Delayed fracture evaluation method, press-molded product manufacturing method, and program

Abstract

Provided is a method capable of quantitatively evaluating the delayed fracture properties of a sheared end surface formed by shearing a three-dimensional rear trim part. This is a delayed fracture property evaluation method for evaluating the delayed fracture properties of a sheared end surface of a metal sheet for press forming. The sheared end surface generated by shearing the metal sheet after press forming into a part having a target part shape is the sheared end surface for evaluating delayed fracture properties, and the delayed fracture evaluation method determines the stress or strain of the sheared end surface or its surroundings to be evaluated in two states: the state of the metal sheet before processing in which the metal sheet after press forming is constrained by the material constraint conditions in shear processing, and the state of the metal sheet after processing in which shearing is completed by shear processing and the constraint conditions of the metal sheet are released, and evaluates the delayed fracture properties based on the difference in stress or strain determined in the two states.

Description

[Technical field]

[0001] The present invention relates to a technique for evaluating delayed fracture properties of a sheared end surface of a metal sheet for press forming. The present invention also relates to a technique for manufacturing a press-formed product using the evaluation technique. The present invention is suitable for manufacturing an automobile part by press forming a metal sheet made of a high-strength steel sheet and further subjecting the press-formed metal sheet to shear processing, for example. [Background technology]

[0002] Currently, automobiles are required to improve fuel efficiency and collision safety by reducing the weight of the vehicle. In order to achieve both weight reduction and protection of passengers in the event of a collision, high-strength steel plates are used in the vehicle body. In particular, in recent years, high-strength steel plates with a tensile strength of 980 MPa or more have been applied to vehicle bodies. One of the issues when high-strength steel plates are applied to vehicle bodies is delayed fracture. In particular, delayed fracture occurring from the end face after shear processing is an important issue for steel plates with a tensile strength of 980 MPa or more. The end face formed by shear processing is also called a sheared end face. For this reason, in this specification, the end face formed by shear processing is also referred to as a sheared end face.

[0003] It is known that a large tensile stress remains at the sheared edge. For this reason, there is concern that delayed fracture may occur at the sheared edge in products manufactured by press forming. As an evaluation method for delayed fracture at the sheared edge, for example, there is a method described in Patent Document 1. The method described in Patent Document 1 describes a method of evaluating hydrogen embrittlement of a steel material by applying a tensile stress to a thin steel plate while charging the steel plate with hydrogen. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5196926 [Patent Document 2] JP 2014-70927 A [Patent Document 3] JP 2023-72460 A [Patent Document 4] WO2023 / 037961 [Patent Document 5] JP 2023-173358 A Summary of the Invention [Problem to be solved by the invention]

[0005] Here, there are two types of sheared end surfaces of an automobile part: a front trim portion and a rear trim portion. The front trim portion is a portion that is sheared before press forming. The rear trim portion is a portion that is sheared after press forming. In this specification, the shearing process for forming the rear trim portion is also referred to as the rear trim. The rear trim portion is generated, for example, by shearing for shaping the part according to the product outline. Shearing for the rear trim portion occurs, for example, when an extra flange portion is required as a plate holding portion for draw forming during press forming. The rear trim portion is a portion formed by shearing along the outline line after press forming. In addition, the rear trim portion is also generated, for example, by processing such as punching after press forming.

[0006] After-trim after press forming is applied to parts that have already been formed by press forming. During after-trim, it is necessary to restrain the part under the restraint condition that the plate is pressed along the part's shape. Then, in after-trim, it is necessary to perform shearing on the part in this restrained state. In other words, the shearing process for the after-trim part needs to be done along the three-dimensional shape of the part. For this reason, the quality of the sheared edge surface of the after-trim part may be worse than that of the before-trim part. Also, in after-trim, unintended stress may be applied to the sheared edge surface.

[0007] From the above findings, the inventors have found that there is a problem in that the risk of delayed fracture at the sheared end face may be higher in the rear trim section than in the front trim section. For example, an example of a rear trim section with a typical three-dimensional shape is a section formed by shearing the entire vertical wall section 1B including the hat-shaped shoulder R section 1C (see Figure 2) (see Figure 2). However, the method of Patent Document 1 does not focus on the evaluation of the sheared edge of the rear trim part. In other words, the method of Patent Document 1 does not allow for delayed fracture evaluation that takes into account the above-mentioned risks. The above-mentioned risks include, for example, deterioration of the quality of the sheared edge that occurs during the shearing process to form the three-dimensional rear trim part.

[0008] The present invention has been made in consideration of the above points. One of the objects of the present invention is to provide a method for quantitatively evaluating the delayed fracture properties of a sheared end surface formed by shear processing of a three-dimensional rear trim portion. Another object of the present invention is to provide a press-formed product having excellent delayed fracture resistance. [Means for solving the problem]

[0009] For actual press-formed products, delayed fracture evaluation related to the rear trim portion as described above is very important. In particular, for automobile parts, delayed fracture evaluation related to the rear trim portion as described above is very important when considering the production process. For this reason, the inventors recognized the need to provide a method capable of quantitatively evaluating the delayed fracture characteristics of the shear end surface of a three-dimensional rear trim portion. The inventor then focused on the changes in stress distribution and strain around the sheared end face before and after shearing during shearing of a part manufactured by press molding. The inventor then developed the present invention based on this focus. Here, the stress distribution before shearing refers to the stress distribution around the sheared end face immediately before shearing. Immediately before shearing refers to the period from when the metal plate is restrained by the plate clamp for shearing until the upper blade comes into contact with the material. The stress distribution after shearing refers to the stress distribution around the sheared end face after the part is sheared and springs back.

[0010] In order to solve the problem, one aspect of the present invention is a delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared end surface of a metal sheet for press forming, in which a sheared end surface generated by performing shear processing on a metal sheet after press forming the metal sheet into a part having a predetermined target part shape is defined as the sheared end surface for evaluating the delayed fracture property, and stress or strain is obtained at or around the sheared end surface to be evaluated in two states: a state of the metal sheet before processing in which the metal sheet after the press forming is constrained under material constraint conditions in the shear processing, and a state of the metal sheet after processing in which shearing is completed by the shear processing and the constraint conditions are released, and the delayed fracture property is evaluated from changes in stress or strain at or around the sheared end surface to be evaluated in the two states obtained. Effect of the Invention

