Steel plates, components, and their manufacturing methods

Abstract

The present invention provides steel plates and components having a tensile strength (TS) of 980 MPa or more, and excellent formability, impact resistance, chemical conversion treatability, and delayed fracture resistance, as well as methods for manufacturing the same. It has a specific chemical composition, and the steel sheet surface coverage rate of Si-based oxides 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, The steel sheet has a structure in which the area fraction of the remaining structure is 10% or less, and the ferrite has 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, and the proportion of the high-Mn ferrite in the ferrite is 20% or more and 90% or less in area fraction.

Description

[Technical Field]

[0001] The present invention relates to steel sheets and members used in various applications such as automobiles and home appliances, and to methods for manufacturing the same. [Background technology]

[0002] In recent years, the application of high-strength steel sheets of 980 MPa or higher to automotive frame components and seat parts has been increasing due to the increasing need for lightweight automobile bodies. However, when high-strength steel sheets of 980 MPa or higher are used in automotive parts, press cracking is likely to occur due to a decrease in El and a decrease in λ. For this reason, these high-strength steel sheets are expected to have better formability than conventional steel sheets. Furthermore, to ensure passenger safety, it is necessary to suppress deformation around the cabin during a collision, so energy-absorbing components such as side members are required to absorb collision energy by deforming during a collision. However, high-strength steel plates with a tensile strength of 780 MPa or higher have a problem in that they are prone to fracture during a collision, originating from areas that have undergone primary forming processing due to reduced axial crushing properties, making it difficult to stably absorb collision energy. Furthermore, when high-strength steel sheets with a tensile strength of 980 MPa or more are formed into parts by cold pressing, bending, or other processes, delayed fracture may occur due to increased residual stress within the part and a deterioration in the delayed fracture resistance of the steel sheet. Here, delayed fracture refers to a phenomenon in which, when a formed part is placed in a hydrogen penetration environment, hydrogen penetrates into the steel sheet that makes up the part, reducing interatomic bonding strength and causing localized deformation, resulting in microcracks, which then propagate and lead to the destruction of the steel sheet.

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

[0004] For example, Patent Document 1 describes the following in mass %: C: 0.15% or more and 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 of Si and Al: 0.70% or more and 2.50% or less in total, one or two of Mn and Cr: 1.50% or more and 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% and 0.0400% or less, with the balance being Fe and impurities, and the area ratios relative to the entire structure are: one or two of ferrite and granular bainite: 10% to 50% in total; one or two of upper bainite and lower bainite: 10% to 50% in total; more than 0% to 30% of tempered martensite; 5% or more of retained austenite; and one or two or more of pearlite, cementite, and martensite: 0% to 10% in total, and by setting the area ratio of ferrite to the total area ratio of ferrite and granular bainite to 25% or less, a steel sheet having a tensile strength of 980 MPa or more and excellent elongation can be obtained. However, Patent Document 1 discloses poor formability and does not take into consideration chemical conversion treatability or delayed fracture resistance.

[0005] Patent Document 2 discloses that a steel sheet having excellent strength, ductility (hereinafter also referred to as El), and hole expandability (hereinafter referred to as λ) can be obtained by containing, by mass%, 0.10 to 0.40% C, 0.5 to 4.0% Mn, 0.005 to 2.5% Si, 0.005 to 2.5% Al, and 0 to 1.0% Cr, with the balance being iron and unavoidable impurities, with P limited to 0.05% or less, S limited to 0.02% or less, and N limited to 0.006% or less, with the steel structure containing, by area fraction, 2 to 30% retained austenite and martensite limited to 20% or less, with the average grain size of cementite being 0.01 μm to 1 μm, and with the cementite containing 30% to 100% cementite having an aspect ratio of 1 to 3. However, Patent Document 2 does not take delayed fracture resistance into consideration. Furthermore, there are cases where a large amount of Si is contained, and in such cases, application to an automotive cold-rolled steel sheet requires pickling technology as described in Patent Document 3 below. Therefore, since construction of pickling equipment is required or running costs are high, there has been a demand for the establishment of other technologies.

[0006] Patent Document 3 discloses a steel containing, by mass%, C: 0.06 to 0.24%, Si: 0.4% or more but less than 1.60%, Mn: 1.5 to 3.2%, P: 0 to 0.02%, S: 0 to 0.01%, sol.Al: less than 1.0%, and N: less than 0.015%, in which the ratio of the Si content in the steel to the Mn content in the steel (Si / Mn) is less than 0.50, with the balance being iron and unavoidable impurities, and the steel structure has an area fraction of polygonal ferrite: 20% to 85%, an area fraction of upper bainite: 9% to 45%, a volume fraction of retained austenite: 3% to 15%, an area fraction of fresh martensite: 3% to 15%, a total area fraction of tempered martensite and lower bainite: 0% to 50%, and The steel sheet, component, and method for manufacturing the same are provided by setting the ratio of the total number of fresh martensite particles and retained austenite particles having an area ratio of 5% or less and an equivalent circle diameter of less than 1.2 μm to the total number of fresh martensite particles and retained austenite particles to be 50% or more, and the ratio of the total number of fresh martensite particles and retained austenite particles having an aspect ratio of 2.5 or more and an equivalent circle diameter of 1.2 μm or more to the number of fresh martensite particles and retained austenite particles having an equivalent circle diameter of 1.2 μm or more to be 40% or more. However, Patent Document 3 provides a steel sheet, component, and method for manufacturing the same that have a tensile strength of 590 MPa or more, high ductility, excellent stretch flangeability, and good chemical conversion treatability. However, Patent Document 3 is insufficient in formability of steel with a strength of 980 MPa or more, and does not take into consideration crash properties and delayed fracture resistance. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6338038 [Patent Document 2] Patent No. 4903915 Publication [Patent Document 3] Patent No. 7294548 Summary of the Invention [Problem to be solved by the invention]

[0008] As described above, the prior art is still insufficient as a technology for producing a steel sheet that has excellent chemical conversion treatability and excellent delayed fracture resistance while ensuring excellent formability and crashworthiness.

[0009] The present invention has been made to solve these problems, and aims to provide a steel plate, a member, and a method for manufacturing the same, which have a tensile strength of 980 MPa or more and realize excellent formability and crashworthiness as well as good chemical conversion treatability and delayed fracture resistance.

[0010] Here, the tensile strength TS is obtained by a tensile test based on JIS Z 2241 (2011). In addition, excellent formability refers to El × λ, 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 based on JIS Z 2256 (2020), of 600% × % or more. Furthermore, excellent impact resistance means that YS × λ, calculated by multiplying YS (MPa) obtained by a tensile test based on JIS Z 2241 (2011) by the limiting hole expansion ratio λ (%) obtained by a hole expansion test in accordance with JIS Z 2256 (2020), is 30,000 MPa × % or more. In addition, good chemical conversion treatability means that the 2 The steel was subjected to sulfuric acid electrolytic pickling at a current density of 1000 for 2 seconds, followed by degreasing and surface conditioning, and then chemical treatment was performed using a zinc phosphate chemical conversion treatment solution. The degreasing process was at a treatment temperature of 40°C for 120 seconds, and spray degreasing was performed. The surface conditioning process was at pH 9.5, room temperature for 20 seconds, and chemical treatment was performed at a chemical conversion treatment solution temperature of 35°C for 120 seconds. The treatment agents used in the degreasing process, surface conditioning process, and chemical conversion treatment process were, in order, Nihon Parkerizing's degreasing agent: FC-E2011, surface conditioning agent: PL-X, and chemical conversion treatment solution: Palbond PB-L3065. The magnification was 2000x and the image was taken at 10,000 μm. 2The surface conversion structure is observed by SEM observation of the above areas, and the area where the conversion coating structure is not formed is 10% or less of the total measured area. Furthermore, "good delayed fracture resistance" means that the delayed fracture resistance is excellent in the corrosive environment specified in the present invention, that is, the number of days to cracking in the evaluation of metallic materials is 63 days or more.

[0011] The steel sheet referred to in the present invention includes not only a steel sheet having no plating layer, but also a steel sheet having a plating layer formed on the surface of a steel sheet (hereinafter also referred to as a base steel sheet), i.e., a steel sheet having a plating layer and a base steel sheet. [Means for solving the problem]

[0012] The present inventors have conducted extensive research to solve the above problems and have come to the following findings. (1) At the 1 / 4 position of the thickness of the steel sheet (base steel sheet), by setting the area fraction of ferrite to 3% or more and 30% or less and the area fraction of tempered martensite to 60% or more and 94% or less, a TS of 980 MPa or more can be achieved. (2) By setting the volume fraction of retained austenite to 3% or more and 15% or less, and by setting the proportion of high-Mn ferrite in the total ferrite at the 1 / 4 position of the plate thickness to 20% or more and 90% or less, it is possible to achieve excellent formability, crashworthiness, and delayed fracture resistance, with El × λ being 600%·% or more and YS × λ being 30,000 MPa·% or more. (3) Based on the above design, by reducing the Si / Mn ratio and setting the steel sheet surface coverage rate of oxides mainly composed of Si (Si-based oxides) to 1% or less, good chemical conversion treatability can be achieved.

[0013] That is, the gist and configuration of the present invention are as follows. [1] 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; The balance has a composition consisting of Fe and unavoidable impurities, The steel sheet surface coverage rate of Si-based oxides is 1% or less, At 1 / 4 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, The area fraction of the remaining structure is 10% or less, The ferrite is low-Mn ferrite having a Mn concentration less than 0.85 times the Mn content of the steel plate; and high-Mn ferrite having a Mn concentration of 0.85 times or more the Mn content of the steel plate, The proportion of high Mn ferrite in the ferrite is 20% or more and 90% or less in terms of area fraction, TS is 980 MPa or more, El×λ is 600%×% or more, A steel plate having a YS×λ of 30,000 MPa×% or more. [2] The component composition further includes, in mass%, 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, 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, Bi: 0.200% or less, The steel sheet according to [1] above, containing at least one element selected from the following: [3] The steel sheet according to [1] or [2] above, having a plating layer on at least one side of the steel sheet. [4] A member made using the steel plate according to any one of [1] to [3] above. [5] A method for producing a steel sheet according to any one of [1] to [3], A steel material having the chemical composition described in [1] or [2] above, Hot rolling is performed, After obtaining the hot rolled steel sheet, The hot-rolled steel sheet is subjected to cold rolling, After obtaining the cold rolled steel sheet, The cold-rolled steel sheet is annealed, The annealing is The annealing temperature is Ac3-100°C or higher and Ac3 or lower, An annealing and holding step of holding the annealing temperature for a holding time of 5 seconds or more; a first cooling step of cooling from the annealing temperature to a temperature T1 of 550°C or higher at a first average cooling rate of less than 11°C / s; a second cooling step of cooling from the temperature T1 to 500°C at a second average cooling rate of 11°C / s or more; a third cooling step of cooling from 500°C to a temperature T2 of Ms or higher and 320°C or higher for a residence time of 10 seconds or longer and 60 seconds or shorter; a fourth cooling step of cooling the material from the temperature T2 to a temperature T3 of 100°C or higher and Ms-80°C or lower at a fourth average cooling rate of 3°C / s or higher and 50°C / s or lower; The tempering temperature is T3 or higher and 450°C or lower, A tempering process in which tempering is performed under the condition that the tempering time is 5 seconds or more and 1000 seconds or less; a fifth cooling step of cooling the steel sheet at a fifth average cooling rate of 1.0°C / h or more and 50.0°C / h or less in a temperature range of 150 to 80°C. [6] The method for producing a steel sheet according to [5], wherein the surface of the steel sheet is subjected to a plating treatment. [7] A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of [1] to [3] above to at least one of forming and joining to form the component. [Effects of the Invention]

