Cold-rolled steel sheet and its manufacturing method

Abstract

This cold-rolled steel sheet has a predetermined chemical composition, wherein: a metallographic structure at a t / 4 position, which is located at 1 / 4 of the sheet thickness t from the surface in a sheet thickness direction, contains, by volume ratio, 2.0% to 8.0% of residual austenite, 80.0% to 98.0% of tempered martensite, a total of 0.0% to 15.0% of ferrite and bainite, and 0.0% to 5.0% of martensite; both in a central region and in an edge region positioned 50 mm from an edge in a width direction, a 20 µm region, which is positioned 20 µm from the surface in the sheet thickness direction, has a metallographic structure containing, by volume ratio, a total of 75.0% to 100.0% of ferrite and bainite and a total of 0.0% to 25.0% of martensite and tempered martensite; and a 75 µm region, which is positioned 75 µm from the surface in the sheet thickness direction, has a metallographic structure containing, by volume ratio, a total of 0.0% to 15.0% of ferrite and bainite.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same. This application claims priority based on Japanese Patent Application No. 2021-038717 filed in Japan on March 10, 2021, and incorporates its content herein.

Background Art

[0002] Today, with the high degree of specialization in industrial technology fields, materials used in each technical field are required to have special and advanced performance. In particular, regarding steel sheets for automobiles, in consideration of the global environment, the demand for thin, high formability, high-tensile cold-rolled steel sheets has significantly increased in order to reduce the vehicle body weight and improve fuel efficiency. Among steel sheets for automobiles, especially cold-rolled steel sheets used for vehicle body frame parts are required to have high strength, and furthermore, high formability for further application expansion is required. Exemplifying the characteristics required for steel sheets for automobiles, the tensile strength (TS) is 1400 MPa or more, and the uniform elongation is 5.0% or more. Or, depending on the processing method and the parts to be applied, it is also required that the limit bending radius R (R / t) normalized by the plate thickness t in 90° V bending is 5.0 or less.

[0003] Although it is effective to have a structure containing ferrite in order to ensure ductility such as uniform elongation, in order to obtain a strength of 1400 MPa or more in a structure containing ferrite, it is necessary to harden the second phase. However, the hard second phase deteriorates the bendability.

[0004] On the other hand, as a technology for realizing high strength, steel sheets having tempered martensite as the main phase have been proposed (for example, Patent Documents 1 and 2). In Patent Documents 1 and 2, it is described that by making the microstructure a single-phase structure of tempered martensite, the bendability is excellent, and since the carbide, which is a hydrogen trap site, is finely dispersed, the hydrogen embrittlement resistance characteristics are excellent. In addition, Patent Document 3 proposes a steel sheet that utilizes the TRIP effect by retained austenite as a technology for achieving both high strength and high formability. Furthermore, Patent Document 4 discloses an alloyed hot-dip galvanized steel sheet that has a high tensile strength of 1470 MPa or more, while also exhibiting excellent uniform deformability (uniform elongation) and local deformability (local elongation).

[0005] However, the steel sheet described in Patent Document 1 has a low tensile strength of less than 1400 MPa. Therefore, in order to achieve higher strength, it is necessary to further improve the workability, bendability, and hydrogen embrittlement resistance that deteriorate as a result. Furthermore, although the steel sheet described in Patent Document 2 can achieve a high strength of 1400 MPa or more, it has the problem that, because it is cooled to near room temperature during the quenching process, the volume fraction of retained austenite is small, and high uniform elongation cannot be obtained. Furthermore, the steel sheet described in Patent Document 3 has a ferrite phase, making it difficult to achieve high strengths of 1400 MPa or more, and it has poor bendability due to strength differences within its structure. Furthermore, Patent Document 4 does not consider hydrogen embrittlement resistance.

[0006] As an invention to solve these technical problems, Patent Document 5 discloses a cold-rolled steel sheet in which the metal structure at a typical location on the steel sheet, one-quarter of the thickness from the surface, is mainly composed of tempered martensite containing retained austenite, and the softening of the surface layer and refinement of the hard phase in the surface layer are achieved by controlling the dew point during annealing, thereby achieving a high level of both formability and hydrogen embrittlement resistance, which are challenges for high-strength steel sheets. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2009-30091 [Patent Document 2] Japanese Patent Application Publication No. 2010-215958 [Patent Document 3] Japanese Patent Application Publication No. 2006-104532 [Patent Document 4] Japanese Patent No. 6187710 [Patent Document 5] Japanese Patent No. 6635236 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, after repeated investigations by the inventors, it was found that in high-strength steel with a tensile strength of 1400 MPa or more, if the softened surface layer is thick, the yield strength of the component (applicable component yield strength) when the steel plate is processed into a component may not increase in proportion to the strength of the steel plate. In other words, it was found that even if the strength of the steel plate is increased, the design load of the component may not be able to be increased. The present invention was made to solve such problems and aims to provide a cold-rolled steel sheet and a method for manufacturing the same that have high strength, excellent formability and yield strength for applicable parts. [Means for solving the problem]

[0009] The inventors of the present invention investigated a method for obtaining sufficient component strength while improving bendability in a cold-rolled steel sheet having a metal structure mainly composed of tempered martensite, which provides high strength and excellent hydrogen embrittlement resistance. As a result, we found that by adjusting the dew point during annealing and the heat treatment conditions during the manufacturing process, we could increase the proportion of relatively soft microstructure at a distance of 20 μm from the surface in the thickness direction, thereby reducing hardness, while maintaining a microstructure equivalent to that inside the steel sheet at a distance of 75 μm from the surface in the thickness direction. This allowed us to improve bendability while suppressing a decrease in the yield strength of the component.

