Cold-rolled steel sheet and method for producing same

Abstract

The cold-rolled steel sheet has a prescribed chemical composition, and has a metallographic structure comprising, in volume fraction, 10-95% of ferrite and the remainder comprising one or more elements selected from martensite, bainite, and retained austenite in a surface layer portion in the range of 10-20 [mu] m from the surface in the sheet thickness direction. At a 1 / 4 thickness position in a range of 1 / 8-3 / 8 of the sheet thickness from the surface in the sheet thickness direction, the metal structure contains, in volume fraction, 0-60% of ferrite, 4-20% of retained austenite, and the remainder contains one or more of martensite and bainite, the volume fraction of tempered martensite in the martensite is 50% or more, and the volume fraction of tempered martensite in the bainite is 10% or more. The ratio of the dislocation density of the surface layer portion to the dislocation density at the 1 / 4 thickness position is 0.20 or more and less than 0.90, and the tensile strength is 980 MPa or more.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same.

[0002] This application claims priority based on Japanese Patent Application No. 2023-007141, filed on January 20, 2023, the contents of which are incorporated herein by reference. Background Art

[0003] In today's highly specialized industrial technology landscape, materials used in each technical field are required to possess specialized and advanced performance. For example, automotive steel sheets require high strength to improve fuel efficiency by reducing vehicle weight, driven by environmental considerations. Applying high-strength steel sheets to automotive bodies allows for a thinner steel sheet, resulting in lighter vehicles, while also providing the desired strength.

[0004] Furthermore, high-strength steel sheets used in automotive parts are required not only to have strength but also to have properties (formability) necessary for part forming, such as uniform elongation. Strength and formability are in a trade-off relationship, and TRIP (Transformation Induced Plasticity) steel sheets are known as a means of achieving both. These high-strength steel sheets utilize the transformation-induced plasticity of retained austenite.

[0005] Furthermore, in the case of automobiles, emphasis is placed on improving collision safety in order to ensure the safety of passengers. When an automobile collides, bending stress is applied to the steel sheets constituting automobile components, and therefore bending properties are required of the steel sheets.

[0006] That is, in order to simultaneously achieve both lightweighting of a vehicle body and improvement in collision safety, a steel sheet having not only high strength and elongation but also excellent bending properties in consideration of collision is required.

[0007] To address such a problem, for example, Patent Document 1 discloses a thin steel sheet having a composition comprising, in mass%, C: 0.10% to 0.35%, Si: 0.01% to 2.0%, Mn: 0.8% to 2.35%, P: 0.05% to 0.05%, S: 0.005% to 0.10%, N: 0.0060% to 0.060%, with the remainder consisting of Fe and unavoidable impurities; and a steel structure having a ferrite area ratio of 30% to 30% (including 0%), a bainite The area ratio of martensite and tempered martensite is 5% or less (including 0%), the area ratio of martensite and tempered martensite is 70% or more (including 100%), the area ratio of retained austenite is 2.0% or less (including 0%), the ratio of the dislocation density in the range of 0 to 20 μm from the surface of the steel plate to the dislocation density in the center of the plate thickness is 90% or more and 110% or less, the average of the cementite grain size within the upper 10% from the surface of the steel plate to a depth of 100 μm is 300 nm or less, and the maximum warping of the steel plate when sheared with a length of 1 m in the longitudinal direction of the steel plate is 15 mm or less.

[0008] Patent Document 1 describes that bendability can be improved by controlling dislocation density and cementite grain size.

[0009] In addition, Patent Document 2 discloses a high-strength steel plate with excellent delayed fracture resistance, characterized in that the chemical composition satisfies C: 0.10-0.40%, Si: 0.6-3.0%, Mn: 1.0-3.5%, Al: 3% or less (excluding 0%), P: 0.15% or less (excluding 0%) and S: 0.02% or less (excluding 0%), the remainder contains iron and inevitable impurities, martensite accounts for 95% by area or more of the entire structure, and, in the structure from a depth of 10 μm in the thickness direction from the steel plate surface to a depth of 1 / 4 of the plate thickness, the old austenite grain size, dislocation density, solute C concentration (mass %) in martensite, and the ratio of the length of carbides precipitated at the old γ grain boundaries to the length of the old γ grain boundaries satisfy the specified relationship, and the tensile strength is 1180 MPa or more.

[0010] Patent Document 2 describes that by refining the prior γ grains, the areas where shear deformation occurs during bending are dispersed in the steel material, thereby avoiding the generation of local stress concentration areas.

[0011] In addition, Patent Document 3 discloses a high-strength galvanized steel sheet with excellent collision absorption energy and a maximum tensile strength of 900 MPa or more, characterized in that the density of dislocations contained in the steel sheet is 8×10 11 (pieces / mm 2 ) or less, the strain rate is 0.0067(s-1 ) under quasi-static strength (FS1) and strain rate 1000 (s -1 ) under the dynamic strength (FS2) is greater than 1.05.

[0012] Prior art literature

[0013] Patent Literature

[0014] Patent Document 1: International Publication No. 2020 / 026838

[0015] Patent Document 2: Japanese Patent Application Laid-Open No. 2013-104081

[0016] Patent Document 3: Japanese Patent No. 5487916 Summary of the Invention

[0017] Problems to be solved by the invention

[0018] However, Patent Document 1 relates to a method for reducing the variation in the bending fracture limit, and is not a method for increasing the average value of the bending fracture limit.

[0019] Patent Document 2 discloses that delayed fracture resistance can be improved by making strain distribution during bending uniform, but does not evaluate bendability in consideration of collision.

[0020] Patent Document 3 considers deformation resistance at high strain rates in order to increase collision absorption energy, but does not consider bending deformation during collision.

[0021] That is, no steel sheet with high strength, excellent elongation, and bendability has been disclosed in the past. The present invention aims to provide a steel sheet with high strength, excellent elongation, and bendability, and a method for producing the same. In the present invention, excellent bendability means high bending resistance and high bending fracture limit.

[0022] Means for solving problems

[0023] The present inventors have studied methods for improving the bendability of TRIP steel sheets having high strength and excellent elongation.

[0024] The results show that the formation of ferrite in the surface layer can suppress bending fracture. On the other hand, it is known that the formation of ferrite in the surface layer reduces bending strength. Therefore, the present inventors conducted further research and found that by controlling the dislocation density in the surface layer within a predetermined range, bending fracture can be suppressed while maintaining bending strength.

[0025] Furthermore, the present inventors have found that controlling the atmosphere during annealing and introducing strain into the surface portion by leveling are effective in forming ferrite in the surface portion and obtaining a predetermined dislocation density.

[0026] The present invention has been accomplished based on the above findings. The gist of the present invention is as follows.

[0027] [1] A cold-rolled steel sheet according to one embodiment of the present invention has a chemical composition comprising, in mass %, C: 0.100-0.400%, Si: 0.01-3.00%, Mn: 1.00-5.00%, sol.Al: 0.001-1.000%, P: 0.100% or less, S: 0.0100% or less, O: 0.100% or less, N: 0.010% or less, Ti: 0-0.200%, B: 0~0.0100%, Cr: 0~1.00%, Mo: 0~1.00%, Ni: 0~1.00%, Cu: 0~1.00%, Sn: 0~0.50%, Nb: 0~0.20%, V: 0~ 0.50%, W: 0~0.50%, Ca: 0~0.010%, Mg: 0~0.010%, Bi: 0~0.010%, Sb: 0~0.100%, Zr: 0~0.010%, REM: 0 to 0.010%, and the remainder: Fe and impurities, Q determined by the following formula (1) is 2.3 or more, SA determined by the following formula (2) is 0.30 or more, in a surface layer portion ranging from 10 to 20 μm in the thickness direction from the surface, the metal structure comprises 10 to 95% by volume of ferrite, and the remainder comprises one or more selected from martensite, bainite and retained austenite, in a surface layer portion ranging from 1 / 8 to 3 / 8 of the thickness of the plate in the thickness direction from the surface 8, the metal structure comprises, by volume, 0 to 60% ferrite and 4 to 20% retained austenite, with the remainder comprising at least one selected from martensite and bainite; the volume fraction of tempered martensite in the martensite is 50% or more; the ratio of the dislocation density in the surface layer to the dislocation density at the 1 / 4 thickness position is 0.20 or more and less than 0.90; and the tensile strength of the cold-rolled steel sheet is 980 MPa or more.

