Method for evaluating delayed fracture characteristics of metal plate for press molding, method for manufacturing press-molded article, and program

Abstract

The purpose of the present invention is to evaluate delayed fracture characteristics of a sheared end surface in consideration of load conditions during press molding. A delayed fracture characteristic evaluation method for evaluating delayed fracture characteristics of a sheared end surface of a metal plate for press molding used in press working, the method comprising: performing shear working on a metal plate to be subjected to press working under the same shear condition, producing a plurality of test pieces having the sheared end surface, and performing a delayed fracture characteristic evaluation on the sheared end surface of the test pieces; stress is applied under load conditions in which the stress distribution in the plate thickness direction is the same or similar, a test is performed by a plurality of load stresses, the test is performed under a plurality of load conditions in which the stress distribution in the plate thickness direction is different from each other, and the stress distribution in the plate thickness direction is determined on the basis of the test under each load condition. A limit stress, which is a limit value of a load stress at which a delayed fracture does not occur, corresponding to the stress distribution in the plate thickness direction is obtained.

Description

Technical Field

[0001] This invention relates to a technique for evaluating the delayed fracture characteristics at the sheared end face of stamped metal sheets. Furthermore, this invention relates to a method and procedure for manufacturing stamped parts using this evaluation technique. This invention is applicable to stamped metal sheets made of high-strength steel sheets. In this specification, high-strength steel sheet refers to a steel sheet with a tensile strength of 980 MPa or higher. Background Technology

[0002] Currently, automobiles require improved fuel efficiency and crash safety achieved through lightweighting. To balance lightweighting and occupant protection in a collision, there is a trend towards using high-strength steel in vehicle body structural components. In recent years, in particular, ultra-high-strength steel with tensile strengths exceeding 1470 MPa has been used in vehicle bodies. One challenge in applying high-strength steel to vehicle bodies is delayed fracture occurring after stamping. Especially with steels possessing tensile strengths exceeding 980 MPa, delayed fracture occurring from the sheared end face has become a significant issue. Furthermore, the sheared end face is also referred to as the sheared end face.

[0003] Here, it is known that a large amount of tensile stress remains on the shear end face. Therefore, there is a concern about delayed fracture occurring at the shear end face. It is known that delayed fracture at this shear end face is further promoted by applying external stress to the shear end face.

[0004] Conventionally, methods described in Patent Documents 1-3 have been proposed as evaluation methods for this delayed fracture. Specifically, Patent Documents 1 and 2 describe test methods that apply stress by fixing a constant displacement based on bending deformation. Patent Document 3 describes a test that applies stress by applying a constant load generated by uniaxial tensile deformation.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2017-142086

[0008] Patent Document 2: Japanese Patent Application Publication No. 2014-70927

[0009] Patent Document 3: Japanese Patent Application Publication No. 2016-57163 Summary of the Invention

[0010] The technical problem that the invention aims to solve

[0011] As mentioned above, there has been a concern in the past about delayed fracture after stamping caused by the shearing end face of high-strength steel plates.

[0012] The inventors conducted in-depth research on delayed fracture occurring at the sheared end face after stamping. The results show that even when the same stress load is applied to the sheared end face, the ultimate stress at which delayed fracture does not occur differs depending on the load conditions. For example, comparing uniaxial tensile deformation and bending deformation reveals differences in the ultimate stress at which delayed fracture does not occur at the sheared end face. Specifically, it is found that the ultimate stress is lower in uniaxial tensile deformation compared to bending deformation.

[0013] Therefore, previous evaluation methods have resulted in poor accuracy in assessing delayed fracture.

[0014] This invention was made with regard to the points mentioned above, and its purpose is to evaluate the delayed fracture characteristics of the shear end face by taking into account the load conditions during stamping.

[0015] Technical solutions for solving technical problems

[0016] The inventors conducted in-depth research on the different ultimate stresses that prevent delayed fracture in uniaxial tensile and bending deformations. The results yielded the following insights: In bending deformation, compared to uniaxial tensile deformation, a stress gradient exists along the plate thickness direction (refer to...). Figure 5 , Figure 6 Therefore, the insight that crack propagation in delayed fracture is suppressed during bending deformation was obtained. Regarding this insight, previous methods failed to consider the stress distribution along the thickness of the plate at the shear end face for delayed fracture evaluation.

[0017] Furthermore, the inventors gained the following insights from actual automotive parts after stamping: In order to predict delayed fracture at the shear end face, the stress distribution along the thickness direction must be considered to set the ultimate stress for delayed fracture.

[0018] Therefore, the inventors considered evaluating the ultimate stress of delayed fracture by taking into account the stress distribution along the plate thickness caused by external loads. Specifically, it is assumed that residual stress in the width direction of the plate at the shear end face is incorporated. Moreover, in delayed fracture tests under stress loading conditions based on bending or uniaxial tension, the following treatment is considered: that is, the ultimate stress of delayed fracture corresponding to the plate thickness distribution of the stress load is determined experimentally, and the plate thickness distribution of the stress load is quantified. It should be noted that "stress loading conditions" and "load conditions" are synonymous.

[0019] It should be noted that the calculation of the ultimate stress for delayed fracture corresponding to the plate thickness distribution of the stress load can refer to the calculated value of the residual stress distribution caused by shear in the plate thickness direction. The calculated value can be obtained, for example, through X-ray residual stress measurement or CAE analysis.

[0020] Furthermore, the ultimate stress for delayed fracture calculated in this way is compared with the plate thickness distribution of the stress load under specific loading conditions. Through this comparison, it is possible to predict whether delayed fracture will occur under that loading condition.

[0021] Load conditions include methods of applying loads such as bending (deformation) and minor axis tension (deformation) to a plate. Generally, a plate deforms due to bending, etc. However, by applying loads, it is possible to produce no definite deformation.

[0022] This invention was completed based on the above considerations.