[0010] This invention was made in view of the above findings. The gist of this invention is as follows. [1] A cold-rolled steel sheet according to one aspect of the present invention has the following composition in mass%, C: 0.180% or more, 0.350% or less, Mn: 2.00% or more, 4.00% 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, Si: 0% or more, 1.00% 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. The chemical composition contains 0% or less of the following elements: 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, 0.0500% or less, and Bi: 0% or more, 0.050% or less, with the remainder being Fe and impurities, and the microstructure at t / 4, which is at 1 / 4 of the plate thickness t in the direction of plate thickness from the surface, is The material comprises, by volume fraction, retained austenite: 2.0% or more, 8.0% or less, tempered martensite: 80.0% or more, 98.0% or less, ferrite and bainite: 0.0% or more, 15.0% or less in total, and martensite: 0.0% or more, 5.0% or less, and in both the edge portion, which is 50 mm from the edge in the width direction, and the central portion in the width direction, the metal structure of the 20 μm portion, which is 20 μm from the surface in the thickness direction, is composed of, by volume fraction, ferrite The material contains bainite in total at 75.0% or more and 100.0% or less, and martensite and tempered martensite in total at 0.0% or more and 25.0% or less, wherein in the 20 μm portion of the metal structure, the average grain size of the martensite and tempered martensite is 5.0 μm or less, and the metal structure in the 75 μm portion, located 75 μm from the surface in the thickness direction of the plate, contains, by volume fraction, ferrite and bainite in total at 0.0% or more and 15.0% or less. [2] The cold-rolled steel sheet described in [1] above has a chemical composition in mass% of: Si: 0.005% or more, 1.00% or less; 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. The following may be included: 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, 0.0500% or less; and Bi: 0.0005% or more, 0.050% or less. [3] The cold-rolled steel sheet described in [1] or [2] above may have a tensile strength of 1400 MPa or more, a uniform elongation of 5.0% or more, and a limit bending radius R in a 90° V bend divided by the sheet thickness t, which is R / t, of 5.0 or less. [4] The cold-rolled steel sheet described in any of [1] to [3] above may have a hot-dip galvanized layer formed on its surface. [5] The cold-rolled steel sheet described in [4] above may have an alloyed hot-dip galvanized layer. [6] A method for manufacturing a cold-rolled steel sheet according to another aspect of the present invention is a method for manufacturing a cold-rolled steel sheet according to any one of [1] to [5], wherein the composition in mass% is: C: 0.180% or more, 0.350% or less, Mn: 2.00% or more, 4.00% 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, Si: 0% or more, 1.00% 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, A hot rolling process to obtain a hot-rolled steel sheet by hot rolling a cast slab having a chemical composition containing 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, 0.0500% or less, and Bi: 0% or more, 0.050% or less, with the remainder being Fe and impurities, after heating as necessary, and then hot rolling the hot-rolled steel sheet, 500℃ or higher The process involves: a winding step of cooling the hot-rolled steel sheet to a winding temperature of 550°C or lower and winding it at the said winding temperature; a cold rolling step of pickling and cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; an annealing step of heating the cold-rolled steel sheet after the cold rolling step to a soaking temperature of 820°C or higher, with the furnace atmosphere during heating being a nitrogen-hydrogen mixed atmosphere containing 1.0% to 20% by volume of hydrogen with a dew point of -20°C or higher and a hydrogen content of 1.0% to 20% by volume, such that the average heating rate from 700°C to the soaking temperature is less than 10.0°C / second, and annealing at the soaking temperature for 30 seconds to 200 seconds; and heating the cold-rolled steel sheet after the annealing step to a temperature range of over 425°C and under 600°C. With an average cooling rate of 5.0°C / second or higher A first cooling step to cool the cold-rolled steel sheet; a holding step after the first cooling step in which the cold-rolled steel sheet is kept in the temperature range of over 425°C and under 600°C for 200 seconds or more and 750 seconds or less; and after the holding step, the cold-rolled steel sheet is brought to a temperature of 50°C or more and 250°C or less. With an average cooling rate of 5.0°C / second or higherA second cooling step for cooling, an annealing step for annealing the cold-rolled steel sheet at a temperature of 200°C or higher and 350°C or lower for 1 second or more after the second cooling step, a third cooling step for cooling to a temperature at which skin pass rolling is possible after the annealing step, and a skin pass step for performing skin pass rolling on the cold-rolled steel sheet after the third cooling step, and the temperature of the hot-rolled steel sheet is made to reach 500°C or lower within 10 hours from the completion of the hot rolling step. [7] In the method for manufacturing a cold-rolled steel sheet according to [6] above, the chemical composition of the cast slab may contain one or more selected from the group consisting of, in mass%, Si: 0.005% or more and 1.00% or less, 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, REM: 0.0005% or more and 0.0500% or less, and Bi: 0.0005% or more and 0.050% or less. [8] In the method for manufacturing a cold-rolled steel sheet according to [6] or [7] above, in the holding step, the cold-rolled steel sheet may be immersed in a plating bath in a state where the temperature of the cold-rolled steel sheet is above 425°C and below 600°C to form a hot-dip galvanized layer on the surface. [9] In the method for manufacturing a cold-rolled steel sheet according to [8] above, in the holding step, an alloying treatment for alloying the hot-dip galvanized layer may be performed. [Advantages of the Invention]

[0011] According to the above aspect of the present invention, it is possible to provide a cold-rolled steel sheet and a method for manufacturing the same, which have high strength, excellent formability, and the strength of the applied parts. [Embodiments for Carrying Out the Invention]

[0012] A cold-rolled steel sheet according to an embodiment of the present invention (the cold-rolled steel sheet according to this embodiment) and a method for manufacturing the same will be described. The cold-rolled steel sheet according to this embodiment has: (a) a predetermined chemical composition; (b) the metallographic structure of the t / 4 part, which is the position of 1 / 4 of the plate thickness t in the plate thickness direction from the surface, is controlled within a predetermined range respectively; (c) in both the edge part, which is the position 50 mm from the end in the width direction, and the central part in the width direction, the metallographic structures of the 20-μm part, which is the position 20 μm in the plate thickness direction from the surface, and the 75-μm part, which is the position 75 μm in the plate thickness direction from the surface, are controlled within a predetermined range respectively. The cold-rolled steel sheet according to this embodiment includes not only a cold-rolled steel sheet without a plating layer on the surface, but also a hot-dip galvanized steel sheet having a hot-dip zinc plating layer on the surface or an alloyed hot-dip galvanized steel sheet having an alloyed hot-dip zinc plating on the surface, and the main conditions of these are common to the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet. However, in the case of a plated steel sheet, the surface serving as a reference for indicating the position defining the metallographic structure means the surface of the base steel sheet excluding the plating. Each will be described below.

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

[0014] C: 0.180% or more and 0.350% or less When the C content is less than 0.180%, it becomes difficult to obtain the above metallographic structure, and the target tensile strength cannot be achieved. Also, the ratio (R / t) of the limit bending radius R to the plate thickness t in 90° V bending deteriorates. Therefore, the C content is 0.180% or more. The C content is preferably more than 0.180%, more preferably 0.200% or more. On the other hand, when the C content exceeds 0.350%, the weldability deteriorates and the bendability deteriorates. Also, the hydrogen embrittlement resistance characteristics deteriorate. Therefore, the C content is 0.350% or less. The C content is preferably less than 0.350%, more preferably 0.300% or less.

[0015] Mn: 2.00% or more, 4.00% or less Mn has the effect of improving the hardenability of steel and is an effective element for obtaining the desired metallic structure described later. If the Mn content is less than 2.00%, it becomes difficult to obtain the desired metallic structure. In this case, sufficient tensile strength cannot be obtained. Therefore, the Mn content should be 2.00% or more. Preferably, the Mn content is greater than 2.00%, more preferably 2.20% or more, and even more preferably 2.40% or more. On the other hand, if the Mn content exceeds 4.00%, the effect of improving hardenability is diminished due to Mn segregation, and material costs increase. Therefore, the Mn content should be 4.00% or less. Preferably, the Mn content is less than 4.00%, more preferably 3.50% or less, and even more preferably 3.25% or less.

[0016] P: 0% or more, 0.100% or less P is an element present in steel as an impurity, and it segregates at grain boundaries, causing the steel to become brittle. Therefore, a lower P content is preferable, and it may even be 0%, but considering the time and cost of removing P, the P content should be 0.100% or less. Preferably, the P content is 0.020% or less, and more preferably 0.015% or less. Considering the costs of refining, etc., the P content may be 0.005% or more.

[0017] S: 0% or more, 0.010% or less S is an element present in steel as an impurity, and it forms sulfide inclusions that degrade the bendability of the steel. Therefore, a lower S content is preferable, and it may even be 0%, but considering the time and cost of removing S, the S content should be 0.010% or less. Preferably, the S content is 0.005% or less, more preferably 0.003% or less, and even more preferably 0.001% or less. Considering the costs of refining, etc., the S content may be 0.0001% or more.

[0018] Al: 0% or more, 0.100% or less Al is an element that has the effect of deoxidizing molten steel. When Al is included for the purpose of deoxidation, the Al content is preferably 0.005% or more, and more preferably 0.010% or more, in order to ensure deoxidation. In addition, Al, like Si, has the effect of increasing the stability of austenite and is an effective element for obtaining the above-mentioned metal structure, so it may also be included. If Al is included, the Al content may be, for example, 0.010% or more. On the other hand, if the Al content is too high, surface defects caused by alumina are more likely to occur, the transformation point rises significantly, and the volume fraction of ferrite increases. In this case, it becomes difficult to obtain the above-mentioned metallic structure, and sufficient tensile strength cannot be obtained. Therefore, the Al content should be 0.100% or less. Preferably, the Al content is 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less. In the cold-rolled steel sheet according to this embodiment, it is not necessarily required to contain Al, and the Al content may be 0%.

