JP6512154B2 - Evaluation method for delayed fracture of metallic materials - Google Patents

Description

  The present invention relates to a method for evaluating delayed fracture of a metal material, and more particularly, to a method for evaluating the delayed fracture of a metallic material simply by which the amount of critical diffusible hydrogen can be defined even in different forming modes.

  For the purpose of achieving both weight reduction in automobiles and improvement in collision safety, strengthening of automobile parts has been promoted. However, when the steel sheet is strengthened, a phenomenon called "hydrogen embrittlement" occurs in which mechanical properties such as elongation deteriorate when hydrogen penetrates into the material. This has long been known as a phenomenon in which a high strength steel component suddenly and brittlely fractures with virtually no plastic deformation when passing for a certain period of time under static load stress. Also called delayed destruction.

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

  The BCC metal such as steel has a low solubility of hydrogen, and most of the hydrogen that has penetrated into the steel is trapped in dislocations, vacancies, precipitates, grain boundaries and the like. However, not all hydrogen in the material affects the embrittlement, and the cause of hydrogen embrittlement is generally interpreted as "diffusive hydrogen" that can diffuse in steel at room temperature. For example, hydrogen which is strongly trapped in precipitates and the like is "non-diffusible hydrogen" which does not diffuse at room temperature, and thus is considered not to affect embrittlement.

  Then, the method of evaluating the delayed fracture of steel materials is proposed based on the quantity of the diffusible hydrogen. Measure both the upper limit of diffusible hydrogen content that does not cause delayed fracture of steel materials: [Hc] (limit diffusible hydrogen content) and the amount of invading hydrogen that penetrates into steel materials from the environment: [He], and [Hc] If the steel is larger than [He], it is determined that delayed fracture does not occur. That is, it means that as the material having a higher limit diffusible hydrogen amount is more excellent in delayed fracture characteristics. Here, the “limit diffusible hydrogen content” refers to a state in which hydrogen is present in the material or a constant load is applied to the test piece while adding hydrogen, and the test piece is broken up to a predetermined limit time. It is the amount of diffusible hydrogen of the test piece which did not generate | occur | produce.

  The amount of critical diffusive hydrogen varies depending on the structure and composition of the material, and even the same material largely varies depending on the strain introduced by deformation. In the field of automotive steel plates, the amount of strain introduced varies depending on the degree of forming, so that the amount of critical diffusible hydrogen can be determined if the amount of strain is determined, etc. There is.

  As a method of measuring the limit diffusive hydrogen amount, there are a U-bending bolting test described in Patent Document 1, a method by constant load tension described in Patent Document 2, and the like.

JP 2005-134152 A 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. Hagiwara, Y. et al., Iron and Steel, 2011, Vol. 97, no. 3, p. 143-151.

  However, the amount of critical diffusive hydrogen can not be uniquely determined because the values measured by the test method are different even with the same structure, amount of strain, and load. Therefore, at present, it is necessary to obtain the limit diffusive hydrogen amount by a test method suitable for each member and to define the limit diffusive hydrogen amount for each member. For example, in steel sheets for automobiles, it is necessary to determine the amount of critical diffusive hydrogen by each test method such as bending and drawing.

  Furthermore, among the test methods, the cup squeeze test method which requires a special forming technique, and the stress to which a member is subjected in an actual environment shown in Patent Document 3 is estimated by stress analysis to quantify the amount of strain, Since there is also a method of conducting a bending test in which strain is applied to a test piece, a great deal of labor and time is required for one test. Therefore, there is a need for a simple test method that can define the amount of critical diffusive hydrogen even in different molding modes.

  The present invention has been made in view of the above situation, and has an object of providing a simple test method capable of defining the amount of critical diffusive hydrogen even in different forming styles in various metal materials including steel materials.

  The inventors of the present invention have found that the amount of hydrogen trapped in a tissue that affects hydrogen embrittlement of a metal material can be uniquely determined if stress and strain are the same, as a result of examination.

