Cold-rolled steel sheet and its manufacturing method

Abstract

This cold-rolled steel sheet has a specified chemical composition, in which the metal structure of a t / 4 part, which is a part located at a depth of 1 / 4 of the thickness t of the cold-rolled steel sheet from the surface of the cold-rolled steel sheet when observed in the thickness direction, contains, in % by volume, retained austenite in an amount of 2.5% to 10.0% inclusive, tempered martensite in an amount of 80.0% to 97.5% inclusive, ferrite and bainite in a total amount of 0.0% to 15.0% inclusive, and martensite in an amount of 0.0% to 3.0% inclusive, the amount of Si contained in the form of a solid solution in a surface part, which is a part located at a depth of 25 μm from the surface of the cold-rolled steel sheet when observed in the thickness direction, is 0.30% by mass to 1.50% by mass inclusive, and the volume ratio of ferrite is 0.0% to 20.0% inclusive and the density of ferrite crystal grains each having a grain diameter of 15 μm or more is 0 grain / mm2 to 3000 grains / mm2 inclusive in the metal structure.

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. 2021-038716, filed on March 10, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] With the highly specialized nature of industrial technology today, materials used in each field are required to have specialized and advanced performance. In particular, with regard to automotive steel sheets, there has been a significant increase in demand for thin, high-tensile cold-rolled steel sheets with excellent formability to reduce vehicle weight and improve fuel efficiency, due to concerns about the global environment. Cold-rolled steel sheets used in automotive body frame components, in particular, are now required to have high strength and, as their applications expand, high formability. Examples of the properties required for automotive steel sheets include a tensile strength (TS) of 1310 MPa or greater and a uniform elongation of 5.0% or greater. Depending on the processing method and the type of part to which they are applied, a critical bending radius R (R / t) normalized by the sheet thickness t in a 90° V-bend of 5.0 or less is also required, as well as excellent hydrogen embrittlement resistance.

[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 with a structure containing ferrite, it is necessary to harden the second phase, but a hard second phase deteriorates bendability.

[0004] On the other hand, as a technology for improving the bendability and hydrogen embrittlement resistance of high-strength steel sheets, steel sheets having tempered martensite as the main phase have been proposed (see, for example, Patent Documents 1 and 2). Patent Documents 1 and 2 disclose that by making the microstructure a single-phase tempered martensite structure, the steel sheets have excellent bendability and also have excellent hydrogen embrittlement resistance because the structure has finely dispersed carbides, which act as hydrogen trapping sites.

[0005] Furthermore, Patent Document 3 proposes a steel sheet that utilizes the TRIP effect of retained austenite as a technology for achieving both high strength and high formability. [Prior art documents] [Patent documents]

[0006] [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 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the steel sheet of Patent Document 1 has a low tensile strength of less than 1310 MPa. Therefore, in order to achieve higher strength, it is necessary to further improve the workability, bendability, and hydrogen embrittlement resistance, which are deteriorated as a result of the increased strength. Furthermore, although the steel plate of Patent Document 2 can achieve a high strength of 1310 MPa or more, it is cooled to near room temperature during quenching, which results in a problem that the volume fraction of retained austenite is small and high uniform elongation cannot be obtained. Furthermore, the steel sheet of Patent Document 3 has difficulty in achieving a high strength of 1310 MPa or more due to the presence of a ferrite phase, and has poor bendability due to differences in strength within the structure.

[0008] That is, the steel sheets proposed so far have not been sufficient in terms of high strength, uniform elongation, bendability, and hydrogen embrittlement resistance to meet the increasingly sophisticated demands of recent years. Therefore, an object of the present invention is to provide a cold-rolled steel sheet having high strength and excellent uniform elongation, bendability, and hydrogen embrittlement resistance, and a method for manufacturing the same. [Means for solving the problem]

[0009] The present inventors have investigated the effects of chemical composition, metal structure, and manufacturing conditions on the strength, uniform elongation, bendability, and hydrogen embrittlement resistance of cold-rolled steel sheets. As a result, they discovered that by making the metal structure inside the steel plate (for example, at a position 1 / 4 of the plate thickness from the surface) a structure mainly composed of tempered martensite containing a specified amount or more of retained austenite, and then controlling the amount of solute Si in the surface layer and the state of ferrite, it is possible to simultaneously improve strength, uniform elongation, bendability, and hydrogen embrittlement resistance. Furthermore, as a result of investigations carried out by the present inventors, it has been found that, in order to control the surface layer portion, it is particularly important to refine the grain size and carbide by applying shear force during hot rolling, to suppress the generation of a Si-depleted layer due to internal oxidation by controlling the coiling temperature, and to suppress the distribution of Si during annealing by controlling the cold rolling and annealing conditions.

[0010] The present invention has been made based on the above findings. The gist of the present invention is as follows. [1] A cold-rolled steel sheet according to one embodiment of the present invention has, by mass%, C: 0.140% or more and 0.400% or less, Si: 0.35% or more and 1.50% or less, Mn: 1.30% or more and 3.50% or less, P: 0% or more and 0.100% or less, S: 0% or more and 0.010% or less, Al: 0% or more and 0.100% or less, N: 0% or more and 0.0100% or less, and Ti: 0% or more and 0.050% or less. , Nb: 0% or more, 0.050% 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, M o: 0% or more, 0.50% or less, B: 0% or more, 0.0100% or less, Ca: 0% or more, 0.010% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.050% and Bi: 0% or more and 0.050% or less, with the balance being Fe and impurities, and the metallographic structure of a t / 4 portion, which is located at 1 / 4 of the sheet thickness t from the surface in the sheet thickness direction, contains, by volume fraction, retained austenite: 2.5% or more and 10.0% or less, tempered martensite: 80.0% or more and 97.5% or less, ferrite and bainite: 0.0% or more and 15.0% or less in total, and martensite: 0.0% or more and 3.0% or less, and in a surface layer portion located 25 μm from the surface in the sheet thickness direction, the amount of solute Si is, by mass%, 0.30% or more and 1.50% or less, the volume fraction of ferrite in the metallographic structure is 0.0% or more and 20.0% or less, and the density of ferrite crystal grains having a grain size of 15 μm or more is 0 grains / mm 2 More than 3000 pieces / mm 2 The following is the result. [2] The cold-rolled steel sheet according to the above item [1], wherein the chemical composition is, in mass%, Ti: 0.001% or more and 0.050% or less, Nb: 0.001% or more and 0.050% or less, 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% and Bi: 0.0005% or more and 0.050% or less. [3] In the cold-rolled steel sheet according to the above [1] or [2], the ratio of the amount of solute Si in the surface layer portion to the amount of solute Si in the t / 4 portion may be 0.85 to 1.10. [4] The cold-rolled steel sheet according to any one of [1] to [3] above may have a tensile strength of 1310 MPa or more, a uniform elongation of 5.0% or more, and an R / t value, which is the value obtained by dividing the limiting bending radius R in a 90° V-bend by the sheet thickness t, of 5.0 or less. [5] The cold-rolled steel sheet according to the above item [4] may have a tensile strength of 1400 MPa or more. [6] The cold-rolled steel sheet according to any one of the above [1] to [5] may have a hot-dip galvanized layer formed on the surface thereof. [7] In the cold-rolled steel sheet according to the above item [6], the hot-dip galvanized layer may be a galvannealed layer. [8] A method for producing a cold-rolled steel sheet according to another aspect of the present invention comprises: [1] A method for producing a cold-rolled steel sheet according to the present invention,In mass%, C: 0.140% or more, 0.400% or less, Si: 0.35% or more, 1.50% or less, Mn: 1.30% or more, 3.50% or less, P: 0% or more, 0.100% or less, S: 0% or more, 0.010% or less, Al: 0% or more, 0.100% or less, N: 0% or more, 0.0100% or less, Ti: 0% or more, 0.050% or less, Nb: 0% or more, 0.050% 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 A cast slab having a chemical composition containing: Mo: 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, 0.010% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more, 0.050% or less, and Bi: 0% or more, 0.050% or less, with the balance being Fe and impurities, is heated as necessary, and then subjected to a rolling process in which the rolling temperature FT in the final stage is 960°C or less, the rolling reduction in the final stage is 10% or more, and the final rolling temperature FT ... and the final rolling reduction in the final stage is 10% or more. a hot rolling step of obtaining a hot rolled steel sheet by hot rolling under conditions in which the coefficient of friction μ at each stage is 0.15 or more; a coiling step of cooling the hot rolled steel sheet to a coiling temperature of 560°C or more and 650°C or less and coiling it at the coiling temperature; a cold rolling step of cold rolling the hot rolled steel sheet after the coiling step under conditions in which the cumulative reduction is 60% or less to obtain a cold rolled steel sheet; and a cold rolling step of heating the cold rolled steel sheet to a soaking temperature of 820°C or more so that the average heating rate up to 750°C is 3.0°C / second or more and a post-annealing cooling process in which the cold-rolled steel sheet after the annealing process is cooled to 50°C or more and 250°C or less so that the average cooling rates in the temperature ranges of 700 to 600°C and 450 to 350°C are both 5.0°C / second or more; and a tempering process in which the cold-rolled steel sheet after the post-annealing cooling process is held at 200°C or more and 350°C or less for 1 second or more, and the temperature of the hot-rolled steel sheet after the hot-rolling process is allowed to reach 500°C or less within 10 hours from the completion of the hot-rolling process. [9] The method for producing a cold-rolled steel sheet according to the above item [8], wherein the chemical composition of the cast slab is, in mass%, Ti: 0.001% or more and 0.050% or less, Nb: 0.001% or more and 0.050% or less, 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, M It may contain one or more selected from the group consisting of O: 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 0.010% or less, Mg: 0.0001% or more and 0.0100% or less, REM: 0.0005% or more and 0.050% or less, and Bi: 0.0005% or more and 0.050% or less.

