Steel sheet, member, and method for producing same

Abstract

Provided are a steel sheet having high strength and excellent delayed fracture resistance, a member, and methods for manufacturing the same. A steel sheet containing, in mass%, 0.15% to 0.45% (inclusive) 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% (inclusive) of B, with the remainder comprising Fe and unavoidable impurities, the steel sheet has a steel structure in which the area ratio of martensite to the entire structure is 85% or more but less than 95%, the area ratio of ferrite to the entire structure is 5% or more but 15% or less, and the average crystal grain size of ferrite is 10 [mu] m or less, and the number density A of precipitates having a circle-equivalent diameter of 500 nm or more satisfies the equation A (particles / mm2) < = 8.5 * 105 * [B].

Description

Technical Field

[0001] The present invention relates to steel sheets such as cold-formed high-strength steel sheets used in automobiles and the like, components using the steel sheets, and methods for producing the same. Background Art

[0002] In recent years, with the goal of reducing vehicle weight and improving collision safety, the use of steel sheets with a tensile strength (TS) of 1310 MPa or higher has been increasing for automotive frame components. Furthermore, the use of steel sheets with a tensile strength (TS) of 1470 MPa or higher has been gaining momentum for bumpers, anti-collision beams, and other components.

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

[0004] Here, delayed fracture refers to the following phenomenon: when a component is placed in a hydrogen intrusion environment under a state where high stress is applied to the component, hydrogen intrudes into the steel sheet constituting the component, reducing the interatomic bonding strength and causing local deformation, thereby generating microcracks, which then propagate and lead to fracture.

[0005] As a technique for improving such delayed fracture resistance, for example, the delayed fracture resistance is improved by reducing coarse precipitates that serve as starting points of delayed fracture. Based on this knowledge, Patent Document 1 discloses a high-strength steel sheet having excellent delayed fracture resistance, characterized in that it contains, in mass %, C: 0.13% to 0.40%, Si: 0.02% to 1.5%, Mn: 0.4% to 1.7%, P: 0.030% to 0.0010%, S: 0.0002% to less than 0.0010%, and sol. Al: 0.1% to 0.40%. A composition comprising 0.01% or more and 0.20% or less of Fe, 0.0055% or less of N, 0.0025% or less of O, 0.002% or more and 0.035% or less of Nb, and 0.002% or more and 0.040% or less of Ti, with the balance being Fe and inevitable impurities, and having a total area ratio of martensite and bainite to the entire structure of 95% or more and 100% or less, with the balance being one or both of ferrite and retained austenite, an average grain size of prior austenite grains exceeding 5 μm, and a group of inclusions having a major axis length of 20 to 80 μm, which satisfies the following conditions and is present at a rate of 5 inclusions / mm 2 The following steel structures exist, and the tensile strength is 1320 MPa or more.

[0006] [%Ti]+[%Nb]>0.007 (1)

[0007] [%Ti]×[%Nb] 2 ≤7.5×10 -6 (2)

[0008] Here, [%Nb] and [%Ti] represent the contents (%) of Nb and Ti.

[0009] In addition, Patent Document 2 discloses a high-strength cold-rolled steel sheet with excellent hydrogen embrittlement resistance and workability, characterized in that it contains, by mass%, C: 0.05-0.30%, 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: 0.01-0.50% or less, and the total is 0.01% or more and satisfies [%C]-[%Nb] / A composition containing one or more of Nb, Ti, and Zr, with the balance consisting of iron and inevitable impurities, in the form of 92.9×12-[%Ti] / 47.9×12-[%Zr] / 91.2×12>0.03, and having a structure containing 50% or more (including 100%) by area ratio of tempered martensite, with the balance consisting of ferrite, wherein the distribution of precipitates in the tempered martensite is such that the number of precipitates having an equivalent circle diameter of 1 to 10 nm is 1 μm 2 The number of tempered martensite particles is 20 or more, and the number of precipitates with an equivalent circle diameter of 20 nm or more, i.e., precipitates containing one or more of Nb, Ti, and Zr, is 20 or more per 1 μm. 2 The number of tempered martensite particles is 10 or less, and the average grain size of ferrite particles surrounded by high-angle grain boundaries having a crystal orientation difference of 15° or more is 5 μm or less.

[0010] Prior art literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent No. 6388085

[0013] Patent Document 2: Japanese Patent No. 4712882 Summary of the Invention

[0014] Problems to be solved by the invention

[0015] However, the conventional technology is not sufficient as a technology for achieving excellent delayed fracture resistance while ensuring a tensile strength TS of 1470 MPa or higher, and the establishment of a new technology is desired.

[0016] The present invention has been made to solve such problems, and an object of the present invention is to provide a steel plate and a member having a tensile strength of 1470 MPa or more (TS ≥ 1470 MPa) and excellent delayed fracture resistance, and a method for producing the same.

[0017] Excellent delayed fracture resistance means that the material is judged to have excellent delayed fracture resistance by the following evaluation.

[0018] (1) First, a strip test piece having a length of 100 mm in the rolling direction and a length of 30 mm in the rolling direction was cut from a position 1 / 4 of the coil width from the width direction end of the obtained steel plate (coil).

[0019] (2) The cutting of the end face of the long side with a length of 100 mm is set as a shearing process. In the shearing process state (without performing mechanical processing to remove burrs), bending is performed in a manner such that the burrs are on the outer peripheral side of the bend. The shape of the test piece during the bending forming is maintained, and the test piece is fixed with bolts.

[0020] The shearing process was performed with a gap of 13% and a rake angle of 1°. The bending process was performed so that the front end had a radius of 10 mm and the angle inside the bend vertex was 90 degrees (V-shaped bend).

[0021] The punch used was a U-shaped punch with a tip radius equal to the aforementioned tip bend radius R (the tip R portion was semicircular and the punch body had a thickness of 2R), and the die used had a corner R of 30mm. The depth of the punch pressing against the steel sheet was adjusted to form the sheet so that the tip bend angle (the angle inside the bend apex) was 90 degrees (V-shaped).

[0022] The test piece was clamped and tightened using a hydraulic jack, so that the distance between the flange ends of the straight piece during bending was the same as during bending (to eliminate the opening of the straight piece due to springback). Bolts were then tightened in this state. The bolts were secured through an elliptical hole (minor axis 10 mm, major axis 15 mm) pre-set 10 mm inward from the short edge of the strip test piece.

[0023] (3) The obtained bolted test piece was immersed in a solution prepared by mixing a 0.1% by mass aqueous solution of ammonium thiocyanate and a McIlvaine buffer solution at a mass ratio of 1:1 and adjusting the pH to 8.0, and in a solution adjusted to 7.4, respectively, to perform a delayed fracture resistance evaluation test. At this time, the temperature of the solution was set at 20°C, and the test piece was immersed in a solution of 1 cm per 1 cm. 3 The liquid volume of the surface area was set to 20 ml.

[0024] (4) After 48 hours, the presence of visually detectable cracks (length 1 mm or greater) was checked. If no cracks were observed in a solution adjusted to pH 8.0, the delayed fracture resistance was judged to be excellent. Furthermore, if no cracks were observed in a solution adjusted to pH 7.4, the delayed fracture resistance was judged to be particularly excellent.

[0025] Methods used to solve problems

[0026] The present inventors have conducted intensive studies to solve the above-mentioned problems and have found that the delayed fracture resistance can be significantly improved by satisfying all of the following conditions.

[0027] i) The area ratio of martensite is 85% or more and less than 95%.

[0028] ii) The area ratio of ferrite is 5% or more and 15% or less, and the average crystal grain size of ferrite is 10 μm or less.

[0029] iii) The number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the following conditions.

[0030] A(pieces / mm 2 )≤8.5×10 5 ×[B]

[0031] Here, [B] represents the B content (mass %).

[0032] The present invention has been completed through further research based on the above findings, and the gist of the present invention is as follows.

