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 the delayed fracture properties of metallic materials, which can accurately evaluate delayed fracture properties caused by hydrogen penetrating into the interior of metallic materials due to atmospheric corrosion in which chlorides are the dominant factor in corrosion. A chloride deposition step (A) of depositing droplets of a chloride-containing aqueous solution on an evaluation surface of a metal material so as to deposit a predetermined amount of chloride, and a corrosion step (B) of performing the cycle one or more times, each cycle including a predetermined drying step (b1), wetting step (b2), transition step (b3), and transition step (b4), are performed one or more times, and the distribution of the droplets on the evaluation surface of the metal material in at least the first chloride deposition step (A) is determined to be such that the average contact area of ​​the droplets is 0.1 mm 2 Over 3.0mm 2 Less than, area ratio of total contact area of ​​droplets to the area of ​​the evaluation surface: 40% or more and 80% or less, standard deviation of contact area of ​​droplets: 3.0 mm 2 The following is a method for evaluating the delayed fracture properties of metallic materials.

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the delayed fracture properties of metallic materials, and in particular to a method for evaluating delayed fracture (presence or absence of delayed fracture and its extent, etc.) of metallic materials used in atmospheric corrosive environments where automobiles run, which is caused by hydrogen that penetrates into the metallic materials due to 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 other hand, automotive parts are used in atmospheric environments. Therefore, in order to properly evaluate the delayed fracture properties of automotive parts, it is important to establish a method for evaluating the delayed fracture properties of hydrogen embrittlement, which is caused by hydrogen that penetrates into automotive parts due to atmospheric corrosion. Therefore, evaluation methods for delayed fracture properties that simulate atmospheric corrosion environments have been proposed.

[0006] For example, Patent Document 1 discloses a method for evaluating the delayed fracture properties of a metallic material by performing a cycle of at least one cycle, which cycle includes a step of attaching a component mainly composed of chlorides to the metallic material, a drying step of drying the surface of the metallic material by changing the relative humidity of the metallic material, and a wetting step of wetting the surface of the metallic material, and performing this cycle at least once. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-180658 [Non-patent literature]

[0008] [Non-Patent Document 1] Shinsaku Matsuyama, Delayed Fracture, Nikkan Kogyo Shimbun, 1989 [Non-patent document 2] Omura et al., Corrosion and Protection Symposium Materials, vol. 170, pp. 47-54, 2010 Summary of the Invention [Problem to be solved by the invention]

[0009] In the atmospheric corrosive environment in which automobiles travel, corrosion is accelerated by the adhesion of salts such as snow-melting salt and airborne salt, and delayed fracture is thought to occur when hydrogen generated by chloride-induced corrosion (atmospheric corrosion in which chlorides are the dominant factor in corrosion) penetrates into metallic materials. Therefore, it is important to evaluate the delayed fracture properties of automotive parts in the range of high chloride adhesion levels, even among the expected chloride adhesion levels. 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, depending on the type of high-strength steel sheet, it may not be possible to accurately determine the superiority or inferiority of the delayed fracture properties.

[0010] 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, such as those in which automobiles run, caused by hydrogen penetrating into the interior of the metallic material as a result of atmospheric corrosion in which chlorides are the dominant factor in corrosion. [Means for solving the problem]

[0011] 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.

[0012] [1] The chloride adhesion amount on the evaluation surface of the metal material is 1000 to 20,000 mg / m 2 a chloride deposition step (A) of depositing droplets of a chloride-containing aqueous solution so that the chloride-containing aqueous solution is a corrosion process (B) in which a cycle including the following drying process (b1), the following wetting process (b2), the following transition process (b3), and the following transition process (b4) is performed one or more times in an atmosphere at a temperature Tb1 of 60°C or less and within a certain range, The distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first chloride deposition step (A) is Average contact area of ​​the droplet on the evaluation surface of the metal material: 0.1 mm 2 Over 3.0mm 2less than, the area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material: 40% or more and 80% or less, and the standard deviation of the contact area of ​​the droplets on the evaluation surface of the metal material: 3.0 mm 2 The following is a method for evaluating the delayed fracture properties of metallic materials. Drying step (b1): A step of drying a metal material by maintaining it in an atmosphere of a relative humidity Hb1 of 45% or less for 1.0 hour or more and 5.0 hours or less; Wetting step (b2): A step of wetting the metal material by holding it in an atmosphere of a relative humidity Hb2 of 80% or more for 1.0 hour or more and 5.0 hours or less; transition step (b3): ​​a step of transitioning from an atmosphere having the relative humidity Hb1 to an atmosphere having the relative humidity Hb2 at a rate of change in relative humidity of 30% / h or less; Transition step (b4): A step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 at a rate of change in relative humidity of 30% / h or less. [2] In the chloride attachment step (A), The method for evaluating the delayed fracture properties of a metallic material according to [1], wherein the chloride-containing aqueous solution is sprayed from a spray nozzle, and a shielding material having an opening is placed between the metallic material and the spray nozzle, and droplets of the chloride-containing aqueous solution sprayed from the spray nozzle are allowed to adhere to an evaluation surface of the metallic material through the opening of the shielding material. [3] The method for evaluating the delayed fracture properties of a metallic material according to [1] or [2], wherein the chloride adhesion step (A) is carried out in an atmosphere of a relative humidity Ha1 of 30% or more and a temperature Ta1 of 50°C or less. [4] 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 [3] above; a selection step of selecting a metal material based on the evaluation results obtained in the evaluation step. [5] A method for manufacturing a component, comprising processing a metal material selected by the method for selecting a metal material according to [4] above to manufacture the component. [6] A metallic material selected by the metallic material selection method described in [4] above. [7] An automotive component using the metal material described in [6] above. [Effects of the Invention]

[0013] The present invention provides a method for evaluating the delayed fracture properties of metallic materials, which can accurately evaluate delayed fracture properties of metallic materials used in atmospheric corrosive environments, such as those in which automobiles run, caused by hydrogen penetrating into the metallic material as a result of atmospheric corrosion in which chlorides are the dominant factor in corrosion.

