Hot-pressed parts and hot-pressed steel plates

Abstract

The object of the present invention is to provide a hot-pressed member using an Al-based plated steel sheet, which has a tensile strength of 1.8 GPa or more, i.e., 1760 MPa or more, and has excellent resistance to hydrogen embrittlement. A steel sheet and a coating layer disposed on at least one surface of the steel sheet, wherein the coating layer has a composition containing, in mass%, at least one selected from Mg: 0.10 to 5.0% and Ca: 0.005 to 1.0%, Si: 3.0 to 15.0%, Fe: 55.0% or less, with the balance being Al and unavoidable impurities, and a length L0 in a direction parallel to the surface of the steel sheet and a surface line length L of the coating layer corresponding to L0. r Ratio to (L r / L0) is L r / L0<1.10, has a tensile strength of 1760 MPa or more, and has a cracking time of 40 days or more in an evaluation of the delayed fracture properties of metallic materials.

Description

[Technical Field]

[0001] The present invention relates to a hot-pressed member and a steel sheet for hot-pressing, and more particularly to a hot-pressed member having excellent resistance to hydrogen embrittlement. [Background technology]

[0002] With the aim of reducing the weight of automobiles and improving collision safety, efforts are being made to increase the strength of automotive steel sheets. In recent years, cold-rolled steel sheets with a tensile strength of 1.5 GPa have been developed, and their application to automotive applications is being considered. However, as the strength of steel sheets increases, forming defects and springback in the press process become an issue in ensuring dimensional accuracy.

[0003] Therefore, the application of hot pressing technology, which forms parts hot rather than cold, is increasing. Hot pressing is a forming method in which the raw steel plate (hot pressing steel plate) is heated to the austenite temperature range, then press-formed while still hot, and simultaneously quenched by contact with a mold. In hot pressing, press forming is performed in a state where formability is improved by heating, and subsequent quenching increases strength, making it possible to manufacture hot-pressed parts with excellent strength and high dimensional accuracy.

[0004] Hot-pressed members obtained by hot-pressing the above-mentioned steel sheets for hot pressing are mainly used for automotive components, particularly for frame structural components (inner panel frames) that require strength. In recent years, hot-pressed members have also come to be used as so-called quasi-exterior panel components, such as components around pillars that are visible when the door is opened. Therefore, hot-pressed members are required to be suitable for painting and to have excellent corrosion resistance in cut areas after painting.

[0005] Therefore, a steel sheet having an Al-based plating layer on the surface (Al-based plated steel sheet) has been proposed as a steel sheet for hot press use suitable for producing hot press members.

[0006] Furthermore, while hot-pressed components have typically had a tensile strength of 1.5 GPa after hot-press forming, in recent years, there has been a demand for higher-strength hot-pressed components with a tensile strength of 1.8 GPa or higher in order to further reduce the weight of automobile bodies. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2017-536472 Summary of the Invention [Problem to be solved by the invention]

[0008] As the strength of steel materials increases, the problem of hydrogen embrittlement becomes more pronounced. Therefore, in order to increase the strength of hot-pressed members, more advanced measures against hydrogen embrittlement are required.

[0009] On the other hand, examples of hydrogen that penetrates during the manufacturing process of hot-pressed members include hydrogen introduced during the manufacturing of hot-press steel sheets, which are the raw material for hot-pressed members, hydrogen that penetrates during the hot-pressing process, hydrogen that penetrates during the painting process, etc. In the case of hot-pressed members made of Al-based plated steel sheets, it is known that the amount of hydrogen that penetrates during the hot-pressing process is extremely large.

[0010] Therefore, when Al-based plated steel sheets are used as materials for hot-pressed members, it is important to reduce the amount of hydrogen that penetrates during the hot-pressing process.

[0011] Furthermore, when Al-plated steel sheets are subjected to hot pressing, the coating layer on the hot-pressed parts becomes an FeAl alloy layer. The formation of the FeAl alloy layer increases the surface roughness, which increases the contact area between the coating surface and the atmosphere during the hot pressing process. The contact area between the coating surface and the atmosphere during the hot pressing process affects hydrogen penetration during the process. Therefore, it is important to reduce the contact area between the coating surface and the atmosphere.

[0012] Patent Document 1 proposes an Al-based plated steel sheet for hot pressing, which comprises a steel substrate and an Al-based plated layer, the Al-based plated layer containing 0.1 to 0.5 wt % of an alkaline earth metal or a transition metal, and an oxide of the alkaline earth metal or the transition metal is formed on the outer surface of the Al-based plated layer during hot pressing.

[0013] In the above Patent Document 1, it is said that by covering the Al-based plating layer with an oxide of an element having high oxygen affinity, such as Mg, the reaction between the Al-based plating layer and moisture is suppressed during the hot pressing process, and the penetration of hydrogen is reduced.

[0014] However, even in the case of hot-pressed members in which the amount of hydrogen penetration is reduced by using the Al-based plated steel sheet disclosed in Patent Document 1, in high-strength hot-pressed members having a tensile strength of 1.8 GPa or more, delayed fracture occurs due to hydrogen embrittlement caused by hydrogen that penetrates due to contact with the air during the hot-pressing process.

[0015] From the above, it is considered important to reduce the surface area of the coating layer of the hot-pressed member as much as possible in order to further reduce contact with the atmosphere during the hot-pressing process and suppress the penetration of hydrogen.

[0016] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a hot-pressed member using an Al-based plated steel sheet, which has a tensile strength of 1.8 GPa or more, i.e., 1760 MPa or more, and has excellent resistance to hydrogen embrittlement. [Means for solving the problem]

[0017] The present inventors have conducted extensive research to solve the above problems and have arrived at the present invention. The present invention has the following features.

