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

Abstract

The present invention aims to provide technology for manufacturing a steel sheet that, after being subjected to cold press working and formed into a component, possesses high impact resistance. A steel sheet having a specific component composition and microstructure, a yield ratio of 0.80 or less, wherein in a tensile test conforming to JIS Z 2241, the retained austenite that disappears between the start of the tensile test and processing that imparts a 1% strain at nominal strain accounts for 0.8% or more of the area of ​​the entire microstructure, and when the processing that imparts a 1% strain and heat treatment at a temperature of 100°C are performed, the increase in YS (YS after processing and heat treatment) minus the YS (YS before processing and heat treatment) is 200 MPa or more, and the El (El after processing and heat treatment) is 0.9 × (El before processing and heat treatment) or more. Note that YS and El represent the yield stress and total elongation of the steel sheet, respectively.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a plated steel sheet having a tensile strength (tensile strength) TS of 1180 MPa or more, which are suitable for use in structural parts of automobile bodies and electrical appliance parts that are subjected to baking coating under a temperature condition lower than the conventional 170°C after any one or more of drawing, bulging and stretch flanging forming by cold pressing, as well as methods for producing the same, members, and methods for producing members. [Background Art]

[0002] In recent years, with increasing public concern over global environmental issues, CO₂ emission regulations have been tightened. In the automotive field, there are demands for improved fuel efficiency through weight reduction of vehicle bodies and for crash safety of large-sized batteries mounted along with the electrification of automobiles. For this purpose, high-strength materials that can provide high body rigidity and crash safety even after being thinned and weight-reduced are being actively applied to steel materials used in automobiles. Many steel manufacturers have expanded their lineups of high-strength materials with tensile strength of 980 MPa class and even higher strength as steel sheets for automobiles. The main challenges for such high-strength materials are (1) achieving both high strength and high ductility, (2) improving delayed fracture resistance (hydrogen embrittlement resistance), which is a problem specific to high-strength materials, (3) good workability when used in cold pressing, and (4) securing high crash characteristics after processing into parts to ensure crash safety for automotive parts and the like. Regarding (1), solid solution strengthening, precipitation strengthening, and dislocation strengthening (transformation strengthening) are generally known as strengthening methods for steel. To increase the strength of steel sheets with tensile strength exceeding 980 MPa, it is necessary to increase the fraction of hard structures such as martensite and bainite through transformation strengthening. However, when a larger amount of these hard phases are included in the steel sheet, the workability of the steel sheet decreases. As a result, the steel sheet cannot be applied to parts requiring complex processing, which has the drawback of limiting the applicable parts.

[0003] One known method for improving the mechanical properties of high-strength steel sheets utilizing such hard phases is to use tempered martensite. For example, Patent Document 1 describes a high-strength cold-rolled steel sheet with a tensile strength exceeding 980 MPa and excellent elongation flangeability, obtained by an ultra-rapid quenching method of 400°C / s or more using water quenching and subsequent tempering heat treatment, which contains more than 70% (tempered) martensite. The method using water quenching has the advantage of being able to increase strength without relying too much on alloying of steel, and it has been found to have excellent hydrogen embrittlement resistance as described in (2), and is also advantageous in terms of weldability as described in (3). Furthermore, because the high yield strength (yield stress) YS of tempered martensite is also advantageous in terms of collision safety as described in (4), its application to automotive parts has been progressing in recent years.

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

[0005] Furthermore, Patent Document 3 describes a method for manufacturing steel sheets that have good bending properties and bendability by plating the steel sheet, cooling it to below 200°C, and then performing a tempering treatment in the temperature range of 100 to 600°C. Although these materials are characterized by excellent ductility, they contain slightly more alloying elements than water-quenched materials, which imposes limitations on product use in terms of weldability as described in (2) and (3), and various countermeasures are being investigated.

[0006] For example, Patent Document 4 describes a method for improving delayed fracture resistance by optimizing the amount of Si in steel.

