STEEL SHEET, MEMBER AND METHODS FOR MANUFACTURING THE SAME

Abstract

One objective is to provide a steel plate which has high strength, good ductility, and good drawability, and in which ductility deterioration is suppressed under a high strain rate, a member obtained from the steel plate, and the manufacturing methods are the same. A steel plate according to the present invention has a specific chemical composition and a steel microstructure which includes, in terms of area fraction, ferrite: 40% or more and 70% or less, total bainite and quenched martensite: 5% or more and 30% or less, retained austenite: 4% or more and 18% or less, fresh martensite: 8% or more and 35% or less, and the remainder: 5% or less.Cementite particles are present in the retained austenite, the ratio of an area fraction of cementite particles in the retained austenite to an area fraction of the retained austenite is 5% or more and 25% or less, and the steel plate has a breaking strength of 780 MPa or greater and less than 980 MPa.

Description

STEEL SHEET, MEMBER AND METHODS FOR MANUFACTURING THE SAME FIELD OF THE INVENTION The present invention relates to a steel sheet that has high strength, good ductility, and good drawability, and in which ductility deterioration is suppressed under a high strain rate, a component, and methods for manufacturing the same. The steel sheet according to the present invention can be suitable for parts used primarily in the automotive field. BACKGROUND OF THE INVENTION In recent years, given global environmental concerns, improved fuel efficiency in automobiles has become a major focus, along with reduced car body weight and enhanced impact resistance. To meet these demands, the demand for high-strength steel sheets, such as those used in automotive applications, has increased. However, generally, increased strength in a steel sheet reduces formability. Therefore, developing a steel sheet that combines both high strength and high formability has been a priority. In the forming of high-strength steel sheets into complex shapes, such as automotive parts, cracking and narrowing in the drawn and flanged portions are serious problems. Therefore, there is a demand for high-strength steel sheets with higher elongation and a higher hole expansion ratio to overcome these cracking and narrowing issues. Furthermore, in actual press forming, steel sheets are formed at high strain rates to improve productivity. Consequently, there is a demand for steel sheets with high elongation that remains constant even at high strain rates, as well as high elongation at low strain rates, which is assessed by a standard tensile test. To improve strength and formability, several multiphase high-strength steel sheets such as two-phase ferrite-martensite steel (two-phase steel (DP)) and TRIP steel, which utilizes the plasticity induced by the transformation of retained austenite, have been manufactured to date. For example, Patent Literature 1 describes a method for manufacturing a high-strength steel sheet that achieves high ductility by adding a large amount of Si, annealing a cold-rolled steel sheet in the two-phase region, and subsequently performing retention in the bainite transformation region from 300°C to 450°C to ensure a large amount of retained austenite. Patent Literature 2 describes a method for manufacturing a high-strength cold-rolled steel sheet that achieves a high hole expansion ratio by providing a microstructure composed of tempered ferrite and martensite by simultaneously adding Si and Mn in large quantities. As a method to increase elongation and hole expansion ratio, a technique has been developed to reduce the hardness difference between microstructures by introducing quenched martensite or bainite. For example, Patent Literature 3 describes a technique for achieving high elongation and a high hole expansion ratio by providing a microstructure composed of ferrite, quenched martensite, and retained austenite. Furthermore, Patent Literature 4 describes a technique for achieving high elongation and a high hole expansion ratio by providing a microstructure composed of ferrite, bainite, and retained austenite. A method for controlling precipitated carbide in steel is also effective. Patent Literature 5 describes a technique for achieving high elongation and a high orifice expansion ratio by providing a microstructure composed of ferrite, a low-temperature transformed phase, and retained austenite, and by reducing the particle size of a carbide in the low-temperature transformed phase. Patent Literature 6 describes a technique for achieving high elongation and a high orifice expansion ratio by optimizing the annealing conditions in steel containing retained austenite to control the size and morphology of the cementite. List of Appointments Patent Literature PTL 1: Publication of Unexamined Japanese Patent Application No. 2-101117 PTL 2: Publication of Unexamined Japanese Patent Application No. 2004-256872 PTL 3: Japanese Patent No. 5463685 PTL 4: Japanese Patent No. 4894863 PTL 5: Publication of Unexamined Japanese Patent Application No. 2008-308717 PTL 6: Japanese Patent No. 4903915 BRIEF DESCRIPTION OF THE INVENTION Technical Problem However, in Patent Literature 1, although ductility is good, draw-forming capacity is not considered. In Patent Literature 2, although draw-forming capacity is good, ductility is insufficient. In Patent Literatures 3, 4, and 5, although high ductility and high draw-forming capacity are achieved, the deterioration of ductility at a high strain rate is not considered. In Patent Literature 6, although high elongation is achieved, the deterioration of ductility at a high strain rate is not considered. In view of the circumstances described above, an objective of the present invention is to provide a steel plate which has high strength, good ductility, and good draw-flanging capability and in which ductility deterioration is suppressed under a high strain rate, a member, and methods for manufacturing the same. The term “high strength” as used herein means that a breaking strength (TS) in a tensile or strain test made on a machined test specimen in a JIS No. 5 test specimen at a cross-cut or cross-cut speed of 10 mm / min in accordance with JIS Z 2241 (2011) is 780 MPa or greater and less than 980 MPa. The term “good ductility” means that a total elongation Eh obtained by the tensile test described above is 23% or more. The term “good stretch flanging capability” means that a hole expansion test is performed on a 100 mm x 100 mm test specimen three times in accordance with the Japan Iron and Steel Federation standard JFS T 1001 with a 60° tapered punch, and an average hole expansion ratio λ is 25% or more. The expression “ductility deterioration under a high strain rate is suppressed” means that a test specimen machined into a JIS No. 5 test specimen is subjected to a high-speed tensile test in which the transverse or crosshead speed of the tensile test described above was modified to 100 mm / min, and a ratio (EI2 / EI1) of a measured value of Ela (total elongation) in the high-speed tensile test to a measured value of El· (total elongation) in the normal tensile test described above is 85% or more. Solution to the Problem The inventors hereof have conducted extensive studies to manufacture a high-strength steel sheet that has good ductility (elongation) and draw-forming capacity (hole expansion ratio) and in which ductility deterioration is suppressed under high strain rates. In particular, they carried out studies to increase elongation and hole expansion ratio by analyzing in detail a microstructural change that occurs during the thermal history of steel sheet manufacturing.In the course of studies conducted by the inventors hereof, a steel sheet obtained by appropriately adjusting the chemical composition was cooled from an annealing temperature at a predetermined cooling rate, subjected to a first holding temperature at 380°C or higher and 420°C or lower to concentrate the carbon in austenite by bainite transformation and quenching and parting (Q&P) treatment, and subsequently subjected to a second holding temperature under predetermined conditions at 440°C or higher and 540°C or lower. As a result, it was found that the above method provides a microstructure in which cementite particles are present in the retained austenite and allows the manufacture of a high-strength steel sheet, which has ductility and drawability and in which ductility deterioration is suppressed under a high strain rate. In general, steel containing large amounts of retained austenite exhibits very high elongation due to the TRIP effect of the retained austenite in a standard tensile test at a low strain rate. However, strain-induced martensite, formed through austenite formation by applying strain, contains a large amount of dissolved carbon and is therefore very hard. Consequently, a significant difference in hardness between microstructures is known to exist, resulting in a decreased hole expansion ratio. It is also known that, in a tensile test at a high strain rate, stable retained austenite does not transform into martensite, resulting in a decrease in elongation.However, in the composition and microstructure of the present invention, the deterioration of drawability and ductility under high strain rates is suppressed while retained austenite is included to achieve good ductility. The details of this are not clear, but it is presumably due to the austenite in which carbon is excessively concentrated, inevitably forming austenite in the first retention stage, which partially precipitates as cementite particles during the second retention stage, thereby increasing the hole expansion ratio. As described above, the retained austenite in which carbon is excessively concentrated, inevitably forming austenite retained by the first retention stage, transforms into very hard martensite due to high stress during part cutting, resulting in a decrease in the hole expansion ratio.Through the second retention process in the present invention, cementite particles precipitate into austenite in which carbon is excessively concentrated, and the amount of austenite in which carbon is excessively concentrated decreases. Specifically, the amount of retained austenite having a relatively lower carbon concentration than the previously described retained austenite in which carbon is excessively concentrated increases. This is considered to increase the amount of retained austenite that contributes to elongation under a high strain rate, and the deterioration of ductility under a high strain rate is suppressed. The present invention is based on the discoveries described above. A brief description of the present invention is as follows. [1] A steel sheet that includes: a chemical composition that contains, in % by mass, C: 0.07% or more and 0.18% or less, If: 0.01% or more and 2.0% or less, Al: 0.01% or more and 2.0% or less, a total of Si and Al: 0.7% or more and 2.5% or less, Mn: 1.5% or more and 2.6% or less, P: 0.1% or less, S: 0.02% or less, and N: 0.010% or less, the remainder being Fe and secondary impurities; and a steel microstructure comprising, in terms of area fraction, ferrite: 40% or more and 70% or less, a total of bainite and quenched martensite: 5% or more and 30% or less, retained austenite: 4% or more and 18% or less, fresh martensite: 8% or more and 35% or less, and the remainder: 5% or less, wherein cementite particles are present in the retained austenite, a ratio of an area fraction of the cementite particles in the retained austenite to an area fraction of the retained austenite is 5% or more and 25% or less, and the steel plate has a tensile strength of 780 MPa or greater and less than 980 MPa. [2] The steel sheet according to [1], wherein the cementite particles in the retained austenite have an average major axis of 30 nm or more and 400 nm or less. [3] The steel sheet according to [1] or [2], wherein the chemical composition further contains, in mass %, at least one selected from Cr, V, Mo, Ni, and Cu in a total amount of 1.0% or less. [4] The steel sheet according to any of [1] to [3], wherein the chemical composition further contains, in % by mass, at least one selected from Ti: 0.20% or less, and Nb: 0.20% or less. [5] The steel sheet according to any of [1] to [4], wherein the chemical composition further contains, in % by mass, B: 0.005% or less. [6] The steel sheet according to any of [1] to [5], wherein the chemical composition ηα^η Ln / zznz / e / γΐΛΐ further contains, in % by mass, at least one selected from Ca: 0.005% or less, and REM: 0.005% or less. [7] The steel sheet according to any of [1] to [6], wherein the chemical composition further contains, in % by mass, at least one selected from Sb: 0.05% or less, and Sn: 0.05% or less. [8] The steel sheet in accordance with any of [1] to [7], which further includes a hot-dip galvanized layer or a hot-dip electro-annealed layer on a surface of the steel sheet. [9] A member obtained by subjecting the steel plate in accordance with any of [1] to [8] to at least one forming and welding process.

