Steel sheet, member, and production methods for these

Abstract

Provided are a steel sheet and a member that have high strength and excellent resistance to delayed fractures, and production methods for these.　The steel sheet contains, in mass%, 0.15% to 0.45% of C, 2.0% or less of Si, 4.0% or less of Mn, 0.10% or less of P, 0.01% or less of S, 0.50% or less of sol. Al, 0.010% or less of N, and 0.0008% to 0.0100% of B, with the remainder consisting of Fe and unavoidable impurities. The steel sheet has a steel structure in which the area ratio of martensite to the whole structure is at least 85% but less than 95%, the area ratio of ferrite to the whole structure is 5% to 15%, and the average crystal grain size of ferrite is 10 μm or less. The number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the formula: A (precipitates / mm2)≤8.5×105×[B].

Description

Steel plates, components, and their manufacturing methods

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

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

[0003] When a high-strength steel sheet having a tensile strength TS of 1470 MPa or more is formed into a part by cold pressing, delayed fracture may occur due to an increase in residual stress within the part or a deterioration in the delayed fracture resistance of the steel sheet itself.

[0004] Here, delayed fracture refers to a phenomenon in which, when a component is placed in a hydrogen penetration environment while high stress is applied to the component, hydrogen penetrates into the steel sheet that makes up the component, reducing the interatomic bonding strength and causing localized deformation, resulting in microcracks, which then propagate and lead to fracture.

[0005] As a technique for improving such delayed fracture resistance, for example, Patent Document 1 discloses a steel sheet containing, in mass %, C: 0.13% or more and 0.40% or less, Si: 0.02% or more and 1.5% or less, Mn: 0.4% or more and 1.7% or less, P: 0.030% or less, S: 0.0002% or more and less than 0.0010%, sol. The alloy contains Al: 0.01% or more and 0.20% or less, N: 0.0055% or less, O: 0.0025% or less, Nb: 0.002% or more and 0.035% or less, and Ti: 0.002% or more and 0.040% or less so as to satisfy the formulas (1) and (2), with the balance being Fe and unavoidable impurities, and the area ratio of martensite and bainite to the entire structure being 95% or more and 100% or less in total, with the balance being one or both of ferrite and retained austenite, the average grain size of prior austenite grains exceeding 5 μm, and the alloy satisfies the following conditions, and the number of inclusions having a major axis length of 20 to 80 μm is 5 / mm ２ The present invention discloses a high-strength steel plate having excellent delayed fracture resistance, characterized by a tensile strength of 1,320 MPa or more, and a steel structure having the following: [%Ti] + [%Nb] > 0.007 (1) [%Ti] × [%Nb] ２ ≦7.5×10 －６ (2) Here, [% Nb] and [% Ti] represent the contents (%) of Nb and Ti.

[0006] Patent Document 2 also describes a steel containing, in mass%, C: 0.05 to 0.30%, Si: 2.0% or less (including 0%), Mn: more than 0.1% and 2.8% or less, P: 0.1% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.01 to 0.50% or less, and one or more of Nb, Ti, and Zr, which are 0.01% or more in total, and [%C] - [%Nb] / 92.9 × 12 - [% The alloy has a composition in which the ratio of [%Ti] / 47.9×12 - [%Zr] / 91.2×12 > 0.03 is satisfied, with the balance being iron and unavoidable impurities, and has a structure in which tempered martensite accounts for 50% or more (including 100%) in area ratio, with the balance being ferrite, and the distribution state of precipitates in the tempered martensite is such that precipitates having a circle equivalent diameter of 1 to 10 nm are present in the tempered martensite at 1 μm or less. ２ The precipitates are 20 or more per 1000 micrometers and have an equivalent circle diameter of 20 nm or more, and contain one or more of Nb, Ti and Zr. ２ The document discloses a high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and workability, characterized in that the average grain size of ferrite surrounded by high-angle grain boundaries with a crystal orientation difference of 15° or more is 5 μm or less, and the number of grains per grain is 10 or less.

[0007] Patent No. 6388085 Patent No. 4712882

[0008] However, the conventional technology is not sufficient to ensure a tensile strength TS of 1470 MPa or more and to provide excellent delayed fracture resistance, and there has been a demand for the establishment of a new technology.

[0009] The present invention has been made to solve such problems, and an object of the present invention is to provide a steel plate, a member, and a method for manufacturing the same, which have a tensile strength of 1470 MPa or more (TS≧1470 MPa) and excellent delayed fracture resistance.

[0010] "Excellent delayed fracture resistance" refers to a judgment that the steel sheet (coil) has excellent delayed fracture resistance based on the following evaluation. (1) First, a strip test piece measuring 100 mm in the direction perpendicular to the rolling direction and 30 mm in the rolling direction is taken from the end of the width direction of the obtained steel sheet (coil) at a position 1 / 4 of the coil width. (2) The end face of the long side with a length of 100 mm is sheared, and the test piece is bent in the as-sheared state (without machining to remove burrs) so that the burrs are on the outer periphery of the bend. The test piece is fixed with bolts while maintaining the shape of the bent piece. The shearing clearance is 13% and the rake angle is 1°. The bending is performed with a tip bending radius of 10 mm and an inner angle of the bend apex of 90 degrees (V-bend). The punch used has a tip radius R equal to the tip bending radius R and is U-shaped (the tip R portion is semicircular and the punch body thickness is 2R), and the die has a corner R of 30 mm. The depth to which the punch pushes the steel plate is then adjusted to form a 90-degree bend angle (the angle at the inner side of the bend apex) (V-shape). The test specimen is clamped and tightened with a hydraulic jack so that the distance between the flange ends of the straight pieces during bending is the same as when the specimen was bent (to cancel out the opening of the straight pieces due to springback), and then bolted in this state. The bolt is passed through an elliptical hole (minor axis 10 mm, major axis 15 mm) pre-drilled 10 mm inward from the short edge of the strip specimen to secure it in place. (3) The bolt-tightened test specimen is then immersed in a solution prepared by mixing 0.1% by mass of ammonium thiocyanate aqueous solution and McIlvaine buffer solution in a 1:1 mass ratio, adjusted to a pH of 8.0, and a solution adjusted to a pH of 7.4, to perform a delayed fracture resistance evaluation test. The solution temperature is 20°C, and the test specimen is immersed in a solution with a surface area of ​​1 cm. ３ The volume of solution per test piece was 20 ml. (4) After 48 hours, the presence or absence of visually observable cracks (lengths of 1 mm or more) was confirmed, and samples in which no cracks were observed in the solution adjusted to pH 8.0 were judged to have excellent delayed fracture resistance. Furthermore, samples in which no cracks were observed in the solution adjusted to pH 7.4 were judged to have particularly excellent delayed fracture resistance.

[0011] The present inventors have conducted extensive research to solve the above problems and have found that delayed fracture resistance can be significantly improved by satisfying all of the following conditions: i) The area fraction of martensite is 85% or more and less than 95%; ii) The area fraction of ferrite is 5% or more and 15% or less, and the average grain size of ferrite is 10 μm or less; and iii) The number density A of precipitates with a circle equivalent diameter of 500 nm or more satisfies the following conditions: A (numbers / mm ２ ) ≦ 8.5 × 10 ５ ×[B] Here, [B] represents the content of B (% by mass).