[0007] Furthermore, an invention using a hot working method to avoid the challenges specific to cold pressing is disclosed in Patent Document 5. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 5151354 [Patent Document 2] Patent No. 6787525 [Patent Document 3] Japanese Patent Publication No. 2017-48412 [Patent Document 4] Patent No. 5423072 [Patent Document 5] Patent No. 7036214 [Overview of the project] [Problems that the invention aims to solve]

[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, methods to avoid this include, for example, using 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 studied and developed a material design focusing on work hardening and baked paint hardening as steel strengthening methods, 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 through 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 TS while having excellent elongation characteristics, and at the same time, a low yield stress correlated with the amount of springback, i.e., a low yield ratio (yield stress / tensile strength), further, to suppress the change in elongation after processing heat treatment and increase the yield stress, and further, in order to ensure collision safety in automobile parts and the like, to increase the amount of stress increase after processing into a part shape and heat treatment at a low temperature, for example, 100°C.

[0014] In this invention, a low yield ratio means a yield ratio of 0.80 or less. Furthermore, suppressing the change in elongation after processing and increasing the yield stress means that, after processing to impart a 1% strain at the nominal strain in a tensile test, heat treatment is performed at a temperature of 100°C. This means that the increase in YS (Yield S) obtained by subtracting the YS (Pre-processing and Heat Treatment) from the YS (Yield S) after processing and heat treatment is 200 MPa or more, and the total elongation (El) after processing and heat treatment is 0.9 × (El before processing and heat treatment) or more. Note that 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 with 1% strain in the tensile test is 75 MPa or more. [Means for solving the problem]

[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] In 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, Contains O: 0.0100% or less, Ti: 0.200% or less, and containing Ti / 48 ≥ N / 14 (Ti and N are components (mass%) contained in the steel sheet), and further, B: 0.0003% or more and 0.0100% or less, It contains P+B such that ≥ 0.0050%, The balance has a component composition consisting of Fe and unavoidable impurities, further, Si / 28+Cr / 52 (Si and Cr are the components (mass%) contained in the steel sheet) satisfies 0.040 or more and 0.052 or less, the steel structure, ferrite has an area ratio of 1% or less, bainite has an area ratio of 0.1% or more, retained austenite has an area ratio of 5% or more and 20% or less, tempered martensite has an area ratio of 70% or more, fresh martensite has an area ratio of 0% or more and 15% or less, the carbon concentration in the retained austenite satisfies 0.45% or more and 1.00% or less, the yield ratio is 0.80 or less, In a tensile test in accordance with JIS Z 2241, the retained austenite that disappears between the start of the tensile test and working that imparts 1% nominal strain has an area ratio of 0.8% or more relative to the entire steel structure, and When performing working that imparts 1% strain and heat treatment at a temperature of 100°C, the increase amount of YS obtained by subtracting YS before (said working and said heat treatment) from YS after (said working and said heat treatment) is 200 MPa or more, the steel sheet wherein El after (said working and said heat treatment) is 0.9×El before (said working and said heat treatment) or more. Here, YS and El each represent the yield stress and total elongation of the steel sheet. [2] The steel sheet according to [1], wherein the component composition further contains, by mass%, Cu: 0.005% or more and 0.500% or less, Sn: 0.005% or more and 0.500% or less, and Sb: 0.001% or more and 0.080% or less, contains at least one selected from the above in a total amount of 0.010% or more and 1.000% or less. [3] The component composition further contains, by mass%, 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, The steel plate according to [1] or [2], containing at least one selected from among the following. [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 described in any of [1] to [3] above. [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 then the cold-rolled sheet obtained by cold rolling is... 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 above the Ac3 point + 30°C, is 1°C / s or less, and the mixture is heated to Ts. 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 then cold-rolled to obtain a cold-rolled sheet. 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 above the Ac3 point + 30°C, is 1°C / s or less, and the mixture is heated to Ts. A method for manufacturing plated steel sheets, comprising the following steps: 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 and below Tm for 1 to 60 seconds; then cooling with an average cooling rate of 2°C / s or more in the temperature range from T1 to T1-10°C; and after a second pickling, skin pass rolling at a plate 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 the cold rolling, by holding the sheet at a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more. [8] A method for manufacturing a plated steel sheet according to [6], wherein pre-cold rolling annealing is performed between the hot rolling and the start of the cold rolling, by holding the plated steel sheet at a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more. [9] The method for manufacturing a steel sheet according to [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 of [1] to [3] above or the plated steel sheet described in [4] above.