[0028] Q=2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+2.0×[Mo] (1)

[0029] SA=[Si]+0.7×[sol.Al] (2)

[0030] In the above formula (1) and the above formula (2), [element symbol] represents the content of the element symbol in mass %, and 0 is substituted when the element is not contained.

[0031] The cold-rolled steel sheet described in [2] and [1] may have a hot-dip galvanizing layer on the surface.

[0032] In the cold-rolled steel sheets described in [3] and [2], the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.

[0033] [4] Another embodiment of the present invention is a method for producing a cold-rolled steel sheet, characterized by comprising:

[0034] The hot rolling process is performed to obtain a chemical composition comprising, by mass%, C: 0.100-0.400%, Si: 0.01-3.00%, Mn: 1.00-5.00%, sol.Al: 0.001-1.000%, P: 0.100% or less, S: 0.0100% or less, O: 0.100% or less, N: 0.010% or less, Ti: 0-0.200%, B: 0-0.0100%, Cr: 0-1.00%, Mo: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, Sn: 0-1.00%. ~0.50%, Nb: 0~0.20%, V: 0~0.50%, W: 0~0.50%, Ca: 0~0.010%, Mg: 0~0.010%, Bi: 0~0.010%, Sb: 0~0.100%, Zr: 0~0.010%, REM: 0~0.010%, and the remainder: Fe and impurities, heating a slab having a Q obtained by the following formula (1) of 2.3 or more and a SA obtained by the following formula (2) of 0.30 or more, and hot rolling the slab so that the finishing temperature is 800°C or more and the Ar3 point or more to obtain a hot-rolled steel sheet;

[0035] a post-hot rolling cooling step, wherein the hot-rolled steel sheet is cooled at an average cooling rate of 5°C / second or more to a coiling temperature of 400 to 750°C after 1.0 second or more has passed since the end of the hot rolling;

[0036] a coiling step of coiling the hot-rolled steel sheet after the hot-rolling cooling step at the coiling temperature;

[0037] a cold rolling step of cold-rolling the hot-rolled steel sheet after the coiling step at a cumulative reduction ratio of 20 to 80% to obtain a cold-rolled steel sheet;

[0038] An annealing step of heating the cold-rolled steel sheet after the cold rolling step to a soaking temperature of 750 to 1000° C. and maintaining the soaking temperature for more than 1 second;

[0039] a first cooling step of cooling the cold-rolled steel sheet after the annealing step from the soaking temperature to a first cooling stop temperature of 600° C. or less at an average cooling rate of 10.0° C. / s or less;

[0040] a second cooling step of cooling the cold-rolled steel sheet after the first cooling step from the first cooling stop temperature to a second cooling stop temperature below the Ms point at an average cooling rate of 1.0°C / s or higher;

[0041] a heat treatment step of maintaining the cold-rolled steel sheet after the second cooling step in a temperature range of 300 to 530° C. for more than 20 seconds; and

[0042] A flattening step is performed to impart a cumulative strain of 1.0% or more to the front and back surfaces of the cold-rolled steel sheet after the heat treatment step, and to make the elongation 0.50% or less.

[0043] In the annealing step, when the heating is performed at a temperature range of at least 650° C. to the soaking temperature, the water vapor partial pressure P in the atmosphere in the furnace is reduced to (H2O) With hydrogen partial pressure P (H2) The ratio is P (H2O) / P (H2) Set it within the range of 0.00010 to 2.00.

[0044] Q=2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+2.0×[Mo] (1)

[0045] SA=[Si]+0.7×[sol.Al] (2)

[0046] In the above formula (1) and the above formula (2), [element symbol] represents the content of the element symbol in mass %, and 0 is substituted when the element is not contained.

[0047] The method for manufacturing the cold-rolled steel sheet described in [5] and [4] may further include a hot-dip galvanizing step after the first cooling step, wherein the cold-rolled steel sheet is set to a temperature of 420 to 530°C and then immersed in a hot-dip galvanizing bath at a bath temperature of 420 to 530°C to form a hot-dip galvanized layer.

[0048] The method for producing a cold-rolled steel sheet described in [6] and [5] may further include an alloying treatment step of maintaining the cold-rolled steel sheet after the hot-dip galvanizing step at a temperature of 460 to 530° C. to alloy the hot-dip galvanized layer.

[0049] Effects of the Invention

[0050] According to the above aspects of the present invention, it is possible to provide a steel sheet having high strength and excellent elongation and bendability, and a method for producing the same. DETAILED DESCRIPTION

[0051] A cold-rolled steel sheet according to one embodiment of the present invention (cold-rolled steel sheet according to this embodiment) and a method for producing the same will be described.

[0052] The cold-rolled steel sheet of this embodiment has a prescribed chemical composition, wherein in a surface portion ranging from 10 to 20 μm from the surface in the plate thickness direction, the metal structure comprises 10 to 95% ferrite by volume, with the remainder comprising one or more selected from martensite, bainite, and retained austenite; at a position at the quarter thickness, ranging from 1 / 8 to 3 / 8 of the plate thickness from the surface and centered at a position at the quarter thickness in the plate thickness direction, i.e., the metal structure comprises 0 to 60% ferrite and 4 to 20% retained austenite by volume, with the remainder comprising one or more selected from martensite and bainite; the volume fraction of tempered martensite in the martensite is 50% or more; the ratio of the dislocation density in the surface portion to the dislocation density at the quarter thickness position is 0.20 or more and less than 0.90; and the tensile strength is 980 MPa or more.

[0053] The cold-rolled steel sheet of the present embodiment includes a plated steel sheet having a plated layer on the surface.

[0054] The following describes each of them separately.

[0055] In the description, the range indicated by "to" includes the values ​​at both ends as the lower limit and the upper limit. However, the numerical values ​​indicated as "exceeding" or "less than" are not included in the range.

[0056] <Chemical Composition>

[0057] First, the chemical composition will be described.

[0058] In the present embodiment, % of the content of each element refers to mass %.

[0059] C:0.100~0.400%

[0060] C (carbon) is an essential element for increasing the strength of steel sheets. From the perspective of increasing strength, the C content is set to 0.100% or more. The C content is preferably 0.120% or more, 0.150% or more, or 0.180% or more.

[0061] On the other hand, if the C content is excessive, bendability, stamping properties, and weldability deteriorate. Therefore, the C content is 0.400% or less. The C content is preferably 0.300% or less or 0.260% or less.

[0062] Si: 0.01~3.00%

[0063] Si (silicon) is a solid solution strengthening element and is effective for increasing the strength of steel sheets. It is also effective for forming retained austenite. To achieve these effects, the Si content is 0.01% or greater. The Si content is preferably 0.10% or greater, more preferably 0.20%, 0.40%, or 0.60%.

[0064] On the other hand, excessive Si content significantly deteriorates not only the chemical conversion treatability and wettability of the steel sheet with hot-dip galvanizing, but also its bendability. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.00% or less, and more preferably 1.70% or less, 1.40% or less, or 1.00% or less.

