COLD ROLLED STEEL SHEETS AND METHODS FOR MAKING THEM

Abstract

The present invention discloses a cold-rolled steel sheet having a predetermined chemical composition, wherein the metallographic structure of the t / 4 portion, located at a position 1 / 4 of the sheet thickness t from the surface of the cold-rolled steel sheet in the sheet thickness direction, includes, based on volume percentage, 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 the surface layer area at a position 25 μm from the surface in the sheet thickness direction, the amount of dissolved Si is 0.30% or more and 1.50% or less by mass %, the volume percentage of ferrite in the metallographic structure is 0.0% or more and 20.0% or less, and the density of ferrite grains having a grain size of 15 μm or more is 0 grains / mm2 or more and 3,000 grains / mm2 or less.

Description

Description COLD ROLLED STEEL SHEETS AND METHODS FOR MAKING THEM Cross-Reference to Related Patent Applications This application claims priority under Japanese Patent Application No. 2021-038716, filed on March 10, 2021, the disclosure of which is incorporated herein by reference in its entirety. Invention Engineering Field The present invention relates to a cold rolled steel sheet and a method for manufacturing the same. Background of the Invention Nowadays, as the industrial technology field is highly segmented, the materials used in each technology field require specialized and advanced performance. In particular, with regard to steel sheets for vehicles, to reduce vehicle body weight and improve fuel efficiency while taking into account the global environment, there is a significantly increasing demand for high-tensile cold-rolled steel sheets with thin sheet thickness and excellent formability. Among steel sheets for vehicles, especially for cold-rolled steel sheets used for vehicle body frame components, high strength is required, and furthermore, high formability is required for wide application. Examples of required properties for steel sheets for vehicles include a tensile strength (TS) of 1,310 MPa or more and a uniform elongation of 5.0% or more.Furthermore, depending on the processing method and the applied component, it is also required that the limit bending radius R (R / t) standardized to the sheet thickness t in 90° V-bending is 5.0 or less and that the hydrogen embrittlement resistance is excellent. Although it is effective to produce a ferrite-containing structure to secure ductility such as uniform elongation, the secondary phase needs to be hardened to obtain a strength of 1,310 MPa or more with the ferrite-containing structure. However, the hard secondary phase deteriorates the bending ability. On the other hand, as a technique for improving the bending ability and hydrogen embrittlement resistance of high-strength steel sheets, steel sheets containing tempered martensite as the main phase have been proposed (see, for example, Patent Document 1 and Patent Document 2). In Patent Document 1 and Patent Document 2, it is disclosed that excellent bending ability is achieved by producing a structure having a single phase of tempered martensite as the microstructure, and excellent hydrogen embrittlement resistance is achieved by producing a structure in which carbides, which are hydrogen trapping sites, are finely dispersed. In addition, Patent Document 3 proposes a steel sheet that utilizes the TRIP effect caused by retained austenite as a technique to achieve high strength and high formability. Previously Disclosed Technology Documents Patent Documents Patent Document 1 Publication of Japanese Patent Application No. 200930091 Patent Document 1 Publication of Japanese Patent Application No. 2010215958 Patent Document 4 Publication of Japanese Patent Application No. 2006104532 Brief Description of the Invention Problems to be Solved by the Invention However, the steel sheet of Patent Document 1 has a tensile strength as low as less than 1,310 MPa. Therefore, in the case of aiming for further high strengthening, it is necessary to further improve the workability, bending ability, and hydrogen embrittlement resistance which deteriorates accordingly. In addition, although the steel sheet of Patent Document 2 can achieve a strength as high as 1,310 MPa or more, because the steel sheet is cooled to near room temperature during quenching during sudden cooling, there is a problem that the volume percentage of retained austenite is small and high uniform elongation cannot be obtained. In addition, because the steel sheet of Patent Document 3 contains ferrite, it is difficult to obtain a strength as high as 1,310 MPa or more, and the difference in strength in the structure causes a decrease in the bending ability with high reinforcement. That is, it cannot be said that the steel sheets that have been proposed in related techniques have high strength and uniform elongation, bending ability, and hydrogen embrittlement resistance sufficient to meet the more advanced requirements in recent years. Therefore, the object of the present invention is to provide a cold-rolled steel sheet having high strength and uniform elongation, bending ability, and excellent hydrogen embrittlement resistance, and a method for manufacturing the same. Ways to Solve Problems The inventors of this invention studied the influence of chemical composition, metallographic structure, and manufacturing conditions on the strength, uniform elongation, bending ability, and hydrogen embrittlement resistance of cold-rolled steel sheets. As a result, it is found that the strength, uniform elongation, bending ability, and hydrogen embrittlement resistance can be simultaneously improved by controlling the metallographic structure inside the steel sheet (e.g., at the position of 1 / 4 sheet thickness from the surface of the steel sheet) which mainly contains tempered martensite with retained austenite in a predetermined amount or more, and then controlling the amount of dissolved Si and the existence state of ferrite in the surface layer area. In addition, as a result of studies by the inventors of the present invention, it was found that to control the surface layer area, refinement of grain size and refinement of carbides by shear force during hot rolling, suppression of formation of Si depletion layer due to internal oxidation by controlling rolling temperature, and suppression of Si partition during annealing by controlling cold rolling and annealing conditions are very important. This invention has been made based on the above findings. The essence of this invention is as follows. 1. A cold-rolled steel sheet according to an aspect of the present invention contains, as a chemical composition, based on 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; 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; 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; REE: 0% or more and 0.050% or less; Bi: 0% or more and 0.050% or less;and the remainder is Fe and impurities, wherein the metallographic structure of the t / 4 section, which is at a position of 1 / 4 of the sheet thickness t from the surface of the cold-rolled steel sheet in the sheet thickness direction, includes, based on volume percentage, retained austenite: 2.5% or more and 10.0% or less, temper 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 the surface layer area at a position of 25 μm from the surface in the thickness direction of the sheet, the amount of dissolved Si is 0.30% or more and 1.50% or less by mass %, the volume percentage of ferrite in the metallographic structure is 0.0% or more and 20.0% or less, and the grain density of ferrite having a grain size of 15 μm or more is 0 grains / mm2or more and 3,000 grains / mm2or less. 2. Cold-rolled steel sheets according to item 1 may contain, as a chemical composition, based on % by mass, one or two or more elements selected from the group consisting only of: 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% 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; LTJ: 0.0005% or more and 0.050% or less; and Bi: 0.0005% or more and 0.050% or less. 3. In cold-rolled steel sheets according to item 1 or 2, the ratio of the amount of dissolved Si in the surface layer area to the amount of dissolved Si in the t / 4 section can be 0.85 to 1.10. 4. In cold-rolled steel sheets according to any one of items 1 to 3, the tensile strength of the cold-rolled steel sheets can be 1310 MPa or more, the uniform elongation of the cold-rolled steel sheets can be 5.0% or more, and R / t, which is the value obtained by dividing the limit bending radius R in 90° V-bending by the sheet thickness t, can be 5.0 or less. 5. In cold rolled steel sheets according to point 4, the tensile strength can be 1,400 MPa or more. 6. On cold rolled steel sheets according to any of points 1 to 5, a hot dip galvanized layer can be formed on the surface. 7. On cold rolled steel sheets according to point 6, the hot dip galvanizing layer may be a hot dip galvanizing layer. 