Method and device for evaluating hydrogen embrittlement

Abstract

The present invention relates to a hydrogen embrittlement evaluation method and an evaluation device, and provides a hydrogen embrittlement evaluation method comprising: a step for applying a stress to a test piece of a steel material so as to have a stress triaxial value pre-selected from the range of 0.30-0.80; and a step for determining the limit hydrogen amount of the steel material. Also provided is a hydrogen embrittlement evaluation device provided with: a holder for holding a test piece of a steel material; the stress applying mechanism is used for applying stress to the test piece; a display unit that displays the amount of displacement of the test piece to which the stress has been applied; and a transmission mechanism for mechanically transmitting the amount of displacement of the test piece to the display unit, in which the application of the stress can be controlled by the stress application mechanism so that the amount of displacement of the test piece corresponds to a predetermined stress triaxial value.

Description

Technical Field

[0001] The present invention relates to a method and an apparatus for evaluating hydrogen embrittlement. Background Art

[0002] In recent years, in the automotive industry, from the viewpoint of improving fuel efficiency, weight reduction of vehicle bodies has been required. In order to achieve both weight reduction of vehicle bodies and collision safety, one effective method is to increase the strength of steel plates used, and against this background, development of high-strength steel materials has been underway. In high-strength steel materials, hydrogen embrittlement cracks (also referred to as delayed fracture, etc.) sometimes become a problem. Hydrogen embrittlement cracks refer to a phenomenon in which a steel member under a high stress in a usage condition or the like suddenly fails due to hydrogen that has invaded the steel from the environment.

[0003] Generally, it is known that the higher the strength of steel, the more likely hydrogen embrittlement cracks occur. Therefore, in the development of high-strength steel materials, it can be said that more accurate evaluation of hydrogen embrittlement is very important in terms of development and selection of appropriate materials.

[0004] In connection with this, Patent Document 1 discloses a method for evaluating the delayed fracture characteristics of a high-strength steel plate, which is characterized in that a test piece obtained by bending a high-strength steel plate into a V shape is prepared, the opening angle of the bent portion of the test piece is changed by a specified angle to be in a state where residual stress is applied to the test piece, and the delayed fracture property of the high-strength steel plate is evaluated based on the occurrence state of cracks in the test piece when the constrained test piece is placed in a hydrogen intrusion environment. The relationship between the maximum residual stress generated on the outermost layer on the front and back surfaces of the plate at the bent portion of the test piece and the change amount is obtained based on the change amount of the opening angle, and based on the obtained relationship, the range of the change amount of the opening angle at which the maximum residual stress exists within the range of the residual stress generated at the bent portion of the product shape when the high-strength steel plate is bent into the target product shape is obtained, and the specified angle is set so as to be within the range of the obtained change amount of the opening angle to perform the above evaluation. In addition, Patent Document 1 teaches that, according to the above method, by always setting the change amount of the opening angle of the test piece bent into a V shape to an appropriate value, the accuracy of evaluating the delayed fracture characteristics at the bent portion of the high-strength steel plate can be improved.

[0005] A method for evaluating the delayed fracture characteristics of a metal plate is described in Patent Document 2. It is a method for evaluating the delayed fracture characteristics of a metal plate. It is characterized in that after a first bending process of bending the metal plate to one side in the plate thickness direction, a second bending process of bending in the opposite direction to the previous bending process in the plate thickness direction is performed one or more times, thereby producing a test piece, and the delayed fracture characteristics are evaluated using the produced test piece. In addition, Patent Document 2 teaches that according to the above method, it is possible to perform a delayed fracture evaluation under conditions where strain and residual stress generated by bending-unbending are introduced into the metal plate. As a result, it is possible to evaluate the delayed fracture characteristics of the metal plate in an environment closer to that of an actual component, and it is possible to easily apply high-strength steel plates to automobile bodies.

[0006] A surface treatment method for a hydrogen embrittlement resistance evaluation test piece is described in Patent Document 3. It is characterized in that electroplating is performed on a test piece made of a hydrogen-containing metal material in a plating bath containing one or both of ZnCl 2 : more than 80 g / l and 300 g / l or less, brightener: 10 - 50 ml / l, NH 4 Cl, KCl in a total amount of 100 - 300 g / l. In addition, Patent Document 3 teaches that according to the above surface treatment method, it is possible to provide a method for evaluating hydrogen embrittlement resistance, which can perform highly accurately and efficiently the determination of the limiting diffusible hydrogen amount, constant load tests, etc. of delayed fracture tests using test pieces plated with zinc, which is safer and has a lower environmental burden than the method using cadmium plating and has a higher hydrogen release prevention ability during the test, and the industrial contribution is extremely significant.

[0007] Prior art documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 2017-142086

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 2022-024814

[0011] Patent Document 3: Japanese Patent Application Laid-Open No. 2008-261742 Summary of the invention

[0012] Problems to be solved by the invention

[0013] The higher the strength of the steel, the more prone it is to hydrogen embrittlement cracks. On the other hand, in the automotive industry and the like, further weight reduction of steel is also required. To achieve such weight reduction, it is necessary to make the steel stronger than before. In connection with this, in an automobile, there are many components made of steel and having different shapes and characteristics, and various stress states may exist depending on these components and the parts of the corresponding components. Therefore, from the viewpoint of appropriately developing and selecting high-strength steel, there is a high demand for an evaluation method of hydrogen embrittlement that takes into account such various stress states.

[0014] Therefore, an object of the present invention is to provide an evaluation method and an evaluation device capable of evaluating hydrogen embrittlement by considering the stress state of steel.

[0015] Means for Solving the Problem

[0016] As a result of research by the present inventors to achieve the above object, it has been found that by applying an appropriate stress to steel while considering the stress triaxiality, various stress states in actual components can be simulated, and thus the present invention has been completed.

[0017] The present invention capable of achieving the above object is as follows.

[0018] (1) An evaluation method of hydrogen embrittlement, comprising: a step of applying a stress to a test piece of steel so as to have a value of stress triaxiality preselected from the range of 0.30 to 0.80; and a step of determining the limiting hydrogen amount of the steel.

[0019] (2) The method according to (1) above, wherein the stress is applied by bending, and the value of the stress triaxiality is controlled by changing the ratio w / t of the width w to the thickness t of the test piece.

[0020] (3) The method according to (1) above, wherein the stress is applied by ball indentation.

[0021] (4) The method according to any one of (1) to (3) above, wherein the step of determining the limiting hydrogen amount of the steel is performed using a test piece into which hydrogen has been introduced and then Zn-plated.

[0022] (5) The method according to any one of (1) to (4) above, wherein, before the step of applying a stress to the test piece of the steel, there is further included a step of calculating the stress triaxiality and the maximum principal stress at a specific part in a component for which hydrogen embrittlement evaluation should be performed,

[0023] The step of applying a stress to the test piece of the steel includes applying a stress corresponding to the calculated stress triaxiality and maximum principal stress to the test piece of the steel.

[0024] (6) An evaluation device for hydrogen embrittlement, comprising:

[0025] A holder for holding a test piece of steel;

[0026] a stress applying mechanism for applying stress to the test piece;

[0027] a display unit that displays the displacement amount of the test piece to which stress is applied; and

[0028] A transmission mechanism for mechanically transmitting the displacement of the test piece to the display unit,

[0029] The application of stress can be controlled by the stress applying mechanism so that the displacement amount of the test piece becomes a displacement amount corresponding to a predetermined value of stress triaxiality.

[0030] (7) The device according to (6) above, wherein the predetermined stress triaxiality value is a value selected from a range of 0.30 to 0.80.

[0031] (8) The device according to (6) or (7) above, wherein the holder for holding the test piece is configured to hold the test piece such that the ratio w / t of the width w to the thickness t varies within a range of 1.00 to 30.00.

