JP2017187441A - Method for evaluating delayed fracture of metal material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simple test method capable of defining a limit amount of diffusible hydrogen even in different molding methods.SOLUTION: A method for evaluating a delayed fracture of metal material includes: a test piece hydrogen introduction step of introducing hydrogen into metal material as a test piece so as to cause a delayed fracture; a test piece hydrogen amount measurement step of measuring the amount of hydrogen in the test piece; a hydrogen capture site identification step of identifying a plurality of hydrogen capture sites existing in the test piece; a delayed fracture structure identification step of identifying structures affecting a delayed fracture based on a correspondence between each hydrogen capture site and a delayed fracture in the test piece; a limit diffusible hydrogen amount acquisition step of acquiring, as the amount of limit diffusible hydrogen, the amount of diffusible hydrogen in the test piece in which breakage did not occurred until a predetermined limit time in a test period, when a constant load was applied to the test piece while making hydrogen exist in the test piece or adding hydrogen to the test piece; and a delayed-fracture-structure limit diffusible hydrogen amount acquisition step of acquiring, out of the amount of limit diffusible hydrogen, only the amount of hydrogen captured by the structures affecting the delayed fracture.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for evaluating delayed fracture of a metal material, and more particularly to a method for evaluating delayed fracture of a metal material that can define the amount of critical diffusible hydrogen even in different forming modes.

  In order to achieve both weight reduction and crash safety improvement in automobiles, the strength of automobile parts is being increased. However, when the strength of the steel plate is increased, a phenomenon called “hydrogen embrittlement” occurs in which mechanical properties such as elongation deteriorate when hydrogen enters the material. This has long been known as a phenomenon in which high-strength steel parts are subjected to static load stress and, after a certain period of time, are suddenly brittlely broken with virtually no plastic deformation. Also called delayed destruction.

  Delayed fracture susceptibility increases as the strength of the material increases. In ultra-high strength steel with a tensile strength of 1000 MPa or higher, embrittlement may occur even in an atmospheric corrosion environment with a small amount of hydrogen penetration. Therefore, in order to actually use ultra high strength steel, it is necessary to accurately grasp and understand delayed fracture evaluation of steel.

  BCC metals, such as steel, have low hydrogen solubility, and most of the hydrogen that has penetrated into the steel is trapped in dislocations, atomic vacancies, precipitates, and grain boundaries. However, not all hydrogen in the material affects the embrittlement, and it is generally interpreted that the cause of hydrogen embrittlement is “diffusible hydrogen” that can diffuse in steel at room temperature. For example, hydrogen that is strongly trapped by precipitates or the like is “non-diffusible hydrogen” that does not diffuse at room temperature, and is therefore considered not to affect embrittlement.

  Thus, a method for evaluating delayed fracture of steel based on the amount of diffusible hydrogen has been proposed. The upper limit of the diffusible hydrogen amount that does not cause delayed fracture of steel: [Hc] (limit diffusible hydrogen amount) and the amount of invading hydrogen that penetrates into the steel from the environment: [He] are measured. If the steel material is larger than [He], it is determined that delayed fracture does not occur. That is, it means that a material with a higher limit diffusible hydrogen content has better delayed fracture characteristics. Here, the “limit diffusible hydrogen amount” means a state where hydrogen is present in the material or a constant load is applied to the test piece while adding hydrogen, and the test piece is not broken until a predetermined limit time. This is the amount of diffusible hydrogen in the test piece that was not generated.

  The amount of critical diffusible hydrogen varies depending on the structure and composition of the material, and also varies greatly depending on the strain introduced by deformation even for the same material. In the field of automotive steel sheets, the amount of strain introduced varies depending on the degree of forming, and therefore, when the strain amount is determined, the amount of critical diffusible hydrogen can be determined. Yes.

  As a method for measuring the limit diffusible hydrogen amount, there are a U-bending bolt tightening test described in Patent Document 1, a method using constant load tension described in Patent Document 2, and the like.

JP-A-2005-134152 JP 2009-069008 A JP 2011-033600 A

S. Takagi, Y. Toji, M. Yoshino, K. Hasegawa, N. Wada, K. Takai, and Y. Hagihara, Steely Hydrogen Conference Proceedings, 2014, 13-20. Kenichi Takai, Materials and Environment, 2011, Vol. 60, p. 230-235. Yukito Sugawara et al., Iron and Steel, 2011, Vol. 97, no. 3, p. 143-151.

