METHOD FOR EVALUATING DELAYED FRACTURE PROPERTIES OF METALLIC MATERIALS, METHOD FOR SELECTING METALLIC MATERIALS, AND METHOD FOR MANUFACTURING COMPONENTS

Abstract

Provided is a method for evaluating delayed fracture properties of a metal material, wherein delayed fracture properties caused by hydrogen penetrating into the interior of the metal material due to atmospheric corrosion can be accurately evaluated and delayed fracture properties in actual usage environments can be simulated. In this method for evaluating delayed fracture properties of a metal material, a step comprising the following steps (A) and (B) is performed at least once. Step (A): a step including a chloride deposition step (a1) for depositing chlorides on the metal material in an atmosphere having a relative humidity Ha1 of 80% or higher and a temperature Ta1 of 60°C or lower. Step (B): a step in which a cycle having a predetermined drying step (b1), a predetermined wetting step (b2), a predetermined transition step (b3), and a predetermined transition step (b4) is performed at least once.

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the delayed fracture properties of metallic materials, and particularly to a method for evaluating the delayed fracture properties (presence or absence of delayed fracture and its degree, etc.) of metallic materials used in atmospheric corrosive environments, which are caused by hydrogen that penetrates into the metallic materials as a result of corrosion. [Background technology]

[0002] In recent years, efforts have been made to reduce the thickness of steel sheets by increasing their strength in order to reduce the weight of automotive structural components. However, this increase in the strength of steel sheets has raised new concerns about delayed fracture, which was not an issue in conventional automotive parts.

[0003] Delayed fracture is a phenomenon in which high-strength steel parts undergo sudden brittle fracture after a certain period of time under static load stress, with little apparent plastic deformation. In a broader sense, this term also includes liquid metal contact cracking and stress corrosion cracking (Non-Patent Document 1). The hydrogen embrittlement type delayed fracture, which is caused by hydrogen that penetrates into the steel due to corrosion, is a problem for automotive parts. Three factors are known to cause delayed fracture: material (strength), processing (strain and stress), and hydrogen. Possible causes of hydrogen penetration into metallic materials include penetration from solutions and solvents that come into contact with the metallic material, and hydrogen generated as the metallic material corrodes in the environment in which it is used.

[0004] In the past, extensive research has been conducted on delayed fracture in the fields of thick plates such as line pipes, which are subject to large amounts of hydrogen penetration from solutions and solvents, and in high-strength steel bolts with tensile strengths of 1200 MPa or more (Non-Patent Document 2). In these fields, methods for evaluating delayed fracture properties have been standardized.

[0005] On the one hand, automotive parts are used in the atmospheric environment. Therefore, in order to properly evaluate the stress corrosion cracking characteristics of automotive parts, it is important to establish a method for evaluating the stress corrosion cracking characteristics caused by hydrogen that penetrates into automotive parts due to atmospheric corrosion. Therefore, a method for evaluating stress corrosion cracking characteristics that simulates the atmospheric corrosion environment has been proposed.

[0006] For example, Patent Document 1 discloses a method for evaluating the stress corrosion cracking characteristics of a metal material by performing a step of attaching a component mainly composed of chloride to the metal material, and a drying step and a wetting step of drying and wetting the surface of the metal material by changing the relative humidity of the metal material, taking one cycle, and performing this cycle at least once.

[0007] Further, Patent Document 2 discloses a method for evaluating hydrogen embrittlement characteristics, which has a salt deposition step of depositing a metal salt containing chloride on the surface of a metal material, a wetting step of exposing the metal material in an atmosphere with a relative humidity of Hh, and a drying step of exposing the metal material in an atmosphere with a relative humidity of Hlo (where Hlo < Hh) as a basic cycle, and a basic process including the basic cycle once or more. The salt deposition step, the wetting step, and the drying step are performed in an atmosphere at 40°C or lower, and at least one drying step in the basic process is performed in an atmosphere at 30°C or lower, with a relative humidity of 0% to 60% and for 1 minute or more and 6 hours or less.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0009]

Non-Patent Document 1

[0010] Temperature is one of the parameters in atmospheric corrosion environments that significantly affect the delayed fracture properties of metallic materials.

[0011] The evaluation method described in Patent Document 1 is an evaluation method in which the temperature in the test process is constant, and it is possible to evaluate the temperature range that affects the delayed fracture properties. However, when the delayed fracture properties of high-strength steel sheets were evaluated using the evaluation method described in Patent Document 1, it was found that there were cases in which the delayed fracture properties in the actual usage environment (real-life environment) could not be reproduced.

[0012] Furthermore, the evaluation method described in Patent Document 2 is not an evaluation method in which the temperature in the test process is constant. Therefore, there is a problem in that the influence of temperature on the delayed fracture properties of metallic materials cannot be accurately evaluated. Furthermore, it is not intended to evaluate delayed fracture properties in a high-temperature environment exceeding 40°C.

[0013] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for evaluating the delayed fracture properties of metallic materials, which can accurately evaluate the delayed fracture properties of metallic materials used in atmospheric corrosive environments, which are caused by hydrogen that penetrates into the interior of the metallic materials due to atmospheric corrosion, and can simulate the delayed fracture properties in actual use environments. [Means for solving the problem]

[0014] As a result of extensive research, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention.

[0015] [1] A method for evaluating the delayed fracture properties of a metallic material, comprising performing a process comprising the following steps (A) and (B) at least once: Step (A): A step including a chloride adhering step (a1) of adhering a chloride to a metal material in an atmosphere of a relative humidity Ha1 of 80% or more and a temperature Ta1 of 60°C or less; Step (B): A step of performing a cycle at least once in an atmosphere at a temperature Tb1 of 60°C or less and within a certain range, the cycle comprising the following drying step (b1), the following wetting step (b2), the following transition step (b3), and the following transition step (b4). Drying step (b1): A step of drying a metal material by maintaining it in an atmosphere of a relative humidity Hb1 of 45% or less for 1.0 hour or more and 5.0 hours or less; Wetting step (b2): A step of wetting the metal material by holding it in an atmosphere of a relative humidity Hb2 of 80% or more for 1.0 hour or more and 5.0 hours or less; transition step (b3): ​​a step of transitioning from an atmosphere of the relative humidity Hb1 to an atmosphere of the relative humidity Hb2 for 1.0 hour or more and 5.0 hours or less; Transition step (b4): A step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 for 1.0 hour or more and 5.0 hours or less. [2] The method for evaluating the delayed fracture properties of a metallic material according to [1], wherein the step (A) further includes, after the chloride adhesion step (a1), a holding step (a2) of holding the sample in an atmosphere having the relative humidity Ha1 and the temperature Ta1. [3] The method for evaluating the delayed fracture properties of a metallic material according to [2], wherein the holding time of the holding step (a2) is set so that the process time of the step (A) is equal to the process time of the wetting step (b2). [4] The method for evaluating delayed fracture properties of a metallic material according to any one of [1] to [3], wherein the metallic material has a coating layer on the surface. [5] An evaluation step of evaluating the delayed fracture properties of a metallic material using the evaluation method for the delayed fracture properties of a metallic material according to any one of [1] to [4] above; a selection step of selecting a metal material based on the evaluation results obtained in the evaluation step. [6] A method for manufacturing a component, comprising processing a metal material selected by the method for selecting a metal material according to [5] above to manufacture the component. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a method for evaluating the delayed fracture properties of metallic materials, which can accurately evaluate the delayed fracture properties of metallic materials used in atmospheric corrosive environments, which are caused by hydrogen that penetrates into the interior of the metallic material due to atmospheric corrosion, and can simulate the delayed fracture properties in actual usage environments.