[0033] [1] A steel plate having a composition comprising, in mass %, 0.15% to 0.45% C, 2.0% to 2.0% Si, 4.0% to 4.0% Mn, 0.1% to 0.1% P, 0.01% to 0.01% S, 0.5% to 0.5% Al, 0.010% to 0.0100% N, 0.0008% to 0.0100% B, with the balance being Fe and unavoidable impurities,

[0034] The steel structure comprises a martensite area ratio of 85% or more and less than 95% relative to the entire structure, a ferrite area ratio of 5% or more and less than 15% relative to the entire structure, and an average ferrite grain size of 10 μm or less.

[0035] The number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the following formula (1).

[0036] A(pieces / mm 2 )≤8.5×10 5 ×[B] ...Formula (1)

[0037] Here, [B] represents the B content (mass %).

[0038] [2] The steel sheet according to [1], wherein 50% or more of the ferrite in terms of area ratio is ferrite precipitated on prior austenite grain boundaries.

[0039] [3] The steel sheet according to [1] or [2], further comprising, as the above-mentioned chemical composition, one or more selected from the group consisting of 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.

[0040] [4] The steel sheet according to any one of [1] to [3], comprising a plating layer on the surface of the steel sheet.

[0041] [5] A member formed using the steel plate according to any one of [1] to [4].

[0042] [6] A method for manufacturing a steel plate, wherein:

[0043] The steel slab having the composition described in [1] or [3] is held at a heating holding temperature of 1100° C. or higher as the surface temperature of the steel slab for 30 minutes or more, and then,

[0044] Hot finish rolling is performed under the conditions that the residence time at 900 to 1000° C. is set to 20 seconds or more and 150 seconds or less and the finish rolling temperature is set to 850° C. or more.

[0045] The average cooling rate in the range from the finishing rolling temperature to 650°C is set to 40°C / s or more.

[0046] Then, the hot rolled steel sheet is produced by coiling at a coiling temperature of 650°C or less.

[0047] The hot-rolled steel sheet is cold-rolled at a reduction ratio of 40% or more to produce a cold-rolled steel sheet.

[0048] Perform continuous annealing as follows:

[0049] The annealing temperature is set to 830-950°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 / s or more.

[0050] The annealing temperature is maintained for 10 seconds or more and 600 seconds or less, and then,

[0051] Cooling from the above annealing temperature to Ar3 point at an average cooling rate of 10°C / s or more,

[0052] Cool from Ar3 point to (Ar3 point - 80°C) at an average cooling rate of 1 to 10°C / s.

[0053] Cooling from (Ar3 point - 80°C) to a cooling stop temperature of 260°C or less at an average cooling rate of 10°C / s or more,

[0054] Then, the temperature is maintained at 150 to 260° C. for 20 to 1500 seconds.

[0055] [7] The method for manufacturing a steel sheet according to [6], wherein the surface of the steel sheet is subjected to a plating treatment after the continuous annealing.

[0056] [8] A method for manufacturing a component, comprising the step of performing at least one of forming and joining on the steel plate according to any one of [1] to [4] to produce the component.

[0057] Effects of the Invention

[0058] According to the present invention, it is possible to provide a steel sheet and a member having high strength and excellent delayed fracture resistance, and a method for producing the same. DETAILED DESCRIPTION

[0059] Hereinafter, embodiments of the present invention will be described.

[0060] The steel sheet of the present invention has a composition containing, in 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 consisting of Fe and inevitable impurities, and has a structure in which the area ratio of martensite with respect to the entire structure is 85% or more and less than 95%, the area ratio of ferrite with respect to the entire structure is 5% or more and 15% or less, the average crystal grain size of ferrite is 10 μm or less, and the number density A of precipitates with an equivalent circle diameter of 500 nm or more satisfies the following formula (1).

[0061] A(pieces / mm 2 )≤8.5×10 5×[B] ...Formula (1)

[0062] Here, [B] represents the B content (mass %).

[0063] Ingredients

[0064] The reasons for limiting the range of the chemical composition of the steel sheet of the present invention are described below. Note that "%" regarding the content of a component means "mass %."

[0065] C: 0.15% or more and 0.45% or less

[0066] C is included to increase the strength of martensite and achieve a tensile strength of 1470 MPa or higher (hereinafter referred to as TS ≥ 1470 MPa). Therefore, to achieve the desired TS, the C content is set to 0.15% or higher. To reduce the weight of automotive frame components through increased strength, the C content is preferably 0.20% or higher, and more preferably 0.25% or higher.

[0067] On the other hand, when 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, and more preferably 0.35% or less.

[0068] Si: 2.0% or less

[0069] Si suppresses the formation of film-like carbides during tempering in a temperature range of 200°C or higher, thereby preventing a decrease in strength and degradation of delayed fracture resistance. While there is no specified lower limit for the Si content, a Si content of 0.02% or higher is preferred. The Si content is preferably 0.10% or higher, and more preferably 0.20% or higher.

[0070] On the other hand, excessive addition of Si leads to deterioration of delayed fracture resistance due to Si segregation. 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.

[0071] Mn: 4.0% or less

[0072] Mn is an element effective in improving the hardenability of steel. There is no lower limit for the Mn content, but in order to stably obtain the desired area ratio of martensite, the Mn content is preferably 0.2% or more. The Mn content is preferably 0.5% or more.

[0073] On the other hand, when excessive Mn is added, the formation of ferrite is excessively suppressed, and the desired ferrite area ratio 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.

[0074] P: less than 0.10%

[0075] P segregates at grain boundaries, reducing grain boundary strength and thus 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 current industrially feasible lower limit is 0.002%. Therefore, the P content is preferably set to 0.002% or more.

[0076] S: 0.01% or less

[0077] S and Mn form coarse inclusions that become the starting point of 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. There is no specified lower limit, but the current industrially feasible lower limit is 0.0002%. Therefore, the S content is preferably set to 0.0002% or more.

[0078] sol.Al: 0.50% or less

[0079] Al is included to ensure sufficient deoxidation and reduce inclusions in steel. While there is no specific lower limit for sol.Al, it is preferably set to 0.005% or higher for stable deoxidation. The sol.Al content is more preferably 0.01% or higher, and even more preferably 0.02% or higher.

[0080] On the other hand, if the sol.Al content exceeds 0.50%, cementite formed during coiling becomes difficult to dissolve during annealing, preventing the number density A of the precipitates from falling within the desired range, significantly deteriorating the delayed fracture resistance. 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.

[0081] N: 0.010% or less

[0082] N forms precipitates such as AlN, which become the starting point of delayed fracture, thereby deteriorating the delayed fracture resistance. In particular, when N exceeds 0.010%, the number density A of the precipitates cannot be kept within the desired range, and the delayed fracture resistance significantly deteriorates. Therefore, the N content is set to 0.010% or less. The N content is preferably 0.005% or less. There is no specified lower limit, but the current industrially feasible lower limit is 0.0006%. Therefore, the N content is preferably set to 0.0006% or more.

[0083] B: 0.0008% or more and 0.0100% or less

[0084] B is an element that improves the hardenability of steel and has the effect of generating martensite at a predetermined area ratio even when the Mn content is low. In addition, B segregates at grain boundaries, thereby increasing the bonding strength of the grain boundaries and suppressing the segregation of P, which reduces the grain boundary strength. In order to utilize these effects to obtain the desired delayed fracture resistance, 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.

[0085] On the other hand, it was found that when excessive B was added, Fe 23 (C, B) 6 and BN become the starting point of delayed fracture, thereby reducing the delayed fracture resistance. Therefore, in order to obtain the effect of improving the delayed fracture resistance brought about by adding B, it is necessary to make the number density A of the precipitates within the desired range while adding B. When the B content exceeds 0.0100%, even if the hot rolling conditions and annealing conditions are controlled, it is difficult to reduce the B-based precipitates and the number density A of the precipitates cannot be made within the 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.

[0086] The steel sheet of the present invention preferably has a composition comprising the above-mentioned elements as basic components, with the balance consisting of iron (Fe) and inevitable impurities.

[0087] In the present invention, the following elements may be contained as component compositions.

[0088] One or more selected from the group consisting of, 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, and REM: 0.020% or less

[0089] Cu: 1.00% or less

[0090] Cu improves the corrosion resistance of steel sheets, reduces hydrogen intrusion into steel sheets, and improves delayed fracture resistance. While there is no specific lower limit for the Cu content, to achieve these effects, the Cu content is preferably 0.01% or more. The Cu content is preferably 0.05% or more, and more preferably 0.10% or more.

