Method and program for evaluating delayed fracture characteristics of sheared end surface

Abstract

To more accurately evaluate delayed fracture characteristics at a sheared end surface.SOLUTION: A delayed fracture characteristic evaluation method for evaluating delayed fracture characteristics of a sheared end surface of a metal sheet comprises: a test including a step for restraining in a state in which preset load stress is applied to the sheared surface of the metal sheet, and a step for placing the metal sheet, in the restrained state, for a preset period in a preset hydrogen intrusion environment; and a step for obtaining, on the basis of results of the test, critical load stress that is load stress at the limit where delayed fracture of the sheared surface of the metal sheet is not generated and setting, on the basis of the obtained critical load stress, a stress margin with respect to the generation of delayed fracture of the sheared end surface of the metal sheet. The obtained stress margin is used as an index for evaluating the delayed fracture characteristics of the sheared end surface of the metal sheet.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present invention relates to a technique for evaluating the delayed fracture properties of a sheared end surface of a metal sheet during press forming.

[0002] In this specification, the end surface of the metal plate subjected to shear processing is called a sheared end surface. The present invention is a technology particularly suitable for high-strength steel plates (high-tensile steel plates) having a tensile strength of 980 MPa or more. In addition, in this specification, among high-strength steel plates, steel plates having a tensile strength of 1470 MPa or more are called ultra-high-strength steel plates. [Background technology]

[0003] Currently, automobiles are required to improve fuel efficiency and collision safety by reducing vehicle weight. To achieve both weight reduction and protection of passengers in the event of a collision, high-strength steel plates are used in vehicle bodies, and in recent years, high-strength steel plates with tensile strengths of 980 MPa or more have begun to be used in vehicle bodies. One of the issues when using high-strength steel plates in vehicle bodies is delayed fracture. In particular, delayed fracture occurring at the shear end face, which is the end face after shear processing, is a major issue with high-strength steel plates with tensile strengths of 980 MPa or more. This issue is particularly problematic with ultra-high-strength steel plates with tensile strengths of 1470 MPa or more.

[0004] It is known that large tensile stress remains at the sheared end surface, which may cause delayed fracture in the metal plate. In order to predict delayed fracture at the sheared edge, it is necessary to prepare a test piece for evaluation and place the test piece in a hydrogen penetration environment. Furthermore, the properties of the sheared edge change due to plastic deformation during shear processing, and the risk of delayed fracture at the edge generally increases. For this reason, for example, in Patent Document 1, the sheared edge is compressed in the plate thickness direction by rolling, and then placed in a hydrogen penetration environment to evaluate the occurrence of delayed fracture.

[0005] On the other hand, even if delayed fracture does not occur when the sheared end face is placed in a hydrogen penetration environment without load, delayed fracture may occur if a test is performed by loading stress from the outside. This is because the external load stress is added to the large tensile stress remaining on the sheared end face. For this reason, for example, in Patent Document 2, a constant load is applied by tension to an evaluation sample including the sheared end face, and the evaluation sample is placed in a hydrogen penetration environment in a restrained state, and the delayed fracture property is evaluated. In Patent Document 3, as a simpler method, the delayed fracture property is evaluated by placing the sample in a hydrogen environment with a load applied by bending. However, Patent Document 3 does not target the sheared end face, but focuses on the evaluation of the delayed fracture property on the surface of the test piece. For this reason, in Patent Document 3, the sheared end face surface of the evaluation sample is sealed with a resin coating and excluded from the evaluation.

[0006] However, as a result of the inventors' investigations, they found that there are further problems to be solved in predicting or preventing the occurrence of delayed fracture in actual automobile parts based on these delayed fracture evaluation methods.

[0007] That is, there is a problem that the strain introduction by rolling as in Patent Document 1, for example, is different from the deformation state in the forming strain introduced by press forming used for automobile parts. In press forming, uniaxial tension and compression, and bending deformation by a combination of these are introduced to the shear end surface, so it is not sufficient as an evaluation method like Patent Document 1. In addition, Patent Documents 2 and 3 do not take into consideration the change in delayed fracture properties due to plastic deformation after shear processing of the shear end surface, and are insufficient for delayed fracture evaluation of press formed products in which various forming strains occur on the shear end surface.

[0008] Furthermore, in all of Patent Documents 1 to 3, the presence or absence and time of delayed fracture occurrence were merely evaluated under individual laboratory hydrogen penetration and stress conditions. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2020-41837 A [Patent Document 2] Patent No. 5196926 [Patent Document 3] Patent No. 5971058 Summary of the Invention [Problem to be solved by the invention]

[0010] Previously, no evaluation had been conducted from the perspective of how much margin there was in the hydrogen penetration environment and stress conditions for the occurrence of delayed fracture, compared with the hydrogen penetration environment and stress in actual automotive parts. The inventors have found that in actual automobile parts, different forming strains are introduced into the processed metal sheets depending on the positions of the sheared edges, and the forming strains cause changes in delayed fracture characteristics due to plastic deformation. Furthermore, at the sheared edges, delayed fracture is more likely to occur because the forming residual stress after press forming is added to the residual stress due to shearing.

[0011] The inventors also found that, in order to avoid delayed fracture at the sheared end faces of automotive parts, it is very important to evaluate how much margin the sheared end faces of press-formed products have against the occurrence of delayed fracture when molding residual stress is loaded on the sheared end faces into which molding strain has been introduced under a certain hydrogen penetration environment.

