Cold-rolled steel sheet and method for manufacturing the same

Abstract

A cold-rolled steel sheet characterized by having a prescribed chemical composition, a steel structure of which is, in area %, martensite: 90.0 to 99.5%, ferrite: 0 to 5%, residual austenite: 0.5 to 7.0%, and the remainder: bainite, and the proportion of tempered martensite in all the martensite is 80 to 100%, the maximum value of the curvature 1 / R represented by the following formula (1) obtained by shape measurement of a region of full width x length 300 mm is 0.010 or less, and the tensile strength of the cold-rolled steel sheet is 1470 MPa or more. 1 / R = MAX{|p1|, |p2|}... (1).

Description

Technical Field

[0001] This disclosure relates to cold-rolled steel sheets and methods for manufacturing the same. Background Technology

[0002] In recent years, from the perspective of limiting greenhouse gas emissions as part of global warming countermeasures, there is a demand for improved fuel efficiency in automobiles. To achieve lighter vehicle bodies and ensure crash safety, the application of high-strength steel sheets is gradually expanding. In particular, the demand for ultra-high-strength steel sheets with tensile strengths exceeding 980 MPa, and even higher tensile strengths, is increasing recently. Furthermore, high-strength hot-dip galvanized steel sheets with rust-resistant surfaces are required for parts of the vehicle body.

[0003] However, when using ultra-high strength steel sheets with such high tensile strength as automotive components, in addition to their press formability, it is also necessary to address the hydrogen embrittlement cracking of the steel sheets.

[0004] Hydrogen embrittlement cracking is a phenomenon in which steel components subjected to high stress under service conditions suddenly fracture due to hydrogen intrusion from the environment into the steel. This phenomenon is also known as delayed fracture based on the fracture morphology. Generally, it is known that the higher the tensile strength of a steel plate, the more prone it is to hydrogen embrittlement cracking. This is believed to be because higher tensile strength results in greater residual stress in the steel plate after component forming. The sensitivity to this hydrogen embrittlement cracking (delayed fracture) is called hydrogen embrittlement resistance.

[0005] To date, various attempts have been made to improve the hydrogen embrittlement resistance of steel plates.

[0006] For example, Patent Document 1 discloses an ultra-high strength cold-rolled steel sheet with a tensile strength of 1300 MPa or more and excellent resistance to hydrogen embrittlement. Its characteristic is that it has a specified chemical composition and the following steel microstructure: the solid solution content solB [mass%] and the original austenite grain diameter Dγ [μm] in the steel satisfy the relationship of equation (1): solB·Dγ≥0.0010. Furthermore, in terms of area ratio, polygonal ferrite is 10% or less, bainite is 30% or less, retained austenite is 6% or less, tempered martensite is 60% or more, and the number density of Fe carbides in the tempered martensite is 1×10⁻⁶. 6 / mm 2 The overall average dislocation density of the steel is 1.0 × 10⁻⁶. 15 / m 2 Above and 2.0×10 16 / m 2 The effective crystal grain size is below 7.0 μm.

[0007] Patent Document 2 discloses a cold-rolled steel sheet having a specified composition and the following microstructure: the combined area ratio of tempered martensite and bainite relative to the overall microstructure is 95% or more and 100% or less; it is composed of inclusion particles with a long axis of 0.3 μm or more extending along the rolling direction and / or distributed in a dotted pattern; when the inclusion particles consist of two or more particles, the distance between the inclusion particles is 30 μm or less; and the number of inclusion groups with a total length of more than 120 μm in the rolling direction is 0.8 particles / mm. 2 Below, the number of Fe-based carbides with an aspect ratio of 2.5 or less and a major axis of 0.20 μm or more but less than 2 μm is 3500 per mm. 2 Below, the carbides with a diameter of 10–50 nm distributed within the interior of the above-mentioned tempered martensite and / or bainite are 0.7 × 10⁻⁶. 7 pcs / mm 2 The original γ grains have an average grain size of less than 18 μm, a plate thickness of 0.5–2.6 mm, and a tensile strength of 1320 MPa or more. Furthermore, Patent Document 2 describes how, based on the above configuration, an ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement and a tensile strength of 1300 MPa or more can be obtained.

[0008] Patent document 3 discloses an ultra-high strength steel plate with excellent resistance to delayed fracture at the cut end. It has a specified composition and a microstructure consisting of more than 90% martensite and more than 0.5% retained austenite in terms of area ratio relative to the whole microstructure. It also has a region in which the local Mn concentration of more than 2% in terms of area ratio is more than 1.1 times the Mn content of the whole steel plate, and the tensile strength is more than 1470 MPa.

[0009] However, in recent years, with increasingly stringent requirements, there is a demand for high tensile strength and total elongation, as well as further improvements in resistance to hydrogen embrittlement. In particular, there is a current need to improve the resistance to hydrogen embrittlement in sheared parts.

[0010] The inventors have made in-depth efforts to improve the hydrogen embrittlement resistance of the shearing section, and as a result, the following approach was found: It was discovered that the better the shape of the steel plate, i.e., the flatter the steel plate, the better the hydrogen embrittlement resistance of the shearing section. This is believed to be because if an uneven steel plate is sheared, an angle is created between the punch and the steel plate during shearing, resulting in greater damage to the shearing section.

[0011] Relatedly, as a technique for improving the shape of high-strength steel plates, there are, for example, the following documents.

[0012] Patent document 4 discloses an ultra-high strength cold-rolled steel sheet and its manufacturing method, which has a specified chemical composition, a single-phase martensitic metal structure, a tensile strength of 980 MPa or more, and a flatness of less than 10 mm.

[0013] Patent document 5 discloses a method for manufacturing a high-strength cold-rolled steel sheet with excellent sheet shape. The method is characterized by having a specified composition and a metallic microstructure with tempered martensite comprising at least 65% area. The method includes the following steps: an annealing step of heating steel with the specified composition in an austenitic single-phase region for 15 to 600 seconds; and after annealing, slowly cooling the steel to a temperature range of 650 to 800°C at an average cooling rate of 10°C / second or less (excluding 0°C / second). The first cooling process is performed up to the first cooling stop temperature; the second cooling process is performed up to the second cooling stop temperature in the temperature range above and below Ms point calculated by the following formula (1) at an average cooling rate of 20 to 100°C / second; the third cooling process is performed up to the second cooling stop temperature at an average cooling rate of more than 100°C / second until room temperature is reached; and the over-aging process is performed by heating to a temperature range of 150 to 300°C and holding it for 30 to 1500 seconds.

[0014] Existing technical documents

[0015] Patent documents

[0016] Patent Document 1: Japanese Patent Application Publication No. 2016-50343

[0017] Patent Document 2; International Publication No. 2016 / 152163

[0018] Patent document 3; Japanese Patent Application Publication No. 2016-153524

[0019] Patent document 4; Japanese Patent Application Publication No. 2011-202195

[0020] Patent document 5; Japanese Patent Application Publication No. 2013-227657 Summary of the Invention

[0021] The problem that the invention aims to solve

[0022] However, the technologies disclosed in Patent Documents 4 and 5 are not techniques for intentionally improving the hydrogen embrittlement resistance of the sheared portion to improve the shape of the steel plate, and therefore are not sufficient for improving the hydrogen embrittlement resistance of the sheared portion. Patent Documents 4 and 5 use "maximum warpage height" as an indicator of the quality of the steel plate shape, but it is known that even if the "maximum warpage height" is within the range of the aforementioned patent documents, the hydrogen embrittlement resistance of the sheared portion may not be excellent.

[0023] Therefore, the object of the present invention is to provide a cold-rolled steel sheet with high tensile strength and total elongation, and improved resistance to hydrogen embrittlement.