[10] A method for manufacturing a steel sheet, the method includes hot rolling or cold rolling a plate having the chemical composition according to any of [1] and [3] to [7]; subsequently holding at an annealing temperature of 700°C or higher and 950°C or lower for 30 seconds or more and 1000 seconds or less; cooling from the annealing temperature to a cooling interruption temperature of 150°C or higher and 420°C or lower at an average cooling rate of 10°C / s or higher; subsequently holding the first under conditions in a temperature range of 380°C or higher and 420°C or lower for 10 seconds or more and 500 seconds or less; and further holding the second under conditions of a temperature of X°C and a holding time of Y seconds satisfying the following formulas 1 to 3. Formula 1: 10000 < (273 + X) (12 + logY) < 11000 Formula 2: 440 < X ​​< 540 Formula 3: Y < 200

[11] The method for manufacturing a steel sheet according to

[10] , wherein the average heating rate from a holding temperature in the first holding to a temperature of X°C in the second holding is 3°C / s greater.

[12] The method for manufacturing a steel sheet according to

[10] , wherein the average heating rate from a holding temperature in the first holding to a temperature of X°C in the second holding is 10°C / s greater.

[13] The method for manufacturing a steel sheet in accordance with any of

[10] to

[12] , which includes, between the first and second retention after completing the second retention, forming a hot-dip galvanized layer or a hot-dip electro-annealed layer on a surface of the steel sheet.

