Cold-rolled steel sheet and method for manufacturing the same

Abstract

This cold rolled steel sheet has a predetermined chemical composition, and a metal structure at a quarter depth position, which is a position at quarter of the sheet thickness from the surface of the sheet, comprising, in volume ratio: greater than 1.0% and less than 8.0% of retained austenite; 80.0% or more of tempered martensite; a total of 0-15.0%, inclusive, of ferrite and bainite; and 0%-5.0%, inclusive, of martensite. In the metal structure, the prior γ particle size is 5.0-25.0 μm, inclusive, and the number density of retained γ on prior γ grain boundaries is at most 100 per mm2.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel sheet and a method for producing the same. This application claims priority based on Japanese Patent Application No. 2022-018412 filed in Japan on February 9, 2022, and incorporates the content thereof herein.

Background Art

[0002] Today, with the highly specialized industrial technology fields, materials used in each technical field are required to have special and advanced performance. In particular, regarding steel sheets for automobiles, in consideration of the global environment, in order to reduce the weight of the vehicle body and improve fuel efficiency, the demand for high-tensile cold-rolled steel sheets with a thin thickness and excellent formability has been significantly increasing. Among the steel sheets for automobiles, especially the cold-rolled steel sheets used for vehicle body frame parts are required to have high strength, and furthermore, high formability for further application expansion is required. In addition, since automotive parts are formed by pressing or the like, even if they have high strength, they are required to have excellent formability (e.g., uniform elongation and bendability). In addition, with the increase in strength, the hydrogen embrittlement susceptibility increases. Steel members used in automobiles have a risk of hydrogen embrittlement cracking due to hydrogen generated during the manufacture and use of automobiles. During manufacture, hydrogen is generated in the material heating process and the electrodeposition coating process, and a part of it is absorbed by the steel member. Also, during use, hydrogen is generated due to the corrosion of the steel member. Therefore, in recent years, as characteristics required for steel sheets for automobiles, it is exemplified that the tensile strength (TS) is 1310 MPa or more, the uniform elongation is 4.0% or more, the ratio of the limit bend (minimum bend radius) R to the plate thickness in 90° V bending, i.e., R / t, is 5.0 or less, and furthermore, it has excellent hydrogen embrittlement resistance characteristics.

[0003] Although it is effective to have a structure containing ferrite to ensure ductility such as uniform elongation, in order to obtain a strength of 1310 MPa or more in a structure containing ferrite, it is necessary to harden the second phase. However, the hard second phase deteriorates the hole expansion property. <00000 As a technique to improve the hole-expanding properties of high-strength steel sheets, steel sheets with tempered martensite as the main phase have been proposed (see, for example, Patent Documents 1 and 2). Patent Documents 1 and 2 show that the hole-expanding properties are excellent when the microstructure is made of a single-phase tempered martensite structure.

[0005] However, the invention described in Patent Document 1 has a low tensile strength of less than 1310 MPa. Therefore, in order to aim for higher strength, it is necessary to further improve the processability, which deteriorates as a result. Furthermore, although the invention described in Patent Document 2 can achieve a high strength of 1310 MPa or more, there is a problem in that the material is cooled to near room temperature during quenching, resulting in a small volume fraction of retained austenite and an inability to obtain high uniform elongation.

[0006] Furthermore, Patent Document 3 proposes a steel sheet that utilizes the TRIP effect due to retained austenite as a technology that achieves both high strength and high formability. However, the steel sheet described in Patent Document 3 has a ferrite phase, making it difficult to obtain high strengths of 1310 MPa or more, and it has poor hole-expanding formability due to strength differences within the structure.

[0007] Furthermore, Patent Document 4 states that by setting the microstructure (metal structure) at a position 1 / 4 of the plate thickness from the surface to a structure mainly composed of tempered martensite containing retained austenite, and then softening the surface layer and refining the hard phase of the surface layer by controlling the dew point during annealing, a high-strength cold-rolled steel sheet can be obtained that has a tensile strength (TS) of 1310 MPa or more, a uniform elongation of 5.0% or more, a ratio of the critical bending radius R to the plate thickness t (R / t) of 5.0 or less in a 90° V bend, and also excellent resistance to hydrogen embrittlement. However, in recent years, there has been a demand for further improvements in performance. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2009-30091 [Patent Document 2] Japanese Patent Publication No. 2010-215958 [Patent Document 3] Japanese Patent Application Publication No. 2006-104532 [Patent Document 4] International Publication No. 2019 / 181950 [Overview of the project] [Problems that the invention aims to solve]

[0009] As mentioned above, in recent years, there has been a demand for steel sheets with high strength, such as a tensile strength (TS) of 1310 MPa or higher, that also possess higher formability and hydrogen embrittlement resistance. The present invention was made to solve the above-mentioned problems, and its objective is to provide a cold-rolled steel sheet and a method for manufacturing the same that have excellent formability, which is a problem with high-strength steel sheets, and excellent resistance to hydrogen embrittlement. Here, cold-rolled steel sheets include not only cold-rolled steel sheets without a plating layer on the surface, but also hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets. [Means for solving the problem]

[0010] The inventors conducted a detailed investigation into the effects of chemical composition, microstructure, and manufacturing conditions on the mechanical properties of high-strength cold-rolled steel sheets. As a result, they found that by creating a microstructure primarily composed of tempered martensite containing a predetermined amount or more of retained austenite, and by removing retained austenite near prior γ (austenite) grain boundaries, the hydrogen embrittlement resistance was improved. Furthermore, we discovered that by controlling the distribution of carbides in hot-rolled steel sheets and adjusting the heat treatment conditions, it is possible to control the presence of retained austenite near prior γ grain boundaries.

