Cold-rolled steel sheet and its manufacturing method

Abstract

This cold-rolled steel sheet has a prescribed chemical composition, and when a range from 1 / 8 to 3 / 8 of the sheet thickness in a sheet thickness direction from a surface is defined as a t / 4 section, and a range of 20 µm from the surface in the sheet thickness direction is defined as a surface layer section, a microstructure in the t / 4 section comprises, in terms of volume fractions, 0-10.0% of residual austenite and 90.0-100% of martensite and / or tempered martensite, the ratio of the dislocation density of the surface layer section to the dislocation density of the t / 4 section is 0.80 or greater, the ratio of the hardness of the surface layer section to the hardness of the t / 4 section is 0.90 or greater, and the tensile strength is 1310 MPa or greater.

Description

[Technical field]

[0001] The present invention relates to a cold-rolled steel sheet and a method for producing the same. This application claims priority based on Japanese Patent Application No. 2021-120895, filed on July 21, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] In today's world, where industrial technology fields have become highly specialized, materials used in each technical field are required to have special and advanced performance. For example, steel sheets for automobiles are required to have high strength in order to improve fuel efficiency by reducing the weight of the car body, in consideration of the global environment. When high-strength steel sheets are used in automobile bodies, the desired strength can be imparted to the car body while reducing the weight of the car body by reducing the thickness of the steel sheet.

[0003] In recent years, the requirements for steel sheets for automobiles have become more advanced. Among the steel sheets for automobiles, particularly the cold-rolled steel sheets used for the body frame parts, are now required to have high strength, and there is a demand for steel sheets having a tensile strength of 1310 MPa or more.

[0004] In response to such demands, for example, Patent Document 1 describes a high-strength steel sheet for use in automobile parts and the like, which has a specified component composition, a specified steel sheet structure mainly composed of martensite and bainite, and an average number of inclusions with an average grain size of 5 μm or more in a cross section perpendicular to the rolling direction of 5.0 pieces / mm 2 The following high-strength steel plate is disclosed, which has excellent delayed fracture resistance and a tensile strength of 1470 MPa or more.

[0005] Patent Document 2 also discloses a thin steel sheet having a steel structure in which the ferrite area ratio is 30% or less (including 0%), the bainite area ratio is 5% or less (including 0%), the martensite and tempered martensite area ratio is 70% or more (including 100%), the retained austenite area ratio is 2.0% or less (including 0%), the ratio of the dislocation density within the range of 0 to 20 μm from the steel sheet surface to the dislocation density at the center of the sheet thickness is 90% to 110%, and the average of the top 10% of cementite particle diameters up to a depth of 100 μm from the steel sheet surface is 300 nm or less, and the maximum warpage of the steel sheet when sheared at a length of 1 m in the longitudinal direction of the steel sheet is 15 mm or less. Patent Document 2 shows that this thin steel sheet has a tensile strength of 980 MPa or more, and can also obtain a tensile strength of 2000 MPa or more.

[0006] Patent Document 3 discloses a high-strength steel plate with excellent resistance to delayed fracture, in which the chemical composition (C, Si, Mn, Al, P, S) satisfies specified ranges with the remainder consisting of iron and unavoidable impurities, martensite accounts for 95% or more by area of ​​the entire structure, the structure from a position 10 μm deep from the steel plate surface in the plate thickness direction to a position ¼ of the plate thickness depth satisfies a specified relational expression, and the tensile strength is 1180 MPa or more. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent No. 6729835 [Patent Document 2] International Publication No. 2020 / 026838 [Patent Document 3] Japanese Patent Application Publication No. 2013-104081 Summary of the Invention [Problem to be solved by the invention]

[0008] As described above, high-strength steel sheets having a tensile strength of 1310 MPa or more have been proposed. In general, the main structure of such high-strength steel sheets is martensite and / or tempered martensite. As a result of investigations by the present inventors, it was found that in a high-strength steel plate whose main structure is martensite or tempered martensite, when a load that causes deformation is applied, the load is removed, and the plate is left for a certain period of time, and then a load is applied again, the flow stress when the load is applied again becomes lower than the flow stress when the load was applied initially (hereinafter, this may simply be referred to as a decrease in flow stress). However, Patent Documents 1 to 3 do not consider at all the decrease in flow stress when a load is applied again, and there is room for improvement.

[0009] The present invention has been made in view of the above. An object of the present invention is to provide a cold-rolled steel sheet having a structure mainly composed of martensite and tempered martensite, which can suppress the flow stress when a load is applied again from becoming lower than the flow stress when a load is initially applied (suppress a decrease in flow stress) when a load is applied, unloaded, left for a certain period of time, and then a load is applied again. [Means for solving the problem]

[0010] The present inventors have investigated the cause of the above-mentioned decrease in flow stress, and have found that even if the structure is martensite and / or tempered martensite throughout the entire sheet thickness direction, the flow stress decreases when the dislocation density in the structure varies depending on the position in the sheet thickness direction. Furthermore, as a result of further investigation by the present inventors, it was found that even when the difference in dislocation density at each position in the sheet thickness direction is small, the flow stress may decrease. The present inventors further investigated the cause of this. As a result, it was found that even when the difference in dislocation density in the sheet thickness direction is small, the flow stress decreases if the dislocations are mainly mobile dislocations.