[0017] Furthermore, according to the method for evaluating the delayed fracture properties of a metallic material of the present invention, it is possible to obtain information necessary for determining whether or not delayed fracture will occur in a metallic material in an actual use environment, such as the fracture threshold stress at which delayed fracture will occur according to the actual use environment. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram illustrating one embodiment of a corrosion test cycle according to the method of evaluating delayed fracture properties of a metallic material of the present invention. [Figure 2] FIG. 2 is a diagram schematically showing a test piece for evaluating delayed fracture properties used in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0019] First, the findings of the present inventors will be described. Patent Document 1 describes a method for evaluating delayed fracture properties of metallic materials used in atmospheric corrosive environments, which are caused by hydrogen penetrating into the interior of the metallic material due to atmospheric corrosion. Therefore, the present inventors evaluated delayed fracture properties under the following corrosion test cycle conditions according to the method described in Patent Document 1, by applying stresses equivalent to YS (yield stress) to commercial 1470 MPa-class cold-rolled steel sheets (sheet thickness: 1.4 mm) at stresses of 1000 MPa, 800 MPa, 600 MPa, and 400 MPa.

[0020] Specifically, commercial 1470 MPa-grade cold-rolled steel sheets (thickness: 1.4 mm) were sheared to a width of 35 mm x length of 100 mm and ground to a width of 30 mm to remove residual stress from shearing. The resulting strip specimens were then immersed in toluene and ultrasonically cleaned for 5 minutes, then bent 180° with a radius of curvature of 4 mm. The specimen shape was then fixed using bolts and nuts. To vary the applied stress, we also prepared strip specimens with inner spacings of 8, 10, 12, 14, and 17 mm. Tests were conducted for up to 63 days, and the specimens were observed daily for cracking. The same specimens were also subjected to a one-year exposure test in Okinawa to evaluate delayed fracture properties. The results are shown in Table 1.

[0021] (Corrosion test cycle conditions) [Process of attaching chlorides to metal materials] Atmospheric atmosphere Chloride species: NaCl Adhesion amount: 10g / m 2 Application method: spray method [A process in which a drying process and a wetting process are considered as one cycle and this cycle is carried out one or more times] Drying process conditions: Temperature 30°C, humidity 30% RH, holding time 2 hours - Transition time from the drying process to the wetting process is 2 hours (excluding the holding time of the humidity holding process below), humidity change rate is 30% RH / hour Wetting process conditions: Temperature 30℃, humidity 90%RH, holding time 2 hours Transition time from the wetting process to the drying process is 2 hours (excluding the holding time of the humidity holding process below), humidity change rate is 30% RH / hour Humidity holding process conditions: 55% RH, holding time 2 hours Cycle order: Drying process → Humidity maintenance process → Wetting process → Humidity maintenance process → Drying process

[0022] [Table 1]

[0023] As can be seen from Table 1, when the method described in Patent Document 1 is used, cracks sometimes occur in the test specimens under conditions of low applied stress (applied stress of 600 MPa), but sometimes do not occur, resulting in variations in the evaluation results of delayed fracture properties. In other words, the method described in Patent Document 1 may not be able to reproduce delayed fracture properties in actual environments. The present inventors conducted a detailed investigation into the cause of this. As a result, they found that when using the method described in Patent Document 1, cracks may occur in the metal material during the step of adhering a component mainly composed of chlorides to the metal material (chloride adhering step). The mechanism by which cracks occur in the metal material during the chloride adhering step is not clearly understood, but the present inventors speculate as follows.

[0024] In atmospheric corrosion environments (real-world environments), chlorides adhere to metal materials due to the adhesion and accumulation of snow-melting salt and airborne salt, and chlorides absorb moisture, forming a water film on the metal material, which accelerates corrosion (atmospheric corrosion). As corrosion progresses, hydrogen generated by corrosion penetrates the interior of the metal material, causing cracks. However, it is difficult to reproduce the chloride adhesion pattern in real-world environments in experiments. In experiments, to adhere an amount of chloride to metal materials equivalent to that in real-world environments, a solution with an adjusted chloride concentration (salt water) is applied to the metal material surface by spraying or atomizing, or the metal material is immersed in salt water, and salt water is applied to achieve the desired amount of chloride adhesion. Furthermore, the application of salt water described above is typically performed in the air.

[0025] However, when the saltwater application method described above is used, the relative humidity at least near the metal surface changes from the relative humidity of the ambient atmosphere to nearly 100%. Therefore, if chlorides are attached to the metal surface, the chlorides absorb moisture, forming a water film containing dissolved chlorides, and the saltwater also forms a thick film of saltwater. Once the saltwater application is complete, the relative humidity near the metal surface changes to the relative humidity of the ambient atmosphere, and the water film begins to dry. The thickness of the water film decreases, and the water film becomes concentrated with chlorides. Furthermore, when the relative humidity near the metal surface falls below the relative humidity at which chlorides deliquesce, the chlorides precipitate, and the water film almost disappears.

[0026] At relative humidities near the deliquescence point of chlorides, chloride ions and metal ions eluted by corrosion increase in the water film. Furthermore, as the relative humidity changes from wet to dry, the pH of the water film decreases with the change in water film thickness, hydrogen atomizes on the surface of the metal material, and the amount of hydrogen penetrating into the metal increases. Therefore, repeating the chloride deposition process in an air atmosphere increases the amount of hydrogen inside the metal material. As a result, the inventors speculate that even under conditions where cracking would not occur in an actual environment, the amount of hydrogen penetrating into the metal material exceeds the fracture limit, resulting in cracking of the metal material. Therefore, the inventors believed that reducing the amount of hydrogen penetrating into the metal material during the chloride deposition process was important to improve the accuracy of the delayed fracture property evaluation method, and after extensive investigations, they completed the present invention. The present invention was made based on the above findings.

[0027] The method for evaluating the delayed fracture properties of a metallic material of the present invention (hereinafter also simply referred to as the evaluation method of the present invention) involves carrying out a process comprising steps (A) and (B) at least once.