  However, the limit diffusible hydrogen amount has a problem that it cannot be uniquely determined because the value measured by the test method is different even with the same structure, strain amount, and load. Therefore, at present, it is necessary to obtain the critical diffusible hydrogen amount by a test method suitable for each member, and to define the critical diffusible hydrogen amount for each member. For example, in the case of an automotive steel sheet, it is necessary to obtain the critical diffusible hydrogen amount by each test method such as bending and drawing.

  Furthermore, among the test methods, a cup drawing test method that requires a special forming technique and a stress that the member receives in the actual environment shown in Patent Document 3 are estimated by stress analysis, and the amount of strain is quantified, which corresponds to that. Since there is also a method of performing a bending test in which strain is applied to a test piece, a large amount of labor and time are required for one test. Therefore, there is a need for a simple test method that can define the amount of critical diffusible hydrogen even in different molding modes.

  This invention is made | formed in view of the said actual condition, and it aims at providing the simple test method which can define the amount of limit diffusible hydrogen even in different shaping | molding styles in various metal materials including steel materials.

  As a result of the study, the present inventor has found that the amount of hydrogen trapped in the structure that affects the hydrogen embrittlement of the metal material can be uniquely determined if the stress and strain are the same even in different forming modes.

This invention is made | formed based on the said knowledge, The summary structure is as follows.
1. A test piece hydrogen introduction process for introducing hydrogen into a metal material as a test piece in order to generate delayed fracture,
A test piece hydrogen amount measuring step for measuring the hydrogen amount in the test piece,
A hydrogen capture site identification step for identifying a plurality of hydrogen capture sites present in the test piece,
Delayed fracture structure identification step for identifying a structure that affects delayed fracture based on the corresponding relationship between each hydrogen capture site in the specimen and delayed fracture,
A test piece in which hydrogen is present in the test piece or a load is applied to the test piece while a constant load is applied to the test piece and no fracture occurs until a predetermined limit time during the test period. A critical diffusible hydrogen amount deriving step for measuring the diffusible hydrogen amount as a critical diffusible hydrogen amount, and a delayed fracture structure for deriving only the hydrogen amount trapped in the tissue that affects delayed fracture among the critical diffusible hydrogen amount The critical hydrogen amount derivation process of
Method for evaluating delayed fracture of metallic materials, including

2. 2. The delayed fracture of the metal material according to 1 above, wherein information on hydrogen derived from a structure involved in delayed fracture is separated from information on the amount of hydrogen, and the delayed fracture characteristics are evaluated particularly by the amount of hydrogen trapped in dislocations. Evaluation method.

3. 3. The method for evaluating delayed fracture of a metal material according to 1 or 2 above, wherein the thickness of the test piece is from 0.1 mm to 5.0 mm.

4). 4. The method for evaluating delayed fracture of a metal material according to any one of 1 to 3, wherein in the step of deriving the limit diffusible hydrogen amount, the amount of diffusible hydrogen is measured by a temperature programmed desorption analysis method.

5. 5. The method for evaluating delayed fracture of a metal material as described in 4 above, wherein, in the temperature programmed desorption analysis method, the temperature range: -50 ° C. or more and 300 ° C. or less of hydrogen is analyzed.

  According to the present invention, if the stress and strain are the same in different molding modes, the critical diffusible hydrogen amount can be uniquely determined, so that it is not necessary to use various test methods, and the test becomes simple. In particular, in members having various deformation modes such as automobile steel plates, the effect of simplification of the test is great, and the industrial contribution is extremely remarkable such as the development of hydrogen-resistant embrittled steel materials being promoted.

It is a flowchart of the delayed fracture evaluation method of steel materials in one embodiment of the present invention. It is a schematic diagram which shows the hydrogen desorption curve obtained when various test pieces are analyzed by a temperature-programmed desorption analysis method. It is an example of the graph which shows the result of a delayed fracture test, and the relationship of hydrogen amount. It is a schematic diagram which shows an example of the procedure which identifies a hydrogen capture site in a hydrogen capture site identification process. It is a figure which shows the amount of hydrogen corresponding to each hydrogen desorption peak of an unbroken material and a fracture material in a delayed fracture structure identification process. It is a graph which shows the relationship between the limit dislocation hydrogen amount in an Example, and the distortion | strain of the limit diffusible hydrogen amount in a comparative example.