The present invention has been made based on the above findings, and the summary of the invention is as follows.
1. A test piece hydrogen introduction step of introducing hydrogen into a metal material as a test piece to cause delayed fracture,
A test piece hydrogen amount measuring step of measuring the amount of hydrogen in the test piece,
A hydrogen trapping site identification step of identifying a plurality of hydrogen trapping sites present in the test piece;
A delayed fracture tissue identification step of identifying a tissue that affects the delayed fracture based on the correspondence relationship between each hydrogen trapping site in the test piece and the delayed fracture;
A state in which hydrogen is present in the test piece or while adding hydrogen to the test piece, a constant load is applied to the test piece, and the test piece in which breakage does not occur until a predetermined limit time during the test period The critical diffusive hydrogen amount deriving step of measuring the diffusive hydrogen content as the critical diffusive hydrogen amount, and the delayed fracture structure that derives only the hydrogen amount trapped in the tissue affecting the delayed fracture among the critical diffusive hydrogen amounts. Critical hydrogen amount derivation process,
Delayed fracture assessment of metallic materials, including:

2. The delayed fracture characteristics of the metal material according to the above 1, wherein the information on hydrogen originating from the tissue involved in delayed fracture is separated from the information on the amount of hydrogen, and in particular, the delayed fracture characteristics are evaluated by the amount of hydrogen trapped in the dislocation. Evaluation method.

3. The delayed fracture evaluation method of the metal material according to 1 or 2, wherein the thickness of the test piece is 0.1 mm or more and 5.0 mm or less.

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

5. The delayed fracture evaluation method for a metal material according to 4 above, wherein an amount of hydrogen in a temperature rising range: −50 ° C. or more and 300 ° C. or less is analyzed in the temperature rising desorption analysis method.

  According to the present invention, it is possible to unambiguously determine the amount of critical diffusible hydrogen if stress and strain are the same in different molding modes, 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 test simplification is great, and industrial contribution such as promotion of development of a hydrogen embrittlement resistant steel is extremely remarkable.

It is a flowchart of the delayed fracture evaluation method of the steel materials in one embodiment of the present invention. It is a schematic diagram which shows the hydrogen desorption curve obtained when analyzing various test pieces by a temperature programmed desorption analysis method. It is an example of the graph which shows the relationship between the result of a delayed destruction test, and the amount of hydrogen. 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 breaking material in a delayed fracture structure identification process. It is a graph which shows the relationship of the amount of critical dislocation hydrogen in an example, and the amount of critical diffusible hydrogen in a comparative example with distortion.

Hereinafter, the method of practicing the present invention will be specifically described.
The delayed fracture evaluation method of the metal material in one embodiment of the present invention includes the following six steps as shown in FIG.
Step S1: Test piece hydrogen introduction step Step S2: Test piece hydrogen amount measurement step Step S3: Hydrogen capture site identification step Step S4: delayed fracture structure identification step Step S5: critical diffusible hydrogen amount derivation step step S6: Process for 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 particular limitation. In the following description, although the case where a steel material is used as a test piece is described as an example, the metal material is not limited to the steel material, and Ti, Al, Mg or their intermetallic compounds, etc. Any metal can be used.

  Since the test piece is used to obtain data serving as a sample / reference for evaluating delayed fracture resistance, it is preferable to form and process the test piece under various conditions. Furthermore, as the test piece, it is preferable to use a steel material into which at least one of a structure capable of functioning as a hydrogen trapping site, for example, dislocation, grain boundary, atomic vacancy, precipitate, carbide, heterophase interface, etc. is introduced. For example, there is a method of increasing or decreasing the rolling reduction ratio in order to create a test piece in which dislocation is introduced.

  The shape and size of the test piece are not particularly limited, and may be appropriately adjusted according to the analysis method and analyzer to be 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), so the thickness of the test piece is 5.0 mm or less It is preferable to do. Moreover, when it becomes difficult to perform a constant load test in the delayed fracture structure identification step (step S4) and the critical diffusive hydrogen amount deriving step (step S5) if the thickness of the test specimen is less than 0.1 mm. Because of this, the thickness of the test piece is preferably 0.1 mm or more. Therefore, the thickness of the test piece is preferably 0.1 mm or more and 5.0 mm or less. Furthermore, if the test piece is sufficiently thin, the release of hydrogen from the test piece is a "delimited rate-controlled process" in which hydrogen desorbed from the capture site is quickly desorbed out of the test piece, and the hydrogen desorption peak temperature The effects of diffusion can be ruled out. From such a viewpoint, it is more preferable to set the thickness of the test piece to 0.3 mm or less.

