Cold-rolled steel sheet

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 10.0% of retained austenite; at least 80.0% of tempered martensite; a total of 0-15.0%, inclusive, of ferrite and bainite; and 0%-3.0%, inclusive, of martensite. The average crystal grain size of the first crystal grains counted from the surface in the sheet thickness direction is at most 20.0 μm when viewed from a cross section parallel to the rolling direction and parallel to the sheet thickness direction, and is at most 30.0 μm when viewed in plan view of the surface.

Description

Technical Field

[0001] The present invention relates to cold-rolled steel sheets. This application claims priority based on Japanese Patent Application No. 2022-018404 filed in Japan on February 9, 2022, and incorporates its content herein.

Background Art

[0002] Today, with the high degree of specialization in industrial technology fields, materials used in each technical field are required to have special and advanced performance. In particular, regarding steel sheets for automobiles, due to considerations for the global environment, in order to reduce the vehicle body weight 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 steel sheets for automobiles, especially for cold-rolled steel sheets used in vehicle body frame parts, high strength has been required, and furthermore, high formability for further application expansion has been required. In addition, since automobile parts are formed by pressing or the like, even if they have high strength, they are required to have excellent formability (for example, uniform elongation and bendability). In addition, with the increase in strength, the hydrogen embrittlement susceptibility increases, so it is also important to have excellent hydrogen embrittlement resistance characteristics. 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 in order 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, a hard second phase deteriorates the hole expansion property.

[0004] 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 properties, particularly in resistance to hydrogen embrittlement. [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 Initiative] [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 that has excellent formability, which is a problem with high-strength steel sheets, and also has 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 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 controlling the shape of the crystal grains on the outermost surface, high levels of strength, formability, and hydrogen embrittlement resistance can all be obtained.

[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: less than 1.00%, Mn: greater than 2.00% and less than 3.50%, 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, 0 0% or less, 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 The material has a chemical composition consisting of % or more, 0.0500% or less, Bi: 0% or more, 0.050% or less, and the remainder being Fe and impurities, and the microstructure at a 1 / 4 depth position, which is 1 / 4 of the thickness from the surface, contains, by volume fraction, retained austenite: more than 1.0% and less than 10.0%, 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 3.0% or less, and the average grain size of the first crystal grain counted from the surface in the thickness direction is 20.0 μm or less when viewed from a cross section parallel to the rolling direction and parallel to the thickness direction, and the average grain size when the surface is viewed from above is 30.0 μm or less. 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 lower, 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 The cold-rolled steel sheet described in [1] or [2] may have a hot-dip galvanized layer formed on its surface. 。 [4][3] The cold-rolled steel sheet described in [4][3] may have a hot-dip galvanized layer which may be an alloyed hot-dip galvanized layer. 。

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.

Mode for Carrying Out the Invention

[0013] The chemical composition, metallographic structure of the cold-rolled steel sheet according to an embodiment of the present invention (hereinafter sometimes simply referred to as the steel sheet according to the present embodiment), and the rolling, annealing conditions, etc. in the manufacturing method capable of efficiently, stably and economically manufacturing the steel sheet will be described in detail below. The steel sheet according to the present embodiment includes not only a cold-rolled steel sheet having no plating layer, but also a hot-dip galvanized steel sheet provided with a hot-dip galvanized layer on the surface of the base steel sheet, or an alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the base steel sheet. The main conditions shown below are also common to the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet.

[0014] <Chemical Composition> First, the chemical composition of the steel sheet according to the present embodiment will be described. “%” indicating the content of each element in the chemical composition means mass % unless otherwise specified.

[0015] [C: more than 0.140% and less than 0.400%] When the C content is 0.140% or less, it becomes difficult to obtain the above metallographic structure, and the target tensile strength cannot be achieved. Also, the bendability decreases. Therefore, the C content is more than 0.140%. The C content is preferably more than 0.160%, more preferably more than 0.180%. On the one hand, when the C content is 0.400% or more, the weldability deteriorates, and the bendability deteriorates. Also, the hydrogen embrittlement resistance deteriorates. Therefore, the C content should be less than 0.400%. The C content is preferably less than 0.350%, more preferably less than 0.300%.