[0065] Mn: 1.00~5.00%

[0066] Mn (manganese) is a strong austenite stabilizing element and is effective in improving the hardenability of steel sheets. To achieve this effect, the Mn content is set to 1.00% or more. The Mn content is preferably 1.50% or more, 1.80% or more, or 2.20% or more.

[0067] On the other hand, if the Mn content is excessive, bendability, weldability, and low-temperature toughness deteriorate. Therefore, the Mn content is 5.00% or less. The Mn content is preferably 3.50% or less, 3.00% or less, or 2.70% or less.

[0068] sol.Al:0.001~1.000%

[0069] Al (aluminum) is an element that is effective for deoxidizing steel. It is also effective for producing retained austenite. To achieve this effect, the sol.Al (acid-soluble Al) content should be at least 0.001%. The sol.Al content is preferably at least 0.005%.

[0070] On the other hand, if the Al content is excessive, the effect is saturated, leading not only to increased costs but also to an increase in the steel's transformation temperature and an increase in the load during hot rolling. Therefore, the sol. Al content is set to 1.000% or less. The sol. Al content is preferably 0.800% or less, 0.500% or less, or 0.200% or less.

[0071] P: 0.100% or less

[0072] P (Phosphorus) is an element that degrades weldability and toughness. If the P content exceeds 0.100%, this adverse effect becomes significant. Therefore, the P content is 0.100% or less. More preferably, the P content is 0.050% or less.

[0073] On the other hand, while the P content is preferably as low as possible, it can be 0%. However, it is sometimes present as an impurity, and extreme reductions in P content increase the cost of P removal. Furthermore, P is a solid solution strengthening element and is effective in increasing the strength of steel sheets. Therefore, the P content can be set to 0.001% or higher.

[0074] S: 0.0100% or less

[0075] Sulfur (S) is an element that forms MnS in steel, degrading toughness and hole expandability. If the S content exceeds 0.0100%, toughness and hole expandability deteriorate significantly. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0050% or less, and more preferably 0.0020% or less.

[0076] On the other hand, the lower the S content, the better, and it can be 0%, but it is sometimes contained as an impurity, and extreme reduction of the S content increases the desulfurization cost. Therefore, from the perspective of economic efficiency, the S content can be set to 0.0005% or more.

[0077] O: 0.100% or less

[0078] O (oxygen) is an element contained as an impurity. If the O content exceeds 0.100%, coarse oxides will form in the steel, reducing bendability and hole expandability. Therefore, the O content is kept at 0.100% or less. The O content is preferably 0.010% or less, and more preferably 0.005% or less. The O content can be 0%, but from the perspective of manufacturing costs, it is advisable to set the O content at 0.0001% or more.

[0079] N: 0.010% or less

[0080] N (nitrogen) is an element contained as an impurity. If the N content exceeds 0.010%, coarse nitrides form in the steel, deteriorating bendability and hole expandability. Therefore, the N content is kept to 0.010% or less. The N content is preferably kept to 0.005% or less. While the N content can be 0%, extreme reductions increase the cost of denitrification. Therefore, the N content can be set to 0.0005% or above, or 0.001% or above.

[0081] The cold-rolled steel sheet of this embodiment may contain the above-mentioned elements, with the remainder being Fe and impurities. However, for the purpose of improving various properties, it may also contain one or more elements (optional elements) selected from the following: Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM. Since the optional elements do not need to be contained, the lower limit is 0%.

[0082] Ti: 0~0.200%

[0083] Ti (titanium) is an element that suppresses the formation of BN, a factor that reduces hardenability, by fixing nitrogen in steel as TiN. Ti also refines the austenite grain size during heating, improving toughness. To achieve this effect, the Ti content is preferably 0.005% or more. A more preferred Ti content is 0.010% or more.

[0084] On the other hand, if the Ti content is excessive, the ductility of the steel sheet decreases. Therefore, when Ti is contained, the Ti content is 0.200% or less. The Ti content is preferably 0.050% or less.

[0085] B:0~0.0100%

[0086] Boron (B) is an element that segregates at austenite grain boundaries or ferrite / austenite grain boundaries during heating of steel sheets, stabilizing the grain boundaries and thereby improving the hardenability of the steel. To achieve this effect, the B content is preferably set at 0.0005% or more, and preferably at 0.0010% or more.

[0087] On the other hand, if the B content is excessive, the hardenability of the steel will be impaired by forming borides. Therefore, when B is contained, the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less or 0.0030% or less.

[0088] Cr: 0~1.00%

[0089] Mo: 0~1.00%

[0090] Ni: 0~1.00%

[0091] Cu: 0~1.00%

[0092] Sn: 0~0.50%

[0093] Cr (chromium), Mo (molybdenum), Ni (nickel), Cu (copper), and Sn (tin) are all elements effective in increasing the strength of the steel sheet and may therefore be contained as needed.

[0094] In order to obtain the above-mentioned effects, it is preferable to contain 0.001% or more of one or more selected from Cr, Mo, Ni, Cu, and Sn, more preferably 0.01% or more, and even more preferably 0.05% or more.

[0095] On the other hand, if these elements are excessively contained, their effects become saturated and costs increase. Therefore, when contained, the contents of Cr, Mo, Ni, and Cu are all kept at 1.00% or less, and the content of Sn is kept at 0.50% or less. The contents of Cr, Mo, Ni, and Cu are all preferably kept at 0.60% or less, and the content of Sn is preferably kept at 0.30% or less.

[0096] Nb: 0~0.20%

[0097] V: 0~0.50%

[0098] W: 0~0.50%

[0099] Nb (niobium), V (vanadium), and W (tungsten) are carbide-forming elements and are effective in increasing the strength of the steel sheet. Therefore, they may be contained as needed.

[0100] In order to obtain the above-mentioned effects, the content of one or more selected from Nb, V, and W is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.01% or more.

[0101] On the other hand, even if these elements are excessively contained, their effects become saturated, and costs increase. Therefore, when contained, the Nb content is set to 0.20% or less, and the V and W contents are both set to 0.50% or less. The Nb content is preferably set to 0.10% or less, and the V and W contents are preferably set to 0.30% or less.

[0102] Ca: 0~0.010%

[0103] Mg: 0~0.010%

[0104] Bi: 0~0.010%

[0105] Sb: 0~0.100%

[0106] Zr: 0~0.010%

[0107] REM: 0~0.010%

[0108] Ca (calcium), Mg (magnesium), Sb (antimony), Zr (zirconium), and REM (rare earth elements) contribute to the fine dispersion of inclusions in steel, while Bi (bismuth) reduces the microsegregation of substitutional alloying elements such as Mn and Si in steel. These elements contribute to improving the bendability of steel sheets. Therefore, they may be included as needed.

[0109] In order to obtain the above-mentioned effects, it is preferable to contain 0.0001% or more of one or more selected from Ca, Mg, Bi, Sb, Zr and REM, and more preferably 0.001% or more.

[0110] On the other hand, excessive amounts of these elements degrade ductility. Therefore, the contents of Ca, Mg, Bi, Zr, and REM are all 0.010% or less, and the Sb content is 0.100% or less. The contents of Ca, Mg, Bi, Zr, and REM are all preferably 0.006% or less, and the Sb content is preferably 0.080% or less.

[0111] Here, REM refers to a total of 17 elements including Sc, Y, and lanthanoids, and the REM content refers to the total content of these elements. Lanthanoids are added industrially as misch metals.

[0112] As described above, the chemical composition of the cold-rolled steel sheet of the present embodiment contains C, Si, Mn, sol.Al, P, S, O, and N, with the remainder containing Fe and impurities, or contains C, Si, Mn, sol.Al, P, S, O, and N, and further contains one or more elements selected from Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM, with the remainder containing Fe and impurities.

