Steel sheet, member, and production methods therefor

Abstract

Provided are: a steel sheet and a member having a tensile strength of 1310 MPa or more, excellent ductility, delayed fracture resistance at sheared edges, and sheared edge properties; and production methods therefor.　The steel sheet has a component composition including specific amounts of C, Si, Mn, P, S, sol. Al, N, and Ti, with the remainder being Fe and unavoidable impurities. The steel sheet has a steel structure in which an area ratio of martensite to the entire structure is 85% or more but less than 95%, an area ratio of ferrite and bainite combined to the entire structure is 0% to 15%, and an area ratio of retained austenite to the entire structure is 0% or more but less than 5%. The steel sheet has average spacing of Ti-based precipitates having an equivalent circle diameter of 5 nm to 30 nm of 1.0 μm or less, a number density of Ti-based precipitates having an equivalent circle diameter of 80 nm to 300 nm of 80 precipitates / mm2 or more, and a tensile strength of 1,310 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 ductility 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 ductility, 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 ductility means that the total elongation (El), as determined by the measurement method compliant with JIS Z2241 (2011), is 8.0% or higher.

[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 the presence of 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] Further, the evaluation of the sheared end face properties was carried out as follows. (1) A strip test piece with a length of 100 mm in the direction perpendicular to rolling and 30 mm in the rolling direction is taken from the end of the obtained steel sheet (coil) in the width direction at a position 1 / 4 of the coil width. At this time, the cutting of the end face on the long side with a length of 100 mm was performed by shearing, the clearance of the shearing was 15%, and the rake angle was 0°. (2) The sheared end face was observed using a stereomicroscope, and the average sheared cross-section ratio in each three-point field of view was determined. (3) For steel sheets with TS: 1310 MPa or more and less than 1600 MPa, those with a sheared cross-section ratio of 25% or more were judged to have excellent sheared end face properties. For steel sheets with TS: 1600 MPa or more and less than 1800 MPa, those with a sheared cross-section ratio of 15% or more were judged to have excellent sheared end face properties. For steel sheets with TS: 1800 MPa or more, those with a sheared cross-section ratio of 5% or more were judged to have excellent sheared end face properties.

[0014] The inventors of the present invention have intensively studied to solve the above problems, and have found that the anti-delayed fracture properties and the sheared end face properties of the sheared end face can be significantly improved by satisfying all of the following conditions. i) The area ratio with respect to the entire martensite structure is 85% or more and less than 95%, the total area ratio of ferrite and bainite with respect to the entire structure is 0% or more and 15% or less, and the area ratio of retained austenite with respect to the entire structure is 0% or more and less than 5%. ii) The average interval of Ti-based precipitates having 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-based precipitates having an equivalent circle diameter of 80 nm or more and 300 nm or less is 80 pieces / mm ２ or more.

[0015] The present invention has been completed based on the above findings, and its gist is as follows. [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 total area ratio of ferrite and bainite to the entire structure is 0% or more and 15% or less, and the area ratio of retained austenite to the entire structure is 0% or more and less than 5%, having a steel structure, the average interval of Ti-based precipitates having 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-based precipitates having an equivalent circle diameter of 80 nm or more and 300 nm or less is 80 pieces / mm ２The 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 [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, Ar from annealing temperature ３ Cool to (°C) at a first average cooling rate of over 10°C / second, Ar ３ (°C) to Ar ３ Cool to -80°C at a second mean cooling rate of 1.0 to 10°C / second, Ar ３Cooling is performed at a third average cooling rate of 10°C / second or more from -80°C to a cooling stop temperature of 260°C or less, heating is performed from the cooling stop temperature to a reheating holding temperature of 150 to 260°C, and continuous annealing is performed by holding at the reheating holding temperature for 20 to 1500 seconds. A method for manufacturing a steel sheet. [6] The method for manufacturing a steel sheet according to [5], wherein plating treatment is performed on the surface of the steel sheet after the continuous annealing. [7] A method for manufacturing a member, comprising a step of forming at least one of a forming process and a joining process on the steel sheet according to any one of [1] to [3] to form a member.

[0016] According to the present invention, there are provided a steel sheet, a member, and a method for manufacturing them, which have high strength, excellent ductility, anti-delayed fracture characteristics of a shear end face, and a shear end face property.