[0016] [Si: less than 1.00%] When the Si content is 1.00% or more, the austenite transformation during heating in the annealing process becomes slow, and sufficient transformation from ferrite to austenite may not occur. In this case, ferrite remains excessively in the structure after annealing, making it impossible to achieve the target tensile strength and deteriorating the bendability. Also, when the Si content is 1.00% or more, the surface properties of the steel sheet deteriorate. Furthermore, the chemical conversion treatment property and plating property deteriorate significantly. Therefore, the Si content should be less than 1.00%. The lower limit of the Si content is not limited and may be 0%. However, Si forms internal oxides in the surface layer of the steel sheet and is an effective element for refining the metal structure in the surface layer due to the pinning effect of these internal oxides. Also, Si is a useful element for increasing the strength of the steel sheet by solid solution strengthening. Also, Si suppresses the formation of cementite, so it is an effective element for promoting the enrichment of C in austenite and generating retained austenite after annealing. To obtain these effects, the Si content is preferably 0.01% or more. The Si content is more preferably 0.05% or more, further preferably 0.10% or more, and still more preferably 0.50% or more.

[0017] [Mn: more than 2.00%, less than 3.50%] 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 2.00% or less, it becomes difficult to obtain the above-described metal structure. In this case, sufficient tensile strength cannot be obtained. Furthermore, Mn forms internal oxides, and the pinning effect of these internal oxides is an effective element for refining the metal structure of the surface layer. To obtain these effects, the Mn content should be greater than 2.00%. Preferably, the Mn content is greater than 2.20%, and more preferably greater than 2.50%. On the other hand, if the Mn content is 3.50% 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 3.50%. Preferably, the Mn content is less than 3.25%, 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 more preferable 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%. On the other hand, if these elements are included in excess, the recrystallization temperature rises, the metal structure of the cold-rolled steel sheet becomes non-uniform, and the bendability is impaired. Therefore, even when they 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%. On the other hand, 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 %". In this embodiment, the reference surface at the 1 / 4 depth position means the surface of the base steel sheet excluding the plating layer (hot-dip galvanized layer, alloyed hot-dip galvanized 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 metallic structure (microstructure) at a depth of 1 / 4 of the way from the surface (1 / 4 of the thickness from the surface) that includes retained austenite: more than 1.0% and less than 10.0%, 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 3.0% or less.

[0029] [Residual austenite: greater than 1.0%, less than 10.0%] Retained austenite improves ductility and contributes to improved uniform elongation through the TRIP effect. Therefore, the volume fraction of retained austenite should be greater than 1.0%. Preferably, the volume fraction of retained austenite is greater than 1.5%, and more preferably greater 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 10.0%. Preferably, the volume fraction of retained austenite is less than 8.0%, and more preferably less than 7.0%.

[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, 3.0% 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 if the volume percentage of martensite is high, the flexibility deteriorates. For this reason, the volume percentage of martensite should be 3.0% or less. Preferably, the volume percentage of martensite is 2.0% 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 (acceptable if within the range of 1 / 8 to 3 / 8 of the thickness from the surface) using a 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 phase 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] [The average grain size of the first crystal grain counted from the surface in the thickness direction, as viewed from a cross-section parallel to the thickness direction, is 20.0 μm or less, and the average grain size when viewing the surface in a planar view is 30.0 μm or less.] The flexibility of a steel sheet is affected by the occurrence of cracks in the outermost layer. Therefore, a fine, uniform structure in the surface layer improves flexibility. Further investigations by the inventors revealed that, in particular, making the first crystal grain counted from the surface in the thickness direction, i.e., the crystal grain in the outermost layer, finer improves the bendability. Therefore, the average grain size when viewed from a cross-section parallel to the thickness direction of the outermost layer of crystal grains is set to 20.0 μm or less, and the average grain size when the surface is viewed from above is set to 30.0 μm or less. The outermost crystal grains are not limited to any particular phase, but due to the effects of decarburization and other factors, they are often ferrite (including bainitic ferrite). To refine the crystal grains in the outermost layer, it is effective to suppress decarburization of the surface layer while promoting austenite transformation using the manufacturing method described later, and to form internal oxides of Si and utilize the pinning effect of these internal oxides. Here, "surface" refers to the surface of a cold-rolled steel sheet without a plating layer, and, in the case of a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, it refers to the surface of the base steel sheet excluding the plating layer (which can also be called the interface between the base steel sheet and the plating layer). Conventionally, the grain size of the surface layer, located several tens of micrometers from the surface, has been controlled. However, our research has shown that even if the crystal grains near the surface (not the outermost layer) are fine, the crystal grains in the outermost layer may coarseen, leading to a decrease in bendability and hydrogen embrittlement resistance. Therefore, controlling the grain size near the surface is not sufficient. For this reason, in the steel sheet according to this embodiment, the grain size of the crystal grains in the outermost layer is defined. Furthermore, the crystal grains of the outermost layer may coarse in both the average grain size when viewed from a cross-section parallel to the thickness direction of the plate and the average grain size when viewed from above the surface. However, in some cases, one coarseness may be significant while the other does not coarse to the same extent. Therefore, it is necessary to satisfy both the average grain size when viewed from a cross-section parallel to the thickness direction of the plate and the average grain size when viewed from above the surface simultaneously.