[11] A method for manufacturing a member, comprising forming and joining a steel sheet according to any one of [1] to [3] or a plated steel sheet according to [4] to form a member. [Effects of the Invention]

[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 application 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. [Modes for carrying out the invention]

[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 decrease, making it difficult to achieve the desired tensile strength (TS). 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 becomes 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 this invention in particular, 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. Furthermore, in hole expansion tests, the hardness of the martensite that transforms from retained austenite during punching increases significantly, leading to increased void formation during punching and hole expansion, and reducing the hole expansion ratio λ, which is an indicator of stretch flangeability. 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 total strength (TS) of more than 980 MPa. Therefore, 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. Therefore, 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 this invention, and 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 presence of Cr increases the residual austenite fraction and has the effect of stabilizing residual austenite, as shown in Patent Document 6. However, in the present invention, it has been found that when utilized under appropriate manufacturing conditions, Cr can destabilize residual austenite while increasing its residual austenite fraction. This increases the amount of residual austenite that undergoes stress-induced transformation during processing, resulting in even better work hardening characteristics. In order to exert this residual γ-destabilizing effect of Cr, the final annealing conditions must be set 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 it is 1.00% or less, the number 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 1.00% or less. The Cr content is preferably 0.90% or less, and more preferably 0.80% or less. The present invention allows for the creation of unprecedentedly unstable retained austenite due to its Cr content. However, in order to utilize this for high strength during processing and heat treatment, particularly under low-temperature heat treatment conditions, the dissolved 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 the conditions of having appropriate amounts of P and dissolved B in the steel sheet, i.e., Ti / 48 ≥ N / 14 (Ti and N are components (mass%) contained in the steel sheet) and P + B ≥ 0.0050%, are effective. Although many aspects of the detailed mechanism remain unclear, it is presumed that when large amounts of P and dissolved B are present, these elements preferentially segregate at grain boundaries and block interfaces instead of dissolved carbon, and as a result, the dissolved carbon fixes mobile dislocations within the grains, leading to increased strength. [Patent Document 6] Patent No. 4688782

[0022] [P:0.1000% or less] P segregates at prior austenite grain boundaries, causing them to become brittle, thus 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 should be 0.1000% or less. Preferably, the P content should be 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 specific 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 to have a P content of 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 specific 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] When Al is present in large quantities, it raises the Ac3 point (Ac3 transformation point) and incorporates a large amount of ferrite into the microstructure, thus hindering the high strength achieved by utilizing the martensitic structure. Therefore, the Al content must be 1.000% or less. Preferably, the Al content is 0.500% or less. More preferably, the Al content is 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, as it suppresses carbide formation during continuous annealing and promotes the formation of retained austenite. More preferably, the Al content is 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 set to 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 specific 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 specific 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 kept below 0.200%. 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 for the Ti content to be 0.001% or more. In addition, the effect of adding B, which will be described later, is further enhanced when the Ti content is within the following range.

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

[0029] [B: 0.0003% or more and 0.0100% or less] If B is present in a concentration of 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. Therefore, 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 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 to 0.052 (Si and Cr are components contained in the steel sheet (mass %))] Si and Cr are useful elements for the formation and stabilization of retained austenite because they suppress carbide formation and secure solid solution carbon; therefore, their total elemental ratio (Si / 28 + Cr / 52) should be 0.040 or higher. Preferably, their total elemental ratio (Si / 28 + Cr / 52) should be 0.042 or higher, more preferably 0.045 or higher, and even more preferably 0.047 or higher. On the other hand, excessive addition will excessively stabilize retained austenite and prevent the desired work hardening characteristics from being obtained; therefore, their total elemental ratio (Si / 28 + Cr / 52) should be 0.052 or lower. More preferably, their total elemental ratio (Si / 28 + Cr / 52) should be 0.050 or lower, and even more preferably 0.049 or lower.