[0032] Effects of the Invention

[0033] According to the present invention, it is possible to provide an evaluation method and an evaluation device capable of evaluating hydrogen embrittlement in consideration of the stress state of a steel material. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a schematic diagram showing a conventional method for evaluating hydrogen embrittlement.

[0035] Figure 2 This is a schematic diagram showing a conventional hydrogen embrittlement evaluation device.

[0036] Figure 3 This is a schematic diagram showing a hydrogen embrittlement evaluation device according to an embodiment of the present invention. DETAILED DESCRIPTION

[0037] <Evaluation method for hydrogen embrittlement>

[0038] The hydrogen embrittlement evaluation method according to the embodiment of the present invention is characterized by comprising: applying stress to a test piece of a steel material so as to obtain a stress triaxiality value preselected from a range of 0.30 to 0.80; and determining a limiting hydrogen amount of the steel material.

[0039] Conventionally, hydrogen embrittlement evaluation methods generally use a method using a strain gauge or the like. Figure 1It is a schematic diagram showing a conventional method for evaluating hydrogen embrittlement. Specifically, in the conventional method for evaluating hydrogen embrittlement, for example, a rectangular test piece having a prescribed length and width is cut out from a steel plate, and the test piece is bent 180° by a punch ( Figure 1 as shown in (a)), and then, after that, a bolt and a nut are used to fasten the U-shaped bent test piece after springback ( Figure 1 as shown in (b)), thereby applying stress ( Figure 1 as shown in (c)). At this time, a strain gauge is pasted on the top of the U-shaped bent test piece, and a prescribed stress is applied by controlling the strain amount. As Figure 1 shown in (d), the applied stress is determined using a stress-strain curve obtained by separately conducting a tensile test using the same strain gauge in advance. However, this time, the present inventors found that in such an evaluation method, in tests such as U-shaped bending tests and separately conducted tensile tests based on uniaxial tension, even when the stress values based on the strain amount are the same, the stress states of the test pieces in each test are sometimes different, and associated therewith, appropriate stress application is sometimes not necessarily performed in the hydrogen embrittlement evaluation test. Furthermore, the present inventors found that there is also a case where the stress states are different between the test piece used in the hydrogen embrittlement evaluation method and the actual component, and in such a case, it is sometimes impossible to appropriately evaluate the steel for hydrogen embrittlement.

[0040] Therefore, in order to achieve a more accurate evaluation of hydrogen embrittlement of steel, the present inventors conducted research from the viewpoint of applying appropriate stress to steel by considering the stress state of the actual component. As a result, the present inventors found that by considering the stress triaxiality and applying appropriate stress to steel instead of the conventional evaluation method using only a strain gauge, various stress states in the actual component can be simulated, and as a result, compared with the case of the conventional evaluation method, the limit hydrogen amount, which is the upper limit of the hydrogen amount at which no hydrogen embrittlement crack occurs in the steel, can be calculated more accurately. Specifically, first, the stress triaxiality is an index indicating the multi-axiality of stress, and as shown in the following formula 1, it is a parameter obtained by dividing the (triaxial) mean stress by the equivalent stress.

[0041] Stress triaxiality = Mean stress / Equivalent stress... Formula 1

[0042] In addition, the mean stress and the equivalent stress are represented by the following formulas 2 and 3 respectively using the three components of the principal stress, σ 1 , σ 2 and σ 3 .

[0043] [Mathematical formula 1]

[0044]

[0045] [Mathematical formula 2]

[0046]

[0047] As can be seen from the above equations (1) to (3), for example, in the case of uniaxial tension, σ 1 = 1, σ 2 = σ 3 = 0, and thus the stress triaxiality is 0.33. On the other hand, for example, in the case where tensile stresses are generated in multiple directions, the mean stress becomes larger, and the equivalent stress is the difference between the principal stresses, and thus becomes smaller. As a result, the stress triaxiality becomes a value higher than 0.33. In this way, the value of the stress triaxiality changes according to the stress state of the material, and the multiaxiality becomes larger compared to 0.33 for uniaxial tension, and its value becomes higher in the positive direction, for example, 0.66 in equibiaxial tension, and higher stress triaxiality can be achieved by performing triaxial tension. On the contrary, it becomes negative when in a compressive stress state, for example, the stress triaxiality is -0.33 in the case of uniaxial compression.

[0048] Here, the generation of hydrogen embrittlement cracks becomes significant mainly when a high tensile stress acts on steel. Therefore, as hydrogen embrittlement, the evaluation under stress states where the stress triaxiality is in the positive range is basically studied. However, regarding actual hydrogen embrittlement cracks, for example, in the case of automotive steel sheets, they may occur under various stress states not only during their manufacturing but also during corrosion in the use environment. For example, in the case of high-strength cold-rolled steel sheets, hydrogen sometimes enters the steel from moisture in the atmosphere during the annealing process during manufacturing. In this case, when the obtained high-strength cold-rolled steel sheet is subsequently cold-formed or assembled into components by welding, there is a risk of generating hydrogen embrittlement cracks due to the tensile stress applied during these operations. In addition, in the case of hot stamping materials, since they are heated to a high temperature of about 900 °C during hot stamping forming, hydrogen sometimes enters the steel from moisture in the atmosphere at this time. In this case, when the obtained hot stamping formed body is assembled into components by spot welding or the like, there is also a risk of generating hydrogen embrittlement cracks due to the tensile stress applied during such operations. Furthermore, in the painting process of the vehicle body, hydrogen sometimes enters the steel during chemical generation and electro-deposition. Moreover, even in the use environment after being assembled into a vehicle body, there is a risk that hydrogen generated during corrosion enters the steel and hydrogen embrittlement cracks are generated due to the load stress during use. As described above, even only during manufacturing, the cases of generating hydrogen embrittlement cracks vary depending on the type of steel, and the stress states at this time are diverse, and the stress state may vary significantly depending on the component using the steel and its location. In addition, in the case of hydrogen embrittlement cracks caused by corrosion in the use environment, the stress state changes according to the pits formed by corrosion, and thus it is of course necessary to consider a stress state different from that during manufacturing.

[0049] Therefore, the present inventors have found that by applying stress to a test piece of steel in such a way that the value of the stress triaxiality is preselected from the range of 0.30 to 0.80, various stress states in actual components can be covered, and by determining the limiting hydrogen amount of the steel in such a stress state, a more appropriate evaluation of hydrogen embrittlement of the steel can be achieved. Conventionally, the evaluation of hydrogen embrittlement of steel has mostly been carried out by only considering the absolute value of the stress applied to the steel. Therefore, by applying appropriate stress considering the stress state of the steel and using stress triaxiality for such stress application, the limiting hydrogen amount of the steel is determined, and thus a more appropriate evaluation of hydrogen embrittlement of the steel can be achieved. Such a fact has been first clarified by the present inventors this time. In addition, according to the hydrogen embrittlement evaluation method of the embodiment of the present invention, stress equivalent to a stress state not included in the conventional evaluation method can also be applied. Therefore, a stress state in which hydrogen embrittlement cracks may occur in the test piece of steel for actual components can be appropriately and reliably generated. Therefore, the hydrogen embrittlement evaluation method of the embodiment of the present invention is very useful in the development and selection of appropriate materials for high-strength steels that need to cope with hydrogen embrittlement. Hereinafter, the hydrogen embrittlement evaluation method of the embodiment of the present invention will be described in more detail.

[0050] [Stress application process]

[0051] In the hydrogen embrittlement evaluation method of the embodiment of the present invention, stress is applied to a test piece of steel in such a way that the value of the stress triaxiality is preselected from the range of 0.30 to 0.80. As described above, in the stress state of uniaxial tension, the stress triaxiality is 0.33. In contrast, as the multiaxiality increases, its value becomes higher. However, in the research conducted by the present inventors, in components such as automobiles, a stress state in which the stress triaxiality exceeds 0.80 has not been confirmed during both manufacturing and use. Therefore, it can be considered that by selecting an appropriate value of stress triaxiality from the range of 0.30 to 0.80 according to the component and its part to which the steel is applied, the stress state in actual components can be appropriately simulated. The lower limit of the stress triaxiality can be, for example, 0.33 or more, 0.36 or more, 0.40 or more, or 0.45 or more. Similarly, the upper limit of the stress triaxiality can be, for example, 0.75 or less, 0.70 or less, 0.66 or less, or 0.60 or less.

