Method for evaluating delayed fracture, method for manufacturing press-molded article, and program

Abstract

Provided is a method capable of quantitatively evaluating delayed fracture characteristics of a sheared edge formed by a shearing process of a three-dimensional rear trim section. This method is a method for evaluating delayed fracture characteristics of a sheared edge of a metal sheet for press-molding. In the method for evaluating delayed fracture characteristics, a sheared edge for evaluating the delayed fracture characteristics is defined as one generated by press-molding a metal sheet into a component having a target component shape and then performing a shearing process on the metal sheet, the values of stress or strain at or around the sheared edge to be evaluated are obtained under two conditions: a pre-processing state of the metal sheet where the press-molded metal sheet is constrained by material constraints in the shearing process; and a post-processing state of the metal sheet where the shearing is completed by the shearing process and the constraints on the metal plate are lifted, and the delayed fracture characteristics are evaluated on the basis of the difference between the values of stress or strain obtained under the two conditions.

Description

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

[0001] The present invention relates to a technology relating to a method for evaluating delayed fracture properties of a sheared edge of a metal sheet for press forming. The present invention also relates to a technology relating to a method for manufacturing a press-formed product using the evaluation technology. The present invention is suitable for, for example, manufacturing an automobile part by press-forming a metal sheet made of a high-strength steel sheet and then subjecting the press-formed metal sheet to shearing.

[0002] Currently, automobiles are required to improve fuel efficiency and collision safety through weight reduction. High-strength steel sheets are used in automobile bodies to achieve both weight reduction and occupant protection in the event of a collision. In particular, in recent years, high-strength steel sheets with a tensile strength of 980 MPa or more have been applied to automobile bodies. One of the issues when high-strength steel sheets are applied to automobile bodies is delayed fracture. Delayed fracture occurring at the end surface after shearing is a significant issue, particularly for steel sheets with a tensile strength of 980 MPa or more. The end surface formed by shearing is also referred to as a sheared end surface. Therefore, in this specification, the end surface formed by shearing is also referred to as a sheared end surface.

[0003] It is known that 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 a method for evaluating 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 for evaluating hydrogen embrittlement of a steel material by applying tensile stress to a thin steel plate while charging it with hydrogen.

[0004] Japanese Patent No. 5196926 JP 2014-70927 A JP 2023-72460 A WO2023 / 037961 JP 2023-173358 A

[0005] Here, there are two types of sheared end surfaces that automotive parts have: front trim portions and rear trim portions. 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 rear trim. The rear trim portion is generated, for example, by shearing to shape the part according to the product outline. The shearing process for this rear trim portion occurs, for example, when an extra flange portion is required as a sheet holding portion for draw forming during press forming. The rear trim portion is a portion that is 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-press trimming is performed on parts that have already been formed by press molding. During after-trimming, the part must be restrained by a constraint that presses the sheet metal along the part's shape. Then, after-trimming requires shearing the part in this restrained state. In other words, the shearing process for the after-trim section must be performed along the three-dimensional shape of the part. Therefore, the quality of the sheared edge of the after-trim section may be worse than that of the before-trim section. Furthermore, after-trimming can impose unintended stress on the sheared edge.

[0007] Based on the above findings, the inventors discovered that there is a problem in that the risk of delayed fracture at the sheared edge may be higher in the rear trim section than in the front trim section. For example, a typical three-dimensional rear trim section is formed by shearing the entire vertical wall section 1B, including the hat-shaped shoulder rounded section 1C (see FIG. 2) (see FIG. 2). However, the method described in Patent Document 1 does not particularly focus on evaluating the sheared edge of such a rear trim section. In other words, the method described in Patent Document 1 does not allow for delayed fracture evaluation that takes the above-mentioned risk into account. The above-mentioned risk may be, for example, deterioration of the sheared edge due to the shearing process used to form the three-dimensional rear trim section.

[0008] The present invention has been made in light of the above-mentioned 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 shearing a three-dimensional rear trim portion. Another object of the present invention is to provide a press-formed product with excellent delayed fracture resistance.

[0009] For actual press-formed products, evaluation of delayed fracture associated with the rear trim portion as described above is extremely important. Especially for automotive parts, evaluation of delayed fracture associated with the rear trim portion as described above is extremely important, considering the production process. For this reason, the inventor recognized the need for a method capable of quantitatively evaluating the delayed fracture characteristics of the sheared edge of a three-dimensional rear trim portion. The inventor then focused on the changes in stress distribution and strain around the sheared edge before and after shearing during shearing of a part manufactured by press forming. Based on this focus, the inventor developed the present invention. Here, the stress distribution before shearing refers to the stress distribution around the sheared edge immediately before shearing. "Immediately before shearing" refers to the period from when the metal sheet is restrained by the sheet clamp for shearing until the upper blade contacts the material. The stress distribution after shearing refers to the stress distribution around the sheared edge after the part is sheared and springback occurs.

[0010] In order to solve the problems, one aspect of the present invention is a delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared edge of a metal sheet for press forming, in which the sheared edge generated by shearing the metal sheet after press forming into a part having a predetermined target part shape is used as the sheared edge for evaluating the delayed fracture property, and the delayed fracture property is evaluated by determining stress or strain at or around the sheared edge for evaluation in two states: a state of the metal sheet before processing in which the press-formed metal sheet is constrained under material constraint conditions in the shearing, and a state of the metal sheet after processing in which shearing is completed by the shearing and the constraint conditions are released, and evaluating the delayed fracture property from changes in stress or strain at or around the sheared edge for evaluation in the two states.