[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. Hereinafter, 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%, at least one of the following must be present in a total of 0.010% to 1.000%: Cu: 0.005% to 0.500%, Sn: 0.005% to 0.500%, and Sb: 0.001% to 0.080%] If the content of Sn and Cu is 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 content of Sn and Cu should be 0.500% or less each. Preferably, the content of Sn and Cu should be 0.100% or less each. Although there is no specific lower limit for the content, since Sn and Cu are elements that improve hardenability, when Sn and Cu are included, the content of Sn and Cu 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 and ferrite, which is unfavorable for high strength, is formed on the surface, 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 number 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. For this reason, when Cu and / or Sn and / or Sb are included within the above range, the total amount of at least one of them should be 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 included 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 the Nb and V content is 0.200% or less, large amounts of coarse precipitates and inclusions will not be formed, 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 be 0.200% or less each. Preferably, the Nb and V content should be 0.100% or less each. More preferably, the Nb and V content should be 0.075% or less each, even more preferably 0.050% or less each, and most preferably 0.040% or less each. 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 be 0.001% or more each. More preferably, it should be 0.005% or more each, and even more preferably 0.010% or more each.

[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 Ta and W content should be 0.10% or less each. Preferably, the Ta and W content should be 0.09% or less each. More preferably, the Ta and W content should be 0.08% or less each. There is no particular lower limit for the Ta and W 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 Ta and W content 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, when Mo is included, the Mo content should be 1.00% or less. Preferably, the Mo content should be 0.80% or less. More preferably, the Mo content should be 0.70% or less, even more preferably 0.60% or less, and most preferably 0.50% or less. There is no specific 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. More preferably, the Mo content should be 0.10% or more, and even more preferably 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, when 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 specific 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 present in amounts of 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, their respective contents should be 0.0100% or less. Preferably, the contents of Ca, Mg, and REM should be 0.0050% or less, more preferably 0.0030% or less, even more preferably 0.0025% or less, and most preferably 0.0020% or less. There is no particular lower limit for the contents of Ca, Mg, and REM, but since these elements spheroidize the shape of nitrides and sulfides and improve the ductility of the steel sheet, it is preferable that the contents of Ca, Mg, and REM be 0.0005% or more, and more preferably 0.0010% or more.

[0040] Zr: 0.100% or less, Te: 0.100% or less If the content of Zr and Te is 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 content of Zr and Te should be 0.100% or less. Preferably, the content of Zr and Te should be 0.080% or less, more preferably 0.050% or less, even more preferably 0.020% or less, and most preferably 0.015% or less. There is no particular lower limit for the content of Zr and Te, but since these elements spheroidize the shape of nitrides and sulfides and improve the ductility of the steel sheet, it is preferable that the content of Zr and Te be 0.001% or more. Preferably, the content of Zr and Te should be 0.005% or more, and even more preferably 0.008% or more.

[0041] Hf: 0.10% or less If Hf is present at a concentration of 0.10% 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 Hf is included, the Hf content should be 0.10% or less. Preferably, the Hf content should be 0.08% or less, more preferably 0.05% or less, and even more preferably 0.02% or less. There is no specific lower limit for the Hf content, but since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ductility of the steel sheet, it is preferable that the Hf content be 0.01% or more.

[0042] Bi:0.200% or less If the Bi content is 0.200% 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 Bi is included, the Bi content should be 0.200% or less. Preferably, the Bi content should be 0.100% or less, more preferably 0.010% or less, even more preferably 0.005% or less, and most preferably 0.003% or less. There is no specific lower limit for the Bi content, but since it is an element that reduces segregation, it is preferable that the Bi content be 0.001% or more, and more preferably 0.002% or more.

[0043] Ni: 0.8% or less If the Ni content is 0.8% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not be impaired. For this reason, when Ni is included, the Ni content should be 0.8% or less. Preferably, the Ni content should be 0.5% or less, more preferably 0.4% or less, even more preferably 0.3% or less, and most preferably 0.2% or less. There is no particular lower limit for the content, but since Ni is an element that improves hardenability, it is preferable that the Ni content be 0.005% or more, more preferably 0.01% or more, and even more preferably 0.03% or more.