[10] In the method for producing a cold-rolled steel sheet according to the above [8] or [9], in the post-annealing cooling step, the cold-rolled steel sheet may be immersed in a coating bath at a temperature of more than 425°C and less than 600°C to form a hot-dip galvanized layer on the surface.

[11] In the method for producing a cold-rolled steel sheet according to the above item

[10] , an alloying treatment for alloying the hot-dip galvanized layer may be carried out in the post-annealing cooling step. [Effects of the Invention]

[0011] According to the above-described aspects of the present invention, it is possible to provide a cold-rolled steel sheet having high strength and excellent uniform elongation, bendability, and hydrogen embrittlement resistance, and a method for manufacturing the same. DETAILED DESCRIPTION OF THE INVENTION

[0012] A cold-rolled steel sheet according to one embodiment of the present invention (a cold-rolled steel sheet according to this embodiment) and a method for producing the same will be described. The cold-rolled steel sheet according to this embodiment (a) has a chemical composition described below, (b) a metallographic structure of a t / 4 portion, which is located at 1 / 4 of the sheet thickness (t) from the surface in the sheet thickness direction, contains, by volume fraction, retained austenite: 2.5% or more and 10.0% or less, tempered martensite: 80.0% or more and 97.5% or less, ferrite and bainite: 0.0% or more and 15.0% or less in total, and martensite: 0.0% or more and 3.0% or less, (c) in a surface layer portion located 25 μm from the surface in the sheet thickness direction, the amount of solute Si is, by mass%, 0.30% or more and 1.50% or less, the volume fraction of ferrite in the metallographic structure is 0.0% or more and 20.0% or less, and the density of ferrite crystal grains having a grain size of 15 μm or more is 0 grains / mm 2 More than 3000 pieces / mm 2 The following is the result. The cold-rolled steel sheet according to this embodiment includes not only a cold-rolled steel sheet having no plating layer on its surface, but also a hot-dip galvanized steel sheet having a hot-dip galvanized layer on its surface, or a galvannealed steel sheet having a galvannealed layer on its surface, and these main conditions are common to both the hot-dip galvanized steel sheet and the galvannealed steel sheet. However, in the case of plated steel sheets, the surface that serves as the reference for indicating the position that defines the metal structure means the surface of the base steel sheet excluding the plating. Each of these will be explained below.

[0013] <Chemical composition> The chemical composition of the cold-rolled steel sheet according to this embodiment will be described below. Hereinafter, "%" indicating the content of each element in the chemical composition means mass % unless otherwise specified.

[0014] C: 0.140% or more, 0.400% or less If the C content is less than 0.140%, it becomes difficult to obtain the above-mentioned metal structure, and the desired tensile strength cannot be achieved. In addition, bendability decreases. Therefore, the C content is set to 0.140% or more. The C content is preferably more than 0.140%, more preferably 0.160% or more, and even more preferably 0.180% or more. On the other hand, if the C content exceeds 0.400%, the weldability and bendability deteriorate. Furthermore, hydrogen embrittlement resistance also deteriorates. Therefore, the C content is set to 0.400% or less. The C content is preferably less than 0.400%, more preferably 0.350% or less, and even more preferably 0.300% or less.

[0015] Si: 0.35% or more, 1.50% or less Si is a useful element for increasing the strength of steel sheets through solid solution strengthening. Furthermore, Si inhibits the formation of cementite, promoting the concentration of C in austenite and effectively forming retained austenite after annealing. If the Si content is less than 0.35%, it becomes difficult to obtain the above-mentioned effects, making it difficult to achieve the target uniform elongation, and hydrogen embrittlement resistance deteriorates. Therefore, the Si content is set to 0.35% or more. The Si content is preferably more than 0.35%, more preferably 0.40% or more, and even more preferably 0.45% or more. On the other hand, if the Si content exceeds 1.50%, the austenite transformation during heating in the annealing process may be slowed, and the transformation from ferrite to austenite may not occur sufficiently. In this case, excessive ferrite remains in the structure after annealing, making it impossible to achieve the target tensile strength and deteriorating bendability. Furthermore, if the Si content exceeds 1.50%, the surface quality of the steel sheet deteriorates. Furthermore, chemical conversion treatability and platability are significantly deteriorated. Therefore, the Si content is set to 1.50% or less. The Si content is preferably less than 1.50%, more preferably 1.25% or less, even more preferably 1.00% or less, and even more preferably 0.90% or less or 0.85% or less. In particular, a Si content of 1.00% or less improves plating adhesion.

[0016] Mn: 1.30% or more, 3.50% or less Mn has the effect of improving the hardenability of steel and is an effective element for obtaining the desired metal structure described below. If the Mn content is less than 1.30%, it becomes difficult to obtain the desired metal structure. In this case, sufficient tensile strength cannot be obtained. Therefore, the Mn content is set to 1.30% or more. The Mn content is preferably more than 1.30%, more preferably 1.50% or more, and even more preferably 2.00% or more. On the other hand, if the Mn content exceeds 3.50%, not only will the effect of improving hardenability be diminished due to Mn segregation, but the material cost will also increase. Therefore, the Mn content is set to 3.50% or less. The Mn content is preferably less than 3.50%, more preferably 3.25% or less, and even more preferably 3.00% or less.