8. A method for making cold rolled steel sheet according to another aspect of the present invention, comprising: a hot rolling process consisting of heating, as required, a cast slab containing, as a chemical composition, based on 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, 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, 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, REE: 0% or more and 0.050% or less,Bi: 0% or more and 0.050% or less, and the remainder is Fe and impurities, and hot rolling the cast slab under conditions where the rolling temperature FT at the final seat is 960°C or lower, the rolling reduction at the final seat is 10% or more, and the friction coefficient μ at the final seat is 0,15 or more to obtain hot rolled steel sheets; the rolling process is in the form of cooling the hot rolled steel sheets to a rolling temperature of 560°C or higher and 650°C or lower and rolling the hot rolled steel sheets at the rolling temperature; the cold rolling process is in the form of performing cold rolling on the hot rolled steel sheets after the rolling process in a condition where the cumulative rolling reduction is 60% or less to obtain cold rolled steel sheets; the annealing process is in the form of heating the cold rolled steel sheets to a soaking temperature of 820°C or higher so that the average heating rate up to 750°C becomes 3.0°C / second or faster,and holding the cold-rolled steel sheet at the soaking temperature; a post-annealing cooling process consisting of cooling the cold-rolled steel sheet after the annealing process to a temperature of 50°C or higher and 250°C or lower so that the average cooling rate in the temperature range of 700°C to 600°C and in the temperature range of 450°C to 350°C is 5.0 °C / second or faster; and a tempering process consisting of holding the cold-rolled steel sheet after the post-annealing cooling process at 200°C or higher and 350°C or lower for 1 second or longer, wherein the temperature of the hot-rolled steel sheet after the hot-rolling process is made to reach 500°C or lower within 10 hours from the completion of the hot-rolling process., 9. In the method for making cold-rolled steel sheets according to clause 8, the cast slab may contain, as a chemical composition, based on % by mass, one or two or more elements selected from the group consisting only of: 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% 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; LTJ: 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 making cold-rolled steel sheets according to item 8 or 9, in the post-annealing cooling process, the cold-rolled steel sheets can be immersed in a galvanizing bath under the circumstances where the temperature of the cold-rolled steel sheets is higher than 425°C and lower than 600°C to form a hot-dip galvanizing layer on the surface of the cold-rolled steel sheets. 11. In the method for making cold-rolled steel sheets according to item 10, the alloying treatment for alloying the hot-dip galvanizing layer may be carried out in the post-annealing cooling process. Effects of Invention According to the above aspects of the present invention, it is possible to produce a cold-rolled steel sheet having high strength and uniform elongation, bending ability, and excellent hydrogen embrittlement resistance, and a method for producing it. Complete Description of the Invention A cold-rolled steel sheet according to an embodiment of the present invention (a cold-rolled steel sheet according to this embodiment) and a method for making it are described. The cold-rolled steel sheet according to this embodiment has (a) the chemical composition described below, wherein (b) the metallographic structure of the t / 4 portion, which is at a position 1 / 4 of the sheet thickness t from the surface of the cold-rolled steel sheet in the sheet thickness direction, includes, based on volume percentage, 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 overall, and martensite: 0.0% or more and 3.0% or less, and (c) in the surface layer area at a position 25 μm from the surface in the sheet thickness direction, the amount of dissolved Si is 0.30% or more and 1.50% or less based on mass %, the volume percentage of ferrite in the metallographic structure is 0.0% or more and 20.0% or less, and the density of ferrite grains is having a grain size of 15 μm or more is 0 grains / mm2 or more and 3,000 grains / mm2 or less. The cold-rolled steel sheet according to this embodiment includes not only a cold-rolled steel sheet having no galvanizing layer on its surface, but also a hot-dip galvanized steel sheet having a hot-dip galvanized layer on its surface or a hot-dip galvanized steel sheet having a hot-dip galvanized layer on its surface, and these main conditions are common to both the hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet. However, in the case of galvanized steel sheets, the surface as a reference indicating the position that determines the metallographic structure means the surface of the base steel sheet excluding the galvanized layer. Each of these will be explained below. Chemical Composition First, the chemical composition of the cold-rolled steel sheet according to this embodiment will be described. Hereinafter, % indicating the amount of each element in the chemical composition means % by mass unless otherwise specified. C: 0.140% or More and 0.400% or Less When the C content is less than 0.140%, it becomes difficult to obtain the metallographic structure described above, and the desired tensile strength cannot be achieved. In addition, the bending ability is reduced. Therefore, the C content is set at 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 is more than 0.400%, the weldability and bending ability are worse. In addition, the hydrogen embrittlement resistance is also worse. Therefore, the C content is set at 0.400% or less. The C content is preferably less than 0.400%, preferably 0.350% or less, and even more preferably 0.300% or less. Si: 0.35% or More and 1.50% or Less Si is a useful element for improving the strength of steel sheets by solid solution strengthening. In addition, Si suppresses the formation of cementite, and thus is an effective element in promoting C concentration in austenite and producing retained austenite after annealing. When the Si content is less than 0.35%, it becomes difficult to obtain the effects of the above actions, it becomes difficult to achieve the target uniform elongation, and the hydrogen embrittlement resistance is deteriorated. Therefore, the Si content is set at 0.35% or more. The Si content is preferably more than 0.35%, preferably 0.40% or more, and even more preferably 0.45% or more. On the other hand, when the Si content is greater than 1.50%, the austenitic transformation during heating in the annealing step is slowed down, and there are cases where the transformation from ferrite to austenite does not occur sufficiently. In this case, an excessive amount of ferrite remains in the microstructure after annealing, and the target tensile strength cannot be achieved, so the bending ability is deteriorated. In addition, when the Si content is greater than 1.50%, the surface properties of the steel sheet are deteriorated. Furthermore, the chemical conversion ability and the plating ability are significantly deteriorated. Therefore, the Si content is set at 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 still more preferably 0.90% or less or 0.85% or less. In particular, when the Si content is set at 1.00% or less, the adhesion of the plating is improved. Mn: 1.30% or More and 3.50% or Less Mn has the action of increasing the hardenability of steel and is an effective element for obtaining the desired metallographic structure described below. When the Mn content is less than 1.30%, it becomes difficult to obtain the desired metallographic structure. In this case, adequate tensile strength cannot be obtained. Therefore, the Mn content is set at 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, when the Mn content is more than 3.50%, the hardenability enhancement effect is reduced due to Mn segregation, and the material cost increases. Therefore, the Mn content is set at 3.50% or less. The Mn content is preferably less than 3.50%, preferably 3.25% or less, and even more preferably 3.00% or less. P: 0% or More and 0.100% or Less P is an element contained in steel as an impurity and is an element that segregates at the grain boundaries and makes steel brittle. Therefore, the P content is preferred to be as small as possible and can be 0%. However, considering the time and cost of removing P, the P content is set at 0.100% or less. The P content is preferred to be 0.020% or less, and more preferably 0.015% or less. The P content can be set at 0.005% or more considering the cost of refining or the like. S: 0% or More and 0.010% or Less S is an element contained in steel as an impurity and is an element that forms sulfide-based inclusions and impairs bending ability. Therefore, the S content is preferably as small as possible and can be 0%. However, considering the time and cost of removing S, the S content is set at 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. The S content can be set at 0.0001% or more considering the cost of refining or the like. Al: 0% or More and 0.100% or Less Al is an element that has a deoxidizing effect on molten steel. In cases where Al is contained for deoxidation purposes, the Al content is preferably 0.005% or more, and preferably 0.010% or more to achieve reliable deoxidation. In addition, Al has a stabilizing effect on austenite like Si and is an effective element for obtaining the metallographic structure described above. Therefore, Al can be contained. In cases where Al is contained, the Al content can be, for example, 0.010% or more. On the other hand, if the Al content is too high, not only the possibility of surface defects caused by alumina, but also the transformation point increases significantly, so that the volume percentage of ferrite increases. In this case, it becomes difficult to obtain the above-mentioned metallographic structure, and sufficient tensile strength cannot be obtained. Therefore, the Al content is set at 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, since Si which has a deoxidizing action is contained like Al, Al does not necessarily have to be contained, and the Al content can be 0%. N: 0% or More and 0.0100% or Less N is an element that can be contained in steel as an impurity and is an element that forms coarse deposits and impairs bending ability. Therefore, the N content is set at 0.0100% or less. The N content is preferably 0.0060% or less, and more preferably 0.0050% or less. The N content is preferably as small as possible, and can be 0%. The N content can be set at 0.0010% or more or 0.0020% or more by considering refining costs or the like. The cold-rolled steel sheet according to this embodiment contains the above-mentioned elements and the remainder is Fe and impurities, and may further contain one or two or more of the elements listed below that affect strength and bending ability as optional elements. However, since these optional elements are not required to be contained, the lower limit is all 0%. 