Method for evaluating delayed fracture characteristics of molded parts and method for manufacturing molded parts

Abstract

To more accurately evaluate delayed fracture characteristics at a sheared end surface of a molded component.SOLUTION: A delayed fracture characteristic evaluation method evaluates delayed fracture characteristics at a sheared end surface of a molded component used by assembling the molded component obtained by molding a metal sheet with another component. The method comprises: a first step 1 for obtaining, on the basis of a result of a test including a step for installing a metal sheet in a hydrogen intrusion environment in a state in which load stress is applied to a sheared surface of the metal sheet and restraining is performed, a stress margin with a strain amount as a variable; a second step 2 for performing molding analysis of molding the metal sheet into a molded component and obtaining the strain amount and residual stress at a sheared end surface of the molded component; a third step 3 for obtaining the load stress applied to the sheared end surface by assembling the molded component with another component; and a fourth step 4 for evaluating, on the basis of the stress margin corresponding to the obtained strain amount and the total stress of the obtained residual stress and the obtained load stress, a margin of delayed fracture of the molded component.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the delayed fracture properties of a formed part, which evaluates the delayed fracture properties at a sheared end surface of a formed part manufactured by molding such as press molding, and a technology related to a method for manufacturing a formed part using the method. In this specification, the edge of a metal sheet that has been sheared is referred to as a sheared edge. The present invention is a technology particularly suitable for formed parts made of high-strength steel sheets (high-tensile steel sheets) with a tensile strength of 980 MPa or more. In addition, in this specification, high-strength steel sheets with a tensile strength of 1470 MPa or more are referred to as ultra-high-strength steel sheets. [Background technology]

[0002] Currently, automobiles are required to improve fuel efficiency and crashworthiness through weight reduction. To achieve both weight reduction and occupant protection in the event of a collision, high-strength steel sheets are used in car bodies. In particular, high-strength steel sheets with a tensile strength of 980 MPa or more have begun to be used in car bodies in recent years. One of the issues when using high-strength steel sheets in car bodies is delayed fracture. Delayed fracture, which occurs at the shear edge, which is the edge surface after shearing, is a particularly serious issue with high-strength steel sheets with a tensile strength of 980 MPa or more. This issue is particularly problematic with ultra-high-strength steel sheets with a tensile strength of 1470 MPa or more. It is known that large tensile stress remains at the sheared edge, and there is concern that the formed parts made from the metal plate may suffer delayed fracture over time.

[0003] In order to predict delayed fracture at the sheared edge, it is necessary to prepare a test specimen for evaluation and place it in a hydrogen penetration environment. Furthermore, the properties of the sheared edge change due to plastic deformation during shearing, which generally increases the risk of delayed fracture at the edge. For this reason, for example, in Patent Document 1, the sheared edge is subjected to compression in the plate thickness direction by rolling, and then placed in a hydrogen penetration environment to evaluate the occurrence of delayed fracture.

[0004] On the other hand, even if delayed fracture does not occur when a sheared edge is placed in a hydrogen entry environment without load, delayed fracture may occur if a test is performed with an externally applied stress. This is because the externally applied stress is added to the large tensile stress remaining at the sheared edge. For this reason, for example, in Patent Document 2, a constant tensile load is applied to an evaluation sample including the sheared edge, and the sample is placed in a hydrogen entry environment under restraint conditions to evaluate delayed fracture properties. In addition, in Patent Document 3, a simpler method is used in which the sample is placed in a hydrogen environment under a bending load to evaluate delayed fracture properties. However, Patent Document 3 focuses on evaluating delayed fracture properties on the surface of the test specimen, not the sheared edge. For this reason, in Patent Document 3, the sheared edge surface of the evaluation sample is sealed with a resin coating and excluded from evaluation.

[0005] However, after investigations, the inventors found that there are further challenges to be overcome in predicting or preventing the occurrence of delayed fracture in actual automobile parts based on these delayed fracture evaluation methods. That is, for example, the introduction of strain by rolling as in Patent Document 1 has the problem of deviating from the deformation state caused by forming strain introduced by press forming used in automobile parts. In press forming, uniaxial tension and compression, as well as bending deformation due to a combination of these, are introduced to the sheared edge, so this is not sufficient as an evaluation method like Patent Document 1. Furthermore, Patent Documents 2 and 3 do not take into account changes in delayed fracture properties due to plastic deformation after shear processing of the sheared edge, and are therefore insufficient for evaluating delayed fracture in formed parts in which various forming strains occur on the sheared edge. Furthermore, in all of Patent Documents 1 to 3, the presence or absence of delayed fracture and the time required for it to occur were evaluated only under individual laboratory hydrogen penetration conditions and stress conditions. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-41837 [Patent Document 2] Patent No. 5196926 [Patent Document 3] Patent No. 5971058 Summary of the Invention [Problem to be solved by the invention]

[0007] Until now, no evaluation has been conducted from the perspective of how much margin there is in the stress conditions regarding the occurrence of delayed fracture by comparing them with the stresses that occur in automotive parts in actual usage environments. The inventors have found that in actual automobile parts, different forming strains are introduced into the processed metal sheets depending on the location of the sheared edge, and that the forming strains cause changes in delayed fracture characteristics due to plastic deformation.Furthermore, they have found that at the sheared edge, delayed fracture may be more likely to occur because the load stress after press forming is added to the residual stress due to shear.

