Steel sheet for cold press forming, plated steel sheet, method for manufacturing steel sheet, method for manufacturing plated steel sheet, member, and method for manufacturing member

Abstract

The purpose of the present invention is to provide a technology relating to a steel sheet for cold press forming and a method for manufacturing same, the steel sheet having high impact resistance after being subjected to cold press forming and being formed into a part.　The steel sheet for cold press forming has a specific component composition and a steel microstructure. The concentration of carbon in retained austenite satisfies 0.45-0.95%. In a tensile test according to JIS Z2241, the retained austenite that disappears during deformation between the start of the tensile test and the application of 1% nominal strain is 0.8% or more in area fraction relative to the total steel microstructure, and the retained austenite present after deformation from the start of the tensile test to uniform elongation is 3% or more in area fraction relative to the total steel microstructure.

Description

Steel sheet for cold press forming, plated steel sheet, method for manufacturing steel sheet, method for manufacturing plated steel sheet, component, and method for manufacturing component

[0001] The present invention relates to steel sheets, plated steel sheets, and methods for manufacturing the same, components, and methods for manufacturing the same, which are suitable for use in structural and electrical components of automobile bodies that are formed by cold pressing to create drawn, stretched, or elongated flanges, and which have a tensile strength (tensile strength) TS of 980 MPa or more.

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

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

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

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

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

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

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

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

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

[0011] The present invention aims to provide technology for cold press forming steel sheets that, after being subjected to cold press forming, have high collision resistance after being formed as parts, and for a method for manufacturing the same. Specifically, the objectives are to provide excellent cold press formability, that is, high tensile strength (TS) and excellent elongation characteristics, while having a low yield strength correlated with the amount of springback, i.e., a low yield ratio (yield stress / tensile strength), further suppressing the change in elongation and increasing the yield stress after processing heat treatment, increasing the increase in stress after heat treatment, and further, to exhibit high collision characteristics after being processed into parts in order to ensure collision safety in automobile parts and the like.

[0012] 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 heat treatment and increasing the yield stress means that, in a tensile test, after processing to impart a 1% strain at nominal strain and then performing heat treatment at a temperature of 170°C, the increase in YS (after processing and heat treatment) minus the YS (before processing and heat treatment) is 250 MPa or more, and the El (total elongation) after processing and heat treatment is 0.9 × (El before processing and heat treatment) or more. Furthermore, increasing the stress increase after heat treatment means that the difference between the stress increase after heat treatment at 170°C without pre-strain in the tensile test and the stress increase after heat treatment at 170°C after processing (tensile processing) to impart a 1% strain in the tensile test is 120 MPa or more.

[0013] The inventors of this invention have diligently conducted research to solve the above problems and have come to design materials that focus on work hardening and baked paint hardening as methods for strengthening steel, in addition to the solid solution strengthening, precipitation strengthening, and dislocation strengthening (transformation strengthening) mentioned above. After sheet parts are cut from a coil-shaped product, strain is introduced into the material (steel sheet) by press forming or the like. At this time, work hardening occurs, and the material generally becomes harder. After forming, the surface is painted after processes such as assembly and welding, but in the paint baking process, the parts are heat-treated in the temperature range of about 170°C. Depending on the material, baked paint hardening occurs after the heat treatment, and it becomes even harder.

[0014] 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 baked paint hardening properties. In other words, the gist of this disclosure is as follows. [1] The composition is such that, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.1000% or less, S: 0.0200% or less, Al: 1.00% or less, Cr: 0.05% or less, N: 0.0200% or less and O: 0.0100% or less, with the remainder being Fe and unavoidable impurities, and the steel structure is such that the total area percentage of ferrite and bainite is 0.1% or more and 15% or less, retained austenite is 5% or more and 20% or less, tempered martensite is 70% or more, and fresh martensite is 0% or more and 16% or less, A steel sheet for cold press forming that satisfies the requirement that the carbon concentration in the retained austenite is 0.45% or more and 0.95% or less, and in a tensile test in accordance with JIS Z2241, the retained austenite that disappears between the start of the tensile test and processing that imparts a strain of 1% nominal strain accounts for 0.8% or more of the area of ​​the entire steel structure, and after processing from the start of the tensile test to uniform elongation in the tensile test, retained austenite accounts for 3% or more of the area of ​​the entire steel structure. [2] The above component composition is further defined in mass percent as follows: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Mo: 1.00% or less, Co: 1.000% or less, Sn: 0.5% or less, Cu: 0.5% 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, Sb: 0.08% or less A steel sheet for cold press forming according to [1], comprising at least one selected from among the following.[3] The component composition further contains Ti in a range of 0.200% or less by mass, satisfying Ti / 48 ≥ N / 14, and B in a range of 0.0003% or more and 0.0100% or less, satisfying B + P ≥ 0.0090%, as described in [2]. The above Ti, N, B, and P represent the components (by mass) in the steel sheet. [4] A plated steel sheet for cold press forming, wherein one of either an electro-galvanized layer or a hot-dip galvanized layer is formed on the surface of the steel sheet described in any of [1] to [3]. [5] A steel slab having the component composition described in any of [1] to [3] above is heated, then hot-rolled at a finish rolling completion temperature of 800°C to 1000°C, then wound up at a temperature between Ms and Bs, the obtained hot-rolled sheet is pickled, then cold-rolled with a reduction ratio of 30% or more, the obtained cold-rolled sheet is heated to Ts with an average heating rate of 2°C / s or less in the temperature range from 650°C to 800°C, and an average heating rate of 1°C / s or less in the temperature range from 800°C to the maximum annealing temperature Ts which is Ac3 point + 30°C or higher, and then cooled to T1 with an average cooling rate of 10°C / s or more to 100°C / s from Ts to 400°C, and an average cooling rate of 5°C / s or more to 20°C / s in the temperature range from 400°C to 280°C, and then from T1 A method for manufacturing steel sheets for cold press forming, comprising raising the temperature to a temperature T2 with an average heating rate of 5°C / s or more in the temperature range T2 between 280°C and 380°C, holding the temperature in the temperature range between T2-10°C and below T2 for 1 second to 60 seconds, then cooling the sheet to a temperature range of T1-10°C or less with an average cooling rate of 2°C / s or more in the temperature range from T1 to T1-10°C, and then performing skin pass rolling on the resulting annealed sheet after a second pickling, with the sheet temperature being 20°C or higher and the elongation being 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 at a finish rolling completion temperature of 800°C to 1000°C, then wound up at a temperature between Ms and Bs, the obtained hot-rolled sheet is pickled, then cold-rolled with a reduction ratio of 30% or more, and the obtained cold-rolled sheet is heated to Ts, with an average heating rate of 2°C / s or less in the temperature range from 650°C to 800°C, and an average heating rate of 1°C / s or less in the temperature range from 800°C to the maximum annealing temperature Ts which is at or above the Ac3 point + 30°C. Furthermore, the method for manufacturing plated steel sheets for cold press forming involves cooling the sheet to T1 with an average cooling rate of 10°C / s or more and 100°C / s or less in the temperature range from Ts to 400°C, an average cooling rate of 5°C / s or more and 20°C / s or less in the temperature range from 400°C to 280°C, then performing a hot-dip galvanizing treatment, raising the temperature to Tm with an average heating rate of 5°C / s or more in the temperature range from T1 to 450°C to 500°C, holding the sheet in the temperature range of Tm-10°C or more and less than Tm for 1 second or more and 60 seconds or less, then cooling the sheet to T1-10°C or less with an average cooling rate of 2°C / s or more in the temperature range from T1 to T1-10°C or less, and after pickling the obtained annealed sheet, performing skin pass rolling under the conditions of a sheet temperature of 20°C or more and an elongation of 0.1% or less (including 0%). [7] A method for manufacturing a steel sheet for cold press forming according to [5], wherein pre-cold rolling annealing is performed by holding the sheet in a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more between the hot rolling and the start of cold rolling. [8] A method for manufacturing a plated steel sheet for cold press forming according to [6], wherein pre-cold rolling annealing is performed by holding the sheet in a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more between the hot rolling and the start of cold rolling. [9] A method for manufacturing a steel sheet for cold press forming according to [5] or [7], wherein the steel sheet is further plated with electrogalvanizing to form a plated steel sheet.

