Steel sheet, member, and methods for producing same

Abstract

Provided is a steel sheet which has a tensile strength TS of 980 MPa or more and has excellent formability, collision characteristics, chemical conversion treatability, and delayed fracture resistance. Also provided are a member and methods for producing the steel sheet and the member.　The steel sheet has a specific composition, and has a steel sheet surface coverage by a Si-based oxide of 1% or less. At the 1 / 4 sheet thickness position, the steel sheet has a structure in which the ferrite area fraction is 3-30%, inclusive, the tempered martensite area fraction is 60-94%, inclusive, the retained austenite volume fraction is 3-15%, inclusive, and the remaining structure area fraction is 10% or less. The ferrite includes low Mn ferrite in which the Mn concentration is less than 0.85 times the Mn content of the steel sheet, and high Mn ferrite in which the Mn concentration is 0.85 or more times the Mn content of the steel sheet. Of the ferrite, the proportion which is the high Mn ferrite is 20-90%, inclusive, in terms of area fraction.

Description

Steel plates, components, and methods for manufacturing them.

[0001] This invention relates to steel plates, components, and methods for manufacturing them, which are used in various applications such as automobiles and home appliances.

[0002] In recent years, the increasing need for lighter automobile bodies has led to the growing application of high-strength steel sheets with a tensile strength of 980 MPa or higher to automotive frame and seat components. However, when high-strength steel sheets with a tensile strength of 980 MPa or higher are applied to automotive parts, press cracks are more likely to occur due to a decrease in El and λ. For this reason, it is desirable that these high-strength steel sheets have superior formability compared to conventional materials. Furthermore, from the perspective of occupant safety, it is necessary to suppress deformation around the cabin during a collision, so energy-absorbing members such as side members are required to absorb collision energy by deforming during a collision. However, with high-strength steel sheets with a tensile strength TS of 780 MPa or higher, the reduced axial crushing characteristics make it easy for the part to fracture at the point of primary processing by forming during a collision, resulting in the inability to stably exhibit collision energy absorption capacity. In addition, when high-strength steel sheets with a tensile strength TS of 980 MPa or higher are formed into parts by cold pressing or bending, there is a risk of delayed fracture due to increased residual stress within the part and deterioration of the delayed fracture resistance characteristics of the steel sheet. Here, delayed fracture refers to the phenomenon in which, when a molded part is placed in a hydrogen-ingression environment, hydrogen penetrates into the steel sheet that makes up the part, reducing interatomic bonding forces and causing localized deformation, leading to the formation of microcracks, and the propagation of these microcracks eventually resulting in the fracture of the steel sheet.

[0003] As a technology to improve the El of steel sheets, TRIP steel has been developed in which retained austenite (hereinafter also referred to as retained γ) is dispersed in the microstructure of the steel sheet.

[0004] For example, Patent Document 1 states that, in mass%, C: 0.15% or more, 0.30% or less, P: 0.040% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0060% or less, one or two types of Si and Al: 0.70% or more, 2.50% or less in total, one or two types of Mn and Cr: 1.50% or more, 3.50% or less in total, Mo: 0 % or more, 1.00% or less, Ni: 0% or more, 1.00% or less, Cu: 0% or more, 1.00% or less, Nb: 0% or more, 0.30% or less, Ti: 0% or more, 0.30% or less, V: 0% or more, 0.30% or less, B: 0% or more, 0.0050% or less, Ca: 0% or more, 0.0400% or less, Mg: 0% or more, 0.0400% or less, and REM: 0% It is disclosed that a steel sheet with a tensile strength of 980 MPa or more and excellent elongation can be obtained by having the above 0.0400% or less, with the remainder being Fe and impurities, and having the following area ratios relative to the total structure: one or two types of ferrite and granular bainite: 10% or more and 50% or less in total, one or two types of upper bainite and lower bainite: 10% or more and 50% or less in total, tempered martensite: more than 0% and 30% or less, retained austenite: 5% or more, and one or more types of pearlite, cementite, and martensite: 0% or more and 10% or less in total, and setting the area ratio of the ferrite to the total area ratio of the ferrite and granular bainite to 25% or less. However, Patent Document 1 has poor formability and does not consider chemical conversion treatment properties or delayed fracture resistance.

[0005] Patent Document 2 discloses that a steel sheet with excellent strength, ductility (hereinafter also referred to as El), and hole-expanding properties (hereinafter referred to as λ) can be obtained by containing, by mass%, C: 0.10 to 0.40%, Mn: 0.5 to 4.0%, Si: 0.005 to 2.5%, Al: 0.005 to 2.5%, and Cr: 0 to 1.0%, with the remainder being iron and unavoidable impurities, and limiting P: 0.05% or less, S: 0.02% or less, and N: 0.006% or less, and having a steel structure that, by area percentage, contains 2 to 30% retained austenite, limits martensite to 20% or less, has an average cementite grain size of 0.01 μm or more and 1 μm or less, and contains 30% to 100% cementite with an aspect ratio of 1 or more and 3 or less. However, Patent Document 2 does not consider delayed fracture resistance. Furthermore, the material may contain a large amount of Si, and in this case, applying it as cold-rolled steel sheet for automobiles requires pickling technology as described in Patent Document 3, which will be discussed later. Therefore, because it requires the construction of pickling equipment or incurs high running costs, there has been a strong demand for the establishment of alternative technologies.

[0006] Patent Document 3 states that the composition, in mass%, is as follows: C: 0.06-0.24%, Si: 0.4% or more and less than 1.60%, Mn: 1.5-3.2%, P: 0-0.02%, S: 0-0.01%, sol. The steel contains less than 1.0% Al and less than 0.015% N, with a Si / Mn ratio of less than 0.50, the remainder consisting of iron and unavoidable impurities, and the steel structure has a polygonal ferrite area ratio of 20% to 85%, an upper bainite area ratio of 9% to 45%, a retained austenite volume ratio of 3% to 15%, a fresh martensite area ratio of 3% to 15%, a combined area ratio of tempered martensite and lower bainite of 0% to 50%, and a remaining structure area ratio of 5% or less, with an equivalent circle diameter of less than 1.2 μm. The present invention provides steel sheets, components, and methods for manufacturing them, which have a tensile strength of 590 MPa or more, high ductility, excellent stretch flange formability, and good chemical conversion treatment properties, by having a ratio of 50% or more of the total number of fresh martensite particles and retained austenite particles to the total number of fresh martensite particles and retained austenite particles with an aspect ratio of 2.5 or more and an equivalent circle diameter of 1.2 μm or more, and a ratio of 40% or more of the total number of fresh martensite particles and retained austenite particles with an equivalent circle diameter of 1.2 μm or more. However, Patent Document 3 describes the insufficient formability of steel with a strength of 980 MPa or more, and does not consider impact characteristics and delayed fracture resistance.

[0007] Patent No. 6338038 Patent No. 4903915 Patent No. 7294548

[0008] Thus, conventional technology has not yet been sufficient to produce steel sheets that possess excellent formability and impact characteristics while also having superior chemical treatment properties and excellent delayed fracture resistance.

[0009] The present invention was made to solve these problems and aims to provide steel sheets, components, and methods for manufacturing them that have a tensile strength of 980 MPa or more, excellent formability and impact characteristics, as well as good chemical treatment properties and delayed fracture resistance.

[0010] Here, the tensile strength TS is obtained by a tensile test based on JIS Z 2241 (2011). Furthermore, excellent formability refers to a value of 600% × % or more, calculated by combining the elongation El (%) obtained by a tensile test based on JIS Z 2241 (2011) and the hole expansion ratio λ (%) obtained by a hole expansion test in accordance with JIS Z 2256 (2020). Furthermore, excellent impact properties refer to a value of 30000 MPa × % or more, calculated by combining the YS (MPa) obtained by a tensile test based on JIS Z 2241 (2011) and the limit hole expansion ratio λ (%) obtained by a hole expansion test in accordance with JIS Z 2256 (2020). Finally, good chemical treatment properties refer to a pressure of 20-35 A / dm ２ The material was subjected to sulfuric acid electrolytic pickling for 2 seconds at a current density to degrease and prepare the surface, followed by a chemical conversion treatment using zinc phosphate chemical conversion solution. The degreasing process was performed at a temperature of 40°C for 120 seconds using spray degreasing, the surface preparation process at a pH of 9.5, at room temperature for 20 seconds, and the chemical conversion treatment process at a chemical conversion solution temperature of 35°C for 120 seconds. The degreasing agent, surface preparation agent, and chemical conversion treatment solution used in each of these processes were, in order, FC-E2011 (manufactured by Nippon Parkerizing Co., Ltd.), PL-X (surface preparation agent), and Palbond PB-L3065 (chemical conversion solution), with a magnification of 2000x and a surface thickness of 10000 μm. ２ By observing the above regions using SEM, the surface chemical conversion structure is observed, and it means that the area where a chemical conversion film structure has not been formed is 10% or less of the total measurement area. Furthermore, good delayed fracture resistance means that the material has excellent delayed fracture resistance in the corrosive environment specified in this invention, that is, in the evaluation of metallic materials, the number of days to crack is 63 days or more.

[0011] Furthermore, the steel sheet referred to in this invention includes not only steel sheets without a plating layer, but also steel sheets in which a plating layer is formed on the surface of a steel sheet (hereinafter also referred to as a base steel sheet), that is, steel sheets having a plating layer and a base steel sheet.

[0012] The inventors have diligently studied to solve the above problems and have obtained the following findings: (1) By setting the area fraction of ferrite to 3% or more and the area fraction of tempered martensite to 60% or more and 94% or less at the 1 / 4 thickness position of the steel plate (base steel plate), a TS of 980 MPa or more can be achieved. (2) By setting the volume fraction of retained austenite to 3% or more and the proportion of high-Mn ferrite in the total ferrite to 20% or more and 90% or less at the 1 / 4 thickness position, excellent formability, impact characteristics, and delayed fracture resistance can be achieved with El×λ of 600%・% or more and YS×λ of 30000 MPa・% or more. (3) Based on the above design, by reducing the Si / Mn ratio and setting the surface coverage rate of Si-based oxides (Si-based oxides) of the steel plate to 1% or less, good chemical conversion treatment properties can be achieved.

[0013] In other words, the gist of the present invention is as follows: [1] The material has a composition in mass%, containing C: 0.030% or more and 0.300%, Si: 0.40% or more and 1.60%, Mn: 1.40% or more and 3.20%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, and Si / Mn < 0.50, with the remainder being Fe and unavoidable impurities, the surface coverage rate of Si-based oxides on the steel sheet is 1% or less, the area fraction of ferrite is 3% or more and 30% or less at the 1 / 4 thickness position, the area fraction of tempered martensite is 60% or more and 94% or less, the volume fraction of retained austenite is 3% or more and 15% or less, and the area fraction of the remaining structure is 10% or less, and the ferrite is A steel sheet comprising low-Mn ferrite having a Mn concentration of less than 0.85 times the Mn content of the steel sheet, and high-Mn ferrite having a Mn concentration of 0.85 times or more the Mn content of the steel sheet, wherein the proportion of high-Mn ferrite among the ferrite is 20% or more and 90% or less in area fraction, TS is 980 MPa or more, El×λ is 600%×% or more, and YS×λ is 30000 MPa×% or more. [2] The above component composition is further defined in mass percent as follows: As: 0.100% or less, 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, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.050% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Zn: 0.100% or less The steel sheet according to [1], which contains at least one element selected from Pb: 0.100% or less, Te: 0.100% or less, Se: 0.020% or less, Ga: 0.020% or less, Ge: 0.020% or less, Sr: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less. [3] The steel sheet according to [1] or [2], which has a plating layer on at least one side of the steel sheet.[4] A member made of the steel sheet according to any one of [1] to [3] above. [5] A method for manufacturing the steel sheet according to any one of [1] to [3] above, wherein a steel material having the component composition described in [1] or [2] above is subjected to hot rolling to obtain a hot-rolled steel sheet, and then the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. After that, annealing is performed on the cold-rolled steel sheet. The annealing includes an annealing holding step of holding under the condition that the annealing temperature is Ac3 - 100°C or higher and Ac3 or lower, and the holding time at the annealing temperature is 5 s or longer; a first cooling step of performing cooling with a first average cooling rate from the annealing temperature to a temperature T1 of 550°C or higher being less than 11°C / s; a second cooling step of performing cooling with a second average cooling rate from the temperature T1 to 500°C being 11°C / s or higher; a third cooling step of performing cooling with a residence time from 500°C to a temperature T2 of Ms or higher and 320°C or higher being 10 s or longer and 60 s or shorter; a fourth cooling step of performing cooling with a fourth average cooling rate from the temperature T2 to a temperature T3 of 100°C or higher and Ms - 80°C or lower being 3°C / s or higher and 50°C / s or lower; a tempering step of performing tempering under the condition that the tempering temperature is T3 or higher and 450°C or lower, and the tempering time is 5 s or longer and 1000 s or shorter; and a fifth cooling step of performing cooling with a fifth average cooling rate in the temperature range of 150 to 80°C being 1.0°C / h or higher and 50.0°C / h or lower. A method for manufacturing a steel sheet. [6] The method for manufacturing a steel sheet according to [5] above, wherein a plating treatment is performed on the surface of the steel sheet. [7] A method for manufacturing a member, including a step of subjecting the steel sheet according to any one of [1] to [3] above to at least one of forming and joining to form a member.