[0008] The inventors also found that, in order to avoid delayed fracture at the sheared edge of an automotive part, it is very important to evaluate the margin of resistance of the sheared edge of a formed part against the occurrence of delayed fracture when forming residual stress is applied to the sheared edge where forming strain has been introduced under a certain hydrogen penetration environment. As described above, the shear edge properties change due to plastic deformation during press forming of automotive parts. However, until now, there has been no index that can predict the occurrence of delayed fracture by comparing it with the stress generated in actual automotive parts, and there has been no method that can evaluate delayed fracture from the perspective of stress margin.

[0009] The present invention focuses on the above points and aims to enable the manufacture of molded parts in which delayed fracture is suppressed by more accurately evaluating the delayed fracture characteristics at the shear end faces of molded parts during use. [Means for solving the problem]

[0010] In order to solve the problem, one aspect of the present invention is a method for evaluating the delayed fracture properties of a formed part, which evaluates the delayed fracture properties of a sheared end surface of a formed part manufactured by forming a metal plate made of a high-strength steel plate, and which evaluates the delayed fracture properties of the formed part at the sheared end surface of the formed part when the formed part is used by assembling it with another part, the method comprising the steps of: restraining the sheared surface of the metal plate while applying a predetermined load stress to the sheared surface of the metal plate; and placing the metal plate in the restrained state in a predetermined hydrogen penetration environment for a predetermined time. The method determines, based on the results of a test, a stress margin, which is an allowable value of external load stress at which delayed fracture does not occur at the sheared surface of the metal plate, using the amount of strain as a variable. a second step of performing a forming analysis of forming the metal plate into the formed part to determine the residual stress and strain amount at the sheared end surface of the formed part that occur when the metal plate is formed into the formed part; a third step of determining the load stress that is applied to the sheared end surface by assembling the formed part to another part; and a fourth step of evaluating the delayed fracture margin of the formed part based on the stress margin of the metal plate using the strain amount determined in the second step as a variable and the total stress of the residual stress determined in the second step and the load stress determined in the third step. The molding is, for example, press molding. [Effects of the Invention]

[0011] According to an aspect of the present invention, it is possible to more accurately evaluate the delayed fracture characteristics at the sheared end surface of a molded part in a usage environment, and to manufacture a molded part in which delayed fracture is suppressed. The stress allowance, which is an index for evaluating delayed fracture, is measured in units of stress, and can be evaluated from the perspective of stress allowance. Therefore, when high-strength steel sheets are used for various parts such as automotive panel parts and structural / skeletal parts, it is possible to predict the occurrence of delayed fracture in formed parts, taking into account the allowance, which has the dimension of stress. Furthermore, for example, by expanding the range of application of ultra-high strength steel sheets, it will be possible to reduce the weight of automobile bodies. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a conceptual diagram showing the relationship between delayed fracture at a sheared end surface and stress margin. [Figure 2] FIG. 1 is a diagram illustrating an example of a configuration according to an embodiment of the present invention. [Figure 3] FIG. 10 is a diagram illustrating a configuration example of a first step. [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 after bending. [Figure 5] FIG. 10 is a diagram showing an example of a processing flow that can be used in evaluating the present method. [Figure 6] FIG. 10 is a diagram illustrating an example of a function of stress margin with respect to the amount of strain. [Figure 7] 1A and 1B are diagrams showing the shape of a molded part (actual part) in this embodiment. [Figure 8] FIG. 1 is a diagram showing an example of delayed fracture determination using stress margin. [Figure 9] FIG. 1 is a diagram showing an example of delayed fracture determination based on the total stress of residual stress and external load stress due to assembly and use. [Figure 10] This figure shows an example of delayed fracture determination based on the total stress of residual stress and external load stress due to assembly and use, when the molding conditions are revised and an intermediate process is added to prevent delayed fracture. DETAILED DESCRIPTION OF THE INVENTION

[0013] (Disclosure details) First, the findings of the present disclosure will be described. The inventors have discovered the following findings (1) to (3) while evaluating delayed fracture at the sheared edge. (1) When a specimen is subjected to a constant load (external load stress) on the sheared edge and constrained, and placed in a hydrogen penetration environment for a certain period of time, there exists a limit (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 shearing and the external load stress on the sheared edge reaches the threshold for delayed fracture on the sheared edge.