Hereinafter, a method for carrying out the present invention will be specifically described.
As shown in FIG. 1, the method for evaluating delayed fracture of a metal material according to an embodiment of the present invention includes the following six steps.
Step S1: Specimen hydrogen introduction process Step S2: Specimen hydrogen amount measurement process Step S3: Hydrogen capture site identification process Step S4: Delayed fracture structure identification process Step S5: Limit diffusible hydrogen amount derivation process S6: Step of deriving critical hydrogen content of delayed fracture structure

[Test pieces]
In the delayed fracture evaluation method of the present invention, any metal material can be used as a test piece without any particular limitation. In the following description, a case where a steel material is used as a test piece is described as an example, but the metal material is not limited to the steel material, and Ti, Al, Mg or an intermetallic compound thereof, Any metal can be used.

  Since the test piece is for obtaining sample / reference data for evaluating delayed fracture resistance, it is preferable that the test piece be formed and processed under various conditions. Further, as the test piece, it is preferable to use a steel material into which at least one kind of structure capable of functioning as a hydrogen trapping site, for example, dislocations, crystal grain boundaries, atomic vacancies, precipitates, carbides, heterophase interfaces, etc. is introduced. For example, in order to create a test piece into which dislocation is introduced, there is a method of increasing or decreasing the rolling reduction rate.

  The shape and dimensions of the test piece are not particularly limited, and may be adjusted as appropriate according to the analysis method and analyzer used. If the thickness of the test piece exceeds 5.0 mm, it may be difficult to identify the hydrogen capture site in the hydrogen capture site identification step (step S3). Therefore, the thickness of the test piece is 5.0 mm or less. It is preferable to do. When the specimen thickness is less than 0.1 mm, it is difficult to perform a constant load test in the delayed fracture structure identification step (step S4) and the limit diffusible hydrogen amount derivation step (step S5). Therefore, the thickness of the test piece is preferably 0.1 mm or more. Therefore, the specimen thickness is preferably 0.1 mm or more and 5.0 mm or less. Furthermore, when the test piece is sufficiently thin, the hydrogen release from the test piece becomes a “desorption rate-determining process” in which the hydrogen desorbed from the capture site is quickly desorbed out of the test piece, and it corresponds to the hydrogen desorption peak temperature. The effects of diffusion can be excluded. From such a viewpoint, it is more preferable that the thickness of the test piece is 0.3 mm or less.

[Step S1: Specimen Hydrogen Introduction Process]
In the test piece hydrogen introduction step, hydrogen is introduced into the steel material as the test piece in order to sufficiently capture hydrogen at the hydrogen supplementation site of the test piece. At this time, as the test piece, it is preferable to use a plurality of test pieces that have been subjected to various molding processes as described above. The introduced hydrogen is captured at a hydrogen capture site present in the test piece.

  Any method can be used for introducing hydrogen as long as hydrogen can be introduced into the steel material. As a hydrogen introduction method that can be suitably used, an acid immersion method in which a test piece is immersed in an acid solution such as hydrochloric acid, a test piece is a cathode in an electrolytic solution, platinum or the like is used as an anode, and a current is applied between the cathode and the anode. The solution is electrolyzed by flowing, and the generated hydrogen is introduced into the steel by cathodic charging, and the neutral chloride aqueous solution is sprayed and the steel is corroded by repeated drying and wetting in a controlled temperature and humidity environment. Combined cycle method in which hydrogen generated due to corrosion reaction is introduced into steel material, test pieces are installed in the actual atmospheric environment, and hydrogen generated due to corrosion reaction occurring in the natural environment is steel material The atmospheric exposure method to be introduced inside. However, the combined cycle method and the atmospheric exposure method require a long period of time to introduce hydrogen, and the amount of hydrogen that penetrates into the steel material due to the corrosion reaction is as small as about 0.1 to 0.9 mass ppm. In some cases, hydrogen trapping sites that are formed are not sufficiently filled with hydrogen. Therefore, from the viewpoint of shortening the test time and controlling the amount of hydrogen in the steel, it is preferable to use an acid immersion method or a cathodic charging method as a method for introducing hydrogen into the test piece.

  Since hydrogen in the test piece may be desorbed out of the test piece due to diffusion, the test piece is quickly stored at a very low temperature such as in liquid nitrogen after hydrogen is introduced to suppress hydrogen desorption from the steel. It is preferable to do.