[0113] Impurities are components that enter steel from raw materials such as ores and scrap, or from other factors, during industrial steel production. These are permitted as long as they do not adversely affect the steel's properties. Examples of impurities include P, S, O, and N, as well as Co, As, Zn, In, Sr, Li, Re, Os, Ir, Tc, Pb, Se, Ta, H, and Hf.

[0114] In the cold-rolled steel sheet of this embodiment, the chemical composition is controlled on the basis of controlling the content of each element within the above-mentioned range so that Q calculated using formula (1) and formula (2) based on the content of C, Si, Mn, Ni, Cr, Mo, and sol.Al is greater than 2.3 and SA is greater than 0.3.

[0115] Q=2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+2.0×[Mo] (1)

[0116] SA=[Si]+0.7×[sol.Al] (2)

[0117] In the above formula (1) and the above formula (2), [element symbol] is the content of the element symbol in mass %.

[0118] Q calculated by formula (1) is an index related to the hardenability of steel. When the value of Q is less than 2.3, the hardenability is insufficient and sufficient strength of the steel plate cannot be obtained. Therefore, Q is greater than 2.3. The upper limit of Q is not limited (the content of each element becomes the upper limit, which becomes the actual upper limit).

[0119] (Q=2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+0.5×[Mo]≥2.3)

[0120] SA calculated by formula (2) is an index related to the formation of retained austenite. If the SA value is less than 0.3, the target volume fraction of retained austenite cannot be obtained. Therefore, SA is 0.3 or more.

[0121] (SA=[Si]+0.7×[sol.Al]≥0.3)

[0122] The upper limit of SA is not limited (the upper limit is the actual upper limit when the content of each element is the upper limit).

[0123] The chemical composition of the steel plate according to the present embodiment can be determined by the following method.

[0124] The chemical composition of the steel plate can be measured according to general chemical composition. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Sol.Al can be measured using the filtrate after the sample is heated and decomposed with acid using ICP-AES. In addition, C and S can be measured using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-non-dispersive infrared absorption method. If the steel plate has a coating on the surface, the chemical composition analysis can be performed after removing the coating by mechanical grinding.

[0125] <Metallic Structure>

[0126] The cold-rolled steel sheet of this embodiment controls the metal structure in the range of 10 to 20 μm from the surface to the plate thickness direction (the position of 10 μm from the surface to the plate thickness direction to the position of 20 μm from the surface to the plate thickness direction), i.e., the surface part, and in the range of 1 / 8 to 3 / 8 from the surface centered on the position of 1 / 4 of the plate thickness from the surface to the plate thickness direction, i.e., the 1 / 4 thickness position, as described below.

[0127] When the cold-rolled steel sheet is a plated steel sheet, the surface referred to here is the surface of the steel sheet (base material steel sheet) excluding the plating layer.

[0128] [Volume Ratio of Each Phase in the Metal Structure of the Surface Layer]

[0129] The cold-rolled steel sheet of this embodiment has a surface layer portion ranging from 10 to 20 μm from the surface to the thickness direction, and the metal structure contains 10 to 95% ferrite, and the remainder contains one or more selected from martensite, bainite and retained austenite.

[0130] If the volume fraction of ferrite is less than 10%, bending fracture cannot be adequately suppressed. The volume fraction of ferrite is preferably 12% or greater, more preferably 20% or greater, 30% or greater, or 40% or greater. On the other hand, if the volume fraction of ferrite exceeds 95%, the tensile strength and bending strength of the steel sheet decrease. Therefore, the volume fraction of ferrite is preferably 95% or less. The volume fraction of ferrite is preferably 80% or less, more preferably 70% or less.

[0131] [Volume ratio of each phase in the metal structure at 1 / 4 thickness position]

[0132] The cold-rolled steel sheet of this embodiment has a metal structure comprising, by volume, 0 to 60% ferrite, 4 to 20% retained austenite at a 1 / 4 thickness position, and the remainder comprising at least one selected from martensite and bainite.

[0133] Retained austenite is a structure that contributes to the improvement of elongation (especially uniform elongation) through the TRIP effect. To achieve this effect, the volume fraction of retained austenite is 4% or more. The volume fraction of retained austenite is preferably 5% or more, and more preferably 7% or more.

[0134] On the other hand, if the volume fraction of retained austenite is excessive, the retained austenite grain size increases. Such retained austenite with a large grain size transforms into coarse and hard martensite after deformation. This is undesirable as it easily becomes the starting point of cracks, causing deterioration in bendability and hole expandability. Therefore, the volume fraction of retained austenite is set to 20% or less. The volume fraction of retained austenite is preferably 18% or less, and more preferably 16% or less.

[0135] Ferrite is not necessarily required, but since it is a soft structure, it is easily deformed and contributes to improved elongation. Therefore, it may be contained. However, if the volume fraction of ferrite exceeds 60%, the volume fraction of the remaining martensite and / or bainite decreases, and sufficient tensile strength cannot be achieved. Therefore, the volume fraction of ferrite is set to 60% or less. The volume fraction of ferrite is preferably 50% or less, more preferably 40% or less, and may be 30% or less.

[0136] In the present embodiment, the martensite includes martensite in a hardened state (so-called fresh martensite) and tempered martensite that has been tempered. Compared with tempered martensite, martensite in a hardened state is more brittle and therefore easily becomes the starting point of damage when plastic deformation such as bending is applied. Therefore, in order to ensure the desired bendability, the proportion of tempered martensite in the entire martensite is 50% or more by volume inside the steel plate (represented by the 1 / 4 thickness position). The proportion of tempered martensite in the entire martensite is preferably 60% or more, 70% or more, or 80% or more. In addition, in the surface layer, the proportion of tempered martensite in the entire martensite is preferably 30% or more by volume. More preferably, it is 40% or more or 50% or more.

[0137] The volume fractions of ferrite, bainite, martensite (tempered martensite and fresh martensite), pearlite, cementite, and retained austenite contained in the metal structure at the surface layer and the 1 / 4 thickness position can be measured using the method shown below.

[0138] A sample was collected from a cross section parallel to the rolling direction and the thickness direction of the steel plate as an observation surface, and the observation surface was polished and etched with Nital.

[0139] Next, when observing the structure at the 1 / 4 thickness position, the magnification is 5000 times, and the field of view is set to 250 μm in the range of 1 / 8 thickness to 3 / 8 thickness from the surface, centered at the 1 / 4 thickness position. 2 A total of five visual fields were observed using a field emission scanning electron microscope (FE-SEM). The area ratios of ferrite, bainite, tempered martensite, fresh martensite, pearlite, cementite, and retained austenite were measured and regarded as volume ratios.

[0140] In addition, when observing the structure of the surface layer, the range of 10 to 20 μm from the surface to the plate thickness direction is observed in the same way as the 1 / 4 thickness position, and the area ratios of ferrite, bainite, tempered martensite, primary martensite, pearlite, cementite, and retained austenite are measured respectively and regarded as the volume ratio.

[0141] Here, regarding the identification of each phase, the region where the lower structure is present in the grain and carbides have multiple variants and precipitate is judged to be tempered martensite. In addition, the region where cementite precipitates in a lamellar shape is judged to be pearlite or cementite. The region where the brightness is small and the lower structure is not confirmed is judged to be ferrite. The region where the brightness is large and the lower structure is not revealed by etching is judged to be primary martensite or retained austenite. The remainder is judged to be bainite. Each volume ratio is calculated by the point counting method, thereby serving as the volume ratio of each structure.

[0142] The volume fraction of fresh martensite can be determined by subtracting the volume fraction of retained austenite determined by the EBSD method described later from the volume fraction of fresh martensite or retained austenite.

