Steel sheet, member, and method for producing same

Abstract

Provided are: a steel sheet having high strength, excellent ductility and hole expandability, and excellent stability of mechanical characteristics in the longitudinal direction of a coil; and a method for manufacturing the steel sheet. A steel sheet having a component composition containing, in mass%, 0.08-0.35% of C, 0.4-3.0% of Si, 1.5-3.5% of Mn, 0.02% or less of P, 0.01% or less of S, 1.0% or less of sol.Al, and 0.015% or less of N, with the remainder comprising Fe and unavoidable impurities, and having a steel structure in which the area ratio of ferrite is 5% or less (including 0%), the total area ratio of tempered martensite and lower bainite is 70% or more, the volume ratio of retained austenite is 5-15, and the area ratio of fresh martensite is 10% or less (including 0%). The standard deviation of the tensile strength (TS) in the longitudinal direction of the coil is 30 MPa or less.

Description

Technical Field

[0001] The present invention relates to steel sheets and members used in various applications such as automobiles and home appliances, and methods for producing the same. Background Art

[0002] In recent years, the strength of automobile components has been increased for the purpose of reducing the weight of automobile bodies. High-strength steel sheets with a tensile strength (TS) of 1180 MPa or more are used in automobile frame parts and seat parts. Generally, the ductility and stretch flange formability of steel sheets decrease with increasing strength, so steel sheets with a TS of 1180 MPa or more are prone to cracking during stamping.

[0003] In order to obtain excellent press formability for a high-strength steel sheet having a TS of 1180 MPa or more, in addition to improving hole expandability by making the steel sheet structure uniformly tempered martensite, it is also important to improve ductility by finely dispersing retained austenite.

[0004] Patent document 1 discloses a high-strength cold-rolled steel sheet with excellent workability and impact resistance, which contains, by mass%, C: 0.05-0.3%, Si: 0.3-2.5%, Mn: 0.5-3.5%, P: 0.003-0.100%, S: less than 0.02%, Al: 0.010-0.5%, and has a steel structure with ferrite: more than 20%, tempered martensite: 10-60%, martensite: 0-10%, retained austenite: 3-15%, and an average grain size of a low-temperature phase transformation phase composed of martensite, tempered martensite, and retained austenite of less than 3 μm. The technology described in Patent Document 1 utilizes the so-called Q&P: Quenching & Partitioning (quenching and carbon distribution from martensite to austenite) process, that is, cooling to a temperature range between the martensite transformation start temperature (Ms) and the martensite transformation completion temperature (Mf) during the cooling process, and then heating and holding to stabilize the retained γ. In recent years, the development of high-strength steel and its manufacturing method with excellent ductility and stretch flange formability using this process has been underway.

[0005] Patent document 2 discloses a high-strength steel plate with excellent workability, which contains, by mass%, C: 0.05-0.5%, Si: 0.01-2.5%, Mn: 0.5-3.5%, P: 0.003-0.100%, S: 0.02% or less, and Al: 0.010-0.5%, and has a steel structure including ferrite of 0-10% by area ratio, 0-10% by area ratio, tempered martensite of 60-95% and retained austenite of 5-20% by a ratio determined by an X-ray diffraction method, a tensile strength of 1200 MPa or more, and a hole expansion ratio of 50% or more.

[0006] Patent document 3 discloses a method for manufacturing a high-strength steel sheet having excellent workability and tensile strength (TS) and excellent stability of mechanical properties, wherein a steel sheet containing, by mass%, C: 0.10% to 0.73%, Si: 3.0% or less, Mn: 0.5% to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, and N: 0.010% or less is heated to an austenite single phase region or an (austenite + ferrite) dual phase region, and then heated to a temperature of 100°C. The martensite transformation starting temperature Ms is used as an indicator, and a target cooling stop temperature is set in a temperature range below Ms and above (Ms-150°C) for cooling, so that a part of the untransformed austenite undergoes martensite transformation, and then the temperature is raised to temper the martensite, thereby manufacturing a high-strength steel plate. At this time, the coldest part of the above-mentioned steel plate in the plate width direction is maintained in a temperature range from the target cooling stop temperature to (cooling stop temperature+15°C) for a time of more than 15 seconds and less than 100 seconds.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent No. 5463685

[0010] Patent Document 2: Japanese Patent No. 5402007

[0011] Patent Document 3: Japanese Patent No. 5333298 Summary of the invention

[0012] Problems to be solved by the invention

[0013] The above-mentioned prior arts respectively have the following problems.

[0014] In the technology described in Patent Document 1, in order to obtain excellent strength, ductility and hole expandability, it is very important to control the cooling stop temperature during annealing. However, when the cooling speed is fast, the cooling stop temperature may easily fluctuate, and the mechanical properties in the length direction of the coil may fluctuate, resulting in a decrease in stamping formability.

[0015] In addition, in the technology described in Patent Document 2, in order to obtain excellent strength, ductility and hole expandability, it is very important to control the cooling stop temperature during annealing. However, since the cooling rate is as fast as 20°C / s or more, the cooling stop temperature may easily fluctuate, and the mechanical properties in the length direction of the coil may fluctuate, resulting in a decrease in stamping formability.

[0016] In addition, in the technology described in Patent Document 3, after being heated to the austenite single-phase region or the (austenite + ferrite) dual-phase region, a high-strength steel plate is manufactured by setting a target cooling stop temperature in a temperature range below Ms and above (Ms-150°C) for cooling. The coldest part of the above-mentioned steel plate in the plate width direction is maintained in a temperature range from the target cooling stop temperature to (cooling stop temperature + 15°C) for a time of more than 15 seconds and less than 100 seconds, thereby providing a high-strength steel plate with excellent stability of mechanical properties in the plate width direction. However, the stability of the mechanical properties in the length direction of the coil is not recorded, and there is a possibility that the mechanical properties in the length direction of the coil fluctuate due to fluctuations in the hot rolling structure, reheating holding temperature, etc., resulting in a decrease in stamping formability.

[0017] The present invention has been made to solve such problems, and an object of the present invention is to provide a steel plate and a member having high strength, excellent ductility and hole expandability, and excellent stability of mechanical properties in the longitudinal direction of the coil, and a method for producing the same.

[0018] In the present invention, high strength means that the tensile strength TS evaluated in accordance with JIS Z2241 (2011) is 1180 MPa or more.

[0019] Excellent ductility means that the total elongation (EL) evaluated in accordance with JIS Z2241 (2011) is 11.0% or more.

[0020] Excellent hole expansion property means that, on a 100mm×100mm steel plate, a hole with a diameter of 10mm is punched out with a gap set to 12% of the plate thickness, a die with an inner diameter of 75mm is used, and a 60° cone punch is pressed into the hole under an anti-wrinkle pressure of 88.2kN. The hole diameter at the limit of cracking is measured, Df is set as the hole diameter (mm) when cracking occurs, D0 is set as the initial hole diameter (mm), and the limit hole expansion rate λ(%)={(Df-D0) / D0}×100 is 40% or more.

