Method for evaluating delayed fracture characteristics of metal material, method for selecting metal material, method for manufacturing member, metal material, and automobile member

Abstract

Provided is a method for evaluating the delayed fracture characteristics of a metal material, the method being capable of accurately evaluating delayed fracture characteristics caused by hydrogen entering the inside of a metal material due to atmospheric corrosion in which a chloride is a dominant factor of corrosion.　This method for evaluating the delayed fracture characteristics of a metal material carries out, at least one time, a process involving: a chloride adhesion step (A) in which droplets of a chloride-containing aqueous solution are adhered to an evaluation surface of a metal material so as to achieve a predetermined adhesion amount of a chloride; and a corrosion step (B) in which a cycle having a predetermined drying step (b1), a wetting step (b2), a transition step (b3), and a transition step (b4) is regarded as one cycle, and the cycle is performed at least one time, wherein, regarding the distribution of the droplets on the evaluation surface of the metal material in at least the first chloride adhesion step (A), the average contact area of the droplets is at least 0.1 mm2 and less than 3.0 mm2, the area ratio of the total contact area of the droplets to the area of the evaluation surface is 40-80%, and the standard deviation of the contact area of the droplets is at most 3.0 mm2.

Description

Method for evaluating delayed fracture properties of metallic materials, method for selecting metallic materials, manufacturing method of components, metallic materials and automotive components

[0001] The present invention relates to a method for evaluating the delayed fracture properties of metallic materials, and particularly to a method for evaluating delayed fracture (presence or absence of delayed fracture and its severity, 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.

[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 without apparent plastic deformation after a certain period of time under static load stress. It also broadly encompasses liquid metal contact cracking and stress corrosion cracking (Non-Patent Document 1). A problem with automotive parts is hydrogen embrittlement-type delayed fracture, which is caused by hydrogen that penetrates into the steel due to corrosion. 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 or 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, research into delayed fracture has been extensively conducted in the fields of thick plates such as line pipes, which are subject to a large amount of hydrogen penetration from solutions and solvents, and in high-strength steel bolts that have achieved a tensile strength 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 adhering a component mainly composed of chlorides to the metallic material, and a step of subjecting the metallic material to a drying step of drying the surface of the metallic material and a wetting step of wetting the surface by changing the relative humidity of the metallic material, and performing this cycle at least once.

[0007] JP 2016-180658 A

[0008] Matsuyama, Shinsaku, Delayed Fracture, Nikkan Kogyo Shimbun, 1989. Omura et al., Corrosion and Protection Symposium Materials, Vol. 170, pp. 47-54, 2010.

[0009] In atmospheric corrosive environments where 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 among the expected chloride adhesion levels. However, as a result of evaluating the delayed fracture properties of high-strength steel sheets using the evaluation method described in Patent Document 1, it was found that it may not be possible to accurately determine the superiority or inferiority of the delayed fracture properties of some types of high-strength steel sheets.

[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.

[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 amount of chloride attached to the evaluation surface of the metal material is 1000 to 20000 mg / m ２ and a corrosion step (B) in which one cycle is performed one or more times in an atmosphere at a temperature Tb1 of 60°C or less and within a certain range, the cycle comprising the following steps: a drying step (b1), a wetting step (b2), a transition step (b3), and a transition step (b4), and the cycle is performed one or more times, and the distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first chloride adhesion step (A) is such that: an average contact area of ​​the droplets on the evaluation surface of the metal material is 0.1 mm ２ More than 3.0 mm ２ less than 1.0 mm, an area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material: 40% or more and 80% or less, and a standard deviation of the contact area of ​​the droplets on the evaluation surface of the metal material: 3.0 mm ２A method for evaluating the delayed fracture properties of a metallic material, comprising: a drying step (b1): a step of drying a metallic material by maintaining an atmosphere of a relative humidity Hb1 of 45% or less for 1.0 hour or more and 5.0 hours or less; a wetting step (b2): a step of wetting a metallic material by maintaining an atmosphere of a relative humidity Hb2 of 80% or more for 1.0 hour or more and 5.0 hours or less; a transition step (b3): ​​a step of transitioning from the atmosphere of the relative humidity Hb1 to the atmosphere of the relative humidity Hb2 at a rate of change of the relative humidity of 30% / h or less; and a transition step (b4): a step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 at a rate of change of the relative humidity of 30% / h or less. [2] The method for evaluating the delayed fracture properties of a metallic material according to [1], wherein in the chloride adhesion step (A), the chloride-containing aqueous solution is sprayed from a spray nozzle, 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 adhered to the 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 performed 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 selecting a metallic material, comprising: 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]; and a selection step of selecting a metallic material based on the evaluation results obtained in the evaluation step. [5] A method for manufacturing a component, comprising processing a metallic material selected by the metallic material selection method according to [4]. [6] A metallic material selected by the metallic material selection method according to [4]. [7] An automotive component using the metal material described in [6] above.