[0018] [1] A steel plate and a coating layer disposed on at least one surface of the steel plate, The coating layer is composed of, in mass %, At least one selected from Mg: 0.10 to 5.0% and Ca: 0.005 to 1.0%; Si: 3.0 to 15.0% Fe: 55.0% or less, the balance being Al and unavoidable impurities, The length L0 in the direction parallel to the surface of the steel plate and the surface line length L of the coating layer corresponding to L0 r Ratio to (L r / L0) is L r / L0<1.10 is satisfied, The tensile strength is 1760 MPa or more, A hot-pressed component that takes 40 days or more to crack in an evaluation of the delayed fracture properties of metallic materials. [2] A steel sheet for hot pressing, comprising a steel sheet and a plating layer disposed on at least one surface of the steel sheet, the plating layer comprises an interface alloy layer disposed on the steel sheet and a metal layer disposed on the interface alloy layer, In a cross section perpendicular to the steel plate surface of the steel plate for hot press use, the standard deviation t of the thickness of the interface alloy layer d But, t d <1.5μm, The metal layer comprises, in mass %, At least one selected from Mg: 0.20 to 7.0% and Ca: 0.01 to 1.5%, Si: 1.0 to 10.0% A steel sheet for hot pressing, containing Fe: 10.0% or less, with the remainder being Al and unavoidable impurities. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a hot-pressed member using an Al-based plated steel sheet, which has a tensile strength of 1.8 GPa or more, i.e., 1760 MPa or more, and has excellent resistance to hydrogen embrittlement. [Brief explanation of the drawings]

[0020] [Figure 1]FIG. 1 is a diagram illustrating the length L0 in the direction parallel to the steel sheet surface and the surface line length Lr of the coating layer corresponding to L0. [Figure 2] FIG. 2 is a graph showing the relationship between the cooling time and the steel sheet temperature in the rapid cooling step in the method for producing a steel sheet for hot press use according to one embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram illustrating the evaluation method (HeTsAce) for the delayed fracture properties of metallic materials. [Figure 4] FIG. 4 is a diagram showing a schematic example of an image of the distribution of droplets on the evaluation surface of a metal material. [Figure 5] FIG. 5 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 6] FIG. 6 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 7] FIG. 7 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 8] FIG. 8 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 9] FIG. 9 is a diagram schematically showing a test piece for evaluating delayed fracture properties used in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, an embodiment of the present invention will be described. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited by the following description. Furthermore, the unit of content, "%", represents "% by mass" unless otherwise specified.

[0022] (1) Hot-pressed parts A hot-pressed member according to one embodiment of the present invention includes a steel plate and a coating layer disposed on at least one surface of the steel plate. Each part will be described below.

[0023] [Steel plate] In the present invention, the above-mentioned problems are solved by controlling the structure of the coating layer as described below. Therefore, the steel plate provided in the hot-pressed member is not particularly limited, and any steel plate can be used.

[0024] The hot-pressed member of the present invention is manufactured by hot-pressing a predetermined steel plate for hot pressing, as described below. Therefore, the steel plate included in the hot-pressed member can also be said to be a steel plate formed by hot pressing. The steel plate included in the hot-pressed member may be either a cold-rolled steel plate or a hot-rolled steel plate.

[0025] From the viewpoint of use as an automotive part, etc., it is preferable that the strength of the hot-pressed part is high. In particular, in order to obtain a hot-pressed part having a tensile strength of 1.8 GPa or more, it is preferable to use a steel material having the following chemical composition as the steel plate provided in the hot-pressed part.

[0026] The composition contains C: 0.26-0.50%, Si: 0.1-1.5%, Mn: 0.5-5.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.10% or less, and N: 0.01% or less, with the remainder being Fe and unavoidable impurities.

[0027] The effects and preferred contents of each element in the above-mentioned preferred component composition will be described below.

[0028] C: 0.26 to 0.50% C is an element that improves strength by forming a structure such as martensite. From the viewpoint of obtaining a strength of 1760 MPa or more, the C content is preferably 0.26% or more, more preferably 0.30% or more, and even more preferably 0.32% or more. On the other hand, if the C content exceeds 0.50%, the toughness of the spot welds deteriorates. Therefore, the C content is preferably 0.50% or less, more preferably 0.45% or less, even more preferably 0.43% or less, and most preferably 0.40% or less.

[0029] Si: 0.1 to 1.5% Silicon is an element that is effective in strengthening steel to obtain good material properties. To achieve this effect, the Si content is preferably 0.1% or more, and more preferably 0.2% or more. On the other hand, if the Si content exceeds 1.5%, ferrite is stabilized, resulting in a decrease in hardenability. Therefore, the Si content is preferably 1.5% or less, more preferably 1.3% or less, and even more preferably 1.1% or less.

[0030] Mn: 0.5 to 5.0% Mn is an effective element for obtaining high strength regardless of the cooling rate. From the viewpoint of ensuring excellent mechanical properties and strength, the Mn content is preferably 0.5% or more, more preferably 0.7% or more, and even more preferably 1.0% or more. On the other hand, if the Mn content exceeds 5.0%, not only will the cost increase, but the effect of containing Mn will saturate. Therefore, the Mn content is preferably 5.0% or less, more preferably 4.5% or less, and even more preferably 4.0% or less.

[0031] P:0.1% or less If the P content is excessive, local ductility deteriorates due to grain boundary embrittlement caused by P segregation to austenite grain boundaries during casting. As a result, the balance between strength and ductility of the steel sheet deteriorates. Therefore, from the viewpoint of improving the balance between strength and ductility of the steel sheet, the P content is preferably 0.1% or less. On the other hand, from the viewpoint of refining costs, the P content is preferably 0.001% or more.

[0032] S: 0.01% or less S becomes inclusions such as MnS, which deteriorates impact resistance and causes cracks along the metal flow of welds. Therefore, it is desirable to reduce the S content as much as possible, and specifically, it is preferable to set it to 0.01% or less. Furthermore, from the viewpoint of ensuring good stretch flangeability, the S content is more preferably set to 0.005% or less, and even more preferably set to 0.001% or less. On the other hand, from the viewpoint of refining costs, the S content is preferably set to 0.0002% or more.

[0033] Al: 0.10% or less Al is an element that acts as a deoxidizer. However, if the Al content exceeds 0.10%, the blanking workability and hardenability of the base steel sheet material will decrease. Therefore, the Al content is preferably 0.10% or less, more preferably 0.07% or less, and even more preferably 0.04% or less. On the other hand, from the viewpoint of ensuring the effect as a deoxidizer, the Al content is preferably 0.01% or more.