[0017] P: 0% or more, 0.100% or less P is an element contained in steel as an impurity, and segregates at grain boundaries to embrittle the steel. Therefore, the lower the P content, the better; even 0% is acceptable. However, taking into consideration the time and cost required to remove P, the P content is set to 0.100% or less. The P content is preferably 0.020% or less, and more preferably 0.015% or less. Taking into consideration the cost of refining, etc., the P content may be set to 0.005% or more.

[0018] S: 0% or more, 0.010% or less S is an element contained in steel as an impurity, and forms sulfide-based inclusions that deteriorate bendability. Therefore, the lower the S content, the better; even 0% is acceptable. However, taking into consideration the time and cost required to remove S, the S content is set to 0.010% or less. The S content is preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.001% or less. Taking into consideration the cost of refining, etc., the P content may be set to 0.0001% or more.

[0019] Al: 0% or more, 0.100% or less Al is an element that has the effect of deoxidizing molten steel. When Al is contained for the purpose of deoxidation, the Al content is preferably 0.005% or more, more preferably 0.010% or more, in order to ensure deoxidation. Furthermore, like Si, Al has the effect of increasing the stability of austenite and is an effective element for obtaining the above-mentioned metal structure, so Al may be contained. When contained, the Al content may be, for example, 0.010% or more. On the other hand, if the Al content is too high, not only will surface defects due to alumina be more likely to occur, but the transformation point will also rise significantly, resulting in a high volume fraction of ferrite. In this case, it will be difficult to obtain the above-mentioned metal structure, and sufficient tensile strength will not be obtained. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less. In the cold-rolled steel sheet according to this embodiment, Si, which has a deoxidizing effect similar to Al, is contained, so Al does not necessarily need to be contained, and the Al content may be 0%.

[0020] N: 0% or more, 0.0100% or less N is an element that can be contained in steel as an impurity and generates coarse precipitates that deteriorate bendability. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0060% or less, and more preferably 0.0050% or less. The lower the N content, the better, and it may even be 0%. In consideration of the cost of refining, etc., the N content may be set to 0.0010% or more, or 0.0020% or more.

[0021] The cold-rolled steel sheet according to this embodiment may contain the above elements with the balance being Fe and impurities, or may further contain one or more optional elements that affect strength and bendability, as listed below. However, the optional elements do not necessarily need to be contained, and the lower limit of each is 0%.

[0022] Ti: 0% or more, 0.050% or less Nb: 0% or more, 0.050% or less V: 0% or more, 0.50% or less Cu: 0% or more, 1.00% or less Ti, Nb, V, and Cu are elements that have the effect of improving the strength of steel sheet through precipitation hardening. Therefore, these elements may be contained. To fully obtain the above effects, the Ti content and Nb content are preferably 0.001% or more, and the V content and Cu content are preferably 0.01% or more. More preferably, the Ti content and Nb content are 0.005% or more, and the V content and Cu content are more preferably 0.05% or more. It is not essential to obtain the above effects. Therefore, there is no need to particularly limit the lower limits of the Ti content, Nb content, V content, and Cu content, and the lower limits for these contents are 0%. On the other hand, if these elements are contained in excess, the recrystallization temperature rises, the metal structure of the cold-rolled steel sheet becomes non-uniform, and bendability is impaired. Therefore, when these elements are contained, the Ti content is 0.050% or less, the Nb content is 0.050% or less, the V content is 0.50% or less, and the Cu content is 1.00% or less. The Ti content is preferably less than 0.050%, more preferably 0.030% or less, and even more preferably 0.020% or less. The Nb content is preferably less than 0.050%, more preferably 0.030% or less, and even more preferably 0.020% or less. The V content is preferably 0.30% or less. The Cu content is preferably 0.50% or less.

[0023] 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 hardenability and contribute to increasing the strength of the steel sheet, and are effective elements for obtaining the above-mentioned metal structure. Therefore, these elements may be contained. To fully obtain the above-mentioned effects, it is preferable that the Ni content, Cr content, and Mo content are each 0.01% or more, and / or the B content is 0.0001% or more. More preferably, the Ni content, Cr content, and Mo content are each 0.05% or more, and the B content is 0.0010% or more. It is not essential to obtain the above-mentioned effects. Therefore, there is no need to particularly limit the lower limits of the Ni content, Cr content, Mo content, and B content, and the lower limits for each are 0%. On the other hand, if these elements are contained in excess, the effects of the above actions will saturate and it will be uneconomical. Therefore, if these elements are contained, the Ni content and Cr content should be 1.00% or less, the Mo content should be 0.50% or less, and the B content should be 0.0100% or less. The Ni content and Cr content are preferably 0.50% or less, the Mo content is preferably 0.20% or less, and the B content is preferably 0.0030% or less.

[0024] Ca: 0% or more, 0.010% or less Mg: 0% or more, 0.0100% or less REM: 0% or more, 0.050% or less Bi: 0% or more, 0.050% or less Ca, Mg, and REM are elements that improve the strength and bendability of steel sheets by adjusting the shape of inclusions. Bi is an element that improves the strength and bendability by refining the solidification structure. Therefore, these elements may be contained. To fully obtain the above effects, the Ca content and Mg content are preferably 0.00010% or more, and the REM content and Bi content are preferably 0.0005% or more. More preferably, the Ca content and Mg content are 0.00080% or more, and the REM content and Bi content are 0.0007% or more. It is not essential to obtain the above effects. Therefore, there is no need to particularly limit the lower limits of the Ca content, Mg content, Bi content, and REM content, and the lower limits are 0%. On the other hand, if these elements are contained in excess, the effects of the above actions saturate and it becomes uneconomical. Therefore, if these elements are contained, the Ca content should be 0.010% or less, the Mg content should be 0.0100% or less, the REM content should be 0.050% or less, and the Bi content should be 0.050% or less. Preferably, the Ca content should be 0.008% or less or 0.002% or less, the Mg content should be 0.0020% or less, the REM content should be 0.010% or less or 0.002% or less, and the Bi content should be 0.010% or less. REM refers to rare earth elements, a collective term for Sc, Y, and lanthanides, a total of 17 elements, and the REM content is the total content of these elements.

[0025] <Metal structure at 1 / 4 of the plate thickness (t / 4 part) from the surface in the plate thickness direction> In the description of the metal structure of the cold-rolled steel sheet according to this embodiment, the structure fraction is expressed as a volume fraction. Therefore, unless otherwise specified, "%" represents "volume %."

[0026] [Residual austenite: 2.5% or more, 10.0% or less] Retained austenite improves the ductility of steel sheet through the TRIP effect, contributing to improved uniform elongation. Therefore, the volume fraction of retained austenite is set to 2.5% or more. The volume fraction of retained austenite is preferably more than 2.5%, more preferably 3.5% or more, and even more preferably 4.5% or more. 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 a large grain size becomes coarse and hard martensite after deformation. In this case, cracks are more likely to start, and bendability deteriorates. For this reason, the volume fraction of retained austenite is set to 10.0% or less. The volume fraction of retained austenite is preferably less than 10.0%, more preferably 8.0% or less, and even more preferably 7.0% or less.