[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.

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

[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, by varying the stress applied to them from a YS (yield stress) equivalent to 0.9YS to 0.5YS. Here, Steel 1 had a TS (tensile strength) of 1480 MPa and a YP (yield point) of 1190 MPa, while Steel 2 had a TS of 1510 MPa and a YP of 1220 MPa.

[0017] Specifically, a 1.4 mm thick cold-rolled steel sheet was 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-shaped test specimens were then immersed in toluene and ultrasonically cleaned for 5 minutes, then bent 180° with a radius of curvature of 4 mmR. The specimen shape was then fixed by restraining the specimen with bolts and nuts. To vary the applied stress, the inner spacing of the bent strip-shaped test specimens was adjusted to a predetermined distance. The test was conducted for a maximum of 63 days, and the test specimens were observed every day to check for cracking. The same test specimens were also subjected to an exposure test (1 year) in a real-world environment (Okinawa) to evaluate delayed fracture properties. The results are shown in Table 1.

[0018] (Conditions of the corrosion test cycle) [Step of attaching chloride to metal material] Air atmosphere Chloride species: NaCl Attachment amount: 1000 to 10000 mg / m ２ Application method: spray method [A cycle consisting of a drying process and a wetting process is performed one or more times] Drying process conditions: temperature 30°C, humidity 30% RH, holding time 2 hours Transition time from drying process to wetting process 2 hours (excluding the holding time of the humidity holding process described below), humidity change rate 30% RH / hour Wetting process conditions: temperature 30°C, humidity 90% RH, holding time 2 hours Transition time from wetting process to drying process 2 hours (excluding the holding time of the humidity holding process described below), humidity change rate 30% RH / hour Humidity holding process conditions: 55% RH, holding time 2 hours Cycle order: drying process → humidity holding process → wetting process → humidity holding process → drying process

[0019]

[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 amount: 7000 to 10000 mg / m ２), which is different 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 significantly 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 initial (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, for this purpose, it is important to control the distribution of droplets of a chloride-containing aqueous solution on the evaluation surface of the metallic material (deposition distribution of droplets) 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 using 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 including a chloride deposition 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 the process including the chloride deposition step (A) and the corrosion step (B) once or more times, the state of the metallic material can be 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 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) will be described. 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. Furthermore, the metallic material may 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. Furthermore, the coating layer may be a single layer or multiple layers. Examples of metallic materials having a coating layer on their surface include metallic materials in which the surface of a metallic material is subjected to a chemical conversion treatment such as zinc phosphate treatment to form a chemical conversion treatment layer, and then further subjected to electrodeposition coating or the like to form a coating layer thereon.

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

[0030] Each step will be described below.

[0031] (Chloride Adhesion Step (A)) In the chloride adhesion step (A), a chloride adhesion amount of 1000 to 20000 mg / m is applied to the evaluation surface of the metal material. ２ In the evaluation method of this embodiment, the distribution of the droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first (first) chloride deposition step (A) (deposition distribution of the droplets) is such that the average contact area of ​​the droplets on the evaluation surface of the metal material is 0.1 mm ２ More than 3.0 mm ２ less than 1.0 mm, the area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material: 40% or more and 80% or less, and the standard deviation of the contact area of ​​the droplets on the evaluation surface of the metal material: 3.0 mm ２ The following applies.

