Steel sheet, plated steel sheet, method for producing steel sheet, method for producing plated steel sheet, member, and method for producing member

Abstract

The purpose of the present invention is to provide a technology relating to: a steel sheet which still has high collision resistance characteristics after being subjected to cold pressing and being formed into a component; and a method for producing the steel sheet.　Disclosed is a steel sheet which has a specific component composition and a specific steel structure, and has a yield ratio of 0.80 or less, wherein: in a tensile test according to JIS Z2241, the retained austenite that disappears during the period from the start of the tensile test to the processing of imparting 1% strain in terms of nominal strain is 0.8% or more in terms of the area ratio with respect to the entire steel structure; and when the processing of imparting 1% strain and a heat treatment at a temperature of 100°C are performed, the amount of increase in YS obtained by subtracting YS before (the processing and the heat treatment) from YS after (the processing and the heat treatment) is 200 MPa or more, and El after (the processing and the heat treatment) is not less than 0.9 × (El before (the processing and the heat treatment)). YS and El represent the yield stress and total elongation of the steel sheet, respectively.

Description

Steel sheet, plated steel sheet, method for manufacturing a steel sheet, method for manufacturing a plated steel sheet, component, and method for manufacturing a component.

[0001] The present invention relates to steel sheets, plated steel sheets, and methods for manufacturing them, as well as components and methods for manufacturing components, which are suitable for use in structural and electrical components of automobile bodies that are formed by cold pressing one or more of the following: drawing, stretching, and elongating flanges, followed by baking paint at a temperature lower than the conventional 170°C.

[0002] In recent years, due to growing concern about global environmental issues, CO2 ２ With stricter emission regulations, the automotive sector is demanding improved fuel efficiency through lighter vehicle bodies and increased crash safety for large batteries used in electrified vehicles. Therefore, there is a growing demand for high-strength materials that provide high body rigidity and crash safety even with thinner and lighter materials for automotive applications. Many steel manufacturers are expanding their lineups of high-strength materials for automotive steel sheets, including those with tensile strengths of 980 MPa and even higher. The main challenges for such high-strength materials are: (1) achieving both high strength and high ductility; (2) improving resistance to delayed fracture (hydrogen embrittlement), a problem specific to high-strength materials; (3) ensuring ease of processing when used in cold press working; and (4) ensuring high crash performance after processing into automotive parts to guarantee crash safety. Regarding (1), generally, solid solution strengthening, precipitation strengthening, and dislocation strengthening (transformation strengthening) are known as methods for strengthening steel. However, to increase the strength of steel sheets with a tensile strength of over 980 MPa, it is necessary to increase the proportion of hard structures such as martensite and bainite through transformation strengthening. However, when more of these hard phases are included in a steel sheet, the workability of the steel sheet decreases. As a result, it has the disadvantage of not being applicable to parts that require complex processing, and the applicable parts are limited.

[0003] As a method for improving the mechanical properties of high-strength steel sheets using such a hard phase, a method of utilizing tempered martensite is known. For example, Patent Document 1 discloses a super-rapid cooling method of 400°C / s or higher using water quenching and subsequent tempering heat treatment, which results in a high-strength cold-rolled steel sheet with a tensile strength exceeding 980 MPa containing 70% or more of (tempered) martensite and excellent elongation flangeability. The method using water quenching has the advantage of achieving high strength with minimal reliance on steel alloying, is excellent in the hydrogen embrittlement resistance property of (2), and is also advantageous in terms of weldability, etc. in relation to (3). Furthermore, due to the high yield strength (yield stress) YS of tempered martensite, it is also advantageous for the collision safety of (4), and thus in recent years, its application to automotive parts has been progressing.

[0004] Also, by further improving these technologies and utilizing the austenite phase responsible for ductility while mainly using tempered martensite, a technology that achieves both high strength and high ductility of steel sheets has been established, as shown in, for example, Patent Document 2. Here, in the final annealing process, by cooling to a temperature below the martensite transformation start temperature and higher than the martensite transformation completion temperature, a structure of martensite and austenite is formed, and then, in order to stabilize the austenite and temper the martensite, re-heating and holding are performed.

[0005] Furthermore, Patent Document 3 discloses a method for manufacturing a steel sheet having good bending characteristics and bendability by performing a tempering treatment on the steel sheet after plating and cooling it to 200°C or lower and then further in the temperature range of 100 to 600°C. Although these materials are characterized by excellent ductility, since they contain slightly more alloying elements than water-quenched materials, there are limitations in product utilization in terms of the weldability of (2) and (3), and various countermeasures are being studied.

[0006] For example, Patent Document 4 discloses a method for improving the stress corrosion cracking resistance property by optimizing the Si content in steel.

[0007] Also, an invention using a hot working method to avoid problems specific to cold pressing is disclosed in Patent Document 5.

[0008] Japanese Patent Publication No. 5151354, Japanese Patent Publication No. 6787525, Japanese Unexamined Patent Publication No. 2017-48412, Japanese Patent Publication No. 5423072, Japanese Patent Publication No. 7036214

[0009] As mentioned above, high-strength steel sheets utilizing the martensitic phase offer excellent impact resistance when applied to automotive parts due to their high yield strength. However, they also have drawbacks, such as cracking during cold pressing and poor shape retention due to significant springback after press forming. Therefore, their application is limited to parts manufactured using processes such as roll forming. While hot rolling is also being considered, as mentioned above, the need for cold-pressed materials remains high in recent years due to the need to reduce environmental impact.

[0010] The high yield strength of the aforementioned high-strength steel sheets is a characteristic that is inevitably obtained by actively utilizing tempered martensite. Therefore, one way to avoid this is to use, for example, untempered martensite (fresh martensite) or tempered martensite with a small degree of tempering, but this significantly reduces ductility such as elongation flangeability. Furthermore, there is the disadvantage that the material becomes unstable due to the large amount of solid solution carbon remaining in the steel sheet.

[0011] The inventors of this invention have diligently conducted research to solve the above problems and have come to design materials that focus on work hardening and baked paint hardening as methods for strengthening steel, in addition to the solid solution strengthening, precipitation strengthening, and dislocation strengthening (transformation strengthening) mentioned above. After sheet parts are cut from a coil-shaped product, strain is introduced into the material by press forming or the like. At this time, work hardening occurs, and the material generally becomes harder. After forming, the surface is painted after processes such as assembly and welding, but in the paint baking process, the parts are heat-treated at a temperature range of about 170°C. Depending on the material, baked paint hardening may occur after heat treatment, causing further hardening.

[0012] Furthermore, in recent years, as part of efforts toward carbon neutrality, research has been underway to lower the temperature of the baking heat treatment as much as possible. Generally, lowering the baking temperature suppresses the increase in strength, so obtaining a high strength increase even with low-temperature heat treatment is becoming an important challenge for steel sheets that utilize baked coating hardening.

[0013] The present invention aims to provide technology for a steel sheet that, after being subjected to cold press working and formed into a part, has high collision resistance, and a method for manufacturing the same. Specifically, the objectives are to provide excellent cold press formability, that is, high tensile strength (TS) and excellent elongation characteristics, while having a low yield stress correlated with the amount of springback, i.e., a low yield ratio (yield stress / tensile strength), further suppressing the change in elongation after heat treatment and increasing the yield stress, and further increasing the stress increase after processing into a part shape and heat treatment at a low temperature, such as 100°C, in order to ensure collision safety in automobile parts and the like.

[0014] In this invention, a low yield ratio means a yield ratio of 0.80 or less. Furthermore, suppressing the change in elongation and increasing the yield stress after processing and heat treatment means that, after processing to impart 1% strain at nominal strain in a tensile test, and then performing heat treatment at a temperature of 100°C, the increase in YS (YS after processing and heat treatment) minus the YS before processing and heat treatment is 200 MPa or more, and the El (total elongation) after processing and heat treatment is 0.9 × (El before processing and heat treatment) or more. Hereinafter, YS and El represent the yield stress and total elongation of the steel plate, respectively. Furthermore, increasing the stress increase after the above heat treatment means that the difference between the stress increase after heat treatment at 100°C without pre-strain in the tensile test and the stress increase after heat treatment at 100°C after processing to impart 1% strain in the tensile test is 75 MPa or more.