[0014] According to the present invention, there are provided a steel sheet, a member, and a method for manufacturing them, which have a tensile strength of 980 MPa or higher, excellent formability and collision characteristics, and good chemical conversion treatment properties and stress corrosion cracking resistance characteristics.

[0015] Figure 1 is a schematic diagram illustrating a method for evaluating the delayed fracture characteristics of a metallic material (HeTsAce). Figure 2 is a schematic diagram showing an example of an image capturing the distribution of droplets on the evaluation surface of a metallic material. Figure 3 is a schematic diagram illustrating the case where a shielding material is placed between the spray nozzle and the metallic material in the method for evaluating the delayed fracture characteristics of a metallic material. Figure 4 is a schematic diagram illustrating how the spray liquid in the atmosphere re-adheses to the evaluation surface when the distance between the shielding material and the evaluation surface of the metallic material is changed. Figure 5 is a diagram illustrating one embodiment of the corrosion test cycle related to the method for evaluating the delayed fracture characteristics of a metallic material. Figure 6 is a diagram illustrating another embodiment of the corrosion test cycle related to the method for evaluating the delayed fracture characteristics of a metallic material. Figure 7 is a schematic diagram showing the test piece used for evaluating the delayed fracture characteristics in the example. Figure 8 is a graph showing the relationship between temperature and time in the manufacturing method of cold-rolled steel sheet according to an embodiment of the present invention.

[0016] (Steel Sheet) Hereinafter, embodiments of the steel sheet and its manufacturing method according to the present invention will be described. The embodiments described below are an example of embodying the present invention, and do not limit the configuration of the present invention by these specific examples. The steel sheet of the present invention contains, in mass%, C: 0.030% or more and 0.300% or less, Si: 0.40% or more and 1.60% or less, Mn: 1.40% or more and 3.20% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, and Si / Mn < 0.50, and the balance consists of Fe and inevitable impurities. It has a component composition, the coverage rate of the Si-based oxide on the steel sheet surface is 1% or less, At the 1 / 4 position of the plate thickness, the area fraction of ferrite is 3% or more and 30% or less, the area fraction of tempered martensite is 60% or more and 94% or less, the volume fraction of retained austenite is 3% or more and 15% or less, and the balance tissue has an area fraction of 10% or less. The ferrite has low-Mn ferrite with a Mn concentration less than 0.85 times the Mn content of the steel sheet and high-Mn ferrite with a Mn concentration of 0.85 times or more the Mn content of the steel sheet. Among the ferrite, the proportion occupied by the high-Mn ferrite is 20% or more and 90% or less in area fraction, TS is 980 MPa or more, El×λ is 600%×% or more, and YS×λ is 30000 MPa×% or more. It has a base steel sheet.

[0017] (Component Composition of Steel Sheet) The component composition of the steel sheet (base steel sheet) of the present invention will be described. In the following description, % indicating the components of steel are all mass% unless otherwise specified.

[0018] [C: 0.030% or more and 0.300% or less] Carbon (C) is one of the important basic components of steel sheets, and in particular, in the present invention, it is an important element that affects the sum of the area fraction of tempered martensite, the area fraction of ferrite, and the volume fraction of retained austenite. If the C content is less than 0.030%, the area fraction of tempered martensite decreases and the sum of the area fractions of ferrite increases, making it difficult to achieve a TS of 980 MPa or more, and in some cases, a sufficient amount of retained austenite cannot be obtained, making it impossible to achieve the desired ductility. Therefore, the C content should be 0.030% or more, preferably 0.040% or more, and more preferably 0.050% or more. The C content should be even more preferably 0.070% or more, and even more preferably 0.080% or more. On the other hand, if the C content exceeds 0.300%, the tempered martensite becomes brittle, and the delayed fracture resistance decreases. Therefore, the C content should be 0.300% or less, and preferably 0.290% or less. The C content is more preferably 0.170% or less, and even more preferably 0.160% or less.

[0019] [Si: 0.40% to 1.60%] Si is included from the viewpoint of achieving high strength in the ferrite structure and from the viewpoint of stabilizing retained austenite (retained γ) by suppressing carbide formation in martensite and bainite, thereby improving ductility. From these viewpoints, the Si content is set to 0.40% or more. From the viewpoint of improving ductility, it is preferable that the Si content be 0.50% or more. More preferably, the Si content is 0.60% or more. On the other hand, if the Si content exceeds 1.60%, the chemical treatment properties deteriorate significantly. For this reason, the Si content is set to 1.60% or less. Preferably, the Si content is less than 1.60%, more preferably 1.30% or less, and even more preferably 1.20% or less. Even more preferably, the Si content is less than 1.00%.

[0020] [Mn: 1.40% to 3.20%] Mn is included to ensure a predetermined hardenability, suppress ferrite transformation, and secure tempered martensite with a desired area fraction, thereby ensuring strength. In addition, Mn is concentrated in γ during ferrite / γ two-phase annealing, stabilizing residual γ by lowering the Ms point of untransformed γ and improving ductility. Mn also increases the volume fraction of residual γ, improving ductility. From these points, Mn is an important element in the present invention. To obtain these effects, the Mn content is 1.40% or more. Preferably, the Mn content is 1.50% or more. From the viewpoint of improving hardenability, it is more preferable that the Mn content be 1.70% or more. It is even more preferable that the Mn content be 1.90% or more. On the other hand, if the Mn content exceeds 3.20%, it may degrade the delayed fracture resistance through the formation of coarse MnS and Mn segregation. Furthermore, if the Mn content exceeds 3.20%, it becomes difficult to suppress the formation of lumpy, coarse γ-ions, and the moldability also deteriorates. Therefore, the Mn content should be 3.20% or less. Preferably, it should be 3.00% or less, and more preferably 2.80% or less.

[0021] [P: 0.100% or less] P segregates at the prior austenite grain boundaries, embrittles the grain boundaries, and thus embrittles the base steel sheet. Therefore, if the P content exceeds 0.100%, the formability and delayed fracture resistance decrease. Accordingly, the P content should be 0.100% or less, preferably 0.090% or less. The P content is more preferably 0.015% or less, and even more preferably 0.013% or less. On the other hand, there is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the base steel sheet, the P content is preferably 0.001% or more. The P content is more preferably 0.005% or more, and even more preferably 0.007% or more.

[0022] [S: 0.0200% or less] S exists as a sulfide and embrittles the underlying steel sheet, so if the S content exceeds 0.0200%, the formability and delayed fracture resistance decrease. Therefore, the S content should be 0.0200% or less, preferably 0.0050% or less. The S content is more preferably 0.0020% or less, and even more preferably 0.0015% or less. On the other hand, there is no particular lower limit for the S content, but due to production technology constraints, the S content is preferably 0.0001% or more. The S content is more preferably 0.0005% or more, and even more preferably 0.0007% or more.

[0023] [Al: 1.000% or less] Al exists as an oxide and can embrittle the underlying steel sheet, so if the Al content exceeds 1.000%, the formability and delayed fracture resistance may decrease. Therefore, the Al content should be 1.000% or less, preferably 0.500% or less. The Al content is more preferably 0.200% or less, and even more preferably 0.100% or less. On the other hand, there is no particular lower limit for the Al content, but since Al suppresses carbide formation during continuous annealing and promotes the formation of retained austenite, the Al content is preferably 0.001% or more. The Al content is more preferably 0.010% or more, and even more preferably 0.020% or more.

[0024] [N: 0.0100% or less] N exists as a nitride, which embrittles the underlying steel sheet, and if the N content exceeds 0.0100%, the formability and delayed fracture resistance decrease. Therefore, the N content should be 0.0100% or less, preferably 0.0085% or less. The N content is more preferably 0.0070% or less, and even more preferably 0.0060% or less. On the other hand, there is no particular lower limit for the N content, but due to production technology constraints, the N content is preferably 0.0001% or more. The N content is more preferably 0.0010% or more, and even more preferably 0.0020% or more.

[0025] [O: 0.0100% or less] O exists as an oxide and embrittles the underlying steel sheet. When the O content exceeds 0.0100%, the formability and delayed fracture resistance decrease. Therefore, the O content should be 0.0100% or less, preferably 0.0090% or less. More preferably, the O content is 0.0080% or less, and even more preferably 0.0070% or less. On the other hand, there is no particular lower limit for the O content, but due to production technology constraints, it is preferable that the O content be 0.0001% or more. More preferably, the O content is 0.0010% or more, and even more preferably 0.0015% or more.

[0026] [Si / Mn < 0.50] Si-based oxides are a surface oxide of steel sheets that significantly degrades the chemical conversion treatment properties. Therefore, in order to form Mn-containing oxides that are readily soluble in acid solutions, the Si / Mn ratio, which is the ratio of Sn content (mass%) to Mn content (mass%), is set to less than 0.50. That is, in the present invention, Si / Mn < 0.50. Preferably, Si / Mn is 0.40 or less, and more preferably 0.35 or less. The lower limit is not particularly limited, but Si / Mn is preferably 0.15 or more, and more preferably 0.20 or more.

[0027] The component composition of the steel sheet in the present invention contains the above-mentioned component elements as basic components, with the remainder being Fe and unavoidable impurities. Here, it is preferable that the steel sheet of the present invention has a component composition consisting of the above-mentioned basic components, with the remainder being Fe and unavoidable impurities.

[0028] (Optional components of steel sheet) In addition to the basic components mentioned above, steel sheet may contain the following components in mass percent: As: 0.100% or less, 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, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.050% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, C It may contain at least one selected from the group consisting of a: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Zn: 0.100% or less, Pb: 0.100% or less, Te: 0.100% or less, Se: 0.020% or less, Ga: 0.020% or less, Ge: 0.020% or less, Sr: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

[0029] [As: 0.100% or less] If the As content exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, reducing the ultimate deformability of the steel and resulting in inferior hole-expanding properties. Therefore, when As is added, the As content should be 0.100% or less. Preferably, the As content is 0.090% or less. More preferably, the As content is 0.070% or less. Adding an appropriate amount of As suppresses the formation of sulfides and improves hole-expanding properties. For this reason, it is preferable that the As content be 0.001% or more. More preferably, it is 0.010% or more.

[0030] [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, a large amount of coarse precipitates or inclusions will not be generated, and the ultimate deformability of the steel sheet (base steel sheet) will not decrease, so the formability and delayed fracture resistance will not decrease. Therefore, when one or more of Ti, Nb, and V are included, the content of each is 0.200% or less, and preferably 0.100% or less. The V content is more preferably 0.080% or less, and even more preferably 0.060% or less. On the other hand, there is no particular lower limit for the content of Ti, Nb, and V, but since these elements increase the strength of the base steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, it is preferable that the content of Ti, Nb, and V be 0.001% or more. The Ti content is more preferably 0.010% or more, and even more preferably 0.020% or more. The Nb content is more preferably 0.015% or more, and even more preferably 0.020% or more. The V content is more preferably 0.005% or more, and even more preferably 0.020% or more.

[0031] [Ta: 0.10% or less, W: 0.10% or less] If the content of Ta and W is 0.10% or less, a large amount of coarse precipitates or inclusions will not be generated, and the underlying steel sheet will not become brittle, so the formability and delayed fracture resistance will not decrease. Therefore, if one or more of Ta and W are included, their content should be 0.10% or less, and preferably 0.08% or less. On the other hand, there is no particular lower limit for the content of Ta and W, but since these elements increase the strength of the underlying steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing, it is preferable that the content of Ta and W be 0.01% or more.

[0032] [B: 0.0100% or less] If the B content is 0.0100% or less, it does not cause cracks to form inside the steel sheet during casting or hot rolling, and does not embrittle the base steel sheet, so the formability and delayed fracture resistance do not decrease. Therefore, when B is included, the B content should be 0.0100% or less, and preferably 0.0080% or less. On the other hand, there is no particular lower limit for the B content, but since B is an element that segregates at the austenite grain boundaries during annealing and improves hardenability, it is preferable that the B content be 0.0003% or more. The B content is more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0033] [Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less] If the content of Cr, Mo, and Ni is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the underlying steel sheet will not become brittle, so the formability and delayed fracture resistance will not decrease. Therefore, when one or more of Cr, Mo, and Ni are included, the content of each is 1.00% or less, and preferably 0.80% or less. The Cr content is more preferably 0.20% or less. On the other hand, there is no particular lower limit for the content of Cr, Mo, and Ni, but since these elements improve hardenability, it is preferable that the content of Cr, Mo, and Ni is 0.01% or more, each. The Cr content is more preferably 0.010% or more, and even more preferably 0.020% or more. The Ni content is more preferably 0.010% or more, and even more preferably 0.030% or more.

[0034] [Co: 0.050% or less] If the Co content is 0.050% or less, the amount of coarse precipitates or inclusions will not increase, and the underlying steel sheet will not become brittle, so the formability and delayed fracture resistance will not decrease. Therefore, when Co is included, the Co content should be 0.050% or less, and preferably 0.010% or less. On the other hand, there is no particular lower limit for the Co content, but since Co improves hardenability, the Co content should preferably be 0.001% or more. The Co content is more preferably 0.003% or more, and even more preferably 0.005% or more.

[0035] [Cu: 1.00% or less] If the Cu content is 1.00% or less, the amount of coarse precipitates or inclusions will not increase, and the underlying steel sheet will not become brittle, so the formability and delayed fracture resistance will not decrease. Therefore, when Cu is included, the Cu content should be 1.00% or less, and preferably 0.80% or less. On the other hand, there is no particular lower limit for the Cu content, but since Cu improves hardenability, it is preferable that the Cu content be 0.01% or more. The Cu content is more preferably 0.03% or more, and even more preferably 0.05% or more.

