JP6287917B2 - Method for evaluating delayed fracture characteristics of metal material and metal material - Google Patents

Description

  The present invention relates to a method for evaluating characteristics (existence and extent of occurrence of delayed fracture) of a metal material used in an atmospheric corrosion environment with respect to delayed fracture caused by hydrogen entering the interior of the metal material due to corrosion. Is.

In recent years, from the viewpoint of reducing the weight of automobile structural members, efforts have been made to reduce the plate thickness by increasing the strength of steel plates to be used. With the increase in strength of such steel sheets, concerns about delayed fracture that has not been a problem with conventional automotive parts have emerged.
Delayed fracture is a phenomenon in which high-strength steel parts suddenly break brittlely with little plastic deformation in appearance when a certain amount of time has passed under the condition of static load stress. In a broad sense, liquid metal contact cracking and stress corrosion cracking are also included (Non-patent Document 1), but the problem with automotive parts is that of hydrogen embrittlement caused by hydrogen entering the steel due to corrosion. It is delayed destruction. It is known that there are three factors that cause delayed fracture: material (strength), processing (strain / stress), and hydrogen. Here, as the cause of the hydrogen intrusion into the metal material, the invasion from the solution / solvent in contact with the metal material or the hydrogen intrusion generated due to the corrosion of the metal material in the environment of use can be considered. .

Conventionally, in the field of thick plates such as line pipes with a large amount of hydrogen intrusion from solutions and solvents, and high strength steel bolts with a tensile strength of 1200 MPa or more (Non-Patent Document 2), research on delayed fracture Has been widely used, and a method for evaluating the occurrence of delayed fracture has been standardized.
In recent years, various proposals have also been made regarding delayed fracture evaluation methods in the field of thin plates, which have been strengthened. For example, Patent Document 1 discloses that a critical diffusible hydrogen amount is being measured in a method of evaluating delayed fracture characteristics of a steel material by causing the steel material to contain diffusible hydrogen by cathodic charging and measuring the critical diffusible hydrogen amount. In order to prevent hydrogen from being released from the steel material, a method of galvanizing the steel material has been proposed.

However, since the technique described in Patent Document 1 performs an accelerated test in which hydrogen is forced to enter steel by cathodic charging, under the conditions different from the actual use environment, Although it is possible to give a superiority or inferiority to the occurrence of delayed fracture depending on the type, it is difficult to estimate whether delayed fracture occurs due to hydrogen intrusion due to corrosion in the actual use environment.
In other words, atmospheric corrosion is corrosion caused by adhesion of chlorides to metal materials and repeated drying and wetting. In order to evaluate whether or not delayed destruction occurs due to hydrogen intrusion accompanying such atmospheric corrosion. It is important to evaluate delayed fracture in a corrosive environment simulating atmospheric corrosion.

Conventionally, for example, the following proposals have been made regarding a corrosion test method simulating atmospheric corrosion.
Patent Document 2 discloses a process (A) in which a salt content containing chloride ions is adhered to the surface of a test object once every 7 days to once a day, and a subsequent process (B), that is, a test object. A process in which the temperature and relative humidity are set in steps on the body and the drying process is performed first, and then the wetting process is performed as one cycle, and this cycle is performed a plurality of times (B) The atmospheric corrosion test method for metal materials for home appliances is repeatedly performed.

Patent Document 3 discloses a method for evaluating the corrosion resistance of a steel material or the like by repeating the process consisting of the process (A) and the process (B) a plurality of times. In the process (A), NaCl, sulfate ion, MgCl _{2 is used.} A solution having a pH of 3.0 to 5.0, a total concentration of chloride of 0.1 to 2.0% by weight, and a NaCl concentration of 3 times or less of the MgCl ₂ concentration is applied to the steel surface for a treatment time of 1 The process (B) consists of a drying process for 3 to 12 hours at a relative humidity of 10 to 50% and a wetting process for 2 to 8 hours at a relative humidity of 80 to 98%. The drying process and the wetting process in which the transition time from the wetting process to the drying process included in the drying process time is set to 20 to 60 minutes as one cycle are set to ˜60 ° C. and the drying process temperature ≧ the wetting process temperature. Resistant to steel, etc. once or multiple times Sexual evaluation method is disclosed.

Patent Document 4 includes (A) a step of attaching a salt containing chloride ions to the surface of a metal material, and a drying step and a wet step set by changing the temperature and relative humidity for the metal material. This is a method of evaluating the corrosion resistance of a metal material by performing the process consisting of the process (B) at least once, repeating this cycle at least once, and in the process (A), the process adheres to the metal material The average particle size of the salt water was 1 to 300 μm, the amount of salt adhered was 0.1 to 10000 mg / m ² , and the required time was within 10 minutes. In step (B), the dew point variation between the drying step and the wetting step was ± 5 A method for evaluating the corrosion resistance of a metal material within a temperature range of ° C. is disclosed.