[0044] Furthermore, regarding the above-mentioned Cu, Sn, Sb, Nb, V, Ta, W, Mo, Co, Ca, Mg, REM, Zr, Te, Hf, Bi, and Ni, if their respective contents are below the preferred lower limit, they will not impair the effects of the present invention and will therefore be included as unavoidable 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: Less than 1% by area] Although ferrite is a soft structure and therefore effective in improving workability, in the steel sheets targeted by this invention, it has a strong effect in reducing the strength of the steel sheet. The ferrite referred to here may be polygonal ferrite, pseudo-polygonal ferrite, or granular bainitic ferrite. For the purpose of achieving high strength, the area percentage of ferrite should 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, a cross-section of the steel plate parallel to the rolling direction (L-section) is polished, then etched with 3 vol.% nital, and 10 fields of view are observed at a position 1 / 4 of the plate thickness (corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate) using a scanning electron microscope (SEM) at 2000x magnification. Note that the surface of the steel plate in the case of zinc plating refers to the interface between the zinc plating layer and the steel plate. Next, using the obtained microstructure images, the area ratio of ferrite for the 10 fields of view is calculated using Media Cybernetics' Image-Pro. The average of these 10 field of view area ratios is taken as the ferrite area ratio. Note that in the above microstructure images, ferrite exhibits a gray structure (underlying structure). Furthermore, the method for measuring the area ratio of martensite, etc., described later, is as follows: A sample is cut from an annealed steel plate (annealed plate), the plate thickness cross section parallel to the rolling direction is polished, and then it is etched with 3 vol.% nital. Ten fields of view are photographed at a magnification of 2000x using an SEM (scanning electron microscope) at a position corresponding to 1 / 4 of the plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate). Additional observations at higher magnifications may be made to confirm the carbides in detail. As above, the area ratio of the structure is determined using the obtained image data with Image-Pro from Media Cybernetics. Fresh martensite is distinguished as a white or light gray area, tempered martensite as a gray or dark gray area containing carbides with misaligned orientations, pearlite as a layered structure of black and white, and bainite as a gray or dark gray area containing carbides with aligned orientations. However, in actual observation, the above classification can be difficult. For example, even in tempered martensite, the orientation of the carbides may appear to be aligned. Therefore, gray or dark gray regions containing aligned carbides, and where the tissue interface extends in a straight line, were classified as bainite, while other gray or dark gray regions containing carbides were classified as tempered martensite.

[0048] [Bainite: 0.1% or more by area] 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 inducing bainite transformation during the tempering process in the final annealing stage, so bainite is formed in an area percentage of 0.1% or more. More preferably, bainite is 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. Furthermore, in order to achieve high strength, it is preferable that the total area ratio of ferrite and bainite be 15% or less. More preferably, the total area ratio of ferrite and bainite is 10% or less, even more preferably 8% or less, and most preferably 7% or less. There is no particular lower limit, but it is preferable that the total area ratio of ferrite and bainite be 1% or more, more preferably 2% or more, and even more preferably 3% or more.

[0049] [Residual austenite: 5% to 20% by area] Retained austenite contributes to the ductility of steel sheets through the TRIP effect and is an essential structural component in this invention. To obtain a sufficient effect, the retained austenite content 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 content 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 equivalent, 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.

number

[0051] [Tempered martensite: over 70% by area] Tempered martensite is characterized by being an aggregate of lath-like crystal grains containing iron-based carbides. Compared to fresh martensite, it has higher ductility and is also characterized by being able to easily obtain the high yield strength necessary to ensure collision safety in automotive components. In order to obtain these effects, the area ratio of tempered martensite must be 70% or more. The area ratio of tempered martensite should be more preferably 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 should preferably be 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. Although steel sheets containing fresh martensite have high strength, their processing characteristics are inferior, so the area ratio of fresh martensite in the steel sheet according to this embodiment is limited to 15% or less. 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 ratio 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 surface of the steel sheet) 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 it is difficult to distinguish fresh martensite from retained austenite which also appears as a white or light gray region, the microstructure fraction is obtained by first determining the area ratio of the white or light gray region and then subtracting the fraction of retained austenite determined by the method above.