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

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

[0015] According to the present invention, a 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 amount of work hardening and baked paint hardening is extremely high, the yield strength of the material is increased significantly after cold press forming and subsequent baked paint heat treatment. For example, when used as an automobile part, it is possible to improve collision performance. As a result, the range of applications to various parts and materials such as automobile body parts that are cold press formed is broadened, and it can also be applied to parts that require complex press processing, and can greatly contribute to the weight reduction of automobile bodies and other electrical components.

[0016] 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 plate of the present invention and the reasons for limiting it will be explained. In the following description, "%" representing the content of component elements of steel means "mass%" unless otherwise specified.

[0017] [C: 0.050% or more and 0.400% 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 0.050%, the fraction of martensite decreases, making it difficult to achieve the desired TS. Therefore, the C content should be 0.050% or more. The C content is preferably 0.080% or more, more preferably 0.100% or more, even more preferably 0.120% or more, and most preferably 0.180% or more. On the other hand, if the C content exceeds 0.400%, the martensite becomes brittle, making it difficult to achieve the desired elongation. Therefore, the C content should be 0.400% or less. The C content should be 0.350% or less. The C content is more preferably 0.300% or less, even more preferably 0.250% or less, and most preferably 0.240% or less.

[0018] [Si: 0.01% or more and 2.50% or less] Si is one of the important basic components of steel, and in particular in this invention, it is an element that affects the hardness of martensite and the fraction of retained austenite by suppressing carbide formation during continuous annealing and promoting 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. Preferably, the Si content is 0.40% or more, more preferably 1.00% or more, even more preferably 1.15% or more, and most preferably 1.20% or more. On the other hand, if the Si content exceeds 2.50%, 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 a decrease in the hole expansion ratio λ, which is an indicator of elongation flangeability. Weldability is also impaired. Therefore, the Si content should be 2.50% or less. Preferably, the Si content should be 2.00% or less, more preferably 1.80% or less, even more preferably 1.65% or less, and most preferably 1.55% or less.

[0019] [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 2.35% 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 4.50% or less, more preferably 4.00% or less, even more preferably 3.50% or less, and most preferably 3.30% or less.

[0020] [P: 0.1000% or less] P segregates at the prior austenite grain boundaries, embrittles them, and impairs the ductility of the steel sheet. It also impairs weldability and delayed fracture resistance. Therefore, the P content must be 0.1000% or less. Preferably, the P content is 0.0700% or less. More preferably, the P content is 0.0200% or less, even more preferably 0.0150% or less, and most preferably 0.0080% or less. There is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable to have a P content of 0.0010% or more. It is even more preferable that the P content be 0.0030% or more, and even more preferably 0.0050% or more.

[0021] [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. Preferably, the S content is 0.0050% or less, more preferably 0.0030% or less, even more preferably 0.0020% or less, and most preferably 0.0010% or less. There is no specific lower limit for the S content, but due to production technology constraints, it is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0022] [Al: 1.00% or less] If Al is present in large quantities, it raises the Ac3 point (Ac3 transformation point) and contains a large amount of ferrite in the microstructure, which hinders the high strength achieved by utilizing the martensitic structure. Therefore, the Al content must be 1.00% or less. The Al content is preferably 0.50% or less. The Al content is more preferably 0.20% or less, even more preferably 0.10% or less, and most preferably 0.060% or less. There is no particular lower limit for the Al content, but it is preferable that the Al content be 0.001% or more in order to suppress carbide formation during continuous annealing and promote the formation of retained austenite. More preferably 0.008% or more, and even more preferably 0.010% or more.

[0023] [Cr: 0.05% or less] If the Cr content is 0.05% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not be impaired. Also, similar to Si, it has the effect of stabilizing retained austenite by promoting carbon concentration, so excessive content is undesirable from the perspective of utilizing unstable retained austenite. For this reason, the Cr content should be 0.05% or less. Preferably, the Cr content is 0.04% or less, and more preferably 0.03% or less. There is no particular lower limit for the Cr content, but since it is an element that improves hardenability, it is preferably 0.01% or more, and more preferably 0.02% or more.

[0024] [N: 0.0200% or less] N exists as a nitride and impairs ductility. Therefore, the N content must be 0.0200% or less. Preferably, the N content is 0.0050% or less. More preferably, the N content is 0.0045% or less, even more preferably 0.0040% or less, and most preferably 0.0035% 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 0.0005% or more, and even more preferably 0.0010% or more.

[0025] [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. Preferably, the O content is 0.0050% or less. More preferably, the O content is 0.0040% or less, even more preferably 0.0020% or less, and most preferably 0.0015% or less. There is no particular lower limit for the O content, but due to production technology constraints, it is preferable that the O content be 0.0001% or more. It is more preferable that the O content be 0.0003% or more, and even more preferably 0.0005% or more.

[0026] A steel sheet according to one embodiment of the present invention has a composition in which the above components are contained, with the remainder being Fe and unavoidable impurities. Preferably, a steel sheet according to one embodiment of the present invention has a composition in which the above components are contained, with the remainder being Fe and unavoidable impurities. Examples of unavoidable impurities include H, Zn, Pb, As, Se, Ge, Sr, and Cs. Unavoidable impurities may be introduced from raw materials such as scrap, and their inclusion is permissible as long as it does not hinder the objective of the present invention. Specifically, it is permissible for these impurities to be present in a total of 0.100% or less.

[0027] Furthermore, in addition to the above component composition, the steel sheet of the present invention may optionally contain, either alone or in combination, at least one selected by mass%, from the following: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Mo: 1.00% or less, Co: 1.000% or less, Sn: 0.5% or less, Cu: 0.5% 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, and Sb: 0.08% or less.

[0028] Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less. If Ti, Nb, and V are each at 0.200% or less, large amounts of coarse precipitates and inclusions will not be generated, and the ductility of the steel sheet will not decrease, nor will the carbon concentration in the retained austenite decrease. For this reason, when Ti, Nb, and V are included, the content of Ti, Nb, and V should each be 0.200% or less. Preferably, the content of Ti, Nb, and V should each be 0.100% or less, more preferably 0.070% or less, even more preferably 0.060% or less, and most preferably 0.050% or less. While there is no specific lower limit for the Ti, Nb, and V content, it is preferable that the Ti, Nb, and V content be 0.001% or more, since the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. It is more preferable that the Ti, Nb, and V content be 0.005% or more, and even more preferable that be 0.010% or more.

[0029] 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 content of Ta and W should be 0.10% or less each. The content of Ta and W should preferably be 0.09% or less each, more preferably 0.08% or less each, even more preferably 0.07% or less each, and most preferably 0.06% or less each. There is no particular lower limit for the content of Ta and W, but since the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, it is more preferable that the content of Ta and W be 0.01% or more each. It is more preferable that the content of Ta and W be 0.02% or more each, and even more preferably 0.03% or more each.

[0030] B: 0.0100% or less. If B is present in a quantity 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, if B is included, 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 0.0050% or less, and most preferably 0.0035% or less. There is no particular lower limit for the B content, but since B is an element that segregates at the austenite grain boundaries during annealing and improves hardenability, it is preferable that the B content be 0.0003% or more. More preferably, the B content should be 0.0008% or more, and even more preferably 0.0010% or more.

[0031] Mo: 1.00% or less. If the Mo content is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not be impaired. Therefore, if Mo is included, the Mo content should be 1.00% or less. It is more preferable that the Mo content be 0.70% or less, even more preferable that it be 0.60% or less, and most preferable that it be 0.50% or less. There is no particular lower limit for the Mo content, but since it is an element that improves hardenability, it is preferable that it be 0.01% or more. It is more preferable that the Mo content be 0.10% or more, and even more preferable that it be 0.15% or more.

[0032] Co: 1.000% or less. If the Co content is 1.000% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not be impaired. Therefore, if Co is included, the Co content should be 1.000% or less. Preferably, the Co content should be 0.500% or less. More preferably, the Co content should be 0.100% or less, even more preferably 0.050% or less, and most preferably 0.030% or less. There is no particular lower limit for the Co content, but since it is an element that improves hardenability, it is preferable that the Co content be 0.001% or more. More preferably, the Co content should be 0.008% or more, and even more preferably 0.010% or more.