[0023] In order to solve the problem, one embodiment of the present invention is a delayed fracture characteristic evaluation method for evaluating the delayed fracture characteristics of the sheared end face of a stamping sheet used in stamping processing. The method is characterized by performing shearing processing on the metal sheet under the same shearing conditions to produce multiple test pieces having the aforementioned sheared end face. Stress is applied to the sheared end face of the test pieces under load conditions with the same or approximately the same stress distribution in the thickness direction. Tests are conducted under multiple load stresses, and these tests are performed under multiple load conditions with different stress distributions in the thickness direction. Based on the tests under each load condition, the limit value of the load stress corresponding to the stress distribution in the thickness direction that does not result in delayed fracture, i.e., the ultimate stress, is determined.

[0024] Invention Effects

[0025] According to embodiments of the present invention, for sheet metal for stamping, the ultimate stress for delayed fracture can be evaluated by considering the stress distribution in the thickness direction caused by external loads. Embodiments of the present invention can be used when manufacturing stamped parts using the evaluated sheet metal for stamping. Furthermore, according to embodiments of the present invention, the delayed fracture characteristics at the shear end face of the sheet can be evaluated in advance during manufacturing. Therefore, according to embodiments of the present invention, a stamping shape and sheet metal having effective delayed fracture characteristics can be selected to manufacture stamped parts.

[0026] Therefore, according to embodiments of the present invention, when high-strength steel sheets are used in various components of an automobile, such as panel components and structural / frame components, delayed fracture characteristics can be improved. Attached Figure Description

[0027] Figure 1 This is a diagram illustrating an example of an evaluation process based on an embodiment of the present invention.

[0028] Figure 2 This is a schematic diagram illustrating the shearing process.

[0029] Figure 3This is a schematic diagram of the shape of the punching residue side of the sheared end face. (a) is a cross-sectional view, and (b) is a plan view.

[0030] Figure 4 This is a schematic diagram showing the distribution of residual stress in the extension direction on a typical shear end face.

[0031] Figure 5 This is a schematic diagram of the stress distribution along the thickness of a plate based on bending deformation.

[0032] Figure 6 This is a schematic diagram of the stress distribution in the thickness direction of a plate based on uniaxial tensile deformation.

[0033] Figure 7 This is a schematic diagram of superimposed stress under bending deformation (the shear surface is outside the bending).

[0034] Figure 8 This is a schematic diagram of superimposed stress under bending deformation (the fracture surface is outside the bending).

[0035] Figure 9 This is a schematic diagram of superimposed stress under uniaxial tensile deformation.

[0036] Figure 10 This is a diagram illustrating the structure of a program.

[0037] Figure 11 This is a graph showing the relationship between the residual stress in the extension direction of the shear end face and the distance from the shear surface.

[0038] Figure 12 It is a graph showing the relationship between the load stress and the distance to the side surface of the shear plane.

[0039] Figure 13 It is a graph showing the relationship between superimposed stress and the distance to the side surface of the self-shearing surface.

[0040] Figure 14 This is a graph showing the relationship between the ultimate stress and the stress gradient τ.

[0041] Figure 15 This is a diagram showing the shape of the stamped part specified in the embodiment. Detailed Implementation

[0042] Next, embodiments based on the present invention will be described with reference to the accompanying drawings.

[0043] (Evaluation method for delayed fracture characteristics)

[0044] This embodiment is an evaluation method for stamping sheet metal used in stamping processes. Specifically, it is a delayed fracture characteristic evaluation method for evaluating the delayed fracture characteristics of the sheared end face of the sheet metal. This embodiment is particularly effective when the sheet metal is a high-strength steel sheet.

[0045] In this embodiment, the process for evaluating the delayed fracture characteristics of the metal plate being evaluated includes a testing step involving actual testing and an evaluation step. Specifically, as follows: Figure 1 As shown, the delayed fracture characteristic evaluation method of this embodiment includes a first step S10, a second step S20, a third step S30, a fourth step S40, and a fifth step S50. The first step S10 to the fourth step S40 correspond to the experimental steps. The fifth step S50 corresponds to the evaluation steps.

[0046] <First Process S10>

[0047] The first step, S10, is a step in which the metal plate to be evaluated is sheared under set shearing conditions. Thus, the first step, S10, produces a test piece with a sheared end face for evaluation.

[0048] Shearing conditions are the processing conditions during shearing operations. For example, shearing conditions can be the value of the gap between the upper and lower cutting edges.

[0049] Figure 2 This is a schematic diagram of a shearing process. (For example...) Figure 2 As shown, the shearing process first involves clamping the metal plate 1 using the lower blade 2 on the blanking residue side and the plate pressing member 3. This constrains the metal plate 1. Under this constrained state, the upper blade 4 moves relatively perpendicularly to the plate. This applies shear deformation to the metal plate 1. If the metal plate 1 is sufficiently deformed by this shear deformation, cracks form at the point where the front end 4a of the upper blade 4 contacts the front end 2a of the lower blade 2. The cracks then merge, thereby separating the metal plate. As a result, the shearing process is completed. It should be noted that the direction of movement of the upper blade 4 is the shearing direction. The reference numeral CR indicates the gap.

[0050] Figure 3 This is a schematic diagram of the shape of the punching residue side of the sheared end face. Figure 4 This represents the residual stress distribution in the extension direction at a typical sheared end face along the thickness of the plate. Here, the sheared end face refers to the region within 1 mm of the end face surface created by shearing. This region is where shear-induced strain and residual stress exist.

[0051] On this sheared end face, there are two regions in the thickness direction of the blanking residue side: the shear surface side and the fracture surface side. The shear surface side refers to the side that becomes a smooth end face after being scraped by the upper cutting edge. The fracture surface side refers to the side that becomes a rougher end face due to the separation caused by the cracking progress from the tips of the upper and lower cutting edges.