JP-A-2005-134152 JP 2012-26945 A JP-A-2005-181102 JP 2011-169918 A

Matsuyama Junsaku, delayed destruction, Nikkan Kogyo Shimbun, 1989 Omura et al., Corrosion and corrosion symposium, Vol.170, p.47-54, 2010 Corrosiveness of various atmospheric test sites as measured by specimens of steel and zinc, in metal corrosion in the Atmosphere, STP 435.pp360-391, American Society for Testing and Materials, Philadelphia, 1968

The techniques described in Patent Documents 2 to 4 are all intended to reproduce the corrosion behavior in an atmospheric corrosion environment in a laboratory, but are they suitable for evaluating delayed fracture characteristics? I didn't know well. For this reason, as a result of the evaluation of delayed fracture characteristics in various corrosive environments, the present inventors cannot accurately evaluate delayed fracture characteristics when the corrosion test cycles as shown in Patent Documents 2 to 4 are used. I found out.
Accordingly, an object of the present invention is to accurately and easily determine the characteristics (existence and degree of occurrence of delayed fracture) of metal materials used in an atmospheric corrosive environment with respect to delayed fracture caused by hydrogen entering the interior of the metal material. It is in providing the evaluation method which can be evaluated to.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have obtained the following knowledge regarding test conditions for evaluating delayed fracture characteristics in a corrosive environment simulating atmospheric corrosion.
(I) By increasing the corrosion rate of the metal material, the amount of hydrogen generated along with the corrosion increases, so the amount of hydrogen intrusion into the metal increases, but increasing the corrosion rate of the metal material It will deviate from the corrosion behavior of the real environment. For this reason, it is necessary to increase the hydrogen penetration amount without increasing the corrosion rate of the metal material, and thus, it is possible to appropriately evaluate the delayed fracture caused by hydrogen that penetrates into the metal due to corrosion.

(Ii) Relative humidity at the time of the process transition between the drying step and the wetting step in the step of performing at least one cycle of applying the drying step and the wetting step by changing the relative humidity with respect to the metal material to which the chloride is adhered. Many hydrogens penetrate into the metal material when changing the temperature, but by providing a step of maintaining the humidity at 50-60% RH during the transition period when changing the relative humidity between the drying step and the wetting step, Without increasing the corrosion rate of the metal material, a large amount of hydrogen can enter the metal material, and it is possible to more accurately determine whether or not the occurrence of delayed fracture in a temperature environment where delayed fracture characteristics should be evaluated. Furthermore, the delayed fracture characteristics can be more appropriately evaluated by setting the humidity change rate when changing the relative humidity during the process transition between the drying process and the wetting process to less than 30% RH / hr.

(Iii) Delayed fracture characteristics change according to the temperature of the environment, and the amount of hydrogen intrusion from the environment also changes greatly. Therefore, in order to appropriately evaluate the delayed fracture characteristics of metal materials in consideration of the application site of the member, It is necessary to evaluate delayed fracture characteristics at a constant temperature. In addition, if the temperature change in the environment is sufficiently small, the amount of hydrogen intrusion from the environment can be evaluated with a sufficiently small fluctuation range with respect to the amount of hydrogen intrusion at the target environment temperature, and the delayed fracture characteristics are accurately evaluated. can do.
The present invention has been made based on the above findings, and the gist thereof is as follows.

[1] A method for evaluating delayed fracture characteristics of a metal material by performing the process consisting of the following process (A) and process (B) at least once, and in the process (A), a chloride adhered to the metal material was a 1~10000mg / ^{m 2} the amount of components mainly, in step (B), with the temperature variations ± 5 ℃ below, changing the relative humidity between the drying step (b1) and wet process (b2) During the transition period (however, “the transition period from the drying step (b1) to the wetting step (b2)” and / or “the transition period from the wetting step (b2) to the drying step (b1)”), the humidity 50 A method for evaluating delayed fracture characteristics of a metal material, characterized by providing a humidity holding step (b3) for holding at -60% RH for 0.5 hours or more.
Step (A): A step of attaching a component mainly composed of chloride to the metal material (including the case where the component consists only of chloride) Step (B): Metal material that has undergone step (A) On the other hand, a process of performing at least one cycle is defined by providing a drying process (b1) for drying the metal material surface by changing the relative humidity and a wetting process (b2) for moistening the metal material surface.

[2] In the evaluation method of the above [1], in the step (B), the drying step (b1), the wetting step (b2) and the humidity maintaining step (b3) are performed under the following conditions. Evaluation method of delayed fracture characteristics.
Drying step (b1) Temperature: 5 to 60 ° C., relative humidity: 40% or less, holding time: 1 to 12 hours Wetting step (b2) Temperature: 5 to 60 ° C., relative humidity: 80 to 98%, holding time : 1-12 hours ・ Humidity holding step (b3) Temperature: 5-60 ° C., relative humidity: 50-60%, holding time: 0.5-4 hours [3] Evaluation method of [1] or [2] above The method for evaluating delayed fracture characteristics of a metal material, wherein the step (A) is performed for 10 minutes or less.

[4] In the evaluation method according to any one of [1] to [3] above, in step (B), a transition period in which the relative humidity is changed between the drying step (b1) and the wetting step (b2) (however, maintaining the humidity) A method for evaluating delayed fracture characteristics of a metal material, characterized in that the humidity change rate during step (b3) is less than 30% RH / hr.
[5] In the evaluation method according to any one of [1] to [4] above, in step (B), the drying step (b1), the wetting step (b2), the humidity maintaining step (b3) and each step are followed. A method for evaluating delayed fracture characteristics of a metal material, wherein one cycle including a transition time for transition to a process is within 24 hours.
[6] A metal material evaluated and selected by the method for evaluating delayed fracture characteristics according to any one of [1] to [5] above.
[7] An automobile member using a metal material evaluated and selected by the method for evaluating delayed fracture characteristics according to any one of [1] to [5] above.