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 Ti, Nb, V, and Cu are elements that have the action of increasing the strength of steel sheets by precipitation hardening. Therefore, these elements can be contained. In order to obtain the above effects sufficiently, it is preferred that each of the Ti content and Nb content is set at 0.001% or more, and each of the V content and Cu content is set at 0.01% or more. Each of the Ti content and Nb content is preferably 0.005% or more, and each of the V content and Cu content is preferably 0.05% or more. It is not essential to obtain the above effects. Therefore, it is not necessary to specifically limit the lower limit of the Ti content, Nb content, V content, and Cu content, and the lower limit is 0%. On the other hand, if these elements are contained in excess, the recrystallization temperature increases, the metallographic structure of the cold-rolled steel sheet becomes non-uniform, and the bending ability is impaired. Therefore, in cases where these elements are contained, the Ti content is set at 0.050% or less, the Nb content is set at 0.050% or less, the V content is set at 0.50% or less, and the Cu content is set at 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. V content is preferred at 0.30% or less. Cu content is preferred at 0.50% or less. Ni: 0% or More and 1.00% or Less Cr: 0% or More and 1.00% or Less Mo: 0% or More and 0.50% or Less B: 0% or More and 0.0100% or Less Ni, Cr, Mo, and B are elements that improve hardenability and contribute to high hardening of steel sheets, and are effective elements for obtaining the metallographic structure described above. Therefore, these elements can be contained. In order to adequately obtain the above effects, it is preferred that each of the Ni content, Cr content, and Mo content is set at 0.01% or more, and / or the B content is set at 0.0001% or more. More preferably, each of the Ni content, Cr content, and Mo content is 0.05% or more, and the B content is 0.0010% or more. It is not essential to obtain the above effects. Therefore, it is not necessary to specifically limit the lower limit of the Ni content, Cr content, Mo content, and B content, and the lower limit is 0%. On the other hand, even if these elements are contained in excess, the effect of the action described above becomes saturated, which is uneconomical. Therefore, in cases where these elements are contained, the Ni content and Cr content are set at 1.00% or less, the Mo content is set at 0.50% or less, and the B content is set at 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. Ca: 0% or More and 0.010% or Less Mg: 0% or More and 0.0100% or Less LTJ: 0% or More and 0.050% or Less Bi: 0% or More and 0.050% or Less Ca, Mg, and REE are elements that have the action of increasing the strength and bending ability of steel sheets by adjusting the shape of inclusions. Bi is an element that has the action of increasing the strength and bending ability by refining the compaction structure. Therefore, these elements can be contained. In order to obtain the above effects sufficiently, it is preferred that each of the Ca content and Mg content is set at 0.00010% or more, and each of the REE content and Bi content is set at 0.0005% or more. More preferably, each of the Ca content and Mg content is 0.00080% or more, and each of the REE content and Bi content is 0.0007% or more. It is not essential to obtain the above effects. Therefore, it is not necessary to specifically limit the lower limit of the Ca content, Mg content, Bi content, and REE content, and the lower limit is 0%. On the other hand, even if these elements are contained in excess, the effect of the above action becomes saturated, which is uneconomical. Therefore, in cases where these elements are contained, the Ca content is set at 0.010% or less, the Mg content is set at 0.0100% or less, the REE content is set at 0.050% or less, and the Bi content is set at 0.050% or less. Preferably, the Ca content is 0.008% or less or 0.002% or less, the Mg content is 0.0020% or less, the REE content is 0.010% or less or 0.002% or less, and the Bi content is 0.010% or less. REE stands for rare earth elements and is a general term for a total of 17 elements Sc, Y and lanthanides, and the REE content is the total amount of these elements. Metallographic Structure at Position 1 / 4 (Section t / 4) Sheet Thickness t from Surface in Sheet Thickness Direction In the description of the metallographic structure of cold-rolled steel sheets according to this embodiment, the microstructure fraction is indicated by volume percentage. Therefore, unless otherwise specified, % indicates %volume. Retained austenite: 2.5% or More and 10.0% or Less Retained austenite improves the ductility of steel sheets by the TRIP effect and contributes to increased uniform elongation. Therefore, the volume percentage of retained austenite is set at 2.5% or more. The volume percentage of retained austenite is preferably more than 2.5%, preferably 3.5% or more, and even more preferably 4.5% or more. On the other hand, if the volume percentage of retained austenite becomes excessive, the grain size of retained austenite increases. The retained austenite with a large grain size becomes coarse and hard martensite after deformation. In this case, crack origination will occur, and the bending ability will deteriorate. Therefore, the volume percentage of retained austenite is set at 10.0% or less. The volume percentage of retained austenite is preferably less than 10.0%, preferably 8.0% or less, and even more preferably 7.0% or less. Temper Martensite: 80.0% or More and 97.5% or Less Temper martensite is a lamellar grain aggregate similar to martensite (called fresh martensite). On the other hand, unlike martensite, temper martensite is a hard microstructure containing fine iron-based carbides in it through tempering. Temper martensite is obtained by tempering the martensite produced by quenching or the like after annealing by heat treatment or the like. Tempered martensite is a microstructure that is less brittle and has ductility compared to martensite. In the cold-rolled steel sheet according to this embodiment, the volume percentage of tempered martensite is set at 80.0% or more to improve strength, bending ability, and hydrogen embrittlement resistance. The volume percentage of tempered martensite is preferably 85.0% or more. To set the volume percentage of retained austenite to 2.5% or more, the volume percentage of tempered martensite is 97.5% or less. Ferrite and Bainite: 0.0% or More and 15.0% or Less Overall Ferrite is a soft phase obtained by two-phase annealing or slow cooling after annealing. When ferrite is mixed with a hard phase such as martensite, the ductility of the steel sheet is improved. However, to achieve strengths as high as 1,310 MPa or higher, it is necessary to limit the volume percentage of ferrite. Bainite is a phase obtained by holding at 350°C or higher and 450°C or lower for a certain period after annealing. Bainite is softer than martensite and has the effect of increasing ductility. However, to achieve strengths as high as 1,310 MPa or more, it is necessary to limit the volume percentage of bainite as in the case of ferrite described above. Therefore, the volume percentage of ferrite and bainite is set at 15.0% or less overall. The volume percentage of ferrite and bainite is preferably 10.0% or less. Since ferrite and bainite may not be contained, the lower limit is 0.0% for each. In addition, because ferrite is softer than bainite, in the case where the total volume percentage of ferrite and bainite is 15.0% or less, the volume percentage of ferrite is preferably less than 10.0% to achieve a strength as high as 1310 MPa or more. Martensite: 0.0% or More and 3.0% or Less Martensite (formed martensite) is a lamellar grain aggregate that can be produced by transformation from austenite during the final cooling after the tempering step. Because martensite is hard and brittle and tends to be the origin of cracks during deformation, a large volume percentage of martensite leads to a deterioration in bending ability. Therefore, the volume percentage of martensite is set at 3.0% or less. The volume percentage of martensite is preferably 2.0% or less, and more preferably 1.0% or less. Because martensite may not be contained, the lower limit of the volume percentage of martensite is 0.0%. The metallographic structure at the 1 / 4 position (t / 4 section) of the sheet thickness t from the surface in the sheet thickness direction may contain pearlite as a residue in the microstructure other than those mentioned above. However, pearlite is a microstructure that has cementite in its microstructure and consumes C (carbon) in the steel which contributes to the increase in strength. When the volume percentage of pearlite is 5.0% or less, the strength of the steel sheet increases. Therefore, the volume percentage of pearlite is preferably set at 5.0% or less. The volume percentage of pearlite is preferably 3.0% or less, and even more preferably 1.0% or less. The volume percentage in the microstructure of a t / 4 section of cold rolled steel sheet according to this embodiment is measured as follows. Namely, the volume percentage of ferrite, bainite, martensite, tempered martensite, and pearlite was measured by a method in which, the test specimen was collected from a certain position in the rolling direction of the steel sheet at the middle position in the width direction of the steel sheet, the longitudinal section (i.e., the cross section parallel to the rolling direction and parallel to the thickness direction) parallel to the rolling direction was polished, and the metallographic structure seen by nital etching at the position of 1 / 4 of the sheet thickness t from the surface in the thickness direction of the sheet was observed using a scanning electron microscope (SEM). In SEM observation, five visual fields measuring 30 pm in the thickness direction of the sheet and 50 pm in the rolling direction were observed at a magnification of 3,000 times so that the position of 1 / 4 of the sheet thickness t from the surface in the thickness direction of the sheet was in the middle.The area ratio of each microstructure was measured from the observed images, and its average value was calculated. Since there was no microstructural change in the direction (width direction of the steel sheet) perpendicular to the rolling direction and the area ratio of the longitudinal section parallel to the rolling direction was equal to the volume percentage, each area ratio obtained by microstructure observation was used as the volume percentage. In measuring the area ratio of each microstructure, a region with no visible substructure and low luminance is defined as ferrite. In addition, a region with a layered structure of ferrite and cementite is defined as pearlite. In addition, a region with