[0143] In the cold-rolled steel sheet of this embodiment, the volume fraction of retained austenite at the surface and at the quarter-thickness position is evaluated by high-resolution crystal structure analysis using the EBSD method (electron backscatter diffraction). Specifically, a sample is collected using a cross section parallel to the rolling direction and the thickness direction of the steel sheet as the observation surface, and the observation surface is polished to a mirror finish. Furthermore, electrolytic polishing or mechanical polishing using colloidal silica is performed to remove the processed layer of the surface.

[0144] Next, the surface and 1 / 4 thickness of the steel plate were respectively observed with a magnification of 5000 times and a field size of 150 μm. 2 The crystal structure analysis was performed on five fields using the EBSD method. The distance between evaluation points (steps) was 0.01 to 0.20 μm.

[0145] Data obtained by the EBSD method were analyzed using "OIM Analysys 6.0" manufactured by TSL. Based on the observation results at various locations, the region identified as FCC iron was identified as retained austenite, and the volume fraction of retained austenite at the surface and at the quarter-thickness position was calculated.

[0146] [Dislocation density]

[0147] In the cold-rolled steel sheet of the present embodiment, the ratio of the dislocation density in the surface portion to the dislocation density at the 1 / 4 thickness position is 0.20 or more and less than 0.90 (20% or more and less than 90%). In order to suppress bending fracture, ferrite with excellent ductility is arranged in the surface portion, and the bending strength is improved by dislocation strengthening the surface portion.

[0148] If 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 1 / 4 thickness position is less than 0.20, the bending strength will be significantly reduced. Therefore, the dislocation density ratio is set to 0.20 or higher. The dislocation density ratio is preferably 0.22 or higher, 0.25 or higher, or 0.30 or higher.

[0149] On the other hand, when the ratio is 0.90 or more, bending fracture cannot be sufficiently suppressed. Therefore, the dislocation density ratio is set to less than 0.90. The dislocation density ratio is preferably 0.70 or less, and more preferably 0.50 or less.

[0150] The dislocation density at the surface layer portion and the 1 / 4 thickness position was determined by the following method.

[0151] Two samples (a sample for measuring the dislocation density of the surface layer portion and a sample for measuring the dislocation density at a 1 / 4 thickness position) were collected so that a surface parallel to the rolled surface of the steel plate became an observation surface.

[0152] For the sample to measure the dislocation density of the surface layer, if a plating layer existed on the surface, the plating layer was peeled off, and then the observation surface was chemically polished to adjust the sample so that the position 10 to 20 μm from the surface could be measured.

[0153] The sample for measuring the dislocation density at the 1 / 4 thickness position was ground from the surface toward the plate thickness to the 1 / 4 thickness position, and then chemically polished to remove the processed layer caused by grinding, adjusting the 1 / 4 thickness position to be exposed on the surface.

[0154] For each sample adjusted as described above, the strain of the steel sheet was measured using an X-ray diffractometer. The measurement was performed using an X-ray diffractometer with CoKα radiation as the X-ray source. The diffraction intensity of the (110), (200), (211), (220), and (310) planes of α-iron was measured. The half-value width of the peak of the reflection intensity for each crystal plane was calculated from the measured graph. The local strain ε' imparted to the steel sheet was determined using the following equations (3) and (4).

[0155] βcosθ / λ=0.9 / D+2ε'sinθ / λ (3)

[0156] in,

[0157] β: Half-value width of the peak (the value corrected by equation (4) is used)

[0158] θ: diffraction angle

[0159] λ: wavelength of CoKα radiation (0.1790 nm)

[0160] D: Crystallite size (size of dislocation unit, grain)

[0161] ε': local strain

[0162] β 2 =βm 2 -β0 2 (4)

[0163] in,

[0164] βm: Half-value width of the peak of the sample for measuring dislocation density

[0165] β0: Half-value width of the peak of a sample without strain.

[0166] βcosθ / λ is plotted against sinθ / λ, and ε' and D are obtained from the slope and intercept. The dislocation density ρ is determined from the obtained local strain ε' using the following equation (5).

[0167] ρ=14.4ε' 2 / b 2 (5)

[0168] in,

[0169] b: Burgers vector (0.248 nm).

[0170] [Plating]

[0171] The cold-rolled steel sheet of the present embodiment described above may have a hot-dip galvanizing layer on the surface of the steel sheet. The presence of the hot-dip galvanizing layer on the surface of the steel sheet improves corrosion resistance.

[0172] For example, when using steel sheets in an environment prone to corrosion, concerns about opening holes may arise, so even with increased strength, it is sometimes impossible to reduce the thickness of the sheet to below a certain thickness. One of the goals of increasing the strength of steel sheets is to achieve lightweighting through thinning. Therefore, even if high-strength steel sheets are developed, if their corrosion resistance is low, their applicable locations are limited. Therefore, it is considered to apply a coating such as hot-dip galvanizing to the steel sheets, which has high corrosion resistance. The coating is, for example, a zinc coating such as a hot-dip galvanizing layer or an electroplated galvanizing layer. In addition, the galvanizing layer may also contain Si, Al, and / or Mg in addition to Zn.

[0173] In addition, the galvanized layer may be alloyed. In an alloyed hot-dip galvanized layer (alloyed hot-dip galvanized layer), Fe is introduced into the hot-dip galvanized layer by alloying treatment, thereby achieving excellent weldability and paintability.

[0174] In addition, for the purpose of improving paintability and weldability, an upper layer plating can be implemented on the galvanized layer. In addition, in the cold-rolled steel sheet of this embodiment, various treatments can be implemented on the hot-dip galvanized layer, such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc.

[0175] <Mechanical properties>

[0176] The cold-rolled steel sheet of the present embodiment has high strength and is excellent in elongation and bendability.

[0177] When used in automobile parts, the tensile strength is 980 MPa or higher, considering its contribution to automobile weight reduction. There is no upper limit for the tensile strength, but from the perspective of ensuring spot weldability, the tensile strength may be 1500 MPa or lower.

[0178] In consideration of application to automobile parts, the thickness of the cold-rolled steel sheet of the present embodiment (the thickness of the base steel sheet excluding the coating layer) is preferably 0.8 to 3.0 mm.

[0179] <Manufacturing method>

[0180] The cold-rolled steel sheet of the present embodiment can be produced by a production method including the following steps.

[0181] (I) a hot rolling step of heating a slab having a predetermined chemical composition and hot rolling it so that the finishing temperature of the finishing rolling is 800° C. or higher and the Ar3 point or higher to obtain a hot-rolled steel sheet;

[0182] (II) a post-hot rolling cooling step, wherein the hot-rolled steel sheet is cooled at an average cooling rate of 5°C / second or more to a coiling temperature of 400 to 750°C after 1.0 second or more has passed since the end of the hot rolling;

[0183] (III) a coiling step of coiling the hot-rolled steel sheet after the hot-rolling cooling step at the coiling temperature;

[0184] (IV) a cold rolling step of cold-rolling the hot-rolled steel sheet after the coiling step at a cumulative reduction ratio of 20 to 80% to obtain a cold-rolled steel sheet;

[0185] (V) an annealing step of heating the cold-rolled steel sheet after the cold rolling step to a soaking temperature of 750 to 1000° C. and maintaining the soaking temperature for more than 1 second;

[0186] (VI) a first cooling step of cooling the cold-rolled steel sheet after the annealing step from the soaking temperature to a first cooling stop temperature of 600° C. or less at an average cooling rate of 10.0° C. / s or less;

[0187] (VII) a second cooling step of cooling the cold-rolled steel sheet after the first cooling step from the first cooling stop temperature to a second cooling stop temperature of less than or equal to the Ms point at an average cooling rate of 1.0°C / s;

[0188] (VIII) a heat treatment step of maintaining the cold-rolled steel sheet after the second cooling step in a temperature range of 300 to 530° C. for 20 seconds or more; and

[0189] (IX) a flattening step of accumulating a strain of 1.0% or more on each of the front and back surfaces of the cold-rolled steel sheet after the heat treatment step and adjusting the elongation to 0.50% or less.