[0017] Hereinafter, embodiments of the present invention will be described.

[0018] Steel Sheet The steel sheet of the present invention contains, 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 unavoidable impurities, has a component composition, the area ratio of the entire martensite structure is 85% or more and less than 95%, the total area ratio of ferrite and bainite to the entire structure is 0% or more and 15% or less, and the area ratio of retained austenite to the entire structure is 0% or more and less than 5%. It has 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 ２ or more, and the tensile strength is 1310 MPa or more. [Ti] ≥ 0.01 + 3.4 × [N] + 1.5 × [S] ··· Formula (1) Here, [Ti], [N], and [S] represent the contents (mass%) of the respective elements of the steel sheet.

[0019] Component Composition The reasons for limiting the range of the component composition of the steel sheet of the present invention are described below. Note that % regarding the component content is "mass%".

[0020] 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. Also, if the C content exceeds 0.45%, the desired ductility cannot be obtained. 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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 it is desirable to have a sol. Al content of 0.005% or more for stable deoxidation. 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.

[0026] 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 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.

[0027] 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. Also, an example of a Ti-based precipitate controlled to have an equivalent circle diameter of 5 nm to 30 nm is TiC.

[0028] 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.

[0029] In this invention, the component composition may include one or more of the following elements as optional elements (selected elements): By 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.

[0030] B: 0.0100% or less. B is an element that improves the hardenability of steel and has the effect of generating martensite of a predetermined area ratio even with a small Mn content. In addition, B improves delayed fracture resistance by increasing the bonding force at grain boundaries through segregation at grain boundaries and by suppressing the segregation of P, which reduces grain boundary strength. To obtain these effects, the B content is preferably 0.0003% or more. The B content is more preferably 0.0008% or more, and even more preferably 0.0013% or more. On the other hand, if B is added in excess, Fe ２３ (C, B) ６ It forms BN and acts as the starting point for delayed fracture, thereby actually reducing the resistance to delayed fracture. Therefore, if B is included, the B content should be 0.0100% or less. Preferably, the B content is 0.0080% or less, and more preferably 0.0050% or less.

[0031] Cu: 1.00% or less Cu has the effect of improving the corrosion resistance of steel sheets, reducing hydrogen penetration into the steel sheets, and improving delayed fracture resistance. There is no lower limit specified for the Cu content, but in order to obtain these effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more, and even more preferably 0.10% or more. On the other hand, if Cu is added in excess, coarse precipitates increase and the delayed fracture resistance deteriorates. Therefore, when Cu is included, the Cu content should be 1.00% or less. The Cu content is preferably 0.50% or less, and more preferably 0.20% or less.

[0032] Cr: 1.00% or less. Cr is an effective element for improving the hardenability of steel. Cr can be added to stably obtain the desired microstructure. There is no particular lower limit for the Cr content, but to obtain these effects, 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, if Cr is added in excess, the solid solution of cementite during annealing is delayed, and a large amount of undissolved cementite remains, which deteriorates the delayed fracture resistance. Therefore, if Cr is included, the Cr content should be 1.00% or less. The Cr content is preferably 0.50% or less, and more preferably 0.20% or less.

[0033] Nb: 0.10% or less. Nb has the effect of improving delayed fracture resistance by refining the prior austenite grain size through a pinning effect by forming fine precipitates such as NbC in the steel. Although there is no lower limit for the Nb content, it is preferable that the Nb content be 0.005% or more in order to obtain this effect. More preferably, the Nb content is 0.01% or more. On the other hand, if Nb is added in excess, the amount of coarse precipitates increases and the delayed fracture resistance deteriorates. Therefore, if Nb is included, the Nb content should be 0.10% or less. The Nb content is preferably 0.05% or less, and more preferably 0.03% or less.

[0034] V: 0.50% or less V has the effect of improving delayed fracture resistance by generating fine carbides containing V that 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 V content, but in order to obtain these effects, the V content is preferably 0.003% or more. The V content is more preferably 0.01% or more, and even more preferably 0.03% or more. On the other hand, if V is added in excess, the amount of coarse precipitates increases and the delayed fracture resistance deteriorates. Therefore, if V is included, the V content should be 0.50% or less. The V content is preferably 0.20% or less, and even more preferably 0.10% or less.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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. The Ta content is preferably 0.004% or more, and 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, when 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.

