Method for introducing hydrogen for evaluating delayed fracture characteristics and method for evaluating delayed fracture characteristics of metallic materials

Abstract

To easily execute introduction of hydrogen into a metal material and evaluate the delay breakage characteristics of the metal material.SOLUTION: A method for introducing hydrogen for evaluating the delay breakage includes: a connection step of electrically connecting a metal material 1 and a metal material 2 to each other; and a hydrogen introduction step of immersing at least a part of the surfaces of the metal material 1 and the metal material 2 electrically connected to each other in an electrolyte and introducing hydrogen into the metal material 1. The immersion potential E1 of the metal material 1 is higher than the immersion potential E2 of the metal material 2.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a hydrogen introduction method for evaluating stress corrosion cracking characteristics and a method for evaluating stress corrosion cracking characteristics of metal materials.

Background Art

[0002] In recent years, from the viewpoints of energy saving and resource saving, steel materials used in various industrial fields have been made stronger. Steel sheets used in automobiles have also been made stronger. For example, high-strength steel sheets with a tensile strength exceeding 1470 MPa are applied to automobile members.

[0003] By the way, stress corrosion cracking may occur in high-strength steel materials. Stress corrosion cracking is a phenomenon in which a metal material suddenly brittlely fractures without almost any plastic deformation visually when a certain period of time has elapsed while receiving a static load stress (load stress below the tensile strength). The stress corrosion cracking of steel materials is more likely to occur as the strength is higher, and particularly, it is more likely to occur in high-strength steel sheets with a tensile strength of 1180 MPa or more.

[0004] The stress corrosion cracking that occurs in steel materials is affected by, for example, the residual stress remaining in the steel material when the steel material is formed into a predetermined shape by processing such as press working and the hydrogen embrittlement of the concentrated portion where the residual stress is concentrated in the steel material. Hydrogen that causes hydrogen embrittlement is, in most cases, hydrogen that has invaded from the external environment into the interior of the steel material and diffused (invasive hydrogen), for example, hydrogen that has occurred during corrosion of the steel material and invaded and diffused into the interior of the steel material.

[0005] Stress corrosion cracking also occurs in metal materials other than steel materials. In order to evaluate the stress corrosion cracking characteristics of a metal material, for example, hydrogen is introduced into the interior of the metal material, and it is determined whether cracks occur in the metal material depending on the amount of introduced hydrogen (invasive hydrogen amount). One method for introducing hydrogen into a metal material is the method described in Patent Document 1. Specifically, Patent Document 1 describes a method for introducing hydrogen into a test piece by immersing an anode and a cathode, which is in contact with a test piece (metal material), in an electrolyte solution, and then passing a constant current through the anode and cathode. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2005-134152 [Overview of the project] [Problems that the invention aims to solve]

[0007] The method described in Patent Document 1 requires a constant current generator such as a potentiometer, resulting in high initial costs, and hydrogen can only be introduced into metal materials in locations with an external power supply.

[0008] Therefore, the objective of the present invention is to easily introduce hydrogen into a metal material and to evaluate the delayed fracture characteristics of the metal material. [Means for solving the problem]

