Steel plate, member and manufacturing method thereof

Abstract

Provided are: a steel sheet having high strength, excellent in terms of ductility and hole expansibility, and having highly stable mechanical properties along the sheet width direction; a member; and methods for producing the steel sheet and the member.　The steel sheet has a composition containing, in terms of mass%, 0.08-0.35% C, 0.4-3.0% Si, 1.5-3.5% Mn, up to 0.02% P, up to 0.01% S, up to 1.0% sol. Al, and up to 0.015% N, the remainder comprising Fe and unavoidable impurities, and has a steel structure in which the areal content of ferrite is 5% or less (including 0%), the total areal content of tempered martensite and lower bainite is 70% or higher, the volume content of retained austenite is 5-15%, and the areal content of fresh martensite is 10% or less (including 0%). The steel sheet has a standard deviation of sheet-width-direction total elongation (EL) of 0.9% 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 to methods for manufacturing the same. [Background technology]

[0002] In recent years, automobile components have become stronger in order to reduce the weight of automobile bodies, and high-strength steel sheets with a tensile strength (TS) of 1180 MPa or more are being used in automobile frame parts and seat parts. In general, the ductility and hole expandability of steel sheets decrease as the strength increases, so steel sheets with a TS of 1180 MPa or more are more likely to crack during press forming.

[0003] In order to obtain excellent press formability in high-strength steel sheets with a TS of 1180 MPa or more, it is important to improve hole expandability by making the steel sheet structure uniformly tempered martensite, and to improve ductility by finely dispersing retained austenite. Since complex heat treatment is required to achieve this steel sheet structure, it is industrially difficult to suppress temperature variation in the steel sheet's width direction, and there is concern that the stability of press forming will decrease due to variation in ductility in the width direction.

[0004] Patent Document 1 discloses a high-strength cold-rolled steel sheet having excellent workability and impact resistance, which contains, by mass%, 0.05-0.3% C, 0.3-2.5% Si, 0.5-3.5% Mn, 0.003-0.100% P, 0.02% or less S, and 0.010-0.5% Al, with 20% or more ferrite, 10-60% tempered martensite, 0-10% martensite, and 3-15% retained austenite, and in which the average crystal grain size of the low-temperature transformation phase consisting of martensite, tempered martensite, and retained austenite is 3 μm or less. The technology described in Patent Document 1 utilizes a process called Q&P;Quenching&Partitioning (quenching and partitioning of carbon from martensite to austenite), in which the material is cooled to a temperature range between the martensite transformation start temperature (Ms) and the martensite transformation completion temperature (Mf) during the cooling process, and then reheated and held to stabilize the residual γ. In recent years, progress has been made in the development of high-strength steels with excellent ductility and stretch flange formability that utilize this process, and methods for manufacturing such steels.

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

[0006] In Patent Document 3, 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) two-phase region, and then, using the martensite transformation start temperature Ms as an index, a target cooling stop temperature is set to a temperature region below Ms and above (Ms-150°C). This publication discloses a method for producing a high-strength steel plate having excellent workability, tensile strength (TS) and stability of mechanical properties, in which a coldest portion of the steel plate in the width direction is held in a temperature range from a target cooling stop temperature to (cooling stop temperature + 15°C) for a time of 15 seconds to 100 seconds. The method involves producing a high-strength steel plate having excellent workability, tensile strength (TS) and stability of mechanical properties, and in which a part of the untransformed austenite is transformed into martensite by heating the plate at a predetermined temperature and then cooled, and the martensite is transformed into martensite by heating the plate at a predetermined temperature and then tempered. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 5463685 [Patent Document 2] Patent No. 5402007 [Patent Document 3] Patent No. 5333298 Summary of the Invention [Problem to be solved by the invention]

[0008] The above-mentioned conventional techniques each have the following problems.

[0009] Patent Document 1 proposes a steel sheet having excellent strength, ductility and hole expandability, with a product of tensile strength and total elongation (TS × El) of 22,000 MPa·% or more and a λ of 70% or more. However, the stability of the mechanical properties in the sheet width direction is not taken into consideration, and there is a possibility that the mechanical properties will vary due to temperature unevenness in the steel sheet width direction during the continuous annealing process.

[0010] Patent Document 2 proposes a steel sheet with excellent workability, in which TS is 1200 MPa or more, λ is 50% or more, and El is 13% or more. However, the variation in ductility in the sheet width direction is not taken into consideration, and the cooling rate during annealing is as fast as 20°C / s or more, so there is a high possibility that the mechanical properties will vary due to temperature unevenness in the steel sheet width direction.

[0011] In addition, Patent Document 3 proposes a method for producing a high-strength steel plate, which is characterized in that, after heating to an austenite single-phase region or an (austenite + ferrite) two-phase region, the steel plate is cooled with a target cooling stop temperature set in a temperature range below Ms and above (Ms-150°C), and the coldest part of the steel plate in the plate width direction is kept in a temperature range from the target cooling stop temperature to (cooling stop temperature + 15°C) for a time of 15 seconds to 100 seconds. This makes it possible to produce a high-strength steel plate with excellent stability of mechanical properties in the plate width direction, in which the standard deviation of tensile strength in the plate width direction is 10 MPa or less and the standard deviation of El in the plate width direction is 2.0%, but the standard deviation of El in the plate width direction is not necessarily sufficient to obtain excellent press formability, and there is room for improvement. In addition, in order to control the coldest part of the steel plate in the plate width direction to a temperature range from the cooling stop temperature to (cooling stop temperature + 15°C), special control is required, such as cooling while checking the temperature distribution of the steel plate, and the hurdle for implementation is high.

[0012] The present invention has been made to solve such problems, and aims to provide a steel plate, a component, and a method for manufacturing the same that are high strength, have excellent ductility and hole expandability, and have excellent stability of mechanical properties in the plate width direction.