no visible substructure and high luminance is defined as martensite or retained austenite. In addition, a region with visible substructure is defined as tempered martensite or bainite. Bainite and tempered martensite can be distinguished from each other by more closely examining the carbides in the grains. Specifically, tempered martensite includes martensite laths and cementite produced within the laths. Here, because there are two or more types of crystal orientation relationships between martensite laths and cementite, cementite belonging to tempered martensite has a number of variants. On the other hand, bainite is classified into upper bainite and lower bainite. Upper bainite includes lath-shaped bainitic ferrite and cementite produced at the interface between the laths and can be easily distinguished from tempered martensite. Lower bainite includes lath-shaped bainitic ferrite and cementite produced within the laths. Here, there is one type of crystal orientation relationship between bainitic ferrite and cementite unlike tempered martensite, and cementite belonging to lower bainite has the same variant. Therefore, lower bainite and tempered martensite can be distinguished from each other based on the cementite variant. On the other hand, martensite and retained austenite cannot be clearly distinguished from each other by observation. SEM. Therefore, the volume percentage of martensite is calculated by subtracting the volume percentage of retained austenite calculated by the method described later from the volume percentage of the microstructure determined to be martensite or retained austenite. The volume percentage of retained austenite was obtained by collecting test specimens from a certain position in the rolling direction of the steel sheet at the middle position in the width direction, chemically polishing the rolled surface of the steel sheet surface to the position of 1 / 4 of the sheet thickness, and calculating the integrated intensity of the (200) and (210) planes of ferrite and the (200), (220), and (311) planes of austenite with MoKa radiation. Metallographic Structure of Surface Layer Area: Ferrite Volume Percentage is 0.0% or More and 20.0% or Less and Ferrite Grain Density Having Grain Size of 15 μm or More is 0 grains / mm2or More and 3,000 grains / mm2or Less In the surface layer area at a position of 25 μm from the surface in the thickness direction of the sheet, when the volume percentage of ferrite is more than 20.0% and the density of ferrite grains having a grain size of 15 μm or more is more than 3,000 grains / mm2, the bending ability is reduced. On the other hand, in the surface layer area at a position of 25 μm from the surface in the thickness direction of the sheet, when the volume percentage of ferrite is 20.0% or less and the density of ferrite grains having a grain size of 15 μm or more is 3,000 grains / mm2or less, the bending ability is enhanced. This is thought to be due to the homogeneous microstructure and the increased bending ability resulting from the less soft phase and the less coarse soft phase. Therefore, in the metallographic structure of the surface layer area, the volume percentage of ferrite is set at 20.0% or less, and the density of ferrite grains having a grain size of 15 μm or more is set at 3,000 grains / mm2 or less. In the metallographic structure of the surface layer area, the volume percentage of ferrite is preferably 18.0% or less, more preferably 15.0% or less, and even more preferably 10.0% or less. In addition, the volume percentage of ferrite can be set at 1.0% or more. In the metallographic structure of the surface layer area, the density of ferrite grains having a grain size of 15 μm or more is preferably 2,500 grains / mm2 or less, and more preferably 2,000 grains / mm2 or less. In addition, the density of ferrite grains having a grain size of 15 μm or more can be set at 100 grains / mm2 or more. The volume percentage of ferrite in the surface layer area was obtained by collecting the test specimens from a certain position in the rolling direction of the steel sheet at the middle position in the width direction, polishing the longitudinal section parallel to the rolling direction, and observing the metallographic structure at the position (specifically, the region 10 to 40 μm from the surface χ region 50 μm in the rolling direction) 25 μm from the surface, which was seen by nital etching, using SEM. In addition, the density of ferrite grains having a grain size of 15 μm or more was calculated by dividing the number (of grains) of ferrite grains having a grain size of 15 μm or more in the cross-section observed with SEM by the observed area (mm2). Amount of dissolved Si in the Surface Layer Area: 0.30%mass or More and 1.50%mass or Less When the amount of dissolved Si in the surface layer area is less than 0.30 mass %, the strength of the soft ferrite decreases, the strength difference from the hard phase increases, and the bending ability deteriorates. Therefore, the amount of dissolved Si in the surface layer area is set at 0.30 mass % or more. The amount of dissolved Si in the surface layer area is preferably 0.35 mass % or more, and more preferably 0.40 mass % or more. In addition, when the amount of dissolved Si in the surface layer area is more than 1.50 mass %, the tempering of martensite by reheating after the post-annealing cooling step is delayed, which results in a hard microstructure and a decrease in bending ability. Therefore, the amount of dissolved Si in the surface layer area is set at 1.50 mass % or less. The amount of dissolved Si in the surface layer area is preferably 1.20 mass % or less, more preferably 1.00 mass % or less, and even more preferably 0.90 mass % or less. In addition, in the cold-rolled steel sheet according to this embodiment, the ratio of the amount of dissolved Si in the surface layer area to the amount of dissolved Si in the t / 4 section is preferably 0.85 to 1.10. When the ratio between the amount of dissolved Si in the t / 4 portion and the amount of dissolved Si in the surface layer area is within the above range, the tempering of martensite by reheating after the post-annealing cooling step is carried out uniformly in the thickness direction of the sheet. Therefore, a uniform structure can be obtained, so that good bending ability and hydrogen embrittlement resistance can be obtained. The ratio of the amount of dissolved Si is preferably 0.87 to 1.05, and more preferably 0.90 to 1.05. The amount of dissolved Si in the surface layer area and in the t / 4 section was measured by the following method. The amount of dissolved Si was obtained by simultaneously measuring Si and O at the target position with EPMA and performing quantitative analysis. The amount of dissolved Si was obtained by collecting test specimens from a certain position in the rolling direction of the steel sheet at the middle position in the width direction, and performing Si and O line analysis in the rolling direction with EPMA on the longitudinal section (i.e., the cross section parallel to the rolling direction and parallel to the thickness direction) parallel to the rolling direction at a position 25 μm from the surface and at a position 1 / 4 of the sheet thickness t in the sheet thickness direction. However, in the case where Si and O were detected simultaneously, Si oxide was present, so the area where O was detected was excluded. These measurements were performed at 10 or more points, and the average value was defined as the amount of dissolved Si. Mechanical Properties Tensile Strength: 1,310 MPa or More Uniform Elongation: 5.0% or More Value (R / t) Obtained by Dividing the Limit Bending Radius R in a 90° V-Bend by the Sheet Thickness t: 5.0 or Less In the cold-rolled steel sheet according to this embodiment, the tensile strength (TS) as a strength that contributes to the reduction of the vehicle body weight is preferably set at 1,310 MPa or more. From the viewpoint of impact absorption, the tensile strength of the steel sheet is preferably 1,350 MPa or more, even more preferably 1,400 MPa or more, and still more preferable. 1,470 MPa or more. There is no need to set an upper limit on tensile strength. However, there are cases where increasing tensile strength results in a decrease in forming ability. Therefore, the tensile strength can be set at 1,900 MPa or less. In addition, from the viewpoint of forming ability, the uniform elongation (uEl) is preferably set at 5.0% or more. To improve forming ability, the uniform elongation (uEl) is preferably 5.5% or more. Although it is not necessary to limit the upper limit of the uniform elongation, the uniform elongation can be set at 30.0% or less or 20.0% or less. In addition, from the viewpoint of bending ability, the value obtained by dividing the limit bending radius R in 90° V-bending by the sheet thickness t (i.e., the limit bending radius R standardized by division by the sheet thickness t) (R / t) is preferably set at 5.0 or less. (R / t) is preferably 4.0 or less and even more preferably 3.0 or less to improve the bending ability. (R / t) can be set at 0.5 or more or 1.0 or more. The tensile strength (TS) and uniform elongation (uEl) were obtained by collecting JIS No. 5 tensile test specimens from steel sheets in the direction perpendicular to the rolling direction and conducting tensile tests according to JIS Z 2241:2011. The standardized limit bending radius (R / t) according to the sheet thickness is obtained by obtaining the minimum bending radius (limit bending radius) R at which no cracking occurs when a 90° V-bending die is used and the radius R is changed at a pitch of 0.5 mm, and dividing the minimum bending radius by the sheet thickness t. Cold-rolled steel sheets according to this embodiment can be provided with a hot-dip galvanized coating on their surfaces. Corrosion resistance is enhanced by forming a galvanized coating on their surfaces. If there is concern about perforation due to corrosion in steel sheets for vehicles, there are cases where the steel sheet cannot be thinned to a certain sheet thickness or less even if high reinforcement is achieved. One of the purposes of high reinforcement of steel sheets is to reduce weight by thinning. Therefore, even if high-strength steel sheets are developed, the application range of steel sheets with low corrosion resistance is limited. As a method to overcome this