[0014] 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 design guidelines (strength, component composition, etc.) for materials that will experience delayed fracture in accordance with the actual use environment. [Brief explanation of the drawings]

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

[0016] 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 metallic material due to atmospheric corrosion. Therefore, the present inventors evaluated delayed fracture properties of two types of high-strength steel plates (Steel 1 and Steel 2) of 1470 MPa class under the following corrosion test cycle conditions according to the method described in Patent Document 1, varying the stress applied to them from YS (yield stress) to 0.9 to 0.5 YS. Here, Steel 1 had a tensile strength (TS) of 1480 MPa and a yield point (YP) of 1190 MPa, while Steel 2 had a TS of 1510 MPa and a YP of 1220 MPa.

[0017] Specifically, 1.4 mm thick cold-rolled steel sheets were sheared to a width of 35 mm x length of 100 mm and then ground to a width of 30 mm to remove residual stress from shearing. 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 in place using bolts and nuts. To vary the applied stress, the inner spacing of the bent strip specimens was adjusted to a predetermined distance. 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.

[0018] (Corrosion test cycle conditions) [Process of attaching chlorides to metal materials] Atmospheric atmosphere Chloride species: NaCl Adhesion amount: 1000-10000mg / 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 / h 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 / h Humidity holding process conditions: 55% RH, holding time 2 hours Cycle order: Drying process → Humidity maintenance process → Wetting process → Humidity maintenance process → Drying process

[0019] [Table 1]

[0020] From Table 1, it can be seen that in the evaluation of delayed fracture properties in an actual environment, the lower limit stress value at which cracks occurred was higher for Steel 1 than for Steel 2, and that Steel 1 was superior in delayed fracture properties. Furthermore, when the delayed fracture properties of high-strength steel sheets were evaluated using the method described in Patent Document 1, the delayed fracture properties were evaluated under conditions where the amount of chloride adhesion was large (chloride adhesion: 7000 to 10000 mg / m 2 ), which differs from the evaluation results of delayed fracture properties in an actual environment, sometimes results show that Steel 2 has better delayed fracture properties, and other times the delayed fracture properties of Steel 1 and Steel 2 are equivalent, and it can be seen that the results of delayed fracture properties vary greatly. In other words, it can be said that the method described in Patent Document 1 may not be able to accurately evaluate delayed fracture properties in an actual environment. The present inventors conducted a detailed investigation into the cause of this. As a result, it was found that the state of chloride adhesion in the process of adhering a component mainly composed of chloride to a metal material (chloride adhesion process) has a significant impact.

[0021] That is, the inventors have found that when a component mainly composed of chlorides is adhered to a metal material, the more uniform the distribution of chlorides on the surface of the metal material (evaluation surface), the less variability in the evaluation results tends to be, and that controlling not only the amount of chlorides adhered but also their distribution is important for conducting an appropriate evaluation. In particular, the distribution of saltwater droplets adhered to the metal material surface at the start of the evaluation test greatly contributes to the uniformity of the entire evaluation test, and the inventors have found that uniformity in the distribution of saltwater droplets adhered to the evaluation surface at the beginning (first time) can reduce variability in the evaluation test results.

[0022] The present inventors have considered that in order to improve the accuracy of a method for evaluating the delayed fracture properties of metallic materials, it is important to uniformly deposit chlorides on the evaluation surface of the metallic material before corrosion, and that to achieve this, it is important to control the distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metallic material (droplet deposition distribution) in the chloride deposition step of the evaluation test, and have completed the present invention after various investigations. The present invention was made based on the above findings.

[0023] 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) comprises carrying out a process including a chloride adhesion process (A) and a corrosion process (B) at least once.

[0024] 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, the metallic material to which stress has been applied as described above can be subjected to a process comprising a chloride adhesion step (A) and a corrosion step (B) at least once (once or twice or more). Furthermore, in the evaluation method of the present invention, after performing a process comprising a chloride adhesion step (A) and a corrosion step (B) once or more times, 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.

[0025] The evaluation method of the present invention will be described below with reference to exemplary embodiments, but the present invention is not limited to the following exemplary embodiments.

[0026] (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.

[0027] 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.

[0028] In the evaluation method of this embodiment, stress is applied to the metal material, and examples of the method for applying stress to the metal material include the methods described above.

[0029] In this embodiment, in order to evaluate the delayed fracture properties of a metallic material, a chloride adhesion step (A) and a corrosion step (B) are carried out while applying stress to the metallic material. After carrying out each of these steps one or more times, the presence or absence and degree of cracking in the metallic material are confirmed, thereby evaluating the delayed fracture properties.

[0030] Each step will be described below.

[0031] (Chloride attachment step (A)) The chloride deposition step (A) is carried out by depositing a chloride on the evaluation surface of the metal material in an amount of 1000 to 20000 mg / m 2 In the evaluation method of this embodiment, the distribution of the droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first (first) chloride deposition step (A) (deposition distribution of the droplets) is such that the average contact area of ​​the droplets on the evaluation surface of the metal material is 0.1 mm 2 Over 3.0mm 2 Less than, the area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material: 40% to 80% and the standard deviation of the contact area of ​​the droplets on the evaluation surface of the metal material: 3.0 mm 2 The following applies.