[0039] The average grain size of the outermost layer of crystal grains, as viewed from a cross-section parallel to the thickness direction of the plate, and the average grain size when viewing the surface from a planar perspective, are determined by the following method. The average grain size, as viewed from a cross-section parallel to the thickness direction, is measured by cutting out a cross-section parallel to both the rolling direction and the thickness direction (longitudinal section), polishing it, and measuring it using EBSD (Electron Back Scattering Diffraction) over three or more fields of view, covering an area of ​​100 μm in the thickness direction and 1000 μm in the longitudinal direction from the surface. Using TSL OIM Analysis, the software included with EBSD, orientation analysis is performed, and grain boundaries are defined as points where the orientation difference between adjacent measurement points is 5° or more, and the average diameter of the crystal grains in the outermost layer is determined. The grain size of the crystals viewed from the surface in a planar view is measured using EBSD over a range of 500 μm in the longitudinal direction and 500 μm in the width direction of the surface (at least one field of view), and the average diameter of the crystal grains is determined using TSL OIM Analysis in the same manner as described above. If the object to be measured is a plated steel sheet, the above measurement should be performed after removing the plating layer with hydrochloric acid or the like.

[0040] <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. 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.

[0041] 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.

[0042] <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.

[0043] The steel sheet according to this embodiment may have a hot-dip galvanized layer on its surface. Providing a plating layer on 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] <Manufacturing method> The steel sheet according to this embodiment can be manufactured by a manufacturing method including the following steps (I) to (VII). (I) Hot rolling process to obtain a hot-rolled steel sheet by heating a cast slab having a predetermined chemical composition and performing hot rolling in the final stage under conditions where the rolling temperature FT is 960°C or less and the reduction ratio is 18% or more. (II) Winding process of hot-rolled steel sheet at a temperature of [Si] × 200 + 500°C or lower. (III) Cold rolling process to descale the hot-rolled steel sheet after the winding process and cold-roll it with a cumulative reduction ratio of 60% or less to produce cold-rolled steel sheet. (IV) Bending and unbending process: The cold-rolled steel sheet is heated to a temperature range of 650°C to 800°C, such that the average heating rate up to 650°C is 3.0°C / second or more, and in this temperature range, while applying a tension of 3.0kN or more, one or more bending and unbending deformations are performed using a roll with a radius of 850mm or less, resulting in a bending angle of 90 degrees or more. (V) Annealing process: The cold-rolled steel sheet after the bending and unbending process is heated to an annealing temperature of 820°C or higher in a nitrogen-hydrogen mixed atmosphere containing 1.0% to 20% by volume of hydrogen with a dew point of -20°C or higher and 20°C or lower, and then uniformly heated at this annealing temperature. (VI) Post-annealing cooling process: Cooling the cold-rolled steel sheet after the annealing process to a temperature of 50°C to 250°C, such that the average cooling rate in the temperature ranges of 700°C to 600°C and 450°C to 350°C is 5°C / second or more. (VII) Tempering process: After the annealing and cooling process, the cold-rolled steel sheet is tempered at a temperature of 200°C to 350°C for at least 1 second.

[0045] In the steel sheet manufacturing method according to this embodiment, each step must simultaneously satisfy the above conditions in order to control the average grain size of the first crystal grain counted from the surface in the thickness direction, which has not been considered conventionally, along with the metal structure. For example, as will be described later, when the crystal grains are refined in the hot rolling step and carbides are finely dispersed in the winding step, and cold rolling is performed with a cumulative reduction ratio of 60% or less, decarburization in the surface layer is sufficiently suppressed in the annealing step. Furthermore, after decarburization is suppressed in this way, the formation of Si internal oxides in the surface layer by the annealing step suppresses the coarsening of the crystal grains in the outermost layer due to the pinning effect of the internal oxides. In other words, since each step affects the conditions of the other steps, it is important to set the conditions for the entire process.

[0046] The following describes each step.