[14] A method for manufacturing a member, the method includes a step of subjecting a steel plate manufactured by the method for manufacturing a steel plate according to any of

[10] to

[13] to at least one of the welding formation. Advantageous Effects of the Invention According to the present invention, a steel sheet is provided which has high strength, good ductility, and good drawability, and in which the deterioration of ductility under a high strain rate is suppressed. Manufacturing elements by subjecting the steel sheet according to the present invention to forming, welding, and the like, and applying the elements to, for example, automotive structural components, reduces the weight of automobile bodies and thus improves fuel efficiency; therefore, the steel sheet according to the present invention provides very high utility from an industrial point of view. DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in more detail below. First, the chemical composition of the steel according to the present invention will be described. Note that “%” used as the unit of content of a component means “% by mass”. C: 0.07% or more and 0.18% or less Carbon (C) is an element that stabilizes austenite and is essential for obtaining retained austenite containing cementite particles. Furthermore, C is necessary to increase the strength of steel sheets because it facilitates the formation of hard microstructures other than ferrite, which are necessary to improve the TS-EL balance by forming a multiphase structure. When the C content is less than 0.07%, the amount of ferrite becomes excessively large, and the desired strength is not achieved. Therefore, the C content is 0.07% or more, preferably 0.08% or more, and even more preferably 0.09% or more. On the other hand, when the C content exceeds 0.18%, the strength increases significantly, and the elongation decreases. Therefore, the C content is 0.18% or less, and preferably 0.17% or less. Yes: 0.01% or more and 2.0% or less Silicon (Si) promotes carbon concentration in austenite and inhibits the formation of carbides such as cementite, while also promoting the formation of retained austenite. Given the cost of desiliconization in steelmaking, the Si content is typically 0.01% or higher. Conversely, when the Si content exceeds 2.0%, surface quality and wettability deteriorate; therefore, the Si content should be 2.0% or lower. Ideally, the Si content should be 1.8% or lower. To: 0.01% or more and 2.0% or less Aluminum promotes carbon concentration in austenite and inhibits the formation of carbides such as cementite, while also promoting the formation of retained austenite. Given the cost of dealuminization in steelmaking, the aluminum content is typically 0.01% or higher. However, when the aluminum content exceeds 2.0%, the risk of cracking in the steel plate increases during continuous casting. Therefore, the aluminum content is 2.0% or lower, and preferably 1.8% or lower. Total Si and Al: 0.7% or more and 2.5% or less Silicon (Si) and aluminum (Al) promote carbon concentration in austenite and inhibit carbide formation as cementite. To obtain a sufficient amount of retained austenite, the total Si and Al content is 0.7% or more, preferably 1.0% or more, and more preferably 1.3% or more. On the other hand, from a manufacturing cost perspective, the total Si and Al content is 2.5% or less, preferably 2.2% or less, and more preferably 2.0% or less. Mn: 1.5% or more and 2.6% or less Manganese (Mn) is an effective element for strengthening steel because it improves the hardenability or tempering capacity and inhibits ferrite and pearlite transformations during cooling after annealing. Mn is an austenite stabilizer and also contributes to the formation of retained austenite. To achieve these effects, the Mn content is 1.5% or more, and preferably 1.7% or more. On the other hand, when the Mn content exceeds 2.6%, the amount of ferrite decreases, the strength becomes excessively high, and the elongation decreases. Therefore, the Mn content is 2.6% or less, and preferably 2.4% or less. P: 0.1% or less Phosphorus (P) is an effective element for strengthening steel. However, when P is added in excessive amounts, such that the P content exceeds 0.1%, it causes grain boundary segregation embrittlement, and the mechanical properties deteriorate. Therefore, the P content is 0.1% or less, preferably 0.05% or less, and more preferably 0.02% or less. Although no lower limit for P content is specified, an industrially feasible lower limit is currently 0.002%. S: 0.02% or less Sulfur (S) causes a deterioration of impact resistance and cracking along the metal flow in a weld zone as a result of the formation of inclusions such as MnS. Therefore, minimizing sulfur content is preferable. Considering manufacturing costs, the sulfur content is 0.02% or less. Ideally, it should be 0.01% or less. Although no lower limit for sulfur content is specified, an industrially feasible lower limit is currently 0.0002%. N: 0.010% or less Nitrogen (N) is an element that significantly impairs the aging resistance of steel, and it is preferable to minimize its content. The deterioration of aging resistance becomes significant when the N content exceeds 0.010%. Therefore, the N content should be 0.010% or less. Although no lower limit for N content is specified, a currently industrially feasible lower limit is 0.0005%. The steel sheet according to the present invention has a chemical composition that includes the above chemical composition as base components, with the remainder including Fe (iron) and secondary impurities. It is preferable that the steel sheet according to the present invention have a chemical composition containing the above-described components as base components, with the remainder being iron and secondary impurities. The steel sheet according to the present invention may contain the optional components described below as appropriate, depending on the desired properties. Note that the lower limits of the following components are not specifically defined because the advantages of the present invention are obtained as long as the content of the components is equal to or less than the upper limits described below.When the content of the following optional elements is less than the preferred lower limits described below, the elements are considered to be contained as secondary impurities. At least one selected from Cr, V, Mo, Ni, and Cu, in total: 1.0% or less Cr, V, Mo, Ni, and Cu inhibit pearlite transformation during cooling from an annealing temperature and effectively act on the formation of retained austenite. However, when the total content of at least one of these selected elements exceeds 1.0%, the effect becomes saturated, and the cost may increase. Therefore, when the steel sheet contains at least one of these elements, the total content of these elements is 1.0% or less. The total content of these elements is preferably 0.50% or less, and more preferably 0.35% or less. The lower limit of the total content is not particularly restricted because the advantages of the present invention are obtained at a total content of 1.0% or less. To more effectively obtain the effect of retained austenite formation due to Cr, V, Mo, Ni, and Cu, the total content is preferably 0.005% or more, and more preferably 0.02% or more. At least one selected from Ti: 0.20% or less and Nb: 0.20% or less Titanium (Ti) and nitride (Nb) form carbides, nitrides, and / or carbonitrides to enhance the strength of steel by strengthening the particle dispersion. However, when Ti and Nb are present in amounts greater than 0.20%, the strength is excessively increased, and ductility decreases. Therefore, when steel sheet contains at least one of Ti and Nb, the content of each element is 0.20% or less. The total content of the elements is preferably 0.15% or less, and more preferably 0.08% or less. The lower limits for the Ti and Nb content are not particularly restricted because the advantages of the present invention are obtained as long as the Ti and Nb content is each 0.20% or less.To more effectively obtain the particle dispersion strengthening effect due to Ti and Nb, the Ti content and Nb content are each preferably 0.01% or more. B: 0.005% or less Boron (B) has a function of increasing strength by inhibiting ferrite formation from the grain boundaries of austenite as a result of segregation at these boundaries. However, when B is present in an amount greater than 0.005%, it precipitates as a boride, and the desired strength-enhancing effect is not achieved. Therefore, when steel sheet contains B, the B content is 0.005% or less. The B content is preferably 