[0024] Methods for solving problems

[0025] The inventors have discovered that, in order to improve the hydrogen embrittlement resistance of the sheared section, it is necessary to improve the quantity representing the curvature of the surface, namely "curvature," rather than improving the "maximum warpage height" of the steel plate. Furthermore, research was conducted on the manufacturing method of the steel plate required to improve its curvature, resulting in the following insights.

[0026] (1) In the hot rolling process, the edges are reheated after rough rolling. This suppresses the variation in strength of the hot-rolled steel sheet in the width direction. Then, the finished steel sheet is coiled within a suitable temperature range. As a result, the shape of the cold-rolled steel sheet is improved.

[0027] (2) In the cold rolling process, the front and rear tensions in each rolling stand as the steel passes through the rolls are controlled within a suitable range based on the yield strength of the hot-rolled steel sheet before cold rolling and the reduction rate in each rolling stand. Furthermore, the cumulative cold rolling reduction rate is controlled within a suitable range. As a result, the shape of the cold-rolled steel sheet is improved.

[0028] (3) In the cooling process following the heat treatment after cold rolling, the average cooling rate below 300°C is limited to a specified range, gas is used as the coolant, and venting to promote heat diffusion is implemented during the cooling process. Furthermore, the average cooling rate between 300 and 700°C and the cooling stop temperature must also be controlled within a suitable range. In addition, the tension of the steel sheet during the cooling process is controlled within an appropriate range. As a result, the shape of the heat-treated steel sheet is improved.

[0029] If all the requirements of (1) to (3) above are met, a steel plate with excellent shape can be obtained at a level that cannot be achieved by existing technology.

[0030] This invention is based on the above insights, and is specifically described below.

[0031] (1) A cold-rolled steel sheet, characterized in that it has the following chemical composition: C: 0.16-0.40% by mass %,

[0032] Si: 0.05~2.00%

[0033] Mn: 0.50~4.00%

[0034] P: below 0.050%

[0035] S: below 0.0100%

[0036] Al: 0.001~1.00%

[0037] N: below 0.0100%

[0038] O: below 0.0050%

[0039] Cr: 0–2.00%

[0040] Mo: 0–1.00%

[0041] Cu: 0–1.00%

[0042] Ni: 0~1.00%

[0043] B: 0~0.0100%

[0044] Co: 0-1.00%

[0045] W: 0~1.00%

[0046] Sn: 0~1.00%

[0047] Sb: 0~1.00%

[0048] Nb: 0~0.100%

[0049] Ti: 0~0.200%

[0050] V: 0~0.50%

[0051] Ca: 0~0.0100%

[0052] Mg: 0~0.0100%

[0053] Ce: 0~0.0100%

[0054] Zr: 0~0.0100%

[0055] La: 0~0.0100%

[0056] Hf: 0~0.0100%

[0057] Bi: 0~0.0100%

[0058] REM concentrations other than Ce and La: 0–0.0100%, and

[0059] The remaining portion consists of Fe and impurities.

[0060] The steel microstructure within a range of 1 / 8 to 3 / 8 of its thickness, centered at 1 / 4 of its thickness from the surface, is expressed as a percentage of area:

[0061] Martensite: 90.0–99.5%

[0062] Ferrite: 0-5%

[0063] Residual austenite: 0.5–7.0%, and

[0064] Remaining portion: bainite,

[0065] Furthermore, tempered martensite accounts for 80%–100% of all martensite.

[0066] The maximum value of the curvature 1 / R, expressed by the following formula (1), is less than or equal to 0.010, obtained by shape measurement of a region with a full width × length of 300 mm.

[0067] The tensile strength of the cold-rolled steel sheet is above 1470 MPa.

[0068] [Mathematical Expression 1]

[0069] 1 / R=MAX{|ρ1|,|ρ2|}…(1)1 / R:curvature

[0070] ρ1 and ρ2: Principal curvatures on the surface

[0071] (2) The cold-rolled steel sheet according to (1) above, characterized in that the above chemical composition, in mass percent, contains one or more elements selected from the group consisting of:

[0072] Cr: 0.001~2.00%

[0073] Mo: 0.001~1.00%

[0074] Cu: 0.001~1.00%

[0075] Ni: 0.001~1.00%

[0076] B: 0.0001~0.0100%

[0077] Co: 0.001~1.00%

[0078] W: 0.001~1.00%

[0079] Sn: 0.001~1.00%

[0080] Sb: 0.001~1.00%

[0081] Nb: 0.001~0.100%

[0082] Ti: 0.001~0.200%

[0083] V: 0.001~0.50%

[0084] Ca: 0.0001~0.0100%

[0085] Mg: 0.0001~0.0100%

[0086] Ce: 0.0001~0.0100%

[0087] Zr: 0.0001~0.0100%

[0088] La: 0.0001~0.0100%

[0089] Hf: 0.0001~0.0100%

[0090] Bi: 0.0001~0.0100%, and

[0091] REM values ​​other than Ce and La: 0.0001–0.0100%.

[0092] (3) The cold-rolled steel sheet according to (1) or (2) above is characterized in that, after the cold-rolled steel sheet is sheared and then subjected to heat treatment at 170°C for 10 minutes, it is immersed in an ammonium thiocyanate aqueous solution with a concentration of 0.3 g / L for 48 hours in a hydrogen embrittlement test, no cracking occurs on the sheared surface.

[0093] (4) The cold-rolled steel sheet according to any one of (1) to (3) above, wherein it has any one of an electro-galvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer on its surface.

[0094] (5) A method for manufacturing a cold-rolled steel sheet according to any one of (1) to (3) above, characterized in that it comprises the following steps:

[0095] (A) A hot rolling process, which includes rough rolling and finish rolling of a slab having the chemical composition described in (1) or (2) above, and satisfying the following conditions (A1) to (A3):

[0096] (A1) The slab heating temperature is above 1150℃;

[0097] (A2) The edge of the width of the rough-rolled steel plate is heated in such a way that the temperature of the edge of the width is 10 to 150°C higher than the temperature of the center of the width;

[0098] (A3) The winding temperature is 450–650℃.

[0099] (B) Cold rolling process, which includes cold rolling the obtained hot-rolled steel sheet using a series rolling mill consisting of N (N≥3) rolling mills, wherein the cumulative cold rolling reduction is 30% or more, and satisfies the following equations (2) and (3):

[0100] [Mathematical Expression 2]

[0101]

[0102] [Mathematical Expression 3]

[0103] σ k = (1.667·σ0)·ε k 0.1 …(3)

[0104] R k : Reduction rate in the k-th rolling stand

[0105] Pb k The rear tension in the k-th rolling mill stand

[0106] Pf k Forward tension in the k-th rolling mill stand

[0107] σ k-1 Flow stress of the steel plate after passing through the (k-1)th rolling mill stand

[0108] σ k Flow stress of the steel plate after passing through the k-th rolling mill stand

[0109] σ0: Yield strength of hot-rolled steel plate

[0110] ε k : Cumulative strain after the kth rolling stand

[0111] (C) A heat treatment process, which includes heat treating the obtained cold-rolled steel sheet and satisfying the following conditions (C1) to (C3):

[0112] (C1) Hold the cold-rolled steel sheet at Ac3 to 950°C for 10 to 500 seconds (heat holding);

[0113] (C2) Implement cooling treatments that satisfy the following (i) to (v):

[0114] (i) The cooling stop temperature T1 is 110–250°C;

[0115] (ii) The average cooling rate between 300 and 700°C is 20 to 150°C / s;

[0116] (iii) The average cooling rate between T1 and 300°C is 1.0 to 20°C / s, and gas is used as the coolant;

[0117] (iv) At least one cooling operation of 0.5 seconds or more shall be carried out between Ms and 700°C and between T1 and below Ms.