[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 from the 1 / 4 position in the thickness to a surface of 0.1 mm, and then chemically polishing it to a further 0.1 mm. The surface exposed at the 1 / 4 position in the 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 amounts added (steel sheet composition) were 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. Preferably, the carbon concentration in retained austenite is 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. Preferably, the carbon concentration in retained austenite is 0.90% or lower, more preferably 0.85% or lower, even more preferably 0.80% or lower, and most preferably 0.75% or lower. [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 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, with a yield ratio of 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 refers to 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 Z 2241, the amount of retained austenite that disappears between the start of the tensile test and the processing that imparts a 1% nominal strain is 0.8% or more in terms of area ratio to the entire steel structure.] The excellent work hardening properties 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 properties with a high yield ratio. Furthermore, the solid solution carbon released when retained austenite transforms has the effect of increasing the yield stress by fixing 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 amount of retained austenite that disappears from the start of the tensile test until processing to impart a 1% strain at nominal strain 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 when processing that applies a 1% strain. In this invention, both can be reconciled, and the method by which this was 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 a tensile test is as follows: A 1.5 cm square steel plate is cut from the center of a JIS No. 5 test specimen after a tensile test. The plate is mechanically ground in the thickness direction (depth direction) to 1 / 4 of its thickness, and then chemically polished with oxalic acid or the like to remove the strain introduced by mechanical polishing, creating an observation surface. X-ray diffraction is performed on this observation surface, and the amount of retained austenite is measured using the method described above.

[0060] [Processing and heat treatment to impart 1% distortion] 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 Z 2241, and the yield stress, uniform elongation, total elongation, and tensile strength were evaluated by averaging the results of the three tests. 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 that applies 1% strain). For the remaining three specimens, tensile processing with a nominal strain of 1% was performed, 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 specimen parallel section, 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] [Increase in YS obtained by subtracting YS before (the processing and heat treatment) from YS after (the processing and heat treatment) ≥ 200 MPa] The processing refers to a process that imparts a 1% strain, and the heat treatment refers to a 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 low heat treatment temperature conditions of 100°C, the material is soft when formed, but when used as a part, it can have a high yield stress and, consequently, high impact characteristics. The increase in YS is preferably 210 MPa or more, and more preferably 220 MPa or more. As described later, under certain conditions, it shows an extremely high increase in yield stress exceeding 240 MPa. There is no particular upper limit, but the increase in YS 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] A feature of the present invention is that the total elongation before and after processing and heat treatment that imparts a 1% strain does not change significantly, and El after (the processing and heat treatment) is 0.9 × ((El before processing and heat treatment)) or more. The above value is preferably 0.95 × ((El before processing and heat treatment)) or more, more preferably ((El before processing and heat treatment)) or more, even more preferably 1.05 × ((El before processing and heat treatment)) or more, and most preferably 1.10 × ((El before 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 increase after treatment. The upper limit is not particularly limited, but it may be 2.00 × ((El before processing and heat treatment)) or less.

[0063] [The difference between the stress increase after 100°C heat treatment without pre-strain and the stress increase after 100°C heat treatment with 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 greater than the stress increase in the unprocessed portion. Therefore, the difference between the stress increase after 100°C heat treatment without pre-strain 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 sheets 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 formed into steel slabs using a conventional ingot-rolling or continuous casting method. Alternatively, thin steel slabs with a thickness of 100 mm or less may be manufactured using a direct casting method.

