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

Abstract

The present invention makes it possible to evaluate delayed fracture characteristics at a sheared end face in a molded member with higher accuracy. The present invention evaluates delayed fracture characteristics at a sheared end surface of a molded member obtained by molding a metal plate, the sheared end surface being used by assembling the molded member to another member. The present invention is provided with: a first step (1) for obtaining a stress margin using a dependent variable as a variable on the basis of a result of a test provided with a step for setting a metal plate in a hydrogen intrusion environment in a state in which a load stress is applied to a shear surface of the metal plate and constraint is performed; a second step (2) in which a molding analysis for molding the metal plate into a molded member is performed, and the residual stress and the strain amount at the sheared end surface of the molded member are obtained; a third step (3) for obtaining a load stress applied to the shear end surface by assembling the molded member to another member; and a fourth step (4) for evaluating the margin of the delayed fracture of the molded member on the basis of the stress margin corresponding to the determined strain amount and the total stress of the determined residual stress and the determined load stress.

Description

Technical Field

[0001] The present invention is a technology for evaluating the delayed fracture characteristics at the shear end surface of a molded part manufactured by stamping, etc. In addition, the present invention is a technology related to a method for evaluating the delayed fracture characteristics of a molded part and a method for manufacturing a molded part using the method.

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

[0003] At present, for automobiles, the pursuit is to improve fuel economy and collision safety by lightweighting. In addition, in order to balance the lightweighting of the vehicle body and the protection of passengers in the event of a collision, high-strength steel plates are used for the vehicle body. In particular, in recent years, high-strength steel plates with a tensile strength of more than 980MPa have begun to be applied to vehicle bodies. One of the issues in the application of high-strength steel plates to vehicle bodies is delayed fracture. In particular, in high-strength steel plates with a tensile strength of more than 980MPa, delayed fracture caused by the shear end face has become an important issue. The shear end face is the end face after shearing. This issue is particularly problematic in ultra-high-strength steel plates with a tensile strength of more than 1470MPa among high-strength steel plates.

[0004] It is known that a large tensile stress remains on the sheared end surface, and this residual stress may cause delayed fracture of a formed part made of a metal sheet over time.

[0005] In order to predict the delayed fracture at the shear end face in advance, it is necessary to prepare a test piece for evaluation and place the test piece in a hydrogen intrusion environment. In addition, the properties of the shear end face will change due to plastic deformation during shearing. In general, the risk of delayed fracture at the shear end face increases. Therefore, for example, in Patent Document 1, the occurrence of delayed fracture is evaluated as follows. That is, in Patent Document 1, compression processing in the thickness direction of the plate based on rolling is applied to the shear end face of the test piece. After that, the test piece is placed in a hydrogen intrusion environment to evaluate the occurrence of delayed fracture.

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

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

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

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

[0010] Prior art literature

[0011] Patent Literature

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

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

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

[0015] Problems to be solved by the invention

[0016] Conventionally, no evaluation has been conducted from the viewpoint of "how much margin exists in the conditions of hydrogen intrusion environment and stress with respect to the occurrence of delayed fracture, when compared with the conditions of hydrogen intrusion environment and stress in actual automobile parts".

[0017] Furthermore, the inventors have obtained the following insights. That is, in actual automobile parts, different forming strains are introduced into the processed metal sheet depending on the forming position of the shear end face. Furthermore, the inventors have obtained the insight that the delayed fracture characteristics based on plastic deformation change due to the forming strain. Furthermore, the inventors have obtained the insight that "in the shear end face, in addition to the residual stress caused by shearing, the load stress after stamping is added, so that delayed fracture sometimes becomes easy to occur."

[0018] In addition, the inventors have obtained the following insights. That is, considering a case where a shear end face into which a forming strain is introduced is loaded with a forming residual stress under a certain hydrogen intrusion environment. In this case, the inventors have obtained the insight that it is very important to evaluate how much margin the shear end face of the formed part has with respect to the occurrence of delayed fracture. In other words, the inventors have obtained the insight that such an evaluation is very important for avoiding delayed fracture at the shear end face of an automobile part.

[0019] As described above, the shear end properties change due to plastic deformation during stamping of automobile parts. On the other hand, in the past, there was no index that could predict the occurrence of delayed fracture by comparing with the stress generated in actual automobile parts, and therefore, there was no method that could evaluate delayed fracture from the perspective of stress margin.

[0020] The present invention focuses on the above-mentioned points and aims to evaluate the delayed fracture characteristics at the shear end surface of a molded component during use with higher accuracy. Another object of the present invention is to enable the production of a molded component in which delayed fracture is suppressed.

[0021] Means for solving problems

[0022] In order to solve the problem, one scheme of the present invention is a method for evaluating delayed fracture characteristics of a formed part, which forms a metal plate composed of a high-strength steel plate to produce a formed part, and evaluates the delayed fracture characteristics of the shear end surface of the formed part used for assembling the formed part to other parts. The gist of the method is to include: a first step, based on the test results including a step of restraining the shear surface of the above-mentioned metal plate in a state where a predetermined load stress is applied and a step of placing the metal plate in a predetermined hydrogen intrusion environment for a predetermined time in the above-mentioned restrained state, an allowable value of the external load stress that does not cause delayed fracture at the shear surface of the above-mentioned metal plate, i.e., The stress margin is calculated with the strain amount as a variable; in the second step, a forming analysis is performed to form the above-mentioned metal plate into the above-mentioned formed component, and the residual stress and the strain amount at the shear end face of the above-mentioned formed component generated when the above-mentioned metal plate is formed into the above-mentioned formed component are calculated; in the third step, a load stress loaded on the above-mentioned shear end face by assembling the above-mentioned formed component to other components is calculated; and in the fourth step, a margin for delayed fracture of the above-mentioned formed component is evaluated based on the stress margin of the above-mentioned metal plate with the strain amount calculated in the above-mentioned second step as a variable and the total stress of the residual stress calculated in the above-mentioned second step and the load stress calculated in the above-mentioned third step.

