Method for evaluating delayed fracture characteristics of sheared end surface, program, and method for manufacturing automobile component

Abstract

According to the invention, the delayed fracture characteristic of the shearing end face can be evaluated with higher precision. Provided is a delayed fracture characteristic evaluation method for evaluating delayed fracture characteristics of a sheared end face of a metal plate. The method comprises: a test having a step for constraining the metal plate in a state in which a predetermined load stress is applied to the shear surface of the metal plate, and a step for setting the metal plate for a predetermined time in a predetermined hydrogen intrusion environment in the constrained state; and a step for determining, on the basis of the results of the test, a limit load stress, which is a limit load stress at which the delayed fracture of the sheared surface of the metal plate does not occur, and setting, on the basis of the determined limit load stress, a stress margin for the occurrence of the delayed fracture of the sheared end surface of the metal plate. The obtained stress margin is used as an index for evaluating the delayed fracture characteristics of the sheared end face of the metal plate.

Description

Technical Field

[0001] The present invention relates to a technology related to evaluation of delayed fracture characteristics of a shear end surface of a metal sheet during press forming and a method for manufacturing an automobile part having the technology related to the evaluation.

[0002] Here, in this specification, the end face of the metal plate subjected to shearing is referred to as a sheared end face. The present invention is a technology particularly suitable for high-strength steel plates (high-tensile steel plates) with a tensile strength of 980 MPa or more. In addition, in this specification, steel plates with a tensile strength of 1470 MPa or more among high-strength steel plates are referred to as ultra-high-strength steel plates. Background Art

[0003] Nowadays, automobiles are required to improve fuel efficiency and collision safety by lightweighting. In addition, in order to balance the lightweighting of the vehicle body with the protection of passengers in the event of a collision, high-strength steel plates are used for the vehicle body. In recent years, high-strength steel plates with a tensile strength of 980 MPa or more have begun to be used for vehicle bodies. One of the issues when high-strength steel plates are used for vehicle bodies is delayed fracture. In particular, for high-strength steel plates with a tensile strength of 980 MPa or more, delayed fracture occurring from the shear end face becomes an important issue. The shear end face is the end face after shearing. The above-mentioned issues are particularly problematic in ultra-high-strength steel plates with a tensile strength of 1470 MPa or more among high-strength steel plates.

[0004] It is known that a large tensile stress remains on the sheared end surface, and this residual stress may cause delayed fracture in the metal plate.

[0005] In order to predict the delayed fracture at the shear end face in advance, it is necessary to prepare a test piece for evaluation and place the test piece in a hydrogen intrusion environment. In addition, the properties of the shear end face change due to the plastic deformation during the shearing process. And, generally, the risk of delayed fracture at the shear end face becomes higher. Therefore, for example, in Patent Document 1, the occurrence of delayed fracture is evaluated in the following manner. That is, in Patent Document 1, the shear end face of the test piece is given a compression process in the plate thickness direction based on rolling. Then, the test piece is placed in a hydrogen intrusion environment, and the occurrence of delayed fracture is evaluated.

[0006] Here, a test is considered in which a shear end face in a shear state is placed in a hydrogen intrusion environment without load. Even if delayed fracture does not occur in this test, delayed fracture may sometimes occur if the test is performed with external load stress. The reason is that the larger tensile stress remaining on the shear end face is further added with the load stress from the outside. Therefore, for example, in Patent Document 2, an evaluation sample including a shear end face is loaded with a constant load based on tension, and is placed in a hydrogen intrusion environment under a constrained state to evaluate the delayed fracture characteristics. In addition, in Patent Document 3, as a simpler method, the delayed fracture characteristics are evaluated by placing the sample in a hydrogen environment under a load based on bending. However, in Patent Document 3, the shear end face is not the object, and the evaluation of the delayed fracture characteristics of the surface of the test piece is mainly focused on. Therefore, in Patent Document 3, the surface of the shear end face of the evaluation sample is sealed with a resin coating, and the shear end face is excluded from the object of evaluation.

[0007] However, the present inventors have conducted various studies and have found the following findings: That is, the present inventors have found that there are further problems in predicting or preventing the occurrence of delayed fracture based on these delayed fracture evaluation methods for actual automobile parts.

[0008] For example, the strain introduction based on rolling as in Patent Document 1 has the following problems. That is, there is a problem of deviation from the deformation state under the forming strain introduced by stamping for automobile parts. In stamping, uniaxial tension and compression, as well as bending deformation caused by their combination, are introduced to the shear end face. Therefore, in the evaluation method such as Patent Document 1, the evaluation is not sufficient. In addition, in Patent Documents 2 and 3, the change in delayed fracture characteristics caused by the plastic deformation of the shear end face after shearing is not considered. Therefore, the delayed fracture evaluation of the stamped product in which various forming strains are generated on the shear end face is insufficient.

[0009] Furthermore, in any of the evaluation methods in Patent Documents 1 to 3, only the presence or absence of delayed fracture occurrence and the time under individual hydrogen intrusion conditions and stress conditions in the laboratory are evaluated.

[0010] Prior art literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Application Publication No. 2020-41837

[0013] Patent Document 2: Japanese Patent No. 5196926

[0014] Patent Document 3: Japanese Patent No. 5971058 Summary of the invention

[0015] Problems to be solved by the invention

[0016] Conventionally, no evaluation has been conducted based on the viewpoint of determining how much margin the conditions of the hydrogen intrusion environment and stress have with respect to the occurrence of delayed fracture, compared with the conditions of the hydrogen intrusion environment and stress of actual automobile parts.

[0017] The inventors of the present invention have obtained the following findings. That is, in actual automobile parts, different forming strains are introduced into the processed metal sheet depending on the forming position of the shear end face. The inventors of the present invention have obtained the following findings: due to the forming strain, the delayed fracture characteristics caused by plastic deformation change. In addition, at the shear end face, the forming residual stress after stamping is added to the residual stress caused by shearing, thereby making it easy for delayed fracture to occur.