[0014] (2) The critical load stress at which the delayed fracture occurs 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. (3) Therefore, the critical load stress of each sheared edge varies depending on the amount of forming strain applied to that sheared edge and the applied stress (external applied stress).The critical load stress at which delayed fracture occurs at 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 amount of forming strain and the applied stress (external applied stress). In this disclosure, the allowable amount of external load stress that a sheared end face has, according to the amount of strain, without causing delayed fracture, is defined as "stress margin."

[0015] Fig. 1 shows a conceptual diagram for explaining the above (1) to (3). Fig. 1(a) illustrates the state of the critical load stress when no forming strain is applied to a metal plate whose edge has been sheared to form a sheared edge. On the other hand, Fig. 1(b) illustrates the state of the critical load stress when forming strain is applied after the sheared edge has been formed. FIG. 1 illustrates an example in which residual stress is reduced by imparting forming strain to a metal sheet before press-forming the metal sheet.

[0016] 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 edge 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 edge and the threshold for delayed fracture, and is the limit of the external load stress at which the sheared edge does not experience delayed fracture. In view of this, in this disclosure, the allowable amount of external load stress at the sheared end surface, which takes into account the forming strain applied, and which does not cause delayed fracture, is defined as the "stress margin." That is, in this embodiment, the allowable amount of external load stress is defined by an index called the stress margin, which uses the forming strain as a variable.

[0017] Here, the residual stress at the sheared edge due to shearing exists only in a very small region of the surface, approximately 100 μm from the sheared edge surface. For this reason, it is difficult to calculate the change in residual stress using CAE or other methods that use conventional shell elements. Stress in small regions can be measured using methods such as X-ray stress measurement, but there are problems with the measured values ​​varying depending on the measurement range and the measurement depth being limited to the outermost layer of the material. Therefore, the magnitude of the measured values ​​may not necessarily correspond to the risk of delayed fracture.

[0018] On the other hand, if the method disclosed in the present disclosure is used to experimentally determine the above-mentioned "stress margin" using forming strain as a variable through delayed fracture testing under stress load, it is possible to obtain an index for directly evaluating the risk of delayed fracture for a formed part without encountering such problems related to calculations and measurements. If this stress margin is evaluated under the conditions of the hydrogen penetration environment to which the automobile part is actually exposed, it can be regarded as the margin until delayed fracture occurs at the sheared end face of the automobile part.

[0019] Moreover, since this stress allowance is expressed in units of stress, it can be assumed that delayed fracture will not occur even if an external load stress applied to a part during assembly or use is added to the residual stress due to molding of the part, as long as the stress does not exceed this stress allowance. Therefore, the concept of stress margin, which is the allowable amount of external load stress that does not cause delayed fracture and is an index corresponding to the amount of strain, is a simple and excellent evaluation index for delayed fracture, which can also be evaluated as a margin having a stress dimension.

[0020] Conversely, a method of changing the hydrogen penetration environment while keeping the stress load constant is also conceivable. However, this method is less useful in that it is not possible to evaluate the margin by comparing the residual stress caused by the molding of the part and the added external load stress caused by deformation of the part during assembly or use. The forming strain is a strain in the direction extending along the shear plane.

[0021] Furthermore, with regard to practical evaluation methods, the inventors have discovered the following findings (4) and (5). (4) By varying the amount of forming strain applied to the sheared end surface after shearing, applying stress to the sheared end surface, and setting it in a hydrogen environment to determine the limit load stress at which delayed fracture occurs, the stress margin can be made a function of the amount of forming strain. (5) Uniaxial tensile or compressive deformation is the preferred method for introducing tensile and compressive forming strains to the sheared edge of the specimen. This is because, with uniaxial forming, springback occurs after forming, so the residual stress after forming in areas other than the sheared edge becomes nearly zero, and its effect can be ignored. Therefore, when the forming strain is applied by uniaxial tension or compression, the external load stress at the limit at which delayed fracture occurs at the sheared edge after additional processing can be evaluated as the "stress margin," which is the simplest method.

[0022] Here, the test piece for evaluating the stress tolerance may be one sheared in a laboratory, or a part cut out from the sheared end surface of a molded part after press molding. Furthermore, the inventors have devised the following method for evaluating and predicting the occurrence of delayed fracture at the shear edge of molded parts, particularly those intended for automobile parts, using the "stress margin" obtained in this way. Examples of this method are shown below in the first to third sections.

[0023] (first) First, a test piece is used to carry out the test using the methods (4) and (5) above, and the stress margin corresponding to the amount of strain due to tension and compression is measured. Then, the stress margin is calculated using the amount of strain as a variable. 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 is equal to the amount of hydrogen that penetrates into an actual automobile part, which is preset as the allowable upper limit.