[0033] Sn: 0.5% or less, Cu: 0.5% or less. If Sn and Cu are present in amounts of 0.5% or less each, cracks will not form inside the steel sheet during casting or hot rolling, and the ductility of the steel sheet will not be impaired. Therefore, when Sn and Cu are included, the Sn and Cu content should be 0.5% or less each. More preferably, the Sn and Cu content should be 0.3% or less each, even more preferably 0.2% or less each, and most preferably 0.1% or less each. There is no particular lower limit for the content, but since these elements improve hardenability, it is preferable that the Sn and Cu content be 0.001% or more each. It is more preferable that the Sn and Cu content be 0.005% or more each, and even more preferable that be 0.008% or more each.

[0034] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less. If Ca, Mg, and REM are each at 0.0100% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not be impaired. Therefore, when Ca, Mg, and REM are included, the content of Ca, Mg, and REM should be 0.0100% or less each. Preferably, the content of Ca, Mg, and REM should be 0.0080% or less each. More preferably, the content of Ca, Mg, and REM should be 0.0060% or less each, and even more preferably 0.0040% or less each. While there is no specific lower limit for the content of Ca, Mg, and REM, it is preferable that the content of Ca, Mg, and REM be 0.0005% or more, since these elements spheroidize the shape of nitrides and sulfides and improve the ductility of steel sheets. It is more preferable that the content of Ca, Mg, and REM be 0.0010% or more, and even more preferable that it be 0.0020% or more.

[0035] 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 each. Preferably, the content of Zr and Te should be 0.080% or less each. More preferably, the content of Zr and Te should be 0.050% or less each, even more preferably 0.020% or less each, and most preferably 0.015% or less each. There is no particular lower limit for the content of Zr and Te, but since they are elements that 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 each. The content of Zr and Te is more preferably 0.005% or more, and even more preferably 0.008% or more.

[0036] Hf: 0.10% or less. If the Hf content is 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, if Hf is included, the Hf content should be 0.10% or less. It is preferable that the Hf content be 0.08% or less. It is more preferable that the Hf content be 0.02% or less, and even more preferable that it be 0.005% or less. There is no particular 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 more preferable that the Hf content be 0.001% or more.

[0037] 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, if Bi is included, the Bi content should be 0.200% or less. Preferably, the Bi content should be 0.100% or less. More preferably, the Bi content should be 0.010% or less, and even more preferably, 0.005% or less. There is no specific lower limit for the Bi content, but since it is an element that reduces segregation, it is more preferable that the Bi content be 0.001% or more. Even more preferably, the Bi content should be 0.002% or more.

[0038] 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, if Ni is included, the Ni content should be 0.8% or less. Preferably, the Ni content should be 0.5% or less. More preferably, the Ni content should be 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, the Ni content should be 0.01% or more, and even more preferably 0.03% or more.

[0039] Sb: 0.08% or less. If the Sb content is 0.08% 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 Sb is included, the Sb content should be 0.08% or less. Preferably, the Sb content should be 0.05% or less, more preferably 0.04% or less, and even more preferably 0.03% or less. There is no particular lower limit for the Sb content, but when it is included to obtain the effect of suppressing surface decarburization in the annealing process, it is preferably 0.001% or more, more preferably 0.004% or more, and even more preferably 0.008% or more.

[0040] Furthermore, the effect of adding B is enhanced when Ti is contained within the following range: Ti: 0.200% or less, satisfying Ti / 48 ≥ N / 14. Note that Ti and N above represent the components (mass %) in the steel sheet. Ti, which has an even higher affinity for N than B, forms nitrides before B, thereby ensuring that B is dissolved in the steel. However, it is presumed that dissolved B, which is a grain boundary segregation element, suppresses the segregation of dissolved C at the grain boundaries and maintains the high bake-on paint hardening properties caused by dissolved C adhering to dislocations within the grains. For this reason, it is preferable that Ti: 0.200% or less, satisfying Ti / 48 ≥ N / 14. More preferably, Ti: 0.100% or less, even more preferably, Ti: 0.060% or less, and most preferably, Ti: 0.050% or less. The lower limit is not particularly limited, but is preferably in the range of Ti: 0.005% or more, and more preferably Ti: 0.015% or more.

[0041] B: in the range of 0.0003% to 0.0100%, and B + P ≥ 0.0090%. Note that B and P above represent the components (mass%) in the steel sheet. Furthermore, in order to further enhance the effect of solid-solution C, it is preferable that the total content of grain boundary segregating elements P and B be 0.0090% or more, with B in the range of 0.0003% to 0.0100%. P and B preferentially segregate at grain boundaries, thereby suppressing C segregation and enhancing the dislocation fixing effect of intragranular solid-solution C. More preferably, it is 0.0100% or more. Even more preferably, it is 0.0110% or more, and most preferably, it is 0.0120% or more. Note that there is no particular upper limit, but it is preferable that the total content of P and B be 0.0200% or less, more preferably 0.0150% or less, and even more preferably 0.0130% or less.

[0042] [Cu: 0.005% or more and 0.5% or less, Sn: 0.005% or more and 0.5% or less, and Sb: 0.001% or more and 0.08% or less, with at least two or more of them in total: 0.01% or more and 1.0% or less] Cu, Sn, and Sb are known as surface segregation elements. By adding a certain amount, decarburization of the surface layer during the annealing process can be suppressed, and thus, it is possible to suppress the non-uniformity of the structure in the plate thickness direction and the reduction in the strength of the base material due to softening of the surface layer of the plate thickness. In particular, in the present invention, since the heating rate during final annealing is small and the maximum temperature reached is high, decarburization of the steel tends to progress during annealing. Therefore, a high softening suppression effect is exerted by adding these elements. Therefore, for the purpose of further improving the strength characteristics, when containing Cu and / or Sn, and / or Sb, Cu: 0.005% or more and 0.5% or less, Sn: 0.005% or more and 0.5% or less, and Sb: 0.001% or more and 0.08% or less, at least two or more of them shall be contained in total at 0.01% or more. More preferably, it is 0.02% or more, still more preferably 0.08% or more, and most preferably 0.09% or more. On the other hand, for the upper limit, when containing Cu and / or Sn, and / or Sb, at least two or more of Cu, Sn, and Sb shall be contained in total at 1.0% or less, preferably 0.4% or less, and more preferably 0.35% or less.

[0043] Regarding Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Ca, Mg, REM, Zr, Te, Hf, Bi, Cu, Ni, and Sb described above, since the effects of the present invention are not impaired when the content of each is less than the preferable lower limit value, they shall be included as inevitable impurities.

[0044] Next, the microstructure will be described. The location defining the structure shall be at the 1 / 4 position in the width direction and the 1 / 4 position in the plate thickness direction. This is because generally, the 1 / 4 position in the width direction and the 1 / 4 position in the plate thickness direction are considered to have the average characteristics and composition of the steel plate.

[0045] [Ferrite + Bainite: Area ratio of 0.1% to 15%] Although ferrite is a soft structure and is effective in improving workability, in the steel sheet targeted by this invention, it has a strong effect on reducing the strength of the steel sheet. Here, ferrite may be polygonal ferrite, pseudopolygonal ferrite, or granular bainitic ferrite. Bainite, like martensite, is an aggregate of lath-like crystal grains, but it is softer than martensite. Furthermore, if the proportion of bainite is high, the amount of retained austenite decreases, and as a result, the carbon concentration in the retained austenite increases excessively, making it impossible to obtain the desired yield ratio or the desired increase in yield stress (YS) after processing heat treatment. Here, bainite includes upper bainite and lower bainite. For the purpose of achieving high strength, the total amount of ferrite and bainite should be 15% or less. Under more preferable conditions, it should be 10% or less. Furthermore, even more preferable conditions are that the ferrite content is less than 2% and the total of ferrite and bainite is 10% or less. The most preferable conditions are that the total of ferrite and bainite is 8% or less. On the other hand, in the present invention, since it is necessary to promote the formation of austenite with different carbon concentration levels by causing an austenite (γ) → ferrite (α) transformation during the tempering process in the final annealing, the total amount of ferrite and bainite is set to be 0.1% or more. The total amount of ferrite and bainite is preferably 1% or more, more preferably 2% or more, even more preferably 3% or more, and most preferably 4% or more.