[0036] [Sn: 0.200% or less] If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the underlying steel sheet will not become brittle, thus not reducing formability or delayed fracture resistance. Therefore, when Sn is included, the Sn content should be 0.200% or less, and preferably 0.100% or less. On the other hand, there is no particular lower limit for the Sn content, but since Sn improves hardenability, the Sn content should preferably be 0.001% or more. The Sn content is more preferably 0.030% or more, and even more preferably 0.050% or more.

[0037] [Sb: 0.200% or less] If the Sb content is 0.200% or less, the amount of coarse precipitates and inclusions does not increase, and the underlying steel sheet does not become brittle, so the formability and delayed fracture resistance do not decrease. Therefore, when Sb is included, its content should be 0.200% or less, and preferably 0.100% or less. On the other hand, there is no particular lower limit for the Sb content, but since Sb controls the surface softening thickness and allows for strength adjustment, it is preferable that the Sb content be 0.001% or more. The Sb content is more preferably 0.010% or more, and even more preferably 0.015% or more.

[0038] [Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less] If the content of Ca, Mg, and REM is 0.0100% or less, the amount of coarse precipitates and inclusions will not increase, and the underlying steel sheet will not become brittle, so the formability and delayed fracture resistance will not decrease. Therefore, if one or more of Ca, Mg, and REM are included, the content of each should be 0.0100% or less, and preferably 0.0050% or less. On the other hand, there is no particular lower limit for the content of Ca, Mg, and REM, but since these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformability of the underlying steel sheet, it is preferable that the content of Ca, Mg, and REM be 0.0005% or more. The Ca content is more preferably 0.0010% or more, and even more preferably 0.0020% or more. The Mg content is more preferably 0.0010% or more, and even more preferably 0.0020% or more. The REM content is more preferably 0.0010% or more, and even more preferably 0.0020% or more. In this invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanide elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this invention, the REM content is the total content of one or more elements selected from the above-mentioned REMs. The REM is not particularly limited, but it is preferably at least one of Sc, Y, Ce, and La.

[0039] [Zr: 0.100% or less, Zn: 0.100% or less, Pb: 0.100% or less, Te: 0.100% or less] If the content of Zr, Zn, Pb, and Te is 0.100% or less, the amount of coarse precipitates or inclusions will not increase, and the underlying steel sheet will not become brittle, so the formability and delayed fracture resistance will not decrease. Therefore, if one or more of Zr, Zn, Pb, and Te are included, the content of each should be 0.100% or less, and preferably 0.080% or less. On the other hand, there is no particular lower limit for the content of Zr, Zn, Pb, and Te, but since these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformability of the underlying steel sheet, it is preferable that the content of Zr, Zn, Pb, and Te be 0.001% or more. The Zr content is more preferably 0.025% or more, and even more preferably 0.040% or more. The Zn content is more preferably 0.008% or more, and even more preferably 0.016% or more. The Pb content is more preferably 0.008% or more, and even more preferably 0.020% or more. The Te content is more preferably 0.010% or more, and even more preferably 0.015% or more.

[0040] [Se: 0.020% or less, Ga: 0.020% or less, Ge: 0.020% or less, Sr: 0.020% or less] If the content of Se, Ga, Ge, and Sr is 0.020% or less, the amount of coarse precipitates or inclusions will not increase, and the underlying steel sheet will not become brittle, so the formability and delayed fracture resistance will not decrease. Therefore, if one or more of Se, Ga, Ge, and Sr are included, their content should be 0.020% or less. On the other hand, there is no particular lower limit for the content of Se, Ga, Ge, and Sr, but since these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformability of the underlying steel sheet, it is preferable that the content of Se, Ga, Ge, and Sr be 0.001% or more. The Ga content is more preferably 0.002% or more.

[0041] [Hf: 0.10% or less] If the Hf content is 0.10% or less, the amount of coarse precipitates or inclusions does not increase, and the underlying steel sheet does not become brittle, so the formability and delayed fracture resistance do not decrease. Therefore, when Hf is included, the Hf content should be 0.10% or less, and preferably 0.08% or less. On the other hand, there is no particular lower limit for the Hf content, but since Hf spheroidizes the shape of nitrides or sulfides, etc., and improves the ultimate deformability of the underlying steel sheet, the Hf content should preferably be 0.01% or more. The Hf content is more preferably 0.02% or more.

[0042] [Bi: 0.200% or less] If the Bi content is 0.200% or less, the amount of coarse precipitates or inclusions will not increase, and the underlying steel sheet will not become brittle, so the formability and delayed fracture resistance will not decrease. Therefore, when Bi is included, the Bi content should be 0.200% or less, and preferably 0.100% or less. On the other hand, there is no particular lower limit for the Bi content, but since Bi reduces segregation, it is preferable that the Bi content be 0.001% or more. The Bi content is more preferably 0.004% or more, and even more preferably 0.008% or more.

[0043] Furthermore, regarding the above-mentioned As, Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Zn, Pb, Te, Se, Ga, Ge, Sr, Hf, and Bi, if the content of each is below its respective preferred lower limit, the effects of the present invention will not be impaired. Therefore, the content of each may be below its respective preferred lower limit, in which case it will be treated as an unavoidable impurity.

[0044] (Surface condition of the steel sheet) Next, the surface condition of the steel sheet of the present invention will be described. [Surface coverage rate of Si-based oxides on the steel sheet: 1% or less] Oxides mainly composed of Si (Si-based oxides) are poorly soluble in chemical conversion treatment solutions, and if Si-based oxides are present on the surface of the steel sheet, the chemical conversion treatment performance is significantly reduced. Therefore, the surface coverage rate of Si-based oxides on the steel sheet is set to 1% or less. Preferably, it is 0%. Here, the surface coverage rate of the steel sheet refers to the area percentage (%) occupied by Si-based oxides on the surface of the steel sheet. Note that Si-based oxides are, for example, SiO ２ MnSiO ３ While these can be cited, in this invention, Mn-rich Si-Mn composite oxides are readily soluble in the chemical conversion treatment solution, and even if they are present on the surface, they do not degrade the chemical conversion treatment properties, so they are not considered to be Si-based oxides. Here, the surface coverage of Si-based oxides is determined by observing five fields of view of the steel plate surface using a SEM at 1000x magnification with a field of view of 87 μm × 128 μm, and analyzing the same fields of view with EDX to identify Si-based oxides, and then determining the coverage using the point counting method.

[0045] (Structure of the steel sheet) Next, the structure of the steel sheet will be described. The steel sheet has a structure at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet (base steel sheet), in which the area fraction of ferrite is 3% or more and 30% or less, the area fraction of tempered martensite is 60% or more and 94% or less, the volume fraction of retained austenite is 3% or more and 15% or less, and the area fraction of the remaining structure is 10% or less. The ferrite described above includes low-Mn ferrite, in which the Mn concentration is less than 0.85 times the Mn content of the steel sheet, and high-Mn ferrite, in which the Mn concentration is 0.85 times or more the Mn content of the steel sheet. Of the ferrite described above, the proportion of high-Mn ferrite is 20% or more and 90% or less in terms of area fraction. The structure described below is the structure at a depth of 1 / 4 of the sheet thickness from the surface of the base steel sheet.

[0046] [Area fraction of ferrite: 3% or more and 30% or less] To ensure high ductility, the area fraction of ferrite should be 3% or more. Preferably, it should be 5% or more, and more preferably 7% or more. On the other hand, to obtain the desired strength, the area fraction of ferrite should be 30% or less. Preferably, it should be 27% or less, and more preferably 25% or less. The method for measuring ferrite is as follows: After polishing the L-section of the steel plate, it is etched with 3 vol. % nital, and the position at 1 / 4 of the plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate) is observed using an SEM at 2000x magnification with a field of view of 44 μm × 64 μm, and 10 fields of view are observed, and the area fraction is measured by image processing. In the above microstructure image, ferrite is a concave structure with a flat interior. The area fraction of ferrite can be determined from the average value of these values.

[0047] [Area fraction of high-Mn ferrite in the ferrite: 20% or more and 90% or less] By generating high-Mn ferrite, the Mn concentration in the matrix martensite can be reduced. This prevents deterioration of delayed fracture resistance due to MnS and Mn segregation. In addition, high-Mn ferrite is a relatively hard ferrite and achieves a high YS. Furthermore, since it grows epitaxially from the interface between low-Mn ferrite and austenite formed during annealing, it reduces the interface between low-Mn ferrite and the matrix martensite, which have a large hardness difference, and suppresses the formation of voids during processing. In order to ensure a high YS, a high hole expansion rate, and excellent delayed fracture resistance, the area fraction of high-Mn ferrite in the ferrite should be 20% or more. Preferably it is 25% or more, and more preferably 30% or more. On the other hand, in order to ensure the desired ductility, the area fraction of high-Mn ferrite in the ferrite should be 90% or less. Preferably it is 85% or less, and more preferably 80% or less.

[0048] High-Mn ferrite refers to ferrite whose Mn concentration is 0.85 times or more the Mn content of the steel sheet. Low-Mn ferrite refers to ferrite whose Mn concentration is less than 0.85 times the Mn content of the steel sheet.

[0049] The method for measuring the Mn concentration in ferrite is as follows: First, the obtained steel sheet was polished using diamond paste so that the cross section parallel to the rolling direction (L section) would be the observation surface. The observation surface was polished to a mirror finish using alumina polishing, and then cleaned using a plasma cleaner to remove hydrocarbon contamination (carbon contamination: referred to as "contamination") from the sample surface.

[0050] The cleaned observation surface was measured using an electron probe microanalyzer (FE-EPMA) equipped with a field emission electron gun. The measurement conditions followed those described in Non-Patent Literature (T. Yamashita, Y. Tanaka, M. Nagoshi and K. Ishida: Sci. Rep., 6 (2016), DOI: 10.1038 / srep29825), with an acceleration voltage of 7 kV and a current of 50 nA. The measured data was converted to Mn concentration using a calibration method, and an elemental mapping image of Mn was obtained. In the mapping measurement, the sample was heated to 100°C and held while being measured, ensuring that the measurement was performed under conditions that prevented contamination. In the obtained elemental mapping images, regions where the Mn concentration was less than 0.85 times or greater than 0.85 times the Mn concentration of the steel plate were identified. More specifically, ferrite grains were identified by referring to the elemental mapping images and SEM images of the same field of view, and low-Mn ferrite and high-Mn ferrite were identified.

[0051] [Area fraction of tempered martensite: 60% or more and 94% or less] In order to ensure a tensile strength of 980 MPa or more, the area fraction of tempered martensite should be 60% or more. Preferably, the area fraction of tempered martensite should be 65% or more, and more preferably 70% or more. On the other hand, if the total area fraction of tempered martensite exceeds 94%, it becomes difficult to achieve excellent ductility. Therefore, the area fraction of tempered martensite should be 94% or less. Preferably, it should be 92% or less, and more preferably 90% or less.

[0052] The measurement method for tempered martensite is as follows: After polishing the L-section of the steel plate, it is etched with 3 vol. % nital. Ten fields of view are observed using a SEM at a magnification of 2000x, with a field of view of 44 μm × 64 μm, at a position corresponding to 1 / 4 of the plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate), and the area fraction is measured by image processing. In the above microstructure image, tempered martensite is observed as a white, fine structure containing carbides. The area fraction of tempered martensite can be determined from the average value of these values.

[0053] [Volume fraction of retained austenite: 3% or more and 15% or less] In order to ensure high ductility, the volume fraction of retained austenite should be 3% or more. Preferably, the volume fraction of retained austenite should be 4% or more, and more preferably 5% or more. On the other hand, if the volume fraction of retained austenite exceeds 15%, the area fraction of tempered martensite decreases and a tensile strength of 980 MPa or more cannot be achieved. Therefore, the volume fraction of retained austenite should be 15% or less. Preferably, it should be 13% or less, and more preferably 10% or less.

[0054] The method for measuring retained austenite is as follows: The retained austenite fraction was determined by polishing a steel plate from 1 / 4 of its thickness down to a surface of 0.1 mm, and then further polishing it by 0.1 mm using chemical polishing. Using an X-ray diffractometer with CoKα rays, the integral intensity ratios of the diffraction peaks of the {200}, {220}, {311} planes of fcc iron and the {200}, {211}, {220} planes of bcc iron were measured, and the nine resulting integral intensity ratios were averaged to determine the retained austenite fraction. Assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite can be expressed as the area fraction of retained austenite.

[0055] [Area fraction of residual structure: 10% or less] The steel structure of the present invention may include pearlite, fresh martensite, and other known steel sheet structures as residual structures. The effects of the present invention will not be impaired if the area fraction of the residual structure is 10% or less. Therefore, the area fraction of the residual structure is set to 10% or less. The lower limit of the area fraction of the residual structure is not particularly limited and may be 0%, but the area fraction of the residual structure may be 1% or more, or 2% or more.

[0056] The method for measuring the area fraction of the remaining structure is as follows: The area fraction of the remaining structure can be determined by measuring the area fraction of ferrite, the area fraction of tempered martensite, and the volume fraction of retained austenite, as mentioned above, and subtracting their sum from 100%. Note that in the subtraction, the volume fraction of retained austenite can be expressed as an area fraction (for example, volume fraction: 10% = area fraction: 10%). Therefore, the volume fraction of retained austenite is also subtracted from 100% along with the area fractions of ferrite and tempered martensite.