[0034] N: 0.01% or less If the N content exceeds 0.01%, nitrides of AlN are formed during hot rolling or heating before hot pressing, which reduces the blanking workability and hardenability of the base steel sheet material. Therefore, the N content is preferably 0.01% or less. On the other hand, from the viewpoint of refining costs, the N content is preferably 0.001% or more.

[0035] The above composition may further optionally contain at least one selected from the group consisting of Nb: 0.10% or less, Ti: 0.05% or less, B: 0.0002 to 0.005%, Cr: 0.1 to 1.0%, and Sb: 0.003 to 0.03%.

[0036] Nb: 0.10% or less Nb is an effective component for strengthening steel, but excessive Nb content increases the rolling load. Therefore, when Nb is contained, the Nb content is preferably 0.10% or less, and more preferably 0.05% or less. On the other hand, the lower limit of the Nb content is not particularly limited and may be 0%, but from the viewpoint of refining costs, the Nb content is preferably 0.005% or more.

[0037] Ti: 0.05% or less Like Nb, Ti is an effective component for strengthening steel, but excessive Ti content reduces shape fixability. Therefore, when Ti is contained, the Ti content is preferably 0.05% or less, and more preferably 0.03% or less. On the other hand, the lower limit of the Ti content is not particularly limited and may be 0%, but from the viewpoint of refining costs, the Ti content is preferably 0.005% or more.

[0038] B: 0.0002 to 0.005% B has the effect of suppressing the formation and growth of ferrite from austenite grain boundaries. When B is contained, in order to obtain this effect, the B content is preferably 0.0002% or more, more preferably 0.0010% or more. On the other hand, an excessive B content reduces formability. Therefore, when B is contained, the B content is preferably 0.005% or less, more preferably 0.003% or less.

[0039] Cr: 0.1 to 1.0% Like Mn, Cr is a useful element for strengthening steel and improving its hardenability. When Cr is contained, the Cr content is preferably 0.1% or more, more preferably 0.2% or more, in order to obtain the above effects. However, since Cr is an expensive element, excessive Cr content significantly increases costs. Therefore, when Cr is contained, the Cr content is preferably 1.0% or less, more preferably 0.6% or less.

[0040] Sb: 0.003 to 0.03% Sb is an element that has the effect of suppressing decarburization of the surface layer of a steel sheet during the annealing process when manufacturing a base steel sheet material. When Sb is contained, in order to obtain this effect, the Sb content is preferably 0.003% or more, and more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.03%, the rolling load increases, resulting in a decrease in productivity. Therefore, when Sb is contained, the Sb content is preferably 0.03% or less, more preferably 0.02% or less, and even more preferably 0.01% or less.

[0041] [Coating layer] A hot-pressed member according to one embodiment of the present invention preferably includes a steel plate as described above. The hot-pressed member further includes a coating layer on at least one surface of the steel plate as described above. The coating layer may be provided on only one surface of the steel plate, but is preferably provided on both surfaces.

[0042] The coating layer is mainly an FeAl alloy layer and contains at least one of Mg and Ca, and Si. As will be described later, such a coating layer is obtained by hot pressing a steel sheet (steel sheet for hot pressing) having an Al-based plating layer. In the hot-pressed member according to this embodiment, the surface of the coating layer is covered with an oxide film of Mg and / or Ca during the hot pressing process, thereby suppressing the penetration of hydrogen from the surface of the coating layer.

[0043] Mg: 0.10 to 5.0% If the Mg content in the coating layer is less than 0.10%, the coating layer cannot be adequately covered with an oxide film of Mg, and the effect of inhibiting hydrogen penetration during the hot pressing process cannot be sufficiently achieved. Therefore, the Mg content is set to 0.10% or more. On the other hand, if the Mg content exceeds 5.0%, lumpy Mg oxides are formed in the outermost layer of the coating layer. The lumpy Mg oxides are likely to react with moisture during the hot pressing process, and if lumpy Mg oxides are formed in the outermost layer of the coating layer, the effect of inhibiting hydrogen penetration cannot be sufficiently achieved. Therefore, the Mg content is set to 5.0% or less. The Mg content is preferably 4.8% or less. Furthermore, the Mg content is preferably 0.5% or more.

[0044] Ca: 0.005 to 1.0% Like Mg, Ca is an element that forms an oxide film on the surface of the coating layer and provides a hydrogen penetration suppression effect. Therefore, the Ca content in the coating layer is set to 0.005% or more, which is sufficient to provide a sufficient hydrogen penetration suppression effect. On the other hand, if the Ca content exceeds 1.0%, as in the case of Mg, lumpy Ca oxides are formed in the outermost layer of the coating layer, which facilitates reaction with moisture during the hot pressing process, and the hydrogen penetration suppression effect is not sufficiently achieved. Therefore, the Ca content is set to 1.0% or less. The Ca content is preferably 0.8% or less. Furthermore, the Ca content is preferably 0.01% or more. The coating layer may contain at least one selected from Mg: 0.10 to 5.0% and Ca: 0.005 to 1.0%.

[0045] Si: 3.0 to 15.0% Si is an element that has the effect of suppressing alloying of the coating layer during the plating process and the heat treatment process before hot pressing when manufacturing a steel sheet for hot pressing. Furthermore, if the Si content in the coating layer is less than 3.0%, the surface roughness of the coating layer on the hot-pressed member increases. Therefore, the Si content is set to 3.0% or more. On the other hand, if the Si content exceeds 15.0%, the amount of Si-based oxides produced increases, inhibiting the formation of oxide films of Mg and Ca. As a result, the hydrogen penetration suppression effect is not sufficiently achieved. Therefore, the Si content is set to 15.0% or less. The Si content is preferably 12.0% or less. Furthermore, the Si content is preferably 8.0% or more.