[0021] The excellent stability of the mechanical properties in the length direction of the coil means that, as a JIS No. 5 tensile test piece in a direction parallel to the rolling direction, a total of 20 pieces are cut at equal intervals in the length direction of the coil, including a test piece cut at a position 10m away from the front and rear ends of the coil, and the standard deviation of TS evaluated in accordance with JIS Z2241 (2011) is 30 MPa or less.

[0022] Methods used to solve problems

[0023] The inventors of the present invention have repeatedly conducted in-depth studies to solve the above problems. As a result, it was found that by controlling the temperature during coiling to homogenize the structure of the hot-rolled sheet in the longitudinal direction of the coil, and by slowly cooling from the vicinity of the Ms point to the cooling stop temperature in the annealing process, the fluctuation of the cooling stop temperature in the longitudinal direction of the coil can be reduced, and as a result, the fluctuation of the mechanical properties can be greatly reduced.

[0024] More specifically, the present invention provides the following solutions.

[0025] [1] A steel sheet having a composition containing, by mass%, C: 0.08 to 0.35%, Si: 0.4 to 3.0%, Mn: 1.5 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol.Al: 1.0% or less, N: 0.015% or less, with the balance being Fe and inevitable impurities, and having a steel microstructure in which the area ratio of ferrite is 5% or less (including 0%), the total area ratio of tempered martensite and lower bainite is 70% or more, the volume ratio of retained austenite is 5 to 15%, and the area ratio of fresh martensite is 10% or less (including 0%), wherein the standard deviation of the tensile strength TS in the longitudinal direction of the coil of the steel sheet is 30 MPa or less.

[0026] [2] The steel sheet according to [1], wherein the above-mentioned component composition contains one or more selected from the group consisting of B: 0.01% or less, Ti: 0.1% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1.5% or less, Mo: 1.0% or less, V: 0.5% or less, Nb: 0.1% or less, Zr: 0.2% or less, and W: 0.2% or less.

[0027] [3] The steel sheet according to [1] or [2], wherein the above-mentioned component composition contains one or more selected from the group consisting of Ca: 0.0040% or less, Ce: 0.0040% or less, La: 0.0040% or less, Mg: 0.0040% or less, Sb: 0.1% or less, and Sn: 0.1% or less in mass %.

[0028] [4] The steel sheet according to any one of [1] to [3], comprising a plating layer on a surface of the steel sheet.

[0029] [5] A member formed using the steel plate according to any one of [1] to [4].

[0030] [6] A method for manufacturing a steel plate, comprising:

[0031] A hot rolling process, wherein a steel slab having the composition described in any one of [1] to [3] is held at a steel slab heating temperature of 1100°C or higher for 1800 seconds or longer, then hot finish rolling is performed at a finish rolling temperature of 850°C or higher, cooled at an average cooling rate of 40°C / s or higher in a temperature range from the finish rolling temperature to 650°C, and coiled at a coiling temperature of 600°C or lower, thereby producing a hot rolled steel sheet;

[0032] a cold rolling step, wherein the hot-rolled steel sheet is cold-rolled at a rolling ratio of 30% or more to form a cold-rolled steel sheet; and

[0033] Annealing step, wherein the cold rolled steel sheet is heated at an average heating rate HR1 of 0.5°C / s or more in a temperature range from 700°C to (Ac3-10°C), then maintained at an annealing temperature of (Ac3-10°C) or more for 30 seconds or more, cooled at an average cooling rate CR1 of 10°C / s or more in a temperature range from the annealing temperature to a slow cooling start temperature T1 of (Ms-30°C) or more and (Ms+30°C) or less, and then cooled at an average cooling rate CR1 of 10°C / s or more in a temperature range from the slow cooling start temperature T1 to (Ms-220°C) or more and (Ms- The method comprises the following steps: cooling the steel sheet at an average cooling rate CR2 of 1 to 10°C / s in a temperature range from the slow cooling stop temperature T2 to a slow cooling stop temperature T2 below 100°C, heating the steel sheet at an average heating rate HR2 of 2°C / s or more in a temperature range from the slow cooling stop temperature T2 to a reheating holding temperature T3 of 300°C to 450°C, maintaining the steel sheet at the reheating holding temperature T3 for 20s to 3000s, and cooling the steel sheet at an average cooling rate CR3 of 0.1°C / s or more in a temperature range from the reheating holding temperature T3 to 50°C.

[0034] [7] The method for manufacturing a steel plate according to [6], wherein, in the annealing step, hot-dip treatment or alloying hot-dip treatment is performed during cooling from the annealing temperature to the slow cooling start temperature T1 or during reheating and holding at the reheating and holding temperature T3.

[0035] [8] The method for manufacturing a steel sheet according to [6], wherein an electroplating treatment is performed after the annealing step.

[0036] [9] A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of [1] to [4] to at least one of a forming process and a joining process to produce the component.

[0037] Effects of the Invention

[0038] According to the present invention, it is possible to provide a steel sheet and a member having high strength, excellent ductility and hole expandability, and excellent stability of mechanical properties in the longitudinal direction of a coil, and a method for producing the same. DETAILED DESCRIPTION

[0039] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the following embodiments.

[0040] The steel sheet of the present invention has a component composition containing, by mass%, C: 0.08-0.35%, Si: 0.4-3.0%, Mn: 1.5-3.5%, P: 0.02% or less, S: 0.01% or less, sol.Al: 1.0% or less, N: 0.015% or less, and the balance consisting of Fe and inevitable impurities, and has a steel structure having an area ratio of ferrite: 5% or less (including 0%), a total area ratio of tempered martensite and lower bainite: 70% or more, a volume ratio of retained austenite: 5-15%, and an area ratio of fresh martensite: 10% or less (including 0%), and a standard deviation of the tensile strength (TS) in the longitudinal direction of the coil is 30 MPa or less.

[0041] First, the chemical composition of the steel sheet of the present invention will be described.

[0042] In the following description of the component composition, "%" as a unit of the content of a component means "mass %." In addition, high strength as referred to in the present invention means that the tensile strength TS is 1180 MPa or more.

[0043] (C: 0.08~0.35%)

[0044] C is contained in order to increase the strength of tempered martensite or lower bainite and ensure a TS of 1180 MPa or more. When the C content is less than 0.08%, the strength of tempered martensite and lower bainite is low, and the desired TS cannot be stably obtained. When the C content is less than 0.08%, the desired ductility cannot be obtained. Therefore, the C content is set to 0.08% or more. The C content is preferably 0.10% or more, and more preferably 0.14% or more.

[0045] On the other hand, excessive addition of C leads to a decrease in hole expandability and ductility due to an increase in the number density of carbides, and deterioration in the shape freezeability of parts due to an excessive increase in YS. Therefore, the C content is set to 0.35% or less. The C content is preferably 0.30% or less, and more preferably 0.25% or less.