[0013] In the present invention, high strength refers to a tensile strength TS evaluated in accordance with JIS Z2241 (2011) of 1180 MPa or more. Excellent ductility refers to a total elongation (EL) of 11.0% or more evaluated in accordance with JIS Z2241 (2011). Excellent hole expandability refers to the following: a 10 mm diameter hole is punched into a 100 mm x 100 mm steel plate with a clearance of 12% of the plate thickness, and a 60° conical punch is pressed into the hole using a die with an inner diameter of 75 mm and a blank holding force of 88.2 kN to measure the hole diameter at the limit for crack initiation; where Df is the hole diameter at the time of crack initiation (mm) and D0 is the initial hole diameter (mm), the limit hole expansion ratio λ (%) = {(Df-D0) / D0} x 100 is 40% or more. Excellent stability of mechanical properties in the plate width direction refers to a standard deviation of 0.9% or less in total elongation (EL) evaluated in accordance with JIS Z2241 (2011) using 20 JIS No. 5 tensile test pieces taken at equal intervals in the plate width direction, including both ends of the plate, parallel to the rolling direction. [Means for solving the problem]

[0014] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that it is possible to significantly reduce the variation in mechanical properties in the sheet width direction and improve the stability of press forming by homogenizing the structure of the hot-rolled sheet in the sheet width direction through temperature control during coiling and slowly cooling the sheet from near the Ms point to the cooling stop temperature in the annealing process.

[0015] More specifically, the present invention provides the following: [1] In mass percent, C: 0.08-0.35%, Si: 0.4 to 3.0%, Mn: 1.5-3.5%, P: 0.02% or less, S: 0.01% or less, Sol.Al: 1.0% or less, N: Contains 0.015% or less, The balance is composed of Fe and unavoidable impurities, Ferrite area ratio: 5% or less (including 0%), Total area ratio of tempered martensite and lower bainite: 70% or more; Volume fraction of retained austenite: 5 to 15%; The area ratio of fresh martensite is 10% or less (including 0%). A steel plate having a standard deviation of total elongation across the plate width of 0.9% or less. [2] The component composition is, 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 The steel sheet according to [1], containing one or more selected from the following: [3] The component composition is, 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 The steel sheet according to [1] or [2], containing one or more selected from the following: [4] The steel sheet according to any one of [1] to [3], which has a plating layer on the surface of the steel sheet. [5] A member made using the steel plate according to any one of [1] to [4]. [6] A steel slab having a chemical composition according to any one of [1] to [3], After holding the slab at a heating temperature of 1100℃ or higher for 1800s or more, Finish hot rolling is performed at a finishing rolling temperature of 850°C or higher. Cooling is performed in a temperature range from the finish rolling temperature to 650°C at an average cooling rate of 40°C / s or more, a hot rolling process in which the coiling temperature is set to 600°C or less and the maximum temperature difference between the coiling temperature and the temperature at the center of the sheet width in the sheet width direction during coiling is set to 50°C or less to produce a hot-rolled steel sheet; The hot-rolled steel sheet, a cold rolling process in which the steel sheet is cold-rolled at a rolling reduction rate of 30% or more to obtain a cold-rolled steel sheet; The cold-rolled steel sheet, After heating the temperature range from 700℃ to (Ac3-10℃) at an average heating rate of HR1 of 0.5℃ / s or more, (Ac3-10℃) or higher annealing temperature for 30 seconds or more. Cooling is performed 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 that is equal to or higher than (Ms-30°C) and equal to or lower than (Ms+30°C), Cooling is performed at an average cooling rate CR2 of 1 to 10°C / s in a temperature range from the annealing start temperature T1 to a annealing stop temperature T2 of (Ms-220°C) or more and (Ms-100°C) or less, The temperature range from the slow cooling stop temperature T2 to a reheating holding temperature T3 of 300°C or more and 450°C or less is heated at an average heating rate HR2 of 2°C / s or more, The reheating holding temperature T3 is held for 20 s or more and 3000 s or less, and an annealing step of cooling the steel sheet in a temperature range from the reheating holding temperature T3 to 50°C at an average cooling rate CR3 of 0.1°C / s or more. [7] The method for producing a steel sheet according to [6], wherein, in the annealing step, during cooling from the annealing temperature to the slow cooling start temperature T1, or during reheating and holding at the reheating holding temperature T3, a hot-dip galvanizing treatment or an alloying hot-dip galvanizing treatment is performed. [8] The method for producing a steel sheet according to [6], further comprising the steps of: performing an electroplating treatment after the annealing step. [9] A method for producing a component, comprising the step of subjecting the steel plate according to any one of [1] to [4] to at least one of forming and joining to produce a component. Effect of the Invention

[0016] According to the present invention, it is possible to provide a steel plate, a member, and a method for manufacturing the same, which have high strength, excellent ductility and hole expandability, and excellent stability of mechanical properties in the plate width direction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, an embodiment of the present invention will be described, however, the present invention is not limited to the following embodiment.

[0018] The steel sheet of the present invention contains, 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, with the balance being Fe and unavoidable impurities, and has a steel structure in which the area fraction of ferrite is 5% or less (including 0%), the total area fraction of tempered martensite and lower bainite is 70% or more, the volume fraction of retained austenite is 5-15%, and the area fraction of fresh martensite is 10% or less (including 0%), and the standard deviation of the total elongation (EL) in the sheet width direction is 0.9% or less.

[0019] First, the chemical composition of the steel sheet of the present invention will be described. In the following description of the composition of the components, the unit of content "%" means "mass %." Furthermore, in the present invention, "high strength" means that the tensile strength TS is 1180 MPa or more.

[0020] (C: 0.08~0.35%) C is contained to increase the strength of tempered martensite or lower bainite and ensure a TS of 1180 MPa or more. If the C content is less than 0.08%, the desired TS cannot be stably obtained, so 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. On the other hand, excessive addition of C leads to a decrease in hole expandability due to an increase in the number density of carbides, a decrease in ductility, and even a deterioration in the shape fixability 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.

[0021] (Si: 0.4 to 3.0%) Silicon improves the strength of steel sheet by solid solution strengthening, and furthermore, by suppressing the coarsening of carbides, suppresses the decrease in strength due to tempering. If the Si content is less than 0.4%, the desired TS may not be stably obtained, and the desired ductility may not be 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. On the other hand, excessive addition of Si leads to a significant decrease in chemical conversion treatability and plating property. 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.