[0032] Chloride adhesion amount: 1000 to 20,000 mg / m 2 > The amount of chloride attached to the metal material (amount of chloride solids not including solvents such as water) is 1000 to 20000 mg / m 2The above-mentioned adhesion amount corresponds to the amount of chloride adhesion assumed to occur in the atmospheric corrosive environment in which an actual automobile runs. 2 In a corrosive environment where the coating weight is less than 20,000 mg / m, corrosion hardly progresses, so hydrogen generation and penetration into the metal material are minimal, making delayed fracture unlikely. 2 If the coating amount exceeds 20,000 mg / m, the corrosion rate will be significantly different from that in the actual environment, resulting in an excessive durability test that will not serve the purpose. 2 In order to simulate the corrosion pattern in the atmospheric corrosive environment in which an actual automobile runs and to promote corrosion, the coating amount is set to 5000 mg / m 2 From the above viewpoint, the amount of adhesion is preferably more than 12000 mg / m 2 The following is preferred:

[0033] The amount of chloride adhesion can be calculated by multiplying the difference in mass of the test specimen (metal material) before and after application of the chloride-containing aqueous solution in the chloride adhesion step (A) by the chloride concentration of the chloride-containing aqueous solution and dividing the result by the area of ​​the test specimen's surface to be evaluated. When measuring the mass difference, if the chloride-containing aqueous solution adheres to areas other than the test specimen's surface to be evaluated, appropriate measures can be taken, such as masking the areas other than the test specimen or wiping off the chloride-containing aqueous solution that has adhered to the areas other than the test specimen. The amount of chloride adhesion can be controlled by, for example, changing the chloride concentration of the chloride-containing aqueous solution or by changing the time for applying the chloride-containing aqueous solution (the process time of the chloride adhesion step (A)) to change the amount of chloride-containing aqueous solution applied to the metal material.

[0034] In the chloride deposition step (A), chloride is deposited on the metal material to obtain a desired chloride deposition amount. The chloride preferably includes one or more chlorides selected from sodium salt (NaCl), potassium salt (KCl), calcium salt (CaCl), and magnesium salt (MgCl), which are present in the atmospheric environment in which typical metal materials are used. In the chloride deposition step (A), a chloride-based component containing chloride and other components may be deposited on the metal material. Here, a chloride-based component refers to a component in which chloride accounts for more than 50 mass% of the total components in terms of solid content. Examples of components other than chloride include, but are not limited to, sulfides and nitrate compounds. Considering actual atmospheric corrosion environments, it is preferable to deposit a NaCl-based component (a component in which NaCl accounts for more than 50 mass% of the total components) on the metal material.

[0035] Furthermore, when simulating delayed fracture characteristics in areas where snow-melting agents are frequently sprayed in winter, it is preferable that the chlorides attached to the metal material have a composition similar to that of the snow-melting agents sprayed in those areas. Examples of components having a composition similar to that of snow-melting agents include a component mainly composed of CaCl2 (a component in which CaCl2 is more than 50 mass% of the total components), a component mainly composed of MgCl2 (a component in which MgCl2 is more than 50 mass% of the total components), and a component mainly composed of NaCl (a component in which NaCl is more than 50 mass% of the total components).

[0036] Furthermore, a component containing a combination of multiple metal salts may be used as the chloride to be attached to the metal material. Examples of a component containing a combination of multiple metal salts include the Society of Automotive Engineers standard (SAE J2334) (0.5% by mass NaCl-0.1% by mass CaCl-0.075% by mass NaHCO), artificial seawater (2.5% by mass NaCl-0.5% by mass MgCl-0.12% by mass CaCl-0.07% by mass KCl, and others (e.g., an aqueous solution of Aquamarine (registered trademark) manufactured by Yashima Pharmaceutical Co., Ltd.)).

[0037] The method for depositing chloride on a metal material (chloride deposition method) is not particularly limited as long as it is a method that can achieve a desired distribution of chloride-containing aqueous solution droplets in terms of evaluating the metal material. Examples of the chloride-containing aqueous solution include a chloride-containing aqueous solution containing a component mainly composed of chloride (usually an aqueous solution such as salt water, hereinafter also referred to as salt water). The following description will be given taking the case where salt water is used as the chloride-containing aqueous solution as an example.

[0038] The spray method is an example of a chloride deposition method. An example of a spray method is a method in which salt water is deposited using a spray nozzle. Types of spray nozzles include one-fluid spray nozzles (nozzles in which a liquid fed under pressure is atomized and sprayed) and two-fluid spray nozzles (nozzles that atomize the liquid using a high-speed fluid such as compressed air). Two-fluid spray nozzles also differ in the liquid supply method, and are classified into liquid pressure types (liquid is pressurized and supplied to the two-fluid nozzle) and suction types (liquid is sucked up and sprayed using the force of compressed air). It is preferable to select a spray nozzle that ensures a uniform deposition distribution of droplets. Furthermore, since salt water is used, it is preferable to use a corrosion-resistant metal such as stainless steel as the material for the spray nozzle.

[0039] The chloride concentration in the saltwater is not particularly limited. However, when controlling the saltwater droplet distribution using a spray nozzle, if saltwater with a chloride concentration of less than 2.0 mass% is used to deposit saltwater on a metal material, the spray time becomes long to obtain an appropriate chloride deposition amount. As a result, it becomes difficult to obtain the desired droplet distribution on the evaluation surface of the metal material. Therefore, the chloride concentration in the saltwater is preferably 2.0 mass% or more, and more preferably 5.0 mass% or more. On the other hand, if saltwater with a chloride concentration of more than 20 mass% is used to deposit saltwater on a metal material, chlorides are likely to precipitate in the spray nozzle, causing clogging and making it difficult to spray saltwater droplets of consistent size. This makes it difficult to obtain the desired droplet distribution on the evaluation surface of the metal material. As a result, the amount of chloride deposition on the evaluation surface of the metal material varies depending on the location, and in areas with a high chloride deposition amount, localized corrosion occurs and the amount of hydrogen penetration increases. This causes the delayed fracture properties to vary within the evaluation surface of the metal material, reducing the accuracy of the delayed fracture property evaluation. This tendency becomes particularly pronounced when the amount of chloride attached is high. Therefore, the chloride concentration in the salt water is preferably 20 mass % or less, and more preferably 15 mass % or less.

[0040] It is recommended to adjust the chloride concentration in the saltwater according to the target chloride deposition amount. It is preferable to use low-concentration saltwater when the chloride deposition amount is relatively low, and high-concentration saltwater when the chloride deposition amount is relatively high. To reduce the chloride deposition amount on the test surface of the metal material using high-concentration saltwater, the total amount of saltwater sprayed must be reduced, which can lead to an undersized average contact area of ​​saltwater droplets on the test surface of the metal material and a ratio of the total contact area of ​​saltwater droplets to the total area of ​​the test surface. On the other hand, to increase the chloride deposition amount on the test surface of the metal material using low-concentration saltwater, the total amount of saltwater sprayed must be increased, which can lead to an oversized average contact area of ​​saltwater droplets on the test surface of the metal material and a ratio of the total contact area of ​​saltwater droplets to the total area of ​​the test surface.