[0032] <Chloride deposition amount: 1,000 to 20,000 mg / m ２ The amount of chloride to be attached to the metal material (the amount of chloride solids not including solvents such as water) is 1,000 to 20,000 mg / m ２ The adhesion amount corresponds to the amount of chlorides that is assumed to be adhered in the atmospheric corrosive environment in which an actual automobile runs. ２ In a corrosive environment where the coating amount is less than 20,000 mg / m, corrosion hardly progresses, and therefore hydrogen generation and penetration into the metal material are minimal, making delayed fracture unlikely to occur. ２If the coating amount exceeds 20,000 mg / m, the corrosion rate will be significantly different from that in the actual environment, resulting in an excessive durability test, which will not serve the purpose. ２ From the viewpoint of simulating the corrosion pattern in the atmospheric corrosive environment in which an actual automobile runs and promoting corrosion, the coating amount is set to 5000 mg / m ２ From the above viewpoint, the amount of adhesion is preferably more than 12,000 mg / m ２ The following is preferred:

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

[0034] In the chloride deposition step (A), chloride is deposited on the metal material to obtain a desired amount of chloride. Examples of chloride include sodium salt (NaCl), potassium salt (KCl), and calcium salt (CaCl), which are present in the atmospheric environment where general metal materials are used. ２ ), magnesium salts (MgCl ２) is preferably included. In the chloride adhering step (A), when adhering chloride to the metal material, a component mainly composed of chloride, which contains chloride and components other than chloride, may be adhered. Here, the component mainly composed of chloride refers to a component in which chloride is more than 50 mass% of all components in terms of solid content. Examples of components other than chloride include, but are not limited to, sulfides and nitrate compounds. In consideration of an actual atmospheric corrosion environment, it is preferable to adhere a component mainly composed of NaCl (a component in which NaCl is more than 50 mass% of all components) to the metal material.

[0035] In addition, when simulating delayed fracture characteristics in an area where snow-melting agents are frequently sprayed in winter, it is preferable that the chlorides to be attached to the metal material have a composition similar to that of the snow-melting agents sprayed in that area. ２ Components mainly composed of (CaCl ２ is more than 50% by mass of all components), MgCl ２ A component mainly composed of MgCl ２ Examples of the component include a component containing NaCl as the main component (a component in which NaCl accounts for more than 50% by mass of all components), a component containing NaCl as the main component (a component in which NaCl accounts for more than 50% by mass of all components), and the like.

[0036] Furthermore, a component containing a combination of multiple metal salts may be used as the chloride to be attached to the metal material. An example of a component containing a combination of multiple metal salts is the SAE J2334 (0.5% by mass NaCl - 0.1% by mass CaCl ２ -0.075% by mass NaHCO ３ ), artificial seawater (2.5% by mass NaCl-0.5% by mass MgCl ２ -0.12% by mass CaCl ２ Examples include -0.07% by mass KCl and others (for example, an aqueous solution of Aquamarine (registered trademark) manufactured by Yashima Pharmaceutical Co., Ltd.).

[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 for evaluation of the metal material. Examples of the chloride-containing aqueous solution include a chloride-containing aqueous solution containing a component mainly composed of chloride (usually an aqueous solution such as salt water, hereinafter also referred to as salt water). The following description will be given taking the case where salt water is used as the chloride-containing aqueous solution as an example.

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

[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 a suitable chloride deposition amount. As a result, it becomes difficult to obtain a desired droplet distribution on the evaluation surface of the metal material. Therefore, the chloride concentration in the saltwater is preferably 2.0 mass% or more, and more preferably 5.0 mass% or more. On the other hand, if saltwater with a chloride concentration of more than 20 mass% is used to deposit saltwater on a metal material, chlorides are likely to precipitate in the spray nozzle, causing clogging and making it difficult to spray saltwater droplets of a consistent size. This makes it difficult to obtain a desired droplet distribution on the evaluation surface of the metal material. As a result, the amount of chloride deposition on the evaluation surface of the metal material varies depending on the location, and in areas with a high chloride deposition amount, localized corrosion occurs and the amount of hydrogen penetration increases. This causes the delayed fracture properties to vary within the evaluation surface of the metal material, reducing the accuracy of the delayed fracture property evaluation. This tendency becomes particularly pronounced when the amount of chloride attached is high. Therefore, the chloride concentration in the salt water is preferably 20 mass % or less, and more preferably 15 mass % or less.