[0047] [Hot rolling process] In the hot rolling process, a cast slab having the same chemical composition as the steel sheet according to the embodiment described above is heated and hot-rolled to produce a hot-rolled steel sheet. If the temperature of the cast slab is high, it may be subjected to hot rolling without first cooling it to near room temperature. The heating conditions for the slab in hot rolling are not limited, but it is preferable to heat it to 1100°C or higher. If the heating temperature is below 1100°C, the homogenization of the material tends to be insufficient. There is no upper limit, but from the viewpoint of economic rationality, it may be 1350°C or lower. The rolling temperature (FT) in the final finishing stage of hot rolling should be 960°C or lower, and the reduction ratio in the final stage should be 18% or higher. By setting the reduction ratio in the final stage and the reduction ratio as described above, the crystal grains can be refined, and carbides can be finely dispersed in the subsequent winding process. With such a structure, decarburization in the surface layer is suppressed in the subsequent annealing process. If the rolling temperature (FT) in the final stage exceeds 960°C, or if the reduction ratio in the final stage is less than 18%, sufficient effects cannot be obtained. Since a lower rolling temperature results in a higher rolling load, it is preferable that the rolling temperature in the final stage be 800°C or higher. Since a higher reduction ratio results in a higher rolling load, it is preferable that the reduction ratio in the final stage be 30% or less.

[0048] Since the chemical composition does not substantially change during the manufacturing process, the chemical composition of the cast slab should be the same as that of the target cold-rolled steel sheet. The method of 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.

[0049] [Winding process] In the winding process, the steel sheet (hot-rolled steel sheet) after the hot-rolling process is wound at a winding temperature CT that satisfies the condition CT ≤ [Si] × 200 + 500 (°C), where CT is the winding temperature and [Si] is the Si content by mass % of the steel sheet. There are no particular limitations on the cooling conditions to the winding temperature after the completion of hot rolling. Normally, it is believed that lowering the winding temperature increases the strength of the hot-rolled steel sheet and reduces its manufacturability. However, in the steel sheet manufacturing method according to this embodiment, the winding temperature is lowered. Specifically, the winding temperature is set to [Si] × 200 + 500 (°C) or lower. This suppresses the formation of a Si-depleted layer. When a Si-depleted layer is formed, internal Si oxides cannot be formed, and the pinning effect by internal oxides cannot be obtained, causing the crystal grains of the outermost layer to become coarser. Therefore, suppressing the formation of a Si-depleted layer is effective in suppressing the crystal grains of the outermost layer. Furthermore, by using the above winding temperature, the carbides can be precipitated in a uniform and finely dispersed state. If the winding temperature exceeds [Si] × 200 + 500 (°C), the above effects cannot be fully obtained.

[0050] [Cold rolling process] In the cold rolling process, the steel sheet (hot-rolled steel sheet) after the coiling process is descaled by pickling or other methods as necessary using known methods, and then cold-rolled to a reduction ratio of 60% or less (cumulative reduction ratio) to produce cold-rolled steel sheet. High reduction ratios during cold rolling promote recrystallization during annealing, making it difficult for γ transformation to occur in the surface layer during the annealing process. In this case, the surface grains become coarser due to annealing. Therefore, the reduction ratio during cold rolling should be kept below 60%. After descaling and before cold rolling, the surface of the steel sheet may be further ground to a thickness of approximately 0.1 μm to 5.0 μm using a brush or similar tool. Grinding has the effect of further refining the crystal grains of the outermost layer due to the grinding strain. Cold-rolled steel sheets after the cold-rolling process may be subjected to degreasing or other treatments according to known methods, if necessary.