[0118] (v) Applicable to cold-rolled steel sheets with a tension of 5 to 20 MPa;

[0119] (C3) Keep at 200-300℃ for 100-1000 seconds (low temperature holding).

[0120] Invention Effects

[0121] According to the present invention, it is possible to provide cold-rolled steel sheets with a tensile strength of 1470 MPa or more, a high total elongation, and improved resistance to hydrogen embrittlement. Attached Figure Description

[0122] Figure 1 This is a schematic diagram of shearing processes associated with hydrogen embrittlement testing. Detailed Implementation

[0123] Chemical composition

[0124] First, the reasons for limiting the chemical composition of the steel plate according to the embodiments of the present invention as described above will be explained. It should be noted that, unless otherwise specified, the "%" in this specification refers to "mass %" for chemical composition. Furthermore, in this specification, the "~" indicating a numerical range is used to mean both the lower and upper limits of the range, including the values ​​described before and after it, unless otherwise specified.

[0125] [C: 0.16~0.40%]

[0126] Carbon (C) is an essential element for ensuring the strength of steel plates. To achieve this effect, the C content is set at 0.16% or more. The C content can also be 0.18% or more, 0.20% or more, or 0.22% or more. On the other hand, excessive C content can sometimes reduce processability, weldability, and resistance to hydrogen embrittlement, such as pressing formability. Therefore, the C content is set at 0.40% or less. The C content can also be 0.37% or less, 0.33% or less, or 0.30% or less.

[0127] [Si: 0.05~2.00%]

[0128] Silicon (Si) is an element that inhibits the formation of iron carbides and contributes to improved strength and formability. To fully obtain these effects, the Si content is set to 0.05% or more. The Si content can also be 0.10% or more, 0.20% or more, or 0.40% or more. On the other hand, excessive addition can sometimes reduce toughness, weldability, and consequently, resistance to hydrogen embrittlement. Therefore, the Si content is set to 2.00% or less. The Si content can also be 1.60% or less, 1.30% or less, or 1.00% or less.

[0129] [Mn: 0.50~4.00%]

[0130] Manganese (Mn) is a strong austenite stabilizing element and is effective in increasing the strength of steel plates. To achieve this effect, the Mn content is set at 0.50% or more. The Mn content can also be 0.80% or more, 1.00% or more, or 1.30% or more. On the other hand, excessive addition can sometimes deteriorate workability (such as pressing formability), weldability, and even resistance to hydrogen embrittlement. Therefore, the Mn content is set at 4.0% or less. The Mn content can also be 3.0% or less, 2.5% or less, or 2.0% or less.

[0131] [P: below 0.050%]

[0132] Phosphorus (P) is a solid solution strengthening element and is effective in increasing the strength of steel plates. However, excessive addition can deteriorate weldability and toughness. Therefore, the P content is limited to 0.050% or less. The preferred P content is 0.045%, 0.035%, or 0.020% or less. The P content can also be 0%, but to minimize the cost of P removal, a lower limit of 0.001% is preferred from an economic point of view.

[0133] [S: below 0.0100%]

[0134] Sulfur (S) is an element present as an impurity, forming MnS in steel and deteriorating its toughness and porosity. Therefore, to minimize the deterioration of toughness and porosity, the S content is limited to 0.0100% or less. The preferred S content is 0.0050%, 0.0040%, or 0.0030% or less. While the S content can be 0%, lowering the S content would increase desulfurization costs; therefore, from an economic point of view, a lower limit of 0.0001% is preferred.

[0135] [Al: 0.001~1.00%]

[0136] For deoxidation of steel, at least 0.001% Al (aluminum) is added. The Al content can also be 0.005%, 0.01%, or 0.02% or more. On the other hand, even with excessive Al addition, the effect saturates, not only unnecessarily increasing costs but also raising the phase transformation temperature of the steel and increasing the load during hot rolling, sometimes resulting in a decrease in the mechanical properties of the steel sheet. Therefore, an upper limit of 1.00% is set for the Al content. The Al content can also be 0.80%, 0.60%, or 0.30% or less.

[0137] [N: below 0.0100%]

[0138] Nitrogen (N) is an element present as an impurity. If its content is high, it can sometimes form large nitrides in steel, deteriorating its flexibility and porosity. Therefore, the N content is limited to 0.0100% or less. Preferably, the N content is 0.0080%, 0.0060%, or 0.0050% or less. The N content can also be 0%, but to minimize the cost of N removal, a lower limit of 0.0001% is preferred from an economic point of view.

[0139] [O: below 0.0050%]

[0140] Oxygen (O) is an element present as an impurity. If its content is high, it can sometimes form large oxides in steel, deteriorating its flexibility and porosity. Therefore, the O content is limited to 0.0100% or less. Preferably, the O content is 0.0080%, 0.0060%, or 0.0050% or less. The O content can also be 0%, but from a manufacturing cost perspective, it is preferable to set the lower limit to 0.0001%.

[0141] The basic chemical composition of the cold-rolled steel sheet and the slab used in its manufacture according to embodiments of the present invention is as described above. Furthermore, the cold-rolled steel sheet and slab may also contain the following optional elements as needed. It should be noted that the lower limit for the content of the optional element not present is 0%.

[0142] [Cr: 0 to 2.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, B: 0 to 0.0100%, Co: 0 to 1.00% , W: 0~1.00%, Sn: 0~1.00%, Sb: 0~1.00%, Nb: 0~0.100%, Ti: 0~0.200% and V: 0~0.50%]

[0143] Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), B (boron), Co (cobalt), W (tungsten), Sn (tin), and Sb (antimony) are all elements that effectively improve the hardenability of steel and thus increase the strength of steel plates. In addition, Nb (niobium), Ti (titanium), and V (vanadium) are alloy carbide-forming elements, contributing to the increase of steel plate strength by precipitating as fine carbides within the steel plate. Therefore, one or more of these elements can be added as needed. However, excessive addition of these elements will result in saturation, unnecessarily increasing costs. Therefore, the content is set as follows: Cr: 0–2.00%, Mo: 0–1.00%, Cu: 0–1.00%, Ni: 0–1.00%, B: 0–0.0100%, Co: 0–1.00%, W: 0–1.00%, Sn: 0–1.00%, Sb: 0–1.00%, Nb: 0–0.100%, Ti: 0–0.200%, and V: 0–0.50%. The content of each element can also be above 0.001%, 0.005%, or 0.010%. In particular, the B content can also be above 0.0001% or 0.0005%.

[0144] [Ca: 0–0.0100%, Mg: 0–0.0100%, Ce: 0–0.0100%, Zr: 0–0.0100%, La: 0–0.0100%, Hf: 0–0.0100%, Bi: 0–0.0100%, and REM (excluding Ce and La): 0–0.0100%]

[0145] Ca (calcium), Mg (magnesium), Ce (cerium), Zr (zirconium), La (lanthanum), Hf (hafnium), and REM (rare earth elements) other than Ce and La are elements that contribute to the fine dispersion of inclusions in steel. Bi (bismuth) is an element that reduces the microsegregation of substitutional alloying elements such as Mn and Si in steel. Since they each contribute to improving the workability of the steel sheet, one or more of these elements can be added as needed. However, excessive addition will cause a deterioration in ductility. Therefore, their content is set to an upper limit of 0.0100%. Alternatively, the content of each element can be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0146] In the cold-rolled steel sheet of the embodiment of the present invention, the remaining portion other than the aforementioned elements consists of Fe and impurities. Impurities are components that are mixed in during the industrial manufacturing of cold-rolled steel sheets, such as raw materials like ores and scrap iron, due to various factors in the manufacturing process.

