Hot-pressed member and steel sheet for hot pressing

Abstract

The purpose of the present invention is to provide a hot-pressed member that uses an Al-system-plated steel sheet and that has a tensile strength at least in the 1.8 GPa class, that is, at least 1760 MPa, and that exhibits an excellent resistance to hydrogen embrittlement.　This hot-pressed member comprises a steel sheet and a coating layer disposed on at least one surface of the steel sheet, wherein the coating layer has a component composition containing, in mass%, Si : 3.0 to 15.0%, Fe : not more than 55.0%, and at least one selection from Mg : 0.10 to 5.0% and Ca : 0.005 to 1.0%, with the balance being Al and unavoidable impurities. For the hot-pressed member, the ratio (Lr / L0) of the length L0 in the direction parallel to the surface of the steel plate divided into the surface line length Lr of the coating layer corresponding to L0 satisfies Lr / L0 < 1.10; the tensile strength is at least 1760 MPa; and the number of days until cracking is at least 40 days in an evaluation of the delayed fracture characteristics of the metal material.

Description

Hot-pressed parts and hot-pressed steel plates

[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.

[0002] In order to reduce the weight of automobiles and improve their crashworthiness, efforts are being made to increase the strength of steel sheets for automobiles. In recent years, cold-rolled steel sheets with a tensile strength of 1.5 GPa have been developed, and their application to automobiles is being considered. However, as the strength of steel sheets increases, problems such as poor forming during press work and springback become a challenge 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 sheet (hot pressing steel sheet) is heated to the austenite temperature range, then press-formed while still at high temperature, 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 the cut areas after painting.

[0005] Therefore, a steel sheet having an Al-based plating layer on its 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, hot-pressed members have mainly had a tensile strength of 1.5 GPa after hot-press forming. In recent years, in order to further reduce the weight of automobile bodies, hot-pressed members are desired to have a high strength, such as a tensile strength of 1.8 GPa or more.

[0007] Special table 2017-536472 publication

[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 steel sheets for hot-pressing, which are the raw materials for the 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 using Al-based plated steel sheets as the raw materials, it is known that the amount of hydrogen that penetrates during the hot-pressing process is extremely large.

[0010] Therefore, when an Al-based plated steel sheet is used as a material for a hot-pressed member, it is important to reduce the amount of hydrogen that penetrates during the hot-pressing process.

[0011] Furthermore, when an Al-based plated steel sheet is subjected to hot pressing, the coating layer of the hot-pressed member becomes an FeAl alloy layer. In this case, the surface roughness increases with the formation of the FeAl alloy layer, and the contact area between the coating layer surface and the atmosphere during the hot pressing process increases. The contact area between the coating layer surface and the atmosphere during the hot pressing process affects hydrogen penetration during the hot pressing process. Therefore, it is important to reduce the contact area between the coating layer 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 plating layer, the Al-based plating 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 plating layer during hot pressing.

[0013] In the above-mentioned 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 in the hot pressing step, and the penetration of hydrogen is reduced.

[0014] However, even in the case of a hot-pressed member in which the amount of hydrogen penetration is reduced by using an Al-based plated steel sheet as disclosed in Patent Document 1, in a high-strength hot-pressed member 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 atmosphere 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-described 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.

[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 sheet and a coating layer disposed on at least one surface of the steel sheet, wherein the coating layer has a component 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 L in a direction parallel to the surface of the steel sheet. 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[2] A hot-pressed member that satisfies the above condition <1.10, has a tensile strength of 1760 MPa or more, and has a number of days to crack of 40 days or more in an evaluation of the delayed fracture properties of a metallic material. [3] A hot-pressed member that satisfies the above condition <1.10, has a tensile strength of 1760 MPa or more, and has a number of days to crack of 40 days or more in an evaluation of the delayed fracture properties of a metallic material. [4] A hot-pressed member that satisfies the above condition <1.10, has a tensile strength of 1760 MPa or more, and has a number of days to crack of 40 days or more in an evaluation of the delayed fracture properties of a metallic material. [5] A hot-pressed member that satisfies the above condition <1.10, has a tensile strength of 1760 MPa or more, and has a number of days to crack of 40 days or more in an evaluation of the delayed fracture properties of a metallic material. [6] A hot-pressed member that satisfies the above condition <1.10, has a tensile strength of 1760 MPa or more, and has a ｄ But, t ｄ <1.5 μm, and the metal layer has a component composition containing, 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%, and Fe: 10.0% or less, with the balance being Al and unavoidable impurities.

[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.