[0011] The present invention has been made in view of the above findings. [1] A cold rolled steel sheet according to one embodiment of the present invention has, by mass%, C: 0.150 to 0.500%, Si: 0.01 to 2.00%, Mn: 0.50 to 3.00%, P: 0.0200% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0200% or less, O: 0.020% or less, Ni: 0 to 1.000%, Mo: 0 to 1.000%, and Cr: 0 to 2. 000%, B:0~0.010%, As:0~0.050%, Co:0~0.500%, Ti:0~0.500%, Nb:0~0.500%, V:0~0.500%, Cu:0 ~0.500%, W:0~0.100%, Ta:0~0.100%, Ca:0~0.050%, Mg:0~0.050%, La:0~0.050%, Ce:0~0.050%, A steel sheet having a chemical composition consisting of Y: 0 to 0.050%, Zr: 0 to 0.050%, Sb: 0 to 0.050%, Sn: 0 to 0.050%, and the balance: Fe and impurities, wherein a range of 1 / 8 to 3 / 8 of the sheet thickness from the surface in the sheet thickness direction is defined as a t / 4 portion, and a range of 20 μm from the surface in the sheet thickness direction is defined as a surface layer portion, the microstructure in the t / 4 portion contains, in volume fractions, 0% or more and 10.0% or less of retained austenite, and 90.0% or more and 100% or less of one or both of martensite and tempered martensite, the ratio of the dislocation density of the surface layer portion to the dislocation density of the t / 4 portion is 0.80 or more, the ratio of the hardness of the surface layer portion to the hardness of the t / 4 portion is 0.90 or more, and the tensile strength is 1310 MPa or more. 2000MPa or less It is. [2] The cold-rolled steel sheet according to [1] may have a coating layer made of zinc, aluminum, or magnesium, or an alloy of at least one of these elements, on the surface. [3] A method for producing a cold-rolled steel sheet according to another aspect of the present invention comprises: A method for producing a cold-rolled steel sheet according to [1],A hot rolling process is performed in which a slab having the chemical composition described in [1] is hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is coiled in a state in which the temperature of the center part in the width direction is higher than 600°C and lower than 700°C and the temperature of an edge part located 20 mm from the end part in the width direction is lower than 600°C; a cold rolling process is performed in which the hot-rolled steel sheet after the hot rolling process is pickled and cold-rolled at a rolling reduction of 30 to 90% to obtain a cold-rolled steel sheet; and a cold rolling process is performed in which the cold-rolled steel sheet is heated to an annealing temperature of more than Ac3°C, held at the annealing temperature, and the cold-rolled steel sheet after the holding is cooled at an average cooling rate of 400°C. the cold-rolled steel sheet after the annealing process is heated to a temperature range of 200 to 350°C and held in the temperature range; and a skin-pass rolling process is performed on the cold-rolled steel sheet after the heat treatment process at a rolling reduction of 0.1% or more, and a difference in thickness between a central portion and an edge portion in the width direction of the cold-rolled steel sheet after the cold rolling process is 10 μm or more. [4] In the method for producing a cold-rolled steel sheet according to [3], in the annealing step, a coating layer made of zinc, aluminum, or magnesium, or an alloy of one or more of these elements may be formed on the front and back surfaces of the cold-rolled steel sheet. Effect of the Invention

[0012] According to the above aspect of the present invention, it is possible to provide a cold-rolled steel sheet having a structure mainly composed of martensite and tempered martensite, which is capable of suppressing the flow stress when a load is applied again from becoming lower than the flow stress when a load is initially applied, when the cold-rolled steel sheet is subjected to a load, unloaded, and then left for a certain period of time, and then a load is applied again, and a manufacturing method thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] A cold-rolled steel sheet according to one embodiment of the present invention (the cold-rolled steel sheet according to this embodiment) and a manufacturing method for obtaining the cold-rolled steel sheet will be described. The cold-rolled steel sheet according to the present embodiment has a predetermined chemical composition, and when a t / 4 portion is a region from the surface to 1 / 8 to 3 / 8 of the sheet thickness in the sheet thickness direction, and a surface layer portion is a region from the surface to 20 μm in the sheet thickness direction, the microstructure (metal structure) in the t / 4 portion contains, in volume fraction, 0% or more and 10.0% or less of retained austenite, and 90.0% or more and 100% or less of one or both of martensite and tempered martensite, the ratio of the dislocation density of the surface layer portion to the dislocation density of the t / 4 portion is 0.80 or more, and the ratio of the hardness of the surface layer portion to the hardness of the t / 4 portion is 0.90 or more. In addition, the tensile strength is 1310 MPa or more. Each of these will be explained below. In the explanations, the ranges indicated with "~" generally include the values ​​at both ends as the lower and upper limits. However, values ​​indicated as "over" or "less than" are not included in the range.

[0014] [Chemical composition] First, the chemical composition will be described. In this embodiment, the "%" for the content of each element means "mass %."

[0015] C: 0.150~0.500% C is related to the hardness of martensite and tempered martensite, and is an element necessary for increasing the strength of the steel sheet. In order to obtain a tensile strength of 1310 MPa or more, the C content must be at least 0.150%. Therefore, the C content is set to 0.150% or more. The C content is preferably 0.180% or more, and more preferably 0.200% or more. On the other hand, if the C content exceeds 0.500%, the weldability and formability deteriorate. Therefore, the C content is set to 0.500% or less. The C content is preferably 0.350% or less, and more preferably 0.300% or less.

[0016] Silicon: 0.01 to 2.00% Silicon is a solid solution strengthening element and is effective in increasing the strength of steel sheets. To obtain this effect, the silicon content is set to 0.01% or more. The silicon content is preferably set to 0.10% or more, and more preferably set to 0.20% or more. On the other hand, if the Si content is excessive, the formability decreases and the wettability with the plating decreases. Therefore, the Si content is set to 2.00% or less. The Si content is preferably set to 1.80% or less, and more preferably set to 1.70% or less.

[0017] Mn: 0.50-3.00% Mn is an element that improves hardenability and promotes the formation of martensite. If the Mn content is less than 0.50%, it is difficult to obtain the target microstructure. Therefore, the Mn content is set to 0.50% or more. On the other hand, if the Mn content is excessive, the effect of improving hardenability is reduced due to segregation of Mn, and the material cost increases. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.80% or less.

[0018] P:0.0200% or less P is an element contained in steel as an impurity, and segregates at grain boundaries to embrittle steel. Therefore, the lower the P content, the better. Although 0% is acceptable, the P content is set to 0.0200% or less, taking into consideration the time and cost required for removing P. The P content is preferably 0.0150% or less, and more preferably 0.0100% or less. From the viewpoint of the cost of refining and the like, the P content may be set to 0.0001% or more.