[Step S1: Test piece hydrogen introduction step]
In the test piece hydrogen introduction step, hydrogen is introduced into a steel material as a test piece in order to cause the hydrogen capture site of the test piece to sufficiently capture hydrogen. At this time, as the test piece, it is preferable to use a plurality of test pieces subjected to various forming processes as described above. The introduced hydrogen is captured at the hydrogen capture site present in the test piece.

  For the introduction of hydrogen, any method can be used as long as hydrogen can be introduced into the steel material. As a hydrogen introducing method which 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 as a cathode in an electrolytic solution, platinum as an anode and a current between the cathode and the anode. The solution is electrolyzed by flowing, and the cathode charge method in which the hydrogen generated at that time is introduced into the steel material, corrosion of the steel material is repeated by repeated application of neutral chloride aqueous solution and drying and wetting in a controlled temperature and humidity environment. Combined cycle method of generating hydrogen and introducing hydrogen generated due to corrosion reaction into steel materials, installing test pieces under actual atmospheric environment, and generating hydrogen caused due to corrosion reaction occurring under natural environment steel materials The air exposure method etc. which are introduced in can be mentioned. However, in the combined cycle method and the atmospheric exposure method, the time required for introducing hydrogen is extended for a long time, and the amount of hydrogen entering the steel material due to the corrosion reaction is as small as about 0.1 to 0.9 mass ppm, There are cases where the formed hydrogen capture site is 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 the acid immersion method or the cathode charge method as a method of introducing hydrogen to the test piece.

  Since hydrogen in the test piece may be released out of the test piece by diffusion, the test piece is stored rapidly in a cryogenic temperature such as in liquid nitrogen after hydrogen introduction, and hydrogen desorption from the steel material is suppressed It is preferable to do.

[Step S2: Test Piece Hydrogen Amount Measurement Step]
In the test piece hydrogen amount measurement step, the amount of hydrogen in the test piece to which hydrogen is introduced in the test piece hydrogen introduction step is measured. For the measurement of the amount of hydrogen, any method can be used as long as it can measure hydrogen in the steel material. As a method of measuring the amount of hydrogen that can be suitably used, for example, a temperature-induced desorption analysis (TDA) in which the temperature of the test piece is increased to desorb hydrogen in the steel material, or from the amount of hydrogen permeating the test piece The electrochemical hydrogen permeation method etc. which ask for hydrogen concentration in steel materials are mentioned. Above all, it is preferable to use a thermal desorption analysis method in which the preparation of the test piece is easy and the hydrogen desorption peak and the hydrogen trapping site can be correlated as shown in Non-patent Document 2.

  In order to prevent the hydrogen introduced into the test piece from escaping, it is preferable to carry out the test piece hydrogen amount measurement step immediately after the test piece hydrogen introduction step. As described above, when the test piece is immersed in liquid nitrogen, it is preferable to immediately carry out the test piece hydrogen amount measuring step after taking it out of the liquid nitrogen.

  In the case of using a thermal desorption analysis to measure the amount of hydrogen in a test piece, gas chromatography can be used as a method of quantifying hydrogen released from the test piece, for example. As an apparatus of the temperature programmed desorption analysis method using gas chromatography for the determination, for example, a temperature rising hydrogen analyzer <gas chromatograph type> (JTF-20A) may be mentioned.

  As measurement conditions in the case of using a temperature programmed desorption analysis method, it is preferable to set a temperature rising rate to 25 ° C./h or more and 400 ° C./h or less. This is because if the heating rate is less than 25 ° C./h, measurement of diffusible hydrogen takes a very long time and the efficiency deteriorates, and if the heating rate exceeds 400 ° C./h, the hydrogen desorption peak is It is because it may be impossible to separate. From the viewpoint of measurement efficiency, it is more preferable to set the heating rate to 50 ° C./h or more and 200 ° C./h or less. In addition, it is preferable that quantitative determination of diffusible hydrogen be performed on the total amount (integrated value) of hydrogen released from the test piece in a temperature range of room temperature to 300 ° C. In addition, since some diffusible hydrogen desorbs from room temperature or less, it is more preferable to set it as the total amount (integrated value) of hydrogen discharge | released in the temperature range to -50 degreeC-300 degreeC.