[0147] Steel structure of cold-rolled steel sheet

[0148] Next, the steel structure of the cold-rolled steel sheet according to an embodiment of the present invention will be described.

[0149] [Martensite: 90.0–99.5%, Ferrite: 0–5%, Retained Austenite: 0.5–7.0%, Remaining portion: Bainite, and the proportion of tempered martensite in the total martensite: 80–100%]

[0150] The steel microstructure, measured in area % as a range of 1 / 8 to 3 / 8 of the thickness centered at 1 / 4 of the thickness of the cold-rolled steel sheet, comprises: martensite 90.0–99.5%, ferrite 0–5%, retained austenite 0.5–7.0%, and the remainder: bainite, with tempered martensite accounting for 80–100% of the total martensite.

[0151] By using martensite (quenched martensite + tempered martensite) as the main component, the desired tensile strength can be obtained. On the other hand, if there is more quenched martensite and less tempered martensite in the martensite, the resistance to hydrogen embrittlement deteriorates. Therefore, the area ratio of martensite is set to 90.0% to 99.5%, and the proportion of tempered martensite in the total martensite is set to 80% to 100%. The lower limit of the area ratio of martensite is preferably 93.0% or more, more preferably 95.0% or more. The upper limit of the area ratio of martensite can also be 99.0% or less or 98.0% or less. The lower limit of the proportion of tempered martensite in the total martensite is preferably 85% or more, more preferably 90% or more. The upper limit of the proportion of tempered martensite in the total martensite can also be 98% or less or 95% or less.

[0152] If ferrite exceeds 5%, it becomes difficult to obtain the desired tensile strength. Furthermore, the presence of soft ferrite within a martensitic microstructure increases the inhomogeneity of the microstructure, thus promoting hydrogen embrittlement. Therefore, the area fraction of ferrite is set to 0–5%. The upper limit of the ferrite area fraction is preferably 4% or less, more preferably 2% or less, and ideally 0%.

[0153] If retained austenite is included in the steel microstructure, the work hardening rate increases through the TRIP (transformation-induced plasticity) effect, thus improving ductility (i.e., increasing total elongation). On the other hand, excessive retained austenite deteriorates hydrogen embrittlement resistance. Therefore, the area fraction of retained austenite is set to 0.5% to 7.0%. The lower limit of the area fraction of retained austenite is preferably 1.0% or more, and may also be 2.0% or more. The upper limit of the area fraction of retained austenite is preferably 6.0% or less, and may also be 5.0% or less or 4.0% or less.

[0154] In addition to martensite, ferrite, and retained austenite, steel microstructure may also contain residual microstructure. Bainite can be exemplified as a residual microstructure, for example. The area ratio of the residual microstructure is 0–9.5%.

[0155] [Methods for determining the area ratio of various tissues]

[0156] The area fractions of the microstructures other than retained austenite were evaluated using SEM-EBSD (electron backscatter diffraction) and SEM secondary electron imaging. First, samples were collected from a section of the plate thickness parallel to the rolling direction of the steel sheet as the observation surface. The observation surface was mechanically ground, polished to a mirror finish, and then electrolytically ground. Next, in one or more observation fields within the observation surface, ranging from 1 / 8 to 3 / 8 of the plate thickness centered at 1 / 4 of the plate's surface thickness, samples were analyzed for a total area of ​​3000 μm. 2 The areas described above were analyzed for crystal structure and orientation using SEM-EBSD. For the analysis of data obtained via EBSD, TSL's "OIM Analysys 7.0" was used. The step distance was set to 0.03–0.20 μm. Grain boundary maps were obtained by defining boundaries with an orientation difference of 15 degrees or more as grain boundaries. Next, the same sample was etched with nitric acid and ethanol. Then, a secondary electron image was captured using FE-SEM for the same field of view as the one used for crystal orientation analysis via EBSD. It is advisable to pre-mark the images using Vickers indentation or similar methods. Finally, the grain boundary map was superimposed on the secondary electron image. For each grain enclosed by grain boundaries with an orientation difference of 15 degrees or more, the microstructure was classified based on the following criteria.

[0157] In secondary electron microscopy (SEEM), grains with no lower microstructure, no iron-based carbides, and a BCC crystal structure are identified as ferrite. Grains showing a lower microstructure and iron-based carbides precipitating in a single variant, or grains without iron-based carbides, are identified as bainite. Grains with cementite precipitating in lamellar form in SEEM are identified as pearlite. However, in this invention, pearlite is generally not included. The remaining portion is identified as martensite and retained austenite. The area ratio of martensite is obtained by subtracting the area ratio of retained austenite (described later) from the area ratio of the remaining portion. Within the remaining portion, grains showing a lower microstructure and two or more iron-based carbides precipitating in multiple variants in the SEEM are identified as tempered martensite.

[0158] The area ratio of retained austenite was calculated using X-ray determination. Specifically, the austenite was removed from the surface of the steel plate by mechanical and chemical grinding up to a depth of 1 / 4 in the thickness direction. Then, the microstructure fraction of retained austenite was calculated from the integral intensity ratio of the diffraction peaks of the bcc phase (200), (211) and the fcc phase (200), (220), (311) obtained by using MoKα1 rays as characteristic X-rays on the ground sample, and this fraction was set as the area ratio of retained austenite.

[0159] [Maximum curvature 1 / R: below 0.010]

[0160] In the cold-rolled steel sheet of the embodiments of the present invention, despite having high strength, for example, 1470 MPa or more, it also has very high flatness. For example, even when the cold-rolled steel sheet is sheared using a punch, the end face shape of the sheared portion is very good, resulting in excellent resistance to hydrogen embrittlement. The shape of the steel sheet with such high flatness in the present invention is defined using the maximum value of curvature 1 / R, which is equivalent to the reciprocal of the radius of curvature R (mm). More specifically, the maximum value of curvature 1 / R in the present invention is defined using the two principal curvatures ρ1 and ρ2 on the surface by the following formula (1). In the embodiments of the present invention, the maximum value of curvature 1 / R is controlled to be 0.010 or less.

[0161] [Mathematical Expression 4]

[0162] 1 / R=MAX{|ρ1|,|ρ2|}···(1)

[0163] Here, the curvature in this invention refers to the larger of the absolute values ​​of the principal curvatures ρ1 and ρ2 on the curved surface. The principal curvatures ρ1 and ρ2 are measured using a general shape measuring machine and evaluated by three-dimensional geometric data with noise suppressed. For example, a representative shape measuring machine can be the ATOS 3D scanner manufactured by GOM Corporation. The curvature distribution within the cold-rolled steel sheet is obtained by measuring each point in a region of full width × 300 mm. In this invention, "full width" refers to the length of the steel sheet in a direction perpendicular to the length direction of the cold-rolled steel sheet (cold-rolled coil). The maximum value of the curvature distribution of the cold-rolled steel sheet obtained by this invention is 0.010 or less. For example, if the maximum value of the curvature distribution exceeds 0.010 when the cold-rolled steel sheet is warped or undulating, it is considered that an angle is formed between the punch and the cold-rolled steel sheet during shearing, resulting in increased damage to the sheared portion and consequently, deterioration of the hydrogen embrittlement resistance of the sheared portion. The maximum value of curvature 1 / R can be, for example, less than 0.008, less than 0.006, less than 0.004, or less than 0.002. The lower limit is not particularly limited, but the maximum value of curvature 1 / R can also be, for example, greater than 0.0005, greater than 0.0006, greater than 0.0007, greater than 0.0008, greater than 0.0009, or greater than 0.001. According to embodiments of the present invention, as described above, even with a high strength of 1470 MPa or more, a very high flatness can be achieved. Even with a very high tensile strength exceeding 1800 MPa, as specifically shown in the embodiments, a flatness with a maximum curvature 1 / R of 0.001 can be achieved. Therefore, it will be readily understood by those skilled in the art that with a lower tensile strength, for example, closer to 1470 MPa, the maximum value of curvature 1 / R can be further reduced, for example, achieving a flatness with a maximum curvature 1 / R of 0.0005.