[0019] S: 0.0200% or less S is an element contained in steel as an impurity, and forms sulfide-based inclusions that deteriorate the formability of steel sheet. For this reason, the lower the S content, the better, and 0% is acceptable, but taking into consideration the time and cost required for removing S, the S content is set to 0.0200% or less. The S content is preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less. From the viewpoint of costs for refining and the like, the S content may be set to 0.0001% or more.

[0020] Al: 0.100% or less Al is an element that has the effect of deoxidizing molten steel. In the cold-rolled steel sheet according to the present embodiment, Al does not necessarily need to be contained, and the Al content may be 0%, but Al may be contained for the purpose of deoxidization. In that case, the Al content is preferably 0.001% or more. Furthermore, Al has the effect of increasing the stability of austenite, similar to Si, and therefore may be contained in order to obtain retained austenite. On the other hand, if the Al content is too high, not only will surface defects caused by alumina be more likely to occur, but the transformation point will rise significantly and the volume fraction of ferrite will increase. In this case, it will be difficult to obtain the desired metal structure, and sufficient tensile strength will not be obtained. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less.

[0021] N: 0.0200% or less N is an element that may be contained in steel as an impurity, and generates coarse precipitates that deteriorate formability. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0100% or less, and more preferably 0.0060% or less. The lower the N content, the better, and it may be 0%, but from the viewpoint of the cost of refining, etc., it may be set to 0.0001% or more.

[0022] O: 0.020% or less O is an element contained as an impurity. If the O content exceeds 0.020%, coarse oxides are formed in the steel, and formability is reduced. Therefore, the O content is set to 0.020% or less. The O content is preferably set to 0.010% or less, and more preferably set to 0.005% or less. The O content may be 0%, but from the viewpoint of costs such as refining, the O content may be set to 0.0001% or more, or may be set to 0.001% or more.

[0023] In the chemical composition of the cold-rolled steel sheet according to the present embodiment, the balance other than the above elements is basically Fe and impurities. Impurities are elements that are mixed in from the steel raw materials and / or during the steelmaking process and whose presence is permitted to the extent that they do not clearly deteriorate the properties of the cold-rolled steel sheet according to the present embodiment. On the other hand, the chemical composition of the cold-rolled steel sheet according to the present embodiment may contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ni, Mo, Cr, B, As, Co, Ti, Nb, V, Cu, W, Ta, Ca, Mg, La, Ce, Y, Zr, Sb, and Sn in the range described below, in order to improve various properties. These elements do not have to be contained, so the lower limit is 0%. Furthermore, as long as the content is within the range described below, even if these elements are contained as impurities, the effect of the cold-rolled steel sheet according to the present embodiment is not impaired.

[0024] Ni: 0 to 1.000% Mo: 0 to 1.000% Cr: 0~2.000% B: 0~0.010% As: 0 to 0.050% Ni, Mo, Cr, B and As are elements that improve hardenability and contribute to increasing the strength of the steel sheet. Therefore, these elements may be contained. In order to fully obtain the above effects, it is preferable that the Ni content, Mo content and Cr content are 0.010% or more, the B content is 0.0001% or more, and / or the As content is 0.001% or more. More preferably, the Ni content, Mo content and Cr content are 0.050% or more, the B content is 0.001% or more, and the As content is 0.005% or more. It is not essential to obtain the above effects. Therefore, there is no need to particularly limit the lower limits of the Ni content, Mo content, Cr content, B content and As content, and the lower limits are 0%. On the other hand, if these elements are contained in excess, the effects of the above actions will saturate and it will be uneconomical. Therefore, when these elements are contained, the Ni content and Mo content should be 1.000% or less, the Cr content should be 2.000% or less, the B content should be 0.010% or less, and the As content should be 0.050% or less. The Ni content and Mo content are each preferably 0.500% or less, the Cr content is preferably 1.000% or less, the B content is preferably 0.0060% or less, and the As content is preferably 0.030% or less.

[0025] Co: 0 to 0.500% Co is an element effective in improving the strength of a steel sheet. The Co content may be 0%, but in order to obtain the above effects, the Co content is preferably 0.010% or more, and more preferably 0.100% or more. On the other hand, if the Co content is too high, the elongation of the steel sheet may decrease, resulting in a risk of deteriorating formability, and therefore the Co content is set to 0.500% or less.

[0026] Ti: 0 to 0.500% Nb: 0 to 0.500% V: 0~0.500% Cu: 0 to 0.500% W: 0~0.100% Ta: 0 to 0.100% Ti, Nb, V, Cu, W, and Ta are elements that have the effect of improving the strength of the steel sheet by precipitation hardening. Therefore, these elements may be contained. In order to fully obtain the above effects, it is preferable that one or more of Ti, Nb, V, Cu, W, and Ta are contained, and each content is 0.001% or more. On the other hand, if these elements are contained in excess, the recrystallization temperature rises, the metal structure of the cold-rolled steel sheet becomes non-uniform, and formability is impaired. Therefore, the Ti content, Nb content, V content, and Cu content are each set to 0.500% or less. In addition, the W content and Ta content are each set to 0.100% or less.

[0027] Ca: 0 to 0.050% Magnesium: 0 to 0.050% La: 0 to 0.050% Ce: 0 to 0.050% Y: 0~0.050% Zr: 0 to 0.050% Sb: 0 to 0.050% Ca, Mg, La, Ce, Y, Zr, and Sb are elements that contribute to finely dispersing inclusions in steel, and contribute to improving the formability of the steel sheet through this fine dispersion. Therefore, these elements may be contained. In order to obtain the above effect, it is preferable to contain one or more of Ca, Mg, La, Ce, Y, Zr, and Sb, and to set the content of each to 0.001% or more. On the other hand, excessive content of these elements deteriorates ductility, therefore the content of Ca, Mg, La, Ce, Y, Zr and Sb is set to 0.050% or less.

[0028] Sn: 0 to 0.050% Sn is an element that suppresses the coarsening of crystal grains and contributes to improving the strength of the steel sheet, and therefore Sn may be contained. On the other hand, Sn is an element that may cause a decrease in cold formability of the steel sheet due to embrittlement of ferrite. If the Sn content exceeds 0.050%, the adverse effects become significant, so the Sn content is set to 0.050% or less. The Sn content is preferably 0.040% or less.