[0028] In the evaluation method of the present invention, stress is preferably applied to a metallic material. Examples of methods for applying stress to the metallic material include methods of processing the metallic material (processing methods). Examples of such processing methods include bending, bulging, tensioning, and twisting. Other examples include methods of fixing the metallic material in a stressed shape using bolts or the like, and methods that utilize residual stress remaining after processing. In the evaluation method of the present invention, a process comprising steps (A) and (B) can be performed at least once (once or twice or more) on the metallic material to which stress has been applied as described above. Furthermore, in the evaluation method of the present invention, after performing the process comprising steps (A) and (B) at least once, the state of the metallic material is confirmed, 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 metallic material for the presence or absence of cracks and the extent of the cracks.

[0029] The evaluation method of the present invention will be described below with reference to exemplary embodiments, although the present invention is not limited to the following exemplary embodiments.

[0030] (metallic material) First, a description will be given of a metallic material to be subjected to the method for evaluating the delayed fracture properties of a metallic material according to this embodiment (hereinafter simply referred to as the evaluation method according to this embodiment). Examples of the metallic material include, but are not limited to, steel materials, and metallic materials such as Ti and Al may also be used. The metallic material may also have a coating layer on its surface. Examples of the coating layer include an organic layer, an inorganic layer, and a mixture layer of an organic material and an inorganic material. The coating layer may also be a single layer or multiple layers. Examples of metallic materials having a coating layer on their surface include metallic materials in which a chemical conversion treatment such as zinc phosphate treatment is performed on the surface of the metallic material to form a chemical conversion treatment layer, and then a coating layer is formed thereon by electro-deposition coating or the like.

[0031] The form of the metal material is not particularly limited and may be, for example, a plate, a bar, or a pipe. However, since the evaluation method of this embodiment is suitable for evaluating plate or pipe materials with a relatively thin thickness, it is preferable to evaluate plate or pipe materials.

[0032] In the evaluation method of this embodiment, stress is applied to the metal material. Examples of the method for applying stress to the metal material include the methods described above.

[0033] In this embodiment, in order to evaluate the delayed fracture properties of a metallic material, steps (A) and (B) are performed while applying stress to the metallic material. After performing each of these steps one or more times, the presence or absence and degree of cracking in the metallic material are confirmed, thereby evaluating the delayed fracture properties.

[0034] Each step will be described below.

[0035] (Process (A)) The step (A) includes a chloride adhering step (a1) of adhering a chloride to a metal material in an atmosphere having a relative humidity Ha1 of 80% or more and a temperature Ta1 of 60°C or less.

[0036] [Chloride deposition step (a1)] In the chloride deposition step (a1), chloride is deposited on a metal material to obtain a desired amount of chloride. The chloride preferably includes one or more chlorides selected from sodium salt (NaCl), potassium salt (KCl), calcium salt (CaCl), and magnesium salt (MgCl), which are present in the atmospheric environment in which typical metal materials are used. In the chloride deposition step (a1), a chloride-based component containing chloride and other components may be deposited on the metal material. Here, a chloride-based component refers to a component in which chloride accounts for more than 50 mass% of the total components in terms of solid content. Examples of components other than chloride include, but are not limited to, sulfides and nitrate compounds. Considering actual atmospheric corrosion environments, it is preferable to deposit a NaCl-based component (a component in which NaCl accounts for more than 50 mass% of the total components) on the metal material.

[0037] Furthermore, when simulating the delayed fracture characteristics in an area where snow-melting agents are frequently sprayed in winter, it is preferable to adhere to the metal material a component having a composition similar to that of the snow-melting agent sprayed in that area. Examples of components having a composition similar to that of snow-melting agents include a component mainly composed of CaCl2 (a component in which CaCl2 is more than 50 mass% of the total components), a component mainly composed of MgCl2 (a component in which MgCl2 is more than 50 mass% of the total components), and a component mainly composed of NaCl (a component in which NaCl is more than 50 mass% of the total components).

[0038] Alternatively, a component containing a combination of multiple metal salts may be attached to the metal material. Examples of the component containing a combination of multiple metal salts include the Society of Automotive Engineers standard (SAE J2334) (0.5% by mass NaCl-0.1% by mass CaCl-0.075% by mass NaHCO), artificial seawater (2.5% by mass NaCl-0.5% by mass MgCl-0.12% by mass CaCl-0.07% by mass KCl, and others (e.g., an aqueous solution of Aquamarine (registered trademark) manufactured by Yashima Pharmaceutical Co., Ltd.)).

[0039] The amount of chloride attached to the metal material (amount of chloride solids not including solvents such as water) is 1 to 100,000 mg / m 2 The deposition amount corresponds to the deposition amount of chlorides assumed to occur in an actual atmospheric corrosion environment. 2 In a corrosive environment below 1 mg / m, corrosion hardly progresses. Therefore, although the delayed fracture properties of metallic materials can be evaluated, the corrosion rate is slow and the evaluation takes time. Furthermore, the amount of hydrogen generated by corrosion is small, so delayed fracture is unlikely to occur, and in many cases it is not necessary to evaluate delayed fracture properties in the first place. Therefore, the coating weight is set to 1 mg / m 2 On the other hand, the amount of adhesion is preferably 100,000 mg / m or more. 2 If the coating amount exceeds 100,000 mg / m, it is highly likely that the actual corrosive environment cannot be simulated. 2 From the viewpoint of simulating the corrosion mode in an actual atmospheric corrosion environment and promoting corrosion, the coating amount is preferably 100 to 30,000 mg / m 2 is more preferred.

[0040] The method for attaching chloride to a metal material (chloride attachment method) is not particularly limited. A typical chloride attachment method involves attaching a solution containing chloride as a solute to the metal material. Specific examples include immersing the metal material in a solution containing a chloride-based component (usually an aqueous solution such as salt water, hereinafter also referred to as salt water), or spraying salt water onto the metal material.

[0041] The chloride concentration in the saltwater is not particularly limited. However, when chlorides are deposited on a metal material using saltwater with a chloride concentration of less than 0.1% by mass, it takes a long time to obtain a suitable chloride deposition amount, or it is difficult to obtain a suitable chloride deposition amount. Therefore, the chloride concentration in the saltwater is preferably 0.1% by mass or more, and more preferably 1% by mass or more. On the other hand, when chlorides are deposited on a metal material by atomizing or spraying using saltwater with a chloride concentration of 20% by mass or more (when applying saltwater), chlorides tend to precipitate in the equipment used to atomize or spray saltwater, causing clogging and making it difficult to apply saltwater with a stable concentration. In addition, it is difficult to uniformly deposit chlorides on the metal material. As a result, the amount of chloride deposition on the metal material varies depending on the location, and in areas with a high chloride deposition amount, localized corrosion occurs and the amount of hydrogen penetration increases. This results in changes in the delayed fracture properties within the evaluation region of the metal material, reducing the accuracy of the evaluation of delayed fracture properties. This tendency is particularly pronounced when the chloride deposition amount is high. Therefore, the chloride concentration in the salt water is preferably 20% by mass or less, and more preferably 15% by mass or less.

[0042] The time for applying salt water to the metal material to adhere the chloride, i.e., the process time for the chloride adhering step (a1), is not particularly limited. The process time for the chloride adhering step (a1) is preferably 0.2 hours or more. The process time for the chloride adhering step (a1) is preferably 1.0 hour or less.