[0052] As a method for preselecting the stress triaxiality, it may be any suitable method known to those skilled in the art, without particular limitation. For example, CAE (Computer Aided Engineering) analysis can be cited. By using analysis methods such as CAE analysis, design evaluation can be repeatedly performed without manufacturing prototypes of components made of application steel. Moreover, by using analysis methods such as CAE analysis, the stress triaxiality of specific parts of components with a specific shape that should be evaluated for hydrogen embrittlement, such as the bending apex of steel that is prone to hydrogen embrittlement cracks, the surface part, etc., can be appropriately calculated. In addition, the stress that needs to be applied to a test piece of steel in order to obtain the value of the preselected stress triaxiality can be relatively easily obtained. Any suitable software (such as Marc manufactured by MSC Software Corporation) can be used, and for example, an analysis model can be created by the finite element method (FEM) to perform CAE analysis. When creating the analysis model, necessary conditions such as the mesh size can be appropriately determined in consideration of the accuracy of the analysis, the time required for the analysis, etc. As another method for preselecting the stress triaxiality, for example, a method based on X-ray diffraction can be cited. More specifically, stress measurement based on X-ray diffraction can also be performed using a prototype of a component made of application steel, etc., and the relationship between the value of the stress triaxiality of a specific part of the component and the stress that needs to be applied in order to obtain this stress triaxiality value can be obtained in advance based on the obtained data.

[0053] [Stress application by bending, etc.]

[0054] The stress applied to the test piece of steel, such as the stress obtained by CAE analysis or the like based on a preselected stress triaxiality, can be applied by any appropriate operation. Although not particularly limited, for example, the application of stress can be carried out by bending, extrusion, tension, pressing, and torsion, etc. Preferably, the application of stress is carried out by bending and extrusion. Most of the stress applied when assembling an automobile is bending stress. In addition, bending is an operation that is very susceptible to the influence of the steel surface. For example, when the steel surface is covered with an Al coating or the like, the surface becomes hard, so hydrogen embrittlement cracks sometimes easily occur. As a conventional method for evaluating hydrogen embrittlement, in addition to the method using a U-shaped bending test piece described above, for example, there is also a method using a notched tensile test piece. However, compared with the case of bending, the evaluation of hydrogen embrittlement using a notched tensile test piece is hardly affected by the steel surface. Therefore, in the conventional methods using such test pieces, if actual components are considered, appropriate evaluation may not sometimes be carried out. Therefore, the application of stress in the method for evaluating hydrogen embrittlement is also carried out by bending, whereby evaluation can be carried out in an environment closer to actual components. However, even if stress is applied only by bending, as described above, if the stress state is different, appropriate evaluation may not sometimes be possible. Therefore, it is important to appropriately apply a bending stress to the test piece of steel so that the value of the stress triaxiality becomes a value preselected from the range of 0.30 to 0.80.

[0055] A further advantage of stress application by bending is that by changing the ratio w / t of the width w (mm) to the thickness t (mm) of the test piece, the value of the stress triaxiality can be relatively easily controlled. Such an advantage was discovered by the present inventors this time. More specifically, for example, when the width w is relatively small with respect to the thickness t of the test piece, the test piece becomes an elongated shape, and even the bending operation substantially becomes a stress state close to uniaxial tension. Therefore, when the w / t value of the test piece is decreased, the stress triaxiality approaches 0.30, especially 0.33. On the other hand, when the width w is sufficiently large with respect to the thickness t, the displacement of the test piece with respect to the stress is constrained, and a stress state such as a plane strain tension with a higher multiaxiality is formed. Therefore, by increasing the w / t value of the test piece, a stress state having a desired stress triaxiality higher than 0.30, especially higher than 0.33, can be achieved.

[0056] Regarding the w / t of the test piece, an appropriate value may be selected according to the stress triaxiality selected in advance, and there is no particular limitation. For example, it is preferably selected from the range of 1.00 to 30.00. From the viewpoint of controlling the stress triaxiality to a higher value, the w / t may be, for example, 2.00 or more, 3.00 or more, 5.00 or more, 10.00 or more, or 15.00 or more. In the case of applying stress by bending, in addition to the w / t of the test piece, the stress state of the test piece is also affected by other conditions such as curvature and the distance between the fulcrums of the test piece. Therefore, the specific value of w / t needs to be appropriately selected in each evaluation method. However, from the viewpoint of controlling the stress triaxiality to a desired value, even if the w / t is increased excessively, the effect will saturate, and it is difficult to control the stress triaxiality higher than a certain value. Therefore, the w / t is preferably 30.00 or less, and may be, for example, 28.00 or less, 25.00 or less, 22.00 or less, or 20.00 or less. The w / t of the test piece can be easily controlled, for example, by changing the width w while keeping the thickness t constant.

[0057] As a specific example of bending, it includes, for example, four-point bending, three-point bending, or U-shaped bending, etc., and an appropriate bending method can be selected according to the desired stress triaxiality. More specifically, in the case of applying stress by four-point bending, by changing the w / t of the test piece, for example, by changing the w / t of the test piece within the range of 1 to 30, the stress triaxiality can be controlled within the range of 0.30 to 0.50, particularly 0.33 to 0.45.

[0058] [Applying stress by ball head extrusion, etc.]

[0059] On the other hand, in the case where the value of the stress triaxiality selected in advance is higher, in the stress application by bending, it may not always be possible to achieve the desired stress triaxiality. In contrast, a stress triaxiality of 0.66 can be achieved by using equibiaxial stretching. However, in order to perform equibiaxial stretching, the test device becomes relatively large, and it may not always be appropriate. Therefore, the present inventor has studied a stress application method that can relatively easily and reliably achieve a higher stress triaxiality. As a result, the present inventor has found that a relatively high stress triaxiality equivalent to that of equibiaxial stretching can be achieved by using ball head extrusion. In addition, ball head extrusion has the advantage of being as easily affected by the steel surface as the above-mentioned bending and being able to evaluate close to actual body components.

[0060] For example, by fixing a test piece to a die having a specified hole diameter and then pressing a punch having a spherical head with a specified diameter that is usually smaller than the hole diameter against the test piece to perform spherical head extrusion, stress application by spherical head extrusion can be carried out. Therefore, stress application by spherical head extrusion can be implemented relatively compactly and easily. In addition, the vertex portion of the spherical head becomes a stress state close to equibiaxial tension, so a higher stress triaxiality that is difficult to reliably achieve in bending can be realized, specifically, a stress triaxiality of 0.55 to 0.65 or 0.57 to 0.65. Further, by appropriately changing the shape of the test piece, the stress state of the test piece can be further changed. For example, by changing the shape of the test piece from a square to a shape in which necks are provided in a pair of opposing parallel portions of the square, the value of the above stress triaxiality can be changed to a lower value, for example, 0.45 to 0.55 or 0.45 to 0.59. In automotive components, for example, in a skeletal member such as a pillar member, a relatively high stress state as described above may occur. Therefore, stress application by spherical head extrusion can form a stress state that cannot be simulated by existing hydrogen embrittlement evaluation methods, and is very useful in the development and selection of appropriate materials for high-strength steel that needs to cope with hydrogen embrittlement.