[0014] According to the present invention, there are provided steel plates and members having a tensile strength of 980 MPa or more, and having excellent formability and crashworthiness as well as good phosphatability and delayed fracture resistance, and methods for producing the same. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram illustrating the method for evaluating the delayed fracture properties of metallic materials (HeTsAce). [Figure 2] FIG. 2 is a diagram showing an example of an image of the droplet distribution on the evaluation surface of a metal material. [Figure 3] FIG. 3 is a schematic diagram illustrating a case where a shielding material is placed between a spray nozzle and a metallic material in a method for evaluating delayed fracture properties of a metallic material. [Figure 4] FIG. 4 is a diagram showing a schematic diagram of how the sprayed liquid in the atmosphere re-adheres to the evaluation surface when the distance between the shielding material and the evaluation surface of the metal material is changed. [Figure 5]FIG. 5 is a diagram illustrating an embodiment of a corrosion test cycle according to a method for evaluating delayed fracture properties of a metallic material. [Figure 6] FIG. 6 is a diagram illustrating another embodiment of a corrosion test cycle according to the method for evaluating delayed fracture properties of a metallic material. [Figure 7] FIG. 7 is a diagram schematically showing a test piece for evaluating delayed fracture properties used in the examples. [Figure 8] FIG. 8 is a graph showing the relationship between temperature and time in the method for producing a cold-rolled steel sheet according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] (steel plate) Hereinafter, embodiments of the steel sheet and the manufacturing method thereof according to the present invention will be described. The embodiment described below is an example of a specific embodiment of the present invention, and the configuration of the present invention is not limited to this specific example. The steel sheet of the present invention contains, by 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, with Si / Mn<0.50 and the balance consisting of Fe and unavoidable impurities, and the steel sheet surface coverage rate of Si-based oxides is 1% or less, At a quarter-thickness position of the plate, 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 comprises low-Mn ferrite having a Mn concentration less than 0.85 times the Mn content of the steel sheet, and high-Mn ferrite having a Mn concentration 0.85 times or more the Mn content of the steel sheet, and the proportion of the high-Mn ferrite in the ferrite is 20% or more and 90% or less in area fraction, and the base steel sheet has TS of 980 MPa or more, El × λ of 600% × % or more, and YS × λ of 30,000 MPa × % or more.

[0017] (Composition of steel plate) The chemical composition of the steel sheet (base steel sheet) of the present invention will be explained below. In the following explanation, all percentages indicating the steel composition are by mass unless otherwise specified.

[0018] [C: 0.030% or more and 0.300% or less] C is one of the important basic components of steel sheet, and in the present invention, it is an important element that affects the total 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, the total area fraction of ferrite increases, making it difficult to achieve a TS of 980 MPa or more, and a sufficient amount of retained austenite may not be obtained, making it impossible to achieve the desired ductility. Therefore, the C content is set to 0.030% or more, preferably 0.040% or more, and more preferably 0.050% or more. The C content is 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 embrittled and the delayed fracture resistance deteriorates. Therefore, the C content is set to 0.300% or less, and preferably 0.290% or less. The C content is more preferably 0.170% or less, and further preferably 0.160% or less.

[0019] [Si: 0.40% or more and 1.60% or less] Si is added from the viewpoints of realizing high strength of the ferrite structure and obtaining the effect of stabilizing retained austenite (residual γ) by suppressing the formation of carbides 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, the Si content is preferably set to 0.50% or more. The Si content is more preferably set to 0.60% or more. On the other hand, if the Si content exceeds 1.60%, the chemical conversion treatability is significantly deteriorated. Therefore, the Si content is set to 1.60% or less. The Si content is preferably 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% or more and 3.20% or less] Mn is contained to ensure a predetermined hardenability, suppress ferrite transformation, ensure a desired area fraction of tempered martensite, and ensure strength. Furthermore, Mn concentrates in γ during ferrite / γ two-phase annealing, lowering the Ms point of untransformed γ, thereby stabilizing the residual γ and improving ductility. Furthermore, Mn increases the volume fraction of the residual γ, improving ductility. From these points of view, Mn is an important element in the present invention. To achieve these effects, the Mn content is set to 1.40% or more. The Mn content is preferably 1.50% or more. From the viewpoint of improving hardenability, the Mn content is more preferably 1.70% or more. The Mn content is even more preferably 1.90% or more. On the other hand, if the Mn content exceeds 3.20%, the delayed fracture resistance may be deteriorated 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 coarse γ aggregates, and formability also deteriorates. Therefore, the Mn content is set to 3.20% or less, preferably 3.00% or less, and more preferably 2.80% or less.

[0021] [P:0.100% or less] P segregates at prior austenite grain boundaries, embrittling the grain boundaries and thus embrittling the substrate steel sheet. Therefore, if the P content exceeds 0.100%, the formability and delayed fracture resistance deteriorate. Therefore, the P content is set to 0.100% or less, and 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, although there is no particular lower limit for the P content, 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, more preferably 0.005% or more, and even more preferably 0.007% or more.

[0022] [S:0.0200% or less] S exists as sulfide and embrittles the base steel sheet, so if the S content exceeds 0.0200%, formability and delayed fracture resistance will decrease. Therefore, the S content is set to 0.0200% or less, and 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, although there is no particular lower limit for the S content, due to constraints on production technology, the S content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0007% or more.

[0023] [Al: 1.000% or less] Since Al exists as an oxide and embrittles the substrate steel sheet, an Al content exceeding 1.000% may result in a decrease in formability and delayed fracture resistance. Therefore, the Al content is set to 1.000% or less, and preferably 0.500% or less. The Al content is more preferably 0.200% or less, and further preferably 0.100% or less. On the other hand, although there is no particular lower limit for the Al content, since Al suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite, the Al content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.

[0024] [N:0.0100% or less] N exists as nitrides and embrittles the substrate steel sheet, and if the N content exceeds 0.0100%, formability and delayed fracture resistance deteriorate. Therefore, the N content is set to 0.0100% or less, and 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, although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0001% or more, 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 substrate steel sheet, and if the O content exceeds 0.0100%, formability and delayed fracture resistance deteriorate. Therefore, the O content is set to 0.0100% or less, and preferably 0.0090% or less. The O content is more preferably 0.0080% or less, and even more preferably 0.0070% or less. On the other hand, although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0026] [Si / Mn<0.50] Examples of surface oxides on steel sheets that significantly deteriorate chemical conversion treatability include Si-based oxides. Therefore, in order to form Mn-containing oxides that are readily soluble in acid solutions, the ratio of Sn content (mass%) to Mn content (mass%), Si / Mn, 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. Although there is no particular lower limit, Si / Mn is preferably 0.15 or more, and more preferably 0.20 or more.

[0027] The steel sheet of the present invention has a composition containing the above-mentioned elemental elements as basic components, with the balance including Fe and unavoidable impurities. Here, the steel sheet of the present invention preferably has a composition containing the above-mentioned basic components, with the balance consisting of Fe and unavoidable impurities.

[0028] (Optional components of steel plate) In addition to the basic components described above, the steel sheet further contains, by mass%, 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 formed, which reduces the ultimate deformability of the steel and results in poor hole expandability. Therefore, when As is added, the As content is set to 0.100% or less. The As content is preferably 0.090% or less. The As content is more preferably 0.070% or less. Addition of an appropriate amount of As suppresses the formation of sulfides and improves hole expandability. Therefore, the As content is preferably 0.001% or more, and more preferably 0.010% or more.

[0030] [Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less] When the content of each of Ti, Nb, and V is 0.200% or less, large amounts of coarse precipitates or inclusions are not formed, and the ultimate deformability of the steel sheet (base steel sheet) is not reduced, so that formability and delayed fracture resistance are not reduced. Therefore, when one or more of Ti, Nb, and V are contained, the content of each is set to 0.200% or less, and preferably 0.100% or less. The V content is more preferably 0.080% or less, and further preferably 0.060% or less. On the other hand, although there are no particular lower limits for the contents of Ti, Nb, and V, these elements form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, thereby increasing the strength of the base steel sheet, so the contents of Ti, Nb, and V are each preferably 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 further preferably 0.020% or more. The V content is more preferably 0.005% or more, and further preferably 0.020% or more.

[0031] [Ta: 0.10% or less, W: 0.10% or less] If the Ta and W contents are 0.10% or less, large amounts of coarse precipitates and inclusions are not formed, and the base steel sheet is not embrittled, so that formability and delayed fracture resistance are not impaired. Therefore, when at least one of Ta and W is contained, the contents thereof are each set to 0.10% or less, and preferably 0.08% or less. On the other hand, although there are no particular lower limits for the contents of Ta and W, these elements increase the strength of the base steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing, so the contents of Ta and W are preferably 0.01% or more, respectively.