[0019] N: 0% or more, 0.0100% or less N is an element that can be contained in steel as an impurity, and it is an element that degrades bendability by forming coarse precipitates. Therefore, the N content should be 0.0100% or less. Preferably, the N content is 0.0060% or less, and more preferably 0.0050% or less. The lower the N content, the better, and it may even be 0%. Considering the costs of refining, etc., the N content may be 0.0010% or more, or 0.0020% or more.

[0020] The cold-rolled steel sheet according to this embodiment may contain the above-mentioned elements, with the remainder being Fe and impurities, or it may further contain one or more of the elements listed below that affect strength and bendability as optional elements. However, the inclusion of optional elements is not required, so the lower limit for each is 0%.

[0021] Si: 0% or more, 1.00% or less Si is a useful element for increasing the strength of steel sheets through solid solution strengthening. Furthermore, Si inhibits cementite formation, thus promoting the enrichment of carbon (C) into austenite, and is a useful element for generating retained austenite after annealing. Therefore, Si may be included. To fully obtain the above effects, the Si content is preferably 0.005% or more. A Si content of 0.005% or more ensures sufficient uniform elongation and excellent hydrogen embrittlement resistance. More preferably, the Si content is 0.01% or more, 0.03% or more, 0.05% or more, 0.10% or more, or 0.30% or more. On the other hand, if the Si content exceeds 1.00%, the austenite transformation during annealing heating will be delayed, and the transformation from ferrite to austenite may not occur sufficiently. In this case, excess ferrite will remain in the microstructure after annealing, and the target tensile strength cannot be achieved. In addition, the bendability may deteriorate, the surface properties of the steel sheet may deteriorate, and the chemical treatment properties and plating properties may deteriorate significantly. Therefore, the Si content should be 1.00% or less. Preferably, the Si content is less than 1.00%, more preferably 0.90% or less or 0.85% or less.

[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 improve the strength of steel sheets through precipitation hardening. Therefore, these elements may be included. To fully obtain the above effects, it is preferable that the Ti content and Nb content be 0.001% or more, and the V content and Cu content be 0.01% or more. More preferable Ti content and Nb content are 0.005% or more, and even more preferable V content and Cu content are 0.05% or more. Obtaining the above effects is not essential. Therefore, there is no need to particularly limit the lower limits of the Ti content, Nb content, V content, and Cu content, and their lower limits are 0%. On the other hand, if these elements are included in excess, the recrystallization temperature rises, the metal structure of the cold-rolled steel sheet becomes non-uniform, and the bendability is impaired. Therefore, when they are included, the Ti content should be 0.050% or less, the Nb content 0.050% or less, the V content 0.50% or less, and the Cu content 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 steel sheets, and are effective elements for obtaining the above-mentioned metal structure. Therefore, these elements may be included. To fully obtain the above effects, it is preferable that the Ni content, Cr content, and Mo content be 0.01% or more, and / or the B content be 0.0001% or more. More preferably, the Ni content, Cr content, and Mo content are 0.05% or more, and the B content is 0.0010% or more. Obtaining the above effects is not essential. Therefore, there is no need to particularly limit the lower limits of the Ni content, Cr content, Mo content, and B content, and their lower limits are 0%. On the other hand, including these elements in excess will saturate the effects of the above-mentioned actions and will also be uneconomical. Therefore, when including them, the Ni content and Cr content should be 1.00% or less, the Mo content 0.50% or less, and the B content 0.0100% or less. Preferably, the Ni content and Cr content are 0.50% or less, the Mo content is 0.20% or less, and the B content is 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, 0.0500% 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 strength and bendability by refining the solidification structure. Therefore, these elements may be included. To fully obtain the above effects, it is preferable that the Ca content be 0.0001% or more, the Mg content be 0.0001% or more, and the REM and Bi content be 0.0005% or more, each. More preferably, the Ca content be 0.0008% or more or 0.0010% or more, the Mg content be 0.0008% or more, and the REM and Bi content be 0.0007% or more, each. Obtaining the above effects is not essential. Therefore, there is no need to particularly limit the lower limits of the Ca, Mg, Bi, and REM content, and their lower limits are 0%. On the other hand, if these elements are included in excess, the effects from the above-mentioned actions will saturate, making it uneconomical. Therefore, when including them, the Ca content should be 0.010% or less, the Mg content 0.0100% or less, the REM content 0.0500% or less, and the Bi content 0.050% or less. Preferably, the Ca content is 0.009% or less or 0.002% or less, the Mg content 0.0020% or less, the REM content 0.0020% or less, and the Bi content 0.010% or less. REM refers to rare earth elements, and is a collective term for a total of 17 elements including Sc, Y, and lanthanides, and the REM content is the total content of these elements.

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

[0026] [Residual austenite: 2.0% or more, 8.0% or less] Retained austenite improves the ductility of steel sheets through the TRIP effect and contributes to improved uniform elongation. Therefore, the volume fraction of retained austenite should be 2.0% or more. Preferably, the volume fraction of retained austenite is greater than 2.0%, more preferably 2.5% or more, and even more preferably 3.5% or more. On the other hand, if the volume fraction of retained austenite becomes excessive, the grain size of the retained austenite increases. Such retained austenite with large grain sizes becomes coarse and hard martensite after deformation. In this case, crack initiation points are more likely to occur, and the flexibility deteriorates. For this reason, the volume fraction of retained austenite should be 8.0% or less. Preferably, the volume fraction of retained austenite is less than 8.0%, more preferably 7.0% or less, and even more preferably 6.0% or less.

[0027] [Tempered martensite: 80.0% or more, 98.0% or less] Tempered martensite, like martensite (so-called fresh martensite), is an aggregate of lath-like crystal grains. However, unlike martensite, it has a hard structure containing fine iron-based carbides due to tempering. Tempered martensite is obtained by tempering martensite formed by cooling after annealing through heat treatment or other methods. Tempered martensite is a structure that is less brittle and more ductile than regular martensite. In the 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 resistance to hydrogen embrittlement. Preferably, the volume fraction of tempered martensite is 85.0% or more. In order to set the volume fraction of retained austenite to 2.0% or more, the volume fraction of tempered martensite is 98.0% or less.

[0028] [Ferrite and bainite: Total of 0.0% or more and 15.0% or less] Ferrite is a soft phase obtained by performing two-phase annealing or slow cooling after annealing. When mixed with hard phases such as martensite, ferrite improves the ductility of steel sheets, but to achieve high strengths of 1400 MPa or more, it is necessary to limit the volume fraction of ferrite. Furthermore, bainite is a phase obtained by holding the material at a temperature between 350°C and 450°C for a certain period of time after annealing. Because bainite is softer than martensite, it has the effect of improving ductility, but in order to achieve high strengths of 1400 MPa or more, it is necessary to limit the volume fraction, similar to ferrite mentioned above. Therefore, the total volume fraction of ferrite and bainite should be 15.0% or less. Preferably, it should be 10.0% or less. Since ferrite and bainite do not need to be included, the lower limit 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, it is preferable that the volume fraction of ferrite be less than 10.0% in order to achieve a high strength of 1400 MPa or more.

[0029] [Martensite: 0.0% or more, 5.0% or less] Martensite (fresh martensite) is an aggregate of lath-like crystal grains that can be formed by a transformation from austenite during the final cooling after the tempering process. Martensite is hard and brittle and easily becomes a crack initiation point during deformation, so if the volume percentage of martensite is high, the flexibility deteriorates. For this reason, the volume percentage of martensite should be 5.0% or less. Preferably, the volume percentage of martensite is 3.0% or less, and more preferably 2.0% or less. Since martensite may not be present at all, the lower limit of the volume percentage of martensite is 0.0%.

[0030] In the metal structure at a position 1 / 4 of the plate thickness t (t / 4 part) from the surface in the thickness direction, in addition to the above, pearlite may be included as the remaining structure. However, pearlite is a structure that contains cementite and consumes C (carbon) in the steel, which contributes to improving strength. When the volume ratio of pearlite is 5.0% or less, the strength of the steel plate can be increased. For this reason, it is preferable that the volume ratio of pearlite be 5.0% or less. Preferably, the volume ratio of pearlite is 3.0% or less, and more preferably 1.0% or less.