[0027] [Tempered martensite: 80.0% or more, 97.5% or less] Tempered martensite, like martensite (so-called fresh martensite), is a collection of lath-shaped crystal grains. However, unlike martensite, it is a hard structure containing fine iron-based carbides inside due to tempering. Tempered martensite is obtained by tempering martensite formed by cooling after annealing, etc., through heat treatment, etc. Tempered martensite is a structure that is less brittle and has ductility compared to martensite. In the cold-rolled 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 hydrogen embrittlement resistance. The volume fraction of tempered martensite is preferably 85.0% or more. In order to set the volume fraction of retained austenite to 2.5% or more, the volume fraction of tempered martensite is 97.5% or less.

[0028] [Ferrite and bainite: 0.0% or more and 15.0% or less in total] Ferrite is a soft phase obtained by annealing in the two-phase region or by slow cooling after annealing. Ferrite improves the ductility of steel sheets when mixed with hard phases such as martensite. However, to achieve high strength of 1310 MPa or more, the volume fraction of ferrite must be limited. Bainite is a phase obtained by holding the temperature between 350°C and 450°C for a certain period of time after annealing. Bainite is softer than martensite and has the effect of improving ductility, but in order to achieve a high strength of 1310 MPa or more, the volume fraction must be limited, just like ferrite above. Therefore, the total volume fraction of ferrite and bainite is set to 15.0% or less, preferably 10.0% or less. Since ferrite and bainite do not necessarily need to be contained, the lower limit for each is 0.0%. Furthermore, since ferrite is softer than bainite, when the total volume fraction of ferrite and bainite is 15.0% or less, the volume fraction of ferrite is preferably less than 10.0% in order to achieve a high strength of 1310 MPa or more.

[0029] [Martensite: 0.0% or more, 3.0% or less] Martensite (fresh martensite) is a collection of lath-shaped crystal grains that can be generated by transformation from austenite during final cooling after the tempering process. Martensite is hard and brittle, and easily becomes the starting point for cracks during deformation, so if the volume fraction of martensite is high, bendability deteriorates. For this reason, the volume fraction of martensite is set to 3.0% or less. The volume fraction of martensite is preferably 2.0% or less, and more preferably 1.0% or less. Since martensite does not necessarily need to be present, the lower limit of the volume fraction of martensite is 0.0%.

[0030] In addition to the above, the metal structure at a position (t / 4 portion) of 1 / 4 of the plate thickness t from the surface in the plate thickness direction may contain pearlite as the remaining structure. However, pearlite is a structure that contains cementite within the structure, and consumes C (carbon) in the steel that contributes to improving strength. If the pearlite volume fraction is 5.0% or less, the strength of the steel plate is increased. Therefore, it is preferable that the pearlite volume fraction be 5.0% or less. The pearlite volume fraction is more preferably 3.0% or less, and even more preferably 1.0% or less.

[0031] The volume fraction in the structure of the t / 4 portion of the cold-rolled steel sheet according to this embodiment is measured as follows. Specifically, the volume fractions of ferrite, bainite, martensite, tempered martensite, and pearlite were determined by taking a test specimen from a position at any point in the rolling direction of the steel sheet and at the center in the width direction. A longitudinal cross section parallel to the rolling direction (i.e., a cross section parallel to both the rolling direction and the thickness direction) was polished, and the metallographic structure revealed by nital etching at a position 1 / 4 of the thickness t from the surface in the thickness direction was observed using an SEM. SEM observations were performed at a magnification of 3000x, with five fields of view (30 μm in the thickness direction and 50 μm in the rolling direction) observed, with the center positioned 1 / 4 of the thickness t from the surface in the thickness direction. The area fractions of each structure were measured from the observed images, and their average values ​​were calculated. Since there was no change in the structure in the direction perpendicular to the rolling direction (the width direction of the steel sheet), and the area fractions of the longitudinal cross section parallel to the rolling direction were equal to the volume fractions, the area fractions obtained by the structure observation were used as the respective volume fractions.

[0032] When measuring the area ratio of each structure, the region where the substructure is not revealed and has low brightness is defined as ferrite. The region with a layered structure of ferrite and cementite is defined as pearlite. The region where the substructure is not revealed and has high brightness is defined as martensite or retained austenite. The region where the substructure is revealed is defined as tempered martensite or bainite.

[0033] Bainite and tempered martensite can be further distinguished by careful observation of intragranular carbides. Specifically, tempered martensite is composed of martensite laths and cementite formed within the laths. Since there are two or more types of crystal orientation relationships between martensite laths and cementite, the cementite that constitutes tempered martensite has multiple variants. On the other hand, bainite is classified into upper bainite and lower bainite. Upper bainite is easily distinguishable from tempered martensite because it is composed of lath-shaped bainitic ferrite and cementite formed at the lath interfaces. Lower bainite is composed of lath-shaped bainitic ferrite and cementite formed within the laths. Unlike tempered martensite, the crystal orientation relationship between bainitic ferrite and cementite is one type, and the cementite that constitutes 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, so the volume fraction of martensite is calculated by subtracting the volume fraction of retained austenite, calculated by the method described below, from the volume fraction of the structure determined to be martensite or retained austenite.

[0034] The volume fraction of retained austenite is determined by taking a test piece from an arbitrary position in the rolling direction of the steel plate and at the center position in the width direction, chemically polishing the rolled surface from the surface of the steel plate to a position 1 / 4 of the plate thickness, and quantifying the volume fraction of retained austenite from the (200) and (210) surface integral intensities of ferrite and the (200), (220), and (311) surface integral intensities of austenite using MoKα radiation.

[0035] <Metal structure of the surface layer: The volume fraction of ferrite is 0.0% or more and 20.0% or less, and the density of ferrite crystal grains with a grain size of 15 μm or more is 0 grains / mm 2 More than 3000 pieces / mm 2 Below> In the surface layer located 25 μm from the surface in the thickness direction, the volume fraction of ferrite is more than 20.0%, or the density of ferrite grains with a grain size of 15 μm or more is 3000 grains / mm2 On the other hand, in the surface layer portion located 25 μm from the surface in the sheet thickness direction, the volume fraction of ferrite is 20.0% or less and the density of ferrite grains having a grain size of 15 μm or more is 3000 grains / mm 2 If the thickness is less than 1000 nm, the bendability is improved. This is thought to be because the soft phase is small and the coarse soft phase is also small, resulting in a homogeneous structure and improved bendability. Therefore, in the metal structure of the surface layer, the volume fraction of ferrite is set to 20.0% or less, and the density of ferrite crystal grains with a grain size of 15 μm or more is set to 3000 grains / mm 2 The following applies. In the metal structure of the surface layer portion, the volume fraction of ferrite is preferably 18.0% or less, more preferably 15.0% or less, and even more preferably 10.0% or less. The volume fraction of ferrite may be 1.0% or more. In the metal structure of the surface layer portion, the density of ferrite crystal grains having a grain size of 15 μm or more is preferably 2500 grains / mm 2 More preferably, 2000 pieces / mm 2 Furthermore, the density of ferrite grains with a grain size of 15 μm or more is 100 grains / mm 2 It may be more than that.