[Step S2: Specimen Hydrogen Content Measurement Process]
In the test piece hydrogen amount measurement step, the amount of hydrogen in the test piece into which hydrogen has been introduced in the test piece hydrogen introduction step is measured. Any method can be used for measuring the hydrogen amount as long as it can measure hydrogen in the steel material. Examples of a method for measuring the amount of hydrogen that can be suitably used include temperature-programmed desorption analysis (TDA) in which the temperature of the test piece is raised to desorb hydrogen in the steel material, and the amount of hydrogen that permeates the test piece. For example, an electrochemical hydrogen permeation method for determining the hydrogen concentration in steel. Among them, it is preferable to use a temperature-programmed desorption analysis method in which preparation of a test piece is easy and as shown in Non-Patent Document 2, a hydrogen desorption peak and a hydrogen capture site can be associated with each other.

  In order to prevent the hydrogen introduced into the test piece from escaping, the test piece hydrogen amount measuring step is preferably performed promptly after the test piece hydrogen introducing step is completed. As described above, when the test piece is immersed in liquid nitrogen, it is preferable to immediately perform the test piece hydrogen amount measurement step after taking out from the liquid nitrogen.

  When using a temperature programmed desorption analysis method for measuring the amount of hydrogen in a test piece, for example, gas chromatography can be used as a method for quantifying the hydrogen released from the test piece. As an apparatus for temperature programmed desorption analysis using gas chromatography for quantification, for example, temperature-programmed hydrogen analyzer <gas chromatograph type> (JTF-20A) can be mentioned.

  As measurement conditions in the case of using the temperature programmed desorption analysis method, it is preferable that the rate of temperature increase be 25 ° C./h or more and 400 ° C./h or less. This is because if the heating rate is less than 25 ° C./h, it takes a very long time to measure diffusible hydrogen, and the efficiency deteriorates. If the heating rate exceeds 400 ° C./h, a hydrogen desorption peak is obtained. This is because separation may not be possible. From the viewpoint of measurement efficiency, it is more preferable that the rate of temperature rise be 50 ° C./h or more and 200 ° C./h or less. Moreover, it is preferable that the quantification of diffusible hydrogen targets the total amount (integrated value) of hydrogen released from the test piece in the temperature range from room temperature to 300 ° C. Since some of the diffusible hydrogen is desorbed from room temperature or lower, it is more preferable to set the total amount (integrated value) of hydrogen released in the temperature range from −50 ° C. to 300 ° C.

[Step S3: Hydrogen Capture Site Identification Step]
In the hydrogen capture site identification step, a plurality of hydrogen capture sites existing in the test piece are identified. A method for identifying a hydrogen capture site in this step will be described with reference to the example of FIG.

  FIG. 2 is a schematic diagram showing hydrogen desorption curves obtained when various test pieces are analyzed by the temperature programmed desorption analysis method. In FIG. 2, the three peaks shown on the left side show hydrogen desorption peaks in three test pieces that are strained by rolling with different rolling reduction ratios, and the peaks increase as the rolling reduction ratio increases. Since the main defects increased by rolling are dislocations, this hydrogen desorption peak is identified to be due to hydrogen trapped in the dislocations. Further, the three peaks shown on the right side in FIG. 2 indicate hydrogen desorption peaks in three test pieces having different crystal grain sizes, and the peaks become larger as the crystal grain size becomes finer. Since the trapping sites that increase due to grain refinement are presumed to be grain boundaries, this hydrogen desorption peak is identified to be due to hydrogen trapped at the grain boundaries.

  As described above, the data of the hydrogen desorption peak temperature and temperature range corresponding to the hydrogen trapping site obtained by the temperature programmed desorption analysis method serve as sample / reference data for evaluating delayed fracture. In order to facilitate the subsequent analysis, it is preferable to fit the hydrogen desorption peak with a normal distribution (Gaussian function).

  In the graph of hydrogen desorption rate and temperature obtained by temperature programmed desorption analysis, a plurality of hydrogen desorption peaks may be observed in different temperature ranges as shown in FIG. Since the desorption temperature of the hydrogen desorption peak depends on the bond energy between each defect and hydrogen, when there are a plurality of hydrogen desorption peaks, there are a plurality of hydrogen capture sites.