[0022] (Mn: 1.5-3.5%) Mn is an element effective in improving hardenability. If the Mn content is less than 1.5%, ferrite or pearlite is formed excessively. As a result, tempered martensite and lower bainite are not obtained sufficiently, and the desired TS cannot be obtained, so the Mn content is set to 1.5% or more. The Mn content is preferably 2.0% or more, and more preferably 2.4% or more. On the other hand, excessive addition of Mn forms coarse MnS, which significantly reduces the hole expandability and bendability. Therefore, the Mn content is set to 3.5% or less. The Mn content is preferably 3.0% or less.

[0023] (P:0.02% or less) P is an effective element for strengthening steel, but excessive addition significantly reduces spot weldability. Therefore, the P content is set to 0.02% or less, and preferably 0.01% or less. Although there is no particular lower limit for the P content, a large amount of cost is required to make the P content less than 0.002%, so the P content is preferably 0.002% or more.

[0024] (S:0.01% or less) 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. Although there is no particular lower limit for the S content, a large amount of cost is required to make the S content less than 0.0002%, so the S content is preferably 0.0002% or more.

[0025] (sol.Al: 1.0% or less) Al is an element added as a deoxidizer in the steelmaking process. If the sol.Al content exceeds 1.0%, inclusions such as Al2O3 and AlN increase, reducing 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. Although there is no particular lower limit for the sol.Al content, in order to obtain a sufficient deoxidizing effect, the sol.Al content is preferably 0.001% or more, more preferably 0.010% or more, and further preferably 0.020% or more.

[0026] (N:0.015% or less) If N is added in excess, it generates a large amount of inclusions such as AlN, which reduces 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. Although there is no particular lower limit for the N content, the production cost increases significantly when the N content is less than 0.001%, so the N content is preferably 0.001% or more.

[0027] The composition of the steel sheet of the present invention contains the above-mentioned components as basic components, and the balance other than this composition contains iron (Fe) and inevitable impurities. In the steel sheet of the present invention, it is preferable that the balance has a composition consisting of Fe and inevitable impurities. Examples of unavoidable impurities include Zn, Co, etc., and in the present invention, even if these elements are contained within the range of a normal steel composition, the effect is not impaired.

[0028] In addition to these basic components, one or more elements selected from B, Ti, Cu, Ni, Cr, Mo, V, Nb, Zr, and W may be added as necessary in place of the iron (Fe) and part of the unavoidable impurities. Furthermore, one or more elements selected from Ca, Ce, La, Mg, Sb, and Sn may be added as necessary. Specifically, the chemical composition of the steel sheet of the present invention may appropriately contain the following (A) and / or (B) as optional elements. (A) one or more selected from, by 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 (B) one or more selected from, by 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

[0029] ([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) These elements may be added for the purpose of improving hardenability, refining crystal grains, and precipitation strengthening to stably obtain the desired TS. On the other hand, if added excessively, coarse precipitates are generated and hole expandability and bendability are deteriorated, so if B is contained, the B content is 0.01% or less, if Ti is contained, the Ti content is 0.1% or less, if Cu is contained, the Cu content is 1% or less, if Ni is contained, the Ni content is 1% or less, if Cr is contained, the Cr content is 1.5% or less, if Mo is contained, the Mo content is 1.0% or less, if V is contained, the V content is 0.5% or less, if Nb is contained, the Nb content is 0.1% or less, if Zr is contained, the Zr content is 0.2% or less, and if W is contained, the W content is 0.2% or less.

[0030] The B content is preferably 0.0050% or less, and more preferably 0.0030% or less. The B content is preferably 0.0003% or more. The Ti content is preferably 0.080% or less, and more preferably 0.050% or less. The Ti content is preferably 0.001% or more, and more preferably 0.010% or more. The Cu content is preferably 0.50% or less, and more preferably 0.20% or less. The Cu content is preferably 0.001% or more, and more preferably 0.030% or more. The Ni content is preferably 0.50% or less, more preferably 0.20% or less. The Ni content is preferably 0.001% or more, more preferably 0.030% or more. The Cr content is preferably 1.2% or less, and more preferably 1.0% or less. The Cr content is preferably 0.001% or more, and more preferably 0.200% or more. The Mo content is preferably 0.50% or less, and more preferably 0.20% or less. The Mo content is preferably 0.001% or more, and more preferably 0.010% or more. The V content is preferably 0.50% or less, and more preferably 0.20% or less. The V content is preferably 0.001% or more, and more preferably 0.010% or more. The Nb content is preferably 0.08% or less, more preferably 0.05% or less. The Nb content is preferably 0.001% or more, more preferably 0.010% or more. The Zr content is preferably 0.1% or less, and more preferably 0.05% or less. The Zr content is preferably 0.001% or more, and more preferably 0.010% or more. The W content is preferably 0.1% or less, more preferably 0.05% or less, and further preferably 0.03% or less. Moreover, the W content is preferably 0.001% or more, and more preferably 0.005% or more.

[0031] ([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) These elements may be added for the purpose of improving hole expandability and bendability by controlling inclusions. When the amount of addition exceeds a certain amount, the effect is saturated, so when Ca is contained, the Ca content is 0.0040% or less, when Ce is contained, the Ce content is 0.0040% or less, when La is contained, the La content is 0.0040% or less, when Mg is contained, the Mg content is 0.0040% or less, when Sb is contained, the Sb content is 0.1% or less, and when Sn is contained, the Sn content is 0.1% or less.

[0032] The Ca content is preferably 0.0030% or less. Also, the Ca content is preferably 0.0003% or more. The Ce content is preferably 0.0030% or less. Also, the Ce content is preferably 0.0003% or more. The La content is preferably 0.0030% or less. Also, the La content is preferably 0.0003% or more. The La content is further preferably 0.0010% or more. The Mg content is preferably 0.0030% or less. Also, the Mg content is preferably 0.0003% or more. The Sb content is preferably 0.05% or less, more preferably 0.02% or less, and more preferably 0.0003% or more. The Sb content is further preferably 0.0020% or more. The Sn content is preferably 0.05% or less, more preferably 0.02% or less, and is preferably 0.0003% or more. The Sn content is further preferably 0.0020% or more.