[0024] The amount of forming strain applied to the sheared end surface is preferably 0.1% or more, considering the amount that has a sufficient effect on the delayed fracture properties. A strain amount of 0.5% or more has a greater effect, and when plastic strain is introduced, the delayed fracture evaluation according to the present invention is particularly effective. Therefore, instead of forming strain, the amount of plastic strain on the sheared end surface can also be used as an evaluation index. Regarding the applied stress to the test specimen, any stress-related parameter, such as the first principal stress or the von Mises stress, can be used in the present disclosure.

[0025] (Second) Second, a CAE forming analysis (computer simulation analysis) of the formed part is performed using a known method, and the amount of forming strain due to tension-compression at each point on the shear end surface of the formed part and the residual stress after forming are calculated. (Third) Third, the total stress of the post-molding residual stress and the external load stress is calculated by adding the external load stress that is expected to be applied to the molded part when it is assembled to other parts and used after assembly to the post-molding residual stress. (Fourth) Fourth, for each location on the sheared edge of the metal plate, the stress margin corresponding to the amount of strain due to tension-compression is compared with the total stress of the above-mentioned residual stress after forming and external load stress in the formed part.

[0026] Any location on the sheared edge where the stress tolerance is exceeded is deemed to be at risk of delayed fracture. However, the stress tolerance may be set lower than the actually measured value, taking into account the safety factor. Furthermore, by referring to the above-mentioned stress margin, it is possible to design the shape of the metal part and the manufacturing process (forming conditions) so that delayed fracture is not predicted to occur. For example, one forming condition is to add a pressing process to relieve residual stress.

[0027] Next, an embodiment of the present invention based on the above disclosure will be described with reference to the drawings. In the following description, it is assumed that press molding is used to form a molded part. The method for evaluating delayed fracture of formed parts according to the present disclosure is suitable for press parts (formed parts) that constitute automotive parts. However, its application is not limited to press parts. It can be applied to various metal parts that have sheared edges and are at risk of delayed fracture. For example, it is expected to be applied to the manufacture of metal parts by various forming methods, including roll forming, incremental forming, bulge forming, hot stamping, hammer forging, and forming of tailored blanks.

[0028] (composition) The method for evaluating the delayed fracture properties of a formed part according to this embodiment is a method for evaluating the delayed fracture properties of a formed part at a sheared end surface of a formed part that is manufactured by press-forming a metal plate made of a high-strength steel plate and that is then assembled to another part for use. The present invention is particularly effective when the metal plate is a high-strength steel plate. The delayed fracture property evaluation method of this embodiment includes a first step 1, a second step 2, a third step 3, and a fourth step 4, as shown in FIG.

[0029] (Step 1) The first step 1 comprises a testing step 1A and a stress margin setting step 1B. Test step 1A is a step of carrying out an actual experiment, and includes a step of restraining a metal plate while applying a predetermined load stress to the shear surface, and a step of placing the metal plate in the restrained state in a predetermined hydrogen penetration environment for a predetermined time. In step 1B of determining the stress margin, the limit load stress at which delayed fracture does not occur on the shear surface of the metal plate is determined for each amount of strain based on the test results in step 1A of the test, and the obtained information is used to determine the stress margin, which is the allowable value of external load stress at which delayed fracture does not occur, according to the amount of strain. That is, in this embodiment, a "stress margin," which is a newly established index in this disclosure, is calculated as an evaluation index for the delayed fracture property evaluation method.

[0030] In the present disclosure, the "stress margin" refers to the allowable external load stress that the sheared end face has, according to the amount of strain, at which delayed fracture does not occur. Here, the external load stress is a stress that occurs when a product is press-molded into a desired shape or when the product is restrained when it is assembled. In the embodiment described below, the stress margin is a value (function) in which the forming strain, which is one of the parameters of the test conditions, is used as a variable. The stress margin based on the critical stress load may be a value obtained by multiplying the critical stress load determined from the test by a predetermined safety factor, or may be a value smaller than the critical stress load determined from the test by a safety margin.

[0031] The first step 1 consists of, for example, five steps as shown in FIG. 1, reference numerals 10 to 13 correspond to test step 1A, and reference numeral 14 corresponds to stress margin setting step 1B. A known method may be used for test step 1A. Each step will be explained below.

[0032] <Shearing process 10> The shearing process 10 is a process for preparing a test piece from a metal plate that has the same conditions as the metal plate used to process the molded part. In the shearing process 10, a metal plate made of the same material and thickness as the metal plate to be evaluated is sheared to prepare a test piece with a sheared end surface for determining the stress margin.

[0033] <Strain introduction process 11> The strain introduction step 11 is a step of applying a forming strain to at least a part of the sheared end surface of the test piece. The applied forming strain is a strain along the extension direction of the sheared end surface. The forming strain to be applied is, for example, 0.1% or more. The forming strain is imparted to the test specimen by, for example, uniaxial tension or uniaxial compression, or by bending the test specimen in the thickness direction.