[0029] The chemical composition of the cold-rolled steel sheet according to this embodiment can be determined by the following method. For example, measurement can be performed using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) for cutting chips in accordance with JIS G1201 (2014). In this case, the chemical composition is the average content over the entire plate thickness. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method. When the steel sheet has a coating layer on its surface, the coating layer may be removed by mechanical grinding or the like before the chemical composition is analyzed. When the coating layer is a plating layer, the plating layer may be removed by dissolving it in an acid solution to which an inhibitor that suppresses corrosion of the steel sheet has been added.

[0030] [Microstructure (metal structure)] In this embodiment, the range from the surface to a position between 1 / 8 and 3 / 8 of the plate thickness, centered at a position at 1 / 4 of the plate thickness in the plate thickness direction from the surface, is referred to as the t / 4 portion ((1 / 4)t portion), and the range from the surface to 20 μm in the plate thickness direction is referred to as the surface layer portion.

[0031] [Microstructure of t / 4 part: Contains, by volume, 0% or more and 10.0% or less of retained austenite, and 90.0% or more and 100% or less of martensite and / or tempered martensite] The retained austenite contributes to improving the formability of the steel sheet by improving the uniform elongation of the steel sheet due to the TRIP effect. Therefore, retained austenite (residual γ) may be contained. To obtain the above effect, the volume fraction of the retained austenite is preferably 1.0% or more. The volume fraction of the retained austenite is more preferably 2.0% or more, and further preferably 3.0% or more. On the other hand, if the volume fraction of the retained austenite is excessive, the grain size of the retained austenite becomes large. Such retained austenite with a large grain size becomes coarse and hard martensite after deformation. In this case, cracks are likely to start and the formability is reduced. For this reason, the volume fraction of the retained austenite is set to 10.0% or less. The volume fraction of the retained austenite is preferably 8.0% or less, and more preferably 7.0% or less. As a structure other than the retained austenite, one or both of martensite and tempered martensite are contained. Martensite (so-called fresh martensite) and tempered martensite are an aggregate of lath-shaped crystal grains and contribute greatly to improving strength, so the cold-rolled steel sheet according to the present embodiment contains martensite and tempered martensite at a total volume ratio of 90.0 to 100%. Unlike martensite, tempered martensite is a hard structure that contains fine iron-based carbides inside due to tempering. Although tempered martensite contributes less to improving strength than martensite, it is not brittle and has ductility, so if you want to improve formability, it is preferable to increase the volume fraction of tempered martensite. For example, the volume fraction of tempered martensite is 85.0% or more. On the other hand, when high strength is desired, it is preferable to increase the volume fraction of martensite. The microstructure may contain bainite in addition to retained austenite, martensite, and tempered martensite, and preferably does not contain ferrite or pearlite.

[0032] The volume fraction of each structure in the microstructure of the t / 4 portion of the cold-rolled steel sheet according to this embodiment is measured as follows. That is, the volume fractions of ferrite, bainite, martensite, tempered martensite, and pearlite are measured by taking a test piece from any position in the rolling direction and width direction of the steel plate, polishing a longitudinal section parallel to the rolling direction (a section parallel to the plate thickness direction), and observing the structure revealed by nital etching in the range of 1 / 8 to 3 / 8 of the plate thickness from the surface (t / 4 part) using a SEM. In the SEM observation, five fields of view of 30 μm × 50 μm are observed at a magnification of 3000 times, and the area fraction of each structure is measured from the observed images, and the average value is calculated. There is no change in the structure in the direction perpendicular to the rolling direction (the plate width direction), and the area fraction of the longitudinal section parallel to the rolling direction is equal to the volume fraction, so the area fractions obtained by the structure observation are the respective volume fractions.

[0033] When measuring the area ratio of each structure, the region where the substructure is not revealed and has low brightness is defined as ferrite, the region where the substructure is not revealed and has high brightness is defined as martensite or retained austenite, and the region where the substructure is revealed is defined as tempered martensite or bainite.

[0034] Bainite and tempered martensite can further be distinguished by careful observation of intragranular carbides. Specifically, tempered martensite is composed of martensite laths and cementite formed inside the laths. At this time, there are two or more types of crystal orientation relationship between martensite laths and cementite, so the cementite that constitutes tempered martensite has multiple variants. On the other hand, bainite is classified into upper bainite and lower bainite. Upper bainite is composed of lath-shaped bainitic ferrite and cementite formed at the lath interface, so it can be easily distinguished from tempered martensite. Lower bainite is composed of lath-shaped bainitic ferrite and cementite formed inside the laths. At this time, the crystal orientation relationship between bainitic ferrite and cementite is one type, unlike tempered martensite, and the cementite that constitutes lower bainite has the same variant. Therefore, lower bainite and tempered martensite can be distinguished based on the variant of cementite. On the other hand, martensite and retained austenite cannot be clearly distinguished by SEM observation, so the volume fraction of martensite is calculated by subtracting the volume fraction of retained austenite, calculated by the method described below, from the volume fraction of the structure determined to be martensite or retained austenite.

[0035] The volume fraction of retained austenite is quantified by taking a test piece from any position on the steel plate, chemically polishing the rolled surface from the surface of the steel plate to a position 1 / 4 of the plate thickness, and measuring the (200), (210) surface integral intensity of ferrite and the (200), (220), and (311) surface integral intensity of austenite using MoKα radiation.