[0057] The steel sheet of the present invention may have a plating layer on its surface (at least one side). Examples of plating layers include zinc plating layers such as a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electroplated zinc layer. Examples of plating layers other than zinc plating layers include aluminum plating layers and alloy plating layers. Examples of alloy plating layers include a hot-dip zinc-aluminum-magnesium alloy plating layer and a Zn-Ni electroalloy plating layer.

[0058] The thickness of the steel plate of the present invention is preferably 0.5 mm or more. Furthermore, the thickness is preferably 3.0 mm or less.

[0059] (Properties of Steel Sheet) [Tensile Strength (TS): 980 MPa or more] The steel sheet has a tensile strength (TS) of 980 MPa or more. The tensile strength can be determined as follows. A JIS No. 5 test piece (gauge length 50 mm, parallel portion width 25 mm) is taken from the test material so that the rolling direction and the perpendicular direction are the longitudinal direction of the test piece, and a tensile test is performed in accordance with JIS Z 2241 (2011). The conditions for the tensile test can be such that the crosshead speed is 1.67×10 －１ mm / second.

[0060] [YS×λ: 30000 MPa×% or more] In order to have excellent collision properties, a high YS-λ balance is required. Therefore, it is preferable that YS×λ is 30000 MPa×% or more. Note that YS can be determined by the tensile test described above, similar to TS. λ can be determined as follows. A hole expansion test is performed in accordance with JIS Z 2256 (2020). After shearing the test material into 100 mm×100 mm, a hole with a diameter of 10 mm is punched out with a clearance of 12.5%. With a die having an inner diameter of 75 mm and the test material being held with a holding force of 9 ton (88.26 kN), a conical punch with a vertex angle of 60° is pushed into the punched hole, and the occurrence of cracks penetrating the plate thickness is confirmed. The hole diameter at the time of crack occurrence is measured, and the limiting hole expansion rate: λ (%) can be determined from the following formula (3). λ (%) = { (D ｆ - D ０ ) / D ０} × 100 ... (3) Here, D ｆ is the hole diameter (mm) at the time of crack occurrence, and D ０ is the initial hole diameter (mm).

[0061] [El×λ: 600%×% or more] In order to have excellent workability, a high El-λ balance is required. Therefore, for the steel sheet of the present invention, El×λ is 600%×% or more. Note that the elongation El can be determined by the tensile test described above, similar to TS.

[0062] [Conversion Property Evaluation] For the steel sheet after annealing, 20 to 35 A / dm ２The material was degreased and surface-conditioned by sulfuric acid electrolytic pickling for 2 seconds at a current density, followed by chemical conversion treatment using zinc phosphate chemical conversion solution. Degreasing process: treatment temperature 40°C, treatment time 120 seconds, spray degreasing. Surface conditioning process: pH 9.5, treatment temperature room temperature, treatment time 20 seconds. Chemical conversion treatment process: chemical conversion solution temperature 35°C, treatment time 120 seconds. The treatment agents used in the degreasing, surface conditioning, and chemical conversion processes were, in order, FC-E2011 (degreasing agent), PL-X (surface conditioning agent), and Palbond PB-L3065 (chemical conversion solution), all manufactured by Nippon Parkerizing Co., Ltd. Magnification: 2000x, 10000 μm ２ Surface chemical formation was observed by SEM observation in the above areas. If the area where chemical formation tissue was not formed was 10% or less of the total measurement area, it was evaluated as ○ (pass), and if the area where chemical formation tissue was not formed was more than 10% of the total measurement area, it was evaluated as × (fail).

[0063] [Delayed Fracture Resistance] In the evaluation method for the delayed fracture characteristics of metallic materials (HeTsAce), the steel sheet of the present invention can be judged to have excellent delayed fracture resistance in a corrosive environment (HeTsAce) in the evaluation method for the delayed fracture characteristics of metallic materials in a corrosive environment (HeTsAce). Specifically, in the evaluation method for the delayed fracture characteristics of metallic materials (HeTsAce) shown below, the steel sheet of the present invention has a cracking period of 63 days or more.

[0064] (HeTsAce) The amount of chloride adhering to the evaluation surface of the metal material being evaluated is 1,000 to 20,000 mg / m². ２ The process comprises: a chloride adhesion step (A) in which droplets of a chloride-containing aqueous solution are attached in such a manner; and a corrosion step (B) in which a cycle consisting of the following steps is performed at least once, in an atmosphere at a temperature Tb1 of 60°C or lower and within a certain range, and the distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first chloride adhesion step (A) is such that the average contact area of ​​the droplets on the evaluation surface of the metal material is 0.1 mm. ２ 3.0 mm ２Less than , area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material: 40% or more and 80% or less, and standard deviation of the contact area of ​​the droplets on the evaluation surface of the metal material: 3.0 mm ２ The following is a method for evaluating the delayed fracture characteristics of a metallic material. Drying step (b1): A step of drying the metallic material by holding it in an atmosphere with a relative humidity Hb1 of 45% or less for 1.0 hour to 5.0 hours. Wetting step (b2): A step of wetting the metallic material by holding it in an atmosphere with a relative humidity Hb2 of 80% or more for 1.0 hour to 5.0 hours. Transition step (b3): ​​A step of transitioning from the atmosphere with relative humidity Hb1 to the atmosphere with relative humidity Hb2 at a rate of change of relative humidity of 30% / h or less. Transition step (b4): A step of transitioning from the atmosphere with relative humidity Hb2 to the atmosphere with relative humidity Hb1 at a rate of change of relative humidity of 30% / h or less.

[0065] In this invention, a metallic material sampled from a steel plate is used for evaluation. The method for sampling the metallic material from the steel plate is not particularly limited. For example, the metallic material can be sampled by shearing the steel plate to a predetermined size. In HeTsAce, it is preferable to apply stress to the metallic material to be evaluated. Methods for applying stress to the metallic material include processing the metallic material (processing method). Examples of such processing methods include bending, bulging, tensioning, and torsion. Other methods include fixing the material in a stressed shape using bolts, etc., and using residual stress remaining after processing. In the evaluation method of this invention, the metallic material to which stress has been applied as described above can be subjected to at least one (one or two or more) steps comprising a chloride deposition step (A) and a corrosion step (B). Furthermore, in the evaluation method of this invention, after performing the steps comprising the chloride deposition step (A) and the corrosion step (B) once or more, the state of the metallic material can be checked, and the delayed fracture characteristics of the metallic material can be evaluated based on the checked state of the metallic material. The aforementioned verification can be carried out, for example, by visually observing the presence and extent of cracks in the metal material.

[0066] In this invention, in order to evaluate the delayed fracture characteristics of a metallic material, a chloride deposition process (A) and a corrosion process (B) are performed on the metallic material while it is under stress. After each of these processes is performed at least once, the presence and extent of cracks in the metallic material are checked to evaluate the delayed fracture characteristics.

[0067] (Chloride deposition process (A)) In the chloride deposition process (A), the amount of chloride deposited on the evaluation surface of the metal material is 1,000 to 20,000 mg / m². ２ This is a step of attaching droplets of a chloride-containing aqueous solution to the metal material. Furthermore, in the evaluation method of the present invention, the distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material (droplet attachment distribution) in at least the first (1st) chloride attachment step (A) is such that the average contact area of ​​the droplets on the evaluation surface of the metal material is 0.1 mm. ２ 3.0 mm ２ Less than , area ratio of total droplet contact area to area of ​​evaluation surface of metal material: 40% or more and 80% or less, and standard deviation of droplet contact area on evaluation surface of metal material: 3.0 mm ２ The following applies:

[0068] <Chloride deposition amount: 1000-20000 mg / m ２ > The amount of chloride to be attached to the metal material (amount of solid chloride excluding solvents such as water) is 1,000 to 20,000 mg / m². ２ The amount of adhesion is assumed to correspond to the amount of chloride adhesion expected in the atmospheric corrosion environment in which actual automobiles operate. The amount of adhesion is 1000 mg / m². ２ In corrosive environments below a certain level, corrosion hardly progresses, resulting in minimal hydrogen generation and hydrogen penetration into the metal material, making delayed fracture less likely. On the other hand, the aforementioned adhesion amount is 20,000 mg / m². ２ If the amount exceeds this limit, it deviates significantly from corrosion in a real environment, resulting in an excessive durability test that does not serve its purpose. Therefore, the aforementioned adhesion amount is 20,000 mg / m². ２ The following applies: From the viewpoint of simulating the corrosion morphology in the atmospheric corrosion environment in which actual automobiles operate, and promoting corrosion, the amount of adhesion is 5000 mg / m². ２ It is preferable that it exceeds 12,000 mg / m². Also, from the above viewpoint, the amount of adhesion is 12,000 mg / m². ２ The following are preferable.

[0069] The amount of chloride deposited can be calculated by multiplying the mass difference of the test specimen (metal material) before and after the application of the chloride-containing aqueous solution in the chloride deposition process (A) by the chloride concentration of the chloride-containing aqueous solution and dividing by the evaluation surface area of ​​the test specimen. When measuring the mass difference, if the chloride-containing aqueous solution adheres to areas other than the evaluation surface of the test specimen, appropriate measures can be taken, such as masking the areas other than the evaluation surface or wiping off the chloride-containing aqueous solution that has adhered to the areas other than the evaluation surface. Furthermore, to change the amount of chloride deposited, it can be controlled by changing the chloride concentration of the chloride-containing aqueous solution or by changing the application time of the chloride-containing aqueous solution (process time of the chloride deposition process (A)) to change the amount of chloride-containing aqueous solution applied to the metal material.

[0070] In the chloride deposition process (A), chlorides are deposited onto the metal material to obtain the desired amount of chloride deposition. The chlorides include sodium salts (NaCl), potassium salts (KCl), and calcium salts (CaCl), which are commonly present in the atmospheric environment in which metal materials are used. ２ ), magnesium salt (MgCl ２ It is preferable to include one or more selected from the above. In the chloride deposition step (A), when depositing chloride onto the metal material, a chloride-based component containing chloride and components other than chloride may be deposited. Here, a chloride-based component refers to a component in which chloride accounts for more than 50% by mass of the total components on a solid content basis. Examples of components other than chloride include sulfides and nitrate compounds, but are not limited to these. Considering the actual atmospheric corrosion environment, it is preferable to deposit a component mainly composed of NaCl (a component in which NaCl accounts for more than 50% by mass of the total components) onto the metal material.

[0071] Furthermore, when simulating delayed fracture characteristics in areas where de-icing agents are frequently applied during winter, it is preferable to use a chloride attached to the metal material that has a composition similar to that of the de-icing agent applied in that area. A component with a composition similar to that of a de-icing agent is CaCl ２ The main component is (CaCl ２(Components that make up more than 50% by mass of all components), MgCl ２ The main component is (MgCl ２ Examples include components in which NaCl makes up more than 50% by mass of the total components, and components mainly composed of NaCl (components in which NaCl makes up more than 50% by mass of the total components).

[0072] Furthermore, a component containing a combination of multiple metal salts may be used as the chloride to be attached to the metal material. An example of a component containing a combination of multiple metal salts is, for example, the Society of Automotive Engineers standard (SAE J2334) (0.5 mass% NaCl-0.1 mass% CaCl ２ -0.075% by mass NaHCO ３ ), artificial seawater (2.5% by mass NaCl-0.5% by mass MgCl ２ -0.12% by mass CaCl ２ Examples include -0.07% by mass of KCl and others (for example, an aqueous solution of Aquamarine® manufactured by Yashima Pharmaceutical Co., Ltd.).

[0073] The method for depositing chlorides onto a metal material (chloride deposition method) is not particularly limited as long as it is a method that can achieve the desired distribution of chloride-containing aqueous solution droplets on the evaluation surface of the metal material. Examples of the chloride-containing aqueous solution include a chloride-containing aqueous solution containing a chloride-based component (usually an aqueous solution such as saltwater, hereinafter also referred to as saltwater). The following explanation will use the case where saltwater is used as the chloride-containing aqueous solution as an example.

[0074] One method for attaching chlorides is the spray method. The spray method involves applying saltwater using a spray nozzle. Types of spray nozzles include single-fluid spray nozzles (nozzles that atomize and spray liquid delivered under pressure) and two-fluid spray nozzles (nozzles that atomize liquid using a high-speed fluid such as compressed air). Two-fluid spray nozzles also have different liquid supply methods, such as pressurized liquid type (liquid is pressurized and supplied to the two-fluid nozzle) and suction type (liquid is drawn up and sprayed using the force of compressed air). It is preferable to select a spray nozzle so that the droplet attachment distribution is uniform. Also, since saltwater is used, it is preferable to use corrosion-resistant metals such as stainless steel for the spray nozzle material.

[0075] The chloride concentration in the saltwater is not particularly limited. However, when controlling the droplet distribution of the saltwater using a spray nozzle, if saltwater with a chloride concentration of less than 2.0% by mass is used to adhere the saltwater to the metal material, the spraying time becomes longer to obtain a suitable amount of chloride adhesion, making it difficult to obtain the desired droplet distribution on the evaluation surface of the metal material. For this reason, it is preferable that the chloride concentration in the saltwater be 2.0% by mass or higher, and more preferably 5.0% by mass or higher. On the other hand, if saltwater with a chloride concentration exceeding 20% ​​by mass is used to adhere the saltwater to the metal material, chloride is more likely to precipitate in the spray nozzle, causing clogging of the spray nozzle, making it difficult to spray saltwater droplets of a stable size, and making it difficult to obtain the desired droplet distribution on the evaluation surface of the metal material. As a result, the amount of chloride adhesion on the evaluation surface of the metal material varies from place to place, and in areas with a large amount of chloride adhesion, localized corrosion occurs and the amount of hydrogen penetration increases. Therefore, the delayed fracture characteristics change within the evaluation surface of the metal material, and the accuracy of evaluating the delayed fracture characteristics decreases. This tendency is particularly pronounced when the amount of chloride adhesion is high. Therefore, the chloride concentration in the saltwater is preferably 20% by mass or less, and more preferably 15% by mass or less.