[0041] [Distribution of droplets of chloride-containing aqueous solution deposited on the evaluation surface of the metal material in the initial chloride deposition step (A)] In the evaluation method of the present invention, it is necessary to control the distribution of saltwater droplets on the evaluation surface of the metal material in at least the first (i.e., performed at least once) chloride deposition step (A) within the range described below. It is believed that the locations where saltwater was present on the evaluation surface of the metal material in the first chloride deposition step primarily become the starting points of corrosion on the evaluation surface. Furthermore, in the second chloride deposition step, saltwater spray occurs on the evaluation surface in a state where corrosion has occurred after the first chloride deposition step and corrosion products have formed on the evaluation surface. Even if saltwater droplets are uniformly deposited on the evaluation surface of the metal material, wetting and spreading occurs due to the influence of the corrosion products. Therefore, it is important to uniformly control the distribution of saltwater droplets on the evaluation surface of the metal material in the first chloride deposition step.

[0042] Fig. 1 is a schematic diagram illustrating an example of the evaluation method of this embodiment. As shown in Fig. 1, in the evaluation method of this embodiment, salt water is sprayed from a spray nozzle to deposit droplets of salt water onto the evaluation surface of the metal material. In this process, in the initial chloride deposition step (A), the distribution of salt water droplets deposited on the evaluation surface of the metal material (deposition distribution of droplets) is controlled within a predetermined range.

[0043] Average contact area of ​​droplets on the evaluation surface of metal materials: 0.1 mm 2 Over 3.0mm 2 Less than> In the evaluation method of this embodiment, in at least the first chloride adhesion step (A), the average contact area of ​​the droplets on the evaluation surface of the metal material (average contact area per droplet) is set to 0.1 mm 2 Over 3.0mm 2 The average contact area is less than 0.1 mm 2 If the average contact area is less than 0.1 mm, the volume of the droplets is too small to achieve the target amount of chloride deposition. 2 The average contact area is 0.5 mm 2 It is preferable that the thickness is 1.0 mm or more. 2 On the other hand, it is more preferable that the average contact area is 3.0 mm or more.2 If the average contact area is more than 3.0 mm, the droplet adhesion distribution becomes non-uniform, resulting in large variations in the delayed fracture evaluation. 2 The average contact area shall be less than 2.8 mm 2 It is preferable that it is 2.5 mm or less. 2 It is more preferable that the average contact area of ​​the droplets on the evaluation surface of the metal material be as follows: The average contact area of ​​the droplets on the evaluation surface of the metal material can be measured by the measurement method described below.

[0044] <Area ratio of the total contact area of ​​droplets to the area of ​​the evaluation surface of the metal material: 40% to 80%> In the evaluation method of this embodiment, in at least the first chloride adhesion step (A), the area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material (total contact area ratio of the droplets) is set to 40% or more and 80% or less. If the total contact area ratio of the droplets is less than 40%, the droplet adhesion distribution will be non-uniform, resulting in large variations in delayed fracture evaluation. Therefore, the total contact area ratio of the droplets is set to 40% or more. The total contact area ratio of the droplets is preferably set to 50% or more, more preferably 55% or more. On the other hand, if the total contact area ratio of the droplets is greater than 80%, adjacent droplets on the evaluation surface will be more likely to bond with each other, and the average contact area of ​​the droplets (average contact area per droplet) will be more likely to coarsen. Therefore, the total contact area ratio of the droplets is set to 80% or less. The total contact area ratio of the droplets is preferably set to 75% or less, more preferably 70% or less. The total contact area ratio of the droplets can be measured using the measurement method described below.

[0045] <Standard deviation of droplet contact area on the evaluation surface of the metal material: 3.0 mm 2 Below> In the evaluation method of this embodiment, in at least the first chloride adhesion step (A), the standard deviation in the distribution of the contact area of ​​the droplet on the evaluation surface of the metal material is 3.0 mm 2 The standard deviation of the contact area of ​​the droplet on the evaluation surface of the metal material is 3.0 mm or less. 2If the contact area is larger than this, the droplets will not adhere uniformly, resulting in a large variation in the delayed fracture evaluation. 2 The standard deviation of the contact area of ​​the droplets is 2.8 mm 2 It is preferable that it is 2.5 mm or less. 2 It is more preferable that the standard deviation of the contact area of ​​the droplets is as follows: The standard deviation of the contact area of ​​the droplets can be measured by the measurement method described below.

[0046] The distribution of saltwater droplets attached to the evaluation surface of the metal material (average droplet contact area, total droplet contact area ratio, and standard deviation of droplet contact area) can be determined by attaching saltwater droplets to the evaluation surface of the metal material in the chloride attachment step (A), acquiring an image of the droplet distribution across the entire evaluation surface of the metal material, and performing image analysis. The image can be acquired using a digital camera, microscope, optical microscope, or the like. The image can also be acquired by photographing the evaluation surface from above (from the direction of the spray nozzle shown in Figure 1 ). The image of the evaluation surface can be acquired inside a test tank equipped with a spray nozzle, or by removing the metal material from the test tank. Preferably, in the latter case, the metal material is removed from the test tank equipped with a spray nozzle and the evaluation surface of the metal material is photographed. The image is also acquired immediately (within 30 seconds) after the saltwater droplets are attached to the evaluation surface of the metal material.

[0047] Figure 2 is a schematic diagram showing an image of the droplet distribution on the evaluation surface of the metal material obtained as described above. In Figure 2, the area indicated by a circle is the contact area of ​​the droplets. From such an image, the average contact area of ​​the droplets (average contact area per droplet), the total contact area ratio of the droplets, and the standard deviation of the contact area of ​​the droplets are determined by image analysis.