[0046] (Si: 0.4~3.0%)

[0047] Si improves the strength of the steel sheet by solid solution strengthening, and further suppresses the reduction in strength due to tempering by suppressing the coarsening of carbides. When Si is less than 0.4%, the desired TS cannot be stably obtained, so the Si content is set to 0.4% or more. The Si content is preferably 1.0% or more, and more preferably 1.4% or more.

[0048] On the other hand, excessive addition of Si leads to a significant decrease in chemical conversion treatability and plating properties. Therefore, the Si content is set to 3.0% or less. The Si content is preferably 2.5% or less, and more preferably 2.0% or less.

[0049] (Mn: 1.5~3.5%)

[0050] Mn is an element effective for improving hardenability. When the Mn content is less than 1.5%, ferrite or pearlite is excessively generated. As a result, tempered martensite and lower bainite etc. cannot be fully obtained sometimes, so the desired TS cannot be obtained. In addition, when the Mn content is less than 1.5%, the desired hole expansion cannot be obtained. Therefore, the Mn content is set to more than 1.5%. The Mn content is preferably more than 2.0%, more preferably more than 2.4%.

[0051] On the other hand, when Mn is added excessively, coarse MnS is formed, and the hole expandability and bendability are greatly reduced. Therefore, the Mn content is set to 3.5% or less. The Mn content is preferably 3.0% or less.

[0052] (P: less than 0.02%)

[0053] P is an element effective in strengthening steel, but excessive addition significantly reduces spot weldability. Therefore, the P content is set to 0.02% or less. The P content is preferably 0.01% or less.

[0054] The lower limit of the P content is not particularly specified, but since a P content less than 0.002% requires a great cost, it is preferably set to 0.002% or more.

[0055] (S: 0.01% or less)

[0056] S forms coarse sulfides with Mn, which reduces hole expandability and bendability. Therefore, the S content is set to 0.01% or less. The S content is preferably 0.002% or less, and more preferably 0.001% or less.

[0057] The lower limit of the S content is not particularly specified, but a content less than 0.0002% requires a lot of cost, so the S content is preferably 0.0002% or more.

[0058] (sol.Al: 1.0% or less)

[0059] Al is an element added as a deoxidizing material in the steelmaking process. When the sol.Al content exceeds 1.0%, inclusions such as Al2O3 and AlN increase, which reduces the hole expandability and bendability. Therefore, the sol.Al content is set to 1.0% or less. The sol.Al content is preferably 0.2% or less, and more preferably 0.05% or less.

[0060] The lower limit of the sol.Al content is not particularly specified, but in order to obtain a sufficient deoxidation effect, the sol.Al content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.

[0061] (N: 0.015% or less)

[0062] When excessive N is added, a large amount of inclusions such as AlN are generated, which reduces the hole expandability and bendability. Therefore, the N content is set to 0.015% or less. The N content is preferably 0.008% or less, and more preferably 0.005% or less.

[0063] The lower limit of the N content is not particularly specified, but it takes a lot of cost to make N less than 0.001%, so the N content is preferably 0.001% or more.

[0064] The steel sheet of the present invention preferably has a composition in which the balance is composed of Fe and inevitable impurities.

[0065] Examples of the inevitable impurities include Zn and Co. In the present invention, even if these elements are contained within the range of a normal steel composition, the effects thereof are not impaired.

[0066] In addition, one or more selected from B, Ti, Cu, Ni, Cr, Mo, V, Nb, Zr, and W may be added as needed to replace part of the above iron (Fe) and inevitable impurities. Furthermore, one or more selected from Ca, Ce, La, Mg, Sb, and Sn may be added as needed.

[0067] Specifically, the chemical composition of the steel sheet of the present invention may appropriately contain the following (A) and / or (B) as optional elements.

[0068] (A) one or more selected from the group consisting of, in mass %, B: 0.01% or less, Ti: 0.1% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1.5% or less, Mo: 1.0% or less, V: 0.5% or less, Nb: 0.1% or less, Zr: 0.2% or less, and W: 0.2% or less

[0069] (B) one or more selected from the group consisting of, in mass %, Ca: 0.0040% or less, Ce: 0.0040% or less, La: 0.0040% or less, Mg: 0.0040% or less, Sb: 0.1% or less, and Sn: 0.1% or less

[0070] ([Group A] B: 0.01% or less, Ti: 0.1% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1.5% or less, Mo: 1.0% or less, V: 0.5% or less, Nb: 0.1% or less, Zr: 0.2% or less, W: 0.2% or less)

[0071] These elements can be added for the purpose of stably obtaining the desired TS by grain refinement and precipitation strengthening. On the other hand, when excessively added, coarse precipitates are generated, which deteriorates the hole expandability and bendability. Therefore, when B is contained, the B content is set to 0.01% or less, when Ti is contained, the Ti content is set to 0.1% or less, when Cu is contained, the Cu content is set to 1% or less, when Ni is contained, the Ni content is set to 1% or less, when Cr is contained, the Cr content is set to 1.5% or less, when Mo is contained, the Mo content is set to 1.0% or less, when V is contained, the V content is set to 0.5% or less, when Nb is contained, the Nb content is set to 0.1% or less, when Zr is contained, the Zr content is set to 0.2% or less, and when W is contained, the W content is set to 0.2% or less.

[0072] The B content is preferably 0.0050% or less, more preferably 0.0030% or less. In addition, the B content is preferably 0.0003% or more.

[0073] The Ti content is preferably 0.080% or less, more preferably 0.050% or less. In addition, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.010% or more.

[0074] The Cu content is preferably 0.50% or less, more preferably 0.20% or less. In addition, the Cu content is preferably 0.001% or more. The Cu content is more preferably 0.030% or more.

[0075] The Ni content is preferably 0.50% or less, more preferably 0.20% or less. In addition, the Ni content is preferably 0.001% or more. The Ni content is more preferably 0.030% or more.

[0076] The Cr content is preferably 1.2% or less, more preferably 1.0% or less. In addition, the Cr content is preferably 0.001% or more. The Cr content is more preferably 0.100% or more.

[0077] The Mo content is preferably 0.50% or less, and more preferably 0.20% or less. The Mo content is further preferably 0.10% or less. In addition, the Mo content is preferably 0.001% or more. The Mo content is more preferably 0.010% or more.

[0078] The V content is preferably 0.50% or less, and more preferably 0.20% or less. The V content is further preferably 0.05% or less. In addition, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more.

[0079] The Nb content is preferably 0.08% or less, more preferably 0.05% or less. In addition, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.010% or more.

[0080] The Zr content is preferably 0.1% or less, more preferably 0.05% or less. In addition, the Zr content is preferably 0.001% or more. The Zr content is more preferably 0.010% or more.

[0081] The W content is preferably 0.1% or less, more preferably 0.05% or less, and further preferably 0.03% or less.

[0082] In addition, the W content is preferably 0.001% or more, and more preferably 0.005% or more.