[0052] Typically, the fracture surface side of a crack propagation zone has higher residual stress than the shear surface side. For example... Figure 4 As shown, there is a point where the cracks from the tips of the upper and lower cutting edges meet. This point is, for example, located on the fracture surface side approximately 1 / 4 of the plate thickness from the lower cutting edge side. Moreover, this is the location where the material finally separates. Therefore, the strongest plastic deformation occurs at this point, resulting in the highest residual tensile stress (see reference). Figure 4 ).

[0053] As described above, in the first step S10, multiple test pieces with the same shearing surface are produced after being sheared under the same shearing processing conditions.

[0054] <Second Process S20>

[0055] The second step, S20, is a method for applying an external load to the shear end face of the test piece. That is, the second step, S20, is the step of selecting the load conditions. In this example, each load condition is a method of applying a load that results in the same or approximately the same stress distribution along the thickness direction of the shear end face.

[0056] As a method of applying load to the shear end face, there are, for example, bending deformation, uniaxial tensile deformation, and tensile-bending deformation, which combines elements of bending and uniaxial tension. Furthermore, the stress distribution in the thickness direction of the plate varies depending on the direction of the bending deformation.

[0057] In this example, the basic load condition is illustrated by selecting from three deformations: a first deformation, a second bending deformation, and a uniaxial tensile deformation. The first bending deformation is a deformation in which the bending occurs as a crest on the shear plane side (tensile side). The second bending deformation is a deformation in which the bending occurs as a crest on the fracture plane side (tensile side). As will be described later, the stress distribution in the thickness direction of these load conditions is a stress distribution in which the stress varies linearly along the thickness direction (see reference). Figure 5 , Figure 6 ).

[0058] <Third Process S30>

[0059] The third step S30 is a step in which a predetermined external load stress is applied to the shear end face of the test piece according to the load conditions selected in the second step S20. Furthermore, the third step S30 is a step in which the test piece is constrained under this load condition.

[0060] As load conditions, when a first bending deformation and a second bending deformation are selected, for example, a method of applying stress to a test piece with a shear end face by four-point bending can be illustrated. Furthermore, when a tensile stress load based on uniaxial tension is selected as the load condition, a method of tensile deformation by gripping both ends of the test piece with a chuck can be illustrated. In summary, the methods for imparting bending deformation and uniaxial tensile deformation can be performed using conventionally known methods.

[0061] Stress adjustment is preferably performed by confirming the value of the load stress. The value of the load stress is confirmed, for example, by using a strain gauge pre-attached to the test specimen.

[0062] Furthermore, prepare multiple test pieces with sufficiently small ranges of load stress, such as 100 MPa scale. That is, prepare multiple test pieces with different load stresses.

[0063] <Fourth Process S40>

[0064] In the third step S30, external load stress is applied to each test piece, and constraints are applied under this condition.

[0065] In the fourth step S40, each test piece in this constrained state is subjected to a predetermined time relative to a predetermined hydrogen intrusion environment. Then, in the fourth step S40, the formation of cracks in the test piece in this state is evaluated.

[0066] At this point, the hydrogen intrusion environment and the setting time are preferably set to conditions that can obtain the same amount of hydrogen intrusion as the amount of hydrogen intruded by the material presumed to be the subject of evaluation in the actual use environment.

[0067] The test specimen is placed in a hydrogen immersion environment, for example, by immersing it in an acid bath containing hydrochloric acid, NH4SCN aqueous solution, or similar acid solutions. The concentration of the acid solution and the immersion time are set in a manner that pre-determines the hydrogen immersion conditions as an allowable upper limit. Furthermore, the maximum load stress that prevents cracking beyond a specified limit is set as the ultimate stress corresponding to the stress distribution in the thickness direction of the plate under that load condition.

[0068] Then, for the test piece produced in the first step S10, the load conditions selected in the second step S20 are changed, and the third to fourth steps S30 and S40 are performed.

[0069] Through the above experiments, the ultimate stress corresponding to the stress distribution in the thickness direction of the plate under each load condition can be determined. The ultimate stress is the limit value of the load stress that prevents delayed fracture. Each ultimate stress can be set as a criterion value for evaluating the limit of the load stress that prevents delayed fracture for each stress distribution in the thickness direction of the plate applied at the shear end face.

[0070] <Fifth Process S50>

[0071] In the fifth step S50, multiple combined data are obtained. The combined data includes the stress distribution in the thickness direction corresponding to the load conditions and the limit value of the load stress corresponding to the stress distribution.

[0072] The stress distribution along the thickness of the plate can also be converted into a function, for example. Furthermore, the stress distribution along the thickness of the plate can also be represented by the corresponding load conditions, for example.

[0073] In this embodiment, the stress distribution along the plate thickness corresponding to each load condition is converted into an index value representing the stress distribution along that plate thickness. This conversion is performed, for example, by a pre-defined function.

[0074] In this embodiment, the index value is represented, for example, by the stress gradient τ, which represents the gradient of the stress distribution in the corresponding plate thickness direction.

[0075] The stress gradient τ can be obtained, for example, by the following equation (1). Here, the stress on the fracture surface is denoted as σf, and the stress on the shear surface is denoted as σb. Furthermore, the stress gradient τ expressed by equation (1) becomes a dimensionless value. Additionally, the stress gradient τ is obtained using the stress distribution under the applied ultimate stress.

[0076] τ=(σf-σb) / |MAX(σf,σb)|···(1)

[0077] The stress distribution can be determined, for example, by the following methods: The first method is to determine it using X-ray residual stress measurement, which measures the residual stress distribution caused by shear in the thickness direction of the plate. The second method is to determine it using calculations from CAE analysis under load conditions applying ultimate stress to the object.

[0078] In addition, the stress gradient τ, which is the gradient of the distribution, can be easily obtained by the following equation (2).