[0015] The inventors have discovered that by utilizing tempered martensite to increase strength, while also appropriately controlling the retained austenite present as a second phase in the steel, it is possible to produce steel sheets with unprecedentedly high work hardening and heat treatment hardening properties. Furthermore, it has been found that such steel sheets have an extremely high strength-increasing effect even at low heat treatment temperatures during baking. In other words, the gist of this disclosure is as follows. [1] The composition is such that, by mass%, C: 0.100% or more and 0.295% or less, Si: 0.01% or more and 1.60% or less, Mn: 0.10% or more and 5.00% or less, Cr: 0.01% or more and 1.00% or less, P: 0.1000% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0200% or less, O: 0.0100% or less, Ti: 0.200% or less, and Ti / 48 ≥ N / 14 (Ti and N are components contained in steel sheets (mass%)), and further, B: 0.0003% or more and 0.0100% or less, and P + B ≥ 0.0050%, with the remainder being Fe and unavoidable impurities. Furthermore, the Si / 28 + Cr / 52 ratio (Si and Cr are components contained in the steel sheet (mass%)) satisfies 0.040 to 0.052, the steel structure is such that ferrite accounts for 1% or less by area percentage, bainite accounts for 0.1% or more by area percentage, retained austenite accounts for 5% to 20% by area percentage, tempered martensite accounts for 70% or more by area percentage, and fresh martensite accounts for 0% to 15% by area percentage, the carbon concentration in the retained austenite satisfies 0.45% to 1.00%, the yield ratio is 0.80 or less, and in a tensile test in accordance with JIS Z2241, the retained austenite that disappears between the start of the tensile test and the processing that imparts 1% strain at nominal strain accounts for 0.8% or more by area percentage of the entire steel structure, and when the processing that imparts 1% strain and heat treatment at a temperature of 100°C are performed, A steel sheet having a yield strength of 200 MPa or more, obtained by subtracting the pre-processing and heat treatment YS from the post-processing and heat treatment YS, and a post-processing and heat treatment El of 0.9 × (pre-processing and heat treatment El) or more. YS and El represent the yield strength and total elongation of the steel sheet, respectively.[2] The steel sheet according to [1], wherein the component composition further contains, by mass%, at least one of the following: Cu: 0.005% to 0.500%, Sn: 0.005% to 0.500%, and Sb: 0.001% to 0.080%, in total amount of 0.010% to 1.000%. [3] The steel sheet according to [1] or [2], wherein the component composition further contains, in mass%, at least one selected from: Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Mo: 1.00% or less, Co: 1.000% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, Ni: 0.8% or less. [4] A plated steel sheet having either an electro-galvanized layer or a hot-dip galvanized layer formed on the surface of the steel sheet according to any one of [1] to [3]. [5] After heating a steel slab having the component composition described in any of [1] to [3] above, hot rolling is performed, the resulting hot-rolled sheet is pickled, and the resulting cold-rolled sheet is heated to Ts such that the average heating rate in the temperature range from 650°C to 800°C is 2°C / s or less, and the average heating rate in the temperature range from 800°C to the maximum annealing temperature Ts which is Ac3 point + 30°C or higher is 1°C / s or less. A method for manufacturing a steel sheet, comprising cooling from 400°C to a temperature T1 between 50°C and 250°C, then raising the temperature from T1 to a temperature T2 between 280°C and 380°C, holding the temperature in the range of T2-10°C or higher and below T2 for 1 to 60 seconds, then cooling with an average cooling rate of 2°C / s or higher in the temperature range from T1 to T1-10°C, and after a second pickling, performing skin pass rolling at a plate temperature of 20°C or higher with an elongation of 0.1% or less (including 0%).[6] A steel slab having the component composition described in any of [1] to [3] above is heated, then hot-rolled, the resulting hot-rolled sheet is pickled, and the resulting cold-rolled sheet is heated to Ts such that the average heating rate in the temperature range from 650°C to 800°C is 2°C / s or less, and the average heating rate in the temperature range from 800°C to the maximum annealing temperature Ts which is Ac3 point + 30°C or higher is 1°C / s or less. A method for manufacturing a plated steel sheet, wherein the sheet is then cooled from 400°C to a temperature T1 between 50°C and 250°C, then hot-dip galvanizing is performed, the temperature is raised from T1 to Tm, which is between 450°C and 500°C, and the sheet is held in the temperature range of Tm-10°C or higher and less than Tm for 1 s to 60 s, and then cooled with an average cooling rate of 2°C / s or higher in the temperature range of T1 to T1-10°C or lower, and after a second pickling, skin pass rolling is performed at a sheet temperature of 20°C or higher with an elongation of 0.1% or less (including 0%). [7] A method for manufacturing a steel sheet according to [5], wherein pre-cold rolling annealing is performed between the hot rolling and the start of cold rolling, by holding the sheet in the temperature range T between 400°C and Ac3 for 10 minutes or more. [8] A method for manufacturing a plated steel sheet according to [6], wherein pre-cold rolling annealing is performed by holding the steel sheet in a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more between the hot rolling and the start of the cold rolling. [9] A method for manufacturing a steel sheet according to [5] or [7], wherein the steel sheet is further electroplated with zinc to make a plated steel sheet.

[10] A component made using at least a part of the steel sheet according to any one of [1] to [3] or the plated steel sheet according to [4].

[11] A method for manufacturing a component, wherein the steel sheet according to any one of [1] to [3] or the plated steel sheet according to [4] is subjected to forming and joining processes to make a component.

[0016] According to the present invention, a high-strength steel sheet with a low yield ratio can be obtained. Because the yield strength is low relative to the tensile strength of the material, it can have high cold press formability, such as a small amount of springback during cold pressing. Furthermore, because the steel sheet of the present invention has an extremely high amount of work hardening and heat treatment hardening, the yield stress of the material is increased significantly after cold press forming and subsequent heat treatment. For example, when used as an automobile part, it is possible to improve collision performance. Moreover, it is possible to obtain an extremely high stress increase effect even at a lower heat treatment temperature than conventional methods. As a result, the range of applications to various parts and components such as automobile body parts that are cold press formed is broadened, and it can be applied to parts that require complex press processing. This can greatly contribute to the weight reduction of automobile bodies and other electrical components, and furthermore, by lowering the heat treatment temperature, it is possible to contribute to the creation of an environment in which steel sheets can be used with a low energy consumption load.

[0017] Embodiments of this disclosure will be described below. However, this disclosure is not limited to the embodiments described below. First, the appropriate range of the component composition of the steel slab used as the material for the steel sheet of the present invention and the reasons for limiting it will be explained. In the following description, "%" representing the content of component elements in the steel sheet means "mass%" unless otherwise specified.

[0018] [C: 0.100% or more and 0.295% or less] Carbon (C) is one of the important basic components of steel, and in particular in this invention, it is an important element that affects the fractions of martensite, ferrite, and retained austenite. If the C content is less than 0.100%, the fraction and strength of martensite will decrease, making it difficult to achieve the desired TS (tensile strength). Therefore, the C content should be 0.100% or more. Preferably, the C content should be 0.120% or more, more preferably 0.150% or more, even more preferably 0.180% or more, and most preferably 0.200% or more. On the other hand, if the C content exceeds 0.295%, the martensite will become brittle, making it difficult to achieve the desired elongation. Therefore, the C content should be 0.295% or less. Preferably, the C content should be 0.275% or less, more preferably 0.260% or less, even more preferably 0.240% or less, and most preferably 0.230% or less.

[0019] [Si: 0.01% or more and 1.60% or less] Si is one of the important basic components of steel, and in particular in this invention, it is an element that affects the hardness of martensite and the fraction of retained austenite because it suppresses carbide formation during continuous annealing and promotes the formation of retained austenite. If the Si content is less than 0.01%, the fraction of retained austenite decreases, making it difficult to achieve the desired El. Therefore, the Si content should be 0.01% or more. A Si content of 0.04% or more is preferable, 0.06% or more is more preferable, 0.40% or more is even more preferable, and 0.85% or more is most preferable. On the other hand, if the Si content exceeds 1.60%, the carbon concentration in the retained austenite increases excessively, making it difficult to secure the unstable retained austenite necessary for this invention. In addition, in hole expansion tests, the hardness of the martensite that transforms from retained austenite during punching increases significantly, which increases the formation of voids during punching and hole expansion, and reduces the hole expansion ratio λ, which is an indicator of stretch flangeability. Furthermore, weldability is also impaired. Therefore, the Si content should be 1.60% or less. Preferably, the Si content should be 1.40% or less, more preferably 1.30% or less, even more preferably 1.25% or less, and most preferably 1.20% or less.

[0020] [Mn: 0.10% or more and 5.00% or less] Mn is one of the important basic components of steel, and in particular in this invention, it is an important element that affects the fraction of martensite. If the Mn content is less than 0.10%, the fraction of martensite decreases, making it difficult to achieve a TS of more than 980 MPa. For this reason, the Mn content should be 0.10% or more. Preferably, the Mn content should be 0.80% or more, more preferably 1.00% or more, even more preferably 1.50% or more, and most preferably 2.00% or more. On the other hand, if the Mn content exceeds 5.00%, weldability and delayed fracture resistance deteriorate. For this reason, the Mn content should be 5.00% or less. Preferably, the Mn content should be 4.50% or less, more preferably 4.00% or less, even more preferably 3.50% or less, and most preferably 3.00% or less.

[0021] [Cr: 0.01% or more and 1.00% or less] Cr is an important element in the present invention. Like Si, it has the effect of suppressing the formation of coarse carbides and securing solid-solution carbon in the steel. Unlike Si, it has a strong effect of improving hardenability, so by including it instead of Si, it is possible to further reduce the content of C and Mn included for high strength. The inclusion of Cr increases the retained austenite fraction and has the effect of stabilizing retained austenite as shown in Patent Document 6. However, in the present invention, it has been found that if used under appropriate manufacturing conditions, it increases the retained austenite fraction while also destabilizing it. As a result, the amount of retained austenite that undergoes stress-induced transformation during processing increases, and even better work hardening characteristics can be obtained. In order to exhibit this retained γ destabilization effect of Cr, it is necessary to set the final annealing conditions to the conditions shown herein, and a Cr content of 0.01% or more is required. The Cr content is preferably 0.10% or more, more preferably 0.50% or more, and even more preferably 0.60% or more. On the other hand, if the Cr content is 1.00% or less, the amount of coarse precipitates and inclusions does not increase, and the ductility of the steel sheet is not impaired. For this reason, the Cr content is set to 1.00% or less. Preferably, the Cr content is 0.90% or less, and more preferably 0.80% or less. By including the amount of Cr according to the present invention, it is possible to create an unprecedentedly unstable retained austenite. However, in order to utilize this for high strength during processing and heat treatment, especially under low-temperature heat treatment conditions, the solid solution carbon released from retained γ during processing must interact with mobile dislocations without segregating at grain boundaries or block interfaces. As a means to achieve this, it has been found that it is effective to satisfy the conditions that the steel sheet contains appropriate amounts of P and solid solution B, that is, Ti / 48 ≥ N / 14 (Ti and N are components contained in the steel sheet (mass%)) and P + B ≥ 0.0050%. Although many aspects of the detailed mechanism remain unclear, it is presumed that when large amounts of P and solid-solution B are present, these elements preferentially segregate at grain boundaries and block interfaces instead of solid-solution carbon, and as a result, the solid-solution carbon fixes mobile dislocations within the grain, thereby increasing strength. [Patent Document 6] Japanese Patent No. 4688782

[0022] [P: 0.1000% or less] P segregates at the prior austenite grain boundaries, causing them to become brittle and impairing the ductility of the steel sheet. It also impairs weldability and delayed fracture resistance. Therefore, the P content must be 0.1000% or less. Accordingly, the P content is set to 0.1000% or less. Preferably, the P content is 0.0700% or less, more preferably 0.0200% or less, even more preferably 0.0100% or less, and most preferably 0.0080% or less. There is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable to have a P content of 0.0010% or more. It is even more preferable for the P content to be 0.0030% or more.