[0083] ([Group B] Ca: 0.0040% or less, Ce: 0.0040% or less, La: 0.0040% or less, Mg: 0.0040% or less, Sb: 0.1% or less, Sn: 0.1% or less)

[0084] These elements can be added for the purpose of improving hole expansion and bendability by controlling inclusions. When the addition amount exceeds a certain amount, the effect is saturated, so when Ca is contained, the Ca content is set to 0.0040% or less, when Ce is contained, the Ce content is set to 0.0040% or less, when La is contained, the La content is set to 0.0040% or less, when Mg is contained, the Mg content is set to 0.0040% or less, when Sb is contained, the Sb content is set to 0.1% or less, and when Sn is contained, the Sn content is set to 0.1% or less.

[0085] The Ca content is preferably 0.0030% or less. The Ca content is more preferably 0.0010% or less. In addition, the Ca content is preferably 0.0003% or more.

[0086] The Ce content is preferably 0.0030% or less. The Ce content is more preferably 0.0010% or less. In addition, the Ce content is preferably 0.0003% or more.

[0087] The La content is preferably 0.0030% or less. The La content is more preferably 0.0010% or less. In addition, the La content is preferably 0.0003% or more.

[0088] The Mg content is preferably 0.0030% or less. In addition, the Mg content is preferably 0.0003% or more. The Mg content is further preferably 0.0010% or more.

[0089] The Sb content is preferably 0.05% or less, more preferably 0.02% or less. In addition, the Sb content is preferably 0.0003% or more. The Sb content is further preferably 0.0020% or more.

[0090] The Sn content is preferably 0.05% or less, more preferably 0.02% or less. In addition, the Sn content is preferably 0.0003% or more. The Sn content is further preferably 0.0020% or more.

[0091] When the optional components are contained in an amount less than the lower limit, the optional elements contained in an amount less than the lower limit do not impair the effects of the present invention. When the optional elements are contained in an amount less than the lower limit, the optional elements are contained as unavoidable impurities.

[0092] Next, the structure (microstructure) of the steel sheet of the present invention will be described.

[0093] (Area ratio of ferrite: 5% or less (including 0%))

[0094] Ferrite contributes to the improvement of ductility, but due to the difference in hardness with hard phases such as tempered martensite, it becomes the starting point of voids during blanking and press forming, which deteriorates hole expandability. When the area ratio of ferrite exceeds 5%, the desired hole expandability may not be obtained.

[0095] Therefore, the area ratio of ferrite is set to 5% or less. The area ratio of ferrite is preferably 3% or less, and more preferably 0%.

[0096] (Total area ratio of tempered martensite and lower bainite: 70% or more)

[0097] In order to stably obtain a TS of 1180 MPa or more, the total area ratio of tempered martensite and lower bainite is set to 70% or more, preferably 80% or more, and more preferably 85% or more. Tempered martensite and lower bainite have different transformation timings, but their effects on mechanical properties as low-temperature transformation products are similar, so they are evaluated by the total area ratio.

[0098] The upper limit is not particularly limited, but the total area ratio of tempered martensite and lower bainite is preferably 95% or less, more preferably 93% or less.

[0099] (Volume ratio of retained austenite: 5-15%)

[0100] The retained austenite contributes to the improvement of uniform elongation through the TRIP effect. In order to obtain the desired ductility, the retained austenite is set to be 5% or more in terms of volume fraction. When the volume fraction of the retained austenite is less than 5%, sometimes the desired ductility cannot be obtained, and sometimes the desired hole expansion cannot be obtained, and sometimes the desired stability of the mechanical properties in the length direction of the coil cannot be obtained. The volume fraction of the retained austenite is preferably 7% or more, and more preferably 9% or more.

[0101] On the other hand, when retained austenite is excessively generated, the hole expandability may be reduced. In addition, when the retained austenite exceeds 15% by volume, the desired ductility cannot be obtained. Therefore, the retained austenite is set to 15% or less.

[0102] (Area ratio of fresh martensite: 10% or less (including 0%))

[0103] Fresh martensite is very hard, so it becomes the starting point of voids during punching and stamping, reducing the hole expandability. When the fresh martensite exceeds 10%, the deterioration of the hole expandability becomes significant, so the fresh martensite is set to 10% or less, preferably 5% or less, and more preferably 3% or less. It should be noted that the fresh martensite may also be 0%.

[0104] In the present invention, the object of the present invention can be achieved as long as the above-mentioned ferrite, tempered martensite, lower bainite, retained austenite, and fresh martensite are satisfied. As the remaining structure other than these, for example, pearlite and upper bainite may be contained if the total is 5% or less.

[0105] In addition, the steel sheet of the present invention may have a coating on the surface of the steel sheet. The type of coating is not particularly limited, and may be a zinc coating, such as an electrogalvanized layer, a hot-dip galvanized layer, or an alloyed hot-dip galvanized layer.

[0106] Next, a method for measuring the microstructure of a steel plate will be described.

[0107] Regarding the area ratio of ferrite, tempered martensite, lower bainite, and fresh martensite, a plate thickness section parallel to the rolling direction was cut out, mirror-polished, and then corroded with a 1 volume % nitric acid ethanol solution. SEM was used to observe 10 visual fields at 5000 times the 1 / 4 thickness position, and the measurement was performed by the point counting method (according to ASTM E562-83 (1988)). In the above observation, ferrite is the area that appears to be the blackest in SEM, and there is almost no carbide inside, which is an equiaxed area. Tempered martensite and lower bainite are areas that appear to be gray in SEM, and are areas where lath-shaped lower structures and carbide precipitation are observed. Fresh martensite is an area that appears to be white and blocky in SEM, and is an area where no lower structure is observed inside.

[0108] The volume fraction of retained austenite was determined by X-ray diffraction using a steel plate that was adjusted to a 1 / 4 surface in the thickness direction by mechanical grinding and 100 μm or more oxalic acid polishing. A Co-Kα ray source was used as the incident X-ray, and the volume fraction of retained austenite was calculated based on the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite. Here, the retained austenite is randomly distributed, so the volume fraction of retained austenite determined by X-ray diffraction is equal to the area fraction.

[0109] The steel sheet of the present invention has a tensile strength TS of 1180 MPa or more evaluated in accordance with JIS Z2241 (2011), which is high strength.

[0110] Furthermore, the steel sheet of the present invention has a total elongation (EL) of 11.0% or more as evaluated in accordance with JIS Z2241 (2011), and is excellent in ductility.

[0111] In addition, regarding the steel plate of the present invention, on a 100mm×100mm steel plate, a hole with a diameter of 10mm is punched out with a gap set to 12% of the plate thickness, a die with an inner diameter of 75mm is used, and a 60° cone punch is pressed into the hole under a wrinkle-proof pressure of 88.2kN, and the hole diameter at the limit of cracking is measured, Df is set as the hole diameter (mm) when cracking occurs, and D0 is set as the initial hole diameter (mm), the limit hole expansion ratio λ(%)={(Df-D0) / D0}×100 is 40% or more, and the hole expandability is excellent.