[0046] Fe:55.0% or less During the manufacturing process of Al-plated steel sheets (steel sheets for hot pressing) and during heating before hot pressing, Fe diffuses from the steel sheet to the plating layer, so Fe is inevitably contained in the coating layer of hot-pressed members. However, as the Fe content increases, the Al concentration in the coating layer decreases, making the coating layer brittle and prone to crack formation and peeling. As a result, the corrosion resistance of the cut portion deteriorates. Therefore, the Fe content is set to 55.0% or less. On the other hand, there is no particular lower limit for the Fe content, but when hot-pressed members are manufactured under general conditions, the Fe content may be 20.0% or more. The Fe content is preferably 50.0% or less. Furthermore, the Fe content is preferably 40.0% or more.

[0047] The coating layer of the hot-pressed member according to this embodiment may have a composition containing the above components, with the remainder being Al and unavoidable impurities.

[0048] The length in the direction parallel to the surface of the steel plate is L0, and the surface line length of the coating layer corresponding to L0 is L r When L r / L0<1.10 The surface line length L of the coating layer corresponding to the length L0 in the direction parallel to the surface of the steel sheet rcan be obtained by measuring the line roughness of the surface of the coating layer of the hot-pressed member using, for example, a surface roughness meter. r is the surface line length of the coating layer in the area corresponding to the length L0 (line segment L0) in the direction parallel to the surface of the steel sheet (see Figure 1). The line roughness is measured by using the surface roughness meter as a parameter, L r If / L0 exists, that parameter can be used. The line roughness can also be calculated from the height profile and measurement interval measured by a surface roughness meter. In this case, the n-th height profile is calculated as h n (μm), the measurement interval is i (μm), and the number of height profiles is m, the line roughness can be calculated using the following formula.

[0049]

number

[0050] In measuring the line roughness, the length L0 (line segment L0) in the direction parallel to the surface of the steel sheet is set to 2 mm or more. The measurement is performed at eight randomly selected points on the surface of the hot-pressed part. At each of the points, L r / L0 is calculated, and the average value is used as the L r / L0. For specific measurement conditions of the line roughness, see the description in the Examples.

[0051] The above L r / L0 is L r When / L0≧1.10, the surface area of the coating layer exposed to the atmosphere increases, and the area where the hydrogen penetration reaction occurs also increases, so that the desired properties cannot be achieved in the evaluation method for delayed fracture properties of metallic materials (HeTsAce) described later. r / L0<1.10 must be satisfied. r / L0 is L r It is more preferable that L0<1.05. rThe lower limit of / L0 is not particularly limited, but as an example, 1.00≦L r / L0.

[0052] The coating weight of the coating layer is not particularly limited. From the viewpoint of corrosion resistance, the coating weight of the coating layer is 60 g / m per side of the steel sheet. 2 On the other hand, from the viewpoint of manufacturing costs, the coating amount of the coating layer is preferably 400 g / m per one side of the steel sheet. 2 The coating weight can be determined by dissolving and removing the coating layer from the surface of the hot-pressed part using an acid solution, and subtracting the mass of the hot-pressed part after removal from the mass of the hot-pressed part before removal. An inhibitor that inhibits dissolution of the steel sheet is preferably added to the acid solution.

[0053] Tensile strength: 1760 MPa or more The tensile strength of the hot-pressed member according to this embodiment is 1760 MPa or more. The tensile strength is preferably 1800 MPa or more. The upper limit of the tensile strength is not particularly limited, but the tensile strength is, for example, 2200 MPa or less. The tensile strength (TS) is measured by taking a JIS No. 5 tensile test piece from the hot-pressed member and conducting a tensile test in accordance with JIS Z 2241:2011.

[0054] In the evaluation of delayed fracture characteristics of metallic materials, the cracking period is 40 days or more. The hot-pressed member according to this embodiment has a cracking period of 40 days or more according to the following evaluation method for delayed fracture properties of metallic materials (HeTsAce).

[0055] (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.

[0056] In this embodiment, a metallic material extracted from a hot-pressed member is evaluated. The method for extracting the metallic material from the hot-pressed member is not particularly limited. For example, the metallic material can be extracted by shearing the hot-pressed member to a predetermined size. In HeTsAce, it is preferable to apply stress to the metallic material to be evaluated. Methods for applying stress to the metallic material include processing (processing) of the metallic material. Examples of processing methods include bending, bulging, stretching, and twisting. Other examples include fixing the stressed shape using bolts or using residual stress remaining after processing. In the evaluation method of this embodiment, the metallic material to which stress has been applied as described above can be subjected to at least one or more processes (one or two or more processes) including a chloride deposition process (A) and a corrosion process (B). Furthermore, in the evaluation method of this embodiment, the state of the metallic material is confirmed after the process including the chloride deposition process (A) and the corrosion process (B) is performed one 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.

[0057] In this embodiment, 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.

[0058] (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 this embodiment, 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.

[0059] 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:

[0060] 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.

[0061] 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.

[0062] 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).

[0063] 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.)).

[0064] 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.

[0065] 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.

[0066] 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 becomes long to obtain an appropriate chloride deposition amount. As a result, it becomes 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, chlorides are likely to precipitate in the spray nozzle, causing clogging and making it difficult to spray saltwater droplets of consistent size. This makes 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 causes the delayed fracture properties to vary within the evaluation surface of the metal material, reducing the accuracy of the delayed fracture property evaluation. This tendency becomes particularly pronounced when the amount of chloride attached is high. Therefore, the chloride concentration in the salt water is preferably 20 mass % or less, and more preferably 15 mass % or less.

[0067] It is recommended to adjust the chloride concentration in the saltwater according to the target chloride deposition amount. Low-concentration saltwater is preferable when the chloride deposition amount is relatively low, while high-concentration saltwater is preferable 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.

[0068] [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 this embodiment, 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, the saltwater spray occurs 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, they will spread 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.

[0069] Fig. 3 is a schematic diagram illustrating an example of the evaluation method of this embodiment. As shown in Fig. 3, in the evaluation method of this embodiment, salt water is sprayed from a spray nozzle to deposit droplets of salt water onto the evaluation surface of the metal material. In this process, in the initial chloride deposition step (A), the distribution of the salt water droplets deposited on the evaluation surface of the metal material (deposition distribution of droplets) is controlled within a predetermined range.

