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

Abstract

The present invention aims to provide technology for cold press forming steel sheets that, after being subjected to cold press forming and formed into parts, possess high impact resistance, and a method for manufacturing the same. A steel sheet for cold press forming having a specific component composition and steel structure, satisfying the requirement that the carbon concentration in the retained austenite is 0.45% or more and 0.95% or less, wherein in a tensile test in accordance with JIS Z 2241, the retained austenite that disappears between the start of the tensile test and processing that imparts a 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, the retained austenite accounts for 3% or more of the area of ​​the entire steel structure.

Description

Technical Field

[0001] The present invention relates to a steel sheet having a tensile strength (tensile strength) TS of 980 MPa or more, a plated steel sheet, methods for producing these, a member, and a method for producing the member, which are suitable for use in structural parts of automobile bodies and electrical appliance parts that are formed by any one or more of drawing, stretch forming, and stretch flange forming by cold pressing.

Background Art

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

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

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

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

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

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

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

[0009] As mentioned above, high-strength steel sheets utilizing the martensitic phase offer excellent impact resistance when applied to automotive parts due to their high yield strength. However, they also have drawbacks, such as cracking during cold pressing and poor shape retention due to significant springback after press forming. Therefore, their application is limited to parts manufactured using processes such as roll forming. While hot 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, methods to avoid this include, for example, using untempered martensite (fresh martensite) or tempered martensite with a small degree of tempering, but this significantly reduces ductility such as elongation flangeability. Furthermore, there is the disadvantage that the material becomes unstable due to the large amount of solid solution carbon remaining in the steel sheet.

[0011] The 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 TS while having 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 (YS after processing and heat treatment) minus the YS (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) that imparts 1% strain in the tensile test is 120 MPa or more. [Means for solving the problem]

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

[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] 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 It contains, The remainder is a component composition consisting of Fe and unavoidable impurities, The steel structure has a total area ratio of ferrite and bainite of 0.1% to 15%. If the area percentage of retained austenite is between 5% and 20%, Tempered martensite accounts for 70% or more by area. Fresh martensite is between 0% and 16% by area. The carbon concentration in the retained austenite satisfies the condition of being 0.45% or more and 0.95% or less. In a tensile test in accordance with JIS Z 2241, the retained austenite that disappears between the start of said tensile test and working that imparts 1% nominal strain has an area ratio of 0.8% or more relative to the entire steel structure, and A steel sheet for cold press forming, wherein retained austenite is present in an area ratio of 3% or more relative to the entire steel structure after working from the start of said tensile test to uniform elongation in said tensile test. [2] The steel sheet for cold press forming according to [1], wherein said component composition further contains, by mass%, 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: 1.00% 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, and contains at least one selected from the above. [3] The steel sheet for cold press forming according to [2], wherein said component composition further contains, by mass%, Ti is contained within a range of 0.200% or less, and in an amount satisfying Ti / 48 ≧ N / 14, and further, B is contained within a range of 0.0003% or more and 0.0100% or less, and is contained in an amount satisfying B + P ≧ 0.0090%. The steel sheet for cold press forming according to [2]. Here, Ti, N, B, and P above represent the components (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] above. [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, and then coiled at a point between Ms and Bs. After pickling the obtained hot-rolled sheet, it is 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. Furthermore, it is cooled to T1, with an average cooling rate of 10°C / s or more and 100°C / s or less in the temperature range from 400°C to 280°C, and 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, from T1... A method for manufacturing steel sheets for cold press forming, comprising raising the temperature to a temperature T2 of 280°C or higher with an average heating rate of 5°C / s or higher in the temperature range T2 to 380°C, holding the temperature in the temperature range of T2-10°C or higher but below T2 for 1 second or more but 60 seconds or less, then cooling the temperature to a temperature range of T1-10°C or lower with an average cooling rate of 2°C / s or higher in the temperature range from T1 to T1-10°C, and then performing skin pass rolling on the obtained annealed sheet after a second pickling, under the conditions of a sheet temperature of 20°C or higher and an elongation of 0.1% or less (including 0%). [6] A steel slab having the component composition described in any of [1] to [3] above is heated, then hot-rolled at a finish rolling completion temperature of 800°C to 1000°C, then coiled at a temperature between the Ms point and the Bs point, the resulting hot-rolled sheet is pickled, then cold-rolled with a reduction ratio of 30% or more, and the resulting cold-rolled sheet is The average heating rate in the temperature range from 650°C to 800°C is set to 2°C / s or less, and the average heating rate in the temperature range from 800°C to the maximum annealing temperature Ts, which is above the Ac3 point + 30°C, is set to 1°C / s or less, and the mixture is heated to Ts. 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 or less in the temperature range from Ts to 400°C, an average cooling rate of 5°C / s or more and 20°C or less in the temperature range from 400°C to 280°C (T1), then performing a hot-dip galvanizing treatment, raising the temperature to Tm (plating temperature Tm, which is 450°C to 500°C) with an average heating rate of 5°C / s or more, holding the sheet in the temperature range of Tm-10°C or more and below 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 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] 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 at a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more between the hot rolling and the start of the cold rolling. [9] A method for manufacturing a steel sheet for cold press forming according to [5] or [7], wherein the steel sheet is further subjected to electro-galvanizing to obtain a plated steel sheet.