[0031] The volume fraction of the microstructure in 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 are determined by taking a test specimen from an arbitrary position relative to the rolling direction of the steel sheet and from the center in the width direction. The longitudinal section parallel to the rolling direction (i.e., a section parallel to both the rolling direction and the thickness direction) is polished, and the metallic structure revealed by nital etching at a position 1 / 4 of the sheet thickness t from the surface is observed using a SEM. In the SEM observation, five fields of view are observed at a magnification of 3000x, with the center being 1 / 4 of the sheet thickness t from the surface, and the field of view is 30 μm in the thickness direction and 50 μm in the rolling direction. From the observed images, the area fraction of each structure is measured, and the average value is calculated. Since there is no change in structure in the direction perpendicular to the rolling direction (steel sheet width direction), and the area fraction of the longitudinal section parallel to the rolling direction is equal to the volume fraction, the area fraction obtained from the microstructure observation is taken as the respective volume fraction.

[0032] When measuring the area ratio of each tissue, areas where the substructure is not exposed and the brightness is low are defined as ferrite. Areas with a layered structure of ferrite and cementite are defined as pearlite. Areas where the substructure is not exposed and the brightness is high are defined as martensite or retained austenite. Areas where the substructure is exposed are defined as tempered martensite or bainite.

[0033] Bainite and tempered martensite can be further distinguished by carefully observing the carbides within the grains. Specifically, tempered martensite is composed of martensite lath and cementite formed within the lath. In this case, there are two or more crystal orientation relationships between the martensite lath and cementite, so the cementite constituting tempered martensite has multiple variants. On the other hand, bainite is classified into upper bainite and lower bainite. Upper bainite is composed of lath-like bainite ferrite and cementite formed at the lath interface, so it can be easily distinguished from tempered martensite. Lower bainite is composed of lath-like bainite ferrite and cementite formed within the lath. In this case, unlike tempered martensite, there is only one crystal orientation relationship between the bainite ferrite and cementite, and the cementite constituting lower bainite has the same variant. Therefore, lower bainite and tempered martensite can be distinguished based on the cementite variant. On the other hand, martensite and retained austenite cannot be clearly distinguished by SEM observation. Therefore, the volume fraction of martensite is calculated by subtracting the volume fraction of retained austenite, calculated using the method described later, from the volume fraction of tissue determined to be either martensite or retained austenite.

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

[0035] In the cold-rolled steel sheet according to this embodiment, the metal structure at a position 20 μm from the surface in the thickness direction (20 μm portion) and at a position 75 μm from the surface in the thickness direction (75 μm portion) are controlled as follows, both at the edge portion located 50 mm from the edge in the width direction and at the center portion in the width direction.

[0036] <Metal structure at a position 20 μm from the surface in the thickness direction (20 μm portion)> The microstructure at a distance of 20 μm from the surface in the thickness direction, specifically at 20 μm, contains, by volume fraction, ferrite and bainite in total at 75.0% to 100.0%, and martensite and tempered martensite in total at 0.0% to 25.0%, with an average grain size of martensite and tempered martensite of 5.0 μm or less. By making the surface layer a phase mainly composed of ferrite and bainite, which are relatively softer than tempered martensite, the flexibility is improved. Therefore, the total volume ratio of ferrite and bainite is set to 75.0% or more. Preferably, the total volume ratio of ferrite and bainite is 77.0% or more, more preferably 80.0% or more. On the other hand, the total volume ratio of ferrite and bainite The upper limit for the volumetric ratio may be set at 100.0%. Furthermore, hard martensite and tempered martensite in the surface microstructure not only harden the surface but also increase the crack initiation points, thus degrading flexurality and hydrogen embrittlement resistance. In other words, the martensite and tempered martensite in the surface microstructure need to have a small volume fraction and be fine. Therefore, in the 20 μm section, the total volume fraction of martensite and tempered martensite should be 25.0% or less, and the average particle size of martensite and tempered martensite should be 5.0 μm or less. Preferably, the total volume fraction of martensite and tempered martensite should be 22.0% or less, and more preferably 20.0% or less. Martensite and tempered martensite do not need to be included, so the lower limit of the total volume fraction may be 0.0%. Furthermore, the average particle size of martensite and tempered martensite should preferably be 4.5 μm or less. On the other hand, there is no lower limit specified for the average particle size of martensite and tempered martensite, but the average particle size may be 0.1 μm or more, or 0.5 μm or more.

[0037] The remainder of the metall structure in the 20 μm portion may include retained austenite and pearlite. Preferably, their total volume percentage is 3.0% or less.

[0038] <Metal structure at a position 75 μm from the surface in the thickness direction (75 μm portion)> The microstructure at the 75 μm point, located 75 μm from the surface in the thickness direction, contains ferrite and bainite in a total volume fraction of 0.0% or more and 15.0% or less. By increasing the proportion of soft tissue in the surface layer and reducing hardness, the flexibility is improved. However, if the thickness of the layer containing a large amount of soft tissue (soft layer) from the surface increases, even if the strength of the steel sheet itself does not decrease, the yield strength of the part after processing (part yield strength) may decrease. As a result of the inventors' investigation, it was found that even if the surface layer (for example, the 20 μm portion mentioned above) mainly consists of a soft structure, if the structure in the 75 μm portion is equivalent to that of the inside of the steel plate (for example, the t / 4 portion) (i.e., there is little soft structure), the strength of the component will not decrease. Therefore, in the cold-rolled steel sheet according to this embodiment, the total volume percentage of ferrite and bainite in the 75 μm portion of the metal structure is set to 15.0% or less. Preferably, the total volume percentage of ferrite and bainite is 10.0% or less. On the other hand, ferrite and bainite are included Since it is not required to be rare, the lower limit of the total volume ratio may be set to 0.0%. Preferably, even at a position 60 μm from the surface in the thickness direction (60 μm portion), the total volume fraction of ferrite and bainite is between 0.0% and 15.0%. In the metallographic structure at 75 μm, the remainder may consist of retained austenite, tempered martensite, and martensite. For example, it may contain retained austenite: 2.0% or more and 8.0% or less, tempered martensite: 80.0% or more and 98.0% or less, and martensite: 0.0% or more and 5.0% or less.

[0039] Conventionally, as described in Patent Document 4 above, it has been considered in some studies to form a soft surface layer where the average fraction of hard structures other than ferrite and pearlite is 0.9 times that of the region from 1 / 4 thickness to 1 / 2 thickness on the surface. However, the soft surface layer in Patent Document 4 does not focus on the total volume fraction of ferrite and bainite, and is based on a different concept from the relationship between the metallic structure of the 20 μm and 75 μm portions of the cold-rolled steel sheet according to this embodiment.

[0040] The volume percentages of ferrite, bainite, martensite, tempered martensite, and pearlite in the metallographic structure of the 20 μm, 60 μm, and 75 μm sections were determined by taking test specimens from arbitrary positions relative to the rolling direction of the steel sheet, the center of the width direction, and 50 mm from the edge in the width direction. The longitudinal sections parallel to the rolling direction of each specimen were polished, and the metallographic structure revealed by nital etching at a position 20 μm from the surface in the thickness direction (range 5 to 35 μm from the surface × 50 μm in the rolling direction), a position 60 μm (range 45 to 75 μm from the surface × 50 μm in the rolling direction), or a position 75 μm (range 60 to 90 μm from the surface × 50 μm in the rolling direction) was observed using a SEM in the same manner as for the observation of the t / 4 section. Furthermore, the volume fraction of retained austenite in the metallographic structure of the 20 μm and 75 μm regions is quantified by taking test specimens from arbitrary positions in the rolling direction of the steel sheet, the center in the width direction, and 50 mm from the edge in the width direction. The rolled surface is chemically polished up to 20 μm or 75 μm from the surface of the steel sheet, and the volume fraction is quantified from the (200), (210) area intensity of ferrite and the (200), (220), and (311) area intensity of austenite using MoKα radiation.