[0046] The area ratio of ferrite is determined by the following method. First, a cross-section of the steel sheet parallel to the rolling direction (L-section) is polished, then etched with 3 vol. % nital, and 10 fields of view are observed at a magnification of 2000x using a scanning electron microscope (SEM) at a position corresponding to 1 / 4 of the sheet thickness (a position corresponding to 1 / 4 of the sheet thickness in the depth direction from the surface of the steel sheet). Note that the surface of the steel sheet in the case of zinc plating refers to the interface between the zinc plating layer and the steel sheet. Next, using the obtained microstructure images, the area ratio of ferrite for 10 fields of view is calculated using Image-Pro from Media Cybernetics. The average of these 10 field of view area ratios is taken as the area ratio of ferrite. Note that in the above microstructure images, ferrite exhibits a gray structure (underlying structure). Furthermore, the measurement method for the area ratio of bainite and martensite, which will be described later, is as follows: A sample of annealed steel sheet is cut out, and a cross-section parallel to the rolling direction is polished, then 3 vol. The plate is corroded with % nital, and 10 fields of view are captured using a scanning electron microscope (SEM) at 2000x magnification 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 performed to confirm the carbides in detail. As described above, the area ratio of the microstructure is determined using Image-Pro from Media Cybernetics using the obtained image data. The gray or dark gray areas containing aligned carbides are bainite. (Aligned carbides refer to carbides whose major axes are in the same direction, and the angle difference between them is within 10°.) Fresh martensite is distinguished as a white or light gray area, tempered martensite as a gray or dark gray area containing misaligned carbides, and perlite as a layered structure of black and white. 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.

[0047] [Retained Austenite: 5% or more and 20% or less in area ratio] Retained austenite contributes to the ductility of the steel sheet by the TRIP effect and is an essential structure in the present invention. In order to obtain sufficient effects, the retained austenite should be 5% or more. The retained austenite is preferably 7% or more, more preferably 8% or more, still more preferably 9% or more, and most preferably 10% or more. Also, if the retained austenite is contained excessively, when it is applied to automotive parts and formed, it may transform into a hard martensite structure, which may impair the processing characteristics. Therefore, the retained austenite should be 20% or less. The retained austenite is preferably less than 16%, more preferably 15% or less, still more preferably 14% or less, and most preferably 13% or less. Note that, as described below, the retained austenite is determined by volume ratio, but since the volume ratio and the area ratio are of the same degree, it is defined as the area ratio here.

[0048] Here, the method for measuring the volume ratio of retained austenite is as follows. After the steel sheet is mechanically ground to 1 / 4 of the sheet thickness in the sheet thickness direction (depth direction), chemical polishing is performed with oxalic acid or the like to remove the strain introduced by mechanical polishing, and the observation surface is obtained. The observation surface is observed by the X-ray diffraction method. As the incident X-ray, a Co Kα ray source is used, and the intensities of the diffraction peaks of the {200}, {220}, and {311} planes of fcc iron (austenite) with respect to the diffraction intensities of the {200}, {211}, and {220} planes of bcc iron are measured, multiplied by the correction coefficient, and the diffraction peaks of martensite and austenite are averaged respectively to eliminate the influence of the preferred direction and calculate the volume ratio of retained austenite.

[0049] [Tempered Martensite: Area Ratio of 70% or More] Tempered martensite is an aggregate of lath-like crystal grains and is characterized by containing iron-based carbides internally. Compared to fresh martensite, it has higher ductility and is also characterized by its ability to easily obtain the high yield strength necessary to ensure collision safety of automotive components. In order to obtain these effects, the area ratio of tempered martensite must be 70% or more. The area ratio of tempered martensite is preferably 75% or more, more preferably 78% or more, even more preferably 80% or more, and most preferably 81% or more. There is no particular upper limit to the area ratio of tempered martensite, but in order to ensure the area ratio of retained austenite, the area ratio of tempered martensite is preferably 95% or less. The area ratio of tempered martensite is more preferably 90% or less, and even more preferably 87% or less. The area ratio of tempered martensite is determined by the method described above.

[0050] [Fresh martensite: 0% to 16% by area percentage] Fresh martensite is martensite that does not contain iron-based carbides. Steel sheets containing fresh martensite have high strength but poor processing characteristics, so the fraction of fresh martensite in the steel sheet according to this embodiment is limited to 16% or less by area percentage. The fraction of fresh martensite is preferably 10% or less by area percentage, more preferably 9% or less, even more preferably 7% or less, and most preferably 5% 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 area percentage of fresh martensite is 0%.

[0051] The method for measuring the microstructure fraction of fresh martensite is as follows: A sample of annealed steel sheet is cut out, the sheet thickness cross section parallel to the rolling direction is polished, and then it is etched with 3 vol. % nital. Ten fields of view are taken at a position 1 / 4 of the sheet thickness (corresponding to 1 / 4 of the sheet thickness in the depth direction from the steel sheet surface) using a scanning electron microscope (SEM) at 2000x magnification. For the purpose of confirming carbides in detail, observation at an additional higher magnification (e.g., 5000x) may be performed. Using the obtained image data, the area ratio is determined using Image-Pro from Media Cybernetics, and the average area ratio of the fields of view determined by the method below is taken as the area ratio of fresh martensite. In the image data, fresh martensite is distinguished as a white or light gray region. Furthermore, since fresh martensite is difficult to distinguish from retained austenite, which similarly appears as a white or light gray region, the tissue fraction is obtained by first determining the area percentage of the white or light gray region and then subtracting the fraction of retained austenite obtained by the above method.

[0052] [Carbon concentration in retained austenite: 0.45% to 0.95%] The quantitative evaluation of the carbon concentration Cγ in retained austenite was performed by cutting a test piece from the 1 / 4 position in the width direction, mechanically polishing it to a surface 0.1 mm from the 1 / 4 position in the plate thickness, and then chemically polishing it to a further 0.1 mm to obtain the surface at the 1 / 4 position in the plate thickness, and then performing 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 equation (2). Here, Mnγ, Alγ, and Siγ are the respective elemental concentrations (mass%) in retained γ. However, in this report, since the distribution of alloy elements before cooling is uniform due to annealing in the γ phase single-phase region, and the distribution of substitutional elements during annealing cooling can be considered negligible, the amount of additive (steel plate composition) was used for Mnγ, Alγ, and Siγ. aγ = 3.5720 + 0.033Cγ + 0.0012Mnγ - 0.00157Siγ + 0.0056Alγ ... Equation (2) The carbon concentration in retained austenite shall be 0.45% or more. 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 more, more preferably 0.65% or more, and even more preferably 0.70% or more. 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 and increases the yield stress and yield ratio, so the carbon concentration in retained austenite shall be 0.95% or less. The carbon concentration in the retained austenite is preferably 0.90% or less, more preferably 0.88% or less, and even more preferably 0.85% or less. [Reference 1] Y. Toji, et al: Acta Materoalia 65 (2014) 215-228)

[0053] Furthermore, in the steel structure of the present invention, in addition to the ferrite, bainite, tempered martensite, fresh martensite, and retained austenite mentioned above, the effects of the present invention will not be impaired even if carbides such as pearlite and cementite, or other structures known as steel sheet structures, are included as long as their total area percentage is within 10% or less. It is more preferable that the total area percentage of the aforementioned structures be 8% or less, and even more preferable that it be 7% or less. There is no particular lower limit, 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.

[0054] [Yield Ratio YS / TS ≤ 0.80] The steel sheet targeted by this invention has a low yield ratio of 0.80 or less. The yield ratio is more preferably 0.75 or less, even more preferably 0.68 or less, and most preferably 0.66 or less. The lower limit is not particularly limited, but the yield ratio is preferably 0.50 or more, more preferably 0.53 or more, and even more preferably 0.56 or more. Here, YS is the 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.

[0055] Below, we will specifically explain the work hardening characteristics and heat treatment hardening characteristics, which are among the challenges.