[0018] In addition, the inventors have obtained the following insights. That is, considering the case where residual stress is formed on the shear end face loaded with forming strain under a certain hydrogen intrusion environment. In this case, the following insights are obtained: it is very important to evaluate the degree of margin that the shear end face of the stamped product has for the occurrence of delayed fracture. That is, the following insights are obtained: such evaluation is very important in avoiding delayed fracture at the shear end face of automotive parts.

[0019] As described above, in the stamping of automobile parts, the shear end properties change due to plastic deformation. On the other hand, there is no index that can be compared with the stress generated in the actual automobile parts to predict the occurrence of delayed fracture. Therefore, there is no method that can evaluate delayed fracture from the perspective of stress margin.

[0020] The present invention focuses on the above-mentioned points, and an object of the present invention is to evaluate the delayed fracture characteristics at the shear end surface with higher accuracy.

[0021] Methods used to solve problems

[0022] In order to solve the problem, one embodiment of the present invention is a method for evaluating the delayed fracture characteristics of a shear end surface, which is a delayed fracture characteristics evaluation method for evaluating the delayed fracture characteristics of a shear end surface of a metal plate. The main purpose is to have: a test having a process of constraining the shear surface of the above-mentioned metal plate under a state where a predetermined load stress is loaded, and a process of placing the metal plate in a predetermined hydrogen intrusion environment for a predetermined time under the above-mentioned constraint state; and a process of calculating the limit load stress, that is, the limit load stress, at which delayed fracture of the shear surface of the above-mentioned metal plate does not occur based on the result of the above-mentioned test, and setting a stress margin for the occurrence of delayed fracture of the shear end surface of the above-mentioned metal plate based on the calculated limit load stress, and using the above-calculated stress margin as an indicator for evaluating the delayed fracture characteristics of the shear end surface of the metal plate.

[0023] The test preferably includes a step of applying a forming strain along the extending direction of the shear surface to the shear surface of the metal plate before the step of constraining. Furthermore, the stress margin is preferably set to a value with the forming strain as a variable.

[0024] Effects of the Invention

[0025] According to the aspect of the present invention, it is possible to apply an index for evaluating delayed fracture occurring at a shear end surface with higher accuracy.

[0026] At this time, the index (stress margin) uses stress as a unit and can be evaluated from the perspective of a margin based on stress. Therefore, for example, when a high-strength steel sheet is applied to various parts such as a panel component, a structure / frame component, etc. of an automobile, the following effects can be achieved. That is, according to the method of the present invention, the occurrence of delayed fracture can be predicted including a margin having a dimension of stress.

[0027] Furthermore, according to the aspects of the present invention, for example, by expanding the application range of ultra-high strength steel sheets, it is also possible to reduce the weight of automobile bodies. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a diagram showing an example of a procedure for evaluating the stress margin corresponding to the strain amount using a test piece.

[0029] Figure 2 This is a diagram showing an example of stress margin with forming strain as a variable.

[0030] Figure 3 This is a conceptual diagram showing the relationship between delayed fracture of a shear end surface and stress margin.

[0031] Figure 4 This is a conceptual diagram showing the relationship between delayed fracture of a shear end surface and stress margin when residual stress remains due to bending.

[0032] Figure 5 It is an explanatory diagram showing an example of a method of applying load stress based on bending.

[0033] Figure 6 It is a graph showing an example of a function of stress margin with respect to strain amount.

[0034] Figure 7 This is a diagram showing an example of delayed fracture determination using stress margin.

[0035] Figure 8 This is a diagram showing an example of a program flow that can be used in the evaluation of the present method. DETAILED DESCRIPTION

[0036] Next, embodiments of the present invention will be described with reference to the drawings.

[0037] (constitute)

[0038] This embodiment is a delayed fracture characteristic evaluation method for evaluating the delayed fracture characteristic of a shear end surface of a metal plate. The present invention is particularly effective when the metal plate is a high-strength steel plate.

[0039] As an evaluation index used in the delayed fracture characteristic evaluation method, the “stress margin” which is an index newly set in the present disclosure is obtained.

[0040] The “stress margin” in the present disclosure refers to the allowable amount of external load stress that a shear end face has without causing delayed fracture.

[0041] In the present embodiment, for example, when a metal plate having a shear cross section is subjected to a processing such as bending to manufacture an automobile part, the delayed fracture characteristic evaluation method of the present embodiment is used. For example, the delayed fracture characteristic at the shear end face is evaluated by the delayed fracture characteristic evaluation method of the present embodiment, and the occurrence of delayed fracture of the manufactured automobile part is predicted. Then, based on the prediction, the shape and material of the automobile part that suppress the occurrence of delayed fracture are selected, and the automobile part is manufactured under the selected conditions.

[0042] In the embodiment shown below, the stress margin is set as a parameter of the test condition, with the forming strain as a variable. The forming strain is the strain given before the external load stress is applied. The stress margin is not limited to the case where the forming strain is used as a variable. Regarding the stress margin, when the parameter of the manufacturing conditions of the metal plate before the external load stress is applied to the metal plate is used as a variable, the application range becomes wider, which is preferred. The parameters here are, for example, the forming strain, the gap during shearing, the wear conditions (shearing processing conditions), etc. The shearing processing conditions are one of the processing conditions of the material to be evaluated.

[0043] The stress margin may be expressed using a test condition other than the step (third step 12 ) of restraining the external load as a variable (parameter) value.

[0044] Here, as test conditions other than the process of applying external loads to constrain, for example, the following test conditions can be cited. That is, as examples of the test conditions, there are the type of material (steel type, plate thickness), shear conditions (gap, wear conditions), conditions in the hydrogen environment for immersion (immersion time), etc. The test conditions excluding the process of applying constraints are to obtain the limit stress load by changing the parameters.