[0023] The above-mentioned forming is, for example, press forming.

[0024] Effects of the Invention

[0025] According to the aspect of the present invention, the delayed fracture characteristics at the shear end surface of the molded component placed in the use environment can be evaluated with higher accuracy. As a result, it is possible to manufacture a molded component in which delayed fracture is suppressed.

[0026] At this time, the stress margin as an indicator for delayed fracture evaluation is based on stress and can be evaluated from the perspective of the margin based on stress. Therefore, for example, when high-strength steel sheets are applied to various parts such as panel parts, structural and frame parts of automobiles, the following effects can be achieved. That is, according to the scheme of the present invention, the occurrence of delayed fracture of formed parts can be predicted in a manner that includes a margin with a dimension of stress.

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

[0028] Figure 1 This is a conceptual diagram showing the relationship between delayed fracture of a shear end surface and stress margin.

[0029] Figure 2 It is a diagram showing a configuration example according to an embodiment of the present invention.

[0030] Figure 3 It is a diagram showing a configuration example of the first step.

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

[0032] Figure 5 This is a diagram showing an example of a process flow that can be used in the evaluation of this method.

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

[0034] Figure 7 It is a figure which shows the shape of the formed part (actual part) in this Example.

[0035] Figure 8 This is a diagram showing an example of delayed fracture determination using stress margin.

[0036] Fig. 9 This 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.

[0037] Fig.10 This figure shows a case where an intermediate step is added to avoid delayed fracture by reviewing the forming conditions. This figure also shows an example of delayed fracture determination based on the total stress of residual stress and external load stress caused by assembly and use. DETAILED DESCRIPTION

[0038] (Details about the disclosure)

[0039] First, the findings of the present disclosure will be described.

[0040] The inventors found the following findings (1) to (3) during the evaluation of delayed fracture of shear end surfaces.

[0041] (1) Consider the case where a load stress (external load stress) based on a fixed load is applied to the shear end face and a predetermined time is set in a hydrogen intrusion environment in a restrained state. In this case, there is a limit load stress (also called limit load stress) at which delayed fracture occurs at the shear end face. This is because delayed fracture occurs when the sum of the residual stress caused by shearing processing and the external load stress at the shear end face reaches the threshold value for delayed fracture of the shear end face.

[0042] (2) The critical load stress causing delayed fracture changes in the shear end surface due to the tensile and compressive strains of the forming strain added after shearing. This is because the residual stress of the shear end surface changes due to the forming strain.

[0043] (3) Therefore, the critical load stress of each shear end face varies depending on the forming strain and load stress (external load stress) applied to the shear end face. Furthermore, when a predetermined setting time is set in a predetermined hydrogen intrusion environment, the critical load stress that causes delayed fracture at the shear end face can be organized as follows. That is, the critical load stress can be organized as an index of "stress margin" that takes into account the forming strain and load stress (external load stress).

[0044] Here, in the present disclosure, the allowable amount of external load stress that does not cause delayed fracture and corresponds to the amount of strain that the shear end surface has is defined as “stress margin”.

[0045] exist Figure 1 A conceptual diagram illustrating the above (1) to (3) is shown in FIG. Figure 1 (a) is a diagram illustrating the state of the critical load stress when no forming strain is applied to a metal plate in which the end portion is sheared to form a shear end surface. Figure 1 (b) is a diagram illustrating a state of a critical load stress when a forming strain is applied after the shear end surface is formed.

[0046] Should Figure 1 The example of reducing the residual stress by applying forming strain to the metal sheet before press-forming the metal sheet is described.

[0047] Here, if the sum of the residual stress caused by shearing and the load stress from the outside reaches the threshold value for delayed fracture, delayed fracture occurs. Therefore, if the residual stress of the shear end face changes due to the forming strain, the critical load stress for delayed fracture also changes. The critical load stress is the difference between the residual stress of the shear end face and the threshold value for delayed fracture, and is the external load stress at the limit where delayed fracture does not occur on the shear end face.

[0048] In view of such a situation, the stress margin index is defined as follows. That is, in the present disclosure, the allowable amount of external load stress that does not cause delayed fracture in the shear end face taking into account the applied forming strain is defined as "stress margin". That is, in the present embodiment, the allowable amount of external load stress is defined by the stress margin index with the forming strain as a variable.

[0049] Here, the residual stress at the shear end face caused by shearing only exists in a tiny area of ​​the extreme surface layer about 100 μm from the shear end face surface. Therefore, the residual stress change is difficult to calculate by CAE using ordinary shell elements. The stress in the tiny area can be measured by X-ray stress measurement, etc. However, there is a problem that the measured value sometimes changes due to the measurement range, and the measurement depth is limited to the outermost layer of the material. Therefore, sometimes the size of the measured value does not necessarily correspond to the risk of delayed fracture.

[0050] On the other hand, if the above-mentioned "stress margin" with forming strain as a variable is experimentally obtained by a delayed fracture test under stress load conditions as in the present disclosure, it can be dealt with. That is, it is possible to obtain an index for directly evaluating the risk of delayed fracture of a formed part without causing such problems related to calculation and measurement.