  According to the present invention, the characteristics (presence and absence of delayed fracture occurrence and the extent of delayed fracture) of a metal material used in an atmospheric corrosive environment with respect to delayed fracture caused by hydrogen entering the interior of the metal material with corrosion are accurately and easily determined. Can be evaluated. For this reason, it is possible to obtain information necessary for determining whether or not delayed fracture occurs due to the amount of hydrogen intrusion due to corrosion in an actual use environment.

Explanatory drawing which shows the process of the corrosion acceleration test performed by this invention method Drawing which shows typically the test piece for delayed fracture evaluation used in the Example

FIG. 1 shows a process of a corrosion acceleration test performed by the method for evaluating delayed fracture characteristics according to the present invention. As shown in FIG. 1, in the present invention, in order to simulate an actual atmospheric corrosion environment, the process consisting of the following process (A) and process (B) is performed one or more times (that is, repeated once or a plurality of times).
Step (A): A component mainly composed of chloride (including the case where the component is composed only of chloride) is attached to the metal material as a test object.
Step (B): One cycle of applying a drying step (b1) to dry the surface of the metal material and a wetting step (b2) to wet the metal material through the step (A) by changing the relative humidity. This cycle is performed at least once and the transition period when changing the relative humidity between the drying step (b1) and the wetting step (b2) (however, “from the drying step (b1) to the wetting step (b2)” During the “transition period” and / or “transition period from the wetting step (b2) to the drying step (b1)”), a humidity holding step (b3) with a humidity of 50 to 60% RH is provided.

Hereinafter, the detail of a process (A) and a process (B) is demonstrated.
・ Process (A)
In this step (A), the component mainly composed of chloride attached to the metal material as a test object includes one or more of NaCl, MgCl ₂ , and CaCl ₂ that are chlorides, and the total of them is all The thing which is more than 50 mass% of an ingredient is pointed out. Therefore, including the case where the component is composed only of chloride _(NaCl, MgCl 2, 1 or more CaCl _2). Examples of components other than chlorides include, but are not limited to, sulfides and nitrate compounds. In consideration of the actual corrosive environment, it is preferable that a component mainly composed of NaCl (a component whose NaCl is more than 50 mass% of all components) is attached to the metal material.

The amount of the component mainly composed of chloride to be attached to the metal material (the amount of solid content not containing a solvent such as water) is 1 to 10,000 mg / m ² . This adhesion amount corresponds to the adhesion amount assumed in an actual corrosive environment. A corrosive environment with an adhesion amount of less than 1 mg / m ² is not suitable for evaluation because corrosion hardly proceeds. For the purpose of simulating an actual corrosive environment and accelerating the corrosion, a preferable adhesion amount is about 100 to 4000 mg / m ² .
The method for attaching the chloride-based component to the surface of the metal material is not particularly limited, but a method of attaching a solution containing a chloride-based component to the surface of the metal material is usually employed. For example, a method of immersing a metal material in a solution containing a component mainly composed of chloride (usually an aqueous solution such as salt water), a method of spray-coating the solution onto the metal material, and the like can be mentioned.

It is preferable that the implementation time of the step of attaching the component mainly composed of chloride to the metal material is within 10 minutes. Implementation time of the step of attaching the chloride-based component to the surface of the metal material, for example, the time for immersing the metal material in a solution containing the chloride-based component, or spraying the solution on the metal material When the time exceeds 10 minutes, the corrosion of the metal material by the solution may proceed, and the correlation with the corrosion in the actual corrosive environment may be lowered.
In addition, the adhesion amount of the component mainly composed of chloride can be calculated by dividing the mass difference between the test object (metal material) before and after the step (A) by the area of the test object. When changing the amount of adhesion, for example, it can be controlled by changing the concentration of a solution containing a chloride-based component or changing the amount of adhesion of the solution to the metal material in the case of a spray coating method. it can.

・ Process (B)
In this step (B), the drying step (b1) for drying the metal material surface by changing the relative humidity and the wetting step (b2) for wetting are performed as one cycle. This is to simulate.
In the step (B), the metal material that has undergone the step (A) is dried in the drying step (b1) and then the metal material surface is wetted in the wetting step (b2). The cycle composed of the drying step (b1) and the wetting step (b2) is repeated once or more.

  In order to investigate in detail about the influence of the temperature of the environment on the amount of hydrogen intrusion that causes delayed fracture, the present inventors conducted an exposure test in an area having a different temperature environment. As a result, it was found that the amount of hydrogen penetrating into the metal material due to corrosion varies greatly with temperature, and the delayed fracture characteristics change. Therefore, in order to appropriately evaluate the delayed fracture characteristics of the metal material in consideration of the application site of the member, not only the adhesion amount of components mainly composed of chloride in the step (A) but also the step (B) It is necessary to set the temperature at the appropriate temperature, and it is necessary to evaluate the delayed fracture characteristics at a constant temperature. Here, the constant temperature may be ± 5 ° C. or less.