[0045] It should be noted that the stress margin may be a value obtained by multiplying the limit stress load by a predetermined safety factor.

[0046] Here, the external load stress is a load stress generated by constraints during press forming into a target product shape or during assembly of the product.

[0047] In this embodiment, the following steps are provided as a process for obtaining the stress margin for the metal plate to be evaluated. That is, in this embodiment, there are steps of testing with actual experiments and steps of setting the stress margin. Specifically, Figure 1 As shown, the process includes a first process 10 to a fifth process 14 .

[0048] Figure 1 Among them, the first step 10 to the fourth step 13 correspond to the test steps. The fifth step 14 corresponds to the step of obtaining the set stress margin.

[0049] <First step 10>

[0050] The first step 10 is a step of making a test piece from a metal plate having the same conditions as the metal plate to be evaluated. The first step 10 is to perform shearing on a metal plate having the same material and thickness as the metal plate to be evaluated. Then, a test piece having a sheared end surface for obtaining a stress margin is made.

[0051] <Second step 11>

[0052] The second step 11 is a step of applying a forming strain to at least a portion of the sheared end surface of the test piece. The applied forming strain is a strain along the extending direction of the sheared end surface.

[0053] The applied forming strain is set to, for example, 0.1% or more.

[0054] The forming strain may be applied by, for example, subjecting the test piece to uniaxial tension or uniaxial compression. Alternatively, the forming strain may be applied by, for example, bending the test piece in the plate thickness direction.

[0055] <Third step 12>

[0056] The third step 12 is a step of applying a pre-set external load stress to the shear end surface of the test piece and constraining it under this load state. The stress loading method is, for example, carried out by tensile stress loading or bending stress loading. In this case, from the perspective of simplicity, a method based on bending stress loading using a fixture is particularly preferred.

[0057] <Fourth step 13>

[0058] In the third step 12, the test piece subjected to the external load stress and restrained is placed in a predetermined hydrogen intrusion environment for a predetermined time. Then, in the third step 12, the test piece in this state is used to perform a process for evaluating the occurrence of cracks.

[0059] At this time, the hydrogen intrusion environment and the setting time are preferably set to conditions that can obtain a target hydrogen intrusion amount. The target hydrogen intrusion amount is, for example, a hydrogen intrusion amount equivalent to the amount of hydrogen that is estimated to intrude into the material to be evaluated under an environment in which it is actually used.

[0060] The test piece is placed in a hydrogen intrusion environment by, for example, immersing the test piece in a bath containing an acid solution such as hydrochloric acid or NH4SCN aqueous solution. The concentration of the acid solution and the immersion time are set so that the amount of hydrogen that is set in advance as the upper limit of the allowable amount intrudes into the test piece.

[0061] For each test piece produced in the first step 10 , the second step 11 to the fourth step 13 were performed while changing the conditions of the applied forming strain and the applied load stress.

[0062] <Fifth step 14>

[0063] In the fifth step 14, first, based on the results of the above test, the limit load stress, i.e., the limit load stress, which is the limit load stress at which delayed fracture does not occur on the shear surface of the metal plate, is evaluated. Then, in the fifth step 14, based on the limit load stress, the stress margin for the occurrence of delayed fracture on the shear end surface of the metal plate is obtained. Specifically, the limit load stress is set as the stress margin under the test conditions.

[0064] For example, based on the test conditions of each test piece and the evaluation results of the presence or absence of cracks at the shear end surface, the values ​​of the external load stress and the ultimate stress load that cause cracks are obtained. The test conditions here refer to the conditions of the forming strain and the external load stress set in the test. In addition, the external load stress that causes cracks refers to the external load stress that causes cracks relative to the same forming strain. In addition, the value of the ultimate stress load refers to the boundary value of the external load stress that does not cause cracks, that is, the value of the ultimate stress load. The value of the ultimate stress load is, for example, the maximum value of the external load stress that does not cause cracks.

[0065] This value is sorted for multiple forming strains to obtain multiple (forming strain, limit stress load) data. Then, find Figure 2 The value (function) of the limit stress load with the forming strain as a variable shown in the figure is used as the data of the stress margin. That is, the stress margin is described as a function of the forming strain based on tension-compression, for example.

[0066] Here, in the above description, the case where the test is performed by changing the forming strain is exemplified, but it is not limited to this. The test can also be performed by limiting the forming strain to a constant (for example, zero). In this case, the case where the forming strain is zero, that is, the second step 11 is not performed, is also included. In this case, the stress margin relative to the specific forming strain is obtained.

[0067] In addition, for example, the shearing condition may be changed, and the stress margin may be obtained with the shearing condition as a variable, together with the forming strain as a variable, or without the forming strain as a variable. The shearing condition refers to the gap or the like.

[0068] It should be noted that Figure 2 The example shown is an example where the residual stress of the shear end face is reduced due to the forming strain. In addition, the example shows that the greater the absolute value of the forming strain, the greater the stress margin for delayed fracture. However, depending on the material and the state of the shear end face, it is also conceivable that the stress margin for delayed fracture is reduced due to the forming strain. Such a situation occurs, for example, when cracks or damage are generated on the shear end face due to the given forming strain.

[0069] Furthermore, in this embodiment, by comparing with the stress margin obtained in the above manner, the following evaluation can be performed. That is, for the test piece with shear end faces that is not used in the evaluation of the stress margin, the possibility of delayed fracture can be evaluated without performing a test. The possibility of delayed fracture is the possibility of delayed fracture relative to the external load (load stress) that is expected to be loaded on the metal plate.

[0070] For example, the evaluation is performed as follows.

[0071] First, for the metal plate to be evaluated, the strain amount based on tension-compression and the load stress on the shear end face for which delayed fracture is to be evaluated are calculated. This calculation is performed by CAE-based forming analysis and elementary mechanics calculation methods.