[0011] According to an aspect of the present invention, the change in stress or strain around the shear end face before and after shearing of the rear trim part of a part manufactured by press forming is taken into consideration. This makes it possible to evaluate and predict the risk of delayed fracture in the rear trim part. Therefore, the aspect of the present invention makes it possible to provide a press-formed product with excellent delayed fracture resistance to which these evaluation and prediction methods are applied. Therefore, the aspect of the present invention can improve delayed fracture properties when high-strength steel plate is applied to various parts such as automotive panel parts, structural and skeletal parts, etc. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is a schematic diagram illustrating an example of rear trim. [Diagram 2] FIG. 13 is a perspective view showing an example of rear trimming of an entire vertical wall portion of a hat-shaped pressed part. [Diagram 3] 1 shows an example of the stress state during deformation in shear processing, where (a) is a diagram in the thickness direction and (b) is a diagram in the width direction. [Figure 4]1 shows an example of the stress state after separation in shear processing, where (a) is a diagram in the thickness direction and (b) is a diagram in the width direction. [Diagram 5] FIG. 2 is a diagram illustrating shear processing when an external tensile stress is applied in the plate width direction. [Figure 6] FIG. 1 is a diagram illustrating shear processing in a state where no external stress is applied in the plate width direction. [Figure 7] FIG. 1 is a diagram illustrating shear processing when an external compressive stress is applied in the plate width direction. [Figure 8] 1 is a schematic diagram for explaining a state in which the stress in the area around the sheared end surface is released by springback after shearing, and a portion other than the sheared end surface is elastically deformed. Specifically, it is a schematic diagram showing a state in which the sheared end surface is subjected to additional deformation due to the elastic deformation around the sheared end surface. [Figure 9] FIG. 1 is a diagram showing an example of a processing procedure of an evaluation method according to an embodiment of the present invention. [Figure 10] FIG. 11 is a diagram illustrating an example of a processing procedure of a determination threshold obtaining step. [Figure 11] FIG. 11 is a diagram showing an example of a processing procedure of a program according to an embodiment based on the present invention. [Figure 12] FIG. 4 is a diagram showing the positions of component locations A to I on the sheared end surface of the rear trim part. [Figure 13] FIG. 1 is a top view showing a location where delayed fracture occurred without any countermeasure in an embodiment. [Figure 14] FIG. 4 is a top view showing a portion where measures are taken to suppress delayed fracture. [Figure 15] FIG. 13 is a diagram showing the amount of movement of the edge at the end of the flange portion after rear trimming and spring back. [Figure 16] FIG. 13 is a diagram showing a rear trim line (dashed line) at regular dimensions and a rear trim line (solid line) at a corrected mold. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Next, an embodiment of the present invention will be described with reference to the drawings. (The idea of ​​the present invention) In order to clarify the principle of the present invention, the mechanism of generation of tensile shear residual stress will be described first. Tensile shear residual stress increases the risk of delayed fracture at the sheared edge. FIG. 1 shows an example of applying rear trim to a metal plate 1 (part 1) produced by press forming. In FIG. 1, reference numeral 1a denotes a part that has become a three-dimensional shape by press forming. Reference numeral TL denotes a cutting line in the rear trim. A typical example of a rear trim part is a part formed by trimming the entire vertical wall part 1B as shown in FIG. 2. The vertical wall part 1B includes a shoulder R part 1C of a hat-shaped part. In FIG. 2, reference numeral 1A denotes a hat-shaped top plate part. Reference numeral 1D denotes a flange part. Reference numeral 3 denotes a scrap side of the metal plate before rear trimming, which is formed by shearing. Reference numeral 2 denotes a product side (press-molded product side) of the metal plate before rear trimming, which is formed by shearing.

[0014] Figure 3 shows a schematic diagram of the stress state during shearing. Figure 4 shows a schematic diagram of the stress state after material separation in shearing. Reference numeral 10 denotes a die (lower die) having a lower blade 10a. Reference numeral 11 denotes a punch (upper die) having an upper blade 11a. As shown in FIG. 3, in shearing, the plate material clamped between the upper blade 11a and the lower blade 10a undergoes extreme shear deformation and tensile deformation of several tens of percent or more in the region between the upper and lower blades 10a, 11a. Then, as shown in FIG. 4, when the deformation of the material reaches its limit, a crack occurs at the contact position with both or one of the upper blade 11a and the lower blade 10a, and the material is separated. Just before the material is separated, the end surface to be sheared is subjected to extreme plastic deformation and tensile stress in the plate thickness direction. In addition, after the material is separated, springback occurs. However, after the material is separated, the inner part is restrained more than the end surface that is not deformed. For this reason, the above tensile stress does not completely spring back and cannot be relaxed. As a result, the rear trim part remains in a state where it is subjected to elastic tensile deformation stress in the plate thickness direction (FIG. 3(a)). Furthermore, around the sheared end face, the deformation in the direction perpendicular to the plate thickness direction (depth direction of the paper) is constrained to a plane strain state (Fig. 3(b)). Therefore, the tensile stress remaining in the plate thickness direction also results in a tensile stress remaining in the plate width direction.

[0015] As described above, the occurrence of shear residual stress is due to extreme deformation of several tens of percent or more. For this reason, there is water around the sheared end before the shearing process begins. The very slight strain that gives the elastic stress field where the strain exists has almost no effect. The very slight strain is, for example, an elastic strain of less than 0.6%.

[0016] At this time, as shown in the schematic diagrams of Figures 5 to 7, the above-mentioned sheared edge surface is generated under, for example, three different conditions. The three conditions are under external tensile stress in the sheet width direction (Figure 5), under a stress-free state (Figure 6), and under compressive stress in the sheet edge direction (Figure 7). Each of the three conditions is a constraint condition. It can be said that there is not much difference in the residual stress described above in the sheared edge surface under any of these three conditions. Such a stress field around the sheared edge surface before shearing can be generated in actual shearing processing in post-trim due to residual stress caused by deformation and forming of the metal sheet 1 by the sheet presser 12.

[0017] On the other hand, as shown in the schematic diagram of Figure 8, after shearing, springback occurs and the stress in the area around the sheared end surface is released. As a result, if the part other than the sheared end surface is elastically deformed, the sheared end surface will be subjected to additional deformation due to the elastic deformation around the sheared end surface. As a result, the sheared end surface may be subjected to tensile or compressive stress. In Figure 8, the symbol 2A indicates the shear residual stress range, and the symbol 2a indicates the sheared end surface.

[0018] At this time, as shown in FIG. 8(a), when the stress in the area around the sheared end surface decreases from tension to zero, the sheared end surface 2a undergoes elastic deformation of compression. In this case, the risk of delayed fracture at the sheared end surface is reduced because the area is additionally subjected to compressive stress. On the other hand, as shown in FIG. 8(b), when the stress in the area around the sheared end surface does not change, the stress remains sheared and does not change. Also, as shown in FIG. 8(c), when the stress in the area around the sheared end surface increases from compression to zero, the sheared end surface 2a undergoes elastic deformation of tension and is inevitably subjected to tensile stress. Therefore, the risk of delayed fracture at the sheared end surface increases. Consider the elastic deformation caused by the release of stress near the sheared end surface 2a after shearing. This elastic deformation can occur when the metal sheet separates from the sheet clamp 12 and springs back during actual shearing in the post-trim.

[0019] Therefore, by adding tensile stress to the sheared edge 2a in addition to the tensile residual stress that is essentially generated, it becomes possible to accurately evaluate delayed fracture as an index of the risk of delayed fracture. The tensile stress to be added is the tensile stress that is additionally applied to the sheared edge 2a by the elastic deformation of the area around the sheared edge 2a before and after shearing. In addition, when the tensile stress applied to the sheared end surface 2a due to the elastic deformation of the area around the sheared end surface 2a before and after shearing reaches a certain value, the risk of delayed fracture is considered to increase. By taking such a thing into consideration, it becomes possible to make a highly accurate prediction of delayed fracture in the rear trim section.