[0079] τ=σf / σb ···(2)

[0080] Then, in the fifth step S50, information on the correlation between the index value and the corresponding limit value of the load stress is obtained by referring to multiple combined data. The combined data consists of (index value, limit stress) data.

[0081] Correlation information can be obtained using well-known computational methods. For example, correlation information can be presented as a multiple regression model or a machine learning-based model. Multiple combined data sets can also be used to represent correlation information. Correlation information can be represented through functions or tables.

[0082] In this way, relevant information can be obtained in advance. Therefore, when stamping metal sheets for stamping into actual stamped parts and forming predetermined stamped part shapes, it is possible to predict whether delayed fracture will occur at the shearing end face.

[0083] For example, consider manufacturing a stamped part by stamping a sheet metal sheet. The stress distribution along the thickness of the sheet at the sheared end face of the stamped part and the residual stress generated during forming are evaluated using the correlation information obtained above.

[0084] Here, the stress distribution along the thickness of the sheet and the residual stress generated by forming can be calculated using conventional CAE-based forming analysis. Furthermore, the stress distribution along the thickness obtained through analysis is converted into an index value, for example. Then, by referring to pre-determined correlation information, it is determined whether the residual stress generated by forming exceeds the ultimate stress under the determined index value. In this determination, the occurrence of delayed fracture at the shear end face is evaluated.

[0085] The evaluation only requires selecting multiple representative locations from the shear end face for evaluation.

[0086] Furthermore, for example, the load conditions that impose external stress on each sheared end face during stamping and the residual stress generated during forming are determined. Then, the ultimate stress of a condition that approximates the determined load conditions and is close to the stress distribution in the thickness direction is used as a criterion value. Based on this criterion value, it can then be evaluated whether delayed fracture has occurred at that location.

[0087] Furthermore, in actual stamping processes, the load conditions on the sheared end face are considered not only for simple bending and uniaxial tension, but also for superimposed composite load conditions. Regarding these composite load conditions, combined data consisting of the stress distribution and ultimate stress along the sheet thickness direction can also be obtained.

[0088] In this case, the ultimate stress can also be determined by conducting the aforementioned tests under the combined load conditions. Furthermore, the ultimate stress can also be determined as follows: The pressure distributions under the ultimate stresses of the superimposed load conditions are superimposed. Then, the stress at the location where the maximum stress occurs in the thickness direction under this superposition is set as the ultimate stress corresponding to the stress distribution in the thickness direction of the plate under the combined load conditions.

[0089] Furthermore, the shearing conditions can be varied to perform the aforementioned processes, and the ultimate stress corresponding to the stress distribution along the thickness direction can be calculated for each shearing condition. First, information regarding the ultimate stress corresponding to the stress distribution along the thickness direction and the shearing conditions used on the metal sheet is determined. Then, this information is used to evaluate whether delayed fracture at the sheared end face has occurred. In summary, in the manufacture of mass-produced products, shearing is mostly performed under the same shearing conditions. Therefore, having only information under these shearing conditions is sufficient.

[0090] (Functions, etc.)

[0091] Figure 5 and Figure 6 This is an example of the stress load distribution along the thickness of a plate, conceived under conditions where stress is applied to the shear end face. That is, Figure 5 and Figure 6 This is an example of a representative stress load distribution in the plate thickness direction relative to the shear end face extension direction, generated by the external load.

[0092] When the load condition is a stress load based on bending deformation, a stress that varies linearly from tension to compression is applied from one side of the plate thickness surface toward the opposite side. Therefore, under a stress load based on bending deformation, it becomes... Figure 5 The stress distribution along the thickness of the plate is shown. Furthermore, depending on the direction of bending deformation, the side where stress becomes tensile is conceived as both the shear plane side and the fracture plane side. The side where stress becomes tensile is the side that bulges out due to bending.

[0093] Furthermore, when the load condition is a stress load based on uniaxial tensile deformation, tensile stress is applied uniformly across the entire plate thickness. Therefore, under a stress load based on uniaxial tensile deformation, it becomes... Figure 6 The stress distribution along the thickness direction of the plate is shown.

[0094] It should be noted that the load conditions are also envisioned as a state of tension-bending, which combines elements of bending and uniaxial tension.

[0095] Furthermore, it is assumed that the stress distribution in the thickness direction of the shear end face when stress is applied to the shear end face is such that the distribution of stress load generated by the load stress is superimposed on the distribution of residual stress generated by the shear.

[0096] Figures 7-9 Yes Figure 4 The diagram shows the residual stress distribution on the shear end face superimposed with the stress load distribution under various loading conditions. Figure 7 This is a schematic diagram showing the load condition where the shear end face is selected as the bending deformation outside the bending. Figure 8 This is a schematic diagram showing the load condition where the fracture surface side is selected as the bending deformation outside the bending direction. Figure 9 This is a schematic diagram showing the load condition of uniaxial tensile deformation.

[0097] As shown in these diagrams (see) Figures 7-9 As shown, even if the maximum plate thickness caused by the external load stress applied to the sheared end face is the same, the superimposed stress is different. That is, the total maximum tensile stress at the sheared end face is different. The total value is the sum of the external load stress and the residual stress at the sheared end face generated by the shearing process.

[0098] Specifically, the relationship of the maximum tensile stress at the shear end face, as a sum, is as follows.

[0099] Uniaxial tension > Bending (tension along the fracture surface) > Bending (tension along the shear surface)

[0100] Furthermore, tensile bending is a combination of uniaxial tension and bending. Therefore, in the case of tensile bending, the maximum tensile stress is considered to be the intermediate between the two.

[0101] Furthermore, if the shear end face is subjected to stress load in a hydrogen-infiltrated environment, cracking and delayed fracture may occur as the hydrogen content increases. It is known that the higher the stress on the shear end face, the more likely this delayed fracture will occur.