[0009] As a result of diligent research, the inventors of this invention discovered that the above objective can be achieved by adopting the following configuration, and thus completed the present invention. In other words, the present invention provides the following [1] to [6]. [1] A hydrogen introduction method for evaluating delayed fracture characteristics, comprising: a connection step of electrically connecting a metal material 1 and a metal material 2; and a hydrogen introduction step of immersing at least a portion of the electrically connected metal material 1 and metal material 2 surfaces in an electrolyte and introducing hydrogen into the metal material 1, wherein the immersion potential E1 of the metal material 1 is higher than the immersion potential E2 of the metal material 2. [2] A method for evaluating the delayed fracture properties of a metallic material, comprising an evaluation step of evaluating the delayed fracture properties of the metallic material 1 into which hydrogen has been introduced using the method described in [1] above. [3] The method for evaluating the delayed fracture characteristics of a metal material according to [2], wherein the metal material 1 is the base material after the coating material has been removed from a coated metal material on which a coating material has been formed, or the base material before the coating material has been formed, and the metal material 2 is a metal material containing the metal M that constitutes the coating material. [4] The method for evaluating the delayed fracture characteristics of a metal material according to [3] above, wherein the coated metal material is a plated steel sheet on which a plating film has been formed, the base material is the base steel sheet, and the coating material is the plating film. [5] A method for evaluating the delayed fracture characteristics of a metal material according to any of [2] to [4] above, wherein the ratio S2 / S1, which is the ratio of the total surface area S2 of the metal material 2 in contact with the electrolyte to the total surface area S1 of the metal material 1 in contact with the electrolyte, is 1.0 or more. [6] A method for evaluating the delayed fracture characteristics of a metallic material according to any of [2] to [5] above, wherein the conductivity of the electrolyte is 2.0 mS / cm or more. [Effects of the Invention]

[0010] According to the present invention, hydrogen can be easily introduced into a metal material, and the delayed fracture characteristics of the metal material can be evaluated. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic diagram showing a hydrogen introduction device. [Figure 2] This is a schematic diagram showing another hydrogen introduction device. [Figure 3] (a) to (c) are perspective views showing the procedure for preparing bending test specimens. [Figure 4] (a) and (b) are plan views showing the procedure for preparing tensile test specimens. [Modes for carrying out the invention]

[0012] [Hydrogen Introduction Method for Evaluating Delayed Fracture Characteristics and Method for Evaluating Delayed Fracture Characteristics of Metal Materials] The hydrogen introduction method for evaluating delayed fracture characteristics in this embodiment includes a connection step of electrically connecting a metal material 1 and a metal material 2, and a hydrogen introduction step of immersing at least a part of the surfaces of the electrically connected metal material 1 and metal material 2 in an electrolyte solution to introduce hydrogen into the metal material 1. The immersion potential E1 of the metal material 1 is higher than the immersion potential E2 of the metal material 2 (the immersion potential E1 is nobler than the immersion potential E2).

[0013] Also, the method for evaluating the delayed fracture characteristics of a metal material in this embodiment includes an evaluation step of evaluating the delayed fracture characteristics of the metal material 1 into which hydrogen has been introduced using the hydrogen introduction method for evaluating delayed fracture characteristics in this embodiment.

[0014] Hereinafter, this embodiment will be described in more detail. The following description also serves as an explanation of the hydrogen introduction method for evaluating delayed fracture characteristics and the method for evaluating the delayed fracture characteristics of a metal material.

[0015] 〈Hydrogen Introduction Device〉 First, the device (hydrogen introduction device) used when introducing hydrogen into the metal material 1 will be described based on FIGS. 1 to 2. FIG. 1 is a schematic diagram showing a hydrogen introduction device 6. It includes a metal material 1, a metal material 2, a conducting wire 3 for electrically connecting the metal material 1 and the metal material 2, and an electrolytic cell 4 for containing an electrolyte solution 5. As shown in FIG. 1, at least a part of the surface of the metal material 1 is immersed in the electrolyte solution 5, and at least a part of the surface of the metal material 2 is immersed in the electrolyte solution 5. The metal material 1 and the metal material 2 may be not only one but also two or more.

[0016] FIG. 2 is a schematic diagram showing another hydrogen introduction device 7. In FIG. 2, two metal materials 1 (metal material 1a and metal material 1b) are shown. That is, one metal material 1a is connected to the metal material 2 by a conducting wire 3a, and the other metal material 1b is connected to the metal material 2 by a conducting wire 3b.