[0043] If the processing time for the chloride deposition step (a1) is less than 0.2 hours, the distribution of the chloride deposition amount on the metal material surface may become non-uniform, which may result in changes in delayed fracture properties within the evaluation region of the metal material surface. Therefore, the processing time for the chloride deposition step (a1) is preferably 0.2 hours or more. The processing time for the chloride deposition step (a1) is more preferably 0.3 hours or more. On the other hand, if the processing time for the chloride deposition step (a1) exceeds 1.0 hours, corrosion behavior may differ from that observed in a real environment. Specifically, when saltwater is supplied to the metal material surface, corrosion products formed on the metal material surface may be washed away with the saltwater. For example, in the case of a galvanized steel sheet, the corrosion products have the effect of suppressing hydrogen penetration into the substrate steel sheet. Therefore, if the corrosion products are washed away with the saltwater, the effects of the corrosion products in a real environment may not be achieved. Furthermore, there is a risk of saltwater corrosion progressing. This may result in a low correlation with corrosion in a real environment, leading to a risk of inconsistency in delayed fracture properties observed in a real environment. Therefore, the time for the chloride attachment step (a1) is preferably 1.0 hour or less, and more preferably 0.7 hour or less.

[0044] The amount of chloride adhesion can be calculated by multiplying the difference in mass of the test piece (metal material) before and after chloride adhesion in the chloride adhesion step (a1) by the chloride concentration in the salt water and dividing the result by the area of ​​the test piece. The amount of chloride adhesion can be controlled by, for example, changing the chloride concentration of the salt water or by changing the time for applying salt water (the process time of the chloride adhesion step (a1)) to change the amount of salt water applied to the metal material.

[0045] When salt water is applied to the surface of a metal material by spraying, the inclination angle of the metal material surface may be, for example, 0° with respect to the horizontal plane. In this case, a water film of the salt water accumulates on the surface of the metal material, so the surface may be inclined with respect to the horizontal plane. The inclination angle of the metal material surface can be similarly set in processes other than the chloride adhesion step (a1). That is, the inclination angle of the metal material surface may be 0° with respect to the horizontal plane or may be inclined in processes other than the chloride adhesion step (a1). By inclining the inclination angle of the metal material with respect to the horizontal plane in processes other than the chloride adhesion step (a1), chlorides are washed away, making it easier to adhere new chlorides in the chloride adhesion step (a1).

[0046] <Relative humidity Ha1 in chloride adhesion process (a1): 80% or more> In the chloride adhesion step (a1), chloride is adhered to the metal material in an atmosphere with a relative humidity Ha1 of 80% or higher. When chloride is adhered to the metal material by atomizing or spraying in the chloride adhesion step (a1), the relative humidity of the environment near the metal material surface rises to nearly 100%. The same is true when the metal material is immersed in salt water. For example, if chloride is adhered to the metal material in an air atmosphere, the relative humidity of the environment after chloride adhesion is completed falls below 80%, forming a water film containing concentrated chloride on the metal material surface, increasing the amount of hydrogen penetrating into the metal material during the chloride adhesion step. In the evaluation method of the present invention, by adhering chloride to the metal material at a relative humidity Ha1 of 80% or higher during the chloride adhesion step (a1), the relative humidity of the environment after chloride adhesion is completed does not fall below 80%. Therefore, the relative humidity does not approach the relative humidity at which chloride deliquescence occurs, and a water film containing concentrated chloride is not formed. This prevents the amount of hydrogen penetrating into the metal material during the chloride adhesion step (a1), thereby suppressing the amount of hydrogen penetrating into the metal material. The upper limit of the relative humidity Ha1 in the chloride adhering step (a1) is not particularly limited. For example, the relative humidity Ha1 can be less than 98%, or may be 95% or less, or may be 90% or less.

[0047] <Temperature Ta1 in chloride adhesion step (a1): 60°C or less> In the chloride adhesion step (a1), chloride is adhered to the metal material in an atmosphere at a temperature Ta1 of 60°C or less. If the temperature Ta1 in the chloride adhesion step (a1) exceeds 60°C, not only will the evaluation be performed in an environment far removed from the corrosive environment in which the metal material will actually be used, but the corrosion mechanism may also be altered. Therefore, the temperature Ta1 in the chloride adhesion step (a1) is set to 60°C or less, preferably 50°C or less. On the other hand, there is no particular lower limit for the temperature Ta1 in the chloride adhesion step (a1). If the temperature Ta1 in the chloride adhesion step (a1) is set to less than 5°C, it may be difficult to control the relative humidity in the corrosion test chamber (constant temperature and humidity chamber) used in the corrosion test. Furthermore, the corrosion rate of the metal material will be significantly reduced, resulting in a longer evaluation time. Therefore, the temperature Ta1 in the chloride adhesion step (a1) is preferably set to 5°C or more, and more preferably 10°C or more.

[0048] [Holding process (a2)] Step (A) preferably includes a holding step (a2) of holding the metal material in an atmosphere at the relative humidity Ha1 and the temperature Ta1 after the chloride-adhering step (a1) of adhering chloride to the metal material. By holding the metal material at the relative humidity Ha1 after the chloride-adhering step (a1), the thickness of the water film formed by the application of salt water changes to a thickness resulting from the absorption of chloride moisture corresponding to the relative humidity Ha1. This is expected to result in the formation of a water film with a uniform chloride concentration on the metal material surface, thereby eliminating unevenness in the amount of chloride adhering to the metal material surface. Therefore, it is preferable to include a holding step (a2) of holding the metal material in an atmosphere at the relative humidity Ha1 and the temperature Ta1 after the chloride-adhering step (a1). The holding time for the holding step (a2), i.e., the holding time at the relative humidity Ha1, is preferably 0.5 hours or more. The upper limit of the holding time is not particularly limited. The holding time of the holding step (a2) is preferably set so that the process time of step (A) (here, the total time of the chloride adhesion step (a1) and the holding step (a2)) is equal to the process time of the wetting step (b2) of step (B), which will be described later. By setting the process time of step (A) in this manner, it is possible to construct a cycle of step (B) in which step (A) is placed in place of the wetting step (b2), as will be described later, making it easier to create a regular corrosion test cycle. Furthermore, by setting the relative humidity Ha1 to the same relative humidity as the relative humidity Hb2 of the wetting step (b2), which will be described later, it is possible to further suppress the amount of hydrogen that penetrates into the metal material due to the chloride adhesion step (a1). Therefore, it is preferable to set the relative humidity Ha1 to the same relative humidity as the relative humidity Hb2 of the wetting step (b2), which will be described later.