[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 Adhered to the Test Surface of the Metallic Material in the Initial Chloride Adhesion Step (A)] In the evaluation method of the present invention, it is necessary to control the distribution of saltwater droplets on the test surface of the metallic material in at least the initial (i.e., at least the first) chloride adhe- sion step (A) within the range described below. It is believed that the locations where saltwater was present on the test surface of the metallic material in the first chloride adhe- sion step primarily become the starting points of corrosion on the test surface. Furthermore, in the second chloride adhe- sion step, the test surface becomes corroded after the first chloride adhe- sion step, forming corrosion products, resulting in saltwater spray. Even if saltwater droplets are uniformly applied to the test surface of the metallic material, wetting and spreading occurs due to the influence of the corrosion products. Therefore, it is important to uniformly control the distribution of saltwater droplets on the test surface of the metallic material in the first chloride adhe- sion step.

[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. During this process, in the initial chloride deposition step (A), the distribution of salt water droplets deposited on the evaluation surface of the metal material (droplet deposition distribution) is controlled within a predetermined range.

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

[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 ​​droplets to the area of ​​the evaluation surface of the metal material (total contact area ratio of droplets) is set to 40% to 80%. If the total contact area ratio of droplets is less than 40%, the droplet adhesion distribution will be non-uniform, resulting in large variations in delayed fracture evaluation. Therefore, the total contact area ratio of droplets is set to 40% or more. The total contact area ratio of droplets is preferably set to 50% or more, more preferably 55% or more. On the other hand, if the total contact area ratio of droplets is greater than 80%, adjacent droplets on the evaluation surface will be more likely to bond with each other, resulting in coarsening of the average contact area of ​​droplets (average contact area per droplet). Therefore, the total contact area ratio of droplets is set to 80% or less. The total contact area ratio of droplets is preferably set to 75% or less, more preferably 70% or less. The total contact area ratio of the droplets can be measured by the measurement method described below.

[0045] <Standard deviation of the contact area of ​​the droplet on the evaluation surface of the metal material: 3.0 mm ２ In the evaluation method of this embodiment, in at least the first chloride adhesion step (A), the standard deviation in the distribution of the contact area of ​​the droplet on the evaluation surface of the metal material is 3.0 mm ２ The standard deviation of the contact area of ​​the droplet on the evaluation surface of the metal material is 3.0 mm or less. ２ If the contact area is larger than this, the droplet adhesion distribution becomes non-uniform, resulting in a large variation in the delayed fracture evaluation. ２The standard deviation of the contact area of ​​the droplets is 2.8 mm ２ It is preferable that the thickness is 2.5 mm or less. ２ It is more preferable that the standard deviation of the contact area of ​​the droplets is as follows: The standard deviation of the contact area of ​​the droplets can be measured by the measurement method described below.

[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, a microscope, an optical microscope, or the like. The image can also be acquired by photographing the evaluation surface from above (from the direction of the spray nozzle shown in Figure 1 ). The image of the evaluation surface can be acquired within a test tank equipped with a spray nozzle, or by removing the metal material from the test tank. Preferably, in the latter case, the metal material is removed from the test tank equipped with the 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 areas indicated by circles are the contact areas of the droplets. From such an image, the average contact area of ​​the droplets (average contact area per droplet), the total contact area ratio of the droplets, and the standard deviation of the contact area of ​​the droplets are determined by image analysis.

[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, as described below, if the evaluation target is a bent metallic material, the evaluation surface can be the surface of the bent portion facing the spray nozzle (see FIG. 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. As the spray nozzle, a two-fluid nozzle is preferable. Examples of the two-fluid nozzle include KSMMS (product name) manufactured by Kyoritsu Alloy Manufacturing Co., Ltd., a two-fluid air atomizing nozzle (product name) manufactured by Spraying Systems Japan LLC, and a fine mist generating nozzle (product name) manufactured by Ikeuchi Co., Ltd.