[0012] The present invention was completed through further studies based on the above findings, and the gist of the present invention is as follows: [1] A steel sheet having a chemical composition containing, by mass%, C: 0.15% or more and 0.45% or less, Si: 2.0% or less, Mn: 4.0% or less, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.5% or less, N: 0.010% or less, B: 0.0008% or more and 0.0100% or less, with the balance consisting of Fe and unavoidable impurities, wherein the area ratio of martensite to the entire structure is 85% or more and less than 95%, and the area ratio of ferrite to the entire structure is 5% or more and 15% or less, and the average grain size of ferrite is 10 μm or less, and wherein the number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the following formula (1): A (numbers / mm ２ ) ≦ 8.5 × 10 ５× [B] Formula (1) Here, [B] represents the content (mass%) of B. [2] The steel sheet according to [1], wherein 50% or more of the ferrite is ferrite precipitated on prior austenite grain boundaries in terms of area ratio. [3] The steel sheet according to [1] or [2], wherein the chemical composition further contains, by mass%, one or more elements selected from Cu: 1.00% or less, Cr: 1.00% or less, Nb: 0.10% or less, Ti: 0.10% or less, V: 0.50% or less, Mo: 0.50% or less, Ni: 1.00% or less, Sb: 0.10% or less, Sn: 0.10% or less, As: 0.10% or less, Ta: 0.10% or less, Ca: 0.020% or less, Mg: 0.020% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, W: 0.020% or less, and REM: 0.020% or less. [4] The steel sheet according to any one of [1] to [3], having a plating layer on the surface of the steel sheet. [5] A member made using the steel sheet according to any one of [1] to [4]. [6] A steel slab having the chemical composition according to [1] or [3] is held at a heating holding temperature of 1100°C or higher at the slab surface temperature for 30 minutes or more, and then hot finish rolling is performed under conditions of a residence time of 20 seconds to 150 seconds at 900 to 1000°C and a finish rolling temperature of 850°C or higher, cooling is performed at an average cooling rate of 40°C / second or higher in the range from the finish rolling temperature to 650°C, and then coiling is performed at a coiling temperature of 650°C or lower to obtain a hot rolled steel sheet, and the hot rolled steel sheet is cold rolled at a reduction of 40% or higher to obtain a cold rolled steel sheet, and an annealing temperature is set to 830 to 950°C, and the cold rolled steel sheet is heated from 400°C to the annealing temperature at an average heating rate of 1.0°C / second or higher, and held at the annealing temperature for 10 seconds to 600 seconds, and then cooled from the annealing temperature to Ar ３ Cooling to a temperature of 10°C / sec or more at an average cooling rate of 10°C / sec or more, ３ From the point (Ar ３ Cooling is performed at an average cooling rate of 1 to 10°C / sec to a temperature of -80°C (Ar ３[7] A method for producing a steel sheet according to [6], comprising cooling the steel sheet from a cooling stop temperature (point −80°C) to a cooling stop temperature of 260°C or less at an average cooling rate of 10°C / second or more, and then performing continuous annealing by holding at a holding temperature of 150 to 260°C for 20 to 1,500 seconds. [7] A method for producing a steel sheet according to [6], wherein a plating treatment is performed on the surface of the steel sheet after the continuous annealing. [8] A method for producing a member, comprising a step of performing at least one of forming and joining on the steel sheet according to any of [1] to [4], to produce a member.

[0013] According to the present invention, there are provided a steel plate and a member having high strength and excellent delayed fracture resistance, and methods for producing the same.

[0014] Hereinafter, an embodiment of the present invention will be described.

[0015] The steel sheet of the present invention has a composition containing, by mass%, C: 0.15% or more and 0.45% or less, Si: 2.0% or less, Mn: 4.0% or less, P: 0.10% or less, S: 0.01% or less, sol. Al: 0.50% or less, N: 0.01% or less, B: 0.0008% or more and 0.0100% or less, with the balance being Fe and unavoidable impurities, and has a structure in which the area ratio of martensite to the entire structure is 85% or more and less than 95%, the area ratio of ferrite to the entire structure is 5% or more and 15% or less, the average grain size of ferrite is 10 μm or less, and the number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the following formula (1): A (numbers / mm ２ ) ≦ 8.5 × 10 ５ ×[B] Formula (1) Here, [B] represents the content of B (% by mass).

[0016] The reasons for limiting the range of the chemical composition of the steel sheet of the present invention will be explained below. Note that % regarding the content of each element is "mass %".

[0017] C: 0.15% or more and 0.45% or less C is contained to increase the strength of martensite and obtain a tensile strength of 1470 MPa or more (hereinafter also referred to as TS≧1470 MPa). Therefore, in order to obtain the desired TS, the C content is set to 0.15% or more. From the viewpoint of reducing the weight of automotive frame parts by increasing strength, the C content is preferably 0.20% or more, more preferably 0.25% or more. On the other hand, if excessive C is added, the formation of ferrite is excessively suppressed, and the desired ferrite area ratio cannot be obtained. Therefore, the C content is set to 0.45% or less. The C content is preferably 0.40% or less, more preferably 0.35% or less.

[0018] Si: 2.0% or less Si suppresses the formation of film-like carbides when tempered at temperatures above 200°C, thereby suppressing a decrease in strength and a deterioration in delayed fracture resistance. Although there is no lower limit for the Si content, it is desirable for Si to be contained in an amount of 0.02% or more. The Si content is preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, excessive addition of Si leads to a deterioration in delayed fracture resistance due to the segregation of Si. Therefore, the Si content is set to 2.0% or less (including 0%). The Si content is preferably 1.5% or less, and more preferably 1.0% or less.

[0019] Mn: 4.0% or less Mn is an element effective in improving the hardenability of steel. Although there is no lower limit for the Mn content, in order to stably obtain the desired area ratio of martensite, the Mn content is desirably 0.2% or more. The Mn content is preferably 0.5% or more. On the other hand, if Mn is added in excess, the formation of ferrite is excessively suppressed, and the desired area ratio of ferrite cannot be obtained. Therefore, the Mn content is set to 4.0% or less. The Mn content is preferably 3.0% or less, and more preferably 1.7% or less.

[0020] P: 0.10% or less P segregates at grain boundaries and reduces grain boundary strength, thereby deteriorating delayed fracture resistance. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.05% or less, more preferably 0.02% or less, and even more preferably 0.01% or less. There is no specified lower limit for the P content, but the lower limit currently industrially feasible is 0.002%. Therefore, the P content is preferably set to 0.002% or more.

[0021] S: 0.01% or less S forms coarse inclusions with Mn, which become the starting points for delayed fracture, thereby deteriorating delayed fracture resistance. Therefore, the S content is set to 0.01% or less. The S content is preferably 0.003% or less, more preferably 0.0015% or less, and even more preferably 0.0008% or less. Although no lower limit is specified, the lower limit currently industrially feasible is 0.0002%. Therefore, the S content is preferably set to 0.0002% or more.

[0022] Sol. Al: 0.50% or less Al is contained to perform sufficient deoxidation and reduce inclusions in steel. Although the lower limit of sol. Al is not particularly specified, in order to perform stable deoxidation, it is desirable that the sol. Al content be 0.005% or more. The sol. Al content is more preferably 0.01% or more, and even more preferably 0.02% or more. On the other hand, if the sol. Al content exceeds 0.50%, cementite formed during coiling is less likely to dissolve in the annealing process, the number density A of precipitates cannot be set within the desired range, and delayed fracture resistance is significantly deteriorated. Therefore, the sol. Al content is set to 0.50% or less. The sol. Al content is preferably 0.20% or less, and more preferably 0.05% or less.

[0023] N: 0.010% or less N forms precipitates such as AlN, which act as initiation sites for delayed fracture, thereby deteriorating delayed fracture resistance. In particular, if the N content exceeds 0.010%, the number density A of precipitates cannot be set within the desired range, resulting in significant deterioration of delayed fracture resistance. Therefore, the N content is set to 0.010% or less. The N content is preferably set to 0.005% or less. Although no lower limit is specified, the lower limit currently industrially feasible is 0.0006%. Therefore, the N content is preferably set to 0.0006% or more.