[0164] Regarding the measurement of the curvature distribution described above, there are no specific limitations on the measurement period, etc. For example, it can be performed on cold-rolled steel sheets that have undergone leveling treatment such as a leveling machine after manufacturing, or it can be performed on cold-rolled steel sheets that have just been manufactured without specific mechanical leveling treatment. For example, in the case of conventional cold-rolled steel sheets with very high tensile strength of 1470 MPa or more, even if leveling treatment is performed simply by a leveling machine or the like, it is extremely difficult to control the maximum value of curvature 1 / R described above to be below 0.010. In the embodiments of the present invention, by using a slab with a specified chemical composition, as detailed below, and by appropriately controlling the conditions of the hot rolling process, the cold rolling process, and the heat treatment process to manufacture cold-rolled steel sheets, such high flatness can be achieved. Furthermore, when the cold-rolled steel sheet has a coating, the coating has no particular effect on the measurement of the curvature distribution, therefore, the above-mentioned measurement of the curvature distribution is performed on cold-rolled steel sheets with a coating without peeling off the coating.

[0165] Next, the mechanical properties of the cold-rolled steel sheet according to the embodiments of the present invention will be described.

[0166] [Tensile Strength (TS)]

[0167] The cold-rolled steel sheet according to embodiments of the present invention can achieve excellent mechanical properties, such as a tensile strength (TS) of 1470 MPa or more. The tensile strength is preferably 1490 MPa or more, more preferably 1500 MPa or more. There is no particular upper limit, but for example, the tensile strength may be 2000 MPa or less, 1900 MPa or less, or 1800 MPa or less.

[0168] [Total elongation (El)]

[0169] According to embodiments of the present invention, the cold-rolled steel sheet can achieve a high total elongation (E1), more specifically, a total elongation of 6.0% or more. The total elongation is preferably 7.0% or more, more preferably 8.0% or more. There is no particular upper limit, but for example, the total elongation may be 20.0% or less or 15.0% or less. Here, the tensile strength and total elongation of the cold-rolled steel sheet are determined by collecting JIS No. 5 tensile test specimens at room temperature (25°C) in atmospheric conditions, from a direction perpendicular to the rolling direction of the steel sheet, and by performing the tensile test specified in JIS Z 2241:2011.

[0170] [Porosity (λ)]

[0171] The cold-rolled steel sheet according to an embodiment of the present invention can achieve high porosity, more specifically, a porosity (λ) of 20% or more. The porosity is preferably 25% or more, more preferably 30% or more. There is no particular upper limit, but for example, the porosity can also be 80.0% or less or 70.0% or less. The porosity (λ) is determined by the Japanese Iron and Steel Federation standard "JFS T 1001:1996 Porosity Test Method".

[0172] [Evaluation using the hydrogen embrittlement test]

[0173] The cold-rolled steel sheet according to embodiments of the present invention is characterized in that it does not crack during a hydrogen embrittlement test using the following method: shearing process... Figure 1 The method shown is as follows. Samples of T (thickness) × 50W (width) × 50L (length) (unit: mm) are collected from the steel plate, including the portion where the maximum curvature 1 / R can be obtained. The shear angle θ is set to 1 degree, and the clearance CL is set to 0.15 × T. The pressure plate is subjected to a load of at least 1 ton. After the above sample is cut by shearing, the steel plate on the product side (pressure plate side) is heat-treated at 170°C for 10 minutes. Afterwards, it is immersed in a 0.3 g / L ammonium thiocyanate aqueous solution at room temperature for 48 hours to introduce the generated hydrogen into the steel plate. The sheared surface is then observed using a microscope to evaluate the presence or absence of cracks. The 10-minute heat treatment at 170°C simulates heat treatments such as painting and baking.

[0174] [Plate thickness]

[0175] The cold-rolled steel sheet of the embodiments of the present invention has, for example, a sheet thickness of 0.5 to 3.0 mm. Although not particularly limited, the sheet thickness may also be 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more. Similarly, the sheet thickness may also be 2.8 mm or less, 2.6 mm or less, or 2.3 mm or less.

[0176] [Board Width]

[0177] The cold-rolled steel sheet in embodiments of the present invention, for example, has a width of 500 mm or more. Although not particularly limited, the width may also be 700 mm or more, 800 mm or more, or 900 mm or more. The upper limit of the width is not particularly limited, but the width may also be 2000 mm or less, 1800 mm or less, 1600 mm or less, 1400 mm or less, 1300 mm or less, 1200 mm or less, or 1100 mm or less.

[0178] [Coating]

[0179] The cold-rolled steel sheet of the embodiments of the present invention may also have a coating on both sides or one side, preferably both sides. Representative examples of the coating include electroplated zinc, hot-dip galvanized, or alloyed hot-dip galvanized layers. These zinc coatings preferably have any composition known to those skilled in the art, and preferably contain additive elements such as Al and Mg in addition to Zn. Furthermore, there is no particular limitation on the amount of coating applied; a general amount is preferred.

[0180] <Manufacturing Method>

[0181] Next, a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention will be described. The purpose of the following description is to illustrate a characteristic method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention, and it is not intended to limit the cold-rolled steel sheet to cold-rolled steel sheets manufactured by the manufacturing method described below.

[0182] (A) Hot rolling process

[0183] First, the hot rolling process will be explained.

[0184] [Slab heating temperature: above 1150℃]

[0185] In the hot rolling process, a slab with the same chemical composition as described above for cold-rolled steel sheet is heated before hot rolling, followed by rough rolling and finish rolling. To fully melt borides, carbides, etc., the heating temperature of the slab must be set to 1150°C or higher, preferably 1200°C or higher. It should be noted that, from a manufacturability point of view, the steel slab used is preferably cast by continuous casting, but it can also be manufactured by ingot casting or thin slab casting.

[0186] [Rough rolling]

[0187] The heated slab is rough-rolled before finishing rolling. There are no particular limitations on the rough-rolling conditions, but it is preferably carried out at 1050°C with a total reduction of 60% or more. If the total reduction is less than 60%, recrystallization during hot rolling becomes insufficient, sometimes resulting in inhomogeneity in the microstructure of the hot-rolled steel sheet. The aforementioned total reduction can also be, for example, 90% or less.

[0188] [The width edge is heated such that the temperature at the edge is 10 to 150°C higher than the temperature at the center.]

[0189] For steel sheets that have undergone rough rolling, the width edges are reheated such that the temperature (Te) at the width edges is 10 to 150°C higher than the temperature (Tc) at the width center. This reheating suppresses strength variations in the width direction of the hot-rolled steel sheet, resulting in a more uniform strength in the width direction. Therefore, in the subsequent cold rolling process, uniform rolling can be performed throughout the width direction, further improving the shape of the cold-rolled steel sheet. Without this reheating, the width edges cool at a higher rate than the width center, causing them to harden. As a result, in the subsequent cold rolling process, a shape defect called "middle wavy" occurs, extending beyond the width center compared to the width edges. Consequently, the curvature in the final product deteriorates. On the other hand, if the width edges are overheated, they become overly soft, resulting in a shape defect called "edge wavy," where the edges extend beyond the center in the subsequent cold rolling process. To avoid these shape defects, the edges are heated such that the temperature of the wide edge portion is 10 to 150°C higher than the temperature of the wide center portion. Preferably, the temperature is 20 to 100°C, more preferably 40 to 90°C. The heating (reheating) of the wide edge portion can be carried out by any suitable means known to those skilled in the art, without particular limitation, but for example, an edge heater can be used.