[0041] 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.

[0042] 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.

[0043] 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. 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. The Zn content is preferably 0.015% or less, and more preferably 0.010% or less.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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 ferrite and bainite combined to the total structure is 0% or more and less than 15%, and the area ratio of retained austenite to the total structure is 0% or more and less than 5%. (Configuration 2) The average spacing of Ti precipitates with an equivalent circle diameter of 5 nm or more and less than 30 nm is 1.0 μm or less, and the number density of Ti precipitates with an equivalent circle diameter of 80 nm or more and less than 300 nm is 80 particles / mm ２ That's all.

[0050] The following describes each component.

[0051] (Structure 1) The structure has a martensite area ratio of 85% or more and less than 95% of the total structure, a ferrite and bainite combined area ratio of 0% or more and less than 15% of the total structure, and a retained austenite area ratio of 0% or more and 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 ratio of martensite must be 85% or more. The area ratio of martensite is preferably 87% or more. On the other hand, it has been found that both excellent delayed fracture resistance and excellent ductility of total elongation ≥ 8.0% can be achieved by using martensite as the main phase and generating a combined area ratio of ferrite and bainite of 0% or more and less than 15%, and retained austenite of 0% or more and less than 5%. In order to obtain these effects, the area ratio of martensite is less than 95%. The area ratio of martensite is preferably less than 93%. Excessive formation of ferrite and bainite reduces strength and degrades delayed fracture resistance. Therefore, to stably obtain a TS ≥ 1310 MPa and excellent delayed fracture resistance, the total area ratio of ferrite and bainite should be 15% or less. The total area ratio of ferrite and bainite is preferably 12% or less, more preferably 10% or less. The total area ratio of ferrite and bainite may be 0%, or it may be 5% or more. Furthermore, excessive formation of retained austenite (retained γ) reduces strength. Therefore, to stably obtain a TS ≥ 1310 MPa and excellent delayed fracture resistance, the area ratio of retained austenite should be less than 5%. The area fraction of retained austenite is preferably 3% or less, more preferably 1% or less. The area ratio of retained austenite may be 0%. Other than these structures are trace amounts of carbides, sulfides, nitrides, and oxides. The area ratio of the remaining structure is preferably 5% or less, more preferably 3% or less, and may be 0%. Furthermore, the martensite includes martensite that has not undergone tempering due to staying at approximately 150°C or above for a certain period of time, including self-tempering during continuous cooling.

[0052] (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. ２ The above is the conclusion. In steel with high strength, TS ≥ 1310 MPa, in order to achieve excellent delayed fracture resistance of the shear end face, 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 starting point for 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, by controlling the average spacing of Ti-based precipitates with an equivalent circle diameter of 5 nm to 30 nm to 1.0 μm or less, the delayed fracture resistance of the base material can be significantly improved by the hydrogen trapping effect of the precipitates and the refinement effect of 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.

[0053] Furthermore, the number density of Ti-based precipitates with an equivalent circular diameter of 80 nm to 300 nm is 80 particles / mm². ２It 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 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. ２ The 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:

[0054] The measurement methods for each component in the above steel microstructure are described below. The area ratios of martensite, bainite, and ferrite are determined 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 shows black contrast in the SEM.

[0055] 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.

[0056] 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.

[0057] 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 area 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 area from 1 / 5 to 4 / 5 of the plate thickness, i.e., from 1 / 5 of the plate thickness from the surface to 4 / 5 of the plate thickness, encompassing the center of the plate thickness, at a density of 0.5 mm 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.

[0058] 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.

[0059] 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).

[0060] 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.

[0061] Next, the steel plate manufacturing method of the present invention will be described. In the steel sheet manufacturing method of the present invention, a steel slab having the above-described component composition 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 cooling is performed with an average cooling rate of 40°C / second or more 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 to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is cold-rolled with a reduction ratio of 30% or more to obtain a cold-rolled steel sheet, the annealing temperature is 800 to 950°C, the cold-rolled steel sheet is heated from 400°C to the above annealing temperature at an average heating rate of 1.0°C / second or more, held at the above annealing temperature for 10 to 600 seconds, and from the above annealing temperature Ar ３ Cool to (°C) at a first average cooling rate of over 10°C / second, Ar ３ (°C) to Ar ３ Cool to -80°C at a second mean cooling rate of 1.0 to 10°C / second, Ar ３Continuous annealing is performed by cooling from -80°C to a cooling stop temperature of 260°C or lower at a third mean cooling rate of 10°C / second or more, heating from the cooling stop temperature to a reheating and holding temperature of 150 to 260°C, and holding at the reheating and holding temperature for 20 to 1500 seconds.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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 20°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)".