[0011] This invention was made in view of the above findings. The gist of this invention is as follows. [1] A cold-rolled steel sheet according to one aspect of the present invention has the following composition in mass%, C: greater than 0.140% and less than 0.400%, Si: 1.00% or less, Mn: greater than 1.30% and less than 4.00%, P: 0.100% or less, S: 0.010% or less, Al: 0.100% or less, N: 0.0100% or less, Ti: 0% or more and less than 0.050%, Nb: 0% or more and less than 0.050%, V: 0% or more and 0.50% or less, Cu: 0% or more and 1.00% or less, Ni: 0% or more and 1.00% or less, Cr: 0% or more and 1.00% or less, Mo: 0% or more and 0.50% or less, B: 0% or more and 0.0100% or less, Ca: 0% or more and 0.0100% or less, Mg: 0 The material has a chemical composition consisting of % or more, 0.0100% or less, REM: 0% or more, 0.0500% or less, Bi: 0% or more, 0.050% or less, and the remainder: Fe and impurities, and the microstructure at the 1 / 4 depth position, which is 1 / 4 of the plate thickness from the surface, contains, by volume fraction, retained austenite: more than 1.0% and less than 8.0%, tempered martensite: 80.0% or more, ferrite and bainite: 0% or more, 15.0% or less in total, and martensite: 0% or more, 5.0% or less, and in the said microstructure, the prior γ grain size is 5.0 μm or more and 25.0 μm or less, and the number density of retained γ on the prior γ grain boundary is 100 particles / mm 2 Below Furthermore, the tensile strength is 1310 MPa or higher, the uniform elongation is 4.0% or higher, the ratio of the limit bending radius (R / t) to the plate thickness in a 90° V-bend is 5.0 or less, and the plate thickness is 0.8 to 2.6 mm. [2][1] The cold-rolled steel sheet described herein has a chemical composition in mass percent of: Ti: 0.001% or more, less than 0.050%, Nb: 0.001% or more, less than 0.050%, V: 0.01% or more, 0.50% or less, Cu: 0.01% or more, 1.00% or less, Ni: 0.01% or more, 1.00% or less, Cr: 0.01% or more, 1.00% or less, Mo: 0.01% or more. It may contain one or more elements selected from the following: 0.50% or less, B: 0.0001% or more, 0.0100% or less, Ca: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less, REM: 0.0005% or more, 0.0500% or less, and Bi: 0.0005% or more, 0.050% or less. [3][1] or [2] The cold-rolled steel sheet described has a number density of retained austenite within 1.0 μm of the prior γ grain boundary of 150 particles / mm². 2 The following is also acceptable. [4][1]~[3] The cold-rolled steel sheet described in any one of the above may have a hot-dip galvanized layer formed on its surface. [5][4] The cold-rolled steel sheet described above may have an alloyed hot-dip galvanized layer. [6] A method for manufacturing cold-rolled steel sheets according to another aspect of the present invention is: A method for manufacturing cold-rolled steel sheets as described in [1],In mass%, the breakdown is as follows: C: greater than 0.140%, less than 0.400%, Si: 1.00% or less, Mn: greater than 1.30%, less than 4.00%, P: 0.100% or less, S: 0.010% or less, Al: 0.100% or less, N: 0.0100% or less, Ti: 0% or more, less than 0.050%, Nb: 0% or more, less than 0.050%, V: 0% or more, 0.50% or less, Cu: 0% or more, 1.00% or less, Ni: 0% or more, 1.00% or less, Cr: 0% or more, 1.00% or less, Mo: 0% or more, 0.50% or less, B: 0% or more, 0.0100% or less, Ca A hot-rolled steel sheet is obtained by hot-rolling a cast slab having a chemical composition consisting of 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.0500% or less, Bi: 0% or more, 0.050% or less, and the remainder being Fe and impurities; hot-rolling is performed on the heated cast slab, either directly or after it has been cooled, to 1100°C or higher; and hot-rolling is performed on the heated cast slab; a winding is performed on the hot-rolled steel sheet at a temperature of 550°C or lower; and after the winding is performed on the hot-rolled steel sheet, descale it and then cold-roll it to obtain a cold-rolled steel sheet. The cold-rolled steel sheet after the cold-rolling process is heated from 700°C to a soaking temperature of 820°C to 880°C at an average heating rate of less than 10.0°C / second, and then annealed by soaking at the soaking temperature for 30 to 200 seconds. The cold-rolled steel sheet after the annealing process is subjected to one or more bending-and-unbending deformations with a bending angle of 90 degrees or more, using a roll with a radius of 850 mm or less while applying a tension of 3.0 kN or more in a temperature range of 700°C or less and 800°C or more, and then heated from 700°C to 60°C. The process includes: a post-annealing cooling step in which the cold-rolled steel sheet after the post-annealing cooling step is cooled so that the average cooling rate at 0°C and the average cooling rate from 450°C to 350°C are both 5.0°C / second or higher, and then subjected to one or more bending-unbending deformations with a bending angle of 90 degrees or higher using a roll with a radius of 850 mm or less while applying a tension of 3.0 kN or more in the temperature range of 350°C or less and 50°C or higher, and then cooled to a cooling stop temperature of 50°C or higher and 250°C or lower; and a tempering step in which the cold-rolled steel sheet after the post-annealing cooling step is tempered at a temperature of 200°C or higher and 350°C or lower for 1 second or more. [7][6]The method for manufacturing the cold-rolled steel sheet described in [reference] may contain one or more of the following chemical components in the casting slab in mass%: Ti: 0.001% or more and less than 0.050%, Nb: 0.001% or more and less than 0.050%, V: 0.01% or more and 0.50% or less, Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 1.00% or less, Cr: 0.01% or more and 1.00% or less, Mo: 0.01% or more and 0.50% or less, B: [0.0001% or more and 0.0100% or less], Ca: 0.0001% or more and 0.0100% or less, Mg: 0.0001% or more and 0.0100% or less, REM: 0.0005% or more and 0.0500% or less, and Bi: 0.0005% or more and 0.050% or less. [8][6] or [7] In the method for manufacturing the cold-rolled steel sheet described in [reference], in the post-annealing cooling process, the average cooling rate from 350°C to the cooling stop temperature may be set to 10°C / second or less. [9][6]~[8] For the method for manufacturing the cold-rolled steel sheet described in any one of [references], in the post-annealing cooling process, the cold-rolled steel sheet may be immersed in a plating bath at a temperature exceeding 425°C and less than 600°C to form a hot-dip galvanized layer on the surface.

[10] [6]~[8] For the method for manufacturing the cold-rolled steel sheet described in any one of [references], in the post-annealing cooling process, the steel sheet may be immersed in a plating bath at a temperature exceeding 425°C and less than 600°C to form a hot-dip galvanized layer on the surface, and the hot-dip galvanized layer may be further alloyed.

Advantages of the Invention

[0012] According to the above aspect of the present invention, it is possible to provide a cold-rolled steel sheet having excellent formability and excellent hydrogen embrittlement resistance characteristics, and a method for manufacturing the same.

Embodiments for Carrying Out the Invention

[0013] A cold-rolled steel sheet (hereinafter sometimes simply referred to as the steel sheet according to this embodiment) and a method for manufacturing the same according to one embodiment of the present invention will be described below. The steel sheet according to this embodiment includes not only cold-rolled steel sheets without a plating layer on the surface, but also hot-dip galvanized steel sheets having a hot-dip galvanized layer on the surface, or alloyed hot-dip galvanized steel sheets having an alloyed hot-dip galvanized layer on the surface, and the main conditions shown below are common to hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets.

[0014] <Chemical composition> First, the chemical composition of the steel sheet according to this embodiment will be described. Unless otherwise specified, the "%" indicating the content of each element in the chemical composition all refer to mass percent.

[0015] [C: Over 0.140%, less than 0.400%] If the carbon content is 0.140% or less, it becomes difficult to obtain the above-described metal structure, and the target tensile strength cannot be achieved. In addition, the bendability decreases. Therefore, the carbon content should be greater than 0.140%. Preferably, the carbon content is greater than 0.160%, and more preferably than 0.180%. On the other hand, if the carbon content is 0.400% or higher, both weldability and bendability deteriorate. Hydrogen embrittlement resistance also deteriorates. Therefore, the carbon content should be less than 0.400%. Preferably, the carbon content is less than 0.350%, and more preferably less than 0.300%.

[0016] [Si:1.00% or less] On the other hand, if the Si content exceeds 1.00%, the surface properties of the steel sheet deteriorate. Furthermore, the chemical conversion treatment properties and plating properties deteriorate significantly. Therefore, the Si content should be 1.00% or less. Preferably, the Si content is 0.80% or less. On the other hand, silicon (Si) is a useful element for increasing the strength of steel sheets through solid solution strengthening. Furthermore, since Si suppresses the formation of cementite, it promotes the enrichment of carbon (C) in austenite, making it an effective element for generating retained austenite after annealing. Therefore, Si may be included. In this case, the Si content is preferably 0.01% or more, more preferably 0.10% or more, and even more preferably 0.50% or more.