[0036] [Ratio of dislocation density in the surface layer to the dislocation density in the t / 4 portion: 0.80 or more] The cold-rolled steel sheet according to this embodiment has a microstructure mainly composed of martensite and / or tempered martensite obtained by tempering martensite. Martensite is obtained by holding a steel plate in the austenite single phase region and then rapidly cooling it, but when cooling is performed using a conventional method, the martensite will have different structural characteristics (e.g., dislocation density) depending on the position in the thickness direction of the steel plate. This difference is due to differences in the timing of transformation. That is, when cooling, the temperature of the area close to the surface of the steel plate drops first, followed by the temperature inside the steel plate. For this reason, the transformation from austenite to martensite occurs first on the surface side of the steel plate. As martensitic transformation is an exothermic reaction, the martensite formed on the surface side is held at a high temperature for a longer period of time than the martensite inside, which allows it to be tempered. Tempering reduces the dislocation density in the martensite. If such a difference in dislocation density exists, when a load that causes deformation is applied, the load is removed, and the steel is left for a certain period of time, and then the load is applied again, the flow stress when the load is applied again is lower than the flow stress when the load was applied initially. Therefore, in the cold-rolled steel sheet according to the present embodiment, the dislocation density (ρ t / 4 ) versus the dislocation density in the surface layer (ρ s ) ratio (ρ s / ρ t / 4 ) should be 0.80 or more. (ρ s / ρ t / 4 ) is preferably 0.85 or more, and more preferably 0.90 or more. The dislocation density at the t / 4 portion is 5.2×10 15 m -2 It is preferable that the above ρ s / ρ t / 4 Considering the preferable range of , the dislocation density in the surface layer is 4.2 × 10 15 m -2 More preferably, it is equal to or greater than this.

[0037] The dislocation density at each position is determined by the following method. Considering the position 20μm from the surface of the steel plate as the representative structure of the surface layer, and the position 1 / 4 of the plate thickness from the surface as the representative structure of the t / 4 part, a sample was prepared by grinding 20μm from the surface and a sample was prepared by grinding 1 / 4 of the plate thickness from the surface. After removing strain from each ground surface by chemical polishing, X-ray diffraction was performed. From the X-ray diffraction profile obtained by X-ray diffraction, the dislocation density at the position 20μm from the surface and the dislocation density at the position 1 / 4 of the plate thickness from the surface were obtained using the modified Williamson-Hall method and the modified Warren-Averbach method. Specifically, the dislocation density was obtained according to the method described in ISIJ Int. vol.50(2010)pp.875-882. The dislocation density at the position 20μm from the surface was the dislocation density of the surface layer, and the dislocation density at the position 1 / 4 of the plate thickness from the surface was the dislocation density of the t / 4 part.

[0038] [Ratio of surface hardness to t / 4 hardness: 0.90 or more] As described above, even if the difference in dislocation density in the sheet thickness direction is reduced, if the dislocations are mainly mobile dislocations, when a load that causes deformation is applied, the load is removed, and the material is left for a certain period of time, and then a load is applied again, the flow stress when the load is applied again will be lower than the flow stress when the load was initially applied. Therefore, in the cold-rolled steel sheet according to the present embodiment, dislocations in the surface layer portion in particular are immobilized. In the present embodiment, the ratio of the hardness of the surface layer portion to the hardness of the t / 4 portion is used as an index of whether or not dislocations are immobilized. (ρ s / ρ t / 4 When the hardness ratio of the surface layer to the t / 4 portion is 0.90 or more, dislocations are immobilized and a decrease in flow stress can be prevented. The hardness ratio is preferably 0.92 or more, more preferably 0.94 or more, and even more preferably 0.95 or more. As described later, dislocation immobilization can be achieved by subjecting a steel sheet having an intentional thickness difference to skin-pass rolling. Since tensile strength and hardness are correlated, the hardness of the t / 4 portion is preferably 360 Hv or more. Therefore, taking into consideration the preferable range of the ratio of the hardness of the surface layer portion to the hardness of the t / 4 portion, the hardness of the surface layer portion is preferably 324 Hv or more.

[0039] The hardness is determined by the following method. A cut surface perpendicular to the rolling direction of the steel plate and parallel to the plate thickness direction is formed, and mirror polished. Next, Vickers hardness is measured on the cut surface at four points, 20 μm from the surface of the steel plate and 1 / 4 of the plate thickness from the surface, based on JIS Z 2244-1 (2020). The load for the Vickers hardness measurement is 2 kgf. The average value of the hardness measurements at a position 20 μm from the surface of the steel plate is defined as the hardness of the surface layer, and the average value of the hardness measurements at a position 1 / 4 of the plate thickness from the surface is defined as the hardness of the t / 4 portion.

[0040] The cold-rolled steel sheet according to this embodiment may have a coating layer on one or both surfaces, which contains zinc, aluminum, or magnesium, or one or more alloys thereof, or is made of zinc, aluminum, or magnesium, or one or more alloys thereof (the inclusion of impurities, etc. is permitted). By providing a coating layer on the surface, corrosion resistance is improved. In the case of automotive steel sheets, there are cases where the sheet thickness cannot be reduced below a certain level even if the sheet is made stronger due to concerns about holes 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, if its corrosion resistance is low, the areas where it can be used will be limited. One method that can be considered to solve these problems is to form a coating layer on the surface to improve corrosion resistance. The coating layer is, for example, a hot-dip galvanized layer, a galvannealed layer, an electrolytic galvanized layer, an aluminum plated layer, a Zn-Al alloy plated layer, an Al-Mg alloy plated layer, or a Zn-Al-Mg alloy plated layer. When a coating layer is present on the surface, the reference surface for the above-mentioned t / 4 portion, etc. is the surface of the base steel excluding the coating layer.

[0041] [Tensile strength] The cold-rolled steel sheet according to the present embodiment is targeted to have a tensile strength (TS) of 1310 MPa or more as a strength that contributes to weight reduction of an automobile body. From the viewpoint of impact absorption, the tensile strength is preferably 1400 MPa or more, and more preferably 1470 MPa or more. There is no need to set an upper limit, but since an increase in tensile strength may result in a decrease in formability, the tensile strength may be set to 2000 MPa or less.