[0033] When the optional components are contained in amounts less than the lower limit, the optional elements contained in amounts less than the lower limit do not impair the effects of the present invention. When the optional elements are contained in amounts less than the lower limit, the optional elements are considered to be contained as unavoidable impurities.

[0034] Next, the structure (microstructure) of the steel sheet of the present invention will be described.

[0035] (Ferrite area ratio: 5% or less (including 0%)) Although ferrite contributes to improving ductility, due to the difference in hardness between ferrite and hard phases such as tempered martensite, it becomes the origin of voids during punching and press forming, degrading press formability. If the area ratio of ferrite exceeds 5%, the deterioration of press formability becomes significant. In addition, if the area ratio of ferrite exceeds 5%, the desired TS cannot be obtained, and the desired stability of mechanical properties in the plate width direction cannot be obtained. Therefore, the area ratio of ferrite is set to 5% or less, preferably 3% or less, and more preferably 0%.

[0036] (Total area ratio of tempered martensite and lower bainite: 70% or more) 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. Although tempered martensite and lower bainite have different transformation timings, they are low-temperature transformation products and have similar effects on mechanical properties, so they are evaluated based on the total area ratio. Although there is no particular upper limit, the total area ratio of tempered martensite and lower bainite is preferably 95% or less, and more preferably 93% or less.

[0037] (Volume fraction of retained austenite: 5-15%) Retained austenite contributes to improving uniform elongation through the TRIP effect. To obtain the desired ductility, the volume fraction of retained austenite must be 5% or more. By making the volume fraction of retained austenite 5% or more, the desired stability of mechanical properties in the sheet width direction can also be obtained. The volume fraction of retained austenite is preferably 7% or more, and more preferably 9% or more. On the other hand, if retained austenite is generated excessively, the hole expandability decreases, so the volume fraction of retained austenite is set to 15% or less.

[0038] (Area ratio of fresh martensite: 10% or less (including 0%)) Fresh martensite is very hard and can become the starting point of cracks during press forming. If the area ratio of fresh martensite exceeds 10%, the desired ductility cannot be obtained, and the desired stability of mechanical properties in the plate width direction cannot be obtained. Therefore, from the viewpoints of suppressing cracking, improving ductility, and furthermore, stability of mechanical properties in the sheet width direction, the area ratio of fresh martensite is set to 10% or less, preferably 5% or less, and more preferably 3% or less. Note that the area ratio of fresh martensite may be 0%.

[0039] In the present invention, one or more of upper bainite, pearlite, etc. may be generated as the remaining structure other than the above-mentioned ferrite, tempered martensite, lower bainite, retained austenite, and fresh martensite, but 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. The remaining structure of pearlite, upper bainite, etc. is preferably 5% or less in total.

[0040] The steel sheet of the present invention may have a plating layer on the surface of the steel sheet. The type of the plating layer is not particularly limited, but may be a zinc plating layer, for example, an electrolytic zinc plating layer, a hot-dip galvanizing layer, or an alloyed hot-dip galvanizing layer.

[0041] Next, a method for measuring the microstructure of a steel sheet will be described.

[0042] The area ratios of ferrite, tempered martensite, lower bainite, and fresh martensite are measured by cutting out a plate cross section parallel to the rolling direction, mirror-polishing it, and then etching it with 1 vol% nital. Using an SEM, 10 fields of view are observed at 5000x magnification at the 1 / 4 thickness position, and the area ratios are measured by the point count method (based on ASTM E562-83 (1988)). In the above observations, ferrite is the region that appears blackest under the SEM, and is an equiaxed region with almost no carbides inside. Tempered martensite and lower bainite are the regions that appear gray under the SEM, and are regions where lath-shaped substructures and carbide precipitation are observed. Fresh martensite is the region that appears white and lumpy under the SEM, and is a region where no substructures are observed inside.

[0043] The volume fraction of retained austenite is determined by X-ray diffraction using steel plates that have been mechanically ground and polished with oxalic acid to a thickness of 100 μm or more so that the measurement position is 1 / 4 of the plate thickness direction. A Co-Kα source is used for the incident X-rays, and the volume fraction of retained austenite is calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite. Here, since the retained austenite is randomly distributed, the volume fraction of retained austenite determined by X-ray diffraction is equal to the area fraction.

[0044] The steel sheet of the present invention has a tensile strength TS of 1180 MPa or more as evaluated in accordance with JIS Z2241 (2011), and thus has high strength. Moreover, the steel sheet of the present invention has a total elongation (EL) evaluated in accordance with JIS Z2241 (2011) of 11.0% or more, and is excellent in ductility. Furthermore, in the steel plate of the present invention, a hole of 10 mm in diameter is punched into a steel plate of 100 mm x 100 mm with a clearance of 12% of the plate thickness, and a 60° conical punch is pressed into the hole using a die of an inner diameter of 75 mm with a blank holding force of 88.2 kN to measure the hole diameter at the limit of crack initiation. Where Df is the hole diameter at the time of crack initiation (mm) and D0 is the initial hole diameter (mm), the limit hole expansion ratio λ (%) = {(Df - D0) / D0} x 100 is 40% or more, and the steel plate has excellent hole expandability. In addition, the steel sheet of the present invention has a standard deviation of total elongation (EL) of 0.9% or less when evaluated in accordance with JIS Z2241 (2011) using 20 JIS No. 5 tensile test pieces taken at equal intervals in the sheet width direction, including both ends of the sheet width, parallel to the rolling direction, and has excellent stability of mechanical properties in the sheet width direction. Furthermore, the steel sheet of the present invention may have a standard deviation of the tensile strength TS in the sheet width direction of 15.0 MPa or less.