[0024] B: 0.0008% or more and 0.0100% or less B is an element that improves the hardenability of steel, and has the effect of generating a predetermined area ratio of martensite even with a small Mn content. In addition, B segregates at grain boundaries to increase the bonding strength of the grain boundaries and suppresses the segregation of P, which reduces grain boundary strength. In order to obtain the desired delayed fracture resistance due to these effects, the B content is set to 0.0008% or more. The B content is preferably 0.0015% or more, and more preferably 0.0020% or more. On the other hand, excessive addition of B increases the Fe content. ２３ (C, B) ６ It has been found that the addition of B results in the formation of BN and BN, which become the starting point of delayed fracture, thereby deteriorating the delayed fracture resistance. Therefore, in order to obtain the effect of improving the delayed fracture resistance by adding B, it is necessary to add B while keeping the number density A of precipitates within a desired range. If the B content exceeds 0.0100%, it is difficult to reduce the B-based precipitates even by controlling the hot rolling conditions and annealing conditions, and the number density A of precipitates cannot be kept within a desired range. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, and more preferably 0.0060% or less.

[0025] The steel sheet of the present invention preferably has a composition containing the above-mentioned basic elements with the balance being iron (Fe) and unavoidable impurities.

[0026] In the present invention, the composition may contain the following: one or more elements selected from, by mass%, Cu: 1.00% or less, Cr: 1.00% or less, Nb: 0.10% or less, Ti: 0.10% or less, V: 0.50% or less, Mo: 0.50% or less, Ni: 1.00% or less, Sb: 0.10% or less, Sn: 0.10% or less, As: 0.10% or less, Ta: 0.10% or less, Ca: 0.020% or less, Mg: 0.020% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, W: 0.020% or less, and REM: 0.020% or less.

[0027] Cu: 1.00% or less Cu has the effects of improving the corrosion resistance of the steel sheet, reducing hydrogen penetration into the steel sheet, and improving delayed fracture resistance. Although there is no lower limit for the Cu content, to achieve these effects, the Cu content is desirably 0.01% or more. The Cu content is preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, excessive addition of Cu increases the number of coarse precipitates, making it impossible to achieve a desired number density A of the precipitates, and deteriorating delayed fracture resistance. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. The Cu content is preferably 0.50% or less, and more preferably 0.30% or less.

[0028] Cr: 1.00% or less Cr is an element effective in improving the hardenability of steel. Cr can be added to stably obtain a desired structure. While the lower limit of the Cr content is not particularly specified, to obtain this effect, the Cr content is desirably 0.01% or more. The Cr content is more preferably 0.05% or more, and even more preferably 0.10% or more. On the other hand, excessive addition of Cr delays the solid solution of cementite during annealing, leaving a large amount of undissolved cementite. This prevents the number density A of precipitates from being within the desired range, resulting in a deterioration in delayed fracture resistance. Therefore, when Cr is contained, the Cr content is set to 1.00% or less. The Cr content is preferably 0.50% or less, and more preferably 0.30% or less.

[0029] Nb: 0.10% or less Nb forms fine precipitates such as NbC in steel, thereby refining the prior austenite grain size through a pinning effect and improving delayed fracture resistance. While there is no lower limit for the Nb content, in order to obtain this effect of improving delayed fracture resistance, the Nb content is desirably 0.005% or more. The Nb content is preferably 0.01% or more. On the other hand, excessive addition of Nb increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, thereby deteriorating delayed fracture resistance. Therefore, when Nb is contained, the Nb content is set to 0.10% or less. The Nb content is preferably 0.08% or less, and more preferably 0.06% or less.

[0030] Ti: 0.10% or less Ti forms fine precipitates such as TiC in steel, thereby refining the prior austenite grain size through a pinning effect and improving delayed fracture resistance. While there is no lower limit for the Ti content, to achieve this effect, the Ti content is desirably 0.005% or more. The Ti content is preferably 0.01% or more. On the other hand, excessive addition of Ti increases the number of coarse precipitates, making it impossible to achieve a desired precipitate number density A, thereby deteriorating delayed fracture resistance. Therefore, when Ti is contained, the Ti content is set to 0.10% or less. The Ti content is preferably 0.08% or less, more preferably 0.06% or less.

[0031] V: 0.50% or less V has the effect of generating fine V-containing carbides that act as hydrogen trapping sites, thereby improving delayed fracture resistance. Furthermore, by forming fine precipitates, the prior austenite grain size is refined through a pinning effect, thereby improving delayed fracture resistance. While there is no specified lower limit for the V content, to achieve this effect, the V content is desirably 0.003% or more. The V content is preferably 0.01% or more, and more preferably 0.03% or more. On the other hand, excessive V addition increases the number of coarse precipitates, making it impossible to achieve a desired precipitate number density A, thereby deteriorating delayed fracture resistance. Therefore, when V is contained, the V content is set to 0.50% or less. The V content is preferably 0.20% or less, more preferably 0.10% or less, and even more preferably 0.06% or less.

[0032] Mo: 0.50% or less Mo has the effect of forming fine carbides containing Mo that act as hydrogen trapping sites, thereby improving delayed fracture resistance. Furthermore, by forming fine precipitates, the prior austenite grain size is refined through a pinning effect, thereby improving delayed fracture resistance. While there is no specified lower limit for the Mo content, in order to achieve this effect of improving delayed fracture resistance, the Mo content is desirably 0.003% or more. The Mo content is preferably 0.01% or more, more preferably 0.03% or more. On the other hand, excessive addition of Mo increases the number of coarse precipitates, making it impossible to achieve a desired precipitate number density A, thereby deteriorating delayed fracture resistance. Therefore, when Mo is contained, the Mo content is set to 0.50% or less. The Mo content is preferably 0.20% or less, more preferably 0.10% or less.

[0033] Ni: 1.00% or less Ni has the effects of improving the corrosion resistance of steel sheets, suppressing hydrogen penetration into steel sheets, and improving delayed fracture resistance. Ni is also an element effective in improving the hardenability of steel and can be added to stably obtain a desired structure. While there is no specified lower limit for the Ni content, to obtain these effects, the Ni content is desirably 0.01% or more. The Ni content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, excessive Ni addition increases the number of coarse precipitates, making it impossible to achieve the desired precipitate number density A within the desired range, resulting in a deterioration in delayed fracture resistance. Therefore, when Ni is contained, the Ni content is set to 1.00% or less. The Ni content is preferably 0.50% or less, more preferably 0.30% or less.

[0034] Sb: 0.10% or less Sb suppresses oxidation and nitriding of the surface layer of the steel sheet, contributing to an increase in strength and an improvement in delayed fracture resistance. Although the lower limit of the Sb content is not specified, in order to obtain such effects as an increase in strength and an improvement in delayed fracture resistance, the Sb content is desirably 0.002% or more. The Sb content is preferably 0.004% or more, and more preferably 0.006% or more. On the other hand, excessive addition of Sb increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating the delayed fracture resistance. Therefore, when Sb is contained, the Sb content is set to 0.10% or less. The Sb content is preferably 0.05% or less, and more preferably 0.02% or less.

[0035] Sn: 0.10% or less Sn suppresses oxidation and nitriding of the surface layer of the steel sheet, contributing to increased strength and improved delayed fracture resistance. While there is no lower limit for the Sn content, in order to achieve such effects as increased strength and improved delayed fracture resistance, the Sn content is desirably 0.002% or more. The Sn content is preferably 0.004% or more, and more preferably 0.006% or more. On the other hand, excessive addition of Sn increases the number of coarse precipitates, making it impossible to achieve a desired number density A of the precipitates, and deteriorating delayed fracture resistance. Therefore, when Sn is contained, the Sn content is set to 0.10% or less. The Sn content is preferably 0.05% or less, and more preferably 0.02% or less.

[0036] As: 0.10% or less As has the effect of increasing the strength of steel. Although there is no lower limit for the As content, in order to obtain this effect of increasing the strength of steel, the As content is desirably 0.002% or more. The As content is preferably 0.004% or more, more preferably 0.006% or more. On the other hand, excessive addition of As increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating the delayed fracture resistance. Therefore, when As is contained, the As content is set to 0.10% or less. The As content is preferably 0.05% or less, more preferably 0.02% or less.