[0065] Hot rolling process The above steel slab is heated and held, then subjected to hot rolling, and hot-rolled sheets are produced by rough rolling and finish rolling and wound into coils. 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 must be completed in the austenite single-phase region to improve elongation and hole-expanding properties after annealing by homogenizing the microstructure within the steel sheet and reducing material anisotropy. Therefore, it is preferable that the finish rolling completion temperature be 800°C or higher. More preferably, it is preferable that the finish rolling completion temperature be 850°C or higher, and even more preferably 900°C or higher. On the other hand, if the finish rolling completion temperature exceeds 1000°C, the microstructure 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. More preferably, it is preferable that the finish rolling completion temperature be 980°C or lower, even more preferably 950°C or lower, and most preferably 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 % of the elements contained in the steel sheet. Ms point (℃)=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 % 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] Cold rolling pre-annealing, held at a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more. Furthermore, between the completion of finish rolling in hot rolling and the start of cold rolling, annealing may be performed for 10 minutes or more at a temperature range T of 400°C or higher and below the Ac3 point in order 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. Also, 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 above 450°C, more preferably above 460°C, even more preferably above 470°C, and most preferably above 480°C. The Ac1 and Ac3 points shown in this specification 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 plate, or elements whose content is below the detection limit of the analysis, should be calculated as 0 mass%. Ac1 point (℃)=723-10.7×[Mn]-16.9×[Ni]+29.1×[Si]+16.9×[Cr]+290×[As]+6.38×[W]...(A) Ac3 points (℃)=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] Cold rolling process Next, 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. 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 should be 2°C / s or less. Next, as an annealing process, the obtained steel sheet is subjected to heat treatment and, if necessary, plating. 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 when the average heating rate is high, the austenite transformation occurs from the unrecrystallized ferrite without ferrite recrystallization. In the present invention, since the ferrite-austenite transformation occurs after the ferrite recrystallization is completed, a balanced retained austenite can be formed, so a low average heating rate is preferable. In the temperature range from 650°C to 800°C, a 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. While there is no particular lower limit, a preferred average heating rate in the temperature range of 650°C to 800°C is 0.5°C / s or higher, and more preferably 0.6°C / s or higher. The above average heating rate can be calculated 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 less than 1°C / s. Next, the steel sheet is heated to Ts, the maximum annealing temperature in the main 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. Furthermore, there is no particular lower limit to the average heating rate, 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 calculated by dividing the temperature difference between 800°C and the maximum annealing temperature Ts by the time required to raise the temperature during that period. In order to complete the austenite transformation with a high amount of recrystallized ferrite and low driving force, Ts must be above the Ac3 point + 30°C, but even better properties can be obtained if it is above 920°C. Therefore, Ts should be above the Ac3 point + 30°C, preferably above 920°C. The higher the 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 works to increase the strength during heat treatment inside the block. Furthermore, Ts is more preferably above 940°C, and even more preferably above 950°C. On the other hand, if the annealing temperature is excessively high, the microstructure becomes excessively large and the toughness deteriorates, so Ts is preferably below 1050°C, and more preferably below 1020°C.

[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. Next, the material is cooled 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 some 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. The cooling means of the present invention can be any known method, 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 mixture is heated from T1 to a temperature T2 of 280°C to 380°C, and then held in a temperature range of T2-10°C to below T2 for 1 to 60 seconds. Next, the material is heated from T1 to a temperature T2 of 280°C to 380°C, and held in a temperature range of T2-10°C to less than T2 for 1 s to 60 s. This step is particularly important for forming the desired retained austenite structure and is referred to herein as the tempering step. In the tempering step, 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 necessary 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 above average heating rate can be determined by dividing the temperature difference between T1 and T2 by the time required to heat from T1 to T2. To promote the transformation, the temperature T2 is set to 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. Therefore, the temperature T2 is set to 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. Furthermore, 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, due to 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. Also, if the holding time is too long, the amount of carbon in the retained austenite increases excessively, and the yield ratio becomes too high, so it needs to be 60 s or less. A preferred holding time is 30 s or less. Since even more favorable properties can be obtained if the holding time is less than 10 s, it is more preferable to have a holding time of less than 10 s, and even more preferable to have a holding time of 9 s or less. The reason for setting the above retention period to a temperature range of T2-10°C or higher and below T2 is that carbon diffusion is relatively significant in this temperature range, and it has a large impact on the material.

[0076] Afterward, final cooling is performed. However, if the temperature range is below T2-10°C, where austenite carbon distribution 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, cooling is performed at an average cooling rate of 2°C / s or higher in the temperature range of T1 to T1-10°C or lower in order to maintain the solid solution state of carbon. It is believed that rapid cooling in the temperature range of T1 to T1-10°C or lower prevents sufficient diffusion of the solid solution carbon produced by the transformation during cooling, thereby reducing the amount of carbon in the retained austenite and maintaining an unstable retained austenite. The average cooling rate mentioned above is preferably 5°C / s or higher, and more preferably 10°C / s or higher. Cooling should preferably be carried out up to 25-85°C, with the pickling temperature (25-85°C) as the lower limit, taking into consideration the subsequent pickling process. That is, the cooling completion temperature (cooling stop temperature) should be equal to or higher than the pickling temperature. The average cooling rate can be determined by dividing the temperature difference between T1 and the cooling completion temperature (cooling stop temperature) by the time required to cool from T1 to the cooling completion 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 meet the above-mentioned cooling rate requirements.