[0102] Based on the above, it is assumed that a limit stress exists in the shear face that leads to delayed fracture. Here, it is assumed that delayed fracture occurs when the maximum sum of the distributions of shear residual stress and load stress in the shear face reaches the limit stress. If this assumption is made, the maximum value of the stress load that can be taken within the range where delayed fracture does not occur is related to the following load conditions. Hereinafter, the maximum value of the stress load that can be taken within the range where delayed fracture does not occur will be simply referred to as the limit stress.

[0103] Uniaxial tension < Bending (tension along the fracture surface) < Bending (tension along the shear surface)

[0104] It should be noted that in the case of tension and bending, since it is a combination of uniaxial tension and bending, it is envisioned as an intermediate state between the two.

[0105] Based on the above, by pre-calculating the ultimate stress of each stress distribution along the sheet thickness, the delayed fracture characteristics of the shear end face in a sheet metal sheet for stamping can be evaluated. Furthermore, the ultimate stress is the limit value of the load stress at which delayed fracture does not occur. For example, the ultimate stress corresponding to the stress distribution along the sheet thickness for each of the multiple stress distributions along the sheet thickness, and the stress corresponding to the load condition, is calculated. Then, it is evaluated whether delayed fracture occurs at a representative location of the shear end face during stamping of the sheet metal sheet for stamping.

[0106] Here, as an evaluation, it is assumed that the stress applied to the shear face of the stamped part can be simply designed within the range where the load conditions do not exceed the ultimate stress of uniaxial tension. However, this does not take into account the stress load caused by bending. Therefore, this evaluation has the problem of being too stringent.

[0107] Therefore, in order to achieve a better evaluation of delayed fracture at the shear end face, the inventors have obtained the following insights.

[0108] (1) Due to the distribution of shear residual stress on the shear end face, the maximum load stress (ultimate stress) of the delayed fracture limit changes. Furthermore, depending on the distribution of stress load within the plate thickness, the maximum load stress (ultimate stress) of the delayed fracture limit changes. Therefore, it is necessary to apply multiple loads with different stress distributions in the plate thickness direction and evaluate the respective ultimate stresses corresponding to each different stress distribution.

[0109] (2) Furthermore, in actual stamping processes, the load conditions on the shear end face are not only simple bending and uniaxial tension, but also the case of superimposed composite load conditions. By calculating the ultimate stress corresponding to the stress distribution corresponding to the composite load condition, the evaluation accuracy is improved.

[0110] (3) The stress distribution index in the thickness direction of the plate is quantified. In this case, the correlation between the stress distribution in the thickness direction and the ultimate stress can be expressed numerically. In this case, the information of this correlation can be represented more easily. As a result, the delayed fracture evaluation at the shear end face after stamping can be performed more simply.

[0111] (4) When the load condition is a stress load caused by uniaxial tension, it is assumed that the delayed fracture limit stress is the lowest. Furthermore, when the load condition is a stress load caused by bending, it is assumed that when tensile stress is applied to the fracture surface side with higher shear residual stress, the delayed fracture limit stress becomes lower. Therefore, the index value can be set to a value that represents the stress gradient in the thickness direction based on the stress distribution in the corresponding plate thickness direction.

[0112] For example, the stress on the fracture surface can be denoted as σf, and the stress on the shear surface can be denoted as σb. In this case, the stress gradient τ, which is the index value, can be defined as equation (1) or a similar mathematical expression.

[0113] (σf-σb) / |MAX (σf, σb)|···(1)

[0114] In this case, the ultimate stress of delayed fracture can be accurately represented by varying the ultimate stress as a function of the stress gradient τ.

[0115] The delayed fracture evaluation method described above can use the analytical information of stamped parts calculated by CAE to evaluate the occurrence of delayed fracture. For example, based on the forming analysis of stamping based on CAE, the stress distribution σ(t) in the thickness direction of the shear end face corresponding to the position t in the thickness direction at each point on the shear end face can be obtained. This obtained stress distribution σ(t) can be used to evaluate the occurrence of delayed fracture.

[0116] Furthermore, the stress distribution σ(t) in the thickness direction of the shear end face is represented by an index value. As a result, the determination of the ultimate stress is generalized, and the delayed fracture characteristics can be evaluated more simply.

[0117] Here, the stress gradient τ, which serves as an example of an index value, is not limited to equation (1) above. For example, (σf / σb) can also be set as the stress gradient τ, which represents the stress gradient in the thickness direction of the plate.

[0118] Furthermore, the load condition itself can be used as an index representing the stress distribution σ(t) in the thickness direction of the shear end face. Moreover, the presence or absence of delayed fracture can be evaluated based on the ultimate stress of a load condition close to the load condition applied by stamping at the evaluated shear end face location. For example, the stress distribution in the thickness direction caused by the external stress load (stamping) at the evaluated location can be determined using CAE analysis. Then, a load condition approximating this stress distribution can be determined. Then, whether delayed fracture occurs can also be evaluated based on whether the ultimate stress under a specific load condition is exceeded.

[0119] However, the stress distribution σ(t) in the thickness direction of the shear end face can be easily evaluated by using an index value.

[0120] (Manufacturing method of stamped parts)

[0121] Stamping is performed on a metal sheet with a sheared end face to produce a stamped part.

[0122] At this point, for the stamping metal sheet used, based on the delayed fracture characteristic evaluation method for stamping metal sheets described above, the correlation between the index value and the ultimate stress is calculated in advance.

[0123] Then, a CAE analysis is performed on the stamped part formed by stamping the aforementioned sheet metal. Based on the stress distribution along the thickness of the sheet at each shear face and the aforementioned correlation, the presence or absence of delayed fracture in the stamped part is evaluated. If delayed fracture is evaluated, the stamping shape of the stamped part is changed, or the material and thickness of the sheet metal used are changed. The delayed fracture evaluation is then performed again. This process is repeated until delayed fracture is evaluated as not occurring.