[0023] [S: 0.0200% or less] S exists as a sulfide and impairs the ductility of the steel sheet. It also impairs weldability and delayed fracture resistance. Therefore, the S content must be 0.0200% or less. Accordingly, the S content is set to 0.0200% or less. Preferably, the S content is 0.0050% or less, more preferably 0.0030% or less, and even more preferably 0.0010% or less. There is no particular lower limit for the S content, but due to production technology constraints, it is preferable to set it to 0.0001% or more. More preferably, the S content is 0.0003% or more, even more preferably 0.0005% or more, and most preferably 0.0006% or more.

[0024] [Al: 1.000% or less] If Al is present in large quantities, it raises the Ac3 point (Ac3 transformation point) and contains a large amount of ferrite in the microstructure, which hinders the high strength achieved by utilizing the martensitic structure. Therefore, the Al content must be 1.000% or less. The Al content is preferably 0.500% or less. The Al content is more preferably 0.200% or less, and even more preferably 0.100% or less. There is no particular lower limit for the Al content, but it is preferable that the Al content be 0.001% or more in order to suppress carbide formation during continuous annealing and promote the formation of retained austenite. The Al content is more preferably 0.004% or more, and even more preferably 0.008% or more.

[0025] [N: 0.0200% or less] N exists as a nitride and impairs ductility. Therefore, the N content must be 0.0200% or less. Accordingly, the N content is 0.0200% or less. Preferably, the N content is 0.0100% or less. More preferably, the N content is 0.0080% or less, even more preferably 0.0060% or less, and most preferably 0.0050% or less. There is no particular lower limit for the N content, but due to production technology constraints, it is preferable that the N content be 0.0001% or more. More preferably, the N content is 0.0005% or more, and even more preferably 0.0010% or more.

[0026] [O: 0.0100% or less] O exists as an oxide and impairs the ductility of the steel sheet. Therefore, the O content must be 0.0100% or less. Accordingly, the O content is set to 0.0100% or less. Preferably, the O content is 0.0050% or less. More preferably, the O content is 0.0040% or less, even more preferably 0.0020% or less, and most preferably 0.0015% or less. There is no particular lower limit for the O content, but due to production technology constraints, it is preferable that the O content be 0.0001% or more. More preferably, the O content is 0.0003% or more, and even more preferably 0.0005% or more.

[0027] [Ti: 0.200% or less] If the Ti content is 0.200% or less, large amounts of coarse precipitates and inclusions will not be generated, and it will not reduce the ductility of the steel sheet or decrease the carbon concentration in the retained austenite. For this reason, it should be 0.200% or less. The Ti content is preferably 0.100% or less, more preferably 0.075% or less, even more preferably 0.050% or less, and most preferably 0.045% or less. Furthermore, since Ti increases the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, it is more preferable to have a Ti content of 0.001% or more. In addition, the effect of adding B, which will be described later, is further enhanced when Ti is contained within the following range.

[0028] Ti / 48 ≥ N / 14 (Ti and N are components (mass%) contained in the steel sheet) Ti, which has an even higher affinity for N than B, forms nitrides before B, thereby ensuring that B is dissolved in the steel. It is presumed that dissolved B, which is a grain boundary segregating element, suppresses the segregation of dissolved C at the grain boundaries and maintains the high heat-treatable hardening properties that result from dissolved C fixing to dislocations within the grains. Therefore, Ti must be contained in an N / 14 × 48 or higher ratio.

[0029] [B: 0.0003% or more and 0.0100% or less] If B is 0.0100% or less, it will not cause cracks to form inside the steel sheet during casting or hot rolling, and will not impair the ductility of the steel sheet. For this reason, the B content should be 0.0100% or less. Preferably, the B content should be 0.0080% or less, more preferably 0.0060% or less, even more preferably 0.0040% or less, and most preferably 0.0030% or less. On the other hand, since B is an element that segregates at the austenite grain boundaries during annealing and improves hardenability, the B content should be 0.0003% or more. Preferably, the B content should be 0.0005% or more, more preferably 0.0008% or more, and even more preferably 0.0010% or more.

[0030] Furthermore, in order to further enhance the effect of solid-solution carbon (C), it is preferable that the total content of grain boundary segregating elements P and B be 0.0050% or more. P and B preferentially segregate at the grain boundaries, thereby suppressing the segregation of carbon (C) and enhancing the dislocation fixing effect of intragranular solid-solution carbon (C). The total content of grain boundary segregating elements P and B is more preferably 0.0070% or more, even more preferably 0.0100% or more, and most preferably 0.0110% or more. There is no particular upper limit, but it is preferable that the total content of P and B be 0.0800% or less, and more preferably 0.0500% or less.

[0031] [Si / 28 + Cr / 52: 0.040 or more and 0.052 or less (Si and Cr are components contained in the steel sheet (mass%))] Si and Cr are elements that suppress the formation of carbides and secure solid solution carbon, and are useful for the formation and stabilization of retained austenite, so the sum of their elemental ratios (Si / 28 + Cr / 52) is set to 0.040 or more. The sum of their elemental ratios (Si / 28 + Cr / 52) is preferably 0.042 or more, more preferably 0.045 or more, and even more preferably 0.047 or more. On the other hand, excessive addition will excessively stabilize retained austenite and will not allow the desired work hardening characteristics to be obtained, so the sum of their elemental ratios (Si / 28 + Cr / 52) is set to 0.052 or less. The sum of their elemental ratios (Si / 28 + Cr / 52) is more preferably 0.050 or less, and even more preferably 0.049 or less.

[0032] A steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the remainder being Fe and unavoidable impurities. Examples of unavoidable impurities include H, Zn, Pb, As, Se, Ge, Sr, and Cs. Unavoidable impurities may be introduced from raw materials such as scrap, and their inclusion is permissible as long as it does not hinder the objective of the present invention. Specifically, it is permissible for these impurities to be present in a total of 0.100% or less.

[0033] Furthermore, in addition to the above component composition, the steel sheet of the present invention may optionally contain, in mass%, at least one of the following: Cu: 0.005% to 0.500%, Sn: 0.005% to 0.500%, and Sb: 0.001% to 0.080%, totaling 0.010% to 1.000%. Optionally, the mixture may also contain, either alone or in combination, at least one element selected from the following by mass%, in the following amounts: Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Mo: 1.00% or less, Co: 1.000% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, and Ni: 0.8% or less.

[0034] [In mass%, Cu: 0.005% to 0.500%, Sn: 0.005% to 0.500%, and Sb: at least one of the following, totaling 0.010% to 1.000%] If Sn and Cu are present in amounts of 0.500% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ductility of the steel sheet will not be impaired. Therefore, when Sn and Cu are included, the Sn and Cu content should be 0.500% or less each. Preferably, the Sn and Cu content should be 0.100% or less each. There is no specific lower limit for the content, but since Sn and Cu are elements that improve hardenability, when Sn and Cu are included, the Sn and Cu content should be 0.005% or more each. Sb has the effect of suppressing surface decarburization during the annealing process. In the high-temperature annealing characteristic of the present invention, decarburization progresses, forming ferrite on the surface which is unfavorable for high strength, making it difficult to obtain the desired strength. Therefore, when Sb is included, it should contain 0.001% or more. The Sb content is preferably 0.004% or more, more preferably 0.005% or more, and even more preferably 0.006% or more. On the other hand, if it is 0.080% or less, the amount of coarse precipitates and inclusions does not increase, and the ductility of the steel sheet is not impaired. Therefore, when Sb is included, the Sb content should be 0.080% or less. The Sb content is preferably 0.050% or less, more preferably 0.020% or less, even more preferably 0.015% or less, and most preferably 0.010% or less. Cu, Sn, and Sb are known as surface segregation elements, and by adding a certain amount, it is possible to suppress surface decarburization during the annealing process, and consequently, to suppress structural non-uniformity in the thickness direction and the decrease in strength of the base material due to softening of the surface of the plate thickness. In particular, in the present invention, the heating rate during the final annealing is small and the maximum temperature reached is high, so decarburization of the steel tends to progress easily during annealing, and the inclusion of these elements provides a high softening suppression effect.Therefore, within the above range, if Cu and / or Sn and / or Sb are included, at least one of them should be present in a total of 0.010% or more, preferably 0.015% or more, more preferably 0.020% or more, even more preferably 0.025% or more, and most preferably 0.030% or more. Furthermore, if Cu and / or Sn and / or Sb are included, at least one of them should be present in a total of 1.000% or less, preferably 0.300% or less, more preferably 0.200% or less, even more preferably 0.150% or less, and most preferably 0.100% or less. It is even more preferable to include two or more of Cu, Sn, and Sb in a total of 0.015% or more. It is even more preferable to include two or more of Cu, Sn, and Sb in a total of 0.080% or less.