[0020] Such elastic deformation of the area around the sheared end surface 2a before and after shearing can be obtained by a known forming analysis (CAE) using a computer. Specifically, the stress in the area around the sheared end surface 2a is obtained when the material is restrained by the sheet holder 12 before shearing and just before the upper blade 11a contacts the material. Also, the stress in the area around the sheared end surface 2a is obtained after the sheet holder 12 is released and the metal sheet springs back after shearing is completed. Then, the tensile stress to be added can be obtained by taking the difference between the two stresses. In addition, not only the stress but also the strain may be adopted. In other words, the same evaluation is possible by taking the difference between the strain before and after shearing in the area around the sheared end surface 2a, since the amount of strain to be added is taken into consideration.

[0021] An example of post-trim for an actual three-dimensional part is post-trim for a hat-shaped part as shown in Fig. 2. In this case, when the material is restrained by the sheet clamp 12, compressive deformation and compressive stress occur on the inside of the shoulder R part 1C, which is the bent part, and on part of the vertical wall part 1B. Then, when the metal sheet leaves the sheet clamp 12 after shearing, springback occurs, and the stress is released and reversed to tensile stress. This causes a large change in stress from compression to tension. It is believed that tensile stress is then applied to the sheared end surface 2a, increasing the risk of delayed fracture. Based on these principles and viewpoints, one of the objects of the present invention is to accurately evaluate and predict the risk of delayed fracture in the rear trim portion of a metal plate that has been formed into a three-dimensional shape by press forming.

[0022] The amount of additional stress that must be applied to the sheared end surface 2a to cause delayed fracture can be determined, for example, as follows. First, the amount of stress that must be applied to the sheared end surface trimmed with a normal plane to cause delayed fracture is evaluated, and this is used as, for example, an index for limit judgment. As a method for obtaining this index, it is possible to use an existing method, for example, a delayed fracture test under stress loading conditions such as the four-point bending described in Patent Document 2 or the uniaxial tension described in Patent Document 3.

[0023] Furthermore, the inventors have found that the load of additional tensile stress can be suppressed by suppressing deformation due to springback after post-trim. The additional tensile stress is additional tensile stress on the shear end surface 2a due to elastic deformation of the region around the shear end surface 2a. As a specific means for achieving this, the inventors have found that it is effective to minimize deformation after trimming by using the shearing device described below. The shearing device is a shearing device in which the metal sheet contact surface of the upper blade 11a, the metal sheet contact surface of the lower blade 10a, and the metal sheet contact surface of the sheet holder 12 are designed with reference to the springback shape after post-trim. By referring to such an index, it is possible to design, for example, the conditions of shear processing necessary to suppress delayed fracture.

[0024] (Evaluation method of this embodiment) Next, a processing example of the embodiment based on the present invention will be described. 9, the evaluation method of this embodiment includes a judgment threshold acquisition step S20, a press forming analysis step S21, a constraint analysis step S22, a post-springback analysis step S23, a change amount acquisition step S24, and an evaluation step S25. The processes of the press forming analysis step S21, the constraint analysis step S22, and the post-springback analysis step S23 are performed by computer-aided forming analysis (CAE).

[0025] <Determination threshold acquisition step S20> The judgment threshold acquisition step S20 is a step of acquiring an index value for evaluation (judgment threshold). The index value for evaluation is an index value for evaluating how much stress is applied to the shear end face 2a obtained by trimming the flat surface to cause delayed fracture. This index value can be acquired by using an existing method, for example, a delayed fracture test under stress loading conditions such as four-point bending described in Patent Document 2 or uniaxial tension described in Patent Document 3. 10, the determination threshold acquisition step S20 includes a shearing step S20a, an external stress application step S20b, a placement step S20c in a hydrogen permeation environment, and a determination threshold calculation step S20d. The shearing step S20a, the external stress application step S20b, and the placement step S20c in a hydrogen permeation environment constitute the testing steps.

[0026] [Shearing process S20a] The shearing step S20a is a step of shearing a flat metal plate to form a test piece having a sheared end surface with a cutting line extending on a plane.

[0027] [External stress application process S20b] The external stress application step S20b is a step of applying a preset external stress to the shear end surface of the test piece and restraining it in the loaded state. The stress application method is, for example, tensile stress application or bending stress application. In this case, the method of applying bending stress using a jig is particularly desirable from the viewpoint of simplicity.

[0028] [Installation process S20c in a hydrogen intrusion environment] The hydrogen entry environment placement step S20c is a step of placing the test specimen restrained under a stress load in a predetermined hydrogen entry environment for a predetermined time, and evaluating the occurrence of cracks in the test specimen under that condition. The hydrogen entry environment and placement time are preferably set to conditions that provide an amount of hydrogen entry equivalent to the amount of hydrogen estimated to enter the material to be evaluated under an environment in which the material is actually used. The test specimen is placed in a hydrogen intrusion environment by immersing it in a bathtub containing an acid such as hydrochloric acid or an aqueous solution of NHSCN. The concentration of the acid and the immersion time are set so that the amount of hydrogen that is preset as the allowable upper limit is absorbed into the test specimen. For each test piece prepared in the shearing step S20a, the external stress application step S20b and the placement step S20c in a hydrogen penetration environment are repeatedly performed while changing the conditions of the applied stress.

[0029] [Decision threshold calculation step S20d] In the evaluation value calculation step, a critical load stress is calculated as an index value (judgment threshold value) based on the results of the above test. The critical load stress is a limit load stress at which delayed fracture does not occur on the sheared end surface of the metal plate.

[0030] Through the above process, in the judgment threshold acquisition step S20, the limit load stress (limit external load stress) that does not cause delayed fracture inherent to the sheared end face is obtained as an index value (judgment threshold). This index value is defined as the "reference limit load stress" for the sheared end face at the post-trim. The "reference limit load stress" can change depending on the shear conditions, for example, the settings of the shear angle and die clearance. The judgment threshold value may be set to a value obtained by reducing the reference limit load stress by a safety margin, taking into account a margin.

[0031] The judgment threshold may be set to a value equal to or less than half the yield strength of the material of the metal plate 1. When setting the judgment threshold in this manner, a simple method can be used to set the judgment threshold to a value equal to or less than the reference limit load stress reduced by a safety margin. Instead of stress, the limit external load strain that does not cause delayed fracture may be calculated, and the calculated value may be used as the "reference limit load strain." The "reference limit load strain" may then be used as the index value (judgment threshold value). In this case as well, the same effect can be obtained.

[0032] <Press molding analysis process S21> In the press forming analysis process S21, an analysis of press forming a metal plate into an evaluation object (part 1) having a target part shape is performed by forming analysis using a computer. The evaluation object of the target part shape is a part immediately before performing post-trim. The forming analysis is performed based on the material conditions and press forming conditions in actual press forming. The forming analysis method using CAE may be a conventionally known forming analysis method.

[0033] <Restraint analysis process S22> In the constraint analysis step S22, the evaluation object obtained in the press molding analysis step S21 is subjected to a forming analysis to analyze the state of being constrained under the constraint conditions in shear processing. This determines the stress-strain distribution occurring in the evaluation object. The determined stress-strain distribution is stored as the analyzed stress-strain distribution immediately before shearing (before processing). For the forming analysis, a conventionally known analysis method may be used. The above-mentioned constraint conditions are, for example, constraint conditions when the evaluation target product is constrained by the die 10 and the plate holder 12 when cutting with the lower blade 10a and the upper blade 11a. In addition, a condition for constraining the scrap side with the punch 11 and the second plate holder may be used. The constraint conditions may be the same as those for the shearing process used in the actual after-trim.