problem, galvanizing such as hot-dip galvanizing with high corrosion resistance can be applied to steel sheets.On cold rolled steel sheets according to this embodiment, since the composition of the steel sheet is controlled as described above, hot dip galvanizing can be carried out. Hot dip galvanizing coating can be a hot dip galvanizing coating. Hot dip galvanized coating and hot dipped galvanil coating may be plating layers formed by ordinary methods. Manufacturing Conditions In particular, cold-rolled steel sheets according to this embodiment may be prepared by a preparation method including the following configurations (I) to (VII): (I) the hot rolling step consists of heating, as required, a cast slab having the chemical composition described above, and hot rolling the cast slab under conditions where the rolling temperature FT at the final seat is 960°C or lower, the rolling reduction at the final seat is 10% or more, and the friction coefficient μ at the final seat is 0.15 or more to obtain a hot-rolled steel sheet; (II) the rolling step consists of cooling the hot rolled steel sheet to a rolling temperature of 560°C or higher and 650°C or lower and rolling the hot rolled steel sheet at that rolling temperature; (III) the cold rolling step consists of carrying out cold rolling on the hot rolled steel sheet after the rolling step under conditions where the cumulative rolling reduction is 60% or less to obtain cold rolled steel sheet; (IV) the annealing step consists of heating the cold-rolled steel sheet to a soaking temperature of 820°C or higher so that the average heating rate up to 750°C becomes 3.0 °C / second or faster, and holding the cold-rolled steel sheet at that soaking temperature; (V) a post-annealing cooling step consisting of cooling the cold-rolled steel sheet after the annealing step to a temperature of 50°C or higher and 250°C or lower so that the average cooling rate in the temperature range of 700°C to 600°C and in the temperature range of 450°C to 350°C is 5.0 °C / second or faster; and (VI) a tempering step consisting 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) wherein the temperature of the hot-rolled steel sheet after the hot-rolling step is made to reach 500°C or lower within 10 hours from the completion of the hot-rolling step. Next, each step will be explained. Hot Rolling Steps In the hot rolling step, the heated cast slab is hot rolled to obtain hot-rolled steel sheets. In cases where the temperature of the cast slab is high, the cast slab can be hot rolled as is without being cooled to about room temperature. The heating conditions of the slab in hot rolling are not limited, but the cast slab is preferably heated to 1,100°C or higher. By setting the heating temperature at 1,100°C or higher, inadequate homogenization of the material can be avoided. In order to control the metallographic structure and the amount of dissolved Si of the surface layer area, the rolling temperature (FT) at the finishing end stand (finishing pass) during hot rolling is set at 960°C or lower, the rolling reduction at the finishing stand is set at 10% or more, and the friction coefficient μ during rolling at the finishing stand is set at 0.15 or more. In the hot rolling step, especially the surface layer area undergoes shear deformation, so that carbides are finely precipitated in the surface layer area in the subsequent rolling step. In this case, by performing cold rolling and subsequent annealing under predetermined conditions, the Si concentration distribution in the surface layer is suppressed, and the amount of dissolved Si and the state of existence of ferrite in the surface layer area are favored. When the rolling temperature is high, the shear deformation effect is reduced, the desired microstructure in the surface layer cannot be formed, and the bending ability and hydrogen embrittlement resistance are not sufficiently improved. Therefore, the rolling temperature (FT) at the final stand is set at 960°C or lower. The rolling temperature at the final stand is preferably 940°C or lower. Although the lower limit of the rolling temperature at the final stand is not limited, the rolling force increases as the rolling temperature decreases. Therefore, the rolling temperature at the final stand can be set at 870°C or higher. In addition, if the rolling reduction at the finishing stand is low or the friction coefficient during the rolling finishing stand is low, the surface layer area does not undergo sufficient shear deformation and the desired microstructure in the surface layer area cannot be formed, and the bending ability and hydrogen embrittlement resistance are not sufficiently improved. Therefore, the friction coefficient μ of the rollers in contact with the steel sheet at the finishing finishing stand during hot rolling is set at 0.15 or more. Preferably, the friction coefficient μ is 0.20 or more. Although the upper limit of the friction coefficient μ is not limited, the rolling force increases as the friction coefficient μ increases. Therefore, the friction coefficient μ can be set at 0.40 or less. In addition, the rolling reduction at the final seat is set at 10% or more. The rolling reduction at the final seat is preferably 12% or more. In addition, the rolling reduction at the final seat need not be limited from the viewpoint of microstructure control of the surface layer area, but is preferably set at 15% or less from the viewpoint of manufacturability such as shape control. Rolling Steps After cooling to the rolling temperature as described above, rolling is performed. The rolling temperature is set at 560°C or higher and 650°C or lower. When the rolling temperature is higher than 650°C, the microstructure of the hot-rolled steel sheet becomes a coarse ferrite-pearlite structure, and a microstructure in which carbides are finely and uniformly dispersed is not obtained. In addition, a Si depletion layer is formed in the surface layer by internal oxidation, and the amount of Si dissolved in the surface layer area is reduced. As a result, the bendability deteriorates. The rolling 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 rolling temperature is lower than 560°C, there is a possibility that the transformation starts before rolling and the microstructure of the steel sheet becomes non-uniform. In this case, in the surface layer area, the density of ferrite grains having a grain size of 15 μm or more does not become 3,000 grains / mm2 or less. By setting the rolling temperature at 560°C or higher and rolling the steel sheet before the start of transformation, the microstructure of the entire steel sheet can be made uniform. In addition, the strength of the hot-rolled steel sheet can be lowered to reduce the load during cold rolling. Therefore, the rolling temperature is 560°C or higher. In cases where the strength of the hot-rolled steel sheet is high, a softening heat treatment such as BAF can be performed before cold rolling. In the method of manufacturing cold-rolled steel sheet according to this embodiment, the temperature of the steel sheet is made to reach 500°C or lower within 10 hours from the completion of the hot-rolling step. By making the temperature of the steel sheet to 500°C or lower within 10 hours, the formation of a Si depletion layer in the surface layer due to internal oxidation of the hot-rolled steel sheet is suppressed (i.e., a constant amount of dissolved Si is obtained in the surface layer area). As a result, good bending ability is obtained after annealing. The time from the completion of the hot rolling step until the steel sheet temperature reaches 500°C or lower is controlled by adjusting the cooling in the rolling step or cooling after rolling. Since heat generation occurs due to transformation at about 500°C, it is not easy to reach 500°C or lower within 10 hours by air cooling in the case of rolling at 560°C or higher, and forced cooling (e.g., water cooling) is preferred. In cases where cooling is performed on hot-rolled (coil-shaped) steel sheets after rolling, the strength increases, the load of cold rolling, which is the next step, increases, or the cost increases. Therefore, in general, forced cooling is not performed on hot-rolled steel sheets after rolling.However, to obtain a cold-rolled steel sheet according to this embodiment, as described above, the time from the completion of the hot-rolling step until the temperature of the steel sheet reaches 500°C or lower, is set at 10 hours or shorter. The time from completion of the hot rolling step until the steel sheet temperature reaches 500°C or lower is preferably 5 hours or shorter. In addition, it is preferred that the temperature of the steel sheet is made to reach 450°C or lower within 10 hours from the completion of the hot rolling step, and it is more preferred that the temperature of the steel sheet is made to reach 450°C or lower within 8 hours from the completion of the hot rolling step. Cold Rolling Steps In the cold rolling step, the hot-rolled steel sheet that has been subjected to hot rolling is descaled by pickling or the like and then cold-rolled under the condition that the rolling reduction (cumulative rolling reduction) is 60% or less to obtain a cold-rolled steel sheet. When the rolling reduction in cold rolling is high, recrystallization during annealing is promoted, coarse ferrite is formed, a uniform microstructure cannot be formed in the surface layer area, so that the bending ability and hydrogen embrittlement resistance are deteriorated. Therefore, the rolling reduction in cold rolling is set at 60% or less. The rolling reduction is preferably 55% or less, and preferably 50% or less. Although the lower limit of rolling reduction is not limited, rolling reduction of 30% or more is preferred from a manufacturability point of view. Annealing Steps The cold-rolled steel sheet after the cold-rolling step is subjected to treatment such as degreasing according to a known method as required, then heated to a soaking temperature of 820°C or higher so that the average heating rate up to 750°C becomes 3.0 °C / s or faster, and held at said soaking temperature. In the annealing step, when the average heating rate up to 750°C is slow, Si is partitioned between ferrite and austenite, and the amount of dissolved Si in the surface layer decreases, so that the bending ability is deteriorated. Therefore, the average heating