[10] A component comprising at least a portion of the steel sheet described in any of [1] to [3] above or the plated steel sheet described in [4] above.

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

[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. [Modes for carrying out the invention]

[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 (Total Strength). Therefore, the C content should be 0.050% or more. Preferably, the C content should be 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. Preferably, the C content should be 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 this invention in particular, it is an element that affects the hardness of martensite and the fraction of retained austenite because it suppresses carbide formation during continuous annealing and promotes the formation of retained austenite. If the Si content is less than 0.01%, the fraction of retained austenite decreases, making it difficult to achieve the desired El. Therefore, the Si content should be 0.01% or more. 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. In addition, 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 stretch 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 prior austenite grain boundaries, causing them to become brittle, thus impairing the ductility of the steel sheet. It also impairs weldability and delayed fracture resistance. Therefore, the P content must be 0.1000% or less. 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 specific lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable to have a P content of 0.0010% or more. It is even more preferable 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] When Al is present in large quantities, it raises the Ac3 point (Ac3 transformation point) and incorporates a large amount of ferrite into the microstructure, thus hindering the high strength achieved by utilizing the martensitic structure. Therefore, the Al content must be 1.00% or less. Preferably, the Al content is 0.50% or less. More preferably, the Al content is 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, as it suppresses carbide formation during continuous annealing and promotes the formation of retained austenite. More preferably, it is 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, impairing 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. Although there is no specific lower limit for the N content, 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 specific lower limit for the O content, but due to production technology constraints, it is preferable that the O content be 0.0001% or more. 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 containing the above-mentioned components, with the remainder being Fe and unavoidable impurities. Preferably, a steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the remainder being Fe and unavoidable impurities. Examples of unavoidable impurities include H, Zn, Pb, As, Se, Ge, Sr, and Cs. Unavoidable impurities may be introduced from raw materials such as scrap, and their inclusion is permissible as long as it does not hinder the objective of the present invention. Specifically, it is permissible for these impurities to be present in a total of 0.100% or less.

[0027] Furthermore, the steel sheet of the present invention optionally contains, in addition to the above component composition, a further component in mass%, At least one of the following elements may be included, either alone or in combination: 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.

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

[0029] Ta: 0.10% or less, W: 0.10% or less If the content of Ta and W is 0.10% or less, large amounts of coarse precipitates and inclusions will not be generated, 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. Preferably, the content of Ta and W should 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 concentration of 0.0100% or less, it will not cause cracks to form inside the steel sheet during casting or hot rolling, and will not impair the ductility of the steel sheet. Therefore, when 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 specific 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, when 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 specific 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, when Co is included, the Co content should be 1.000% or less. Preferably, the Co content should be 0.500% or less. More preferably, the Co content should be 0.100% or less, even more preferably 0.050% or less, and most preferably 0.030% or less. There is no specific lower limit for the Co content, but since it is an element that improves hardenability, it is preferable that the Co content be 0.001% or more. More preferably, the Co content should be 0.008% or more, and even more preferably 0.010% or more.

[0033] Sn: 0.5% or less, Cu: 0.5% or less If the content of Sn and Cu is 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 content of Sn and Cu should be 0.5% or less each. More preferably, the content of Sn and Cu 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 content of Sn and Cu be 0.001% or more each. It is more preferable that the content of Sn and Cu be 0.005% or more each, and even more preferably 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 present in amounts of 0.0100% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not be impaired. Therefore, when Ca, Mg, and REM are included, their respective contents should be 0.0100% or less. Preferably, the contents of Ca, Mg, and REM should be 0.0080% or less. More preferably, the contents of Ca, Mg, and REM should be 0.0060% or less, and even more preferably, 0.0040% or less. There is no specific lower limit for the contents of Ca, Mg, and REM, but since these elements spheroidize the shape of nitrides and sulfides and improve the ductility of the steel sheet, it is preferable that the contents of Ca, Mg, and REM be 0.0005% or more. More preferably, the contents of Ca, Mg, and REM should be 0.0010% or more, and even more preferably, 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. More preferably, the content of Zr and Te should be 0.005% or more each, and even more preferably 0.008% or more each.

[0036] Hf: 0.10% or less If Hf is present at a concentration of 0.10% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not be impaired. Therefore, when Hf is present, the Hf content should be 0.10% or less. Preferably, the Hf content should be 0.08% or less. More preferably, the Hf content should be 0.02% or less, and even more preferably, 0.005% or less. Although there is no specific lower limit for the Hf content, it is more preferable that the Hf content be 0.001% or more, as Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ductility of the steel sheet.

