Delayed fracture characteristic evaluation method, delayed fracture prediction method, program, press-molded article production method, and shearing device

Abstract

The present invention provides a technique with which it is possible to evaluate the influence of a shear condition for shearing to form a shear end surface on a delayed fracture characteristic of the shear end surface. Provided is a delayed fracture characteristic evaluation method for evaluating a delayed fracture characteristic of a shear end surface of a metal plate (10) made of a high-strength steel sheet, wherein: a test piece of the metal plate (10) having the shear end surface is prepared by shearing the metal plate (10); and then, the delayed fracture characteristic of the shear end surface with respect to a shear condition of the shearing is evaluated by setting the test piece in a hydrogen intrusion environment in a state in which a load stress is applied to the shear end surface of the test piece.

Description

Delayed fracture property evaluation method, delayed fracture prediction method, program, press-molded product manufacturing method, and shearing device

[0001] The present invention relates to a technique suitable for evaluating delayed fracture at a sheared edge of a press-formed product manufactured from a metal plate made of high-strength steel plate, and a technique utilizing the evaluation technique. In this specification, high-strength steel plate refers to a steel plate having a tensile strength of 980 MPa or more.

[0002] Currently, automobiles are required to improve fuel efficiency and crashworthiness through weight reduction. To achieve both weight reduction and occupant protection in the event of a collision, there is a trend toward using high-strength steel sheets for the components that make up the vehicle body. In recent years, ultra-high-strength steel sheets with a tensile strength of 1,470 MPa or more have also been applied to vehicle bodies. One of the issues when using high-strength steel sheets in vehicle bodies is delayed fracture. In particular, for high-strength steel sheets with a tensile strength of 980 MPa or more, delayed fracture that occurs at the edge after shearing is a significant issue. The edge after shearing is also referred to as the sheared edge. It is known that large tensile stress remains at this sheared edge. Therefore, there is concern about the occurrence of delayed fracture at the sheared edge.

[0003] A method for evaluating delayed fracture at a sheared edge is, for example, the method described in Patent Document 1. The method described in Patent Document 1 describes evaluating hydrogen embrittlement of a thin steel plate by injecting hydrogen into a stressed test specimen.

[0004] Patent No. 5196926

[0005] Here, there are two types of sheared end surfaces of automotive parts: front trim and rear trim. The front trim is a portion sheared before press forming. The rear trim is a portion sheared after press forming. The latter, rear trim, occurs, for example, when a sheet clamp is required during draw forming. In this case, for example, after press forming, the excess flange for draw forming is sheared along the product outline (rear trimming) (see Figure 9). In the case of such rear trimming, the pressed part, which has already been shaped by press forming, is restrained by a lower die and a sheet clamp along the part shape. Then, in the case of rear trimming, the shearing process must be performed in this restrained state.

[0006] However, the shape of the constrained press part is not consistent due to variations, such as deviation from the target part shape. This deviation occurs, for example, due to springback. Furthermore, the inconsistent shape often results in insufficient contact between the lower die and the sheet clamp and the part (metal sheet). This can result in areas of the sheet clamp and lower die surfaces that do not make contact with the constrained part (metal sheet). In other words, there is a possibility that gaps will form in the constrained portion during shearing. Based on these findings, the inventors have discovered that the properties of the sheared end surface may differ between pre-trim, which shears a flat metal sheet before press forming, and post-trim, which shears the metal sheet after forming.

[0007] However, conventional evaluation methods for delayed fracture properties, such as those described in Patent Document 1, do not particularly focus on evaluating the sheared edge of such a rear trim portion. Considering the production process of actual press-formed products, particularly mass-produced automobile parts, evaluation of delayed fracture related to such a rear trim portion is extremely important. The present invention has been made with this in mind. An object of the present invention is to provide a technology that can evaluate the influence of shearing conditions in the shearing process that forms the sheared edge on the delayed fracture properties of the sheared edge.

[0008] Based on the above findings, the inventors focused on the difference in shear conditions caused by the degree of improper contact of the metal sheet with the sheet clamp or lower die due to the shape change of the rear trim section before trim forming. The inventors then considered providing a method capable of quantitatively evaluating the influence of the shear edge on the delayed fracture characteristics. Furthermore, the inventors discovered that, as mentioned above, the sheet restraint conditions are important as shear conditions for rear trim.