[0072] Next, the stress margin is matched according to the strain amount caused by tension-compression of the shear end face, and it is determined whether the load stress on the shear end face exceeds the stress margin. Then, if it exceeds, it is determined that there is a possibility of delayed fracture. At this time, considering the safety factor, the stress margin may also be smaller than the actually measured value.

[0073] The delayed fracture evaluation of the shear end surface by comparison with the stress margin described above can also be performed more efficiently by using a procedure described later.

[0074] In the above description, a case where no safety factor is provided for the stress margin is shown. However, by setting the stress margin to be low, a safety factor may be provided for the stress margin.

[0075] In the above description, the forming strain is actually applied to the test piece in the second step 11 in the test, and the stress margin is described as a value with the forming strain as a variable. However, the present invention is not limited to this.

[0076] For example, a process of bending the shear end face of the metal plate before the constraint process is performed is provided. Thus, a process (called the sixth process) is provided for obtaining the forming residual stress generated on the shear end face and the forming strain generated on the shear end face. The sixth process can be obtained by an experiment performed separately. It can also be obtained by structural analysis such as CAE analysis. Then, the forming residual stress obtained in the sixth process is added to the limit load stress (stress margin) obtained in the above-mentioned test without performing the second process 11. The added value can be used as a stress margin with the forming strain as a variable.

[0077] (Details about the disclosure)

[0078] The present disclosure will be described in more detail.

[0079] The present inventors found the following findings (1) to (3) during the process of evaluating delayed fracture of sheared end surfaces.

[0080] (1) Consider the case where a predetermined time is set in a hydrogen intrusion environment while the shear end face load is constrained based on a load stress (external load stress) of a constant load. In this case, regarding the load stress, there is a limit load stress (also called limit load stress) at which delayed fracture occurs at the shear end face. This is because delayed fracture occurs when the sum of the residual stress caused by shearing processing at the shear end face and the load stress from the outside reaches the threshold value for the occurrence of delayed fracture at the shear end face.

[0081] (2) The limit load stress at which delayed fracture occurs changes at the shear end surface according to the tensile and compressive strains of the forming strain applied after shearing. This is because the residual stress at the shear end surface changes due to the forming strain.

[0082] (3) Therefore, the limit load stress of each shear end face varies according to the forming strain and load stress (external load stress) applied to the shear end face. Furthermore, when the predetermined setting time is set in a predetermined hydrogen intrusion environment, the limit load stress at which delayed fracture occurs at the shear end face can be organized as follows. That is, the limit load stress can be organized as an index such as "stress margin" that takes into account the forming strain and load stress (external load stress).

[0083] Here, in the present disclosure, the allowable amount of external load stress that does not cause delayed fracture on the shear end surface is defined as "stress margin".

[0084] Figure 3 A conceptual diagram illustrating the above (1), (2), and (3) is shown in FIG. Figure 3 (a) shows an example of the state of the limit load stress when no forming strain is given to a metal plate having a sheared end surface formed by shearing the end. Figure 3 (b) shows an example of the state of the limit load stress when the forming strain is applied after the shear end surface is formed. Figure 3 The case where the residual stress is reduced by applying forming strain to the metal sheet before press-forming the metal sheet is exemplified.

[0085] Here, delayed fracture occurs when the sum of the residual stress caused by shearing and the load stress from the outside reaches the threshold value for delayed fracture. Therefore, when the residual stress of the shear end face changes due to the forming strain, the limit load stress for delayed fracture also changes. The limit load stress is the difference between the residual stress of the shear end face and the threshold value for delayed fracture, and is the limit external load stress for the shear end face not to cause delayed fracture.

[0086] In view of the above, in the present disclosure, the allowable amount of external load stress that does not cause delayed fracture taking into account the forming strain applied to the shear end surface is defined as "stress margin". In the present embodiment, the allowable amount of external load stress is specified using an indicator such as stress margin with forming strain as a variable.

[0087] Here, the residual stress at the shear end face caused by shearing only exists in a tiny area of ​​the extreme surface layer of about 100μm from the shear end face surface. Therefore, the residual stress change is difficult to calculate by CAE using shell elements. The stress in the fine area can be measured by X-ray stress measurement. However, there is a problem that the measured value sometimes changes due to the measurement range, and the measurement depth is limited to the outermost layer of the material. Therefore, sometimes the size of the measured value does not necessarily correspond to the risk of delayed fracture.

[0088] On the other hand, if the method of experimentally obtaining the above-mentioned "stress margin" with forming strain as a variable through delayed fracture test under stress load state is used as disclosed in the present invention, it can be dealt with. That is, such problems related to calculation and measurement will not occur, and the index for evaluating the risk of delayed fracture can be directly obtained.

[0089] It is considered that the stress margin is evaluated under the conditions of the hydrogen intrusion environment to which the automobile parts are actually exposed. In this case, it can be regarded as the margin until the delayed fracture occurs at the shear end surface of the automobile parts.

[0090] Moreover, the stress margin is expressed by stress in units of stress. Therefore, even when the external load stress applied to the component during assembly, use, etc. is added in addition to the residual stress caused by the forming of the component, it can be estimated. That is, as long as the stress margin is not exceeded, it can be estimated that delayed fracture will not occur.

[0091] Therefore, the stress margin is a simple and excellent delayed fracture evaluation index that can evaluate the margin having the dimension of stress.

[0092] On the contrary, a method of setting the stress load value to a constant value and changing the hydrogen intrusion environment can also be considered. However, this has problems with the addition of the residual stress caused by the forming of the above-mentioned components and the external load stress caused by the deformation of the components during assembly and use. That is, this is less useful in that it is impossible to evaluate the margin compared with the case where stress is used as a scale.

[0093] It should be noted that the above-mentioned forming strain is the strain in the extension direction of the shear plane.