[0190] Hereinafter, each step will be described.

[0191] [Hot rolling process]

[0192] In the hot rolling step, a slab having the same chemical composition as that of the cold-rolled steel sheet according to the present embodiment is heated and hot-rolled to produce a hot-rolled steel sheet.

[0193] The slabs used for hot rolling may be those produced by continuous casting, thin slab continuous casting, or the like.

[0194] During the hot rolling process, the slab heating temperature is preferably 1050°C or higher. If the slab heating temperature is too low, the finishing temperature will also decrease. This decrease in finishing temperature will lead to an excessive increase in rolling load, making rolling difficult or causing poor shape of the rolled steel sheet.

[0195] The upper limit of the slab heating temperature is not particularly limited, and the effects of the present invention can be exhibited. However, excessively high heating temperatures are not economically preferable, so the slab heating temperature is preferably 1350° C. or lower.

[0196] Furthermore, when a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting is applied, if hot rolling can be started at a slab temperature of 1050° C. or higher, heating of the slab is unnecessary.

[0197] Hot rolling is performed so that the finishing temperature is 800° C. or higher and the Ar3 point or higher. If the finishing temperature is lower than 800° C. or lower than the Ar3 point, the rolling load during finishing rolling increases, making hot rolling difficult.

[0198] On the other hand, there is no upper limit for the finishing rolling temperature. However, if the finishing rolling temperature is too high, the slab heating temperature must be too high to ensure the temperature. Therefore, the finishing rolling temperature is preferably 1000°C or lower.

[0199] The temperature of the Ar3 point is calculated by the following formula.

[0200] Ar3=901-325×[C]+33×[Si]-92×([Mn]+[Ni] / 2+[Cr] / 2+[Cu] / 2+[Mo] / 2)+52×[sol.Al]

[0201] In the above formula, [element symbol] represents the content [mass %] of each element. If the element is not contained, 0 is substituted.

[0202] [Cooling process after hot rolling]

[0203] [Coiling process]

[0204] For the hot-rolled steel sheet after the hot rolling process, it is preferred to start cooling after 1.0 second or more from the end of hot rolling, and cool it to a coiling temperature of 400 to 750°C at an average cooling rate of 5°C / second or more (post-hot rolling cooling process), and coil the hot-rolled steel sheet at the coiling temperature (coiling process).

[0205] If the time from the end of hot rolling to the start of cooling is less than 1.0 second, the recrystallization of austenite becomes insufficient, and the anisotropy of the steel sheet becomes pronounced, which is not preferred. Furthermore, if the average cooling rate from the end of finish rolling to the coiling temperature is less than 5°C / second, ferrite transformation in the high temperature range is accelerated, resulting in coarsening of the hot-rolled sheet structure, which is also not preferred.

[0206] Furthermore, coiling temperatures exceeding 750°C are not preferred because the thickness of oxides formed on the steel sheet surface increases excessively, deteriorating pickling properties. To improve pickling properties, the coiling temperature is more preferably 720°C or lower, and even more preferably 700°C or lower. On the other hand, coiling temperatures below 400°C are not preferred because the strength of the hot-rolled steel sheet becomes excessively high, making cold rolling difficult.

[0207] The hot rolled coil after the coiling process may be pickled by conventional methods as needed. In addition, skin pass rolling may be performed to improve the shape correction of the hot rolled coil and improve pickling resistance.

[0208] [Cold rolling process]

[0209] In the cold rolling process, the hot-rolled steel sheet is cold-rolled to obtain the cold-rolled steel sheet. The cumulative reduction during cold rolling is preferably 20-80%. A cumulative reduction of 20% or more allows for the refinement of the austenite grain size during heating in the annealing process described later. The cumulative reduction during cold rolling is more preferably 30% or more. On the other hand, excessive reduction results in excessive rolling loads, increasing the load on the cold rolling mill. The cumulative reduction is more preferably 70% or less.

[0210] [Annealing process]

[0211] In the annealing process, the cold rolled steel sheet after the cold rolling process is heated to a soaking temperature (annealing temperature) of 750 to 1000°C and maintained at the soaking temperature for more than 1 second. In addition, in the annealing process, when heating is performed in a temperature range of at least 650°C to the soaking temperature, the water vapor partial pressure P in the atmosphere in the furnace is reduced to (H2O)With hydrogen partial pressure P (H2) The ratio is P (H2O) / P (H2) Set it within the range of 0.00010 to 2.00.

[0212] When the soaking temperature is lower than 750°C, the amount of austenite during soaking becomes insufficient, and the phase transformation during subsequent cooling cannot ensure a sufficient amount of hard structure (martensite and bainite). Therefore, the soaking temperature is 750°C or higher. The soaking temperature is preferably 770°C or higher. On the other hand, if the soaking temperature exceeds 1000°C, the grain size of the austenite becomes coarse, the phase transformation during cooling is difficult to proceed, and it is difficult to fully obtain a soft ferrite structure. Therefore, the soaking temperature is 1000°C or lower. The soaking temperature is preferably 900°C or lower.

[0213] The hold time during soaking is 1 second or longer. If the hold time is less than 1 second, austenite transformation may not occur sufficiently. There is no particular upper limit on the hold time, but if the hold time is too long, the manufacturability of the steel sheet is impaired. Therefore, the hold time can be set to 1000 seconds or less.

[0214] The average heating rate from 650° C. to the soaking temperature is not limited, but is preferably 5.0° C. / second or less in order to ensure an appropriate decarburization amount.

[0215] Furthermore, in the heating process of the annealing step, by controlling the atmosphere in the furnace when heating from 650° C. to the soaking temperature, a large amount of soft ferrite can be produced in the surface layer portion in the subsequent first cooling step due to a decarburization reaction.

[0216] P in the furnace during heating from 650°C to the soaking temperature (H2O) / P (H2) When the volume ratio is less than 0.00010, ferrite cannot be generated sufficiently to improve the bendability. (H2O) / P (H2) 0.00010 or more. (H2O) / P (H2) It is preferably 0.00020 or more, and more preferably 0.01 or more.

[0217] On the other hand, if P (H2O) / P (H2) If the value exceeds 2.00, decarburization will be excessive, the thickness of the decarburized layer will increase, and the bending strength may decrease. (H2O) / P (H2) Below 2.00. (H2O) / P (H2) It is preferably 1.50 or less, and more preferably 1.20 or less.

[0218] [First Cooling Step]

[0219] [Second Cooling Step]

[0220] In the annealing step, the cold-rolled steel sheet is held at the soaking temperature and then cooled in two stages.

[0221] That is, after the annealing step, a process including a first cooling step from the soaking temperature to a first cooling stop temperature and a second cooling step from a second cooling start temperature equal to the first cooling stop temperature to the second cooling stop temperature is performed.

[0222] In the first cooling step, the cold-rolled steel sheet is cooled from the soaking temperature to a first cooling stop temperature of 600° C. or less at an average cooling rate of 10.0° C. / s or less. This cooling promotes ferrite transformation in the surface layer of the steel sheet.

[0223] If the average cooling rate exceeds 10.0° C. / second, or the cooling stop temperature exceeds 600° C., ferrite cannot be generated in a sufficient volume fraction in the surface layer.

[0224] In the second cooling step performed subsequent to the first cooling step, the cold-rolled steel sheet is cooled from a second cooling start temperature equal to the first cooling stop temperature to a temperature below the Ms point (second cooling stop temperature) at an average cooling rate of 1.0°C / s or more.