[0061] As described above, by applying stress application by bending and stress application by spherical head extrusion, at least the stress states from a stress state equivalent to uniaxial tension to a stress state equivalent to equibiaxial tension can be comprehensively covered. Therefore, according to an embodiment of the present invention, by appropriately differentiating the use of these stress applications according to the component for which hydrogen embrittlement evaluation should be performed, and further appropriately changing the shape of each test piece, more accurate hydrogen embrittlement evaluation can be performed in an environment closer to the actual component. In addition, by performing triaxial tension, a stress state higher than the stress state equivalent to equibiaxial tension can be formed, and more specifically, a stress state equivalent to a stress triaxiality of 0.65 to 0.80 can be formed. Therefore, the hydrogen embrittlement evaluation method according to the embodiment of the present invention can of course be usefully applied in the automotive field, and can also be usefully applied in the evaluation of steel in other technical fields that require both high strength and excellent hydrogen embrittlement resistance characteristics.

[0062] [Procedure for determining the limiting hydrogen amount]

[0063] In the hydrogen embrittlement evaluation method according to an embodiment of the present invention, the limiting hydrogen amount (Hc) of the steel is determined in the next process for a test piece that has been stressed in the stress application process. In the present invention, the limiting hydrogen amount (Hc) refers to the upper limit hydrogen amount at which the steel does not develop hydrogen embrittlement cracks. The determination of the limiting hydrogen amount of the steel is not particularly limited, and can be performed by any appropriate method known to those skilled in the art.

[0064] For example, first, a prescribed shape is cut out in a manner suitable for a specific test method. For a test piece into which hydrogen has been introduced by any appropriate method, stress is applied in the stress application process described above. In the subsequent process of determining the limiting hydrogen amount, when evaluating hydrogen embrittlement of steel materials used in the manufacture of automobiles or the like, the test piece to which stress has been applied as described above is placed indoors for 1 to 5 days, for example. After allowing hydrogen to fully diffuse into the test piece by placing it indoors for 1 to 5 days, the test piece is observed for cracks. At the same time, the test piece is heated by the temperature-programmed desorption method, and the amount of hydrogen introduced is measured based on the amount of hydrogen released at this time. The temperature-programmed desorption can be carried out by heating the test piece from room temperature to 250°C or 300°C at a prescribed heating rate, for example, 100°C / h. The test piece for measuring the amount of hydrogen introduced can be a test piece different from the test piece to which stress has been applied, that is, as long as it is a test piece into which hydrogen has been introduced by the same method as the test piece to which stress has been applied, stress may not be applied. However, since the amount of hydrogen introduced sometimes varies due to stress, when comparing between steel materials, it is preferable to measure the hydrogen amount using a test piece to which no stress has been applied. On the other hand, when evaluating hydrogen embrittlement of steel materials during the use of automobiles or the like, after placing the test piece to which stress has been applied in an outdoor exposure test for up to 10 years according to the assumed corrosion environment or conducting an accelerated test such as a cyclic corrosion test (CCT) under appropriate conditions, the test piece is observed for cracks, and similarly, the maximum amount of hydrogen introduced at which no cracks occur can be determined as the limiting hydrogen amount.

[0065] The timing of hydrogen introduction need not be limited. For example, hydrogen can be introduced before stress is applied to a test piece cut out from a steel material. Alternatively, hydrogen can be introduced into the steel material before the test piece is cut out. However, in this case, it is preferable to cool the test piece with dry ice or the like during processing of the test piece to prevent hydrogen from escaping. Or, from the viewpoint of simulating the painting process, for example, hydrogen can also be introduced in a state where stress has been applied to a test piece after stress has been applied by four-point bending or the like. However, in this case, it is preferable not to perform hydrogen encapsulation based on conventionally known Cd plating or Zn plating described in detail later. When simulating hydrogen introduction during high-temperature heating for hot stamping forming, it is preferable to introduce hydrogen into the steel material before the test piece is cut out. On the other hand, when simulating hydrogen embrittlement cracks during the manufacturing process of automobiles or the like, hydrogen can also be introduced into a test piece before stress is applied cut out from a steel material or a test piece after stress has been applied by bending or the like.

[0066] The method for introducing hydrogen is not particularly limited, and any appropriate method known to those skilled in the art can be adopted. For example, the introduction of hydrogen can be carried out by immersing steel materials or test pieces in a solution containing a hydrogen supply source such as hydrochloric acid or thiocyanic acid, or by chemical generation or electrodeposition treatment of steel materials or test pieces. In the case of hot stamping materials, for example, in addition to the above-introduced methods, the introduction of hydrogen can also be carried out by dew point control under heat treatment. More specifically, it is also possible to heat the steel material to a specified temperature, such as about 900 °C or a temperature higher than that, and hold it for, for example, 120 to 1000 seconds in a heating furnace with the dew point controlled to a specified value within the range of -30 to +50 °C, thereby introducing hydrogen from the moisture in the atmosphere into the steel material. Thus, the introduction of hydrogen during high-temperature heating in hot stamping forming can be simulated. In addition, by changing the dew point in the heating furnace and / or the holding time of the steel material, the amount of hydrogen introduced into the steel material can be controlled. More specifically, by increasing the dew point and / or extending the holding time, the amount of hydrogen introduced into the steel material can be increased, and similarly, by decreasing the dew point and / or shortening the holding time, the amount of hydrogen introduced into the steel material can be reduced.

[0067] On the other hand, in the case of high-strength cold-rolled steel sheets, when heat treatment at a high temperature as described in relation to hot stamping materials is carried out, sometimes the metal structure changes and the strength decreases. Therefore, from the viewpoint of simulating hydrogen embrittlement cracks when cold-forming high-strength cold-rolled steel sheets or assembling them into components by welding, for example, it is preferable to introduce hydrogen by immersing steel materials or test pieces in a solution containing a hydrogen supply source such as hydrochloric acid or thiocyanic acid. In this case, the immersion of test pieces, etc. in a solution containing a hydrogen supply source can be carried out, for example, as follows: A test piece stressed by four-point bending or U-bending, etc. using a specified fixture is immersed together with the fixture in a solution containing a hydrogen supply source such as hydrochloric acid or thiocyanic acid and having a specified pH or concentration for, for example, 24 to 72 hours. In the case of immersion in hydrochloric acid, for example, the amount of hydrogen introduced can be controlled by changing the pH of the hydrochloric acid aqueous solution. More specifically, the amount of hydrogen introduced can be increased by forming a more acidic solution by decreasing the pH. Similarly, in the case of immersion in thiocyanic acid, the amount of hydrogen introduced can be controlled by changing the concentration of the solution containing thiocyanic acid, such as ammonium thiocyanate solution. More specifically, the amount of hydrogen introduced can be increased by increasing the concentration of the ammonium thiocyanate solution.

[0068] The cutting (processing) of the test piece is not particularly limited, and any appropriate method such as wire electrical discharge machining, laser cutting, etc. can be used. However, after cutting by wire electrical discharge machining, laser cutting, etc., the end face is preferably finish ground with a grinding tool or the like to remove the heat-affected part generated by cutting. In addition, for example, when hydrogen is introduced before processing the test piece, there is a problem that hydrogen will escape from the cut end face after processing the test piece. On the other hand, when hydrogen is introduced after processing the test piece, there is also a problem that hydrogen escapes from the cut end face of the test piece after hydrogen is introduced. When such hydrogen escape occurs, it may not be possible to more accurately simulate the hydrogen intrusion state in the actual component. To elaborate, generally, the surface of the steel material is plated, but the cut end face after processing is not plated. Therefore, the hydrogen that intrudes into the steel during manufacturing is mainly likely to escape from the unplated cut end face. However, in actual components such as in automobiles, when comparing the area of the entire component with the area of the cut end face, the area of the entire component is very large, so the amount of hydrogen intruded into the component is not necessarily greatly affected by the hydrogen escape from the cut end face. In contrast, the test piece used in the hydrogen embrittlement evaluation method is very small in size compared to the actual component. Nevertheless, for example, in the case of a rectangular test piece, since the four sides are surrounded by cut end faces having the same thickness (plate thickness) as the actual component, the proportion of the area of the cut end face to the area of the entire test piece becomes very large. Therefore, when compared with the actual component, the amount of hydrogen introduced into the test piece is relatively greatly affected by the hydrogen escape from the cut end face. Therefore, in order to more accurately simulate the hydrogen intrusion state in the actual component, it is preferable to suppress or prevent hydrogen from escaping from the cut end face of the test piece.