[0017] <Connection Steps> In the connection step, metal material 1 and metal material 2 are electrically connected. One method for electrically connecting metal material 1 and metal material 2 is to connect them using a conductor. Examples of conductors include copper wire, and commercially available wires may be used. Methods for connecting a wire to a metal material (metal material 1 or metal material 2) include, for example, using a clip such as an alligator clip to clamp the wire to the metal material; soldering the wire to the metal material; or clamping the metal material with an alligator clip to which the wire has been attached.

[0018] <Hydrogen introduction steps> In the hydrogen introduction step, at least a portion of the electrically connected metal material 1 and metal material 2 surfaces are immersed in the electrolyte to introduce hydrogen into metal material 1.

[0019] Immersion potential E1 and immersion potential E2 In this embodiment, the immersion potential E1 of metal material 1 in the electrolyte used is higher than the immersion potential E2 of metal material 2 (E1 > E2). As a result, a potential difference is created between metal material 1 and metal material 2. In this embodiment, by using this potential difference, hydrogen can be easily introduced into the metal material 1 without using an external power source. Specifically, using the potential difference (E1-E2) as the driving force, a dissolution reaction (anodic reaction) of metal material 2 occurs at the surface of metal material 2 in contact with the electrolyte (immersion surface s2), and a hydrogen evolution reaction (cathode reaction) occurs at the surface of metal material 1 in contact with the electrolyte (immersion surface s1). In this way, hydrogen is stably introduced into metal material 1.

[0020] More specifically, hydrogen gas (H2) is generated by the cathode reaction, but as a preliminary step, hydrogen atoms are generated, and it is thought that some of them become solid-dissolved in the metal (e.g., Fe) that makes up metal material 1. In other words, most of the hydrogen is discharged out of the system as hydrogen gas, but some is introduced into metal material 1 and is thought to gradually diffuse from the surface into the interior of metal material 1.

[0021] Furthermore, as long as the elements constituting the metallic materials (metallic material 1 and metallic material 2) do not change to other elements along the way, the potential difference (E1-E2) will remain constant. If the potential difference is constant, the current value will also be constant, and the hydrogen evolution reaction that occurs in that case will also be constant and reproducible. This is what is meant by "stable". Generally, various evaluation tests require reproducibility, and according to this embodiment, the introduction of hydrogen into the metal material 1 is stable and reproducible. For this reason, this embodiment is suitably used for evaluating delayed fracture characteristics.

[0022] <Evaluation Steps> In the evaluation step, the delayed fracture characteristics of metal material 1 into which hydrogen has been introduced are evaluated. For example, the delayed fracture characteristics of metal material 1 are evaluated based on the time until cracking occurs in metal material 1 upon introduction of hydrogen, the amount of hydrogen, stress, etc. Specifically, for example, the delayed fracture characteristics of metal material 1 are evaluated as follows.

[0023] A stress is applied to a metal material 1 to form a stress-loaded area. Hydrogen is introduced into the metal material 1 with the stress-loaded area. At this time, the conditions for introducing hydrogen (amount of hydrogen) are changed, and the delayed fracture characteristics of the metal material 1 are evaluated based on the amount of hydrogen at which cracks occur in the metal material 1. A stress-loaded area is formed by processing, and a predetermined amount (a fixed amount) of hydrogen is introduced into multiple metal materials 1, each with different stresses in the stress-loaded area. The delayed fracture characteristics are then evaluated based on the stress in the stress-loaded area where cracking occurs. Hydrogen is introduced into a metal material 1 that does not have a stress-loaded area, and stress is applied after the introduction of hydrogen. The delayed fracture characteristics are evaluated based on the stress at which cracking occurs. A load is applied to a flat metal material 1 while introducing hydrogen, and the delayed fracture characteristics are evaluated based on the load at which cracking occurs. A predetermined amount of hydrogen is introduced into a metal material 1 with a stress-loaded section, and the delayed fracture characteristics are evaluated based on the time required until crack initiation.