[0094] Furthermore, the present inventors have found the following findings (4) to (7) regarding actual evaluation methods.

[0095] (4) As a method for introducing the forming strain for the tension and compression of the shear end face into the evaluation test piece, it is preferred to use uniaxial tensile deformation and compressive deformation. This is because of the following reasons. This is because, in the uniaxial forming, the residual stress of the part other than the shear end face after forming is almost 0 due to rebound after forming. Therefore, its influence can be ignored. Therefore, when the forming strain is given based on uniaxial tension and compression, it has the following advantages. That is, the external load stress of the shear end face after additional processing, which is the limit of delayed fracture, can be directly evaluated as "stress margin", which is the simplest.

[0096] (5) The bending deformation introduced into the actual stamped product can be described as follows. That is, from the surface to the back of the plate, the "stress margin" at the shear end surface after uniaxial tension / compression in the method of (4) above is obtained respectively. The "stress margin" obtained above can be applied according to the strain amount and the sign of the strain. "From the surface to the back of the plate" means "in the thickness direction of the plate".

[0097] (6) Stress margin can also be evaluated by using bending deformation as a method of imparting forming strain to the shear end face. In this case, due to bending deformation, residual stress caused by bending may be generated in the entire test piece including the shear end face. Therefore, in this case, it is necessary to add the value obtained by adding the residual stress caused by bending to the external load stress at the limit of delayed fracture as the "stress margin". It should be noted that the residual stress caused by bending deformation can be evaluated by computer analysis based on CAE, elementary mechanics calculation, etc.

[0098] Figure 4 is a conceptual diagram illustrating (6). It should be noted that Figure 4 The figure shows the case where the residual stress caused by bending is positive. The sign of the residual stress caused by bending is reversed on the front and back of the plate. Therefore, for the part where the residual stress caused by bending is negative, the amount of residual stress caused by bending and the residual stress caused by shearing need to be subtracted. Subtraction is to add negative values.

[0099] (7) As a method for applying external load stress to the evaluation test piece, there are a method based on stretching under a constant load and a method based on bending. The method based on bending can be performed with a smaller load and using a smaller fixture, so it is simple. The external load stress can be controlled by adjusting the load and deformation applied to the test piece. The load on the shear end face of the test piece can be calculated by CAE and elementary analysis.

[0100] Figure 5 This is a diagram showing an example of a method of applying stress load based on bending in (7). Figure 5 In FIG. 1 , a method of stress loading based on four-point bending is exemplified. Figure 5 In the figure, reference numeral 1 indicates a test piece, reference numeral 2 indicates a fulcrum for load bending, and reference numeral 3 indicates a screw for adjusting the bending amount.

[0101] Here, as for the test piece for evaluating the stress margin, a test piece sheared in a laboratory can be used. In addition, as the test piece, a part of the sheared end surface of the stamped product after stamping can also be cut out.

[0102] Furthermore, the present inventors devised the following method as a method for evaluating and predicting the occurrence of delayed fracture of a shear end surface using the thus obtained "stress margin". Examples thereof are shown in the following first to third examples.

[0103] (First)

[0104] First, the stress margin is measured based on the amount of strain due to tension and compression applied to the test piece in the laboratory by the methods (4) to (7) above.

[0105] Here, the hydrogen intrusion environment and the time of placement in the environment are preferably set so that the amount of hydrogen intruding the test piece reaches a target amount of hydrogen intrusion. The target amount of hydrogen intrusion is the amount of hydrogen intrusion that is set in advance as the upper limit allowed in actual automobile parts.

[0106] As the strain amount of the forming strain given to the shear end face, if the amount that fully affects the delayed fracture characteristics is considered, the strain amount is preferably set to 0.1% or more. As a strain amount with a greater degree of influence, the strain amount is 0.5% or more. In the case of introducing plastic strain, the delayed fracture evaluation of the present invention is particularly effective. Therefore, the plastic strain amount on the shear end face can also be used as an evaluation index instead of the forming strain. Regarding the load stress on the test piece, as long as it is a stress-related parameter such as the first principal stress and Mises stress, it can be used in the present disclosure.

[0107] (second)

[0108] Second, for a test piece having a shear end face for which the possibility of delayed fracture is to be evaluated, the amount of forming strain due to tension-compression and the load stress on the shear end face are calculated. The calculation is performed, for example, by forming analysis based on CAE or elementary mechanics methods.

[0109] (third)

[0110] Third, consider a test piece with a shear end face for which the possibility of delayed fracture is to be evaluated. The forming strain and residual stress after forming of the test piece are compared with the stress margin corresponding to the strain caused by tension-compression. Then, for the part exceeding the stress margin, it is determined that there is a possibility of delayed fracture. However, considering the safety factor, the stress margin may be smaller than the actually measured value.

[0111] (program)

[0112] An example of a procedure used in the above evaluation method is shown.

[0113] In the program of the first example, the relationship between the stress margin and the forming strain obtained by the delayed fracture characteristic evaluation method as described above is stored in the storage unit. The program of the first example causes the computer to refer to the stored relationship between the stress margin and the forming strain. Then, the program of the first example causes the computer to execute a process of determining the stress margin corresponding to the strain amount of the input forming strain.

[0114] In another example of the program, the relationship between the stress margin, the forming strain, and the external load stress obtained by the delayed fracture characteristic evaluation method as described above is stored in the storage unit. Another example of the program causes the computer to refer to the stored relationship between the stress margin, the forming strain, and the external load stress. Then, another example of the program performs a process of evaluating the possibility of delayed fracture for the input strain amount of the forming strain and the external load stress.

[0115] Next, refer to Figure 8 The following describes an example of processing based on the program of the evaluation method using the stress margin described above. Figure 8 Such processing and evaluation can perform more efficient delayed fracture evaluation.