[0020] FIG. 1 shows the length L in the direction parallel to the steel plate surface. ０ and the L ０ The surface line length L of the coating layer corresponding to ｒFIG. 2 is a graph showing the relationship between the cooling time and the steel sheet temperature in the rapid cooling step in the manufacturing method of a steel sheet for hot press use according to one embodiment of the present invention. FIG. 3 is a schematic diagram showing a method for evaluating the delayed fracture properties of a metallic material (HeTsAce). FIG. 4 is a schematic diagram showing an example of an image photographed showing the distribution of droplets on the evaluation surface of a metallic material. FIG. 5 is a schematic diagram showing a case where a shielding material is placed between the spray nozzle and the metallic material in the method for evaluating the delayed fracture properties of a metallic material. FIG. 6 is a schematic diagram showing 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 metallic material is changed. FIG. 7 is a diagram showing one embodiment of a corrosion test cycle according to the method for evaluating the delayed fracture properties of a metallic material. FIG. 8 is a diagram showing another embodiment of a corrosion test cycle according to the method for evaluating the delayed fracture properties of a metallic material. FIG. 9 is a schematic diagram showing a test piece for evaluating delayed fracture properties used in the examples.

[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 "mass %" unless otherwise specified.

[0022] (1) Hot-pressed member 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 composition as the steel plate provided in the hot-pressed part.

[0026] The composition contains C: 0.26 to 0.50%, Si: 0.1 to 1.5%, Mn: 0.5 to 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 balance 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% Si is an element 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 the weld. Therefore, it is desirable to reduce the S content as much as possible, 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] Furthermore, the above-mentioned 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 if it is contained in excess, shape fixability decreases. 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, and 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, and more preferably 0.003% or less.

[0039] Cr: 0.1 to 1.0% Cr, like Mn, is a useful element for strengthening steel and improving hardenability. When Cr is contained, the Cr content is preferably 0.1% or more, and more preferably 0.2% or more, in order to obtain the above effects. On the other hand, since Cr is an expensive element, the inclusion of excessive Cr leads to a significant increase in costs. Therefore, when Cr is contained, the Cr content is preferably 1.0% or less, and 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 has an FeAl alloy layer as a main layer and contains at least one of Mg and Ca, and Si. As described below, 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 hydrogen penetration 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 Mg oxide film does not adequately cover the coating layer, and the hydrogen penetration suppression effect during the hot pressing process is not sufficiently obtained. 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 hydrogen penetration suppression effect is not sufficiently obtained. 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 forms an oxide film on the surface of the coating layer, thereby providing 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 form in the outermost layer of the coating layer, which facilitates reaction with moisture during the hot pressing process, resulting in insufficient hydrogen penetration suppression effect. 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 element 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 plating layer during the plating process and the heat treatment process prior to 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, prone to crack formation, and peeling of the coating layer. 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, although the lower limit of the Fe content is not particularly limited, 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 parallel to the surface of the steel plate is L ０ , said L ０ The surface line length of the coating layer corresponding to ｒ When this is done, L ｒ / L ０ <1.10 Length L in the direction parallel to the surface of the steel plate ０ The surface line length L of the coating layer corresponding to ｒ The surface line length L of the coating layer can be determined 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 length L in the direction parallel to the surface of the steel plate ０ (Line segment L ０ ) (see Figure 1). The line roughness is measured by a surface roughness meter using the parameter L ｒ / L ０ If there exists a parameter, the parameter can be used. The line roughness may also be calculated from the height profile measured by a surface roughness meter and the measurement interval. 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 by the following formula.

[0049]

[0050] In the measurement of the line roughness, the length L in the direction parallel to the surface of the steel plate ０ (Line segment L ０ The width of the area is 2 mm or more. The measurement is carried out at eight randomly selected points on the surface of the hot-pressed part. At each of the points, L ｒ / L ０ The average value is calculated as the L of the hot press part. ｒ / L ０ For specific conditions for measuring the line roughness, see the description in the Examples.

[0051] The above L ｒ / L ０ But, L ｒ / L ０ If the ratio is ≥ 1.10, the surface area of ​​the coating layer exposed to the atmosphere increases, and the area where the hydrogen penetration generation reaction occurs also increases, and the desired properties cannot be satisfied in the evaluation method for delayed fracture properties of metallic materials (HeTsAce) described later. ｒ / L ０ <1.10. ｒ / L ０ Is, L ｒ / L ０ It is more preferable that L<1.05. ｒ / L ０ The lower limit of L is not particularly limited, but as an example, 1.00≦L ｒ / L ０ It may be.

[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 one 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. ２The 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 properties of metal materials, the number of days to crack is 40 days or more. The hot-pressed member of this embodiment has a number of days to crack of 40 days or more in the evaluation method for delayed fracture properties of metal materials (HeTsAce) shown below.