[0048] Here, the evaluation surface of a metallic material refers to the surface of the metallic material for evaluating the delayed fracture properties. The evaluation surface can be determined appropriately depending on the metallic material to be evaluated. For example, if the metallic material is a plate, the evaluation surface can be the surface of the plate facing the spray nozzle (see FIG. 1). Furthermore, if stress is applied to the metallic material, the evaluation surface can be the surface of the stressed portion facing the spray nozzle (the surface corresponding to the plan view (top view) of the metallic material when the direction in which the spray nozzle is installed relative to the metallic material is upward). More specifically, for example, if a bent metallic material is to be evaluated, as described below, the evaluation surface can be the surface of the bent portion facing the spray nozzle (see FIG. 7).

[0049] As a method for achieving the above-mentioned distribution of saltwater droplets, there is a method in which saltwater is applied to the evaluation surface of the metal material using a spray nozzle, as described above. A two-fluid nozzle is preferable as the spray nozzle. Examples of two-fluid nozzles include KSMMS (product name) manufactured by Kyoritsu Alloy Manufacturing Co., Ltd., Two-fluid Air Atomizing Nozzle (product name) manufactured by Spraying Systems Japan LLC, and Fine Mist Generating Nozzle (product name) manufactured by Ikeuchi Co., Ltd.

[0050] As an example of specific conditions when using a spray nozzle, the distance from the tip of the spray nozzle to the evaluation surface of the metal material (X in Figure 1) is preferably 10 to 30 cm. The spray pressure of the spray nozzle is preferably 0.05 to 0.7 MPa. The spray angle (θ in Figure 1) is preferably 30 to 120°. The salt water spray time is preferably 10 seconds or less. As mentioned above, it is also preferable to adjust the chloride concentration in the salt water according to the target chloride adhesion amount. Note that, as shown in Figure 7, when a bent metal material is to be evaluated, the distance from the tip of the spray nozzle to the evaluation surface of the metal material is the shortest distance from the tip of the spray nozzle to the evaluation surface of the metal material.

[0051] A particularly preferred method for achieving the above-described saltwater droplet distribution is to place a shielding material having an opening between the spray nozzle and the metal material, and allow droplets of the chloride-containing aqueous solution sprayed from the spray nozzle to adhere to the evaluation surface of the metal material through the opening of the shielding material. The opening of the shielding material preferably has a shape and size substantially equivalent to the shape and size of the evaluation surface of the metal material. "Substantially equivalent" means that the opening of the shielding material is equivalent to the peripheral shape and size of the evaluation surface of the metal material when viewed from above, or that the area of ​​the opening of the shielding material is within ±10% of the area of ​​the evaluation surface of the metal material. Furthermore, it is preferable that the shielding material be capable of shielding areas other than the evaluation surface of the metal material. That is, when viewed from above, the evaluation surface of the metal material can be seen through the opening of the shielding material, while other areas are not visible (are shielded).

[0052] Fig. 3 is a schematic diagram illustrating the case where a shielding material is placed between the spray nozzle and the metal material in the evaluation method of this embodiment. As shown in Fig. 3, by placing the above-mentioned shielding material between the spray nozzle and the metal material, the sprayed liquid (atomized salt water) that was sprayed from the spray nozzle but floats in the atmosphere without adhering to the evaluation surface of the metal material can be prevented from adhering (re-adhering) to the evaluation surface of the metal material to which droplets have already adhered after the salt water spray has ended. As a result, it is possible to achieve a desired droplet distribution on the evaluation surface of the metal plate with high precision.

[0053] When a shielding material is used, the distance X from the tip of the spray nozzle to the evaluation surface of the metal material is preferably 10 to 30 cm. The distance between the shielding material and the evaluation surface of the metal material (Y in FIG. 3) is preferably 1 cm or more and 0.3X cm or less. If the distance Y is less than 1 cm, the sprayed liquid that does not adhere to the evaluation surface of the metal material and floats in the atmosphere remains near the evaluation surface of the metal material, reducing the effect of suppressing redeposition (FIG. 4(a)). On the other hand, if the distance Y is greater than 0.3X cm, the sprayed liquid that passes through the opening of the shielding material scatters below the shielding material, reducing the effect of suppressing redeposition (FIG. 4(b)). Note that the distance Y is the shortest distance from the shielding material to the evaluation surface of the metal material. The material of the shielding material is not limited as long as it can prevent the transmission of the sprayed liquid. Examples of materials include resin, ceramic, metal, and wood. These materials can be processed and used as the shielding material.

[0054] As described above, in the evaluation method of this embodiment, it is important to uniformly control the distribution of saltwater droplets on the evaluation surface of the metal material during the initial (first) chloride deposition step (A). The droplet distribution control described above is performed at least during the initial chloride deposition step (A). The average droplet contact area, total droplet contact area ratio, and standard deviation of droplet contact area can be measured at the end of the initial chloride deposition step (A) (within 30 seconds after the end of saltwater spraying). Alternatively, a test specimen other than the one being tested may be pre-set to achieve a predetermined average droplet contact area, total droplet contact area ratio, and standard deviation of droplet contact area, and the test specimen may be evaluated under the same conditions. In the second and subsequent chloride deposition steps (A), chloride deposition may be performed under the same conditions as the initial chloride deposition step (A), or under different conditions from the initial chloride deposition step (A) as long as the desired chloride deposition amount is achieved. Preferably, conditions for achieving a predetermined average droplet contact area, total droplet contact area ratio, and standard deviation of droplet contact area are set in advance, and the first chloride deposition step (A) is carried out under the set conditions. When the chloride deposition step (A) is carried out two or more times, it is preferable to carry out the second and subsequent chloride deposition steps (A) under the set conditions.

[0055] The chloride adhesion step (A) is preferably carried out in an atmosphere with a relative humidity Ha1 of 30% or more. If the relative humidity Ha1 in the chloride adhesion step (A) is less than 30%, the droplets sprayed from the spray nozzle, particularly when spraying droplets of a chloride-containing aqueous solution using a spray nozzle, tend to dry before reaching the evaluation surface of the metal material. As a result, it may be difficult to control the droplet distribution to obtain the desired distribution on the evaluation surface of the metal material. Furthermore, the chloride adhesion step (A) is preferably carried out in an atmosphere with a relative humidity of 80% or less. If the relative humidity in the chloride adhesion step (A) is greater than 80%, the droplets that adhere to the metal material tend to become coarse.