[0190] [Precision rolling]

[0191] After reheating the edges, the steel is finished rolled. The conditions are not particularly limited, but it is preferably carried out within the range of a finishing roll entry temperature of 950–1050°C, a finishing roll exit temperature of 850–1000°C, and a total reduction rate of 70–95%. When the finishing roll entry temperature is below 950°C, or the finishing roll exit temperature is below 850°C, or the total reduction rate exceeds 95%, the anisotropy in the final product sheet may become more pronounced due to the well-developed texture of the hot-rolled steel sheet. On the other hand, when the finishing roll entry temperature exceeds 1050°C, or the finishing roll exit temperature exceeds 1000°C, or the total reduction rate is below 70%, the grain size of the hot-rolled steel sheet may become coarser, leading to a coarser microstructure in the final product sheet.

[0192] [Winding temperature: 450~650℃]

[0193] In this method, by coiling the finished steel sheet at a coiling temperature of 450–650°C, the shape of the cold-rolled steel sheet can be improved. At coiling temperatures below 450°C, the hot-rolled steel sheet exhibits high strength, thus deteriorating the shape of the cold-rolled steel sheet. On the other hand, at coiling temperatures exceeding 650°C, the cementite coarsens, leaving behind unmelted cementite, which sometimes impairs processability.

[0194] [Pickling]

[0195] After hot rolling, pickling is performed as needed to remove the oxide scale. The pickling method can follow standard procedures. Alternatively, to correct the shape of the hot-rolled coil or improve its pickling properties, pretreatments such as surface finishing or shot peening can be performed before pickling.

[0196] (B) Cold rolling process

[0197] Next, the cold rolling process will be explained.

[0198] [Cold rolling was performed using a series rolling mill consisting of N (N≥3) rolling mills.]

[0199] In this method, a cold rolling process is implemented, which includes cold rolling the obtained hot-rolled steel plate using a series rolling mill consisting of N (N≥3) rolling mills, wherein the cumulative cold rolling reduction rate is more than 30%, and satisfies the following equations (2) and (3).

[0200] [Mathematical Expression 5]

[0201]

[0202] [Mathematical Expression 6]

[0203] σ k = (1.667·σ0)·ε k 0.1 ···(3)

[0204] R k : Reduction rate in the k-th rolling stand

[0205] Pb k The rear tension in the k-th rolling mill stand

[0206] Pf k Forward tension in the k-th rolling mill stand

[0207] σ k-1 Flow stress of the steel plate after passing through the (k-1)th rolling mill stand

[0208] σ k Flow stress of the steel plate after passing through the k-th rolling mill stand

[0209] σ0: Yield strength of hot-rolled steel plate

[0210] ε k : Cumulative strain after the kth rolling stand

[0211] In the cold rolling process of this invention, the reduction rate, front tension / flow stress, and rear tension / flow stress in each rolling stand must be controlled in accordance with the above formula (2). The front tension and rear tension in each rolling stand of a tandem rolling mill are parameters that are generally measured. For example, as described in "Special Report No. 36 Theory and Practice of Plate Rolling (Revised Edition), Compilation of Rolling Theory Department, Production Technology Division, Japan Iron and Steel Association, 2010, p. 264", test rolls are placed on the cold-rolled steel plate, and the tension is measured by a load in the vertical direction. In addition, the flow stress of the cold-rolled steel plate is given by formula (3). Here, σ0 is the flow stress of the steel plate before the first stand passes through, that is, the yield strength of the hot-rolled steel plate. σ0 is obtained by taking JIS No. 5 tensile test pieces from the center of the width of the hot-rolled steel plate along the rolling direction and performing a tensile test according to JIS Z 2241:2011. Equation (2) means that if a large reduction is applied when the difference between the forward tension / flow stress and the backward tension / flow stress is large, the value will increase. To reduce Equation (2), the difference between the forward tension / flow stress and the backward tension / flow stress must be reduced. By reducing the difference between the forward tension / flow stress and the backward tension / flow stress to satisfy Equation (2), the phenomenon of steel plate slippage relative to the rolls, the so-called slippage, can be reliably suppressed, and more stable cold rolling can be achieved. As a result, the shape of the cold-rolled steel plate can be improved.

[0212] When the left side of equation (2) is 3.0 or higher, the shape of the cold-rolled steel sheet deteriorates significantly, and the curvature in the final product no longer satisfies equation (1). The smaller the left side of equation (2), the better; for example, it is preferable to be below 2.5 or below 2.0, and even more preferably below 1.0. Although the lower limit is not particularly limited, the left side of equation (2) can also be above 0.1 or above 0.2. Equation (2) is a preferred index for achieving stable cold rolling without slippage or other rolling defects by balancing the tension and yield strength of the hot-rolled steel sheet well at the front and back of each rolling stand. Therefore, in order to achieve such stable cold rolling, other control methods can be used instead of the control method based on equation (2).

[0213] In the cold rolling process, in addition to satisfying equation (2), setting the cumulative cold rolling reduction rate to 30% or more is also important for obtaining a good steel sheet shape with high flatness. If the cumulative cold rolling reduction rate is less than 30%, the shape of the cold-rolled steel sheet is not sufficiently improved, resulting in the curvature in the final product not satisfying equation (1). The cumulative cold rolling reduction rate can also be 40% or more or 50% or more. There is no particular upper limit, but excessive reduction will make the rolling load too large and increase the burden on the cold rolling mill. Therefore, for example, the cumulative cold rolling reduction rate can also be 75% or less or 70% or less.

[0214] (C) Heat treatment process

[0215] Next, the heat treatment process will be explained.

[0216] [Heating hold: Hold at Ac3~950℃ for 10 seconds~500 seconds]

[0217] The resulting cold-rolled steel sheet is then subjected to a specified heat treatment process. First, to ensure sufficient austenitization, heating is performed at or above Ac3°C for at least 10 seconds. If the heating temperature is below Ac3°C or the holding time is less than 10 seconds, austenitization is insufficient, and the desired steel microstructure dominated by martensite cannot be obtained, resulting in insufficient strength. On the other hand, if the heating temperature exceeds 950°C or the holding time exceeds 500 seconds, in addition to coarsening of the grain size, it also leads to increased fuel costs and equipment damage. Ac3 (°C) is calculated using the following formula. Substitute the mass percentage of the element in the element symbols in the following formula. For elements not present, substitute 0 by mass.

[0218] Ac3(℃)=912-230.5×C+31.6×Si-20.4×Mn-39.8×Cu-18.1×Ni-14.8×Cr+16.8×Mo+100.0×Al

[0219] [Cooling stop temperature T1: 110~250℃]

[0220] After heating, the temperature is cooled to a range of 110–250°C. When T1 is below 110°C, the retained austenite is less than 0.5% by area, resulting in a decrease in total elongation. On the other hand, when the temperature exceeds 250°C, the proportion of tempered martensite in the martensite becomes less than 80%, resulting in reduced resistance to hydrogen embrittlement. The cooling stop temperature can also be above 120°C and / or below 220°C.

[0221] [Average cooling rate between 300 and 700°C: 20–150°C / s]

[0222] By controlling the average cooling rate (average cooling rate 1) between 300 and 700°C to a range of 20 to 150°C / s, the increase in temperature deviation within the steel sheet can be suppressed, thus improving the curvature of the steel sheet. When the average cooling rate in the aforementioned range is below 20°C / s, the martensite fraction decreases, making it impossible to obtain the desired tensile strength. On the other hand, when the rate exceeds 150°C / s, the curvature of the steel sheet deteriorates due to the increased temperature deviation within the steel sheet. It should be noted that the average cooling rate in this invention includes the cooling time described later.