[[Hydrogen desorption curve fitting]]
As described above, since the temperature of the hydrogen desorption peak appearing in the hydrogen desorption curve varies depending on the hydrogen capture site, the hydrogen capture site can be identified from the hydrogen desorption peak. However, since the hydrogen desorption peak has a certain temperature range, when the temperature zone of each hydrogen desorption peak is close, the hydrogen desorption curve may be measured as a superposition of each hydrogen desorption peak. . In that case, as shown in Non-Patent Document 3, it is preferable to fit a hydrogen desorption curve as a superposition of a normal distribution in which a hydrogen desorption peak already obtained and a hydrogen capture site are associated with each other. Even when the hydrogen desorption curve is measured as a superposition of the respective hydrogen desorption peaks, the amount of hydrogen at each hydrogen desorption peak can be calculated numerically by fitting as a normal distribution.

  When fitting a hydrogen desorption curve as several normally distributed hydrogen desorption peaks, for example, on a spreadsheet software such as Microsoft (registered trademark) Excel (registered trademark), arbitrary temperature and peak temperature And the peak width can be determined and fitted to match the actual hydrogen desorption curve. Also, software that can automatically perform fitting may be used. In the case of fitting as a normal distribution, it is not always necessary to match completely, and a difference that does not adversely affect the evaluation result is allowed.

  By using the above method, it is possible to identify various hydrogen trapping sites in steel materials such as dislocations, grain boundaries, atomic vacancies, precipitates, carbides, heterogeneous interfaces, and the like. The amount of hydrogen at the desorption peak can be measured and calculated independently.

  The hydrogen desorption peak temperature and temperature range in the temperature programmed desorption analysis method depend on the temperature rise rate at the time of measurement and the surface condition such as the thickness of the test piece, the surface coating, and plating. Therefore, when identifying hydrogen capture sites using data of known hydrogen desorption peak temperatures and temperature ranges, use steel materials with the same temperature conditions, such as the heating rate, specimen thickness, surface coating, and plating. It is preferable to do.

[Step S4: Delayed Fracture Structure Identification Step]
In the delayed fracture structure identification step, a structure that influences delayed fracture (hereinafter also referred to as “delayed fracture structure”) is identified based on the correspondence between each hydrogen trapping site in the specimen and delayed fracture. For example, a delayed fracture test (a test in which hydrogen is present in the material or a constant load is applied to the test piece while hydrogen is added, and the test piece is checked for breakage up to a predetermined limit time) ), The hydrogen content of the broken and unbroken test pieces is measured. The delayed fracture structure can be estimated by separating the hydrogen amounts of the two test pieces for each structure of the hydrogen capture site using the method as described above and comparing them.

  A method for identifying a delayed fracture structure in a test piece will be described with reference to FIG. FIG. 3 is a graph comparing the amounts of hydrogen trapped at each trapping site after measuring the hydrogen amount of the test piece that was broken in the delayed fracture test and the unbroken test piece, separated for each structure of the hydrogen trapping site. . Note that there are two main trapping sites of this test piece: dislocation and martensite structure interface, the strain amount and load stress of both test pieces are the same, and only the amount of introduced hydrogen is different. Comparing the amount of hydrogen between the unbroken material and the broken material, it can be seen that hydrogen is trapped only at the martensite structure interface. On the other hand, hydrogen is trapped by dislocations in the fracture material. That is, it is considered that the material is easily embrittled by trapping hydrogen in the dislocation, and it can be estimated that the structure that affects the delayed fracture of this test piece is the dislocation.

  In the above example, dislocations are capture sites that affect embrittlement, but in the case of other test pieces, other structures may be structures that affect embrittlement. In addition, there may be a plurality of structures that affect embrittlement. By using the above-described method, the hydrogen amount at the hydrogen capture site is correlated with the presence or absence of delayed fracture, so that other structures that affect embrittlement can be obtained. It is possible to estimate. Thus, until now, the presence or absence of delayed fracture was evaluated only by the amount of hydrogen, but by using the method of the present invention, delayed fracture can be evaluated based on the amount of hydrogen only in the structure that affects embrittlement. .

  In addition, when evaluating about several sample pieces which consist of the same kind of material, since the delayed fracture structure is the same in the same kind of material, after identifying the delayed fracture structure, steps S1 to S4 are omitted, It is also possible to implement only S5 and subsequent steps.

[Step S5: Step of Deriving Critical Diffusible Hydrogen]
In the process of deriving the limit diffusible hydrogen amount, the test piece is in a state where hydrogen is present, or while adding hydrogen to the test piece, a constant load is applied to the test piece, and the test piece breaks to a predetermined limit time during the test period. The amount of diffusible hydrogen in the test piece in which no slag occurred was measured as the limit diffusible hydrogen amount. By this step, the amount of critical diffusible hydrogen that leads to the fracture of steel at a certain load stress can be determined.