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

[0012] FIG. 1 is a schematic diagram illustrating an example of after-trimming. FIG. 2 is a perspective view showing an example of after-trimming the entire vertical wall portion of a hat-shaped press part. FIG. 3 is a diagram showing an example of a stress state during deformation in shearing. (a) is a diagram in the plate thickness direction, and (b) is a diagram in the plate width direction. FIG. 4 is a diagram showing an example of a stress state after separation in shearing. (a) is a diagram in the plate thickness direction, and (b) is a diagram in the plate width direction. FIG. 5 is a diagram explaining shearing in a state where an external tensile stress is applied in the plate width direction. FIG. 6 is a diagram explaining shearing in a state where an external stress is not applied in the plate width direction. FIG. 7 is a diagram explaining shearing in a state where an external compressive stress is applied in the plate width direction. FIG. 8 is a schematic diagram explaining a situation where springback occurs after shearing, releasing stress in a region around the sheared end face, and a portion other than the sheared end face elastically deforms. Specifically, it is a schematic diagram showing a situation where the sheared end face undergoes additional deformation due to elastic deformation around the sheared end face. FIG. 9 is a diagram showing an example of a processing procedure of an evaluation method according to an embodiment based on the present invention. FIG. 10 is a diagram showing an example of a processing procedure of a judgment threshold acquisition step. FIG. 11 is a diagram showing an example of a processing procedure of a program according to an embodiment based on the present invention. 1 is a diagram showing the positions of each component location A to I on the sheared end surface of the rear trim portion; FIG. 2 is a top view showing the location where delayed fracture occurred without any countermeasures in the embodiment; FIG. 3 is a top view showing the portion where countermeasures for suppressing delayed fracture were taken; FIG. 4 is a diagram showing the amount of movement of the edge at the end of the flange portion after rear trimming and springback; and FIG. 5 is a diagram showing the rear trim line (dashed line) at the correct size and the rear trim line (solid line) with a corrected mold.

[0013] Next, an embodiment of the present invention will be described with reference to the drawings. (Concept of the Present Invention) 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 post-trim processing on a metal sheet 1 (part 1) after press forming. In FIG. 1, reference numeral 1a denotes a portion that has a three-dimensional shape due to press forming. Reference numeral TL denotes a cutting line in the post-trim processing. A typical example of a post-trim processing portion is a portion formed by trimming the entire vertical wall portion 1B, as shown in FIG. 2. The vertical wall portion 1B includes a shoulder R portion 1C of a hat-shaped part. In FIG. 2, reference numeral 1A denotes a top plate portion of the hat shape. Reference numeral 1D denotes a flange portion. Reference numeral 3 denotes the scrap side of the metal sheet before post-trim processing, resulting from the shearing process. Reference numeral 2 denotes the product side (press-formed product side) of the metal sheet before post-trim processing, resulting from the shearing process.

[0014] FIG. 3 shows a schematic diagram of the stress state during shearing. FIG. 4 shows a schematic diagram of the stress state after material separation during 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 , during shearing, a 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 material deformation reaches its limit, cracks initiate at the contact points with both or one of the upper and lower blades 11a and 10a, causing the material to separate. Just before this separation, the sheared end surface is subjected to extreme plastic deformation and tensile stress in the thickness direction. Furthermore, after the separation of the material, springback occurs. However, after the separation of the material, the inner portion is more constrained than the undeformed end surface. For this reason, the tensile stress does not completely spring back and cannot be relaxed. As a result, the rear trim section remains in a state where it is subjected to elastic tensile deformation stress in the thickness direction (Fig. 3(a)). Furthermore, the area around the sheared end surface is in a plane strain state where deformation is constrained in the direction perpendicular to the thickness direction (depth direction of the page) (Fig. 3(b)). Therefore, the tensile stress remaining in the thickness direction also results in tensile stress remaining in the width direction.

[0015] As described above, the occurrence of shear residual stress is due to extreme deformation of several tens of percent or more. Therefore, before the shearing process begins, there is a sheared edge around the sheared surface. A very small strain that creates an elastic stress field with this strain has almost no effect. A very small strain is, for example, an elastic strain of less than 0.6%.

[0016] As shown in the schematic diagrams of Figures 5 to 7, the sheared edge occurs under three different conditions. The three conditions are external tensile stress in the sheet width direction (Figure 5), no stress (Figure 6), and compressive stress in the sheet edge direction (Figure 7). Each of the three conditions acts as a constraint. It can be said that there is not much difference in the residual stresses at the sheared edge under any of these three conditions. Such a stress field around the sheared edge before shearing can occur during actual shearing in post-trim due to residual stresses caused by deformation and forming of the metal sheet 1 by the sheet clamp 12.

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

[0018] At this time, as shown in Figure 8(a), if the stress in the region around the sheared edge decreases from tensile to zero, the sheared edge 2a undergoes elastic compressive deformation. In this case, the additional compressive stress reduces the risk of delayed fracture at the sheared edge. On the other hand, as shown in Figure 8(b), if the stress in the region around the sheared edge does not change, the shear stress remains unchanged. Furthermore, as shown in Figure 8(c), if the stress in the region around the sheared edge increases from compressive to zero, the sheared edge 2a undergoes elastic tensile deformation, and is inevitably subjected to tensile stress. Therefore, the risk of delayed fracture at the sheared edge increases. Consider the elastic deformation that accompanies the release of stress near the sheared edge 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 after-trim process.