[0112] In addition, regarding the steel plate of the present invention, as JIS No. 5 tensile test pieces in a direction parallel to the rolling direction, including test pieces cut at a position 10m away from the front and rear ends of the coil, a total of 20 pieces are cut at equal intervals in the length direction of the coil, and the standard deviation of TS evaluated in accordance with JIS Z2241 (2011) is 30MPa or less, and the stability of mechanical properties in the length direction of the coil is excellent.

[0113] In addition, the standard deviation of EL in the coil length direction of the steel sheet of the present invention may be 1.5% or less.

[0114] It should be noted that, unless otherwise specified, the temperatures for heating or cooling a steel slab (steel material), steel plate, etc. shown below refer to the surface temperatures of the steel slab (steel material), steel plate, etc.

[0115] The method for manufacturing a steel sheet of the present invention comprises: a hot rolling process, wherein a steel sheet having the above-mentioned composition is held at a steel sheet heating temperature of 1100°C or higher for 1800s or more, then hot finish rolling is performed at a finish rolling temperature of 850°C or higher, cooled at an average cooling rate of 40°C / s or higher in a temperature range from the finish rolling temperature to 650°C, and coiled at a coiling temperature of 600°C or lower, thereby producing a hot-rolled steel sheet; a cold rolling process, wherein the hot-rolled steel sheet is cold-rolled at a rolling ratio of 30% or higher to produce a cold-rolled steel sheet; and an annealing process, wherein the cold-rolled steel sheet is heated at an average heating rate HR1 of 0.5°C / s or higher in a temperature range from 700°C to (Ac3-10°C), then held at an annealing temperature of (Ac3-10°C) or higher for 30s or more, and then the annealing temperature is heated at an average heating rate HR1 of 0.5°C / s or higher to produce a cold-rolled steel sheet. In the temperature range from the ignition temperature to the slow cooling start temperature T1 of (Ms-30°C) or higher and (Ms+30°C) or lower, cooling is performed at an average cooling rate CR1 of 10°C / s or higher; in the temperature range from the above-mentioned slow cooling start temperature T1 to the slow cooling stop temperature T2 of (Ms-220°C) or higher and (Ms-100°C) or lower, cooling is performed at an average cooling rate CR2 of 1 to 10°C / s; in the temperature range from the above-mentioned slow cooling stop temperature T2 to the reheating holding temperature T3 of 300°C or higher and 450°C or lower, heating is performed at an average heating rate HR2 of 2°C / s or higher; the reheating holding temperature T3 is maintained for 20s or higher and 3000s or lower; and in the temperature range from the above-mentioned reheating holding temperature T3 to 50°C, cooling is performed at an average cooling rate CR3 of 0.1°C / s or higher.

[0116] In the present invention, the steelmaking process can be carried out according to a conventional method.

[0117] Hereinafter, the hot rolling process, the pickling process, the cold rolling process, and the annealing process will be described.

[0118] [Hot rolling process]

[0119] As a method for hot rolling a steel billet, there are a method of reheating a steel billet cooled to room temperature and then rolling it, a method of directly rolling a steel billet without heating it after continuous casting, and a method of heating the steel billet after continuous casting for a short time and then rolling it. The present invention uses any of the above methods to keep the steel billet at a steel billet heating temperature of more than 1100°C for more than 1800s and then perform hot finish rolling at a finishing temperature of more than 850°C. Then, the steel billet is cooled at an average cooling rate of more than 40°C / s in the temperature range from the finishing temperature to 650°C, and is coiled at a coiling temperature of less than 600°C, thereby producing a hot rolled steel sheet.

[0120] (Bill heating temperature: above 1100℃)

[0121] (Bill heating holding time: more than 1800s)

[0122] When the billet heating temperature is lower than 1100° C., inclusions such as MnS remain and hole expandability decreases. Therefore, the billet heating temperature is set to 1100° C. or higher. The billet heating temperature is preferably 1180° C., and more preferably 1200° C. or higher.

[0123] In addition, when the billet heating holding time is less than 1800 seconds, a large amount of inclusions such as MnS may remain, and the hole expandability may be reduced. Therefore, the billet heating holding time is set to 1800 seconds or more.

[0124] The upper limits of the slab heating temperature and the slab heating holding time are not specified, but from the viewpoint of production cost, the slab heating temperature is preferably 1300° C. or lower, and the slab heating holding time is preferably 3 hours or less.

[0125] (Finishing rolling temperature: above 850℃)

[0126] When the finishing rolling temperature is lower than 850°C, ferrite is generated during hot rolling, and the structure after rolling becomes non-uniform, so that the mechanical properties in the longitudinal direction of the coil after annealing may vary. Therefore, the finishing rolling temperature is set to 850°C or higher.

[0127] The upper limit is not particularly limited, but is preferably set to 950° C. or lower.

[0128] (Average cooling rate from finishing temperature to 650°C: 40°C / s or more)

[0129] When the average cooling rate from the finishing temperature to 650°C is less than 40°C / s, ferrite and pearlite are generated during cooling, and the hot rolled structure tends to become uneven. In this case, the grain size and the amount of retained austenite in the length direction of the coil after annealing fluctuate, resulting in fluctuations in strength and ductility. Therefore, the average cooling rate from the finishing temperature to 650°C is set to 40°C / s or more. The average cooling rate is preferably 60°C / s or more.

[0130] It should be noted that the average cooling rate here is "(finishing rolling temperature (° C.) - 650° C.) / cooling time from the finishing rolling temperature to 650° C. (seconds)".

[0131] (Coiling temperature: below 600℃)

[0132] When the coiling temperature exceeds 600°C, ferrite and pearlite are easily generated, and the hot-rolled structure becomes uneven due to the fluctuation of the coiling temperature in the length direction of the coil. In this case, the grain size in the length direction of the coil after annealing fluctuates, resulting in fluctuations in strength and ductility. Therefore, the coiling temperature is set to 600°C or less. The coiling temperature is preferably 550°C or less. There is no particular lower limit for the coiling temperature. When the coiling temperature is lower than 400°C, the hot-rolled structure is sometimes hardened due to the formation of martensite, and the cold rolling load is excessively increased. Therefore, the coiling temperature is preferably above 400°C.

[0133] Furthermore, after the hot rolling step, the hot rolled steel sheet may be subjected to a heating treatment as necessary from the viewpoint of reducing the cold rolling load.

[0134] [Pickling process]

[0135] After the hot rolling process, pickling can be performed to remove the oxide scale on the surface of the hot rolled sheet. The pickling treatment method is not particularly specified and can be performed according to a conventional method.

[0136] [Cold rolling process]

[0137] (Rolling rate (cold rolling rate): 30% or more)

[0138] From the viewpoint of promoting recrystallization in the annealing heating afterwards, stabilizing the material, the cold rolling rate (cumulative cold rolling rate) is set to more than 30%. The upper limit of the cold rolling rate is not particularly specified, and when it exceeds 95%, the cold rolling load sometimes increases excessively. Therefore, the cold rolling rate is preferably below 95%.