[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 metal material that has been bent is to be evaluated, the distance from the tip of the spray nozzle to the evaluation surface of the metal material is the shortest distance from the tip of the spray nozzle to the evaluation surface of the metal material.

[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 identical to the shape and size of the evaluation surface of the metal material. "Substantially identical" means that the opening of the shielding material is equivalent to the peripheral shape and size of the evaluation surface of the metal material when viewed from above, or that the area of ​​the opening of the shielding material is within ±10% of the area of ​​the evaluation surface of the metal material. Furthermore, it is preferable that the shielding material be capable of shielding areas other than the evaluation surface of the metal material. That is, when viewed from above, the evaluation surface of the metal material can be seen through the opening of the shielding material, while other areas are not visible (are shielded).

[0052] 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 Figure 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 floating in the atmosphere without adhering to the evaluation surface of the metal material remains near the evaluation surface of the metal material, thereby reducing the effect of suppressing redeposition (Figure 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 will scatter below the shielding material, reducing the effect of suppressing redeposition (Figure 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 in the initial (first) chloride deposition step (A). The droplet distribution control on the evaluation surface as described above is performed at least in the initial chloride deposition step (A). The average droplet contact area, total droplet contact area ratio, and standard deviation of the droplet contact area can be measured at the end of the initial chloride deposition step (A) (within 30 seconds after the end of saltwater spraying). Alternatively, a test specimen other than the one to be tested may be used, and the respective conditions may be set in advance to achieve a predetermined average droplet contact area, total droplet contact area ratio, and standard deviation of the droplet contact area. The evaluation test of the test specimen may then be performed under the same conditions. In the second or subsequent chloride deposition steps (A), chloride deposition may be performed under the same conditions as in the initial chloride deposition step (A), or may be performed under different conditions from the initial chloride deposition step (A) as long as the desired chloride deposition amount is achieved. Preferably, conditions for achieving a predetermined average contact area of ​​droplets, a total contact area ratio of droplets, and a standard deviation of the contact areas of droplets are set in advance, and the first chloride deposition step (A) is carried out under the set conditions. When the chloride deposition step (A) is carried out two or more times, it is preferable to carry out the second and subsequent chloride deposition steps (A) under the set conditions.

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

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

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

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

[0059] <Temperature Tb1 of Corrosion Step (B): 60°C or Less and Within a Certain Range> The corrosion step (B) is performed in an atmosphere with a temperature Tb1 of 60°C or less and within a certain range. If the temperature Tb1 of the corrosion step (B) exceeds 60°C, not only will the evaluation be performed in an environment far removed from the corrosive environment in which the metal material is actually used, but the corrosion mechanism may also change. Therefore, the temperature Tb1 of the corrosion step (B) is set to 60°C or less, preferably 50°C or less. On the other hand, the lower limit of the temperature Tb1 of the corrosion step (B) is not particularly limited. If the temperature Tb1 of the corrosion step (B) is less than 5°C, it may be difficult to control the relative humidity in the corrosion test chamber (constant temperature and humidity chamber) used when conducting the corrosion test. In addition, the corrosion rate of the metal material will be significantly reduced, resulting in a longer evaluation time. Therefore, the temperature Tb1 of the corrosion step (B) is preferably set to 5°C or more, and more preferably 10°C or more.

[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 of the corrosion step (B) within a certain range. When the temperature Tb1 of the corrosion step (B) fluctuates within ±5°C, the amount of hydrogen penetrating from the environment into the metallic material (hydrogen penetration) can be evaluated within a fluctuation range of 30% of the hydrogen penetration amount at the target environmental temperature, allowing for accurate evaluation of delayed fracture properties. When the temperature Tb1 of the corrosion step (B) fluctuates within ±2°C, the fluctuation range of the hydrogen penetration amount is within 15%. Therefore, the fluctuation range of the temperature Tb1 of the step (B) is preferably within ±5°C, and more preferably within ±2°C.