rate is set at 3.0°C / s or higher. On the other hand, by setting the average heating rate in the above temperature range to 50.0 °C / s or slower, the excessive promotion of ferritic transformation due to austenite refinement can be suppressed, which is advantageous in improving the strength and bending ability. Therefore, said average heating rate is preferably set at 50.0 °C / s or slower. Said average heating rate is preferably 30.0 °C / s or slower, and even more preferably 10.0 °C / s or slower. The soaking temperature (annealing temperature) in the annealing step is set at 820°C or higher. If the soaking temperature is low, single-phase annealing of austenite is not carried out, the volume percentage of ferrite increases, and the bending ability is deteriorated. The soaking temperature is preferably 830°C or higher or 835°C or higher. Bending ability can be easily obtained with a high soaking temperature. However, if the soaking temperature is too high, the production cost is increased. Therefore, 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 30 to 450 seconds is preferred. By setting the soaking time to 30 seconds or longer, austenitization can proceed adequately. Therefore, a soaking time of 30 seconds or longer is preferred. On the other hand, from a productivity standpoint, a soaking time of 450 seconds or shorter is preferred. Post-Annealing Cooling Steps In order to obtain the metallographic structure described above, the cold-rolled steel sheet after annealing is cooled to a temperature of 50°C or higher and 250°C or lower (cooling stop temperature) so that the average cooling rate in the ferritic transformation temperature range of 700°C to 600°C and the average cooling rate in the bainitic transformation temperature range of 450°C to 350°C is 5.0°C / s or faster. When the cooling rate in the above temperature range is slow, the volume percentage of ferrite and bainite at the position of 1 / 4 of the sheet thickness from the surface increases, and the volume percentage of tempered martensite decreases. As a result, the tensile strength decreases, and the bending ability and hydrogen embrittlement resistance deteriorate. Therefore, the average cooling rate from 700°C to 600°C and from 450°C to 350°C is set at 5.0°C / s or faster.Each of the average cooling rates in the above temperature range is preferably 10.0 °C / s or higher and more preferably 20.0 °C / s or higher. The upper limit of the average cooling rate in the above temperature range is not limited. However, if the cooling rate is fast, it is difficult to perform uniform cooling in the width direction, and the shape of the steel sheet is deteriorated. Therefore, each of the average cooling rates is preferably 100 °C / s or slower. The cooling stop temperature is set at 50°C or higher and 250°C or lower. When the cooling stop temperature is high, the (untempered) martensite increases in cooling after the subsequent tempering step, and the bending ability and hydrogen embrittlement resistance deteriorate. Therefore, the cooling stop temperature is set at 250°C or lower. On the other hand, when the cooling stop temperature is low, the retained austenite fraction decreases, and the uniform elongation is reduced. Therefore, the cooling stop temperature is set at 50°C or higher. The cooling stop temperature is preferably 75°C or higher, and preferably 100°C or higher. In the case of manufacturing cold-rolled steel sheets (hot-dip galvanized steel sheets) equipped with a hot-dip galvanized layer on the surface, in the post-annealing cooling step, in the case where the temperature of the cold-rolled steel sheets is higher than 425°C and lower than 600°C, the cold-rolled steel sheets can be further immersed in a hot-dip galvanizing bath at the same temperature to form a hot-dip galvanized layer on their surface (hot-dip galvanizing step). In addition, in the case of manufacturing cold-rolled steel sheets (hot-dip galvanized steel sheets) having a hot-dip galvanized layer on the surface, an alloying treatment can be performed after the hot-dip galvanizing step described above to fuse the galvanizing layer into a hot-dip galvanized layer (alloying step). Tempering Steps The cold-rolled steel sheet after the post-annealing cooling step is cooled to a temperature of 50°C or higher and 250°C or lower so that the untransformed austenite transforms into martensite. After that, the cold-rolled steel sheet is tempered at a temperature of 200°C or higher and 350°C or lower for 1 second or longer (tempering step), so that the microstructure mainly containing tempered martensite is obtained in the t / 4 section. In cases where hot-dip galvanizing and / or alloying steps are performed, the cold-rolled steel sheet after the hot-dip galvanizing step or the cold-rolled steel sheet after the hot-dip galvanizing step and the alloying step is cooled to a temperature of 50°C or higher and 250°C or lower, and then tempered at a temperature of 200°C or higher and 350°C or lower for 1 second or longer. If the tempering temperature is higher than 350°C, the strength of the steel sheet is reduced. Therefore, the tempering temperature is set at 350°C or lower. The tempering temperature is preferably 325°C or lower, and preferably 300°C or lower. In cases where it is desired to further increase the tensile strength, it is preferred to lower the tempering temperature. For example, in the case where the tensile strength is set at 1,400 MPa or more, the tempering temperature is preferably set at 275°C or lower, and in the case where the tensile strength is set at 1,400 MPa or more, the tempering temperature is preferably set at 275°C or lower.470 MPa or more, the tempering temperature is preferably set at 250°C or lower. On the other hand, if the tempering temperature is lower than 200°C, the tempering becomes inadequate, and the bending ability and hydrogen embrittlement resistance deteriorate. Therefore, the tempering temperature is set at 200°C or higher. From the viewpoint of bending ability and hydrogen embrittlement resistance, the tempering temperature is preferably 220°C or higher, and preferably 250°C or higher. The tempering time can be 1 second or longer, but 5 seconds or longer is preferred, and 10 seconds or longer is preferred to achieve stable tempering treatment. On the other hand, to avoid reducing the strength of the steel sheet, the tempering time is preferably 750 seconds or shorter, and 500 seconds or shorter is preferred. In this embodiment, the term tempering means cooling to the tempering temperature described above in a post-annealing cooling step and then holding at that temperature, or cooling to a temperature lower than the tempering temperature in a post-annealing cooling step, then raising the temperature to the tempering temperature, and holding at that temperature. In addition, the term holding means not only maintaining at a constant temperature but also allowing a temperature change of 1.0 °C / sec or less within the tempering temperature range described above (i.e., 200°C or higher and 350°C or lower). Example The present invention will be explained more particularly with reference to examples. A slab having the chemical composition shown in Table 1 was cast. The slab after casting was heated to 1,100°C or higher, hot-rolled to 2.8 mm, rolled, and then cooled to room temperature. The hot-rolling conditions and rolling temperatures were as shown in Tables 2A and 2B. In addition, the time from the completion of hot-rolling to reaching 500°C or lower and the time to reach 450°C or lower were as shown in Tables 2A and 2B. After that, scale removal was carried out by acidification, cold rolling to 1.4 mm was carried out, and then annealing was carried out at the soaking temperature shown in Tables 2A and 2B for 120 seconds. The average heating rate up to 750°C during annealing heating was set as shown in Tables 2A and 2B. After annealing, cooling to a cooling stop temperature of 50°C or higher and 250°C or lower is carried out so that the average cooling rate in the temperature range of 700°C to 600°C and in the temperature range of 450°C to 350°C is 20°C / s or faster, and thereafter a tempering heat treatment is carried out for 1 to 500 seconds at the tempering temperature shown in Tables 2A and 2B. In the case where the cooling stop temperature is lower than the tempering temperature, tempering is carried out by heating to the tempering temperature shown in Tables 2A and 2B and holding at that temperature, and in the case where the cooling stop temperature is the same as the tempering temperature, tempering is carried out by cooling and then holding at that temperature. In some examples, hot-dip galvanizing and / or alloying is performed during post-annealing cooling. CR shown in Table 5 indicates ungalvanized cold-rolled steel sheet, GI is hot-dip galvanized steel sheet, and GA is hot-dip galvanized steel sheet. Hot-dip galvanized steel sheet is subjected to hot-dip galvanization of approximately 35 to 65 g / m2 at a temperature higher than 425°C and lower than 600°C. Hot-dip galvanized steel sheet is subjected to hot-dip galvanization of approximately 35 to 65 g / m2 at a temperature higher than 425°C and lower than 600°C and then further alloying at a temperature higher than 425°C and lower than 600°. From the cold rolled steel sheets obtained, the metallographic structure of the t / 4 section, the amount of dissolved Si in the surface layer area, the percentage of ferrite volume in the surface layer area, the density of ferrite grains having a grain size of 15 μm or more in the surface layer area, the ratio of the amount of dissolved Si in the surface layer area to the amount of The dissolved Si in the t / 4 part is obtained by the method described above. The results are shown in Tables 3 and 4. In addition, tensile strength (TS), uniform elongation (uEl), bending ability (R / t), and hydrogen embrittlement resistance were evaluated as described below. The results are shown in Table 5. The tensile strength (TS) and uniform elongation (uEl) were obtained by collecting JIS No. 5 tensile test specimens from cold-rolled steel sheets obtained in the direction perpendicular to the rolling direction, and conducting tensile tests according to JIS Z 2241:2011. The results are shown in Table 5. The ultimate bending radius (R / t), which is an index of bending ability, is obtained by obtaining the