[0032] [B:0.0100% or less] If the B content is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the base steel sheet will not be embrittled, so that formability and delayed fracture resistance will not be impaired. Therefore, if B is contained, the B content is set to 0.0100% or less, and preferably 0.0080% or less. On the other hand, although there is no particular lower limit for the B content, since B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, the B content is preferably 0.0003% or more, 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] When the content of each of Cr, Mo, and Ni is 1.00% or less, the amount of coarse precipitates and inclusions does not increase, and the base steel sheet is not embrittled, so that formability and delayed fracture resistance are not impaired. Therefore, when one or more of Cr, Mo, and Ni are contained, the content of each is set to 1.00% or less, and preferably 0.80% or less. The Cr content is more preferably 0.20% or less. On the other hand, although there is no particular lower limit for the content of Cr, Mo, and Ni, these elements improve hardenability, so the content of Cr, Mo, and Ni is preferably 0.01% or more. The Cr content is more preferably 0.010% or more, and further preferably 0.020% or more. The Ni content is more preferably 0.010% or more, and further 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 does not increase, and the base steel sheet is not embrittled, so that the formability and delayed fracture resistance are not deteriorated. Therefore, if Co is contained, the Co content is set to 0.050% or less, and preferably 0.010% or less. On the other hand, although there is no particular lower limit for the Co content, since Co improves hardenability, the Co content is preferably 0.001% or more, 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 base steel sheet will not be embrittled, so that the formability and delayed fracture resistance will not be reduced. Therefore, if Cu is contained, the Cu content should be 1.00% or less, and preferably 0.80% or less. On the other hand, although there is no particular lower limit for the Cu content, since Cu improves hardenability, the Cu content is preferably 0.01% or more, 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 Sn content will not embrittle the base steel sheet, so that formability and delayed fracture resistance will not deteriorate. Therefore, if Sn is contained, the Sn content is set to 0.200% or less, and preferably 0.100% or less. On the other hand, although there is no particular lower limit for the Sn content, since Sn improves hardenability, the Sn content is preferably 0.001% or more, 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 will not increase, and the Sb content will not embrittle the base steel sheet, so that the formability and delayed fracture resistance will not deteriorate. Therefore, if Sb is contained, its content should be 0.200% or less, and preferably 0.100% or less. On the other hand, although there is no particular lower limit for the Sb content, since Sb controls the softened surface thickness and enables strength adjustment, the Sb content is preferably 0.001% or more, 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] When the Ca, Mg, and REM contents are each 0.0100% or less, coarse precipitates and inclusions do not increase, the base steel sheet is not embrittled, and formability and delayed fracture resistance are not impaired. Therefore, when one or more of Ca, Mg, and REM are contained, their contents should each be 0.0100% or less, and preferably 0.0050% or less. On the other hand, although there are no particular lower limits for the contents of Ca, Mg, and REM, these elements spheroidize the shape of nitrides, sulfides, etc., and improve the ultimate deformability of the base steel sheet, so it is preferable that the contents of Ca, Mg, and REM are each 0.0005% or more. The Ca content is more preferably 0.0010% or more, and further preferably 0.0020% or more. The Mg content is more preferably 0.0010% or more, and further preferably 0.0020% or more. The REM content is more preferably 0.0010% or more, and further preferably 0.0020% or more. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanoid elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content in the present invention refers to the total content of one or more elements selected from the above-mentioned REM. The REM is not particularly limited, but 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] When the content of Zr, Zn, Pb, and Te is 0.100% or less, the amount of coarse precipitates or inclusions does not increase, the base steel sheet is not embrittled, and formability and delayed fracture resistance are not impaired. Therefore, when one or more of Zr, Zn, Pb, and Te are contained, the content of each is set to 0.100% or less, and preferably 0.080% or less. On the other hand, although there are no particular lower limits for the contents of Zr, Zn, Pb, and Te, these elements spheroidize the shape of nitrides, sulfides, etc., and improve the ultimate deformability of the base steel sheet, so the contents of Zr, Zn, Pb, and Te are each preferably 0.001% or more. The Zr content is more preferably 0.025% or more, and further preferably 0.040% or more. The Zn content is more preferably 0.008% or more, and further preferably 0.016% or more. The Pb content is more preferably 0.008% or more, and further preferably 0.020% or more. The Te content is more preferably 0.010% or more, and further 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 each of Se, Ga, Ge, and Sr is 0.020% or less, the amount of coarse precipitates or inclusions will not increase and the base steel sheet will not become embrittled, so formability and delayed fracture resistance will not deteriorate. Therefore, if one or more of Se, Ga, Ge, and Sr are contained, the content of each should be 0.020% or less. On the other hand, although there are no particular lower limits for the contents of Se, Ga, Ge, and Sr, these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformability of the substrate steel sheet, so the contents of Se, Ga, Ge, and Sr are each preferably 0.001% or more, and 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 will not increase and the Hf content will not embrittle the base steel sheet, so that the formability and delayed fracture resistance will not deteriorate. Therefore, if Hf is contained, the Hf content should be 0.10% or less, and preferably 0.08% or less. On the other hand, although there is no particular lower limit for the Hf content, since Hf spheroidizes the shape of nitrides or sulfides and improves the ultimate deformability of the substrate steel sheet, the Hf content is preferably 0.01% or more, and 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 base steel sheet will not be embrittled, so that the formability and delayed fracture resistance will not be deteriorated. Therefore, if Bi is contained, the Bi content is set to 0.200% or less, and preferably 0.100% or less. On the other hand, although there is no particular lower limit for the Bi content, since Bi reduces segregation, the Bi content is preferably set to 0.001% or more. The Bi content is more preferably 0.004% or more, and further preferably 0.008% or more.

[0043] In addition, when the content of each of 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 is less than the respective preferable lower limit, the effect of the present invention is not impaired. Therefore, each content may be less than the respective preferable lower limit, in which case it is treated as an unavoidable impurity.

[0044] (Surface condition of steel plate) Next, the surface state of the steel sheet of the present invention will be described. [Steel sheet surface coverage rate of Si-based oxides: 1% or less] Oxides mainly containing Si (Si-based oxides) are poorly soluble in chemical conversion treatment solutions, and the presence of Si-based oxides on the steel sheet surface significantly reduces chemical conversion treatability. Therefore, the coverage of the steel sheet surface with Si-based oxides is set to 1% or less, preferably 0%. Here, the steel sheet surface coverage refers to the area ratio (%) of Si-based oxides to the steel sheet surface. Examples of oxides mainly containing Si include SiO2 and MnSiO3, but in the present invention, Mn-rich Si-Mn composite oxides are not considered to be oxides mainly containing Si because they are readily soluble in chemical conversion treatment solutions and do not deteriorate chemical conversion treatability even when present in the surface layer. Here, the surface coverage of oxides mainly containing Si is determined by observing the steel sheet surface at 1000x magnification using an SEM with a field of view of 87 μm × 128 μm in five fields, and analyzing the same fields of view with EDX to identify oxides mainly containing Si, and then determining the coverage using the point counting method.

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

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

[0047] [Area fraction of high-Mn ferrite in ferrite: 20% to 90%] By generating high-Mn ferrite, the Mn concentration in the parent martensite can be reduced. This prevents the deterioration of delayed fracture resistance due to MnS and Mn segregation. Furthermore, high-Mn ferrite is a relatively hard ferrite that 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 the low-Mn ferrite and the parent martensite, which has a large difference in hardness, and suppresses the formation of voids during processing. To ensure a high YS, a high hole expansion ratio, and excellent delayed fracture resistance, the area fraction of high-Mn ferrite in the ferrite is set to 20% or more. It is preferably 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 is set to 90% or less, preferably 85% or less, and more preferably 80% or less.

[0048] High-Mn ferrite refers to ferrite with a Mn concentration of 0.85 times or more the Mn content of the steel sheet, and low-Mn ferrite refers to ferrite with a Mn concentration of less than 0.85 times the Mn content of the steel sheet.

[0049] The Mn concentration in ferrite was measured as follows. First, the obtained steel sheet was polished using diamond paste so that the cross section (L cross section) parallel to the rolling direction would be the observation surface. The observation surface was mirror-finished 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 beam microanalyzer (FE-EPMA) equipped with a field emission electron gun. Measurement conditions were as follows: an acceleration voltage of 7 kV and a current of 50 nA (T. Yamashita, Y. Tanaka, M. Nagoshi and K. Ishida: Sci. Rep., 6 (2016), DOI: 10.1038 / srep29825). The measured data was converted to Mn concentration using a calibration method, and an elemental mapping image of Mn was obtained. The mapping measurement was performed by heating the sample to 100°C and maintaining it there, under conditions that prevented contamination. In the obtained elemental mapping image, areas where the Mn concentration was less than 0.85 times or more than 0.85 times the Mn concentration of the steel sheet were identified. More specifically, ferrite grains were identified by referring to SEM images of the same field of view as the element mapping images, and low-Mn ferrite and high-Mn ferrite were identified.

[0051] [Area fraction of tempered martensite: 60% to 94%] To ensure a tensile strength of 980 MPa or more, the area fraction of tempered martensite is set to 60% or more, preferably 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 is set to 94% or less, preferably 92% or less, and more preferably 90% or less.

[0052] The tempered martensite content is measured as follows: After polishing the L-section of the steel sheet, it is corroded with 3 vol.% nital, and the 1 / 4 position in the sheet thickness direction (the position corresponding to 1 / 4 of the sheet thickness in the depth direction from the steel sheet surface) is observed using an SEM at 2000x magnification with a field of view of 44 μm × 64 μm, with 10 fields of view, and the area fraction is measured by image processing. In the above structural image, tempered martensite is observed as a white fine structure containing carbides. The area fraction of tempered martensite can be calculated from the average of these values.

[0053] [Volume fraction of retained austenite: 3% to 15%] In order to ensure high ductility, the volume fraction of retained austenite is set to 3% or more, preferably 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, making it impossible to achieve a tensile strength of 980 MPa or more. Therefore, the volume fraction of retained austenite is set to 15% or less, preferably 13% or less, and more preferably 10% or less.

[0054] The method for measuring retained austenite is as follows: The steel plate was polished from 1 / 4 of the plate thickness down to a surface 0.1 mm deep, and then chemically polished a further 0.1 mm down to a surface. The retained austenite fraction was determined by measuring the integrated intensity ratios of the diffraction peaks of the {200}, {220}, and {311} planes of fcc iron and the {200}, {211}, and {220} planes of bcc iron using CoKα radiation in an X-ray diffractometer, and then averaging the nine integrated intensity ratios obtained. Considering that the retained austenite is three-dimensionally homogeneous, the volume fraction of the retained austenite can be taken as the area fraction of the retained austenite.

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

[0056] Here, 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 described above, and subtracting their total from 100%. 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 ferrite and tempered martensite, which are expressed as area fractions.

[0057] The steel sheet of the present invention may have a plating layer on its surface (at least one side). Examples of the plating layer include a zinc plating layer such as a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, and an electrogalvanized layer. Examples of plating layers other than a zinc plating layer include an aluminum plating layer and an alloy plating layer. 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 sheet of the present invention is preferably 0.5 mm or more, and more preferably 3.0 mm or less.

[0059] (Steel plate characteristics) [Tensile strength (TS): 980 MPa or more] The steel plate 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 part width 25 mm) is taken from the test material so that the longitudinal direction of the test piece is perpendicular to the rolling direction, and a tensile test is performed in accordance with JIS Z 2241 (2011). The conditions for the tensile test are a crosshead speed of 1.67 x 10 -1 It can be mm / sec.

[0060] [YS×λ: 30,000 MPa×% or more] To have excellent crashworthiness, a high YS-λ balance is required, and therefore, a YS×λ of 30,000 MPa×% or more is preferred. YS can be determined by the tensile test described above, just like 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 to 100 mm x 100 mm, a 10 mm diameter hole was punched with a clearance of 12.5%. A 75 mm inner diameter die was used to hold the test material down with a blank holding force of 9 tons (88.26 kN), and a conical punch with an apex angle of 60° was pressed into the punched hole to check for the occurrence of a crack penetrating the plate thickness. The hole diameter when a crack occurred was measured, and the limiting hole expansion ratio: λ (%) could be calculated using the following equation (3). λ(%)={(D f -D0) / D0}×100 (3) where D f is the hole diameter (mm) when the crack occurs, and D0 is the initial hole diameter (mm).