[0012] As described above, the shear edge properties change due to plastic deformation caused by press forming of automotive parts. However, there was no index that could predict the occurrence of delayed fracture by comparing with the stress generated in actual automotive parts, and there was no method that could evaluate delayed fracture from the viewpoint of stress margin.

[0013] The present invention has been made in view of the above points, and has an object to enable more accurate evaluation of delayed fracture properties at a sheared edge. [Means for solving the problem]

[0014] In order to solve the problems, one aspect of the present invention is a method for evaluating the delayed fracture properties of a sheared edge of a metal plate, the method comprising: a test including a step of restraining the sheared surface of the metal plate while applying a predetermined load stress to the sheared surface of the metal plate; and a step of placing the metal plate in a predetermined hydrogen penetration environment for a predetermined time period in the restrained state; and a step of determining a critical load stress, which is a limit load stress at which delayed fracture does not occur in the sheared surface of the metal plate, based on a result of the test, and setting a stress margin for the occurrence of delayed fracture in the sheared edge of the metal plate based on the determined critical load stress, wherein the determined stress margin is used as an index for evaluating the delayed fracture properties of the sheared edge of the metal plate.

[0015] It is preferable that the test includes a step of applying a forming strain to the shear surface of the metal plate along the extension direction of the shear surface prior to the restraining step, and the stress margin is a value having the forming strain as a variable. Effect of the Invention

[0016] According to an aspect of the present invention, it is possible to apply an index for evaluating delayed fracture occurring at a sheared edge with higher accuracy.

[0017] In this case, the index (stress margin) is expressed in units of stress, and can be evaluated from the viewpoint of margin due to stress. Therefore, for example, when high-strength steel plates are applied to various parts such as panel parts, structural and skeletal parts of automobiles, it becomes possible to predict the occurrence of delayed fracture including margin having a dimension of stress. For example, expanding the range of application of ultra-high strength steel sheets can also make it possible to reduce the weight of automobile bodies. [Brief description of the drawings]

[0018] [Figure 1]FIG. 1 is a diagram showing an example of a procedure for evaluating a stress margin according to an amount of strain using a test specimen. [Diagram 2] FIG. 13 is a diagram showing an example of stress margin using forming strain as a variable. [Diagram 3] FIG. 2 is a conceptual diagram showing the relationship between delayed fracture at a sheared end surface and stress margin. [Figure 4] FIG. 1 is a conceptual diagram showing the relationship between delayed fracture at a sheared end surface and stress margin when residual stress remains due to bending. [Diagram 5] FIG. 13 is an explanatory diagram showing an example of a method for applying a load stress by bending. [Figure 6] FIG. 13 is a diagram illustrating an example of a function of stress margin with respect to the amount of strain. [Figure 7] FIG. 1 is a diagram illustrating an example of delayed fracture determination using stress margin. [Figure 8] FIG. 13 is a diagram showing an example of a program flow that can be used in evaluating the present technique. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Next, an embodiment of the present invention will be described with reference to the drawings. (composition) The present embodiment relates to a delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared end surface of a metal plate. The present invention is particularly effective when the metal plate is a high-strength steel plate. As an evaluation index for the delayed fracture property evaluation method, a "stress margin," which is a newly established index in this disclosure, is obtained. In the present disclosure, the term "stress margin" refers to the allowable amount of external load stress that a sheared end surface has and that does not cause delayed fracture.

[0020] In the embodiment described below, the stress allowance is a value with the forming strain, which is one parameter of the test conditions, as a variable. The forming strain is a strain that is applied before an external load stress is applied. The stress allowance is not limited to the case where the forming strain is used as a variable. However, it is preferable to use the parameters of the manufacturing conditions of the metal plate before the external load stress is applied to the metal plate, such as the forming strain, the clearance during shearing, and the wear conditions (shear processing conditions), as variables of the stress allowance, as this will broaden the scope of application. The shear processing conditions are one of the processing conditions of the material to be evaluated. The stress margin may be expressed as a value in which test conditions other than the step of applying an external load and restraining the specimen (third step 12) are used as variables (parameters). Here, test conditions other than the process of applying an external load and restraining include, for example, the type of material (steel type and plate thickness), shear conditions (clearance and wear conditions), conditions in the hydrogen environment for immersion (immersion time), etc. The reason why the test conditions for the restraining process are removed is because the limit stress load can be found by changing this parameter. The stress margin may be a value obtained by multiplying the critical stress load by a predetermined safety factor.

[0021] The external load stress mentioned above is a load stress that occurs when a product is press-molded into a desired shape or when the product is assembled.

[0022] In this embodiment, the process for determining the stress margin for a metal plate to be evaluated includes a test step having an actual experiment and a step for setting the stress margin. Specifically, as shown in FIG. 1, the process includes a first step 10 to a fifth step 14. In FIG. 1, the first step 10 to the fourth step 13 correspond to the testing steps, and the fifth step 14 corresponds to the step of determining the stress margin.

[0023] <First step 10> The first step 10 is a step of preparing a test piece from a metal plate having the same conditions as the metal plate to be evaluated. In the first step 10, a metal plate made of the same material and thickness as the metal plate to be evaluated is subjected to shear processing to prepare a test piece having a sheared end surface for determining the stress margin.

[0024] <Second step 11> The second step 11 is a step of imparting a forming strain to at least a part of the sheared end surface of the test piece. The forming strain to be imparted is a strain along the extending direction of the sheared end surface.