[0024] Methods for forming stress-loaded areas in metal materials include, for example, bending, tensile, drawing, shearing, and welding. Examples of bending processes include hat-shaped bending, U-shaped bending, and V-shaped bending. U-shaped or V-shaped bending is simpler than hat-shaped bending, and the load stress on the bent part can be changed by adjusting the tightening amount using bolts. Automotive components are often manufactured by bending, shearing, and / or welding. Therefore, when evaluating the delayed fracture characteristics of metal materials used as automotive components, it is preferable to form stress-loaded sections by bending, shearing, and / or welding the metal material. Before forming the stress-loaded sections, strain (plastic deformation) may be applied by rolling, tensile working, or other methods.

[0025] <Electrolyte> The electrolyte consists of a solute and a solvent, and has the function of conducting an electric current (galvanic current) based on the potential difference between metal material 1 and metal material 2. The electrolyte is not particularly limited as long as it is a substance through which galvanic current flows. However, if the conductivity of the electrolyte is too low, the galvanic current will not flow easily, and the hydrogen evolution reaction in metal material 1 may not proceed stably. For this reason, from the viewpoint of efficiently passing a galvanic current between metal material 1 and metal material 2, the conductivity of the electrolyte is preferably 2.0 mS / cm or higher, more preferably 3.5 mS / cm or higher, even more preferably 7.0 mS / cm or higher, and particularly preferably 10.0 mS / cm or higher.

[0026] There is no particular upper limit to the conductivity of the electrolyte. However, since the effect saturates, the conductivity of the electrolyte is preferably 100.0 mS / cm or less, and more preferably 50.0 mS / cm or less.

[0027] The solvent constituting the electrolyte is not particularly limited as long as it can conduct a galvanic current and dissolve the electrolyte, but water is preferred from the viewpoint of producing a hydrogen evolution reaction, which is a cathode reaction.

[0028] Examples of solutes constituting the electrolyte include chlorides (hydrochloric acid, sodium chloride, etc.) that promote the dissolution reaction (anodic reaction) of metal material 2; acidic or alkaline substances that adjust the pH; and thiosulfate compounds (ammonium thiosulfate, etc.) that promote hydrogen introduction. These may be used individually or in combination of two or more.

[0029] <Area ratio (S2 / S1)> The area ratio (S2 / S1) is the ratio of the total area S2 of the surface of metal material 2 that comes into contact with the electrolyte (immersion surface s1) to the total area S1 of the surface of metal material 1 that comes into contact with the electrolyte (immersion surface s1).

[0030] If the area ratio (S2 / S1) is too small, the dissolution reaction of metal material 2 becomes the rate-determining reaction (rate-determining process), and the hydrogen evolution reaction at the immersion surface s1 of metal material 1 may not proceed sufficiently. In this case, the amount of hydrogen introduced into metal material 1 is small. Therefore, from the viewpoint of ensuring that the hydrogen evolution reaction proceeds sufficiently, the area ratio (S2 / S1) is preferably 1.0 or higher, and more preferably 2.0 or higher.

[0031] There is no particular upper limit to the area ratio (S2 / S1). However, if the area ratio (S2 / S1) is too large, it may become necessary to enlarge the electrolytic cell or prepare a large amount of electrolyte, which can be disadvantageous from the standpoint of work efficiency and cost. Therefore, the area ratio (S2 / S1) is preferably 20.0 or less, and more preferably 15.0 or less.

[0032] The area ratio (S2 / S1) can be adjusted, for example, by changing the size of the metal materials (metal material 1 and metal material 2). Specifically, one method for increasing the area ratio (S2 / S1) is to increase the total area S2 of the metal material 2 by connecting multiple metal materials 2 to the metal material 1.

[0033] If the metal materials (metal material 1 and metal material 2) are coated metal materials such as plated steel sheets, they will have surfaces of different metal types. In this case, when adjusting the area ratio (S2 / S1), it is preferable to coat the surfaces of the metal types not being evaluated with resin or the like. In this case, it is preferable that the connection point between the conductor and the metal material (metal material 1 and metal material 2) be located in a position that is not immersed in the electrolyte, or that it be covered with resin or the like.