[0051] [Bending-returning process] In the bending-and-returning process, the cold-rolled steel sheet is heated to a temperature range of 650°C to 800°C, such that the average heating rate up to 650°C is 3.0°C / second or more. In this temperature range, while applying a tension of 3.0kN or more, the sheet is subjected to one or more bending-and-returning deformations using a roll with a radius of 850mm or less, resulting in a bending angle of 90 degrees or more. For example, by using a roll with a radius of 850 mm or less (along the roll), bending the material at a bending angle of 90 degrees or more with the surface facing inward, and then bending it again at a bending angle of 90 degrees or more with the back surface facing inward, the desired bending-bending-unbending can be achieved. This bending-bending-unbending process applies strain to the surface layer during annealing heating, promoting austenite transformation and suppressing decarburization, thereby preventing the surface layer from becoming a single phase of ferrite that is prone to coarse grains. As a result, the surface layer and the crystal grains on the surface become finer, resulting in high bendability and resistance to hydrogen embrittlement. When the radius of the roll used for bending is large (large bending radius) or the bending angle is small, the strain introduced into the surface layer becomes insufficient, resulting in coarse grains in the surface layer and surface, and high bendability and hydrogen embrittlement resistance cannot be obtained. Furthermore, if the bending and unbending temperature is below 650°C, the yield strength of the steel is high, resulting in elastic deformation rather than plastic deformation, and thus the above effects cannot be fully obtained. On the other hand, if the temperature exceeds 800°C, the ferrite becomes coarser before bending and unbending can occur, so the refining effect cannot be obtained. If the average heating rate up to 650°C is slow, recrystallization will progress, making it difficult for the gamma transformation of the surface layer to occur during annealing, which can lead to coarse grain formation of the surface layer. Therefore, the average heating rate up to 650°C should be 3.0°C / second or higher. An average heating rate of 5.0°C / second or higher is preferable, and 7.0°C / second or higher is more preferable. The bending-rebound tension is preferably 6.0 kN or more, and preferably 8.0 kN or more, in order to reliably apply strain to the surface layer.

[0052] [Annealing process] In the annealing process, the steel sheet (cold-rolled steel sheet) after the bending and unbending process is heated to an annealing temperature of 820°C or higher in a nitrogen-hydrogen mixed atmosphere containing 1.0% to 20.0% by volume of hydrogen, with a dew point of -20°C or higher and 20°C or lower, and then soaked at this annealing temperature (soaking temperature). By using the above-described atmosphere during annealing heating, fine internal oxides can be formed, and the crystal grains in the surface layer can be refined. The atmosphere during soaking is not limited, but it may be the same as the atmosphere during heating. Furthermore, if the soaking temperature is too low, austenite single-phase annealing will not occur, the volume fraction of ferrite will increase, and the flexibility 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 flexibility, but if the soaking temperature is too high, the manufacturing cost will increase, so a soaking temperature of 900°C or lower is preferable. A soaking temperature of 880°C or lower is more preferable, and 870°C or lower is even more preferable. The soaking time is preferably 30 to 450 seconds. If the soaking time is less than 30 seconds, austenitization may not proceed sufficiently. Therefore, a soaking time of 30 seconds or more is preferable. On the other hand, if the soaking time exceeds 450 seconds, productivity will decrease, so a soaking time of 450 seconds or less is preferable.

[0053] [Post-annealing cooling process] In the post-annealing cooling process, the cold-rolled steel sheet after the annealing process is cooled to a temperature between 50°C and 250°C, such that the average cooling rate in both the ferrite transformation temperature range of 700°C to 600°C and the bainite transformation temperature range of 450°C to 350°C is 5°C / second or more, in order to obtain the above-mentioned microstructure. If the cooling rate is slow in the above temperature range, 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 from 700°C to 600°C and from 450°C to 350°C should be 5°C / second or higher. An average cooling rate of 10°C / second or higher is preferable, and 20°C / second or higher is even more preferable. The cooling stop temperature and holding temperature should be 50°C or higher and 250°C or lower. If the cooling stop temperature is too high, the amount of (untempered) martensite will increase during the subsequent tempering process, degrading the flexibility and hydrogen embrittlement resistance. Therefore, the cooling stop temperature should be 250°C or lower. Preferably, the cooling stop temperature should be 220°C or lower, and more preferably 200°C or lower. On the other hand, if the cooling stop temperature is too low, the residual austenite fraction decreases, and the desired uniform elongation cannot be obtained. Therefore, the cooling stop temperature should be 50°C or higher. Preferably, the cooling stop temperature should be 75°C or higher, and more preferably 100°C or higher.

[0054] [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 450°C and below 600°C, following the hot-dip galvanizing process, to obtain alloyed hot-dip galvanizing.

[0055] [Tempering process] After the annealing and cooling process, the cold-rolled steel sheet is cooled to a temperature between 50°C and 250°C, during which the un-transformed austenite transforms into martensite. In the tempering process, the cold-rolled steel sheet is tempered at a temperature of 200°C to 350°C for at least 1 second to obtain a structure mainly 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.