[0070] 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 this embodiment, 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.

[0071] <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 this embodiment, 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 the droplets) is set to 40% or more and 80% or less. If the total contact area ratio of the 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 the droplets is set to 40% or more. The total contact area ratio of the droplets is preferably set to 50% or more, more preferably 55% or more. On the other hand, if the total contact area ratio of the 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 the droplets is set to 80% or less. The total contact area ratio of the droplets is preferably set to 75% or less, more preferably 70% or less. The total contact area ratio of the droplets can be measured using the measurement method described below.

[0072] <Standard deviation of droplet contact area on the evaluation surface of the metal material: 3.0 mm 2 Below> In the evaluation method of this embodiment, in at least the first chloride adhesion step (A), the standard deviation in the distribution of the contact area of the droplet 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.

[0073] 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 3). 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.

[0074] Figure 4 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 4, 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.

[0075] 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. 3). 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. 9).

[0076] 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.

[0077] 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 3) 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 3) 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 9, 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.

[0078] 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).

[0079] Fig. 5 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 this embodiment. As shown in Fig. 5, by placing the above-mentioned shielding material between the spray nozzle and the metal material, the sprayed liquid (atomized salt water) that was sprayed from the spray nozzle but remains floating in the atmosphere without adhering to the evaluation surface of the metal material can be prevented 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 accurately achieve the desired distribution of droplets on the evaluation surface of the metal plate.

[0080] 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. 5) 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. 6(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. 6(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.

[0081] As described above, in the evaluation method of this embodiment, it is important to uniformly control the distribution of saltwater droplets on the evaluation surface of the metal material in the initial (first) chloride deposition step (A). Therefore, the droplet distribution on the evaluation surface as described above is controlled at least in the first chloride deposition step (A). The average droplet contact area, total droplet contact area ratio, and standard deviation of the droplet contact area can be measured at the end of the first chloride deposition step (A) (within 30 seconds after the end of saltwater spraying). Alternatively, a test specimen other than the one to be tested may be used. The conditions for the average droplet contact area, total droplet contact area ratio, and standard deviation of the droplet contact area may be set in advance, 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 in the first chloride deposition step (A), or under different conditions from the first 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.

[0082] 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.

[0083] 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 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 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 can be 20°C or 25°C.

[0084] 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.

[0085] (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.

[0086] <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.

[0087] 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.

[0088] [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.

[0089] 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.

[0090] [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%.

[0091] 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.

[0092] [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 the atmosphere with relative humidity Hb2 to the atmosphere with relative humidity 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, the rate of change in relative humidity during the transition steps (b3) and (b4) is set to 30% / h or less. A rate of change in relative humidity of 30% / h or less during the transition steps (b3) and (b4) allows sufficient penetration of hydrogen generated by corrosion into the metal material, enabling appropriate evaluation of delayed fracture properties. While there is no specific lower limit for the rate of change in relative humidity, excessively long transition steps (b3) and (b4) can result in a long delay in evaluating 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.

[0093] The purpose of the method for evaluating the delayed fracture properties of a metallic material according to this embodiment is to simulate the daytime and nighttime changes in relative humidity in an actual environment. Therefore, if the processing 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 processing time of the corrosion step (B) is preferably set to 24 hours or less. To expedite the evaluation, the processing time of the corrosion step (B) is more preferably set to 12 hours or less. On the other hand, if the processing time of the corrosion step (B) is shortened, the relative humidity will change rapidly, reducing the correlation with corrosion in an actual environment and resulting in inconsistencies in the delayed fracture properties in an actual environment. Therefore, the processing time of the corrosion step (B) is preferably set to 5 hours or more.

[0094] In the evaluation method of this embodiment, 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.

[0095] Next, a process including a chloride adhesion step (A) and a corrosion step (B) will be described. FIG. 7 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. 7 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.

[0096] 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).

[0097] 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 8 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, which may make 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 metal material is immersed in water to wash the evaluation surface.

[0098] In the evaluation method of this embodiment, after performing the process comprising the chloride adhesion process (A) and the corrosion process (B) as described above at least once, the state of the metallic material (presence or absence of cracks in the metallic material and the extent of cracks, etc.) is confirmed, and the delayed fracture properties of the metallic material are evaluated based on the confirmed state of the metallic material.

[0099] Specifically, the hot-pressed member of the present invention has a cracking period of 40 days or more under the following conditions in the evaluation method for delayed fracture properties of metallic materials (HeTsAce).

[0100] ·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 3): 30 cm Distance between the shielding material and the evaluation surface (Y in Figure 5): 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 hours Wetting step (b2) Temperature: 30°C, relative humidity: 90%, retention time: 2 hours Transition step (b3) Relative humidity change rate: 25% / h Transition step (b4) Relative humidity change rate: 25% / h Temperature fluctuation range: within ±5℃ 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. This cycle is repeated four times before process (A) is performed. Metallic material used for evaluation: Hot-pressed components were sheared to a width of 35 mm x length of 100 mm, and then ground to a width of 30 mm to remove residual stress from shearing, creating strip-shaped test specimens. The resulting strip-shaped test specimens were immersed in toluene and ultrasonically cleaned for 5 minutes, then bent 180° with a radius of curvature of 4 mmR. In this state, the specimen shape was fixed by restraining with bolts and nuts, resulting in a test specimen for evaluation of delayed fracture properties, as shown in Figure 9. This test specimen for evaluation of delayed fracture properties was created by bending the strip-shaped test specimen 180° with a radius of curvature of 4 mmR and adjusting the inner spacing of the bent strip-shaped test specimen to a load stress equivalent to 0.7 YS.

[0101] In the above-mentioned evaluation method for the delayed fracture properties of metallic materials (HeTsAce), specimens that take a long time to crack tend to have a low amount of diffusible hydrogen in the steel. This is because the amount of hydrogen that penetrates into the steel during the evaluation of the delayed fracture properties of the metallic material correlates with the amount of hydrogen that penetrates during the hot pressing process.