[0061] [Drying Step (b1)] The drying step (b1) is a step of drying the metal material in an atmosphere with a relative humidity Hb1 of 45% or less for 1.0 to 5.0 hours. The relative humidity Hb1 in the drying step (b1) is set to 45% or less. This is to simulate the dry state, which is one of the characteristics of an atmospheric corrosive environment. Furthermore, if the relative humidity Hb1 in the drying step (b1) exceeds 45%, a long period of time is required to sufficiently dry the metal material surface, resulting in a longer evaluation time. The relative humidity Hb1 in the drying step (b1) is preferably 40% or less. On the other hand, the lower limit of the relative humidity Hb1 in the drying step (b1) is not particularly limited. From the viewpoint of relative humidity controllability, the relative humidity Hb1 in the drying step (b1) is preferably 20% or more. Furthermore, if the components to be attached to the metal material surface contain substances that exhibit deliquescent properties at lower relative humidities, such as magnesium chloride or calcium chloride, it is preferable to set the relative humidity Hb1 in the drying step (b1) low.

[0062] The process time of the drying step (b1) (the time for maintaining the sample in an atmosphere of relative humidity Hb1) is 1.0 hour or more and 5.0 hours or less. If the process time of the drying step (b1) is less than 1.0 hour, an actual corrosive environment cannot be simulated. On the other hand, if the process time of the drying step (b1) is more than 5.0 hours, an actual corrosive environment can be simulated, but it takes a long time to evaluate the delayed fracture properties.

[0063] [Wetting Step (b2)] The wetting step (b2) is a step of wetting a metal material by maintaining an atmosphere with a relative humidity Hb2 of 80% or higher for 1.0 to 5.0 hours. The relative humidity Hb2 in the wetting step (b2) is set to 80% or higher. This is to simulate the wet state, which is one of the characteristics of an atmospheric corrosive environment. If the relative humidity Hb2 in the wetting step (b2) is less than 80%, the effect of wetting will be insufficient, making it impossible to simulate an actual corrosive environment. Among chlorides, sodium chloride has the highest saturated critical vapor pressure, which is approximately 75 to 78% in relative humidity terms. Therefore, for any chloride, if the relative humidity is set to 80% or higher, a water film will form on the metal material surface due to moisture absorption by the chloride, allowing the metal material to maintain a wet state. Therefore, the relative humidity Hb2 in the wetting step (b2) is set to 80% or higher. On the other hand, although there is no particular upper limit for the relative humidity Hb2 in the wetting step (b2), it is preferable that the relative humidity Hb2 in the wetting step (b2) be less than 98%. This is because if the relative humidity Hb2 is 98% or higher, the water film formed by condensation becomes too thick, making it easier for the attached chlorides to be washed away. This phenomenon is particularly likely to occur when evaluating processed test specimens. Therefore, when evaluating processed test specimens, it is preferable that the relative humidity Hb2 in the wetting step (b2) be less than 98%.

[0064] The process time of the wetting step (b2) (the time for maintaining the specimen in an atmosphere with a relative humidity Hb2 of 80% or more) is set to 1.0 hour or more and 5.0 hours or less. If the process time of the wetting step (b2) is less than 1.0 hour, it is not possible to simulate an actual corrosive environment. On the other hand, if the process time of the wetting step (b2) is more than 5.0 hours, it is possible to simulate an actual corrosive environment, but it takes a long time to evaluate the delayed fracture properties.