0.004% or less, and more preferably 0.003% or less. The lower limit of the B content is not particularly restricted because the advantages of the present invention are obtained as long as the B content is 0.005% or less. To more effectively achieve the strength-enhancing effect of B, the B content is preferably 0.0003% or more. At least one selected from Ca: 0.005% or less and REM: 0.005% or less Calcium (Ca) and REM each have a forming capacity-enhancing effect through the morphological control of sulfides. However, excessive addition can adversely affect cleanability. Therefore, when the steel sheet contains at least one of Ca and REM, the content of each element is 0.005% or less. The content of each element is preferably 0.004% or less, and more preferably 0.003% or less. The lower limits of the Ca and REM content are not particularly restricted because the advantages of the present invention are obtained as long as the Ca and REM content is each 0.005% or less. To more effectively obtain the forming capacity-enhancing effect of Ca and REM, the Ca and REM content is preferably each 0.0001% or more. At least one selected from Sb: 0.05% or less and Sn: 0.05% or less Sb and Sn have the function of suppressing a decrease in the strength of steel by inhibiting, for example, decarbonization, denitrification, and defleshing. However, excessive addition can impair the flanging capacity under draw. Therefore, when the steel sheet contains at least one of Sb and Sn, the content of each element is 0.05% or less. The content of each element is preferably 0.04% or less, and more preferably 0.03% or less. The lower limits of the Sb and Sn content are not particularly restricted because the advantages of the present invention are obtained as long as the Sb and Sn content is each 0.05% or less. To more effectively achieve the effect of suppressing a decrease in strength due to Sb and Sn, the Sb and Sn content is preferably each 0.003% or more. The microstructure of the steel sheet will be described below. The steel sheet according to the present invention has a steel microstructure comprising, in terms of area fraction, ferrite: 40% or more and 70% or less, a total of bainite and quenched martensite: 5% or more and 30% or less, retained austenite: 4% or more and 18% or less, fresh martensite: 8% or more and 35% or less, and the remainder: 5% or less. In addition, cementite particles are present in the retained austenite, and the ratio of the area fraction of the cementite particles in the retained austenite to the area fraction of the retained austenite is 5% or more and 25% or less. Fraction of ferrite area: 40% or more and 70% or less To ensure good ductility, a ferrite area fraction of 40% or more is required. Ferrite is relatively soft, and this is necessary in terms of area fraction. The ferrite area fraction is preferably 45% or more. On the other hand, to ensure strength, the ferrite area fraction needs to be 70% or less. The ferrite area fraction is preferably 65% ​​or less. Total area fraction of bainite and tempered martensite: 5% or more and 30% or less Carbon is concentrated in the austenite due to the transformation of bainite, and carbon is partitioned from the martensite to form retained austenite. Therefore, the total area fraction of bainite is 5% or more, and preferably 7% or more. On the other hand, to ensure good ductility, the total area fraction is 30% or less, and preferably 28% or less. Note that it is sufficient for the total area fraction of bainite and quenched martensite to be within the range described above, and any area fraction can be 0%. Fraction of fresh martensite area: 8% or more and 35% or less From the standpoint of achieving the strength required by the present invention, the area fraction of fresh martensite needs to be 8% or more, and is preferably 10% or more. When the area fraction of fresh martensite exceeds 35%, the strength increases, and the elongation decreases. Therefore, the area fraction of fresh martensite is 35% or less, and preferably 33% or less. The area fractions of ferrite, bainite, quenched martensite, and fresh martensite in the present invention are determined by a point-counting method. A cross-section through the thickness of the sheet parallel to a rolling direction of the steel sheet is cut and heat-treated at 200°C for two hours. As a result, the fresh martensite is slightly quenched. The cross-section through the thickness of the sheet (cross-section L) of this sample is polished, then etched with 1% by volume nital, and observed with a scanning electron microscope (SEM) at a position 1 / 4 of the thickness of a surface of the steel sheet at a magnification of 1500x by two fields of view. The area fractions can be determined by plotting a grid over an image obtained by observation and performing a point count at 240 points in each field of view.Ferrite is a black microstructure, and bainite is a gray microstructure with a lattice-like morphology. In both the quenched martensite and the fresh martensite (after heat treatment at 200°C for two hours), a hierarchical structure, including blocks and bundles, is observed, along with a precipitate. Since the hierarchical structure and precipitate in the quenched martensite are apparently coarser than those in the fresh martensite (after heat treatment at 200°C for two hours), the area fraction of the quenched martensite and the area fraction of the fresh martensite can be clearly determined. The retained austenite contains cementite, distinguishable from the other microstructures because a hierarchical structure is not observed in this phase under the sample preparation and observation conditions described above. Fraction of retained austenite area: 4% or more and 18% or less To ensure good ductility, the TRIP effect of retained austenite is utilized. To increase elongation through the TRIP effect, the area fraction of retained austenite needs to be 4% or more. The area fraction of retained austenite is preferably 5% or more, and more preferably 6% or more. From the standpoint of achieving strength in the present invention, the area fraction of retained austenite is 18% or less, preferably 17% or less, and more preferably 16% or less. In the present invention, a volume fraction of retained austenite determined by a measurement method described below is considered the area fraction of retained austenite. The volume fraction can be determined by polishing the steel sheet in one direction of its thickness until a surface is exposed at a position of 1 / 4 of the thickness, and subjecting the surface at the 1 / 4 thickness position to X-ray diffraction intensity measurement. Mo-Kα radiation is used as the incident X-ray; intensity ratios are determined with respect to all combinations of integral intensities of the peaks of the {111}, {200}, {220}, and {311} planes of the retained austenite and the {110}, {200}, and {211} planes of the ferrite, and the average of these is defined as the volume fraction of retained austenite. Ratio of cementite particle area fraction in retained austenite to retained austenite area fraction (Cementite particle area fraction in retained austenite / Retained austenite area fraction): 5% or more and 25% or less. Cementite particles are present in the retained austenite. The expression “cementite particles are present in the retained austenite,” as used herein, is defined as a state where the cementite has at least a portion of its interface with the retained austenite. Consequently, other portions may interface with other phases such as ferrite, bainite, quenched martensite, and fresh martensite, as long as the cementite has an interface with the retained austenite in some portion. When the retained austenite contains cementite particles, a portion with an excessively high C solute concentration in the retained austenite, which decreases the orifice expansion ratio, can be reduced to increase the orifice expansion ratio.This effect is obtained when the ratio of the cementite particle area fraction in the retained austenite to the retained austenite area fraction is 5% or higher. Conversely, when the ratio exceeds 25%, the stability of the retained austenite decreases significantly, resulting in a decrease in elongation. Therefore, the ratio is 5% or higher, and the ratio is 25% or lower. In the present invention, the ratio of the cementite particle area fraction in the retained austenite to the retained austenite area fraction is determined by transmission electron microscopy on an observation surface located at a depth of 1 / 4 of the steel sheet thickness. Specifically, the ratio is determined by observing five grains of retained