ratios of martensite, bainite, and ferrite were determined by polishing the L-section (a perpendicular section parallel to the rolling direction) of the steel plate, etching it with nital, and observing four fields of view with a SEM at 2000x magnification at a thickness of 48 μm × 60 μm from the surface of the steel plate. The microstructure images were then analyzed for measurement. Here, martensite and bainite refer to structures that appear gray or white on the SEM. Bainite has the following characteristics: it has an aspect ratio of 2.5 or more, exhibits a plate-like morphology, and is slightly darker in color than 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. ２On the other hand, ferrite is a region that exhibits black contrast in SEM. Note that while martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides, it is difficult to exclude these, so the area percentage of the region including these was used as the area percentage.

[0090] The residual austenite (residual γ) was measured by chemically polishing the top 200 μm of the steel plate with oxalic acid and determining its concentration by X-ray diffraction intensity spectroscopy. It was calculated from the integrated intensities of the (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ diffraction plane peaks measured by Mo-Kα radiation.

[0091] The average spacing of Ti precipitates with an equivalent circle diameter of 5 nm to 30 nm was calculated by taking a thin film sample from the region at 1 / 4 of the steel plate thickness and observing it at TEM at a magnification of 20,000x with a field of view of 830 nm × 830 nm. A total of 10 consecutive fields of view were used for observation, and component analysis was performed on individual precipitate particles. The average spacing was calculated by measuring the spacing between the nearest Ti precipitates for each observed Ti precipitate with an equivalent circle diameter of 5 nm to 30 nm, and dividing the sum of the measured precipitate spacings (μm) by the number of measured spacings. The number density of precipitates with an equivalent circle diameter of 80 nm to 300 nm was calculated by polishing the L-section (perpendicular section parallel to the rolling direction) of the steel plate and observing the region from 1 / 5 to 4 / 5 of the steel plate thickness, i.e., from 1 / 5 of the thickness from the surface to 4 / 5 of the thickness, encompassing the center of the thickness, within a 0.5 mm area. ２ The region was continuously imaged using FE-SEM, and the number of such precipitates was determined by measuring the number of precipitates from the captured SEM images. The magnification used for imaging was 10,000x. Component analysis was also performed on individual precipitate particles. The number density was calculated by measuring the total number of Ti-based precipitates (particles) with an equivalent circle diameter of 80 nm to 300 nm within the measurement region (mm²). ２ It was calculated by dividing by ).

[0092] For the tensile test, a JIS No. 5 tensile test specimen was cut out at a point where the coil width was 1 / 4 of the way through, with the direction perpendicular to the rolling direction being the longitudinal direction. The tensile test (in accordance with JIS Z2241) was then performed and the TS and El values ​​were evaluated.

[0093] For the bending test, a JIS No. 3 test specimen was taken from each steel plate with the longitudinal direction perpendicular to the rolling direction (coil width direction), and a 90° V-bending test was performed using the V-block method in accordance with the provisions of JIS Z 2248 (2022) with varying bending radii. The bendability was evaluated by the value (R / t) obtained by dividing the minimum bending radius R, which does not produce cracks of 0.3 mm or more on the surface of the test specimen, by the plate thickness t. The bending ridge direction was made parallel to the rolling direction. In this invention, steel plates with an R / t of 3.0 or less are evaluated as having excellent bendability and are indicated with "○" in Table 3. Steel plates with an R / t greater than 3.0 are evaluated as having inferior bendability and are indicated with "×" in Table 3.

[0094] The shear end surface properties were evaluated as follows: (1) A strip of test material was taken from the widthwise end of the obtained steel plate (coil) at a position 1 / 4 of the coil width, with a length of 100 mm perpendicular to the rolling direction and 30 mm in the rolling direction. At this time, the end surface on the longer side, which has a length of 100 mm, was cut by shearing, with a shearing clearance of 15% and a rake angle of 0°. (2) The shear end surface was observed using a stereomicroscope, and the average shear surface ratio in each of the three fields of view was determined. (3) For steel plates with a TS of 1310 MPa or more and less than 1600 MPa, those with a shear surface ratio of 25% or more were judged to have excellent shear end surface properties. For steel plates with a TS of 1600 MPa or more and less than 1800 MPa, those with a shear surface ratio of 15% or more were judged to have excellent shear end surface properties. For steel plates with a TS of 1800 MPa or higher, those with a shear surface ratio of 5% or higher were judged to have superior shear surface properties.