[0037] Ta: 0.10% or less Ta has the effect of increasing the strength of steel. Although there is no lower limit for the Ta content, in order to obtain this effect of increasing the strength of steel, the Ta content is desirably 0.002% or more. The Ta content is preferably 0.004% or more, more preferably 0.006% or more. On the other hand, if Ta is added in excess, the number density A of the precipitates increases, making it impossible to keep the desired range, and the delayed fracture resistance deteriorates. Therefore, when Ta is contained, the Ta content is set to 0.10% or less. The Ta content is preferably 0.05% or less, more preferably 0.02% or less.

[0038] Ca: 0.020% or less Ca reduces the initiation points of delayed fracture by spheroidizing the shape of sulfides, thereby improving delayed fracture resistance. To achieve this effect of improving delayed fracture resistance, the Ca content is desirably 0.0002% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, excessive addition of Ca increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating delayed fracture resistance. Therefore, when Ca is contained, the Ca content is set to 0.020% or less. The Ca content is preferably 0.015% or less, and more preferably 0.010% or less.

[0039] Mg: 0.020% or less Mg reduces the initiation points of delayed fracture by spheroidizing the shape of sulfides, thereby improving delayed fracture resistance. To achieve this effect of improving delayed fracture resistance, the Mg content is desirably 0.0002% or more. The Mg content is preferably 0.001% or more, and more preferably 0.003% or more. On the other hand, excessive addition of Mg increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating delayed fracture resistance. Therefore, when Mg is contained, the Mg content is set to 0.020% or less. The Mg content is preferably 0.015% or less, and more preferably 0.010% or less.

[0040] Zn: 0.020% or less Zn improves delayed fracture resistance by refining the prior austenite grain size and spheroidizing the shape of inclusions. To achieve this effect of improving delayed fracture resistance, the Zn content is desirably 0.001% or more. The Zn content is preferably 0.003% or more. On the other hand, excessive addition of Zn increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating delayed fracture resistance. Therefore, when Zn is contained, the Zn content is set to 0.020% or less. The Zn content is preferably 0.015% or less, and more preferably 0.010% or less.

[0041] Co: 0.020% or less Co improves delayed fracture resistance by refining the prior austenite grain size and spheroidizing the shape of inclusions. To achieve this effect of improving delayed fracture resistance, the Co content is desirably 0.001% or more. The Co content is preferably 0.003% or more. On the other hand, excessive addition of Co increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, and deteriorating delayed fracture resistance. Therefore, when Co is contained, the Co content is set to 0.020% or less. The Co content is preferably 0.015% or less, and more preferably 0.010% or less.

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

[0043] W: 0.020% or less W refines the prior austenite grain size through the formation of precipitates, thereby improving delayed fracture resistance. To achieve this effect of improving delayed fracture resistance, the W content is desirably 0.001% or more. The W content is preferably 0.003% or more. On the other hand, excessive addition of W increases the number of coarse precipitates, making it impossible to keep the number density A of the precipitates within the desired range, thereby deteriorating delayed fracture resistance. Therefore, when W is contained, the W content is set to 0.020% or less. The W content is preferably 0.015% or less, and more preferably 0.010% or less.

[0044] REM: 0.020% or less. REM also contributes to improving delayed fracture resistance by spheroidizing inclusions. To achieve this effect of improving delayed fracture resistance, the REM content is desirably 0.0002% or more. The REM content is preferably 0.001% or more, more preferably 0.003% or more. On the other hand, excessive addition of REM increases the number of coarse precipitates, making it impossible to achieve the desired number density A of the precipitates, and deteriorating delayed fracture resistance. Therefore, when REM is contained, the REM content is set to 0.020% or less. The REM content is preferably 0.015% or less, more preferably 0.010% or less. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REMs. The REMs are not particularly limited, but are preferably La and / or Ce.

[0045] When the optional element is contained in an amount less than the preferable lower limit, the optional element is considered to be contained as an unavoidable impurity.

[0046] The steel structure of the steel sheet of the present invention has the following features: (Feature 1) The area ratio of martensite to the entire structure is 85% or more and less than 95%, the area ratio of ferrite to the entire structure is 5% or more and 15% or less, and the average grain size of ferrite is 10 μm or less. (Feature 2) The number density A of precipitates having a circle equivalent diameter of 500 nm or more satisfies the following formula (1): A (numbers / mm ２ ) ≦ 8.5 × 10 ５ ×[B] Formula (1) Here, [B] represents the content of B (% by mass).

[0047] Furthermore, by providing the following configuration in addition to (Configuration 1) and (Configuration 2), even better delayed fracture resistance can be obtained: (Configuration 3) In terms of area ratio, 50% or more of the ferrite is ferrite precipitated on prior austenite grain boundaries.

[0048] Each component will be described below.

[0049] (Configuration 1) Area fraction of martensite: 85% or more and less than 95%, area fraction of ferrite: 5% or more and 15% or less, average grain size of ferrite: 10 μm or less. In the present invention, by using martensite as the main phase, it is possible to achieve high strength of TS≧1470 MPa. To achieve such an effect of achieving high strength, the area fraction of martensite needs to be 85% or more. The area fraction of martensite is preferably 88% or more. It has also been found that excellent delayed fracture resistance can be obtained by using martensite as the main phase and generating ferrite at an area fraction of 5% or more and 15% or less. To obtain excellent delayed fracture resistance, the area fraction of martensite is less than 95% and the area fraction of ferrite is 5% or more. The area fraction of martensite is preferably less than 93%, and the area fraction of ferrite is preferably 7% or more. On the other hand, excessive generation of ferrite reduces strength and deteriorates delayed fracture resistance. Therefore, in order to stably obtain excellent delayed fracture resistance such as TS≧1470 MPa, the area ratio of ferrite is set to 15% or less, and preferably 12% or less.

[0050] In addition, in the present invention, the morphology of ferrite is also controlled to obtain the effect of improving delayed fracture resistance by 5% to 15% ferrite. If the ferrite is coarse, the effect of improving delayed fracture resistance by ferrite cannot be obtained, and the formation of ferrite actually deteriorates the delayed fracture resistance. In order to obtain the effect of improving delayed fracture resistance by ferrite, the average crystal grain size of ferrite is set to 10 μm or less. The average crystal grain size of ferrite is preferably 6 μm or less, and more preferably 4 μm or less. On the other hand, if the ferrite is excessively fine, the effect of improving delayed fracture resistance may not be sufficiently obtained. Therefore, the average crystal grain size of ferrite is preferably 0.5 μm or more, and more preferably 1.0 μm or more.

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

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

[0053] (Configuration 3) In terms of area ratio, 50% or more of the ferrite is precipitated on prior austenite grain boundaries. The main methods for dispersing ferrite in martensite include holding in the austenite-ferrite two-phase region and then quenching to transform the austenite to martensite, and holding in the austenite single-phase region and then quenching in the ferrite formation temperature region to form ferrite, and then quenching to transform the remaining austenite to martensite. The ferrite formed by holding in the austenite single-phase region and then quenching in the ferrite formation temperature region is called proeutectoid ferrite, which preferentially forms on grain boundaries to suppress intergranular fracture, thereby significantly improving delayed fracture resistance. Having 50% or more of the total ferrite precipitated on prior austenite grain boundaries can significantly improve delayed fracture resistance. Therefore, it is desirable for the area ratio of ferrite precipitated on prior austenite grain boundaries to be 50% or more of the total ferrite. Preferably, the area ratio of ferrite precipitated on the prior austenite grain boundaries to the total amount of ferrite is 70% or more, more preferably 90% or more, or the area ratio of ferrite precipitated on the prior austenite grain boundaries to the total amount of ferrite may be 100%.

[0054] The method for measuring each component in the above steel structure will be described. The area ratios of martensite, bainite, and ferrite are measured by polishing an L-section of a steel sheet (a section parallel to the rolling direction and perpendicular to the steel sheet surface (hereinafter also referred to as a perpendicular section parallel to the rolling direction)) and then etching it with nital. Four fields of view within a range of 50 μm × 65 μm are observed with an SEM at a magnification of 2000 times at a position ¼ of the thickness from the steel sheet surface, and the photographed structure is analyzed by image analysis. Here, martensite and bainite refer to structures that appear gray or white in SEM. On the other hand, ferrite is a region that appears in black contrast in SEM. Note that martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides inside, but since it is difficult to exclude these, the area ratio of the region including these is defined as the area ratio.