[0091] On the other hand, when Cu is added excessively, the number of coarse precipitates increases, the number density A of the precipitates cannot be brought to the desired range, and the delayed fracture resistance deteriorates. 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.

[0092] Cr: less than 1.00%

[0093] Cr is an element effective in improving the hardenability of steel. Cr can be added to stably obtain the desired structure. There is no particular lower limit for the Cr content, but to achieve this effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.05% or more, and even more preferably 0.10% or more.

[0094] On the other hand, when excessive Cr is added, the solid solution of cementite is delayed during annealing, and a large amount of undissolved cementite remains, which makes it impossible to make the number density A of precipitates within the desired range, and the delayed fracture resistance deteriorates. 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.

[0095] Nb: 0.10% or less

[0096] Nb forms fine precipitates such as NbC in steel, which, through a pinning effect, refines the prior austenite grain size and improves delayed fracture resistance. While there is no specified lower limit for the Nb content, to achieve this effect of improving delayed fracture resistance, the Nb content is preferably 0.005% or more. The Nb content is preferably 0.01% or more.

[0097] On the other hand, excessive addition of Nb increases the number of coarse precipitates, preventing the number density A of the precipitates from falling within the desired range, and deteriorating the 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.

[0098] Ti: less than 0.10%

[0099] Ti forms fine precipitates such as TiC in steel, which, through a pinning effect, refines the prior austenite grain size and improves delayed fracture resistance. While there is no specified lower limit for the Ti content, to achieve this effect, the Ti content is preferably 0.005% or more. The Ti content is preferably 0.01% or more.

[0100] On the other hand, when Ti is added excessively, coarse precipitates increase, the number density A of the precipitates cannot be brought to the desired range, and the delayed fracture resistance deteriorates. Therefore, when Ti is contained, the Ti content is set to 0.10% or less. The Ti content is preferably 0.08% or less, and more preferably 0.06% or less.

[0101] V: 0.50% or less

[0102] V generates fine V-containing carbides that serve as hydrogen trapping sites, improving delayed fracture resistance. Furthermore, by forming fine precipitates, it refines the prior austenite grain size through a pinning effect, improving delayed fracture resistance. While there is no specified lower limit for the V content, to achieve this effect, the V content is preferably 0.003% or greater. The V content is preferably 0.01% or greater, and more preferably 0.03% or greater.

[0103] On the other hand, when V is added excessively, coarse precipitates increase, and the number density A of the precipitates cannot be brought into the desired range, resulting in deterioration of 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.

[0104] Mo: 0.50% or less

[0105] Mo generates fine Mo-containing carbides that serve as hydrogen trapping sites, improving delayed fracture resistance. Furthermore, by forming fine precipitates, it refines the prior austenite grain size through a pinning effect, improving delayed fracture resistance. While there is no specified lower limit for the Mo content, to achieve this effect of improving delayed fracture resistance, the Mo content is preferably 0.003% or greater. The Mo content is preferably 0.01% or greater, and more preferably 0.03% or greater.

[0106] On the other hand, when Mo is added excessively, the number of coarse precipitates increases, and the number density A of the precipitates cannot be brought to the desired range, resulting in deterioration of 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, and more preferably 0.10% or less.

[0107] Ni: less than 1.00%

[0108] Ni has the effect of improving the corrosion resistance of the steel plate, suppressing the intrusion of hydrogen into the steel plate, and improving the delayed fracture resistance. In addition, Ni is an element effective in improving the hardenability of steel and can be added in order to stably obtain the desired structure. The lower limit of Ni content is not specified, but in order to obtain such an effect, Ni content is preferably 0.01% or more. Ni content is preferably 0.05% or more, more preferably 0.10% or more.

[0109] On the other hand, when excessive Ni is added, coarse precipitates increase, and the number density A of the precipitates cannot be brought into the desired range, resulting in deterioration of 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, and more preferably 0.30% or less.

[0110] Sb: 0.10% or less

[0111] Sb inhibits oxidation and nitridation of the steel sheet surface, contributing to increased strength and delayed fracture resistance. While there is no specified lower limit for the Sb content, to achieve these effects of increased strength and delayed fracture resistance, the Sb content is preferably 0.002% or greater. The Sb content is preferably 0.004% or greater, and more preferably 0.006% or greater.

[0112] On the other hand, excessive addition of Sb increases the number of coarse precipitates, preventing the number density A of the precipitates from falling 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.

[0113] Sn: 0.10% or less

[0114] Sn inhibits oxidation and nitridation of the steel sheet surface, contributing to increased strength and delayed fracture resistance. While there is no specified lower limit for the Sn content, to achieve these effects of increased strength and delayed fracture resistance, the Sn content is preferably 0.002% or greater. The Sn content is preferably 0.004% or greater, and more preferably 0.006% or greater.

[0115] On the other hand, when Sn is added excessively, the number of coarse precipitates increases, and the number density A of the precipitates cannot be brought to the desired range, resulting in deterioration of 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.

[0116] As: 0.10% or less

[0117] As has the effect of increasing the strength of steel. While there is no specified lower limit for the As content, to achieve this effect of increasing the strength of steel, the As content is preferably 0.002% or more. The As content is preferably 0.004% or more, and more preferably 0.006% or more.

[0118] On the other hand, excessive addition of As increases the number of coarse precipitates, preventing the precipitate number density A from falling within the desired range, and deteriorating 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, and more preferably 0.02% or less.

[0119] Ta: less than 0.10%

[0120] Ta has the effect of increasing the strength of steel. While there is no specified lower limit for the Ta content, to achieve this effect, the Ta content is preferably 0.002% or more. The Ta content is preferably 0.004% or more, and more preferably 0.006% or more.

[0121] On the other hand, when Ta is added excessively, the number of coarse precipitates increases, and the number density A of the precipitates cannot be kept within the desired range, resulting in deterioration of delayed fracture resistance. Therefore, when Ta is contained, the Ta content is set to 0.10% or less. The Ta content is preferably 0.05% or less, and more preferably 0.02% or less.

[0122] Ca: 0.020% or less

[0123] Ca spheroidizes the shape of sulfides, reducing the starting point of delayed fracture and improving delayed fracture resistance. To achieve this effect of improving delayed fracture resistance, the Ca content is preferably 0.0002% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0124] On the other hand, when Ca is added excessively, the number of coarse precipitates increases, and the number density A of the precipitates cannot be brought to the desired range, resulting in deterioration of 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.

[0125] Mg: 0.020% or less

[0126] Mg spheroidizes the shape of sulfides, reducing the starting point of delayed fracture and improving delayed fracture resistance. To achieve this effect of improving delayed fracture resistance, the Mg content is preferably 0.0002% or more. The Mg content is preferably 0.001% or more, and more preferably 0.003% or more.

[0127] On the other hand, excessive addition of Mg increases the number of coarse precipitates, preventing the precipitate number density A from falling within the desired range, and deteriorating delayed fracture resistance. Therefore, when Mg is included, 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.

[0128] Zn: 0.020% or less

[0129] Zn improves delayed fracture resistance by reducing the size of prior austenite grains and spheroidizing inclusions. To achieve this effect, the Zn content is preferably 0.001% or more. The Zn content is preferably 0.003% or more.

[0130] On the other hand, when Zn is added excessively, the number of coarse precipitates increases, and the number density A of the precipitates cannot be brought to the desired range, resulting in deterioration of 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.

[0131] Co: 0.020% or less

[0132] Co improves delayed fracture resistance by reducing the size of prior austenite grains and spheroidizing inclusions. To achieve this effect, the Co content is preferably 0.001% or more. The Co content is preferably 0.003% or more.

[0133] On the other hand, when Co is added excessively, coarse precipitates increase, the number density A of the precipitates cannot be brought to the desired range, and the delayed fracture resistance deteriorates. 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.

[0134] Zr: 0.020% or less

[0135] Zr improves delayed fracture resistance by reducing the size of prior austenite grains and spheroidizing inclusions. To achieve this effect, the Zr content is preferably 0.001% or more. The Zr content is preferably 0.003% or more.