[0017] [Mn: greater than 1.30%, less than 4.00%] Mn has the effect of improving the hardenability of steel and is an effective element for obtaining the above-described metal structure. If the Mn content is 1.30% or less, it becomes difficult to obtain the above-described metal structure. In this case, sufficient tensile strength cannot be obtained. Therefore, the Mn content should be greater than 1.30%. The Mn content is preferably greater than 2.00%, and more preferably greater than 2.50%. On the other hand, if the Mn content is 4.00% or higher, the effect of improving hardenability is diminished due to Mn segregation, and material costs increase. Therefore, the Mn content should be less than 4.00%. Preferably, the Mn content is less than 3.50%, and more preferably less than 3.00%.

[0018] [P:0.100% or less] P is an element present in steel as an impurity, and it segregates at grain boundaries, causing the steel to become brittle. For this reason, a low P content is preferable, and it may even be 0%, but considering the time and cost of removing P, the P content is set to 0.100% or less. Preferably, the P content is 0.020% or less, and more preferably 0.015% or less.

[0019] [S:0.010% or less] S is an element that is present in steel as an impurity and forms sulfide inclusions that degrade the flexibility of the steel. For this reason, a lower S content is preferable, and it may even be 0%, but considering the time and cost of removing S, the S content should be 0.010% or less. Preferably, the S content is 0.005% or less, more preferably 0.003% or less, and even more preferably 0.001% or less.

[0020] [Al:0.100% or less] If the Al content is too high, surface defects caused by alumina are more likely to occur, the transformation point rises significantly, and the volume fraction of ferrite increases. In this case, it becomes difficult to obtain the above-mentioned metallic structure, and sufficient tensile strength cannot be obtained. Therefore, the Al content should be 0.100% or less. Preferably, the Al content is 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less. On the other hand, Al is an element that has the effect of deoxidizing molten steel. In the steel sheet according to this embodiment, since Si, which has a deoxidizing effect similar to Al, is included, it is not necessarily required to include Al, and the Al content may be 0%. However, if Al is included for the purpose of deoxidation, the Al content is preferably 0.005% or more, and more preferably 0.010% or more, in order to ensure deoxidation. Furthermore, Al, like Si, has the effect of increasing the stability of austenite and is an effective element for obtaining the above-mentioned metal structure, so it may be included for this reason as well.

[0021] [N:0.0100% or less] N is an element present in steel as an impurity, and it is an element that degrades bendability by forming coarse precipitates. Therefore, the N content should be 0.0100% or less. Preferably, the N content is 0.0060% or less, and more preferably 0.0050% or less. The lower the N content, the better, and it may even be 0%.

[0022] The steel sheet according to this embodiment may contain the above-mentioned elements, with the remainder being Fe and impurities. However, it may also contain one or more of the elements listed below that affect strength and bendability as optional elements. However, these elements do not necessarily need to be included, so the lower limit for each is 0%.

[0023] [Ti: 0% or more, less than 0.050%] [Nb: 0% or more, less than 0.050%] [V: 0% or more, 0.50% or less] [Cu: 0% or more, 1.00% or less] Ti, Nb, V, and Cu are elements that improve the strength of steel sheets through precipitation hardening. Therefore, these elements may be included. To fully obtain the above effects, it is preferable that the Ti content and Nb content be 0.001% or more, and the V content and Cu content be 0.01% or more. More preferable Ti content and Nb content are 0.005% or more, and the V content and Cu content are 0.05% or more. Obtaining the above effects is not essential. Therefore, there is no need to particularly limit the lower limits of the Ti, Nb, V, and Cu content, and their lower limits are 0%. However, if these elements are included in excess, the recrystallization temperature rises, the metal structure of the cold-rolled steel sheet becomes non-uniform, and its bendability is impaired. Therefore, even when these are included, the Ti content should be less than 0.050%, the Nb content less than 0.050%, the V content 0.50% or less, and the Cu content 1.00% or less. The Ti content is preferably less than 0.030%, more preferably less than 0.020%. The Nb content is preferably less than 0.030%, more preferably less than 0.020%. The V content is preferably 0.30% or less. The Cu content is preferably 0.50% or less.

[0024] [Ni: 0% or more, 1.00% or less] [Cr: 0% or more, 1.00% or less] [Mo: 0% or more, 0.50% or less] [B: 0% or more, 0.0100% or less] Ni, Cr, Mo, and B are elements that improve the hardenability of steel and contribute to its high strength, and are effective elements for obtaining the above-mentioned metallic structure. Therefore, these elements may be included. To fully obtain the above effects, it is preferable that the Ni content, Cr content, and Mo content be 0.01% or more, and / or the B content be 0.0001% or more. More preferably, the Ni content, Cr content, and Mo content are 0.05% or more, and the B content is 0.0010% or more. Obtaining the above effects is not essential. Therefore, there is no need to particularly limit the lower limits of the Ni content, Cr content, Mo content, and B content, and their lower limits are 0%. However, including these elements in excess will saturate the effects of the above-mentioned actions and will also be uneconomical. Therefore, even when including them, the Ni content and Cr content should be 1.00% or less, the Mo content 0.50% or less, and the B content 0.0100% or less. Preferably, the Ni and Cr content is 0.50% or less, the Mo content is 0.20% or less, and the B content is 0.0030% or less.

[0025] [Ca: 0% or more, 0.0100% or less] [Mg: 0% or more, 0.0100% or less] [REM: 0% or more, 0.0500% or less] [Bi:0% or more, 0.050% or less] Ca, Mg, and REM are elements that improve strength and flexibility by adjusting the shape of inclusions. Bi is an element that improves strength and flexibility by refining the solidification structure. Therefore, these elements may be included. To fully obtain the above effects, it is preferable that the Ca and Mg content be 0.0001% or more, and the REM and Bi content be 0.005% or more. More preferably, the Ca and Mg content be 0.0008% or more, and the REM and Bi content be 0.007% or more. Obtaining the above effects is not essential. Therefore, there is no need to particularly limit the lower limits of the Ca, Mg, Sb, Zr, and REM content; their lower limits are 0%. On the other hand, if these elements are included in excess, the effects from the above-mentioned action will saturate, making it uneconomical. Therefore, even when they are included, the Ca content should be 0.0100% or less, the Mg content 0.0100% or less, the REM content 0.0500% or less, and the Bi content 0.050% or less. Preferably, the Ca content is 0.0020% or less, the Mg content 0.0020% or less, the REM content 0.0020% or less, and the Bi content 0.010% or less. REM refers to rare earth elements, and is a collective term for a total of 17 elements including Sc, Y, and lanthanides, and the REM content is the total content of these elements.

[0026] The chemical composition of the steel sheet according to this embodiment can be measured by a general method. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on chips in accordance with JIS G 1201:2014. In this case, the chemical composition is the average content over the entire sheet thickness. For C and S, which cannot be measured by ICP-AES, the combustion-infrared absorption method can be used, and for N, the inert gas fusion-thermal conductivity method can be used. If the steel sheet has a coating such as plating on its surface, the coating should be removed by mechanical grinding or the like before the chemical composition analysis can be performed. If the coating is a plating layer, it may also be removed by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel sheet.