[0034] <Post-springback analysis process S23> In the post-springback analysis process S23, following the analysis process under the constraint conditions in the constraint analysis process S22, an analysis is performed of cutting with the lower blade 10a and upper blade 11a. Then, an analysis is performed to exclude the area sheared by post-trim. Furthermore, a springback analysis is performed on the remaining area (area that will become the product). Then, in the post-springback analysis process S23, after the springback analysis, a forming analysis is performed and the stress-strain around the post-trim portion is stored.

[0035] <Change amount acquisition step S24> The stress-strain value around the post-trimmed portion before processing, obtained in the constraint analysis process S22, is compared with the stress-strain value around the post-trimmed portion after processing, obtained in the post-springback analysis process S23. From this comparison, the amount of change in stress or strain at each detection point is calculated. In this embodiment, the difference is calculated as the amount of change. When the difference in stress or strain is a positive value (tensile side), it is considered that there is a high risk of delayed fracture. For points where the difference is positive, the difference in stress or strain amount is recorded as the "delayed fracture driving stress" or "delayed fracture driving strain amount." The recorded value is then used as an index of the risk of delayed fracture.

[0036] <Evaluation process S25> In the evaluation process S25, if the "delayed fracture driving stress" or "delayed fracture driving strain" obtained in the change acquisition process S24 exceeds the "reference limit load stress" or "reference limit load strain", it is determined that there is a high risk of delayed fracture. However, cutting the vertical wall of a part generally results in poorer end surface quality than cutting a flat surface. For this reason, when judging the risk of delayed fracture against the standard limit, it is desirable to provide a margin (safety allowance) on the lower side of the stress or strain.

[0037] Here, the press forming analysis process S21, the restraint analysis process S22, and the post-springback analysis process S23 are executed as a series of forming analysis processes using a computer. Then, the stress-strain distribution of the rear trim part (shear end surface 2a) and its surroundings may be stored. The area around the rear trim part is within 3 mm from the cutting line (shear end surface 2a) of the rear trim, and preferably is a finite element close to the cutting line. In the above description, the case of shearing processing in which a part is sheared after press forming to shape the part has been exemplified as the post-trim. The applicable post-trim portion is not limited to this. The present invention can be similarly applied to general post-trim portions, such as a hole punching trim portion of a part after press forming, or an outer diameter trim portion having a gently uneven portion.

[0038] (Method of manufacturing press-molded products) <First manufacturing method> Assume that a metal plate is press-formed into a part having a target part shape, and then post-trimming is performed to produce a product (press-formed part). Before actual manufacturing, the evaluation method of this embodiment described above is used to evaluate delayed fracture of the sheared end surface 2a of the rear trim portion formed by the rear trim. Then, in the trim portion sheared after press forming, a portion evaluated to be at risk of delayed fracture is identified. The above product is manufactured for the identified portion using a manufacturing method that incorporates a suppression treatment (suppression method) for suppressing delayed fracture.

[0039] Next, examples of suppression methods will be described. The suppression methods shown below may be used in combination. <First suppression method> The first prevention method is to cut the area evaluated as having a risk of delayed fracture and its surroundings in the post-trim using a cutting method that has a low risk of delayed fracture, rather than shearing. An example of a cutting method that has a low risk of delayed fracture is laser cutting. This makes it possible to manufacture a press-formed product in which the risk of delayed fracture in the rear trim portion is reduced. The press-formed product to be manufactured may be, for example, an automobile part. Furthermore, in the case of a manufacturing method that applies this first suppression method, laser cutting or the like is used only on a portion of the rear trim portion. This results in a shorter cutting length than if the entire end face of the rear trim portion were laser cut. This makes it possible to produce parts more efficiently with a manufacturing method that applies the first suppression method.

[0040] <Second suppression method> The second suppression method is, for example, to change the shear processing conditions for a portion evaluated to be at risk of delayed fracture and its surroundings to shear conditions that can further suppress delayed fracture. This is a method of changing the shear conditions of the post-trim for a portion determined to have a high risk of delayed fracture. For example, the shape after post-trim and springback in forming analysis, or the shape after springback of the post-trim portion of an actual press-formed product is obtained. Then, based on the shape, the metal sheet contact surface of the punch 11, the metal sheet contact surface of the die 10, and the metal sheet contact surface of the sheet holder 12 in shear processing are made as close as possible to the above-mentioned obtained shape. In this case, the "delayed fracture driving stress" or the "delayed fracture driving strain amount" is reduced. As a result, it is possible to reduce the risk of delayed fracture. The punch 11 has an upper blade 11a. The die 10 has a lower blade 10a.

[0041] <Third suppression method> The third suppression method is to introduce plastic strain into the area determined to have a high risk of delayed fracture after post-trim. The process of introducing plastic strain is, for example, to introduce plastic strain so that the line length in the shear direction of the sheared end surface 2a is changed by 1% or more compared to the target line length. The sheared end surface 2a into which the plastic strain is introduced is the sheared end surface of the region including the sheared end surface 2a into which the plastic strain is introduced. Here, Patent Document 4 discloses a method and its principle for reducing the residual stress on the sheared end surface 2a by subjecting it to plastic processing by press forming, thereby suppressing the occurrence of delayed fracture. The above-mentioned plastic strain can be introduced by such a known method. Even if the springback stress after post-trim is added to the above-mentioned shear residual stress, plastic deformation is introduced after post-trim, which makes it possible to relieve the stress around the sheared end surface 2a.

[0042] When performing processing that introduces such plastic strain, the trim line TL of the post-trim is changed compared to the part's actual dimensions. In this case, it is sufficient to design the part so that the plastic strain is introduced to the post-trim line TL by restriking after the post-trim, and the final part shape is sufficiently close to the part's actual dimensions. In this way, the amount of additional stress applied to the sheared end surface 2a after post-trimming that will cause delayed fracture can be evaluated and used as an index to determine the amount of stress that will cause delayed fracture to occur when the sheared end surface 2a is trimmed in a normal plane, as described above. Using this index as a reference, it is possible to design a shearing device that is necessary to suppress delayed fracture.

[0043] Furthermore, if plastic strain is applied after post-trim, the stress at the sheared end 2a is relaxed, increasing the critical load stress at which delayed fracture occurs from the sheared end 2a. Therefore, as in Patent Document 5, for example, a delayed fracture test should be performed on the strained sheared end 2a under the stress load conditions described in Patent Documents 2 and 3. In this case, it can be used as an index of the critical load stress at which delayed fracture occurs when plastic strain is applied to the post-trim section. Furthermore, in this example, for locations determined to have a high risk of delayed fracture, the rear trim line TL is selectively changed from the component's final size. This allows plastic strain to be introduced into the rear trim line TL in the restriking after the rear trim. Preferably, the length of the rear trim line TL is changed by 1% or more from the component's final size in an area including locations with a high risk of delayed fracture so that sufficient plastic strain, 0.2% or more, is introduced.