[0095] Furthermore, the delayed fracture resistance characteristics of the shear end face were evaluated as follows: (1) First, a strip test piece was taken from the widthwise end of the obtained steel plate (coil) at a position 1 / 4 of the coil width, with a length of 100 mm perpendicular to the rolling direction and 30 mm in the rolling direction. (2) The end face on the longer side, which is 100 mm in length, was cut out by shearing, and while still in the sheared state (without machining to remove burrs), it was bent so that the burrs were on the outer circumference of the bend, and the test piece was fixed with bolts while maintaining the shape of the test piece during bending. The clearance for shearing was set to 15%, and the rake angle was set to 0°. The bending was performed with a tip bending radius of 10 mm, and the angle on the inside of the bend apex was 90 degrees (V-bend). A V-shaped punch with a tip radius the same as the tip bending radius R was used, and a die with a corner R of 30 mm was used. Then, the depth to which the punch presses into the steel plate is adjusted, and the tip is shaped so that the bending angle (angle on the inside of the bending apex) is 90 degrees (V-shape). The test piece is clamped and tightened with a hydraulic jack so that the distance between the flange ends of the straight section during bending is the same as the distance when bending (to cancel out the opening of the straight section due to springback), and then bolted in that state. The bolts are fixed by passing them through elliptical holes (minor axis 10 mm, major axis 15 mm) that have been made 10 mm inward from the short edge of the strip test piece. (3) The obtained bolted test piece is immersed in a solution prepared by mixing 0.1 mass% ammonium thiocyanate aqueous solution and McIlvaine buffer in a 1:1 mass ratio and adjusting the pH to 8.0, and a delayed fracture resistance evaluation test is performed. At this time, the temperature of the solution is 20°C, and the surface area of ​​the test piece is 11 cm². ２ The liquid volume per sample was set at 20 ml. (4) After 100 hours, the presence or absence of cracks that could be visually confirmed (length of 1 mm or more) was checked, and samples in which no cracks were observed (or in which cracks were present but less than 1 mm in length) were judged to have excellent delayed fracture resistance.

[0096]

[0097] Furthermore, it was found that members obtained by forming and joining using the steel plate of the present invention example exhibited similar strength, bendability, delayed fracture resistance of the shear end face, and shear end face characteristics to the steel plate of the present invention example.

Claims

1. In mass%, C: 0.10% or more and 0.45% or less, Si: 1.5% or less, Mn: more than 1.7% and 4.0% or less, P: 0.10% or less, S: 0.010% or less, sol. The steel has a microstructure containing Al: 0.50% or less, N: 0.0200% or less, with a Ti content satisfying the following formula (1), the remainder consisting of Fe and unavoidable impurities, with a martensite area ratio of 85% or more and less than 95% of the total microstructure, a retained austenite area ratio of 5% or more and less than 15% of the total microstructure, and a remainder consisting of one or more of bainite and ferrite with an area ratio of less than 5% of the total microstructure, with an average spacing of Ti precipitates with an equivalent circle diameter of 5 nm or more and less than 30 nm being 1.0 μm or less, and a number density of Ti precipitates with an equivalent circle diameter of 80 nm or more and less than 300 nm being 80 particles / mm². ２ The above is true, and the steel plate has a tensile strength of 1310 MPa or more. [Ti] ≥ 0.01 + 3.4 × [N] + 1.5 × [S] ... Equation (1) Here, [Ti], [N], and [S] represent the content (mass %) of each element.

2. The steel sheet according to claim 1, further comprising, in mass percent, one or more selected from the following as the component composition: B: 0.0100% or less, Cu: 1.00% or less, Cr: 1.00% or less, Nb: 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: 0.50% or less, Zr: 0.020% or less, W: 0.020% or less, REM: 0.020% or less.

3. The steel sheet according to claim 1 or 2, wherein the steel sheet surface has a plating layer.

4. A member made using the steel plate described in any one of claims 1 to 3.

5. A steel slab having the component composition described in claim 1 or 2 is heated and held at a slab surface temperature of 1100°C or higher for 10 to 60 minutes, then hot finish rolling is performed under conditions where the residence time at 1000 to 1100°C is 20 to 120 seconds and the finish rolling temperature is 850°C or higher, and the slab is cooled at an average cooling rate of 40°C / second or more in the range from the finish rolling temperature to 650°C, then wound at a winding temperature of 650°C or lower, and after winding is held at a temperature of 500 to 650°C for 40 minutes or more to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is cold-rolled with a reduction ratio of 30% or more, and the cold-rolled steel sheet is heated at an average heating rate of 1.0°C / second or more from 400°C to the annealing temperature with an annealing temperature of 800 to 950°C, and held at the annealing temperature for 10 to 600 seconds, A method for manufacturing 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 and holding temperature of 250 to 450°C; and then performing continuous annealing, holding at the reheating and holding temperature for 20 to 1500 seconds.

6. The method for manufacturing a steel sheet according to claim 5, wherein, after continuous annealing, a plating treatment is performed on the surface of the steel sheet.

7. A method for manufacturing a component, comprising the step of forming and joining a steel plate according to any one of claims 1 to 3 to form a component.

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