[0019] Therefore, by taking into account the tensile stress in addition to the tensile residual stress inherently generated at the sheared edge 2a, it becomes possible to accurately evaluate delayed fracture as an indicator of the risk of delayed fracture. The tensile stress to be taken into account is the tensile stress additionally applied to the sheared edge 2a due to the elastic deformation of the area around the sheared edge 2a before and after shearing. Furthermore, it is believed that the risk of delayed fracture increases when the tensile stress additionally applied to the sheared edge 2a due to the elastic deformation of the area around the sheared edge 2a before and after shearing reaches a certain value. By taking these factors into consideration, it becomes possible to accurately predict delayed fracture in the after-trim section.

[0020] Such elastic deformation of the region around the sheared end surface 2a before and after shearing can be calculated using a known computer-aided forming analysis (CAE). Specifically, the stress in the region around the sheared end surface 2a is calculated before shearing, when the material is restrained by the sheet clamp 12 and just before the upper blade 11a contacts the material. The stress in the region around the sheared end surface 2a is also calculated after shearing is completed and the sheet clamp 12 is released, causing the metal sheet to spring back. The tensile stress to be added can then be calculated by taking the difference between the two stresses. Furthermore, strain may be used in addition to stress. In other words, a similar evaluation can be performed by taking the difference between the strain in the region around the sheared end surface 2a before and after shearing, since the amount of strain to be added is taken into account.

[0021] An example of rear trim for an actual three-dimensional part is the rear 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 in the inner side of the shoulder R portion 1C and part of the vertical wall portion 1B, which are the bent portions. Subsequently, when the metal sheet leaves the sheet clamp 12 after shearing, springback occurs, releasing the stress and reversing it to tensile stress. This increases the change in stress from compression to tension. Tensile stress is then applied to the sheared end surface 2a, which is thought to increase the risk of delayed fracture. Based on these principles and perspectives, one of the objectives of the present invention is to accurately evaluate and predict the risk of delayed fracture in the rear trim portion of a metal sheet 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 edge 2 a to cause delayed fracture can be determined, for example, as follows: First, the amount of stress that must be applied to the sheared edge trimmed to a normal flat surface to cause delayed fracture is evaluated, and this is used as an index for limit judgment. This index can be obtained by using existing methods, such as delayed fracture tests under stress loading conditions such as four-point bending described in Patent Document 2 or uniaxial tension described in Patent Document 3.

[0023] Furthermore, the inventors have found that suppressing deformation due to springback after post-trim can suppress the load of additional tensile stress. The additional tensile stress is additional tensile stress on the sheared end surface 2a due to elastic deformation of the region around the sheared end surface 2a. Specifically, the inventors have found that using the shearing device described below is effective in minimizing deformation after trimming. This shearing device is designed with the metal sheet contact surfaces of the upper blade 11a, the lower blade 10a, and the sheet holder 12 in mind, taking into account the springback shape after post-trim. Using these indicators as a reference, it is possible to design shearing conditions necessary to suppress delayed fracture, for example.

[0024] (Evaluation Method of the Present Embodiment) Next, a processing example of the embodiment based on the present invention will be described. As shown in Fig. 9, the evaluation method of the present embodiment includes a judgment threshold value 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 engineering (CAE).

[0025] <Decision Threshold Acquisition Step S20> The decision threshold acquisition step S20 is a step of acquiring an index value (decision threshold) for evaluation. The evaluation index value is an index value for evaluating the amount of stress that must be applied to the sheared end face 2a, which is obtained by trimming the flat surface, before delayed fracture occurs. This index value can be acquired using an existing method, for example, a delayed fracture test under stress loading conditions, such as the four-point bending test described in Patent Document 2 or the uniaxial tensile test described in Patent Document 3. The decision threshold acquisition step S20 includes, for example, a shearing step S20a, an external stress application step S20b, an exposure step S20c in a hydrogen penetration environment, and a decision threshold calculation step S20d, as shown in FIG. 10 . The shearing step S20a, the external stress application step S20b, and the exposure step S20c in a hydrogen penetration environment constitute the testing steps.

[0026] [Shearing Step S20a] The shearing step S20a is a step of shearing a flat metal plate to form a test piece having a sheared end surface along which a cutting line extends on a plane.

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

[0028] [Step S20c of placing in a hydrogen entry environment] The step S20c of placing in a hydrogen entry environment 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 the placement time are preferably set to conditions that result in an amount of hydrogen entry equivalent to the amount of hydrogen that is estimated to enter the material to be evaluated under the environment in which it is actually used. The test specimen is placed in a hydrogen entry environment, for example, in a hydrogen atmosphere containing hydrochloric acid or NH ４The test specimen is immersed in a bath containing an acid solution such as an SCN solution. The concentration of the acid solution and the immersion time are set so that the amount of hydrogen that is pre-set as the allowable upper limit is penetrated into the test specimen. For each test specimen prepared in the shearing step S20a, the external stress application step S20b and the installation step S20c in a hydrogen penetration environment are repeatedly performed by changing the conditions of the applied load stress.

[0029] In the evaluation value calculation step, a critical load stress is calculated as an index value (determination threshold) based on the results of the above test. The critical load stress is the limit load stress at which delayed fracture does not occur at 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) inherent in the sheared end face at which delayed fracture does not occur is obtained as an index value (judgment threshold). This index value is defined as the "reference limit load stress" for the sheared end face in the after-trim. The "reference limit load stress" can change depending on the shear conditions, for example, the shear angle and die clearance settings. The judgment threshold may be a value obtained by reducing the reference limit load stress by a safety margin, taking into account a margin.

[0031] The judgment threshold may also be set to a value equal to or less than half the yield strength of the material of the metal plate 1. In this case, the judgment threshold can be set to a value equal to or less than the reference limit load stress reduced by a safety margin using a simple method. Instead of stress, the limit external load strain at which delayed fracture does not occur may be determined, and the determined value may be used as the "reference limit load strain." Then, the "reference limit load strain" may be used as the index value (judgment threshold). In this case, the same effect can be obtained.