[0027] <Metallic structure (microstructure)> First, the metallographic structure of the steel sheet according to this embodiment will be described. In describing the metallographic structure of the steel sheet according to this embodiment, the microstructure fraction is expressed as a volume fraction. Therefore, unless otherwise specified, "%" represents "volume %". Also, in this embodiment, the reference surface at the 1 / 4 depth position refers to the surface of the base steel sheet excluding the plating layer in the case of plated steel sheets.

[0028] The steel sheets according to this embodiment (including cold-rolled steel sheets, hot-dip galvanized steel sheets, and alloyed hot-dip galvanized steel sheets) have a microstructure at a 1 / 4 depth position (1 / 4 of the thickness from the surface (the surface of the base steel sheet in the case of a galvanized steel sheet)) that, by volume fraction, includes: retained austenite: greater than 1.0% and less than 8.0%, tempered martensite: 80.0% or more, ferrite and bainite: 0% or more and 15.0% or less in total, and martensite: 0% or more and 5.0% or less.

[0029] [Residual austenite: greater than 1.0%, less than 8.0%] Retained austenite improves ductility and contributes to improved uniform elongation through the TRIP effect. Therefore, the volume fraction of retained austenite in the microstructure at a position 1 / 4 of the plate thickness from the surface in the thickness direction should be greater than 1.0%. Preferably, the volume fraction of retained austenite is greater than 1.5%, and more preferably than 2.0%. On the other hand, if the volume fraction of retained austenite is excessive, the grain size of the retained austenite becomes large. Such retained austenite with large grain sizes becomes coarse and hard martensite after deformation. In this case, crack initiation points are more likely to occur, and the flexibility deteriorates. For this reason, the volume fraction of retained austenite should be less than 8.0%. Preferably, the volume fraction of retained austenite is less than 7.0%, and more preferably less than 6.0%.

[0030] [Tempered martensite: 80.0% or more] Tempered martensite, like martensite (so-called fresh martensite), is an aggregate of lath-like crystal grains. However, unlike martensite, it has a hard structure containing fine iron-based carbides due to tempering. Tempered martensite is obtained by tempering martensite generated by cooling after annealing through heat treatment or other methods. Tempered martensite is a structure that is less brittle and more ductile than regular martensite. In the steel sheet according to this embodiment, the volume fraction of tempered martensite is set to 80.0% or more in order to improve strength, bendability, and resistance to hydrogen embrittlement. Preferably, the volume fraction is 85.0% or more. The volume fraction of tempered martensite is less than 99.0%.

[0031] [Ferrite and bainite: Total 0% or more, 15.0% or less] Ferrite is a soft phase formed during two-phase annealing or slow cooling after holding during the annealing process. When mixed with hard phases such as martensite, ferrite improves the ductility of steel sheets, but to achieve high strengths of 1310 MPa or higher, it is necessary to limit the volume fraction of ferrite. Furthermore, bainite is a phase formed during the cooling process after holding at the annealing temperature, by holding the material at a temperature between 350°C and 450°C for a certain period of time. Because bainite is softer than martensite, it has the effect of improving ductility, but in order to achieve high strengths of 1310 MPa or more, it is necessary to limit its volume fraction, similar to ferrite mentioned above. Therefore, the total volume fraction of ferrite and bainite should be 15.0% or less. Preferably, it should be 10.0% or less. Since ferrite and bainite do not need to be included, the lower limit is 0%. Furthermore, the individual volume fractions of ferrite and bainite are not limited.

[0032] [Martensite: 0% or more, 5.0% or less] Martensite (fresh martensite) is an aggregate of lath-like crystal grains that is formed by a transformation from austenite during final cooling. Martensite is hard and brittle, and is prone to cracking during deformation, so a high volume percentage of martensite degrades the flexibility. For this reason, the volume percentage of martensite should be 5.0% or less. Preferably, the volume percentage of martensite is 3.0% or less, and more preferably 1.0% or less. Since martensite does not need to be present, the lower limit is 0%.

[0033] [Remaining tissue] In the metallographic structure at a depth of 1 / 4, in addition to the above, pearlite may be included as the remaining structure. However, pearlite is a structure that contains cementite and consumes carbon (C) in the steel, which contributes to improving strength. Therefore, if the volume ratio of pearlite exceeds 5.0%, the strength of the steel sheet decreases. For this reason, the volume ratio of pearlite should be 5.0% or less. Preferably, the volume ratio of pearlite is 3.0% or less, and more preferably 1.0% or less.

[0034] The volume fraction of each phase in the metallographic structure at a depth of 1 / 4 of the steel plate according to this embodiment is measured as follows. Specifically, the volume fractions of ferrite, bainite, martensite, tempered martensite, and pearlite are determined by taking a test piece from an arbitrary position relative to the rolling direction and width direction of the steel sheet, polishing the longitudinal section parallel to the rolling direction (a section parallel to the thickness direction), and observing the metallographic structure revealed by nital etching at a depth of 1 / 4 using a scanning electron microscope (SEM). In the SEM observation, five fields of view of 30 μm × 50 μm are observed at a magnification of 3000x, and the area fraction of each structure is measured from the observed images, and the average value is calculated. In the steel sheet according to this embodiment, the area fraction of the longitudinal section parallel to the rolling direction can be considered equal to the volume fraction, so the area fractions obtained from the microstructure observation are taken as the respective volume fractions.

[0035] When measuring the area ratio of each phase (structure), areas where the substructure is not visible and the brightness is low are considered ferrite. Areas where the substructure is not visible and the brightness is high are considered martensite or retained austenite. Areas where the substructure is visible are considered tempered martensite or bainite.

[0036] Bainite and tempered martensite can be further distinguished by carefully observing the carbides within the grains. Specifically, tempered martensite is composed of a martensite lath and cementite formed within the lath. In this case, there are two or more possible crystal orientation relationships between the martensite lath and the cementite, so the cementite constituting the tempered martensite has multiple variants. Bainite is classified into upper bainite and lower bainite. Upper bainite is composed of lath-like bainite ferrite and cementite formed at the lath interface, and can therefore be easily distinguished from tempered martensite. Lower bainite is composed of lath-like bainite ferrite and cementite formed inside the lath. In this case, the crystal orientation relationship of bainite ferrite and cementite is unique, unlike in tempered martensite, and the cementite constituting lower bainite has the same variant. Therefore, lower bainite and tempered martensite can be distinguished based on the cementite variant. On the other hand, martensite and retained austenite cannot be clearly distinguished by SEM observation. Therefore, the volume fraction of martensite is calculated by subtracting the volume fraction of retained austenite, calculated using the method described later, from the volume fraction of tissue determined to be either martensite or retained austenite.

[0037] The volume fraction of retained austenite is quantified by taking a test specimen from any position on the steel sheet, chemically polishing the rolled surface from the surface down to a position 1 / 4 of the sheet thickness (1 / 4 depth), and measuring the (200), (210), and (311) area fraction intensities of ferrite and austenite using MoKα radiation.