[0124] This enables the manufacture of stamped parts that do not experience delayed fracture.

[0125] (program)

[0126] This section presents an example of the procedure used in the above methods for evaluating delayed fracture characteristics. For example... Figure 10 As shown, the program is stored in the computer's RAM, ROM, or other storage units 40. The program is executed by the computer.

[0127] In this example, the program pre-calculates and stores the information 40A related to the index value and the ultimate stress in the storage unit 40. The index value is an index value that represents the stress distribution in the thickness direction of the metal plate caused by applying an external load. The ultimate stress is the ultimate stress under the load condition that constitutes the stress distribution in the thickness direction of the plate. This information 40A related to the stress distribution can be pre-calculated by processing the delayed fracture characteristic evaluation method as described above. For example, the index value is calculated using equation (1).

[0128] In this example, the program takes stress distribution information as input and outputs the limit value of the load stress that will not cause delayed fracture, i.e., the ultimate stress.

[0129] The input stress distribution information is obtained by performing a CAE analysis to form the metal sheet into the shape of the stamped part, and then substituting the values ​​obtained from this analysis into the formula for calculating the index value. Figure 10 The figure 20 indicates the CAE analysis unit.

[0130] In this example, program 30 is executed in the following manner. First, in the index value calculation step 30A, the index value is calculated based on the input stress distribution information, and the stress values ​​at both ends of the plate in the thickness direction are substituted into equation (1) for evaluation.

[0131] Next, in the ultimate stress calculation step 30B, the ultimate stress corresponding to the calculated index value is obtained by referring to the relevant information 40A.

[0132] Then, program 30 in this example outputs the calculated ultimate stress value. Alternatively, given the input stress distribution information and the stress value generated at the evaluation location, the program can compare the calculated ultimate stress with the input stress value and output information on whether delayed fracture has occurred.

[0133] (other)

[0134] This disclosure may also take the following forms.

[0135] (1) Scheme 1: A method for evaluating the delayed fracture characteristics of stamping metal sheets, wherein the delayed fracture characteristics of the sheared end face of stamping metal sheets used in stamping processes are evaluated, characterized in that,

[0136] Multiple test pieces with the aforementioned sheared end faces were produced by performing shearing processing on a metal plate under the same shearing conditions.

[0137] For the shear end face of the above test piece, stress was applied under load conditions with the same or approximately the same stress distribution in the thickness direction, and tests were conducted under multiple load stresses.

[0138] The above test was performed under multiple load conditions with different stress distributions along the plate thickness.

[0139] Based on the above tests for each load condition, the limit value of the load stress that does not cause delayed fracture, corresponding to the stress distribution in the plate thickness direction, is determined, i.e., the ultimate stress.

[0140] (2) Scheme 2: The test piece under stress is placed in a hydrogen intrusion environment, and the upper limit of the load stress that does not cause delayed fracture in the hydrogen intrusion environment is taken as the ultimate stress.

[0141] (3) Scheme 3: The above multiple load conditions include loads caused by bending that protrudes in the shear direction, loads caused by bending that is concave in the shear direction, and stress loads caused by uniaxial tension.

[0142] (4) Scheme 4: A composite load condition that superimposes two or more load conditions that cause different stress distributions in the thickness direction of the plate.

[0143] Then, the ultimate stress corresponding to the stress distribution in the thickness direction of the plate under the above-mentioned combined load conditions is determined.

[0144] (5) Scheme 5: The metal sheet used for stamping is made of high-strength steel sheet.

[0145] (6) Scheme 6: Convert the stress distribution in the thickness direction of the plate into an index value that represents the distribution state of the stress.

[0146] Based on a set of data consisting of the aforementioned index values ​​and the aforementioned ultimate stress, the correlation between the aforementioned index values ​​and the aforementioned ultimate stress is determined.

[0147] (7) Scheme 7: The above index values ​​are the values ​​that represent the gradient of stress distribution in the thickness direction of the plate.

[0148] (8) Scheme 8: When the stress on the fracture surface of the plate in the stress distribution is set as σf and the stress on the shear surface of the plate is set as σb, the above index value τ is expressed by the following formula (1).

[0149] τ=(σf-σb) / |MAX(σf, σb)|···(1).

[0150] (9) Scheme 9: A method for manufacturing a stamped part, wherein a stamped metal sheet having a sheared end face is stamped to manufacture the stamped part, wherein,

[0151] For the metal sheet being tested, based on the delayed fracture characteristic evaluation method for stamping metal sheets of this disclosure, the correlation between the aforementioned index values ​​and the aforementioned ultimate stress is calculated in advance.

[0152] A CAE analysis was performed on the stamped parts formed by stamping the aforementioned metal sheet.

[0153] Based on the stress distribution along the thickness of the sheet at various points on the shear end face according to this analysis and the aforementioned correlation, an evaluation is made as to whether the stamped part has experienced delayed fracture.

[0154] The stress distribution along the thickness of the plate at each of the shear end faces is converted into index values ​​for the above evaluation.

[0155] (10) Solution 10: A program that stores information on the correlation between the above-mentioned index values ​​and the above-mentioned ultimate stress, obtained based on the delayed fracture characteristics evaluation method for stamping sheet metal of this disclosure, in a storage unit.

[0156] When executed by a computer, the following steps are performed: the above-mentioned index value is calculated based on the stress distribution information in the thickness direction of the plate; based on the above-mentioned correlation information and the calculated index value, the limit value of the load stress that does not cause delayed fracture, i.e., the ultimate stress, is determined.

[0157] Example

[0158] Next, embodiments based on this implementation will be described.