minimum bending radius R at which no cracking occurs when a 90° V-bending die is used and the radius R is changed at a division distance of 0.5 mm, and dividing the minimum bending radius by the sheet thickness (1.4 mm). The following tests were performed to evaluate hydrogen embrittlement resistance. That is, the test specimen having a mechanically ground end surface was bent into a U shape by a compression bending method to produce a U-bend specimen having a radius of 5R, the U-bend specimen was tightened with bolts to deform elastically so that the unbent portions were parallel to each other, and after that a delayed fracture acceleration test in which hydrogen was allowed to penetrate into the steel sheet was carried out by immersing the U-bend specimen in hydrochloric acid having a pH of 1. The specimen in which cracking did not occur even when the immersion time was 100 hours was evaluated as a steel sheet having good delayed fracture resistance properties (OK), and the specimen in which cracking occurred was evaluated as defective (NG).To eliminate the influence of plating, regarding the plating material, the plating layer was removed with hydrochloric acid containing an inhibitor before testing, and after that the hydrogen embrittlement resistance was evaluated. As can be seen from Tables 1 to 5, all steels of the present invention have a TS of 1310 MPa or more, a uEl of 5.0% or more, a limit bending radius (R / t) of 5.0 or less, and good hydrogen embrittlement resistance. In contrast to this, in the test number (comparative example) where any of the chemical composition and manufacturing method are outside the range of the present invention and the metallographic structure and texture are outside the range of the present invention, any of the tensile strength, uniform elongation, and ultimate bending radius, and hydrogen embrittlement resistance do not reach the target. Table 1 Baj a Chemical composition (%mass) (rest: Fe and impurities) C Si Mn PS Al N Other A 0.137 0.43 1.81 0.009 0.001 0.032 0.0033 B 0.226 0.34 2.44 0.010 0.031 0.031 0.0035 C 0.234 0.99 1.27 0.009 0.001 0.029 0.0028 D 0.403 0.42 3.44 0.008 0.001 0.031 0.0027 E 0.222 1.091 .091 0.001 0.037 0.0029 F 0.326 0.82 3.64 0.010 0.001 0.032 0.0026 G 0.224 0.77 2.12 0.008 0.001 0.114 0.027 H 0.737 2.66 0.009 0.001 0.033 0.0032 I 0.245 0.74 2.55 0.008 0.001 0.032 0.0031 J 0.223 0.74 2.57 0.010 0.001 0.030 K 0.167 0.75 3.15 0.009 0.001 0.032 0.0034 L 0.228 0.75 2.55 0.009 0.001 0.032 0.0031 Ni: 0.10 Cu: 0.122 V: 0.24 M 0.74 2.51 0.009 0.001 0.034 0.0033 Ti: 0.017 Nb: 0.013 B: 0.0015 N 0.342 0.74 1.81 0.010 0.001 0.029 0.0032 Mo: O 0.231 0.73 2.61 0.009 0.001 0.028 0.0029 Bi: 0.008 LTJ: 0.008 P 0.234 0.76 2.47 0.009 0.001 0.031 0.003 Ca: 0.07 Mg: 0.0007 Table 2A No. Steel Test Hot rolling step Rolling step Time from completion of hot rolling until 500°C or lower is reached (hours) Time from completion of hot rolling until 450°C or lower is reached (hours) Cold rolling step Annealing step Tempering step Final seat rolling temperature (°C) Final seat rolling reduction (%) Final seat reduction friction coefficient μ Rolling temperature (°C) Cumulative rolling reduction (%) Average heating rate to 750°C (°C / sec) Soaking temperature (°C) Tempering temperature (°C) 1 A 930 12 0.25 610 5 7 50 5.0 825 260 2 B 930 11 0.25 590 5 7 50 5.0 845 235 3 B 930 11 0.25 640 5 7 50 5.0 845 235 4 C 935 12 0.25 630 5 7 50 5.0 820 265 5 D 940 11 0.25 615 5 7 50 5.0 820 210 6 E 940 12 0.25 640 5 7 50 5.0 830 270 7 E 940 12 0.25 570 5 7 50 5.0 830 275 8 F 930 13 0.25 600 5 7 50 5.0 835 250 9G 930 12 0.25 610 5 7 50 5.0 830 225 10 H 945 12 0.25 575 5 7 50 5.0 855 260 11 H 940 12 0.25 610 5 7 50 5,0 810 265 12 H 965 12 0,25 610 5 7 50 5,0 825 260 13 H 940 8 0,25 620 5 7 50 5,0 825 265 14 H 930 11 0,10 615 5 7 50 5,0 825 260 15 H 930 12 0,25 660 5 7 50 5,0 830 280 16 H 930 13 0,25 680 5 7 50 5,0 825 285 17 H 960 12 0,20 570 11 14 50 5,0 840 245 18 H 955 11 0,20 550 5 7 50 5,0 835 245 19 H 935 13 0,20 570 5 7 50 5,0 850 245 20 H 935 14 0,25 570 5 7 50 5,0 850 245, Tabel 2B No. Steel Test Hot rolling step Rolling step Time from completion of hot rolling until 500°C or lower is reached (hours) Time from completion of hot rolling until 450°C or lower is reached (hours) Cold rolling step Annealing step Tempering step Final seat rolling temperature (°C) Final seat rolling reduction (%) Final seat reduction friction coefficient μ Rolling temperature (°C) Cumulative rolling reduction (%) Average heating rate to 750°C (°C / sec) Soaking temperature (°C) Tempering temperature (°C) 21 H 930 13 0.25 590 5 7 50 5.0 825 250 22 H 920 11 0.25 610 5 7 50 5.0 830 255 23 H 920 12 0.25 600 5 7 50 5.0 830 300 24 H 935 12 0.25 590 5 7 50 5.0 850 220 25 H 940 13 0.25 590 5 7 50 5.0 835 280 26 H 930 11 0.25 595 5 7 50 5.0 845 260 27 H 930 12 0.25 595 5 7 50 5.0 840 250 28 H 935 13 0.25 580 5 7 50 5.0 830 305 29 I 935 13 0.25 575 5 7 50 5.0 840 245 30 J 940 12 0.25 595 5 7 50 5.0 845 230 31 K 930 12 0.25 610 5 7 50 5,0 855 325 32 L 920 12 0.25 600 5 7 50 5.0 845 240 33 M 930 13 0.25 620 5 7 50 5.0 845 240 5 5.0 840 245 35 N 920 13 0.20 595 5 7 50 5.0 845 210 36 O 930 12 0.25 615 5 7 50 5.0 5.0 840 240 38 P 930 13 0.25 625 5 7 50 5.0 840 230 39 P 935 12 0.25 600 5 7 50 5.0 840 235 5 5.0,845,230, Table 3 No. Test Section t / 4 Metallographic structure (% indicates volume percentage) Ferrite (%) Bainite (%) Ferrite and bainite as a whole (%) Retained austenite (%) Martensite (%) Tempered martensite (%) Remainder (%) 1 10.8 6.1 16.9 3.4 0.0 79.7 0.0 2 0.0 6.2 6.2 2.2 0.0 91.6 0.0 3 0.0 6.3 6.3 2.1 0.0 91.6 0.0 4 5.6 17.2 22.8 5.3 0.0 71.9 0.0 5 0.0 0.0 0.0 7.6 5.3 87.1 0.0 6 22.2 22.9 45.1 10.5 7.8 36.6 0.0 7 21.3 24.1 45.4 10.3 7.5 36.8 0.0 8 0.0 0.0 0.0 6.2 6.2 87.6 0.0 9 24.9 0.0 24.9 6.1 0.0 69.0 0.0 10 0.0 8.2 8.2 5.2 0.0 86.6 0.0 11 20.4 0.0 20.4 5.6 1.3 67.6 5.1 12 8.6 3.8 12.4 6.3 1.2 80.1 0.0 13 8.4 3.6 12.0 6.2 1.3 80.5 0.0 14 8.5 4.1 12.6 5.2 1.4 80.8 0.0 15 6.4 2.5 8.9 5.3 0.9 84.9 0.0 16 6.6 2.9 9.5 5.4 0.8 84.3 0.0 17 0.0 2.4 2.4 5.4 0.0 92.2 0.0 18 0.0 3.7 3.7 5.5 0.0 90.8 0.0 19 0.0 0.5 0.5 5.3 0.0 94.2 0.0 20 0.0 0.0 0.0 5.1 0.0 94.9 0.0 21 8.2 3.9 12.1 5.2 1.4 80.2 1.1 22 6.3 2.8 9.1 5.3 0.0 85.0 0.6 23 6.2 2.6 8.8 5.4 0.0 85.8 0.0 24 0.0 0.0 0.0 5.5 0.0 94.5 0.0 25 4.5 2.4 6.9 5.4 0,0 87.7 0.0 26 1.9 1.2 3.1 5.2 0.0 91.7 0.0 27 3.1 1.5 4.6 5.1 0.0 90.3 0.0 28 6.1 2.8 8.9 5.2 0.0 83.7 2.2 29 2.1 0.6 2.7 5.2 0.0 92.1 0.0 30 0.4 0.0 0.4 5.4 0.0 94.2 0.0 31 0.0 0.5 0.5 5.5 0.0 94.0 0.0 32 2.4 0.7 3.1 5.3 0.0 91.6 0.0 33 2.6 1.3 3.9 5.1 0.0 91.0 0.0 34 2.3 0.7 3.0 5.2 0.0 91.8 0.0 35 3.3 1.7 5.0 7.4 2.7 83.8 1.1 36 6.8 3.2 10.0 5.2 0.0 84.8 0.0 37 6.3 3.1 9.4 5.4 0.0 85.2 0.0 38 3.6 2.6 6.2 5.3 0.0 88.5 0.0 39 3.5 2.4 5.9 5.2 0.0 88.9 0.0 40 3.8 2.6 6.4 5.0 0.0 88.6 0.0, Table 4 Test No. Surface layer area (position 25 μη) Amount of dissolved Si (%mass) Ratio of the amount of dissolved Si in the surface layer area to the amount of dissolved Si in the t / 4 part Metallographic structure (% indicates volume percentage) Ferrite (%) Density of ferrite grains having a grain size of 15 μη or more (grains / mm2) 1 0.40 0.93 16.1 2560 2 0.33 0.97 0.0 0 3 0.28 0.82 14.3 2430 4 0.81 0.82 9.7 1500 5 0.41 0.98 0.0 0 6 1.21 0.79 36.7 3970 7 1.51 0.98 21.8 3120 8 0.81 0.99 0.0 0 9 0.71 0.92 30.9 3480 10 0.71 0.97 0.0 0 11 0.66 0.90 32.5 3800 12 0.60 0.82 21.4 3030 13 0.61 0.84 20.9 3060 14 0.60 0.82 20.6 3050 15 0.60 0.82 20.4 3100 16 0.55 0.75 20.4 3020 17 0.57 0.78 20.3 3050 18 0.68 0.93 16.6 3020 19 0.70 0.96 0.0 0 20 0.70 0.96 0.0 0 21 0.70 0.96 9.8 1500 22 0.69 0. 95 7.9 1200 23 0.68 0.93 7.8 1150 24 0.70 0.96 0.0 0 25 0.70 0.96 6.1 950 26 0.70 0.96 2.4 510 27 0.70 0.96 5.6 920 28 0.70 0.96 7.8 990 29 0.70 0.96 3.6 780 30 0.70 0.95 3.5 730 31 0.70 0.93 0.0 0 32 0.70 0.93 3,6 750 33 0.65 0.90 3.8 720 34 0.70 0.97 3.5 700 35 0.70 0.95 4.6 760 36 0.67 0.92 8.1 1030, 37 0.63 0.86 8.0 1050 38 0.65 0.89 5.1 750 39 0.72 0.95 5.0 720 40 0.71 0.93 4.7 680 Table 5 No. Test Presence or absence of plating Mechanical Properties Hydrogen embrittlement resistance Remarks CR / GI / GA TS (MPa) Uniform elongation (%) Ultimate bending radius (R / t) 1 CR 1225 5.3 5.4 OK Comparative Sample 2 CR 1560 4.8 4.6 OK Comparative Sample 3 CR 1545 4.9 5.4 NG Comparative Sample 4 CR 1305 7.2 5.4 OK Comparative Sample 5 CR 1976 6.6 6.1 NG Comparative Sample 6 CR 891 16.4 5.4 OK Comparative Sample 7 CR 897 16.1 5.4 OK Comparative Sample 8 GA 1614 6.3 6.1 NG Comparative Sample 9 CR 1306 7.7 3.9 OK Comparative Sample 10 CR 1473 6.4 2.5 OK Invention Example 11 GA 1308 7.2 6.1 OK Comparative Example 12 CR 1441 6.8 5.4 NG Comparative Example 13 GA 1446 6.6 5.4 NG Comparative Example 14 CR 1449 6.7 5.4 NG Comparative Example 15 GA 1455 6.5 5.4 NG Comparative Example 16 CR 1450 6.7 5.4 NG Comparative Example 17 CR 1557 6.4 5.4 NG Comparative Example 18 GA 1552 6.5 5.4 NG Comparative Example 19 GA 1566 6.4 4.3 OK Invention Example 20 CR 1558 6.2 2.5 OK Example of Invention 21 GA 1456 6.6 2.9 OK Example of Invention 22 CR 1472 6.4 2,5 OK Example of Invention 23 GA 1450 6.4 4.3 OK Example of Invention 24 CR 1577 6.0 2.1 OK Example of Invention 25 CR 1466 6.8 4.3 OK Example of Invention 26 GA 1495 6.5 2.9 OK Example of Invention 27 CR 1480 6.4 2.5 OK Example of Invention 28 CR 1443 6.4 2.9 OK Example of Invention 29 GA 1475 6.3 2.5 OK Example of Invention 30 GA 1572 6.2 2.1 OK Example of Invention 31 CR 1379 7.1 2.9 OK Example of Invention, 32 CR 1479 6.5 2.5 OK Example of the Invention 33 GA 1480 6.3 2.5 OK Example of the Invention 34 CR 1489 6.2 2.5 OK Example of the Invention 35 CR 1860 6.2 3.9 OK Example of the Invention 36 GA 1475 6.3 3.2 OK Example of the Invention 37 CR 1493 6.3 3.2 OK Example of the Invention 38 GA 1485 6.3 2.5 OK Example of the Invention 39 GI 1483 6.5 2.9 OK Example of the Invention 40 GI 1493 6.2 3.2 OK Example of the Invention Applicability in Industry According to the present invention, a cold-rolled steel sheet having high strength as well as uniform elongation, bending ability, and excellent hydrogen embrittlement resistance and a method for producing it can be obtained. Since the steel sheet has sufficient formability that can be applied to processing such as press forming, the present invention can contribute to solving global environmental problems by reducing the weight of vehicle bodies, and thereby make a major contribution to industrial development.