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

[0079] Plating Furthermore, during the annealing process, hot-dip galvanizing may be applied to produce hot-dip galvanized steel sheets. 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, 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. Furthermore, the range of Tm is 500°C or lower, preferably 495°C or lower, and more preferably 490°C or lower. Furthermore, 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 more, more preferably 7°C / s or more, even more preferably 10°C / s or more, and most preferably 15°C / s or more. The upper limit of the average heating rate is not particularly limited, but it may be 60°C / s or less. In addition, the holding time in the temperature range of Tm-10°C or more and less than Tm should be 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 should be 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, regarding the final cooling, T2 is replaced with the plating temperature Tm, and the explanation is as described above.

[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. [Examples]

[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 down and formed into steel slabs after bloc rolling. 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. Some of the slabs were 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, cold rolling was performed. Subsequently, heat treatment (annealing and tempering) was performed in a nitrogen atmosphere. Some of the slabs were then hot-dip galvanized and electro-galvanized. From the obtained steel sheets, JIS No. 5 test pieces (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] [Table 1]

[0086] [Table 2-1]

[0087] [Table 2-2]

[0088] [Table 3-1]

[0089] [Table 3-2]

Claims

1. In mass percent, 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: Contains 0.0100% or less, Ti: 0.200% or less, and containing Ti / 48 ≥ N / 14 (Ti and N are components (mass%) contained in the steel sheet), and further, B: 0.0003% or more and 0.0100% or less, It contains such a ratio that P + B ≥ 0.0050%, The remainder is a component composition consisting of Fe and unavoidable impurities, Furthermore, the Si / 28 + Cr / 52 ratio (Si and Cr are components contained in the steel sheet (mass%)) satisfies the condition of being between 0.040 and 0.

052. Steel structure, Ferrite is less than 1% by area percentage. If bainite is present in an area of ​​0.1% or more, If the area percentage of retained austenite is between 5% and 20%, Tempered martensite accounts for 70% or more by area. Fresh martensite is between 0% and 15% by area. The carbon concentration in the retained austenite satisfies the condition of being 0.45% or more and 1.00% or less. The yield ratio is 0.80 or less, In a tensile test in accordance with JIS Z 2241, the retained austenite that disappears between the start of the tensile test and the processing that imparts a 1% strain at nominal stress is 0.8% or more in terms of area relative to the entire steel structure, and When the aforementioned 1% strain is applied and heat treatment is performed at a temperature of 100°C, The increase in YS obtained by subtracting the YS before (the processing and heat treatment) from the YS after (the processing and heat treatment) is 200 MPa or more. A steel sheet having an El after (the processing and heat treatment) of 0.9 × (El before (the processing and heat treatment)) or greater. Note that YS and El represent the yield stress and total elongation of the steel sheet, respectively.

2. The aforementioned component composition is further expressed in mass%, Cu: 0.005% or more and 0.500% or less, Sn: 0.005% or more and 0.500% or less, Sb: 0.001% or more and 0.080% or less, The steel plate according to claim 1, containing at least one of the above in a total amount of 0.010% or more and 1.000% or less.

3. The aforementioned component composition is further expressed in mass%, 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, The steel plate according to claim 1, which contains at least one selected from among the following.

4. The aforementioned component composition is further expressed in mass%, 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, The steel plate according to claim 2, which contains at least one selected from among the following.

5. 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 4.

6. A method for manufacturing a steel sheet according to any one of claims 1 to 4, wherein a steel slab having the above-mentioned component composition is heated, then hot-rolled, the resulting hot-rolled sheet is pickled, and then cold-rolled to obtain a cold-rolled sheet, 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, and the mixture is heated to Ts. 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%).

7. A method for manufacturing a plated steel sheet according to Claim 5, wherein a steel slab having the component composition according to any one of Claims 1 to 4 is heated, then hot-rolled, the resulting hot-rolled sheet is pickled, and then cold-rolled to obtain a cold-rolled sheet, 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, and the mixture is heated to Ts. 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%).

8. A method for manufacturing a 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. A method for manufacturing a plated steel sheet according to claim 7, 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.

10. The method for manufacturing a steel sheet according to claim 6, wherein the steel sheet is further subjected to electro-galvanizing to obtain a plated steel sheet.

11. The method for manufacturing a steel sheet according to claim 8, wherein the steel sheet is further subjected to electro-galvanizing to obtain a plated steel sheet.

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