In addition, the present inventors have investigated in detail the influence of environmental temperature on the amount of hydrogen intrusion that causes delayed fracture. As a result, if the environmental temperature change is ± 5 ° C or less, the amount of hydrogen intrusion from the environment can be evaluated with a fluctuation range of 30% or less with respect to the amount of hydrogen intrusion at the target environment temperature. It was found that the characteristics can be accurately evaluated. The more preferable temperature fluctuation range is ± 2 ° C. or less, and the hydrogen penetration amount in this case can be evaluated with a fluctuation range of 15% or less with respect to the hydrogen penetration amount at the temperature of the target environment. I understood.
For the above reasons, in the step (B), the temperature fluctuation is set to ± 5 ° C. or less, and the cycle of applying the drying step (b1) and the wetting step (b2) in a substantially constant temperature environment is performed at least once.

  Furthermore, the present inventors have found the fact that the amount of hydrogen intrusion into a metal material, particularly a steel material, increases at the timing when humidity changes. As a result of further investigation based on such knowledge, the process transitions from the drying process (b1) to the wetting process (b2) (transition period), and the process proceeds from the wetting process (b2) to the drying process (b1). It has been clarified that a large amount of hydrogen enters the metal material during the process transition (transition period). In addition, among them, in the region where the relative humidity is 50 to 60% RH, the corrosion of the metal material is hardly caused, but it has been clarified that the hydrogen penetration amount into the metal material is the largest.

  Although the reason why the amount of hydrogen intrusion into the metal material increases at the timing when the humidity changes as described above is not necessarily clear, it can be considered as follows. First, when shifting from the drying step (b1) to the wetting step (b2), moisture begins to be absorbed due to deliquescence of chloride present on the surface of the metal material, and corrosion of the metal material is started. It is known that the corrosion product existing on the surface of the metal material at this time changes, and it is considered that hydrogen is generated with the change of the corrosion product. In addition, when shifting from the wetting step (b2) to the drying step (b1), it is considered that the water becomes a concentrated solution containing a large amount of chloride and iron ions eluted by corrosion, and the pH of the solution is lowered. That is, it is considered that a large amount of hydrogen ions are contained in the solution in the drying process, and hydrogen easily enters the metal material.

  As described above, hydrogen intrusion into the metal material occurs when a part of the hydrogen generated by the corrosion of the metal material is taken into the metal material. Therefore, the amount of hydrogen intrusion can be increased by increasing the corrosion rate of the metal material. can do. However, increasing the corrosion rate of the metal material deviates from the actual corrosion behavior. Here, as a method of knowing the actual corrosion behavior, it can be estimated by using the corrosion rate ratio of iron and zinc. In a general atmospheric corrosion environment, the corrosion rate ratio of iron and zinc is 10 to 100. It has been reported (Non-Patent Document 3). For example, in the case of a salt spray test, which is a test method significantly deviating from the atmospheric corrosive environment, the corrosion rate ratio of iron and zinc is about 0.3 from the results of the present inventors. From this, it is considered that a corrosion test in which the corrosion rate ratio of iron and zinc is 10 to 100 is suitable as a method for reproducing the corrosion result in the actual environment.

  In other words, if the generated hydrogen can be efficiently drawn into the metal material without increasing the corrosion rate of the metal material, the corrosion behavior in the real environment can be simulated and the metal material that can occur in the real environment can be simulated. Since a large amount of hydrogen can enter, delayed fracture can be accurately evaluated. As described above, it has been clarified that the region where the relative humidity is 50 to 60% RH is a humidity region in which the metal material hardly corrodes, but the amount of hydrogen entering the metal material increases. . Therefore, in the present invention, a transition period when the relative humidity is changed between the drying step (b1) and the wetting step (b2) (provided that “the transition period from the drying step (b1) to the wetting step (b2)” and / or Alternatively, a humidity maintaining step (b3) with a humidity of 50 to 60% RH is provided during the “transition period from the wetting step (b2) to the drying step (b1)”.

  The time for the humidity maintaining step (b3), that is, the time for maintaining the humidity at 50 to 60% RH is 0.5 hour or more. If the humidity maintaining step (b3) is less than 0.5 hours, hydrogen penetration is not efficiently performed, and thus a sufficient effect cannot be obtained. On the other hand, as a result of examining the change over time in the amount of hydrogen intrusion being held by the present inventors, the amount of hydrogen intrusion tends to gradually decrease with time. In particular, if the humidity holding step (b3) exceeds 4 hours, the efficiency of the hydrogen penetration amount is reduced compared to the corrosion amount, so that the time for evaluating delayed fracture becomes longer. For this reason, the humidity holding step (b3) is preferably 4 hours or less.

  The transition period between the drying step (b1) and the wetting step (b2) in which the humidity holding step (b3) is performed is “the transition period from the drying step (b1) to the wetting step (b2)”, “the wetting step ( Either “a transition period from b2) to drying step (b1)” or both may be used. Therefore, when step (B) is repeated two or more times after step (A), which is a chloride application step, that is, when a cycle consisting of drying step (b1) and wetting step (b2) is repeated two or more times, The humidity holding step (b3) can be performed during the transition period from the wetting step (b2) to the drying step (b1).