[0069] [Enclosure of the introduced hydrogen]

[0070] In this regard, in a preferred embodiment of the present invention, Zn plating is performed on the test piece into which hydrogen has been introduced. By performing Zn plating on at least the cut end face of the test piece into which hydrogen has been introduced, and preferably on the entire surface of the test piece, it is possible to reliably suppress or prevent the hydrogen introduced from escaping from the cut end face or the like. The Zn plating is preferably electroplating. In a preferred embodiment of the present invention, the Zn plating is performed not for the purpose of corrosion resistance but for the purpose of encapsulating hydrogen. Therefore, as long as the Zn plating can be uniformly performed on the entire test piece at a thickness sufficient to encapsulate the hydrogen introduced into the test piece. From this perspective, electroplating is more preferably used as the Zn plating compared to hot-dip galvanizing. The thickness of the Zn plating is, for example, preferably 5 μm or more, more preferably 10 μm or more or 15 μm or more, and most preferably 20 μm or more. In this regard, in the experiments conducted by the present inventors, it was confirmed that by performing Zn plating with a thickness of 20 μm or more on the entire surface of the test piece, hydrogen escape from the cut end face or the like of the test piece can be completely or substantially completely prevented. In this case, considering the fact that the cut end face in the actual component is not Zn-plated, etc., in the actual component, as described above, although the overall area of the component is much larger than the area of the cut end face, hydrogen escape itself may occur from the cut end face. In contrast, according to a preferred embodiment of the present invention, when Zn plating with a thickness of 20 μm or more is performed on the test piece, it is possible to evaluate in a state where the initially introduced hydrogen is completely or substantially completely encapsulated in the test piece. Therefore, compared with the case of the actual component, a more stringent evaluation or at least an equivalent evaluation can be performed for hydrogen embrittlement.

[0071] In order to make the above effects significant when Zn plating is performed, the test piece into which hydrogen has been introduced is preferably subjected to Zn plating treatment as soon as possible before hydrogen escapes from the cut end face or the like. For example, when hydrogen is introduced after the processing of the test piece, it is preferable to perform Zn plating treatment on the test piece within 30 minutes, preferably within 10 minutes, after the introduction of hydrogen. Similarly, when the test piece is processed after hydrogen is introduced into the steel, it is preferable to perform Zn plating treatment on the test piece within 30 minutes, preferably within 10 minutes, after the processing of the test piece.

[0072] On the basis of or in place of the Zn plating treatment, after hydrogen is introduced into the test piece, during each operation such as subsequent processing operations, hydrogen encapsulation operations, and stress application operations of the test piece, the test piece is preferably cooled and stored (frozen treatment) by immersion in dry ice (more specifically, solid dry ice), an organic solvent cooled with dry ice, or liquid nitrogen. By cooling and storing the test piece with dry ice or the like, hydrogen escape from the cut end face or the like of the test piece can be suppressed or reduced. Therefore, the intrusion state of hydrogen in the actual component can be more accurately simulated.

[0073] [Steel]

[0074] The steel material to which the hydrogen embrittlement evaluation method according to the embodiment of the present invention is applied may be any steel material that may undergo hydrogen embrittlement. Although not particularly limited, for example, the steel material may be a so-called high-strength cold-rolled steel sheet, more specifically, a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, or may be a steel material for hot stamping or a hot-stamped formed body formed by hot stamping. Generally speaking, as the steel material becomes higher in strength, it becomes difficult to evaluate hydrogen embrittlement in an environment closer to that of actual components such as automobiles. However, in the hydrogen embrittlement evaluation method according to the embodiment of the present invention, by considering the stress state of the steel material and applying an appropriate stress to the test piece of the steel material using the stress triaxiality, even for high-strength steel materials, a more appropriate evaluation of hydrogen embrittlement can be achieved. Therefore, the hydrogen embrittlement evaluation method according to the embodiment of the present invention can be applied to steel materials having a very high strength, for example, it can be applied to steel materials having a tensile strength of 980 MPa or more, 1180 MPa or more, 1470 MPa or more, 1600 MPa or more, 1800 MPa or more, or 2000 MPa or more. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the steel material may be 3150 MPa or less, 2850 MPa or less, or 2500 MPa or less. The tensile strength of the steel material is measured by fabricating a No. 5 test piece in accordance with JIS Z 2241:2022 and conducting a tensile test. When it is not possible to obtain a JIS No. 5 test piece due to restrictions on the size of the steel material, an ASTM 1 / 2 test piece may be used in accordance with ASTM standards E8.

[0075] [Coating]

[0076] The steel material may have a coating on a part or all of its surface. The coating is also referred to as a film, plating, alloyed plating, or intermetallic compound layer. For example, by applying the same coating as that applied to the actual component to the test piece, a more accurate evaluation of hydrogen embrittlement can be carried out in an environment closer to that of the actual component. The coating may be a coating mainly composed of an Fe-Al alloy or a coating mainly composed of an Fe-Zn alloy. A coating mainly composed of an Fe-Al alloy refers to a coating containing 70 mass% or more of Fe and Al in total, and a coating mainly composed of an Fe-Zn alloy refers to a coating containing 70 mass% or more of Fe and Zn in total. In addition, the coating may also be an aluminum plating, an aluminum-zinc plating, an aluminum-silicon plating, a hot-dip galvanized layer, an electrogalvanized layer, an alloyed hot-dip galvanized layer, a zinc-nickel plating, or an aluminum-magnesium-zinc-based plating. When the coating contains Al, the Fe-Al alloy formed at the interface between the coating and the steel material functions as a barrier to hydrogen, thereby reducing or suppressing the hydrogen dissipation from the steel material to the outside. Therefore, from the viewpoint of conducting a more accurate evaluation of hydrogen embrittlement, the steel material may also have a coating containing Al. The thickness of the coating is not particularly limited, and is usually preferably 10 to 100 μm.

[0077] [Calculation process of stress conditions]

[0078] In the method for evaluating hydrogen embrittlement according to the preferred embodiment of the present invention, before the step of applying stress to a test piece of steel, it further includes a step of calculating the stress triaxiality and the maximum principal stress of a specific part of a component to be evaluated for hydrogen embrittlement, particularly in a steel component, and the step of applying stress to the test piece of steel includes applying stress corresponding to the calculated stress triaxiality and the maximum principal stress to the test piece of steel.

[0079] As described above, as shown in the following formula 1, the stress triaxiality is a parameter obtained by dividing the (triaxial) mean stress by the equivalent stress, and the mean stress and the equivalent stress are calculated using the three components of the principal stress, namely σ 1 , σ 2 and σ 3 which are respectively represented by the following formulas 2 and 3.

[0080] Stress triaxiality = Mean stress / Equivalent stress... Formula 1

[0081] [Formula 3]

[0082]

[0083] [Formula 4]

[0084]

[0085] Here, the maximum principal stress refers to the maximum value among the three components of the principal stress, namely σ 1 , σ 2 and σ 3 . Alternatively, the maximum principal stress (σ 1 ), the intermediate principal stress (σ 2 ) and the minimum principal stress (σ 3 ) can be set in order from the larger value, that is, σ 1 is defined as the maximum principal stress. In the preferred embodiment of the present invention, by using analysis methods such as CAE analysis, the stress triaxiality and the maximum principal stress of a specific part of a component to be evaluated for hydrogen embrittlement, particularly in a steel component, such as the bending apex and the surface part of a steel material prone to hydrogen embrittlement cracks, are calculated, and the stress corresponding to the calculated stress triaxiality and the maximum principal stress is applied to the test piece of steel. At this time, the present inventors have found that when applying stress in the stress application step, especially when applying stress corresponding to the stress triaxiality and the maximum principal stress to the test piece, it is not greatly affected by the stress state of the steel material, and the limiting hydrogen amount of the steel material can be determined more stably.