[0045] Next, the manufacturing method of the steel plate of the present invention will be described. Note that the temperatures when heating or cooling the steel slab (steel material), steel plate, etc. shown below refer to the surface temperatures of the steel slab (steel material), steel plate, etc., unless otherwise specified. The method for producing a steel sheet of the present invention includes a hot rolling step in which a steel slab having the above-mentioned composition is held at a slab heating temperature of 1100°C or more for 1800s or more, followed by finish hot rolling at a finish rolling temperature of 850°C or more, cooling at an average cooling rate of 40°C / s or more in the temperature range from the finish rolling temperature to 650°C, setting the coiling temperature to 600°C or less, and coiling under conditions in which the maximum temperature difference with the temperature at the center of the sheet width in the sheet width direction during coiling is 50°C or less to obtain a hot rolled steel sheet, a cold rolling step in which the above hot rolled steel sheet is cold rolled at a rolling reduction rate of 30% or more to obtain a cold rolled steel sheet, and a heating step in which the above cold rolled steel sheet is heated at an average heating rate of HR1 of 0.5°C / s or more in the temperature range from 700°C to (Ac3-10°C), followed by annealing at an annealing temperature of (Ac3-10°C) or more for 30 minutes. and an annealing process of cooling the annealed 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, cooling the annealed steel sheet at an average cooling rate CR2 of 1 to 10°C / s in a temperature range from the slow cooling start temperature T1 to a slow cooling stop temperature T2 of (Ms-220°C) or more and (Ms-100°C) or less, heating the annealed 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 or more and 450°C or less, holding the annealing temperature T3 at 20 s or more and 3000 s or less, and cooling the annealed 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.

[0046] In the present invention, the steelmaking process can be carried out according to a conventional method. The hot rolling process, the pickling process, the cold rolling process, and the annealing process will be described below.

[0047] [Hot rolling process] Methods for hot rolling a steel slab include a method of reheating a steel slab cooled to room temperature and then rolling it, a method of directly rolling a steel slab after continuous casting without heating it, and a method of rolling a steel slab after continuous casting by subjecting it to a short-term heat treatment. In the present invention, the steel slab is held at a slab heating temperature of 1100°C or higher for 1800s or more by any of the above methods, and then finish hot rolling is performed at a finish rolling temperature of 850°C or higher. Then, the steel slab is cooled at an average cooling rate of 40°C / s or higher in the temperature range from the finish rolling temperature to 650°C, and coiled under conditions of a coiling temperature of 600°C or lower and a maximum temperature difference in the sheet width direction during coiling of 50°C or lower to obtain a hot-rolled steel sheet.

[0048] (Slab heating temperature: 1100℃ or higher) (Slab heating hold time: 1800s or more) If the slab heating temperature is less than 1100° C., inclusions such as MnS remain and the hole expandability decreases. Therefore, the slab heating temperature is set to 1100° C. or higher. The slab heating temperature is preferably 1180° C., and more preferably 1200° C. or higher. Similarly, when the slab heating holding time is less than 1800 seconds, a large amount of inclusions such as MnS remains, and the hole expandability is reduced. Therefore, the slab heating holding time is set to 1800 seconds or more. Although there are no upper limits for the slab heating temperature and the slab heating holding time, from the viewpoint of production costs, the slab heating temperature is preferably 1300° C. or less, and the slab heating holding time is preferably 3 hours or less.

[0049] (Finish rolling temperature: 850℃ or higher) If the finish rolling temperature is less than 850°C, ferrite will form during hot rolling, resulting in a non-uniform structure after rolling, which may lead to variations in the mechanical properties in the sheet width direction after annealing. Therefore, the finish rolling temperature is set to 850°C or higher. There is no particular upper limit, but it is preferably 950° C. or lower.

[0050] (Average cooling rate from finish rolling temperature to 650℃: 40℃ / s or more) If the average cooling rate from the finish rolling temperature to 650°C is less than 40°C / s, ferrite and pearlite are generated during cooling, and the hot-rolled structure is likely to become non-uniform. In this case, the grain size in the sheet width direction after annealing varies, which causes variations in strength and ductility. Therefore, the average cooling rate from the finish rolling temperature to 650°C is set to 40°C / s or more. This average cooling rate is preferably 60°C / s or more. The average cooling rate here is "(finish rolling temperature (°C)-650°C) / cooling time from the finish rolling temperature to 650°C (seconds)".

[0051] (Wind-up temperature: 600℃ or less) (Maximum temperature difference between the center of the strip width and the strip width direction during winding: 50°C or less) If the coiling temperature exceeds 600°C, ferrite and pearlite are likely to be generated, and even if the maximum temperature difference in the sheet width direction during coiling is small, the hot-rolled structure may become non-uniform. In this case, the grain size in the sheet width direction after annealing varies, which causes variations in strength and ductility. Therefore, the coiling temperature is set to 600°C or less. The coiling temperature is preferably 550°C or less. Although the lower limit of the coiling temperature is not particularly specified, if the coiling temperature is less than 400°C, the hot-rolled structure may become hard due to the generation of martensite, and the cold rolling load may increase excessively. Therefore, the coiling temperature is preferably 400°C or more. Here, the maximum temperature difference is the maximum value of the temperature difference between the temperature at the center of the sheet width and any position in the sheet width direction, i.e., the maximum temperature difference refers to the difference between the temperature at the center of the sheet width and the lowest temperature in the sheet width direction.

[0052] Furthermore, even if the coiling temperature is 600°C or less, if the maximum temperature difference in the sheet width direction during coiling exceeds 50°C, the difference in the structure morphology in the sheet width direction becomes large, which causes variations in strength and ductility in the sheet width direction after annealing. Therefore, the maximum coiling temperature difference in the sheet width direction is set to 50°C or less. The maximum coiling temperature difference in the sheet width direction is preferably 30°C or less, and more preferably 20°C or less.

[0053] Furthermore, after the hot rolling step, the hot-rolled steel sheet may be subjected to a heat treatment as necessary in order to reduce the cold rolling load.

[0054] [Pickling process] After the hot rolling step, pickling may be carried out to remove scale from the surface layer of the hot-rolled sheet. The pickling method is not particularly limited, and may be carried out according to a conventional method.