[0025] The forming strain to be applied is, for example, 0.1% or more. The forming strain is imparted, for example, by subjecting the test piece to uniaxial tension or uniaxial compression, or by bending the test piece in the plate thickness direction.

[0026] <Third step 12> The third step 12 is a step of applying a preset external load stress to the shear end surface of the test piece and restraining it in the loaded state. The stress application method is, for example, tensile stress application or bending stress application. In this case, the method of applying bending stress using a jig is particularly desirable from the viewpoint of simplicity.

[0027] <4th step 13> The third step 12 is a step in which the test specimen that has been subjected to an external load stress and restrained is placed in a predetermined hydrogen penetration environment for a predetermined period of time, and the occurrence of cracks in the test specimen in that state is evaluated. In this case, the hydrogen penetration environment and the installation time are preferably set to conditions that result in an amount of hydrogen penetration equivalent to the amount of hydrogen that is estimated to penetrate in the environment in which the material to be evaluated is actually used.

[0028] The test specimen was placed in an environment where hydrogen was absorbed, for example, hydrochloric acid or NH 4This is done by immersing the test piece in a bath containing an acid solution such as an SCN solution. The concentration of the acid solution and the immersion time are set so that the amount of hydrogen that is preset as the allowable upper limit penetrates into the test piece.

[0029] For each test piece prepared in the first step 10, the above-mentioned second step 11 to fourth step 13 are carried out while changing the conditions of the applied forming strain and the applied load stress.

[0030] <5th step 14> In the fifth step 14, a critical load stress, which is a limit load stress at which delayed fracture does not occur on the sheared surface of the metal plate, is evaluated based on the results of the test, and a stress margin against delayed fracture on the sheared end surface of the metal plate is calculated based on the critical load stress. Specifically, the critical load stress is taken as the stress margin under the test conditions.

[0031] For example, based on the test conditions for each test piece (forming strain and external load stress) and the evaluation results for the presence or absence of cracks at the shear end surface, the critical stress load value is determined, which is the boundary value between the external load stress at which cracks occur for the same forming strain and the external load stress at which cracks do not occur (such as the maximum external load stress at which cracks do not occur).

[0032] This is organized for multiple forming strains, multiple data (forming strain, critical stress load) are obtained, and the value (function) of the critical stress load with the forming strain as a variable is obtained as stress margin data, as shown in the graph in Figure 2. In other words, the stress margin is described as a function of forming strain due to tension-compression, for example.

[0033] Here, in the above explanation, the case where the test is performed by changing the forming strain is exemplified, but the present invention is not limited to this. The forming strain may be limited to a constant value (for example, zero) to perform the test. In this case, the forming strain may be zero, that is, the second step 11 may not be performed. In this case, the stress margin for a specific forming strain is obtained.

[0034] Also, for example, the shear conditions (clearance, etc.) may be changed, and the stress margin may be calculated using the shear conditions as variables together with or without the forming strain as a variable.

[0035] In addition, Fig. 2 shows an example in which the residual stress at the sheared edge is reduced by the forming strain, and 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 sheared edge, it is also possible that the forming strain may cause cracks or damage at the sheared edge, and thus the forming strain may actually reduce the stress margin for delayed fracture.

[0036] In this embodiment, by comparing the stress margin obtained as described above, it is possible to evaluate the possibility of delayed fracture of the test piece having the shear end face that was not used in the evaluation of the stress margin against the external load (load stress) to be applied to the metal plate without conducting a test.

[0037] For example, the evaluation is performed as follows: First, for the metal plate being evaluated, the amount of strain due to tension and compression and the load stress on the shear end surface for which the occurrence of delayed fracture is to be evaluated are calculated using CAE forming analysis or elementary mechanical calculation methods.

[0038] Next, the stress margin is adjusted according to the amount of strain caused by tension and compression at the sheared end, and it is judged whether the load stress on the sheared end exceeds the stress margin. If it does, it is judged that there is a possibility of delayed fracture. At this time, the stress margin can be set smaller than the actually measured value, taking into account the safety factor. The delayed fracture evaluation of the sheared end surface in comparison with the above-mentioned stress margin can be carried out more efficiently by using a program as described below.

[0039] In the above explanation, a case has been shown in which no safety factor is provided for the stress margin, but it is also possible to provide a safety factor for the stress margin by setting the stress margin at a low level.

[0040] In the above description, in the second step 11 in the test, a forming strain is actually applied to the test piece, and the stress margin is calculated using the forming strain as a variable, but the present invention is not limited to this.

[0041] For example, a separate step (called the sixth step) is provided in which the forming residual stress and forming strain generated at the sheared end surface are obtained by performing bending on the sheared end surface of the metal plate before the restraining step. This sixth step may be obtained by a separately performed experiment, or by structural analysis such as CAE analysis. The value obtained by adding the forming residual stress obtained in the sixth step to the limit load stress (stress margin) obtained in the above test without performing the second step 11 may be used as the stress margin with the forming strain as a variable.

[0042] (Details of disclosure) The present disclosure will now be described in further detail. The inventors have discovered the following findings (1) to (3) while evaluating delayed fracture on the sheared end surface.

[0043] (1) When a constant load stress (external load stress) is applied to the sheared edge and the sheared edge is placed in a hydrogen penetration environment for a certain period of time, there exists a limit load stress (also called critical load stress) at which delayed fracture occurs on the sheared edge. This is because delayed fracture occurs when the sum of the residual stress due to shear processing and the external load stress on the sheared edge reaches the threshold value for delayed fracture on the sheared edge.