[0034] As described above, an anodic reaction occurs at the immersion surface s2 of the metal material 2. Therefore, if a strong oxide film is present on the surface of the metal material 2, a stable anodic reaction may not occur. In such cases, it is preferable to remove the oxide film on the surface of the metal material 2 mechanically or electrically.

[0035] <Metal materials> The shapes of metal material 1 and metal material 2 are not particularly limited, and as described above, they may form a stress-loading portion.

[0036] As metal material 1, we use a metal material (for example, steel) whose delayed fracture characteristics we want to evaluate. As mentioned above, delayed fracture of steel materials is more likely to occur as their strength increases; therefore, it is preferable to use a high-strength steel plate with a tensile strength of 1180 MPa or higher as the metal material 1.

[0037] The combination of metal material 1 and metal material 2 is not particularly limited, as long as the immersion potential E1 of metal material 1 is higher than the immersion potential E2 of metal material 2. For example, if steel plate is used as metal material 1, then metal material 2 may be a metal material with a lower immersion potential than steel plate, such as zinc; an alloy of zinc and other elements; aluminum; an alloy of aluminum and other elements; etc. However, depending on factors such as the oxidation state of the metal material, the content of other elements in the metal material, and the conditions of the electrolyte (composition, temperature, etc.), it may exhibit a higher immersion potential than steel plates. Therefore, it is preferable to confirm in advance that E1 > E2 for the electrolyte to be used. The immersion potential can be read from a potential-pH diagram, or it can be measured beforehand using a potentiometer or similar device.

[0038] As the metal material, a coated metal material in which a coating material is formed on a base material may be used. A specific example of a coated metal material is a plated steel sheet in which a plating film (coating material) is formed on a base steel sheet (base material).

[0039] The inventors of this invention have diligently studied the corrosion of plated steel sheets. As a result, they found that initially, corrosion of the plating film (e.g., zinc plating film) progresses, followed by the exposure of the underlying steel sheet in some areas, and galvanic corrosion progresses between the plating film and the underlying steel sheet, after which corrosion of the underlying steel sheet progresses; and that in plated steel sheets, the introduction of hydrogen during galvanic corrosion has the greatest impact on the delayed fracture characteristics.

[0040] In other words, we found that simulating hydrogen introduction due to galvanic corrosion between the plating film and the underlying steel sheet by introducing hydrogen into the underlying steel sheet and evaluating the delayed fracture characteristics of the underlying steel sheet is important for accurately evaluating the delayed fracture characteristics of the plated steel sheet.

[0041] Based on this finding, the inventors conceived of using a base material (e.g., a base steel sheet) of a coated metal material (e.g., a plated steel sheet) as metal material 1, and a coating material (e.g., a plating film) of the coated metal material as metal material 2. The following explanation will use plated steel sheets as an example of a coated metal material, but it is not limited to this.

[0042] Specifically, as the metal material 1, it is preferable to use a base steel sheet from which the plating film has been removed from a plated steel sheet. The method for removing the plating film is not particularly limited, and examples include immersing the plated steel sheet in an acid or alkali to dissolve and remove the plating film (chemical treatment), or mechanically removing the plating film by polishing, grinding, etc. (mechanical treatment), depending on the composition of the plating film. Furthermore, the base steel sheet before the plating film is formed may be used as the metal material 1. This eliminates the influence of changes in the surface condition of the base steel sheet caused by the removal of the plating film on the delayed fracture characteristics, allowing for a more accurate evaluation of the delayed fracture characteristics.