Claims

Claim 1. A cold rolled steel sheet containing, as a chemical composition, 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; 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; 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; REE: 0% or more and 0.050% or less; Bi: 0% or more and 0.050% or less;and the remainder is Fe and impurities, wherein the metallographic structure of the t / 4 portion, which is at a position of 1 / 4 of the sheet thickness t from the surface of the cold-rolled steel sheet in the sheet thickness direction, includes, based on volume percentage, 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 the surface layer area at a position of 25 μm from the surface in the sheet thickness direction, the amount of dissolved Si is 0.30% or more and 1.50% or less based on mass %, the volume percentage of ferrite in the metallographic structure is 0.0% or more and 20.0% or less, and the density of ferrite grains having a grain size of 15 μm or more is 0 granules / mm2 or more and 3,000 granules / mm2 or less; 2. Cold rolled steel sheet according to claim 1, containing, as a chemical composition, based on % by mass, one or two or more elements selected from the group consisting only of: 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% 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; LTJ: 0.0005% or more and 0.050% or less; and Bi: 0.0005% or more and 0.050% or less.

3. Cold rolled steel sheet according to claim 1 or 2, wherein the ratio of the amount of dissolved Si in the surface layer area to the amount of dissolved Si in the t / 4 section is 0.85 to 1.

10.

4. A cold-rolled steel sheet according to any one of claims 1 to 3, wherein the tensile strength of the cold-rolled steel sheet is 1310 MPa or more, the uniform elongation of the cold-rolled steel sheet is 5.0% or more, and R / t, which is a value obtained by dividing the ultimate bending radius R in a 90° V-bend by the sheet thickness t, is 5.0 or less.

5. Cold rolled steel sheet according to claim 4, wherein the tensile strength is 1,400 MPa or more.

6. Cold rolled steel sheet according to any one of claims 1 to 5, wherein a hot dip galvanized layer is formed on the surface.

7. Cold rolled steel sheet according to claim 6, wherein the hot dip galvanized layer is a hot dip galvanized layer.

8. A method for making cold rolled steel sheet, comprising: a hot rolling process of heating, as required, a cast slab containing, as a chemical composition, 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, 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, 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, REE: 0% or more and 0.050% or less, Bi: 0% or more and 0.050% or less, and the remainder is Fe and impurities, and hot rolling the cast slab under conditions where the rolling temperature FT at the final seat is 960°C or lower, the rolling reduction at the final seat is 10% or more, and the friction coefficient μ at the final seat is 0.15 or more to obtain a hot-rolled steel sheet; the rolling process consists of cooling the hot-rolled steel sheet to a rolling temperature of 560°C or higher and 650°C or lower and rolling the hot-rolled steel sheet at that rolling temperature; the cold rolling process consists of cold rolling the hot-rolled steel sheet after the rolling process under conditions where the cumulative rolling reduction is 60% or less to obtain a cold-rolled steel sheet; the annealing process consists of heating the cold-rolled steel sheet to a soaking temperature of 820°C or higher so that the average heating rate up to 750°C becomes 3,0 °C / second or faster, and holding the cold-rolled steel sheet at said soaking temperature; a post-annealing cooling process consisting of cooling the cold-rolled steel sheet after the annealing process at a temperature of 50°C or higher and 250°C or lower so that the average cooling rate in the temperature range of 700°C to 600°C and in the temperature range of 450°C to 350°C is 5.0 °C / second or faster; and a tempering process consisting of holding the cold-rolled steel sheet after the post-annealing cooling process at a temperature of 200°C or higher and 350°C or lower for 1 second or longer, wherein the temperature of the hot-rolled steel sheet after the hot-rolling process is made to reach 500°C or lower within 10 hours from the completion of the hot-rolling process., 9. A method for making cold-rolled steel sheet according to claim 8, wherein the cast slab contains, as a chemical composition, based on % by mass, one or two or more elements selected from the group consisting only of: 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% 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; LTJ: 0.0005% or more and 0.050% or less; and Bi: 0.0005% or more and 0.050% or less.

10. A method for making a cold-rolled steel sheet according to claim 8 or 9, wherein, in the post-annealing cooling process, the cold-rolled steel sheet is immersed in a galvanizing bath in state 5 wherein the temperature of the cold-rolled steel sheet is higher than 425°C and lower than 600°C to form a hot-dip galvanizing layer on the surface of the cold-rolled steel sheet.

11. A method for producing cold-rolled steel sheet according to claim 10, wherein the alloying treatment for alloying the hot-dip galvanized layer is carried out in a post-annealing cooling process.