[0065] [Transition Step (b3) and Transition Step (b4)] The transition step (b3) is a step of transitioning from an atmosphere with the relative humidity Hb1 to an atmosphere with the relative humidity Hb2, and the transition step (b4) is a step of transitioning from an atmosphere with the relative humidity Hb2 to an atmosphere with the relative humidity Hb1. It is known that the amount of hydrogen penetration into a metal material, particularly a steel material, increases when the relative humidity changes. That is, a large amount of hydrogen penetrates into the steel material during the transition steps (b3) and (b4). The reason why the amount of hydrogen penetration into the metal material increases when the relative humidity changes is not entirely clear, but it can be considered as follows. In the transition step (b3), which is a step of transitioning from an atmosphere with the relative humidity Hb1 to an atmosphere with the relative humidity Hb2, moisture absorption begins due to deliquescence of chlorides present on the surface of the metal material, and corrosion of the metal material begins. It is known that corrosion products present on the surface of the metal material change at this time, and hydrogen is thought to be generated along with this change in corrosion products. Furthermore, in the transition step (b4), which is a step of transitioning from an atmosphere with the relative humidity Hb2 to an atmosphere with the relative humidity Hb1, the moisture becomes a concentrated solution containing a large amount of chlorides and metal ions eluted by corrosion, and in the case of steel materials, iron ions, which is thought to lower the pH of the solution. In other words, a large amount of hydrogen ions is contained in the solution during the drying process, which is thought to facilitate hydrogen penetration into the metal material. For this reason, in the transition steps (b3) and (b4), the rate of change of the relative humidity when changing the relative humidity is set to 30% / h or less. If the rate of change of the relative humidity in the transition steps (b3) and (b4) is 30% / h or less, hydrogen generated by corrosion can be sufficiently penetrated into the metal material, enabling appropriate evaluation of delayed fracture properties. On the other hand, although there is no particular lower limit for the rate of change of the relative humidity, if the transition steps (b3) and (b4) are too long, it will take a long time to evaluate delayed fracture properties. Therefore, the rate of change of the relative humidity is preferably 1.5% / h or more, and more preferably 10% / h or more.

[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, 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, reducing the correlation with corrosion in an actual environment and resulting in inconsistencies in the delayed fracture properties in an actual environment. Therefore, the process time of the corrosion step (B) is preferably set to 5 hours or more.

[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 every random number of cycles of the corrosion step (B) or every predetermined number of cycles of the corrosion step (B). The upper limit of the number of times the process including the chloride adhesion step (A) and the corrosion step (B) is performed is not particularly limited. For example, the process including the chloride adhesion step (A) and the corrosion step (B) may be performed until cracks occur in the metal material. Alternatively, the number of test days may be determined in advance, and the process may be performed for a number of 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) includes 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 the variability in the evaluation of delayed fracture properties. If the corrosion process (B) cycle begins with the transition process (b3), the wetting process (b2), or the transition process (b4), the saltwater applied in the chloride deposition process (A) may not be sufficiently dried, or the saltwater applied in a high-humidity environment may absorb moisture and become coarse, resulting in uneven dispersion of condensation initiation points. Therefore, it is best to avoid starting the corrosion process (B) cycle from any process other than the drying process (b1).

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

[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 (the presence or absence of cracks in the metallic material, the extent of cracks, etc.) is confirmed, and the delayed fracture properties of the metallic material are evaluated based on the confirmed state of the metallic material.

[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 corrosion test conditions (temperature, relative humidity, chloride adhesion amount, etc.) and delayed fracture properties (presence or absence of delayed fracture and its severity (e.g., 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 for shipment, 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.

[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 cold-rolled steel sheets were used as the targets for evaluation of delayed fracture properties 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, and then ground to a width of 30 mm to remove residual stress during shearing, to prepare rectangular test specimens. The obtained rectangular test specimens were immersed in toluene and ultrasonically cleaned for 5 minutes, then bent 180° with a curvature radius of 4 mmR. In this state, the test specimen shape was fixed by restraining with bolts and nuts, and a test specimen for evaluation of delayed fracture properties, as shown in FIG. 7, was obtained. As test pieces for evaluating the delayed fracture properties (hereinafter, for convenience of explanation, simply referred to as "test pieces"), five strip-shaped test pieces were prepared by bending them by 180° with a curvature radius of 4 mmR, and adjusting the inner spacing of the strip-shaped test pieces after bending to make the load stress equivalent to 0.7 YS.

[0076]

[0077] (Corrosion Test Cycle) The above-described test specimens were subjected to a corrosion test cycle (corrosion test) consisting of the following chloride adhesion step (A) and corrosion step (B). Details of the conditions for this corrosion test cycle are shown in Table 3. In the chloride adhesion step (A), a chloride-containing aqueous solution containing the chloride species shown in Table 3 and adjusted to the concentration shown in Table 3 was sprayed using a two-fluid spray. The distance between the spray nozzle and the evaluation surface of the test specimen (X in Figure 3) was 30 cm. The spray pressure of the spray nozzle was 0.2 MPa. The saltwater spray time 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 the distance between the shielding material and the evaluation surface of the metal material (Y in Figure 3) varied. The shielding material had an opening with a shape and size equivalent to the shape and size of the evaluation surface of the test specimen, and was capable of shielding the entire surface of the test specimen other than the evaluation surface when viewed from above (the position of the spray nozzle). The No. 10 ... For No. 24, the chloride-containing aqueous solution was applied to the evaluation surface by a hand sprayer instead of the two-fluid sprayer.