[0139] [Annealing process]

[0140] (Average heating rate HR1 from 700℃ to (Ac3-10℃): 0.5℃ / s or more)

[0141] When the average heating rate HR1 from 700°C to (Ac3-10°C) is less than 0.5°C / s, the enrichment of C from ferrite to austenite during heating progresses, and the C concentration distribution in the steel sheet deviates, so the material becomes uneven. In addition, when the C concentration distribution in the steel sheet deviates, the fluctuation of mechanical properties becomes larger due to the change of the cooling stop temperature and the reheating temperature in the length direction of the coil. Therefore, the average heating rate HR1 from 700°C to (Ac3-10°C) is set to 0.5°C / s or more. The average heating rate HR1 from 700°C to (Ac3-10°C) is preferably 1.0°C / s or more, and more preferably 1.5°C / s or more.

[0142] The average heating rate HR1 is preferably 50° C. / s or less, more preferably 20° C. / s or less.

[0143] It should be noted that the average heating rate HR1 is "(Ac3-10°C)-700°C) / heating time (seconds) from 700°C to (Ac3-10°C)".

[0144] (Annealing temperature: (Ac3-10℃) or above)

[0145] (Holding time (annealing time): more than 30s)

[0146] In order to control the area ratio of ferrite to a desired range, the annealing temperature is set to (Ac3-10° C.) or higher. When the annealing temperature is lower than (Ac3-10° C.), the desired stability of mechanical properties in the longitudinal direction of the coil may not be obtained.

[0147] The upper limit of the annealing temperature is not specified, but when it exceeds (Ac3+50°C), the austenite grain size may be significantly coarsened, and the balance between strength and ductility may be reduced.

[0148] Therefore, the annealing temperature is preferably not more than (Ac3+50°C).

[0149] When the holding time (annealing time) is less than 30 seconds, carbides are not dissolved and remain, and hole expansion and bendability are reduced. Therefore, the holding time is set to 30 seconds or more. The holding time is preferably 60 seconds or more.

[0150] It should be noted that Ac3 is calculated by the following formula. In the following formula, [element symbol] refers to the content (mass %) of each element. ("Leslie Iron and Steel Materials" (Maruzen Co., Ltd., published on May 31, 1985, page 273))

[0151] Ac3(℃)=910-203×[C] 1 / 2-15.2×[Ni]+44.7×[Si]+104×[V]+31.5×[Mo]+13.1×[W]-(30×[Mn]+11×[Cr]+20×[Cu]-700×[P]-400×[sol.Al]-120×[As]-400×[Ti])

[0152] (Average cooling rate CR1 from annealing temperature to slow cooling start temperature T1: 10°C / s or more)

[0153] (Slow cooling start temperature T1: martensitic transformation start temperature Ms±30°C ((Ms-30°C) or higher and (Ms+30°C) or lower))

[0154] When CR1 is less than 10°C / s, ferrite is excessively generated, and desired tempered martensite and lower bainite may not be obtained, and desired strength may not be obtained. Therefore, CR1 is set to 10°C / s or more. CR1 is preferably 15°C / s or more.

[0155] There is no upper limit for CR1, but excessive increase in the average cooling rate may promote uneven cooling in the coil length direction, resulting in reduced material uniformity in the coil length direction. Therefore, CR1 is preferably 100°C / s or less.

[0156] The average cooling rate CR1 is "(annealing temperature (° C.) - slow cooling start temperature (T1) (° C.)) / cooling time (seconds) from the annealing temperature to the slow cooling start temperature (T1)".

[0157] When T1 exceeds (Ms+30°C), ferrite and pearlite are sometimes excessively generated, and the desired tempered martensite and lower bainite cannot be obtained, and the desired strength cannot be obtained. In addition, when T1 exceeds (Ms+30°C), the desired hole expandability cannot be obtained. Therefore, T1 is set to (Ms+30°C) or less. T1 is preferably (Ms+20°C) or less, and more preferably (Ms+10°C) or less.

[0158] On the other hand, when T1 is lower than (Ms-30°C), the desired amount of retained austenite may not be obtained, and the desired ductility may not be obtained. In addition, when T1 is lower than (Ms-30°C), the desired stability of mechanical properties in the length direction of the coil may not be obtained. Therefore, T1 is set to (Ms-30°C) or higher. T1 is preferably (Ms-20°C) or higher, and more preferably (Ms-10°C) or higher.

[0159] It should be noted that the martensitic transformation start temperature Ms (°C) can be calculated as follows: using a Formaster testing machine, a cylindrical test piece (3 mm in diameter × 10 mm in height) is maintained at an annealing temperature above (Ac3-10°C), and then rapidly cooled using helium at a cooling rate of more than 30°C / s. The volume change at this time is measured, and the martensitic transformation start temperature Ms (°C) is calculated.

[0160] (Average cooling rate CR2 from slow cooling start temperature T1 to slow cooling stop temperature T2: 1 to 10°C / s)

[0161] (Slow cooling stop temperature T2: (Ms-220°C) or higher and (Ms-100°C) or lower)

[0162] By setting the average cooling rate CR2 from T1 to T2 to 10°C / s or less, the fluctuation of the cooling stop temperature is reduced, and the phase changes of martensite and lower bainite in the length direction of the coil become uniform, thereby suppressing the fluctuation of the mechanical properties in the length direction of the coil. In addition, by setting CR2 to 10°C / s or less, C is distributed from martensite and lower bainite to austenite during cooling, and austenite is stabilized. As a result, even if the reheating temperature fluctuates, the decomposition of retained austenite is suppressed, and the fluctuation of mechanical properties in the length direction of the coil is suppressed. Therefore, CR2 is set to 10°C / s or less. When CR2 is less than 1°C / s, the production line length increases and the manufacturing efficiency decreases, so CR2 is set to 1°C / s or more.

[0163] The average cooling rate CR2 is "(slow cooling start temperature T1 (° C.)−slow cooling stop temperature T2 (° C.)) / cooling time (seconds) from the slow cooling start temperature T1 to the slow cooling stop temperature T2".

[0164] When T2 is lower than (Ms-220°C), martensitic transformation proceeds excessively, the desired amount of retained austenite cannot be obtained, and ductility decreases. Therefore, T2 is set to (Ms-220°C) or higher. T2 is preferably (Ms-200°C) or higher, and more preferably (Ms-180°C) or higher.

[0165] On the other hand, when T2 exceeds (Ms-100°C), the distribution of C from martensite and lower bainite to austenite does not occur sufficiently during slow cooling, so decomposition of austenite occurs during the reheating and holding process, resulting in fluctuations in the mechanical properties in the longitudinal direction of the coil. Therefore, T2 is set to (Ms-100°C) or less.

[0166] (Average heating rate HR2 from slow cooling stop temperature T2 to reheating holding temperature T3: 2°C / s or more)

[0167] By heating from the slow cooling stop temperature T2 to the reheating holding temperature T3 in a short time, the precipitation of carbides can be suppressed and high ductility can be ensured. Therefore, the average heating rate HR2 is set to 2°C / s or more. HR2 is preferably 5°C / s or more, and more preferably 10°C / s or more. The upper limit of the average heating rate HR2 is not particularly limited, but there is a case where the higher the average heating rate HR2, the more difficult it is to maintain the uniformity of the steel plate temperature. Therefore, HR2 is preferably 50°C / s or less, and more preferably 20°C / s or less.