[0076] Furthermore, it is preferable to adjust the chloride concentration in the brine according to the desired amount of chloride deposition. A low-concentration brine is preferable under conditions where the amount of chloride deposition is relatively small, while a high-concentration brine is preferable under conditions where the amount of chloride deposition is relatively large. In order to reduce the amount of chloride deposition on the evaluation surface of the metal material using a high-concentration brine, the total amount of brine sprayed must be reduced, which tends to result in an underestimation of the average contact area of ​​the brine droplets adhering to the evaluation surface of the metal material and the ratio of the total contact area of ​​the brine droplets to the total area of ​​the evaluation surface. On the other hand, in order to increase the amount of chloride deposition on the evaluation surface of the metal material using a low-concentration brine, the total amount of brine sprayed must be increased, which tends to result in an underestimation of the average contact area of ​​the brine droplets adhering to the evaluation surface of the metal material and the ratio of the total contact area of ​​the brine droplets to the area of ​​the evaluation surface.

[0077] [Distribution of chloride-containing aqueous solution droplets attached to the evaluation surface of the metal material in the first chloride attachment step (A)] In the evaluation method of the present invention, it is necessary to control the distribution of saltwater droplets on the evaluation surface of the metal material in at least the first (i.e., performed at least once) chloride attachment step (A) to the range described below. In the first chloride attachment step, the areas where saltwater was present on the evaluation surface of the metal material are considered to be the starting points of corrosion on that evaluation surface. Then, in the second chloride attachment step, the saltwater spray occurs in a state where corrosion has occurred after the first chloride attachment step and corrosion products have formed on the evaluation surface, and even if the saltwater droplets are uniformly attached to the evaluation surface of the metal material, wetting will spread due to the influence of the corrosion products. For this reason, it is important to uniformly control the distribution state of saltwater droplets on the evaluation surface of the metal material in the first chloride attachment step.

[0078] Figure 1 is a schematic diagram illustrating an example of the evaluation method of the present invention. As shown in Figure 1, in the evaluation method of the present invention, saltwater is sprayed from a spray nozzle to adhere droplets of saltwater to the evaluation surface of a metal material. At that time, in the first chloride adhesion step (A), the distribution of saltwater droplets (droplet adhesion distribution) to be adhered to the evaluation surface of the metal material is controlled to a predetermined range.

[0079] <Average contact area of ​​droplets on the evaluation surface of the metal material: 0.1 mm ２ 3.0 mm ２ Less than > In the evaluation method of the present invention, in at least the first chloride deposition step (A), the average contact area of ​​the droplets on the evaluation surface of the metal material (average contact area per droplet) is set to 0.1 mm ２ 3.0 mm ２ The average contact area shall be less than 0.1 mm². ２ If the amount is less than 0.1 mm, the droplet volume is too small to reach the target amount of chloride adhesion. Therefore, the average contact area is 0.1 mm. ２ The above is the end of the explanation. The average contact area is 0.5 mm². ２ Preferably, it should be 1.0 mm or more. ２ It is more preferable to have the above. On the other hand, the average contact area is 3.0 mm ２If the above is true, the droplet adhesion distribution becomes uneven, and the variability in the delayed fracture evaluation increases. Therefore, the average contact area is 3.0 mm². ２ It shall be less than 2.8 mm². The average contact area is 2.8 mm². ２ Preferably, it should be 2.5 mm ２ The following is more preferable. The average contact area of ​​the droplet on the evaluation surface of the metal material can be measured by the measurement method described later.

[0080] <Area ratio of total droplet contact area to area of ​​evaluation surface of metal material: 40% or more and 80% or less> In the evaluation method of the present invention, in at least the first chloride adhesion step (A), the area ratio of the total droplet contact area to the area of ​​evaluation surface of metal material (total droplet contact area ratio) is set to 40% or more and 80% or less. If the total droplet contact area ratio is less than 40%, the droplet adhesion distribution becomes uneven, and the variability of the delayed fracture evaluation increases. For this reason, the total droplet contact area ratio is set to 40% or more. Preferably, the total droplet contact area ratio is 50% or more, and more preferably 55% or more. On the other hand, if the total droplet contact area ratio is greater than 80%, bonding between adjacent droplets is more likely to occur on the evaluation surface, and the average contact area of ​​the droplets (average contact area per droplet) tends to become coarser. For this reason, the total droplet contact area ratio is set to 80% or less. Preferably, the total droplet contact area ratio is 75% or less, and more preferably 70% or less. The total contact area ratio of the above droplets can be measured by the measurement method described later.

[0081] Standard deviation of the contact area of ​​liquid droplets on the evaluation surface of a metallic material: 3.0 mm ２ > In the evaluation method of the present invention, in at least the first chloride deposition step (A), the standard deviation of the distribution of the contact area of ​​the droplets on the evaluation surface of the metal material is 3.0 mm. ２ The following conditions apply: The standard deviation of the contact area of ​​the droplet on the evaluation surface of the metal material is 3.0 mm. ２ If the standard deviation is larger, the droplet adhesion distribution becomes uneven, leading to greater variability in the delayed fracture evaluation. Therefore, the standard deviation of the droplet contact area is 3.0 mm. ２The following applies: The standard deviation of the contact area of ​​the droplet is 2.8 mm. ２ Preferably, it should be 2.5 mm ２ The following is more preferable. The standard deviation of the contact area of ​​the above droplets can be measured by the measurement method described later.

[0082] The distribution of saltwater droplets adhering to the evaluation surface of a metal material (average contact area of ​​droplets, total contact area ratio of droplets, and standard deviation of the contact area of ​​droplets) can be determined in the chloride adhesion process (A) by acquiring an image of the droplet distribution across the entire evaluation surface of the metal material after the saltwater droplets have adhered to the evaluation surface of the metal material, and then performing image analysis. The image can be acquired using a digital camera, microscope, optical microscope, etc. The image can also be acquired by photographing the evaluation surface from above (from the direction of the spray nozzle shown in Figure 1). The photograph of the evaluation surface may be taken inside the test tank where the spray nozzle is installed, or it may be taken after removing the metal material from the test tank. Preferably, the latter is used, where the metal material is removed from the test tank where the spray nozzle is installed, and the evaluation surface of the metal material is photographed. The image should be acquired immediately after the saltwater droplets have adhered to the evaluation surface of the metal material (within 30 seconds).

[0083] Figure 2 schematically shows an image of the droplet distribution on the evaluation surface of the metal material acquired as described above. In Figure 2, the area indicated by a circle represents the droplet contact area. From such an image, the average contact area of ​​the droplets (average contact area per droplet), the total contact area ratio of the droplets, and the standard deviation of the droplet contact area are determined by image analysis.

[0084] Here, the evaluation surface of the metallic material is the surface on which the delayed fracture characteristics of the metallic material are evaluated. The evaluation surface can be appropriately determined depending on the metallic material being evaluated. For example, if the metallic material is a plate, the surface of the plate facing the spray nozzle can be used as the evaluation surface (see Figure 1). Also, if stress is applied to the metallic material, the surface of the stressed area facing the spray nozzle (the surface corresponding to the plan view (top view) of the metallic material when the direction in which the spray nozzle is installed relative to the metallic material is considered upward) can be used as the evaluation surface. More specifically, for example, when evaluating a bent metallic material as described later, the surface of the bent area facing the spray nozzle can be used as the evaluation surface (see Figure 7).

[0085] As a method for achieving the above-mentioned distribution of saltwater droplets, one method is to apply saltwater to the evaluation surface of the metal material using a spray nozzle, as described above. A two-fluid nozzle is preferred as the spray nozzle. Examples of two-fluid nozzles include KSMMS (product name) manufactured by Kyoritsu Alloy Manufacturing Co., Ltd., two-fluid air atomizing nozzle (product name) manufactured by Spraying Systems Japan LLC, and fine mist generating nozzle (product name) manufactured by Ikeuchi Co., Ltd.

[0086] As an example of specific conditions when using a spray nozzle, the distance from the tip of the spray nozzle to the evaluation surface of the metal material (X in Figure 1) is preferably 10 to 30 cm. The spray pressure of the spray nozzle is preferably 0.05 to 0.7 MPa. The spray angle (θ in Figure 1) is preferably 30 to 120°. The spray time of the saltwater is preferably within 10 seconds. Furthermore, as mentioned above, it is preferable to adjust the chloride concentration in the saltwater according to the target amount of chloride adhesion. Note that, as shown in Figure 7, when a bent metal material is to be evaluated, the distance from the tip of the spray nozzle to the evaluation surface of the metal material is the shortest distance from the tip of the spray nozzle to the evaluation surface of the metal material.

[0087] As a method for achieving the above-mentioned distribution of saltwater droplets, it is particularly preferable to place a shielding material having an opening between the spray nozzle and the metal material, and to allow droplets of the chloride-containing aqueous solution sprayed from the spray nozzle to adhere to the evaluation surface of the metal material through the opening of the shielding material. It is preferable that the opening of the shielding material be approximately the same shape and size as the evaluation surface of the metal material. Approximately the same means that the opening of the shielding material is equivalent to the outer circumference shape and size of the evaluation surface of the metal material when viewed from above from the spray side, or that the area of ​​the opening of the shielding material is within ±10% of the area of ​​the evaluation surface of the metal material. Furthermore, it is preferable that the shielding material be capable of shielding areas other than the evaluation surface of the metal material. That is, it is preferable that when the shielding material is viewed from above, the evaluation surface of the metal material can be seen through the opening of the shielding material, while other parts are not visible (shielded).

[0088] Figure 3 is a schematic diagram illustrating the case in the evaluation method of the present invention in which a shielding material is placed between the spray nozzle and the metal material. As shown in Figure 3, by placing the above-mentioned shielding material between the spray nozzle and the metal material, it is possible to suppress the adhesion (re-adhesion) of the spray liquid (atomized salt water) that is ejected from the spray nozzle but floats in the atmosphere without adhering to the evaluation surface of the metal material, after the salt water spraying is finished, i.e., to the evaluation surface of the metal material to which droplets have already adhered. As a result, it becomes possible to achieve a desired droplet distribution on the evaluation surface of the metal plate with high accuracy.

[0089] When using a shielding material, the distance X from the tip of the spray nozzle to the evaluation surface of the metal material is preferably 10 to 30 cm. Furthermore, the distance between the shielding material and the evaluation surface of the metal material (Y in Figure 3) is preferably 1 cm or more and 0.3X cm or less. If the distance Y is less than 1 cm, the spray liquid that does not adhere to the evaluation surface of the metal material remains in the vicinity of the evaluation surface of the metal material, so the effect of suppressing re-adhesion is reduced (Figure 4(a)). On the other hand, if the distance Y is greater than 0.3X cm, the spray liquid that passes through the opening of the shielding material scatters below the shielding material, so the effect of suppressing re-adhesion is reduced (Figure 4(b)). Note that the distance Y is the shortest distance from the shielding material to the evaluation surface of the metal material. Note that the material of the shielding material is not limited as long as it can prevent the permeation of the spray liquid. Examples of materials include resin, ceramic, metal, and wood. The shielding material can be made by processing these materials.

[0090] As described above, in the evaluation method of the present invention, it is important to uniformly control the distribution of saltwater droplets on the evaluation surface of the metal material during the first (1st) chloride deposition step (A), and this control of droplet distribution on the evaluation surface is performed at least during the first chloride deposition step (A). Furthermore, the average contact area of ​​the droplets, the total contact area ratio of the droplets, and the standard deviation of the droplet contact area can be measured at the time the first chloride deposition step (A) is completed (within 30 seconds after the end of saltwater spraying). Alternatively, conditions may be set in advance using a test specimen separate from the test specimen to be used for the test, so that the average contact area of ​​the droplets, the total contact area ratio of the droplets, and the standard deviation of the droplet contact area are predetermined, and the evaluation test of the test specimen may be performed under the same conditions. In the second and subsequent chloride deposition steps (A), chloride deposition may be performed under the same conditions as the first chloride deposition step (A), or chloride deposition may be performed under different conditions from the first chloride deposition step (A), as long as the predetermined amount of chloride deposition can be obtained. Preferably, conditions are set in advance to determine the average contact area of ​​the droplets, the total contact area ratio of the droplets, and the standard deviation of the contact area of ​​the droplets, and the first chloride adhesion step (A) is performed under these set conditions. If the chloride adhesion step (A) is performed two or more times, it is preferable to perform the second and subsequent chloride adhesion steps (A) under the same set conditions.

[0091] The chloride deposition process (A) is preferably carried out in an atmosphere with a relative humidity of 30% or higher (Ha1). If the relative humidity Ha1 in the chloride deposition process (A) is less than 30%, the droplets of the chloride-containing aqueous solution tend to dry out before reaching the evaluation surface of the metal material, especially when droplets are sprayed using a spray nozzle. As a result, it may become difficult to control the distribution of droplets to obtain the desired distribution on the evaluation surface of the metal material. Furthermore, the chloride deposition process (A) is preferably carried out in an atmosphere with a relative humidity of 80% or lower. If the relative humidity in the chloride deposition process (A) is greater than 80%, the droplets that adhere to the metal material tend to become coarser.