[0056] Furthermore, the chloride deposition step (A) is preferably performed in an atmosphere with a temperature Ta1 of 50°C or less. If the temperature Ta1 in the chloride deposition step (A) exceeds 50°C, the droplets sprayed from the spray nozzle, particularly when spraying droplets of a chloride-containing aqueous solution using a spray nozzle, tend to dry before reaching the evaluation surface of the metal material. As a result, it may be difficult to control the droplet distribution to obtain the desired droplet distribution on the evaluation surface of the metal material. On the other hand, the lower limit of the temperature Ta1 is not limited as long as it is a temperature at which the saltwater state can be maintained. As an example, the lower limit of the temperature Ta1 can be 20°C or 25°C.

[0057] A characteristic of atmospheric corrosion environments is the repeated alternation of wet (humid) and dry (dry) states, and simulating this environmental change is important in order to approximate the corrosion patterns found in the actual environment in which a vehicle runs. For example, in the case of steel materials, it is known that the corrosion products formed on the steel material change depending on the wet and dry states, and hydrogen is generated during the process of changing from a wet state to a dry state, or from a dry state to a wet state. Therefore, the conditions during the cycle of relative humidity change (corrosion process (B)) are also important in evaluating delayed fracture properties.

[0058] (Corrosion process (B)) The corrosion process (B) is a process in which a cycle including the following drying process (b1), the following wetting process (b2), the following transition process (b3), and the following transition process (b4) is performed at least once (once or twice or more) in an atmosphere of 60°C or less and within a certain temperature range, Tb1.

[0059] <Temperature Tb1 of corrosion process (B): 60°C or less and within a certain range> The corrosion step (B) is performed in an atmosphere at a temperature Tb1 of 60°C or less and within a certain range. If the temperature Tb1 in the corrosion step (B) exceeds 60°C, not only will the evaluation be performed in an environment far removed from the corrosive environment in which the metal material will actually be used, but the corrosion mechanism may also change. Therefore, the temperature Tb1 in the corrosion step (B) is set to 60°C or less, preferably 50°C or less. On the other hand, there is no particular lower limit for the temperature Tb1 in the corrosion step (B). If the temperature Tb1 in the corrosion step (B) is less than 5°C, it may be difficult to control the relative humidity in the corrosion test chamber (constant temperature and humidity chamber) used in the corrosion test. Furthermore, the corrosion rate of the metal material will be significantly reduced, resulting in a longer evaluation time. Therefore, the temperature Tb1 in the corrosion step (B) is preferably set to 5°C or more, and more preferably 10°C or more.

[0060] In addition, delayed fracture properties are strongly affected by the temperature of the environment (atmosphere). Therefore, in order to properly evaluate the delayed fracture properties of metallic materials, taking into account the application location and the environment in which the metallic material is used, it is necessary to keep the temperature Tb1 in the corrosion step (B) within a certain range. When the temperature Tb1 in the corrosion step (B) fluctuates within ±5°C, the amount of hydrogen penetrating from the environment into the metallic material (hydrogen penetration) can be evaluated within a fluctuation range of 30% of the hydrogen penetration amount at the target environmental temperature, allowing for accurate evaluation of delayed fracture properties. When the temperature Tb1 in the corrosion step (B) fluctuates within ±2°C, the fluctuation range of the hydrogen penetration amount is within 15%. Therefore, the fluctuation range of the temperature Tb1 in step (B) is preferably within ±5°C, and more preferably within ±2°C.

[0061] [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.

[0062] 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.

[0063] [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%.

[0064] 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.

[0065] [Transition process (b3), transition process (b4)] The transition step (b3) is a step of transitioning from an atmosphere with the relative humidity Hb1 to an atmosphere with the relative humidity Hb2, and the transition step (b4) is a step of transitioning from an atmosphere with the relative humidity Hb2 to an atmosphere with the relative humidity Hb1. It is known that the amount of hydrogen penetration into metal materials, particularly steel materials, increases when the relative humidity changes. That is, a large amount of hydrogen penetrates into the steel material during the transition steps (b3) and (b4). The reason why the amount of hydrogen penetration into the metal material increases when the relative humidity changes is not entirely clear, but it can be considered as follows. In the transition step (b3), which is a step of transitioning from an atmosphere with the relative humidity Hb1 to an atmosphere with the relative humidity Hb2, chlorides present on the surface of the metal material begin to absorb moisture due to deliquescence, and corrosion of the metal material begins. It is known that corrosion products present on the surface of the metal material change during this process, and hydrogen is thought to be generated along with this change in corrosion products. Furthermore, in the transition step (b4), which transitions from the atmosphere with relative humidity Hb2 to the atmosphere with relative humidity Hb1, the moisture becomes a concentrated solution containing chlorides and metal ions eluted by corrosion, and in the case of steel materials, iron ions, which is thought to lower the solution's pH. In other words, the solution contains a large amount of hydrogen ions during the drying process, which is thought to facilitate hydrogen penetration into the metal material. For this reason, the rate of change in relative humidity during the transition steps (b3) and (b4) is set to 30% / h or less. A rate of change in relative humidity of 30% / h or less during the transition steps (b3) and (b4) allows sufficient penetration of hydrogen generated by corrosion into the metal material, enabling appropriate evaluation of delayed fracture properties. While there is no specific lower limit for the rate of change in relative humidity, excessively long transition steps (b3) and (b4) can result in a long delay in evaluating delayed fracture properties. Therefore, the rate of change of the relative humidity is preferably 1.5% / h or more, and more preferably 10% / h or more.