[0041] The average particle size of the martensite and tempered martensite in the 20 μm portion is determined by the following method. Test specimens are taken from arbitrary positions in the rolling direction of the steel sheet, and from the center and 50 mm from the edge in the width direction. The longitudinal section parallel to the rolling direction of each specimen is polished, and the microstructure revealed by nital etching at a position 20 μm from the surface of the steel sheet in the thickness direction is observed using a SEM. The equivalent average diameter of the circle of the microstructure determined to be martensite or tempered martensite is calculated from this microstructure using the cutting method described in JIS G 0551 (2013), and this is taken as the average grain size of martensite and tempered martensite.

[0042] In the cold-rolled steel sheet according to this embodiment, as described above, the metal structure of the 20 μm portion and the 75 μm portion are present not only in the central portion in the width direction, but also in the edge portion 50 mm from the edge in the width direction, and each of the phases has the volume fraction and average grain size described above. In this case, there is no need to trim the edges when applying the steel sheet to the parts, which improves the yield of the steel sheet.

[0043] <Mechanical properties> [Tensile strength: 1400 MPa or higher] [Uniform spread: 5.0% or more] [Value obtained by dividing the limiting bending radius R in a 90° V-bend by the plate thickness t (R / t): 5.0 or less] In the cold-rolled steel sheet according to this embodiment, it is preferable that the tensile strength (TS) be 1400 MPa or higher, as this strength contributes to the weight reduction of automobile bodies. From the viewpoint of impact absorption, the tensile strength of the steel sheet is more preferably 1470 MPa or higher. There is no need to limit the upper limit of the tensile strength, but since formability may decrease as the tensile strength increases, the tensile strength may be 1900 MPa or lower. Furthermore, from the viewpoint of moldability, it is preferable that the uniform elongation (uEl) be 5.0% or higher. To further improve moldability, the uniform elongation (uEl) is more preferably 5.5% or higher. There is no need to limit the upper limit of the uniform elongation, but it may be 30.0% or less, or 20.0% or less. Furthermore, from the viewpoint of bendability, the value obtained by dividing the limit bending radius R in a 90° V bend by the plate thickness t (i.e., the limit bending radius R normalized by dividing by the plate thickness t) (R / t) is preferably 5.0 or less. (R / t) is more preferably 4.0 or less, and even more preferably 3.0 or less, in order to further improve bendability. (R / t) may be 0.5 or more, or 1.0 or more.

[0044] Tensile strength (TS) and uniform elongation (uEl) are determined by taking a JIS No. 5 tensile test specimen from the steel sheet perpendicular to the rolling direction and performing a tensile test in accordance with JIS Z 2241:2011. Furthermore, the standardized limit bending radius (R / t) for each plate thickness is determined by using a 90° V-bending die, varying the radius R in 0.5 mm increments, finding the minimum bending radius (limit bending radius) R at which cracking does not occur, and then dividing it by the plate thickness t.

[0045] In the cold-rolled steel sheet according to this embodiment, a hot-dip galvanizing layer may be provided on the surface. Providing a plating layer on the surface improves corrosion resistance. Automotive steel sheets may not be able to be thinned below a certain thickness even if their strength is increased due to concerns about perforation caused by corrosion. One of the purposes of increasing the strength of steel sheets is to reduce weight by making them thinner, so even if a high-strength steel sheet is developed, its application is limited if its corrosion resistance is low. As a method to solve these problems, it is conceivable to apply a plating such as hot-dip galvanizing, which has high corrosion resistance, to the steel sheet. In the cold-rolled steel sheet according to this embodiment, the steel sheet components are controlled as described above, so hot-dip galvanizing is possible. The hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.

[0046] <Manufacturing conditions> Specifically, the cold-rolled steel sheet according to this embodiment can be manufactured by a manufacturing method that includes the following configurations (I) to (XI). (I) A hot rolling process in which a cast slab having the above-mentioned chemical composition is heated as necessary and then hot-rolled to produce a hot-rolled steel sheet, (II) A winding step in which the hot-rolled steel sheet is cooled to a winding temperature of 550°C or less and wound at the winding temperature, (III) A cold rolling step in which the hot-rolled steel sheet is pickled and cold-rolled to obtain a cold-rolled steel sheet; (IV) An annealing step in which the cold-rolled steel sheet after the cold rolling step is heated to a soaking temperature of 820°C or higher, with the furnace atmosphere during heating being a nitrogen-hydrogen mixed atmosphere containing 1.0% to 20% by volume of hydrogen with a dew point of -20°C or higher and a hydrogen content of 1.0% to 20% by volume, such that the average heating rate from 700°C to the soaking temperature is less than 10.0°C / second, and then annealed at the soaking temperature for 30 seconds to 200 seconds; (V) A first cooling step in which the cold-rolled steel sheet after the annealing step is cooled to a temperature range of over 425°C and less than 600°C, (VI) After the first cooling step, a holding step in which the cold-rolled steel sheet is kept in the temperature range of over 425°C and under 600°C for 200 seconds or more and 750 seconds or less, (VII) A second cooling step in which the cold-rolled steel sheet is cooled to a temperature of 50°C or higher and 250°C or lower after the holding step, (VIII) After the second cooling step, a tempering step is performed in which the cold-rolled steel sheet is tempered at a temperature of 200°C to 350°C for 1 second or more, (IX) A third cooling step in which the temperature is cooled to a temperature suitable for skin pass rolling after the tempering step, (X) A skin pass step in which skin pass rolling is performed on the cold-rolled steel sheet after the third cooling step, (XI) The temperature of the hot-rolled steel sheet is brought to 500°C or below within 10 hours of the completion of the hot rolling process. The following explains each of these points.

[0047] [Hot rolling process] In the hot rolling process, the cast slab having the above-mentioned chemical composition is heated and hot-rolled to produce a hot-rolled steel sheet. If the temperature of the cast slab is high, it may be subjected to hot rolling without first cooling it to near room temperature. While there are no limitations on the conditions for hot rolling, it is preferable to heat the material to 1100°C or higher and perform hot rolling so that the exit temperature at the finish rolling stage is above the Ar3 transformation point. Heating to 1100°C or higher helps to avoid insufficient homogenization of the material. Furthermore, if the exit temperature at the finish rolling stage is above the Ar3 transformation point, ferrite structures will not form, making it easier to obtain a uniform structure, which is advantageous for improving bendability. The Ar3 transformation point (°C) can be simply expressed in relation to the elemental content in the steel using the following formula. That is, it is written as shown in equation (1) below. Ar3=910-310×[C]+25×{[Si]+2×[Al]}-80×[Mn eq ] ... Formula (1) Here, if B is not present, [Mn eq ] is shown by the following equation (2a). [Mn eq ]=[Mn]+[Cr]+[Cu]+[Mo]+[Ni] / 2+10([Nb]-0.02) ··· Formula (2a) Also, if B is present, [Mn eq ] is shown by the following equation (2b). [Mn eq ]=[Mn]+[Cr]+[Cu]+[Mo]+[Ni] / 2+10([Nb]-0.02)+1 ··· Formula (2b) In the formula, [C] represents the C content (mass%), [Si] represents the Si content (mass%), [Al] represents the Al content (mass%), [Mn] represents the Mn content (mass%), [Cr] represents the Cr content (mass%), [Cu] represents the Cu content (mass%), [Mo] represents the Mo content (mass%), [Ni] represents the Ni content (mass%), and [Nb] represents the Nb content (mass%).