[0086] More specifically, as described above, the limiting hydrogen amount refers to the upper limit of the hydrogen amount at which no hydrogen embrittlement cracks occur in the steel. Generally, the higher the strength of the steel, the smaller this value. Therefore, even when the steel is strengthened to a high strength, as long as the limiting hydrogen amount of the steel can be maintained above a certain constant high value, the steel can be evaluated as having excellent hydrogen embrittlement resistance. On the other hand, if the stress conditions applied are not constant, the resulting limiting hydrogen amount will deviate, and thus the hydrogen embrittlement resistance of the steel cannot be accurately evaluated. For example, when testing the same steel, if a higher stress is applied in one test and a lower stress is applied in the other test, the calculated limiting hydrogen amount of the steel will be smaller in the test with the higher applied stress. Therefore, in the conventional hydrogen embrittlement evaluation method, a specified stress is applied to the test piece through a U-bending test or the like using a strain gauge, etc., so that the calculated limiting hydrogen amount of the steel does not change due to the application of stress. However, as described above, even in such an evaluation method, there are sometimes differences in the stress states among tests such as the U-bending test, a tensile test based on uniaxial tension performed separately therefrom, and actual components. In such cases, appropriate stress application may not necessarily be carried out in the hydrogen embrittlement evaluation test. In contrast, according to an embodiment of the present invention, instead of the conventional evaluation method that only uses a strain gauge, appropriate stress is applied to the steel considering the stress triaxiality, whereby various stress states in actual components can be simulated. As a result, compared with the case of the conventional evaluation method, the limiting hydrogen amount, that is, the upper limit of the hydrogen amount at which no hydrogen embrittlement cracks occur in the steel, can be calculated more accurately.

[0087] According to a preferred embodiment of the present invention, by applying to the test piece of the steel a stress related to the stress triaxiality and the principal stresses σ 1 , σ 2 and σ 3The stress corresponding to the highest principal stress (i.e., the maximum principal stress) among them. Thus, for example, the limiting hydrogen content of steel will not vary significantly due to the value of the stress triaxiality, and the limiting hydrogen content can be determined more stably. Although not intending to be bound by any specific theory, in actual components, the stress states are diverse, and it can be considered that the hydrogen embrittlement characteristics of steel are determined by the highest stress among the load stresses in various directions that make up such a stress state. Therefore, in the evaluation method of hydrogen embrittlement, it can also be said that the absolute value of the applied stress is not important, and it is more important to use the stress triaxiality to more specifically grasp the stress state of a specific part of the component for which hydrogen embrittlement evaluation should be carried out. In the calculation process of the stress conditions, the stress triaxiality and the maximum principal stress that make up such a stress state can be calculated, and the stress corresponding to the calculated stress triaxiality and maximum principal stress can be obtained. More specifically, it is the stress (or the displacement amount of the test piece) required to reproduce the calculated stress triaxiality and maximum principal stress in the test piece in the hydrogen embrittlement evaluation test. When the stress thus obtained is applied to the test piece in the subsequent stress application process, it can be considered that the stress is applied appropriately in an environment closer to the actual component, and thus a more accurate evaluation of hydrogen embrittlement can be carried out.

[0088] The grasp of the stress state in the actual component, that is, the calculation of the stress triaxiality and the maximum principal stress, and the calculation of the stress required to reproduce the stress triaxiality and the maximum principal stress in the test piece, etc., are not particularly limited, but can be relatively easily carried out by using analysis methods such as CAE analysis.

[0089] <Hydrogen Embrittlement Evaluation Device>

[0090] Next, a hydrogen embrittlement evaluation device suitable for implementing the hydrogen embrittlement evaluation method described above will be described. However, this evaluation device is not limited to being used in such a specific evaluation method, and of course, it can also be applied to any evaluation method for evaluating the hydrogen embrittlement resistance characteristics of steel.

[0091] The hydrogen embrittlement evaluation device according to an embodiment of the present invention is characterized by including: a holder for holding a test piece of steel; a stress application mechanism for applying stress to the test piece; a display unit for displaying the displacement amount of the test piece to which stress has been applied; and a transmission mechanism for mechanically transmitting the displacement amount of the test piece to the display unit, and the application of stress can be controlled by the stress application mechanism so that the displacement amount of the test piece becomes a displacement amount corresponding to a specified value of the stress triaxiality.

[0092] Figure 2 It is a schematic diagram showing a conventional hydrogen embrittlement evaluation device. Refer to Figure 2, a conventional hydrogen embrittlement evaluation apparatus 1 includes a holder 3 for holding a test piece 2 of steel, a stress application mechanism 4 for applying stress to the test piece 2, and a strain gauge 5 attached to the bending apex of the test piece 2. The strain gauge 5 generally has a structure in which a resistor made of metal arranged in a zigzag shape or the like is mounted on a thin insulator, and can measure the resistance change accompanying the deformation of the resistor to calculate the strain of the object to be measured (test piece). In addition, in Figure 2 , a structure composed of a load clamp 3a and a support clamp 3b is shown as the holder 3, and a four-point bending test in which the test piece is bent by two fulcrums of the load clamp 3a and two fulcrums of the support clamp 3b can be performed. In the conventional hydrogen embrittlement evaluation apparatus 1, as described in connection with Figure 1 (d), the stress applied to the test piece 2 is determined using a stress-strain curve obtained by performing a tensile test or the like separately using the same strain gauge in advance. However, in this case, even if the stress values based on the strain are the same in the evaluation test and the separately performed tensile test or the like, the stress states of the test pieces in each test may be different, and the stress states may also be different between the test piece used in the evaluation test and the actual component. In such a case, of course, the evaluation of steel for hydrogen embrittlement cannot be appropriately performed.

[0093] Figure 3 is a schematic diagram showing a hydrogen embrittlement evaluation apparatus according to an embodiment of the present invention. Referring to Figure 3 , the hydrogen embrittlement evaluation apparatus 10 according to the embodiment of the present invention includes: a holder 3 for holding a test piece 2 of steel; a stress application mechanism 4 for applying stress to the test piece 2; a display unit 6 for displaying the displacement amount of the test piece 2 to which stress is applied; and a transmission mechanism 7 for mechanically transmitting the displacement amount of the test piece 2 to the display unit 6. In Figure 3Similarly, in this case, a structure composed of a load clamp 3a and a support clamp 3b is shown as the retainer 3, and a four-point bending test capable of bending the test piece by two fulcrums of the load clamp 3a and two fulcrums of the support clamp 3b can be performed. In the hydrogen embrittlement evaluation device 10 according to the embodiment of the present invention, when the test piece 2 is displaced downward by such a four-point bending test, the transfer mechanism 7 disposed in contact with the displacement portion of the test piece 2 presses down the display portion 6 along the axial direction according to the displacement amount, whereby the displacement amount is mechanically transmitted to the display portion 6, and the displacement amount of the test piece 2 is displayed in the display portion 6. Therefore, in the hydrogen embrittlement evaluation device 10 according to the embodiment of the present invention, unlike the case of the conventional hydrogen embrittlement evaluation device 1, it is not necessarily required to use a strain gauge 5 for electrically detecting the displacement amount (strain amount) of the test piece 2. That is, the hydrogen embrittlement evaluation device 10 according to the embodiment of the present invention may not include the strain gauge 5. More specifically, in the evaluation device 10, first, by using an analysis method such as CAE analysis, the displacement amount of the test piece 2 corresponding to a specified value of stress triaxiality is calculated in advance. Then, a stress is applied to the test piece 2 by the stress application mechanism 4 so that the displacement amount of the test piece 2 displayed on the display portion 6 becomes the displacement amount calculated in advance. In Figure 3 this case, a bolt is shown as the stress application mechanism 4, and the bolt penetrates the outer wall of the retainer 3 and contacts the surface of the load clamp 3a in the retainer 3 on the side opposite to the surface having two fulcrums. By adopting such a structure, the load clamp 3a can be pressed in by screwing in the bolt as the stress application mechanism 4, and the test piece 2 can be subjected to four-point bending by two fulcrums of the load clamp 3a and two fulcrums of the support clamp 3b. According to the evaluation device 10, while confirming the displacement amount of the test piece displayed on the display portion 6 (equivalent to the "press-in amount" in the embodiment shown in Figure 3 ), the application of stress can be controlled or adjusted by the stress application mechanism 4, so that precise stress application can be achieved. In addition, from the viewpoints of improving the evaluation accuracy of hydrogen embrittlement and reducing the burden of condition setting in analysis methods such as CAE analysis, a strain gauge may also be used in combination to confirm whether the displacement amount calculated by CAE analysis or the like is the target displacement amount. Hereinafter, each component of the hydrogen embrittlement evaluation device according to the embodiment of the present invention will be described in more detail.