[0092] Furthermore, the chloride deposition process (A) is preferably carried out in an atmosphere with a temperature Ta1 of 50°C or lower. If the temperature Ta1 in the chloride deposition process (A) exceeds 50°C, the droplets of the chloride-containing aqueous solution tend to dry out before reaching the evaluation surface of the metal material, especially when droplets are sprayed using a spray nozzle. As a result, it may become difficult to control the distribution of droplets to obtain the desired distribution on the evaluation surface of the metal material. On the other hand, the lower limit of the temperature Ta1 is not limited as long as the saltwater state can be maintained. As an example, the lower limit of the temperature Ta1 is 25°C.

[0093] A characteristic of atmospheric corrosion environments is the repeated cycles of wetting (humid) and drying (dry) states. Simulating these environmental changes is crucial for simulating corrosion patterns in real-world environments where vehicles operate. For example, in the case of steel materials, the corrosion products formed on the steel material change depending on whether it is wet or dry, and it is known that hydrogen is generated during the transition from wet to dry or vice versa. Therefore, the conditions in the cycle of relative humidity change (corrosion process (B)) are also important in evaluating delayed fracture characteristics.

[0094] (Corrosion process (B)) The corrosion process (B) is a process in which a cycle consisting of the following drying process (b1), the following wetting process (b2), the following transition process (b3), and the following transition process (b4) is performed in an atmosphere with a temperature Tb1 of 60°C or less and within a certain range, and this cycle is performed at least once (once or twice or more).

[0095] <Temperature Tb1 of corrosion process (B): 60°C or less and within a certain range> The corrosion process (B) is carried out in an atmosphere with a temperature Tb1 of 60°C or less and within a certain range. If the temperature Tb1 of the corrosion process (B) exceeds 60°C, not only will the evaluation be conducted in an environment far removed from the corrosive environment in which the metal material is actually used, but the corrosion mechanism may also change. For this reason, the temperature Tb1 of the corrosion process (B) should be 60°C or less. Preferably, it should be 50°C or less. On the other hand, there is no particular lower limit to the temperature Tb1 of the corrosion process (B). If the temperature Tb1 of the corrosion process (B) is less than 5°C, it may be difficult to control the relative humidity in the corrosion test chamber (constant temperature and humidity chamber) used when conducting the corrosion test. Also, the evaluation time will be longer because the corrosion rate of the metal material will be significantly reduced. For this reason, the temperature Tb1 of the corrosion process (B) should preferably be 5°C or higher, and more preferably 10°C or higher.

[0096] Furthermore, delayed fracture characteristics are strongly influenced by the temperature of the environment (atmosphere). Therefore, in order to properly evaluate the delayed fracture characteristics of a metallic material, taking into account the application site of the metallic material component and the environment in which it is used, it is necessary to keep the temperature Tb1 of the corrosion process (B) within a certain range. When the fluctuation range of the temperature Tb1 of the corrosion process (B) is within ±5°C, the amount of hydrogen that penetrates into the interior of the metallic material from the environment (hydrogen intrusion amount) can be evaluated with a fluctuation range of within 30% of the hydrogen intrusion amount at the target environmental temperature, and the delayed fracture characteristics can be accurately evaluated. When the fluctuation range of the temperature Tb1 of the corrosion process (B) is within ±2°C, the fluctuation range of the hydrogen intrusion amount is within 15%. For this reason, the fluctuation range of the temperature Tb1 of process (B) is preferably within ±5°C, and more preferably within ±2°C.

[0097] [Drying Process (b1)] The drying process (b1) is a process of drying the metal material by holding it in an atmosphere with a relative humidity Hb1 of 45% or less for 1.0 hour to 5.0 hours. The relative humidity Hb1 in the drying process (b1) is set to 45% or less. This is to simulate the dry state, which is one of the characteristics of an atmospheric corrosion environment. Furthermore, if the relative humidity Hb1 in the drying process (b1) exceeds 45%, it will be necessary to hold it for a long time in order to sufficiently dry the surface of the metal material, and the evaluation time will be longer. The relative humidity Hb1 in the drying process (b1) is preferably 40% or less. On the other hand, there is no particular lower limit to the relative humidity Hb1 in the drying process (b1). From the viewpoint of controllability of relative humidity, the relative humidity Hb1 in the drying process (b1) is preferably 20% or more. In addition, if the components to be attached to the surface of the metal material include substances that exhibit deliquescence at lower relative humidity, such as magnesium chloride or calcium chloride, it is preferable to set the relative humidity Hb1 in the drying process (b1) lower.

[0098] The drying process (b1) duration (the time spent in an atmosphere with relative humidity Hb1) shall be between 1.0 hour and 5.0 hours. If the drying process (b1) duration is less than 1.0 hour, it is not possible to simulate an actual corrosion environment. On the other hand, if the drying process (b1) duration exceeds 5.0 hours, an actual corrosion environment can be simulated, but it takes time to evaluate the delayed fracture characteristics.

[0099] [Wetting Process (b2)] The wetting process (b2) is a process in which the metal material is wetted by maintaining it in an atmosphere with a relative humidity of 80% or more Hb2 for 1.0 hour to 5.0 hours. The relative humidity Hb2 in the wetting process (b2) is set to 80% or more. This is to simulate the wet state, which is one of the characteristics of an atmospheric corrosion environment. If the relative humidity Hb2 in the wetting process (b2) is less than 80%, the effect of wetting will be insufficient, and as a result, it will not be possible to simulate an actual corrosion environment. Among chlorides, sodium chloride has the highest saturated critical vapor pressure, which is approximately 75-78% in terms of relative humidity. Therefore, if the relative humidity is set to 80% or more for any chloride, a water film will be formed on the surface of the metal material due to moisture absorption by the chloride, and the wet state can be maintained. For this reason, the relative humidity Hb2 in the wetting process (b2) is set to 80% or more. On the other hand, there is no particular upper limit to the relative humidity Hb2 in the wetting process (b2), but it is preferable that the relative humidity Hb2 in the wetting process (b2) be less than 98%. This is because if the relative humidity Hb2 is 98% or higher, the water film thickness generated by condensation becomes too thick, making it easier for the attached chlorides to be washed away. This phenomenon is particularly likely to occur when evaluating the processed test specimen. Therefore, when evaluating the processed test specimen, it is preferable that the relative humidity Hb2 in the wetting process (b2) be less than 98%.

[0100] The process time for the wetting process (b2) (the time spent in an atmosphere with a relative humidity of 80% or higher Hb2) shall be between 1.0 hour and 5.0 hours. If the process time for the wetting process (b2) is less than 1.0 hour, it is not possible to simulate an actual corrosive environment. On the other hand, if the process time for the wetting process (b2) exceeds 5.0 hours, an actual corrosive environment can be simulated, but it takes time to evaluate the delayed fracture characteristics.

[0101] [Transition process (b3), Transition process (b4)] Transition process (b3) is a process of transitioning from an atmosphere with relative humidity Hb1 to an atmosphere with relative humidity Hb2, and transition process (b4) is a process of transitioning from an atmosphere with relative humidity Hb2 to an atmosphere with relative humidity Hb1. It is known that the amount of hydrogen entering the interior of metallic materials, especially steel materials, increases when the relative humidity changes. In other words, a large amount of hydrogen enters the interior of the steel material in transition process (b3) and transition process (b4). The reason why the amount of hydrogen entering the metallic material increases when the relative humidity changes is not entirely clear, but it can be considered as follows. In the case of transition process (b3), which is a process of transitioning from an atmosphere with relative humidity Hb1 to an atmosphere with relative humidity Hb2, moisture is absorbed due to the deliquescence of chlorides present on the surface of the metallic material, and corrosion of the metallic material begins. It is known that the corrosion products present on the surface of the metallic material change at this time, and it is thought that hydrogen is generated along with this change in corrosion products. Furthermore, in the transition step (b4), which is the process of transitioning from an atmosphere with relative humidity Hb2 to an atmosphere with relative humidity Hb1, the moisture becomes a concentrated solution containing a large amount of chlorides and metal ions dissolved by corrosion, and in the case of steel materials, a large amount of iron ions, so the pH of the solution is thought to decrease. In other words, a large amount of hydrogen ions will be contained in the solution during the drying process, and it is thought that hydrogen will easily penetrate into the metal material. For this reason, in the transition steps (b3) and (b4), the rate of change of relative humidity when changing the relative humidity should be 30% / h or less. If the rate of change of relative humidity in the transition steps (b3) and (b4) is 30% / h or less, it will be possible to sufficiently penetrate the metal material with hydrogen generated by corrosion, and it will be possible to properly evaluate the delayed fracture characteristics. On the other hand, there is no specific lower limit for the rate of change of relative humidity, but if the transition process (b3) and the transition process (b4) are too long, it will take a long time to evaluate the delayed failure characteristics. Therefore, the rate of change of relative humidity is preferably 1.5% / h or more, and more preferably 10% / h or more.

[0102] The present invention's method for evaluating the delayed fracture characteristics of a metal material aims to simulate the diurnal relative humidity changes in a real environment. Therefore, if the process time (time of one cycle) of the corrosion process (B), which simulates the diurnal relative humidity changes in a real environment, exceeds 24 hours, it means that the corrosion will be slower than in a real environment, and thus the evaluation of the delayed fracture characteristics will take a long time. In other words, it is preferable to set the process time of the corrosion process (B) to 24 hours or less. To expedite the evaluation, it is even more preferable to set the process time of the corrosion process (B) to 12 hours or less. On the other hand, if the process time of the corrosion process (B) is shortened, the relative humidity will change rapidly, resulting in a low correlation with corrosion in a real environment and a discrepancy with the results of delayed fracture characteristics in a real environment. For this reason, it is preferable to set the process time of the corrosion process (B) to 5 hours or more.

[0103] In the evaluation method of the present invention, the chloride deposition step (A) and the corrosion step (B) are each performed at least once. The chloride deposition step (A) may be placed at random intervals of the corrosion step (B) cycles, or at predetermined intervals of the corrosion step (B) cycles. There is no particular upper limit to the number of times the process comprising the chloride deposition step (A) and the corrosion step (B) is performed. For example, the process comprising the chloride deposition step (A) and the corrosion step (B) can be performed until cracks occur in the metal material. Alternatively, the number of test days may be predetermined, and the process may be performed for a number of days corresponding to that test period. The number of times the process is performed can be set appropriately, for example, by considering the simulation of corrosion patterns in a real environment. As an example, the process may be 200 times or less, or 100 times or less.

[0104] Next, a process comprising a chloride adhesion process (A) and a corrosion process (B) will be described. Figure 5 is a diagram illustrating one embodiment of a corrosion test cycle according to the evaluation method of the present invention. The corrosion test cycle shown in Figure 5 is an example of a corrosion test cycle in which the chloride adhesion process (A) and the corrosion process (B) are performed once each. In this example, the corrosion process (B) has a process consisting of a drying process (b1), a migration process (b3), a wetting process (b2), and a migration process (b4) as one cycle.

[0105] It is preferable that the corrosion cycle (B) following the chloride deposition process (A) begin with the drying process (b1). By drying the saltwater deposited in the chloride deposition process (A) during the drying process (b1), the points at which condensation occurs when humidity rises are uniformly dispersed, reducing variability in the evaluation of delayed fracture characteristics. If the corrosion cycle (B) is started with the transition process (b3), the wetting process (b2), or the transition process (b4), the saltwater deposited in the chloride deposition process (A) may not be sufficiently dried, or the saltwater deposited in a high-humidity environment may absorb moisture and become coarser, leading to an uneven dispersion of condensation points. Therefore, it is best to avoid starting the corrosion cycle (B) with any process other than the drying process (b1).

[0106] When performing the chloride deposition process (A) followed by the corrosion process (B) cycle, and then repeating the chloride deposition process (A), it is preferable to include a water washing process (C) before the chloride deposition process (A). Figure 6 shows an example of a corrosion test cycle when performing the chloride deposition process (A), followed by the corrosion process (B), and then repeating the chloride deposition process (A) with a water washing process (C). If the chloride deposition process (A) is repeated without including the water washing process (C), the amount of chloride deposited on the surface of the metal material tends to increase as the amount of chloride deposition increases, and it may become impossible to maintain the same corrosion environment. Therefore, there is a possibility that the delayed fracture characteristics will be evaluated in an environment different from the assumed corrosion environment. For this reason, it is preferable to include a water washing process (C) before repeating the chloride deposition process (A). Note that the water washing process (C) is a process of washing the evaluation surface of the metal material with water. The washing method in the washing step (C) is not particularly limited, but examples include a method of washing the evaluation surface of a metal material by spraying water from a spray nozzle, or a method of washing the evaluation surface by immersing the metal material in water.

[0107] In the evaluation method of the present invention, the process involves performing a cycle of chloride deposition (A) and corrosion (B) as described above, and then, before performing the chloride deposition (A) again, performing a water washing (C) step at least once to complete the corrosion test cycle. After this, the condition of the metal material (such as the presence and extent of cracks in the metal material) is checked, and the delayed fracture characteristics of the metal material are evaluated based on the checked condition of the metal material.

[0108] Specifically, the steel sheet of the present invention has a cracking time of 63 days or more under the following conditions in the evaluation method for the delayed fracture characteristics of the above-mentioned metal material (HeTsAce).