[0056] [In a tensile test conforming to JIS Z 2241, retained austenite that disappears between the start of the tensile test and processing that imparts a 1% strain at nominal value: 0.8% or more in terms of area fraction of the entire steel structure] The excellent work hardening characteristics of the present invention are thought to be due to stress-induced transformation of unstable retained austenite during processing. That is, under low stress conditions, unstable fcc crystals transform into bcc crystals, and deformation progresses with volume expansion, resulting in stress-strain characteristics with a high yield ratio. Furthermore, the solid solution carbon released when retained austenite transforms has the effect of increasing the yield stress by adhering to dislocations introduced during processing during subsequent heat treatment. Therefore, the higher the fraction of unstable retained austenite that transforms in the early stages of deformation, the more advantageous it is for increasing the amount of work hardening and the amount of hardening after heat treatment. Thus, the amount of retained austenite that disappears between the start of the tensile test and processing that imparts a 1% strain at nominal value is set to 0.8% or more. The amount of retained austenite that disappears between the start of the tensile test and the processing that imparts a 1% strain at the nominal value is preferably 0.9% or more, more preferably 1.0% or more, even more preferably 1.5% or more, and most preferably 1.7% or more. There is no particular upper limit, but since it is necessary to ensure that 3% or more of austenite remains even after uniform elongation, it is preferable that the amount of retained austenite that disappears between the start of the tensile test and the processing that imparts a 1% strain at the nominal value be 5.0% or less, more preferably 4.0% or less, and even more preferably 3.0% or less. It should be noted that this constituent requirement and the above requirement of "carbon concentration in retained austenite ≥ 0.60%" were difficult to reconcile. This is because increasing the carbon concentration to stabilize the retained austenite makes it difficult for the retained austenite to undergo stress-induced transformation during processing that imparts a 1% strain. In this invention, both requirements are reconciled, and the method by which this was achieved will be described later. The amount of retained austenite that disappears during the above processing was evaluated by the difference in the amount of retained austenite before and after the tensile test. The amount of retained austenite after the tensile test was evaluated using a sample cut from the central portion of the tensile test specimen after a predetermined strain had been introduced and then unloaded. The method for measuring the amount of retained austenite after the tensile test is as follows:A 1.5 cm square steel plate was cut from the center of a JIS No. 5 test specimen after a tensile test. The plate was 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 was performed on this observation surface, and the retained austenite was measured using the method described above.

[0057] [Retained austenite present after processing from the start of the tensile test to uniform elongation in the tensile test: 3% or more in terms of area ratio to the entire steel structure] Retained austenite is a slightly softer structure compared to martensite and is less likely to cause crack initiation during deformation. Therefore, retaining more of it until the later stages of deformation after uniform elongation is advantageous in terms of improving the impact characteristics of the part. Accordingly, retained austenite should be 3% or more after processing to uniform elongation. More preferably, retained austenite should be 4% or more after processing to uniform elongation. There is no particular upper limit, but in order to improve ductility from the middle to the later stages of deformation, it is preferable that retained austenite present from the start of the tensile test to uniform elongation be 5% or less.

[0058] [Processing and Heat Treatment to Infuse 1% Strain] Work hardening and heat treatment hardening characteristics were evaluated by tensile testing using JIS No. 5 tensile test specimens in the C direction (gauge length 50 mm, parallel section width 25 mm). The C direction refers to the direction perpendicular to the rolling direction. First, six JIS No. 5 tensile test specimens in the C direction were prepared, cut from the same width direction position of the steel plate. Three of these specimens were subjected to tensile testing until fracture, according to JIS Z2241, and the yield stress, uniform elongation, total elongation, and tensile strength were evaluated by averaging the test results of the three specimens. The yield stress and total elongation here are considered to be the steel plate characteristics, i.e., the yield stress and total elongation before (processing and heat treatment to infuse 1% strain). For the remaining three specimens, tensile processing was performed to a nominal strain of 1%, then the tensile load was removed, and after heat treatment at 170°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, and the thickness and width of the initial parallel section of the specimen, necessary for stress calculation, are measured again. The reason for setting the processing amount to 1% is that if it 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. The heat treatment described above is, for example, a heat treatment equivalent to the paint-baking process as described in JIS G 3135, Annex A.

[0059] [Increase in YS after (processing and heat treatment that imparts 1% strain) ≥ 250 MPa] The objective of this invention is to ensure that the yield stress after (processing and heat treatment that imparts 1% strain) is 250 MPa or more higher than the yield stress before (processing and heat treatment that imparts 1% strain). This makes it possible for the material to be soft when formed, but to have high yield stress and, consequently, high impact characteristics when used as a part. Although there are differences depending on the chemical composition of the steel and the heat treatment conditions, work hardening of at least 70 MPa occurs with processing that imparts 1% strain, and a stress increase of 180 MPa or more is observed due to the effect of subsequent heat treatment. YS is the yield stress. As described later, under certain conditions, an extremely high increase in yield stress exceeding 300 MPa is observed. There is no particular upper limit to the amount of YS increase, but it is preferable that it be 1000 MPa or less.

[0060] [El after (processing and heat treatment to impart 1% strain): (0.9 × El before (processing and heat treatment)) or more] Another feature of the present invention is that the total elongation before and after (processing and heat treatment to impart 1% strain) does not change significantly, and El after (1% processing and heat treatment) is (0.9 × El before (processing and heat treatment)) or more. Note that El is the total elongation. Normally, after processing and heat treatment, the ductility decreases as the material hardens, but in the steel of the present invention, no significant decrease in total elongation is observed, and as will be described later, depending on the conditions, the total elongation may even increase after treatment. Furthermore, it is preferable that El after (1% processing and heat treatment) is (1.0 × El before (processing and heat treatment)) or more. Furthermore, it is more preferable that El after (1% processing and heat treatment) is (1.1 × El before (processing and heat treatment)) or greater, and even more preferable that El after (1% processing and heat treatment) is (1.2 × El before (processing and heat treatment)) or greater. There is no particular upper limit, but it is preferable that El after (1% processing and heat treatment) is (1.5 × El before (processing and heat treatment)) or less, and more preferable that El after (1% processing and heat treatment) is (1.4 × El before (processing and heat treatment)) or less.

[0061] [The difference between the stress increase after 170°C heat treatment without pre-straining and the stress increase after 170°C heat treatment after processing (tensile processing) that imparts 1% strain is 120 MPa or more.] The steel sheet shown in the present invention is a suitable material for cold-pressed materials, which are based on the premise of introducing processing strain. A major feature is that the stress increase in the processed portion is significantly greater than the stress increase in the unprocessed portion. Therefore, as stated in the object of the present invention, the difference between the stress increase after 170°C heat treatment without pre-straining and the stress increase after 170°C heat treatment after 1% processing (tensile processing) is 120 MPa or more. Preferably, the difference between the stress increase after 170°C heat treatment without pre-straining and the stress increase after 170°C heat treatment after 1% processing (tensile processing) is 150 MPa or more, and more preferably 170 MPa or more. While there is no particular upper limit, it is preferable that the difference between the stress increase after 170°C heat treatment without pre-strain and the stress increase after 170°C heat treatment after 1% processing (tensile processing) be 300 MPa or less. Note that "after 170°C heat treatment without pre-strain" refers to the state in which 170°C heat treatment is performed without applying strain (processing). Note that the stress increase as used herein refers to the difference (σa - σb) between the yield stress 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 by performing a tensile test again after heat treatment at a specified temperature for 20 minutes on a test specimen obtained by removing the tensile test load from the tensile test specimen from which the σb was obtained (or the test specimen as it was cut from the original plate if there is no pre-strain), and repeating the tensile test.

[0062] Next, the method for manufacturing steel sheets according to the present invention will be described. A steel material having the above-mentioned component composition is melted down in a conventional refining process, and then formed into a steel slab by a conventional ingot-blowing or continuous casting method. Alternatively, a thin steel slab with a thickness of 100 mm or less may be manufactured by a direct casting method. After heating and holding the steel slab, it is subjected to hot rolling, and a hot-rolled sheet is manufactured by rough rolling and finish rolling and wound into a coil. The thickness of the hot-rolled sheet is preferably 0.8 mm or more. More preferably 1.0 mm or more, even more preferably 1.4 mm or more, and most preferably 1.6 mm or more. Also, the thickness is preferably 4.0 mm or less. More preferably 3.5 mm or less, even more preferably 3.2 mm or less, and most preferably 3.0 mm or less.