[0055] (HeTsAce) The amount of chloride attached to the evaluation surface of the metal material to be evaluated is 1000 to 20000 mg / m ２ and a corrosion step (B) in which one cycle is performed one or more times in an atmosphere at a temperature Tb1 of 60°C or less and within a certain range, the cycle comprising the following steps: a drying step (b1), a wetting step (b2), a transition step (b3), and a transition step (b4), and the cycle is performed one or more times, and the distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first chloride adhesion step (A) is such that: an average contact area of ​​the droplets on the evaluation surface of the metal material is 0.1 mm ２ More than 3.0 mm ２ less than 1.0 mm, an 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 a standard deviation of the contact area of ​​the droplets on the evaluation surface of the metal material: 3.0 mm ２A method for evaluating the delayed fracture properties of a metallic material, comprising: a drying step (b1): a step of drying a metallic material by maintaining an atmosphere of a relative humidity Hb1 of 45% or less for 1.0 hour or more and 5.0 hours or less; a wetting step (b2): a step of wetting a metallic material by maintaining an atmosphere of a relative humidity Hb2 of 80% or more for 1.0 hour or more and 5.0 hours or less; a transition step (b3): ​​a step of transitioning from the atmosphere of the relative humidity Hb1 to the atmosphere of the relative humidity Hb2 at a rate of change of the relative humidity of 30% / h or less; and a 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 of the relative humidity of 30% / h or less.

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

[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 of cracks in the metallic material and the extent of the cracks are confirmed, thereby evaluating the delayed fracture properties.

[0058] (Chloride Adhesion Step (A)) In the chloride adhesion step (A), a chloride adhesion amount of 1000 to 20000 mg / m is applied to the evaluation surface of the metal material. 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 More than 3.0 mm ２ less than 1.0 mm, 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 applies.

[0059] <Chloride deposition amount: 1,000 to 20,000 mg / m ２ The amount of chloride to be attached to the metal material (the amount of chloride solids not including solvents such as water) is 1,000 to 20,000 mg / m ２ The adhesion amount corresponds to the amount of chloride that is assumed to be adhered in the atmospheric corrosive environment in which an actual automobile runs. ２ In a corrosive environment of less than 20,000 mg / m, corrosion hardly progresses, so hydrogen generation and hydrogen penetration into the metal material are minimal, and delayed fracture is unlikely to occur. 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, which will not serve the purpose. ２ From the viewpoint of simulating the corrosion pattern in the atmospheric corrosive environment in which an actual automobile runs and promoting corrosion, the coating amount is set to 5000 mg / m ２ From the above viewpoint, the amount of adhesion is preferably more than 12,000 mg / m ２ The following is preferred:

[0060] The amount of chloride adhesion can be calculated by multiplying the mass difference between the test piece (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 piece's evaluation surface. When measuring the mass difference, if the chloride-containing aqueous solution adheres to areas other than the evaluation surface of the test piece, appropriate measures can be taken, such as masking the areas other than the evaluation surface or wiping off the chloride-containing aqueous solution that has adhered to the areas other than the evaluation surface. The amount of chloride adhesion can be controlled, for example, by 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 amount of chloride. Examples of chloride include sodium salt (NaCl), potassium salt (KCl), and calcium salt (CaCl), which are present in the atmospheric environment where general metal materials are used. ２ ), magnesium salts (MgCl ２ ) is preferably included. In the chloride adhering step (A), when adhering chloride to the metal material, a component mainly composed of chloride, which contains chloride and components other than chloride, may be adhered. Here, the component mainly composed of chloride refers to a component in which chloride is more than 50 mass% of all components in terms of solid content. Examples of components other than chloride include, but are not limited to, sulfides and nitrate compounds. In consideration of an actual atmospheric corrosion environment, it is preferable to adhere a component mainly composed of NaCl (a component in which NaCl is more than 50 mass% of all components) to the metal material.

[0062] In addition, when simulating delayed fracture characteristics in an area where snow-melting agents are frequently sprayed in winter, it is preferable that the chlorides to be attached to the metal material have a composition similar to that of the snow-melting agents sprayed in that area. ２ Components mainly composed of (CaCl ２ is more than 50% by mass of all components), MgCl ２A component mainly composed of MgCl ２ Examples of the component include a component containing NaCl as the main component (a component in which NaCl accounts for more than 50% by mass of all components), a component containing NaCl as the main component (a component in which NaCl accounts for more than 50% by mass of all components), and the like.