[0035] Nb: 0.200% or less, V: 0.200% or less If Nb and V are each 0.200% or less, large amounts of coarse precipitates and inclusions will not be generated, and the ductility of the steel sheet will not decrease, nor will the carbon concentration in the retained austenite decrease. For this reason, when Nb and V are included, the Nb and V content should each be 0.200% or less. Preferably, the Nb and V content should each be 0.100% or less. More preferably, the Nb and V content should each be 0.075% or less, even more preferably 0.050% or less, and most preferably 0.040% or less. There is no particular lower limit for the Nb and V content, but since the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, it is preferable that the Nb and V content each be 0.001% or more. More preferably, each component is 0.005% or more, and even more preferably, each component is 0.010% or more.

[0036] Ta: 0.10% or less, W: 0.10% or less. If the amounts of Ta and W are 0.10% or less, large amounts of coarse precipitates and inclusions will not be formed, and the ductility of the steel sheet will not be impaired. For this reason, when Ta and W are included, the amounts of Ta and W should be 0.10% or less each. Preferably, the amounts of Ta and W should be 0.09% or less each. More preferably, the amounts of Ta and W should be 0.08% or less each. There is no particular lower limit for the amounts of Ta and W, but since the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, it is preferable that the amounts of Ta and W be 0.01% or more each, and more preferably 0.02% or more each.

[0037] Mo: 1.00% or less. If the Mo content is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not be impaired. Therefore, if Mo is included, the Mo content should be 1.00% or less. It is preferable that the Mo content be 0.80% or less. It is more preferable that the Mo content be 0.70% or less, even more preferable that it be 0.60% or less, and most preferable that it be 0.50% or less. There is no particular lower limit for the Mo content, but since it is an element that improves hardenability, it is preferable that the Mo content be 0.01% or more. It is more preferable that the Mo content be 0.10% or more, and even more preferable that it be 0.15% or more.

[0038] Co: 1.000% or less. If the Co content is 1.000% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not be impaired. Therefore, if Co is included, the Co content should be 1.000% or less. Preferably, the Co content should be 0.500% or less. More preferably, the Co content should be 0.100% or less, even more preferably 0.050% or less, and most preferably 0.030% or less. There is no particular lower limit for the Co content, but since it is an element that improves hardenability, it is preferable that the Co content be 0.001% or more. More preferably, the Co content should be 0.008% or more, and even more preferably 0.010% or more.

[0039] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less. If Ca, Mg, and REM are each at 0.0100% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not be impaired. Therefore, when Ca, Mg, and REM are included, the content of Ca, Mg, and REM should be 0.0100% or less each. Preferably, the content of Ca, Mg, and REM should be 0.0050% or less each, more preferably 0.0030% or less each, even more preferably 0.0025% or less each, and most preferably 0.0020% or less each. Although there is no specific lower limit for the content of Ca, Mg, and REM, these elements spheroidize the shape of nitrides and sulfides and improve the ductility of steel sheets. Therefore, it is preferable that the content of Ca, Mg, and REM be 0.0005% or more, and more preferable that it be 0.0010% or more, for each element.

[0040] Zr: 0.100% or less, Te: 0.100% or less If Zr and Te are each at 0.100% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not be impaired. Therefore, when Zr and Te are included, the Zr and Te content should be 0.100% or less each. Preferably, the Zr and Te content should be 0.080% or less each, more preferably 0.050% or less each, even more preferably 0.020% or less each, and most preferably 0.015% or less each. There is no particular lower limit for the Zr and Te content, but since these elements spheroidize the shape of nitrides and sulfides and improve the ductility of the steel sheet, it is preferable that the Zr and Te content be 0.001% or more each. Preferably, the Zr and Te content should be 0.005% or more each, and even more preferably 0.008% or more each.

[0041] Hf: 0.10% or less. If Hf is 0.10% or less, no coarse precipitates or inclusions will increase, and the ductility of the steel sheet will not be impaired. Therefore, when Hf is contained, the Hf content should be 0.10% or less. The Hf content is preferably 0.08% or less, more preferably 0.05% or less, and even more preferably 0.02% or less. Although the lower limit of the Hf content is not particularly specified, since it is an element that spheroidizes the shape of nitrides and sulfides and improves the ductility of the steel sheet, the Hf content is preferably 0.01% or more.

[0042] Bi: 0.200% or less. If Bi is 0.200% or less, no coarse precipitates or inclusions will increase, and the ductility of the steel sheet will not be impaired. Therefore, when Bi is contained, the Bi content should be 0.200% or less. The Bi content is preferably 0.100% or less, more preferably 0.010% or less, even more preferably 0.005% or less, and most preferably 0.003% or less. Although the lower limit of the Bi content is not particularly specified, since it is an element that reduces segregation, the Bi content is preferably 0.001% or more, and more preferably 0.002% or more.

[0043] Ni: 0.8% or less. If Ni is 0.8% or less, no coarse precipitates or inclusions will increase, and the ductility of the steel sheet will not be impaired. Therefore, when Ni is contained, the Ni content should be 0.8% or less. The Ni content is preferably 0.5% or less, more preferably 0.4% or less, even more preferably 0.3% or less, and most preferably 0.2% or less. Although the lower limit of the content is not particularly specified, since it is an element that improves hardenability, the Ni content is preferably 0.005% or more, more preferably 0.01% or more, and even more preferably 0.03% or more.

[0044] Regarding Cu, Sn, Sb, Nb, V, Ta, W, Mo, Co, Ca, Mg, REM, Zr, Te, Hf, Bi, and Ni described above, if each content is less than the preferred lower limit value, it will not harm the effects of the present invention, so they are included as inevitable impurities.

[0045] Next, we will explain the steel structure. The locations that define the structure are at the 1 / 4 position in the width direction and the 1 / 4 position in the thickness direction. This is because, generally, the 1 / 4 position in the width direction and the 1 / 4 position in the thickness direction are considered to be the locations that have the average characteristics and composition of the steel sheet.

[0046] [Ferrite: Area percentage of 1% or less] Although ferrite is a soft structure and is effective in improving workability, in the steel sheets targeted by this invention, it has a strong effect on reducing the strength of the steel sheet. The ferrite referred to here may be polygonal ferrite, pseudopolygonal ferrite, or granular bainitic ferrite. In order to achieve high strength, the area percentage of ferrite shall be 1% or less. It may be 0%, and a more preferable condition is 0%.

[0047] The area ratio of ferrite is determined by the following method. First, after polishing the plate thickness cross-section (L cross-section) parallel to the rolling direction of the steel plate, it is corroded with 3 vol.% nital, and the position at 1 / 4 of the plate thickness (the position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface) is observed at a magnification of 2000 times in 10 fields of view using a SEM (scanning electron microscope). Note that the steel plate surface in the case of having zinc plating indicates the interface between the zinc plating layer and the steel plate. Then, using the obtained tissue images, the area ratio of ferrite is calculated for 10 fields of view using Image-Pro of Media Cybernetics. The average of the area ratios of these 10 fields of view is taken as the area ratio of ferrite. Note that in the above tissue images, ferrite exhibits a gray tissue (base tissue). Also, the method for measuring the area ratio of martensite and the like, which will be described later, is as follows. A sample is cut out from the annealed steel plate (annealed sheet), after polishing the plate thickness cross-section parallel to the rolling direction, it is corroded with 3 vol.% nital, and the position at 1 / 4 of the plate thickness (the position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface) is photographed at a magnification of 2000 times in 10 fields of view using a SEM (scanning electron microscope). For the purpose of confirming carbides in detail, additional observation at a higher magnification may be added. Similarly to the above, using the obtained image data, the area ratio of the tissue is determined by Image-Pro manufactured by Media Cybernetics. Fresh martensite is a white or light gray region, tempered martensite is a gray or dark gray containing non-aligned carbides, pearlite is a black and white layered structure, and bainite is a gray or dark gray region containing aligned carbides. However, in actual observation, it may be difficult to make the above classification. For example, even in the case of tempered martensite, it may be observed that the carbides are aligned. Therefore, a gray or dark gray region containing aligned carbides, and further, a tissue whose tissue interface extends linearly is regarded as bainite, and a gray or dark gray region containing other carbides is regarded as tempered martensite.

[0048] [Bainite: 0.1% or more by area percentage] Bainite, like martensite, is an aggregate of lath-like crystal grains, but is softer than martensite. Here, bainite includes upper bainite and lower bainite. In the present invention, it is necessary to promote the formation of austenite with different carbon concentration levels by causing bainite transformation during the tempering process in the final annealing stage, so bainite is formed in an area percentage of 0.1% or more. The bainite is more preferably 1% or more, even more preferably 2.5% or more, and most preferably 3% or more. Furthermore, the upper limit of bainite is preferably 12% or less, more preferably 11% or less, and even more preferably 10% or less. Also, in order to achieve high strength, it is preferable that the total area percentage of ferrite and bainite is 15% or less. The total area percentage of ferrite and bainite is more preferably 10% or less, even more preferably 8% or less, and most preferably 7% or less. While there is no particular lower limit, the total amount of ferrite and bainite is preferably 1% or more, more preferably 2% or more, and even more preferably 3% or more.

[0049] [Retained Austenite: 5% to 20% by area fraction] Retained austenite contributes to the ductility of the steel sheet through the TRIP effect and is an essential structure in this invention. To obtain a sufficient effect, it should be 5% or more. Preferably, it should be 7% or more. More preferably, it should be 9% or more, even more preferably 10% or more, and most preferably 11% or more. Furthermore, if retained austenite is present in excess, it may transform into a hard martensitic structure when applied to and formed for automotive parts, etc., potentially impairing the processing characteristics. For this reason, the retained austenite should be 20% or less. Preferably, it should be less than 16%. More preferably, it should be 15% or less, even more preferably 14% or less, and most preferably 13% or less. As explained below, retained austenite is determined by volume fraction, but since the volume fraction and area fraction are roughly the same, it will be defined as an area fraction here.