[0032] <Press-forming analysis step S21> In the press-forming analysis step S21, 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 for the target part shape is a part immediately before post-trim is performed. The forming analysis is performed based on the material conditions and press-forming conditions in actual press-forming. Furthermore, the forming analysis method using CAE may be a conventionally known forming analysis method.

[0033] <Constraint Analysis Step S22> In the constraint analysis step S22, a forming analysis is performed to analyze the state in which the evaluation object determined in the press forming analysis step S21 is constrained by the constraint conditions in shearing. 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). A conventionally known analytical method may be used for the forming analysis. The above constraint conditions are, for example, the constraint conditions when the evaluation object is constrained by the die 10 and the sheet holder 12 when cutting with the lower blade 10a and the upper blade 11a. In addition, conditions for constraining the scrap side with the punch 11 and the second sheet holder may also be used. The constraint conditions may be the shearing constraint conditions used in actual post-trim.

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

[0035] <Change Acquisition Step S24> The stress-strain values ​​around the post-trimmed portion before processing, obtained in the constraint analysis step S22, are compared with the stress-strain values ​​around the post-trimmed portion after processing, obtained in the post-springback analysis step 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 is recorded as the "delayed fracture driving stress" or "delayed fracture driving strain." The recorded value is then used as an index of the risk of delayed fracture.

[0036] <Evaluation Step S25> In the evaluation step S25, if the "delayed fracture driving stress" or "delayed fracture driving strain" obtained in the change amount acquisition step 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, when cutting a vertical wall portion of a part, the end surface quality is generally often poorer than when cutting a flat surface. For this reason, when determining the risk of delayed fracture relative to the reference limit amount, it is desirable to provide a margin (safety margin) on the lower side of the stress or strain.

[0037] Here, the press forming analysis step S21, the constraint analysis step S22, and the post-springback analysis step S23 are executed as a series of forming analysis processes using a computer. The stress-strain distribution of the rear trim portion (sheared end surface 2a) and its surroundings is then stored. The area around the rear trim portion is defined as a finite element within 3 mm of the cutting line (sheared end surface 2a) of the rear trim portion, preferably close to the cutting line. The above description exemplifies the case of shearing, in which a part is sheared after press forming to shape the part. Applicable rear trim portions are not limited to this. The present invention can also be applied to general rear trim portions, such as punched trim portions and outer diameter trim portions with gently uneven portions, on parts after press forming.

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

[0039] Next, examples of the suppression method will be described. The suppression methods described below may be used in combination. <First suppression method> The first suppression method is a method in which a portion of the rear trim evaluated as having a risk of delayed fracture and its surrounding area is cut using a cutting method that reduces the risk of delayed fracture, rather than shearing. An example of a cutting method that reduces the risk of delayed fracture is laser cutting. This makes it possible to manufacture a press-formed product with a reduced risk of delayed fracture in the rear trim portion. The manufactured press-formed product may be, for example, an automobile part. Furthermore, in a manufacturing method that applies this first suppression method, laser cutting or the like is applied only to a portion of the rear trim portion. Therefore, the cutting length of the laser cut is shorter than when the entire end face of the rear trim portion is formed by laser cutting. Therefore, a manufacturing method that applies the first suppression method enables more efficient production of parts.

[0040] <Second Suppression Method> The second suppression method involves, for example, changing the shearing conditions for a portion evaluated as having a risk of delayed fracture and its surroundings to shearing conditions during shearing that can more effectively suppress delayed fracture. This method involves changing the shearing conditions for post-trim for a portion determined to have a high risk of delayed fracture. For example, the shape after post-trim and springback in a forming analysis or the shape after springback of the post-trim portion of an actual press-formed product is calculated. Then, based on this shape, the metal sheet contact surfaces of the punch 11, the die 10, and the sheet holder 12 are adjusted as close as possible to the calculated shapes during shearing. In this case, the "delayed fracture driving stress" or "delayed fracture driving strain" is reduced. As a result, the risk of delayed fracture can be reduced. The punch 11 has an upper blade 11a. The die 10 has a lower blade 10a.

[0041] <Third Suppression Method> The third suppression method involves introducing plastic strain into a portion determined to be at high risk of delayed fracture after post-trim. The plastic strain introduction process involves, for example, introducing plastic strain so as to change the line length of the shear end face 2a in the shear direction by 1% or more compared to the target line length. The shear end face 2a into which plastic strain is introduced is the shear end face in the region including the shear end face 2a into which plastic strain is introduced. Patent Document 4 discloses a method and its principle for reducing residual stress at the shear end face 2a by subjecting the shear end face 2a to plastic processing through press forming, thereby reducing the occurrence of delayed fracture. The plastic strain can be introduced using such a known method. Furthermore, even if springback after post-trim adds stress to the shear residual stress, plastic deformation is introduced after post-trim. This method allows stress reduction around the shear end face 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 original dimensions. In this case, the restriking after the post-trim is designed to introduce plastic strain into the post-trim line TL, while ensuring that the final part shape is sufficiently close to the part's original dimensions. In this way, the amount of additional stress applied to the sheared end surface 2a after the post-trim to cause delayed fracture can be determined by evaluating, as described above, the amount of stress applied to the sheared end surface 2a trimmed to a normal plane to cause delayed fracture. This index can be used as a reference for designing a shearing device necessary to suppress delayed fracture.