[0102] (2) Steel sheets for hot pressing As will be described later, a hot-pressed member according to one embodiment of the present invention can be manufactured by hot-pressing a steel sheet (a steel sheet for hot-pressing) having a plating layer. Hereinafter, a hot-pressed steel sheet that can be used for manufacturing the hot-pressed member of the present invention will be described.

[0103] The steel sheet for hot press use comprises a steel sheet and a plating layer disposed on at least one surface of the steel sheet.

[0104] [Steel plate] The steel sheet (base steel sheet) is not particularly limited and any steel sheet can be used. The steel sheet may be either a cold-rolled steel sheet or a hot-rolled steel sheet. The composition of the steel sheet is also not particularly limited, but it is preferable to use a steel sheet having the component composition described above in the description of the hot-pressed member.

[0105] [Plating layer] The steel sheet for hot press use of the present invention has a plating layer on at least one surface of the steel sheet. The plating layer has an interfacial alloy layer disposed on the steel sheet and a metal layer disposed on the interfacial alloy layer. The plating layer may be provided on only one surface of the steel sheet, but is preferably provided on both surfaces.

[0106] [Interfacial alloy layer] A steel sheet for hot pressing is typically produced by hot-dip galvanizing a base steel sheet, as described below. During this process, Fe, Mn, etc. contained in the base steel sheet react with components such as Al, Si, etc. contained in the plating bath, forming an interfacial alloy layer at the interface between the base steel sheet and the metal layer. In the present invention, the composition of the interfacial alloy layer is not particularly limited. On the other hand, in the hot-pressed member finally obtained after the hot pressing process, the above-mentioned L r In order to satisfy / L0<1.10, the interface alloy layer must satisfy the following conditions.

[0107] The standard deviation of the thickness of the interfacial alloy layer in the cross section perpendicular to the steel plate surface of the hot-pressed steel plate is t d But, t d Meets <1.5μm. The surface line length of the coating layer of the hot-pressed member tends to decrease as the standard deviation of the thickness of the interface alloy layer of the hot-press steel plate that is the raw material for the hot-pressed member decreases. r To satisfy / L0<1.10, the standard deviation of the thickness of the interface alloy layer of the hot-press steel sheet t d is t d <1.5μm must be satisfied. d is t d It is more preferable that the above t<1.2 μm is satisfied. d The smaller the better. For example, t d , 0.5μm≦t d It may be.

[0108] The standard deviation of the thickness of the interfacial alloy layer, t d can be determined by observing a cross section of a steel sheet for hot pressing perpendicular to the surface of the steel sheet using a scanning electron microscope (SEM). At this time, the observation is carried out over a width of 200 μm or more in a direction parallel to the surface of the steel sheet. The thickness of the interface alloy layer obtained from the cross section observation is measured at 10 μm intervals, and the standard deviation t of the thickness of the interface alloy layer is calculated. d The observation is performed by measuring (observing) eight randomly selected points on the steel plate. d For specific measurement conditions, please refer to the description in the Examples.

[0109] [Metal layer] In order for the component composition of the coating layer after hot pressing to satisfy the above-mentioned conditions, the metal layer of the steel sheet for hot pressing preferably has the following component composition.

[0110] The composition contains at least one selected from Mg: 0.20-7.0% and Ca: 0.01-1.5%, Si: 1.0-10.0%, Fe: 10.0% or less, and the balance being Al and unavoidable impurities. The Mg content is preferably 6.0% or less. The Ca content is preferably 1.0% or less. The Si content is preferably 8.0% or less. The Fe content is preferably 6.0% or less. There is no particular lower limit for the Fe content, but the Fe content may be 2.0% or more.

[0111] On the other hand, the surface line length of the coating layer of the hot-pressed member does not depend on the surface roughness of the steel plate for hot pressing. This is because the surface roughness of the steel plate for hot pressing changes significantly during the alloying process of the coating layer in the hot pressing process. Therefore, even if the surface roughness of the steel plate for hot pressing is reduced, the L of the hot-pressed member after hot pressing r In order to satisfy the above requirement, the standard deviation t d But, t d The condition of <1.5 μm must be met.

[0112] In order for the standard deviation of the thickness of the interface alloy layer in a steel sheet for hot pressing to satisfy the above condition, it is necessary to perform cooling under specific conditions during the production of the steel sheet for hot pressing. Specifically, a rapid cooling process is performed after the steel sheet is pulled up from the coating bath, and in this process, the cooling time of the steel sheet from 660°C to 200°C is set to 10 seconds or less (see Figure 2). By performing the rapid cooling process, the growth of the interface alloy layer during the solidification process is suppressed, and the standard deviation of the thickness of the interface alloy layer can be reduced. As a result, the standard deviation t of the thickness of the interface alloy layer that satisfies the above condition is d Furthermore, as a result, the surface line length of the coating layer of the hot-pressed member after the hot-pressing process can be reduced. Note that, when the temperature of the steel sheet when pulled out of the coating bath is less than 660°C, the cooling time of the steel sheet from the temperature when pulled out to 200°C may be set to 10 seconds or less.

[0113] (3) Manufacturing method of hot-pressed parts Next, a preferred method for manufacturing a hot-pressed member according to one embodiment of the present invention will be described.

[0114] The hot-pressed member according to this embodiment can be manufactured by hot-pressing a steel sheet for hot-pressing that satisfies the above-mentioned conditions, for example. In particular, as described above, it is important to use an Al-based plated steel sheet that has been manufactured under specific conditions after plating as the steel sheet for hot-pressing that is used as the raw material.

[0115] The method of hot pressing is not particularly limited and can be carried out according to a conventional method. Typically, the steel sheet for hot pressing is heated to a predetermined heating temperature (heat treatment step), and then the steel sheet for hot pressing heated in the heat treatment step is hot pressed (hot pressing step). Preferred conditions for each step are described below.