[0050] The method for measuring the volume fraction of retained austenite is as follows: After mechanically grinding a steel plate in the thickness direction (depth direction) to 1 / 4 of its thickness, chemical polishing with oxalic acid or the like is performed to remove the strain introduced by mechanical polishing, creating an observation surface. This observation surface is observed by X-ray diffraction. A Co Kα source is used as the incident X-ray source, and the intensity of the diffraction peaks of the {200}, {211}, {220} planes of bcc iron and the {200}, {220}, {311} planes of fcc iron (austenite) is measured. By multiplying these by a correction factor and averaging the diffraction peaks of martensite and austenite, the influence of the preferred direction is eliminated, and the volume fraction of retained austenite is calculated.

[0051] [Tempered Martensite: Area Ratio of 70% or More] Tempered martensite is an aggregate of lath-like crystal grains and is characterized by containing iron-based carbides internally. Compared to fresh martensite, it has higher ductility and is also characterized by its ability to easily obtain the high yield strength necessary to ensure collision safety of automotive components. In order to obtain these effects, the area ratio of tempered martensite must be 70% or more. More preferably, the area ratio of tempered martensite should be 75% or more, even more preferably 80% or more, and most preferably 83% or more. There is no particular upper limit to the area ratio of tempered martensite, but in order to ensure the area ratio of retained austenite, the area ratio of tempered martensite is preferably 95% or less, more preferably 93% or less, and even more preferably 91% or less.

[0052] [Fresh martensite: 0% to 15% by area] Fresh martensite is martensite that does not contain iron-based carbides. Steel sheets containing fresh martensite have high strength but poor processing characteristics, so the fresh martensite in the steel sheet according to this embodiment is limited to 15% or less by area. Preferably it is 10% or less. More preferably it is 9% or less, even more preferably 8% or less, and most preferably 7% or less. On the other hand, even if the fraction of fresh martensite is 0%, the steel sheet according to this embodiment has sufficient strength. Therefore, the steel sheet according to this embodiment does not need to contain fresh martensite, so the lower limit of the volume fraction of fresh martensite is 0%.

[0053] The method for measuring the microstructure fraction of fresh martensite is as follows: A sample of annealed steel sheet is cut out, the sheet thickness cross section parallel to the rolling direction is polished, and then it is etched with 3 vol. % nital. Ten fields of view are taken at a position 1 / 4 of the sheet thickness (corresponding to 1 / 4 of the sheet thickness in the depth direction from the steel sheet surface) using a scanning electron microscope (SEM) at 2000x magnification. For the purpose of confirming carbides in detail, observation at an additional higher magnification (e.g., 5000x) may be performed. Using the obtained image data, the area ratio is determined using Image-Pro from Media Cybernetics, and the average area ratio of the fields of view determined by the method below is taken as the area ratio of fresh martensite. In the image data, fresh martensite is distinguished as a white or light gray region. Furthermore, since fresh martensite is difficult to distinguish from retained austenite, which similarly appears as a white or light gray region, the tissue fraction is obtained by first determining the area percentage of the white or light gray region and then subtracting the fraction of retained austenite obtained by the above method.

[0054] [Carbon concentration in retained austenite: 0.45% to 1.00%] The quantitative evaluation of the carbon concentration Cγ in retained austenite was performed by cutting a specimen from the 1 / 4 position in the width direction, mechanically polishing it to a surface 0.1 mm from the 1 / 4 position in the plate thickness, and then chemically polishing it to a further 0.1 mm. The surface exposed at the 1 / 4 position in the plate thickness was then subjected to X-ray diffraction using a Co-Kα source (Empyrean, Spectris). Based on Reference 1, the lattice constant aγ [Å] obtained from the (220)γ diffraction peak position was substituted into the following formula. Here, Mnγ, Alγ, and Siγ are the respective elemental concentrations (mass%) in retained γ. However, in this report, since the distribution of alloy elements before cooling is uniform due to annealing in the γ phase single-phase region, and the distribution of substitutional elements during annealing cooling can be considered negligible, the amount of additive (steel plate composition) was used for Mnγ, Alγ, and Siγ. aγ = 3.5720 + 0.033Cγ + 0.0012Mnγ - 0.00157Siγ + 0.0056Alγ ... Equation (2) The carbon concentration in retained austenite must be 0.45% or higher. If it is less than 0.45%, the retained austenite becomes excessively unstable, and much of it disappears by stress-induced transformation in the early stages of deformation, impairing ductility from the middle to the later stages of deformation. The carbon concentration in retained austenite is preferably 0.60% or higher, more preferably 0.65% or higher, and even more preferably 0.70% or higher. On the other hand, if it is excessively high, the retained austenite becomes too stable, preventing stress-induced transformation even in the later stages of deformation, which also impairs ductility, so the carbon concentration in retained austenite should be 1.00% or lower. The carbon concentration in the retained austenite is preferably 0.90% or less, more preferably 0.85% or less, even more preferably 0.80% or less, and most preferably 0.75% or less. [Reference 1] Y. Toji, et al: Acta Materoalia 65 (2014) 215-228)

[0055] Furthermore, in the steel structure of the present invention, in addition to the ferrite, bainite, tempered martensite, fresh martensite, and retained austenite mentioned above, the effects of the present invention will not be impaired even if carbides such as pearlite and cementite, or other structures known as steel sheet structures, are included as long as their total area percentage is within 10% or less. The lower limit is not particularly limited, but the area percentage of the aforementioned structures may be 0% or more. The other structures of the steel sheet (the remaining structure) can be confirmed and determined, for example, by SEM observation.

[0056] [Yield Ratio YS / TS ≤ 0.80] The steel sheet targeted by this invention has a low yield ratio, which is 0.80 or less. Preferably, the yield ratio is 0.78 or less, more preferably 0.75 or less, even more preferably 0.74 or less, and most preferably 0.72 or less. The lower limit is not particularly limited, but it is preferable that the yield ratio be 0.50 or more, more preferably 0.52 or more, and even more preferably 0.53 or more. Here, YS is the yield strength (yield stress), but if a clear upper yield point occurs, it is the upper yield point strength, and if it does not occur, it is the 0.2% proof load strength.

[0057] The following provides a detailed explanation of work hardening characteristics and heat treatment hardening characteristics.

[0058] [In a tensile test conforming to JIS Z2241, retained austenite that disappears between the start of the tensile test and processing that imparts a 1% strain at nominal value: 0.8% or more in terms of area ratio to the entire steel structure] The excellent work hardening characteristics of the present invention are thought to be due to stress-induced transformation of unstable retained austenite during processing. That is, under low stress conditions, unstable fcc (austenite) crystals transform into bcc (martensite) crystals, and deformation progresses with volume expansion, resulting in stress-strain characteristics with a high yield ratio. Furthermore, the solid solution carbon released when retained austenite transforms has the effect of increasing the yield stress by adhering to dislocations introduced during processing during subsequent heat treatment. Therefore, the higher the fraction of unstable retained austenite that transforms in the early stages of deformation, the more advantageous it is for increasing the amount of work hardening and the amount of hardening after heat treatment. Thus, the retained austenite that disappears between the start of the tensile test and processing that imparts a 1% strain at nominal value is set to 0.8% or more in terms of area ratio to the entire steel structure. The amount of retained austenite lost between the start of the tensile test and the processing that applies a 1% nominal strain is preferably 1.0% or more, more preferably 1.2% or more, even more preferably 1.5% or more, and most preferably 1.7% or more, in terms of area ratio relative to the entire steel structure. There is no particular upper limit, but since it is necessary to ensure 3% or more of austenite even after uniform elongation, it is preferable that the amount of retained austenite lost between the start of the tensile test and the processing that applies a 1% nominal strain is 5.0% or less, more preferably 4.7% or less, and even more preferably 4.4% or less, in terms of area ratio relative to the entire steel structure. It should be noted that it was difficult to reconcile this constituent requirement with the aforementioned characteristic of "carbon concentration in retained austenite ≥ 0.60%". This is because increasing the carbon concentration to stabilize the retained austenite makes it less likely for the retained austenite to undergo stress-induced transformation during processing that applies a 1% strain. The present invention makes it possible to reconcile both, and the method by which this is achieved will be described later.

[0059] The retained austenite lost during the above processing was evaluated by the difference in the amount of retained austenite before and after the tensile test. The retained austenite after the tensile test was evaluated using a sample cut from the central part of the tensile test specimen after a predetermined strain was introduced and then unloaded. The method for measuring the amount of retained austenite after the tensile test is as follows: A 1.5 cm square steel plate was cut from the central part of the JIS No. 5 test specimen after the tensile test, and after mechanical grinding in the thickness direction (depth direction) to 1 / 4 of the plate thickness, chemical polishing with oxalic acid or the like was performed to remove the strain introduced by mechanical polishing, creating an observation surface. X-ray diffraction was performed on this observation surface, and the retained austenite was measured using the method described above.

[0060] [Processing and Heat Treatment to Infuse 1% Strain] Work hardening and heat treatment hardening characteristics were evaluated by tensile testing using JIS No. 5 tensile test specimens in the C direction (gauge length 50 mm, parallel section width 25 mm). The C direction refers to the direction perpendicular to the rolling direction. First, six JIS No. 5 tensile test specimens in the C direction were prepared, cut from the same width direction position of the steel plate. Three of these specimens were subjected to tensile testing until fracture, according to JIS Z2241, and the yield stress, uniform elongation, total elongation, and tensile strength were evaluated by averaging the test results of the three specimens. The yield stress and total elongation here are considered to be the steel plate characteristics, i.e., the yield stress and total elongation before (processing and heat treatment to infuse 1% strain). For the remaining three specimens, tensile processing was performed to a nominal strain of 1%, then the tensile load was removed, and after heat treatment at 100°C for 20 minutes, tensile testing was performed again to evaluate the yield stress, uniform elongation, total elongation, and tensile strength. The yield stress and total elongation shown here are defined as the yield stress and total elongation after (processing and heat treatment to impart 1% strain). For the re-tensile test, the gauge length is set to 50 mm again, and the plate thickness and width of the initial parallel section of the specimen, which are necessary for stress calculation, are measured. The reason for setting the processing amount to 1% is that if the processing amount exceeds 2%, the specimen may fracture during the yielding process during re-tensile testing, making it impossible to obtain a stress-strain curve with low variability.

