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

Abstract

Provided is a method for evaluating the delayed fracture characteristics of a metal material, with which it is possible to accurately evaluate the delayed fracture characteristics caused by hydrogen intruding into the interior of the metal material due to atmospheric corrosion and to simulate the delayed fracture characteristics in an actual use environment. The method for evaluating the delayed fracture characteristics of the metal material performs at least one step comprising the following step (A) and step (B). Step (A): a step having a chloride adhesion step (a1) in which a chloride is adhered to the metal material in an atmosphere having a relative humidity (Ha1) of 80% or more and a temperature (Ta1) of 60 DEG C or less. Step (B): a step in which a cycle having a prescribed drying step (b1), a prescribed wetting step (b2), a prescribed transition step (b3), and a prescribed transition step (b4) is performed once or more.

Description

Technical Field

[0001] The present invention relates to a method for evaluating delayed fracture characteristics of metal materials, and in particular to a method for evaluating delayed fracture characteristics (presence and degree of delayed fracture, etc.) of metal materials used in atmospheric corrosion environments caused by hydrogen penetrating into the metal materials accompanying corrosion. Background Art

[0002] In recent years, efforts have been made to reduce the thickness of steel sheets used by increasing the strength of the steel sheets in order to reduce the weight of the structural members of automobiles. With the increase in the strength of the steel sheets, new concerns have arisen about delayed fracture, which has not been a problem in conventional automobile parts.

[0003] Delayed fracture refers to the phenomenon that high-strength steel parts suddenly undergo brittle fracture after a certain period of time when subjected to static load stress, with almost no plastic deformation in appearance. In a broad sense, it also includes liquid metal contact cracking, stress corrosion cracking, etc. (Non-patent document 1). The problem in automotive parts is hydrogen embrittlement-type delayed fracture caused by hydrogen that penetrates into the steel along with corrosion. The three factors known to cause delayed fracture are material (strength), processing (strain / stress), and hydrogen. Here, as the cause of hydrogen intrusion into metal materials, the intrusion of solutions / solvents in contact with the metal materials and the intrusion of hydrogen generated by the corrosion of the metal materials in the environment in which they are used are considered.

[0004] In the past, extensive research on delayed fracture has been conducted in the field of thick plates such as line pipes where a large amount of hydrogen enters from solutions and solvents, and in high-strength steel bolts with a tensile strength of 1200 MPa or more (Non-Patent Document 2). In addition, in these fields, methods for evaluating delayed fracture characteristics have been standardized.

[0005] On the other hand, automotive parts are used in an atmospheric environment. Therefore, in order to properly evaluate the delayed fracture characteristics of automotive parts, it is important to establish a method for evaluating the delayed fracture characteristics of hydrogen embrittlement caused by hydrogen that penetrates into automotive parts along with atmospheric corrosion. Therefore, a method for evaluating the delayed fracture characteristics in a simulated atmospheric corrosion environment is proposed.

[0006] For example, Patent Document 1 discloses a method for evaluating delayed fracture characteristics of a metal material by performing the following steps one or more times, the steps including: a step of causing a component mainly composed of chloride to adhere to the metal material, and a step of subjecting the metal material to a drying step of drying the surface of the metal material by changing the relative humidity and a wetting step of wetting the surface of the metal material as one cycle and performing the cycle at least once.

[0007] In addition, Patent Document 2 discloses a method for evaluating hydrogen embrittlement characteristics, which comprises: a salt adhesion step, in which a metal salt containing chloride is adhered to the surface of a metal material; and a basic step, in which a wetting step of exposing the metal material to an atmosphere with a relative humidity of Hh and a drying step of exposing the metal material to an atmosphere with a relative humidity of Hlo (wherein Hlo<Hh) are each performed once as a basic cycle, and the basic cycle includes one or more of the above basic cycles; the salt adhesion step, the wetting step and the drying step are performed in an atmosphere below 40°C, and at least one drying step in the above basic steps is performed in an atmosphere with a temperature below 30°C, a relative humidity of 0% to 60%, and a period of 1 minute to 6 hours.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Publication No. 2016-180658

[0011] Patent Document 2: International Publication No. 2019 / 186940

[0012] Non-patent literature

[0013] Non-patent document 1: Matsuyama Shinsaku, Delayed Fracture, Nikkan Kogyo Shimbun, 1989

[0014] Non-patent document 2: Omura et al., Corrosion and Corrosion Prevention Symposium Materials, vol. 170, pp. 47-54, 2010

[0015] Non-patent document 3: Corrosiveness of various atmospheric test sites as measured by specimens of steel and zinc, in metalcorrosion in the Atmosphere, STP 435.pp360-391, American Society for Testing and Materials, Philadelphia, 1968 Summary of the invention

[0016] One of the parameters of the atmospheric corrosion environment that has a great influence on the delayed fracture characteristics of metal materials is temperature.

[0017] The evaluation method described in Patent Document 1 is an evaluation method that makes the temperature of the test process constant, and can evaluate the temperature range that affects the delayed fracture characteristics. However, the results of evaluating the delayed fracture characteristics of high-strength steel sheets using the evaluation method described in Patent Document 1 show that the delayed fracture characteristics in the actual use environment (real environment) may not be reproduced.

[0018] In addition, the evaluation method described in Patent Document 2 is not an evaluation method that keeps the temperature of the test process constant. Therefore, there is a problem that the influence of temperature on the delayed fracture characteristics of metal materials cannot be accurately evaluated. Furthermore, it is not assumed to evaluate the delayed fracture characteristics in a high temperature environment exceeding 40°C.

[0019] The present invention has been completed in view of the above situation, and its purpose is to provide a method for evaluating the delayed fracture characteristics of metal materials, which can evaluate the delayed fracture characteristics of metal materials used in an atmospheric corrosion environment with high accuracy, which are caused by hydrogen that penetrates into the interior of the metal material along with atmospheric corrosion, and can simulate the delayed fracture characteristics in the actual use environment.

[0020] The present inventors have conducted intensive studies and, as a result, have found that the above-mentioned object can be achieved by adopting the following configuration, thereby completing the present invention.

[0021] [1] A method for evaluating delayed fracture characteristics of a metal material, comprising performing at least one of the following steps (A) and (B).

[0022] Step (A): a step comprising a chloride attaching step (a1) of attaching chloride to a metal material in an atmosphere having a relative humidity Ha1 of 80% or more and a temperature Ta1 of 60° C. or less,

[0023] Step (B): In an atmosphere of temperature Tb1 below 60°C and within a certain range, a cycle comprising the following drying step (b1), the following wetting step (b2), the following transition step (b3) and the following transition step (b4) is set as one cycle, and the cycle is performed at least once.

[0024] Drying step (b1): A step of drying the metal material by maintaining it in an atmosphere of relative humidity Hb1 of 45% or less for 1.0 to 5.0 hours.

[0025] Wetting step (b2): a step of wetting the metal material by maintaining it in an atmosphere of relative humidity Hb2 of 80% or more for 1.0 to 5.0 hours,

[0026] Transition step (b3): ​​a step of transitioning from the atmosphere of the relative humidity Hb1 to the atmosphere of the relative humidity Hb2 in 1.0 to 5.0 hours,

[0027] Transition step (b4): a step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 over 1.0 hour to 5.0 hours.

[0028] [2] The method for evaluating the delayed fracture characteristics of a metal material according to [1], wherein the step (A) further comprises a holding step (a2) of holding the material in an atmosphere of the relative humidity Ha1 and the temperature Ta1 after the chloride adhesion step (a1).