[0048] [Winding process] After the hot rolling process, the sheet is cooled to the winding temperature before winding. The winding temperature should be 550°C or lower. If the winding temperature exceeds 550°C, the structure of the hot-rolled steel sheet becomes a coarse ferrite-pearlite structure, resulting in an uneven metal structure after annealing and a deterioration in bendability. In addition, surface decarburization increases during winding, making it impossible to control the metal structure of the 75μm portion after annealing within the above range. Furthermore, because the cooling rate is faster at the edges, the difference in structure in the width direction of the hot-rolled steel sheet becomes larger, resulting in an uneven metal structure in the width direction after annealing. A winding temperature of 525°C or lower is preferable. There is no lower limit to the winding temperature, but since low winding temperatures can make temperature control in the width direction difficult, the winding temperature may be set to 450°C or higher, 500°C or higher, or 510°C or higher. If the strength of the hot-rolled steel sheet is high, a softening heat treatment such as BAF may be applied before cold rolling.

[0049] In the cold-rolled steel sheet manufacturing method according to this embodiment, the temperature of the steel sheet is brought down to 500°C or below within 10 hours of the completion of the hot rolling process. By bringing the steel sheet temperature down to 500°C or below within 10 hours, the metal structure becomes the same in the center and edge portions of the sheet width. The time from the completion of the hot rolling process until the temperature of the steel sheet drops below 500°C can be controlled by adjusting the cooling during the winding process and the cooling after winding. Preferably, the time required to bring the temperature of the steel sheet down to 500°C or below after the completion of the hot rolling process is within 5 hours. Furthermore, it is preferable to raise the temperature of the steel sheet to 450°C or below within 10 hours of the completion of the hot rolling process, and more preferably to raise the temperature of the steel sheet to 450°C or below within 8 hours of the completion of the hot rolling process.

[0050] [Cold rolling process] In the cold rolling process, hot-rolled steel sheets are descaled by pickling or other methods, and then cold-rolled to produce cold-rolled steel sheets. The cold rolling conditions are not particularly limited, but promoting recrystallization and homogenizing the metal structure after cold rolling and annealing improves bendability. For this reason, it is preferable to have a cold reduction ratio (cumulative reduction ratio) of 40% or more. A cold rolling ratio of 45% or more is more preferable, and 50% or more is even more preferable. If the cold compression ratio is too high, the rolling load increases, making rolling difficult. Therefore, a cold compression ratio of less than 70% is preferable. A cold rolling ratio of less than 65% is more preferable, and less than 60% is even preferable.

[0051] [Annealing process] In the annealing process, the cold-rolled steel sheet after the cold rolling process is subjected to degreasing and other treatments as necessary according to known methods. Then, the furnace atmosphere during heating is a nitrogen-hydrogen mixed atmosphere containing 1.0% to 20% by volume of hydrogen with a dew point of -20°C to 20°C. The sheet is heated from 700°C to a soaking temperature of 820°C or higher, such that the average heating rate from 700°C to the soaking temperature is less than 10.0°C / second. The sheet is then annealed at the soaking temperature for 30 to 200 seconds. By creating a nitrogen-hydrogen mixed atmosphere in the furnace (heating zone and homogenized zone) with a dew point of -20°C to 20°C and containing 1.0% to 20% by volume of hydrogen, with the remainder being nitrogen and impurities, and performing annealing under this atmosphere, appropriate decarburization occurs in the surface layer of the steel sheet. As a result, the 20 μm and 75 μm sections can be made to the desired metallic structure. In other words, because the surface layer, which has a low carbon content due to decarburization, undergoes ferrite and bainite transformations before the transformation initiation of the core, which has a high carbon content, only the surface layer becomes soft, and by suppressing excessive decarburization, the metallic structure of the 75 μm section can be made equivalent to that of the t / 4 section. However, since decarburization is likely to occur in the temperature range above 700°C, heating to the soaking temperature is performed so that the average heating rate from 700°C to the soaking temperature is less than 10.0°C / second to promote decarburization. The average heating rate in this temperature range is preferably less than 8.0°C / second, and more preferably less than 5.0°C / second. There is no lower limit to the average heating rate from 700°C to the soaking temperature, but from an operational standpoint, the average heating rate may be 1.0°C / second or higher.

[0052] The soaking temperature in the annealing process should be 820°C or higher. If the soaking temperature is below 820°C, the volume fraction of ferrite in the t / 4 portion from the surface will be high, resulting in an insufficient proportion of tempered martensite, making it difficult to ensure sufficient bendability. The soaking temperature is preferably 840°C or higher, and more preferably 850°C or higher. A higher soaking temperature makes it easier to ensure strength, but if the soaking temperature is too high, the manufacturing cost will increase, so the soaking temperature is preferably 900°C or lower. More preferably 880°C or lower, and even more preferably 870°C or lower. The soaking time should be between 30 seconds and 200 seconds. A soaking time of 30 seconds or more allows for sufficient austenitization. On the other hand, from a productivity standpoint, the soaking time should be 200 seconds or less.

[0053] [First cooling process] [Holding process] After the annealing process, the cold-rolled steel sheet is cooled to a temperature range of over 425°C and under 600°C (first cooling step) in order to obtain the gradient structure described above (i.e., a structure in which the total volume fraction of ferrite and bainite differs between the 20 μm and 75 μm sections). In this temperature range (over 425°C and under 600°C), the sheet is held for a residence time of 200 seconds or more and 750 seconds or less (holding step). If the cooling stop temperature and the subsequent holding temperature are below 425°C, the volume fraction of bainite in the t / 4 section increases, and the volume fraction of tempered martensite decreases. As a result, the tensile strength decreases and the bendability deteriorates. On the other hand, if the cooling stop temperature and the subsequent holding temperature are above 600°C, the ferrite fraction increases in the center of the steel sheet, and the volume fraction of tempered martensite decreases. As a result, the tensile strength decreases and the bendability deteriorates. Therefore, the cooling stop temperature and holding temperature should be greater than 425°C and less than 600°C. The holding temperature is preferably greater than 440°C and less than 580°C, and more preferably greater than 450°C and less than 560°C. Within this temperature range, it is acceptable to change the temperature during the residency period. In the first cooling step, it is preferable to cool at an average cooling rate of 5.0°C / second or higher in order to suppress ferrite transformation during cooling. An average cooling rate of 10.0°C / second or higher is more preferable. If the residence time at temperatures above 425°C and below 600°C is less than 200 seconds, the ferrite and bainite transformations in the surface layer (e.g., the 20 μm portion) do not proceed, and the untransformed austenite becomes martensite and tempered martensite after final cooling. As a result, not only does the volume fraction of martensite and tempered martensite increase, but the grain size also increases. Therefore, the residence time at temperatures above 425°C and below 600°C during the holding process should be 200 seconds or more. A residence time of 300 seconds or more is preferable, and 350 seconds or more is more preferable. On the other hand, if the residence time is too long, ferrite and bainite transformations occur in the 75 μm and t / 4 sections, resulting in a failure to obtain the desired microstructure, a decrease in steel sheet strength, and deterioration of bendability. Therefore, the upper limit of the residence time between 425°C and 600°C should be 750 seconds or less. A residence time of 650 seconds or less is preferable, and 550 seconds or less is even more preferable. In the holding process, it is preferable to create a reducing atmosphere in the furnace from the viewpoint of the chemical treatment properties of the steel sheet or the adhesion of the plating.

[0054] When manufacturing cold-rolled steel sheets (hot-dip galvanized steel sheets) with a hot-dip galvanized surface, the cold-rolled steel sheets may be immersed in a hot-dip galvanizing bath during the holding process to perform hot-dip galvanizing (hot-dip galvanizing process). Alternatively, when manufacturing cold-rolled steel sheets (alloyed hot-dip galvanized steel sheets) with an alloyed hot-dip galvanized surface, an alloying process may be performed following the hot-dip galvanizing process to obtain alloyed hot-dip galvanizing (alloying process).