[0055] [Cold rolling process] (Rolling ratio (cold rolling ratio): 30% or more) The cold rolling reduction (cumulative cold rolling reduction) is set to 30% or more from the viewpoint of controlling the recrystallization behavior in the subsequent annealing and stabilizing the material. Although there is no particular upper limit for the cold rolling reduction, if it exceeds 95%, the cold rolling load may increase excessively. Therefore, the cold rolling reduction is preferably 95% or less.

[0056] [Annealing process] (Average heating rate HR1 from 700℃ to (Ac3-10℃): 0.5℃ / s or more) If the average heating rate HR1 from 700°C to (Ac3-10°C) is less than 0.5°C / s, C concentration from ferrite to austenite progresses during heating, and the C concentration distribution in the steel sheet becomes uneven, resulting in non-uniform material properties. In addition, if the C concentration distribution in the steel sheet becomes uneven, the mechanical properties will vary even more due to variations in the cooling stop temperature and reheating temperature in the sheet width direction. 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. The average heating rate HR1 is preferably 50° C. / s or less, and more preferably 20° C. / s or less. The average heating rate HR1 is "(Ac3-10°C)-700°C) / heating time (seconds) from 700°C to (Ac3-10°C)."

[0057] (Annealing temperature: (Ac3-10℃) or higher) (Holding time (annealing time): 30 seconds or more) In order to control the area ratio of ferrite within a desired range, the annealing temperature is set to (Ac3-10°C) or higher. Although there is no upper limit for the annealing temperature, if it exceeds (Ac3+50°C), the austenite grain size may significantly coarsen, and the balance between strength and ductility may deteriorate. Therefore, the annealing temperature is preferably (Ac3+50°C) or lower.

[0058] If the holding time (annealing time) is less than 30 seconds, the carbides remain in an undissolved state, and the hole expandability and bendability may decrease. If the holding time (annealing time) is less than 30 seconds, the desired stability of the mechanical properties in the sheet width direction cannot be obtained. Therefore, the holding time is set to 30 seconds or more. The holding time is preferably 60 seconds or more.

[0059] Ac3 is calculated by the following formula. In the formula, the [element symbol] means the content (mass%) of each element. (Leslie Steel Materials Science, Maruzen Co., Ltd., published May 31, 1985, p. 273) 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])

[0060] (Average cooling rate CR1 from annealing temperature to slow cooling start temperature T1: 10℃ / s or more) (Slow cooling start temperature T1: Martensitic transformation start temperature Ms±30℃ ((Ms-30℃) or higher, (Ms+30℃) or lower)) If CR1 is less than 10°C / s, ferrite and pearlite are excessively generated. As a result, the desired tempered martensite and lower bainite may not be obtained, and the desired strength may not be obtained. In addition, the desired hole expandability may not be obtained, and the desired stability of mechanical properties in the plate width direction may not be obtained. Therefore, CR1 is set to 10°C / s or more. CR1 is preferably 15°C / s or more. Although there is no upper limit for CR1, an excessive increase in the average cooling rate may promote uneven cooling in the sheet width direction, which may lead to a decrease in material uniformity in the sheet width direction. Therefore, CR1 is preferably 1000°C / s or less, and more preferably 100°C / s or less. 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".

[0061] When T1 exceeds (Ms+30°C), ferrite and pearlite are also excessively generated. As a result, the desired tempered martensite and lower bainite are not obtained, and the desired strength may not be obtained. When T1 exceeds (Ms+30°C), the area ratio of fresh martensite exceeds 10%, and the desired ductility is not obtained, and furthermore, the desired stability of mechanical properties in the sheet width direction is not 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. On the other hand, if T1 is less than (Ms-30°C), the desired amount of retained austenite may not be obtained, and the desired ductility may not be obtained. In addition, if T1 is less than (Ms-30°C), the desired stability of mechanical properties in the sheet width direction may not be obtained. Therefore, T1 is set to be equal to or greater than (Ms-30° C.). T1 is preferably equal to or greater than (Ms-20° C.), and more preferably equal to or greater than (Ms-10° C.).

[0062] The martensitic transformation start temperature Ms (°C) can be determined by using a Formaster testing machine to measure the volume change when a cylindrical test piece (diameter 3 mm x height 10 mm) is held at an annealing temperature of (Ac3-10°C) or higher and then quenched at a cooling rate of 30°C / s or higher using helium gas.

[0063] (Average cooling rate CR2 from the annealing start temperature T1 to the annealing stop temperature T2: 1-10℃ / s) (Slow cooling stop temperature T2: (Ms-220℃) or higher, (Ms-100℃) or lower) By setting the average cooling rate CR2 from T1 to T2 to 10°C / s or less, the temperature variation in the plate width direction caused by the heat generated by transformation to martensite and lower bainite is reduced, and the transformation amount of martensite and lower bainite in the plate width direction is uniform, thereby suppressing the variation in the mechanical properties in the plate width direction. In addition, by setting CR2 to 10°C / s or less, carbon is distributed from martensite and lower bainite to austenite during cooling, stabilizing the austenite. As a result, even if the edge of the steel plate is overheated during reheating, the decomposition of the retained austenite is suppressed, and the variation in the mechanical properties in the plate width direction including the edge of the steel plate is suppressed. Therefore, CR2 is set to 10°C / s or less. If CR2 is less than 1°C / s, the line length increases and the production efficiency decreases, so CR2 is set to 1°C / s or more. 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."

[0064] If T2 is less 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 be (Ms-220°C) or more. T2 is preferably (Ms-200°C) or more, and more preferably (Ms-180°C) or more. On the other hand, if T2 exceeds (Ms-100°C), carbon does not sufficiently distribute from martensite and lower bainite to austenite during slow cooling, which can cause decomposition of austenite during the reheating and holding process, resulting in variations in the mechanical properties in the sheet width direction. Furthermore, if T2 exceeds (Ms-100°C), the area ratio of fresh martensite exceeds 10%, and the desired ductility cannot be obtained, and furthermore, the desired stability of mechanical properties in the sheet width direction cannot be obtained. Therefore, T2 is set to (Ms-100°C) or less.