[0116] [Heat treatment process] In the heat treatment process, the steel sheet for hot pressing is heated to a heating temperature equal to or higher than the Ac3 transformation point. By setting the heating temperature equal to or higher than the Ac3 transformation point, the structure of the steel sheet can be austenitized. The austenite is then rapidly cooled during hot pressing to form a hard phase such as martensite, thereby increasing the strength of the hot-pressed part. If the heating temperature is lower than the Ac3 transformation point, the austenite fraction in the heated steel sheet decreases, resulting in an insufficient volume fraction of martensite after hot pressing, making it impossible to ensure sufficient tensile strength.

[0117] The Ac3 transformation point can be determined by the following formula (1). Ac3 transformation point (℃) = 881 - 206C + 53Si - 15Mn - 20Ni - 1Cr - 27Cu + 41Mo (1) In formula (1), the element symbols represent the content (mass%) of each element in the steel sheet. The content of elements that are not contained is set to 0 in the calculation.

[0118] The method for heating the steel sheet for hot press use in the heat treatment step is not particularly limited, and any method can be used. The heating can be performed, for example, by heating in a heating furnace, electrical heating, induction heating, high-frequency heating, flame heating, etc. Any heating furnace can be used, such as an electric furnace or a gas furnace.

[0119] [Hot pressing process] After the heating, the steel sheet for hot pressing is hot pressed to obtain a hot pressed member. In the hot pressing, cooling is carried out using a mold or a coolant such as water simultaneously with or immediately after the working. In the present invention, the hot pressing conditions are not particularly limited. For example, pressing can be carried out at a general hot pressing temperature range of 600 to 800°C. Note that the temperature in this embodiment is the temperature of the steel sheet surface.

[0120] The manufacturing method of a steel sheet for hot press use is not particularly limited except for the above-mentioned rapid cooling step. For example, a steel sheet for hot press use can be manufactured by the following method. First, a steel slab having the above-mentioned component composition of the steel sheet is heated in a hot rolling step, and then subjected to rough rolling and finish rolling to obtain a hot-rolled steel sheet. Thereafter, scale on the surface of the hot-rolled steel sheet is removed in a pickling step, and the hot-rolled steel sheet is cold-rolled in a cold rolling step as necessary. The steps from the hot rolling step to the cold rolling step are not particularly limited, and for example, a known manufacturing method can be adopted.

[0121] Next, the hot-rolled steel sheet, or the cold-rolled steel sheet that has been cold-rolled as necessary, is subjected to an annealing treatment to produce a base steel sheet, which is then subjected to a plating treatment.

[0122] In the present invention, the conditions for the plating treatment are not particularly limited, and known conditions can be used, for example. The plating treatment can be performed, for example, by immersing a base steel sheet in a plating bath at 640°C or higher. The plating bath contains Mg and / or Ca and Si, with the balance consisting of Al and unavoidable impurities. The composition of the plating bath may be adjusted so as to obtain the plating layer of the steel sheet for hot press use described above. For example, when Mg is contained, the Mg concentration in the bath can be 0.2 to 7.0 mass%, and when Ca is contained, the Ca concentration can be 0.01 to 1.5 mass%. Furthermore, the Si concentration in the bath can be 1.0 to 15.0 mass%. The steel sheet for hot press use is then obtained by subjecting the steel sheet removed from the plating bath to the rapid cooling process described above. [Example]

[0123] In order to confirm the effects of the present invention, a steel plate for hot press use and a hot press member using the steel plate for hot press use were produced, and their properties were evaluated. The results are shown below. However, the present invention is not limited to the following examples.

[0124] [Steel sheets for hot pressing] A steel sheet for hot pressing was prepared by forming a coating layer on the surface of the steel sheet using the following procedure. Specifically, a 1.4 mm thick steel sheet (base steel sheet) was subjected to a coating process using continuous hot-dip galvanizing equipment to form a coating layer on both sides. The steel sheet used was a cold-rolled steel sheet containing, by mass%, 0.34% C, 0.25% Si, 1.20% Mn, 0.005% P, 0.001% S, 0.03% Al, 0.004% N, 0.02% Ti, 0.002% B, 0.18% Cr, and 0.008% Sb, with the balance consisting of Fe and unavoidable impurities. The Ac3 transformation point of the cold-rolled steel sheet was 806°C. The temperature of the coating bath was 640°C, and the coating weight of the coating layer was 60 g / m per side of the steel sheet. 2 , i.e., 120 g / m on both sides 2 In the coating treatment, a rapid cooling step was provided under the conditions shown in Table 1, and the steel sheet was rapidly cooled after being pulled out of the coating bath. The rapid cooling was performed using helium gas.

[0125] (Composition of metal layer) The composition of the metal layer of the obtained steel sheet for hot pressing was measured by area analysis using SEM (scanning electron microscope)-EDX (energy dispersive X-ray analysis). The SEM-EDX analysis was performed using a JEOL SEM (JSM-7200F) and a Thermo Fisher EDX detector (UltraDry) at an acceleration voltage of 15.0 kV. The results of the composition of the obtained metal layer are shown in Table 1.

[0126] (Standard deviation of the thickness of the interfacial alloy layer t d ) The standard deviation of the thickness of the interfacial alloy layer of the obtained hot-press steel sheet, td was measured by observation using a scanning electron microscope (SEM) according to the method described above. The SEM observation was also carried out using a JEOL SEM (JSM-7200F) at an accelerating voltage of 15.0 kV. The standard deviation t d The results are shown in Table 1. ◎:t d <1.2 μm ○: 1.2 μm≦t d <1.5 μm ×: 1.5 μm≦t d

[0127] [Hot-pressed parts] Next, each of the obtained steel plates for hot pressing was hot pressed under the following conditions to produce hot-pressed members. First, the steel plates for hot pressing were cut into a size of 70 mm x 150 mm and subjected to heat treatment in an electric furnace. The heating temperature in the heat treatment was 900°C, and the holding time at this heating temperature was 1.5 minutes. The heating was performed in an atmosphere with a dew point of 10°C. Next, the steel plates for hot pressing were removed from the electric furnace and hot-pressed using a flat die. The forming start temperature was 700°C. The tensile strength of the obtained hot-pressed members was 1765 MPa to 1850 MPa, and all of them satisfied the tensile strength requirement of 1760 MPa or more.