[0061] [YS increase obtained by subtracting the YS before (the processing and heat treatment) from the YS after (the processing and heat treatment) ≥ 200 MPa] The processing refers to processing that imparts a 1% strain, and the heat treatment refers to heat treatment at 100°C for 20 minutes. A feature of the present invention is that the yield stress after (the processing and heat treatment) is 200 MPa or more higher than the yield stress before (the processing and heat treatment). As a result, even under conditions where the heat treatment temperature is as low as 100°C, the material is soft when formed, but when used as a part, it is possible to have a high yield stress and, consequently, high impact characteristics. The YS increase is preferably 210 MPa or more, and more preferably 220 MPa or more. As described later, under certain conditions, it shows an extremely high yield stress increase exceeding 240 MPa. There is no particular upper limit, but the YS increase is preferably 500 MPa or less, more preferably 480 MPa or less, and even more preferably 460 MPa or less.

[0062] [El after (the processing and heat treatment): 0.9 × (El before (the processing and heat treatment)) or more] Another feature of the present invention is that the total elongation before and after processing and heat treatment that imparts 1% strain does not change significantly, and El after (the processing and heat treatment) is 0.9 × (El before (the processing and heat treatment)) or more. The above value is preferably 0.95 × (El before (the processing and heat treatment)) or more, more preferably (El before (the processing and heat treatment)) or more, even more preferably 1.05 × (El before (the processing and heat treatment)) or more, and most preferably 1.10 × (El before (the processing and heat treatment)) or more. Normally, after processing and heat treatment, the ductility decreases as the material hardens, but in the steel of the present invention, no significant decrease in total elongation is observed, and as will be described later, depending on the conditions, the total elongation may even increase after treatment. There is no particular upper limit, but it may be 2.00 × (El before (the processing and the heat treatment)) or less.

[0063] [The difference between the stress increase after 100°C heat treatment without pre-straining and the stress increase after 100°C heat treatment after 1% processing (tensile processing) is 75 MPa or more.] The steel sheet shown in the present invention is a suitable material for cold-pressed materials where the introduction of processing strain is a prerequisite, and a major feature is that the stress increase in the processed portion is significantly larger than the stress increase in the unprocessed portion. Therefore, the difference between the stress increase after 100°C heat treatment without pre-straining and the stress increase after 100°C heat treatment after 1% processing (tensile processing) is 75 MPa or more. The difference in stress increase is preferably 100 MPa or more, more preferably 110 MPa or more, and even more preferably 120 MPa or more. There is no particular upper limit, but the difference in stress increase is preferably 200 MPa or less, more preferably 190 MPa or less, and even more preferably 180 MPa or less. The stress increase refers to the difference (σa-σb) between the stress value σb obtained when a specified pre-strain is applied to a JIS No. 5 tensile test specimen cut from the original plate (or the yield stress of the original plate if there is no pre-strain), and the yield stress σa obtained when the same tensile test specimen is subjected to heat treatment at a specified temperature for 20 minutes, after applying the specified pre-strain and then removing the tensile test load (or the specimen as it was cut from the original plate if there is no pre-strain), and then subjected to another tensile test.

[0064] Next, the method for manufacturing steel plates according to the present invention will be described. After melting the steel material having the aforementioned component composition using a conventional refining process, it is made into a steel slab using a conventional ingot-parting rolling method or continuous casting method. Alternatively, a thin steel slab with a thickness of 100 mm or less may be manufactured by a direct casting method.

[0065] Hot Rolling Process After heating and holding the steel slab, it is subjected to hot rolling, and a hot-rolled sheet is produced by rough rolling and finish rolling and wound into a coil. The thickness of the hot-rolled sheet is preferably 0.8 mm or more, more preferably 1.0 mm or more, even more preferably 1.4 mm or more, and most preferably 1.6 mm or more. The thickness after hot rolling is preferably 4.0 mm or less, more preferably 3.5 mm or less, and even more preferably 3.2 mm or less. Here, the finish rolling completion temperature (finish hot rolling temperature) is preferably 800 to 1000°C, and for this purpose, the slab heating temperature is preferably 1080°C or more, more preferably 1140°C or more, and even more preferably 1160°C or more. Also, for the above reasons, the slab heating temperature is preferably 1300°C or less, more preferably 1250°C or less, and even more preferably 1240°C or less. Hot rolling needs to be completed in the austenite single-phase region in order to improve elongation and hole-expanding properties after annealing by homogenizing the structure within the steel sheet and reducing material anisotropy. Therefore, it is preferable that the finish rolling completion temperature be 800°C or higher. It is more preferable that the finish rolling completion temperature be 850°C or higher, and even more preferable that it be 900°C or higher. On the other hand, if the finish rolling completion temperature exceeds 1000°C, the structure of the hot-rolled sheet becomes coarse, and the properties after annealing deteriorate. For this reason, it is preferable that the finish rolling completion temperature be 1000°C or lower. It is more preferable that the finish rolling completion temperature be 980°C or lower, even more preferable that it be 950°C or lower, and most preferable that it be 930°C or lower.

[0066] After the finish rolling is complete, it is preferable to cool the sheet and then wind it up at a winding temperature between the Ms point and the Bs point to complete the hot rolling process. If the winding temperature is below the Ms point, the hot-rolled sheet becomes excessively hard, making it difficult to wind up, and also suppresses the formation of scale on the surface of the steel sheet, which is useful for the present invention. For this reason, it is preferable to set the winding temperature to be above the Ms point. It is more preferable to set the winding temperature to be above the Ms point + 10°C, even more preferable to be above the Ms point + 20°C, and most preferably to be above the Ms point + 25°C. Here, the Ms point (°C) is calculated by the following formula. The values ​​in brackets in the following formula are the mass percentages of the elements contained in the steel sheet. Ms point (°C) = 561 - 474 [C] - 33 [Mn] - 17 [Ni] - 17 [Cr] - 21 [Mo] Bs point (°C) = 830 - 270 [C] - 90 [Mn] - 37 [Ni] - 70 [Cr] - 83 [Mo] On the other hand, if the winding temperature is excessively high, the ferrite phase fraction increases and the microstructure becomes non-uniform, which increases gauge fluctuations during cold rolling and variations in the quality of the final product sheet. Therefore, it is preferable to set the upper limit to the Bs point. It is more preferable to set the winding temperature to Bs point - 10°C or lower, even more preferable to set it to Bs point - 20°C or lower, and most preferable to set it to Bs point - 30°C or lower. Here, the Bs point (°C) is calculated using the above formula. The values ​​in brackets are the mass percentages of the elements contained in the steel sheet.

[0067] The above-mentioned finish rolling completion temperature and winding temperature are values ​​measured at the center of the width of the steel sheet. Furthermore, since the above-mentioned finish rolling completion temperature and winding temperature are known to affect the material properties of the final product, it is preferable that the maximum temperature range of the average finish rolling completion temperature at the center of the width direction of the hot-rolled sheet, one edge (OP side), and the other edge (DR side) is 50°C or less, and the maximum temperature range of the average winding temperature is 30°C or less. More preferably, the maximum temperature range of the average finish rolling completion temperature is 30°C or less, and the maximum temperature range of the average winding temperature is 20°C or less. It is preferable that the maximum temperature range of the average finish rolling completion temperature at the above-mentioned locations is 5°C or more, and the maximum temperature range of the average winding temperature is 3°C or more.

[0068] Next, after the finish rolling is completed, skin pass rolling may be performed. Skin pass rolling can correct the shape of the steel sheet and break down the scale formed on the hot-rolled sheet, thereby increasing the scale removal efficiency in the subsequent pickling process. When skin pass rolling is performed, the reduction ratio is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.01% or more. Furthermore, the reduction ratio is preferably 2.0% or less, more preferably 1.0% or less, even more preferably 0.2% or less, and most preferably 0.1% or less.

[0069] Pre-cold rolling annealing is performed by holding the material at a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more. Alternatively, after the completion of finish rolling in hot rolling and before the start of cold rolling, annealing may be performed at a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more to reduce the cold rolling load. If the annealing temperature exceeds the Ac3 point, a hard structure will form from the austenite phase during cooling, so if annealing is performed, the annealing temperature should be below the Ac3 point. The annealing temperature is preferably below the Ac3 point - 10°C, more preferably below the Ac1 point, even more preferably below the Ac1 point - 10°C, and most preferably below the Ac1 point - 20°C. Furthermore, if the temperature is below 400°C, there is almost no softening and the effect of reducing the cold rolling load is small, so the annealing temperature should be 400°C or higher. The annealing temperature is preferably 450°C or higher, more preferably 460°C or higher, even more preferably 470°C or higher, and most preferably 480°C or higher. The Ac1 and Ac3 points shown herein can be calculated from the following equations (A) and (B) described in "Leslie's Iron and Steel Science" (Maruzen Co., Ltd., translated under the supervision of Naruyasu Koda, published May 31, 1985, p. 273). In the equations, [ ] indicates the content (mass%) of each element, and elements not contained in the steel sheet, or elements whose content is below the detection limit of the analysis, can be calculated as 0 mass%. Ac1 point (°C) = 723-10.7×[Mn]-16.9×[Ni]+29.1×[Si]+16.9×[Cr]+290×[As]+6.38×[W]...(A) Ac3 point (°C) = 910-203×√[C]-15.2×[Ni]+44.7×[Si]+104×[V]+31.5×[Mo]+13.1×[W ]-30×[Mn]-11×[Cr]-20×[Cu]+700×[P]+400×[Al]+120×[As]+400×[Ti]...(B)

[0070] Following the cold rolling process, the steel sheet after hot rolling, or the steel sheet after soft annealing (annealed before cold rolling), is pickled and then cold-rolled to obtain a cold-rolled sheet of the desired thickness. The cold rolling can be performed by tandem rolling (unidirectional rolling) or reverse rolling, and known hot rolling techniques or inter-pass aging techniques may be used. If the reduction ratio is low, the recrystallization driving force is low, so sufficient recrystallization does not occur in the subsequent annealing process, leading to a decrease in ductility. Also, if the reduction ratio is high, high local ductility is easily obtained, so preferably the reduction ratio is 30% or more. The reduction ratio is more preferably 40% or more, even more preferably 45% or more, and most preferably 50% or more. There is no particular upper limit, but from the viewpoint of rolling load, the reduction ratio is preferably 95% or less, more preferably 85% or less, and even more preferably 75% or less.