[0223] [Average cooling rate between T1 and 300℃: 1.0–20℃ / s and coolant: gas]

[0224] By setting the average cooling rate (average cooling rate 2) between T1 and 300°C to 1.0–20°C / s and using a gas (e.g., nitrogen) as the coolant to achieve a relatively stable cooling process, the increase in temperature deviation within the steel sheet can be suppressed, thus improving the curvature of the steel sheet. When the average cooling rate in the aforementioned range is below 1.0°C / s, the martensite fraction decreases, making it impossible to obtain the desired tensile strength. On the other hand, when the rate exceeds 20°C / s, the curvature of the steel sheet deteriorates due to the increased temperature deviation within the steel sheet. Furthermore, from the viewpoint of reliably suppressing the increase in temperature deviation within the steel sheet, a gas must be used as the coolant.

[0225] [Perform at least one cooling cycle of at least 0.5 seconds between Ms and 700°C, and between T1 and below Ms.]

[0226] Cooling is temporarily halted in the ranges between Ms and 700°C and between T1 and below Ms, with a cooling period of at least 0.5 seconds. This treatment promotes heat transfer within the steel plate and improves temperature uniformity, thereby improving the plate's curvature. Ms (°C) is calculated using the following formula. Substitute the mass percentage of the element into the element symbols in the following formula. For elements not present, substitute 0% by mass.

[0227] Ms(℃)=561-474×C-33×Mn-17×Cr-21×Mo-7.5×Si+10×Co

[0228] [Suitable tension for cold-rolled steel sheets: 5~20MPa]

[0229] Between the aforementioned cooling processes, the tension applied to the cold-rolled steel sheet must be limited to 6–20 MPa. By controlling the tension within this range, the flatness of the cold-rolled steel sheet can be improved, thus enhancing the curvature of the final cold-rolled steel sheet. Conversely, outside this tension range, the curvature of the cold-rolled steel sheet deteriorates. The tension can also be 8 MPa or higher. Similarly, the tension can be 16 MPa or lower.

[0230] [Low Temperature Hold: Maintain at 200–300°C for 100–1000 seconds]

[0231] After cooling to the cooling stop temperature T1, the temperature is held between 200 and 300°C for 100 to 1000 seconds. This allows carbon to be distributed within the untransformed austenite, resulting in retained austenite. At temperatures below 200°C or holding times below 100 seconds, the desired amount of retained austenite cannot be obtained. Conversely, at temperatures above 300°C or holding times exceeding 1000 seconds, the desired steel microstructure cannot be obtained, resulting in the unattainable tensile strength and total elongation.

[0232] For cold-rolled steel sheets obtained by the manufacturing method of cold-rolled steel sheets according to embodiments of the present invention, subsequent processes such as plating processes that form a coating on one or both sides of the cold-rolled steel sheet can also be performed. These subsequent processes, such as plating processes, can be carried out using conventional methods.

[0233] Example

[0234] The following describes embodiments of the cold-rolled steel sheet according to the present invention. The conditions described in the embodiments are examples adopted to confirm the feasibility and effectiveness of the present invention. The present invention is not limited to these specific conditions. Various conditions can be used in the present invention as long as they do not depart from the spirit and purpose of the invention.

[0235] First, steel with the chemical composition shown in Table 1 was cast to produce slabs. The remainder other than the components shown in Table 1 consisted of Fe and impurities. These slabs were then hot-rolled under the conditions shown in Table 2, including roughing and finishing, to produce hot-rolled steel sheets. Heating (reheating) of the width edges after roughing was performed using edge heaters. Next, the hot-rolled steel sheets were pickled to remove surface oxide scale, and then cold-rolled using a tandem rolling mill consisting of five rolling mills under the conditions shown in Table 2. The cold-rolled sheets were all 1.6 mm thick and 1000 mm wide. Finally, the resulting cold-rolled steel sheets were heat-treated under the conditions shown in Table 2. Cooling between cooling stop temperatures T1 and 300°C was performed using nitrogen (water in Comparative Example 24) as the coolant, at a predetermined average cooling rate (average cooling rate 2 in Table 2).

[0236] In steel plates obtained in this manner, JIS No. 5 tensile test specimens were collected at room temperature (25°C) in an atmospheric environment, perpendicular to the rolling direction of the steel plate. Tensile tests were conducted according to JIS Z 2241:2011 to determine the tensile strength (TS) and total elongation (El). In addition, the hole expansion ratio (λ) was determined according to the Japanese Iron and Steel Federation standard "JFS T 1001:1996 Hole Expansion Test Method".

[0237] The maximum value of curvature 1 / R is determined as follows: First, for freshly manufactured cold-rolled steel sheets without specific mechanical planarization, measurements are taken at various points along a 300mm length region using an ATOS 3D scanner manufactured by GOM, thereby obtaining the curvature distribution within the cold-rolled steel sheet. Next, the larger of the absolute values ​​of the principal curvatures ρ1 and ρ2 in this measured curvature distribution is determined as the maximum value of curvature 1 / R.

[0238] Hydrogen embrittlement resistance is achieved by utilizing Figure 1 The hydrogen embrittlement test of the shearing process shown is used for evaluation. Specifically, firstly, samples of T (thickness) × 50W (width) × 50L (length) (unit: mm) are collected from the steel plate, including the part where the maximum curvature 1 / R is obtained. The shearing angle θ is set to 1 degree, the clearance CL is set to 0.15 × T, and the pressure plate is set to a load of 1 ton or more. After the above samples are cut by shearing, the steel plate on the product side (pressure plate side) is heat-treated at 170°C for 10 minutes. Then, hydrogen is introduced into the steel plate by immersion in ammonium thiocyanate aqueous solutions with concentrations of 0.3 g / L and 3 g / L at room temperature for 48 hours. Afterward, the sheared surface is observed under a microscope to evaluate the presence or absence of cracks. The presence of cracks at 0.3 g / L is judged as × (unqualified); the presence of cracks at 3 g / L but not at 0.3 g / L is judged as 〇 (qualified); and the presence of cracks at both 0.3 g / L and 3 g / L is judged as ◎ (qualified).

[0239] Cold-rolled steel sheets with a tensile strength (TS) of 1470 MPa or higher and an e1 of 6.0% or higher, thus meeting the requirements for hydrogen embrittlement resistance, are evaluated as having high tensile strength and total elongation, and improved hydrogen embrittlement resistance. The results are shown in Table 3.

[0240]

[0241] Table 2-1

[0242]

[0243] The underlined part indicates that it is outside the scope of this invention.

[0244] Table 2-2

[0245]

[0246] The underlined part indicates that it is outside the scope of this invention.

[0247] Table 3

[0248]

[0249] The underlined part indicates that it is outside the scope of this invention.

[0250] Referring to Table 3, in Comparative Example 2, because Equation (2) is not satisfied in the cold rolling process, the maximum value of curvature 1 / R is higher, and the hydrogen embrittlement resistance is lower. In Comparative Examples 3 and 12, because the temperature difference between the width edge and the width center of the steel plate after rough rolling in the hot rolling process is unsuitable, the maximum value of curvature 1 / R is higher, and the hydrogen embrittlement resistance is lower. In Comparative Example 4, because the accumulated cold rolling reduction rate in the cold rolling process is low, the maximum value of curvature 1 / R is higher, and the hydrogen embrittlement resistance is lower. In Comparative Example 5, because the cooling stop temperature T1 in the heat treatment process is low, retained austenite is not sufficiently generated, and El decreases. In Comparative Example 6, because the average cooling rate (average cooling rate 1) between 300 and 700°C in the heat treatment process is slow, martensite is not sufficiently generated, and TS decreases. In Comparative Example 7, the maximum value of curvature 1 / R is higher and the hydrogen embrittlement resistance is lower because the average cooling rate (average cooling rate 2) between T1 and 300°C is fast during the heat treatment process. In Comparative Examples 8 and 19, the maximum value of curvature 1 / R is higher and the hydrogen embrittlement resistance is lower because the tension applied to the cold-rolled steel sheet during the heat treatment process is unsuitable. In Comparative Examples 9, 10, and 23, the desired steel microstructure cannot be obtained due to unsuitable low-temperature holding temperature or time during the heat treatment process, and TS and / or E1 decrease. In Comparative Example 11, the TS decreases because the heating and holding temperature is low during the heat treatment process, resulting in insufficient martensite formation.