[0044] (2) The critical load stress at which the delayed fracture occurs at the sheared edge varies depending on the amount of tensile and compressive strain applied after shearing. This is because the residual stress at the sheared edge varies depending on the forming strain.

[0045] (3) Therefore, the critical load stress of each sheared edge varies depending on the forming strain and the load stress (external load stress) applied to the sheared edge. The critical load stress at which delayed fracture occurs on the sheared edge when placed in a predetermined hydrogen penetration environment for a predetermined installation time can be organized as an index called "stress margin" that takes into account the forming strain and the load stress (external load stress).

[0046] In this disclosure, the allowable amount of external load stress that a sheared end surface has without causing delayed fracture is defined as "stress margin."

[0047] Fig. 3 shows a conceptual diagram for explaining the above (1), (2), and (3). Fig. 3(a) illustrates the state of the limit load stress when no forming strain is applied to a metal plate whose end has been sheared to form a sheared end face. On the other hand, Fig. 3(b) illustrates the state of the limit load stress when forming strain is applied after the sheared end face is formed. Fig. 3 illustrates the case where residual stress is reduced by applying forming strain to a metal plate before press forming the metal plate.

[0048] Here, delayed fracture occurs when the sum of the residual stress due to shear and the external load stress reaches the threshold for delayed fracture. Therefore, when the residual stress at the sheared end surface changes due to forming strain, the critical load stress at which delayed fracture occurs also changes. The critical load stress is the difference between the residual stress at the sheared end surface and the threshold for delayed fracture, and is the limit of the external load stress at which the sheared end surface does not experience delayed fracture.

[0049] In view of this, in this 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 face, is defined as the "stress margin." In this embodiment, the allowable amount of external load stress is specified by an index called the stress margin, which uses the forming strain as a variable.

[0050] Here, the residual stress at the sheared edge due to shearing exists only in a very small area of ​​the surface layer, about 100 μm from the sheared edge surface. For this reason, it is difficult to calculate the change in residual stress using CAE that uses normal shell elements. Stress in a small area can be measured using X-ray stress measurement, but there are problems in that the measured value may vary depending on the measurement range and that the measurement depth is limited to the outermost layer of the material. Therefore, the magnitude of the measured value may not necessarily correspond to the risk of delayed fracture.

[0051] On the other hand, if a method is used in which the above-mentioned "stress margin" using forming strain as a variable is experimentally determined by a delayed fracture test under stress loading, as disclosed herein, it is possible to obtain an index for directly evaluating the risk of delayed fracture without encountering such problems related to calculations and measurements.

[0052] If this stress margin is evaluated under conditions of a hydrogen penetration environment to which an automobile part is actually exposed, it can be regarded as a margin until the occurrence of delayed fracture at the sheared end surface of the automobile part.

[0053] Moreover, since this stress margin is expressed in units of stress, it can be assumed that delayed fracture will not occur unless this stress margin is exceeded, even if an external load stress applied to the part during assembly or use is added to the residual stress due to molding of the part.

[0054] Therefore, this index of stress margin is a simple and convenient index for evaluating delayed fracture, which also allows evaluation as a margin having a stress dimension. Conversely, a method of changing the hydrogen penetration environment while keeping the stress load constant is also conceivable. However, this is less useful in that it is not possible to evaluate the margin by comparing the residual stress due to the molding of the parts mentioned above and the added external load stress due to deformation of the parts during assembly and use, etc., using stress as a measure. The forming strain is a strain in the direction extending along the shear plane.

[0055] Furthermore, regarding practical evaluation methods, the inventors have discovered the following findings (4) to (7). (4) Uniaxial tensile or compressive deformation is desirable as a method for introducing tensile and compressive forming strains to the sheared end surface into the evaluation test piece. This is because, in uniaxial forming, the residual stress after forming in parts other than the sheared end surface becomes almost zero due to springback after forming, and its effect can be ignored. Therefore, when forming strain is applied by uniaxial tension or compression, the external load stress at the limit at which delayed fracture occurs at the sheared end surface after additional processing can be evaluated as it is as the "stress margin," which is the simplest method.

[0056] (5) For the bending deformation introduced into an actual press-formed product, the “stress margin” obtained at the shear end surface after uniaxial tension / compression by the method described above in (4) can be applied from the front to the back of the plate (in the thickness direction of the plate) according to the amount of strain and the sign of the strain.

[0057] (6) It is also possible to evaluate the stress margin by using bending deformation as a method of applying forming strain to the sheared end surface. In this case, bending deformation may cause residual stress due to bending in the entire test piece, including the sheared end surface. Therefore, in this case, the residual stress due to bending should be added to the external load stress at the limit at which delayed fracture occurs to determine the "stress margin." Note that residual stress due to bending deformation can be evaluated by computer analysis using CAE, etc., or elementary mechanical calculations.

[0058] Figure 4 is a conceptual diagram explaining (6). Note that Figure 4 shows the case where the residual stress due to bending is positive, but the sign of the residual stress due to bending is reversed on the front and back of the plate. Therefore, for parts where the residual stress due to bending is negative, it is necessary to subtract the residual stress due to shear (add a negative value) by the amount of the residual stress due to bending.