In addition, based on the knowledge that a lot of hydrogen penetrates into the metal material when changing the relative humidity during the process transition between the drying process and the wet process, as a result of investigating the relationship between the humidity transition speed and the hydrogen penetration amount, It was found that if the humidity change rate at the time of transition to each of the above steps is less than 30% RH / hr, the delayed fracture characteristics can be more appropriately evaluated. Therefore, in the step (B), the humidity change rate during the transition period (excluding the period of the humidity holding step (b3)) in which the relative humidity is changed between the drying step (b1) and the wetting step (b2) is 30. It is preferable to be less than% RH / hr.
The lower limit of the humidity change rate is not particularly specified, but if the transition time between the drying step (b1) and the wetting step (b2) is too long, it takes a long time to evaluate delayed fracture characteristics. % RH / hr or more is preferable.

  The process transition between the drying process (b1) and the wetting process (b2) for controlling the humidity change rate as described above refers to the process transition when shifting from the drying process (b1) to the wetting process (b2). And the time of the process transition when shifting from the wetting process (b2) to the drying process (b1). Therefore, when step (B) is repeated two or more times after step (A), which is a chloride application step, that is, when a cycle consisting of drying step (b1) and wetting step (b2) is repeated two or more times, The humidity change rate when changing the relative humidity is also controlled as described above at the time of the process transition from the wetting process (b2) to the drying process (b1).

  The temperature in the drying step (b1) and the wetting step (b2) is preferably 5 to 60 ° C. Temperatures exceeding 60 ° C. are not appropriate because not only will the evaluation be far from the actual corrosive environment, but the corrosion mechanism may change. On the other hand, there is no problem with the technical idea of the present invention for evaluation at temperatures below 5 ° C., but it is difficult to control humidity at temperatures below 5 ° C. in current corrosion test tanks that carry out corrosion tests. In order to significantly reduce the corrosion rate of the steel, it is better to avoid it if possible from the viewpoint that the evaluation days become long.

The relative humidity in the drying step (b1) is preferably 40% RH or less. If the relative humidity exceeds 40% RH, it is necessary to keep the surface of the metal material for a long time, which is not preferable because the evaluation period becomes longer. In addition, when the component attached to the surface of the metal material includes a substance that exhibits deliquescence at a lower humidity such as magnesium chloride or calcium chloride, it is preferable to lower the humidity setting.
The relative humidity in the wetting step (b2) is preferably 80 to 98% RH. When the relative humidity in the wetting process is less than 80% RH, the influence of wetting becomes insufficient, and it takes time to evaluate. On the other hand, even if the relative humidity is over 98% RH, there is no problem in the technical idea of the present invention. However, when evaluating the processed specimen, the amount of water absorbed by the adhered components increases, and a thick liquid film is formed. Since adhering components may be washed away to form, it should be avoided if possible.

  The holding time of the drying step (b1) and the wetting step (b2) is preferably 1 to 12 hours. If the holding time is less than 1 hour, the surface of the metal material may not be sufficiently dried or moistened, and the humidity distribution in the corrosion test tank may increase, which may lead to variations in test results. . Further, if the holding time exceeds 12 hours, it takes a long time to evaluate the delayed fracture characteristics.

As described above, the drying step (b1) and the wetting step (b2) are aimed at simulating a change in humidity day and night. Therefore, when one cycle including the drying step (b1), the wetting step (b2), the humidity holding step (b3) and the transition time for shifting from each step to the next step exceeds 24 hours, it becomes slower than the actual corrosion. Therefore, it takes a long time to evaluate delayed fracture characteristics. From such a viewpoint, it is preferable that one cycle including the drying step (b1), the wetting step (b2), the humidity maintaining step (b3), and the transition time for shifting from each step to the next step is 24 hours or less. .
The process consisting of the above-mentioned process (A) and process (B) may repeat process (A) and process (B), or may repeat process (B) following process (A) several times. Good.

In the present invention, in order to specifically evaluate the delayed fracture characteristics, it is necessary to apply stress to the metal material by processing the metal material. Examples of the processing method include bending, overhanging, and tensile processing. In addition, to evaluate delayed fracture characteristics, it is necessary to apply stress to the metal material, such as a method of fixing with a stressed shape using bolts, a method of evaluating using residual stress remaining after processing, etc. Is mentioned.
The metal material to be evaluated in the present invention is usually a steel material such as a steel plate, but is not limited thereto, and may be a metal material such as Ti or Al.
Since the delayed fracture property evaluation method of the present invention can accurately evaluate the delayed fracture property of a metal material, the selected metal material (particularly steel such as a steel plate) has excellent delayed fracture property. Various members (for example, automobile members) having excellent delayed fracture characteristics can be stably manufactured by using this metal material.

A commercial 1.6mm-thick 1470 MPa grade steel cold-rolled steel sheet was used as an object, and delayed fracture characteristics were evaluated according to invention examples and comparative examples.
The target cold-rolled steel sheet was sheared to a width of 35 mm and a length of 100 mm, and subjected to grinding until the width became 30 mm in order to remove residual stress at the time of shearing, and a test piece (steel sheet) was produced. This test piece is immersed in toluene and subjected to ultrasonic cleaning for 5 minutes, and then bent 180 °. In this state, the shape of the test piece is fixed by restraining it with bolts and nuts, and for delayed fracture evaluation as shown in FIG. A specimen was obtained. This delayed fracture evaluation test piece includes (i) a test piece (steel plate) bent by 180 ° with a radius of curvature of 4 mmR and an inner interval of the bent test pieces set to 8 mm, and (ii) a test piece (steel plate). Bending was performed 180 ° with a radius of curvature of 5 mmR, and a bending test piece having an inner interval of 10 mm was prepared.