[0036] The volume fraction of ferrite in the surface layer can be obtained by taking a test piece from an arbitrary position in the rolling direction of the steel plate and from the center position in the width direction, polishing a longitudinal cross section parallel to the rolling direction, and observing the metal structure revealed by nital etching at a position 25 μm from the surface (specifically, a region in the range of 10 to 40 μm from the surface x 50 μm in the rolling direction) using an SEM. The density of ferrite grains with a grain size of 15 μm or more was calculated by dividing the number of ferrite grains with a grain size of 15 μm or more in the cross section observed by the SEM by the observation area (mm 2 ) is calculated by dividing by

[0037] <Solute Si content in the surface layer: 0.30 mass% or more, 1.50 mass% or less> If the amount of solute Si in the surface layer is less than 0.30% by mass, the strength of the soft ferrite decreases, increasing the difference in strength with the hard phase and deteriorating bending properties. Therefore, the amount of solute Si in the surface layer is set to 0.30% by mass or more, preferably 0.35% by mass or more, and more preferably 0.40% by mass or more. Furthermore, if the amount of solute Si in the surface layer exceeds 1.50 mass%, the tempering of martensite by reheating after the annealing cooling step is delayed, resulting in a hard structure and degraded bending properties. Therefore, the amount of solute Si in the surface layer is set to 1.50 mass% or less. It is preferably 1.20 mass% or less, more preferably 1.00 mass% or less, and even more preferably 0.90 mass% or less.

[0038] In addition, in the cold-rolled steel sheet according to this embodiment, the ratio of the amount of solute Si in the surface layer portion to the amount of solute Si in the t / 4 portion is preferably 0.85 to 1.10. When the ratio of the amount of solute Si in the t / 4 portion to the amount of solute Si in the surface layer portion is within the above range, the martensite is uniformly tempered in the thickness direction by reheating after the cooling step after annealing, resulting in a uniform structure and providing good bending properties and hydrogen embrittlement resistance. The ratio of the amount of solute Si is preferably 0.87 to 1.05, and more preferably 0.90 to 1.05.

[0039] The amount of solid solution Si in the surface layer portion and the t / 4 portion is measured by the following method. The amount of solute Si is determined by simultaneously measuring and quantitatively analyzing Si and O at the target position using an EPMA. A test piece is taken from an arbitrary position in the rolling direction of the steel plate and from the center in the width direction, and Si and O are determined by linear analysis using an EPMA in the rolling direction at a position 25 μm from the surface and at a position 1 / 4 of the plate thickness t from the surface in the plate thickness direction on a longitudinal cross section parallel to the rolling direction (i.e., a cross section parallel to the rolling direction and the thickness direction). However, if Si and O are detected simultaneously, this is Si oxide, so the area where O is detected is excluded. This measurement is performed at 10 or more positions, and the average value is taken as the amount of solute Si.

[0040] <Mechanical properties> [Tensile strength: 1310 MPa or more] [Uniform elongation: 5.0% or more] [The limiting bending radius R at a 90° V-bend divided by the plate thickness t (R / t): 5.0 or less] In the cold-rolled steel sheet according to this embodiment, the tensile strength (TS) is preferably 1310 MPa or more as a strength that contributes to reducing the weight of an automobile body. From the viewpoint of impact absorption, the tensile strength of the steel sheet is more preferably 1350 MPa or more, even more preferably 1400 MPa or more, and even more preferably 1470 MPa or more. There is no need to set an upper limit for the tensile strength, but if the tensile strength is too high, formability may be reduced, so the tensile strength may be 1900 MPa or less. From the viewpoint of formability, the uniform elongation (uEl) is preferably 5.0% or more. To further improve formability, the uniform elongation (uEl) is more preferably 5.5% or more. There is no need to set an upper limit for the uniform elongation, but the uniform elongation may be 30.0% or less, or 20.0% or less. Furthermore, from the viewpoint of bendability, the value obtained by dividing the limiting bending radius R in a 90° V-bend by the plate thickness t (i.e., the limiting bending radius R normalized by dividing by the plate thickness t) (R / t) is preferably 5.0 or less. To improve bendability, (R / t) is more preferably 4.0 or less, and even more preferably 3.0 or less. (R / t) may be 0.5 or more, or may be 1.0 or more.

[0041] The tensile strength (TS) and uniform elongation (uEl) are determined by taking a JIS No. 5 tensile test piece from the steel sheet in the direction perpendicular to the rolling direction and conducting a tensile test in accordance with JIS Z 2241:2011. In addition, the critical bending radius (R / t) normalized by plate thickness is calculated by using a 90° V-bending die, varying the radius R in 0.5 mm increments, determining the minimum bending radius (critical bending radius) R at which cracks do not occur, and dividing this by the plate thickness t.

[0042] The cold-rolled steel sheet according to this embodiment may have a hot-dip galvanized layer on the surface. Providing a plated layer on the surface improves corrosion resistance. For automotive steel sheets, there are cases where the sheet thickness cannot be reduced below a certain level even if the strength is increased due to concerns about holes due to corrosion. Since one of the purposes of increasing the strength of steel sheets is to reduce weight by reducing the thickness, even if a high-strength steel sheet is developed, its application areas will be limited if its corrosion resistance is low. One possible method for solving these problems is to apply a highly corrosion-resistant coating, such as hot-dip galvanized coating, to the steel sheet. The cold-rolled steel sheet according to this embodiment is capable of hot-dip galvanized coating because the steel sheet composition is controlled as described above. The hot-dip galvanized layer may be a galvannealed layer. The hot-dip galvanized layer and the alloyed hot-dip galvanized layer may be a plated layer formed by a conventional method.

[0043] <Manufacturing conditions> Specifically, the cold-rolled steel sheet according to this embodiment can be produced by a production method including the following features (I) to (VII). (I) a hot rolling step in which a cast slab having the above-mentioned chemical composition is heated as necessary and then hot-rolled under conditions in which the rolling temperature FT in the final stage is 960°C or less, the rolling reduction in the final stage is 10% or more, and the friction coefficient μ in the final stage is 0.15 or more to obtain a hot-rolled steel sheet; (II) a coiling step of cooling the hot-rolled steel sheet to a coiling temperature of 560°C or higher and 650°C or lower and coiling the hot-rolled steel sheet at the coiling temperature; (III) a cold rolling step of cold rolling the hot-rolled steel sheet after the coiling step under conditions where a cumulative rolling reduction is 60% or less to obtain a cold-rolled steel sheet; (IV) an annealing step of heating the cold-rolled steel sheet to a soaking temperature of 820°C or higher at an average heating rate of 3.0°C / second or higher up to 750°C and maintaining the soaking temperature; (V) a post-annealing cooling step of cooling the cold-rolled steel sheet after the annealing step to 50°C or higher and 250°C or lower so that the average cooling rate in both a temperature range of 700 to 600°C and a temperature range of 450 to 350°C is 5.0°C / sec or higher; (VI) a tempering step of holding the cold-rolled steel sheet after the post-annealing cooling step at 200°C or higher and 350°C or lower for 1 second or longer, (VII) The temperature of the hot-rolled steel sheet after the hot-rolling step is allowed to reach 500°C or less within 10 hours after the completion of the hot-rolling step. Each of these will be explained below.