[0042] <Manufacturing method> The cold-rolled steel sheet according to the present embodiment can obtain the above-mentioned effects regardless of the manufacturing method, but can be stably manufactured by the manufacturing method described below. Specifically, the cold-rolled steel sheet according to this embodiment can be produced by a production method including the following steps (I) to (VI). (I) A hot rolling process in which a slab having a predetermined chemical composition is hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is coiled in a state in which the temperature of the center in the width direction is higher than 600°C and is 700°C or less, and the temperature of the edge portion located 20 mm from the end in the width direction is 600°C or less. (III) A cold rolling step in which the hot-rolled steel sheet after the hot rolling step is pickled and cold-rolled at a rolling reduction of 30 to 90% to obtain a cold-rolled steel sheet. (IV) An annealing process in which the cold-rolled steel sheet is heated to an annealing temperature exceeding Ac3°C, held at the annealing temperature, and cooled to a cooling stop temperature after the holding step such that the average cooling rate to 400°C is 10°C / sec or more and the average cooling rate from 400°C to a cooling stop temperature of 100°C or less is 15°C / sec or more. (V) A heat treatment process for heating the cold-rolled steel sheet after the annealing process to a temperature range of 200 to 350° C. and maintaining the same in the temperature range. (VI) a skin-pass rolling step in which the cold-rolled steel sheet after the heat treatment step is subjected to skin-pass rolling at a rolling reduction rate of 0.1% or more. In the method for producing a cold-rolled steel sheet according to the present embodiment, the difference in thickness between the central portion and the edge portions in the width direction of the cold-rolled steel sheet after the cold rolling step is set to 10 μm or more. Each step will be described below.

[0043] [Hot rolling process] In the hot rolling process, a slab having the same chemical composition as the cold rolled steel sheet according to the present embodiment is hot rolled to obtain a hot rolled steel sheet. The hot rolling is preferably performed under conditions such that the finish rolling completion temperature is Ac3°C or higher in order to satisfy the coiling temperature described below. The upper limit of the finish rolling completion temperature is not particularly limited, but is generally 950°C or lower. This hot-rolled steel sheet is wound up in a state where the temperature of the widthwise central portion is higher than 600°C and not higher than 700°C, and the temperature of the edge portion located 20 mm from the widthwise end portion is not higher than 600°C. In order to make the coiling temperature of the edge portion lower than that of the center portion in the width direction, the edge portion is cooled so that the cooling rate is faster than that of the center portion. For example, when only the edge portion of the hot-rolled steel sheet is water-cooled, or when the entire steel sheet is water-cooled, the amount of cooling water at the edge portion may be set to be greater than that at the center portion in the width direction. After being water-cooled, the edge portions are tempered by heat transferred from the widthwise center portion, which has a higher temperature, during winding, and become softer than the widthwise center portion. As a result, when cooled to near room temperature, the strength of the edge portions is lower than that of the widthwise center portion. By subjecting a steel plate having such a strength difference in the width direction to cold rolling, which will be described later, a thickness difference occurs between the center portion and the edge portions in the width direction of the steel plate. If the coiling temperature of the widthwise center exceeds 700°C, the widthwise center becomes soft. If the coiling temperature of the widthwise center is 600°C or less, the temperature difference with the edge parts becomes small, or the edge parts cannot be sufficiently tempered. The coiling temperature of the center is preferably 620°C or more. In addition, if the coiling temperature of the edge portion exceeds 600°C, the softening effect by tempering cannot be sufficiently obtained. In addition, if the coiling temperature of the edge portion is 400°C or less, the edge portion is tempered by heat transfer from the center portion in the width direction, but the strength increases, so the cold rolling load increases, and in some cases cracks occur. Therefore, the coiling temperature of the edge portion is preferably more than 400°C, more preferably 450°C or more. When the difference in sheet thickness between the width direction center and the edge portions is made larger, the difference in coiling temperature between the width direction center and the edge portions is preferably 50°C or more, more preferably 75°C or more, and even more preferably 100°C or more.

[0044] The method for producing the slab to be subjected to hot rolling is not particularly limited. In a preferred method for producing the slab, the steel having the above-mentioned chemical composition is melted by a known means, and then made into a steel ingot by a continuous casting method, or made into a steel billet by a method of making a steel ingot by any casting method and then blooming it. In the continuous casting process, in order to suppress the occurrence of surface defects caused by inclusions, it is preferable to cause an external additional flow such as electromagnetic stirring in the molten steel in the mold. The steel ingot or billet may be once cooled and then reheated for hot rolling, or the steel ingot in a high temperature state after continuous casting or the steel billet in a high temperature state after blooming may be used for hot rolling as it is, or after keeping warm, or after supplementary heating. In this embodiment, such steel ingots and billets are collectively referred to as "slabs" as materials for hot rolling.

[0045] [Cold rolling process] In the cold rolling process, the hot-rolled steel sheet after the hot rolling process is pickled and cold-rolled at a rolling reduction of 30 to 90% to obtain a cold-rolled steel sheet. The pickling conditions are not particularly limited and may be any known conditions. In the cold rolling process, a steel sheet having a difference in strength in the width direction is cold rolled to obtain a steel sheet having a difference in sheet thickness in the width direction (cold-rolled steel sheet). If the cold rolling reduction (cumulative rolling reduction) is less than 30%, a sufficient thickness difference cannot be achieved, and if the cold rolling reduction exceeds 90%, the cold rolling load becomes too large, making cold rolling difficult.

[0046] In the method for producing a cold-rolled steel sheet according to the present embodiment, the above-mentioned hot rolling step and cold rolling step are performed to obtain a cold-rolled steel sheet having a difference in thickness between the center part and the edge part in the width direction of the cold-rolled steel sheet after the cold rolling step of 10 μm or more. The difference in thickness is preferably 15 μm or more. Although there is no upper limit to the thickness difference, if the thickness difference is large, cracks may occur from the thinner parts and the hole expandability may decrease. Therefore, from the viewpoint of formability, the thickness difference may be set to 55 μm or less.

[0047] The thickness of the sheet at the center in the width direction and the thickness of the sheet at the edge portions can be measured by installing a scanning thickness gauge at the exit side of the cold rolling mill.

[0048] [Width trimming process] If the difference in thickness between the widthwise center and edge portions after cutting is 10 μm or more, width trimming may be performed to cut off an arbitrary width from the widthwise end portions of the steel plate. By performing width trimming, even if cracks or scratches occur at the ends of the cold-rolled steel sheet, the steel sheet can be subjected to the next process by cutting off the cracks, which is preferable in terms of cost and yield.