[0034] <Load process 12> The loading step 12 is a step of applying a predetermined external load stress to the shear end face of the test piece and restraining it in that loaded state. The stress application method is, for example, tensile stress application or bending stress application. In this case, a method of applying bending stress using a jig is particularly desirable from the viewpoint of simplicity.

[0035] <Hydrogen penetration step 13> This is a process in which the test piece that has been subjected to external load stress and restrained in the loading process 12 is placed in a predetermined hydrogen penetration environment for a predetermined time, and the occurrence of cracks in the test piece in that state is evaluated. In this case, the hydrogen penetration environment and 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 under the environment in which the material to be evaluated is actually used. The test specimen is placed in a hydrogen penetration environment by immersing it in a bath containing an acid solution such as hydrochloric acid or an aqueous solution of NHSCN. The concentration of the acid solution and the immersion time are set so that the amount of hydrogen that is pre-set as the allowable upper limit is penetrated into the test specimen. For each test piece prepared in the shearing process 10, the above-mentioned strain introduction process 11 to hydrogen penetration process 13 are carried out while changing the conditions of the forming strain to be applied and the load stress to be applied.

[0036] <Step 14: Determining stress margin> In the stress margin determination step 14, a critical load stress, which is the limit load stress at which delayed fracture does not occur on the sheared surface of the metal plate, is evaluated based on the test results, and the stress margin against the occurrence of delayed fracture on the sheared end surface of the metal plate is determined based on the critical load stress. Specifically, the critical load stress is taken as the stress margin under the test conditions. For example, based on the test conditions for each test piece (forming strain and external load stress) and the evaluation results for whether or not cracks occur 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).

[0037] 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 forming strain as a variable is obtained as data expressing the stress margin, which is the allowable value of external load stress that does not cause delayed fracture according to strain, as shown in the graph in Figure 4. In other words, the stress margin is written as a function with forming strain due to tension-compression as a variable, for example.

[0038] Figure 4 shows an example in which the residual stress at the sheared edge increases or decreases depending on 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 condition of the material and the sheared edge, it is possible that the applied forming strain may cause cracks or damage at the sheared edge, which may actually reduce the stress margin for delayed fracture. 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 sheared 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.

[0039] (Second step 2) The second step 2 involves performing a forming analysis (CAE analysis) on the process of press-forming the metal plate into the desired formed part, and determining the residual stress and strain at each point on the shear end surface of the formed part that occurs when the metal plate is formed into the formed part.

[0040] (Step 3 of 3) In the third step, the molded part is assembled to another part to determine the load stress applied to each point on the sheared end surface of the molded part. For example, load stress is generated by assembling the part while deforming it by the amount of springback that occurs when it is released from the mold. Regarding the end faces, the deformation of the molded parts that occurs during assembly is smaller than the deformation during press molding, so the strain that occurs during assembly is ignored. In the case of a molded part that is an automotive structural part, it is assembled to the automobile body either alone or after being assembled with other parts. In this way, when the molded part is assembled with other parts or to the automobile body, a predetermined load stress may be applied as an external load. This external load is calculated as the load stress.

[0041] The load stress can be measured, for example, by attaching a gauge or other sensor to an actually manufactured molded part and assembling it. Alternatively, the load stress can be determined by performing a known CAE analysis to determine the stress input when the target molded part is assembled to another part. The delayed fracture that is the subject of this disclosure occurs over time as the molded part is used, and is not a phenomenon that occurs immediately after the molded part is assembled with another part. Therefore, it is possible to actually assemble the molded part and determine the external load.

[0042] (4th step 4) In the fourth step 4, the margin of delayed fracture of the molded part is evaluated based on the stress margin of the metal plate, using the strain amount determined in the second step 2 as a variable, and the total stress of the residual stress determined in the second step 2 and the load stress determined in the third step 3. Here, the residual stress and applied stress used to calculate the total stress are the sum of the residual stress and applied stress occurring in the same region (same location) on the sheared edge. In other words, the total stress is calculated for each location on the sheared edge. The total stress may be calculated only for portions where the strain amount is equal to or greater than a preset threshold value.

[0043] In the fourth step 4, for example, the occurrence of delayed fracture is evaluated by comparing the stress margin corresponding to the strain amount determined in the second step 2 at each location on the sheared edge with the total stress of the residual stress determined by CAE of the molded part and the external load stress during assembly. Alternatively, the stress margin, which is the difference between the stress margin and the total stress, may be calculated to evaluate the degree of stress margin against delayed fracture.

[0044] For example, for an automotive part for which delayed fracture occurrence is desired, a CAE forming analysis is performed to calculate the amount of tension-compression strain at each shear edge and the post-forming residual stress. The external load stress expected during assembly and use of the part is then added to the post-forming residual stress to determine the total. The sum of the post-forming residual stress and the external load stress during assembly and use is then determined to be in excess of the stress margin at each shear edge. If it is exceeded, the risk of delayed fracture is deemed high. However, the stress margin can be set lower than the actual measured value, taking into account a safety factor. Furthermore, the stress margin can be used as a reference to design metal part shapes and manufacturing processes that predict delayed fracture will not occur.