[0049] Furthermore, the amount of hydrogen that penetrates into a metallic material due to corrosion in an atmospheric corrosive environment varies significantly depending on the temperature of the environment (atmosphere). Therefore, the delayed fracture properties are also strongly affected by the temperature of the environment. Therefore, to properly evaluate the delayed fracture properties of a metallic material, taking into account the application location and the environment of the metallic material component (a component made of a metallic material), it is preferable to maintain a constant atmospheric temperature in step (A) (here, the chloride deposition step (a1) or the chloride deposition step (a1) and the holding step (a2)) and step (B), which will be described later. However, the atmospheric temperature may change in the chloride deposition step (a1). This is thought to be due to the influence of the added moisture, and strict control of the atmospheric temperature may be difficult. However, if the atmospheric temperature Ta1 in step (A) can be controlled to within ±10°C of the atmospheric temperature Tb1 in step (B), the effect on the delayed fracture properties can be significantly reduced. Therefore, it is preferable to control the atmospheric temperature Ta1 in step (A) to within ±10°C of the atmospheric temperature Tb1 in step (B).

[0050] A characteristic of atmospheric corrosion environments is the repeated alternation of wet (moist) and dry (dry) states, and simulating this environmental change is important in approximating the corrosion pattern in real environments. For example, in the case of steel materials, it is known that the corrosion products formed on the steel material change depending on the wet and dry states, and hydrogen is generated during the process of changing from wet to dry, or from dry to wet. Therefore, the conditions in the cycle of relative humidity change (process (B)) are also important in evaluating delayed fracture properties.

[0051] (Process (B)) 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 lower and within a certain range, the cycle comprising the following drying step (b1), the following wetting step (b2), the following transition step (b3), and the following transition step (b4).

[0052] <Temperature Tb1 of step (B): 60°C or less and within a certain range> Step (B) is performed in an atmosphere at a temperature Tb1 of 60°C or less and within a certain range. If the temperature Tb1 in step (B) exceeds 60°C, not only will the evaluation be performed in an environment far removed from the corrosive environment in which the metal material will actually be used, but the corrosion mechanism may also change. Therefore, the temperature Tb1 in step (B) is set to 60°C or less, preferably 50°C or less. On the other hand, there is no particular lower limit for the temperature Tb1 in step (B). If the temperature Tb1 in step (B) is less than 5°C, it may be difficult to control the relative humidity in the corrosion test chamber (constant temperature and humidity chamber) used in the corrosion test. Furthermore, the corrosion rate of the metal material will be significantly reduced, resulting in a longer evaluation time. Therefore, the temperature Tb1 in step (B) is preferably set to 5°C or more, and more preferably 10°C or more.

[0053] Furthermore, as mentioned above, delayed fracture properties are strongly affected by the temperature of the environment (atmosphere). Therefore, in order to properly evaluate the delayed fracture properties of metallic materials, taking into account the application location and the environment in which the metallic material is used, it is necessary to keep the temperature Tb1 in step (B) within a certain range. When the fluctuation range of the temperature Tb1 in step (B) is within ±5°C, the amount of hydrogen that penetrates into the metallic material from the environment (hydrogen penetration amount) 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 fluctuation range of the temperature Tb1 in step (B) is within ±2°C, the fluctuation range of the hydrogen penetration amount is within 15%. Therefore, the fluctuation range of the temperature Tb1 in step (B) is preferably within ±5°C, and more preferably within ±2°C.

[0054] [Drying process (b1)] The drying step (b1) is a step of drying the metal material in an atmosphere with a relative humidity Hb1 of 45% or less for 1.0 to 5.0 hours. The relative humidity Hb1 in the drying step (b1) is set to 45% or less. This is to simulate the dry state, which is one of the characteristics of an atmospheric corrosion environment. Furthermore, if the relative humidity Hb1 in the drying step (b1) exceeds 45%, a long period of time is required to sufficiently dry the metal material surface, which increases the evaluation time. The relative humidity Hb1 in the drying step (b1) is preferably 40% or less. On the other hand, there is no particular lower limit for the relative humidity Hb1 in the drying step (b1). From the viewpoint of relative humidity controllability, the relative humidity Hb1 in the drying step (b1) is preferably 20% or more. Furthermore, if the components to be attached to the metal material surface contain substances that exhibit deliquescent properties at lower relative humidities, such as magnesium chloride or calcium chloride, it is preferable to set the relative humidity Hb1 in the drying step (b1) low.

[0055] The processing time for the drying process (b1) (the time for which the specimen is maintained in an atmosphere of relative humidity Hb1) should be between 1.0 and 5.0 hours. If the processing time for the drying process (b1) is less than 1.0 hour, it is not possible to simulate an actual corrosive environment. On the other hand, if the processing time for the drying process (b1) is more than 5.0 hours, it is possible to simulate an actual corrosive environment, but it takes a long time to evaluate the delayed fracture properties.

[0056] [Wetting process (b2)] The wetting step (b2) is a process in which the metal material is wetted in an atmosphere with a relative humidity Hb2 of 80% or higher for 1.0 to 5.0 hours. The relative humidity Hb2 in the wetting step (b2) is set to 80% or higher. This is to simulate the wet state, which is one of the characteristics of an atmospheric corrosion environment. If the relative humidity Hb2 in the wetting step (b2) is less than 80%, the effect of wetting will be insufficient, making it impossible to simulate an actual corrosion environment. Among chlorides, sodium chloride has the highest saturation critical vapor pressure, which is approximately 75 to 78% in relative humidity terms. Therefore, for any chloride, if the relative humidity is kept at 80% or higher, a water film will form on the metal material surface due to moisture absorption by the chloride, thereby maintaining a wet state. Therefore, the relative humidity Hb2 in the wetting step (b2) is set to 80% or higher. On the other hand, although there is no particular upper limit for the relative humidity Hb2 in the wetting step (b2), it is preferable that the relative humidity Hb2 in the wetting step (b2) be less than 98%. This is because when the relative humidity Hb2 is 98% or higher, the water film formed by condensation becomes too thick, making it easier for the attached chlorides to be washed away. This phenomenon is particularly likely to occur when evaluating processed test specimens. Therefore, when evaluating processed test specimens, it is preferable that the relative humidity Hb2 in the wetting step (b2) be less than 98%.

[0057] The process time for the wetting step (b2) (the time for maintaining the specimen in an atmosphere with a relative humidity Hb2 of 80% or higher) is 1.0 hour or more and 5.0 hours or less. If the process time for the wetting step (b2) is less than 1.0 hour, it is not possible to simulate an actual corrosive environment. On the other hand, if the process time for the wetting step (b2) is more than 5.0 hours, it is possible to simulate an actual corrosive environment, but it takes a long time to evaluate the delayed fracture properties.