[0038] [Previous γ (austenite) grain size is 5.0 μm or larger and 25.0 μm or smaller] [The number density of retained austenite on the former γ (austenite) grain boundary is 100 particles / mm².] 2 below] Retained austenite is a necessary structure for improving formability, but the inventors have found that when retained austenite is present on prior γ grain boundaries, hydrogen embrittlement resistance decreases. The reason for this is not clear, but since hydrogen embrittlement often involves cracking at prior γ grain boundaries, it is presumed that when austenite grains with a high hydrogen solid solubility limit are present on prior γ grain boundaries, they become a source of hydrogen during martensitic transformation in processing, making the material more prone to cracking. Furthermore, it is presumed that the formation of a hard martensitic structure on prior γ grain boundaries due to processing makes them prone to crack initiation, which is the cause of the deterioration of bendability and hydrogen embrittlement resistance. Therefore, in the steel sheet according to this embodiment, the number density of retained austenite on prior γ grain boundaries is limited. Specifically, even if the volume fraction of retained austenite exceeds 1.0%, excellent hydrogen embrittlement resistance is obtained by setting the number density of retained austenite on the prior γ grain boundaries to 100 particles / mm³. 2 The following applies: Furthermore, if the prior γ grain size is less than 5.0 μm, the grain size is too small, leading to an increase in retained austenite on or near the grain boundaries, thus degrading the flexibility and hydrogen embrittlement resistance. On the other hand, if the prior γ grain size is greater than 25.0 μm, although the proportion of prior γ grain boundaries, which are prone to becoming starting points, decreases, strain concentration is more likely to occur during processing, resulting in a degradation of flexibility and hydrogen embrittlement resistance.

[0039] Preferably, the number density of retained austenite within 1.0 μm from the prior γ grain boundary is 150 particles / mm². 2 below] By reducing the number density of retained austenite not only on the prior γ grain boundaries but also near the prior γ grain boundaries, hydrogen embrittlement resistance can be further improved. Therefore, to obtain superior hydrogen embrittlement resistance, the number density of retained austenite within 1.0 μm of the prior γ grain boundaries (including retained austenite on the prior γ grain boundaries) should be set to 150 particles / mm².2 The following is preferable.

[0040] The prior γ (austenite) grain size, the number density of retained austenite on the prior γ grain boundary, and the number density of retained austenite within 1.0 μm of the prior γ grain boundary are determined by the following method: A longitudinal section parallel to the rolling direction (a section parallel to the thickness direction) is cut out and polished, and at a depth of 1 / 4, measurements are taken using EBSD (Electron Back Scattering Diffraction) in a range (field of view) of 200 μm in the thickness direction and 200 μm in the longitudinal direction for at least three fields of view. Using TSL OIM Analysis, the software included with EBSD, orientation analysis is performed, and grain boundaries are defined as boundaries where the orientation difference between adjacent measurement points is 5° or more, thereby defining the crystal grains. If the average crystal orientation of this crystal grain has a KS (Kurdjumov-Sachs) orientation relationship, assuming that the average crystal orientation of adjacent crystal grains allows for an orientation difference of 3°, it is defined as the same prior γ grain, and the prior γ grains are defined by repeating the orientation analysis with adjacent crystal grains for each crystal grain. Furthermore, the number of crystal grains adjacent to these prior γ grain boundaries, which were determined to be γ phase by EBSD measurement, is measured, and the retained austenite density on the prior γ grains is calculated. The number density of retained austenite within 1.0 μm of the grain boundary can be similarly determined. If ferrite is present during microstructural observation before EBSD measurement, the prior γ grain boundary becomes unclear. Therefore, the position of the ferrite in the same field of view is coordinated and excluded from the range of the prior γ grain boundary in the EBSD measurement.

[0041] <Mechanical properties> [Tensile strength of 1310 MPa or higher] [Uniform spread of 4.0% or more] [The ratio of the limit bending radius (R) to the plate thickness (R / t) in a 90° V-bend is 5.0 or less.] In the steel plate according to this embodiment, the target tensile strength (TS) is 1310 MPa or higher, which contributes to the weight reduction of the automobile body. From the viewpoint of impact absorption, the strength of the steel plate is preferably 1400 MPa or higher, and more preferably 1470 MPa or higher. There is no upper limit, but it may be 1960 MPa or lower. Furthermore, from the viewpoint of moldability, the target uniform elongation (uEl) is 4.0% or higher. To further improve moldability, the uniform elongation (uEl) is preferably 4.5% or higher, and more preferably 5.0% or higher. Furthermore, from the viewpoint of formability, the ratio of the limit bending radius (R) to the sheet thickness (t) in a 90° V-bend (R / t) is targeted to be 5.0 or less. (R / t) is preferably 4.0 or less, and more preferably 3.0 or less, in order to further improve formability.

[0042] Tensile strength (TS) and uniform elongation (uEl) are determined by taking a JIS No. 5 tensile test specimen from the steel sheet perpendicular to the rolling direction and performing a tensile test in accordance with JIS Z 2241:2011. Furthermore, (R / t) is determined by using a 90° V bending die, varying the radius R in 0.5 mm increments, finding the minimum bending radius R at which cracking does not occur, and then dividing it by the plate thickness t.

[0043] The steel sheet according to this embodiment may have a hot-dip galvanized layer on its surface. Adding a plating layer to the surface improves corrosion resistance. Automotive steel sheets, even with increased strength, may not be able to be thinned below a certain thickness due to concerns about perforation caused by corrosion. One of the purposes of increasing the strength of steel sheets is weight reduction through thinning; therefore, even if a high-strength steel sheet is developed, its application is limited if its corrosion resistance is poor. As a method to solve these problems, applying a highly corrosion-resistant plating, such as hot-dip galvanizing, to the steel sheet is conceivable. Since the steel sheet components of the steel sheet according to this embodiment are controlled as described above, hot-dip galvanizing is possible. The hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.

[0044] <plate thickness> The thickness of the steel plate according to this embodiment is not limited, but considering the products to which it is intended to be applied, 0.8 to 2.6 mm is preferred.

[0045] <Manufacturing method> The steel sheet according to this embodiment will be effective as long as it has the above configuration, so the manufacturing method is not limited, but it can be manufactured by a manufacturing method including the following steps (I) to (VI). (I) A hot rolling process in which a cast slab is heated to 1100°C or higher either directly or after being cooled, and the heated cast slab is hot-rolled to obtain a hot-rolled steel sheet, (II) A winding process in which the hot-rolled steel sheet is wound at a temperature of 550°C or lower, (III) A cold rolling process in which the hot-rolled steel sheet after the winding process is descaled and then cold-rolled to produce a cold-rolled steel sheet, (IV) An annealing step in which the cold-rolled steel sheet after the cold-rolling step is heated from 700°C to a soaking temperature of 820°C or higher and 880°C or lower, such that the average heating rate to the soaking temperature is less than 10.0°C / second, and then annealed by soaking at the soaking temperature for 30 to 200 seconds, (V) The cold-rolled steel sheet after the annealing process is subjected to one or more bending-and-return deformations with a bending angle of 90 degrees or more, using a roll with a radius of 850 mm or less while applying a tension of 3.0 kN or more in a temperature range of 800°C or less and 700°C or more, and then cooled so that the average cooling rate from 700°C to 600°C and the average cooling rate from 450°C to 350°C are both 5.0°C / second or more, and subjected to one or more bending-and-return deformations with a bending angle of 90 degrees or more, using a roll with a radius of 850 mm or less while applying a tension of 3.0 kN or more in a temperature range of 350°C or less and 50°C or more, and then cooled to a cooling stop temperature of 50°C or more and 250°C or less, in a post-annealing cooling process, (VI) A tempering step in which the cold-rolled steel sheet after the annealing and cooling step is tempered at a temperature of 200°C to 350°C for 1 second or more. The following describes each step.