[0061] [El×λ: 600%×% or more] To have excellent workability, a high El-λ balance is required. For this reason, the steel sheet of the present invention has El×λ of 600%×% or more. Note that elongation El can be determined by the above-mentioned tensile test, similar to TS.

[0062] [Conversion evaluation] For annealed steel sheets, 20 to 35 A / dm 2The steel was subjected to sulfuric acid electrolytic pickling at a current density of 1000 kJ for 2 seconds, followed by degreasing and surface conditioning, and then chemical conversion treatment was performed using a zinc phosphate chemical conversion treatment solution. The chemical conversion treatment was performed as follows: degreasing step: treatment temperature 40°C, treatment time 120 seconds, spray degreasing; surface conditioning step: pH 9.5, treatment temperature room temperature, treatment time 20 seconds; chemical conversion treatment step: chemical conversion treatment solution temperature 35°C, treatment time 120 seconds. The treatment agents used in the degreasing step, surface conditioning step, and chemical conversion treatment step, respectively, were degreaser FC-E2011, surface conditioner PL-X, and chemical conversion treatment solution Palbond PB-L3065, all manufactured by Nihon Parkerizing Co., Ltd. Magnification: 10,000 μm at 2,000x 2 The surface conversion structure was observed by SEM observation of the above areas, and the area where the conversion coating structure was not formed was evaluated as ○ (pass) if the area where the conversion coating structure was not formed was 10% or less of the total measured area, and the area where the conversion coating structure was not formed was evaluated as × (fail) if the area where the conversion coating structure was not formed was more than 10% of the total measured area.

[0063] [Delayed fracture resistance] In the evaluation method for delayed fracture properties of metallic materials (HeTsAce), the cracking period is 63 days or more. The steel sheet of the present invention can be judged to have excellent delayed fracture resistance in a corrosive environment, according to the method for evaluating delayed fracture resistance in a corrosive environment (HeTsAce) of metallic materials. Specifically, the steel sheet of the present invention has a cracking period of 63 days or more according to the following evaluation method for delayed fracture properties of metallic materials (HeTsAce).

[0064] (HeTsAce) The amount of chloride attached to the evaluation surface of the metal material being evaluated is 1000 to 20,000 mg / m 2 a chloride deposition step (A) of depositing droplets of a chloride-containing aqueous solution so that the chloride-containing aqueous solution is a corrosion process (B) in which a cycle including the following drying process (b1), the following wetting process (b2), the following transition process (b3), and the following transition process (b4) is performed one or more times in an atmosphere at a temperature Tb1 of 60°C or less and within a certain range, The distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first chloride deposition step (A) is Average contact area of ​​the droplet on the evaluation surface of the metal material: 0.1 mm 2 Over 3.0mm 2 less than, the 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 the 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 properties of metallic materials. Drying step (b1): A step of drying a metal material by maintaining it in an atmosphere of a relative humidity Hb1 of 45% or less for 1.0 hour or more and 5.0 hours or less; Wetting step (b2): A step of wetting the metal material by holding it in an atmosphere of a relative humidity Hb2 of 80% or more for 1.0 hour or more and 5.0 hours or less; transition step (b3): ​​a step of transitioning from an atmosphere having the relative humidity Hb1 to an atmosphere having the relative humidity Hb2 at a rate of change in relative humidity of 30% / h or less; Transition step (b4): A step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 at a rate of change in relative humidity of 30% / h or less.

[0065] In the present invention, a metallic material extracted from a steel plate is evaluated. The method for extracting the metallic material from the steel plate is not particularly limited. For example, the metallic material can be extracted by shearing the steel plate to a predetermined size. In HeTsAce, it is preferable to apply stress to the metallic material to be evaluated. Examples of methods for applying stress to the metallic material include a method of processing the metallic material (processing method). Examples of processing methods include bending, bulging, stretching, and twisting. Other examples include a method of fixing the metallic material in a stressed shape using bolts or the like, and a method of using residual stress remaining after processing. In the evaluation method of the present invention, a process including a chloride adhesion process (A) and a corrosion process (B) can be performed at least once (once or twice or more) on the metallic material to which stress has been applied as described above. Furthermore, in the evaluation method of the present invention, the state of the metallic material is confirmed after the process including the chloride adhesion process (A) and the corrosion process (B) is performed once or more times, and the delayed fracture properties of the metallic material can be evaluated based on the confirmed state of the metallic material. The confirmation can be carried out, for example, by visually observing the presence or absence of cracks in the metal material and the extent of the cracks.

[0066] In the present invention, in order to evaluate the delayed fracture properties of a metallic material, a chloride adhesion step (A) and a corrosion step (B) are carried out while applying stress to the metallic material. After carrying out each of these steps one or more times, the presence or absence and degree of cracking in the metallic material are confirmed, thereby evaluating the delayed fracture properties.

[0067] (Chloride attachment step (A)) The chloride deposition step (A) is carried out by depositing a chloride on the evaluation surface of the metal material in an amount of 1000 to 20000 mg / m 2 In the evaluation method of the present invention, the distribution of the droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first (first) chloride deposition step (A) (deposition distribution of the droplets) is such that the average contact area of ​​the droplets on the evaluation surface of the metal material is 0.1 mm 2 Over 3.0mm 2Less than, the area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material: 40% to 80% and the standard deviation of the contact area of ​​the droplets on the evaluation surface of the metal material: 3.0 mm 2 The following applies.

[0068] Chloride adhesion amount: 1000 to 20,000 mg / m 2 > The amount of chloride attached to the metal material (the amount of solid chloride not including solvents such as water) is 1000 to 20000 mg / m 2 The above-mentioned adhesion amount corresponds to the amount of chloride adhesion assumed to occur in the atmospheric corrosive environment in which an actual automobile runs. 2 In a corrosive environment where the coating weight is less than 20,000 mg / m, corrosion hardly progresses, so hydrogen generation and penetration into the metal material are minimal, making delayed fracture unlikely. 2 If the coating amount exceeds 20,000 mg / m, the corrosion rate will be significantly different from that in the actual environment, resulting in an excessive durability test that will not serve the purpose. 2 In order to simulate the corrosion pattern in the atmospheric corrosive environment in which an actual automobile runs and to promote corrosion, the coating amount is set to 5000 mg / m 2 From the above viewpoint, the amount of adhesion is preferably more than 12000 mg / m 2 The following is preferred:

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

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

[0071] Furthermore, when simulating delayed fracture characteristics in areas where snow-melting agents are frequently sprayed in winter, it is preferable that the chlorides attached to the metal material have a composition similar to that of the snow-melting agents sprayed in those areas. Examples of components having a composition similar to that of snow-melting agents include a component mainly composed of CaCl2 (a component in which CaCl2 is more than 50 mass% of the total components), a component mainly composed of MgCl2 (a component in which MgCl2 is more than 50 mass% of the total components), and a component mainly composed of NaCl (a component in which NaCl is more than 50 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. Examples of a component containing a combination of multiple metal salts include the Society of Automotive Engineers standard (SAE J2334) (0.5% by mass NaCl-0.1% by mass CaCl-0.075% by mass NaHCO), artificial seawater (2.5% by mass NaCl-0.5% by mass MgCl-0.12% by mass CaCl-0.07% by mass KCl, and others (e.g., an aqueous solution of Aquamarine (registered trademark) manufactured by Yashima Pharmaceutical Co., Ltd.)).

[0073] The method for depositing chloride on a metal material (chloride deposition method) is not particularly limited as long as it is a method that can achieve a desired distribution of chloride-containing aqueous solution droplets in terms of evaluating the metal material. Examples of the chloride-containing aqueous solution include a chloride-containing aqueous solution containing a component mainly composed of chloride (usually an aqueous solution such as salt water, hereinafter also referred to as salt water). The following description will be given taking the case where salt water is used as the chloride-containing aqueous solution as an example.

[0074] The spray method is an example of a chloride deposition method. An example of a spray method is a method in which salt water is deposited using a spray nozzle. Types of spray nozzles include one-fluid spray nozzles (nozzles in which a liquid fed under pressure is atomized and sprayed) and two-fluid spray nozzles (nozzles that atomize the liquid using a high-speed fluid such as compressed air). Two-fluid spray nozzles also differ in the liquid supply method, and are classified into liquid pressure types (liquid is pressurized and supplied to the two-fluid nozzle) and suction types (liquid is sucked up and sprayed using the force of compressed air). It is preferable to select a spray nozzle that ensures a uniform deposition distribution of droplets. Furthermore, since salt water is used, it is preferable to use a corrosion-resistant metal such as stainless steel as the material for the spray nozzle.

[0075] The chloride concentration in the saltwater is not particularly limited. However, when controlling the saltwater droplet distribution using a spray nozzle, if saltwater with a chloride concentration of less than 2.0 mass% is used to deposit saltwater on a metal material, the spray time is long to achieve a suitable chloride deposition amount, making it difficult to obtain the desired droplet distribution on the evaluation surface of the metal material. Therefore, the chloride concentration in the saltwater is preferably 2.0 mass% or more, and more preferably 5.0 mass% or more. On the other hand, if saltwater with a chloride concentration of more than 20 mass% is used to deposit saltwater on a metal material, chloride precipitation is likely to occur in the spray nozzle, causing clogging, making it difficult to spray saltwater droplets of consistent 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 deposition on the evaluation surface of the metal material varies depending on the location, and in areas with a high chloride deposition amount, localized corrosion occurs and the amount of hydrogen penetration increases. This results in changes in the delayed fracture properties within the evaluation surface of the metal material, reducing the accuracy of the evaluation of delayed fracture properties. This tendency is particularly pronounced when the chloride deposition amount is high. Therefore, the chloride concentration in the salt water is preferably 20% by mass or less, and more preferably 15% by mass or less.

[0076] It is recommended to adjust the chloride concentration in the saltwater according to the target chloride deposition amount. It is preferable to use low-concentration saltwater when the chloride deposition amount is relatively low, and high-concentration saltwater when the chloride deposition amount is relatively high. To reduce the chloride deposition amount on the test surface of the metal material using high-concentration saltwater, the total amount of saltwater sprayed must be reduced, which can lead to an undersized average contact area of ​​saltwater droplets on the test surface of the metal material and a ratio of the total contact area of ​​saltwater droplets to the total area of ​​the test surface. On the other hand, to increase the chloride deposition amount on the test surface of the metal material using low-concentration saltwater, the total amount of saltwater sprayed must be increased, which can lead to an oversized average contact area of ​​saltwater droplets on the test surface of the metal material and a ratio of the total contact area of ​​saltwater droplets to the total area of ​​the test surface.