[0066] The purpose of the method for evaluating the delayed fracture properties of metallic materials of the present invention is to simulate the daytime and nighttime changes in relative humidity in an actual environment. Therefore, if the process time (time for one cycle) of the corrosion step (B), which simulates the daytime and nighttime changes in relative humidity in an actual environment, exceeds 24 hours, this means that corrosion will be slower than in an actual environment, and the evaluation of delayed fracture properties will require a long time. In other words, the process time of the corrosion step (B) is preferably set to 24 hours or less. To expedite the evaluation, the process time of the corrosion step (B) is more preferably set to 12 hours or less. On the other hand, if the process time of the corrosion step (B) is shortened, the relative humidity will change rapidly, which will reduce the correlation with corrosion in an actual environment and may result in a discrepancy with the delayed fracture properties in an actual environment. Therefore, the process time of the corrosion step (B) is preferably set to 5 hours or more.

[0067] In the evaluation method of the present invention, the chloride adhesion step (A) and the corrosion step (B) are each performed at least once. The chloride adhesion step (A) may be performed after a random number of cycles of the corrosion step (B) or after a predetermined number of cycles of the corrosion step (B). The upper limit of the number of times the process including the chloride adhesion step (A) and the corrosion step (B) is performed is not particularly limited. For example, the process including the chloride adhesion step (A) and the corrosion step (B) may be performed until cracks occur in the metal material. Alternatively, the number of test days may be determined in advance, and the process may be performed for a number of test days corresponding to the number of test days. The number of times the process is performed can be appropriately set, taking into consideration, for example, simulating corrosion patterns in an actual environment. As an example, the process may be performed 200 times or less, or may be performed 100 times or less.

[0068] Next, a process including a chloride adhesion step (A) and a corrosion step (B) will be described. Fig. 5 is a diagram illustrating one embodiment of a corrosion test cycle according to the evaluation method of the present invention. The corrosion test cycle shown in Fig. 5 shows an example of a corrosion test cycle in which the chloride adhesion step (A) and the corrosion step (B) are each performed once. In this example, the corrosion step (B) has a drying step (b1), a transition step (b3), a wetting step (b2), and a transition step (b4) as one cycle.

[0069] The corrosion process (B) cycle following the chloride deposition process (A) preferably begins with the drying process (b1). By drying the saltwater applied in the chloride deposition process (A) in the drying process (b1), condensation initiation points are uniformly dispersed when humidity increases, reducing variability in the evaluation of delayed fracture properties. Starting the corrosion process (B) cycle from the transition process (b3), wetting process (b2), or transition process (b4) can lead to uneven dispersion of condensation initiation points due to factors such as insufficient drying of the saltwater applied in the chloride deposition process (A) or the saltwater applied in a high-humidity environment absorbing moisture and becoming coarse. Therefore, it is best to avoid starting the corrosion process (B) cycle from any process other than the drying process (b1).

[0070] When performing a cycle of the chloride deposition step (A) followed by the corrosion step (B) and then performing the chloride deposition step (A) again, it is preferable to include a water rinsing step (C) before the chloride deposition step (A). Figure 6 shows an example of a corrosion test cycle in which a cycle of the chloride deposition step (A) followed by the corrosion step (B) is performed, and then a water rinsing step (C) is performed before performing the chloride deposition step (A) again. If the chloride deposition step (A) is performed again without the water rinsing step (C), the amount of chloride deposited on the surface of the metal material tends to increase as the amount of chloride deposition increases, making it impossible to continue the same corrosive environment. This may result in the possibility of evaluating delayed fracture properties in an environment different from the intended corrosive environment. Therefore, it is preferable to include a water rinsing step (C) before performing the chloride deposition step (A) again. The water rinsing step (C) is a step in which the evaluation surface of the metal material is rinsed with water. The water-washing method in the water-washing step (C) is not particularly limited, but examples include a method in which water is sprayed onto the evaluation surface of the metal material from a spray nozzle to wash the evaluation surface, and a method in which the evaluation surface is immersed in water to wash the evaluation surface.

[0071] In the evaluation method of the present invention, after the process comprising the chloride adhesion process (A) and the corrosion process (B) as described above is performed at least once, the state of the metallic material (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.

[0072] 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., the test time until cracking occurs)) can be obtained. Furthermore, the stress applied to the metallic material may be varied 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 to be shipped, etc., can then be selected from the classification based on the environment in which the metallic material will be used and the applied stress of the component. The selection step may also be performed based on whether the metallic material subjected to the evaluation step satisfies predetermined criteria. In this case, for example, the selection step can select a metallic material if the metallic material subjected to the evaluation step satisfies the predetermined criteria. Examples of the criteria include the presence or absence of cracking and the number of days until cracking occurs. A metallic material selected by the metallic material selection method of the present invention has excellent adaptability to, for example, the environment and applied stress in which the metallic material is used. Furthermore, an automotive component using the metallic material has excellent adaptability to, for example, the environment and applied stress in which the automotive component is used.

[0073] 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 above-mentioned method for selecting a metal material. The processing is not particularly limited, and examples thereof include various metal processing such as molding. The component is preferably an automotive component. [Example]

[0074] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

[0075] (metallic material) Cold-rolled steel sheets (Steel A, Steel B, and Steel C) with different strengths shown in Table 2 were prepared as metallic materials. The sheet thickness was 1.4 mm. Here, the delayed fracture properties of the cold-rolled steel sheets were evaluated in the invention examples and comparative examples. Each of the cold-rolled steel sheets was sheared to a width of 35 mm and a length of 100 mm. To remove residual stress from shearing, the cold-rolled steel sheets were ground to a width of 30 mm to prepare rectangular test specimens. The obtained rectangular test specimens were immersed in toluene and ultrasonically cleaned for 5 minutes, then bent 180° with a curvature radius of 4 mmR. In this state, the test specimen shape was fixed by restraining with bolts and nuts, and the test specimen for evaluation of delayed fracture properties, as shown in Figure 7, was obtained. The test pieces for evaluating the delayed fracture properties (hereinafter simply referred to as "test pieces" for the sake of convenience) were prepared by bending rectangular test pieces through 180° with a curvature radius of 4 mmR, and adjusting the inner spacing of the rectangular test pieces after bending to prepare five pieces each with a load stress equivalent to 0.7 YS.