[0063] Furthermore, a component containing a combination of multiple metal salts may be used as the chloride to be attached to the metal material. An example of a component containing a combination of multiple metal salts is the SAE J2334 (0.5% by mass NaCl - 0.1% by mass CaCl ２ -0.075% by mass NaHCO ３ ), artificial seawater (2.5% by mass NaCl-0.5% by mass MgCl ２ -0.12% by mass CaCl ２ Examples include 0.07% by mass KCl and others (for example, 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 for evaluation of 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. One example of a spray method is the deposition of salt water using a spray nozzle. Types of spray nozzles include single-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 uniform deposition distribution of droplets. Furthermore, since salt water is used, it is preferable to use a corrosion-resistant metal such as stainless steel for the spray nozzle material.

[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 a suitable chloride deposition amount. As a result, it becomes difficult to obtain a 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 a consistent size. This makes it difficult to obtain a 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 Adhered to the Test Surface of the Metallic Material in the Initial Chloride Adhesion Step (A)] In the evaluation method of this embodiment, it is necessary to control the distribution of saltwater droplets on the test surface of the metallic material in at least the first (i.e., performed at least the first time) chloride adhe- sion step (A) within the range described below. It is believed that the locations where saltwater was present on the test surface of the metallic material in the first chloride adhe- sion step primarily become corrosion initiation sites on the test surface. Furthermore, in the second chloride adhe- sion step, the test surface becomes corroded after the first chloride adhe- sion step, forming corrosion products, resulting in saltwater spray. Even if saltwater droplets are uniformly applied to the test surface of the metallic material, wetting and spreading occurs due to the influence of the corrosion products. Therefore, it is important to uniformly control the distribution of saltwater droplets on the test surface of the metallic material in the first chloride adhe- sion step.

[0069] 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 salt water droplets deposited on the evaluation surface of the metal material (droplet deposition distribution) is controlled within a predetermined range.

[0070] <Average contact area of ​​droplets on the evaluation surface of the metal material: 0.1 mm ２ More than 3.0 mm ２ 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 More than 3.0 mm 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 chloride deposition amount. 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. ２If the average contact area is more than 3.0 mm, the droplet adhesion distribution becomes non-uniform, and the delayed fracture evaluation results vary widely. ２ The average contact area is less than 2.8 mm ２ It is preferable that the thickness 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 ​​droplets to the area of ​​the evaluation surface of the metal material (total contact area ratio of droplets) is set to 40% to 80%. If the total contact area ratio of droplets is less than 40%, the droplet adhesion distribution will be non-uniform, resulting in large variations in delayed fracture evaluation. Therefore, the total contact area ratio of droplets is set to 40% or more. The total contact area ratio of droplets is preferably set to 50% or more, more preferably 55% or more. On the other hand, if the total contact area ratio of droplets is greater than 80%, adjacent droplets on the evaluation surface will be more likely to bond with each other, resulting in coarsening of the average contact area of ​​droplets (average contact area per droplet). Therefore, the total contact area ratio of droplets is set to 80% or less. The total contact area ratio of droplets is preferably set to 75% or less, more preferably 70% or less. The total contact area ratio of the droplets can be measured by the measurement method described below.

[0072] <Standard deviation of the contact area of ​​the droplet on the evaluation surface of the metal material: 3.0 mm ２ 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 droplet adhesion distribution becomes non-uniform, resulting in a large variation in the delayed fracture evaluation. ２The standard deviation of the contact area of ​​the droplets is 2.8 mm ２ It is preferable that the thickness 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, a microscope, an 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 evaluation surface of the metal material. The image of the evaluation surface can be acquired within 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 areas indicated by circles are the contact areas 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, as described below, if the evaluation target is a bent metallic material, 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. As the spray nozzle, a two-fluid nozzle is preferable. Examples of the two-fluid nozzle include KSMMS (product name) manufactured by Kyoritsu Alloy Manufacturing Co., Ltd., a two-fluid air atomizing nozzle (product name) manufactured by Spraying Systems Japan LLC, and a 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 metal material that has been bent 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 identical to the shape and size of the evaluation surface of the metal material. "Substantially identical" 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] 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 floats 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 droplet distribution 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 Figure 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 floating in the atmosphere without adhering to the evaluation surface of the metal material remains near the evaluation surface of the metal material, thereby reducing the effect of suppressing redeposition (Figure 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 will scatter below the shielding material, reducing the effect of suppressing redeposition (Figure 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 initial 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 initial 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, and the conditions may be set in advance to achieve a predetermined average droplet contact area, total droplet contact area ratio, and standard deviation of the droplet contact area. The evaluation test of the test specimen may then be performed under the same conditions. In the second or subsequent chloride deposition steps (A), chloride deposition may be performed under the same conditions as the initial chloride deposition step (A), or may be performed under different conditions from the initial chloride deposition step (A) as long as the desired chloride deposition amount is achieved. Preferably, conditions for achieving a predetermined average contact area of ​​droplets, a total contact area ratio of droplets, and a standard deviation of the contact areas of droplets 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%, particularly when droplets of a chloride-containing aqueous solution are sprayed using a spray nozzle, the droplets sprayed from the 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 a 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 adhering to the metal material tend to become coarse.