  The amount of limit diffusible hydrogen can be measured by a method using a U-bend bolting test or a constant load tension such as a uniaxial tension as described in Patent Document 1 and Patent Document 2, for example. Measurement of the amount of hydrogen in the steel can be achieved by temperature desorption analysis that desorbs hydrogen in the steel by raising the temperature of the test piece, or by measuring the hydrogen concentration in the steel from the hydrogen that passes through the test piece. Although there are electrochemical hydrogen permeation methods and the like, a temperature-programmed desorption analysis method that allows easy preparation of a test piece and enables correspondence between a hydrogen desorption peak and a hydrogen capture site is preferable. A preferred embodiment of the temperature-programmed desorption analysis method is the same as that described in the description of the test piece hydrogen amount measurement step.

[Step S6: Step of Deriving Critical Hydrogen Content of Delayed Fracture Structure]
In the process of deriving the critical hydrogen content of the delayed fracture structure, the delayed fracture structure identified in the delayed fracture structure identification process (step S4) is obtained from the critical diffusible hydrogen quantity obtained in the critical diffusible hydrogen quantity derivation process (step S5). Only the amount of trapped hydrogen is extracted. By this step, the critical hydrogen content of the structure that affects delayed fracture is determined. That is, it is required how much hydrogen penetrates into a delayed fracture structure of a material under a certain stress to cause delayed fracture. In addition, when there exist two or more structures | tissues which influence the embrittlement in a test piece, it is preferable to make the total amount of the hydrogen amount into a limit hydrogen amount.

  The method for extracting only the amount of hydrogen trapped in the delayed fracture structure is not particularly limited, and can be any method. For example, when the amount of diffusible hydrogen is measured by the temperature programmed desorption analysis method in the limiting diffusible hydrogen amount deriving step (step S5), the hydrogen desorption curve (hydrogen desorption) obtained by the temperature programmed desorption analysis method is used. Separation rate-temperature). Specifically, the above-mentioned “hydrogen desorption curve fitting” is applied to the hydrogen desorption curve obtained in the limiting diffusible hydrogen amount derivation step (step S5), and the hydrogen desorption peak at each capture site is obtained. To separate. Thereafter, the amount of hydrogen in the tissue captured by the delayed fracture tissue determined in the delayed fracture tissue identification step (step S4) is calculated. This value is the limit amount of hydrogen in the test piece, and it is a feature of the present invention that this value is used to compare with other strain amounts and other steel types.

  Furthermore, as a result of various investigations, the present inventor has found that the amount of hydrogen trapped only in the structure that affects the hydrogen embrittlement of the steel material is the limit that causes delayed fracture if the stress and strain are the same even in different forming modes. The amount of hydrogen was found to be equivalent. In other words, if the stress and strain are the same, the same critical hydrogen amount can be defined even in different molding modes. Therefore, it is not necessary to determine the critical diffusible hydrogen amount by various methods, which is a problem, and to define it for each material. It is easy to use.

  If the stress and strain are the same, the same critical hydrogen amount can be determined even in different molding modes. Therefore, in the critical diffusible hydrogen amount derivation step (step S5), the calculation of the strain amount and the processing of the specimen are simple. It is preferable to measure the critical hydrogen amount by axial tension. However, the present invention is not limited to the calculation of the critical hydrogen amount in the uniaxial tensile test method, but a constant load is tested while hydrogen is present in the material, such as U-bending or bolting test, or hydrogen is added. Any method can be used as long as the limit hydrogen amount can be calculated from the hydrogen amount of the test piece that was loaded on the piece and did not break until a predetermined limit time.

  Next, based on an Example, it demonstrates further more concretely. In this example, TS: 1180 MPa class ferrite-martensitic steel (DP steel) was used as a test piece, and ferritic steel (IF steel) was used as a reference test piece. The thickness of the said test piece and the reference test piece was 0.3 mm. A strain of 0 to 0.5 was applied as a true strain to these test pieces.

Hydrogen introduction in the test piece hydrogen introduction step (step S1) was performed by a cathode charge method. The cathode charge was performed by electrolysis at a current density of 30 A / m ² in a 3% by mass NaCl + 3% by mass NH ₄ SCN solution for 24 hours, whereby hydrogen was introduced into the test piece. Since hydrogen in the test piece may be desorbed out of the test piece due to diffusion, the test piece was quickly stored in the cryogenic temperature of liquid nitrogen after hydrogen introduction.