[0078] The amount of chloride deposition was calculated by calculating the difference in mass of the test specimen before and after saltwater spraying, dividing the mass by the area of ​​the test specimen's surface to be evaluated, and then calculating the amount of aqueous solution deposition from the chloride concentration of the aqueous solution. The droplet distribution on the test surface was determined by capturing an image 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 spraying of the chloride-containing aqueous solution) and analyzing the image. 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) was performed in the following order: drying step (b1), transition step (b3), wetting step (b2), and transition step (b4), with one cycle consisting of these four steps. In this example, the temperature fluctuation range in the corrosion step (B) was within ±5°C. In this example, the corrosion test cycle consisted of the chloride deposition step (A) followed by the corrosion step (B), followed by the water washing step (C) before performing the chloride deposition step (A) again. That is, in this example, the delayed fracture property was evaluated using the following corrosion test cycle: a cycle in which the chloride adhesion step (A) → corrosion step (B) → water washing step (C) were repeatedly performed

[0079] (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 above-mentioned corrosion test, the test specimens were visually observed 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 checked 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 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 in which cracks appeared 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 among 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) as those used in the corrosion test of this example were subjected to an exposure test in Okinawa Prefecture conducted by the present inventors. As a result, it was found that the delayed fracture properties of the cold-rolled steel sheets tested were (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 (low) Steel A < Steel B < Steel C (high), the method is deemed suitable as an evaluation method for delayed fracture properties.

[0082] As shown in Table 3, the evaluation method of the present invention makes it possible to 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 interior of the metallic material as a result of atmospheric corrosion in which chlorides are the dominant factor in corrosion, and to simulate delayed fracture characteristics in actual usage environments.

[0083]

Claims

1. The amount of chloride attached to the evaluation surface of the metal material is 1,000 to 20,000 mg / m ２ and a corrosion step (B) in which one cycle is performed one or more times in an atmosphere at a temperature Tb1 of 60°C or less and within a certain range, the cycle comprising the following steps: a drying step (b1), a wetting step (b2), a transition step (b3), and a transition step (b4), and the cycle is performed one or more times, and the distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first chloride adhesion step (A) is such that: an average contact area of ​​the droplets on the evaluation surface of the metal material is 0.1 mm ２ More than 3.0 mm ２ less than 1.0 mm, an area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material: 40% or more and 80% or less, and a standard deviation of the contact area of ​​the droplets on the evaluation surface of the metal material: 3.0 mm ２ A method for evaluating the delayed fracture properties of a metallic material, comprising: a drying step (b1): a step of drying a metallic material by maintaining an atmosphere of a relative humidity Hb1 of 45% or less for 1.0 hour or more and 5.0 hours or less; a wetting step (b2): a step of wetting a metallic material by maintaining an atmosphere of a relative humidity Hb2 of 80% or more for 1.0 hour or more and 5.0 hours or less; a transition step (b3): ​​a step of transitioning from the atmosphere of the relative humidity Hb1 to the atmosphere of the relative humidity Hb2 at a rate of change of the relative humidity of 30% / h or less; and a transition step (b4): a step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 at a rate of change of the relative humidity of 30% / h or less.

2. A method for evaluating the delayed fracture properties of a metallic material as described in claim 1, wherein in the chloride adhesion step (A), 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 adhered to the evaluation surface of the metallic material through the opening of the shielding material.

3. A method for evaluating the delayed fracture properties of a metallic material according to claim 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. A method for selecting a metallic material, comprising: 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 described in any one of claims 1 to 3; and a selection step of selecting a metallic material based on the evaluation results obtained in the evaluation step.

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

6. A metallic material selected by the metallic material selection method according to claim 4.

7. An automobile component made of the metal material according to claim 6.

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