[0066] After cooling to 650°C as described above, further cooling is performed and winding is carried out. 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.

[0067] 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.

[0068] 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.

[0069] 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)".

[0070] 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. As a result, the desired precipitate state cannot be obtained, and the delayed fracture resistance deteriorates. 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 less than 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, if the soaking time (holding time) is excessively long, the Ti precipitates also become excessively coarse, and the amount of Ti 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 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 60 seconds or more.

[0071] Subsequently, in order to obtain a structure in which the area ratio of martensite is 85% or more and less than 95%, and the total area ratio of ferrite and bainite relative to the whole structure is 0% or more and 15% or less, the annealing temperature is set from Ar ３ Cool to (°C) at a first average cooling rate of over 10°C / second, Ar ３ (°C) to Ar ３Cool to -80°C at a second mean cooling rate of 1.0 to 10°C / second, Ar ３ Cooling is performed from -80°C to a cooling stop temperature of 260°C or lower at a third mean cooling rate of 10°C / second or higher.

[0072] Here, the mean cooling rate is defined as follows: First, the first mean cooling rate (°C / sec) is given by "(annealing temperature (°C) - Ar ３ (°C) / Ar from annealing temperature ３ The cooling time until (seconds) is given by the second mean cooling rate (°C / second), which is given by the second mean cooling rate (°C / second). ３ (°C) - (Ar ３ -80)(℃)) / Ar ３ From Ar ３ The cooling time to -80°C (seconds) is given by the third mean cooling rate (°C / second), which is given by the third mean cooling rate (°C / second). ３ -80°C (°C) / Cooling stop temperature below -260°C (°C) / Ar ３ This is the cooling time (in seconds) from -80°C to a cooling stop temperature of 260°C or lower. Also, Ar ３ The transformation point can be found by the following formula: Ar ３ Transformation point (°C) = 910 - 310 × C - 80 × Mn - 20 × Cu - 15 × Cr - 55 × Ni - 80 × Mo. Note that the element symbols in the formula represent the mass %) content of each element in the steel slab (steel plate). Elements that are not present are calculated as 0 (zero).

[0073] Ar from annealing temperature ３ If the average cooling rate up to Ar is 10°C / second or less, the Ti precipitates become coarser, and the number of Ti precipitates with an equivalent circle diameter of 5 nm to 30 nm decreases, resulting in the failure to obtain the desired precipitates and a deterioration in delayed fracture resistance. Therefore, from the annealing temperature to Ar ３ The first average cooling rate up to a certain point shall be greater than 10°C / second. The first average cooling rate is preferably 15°C / second or higher, and more preferably 20°C / second or higher. There is no particular upper limit to the first average cooling rate, but the first average cooling rate is preferably 200°C / second or lower, and more preferably 100°C / second or lower.

[0074] Ar ３ From Ar ３By keeping the average cooling rate down to -80°C (second mean cooling rate) below 10°C / second and generating small amounts of ferrite and bainite, the delayed fracture resistance can be improved. Therefore, Ar ３ From Ar ３ The second mean cooling rate down to -80°C shall be 10°C / second or less. Preferably, the second mean cooling rate is 8°C / second or less. On the other hand, Ar ３ From Ar ３ If the second mean cooling rate down to -80°C is less than 1.0°C / second, excessive ferrite and bainite are formed, so Ar ３ From Ar ３ The second mean cooling rate down to -80°C shall be 1.0°C / second or higher. Preferably, the second mean cooling rate is 3°C / second or higher, and more preferably 5°C / second or higher.

[0075] Ar ３ If the average cooling rate (third mean cooling rate) from -80°C to a cooling stop temperature of 260°C or lower is less than 10°C / second, a large amount of ferrite and bainite will be formed, and sufficient strength and delayed fracture resistance cannot be obtained. Therefore, Ar ３ The third mean cooling rate from -80°C or above to the cooling stop temperature of 260°C or below shall be 10°C / second or more. The mean cooling rate is preferably 15°C / second or more, and more preferably 70°C / second or more. There is no particular upper limit to the third mean cooling rate, but the third mean cooling rate is preferably 1000°C / second or less, and more preferably 800°C / second or less.