austenite and using a point-counting method. A sample for transmission electron microscopy is prepared by electropolishing. The use of a transmission electron microscope allows for the easy identification of retained austenite from information obtained through electron diffraction patterns, stacking faults, or similar features. A bright-field image of the retained austenite is captured at a magnification of 50,000x to include the surrounding interface.The determination is made by plotting a grid over the obtained image, counting 240 points in each field of view, and dividing the number of intersection points corresponding to cementite particles by the number of intersection points corresponding to retained austenite. The grid is a 0.1 pm x 0.1 pm grid with respect to the image. Electron diffraction is used to identify the cementite particles. Cementite particles are also present in quenched martensite. However, cementite particles present in retained austenite and cementite particles present in quenched martensite are easily distinguishable from each other in an electron diffraction pattern of a selected area or substructure. Average major axis of cementite particles in retained austenite: 30 nm or more and 400 nm or less (preferred range) To ensure a high hole expansion ratio, the cementite particles in the retained austenite preferably have an average major axis of 30 nm or more. When the average major axis is 30 nm or more, fine voids are less likely to form during shearing, and a high hole expansion ratio is easily achieved. When the cementite particles in the retained austenite have an average major axis of 400 nm or less, the carbon concentration in the retained austenite near the cementite particles is less likely to decrease, the stability of the retained austenite is less likely to increase, and high elongation is easily achieved. Thus, to ensure better elongation, the average major axis of the cementite particles in the retained austenite is preferably 400 nm or less.The average major axis of the cementite particles is determined by measuring the maximum lengths of 10 cementite particles from an image of cementite particles present in the retained austenite, the image being captured by a transmission electron microscope, and calculating the average of the maximum lengths. The rest: 5% or less The remainder, in addition to ferrite, bainite, quenched martensite, fresh martensite, and retained austenite, is 5% or less, and preferably 4% or less to obtain the advantages of the present invention. The remaining microstructure may include, for example, carbides that remain undissolved after annealing, precipitate due to alloying elements, and pearlite. Note that the cementite particles present in the retained austenite are included in the remainder. The steel sheet according to the present invention may have a hot-dip galvanized layer or a hot-dip electro-annealed layer on one of its surfaces. The thickness of the steel sheet according to the present invention is preferably 0.2 mm or more and 3.2 mm or less from the point of view of effectively obtaining the advantages of the present invention. The following will describe one embodiment of a method for manufacturing a steel sheet according to the present invention. One embodiment of a method for manufacturing a steel sheet according to the present invention includes, for example, holding a hot-rolled and cold-rolled steel sheet having the chemical composition described above at an annealing temperature of 700°C or higher and 950°C or lower for 30 seconds or more and 1000 seconds or less, cooling from the annealing temperature to a cooling interruption temperature of 150°C or higher and 420°C or lower at an average cooling rate of 10°C / s or higher, subsequently performing the first holding under conditions in a temperature range of 380°C or higher and 420°C or lower for 10 seconds or more and 500 seconds or less, and further performing the second holding under conditions of a temperature of X°C and a holding time of Y seconds that satisfies formulas 1 to 3 below. Formula 1: 10000 < (273 + X) (12 + logY) < 11000 Formula 2: 440 < X ​​< 540 Formula 3: Y < 200 A specific embodiment of the method for manufacturing a steel sheet according to the present invention will be described in detail below. Note that the temperatures described below for heating or cooling a plate (steel material), a steel sheet, or the like refer to surface temperatures of the plate (steel material), the steel sheet, or the like, unless otherwise stated. Steel with the chemical composition described above is obtained by steelmaking through a common, publicly available process, then by casting into a slab through bloom or continuous casting, and the slab is hot-rolled to obtain a hot coil. In hot rolling, preferably, the slab is heated to a temperature in the range of 1100°C to 1300°C, hot-rolled to a final finishing temperature of 850°C or higher, and coiled at a temperature in the range of 400°C to 750°C. When the coiling temperature exceeds 750°C, a carbide such as cementite in the hot-rolled steel sheet becomes oily and does not fully melt during the short-term soaking in annealing after cold rolling, and thus the required strength cannot be achieved.Subsequently, the hot-rolled steel sheet undergoes preliminary treatment, such as pickling or degreasing, by a common, publicly available method, and is then cold-rolled. In cold rolling, cold rolling is preferably carried out at a cold rolling reduction of 30% or more. At a low cold rolling reduction, ferrite recrystallization is not promoted, and unrecrystallized ferrite remains, which can result in impaired ductility (elongation) and flanging ability. Retention at annealing temperature of 700°C or higher and 950°C or lower for 30 seconds or more and 1000 seconds or less In the present invention, annealing (holding) is performed within a temperature range of 700°C or higher and 950°C or lower, specifically in a single-phase austenite region or a two-phase austenite-ferrite region, for 30 seconds or more and 1000 seconds or less. When the annealing temperature is lower than 700°C or the holding time is less than 30 seconds, ferrite recrystallization or the reverse transformation to austenite does not occur sufficiently, and the desired microstructure is not formed, which may result in insufficient strength. On the other hand, when the annealing temperature exceeds 950°C, the austenite grains grow significantly, which may lead to a decrease in nucleation sites for ferrite transformation caused by subsequent cooling.When the holding time (annealing) exceeds 1000 seconds, the austenite becomes oily, and this can lead to increased costs due to high energy consumption. The annealing temperature is preferably 750°C or higher. The annealing temperature is preferably 900°C or lower. The holding time at the annealing temperature is preferably 40 seconds or more. The holding time at the annealing temperature is preferably 500 seconds or less. Cooling from the annealing temperature to the cooling interruption temperature of 150°C or higher and 420°C or lower at an average cooling rate of 10°C / s or higher When the average cooling rate of the annealing temperature is less than 10°C / s, ferrite and pearlite form in large quantities, and insufficient retained austenite is obtained, resulting in decreased elongation. Therefore, the average cooling rate of the annealing temperature is 10°C / s higher. The average cooling rate is preferably 15°C / s higher. The upper limit of the average cooling rate is not particularly restricted, but it is preferably 200°C / s lower from the standpoint of reducing equipment investment costs. When the cooling stop temperature is above 420°C, the driving force for bainite transformation decreases, resulting in insufficient retained austenite. Conversely, when the cooling stop temperature is below 150°C, martensite transformation proceeds, the amount of unretained austenite decreases, and insufficient retained austenite is obtained. Therefore, the cooling stop temperature is either 150°C or higher and 420°C or lower. First retention under the condition in the temperature range of 380°C or higher and 420°C or lower for 10 seconds or more and 500 seconds or less. Retention within this temperature range is one of the important requirements of the present invention. When the retention temperature is below 380°C, exceeds 420°C, or the retention time is less than 10 seconds, the concentration of carbon in the untransformed austenite is not promoted by bainite transformation or carbon partitioning from martensite to untransformed austenite. Therefore, a sufficient amount of