[0045] In this disclosure, the strain in the direction parallel to the sheared end surface is preferably used as an example of the strain output from the CAE in the second step 2. The strain used may also be a parameter related to other strains, such as plastic strain. Similarly, it is preferable to use the first principal stress for the residual stress and the applied stress, but other stress-related parameters such as the von Mises stress may also be used.

[0046] (Processing flow) An example of a processing flow used in the evaluation method of the present disclosure will be shown below. In this processing flow, the stress allowance corresponding to the forming strain obtained in the first step 1 is stored in a storage unit, and the computer is made to refer to the stored stress allowance to determine the value of the stress allowance corresponding to the strain amount of the input forming strain. Then, the computer is made to execute a process to evaluate the possibility of delayed fracture for the strain amount of the input forming strain and the external load stress. This processing flow will be explained with reference to Fig. 5. If evaluation is performed using the processing shown in Fig. 5, it becomes possible to more efficiently evaluate delayed fracture of formed parts. 5 includes a stress margin calculation unit 20, an evaluation main unit 30, a storage unit 40, and a review unit 50. The processing flow for performing the processes of the stress margin calculation unit 20 and the evaluation main unit 30 is stored in the storage unit 40, such as a RAM or ROM, of a computer, and is executed by the computer.

[0047] <Storage section 40> The storage unit 40 is made up of a recording medium such as a database. The memory unit 40 stores data on the stress margin d obtained by repeating the tests from the shearing process 10 to the stress margin determination process 14 while changing the amount of forming strain in various ways, for each of the material conditions of the metal plate, the hydrogen environment conditions, and the shear conditions, using the test conditions as variables and the forming strain as variables.

[0048] <Stress margin calculation unit 20> The stress margin calculation unit 20 corresponds to the first step 1. 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. Next, in step S20, the operator is prompted to input shear conditions, and the input is acquired through input operations by the operator.

[0049] Next, in step S30, a data group of stress margins for each strain amount (a collection of data (strain amount, stress margin values)) that matches the conditions input in steps S10 and S20 is obtained from memory unit 40. Alternatively, the operator is prompted to input a group of data on the stress margin for each strain amount obtained by the test, and the input information is acquired through the operator's input operation. The acquired data is stored in the storage unit 40. Next, in step S40, the data group of stress margins for the strain amount acquired in step S30 is referenced, and a calculation process is performed using a known processing method to obtain the stress margin d as a function f(x) with the strain amount x as a variable.

[0050] Next, in step S50, the function of the stress margin d obtained in step S40 is changed to an expression considering the safety factor s (: 0 < s ≦ 1) as shown in the following equation. d = s·f(x) Then, the information on the function of the obtained stress margin d is stored in the storage unit 40 using the test conditions as a key.

[0051] <Evaluation main body unit 30> In the evaluation main body unit 30, first, in step S100, the operator is prompted to input the type of material of the actual part to be evaluated (steel type, thickness, etc.), part conditions such as the formed shape, conditions of the metal sheet such as shearing conditions, forming conditions, and conditions of the hydrogen environment (acidity, installation time), which are the conditions for delayed fracture. The above inputs are acquired through the operator's input operation. Next, in step S110, based on the conditions of the metal sheet and shearing process and the shape information of the actual part to be formed acquired in step S100, a forming analysis is performed by CAE. Next, in step S120, from the processing results of the forming analysis in step S110, the strain amount x and the residual stress g are obtained for all the sheared end face parts of the actual part.

[0052] In step S130, the operator is prompted to input the load stress h during assembly and use, and the load stress h is acquired through the operator's input. In step S140, the 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. For each sheared end face location, the stress margin d corresponding to the strain amount x input in step S110 is compared with the sum of the residual stress g output in step S120 and the load stress h input in step S130, and a determination is made as to whether there is a risk of delayed fracture.

[0053] Here, in step S140 of FIG. 5, a determination is made as to whether there is a risk of delayed fracture, but the margin of delayed fracture (= d - (g + h)) may also be output together. Furthermore, 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 storage unit 40 as data using the input values ​​in steps S10 to S20 as keys.