[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, when Bi is included, the Bi content should be 0.200% or less. Preferably, the Bi content should be 0.100% or less. More preferably, 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, when Ni is included, the Ni content should be 0.8% or less. Preferably, the Ni content should be 0.5% or less. More preferably, 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: Within the range of 0.200% or less, the condition Ti / 48 ≥ N / 14 is satisfied. Note that the above values ​​of Ti and N represent the components (mass %) in the steel sheet. Ti, which has an even higher affinity for N than B, forms nitrides before B, thus ensuring that B is dissolved in the steel. However, it is presumed that the 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 the solid-solution C adhering to dislocations within the grains. Therefore, it is preferable that Ti / 48 ≥ N / 14 is satisfied in the range of Ti: 0.200% or less. More preferably, it is in the range of Ti: 0.100% or less, even more preferably in the range of Ti: 0.060% or less, and most preferably in the range of Ti: 0.050% or less. The lower limit is not particularly limited, but it is preferably in the range of Ti: 0.005% or more, and more preferably in the range of 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 indicate the components (mass %) in the steel plate. Furthermore, in order to further enhance the effect of solid-solution carbon (C), it is preferable that the total content of grain boundary segregating elements P and B be 0.0090% or more, with B being 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 carbon. More preferably, it is 0.0100% or more. Even more preferably, it is 0.0110% or more, and most preferably, 0.0120% or more. 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% to 0.5%, Sn: 0.005% to 0.5%, and Sb: 0.001% to 0.08%, with at least two of these materials totaling 0.01% to 1.0%] Cu, Sn, and Sb are known as surface segregation elements, and by adding a certain amount, it is possible to suppress surface decarburization during the annealing process, and consequently suppress the reduction in strength of the base material due to non-uniformity of the structure in the thickness direction and softening of the surface of the plate thickness. In particular, in the present invention, the heating rate during the final annealing is small and the maximum temperature reached is high, so decarburization of the steel tends to progress easily during annealing, and the addition of these elements exhibits a high softening suppression effect. For this reason, in order to further improve strength properties, when Cu and / or Sn and / or Sb are included, at least two of the following should be included in a total of 0.01% or more: Cu: 0.005% to 0.5%, Sn: 0.005% to 0.5%, and Sb: 0.001% to 0.08%. More preferably 0.02% or more, even more preferably 0.08% or more, and most preferably 0.09% or more. On the other hand, regarding the upper limit, if Cu and / or Sn and / or Sb are included, at least two of Cu, Sn, and Sb should be included in a total of 1.0% or less, preferably 0.4% or less, and more preferably 0.35% or less.

[0043] Furthermore, regarding the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Ca, Mg, REM, Zr, Te, Hf, Bi, Cu, Ni, and Sb, if their respective contents are below the preferred lower limit, they will not impair the effects of the present invention and will therefore be included as unavoidable impurities.

[0044] Next, we will explain the microstructure. The locations that define the structure are at the 1 / 4 position in the width direction and the 1 / 4 position in the thickness direction. This is because, generally speaking, the 1 / 4 position in the width direction and the 1 / 4 position in the thickness direction are considered to be the locations that have the average characteristics and composition of the steel sheet.

[0045] [Ferrite + Bainite: 0.1% to 15% by area] Although ferrite is a soft structure and therefore effective in improving workability, in the steel sheets targeted by this invention, it has a strong effect in reducing the strength of the steel sheet. Here, ferrite may be polygonal ferrite, pseudopolygonal ferrite, or granular bainite 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, resulting in an excessive increase in carbon concentration in the retained austenite, 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. A more preferable condition is 10% or less. An even more preferable condition is when the amount of ferrite is less than 2% and the total amount of ferrite and bainite is 10% or less. The most preferable condition is that the total amount 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. Preferably, the total amount of ferrite and bainite is 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 plate parallel to the rolling direction (L-section) is polished, then etched with 3 vol.% nital, and 10 fields of view are observed at a position 1 / 4 of the plate thickness (corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate) using a scanning electron microscope (SEM) at 2000x magnification. Note that the surface of the steel plate in the case of zinc plating refers to the interface between the zinc plating layer and the steel plate. Next, using the obtained microstructure images, the area ratio of ferrite for the 10 fields of view is calculated using Media Cybernetics' Image-Pro. The average of these 10 field of view area ratios is taken as the ferrite area ratio. Note that in the above microstructure images, ferrite exhibits a gray structure (underlying structure). Furthermore, the method for measuring the area ratio of bainite and martensite, which will be described later, is as follows: A sample of annealed steel sheet is cut out, the thickness cross section parallel to the rolling direction is polished, and then it is etched with 3 vol.% nital. Ten fields of view are photographed at a magnification of 2000x using an SEM (scanning electron microscope) at a position corresponding to 1 / 4 of the sheet thickness (a position corresponding to 1 / 4 of the sheet thickness in the depth direction from the surface of the steel sheet). Additional observations at higher magnifications may be made in order to confirm the carbides in detail. In the same manner as above, the area ratio of the microstructure is determined using Image-Pro from Media Cybernetics using the obtained image data. Gray or dark gray areas containing oriented carbides are bainite. (Oriented carbides refer to carbides whose major axes are aligned in the same direction, and the angle difference between them is within 10°.) Furthermore, fresh martensite is distinguished as a white or light gray region, tempered martensite as a gray or dark gray region containing carbides with misaligned orientations, 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 carbides may appear to be aligned. Therefore, gray or dark gray regions containing carbides with misaligned orientations, and where the interface of the structure extends in a straight line, are classified as bainite, while gray or dark gray regions containing other carbides are classified as tempered martensite.