[0029] [3] The method for evaluating the delayed fracture characteristics of a metal material according to [2], wherein the holding time of the holding step (a2) is set so that the process time of the step (A) is equal to the process time of the wetting step (b2).

[0030] [4] The method for evaluating delayed fracture characteristics of a metal material according to any one of [1] to [3], wherein the metal material has a coating layer on the surface.

[0031] [5] A method for selecting a metal material, comprising the following steps:

[0032] An evaluation step of evaluating the delayed fracture characteristics of the metal material using the method for evaluating the delayed fracture characteristics of the metal material described in any one of [1] to [4] above; and

[0033] The selection step is to select a metal material based on the evaluation result obtained in the above evaluation step.

[0034] [6] A method for manufacturing a component, comprising manufacturing the component by processing a metal material selected by the method for selecting a metal material described in [5] above.

[0035] According to the present invention, a method for evaluating the delayed fracture characteristics of a metal material can be provided, which can evaluate with high accuracy the delayed fracture characteristics of a metal material used in an atmospheric corrosion environment, caused by hydrogen that penetrates into the interior of the metal material accompanying atmospheric corrosion, and can simulate the delayed fracture characteristics in an actual use environment.

[0036] Furthermore, according to the method for evaluating delayed fracture characteristics of a metal material of the present invention, it is possible to obtain information necessary for determining whether delayed fracture occurs in a metal material in an actual use environment, for example, the fracture limit stress at which delayed fracture occurs in an actual use environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a diagram for explaining one embodiment of a corrosion test cycle of a method for evaluating delayed fracture characteristics of a metal material according to the present invention.

[0038] Figure 2 This is a diagram schematically showing a test piece for evaluating delayed fracture characteristics used in Examples. DETAILED DESCRIPTION

[0039] First, the findings obtained by the present inventors are described. Patent Document 1 describes a method for evaluating the delayed fracture characteristics of a metal material used in an atmospheric corrosion environment, which is caused by hydrogen that penetrates into the interior of the metal material along with atmospheric corrosion. Therefore, the present inventors evaluated the delayed fracture characteristics by changing the stress applied to a commercially available 1470MPa grade cold-rolled steel sheet (sheet thickness: 1.4mm) to 1000MPa, 800MPa, 600MPa, and 400MPa corresponding to YS (yield stress) under the conditions of the following corrosion test cycle according to the method described in Patent Document 1.

[0040] Specifically, a commercially available 1470MPa grade cold-rolled steel sheet (plate thickness: 1.4mm) was cut into a width of 35mm × length of 100mm, and grinding was performed until the width reached 30mm to remove the residual stress during shearing to make a long strip test piece. The obtained long strip test piece was immersed in toluene and ultrasonically cleaned for 5 minutes, and then bent 180° with a radius of curvature of 4mmR. In this state, bolts and nuts were used to constrain and fix the shape of the test piece. In addition, in order to change the applied stress, test pieces with inner spacing of 8, 10, 12, 14, and 17mm after bending were made respectively. The test was carried out for up to 63 days, and the cracks of the test piece were observed every day to confirm whether cracks occurred. In addition, for the same test piece, the delayed fracture characteristics were also evaluated in the real environment (Okinawa) (exposure test (1 year)). The results are shown in Table 1.

[0041] (Conditions of corrosion test cycle)

[0042] [Step of attaching chloride to metal material]

[0043] Atmosphere

[0044] Chlorinated species: NaCl

[0045] Adhesion amount: 10g / m 2

[0046] Attachment method: spraying

[0047] [A process in which a drying process and a wetting process are set as one cycle and the cycle is performed one or more times]

[0048] Drying process conditions: temperature 30°C, humidity 30% RH, holding time 2 hours

[0049] · 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 / hr

[0050] Wetting process conditions: temperature 30°C, humidity 90% RH, holding time 2 hours

[0051] · Transition time from the wetting process to the drying process: 2 hours (excluding the holding time of the humidity holding process described below), humidity change rate: 30% RH / hr

[0052] Humidity maintenance process conditions: 55% RH, maintenance time 2 hours

[0053] Cycle sequence: Drying process → Humidity maintenance process → Wetting process → Humidity maintenance process → Drying process

[0054] [Table 1]

[0055]

[0056] *1The denominator indicates the number of test pieces submitted for testing, and the numerator indicates the number of test pieces in which cracks occurred.

[0057] As can be seen from Table 1, in the method described in Patent Document 1, under the condition that the load stress on the test piece is low (additional stress 600MPa), the test piece has the situation that cracks are generated and the situation that cracks are not generated, and there is a deviation in the evaluation result of the delayed fracture characteristics. That is, in the method described in Patent Document 1, it can be said that the delayed fracture characteristics in the real environment cannot be reproduced sometimes. The present inventors have conducted a detailed investigation on the reasons. It was found that in the method described in Patent Document 1, in the process of making the metal material adhere to the component mainly composed of chloride (chloride adhesion process), the metal material sometimes cracks. The mechanism of the metal material cracking in the chloride adhesion process is not yet clear, but the present inventors speculate as follows.

[0058] In an atmospheric corrosion environment (real environment), chlorides adhere to and accumulate on metal materials due to snow-melting salt and flying salt. Water films are formed on metal materials due to the moisture absorption of chlorides, and corrosion of metal materials (atmospheric corrosion) proceeds. If corrosion proceeds, hydrogen generated by corrosion invades the interior of the metal material and cracks are generated. However, it is difficult to reproduce the adhesion form of chlorides in a real environment through experiments. In the experiment, in order to make the metal material adhere to the same amount of chloride as in the real environment, a solution (brine) with an adjusted chloride concentration is applied to the surface of the metal material by spraying, spraying, etc.; or the metal material is immersed in brine, etc., and brine is applied in a manner to obtain the desired amount of chloride adhesion. In addition, the application of the above-mentioned brine is generally carried out in an atmospheric atmosphere.

[0059] However, in the case of using the above-mentioned method of imparting salt water, at least the relative humidity near the surface of the metal material changes from the relative humidity of the atmospheric atmosphere to a relative humidity of almost 100%. Therefore, in the case where chloride is attached to the surface of the metal material, the chloride absorbs moisture, a water film in which chloride is dissolved is formed, and a thick water film of salt water is formed due to the attachment of salt water. When the imparting of salt water is completed, the relative humidity near the surface of the metal material changes to the relative humidity of the atmospheric atmosphere, so the water film begins to dry, the thickness of the water film decreases, and it becomes a water film in which chloride is concentrated. Furthermore, when the relative humidity near the surface of the metal material is lower than the relative humidity at which chloride deliquesces, chloride precipitates and the water film almost disappears.

[0060] Near the relative humidity at which chloride deliquesces, chloride ions and metal ions dissolved by corrosion increase in the water film. Moreover, as the thickness of the water film changes when the relative humidity changes from wet to dry, the pH of the water film decreases, hydrogen atomizes on the surface of the metal material, and the amount of hydrogen intruding into the interior of the metal material increases. Therefore, by repeating the chloride attachment process under an atmospheric atmosphere, the amount of hydrogen in the interior of the metal material increases. As a result, the inventors of the present invention speculate that even under conditions where no cracks occur in a real environment, the amount of hydrogen intruding into the interior of the metal material exceeds the fracture limit hydrogen amount, and cracks occur in the metal material. Therefore, the inventors of the present invention believe that in order to improve the accuracy of the evaluation method of delayed fracture characteristics, it is important to reduce the amount of hydrogen intruding into the interior of the metal material during the chloride attachment process, and various studies have been conducted to complete the present invention. The present invention is completed based on the above insights.