[0054] <Review Section 50> The review unit 50 executes a review process on the molding conditions and part shapes, and outputs the modified molding conditions and part shape conditions changed by the review to step S120. For example, for the sheared end surface where the delayed fracture margin (= d - (g + h)) is a negative value, the process involves changing the forming conditions or part shape so that the residual stress is reduced by the amount of stress that exceeds the absolute value of the margin. For example, the number of press steps can be increased to reduce the residual stress. When the part is large and it is difficult to manually evaluate all of the sheared end faces, evaluation using the processing flow shown in Figure 5 makes it possible to automatically and efficiently evaluate the part. If it is determined that there is a risk of delayed fracture, it is possible to repeatedly review the molding conditions until it is determined that there is no risk of delayed fracture. [Example]

[0055] An example of this embodiment will be described. Example 1 In this example, 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. The present invention is not limited to the plate conditions of this test material X, but can be applied to metal materials including high-strength steel plates with a tensile strength of 980 MPa or more, which cause delayed fracture at the shear edge. First, the test material X was sheared by shearing to prepare a test piece having a linear sheared end surface with a length of 100 mm. The width of the test piece when shearing was set to 30 mm, and the test piece was in the shape of a 100 mm x 30 mm strip. The clearance during shearing was set to 12% of the plate thickness. Note that, although the above embodiment has been described using an example in which the shearing condition is the same, evaluation corresponding to changes in shearing conditions, such as the clearance during shearing, is also possible. In other words, it is sufficient to determine the stress margin under those shearing conditions.

[0056] 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 using 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 when the forming strain is deformation by bending. In addition, test pieces that were sheared without applying forming strain were also prepared. 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 at the outside of the bend so that tensile stress was applied. The magnitude of the applied stress was determined by determining the first principal stress-first principal strain relationship at the center and vertex of the test piece using CAE, and then measuring the amount of strain when the test piece was actually bent.

[0057] In this example, four-point bending was used as the stress application method. 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. 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. The test piece to which the load stress was applied was then immersed in a bath of thiocyanic acid solution of pH 6 for 96 hours, and the delayed fracture properties were evaluated based on whether or not cracks occurred due to delayed fracture after 96 hours.

[0058] The above conditions and evaluation results are shown in Tables 1 to 11. Each table is a summary of the amount of forming strain.

[0059] [Table 1]

[0060] [Table 2]

[0061] [Table 3]

[0062] [Table 4]

[0063] [Table 5]

[0064] [Table 6]

[0065] [Table 7]

[0066] [Table 8]

[0067] [Table 9]

[0068] [Table 10]

[0069] [Table 11]

[0070] 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. 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 critical load stress. In this example, the critical load stress with the forming strain as a variable becomes the stress margin.

[0071] Figure 6 shows the stress margin calculated from the limit load stress at which delayed fracture did not occur, and plots the stress margin as a function of strain. In this way, it is possible to describe the stress margin according to the forming strain after shearing. In this example, we have shown the case where the stress margin increases due to the forming strain after shearing, but the same evaluation is also possible when the stress margin decreases due to the forming strain after shearing.

[0072] Example 2 Next, an example of the delayed fracture determination using the stress margin according to the forming strain after shearing, which was obtained in Example 1, will be described. In Example 2, assuming an actual automobile part having a sheared end face of the shape shown in Fig. 7, a test material X was press-formed into the shape of the actual part. In the part shape shown in Fig. 7, strain is input to the end face. At this time, CAE was used to calculate the forming strain and residual stress on the surface on the burr side at the time of shearing at 10 representative locations A to J (not shown) of the representative sheared end face portion of the formed part. For CAE, 1.0 mm square shell elements were used and the forming and springback processes were calculated using the dynamic explicit method.

[0073] In this example, the actual part shown in FIG. 7 is used as an example, but the present invention is not limited to this part, and similar evaluation is possible for any part that has a sheared end surface of a material that is at risk of delayed fracture. After that, the actual molded parts were subjected to a delayed fracture test, in which they were immersed in a thiocyanic acid solution with a pH of 6, and the delayed fracture was evaluated based on whether or not cracks occurred due to delayed fracture after 96 hours. The CAE results of the forming strain and residual stress after shearing are shown in Table 12, along with whether or not delayed fracture occurred in the test.

[0074] [Table 12]

[0075] Example 3 Next, in Figure 8, the forming strain and residual stress after shearing shown in Table 12, and the occurrence of delayed fracture are plotted and compared with the stress allowance shown in Figure 6. Figure 8 shows that the occurrence of delayed fracture at the sheared edge of an actual part can be predicted by whether or not the stress allowance line is exceeded. Furthermore, we assumed that the external stress load during assembly and use of the actual part would be a maximum of 300 MPa at all points. The maximum amount of external load stress during assembly and use was estimated by using strain gauges to measure the amount of elastic deformation of the actual part during assembly and assumed use. When this stress was added, the forming strain and residual stress after shearing were calculated at representative points A to J. Figure 9 shows these, plotted against the stress margin in Figure 6, along with whether or not delayed fracture occurred while the material was formed.

[0076] In Figure 9, even when delayed fracture did not occur in the delayed fracture test after the part was formed, there were some locations where the stress margin was exceeded when the external stress load during assembly and use was taken into consideration. These locations did not experience delayed fracture in the delayed fracture test while still formed, but when the load stress during assembly and use was taken into consideration, they were found to be potential delayed fracture risk locations where delayed fracture could occur.