[0047] [Residual austenite: 5% to 20% by area] Retained austenite contributes to the ductility of steel sheets through the TRIP effect and is an essential structural component in this invention. To obtain a sufficient effect, the retained austenite content should be 5% or more. Preferably, it should be 7% or more, more preferably 8% or more, even more preferably 9% or more, and most preferably 10% or more. Furthermore, if retained austenite is present in excessive amounts, it may transform into a hard martensitic structure when applied to and formed for automotive parts, potentially impairing the processing characteristics. For this reason, the retained austenite content should be 20% or less. Preferably, it should be less than 16%, more preferably 15% or less, even more preferably 14% or less, and most preferably 13% or less. As explained below, retained austenite is determined by volume fraction, but since volume fraction and area fraction are roughly equivalent, it will be defined as area fraction here.

[0048] The method for measuring the volume fraction of retained austenite is as follows: After mechanically grinding a steel plate in the thickness direction (depth direction) to 1 / 4 of its thickness, chemical polishing with oxalic acid or the like is performed to remove the strain introduced by mechanical polishing, creating an observation surface. This observation surface is observed by X-ray diffraction. A Co Kα source is used as the incident X-ray source, and the intensity of the diffraction peaks of the {200}, {220}, and {311} surfaces of fcc iron (austenite) is measured relative to the diffraction intensity of the {200}, {211}, and {220} surfaces of bcc iron. By multiplying these by a correction factor and averaging the diffraction peaks of martensite and austenite, the influence of the preferred direction is eliminated, and the volume fraction of retained austenite is calculated.

number

[0049] [Tempered martensite: over 70% by area] 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 in automotive components. In order to obtain these effects, the area ratio of tempered martensite must be 70% or more. The area ratio of tempered martensite 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 using the method described above.

[0050] [Fresh martensite: 0% to 16% by area] Fresh martensite is martensite that does not contain iron-based carbides. Although steel sheets containing fresh martensite have high strength, their processing characteristics are inferior, so the fraction of fresh martensite in the steel sheet according to this embodiment is limited to 16% or less by area ratio. The fraction of fresh martensite is preferably 10% or less by area ratio, 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 ratio 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 surface of the steel sheet) using a scanning electron microscope (SEM) at 2000x magnification. For the purpose of confirming carbides in detail, observation at an additional higher magnification (e.g., 5000x) may be performed. Using the obtained image data, the area ratio is determined using Image-Pro from Media Cybernetics, and the average area ratio of the fields of view determined by the method below is taken as the area ratio of fresh martensite. In the image data, fresh martensite is distinguished as a white or light gray region. Furthermore, since it is difficult to distinguish fresh martensite from retained austenite which also appears as a white or light gray region, the microstructure fraction is obtained by first determining the area ratio of the white or light gray region and then subtracting the fraction of retained austenite determined by the method above.

[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 specimen from the 1 / 4 position in the width direction, mechanically polishing it to a surface 0.1 mm thick from the 1 / 4 position in the thickness direction, and then chemically polishing it to a further 0.1 mm thick. The surface exposed at the 1 / 4 position in the thickness direction was then subjected to X-ray diffraction using a Co-Kα source (Empyrean, Spectris). Based on Reference 1, the lattice constant aγ [Å] obtained from the (220)γ diffraction peak position was substituted into the following 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 amounts added (steel sheet composition) were used for Mnγ, Alγ, and Siγ. aγ=3.5720+0.033Cγ+0.0012Mnγ-0.00157Siγ+0.0056Alγ...Equation (2) The carbon concentration in retained austenite should be 0.45% or higher. If it is less than 0.45%, the retained austenite becomes excessively unstable, and much of it disappears by stress-induced transformation in the early stages of deformation, impairing ductility from the middle to the later stages of deformation. Preferably, the carbon concentration in retained austenite is 0.60% or higher, more preferably 0.65% or higher, and even more preferably 0.70% or higher. On the other hand, if it is excessively high, the retained austenite becomes too stable, preventing stress-induced transformation even in the later stages of deformation, which also impairs ductility and increases the yield stress and yield ratio. Therefore, the carbon concentration in retained austenite should be 0.95% or lower. Preferably, the carbon concentration in retained austenite is 0.90% or lower, more preferably 0.88% or lower, and even more preferably 0.85% or lower. [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 refers to the yield stress, but if a clear upper yield point occurs, it is the upper yield point strength, and if not, it is the 0.2% proof load strength.

[0055] Below, we will specifically explain one of the challenges: work hardening characteristics and heat treatment hardening characteristics.