[0061] The method for evaluating delayed fracture characteristics of a metal material of the present invention (hereinafter also referred to simply as the evaluation method of the present invention) is performed at least once including the steps (A) and (B).

[0062] In the evaluation method of the present invention, it is preferred to apply stress to the metal material. As the above-mentioned method of applying stress to the metal material, a method of processing the metal material (processing method) can be cited. As the above-mentioned processing method, for example, bending processing, drawing processing, stretching processing, torsion processing, etc. can be cited. In addition, a method of fixing in a shape to which stress is applied using bolts, etc., a method of using residual stress remaining after processing, etc. can be cited. In the evaluation method of the present invention, the metal material to which stress is applied as described above can be subjected to a process having step (A) and step (B) at least once (once or twice or more). In addition, in the evaluation method of the present invention, the state of the metal material can be confirmed after performing the process having step (A) and step (B) at least once, and the delayed fracture characteristics of the metal material can be evaluated based on the confirmed state of the metal material. The above-mentioned confirmation can be carried out, for example, by visually observing the presence or absence of cracks in the metal material and its degree.

[0063] Hereinafter, the evaluation method of the present invention will be described with reference to the embodiment examples, but the present invention is not limited to the following embodiment examples.

[0064] (Metal material)

[0065] First, the metal material provided for the method for evaluating the delayed fracture characteristics of the metal material of the present embodiment (hereinafter also referred to as the evaluation method of the present embodiment) is described. As the above-mentioned metal material, for example, steel materials can be cited, but it is not limited to this, and metal materials such as Ti and Al can also be cited. In addition, the metal material can have a coating layer on the surface. As the coating layer, an organic layer, an inorganic layer, and a mixture layer of organic and inorganic substances can be cited. In addition, the coating layer can be a single layer or a multilayer. As the metal material having a coating layer on the surface, for example, a metal material in which a chemical conversion treatment layer is formed by subjecting the surface of the metal material to a chemical conversion treatment layer such as zinc phosphate treatment, and an electroplating coating is further performed thereon to form a coating layer can be cited.

[0066] The form of the metal material is not particularly limited, and may be, for example, a plate, a rod, or a pipe. However, the evaluation method of this embodiment is suitable for evaluating a plate or a pipe with a relatively thin plate thickness, and therefore, a plate or a pipe is preferably used as the evaluation object.

[0067] In the evaluation method of the present embodiment, stress is applied to the metal material. Examples of the method for applying stress to the metal material include the above-mentioned method.

[0068] In this embodiment, in order to evaluate the delayed fracture characteristics of the metal material, the process (A) and the process (B) are performed under stress on the metal material. Then, after performing each of these processes once or more, the presence or absence of cracks in the metal material and the degree of cracks are confirmed, thereby evaluating the delayed fracture characteristics.

[0069] Hereinafter, each step will be described.

[0070] (Process (A))

[0071] The step (A) includes a chloride attaching step (a1) of attaching chloride to the metal material in an atmosphere having a relative humidity Ha1 of 80% or more and a temperature Ta1 of 60°C or less.

[0072] [Chloride Adhering Step (a1)]

[0073] In the chloride attachment process (a1), in order to obtain the desired chloride attachment amount, chloride is attached to the metal material. As chloride, it is preferred to contain one or more of sodium salt (NaCl), potassium salt (KCl), calcium salt (CaCl2), and magnesium salt (MgCl2) selected from the atmospheric environment where general metal materials are used. It should be noted that in the chloride attachment process (a1), when the metal material is attached with chloride, a chloride-based component containing chloride and components other than chloride can be attached. Here, a chloride-based component refers to a component in which chloride exceeds 50% by mass of all components in terms of solid content. As components other than chloride, sulfides, nitric acid compounds, etc. can be cited, but are not limited to them. Considering the actual atmospheric corrosion environment, it is preferred to attach a component in which NaCl is the main component (a component in which NaCl exceeds 50% by mass of all components) to the metal material.

[0074] In addition, when simulating the delayed fracture characteristics of an area where deicing agents are frequently spread in winter, it is preferred to attach a component having a composition similar to that of the deicing agent spread in the area to the metal material. As components having a composition similar to that of the deicing agent, components mainly composed of CaCl2 (components in which CaCl2 exceeds 50% by mass of all components), components mainly composed of MgCl2 (components in which MgCl2 exceeds 50% by mass of all components), components mainly composed of NaCl (components in which NaCl exceeds 50% by mass of all components), etc. can be exemplified.

[0075] In addition, the metal material can be attached with a combination of components containing multiple metal salts. Examples of components containing multiple metal salts include the American Automotive Engineering Association standard (SAE J2334) (0.5 mass% NaCl-0.1 mass% CaCl2-0.075 mass% NaHCO3), artificial seawater (2.5 mass% NaCl-0.5 mass% MgCl2-0.12 mass% CaCl2-0.07 mass% KCl, etc. (for example, the aqueous solution of Aquamarine (registered trademark) manufactured by Yashio Pharmaceutical Co., Ltd.)), etc.

[0076] The amount of chloride attached to the metal material (the amount of the solid content of the chloride excluding solvents such as water) is preferably 1 to 100,000 mg / m 2 The above adhesion amount corresponds to the adhesion amount of chloride assumed in the actual atmospheric corrosion environment. 2 In a corrosive environment, corrosion hardly occurs. Therefore, although the delayed fracture characteristics of metal materials can be evaluated, the corrosion rate is slow and the evaluation takes time. Furthermore, since the amount of hydrogen generated by corrosion is small, delayed fracture is not likely to occur, and in many cases, it is not necessary to evaluate the delayed fracture characteristics. Therefore, the above-mentioned adhesion amount is preferably 1 mg / m2 On the other hand, if the above adhesion amount exceeds 100000 mg / m 2 , it is very likely that the actual corrosion environment cannot be simulated. Therefore, the above adhesion amount is preferably 100000mg / m 2 From the viewpoint of simulating the corrosion morphology in an actual atmospheric corrosion environment and promoting corrosion, the above-mentioned adhesion amount is more preferably 100 to 30,000 mg / m 2 .

[0077] The method for attaching chloride to the metal material (chloride attachment method) is not particularly limited. As the chloride attachment method, a method of attaching a solution containing chloride as a solute to the metal material is generally adopted. As a specific example, for example, a method of immersing the metal material in a solution containing a chloride as a main component (generally an aqueous solution such as salt water, hereinafter also referred to as salt water), a method of spraying or spraying salt water on the metal material, etc. can be cited.

[0078] The chloride concentration in the salt water is not particularly limited. However, when the chloride is attached to the metal material using a salt water with a chloride concentration of less than 0.1% by mass, it takes time to obtain a suitable amount of chloride, or it is difficult to obtain a suitable amount of chloride. Therefore, the chloride concentration in the salt water is preferably 0.1% by mass or more, and more preferably 1% by mass or more. On the other hand, when the chloride is attached to the metal material by spraying, spraying, etc. using a salt water with a chloride concentration of more than 20% by mass (when the salt water is applied), the chloride is easily precipitated in the apparatus for spraying or spraying the salt water, resulting in clogging of the apparatus, etc., and it is sometimes difficult to apply a salt water with a stable concentration. In addition, it is difficult to uniformly attach chloride to the metal material. As a result, the amount of chloride attached to the metal material varies according to the position, local corrosion occurs in the area with a large amount of chloride attached, and the amount of hydrogen intrusion increases. Therefore, the delayed fracture characteristics change in the evaluation area of ​​the metal material, and the evaluation accuracy of the delayed fracture characteristics decreases. Especially when the amount of chloride attached is high, this tendency becomes significant. Therefore, the chloride concentration in the salt water is preferably 20% by mass or less, and more preferably 15% by mass or less.