[0058] [Transition process (b3), transition process (b4)] The transition step (b3) is a step of transitioning from an atmosphere with the relative humidity Hb1 to an atmosphere with the relative humidity Hb2, and the transition step (b4) is a step of transitioning from an atmosphere with the relative humidity Hb2 to an atmosphere with the relative humidity Hb1. It is known that the amount of hydrogen penetration into metal materials, particularly steel materials, increases when the relative humidity changes. That is, a large amount of hydrogen penetrates into the steel material during the transition steps (b3) and (b4). For example, consider the transition step (b4) in which an atmosphere with the relative humidity Hb2 in the wetting step (b2) is transitioned to an atmosphere with the relative humidity Hb1 in the drying step (b1). In the wetting step (b2), chlorides deliquesce, forming a water film with a thickness corresponding to the relative humidity of the environment, and corrosion of the metal material progresses. On the other hand, in the drying step (b1), most chlorides do not deliquesce, so the non-deliquescent chlorides precipitate, the water film almost disappears, and corrosion hardly progresses. When such a change in water film thickness occurs, chloride ions and metal ions eluted by corrosion become concentrated in the water film near the relative humidity at which chlorides deliquesce, and a hydrolysis reaction occurs in the concentrated chloride water film. This causes a decrease in the pH of the water film, hydrogen atomization on the metal material surface, and an increase in the amount of hydrogen penetrating into the metal material. Meanwhile, because transition step (b3) is the reverse process of transition step (b4), a concentrated chloride water film is formed near the relative humidity at which chlorides deliquesce, but the amount of metal ions eluted by corrosion is relatively smaller than in transition step (b4). Therefore, the amount of hydrogen penetrating in transition step (b3) is less than in transition step (b4). To ensure sufficient reaction time for the hydrolysis and the amount of hydrogen penetrating, the process times for transition steps (b3) and (b4) must each be 1.0 hour or longer. Preferably, each is 1.5 hours or longer. Meanwhile, the process times for transition steps (b3) and (b4) must each be 5.0 hours or shorter. If the time for each of the above steps exceeds 5.0 hours, the concentration rate of chlorides slows down and the progress of the hydrolysis reaction becomes slow, so that evaluation of delayed fracture properties takes too long and is not an appropriate evaluation method.

[0059] Furthermore, when the chloride contains NaCl, the amount of hydrogen penetration into the metal material is greatest in a relative humidity range of 55% to 75%, which is near the deliquescence humidity of NaCl, but corrosion of the metal material is hardly caused in this range. Therefore, a holding step (holding step at a relative humidity of 55% to 75%) may be provided during either or both of the transition steps (b3) and (b4), in which the material is held for a predetermined time in a relative humidity range of 55% to 75%. By providing this holding step, the amount of hydrogen penetration into the metal material can be increased. The process time for this holding step is preferably 0.5 hours or more. Note that the process time for the transition steps (b3) and (b4) also includes the process time for the holding step at a relative humidity of 55% to 75%.

[0060] The purpose of the method for evaluating the delayed fracture properties of metallic materials of the present invention is to simulate the daytime and nighttime changes in relative humidity in an actual environment. Therefore, if the process time (time for one cycle) of step (B), which simulates the daytime and nighttime changes in relative humidity in an actual environment, exceeds 24 hours, this means that corrosion will be slower than in an actual environment, and the evaluation of delayed fracture properties will require a long time. In other words, the process time of step (B) is preferably set to 24 hours or less. To expedite the evaluation, the process time of step (B) is more preferably set to 12 hours or less. On the other hand, if the process time of step (B) is shortened, the relative humidity will change rapidly, which will reduce the correlation with corrosion in an actual environment and may result in a discrepancy with the delayed fracture properties in an actual environment. Therefore, the process time of step (B) is preferably set to 5 hours or more.

[0061] In the evaluation method of the present invention, the steps (A) and (B) are each performed at least once. For the purpose of simulating corrosion patterns in an actual environment and for the convenience of setting up a corrosion test cycle, the step (A) is preferably set as a cycle in which the wetting step (b2) in the step (B) is replaced with the step (A), as described below. The step (A) may be performed every random number of cycles of the step (B) or every predetermined number of cycles of the step (B). The upper limit of the number of times the step comprising the steps (A) and (B) is performed is not particularly limited. For example, the step comprising the steps (A) and (B) may be performed until cracks appear in the metal material. Alternatively, the number of test days may be determined in advance, and the step may be performed for a number of test days corresponding to the number of test days. The number of times the step is performed can be appropriately set, taking into account, for example, the need to simulate corrosion patterns in an actual environment. For example, the step may be performed 1,500 times or less. The number of times the process is repeated can be set depending on the quality and type of the metallic material to be evaluated. For example, when evaluating the delayed fracture properties of a metallic material having a coating layer on its surface, the upper limit of the number of times the process is repeated can be set to 1,500, and when evaluating the delayed fracture properties of a metallic material having no coating layer on its surface, the upper limit of the number of times the process is repeated can be set to 420.

[0062] Next, a process including steps (A) and (B) will be described. FIG. 1 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. 1 shows an example of a corrosion test cycle in which steps (A) and (B) are each performed once. In this example, step (A) includes a chloride adhesion step (a1) and a holding step (a2). Step (B) includes a wetting step (b2), a transition step (b4), a drying step (b1), and a transition step (b3) as one cycle.

[0063] In addition, in the corrosion test cycle shown in FIG. 1, after performing step (A), a cycle including a step of transitioning from the atmosphere of step (A) to an atmosphere of relative humidity Hb1 (transition step (c1)), a step of maintaining the atmosphere at relative humidity Hb1 (maintenance step (c2)), and a step of transitioning from the atmosphere of relative humidity Hb1 to an atmosphere of relative humidity Hb2 (transition step (c3)) is performed (step (B) * ) is provided.

[0064] In this embodiment, step (B) * is set as a cycle in which the step (A) is arranged instead of the wetting step (b2) in the cycle of the step (B). That is, the step (B) * The process time of step (A) in the present invention (the sum of the process times of the chloride attachment step (a1) and the holding step (a2)) is set equal to the process time of the wetting step (b2) in step (B). * The transition step (c1), the holding step (c2), and the transition step (c3) of step (B) are set under the same conditions as the transition step (b4), the drying step (b1), and the transition step (b3) of step (B), respectively. As a result, in the corrosion test cycle shown in FIG. * The process time of step (A) and the process time of step (B) are set to be equal. In this way, by setting the cycle in which step (A) is placed in place of the wetting process (b2) in the cycle of step (B), it becomes easier to create a regular corrosion test cycle. Furthermore, it is possible to create a corrosion test cycle that simulates the hydrogen that penetrates due to actual atmospheric corrosion while more closely simulating an actual corrosion environment (atmospheric corrosion environment).

[0065] In the evaluation method of this embodiment, step (B) * and step (B) may be performed at least once or twice or more times to form a corrosion test cycle. * may be arranged for every predetermined number of cycles of step (B), or may be arranged for every random number of cycles of step (B). * The corrosion test cycle may be one in which the step (A) and the step (B) are carried out at least once.

[0066] In the corrosion test cycle shown in Figure 1, step (B) * In the method, the transition step (c1), the holding step (c2), and the transition step (c3) are set under the same conditions as the transition step (b4), the drying step (b1), and the transition step (b3) of step (B), respectively, but are not limited thereto. For example, the transition step (c1) may be different from the transition step (b4) of step (B) in terms of processing time and other conditions, and for example, the transition step (c1) may have a processing time (transition time) of less than 1.0 hour. Similarly, the transition step (c3) may be different from the transition step (b3) of step (B) in terms of processing time and other conditions, and for example, the transition step (c3) may have a processing time of less than 1.0 hour.