[0063] The finishing rolling temperature for the hot rolling process is 800°C or higher and 1000°C or lower. Here, the finishing rolling temperature (finishing hot rolling temperature) is 800°C or higher and 1000°C or lower. To achieve this, it is preferable that the slab heating temperature be 1080°C or higher. It is more preferable that the slab heating temperature be 1120°C or higher, even more preferable that it be 1140°C or higher, and most preferable that it be 1160°C or higher. It is also preferable that the slab heating temperature be 1300°C or lower. It is more preferable that the slab heating temperature be 1260°C or lower, even more preferable that it be 1250°C or lower, and most preferable that it be 1240°C or lower. Hot rolling needs to be completed in the austenite single-phase region in order to improve elongation and hole-expanding properties after annealing by homogenizing the structure within the steel sheet and reducing material anisotropy, so the finishing rolling temperature is 800°C or higher. The finish rolling completion temperature is preferably 850°C or higher, more preferably 880°C or higher, even more preferably 890°C or higher, and most preferably 900°C or higher. On the other hand, if the finish rolling completion temperature exceeds 1000°C, the hot-rolled structure becomes coarse, and the properties after annealing deteriorate. Therefore, the finish rolling completion temperature should be 1000°C or lower. A preferred rolling completion temperature is 950°C or lower, more preferably 940°C or lower, and even more preferably 930°C or lower.

[0064] The winding temperature is between Ms and Bs. After the finish rolling is completed and the sheet is cooled, the sheet is wound at a temperature between Ms and Bs to complete the hot rolling process. If the winding temperature is below Ms, the hot-rolled sheet becomes excessively hard, making winding and subsequent cold rolling difficult, and also suppressing scale formation on the surface of the steel sheet, which is useful for the present invention. Therefore, the winding temperature is set to be above Ms. It is more preferable that the winding temperature be Ms + 10°C or higher, even more preferable that it be Ms + 20°C or higher, and most preferable that it be Ms + 25°C or higher. Here, Ms (°C) is calculated by the following formula. The values ​​in brackets are the mass percentages of each component. Ms point (°C) = 561 - 474 [C] - 33 [Mn] - 17 [Ni] - 17 [Cr] - 21 [Mo] Bs point (°C) = 830 - 270 [C] - 90 [Mn] - 37 [Ni] - 70 [Cr] - 83 [Mo] On the other hand, if the winding temperature is excessively high, the ferrite phase fraction increases and the structure becomes non-uniform, which increases gauge fluctuations during cold rolling and variations in the quality of the final product sheet. Therefore, the upper limit of the winding temperature is set to the Bs point. The winding temperature is preferably Bs point - 10°C or lower, more preferably Bs point - 20°C or lower, even more preferably Bs point - 25°C or lower, and most preferably Bs point - 30°C or lower.

[0065] 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. There is no particular lower limit, but it is most preferable that both be 0°C.

[0066] 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.

[0067] Cold rolling pre-annealing is performed by holding the material in a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more. Alternatively, after the completion of finish rolling in hot rolling and before the start of cold rolling, cold rolling pre-annealing may be performed in a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more to reduce the cold rolling load. If the annealing temperature exceeds the Ac3 point, a hard structure will be formed from the austenite phase during cooling. Therefore, when cold rolling pre-annealing is performed, the cold rolling pre-annealing temperature should be below the Ac3 point. Preferably, it should be below the Ac1 point. Furthermore, if the temperature is below 400°C, the material will not soften sufficiently, and the effect of reducing the cold rolling load will be small. Therefore, when cold rolling pre-annealing is performed, the cold rolling pre-annealing temperature should be 400°C or higher. Preferably, the cold rolling pre-annealing temperature is 450°C or higher. Note that the Ac1 point and Ac3 point shown in this specification can be calculated from the following equations (A) and (B) described in Reference 2. In the formula, the values ​​in brackets [ ] indicate the content (mass%) of each element. Elements not present in the steel plate, or elements whose content is below the detection limit for analysis, should be calculated as 0 mass%. Ac1 point (°C) = 723 - 10.7 × [Mn] - 16.9 × [Ni] + 29.1 × [Si] + 16.9 × [Cr] + 290 × [As] + 6.38 × [W] ... (A) Ac3 point (°C) = 910 - 203 × √ [C] - 15.2 × [Ni] + 44.7 × [Si] + 104 × [V] + 31.5 × [Mo] + 13.1 × [W] - 30 × [Mn] - 11 × [Cr] - 20 × [Cu] + 700 × [P] + 400 × [Al] + 120 × [As] + 400 × [Ti] ... (B) [Reference 2] "Leslie's Science of Iron and Steel" (Maruzen Co., Ltd., translated under the supervision of Naruyasu Koda, published May 31, 1985, p. 273)

[0068] Next, the hot-rolled sheet after the hot-rolling process described above, or the steel sheet after soft annealing (annealing before cold rolling), is pickled and then cold-rolled at a reduction ratio of 30% or more to obtain a cold-rolled sheet of the desired thickness. Cold rolling may 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 and an increase in yield stress, resulting in a higher yield ratio. For this reason, the reduction ratio for cold rolling should be 30% or more. Furthermore, since high local ductility is easily obtained when the reduction ratio is high, it is more preferable to have a reduction ratio of 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, it is preferable to have a reduction ratio of 95% or less, more preferably 85% or less, and even more preferably 75% or less.

[0069] Annealing Process The average heating rate in the temperature range from 650°C to 800°C is 2°C / s or less. Then, as an annealing process, the obtained steel sheet is heat-treated and, if necessary, plated. In the heating process performed at the beginning of the heat treatment, the average heating rate in the temperature range from 650°C to 800°C is set to 2°C / s or less. In this temperature range, competition occurs between ferrite recrystallization and ferrite-austenite transformation from the unrecrystallized structure after cold rolling. Under conditions of a low average heating rate, ferrite recrystallization occurs before the austenite transformation, but it has been found that under conditions of a high average heating rate, the austenite transformation occurs from unrecrystallized ferrite without ferrite recrystallization. Furthermore, austenite produced from recrystallized ferrite is more easily formed in the pre-heat treatment stage than austenite produced from unrecrystallized ferrite, and carbon tends to become excessively concentrated during the heat treatment. In the present invention, after the recrystallization of ferrite is completed, a transformation from ferrite to austenite occurs, thereby forming retained austenite containing an appropriate amount of carbon. Therefore, a low average heating rate is preferable. The average heating rate in the temperature range from 650°C to 800°C is preferably 1.8°C / s or less, more preferably 1.5°C / s or less, and even more preferably 1.3°C / s or less. The lower limit is not particularly limited, but it is preferably 0.5°C / s or more, more preferably 0.6°C / s or more, and even more preferably 0.7°C / s or more. The above average heating rate can be determined by dividing the temperature difference between 650°C and 800°C (150°C) by the time required to raise the temperature from 650°C to 800°C.

[0070] The average heating rate in the temperature range from 800°C to the maximum annealing temperature Ts is 1°C / s or less. Subsequently, the steel sheet is heated to Ts, which is the maximum annealing temperature in this annealing process. Here, mainly an austenite transformation occurs from ferrite, but since the ferrite has already recrystallized and become relatively coarse grains, the driving force for the austenite transformation is somewhat low. In order to completely recrystallize the unrecrystallized ferrite and reduce residual ferrite, the heating rate in the temperature range from 800°C to the maximum annealing temperature Ts is set to 1°C / s or less. More preferably, it is set to 0.8°C / s or less, even more preferably to 0.7°C / s or less, and most preferably to 0.6°C / s or less. The lower limit is not particularly limited, but it is preferably 0.01°C / s or more, more preferably 0.05°C / s or more, and even more preferably 0.1°C / s or more. Near Ts, the austenite transformation is completed in order to minimize the ferrite fraction in the subsequent cooling process and achieve the highest possible strength. The average heating rate mentioned above can be determined by dividing the temperature difference between 800°C and the maximum annealing temperature Ts by the time required to raise the temperature during this period. In order to complete the austenite transformation with a large amount of recrystallized ferrite and low driving force, Ts must be at least Ac3 + 30°C. Preferably, it is at least Ac3 + 40°C, and more preferably at least Ac3 + 50°C. On the other hand, if the annealing temperature is higher than 1050°C, the microstructure becomes excessively large and the toughness deteriorates, so it is preferably 1050°C or lower, more preferably 1020°C or lower, even more preferably 1000°C or lower, and most preferably 980°C or lower.