[0077] [Distribution of droplets of chloride-containing aqueous solution deposited on the evaluation surface of the metal material in the initial chloride deposition 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 deposition step (A) within the range described below. It is believed that the locations where saltwater was present on the evaluation surface of the metal material in the first chloride deposition step primarily become the starting points of corrosion on the evaluation surface. Furthermore, in the second chloride deposition step, saltwater spray occurs on the evaluation surface in a state where corrosion has occurred after the first chloride deposition step and corrosion products have formed on the evaluation surface. Even if saltwater droplets are uniformly deposited on the evaluation surface of the metal material, wetting and spreading occurs due to the influence of the corrosion products. Therefore, it is important to uniformly control the distribution of saltwater droplets on the evaluation surface of the metal material in the first chloride deposition step.

[0078] Fig. 1 is a schematic diagram illustrating an example of the evaluation method of the present invention. As shown in Fig. 1, in the evaluation method of the present invention, salt water is sprayed from a spray nozzle to deposit droplets of salt water onto the evaluation surface of a metal material. In this process, in the initial chloride deposition step (A), the distribution of salt water droplets deposited on the evaluation surface of the metal material (deposition distribution of droplets) is controlled within a predetermined range.

[0079] Average contact area of ​​droplets on the evaluation surface of metal materials: 0.1 mm 2 Over 3.0mm 2 Less than> In the evaluation method of the present invention, in at least the first chloride adhesion 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 Over 3.0mm 2 The average contact area is less than 0.1 mm 2 If the average contact area is less than 0.1 mm, the volume of the droplets is too small to achieve the target amount of chloride deposition. 2 The average contact area is 0.5 mm 2 It is preferable that the thickness is 1.0 mm or more. 2 On the other hand, it is more preferable that the average contact area is 3.0 mm or more. 2 If the average contact area is more than 3.0 mm, the droplet adhesion distribution becomes non-uniform, resulting in large variations in the delayed fracture evaluation. 2 The average contact area shall be less than 2.8 mm 2 It is preferable that it is 2.5 mm or less. 2 It is more preferable that the average contact area of ​​the droplets on the evaluation surface of the metal material be as follows: The average contact area of ​​the droplets on the evaluation surface of the metal material can be measured by the measurement method described below.

[0080] <Area ratio of the total contact area of ​​droplets to the area of ​​the evaluation surface of the metal material: 40% to 80%> In the evaluation method of the present invention, in at least the first chloride adhesion step (A), the area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material (total contact area ratio of droplets) is set to 40% or more and 80% or less. If the total contact area ratio of droplets is less than 40%, the droplet adhesion distribution will be non-uniform, resulting in large variations in delayed fracture evaluation. Therefore, the total contact area ratio of droplets is set to 40% or more. The total contact area ratio of droplets is preferably set to 50% or more, more preferably 55% or more. On the other hand, if the total contact area ratio of droplets is greater than 80%, adjacent droplets on the evaluation surface will be more likely to bond with each other, and the average contact area of ​​the droplets (average contact area per droplet) will be more likely to coarsen. Therefore, the total contact area ratio of droplets is set to 80% or less. The total contact area ratio of droplets is preferably set to 75% or less, more preferably 70% or less. The total contact area ratio of droplets can be measured using the measurement method described below.

[0081] <Standard deviation of droplet contact area on the evaluation surface of the metal material: 3.0 mm 2 Below> In the evaluation method of the present invention, in at least the first chloride adhesion step (A), the standard deviation in the distribution of the contact area of ​​the droplets on the evaluation surface of the metal material is 3.0 mm 2 The standard deviation of the contact area of ​​the droplet on the evaluation surface of the metal material is 3.0 mm or less. 2 If the contact area is larger than this, the droplets will not adhere uniformly, resulting in a large variation in the delayed fracture evaluation. 2 The standard deviation of the contact area of ​​the droplets is 2.8 mm 2 It is preferable that it is 2.5 mm or less. 2 It is more preferable that the standard deviation of the contact area of ​​the droplets is as follows: The standard deviation of the contact area of ​​the droplets can be measured by the measurement method described below.

[0082] The distribution of saltwater droplets attached to the evaluation surface of the metal material (average droplet contact area, total droplet contact area ratio, and standard deviation of droplet contact area) can be determined by attaching saltwater droplets to the evaluation surface of the metal material in the chloride attachment step (A), acquiring an image of the droplet distribution across the entire evaluation surface of the metal material, and performing image analysis. The image can be acquired using a digital camera, microscope, optical microscope, or the like. 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 image of the evaluation surface can be acquired inside a test tank equipped with a spray nozzle, or by removing the metal material from the test tank. Preferably, in the latter case, the metal material is removed from the test tank equipped with a spray nozzle and the evaluation surface of the metal material is photographed. The image is also acquired immediately (within 30 seconds) after the saltwater droplets are attached to the evaluation surface of the metal material.

[0083] Figure 2 is a schematic diagram showing an image of the droplet distribution on the evaluation surface of the metal material obtained as described above. In Figure 2, the area indicated by a circle is the contact area of ​​the droplets. 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 contact area of ​​the droplets are determined by image analysis.

[0084] Here, the evaluation surface of a metallic material refers to the surface of the metallic material for evaluating the delayed fracture properties. The evaluation surface can be determined appropriately depending on the metallic material to be evaluated. For example, if the metallic material is a plate, the evaluation surface can be the surface of the plate facing the spray nozzle (see FIG. 1). Furthermore, if stress is applied to the metallic material, the evaluation surface can be the surface of the stressed portion 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 upward). More specifically, for example, if a bent metallic material is to be evaluated, as described below, the evaluation surface can be the surface of the bent portion facing the spray nozzle (see FIG. 7).

[0085] As a method for achieving the above-mentioned distribution of saltwater droplets, there is a method in which saltwater is applied to the evaluation surface of the metal material using a spray nozzle, as described above. A two-fluid nozzle is preferable 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 salt water spray time is preferably 10 seconds or less. As mentioned above, it is also preferable to adjust the chloride concentration in the salt water according to the target chloride adhesion amount. 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] A particularly preferred method for achieving the above-described saltwater droplet distribution is to place a shielding material having an opening between the spray nozzle and the metal material, and 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. The opening of the shielding material preferably has a shape and size substantially equivalent to the shape and size of the evaluation surface of the metal material. "Substantially equivalent" means that the opening of the shielding material is equivalent to the peripheral shape and size of the evaluation surface of the metal material when viewed from above, 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, when viewed from above, the evaluation surface of the metal material can be seen through the opening of the shielding material, while other areas are not visible (are shielded).

[0088] Figure 3 is a schematic diagram illustrating the case where a shielding material is placed between the spray nozzle and the metal material in the evaluation method of the present invention. By placing the above-mentioned shielding material between the spray nozzle and the metal material as shown in Figure 3, it is possible to prevent the spray liquid (atomized salt water) that is sprayed from the spray nozzle but floats in the atmosphere without adhering to the evaluation surface of the metal material from adhering (re-adhering) to the evaluation surface of the metal material to which droplets have already adhered after the salt water spray has ended. As a result, it is possible to achieve a desired droplet distribution on the evaluation surface of the metal plate with high precision.

[0089] When a shielding material is used, the distance X from the tip of the spray nozzle to the evaluation surface of the metal material is preferably 10 to 30 cm. The distance between the shielding material and the evaluation surface of the metal material (Y in FIG. 3) is preferably 1 cm or more and 0.3X cm or less. If the distance Y is less than 1 cm, the sprayed liquid that does not adhere to the evaluation surface of the metal material and floats in the atmosphere remains near the evaluation surface of the metal material, reducing the effect of suppressing redeposition (FIG. 4(a)). On the other hand, if the distance Y is greater than 0.3X cm, the sprayed liquid that passes through the opening of the shielding material scatters below the shielding material, reducing the effect of suppressing redeposition (FIG. 4(b)). Note that the distance Y is the shortest distance from the shielding material to the evaluation surface of the metal material. The material of the shielding material is not limited as long as it can prevent the transmission of the sprayed liquid. Examples of materials include resin, ceramic, metal, and wood. These materials can be processed and used as the shielding material.

[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 initial (first) chloride deposition step (A). The droplet distribution control described above is performed at least during the initial chloride deposition step (A). The average droplet contact area, total droplet contact area ratio, and standard deviation of droplet contact area can be measured at the end of the initial chloride deposition step (A) (within 30 seconds after the end of saltwater spraying). Alternatively, a test specimen other than the one being tested may be pre-set to achieve a predetermined average droplet contact area, total droplet contact area ratio, and standard deviation of droplet contact area, and the test specimen may be evaluated under the same conditions. In the second and subsequent chloride deposition steps (A), chloride deposition may be performed under the same conditions as the initial chloride deposition step (A), or under different conditions from the initial chloride deposition step (A) as long as the desired chloride deposition amount is achieved. Preferably, conditions for achieving a predetermined average droplet contact area, total droplet contact area ratio, and standard deviation of droplet contact area are set in advance, and the first chloride deposition step (A) is carried out under the set conditions. When the chloride deposition step (A) is carried out two or more times, it is preferable to carry out the second and subsequent chloride deposition steps (A) under the set conditions.

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

[0092] Furthermore, the chloride deposition step (A) is preferably performed in an atmosphere with a temperature Ta1 of 50°C or less. If the temperature Ta1 in the chloride deposition step (A) exceeds 50°C, the droplets sprayed from the spray nozzle, particularly when droplets of a chloride-containing aqueous solution are sprayed using a spray nozzle, tend to dry before reaching the evaluation surface of the metal material. As a result, it may be difficult to control the droplet distribution to obtain the desired droplet 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 it is a temperature at which 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 alternation of wet (humid) and dry (dry) states, and simulating this environmental change is important in order to approximate the corrosion patterns found in the actual environment in which a vehicle runs. For example, in the case of steel materials, it is known that the corrosion products formed on the steel material change depending on the wet and dry states, and hydrogen is generated during the process of changing from a wet state to a dry state, or from a dry state to a wet state. Therefore, the conditions during the cycle of relative humidity change (corrosion process (B)) are also important in evaluating delayed fracture properties.

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

[0095] <Temperature Tb1 of corrosion process (B): 60°C or less and within a certain range> The corrosion step (B) is performed in an atmosphere at a temperature Tb1 of 60°C or less and within a certain range. If the temperature Tb1 in the corrosion step (B) exceeds 60°C, not only will the evaluation be performed in an environment far removed from the corrosive environment in which the metal material will actually be used, but the corrosion mechanism may also change. Therefore, the temperature Tb1 in the corrosion step (B) is set to 60°C or less, preferably 50°C or less. On the other hand, there is no particular lower limit for the temperature Tb1 in the corrosion step (B). If the temperature Tb1 in the corrosion step (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 in the corrosion test. Furthermore, the corrosion rate of the metal material will be significantly reduced, resulting in a longer evaluation time. Therefore, the temperature Tb1 in the corrosion step (B) is preferably set to 5°C or more, and more preferably 10°C or more.