[0065] (Average heating rate HR2 from slow cooling stop temperature T2 to reheating holding temperature T3: 2℃ / s or more) By heating from the slow cooling stop temperature T2 to the reheating holding temperature T3 in a short time, carbide precipitation 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. There is no particular upper limit to the average heating rate HR2, but as the average heating rate HR2 increases, it may become more difficult to maintain the uniform temperature of the steel sheet. Therefore, HR2 is preferably 50°C / s or less, and more preferably 20°C / s or less. The average heating rate HR2 is "reheating holding temperature T3 (°C) - gradual cooling stop temperature T2 (°C)) / heating time (seconds) from the gradual cooling stop temperature T2 to the reheating holding temperature T3".

[0066] (Reheating holding temperature T3: 300℃ or higher, 450℃ or lower) (Reheating holding time: 20s or more, 3000s or less) The reheating is performed to stabilize the austenite by C distribution. If the reheating temperature T3 is less than 300°C, C distribution is not sufficient and the desired amount of retained austenite cannot be obtained, so there is a concern that the ductility may decrease. If the reheating temperature T3 is less than 300°C, the desired stability of the mechanical properties in the sheet width direction cannot be obtained. Therefore, the reheating temperature T3 is set to 300°C or higher. T3 is preferably 330°C or higher, and more preferably 350°C or higher. On the other hand, if the reheating holding temperature T3 exceeds 450°C, transformation from austenite to pearlite occurs, and the desired amount of retained austenite cannot be obtained, which may result in a decrease in ductility. Moreover, if the reheating holding temperature T3 exceeds 450° C., the desired stability of mechanical properties in the sheet width direction cannot be obtained. Therefore, the reheating holding temperature T3 is set to 450° C. or less. T3 is preferably 420° C. or less.

[0067] Furthermore, if 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. Since the effect of carbon distribution by reheating and holding is saturated at more than 3000 s, the reheating and holding time is set to 3000 s or less, preferably 1500 s or less, and more preferably 600 s or less.

[0068] (Average cooling rate CR3 from reheating holding temperature T3 to 50℃: 0.1℃ / s or more) If 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 the ductility will decrease due to softening caused by excessive tempering and carbide precipitation. 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. CR3 is preferably 100° C. / s or less, and more preferably 50° C. / s or less. The average cooling rate CR3 is "(reheating holding temperature T3 (°C) - 50°C) / cooling time (seconds) from reheating holding temperature T3 to 50°C."

[0069] [Hot-dip plating process] In the present invention, in the annealing step, hot-dip galvanizing treatment can be 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. The hot-dip galvanizing treatment may be hot-dip galvanizing treatment. When hot-dip galvanizing treatment is performed, it is preferable to immerse the steel sheet in a zinc plating bath at 440°C or more and 500°C or less to perform hot-dip galvanizing treatment, and then adjust the coating weight by gas wiping or the like. For hot-dip galvanizing, it is preferable to use a zinc plating bath having an Al content of 0.10% or more and 0.22% or less. In addition, after the hot dip galvanizing treatment, a galvanizing alloying treatment can be performed. When performing a galvanizing alloying treatment, it is preferable to perform the treatment in a temperature range of 480° C. or higher and 600° C. or lower after immersion in the galvanizing bath.

[0070] [Temper rolling] In the present invention, from the viewpoint of stabilizing press formability and increasing YS, the steel sheet after annealing can be subjected to temper rolling. The elongation rate is preferably 0.1% or more. Also, the elongation rate is preferably 0.5% or less.

[0071] [Leveller correction] In the present invention, in order to straighten the sheet shape, the steel sheet after annealing may be subjected to leveller straightening. The leveller straightening method is not particularly specified, and may be carried out according to a conventional method.

[0072] [Electroplating process] In the present invention, after the annealing step, an electroplating treatment such as electrogalvanizing can be carried out as a surface treatment.

[0073] The thickness of the steel plate of the present invention thus obtained is preferably 0.5 mm or more, and more preferably 2.0 mm or less. The plate width is preferably 600 mm or more, and more preferably 1700 mm or less.

[0074] Next, the member of the present invention and the method for producing the same will be described.

[0075] The member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining. Also, the method for producing the member of the present invention includes a step of subjecting the steel plate of the present invention to at least one of forming and joining to obtain the member.

[0076] The steel plate of the present invention has a tensile strength of 1180 MPa or more, excellent ductility and hole expandability, and excellent stability of mechanical properties in the plate width direction. Therefore, the member obtained by using the steel plate of the present invention also has high strength, excellent ductility and hole expandability, and excellent stability of mechanical properties in the plate width direction. In addition, the member of the present invention can be made lighter. Therefore, the member of the present invention can be suitably used, for example, for vehicle body frame parts. The member of the present invention also includes a welded joint.

[0077] The forming process may be performed by a general processing method such as pressing, without any restrictions. The joining process may be performed by a general welding method such as spot welding or arc welding, riveting, or crimping, without any restrictions. EXAMPLES

[0078] The present invention will be specifically described with reference to examples, however, the scope of the invention is not limited to the examples.

[0079] A slab having the composition shown in Table 1 was held at a slab heating temperature of 1230°C for 3000s, then hot rolled at a finish rolling temperature of 870°C, cooled at an average cooling rate of 65°C / s in the temperature range from the finish rolling temperature to 650°C, and coiled at the coiling temperature and with the maximum temperature difference in the sheet width direction during coiling shown in Table 2 to produce a hot-rolled steel sheet with a thickness of 2.8 mm and a width of 1100 mm. The hot-rolled steel sheet was cold rolled at a rolling reduction of 50% to produce a cold-rolled steel sheet with a thickness of 1.4 mm and a width of 1100 mm.

[0080] Thereafter, the cold-rolled steel sheet was annealed under the conditions shown in Table 2. Among the annealing conditions, the average heating rate HR1 in heating from 700° C. to (Ac3-10° C.) was set to 2.0° C. / s. In addition, No. 11 had its steel sheet surface electroplated with zinc (EG), and No. 12 had its steel sheet surface hot-dip galvanized. In addition, No. 12 had its steel sheet surface hot-dip galvanized by holding at 510°C for 10 seconds to turn the plated layer into a galvannealed hot-dip galvanized layer.