[0059] (7) There are two methods for applying an external load stress to the evaluation specimen: tension at a constant load and bending. The bending method is simpler because it can be performed with a smaller load and a smaller jig. The external load stress can be controlled by adjusting the load and deformation applied to the specimen. The load on the shear end surface of the specimen can be calculated by CAE or elementary analysis.

[0060] Fig. 5 is a diagram showing an example of a method of applying stress by bending in (7). Fig. 5 illustrates a method of applying stress by four-point bending. In Fig. 5, reference numeral 1 indicates a test piece, reference numeral 2 indicates a fulcrum for applying bending, and reference numeral 3 indicates a screw for adjusting the amount of bending.

[0061] Here, the test piece for evaluating the stress margin may be one that has been sheared in a laboratory, or may be a part cut out from the sheared end surface of a press-molded product after press molding. Furthermore, the inventors have devised the following method for evaluating and predicting the occurrence of delayed fracture at the sheared edge using the "stress margin" thus obtained. Examples of this method are shown in the following first to third examples.

[0062] (first) First, the stress margin is measured in a laboratory according to the amount of strain caused by tension and compression using a test piece by the above methods (4) to (7).

[0063] Here, it is preferable to set the hydrogen penetration environment and the exposure time in that environment under conditions such that the amount of hydrogen that penetrates into the test specimen becomes equal to the amount of hydrogen that penetrates into an actual automobile part, which is preset as an allowable upper limit.

[0064] The amount of forming strain applied to the sheared end surface is preferably 0.1% or more, considering the amount that sufficiently affects the delayed fracture properties. A strain amount of 0.5% or more has a greater effect, and the delayed fracture evaluation according to the present invention is particularly effective when plastic strain is introduced. Therefore, instead of forming strain, it is also possible to use the amount of plastic strain on the sheared end surface as an evaluation index. Regarding the load stress on the test piece, any parameter related to stress, such as the first principal stress or the Mises stress, can be used in the present disclosure.

[0065] (Second) Secondly, for test pieces with sheared ends for which the possibility of delayed fracture is to be evaluated, the forming strain due to tension-compression and the load stress on the sheared ends are calculated using CAE forming analysis and elementary mechanical methods.

[0066] (Third) Thirdly, the forming strain and the residual stress after forming of the test piece with the sheared end surface to be evaluated for the possibility of delayed fracture are compared with the stress margin corresponding to the strain due to tension-compression, and the parts that exceed the stress margin are judged to have the possibility of delayed fracture. However, the stress margin can be set smaller than the actually measured value, taking into account the safety factor.

[0067] (program) An example of a program used for the above evaluation method is shown below. The first example program is a program for storing in a memory unit the relationship between stress allowance and forming strain obtained by the above-mentioned delayed fracture property evaluation method, and for causing a computer to execute a process of determining a stress allowance corresponding to an input amount of forming strain by referring to the stored relationship between stress allowance and forming strain.

[0068] Another example of the program is a program for storing in a memory unit the relationship between the stress margin and the forming strain and the externally applied stress, which is obtained by the above-mentioned delayed fracture property evaluation method, and for causing a computer to execute a process of evaluating the possibility of delayed fracture for the input amount of forming strain and the externally applied stress, by referring to the stored relationship between the stress margin and the forming strain and the externally applied stress.

[0069] Next, an example of processing by a program of the evaluation method using the stress margin as described above will be described with reference to Fig. 8. If evaluation is performed by the processing as shown in Fig. 8, delayed fracture can be evaluated more efficiently.

[0070] 8 includes a stress margin calculation section 20, an evaluation main section 30, and a storage section 40. Programs for performing the processes of the stress margin calculation section 20 and the evaluation main section 30 are stored in the storage section 40, such as a RAM or ROM of a computer, and are executed by the computer.

[0071] <Storage section 40> The storage unit 40 is composed of a recording medium such as a database. The memory unit 40 stores data on the stress margin d obtained for the forming strain for each of the material conditions of the metal plate, the hydrogen environment conditions, and the shear conditions by repeating the tests of the first step 10 to the fifth step 14 while varying the material conditions of the metal plate, the hydrogen environment conditions, the shear conditions, and the amount of forming strain in various ways.

[0072] <Stress margin calculation unit 20> In the stress margin calculation unit 20, first, in step S10, the operator is prompted to input the basic conditions for evaluation, such as the type of material (steel type and thickness) and the hydrogen environment conditions (acidity and installation time), which are the conditions for delayed fracture, and the above input is obtained through the operator's input operation.

[0073] Next, in step S20, the operator is prompted to input the shear conditions, and the input is acquired through the operator's input operation. Next, in step S30, a data group of stress margins for each strain amount ((a collection of data of (strain amount, stress margin)) that matches the conditions input in steps S10 and 20) is acquired from the storage unit 40. Alternatively, an input of a data group of stress margins for each strain amount obtained by testing is prompted, and the above input information is acquired by the input operation of an operator. The acquired data is stored in the storage unit 40.

[0074] Next, in step S40, referring to the data group of stress margins for the strain amount acquired in step S30, arithmetic processing is executed to obtain the stress margin d as a function f(x) with the strain amount x as a variable by a known processing method. Next, in step S50, the function of the stress margin d obtained in step S40 is changed to an expression considering a safety factor s (: 0 < s ≤ 1) as shown in the following expression. d = s·f(x) Then, the information of the function of the obtained stress margin d is stored in the storage unit 40 using the test conditions as a key.