[0159] In this example, a 1.4 mm thick steel sample with a tensile strength of 1470 MPa is used as an example for illustrating the metal sheet used in stamping. However, the present invention is not limited to such a sample A. The present invention can be appropriately applied to metallic materials, such as ultra-high tensile steel with a tensile strength of 980 MPa or more, which exhibit delayed fracture at the shear end face.

[0160] In this example, the shearing gap was set to 12% of the plate thickness as the shearing processing condition. Then, under these shearing conditions, sample A was sheared to produce a test piece. The test piece had a straight sheared end face with a length of 500 mm.

[0161] Figure 11 This represents the stress distribution within the plate thickness in the direction parallel to the shear end face of the test piece, calculated based on X-rays. Furthermore, Figure 11 The calculated values ​​represent the CAE-based FEM analysis of the shearing process of the test piece's shear end face.

[0162] Depend on Figure 11 It can be seen that the stress measurements based on X-rays and the CAE analysis results show the same trend. Furthermore, it can be seen that the residual stress on the fracture surface side is generally higher than that on the shear end face. In particular, the maximum tensile residual stress is found at the 1 / 4 position of the plate thickness on the fracture surface side.

[0163] Next, a stress load is applied to the shear end face of the test piece by a bending load based on four-point bending. Then, a uniaxial tensile stress load is applied by clamping the long strip-shaped test piece with a chuck and applying tension. That is, bending deformation and uniaxial tensile deformation are illustrated as load conditions. Then, under each load condition, external stress is applied to multiple test pieces separately.

[0164] The applied stress was adjusted as described below. A strain gauge was attached near the shear end face to measure the strain. Then, the maximum load stress within the plate thickness under each condition was determined by multiplying the measured strain by Young's modulus of 205 GPa. Based on this determination, the external stress load applied to each test piece was adjusted. Then, by applying the load stress at 100 MPa intervals, multiple test pieces with different load stresses under each load condition were prepared. However, for the stress load caused by the bending load generated by four-point bending, two types of test pieces were prepared. That is, test pieces were prepared with external loads applied under two conditions: one where the shear surface side was tensile (convex) and the other where the fracture surface side was tensile (convex). Tensile (convex) refers to bending deformation with the tensile side convex.

[0165] Figure 12A schematic diagram showing the load stress distribution within the estimated plate thickness under various load conditions. Figure 12 The example shown is for a maximum load stress of 1000 MPa.

[0166] Next, each test piece under stress loading was immersed in a hydrochloric acid bath with a constant pH of 2.5 for 100 hours. Then, the presence or absence of penetrating cracks along the thickness direction was determined at the end of the immersion. This determination investigated the occurrence of delayed fracture. It should be noted that the maximum external stress at which no such cracks occurred, i.e., no delayed fracture, was defined as the ultimate stress.

[0167] The ultimate stresses for each load condition are shown in Table 1.

[0168] [Table 1]

[0169]

[0170] Table 1 shows that the ultimate stress is lowest under uniaxial tension. Furthermore, regarding ultimate stress, bending (tension along the fracture surface) is high, while bending (tension along the shear surface) is the highest. Bending (tension along the fracture surface) refers to the stretching caused by bending deformation along the fracture surface. Bending (tension along the shear surface) refers to the stretching caused by bending deformation along the shear surface.

[0171] The stress distribution of each load under uniaxial tension, bending (fracture plane side tension), and bending (shear plane side tension) at each ultimate stress is compared with... Figure 5 The sum of the X-ray stress measurements shown is as follows: Figure 13 As shown. The ultimate stress, which represents the maximum load stress within the plate thickness under various load conditions, differs. However, as... Figure 13 As shown, when combined with the residual shear stress, the following relationship emerges: Bending (tensile stress on the fracture surface) has a maximum stress of 1632 MPa at the outermost surface of the fracture surface. Bending (tensile stress on the shear surface) has a maximum stress of 1665 MPa at the outermost surface of the shear surface. Uniaxial tension has a maximum stress of 1613 MPa at the shear surface. In other words, in any case, the local maximum stress is approximately the same.

[0172] This indicates that the following situation exists due to the distribution of residual stress within the sheet thickness caused by shearing. Specifically, it indicates that there are different maximum stresses for delayed fracture limit loads depending on the stress distribution σ(t). The stress distribution σ(t) is the stress distribution along the sheet thickness direction of the shear end face corresponding to the position t in the sheet thickness direction during stress application to the shear end face. Moreover, this evaluation can be performed through the aforementioned test. In short, the stress applied to the shear end face of the stamped part is designed within the range not exceeding the delayed fracture limit stress of uniaxial tension. However, this does not consider the stress load caused by bending. Therefore, there is a problem of overly stringent requirements.

[0173] It should be noted that the ultimate stress at delayed fracture is the lowest under uniaxial tensile load. Furthermore, even under the same bending load, the ultimate stress differs between shear-side tension and fracture-side tension. Therefore, it is necessary to distinguish between shear-side tension and fracture-side tension.

[0174] In this example, the stress gradient τ specified in (1) above is used as an example value. In this case, the ultimate stress corresponding to the stress distribution in the plate thickness direction can be accurately represented by changing the ultimate stress σlim as a function of the stress gradient τ.

[0175] Figure 8 Based on the results in Table 1, a graph was drawn showing the stress gradient τ and the information on the three ultimate stresses: uniaxial tension, bending (shear plane tension), and bending (fracture plane tension). Furthermore, Figure 8 It represents the linear interpolation between plotted points, which is equivalent to the state of stretching and bending.

[0176] According to this method, the ultimate stress σlim is expressed by the following formula.

[0177] σlim=-550×τ+400(τ≤0)

[0178] σlim=200×τ+400(τ>0)

[0179] By using the above formula, the dependence of the ultimate stress on the load stress distribution can be represented more accurately. It should be noted that separate experiments can be conducted for the tensile-bending state, and the results can be plotted on [date / time]. Figure 14 .