[0136] On the other hand, excessive addition of Zr increases the number of coarse precipitates, preventing the precipitate number density A from falling within the desired range, and 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.

[0137] W: 0.020% or less

[0138] W forms precipitates, which refine the prior austenite grain size and improve delayed fracture resistance. To achieve this effect, the W content is preferably 0.001% or more. The W content is preferably 0.003% or more.

[0139] On the other hand, excessive addition of W increases the number of coarse precipitates, preventing the precipitate number density A from falling within the desired range, and 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.

[0140] REM: 0.020% or less

[0141] REM also contributes to improved delayed fracture resistance by spheroidizing inclusions. To achieve this effect of improving delayed fracture resistance, the REM content is preferably 0.0002% or more. The REM content is preferably 0.001% or more, and more preferably 0.003% or more.

[0142] On the other hand, excessive addition of REM increases the number of coarse precipitates, preventing the precipitate number density A from falling within the desired range, 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, and more preferably 0.010% or less.

[0143] It should be noted that REMs in the present invention refer to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanide series 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 REMs. REMs are not particularly limited, but are preferably La and / or Ce.

[0144] In addition, when the above-mentioned optional elements are contained in an amount less than the preferred lower limit, the above-mentioned optional elements are considered to be contained as unavoidable impurities.

[0145] Steel structure

[0146] The steel structure of the steel plate of the present invention has the following structure.

[0147] (Configuration 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 crystal grain size of the ferrite is 10 μm or less.

[0148] (Configuration 2) The number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the following formula (1).

[0149] A(pieces / mm 2 )≤8.5×10 5 ×[B] ...Formula (1)

[0150] Here, [B] represents the B content (mass %).

[0151] Furthermore, by providing the following configuration in addition to (Configuration 1) and (Configuration 2), even better delayed fracture resistance can be obtained.

[0152] (Configuration 3) 50% or more of the ferrite in terms of area ratio is ferrite precipitated on the prior austenite grain boundaries.

[0153] Hereinafter, each configuration will be described.

[0154] (Configuration 1) Martensite area ratio: 85% or more and less than 95%, ferrite area ratio: 5% or more and 15% or less, average ferrite grain size: 10 μm or less

[0155] In the present invention, by using martensite as the main phase, a high strength of TS ≥ 1470 MPa can be achieved. To achieve this high strength effect, the martensite area ratio must be set to 85% or more. The area ratio of martensite is preferably 88% or more.

[0156] Furthermore, it was discovered that excellent delayed fracture resistance can be achieved by using martensite as the main phase and forming ferrite at an area ratio of 5% to 15%. To achieve excellent delayed fracture resistance, the area ratio of martensite is set to less than 95%, and the area ratio of ferrite is set to 5% or more. The area ratio of martensite is preferably less than 93%, and the area ratio of ferrite is preferably 7% or more.

[0157] On the other hand, excessive ferrite formation reduces strength and degrades delayed fracture resistance. Therefore, to stably achieve TS ≥ 1470 MPa and excellent delayed fracture resistance, the ferrite area fraction is set to 15% or less. The ferrite area fraction is preferably 12% or less.

[0158] Furthermore, in the present invention, the ferrite morphology is also controlled to achieve the effect of improving delayed fracture resistance due to ferrite content of 5% to 15%. If the ferrite is coarse, the effect of improving delayed fracture resistance due to ferrite is lost, and the delayed fracture resistance is actually degraded due to the formation of ferrite. To achieve the effect of improving delayed fracture resistance due to ferrite, the average ferrite grain size is set to 10 μm or less. The average ferrite grain size is preferably 6 μm or less, and more preferably 4 μm or less.

[0159] On the other hand, if the ferrite is too fine, the effect of improving the delayed fracture resistance may not be sufficiently obtained. Therefore, the average crystal grain size of the ferrite is preferably 0.5 μm or more, more preferably 1.0 μm or more.

[0160] It should be noted that when structures other than martensite and ferrite are present, the balance is preferably composed of bainite, retained austenite (retained γ), and pearlite. Other than these structures, trace amounts of carbides, sulfides, nitrides, and oxides may be present. The balance structure is 10% or less, preferably 5% or less, and more preferably 3% or less, in terms of area ratio.

[0161] The remainder structure can be 0%. That is, the steel structure can be composed only of martensite and ferrite.

[0162] The martensite includes both martensite that has been self-tempered during continuous cooling and martensite that has not been tempered by being held at approximately 150° C. or higher for a certain period of time.

[0163] (Configuration 2) The number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the following formula.

[0164] A(pieces / mm 2 )≤8.5×10 5 ×[B] ...Formula (1)

[0165] Here, [B] represents the B content (mass %).

[0166] In order to suppress delayed fracture in high-strength steel with TS ≥ 1470 MPa, it is necessary to control the area ratio of martensite and ferrite and the average grain size of ferrite, and it is also necessary to strengthen the grain boundaries and suppress intergranular fracture by using B. However, it was found that simply increasing the B content not only increases the B segregation at the grain boundaries, but also increases the Fe content, which is the starting point of delayed fracture. 23The increase in B-based precipitates, mainly (C,B)6, conversely reduces the delayed fracture resistance. The present inventors have discovered that by controlling the hot rolling conditions, etc., and reducing the number density A of precipitates with an equivalent circle diameter of 500 nm or more to achieve the following conditions, it is possible to achieve both an improvement in delayed fracture resistance due to grain boundary strengthening of B and suppression of fracture originating from precipitates.

[0167] A(pieces / mm 2 )≤8.5×10 5 ×[B]

[0168] The number density A of the precipitates is preferably A (pieces / mm 2 )≤6.5×10 5 ×[B], more preferably A(pieces / mm 2 )≤5.0×10 5 ×[B].

[0169] In the present invention, the lower limit of A is not particularly limited, and A can be 0 or A (pieces / mm 2 )≥0.5×10 5 ×[B].

[0170] (Configuration 3) 50% or more of the ferrite in terms of area ratio is ferrite precipitated on the prior austenite grain boundaries.

[0171] The main methods for dispersing ferrite in martensite include a method of maintaining austenite in a dual-phase region of austenite and ferrite and then transforming austenite into martensite by quenching, and a method of maintaining austenite in a single-phase region and then maintaining it within a ferrite formation temperature range to generate ferrite and then transforming the remaining austenite into martensite by quenching.

[0172] The ferrite generated by maintaining in the ferrite formation temperature range after maintaining in the austenite single phase region is called primary ferrite, which suppresses intergranular fracture by preferentially generating at the grain boundaries, and thus has a particularly large effect on improving the delayed fracture resistance. By making more than 50% of the total ferrite amount be ferrite precipitated on the original austenite grain boundaries, a significant improvement in the delayed fracture resistance can be obtained. Therefore, the area ratio of the ferrite precipitated on the original austenite grain boundaries relative to the total ferrite amount is preferably 50% or more. Preferably, the area ratio of the ferrite precipitated on the original austenite grain boundaries relative to the total ferrite amount is 70% or more, and more preferably, the area ratio of the ferrite precipitated on the original austenite grain boundaries relative to the total ferrite amount is 90% or more. It should be noted that the area ratio of the ferrite precipitated on the original austenite grain boundaries relative to the total ferrite amount can be 100%.

[0173] The measurement method of each component in the above steel structure will be described.

[0174] The area ratios of martensite, bainite and ferrite are determined as follows: the L section of the steel plate (a section parallel to the rolling direction and perpendicular to the surface of the steel plate (hereinafter also referred to as a perpendicular section parallel to the rolling direction)) is ground and then corroded with a nitric acid solution, and four fields of view within a range of 50 μm × 65 μm are observed at a magnification of 2000 times using an SEM at a position 1 / 4 of the thickness from the surface of the steel plate. The image analysis of the taken tissue photographs is performed to determine the area ratios of martensite, bainite and ferrite. Here, martensite and bainite refer to tissues that appear gray or white in SEM. On the other hand, ferrite is an area with a black contrast in SEM. It should be noted that trace amounts of carbides, nitrides, sulfides and oxides are contained in the interior of martensite and bainite, but it is difficult to exclude them, so the area ratio of the area containing them is taken as its area ratio.