[0056] [In a tensile test conforming to JIS Z 2241, the amount of retained austenite that disappears between the start of the tensile test and the processing that imparts a 1% nominal strain is 0.8% or more in terms of area ratio to the entire steel structure.] The excellent work hardening properties of the present invention are thought to be due to stress-induced transformation of unstable retained austenite during processing. That is, under low stress conditions, unstable fcc crystals transform into bcc crystals, and deformation progresses with volume expansion, resulting in stress-strain properties with a high yield ratio. Furthermore, the solid solution carbon released when retained austenite transforms has the effect of increasing the yield stress by fixing to dislocations introduced during processing during subsequent heat treatment. Therefore, the higher the fraction of unstable retained austenite that transforms in the early stages of deformation, the more advantageous it is for increasing the amount of work hardening and the amount of hardening after heat treatment. Thus, the amount of retained austenite that disappears between the start of the tensile test and processing that imparts a 1% strain at nominal stress is set to 0.8% or more. Preferably, the amount of retained austenite that disappears between the start of the tensile test and processing that imparts a 1% strain at nominal stress is 0.9% or more, more preferably 1.0% or more, even more preferably 1.5% or more, and most preferably 1.7% or more. While no upper limit is specifically set, it is necessary to ensure at least 3% austenite even after uniform elongation. Therefore, the amount of retained austenite lost between the start of the tensile test and the processing that applies 1% strain at nominal stress is preferably 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 aforementioned 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 less likely for the retained austenite to undergo stress-induced transformation during processing that applies 1% strain. The present invention makes it possible to reconcile both requirements, and the method by which this is achieved will be described later. The retained austenite lost during the above processing was evaluated by the difference in the amount of retained austenite before and after the tensile test. The retained austenite after the tensile test was evaluated using a sample cut from the central part of the tensile test specimen after a predetermined strain 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 central part of the JIS No. 5 test specimen after the tensile test, and after mechanical grinding in the thickness direction (depth direction) to 1 / 4 of the plate thickness, chemical polishing with oxalic acid or the like was performed to remove the strain introduced by mechanical polishing, creating an observation surface. X-ray diffraction was performed on this observation surface, and the retained austenite was measured using the method described above.

[0057] [Residual austenite present after processing from the start of the tensile test to the point of uniform elongation in the aforementioned tensile test: 3% or more in terms of area ratio relative to the entire steel structure] Retained austenite has a slightly softer structure compared to martensite and is less likely to cause cracks 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 achieve uniform elongation. More preferably, retained austenite should be 4% or more after processing to achieve 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 until processing to achieve uniform elongation be 5% or less.

[0058] [Processing and heat treatment to impart 1% distortion] Work hardening and heat treatment hardening characteristics were evaluated by tensile testing using JIS No. 5 tensile test specimens in the C direction (gauge length 50 mm, parallel section width 25 mm). The C direction refers to the direction perpendicular to the rolling direction. First, six JIS No. 5 tensile test specimens in the C direction were prepared, cut from the same width direction position of the steel plate. Three of these specimens were subjected to tensile testing until fracture, according to JIS Z 2241, and the yield stress, uniform elongation, total elongation, and tensile strength were evaluated by averaging the test results of the three specimens. The yield stress and total elongation here are considered to be the characteristics of the steel plate, i.e., the yield stress and total elongation before (processing and heat treatment that imparts 1% strain). For the remaining three test specimens, tensile processing was performed to a nominal strain of 1%, after which the tensile load was removed, and heat treatment was performed at 170°C for 20 minutes. Then, a tensile test was performed again to evaluate the yield stress, uniform elongation, total elongation, and tensile strength. The yield stress and total elongation at this stage are defined as the yield stress and total elongation after (processing to apply 1% strain and heat treatment). For the re-tensile test, the gauge length was set to 50 mm again, and the plate thickness and width of the parallel section of the initial test specimen, which are necessary for stress calculation, were measured again. The reason for setting the processing amount to 1% is that if it exceeds 2%, the test specimen may break during the yielding process during re-tensile testing, making it impossible to obtain a stress-strain curve with small variation. 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 machining and heat treatment that imparts 1% distortion) ≥ 250 MPa] The present invention aims to achieve a yield stress that is at least 250 MPa higher than the yield stress before processing (1% strain application and heat treatment) after processing (1% strain application and heat treatment). This makes it possible for the material to be soft during forming but to have high yield stress and, consequently, high impact characteristics when used as a component. Although there are differences depending on the chemical composition of the steel and the heat treatment conditions, processing that applies 1% strain results in work hardening of at least 70 MPa, and a stress increase of 180 MPa or more is observed as a result of the subsequent heat treatment. YS is the yield stress. As described later, under certain conditions, an extremely high yield stress increase of over 300 MPa is observed. There is no particular upper limit to the amount of YS increase, but it is preferable to keep it below 1000 MPa.