[0067] In addition, in the corrosion test cycle shown in Fig. 1, step (A) includes a chloride attachment step (a1) and a holding step (a2), but is not limited thereto. Step (A) does not necessarily have to include the holding step (a2).

[0068] Furthermore, in the corrosion test cycle shown in FIG. 1, after the step (A), a step (B) having a transition step (c1), a holding step (c2), and a transition step (c3) is performed. * ), but is not limited thereto. A corrosion test cycle may be such that step (B) is performed immediately after step (A) without providing the transition step (c1), the holding step (c2), and the transition step (c3). The cycle of step (B) may start with the wetting step (b2) or with another step (such as the transition step (b4)).

[0069] In the evaluation method of the present invention, after performing the process comprising the above-described steps (A) and (B) at least once, the state of the metallic material (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.

[0070] The method for selecting a metallic material of the present invention includes an evaluation step of evaluating the delayed fracture properties of a metallic material using the above-described evaluation method for the delayed fracture properties of a metallic material, and a selection step of selecting a metallic material based on the evaluation results obtained in the evaluation step. By performing the evaluation step on a stressed metallic material, the relationship between the corrosion test conditions (temperature, relative humidity, amount of chloride adhesion, etc.) and the delayed fracture properties (presence or absence of delayed fracture and its severity (e.g., test time at which cracks occur)) can be obtained. Furthermore, the stress applied to the metallic material may be changed to obtain the relationship under one or more of the corrosion test conditions. These relationships can then be used to classify metallic materials. Metallic materials for shipment, etc. can then be selected from the classification according to the environment in which the metallic material will be used and the applied stress of the components.

[0071] The method for manufacturing a component of the present invention includes a step of manufacturing a component by processing a metal material selected by the method for selecting a metal material described above. The processing is not particularly limited, and examples thereof include various metal processing methods such as forming. The component is preferably an automotive component. [Example]

[0072] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

[0073] (metallic material) The metallic materials used were commercial 1.4 mm thick 1470 MPa class cold rolled steel sheets and cold rolled steel sheets having a coating layer on their surface, and the delayed fracture properties were evaluated in accordance with the invention examples and comparative examples. The cold rolled steel sheets having a coating layer on their surface were prepared as follows: First, the surface of the cold rolled steel sheets was degreased with alkali. Then, a zinc phosphate conversion treatment was carried out to form a conversion treatment layer (adhesion weight: 2 g / m 2), and then electrodeposition coating was applied on the chemical conversion coating layer to form a coating layer (film thickness: 15 μm). PB-SX35 manufactured by Nihon Parkerizing Co., Ltd. was used as the chemical conversion treatment agent for the zinc phosphate chemical conversion treatment. GT-100 manufactured by Kansai Paint Co., Ltd. was used as the electrodeposition paint for the electrodeposition coating. The cold-rolled steel sheets and cold-rolled steel sheets with a coating layer on their surfaces were each sheared to a width of 35 mm and a length of 100 mm, and ground to a width of 30 mm to remove residual stress from shearing, to prepare strip-shaped test pieces (steel sheets). Of the resulting strip-shaped test pieces, those without a coating layer on their surfaces were immersed in toluene and ultrasonically cleaned for 5 minutes, while those with a coating layer on their surfaces were bent 180° without cleaning. In this state, the test piece shape was fixed by restraining with bolts and nuts, and test pieces for evaluating delayed fracture properties, as shown in Figure 2, were obtained. The test pieces for evaluating the delayed fracture properties (hereinafter simply referred to as "test pieces" for the sake of convenience) were prepared as follows: (i) strip-shaped test pieces (steel plates) that were bent 180° with a curvature radius of 4 mmR, with the inner spacing of the strip-shaped test pieces after bending being 8 mm; and (ii) strip-shaped test pieces (steel plates) that were bent 180° with a curvature radius of 5 mmR, with the inner spacing of the strip-shaped test pieces after bending being 10 mm.

[0074] (Corrosion test cycle) The above test piece is subjected to the following step (B): * A corrosion test cycle (corrosion test) consisting of step (b2) and step (b3) was carried out. Details of the conditions of this corrosion test cycle are shown in Tables 2-1 and 2-2. Step (B) is a process in which the following four steps were carried out in order: wetting step (b2) → transition step (b4) → drying step (b1) → transition step (b3). One cycle is defined as the execution of the four steps described above. * is a process in which the wetting step (b2) of step (B) is replaced with step (A). Specifically, step (B) *is a process in which the following steps are carried out in order: Step (A) [chloride adhesion step (a1) → holding step (a2)] → transition step (b4) → drying step (b1) → transition step (b3). The delayed fracture properties were evaluated using the following corrosion test cycles (I) to (IV). The symbol "-" in Tables 2-1 and 2-2 indicates that the corresponding step was not carried out. Cycle (I): Step (B) * → A cycle in which process (B) is repeated Cycle (II): Step (B) * → A cycle of repeating process (B) → process (B) → process (B) Cycle (III): A cycle in which cycle (I) → cycle (II) is repeatedly performed, i.e., step (B) * →Process (B)→Process (B) * → A cycle of repeating process (B) → process (B) → process (B) Cycle (IV): Step (B) * Repeated cycle of only

[0075] (1) Evaluation of delayed fracture properties The delayed fracture properties were evaluated by the number of days until cracks appeared in the test specimens. Specifically, during the corrosion test described above, the test specimens were visually inspected once a day for cracks at the 180° bent portion, and the number of days until cracks appeared (number of days until cracks appeared) was measured for a maximum of 63 days. Here, cracks were judged to have occurred when a new crack had developed from the processed surface state before the corrosion test to a size of 1 mm or more. The corrosion test for evaluating the delayed fracture properties was performed on three specimens per example (invention example, comparative example), and the number of days until cracks appeared in two or more specimens was regarded as the number of days until cracks appeared.

[0076] The evaluation results of this delayed fracture property are shown in Table 3. In addition, if cracks ultimately occurred in only one specimen, this is noted in Table 3.

[0077] The same test specimens (test specimens for delayed fracture evaluation) used in the corrosion test of this example were subjected to an exposure test conducted by the present inventors in Okinawa Prefecture. As a result, among the test specimens, those made from cold-rolled steel sheets with a curvature radius of 4 mmR cracked on the 24th day, while those with a curvature radius of 5 mmR did not crack even after one year of exposure. Furthermore, among the test specimens made from cold-rolled steel sheets with a coating layer on the surface, those with a curvature radius of 4 mmR cracked on the 210th day, while those with a curvature radius of 5 mmR did not crack even after one year of exposure. The average temperature during this test was 26°C during the period when cracks occurred in the test specimen with a curvature radius of 4 mmR and 20°C during the one-year exposure period for the test specimen with a curvature radius of 5 mmR. Based on these results, in the above-mentioned corrosion test, if cracks occurred in two or more specimens with a curvature radius of 4 mmR and no cracks occurred in any specimens with a curvature radius of 5 mmR, the evaluation result for delayed fracture properties was rated as "Good" (excellent evaluation accuracy for delayed fracture properties), and otherwise it was rated as "Poor evaluation accuracy for delayed fracture properties."