[0175] Bainite has the following characteristics. Specifically, it has an aspect ratio of 2.5 or greater and is in the form of flakes, with a slightly darker structure than martensite. The flakes are 0.3 to 1.7 μm wide. The density of carbides with diameters of 10 to 200 nm within the bainite is 0 to 3 per μm. 2 .

[0176] Retained austenite (retained γ) was measured as follows: The surface layer (200 μm) of the steel plate was chemically polished with oxalic acid, and the plate surface was used as the target. Retained austenite (retained γ) was determined by X-ray diffraction intensity. The retained austenite (retained γ) was calculated based on 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.

[0177] The average crystal grain size of ferrite is determined as follows: the L section (vertical section parallel to the rolling direction) of the steel plate is ground and then corroded with nitric acid solution. Ten fields of view are observed in a range of 50 μm × 65 μm at a magnification of 2000 times using an SEM at a position 1 / 4 of the thickness from the surface of the steel plate. The image analysis of the taken tissue photographs is performed to determine the equivalent circle diameter. It should be noted that the equivalent circle diameter refers to the diameter of a perfect circle having the area of ​​the region judged to be ferrite based on the SEM photograph. The value obtained by dividing the sum of the above diameters of the 10 fields of view by the number of ferrites whose diameters were measured is taken as the average crystal grain size of ferrite.

[0178] The method for measuring the area ratio of ferrite precipitated on the prior austenite grain boundaries is as follows.

[0179] First, the L-section of the steel plate was mirror-polished using colloidal silica vibration polishing. Electron beam backscatter diffraction (EBSD) was then performed to obtain local crystal orientation data for 10 fields at a depth of one-quarter the thickness of the steel plate surface. The step size was set to 0.10 μm, and the measurement area was 50 μm x 50 μm. After cleaning using the analysis software OIMAnalysis7, the resulting local orientation data was analyzed to identify prior austenite grain boundaries.

[0180] The cleanup process replaces the orientation and CI value of pixels having a CI value of 0.2 or less with the orientation and CI value of the pixel having the highest CI value among adjacent pixels.

[0181] Specifically, the Neighbor CI Correlation function of the analysis software was used to perform the cleaning process with the parameter Minimum Coufidence Index set to 0.2.

[0182] A map of grain boundaries having a crystal orientation difference of 20° or more and 50° or less was prepared, and these interfaces were determined to be prior austenite grain boundaries.

[0183] After EBSD measurement, the L-section of the steel plate was etched with Nital and the same 10 fields of view used for EBSD were photographed using an SEM at a magnification of 3000x. By superimposing the prior austenite grain boundary map for the same field of view with the captured SEM image, ferrite present at the prior austenite grain boundaries was identified and the area fraction was measured.

[0184] As a method for observing the same visual field using EBSD and SEM, there is a method in which an indentation is made in advance at the observation position of the steel plate using a Vickers hardness tester to serve as a mark of the observation position.

[0185] The number density A of precipitates having an equivalent circle diameter of 500 nm or more is determined as follows: After grinding the L section (a vertical section parallel to the rolling direction) of the steel plate, 2 mm long sections are continuously photographed using a SEM in the region from 1 / 5 to 4 / 5 of the thickness of the steel plate, i.e., from the position at 1 / 5 of the thickness of the steel plate surface to the position at 4 / 5 of the thickness. 2 The number of such precipitates was measured from the SEM photograph taken in the region, and the number density A of precipitates with an equivalent circle diameter of 500 nm or more was obtained. In addition, the magnification for the photograph was 2000 times. In addition, when performing the component analysis of each inclusion particle, each inclusion particle was magnified to 10000 times and the above precipitates were analyzed. Here, the precipitates with an equivalent circle diameter of 500 nm or more are Fe 23The precipitates containing B such as (C, B) 6 were examined for the presence of a B peak by elemental analysis using energy dispersive X-ray spectroscopy (EDS) at an accelerating voltage of 3 kV. If a B peak was present, it was evaluated that the precipitates were present.

[0186] In addition, the equivalent circle diameter refers to the diameter of a perfect circle having the area of ​​each precipitate calculated from the SEM photograph.

[0187] Tensile strength (TS): 1470MPa or more

[0188] When the tensile strength of the steel sheet exceeds 1470 MPa, the delayed fracture resistance significantly deteriorates. One of the characteristics of the present invention is that the delayed fracture resistance is excellent even at a tensile strength of 1470 MPa or above. From the perspective of lightweighting automotive frame components, the tensile strength is preferably 1700 MPa or above.

[0189] It should be noted that the tensile strength of the steel plate of the present invention can be set to 2100 MPa or less.

[0190] The tensile strength of the steel sheet of the present invention can be set to 1900 MPa or less. Furthermore, the tensile strength of the steel sheet of the present invention can also be set to 1800 MPa or less. The steel sheet of the present invention can achieve extremely excellent delayed fracture resistance by having a tensile strength of 1800 MPa or less. Furthermore, even if the tensile strength of the steel sheet of the present invention is greater than 1800 MPa and less than 1900 MPa, extremely excellent delayed fracture resistance can be achieved by ensuring that the area ratio of ferrite precipitated on the prior austenite grain boundaries relative to the total ferrite is 50% or more.

[0191] The tensile strength can be measured by cutting a JIS No. 5 tensile test piece at a position 1 / 4 of the coil width with the rolling direction perpendicular to the longitudinal direction, and measuring the tensile strength by a tensile test according to JIS Z2241 (2022).

[0192] The steel sheet of the present invention may also have a coating on the surface. The coating may be a Zn coating or a coating of another metal. In addition, it may be a hot-dip coating or an electroplated coating.

[0193] Next, the method for producing the steel sheet of the present invention will be described.

[0194] The method for manufacturing a steel plate of the present invention is a method for manufacturing a steel plate as follows, wherein a steel slab having the above-mentioned composition is held at a heating holding temperature of 1100° C. or higher as measured by the slab surface temperature for 30 minutes or longer, and then, the steel slab is subjected to hot finish rolling at a finish rolling temperature of 850° C. or higher with a dwell time of 20 seconds or longer and 150 seconds or shorter at a temperature of 900 to 1000° C., and then, the steel slab is cooled at an average cooling rate of 40° C. / second or higher from the finish rolling temperature to 650° C., and then, the steel slab is coiled at a coiling temperature of 650° C. or lower to produce a hot-rolled steel plate, and the hot-rolled steel plate is cold-rolled at a reduction ratio of 40% or higher to produce a cold-rolled steel plate. The rolled steel sheet is continuously annealed as follows: the annealing temperature is set to 830-950°C, the cold-rolled steel sheet is heated from 400°C to the annealing temperature at an average heating rate of 1.0°C / s or more, maintained at the annealing temperature for 10 seconds or more and 600 seconds or less, then cooled from the annealing temperature to the Ar3 point at an average cooling rate of 10°C / s or more, cooled from the Ar3 point to (Ar3 point-80°C) at an average cooling rate of 1-10°C / s, cooled from (Ar3 point-80°C) to a cooling stop temperature of 260°C or less at an average cooling rate of 10°C / s or more, and then maintained at a holding temperature of 150-260°C for 20-1500 seconds.

[0195] It should be noted that the temperature specified in each step of the present invention refers to the surface temperature of the slab or steel plate.

[0196] Hot Rolling

[0197] Heating and holding temperature: above 1100℃ based on billet surface temperature

[0198] Heating holding time (billet heating holding time): more than 30 minutes

[0199] During slab heating before hot rolling, the slab is held at a heating and holding temperature (slab heating and holding temperature) of 1100°C or higher, as measured by the slab surface temperature, for 30 minutes or longer. This promotes the solid solution of precipitates such as B-based precipitates, thereby reducing the size and number of precipitates. The heating and holding temperature is preferably 1150°C or higher. The heating and holding temperature is preferably 1250°C or lower.

[0200] Furthermore, the holding time at the heating and holding temperature (holding time (slab heating and holding time)) is preferably 40 minutes or longer, and preferably 50 minutes or shorter.