[0079] The time for applying salt water to the metal material in order to attach chloride, that is, the process time of the chloride attachment step (a1), is not particularly limited. The process time of the chloride attachment step (a1) is preferably 0.2 hours or more. In addition, the process time of the chloride attachment step (a1) is preferably 1.0 hours or less.

[0080] When the process time of the chloride attachment process (a1) is less than 0.2 hours, the distribution of the amount of chloride attached on the surface of the metal material sometimes becomes uneven, and sometimes the delayed fracture characteristics change in the evaluation area of ​​the surface of the metal material. Therefore, the process time of the chloride attachment process (a1) is preferably more than 0.2 hours. The process time of the chloride attachment process (a1) is more preferably more than 0.3 hours. On the other hand, when the process time of the above-mentioned chloride attachment process (a1) exceeds 1.0 hours, it sometimes becomes a corrosion behavior different from the real environment. Specifically, when salt water is supplied to the surface of the metal material, the corrosion products generated on the surface of the metal material are sometimes washed away together with the salt water. For example, in the case of Zn-plated steel plates, the above-mentioned corrosion products have the effect of inhibiting hydrogen from invading the interior of the base steel plate. Therefore, when the above-mentioned corrosion products are washed away together with the salt water, the effect of the above-mentioned corrosion products in the real environment cannot be obtained. In addition, there is also a risk of corrosion caused by salt water. Therefore, the correlation with the corrosion in the real environment becomes low, and there is a risk of producing a situation inconsistent with the results of the delayed fracture characteristics in the real environment. Therefore, the time of the chloride deposition step (a1) is preferably 1.0 hour or less, more preferably 0.7 hour or less.

[0081] The amount of chloride deposited can be calculated by multiplying the mass difference of the test object (metal material) before and after the chloride deposited in the chloride depositing step (a1) by the chloride concentration in the salt water and dividing the result by the area of ​​the test object. When the amount of chloride deposited is changed, the amount of salt water deposited on the metal material can be controlled by changing the chloride concentration of the salt water or by changing the time for applying the salt water (the process time of the chloride depositing step (a1)).

[0082] When salt water is applied to the surface of a metal material by spraying, the inclination angle of the surface of the metal material can be made, for example, 0° relative to the horizontal plane. It should be noted that, at this time, since a water film of the salt water accumulates on the surface of the metal material, the surface can be made inclined relative to the horizontal plane. It should be noted that the inclination angle of the surface of the metal material can also be adjusted in the same manner in processes other than the chloride attachment process (a1). That is, the inclination angle of the surface of the metal material can also be 0° or inclined relative to the horizontal plane in processes other than the chloride attachment process (a1). In processes other than the chloride attachment process (a1), by making the inclination angle of the metal material inclined relative to the horizontal plane, chloride washing away also occurs, and new chloride is easily attached in the chloride attachment process (a1).

[0083] <Relative humidity Ha1 in the chloride adhesion step (a1): 80% or more>

[0084] In the chloride attachment process (a1), chloride is attached to the metal material in an atmosphere with a relative humidity Ha1 of 80% or more. In the chloride attachment process (a1), when chloride is attached to the metal material by spraying or sprinkling, the relative humidity of the environment near the surface of the metal material rises to nearly 100%. In addition, the same is true when immersed in salt water. Here, for example, when chloride is attached to the metal material in an atmospheric atmosphere, the relative humidity of the environment after the chloride attachment is completed is less than 80%, a water film in which chloride is concentrated is formed on the surface of the metal material, and the amount of hydrogen that intrudes into the interior of the metal material during the chloride attachment process increases. In the evaluation method of the present invention, by making the relative humidity Ha1 of the chloride attachment process (a1) more than 80% and attaching chloride to the metal material, the relative humidity of the environment after the chloride attachment is completed is not less than 80%. Therefore, a water film concentrated with chloride is not formed near the relative humidity at which the chloride deliquesces, so the amount of hydrogen that intrudes into the interior of the metal material during the chloride attachment process (a1) can be suppressed. The upper limit of the relative humidity Ha1 in the chloride adhering step (a1) is not particularly limited. For example, the relative humidity Ha1 may be less than 98%, 95% or less, or 90% or less.

[0085] <Temperature Ta1 of the chloride deposition step (a1): 60°C or less>

[0086] In the chloride attachment step (a1), chloride is attached to the metal material in an atmosphere of a temperature Ta1 below 60°C. If the temperature Ta1 of the chloride attachment step (a1) exceeds 60°C, it is considered that not only is the evaluation performed in an environment far away from the corrosive environment in which the metal material is actually used, but the corrosion mechanism also changes. Therefore, the temperature Ta1 of the chloride attachment step (a1) is below 60°C. Preferably, it is below 50°C. On the other hand, the lower limit of the temperature Ta1 of the chloride attachment step (a1) is not particularly limited. If the temperature Ta1 of the chloride attachment step (a1) is less than 5°C, it is sometimes difficult to control the relative humidity in the corrosion test tank (constant temperature and humidity tank) used when performing the corrosion test. In addition, the corrosion rate of the metal material is significantly reduced, so the evaluation time becomes longer. Therefore, the temperature Ta1 of the chloride attachment step (a1) is preferably above 5°C, and more preferably above 10°C.

[0087] [Maintaining step (a2)]

[0088] The process (A) preferably further comprises a holding process (a2) of maintaining in an atmosphere of the above-mentioned relative humidity Ha1 and the above-mentioned temperature Ta1 after the chloride is attached to the metal material in the above-mentioned chloride attachment process (a1). By maintaining at the relative humidity Ha1 after the chloride attachment process (a1), the thickness of the water film generated by the application of salt water changes to the thickness of the water film generated by the moisture absorption of the chloride corresponding to the relative humidity Ha1. Thus, it is expected that a water film with a uniform chloride concentration is formed on the surface of the metal material, and the uneven amount of chloride attached to the surface of the metal material is eliminated. Therefore, it is preferred to set a holding process (a2) of maintaining in an atmosphere of the relative humidity Ha1 and the above-mentioned temperature Ta1 after the chloride attachment process (a1). The holding process (a2), i.e., the holding time of maintaining at the relative humidity Ha1, is preferably 0.5 hours or more. The upper limit of the above-mentioned holding time is not particularly limited. The holding time of the holding step (a2) is preferably set to be equal to the process time of the process (A) (here, the total time of the chloride attachment step (a1) and the holding step (a2)) and the process time of the wetting step (b2) of the process (B) described later. By setting the process time of the process (A) in this way, as described later, it is possible to configure the cycle of the wetting step (b2) in the cycle of the process (B) by configuring the process (A) to replace the cycle of the wetting step (b2), and it is easy to establish a regular corrosion test cycle. Furthermore, by setting the relative humidity Ha1 to the same relative humidity as the relative humidity Hb2 of the wetting step (b2) described later, the amount of hydrogen intrusion into the metal material caused by the chloride attachment step (a1) can be further suppressed. Therefore, it is preferred to set the relative humidity Ha1 to the same relative humidity as the relative humidity Hb2 of the wetting step (b2) described later.