[0083] Furthermore, the chloride deposition step (A) is preferably performed in an atmosphere having a temperature Ta1 of 50°C or less. If the temperature Ta1 in the chloride deposition step (A) exceeds 50°C, particularly when droplets of a chloride-containing aqueous solution are sprayed using a spray nozzle, the droplets sprayed from the 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 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) conditions, and simulating this environmental change is important for approximating the corrosion patterns 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 conditions, 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 in the cycle of relative humidity change (corrosion process (B)) are also important for evaluating delayed fracture properties.

[0085] (Corrosion Step (B)) The corrosion step (B) is a step of performing a cycle at least once (once or twice or more) in an atmosphere at a temperature Tb1 that is 60°C or less and within a certain range, the cycle including the following drying step (b1), the following wetting step (b2), the following transition step (b3), and the following transition step (b4).

[0086] <Temperature Tb1 of Corrosion Step (B): 60°C or Less and Within a Certain Range> The corrosion step (B) is performed in an atmosphere with a temperature Tb1 of 60°C or less and within a certain range. If the temperature Tb1 of 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 is actually used, but the corrosion mechanism may also change. Therefore, the temperature Tb1 of the corrosion step (B) is set to 60°C or less, preferably 50°C or less. On the other hand, the lower limit of the temperature Tb1 of the corrosion step (B) is not particularly limited. If the temperature Tb1 of 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 when conducting the corrosion test. In addition, the corrosion rate of the metal material will be significantly reduced, resulting in a longer evaluation time. Therefore, the temperature Tb1 of 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 of the corrosion step (B) within a certain range. When the temperature Tb1 of 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 of 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 of the step (B) is preferably within ±5°C, and more preferably within ±2°C.

[0088] [Drying Step (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 corrosive 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, resulting in a longer evaluation time. The relative humidity Hb1 in the drying step (b1) is preferably 40% or less. On the other hand, the lower limit of the relative humidity Hb1 in the drying step (b1) is not particularly limited. 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 process time of the drying step (b1) (the time for maintaining the sample in an atmosphere of relative humidity Hb1) is 1.0 hour or more and 5.0 hours or less. If the process time of the drying step (b1) is less than 1.0 hour, an actual corrosive environment cannot be simulated. On the other hand, if the process time of the drying step (b1) is more than 5.0 hours, an actual corrosive environment can be simulated, but it takes a long time to evaluate the delayed fracture properties.

[0090] [Wetting Step (b2)] The wetting step (b2) is a step of wetting a metal material by maintaining 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 corrosive 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 corrosive environment. Among chlorides, sodium chloride has the highest saturated critical vapor pressure, which is approximately 75 to 78% in relative humidity terms. Therefore, for any chloride, if the relative humidity is set to 80% or higher, a water film will form on the metal material surface due to moisture absorption by the chloride, allowing the metal material to maintain 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 if 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 of the wetting step (b2) (the time for maintaining the specimen in an atmosphere with a relative humidity Hb2 of 80% or more) is set to 1.0 hour or more and 5.0 hours or less. If the process time of 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 of 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 Step (b3) and Transition Step (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 a metal material, particularly a steel material, 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, moisture absorption begins due to deliquescence of chlorides present on the surface of the metal material, and corrosion of the metal material begins. It is known that corrosion products present on the surface of the metal material change at this time, and hydrogen is thought to be generated along with this change in corrosion products. Furthermore, in the transition step (b4), which is a step of transitioning from an atmosphere with the relative humidity Hb2 to an atmosphere with the relative humidity Hb1, the moisture becomes a concentrated solution containing a large amount of chlorides and metal ions eluted by corrosion, and in the case of steel materials, iron ions, which is thought to lower the pH of the solution. In other words, a large amount of hydrogen ions is contained in the solution during the drying process, which is thought to facilitate hydrogen penetration into the metal material. For this reason, in the transition steps (b3) and (b4), the rate of change of the relative humidity when changing the relative humidity is set to 30% / h or less. If the rate of change of the relative humidity in the transition steps (b3) and (b4) is 30% / h or less, hydrogen generated by corrosion can be sufficiently penetrated into the metal material, enabling appropriate evaluation of delayed fracture properties. On the other hand, although there is no particular lower limit for the rate of change of the relative humidity, if the transition steps (b3) and (b4) are too long, it will take a long time to evaluate 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 process time (time for one cycle) of the corrosion step (B), which simulates the daytime and nighttime changes in relative humidity in an actual environment, exceeds 24 hours, the corrosion will be slower than in an actual environment, and the evaluation of the delayed fracture properties will require a long time. In other words, the process time of the corrosion step (B) is preferably set to 24 hours or less. To expedite the evaluation, the process time of the corrosion step (B) is more preferably set to 12 hours or less. On the other hand, if the process time of the corrosion step (B) is shortened, the relative humidity will change rapidly, reducing the correlation with corrosion in an actual environment and resulting in a discrepancy in the delayed fracture properties in an actual environment. Therefore, the process time of the corrosion step (B) is preferably set to 5 hours or more.