[0044] (program) An example of a program used in the above-described evaluation method will be described below. In advance, the limit value of the stress or strain at which delayed fracture occurs is obtained for the shear end surface 2a of the flat metal plate to be evaluated by a conventionally known method. Then, a judgment threshold value is obtained from the limit value as an index value to be referenced by the program. The obtained judgment threshold value is stored in the storage unit in advance. The judgment threshold value is set to, for example, a value smaller than the obtained limit value by a safety margin. This program is a program for evaluating the delayed fracture properties of a sheared end surface 2a of a metal plate for press forming. Specifically, this program is used in a manufacturing method for a press-formed product. The manufacturing method for a press-formed product is a method for manufacturing a press-formed product by press-forming a metal plate made of a known material into a part having a preset target part shape, and then subjecting it to shear processing. This program is a program for evaluating the delayed fracture properties of a sheared end surface 2a formed by the above shear processing, which is post-trim.

[0045] As described in the above evaluation method, the evaluation method in this embodiment is executed. That is, a forming analysis of press forming is performed. A forming analysis of the subsequent shear processing and the subsequent springback is performed. A process of calculating the difference between the values ​​of the locations obtained by the forming analysis is performed. A process of comparing the calculated difference with a threshold value stored in a storage unit is performed. The above forming analysis processes can be processed by CAE using a conventionally known computer by referring to the conditions of the metal plate, the conditions of press forming, and the conditions of shear processing. In addition, the process of calculating the difference between the values ​​of the locations obtained by the forming analysis and the process of comparing the calculated difference with a threshold value stored in a storage unit are also realized by arithmetic processing using a computer.

[0046] In this manner, the evaluation method of the present embodiment can be realized as a computer that is executed by a computer. An example of such a program is described below. As shown in FIG. 11, the program of this embodiment is made up of a first step S30 to a fifth step S34, and the processes are executed in this order. The first step S30 is a step of performing forming analysis using a computer to press-form a metal plate into a part having a preset target part shape. In the second step S31, a forming analysis is performed using a computer to analyze the part having the target part shape analyzed in the first step S30 while the part is restrained under the restraining conditions for the shearing process. As a result, in the second step S31, the distribution of stress or strain at or around the sheared end surface 2a is calculated.

[0047] The third step S32 is a step of obtaining the springback state of the metal plate after the shearing process is performed on the part having the target part shape analyzed in the first step S30 by forming analysis using a computer, and calculating the distribution of stress or strain at or around the sheared end surface 2a. The fourth step S33 refers to the distribution of each stress or strain obtained in the second step S31 and the third step S32. The fourth step S33 is a step of obtaining a difference in stress or strain at the sheared end surface 2a to be evaluated or its periphery. This step is performed for two states. The two states are the constraint conditions for the shearing process and the state of the metal plate after the shearing process is completed and the constraint conditions of the metal plate are released. The fifth step S34 is a step of determining the presence or absence of delayed fracture from the difference obtained in the fourth step S33. For example, the difference at each location is compared with a threshold value stored in the storage unit to determine locations where the difference is equal to or greater than the threshold value.

[0048] By having such a program, it becomes possible to automatically predict whether there is a risk of delayed fracture in the rear trim portion, and the locations where there is a risk of delayed fracture. It is also possible to display risk assessment information. For example, (difference / threshold) may be displayed as the degree of risk.

[0049] (others) The present disclosure may also have the following configuration. (1) Disclosure 1 is a method for evaluating delayed fracture properties of a shear end surface of a metal plate for press forming, comprising: A sheared end surface generated by shearing a metal sheet after press-forming the metal sheet into a part having a predetermined target part shape is defined as a sheared end surface for evaluating the delayed fracture property, The stress or strain at or around the sheared end surface to be evaluated is obtained in two states: a state of the metal sheet before processing in which the metal sheet after press forming is restrained under the material restraint conditions in the shear processing, and a state of the metal sheet after processing in which shearing is completed by the shear processing and the restraint conditions are released; and the delayed fracture property is evaluated from the change in stress or strain at or around the sheared end surface to be evaluated in the two states obtained. Delayed fracture evaluation method. The change is calculated based on, for example, the difference between the stress or strain calculated in the two states. (2) Disclosure 2 calculates the stress or strain in the above two states by forming analysis using a computer. (3) Disclosure 3 assesses that there is a concern that delayed fracture may occur if the difference in the above stress or strain exceeds the set judgment threshold. (4) Disclosure 4 performs a delayed fracture test in which a tensile stress is applied to a sheared end surface formed by shearing a flat metal plate, thereby determining the limit value of stress or strain at which delayed fracture does not occur, and sets the above judgment threshold value based on the determined limit value. (5) In Disclosure 5, the above judgment threshold value is determined by subtracting a safety allowance from the determined limit value. (6) Disclosure 6 sets the judgment threshold to be equal to or less than half the yield strength of the material of the metal plate. (7) Disclosure 7 discloses a method for manufacturing a press-molded product, in which a metal plate is press-molded into a part having a target part shape, and then the part is subjected to a shearing process to manufacture the press-molded product, In the delayed fracture evaluation method described in the present disclosure, the difference at the sheared end surface generated by the shear processing is obtained, and if the difference exceeds a set judgment threshold, a constraint condition in the shear processing is obtained such that the difference at the point where the difference exceeds the judgment threshold is within the judgment threshold; The above shearing process is carried out under the obtained constraint conditions. A manufacturing method for press-molded products. (8) Disclosure 8 discloses a method for manufacturing a press-molded product, in which a metal plate is press-molded into a part having a target part shape, and then the part is subjected to a shearing process to manufacture the press-molded product, In the delayed fracture evaluation method described in the present disclosure, the difference at the sheared end surface generated by the shear processing is obtained, and if the difference exceeds a set judgment threshold, a process of introducing plastic strain after the shear processing is performed on the part of the sheared end surface where the difference exceeds the judgment threshold. A manufacturing method for press-molded products. (9) Disclosure 9 describes a process for introducing the above-mentioned plastic strain, which is performed so as to change the line length in the shear direction of the shear end face in the region including the shear end face where the above-mentioned difference exceeds the judgment threshold by 1% or more compared to the target line length. (10) Disclosure 10 discloses a method for manufacturing a press-molded product, in which a metal plate is press-molded into a part having a target part shape, and then the part is subjected to a shearing process to manufacture the press-molded product, In the delayed fracture evaluation method described in the present disclosure, the difference at the sheared end surface generated by the shearing process is determined, and if the difference exceeds a set judgment threshold, cutting is performed by laser cutting instead of the shearing process at the location of the sheared end surface where the difference exceeds the judgment threshold. (11) Disclosure 11 is a program for evaluating delayed fracture properties of a sheared end surface formed by the shearing process in a manufacturing method of a press-molded product in which a metal plate is press-molded into a part having a preset target part shape and then sheared to manufacture a press-molded product, A first step of performing a forming analysis using a computer to perform an analysis of press forming a metal plate into a part having a preset target part shape; A second step of performing a forming analysis using a computer to analyze a state in which the part having the target part shape analyzed in the first step is restrained under the restraint conditions for the shearing process, and calculating the distribution of stress or strain at or around the sheared end face; A third step of performing a forming analysis using a computer to analyze the state after the shearing process and springback of the part having the target part shape analyzed in the first step, and calculating the distribution of stress or strain at or around the sheared end face; A program for causing a computer to execute the following. (12) A fourth step of calculating a difference in stress or strain at or near the sheared end surface to be evaluated between two states, a state where the sheared end surface is restrained under the restraining conditions for the shearing process and a state where the shearing process is completed and the shearing process is completed after springback, from the stress or strain distributions calculated in the second step and the third step; a fifth step of determining whether or not delayed fracture occurs based on the difference obtained in the fourth step; Disclosure 11: A program for causing a computer to execute the above. EXAMPLES

[0050] Next, examples based on this embodiment will be described. In this example, a test material made of a 1470 MPa-class steel sheet having a thickness of 1.4 mm is used as the metal sheet to be evaluated. The yield strength of the test material is about 1200 MPa. The present invention is not limited to the material or thickness of such test material. The present invention is applicable to metal plates made of a metal material that generates delayed fracture at the sheared end surface 2a. An example of such a metal material is a high-tensile steel having a tensile strength of 980 MPa or more. The present invention is particularly suitable for steel plates having a tensile strength of 980 MPa or more. In this example, the test material is press-formed into a hat shape as shown in FIG. 2, and then the vertical wall portion 1B including the shoulder R portion 1C, which is the bent portion, is sheared to form a rear trim portion to produce a press-formed product.