[0116] Figure 8 The example shown includes a stress margin calculator 20, an evaluation main unit 30, and a storage unit 40. A program for performing processing of the stress margin calculator 20 and the evaluation main unit 30 is stored in the storage unit 40 such as RAM or ROM of the computer and executed by the computer.

[0117] <Storage Unit 40>

[0118] The storage unit 40 is composed of a recording medium such as a database.

[0119] The storage unit 40 stores data of stress margin d obtained for forming strain using test conditions as variables according to the material conditions of the metal plate, the conditions of the hydrogen environment, and the shearing conditions. The data is obtained by repeatedly performing the tests of the first step 10 to the fifth step 14 while making various changes to the material conditions of the metal plate, the conditions of the hydrogen environment, the shearing conditions, and the amount of forming strain.

[0120] <Stress margin calculation section 20>

[0121] In the stress margin calculation unit 20, first, in step S10, input of basic conditions for evaluation is prompted, and the input is obtained through input operation by the operator. The basic conditions for evaluation are conditions of material type (steel type, thickness) and conditions of hydrogen environment (acidity, setting time) as conditions for delayed fracture.

[0122] Next, in step S20, the input of the cutting condition is prompted, and the input is obtained through the input operation of the operator.

[0123] Next, in step S30, a data group matching the conditions input in steps S10 and 20 is obtained from the storage unit 40. The data group is a data group of stress margins for each strain amount. In addition, the data group is a collection of data of (strain amount, stress margin).

[0124] Alternatively, the input of a data group of stress margins for each strain amount obtained by experiment is prompted, and the input information is obtained through input operation by the operator. The obtained data is stored in the storage unit 40 .

[0125] Next, in step S40, the data group of stress margin for strain amount obtained in step S30 is referred to. Then, in step S40, a calculation process is performed to obtain stress margin d as a function f(x) with strain amount x as a variable by a known processing method.

[0126] Next, in step S50 , the function of the stress margin d obtained in step S40 is changed to an equation taking into account a safety factor s (0<s≤1) as shown in the following equation.

[0127] d = s f(x)

[0128] Then, information on the function of the obtained stress margin d is stored in the storage unit 40 using the test conditions as keywords.

[0129] <Evaluation body 30>

[0130] In the evaluation main body 30, first, in step S100, input of the type of material to be evaluated and the condition of the hydrogen environment as a condition for delayed fracture is prompted. Then, the above input is obtained through the input operation of the operator. The type of material is, for example, the steel type and thickness. The condition of the hydrogen environment is, for example, the acidity and the setting time.

[0131] Next, in step S110 , input of a strain amount x and a load stress g predetermined during machining is prompted, and the above input is obtained through input operation by the operator.

[0132] In step S120, information on the function "s·f(x)" of the stress margin d that matches the condition input in step S100 is obtained from the storage unit 40. Then, in step S120, the stress margin d corresponding to the strain amount x input in step S110 is compared with the load stress g input in step S110. Then, through this comparison, it is determined whether there is a risk of delayed fracture.

[0133] Here, in Figure 8 In step S120, it is determined whether there is a risk of delayed fracture. The margin of delayed fracture (=dg) may also be output.

[0134] In addition, the stress margin calculation unit 20 may perform another calculation process to obtain a function of the stress margin d with the input values ​​in steps S10 to S20 as conditions. Then, the obtained function may be input to the storage unit 40 as data with the input values ​​in steps S10 to S20 as keywords.

[0135] (other)

[0136] The present disclosure may also take the following configurations.

[0137] (1) A delayed fracture characteristic evaluation method for evaluating the delayed fracture characteristics of a shear end face of a metal plate, comprising: a test including a step of constraining the shear face of the metal plate under a predetermined load stress and a step of placing the metal plate in a predetermined hydrogen intrusion environment for a predetermined time under the constrained state; and a step of determining a limit load stress, i.e., a limit load stress, at which delayed fracture of the shear face of the metal plate does not occur based on the test result, and setting a stress margin for the occurrence of delayed fracture of the shear face of the metal plate based on the determined limit load stress, and using the determined stress margin as an indicator for evaluating the delayed fracture characteristics of the shear face of the metal plate.

[0138] (2) The test is performed by changing one or more test conditions, i.e., selected conditions, selected from test conditions other than the above-mentioned constraint-performing step, and obtaining the above-mentioned limit load stress under each selected condition as the above-mentioned stress margin, and expressing the above-mentioned stress margin as a value with the above-mentioned selected condition as a variable.

[0139] Here, as test conditions other than the above-mentioned step of applying restraint, there are shearing conditions (gap, etc.), forming strain when applying forming strain, setting time in hydrogen intrusion environment, etc.

[0140] (3) The test includes the step of applying a forming strain along the extending direction of the shear surface to the shear surface of the metal plate before the step of constraining, and the stress margin is set to a value with the forming strain as a variable.

[0141] (4) The forming strain imparted in the step of imparting the forming strain is set to 0.1% or more.

[0142] (5) In the step of imparting the forming strain, the forming strain is imparted by uniaxial stretching or uniaxial compression.

[0143] (6) In the step of imparting the forming strain, the forming strain is imparted by bending.

[0144] (7) A process for obtaining a forming residual stress generated on a shear end face by bending the shear end face of a metal plate before the above-mentioned constraining process is provided, and in the process for setting the above-mentioned stress margin, a value obtained by adding the above-mentioned ultimate load stress obtained and the forming residual stress in the above-mentioned bending process obtained in the process for obtaining the above-mentioned forming residual stress is used as the above-mentioned stress margin.

[0145] (8) In the step of obtaining the forming residual stress, the relationship between the forming strain generated on the shear end surface due to bending and the forming residual stress is obtained, and the stress margin is set to a value with the forming strain as a variable.