[0225] If the average cooling rate is less than 1.0°C / s, the volume fraction of martensite decreases, and sufficient tensile strength may not be achieved. The upper limit of the average cooling rate does not need to be specifically defined, but special equipment is required to achieve a cooling rate exceeding 300°C / s. Therefore, the average cooling rate can be set to 300°C / s or less.

[0226] Furthermore, if the second cooling stop temperature exceeds the Ms point, it becomes difficult for the volume ratio of the tempered martensite in the entire martensite to be 50% or more, and there is a possibility that the bendability may deteriorate.

[0227] Ms point (°C) can be calculated using the following formula.

[0228] Ms=550-361×[C]-39×[Mn]-35×[V]-20×[Cr]-17×[Ni]-10×[Cu]-5×[Mo]+30×[sol.Al]

[0229] The [element symbol] in the formula represents the content (mass %) of each element contained in the steel. When the content is 0, 0 is substituted into the formula for calculation.

[0230] [Heat treatment process]

[0231] After the second cooling step, the cold-rolled steel sheet is subjected to a heat treatment held at a temperature of 300-530°C for 20 seconds or longer. This heat treatment generates bainite at least at a quarter thickness, stabilizes retained austenite, and further tempers martensite.

[0232] If the holding time is less than 20 seconds, bainite transformation is insufficient and a sufficient amount of retained austenite cannot be obtained. The upper limit of the holding time is not particularly specified, but from the viewpoint of productivity, the holding time is preferably 1000 seconds or less.

[0233] Furthermore, if the heat treatment temperature (holding temperature) is lower than 300°C, bainite transformation does not occur sufficiently, and a sufficient amount of retained austenite cannot be obtained, resulting in reduced elongation. On the other hand, if the heat treatment temperature exceeds 530°C, martensite is excessively tempered, making it difficult to ensure sufficient tensile strength.

[0234] [Galvanizing process]

[0235] When the cold-rolled steel sheet is a hot-dip galvanized steel sheet (when a hot-dip galvanized layer is formed on the surface), the hot-dip galvanized layer can be formed by immersing the cold-rolled steel sheet in a coating bath in any step after the first cooling step and before the leveling step described below. Specifically, any of the steps can be in the middle of the second cooling step, between the second cooling step and the heat treatment step, in the middle of the heat treatment step, or between the heat treatment step and the leveling step.

[0236] When forming a hot-dip galvanizing layer, the cold-rolled steel sheet is heated to 420 to 530° C. and then immersed in a hot-dip galvanizing bath at a bath temperature of 420 to 530° C. to form the hot-dip galvanizing layer.

[0237] If the temperature of the cold-rolled steel sheet during immersion in the galvanizing bath is lower than 420°C, heat dissipation from the hot-dip galvanizing bath increases, hindering productivity. Furthermore, if the temperature of the steel sheet or the galvanizing bath during immersion in the galvanizing bath exceeds 530°C, pearlite transformation occurs, making it difficult to obtain the desired steel structure.

[0238] The time from when the steel sheet is heated to 420 to 530° C. to when it is immersed in the hot-dip galvanizing bath is not particularly limited, but is preferably 100 seconds or less from the viewpoint of productivity.

[0239] In order to form a zinc plating layer, when electrogalvanizing is performed, it can be performed after the heat treatment step. In addition, regarding the conditions of electrogalvanizing, conventional methods can be followed.

[0240] [Alloying process]

[0241] When the hot-dip galvanized layer is subjected to alloying treatment to obtain an alloyed hot-dip galvanized layer, the cold-rolled steel sheet on which the hot-dip galvanized layer is formed is maintained at a temperature of 460 to 530°C.

[0242] When the alloying treatment temperature (holding temperature) is lower than 460°C, the alloying reaction takes a long time, thereby hindering productivity. On the other hand, when the alloying treatment temperature exceeds 530°C, pearlite transformation occurs, making it difficult to obtain the desired steel structure.

[0243] [Leveling process]

[0244] The cold-rolled steel sheet after each of the above steps (after the heat treatment step, after the hot-dip galvanizing step, or after the alloying step) is subjected to bending recovery deformation to impart cumulative strain of 1.0% or more to the front and back surfaces.

[0245] This allows dislocations to be introduced into the front and back surfaces of the cold-rolled steel sheet, improving its bending strength. If the cumulative strain on the front and back surfaces is less than 1.0%, good bending strength cannot be achieved. The cumulative strain is preferably 3.0% or greater, and more preferably 10.0% or greater.

[0246] The upper limit of the cumulative strain does not need to be limited, but is preferably 30.0% or less from the viewpoint of fatigue strength.

[0247] However, the overall elongation in the leveling process must be 0.50% or less. If the elongation exceeds 0.50%, the elongation of the cold-rolled steel sheet decreases. The elongation in the leveling process is preferably 0.20% or less.

[0248] The cumulative strain is calculated by accumulating the strain history obtained by attaching strain gauges to the front and back surfaces. If the cumulative strain is previously investigated using steel plates of equivalent strength and thickness, the target cumulative strain can be achieved by performing bending and recovery under the same conditions.

[0249] [Quenching and tempering rolling process]

[0250] After the leveling step, temper rolling may be performed for further shape adjustment, etc. However, when temper rolling is performed, the total elongation in the leveling step and the temper rolling is 0.50% or less.

[0251] Example

[0252] Slabs having the chemical compositions described in Tables 1-1 and 1-2 were hot-rolled and coiled under the conditions described in Tables 2-1 and 2-2 to obtain hot-rolled steel sheets.

[0253] Furthermore, the hot-rolled steel sheets were cold-rolled under the conditions described in Tables 2-1 and 2-2 to obtain cold-rolled steel sheets.

[0254] The cold-rolled steel sheets were annealed under the conditions listed in Tables 3-1 and 3-2, and then subjected to first and second cooling. During the second cooling period, some of the cold-rolled steel sheets were hot-dip galvanized by immersion in a 420-520°C galvanizing bath under the conditions listed in Tables 3-3 and 3-4. Furthermore, some of the cold-rolled steel sheets that had been hot-dip galvanized were alloyed at the alloying temperatures listed in Tables 3-3 and 3-4. The coating that had only been hot-dip galvanized was designated GI, while the coating that had been alloyed was designated GA.

[0255] Then, heat treatment, leveling and tempering rolling were carried out under the conditions shown in Table 3-3 and Table 3-4.

[0256]

[0257]

[0258] Table 2-1

[0259]

[0260] Table 2-2

[0261]

[0262]

[0263]

[0264]

[0265]

[0266] The resulting cold-rolled steel sheets were observed for their microstructure at the surface and at the quarter-thickness position using the above method, and the volume fractions of each phase were measured. The results are shown in Tables 4-1 and 4-2. In the tables, Vα represents the volume fraction of ferrite, VB represents the volume fraction of bainite, VM represents the volume fraction of martensite, Vγ represents the volume fraction of retained austenite, and VTM / VM represents the volume ratio of tempered martensite to martensite.

[0267] Furthermore, for the obtained cold-rolled steel sheets, the ratio of the dislocation density in the surface layer portion to the dislocation density at the 1 / 4 thickness position was measured by the above-mentioned method.

[0268] The results are shown in Tables 4-1 and 4-2.

[0269] Furthermore, the tensile properties and bendability of the obtained cold-rolled steel sheets were evaluated according to the following procedures. The results are shown in Tables 4-3 and 4-4.