[0081] [Table 1]

[0082] [Table 2]

[0083] The steel structure was measured by the above-mentioned method, and the measurement results are shown in Table 3.

[0084] Tensile strength (TS) and total elongation (EL) were evaluated in accordance with JIS Z2241 (2011). JIS No. 5 tensile test pieces were prepared from the obtained steel plates and subjected to tensile tests. Steels with a TS of 1180 MPa or more were judged to have excellent strength, and steels with an EL of 11.0% or more were judged to have excellent ductility.

[0085] The hole expandability was evaluated in accordance with the Japan Iron and Steel Federation standard JFST1001. After cutting each of the obtained steel plates to 100 mm x 100 mm, a hole with a diameter of 10 mm was punched out with a clearance of 12% of the plate thickness, and a 60° conical punch was pressed into the hole while applying a blank holding force of 88.2 kN using a die with an inner diameter of 75 mm to measure the hole diameter at the crack initiation limit, and the limit hole expansion ratio λ (%) was calculated from the formula (1) and evaluated, with λ of 40% or more being judged to be excellent in hole expandability. Limit hole expansion rate λ(%)={(Df-D0) / D0}×100 ···(1) where Df is the hole diameter when the crack occurs (mm) and D0 is the initial hole diameter (mm).

[0086] The stability of mechanical properties in the plate width direction was evaluated by taking a total of 20 JIS No. 5 tensile test pieces parallel to the rolling direction at equal intervals in the plate width direction, including both ends of the plate width, conducting the tensile test described above, and calculating the standard deviation of TS and EL. Those with a standard deviation of EL of 0.9% or less were judged to have excellent stability of mechanical properties in the plate width direction. There was no particular regulation on the standard deviation of TS, but a standard deviation of TS of 15.0 MPa or less was judged to have excellent stability of mechanical properties in the plate width direction.

[0087] [Table 3]

[0088] The examples of the present invention shown in Tables 2 and 3 are excellent in strength, ductility, hole expandability, and stability of mechanical properties, whereas the comparative examples are inferior in at least one of them. In addition, the standard deviation of the tensile strength TS of the examples of the present invention was 15.0 MPa or less.

[0089] In addition, using the steel plate of the present invention, it was found that the components obtained by molding, the components obtained by joining, and the components obtained by further molding and joining have high strength and excellent ductility, hole expandability, and stability of mechanical properties in the plate width direction, similar to the steel plate of the present invention, because the steel plate of the present invention has high strength and excellent ductility, hole expandability, and stability of mechanical properties in the plate width direction.

Claims

1. In mass percent, 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; The balance has a composition consisting of Fe and unavoidable impurities, Ferrite area ratio: 5% or less (including 0%), Total area ratio of tempered martensite and lower bainite: 70% or more; Volume fraction of retained austenite: 5 to 15%; The area ratio of fresh martensite is 10% or less (including 0%). The standard deviation of the total elongation in the width direction is 0.9% or less; The tensile strength TS is 1180 MPa or more, The total elongation EL is 11.0% or more, The limit hole expansion ratio λ is 40% or more, A steel plate having a width of 600 to 1700 mm.

2. The steel sheet according to claim 1, comprising the following (A) and / or (B) as the component composition: (A) 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; W: 0.2% or less One or more selected from the following: (B) 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; Sn: 0.1% or less One or more selected from

3. The steel sheet according to claim 1 , which has a plating layer on a surface of the steel sheet.

4. The steel sheet according to claim 2 , which has a plating layer on a surface of the steel sheet.

5. A member made using the steel plate according to any one of claims 1 to 4.

6. A steel slab having the composition according to claim 1 or 2, After holding the slab at a heating temperature of 1100°C or higher for 1800s or more, Finish hot rolling is performed at a finish rolling temperature of 850°C or higher, Cooling is performed at an average cooling rate of 40 ° C. / s or more in a temperature range from the finish rolling temperature to 650 ° C., a hot rolling process in which the coiling temperature is set to 600° C. or less and the maximum temperature difference between the temperature at the center of the sheet width in the sheet width direction during coiling is set to 50° C. or less to obtain a hot-rolled steel sheet; The hot-rolled steel sheet, A cold rolling process in which the steel sheet is cold-rolled at a rolling ratio of 30% or more to obtain a cold-rolled steel sheet; The cold-rolled steel sheet, From 700°C (Ac ３ -10 ° C.) at an average heating rate HR1 of 0.5 ° C. / s or more, (Ac ３ -10 ° C) or higher annealing temperature for 30 seconds or more, Cooling is performed 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 which is equal to or higher than (Ms-30 ° C.) and equal to or lower than (Ms+30 ° C.), The temperature range from the annealing start temperature T1 to the annealing stop temperature T2, which is equal to or higher than (Ms-220°C) and equal to or lower than (Ms-100°C), is cooled at an average cooling rate CR2 of 1 to 10°C / s, The temperature range from the slow cooling stop temperature T2 to a reheating holding temperature T3 of 300° C. or more and 450° C. or less is heated at an average heating rate HR2 of 2° C. / s or more, The reheating holding temperature T3 is held for 20 s or more and 3000 s or less, An annealing process of cooling the temperature range from the reheating holding temperature T3 to 50 ° C. at an average cooling rate CR3 of 0.1 ° C. / s or more. Ferrite area ratio: 5% or less (including 0%), Total area ratio of tempered martensite and lower bainite: 70% or more; Volume fraction of retained austenite: 5 to 15%; The area ratio of fresh martensite is 10% or less (including 0%). The standard deviation of the total elongation in the width direction is 0.9% or less; The tensile strength TS is 1180 MPa or more, The total elongation EL is 11.0% or more, The limit hole expansion ratio λ is 40% or more, A method for manufacturing a steel plate, wherein the plate width is 600 to 1700 mm.

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

8. The method for producing a steel sheet according to claim 6, further comprising the step of: performing an electroplating treatment after the annealing step.

9. A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining to form a component.

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