  The corrosion acceleration test consisting of the following step (A) and step (B) was performed on the above test pieces for delayed fracture evaluation. Details of the test conditions are shown in Tables 1 to 3. In the test, “Step (A)” → “Step (B)” was repeatedly carried out for a maximum of 60 days. The process (B) is “drying process (b1), transition from drying process (b1) to wetting process (b2), wetting process (b2), transition from wetting process (b2) to drying process (b1)” ("Moisture holding step (b3) is provided during" transition period from drying step (b1) to wetting step (b2) "and / or" transition period from wetting step (b2) to drying step (b1) ") The process (B) was repeated once or a plurality of times (2 to 4 times), and then the “process (A)” which is a chloride application process was performed. “Adhesion frequency (number of steps of process (B) / times)” in Table 1 is the number of cycles of process (B) performed for each process (A) (= “process (b1) → process (b2)”. Cycle number). “Adhesion frequency (day / time)” is the frequency (day) at which the step (A) is performed once. “1 (day / time)” means “1 (day / time)” once, “2 (day / time)”. “Times” means once every two days.

・ Process (A)
Chloride was deposited on the surface of a delayed fracture evaluation test piece (hereinafter simply referred to as “test piece” for convenience of explanation) in various amounts. Adhesion methods include mixing chloride (in some examples, chloride + sodium sulfate) with pure water to make an aqueous solution, (i) immersing the test piece in the aqueous solution, and (ii) spraying the aqueous solution onto the test piece. Either method of coating was used. In the dipping method (i), the dipping time (time for carrying out the step (A)) is 5 minutes or 15 minutes. However, in the spray coating method (ii), since the instantaneous spray coating is performed, The application time (time for carrying out the step (A)) is several seconds. About the amount of adhesion, the amount of aqueous solution adhering was computed by dividing the test piece mass difference before and after making chloride adhere by the surface area of the specimen, and the amount of solid content was computed from the component concentration of the aqueous solution.

・ Process (B)
A cycle in which a drying step (b1) for drying the surface of the test piece by changing the relative humidity and a wetting step (b2) for moistening are performed on the test piece (test piece to which an aqueous solution is attached) that has undergone the step (A). Was repeated. In most test examples (invention examples), in the step (B), “the transition period from the drying step (b1) to the wetting step (b2)”, “from the wetting step (b2) to the drying step (b1)”. The humidity holding step (b3) was performed both in the “transition period”, but in some of the test examples (invention examples), the humidity holding step (b3) was performed on either side. In the step (B), the temperature fluctuation (Table 2 and Table 3) has a predetermined width with respect to the set temperature (Tables 2 and 3), the drying step (b1), the wetting step (b2), and the humidity maintaining step (b3). Each humidity and holding time were changed. In some test examples, a heating device was arranged in the test tank, and the test was performed under conditions that caused temperature fluctuations in the tank by changing the heating output and the heating time. The temperature increase at this time was performed once per hour.

(1) Evaluation of delayed fracture characteristics While conducting the above-mentioned corrosion acceleration test, the presence or absence of cracks in the 180 ° bent portion of the test piece is visually observed once a day, and the number of days until cracks occur ( The number of crack occurrence days) was investigated. Here, regarding the determination of the occurrence of cracking, it was determined that cracking occurred when a newly generated crack was 1 mm or more from the surface state of the processed part before the corrosion test after the processing test. In addition, the corrosion acceleration test for evaluating the delayed fracture characteristics was performed for each of three samples for one example (invention example, comparative example), and the number of days in which cracks occurred in two or more samples was defined as “number of crack generation days”. .

Table 4 shows the evaluation results of the delayed fracture characteristics. When only one specimen is finally cracked, this is described in Table 4.
The same test piece (delayed fracture evaluation test piece) used in the corrosion promotion test of this example is used for an exposure test in Okinawa conducted by the present inventors and has a curvature radius of 4 mmR. Then, cracks occurred on the 24th day, and no cracks occurred in the test piece having a radius of curvature of 5 mmR even during the one year exposure period. The average temperature at this time was 26 ° C. during a period when the test piece having a curvature radius of 4 mmR was cracked, and 20 ° C. during one year which was an exposure period of the test piece having a curvature radius of 5 mmR. Based on this result, the test cycle that required a crack generation period of 2 times or more (48 days or more) was judged to be inappropriate and evaluated as “X”, and the others were evaluated as “O” (appropriate). .