[0089] In addition, the amount of hydrogen that penetrates into the interior of the metal material due to corrosion of the metal material in an atmospheric corrosion environment varies greatly depending on the temperature of the environment (atmosphere). Therefore, the delayed fracture characteristics are also strongly affected by the ambient temperature. Therefore, in order to properly evaluate the delayed fracture characteristics of the metal material taking into account the application location of the metal material component (component formed by the metal material) and the environment in which it is used, it is preferred to keep the temperature of the atmosphere of the process (A) (herein, the chloride adhesion process (a1) or the chloride adhesion process (a1) and the holding process (a2)) and the process (B) described later constant. However, in the chloride adhesion process (a1), the temperature of the atmosphere sometimes changes. This is considered to be the influence of the moisture imparted, and it is sometimes difficult to strictly control the temperature of the atmosphere. However, if the temperature Ta1 of the atmosphere of the process (A) can be controlled to fluctuate within ±10°C relative to the temperature Tb1 of the atmosphere of the process (B), the influence on the delayed fracture characteristics can be greatly reduced. Therefore, it is preferred to control the temperature Ta1 of the atmosphere of the process (A) to fluctuate within ±10°C relative to the temperature Tb1 of the atmosphere of the process (B).

[0090] The characteristic of atmospheric corrosion environment is that the wet (humid) state and the dry (dry) state are repeated, and simulating this environmental change is important to approach the corrosion morphology in the real environment. For example, in the case of steel materials, it is known that the corrosion products formed on the steel materials are deteriorated according to the wet state and the dry state, and hydrogen is generated in the process of changing from the wet state to the dry state or from the dry state to the wet state. Therefore, the conditions in the cycle of relative humidity change (step (B)) also become important in the evaluation of delayed fracture characteristics.

[0091] (Process (B))

[0092] Process (B) is a process in which a cycle having the following drying process (b1), the following wetting process (b2), the following transition process (b3) and the following transition process (b4) is set as one cycle in an atmosphere of temperature Tb1 below 60°C and within a certain range, and the cycle is performed at least once (once or twice or more).

[0093] <Temperature Tb1 of step (B): 60°C or less and within a certain range>

[0094] Process (B) is carried out in an atmosphere of temperature Tb1 below 60°C and within a certain range. If the temperature Tb1 of process (B) exceeds 60°C, it is considered that not only is the evaluation carried out in an environment far away from the corrosive environment in which the metal material is actually used, but the corrosion mechanism also changes. Therefore, the temperature Tb1 of process (B) is below 60°C. Preferably, it is below 50°C. On the other hand, the lower limit of the temperature Tb1 of process (B) is not particularly limited. If the temperature Tb1 of process (B) is less than 5°C, it is sometimes difficult to control the relative humidity in the corrosion test tank (constant temperature and humidity tank) used when performing corrosion tests. In addition, the corrosion rate of the metal material is significantly reduced, so the evaluation time becomes longer. Therefore, the temperature Tb1 of process (B) is preferably above 5°C, and more preferably above 10°C.

[0095] In addition, as mentioned above, the delayed fracture characteristics are strongly affected by the temperature of the environment (atmosphere). Therefore, in order to properly evaluate the delayed fracture characteristics of the metal material taking into account the application location of the metal material component and the environment in which it is used, it is necessary to make the temperature Tb1 of the above-mentioned process (B) within a certain range. When the temperature Tb1 of the process (B) varies within ±5°C, the amount of hydrogen that invades the interior of the metal material from the environment (hydrogen intrusion amount) can be evaluated with a variation range of within 30% relative to the hydrogen intrusion amount at the temperature of the target environment, and the delayed fracture characteristics can be correctly evaluated. When the temperature Tb1 of the process (B) varies within ±2°C, the variation range of the hydrogen intrusion amount is within 15%. Therefore, the variation range of the temperature Tb1 of the process (B) is preferably within ±5°C, and more preferably within ±2°C.

[0096] [Drying step (b1)]

[0097] The drying process (b1) is a process of drying the metal material by keeping it in an atmosphere with a relative humidity Hb1 of less than 45% for 1.0 hour to 5.0 hours. The relative humidity Hb1 of the drying process (b1) is less than 45%. This is to simulate the dry state which is one of the characteristics of the atmospheric corrosion environment. Furthermore, if the relative humidity Hb1 of the drying process (b1) exceeds 45%, it is necessary to keep it for a long time in order to fully dry the surface of the metal material, and the evaluation time becomes longer. The relative humidity Hb1 of the drying process (b1) is preferably less than 40%. On the other hand, the lower limit of the relative humidity Hb1 of the drying process (b1) is not particularly limited. From the viewpoint of the controllability of the relative humidity, the relative humidity Hb1 of the drying process (b1) is preferably more than 20%. In addition, when the components attached to the surface of the metal material include substances such as magnesium chloride and calcium chloride that show deliquescent properties at a lower relative humidity, it is preferred to set the relative humidity Hb1 of the drying process (b1) to a lower level.

[0098] The process time of the drying process (b1) (the time maintained in the atmosphere of relative humidity Hb1) is 1.0 hour to 5.0 hours. If the process time of the drying process (b1) is less than 1.0 hour, the actual corrosive environment cannot be simulated. On the other hand, if the process time of the drying process (b1) exceeds 5.0 hours, the actual corrosive environment can be simulated, but the evaluation of the delayed fracture characteristics takes time.

[0099] [Wetting step (b2)]

[0100] The wetting process (b2) is a process of wetting the metal material by keeping it in an atmosphere with a relative humidity Hb2 of 80% or more for 1.0 hour to 5.0 hours. The relative humidity Hb2 of the wetting process (b2) is 80% or more. This is to simulate the wet state, which is one of the characteristics of the atmospheric corrosion environment. If the relative humidity Hb2 of the wetting process (b2) is less than 80%, the influence of the wetness becomes insufficient, and the actual corrosion environment cannot be simulated as a result. The saturated critical vapor pressure of sodium chloride is the highest among chlorides, and the saturated critical vapor pressure of sodium chloride is about 75 to 78% when converted to relative humidity. Therefore, if any chloride makes the relative humidity above 80%, the surface of the metal material forms a water film through the moisture absorption of the chloride, and can maintain a wet state. Therefore, the relative humidity Hb2 of the wetting process (b2) is 80% or more. On the other hand, the upper limit of the relative humidity Hb2 of the wetting process (b2) is not particularly limited, but the relative humidity Hb2 of the wetting process (b2) is preferably less than 98%. This is because if the relative humidity Hb2 is 98% or more, the thickness of the water film generated by condensation becomes too thick, and the attached chloride is easily washed away. This phenomenon is particularly likely to occur when evaluating processed test objects. Therefore, when evaluating processed test objects, the relative humidity Hb2 of the wetting step (b2) is preferably less than 98%.

[0101] The process time of the wetting process (b2) (the time maintained in an atmosphere of relative humidity Hb2 of 80% or more) is 1.0 to 5.0 hours. If the process time of the wetting process (b2) is less than 1.0 hour, the actual corrosive environment cannot be simulated. On the other hand, if the process time of the wetting process (b2) exceeds 5.0 hours, the actual corrosive environment can be simulated, but the evaluation of the delayed fracture characteristics takes time.