[0051] (Regarding the actual manufacturing of press-molded products) First, a metal plate made of the test material was press-formed using a die to produce a hat-shaped part as shown in Fig. 2. This hat-shaped part has a shoulder R part 1C (ridge line part) which is a bent part connecting a top plate part 1A and a vertical wall part 1B. Next, the above press-formed parts were subjected to shearing at the rear trim line TL shown in FIG.

[0052] The shearing process was performed under a restraining condition where the part was restrained by the die 10 having the lower blade 10a and the plate holder 12. Specifically, in this restraining state, the shearing process was performed by moving the punch 11 having the upper blade 11a relative to the lower blade 10a. At this time, the clearance between the upper blade 11a and the lower blade 10a during shearing was set to 10% of the plate thickness. The shapes of the blade tips of the upper blade 11a and the lower blade 10a were set to the same shape as the rear trim line TL, and were designed so that the blades would contact each part of the part to be sheared as simultaneously as possible. In other words, the contact surfaces of the die and plate holder 12 that face the product were shaped to conform as closely as possible to the shape of the product.

[0053] When the product manufactured as described above is released from the die for shearing, spring bag occurs in the product. Next, nine locations on the shear end surface 2a of the rear trim section of the product after springback, namely, A, B, C, D, E, F, G, H, and I shown in Figure 12, were evaluated for delayed fracture. The product after springback is a hat-shaped press-formed product after post-trimming. The hat-shaped press-formed product after post-trimming was immersed in hydrochloric acid of pH 2 for 96 hours. Then, the occurrence of delayed fracture was investigated before and after immersion for each delayed fracture evaluation point A, B, C, D, E, F, G, H, and I. The criterion for the occurrence of delayed fracture here was whether or not a visible crack was present over a length of half or more of the plate thickness.

[0054] Table 1 shows the locations where delayed fracture was evaluated and whether or not delayed fracture occurred. In Table 1, the locations where delayed fracture was evaluated are listed as product locations.

[0055] [Table 1]

[0056] (Calculation of standard stress) In this example, the reference stress (reference limit load stress) serving as the judgment threshold value was calculated based on a known method. The calculation method is as follows. First, a flat test material (metal plate) before press forming was subjected to shearing to prepare a test piece having a sheared end surface extending linearly. The shearing conditions were the same as those for the rear trim. In other words, the shearing process was carried out by moving a punch with an upper blade relative to the lower blade while the test piece was restrained under the restraining conditions of restraining the molded product with a die with a lower blade and a plate holder. The clearance between the upper blade and the lower blade during shearing was set to 10% of the plate thickness. This is the same shearing condition as above. In addition, since the test material was flat, the contact surfaces of the die and plate holder that face the product were also flat.

[0057] Next, the flat cut end face of the test piece was restrained in a state where a load stress was applied to the shear end face of the test piece using four-point bending as described in Patent Document 2. Then, the test piece was subjected to an immersion test. At this time, the test was performed on a plurality of test pieces in which the load stress was changed in increments of 100 MPa. Then, the occurrence of delayed fracture after immersion was investigated, and the limit load stress at which delayed fracture would not occur was obtained. Then, it was confirmed that the limit load stress obtained was 700 MPa. The confirmed load stress was defined as the index value (reference stress) for the occurrence of delayed fracture. However, if it is expected that the properties of the sheared end surface 2a of the vertical cut will be worse than those of the flat cut, the standard stress for the occurrence of delayed fracture may be set lower by a margin (safety margin). For example, a margin of 200 MPa may be provided and the standard stress may be set at 500 MPa. This is to take into account the fact that there will be variations in shear conditions and forming conditions in the actual mass production process.

[0058] Moreover, the yield strength of the ultra-high strength steel used in this study is approximately 1200 MPa. Therefore, if the residual stress is reduced to 600 MPa or less, which is less than the standard stress of 700 MPa, delayed fracture is suppressed. Therefore, a value of half the yield strength or less can be cited as one guideline for the stress reduction required to suppress delayed fracture. In this example, half the yield strength or less is 600 MPa or less.

[0059] (Evaluation of rear trim) Based on the material conditions of the test material and the above-mentioned actual press forming and post-trim shear processing conditions, a forming analysis was performed by computer aided engineering (CAE) to press the test material and perform post-trim. A known method can be used for forming analysis by CAE. The CAE evaluation used seven integration points on the shell element, and the integration points on the front surface (top surface of the part), center surface, and back surface were used as stress evaluation points. The evaluation points correspond to the evaluation points for delayed fracture.

[0060] First, a forming analysis is performed to form the test material into the formed product shown in Fig. 2 under the same press conditions as in the actual press forming. Next, an analysis is performed under the constraint conditions in which the plate pressing force during shearing is applied to the formed product shown in Fig. 2. In other words, an analysis is performed in which the load stress due to the constraint during shearing is applied to the part. Then, the stress in the state before the upper blade 11a contacts the part during shearing is calculated at the above three integration points for each of the part locations A to I. The state before contacting the part is the state before processing where the part is restrained under the constraint conditions during shearing.

[0061] Next, an analysis was performed on the shearing process to remove the scrap elements of the area divided by post-trim while restrained by the sheet clamp 12. Furthermore, a springback analysis was performed on the remaining area (area on the product side). After springback, calculations were performed again at the above three integration points for each of the part locations A to I. After that, the difference between the stress at restraint under the restraint conditions just before shearing and the stress after springback was calculated at each integral point at each location. The stress at restraint under the restraint conditions just before shearing is the stress at restraint at the plate holder 12. The calculated difference was taken as the delayed fracture driving stress of the post-trim shear end face 2a. At this time, it has been confirmed that a similar evaluation is possible even if the change in strain is used as an index of the driving force using the first principal strain or equivalent strain instead of the driving stress.

[0062] The results are shown in Table 2.

[0063] [Table 2]

[0064] Table 2 shows the values ​​found at three integral points on the front surface (top surface side of the part), center surface, and back surface for each of the part locations A to I. The values ​​shown are the delayed fracture driving stress, which is the difference between the stress when the sheet is held down and the stress after post-trim and springback. They are also the results of a CAE determination of whether delayed fracture will occur. However, for simplicity, the calculation results shown here are for each location, at the integral point with the highest delayed fracture driving stress in the three integral transitions.