[0055] Bainite has the following characteristics: It has an aspect ratio of 2.5 or more, exhibits a plate-like form, and is a slightly black structure compared to martensite. The width of the plates is 0.3 to 1.7 μm. The distribution density of carbides with a diameter of 10 to 200 nm inside the bainite is 0 to 3 particles / μm. ２ is.

[0056] The retained austenite (retained gamma) is measured by chemically polishing the surface of the steel sheet with oxalic acid for 200 μm, and then measuring the surface using the X-ray diffraction intensity method. It is calculated from the integrated intensity of the (200) α, (211) α, (220) α, (200) γ, (220) γ, and (311) γ diffraction peaks measured using Mo-Kα radiation.

[0057] To measure the average grain size of ferrite, an L-section of the steel sheet (a vertical section parallel to the rolling direction) is polished and then corroded with nital. Ten fields of view covering a 50 μm × 65 μm area are observed at a 1 / 4 thickness position from the surface of the steel sheet using an SEM at a magnification of 2000 times. The microstructure photographs are then analyzed to determine the equivalent circle diameter. The equivalent circle diameter refers to the diameter of a perfect circle having the area of ​​a region determined to be ferrite from the SEM photograph. The value obtained by dividing the sum of the diameters in the 10 fields of view by the number of ferrite particles whose diameters are measured is taken as the average grain size of ferrite.

[0058] The area ratio of ferrite precipitated on prior austenite grain boundaries is measured as follows. First, the L-section of the steel sheet is mirror-polished using colloidal silica vibration polishing, and then electron backscatter diffraction (EBSD) measurement is performed to obtain local crystal orientation data for 10 fields of view at a quarter-thickness position from the steel sheet surface. The step size is 0.10 μm, and the measurement area is 50 μm × 50 μm. After cleanup processing using analysis software OIM Analysis 7, the obtained local orientation data is analyzed to determine the prior austenite grain boundaries. The cleanup processing involves replacing the orientation and CI value of pixels with a CI value of 0.2 or less with that of the pixel with the highest CI value among neighboring pixels. Specifically, the cleanup processing is performed using the Neighbor CI Correlation function of the analysis software, with the parameter Minimum Coefficient of Coefficient set to 0.2. A map of grain boundaries having a crystal orientation difference of 20° or more and 50° or less is created, and these interfaces are determined to be prior austenite grain boundaries. After EBSD measurement, the L-section of the steel sheet is corroded with nital, and the same 10 fields of view as those used for EBSD are photographed with an SEM at a magnification of 3000x. When the grain boundary map of prior austenite in the same field of view and the photographed SEM image are superimposed, the ferrite present on the prior austenite grain boundaries is identified and its area fraction is measured. One method for observing the same field of view with EBSD and SEM is to make an indentation in advance with a Vickers hardness tester at the observation position on the steel sheet to serve as a marker for the observation position.

[0059] The number density A of precipitates having a circle equivalent diameter of 500 nm or more is measured after polishing the L cross section (vertical cross section parallel to the rolling direction) of the steel sheet, in the region from the 1 / 5 position to the 4 / 5 position of the sheet thickness of the steel sheet, that is, in the region from the 1 / 5 position of the sheet thickness from the surface of the steel sheet to the 4 / 5 position, sandwiching the center of the sheet thickness. ２ The area is photographed continuously with an SEM, and the number of such precipitates is counted from the photographed SEM photographs. The magnification for photographing is 2000 times. When analyzing the components of individual inclusion particles, the individual inclusion particles are magnified 10000 times and the above precipitates are analyzed. Here, precipitates with a circle equivalent diameter of 500 nm or more are considered to be Fe２３ (C, B) ６ The presence or absence of a peak of B is examined by elemental analysis using energy dispersive X-ray spectroscopy (EDS) at an acceleration voltage of 3 kV, and if a peak of B is present, it is evaluated that the above-mentioned precipitates are present. The circle equivalent diameter refers to the diameter of a perfect circle having the area of ​​each precipitate calculated from an SEM photograph.

[0060] Tensile strength (TS): 1470 MPa or more When the tensile strength of a steel sheet is 1470 MPa or more, the delayed fracture resistance is significantly deteriorated. In this regard, one of the features of the present invention is that the delayed fracture resistance is good even when the tensile strength is 1470 MPa or more. From the viewpoint of reducing the weight of automotive frame parts, the tensile strength is preferably 1700 MPa or more. The tensile strength of the steel sheet of the present invention may be 2100 MPa or less. The tensile strength of the steel sheet of the present invention may be 1900 MPa or less. Furthermore, the tensile strength of the steel sheet of the present invention may be 1800 MPa or less. When the tensile strength is 1800 MPa or less, the steel sheet of the present invention can obtain extremely excellent delayed fracture resistance. Furthermore, even when the tensile strength is more than 1800 MPa but not more than 1900 MPa, the area ratio of ferrite precipitated on prior austenite grain boundaries to the total ferrite is 50% or more, so that the steel sheet of the present invention can obtain extremely excellent delayed fracture resistance.

[0061] The tensile strength can be measured by cutting a JIS No. 5 tensile test piece at a position 1 / 4 of the coil width so that the direction perpendicular to the rolling direction is the longitudinal direction, and performing a tensile test in accordance with JIS Z2241 (2022).

[0062] The steel sheet of the present invention may have a plating layer on its surface. The plating layer may be Zn plating or other metal plating. The plating layer may be either a hot-dip plating layer or an electroplated layer.

[0063] Next, a method for producing a steel sheet according to the present invention will be described. In the method for producing a steel sheet according to the present invention, a steel slab having the above-described chemical composition is held at a heating holding temperature of 1100°C or higher for 30 minutes or more, and then subjected to hot finish rolling under conditions of a slab surface temperature of 900 to 1000°C for 20 to 150 seconds, a finish rolling temperature of 850°C or higher, cooling at an average cooling rate of 40°C / second or more in the range from the finish rolling temperature to 650°C, and then coiling at a coiling temperature of 650°C or lower to obtain a hot-rolled steel sheet, and cold-rolling the hot-rolled steel sheet at a rolling reduction of 40% or more to obtain a cold-rolled steel sheet, an annealing temperature of 830 to 950°C, heating the cold-rolled steel sheet from 400°C to the annealing temperature at an average heating rate of 1.0°C / second or more, holding at the annealing temperature for 10 to 600 seconds, and then cooling from the annealing temperature to Ar. ３ The temperature was then cooled to the temperature point at an average cooling rate of 10°C / sec or more, and Ar ３ From the point (Ar ３ The temperature was cooled to a temperature of −80° C. at an average cooling rate of 1 to 10° C. / sec. (Ar ３ This is a method for producing a steel sheet, in which the steel sheet is cooled from a temperature (point −80°C) to a cooling stop temperature of 260°C or less at an average cooling rate of 10°C / second or more, and then continuously annealed by holding the steel sheet at a holding temperature of 150 to 260°C for 20 to 1500 seconds. Note that the temperatures specified in each step in the present invention refer to the surface temperatures of the slab (steel slab) or steel sheet.

[0064] Hot rolling Heating holding temperature: slab surface temperature of 1100°C or higher Heating holding time (slab heating holding time): 30 minutes or longer In slab heating before hot rolling, the steel slab is held at a heating holding temperature (slab heating holding temperature) of 1100°C or higher at the slab surface for 30 minutes or longer, which promotes solid solution of precipitates such as B-based precipitates and reduces the size and number of precipitates. The heating holding temperature is preferably 1150°C or higher. The heating holding temperature is preferably 1250°C or lower. Furthermore, the time held at the heating holding temperature (holding time (slab heating holding time)) is preferably 40 minutes or longer. The holding time is preferably 50 minutes or shorter.