[0201] Dwell time at 900-1000°C: 20 seconds or more and 150 seconds or less

[0202] During hot rolling, the steel billet is kept at 900-1000°C for more than 20 seconds and less than 150 seconds. The increase in the residence time in the temperature range of 900-1000°C causes precipitates mainly composed of BN to be generated, which coarsens the precipitates. The precipitates generated in these temperature ranges are not easily dissolved by annealing heating, which reduces the amount of dissolved B after annealing. When the residence time exceeds 150 seconds, the amount of dissolved B that is effective in suppressing delayed fracture cannot be obtained. Therefore, the above-mentioned residence time is less than 150 seconds, preferably less than 120 seconds, and more preferably less than 100 seconds.

[0203] On the other hand, when the above-mentioned residence time is less than 20 seconds, there is a possibility that the structure becomes uneven. Therefore, the above-mentioned residence time is 20 seconds or more. The above-mentioned residence time is preferably 30 seconds or more.

[0204] Finishing rolling temperature: above 850℃

[0205] In the hot finish rolling, the finish rolling temperature (FT) is set to 850° C. or higher in order to suppress unevenness of the hot rolled structure. The finish rolling temperature is preferably 870° C. or higher.

[0206] The finishing rolling temperature is preferably 930° C. or lower.

[0207] Average cooling rate from the finishing temperature to 650°C (first average cooling rate): 40°C / s or more

[0208] During the cooling after hot finishing rolling, the average cooling rate in the range from the finishing rolling temperature to 650°C is set to 40°C / s or more. In the temperature range from the finishing rolling temperature to 650°C, B segregates at the grain boundaries along with the recrystallization of austenite, and Fe 23 (C,B)6 precipitation. In order to suppress Fe 23 For the precipitation of (C, B) 6, the average cooling rate (first average cooling rate) is set to 40°C / s or higher. The average cooling rate is preferably 60°C / s or higher.

[0209] The average cooling rate is preferably 500° C. / s or less, more preferably 300° C. / s or less.

[0210] It should be noted that the average cooling rate in the hot rolling process (first average cooling rate) refers to "(temperature at the start of cooling (finishing rolling temperature) (°C) - temperature at the end of cooling (°C) (650°C)) / cooling time from the start of cooling to the end of cooling (seconds)".

[0211] Coiling temperature: below 650℃

[0212] After the above cooling to 650°C, further cooling is required before coiling. 23The precipitation of (C,B)6 is accelerated, and thus the delayed fracture resistance is deteriorated. Therefore, the coiling temperature is set to 650°C or lower. The coiling temperature is preferably 600°C or lower. In addition, the coiling temperature is preferably 500°C or higher.

[0213] Cold Rolling

[0214] Reduction rate: more than 40%

[0215] During cold rolling, setting the reduction ratio (cumulative reduction ratio (cold rolling ratio)) to 40% or more stabilizes the recrystallization behavior and texture orientation during subsequent continuous annealing. If the reduction ratio is less than 40%, some austenite grains may become coarse during annealing, potentially reducing strength. Furthermore, the reduction ratio is preferably 80% or less.

[0216] Continuous annealing

[0217] Average heating rate from 400°C to annealing temperature: 1.0°C / s or more

[0218] The cold-rolled steel sheets are subjected to annealing and tempering treatments in a continuous annealing line (CAL), and further subjected to skin-pass rolling as needed.

[0219] Fe 23 (C,B)6 is generated and coarsened in the ferrite region during annealing heating. Therefore, in order to make Fe 23 To reduce (C,B)6 and fully achieve the grain boundary strengthening effect of 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 temperatures above 400°C 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.

[0220] The average heating rate herein refers to "annealing temperature (° C.) - 400° C. described later / heating time (minutes) from 400° C. to the annealing temperature."

[0221] Annealing temperature: 830~950℃

[0222] Soaking time (holding time at annealing temperature): 10 seconds or more and 600 seconds or less

[0223] In order to fully reduce the amount of Fe remaining after annealing due to unsolvated 23 To prevent the formation of precipitates such as (C, B)6, annealing must be performed at high temperatures for a long period of time. Specifically, the annealing temperature must be set to 830°C or higher. The annealing temperature is preferably 840°C or higher, and more preferably 850°C or higher.

[0224] On the other hand, annealing at temperatures exceeding 950°C causes the prior austenite grain size to coarsen, and the ferrite grain size to coarsen. When the annealing temperature exceeds 950°C, the desired average ferrite grain size cannot be achieved, so the annealing temperature is set to 950°C or lower. The annealing temperature is preferably 900°C or lower.

[0225] Furthermore, excessively long soaking times (holding times) also lead to coarsening of the prior austenite grain size, which in turn increases the average ferrite grain size. Therefore, the soaking time is set to 600 seconds or less. The soaking time is preferably 540 seconds or less, and more preferably 480 seconds or less.

[0226] On the other hand, in order to fully reduce the Fe remaining due to the lack of solid solution 23 For precipitates such as (C, B) 6, the soaking time is set to 10 seconds or longer, preferably 30 seconds or longer, and more preferably 60 seconds or longer.

[0227] Average cooling rate from annealing temperature to Ar3 point (second average cooling rate): 10°C / s or more

[0228] Average cooling rate from Ar3 point to (Ar3 point - 80°C) (third average cooling rate): 1-10°C / sec

[0229] Average cooling rate from (Ar3 point - 80°C) to the cooling stop temperature of 260°C or less (fourth average cooling rate): 10°C / s or more

[0230] In order to obtain a structure in which the number density A of precipitates is within the desired range and the area ratio of martensite is 85% or more and less than 95%, and the area ratio of ferrite is 5% or more and 15% or less, cooling is performed from the annealing temperature to the Ar3 point (°C) at an average cooling rate of 10°C / second or more (second average cooling rate), cooling is performed from the Ar3 point (°C) to (Ar3 point - 80°C) at an average cooling rate of 1 to 10°C / second (third average cooling rate), and cooling is performed from (Ar3 point - 80°C) to a cooling stop temperature of 260°C or less at an average cooling rate of 10°C / second or more (fourth average cooling rate).

[0231] Here, the second average cooling rate means "(annealing temperature (° C.)−Ar3 point (° C.)) / cooling time from annealing temperature to Ar3 point (seconds)".

[0232] In addition, the third average cooling rate means "(Ar3 point (°C) - (Ar3 point - 80 (°C)) / cooling time (seconds) from Ar3 point (°C) to (Ar3 point - 80 (°C)"".

[0233] In addition, the fourth average cooling rate refers to "((Ar3 point-80)(°C)-cooling stop temperature below 260°C (°C)) / cooling time from (Ar3 point-80°C) to cooling stop temperature below 260°C (seconds)".

[0234] In addition, the Ar3 point can be obtained by the following formula.

[0235] Ar3 point (℃) = 910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo

[0236] The element symbols in the formula represent the content (mass %) of each element. Elements not contained are calculated as 0 (zero).

[0237] When the average cooling rate from the annealing temperature to the Ar3 point (the second average cooling rate) is less than 10°C / sec, Fe 23 (C,B)6 is excessively generated, and the delayed fracture resistance is deteriorated. Therefore, the average cooling rate from the annealing temperature to the Ar3 point is set to 10°C / second or more.

[0238] The second average cooling rate is preferably 20° C. / second or higher, more preferably 30° C. / second or higher.

[0239] The second average cooling rate is preferably 150° C. / second or less, more preferably 100° C. / second or less.

[0240] If the average cooling rate from the Ar3 point to (Ar3 point - 80°C) (third average cooling rate) exceeds 10°C / second, ferrite is not sufficiently formed, and delayed fracture resistance deteriorates. Therefore, the average cooling rate from the Ar3 point to (Ar3 point - 80°C) (third average cooling rate) is set to 10°C / second or less.

[0241] On the other hand, if the average cooling rate from the Ar3 point to (Ar3 point - 80°C) (the third average cooling rate) is less than 1°C / second, ferrite is excessively formed. Therefore, the average cooling rate from the Ar3 point to (Ar3 point - 80°C) (the third average cooling rate) is set to 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.