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

  FIG. 2 is a schematic view showing hydrogen desorption curves obtained when various test pieces are analyzed by thermal desorption analysis. In FIG. 2, three peaks shown on the left side show hydrogen desorption peaks in three test pieces to which strain is applied by rolling at different reduction ratios, and the peaks become larger as the reduction ratio increases. Since the main defect that is increased by rolling is dislocation, this hydrogen desorption peak is identified as attributed to hydrogen trapped in the dislocation. Further, 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 capture site increased by grain refinement is presumed to be a grain boundary, this hydrogen desorption peak is identified as originating from hydrogen trapped at the grain boundary.

  As described above, the data of the hydrogen desorption peak temperature and the temperature width corresponding to the hydrogen trapping site obtained by the temperature programmed desorption analysis method become data serving as a sample / reference 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).

  Further, in the graph of hydrogen desorption rate and temperature obtained by the temperature programmed desorption analysis method, 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 binding energy of each defect and hydrogen, when there are a plurality of hydrogen desorption peaks, there will be a plurality of hydrogen trapping sites.

[[Fitting of hydrogen desorption curve]]
As described above, since the temperature of the hydrogen desorption peak appearing in the hydrogen desorption curve differs depending on the hydrogen trapping site, it is possible to identify the hydrogen trapping site from the hydrogen desorption peak. However, since the hydrogen desorption peak has a constant temperature range, the hydrogen desorption curve may be measured as the superposition of the respective hydrogen desorption peaks when the temperature zones of the respective hydrogen desorption peaks are close. . In that case, it is preferable to fit the hydrogen desorption curve as superposition of the normal distribution in which the hydrogen desorption peak and the hydrogen trapping site are already obtained as shown in Non-Patent Document 3. Even when the hydrogen desorption curve is measured as superposition of the respective hydrogen desorption peaks, it is possible to numerically calculate the hydrogen amount of each hydrogen desorption peak by fitting as a normal distribution.

  When fitting a hydrogen desorption curve as several normally distributed hydrogen desorption peaks, any temperature and peak temperature may be used, for example, on spreadsheet software such as Microsoft® Excel®. And peak width can be determined and fitted to match the actual hydrogen desorption curve. Alternatively, software that can automatically perform fitting may be used. In addition, when fitting as a normal distribution, it is not necessary to make it completely correspond, and the difference of the grade which does not have a bad influence on an evaluation result is accept | permitted.

  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, heterophase interfaces, etc. The amount of hydrogen of the desorption peak can be measured and calculated independently.

  The hydrogen desorption peak temperature and temperature width in the temperature programmed desorption analysis method depend on the temperature rising rate at the time of measurement and the surface condition of the plate thickness of the test piece, surface coating, plating and the like. Therefore, when identifying hydrogen trapping sites using known hydrogen desorption peak temperature and temperature width data, steel materials with the same temperature conditions such as temperature rise rate, plate thickness of test pieces, surface coating, plating, etc. are used It is preferable to do.

[Step S4: Delayed fracture tissue identification process]
In the delayed fracture tissue identification step, a tissue that influences delayed fracture (hereinafter also referred to as “delayed fracture tissue”) is identified based on the correspondence between each hydrogen capture site in the test piece and the delayed fracture. For example, a delayed fracture test (a test in which a constant load is applied to a test piece while hydrogen is present in a material or in the presence of hydrogen, and the presence or absence of breakage of the test piece to a predetermined limit time is confirmed) Measure the hydrogen content of the broken test piece and the unbroken test piece. It is possible to estimate the delayed fractured structure by separating the hydrogen content of both test pieces for each tissue of the hydrogen capture site using the method as described above and comparing them.