[0065] Residence time at 900 to 1000°C: 20 seconds or more and 150 seconds or less During hot rolling, the slab is retained at 900 to 1000°C for 20 seconds or more and 150 seconds or less. Increasing the residence time in the 900 to 1000°C temperature range generates BN-based precipitates, which coarsen. Precipitates formed in this temperature range are difficult to dissolve during annealing, reducing the amount of dissolved B after annealing. A residence time of more than 150 seconds fails to provide an amount of dissolved B effective in suppressing delayed fracture. Therefore, the residence time is 150 seconds or less, preferably 120 seconds or less, and more preferably 100 seconds or less. On the other hand, a residence time of less than 20 seconds may result in a non-uniform structure. Therefore, the residence time is 20 seconds or more. The residence time is preferably 30 seconds or more.

[0066] Finish rolling temperature: 850°C or higher In hot finish rolling, the finish rolling temperature (FT) is set to 850°C or higher to suppress non-uniformity of the hot rolled texture. The finish rolling temperature is preferably 870°C or higher. The finish rolling temperature is preferably 930°C or lower.

[0067] Average cooling rate in the range from the finish rolling temperature to 650°C (first average cooling rate): 40°C / sec or more In the cooling after hot finish rolling, cooling is performed at an average cooling rate of 40°C / sec or more in the range from the finish rolling temperature to 650°C. In the temperature range from the finish rolling temperature to 650°C, B segregates at the grain boundaries with the recrystallization of austenite, and Fe ２３ (C, B) ６ is precipitated. ２３ (C, B) ６ In order to minimize the precipitation of , the average cooling rate (first average cooling rate) is set to 40°C / second or more. The average cooling rate is preferably 60°C / second or more. The average cooling rate is preferably 500°C / second or less, and more preferably 300°C / second or less. The average cooling rate (first average cooling rate) in the hot rolling process is "(temperature at the start of cooling (finish rolling temperature) (°C) - temperature at the end of cooling (°C) (650°C)) / cooling time (seconds) from the start of cooling to the end of cooling."

[0068] Coiling temperature: 650°C or less After cooling to 650°C as described above, the wire is further cooled as necessary and then coiled. At this time, if the coiling temperature exceeds 650°C, the Fe ２３ (C, B) ６ Therefore, the coiling temperature is set to 650°C or lower. Preferably, the coiling temperature is 600°C or lower. Also, the coiling temperature is preferably 500°C or higher.

[0069] Cold rolling reduction: 40% or more If the reduction (cumulative reduction (cold rolling reduction)) in cold rolling is 40% or more, the recrystallization behavior and texture orientation in the subsequent continuous annealing can be stabilized. If the reduction is less than 40%, some austenite grains may become coarse during annealing, which may result in a decrease in strength. In addition, the reduction is preferably 80% or less.

[0070] Continuous annealing Average heating rate from 400°C to the annealing temperature: 1.0°C / sec or more After cold rolling, the steel sheet is annealed and tempered in a continuous annealing line (CAL), and further subjected to temper rolling as needed. ２３ (C, B) ６ is generated in the ferrite region during annealing and coarsens, so Fe ２３ (C, B) ６ In order to reduce the amount of B and fully obtain the effect of grain boundary strengthening by B, it is necessary to increase the average heating rate from 400°C to the annealing temperature. From this perspective, the average heating rate at 400°C or higher is 1.0°C / second or higher. The average heating rate from 400°C to the annealing temperature is preferably 1.5°C / second or higher, and more preferably 3.0°C / second or higher. Furthermore, the average heating rate is preferably 10°C / second or lower. The average heating rate here is "annealing temperature (°C) - 400 (°C) (described below) / heating time (minutes) from 400°C to the annealing temperature".

[0071] Annealing temperature: 830 to 950°C. Soaking time (holding time at annealing temperature): 10 seconds or more and 600 seconds or less. Undissolved Fe remaining after annealing ２３ (C, B) ６In order to sufficiently reduce precipitates such as Fe, annealing is performed at a high temperature for a long time. Specifically, the annealing temperature needs to be 830°C or higher. The annealing temperature is preferably 840°C or higher, more preferably 850°C or higher. On the other hand, annealing at a temperature higher than 950°C coarsens the prior austenite grain size, which in turn coarsens the grain size of the subsequently generated ferrite. If the annealing temperature exceeds 950°C, the desired average grain size of ferrite cannot be obtained, so the annealing temperature is set to 950°C or lower. The annealing temperature is preferably 900°C or lower. In addition, an excessively long soaking time (holding time) also leads to coarsening of the prior austenite grain size, which in turn coarsens the average grain size of ferrite. Therefore, the soaking time is set to 600 seconds or lower. The soaking time is preferably 540 seconds or lower, more preferably 480 seconds or lower. On the other hand, Fe remaining in a solid solution is not coarsened. ２３ (C, B) ６ In order to sufficiently reduce the amount of precipitates such as ZnO, the soaking time is set to 10 seconds or more, preferably 30 seconds or more, and more preferably 60 seconds or more.

[0072] Annealing temperature to Ar ３ Average cooling rate to the point (second average cooling rate): 10 ° C. / sec or more Ar ３ From the point (Ar ３ Average cooling rate (third average cooling rate) to the point −80°C: 1 to 10°C / sec (Ar ３ Average cooling rate from the annealing temperature (point −80 ° C.) to the cooling stop temperature of 260 ° C. or less (fourth average cooling rate): 10 ° C. / second or more. In order to obtain a structure in which the area fraction of martensite is 85% or more and less than 95% and the area fraction of ferrite is 5% or more and 15% or less while keeping the number density A of precipitates in the desired range, ３ The temperature is then cooled to a temperature (°C) at an average cooling rate of 10°C / sec or more (second average cooling rate), and the ３ Point (°C) to (Ar ３ The temperature is then cooled to a temperature of −80° C. at an average cooling rate of 1 to 10° C. / second (third average cooling rate), and the temperature is then cooled to a temperature of −80° C. ３ The material is cooled from the cooling stop temperature (point −80° C.) to a cooling stop temperature of 260° C. or lower at an average cooling rate of 10° C. / second or higher (fourth average cooling rate).

[0073] Here, the second average cooling rate is defined as "(annealing temperature (°C) - Ar ３ Point (°C) / Annealing temperature to Ar ３ The third average cooling rate is the cooling time (seconds) to reach the point where the temperature is ３ Point (°C) - (Ar ３ Point -80) (°C)) / Ar ３ Point (°C) to (Ar ３ The fourth average cooling rate is the cooling time (seconds) to the point −80°C. ３ Point -80) (°C) - Cooling stop temperature below 260°C (°C)) / (Ar ３ The cooling time (seconds) from the point (-80°C) to the cooling stop temperature of 260°C or less. ３ The point can be calculated by the following formula: Ar ３ Point (°C) = 910 - 310 x C - 80 x Mn - 20 x Cu - 15 x Cr - 55 x Ni - 80 x Mo. The element symbols in the formula represent the content (mass%) of each element. Elements that are not contained are calculated as 0 (zero).

[0074] Annealing temperature to Ar ３ When the average cooling rate to the point (second average cooling rate) is less than 10°C / sec, Fe ２３ (C, B) ６ Therefore, the annealing temperature is increased by 100°C. ３ The average cooling rate to the first cooling point is 10°C / s or more. The second average cooling rate is preferably 20°C / s or more, more preferably 30°C / s or more. The second average cooling rate is preferably 150°C / s or less, more preferably 100°C / s or less.

[0075] Ar ３ From the point (Ar ３ If the average cooling rate (third average cooling rate) to the point (-80°C) exceeds 10°C / sec, ferrite is not sufficiently formed, and the delayed fracture resistance deteriorates. ３ From the point (Ar ３ The average cooling rate (third average cooling rate) to the point (-80°C) is 10°C / sec or less. ３ From the point (Ar ３If the average cooling rate to the point −80° C. (third average cooling rate) is less than 1° C. / sec, excessive ferrite is generated. ３ From the point (Ar ３ The average cooling rate to the temperature (point −80° C.) (third average cooling rate) is 1° C. / second or more. The third average cooling rate is preferably 3° C. / second or more, and more preferably 5° C. / second or more.