[0049] [Annealing process] In the annealing process, the cold-rolled steel sheet after the cold rolling process, or after a width trimming process as required, is heated to an annealing temperature of more than Ac3° C. and held at this annealing temperature. If the annealing temperature is Ac3° C. or lower, the structure will not be sufficiently transformed into austenite, and the desired microstructure mainly composed of martensite will not be obtained after the annealing process. On the other hand, excessively high heating temperatures such as annealing at temperatures exceeding 900° C. will increase the manufacturing costs, and therefore the annealing temperature is preferably set to 900° C. or lower. The temperature (°C) of the Ac3 point can be determined by the following method. Ac3 = 910 - (203 x C 1 / 2 )+44.7×Si-30×Mn+700×P-20×Cu-15.2×Ni-11×Cr+31.5×Mo+400×Ti+104×V+120×Al Here, the element symbols included in the formulas represent the contents, in unit mass %, of the elements included in the steel sheet. The holding time at the annealing temperature is preferably 40 to 135 seconds. If the holding time is less than 40 seconds, austenitization may not proceed sufficiently, and if the holding time is more than 135 seconds, productivity decreases.

[0050] The cold-rolled steel sheet after the above-mentioned holding is cooled to a cooling stop temperature such that the average cooling rate to 400°C is 10°C / sec or more and the average cooling rate from 400°C to a cooling stop temperature of 100°C or less is 15°C / sec or more. If the average cooling rate to 400°C is less than 10°C / s, ferrite may form in the microstructure. If the average cooling rate from 400°C to the cooling stop temperature (100°C or less) is less than 15°C / s, or if the cooling stop temperature exceeds 100°C / s, bainite may form in the microstructure. In these cases, the desired microstructure cannot be obtained.

[0051] In the annealing step, a coating layer made of zinc, aluminum, or magnesium, or an alloy of one or more of these elements may be formed on one or both surfaces of the cold-rolled steel sheet. When forming a coating layer, for example, in the case of hot-dip plating, the steel sheet is immersed in a plating bath during cooling within a range in which the average cooling rate to 400°C is 10°C / sec or more and the average cooling rate from 400°C to a cooling stop temperature of 100°C or less is 15°C / sec or more to form a hot-dip coating on the surface, and the temperature is held at about 450 to 470°C for 10 to 40 seconds. When alloying the hot-dip coating layer, it is preferable to immerse the steel sheet in a coating bath to form a hot-dip galvanized layer, heat the steel sheet to a temperature range (alloying temperature) of 470 to 550°C, and hold the steel sheet at that temperature range for 10 to 40 seconds. If the alloying temperature is less than 470°C, the alloying may not proceed sufficiently. On the other hand, if the alloying temperature exceeds 550°C, the alloying may proceed too much, and the Fe concentration in the coating layer may exceed 15% due to the formation of Γ phase, which may deteriorate the corrosion resistance. The alloying temperature is more preferably 480°C or higher. Also, the alloying temperature is more preferably 540°C or lower.

[0052] [Heat treatment process] In the annealing process, cooling (cooling after holding) transforms the untransformed austenite into martensite in the microstructure of the cold-rolled steel sheet. However, some austenite may not transform and may become retained austenite. Such cold-rolled steel sheet is subjected to a heat treatment in which it is heated to a temperature range of 200 to 350° C. and maintained at this temperature range. This heat treatment causes a part or the whole of the martensite to become tempered martensite. In order to obtain a microstructure mainly consisting of tempered martensite, the holding time is preferably 1 second or more. If the heating temperature is less than 200°C, the martensite is not sufficiently tempered, and the microstructure and mechanical properties may not be sufficiently changed.If the heating temperature is more than 350°C, the dislocation density in the tempered martensite may decrease, resulting in a decrease in tensile strength.

[0053] [Skin pass rolling process] In the skin pass rolling process, the cold rolled steel sheet after the heat treatment process is subjected to skin pass rolling at a rolling reduction of 0.1% or more. As described above, the cold-rolled steel sheet after the heat treatment step has a thickness difference of 10 μm or more between the center portion in the width direction and the edge portions. When such a cold-rolled steel sheet is subjected to skin pass rolling, the longitudinal direction of the steel sheet is not perpendicular to the axial direction of the rolling rolls but is bitten at a certain angle due to the difference in thickness. The reduction ratio can be arbitrarily selected by setting the skin pass rolling mill, but when there is a difference in thickness, the amount of strain introduced into the surface layer can be increased more than the amount of strain that is assumed to be introduced by the reduction ratio set when the thickness is uniform. In this embodiment, by subjecting a cold-rolled steel sheet having a predetermined thickness difference to skin-pass rolling to introduce strain into the surface layer portion, it is possible to increase the dislocation density in the surface layer portion and immobilize the dislocations. However, if the rolling reduction is less than 0.1%, sufficient effect cannot be obtained, so the rolling reduction is set to 0.1% or more. Although there is no upper limit for the rolling reduction, if it exceeds 1.5%, productivity decreases significantly, so it is preferably set to less than 1.5%. Generally, skin pass rolling with a reduction ratio of 0.1% or more is not performed on steel plate having a tensile strength of 1310 MPa or more. However, in the present embodiment, based on the above-mentioned new findings discovered by the inventors, skin pass rolling with a reduction ratio of 0.1% or more is performed. EXAMPLES

[0054] Slabs (steel types A to W) having the chemical compositions (units are mass %, the remainder being Fe and impurities) shown in Tables 1-1 and 1-2 were produced by continuous casting. These slabs were hot-rolled so that the completion temperature of the finish rolling was Ac3°C or higher, and the cooling conditions for the center and edges were changed to coil the slabs under the conditions shown in Table 2-1, thereby obtaining hot-rolled steel sheets. These hot-rolled steel sheets were cold-rolled under the conditions shown in Table 2-1 to obtain cold-rolled steel sheets having the thickness difference shown in Table 2-1. These cold rolled steel sheets were subjected to annealing, heat treatment and skin pass rolling under the conditions shown in Table 2-2. In addition, some cold-rolled steel sheets were subjected to hot-dip galvanization during annealing. Some of the coating layers were alloyed by changing the holding temperature after immersion in the coating bath. The holding time was 10 to 40 seconds in all cases. In Table 3-1, GI indicates that a hot-dip galvanized layer was formed, and GA indicates that an alloyed hot-dip galvanized layer was formed.