retained austenite is not obtained, and high elongation is not achieved. When the retention time exceeds 500 seconds, pearlite transformation occurs, the area fraction of retained austenite decreases, and thus high elongation is not achieved. Second retention under temperature conditions of X°C and retention time of Y seconds that satisfy formulas 1 to 3 below Formula 1: 10000 < (273 + X) (12 + logY) < 11000 Formula 2: 440 < X ​​< 540 Formula 3: Y < 200 Retention within a temperature range that satisfies the above conditions is also one of the important requirements of the present invention. During the second retention stage, cementite particles precipitate into austenite formed during the first retention stage, in which carbon is excessively concentrated. This allows the orifice expansion ratio to increase and inhibits a decrease in elongation under a high strain rate. This precipitation of cementite particles from the austenite in which carbon is excessively concentrated has been scarcely studied to date.As a result of extensive studies on this precipitation phenomenon, it was found that when the parameter “(273 + X) (12 + logY)” in Formula 1, which depends on temperature and time, satisfies 10000 or more and 11000 or less, the fraction of retained austenite area becomes 4% or more, and cementite particles can become appropriately present in the retained austenite. “(273 + X) (12 + logY)” is a parameter in which the constant is set to 12 in the tempering parameter of martensitic steel, and depends on the temperature of X°C and the holding time of Y seconds in the second holding. In the case of X < 440 or (273 + X) (12 + logY) < 10000, the precipitation of cementite particles does not occur sufficiently, the retained austenite in which C is excessively concentrated remains, resulting in a decrease in the hole expansion ratio and a decrease in elongation under a high strain rate.On the other hand, in the case of 540 < X ​​or 11000 < (273 + X) (12 + logY), high elongation is not achieved because the cementite particles precipitate excessively, or the amount of retained austenite decreases significantly due to pearlite transformation. In the case of Y > 200, elongation decreases due to increased precipitated cementite or the occurrence of pearlite transformation. Therefore, the second retention test needs to be carried out under conditions of a temperature of X°C and a retention time of Y seconds that satisfy formulas 1 to 3 above. The average heating rate from the holding temperature in the first holding to the temperature of X°C in the second holding is 3°C / s greater (preferred interval). When the average heating rate from the holding temperature at the first holding to the temperature of X°C at the second holding is 3°C / s higher, the cementite particles tend to precipitate uniformly, and high elongation is readily achieved. Therefore, the average heating rate is preferably 3°C / s higher. The average heating rate is more preferably 10°C / s higher. The average heating rate is even more preferably 20°C / s higher. The upper limit of the average heating rate is not particularly restricted, but it is preferably 200°C / s lower from the standpoint of reducing equipment investment costs. Formation of the hot-dip galvanized layer or hot-dip electroannealed layer. A hot-dip galvanized or hot-dip annealed coating may be formed on a steel sheet surface between the first and second holding stages (after completing the first holding stage and before starting the second) or after completing the second holding stage. If the hot-dip galvanized coating is formed on a steel sheet surface between the first and second holding stages, or after completing the second holding stage, the steel sheet is immersed in an electroplating bath at normal bath temperature and subjected to coating treatment. The coating weight is adjusted, for example, by gas cleaning. There are no particular limitations on the electroplating bath temperature, but it is preferably between 450°C and 500°C.In the case where a hot-dip galvanized layer is formed on a surface of the steel sheet, after a hot-dip galvanized layer is formed, the hot-dip galvanized layer is subjected to alloy treatment to form a hot-dip galvanized layer. To improve corrosion resistance in practical applications, a steel sheet surface can undergo hot-dip galvanizing, as described above. In such cases, to ensure press formability, spot welding capability, and paint adhesion, hot-dip electro-annealing is often used. This treatment is performed after the coating to diffuse the iron from the steel sheet into the coated layer. In a series of heat treatments in the manufacturing method according to the present invention, the holding temperature need not be constant as long as the temperature remains within the range described above. Even if the cooling rate changes during cooling, the objective of the present invention is not affected as long as the cooling rate remains within the specified range. Furthermore, the steel sheet can be subjected to heat treatment in any equipment, provided only the thermal history is satisfied. Additionally, it is also included within the scope of the present invention that the steel sheet according to the present invention be subjected to temper rolling after heat treatment for the purpose of shape correction. A member according to the present invention and a method for manufacturing the same will now be described. A member according to the present invention is obtained by subjecting the steel sheet according to the present invention to at least one forming and welding step. A method for manufacturing a member according to the present invention includes a step of subjecting a steel sheet manufactured by the method for manufacturing a steel sheet according to the present invention to at least one of the forming and welding steps. The steel sheet according to the present invention has high strength, good ductility, and good draw-forming capacity, and is less likely to undergo ductility deterioration under a high strain rate. Therefore, a member obtained using the steel sheet according to the present invention has high strength, in which cracking and narrowing rarely occur in a drawn portion and a drawn-formed draw-formed draw-formed portion. Thus, the member according to the present invention can be suitable for, for example, a part obtained by forming a steel sheet into a complex shape. The member according to the present invention can be suitable for, for example, an automotive part. For forming, any common forming method, such as press forming, may be used without limitation. For welding, any common welding method, such as spot welding or arc welding, may be used without limitation. EXAMPLES The present invention will be described specifically with reference to Examples. The scope of the present invention is not limited by the following examples. Example 1 Steels having the chemical compositions shown in Table 1 were each obtained by steelmaking in a vacuum melting furnace, heated and held at a temperature of 1250°C for one hour, and rolled to a sheet thickness of 4.0 mm at a finish rolling temperature of 900°C. The hot-rolled steel sheets were held at 540°C for one hour and then furnace quenched. Note that the holding treatment of the steel sheet after hot rolling at 540°C for one hour, followed by furnace quenching, is equivalent to the coiling treatment of a steel sheet after hot rolling at 540°C. Subsequently, the resulting hot-rolled steel sheets were each pickled and then cold-rolled to a sheet thickness of 1.4 mm.Subsequently, the cold-rolled steel sheets after cold rolling were treated under the conditions shown in Table 2 to manufacture steel sheets. nofrn ιη / ζζηζ / Β / γίΛΐ § > NCNN C Table 1 Observations Suitable steel Suitable steel Suitable steel Suitable steel Comparative steel Comparative steel Suitable steel Suitable steel Suitable steel Comparative steel Suitable steel Suitable steel Comparative steel Suitable steel Suitable steel Comparative steel Suitable steel Suitable steel Suitable steel Suitable steel Suitable steel Suitable steel Suitable steel Suitable steel Suitable steel Suitable steel Suitable steel Suitable steel Suitable steel Chemical composition (% by mass) CO 800.0 qs • • cq 10.0 REM • • 0.002 100.0 05 0.002 CQ 0.0011 0.0012 0.03 0.02 0.02 <o CM ó • 0.05 • Mo • o 0.05 § 90'0 o LO 0.0033 0.0022 0.0028 0.0025 0.0022 0.0025 0.0031 0.0035 0.0033 0.0035 0.0036 0.0035 0.0035 9800'0 0.0035 0.0038 0.0035 0.0022 0.0035 0.0022 9800'0 0.0022 0.0035 0.0031 0.0041 0.0037 co 0.0020 0.0015 0.0014 0100'0 0.0033 0.0012 0.0018 1100'0 0.0012 0.0033 0.0021 0100'0 0.0023 0.0012 0.0009 0900'0 0900'0 0100'0 0100'0 0.0012 6000'0 0900'0 0.0032 0.0021 0.0011 0.0014 Q_ 0.011 0.011 0.016 010'0 0.011 0.010 0.014 0.011 0.012 010'0 110'0 0.014 0.017 110'0 0.011 0.012 0.010 0.014 0.011 0.016 0.012 0.011 0.012 0.011 600'0 80'0 CMn 2040. 