[0046] [Hot rolling process] The cast slab is heated to 1100°C or higher, either directly or after it has been cooled, and the heated cast slab is hot-rolled to obtain a hot-rolled steel sheet. The hot rolling conditions are not limited. Since the chemical composition does not change substantially during the manufacturing process, the chemical composition of the cast slab should be the same as that of the target cold-rolled steel sheet.

[0047] The method for manufacturing the cast slab is not limited. From the viewpoint of productivity, it is preferable to cast it by continuous casting, but it may also be manufactured by ingot casting or thin slab casting. If the steel billet obtained by continuous casting can be subjected to the hot rolling process at a sufficiently high temperature, the heating step may be omitted.

[0048] [Winding process] The hot-rolled steel sheet is wound at a temperature of 550°C or lower. By winding the sheet at a temperature of 550°C or lower, the formation of a coarse ferrite structure without carbides can be suppressed, allowing for the fine dispersion and precipitation of carbides. These carbides serve as the starting point for austenite transformation during subsequent annealing, and after the austenite transformation, they dissolve, resulting in an austenite grain structure with a uniform carbon concentration. By cooling this austenite with a uniform carbon concentration while controlling the cooling rate, etc., a steel sheet with excellent bendability and hydrogen embrittlement resistance can be obtained, in which no retained austenite remains on the prior γ grain boundaries. If the winding temperature exceeds 550°C, the carbides become coarse, and sufficient effect cannot be obtained.

[0049] [Cold rolling process] After the winding process, the hot-rolled steel sheet is descaled and then cold-rolled to produce a cold-rolled steel sheet. The cold-rolling conditions are not limited, but in terms of promoting the γ transformation in the annealing process, a reduction ratio (cumulative reduction ratio) of 30% or more is preferable. On the other hand, since achieving a reduction ratio of more than 70% would require a high cold-rolling load, the reduction ratio may be 70% or less.

[0050] [Annealing process] In the annealing process, the cold-rolled steel sheet after the cold rolling process is heated to a soaking temperature (annealing temperature) of 820-880°C, with an average heating rate of less than 10.0°C / second from 700°C to the soaking temperature, and then annealed by soaking at the soaking temperature for 30-200 seconds. If the average heating rate from 700°C to the soaking temperature exceeds 10.0°C / second, the carbides formed in the hot-rolled steel sheet may not be fully dissolved, or the diffusion of dissolved carbon may be insufficient, resulting in an uneven carbon concentration. Consequently, it becomes impossible to obtain a steel sheet with excellent bendability and hydrogen embrittlement resistance, as retained austenite does not remain on the prior γ grain boundaries. Furthermore, if the soaking temperature is too low, austenite single-phase annealing will not occur, the volume fraction of ferrite will increase, and the bendability will deteriorate. Therefore, the soaking temperature should be 820°C or higher. A soaking temperature of 830°C or higher is preferable. A higher soaking temperature makes it easier to ensure bendability, but if the soaking temperature is too high, the prior austenite will coarseen, so retained austenite will not remain on the prior γ grain boundaries, making it impossible to obtain a steel sheet with excellent bendability and hydrogen embrittlement resistance. Therefore, the soaking temperature should be 880°C or lower. A soaking temperature of 870°C or lower is preferable. If the soaking time is less than 30 seconds, austenitization may not proceed sufficiently. On the other hand, if the soaking time exceeds 200 seconds, productivity will decrease, so the soaking time should be 200 seconds or less.

[0051] [Post-annealing cooling process] In the post-annealing cooling process, the cold-rolled steel sheet after annealing is cooled to a temperature of 50°C to 250°C (cooling stop temperature) so that the average cooling rate in the ferrite transformation temperature range of 700°C to 600°C and the average cooling rate in the bainite transformation temperature range of 450°C to 350°C are both 5.0°C / second or higher in order to obtain the above-mentioned microstructure. If the cooling rate in the above temperature range is slow, the volume fraction of ferrite and bainite at the 1 / 4 depth position increases, and the volume fraction of tempered martensite decreases. As a result, the tensile strength decreases, and the bendability and hydrogen embrittlement resistance deteriorate. Therefore, the average cooling rate in the 700°C to 600°C and 450°C to 350°C ranges are both 5.0°C / second or higher. Preferably, the average cooling rate in the above temperature ranges is 10.0°C / second or higher, more preferably 15.0°C / second or higher, and even more preferably 20.0°C / second or higher. Furthermore, during this cooling process, while satisfying the above average cooling rate, in the temperature range of 800°C or below and 700°C or above, a tension of 3.0kN or more is applied, and a roll with a radius of 850mm or less is used to apply one or more bending-bending deformations resulting in a bending angle of 90 degrees or more. In addition, in the temperature range of 350°C or below and 50°C or above, a tension of 3.0kN or more is applied, and a roll with a radius of 850mm or less is used to apply one or more bending-bending deformations resulting in a bending angle of 90 degrees or more. By applying bending-bending deformation in the temperature range of 800°C or below and 700°C or above, strain is introduced into the austenite, thereby introducing martensitic transformation nuclei. The strain enters particularly near the grain boundaries of the austenite, and then by applying bending-bending deformation in the temperature range of 350°C or below and 50°C or above, martensitic transformation near the grain boundaries is promoted. This makes it possible to reduce the number density of retained austenite on the prior γ grain boundaries. Bending and unbending in only one of the temperature ranges, either the low temperature range of 50°C or below 350°C, or the high temperature range of 700°C or below 800°C, does not sufficiently reduce the number density of retained austenite on the prior γ grain boundaries. By integrally controlling the bending and unbending in both the high and low temperature ranges, including controlling the average cooling rate in the temperature ranges mentioned above, it is possible to promote martensitic transformation near the grain boundaries and reduce the number density of retained austenite on the prior γ grain boundaries. For example, within the target temperature range, the desired bend-and-unbend process can be achieved by using a roll with a radius of 850 mm or less (along the roll), bending the material at a bend angle of 90 degrees or more with the surface facing inward, and then bending it again at a bend angle of 90 degrees or more with the back surface facing inward. The tension applied during bending and unbending is preferably 5.0 kN or more, and more preferably 8.0 kN or more, in order to apply strain near the grain boundaries to ensure sufficient martensitic transformation and to stabilize the sheet. Furthermore, the tension applied during bending and unbending in the 350-50°C temperature range may be higher than the tension applied during bending and unbending in the 800-700°C temperature range. By not applying excessive tension in the high-temperature range, deformation of the steel sheet can be suppressed, while applying strong tension in the low-temperature range can sufficiently promote martensitic transformation near the grain boundaries.

[0052] In the post-annealing cooling process, it is preferable to set the average cooling rate in the temperature range below 350°C to 10°C / second or less. More preferably, it is 7°C / second or less. As described above, by applying bending and unbending, and then reducing the cooling rate below 350°C, which is the temperature range in which martensite is formed, the martensitic transformation from strain near the grain boundaries is promoted, and the number density of retained austenite near the prior austenite grain boundaries (for example, within 1.0 μm from the prior austenite grain boundaries) can be reduced.