[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 every random number of cycles of the corrosion step (B) or every 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) includes 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 the variability in the evaluation of delayed fracture properties. If the corrosion process (B) cycle begins with the transition process (b3), the wetting process (b2), or the transition process (b4), the saltwater applied in the chloride deposition process (A) may not be sufficiently dried, or the saltwater applied in a high-humidity environment may absorb moisture and become coarse, resulting in uneven dispersion of condensation initiation points. Therefore, it is best to avoid starting the corrosion process (B) cycle from any process other than the drying process (b1).

[0097] When a cycle of the chloride deposition step (A) followed by the corrosion step (B) is performed and then the chloride deposition step (A) is performed 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 the chloride deposition step (A) is performed 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 of rinsing the evaluation surface of the metal material with water. The water-washing method in the water-washing step (C) is not particularly limited, but examples thereof include a method in which water is sprayed onto the evaluation surface of the metal material from a spray nozzle to wash the evaluation surface, and a method in which the evaluation surface is immersed in water to wash the evaluation surface.

[0098] In the evaluation method of this embodiment, after the process comprising the chloride adhesion process (A) and the corrosion process (B) as described above is performed at least once, the state of the metallic material (the presence or absence of cracks in the metallic material, 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 the above-mentioned metallic material (HeTsAce).

[0100] Step (A) Chloride-containing aqueous solution: 15 mass% NaCl aqueous solution sprayed by a spray method. Chloride adhesion amount (solid content equivalent): 10,000 mg / m ２ Distance between the nozzle and the 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% Distribution of droplets on the evaluation surface Average contact area of ​​droplets: 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 process (b2) Temperature: 30°C, relative humidity: 90%, retention time: 2 hours Transition process (b3) Relative humidity change rate: 25% / h Transition process (b4) Relative humidity change rate: 25% / h Temperature fluctuation range: within ±5°C Corrosion test cycle After performing the first process (A), the distribution of droplets on the evaluation surface was confirmed. Subsequent processes (A) were performed under the same conditions as the first process (A). Process (B) consists of one cycle of the drying process (b1) → transition process (b3) → wetting process (b2) → transition process (b4) in this order, and this cycle was repeated four times before performing process (A). Metallic materials used for evaluation: Hot-pressed members 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, to prepare rectangular test specimens. The obtained rectangular 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, to obtain a test specimen for evaluation of delayed fracture properties as shown in Figure 9. This test specimen for evaluation of delayed fracture properties was prepared by bending the rectangular test specimen 180° with a radius of curvature of 4 mmR and adjusting the inner spacing of the rectangular test specimen after bending to prepare one with a load stress equivalent to 0.7 YS.

[0101] In the above-mentioned evaluation method for the delayed fracture property of metallic materials (HeTsAce), specimens that show a long number of days until cracks occur tend to have a small amount of diffusible hydrogen in the steel, because the amount of hydrogen that penetrates into the steel during the evaluation of the delayed fracture property of the metallic material correlates with the amount of hydrogen that penetrates during the hot pressing process.

[0102] (2) Steel Sheet for Hot Pressing A hot-pressed member according to one embodiment of the present invention can be produced by hot pressing a steel sheet (steel sheet for hot pressing) provided with a plating layer, as will be described later. Hereinafter, a steel sheet for hot pressing that can be used for producing 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 plate (base steel plate) is not particularly limited and any steel plate can be used. The steel plate may be either a cold-rolled steel plate or a hot-rolled steel plate. The composition of the steel plate is also not particularly limited, but it is preferable to use a steel plate 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 interface alloy layer disposed on the steel sheet and a metal layer disposed on the interface alloy layer. The plating layer may be provided on only one surface of the steel sheet, but is preferably provided on both surfaces.