[0043] Furthermore, if plastic strain is applied after post-trim, the stress at the sheared edge 2a is relaxed, increasing the critical load stress at which delayed fracture occurs from the sheared edge 2a. Therefore, as in Patent Document 5, for example, a delayed fracture test under the stress loading conditions described in Patent Documents 2 and 3 may be performed on the strained sheared edge 2a. In this case, the results can serve as an indicator of the critical load stress at which delayed fracture occurs when plastic strain is applied to the post-trim portion. Furthermore, in this example, for locations determined to be at high risk of delayed fracture, the post-trim line TL is selectively changed from the component's final dimension. This allows plastic strain to be introduced into the post-trim line TL during restriking after post-trim. Preferably, the length of the post-trim line TL is changed by 1% or more from the component's final dimension in areas including locations at high risk of delayed fracture so that sufficient plastic strain, i.e., 0.2% or more, is introduced.

[0044] (Program) An example of a program used in the evaluation method described above is shown. A limit value of stress or strain at which delayed fracture occurs is determined in advance for a sheared edge 2a of a flat metal plate to be evaluated, using a conventionally known method. Then, a judgment threshold is determined as an index value referenced by the program from the limit value. The determined judgment threshold is stored in advance in a storage unit. The judgment threshold is, for example, a value smaller than the determined limit value by a safety margin. This program is a program for evaluating the delayed fracture properties of the sheared edge 2a of a metal plate for press forming. Specifically, this program is used in a method for manufacturing a press-formed product. The method for manufacturing a press-formed product involves press-forming a metal plate made of a known material into a part having a predetermined target part shape, followed by shearing to produce the press-formed product. This program is a program for evaluating the delayed fracture properties of the sheared edge 2a, which is a post-trim, formed by the shearing.

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

[0046] As described above, the evaluation method of this embodiment can be realized as a computer program for executing the method. An example of the program is described below. As shown in FIG. 11, the program of this embodiment comprises 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 a forming analysis using a computer to press-form a metal plate into a part having a predetermined target part shape. In the second step S31, a forming analysis using a computer is performed on the part having the target part shape analyzed in the first step S30, in which the part is constrained under the constraint 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 using a computer-assisted forming analysis to determine the springback state of the metal sheet after shearing the part having the target part shape analyzed in the first step S30. Then, the stress or strain distribution at the sheared end surface 2a or its periphery is calculated. The fourth step S33 refers to the stress or strain distributions determined in the second step S31 and the third step S32. The fourth step S33 is a step of determining the 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 constraint conditions for the shearing and the state of the metal sheet after shearing is completed by the shearing and the constraint conditions for the metal sheet are released. The fifth step S34 is a step of determining the presence or absence of delayed fracture based on the difference determined in the fourth step S33. For example, the difference at each location is compared with a threshold value stored in the storage unit, and locations where the difference is equal to or greater than the threshold value are found.

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

[0049] (Other) The present disclosure may also have the following configurations. (1) Disclosure 1 provides a delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared edge of a metal sheet for press forming, wherein the sheared edge generated by shearing the metal sheet after press forming into a part having a predetermined target part shape is the sheared edge to be evaluated for the delayed fracture property; The method calculates stress or strain at or around the sheared edge to be evaluated in two states: a pre-processing state of the metal sheet in which the press-formed metal sheet is constrained by the material constraint conditions for the shearing, and a post-processing state of the metal sheet in which shearing is completed by the shearing and the constraint conditions are released; and evaluates the delayed fracture property from changes in stress or strain at or around the sheared edge to be evaluated in the two states. The changes are calculated, for example, based on the difference in stress or strain calculated in the two states. (2) Disclosure 2 provides a delayed fracture evaluation method in which the stress or strain in the two states is calculated by forming analysis using a computer. (3) Disclosure 3 assesses that there is a risk of delayed fracture occurring if the difference in stress or strain exceeds a set judgment threshold. (4) Disclosure 4 conducts a delayed fracture test in which tensile stress is applied to the 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 judgment threshold based on the determined limit value. (5) Disclosure 5 determines the judgment threshold as the determined limit value minus a safety margin. (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 provides a method for manufacturing a press-formed product, in which a metal plate is press-formed into a part having a target part shape, and then the part is sheared to manufacture the press-formed product, the method comprising: determining the difference at the sheared end surface caused by the shearing process using the delayed fracture evaluation method described in the present disclosure; and, if the difference exceeds a set judgment threshold, determining constraint conditions for the shearing process such that the difference at the point where the difference exceeds the judgment threshold is within the judgment threshold; and performing the shearing process under the determined constraint conditions.(8) Disclosure 8 discloses a method for manufacturing a press-formed product, in which a metal plate is press-formed into a part having a target part shape, and then the part is sheared to manufacture the press-formed product, the method comprising: determining the difference at a sheared end surface caused by the shearing process using a delayed fracture evaluation method described in the present disclosure; and, if the difference exceeds a set judgment threshold, performing a process of introducing plastic strain after the shearing process on a portion of the sheared end surface where the difference exceeds the judgment threshold. (9) Disclosure 9 discloses a method for introducing plastic strain, in which plastic strain is introduced so as to change the line length in the shear direction of the sheared end surface in a region including the sheared end surface where the difference exceeds the judgment threshold by 1% or more compared to a target line length. (10) Disclosure 10 provides a method for manufacturing a press-formed product in which a metal plate is press-formed into a part having a target part shape, and then the part is subjected to shearing to manufacture the press-formed product, wherein the delayed fracture evaluation method described in the present disclosure determines the difference at the sheared end surface caused by the shearing, and if the difference exceeds a set judgment threshold, the part of the sheared end surface where the difference exceeds the judgment threshold is cut by laser cutting instead of the shearing. (11) Disclosure 11 is a program for evaluating delayed fracture properties of a sheared end surface formed by shearing in a manufacturing method for a press-molded product, in which a metal plate is press-molded into a part having a predetermined target part shape and then sheared to manufacture the press-molded product, the program comprising: a first step of performing a forming analysis using a computer to analyze press-molding of a metal plate into a part having a predetermined 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 constrained by constraint conditions for the shearing, and calculating a stress or strain distribution at the sheared end surface or its periphery; and a third step of performing a forming analysis using a computer to analyze a state after the part having the target part shape analyzed in the first step is sheared by the shearing and springback, and calculating a stress or strain distribution at the sheared end surface or its periphery.(12) The program of disclosure 11 for causing a computer to execute the following steps: a fourth step of determining, from the stress or strain distributions determined in the second step and the third step, a difference in stress or strain at or near the sheared end face to be evaluated between two states: a state restrained by the restraining conditions for the shearing process, and a state after springback when shearing is completed by the shearing process; and a fifth step of determining whether or not delayed fracture occurs from the difference determined in the fourth step.