[0146] (9) A delayed fracture characteristic evaluation method for evaluating the possibility of delayed fracture at the shear end face of a metal plate under the same conditions as the metal plate used in the above-mentioned test, i.e., a metal plate to be evaluated, wherein the possibility of delayed fracture at the end face of the metal plate to be evaluated is evaluated by utilizing the forming strain and load stress imparted to the end face of the metal plate to be evaluated based on the above-mentioned stress margin calculated by the delayed fracture characteristic evaluation method disclosed herein.

[0147] (10) The metal plate is a steel plate having a tensile strength of 980 MPa or more.

[0148] (11) A program in which the relationship between the stress margin and the forming strain obtained by the delayed fracture characteristic evaluation method disclosed in the present invention is stored in a storage unit, and a computer is caused to perform the following processing: referring to the above-stored relationship between the stress margin and the forming strain, the stress margin corresponding to the strain amount of the input forming strain is determined.

[0149] (12) A program in which the relationship between the stress margin, forming strain and external load stress obtained by the delayed fracture characteristic evaluation method disclosed in the present invention is stored in a storage unit, and a computer is caused to perform the following processing: referring to the stored relationship between the stress margin, forming strain and external load stress, the possibility of delayed fracture for the input strain amount of the forming strain and the external load stress is evaluated.

[0150] (13) A method for producing an automobile part, comprising producing an automobile part from a metal plate having a sheared end surface, the method comprising the method for evaluating delayed fracture characteristics of a sheared end surface disclosed herein.

[0151] (14) The delayed fracture characteristics evaluation method of the shear end surface disclosed in the present invention is used to predict the occurrence of delayed fracture of an automobile part being manufactured.

[0152] Example

[0153] Examples of this embodiment will be described.

[0154] (Example 1)

[0155] In the present example, a test material X composed of a metal plate having a tensile strength of 1470 MPa grade steel plate and a thickness of 1.0 mm is described as a metal plate to be evaluated.

[0156] It should be noted that the present invention is not limited to the metal plate of the test material, but can be applied to various metal materials represented by high-strength steel plates having a tensile strength of 980 MPa or more that causes delayed fracture at the shear end surface.

[0157] First, the test material X is sheared by shearing to make a test piece with a straight shear end face of 100 mm in length. The width of the test piece during shearing is set to 30 mm, and the test piece is set to a strip shape of 100 mm × 30 mm. The gap during shearing is set to 12% relative to the plate thickness. It should be noted that in the description of the above embodiment, the case of a single shearing condition is used as an example. However, when the shearing conditions such as the gap during shearing change, the corresponding evaluation can also be performed. That is, the stress margin under the shearing condition can be obtained.

[0158] Next, a forming strain based on tension or compression is applied to the shear end face of the test piece along the extension direction of the shear end face. In this example, the forming strain is applied by a uniaxial load tester while the two ends of the test piece are clamped. It should be noted that in this example, the forming strain is described for the case of tension or compression. It was confirmed that the same result was obtained when the forming strain was a deformation based on bending. In addition, a test piece that was not given a forming strain and maintained in a shearing state was also prepared.

[0159] Next, external constraints were applied to each test piece by four-point bending using a jig, and stress was applied to the center of the shear end surface of the test piece. However, the burr side during shearing was set to the outer side of the bend, and tensile stress was applied.

[0160] Here, the magnitude of the load stress is determined as follows: The first principal stress-first principal strain relationship at the center and top of the width of the test piece is obtained by CAE. Then, the strain amount when the test piece is actually bent is measured and measured by establishing a correspondence.

[0161] In this example, the case of four-point bending is described as a stress loading method. The same trend of results can be obtained by other loading methods such as bending loading methods and uniaxial tension. In addition, in this example, when stress loading, the burr side during shearing is set as the outer side of the bend, and tensile stress is loaded. Similarly, the surface on the side opposite to the burr side can also be evaluated.

[0162] In this example, the load stress applied to each test piece was changed by 100 MPa as shown in the table. Then, a plurality of test pieces were prepared according to the conditions of each forming strain.

[0163] The test piece loaded with the load stress was immersed in a bath of a thiocyanate solution at pH 6 for 96 hours. Then, the delayed fracture characteristics were evaluated by the presence or absence of cracks caused by delayed fracture after 96 hours.

[0164] The above conditions and evaluation results are shown in Tables 1 to 11.

[0165] Each table is summarized according to the strain amount of forming strain.

[0166]

[0167]

[0168]

[0169]

[0170]

[0171]

[0172]

[0173]

[0174]

[0175]

[0176]

[0177] Each table shows the presence or absence of delayed fracture for each load stress that differs according to the amount of strain when tension is positive and compression is negative.

[0178] As can be seen from Tables 1 to 11, the greater the absolute value of the strain amount of the forming strain, the greater the limit load stress. In this example, the limit load stress with the forming strain as a variable is referred to as the stress margin.

[0179] Figure 6 This is a graph in which the stress margin is obtained from the limit load stress at which delayed fracture does not occur and the stress margin is described as a function of strain.

[0180] It is known that the stress margin corresponding to the forming strain after shearing can be described. It should be noted that in this example, the case where the stress margin increases due to the forming strain after shearing is shown, but conversely, the same evaluation can be performed even when the stress margin decreases due to the forming strain after shearing.

[0181] (Example 2)

[0182] Next, an example of delayed fracture determination using the stress margin corresponding to the forming strain after shearing obtained in Example 1 will be described.

[0183] In Example 2, for the test material X, test pieces consisting of A, B, C, D, E, F, G, and H having different amounts of forming strain and load stress were prepared by the same process as in Example 1.

[0184] Each test piece was immersed in a bath of a thiocyanate solution at pH 6 for 96 hours, and delayed fracture was evaluated by the presence or absence of cracks caused by delayed fracture after 96 hours.

[0185] Table 12 shows the evaluation results of the forming strain amount, load stress, and delayed fracture of each test piece.

[0186]

[0187] Next, the results of the test pieces A to G were compared with the stress margins obtained in Example 1 (see Figure 6 ) to draw and compare. The comparison result is Figure 7 .