[0044] [Hot rolling process] In the hot rolling process, the heated cast slab is 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 as is without first cooling it to near room temperature. The slab heating conditions in hot rolling are not limited, but it is preferable to heat it to 1100°C or higher. By setting the heating temperature to 1100°C or higher, it is possible to avoid insufficient homogenization of the material. In order to control the metal structure and amount of solute Si in the surface layer, the rolling temperature (FT) in the final finishing stage (final pass) of hot rolling is set to 960°C or lower, the reduction rate in the final stage is set to 10% or higher, and the friction coefficient μ during the final stage of rolling is set to 0.15 or higher. By shearing the surface layer, in particular, in the hot rolling process, carbides are finely precipitated in the surface layer in the subsequent coiling process. In this case, by performing the subsequent cold rolling and annealing under specified conditions, the distribution of Si concentration in the surface layer is suppressed, and the amount of solute Si and the state of ferrite in the surface layer are favorable. If the rolling temperature is too high, the effect of shear deformation is reduced, the surface structure cannot be developed, and bendability and hydrogen embrittlement resistance are not sufficiently improved. Therefore, the final stage rolling temperature (FT) is set to 960°C or lower. The final stage rolling temperature is preferably 940°C or lower. There is no lower limit for the final stage rolling temperature, but since a lower rolling temperature increases the rolling load, the final stage rolling temperature may be set to 870°C or higher. Furthermore, if the reduction ratio in the final stage is low or the coefficient of friction during final stage rolling is low, the surface layer does not receive sufficient shear deformation and the structure of the surface layer cannot be developed, resulting in insufficient improvement in bendability and hydrogen embrittlement resistance. Therefore, the coefficient of friction μ of the roll that comes into contact with the steel sheet in the final stage of finishing during hot rolling is set to 0.15 or more. More preferably, the coefficient of friction μ is 0.20 or more. There is no upper limit for the coefficient of friction μ, but since an increase in the coefficient of friction μ increases the rolling load, the coefficient of friction μ may be set to 0.40 or less. The rolling reduction in the final stage is set to 10% or more. The rolling reduction in the final stage is preferably set to 12% or more. The rolling reduction in the final stage does not need to be limited from the viewpoint of controlling the structure of the surface layer portion, but is preferably set to 15% or less from the viewpoint of manufacturability such as shape control.

[0045] [Winding process] After cooling to the coiling temperature as described above, coiling is performed. The coiling temperature is 560°C or higher and 650°C or lower. If the coiling temperature exceeds 650°C, the structure of the hot-rolled steel sheet becomes a coarse ferrite-pearlite structure, and the carbides do not become finely and uniformly dispersed. In addition, an Si-deficient layer is formed in the surface layer due to internal oxidation, and the amount of solid-solution Si in the surface layer decreases. As a result, bendability deteriorates. The coiling temperature is preferably 630°C or lower, more preferably 620°C or lower, and even more preferably 600°C or lower. On the other hand, if the coiling temperature is less than 560°C, transformation may begin before coiling, which may result in a non-uniform structure of the steel sheet. In this case, the density of ferrite crystal grains with a grain size of 15 μm or more in the surface layer may be 3000 grains / mm 2 The temperature must not be lower than this. By setting the coiling temperature to 560°C or higher and coiling before the start of transformation, the structure of the entire steel sheet can be made uniform. It is also possible to reduce the strength of the hot-rolled steel sheet and reduce the load during cold rolling. For this reason, the coiling temperature is 560°C or higher. If the strength of the hot-rolled steel sheet is high, softening heat treatment such as BAF may be performed before cold rolling.

[0046] In the method for producing a cold-rolled steel sheet according to this embodiment, the temperature of the steel sheet is raised to 500°C or less within 10 hours after the completion of the hot rolling step. By lowering the steel sheet temperature to 500°C or less within 10 hours, the formation of a surface Si-depleted layer due to internal oxidation in the hot-rolled steel sheet is suppressed (i.e., a certain amount of solute Si is secured in the surface layer), and as a result, good bendability is obtained after annealing. The time from the completion of the hot rolling step until the temperature of the steel sheet reaches 500°C or less is controlled by adjusting the cooling in the coiling step and the cooling after coiling. Since heat generation due to transformation occurs around 500°C, when coiling is performed at 560°C or more, it is not easy to reach 500°C or less within 10 hours by natural cooling, and forced cooling (for example, water cooling) is preferable. If the hot-rolled steel sheet (coiled) after coiling is cooled, the strength increases, which increases the load on the subsequent cold rolling step and increases costs. Therefore, forced cooling is not generally performed on the hot-rolled steel sheet after coiling. However, in order to obtain the cold-rolled steel sheet according to this embodiment, the time from the completion of the hot rolling step until the temperature of the steel sheet reaches 500°C or less is set to within 10 hours, as described above. The time required for raising the temperature of the steel sheet to 500°C or less after the completion of the hot rolling step is preferably 5 hours or less. Furthermore, it is preferable to allow the temperature of the steel sheet to reach 450°C or less within 10 hours after the completion of the hot rolling process, and it is more preferable to allow the temperature of the steel sheet to reach 450°C or less within 8 hours after the completion of the hot rolling process.

[0047] [Cold rolling process] In the cold rolling process, the hot-rolled steel sheet is descaled by pickling or the like, and then cold-rolled at a rolling reduction (cumulative rolling reduction) of 60% or less to produce a cold-rolled steel sheet. If the rolling reduction in cold rolling is high, recrystallization during annealing is promoted, coarse ferrite is generated, and the surface layer structure cannot be made uniform, resulting in deterioration of bendability and hydrogen embrittlement resistance. Therefore, the rolling reduction in cold rolling is set to 60% or less. A rolling reduction of 55% or less is preferred, and 50% or less is more preferred. There is no lower limit to the rolling reduction, but from the viewpoint of manufacturability, a rolling reduction of 30% or more is preferred.

[0048] [Annealing process] The cold-rolled steel sheet after the cold rolling step is subjected to treatment such as degreasing according to a known method as necessary, and then heated to a soaking temperature of 820°C or higher so that the average heating rate up to 750°C is 3.0°C / sec or higher, and is held at the soaking temperature. In the annealing process, if the average heating rate up to 750°C is slow, Si will be distributed between the ferrite and austenite, reducing the amount of solute Si in the surface layer and deteriorating bendability. Therefore, the average heating rate should be 3.0°C / sec or more. On the other hand, by setting the average heating rate within the above temperature range to 50.0°C / s or less, excessive promotion of ferrite transformation due to austenite grain refinement can be suppressed, which is advantageous for improving strength and bendability. Therefore, the average heating rate is preferably 50.0°C / s or less. The average heating rate is more preferably 30.0°C / s or less, and even more preferably 10.0°C / s or less. The soaking temperature (annealing temperature) in the annealing process is 820°C or higher. If the soaking temperature is low, austenite single-phase annealing will not occur, and the volume fraction of ferrite will increase, resulting in poor bendability. The soaking temperature is preferably 830°C or higher or 835°C or higher. A higher soaking temperature makes it easier to ensure bendability, but if the soaking temperature is too high, the manufacturing cost will increase, so the soaking temperature is preferably 900°C or lower. The soaking temperature is more preferably 880°C or lower, and even more preferably 870°C or lower. The soaking time is not limited, but is preferably 30 to 450 seconds. By setting the soaking time to 30 seconds or more, austenitization can be sufficiently promoted. Therefore, the soaking time is preferably 30 seconds or more. On the other hand, from the viewpoint of productivity, the soaking time is preferably 450 seconds or less.