[0056] [Skin Pass 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 0.05% or higher, more preferably 0.10% 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]

[0057] 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, and then coiled and cooled to room temperature. The hot-rolling conditions and coiling temperatures are as shown in Tables 2A and 2B. Subsequently, the scale was removed by pickling, and the material was cold-rolled to 1.4 mm. Then, it was heated in a temperature range of 650°C to 800°C, such that the average heating rate up to 650°C was as shown in Tables 2A and 2B. In this temperature range, the material was bent at a bending angle of 90 degrees or more along rolls of the radii shown in Tables 2A and 2B, with the surface facing inward, and then bent again at a bending angle of 90 degrees or more with the back facing inward, thereby performing a bend-and-rebend procedure. Subsequently, without cooling, the material was heated to the annealing temperatures shown in Table 2A and Table 2B in a nitrogen-hydrogen mixed atmosphere containing 1.0% to 20% by volume of hydrogen, with a dew point between -20°C and 20°C, and annealed for 120 seconds at the annealing temperature. After annealing, the material was cooled to a cooling stop temperature of 50°C to 250°C, such that the average cooling rate in the 700°C to 600°C temperature range and the 450°C to 350°C temperature range was 20°C / second or more. Then, a heat treatment was performed to temper the material at 200°C to 350°C for 1 to 500 seconds. In some cases, hot-dip galvanizing and alloying were performed during cooling after annealing. In Tables 3A and 3B, "CR" indicates cold-rolled steel sheet without galvanizing, "GI" indicates hot-dip galvanized steel sheet, and "GA" indicates alloyed hot-dip galvanized steel sheet. For alloyed hot-dip galvanized steel sheets, the galvanizing rate was 35-65 g / m² at temperatures between 450°C and 600°C. 2 After applying a moderate amount of hot-dip galvanizing, the material was further alloyed at a temperature between 450°C and 600°C.

[0058] [Table 1]

[0059] [Table 2A]

[0060] [Table 2B]

[0061] From the obtained cold-rolled steel sheet, specimens for SEM observation were taken as described above. After polishing the longitudinal section parallel to the rolling direction, the metallographic structure at a depth of 1 / 4 was observed, and the volume fraction of each structure was measured by image processing. In addition, specimens for X-ray diffraction were taken, and the volume fraction of retained austenite was measured by X-ray diffraction on a surface that had been chemically polished to a depth of 1 / 4 from the surface as described above. As a result, the volume fractions of ferrite, bainite, martensite, tempered martensite, pearlite, and retained austenite were obtained. Furthermore, using the methods described above, the average grain size of the outermost crystal grains when viewed from a cross section (L-section) parallel to both the rolling direction and the thickness direction, and the average grain size when viewing the surface from a planar perspective, were determined using EBSD and the accompanying software, TSL OIM Analysis. The results are shown in Tables 3A and 3B.

[0062] [Table 3A]

[0063] [Table 3B]

[0064] Furthermore, the tensile strength (TS), uniform elongation (uEl), (R / t), and hydrogen embrittlement resistance were evaluated according to the following procedure.

[0065] Tensile strength (TS) and uniform elongation (uEl) were determined by taking JIS No. 5 tensile test specimens perpendicular to the rolling direction from cold-rolled steel sheets and performing tensile tests in accordance with JIS Z 2241:2011.

[0066] 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 (t=1.4 mm).

[0067] 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.

[0068] The results for each mechanical property are shown in Table 4.

[0069] [Table 4]

[0070] As can be seen from Tables 1-4, all of the steels of the present invention (test numbers 3, 10, 17-35) had a TS of 1310 MPa or higher, a uEl of 4.0% or higher, and an (R / t) of 5.0 or lower, and also exhibited good hydrogen embrittlement resistance. In contrast, in the test numbers (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 and the average grain size of the crystal grains on the outermost layer 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. [Industrial applicability]

[0071] According to the present invention, it is possible to provide a cold-rolled steel sheet that has excellent formability and excellent resistance to hydrogen embrittlement. When used as a steel sheet for automobiles, this steel sheet contributes to the weight reduction of the vehicle body, and therefore has high industrial applicability.

Claims

1. In mass percent, C: Over 0.140%, less than 0.400% Si: Less than 1.00%, Mn: greater than 2.00%, less than 3.50% 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 10.0% 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 3.0% or less. The average grain size of the first crystal grain counted from the surface in the thickness direction is 20.0 μm or less when viewed from a cross section parallel to the rolling direction and also parallel to the thickness direction, and the average grain size when the surface is viewed from above is 30.0 μm or less. 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 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, Contains one or more selected from, The cold-rolled steel sheet according to feature 1.

3. The cold-rolled steel sheet according to claim 1 or 2, characterized in that a hot-dip galvanized layer is formed on the surface.

4. The cold-rolled steel sheet according to claim 3, characterized in that the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

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