[0168] The average heating rate HR2 is "reheating holding temperature T3 (° C.) - slow cooling stop temperature T2 (° C.)) / heating time (seconds) from slow cooling stop temperature T2 to reheating holding temperature T3".

[0169] (Reheating holding temperature T3: 300°C or higher and 450°C or lower)

[0170] (Reheating holding time: 20s or more and 3000s or less)

[0171] Reheating and holding are performed to stabilize austenite due to C distribution. When the reheating and holding temperature is lower than 300°C, C distribution does not occur sufficiently, and the desired amount of retained austenite cannot be obtained, so there is a concern that ductility will decrease. Therefore, the reheating and holding temperature T3 is set to 300°C or higher. T3 is preferably 330°C or higher, and more preferably 350°C or higher.

[0172] On the other hand, when the reheating holding temperature T3 exceeds 450°C, a phase transformation from austenite to pearlite occurs, and the desired amount of retained austenite cannot be obtained, which may cause a decrease in ductility. In addition, when the reheating holding temperature T3 exceeds 450°C, the desired tensile strength cannot be obtained. Therefore, the reheating holding temperature T3 is set to 450°C or less. T3 is preferably 420°C or less.

[0173] In addition, when the reheating holding time (holding time (residence time) at the reheating holding temperature T3) is less than 20 seconds, sufficient C distribution does not occur, and the desired amount of retained austenite cannot be obtained. Therefore, the reheating holding time is set to 20 seconds or more. The reheating holding time is preferably 50 seconds or more, and more preferably 100 seconds or more.

[0174] The effect of C distribution by reheating and holding is saturated when it exceeds 3000 s, so the reheating and holding time is set to 3000 s or less. The reheating and holding time is preferably 1500 s or less, and more preferably 600 s or less.

[0175] (Average cooling rate CR3 from reheating holding temperature T3 to 50°C: 0.1°C / s or more)

[0176] When the average cooling rate CR3 from the reheating holding temperature T3 to 50°C is less than 0.1°C / s, there is a concern that ductility may be reduced due to softening and carbide precipitation caused by excessive tempering. Therefore, the average cooling rate CR3 from the reheating holding temperature T3 to 50°C is set to 0.1°C / s or more. CR3 is preferably 5°C / s or more, and more preferably 8°C / s or more.

[0177] CR3 is preferably 100° C. / s or less, more preferably 50° C. / s or less.

[0178] The average cooling rate CR3 is "(reheating holding temperature T3 (° C.) - 50° C.) / cooling time from the reheating holding temperature T3 to 50° C. (seconds)".

[0179] [Hot dip treatment]

[0180] In the present invention, in the annealing process, hot-dip treatment may be performed during cooling from the annealing temperature to the slow cooling start temperature T1, or during reheating and holding at the reheating holding temperature T3. The hot-dip treatment may be a hot-dip galvanizing treatment. When hot-dip galvanizing treatment is performed, it is preferred that the steel sheet be immersed in a zinc plating bath at 440°C or higher and 500°C or lower, hot-dip galvanizing treatment is performed, and then the coating adhesion amount is adjusted by gas wiping or the like. Hot-dip galvanizing preferably uses a zinc plating bath having an Al content of 0.10% or higher and 0.22% or lower.

[0181] In addition, the zinc coating may be subjected to alloying treatment after the hot-dip galvanizing treatment. When the zinc coating is subjected to alloying treatment, it is preferably conducted at a temperature within a range of 480° C. to 600° C. after immersion in the coating bath.

[0182] [Surface rolling]

[0183] In the present invention, from the viewpoint of stabilizing the press formability and improving the YS, the steel sheet after annealing may be subjected to surface temper rolling. The elongation is preferably set to 0.1% or more. In addition, the elongation is preferably set to 0.5% or less.

[0184] [Leveling machine correction]

[0185] In the present invention, the annealed steel sheet may be subjected to leveling machine correction in order to correct the sheet shape. The leveling machine correction method is not particularly specified and may be performed according to a conventional method.

[0186] [Plating treatment]

[0187] In the present invention, after the annealing step, surface treatment such as electroplating may be performed.

[0188] The thickness of the steel sheet of the present invention obtained as described above is preferably set to 0.5 mm or more. In addition, the thickness of the steel sheet of the present invention is preferably set to 2.0 mm or less.

[0189] In addition, the plate width is preferably set to 600 mm or more. In addition, the plate width is preferably set to 1700 mm or less.

[0190] The steel plate of the present invention is not particularly limited, and the plate length (length in the coil length direction) may be 100 m or more, and the plate thickness may be 4000 m or less.

[0191] Next, the member of the present invention and a method for producing the same will be described.

[0192] The component of the present invention is a component formed by subjecting the steel plate of the present invention to at least one of forming and joining. In addition, the method for manufacturing the component of the present invention includes the step of subjecting the steel plate of the present invention to at least one of forming and joining to form the component.

[0193] The tensile strength of the steel plate of the present invention is 1180 MPa or more, and the ductility and hole expansion are excellent, and the stability of the mechanical properties in the length direction of the coil is excellent. Therefore, the component obtained by using the steel plate of the present invention is also high-strength, ductility and hole expansion are excellent, and the stability of the mechanical properties in the length direction of the coil is excellent. In addition, if the component of the present invention is used, lightweight can be achieved. Therefore, the component of the present invention can be suitable for use in, for example, a vehicle body frame component. The component of the present invention also includes a welded joint.

[0194] The forming process can use general processing methods such as press working without limitation. In addition, the joining process can use general welding such as spot welding and arc welding, rivet joining, caulking joining, etc. without limitation.

[0195] Example

[0196] The present invention will be described in detail with reference to the Examples, but the scope of the invention is not limited to the Examples.

[0197] The steel slabs having the component composition shown in Table 1 were held at a slab heating temperature of 1210°C for 3000s, then hot rolled at a finishing temperature of 880°C, cooled at an average cooling rate of 65°C / s in a temperature range from the finishing temperature to 650°C, and coiled at a coiling temperature shown in Table 2 to produce a hot-rolled steel sheet having a thickness of 2.8 mm. The hot-rolled steel sheet was cold rolled at a reduction ratio of 50% to produce a cold-rolled steel sheet having a thickness of 1.4 mm and a total length of 1500 m.

[0198] Then, the cold-rolled steel sheets were annealed under the conditions shown in Table 2. In the annealing conditions, the average heating rate HR1 in heating from 700° C. to (Ac3-10° C.) was set to 2.0° C. / s.

[0199] In addition, the surface of the steel sheet No. 11 was subjected to electrogalvanizing (EG), and the surface of the steel sheet No. 12 was subjected to hot-dip galvanizing. In addition, in No. 12, an alloying treatment (GA) was performed at 510° C. for 10 seconds in order to make the coating layer an alloyed hot-dip galvanneal layer.