[0109] Process (A) Chloride-containing aqueous solution: 15% by mass NaCl aqueous solution is sprayed. Chloride adhesion amount (in terms of solid content): 10,000 mg / m ２ Distance between nozzle and evaluation surface (X in Figure 1): 30 cm Distance between shielding material and evaluation surface (Y in Figure 4): 5 cm Temperature: 22°C, relative humidity: 50% Droplet distribution on evaluation surface Average contact area of ​​droplets: 1.3 mm ２ Standard deviation of the contact area of ​​the droplet: 1.3 mm ２ Total contact area ratio of droplets: 71% Process (B) Drying process (b1) Temperature: 30°C, relative humidity: 40%, holding time: 2.0 hours Wetting process (b2) Temperature: 30°C, relative humidity: 90%, holding time: 2.0 hours Transition process (b3) Rate of change in relative humidity: 25% / h Transition process (b4) Rate of change in relative humidity: 25% / h Temperature fluctuation range: within ±5°C

[0110] The water washing step (C) is a step of washing the evaluation surface of the metal material with water. The water washing method in the water washing step (C) is not particularly limited, but water is sprayed onto the evaluation surface of the metal material from a spray nozzle to wash the evaluation surface.

[0111] - After performing the first step (A) of the corrosion test cycle, the distribution of droplets on the evaluation surface is checked. Subsequent steps (A) are performed under the same conditions as the first step (A). Step (B) consists of a cycle in which drying step (b1) → transition step (b3) → wetting step (b2) → transition step (b4) are performed in that order. After repeating this cycle four times, a water washing step (C) is added before performing step (A), and then step (A) is performed. - Metal material used for evaluation: The obtained steel plate is sheared to a size of 16 mm x 75 mm with the longitudinal side perpendicular to the rolling direction, and test specimens are prepared. The clearance during shearing is 15% in all cases. Then, a four-point bending test is performed according to ASTM (G39-99), and stresses equivalent to YS and TS are applied to the bending apex of the test specimen. Then, the above corrosion environment cycle (HeTsAce) is carried out for 63 days. After the test, each test specimen is visually inspected for cracks. If no cracks are found in a sample subjected to a stress equivalent to YS, it is determined that the material exhibits excellent delayed fracture resistance in corrosive environments.

[0112] In the aforementioned evaluation method for the delayed fracture characteristics of metallic materials (HeTsAce), materials with a longer time until cracking occurs tend to have a lower amount of diffusible hydrogen in the steel.

[0113] (Method for Manufacturing Steel Sheets) Next, the method for manufacturing steel sheets according to the present invention will be described. Figure 8 is a graph showing the relationship between temperature and time in the method for manufacturing cold-rolled steel sheets according to an embodiment of the present invention. As shown in Figure 8, the present invention provides a method for manufacturing a steel sheet, in which a steel having the aforementioned component composition is subjected to hot rolling to obtain a hot-rolled steel sheet, the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet, the cold-rolled steel sheet is annealed, the annealing is performed as follows: an annealing holding step in which the annealing temperature is between Ac3 and 100°C and below Ac3, and the holding time at the annealing temperature is 5 s or more; a first cooling step in which cooling is performed from the annealing temperature to a temperature T1 of 550°C or higher with a first average cooling rate of less than 11°C / s; a second cooling step in which cooling is performed from temperature T1 to 500°C with a second average cooling rate of 11°C / s or higher; and a third cooling step in which cooling is performed from 500°C to a temperature T2 of Ms or higher and 320°C or higher with a residence time of 10 s or more and 60 s or less. The process includes: a fourth cooling step in which cooling is performed from the above temperature T2 to a temperature T3 between 100°C and Ms-80°C, with a fourth average cooling rate of 3°C / s or more and 50°C / s or less; a tempering step in which tempering is performed under conditions where the tempering temperature is between T3 and 450°C and the tempering time is between 5s and 1000s; and a fifth cooling step in which cooling is performed in a temperature range of 150 to 80°C with a fifth average cooling rate of 1.0°C / h or more and 50.0°C / h or less.

[0114] Steel Material (Steel Slab) In this invention, the method of melting the steel material (steel slab) is not particularly limited, and any known melting method, such as a converter or electric furnace, is suitable. The steel slab (slab) is preferably manufactured by a continuous casting method in order to prevent macrosegregation.

[0115] The method for manufacturing hot-rolled hot-rolled sheets is not particularly limited and can be carried out according to conventional methods. For example, it is preferable that the slab heating temperature be 1100°C or higher. It is preferable that the slab heating temperature be 1300°C or lower. It is also preferable that the soaking time be 20 min or higher. It is also preferable that the soaking time be 30 min or lower. Furthermore, the finish rolling temperature is Ar ３ It is preferable to set the temperature above the transformation point. The finishing rolling temperature is Ar ３It is preferable that the temperature be below the transformation point + 200°C. Furthermore, it is preferable that the winding temperature be 400°C or higher. It is preferable that the winding temperature be 720°C or lower. It is desirable that the winding temperature suppresses plate thickness fluctuations and stably ensures high strength. From this viewpoint, it is preferable that the winding temperature be 430°C or higher. Furthermore, it is preferable that the winding temperature be 530°C or lower.

[0116] The method for manufacturing cold-rolled steel sheets is not particularly limited and can be carried out according to conventional methods. For example, the rolling ratio (cumulative reduction ratio) should be 30% or more. Alternatively, the rolling ratio may be 85% or less. From the viewpoint of stably ensuring high strength and minimizing anisotropy, it is preferable that the rolling ratio be 35% or more, and more preferably between 35% and 85%. If the rolling load is high, it is possible to perform softening annealing treatment at 450 to 730°C using CAL (Continuous Annealing Line) or BAF (Box Annealing). The obtained hot-rolled steel sheet may be cold-rolled as is, or it may be cold-rolled after pickling treatment of the hot-rolled steel sheet. Pickling treatment is preferable because it removes oxides from the surface of the steel sheet, resulting in a suitable plating quality for the final steel sheet product. Pickling treatment may be performed once or in multiple stages.

[0117] Annealing (Annealing and Holding Process) [Annealing Temperature: Ac3-100°C or higher, and Ac3 or lower] The obtained cold-rolled steel sheet is subjected to the annealing and holding process. If the annealing temperature exceeds Ac3 (°C) as defined by the following formula (1), the area fraction of low-Mn ferrite decreases, making it difficult to achieve high formability. Therefore, the annealing temperature should be Ac3 or lower. The annealing temperature is preferably Ac3-10°C or lower, and more preferably Ac3-20°C or lower. On the other hand, if the annealing temperature is too low, the area fraction of tempered martensite decreases, making it difficult to achieve a TS of 980 MPa or higher, so the annealing temperature should be Ac3-100°C or higher. The annealing temperature is preferably Ac3-90°C or higher, and more preferably Ac3-80°C or higher. Ac3 (°C) = 881 - 205.7 × [%C] + 53.1 × [%Si] - 15 × [%Mn] - 27 × [%Cu] - 20.1 × [%Ni] - 0.7 × [%Cr] + 41.1 × [%Mo] ... (1) Here, [%X] indicates the content (mass%) of element X in the above component composition, and is set to 0 if the above component composition does not contain element X.

[0118] [Holding time at annealing temperature: 5 s or more] In the annealing holding process, if the holding time at annealing temperature is less than 5 s, it becomes difficult to control the fraction of low-Mn ferrite and tempered martensite. Therefore, the holding time at annealing temperature should be 5 s or more, preferably 50 s or more. The holding time at annealing temperature is more preferably 80 s or more, and even more preferably 100 s or more. On the other hand, there is no upper limit on the holding time at annealing temperature, but since productivity decreases due to increased heating costs and longer manufacturing times, the holding time at annealing temperature is preferably 1000 s or less. The holding time at annealing temperature is more preferably 800 s or less, and even more preferably 600 s or less. When the annealing time is short, it is desirable to perform heating by a highly responsive method such as IH.

[0119] (First Cooling Step) [First average cooling rate from annealing temperature to temperature T1 of 550°C or higher: less than 11°C / s] If the average cooling rate (first average cooling rate) from the annealing temperature to any temperature T1 (cooling stop temperature T1) between 550°C and the annealing temperature is 11°C / s or higher, the precipitation and growth of high Mn ferrite will be suppressed, making it difficult to achieve a high λ. Therefore, the first average cooling rate from the annealing temperature to any temperature T1 (cooling stop temperature T1) of 550°C or higher will be less than 11°C / s. The first average cooling rate is preferably 10°C / s or lower, and more preferably 8°C / s or lower. Also, if the cooling rate is drastically reduced, the production efficiency will decrease, so the first average cooling rate is preferably 1°C / s or higher, and more preferably 5°C / s or higher. If the cooling stop temperature T1 of the first cooling step is less than 550°C, a large amount of residual structure will precipitate, and the desired moldability cannot be obtained. Therefore, the cooling stop temperature T1 of the first cooling step is set to 550°C or higher. The cooling stop temperature T1 is preferably 560°C or higher, and more preferably 580°C or higher. The cooling stop temperature T1 is preferably 800°C or lower, and more preferably 700°C or lower. Here, the first average cooling rate is determined by "(cooling start temperature (annealing temperature) (°C) - cooling stop temperature T1 (°C)) / cooling time from cooling start temperature to cooling stop temperature (s)".

[0120] (Second Cooling Step) [Second average cooling rate from temperature T1 to 500°C: 11°C / s or more] If the average cooling rate from temperature T1 to 500°C (second average cooling rate) is less than 11°C / s, a large amount of residual tissue will precipitate, and the effects of the present invention will be lost. Therefore, the second average cooling rate in the second cooling step should be 11°C / s or more. Furthermore, the second average cooling rate is preferably 12°C / s or more, and more preferably 13°C / s or more. If the second average cooling rate is too high, it will be difficult to control the temperature T2 described later. Therefore, the second average cooling rate is preferably 100°C / s or less, and more preferably 50°C / s or less. Here, the second average cooling rate is determined by "(cooling start temperature (temperature T1) (°C) - cooling stop temperature (500°C)) / cooling time from cooling start temperature to cooling stop temperature (s)".

[0121] (Third Cooling Step) [Residence time from 500°C to a temperature T2 of Ms or higher and 320°C or higher: 10s to 60s] If the residence time from 500°C to any temperature T2 (residence stop temperature T2) of Ms or higher and 320°C to 500°C is less than 10s, sufficient retained austenite cannot be generated, and the desired moldability cannot be obtained. Therefore, the residence time should be 10s or more. Preferably it is 15s or more, and more preferably 20s or more. Note that Ms is represented by formula (2) described later. On the other hand, if the residence time exceeds 60s, the concentration of C in the untransformed austenite proceeds excessively, increasing the amount of fresh martensite generated during cooling, and the desired moldability and impact properties cannot be achieved. Therefore, the residence time should be 60s or less. Preferably it is 55s or less, and more preferably 50s or less. Note that if the temperature T2 exceeds 500°C, the remaining structure precipitates excessively, and the desired properties cannot be obtained. Furthermore, if the temperature T2 is less than Ms or less than 320°C, sufficient retained austenite cannot be generated, and the desired moldability cannot be obtained. Therefore, the temperature T2 should be Ms or greater and between 320°C and 500°C. Also, the cooling rate in the third cooling step (the average cooling rate of the third step) is not particularly limited as long as the above conditions for the third cooling step are met.

[0122] Ms = 519 - 474 × [%C] - 30.4 × [%Mn] - 12.1 × [%Cr] - 7.5 × [%Mo] - 17.7 × [%Ni] ... (2) Here, [%X] indicates the content (mass%) of element X in the above component composition, and is set to 0 if the above component composition does not contain element X.

[0123] (Fourth Cooling Step) [Fourth average cooling rate from temperature T2 to temperature T3 of 100°C or higher and Ms-80°C or lower: 3°C / s or higher and 50°C / s or lower] If the average cooling rate (fourth average cooling rate) from T2 to any temperature T3 of 100°C or higher and Ms-80°C or lower (cooling stop temperature T3) is less than 3°C / s, the concentration of C in the untransformed austenite will proceed excessively, increasing the amount of fresh martensite in the remaining structure generated during cooling, and high moldability cannot be achieved. Therefore, the average cooling rate from T2 to T3 should be 3°C / s or higher. Preferably it should be 5°C / s or higher, and more preferably 7°C / s or higher. On the other hand, if the cooling rate from T2 to T3 is too high, it becomes difficult to control the temperature T3, so the fourth average cooling rate in this temperature range should be 50°C / s or lower. Preferably it should be 40°C / s or lower, and more preferably 30°C / s or lower. Furthermore, if T3 is higher than Ms-80°C, the fraction of retained austenite increases, and a sufficient fraction of tempered martensite cannot be obtained, making it difficult to achieve a TS of 980 MPa or higher. On the other hand, if T3 is below 100°C, a sufficient amount of retained austenite cannot be obtained, and the desired formability cannot be achieved. Therefore, T3 should be between 100°C and Ms-80°C. T3 is preferably 110°C or higher, and more preferably 120°C or higher. Also, T3 is preferably Ms-90°C or lower, and more preferably Ms-120°C or lower. Here, the fourth average cooling rate is determined by "(cooling start temperature (temperature T2) (°C) - cooling stop temperature (temperature T3) (°C)) / cooling time from cooling start temperature to cooling stop temperature (s)".