[0051] It is considered that the stress margin is evaluated under the conditions of the hydrogen intrusion environment to which the automobile parts are actually exposed. In this case, it can be regarded as the margin until the delayed fracture occurs at the shear end surface of the automobile parts.

[0052] Moreover, the stress margin is expressed by stress in units of stress. Therefore, even when the external load stress applied to the component during assembly, use, etc. is added in addition to the residual stress caused by the forming of the component, it can be estimated. That is, as long as the stress margin is not exceeded, it can be estimated that delayed fracture will not occur.

[0053] Therefore, the concept of stress margin, which is an allowable amount of external load stress that does not cause delayed fracture and an index corresponding to the amount of strain, is simple. Moreover, this index is an excellent delayed fracture evaluation index that can also evaluate the margin as a dimension of stress.

[0054] On the contrary, it is also possible to consider a method of making the stress load value constant and changing the hydrogen intrusion environment. However, this has problems with the addition of the residual stress caused by the forming of the components and the external load stress caused by the deformation of the components during assembly and use. That is, compared with the case where stress is used as a scale, this is less useful in that the margin cannot be evaluated.

[0055] It should be noted that the above-mentioned forming strain is the strain in the extension direction of the shear plane.

[0056] Furthermore, regarding the actual evaluation method, the inventors have found the following findings (4) and (5).

[0057] (4) For the sheared end surface, the amount of forming strain applied after shearing is changed, and the upper stress is applied and set in a hydrogen environment to find the critical load stress that causes delayed fracture. In this way, the stress margin can be made a function of the amount of forming strain.

[0058] (5) As a method for introducing the tensile and compressive forming strains relative to the shear end face into the evaluation test piece, it is preferred to use uniaxial tensile deformation and compressive deformation. This is for the following reasons. In uniaxial forming, the residual stress after forming of the part other than the shear end face becomes approximately 0 due to springback after forming. Therefore, its influence can be ignored. Therefore, when the imparting of the forming strain is based on uniaxial tensile and compression, there is the following advantage. That is, the external load stress that produces the limit of delayed fracture in the shear end face after additional processing can be directly evaluated as "stress margin", which is the simplest.

[0059] Here, as for the test piece for evaluating the stress margin, a test piece cut out in a laboratory can be used. In addition, as the test piece, a part of the shear end surface of the formed part after stamping can also be cut out.

[0060] The inventors have devised the following method as a method for evaluating and predicting the occurrence of delayed fracture at shear end faces in a molded component that takes automotive components into consideration, using the thus obtained "stress margin". Examples thereof are shown in the first to third examples below.

[0061] (First)

[0062] First, the test piece is tested by the methods (4) and (5) above to measure the stress margin corresponding to the strain amount caused by tension and compression. Then, the stress margin with the strain amount as a variable is obtained.

[0063] Here, the hydrogen intrusion environment and the time of setting in the environment are preferably set so that the amount of hydrogen intruding into the test piece reaches the target amount of hydrogen intrusion. The target amount of hydrogen intrusion is the amount of hydrogen intrusion previously set as the upper limit of tolerance in actual automobile parts.

[0064] As the strain amount of the forming strain given to the shear end face, if the amount that has a sufficient impact on the delayed fracture characteristics is considered, the strain amount is preferably set to 0.1% or more. As a strain amount with a greater degree of influence, the strain amount is 0.5% or more. In the case of introducing plastic strain, the delayed fracture evaluation of the present invention is particularly effective. Therefore, it is also possible to replace the forming strain and use the plastic strain amount to the shear end face as an evaluation index. Regarding the load stress to the test piece, as long as it is a stress-related parameter such as the first principal stress and the von Mises stress, it can be used in the present disclosure.

[0065] (second)

[0066] Second, a forming analysis based on CAE (computer simulation analysis) of the formed part is performed by a known method, and the forming strain amount due to tension and compression at each shear end surface in the formed part and the residual stress after forming are calculated.

[0067] (third)

[0068] Third, the external load stress assumed to be applied to the molded part during the assembly of the molded part to other parts and use after the assembly is added to the residual stress after molding. Thus, the total stress of the residual stress after molding and the external load stress is obtained.

[0069] (fourth)

[0070] Fourthly, the stress margin corresponding to the strain amount due to tension and compression and the total stress of the residual stress after forming and the external load stress in the formed component are compared at each location on the shear end surface of the metal plate.

[0071] The portions of the shear end surface that exceed the stress margin are judged to be at risk of delayed fracture. However, the stress margin may be smaller than the actually measured value in consideration of the safety factor.

[0072] Furthermore, the shape of the metal component and the manufacturing process (forming conditions) that are predicted not to cause delayed fracture can be designed with reference to the above-mentioned stress margin. As the forming conditions, an example of adding a press process for relaxing the residual stress can be given.

[0073] Next, embodiments of the present invention based on the above disclosure will be described with reference to the drawings.

[0074] Here, in the following description, press forming will be assumed as the forming of the formed part.

[0075] The evaluation method of delayed fracture of formed parts disclosed in the present invention is suitable for stamped parts (formed parts) constituting automobile parts. However, the application objects are not limited to stamped parts. It can be applied to various metal parts with shear end faces that are at risk of delayed fracture. It is envisaged to apply to the manufacture of metal parts based on various forming methods including, for example, roll forming, incremental forming, bulging forming, hot stamping forming, hammer forging, forming of tailor-welded blanks, etc.