05 2.30 1.99 2.33 2.22 co CM CO LO CM co CM cq CM co cq s co cq 2.36 CM CM cq 1.92 1.89 2.00 S y- 954 004 LO co LO 1.52 LO cq 10'8 50 S. 674 LO LO 86'0 en co 874 LO 0.98 674 1.59 en LO 0.04 0.06 0.04 lo 0.04 0.04 0.04 0.04 co 0.03 0.04 LO 0.04 0.04 0.030 0.040 en 4040. 0.04 0.04 0.03 0.03 LO 0.03 c / 5 CM 0.94 194 0.33 LO 874 CO 09'0 CO s 1.72 CM LO LO 1.51 0.95 0.40 CM 974 CM LO 0.40 974 974 CM. o 0.130 CD 0.078 0.172 0.058 0.203 0.127 0.145 0.135 0.139 0.132 0.124 0.102 0.133 0.127 0.122 0.165 CD 0.125 0.129 0.130 co 0.132 0.124 0.115 Steel type<c CQ O LU Ll— O ΞΕ — -□ ____________I O O- O CE CO 1— s X > - IXJ. The rest besides the above is Fe and secondary impurities. Table 2 OQbO ίΠ / ΖΖηΖ / Β / ΥΙΛΙ Table 2 (continued) Evaluation of Microstructure Area fractions of ferrite, bainite, tempered martensite, and fresh martensite The area fractions of ferrite, bainite, quenched martensite, and fresh martensite were determined by a point-counting method. From each of the steel sheets produced by the method described above, a cross-section through the sheet thickness was cut parallel to the rolling direction and heat-treated at 200°C for two hours. As a result, the fresh martensite was slightly quenched. The resulting cross-section through the sheet thickness (cross-section L) sample was polished, then etched with 1% by volume nital, and observed with a scanning electron microscope (SEM) at a position 1 / 4 of the sheet thickness at 1500x magnification using two fields of view. The area fractions were determined by plotting a grid over the image obtained by observation and counting 240 points in each field of view.Ferrite is a black microstructure, and bainite is a gray microstructure with a lattice-like morphology. In each of the quenched martensite samples, and in the fresh martensite sample after heat treatment at 200°C for two hours, a hierarchical structure including blocks and bundles, and a precipitate were observed. Since the hierarchical structure and precipitate in the quenched martensite are apparently coarser than those in the fresh martensite after heat treatment at 200°C for two hours, the area fraction of the quenched martensite and the area fraction of the fresh martensite can be clearly distinguished. The retained austenite containing cementite is distinguishable from other microstructures because the hierarchical structure is not observed in this phase under the sample preparation and observation conditions described above. Fraction of retained austenite area The volume fraction of retained austenite determined by a measurement method described below was considered to be the area fraction of retained austenite. The volume fraction of retained austenite was determined by polishing each of the steel sheets manufactured by the method described above in one direction of their thickness until a surface was exposed at a position of 1 / 4 of the thickness, and subjecting the surface at the 1 / 4 thickness position to X-ray diffraction intensity measurement. Mo-Kα radiation was used as the incident X-ray, the intensity ratios were determined with respect to all combinations of integral intensities of the peaks of the {111}, {200}, {220}, and {311} planes of retained austenite and the {110}, {200}, and {211} planes of ferrite, and the average of these was defined as the volume fraction of retained austenite. Area fraction of the remainder besides ferrite, bainite, tempered martensite, fresh martensite, and retained austenite The area fraction of the remainder was calculated by subtracting each of the area fractions of ferrite, bainite, quenched martensite, fresh martensite, and retained austenite calculated by the methods described above from 100%. Ratio of cementite particle area fraction in retained austenite to retained austenite area fraction Five retained austenite grains were observed by transmission electron microscopy on an observation surface, which was a surface located at a position 1 / 4 of the thickness of each of the steel sheets fabricated by the method described above. The ratio of the cementite particle area fraction in the retained austenite to the area fraction of the retained austenite was determined by the dot-counting method. A sample for transmission electron microscopy was prepared by electropolishing. A bright-field image of the retained austenite was captured at 50,000x magnification to include the surrounding interface.A grid was overlaid on the image, and 240 points were counted in each field of view. The number of intersection points corresponding to cementite particles was divided by the number of intersection points corresponding to retained austenite to determine the area fraction of the cementite particles. The grid had a shape similar to a square, with a length × width of 0.1 pm relative to the image. Electron diffraction was used to identify the cementite particles. Average major axis of cementite particles in retained austenite The average major axis of the cementite particles in the retained austenite was determined by measuring the maximum lengths of 10 cementite particles from the image described above of the cementite particles present in the retained austenite, the image being captured by a transmission electron microscope, and calculating the average of the maximum lengths. Note that, for samples in which the area fraction of retained austenite was less than 4%, measurements of the area fraction and average major axis of cementite particles were not performed using the transmission electron microscope. Tension Properties A tensile test was performed to measure TS (tensile strength) and Eh (total elongation). A test specimen machined from a JIS No. 5 test specimen was subjected to the tensile test at a crosshead or transverse speed of 10 mm / min in accordance with JIS Z 2241 (2011). In the present invention, where the tensile strength was 780 MPa or greater and less than 980 MPa, and Eh > 23%, the ductility was evaluated as good. Stretching Flaring Capacity The draw-forming capability was evaluated by a hole expansion test. A 100 mm x 100 mm test specimen was subjected to a hole expansion test three times according to the Japan Iron and Steel Federation Standard JFS T 1001 using a 60° conical punch, and a hole expansion ratio λ (%) was determined. In the present invention, in the case of λ > 25 (%), the draw-forming capability was evaluated as good. Elongation at a High Strain Rate A high-speed tensile test was performed to measure Eh (total elongation). A test specimen machined from a JIS No. 5 test specimen was subjected to a high-speed tensile test in which the crosshead or transverse speed of the tensile test described above was changed to 100 mm / min. In the present invention, the case where the ratio of the measured EI2 (total elongation) value in the high-speed tensile test to a measured Eh (total elongation) value in the normal tensile test described above was 85% or more was considered acceptable. That is, an EI2 / EI1 ratio of 0.85 or more was considered indicative of ductility deterioration under high strain rates. nofrn Ln / zznz / e / γΐΛΐ Table 3 OQbO ίΠ / ΖΖηΖ / Β / ΥΙΛΙ Table 3 (continued) Ek / Eh: Ratio of total elongation (EI2) in the high-speed tensile test to total elongation (Eh) in the normal tensile test The steel plates of the Examples of the Invention each have high strength, namely a tensile strength (TS) of 780 MPa or more and less than 980 MPa, good ductility, and good drawability. In the steel plates of the Examples of the Invention, the deterioration of ductility under a high strain rate was suppressed. In contrast, the steel plates of the Comparative Examples were inferior to those of the Examples of the Invention in at least one of these respects. Example 2 Steel sheet No. 1 in Table 3 of Example 1 was formed by press forming to manufacture a member of the Example of the Invention. Furthermore, steel sheet No. 1 in Table 3 of Example 1 and steel sheet No. 9 in Table 3 of Example 1 were joined by spot welding to manufacture a member of the Example of the Invention. It was confirmed that because the members of the Examples of the Invention have high strength, and because cracking and narrowing in the drawn and flanged portions of the members rarely occur, and because ductility deterioration under a high strain rate is suppressed, the members of the Examples of the Invention can be used appropriately for, for example, automotive parts.