[0106] [Interface alloy layer] A steel sheet for hot pressing is typically produced by hot-dip galvanizing a base steel sheet as described below. At that time, Fe, Mn, etc. contained in the base steel sheet react with components such as Al, Si, etc. contained in the plating bath, forming an interface alloy layer at the interface between the base steel sheet and the metal layer. In the present invention, the composition of the interface 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 / L ０ In order to satisfy the condition <1.10, the interface alloy layer must satisfy the following conditions.

[0107] In a cross section perpendicular to the steel plate surface of the hot press steel plate, the standard deviation t ｄ But, t ｄ 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 / L ０ In order to satisfy <1.10, the standard deviation t ｄ is t ｄ <1.5 μm. ｄ is t ｄ It is more preferable that the t<1.2 μm is satisfied. ｄ The smaller the better. For example, t ｄ 0.5 μm≦t ｄ It may be.

[0108] The standard deviation t of the thickness of the interface alloy layer 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 performed 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 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] A component composition containing 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, with 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. The lower limit of the Fe content is not particularly limited, 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 press-formed member does not depend on the surface roughness of the steel plate for hot press-formed. This is because the surface roughness of the steel plate for hot press-formed. In the process of alloying the coating layer in the hot press process, the surface roughness of the steel plate for hot press-formed. Therefore, even if the surface roughness of the steel plate for hot press-formed. is reduced, the L of the hot press-formed member after hot press-formed. ｒ / L ０ 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 press-forming to satisfy the above condition, it is necessary to perform cooling under specific conditions during the production of the steel sheet for hot press-forming. 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 up from the coating bath is less than 660°C, the cooling time of the steel sheet from the temperature of the steel sheet when pulled up to 200°C may be set to 10 seconds or less.

[0113] (3) Method for Manufacturing Hot-Pressed Member 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 will be described below.

[0116] [Heat Treatment Step] In the heat treatment step, the steel sheet for hot pressing is ３ Heat to a temperature above the transformation point. ３ By heating the steel sheet at a temperature equal to or higher than the transformation point, the structure of the steel sheet can be austenitized. The austenite is converted into a hard phase such as a martensite phase by rapid cooling during the subsequent hot pressing, and as a result, the strength of the hot-pressed member can be increased. ３ If the temperature is lower than the transformation point, the austenite fraction in the heated steel sheet will decrease, resulting in an insufficient volume fraction of martensite after hot pressing, making it impossible to ensure sufficient tensile strength.

[0117] In addition, Ac ３ The transformation point can be calculated by the following formula (1): Ac ３ Transformation point (°C) = 881 - 206C + 53Si - 15Mn - 20Ni - 1Cr - 27Cu + 41Mo (1) where the element symbols in formula (1) represent the content (mass%) of each element in the steel sheet. The content of elements that are not contained is calculated as 0.

[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 above 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 processing. 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 employed, 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, if Mg is contained, the Mg concentration in the bath may be 0.2 to 7.0 mass%, and if Ca is contained, the Ca concentration may be 0.01 to 1.5 mass%. Furthermore, the Si concentration in the bath may 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.

[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 Sheet for Hot Pressing] A steel sheet for hot pressing was prepared by forming a plating layer on the surface of the steel sheet according to the following procedure. Specifically, a plating process was performed on both sides of a steel sheet (base steel sheet) with a thickness of 1.4 mm using a continuous hot-dip plating facility to form a plating layer. The steel sheet used was a cold-rolled steel sheet having a composition containing, in mass %, C: 0.34%, Si: 0.25%, Mn: 1.20%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Ti: 0.02%, B: 0.002%, Cr: 0.18%, and Sb: 0.008%, with the balance being Fe and unavoidable impurities. The Ac of the cold-rolled steel sheet ３ The transformation point is 806°C. The temperature of the plating bath is 640°C, and the coating weight of the plating layer is 60 g / m per side of the steel sheet. ２ , i.e., 120 g / m on both sides in total 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 press use was measured by area analysis using SEM (scanning electron microscope)-EDX (energy dispersive X-ray analysis). In the SEM-EDX analysis, an SEM (JSM-7200F) manufactured by JEOL Ltd. and an EDX detector (UltraDry) manufactured by Thermo Fisher were used, and the analysis was performed 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 t of the thickness of the interface alloy layer of the obtained steel sheet for hot pressing ｄ was measured by observation with 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 acceleration voltage of 15.0 kV. The standard deviation t ｄ The results are shown in Table 1. ｄ <1.2μm ○:1.2μm≦t ｄ <1.5μm ×: 1.5μm≦t ｄ

[0127] [Hot-pressed members] Next, each of the obtained hot-pressed steel plates was hot-pressed under the following conditions to produce hot-pressed members. First, the hot-pressed steel plates were cut into 70 mm x 150 mm pieces and heat-treated 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 hot-pressed steel plates were removed from the electric furnace and hot-pressed using a flat die. The forming start temperature was 700°C. The tensile strengths of the obtained hot-pressed members were 1765 MPa to 1850 MPa, and all satisfied the tensile strength requirement of 1760 MPa or higher.