[0071] Annealing Process The average heating rate in the temperature range from 650°C to 800°C is 2°C / s or less. Then, as an annealing process, the obtained steel sheet is heat-treated and, if necessary, plated. In the heating process performed at the beginning of the heat treatment, the average heating rate is set to 2°C / s or less in the temperature range from 650°C to 800°C. In this temperature range, competition occurs between ferrite recrystallization and ferrite-austenite transformation from the unrecrystallized structure after cold rolling. Under conditions of a low average heating rate, ferrite recrystallization occurs before the austenite transformation, but it has been found that under conditions of a high average heating rate, the austenite transformation occurs from the unrecrystallized ferrite without ferrite recrystallization. In the present invention, since a balanced retained austenite can be formed by the ferrite-austenite transformation occurring after the ferrite recrystallization is completed, a low average heating rate is preferable. In the temperature range from 650°C to 800°C, the preferred average heating rate is 1.5°C / s or less, more preferably 1.4°C / s or less, and even more preferably 1.3°C / s or less. The lower limit is not particularly limited, but in the temperature range from 650°C to 800°C, the preferred average heating rate is 0.5°C / s or more, and more preferably 0.6°C / s or more. The above average heating rate can be determined by dividing the temperature difference between 650°C and 800°C (150°C) by the time required to raise the temperature from 650°C to 800°C.

[0072] The average heating rate in the temperature range from 800°C to the maximum annealing temperature Ts is 1°C / s or less. Subsequently, the steel sheet is heated to Ts, which is the maximum annealing temperature in this annealing process. Here, mainly an austenite transformation occurs from ferrite, but since the ferrite has already recrystallized and become relatively coarse grains, the driving force for the austenite transformation is somewhat low. In order to completely recrystallize the unrecrystallized ferrite, the average heating rate in the temperature range from 800°C to the maximum annealing temperature Ts is set to 1°C / s or less. The average heating rate is preferably 0.8°C / s or less, more preferably 0.7°C / s or less, and even more preferably 0.6°C / s or less. Near Ts, the austenite transformation is completed in order to minimize the ferrite fraction in the subsequent cooling process and achieve the highest possible strength. The lower limit of the average heating rate is not particularly limited, but it is preferably 0.005°C / s or more, and more preferably 0.1°C / s or more. The average heating rate mentioned above can be determined by dividing the temperature difference between 800°C and the maximum annealing temperature Ts by the time required to heat the material during this period. In order to complete the austenite transformation with a large amount of recrystallized ferrite and low driving force, Ts must be at least Ac3 point + 30°C, but even better properties can be obtained if it is 920°C or higher. For this reason, Ts should be at least Ac3 point + 30°C, preferably at 920°C or higher. The higher Ts, the coarser the austenite grains become, and the larger the martensite block diameter becomes in the subsequent cooling process. Within the scope of the present invention, it has been confirmed that the effect of increasing the yield stress after heat treatment tends to increase as the block diameter increases. Although there are many unknowns regarding the detailed mechanism, it is presumed that when the block diameter is large, the area of ​​the block interface where carbon is likely to segregate decreases, resulting in an increase in the amount of carbon that effectively contributes to high strength during heat treatment within the block. More preferably, Ts is 940°C or higher, and even more preferably 950°C or higher. On the other hand, if the annealing temperature is excessively high, the tissue will become excessively large and the toughness will deteriorate. Therefore, it is preferable that Ts be 1050°C or lower, and more preferably 1020°C or lower.

[0073] In the subsequent cooling process, the average cooling rate from the maximum annealing temperature Ts to 400°C is not particularly limited, but it is preferable to cool at an average cooling rate of 10°C / s or more and 100°C / s or less. This is because a higher average cooling rate is preferable for forming a hard structure. Therefore, the average cooling rate is preferably 10°C / s or more, more preferably 15°C / s or more, even more preferably 20°C / s or more, and most preferably 40°C / s or more. On the other hand, if the cooling rate is excessively high, the uneven cooling will cause uneven distortion in the steel plate and impair its flatness, so it is preferable to set the upper limit to 100°C / s or less, more preferably 80°C / s or less, and even more preferably 70°C / s or less. The above average cooling rate can be determined by dividing the temperature difference between the maximum annealing temperature Ts and 400°C by the time required to cool from Ts to 400°C.

[0074] Cooling from 400°C to a temperature T1 between 50°C and 250°C. The average cooling rate during this period is not particularly limited, but it is preferable that the average cooling rate be between 5°C / s and 20°C / s. Cooling to 250°C or below causes a portion of the austenite produced by annealing to transform into martensite. Below 50°C, much of the austenite necessary for improving ductility transforms into hard structures such as martensite and disappears, so the temperature T1 should be 50°C or higher. Preferably, the temperature T1 is 75°C or higher. More preferably, the temperature T1 is 100°C or higher, even more preferably 125°C or higher, and most preferably 130°C or higher. Also, above 250°C, the martensite transformation does not occur sufficiently, and the fraction of martensite structure necessary for high strength decreases. Therefore, the temperature T1 should be 250°C or lower. The temperature T1 is preferably 230°C or lower, more preferably 220°C or lower, even more preferably 210°C or lower, and most preferably 200°C or lower. Known methods can be used as the cooling means of the present invention, and gas cooling, oil cooling, mist cooling, etc., can be applied. The average cooling rate can be determined by dividing the temperature difference between 400°C and temperature T1 by the time required to cool from 400°C to T1.

[0075] Tempering Process The material is heated from T1 to a temperature T2 of 280°C to 380°C, and held in the temperature range of T2-10°C to less than T2 for 1 s to 60 s. Subsequently, the material is heated from T1 to a temperature T2 of 280°C to 380°C, and held in the temperature range of T2-10°C to less than T2 for 1 s (seconds) to 60 s. This process is particularly important for forming the desired retained austenite structure and is referred to herein as the tempering process. In the tempering process, high-temperature annealing transforms a very small amount of austenite into bainite, thereby increasing the carbon concentration in the austenite near the transformed bainite, while preventing excessive carbon enrichment in the rest of the austenite. Rapid heating and short processing time are required for this purpose. The average heating rate from T1 to temperature T2 is preferably 5°C / s or more, more preferably 7°C / s or more, and even more preferably 9°C / s or more. There is no particular upper limit, but it is preferably 100°C / s or less. The average heating rate mentioned above can be determined by dividing the temperature difference between T1 and T2 by the time required to raise the temperature from T1 to T2. To promote the transformation, the temperature T2 should be 280°C or higher. Preferably, the temperature T2 is 290°C or higher, more preferably 300°C or higher, even more preferably 310°C or higher, and most preferably 320°C or higher. On the other hand, if the temperature is too high, the transformation to bainite, which is one form of the bcc phase, becomes difficult to occur, and it becomes impossible to produce austenite with a low carbon concentration, so the temperature T2 should be 380°C or lower. Preferably, the temperature T2 is 370°C or lower, more preferably 360°C or lower, even more preferably 350°C or lower, and most preferably 340°C or lower. In addition, the holding time in the temperature range of T2-10°C or higher and below T2, where carbon diffuses relatively well, needs to be 1 s or more for reasons such as promoting the transformation from austenite to bainite. The holding time is preferably 2 s or more, more preferably 5 s or more, and even more preferably 7 s or more. Furthermore, if the holding time is too long, the amount of carbon in the retained austenite will increase excessively, and the yield ratio will become too high, so it is necessary to keep it at 60 s or less. A preferred holding time is 30 s or less.If the holding time is less than 10 seconds, even more favorable properties can be obtained, so it is more preferable to set the holding time to less than 10 seconds, and even more preferable to set it to 9 seconds or less. The reason for setting the above holding time in the temperature range of T2-10°C or higher and less than T2 is that carbon diffusion is relatively significant in this temperature range, and it is a temperature range in which the effect on the material is large.

[0076] Subsequently, final cooling is performed. However, if the temperature range is below T2-10°C, where carbon distribution in austenite is relatively slow, and above T1, where austenite transformation is slow, then it is acceptable to hold the temperature in the range of T2-10°C to below T2, and before final cooling. In the final cooling step, in order to maintain the solid solution state of carbon, cooling is performed in the temperature range of T1 to T1-10°C or lower, at an average cooling rate of 2°C / s or higher. It is thought that rapid cooling in the temperature range of T1 to T1-10°C or lower prevents sufficient diffusion of solid solution carbon generated by transformation during cooling, thus reducing the amount of carbon in the retained austenite and maintaining unstable retained austenite. The average cooling rate mentioned above is preferably 5°C / s or higher, more preferably 10°C / s or higher. Cooling is preferably carried out up to 25-85°C, with the pickling temperature (25-85°C) as the lower limit, taking into consideration the subsequent pickling properties. That is, the cooling end temperature (cooling stop temperature) is set to or above the pickling temperature. The average cooling rate mentioned above can be calculated by dividing the temperature difference between T1 and the cooling end temperature (cooling stop temperature) by the time required to cool from T1 to the cooling end temperature (cooling stop temperature). Note that the final cooling can be performed in two stages, for example, a gas-cooled section and a water-cooled section, but in that case, the cooling rate of each stage must satisfy the above-mentioned cooling rate.