[0065] The following can be seen from Table 2. That is, in the flange portion 1D of part location A and the top plate portion 1A of part location I, there is little change in shape before and after the plate is clamped. For this reason, the driving stress is small at each of the integration points at those locations. On the other hand, in the shoulder R portion 1C of part locations C and G, the driving force for delayed fracture is very large due to the effects of bending. Also, in the vertical wall portion 1B of part location E and around the R ends of part locations B, D, F, and H, bending back deformation occurs which flattenes the curved shape. For this reason, there is a moderate driving stress at those locations.

[0066] The stress was then compared with 700 MPa to determine whether delayed fracture would occur. 700 MPa is the reference stress (determination threshold) for the occurrence of delayed fracture determined above. In making this determination, it was determined that delayed fracture would occur if any integral point within the plate thickness exceeded the reference stress. The determination results are also shown in Table 2. However, if it is expected that the properties of the sheared end surface 2a of the vertical cut will be worse than those of the flat cut, a margin (allowance) may be provided in the reference stress for the occurrence of delayed fracture, and the reference stress may be set to, for example, 500 MPa.

[0067] As can be seen from a comparison between Tables 1 and 2, the location predicted by CAE (forming analysis) as the occurrence of delayed fracture coincides with the location where delayed fracture occurred in the actual delayed fracture test. Therefore, it was shown that the evaluation based on the present invention can be used to evaluate the shear end surface 2a of the rear trim part for the purpose of predicting the risk of delayed fracture.

[0068] (Measures to prevent delayed fracture of rear trim parts after molding 1) Next, the test material was press-formed into the shape shown in Fig. 13, and then cut along the rear trim line TL shown by the dashed line in Fig. 13. An evaluation test was then carried out to evaluate delayed fracture of the rear trim portion after forming. The evaluation method was the same as that shown in the above-mentioned embodiment. The evaluation test predicted a risk of delayed fracture at the location indicated by arrow 40 in Fig. 13, that is, at the three-dimensional R portion. Furthermore, when an evaluation of delayed fracture was carried out on an actual product, delayed fracture occurred at the same location.

[0069] Therefore, as shown in Figure 14, only the region 50 including the portion where the risk of delayed fracture was predicted (judged) in advance was cut by laser cutting, which has a low risk of delayed fracture, as post-trim. When the delayed fracture of the actual product cut by this laser cutting was evaluated, it was confirmed that the delayed fracture was suppressed in the portion where the risk of delayed fracture was predicted (judged). In other words, it was found that press-formed products such as automobile parts in which delayed fracture is suppressed can be obtained by processing with a minimum laser trimming length. In this way, it was found that by using the method based on the present invention, it is possible to manufacture automobile parts in which delayed fracture is effectively suppressed.

[0070] (Measures to prevent delayed fracture of rear trim parts after molding 2) In this example, a shearing device was used in which the metal sheet contact surface of the punch 11 having the upper blade 11a, the metal sheet contact surface of the die 10 having the lower blade 10a, and the metal sheet contact surface of the sheet holder 12 were designed with reference to the springback shape after post-trim. This was an attempt to minimize deformation after trimming. FIG. 15 is a diagram showing a comparison of the shapes of the trim line TL before and after springback, and a definition of the springback mid-shape die in the embodiment. The inventors focused on the edge of the flange of the metal plate, which has the longest springback distance. Then, referring to the shape during springback, the die after measures were designed so that the flange movement amount ME of the edge of the metal plate is reduced. Specifically, without measures, the flange movement amount ME of the edge was 13.6 mm. In response to this, as a countermeasure, a die of a shearing device consisting of an upper blade 11a, a lower blade 10a, and a plate holder 12 was designed so that the flange movement amount ME was 9 mm, 6 mm, 3 mm, 1 mm, and 0.2 mm, respectively.

[0071] Table 3 shows the delayed fracture driving force when the flange movement amount ME of the metal plate edge part was changed for the location where the delayed fracture occurred, and whether or not the CAE determined that delayed fracture occurred when the standard stress was set to 700 MPa. In this die, the flange movement amount ME of the edge of the metal sheet is equal to the maximum distance among the distances between the metal sheet contact surface of the upper blade 11a, the metal sheet contact surface of the lower blade 10a, and the metal sheet contact surface of the sheet holder 12 and the upper and lower surface shapes of the metal sheet. The state of a movement amount of 0.2 mm is intended to be a case where the flange movement amount ME of the edge of the metal sheet is reduced as much as possible within the dimensional tolerance.

[0072] [Table 3]

[0073] Furthermore, dies corresponding to each flange movement amount ME were fabricated, and test pieces including the rear trim part were subjected to delayed fracture tests. Table 4 shows the presence or absence of delayed fracture.

[0074] [Table 4]

[0075] The following was found from Table 4. That is, the effect of suppressing delayed fracture was partially confirmed when the flange movement amount ME was 9.0 mm or less. On the other hand, when the flange movement amount ME was 3.0 mm or less, delayed fracture was completely suppressed, and it was found that it is possible to reduce the risk of delayed fracture of the shear end surface 2a of the rear trim portion.

[0076] Furthermore, when manufacturing a product by executing the press forming shape and post-trim similar to that shown in Figure 13 above, the method of response 2 of this example was applied. Specifically, the target metal sheet was a metal sheet having a shape in which it had left the sheet holder 12 after completion of shearing and had spring back. Then, measures against delayed fracture were taken in the shearing device of the rear trim so that any of the following metal sheet contact surfaces matched the upper and lower surface shapes of the metal sheet within ±3 mm. The following metal sheet contact surfaces were the metal sheet contact surface of the punch 11 having the upper blade 11a, the metal sheet contact surface of the die 10 having the lower blade 10a, and the metal sheet contact surface of the sheet holder 12. In this case, delayed fracture was suppressed, making it possible to manufacture an automobile part with excellent delayed fracture resistance.

[0077] (Measures to prevent delayed fracture of rear trim parts after molding 3) Furthermore, the inventors performed post-trim on the risky areas where delayed fracture occurred, as shown in Table 2, using a shearing machine in which the post-trim line TL was changed to be 1% or more shorter than the final dimensions of the part, as shown in the schematic diagram of Figure 16. As a result, restriking was performed using a die that was adjusted to approach the final dimensions while introducing plastic deformation by applying a deformation of 1% or more in the restriking after post-trim. In this die, the shapes of the upper and lower dies in the forming process before post-trim were also changed to match the post-trim line TL. In addition, when restriking to the final dimensions after post-trim, an allowance is made in the die so that the part will be the final dimensions after forming.

[0078] Table 5 shows the results of the delayed fracture occurrence determination for the location where the delayed fracture had occurred, based on the amount of plastic strain introduced when restriking after post-trim, the residual first principal stress, and the reference stress of 1,400 MPa when plastic strain is introduced. That is, Table 5 shows the amount of plastic strain introduced when restriking after post-trim to the part locations B, C, F, G, and H, which were at risk of delayed fracture. Furthermore, Table 5 shows the results of delayed fracture occurrence judgment compared with the residual first principal stress and the reference stress of 1400 MPa when plastic strain is introduced. This result is calculated by CAE. The introduction of plastic strain relieves the residual shear stress and the additional external tensile stress introduced by springback after post-trim. Therefore, the occurrence of delayed fracture can be judged by comparing the residual first principal stress with the reference stress when plastic strain is introduced. This CAE study determined that delayed fracture would not occur.