[0078] (2) Evaluation of suitability for corrosive environments In the present invention, in order to evaluate delayed fracture properties under an actual usage environment (actual corrosive environment), it is important to be able to simulate the actual usage environment (corrosive environment). Therefore, to determine whether a corrosion test can simulate an actual corrosion environment (suitability of the corrosive environment), a cold-rolled steel sheet and a zinc ingot were used, and the ratio of the corrosion amount of the cold-rolled steel sheet to that of the zinc ingot was evaluated according to the following criteria. In this evaluation test, a commercially available cold-rolled steel sheet with low impurities (JIS standard SPCE) was used as the cold-rolled steel sheet, and a zinc sheet with a purity of 99% was used as the zinc ingot. The corrosion amount ratio used as the standard was based on the results of the corrosion amount ratio between steel and zinc obtained in an exposure test under an actual environment, as described in Non-Patent Document 3. If the ratio of the corrosion amount of the cold-rolled steel sheet to the corrosion amount of the zinc ingot in the corrosion test deviates from the standard corrosion amount ratio, it means that the corrosion test does not simulate an actual corrosion environment (deviates from the actual corrosion environment). In other words, if the ratio of the amount of corrosion of the cold-rolled steel sheet to the amount of corrosion of the zinc block in the corrosion test deviates from the standard corrosion ratio, the cold-rolled steel sheet (metal material) will be evaluated in a corrosion state different from the actual corrosion environment, and the generation of hydrogen, which is generated by corrosion and causes delayed fracture, will also be significantly different from the actual corrosion environment. Therefore, it is necessary for the corrosion test to simulate the actual corrosion environment.

[0079] The amount of corrosion of the cold-rolled steel sheet was calculated by dividing the difference in mass before and after the corrosion test by the corroded area. The mass of the cold-rolled steel sheet after the corrosion test was measured after removing the corrosion products by immersing the cold-rolled steel sheet in a 5% HCl solution. The amount of corrosion of the zinc block was also calculated by dividing the difference in mass before and after the corrosion test by the corroded area. The mass of the zinc block after the corrosion test was measured after removing the corrosion products by immersing the zinc block in ammonium dichromate for 15 minutes. The ratio of the amount of corrosion between the cold-rolled steel sheet and the zinc block in the corrosion test was then calculated using the following formula, and the suitability of the corrosive environment was evaluated using the following evaluation criteria. [Corrosion ratio of cold-rolled steel sheet to zinc block] = [Corrosion amount of cold-rolled steel sheet] / [Corrosion amount of zinc block]

[0080] <Evaluation criteria> ○ (Favorable corrosion environment): 10≦[Cold-rolled steel sheet / zinc block corrosion amount ratio]≦100 △ (suitable corrosion environment): 3≦[Corrosion amount ratio of cold-rolled steel sheet to zinc block]<10, or 100<[Corrosion ratio of cold-rolled steel sheet to zinc block]≦500 × (inappropriate corrosion environment): [Cold-rolled steel sheet / zinc block corrosion ratio] < 3, or 500 < [Cold-rolled steel sheet / zinc block corrosion ratio] Based on this evaluation standard, "◯" or "△" was evaluated as a product that was able to simulate the actual usage environment (corrosive environment), and "×" was evaluated as a product that was not able to simulate the actual usage environment.

[0081] Then, (1) a rating of "Good" in the evaluation of delayed fracture characteristics and (2) a rating of "Good" or "Poor" in the evaluation of suitability for the corrosive environment were given an overall rating of "Good" (the delayed fracture characteristics caused by hydrogen penetrating into the interior of metal materials due to atmospheric corrosion can be accurately evaluated, and the delayed fracture characteristics in actual usage environments can be simulated), and all other ratings were given an "Poor" (poor evaluation accuracy of the delayed fracture characteristics and / or inability to simulate the delayed fracture characteristics in actual usage environments).

[0082] As shown in Tables 2-1, 2-2, and 3, the evaluation method of the present invention makes it possible to accurately evaluate delayed fracture characteristics caused by hydrogen penetrating into the interior of metallic materials due to atmospheric corrosion, and to simulate delayed fracture characteristics in actual usage environments.

[0083] [Table 2-1]

[0084] [Table 2-2]

[0085] [Table 3]

Claims

1. A process comprising the following step (A) and the following step (B) is carried out at least once, wherein: Step (A): A step including a chloride adhering step (a1) of adhering a chloride to a metal material in an atmosphere of a relative humidity Ha1 of 80% or more and a temperature Ta1 of 60°C or less; Step (B): A step of performing a cycle at least once or more in an atmosphere of a temperature Tb1 that is 60° C. or less and within a certain range, the cycle including a drying step (b1), a wetting step (b2), a transition step (b3), and a transition step (b4), Drying step (b1): A step of drying the metal material by maintaining it in an atmosphere having a relative humidity Hb1 of 45% or less for 1.0 hour or more and 5.0 hours or less; Wetting step (b2): a step of wetting the metal material by holding it in an atmosphere of a relative humidity Hb2 of 80% or more for 1.0 hour or more and 5.0 hours or less; transition step (b3): ​​a step of transitioning from the atmosphere of the relative humidity Hb1 to the atmosphere of the relative humidity Hb2 for 1.0 hour or more and 5.0 hours or less; transition step (b4): a step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 for 1.0 hour or more and 5.0 hours or less; and, The process time of the chloride attachment step (a1) is 0.7 hours or less, The method for evaluating delayed fracture properties of a metallic material, wherein the step (A) further includes, after the chloride adhesion step (a1), a holding step (a2) of holding the sample in an atmosphere having the relative humidity Ha1 and the temperature Ta1.

2. 2. The method for evaluating delayed fracture properties of a metallic material according to claim 1, wherein the holding time of the holding step (a2) is set so that the process time of the step (A) and the process time of the wetting step (b2) are equal.

3. 3. The method for evaluating delayed fracture properties of a metallic material according to claim 1, wherein the metallic material has a coating layer on a surface thereof.

4. an evaluation step of evaluating the delayed fracture properties of a metallic material using the method for evaluating the delayed fracture properties of a metallic material according to claim 1 or 2; a selection step of selecting a metal material based on the evaluation results obtained in the evaluation step.

5. an evaluation step of evaluating the delayed fracture properties of a metallic material using the method for evaluating the delayed fracture properties of a metallic material according to claim 3; a selection step of selecting a metal material based on the evaluation results obtained in the evaluation step.

6. A method for manufacturing a component, comprising processing a metal material selected by the method for selecting a metal material according to claim 4 to manufacture the component.

7. A method for manufacturing a component, comprising processing a metal material selected by the method for selecting a metal material according to claim 5 to manufacture the component.

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