[0076] (constitute)

[0077] In the delayed fracture characteristics evaluation method of a formed part of the present embodiment, a metal plate composed of a high-strength steel plate is press-formed to produce a formed part. In addition, the delayed fracture characteristics at the shear end surface of the formed part used for assembling the formed part to other parts are evaluated. The present invention is particularly effective when the metal plate is a high-strength steel plate.

[0078] like Figure 2 As shown, the delayed fracture characteristic evaluation method of the present embodiment includes a first step 1 , a second step 2 , a third step 3 , and a fourth step 4 .

[0079] (First process 1)

[0080] The first step 1 includes a testing step 1A and a stress margin setting step 1B.

[0081] The test process 1A is a process for performing an actual experiment, and includes a process of restraining the metal plate in a state where a predetermined load stress is applied to the shear surface of the metal plate. Furthermore, the test process 1A includes a process of placing the metal plate in a predetermined hydrogen intrusion environment for a predetermined time in the restrained state.

[0082] The stress margin setting step 1B obtains the critical load stress that does not cause delayed fracture of the shear surface of the metal plate for each strain amount based on the test results of the test step 1A. Furthermore, the stress margin setting step 1B uses the obtained information to obtain the allowable value of the external load stress that does not cause delayed fracture corresponding to the strain amount, that is, the stress margin.

[0083] That is, in the present embodiment, as an evaluation index for the delayed fracture characteristic evaluation method, the “stress margin” which is an index newly set in the present disclosure is obtained.

[0084] The "stress margin" in the present disclosure refers to an allowable amount of external load stress that a shear end surface has, corresponding to the amount of strain, without causing delayed fracture.

[0085] Here, the external load stress is stress generated by constraints when the product is press-formed into a target product shape or when the product is assembled.

[0086] In the embodiment described below, the stress margin is set as a value (function) with forming strain, which is one parameter of the test condition, as a variable.

[0087] It should be noted that the stress margin formed by the limit stress load may also be a value obtained by multiplying the limit stress load obtained by the test by a predetermined safety factor. In addition, the stress margin formed by the limit stress load may also be set to a value smaller than the limit stress load obtained by the test by a safety margin.

[0088] The first step 1 is, for example, Figure 3 The five steps shown are composed of:

[0089] exist Figure 1 In the figure, reference numerals 10 to 13 correspond to the test process 1A, and reference numeral 14 corresponds to the stress margin setting process 1B. The test process 1A may also adopt a known method.

[0090] Each of these steps will be described.

[0091] <Shearing process 10>

[0092] The shearing process 10 is a process for preparing a test piece using a metal plate having the same conditions as the metal plate to be processed into the formed part. The shearing process 10 prepares a test piece having a sheared end surface. The process 10 is performed to perform shearing on a metal plate having the same material and thickness as the metal plate to be evaluated and to obtain a stress margin.

[0093] <Strain introduction step 11>

[0094] The strain introduction step 11 is a step of applying a forming strain to at least a portion of the shear end surface of the test piece. The applied forming strain is set as a strain along the extending direction of the shear end surface.

[0095] The applied forming strain is set to a magnitude of, for example, 0.1% or more.

[0096] The forming strain may be applied by, for example, uniaxially stretching or uniaxially compressing the test piece. Alternatively, the forming strain may be applied by, for example, bending the test piece in the plate thickness direction.

[0097] <Loading process 12>

[0098] The loading process 12 is a process of applying an external load stress to the shear end surface load of the test piece in advance and restraining it in this load state. The stress loading method is carried out, for example, by tensile stress loading or bending stress loading. In this case, the bending stress loading method using a fixture is particularly preferred from the perspective of simplicity.

[0099] <Hydrogen intrusion step 13>

[0100] The hydrogen penetration step 13 places the test piece subjected to the external load stress and restrained in the loading step 12 in a predetermined hydrogen penetration environment for a predetermined time. The hydrogen penetration step 13 is a step for evaluating the occurrence of cracks in the test piece in this state.

[0101] At this time, the hydrogen intrusion environment and the setting time are preferably set to conditions that obtain a target hydrogen intrusion amount. The target hydrogen intrusion amount is, for example, the same amount of hydrogen estimated to intrude under an environment where the material to be evaluated is actually used.

[0102] The test piece is placed in a hydrogen intrusion environment by, for example, immersing the test piece in a bath containing an acid solution such as hydrochloric acid or NH4SCN aqueous solution. The concentration of the acid solution and the immersion time are set so as to achieve the conditions under which the amount of hydrogen set in advance as the upper limit of the allowable value intrudes into the test piece.

[0103] For each test piece produced in the shearing process step 10 , the strain introduction step 11 to the hydrogen penetration step 13 were performed by changing the conditions of the applied forming strain and the load stress.

[0104] <Stress margin determination process 14>

[0105] The stress margin determination step 14 evaluates the critical load stress that does not cause delayed fracture of the shear surface of the metal plate based on the test results. And, based on the critical load stress, the stress margin for delayed fracture of the shear end surface of the metal plate is obtained. Specifically, the critical load stress is used as the stress margin under the test conditions.

[0106] For example, based on the test conditions of each test piece and the evaluation results of the presence or absence of cracks at the shear end surface, the values ​​of the external load stress and the limit stress load that cause the cracks are obtained. The test conditions here are, for example, the conditions of the forming strain and the external load stress. In addition, the external load stress that causes the cracks is the external load stress that causes the cracks relative to the same forming strain. In addition, the value of the limit stress load is, for example, the boundary value of the external load stress that does not cause the cracks, that is, the value of the limit stress load. The boundary value is, for example, the maximum value of the external load stress that does not cause the cracks.