[0076] (Ar ３ If the average cooling rate (fourth average cooling rate) from the cooling stop temperature (point -80°C) to 260°C or less is less than 10°C / sec, bainite, ferrite and pearlite are formed in large amounts, and sufficient strength and delayed fracture resistance cannot be obtained. ３ The fourth average cooling rate from the first temperature (point −80° C.) to 260° C. is 10° C. / second or more. The fourth average cooling rate is preferably 70° C. / second or more. The fourth average cooling rate is more preferably 100° C. / second or more, and even more preferably 200° C. / second or more. The fourth average cooling rate is preferably 1000° C. / second or less, and more preferably 750° C. / second or less.

[0077] Furthermore, if the cooling stop temperature exceeds 260°C, upper and lower bainite are formed, retained austenite and fresh martensite increase, and sufficient delayed fracture resistance cannot be obtained. Therefore, the cooling stop temperature is set to 260°C or lower. The cooling stop temperature is preferably 250°C or lower, and more preferably 240°C or lower.

[0078] Holding temperature: 150 to 260°C Holding time: 20 to 1500 seconds The carbides distributed within martensite are carbides that are formed during holding in the low-temperature region after quenching. To ensure excellent delayed fracture resistance and a tensile strength of 1470 MPa or higher (TS≧1470 MPa), it is necessary to appropriately control the formation of these carbides. To achieve this, a holding temperature of 150 to 260°C and a holding time of 20 to 1500 seconds are required. If the holding temperature is lower than the lower limit of 150°C or if the holding time is too short, the carbide distribution density within the transformed phase becomes insufficient, resulting in a deterioration of delayed fracture resistance. On the other hand, if the holding temperature is higher than the upper limit of 260°C, significant coarsening of carbides within grains and at block grain boundaries may occur, potentially resulting in a deterioration of delayed fracture resistance. Furthermore, if the holding time exceeds 1500 seconds, coarsening of carbides within grains and at block grain boundaries becomes significant, which may result in deterioration of delayed fracture resistance. Therefore, in the present invention, continuous annealing is performed at a holding temperature of 150 to 260°C for 20 to 1500 seconds. The holding temperature is preferably 250°C or less, more preferably 240°C or less. The holding time is preferably 50 seconds or more, more preferably 100 seconds or more. The holding time is preferably 1300 seconds or less, more preferably 1000 seconds or less.

[0079] The steel sheet obtained in this manner can be subjected to skin-pass rolling from the viewpoint of stabilizing press formability, such as adjusting the surface roughness and flattening the sheet shape. In this case, the skin-pass elongation is preferably 0.1% or more. Also, the skin-pass elongation is preferably 1.0% or less. In this case, it is preferable to use a dull roll as the skin-pass roll and adjust the roughness Ra of the steel sheet to 0.8 μm or more from the viewpoint of flattening the shape. Also, it is preferable to adjust the roughness Ra of the steel sheet to 1.8 μm or less.

[0080] The obtained steel sheet may be subjected to a plating treatment. That is, after continuous annealing, the surface of the steel sheet may be subjected to a plating treatment. By performing the plating treatment, a steel sheet having a plating layer on the surface can be obtained.

[0081] As described above, according to the present invention, the delayed fracture resistance of high-strength cold-rolled steel sheets is significantly improved, and the application of high-strength steel sheets contributes to improving the strength of parts and reducing their weight. The steel sheets of the present invention preferably have a thickness of 0.5 mm or more. Also, the thickness is preferably 2.0 mm or less.

[0082] Next, the member of the present invention and the method for manufacturing the same will be described.

[0083] The member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining. Also, the method for manufacturing the member of the present invention includes a step of subjecting the steel plate of the present invention to at least one of forming and joining to form the member.

[0084] The steel sheet of the present invention has a tensile strength of 1470 MPa or more and excellent delayed fracture resistance. Therefore, members obtained using the steel sheet of the present invention also have high strength and are superior in delayed fracture resistance compared to conventional high-strength members. Furthermore, the use of the member of the present invention enables weight reduction. Therefore, the member of the present invention can be suitably used, for example, in vehicle body frame parts.

[0085] The forming process can be performed using a general processing method such as press working without any restrictions, and the joining process can be performed using general welding methods such as spot welding and arc welding, riveting, crimping, etc. without any restrictions.

[0086] Examples of the present invention will be described below. Steels having the chemical compositions shown in Table 1 were melted and then cast into slabs. These slabs were subjected to the heat treatment and rolling shown in Table 2 to obtain steel plates having a thickness of 1.4 mm.

[0087] Specifically, a slab having each component composition was held at the heating holding temperature shown in Table 2 for the heating holding time shown in Table 2, then subjected to hot finish rolling at the finish rolling temperature shown in Table 2 for the residence time at 900 to 1000°C shown in Table 2, cooled at the first average cooling rate shown in Table 2, and coiled at the coiling temperature shown in Table 2 to obtain a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was cold-rolled at the reduction (cold rolling reduction) shown in Table 2 to obtain a cold-rolled steel sheet. Thereafter, the cold-rolled steel sheet was heated to the annealing temperature shown in Table 2 at the average heating rate shown in Table 2, held for the soaking time shown in Table 2, and then cooled to 260°C or less under the cooling conditions (second average cooling rate, third average cooling rate, fourth average cooling rate, cooling stop temperature) shown in Table 2, reheated as necessary, and subjected to continuous annealing at the holding temperature shown in Table 2 for the holding time shown in Table 2.

[0088] For No. 2, the obtained steel sheet was subjected to electroplating to obtain a steel sheet on which a Zn plating layer was formed.

[0089]

[0090]

[0091] The obtained steel sheets were subjected to quantification of the metallographic structure using the method described above, and further to tensile tests and delayed fracture resistance evaluation tests. Specifically, the microstructure measurement method was as follows. The L-section of the steel sheet (a vertical section parallel to the rolling direction) was polished and then corroded with nital, and four fields of view within a 50 μm × 65 μm area were observed with an SEM at a magnification of 2000 times at a position ¼ thickness from the steel sheet surface, and the photographed microstructure was measured by image analysis. Here, martensite and bainite refer to structures that appear gray or white under an SEM. Bainite has the following characteristics. That is, it has an aspect ratio of 2.5 or more, exhibits a plate-like morphology, and is a slightly black structure compared to martensite. The width of the above plates is 0.3 to 1.7 μm. The distribution density of carbides with a diameter of 10 to 200 nm inside the bainite is 0 to 3 / μm. ２On the other hand, ferrite is the region that shows a black contrast in the SEM. Note that martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides, but since it is difficult to exclude these, the area ratio of the region including these was used as the area ratio.

[0092] The retained austenite (retained γ) was measured by chemically polishing the surface of the steel sheet (200 μm) with oxalic acid, and the X-ray diffraction intensity was measured on the sheet surface. It was calculated from the integrated intensity of the (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ diffraction peaks measured with Mo-Kα radiation.

[0093] The average grain size of ferrite was obtained by polishing an L-section of the steel sheet (a vertical section parallel to the rolling direction) and then etching it with nital, observing it at a position ¼ of the thickness from the surface of the steel sheet with an SEM at 2000x magnification in 10 fields of view within an area of ​​50 μm × 65 μm, and performing image analysis on the photographed structure to determine the circle equivalent diameter.