Claims

1. A steel sheet characterized in that it comprises: a chemical composition containing, in % by mass, C: 0.07% or more and 0.18% or less, Si: 0.01% or more and 2.0% or less, Al: 0.01% or more and 2.0% or less, a total of Si and Al: 0.7% or more and 2.5% or less, Mn: 1.5% or more and 2.6% or less, P: 0.1% or less, S: 0.02% or less, and N: 0.0.10% or less, with the remainder being Fe and secondary impurities; and a steel microstructure that includes, in terms of area fraction, ferrite: 40% or more and 70% or less, a total of quenched bainite and martensite: 5% or more and 30% or less, retained austenite: 4% or more and 18% or less, fresh martensite: 8% or more and 35% or less, and the remainder: 5% or less, wherein cementite particles are present in the retained austenite, a ratio of an area fraction of the cementite particles in the retained austenite to an area fraction of the retained austenite is 5% or more and 25% or less, and the steel plate has a tensile strength of 780 MPa or greater and less than 980 MPa.

2. The steel sheet according to claim 1, further characterized in that the cementite particles in the retained austenite have an average major axis of 30 nm or more and 400 nm or less.

3. The steel sheet according to claim 1 or 2, further characterized in that the chemical composition additionally contains, by mass %, at least one selected from Cr, V, Mo, Ni, and Cu in a total amount of 1.0% or less.

4. The steel sheet according to any of claims 1 to 3, further characterized in that the chemical composition additionally contains, in % by mass, at least one selected from Ti: 0.20% or less, and Nb: 0.20% or less.

5. The steel sheet according to any of claims 1 to 4, further characterized in that the chemical composition additionally contains, in % by mass, B: 0.005% or less.

6. The steel sheet according to any of claims 1 to 5, further characterized in that the chemical composition additionally contains, in % by mass, at least one selected from Ca: 0.005% or less, and REM: 0.005% or less.

7. The steel sheet according to any of claims 1 to 6, further characterized in that the chemical composition additionally contains, in % by mass, the nofrn Ln / zznz / e / γALA less one selected from Sb: 0.05% or less, and Sn: 0.05% or less.

8. The steel sheet according to any of claims 1 to 7, further characterized in that it additionally comprises a hot-dip galvanized layer or a hot-dip electro-annealed layer on a surface of the steel sheet.

9. A member characterized in that it is obtained by subjecting the steel sheet as claimed in any of claims 1 to 8 to at least one forming and welding process.

10. A method for manufacturing a steel sheet, characterized in that the method comprises hot rolling or cold rolling a sheet having the chemical composition according to any of claims 1 and 3 to 7; subsequently holding it at an annealing temperature of 700°C or higher and 950°C or lower for 30 seconds or more and 1000 seconds or less; cooling it from the annealing temperature to a cooling interruption temperature of 150°C or higher and 420°C or lower at an average cooling rate of 10°C / s or higher; subsequently holding it a first time under conditions in a temperature range of 380°C or higher and 420°C or lower for 10 seconds or more and 500 seconds or less; and further holding it a second time under conditions of a temperature of X°C and a holding time of Y seconds satisfying the following formulas 1 to 3.Formula 1: 10000 < (273 + X) (12 + logY) <11000 Formula 2: 440 < 11. The method for manufacturing a steel sheet according to claim 10, further characterized in that an average heating rate from a holding temperature in the first holding to a temperature of X°C in the second holding is 3°C / s greater.

12. The method for manufacturing a steel sheet according to claim 10, further characterized in that an average heating rate from a holding temperature in the first holding to a temperature of X°C in the second holding is 10°C / s greater.

13. The method for manufacturing a steel sheet according to any of claims 10 to 12, further characterized in that it comprises, between the first retention and the second retention or after completing the second retention, forming a hot-dip galvanized layer or a hot-dip electro-annealed layer on a surface of the steel sheet.

14. A method for manufacturing a member, characterized in that the method comprises a step of subjecting a steel plate manufactured by the method for manufacturing a steel plate in accordance with any of claims 10 to 13 to at least one forming and welding step.