[0107] The data on multiple forming strains are sorted out to obtain multiple (forming strain, critical stress load) data. Figure 4 The value (function) of the limit stress load represented by the curve graph with the forming strain as a variable is obtained as data representing the stress margin. The stress margin is the allowable value of the external load stress that does not cause delayed fracture corresponding to the strain. That is, the stress margin is described as a function with the forming strain caused by tension-compression as a variable, for example.

[0108] It should be noted that in Figure 4 In the case of the shear end surface, the residual stress increases or decreases according to the forming strain. Figure 4In the example, the stress margin for delayed fracture increases as the absolute value of the forming strain increases. However, depending on the state of the material and the shear end surface, it is also conceivable that the stress margin for delayed fracture decreases due to the forming strain. Such a situation occurs, for example, when cracks or damage are generated on the shear end surface due to the applied forming strain.

[0109] Furthermore, in this embodiment, by comparing with the stress margin obtained as described above, the following evaluation can be performed. That is, for the test piece with shear end surface that is not used for the evaluation of the stress margin, the possibility of delayed fracture can also be evaluated without testing. The possibility of delayed fracture is the possibility of delayed fracture relative to the external load (load stress) predetermined to be applied to the metal plate.

[0110] (Second process 2)

[0111] The second step 2 performs forming analysis (CAE analysis) on the process of press-forming the metal sheet into the target formed part, and also performs the process of obtaining the residual stress and strain amount at each location of the shear end surface of the formed part caused by forming the metal sheet into the formed part.

[0112] (Third process 3)

[0113] The third step 3 is to obtain the load stress applied to each location of the shear end surface of the molded component by assembling the molded component to other components. For example, the load stress is generated by deforming the molded component by the springback amount during demolding and assembling the molded component.

[0114] It should be noted that, regarding the end surface, the deformation of the formed component generated during assembly is smaller than the deformation during press forming. Therefore, the strain generated during assembly is ignored.

[0115] When the molded component is a structural component for an automobile, it is assembled to the body of the automobile in a single state or after being assembled with other components. In this way, the molded component is assembled with other components or assembled to the body of the automobile. At this time, it is sometimes assembled in a state where a predetermined load stress is applied as an external load. This external load is obtained as the load stress.

[0116] The load stress is measured by, for example, attaching an instrument or other sensor to the molded part actually manufactured. Alternatively, the load stress can be obtained by performing and analyzing the stress input when the target molded part is assembled to other parts through a known CAE analysis.

[0117] It should be noted that the delayed fracture targeted by the present disclosure is caused by the use of the formed part over time. However, the delayed fracture targeted by the present disclosure is not a phenomenon that occurs immediately after the formed part is assembled with other parts. Therefore, the external load can also be obtained by actual assembly.

[0118] (Fourth process 4)

[0119] The fourth step 4 evaluates the margin for delayed fracture of the formed part based on the stress margin and the total stress of the metal plate. The stress margin of the metal plate is the stress margin of the metal plate with the strain amount obtained in the second step 2 as a variable. The total stress is the total stress of the residual stress obtained in the second step 2 and the load stress obtained in the third step 3.

[0120] Here, regarding the residual stress and the load stress for calculating the total stress, the residual stress and the load stress generated in the same region (same location) on the shear end surface are added together. That is, the total stress is calculated for each location on the shear end surface.

[0121] In addition, the total stress may be calculated only for the portion where the strain amount is equal to or greater than a preset threshold value.

[0122] The fourth step 4 is performed, for example, at various locations of the shear end surface. In the fourth step 4, the stress margin corresponding to the strain amount obtained in the second step 2 is compared with the residual stress of the formed part based on CAE and the total stress of the external load stress during assembly. And the presence or absence of delayed fracture is evaluated by this comparison. In addition, the stress margin composed of the difference between the stress margin and the total stress can also be calculated to evaluate the degree of stress margin with respect to delayed fracture.

[0123] For example, a forming analysis based on CAE is performed on an automobile part for which delayed fracture is to be evaluated. Furthermore, the strain caused by tension-compression at each location of the shear end surface and the residual stress after forming are calculated. Furthermore, the total is calculated by adding the external load stress assumed during assembly and use of the part to the residual stress after forming. It is determined whether the total of the residual stress after forming and the external load stress during assembly and use exceeds the stress margin at each location of the shear end surface. Furthermore, if it exceeds, it is determined that the risk of delayed fracture is high. However, the stress margin can also take a value smaller than the actually measured value in consideration of the safety factor. Furthermore, the shape of the metal part and the manufacturing process that are predicted to not cause delayed fracture can also be designed with reference to the aforementioned stress margin.

[0124] Here, in the present disclosure, it is preferable to use strain in a direction parallel to the shear end face as an example of strain output from CAE in the second step 2. As the strain to be used, other parameters related to strain such as plastic strain may be used.

[0125] Similarly, for residual stress and load stress, it is also preferable to use the first principal stress. In addition, other stress-related parameters such as von Mises stress may also be used.

[0126] (Processing Flow)

[0127] An example of a process flow used in the above evaluation method of the present disclosure is shown.

[0128] This processing flow stores the stress margin corresponding to the forming strain obtained in the first step 1 in advance in the storage unit. Then, the computer is caused to refer to the stored stress margin to determine the value of the stress margin corresponding to the input strain amount of the forming strain. Then, the computer is caused to perform a process of evaluating the possibility of delayed fracture with respect to the input strain amount of the forming strain and the external load stress.