[0076] Furthermore, if the cooling stop temperature exceeds 260°C, a large amount of bainite is generated, increasing the amount of retained austenite and fresh martensite, and sufficient delayed fracture resistance cannot be obtained. Therefore, the cooling stop temperature should be 260°C or lower. The cooling stop temperature is preferably 240°C or lower, and more preferably 230°C or lower. The lower limit of the cooling stop temperature is not particularly limited, but it is preferably 100°C or higher, and more preferably 150°C or higher.

[0077] If the reheating and holding temperature exceeds 260°C, carbides may precipitate on the grain boundaries, potentially reducing strength, ductility, and delayed fracture resistance. Similarly, if the holding time at the reheating and holding temperature exceeds 1500 seconds, carbides may precipitate on the grain boundaries, potentially reducing strength, ductility, and delayed fracture resistance. On the other hand, if the reheating and holding temperature is below 150°C, tempering may be insufficient, potentially reducing delayed fracture resistance and ductility. Furthermore, if the holding time at the reheating and holding temperature is 20 seconds or less, tempering may be insufficient, potentially degrading delayed fracture resistance and ductility. Therefore, in this invention, continuous annealing is performed at a reheating and holding temperature of 150 to 260°C for 20 to 1500 seconds. The reheating and holding temperature is preferably 250°C or lower, more preferably 240°C or lower. The reheating and holding temperature is also preferably 160°C or higher, more preferably 170°C or higher. The holding time at the reheating and holding temperature is preferably 1200 seconds or lower, more preferably 900 seconds or lower. Furthermore, the holding temperature at the reheating and holding temperature is preferably 60 seconds or more, and more preferably 240 seconds or more.

[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 possesses excellent ductility, 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 ductility, 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 then 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 for the heating and holding time shown in Table 2, then hot finish rolling was performed at the residence time of 1000-1100°C and the 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 conditions shown in Table 2, reheated as necessary, and continuously annealed by holding at the holding temperature and 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 1 / 4 of the way from the surface of the steel plate, with a field of view of 48 μm × 60 μm. 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 a slightly darker 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 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 area 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 area from 1 / 5 to 4 / 5 of the plate thickness, i.e., from 1 / 5 of the plate thickness from the surface to 4 / 5 of the plate thickness, encompassing the center of the plate thickness, at a density of 0.5 mm 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 position 1 / 4 of the coil width, with the direction perpendicular to the rolling direction being the longitudinal direction. The tensile test (in accordance with JIS Z2241) was performed to evaluate the tensile strength TS and total elongation El. In this invention, steel sheets with El of 8.0% or more are evaluated as having excellent ductility and are indicated with "○" in Table 3. Steel sheets with El less than 8.0% are evaluated as having inferior ductility and are indicated with "×" in Table 3.

[0093] 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.

[0094] 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 punch is adjusted to the depth into which it presses the steel plate, 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 it is bent (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 ３ 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.

[0095]

[0096] Furthermore, it was found that members obtained by forming and joining using the steel plate of the present invention example exhibited similar strength, ductility, 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 in which Al is 0.50% or less, N is 0.0200% or less, Ti content satisfies the following formula (1), the remainder being Fe and unavoidable impurities, the area ratio of martensite to the total microstructure is 85% or more and less than 95%, the area ratio of ferrite and bainite combined to the total microstructure is 0% or more and less than 15%, and the area ratio of retained austenite to the total microstructure is 0% or more and less than 5%, 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 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. From the aforementioned annealing temperature, Ar ３ Cool to (°C) at a first average cooling rate of over 10°C / second, Ar ３ (°C) to Ar ３ Cool to -80°C at a second mean cooling rate of 1.0 to 10°C / second, Ar ３ A method for manufacturing a steel sheet, comprising: cooling from -80°C to a cooling stop temperature of 260°C or lower at a third mean cooling rate of 10°C / second or more; heating from the cooling stop temperature to a reheating and holding temperature of 150 to 260°C; and performing continuous annealing by 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.

Images