[0242] If the average cooling rate (fourth average cooling rate) from (Ar3 point - 80°C) to the cooling stop temperature of 260°C or less is less than 10°C / second, bainite, ferrite, and pearlite are generated in large quantities, and sufficient strength and delayed fracture resistance cannot be achieved. Therefore, the average cooling rate from (Ar3 point - 80°C) to 260°C or less is set to 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.

[0243] The fourth average cooling rate is preferably 1000° C. / s or less, and more preferably 750° C. / s or less.

[0244] Furthermore, if the cooling stop temperature exceeds 260°C, upper bainite / lower bainite is formed, which increases retained austenite and fresh martensite, and fails to achieve sufficient delayed fracture resistance. 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.

[0245] Maintaining temperature: 150-260°C

[0246] Holding time: 20 to 1500 seconds

[0247] The carbides distributed within the martensite are formed during the low-temperature holding period after quenching. To ensure excellent delayed fracture resistance and a tensile strength of 1470 MPa or higher (TS ≥ 1470 MPa), the formation of these carbides must be properly controlled. Therefore, the holding temperature must be between 150°C and 260°C, and the holding time must be controlled to between 20 and 1500 seconds.

[0248] If the holding temperature is lower than the lower limit of 150° C. or the holding time is short, the distribution density of carbides inside the transformation phase becomes insufficient, and the delayed fracture resistance deteriorates.

[0249] On the other hand, if the holding temperature is higher than the upper limit of 260°C, the carbides within the grains and at the bulk boundaries may coarsen significantly, deteriorating the delayed fracture resistance. Furthermore, if the holding time exceeds 1500 seconds, the carbides within the grains and at the bulk boundaries may coarsen significantly, deteriorating the delayed fracture resistance. Therefore, in the present invention, the continuous annealing is performed at a holding temperature of 150 to 260°C for 20 to 1500 seconds.

[0250] The holding temperature is preferably 250°C or lower, more preferably 240°C or lower.

[0251] The holding time is preferably 50 seconds or longer, more preferably 100 seconds or longer.

[0252] The holding time is preferably 1300 seconds or less, more preferably 1000 seconds or less.

[0253] From the perspective of stabilizing stamping properties by adjusting the surface roughness and flattening the plate shape, the steel sheet thus obtained can be subjected to temper rolling. In this case, the temper rolling elongation is preferably set to 0.1% or more. In addition, the temper rolling elongation is preferably set to 1.0% or less. In this case, from the perspective of flattening the shape, it is preferable that the temper rolling rolls are roughened and the roughness Ra of the steel sheet is adjusted to 0.8μm or more. In addition, the roughness Ra of the steel sheet is preferably adjusted to 1.8μm or less.

[0254] Furthermore, the obtained steel sheet can be plated. That is, the surface of the steel sheet can be plated after continuous annealing. By performing the plating treatment, a steel sheet having a plated layer on the surface can be obtained.

[0255] As described above, according to the present invention, the delayed fracture resistance of high-strength cold-rolled steel sheet is significantly improved, contributing to the increased strength and lightweighting of components achieved by the use of high-strength steel sheet. The steel sheet of the present invention preferably has a thickness of 0.5 mm or greater. Alternatively, the thickness is preferably 2.0 mm or less.

[0256] Next, the member of the present invention and its manufacturing method will be described.

[0257] The component of the present invention is formed by subjecting the steel sheet of the present invention to at least one of forming and joining. Furthermore, the method for manufacturing the component of the present invention includes the step of subjecting the steel sheet of the present invention to at least one of forming and joining to form the component.

[0258] The steel sheet of the present invention has a tensile strength of 1470 MPa or higher and exhibits excellent delayed fracture resistance. Consequently, components produced using the steel sheet of the present invention are also high-strength and exhibit superior delayed fracture resistance compared to conventional high-strength components. Furthermore, the use of the components of the present invention can achieve weight reduction. Therefore, the components of the present invention are suitable for use, for example, in vehicle body frame components.

[0259] The forming process can use a common working method such as press working without limitation. In addition, the joining process can use a common welding method such as spot welding, arc welding, rivet joining, caulking joining, etc. without limitation.

[0260] Example

[0261] Hereinafter, embodiments of the present invention will be described.

[0262] Steel having the chemical composition shown in Table 1 was melted and then cast into steel billets.

[0263] The steel slabs were subjected to the heat treatment and rolling shown in Table 2 to obtain steel plates having a thickness of 1.4 mm.

[0264] Specifically, the steel slabs having the various component compositions were held at the heating holding temperature shown in Table 2 for the heating holding time shown in Table 2, then the holding time was set at 900 to 1000° C. as shown in Table 2, and hot finish rolling was performed at the finishing temperature shown in Table 2. The steel slabs were cooled at the first average cooling rate shown in Table 2, and coiled at the coiling temperature shown in Table 2 to produce hot-rolled steel sheets.

[0265] Then, the hot-rolled steel sheets were cold-rolled at the reduction ratios (cold-rolling reduction ratios) shown in Table 2 to prepare cold-rolled steel sheets.

[0266] Then, continuous annealing was performed as follows: the cold-rolled steel sheet was heated to the annealing temperature shown in Table 2 at the average heating rate shown in Table 2, and maintained for the soaking time shown in Table 2. Then, the cold-rolled steel sheet was cooled to 260° C. or less under the cooling conditions shown in Table 2 (second average cooling rate, third average cooling rate, fourth average cooling rate, cooling stop temperature), and reheated as needed, and maintained at the holding temperature shown in Table 2 for the holding time shown in Table 2.

[0267] In addition, regarding No. 2, the obtained steel sheet was subjected to electroplating to obtain a steel sheet having a Zn plating layer formed thereon.

[0268]

[0269]

[0270] The obtained steel sheets were subjected to quantitative analysis of their metal structures by the above-mentioned method, and further subjected to tensile tests and delayed fracture resistance evaluation tests.

[0271] Specifically, the tissue measurement method was performed as follows.

[0272] The area ratio of martensite, bainite and ferrite is measured as follows: the L section (vertical section parallel to the rolling direction) of the steel plate is ground and then corroded with nitric acid ethanol solution, and four fields of view within a range of 50μm×65μm are observed with a SEM at a magnification of 2000 times at a position 1 / 4 of the thickness from the surface of the steel plate. The image analysis of the taken tissue photographs is performed to measure the area ratio of martensite, bainite and ferrite. Here, martensite and bainite refer to tissues that appear gray or white in SEM. Bainite has the following characteristics. That is, it has an aspect ratio of 2.5 or more and is in a flaky form, and is a slightly black tissue compared to martensite. The width of the above-mentioned sheet is 0.3~1.7μm. The distribution density of carbides with a diameter of 10~200nm inside the bainite is 0~3 pieces / μm 2On the other hand, ferrite is a region that appears black in contrast in SEM. It should be noted that trace amounts of carbides, nitrides, sulfides, and oxides are contained within martensite and bainite, but it is difficult to exclude them, so the area ratio of the region containing them is used as the area ratio.

[0273] Retained austenite (retained γ) was measured as follows: The surface layer (200 μm) of the steel plate was chemically polished with oxalic acid, and the plate surface was used as the target. Retained austenite (retained γ) was determined by X-ray diffraction intensity. The retained austenite (retained γ) was calculated based on 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.

[0274] The average grain size of ferrite is obtained as follows: the L-section of the steel plate (a vertical section parallel to the rolling direction) is ground and then corroded with a nitric acid alcohol solution. Ten fields of view within a range of 50 μm × 65 μm are observed at a magnification of 2000 times using an SEM at a position 1 / 4 of the thickness from the surface of the steel plate. The image analysis of the taken tissue photographs is performed to determine the equivalent circle diameter, thereby obtaining the average grain size of ferrite.

[0275] The method for measuring the area ratio of ferrite precipitated on the prior austenite grain boundaries is as follows.