[0096] In addition, delayed fracture properties are strongly affected by the temperature of the environment (atmosphere). Therefore, in order to properly evaluate the delayed fracture properties of metallic materials, taking into account the application location and the environment in which the metallic material is used, it is necessary to keep the temperature Tb1 in the corrosion step (B) within a certain range. When the temperature Tb1 in the corrosion step (B) fluctuates within ±5°C, the amount of hydrogen penetrating from the environment into the metallic material (hydrogen penetration) can be evaluated within a fluctuation range of 30% of the hydrogen penetration amount at the target environmental temperature, allowing for accurate evaluation of delayed fracture properties. When the temperature Tb1 in the corrosion step (B) fluctuates within ±2°C, the fluctuation range of the hydrogen penetration amount is within 15%. Therefore, the fluctuation range of the temperature Tb1 in step (B) is preferably within ±5°C, and more preferably within ±2°C.

[0097] [Drying process (b1)] The drying step (b1) is a step of drying the metal material in an atmosphere with a relative humidity Hb1 of 45% or less for 1.0 to 5.0 hours. The relative humidity Hb1 in the drying step (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 step (b1) exceeds 45%, a long period of time is required to sufficiently dry the metal material surface, which increases the evaluation time. The relative humidity Hb1 in the drying step (b1) is preferably 40% or less. On the other hand, there is no particular lower limit for the relative humidity Hb1 in the drying step (b1). From the viewpoint of relative humidity controllability, the relative humidity Hb1 in the drying step (b1) is preferably 20% or more. Furthermore, if the components to be attached to the metal material surface contain substances that exhibit deliquescent properties at lower relative humidities, such as magnesium chloride or calcium chloride, it is preferable to set the relative humidity Hb1 in the drying step (b1) low.

[0098] The processing time for the drying process (b1) (the time for which the specimen is maintained in an atmosphere of relative humidity Hb1) should be between 1.0 and 5.0 hours. If the processing time for the drying process (b1) is less than 1.0 hour, it is not possible to simulate an actual corrosive environment. On the other hand, if the processing time for the drying process (b1) is more than 5.0 hours, it is possible to simulate an actual corrosive environment, but it takes a long time to evaluate the delayed fracture properties.

[0099] [Wetting process (b2)] The wetting step (b2) is a process in which the metal material is wetted in an atmosphere with a relative humidity Hb2 of 80% or higher for 1.0 to 5.0 hours. The relative humidity Hb2 in the wetting step (b2) is set to 80% or higher. 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 step (b2) is less than 80%, the effect of wetting will be insufficient, making it impossible to simulate an actual corrosion environment. Among chlorides, sodium chloride has the highest saturation critical vapor pressure, which is approximately 75 to 78% in relative humidity terms. Therefore, for any chloride, if the relative humidity is kept at 80% or higher, a water film will form on the metal material surface due to moisture absorption by the chloride, thereby maintaining a wet state. Therefore, the relative humidity Hb2 in the wetting step (b2) is set to 80% or higher. On the other hand, although there is no particular upper limit for the relative humidity Hb2 in the wetting step (b2), it is preferable that the relative humidity Hb2 in the wetting step (b2) be less than 98%. This is because when the relative humidity Hb2 is 98% or higher, the water film formed 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 processed test specimens. Therefore, when evaluating processed test specimens, it is preferable that the relative humidity Hb2 in the wetting step (b2) be less than 98%.

[0100] The process time for the wetting step (b2) (the time for maintaining the specimen in an atmosphere with a relative humidity Hb2 of 80% or higher) is 1.0 hour or more and 5.0 hours or less. If the process time for the wetting step (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 step (b2) is more than 5.0 hours, it is possible to simulate an actual corrosive environment, but it takes a long time to evaluate the delayed fracture properties.

[0101] [Transition process (b3), transition process (b4)] The transition step (b3) is a step of transitioning from an atmosphere with the relative humidity Hb1 to an atmosphere with the relative humidity Hb2, and the transition step (b4) is a step of transitioning from an atmosphere with the relative humidity Hb2 to an atmosphere with the relative humidity Hb1. It is known that the amount of hydrogen penetration into metal materials, particularly steel materials, increases when the relative humidity changes. That is, a large amount of hydrogen penetrates into the steel material during the transition steps (b3) and (b4). The reason why the amount of hydrogen penetration into the metal material increases when the relative humidity changes is not entirely clear, but it can be considered as follows. In the transition step (b3), which is a step of transitioning from an atmosphere with the relative humidity Hb1 to an atmosphere with the relative humidity Hb2, chlorides present on the surface of the metal material begin to absorb moisture due to deliquescence, and corrosion of the metal material begins. It is known that corrosion products present on the surface of the metal material change during this process, and hydrogen is thought to be generated along with this change in corrosion products. Furthermore, in the transition step (b4), which transitions from an atmosphere with a relative humidity of Hb2 to an atmosphere with a relative humidity of Hb1, the moisture becomes a concentrated solution containing chlorides and metal ions eluted by corrosion, and in the case of steel materials, iron ions, which is thought to lower the solution's pH. In other words, the solution contains a large amount of hydrogen ions during the drying process, which is thought to facilitate hydrogen penetration into the metal material. For this reason, in the transition steps (b3) and (b4), the rate of change in relative humidity is set to 30% / h or less. If the rate of change in relative humidity during the transition steps (b3) and (b4) is 30% / h or less, hydrogen generated by corrosion can be sufficiently absorbed into the metal material, enabling appropriate evaluation of delayed fracture properties. On the other hand, the lower limit of the rate of change of the relative humidity is not particularly specified. However, if the transition steps (b3) and (b4) are too long, it will take a long time to evaluate the delayed fracture properties. Therefore, the rate of change of the relative humidity is preferably 1.5% / h or more, and more preferably 10% / h or more.

[0102] The purpose of the method for evaluating the delayed fracture properties of metallic materials of the present invention is to simulate the daytime and nighttime changes in relative humidity in an actual environment. Therefore, if the process time (time for one cycle) of the corrosion step (B), which simulates the daytime and nighttime changes in relative humidity in an actual environment, exceeds 24 hours, this means that corrosion will be slower than in an actual environment, and the evaluation of delayed fracture properties will require a long time. In other words, the process time of the corrosion step (B) is preferably set to 24 hours or less. To expedite the evaluation, the process time of the corrosion step (B) is more preferably set to 12 hours or less. On the other hand, if the process time of the corrosion step (B) is shortened, the relative humidity will change rapidly, which will reduce the correlation with corrosion in an actual environment and may result in a discrepancy with the delayed fracture properties in an actual environment. Therefore, the process time of the corrosion step (B) is preferably set to 5 hours or more.

[0103] In the evaluation method of the present invention, the chloride adhesion step (A) and the corrosion step (B) are each performed at least once. The chloride adhesion step (A) may be performed after a random number of cycles of the corrosion step (B) or after a predetermined number of cycles of the corrosion step (B). The upper limit of the number of times the process including the chloride adhesion step (A) and the corrosion step (B) is performed is not particularly limited. For example, the process including the chloride adhesion step (A) and the corrosion step (B) may be performed until cracks occur in the metal material. Alternatively, the number of test days may be determined in advance, and the process may be performed for a number of test days corresponding to the number of test days. The number of times the process is performed can be appropriately set, taking into consideration, for example, simulating corrosion patterns in an actual environment. As an example, the process may be performed 200 times or less, or may be performed 100 times or less.

[0104] Next, a process including a chloride adhesion step (A) and a corrosion step (B) will be described. Fig. 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 Fig. 5 shows an example of a corrosion test cycle in which the chloride adhesion step (A) and the corrosion step (B) are each performed once. In this example, the corrosion step (B) has a drying step (b1), a transition step (b3), a wetting step (b2), and a transition step (b4) as one cycle.

[0105] The corrosion process (B) cycle following the chloride deposition process (A) preferably begins with the drying process (b1). By drying the saltwater applied in the chloride deposition process (A) in the drying process (b1), condensation initiation points are uniformly dispersed when humidity increases, reducing variability in the evaluation of delayed fracture properties. Starting the corrosion process (B) cycle from the transition process (b3), wetting process (b2), or transition process (b4) can lead to uneven dispersion of condensation initiation points due to factors such as insufficient drying of the saltwater applied in the chloride deposition process (A) or the saltwater applied in a high-humidity environment absorbing moisture and becoming coarse. Therefore, it is best to avoid starting the corrosion process (B) cycle from any process other than the drying process (b1).

[0106] When performing a cycle of the chloride deposition step (A) followed by the corrosion step (B) and then performing the chloride deposition step (A) again, it is preferable to include a water rinsing step (C) before the chloride deposition step (A). Figure 6 shows an example of a corrosion test cycle in which a cycle of the chloride deposition step (A) followed by the corrosion step (B) is performed, and then a water rinsing step (C) is performed before performing the chloride deposition step (A) again. If the chloride deposition step (A) is performed again without the water rinsing step (C), the amount of chloride deposited on the surface of the metal material tends to increase as the amount of chloride deposition increases, making it impossible to continue the same corrosive environment. This may result in the possibility of evaluating delayed fracture properties in an environment different from the intended corrosive environment. Therefore, it is preferable to include a water rinsing step (C) before performing the chloride deposition step (A) again. The water rinsing step (C) is a step in which the evaluation surface of the metal material is rinsed with water. The water-washing method in the water-washing step (C) is not particularly limited, but examples include a method in which water is sprayed onto the evaluation surface of the metal material from a spray nozzle to wash the evaluation surface, and a method in which the evaluation surface is immersed in water to wash the evaluation surface.

[0107] In the evaluation method of the present invention, a cycle of the chloride adhesion step (A) and the corrosion step (B) as described above is carried out, and then a corrosion test cycle in which the water rinsing step (C) is carried out before the chloride adhesion step (A) is carried out again is carried out at least once. After that, the condition of the metallic material (the presence or absence of cracks in the metallic material, the degree of cracks, etc.) is confirmed, and the delayed fracture properties of the metallic material are evaluated based on the confirmed condition of the metallic material.

[0108] Specifically, the steel sheet of the present invention has a cracking period of 63 days or more under the following conditions in the evaluation method for delayed fracture properties of metallic materials (HeTsAce).

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

[0110] The water-washing step (C) is a step of rinsing the evaluation surface of the metal material with water. The water-washing method in the water-washing step (C) is not particularly limited, but the evaluation surface of the metal material is rinsed with water by spraying water onto the evaluation surface from a spray nozzle.