[0075] <Evaluation main body unit 30> In the evaluation main body unit 30, first, in step S100, an input of the conditions of the material type (steel type and thickness) of the evaluation target and the conditions of the hydrogen environment (acidity and installation time), which are the conditions of delayed fracture, is prompted, and the above input is acquired by the input operation of an operator.

[0076] Next, in step S110, an input of the strain amount x and the load stress g planned during processing is prompted, and the above input is acquired by the input operation of an operator. In step S120, information on the function "s·f(x)" of the stress margin d that matches the conditions input in step S100 is acquired from the storage unit 40, and 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 to determine whether there is a risk of delayed fracture.

[0077] Here, in step S120 in FIG. 8, it is determined whether or not there is a risk of delayed fracture, but the margin of delayed fracture (=dg) may also be output together. In addition, the stress margin calculation unit 20 may execute a separate calculation process to find a function of the stress margin d using the input values ​​in steps S10 to S20 as conditions, and input the found function to the memory unit 40 as data using the input values ​​in steps S10 to S20 as keys.

[0078] (others) The present disclosure may also have the following configuration. (1) A method for evaluating the delayed fracture properties of a sheared edge of a metal plate, comprising: a test including a step of restraining the sheared surface of the metal plate while applying a predetermined load stress to the sheared surface of the metal plate; and a step of placing the metal plate in a predetermined hydrogen penetration environment for a predetermined time period while being restrained; and a step of determining a limit load stress, which is a limit load stress at which delayed fracture does not occur on the sheared surface of the metal plate, based on the results of the test; and setting a stress margin for the occurrence of delayed fracture on the sheared edge of the metal plate based on the determined limit load stress, wherein the determined stress margin is used as an index for evaluating the delayed fracture properties of the sheared edge of the metal plate. (2) The test is carried out by changing one or more selection conditions selected from the test conditions other than the constraining process, and the limit load stress under each selection condition is obtained as the stress margin, and the stress margin is expressed as a value having the selection condition as a variable. Here, the test conditions other than the above-mentioned restraining step include shear conditions (clearance, etc.), forming strain when forming strain is applied, and placement time in a hydrogen penetration environment. (3) The above test includes a step of applying a forming strain to the shear surface of the metal plate along the extension direction of the shear surface prior to the restraining step, and the above stress margin is a value having the forming strain as a variable.

[0079] (4) The forming strain imparted in the above-mentioned forming strain imparting step shall be 0.1% or more. (5) In the step of applying forming strain, the forming strain is applied by uniaxial tension or uniaxial compression. (6) In the step of applying forming strain, forming strain is applied by bending.

[0080] (7) The method further includes a step of determining a forming residual stress generated at the sheared end surface by performing bending processing on the sheared end surface of the metal plate before the restraining step, and the step of setting the stress margin defines the stress margin as a value obtained by adding the forming residual stress in the bending processing determined in the step of determining the forming residual stress to the determined limit load stress. (8) In the step of determining the above-mentioned molding residual stress, the relationship between the molding strain generated at the shear end surface due to bending and the molding residual stress is determined, and the above-mentioned stress margin is set as a value with the molding strain as a variable.

[0081] (9) A delayed fracture property evaluation method for evaluating the possibility of delayed fracture at the shear end surface of a metal plate to be evaluated, which is a metal plate under the same conditions as the metal plate used in the above test, and based on the stress margin obtained by the delayed fracture property evaluation method disclosed herein, the possibility of delayed fracture at the end surface of the metal plate to be evaluated is evaluated by the forming strain and load stress applied to the end surface of the metal plate to be evaluated.

[0082] (10) The metal plate is a steel plate having a tensile strength of 980 MPa or more. (11) A program for storing in a memory unit the relationship between stress margin and forming strain obtained by the disclosed method for evaluating delayed fracture properties, and for causing a computer to execute a process of determining a stress margin corresponding to an input amount of forming strain by referring to the stored relationship between stress margin and forming strain.

[0083] (12) A program for storing in a memory unit the relationship between the stress margin and the forming strain and the external load stress obtained by the method for evaluating delayed fracture properties disclosed herein, and for causing a computer to execute a process of evaluating the possibility of delayed fracture for an input amount of strain of the forming strain and an external load stress by referring to the stored relationship between the stress margin and the forming strain and the external load stress. EXAMPLES

[0084] An example of this embodiment will be described. Example 1 In this embodiment, a test material X made of a steel plate with a tensile strength of 1470 MPa and a thickness of 1.0 mm will be described as the metal plate to be evaluated. It should be noted that the present invention is not limited to the metal plate used as the test material, but can be applied to metal materials including high-strength steel plates having a tensile strength of 980 MPa or more, which are susceptible to delayed fracture at the shear end surface.

[0085] First, the test material X was sheared by shear processing to prepare a test piece having a linear shear end surface with a length of 100 mm. The width of the test piece when shearing was 30 mm, and the test piece was in the shape of a strip of 100 mm x 30 mm. The clearance during shear processing was set to 12% of the plate thickness. In the above embodiment, the case where the shear condition is a single one was described as an example, but even if the shear condition such as the clearance during shear processing is changed, an evaluation corresponding to the change is possible. In other words, it is sufficient to obtain the stress margin under the shear condition.

[0086] Next, a forming strain was applied to the sheared end of the test piece by tension or compression along the extension direction of the sheared end. In this example, the forming strain was applied by a uniaxial load testing machine with both ends of the test piece clamped. Note that, although this example describes the case where the forming strain is tension or compression, it has been confirmed that similar results can be obtained even when the forming strain is deformation by bending. In addition, a test piece that was sheared without applying forming strain was also prepared.