[0129] Reference Figure 5 To illustrate the process flow. Figure 5 By performing the evaluation by such a treatment, it is possible to perform a more efficient delayed fracture evaluation on the molded part.

[0130] Figure 5 The example shown includes a stress margin calculation unit 20, an evaluation main unit 30, a storage unit 40, and a review unit 50. The processing flow of the stress margin calculation unit 20 and the evaluation main unit 30 is stored in the storage unit 40 such as RAM and ROM of the computer. Each process is executed by the computer.

[0131] <Storage Unit 40>

[0132] The storage unit 40 is composed of a recording medium such as a database.

[0133] The storage unit 40 stores data of stress margin d obtained by taking forming strain as a variable according to material conditions of the metal plate, hydrogen environment conditions, shearing conditions, and test conditions. The data is obtained by repeatedly performing the test of the shearing process 10 to the stress margin determination process 14 while variously changing the forming strain amount.

[0134] <Stress margin calculation unit 20>

[0135] The stress margin calculation unit 20 corresponds to the first step 1 .

[0136] In the stress margin calculation unit 20, first, in step S10, the input of the basic conditions for evaluation is prompted, and the above input is obtained through the input operation of the operator. The basic conditions for evaluation are, for example, conditions such as the type of material (steel type, thickness) and the conditions of the hydrogen environment as the condition for delayed fracture (acidity, setting time), etc.

[0137] Next, in step S20, the input of the shearing conditions is prompted, and the above input is obtained through the input operation of the operator.

[0138] Next, in step S30, a data set corresponding to the conditions input in steps S10 and S20 is obtained from the storage unit 40. The data set is a data set of stress margins with respect to each strain. In addition, the data set is a collection of data of (strain, stress margin value).

[0139] Alternatively, the input of a data set of stress margins with respect to each strain obtained through experiments is prompted, and the above input information is obtained through the input operation of the operator. The obtained data is stored in the storage unit 40.

[0140] Next, in step S40, the data set of stress margins with respect to the strain obtained in step S30 is referred to. And in step S40, by a known processing method, an arithmetic process of obtaining the stress margin d as a function f(x) with the strain x as a variable is performed.

[0141] Next, in step S50, the function of the stress margin d obtained in step S40 is changed to an expression that takes into account the safety factor s (: 0 < s ≤ 1) as follows.

[0142] d = s·f(x)

[0143] Then, the information of the function of the obtained stress margin d is stored in the storage unit 40 with the test conditions as the keyword.

[0144] <Evaluation main body unit 30>

[0145] In the evaluation main body unit 30, first, in step S100, the input of the conditions of the actual component to be evaluated is prompted. The conditions of the actual component are, for example, component conditions such as the type of material of the actual component, the formed shape, etc., conditions of the metal plate such as shearing conditions and forming conditions, and the conditions of the hydrogen environment as the condition for delayed fracture. And the above input is obtained through the input operation of the operator. As the type of material, for example, it is the steel type and thickness. The conditions of the hydrogen environment are, for example, acidity and setting time.

[0146] Next, in step S110 , forming analysis using CAE is performed based on the metal plate and shearing conditions acquired in step S100 , shape information of the actual formed component, and the like.

[0147] Next, in step S120 , based on the processing result of the forming analysis in step S110 , the strain amount x and the residual stress g are obtained for all the shear end surface portions of the actual component.

[0148] In step S130 , input of the load stress h during assembly and use is urged, and the load stress h is acquired through the input of the operator.

[0149] In step S140, information of the function "s·f(x)" of the stress margin d that matches the condition input in step S100 is obtained from the storage unit 40. And, for each shear end surface part, the stress margin d and the total stress (g+h) are compared. The stress margin d is the stress margin d corresponding to the strain x input in step S110. The total stress (g+h) is the total stress of the residual stress g output in step S120 and the load stress h input in step S130. And, through this comparison, it is determined whether there is a risk of delayed fracture.

[0150] Here, in Figure 5 In step S140, it is determined whether there is a risk of delayed fracture. The margin of delayed fracture (=d-(g+h)) may also be output.

[0151] In addition, the stress margin calculation unit 20 may perform a separate calculation process to obtain a function of the stress margin d based on the input values ​​in steps S10 to S20. Furthermore, the obtained function may be input to the storage unit 40 as data based on the input values ​​in steps S10 to S20 as keywords.

[0152] <Reexamination Section 50>

[0153] The review unit 50 performs a review process on the molding conditions and the component shape, and outputs the changed molding conditions and the component shape after the review to step S120.

[0154] The review process is performed, for example, on the shear end surface where the margin for delayed fracture (=d-(g+h)) is a negative value. For example, the forming conditions and part shape are changed in such a way that the residual stress is reduced by the stress amount exceeding the absolute value of the margin. Regarding the review of the forming conditions, for example, the number of stamping steps is increased to seek to relax the residual stress.

[0155] If the component is large and it is difficult to evaluate the entire shear end surface manually, Figure 5 Such a process flow evaluation can be automatically and efficiently evaluated. Thus, even if it is determined that there is a risk of delayed fracture, the forming conditions can be repeatedly reviewed until it is determined that there is no risk of delayed fracture.

[0156] Example

[0157] Examples of this embodiment will be described.

[0158] (Example 1)

[0159] In the present example, a test material X composed of a metal plate having a tensile strength of 1470 MPa grade steel plate and a thickness of 1.0 mm is described as a metal plate to be evaluated.

[0160] It should be noted that the present invention is not limited to the plate condition of the test material X. The present invention can be applied to various metal materials including high-strength steel plates having a tensile strength of 980 MPa or more that cause delayed fracture at shear end surfaces.