[0111] Corrosion test cycle: After the first process (A) is performed, the distribution of droplets on the evaluation surface is checked. Subsequent processes (A) are performed under the same conditions as the first process (A). Process (B) consists of the drying process (b1) → transition process (b3) → wetting process (b2) → transition process (b4) performed in this order, which is considered one cycle. After repeating this cycle four times, a water washing process (C) is performed before process (A), and then process (A) is performed. Metallic materials used for evaluation: The obtained steel plates were sheared into 16 mm x 75 mm specimens, with the longitudinal direction perpendicular to the rolling direction. The clearance during shearing was 15% for all specimens. A four-point bending test was then conducted in accordance with ASTM (G39-99), and stresses equivalent to YS and TS were applied to the apex of the bend of the specimen. The above-mentioned corrosive environment cycle (HeTsAce) was then carried out for 63 days. After the test, each specimen was visually inspected for cracks, and samples that were subjected to stress equivalent to YS and showed no cracks were deemed to have excellent delayed fracture resistance in a corrosive environment.

[0112] In the above-mentioned evaluation method for delayed fracture properties of metallic materials (HeTsAce), the longer the number of days until cracks occur, the smaller the amount of diffusible hydrogen in the steel tends to be.

[0113] (Steel plate manufacturing method) Next, a method for manufacturing a steel sheet according to the present invention will be described. Fig. 8 is a graph showing the relationship between temperature and time in a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention. As shown in Fig. 8, the method for manufacturing a steel sheet according to the present invention comprises the steps of: After hot rolling to obtain hot rolled steel sheets, After cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet, The cold-rolled steel sheet is annealed, Annealing is The annealing temperature is Ac3-100°C or higher and Ac3 or lower, An annealing holding step of holding the annealing temperature for a holding time of 5 seconds or more; a first cooling step of cooling from the annealing temperature to a temperature T1 of 550°C or higher at a first average cooling rate of less than 11°C / s; a second cooling step of cooling from temperature T1 to 500°C at a second average cooling rate of 11°C / s or more; a third cooling step of cooling from 500°C to a temperature T2 of Ms or higher and 320°C or higher for a residence time of 10 seconds or longer and 60 seconds or shorter; a fourth cooling step of cooling from the temperature T2 to a temperature T3 of 100°C or higher and Ms-80°C or lower at a fourth average cooling rate of 3°C / s or higher and 50°C / s or lower; The tempering temperature is T3 or higher and 450°C or lower, A tempering process in which tempering is performed under the condition that the tempering time is 5 seconds or more and 1000 seconds or less; and a fifth cooling step of cooling the mixture at a fifth average cooling rate of 1.0°C / h or more and 50.0°C / h or less in the temperature range of 150 to 80°C.

[0114] Steel material (steel slab) In the present invention, the method for producing the steel material (steel slab) is not particularly limited, and any known method such as a converter or an electric furnace is suitable. The steel slab (slab) is preferably produced by a continuous casting method to prevent macrosegregation.

[0115] hot rolling The method for producing the hot-rolled sheet is not particularly limited and may be carried out according to a conventional method, and for example, the slab heating temperature is preferably 1100°C or higher and 1300°C or lower. The soaking time is preferably 20 minutes or more, and 30 minutes or less. The finish rolling temperature is preferably the Ar3 transformation point or higher, and more preferably the Ar3 transformation point + 200°C or lower. The coiling temperature is preferably 400°C or higher. The coiling temperature is preferably 720°C or lower. The coiling temperature is preferably set to suppress thickness fluctuations and ensure high strength stably. From this viewpoint, the coiling temperature is preferably 430°C or higher. The coiling temperature is preferably 530°C or lower.

[0116] cold rolling The method for producing a cold-rolled steel sheet is not particularly limited and may be carried out according to a conventional method. For example, the rolling ratio (cumulative rolling reduction) may be 30% or more. Alternatively, the rolling ratio may be 85% or less. From the viewpoint of stably ensuring high strength and reducing anisotropy, the rolling ratio is preferably 35% or more, and more preferably 35 to 85%. When the rolling load is high, softening annealing can be performed at 450 to 730°C using a continuous annealing line (CAL) or box annealing (BAF). The obtained hot-rolled steel sheet may be subjected to cold rolling as is, or may be subjected to pickling before cold rolling. Pickling can remove oxides from the steel sheet surface, thereby achieving favorable plating quality in the final steel sheet product, so pickling is preferred. Pickling may be performed once or multiple times.

[0117] annealing (Annealing holding process) [Annealing temperature: Ac3 - 100℃ or higher, Ac3 or lower] The obtained cold-rolled steel sheet is subjected to an annealing and holding step. 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 is set to Ac3 or less. The annealing temperature is preferably Ac3-10°C or less, and more preferably Ac3-20°C or less. 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 more, so the annealing temperature is Ac3-100°C or higher, preferably Ac3-90°C or higher, and more preferably Ac3-80°C or higher. Ac3(℃)=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 composition, and is set to 0 when the composition does not contain element X.

[0118] [Holding time at annealing temperature: 5 seconds or more] In the annealing and holding step, if the holding time at the annealing temperature is less than 5 seconds, it becomes difficult to control the fractions of low-Mn ferrite and tempered martensite, so the holding time at the annealing temperature is set to 5 seconds or more, preferably 50 seconds or more, more preferably 80 seconds or more, and further preferably 100 seconds or more. On the other hand, although there is no upper limit to the holding time at the annealing temperature, the holding time at the annealing temperature is preferably 1000 seconds or less, more preferably 800 seconds or less, and even more preferably 600 seconds or less, because productivity decreases as the heating cost and manufacturing time increase. When the annealing time is short, it is desirable to use a highly responsive heating method such as induction heating.

[0119] (First cooling step) [First average cooling rate from the annealing temperature to a 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 an arbitrary temperature T1 (cooling stop temperature T1) that is 550°C or higher and lower than the annealing temperature is 11°C / s or higher, the precipitation and growth of high-Mn ferrite are suppressed, making it difficult to achieve a high λ. Therefore, the first average cooling rate from the annealing temperature to an arbitrary temperature T1 (cooling stop temperature T1) of 550°C or higher is set to less than 11°C / s. The first average cooling rate is preferably 10°C / s or less, and more preferably 8°C / s or less. Furthermore, if the cooling rate is reduced too much, production efficiency will decrease, so the first average cooling rate is preferably 1°C / s or more, and more preferably 5°C / s or more. If the cooling stop temperature T1 in the first cooling step is less than 550°C, a large amount of residual structure will precipitate, making it impossible to obtain the desired formability. Therefore, the cooling stop temperature T1 in 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 calculated by "(cooling start temperature (annealing temperature) (°C) - cooling stop temperature T1 (°C)) / cooling time (s) from the cooling start temperature to the cooling stop temperature".

[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 structure will precipitate, and the effects of the present invention will be lost. Therefore, the second average cooling rate in the second cooling step is set to 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 becomes difficult to control the temperature T2 described below. 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 calculated by "(cooling start temperature (temperature T1) (°C) - cooling stop temperature (500°C)) / cooling time (s) from the cooling start temperature to the cooling stop temperature."

[0121] (Third cooling step) [Dwell time from 500°C to temperature T2, which is above Ms and above 320°C: 10 seconds to 60 seconds] If the residence time from 500°C to an arbitrary temperature T2 (retention stop temperature T2) that is equal to or higher than Ms and equal to or higher than 320°C and equal to or lower than 500°C is less than 10 seconds, retained austenite cannot be sufficiently generated, and the desired formability cannot be obtained. Therefore, the residence time is set to 10 seconds or more, preferably 15 seconds or more, and more preferably 20 seconds or more. Ms is expressed by the formula (2) described below. On the other hand, if the holding time exceeds 60 seconds, the concentration of C in the untransformed austenite will proceed excessively, and the amount of fresh martensite that forms during cooling will increase, making it impossible to achieve the desired formability and impact properties. Therefore, the residence time is set to 60 seconds or less, preferably 55 seconds or less, and more preferably 50 seconds or less. If the temperature T2 exceeds 500°C, the residual structure will precipitate excessively, making it impossible to obtain the desired properties. If the temperature T2 is lower than Ms or lower than 320°C, it will be impossible to generate sufficient retained austenite, making it impossible to obtain the desired formability. Therefore, the temperature T2 is set to be equal to or higher than Ms and between 320°C and 500°C. The cooling rate in the third cooling step (third average cooling rate) is not particularly limited as long as it satisfies the above-mentioned conditions for the third cooling step.

[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 composition, and is set to 0 when the composition does not contain element X.

[0123] (Fourth cooling step) [Fourth average cooling rate from temperature T2 to temperature T3 of 100°C or more and Ms-80°C or less: 3°C / s or more and 50°C / s or less] If the average cooling rate (fourth average cooling rate) from T2 to an arbitrary temperature T3 (cooling stop temperature T3) between 100°C and Ms-80°C is less than 3°C / s, C will be excessively concentrated in the untransformed austenite, and the amount of fresh martensite formed in the remaining structure during cooling will increase, making it impossible to achieve high formability. Therefore, the average cooling rate from T2 to T3 is set to 3°C / s or more, preferably 5°C / s or more, and more preferably 7°C / s or more. On the other hand, if the cooling rate from T2 to T3 becomes too high, it becomes difficult to control the temperature T3, so the fourth average cooling rate in this temperature range is set to 50°C / s or less, preferably 40°C / s or less, and more preferably 30°C / s or less. Furthermore, if T3 is higher than Ms-80°C, the fraction of retained austenite increases and the fraction of tempered martensite is insufficient, making it difficult to achieve a TS of 980 MPa or more. On the other hand, if T3 is less than 100°C, a sufficient amount of retained austenite cannot be obtained, making it impossible to achieve the desired formability. Therefore, T3 is set to 100° C. or higher and Ms-80° C. or lower. T3 is preferably 110° C. or higher, and more preferably 120° C. or higher. Furthermore, T3 is preferably Ms-90° C. or lower, and more preferably Ms-120° C. or lower. Here, the fourth average cooling rate is calculated by (cooling start temperature (temperature T2) (°C) - cooling stop temperature (temperature T3) (°C)) / cooling time (s) from the cooling start temperature to the cooling stop temperature.

[0124] (Tempering process) [Tempering temperature: T3 or higher, 450°C or lower] After the fourth cooling step, the steel is either held at that temperature or reheated and held at a temperature of 450°C or lower to stabilize the retained austenite. If the tempering temperature is lower than T3, the concentration of C in the untransformed austenite will be insufficient, and the amount of fresh martensite formed in the residual structure during cooling will increase, making it impossible to achieve high formability. On the other hand, if the tempering temperature exceeds 450°C, the tempering of martensite proceeds excessively, making it difficult to achieve a TS of 980 MPa or more. Therefore, the tempering temperature is set to T3 or higher and 450° C. or lower. The tempering temperature is preferably T3+10° C. or higher, and more preferably T3+20° C. or higher. The tempering temperature is preferably 420° C. or lower, and more preferably 400° C. or lower.