[0251] In Comparative Example 13, due to the high T1 during the heat treatment process, the proportion of tempered martensite in the martensite decreased, resulting in reduced resistance to hydrogen embrittlement. In Comparative Example 14, due to the fast average cooling rate 1 during the heat treatment process, the temperature deviation within the steel sheet increased, resulting in a higher maximum value of curvature 1 / R and reduced resistance to hydrogen embrittlement. In Comparative Example 15, due to the slow average cooling rate 2 during the heat treatment process, insufficient martensite formation occurred, leading to a decrease in TS. In Comparative Example 16, it is believed that the low coiling temperature during the hot rolling process resulted in a high-strength hot-rolled steel sheet. As a result, the shape of the cold-rolled steel sheet deteriorated, and resistance to hydrogen embrittlement decreased. In Comparative Examples 17 and 18, it is believed that the lack of proper cooling between Ms and 700°C or between T1 and below Ms during the heat treatment process resulted in temperature unevenness within the steel sheet. As a result, the maximum value of curvature 1 / R increased, and resistance to hydrogen embrittlement decreased. In Comparative Example 24, because water was used as a coolant in the heat treatment process to cool between T1 and 300°C, the average cooling rate 2 increased, and proper venting was not performed between T1 and below Ms. As a result, the temperature deviation within the steel plate increased, the maximum value of curvature 1 / R increased, and the hydrogen embrittlement resistance decreased. In Comparative Example 45, due to the low Si content, retained austenite was not sufficiently formed, resulting in a decrease in El. In Comparative Example 46, due to the low Mn content, martensite was not sufficiently formed, resulting in a decrease in TS. In Comparative Example 47, due to the low C content, TS decreased. In Comparative Examples 48-50, due to the high C, Mn, or Si content, the hydrogen embrittlement resistance decreased.

[0252] In contrast, in Examples 1, 20-22 and 25-44 of the present invention, by having a specified chemical composition and steel structure, and thereby controlling the maximum value of curvature 1 / R to below 0.010, it is possible to obtain cold-rolled steel sheets with high tensile strength and total elongation, and improved resistance to hydrogen embrittlement.

Claims

1. A cold-rolled steel sheet, characterized in that, It has the following chemical composition: in mass% C：0.16～0.40%、 Si: 0.05~2.00% Mn: 0.50~4.00%, P: below 0.050% S: Below 0.0100% Al：0.001～1.00%、 N: below 0.0100% O: Below 0.0050% Cr：0～2.00%、 Mo: 0~1.00%, Cu: 0~1.00%, Ni: 0~1.00%, B：0～0.0100%、 Co: 0~1.00%, W：0～1.00%、 Sn: 0~1.00% Sb: 0~1.00%, Nb: 0~0.100%, Ti: 0~0.200%, V：0～0.50%、 Ca: 0~0.0100%, Mg: 0~0.0100%, Ce: 0~0.0100% Zr：0～0.0100%、 La: 0~0.0100% Hf: 0~0.0100% Bi: 0~0.0100% REM levels other than Ce and La: 0–0.0100%, and The remaining portion consists of Fe and impurities. The steel microstructure within a range of 1 / 8 to 3 / 8 of its thickness, centered at 1 / 4 of the thickness from the surface, is expressed as a percentage of area: Martensite: 90.0–99.5% Ferrite: 0-5% Residual austenite: 0.5–7.0%, and Remaining portion: bainite, Furthermore, tempered martensite accounts for 80%–100% of all martensite. The maximum value of the curvature 1 / R, expressed by the following formula (1), is less than or equal to 0.010, obtained by shape measurement of a region with a full width × length of 300 mm. The tensile strength of the cold-rolled steel sheet is above 1470 MPa. 1 / R: Curvature ρ1 and ρ2: Principal curvatures on the surface.

2. The cold-rolled steel sheet according to claim 1, characterized in that, The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the group consisting of: Cr：0.001～2.00%、 Mo: 0.001~1.00%, Cu: 0.001~1.00%, Ni: 0.001~1.00% B：0.0001～0.0100%、 Co: 0.001~1.00%, W：0.001～1.00%、 Sn: 0.001~1.00% Sb: 0.001~1.00% Nb: 0.001~0.100% Ti: 0.001~0.200% V：0.001～0.50%、 Ca: 0.0001~0.0100% Mg: 0.0001~0.0100%, Ce: 0.0001~0.0100% Zr：0.0001～0.0100%、 La: 0.0001~0.0100% Hf: 0.0001~0.0100% Bi: 0.0001~0.0100%, and REM values ​​other than Ce and La: 0.0001–0.0100%.

3. The cold-rolled steel sheet according to claim 1 or 2, characterized in that, The cold-rolled steel sheet was sheared, then subjected to a heat treatment at 170°C for 10 minutes, and then immersed in a 0.3 g / L ammonium thiocyanate aqueous solution for 48 hours in a hydrogen embrittlement test. No cracking occurred on the sheared surface.

4. The cold-rolled steel sheet according to claim 1 or 2, wherein, The surface has any one of the following: an electroplated zinc layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer.

5. A method for manufacturing a cold-rolled steel sheet according to any one of claims 1 to 3, characterized in that, It includes the following processes: (A) A hot rolling process comprising rough rolling and finish rolling of a slab having the chemical composition of claim 1 or 2, and satisfying the following conditions (A1) to (A3): (A1) The slab heating temperature is above 1150℃; (A2) The edge of the width of the rough-rolled steel plate is heated in such a way that the temperature of the edge of the width is 10 to 150°C higher than that of the center of the width; (A3) The winding temperature is 450–650℃. (B) Cold rolling process, which includes cold rolling the obtained hot-rolled steel sheet using a series rolling mill consisting of N rolling mills, where N≥3, and the cumulative cold rolling reduction is 30% or more, and satisfies the following equations (2) and (3): R k : Reduction rate in the k-th rolling stand Pb k The rear tension in the k-th rolling mill stand Pf k Forward tension in the k-th rolling mill stand σ k-1 Flow stress of the steel plate after passing through the (k-1)th rolling mill stand σ k Flow stress of the steel plate after passing through the k-th rolling mill stand σ0: Yield strength of hot-rolled steel plate ε k : Cumulative strain after the kth rolling stand (C) A heat treatment process, which includes heat treating the obtained cold-rolled steel sheet and satisfying the following conditions (C1) to (C3): (C1) Heating and holding: Hold the cold-rolled steel sheet at Ac3 to 950°C for 10 to 500 seconds; (C2) Implement cooling treatments that satisfy (i) to (v) below: (i) The cooling stop temperature T1 is 110~250℃; (ii) The average cooling rate between 300 and 700°C is 20 to 150°C / s; (iii) The average cooling rate between T1 and 300°C is 1.0 to 20°C / s, and gas is used as the coolant; (iv) At least one cooling operation of 0.5 seconds or more shall be carried out between Ms and 700°C and between T1 and below Ms. (v) Applicable tension for cold-rolled steel sheets is 5–20 MPa; (C3) Low temperature holding: Hold at 200-300℃ for 100-1000 seconds.

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