[0049] [Cooling process after annealing] To obtain the above-described metal structure, the annealed cold-rolled steel sheet is cooled to a temperature (cooling stop temperature) of 50°C to 250°C so that the average cooling rate in the ferrite transformation temperature range from 700°C to 600°C and the average cooling rate in the bainite transformation temperature range from 450°C to 350°C are both 5.0°C / s or higher. If the cooling rate in these temperature ranges is slow, the volume fraction of ferrite and bainite at a position from the surface to 1 / 4 of the sheet thickness increases, and the volume fraction of tempered martensite decreases. As a result, tensile strength decreases, and bendability and hydrogen embrittlement resistance deteriorate. Therefore, the average cooling rates from 700°C to 600°C and from 450°C to 350°C are both 5.0°C / s or higher. The average cooling rates in the above temperature ranges are preferably 10.0°C / s or higher, and more preferably 20.0°C / s or higher. Although there is no upper limit to the average cooling rate in the above temperature range, if the cooling rate is too high, it becomes difficult to achieve uniform cooling in the width direction, which may cause deterioration in the shape of the steel sheet. Therefore, the average cooling rate is preferably 100°C / sec or less. The cooling stop temperature is set to 50°C or higher and 250°C or lower. If the cooling stop temperature is high, the amount of (untempered) martensite increases during cooling after the subsequent tempering process, deteriorating bendability and hydrogen embrittlement resistance. Therefore, the cooling stop temperature is set to 250°C or lower. On the other hand, if the cooling stop temperature is low, the fraction of retained austenite decreases and uniform elongation decreases. Therefore, the cooling stop temperature is set to 50°C or higher. The cooling stop temperature is preferably 75°C or higher, and more preferably 100°C or higher.

[0050] When producing a cold-rolled steel sheet having a hot-dip galvanized layer on its surface (hot-dip galvanized steel sheet), the cold-rolled steel sheet may be further immersed in a hot-dip galvanizing bath at a temperature of more than 425°C and less than 600°C in the post-annealing cooling step to form a hot-dip galvanized layer on its surface (hot-dip galvanizing step).Furthermore, when producing a cold-rolled steel sheet having a galvannealed hot-dip coating on its surface (galvannealed hot-dip galvanized steel sheet), subsequent to the above-mentioned hot-dip galvanizing step, an alloying treatment may be performed to turn the coating layer into an alloyed hot-dip galvanized layer (alloying step).

[0051] [Tempering process] After the annealing and cooling process, the cold-rolled steel sheet is cooled to a temperature of 50°C to 250°C, which transforms the untransformed austenite into martensite.The cold-rolled steel sheet is then tempered at a temperature of 200°C to 350°C for at least 1 second (tempering process), resulting in a structure mainly composed of tempered martensite in the t / 4 section. When 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 the hot-dip galvanizing process and the alloying process is cooled to a temperature of 50°C or more and 250°C or less, and then tempered at a temperature of 200°C or more and 350°C or less for 1 second or more. If the tempering temperature exceeds 350°C, the strength of the steel sheet decreases. Therefore, the tempering temperature is set to 350°C or less. The tempering temperature is preferably 325°C or less, and more preferably 300°C or less. If the tensile strength is to be further increased, it is preferable to lower the tempering temperature. For example, if the tensile strength is to be 1400 MPa or more, the tempering temperature is preferably 275°C or less, and if the tensile strength is to be 1470 MPa or more, the tempering temperature is preferably 250°C or less. On the other hand, if the tempering temperature is less than 200°C, the tempering will be insufficient, resulting in deterioration of bendability and hydrogen embrittlement resistance. Therefore, the tempering temperature is set to 200°C or higher. From the viewpoint of bendability and hydrogen embrittlement resistance, the tempering temperature is preferably 220°C or higher, and more preferably 250°C or higher. The tempering time should be 1 second or more, but is preferably 5 seconds or more, more preferably 10 seconds or more, to ensure stable tempering treatment. On the other hand, to avoid a decrease in the strength of the steel sheet, the tempering time is preferably 750 seconds or less, more preferably 500 seconds or less. In this embodiment, tempering means cooling to the tempering temperature in the post-annealing cooling step and then holding at that temperature, or cooling to a temperature below the tempering temperature in the post-annealing cooling step and then raising the temperature to the tempering temperature and holding at that temperature. Holding does not only mean maintaining a constant temperature, but also means allowing a temperature change of 1.0°C / second or less within the tempering temperature range (i.e., 200°C or higher and 350°C or lower). [Example]

[0052] The present invention will now be described more specifically with reference to examples. Slabs having the chemical compositions shown in Table 1 were cast. The cast slabs were heated to 1100°C or higher, hot rolled to 2.8 mm, coiled, and then cooled to room temperature. The hot rolling conditions and coiling temperatures were as shown in Tables 2A and 2B. The times from the end of hot rolling to reaching 500°C or less and 450°C or less were also as shown in Tables 2A and 2B. Thereafter, the steel sheets were pickled to remove scale, cold rolled to 1.4 mm, and then annealed for 120 seconds at the soaking temperatures shown in Tables 2A and 2B. The average heating rate up to 750°C during annealing heating was as shown in Tables 2A and 2B. After annealing, the steel was cooled to a cooling stop temperature of 50°C or higher and 250°C or lower so that the average cooling rate in both the temperature ranges of 700°C to 600°C and 450°C to 350°C was 20°C / second or higher, and then subjected to a heat treatment in which the steel was tempered for 1 to 500 seconds at the tempering temperature shown in Tables 2A and 2B. When the cooling stop temperature was lower than the tempering temperature, tempering was performed by heating to the tempering temperature shown in Tables 2A and 2B and holding at that temperature, and when the cooling stop temperature was the same as the tempering temperature, tempering was performed by cooling and then holding at that temperature. In some cases, hot-dip galvanizing and alloying were performed during cooling after annealing. In Table 5, "CR" indicates cold-rolled steel sheets that were not galvanized, "GI" indicates hot-dip galvanized steel sheets, and "GA" indicates alloyed hot-dip galvanized steel sheets. Hot-dip galvanized steel sheets were coated with a galvannealed coating of 35 to 65 g / m at a temperature of more than 425°C and less than 600°C. 2 For alloyed hot-dip galvanized steel sheets, the coating thickness is 35 to 65 g / m at temperatures exceeding 425°C and less than 600°C. 2 After being subjected to hot dip galvanizing to a degree, it was further alloyed at a temperature above 425°C and below 600°C.

[0053] From the obtained cold-rolled steel sheet, the metal structure of the t / 4 part, the amount of solute Si in the surface layer part, the ferrite volume fraction in the surface layer part, the density of ferrite crystal grains having a grain size of 15 μm or more in the surface layer part, and the ratio of the amount of solute Si in the surface layer part to the amount of solute Si in the t / 4 part were determined by the methods described above. The results are shown in Tables 3 and 4.

[0054] The tensile strength (TS), uniform elongation (uEl), bendability (R / t), and hydrogen embrittlement resistance were evaluated as follows. The results are shown in Table 5.

[0055] The tensile strength (TS) and uniform elongation (uEl) were determined by taking JIS No. 5 tensile test pieces from the obtained cold-rolled steel sheets in a direction perpendicular to the rolling direction and conducting tensile tests in accordance with JIS Z 2241:2011. The results are shown in Table 5.

[0056] The limiting bending radius (R / t), an index of bendability, was determined by using a 90° V-bending die, varying the radius R in 0.5 mm increments, determining the minimum bending radius R at which cracks did not occur, and dividing this by the plate thickness (1.4 mm).