[0050] Next, an example based on this embodiment will be described. In this example, a test material made of 1470 MPa-class steel with a thickness of 1.4 mm will be used as the metal sheet to be evaluated. The yield strength of the test material is approximately 1200 MPa. Note that the present invention is not limited to such a test material material and thickness. The present invention can be applied to metal sheets 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 with a tensile strength of 980 MPa or more. The present invention is particularly suitable for steel sheets with a tensile strength of 980 MPa or more. In this example, a test material is press-formed into the hat shape 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, thereby producing a press-formed product.

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

[0052] The shearing process was performed under constraint conditions in which the part was restrained by a die 10 having a lower blade 10a and a sheet holder 12. Specifically, in this constraint state, the shearing process was performed by moving a punch 11 having an 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 sheet thickness. The shapes of the cutting edges 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 location on the part to be sheared as simultaneously as possible. In other words, the contact surfaces of the die and sheet holder 12 that face the product were shaped to conform as closely as possible to the product shape.

[0053] When the product manufactured as described above is released from the shearing mold, springback occurs in the product. Next, nine locations on the sheared end surface 2a of the rear trim portion of the product after springback, namely, A, B, C, D, E, F, G, H, and I shown in FIG. 12 , were evaluated for delayed fracture. The product after springback was a hat-shaped press-formed product after rear trimming. The hat-shaped press-formed product after rear trimming was immersed in hydrochloric acid with a pH of 2 for 96 hours. Then, the occurrence of delayed fracture was investigated for each of the delayed fracture evaluation locations A, B, C, D, E, F, G, H, and I before and after immersion. The criterion for the occurrence of delayed fracture here was whether or not a visible crack existed over a length of at least half 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 referred to as product locations.

[0055]

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

[0057] Next, the flat-cut end face of the test specimen was restrained while applying a load stress to the shear end face of the test specimen using a four-point bending test as described in Patent Document 2. The test specimens were then subjected to an immersion test. The test was performed on multiple test specimens with the load stress varied in 100 MPa increments. The occurrence of delayed fracture after immersion was investigated to determine the limit load stress at which delayed fracture did not occur. The determined limit load stress was confirmed to be 700 MPa. This confirmed load stress was defined as the index value (reference stress) for delayed fracture occurrence. However, if the properties of the shear end face 2a of the vertical cut are expected to be worse than those of the flat cut, the reference stress for delayed fracture occurrence may be set lower by a margin (safety margin). For example, a 200 MPa margin may be provided, and the reference stress may be set to 500 MPa. This is to take into account variations in shear conditions and molding conditions that may occur in actual mass production processes.

[0058] Furthermore, 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 or less of the yield strength can be cited as one guideline for the stress reduction required to suppress delayed fracture. In this example, half or less of the yield strength corresponds to 600 MPa or less.

[0059] (Evaluation of rear trim portion) Based on the material conditions of the test material and the actual conditions for press forming and shearing the rear trim, a forming analysis was performed using computer-aided engineering (CAE) to press-form the test material and perform rear trimming. A known method can be used for forming analysis using CAE. For the CAE evaluation, of the seven integration points provided for the shell element, the integration points on the surface (top surface side of the part), center surface, and rear surface were used as stress evaluation points. The evaluation points correspond to the evaluation locations for delayed fracture.

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

[0061] Next, a shearing analysis was performed to remove the scrap-side elements from the area divided by the post-trim while the shearing was restrained by the sheet clamp 12. Furthermore, a springback analysis was performed on the remaining area (the product-side area). After springback, calculations were performed again at the three integration points for each of the component locations A to I. The difference between the stress at restraint under the restraint conditions just before shearing and the stress after springback was then calculated at each integration point for each location. The stress at restraint under the restraint conditions just before shearing is the stress at restraint by the sheet clamp 12. This calculated difference was used as the delayed fracture driving stress of the post-trim sheared edge 2a. It has been confirmed that a similar evaluation can be performed using the first principal strain or equivalent strain, etc., instead of the driving stress, as an indicator of the driving force.

[0062] The results are shown in Table 2.