  In the test piece hydrogen amount measurement step (step S2), a temperature programmed desorption analysis method was used. For the temperature programmed desorption analysis, a low temperature temperature programmed hydrogen analyzer was used. Thermal desorption analysis was performed in a temperature range from −50 ° C. to 300 ° C. at a temperature rising rate of 200 ° C./h. At that time, in order to prevent the hydrogen introduced into the test piece from escaping, a sample frozen in liquid nitrogen was immediately measured with a low temperature type temperature rising type hydrogen analyzer.

  The derivation of the limit diffusible hydrogen amount in the limit diffusible hydrogen amount derivation step (step S5) was performed by two test methods, a uniaxial tensile test method and a cup drawing test method. The strain of the uniaxial tensile test piece was applied by rolling, and in the cup drawing test, the strain was applied by drawing. The strain amount of the cup drawing was calculated by the finite element method. The fracture starting stress of the uniaxial tensile test method and the cup drawing test method was set to 900 MPa, and the load was 2.6 kN in the uniaxial tensile test.

In the critical diffusible hydrogen amount derivation step (step S5), the hydrogen introduction condition is 1 to 10 A / m ² in a 3 mass% NaCl + 3 g / L NH ₄ SCN solution in the case of the cathodic hydrogen charging method, and pH 1 to ₄ in the case of hydrochloric acid immersion. Hydrogen was introduced in the solution. First, hydrogen is introduced to such an extent that delayed fracture occurs at this load, and the hydrogen introduction condition is gradually reduced from this condition, and the amount of hydrogen in the test piece introduced under the condition that no fracture occurs even after 100 hours or more is temperature-escalated. A hydrogen desorption curve was obtained from a test piece having a hydrogen content at the fracture limit as measured by a separation analysis method.

  For the temperature programmed desorption analysis, a low temperature type temperature programmed hydrogen analyzer was used. Thermal desorption analysis was performed in a temperature range from −50 ° C. to 300 ° C. at a temperature rising rate of 200 ° C./h.

  Next, a hydrogen desorption curve from a test piece having a hydrogen content at the fracture limit is fitted with some normal distribution on a spreadsheet software such as Microsoft (registered trademark) Excel (registered trademark). Only the amount of hydrogen from the tissue that affected delayed fracture was extracted. Finally, the relationship between the amount of hydrogen and strain from the microstructure that affects delayed fracture obtained was compared between the uniaxial tensile test method and the cup drawing test method. In addition, this invention is not restricted to this, It is a delayed fracture evaluation method which can be evaluated also in other steel materials and hydrogenation methods.

  FIG. 4 shows a procedure for identifying a hydrogen capture site in the hydrogen capture site identification step (step S3). FIG. 4A is a hydrogen desorption curve in ferritic steel (IF). In ferritic steel, Peak A suddenly rises due to rolling, so Peak A was identified as a “dislocation” hydrogen desorption peak. Further, although the amount of hydrogen was small, a second peak (Peak B) was present, and the size of Peak B was increased by rolling. Since Peak B disappears by tempering at a low temperature (200 ° C.), it was identified as a “vacancy cluster” that is regarded as a thermally unstable defect. Thus, it is also possible to identify the hydrogen trapping site by increasing or decreasing the hydrogen trapping site by heat treatment or the like.

  Next, as shown in FIG. 4B, the hydrogen desorption curve was separated into two hydrogen desorption peaks by fitting each of the identified hydrogen desorption peaks with a Gaussian function.

  Next, the hydrogen desorption curve in DP steel is analyzed based on these information obtained by measuring IF steel. FIG. 4C is a hydrogen desorption curve in DP steel. When fitting with two hydrogen desorption peaks obtained by measurement of IF steel, FIG. 4D is obtained. At this time, the structure of IF steel is different from that of DP steel. Therefore, the peak of DP steel cannot be reproduced using only the hydrogen desorption peak of IF steel. However, as shown in FIG. It becomes a hydrogen desorption peak due to the capture site. This trapping site that does not exist in IF steel is prominent in the two-phase structure of ferrite-martensitic steel, and is thus identified as the “interface between the martensite structure and the ferrite structure” (hereinafter referred to as the martensite structure interface). ). In the 1180 MPa class ferritic-martensitic steel thus imparted with strain, at least dislocations, martensite structure interfaces, and vacancy clusters existed as hydrogen trapping sites.