[0102] [Transition Step (b3), Transition Step (b4)]

[0103] The transition process (b3) is a process for transitioning from the atmosphere of the relative humidity Hb1 to the atmosphere of the relative humidity Hb2, and the transition process (b4) is a process for transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1. It is known that the amount of hydrogen intrusion into the interior of metal materials, especially steel materials, increases at the moment of relative humidity change. That is, a large amount of hydrogen intrudes into the interior of the steel material in the transition process (b3) and the transition process (b4). For example, consider the transition process (b4) of transitioning from the atmosphere of the relative humidity Hb2 of the wetting process (b2) to the atmosphere of the relative humidity Hb1 of the drying process (b1). In the wetting process (b2), the chloride deliquesces to form a water film with a water film thickness corresponding to the relative humidity of the environment, and the corrosion of the metal material proceeds. On the other hand, in the drying process (b1), most of the chlorides do not deliquesce, so the chlorides that do not deliquesce precipitate, the water film almost disappears, and corrosion hardly proceeds. When such a change in the thickness of the water film occurs, near the relative humidity of chloride deliquescence, chloride ions and metal ions dissolved by corrosion are enriched in the water film, and a hydrolysis reaction occurs in the thick chloride water film. As a result, the pH of the water film is reduced, hydrogen is atomized on the surface of the metal material, and the amount of hydrogen intruding into the interior of the metal material increases. On the other hand, since the transition process (b3) is the opposite process to the transition process (b4), a thick chloride water film is formed near the relative humidity of chloride deliquescence, but the amount of metal ions dissolved by corrosion is less than that of the transition process (b4). Therefore, in the transition process (b3), the amount of intrusive hydrogen is less than that of the transition process (b4). Here, in order to fully ensure the reaction time of the above-mentioned hydrolysis and ensure the amount of intrusive hydrogen, the process time of the transition process (b3) and the transition process (b4) needs to be more than 1.0 hours respectively. Preferably, it is more than 1.5 hours respectively. On the other hand, the process time of the transition process (b3) and the transition process (b4) is less than 5.0 hours respectively. If the time of each of the above steps exceeds 5.0 hours, the concentration rate of the chloride becomes slow and the hydrolysis reaction proceeds slowly, so that the evaluation of the delayed fracture property takes a long time and is not suitable as an evaluation method.

[0104] Furthermore, when the chloride contains NaCl, the amount of hydrogen intrusion into the metal material is the largest in the region of relative humidity of 55% to 75% near the deliquescent humidity of NaCl, but corrosion of the metal material is hardly caused in the above region. Therefore, in either or both of the transition process (b3) and the transition process (b4), a holding process (holding process of relative humidity of 55% to 75%) can be provided in a region of relative humidity of 55% to 75% for a prescribed time. By providing the above holding process, the amount of hydrogen intrusion into the metal material can be increased. The process time of the above holding process is preferably more than 0.5 hours. It should be noted that the process time of the transition process (b3) and the transition process (b4) also includes the process time of the above holding process of relative humidity of 55% to 75%.

[0105] The purpose of the method for evaluating the delayed fracture characteristics of a metal material of the present invention is to simulate the relative humidity changes during the day and night in a real environment. Therefore, if the process time (one cycle time) of the above-mentioned process (B) that simulates the relative humidity changes during the day and night in a real environment exceeds 24 hours, it means that the corrosion is slower than in the real environment, so the evaluation of the delayed fracture characteristics requires a long time. That is, the process time of process (B) is preferably set to less than 24 hours. In order to facilitate the evaluation, the process time of process (B) is more preferably set to less than 12 hours. On the other hand, if the process time of process (B) is shortened, the relative humidity changes dramatically, the correlation with the corrosion in the real environment becomes lower, and a situation inconsistent with the results of the delayed fracture characteristics in the real environment occurs. Therefore, the process time of process (B) is preferably set to more than 5 hours.

[0106] In the evaluation method of the present invention, the above-mentioned process (A) and the above-mentioned process (B) are respectively performed at least once. In order to simulate the corrosion morphology in the real environment and set the corrosion test cycle, the above-mentioned process (A) is preferably set as described later to configure the process (A) in the cycle of the process (B) instead of the cycle of the wetting process (b2). In addition, the above-mentioned process (A) can be configured according to the cycle of each random number of the above-mentioned process (B), or it can be configured according to the cycle of each specified number of the above-mentioned process (B). It should be noted that the upper limit of the number of times the process having the above-mentioned process (A) and the above-mentioned process (B) is performed is not particularly limited. For example, the process having the above-mentioned process (A) and the above-mentioned process (B) can be performed until cracks are generated in the metal material. In addition, the number of test days can be predetermined, and the above-mentioned process is performed according to the number of test days. The number of the above-mentioned processes can be appropriately set, for example, considering the simulation of the corrosion morphology in the real environment. As an example, the above-mentioned process can be less than 1500 times. In addition, the number of the above-mentioned processes can be set according to the material, type, etc. of the metal material as the evaluation object. For example, when evaluating the delayed fracture characteristics of a metal material having a coating layer on its surface, the upper limit of the above process can be set to 1500 times, and when evaluating the delayed fracture characteristics of a metal material having no coating layer on its surface, the upper limit of the above process can be set to 420 times.

[0107] Next, the process including the step (A) and the step (B) will be described. Figure 1 This is a diagram for explaining one embodiment of a corrosion test cycle of the evaluation method of the present invention. Figure 1The corrosion test cycle shown is an example of a corrosion test cycle when the step (A) and the step (B) are performed once respectively. In this example, the step (A) includes a chloride adhesion step (a1) and a holding step (a2). The step (B) includes a wetting step (b2), a transition step (b4), a drying step (b1), and a transition step (b3) as one cycle.

[0108] In addition, Figure 1 In the corrosion test cycle shown, after step (A), a step (step (B) ＊ ), the process (process (B) ＊ ) carries out a cycle having the following steps: a step of transitioning from the atmosphere of step (A) to an atmosphere of relative humidity Hb1 (transition step (c1)), a step of maintaining in the atmosphere of relative humidity Hb1 (maintaining step (c2)), and a step of transitioning from the atmosphere of relative humidity Hb1 to the atmosphere of relative humidity Hb2 (transition step (c3)).

[0109] In this embodiment, step (B) ＊ The process (B) is set to include the process (A) instead of the wetting process (b2) in the process (B) cycle. ＊ The process time of step (A) in the step (the total process time of the chloride adhesion step (a1) and the holding step (a2)) is set to be equal to the process time of the wetting step (b2) of step (B). ＊ The transition step (c1), the holding step (c2), and the transition step (c3) of step (B) are respectively set to the same conditions as the transition step (b4), the drying step (b1), and the transition step (b3) of step (B). Figure 1 In the corrosion test cycle shown, step (B) ＊ The process time of step (A) is set equal to the process time of step (B). In this way, by configuring step (A) in the cycle of step (B) instead of the cycle of wetting step (b2), it is easy to establish a regular corrosion test cycle. Furthermore, it can be set to further simulate the actual corrosion environment (atmospheric corrosion environment) while simulating the corrosion test cycle of hydrogen intruding with actual atmospheric corrosion.

[0110] In the evaluation method of this embodiment, the step (B) may be performed at least once (once or twice) ＊ The corrosion test cycle of the process (B) is the same as that of the process (B). ＊ It can be configured for each predetermined number of cycles of step (B), or it can be configured for each random number of cycles of step (B). In addition, in the evaluation method of this embodiment, it can be configured to perform at least one or more cycles of step (B).＊ A corrosion test cycle consisting of a process (B) and a process (B).