[0060] [El after (machining and heat treatment that imparts 1% distortion): (0.9 × El before (machining and heat treatment)) or more] A feature of the present invention is that the total elongation before and after (processing and heat treatment that imparts 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, under certain 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 more, and even more preferable that El after (1% processing and heat treatment) is (1.2 × (El before processing and heat treatment)) or more. While there are no particular upper limits, it is preferable that El after (1% processing and heat treatment) be (1.5 × El before (processing and heat treatment)) or less, and more preferably that El after (1% processing and heat treatment) be (1.4 × El before (processing and heat treatment)) or less.

[0061] [The difference between the stress increase after 170°C heat treatment without pre-strain and the stress increase after 170°C heat treatment after processing with 1% strain (tensile processing) is 120 MPa or more.] The steel sheet shown in the present invention is a suitable material for cold-pressed materials where the introduction of processing strain is a prerequisite, and a major feature is that the stress increase in the processed portion is significantly greater than the stress increase in the unprocessed portion. Therefore, 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. 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 preferably 150 MPa or more, and more preferably 170 MPa or more. There is no particular upper limit, but it is preferable that 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 300 MPa or less. Note that "after 170°C heat treatment without pre-straining" refers to the state after 170°C heat treatment without applying any strain (processing). In this specification, the stress increase 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 removing the tensile test load from the tensile test specimen from which the σb was obtained (or the specimen as it was cut from the original plate if there is no pre-strain), performing heat treatment at a specified temperature for 20 minutes, and then performing another tensile test.

[0062] Next, the method for manufacturing steel sheets according to the present invention will be described. The steel material having the aforementioned component composition is melted down using a conventional refining process, and then formed into steel slabs using a conventional ingot-parting rolling method or continuous casting method. Alternatively, thin steel slabs with a thickness of 100 mm or less may be produced by direct casting. After heating and holding the steel slabs, they are subjected to hot rolling, and hot-rolled sheets are produced by rough rolling and finish rolling and wound into coils. The thickness of the hot-rolled sheets 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. Furthermore, 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] Hot rolling process The finishing rolling temperature is between 800°C and 1000°C. Here, the finish rolling completion temperature (finishing hot rolling temperature) shall be between 800°C and 1000°C. To achieve this, the slab heating temperature is preferably 1080°C or higher. More preferably 1120°C or higher, even more preferably 1140°C or higher, and most preferably 1160°C or higher. Furthermore, the slab heating temperature is preferably 1300°C or lower. More preferably 1260°C or lower, even more preferably 1250°C or lower, and most preferably 1240°C or lower. Hot rolling must be completed in the austenite single-phase region in order to improve elongation and hole-expanding properties after annealing by homogenizing the structure within the steel sheet and reducing material anisotropy. Therefore, the finish rolling completion temperature should be 800°C or higher. Preferably, the finish rolling completion temperature should be 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] Winding temperature is above Ms point and below Bs point After the finish rolling is complete, the sheet is cooled and then wound at a winding temperature between the Ms point and the Bs point to complete the hot rolling process. If the winding temperature is below the Ms point, the hot-rolled sheet becomes excessively hard, making winding and subsequent cold rolling difficult, and also suppressing scale formation on the steel sheet surface, which is useful for the present invention. Therefore, the winding temperature is set to be above the Ms point. It is more preferable that the winding temperature be Ms point + 10°C or higher, even more preferable that it be Ms point + 20°C or higher, and most preferable that it be Ms point + 25°C or higher. Here, the Ms point (°C) is calculated by the following formula. [ ] represents the mass % of each component. Ms point (℃)=561-474[C]-33[Mn]-17[Ni]-17[Cr]-21[Mo] Bs point (°C) = 830 - 270 [C] - 90 [Mn] - 37 [Ni] - 70 [Cr] - 83 [Mo] On the other hand, if the winding temperature is excessively high, the ferrite phase fraction increases, and the microstructure becomes non-uniform, which increases gauge fluctuations during cold rolling and variations in the quality of the final product sheet. Therefore, 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, held at a temperature range T of 400°C or higher and below the Ac3 point for 10 minutes or more. Furthermore, between the completion of the finish rolling in hot rolling and the start of cold rolling, pre-cold rolling annealing may be performed for 10 minutes or more at a temperature range T of 400°C or higher and below the Ac3 point in order to reduce the cold rolling load. If the annealing temperature exceeds the Ac3 point, a hard structure will form from the austenite phase during cooling. Therefore, if pre-cold rolling annealing is performed, the pre-cold rolling annealing temperature should be below the Ac3 point. Preferably, it should be below the Ac1 point. Also, 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, if pre-cold rolling annealing is performed, the pre-cold rolling annealing temperature should be 400°C or higher. Preferably, the pre-cold rolling annealing temperature is 450°C or higher. The Ac1 and Ac3 points shown in this specification can be calculated from the following equations (A) and (B) described in Reference 2. In the equations, [ ] indicates the content (mass%) of each element, and elements not present in the steel plate, or elements whose content is below the detection limit of the analysis, should be calculated as 0 mass%. Ac1 point (℃)=723-10.7×[Mn]-16.9×[Ni]+29.1×[Si]+16.9×[Cr]+290×[As]+6.38×[W]...(A) Ac3 points (℃)=910-203×√[C]-15.2×[Ni]+44.7×[Si]+104×[V]+31.5×[Mo]+13.1×[W ]-30×[Mn]-11×[Cr]-20×[Cu]+700×[P]+400×[Al]+120×[As]+400×[Ti]...(B) [Reference 2] "Leslie's Science of Steel Materials" (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 to 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 should be 2°C / s or less. Next, as an annealing process, the obtained steel sheet is subjected to heat treatment and, if necessary, plating. In the heating process performed at the beginning of the heat treatment, the temperature range from 650°C to 800°C is maintained at an average heating rate of 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 tends to become excessively concentrated with carbon during heat treatment. In this invention, since retained austenite containing an appropriate amount of carbon can be formed by the transformation from ferrite to austenite occurring after the recrystallization of ferrite is completed, 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 less than 1°C / s. Next, the steel sheet is heated to Ts, the maximum annealing temperature in the main annealing process. Here, mainly an austenite transformation occurs from ferrite, but since the ferrite has already recrystallized into 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. There is no particular lower limit, 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 calculated by dividing the temperature difference between 800°C and the maximum annealing temperature Ts by the time required to raise the temperature during that period. In order to complete the austenite transformation with a high amount of recrystallized ferrite and low driving force, Ts must be above Ac3 point + 30°C. Preferably, it is above Ac3 point + 40°C, and more preferably above Ac3 point + 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 below 1050°C, more preferably below 1020°C, even more preferably below 1000°C, and most preferably below 980°C.