[0200]

[0201]

[0202] The steel structure was measured by the above method, and the measurement results are shown in Table 3.

[0203] The tensile strength (TS) and total elongation (EL) were evaluated according to JIS Z2241 (2011). JIS No. 5 tensile test pieces were made from the obtained steel sheets and subjected to tensile tests. The samples with a TS of 1180 MPa or more were judged to have excellent strength, and the samples with an EL of 11.0% or more were judged to have excellent ductility.

[0204] In addition, the hole expansion property was evaluated according to the Japan Iron and Steel Federation Standard JFST1001. After each obtained steel plate was cut into 100 mm × 100 mm, a hole with a diameter of 10 mm was punched out under the condition that the gap was 12% of the plate thickness. A punch with an inner diameter of 75 mm was used, and a 60° cone punch was pressed into the hole under a wrinkle-proof pressure of 88.2 kN. The hole diameter at the limit of crack generation was measured, and the limit hole expansion rate λ (%) was calculated according to formula (1). The evaluation was performed, and the sample with λ of 40% or more was judged to have excellent hole expansion property.

[0205] Limit hole expansion ratio λ(%)={(Df-D0) / D0}×100 …(1)

[0206] Here, Df is the hole diameter (mm) when cracks occur, and D0 is the initial hole diameter (mm).

[0207] The stability of the mechanical properties in the length direction of the coil is evaluated as follows. First, as a JIS No. 5 tensile test piece in a direction parallel to the rolling direction, including a test piece cut at a position 10m away from the front and rear ends of the coil, test pieces located between these test pieces are cut at equal intervals in the length direction of the coil, and a total of 20 pieces are cut. Then, the above-mentioned tensile test is performed on these 20 test pieces, and the standard deviations of TS and EL are calculated and evaluated. The sample with a standard deviation of TS in the length direction of the coil of less than 30MPa is judged to have excellent stability of the mechanical properties in the length direction of the coil. The standard deviation of EL in the length direction of the coil is not specifically specified, but it is judged that the stability of the mechanical properties in the length direction of the coil is better when it is less than 1.5%.

[0208]

[0209] The examples of the present invention shown in Tables 2 and 3 are excellent in strength, ductility, hole expansion and stability of mechanical properties, while the comparative examples are inferior in one or more items. In addition, in the examples of the present invention, the standard deviation of TS in the longitudinal direction of the coil can be made 30 MPa or less, and the standard deviation of EL in the longitudinal direction of the coil can be made 1.5% or less.

[0210] In addition, it can be seen that since the steel plate of the example of the present invention has high strength and excellent ductility, hole expandability and stability of mechanical properties in the length direction of the coil, the components obtained by forming the steel plate of the example of the present invention, the components obtained by joining, and the components obtained by forming and joining are high strength and excellent in ductility, hole expandability and stability of mechanical properties in the length direction of the coil, just like the steel plate of the example of the present invention.

Claims

1. A steel sheet having a composition comprising, by mass%, C: 0.08-0.35%, Si: 0.4-3.0%, Mn: 1.5-3.5%, P: 0.02% or less, S: 0.01% or less, sol.Al: 1.0% or less, N: 0.015% or less, and the balance being Fe and inevitable impurities, The steel structure has an area ratio of ferrite of 5% or less (including 0%), a total area ratio of tempered martensite and lower bainite of 70% or more, a volume ratio of retained austenite of 5 to 15%, and an area ratio of fresh martensite of 10% or less (including 0%), The standard deviation of the tensile strength TS of the steel plate in the coil longitudinal direction is 30 MPa or less.

2. The steel plate according to claim 1, wherein: The component composition contains, in mass %, one or more selected from the group consisting of B: 0.01% or less, Ti: 0.1% or less, Cu: 1% or less, Ni: 1% or less, Cr: 1.5% or less, Mo: 1.0% or less, V: 0.5% or less, Nb: 0.1% or less, Zr: 0.2% or less, and W: 0.2% or less.

3. The steel plate according to claim 1 or 2, wherein: The component composition contains, in mass %, one or more selected from the group consisting of Ca: 0.0040% or less, Ce: 0.0040% or less, La: 0.0040% or less, Mg: 0.0040% or less, Sb: 0.1% or less, and Sn: 0.1% or less.

4. The steel plate according to any one of claims 1 to 3, wherein The steel plate has a coating on its surface. 5 . A member formed using the steel plate according to claim 1 .

6. A method for manufacturing a steel plate, comprising: A hot rolling process, wherein a steel slab having the composition described in any one of claims 1 to 3 is held at a steel slab heating temperature of 1100°C or higher for more than 1800 seconds, then hot finish rolling is performed at a finish rolling temperature of 850°C or higher, cooled at an average cooling rate of 40°C / s or higher in a temperature range from the finish rolling temperature to 650°C, and coiled at a coiling temperature of 600°C or lower, thereby producing a hot rolled steel sheet; a cold rolling step, wherein the hot-rolled steel sheet is cold-rolled at a rolling ratio of 30% or more to form a cold-rolled steel sheet; and The annealing step comprises heating the cold-rolled steel sheet at an average heating rate HR1 of 0.5°C / s or more in a temperature range from 700°C to (Ac3-10°C), then maintaining the cold-rolled steel sheet at an annealing temperature of (Ac3-10°C) or more for 30 seconds or more, cooling the cold-rolled steel sheet at an average cooling rate CR1 of 10°C / s or more in a temperature range from the annealing temperature to a slow cooling start temperature T1 of (Ms-30°C) or more and (Ms+30°C) or less, and cooling the cold-rolled steel sheet at an average cooling rate CR1 of 10°C / s or more in a temperature range from the slow cooling start temperature T1 to a slow cooling start temperature T1 of (Ms-220°C) or more and (Ms- The present invention relates to a method for cooling the reheated carbon steel sheet at an average cooling rate CR2 of 1 to 10°C / s in a temperature range from the slow cooling stop temperature T2 to a slow cooling stop temperature T2 below 100°C, heating at an average heating rate HR2 of 2°C / s or more in a temperature range from the slow cooling stop temperature T2 to a reheating holding temperature T3 of 300°C to 450°C, maintaining the reheating holding temperature T3 for 20s to 3000s, and cooling at an average cooling rate CR3 of 0.1°C / s or more in a temperature range from the reheating holding temperature T3 to 50°C.

7. The method for manufacturing a steel plate according to claim 6, wherein: In the annealing step, hot-dip treatment or alloying hot-dip treatment is performed during cooling from the annealing temperature to the slow cooling start temperature T1 or during reheating and holding at the reheating and holding temperature T3.

8. The method for manufacturing a steel plate according to claim 6, wherein: After the annealing process, electroplating is performed. 9 . A method for manufacturing a member, comprising the step of subjecting the steel sheet according to claim 1 to at least one of a forming process and a joining process to produce a member.

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