[0077] Table 13 shows an example of adding an intermediate forming process to the same molded part before the final forming process to prevent delayed fracture, even when taking into account the stress during assembly and use, with reference to the stress margin.The figure shows the forming strain and residual stress at representative points A to J of the part when the stress and strain amount are changed by adding an intermediate forming process. Furthermore, Figure 10 shows a comparison of the stress allowance in Figure 6 when an assembly stress of up to 300 MPa is applied to the part locations in Table 13. In Figure 10, even when considering the stress during assembly and use, the stress allowance is not exceeded, so it is predicted that the risk of delayed fracture is small. By referring to the stress allowance, it has been found that it is possible to design automotive parts and their manufacturing processes that do not cause delayed fracture.

[0078] [Table 13]

[0079] (others) The present disclosure may also have the following configuration. (1) A method for evaluating the delayed fracture properties of a formed part, in which a formed part is manufactured by forming a metal plate made of a high-strength steel plate, and the formed part is assembled to another part for use, and the delayed fracture properties of the formed part at a sheared end surface are evaluated, a first step of determining, based on the results of a test including a step of restraining 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 of placing the metal plate in a predetermined hydrogen penetration environment for a predetermined time while being restrained, a stress margin, which is an allowable value of external load stress at which delayed fracture does not occur on the shear surface of the metal plate, using the amount of strain as a variable; a second step of performing a forming analysis of forming the metal plate into the formed part to determine residual stress and strain at the sheared end surface of the formed part that occur when forming the metal plate into the formed part; a third step of assembling the molded part to another part to determine the load stress applied to the shear end surface; a fourth step of evaluating the margin of delayed fracture of the molded part based on the stress margin of the metal plate using the amount of strain determined in the second step as a variable and the total stress of the residual stress determined in the second step and the load stress determined in the third step; Equipped with. (2) A fifth step of changing either the shape of the formed part or the forming conditions when it is determined in the fourth step that the residual stress and strain amount determined in the second step will cause delayed fracture, In the fifth step, the location evaluated as being at risk of delayed fracture is modified in a direction that reduces the residual stress determined in the second step. (3) The metal plate is a steel plate having a tensile strength of 980 MPa or more. (4) A method for manufacturing a molded part, which includes determining the molding conditions under which the molded part is evaluated as not causing delayed fracture using the method for evaluating the delayed fracture properties of a molded part disclosed herein, and manufacturing the molded part by molding under the determined molding conditions. [Explanation of symbols]

[0080] 1. First step 2. Second step 3. Third step 4. Fourth step 10 Shearing process 11 Strain introduction process 12 Load process 13 Hydrogen penetration process 14 Stress allowance determination process 20 Stress margin calculation section 30 Evaluation Body 40 Storage section 50 Review Department d Stress margin g Residual stress h Load stress x strain amount

Claims

1. A method for evaluating the delayed fracture properties of a formed part, which comprises forming a metal plate made of a high-strength steel plate to produce a formed part, and evaluating the delayed fracture properties of the formed part at a sheared end surface of the formed part which is used by assembling the formed part to another part, comprising: a first step of determining, based on the results of a test including a step of restraining 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 of placing the metal plate in a predetermined hydrogen penetration environment for a predetermined time while being restrained, a stress margin, which is an allowable value of external load stress at which delayed fracture does not occur on the shear surface of the metal plate, using the amount of strain as a variable; a second step of performing a forming analysis of forming the metal plate into the formed part to determine residual stress and strain at a sheared end surface of the formed part that occur when forming the metal plate into the formed part; a third step of assembling the molded part to another part to determine a load stress applied to the shear end surface; a fourth step of evaluating the margin of delayed fracture of the molded part based on the stress margin of the metal plate using the amount of strain determined in the second step as a variable and the total stress of the residual stress determined in the second step and the load stress determined in the third step; A method for evaluating delayed fracture properties of a molded part, comprising:

2. a fifth step of changing either the shape of the formed part or the forming conditions when it is determined in the fourth step that the residual stress and the amount of strain obtained in the second step will cause delayed fracture; In the fifth step, the portion evaluated as being at risk of delayed fracture is changed in a direction that reduces the residual stress determined in the second step.

2. The method for evaluating delayed fracture properties of a molded part according to claim 1.

3. 2. The method for evaluating delayed fracture properties of a formed part according to claim 1, wherein the metal plate is a steel plate having a tensile strength of 980 MPa or more.

4. The molding conditions under which the molded part is evaluated as not causing delayed fracture are determined by the method for evaluating the delayed fracture properties of a molded part according to any one of claims 1 to 3, and the molded part is manufactured by molding under the determined molding conditions. A method for producing a molded part, comprising:

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