[0063]

[0064] Table 2 shows the values ​​obtained at three integral points on the surface (top surface side of the part), center surface, and back surface of each of the part locations A to I. The values ​​shown are the delayed fracture driving stress, which is the difference between the stress at the time of sheet clamping and the stress after post-trim and springback. They also show the results of CAE's determination of whether delayed fracture will occur. However, for simplicity's sake, 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 component A and the top plate portion 1A of component I, there is little change in shape before and after the sheet is pressed down. Therefore, the driving stress at these locations is small at each integration point. On the other hand, in the shoulder R portion 1C of component C and G, the delayed fracture driving force is very large due to the influence of bending. Furthermore, in the vertical wall portion 1B of component E and around the R ends of component B, D, F, and H, bending back deformation occurs, flattening the curved shape. Therefore, there is a moderate driving stress at these 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 delayed fracture occurrence determined above. In this determination, delayed fracture was deemed to 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 a vertical cut will be worse than those of a flat cut, a margin (allowance) may be provided in the reference stress for delayed fracture occurrence, and the reference stress may be set to, for example, 500 MPa.

[0067] As can be seen from a comparison of Tables 1 and 2, the locations predicted by CAE (forming analysis) as the occurrence of delayed fracture coincide with the locations where delayed fracture occurred in the actual delayed fracture test. Therefore, it has been demonstrated that the evaluation based on the present invention can be used to evaluate the sheared end surface 2a of the rear trim portion in order to predict the risk of delayed fracture.

[0068] (Measure 1 for preventing delayed fracture in the rear trim portion after forming) Next, a 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 conducted to evaluate delayed fracture in the rear trim portion after forming. The evaluation method was the same as that shown in the above-mentioned example. The evaluation test predicted a risk of delayed fracture at the location indicated by arrow 40 in FIG. 13, i.e., the three-dimensional R portion. Furthermore, when an evaluation of delayed fracture was conducted 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 previously predicted (judged) to be at risk of delayed fracture was post-trimmed by laser cutting, which has a low risk of delayed fracture. When the actual product cut by this laser cutting was evaluated for delayed fracture, it was confirmed that delayed fracture was suppressed in the portion predicted (judged) to be at risk of delayed fracture. In other words, it was found that press-formed products such as automotive parts in which delayed fracture is suppressed can be obtained by processing with a minimum laser trim length. As such, it was found that the method according to the present invention can effectively produce automotive parts in which delayed fracture is suppressed.

[0070] (Measure 2 for preventing delayed fracture of the post-trim portion after forming) 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 based on the springback shape after post-trim. This attempted to minimize deformation after trimming. Figure 15 shows a comparison of the shapes of the trim line TL before and after springback and the definition of the mold for the mid-springback shape in the example. The inventors focused on the edge of the flange portion of the metal sheet, which has the greatest springback distance. Then, based on the shape during springback, a mold for reducing the flange movement amount ME at the edge of the metal sheet was designed. Specifically, without the measures, the flange movement amount ME at the edge was 13.6 mm. To address this issue, the mold for the shearing device, consisting of the upper blade 11a, the lower blade 10a, and the plate holder 12, was designed so that the flange movement amounts ME were 9 mm, 6 mm, 3 mm, 1 mm, and 0.2 mm, respectively.

[0071] Table 3 shows the delayed fracture driving force and the CAE judgment of delayed fracture occurrence when the flange movement ME at the edge of the metal sheet was changed and the reference stress was set to 700 MPa for the position where delayed fracture had occurred. In this die, the flange movement ME at 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 0.2 mm movement is intended to be the case where the flange movement ME at the edge of the metal sheet is reduced as much as possible within the dimensional tolerances.

[0072]

[0073] Furthermore, molds corresponding to each flange movement amount ME were fabricated, and test pieces including the rear trim portion were subjected to delayed fracture tests. Table 4 shows whether or not delayed fracture occurred.

[0074]

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

[0076] Furthermore, the method of Response 2 of this example was applied when a product was manufactured by performing a press-formed shape and post-trim similar to those shown in Figure 13 above. Specifically, the target metal sheet was a metal sheet in a state in which the metal sheet had released the sheet holder 12 and spring back after shearing was completed. Then, measures to prevent delayed fracture were implemented in the post-trim shearing device 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 automotive part with excellent delayed fracture resistance.

[0077] (Measure 3 for preventing delayed fracture in the rear trim portion after forming) Furthermore, the inventors performed post-trimming on the risk areas where delayed fracture occurred, as shown in Table 2, using a shearing machine in which the rear trim line TL was shortened by 1% or more relative to the final part dimensions, as shown in the schematic diagram of Figure 16. This resulted in restriking using a die 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 used in the forming process before post-trimming were also changed to match the rear trim line TL. Furthermore, when restriking to the final dimensions after post-trim, the die was adjusted to allow for the final dimensions of the part after forming.

[0078] Table 5 shows the results of delayed fracture occurrence assessment, comparing the amount of plastic strain introduced during restriking after post-trim with the residual first principal stress and the reference stress of 1400 MPa when plastic strain is introduced, for the locations where delayed fracture had occurred. That is, Table 5 shows the amount of plastic strain introduced during restriking after post-trim for 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 assessment, comparing the residual first principal stress and the reference stress of 1400 MPa when plastic strain is introduced. These results are calculated using CAE. The introduction of plastic strain relieved the residual shear stress and the additional external tensile stress introduced by springback after post-trim. Therefore, the occurrence of delayed fracture can be determined 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]

[0080] Furthermore, a mold corresponding to Table 6 was prepared, and test pieces including the end faces where the rear trim portion had been restriked were subjected to a delayed fracture test. Table 4 shows whether or not delayed fracture occurred.

[0081]

[0082] As can be seen from Table 6, delayed fracture did not occur in each area where it had occurred without any countermeasures, as a result of applying this measure 3. As described above, delayed fracture was prevented by predicting delayed fracture-prone areas in the rear trim section from stress changes during springback and selectively restriking them. Note that when the rear trim line TL was shortened by 0.5% relative to the actual part dimensions, these delayed fracture prevention effects were not achieved. This is thought to be because a deformation of about 0.5% only causes elastic deformation at the sheared end surface 2a, preventing the intended stress relaxation effect from plastic deformation from being fully achieved.