[0094] [Test Piece]

[0095] The test piece may be made of any steel material that may cause hydrogen embrittlement. Specifically, it can be made of a steel material described in connection with the hydrogen embrittlement evaluation method according to the embodiment of the present invention, for example, a steel material having a tensile strength of 980 MPa or more. The shape of the test piece can be any shape suitable for a specific test method. Although not particularly limited, for example, in Figure 3In the case of a test based on four-point bending, three-point bending, U-shaped bending, etc., the test piece may have a rectangular shape. Further, in the case of a test based on ball head extrusion, the test piece may have a rectangular or square shape.

[0096] [Holder]

[0097] The holder for holding the test piece of steel is not particularly limited and may have any structure configured to hold the test piece and apply a prescribed stress, such as a stress based on three-point bending, four-point bending, or ball head extrusion. For example, in the case of applying a stress by four-point bending, as Figure 3 shown, the holder may be composed of a load jig and a support jig each having two fulcrums, and the test piece is held between the side having two fulcrums of the load jig and the side having two fulcrums of the support jig. Similarly, for example, in the case of applying a stress by three-point bending, the holder may be composed of a load jig having one fulcrum and a support jig having two fulcrums, and the test piece is held between the side having one fulcrum of the load jig and the side having two fulcrums of the support jig.

[0098] The holder may also be configured to be able to hold a test piece in which the ratio w / t of the width w (mm) to the thickness t (mm) varies within the range of 1.00 to 30.00. For example, in the case of applying a stress by bending such as four-point bending and three-point bending, a rectangular test piece is usually used, and a wide test piece is not used. Therefore, a conventional holder for applying a bending stress is not configured to be able to hold a test piece in which the ratio w / t of the width w to the thickness t varies within the range of 1.00 to 30.00, particularly a wide test piece. As described above regarding the evaluation method of hydrogen embrittlement, by varying the ratio w / t of the width w to the thickness t of the test piece, the value of the stress triaxiality can be relatively easily controlled. Therefore, in the case where the holder for holding the test piece can hold not only the test piece having a normal w / t used in the past but also a wide test piece having a relatively large w / t, various stress states in actual components can be simulated in the same hydrogen embrittlement evaluation device only by appropriately selecting the shape of the test piece. w / t may be, for example, 2.00 or more, 3.00 or more, 5.00 or more, 10.00 or more, or 15.00 or more. Similarly, w / t may be, for example, 28.00 or less, 25.00 or less, 22.00 or less, or 20.00 or less. The shape of the holder for holding a test piece in which w / t varies within the range of 1 to 30.00 is not particularly limited, but for example, the size of the holder in the direction corresponding to the width direction of the test piece, more specifically Figure 3 shown, the depth dimension of the holder is large enough.

[0099] In the case of applying stress by ball head extrusion, a holder for holding a test piece of steel can, for example, include a die having a specified hole diameter that is commonly used in ball head extrusion molding and a punch with a ball head suitable for this hole diameter. For example, Figure 3 the load clamp 3a of the holder shown can be replaced with a punch of a ball head, a die having a specified hole diameter for ball head extrusion molding can be arranged inside this holder, and a test piece can be held between them. As described above regarding the evaluation method of hydrogen embrittlement, by applying stress application based on bending and stress application based on ball head extrusion, it is possible to at least comprehensively cover the stress states from a stress state equivalent to uniaxial tension to a stress state equivalent to equi-biaxial tension. Therefore, by making Figure 3 the holder shown can also be applied to stress application based on ball head extrusion, and thus various stress states can be formed in one hydrogen embrittlement evaluation device.

[0100] [Stress application mechanism and transmission mechanism]

[0101] The stress application means for applying stress to the test piece is not particularly limited and can be any suitable means commonly used in the evaluation of hydrogen embrittlement. For example, as a stress application mechanism, as Figure 3 illustrated in, a screwing-in mechanism such as a bolt can be used. A screwing-in mechanism such as a bolt can reduce the axial displacement amount with respect to the screwing-in rotation amount. Therefore, a screwing-in mechanism such as a bolt can perform precise control of the displacement amount and is useful for application as a stress application mechanism. The transmission mechanism for mechanically transmitting the displacement amount of the test piece to the display unit is not particularly limited and can be any structure that can mechanically transmit the displacement amount of the test piece based on the stress application mechanism to the display unit. For example, in the case of applying stress by four-point bending, three-point bending, and ball head extrusion, as Figure 3 shown, the displacement amount of the test piece can correspond to the press-in amount. Therefore, in the case of such an embodiment, the transmission mechanism is preferably a columnar body such as a cylinder or a prism having an axial length. By arranging such a columnar body as the transmission mechanism in contact with the displacement portion of the test piece, as Figure 3 shown, the transmission mechanism presses down on the display unit along the axial direction according to the displacement amount of the test piece, and thus the displacement amount can be mechanically and reliably transmitted to the display unit.

[0102] [Display unit]

[0103] The display unit for displaying the displacement amount of the test piece to which stress is applied is not particularly limited, and it can be any device that can convert the displacement amount mechanically transmitted by the transmission mechanism into an electrical signal or the like for display. In the hydrogen embrittlement evaluation device according to an embodiment of the present invention, for example, the displacement amount of the test piece corresponding to a value of a prescribed stress triaxiality can be calculated in advance by CAE analysis or the like, and while confirming the displacement amount of the test piece displayed by the display unit, the application of stress is controlled by the stress application mechanism so that this displacement amount becomes the pre-calculated displacement amount, and thus precise stress application can be achieved.

[0104] Although not shown in Figure 3 , the hydrogen embrittlement evaluation device according to an embodiment of the present invention may also include a stand for setting up this evaluation device. The stand can be any structure that can fix the positional relationship between the holder and the display unit in the hydrogen embrittlement evaluation device. By fixing the positional relationship between the holder and the display unit, even when stress is applied to the test piece by a pressing operation or the like based on the stress application mechanism, the holder and the display unit do not move, and their positional relationship can be maintained. Therefore, precise stress application can be reliably achieved. The fixing method is not particularly limited. For example, a placement part for placing the holder and / or the display unit can be provided on the stand, and the holder and / or the display unit can be fixed on this placement part, or the holder and / or the display unit can be fixed to the side wall part of the stand.

[0105] Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited by any of these examples.

[0106] Examples

[0107] In the following examples, the hydrogen embrittlement of steel materials was evaluated under various stress states using the hydrogen embrittlement evaluation method and evaluation device according to an embodiment of the present invention, and the limiting hydrogen amount (Hc) was calculated in each evaluation.