[0111] It should be noted that Figure 1 In the corrosion test cycle shown, in step (B) ＊ In the embodiment, the transition step (c1), the holding step (c2), and the transition step (c3) are respectively set to the same conditions as the transition step (b4), the drying step (b1), and the transition step (b3) of the step (B), but are not limited thereto. For example, the process time and other conditions of the transition step (c1) may be different from those of the transition step (b4) of the step (B), for example, the process time (transition time) of the transition step (c1) may be less than 1.0 hour. Similarly, the process time and other conditions of the transition step (c3) may be different from those of the transition step (b3) of the step (B), for example, the process time of the transition step (c3) may be less than 1.0 hour.

[0112] In addition, Figure 1 In the corrosion test cycle shown, step (A) includes the chloride adhesion step (a1) and the holding step (a2), but the present invention is not limited thereto. Step (A) may not include the holding step (a2).

[0113] Furthermore, in Figure 1 In the corrosion test cycle shown in FIG. 1 , a process (process (B)) including a transition process (c1), a holding process (c2), and a transition process (c3) is provided after process (A). ＊ ), but not limited to this. It is also possible to set the corrosion test cycle of step (B) to be immediately performed without providing the transition step (c1), the holding step (c2), and the transition step (c3) after step (A). It should be noted that the cycle of step (B) can be started from the wetting step (b2) or from other steps (such as the transition step (b4)).

[0114] In the evaluation method of the present invention, after performing at least one step including the steps (A) and (B) as described above, the state of the metal material (the presence or absence of cracks in the metal material and the extent of cracks, etc.) is confirmed, and the delayed fracture characteristics of the metal material are evaluated based on the confirmed state of the metal material.

[0115] In addition, the method for selecting a metal material of the present invention comprises: an evaluation step of evaluating the delayed fracture characteristics of a metal material using the evaluation method for the delayed fracture characteristics of the metal material described above, and a selection step of selecting a metal material based on the evaluation result obtained in the evaluation step described above. By performing the evaluation step described above on the metal material to which stress is applied, the relationship between the conditions of the corrosion test (temperature, relative humidity, amount of chloride adhesion, etc.) and the delayed fracture characteristics (whether or not delayed fracture occurs and its degree (test time for crack generation, etc.)) is obtained. In addition, the stress applied to the metal material can be changed to obtain the above relationship under one or more conditions of the corrosion test. Then, the metal materials can be classified using these relationships. Then, the metal material to be shipped can be selected from the above classification based on the environment in which the metal material is used and the additional stress of the component.

[0116] The manufacturing method of the component of the present invention has a step of manufacturing the component by processing the metal material selected by the above-mentioned metal material selection method. The above-mentioned processing is not particularly limited, and various metal processing such as molding processing can be cited. As the above-mentioned component, the automotive component is preferably used.

[0117] Example

[0118] The present invention will be described below with reference to Examples, but the present invention is not limited to the following Examples.

[0119] (Metal material)

[0120] As metal materials, commercially available 1.4 mm thick 1470 MPa grade cold-rolled steel sheets and cold-rolled steel sheets with a coating layer on the surface were used as objects, and the delayed fracture characteristics of the invention examples and comparative examples were evaluated. The cold-rolled steel sheets with a coating layer on the surface were prepared as follows. First, the surface of the cold-rolled steel sheets was subjected to alkali degreasing. Then, a chemical conversion treatment layer (adhesion amount: 2 g / m 2), electroplating coating is performed on the chemical conversion treatment layer to form a coating layer (film thickness: 15 μm). As the chemical conversion treatment agent in the above-mentioned zinc phosphate chemical conversion treatment, PB-SX35 manufactured by Nippon Parkersei Co., Ltd. is used. In addition, as the electroplating paint in the above-mentioned electroplating coating, GT-100 manufactured by Kansai Paint Co., Ltd. is used. The above-mentioned cold-rolled steel plate and the cold-rolled steel plate with a coating layer on the surface as the object are cut into a width of 35 mm × a length of 100 mm, respectively. In order to remove the residual stress during shearing, grinding is performed until the width reaches 30 mm to prepare a long strip test piece (steel plate). Among the obtained long strip test pieces, the test piece without a coating layer on the surface is immersed in toluene and ultrasonically cleaned for 5 minutes; the test piece with a coating layer on the surface is not cleaned as above; it is bent 180°, and in this state, it is constrained with bolts and nuts to fix the shape of the test piece to obtain the following Figure 2 As the test pieces for evaluating the delayed fracture characteristics (hereinafter referred to as "test pieces" for the sake of convenience), (i) a long strip test piece (steel plate) was bent 180° with a curvature radius of 4 mmR, and the inner spacing of the long strip test piece after the bending was 8 mm; (ii) a long strip test piece (steel plate) was bent 180° with a curvature radius of 5 mmR, and the inner spacing of the long strip test piece after the bending was 10 mm.

[0121] (Corrosion test cycle)

[0122] The above test piece is subjected to the following step (B): ＊ The corrosion test cycle (corrosion test) consisting of step (B) is shown in detail in Table 2-1 and Table 2-2. Step (B) is a process in which the wetting step (b2) → transition step (b4) → drying step (b1) → transition step (b3) are performed in sequence and the above four steps are set as one cycle. In addition, step (B) ＊ This is a step in which the wetting step (b2) of step (B) is replaced by step (A). Specifically, step (B) ＊ The process is to sequentially carry out the steps (A) [chloride adhesion step (a1) → holding step (a2)] → transition step (b4) → drying step (b1) → transition step (b3). Then, the delayed fracture characteristics are evaluated using the following corrosion test cycles (I) to (IV). It should be noted that the symbol "-" in Table 2-1 and Table 2-2 indicates that the step was not carried out.

[0123] Cycle (I): Repeat step (B) ＊ → Cycle of process (B)

[0124] Cycle (II): Repeat step (B) ＊ →Process (B) →Process (B) →Process (B) cycle

[0125] Cycle (III): Repeat the cycle of cycle (I) → cycle (II), that is, repeat the step (B) ＊ →Process (B) →Process (B) ＊ →Process (B) →Process (B) →Process (B) cycle

[0126] Cycle (IV): Repeat only step (B) ＊ Cycle

[0127] (1) Evaluation of delayed fracture characteristics

[0128] The delayed fracture characteristics are evaluated by the number of days until cracks appear on the test piece. Specifically, during the implementation of the above-mentioned corrosion test, the 180° bent portion of the test piece is visually observed once a day for the presence or absence of cracks, and the number of days until cracks appear (the number of days until cracks appear) is investigated, with a maximum of 63 days. Here, for the determination of crack generation, when the newly generated cracks from the surface state of the processed portion before the corrosion test are greater than 1 mm, it is determined that cracks have occurred. It should be noted that the corrosion test for evaluating the delayed fracture characteristics is carried out on 3 samples for each embodiment (inventive example, comparative example), and the number of days until cracks appear in more than 2 samples is regarded as the number of days until cracks appear.

[0129] The evaluation results of the delayed fracture characteristics are shown in Table 3. In addition, when cracks were finally generated in only one sample, this was recorded in Table 3.