[0083] Furthermore, we applied the method of Response 3 of this example to a product manufactured by executing a press-formed shape and rear trim similar to those shown in Figure 13 above. Specifically, we examined the case where delayed fracture countermeasures were implemented in the shear risk area of ​​the rear trim, including changing the rear trim line TL from the normal size and introducing plastic strain through restriking. The following was found from this investigation: Delayed fracture occurred near the R portion of the vertical wall portion 1B of the rear trim. On the other hand, with the countermeasures in place, delayed fracture was suppressed, making it possible to manufacture an automotive part with excellent delayed fracture resistance. Without the countermeasures in place, delayed fracture occurred near the shoulder R portion 1C of the vertical wall portion 1B of the rear trim.

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

[0085] 1 Metal plate 10 Die 10a Lower blade 11 Punch 11a Upper blade 12 Plate holder ME Flange movement amount of edge portion S20 Judgment threshold acquisition process S20a Shearing process S20b External stress loading process S20c Installation process in hydrogen penetration environment S20d Judgment threshold calculation process S21 Press forming analysis process S22 Constraint analysis process S23 Post-springback analysis process S24 Change amount acquisition process S25 Evaluation process S30 First process S31 Second process S32 Third process S33 Fourth process S34 Fifth process TL Post-trim trim line (cutting line)

Claims

1. A delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared edge of a metal sheet for press forming, comprising the steps of: defining the sheared edge generated by shearing the metal sheet after press forming the metal sheet into a part having a predetermined target part shape as the sheared edge for evaluating the delayed fracture property; determining stress or strain at or around the sheared edge 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 the shearing processing, and the state of the metal sheet after processing, in which shearing is completed by the shearing processing and the constraint conditions are released; and evaluating the delayed fracture property from the change in stress or strain at or around the sheared edge to be evaluated in the two states obtained.

2. A delayed fracture evaluation method as set forth in claim 1, wherein the stress or strain in the two states is calculated by forming analysis using a computer.

3. A delayed fracture evaluation method according to claim 1 or claim 2, wherein the delayed fracture properties are evaluated based on the difference in stress or strain determined in the two states.

4. A delayed fracture evaluation method as set forth in claim 3, wherein if the difference in stress or strain exceeds a set judgment threshold, it is evaluated that there is a risk of delayed fracture occurring.

5. A delayed fracture evaluation method as described in claim 4, in which a delayed fracture test is carried out by applying tensile stress to the 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 setting the judgment threshold value based on the determined limit value.

6. A delayed fracture evaluation method according to claim 5, wherein the value obtained by subtracting a safety margin from the determined limit value is set as the judgment threshold value.

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

8. A manufacturing method for press-formed products in which a metal plate is press-formed into a part having a target part shape, and then the part is sheared to manufacture a press-formed product, the manufacturing method for press-formed products comprising: determining the difference at the sheared end surface caused by the shearing process using the delayed fracture evaluation method set forth in any one of claims 3 to 7; and, if the difference exceeds a set judgment threshold, determining constraint conditions for the shearing process such that the difference at the point where the difference exceeds the judgment threshold is within the judgment threshold; and performing the shearing process under the determined constraint conditions.

9. A method for manufacturing a press-formed product in which a metal plate is press-formed into a part having a target part shape, and then the part is sheared to manufacture the press-formed product, the method comprising: determining the difference at the sheared end surface caused by the shearing process using the delayed fracture evaluation method set forth in any one of claims 3 to 7; and, if the difference exceeds a set judgment threshold, performing a process to introduce plastic strain after the shearing process on the part of the sheared end surface where the difference exceeds the judgment threshold.

10. A method for manufacturing a press-formed product as described in claim 9, wherein the process of introducing the plastic strain is carried out by introducing plastic strain so as to change the line length in the shear direction of the sheared end face in the region including the sheared end face where the difference exceeds the judgment threshold by 1% or more compared to the target line length.

11. A method for manufacturing a press-formed product in which a metal plate is press-formed into a part having a target part shape, and then the part is sheared to manufacture the press-formed product, the method comprising: determining the difference at the sheared end surface caused by the shearing process using the delayed fracture evaluation method set forth in any one of claims 3 to 7; and, if the difference exceeds a set judgment threshold, performing laser cutting instead of shearing on the part of the sheared end surface where the difference exceeds the judgment threshold.

12. A program for evaluating the delayed fracture properties of a sheared edge formed by shearing in a manufacturing method of a press-formed product, in which a metal plate is press-formed into a part having a predetermined target part shape and then sheared, said program having a computer-implemented first step of performing a forming analysis using a computer to analyze the press-forming of a metal plate into a part having a predetermined target part shape; a second step of performing a forming analysis using a computer to analyze the part having the target part shape analyzed in the first step in a state where the part is restrained under the restraint conditions for the shearing, and calculating the stress or strain distribution at or around the sheared edge; and a third step of performing a forming analysis using a computer to analyze the state of the part having the target part shape analyzed in the first step after shearing and springback, and calculating the stress or strain distribution at or around the sheared edge.

13. A program as recited in claim 12 for causing a computer to execute the following steps: a fourth step of determining the difference in stress or strain at or around the sheared end face to be evaluated between two states, one in which the end face is constrained by the constraint conditions for the shearing process and the other in which the shearing is completed by the shearing process and springback has occurred, from the stress or strain distributions determined in the second and third steps; and a fifth step of determining the presence or absence of delayed fracture from the difference determined in the fourth step.

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