[0071] In the cooling process that continues to cool up to temperature T1, the average cooling rate in the temperature range from Ts to 400°C is set to 10°C / s or more and 100°C / s or less, and the average cooling rate in the temperature range T1 from 400°C to 50°C to 280°C is set to 5°C / s or more and 20°C / s or less. In the cooling process that continues to cool up to temperature T1, the average cooling rate from Ts to 400°C is set to 10°C / s or more and 100°C / s or less. A higher average cooling rate is preferable for forming a hard structure, and is set to 10°C / s or more. Preferably, the average cooling rate is 12°C / s or more, more preferably 15°C / s or more, even more preferably 20°C / s or more, and most preferably 25°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 the average cooling rate is set to 100°C / s or less. The average cooling rate is preferably 80°C / s or less, more preferably 70°C / s or less, even more preferably 60°C / s or less, and most preferably 50°C / s or less. The average cooling rate is determined by dividing the temperature difference (cooling start temperature - cooling end temperature), where the cooling start temperature is Ts and the cooling end temperature is 400°C, by the cooling time required during this period.

[0072] Next, the material is cooled from 400°C to a temperature T1 between 50°C and 280°C at an average cooling rate of 5°C / s to 20°C / s. The reason for setting the temperature T1 between 50°C and 280°C is as follows: Cooling to 280°C or below causes some of the austenite produced by annealing to transform into martensite. However, if the temperature falls below 50°C, much of the austenite necessary for improving ductility will transform into hard structures such as martensite and disappear, so the lower limit is 50°C. For this reason, the temperature T1 is set to 50°C or higher. Preferably, the temperature T1 is 75°C or higher, more preferably 100°C or higher, even more preferably 110°C or higher, and most preferably 120°C or higher. Furthermore, if the temperature exceeds 280°C, the martensite transformation will not occur sufficiently, not only reducing the fraction of tempered martensite structure, but also causing the remaining austenite to transform into martensite after final cooling, increasing the fresh martensite fraction. Therefore, the temperature T1 should be 280°C or lower. Preferably, it should be 250°C or lower, more preferably 240°C or lower, even more preferably 230°C or lower, and most preferably 220°C or lower. The average cooling rate should be 5°C / s or higher, because if it is lower than 5°C / s, excessive ferrite and bainite will form. Preferably, the average cooling rate should be 7°C / s or higher. More preferably, the average cooling rate should be 10°C / s or higher, even more preferably 11°C / s or higher, and most preferably 12°C / s or higher. On the other hand, if it exceeds 20°C / s, it becomes difficult to stop cooling at the target temperature (T1), so the average cooling rate during this period should be 20°C / s or lower. Preferably, it should be 18°C / s or lower, more preferably 17°C / s or lower, even more preferably 16°C / s or lower, and most preferably 15°C / s or lower. The average cooling rate mentioned above 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. The cooling means of this invention can be any known method, such as gas cooling, oil cooling, or mist cooling.

[0073] The temperature is raised from T1 to a temperature T2 of 280°C to 380°C at an average heating rate of 5°C / s or more, and then held in the temperature range of T2-10°C to less than T2 for 1 second to 60 seconds. Subsequently, the temperature is raised from T1 to a temperature T2 of 280°C to 380°C at an average heating rate of 5°C / s or more, and held in the temperature range of T2-10°C to less than T2 for 1 second (s) to 60 seconds. This process is particularly important for forming the desired retained austenite structure and is called the tempering process in this invention. In the tempering process, high-temperature annealing transforms a very small amount of austenite into the bcc phase, thereby increasing the carbon concentration in the austenite near the transformed bcc (ferrite or bainite) phase, while preventing excessive carbon enrichment in the rest of the austenite. Rapid heating and short processing time are necessary for this purpose. For this reason, the average heating rate is set to 5°C / s or more, and no upper limit is specifically set. The average heating rate is preferably 7°C / s or higher, more preferably 10°C / s or higher, even more preferably 15°C / s or higher, and most preferably 17°C / s or higher. There is no particular upper limit, but it may be 100°C / s or lower. The above average heating rate can be determined by dividing the temperature difference between T1 and T2 by the time required to raise the temperature from T1 to T2. To promote the transformation from austenite to bainite, the temperature range of T2 should be 280°C or higher. The temperature range of T2 is preferably 300°C or higher, more preferably 320°C or higher, and even more preferably 330°C or higher. On the other hand, if it is too high, the transformation to bainite, which is one form of the bcc phase, will not occur easily, and it will not be possible to produce austenite with a low carbon concentration, so it should be 380°C or lower. The temperature range of T2 is preferably 370°C or lower, more preferably 360°C or lower, and even more preferably 350°C or lower. Furthermore, if the holding time in the temperature range from T2-10°C to below T2 exceeds 60 s, the carbon concentration in the retained austenite becomes too high, resulting in an excessively high yield ratio. Therefore, the holding time should be 60 s or less. A preferred holding time in the temperature range from T2-10°C to below T2 is 45 s or less, and more preferably 30 s or less.To obtain even more favorable properties, the holding time in the temperature range of T2-10°C to less than T2 is more preferably less than 10 seconds, and most preferably 9 seconds or less. Also, for reasons such as promoting the austenite-to-bainite transformation, the holding time is 1 second or more. The holding time is preferably 2 seconds or more, more preferably 4 seconds or more, and even more preferably 5 seconds or more. The reason for holding in the temperature range of T2-10°C to less than T2 is that carbon diffusion is relatively significant in this temperature range, and it is a temperature range that has a large impact on the material.

[0074] Subsequently, final cooling is performed. However, if the temperature range is below T2-10°C, where carbon distribution in austenite is relatively slow, and above T1, where austenite transformation is slow, then it is acceptable to hold the temperature in the range of T2-10°C to below T2, and before final cooling. In the final cooling step, in order to maintain the solid solution state of carbon, cooling is performed in the temperature range of T1 to T1-10°C or lower, at an average cooling rate of 2°C / s or higher. It is believed that rapid cooling in the above temperature range prevents sufficient diffusion of solid solution carbon generated by transformation during cooling, thus reducing the amount of carbon in the retained austenite and maintaining unstable retained austenite. The average cooling rate is preferably 5°C / s or higher, more preferably 10°C / s or higher. Cooling is preferably set to 25-85°C, with the pickling temperature (25-85°C) as the lower limit, taking into consideration the subsequent pickling properties. That is, the above cooling end temperature (cooling stop temperature) is set to or above the pickling temperature. The average cooling rate mentioned above can be calculated by dividing the temperature difference between T1 and the cooling end temperature (cooling stop temperature) by the time required to cool from T1 to the cooling end temperature (cooling stop temperature). Note that the final cooling can be performed in two stages, for example, a gas-cooled section and a water-cooled section, but in that case, the cooling rate of each stage must satisfy the above-mentioned cooling rate.

[0075] After cooling, the resulting annealed sheet is pickled a second time and then subjected to skin pass rolling. Skin pass rolling must be performed at a sheet temperature (steel sheet temperature) of 20°C or higher and an elongation of 0.1% or less (including 0%). If the sheet 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. More preferable conditions are a sheet 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 sheet 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 to correct the shape is preferably not performed from the viewpoint of retaining residual austenite.

[0076] Alternatively, the steel sheet obtained after the annealing process described above may be electro-galvanized to obtain an electro-galvanized steel sheet.