[0130] The same test pieces (test pieces for delayed fracture evaluation) as the test pieces used in the corrosion test of this embodiment were provided for exposure tests conducted by the inventors of the present invention in Okinawa Prefecture. As a result, among the test pieces, the test piece with a curvature radius of 4 mmR produced cracks on the 24th day, and the test piece with a curvature radius of 5 mmR did not produce cracks during the exposure period of 1 year. In addition, among the test pieces, the test piece with a curvature radius of 4 mmR produced cracks on the 210th day, and the test piece with a curvature radius of 5 mmR did not produce cracks during the exposure period of 1 year. The average temperature at this time was 26°C during the period when the test piece with a curvature radius of 4 mmR produced cracks, and was 20°C during the exposure period of the test piece with a curvature radius of 5 mmR, i.e., 1 year. According to the results, in the above-mentioned corrosion test, the case where cracks occurred in more than two samples in the test piece with a curvature radius of 4 mmR and no cracks occurred in one sample in the test piece with a curvature radius of 5 mmR was evaluated as an evaluation result of the delayed fracture characteristics of "0" (excellent evaluation accuracy of the delayed fracture characteristics), and the other cases were evaluated as "×" (poor evaluation accuracy of the delayed fracture characteristics).

[0131] (2) Evaluation of suitability for corrosive environments

[0132] In the present invention, in order to evaluate the delayed fracture characteristics under the actual use environment (actual corrosion environment), it is important to be able to simulate the actual use environment (corrosion environment). Therefore, whether the corrosion test can simulate the actual corrosion environment (suitability of the corrosion environment) is evaluated according to the following standards based on the ratio of the corrosion amount of the cold-rolled steel sheet to the corrosion amount of the zinc block using a cold-rolled steel sheet and a zinc block. In this evaluation test, a commercially available cold-rolled steel sheet with few impurities (SPCE of the JIS standard) is used as the cold-rolled steel sheet, and a zinc sheet with a purity of 99% is used as the zinc block. It should be noted that the standard corrosion amount ratio is based on the results of the corrosion amount ratio of steel to zinc obtained by exposure tests in real environments such as those described in non-patent document 3. The deviation of the ratio of the corrosion amount of the cold-rolled steel sheet to the corrosion amount of the zinc block in the corrosion test from the standard corrosion amount ratio means that the corrosion test cannot simulate the actual corrosion environment (deviates from the actual corrosion environment). That is, if the ratio of the corrosion amount of the cold-rolled steel sheet to the corrosion amount of the zinc block in the corrosion test deviates from the standard corrosion amount ratio, the cold-rolled steel sheet (metal material) is evaluated under a corrosion condition different from the actual corrosion environment, so it is believed that the generation condition of hydrogen generated by corrosion and causing delayed fracture is also greatly different from the actual corrosion environment. Therefore, it is necessary for the corrosion test to simulate the actual corrosion environment.

[0133] The amount of corrosion of the cold-rolled steel sheet is calculated by dividing the mass difference before and after the corrosion test (before and after corrosion) by the corrosion area. The mass of the cold-rolled steel sheet after the corrosion test is measured after removing the corrosion products by immersing the cold-rolled steel sheet after the corrosion test in a 5% HCl solution. The amount of corrosion of the zinc block is also calculated by dividing the mass difference before and after the corrosion test by the corrosion area. The mass of the zinc block after the corrosion test is measured after removing the corrosion products by immersing the zinc block after the corrosion test in ammonium dichromate for 15 minutes. Then, the [corrosion amount ratio of cold-rolled steel sheet to zinc block] in the corrosion test is calculated by the following formula, and the suitability of the corrosion environment is evaluated according to the following evaluation criteria.

[0134] [Corrosion amount ratio of cold-rolled steel sheet and zinc block] = [Corrosion amount of cold-rolled steel sheet] / [Corrosion amount of zinc block]

[0135] <Evaluation Criteria>

[0136] ○(Suitable corrosion environment): 10≤[corrosion ratio of cold-rolled steel sheet and zinc block]≤100

[0137] △(Appropriate corrosion environment): 3≤[corrosion ratio of cold-rolled steel sheet to zinc block]<10 or 100<[corrosion ratio of cold-rolled steel sheet to zinc block]≤500

[0138] × (Inappropriate corrosive environment): [Corrosion ratio of cold-rolled steel sheet to zinc block] < 3 or

[0139] 500<[Corrosion ratio of cold-rolled steel sheet and zinc block]

[0140] According to this evaluation standard, "0" or "△" was evaluated as being able to simulate the actual use environment (corrosive environment), and "×" was evaluated as being unable to simulate the actual use environment.

[0141] Then, the cases where (1) the delayed fracture characteristics were evaluated as "0" and (2) the suitability for the corrosive environment was evaluated as "0" or "△" were evaluated as a comprehensive evaluation of "0" (the delayed fracture characteristics caused by hydrogen intruding into the interior of the metal material accompanying atmospheric corrosion can be evaluated with high accuracy, and the delayed fracture characteristics in the actual use environment can be simulated), and the other cases were evaluated as "×" (the evaluation accuracy of the above-mentioned delayed fracture characteristics was poor and / or the delayed fracture characteristics in the actual use environment could not be simulated).

[0142] As shown in Tables 2-1, 2-2, and 3, the evaluation method of the present invention can accurately evaluate the delayed fracture characteristics caused by hydrogen that penetrates into the interior of a metal material along with atmospheric corrosion, and can simulate the delayed fracture characteristics in an actual use environment.

[0143]

[0144]

[0145] [Table 3]

[0146]

[0147] *1The numbers in brackets are the days it took for cracks to appear in “one sample”

Claims

1. A method for evaluating delayed fracture characteristics of a metal material, comprising performing at least one of the following steps (A) and (B), Step (A): a step comprising a chloride attaching step (a1) of attaching chloride to a metal material in an atmosphere having a relative humidity Ha1 of 80% or more and a temperature Ta1 of 60° C. or less, Step (B): a step of performing a cycle comprising the following drying step (b1), the following wetting step (b2), the following transition step (b3) and the following transition step (b4) at least once in an atmosphere at a temperature Tb1 within a certain range and not more than 60°C, Drying step (b1): a step of drying the metal material by maintaining it in an atmosphere of relative humidity Hb1 of 45% or less for 1.0 to 5.0 hours, Wetting step (b2): a step of wetting the metal material by maintaining it in an atmosphere of relative humidity Hb2 of 80% or more for 1.0 to 5.0 hours, Transition step (b3): ​​a step of transitioning from the atmosphere of the relative humidity Hb1 to the atmosphere of the relative humidity Hb2 in 1.0 hour to 5.0 hours, Transition step (b4): a step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 in 1.0 hour to 5.0 hours.

2. The method for evaluating delayed fracture characteristics of a metal material according to claim 1, wherein: The step (A) further comprises, after the chloride attaching step (a1), a holding step (a2) of holding the mixture in the atmosphere at the relative humidity Ha1 and the temperature Ta1.

3. The method for evaluating delayed fracture characteristics of a metal material according to claim 2, wherein: The holding time of the holding step (a2) is set so that the process time of the step (A) is equal to the process time of the wetting step (b2).

4. The method for evaluating delayed fracture characteristics of a metal material according to any one of claims 1 to 3, wherein: The metal material has a coating layer on the surface.

5. A method for selecting a metal material, comprising the following steps: An evaluation step of evaluating the delayed fracture characteristics of the metal material using the delayed fracture characteristics evaluation method of the metal material according to any one of claims 1 to 4; and A selection step is to select a metal material according to the evaluation result obtained in the evaluation step.

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

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