[0188] By Figure 7 It can be seen that the presence or absence of delayed fracture at the shear end surface of an actual component can be predicted by the line of whether or not the stress margin is exceeded.

[0189] Therefore, if the stress margin disclosed in the present invention is used, it is possible to determine the location of the shear end surface that is at risk of delayed fracture.

[0190] It should be noted that although the variable of the stress margin is set to the forming strain, the stress margin may be represented by using shearing conditions such as the gap in the shearing process as a variable.

[0191] The entire contents of Japanese Patent Application 2022-085550 (filed on May 25, 2022) to which this application claims priority are incorporated herein by reference. Although a limited number of embodiments are described herein, the scope of rights is not limited thereto, and changes based on the above disclosed embodiments are obvious to those skilled in the art.

[0192] Explanation of symbols

[0193] 10 The first step (cutting step)

[0194] 11 Second step (step of imparting forming strain)

[0195] 12 The third step (step of restraining under load)

[0196] 13. The fourth step (step of setting a predetermined time in a hydrogen intrusion atmosphere)

[0197] 14 Fifth step (stress margin setting step)

[0198] 20 Stress margin calculation section

[0199] 30 Evaluation Body

[0200] 40 Storage

[0201] d Stress margin

Claims

1. A method for evaluating delayed fracture characteristics of a sheared end surface, which is a method for evaluating delayed fracture characteristics of a sheared end surface of a metal plate, characterized in that: have: A test comprising the steps of constraining the metal plate in a state where a predetermined load stress is applied to the shear surface of the metal plate and placing the metal plate in a predetermined hydrogen intrusion atmosphere for a predetermined time in the constrained state; and a step of determining a limit load stress at which delayed fracture of the shear surface of the metal plate does not occur based on the test result, i.e., a limit load stress, and setting a stress margin for delayed fracture of the shear end surface of the metal plate based on the determined limit load stress; The obtained stress margin is used as an index for evaluating the delayed fracture characteristics of the shear end surface of the metal plate.

2. The method for evaluating delayed fracture characteristics of a sheared end surface according to claim 1, characterized in that: The test is performed by changing one or more test conditions (selected conditions) selected from the test conditions other than the step of constraining, and the limit load stress under each selected condition is obtained as the stress margin. The stress margin is expressed as a value with the selected condition as a variable.

3. The method for evaluating delayed fracture characteristics of a sheared end surface according to claim 1 or claim 2, characterized in that: The test includes a step of applying a forming strain along the extending direction of the shear surface to the shear surface of the metal plate before the step of constraining. The stress margin is set to a value with the forming strain as a variable.

4. The method for evaluating delayed fracture characteristics of a sheared end surface according to claim 3, characterized in that: The forming strain imparted in the step of imparting the forming strain is set to be 0.1% or more.

5. The method for evaluating delayed fracture characteristics of a sheared end surface according to claim 3 or claim 4, characterized in that: In the step of imparting the forming strain, the forming strain is imparted by uniaxial tension or uniaxial compression.

6. The method for evaluating delayed fracture characteristics of a sheared end surface according to claim 3 or claim 4, characterized in that: In the step of imparting the forming strain, the forming strain is imparted by bending.

7. The method for evaluating delayed fracture characteristics of a sheared end surface according to any one of claims 1 to 6, characterized in that: The method further comprises the step of obtaining a forming residual stress generated on the sheared end surface by bending the sheared end surface of the metal plate before the step of constraining. In the step of setting the stress margin, a value obtained by adding the obtained limit load stress to the forming residual stress in the bending process obtained in the step of obtaining the forming residual stress is set as the stress margin.

8. The method for evaluating delayed fracture characteristics of a sheared end surface according to claim 7, characterized in that: In the step of obtaining the forming residual stress, the relationship between the forming strain generated on the shear end surface due to the bending process and the forming residual stress is obtained. The stress margin is set to a value with the forming strain as a variable.

9. A method for evaluating delayed fracture characteristics of a sheared end surface, the method for evaluating the possibility of delayed fracture at a sheared end surface of a metal plate under the same conditions as the metal plate used in the test, i.e., a metal plate to be evaluated, characterized in that: Based on the stress margin obtained by the delayed fracture characteristic evaluation method of the shear end surface according to any one of claims 3 to 6, the possibility of delayed fracture at the end surface of the metal plate to be evaluated is evaluated using the forming strain and load stress given to the end surface of the metal plate to be evaluated.

10. The method for evaluating delayed fracture characteristics of a sheared end surface according to any one of claims 1 to 8, wherein: The metal plate is a steel plate having a tensile strength of 980 MPa or more.

11. A program, wherein: The relationship between the stress margin and the forming strain obtained by the delayed fracture characteristic evaluation method of the shear end surface according to any one of claims 3 to 6 is stored in a storage unit, The computer is caused to execute a process of referring to the stored relationship between the stress margin and the forming strain to determine the stress margin corresponding to the strain amount of the input forming strain.

12. A program, wherein: storing the relationship between the stress margin, the forming strain, and the external load stress obtained by the delayed fracture characteristic evaluation method of the shear end surface according to any one of claims 3 to 6 in a storage unit, The computer is caused to execute a process of evaluating the possibility of delayed fracture with respect to the input strain amount of the forming strain and the external load stress by referring to the stored relationship between the stress margin, the forming strain, and the external load stress.

13. A method for manufacturing an automobile part, which is a method for manufacturing an automobile part from a metal plate having a sheared end surface, wherein: A method for evaluating delayed fracture characteristics of a sheared end surface according to any one of claims 1 to 10.

14. The method for manufacturing an automobile component according to claim 13, wherein: The occurrence of delayed fracture of an automobile part to be manufactured is predicted using the method for evaluating delayed fracture characteristics of a sheared end surface according to any one of claims 1 to 10.

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