[0077] Furthermore, in the annealing process, hot-dip galvanizing may be applied to produce a hot-dip galvanized steel sheet. T1 is as described above. In this case, T2 can be read as the plating temperature Tm, but the range of Tm is not limited to the manufacturing conditions of the cold-rolled steel sheet described above, i.e., 280°C to 380°C, but rather to the normal plating temperature of 450°C to 500°C. That is, the range of Tm is 450°C or higher, preferably 455°C or higher, and more preferably 460°C or higher. Also, the range of Tm is 500°C or lower, preferably 495°C or lower, and more preferably 490°C or lower. The reason for setting the average heating rate in the temperature range from temperature T1 to temperature Tm to 5°C / s or higher is to obtain the desired total area ratio of ferrite and bainite, and the average heating rate is preferably 7°C / s or higher, more preferably 10°C / s or higher, and even more preferably 15°C / s or higher. There is no particular upper limit, but it may be 60°C / s or lower. The holding time in the temperature range of Tm -10°C or higher and below Tm is 1 s or more in order to obtain the desired area ratio of retained austenite and the desired carbon concentration in the retained austenite. Preferably, the holding time is 3 s or more, more preferably 5 s or more, and even more preferably 10 s or more. Also, the holding time is 60 s or less. Preferably, the holding time is 50 s or less, more preferably 45 s or less, and even more preferably 40 s or less. The average heating rate described above can be determined by dividing the temperature difference between T1 and Tm by the time required to heat from T1 to Tm. Furthermore, for the final cooling, T2 is read as the plating temperature Tm, and the process is as described above.

[0078] Furthermore, manufacturing conditions other than those described above can be carried out by conventional methods. The technology described herein makes it possible to manufacture steel sheets for cold press forming with excellent collision resistance.

[0079] 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.

[0080] 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.

[0081] The steel material having the chemical 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 these slabs were then heat-treated in a nitrogen atmosphere and cooled in the air or furnace-cooled in a nitrogen atmosphere. After pickling, cold rolling was performed. Further heat treatment was carried out in a nitrogen atmosphere. Some of the slabs were then hot-dip galvanized as a post-heat treatment step. From the obtained steel sheets, JIS No. 5 test specimens (gauge length 50 mm, parallel section width 25 mm) were cut perpendicular to the rolling direction, and tensile tests were performed according to JIS Z2241. The microstructure, work hardening characteristics, and heat treatment characteristics of the steel sheets were measured based on the methods described above. Furthermore, using the obtained steel plate, two hat-shaped members with an M-shaped cross-section (hat members) were fabricated. One was subjected to a three-point bending test without heat treatment, and the other was subjected to a three-point bending test after heat treatment at 170°C for 20 minutes, and the maximum load (unit: kN) was determined. Here, the thickness of the steel plate was 1.4 mm, and the punch stroke speed was 0.1 m / s. The maximum stroke length was 50 mm, and a deformation load-punch displacement curve was obtained. Other detailed test conditions followed the following reference 3. [Reference 3] Kentaro Sato, Takayuki Futatsuka, Tomohiro Sakaitani, Shinpei Yoshioka & Yoshikiyo Tamai. (2022), Influence of material properties of ultra-high-strength steel plate on the impact performance of hat parts. Transactions of the Society of Automotive Engineers of Japan, 53(3), 675-680. For the above maximum load, if the value obtained after heat treatment at 170°C for 20 minutes increased by 2.0% or more compared to the value obtained without heat treatment at 170°C for 20 minutes, it was determined that good collision resistance characteristics could be obtained by baked coating hardening. Tables 3-1 and 3-2 show the characteristics obtained above.

[0082]

[0083]

[0084]

[0085]

[0086]

Claims

1. The composition is such that, by mass%, C: 0.050% to 0.400%, Si: 0.01% to 2.50%, Mn: 0.10% to 5.00%, P: 0.1000% or less, S: 0.0200% or less, Al: 1.00% or less, Cr: 0.05% or less, N: 0.0200% or less, and O: 0.0100% or less, with the remainder being Fe and unavoidable impurities, and the steel structure is such that the total area percentage of ferrite and bainite is 0.1% to 15%, the area percentage of retained austenite is 5% to 20%, the area percentage of tempered martensite is 70% or more, and the area percentage of fresh martensite is 0% to 16%. A steel sheet for cold press forming that satisfies the requirement that the carbon concentration in the retained austenite is 0.45% or more and 0.95% or less, and in a tensile test in accordance with JIS Z2241, the retained austenite that disappears between the start of the tensile test and processing that imparts a strain of 1% nominal strain accounts for 0.8% or more of the area of ​​the entire steel structure, and after processing from the start of the tensile test to uniform elongation in the tensile test, retained austenite accounts for 3% or more of the area of ​​the entire steel structure.

2. The above component composition is further defined in mass percent as follows: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Mo: 1.00% or less, Co: 1.000% or less, Sn: 0.5% or less, Cu: 0.5% 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, Sb: 0.08% or less. A steel sheet for cold press forming according to claim 1, comprising at least one selected from among the following.

3. The steel sheet for cold press forming according to claim 2, wherein the component composition further contains Ti in mass% of 0.200% or less and satisfies Ti / 48 ≥ N / 14, and further contains B in mass% of 0.0003% or more and 0.0100% or less and satisfies B + P ≥ 0.0090%. The above Ti, N, B, and P represent the components (mass%) in the steel sheet.

4. A plated steel sheet for cold press forming, wherein one of the following is formed on the surface of the steel sheet according to any one of claims 1 to 3: an electro-galvanized layer or a hot-dip galvanized layer.

5. A steel slab having the component composition described in any one of claims 1 to 3 is heated, then hot-rolled at a finish rolling completion temperature of 800°C to 1000°C, then wound up at a temperature between Ms and Bs, the obtained hot-rolled sheet is pickled, then cold-rolled with a reduction ratio of 30% or more, the obtained cold-rolled sheet is heated to Ts with an average heating rate of 2°C / s or less in the temperature range from 650°C to 800°C, and an average heating rate of 1°C / s or less in the temperature range from 800°C to the maximum annealing temperature Ts which is above the Ac3 point + 30°C, and then cooled to T1 with an average cooling rate of 10°C / s or more to 100°C / s from Ts to 400°C, and an average cooling rate of 5°C / s or more to 20°C / s in the temperature range from 400°C to 280°C, and then from T1 A method for manufacturing steel sheets for cold press forming, comprising raising the temperature to a temperature T2 with an average heating rate of 5°C / s or more in the temperature range T2 between 280°C and 380°C, holding the temperature in the temperature range between T2-10°C and below T2 for 1 second to 60 seconds, then cooling the sheet to a temperature range of T1-10°C or less with an average cooling rate of 2°C / s or more in the temperature range from T1 to T1-10°C, and then performing skin pass rolling on the resulting annealed sheet after a second pickling, with the sheet temperature being 20°C or higher and the elongation being 0.1% or less (including 0%).

6. A steel slab having the component composition described in any one of claims 1 to 3 is heated, then hot-rolled at a finish rolling completion temperature of 800°C to 1000°C, then wound up at a temperature between Ms and Bs, the obtained hot-rolled sheet is pickled, then cold-rolled with a reduction ratio of 30% or more, and the obtained cold-rolled sheet is heated to Ts, with an average heating rate of 2°C / s or less in the temperature range from 650°C to 800°C, and an average heating rate of 1°C / s or less in the temperature range from 800°C to the maximum annealing temperature Ts which is at or above the Ac3 point + 30°C. Furthermore, the method for manufacturing plated steel sheets for cold press forming involves cooling the sheet to T1 with an average cooling rate of 10°C / s or more and 100°C / s or less in the temperature range from Ts to 400°C, an average cooling rate of 5°C / s or more and 20°C / s or less in the temperature range from 400°C to 280°C, then performing a hot-dip galvanizing treatment, raising the temperature to Tm with an average heating rate of 5°C / s or more in the temperature range from T1 to 450°C to 500°C, holding the sheet in the temperature range of Tm-10°C or more and less than Tm for 1 second or more and 60 seconds or less, then cooling the sheet to T1-10°C or less with an average cooling rate of 2°C / s or more in the temperature range from T1 to T1-10°C or less, and after pickling the obtained annealed sheet, performing skin pass rolling under the conditions of a sheet temperature of 20°C or more and an elongation of 0.1% or less (including 0%).

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

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

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

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

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

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