[0071] 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 from 400°C to 50°C to 280°C (T1) is set to 5°C / s or more and 20°C or less, and the mixture is cooled down to T1. In the subsequent cooling process, 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 harder 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, 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. Preferably, the average cooling rate is 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 calculated by dividing the temperature difference (cooling start temperature - cooling end temperature), where Ts is the cooling start temperature and 400°C is the cooling end temperature, 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 to between 50°C and 280°C is as follows: Cooling to below 280°C transforms some of the austenite produced by annealing into martensite. However, below 50°C, much of the austenite necessary for improving ductility transforms into hard structures such as martensite and disappears, 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, above 280°C, the martensite transformation does not occur sufficiently, not only reducing the fraction of tempered martensite structure, but also increasing the fresh martensite fraction as the remaining austenite transforms into martensite after final cooling. Therefore, the temperature T1 is set to 280°C or lower. Preferably it is 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, as a rate below 5°C / s will result in excessive formation of ferrite and bainite. 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 the rate 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 above average cooling rate can be determined by dividing the temperature difference between 400°C and temperature T1 by the time required to cool from 400°C to T1. The cooling means of the present invention can be any known method, and gas cooling, oil cooling, mist cooling, etc., can be applied.

[0073] The temperature is raised from T1 to T2 with an average heating rate of 5°C / s or more in the temperature range of 280°C to 380°C, and then held at a temperature range of T2-10°C or higher but below T2 for 1 second to 60 seconds. Next, the temperature is increased from the aforementioned temperature T1 to a temperature T2 of 280°C to 380°C at an average heating rate of 5°C / s or more, and the temperature is held in the range of T2-10°C to less than T2 for 1 second to 60 seconds. This step is particularly important for forming the desired retained austenite structure and is called the tempering step in this invention. In the tempering step, 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. Therefore, rapid heating and short processing time are necessary. For this reason, the average heating rate is set to 5°C / s or more, with no particular upper limit. The average heating rate is preferably 7°C / s or more, more preferably 10°C / s or more, even more preferably 15°C / s or more, and most preferably 17°C / s or more. There is no particular upper limit, but it should be 100°C / s or less. The average heating rate mentioned above can be calculated by dividing the temperature difference between T1 and T2 by the time it took to raise the temperature from T1 to T2. To promote the transformation from austenite to bainite, the T2 temperature range should be 280°C or higher. Preferably, the T2 temperature range should be 300°C or higher, more preferably 320°C or higher, and even more preferably 330°C or higher. On the other hand, if the temperature is too high, the transformation to bainite, which is one form of the bcc phase, becomes difficult to occur, and it becomes impossible to produce austenite with a low carbon concentration, so it should be 380°C or lower. Preferably, the T2 temperature range should be 370°C or lower, more preferably 360°C or lower, and even more preferably 350°C or lower. Furthermore, the holding time in the temperature range from T2-10°C to below T2 should be 60s or less, because if it exceeds 60s, the carbon concentration in the retained austenite becomes too high, resulting in an excessively high yield ratio. A preferred holding time in the temperature range from T2-10°C to below T2 is 45s or less, and more preferably 30s or less. To obtain even more favorable properties, it is even more preferable that the holding time in the temperature range from T2-10°C to below T2 be less than 10s, and most preferably 9s or less. Also, for reasons such as promoting the transformation from austenite to bainite, the holding time should be 1s or more. The holding time is preferably 2s or more, more preferably 4s or more, and even more preferably 5s or more. The reason for setting the above retention period to a temperature range of T2-10°C or higher and below T2 is that carbon diffusion is relatively significant in this temperature range, and it has a large impact on the material.