[0079] [Table 5]

[0080] Furthermore, a mold corresponding to Table 6 was fabricated, and a test piece including the end face where the rear trim part was restrike was subjected to a delayed fracture test. Table 4 shows whether or not delayed fracture occurred.

[0081] [Table 6]

[0082] As can be seen from Table 6, delayed fracture did not occur in each area where delayed fracture had occurred without any countermeasures, by implementing Measure 3. As described above, delayed fracture can be prevented by predicting areas at risk of delayed fracture in the rear trim from stress changes during springback and selectively restriking. However, when the rear trim line TL was made 0.5% shorter than the component dimensions, the delayed fracture suppression effect was not achieved. This is thought to be because only elastic deformation occurs at the sheared end surface 2a with a deformation of about 0.5%, and the intended effect of stress relaxation by plastic deformation cannot be fully achieved.

[0083] Furthermore, when manufacturing a product by performing press forming shape and post-trim similar to that shown in Figure 13 above, the method of response 3 of this example was applied. Specifically, we investigated the case where delayed fracture countermeasures were taken in the shear risk area of ​​the rear trim by changing the rear trim line TL from the normal dimension and introducing plastic strain by restriking. The investigation revealed the following. That is, delayed fracture occurred near the R part of the vertical wall part 1B of the rear trim part. On the other hand, when countermeasures were taken, delayed fracture was suppressed, making it possible to manufacture an automotive part with excellent delayed fracture resistance. Furthermore, when no countermeasures were taken, delayed fracture occurred near the shoulder R part 1C of the vertical wall part 1B of the rear trim part.

[0084] The entire contents of Japanese Patent Application No. 2024-064089 (filed on April 11, 2024), from which this application claims priority, are incorporated herein by reference. Although the present application has been described with reference to a limited number of embodiments, the scope of the rights is not limited thereto, and modifications of each embodiment based on the above disclosure would be obvious to one skilled in the art. [Explanation of symbols]

[0085] 1 metal plate 10 Die 10a lower blade 11 Punch 11a Upper blade 12 Board Holder ME Edge flange movement S20: Determination threshold acquisition process S20a Shearing process S20b External stress loading process S20c Installation process in hydrogen intrusion environment S20d Judgment threshold calculation step S21 Press molding analysis process S22 Constraint analysis process S23 Post-springback analysis process S24: Change amount acquisition process S25 Evaluation process S30 1st process S31 Second process S32 Third process S33 Fourth step S34 The fifth step TL Rear trim trim line (cutting line)

Claims

1. A method for evaluating delayed fracture properties of a sheared end surface of a metal plate for press forming, comprising: A sheared end surface generated by shearing a metal sheet after press-forming the metal sheet into a part having a predetermined target part shape is defined as a sheared end surface for evaluating the delayed fracture property, The stress or strain of the sheared end surface or its periphery is calculated in two states: a state of the metal plate before processing in which the metal plate after the press forming is restrained under the material restraint conditions in the shear processing, and a state of the metal plate after processing in which the shearing is completed by the shear processing and the restraint conditions are released; The delayed fracture properties are evaluated based on the change in stress or strain at the shear end surface to be evaluated or its periphery in the two states obtained. Delayed fracture evaluation method.

2. Calculate the stress or strain in the above two states by forming analysis using a computer. The delayed fracture evaluation method according to claim 1.

3. The delayed fracture properties are evaluated based on the difference in stress or strain obtained in the two states. The delayed fracture evaluation method according to claim 1.

4. If the difference in the stress or strain exceeds the set judgment threshold, it is evaluated that there is a risk of delayed fracture occurring. The delayed fracture evaluation method according to claim 3.

5. A delayed fracture test is carried out by applying a tensile stress to a sheared end surface formed by shearing a flat metal plate, thereby determining a limit value of stress or strain at which delayed fracture does not occur, and setting the judgment threshold value based on the determined limit value. The delayed fracture evaluation method according to claim 4.

6. The value obtained by subtracting the safety allowance from the obtained limit value is set as the judgment threshold value. The delayed fracture evaluation method according to claim 5.

7. The judgment threshold is set to half or less of the yield strength of the material of the metal plate. The delayed fracture evaluation method according to claim 4.

8. A method for manufacturing a press-molded product, comprising the steps of: press-molding a metal plate into a part having a target part shape; and then subjecting the part to a shearing process to manufacture the press-molded product, In the delayed fracture evaluation method according to any one of claims 3 to 7, the difference at the sheared end surface generated by the shearing process is obtained, and if the difference exceeds a set judgment threshold, a constraint condition in the shearing process is obtained such that the difference at the point where the difference exceeds the judgment threshold is within the judgment threshold, The above shearing process is carried out under the obtained constraint conditions. A manufacturing method for press-molded products.

9. A method for manufacturing a press-molded product, comprising the steps of: press-molding a metal plate into a part having a target part shape; and then subjecting the part to a shearing process to manufacture the press-molded product, In the delayed fracture evaluation method according to any one of claims 3 to 7, the difference at the sheared end surface generated by the shearing process is obtained, and when the difference exceeds a set judgment threshold, a process of introducing plastic strain after the shearing process is performed on the part of the sheared end surface where the difference exceeds the judgment threshold. A manufacturing method for press-molded products.

10. The process of introducing the plastic strain is performed by introducing plastic strain so that the line length in the shear direction of the shear end face in the region including the shear end face where the difference exceeds the judgment threshold is changed by 1% or more compared to the target line length. A method for producing a press-molded product according to claim 9.

11. A method for manufacturing a press-molded product, comprising the steps of: press-molding a metal plate into a part having a target part shape; and then subjecting the part to a shearing process to manufacture the press-molded product, In the delayed fracture evaluation method according to any one of claims 3 to 7, the difference at the sheared end surface generated by the shearing process is obtained, and when the difference exceeds a set judgment threshold, a portion of the sheared end surface where the difference exceeds the judgment threshold is cut by laser cutting instead of the shearing process. A manufacturing method for press-molded products.

12. A program for evaluating delayed fracture properties of a sheared end surface formed by the shearing process in a method for producing a press-formed product, the method comprising the steps of: press-forming a metal plate into a part having a predetermined target part shape, and then shearing the metal plate to produce a press-formed product; A first step of performing a forming analysis using a computer to perform an analysis of press forming a metal plate into a part having a preset target part shape; A second step of performing a forming analysis using a computer to analyze a state in which the part having the target part shape analyzed in the first step is restrained under the restraint conditions for the shearing process, and calculating a distribution of stress or strain at or around the sheared end face; A third step of performing a forming analysis using a computer to analyze the state after the shearing process is performed on the part having the target part shape analyzed in the first step and springback, thereby calculating the distribution of stress or strain at or around the sheared end face; A program for causing a computer to execute the following.

13. Further, a fourth step of calculating the difference in stress or strain at or around the sheared end surface to be evaluated in two states, a state where the sheared end surface is restrained under the restraining conditions for the shearing process and a state where the shearing process is completed and the shearing process is completed after springback, from the stress or strain distribution calculated in the second step and the third step; a fifth step of determining whether or not delayed fracture occurs based on the difference obtained in the fourth step; 13. The program according to claim 12, for causing a computer to execute the above steps.

Images