[0055] [Second cooling process] [Tempering process] After the holding process, the cold-rolled steel sheet is cooled to a temperature of 50°C to 250°C (second cooling process), during which the un-transformed austenite transforms into martensite. In the second cooling process, it is preferable to cool at an average cooling rate of 5.0°C / second or higher in order to suppress bainite transformation during cooling. An average cooling rate of 10.0°C / second or higher is more preferable. Subsequently, the cold-rolled steel sheet is tempered at a temperature of 200°C to 350°C for 1 second or more (tempering process), resulting in a structure mainly composed of tempered martensite at a position 1 / 4 of the sheet thickness from the surface. If a hot-dip galvanizing process and / or an alloying process is performed, the cold-rolled steel sheet after the hot-dip galvanizing process, or the cold-rolled steel sheet after both the hot-dip galvanizing and alloying processes, shall be cooled to a temperature of 50°C to 250°C, and then tempered at a temperature of 200°C to 350°C for at least 1 second.

[0056] If the cooling stop temperature in the second cooling step exceeds 250°C, the martensitic transformation will be insufficient, increasing the volume fraction of untempered martensite and degrading the bendability. On the other hand, if the cooling stop temperature in the second cooling step is below 50°C, no retained austenite will remain, degrading the ductility. Therefore, the cooling stop temperature should be between 50°C and 250°C. Preferably, the cooling stop temperature is between 75°C and 225°C, and more preferably between 100°C and 200°C. In the subsequent tempering process, if the tempering temperature exceeds 350°C, the strength of the steel sheet decreases. Therefore, the tempering temperature should be 350°C or lower. Preferably, the tempering temperature is 330°C or lower, and more preferably 310°C or lower. On the other hand, if the tempering temperature is less than 200°C, the tempering will be insufficient and the flexibility will deteriorate. Therefore, the tempering temperature should be 200°C or higher. Preferably, the tempering temperature should be 250°C or higher, more preferably 260°C or higher, and even more preferably 270°C or higher. A tempering time of 1 second or more is sufficient, but 5 seconds or more is preferable for stable tempering, and 10 seconds or more is even preferable. On the other hand, to avoid a decrease in the strength of the steel sheet, a tempering time of 90 seconds or less is preferable, and 60 seconds or less is even preferable. In this embodiment, tempering means either cooling to the tempering temperature in the second cooling step and then holding it at that temperature, or cooling to below the tempering temperature in the second cooling step, then raising the temperature to the tempering temperature and holding it at that temperature. Furthermore, holding in the tempering step does not only mean maintaining a constant temperature, but also allows for temperature changes of 1.0°C / second or less within the tempering temperature range (i.e., 200°C to 350°C).

[0057] [Third cooling process] [Skin Pass Process] After the tempering process, the cold-rolled steel sheet is cooled to a temperature suitable for skin-pass rolling (third cooling step), and then skin-pass rolling is performed (skin-pass step). If the cooling after annealing (first cooling step) is performed using water spray cooling, dip cooling, or steam-water cooling, pickling and plating with one or more of the following elements (Ni, Fe, Co, Sn, Cu) may be applied before skin-pass rolling to remove the oxide film formed by contact with water at high temperatures and to improve the chemical conversion treatment properties of the steel sheet. Here, "trace amount" refers to 3 to 30 mg / m² on the surface of the steel sheet. 2 It refers to the degree of plating. The shape of the steel sheet can be adjusted by skin pass rolling. The elongation rate of skin pass rolling is preferably 0.1% or higher. More preferably 0.2% or higher, and even more preferably 0.3% or higher. On the other hand, if the elongation rate of skin pass rolling is high, the volume fraction of retained austenite decreases and the ductility deteriorates. Therefore, the elongation rate is preferably 1.0% or lower. The elongation rate is more preferably 0.8% or lower, even more preferably 0.6% or lower, and even more preferably 0.5% or lower. [Examples]

[0058] The present invention will be described in more detail with reference to examples. Slabs having the chemical composition shown in Table 1 were cast. After casting, the slabs were heated to over 1100°C and hot-rolled to 2.8 mm so that the finish rolling exit temperature was above the Ar3 transformation point. They were then wound at the winding temperatures shown in Tables 2A and 2B and cooled to room temperature. The time it took for the steel sheet temperature to reach below 500°C and below 450°C after the completion of hot rolling is shown in Tables 2A and 2B. Subsequently, the scale was removed by pickling, and the material was cold-rolled to 1.4 mm. After that, annealing was performed under the conditions shown in Tables 2A and 2B. The holding time at soaking temperature was 120 seconds. The furnace atmosphere was a nitrogen-hydrogen mixed atmosphere with a dew point of -20°C to 20°C and containing 1.0% to 20% by volume of hydrogen. After annealing, the material was cooled at 10°C / second to the holding temperatures shown in Tables 2A and 2B (first cooling), and then kept at those temperatures for the times shown in Tables 2A and 2B. In some cases, hot-dip galvanizing and alloying were performed during the holding period. In Table 6, CR represents cold-rolled steel sheet without galvanizing, GI represents hot-dip galvanized steel sheet, and GA represents alloyed hot-dip galvanized steel sheet. For hot-dip galvanized steel sheets, the density range is 35-65 g / m². 2 A degree of hot-dip galvanizing was applied. For alloyed hot-dip galvanized steel sheets, the rate was 35-65 g / m². 2 After applying a moderate amount of hot-dip galvanizing, the alloy was formed at a temperature of less than 600°C. In this example, the temperature was kept constant during the residency period between 425°C and 600°C, but as mentioned above, it is not a problem to change the temperature during the residency period as long as it is within this temperature range. Furthermore, after holding, the material was cooled at a rate of 10.0°C / second or more to a cooling stop temperature of 50°C to 250°C (second cooling), followed by a heat treatment of tempering at a tempering temperature of 250°C to 350°C for at least 1 second. If the cooling stop temperature was lower than the tempering temperature, tempering was performed by heating to the tempering temperature and holding at that temperature. If the cooling stop temperature was the same as the tempering temperature, tempering was performed by cooling and then holding at that temperature. Subsequently, the material was cooled to 50°C (third cooling) and subjected to skin pass rolling with an elongation of 0.1-1.0%.

[0059] As described above, test specimens for SEM observation were taken from the obtained annealed steel sheet (cold-rolled steel sheet). After polishing the longitudinal section parallel to the rolling direction, the metallographic structure was observed at 20 μm, 60 μm, 75 μm, and t / 4 sections, and the volume fraction of each structure was measured in the manner described above. The average grain size of martensite and tempered martensite at 20 μm was also determined. Furthermore, X-ray diffraction specimens were taken, and as described above, the volume fraction of retained austenite was measured by X-ray diffraction on surfaces that had been chemically polished to depths of 20 μm, 75 μm, and 1 / 4 of the plate thickness from the surface. The volume fraction of each microstructure in the t / 4 section was determined for the central part in the width direction. On the other hand, the volume fraction of each microstructure in the 20 μm, 60 μm, and 75 μm sections, as well as the average grain size of martensite and tempered martensite in the 20 μm section, were determined for the edge section (50 mm from the edge in the width direction of the steel sheet) and the central part in the width direction, respectively. The results are shown in Tables 3, 4A, 4B, 5A, and 5B.

[0060] The tensile strength (TS) and uniform elongation (uEl) were determined by taking a JIS No. 5 tensile test specimen perpendicular to the rolling direction from the center of the width direction of the obtained cold-rolled steel sheet and performing a tensile test according to JIS Z 2241 (2011). The results are shown in Table 6.

[0061] The critical bending radius (R / t) was determined by varying the radius R in 0.5 mm increments using a 90° V bending die at the center of the width direction of the obtained cold-rolled steel sheet, finding the minimum bending radius R at which cracking did not occur, and then dividing by the sheet thickness (1.4 mm). The results are shown in Table 6.