[0043] As the metal material 2, it is preferable to use a metal material containing the metal M that constitutes the plating film. Specifically, metal M is, for example, a metal that accounts for 95% or more (preferably 98% or more by mass) of the plating film. In other words, the metal material 2 is not composed of impurities (unavoidable impurities) contained in the plating film. If the metal M constituting the plating film is an alloy, it is preferable to also reflect its composition ratio (alloy ratio) in the metal material 2. The content of metal M in metal material 2 is preferably 96% by mass or more, and more preferably 98% by mass or more.

[0044] As the metal material 2, a test piece may be used in which only the plated surface of a plated steel sheet is exposed (the base steel sheet is covered with resin or the like). In this case, if the plating film is thin, there is a possibility that the plating film may disappear (dissolve) during the hydrogen introduction step, exposing the underlying steel sheet. Therefore, it is preferable to use a plated steel sheet with a plating film of sufficient thickness. Alternatively, multiple test pieces may be prepared, and when the plating film on one test piece disappears, it may be connected to the metal material 1 of another test piece.

[0045] As the metal material 2, a metal plate obtained by melting and solidifying a metal so that it has the same composition ratio as the metal M that constitutes the plating film may be used. If the plating film is a molten-dip galvanized film, a test piece prepared by solidifying the plating bath used to form the plating film may be used as the metal material 2. If the plating film is an electroplated film, a test piece prepared by electrodepositing the metal M constituting the plating film may be used as the metal material 2. [Examples]

[0046] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0047] <Plated steel sheets A, B, and C> As coated metal materials (plated steel sheets) comprising a base steel sheet and a plating film formed on the base steel sheet, plated steel sheets A, B, and C (thickness: 1.6 mm) shown in Table 1 below were prepared.

[0048] [Table 1]

[0049] <Metal materials A-1~C-3> Metal materials A-1 to C-3 were prepared to be used as metal material 1. First, the plated steel sheets A, B, and C described above were used as they were, and designated as metal materials A-1, B-1, and C-1, respectively, as shown in Table 2 below.

[0050] Next, the plated steel sheet was subjected to chemical treatment to remove the plating film, thereby obtaining metal materials A-2, B-2, and C-2 consisting of the base steel sheet, as shown in Table 2 below. As a chemical treatment, the plated steel sheet was immersed in a 5% by mass hydrochloric acid aqueous solution to which an inhibitor (Hibilon, manufactured by Sugimura Chemical Industry Co., Ltd.) was added, in order to dissolve the plating film. After the plating film was removed, the base steel sheet was immersed in a saturated sodium bicarbonate aqueous solution to neutralize any remaining acid components on the surface of the base steel sheet, and then washed with water and dried.

[0051] Furthermore, by mechanically processing the plated steel sheet to remove the plating film, metal materials A-3, B-3, and C-3 consisting of the base steel sheet were obtained, as shown in Table 2 below. For the mechanical treatment, the plating film was removed by surface grinding using a grinding wheel with a grit size of #220.

[0052] [Table 2]

[0053] Metal materials D and E were prepared to be used as metal material 2. For metal material D, a zinc plate (Zn content: 99.9% by mass) was used, and for metal material E, an aluminum plate (Al content: 99.0% by mass) was used.

[0054] <Processing (Formation of stress-loaded area)> Bending or tensile testing specimens were prepared by applying bending or tensile processing to metal material 1 (more specifically, metal materials A-1 to C-3 used as metal material 1) as described below, thereby forming stress-loaded sections.

[0055] Figures 3(a) to 3(c) are perspective views showing the procedure for preparing bending test specimens. First, the metal material 1 was sheared to a size of 35 mm in width and 100 mm in length, and then two holes 11 with a diameter of 10 mm were formed as shown in Figure 3(a). After that, in order to remove the residual stress from shearing, the metal material 1 was ground down until its width was 30 mm. Next, ultrasonic cleaning was performed in toluene for 5 minutes, and then the metal material 1 was bent as shown in Figure 3(b). Specifically, the metal material 1 was placed between the die and the punch with the fracture surface of the shear end face 12 facing the die side and the shear surface of the shear end face 12 facing the punch side, and a 180° bend was performed with a radius of curvature R of 5 mm. As a result, a bending test specimen with a stress-loading section 13 was fabricated, as shown in Figure 3(c). The bending test specimen was restrained using bolts 14 and nuts 15. At this time, the inner spacing of the bending test specimens was adjusted within the range of 10 to 20 mm to vary the stress applied to the stress-loading section 13 within the range of 300 to 1700 MPa. The narrower the inner spacing of the bending test specimens, the higher the stress applied to the stress-loading section 13.