[0087] Next, each test piece was subjected to external restraint by four-point bending using a jig, and stress was applied to the center of the sheared end face of the test piece, with the burr side being on the outside of the bend so that a tensile stress was applied. Here, the magnitude of the applied stress was measured by determining the first principal stress-first principal strain relationship at the center and apex of the width of the test piece using CAE, and then measuring the amount of strain when the test piece was actually bent.

[0088] In this example, the method of stress application was explained as four-point bending. Similar results can be obtained with other bending load methods and loading methods such as uniaxial tension. In this example, the burr side at the time of shearing was placed on the outside of the bend when applying stress, and tensile stress was applied, but it is also possible to perform an evaluation on the surface opposite the burr side in a similar manner.

[0089] In this example, the load stress applied to each test piece was changed in increments of 100 MPa as shown in the table, and multiple test pieces were prepared for each forming strain condition.

[0090] The test pieces to which the load stress was applied were then immersed in a bath of a pH 6 thiocyanate solution for 96 hours, and the delayed fracture properties were evaluated based on the presence or absence of cracks due to delayed fracture after 96 hours. The above conditions and evaluation results are shown in Tables 1 to 11. Each table is organized by the amount of forming strain.

[0091] [Table 1]

[0092] [Table 2]

[0093] [Table 3]

[0094] [Table 4]

[0095] [Table 5]

[0096] [Table 6]

[0097] [Table 7]

[0098] [Table 8]

[0099] [Table 9]

[0100] [Table 10]

[0101] [Table 11]

[0102] Each table shows whether or not delayed fracture occurs for each applied stress, which varies depending on the amount of strain, when tension is positive and compression is negative.

[0103] As can be seen from Tables 1 to 11, the greater the absolute value of the amount of forming strain, the greater the critical load stress. In this example, the critical load stress with the forming strain as a variable is the stress margin. FIG. 6 shows the stress margin calculated from the limit load stress at which delayed fracture did not occur, and plotted as a function of strain.

[0104] In this way, it is possible to describe the stress margin according to the forming strain after shearing. In this example, the case where the stress margin increases due to the forming strain after shearing is shown, but the same evaluation is also possible in the case where the stress margin decreases due to the forming strain after shearing.

[0105] Example 2 Next, an example of delayed fracture judgment using the stress margin corresponding to the forming strain after shearing, obtained in Example 1, will be described. In Example 2, test pieces A, B, C, D, E, F, G, and H were prepared from the test material X by the same process as in Example 1, and had different forming strain amounts and load stresses.

[0106] Each test piece was immersed in a bath of a thiocyanic acid solution of pH 6 for 96 hours, and delayed fracture was evaluated based on the presence or absence of cracks due to delayed fracture after 96 hours. Table 12 shows the evaluation results of the forming strain, applied stress, and delayed fracture of each test piece.

[0107] [Table 12]

[0108] Next, the results of each of the test pieces A to G were plotted and compared against the stress margin (see FIG. 6) obtained in Example 1. The comparison results are shown in FIG. As can be seen from FIG. 7, it was found that the occurrence of delayed fracture at the shear end face of an actual part can be predicted by whether or not the stress margin line is exceeded.

[0109] Therefore, by using the stress margin disclosed herein, it is possible to determine locations on the sheared end surface that are at risk of delayed fracture. Although the variable of the stress allowance is the forming strain, the stress allowance may be expressed by using the shearing conditions such as the clearance in the shearing process as a variable. [Explanation of symbols]

[0110] 10 First process (shearing process) 11 Second process (process of applying forming strain) 12 Third step (restraining under load) 13 Fourth step (step of placing in hydrogen intrusion environment for a preset time) 14 Fifth step (step of setting stress margin) 20 Stress margin calculation section 30 Evaluation Body 40 Storage section d Stress margin

Claims

1. A delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared end surface of a metal plate, comprising: a test including a step of restraining the metal plate in a state in which a preset load stress is applied to the shear surface of the metal plate, and a step of placing the metal plate in the restrained state in a preset hydrogen penetration environment for a preset time; A step of determining a critical load stress, which is a limit load stress at which delayed fracture does not occur on the sheared surface of the metal plate, based on the results of the test, and setting a stress margin for delayed fracture on the sheared end surface of the metal plate based on the determined critical load stress, The stress margin obtained above is used as an index for evaluating the delayed fracture properties of the sheared end surface of the metal plate, The test includes a step of applying a forming strain to the shear surface of the metal plate along an extension direction of the shear surface before the restraining step, In the step of applying the forming strain, the forming strain is applied by uniaxial tension or uniaxial compression. A method for evaluating delayed fracture characteristics of a sheared end surface.

2. The test is carried out by changing one or more selected test conditions from among the test conditions other than the restraining step, and the critical load stress under each selected test condition is obtained as the stress margin; The stress margin is expressed as a value having the selection conditions as variables.

2. The method for evaluating delayed fracture properties of a sheared end surface according to claim 1.

3. The forming strain imparted in the forming strain imparting step is 0.1% or more.

2. The method for evaluating delayed fracture properties of a sheared end surface according to claim 1.

4. A delayed fracture property evaluation method for evaluating the possibility of delayed fracture at a shear end surface of a metal plate to be evaluated, the metal plate being a metal plate under the same conditions as the metal plate used in the above test, comprising: A method for evaluating the delayed fracture properties of a sheared end surface, characterized in that the possibility of delayed fracture at the end surface of the metal plate to be evaluated is evaluated based on the forming strain and load stress applied to the end surface of the metal plate to be evaluated, based on the stress margin obtained by the method for evaluating the delayed fracture properties of the sheared end surface described in any one of claims 1 to 3.