[0053] [Hot-dip galvanizing] [Alloying] When manufacturing cold-rolled steel sheets (hot-dip galvanized steel sheets) having a hot-dip galvanized layer on the surface, the steel sheets may be further immersed in a plating bath at a similar temperature while the steel sheet temperature is above 425°C and below 600°C during the annealing and cooling process to perform hot-dip galvanizing. The composition of the plating bath may be within a known range. Furthermore, when manufacturing cold-rolled steel sheets (alloyed hot-dip galvanized steel sheets) having an alloyed hot-dip galvanized layer on the surface, the steel sheets may be subjected to an alloying heat treatment, for example, by heating to above 425°C and below 600°C, following the hot-dip galvanizing process, to obtain alloyed hot-dip galvanizing. If performed during the annealing and cooling process, it should be carried out within a range that satisfies the average cooling rate (5.0°C / second or more) in the bainite transformation temperature range of 450°C to 350°C mentioned above.

[0054] [Tempering process] In the tempering process, the cold-rolled steel sheet, after the annealing and cooling process, is tempered at a temperature of 200°C to 350°C for at least 1 second. After the annealing and cooling process, the cold-rolled steel sheet is cooled to a temperature between 50°C and 250°C, causing the un-transformed austenite to transform into martensite. In the tempering process, the cold-rolled steel sheet is tempered at a temperature between 200°C and 350°C for at least one second, resulting in a structure primarily composed of tempered martensite at a depth of 1 / 4. If a hot-dip galvanizing process and / or an alloying process is performed, the cold-rolled steel sheet after the hot-dip galvanizing process, or the cold-rolled steel sheet after both the hot-dip galvanizing and alloying processes, is cooled to a temperature of 50°C to 250°C, and then tempered at a temperature of 200°C to 350°C for at least 1 second. If the tempering temperature exceeds 350°C, the strength of the steel sheet decreases. Therefore, the tempering temperature should be 350°C or lower. A tempering temperature of 325°C or lower is preferable, and 300°C or lower is more preferable. On the other hand, if the tempering temperature is below 200°C, the tempering will be insufficient, and the flexibility and hydrogen embrittlement resistance will deteriorate. Therefore, the tempering temperature should be 200°C or higher. A tempering temperature of 220°C or higher is preferable, and 250°C or higher is more preferable. A tempering time of 1 second or more is sufficient, but 5 seconds or more is preferable for stable tempering, and 10 seconds or more is even preferable. On the other hand, since prolonged tempering may reduce the strength of the steel sheet, a tempering time of 750 seconds or less is preferable, and 500 seconds or less is even preferable.

[0055] [Skin pass rolling process] After the tempering process, the cold-rolled steel sheet may be cooled to a temperature suitable for skin-pass rolling before skin-pass rolling. If the cooling after annealing is done using water, such as water spray cooling, dip cooling, or steam-water cooling, it is preferable to perform pickling and then plating with one or more of the following elements (Ni, Fe, Co, Sn, Cu) before skin-pass rolling to remove the oxide film formed by contact with water at high temperatures and to improve the chemical conversion treatment properties of the steel sheet. Here, "trace amount" refers to 3 to 30 mg / m² on the surface of the steel sheet. 2 It refers to the degree of plating. Skin pass rolling can be used to shape steel sheets. The elongation rate of skin pass rolling is preferably 0.10% or higher, more preferably 0.15% or higher. On the other hand, if the elongation rate of skin pass rolling is high, the volume fraction of retained austenite decreases and the ductility deteriorates. Therefore, the elongation rate is preferably 1.00% or lower, more preferably 0.75% or lower, and even more preferably 0.50% or lower. [Examples]

[0056] The present invention will be described in more detail with reference to examples. Slabs having the chemical composition shown in Table 1 were cast. The cast slabs were heated to over 1100°C, hot-rolled to 2.8 mm, wound at the winding temperature shown in Table 2, and cooled to room temperature. Subsequently, the scale was removed by pickling, and the material was cold-rolled to 1.4 mm. After that, it was annealed for 120 seconds at the soaking temperature shown in Table 2. The average heating rate from 700°C to the soaking temperature during annealing was as shown in Table 2. After annealing, the material was cooled to a cooling stop temperature between 50°C and 250°C, while performing bending and unbending of 90°C or more in the temperature ranges of 800°C to 700°C and 350°C to 50°C, so that the average cooling rate in the temperature ranges of 700°C to 600°C and 450°C to 350°C was 20°C / second or more in both cases. Bending and unbending was performed by applying the tension shown in Table 2, bending the material at an angle of 90°C or more with the surface facing inward, and then bending it again at an angle of 90°C or more with the back surface facing inward, along a roll of the radius shown in Table 2 in the target temperature range. The average cooling rate in the temperature range below 350°C was as shown in Table 2. Afterward, tempering was performed by heating the material at 200-350°C for 1-500 seconds. In some cases, hot-dip galvanizing and alloying were performed during the cooling process after annealing. In Table 4, CR represents cold-rolled steel sheet without galvanizing, GI represents hot-dip galvanized steel sheet, and GA represents alloyed hot-dip galvanized steel sheet. Hot-dip galvanizing involves a process where 35-65 g / m² is applied during the cooling process after annealing. 2 A hot-dip galvanized layer was formed. For the alloyed hot-dip galvanized steel sheet, hot-dip galvanizing was performed by immersion in a plating bath at a temperature of over 425°C and under 600°C, and then alloying was performed at a temperature of over 425°C and under 600°C.

[0057] [Table 1]

[0058] [Table 2]

[0059] From the obtained cold-rolled steel sheets, the volume fraction of the metal structure at a depth of 1 / 4 (retained austenite, tempered martensite, ferrite, bainite, martensite, pearlite), prior γ grain size, number density of retained austenite on prior γ grain boundaries, and number density of retained austenite in the range of 1.0 μm from the prior γ grain boundaries were measured using the method described above. The results are shown in Table 3.

[0060] Furthermore, tensile strength (TS) and uniform elongation (uEl) were determined by taking JIS No. 5 tensile test specimens perpendicular to the rolling direction from the cold-rolled steel sheet and conducting tensile tests in accordance with JIS Z 2241:2011.

[0061] The following tests were conducted to evaluate the hydrogen embrittlement resistance. Specifically, test specimens with mechanically ground ends were bent into a U-shape using the press-bending method to create U-bend test specimens with the smallest possible bending radius R. After elastic deformation by tightening bolts so that the unbent sections were parallel, the specimens were immersed in pH 1 hydrochloric acid to conduct a delayed fracture acceleration test in which hydrogen penetrated the steel plate. Steel plates that did not crack after 100 hours of immersion were evaluated as having good (○: OK) delayed fracture resistance, while those that cracked were evaluated as poor (×: NG). To eliminate the effect of plating, for plated materials, the plating layer was removed with hydrochloric acid containing an inhibitor before the test, and then the hydrogen embrittlement resistance was evaluated. The results are shown in Table 4.

[0062] The bendability index (R / t) was determined by using a 90° V bending die, varying the radius R in 0.5 mm increments, finding the minimum bending radius R at which cracking did not occur, and then dividing by the plate thickness of 1.4 mm. The results are shown in Table 4.