[0270] Table 4-1

[0271]

[0272] Table 4-2

[0273]

[0274] Table 4-3

[0275]

[0276] Table 4-4

[0277]

[0278] <Tensile Properties>

[0279] JIS No. 5 tensile test pieces were taken from the direction perpendicular to the rolling direction and thickness direction of the cold-rolled steel sheet (width direction), and tensile tests were performed in accordance with JIS Z 2241:2011 to measure tensile strength (TS), yield strength (YS), and total elongation (EL).

[0280] If the tensile strength is 980 MPa or more, it is judged to be high strength.

[0281] In addition, (tensile strength / 1000) 2 When ×EL is 16 or more, it is judged to have excellent elongation (the EL column in the table is 0).

[0282] <Flexibility>

[0283] The bending test was conducted according to 238-100 established by VDA (German Association of the Automotive Industry) to evaluate the bending angle.

[0284] If the bending angle is 80° or more, it is judged to have a sufficient bending angle (0 in the table).

[0285] In addition, the reaction force of the stroke 2mm is set as the load of the VDA bending stroke 2mm. If the load of the VDA bending stroke 2mm (kN) / tensile strength (MPa) / 1000 is 5.0 or more, it is judged that the bending strength is excellent (0 in the table).

[0286] As can be seen from Tables 1-1 to 4-4, in Experiments No. 1 to 8, 16, 17, 20, 23 to 26, 30 to 39, 45, and 46, which are inventive examples, the chemical composition, volume ratio of the metal structure, and dislocation density ratio are within the range of the present invention, and they have high strength and excellent elongation and bendability.

[0287] On the other hand, in experiments No. 9 to 15, 18, 19, 21, 22, 27 to 29, 40 to 44, and 47 to 49, which are comparative examples, one or more of the chemical composition, volume ratio of the metal structure, and dislocation density ratio deviate from the scope of the present invention, and one or more of the strength (tensile strength), elongation, and bendability are poor.

[0288] Industrial applicability

[0289] According to the present invention, a steel sheet having high strength and excellent elongation and bendability and a method for producing the same can be provided, and thus the steel sheet has high industrial applicability.

Claims

1. A cold-rolled steel sheet having a chemical composition comprising, by mass %, C: 0.100 to 0.400%, Si: 0.01~3.00%, Mn: 1.00~5.00%, sol.Al:0.001~1.000%, P: 0.100% or less, S: 0.0100% or less, O: 0.100% or less, N: 0.010% or less, Ti: 0~0.200%, B:0～0.0100％、 Cr:0～1.00％、 Mo: 0~1.00%, Ni: 0~1.00%, Cu: 0~1.00%, Sn: 0~0.50%, Nb: 0~0.20%, V:0～0.50％、 W:0～0.50％、 Ca: 0~0.010%, Mg: 0~0.010%, Bi: 0~0.010%, Sb: 0~0.100%, Zr:0～0.010％、 REM: 0~0.010%, and The rest: Fe and impurities, Q calculated from the following formula (1) is 2.3 or more, The SA obtained by the following formula (2) is 0.30 or more, In the surface layer portion ranging from 10 to 20 μm in the thickness direction from the surface, the metal structure comprises 10 to 95% by volume of ferrite, and the remainder comprises at least one selected from martensite, bainite, and retained austenite. At a position of 1 / 4 thickness in a range of 1 / 8 to 3 / 8 of the thickness in the thickness direction of the plate from the surface, the metal structure comprises, by volume, 0 to 60% ferrite and 4 to 20% retained austenite, with the remainder comprising at least one selected from martensite and bainite, and the volume fraction of tempered martensite in the martensite being 50% or more. The ratio of the dislocation density of the surface layer portion to the dislocation density at the 1 / 4 thickness position is 0.20 or more and less than 0.90, The tensile strength of the cold-rolled steel sheet is above 980 MPa. Q=2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+2.0×[Mo] (1) SA=[Si]+0.7×[sol.Al] (2) in, In the above formula (1) and the above formula (2), [element symbol] represents the content of the element symbol in mass %, and 0 is substituted when the element is not contained.

2. The cold-rolled steel sheet according to claim 1, wherein: The surface is provided with a hot-dip galvanized layer.

3. The cold-rolled steel sheet according to claim 2, wherein: The hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

4. A method for manufacturing a cold-rolled steel sheet, characterized in that: have: The hot rolling process is performed to obtain a chemical composition comprising, by mass%, C: 0.100-0.400%, Si: 0.01-3.00%, Mn: 1.00-5.00%, sol.Al: 0.001-1.000%, P: 0.100% or less, S: 0.0100% or less, O: 0.100% or less, N: 0.010% or less, Ti: 0-0.200%, B: 0-0.0100%, Cr: 0-1.00%, Mo: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, Sn: 0-1.00%. ~0.50%, Nb: 0~0.20%, V: 0~0.50%, W: 0~0.50%, Ca: 0~0.010%, Mg: 0~0.010%, Bi: 0~0.010%, Sb: 0~0.100%, Zr: 0~0.010%, REM: 0~0.010%, and the remainder: Fe and impurities, heating a slab having a Q obtained by the following formula (1) of 2.3 or more and a SA obtained by the following formula (2) of 0.30 or more, and hot rolling the slab so that the finishing temperature is 800°C or more and the Ar3 point or more to obtain a hot-rolled steel sheet; a post-hot rolling cooling step, wherein the hot-rolled steel sheet is cooled at an average cooling rate of 5°C / second or more to a coiling temperature of 400 to 750°C after 1.0 second or more has passed since the end of the hot rolling; a coiling step of coiling the hot-rolled steel sheet after the hot-rolling cooling step at the coiling temperature; a cold rolling step of cold-rolling the hot-rolled steel sheet after the coiling step at a cumulative reduction ratio of 20 to 80% to obtain a cold-rolled steel sheet; an annealing step of heating the cold-rolled steel sheet after the cold rolling step to a soaking temperature of 750 to 1000° C. and maintaining the soaking temperature for more than 1 second; a first cooling step of cooling the cold-rolled steel sheet after the annealing step from the soaking temperature to a first cooling stop temperature of 600° C. or less at an average cooling rate of 10.0° C. / s or less; a second cooling step of cooling the cold-rolled steel sheet after the first cooling step from the first cooling stop temperature to a second cooling stop temperature below the Ms point at an average cooling rate of 1.0°C / s or higher; a heat treatment step of maintaining the cold-rolled steel sheet after the second cooling step in a temperature range of 300 to 530° C. for more than 20 seconds; as well as A flattening step is performed to apply a cumulative strain of 1.0% or more to the front and back surfaces of the cold-rolled steel sheet after the heat treatment step, and to make the elongation 0.50% or less. In the annealing step, when the heating is performed in a temperature range of at least 650° C. to the soaking temperature, the water vapor partial pressure P in the atmosphere in the furnace is reduced to (H2O) With hydrogen partial pressure P (H2) The ratio is P (H2O) / P (H2) Set it within the range of 0.00010 to 2.

00. Q=2.7×[C]+0.4×[Si]+[Mn]+0.45×[Ni]+0.8×[Cr]+2.0×[Mo] (1) SA=[Si]+0.7×[sol.Al] (2) In the above formula (1) and the above formula (2), [element symbol] represents the content of the element symbol in mass %, and 0 is substituted when the element is not contained.

5. The method for manufacturing a cold-rolled steel sheet according to claim 4, wherein: After the first cooling step, a hot-dip galvanizing step is further provided in which the cold-rolled steel sheet is heated to 420 to 530° C. and then immersed in a hot-dip galvanizing bath at a bath temperature of 420 to 530° C. to form a hot-dip galvanized layer. 6 . The method for producing a cold-rolled steel sheet according to claim 5 , further comprising an alloying treatment step of maintaining the cold-rolled steel sheet after the hot-dip galvanizing step at a temperature of 460 to 530° C. to alloy the hot-dip galvanized layer.

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