[0125] [Tempering time: 5 seconds or more and 1000 seconds or less] If the holding time at the tempering temperature (tempering time) is less than 5 seconds, the concentration of C in the untransformed austenite will be insufficient, and fresh martensite will form during final cooling, making it difficult to achieve high ductility and high hole expandability. If the holding time at the tempering temperature (tempering time) exceeds 1000 seconds, the tempering of martensite proceeds excessively, making it difficult to achieve a TS of 980 MPa or more. Therefore, the holding time at the tempering temperature (tempering time) is set to 5 seconds or more and 1000 seconds or less. The holding time at the tempering temperature (tempering time) is preferably 50 seconds or more, and more preferably 60 seconds or more. The holding time at the tempering temperature (tempering time) is preferably 800 seconds or less, and more preferably 700 seconds 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 (the fifth average cooling rate) exceeds 50.0°C / h, the concentration of C in austenite during cooling becomes insufficient, and fresh martensite is formed during final cooling, making it difficult to achieve high formability. 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 constraints on production technology. 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 the steel sheet is to be traded, it is usually cooled to room temperature before being traded. The steel sheet may be plated during or after annealing. Examples of plating treatment during annealing include hot dip galvanizing treatment according to a conventional method after the annealing holding step, or further alloying treatment after the hot dip galvanizing treatment. Furthermore, examples of plating treatments after annealing include electroplating treatments such as Zn-Ni electroplating or pure Zn electroplating after tempering. The plating layer may be formed by electroplating. Alternatively, hot-dip zinc-aluminum-magnesium alloy plating may be applied. While the above plating process has been described primarily in connection with zinc plating, the type of plating metal is not particularly limited, and Al plating, etc., may also be used. The other manufacturing method conditions are not particularly limited, but from the viewpoint of productivity, it is preferable that the series of processes, such as the annealing, hot-dip galvanizing, and zinc alloying treatment, be carried out in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping can be performed to adjust the coating weight of the plating. Plating conditions other than those described above may be those of conventional methods, such as hot-dip galvanizing.

[0128] In addition, conventional methods can be used for steps and conditions not described in the present invention.

[0129] (Components and manufacturing methods for components) The member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining. Also, the method for manufacturing the member of the present invention includes a step of subjecting the steel plate of the present invention to at least one of forming and joining to form the member.

[0130] The steel sheet of the present invention has a tensile strength TS of 980 MPa or more, and is excellent in formability, crashworthiness, chemical conversion treatability, and delayed fracture resistance. Therefore, members obtained using the steel sheet of the present invention also have a tensile strength TS of 980 MPa or more, and are excellent in formability, crashworthiness, chemical conversion treatability, and delayed fracture resistance. Use of the member of the present invention enables weight reduction. Therefore, the member of the present invention can be suitably used, for example, in automotive structural members.

[0131] The forming process can be performed using a general processing method such as press working without any restrictions, and the joining process can be performed using general welding methods such as spot welding and arc welding, riveting, crimping, etc. without any restrictions. [Example]

[0132] Steel having the chemical composition shown in Table 1, with the balance being Fe and unavoidable impurities, was melted in a converter and formed into a steel slab by continuous casting. The obtained steel slab was then subjected to hot rolling and pickling, followed by cold rolling. The annealing process, first cooling process, second cooling process, third cooling process, fourth cooling process, tempering process, and fifth cooling process were then carried out under the conditions shown in Table 2 (Table 2-1, Table 2-2). As a result, steel sheets with thicknesses of 0.8 to 2.4 mm were obtained. Regarding the coating treatments shown in Table 2, hot-dip galvanizing is indicated as "GI," galvannealed hot-dip galvanizing is indicated as "GA," and electrogalvanizing is indicated as "EG." Furthermore, the absence of any coating treatment is indicated as "CR."

[0133] Using the obtained steel sheet as a test material, the area fraction of ferrite, the proportion (area fraction) of high-Mn ferrite in the ferrite, the area fraction of tempered martensite, the volume fraction of retained austenite, and the area fraction of the remaining structure were determined by the above-mentioned methods. The results are shown in Table 3. Next, the obtained steel sheets were used as test materials and evaluated for tensile strength TS, yield strength YS, elongation El, hole expansion ratio λ, chemical conversion treatability, and delayed fracture resistance by the methods described above. More specifically, the evaluation of delayed fracture resistance was carried out as follows.

[0134] (Delayed fracture resistance) The obtained steel sheets were sheared into 16 mm x 75 mm specimens, with the longitudinal direction perpendicular to the rolling direction. The clearance during shearing was 15%. Four-point bending tests were then performed according to ASTM (G39-99), and stresses equivalent to YS and TS were applied to the apex of the bend. Delayed fracture tests were then performed using the corrosive environment cycle described below.

[0135] (Corrosion test cycle) The above test pieces were subjected to the following corrosion test cycle (corrosion test) consisting of a chloride adhesion step (A) and a corrosion step (B), and a water washing step (C). In the chloride deposition process (A), the amount of chloride deposition (solid content equivalent) is 10,000 mg / m 2 Droplets of the chloride-containing aqueous solution were deposited so that the surface area was 30 cm. The distance between the spray nozzle and the test surface of the test piece (X in Figure 1) was 30 cm. The spray pressure of the spray nozzle was 0.2 MPa. The salt water spray time was 5 seconds. The amount of chloride attached was calculated by calculating the difference in mass of the test piece before and after salt water spraying, and dividing this by the area of ​​the test piece's test surface, which was then used to calculate the amount of aqueous solution attached, and then calculated from 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 the spray method Distance between the shielding material and the evaluation surface (Y in Figure 4): 5 cm Temperature: 22°C, Relative humidity: 50% Droplet distribution on the evaluation surface Average contact area of ​​the droplet: 1.3 mm 2 , Standard deviation of droplet contact area: 1.3 mm 2 , Total contact area rate of droplets: 71% In the second and subsequent chloride deposition steps (A), chloride deposition was carried out under the same conditions as in the first chloride deposition step (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 this order, and one cycle is defined as the execution of the four processes. In this example, the temperature fluctuation range of the corrosion process (B) was set within 30±5°C. Drying process (b1): A process of drying the metal material by holding it in an atmosphere of 40% relative humidity Hb1 for 2.0 hours. Wetting step (b2): A step of wetting the metal material by holding it in an atmosphere of 90% relative humidity Hb2 for 2.0 hours. Transition step (b3): ​​A step of transitioning from an atmosphere of the relative humidity Hb1 to an atmosphere of the relative humidity Hb2 at a relative humidity change rate of 25% / h. Transition step (b4): A step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 at a relative humidity change rate of 25% / h.

[0137] The water-washing step (C) is a step of rinsing the evaluation surface of the metal material with water. The water-washing method in the water-washing step (C) is not particularly limited, but the evaluation surface of the metal material was washed with water by spraying water onto the evaluation surface from a spray nozzle.

[0138] In this example, the corrosion test cycle was such that the chloride adhesion step (A) was followed by the corrosion step (B), and then the water-rinsing step (C) was performed before the chloride adhesion step (A) was performed again. That is, in this example, the delayed fracture properties were evaluated using the following corrosion test cycle. A cycle in which the chloride adhesion process (A) → corrosion process (B) → water washing process (C) are repeated In this example, the period for evaluating delayed fracture resistance using a corrosive environment cycle was 63 days.

[0139] After the test, each test piece was visually inspected for the presence or absence of cracks. The delayed fracture resistance in a corrosive environment was evaluated according to the following criteria. In the present invention, a sample loaded with a stress equivalent to YS and free of cracks was deemed to have excellent delayed fracture resistance in a corrosive environment. ◎ (Pass, particularly excellent): No cracks in samples loaded with stress equivalent to YS and TS. ○ (Pass, Excellent): No cracks in the sample loaded with stress equivalent to YS × (Fail): Cracks were observed in both samples loaded with stress equivalent to YS and TS.

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

[0141] [Table 1]

[0142] [Table 2-1]

[0143] [Table 2-2]

[0144] [Table 3-1]

[0145] [Table 3-2]

[0146] The steel sheets of the present invention had a tensile strength (TS) of 980 MPa or more and were excellent in all of formability, crashworthiness, chemical conversion treatability, and delayed fracture resistance. On the other hand, the steel sheets of the comparative examples were inferior in at least one of TS, formability, crashworthiness, chemical conversion treatability, and delayed fracture resistance.

[0147] Furthermore, the components obtained by forming and joining the steel plates of the present invention have a tensile strength TS of 980 MPa or more, and are excellent in formability, impact resistance, chemical conversion treatability, and delayed fracture resistance, just like the steel plates of the present invention.

Claims

1. 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; The balance has a composition consisting of Fe and unavoidable impurities, The steel sheet surface coverage rate of Si-based oxides 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, The area fraction of the remaining structure is 10% or less, The ferrite is low-Mn ferrite having a Mn concentration less than 0.85 times the Mn content of the steel plate; and high Mn ferrite having a Mn concentration of 0.85 times or more the Mn content of the steel plate, The proportion of high Mn ferrite in the ferrite is 20% or more and 90% or less in terms of area fraction, TS is 980 MPa or more, El×λ is 600%×% or more, A steel plate having YS×λ of 30000 MPa×% or more.

2. The component composition further includes, in mass %, 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, 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, Bi: 0.200% or less, The steel sheet according to claim 1, further comprising at least one element selected from the group consisting of:

3. The steel sheet according to claim 1 , which has a plating layer on at least one surface thereof.

4. A steel plate as described in claim 2, having a plating layer on at least one side of the steel plate.

5. A member made using the steel plate according to any one of claims 1 to 4.

6. The method for producing a steel sheet according to any one of claims 1 to 4, A steel material having the chemical composition according to claim 1 or 2, Hot rolling is carried out, After obtaining the hot rolled steel sheet, The hot-rolled steel sheet is subjected to cold rolling, After obtaining the cold rolled steel sheet, The cold-rolled steel sheet is annealed, The annealing is The annealing temperature is Ac3-100°C or higher and Ac3 or lower, An annealing holding step of holding the annealing temperature for a holding time of 5 seconds or more; a first cooling step of cooling the steel sheet from the annealing temperature to a temperature T1 of 550°C or higher at a first average cooling rate of less than 11°C / s; a second cooling step of cooling from the temperature T1 to 500°C at a second average cooling rate of 11°C / s or more; a third cooling step of cooling from 500°C to a temperature T2 that is equal to or higher than Ms and equal to or higher than 320°C, with a residence time of 10 seconds to 60 seconds; a fourth cooling step of cooling the steel sheet from the temperature T2 to a temperature T3 of 100°C or higher and Ms-80°C or lower at a fourth average cooling rate of 3°C / s or higher and 50°C / s or lower; The tempering temperature is T3 or higher and 450°C or lower, A tempering process in which tempering is performed under the condition that the tempering time is 5 seconds or more and 1000 seconds or less; a fifth cooling step of cooling the steel sheet at a fifth average cooling rate of 1.0°C / h or more and 50.0°C / h or less in a temperature range of 150 to 80°C.

7. The method for producing a steel sheet according to claim 6, further comprising the step of plating a surface of the steel sheet.

8. A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining to form a component.

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