[0161] First, the test material X is sheared by shearing to produce a test piece with a straight shear end face of 100 mm in length. The width of the test piece during shearing is 30 mm, making the test piece into a long strip shape of 100 mm × 30 mm. The interval during shearing is set to 12% relative to the plate thickness. It should be noted that in the description of the above-mentioned embodiment, the case where the shearing condition is a single one is used as an example. However, in the case where the shearing conditions such as the interval during shearing change, the corresponding evaluation can also be performed. That is, the stress margin under the shearing condition can be obtained.

[0162] Next, the shear end face of the test piece is given a forming strain caused by stretching or compression along the extension direction of the shear end face. In this example, the forming strain is given by a uniaxial load tester in a state where both ends of the test piece are clamped. It should be noted that in this example, the case where the forming strain is caused by stretching or compression is described. It was confirmed that the same result was obtained even in the case where the forming strain is a deformation caused by bending. In addition, a test piece in a shearing state without giving a forming strain was also prepared.

[0163] Next, external restraint was applied to each test piece by four-point bending using a jig, and stress was applied to the center of the sheared end surface of the test piece. However, the tensile stress was applied with the burr side during shearing being the outer side of the bend.

[0164] Here, the magnitude of the load stress is determined as follows. The first principal stress-first principal strain relationship at the center and top of the width of the test piece is obtained by CAE. Then, the strain amount when the test piece is actually bent is measured and measured by establishing a correspondence.

[0165] In this example, as a stress loading method, the case of four-point bending is described. Even other loading methods such as bending loading methods and uniaxial tension loading methods can obtain the same trend of results. In addition, in this example, when stress loading, the burr side during shearing is made the outer side of the bend and the tensile stress is loaded. The surface on the opposite side to the burr side can also be evaluated in the same way.

[0166] In this example, the load stress applied to each test piece was changed every 100 MPa as shown in the table. In addition, a plurality of test pieces were prepared for each forming strain condition.

[0167] Furthermore, the test piece loaded with the load stress was immersed in a bath of a thiocyanate solution at pH 6 for 96 hours, and the delayed fracture characteristics were evaluated by the presence or absence of cracks caused by delayed fracture after 96 hours.

[0168] The above conditions and evaluation results are shown in Tables 1 to 11.

[0169] Each table summarizes the strain amount for each forming strain.

[0170]

[0171]

[0172]

[0173]

[0174]

[0175]

[0176]

[0177]

[0178]

[0179]

[0180]

[0181] Each table shows the presence or absence of delayed fracture for each load stress different in response variable when tension is positive and compression is negative.

[0182] As can be seen from Tables 1 to 11, the greater the absolute value of the strain amount of the forming strain, the greater the limit load stress. In this example, the limit load stress with the forming strain as a variable becomes the stress margin.

[0183] Figure 6 This is a graph in which the stress margin is obtained from the critical load stress at which delayed fracture does not occur, and the stress margin is described as a function of strain.

[0184] Thus, it can be seen that the stress margin corresponding to the forming strain after shearing can be described. It should be noted that in this example, the case where the stress margin increases due to the forming strain after shearing is shown. Even if the stress margin decreases due to the forming strain after shearing, the same evaluation can be performed.

[0185] (Example 2)

[0186] Next, an example of delayed fracture determination using the stress margin corresponding to the forming strain after shearing obtained in Example 1 will be described.

[0187] In Example 2, an actual component in an automobile component is assumed, and a Figure 7 The actual part with the sheared end face of the shape shown. And, the test material X is used to stamp and form into the shape of the actual part. Figure 7 In the component geometry shown, the strains are entered on the end faces.

[0188] At this time, CAE was used to calculate 10 representative locations A to J (not shown) of representative sheared end surfaces in the formed component. That is, CAE was used to calculate the forming strain and residual stress on the surface of the burr side during shearing.

[0189] The CAE used a shell element with a side length of 1.0 mm square and calculated the forming and springback processes using the dynamic explicit method.

[0190] It should be noted that, in this embodiment, Figure 7 The actual component shown is not limited to this component, and similar evaluation can be performed as long as it is a component having a shear end surface of a material that is likely to have delayed fracture.

[0191] After that, the actual parts after forming were subjected to a delayed fracture test. In the delayed fracture test, however, the parts were immersed in a thiocyanate solution of pH 6, and the delayed fracture was evaluated by the presence or absence of cracks caused by the delayed fracture after 96 hours.

[0192] The CAE results of the forming strain and residual stress after shearing are shown in Table 12 together with the presence or absence of the delayed fracture test.

[0193]

[0194] (Example 3)

[0195] Then, in Figure 8 The forming strain and residual stress after shearing and the presence or absence of delayed fracture shown in Table 12 were compared with Figure 6 The stress margins of the Figure 8 It is found that the presence or absence of delayed fracture of the shear end surface in an actual component can be predicted based on whether or not the stress margin line is exceeded.

[0196] Furthermore, the external stress load during assembly and use of the actual component is assumed to be a maximum of 300 MPa at all locations. The maximum amount of external load stress during assembly and use is estimated by measuring the elastic deformation of the actual component during assembly and assumed use using a strain gauge.

[0197] The forming strain and residual stress after shearing at representative locations of A to J were calculated when the corresponding stress was added. The residual stress was compared with the occurrence of delayed fracture in the state after forming. Figure 6 The stress margins of Fig. 9 .