[0124] (Tempering process) [Tempering temperature: T3 or higher and 450°C or lower] After the fourth cooling process, the material is either held at the same temperature or reheated and held at a temperature of 450°C or lower to stabilize the retained austenite. If the tempering temperature is less than T3, the concentration of C in the untransformed austenite will be insufficient, increasing the amount of fresh martensite in the remaining structure generated during cooling, and high formability cannot be achieved. On the other hand, if the tempering temperature is higher than 450°C, the tempering of the martensite will proceed excessively, making it difficult to achieve a TS of 980 MPa or higher. Therefore, the tempering temperature should be T3 or higher and 450°C or lower. Preferably, the tempering temperature is T3 + 10°C or higher, and more preferably T3 + 20°C or higher. Preferably, the tempering temperature is 420°C or lower, and more preferably 400°C or lower.

[0125] [Tempering time: 5 s to 1000 s] If the holding time at the tempering temperature (tempering time) is less than 5 s, the concentration of C in the untransformed austenite will be insufficient, and fresh martensite will be formed during final cooling, making it difficult to achieve high ductility and high hole expansion. If the holding time at the tempering temperature (tempering time) is more than 1000 s, the tempering of the martensite will proceed excessively, making it difficult to achieve a TS of 980 MPa or higher. Therefore, the holding time at the tempering temperature (tempering time) should be 5 s to 1000 s. The holding time at the tempering temperature (tempering time) is preferably 50 s or more, more preferably 60 s or more. The holding time at the tempering temperature (tempering time) is preferably 800 s or less, more preferably 700 s or less.

[0126] (Fifth Cooling Step) [Fifth average cooling rate in the temperature range of 150 to 80°C: 1.0°C / h or more and 50.0°C / h or less] If the average cooling rate in the temperature range of 150 to 80°C (fifth average cooling rate) exceeds 50.0°C / h, the concentration of C in austenite during cooling becomes insufficient, and fresh martensite is formed at the final cooling stage, making it difficult to achieve high moldability. On the other hand, as for the lower limit, the average cooling rate in the temperature range of 150 to 80°C (fifth average cooling rate) is 1.0°C / h or more due to production technology constraints. The fifth average cooling rate is preferably 1.2°C / h or more. The fifth average cooling rate is preferably 45.0°C / h or less. The fifth average cooling rate is more preferably 30.0°C / h or less, and even more preferably 20.0°C / h or less.

[0127] When steel sheets are the subject of trade, they are usually cooled to room temperature before being traded. Plating treatment may be applied to the steel sheets during or after annealing. Examples of plating treatment during annealing include, for example, hot-dip galvanizing after the annealing holding process according to a conventional method, or alloying after hot-dip galvanizing. Examples of plating treatment after annealing include electroplating such as Zn-Ni electroalloy plating or pure Zn electroplating after tempering. A plating layer may be formed by electroplating. Alternatively, hot-dip zinc-aluminum-magnesium alloy plating may be applied. While the above explanation focuses on zinc plating, the type of plating metal is not particularly limited, and Al plating, etc., can also be used. Other manufacturing conditions are not particularly limited, but from the viewpoint of productivity, it is preferable that the above-mentioned series of processes, such as annealing, hot-dip galvanizing, and alloying of zinc plating, be carried out on a CGL (Continuous Galvanizing Line), which is a hot-dip galvanizing line. After hot-dip galvanizing, wiping is possible to adjust the plating thickness. Plating conditions other than those described above can be handled according to conventional methods for hot-dip galvanizing.

[0128] Furthermore, conventional methods can be used for processes and conditions not described in this invention.

[0129] (Methods and Methods for Manufacturing Members) The members of the present invention are obtained by subjecting a steel sheet of the present invention to at least one of forming and joining processes. Furthermore, the method for manufacturing the members of the present invention includes the step of subjecting a steel sheet of the present invention to at least one of forming and joining processes to obtain a member.

[0130] The steel sheet of the present invention has a tensile strength TS of 980 MPa or higher and exhibits excellent formability, impact characteristics, chemical treatment properties, and delayed fracture resistance. Therefore, components obtained using the steel sheet of the present invention also have a tensile strength TS of 980 MPa or higher and exhibit excellent formability, impact characteristics, chemical treatment properties, and delayed fracture resistance. Weight reduction is possible by using the components of the present invention. Accordingly, the components of the present invention can be suitably used, for example, in automotive structural components.

[0131] Forming processes can utilize general processing methods such as press working without restriction. Joining processes can utilize general welding methods such as spot welding and arc welding, as well as riveting and crimping without restriction.

[0132] Steel having the component composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a converter and formed into steel slabs by continuous casting. Next, the obtained steel slabs were subjected to hot rolling and pickling, followed by cold rolling. Then, under the conditions shown in Table 2 (Table 2-1, Table 2-2), the annealing process, the first cooling process, the second cooling process, the third cooling process, the fourth cooling process, the tempering process, and the fifth cooling process were carried out sequentially. As a result, steel plates with thicknesses of 0.8 to 2.4 mm were obtained. In Table 2, the plating treatments are denoted as follows: hot-dip galvanizing is "GI", alloyed hot-dip galvanizing is "GA", and electro-galvanizing is "EG". If no plating treatment is applied, it is indicated as "CR".

[0133] Table 3 shows the results obtained by using the obtained steel sheets as test materials to determine the area fraction of ferrite, the proportion of high-Mn ferrite within the ferrite (area fraction), the area fraction of tempered martensite, the volume fraction of retained austenite, and the area fraction of the remaining structure, respectively, using the method described above. Next, using the obtained steel sheets as test materials, the tensile strength TS, yield strength YS, elongation El, hole expansion ratio λ, chemical conversion treatment properties, and delayed fracture resistance properties were evaluated using the method described above. More specifically, the evaluation of delayed fracture resistance properties was performed as follows.

[0134] (Delayed fracture resistance) The obtained steel plate was sheared to a size of 16 mm x 75 mm with the longitudinal side perpendicular to the rolling direction, and test specimens were prepared. The clearance during shearing was 15% in all cases. Next, a four-point bending test was performed according to ASTM (G39-99), and stresses equivalent to YS and TS were applied to the bending apex of the test specimen. Then, a delayed fracture test was performed using the corrosion environment cycle described later.

[0135] (Corrosion Test Cycle) The above-mentioned test specimens were subjected to the following corrosion test cycle (corrosion test) consisting of a chloride deposition step (A), a corrosion step (B), and a water washing step (C). In the chloride deposition step (A), the amount of chloride deposited (in terms of solid content) was 10,000 mg / m ２ A chloride-containing aqueous solution was applied to the specimen in the following manner. The distance between the spray nozzle and the evaluation surface of the specimen (X in Figure 1) was 30 cm. The spray pressure of the spray nozzle was 0.2 MPa. The spraying time of the saltwater was 5 seconds. The amount of chloride deposited was calculated by determining the mass difference of the specimen before and after saltwater spraying, dividing it by the evaluation surface area of ​​the specimen to calculate the amount of aqueous solution deposited, and then calculating the chloride concentration of the aqueous solution. In addition, the following conditions were adopted: Chloride-containing aqueous solution: 15% by mass NaCl aqueous solution sprayed by spraying Distance between shielding material and evaluation surface (Y in Figure 4): 5 cm Temperature: 22°C, relative humidity: 50% Droplet distribution on the evaluation surface Average contact area of ​​droplets: 1.3 mm ２ Standard deviation of the contact area of ​​the droplet: 1.3 mm ２The total contact area ratio of the droplets was 71%. In addition, the chloride deposition process (A) from the second time onward was carried out under the same conditions as the first chloride deposition process (A).

[0136] The corrosion process (B) is a process in which the drying process (b1) → transition process (b3) → wetting process (b2) → transition process (b4) are performed in order, with the completion of the four aforementioned processes considered as one cycle. In this embodiment, the temperature fluctuation range of the corrosion process (B) was set to within 30 ± 5°C. Drying process (b1): A process in which the metal material is dried by holding it in an atmosphere of 40% relative humidity Hb1 for 2.0 hours. Wetting process (b2): A process in which the metal material is wet by holding it in an atmosphere of 90% relative humidity Hb2 for 2.0 hours. Transition process (b3): ​​A process in which the relative humidity changes from the atmosphere of Hb1 to the atmosphere of Hb2 at a rate of change of relative humidity of 25% / h. Transition process (b4): A process in which the relative humidity changes from the atmosphere of Hb2 to the atmosphere of Hb1 at a rate of change of relative humidity of 25% / h.

[0137] The water washing step (C) is a step of washing the evaluation surface of the metal material with water. The water washing method in the water washing step (C) is not particularly limited, but in this case, water was sprayed onto the evaluation surface of the metal material from a spray nozzle to wash the evaluation surface.

[0138] Furthermore, in this embodiment, the corrosion test cycle consisted of a chloride deposition process (A), followed by a corrosion process (B), and then a water washing process (C) before repeating the chloride deposition process (A). In other words, in this embodiment, the evaluation of delayed fracture characteristics was performed using the following corrosion test cycle: a cycle of repeated chloride deposition process (A) → corrosion process (B) → water washing process (C). In this embodiment, the period for evaluating delayed fracture resistance using the corrosion environment cycle was 63 days.

[0139] After the test, each test specimen was visually inspected for cracks. The delayed fracture resistance in a corrosive environment was then evaluated according to the following criteria. In this invention, a sample that showed no cracks when subjected to a stress equivalent to YS was judged to have excellent delayed fracture resistance in a corrosive environment. ◎ (Pass, particularly excellent): No cracks in samples subjected to stresses equivalent to YS and TS ○ (Pass, excellent): No cracks in samples subjected to stresses equivalent to YS × (Fail): Cracks present in both samples subjected to stresses equivalent to YS and TS

[0140] The evaluation results are shown in Table 3 (Table 3-1, Table 3-2).

[0141]

[0142]

[0143]

[0144]

[0145]

[0146] The steel sheet of the present invention had a tensile strength (TS) of 980 MPa or higher and was excellent in formability, impact characteristics, chemical treatment properties, and delayed fracture resistance. On the other hand, the comparative steel sheet was inferior in at least one of the following: TS, formability, impact characteristics, chemical treatment properties, and delayed fracture resistance.

[0147] Furthermore, it was found that members obtained by forming and joining using the steel plate of the present invention example have a tensile strength TS of 980 MPa or more, and exhibit excellent formability, impact characteristics, chemical treatment properties, and delayed fracture resistance, similar to the steel plate of the present invention example.

Claims

1. The material has a composition in mass%, containing C: 0.030% or more and 0.300% or less, Si: 0.40% or more and 1.60% or less, Mn: 1.40% or more and 3.20% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, and having a Si / Mn < 0.50, with the remainder being Fe and unavoidable impurities, the surface coverage rate of Si-based oxides on the steel sheet is 1% or less, the area fraction of ferrite is 3% or more and 30% or less at the 1 / 4 thickness position, the area fraction of tempered martensite is 60% or more and 94% or less, the volume fraction of retained austenite is 3% or more and 15% or less, and the area fraction of the remaining structure is 10% or less, and the ferrite is A steel sheet comprising low-Mn ferrite having a Mn concentration of less than 0.85 times the Mn content of the steel sheet, and high-Mn ferrite having a Mn concentration of 0.85 times or more the Mn content of the steel sheet, wherein the proportion of high-Mn ferrite among the ferrite is 20% or more and 90% or less in area fraction, TS is 980 MPa or more, El×λ is 600%×% or more, and YS×λ is 30000 MPa×% or more.

2. The above component composition is further defined in mass percent as follows: As: 0.100% or less, 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, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.050% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Zn: 0.100% or less. The steel sheet according to claim 1, containing at least one element selected from Pb: 0.100% or less, Te: 0.100% or less, Se: 0.020% or less, Ga: 0.020% or less, Ge: 0.020% or less, Sr: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

3. The steel sheet according to claim 1 or 2, wherein the steel sheet has a plating layer on at least one side.

4. A member made using the steel plate described in any one of claims 1 to 3.

5. A method for manufacturing a steel sheet according to any one of claims 1 to 3, comprising: hot rolling a steel material having the component composition according to claim 1 or 2 to obtain a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet, the annealing comprising: an annealing holding step in which the annealing temperature is between Ac3 and 100°C and below Ac3, and the holding time at the annealing temperature is 5 s or more; a first cooling step in which cooling is performed from the annealing temperature to a temperature T1 of 550°C or higher with a first average cooling rate of less than 11°C / s; a second cooling step in which cooling is performed from the temperature T1 to 500°C with a second average cooling rate of 11°C / s or higher; and a third cooling step in which cooling is performed from 500°C to a temperature T2 of Ms or higher and 320°C or higher with a residence time of 10 s or more and 60 s or less. A method for manufacturing a steel sheet, comprising: a fourth cooling step of performing cooling from a temperature T2 to a temperature T3 between 100°C and Ms-80°C, with a fourth average cooling rate of 3°C / s or more and 50°C / s or less; a tempering step of performing tempering under conditions where the tempering temperature is between T3 and 450°C and the tempering time is between 5s and 1000s; and a fifth cooling step of performing cooling in a temperature range of 150 to 80°C with a fifth average cooling rate of 1.0°C / h or more and 50.0°C / h or less.

6. The method for manufacturing a steel sheet according to claim 5, wherein a plating treatment is applied to the surface of the steel sheet.

7. A method for manufacturing a component, comprising the step of forming and joining a steel plate according to any one of claims 1 to 3 to form a component.

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