[0076] [Table 2]

[0077] (Corrosion test cycle) The above-mentioned test specimens were subjected to a corrosion test cycle (corrosion test) consisting of the following chloride deposition process (A) and corrosion process (B). The detailed conditions of this corrosion test cycle are shown in Table 3. In the chloride deposition process (A), a chloride-containing aqueous solution containing the chloride species and adjusted to the concentrations shown in Table 3 was sprayed using a two-fluid sprayer. The distance between the spray nozzle and the test specimen's evaluation surface (X in Figure 3) was 30 cm. The spray pressure of the spray nozzle was 0.2 MPa. The saltwater spray duration was 5 seconds. Furthermore, a shielding material was placed between the spray nozzle and the test specimen, and the chloride-containing aqueous solution was sprayed with various distances between the shielding material and the metal material's evaluation surface (Y in Figure 3). The shielding material had an opening with the same shape and size as the test specimen's evaluation surface and was able to shield all areas of the test specimen except for the evaluation surface when viewed from above (the spray nozzle position). For No. 24 in Table 3, the chloride-containing aqueous solution was applied to the evaluation surface using a hand sprayer instead of the two-fluid sprayer.

[0078] The amount of chloride deposition was calculated by dividing the difference in mass of the test specimen before and after saltwater spraying by the area of ​​the test specimen to calculate the amount of aqueous solution deposition, and then calculating the chloride concentration of the aqueous solution. The droplet distribution on the test surface was determined by capturing images of the test surface from above using a digital camera after the first chloride deposition step (A) (within 30 seconds after the end of the chloride-containing aqueous solution spraying) and analyzing the images. Note that the second and subsequent chloride deposition steps (A) were performed under the same conditions as the first chloride deposition step (A). The corrosion step (B) consisted of the drying step (b1), the transition step (b3), the wetting step (b2), and the transition step (b4), in that order. Each of these four steps constituted one cycle. In this example, the temperature fluctuation range during the corrosion step (B) was limited to ±5°C. In this example, the corrosion test cycle was such that the chloride adhesion step (A) was followed by the corrosion step (B), and then the water-rinsing step (C) was performed before the chloride adhesion step (A) was performed again. That is, in this example, the delayed fracture properties were evaluated using the following corrosion test cycle. A cycle in which the chloride adhesion process (A) → corrosion process (B) → water washing process (C) are repeated

[0079] (Evaluation of delayed fracture properties) The delayed fracture properties were evaluated by the number of days until cracks appeared on the test specimens. Specifically, during the corrosion test, 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 monitored for up to 63 days. The occurrence of cracks was judged when a new crack appeared from the processed surface state before the corrosion test and exceeded 1 mm. The corrosion test for evaluating the delayed fracture properties was performed on five specimens per Example (invention example, comparative example), and the number of test specimens that had cracks by the 63rd day (number of cracks appeared) was used to judge the superiority or inferiority of the delayed fracture properties.

[0080] The evaluation results of the delayed fracture properties are shown in Table 3. When the difference in the number of cracks generated between steels A, B, and C was 1 or less, they were treated as the same result.

[0081] The same test specimens (test specimens for evaluating delayed fracture) 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, it was found that the delayed fracture properties of the cold-rolled steel sheets tested were in the order of (superior) Steel A > Steel B > Steel C (poor). In other words, when the delayed fracture properties of Steel A, Steel B, and Steel C are evaluated under the same conditions, if the number of cracks occurring is in the order of (low) Steel A < Steel B < Steel C (high), then the method is deemed suitable for evaluating delayed fracture properties.

[0082] As shown in Table 3, the evaluation method of the present invention can accurately evaluate delayed fracture characteristics of metallic materials used in atmospheric corrosive environments, such as those in which automobiles are driven, caused by hydrogen penetrating into the metallic material as a result of atmospheric corrosion in which chlorides are the dominant factor in corrosion, and can simulate delayed fracture characteristics in actual usage environments.

[0083] [Table 3]

Claims

1. The amount of chloride attached to the evaluation surface of the metal material is 1000 to 20000 mg / m ２ a chloride deposition step (A) of depositing droplets of a chloride-containing aqueous solution so that the chloride-containing aqueous solution is a corrosion process (B) in which a cycle including the following drying process (b1), the following wetting process (b2), the following transition process (b3), and the following transition process (b4) is performed one or more times in an atmosphere of a temperature Tb1 that is 60° C. or less and within a certain range, The distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first chloride deposition step (A) is Average contact area of ​​the droplet on the evaluation surface of the metal material: 0.1 mm ２ More than 3.0 mm ２ less than 1.0 mm, an area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material: 40% or more and 80% or less, and a standard deviation of the contact area of ​​the droplets on the evaluation surface of the metal material: 3.0 mm ２ The following is a method for evaluating the delayed fracture properties of metallic materials. Drying step (b1): A step of drying a metal material by maintaining it in an atmosphere 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 having the relative humidity Hb1 to the atmosphere having the relative humidity Hb2 at a rate of change of the relative humidity of 30% / h or less; Transition step (b4): A step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 at a rate of change in relative humidity of 30% / h or less.

2. In the chloride attachment step (A), 2. The method for evaluating the delayed fracture properties of a metallic material according to claim 1, wherein the chloride-containing aqueous solution is sprayed from a spray nozzle, and at that time, a shielding material having an opening is placed between the metallic material and the spray nozzle, and droplets of the chloride-containing aqueous solution sprayed from the spray nozzle are caused to adhere to the evaluation surface of the metallic material through the opening of the shielding material.

3. 2. The method for evaluating delayed fracture properties of a metallic material according to claim 1, wherein the chloride adhesion step (A) is carried out in an atmosphere of a relative humidity Ha1 of 30% or more and a temperature Ta1 of 50° C. or less.

4. A method for evaluating the delayed fracture properties of a metallic material described in claim 2, wherein the chloride adhesion process (A) is carried out in an atmosphere of relative humidity Ha1 of 30% or more and temperature Ta1 of 50°C or less.

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 claims 1 to 4; 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 5 to manufacture the component.

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