5. A relationship between stress margin and forming strain obtained by the method for evaluating delayed fracture properties of a sheared end surface according to any one of claims 1 to 3 is stored in a storage unit; A program for causing a computer to execute a process of determining a stress allowance corresponding to an input amount of forming strain by referring to the stored relationship between the stress allowance and forming strain.

6. A relationship between the stress margin and the forming strain and the external load stress, which is obtained by the method for evaluating the delayed fracture properties of a sheared end surface according to any one of claims 1 to 3, is stored in a storage unit; A program for causing a computer to execute a process for evaluating the possibility of delayed fracture for the input amount of forming strain and external load stress by referring to the relationship between the stored stress margin and the forming strain and external load stress.

7. A delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared end surface of a metal plate, comprising: a test including a step of restraining the metal plate in a state in which a preset load stress is applied to the shear surface of the metal plate, and a step of placing the metal plate in the restrained state in a preset hydrogen penetration environment for a preset time; A step of determining a critical load stress, which is a limit load stress at which delayed fracture does not occur on the sheared surface of the metal plate, based on the results of the test, and setting a stress margin for delayed fracture on the sheared end surface of the metal plate based on the determined critical load stress, The stress margin obtained above is used as an index for evaluating the delayed fracture properties of the sheared end surface of the metal plate, The method includes a step of determining a forming residual stress generated on the sheared end surface by performing bending on the sheared end surface of the metal plate before the restraining step, The step of setting the stress margin includes setting the stress margin to a value obtained by adding the critical load stress obtained above to the molding residual stress obtained in the step of obtaining the molding residual stress. A method for evaluating delayed fracture characteristics of a sheared end surface.

8. In the step of determining the forming residual stress, a relationship between forming strain generated on the shear end surface due to bending and the forming residual stress is determined, The above stress margin is a value with forming strain as a variable.

8. The method for evaluating delayed fracture properties of a sheared end surface according to claim 7.

9. A test comprising a step of restraining a metal plate while applying a predetermined load stress to a shear surface of the metal plate, and a step of placing the metal plate in the restrained state in a predetermined hydrogen penetration environment for a predetermined time. A step of determining a critical load stress, which is a limit load stress at which delayed fracture does not occur on the sheared surface of the metal plate, based on the results of the test, and setting a stress margin for delayed fracture on the sheared end surface of the metal plate based on the determined critical load stress, The stress margin obtained above is used as an index for evaluating the delayed fracture properties of the sheared end surface of the metal plate, The test includes a step of applying a forming strain to the shear surface of the metal plate along an extension direction of the shear surface before the restraining step, The stress margin is a value that uses the forming strain as a variable. A first method for evaluating delayed fracture properties of a sheared edge of a metal plate, comprising: A delayed fracture property evaluation method for evaluating the possibility of delayed fracture at a sheared edge of a metal plate to be evaluated, the metal plate being a metal plate under the same conditions as the metal plate used in the test, using the first delayed fracture property evaluation method for the sheared edge, Based on the stress margin obtained by the first method for evaluating delayed fracture characteristics of a sheared end face, a forming strain and a load stress are applied to the end face of the metal plate to be evaluated, and a possibility of delayed fracture at the end face of the metal plate to be evaluated is evaluated. A method for evaluating delayed fracture characteristics of a sheared end surface.

10. The forming strain imparted in the forming strain imparting step is 0.1% or more.

10. The method for evaluating delayed fracture properties of a sheared end surface according to claim 9.

11. 10. The method for evaluating delayed fracture properties of a sheared end surface according to claim 9, wherein in the step of applying forming strain, the forming strain is applied by bending.

12. The method for evaluating delayed fracture properties of a sheared end surface according to any one of claims 1 to 3 and claims 7 to 11, wherein the metal plate is a steel plate having a tensile strength of 980 MPa or more.

13. A test comprising the steps of restraining a sheared surface of a metal plate under a preset load stress, and placing the metal plate in the restrained state in a preset hydrogen penetration environment for a preset time, and determining a limit load stress, which is a limit load stress at which delayed fracture of the sheared surface of the metal plate does not occur, based on the results of the test, and setting a stress margin for delayed fracture of the sheared end surface of the metal plate based on the determined limit load stress, wherein the determined stress margin is used as an index for evaluating the delayed fracture characteristics of the sheared end surface of the metal plate, and the test comprises the steps of applying a forming strain to the sheared surface of the metal plate along the extension direction of the sheared surface prior to the restraining step, and the stress margin is set to a value with the forming strain as a variable, wherein the relationship between the stress margin, the forming strain, and an external load stress determined by the delayed fracture characteristics evaluation method for the sheared end surface of a metal plate is stored in a memory unit, A program for causing a computer to execute a process for evaluating the possibility of delayed fracture for the input amount of forming strain and external load stress by referring to the relationship between the stored stress margin and the forming strain and external load stress.

14. The forming strain imparted in the forming strain imparting step is 0.1% or more.

14. The program according to claim 13.

15. 14. The program according to claim 13, wherein in the step of applying forming strain, the forming strain is applied by bending.

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