[0128] The composition of the coating layer of the hot-pressed member 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 UltraDry EDX detector 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) along the length L ０ The measurement was performed at 8 locations (N8) with the following conditions: λc: 2 mm, high-pass filter cutoff value λc: 0.8 mm, low-pass filter cutoff value λs: 2.5 μm. 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 / L ０ The average value is calculated as the L of the hot press part. ｒ / L ０ The results are shown in Table 1. ｒ / L ０ <1.05 ○: 1.05≦L ｒ / L ０ <1.10 ×: 1.10≦L ｒ / L ０

[0130] Evaluation of delayed fracture properties of metal materials (hot-pressed members) (HeTsAce) The obtained hot-pressed members 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 during shearing, to prepare rectangular test specimens. These rectangular 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 test specimen shape was fixed by restraining with bolts and nuts to obtain test specimens for delayed fracture evaluation as shown in Figure 9. For these delayed fracture evaluation test specimens, three rectangular test specimens were bent 180° with a radius of curvature of 4 mmR, and three specimens were prepared by adjusting the inner spacing of the rectangular test specimens after bending to a load stress equivalent to 0.7 YS.

[0131] The above test pieces for evaluating delayed fracture were subjected to a corrosion test under the following conditions: Step (A) Chloride-containing aqueous solution: 15 mass % NaCl aqueous solution sprayed using a two-fluid spray nozzle Adhesion amount of chloride (solid content equivalent): 10,000 mg / m ２ Distance between the nozzle and the 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% Distribution of droplets on the evaluation surface Average contact area of ​​droplets: 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 process (b2) Temperature: 30°C, relative humidity: 90%, retention time: 2 hours Transition process (b3) Relative humidity change rate: 25% / h Transition process (b4) Relative humidity change rate: 25% / h Temperature fluctuation range: within ±5°C Test cycle After performing the first process (A), the distribution of droplets on the evaluation surface was confirmed. Subsequent processes (A) were performed under the same conditions as the first process (A). Process (B) consists of one cycle of the drying process (b1) → transition process (b3) → wetting process (b2) → transition process (b4) in this order, and this cycle was repeated four times before process (A) was performed.

[0132] (1) Evaluation of Delayed Fracture Properties During the corrosion test described above, the presence or absence of cracks at the 180° bend of the test specimen was visually observed once a day, and the number of days until cracks occurred (number of days until cracks occurred) was checked for a maximum of 63 days. Here, crack occurrence was judged as occurring when a newly generated crack grew to 1 mm or more from the evaluation surface state before the corrosion test. The corrosion test for evaluating this delayed fracture property 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 obtained 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] (Amount of diffusible hydrogen in steel) The amount of diffusible hydrogen in the obtained hot-pressed member immediately after pressing was measured using the following method. A small piece of 10 x 15 mm was cut from the flat portion of the hot-pressed member, and the coating layers on both sides were removed by grinding with a precision mill. Then, a thermal desorption analysis was performed, and the integrated value of the amount of hydrogen when the temperature was raised from room temperature to 200°C was taken as the amount of diffusible hydrogen. For the thermal desorption analysis, a thermal desorption analyzer manufactured by J Science Corporation was used, with argon as the carrier gas and a heating rate of 200°C / s. The obtained amount of diffusible hydrogen in the 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 ppm by mass or more but less than 0.18 ppm by mass ×: 0.18 ppm by mass 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]

Claims

1. A steel sheet comprising a steel plate and a coating layer disposed on at least one surface of the steel plate, 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 L in a direction parallel to the surface of the steel sheet. 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, has a tensile strength of 1760 MPa or more, and has a cracking period of 40 days or more in an evaluation of the delayed fracture properties of the metal material.

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, wherein the plating layer comprises an interface alloy layer disposed on the steel sheet and a metal layer disposed on the interface alloy layer, and the standard deviation t of the thickness of the interface alloy layer in a cross section perpendicular to the steel sheet surface of the steel sheet for hot pressing ｄ But, t ｄ <1.5 μm, and the metal layer has a component composition containing, 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%, and Fe: 10.0% or less, with the balance being Al and unavoidable impurities.

Images