Next, the delayed fracture structure identification step (step S4) will be described. The DP steel was subjected to a constant load test in which hydrogen was generated at a current density of 0.5 to 5 A / m ² in a 3% by mass NaCl + 3 g / L NH ₄ SCN solution to give 1125 kN. As a result, it did not break at a current density of 0.5 A / m ² , but it broke at 5 A / m ² . FIG. 5 shows the results of performing temperature programmed desorption analysis under the same conditions as described above, measuring the hydrogen content of these test pieces, and calculating the hydrogen content corresponding to each hydrogen desorption peak. Comparing the amount of hydrogen between the unbroken material and the broken material, it can be seen that hydrogen is trapped only at the martensite structure interface. On the other hand, a lot of hydrogen is captured by dislocations in the fractured material. That is, it is considered that the material is easily embrittled by capturing hydrogen in the dislocation, and it can be estimated that the structure that affects the delayed fracture of the specimen is the dislocation. Based on this result, the amount of hydrogen trapped in the dislocation was extracted. Hereinafter, this value is referred to as “limit dislocation hydrogen amount”.

  For comparison, the amount of limit diffusible hydrogen was measured by a conventional method for the same test piece as in the above Example (Comparative Example). FIG. 6 shows the relationship between the amount of critical dislocation hydrogen in the above example and the strain of the critical diffusible hydrogen amount in the comparative example.

  As can be seen from FIG. 6, in the comparative example, the amount of critical diffusible hydrogen is different even when the applied strain is the same between the cup drawing method and the uniaxial tension. On the other hand, the “limit dislocation hydrogen content” obtained by the method of the present invention as described above was about 0.5 ppm by mass regardless of the strain. That is, the steel material used as the test piece in this example does not break when the amount of hydrogen trapped by the dislocation is 0.5 mass ppm or less. Furthermore, in the examples, since the critical dislocation hydrogen amount in the cup drawing method and the critical dislocation hydrogen amount in the uniaxial tensile test method are the same, the same evaluation can be made by either one of the load application methods. In general, since the cup drawing method requires a special device, it is easy and preferable to evaluate by a uniaxial tensile test method.

  As described above, in the method of the present invention, if the stress and strain are the same, the same critical hydrogen amount can be defined even in different molding modes. Therefore, it is not necessary to obtain the limit diffusible hydrogen amount in each test method, and the test can be simplified. Furthermore, if the relationship between the amount of hydrogen that can enter in the actual environment and the amount of strain is known, the presence or absence of delayed fracture can be predicted, and an effect that serves as a guideline for material design can be considered.

Claims (5)

A test piece hydrogen introduction process for introducing hydrogen into a metal material as a test piece in order to generate delayed fracture,
A test piece hydrogen amount measuring step for measuring the hydrogen amount in the test piece,
A hydrogen capture site identification step for identifying a plurality of hydrogen capture sites present in the test piece,
Delayed fracture structure identification step for identifying a structure that affects delayed fracture based on the corresponding relationship between each hydrogen capture site in the specimen and delayed fracture,
A test piece in which hydrogen is present in the test piece or a load is applied to the test piece while a constant load is applied to the test piece and no fracture occurs until a predetermined limit time during the test period. A critical diffusible hydrogen amount deriving step for measuring the diffusible hydrogen amount as a critical diffusible hydrogen amount, and a delayed fracture structure for deriving only the hydrogen amount trapped in the tissue that affects delayed fracture among the critical diffusible hydrogen amount The critical hydrogen amount derivation process of
Method for evaluating delayed fracture of metallic materials, including   2. The delay of the metal material according to claim 1, wherein information on hydrogen derived from a structure involved in delayed fracture is separated from information on the amount of hydrogen, and the delayed fracture characteristics are evaluated in particular by the amount of hydrogen trapped in dislocations. Destructive evaluation method.   The method for evaluating delayed fracture of a metal material according to claim 1 or 2, wherein the thickness of the test piece is from 0.1 mm to 5.0 mm.   The method for evaluating delayed fracture of a metal material according to any one of claims 1 to 3, wherein in the step of deriving the critical diffusible hydrogen amount, the amount of diffusible hydrogen is measured by a temperature programmed desorption analysis method. 5. The delayed fracture evaluation method for a metal material according to claim 4, wherein in the temperature-programmed desorption analysis, the amount of hydrogen in a temperature-raising range: −50 ° C. to 300 ° C. is analyzed.

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