[0077] After cooling, a second pickling may be performed, followed by skin pass rolling. Skin pass rolling must be performed at a plate temperature (steel plate temperature) of 20°C or higher and an elongation of 0.1% or less (including 0%). If the steel plate temperature is low, or if the elongation is higher than 0.1%, an excessive amount of austenite will be transformed and lost during skin pass rolling, and the amount of retained austenite lost during subsequent processing up to 1% strain will be small. More preferable conditions are a plate temperature of 30°C or higher and an elongation of 0.05% or less, and even more preferable conditions are an elongation of 0.01% or less. The upper limit of the plate temperature is not particularly limited, but it is preferable to keep it at 80°C or lower, and the lower limit of the elongation may be 0%. Furthermore, leveling processing to correct the shape is preferable not to be performed from the viewpoint of retaining retained austenite.

[0078] Alternatively, the steel sheet obtained after the annealing process described above may be electro-galvanized to obtain an electro-galvanized steel sheet.

[0079] In the plating and annealing processes, hot-dip galvanizing may be applied to produce a hot-dip galvanized steel sheet. T1 is as described above. In this case, T2 can be read as the plating temperature Tm, but the range of Tm is not limited to the T2 conditions of the cold-rolled steel sheet described above, i.e., 280°C to 380°C, but rather to the normal plating temperature of 450°C to 500°C. That is, the range of Tm is 450°C or higher, preferably 455°C or higher, and more preferably 460°C or higher. Also, the range of Tm is 500°C or lower, preferably 495°C or lower, and even more preferably 490°C or lower. In order to obtain the desired total area ratio of ferrite and bainite, it is preferable that the average heating rate in the temperature range from temperature T1 to temperature Tm be 5°C / s or higher, more preferably 7°C / s or higher, even more preferably 10°C / s or higher, and most preferably 15°C / s or higher. The upper limit of the average heating rate is not particularly limited, but it may be 60°C / s or less. Furthermore, the holding time in the temperature range of Tm - 10°C or higher and less than Tm is set to 1 s or more, preferably 3 s or more, more preferably 5 s or more, and even more preferably 10 s or more, in order to obtain the desired area ratio of retained austenite and the desired carbon concentration in the retained austenite. The above holding time is set to 60 s or less, preferably 50 s or less, more preferably 45 s or less, and even more preferably 40 s or less. The above average heating rate can be determined by dividing the temperature difference between T1 and Tm by the time required to raise the temperature from T1 to Tm. Furthermore, for the final cooling, T2 is read as the plating temperature Tm, and the above explanation applies.

[0080] Furthermore, manufacturing conditions other than those mentioned above can be met by conventional methods.

[0081] The steel sheet of the present invention can be manufactured using the technology described herein.

[0082] Furthermore, the present invention provides a component characterized by using at least a portion of the steel sheet or plated steel sheet obtained above. In addition, a method for manufacturing a component is also provided, in which at least one of forming and joining processes is applied to the steel sheet or plated steel sheet obtained above to form the component.

[0083] Examples of the present invention are shown in the table below. However, the present invention is not limited by the following examples, and it is possible to implement the invention with appropriate modifications within the scope that is consistent with the spirit of the invention, and all such modifications shall be considered to fall within the technical scope of the present invention.

[0084] The steel material, having the component composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted and, after bloc rolling, formed into steel slabs. These slabs were then hot-rolled under the conditions shown in Tables 2-1 and 2-2, cooled, and coiled. Subsequently, both sides were ground uniformly by 0.2 mm. A portion of the slabs was then heat-treated in a nitrogen atmosphere (annealing before cold rolling), and cooled in the air or furnace-cooled in a nitrogen atmosphere. After pickling, the slabs were cold-rolled. A portion of the slabs was then heat-treated in a nitrogen atmosphere (annealing and tempering). A portion of the slabs was then hot-dip galvanized and electro-galvanized. From the obtained steel sheets, JIS No. 5 test specimens (gauge length 50 mm, parallel section width 25 mm) were cut perpendicular to the rolling direction, and tensile tests were performed according to JIS Z2241. The microstructure, work hardening characteristics, and heat treatment characteristics of the processed paint baked steel sheets were measured based on the methods described above. The results are shown in Tables 3-1 and 3-2.

[0085]

[0086]

[0087]

[0088]

[0089]

Claims

1. The composition is such that, by mass%, C: 0.100% to 0.295%, Si: 0.01% to 1.60%, Mn: 0.10% to 5.00%, Cr: 0.01% to 1.00%, P: 0.1000% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0200% or less, O: 0.0100% or less, Ti: 0.200% or less, and Ti / 48 ≥ N / 14 (Ti and N are components contained in steel sheets (mass%)), and further, B: 0.0003% to 0.0100%, and P + B ≥ 0.0050%, with the remainder being Fe and unavoidable impurities. Furthermore, the Si / 28 + Cr / 52 ratio (Si and Cr are components contained in the steel sheet (mass%)) satisfies 0.040 to 0.052, the steel structure is such that ferrite accounts for 1% or less by area percentage, bainite accounts for 0.1% or more by area percentage, retained austenite accounts for 5% to 20% by area percentage, tempered martensite accounts for 70% or more by area percentage, and fresh martensite accounts for 0% to 15% by area percentage, the carbon concentration in the retained austenite satisfies 0.45% to 1.00%, the yield ratio is 0.80 or less, and in a tensile test in accordance with JIS Z2241, the retained austenite that disappears between the start of the tensile test and the processing that imparts 1% strain at nominal strain accounts for 0.8% or more by area percentage of the entire steel structure, and when the processing that imparts 1% strain and heat treatment at a temperature of 100°C are performed, A steel sheet having a yield strength of 200 MPa or more, obtained by subtracting the pre-processing and heat treatment YS from the post-processing and heat treatment YS, and a post-processing and heat treatment El of 0.9 × (pre-processing and heat treatment El) or more. YS and El represent the yield strength and total elongation of the steel sheet, respectively.

2. The steel sheet according to claim 1, wherein the component composition further contains, by mass%, at least one of the following: Cu: 0.005% to 0.500%, Sn: 0.005% to 0.500%, and Sb: 0.001% to 0.080%, in total of 0.010% to 1.000%.

3. The steel sheet according to claim 1 or 2, wherein the component composition further contains, in mass%, at least one selected from: Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Mo: 1.00% or less, Co: 1.000% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, Ni: 0.8% or less.

4. A plated steel sheet having either an electro-galvanized layer or a hot-dip galvanized layer formed on the surface of the steel sheet according to any one of claims 1 to 3.

5. After heating a steel slab having the component composition described in any one of claims 1 to 3, hot rolling is performed, the resulting hot-rolled sheet is pickled, and the cold-rolled sheet obtained by cold rolling is heated to the temperature Ts such that the average heating rate in the temperature range from 650°C to 800°C is 2°C / s or less, and the average heating rate in the temperature range from 800°C to the maximum annealing temperature Ts which is Ac3 point + 30°C or higher is 1°C / s or less. A method for manufacturing a steel sheet, comprising cooling from 400°C to a temperature T1 between 50°C and 250°C, then raising the temperature from T1 to a temperature T2 between 280°C and 380°C, holding the temperature in the range of T2-10°C or higher and below T2 for 1 to 60 seconds, then cooling with an average cooling rate of 2°C / s or higher in the temperature range from T1 to T1-10°C, and after a second pickling, performing skin pass rolling at a plate temperature of 20°C or higher with an elongation of 0.1% or less (including 0%).

6. A steel slab having the component composition described in any one of claims 1 to 3 is heated, then hot-rolled, the resulting hot-rolled sheet is pickled, and the resulting cold-rolled sheet is heated to the temperature Ts such that the average heating rate in the temperature range from 650°C to 800°C is 2°C / s or less, and the average heating rate in the temperature range from 800°C to the maximum annealing temperature Ts which is Ac3 point + 30°C or higher is 1°C / s or less. A method for manufacturing plated steel sheets, comprising: cooling from 400°C to a temperature T1 between 50°C and 250°C; then performing hot-dip galvanizing; raising the temperature from T1 to Tm, which is between 450°C and 500°C; holding the temperature in the range of Tm-10°C or higher and below Tm for 1 s to 60 s; then cooling with an average cooling rate of 2°C / s or higher in the temperature range from T1 to T1-10°C; and after a second pickling, performing skin pass rolling at a plate temperature of 20°C or higher with an elongation of 0.1% or less (including 0%).

7. The method for manufacturing a steel sheet according to claim 5, wherein pre-cold rolling annealing is performed, in which the sheet is held at a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more between the hot rolling and the start of cold rolling.

8. The method for manufacturing a plated steel sheet according to claim 6, wherein pre-cold rolling annealing is performed, in which the sheet is held at a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more between the hot rolling and the start of the cold rolling.

9. The method for manufacturing a steel sheet according to claim 5 or 7, wherein the steel sheet is further subjected to electro-galvanizing to obtain a plated steel sheet.

10. A component comprising at least a portion of the steel sheet described in any one of claims 1 to 3 or the plated steel sheet described in claim 4.

11. A method for manufacturing a member, comprising forming and joining a steel sheet according to any one of claims 1 to 3 or a plated steel sheet according to claim 4.

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