[0062] Furthermore, the following tests were conducted to evaluate hydrogen embrittlement resistance. Specifically, test specimens with mechanically ground ends were bent into a U-shape using the pressure bending method to create U-bend test specimens with a radius of 5R. After elastic deformation by tightening bolts so that the unbent portion was parallel, the specimens were immersed in hydrochloric acid at pH 1 to conduct an accelerated delayed fracture test in which hydrogen penetrated the steel plate. Steel plates that did not crack after 100 hours of immersion were evaluated as having good (OK) delayed fracture resistance, and those that cracked were evaluated as poor (NG). To eliminate the effect of plating, the plating layer of plated materials was removed with hydrochloric acid containing an inhibitor before the test, and then the hydrogen embrittlement resistance was evaluated. The results are shown in Table 6.

[0063] The yield strength of the component was determined using the following method. The obtained cold-rolled steel sheet was press-bent at an R5 radius to form a hat shape with a height of 50 mm, a top edge of 70 mm, a bottom edge of 120 mm, and a length of 900 mm. A steel sheet of the same size was then attached to the bottom edge and the flange portion was spot-welded to create a model part. A comparison part was also created in the same manner as the model part, using a cold-rolled steel sheet in which the metal structure of the surface layer (i.e., the 20 μm portion) was equivalent to that of the t / 4 portion. For both the model part and the comparison part, the maximum load when the center portion was pressed into a circular indenter and bent was defined as the part's yield strength. The model part was considered acceptable if its yield strength was 95% or more of that of the comparison part. However, the component yield strength tests were conducted only on cold-rolled steel sheets with a tensile strength of 1400 MPa or higher and a critical bending radius (R / t) of 5.0 or less. The results are shown in Table 6.

[0064] [Table 1]

[0065] [Table 2A]

[0066] [Table 2B]

[0067] [Table 3]

[0068] [Table 4A]

[0069] [Table 4B]

[0070] [Table 5A]

[0071] [Table 5B]

[0072] [Table 6]

[0073] As can be seen from Tables 1 to 6, the inventive examples in which the chemical composition and the metallographic structure of the t / 4, 20 μm, and 75 μm parts fall within the scope of the present invention exhibited high strength, excellent bendability, and sufficient component yield strength. In contrast, in comparative examples where the chemical composition and one or more of the metallographic structures in the t / 4, 20 μm, and 75 μm parts fell outside the scope of the present invention, one or more of the strength, bendability, and component yield strength failed to satisfy the target. In test number 11 (comparative example), where the "total amount of ferrite and bainite (%)", "total amount of martensite and tempered martensite (%)", and "average grain size of martensite and tempered martensite (μm)" at a position 20 μm from the surface of the widthwise edge portion fell outside the scope of the present invention, although not shown in the table, the R / t (value obtained by dividing the limit bending radius R in a 90° V bend by the plate thickness t) at the widthwise edge portion (50 mm from the end of the steel plate in the widthwise direction) was measured to be high at over 5.0, and the steel plate as a whole did not meet the quality standards. As a result, the yield decreased significantly.

Claims

1. In mass percent, C: 0.180% or more, 0.350% or less, Mn: 2.00% or more, 4.00% 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, Si: 0% or more, 1.00% 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, 0.0500% or less, and Bi: 0% or more, 0.050% or less, It contains and has a chemical composition consisting of Fe and impurities as the remainder, The metallic structure at the t / 4 point, which is one-quarter of the plate thickness t in the direction of plate thickness from the surface, has a volume fraction of, Residual austenite: 2.0% or more, 8.0% or less. Tempered martensite: 80.0% or more, 98.0% or less. Ferrite and bainite: 0.0% or more, 15.0% or less in total, Martensite: 0.0% or more, 5.0% or less Includes, At the edge portion located 50 mm from the end in the width direction, and at the central portion in the width direction, The metallic structure at a position 20 μm from the surface in the thickness direction of the plate, at a distance of 20 μm, is defined by volume fraction as follows: Ferrite and bainite: 75.0% or more, 100.0% or less in total. Martensite and tempered martensite: total of 0.0% or more and 25.0% or less. Includes, In the metal structure of the 20 μm portion, the average particle size of the martensite and the tempered martensite is 5.0 μm or less. The metallic structure at a position 75 μm from the surface in the thickness direction of the plate, at a distance of 75 μm, is, by volume fraction, Ferrite and bainite: Including a total of 0.0% or more and 15.0% or less. Cold rolled steel plate.

2. The aforementioned chemical composition is, in mass%, Si: 0.005% or more, 1.00% or less, 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, 0.0500% or less, and Bi: 0.0005% or more, 0.050% or less, It contains one or more selected from the group consisting of, The cold-rolled steel sheet according to claim 1.

3. The tensile strength is 1400 MPa or more. The uniform elongation is 5.0% or more. The limit bending radius R in a 90° V-bend is divided by the plate thickness t, and the value R / t is 5.0 or less. The cold-rolled steel sheet according to claim 1 or 2.

4. A hot-dip galvanized layer is formed on the aforementioned surface. A cold-rolled steel sheet according to any one of claims 1 to 3.

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

6. A method for manufacturing a cold-rolled steel sheet according to any one of claims 1 to 5, wherein the composition is as follows in mass%, C: 0.180% or more, 0.350% or less, Mn: 2.00% or more, 4.00% 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, Si: 0% or more, 1.00% 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 A hot rolling process is performed to obtain a hot-rolled steel sheet by hot-rolling a cast slab having a chemical composition containing 1.00% or less of the following: 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, 0.0500% or less, and Bi: 0% or more, 0.050% or less, with the remainder being Fe and impurities, after heating as necessary. The hot-rolled steel sheet is cooled to a winding temperature of 500°C or higher and 550°C or lower, and then wound at the winding temperature; The aforementioned hot-rolled steel sheet is pickled and cold-rolled to produce a cold-rolled steel sheet in a cold-rolling process, The cold-rolled steel sheet after the cold rolling process is heated in a nitrogen-hydrogen mixed atmosphere with a dew point of -20°C to 20°C and containing 1.0% to 20% by volume of hydrogen, to a soaking temperature of 820°C or higher, such that the average heating rate from 700°C to the soaking temperature is less than 10.0°C / second, and then annealed at the soaking temperature for 30 seconds to 200 seconds. A first cooling step in which the cold-rolled steel sheet after the annealing step is cooled to a temperature range of over 425°C but less than 600°C at an average cooling rate of 5.0°C / second or more, After the first cooling step, a holding step is performed in which the cold-rolled steel sheet is kept in the temperature range of over 425°C and under 600°C for 200 seconds or more and 750 seconds or less. A second cooling step is performed after the holding step, in which the cold-rolled steel sheet is cooled to a temperature of 50°C to 250°C at an average cooling rate of 5.0°C / second or more. After the second cooling step, a tempering step is performed on the cold-rolled steel sheet, in which it is tempered at a temperature of 200°C to 350°C for 1 second or more. A third cooling step is performed after the tempering step to cool the material to a temperature at which skin pass rolling is possible. A skin pass step in which skin pass rolling is performed on the cold-rolled steel sheet after the third cooling step, Equipped with, The temperature of the hot-rolled steel sheet is raised to 500°C or below within 10 hours of the completion of the hot rolling process. A method for manufacturing cold-rolled steel sheets.

7. The chemical composition of the aforementioned cast slab is, in mass%, Si: 0.005% or more, 1.00% or less, 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, 0.0500% or less, and Bi: 0.0005% or more, 0.050% or less, It contains one or more selected from the group consisting of, The method for manufacturing a cold-rolled steel sheet according to claim 6.

8. In the holding step, the cold-rolled steel sheet is immersed in a plating bath when its temperature is above 425°C and below 600°C to form a hot-dip galvanized layer on its surface. A method for manufacturing a cold-rolled steel sheet according to claim 6 or 7.

9. In the holding step, an alloying treatment is performed to alloy the hot-dip galvanized layer. The method for manufacturing a cold-rolled steel sheet according to claim 8.

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