[0108] [B1~B10]

[0109] First, a steel material having the chemical composition shown in Table 1 is charged into a heating furnace in which the dew point is controlled to a specified value within the range of -30 to +50°C and heated to 920°C. Then, it is held for a specified time within the range of 120 to 1000 seconds, whereby hydrogen is introduced from the moisture in the atmosphere into the steel material. Next, the steel material is taken out of the heating furnace, clamped with a flat die at a temperature around room temperature, and quenched to obtain a hot stamping material. A rectangular four-point bending test piece or a square ball head extrusion test piece having a w / t (ratio of the width w to the thickness t of the test piece) shown in B1 to B10 of Table 2 is produced from the hot stamping material into which hydrogen has been introduced. During the processing, it is cryogenically treated with dry ice. Next, the test piece is electro-galvanized to encapsulate the hydrogen introduced into the test piece. The test piece encapsulating hydrogen is set in the holder of the hydrogen embrittlement evaluation device shown in Figure 3 , and stress is applied to the test piece by four-point bending or ball head extrusion so as to achieve the values of the stress triaxiality and the maximum principal stress shown in Table 2. Then, it is held for up to 72 hours and the presence or absence of cracks is confirmed. The values of the stress triaxiality and the maximum principal stress are pre-calculated using CAE analysis.

[0110] More specifically, when calculating the value of the stress triaxiality, a tensile test of the steel material is carried out, and CAE analysis is performed using the obtained strain-stress curve. The tensile test is carried out in accordance with ISO standard 6892-1:2009. The CAE analysis is carried out using Marc produced by MSC Software Corporation, and four-point bending and ball head extrusion are modeled by the finite element method. When modeling based on the finite element method, the mesh size is set to 0.2 mm, and the friction coefficient μ at the contact part between the test piece and the evaluation device is set to 0.05.

[0111] The test is repeated by changing the conditions of the heating furnace to change the amount of hydrogen introduced, and the presence or absence of cracks in the test piece after the test is observed. The maximum amount of hydrogen introduced without generating cracks is determined as the limiting hydrogen amount (Hc). Regarding the amount of hydrogen introduced, the test piece into which hydrogen has been introduced under the same conditions is heated by the temperature rise desorption method, and the amount of hydrogen released at this time is measured. The temperature rise desorption is carried out by heating the test piece from room temperature to 250°C at a heating rate of 100°C / h. The obtained results are shown in Table 2.

[0112] [B11 and B12]

[0113] In this example, from the perspective of the simulated painting process, the hydrogen embrittlement of steel was evaluated in the case where the test piece was processed and hydrogen was introduced after applying stress. First, different from the cases of B1 to B10, the steel having the chemical composition shown in Table 1 was heated and held without introducing hydrogen. More specifically, the above-mentioned steel was loaded into a heating furnace with the dew point controlled at -30°C and heated to 920°C, and then held for 120 seconds. Then, the steel was taken out from the heating furnace, clamped by a flat die at a temperature around room temperature, and quenched to obtain a hot stamping material. A rectangular four-point bending test piece or a square ball head extrusion test piece with w / t (the ratio of the width w to the thickness t of the test piece) shown in B11 and B12 of Table 2 was made from the obtained hot stamping material. Then, the fabricated test piece was set on the holder of the hydrogen embrittlement evaluation device shown in Figure 3 and stress was applied to the test piece by four-point bending or ball head extrusion so as to achieve the values of stress triaxiality and maximum principal stress shown in Table 2. The stressed test piece and the fixture were immersed in an ammonium thiocyanate solution with a specified concentration of 0.1 to 10%, and then held for 72 hours to confirm the presence or absence of cracks. Similar to the cases of B1 to B10, the values of stress triaxiality and maximum principal stress were pre-calculated using CAE analysis.

[0114] The test was repeated by changing the concentration of the ammonium thiocyanate solution to change the amount of hydrogen introduced, and the presence or absence of cracks in the test piece after the test was observed. The maximum amount of hydrogen introduced without generating cracks was determined as the limiting hydrogen amount (Hc). Regarding the amount of hydrogen introduced, the test piece into which hydrogen was introduced under the same conditions was heated by the temperature rise desorption method, and the amount of hydrogen released at this time was measured. The temperature rise desorption was carried out by heating the test piece from room temperature to 250°C at a heating rate of 100°C / h. The obtained results are shown in Table 2.

[0115] [Tensile strength]

[0116] Regarding the tensile strength, No. 5 test pieces were made from the hot stamping materials obtained according to B1 to B12 in accordance with JIS Z2241:2022, and tensile tests were carried out to measure. The obtained results are shown in Table 2.

[0117] [Table 1]

[0118]

[0119] [Table 2]

[0120]

[0121] Referring to Table 2, in each of the steel materials with a tensile strength of 1820 MPa and 2015 MPa, a tendency was observed that the limiting hydrogen amount (Hc) decreased as the maximum principal stress increased. In addition, for the steel material A1 with a tensile strength of 1820 MPa, in B3 - B5 where the test piece was stressed in such a way that the maximum principal stress became 1260 MPa, although the values of the stress triaxiality were different, the same value of the limiting hydrogen amount was obtained. This result indicates that by the hydrogen embrittlement evaluation method of the present invention, while simulating various stress states in actual components, the limiting hydrogen amount of the steel material can be accurately calculated. In addition, the value of the stress triaxiality can be controlled by changing w / t of the four-point bending test piece. More specifically, the value of the stress triaxiality can be increased by increasing the value of w / t of the four-point bending test piece. Furthermore, by using ball head extrusion instead of four-point bending, the value of the stress triaxiality can be further increased. As a result, the value of the stress triaxiality can be controlled within a larger range, and thus the limiting hydrogen amount of the steel material under various stress states can be determined.

[0122] Explanation of symbols

[0123] 1, 10: Hydrogen embrittlement evaluation device

[0124] 2: Test piece of steel material

[0125] 3: Retainer

[0126] 3a: Load fixture

[0127] 3b: Support fixture

[0128] 4: Stress application mechanism

[0129] 5: Strain gauge

[0130] 6: Display unit

[0131] 7: Transmission mechanism

Claims

1. A method for evaluating hydrogen embrittlement, comprising: a step of applying stress to a test piece of steel in such a manner that the value of the stress triaxiality becomes a value preselected from the range of 0.30 to 0.80; and a step of determining the limiting hydrogen amount of the steel.

2. The method according to claim 1, wherein the application of stress is carried out by bending, and the value of the stress triaxiality is controlled by varying the ratio w / t of the width w to the thickness t of the test piece.

3. The method according to claim 1, wherein the application of stress is carried out by ball head extrusion.

4. The method according to any one of claims 1 to 3, wherein the step of determining the limiting hydrogen amount of the steel is carried out using a test piece into which hydrogen has been introduced and then Zn plating has been performed.

5. The method according to any one of claims 1 to 4, wherein before the step of applying stress to the test piece of the steel, there is further included a step of calculating the stress triaxiality and the maximum principal stress at a specific part in the component for which hydrogen embrittlement evaluation should be carried out, and the step of applying stress to the test piece of the steel includes applying stress corresponding to the calculated stress triaxiality and maximum principal stress to the test piece of the steel.

6. An apparatus for evaluating hydrogen embrittlement, comprising: a holder for holding a test piece of steel; a stress application mechanism for applying stress to the test piece; a display unit for displaying the displacement amount of the test piece to which stress has been applied; and a transmission mechanism for mechanically transmitting the displacement amount of the test piece to the display unit, and the application of stress can be controlled by the stress application mechanism so that the displacement amount of the test piece becomes a displacement amount corresponding to a specified value of the stress triaxiality.

7. The apparatus according to claim 6, wherein the specified value of the stress triaxiality is a value selected from the range of 0.30 to 0.

80.

8. The apparatus according to claim 6 or 7, wherein the holder for holding a test piece of steel is configured to be able to hold a test piece in which the ratio w / t of the width w to the thickness t varies within the range of 1.00 to 30.00.

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