  A method for identifying delayed fractured tissue in a test piece will be described with reference to FIG. FIG. 3 is a graph showing the amount of hydrogen in a broken test piece and a non-broken test piece in the delayed fracture test, and separating the hydrogen trapping site by tissue and comparing the amount of hydrogen trapped in each trapping site . The main capture sites of this test piece are two, dislocation and martensitic structure interface, the strain amount of the test piece and the load stress of the load are the same, and only the introduced hydrogen amount is different. Comparing the amount of hydrogen between the unbroken material and the broken material, it can be seen that the unbroken material has hydrogen trapped only at the martensitic structure interface. On the other hand, in the fracture material, hydrogen is also trapped in dislocations. That is, it is considered that the material is easily embrittled by the capture of hydrogen in dislocations, and the structure that affects the delayed fracture of the present test piece can be estimated to be dislocations.

  In the above example, dislocations are capture sites that affect embrittlement, but in the case of other specimens, other tissues may be tissues that affect embrittlement. In addition, there may be a plurality of tissues that affect embrittlement, but using the method described above, by correlating the amount of hydrogen at the hydrogen capture site with the presence or absence of delayed fracture, the other tissues that affect embrittlement are It is possible to estimate. Thus, although the presence or absence of delayed fracture has been evaluated based on only the amount of hydrogen so far, the method of the present invention can be used to evaluate delayed fracture based on the amount of hydrogen of only a tissue that affects embrittlement. .

  When evaluating a plurality of sample pieces of the same type of material, since the delayed fractured tissue is the same for the same kind of material, steps S1 to S4 are omitted after the delayed fractured tissue is identified once, and the steps are performed. It is also possible to implement only S5 or later.

[Step S5: Process of Deriving Critical Diffusible Hydrogen Amount]
In the critical diffusive hydrogen amount deriving step, a constant load is applied to the test piece while hydrogen is present in the test piece or in a state where hydrogen is present in the test piece, and the test piece is broken up to a predetermined limit time. The amount of diffusible hydrogen of the test piece which did not generate is measured as the amount of diffusible hydrogen. By this process, the amount of critical diffusible hydrogen leading to the fracture of the steel under a certain applied stress can be determined.

  The measurement of the amount of critical diffusive hydrogen can be performed, for example, by a U-bending bolting test as described in Patent Document 1 and Patent Document 2 or a method using constant load tension such as uniaxial tension. The amount of hydrogen in the steel is measured by thermal desorption analysis, which raises the temperature of the test piece to desorb the hydrogen in the steel, and measures the hydrogen concentration in the steel from the hydrogen that permeates the test piece. Although there are electrochemical hydrogen permeation methods and the like, a thermal desorption analysis method is preferred, in which the preparation of the test piece is easy and the correspondence between the hydrogen desorption peak and the hydrogen trapping site is possible. Preferred embodiments of the thermal desorption analysis method are the same as those described in the description of the test piece hydrogen amount measurement step.

[Step S6: Process of Deriving Critical Hydrogen Amount of Delayed Fracture Structure]
In the critical hydrogen content deriving process of the delayed fracture structure, the delayed diffusive texture identified in the delayed fracture structure identification process (step S4) is obtained from the critical diffusible hydrogen content obtained in the critical diffusible hydrogen content deriving process (step S5). Extract only the amount of captured hydrogen. By this process, the limit hydrogen amount of the tissue that affects the delayed fracture is obtained. That is, how much hydrogen penetrates into the delayed fracture structure of the material under a certain stress is required to cause delayed fracture. In addition, when there exist multiple structures which affect the embrittlement in a test piece, it is preferable to make the total amount of the amount of hydrogen into a limit hydrogen amount.

  The method for extracting only the amount of hydrogen trapped in the delayed fractured tissue is not particularly limited, and may be any method. For example, in the critical diffusive hydrogen amount deriving step (step S5), when the measurement of the diffusible hydrogen amount is performed by a thermal desorption analysis method, a hydrogen desorption curve (hydrogen It can be carried out using the release rate-temperature). Specifically, the above-mentioned "fitting of the hydrogen desorption curve" is applied to the hydrogen desorption curve obtained in the critical diffusive hydrogen amount deriving step (step S5), and the hydrogen desorption peak for each capture site is obtained. To separate. Thereafter, the amount of hydrogen in the tissue captured by the delayed fractured tissue determined in the delayed fractured tissue identification step (step S4) is calculated. This value is the limit hydrogen amount in the present test piece, and it is a feature of the present invention to use this value to compare with other strain amounts and other steel types.