[0128] The composition of the coating layer of the hot-pressed part was then measured by area analysis using SEM (scanning electron microscope)-EDX (energy dispersive X-ray analysis). The SEM-EDX analysis was also performed using a JEOL SEM (JSM-7200F) and a Thermo Fisher EDX detector (UltraDry) at an accelerating voltage of 15.0 kV.

[0129] Next, the line roughness of the coating layer of the obtained hot-pressed member was measured using a surface roughness meter. The line roughness was measured using a Mitutoyo surface roughness meter (SJ210), with a length L0 in the direction parallel to the surface of the steel sheet: 2 mm, a high-pass filter cutoff value λc: 0.8 mm, a low-pass filter cutoff value λs: 2.5 μm, and eight measurement locations (N8). From the obtained line roughness height profile and measurement interval, the line length L of the coating layer was calculated as described above. r was calculated. r / L0 is calculated, and the average value is used as the L r The evaluation results are shown in Table 1. ◎:L r / L0<1.05 ○:1.05≦L r / L0<1.10 ×:1.10≦L r / L0

[0130] Evaluation of delayed fracture properties of metal materials (hot-pressed components) (HeTsAce) The hot-pressed specimens were sheared to a width of 35 mm and a length of 100 mm, and then ground to a width of 30 mm to remove residual stress from shearing. These rectangular specimens were then immersed in toluene for 5 minutes and ultrasonically cleaned. They were then bent 180° with a radius of curvature of 4 mmR and restrained with bolts and nuts to fix the specimen shape. The delayed fracture evaluation specimens shown in Figure 9 were obtained. The rectangular specimens were bent 180° with a radius of curvature of 4 mmR, and three specimens were prepared for a delayed fracture evaluation by adjusting the inner spacing of the bent rectangular specimens to achieve a load stress equivalent to 0.7 YS.

[0131] The above test pieces for evaluation of delayed fracture were subjected to a corrosion test under the following conditions. ·Process (A) Chloride-containing aqueous solution: 15 mass% NaCl aqueous solution sprayed using a two-fluid spray nozzle Chloride adhesion amount (solid content equivalent): 10,000 mg / m 2 Distance between nozzle and evaluation surface (X in Figure 3): 30 cm Distance between the shielding material and the evaluation surface (Y in Figure 5): 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 hours Wetting step (b2) Temperature: 30°C, relative humidity: 90%, retention time: 2 hours Transition step (b3) Relative humidity change rate: 25% / h Transition step (b4) Relative humidity change rate: 25% / h Temperature fluctuation range: within ±5℃ 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) is considered one cycle when the drying process (b1) → transition process (b3) → wetting process (b2) → transition process (b4) are performed in this order, and this cycle is repeated four times before process (A) is performed.

[0132] (1) Evaluation of delayed fracture properties During the corrosion test, the test specimens were visually inspected once a day for cracks at the 180° bend, and the number of days until cracks occurred (number of days until cracks occurred) was monitored for up to 63 days. Cracking was judged to have occurred when a newly generated crack measured 1 mm or more from the evaluation surface state before the corrosion test. The corrosion test for evaluating delayed fracture properties was performed on three specimens per Example, and the number of days until cracks occurred in two or more specimens by the 63rd day was defined as the "number of days until cracks occurred." The number of days until cracks occurred was evaluated according to the following criteria. The evaluation results are shown in Table 1. ○: 40 days or more or no cracks ×: Less than 40 days

[0133] The amount of diffusible hydrogen in the steel of the hot-pressed member thus obtained was measured.

[0134] (Diffusible hydrogen content in steel) The diffusible hydrogen content of the resulting hot-pressed parts immediately after pressing was measured using the following method. A small piece measuring 10 x 15 mm was cut from the flat portion of the hot-pressed part, and the coating layers on both sides were removed by grinding with a precision mill. Thermal desorption analysis was then performed, and the integrated value of the hydrogen content when heated from room temperature to 200°C was taken as the diffusible hydrogen content. For the thermal desorption analysis, a J-Science Corporation thermal desorption analyzer was used, with argon as the carrier gas and a heating rate of 200°C / s. The diffusible hydrogen content in the resulting steel was evaluated according to the following criteria. The evaluation results are shown in Table 1. ◎: Less than 0.15 ppm by mass ○: 0.15 mass ppm or more, less than 0.18 mass ppm ×: 0.18 mass ppm or more

[0135] As can be seen from the results shown in Table 1, the hot-pressed members satisfying the conditions of the present invention had a small amount of diffusible hydrogen immediately after hot pressing and were excellent in hydrogen embrittlement resistance.

[0136] [Table 1]

Claims

1. A steel plate and a coating layer disposed on at least one surface of the steel plate, The coating layer comprises, in mass %, At least one selected from Mg: 0.10 to 5.0% and Ca: 0.005 to 1.0%, Si: 3.0 to 15.0%, Fe: 55.0% or less, the balance being Al and unavoidable impurities; Length L in the direction parallel to the surface of the steel plate ０ and the L ０ The surface line length L of the coating layer corresponding to ｒ The ratio (L ｒ / L ０ ) but L ｒ / L ０ <1.10, The tensile strength is 1760 MPa or more, A hot-pressed member that has a cracking time of 40 days or more in an evaluation of the delayed fracture characteristics of a metallic material.

2. A steel sheet for hot press use, comprising a steel sheet and a plating layer disposed on at least one surface of the steel sheet, the plating layer comprises an interface alloy layer disposed on the steel sheet and a metal layer disposed on the interface alloy layer, In a cross section perpendicular to the steel plate surface of the hot press steel plate, the standard deviation t ｄ But, t ｄ <1.5 μm, The metal layer comprises, in mass %, At least one selected from Mg: 0.20 to 7.0% and Ca: 0.01 to 1.5%, Si: 1.0 to 10.0%, A steel sheet for hot pressing, having a chemical composition containing Fe: 10.0% or less, with the balance being Al and unavoidable impurities.

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