[0276] First, the L-section of the steel plate was mirror-polished using colloidal silica vibration polishing. Electron beam backscatter diffraction (EBSD) was then performed to obtain local crystal orientation data for 10 fields at a depth of one-quarter the thickness of the steel plate surface. The step size was set to 0.10 μm, and the measurement area was 50 μm x 50 μm. After cleaning using the analysis software OIMAnalysis7, the resulting local orientation data was analyzed to identify prior austenite grain boundaries.

[0277] The cleanup process replaces the orientation and CI value of pixels having a CI value of 0.2 or less with the orientation and CI value of the pixel having the highest CI value among adjacent pixels.

[0278] Specifically, the Neighbor CI Correlation function of the analysis software was used to perform the cleaning process with the parameter Minimum Coufidence Index set to 0.2.

[0279] A map of grain boundaries having a crystal orientation difference of 20° or more and 50° or less in rotation angle was prepared, and these interfaces were determined to be prior austenite grain boundaries.

[0280] After EBSD measurement, the L-section of the steel plate was etched with Nital and the same 10 fields of view used for EBSD were photographed using an SEM at a magnification of 3000x. By superimposing the prior austenite grain boundary map for the same field of view with the captured SEM image, ferrite present at the prior austenite grain boundaries was identified and the area fraction was measured.

[0281] As a method for observing the same visual field by EBSD and SEM, an indentation is previously made at the observation position of the steel plate using a Vickers hardness tester to serve as a mark of the observation position.

[0282] The number density A of precipitates having an equivalent circle diameter of 500 nm or more is determined as follows: After grinding the L section (a vertical section parallel to the rolling direction) of the steel plate, 2 mm long sections are continuously photographed using a SEM in the region from 1 / 5 to 4 / 5 of the thickness of the steel plate, i.e., from the position at 1 / 5 of the thickness of the steel plate surface to the position at 4 / 5 of the thickness. 2 The number of such precipitates was measured from the SEM photograph taken in the region, and the number density A of precipitates with an equivalent circle diameter of 500 nm or more was obtained. In addition, the magnification for the photograph was 2000 times. In addition, when performing the component analysis of each inclusion particle, each inclusion particle was magnified to 10000 times and the above precipitates were analyzed. Here, the precipitates with an equivalent circle diameter of 500 nm or more are Fe 23 The precipitates containing B such as (C, B) 6 were examined for the presence of a B peak by elemental analysis using energy dispersive X-ray spectroscopy (EDS) at an accelerating voltage of 3 kV. If a B peak was present, it was evaluated that the precipitates were present.

[0283] In the tensile test, a JIS No. 5 tensile test piece was cut at a position 1 / 4 of the coil width, with the rolling direction perpendicular to the longitudinal direction. Tensile testing (according to JIS Z2241 (2022)) was performed to evaluate the tensile strength TS. A tensile strength TS of 1470 MPa or greater was considered acceptable.

[0284] The delayed fracture resistance was evaluated as follows.

[0285] The test piece is cut from the obtained steel plate (coil) at a position 1 / 4 of the width of the coil in the width direction, with a strip test piece of 100 mm in the rolling right angle direction and 30 mm in the rolling direction. The cutting of the end face of the long side with a length of 100 mm is set as shearing processing. Under the shearing processing state (no mechanical processing for removing burrs is performed), bending processing is performed in a manner that the burrs are on the outer peripheral side of the bend, and the shape of the test piece during the bending forming is maintained, and the test piece is fixed with bolts. The gap of the shearing processing is set to 13%, and the front angle is set to 1°. During the bending processing, the front bending radius is set to 10 mm and the angle of the inner side of the bending vertex is set to 90 degrees (V-shaped bending). The punch uses a punch with a front radius the same as the above-mentioned front bending radius R and a U-shaped punch (the front end R part is semicircular and the thickness of the punch body is 2R), and the die uses a die with a corner R of 30 mm. Then, the depth of the punch pressing the steel plate is adjusted, and the forming is performed in such a manner that the bending angle of the front end (the angle inside the bending apex) is 90 degrees (V-shaped). The distance between the flange ends of the straight piece during bending is the same as that during bending (in such a manner that the opening of the straight piece caused by springback is eliminated), and the test piece is clamped and tightened with a hydraulic jack, and bolts are tightened in this state. The bolts are fixed by pre-setting an elliptical hole (short axis 10 mm, long axis 15 mm) 10 mm inside the short side edge of the strip test piece. The obtained bolted test piece is immersed in a solution prepared by mixing a 0.1% by mass ammonium thiocyanate aqueous solution and a McIlvaine buffer solution in a mass ratio of 1:1 and adjusting the pH to 8.0, and in a solution prepared by adjusting the pH to 7.4, and a delayed fracture resistance evaluation test is performed. At this time, the temperature of the solution is set to 20°C, and every 1 cm of the test piece is fixed. 3 The amount of liquid in the surface area is set to 20 ml. After 48 hours, it is confirmed whether there is a crack that can be visually confirmed at the level (length 1 mm or more). When the pH is adjusted to 8.0, no crack is observed and it is judged that the delayed fracture resistance is excellent. Furthermore, when the pH is adjusted to 7.4, no crack is observed and it is judged that the delayed fracture resistance is particularly excellent. In Table 3, when the pH is adjusted to 8.0, no crack is observed and it is represented by "0 (qualified)", and when the pH is adjusted to 7.4, no crack is observed and it is represented by "◎ (qualified)".

[0286] On the other hand, the case where cracks were observed in the solution adjusted to pH 8.0 was indicated as "× (failed)".

[0287] Table 3 shows the structures and properties of the obtained steel plates.

[0288]

[0289] The steel sheet within the scope of the present invention has high strength and excellent delayed fracture resistance.

[0290] 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)" with respect to delayed fracture resistance, indicating particularly excellent performance.

[0291] In addition, even for steel plates with a tensile strength of more than 1800 MPa and less than 1900 MPa, the area ratio of ferrite precipitated on the prior austenite grain boundaries relative to the total ferrite is 50% or more, and the evaluation result of delayed fracture resistance is "◎ (pass)", which is particularly excellent.

[0292] On the other hand, in the comparative examples, at least one of the tensile strength and the delayed fracture resistance was insufficient.

[0293] Furthermore, it can be seen that since the steel plates of the examples of the present invention have high strength and excellent delayed fracture resistance, components obtained by forming or joining the steel plates of the examples of the present invention have high strength and excellent delayed fracture resistance, just like the steel plates of the examples of the present invention.

Claims

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

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

3. The steel plate according to claim 1 or 2, wherein: The component composition further contains, in mass %, one or more selected from the group consisting of 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 plate according to any one of claims 1 to 3, wherein There is a coating on the surface of the steel plate. 5 . A member formed using the steel plate according to claim 1 .

6. A method for manufacturing a steel plate, wherein: The steel slab having the composition of claim 1 or 3 is held at a heating holding temperature of 1100° C. or higher as the surface temperature of the steel slab for 30 minutes or longer, and then Hot finish rolling is performed under the conditions that the residence time at 900 to 1000° C. is set to 20 seconds or more and 150 seconds or less and the finish rolling temperature is set to 850° C. or more. The average cooling rate in the range from the finishing rolling temperature to 650°C is set to 40°C / s or more. Then, the hot rolled steel sheet is produced by coiling at a coiling temperature of 650°C or less. The hot-rolled steel sheet is cold-rolled at a reduction ratio of 40% or more to produce a cold-rolled steel sheet. Perform continuous annealing as follows: The annealing temperature is set to 830-950° 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. / s or more. The annealing temperature is maintained for 10 seconds or more and 600 seconds or less, and then, Cooling from the annealing temperature to the Ar3 point at an average cooling rate of 10°C / s or more, Cool from Ar3 point to (Ar3 point - 80°C) at an average cooling rate of 1 to 10°C / s. Cooling from (Ar3 point - 80°C) to a cooling stop temperature of 260°C or less at an average cooling rate of 10°C / s or more, Then, the temperature is maintained at 150 to 260° C. for 20 to 1500 seconds.

7. The method for manufacturing a steel plate according to claim 6, wherein: After the continuous annealing, the surface of the steel sheet is plated. 8 . A method for producing a member, comprising the step of subjecting the steel sheet according to claim 1 to at least one of forming and joining to produce the member.

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