  Furthermore, as a result of various studies by the inventor, the amount of hydrogen trapped only in the structure that affects hydrogen embrittlement of steel materials is a limit that causes delayed fracture if stress and strain are the same, even in different forming modes. It was found that the amount of hydrogen was the same value. That is, since the same limit hydrogen amount can be defined even in different forming styles as long as stress and strain are the same, it is not necessary to obtain the limit diffusible hydrogen amount by various methods which is a task, and to define for each material. It is characterized by being simple.

  If the stress and strain are the same, the same limit hydrogen amount can be determined even in different forming styles. Therefore, in the critical diffusive hydrogen amount deriving step (step S5), it is simple to calculate the strain amount and to process the test piece. It is preferable to measure the limit hydrogen amount by axial tension. However, the present invention is not limited to the calculation of the limit hydrogen amount in the uniaxial tension test method, and tests a constant load while hydrogen is present or hydrogen is added to the material, such as a U-bending or bolting test. Any method can be used as long as it is a method of loading the piece and calculating the limit amount of hydrogen from the amount of hydrogen of the test piece which did not generate breakage until a predetermined limit time.

  Next, based on an example, it explains still more concretely. In this example, a TS: 1180 MPa grade ferrite-martensitic steel (DP steel) was used as a test piece, and a ferritic steel (IF steel) was used as a reference test piece. The thickness of the test piece and the reference test piece was 0.3 mm. Strains of 0 to 0.5 were applied to these test pieces in true strain.

Hydrogen introduction in the test piece hydrogen introduction step (step S1) was performed by the cathode charge method. The cathode charge was performed by electrolysis for 24 hours at a current density of 30 A / m ² in a 3 mass% NaCl + 3 mass% NH ₄ SCN solution, whereby hydrogen was introduced to the test piece. Since hydrogen in the test piece may be released to the outside of the test piece by diffusion, the test piece was stored quickly 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 type temperature rising hydrogen analyzer was used. Thermal desorption analysis was performed at a temperature rising rate of 200 ° C./h in a temperature range of −50 ° C. to 300 ° C. In that case, in order to prevent the hydrogen introduced into the test piece from escaping, the sample frozen in liquid nitrogen was promptly measured by a low-temperature temperature rising hydrogen analyzer.

  Derivation of the limit diffusible hydrogen amount in the limit diffusible hydrogen amount deriving step (step S5) was performed by two types of test methods of the uniaxial tensile test method and the cup reduction test method. The strain of the uniaxial tensile test specimen was applied by rolling, and the cup squeeze test applied strain by squeezing. The amount of strain in the cup reduction was calculated by the finite element method. The fracture starting point stress in 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.

The hydrogen introduction conditions in the critical diffusive hydrogen amount deriving step (step S5) are 1 to 10 A / m ^{2 in} 3% by mass NaCl + 3 g / L NH ₄ SCN solution in the cathodic hydrogen charging method, and pH 1 to ₄ in hydrochloric acid immersion The hydrogen was introduced in the solution of First, hydrogen is introduced to the extent that delayed fracture occurs under this load, the hydrogen introduction condition is gradually reduced from this condition, and the hydrogen amount of the specimen introduced under the condition that destruction does not occur even for 100 hours or more is increased by heating The hydrogen desorption curve was obtained from a test piece having a hydrogen content at the fracture limit, which was measured by desorption analysis.

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

  Next, a hydrogen desorption curve from a test piece having a hydrogen content at the limit of destruction is fitted with several normal distributions on spreadsheet software such as Microsoft (registered trademark) Excel (registered trademark), Among them, only the amount of hydrogen from the tissue affecting the delayed destruction was extracted. Finally, the relationship between the amount of hydrogen and the amount of strain from the structure that affects the delayed fracture obtained was compared in the uniaxial tension test method and the cup reduction test method. The present invention is not limited to this, and is a delayed fracture evaluation method that can be evaluated also with other steel materials and hydrogenation methods.