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

[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 more is to obtain the desired total area ratio of ferrite and bainite. The average heating rate is preferably 7°C / s or more, more preferably 10°C / s or more, and even more preferably 15°C / s or more. There is no particular upper limit, but it may be 60°C / s or less. The holding time in the temperature range of Tm-10°C or more and less than 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. The holding time is preferably 3 s or more, more preferably 5 s or more, and even more preferably 10 s or more. Also, the holding time should be 60 s or less. The holding time is preferably 50 s or less, more preferably 45 s or less, and even more preferably 40 s or less. The average heating rate can be determined by dividing the temperature difference between T1 and Tm by the time required to heat from T1 to Tm. Furthermore, regarding the final cooling, T2 is replaced with the plating temperature Tm, and the explanation is as described above.

[0078] Furthermore, manufacturing conditions other than those mentioned above can be met 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. [Examples]

[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, rolled into a slab, and then hot-rolled under the conditions shown in Tables 2-1 and 2-2. After cooling, it was coiled. Subsequently, both sides were ground evenly by 0.2 mm. Some of the slabs were then heat-treated in a nitrogen atmosphere and cooled in the air or furnace-cooled in a nitrogen atmosphere. After that, they were pickled and cold-rolled. After that, they were heat-treated in a nitrogen atmosphere. Some of the slabs were hot-dip galvanized as a post-heat treatment step. From the obtained steel sheet, a JIS No. 5 test specimen (gauge length 50 mm, parallel section width 25 mm) was cut perpendicular to the rolling direction, and a tensile test was performed according to JIS Z2241. In addition, the microstructure, work hardening characteristics, and heat treatment characteristics of the steel sheet were measured based on the method described above. Furthermore, using the obtained steel plate, two hat-shaped members with an M-shaped cross-section (hat members) were fabricated. One member underwent a three-point bending test without heat treatment, while the other member underwent 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 reference 3 below. [Reference 3] Sato, Kentaro, Futatsuka, Takayuki, Sakaitani, Tomohiro, Yoshioka, Shinpei & Tamai, Yoshikiyo. (2022), Influence of material properties of ultra-high-strength steel sheets on the collision performance of hat components. Transactions of the Society of Automotive Engineers of Japan, 53(3), 675-680. For the maximum load mentioned above, 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 impact resistance characteristics could be obtained through baked coating hardening. Tables 3-1 and 3-2 show the characteristics obtained above.

[0082] [Table 1]

[0083] [Table 2-1]

[0084] [Table 2-2]

[0085] [Table 3-1]

[0086] [Table 3-2]

Claims

1. In mass percent, 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 It contains, The remainder is a component composition consisting of Fe and unavoidable impurities, The steel structure has a total area ratio of ferrite and bainite of 0.1% to 15%. If the area percentage of retained austenite is between 5% and 20%, Tempered martensite accounts for 70% or more by area. Fresh martensite is between 0% and 16% by area. The carbon concentration in the retained austenite satisfies the condition of being 0.45% or more and 0.95% or less. In a tensile test in accordance with JIS Z 2241, the retained austenite that disappears between the start of the tensile test and the processing that imparts a 1% strain at nominal strain is 0.8% or more in area relative to the entire steel structure, and A steel sheet for cold press forming in which retained austenite accounts for 3% or more of the total area of ​​the steel structure after processing from the start of the tensile test to the point of uniform elongation in the tensile test.

2. The aforementioned component composition is further expressed in mass%, 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 aforementioned component composition is further expressed in mass%, Ti: Contains in the range of 0.200% or less, and satisfies Ti / 48 ≥ N / 14, and further, B: In the range of 0.0003% or more and 0.0100% or less, The condition B + P ≥ 0.0090% is satisfied. A steel sheet for cold press forming according to claim 2, comprising the features described in claim 2. Note that 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 method for manufacturing a steel sheet for cold press forming according to any one of claims 1 to 3, wherein a steel slab having the above-mentioned component composition is heated, then hot-rolled at a finish rolling completion temperature of 800°C to 1000°C, and then wound up at a temperature between the Ms point and the Bs point, After pickling the obtained hot-rolled sheet, it is 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. Furthermore, it is cooled to T1, with an average cooling rate of 10°C / s or more and 100°C / s or less from Ts to 400°C, and 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, 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 method for manufacturing a plated steel sheet for cold press forming according to Claim 4, wherein a steel slab having the component composition according to 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 at an Ms point to a Bs point, 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 The average heating rate in the temperature range from 650°C to 800°C is set to 2°C / s or less, and the average heating rate in the temperature range from 800°C to the maximum annealing temperature Ts, which is above the Ac3 point + 30°C, is set to 1°C / s or less, and the mixture is heated to Ts. 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 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. A 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, wherein the steel sheet is further subjected to electro-galvanizing to obtain a plated steel sheet.

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

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