[0057] To evaluate hydrogen embrittlement resistance, the following tests were carried out. Specifically, test specimens with mechanically ground edges were bent into a U shape using the push-bending method to create U-bend test specimens with a radius of 5R. The specimens were then tightened with bolts so that the unbent portions were parallel, causing elastic deformation. They were then immersed in hydrochloric acid with a pH of 1 to conduct a delayed fracture accelerated test in which hydrogen penetrated the steel sheet. Steel sheets that showed no cracks after 100 hours of immersion were rated as having good (OK) delayed fracture resistance, while steel sheets that showed cracks were rated as poor (NG). To eliminate the influence of the plating, the plating layer of plated materials was removed with hydrochloric acid containing an inhibitor before the test, and then hydrogen embrittlement resistance was evaluated.

[0058] As can be seen from Tables 1 to 5, all of the steels of the present invention had a TS of 1310 MPa or more, a uEl of 5.0% or more, and a critical bending radius (R / t) of 5.0 or less, and also had good hydrogen embrittlement resistance. In contrast, in the test numbers (comparative examples) in which either the chemical composition or the manufacturing method was outside the range of the present invention, and the metal structure and texture were outside the range of the present invention, the targets were not achieved in any of the tensile strength, uniform elongation, limiting bending radius, and hydrogen embrittlement resistance.

[0059] [Table 1]

[0060] [Table 2A]

[0061] [Table 2B]

[0062] [Table 3]

[0063] [Table 4]

[0064] [Table 5] [Industrial Applicability]

[0065] According to the present invention, a cold-rolled steel sheet having high strength and excellent uniform elongation, bendability, and hydrogen embrittlement resistance, and a method for manufacturing the same can be obtained. Such a steel sheet has sufficient formability that makes it applicable to processing such as press forming. Therefore, the present invention will greatly contribute to industrial development, such as by helping to solve global environmental problems through weight reduction of automobile bodies.

Claims

1. In mass%, C: 0.140% or more, 0.400% or less, Si: 0.35% or more, 1.50% or less, Mn: 1.30% or more, 3.50% or less, P: 0% or more, 0.100% or less, S: 0% or more, 0.010% or less, Al: 0% or more, 0.100% or less, N: 0% or more, 0.0100% or less, Ti: 0% or more, 0.050% or less, Nb: 0% or more, 0.050% 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, 0.010% or less, Mg: 0% or more, 0.0100% or less, REM: 0% or more and 0.050% or less, and Bi: 0% or more, 0.050% or less, and the balance being Fe and impurities, The metal structure of the t / 4 portion, which is located at 1 / 4 of the plate thickness t from the surface in the plate thickness direction, is, in terms of volume fraction, Retained austenite: 2.5% or more and 10.0% or less, Tempered martensite: 80.0% or more and 97.5% or less, Ferrite and bainite: 0.0% or more and 15.0% or less in total; and Martensite: 0.0% or more and 3.0% or less, Including, In a surface layer portion located 25 μm from the surface in the plate thickness direction, The amount of solute Si is, in mass%, 0.30% or more and 1.50% or less, The volume fraction of ferrite in the metal structure is 0.0% or more and 20.0% or less, The density of ferrite grains having a grain size of 15 μm or more is 0 grains / mm ２ More than 3000 pieces / mm ２ Below is the Cold rolled steel plate.

2. The chemical composition is, in mass %, Ti: 0.001% or more, 0.050% or less, Nb: 0.001% or more, 0.050% or less, 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, 0.010% or less, Mg: 0.0001% or more, 0.0100% or less, REM: 0.0005% or more and 0.050% or less, and Bi: 0.0005% or more, 0.050% or less, Contains one or more selected from the group consisting of The cold rolled steel sheet according to claim 1.

3. the ratio of the amount of solute Si in the surface layer portion to the amount of solute Si in the t / 4 portion is 0.85 to 1.10; The cold-rolled steel sheet according to claim 1 or 2.

4. The tensile strength is 1310 MPa or more, The uniform elongation is 5.0% or more, R / t, which is the value obtained by dividing the limit bending radius R at 90 ° V bending by the plate thickness t, is 5.0 or less, The cold-rolled steel sheet according to any one of claims 1 to 3.

5. The tensile strength is 1400 MPa or more. The cold rolled steel sheet according to claim 4.

6. A hot-dip galvanized layer is formed on the surface. The cold-rolled steel sheet according to any one of claims 1 to 5.

7. The hot-dip galvanized layer is a galvannealed layer. The cold rolled steel sheet according to claim 6.

8. A method for manufacturing a cold rolled steel sheet according to claim 1, comprising the steps of: C: 0.140% or more and 0.400% or less, Si: 0.35% or more and 1.50% or less, Mn: 1.30% or more and 3.50% or less, P: 0% or more and 0.100% or less, S: 0% or more and 0.010% or less, Al: 0% or more and 0.100% or less, N: 0% or more and 0.0100% or less, Ti: 0% or more and 0.050% or less, Nb: 0% or more and 0.050% or less, 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 a hot rolling process in which a cast slab having a chemical composition containing Mo: 0% or more and 0.50% or less, B: 0% or more and 0.0100% or less, Ca: 0% or more and 0.010% or less, Mg: 0% or more and 0.0100% or less, REM: 0% or more and 0.050% or less, and Bi: 0% or more and 0.050% or less, with the balance being Fe and impurities, is heated as necessary, and then hot-rolled under conditions in which a rolling temperature FT in a final stage is 960°C or less, a rolling reduction in the final stage is 10% or more, and a friction coefficient μ in the final stage is 0.15 or more, to obtain a hot-rolled steel sheet; a coiling step of cooling the hot-rolled steel sheet to a coiling temperature of 560°C or higher and 650°C or lower and coiling the hot-rolled steel sheet at the coiling temperature; a cold rolling step of cold rolling the hot-rolled steel sheet after the coiling step under conditions where a cumulative rolling reduction is 60% or less to obtain a cold-rolled steel sheet; An annealing process in which the cold-rolled steel sheet is heated to a soaking temperature of 820°C or higher so that the average heating rate up to 750°C is 3.0°C / sec or higher and maintained at the soaking temperature; A post-annealing cooling process in which the cold-rolled steel sheet after the annealing process is cooled to 50°C or higher and 250°C or lower so that the average cooling rate in both the temperature range of 700 to 600°C and the temperature range of 450 to 350°C is 5.0°C / sec or higher; A tempering process in which the cold-rolled steel sheet after the post-annealing cooling process is held at 200 ° C. or higher and 350 ° C. or lower for 1 second or more; Equipped with The temperature of the hot-rolled steel sheet after the hot rolling step is allowed to reach 500°C or less within 10 hours from the completion of the hot rolling step. Manufacturing method of cold rolled steel sheet.

9. The chemical composition of the cast slab, in mass %, is Ti: 0.001% or more, 0.050% or less, Nb: 0.001% or more, 0.050% or less, 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, 0.010% or less, Mg: 0.0001% or more, 0.0100% or less, REM: 0.0005% or more and 0.050% or less, and Bi: 0.0005% or more, 0.050% or less, Contains one or more selected from the group consisting of The method for producing a cold-rolled steel sheet according to claim 8.

10. In the post-annealing cooling step, the cold-rolled steel sheet is immersed in a plating bath at a temperature of more than 425°C and less than 600°C to form a hot-dip galvanized layer on the surface. The method for producing a cold-rolled steel sheet according to claim 8 or 9.

11. The method for producing a cold-rolled steel sheet according to claim 10, wherein an alloying treatment is performed to alloy the hot-dip galvanized layer in the post-annealing cooling step.

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