[0056] Figures 4(a) and 4(b) are plan views showing the procedure for preparing tensile test specimens. First, as shown in Figure 4(a), the metal material 1 was sheared to a size of 35 mm in width and 100 mm in length. Then, in order to remove the residual stress from shearing, the metal material 1 was ground down to a width of 30 mm. After that, the central part 21 of the metal material 1 was ground down to a size of 12 mm in length and 5 mm in width, with a radius of curvature R of 12.5 mm. Next, as shown in Figure 4(b), a tensile test specimen was prepared by subjecting the metal material 1 to tensile processing to form a stress-loaded section 22. At this time, the stress applied to the stress-loaded section 22 was varied within the range of 500 to 1400 MPa.

[0057] <Electrolyte> Pure water was used as the solvent for the electrolyte. Sodium chloride (NaCl), hydrochloric acid (HCl), and ammonium thiocyanate (NH4SCN) were used as solutes in the electrolyte. One or two solutes were dissolved in a solvent at the concentrations (unit: mass%) shown in Table 3 below to prepare the electrolyte. The conductivity (unit: mS / cm) of the electrolyte is shown in Table 3 below.

[0058] <Connection and hydrogen introduction> A commercially available vinyl-coated copper wire (conductor) was soldered to metal material 1 and metal material 2, and the two were connected. Next, metal material 1 (more specifically, the stress-loaded part of metal material 1) and metal material 2 were immersed in an electrolyte solution, and hydrogen was introduced into metal material 1.

[0059] Metallic Material 1 and Metallic Material 2 The metal materials 1 and 2 used are shown in Table 3 below. Specifically, when evaluating the delayed fracture characteristics of plated steel sheet A (plating film: Zn), metal materials A-2 to A-3 were used as metal material 1, and metal material D (metal type: Zn) was used as metal material 2. When evaluating the delayed fracture characteristics of plated steel sheet B (plating film: Al), metal materials B-2 to B-3 were used as metal material 1, and metal material E (metal type: Al) was used as metal material 2. When evaluating the delayed fracture characteristics of plated steel sheet C (plating film: Zn), metal materials C-2 to C-3 were used as metal material 1, and metal material D (metal type: Zn) was used as metal material 2.

[0060] However, in Nos. 1-3, metal materials A-1, B-1, and C-1 were used as metal material 1, and metal material 2 was not used. In Nos. 4-6, metal materials A-1, B-1, and C-1 were used as metal material 1, and the same metal materials A-1, B-1, and C-1 as metal material 1 were used as metal material 2. In samples No. 34-36, metal material D and metal material E were used interchangeably.

[0061] For the metal material 1, a bending test specimen or a tensile test specimen with a stress-loaded section was used. When a bending test specimen was used, "bending" was indicated in the "processing" column of Table 3 below; when a tensile test specimen was used, "tensile" was indicated. Furthermore, the size and number of metal material 1 and / or metal material 2 were adjusted so that the area ratio (S2 / S1) shown in Table 3 below was obtained. Furthermore, Table 3 below shows the immersion potential E1 (unit: V vs. SHE) of metal material 1 and the immersion potential E2 (unit: V vs. SHE) of metal material 2.