[0180] Figure 15 This is a schematic diagram of a stamped part with a sheared end face, as shown in this example. The stamping process is... Figure 15 The evaluation results for the stamped parts shown are presented in Table 2.

[0181] Table 2 shows the maximum load stress, stress gradient τ, ultimate stress dependent on τ, and delayed fracture test results for the sample portion of the shear end face based on CAE analysis, representing the residual stress of stamping.

[0182] [Table 2]

[0183]

[0184] As shown in Table 2, for stamped parts with shear end faces, delayed fracture can be accurately predicted by evaluating the delayed fracture based on the stress distribution within the sheet thickness.

[0185] In addition, the results of taking measures to reduce stress at locations C, E, and F where delayed fracture occurred are shown in Table 3.

[0186] [Table 3]

[0187]

[0188] As shown in Table 3, by referencing the ultimate stress, which is an indicator of the stress gradient τ, to reduce the stress at the location of delayed fracture, delayed fracture is suppressed. This allows for the fabrication of stamped parts with shear-faced surfaces exhibiting excellent resistance to delayed fracture.

[0189] This disclosure is formed in part by reference to all contents of Japanese Application No. 2023-121673 (filed July 26, 2023), which claims priority to this application. A limited number of embodiments have been described herein, but the scope of the claims is not limited thereto, and modifications based on the embodiments disclosed above will be readily apparent to those skilled in the art.

[0190] Explanation of reference numerals in the attached figures

[0191] 1: Metal plate;

[0192] 2: Lower blade;

[0193] 2a: Front-end;

[0194] 4: Upper blade;

[0195] 4a: Front-end;

[0196] 30: Program;

[0197] 30A: Steps for calculating indicator values;

[0198] 30B: Steps for calculating ultimate stress;

[0199] 40: Storage Department;

[0200] 40A: Relevance information;

[0201] S10: First process;

[0202] S20: Second process;

[0203] S30: Third process;

[0204] S40: Fourth process;

[0205] S50: Fifth process;

[0206] τ: Stress gradient (index value).

Claims

1. A method for evaluating the delayed fracture characteristics of stamped metal sheets, comprising evaluating the delayed fracture characteristics of the sheared end face of stamped metal sheets used in stamping processes, characterized in that, Multiple test pieces with the aforementioned sheared end faces were produced by performing shearing processing on a metal plate under the same shearing conditions. For the shear end face of the above test piece, stress was applied under load conditions with the same or approximately the same stress distribution in the thickness direction, and tests were conducted under multiple load stresses. The above test was performed under multiple load conditions with different stress distributions along the plate thickness. Based on the above tests for each load condition, the limit value of the load stress that does not cause delayed fracture, corresponding to the stress distribution in the plate thickness direction, is determined, i.e., the ultimate stress.

2. The method for evaluating the delayed fracture characteristics of stamped metal sheets according to claim 1, The above test involves placing a stressed test piece in a hydrogen immersion environment, and the upper limit of the load stress that does not cause delayed fracture in this hydrogen immersion environment is taken as the ultimate stress.

3. The method for evaluating the delayed fracture characteristics of stamped metal sheets according to claim 1 or 2, The aforementioned load conditions include loads caused by bending that protrudes in the shear direction, loads caused by bending that is concave in the shear direction, and stress loads caused by uniaxial tension.

4. The method for evaluating the delayed fracture characteristics of stamped metal sheets according to any one of claims 1 to 3. A composite load condition consisting of two or more load conditions that result in different stress distributions along the thickness direction of the plate. Then, the ultimate stress corresponding to the stress distribution in the thickness direction of the plate under the above-mentioned combined load conditions is determined.

5. The method for evaluating the delayed fracture characteristics of stamped metal sheets according to any one of claims 1 to 4, The aforementioned stamping metal sheet is made of high-strength steel sheet.

6. The method for evaluating the delayed fracture characteristics of stamped metal sheets according to any one of claims 1 to 5, The stress distribution along the thickness of the plate is converted into index values ​​that represent the distribution state of this stress. Based on a set of data consisting of the aforementioned index values ​​and the aforementioned ultimate stress, the correlation between the aforementioned index values ​​and the aforementioned ultimate stress is determined.

7. The method for evaluating the delayed fracture characteristics of stamped metal sheets according to claim 6. The above index values ​​represent the stress values ​​that reflect the stress distribution along the thickness direction of the plate.

8. The method for evaluating the delayed fracture characteristics of stamped metal sheets according to claim 6 or 7. When the stress on the fracture surface of the plate in the stress distribution is set as σf and the stress on the shear surface of the plate is set as σb, the above index value τ is expressed by the following equation (1). τ=(σf-σb) / |MAX(σf, σb)|···(1).

9. A method for manufacturing a stamped part, comprising stamping a metal sheet having a sheared end face to manufacture the stamped part, characterized in that, For the metal sheet being tested, based on the delayed fracture characteristic evaluation method for stamping metal sheets according to any one of claims 6 to 8, the correlation between the aforementioned index values ​​and the aforementioned ultimate stress is calculated in advance. A CAE analysis was performed on the stamped parts formed by stamping the aforementioned metal sheet. Based on the stress distribution along the thickness of the sheet at various points on the shear end face according to this analysis and the aforementioned correlation, an evaluation is made as to whether the stamped part has experienced delayed fracture. The stress distribution along the thickness of the plate at each of the shear end faces is converted into index values ​​for the above evaluation.

10. A program that stores information on the correlation between the index values ​​and the ultimate stress obtained based on the delayed fracture characteristics evaluation method for stamping sheet metal according to any one of claims 6 to 8 in a storage unit. When executed by a computer, the following steps are performed: the above-mentioned index value is calculated based on the stress distribution information in the thickness direction of the plate; based on the above-mentioned correlation information and the calculated index value, the limit value of the load stress that does not cause delayed fracture, i.e., the ultimate stress, is determined.

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