(2) Evaluation of suitability of corrosive environment In the present invention, it is important to be able to simulate an actual corrosive environment in order to evaluate delayed fracture characteristics. Therefore, whether the accelerated corrosion test can simulate the actual corrosive environment (appropriate corrosive environment) is as follows according to the ratio of the amount of corrosion of the cold rolled steel sheet and the amount of zinc corroded using cold rolled steel sheet and zinc lump. Evaluation was performed according to the criteria. In this evaluation test, a cold rolled steel sheet (JIS standard SPCE) with few commercial impurities was used as a cold rolled steel sheet, and a zinc sheet with a purity of 99% was used as a zinc ingot. The reference corrosion amount ratio is based on the results obtained in an actual environment exposure test described in Non-Patent Document 3. If the ratio of the corrosion amount of the cold rolled steel sheet to the corrosion amount of the zinc in the corrosion test cycle is outside the standard corrosion amount ratio, it means that it is out of the corrosive environment of the actual environment, including the galvanized steel sheet. Since steel materials will be evaluated in different corrosion situations, it is possible that the generation situation of hydrogen that is caused by corrosion and causes delayed fracture is also greatly different, so it is necessary to simulate the actual corrosive environment.

The corrosion amount of the cold-rolled steel sheet was calculated by dividing the mass difference before and after the corrosion by the corrosion area. The mass after corrosion was measured after removing the corrosion products by immersing the corroded steel sheet in a 5% HCl solution. Similarly, the amount of corrosion of the zinc mass is calculated by dividing the mass difference before and after the corrosion by the corrosion area. The mass after corrosion is obtained by immersing the zinc mass after corrosion in ammonium dichromate for 15 minutes. Measured after removal.
[Corrosion amount ratio of cold rolled steel sheet / zinc lump] = [Corrosion amount of cold rolled steel sheet] / [Corrosion amount of zinc lump]
<Evaluation criteria>
○ (preferred corrosive environment): 5 ≦ [Corrosion ratio of cold-rolled steel sheet / zinc lump] ≦ 500
× (Inappropriate corrosion environment): [Corrosion ratio of cold-rolled steel sheet / zinc ingot] <5 or 500 <[Corrosion ratio of cold-rolled steel sheet / zinc ingot]

In Tables 1 to 4, no. 1 and No. No. 2 is a comparative example and invention example in which the presence or absence of the humidity holding step (b3) in the step (B) was evaluated. In the comparative example 1, it can be seen that the crack occurrence state is the same as the actual environment, but the crack generation period is prolonged. For this reason, the evaluation result is judged to be inappropriate. On the other hand, No. with a humidity maintaining step (b3). In Example 2 of the invention, it can be seen that cracking occurred early in a proper corrosive environment.
No. 2 and No. No. 6 is an example of an invention in which the method of depositing chloride differs, but it can be seen that the evaluation result of delayed fracture characteristics and the suitability evaluation of corrosive environment do not change depending on the method of depositing chloride.

No. Examples 3 to 8 are examples in which chlorides were deposited in different deposition amounts in the method of depositing chlorides by spray coating. Among these, No. From 4 to 7, it can be seen that the evaluation results of delayed fracture characteristics are consistent with the results in the real environment (results of the exposure test in Okinawa Prefecture), although there is a change in the days of delayed fracture occurrence. On the other hand, no. It can be seen that the comparative example 3 is not suitable for evaluation of delayed fracture characteristics because no crack was generated even in a test piece having a curvature radius of 4 mmR. Furthermore, the amount of chloride attached is too large. The comparative example No. 8 is an inappropriate corrosive environment, and further, it can be seen that the evaluation results of delayed fracture characteristics do not coincide with the results in the actual environment (results of the exposure test in Okinawa Prefecture).
No. No. 9 shows the process transition time between the drying step (b1) and the wetting step (b2). This is an example of an invention in which the humidity change rate between the drying step (b1) and the wetting step (b2) is increased compared to the case of No. 2, but the delayed fracture evaluation results are the same as in the actual environment. It can be seen that the crack generation time is prolonged compared to 2. From this, it can be seen that it is preferable to delay the humidity transition time.

No. 10 and no. Although 11 is an invention example in which the chloride species is changed, it can be seen that the evaluation results of delayed fracture characteristics and the suitability evaluation of corrosive environment are not changed and are suitable.
No. 12 and No. No. 13 shows the retention times of the drying step (b1) and the wetting step (b2) as No. 13. Although it is an invention example longer than 2, although the number of crack occurrence days increases, the evaluation result of delayed fracture characteristics and the suitability evaluation of the corrosive environment do not change, and it is understood that it is preferable.
No. No. 14 is an invention example in which the humidity maintaining step (b3) is set only during the transition period from the drying step (b1) to the wetting step (b2), No. 14 No. 15 is an invention example in which the humidity maintaining step (b3) is set only during the transition period from the wetting step (b2) to the drying step (b1). Compared with the invention example 2, the number of times the humidity holding step (b3) is reduced, the number of days of cracking is delayed, but the evaluation results of delayed fracture characteristics and the suitability evaluation of the corrosive environment are unchanged, and it is understood that it is preferable. .

No. 2, No. 16, no. 17 is an example of the invention in which the set temperature of the step (B) is changed. The lower the temperature, the longer the number of crack generation days, but the evaluation result of delayed fracture characteristics and the suitability evaluation of the corrosive environment are not changed, and it is understood that it is suitable. .
No. 2, No. 18, no. 19, no. No. 20 and No. 21 are invention examples and comparative examples in which the holding time of the humidity holding step (b3) is changed, and No. 21 in which the holding time of the humidity holding step (b3) is less than 0.5 hours. The comparative example of No. 18 is the same cracking situation as in the actual environment, but No. No humidity holding step (b3). It can be seen that the crack generation period is prolonged as in the comparative example 1. For this reason, the evaluation result is judged to be inappropriate. No. 20 and no. If the holding time of the humidity holding step (b3) is long as in the invention example 21, the crack generation period is prolonged, but the evaluation result of the delayed fracture characteristics and the suitability evaluation of the corrosive environment are not changed, which is preferable. I understand.