[0055] The microstructure of the t / 4 portion of the obtained cold rolled steel sheet was observed by the above-mentioned method, and the volume fractions of martensite, tempered martensite, bainite, retained austenite, ferrite, and pearlite were measured. The measurement results of the volume fractions of martensite, tempered martensite, bainite, and retained austenite are shown in Table 3-1. Although not shown in the table, in No. 38 and No. 39, ferrite was formed in addition to martensite, tempered martensite, bainite, and retained austenite.

[0056] In addition, the dislocation density and hardness of the surface layer portion and t / 4 portion of the obtained cold-rolled steel sheet were measured by the above-mentioned methods. The results are shown in Table 3-2. Note that the dislocation density notation "5.0E+15" in the table is 5.0×10 15 When it says YE+X, the other numbers are also Y×10. x means.

[0057] The tensile strength of the obtained cold-rolled steel sheets was measured by the following method. Tensile strength (TS) was measured by taking a JIS No. 5 test piece so that the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the steel plate, and conducting a tensile test in accordance with JIS Z 2241 (2011). The results are shown in Table 3-2.

[0058] In addition, in order to evaluate the decrease in flow stress, a tensile test was carried out after pre-straining as follows. The change in flow stress was evaluated by taking a JIS No. 5 test piece so that the longitudinal direction of the test piece was parallel to the direction perpendicular to the rolling direction of the steel plate, and comparing the stress when the test piece was pulled again after being given a pre-strain of 0.1% and then left for one day after unloading in accordance with JIS Z 2241 (2011) with the stress when it was pre-strained by 0.1%. If the flow stress when strained again increases, is the same, or the drop is less than 40 MPa, it was rated A (excellent), if the drop was 40 MPa or more but less than 80 MPa it was rated B (good), and if the drop was 80 MPa or more it was rated C (does not meet the target). The results are shown in Table 3-2.

[0059] [Table 1-1]

[0060] [Table 1-2]

[0061] [Table 2-1]

[0062] [Table 2-2]

[0063] [Table 3-1]

[0064] [Table 3-2]

[0065] As can be seen from Tables 1-1 to 3-2, inventive examples Nos. 1 to 30 had tensile strengths of 1310 MPa or more, and the decrease in flow stress was suppressed to less than 80 MPa. On the other hand, in the comparative examples Nos. 31 to 45, a tensile strength of 1310 MPa or more was not obtained, or a decrease in flow stress of 80 MPa or more was observed.

Claims

1. In mass percent, C: 0.150-0.500%, Si: 0.01 to 2.00%, Mn: 0.50-3.00%, P: 0.0200% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0200% or less, O: 0.020% or less, Ni: 0-1.000%, Mo: 0-1.000%, Cr: 0-2.000%, B: 0 to 0.010%, As: 0 to 0.050%, Co: 0 to 0.500%, Ti: 0 to 0.500%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, Ca: 0-0.050%, Mg: 0 to 0.050%, La: 0 to 0.050%, Ce: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, Sb: 0 to 0.050%, Sn: 0 to 0.050%, and The balance: Fe and impurities, having a chemical composition consisting of When the range of 1 / 8 to 3 / 8 of the plate thickness from the surface in the plate thickness direction is defined as the t / 4 part, and the range of 20 μm from the surface in the plate thickness direction is defined as the surface layer part, The microstructure in the t / 4 portion includes, by volume fraction, 0% or more and 10.0% or less of retained austenite, and 90.0% or more and 100% or less of one or both of martensite and tempered martensite, a ratio of the dislocation density of the surface layer portion to the dislocation density of the t / 4 portion is 0.80 or more; The ratio of the hardness of the surface layer portion to the hardness of the t / 4 portion is 0.90 or more; The tensile strength is 1310 MPa or more and 2000 MPa or less.

1. A cold-rolled steel sheet comprising:

2. The surface has a coating layer made of zinc, aluminum, or magnesium, or an alloy of one or more of these. The cold rolled steel sheet according to claim 1 .

3. A method for producing the cold-rolled steel sheet according to claim 1, A hot rolling process in which a slab having the chemical composition according to claim 1 is hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is coiled in a state in which the temperature of the width direction central portion is higher than 600 ° C. and 700 ° C. or less, and the temperature of the edge portion located 20 mm from the width direction end portion is 600 ° C. or less; A cold rolling process in which the hot-rolled steel sheet after the hot rolling process is pickled and cold-rolled at a rolling reduction of 30 to 90% to obtain a cold-rolled steel sheet; An annealing step of heating the cold-rolled steel sheet to an annealing temperature exceeding Ac3 ° C., holding the cold-rolled steel sheet at the annealing temperature, and cooling the cold-rolled steel sheet after the holding to a cooling stop temperature such that the average cooling rate to 400 ° C. is 10 ° C. / sec or more and the average cooling rate from 400 ° C. to a cooling stop temperature of 100 ° C. or less is 15 ° C. / sec or more; A heat treatment process in which the cold-rolled steel sheet after the annealing process is heated to a temperature range of 200 to 350 ° C. and maintained at the temperature range; A skin pass rolling process in which the cold rolled steel sheet after the heat treatment process is skin pass rolled at a rolling reduction rate of 0.1% or more; having After the cold rolling process, the difference between the thickness of the center part and the thickness of the edge part in the width direction of the cold-rolled steel sheet is 10 μm or more. A method for producing a cold-rolled steel sheet.

4. In the annealing step, a coating layer made of zinc, aluminum, or magnesium, or an alloy of one or more of these, is formed on the front and back surfaces of the cold-rolled steel sheet. The method for producing a cold rolled steel sheet according to claim 3 .

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