[0094] The area ratio of ferrite precipitated on prior austenite grain boundaries was measured as follows. First, the L-section of the steel sheet was mirror-polished using colloidal silica vibration polishing, and then electron backscatter diffraction (EBSD) measurement was performed to obtain local crystal orientation data for 10 fields of view at a quarter-thickness position from the steel sheet surface. The step size was 0.10 μm, and the measurement area was 50 μm × 50 μm. After cleanup processing using analysis software OIM Analysis 7, the obtained local orientation data was analyzed to determine prior austenite grain boundaries. The cleanup processing involved replacing the orientation and CI value of pixels with a CI value of 0.2 or less with that of the pixel with the highest CI value among neighboring pixels. Specifically, the cleanup processing was performed using the Neighbor CI Correlation function of the analysis software, with the parameter Minimum Coefficient of Coefficient Index set to 0.2. A map of grain boundaries having a crystal orientation difference of 20° or more and 50° or less was created, and these interfaces were determined to be prior austenite grain boundaries. After EBSD measurement, the L-section of the steel sheet was corroded with nital, and the same 10 fields of view as those used for EBSD were photographed with an SEM at a magnification of 3000x. When the grain boundary map of prior austenite in the same field of view and the photographed SEM image were superimposed, the ferrite present on the prior austenite grain boundaries was identified and the area ratio was measured. As a method for observing the same field of view with EBSD and SEM, an indentation was made in advance with a Vickers hardness tester at the observation position on the steel sheet to serve as a mark for the observation position.

[0095] The number density A of precipitates having a circle equivalent diameter of 500 nm or more is measured after polishing the L cross section (vertical cross section parallel to the rolling direction) of the steel sheet, in the region from the 1 / 5 position to the 4 / 5 position of the sheet thickness of the steel sheet, that is, in the region from the 1 / 5 position of the sheet thickness from the surface of the steel sheet to the 4 / 5 position, sandwiching the center of the sheet thickness. ２ The area was photographed continuously with an SEM, and the number of such precipitates was counted from the photographed SEM photographs. The magnification for photographing was 2000 times. When analyzing the components of individual inclusion particles, the individual inclusion particles were magnified 10000 times and the above precipitates were analyzed. Here, precipitates with a circle equivalent diameter of 500 nm or more were considered to be Fe２３ (C, B) ６ The presence or absence of a B peak was examined by elemental analysis using energy dispersive X-ray spectroscopy (EDS) at an acceleration voltage of 3 kV, and when a B peak was present, it was evaluated that the above-mentioned precipitates were present.

[0096] For the tensile test, a JIS No. 5 tensile test piece was cut out at a quarter position of the coil width so that the direction perpendicular to the rolling direction was the longitudinal direction, and a tensile test (in accordance with JIS Z2241 (2022)) was carried out to evaluate the tensile strength TS. A tensile strength TS of 1470 MPa or more was deemed to have passed.

[0097] The delayed fracture resistance was evaluated as follows. Strip test pieces measuring 100 mm in the direction perpendicular to the rolling direction and 30 mm in the rolling direction were taken from the width direction of the obtained steel sheet (coil) at a position ¼ of the coil width. The end face of the long side with a length of 100 mm was sheared, and the sheared specimen was bent in its as-sheared state (without machining to remove burrs) so that the burrs were on the outer periphery of the bend. The specimen was fixed with bolts while maintaining the shape of the bent specimen. The shear clearance was 13% and the rake angle was 1°. The bending was performed with a tip bend radius of 10 mm and a 90° angle inside the bend apex (V-bend). The punch used had a tip radius R equal to the tip bend radius R and was U-shaped (semicircular tip R portion and punch body thickness of 2R), and the die had a corner R of 30 mm. The depth to which the punch pressed the steel plate was adjusted so that the bending angle at the tip (the angle at the inner side of the bend apex) was 90 degrees (V-shaped). The test specimen was clamped and tightened with a hydraulic jack so that the distance between the flange ends of the straight pieces during bending was the same as when the specimen was bent (to cancel out the opening of the straight pieces due to springback), and then bolted in this state. The bolt was fixed by passing it through an elliptical hole (minor axis 10 mm, major axis 15 mm) pre-installed 10 mm inside the short edge of the strip specimen. The obtained bolt-tightened test specimen was immersed in a solution prepared by mixing 0.1 mass% ammonium thiocyanate aqueous solution and McIlvaine buffer solution in a mass ratio of 1:1, adjusted to a pH of 8.0, and a solution adjusted to a pH of 7.4 to evaluate delayed fracture resistance. The solution temperature was 20°C, and the test specimens were immersed in a solution with a surface area of ​​1 cm. ３The amount of solution per test piece was 20 ml. After 48 hours, the presence or absence of visually observable cracks (lengths of 1 mm or more) was confirmed, and specimens in which no cracks were observed in the solution adjusted to pH 8.0 were judged to have excellent delayed fracture resistance. Furthermore, specimens in which no cracks were observed in the solution adjusted to pH 7.4 were judged to have particularly excellent delayed fracture resistance. In Table 3, specimens in which no cracks were observed in the solution adjusted to pH 8.0 are indicated by "◯ (pass)," and specimens in which no cracks were observed even in the solution adjusted to pH 7.4 are indicated by "◎ (pass)." On the other hand, specimens in which cracks were observed in the solution adjusted to pH 8.0 are indicated by "× (fail)."

[0098] The structure and properties of the obtained steel sheets are shown in Table 3.

[0099]

[0100] The steel sheets within the scope of the present invention had high strength and excellent delayed fracture resistance. Among the steel sheets within the scope of the present invention, those having a tensile strength of 1800 MPa or less were evaluated as "◎ (pass)" and were particularly excellent in delayed fracture resistance. Furthermore, even if the tensile strength was greater than 1800 MPa and less than 1900 MPa, those having an area ratio of ferrite precipitated on prior austenite grain boundaries of 50% or more relative to the total ferrite were evaluated as "◎ (pass)" and were particularly excellent in delayed fracture resistance. On the other hand, in the comparative examples, at least one of the tensile strength and the delayed fracture resistance was insufficient.

[0101] Furthermore, it was found that the members obtained by forming and joining the steel plates of the present invention have high strength and excellent delayed fracture resistance, similar to the steel plates of the present invention, because the steel plates of the present invention have high strength and excellent delayed fracture resistance.

Claims

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

2. 2. The steel sheet according to claim 1, wherein 50% or more of the ferrite is precipitated on prior austenite grain boundaries, in terms of area ratio.

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

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

5. The steel sheet according to any one of claims 1 to 4, which has a plating layer on a surface of the steel sheet.

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

7. A component made using the steel plate described in claim 5.

8. A steel slab having the composition according to claim 1 or 3 is heated and held at a slab surface temperature of 1100° C. or higher for 30 minutes or more, and then Hot finish rolling is performed under conditions of a residence time at 900 to 1000°C of 20 seconds or more and 150 seconds or less and a finish rolling temperature of 850°C or more; Cooling is performed at an average cooling rate of 40° C. / sec or more in the range from the finish rolling temperature to 650° C., Thereafter, the hot-rolled steel sheet is obtained by coiling the steel sheet at a coiling temperature of 650°C or less. The hot-rolled steel sheet is cold-rolled at a rolling reduction of 40% or more to obtain a cold-rolled steel sheet; The annealing temperature is set to 830 to 950 ° C., and the cold-rolled steel sheet is heated from 400 ° C. to the annealing temperature at an average heating rate of 1.0 ° C. / sec or more, After holding at the annealing temperature for 10 seconds or more and 600 seconds or less, From the annealing temperature to Ar ３ Cooling to a temperature of 10° C. / s or more at an average cooling rate of 10° C. / s or more; Ar ３ From the point (Ar ３ Cooling to a temperature of −80° C. at an average cooling rate of 1 to 10° C. / sec, (Ar ３ Cooling is performed at an average cooling rate of 10°C / sec or more from the cooling stop temperature (point -80°C) to a cooling stop temperature of 260°C or less, This is followed by continuous annealing at a holding temperature of 150 to 260°C for 20 to 1500 seconds.

9. The method for producing a steel sheet according to claim 8, further comprising the step of: plating a surface of the steel sheet after the continuous annealing.

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

11. A method for manufacturing a component, comprising the step of subjecting the steel plate described in claim 5 to at least one of forming processing and joining processing to form a component.