[0063] [Table 3]

[0064] [Table 4]

[0065] As can be seen from Tables 1-4, all of the examples of the present invention (test numbers 2, 10, 12, 17-34) had a TS of 1310 MPa or higher, a uEl of 4.0% or higher, and (R / t) of 5.0 or lower, and also exhibited good hydrogen embrittlement resistance. In contrast, in test numbers 1, 3-9, 11, and 13-16 (comparative examples), where either the chemical composition or the manufacturing method was outside the scope of the present invention, and the metal structure at 1 / 4 depth, prior γ grain size, and the number density texture of residual γ on the prior γ grain boundaries were outside the scope of the present invention, one or more of the tensile strength, uniform elongation, R / t, and hydrogen embrittlement resistance properties did not meet the target.

Claims

1. In mass percent, C: Over 0.140%, less than 0.400% Si: 1.00% or less, Mn: greater than 1.30%, less than 4.00% P: 0.100% or less, S: 0.010% or less, Al: 0.100% or less, N: 0.0100% or less, Ti: 0% or more, less than 0.050% Nb: 0% or more, less than 0.050% V: 0% or more, 0.50% or less, Cu: 0% or more, 1.00% or less, Ni: 0% or more, 1.00% or less, Cr: 0% or more, 1.00% or less, Mo: 0% or more, 0.50% or less, B: 0% or more, 0.0100% or less, Ca: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.0500% or less, Bi: 0% or more, 0.050% or less, and Remainder: Fe and impurities It has a chemical composition consisting of, The microstructure at the 1 / 4 depth position, which is 1 / 4 of the plate thickness from the surface, is, by volume fraction, Residual austenite: over 1.0%, less than 8.0% Tempered martensite: 80.0% or more Ferrite and bainite: 0% or more, 15.0% or less in total, and Martensite: Contains 0% or more and 5.0% or less. In the aforementioned metallographic structure, the prior γ grain size is 5.0 μm or larger and 25.0 μm or smaller, and the number density of residual γ on the prior γ grain boundaries is 100 particles / mm². ２ The following: The tensile strength is 1310 MPa or higher, the uniform elongation is 4.0% or higher, and the ratio of the limit bending radius (R) to the plate thickness (R / t) in a 90° V-bend is 5.0 or lower. The plate thickness is 0.8 to 2.6 mm. A cold-rolled steel sheet characterized by the following features.

2. The aforementioned chemical composition, in mass%, Ti: 0.001% or more, less than 0.050% Nb: 0.001% or more, less than 0.050% V: 0.01% or more, 0.50% or less, Cu: 0.01% or more, 1.00% or less, Ni: 0.01% or more, 1.00% or less, Cr: 0.01% or more, 1.00% or less, Mo: 0.01% or more, 0.50% or less, B: 0.0001% or more, 0.0100% or less, Ca: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less, REM: 0.0005% or more, 0.0500% or less, and Bi: 0.0005% or more, 0.050% or less, Contains one or more selected from, The cold-rolled steel sheet according to feature 1.

3. The number density of retained austenite within 1.0 μm of the aforementioned prior γ grain boundary is 150 particles / mm². ２ The following is: The cold-rolled steel sheet according to claim 1 or 2, characterized by its features.

4. A hot-dip galvanized layer is formed on the aforementioned surface. The cold-rolled steel sheet according to claim 1 or 2, characterized by its features.

5. The aforementioned hot-dip galvanized layer is an alloyed hot-dip galvanized layer. The cold-rolled steel sheet according to feature 4.

6. A method for manufacturing a cold-rolled steel sheet according to Claim 1, In mass percent, C: greater than 0.140% and less than 0.400%, Si: 1.00% or less, Mn: greater than 1.30% and less than 4.00%, P: 0.100% or less, S: 0.010% or less, Al: 0.100% or less, N: 0.0100% or less, Ti: 0% or more and less than 0.050%, Nb: 0% or more and less than 0.050%, V: 0% or more and 0.50% or less, Cu: 0% or more and 1.00% or less, Ni: 0% or more and 1.00% or less, Cr: 0% or more and 1.00% or less, M A hot rolling process is performed to obtain a hot-rolled steel sheet by directly or after cooling a cast slab having a chemical composition consisting of o: 0% or more, 0.50% or less, B: 0% or more, 0.0100% or less, Ca: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.0500% or less, Bi: 0% or more, 0.050% or less, and the remainder being Fe and impurities. A winding process in which the hot-rolled steel sheet is wound at a temperature of 550°C or lower, The hot-rolled steel sheet after the winding process is descaled and then cold-rolled to produce a cold-rolled steel sheet in a cold-rolling process, The cold-rolled steel sheet after the cold-rolling process is heated from 700°C to a soaking temperature of 820°C to 880°C at an average heating rate of less than 10.0°C / second, and then annealed by soaking at the soaking temperature for 30 to 200 seconds, in an annealing process. The cold-rolled steel sheet after the annealing process is subjected to one or more bending-and-return deformations with a bending angle of 90 degrees or more, using a roll with a radius of 850 mm or less while applying a tension of 3.0 kN or more in a temperature range of 800°C or less and 700°C or more, and then cooled so that the average cooling rate from 700°C to 600°C and the average cooling rate from 450°C to 350°C are both 5.0°C / second or more, and then subjected to one or more bending-and-return deformations with a bending angle of 90 degrees or more, using a roll with a radius of 850 mm or less while applying a tension of 3.0 kN or more in a temperature range of 350°C or less and 50°C or more, and then cooled to a cooling stop temperature of 50°C or more and 250°C or less in a post-annealing cooling process. The cold-rolled steel sheet after the annealing and cooling process is tempered at a temperature of 200°C to 350°C for at least one second, in a tempering process. Equipped with, A method for manufacturing cold-rolled steel sheets, characterized by the following features.

7. The chemical composition of the aforementioned cast slab is, in mass%, Ti: 0.001% or more, less than 0.050% Nb: 0.001% or more, less than 0.050% V: 0.01% or more, 0.50% or less, Cu: 0.01% or more, 1.00% or less, Ni: 0.01% or more, 1.00% or less, Cr: 0.01% or more, 1.00% or less, Mo: 0.01% or more, 0.50% or less, B: 0.0001% or more, 0.0100% or less, Ca: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less, REM: 0.0005% or more, 0.0500% or less, and Bi: 0.0005% or more, 0.050% or less, Containing one or more of the following: A method for manufacturing a cold-rolled steel sheet according to claim 6, characterized in that

8. In the post-annealing cooling process, the average cooling rate from 350°C to the cooling stop temperature is set to 10°C / second. A method for manufacturing cold-rolled steel sheets according to claim 6 or 7, characterized by the present invention.

9. In the annealing and cooling process, the cold-rolled steel sheet is immersed in a plating bath when its temperature is above 425°C and below 600°C to form a hot-dip galvanized layer on its surface. A method for manufacturing cold-rolled steel sheets according to claim 6 or 7, characterized by the present invention.

10. In the annealing and cooling process, the steel sheet is immersed in a plating bath while its temperature is above 425°C and below 600°C to form a hot-dip galvanized layer on its surface, and the hot-dip galvanized layer is further alloyed. A method for manufacturing cold-rolled steel sheets according to claim 6 or 7, characterized by the present invention.

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