No. 6 and no. No. 22 is an invention example in which the temperature fluctuation in the step (B) is different, but since the temperature fluctuation is ± 5 ° C. or less, the evaluation results of the delayed fracture characteristics and the suitability evaluation of the corrosive environment are not changed, which is preferable. I understand that. On the other hand, no. No. 23 shows that the delayed fracture characteristics do not match the actual environment and is inappropriate.
No. 6 and no. 24-No. 27 is an example in which the humidity in the humidity holding step (b3) was changed. 6, no. 24, no. No. 26 shows that the evaluation results of delayed fracture characteristics and the suitability evaluation of the corrosive environment remain unchanged and are suitable. On the other hand, No. in which the humidity maintaining step (b3) is not within the scope of the present invention. 25 and no. No. 27 is the same cracking situation as in the actual environment, but No. No humidity holding step (b3). It can be seen that the crack generation period is prolonged as in the comparative example 1. For this reason, the evaluation result is judged to be inappropriate.
No. 6 and no. 28, no. No. 29 is an invention example in which the humidity in the wetting step (b2) was changed, but in any case, the evaluation results of the delayed fracture characteristics and the suitability evaluation of the corrosive environment do not change, and it can be seen that they are suitable.

Claims (5)

A method for evaluating delayed fracture characteristics of a metal material by performing the process consisting of the following process (A) and process (B) twice or more,
Step (A): A step of attaching a component mainly composed of chloride (including the case where the component is composed only of chloride) to the metal material.
Step (B): One cycle of applying a drying step (b1) to dry the surface of the metal material and a wetting step (b2) to wet the metal material through the step (A) by changing the relative humidity. And performing this cycle at least once.
In the step (A), the amount of the component mainly composed of chloride attached to the metal material is 1 to 10,000 mg / m ² ,
In the step (B), the temperature fluctuation is ± 5 ° C. or less, and the transition period when changing the relative humidity between the drying step (b1) and the wetting step (b2) (however, “wet from the drying step (b1)” Humidity holding step of holding at a humidity of 50 to 60% RH for 0.5 hours or more during the transition period to the step (b2) and / or "the transition period from the wetting step (b2) to the drying step (b1)") A method for evaluating delayed fracture characteristics of a metal material, characterized in that (b3) is provided and the drying step (b1), the wetting step (b2) and the humidity maintaining step (b3) are performed under the following conditions.
Drying step (b1) Temperature: 5 to 60 ° C., relative humidity: 40% or less, holding time: 1 to 12 hours Wetting step (b2) Temperature: 5 to 60 ° C., relative humidity: 80 to 98%, holding time : 1-12 hours ・ Humidity holding step (b3) Temperature: 5-60 ° C., relative humidity: 50-60%, holding time: 0.5-4 hours A method for evaluating delayed fracture characteristics of a metal material by performing the process consisting of the following process (A) and process (B) at least once,
Step (A): A step of attaching a component mainly composed of chloride (including the case where the component is composed only of chloride) to the metal material.
Step (B): One cycle of applying a drying step (b1) to dry the surface of the metal material and a wetting step (b2) to wet the metal material through the step (A) by changing the relative humidity. And performing this cycle at least twice .
In the step (A), the amount of the component mainly composed of chloride attached to the metal material is 1 to 10,000 mg / m ² ,
In the step (B), the temperature fluctuation is ± 5 ° C. or less, and the transition period when changing the relative humidity between the drying step (b1) and the wetting step (b2) (however, “wet from the drying step (b1)” Humidity holding step of holding at a humidity of 50 to 60% RH for 0.5 hours or more during the transition period to the step (b2) and / or "the transition period from the wetting step (b2) to the drying step (b1)") A method for evaluating delayed fracture characteristics of a metal material, characterized in that (b3) is provided and the drying step (b1), the wetting step (b2) and the humidity maintaining step (b3) are performed under the following conditions.
Drying step (b1) Temperature: 5 to 60 ° C., relative humidity: 40% or less, holding time: 1 to 12 hours Wetting step (b2) Temperature: 5 to 60 ° C., relative humidity: 80 to 98%, holding time : 1-12 hours ・ Humidity holding step (b3) Temperature: 5-60 ° C., relative humidity: 50-60%, holding time: 0.5-4 hours The method for evaluating delayed fracture characteristics of a metal material according to claim 1 or 2, wherein the step (A) is carried out within 10 minutes. In the step (B), the humidity change rate during the transition period (excluding the period of the humidity holding step (b3)) in which the relative humidity is changed between the drying step (b1) and the wetting step (b2) is 30% RH. The method for evaluating delayed fracture characteristics of a metal material according to any one of claims 1 to 3, wherein the value is less than / hr. In the step (B), one cycle including the drying step (b1), the wetting step (b2), the humidity holding step (b3) and the transition time from each step to the next step is within 24 hours. The method for evaluating delayed fracture characteristics of a metal material according to any one of claims 1 to 4 .

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