[0062] <Evaluation of delayed failure characteristics> By introducing hydrogen into metal material 1, cracks were induced in the stress-loaded area of ​​metal material 1, and its delayed fracture characteristics were evaluated. The minimum stress at which cracking occurred among the stress-loaded stresses was defined as the crack initiation limit stress (unit: MPa) and is shown in Table 3 below. If no cracks occurred, a "-" was indicated.

[0063] A higher value for the lower limit stress at which cracking occurs indicates a lower likelihood of delayed fracture. Furthermore, it had already been confirmed through separate tests that plated steel sheets A, B, and C were most prone to delayed fracture, and plated steel sheet A was least prone to delayed fracture. Therefore, if the values ​​of the lower limit stress for crack initiation are A > B > C, it can be determined that the delayed fracture characteristics have been properly evaluated.

[0064] [Table 3] TIFF0007871756000004.tif228125

[0065] <Summary of Evaluation Results> As shown in Table 3 above, for samples No. 7-21 and 25-30, the order of the lower limit stress for crack initiation was A > B > C, indicating that the delayed fracture characteristics were properly evaluated.

[0066] In contrast, in experiments No. 1-3, where metal material 2 was not used, cracks did not occur in metal material 1 in some cases, making it impossible to properly evaluate the delayed fracture characteristics.

[0067] In experiments No. 4-6, where metal materials A-1, B-1, and C-1 were used as metal material 2 (i.e., E1=E2), cracking did not occur in metal material 1 in some cases, making it impossible to properly evaluate the delayed fracture characteristics.

[0068] In samples No. 22-24, where the electrolyte conductivity was 1.2 mS / cm, no cracks occurred in metal material 1, making it impossible to properly evaluate the delayed fracture characteristics.

[0069] In samples No. 31-33, where the area ratio (S2 / S1) was 0.5, cracks did not occur in metal material 1 in some cases, making it impossible to properly evaluate the delayed fracture characteristics.

[0070] In experiments No. 34-36, where metal material D and metal material E were used interchangeably, the order of the lower limit stress for crack initiation was not A>B>C, and the delayed fracture characteristics could not be properly evaluated. [Explanation of symbols]

[0071] 1: Metal material 2: Metal material 3: Conductor 4: Electrolytic cell 5: Electrolyte 6: Hydrogen introduction device 7: Hydrogen introduction device 11: Hole 12: Shear end face 13: Stress Loading Section 14: Bolt 15: Nut 21: Central part 22: Stress Loading Section

Claims

1. A connection step of electrically connecting metal material 1 and metal material 2, The system includes a hydrogen introduction step of immersing at least a portion of the electrically connected metal material 1 and metal material 2 surfaces in an electrolyte and introducing hydrogen into the metal material 1, The immersion potential E1 of the metal material 1 is higher than the immersion potential E2 of the metal material 2. The evaluation step includes evaluating the delayed fracture characteristics of the metal material 1 into which hydrogen has been introduced using a hydrogen introduction method for evaluating delayed fracture characteristics. A method for evaluating the delayed fracture characteristics of a metal material, wherein the ratio S2 / S1, which is the ratio of the total surface area S2 of the metal material 2 in contact with the electrolyte to the total surface area S1 of the metal material 1 in contact with the electrolyte, is 1.0 or more.

2. The metal material 1 is the base material after the coating material has been removed from a coated metal material on which a coating material has been formed, or the base material before the coating material has been formed. The method for evaluating the delayed fracture characteristics of a metal material according to claim 1, wherein the metal material 2 is a metal material containing the metal M that constitutes the coating material.

3. The aforementioned coated metal material is a plated steel sheet on which a plating film is formed on a base steel sheet. The aforementioned base material is the aforementioned base steel plate, The method for evaluating the delayed fracture characteristics of a metallic material according to claim 2, wherein the coating material is the plating film.

4. A method for evaluating the delayed fracture characteristics of a metallic material according to any one of claims 1 to 3, wherein the conductivity of the electrolyte is 2.0 mS / cm or more.

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