  FIG. 4 shows the procedure for identifying the hydrogen capture site in the hydrogen capture site identification step (step S3). FIG. 4 (a) is a hydrogen desorption curve in a ferritic steel (IF). In the ferritic steel, Peak A was identified as the “dislocation” hydrogen desorption peak because Peak A was rapidly raised by rolling. 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 this Peak B disappears by being tempered (200 ° C.) at a low temperature, it was identified as a “vacancy cluster” which is regarded as a thermally unstable defect. Thus, it is also possible to identify a hydrogen capture site by increasing or decreasing the hydrogen capture site by heat treatment or the like.

  Next, as shown in FIG. 4 (b), 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 the information obtained by measurement of IF steel. FIG. 4 (c) is a hydrogen desorption curve in DP steel, and fitting with two hydrogen desorption peaks obtained by measurement of IF steel is as shown in FIG. 4 (d). At this time, since the structures of IF steel and DP steel are different, the hydrogen desorption peak of IF steel can not reproduce the DP steel peak alone, but as shown in FIG. 4 (e), the difference is hydrogen specific to DP steel It becomes a hydrogen desorption peak due to the capture site. Since the capture sites not present in this IF steel are prominent in the dual phase structure of the ferrite-martensitic steel, they are identified as "the interface between the martensitic structure and the ferrite structure" (hereinafter, the martensitic structure interface Note). At least a dislocation, a martensitic structure interface, and a vacancy cluster are present as hydrogen capture sites in the strained 1180 MPa class ferrite-martensitic steel.

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

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

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

  As described above, in the method of the present invention, if the stress and strain are the same, the same limit hydrogen amount can be defined even in different forming modes. Therefore, it is not necessary to determine the amount of critical diffusive hydrogen in each test method, and the test can be simplified. Furthermore, if the relationship between hydrogen and strain that can enter in a real environment is known, the presence or absence of delayed fracture can be predicted, and the effect of guiding material design can also be considered.

Claims (5)

A test piece hydrogen introduction step of introducing hydrogen into a metal material as a test piece to cause delayed fracture,
A test piece hydrogen amount measuring step of measuring the amount of hydrogen in the test piece,
A hydrogen trapping site identification step of identifying a plurality of hydrogen trapping sites present in the test piece;
A delayed fracture tissue identification step of identifying a tissue that affects the delayed fracture based on the correspondence relationship between each hydrogen trapping site in the test piece and the delayed fracture;
A state in which hydrogen is present in the test piece or while adding hydrogen to the test piece, a constant load is applied to the test piece, and the test piece in which breakage does not occur until a predetermined limit time during the test period The critical diffusive hydrogen amount deriving step of measuring the diffusive hydrogen content as the critical diffusive hydrogen amount, and the delayed fracture structure that derives only the hydrogen amount trapped in the tissue affecting the delayed fracture among the critical diffusive hydrogen amounts. look free limit hydrogen amount derivation step, and
In the delayed fracture tissue identification step, a delayed fracture test is performed to measure the amount of hydrogen of the broken test piece and the non-broken test piece, and the hydrogen amount is separated for each structure of the hydrogen capture site and compared. A delayed fracture evaluation method for metallic materials that identifies the tissue that affects the delayed fracture.   2. The delay of the metallic material according to claim 1, wherein information of hydrogen originating from tissues involved in delayed destruction is separated from the information related to the amount of hydrogen, and in particular, the delayed fracture characteristics are evaluated by the amount of hydrogen trapped in dislocations. Failure assessment method.   The delayed fracture evaluation method of the metal material according to claim 1 or 2, wherein the thickness of the test piece is 0.1 mm or more and 5.0 mm or less.   The method for evaluating delayed fracture of a metal material according to any one of claims 1 to 3, wherein in the critical diffusive hydrogen amount deriving step, the amount of diffusible hydrogen is measured by a thermal desorption analysis method.   The method for evaluating delayed fracture of a metal material according to claim 4, wherein an amount of hydrogen in a temperature rising range: -50 ° C to 300 ° C is analyzed in the temperature rising desorption analysis method.

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