[0198] exist Fig. 9 Even if delayed fracture does not occur in the delayed fracture test after component forming, there are parts that exceed the stress margin if the external stress load during assembly and use is considered. These parts do not cause delayed fracture in the delayed fracture test in the state after forming. However, it can be seen that if the load stress during assembly and use is considered, it can be said that these parts are potential delayed fracture risk parts that may cause delayed fracture.

[0199] Table 13 is an example of preventing delayed fracture from occurring even when stress during assembly and use is taken into account by using stress margin as a reference. Therefore, it is an example of setting an intermediate forming process before the final forming process for the same formed part. The forming strain and residual stress of representative parts A to J of the part are shown when the stress and strain amount are changed by setting the intermediate forming process.

[0200] In addition, the component loads in Table 13 are assumed to be a maximum assembly stress of 300 MPa and Figure 6 The stress margins of Fig.10 .exist Fig.10 In the figure, even if the stress during assembly and use is considered, it does not exceed the stress margin. Therefore, the risk of delayed fracture is predicted to be small. It can be seen that by using the stress margin as a reference, it is possible to design such automotive parts that do not cause delayed fracture and their manufacturing processes.

[0201]

[0202] (other)

[0203] The present disclosure can also adopt the following structures.

[0204] (1) A method for evaluating delayed fracture characteristics of a formed part, wherein a metal plate composed of a high-strength steel plate is formed to produce a formed part, and the delayed fracture characteristics of a shear end surface of the formed part used for assembling the formed part into another part are evaluated, wherein the method comprises:

[0205] The first step is to obtain an allowable value of the external load stress that does not cause delayed fracture at the shear surface of the metal plate, i.e., a stress margin, based on the results of a test including a step of restraining the metal plate in a state where 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 intrusion environment for a predetermined time in the restrained state, using the strain amount as a variable;

[0206] In a second step, a forming analysis is performed to form the metal sheet into the formed component, and a residual stress and strain amount at a shear end surface of the formed component generated when the metal sheet is formed into the formed component are obtained;

[0207] A third step is to determine a load stress applied to the shear end surface by assembling the molded component to another component; and

[0208] The fourth step is to evaluate the margin for delayed fracture of the formed part based on the stress margin of the metal plate with the strain amount obtained in the second step as a variable and the total stress of the residual stress obtained in the second step and the load stress obtained in the third step.

[0209] (2) comprising the following fifth step: when it is evaluated in the fourth step that delayed fracture will occur under the residual stress and strain amount obtained in the second step, changing any one of the shape of the formed part and the forming conditions;

[0210] In the fifth step, the portion evaluated to cause delayed fracture is changed in a direction in which the residual stress obtained in the second step becomes smaller.

[0211] (3) The metal plate is a steel plate having a tensile strength of 980 MPa or more.

[0212] (4) A method for manufacturing a molded part, comprising determining the molding conditions that are evaluated as the molded part not causing delayed fracture by the delayed fracture characteristic evaluation method of the molded part disclosed in the present invention, and performing molding under the determined molding conditions to manufacture the molded part.

[0213] Here, the entire contents of Japanese Patent Application 2022-085553 (filed on May 25, 2022) to which this application claims priority constitute a part of the present disclosure by reference. Here, the description is made with reference to a limited number of embodiments, but the scope of rights is not limited to them, and changes based on the above-disclosed embodiments are obvious to those skilled in the art.

[0214] Description of Reference Numerals

[0215] 1. First process

[0216] 2 Second process

[0217] 3. The third process

[0218] 4. The fourth process

[0219] 10 Cutting process

[0220] 11 Strain introduction process

[0221] 12 Loading process

[0222] 13 Hydrogen Intrusion Process

[0223] 14 Stress margin determines the process

[0224] 20 Stress margin calculation part

[0225] 30 Evaluation Body

[0226] 40 Storage

[0227] 50 Retrial Department

[0228] d stress margin

[0229] g Residual stress

[0230] h Load stress

[0231] x is the dependent variable.

Claims

1. A method for evaluating delayed fracture characteristics of a formed part, wherein a metal plate composed of a high-strength steel plate is formed to produce a formed part, and the delayed fracture characteristics at a shear end surface of the formed part used for assembling the formed part to other parts are evaluated, characterized in that: include: The first step is to obtain an allowable value of the external load stress that does not cause delayed fracture at the shear surface of the metal plate, i.e., a stress margin, based on the results of a test including a step of restraining the metal plate in a state where 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 intrusion environment for a predetermined time in the restrained state, using the strain amount as a variable; In a second step, a forming analysis is performed to form the metal sheet into the formed component, and a residual stress and strain amount at a shear end surface of the formed component generated when the metal sheet is formed into the formed component are obtained; A third step is to determine a load stress applied to the shear end surface by assembling the molded component to another component; and The fourth step is to evaluate the margin for delayed fracture of the formed part based on the stress margin of the metal plate with the strain amount obtained in the second step as a variable and the total stress of the residual stress obtained in the second step and the load stress obtained in the third step.

2. The method for evaluating delayed fracture characteristics of a molded part according to claim 1, wherein: The method comprises the following fifth step: when it is evaluated in the fourth step that delayed fracture will occur under the residual stress and strain amount obtained in the second step, changing any one of the shape of the formed part and the forming conditions; In the fifth step, the portion evaluated to cause delayed fracture is changed in a direction in which the residual stress obtained in the second step becomes smaller.

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

4. A method for manufacturing a formed part, characterized in that: The forming conditions are determined so that the formed part will not experience delayed fracture as evaluated by the delayed fracture characteristic evaluation method of a formed part according to any one of claims 1 to 3, and the formed part is manufactured by forming under the determined forming conditions.

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