[0009] To solve the problems, one aspect of the present invention is a method for evaluating delayed fracture properties of a sheared edge of a metal plate made of a high-strength steel plate, which comprises: shearing the metal plate to prepare a metal plate test piece having the sheared edge; placing the test piece in a hydrogen penetration environment while applying a load stress to the sheared edge of the prepared test piece; and evaluating the delayed fracture properties of the sheared edge for the shearing conditions. The shear conditions used as variables may be, for example, constraint conditions of the metal plate.

[0010] According to an aspect of the present invention, the delayed fracture properties of a sheared edge are evaluated according to the shearing conditions of shearing. Therefore, according to an aspect of the present invention, it is possible to evaluate the influence of the shearing conditions during the forming of a rear trim portion on the delayed fracture properties of the sheared edge. Note that this aspect of the present invention is suitable for evaluating the shearing conditions of a sheared edge caused by a rear trim that is cut after press forming. However, the present invention can also be applied to evaluating the shearing conditions of a sheared edge caused by a front trim.

[0011] Here, the shearing conditions during trim forming of the rear trim section vary depending on the state of constraint of the metal sheet during shearing, etc. According to an aspect of the present invention, for example, it is possible to evaluate the critical stress for delayed fracture and the occurrence of delayed fracture by taking into account the state of constraint of the metal sheet during shearing. As a result, according to an aspect of the present invention, it is possible to further improve the delayed fracture properties when applying high-strength steel sheets to various parts such as panel parts, structural / framework parts, etc. of automobiles.

[0012] Furthermore, according to this aspect of the present invention, the shear conditions during shear processing in preparing the test specimen can be adjusted using a spacer, thereby easily simulating the restraint state (shear conditions) during post-trim. As a result, according to this aspect of the present invention, the shear conditions can be easily quantified and changes (adjustments) in the shear conditions can be easily reproduced.

[0013] FIG. 1 is a diagram illustrating a processing flow for evaluating delayed fracture properties according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a case where a lower die having a sheet holder and a lower blade is in contact with a metal sheet, but the sheet holder is separated from the lower blade contact portion in a plan view (shear condition A). FIG. 3 is a diagram illustrating an example configuration of a shearing device for shearing a flat metal sheet, simulating shear condition A. FIG. 4 is a diagram illustrating a case where, in a shear standby state, the sheet holder is not in contact with the metal sheet, but the lower die is in contact with the metal sheet (shear condition B). FIG. 5 is a diagram illustrating an example configuration of a shearing device for shearing a flat metal sheet, simulating shear condition B. FIG. 6 is a diagram illustrating a case where, in a shear standby state, the sheet holder is in contact with the metal sheet, but the lower die is not in contact with the metal sheet near the lower blade contact portion (shear condition C). FIG. 7 is a diagram illustrating an example configuration of a shearing device for shearing a flat metal sheet, simulating shear condition C. FIG. 8 is a diagram illustrating an example processing example of a program according to an embodiment of the present invention. FIG. 9 is a top view showing a rear trim of a press-formed product in an example.

[0014] Next, an embodiment of the present invention will be described with reference to the drawings. (Summary) First, an overview of this embodiment will be described. The inventors focused on the manner in which the sheet clamp contacts the metal sheet, which is the workpiece. They then classified the sheet restraint defects during shearing with post-trim into the following three patterns: A, B, and C. The sheet clamp contact pattern refers to the state in which the metal sheet is restrained by the lower die and the sheet clamp. These defects are likely to occur when a press-formed product is sheared (during post-trim). In other words, the present invention is a technology suitable for evaluating the delayed fracture properties of a sheared edge formed by post-trim.

[0015] A: When both surface 2B of sheet holder 2 and surface 1B of lower die 1 are in contact with metal sheet 10 and restraining it, but sheet holder 2 is separated from lower blade 1A in plan view (see Figure 2). B: When surface 2B of sheet holder 2 is not in contact with metal sheet 10 in a shear standby state, but surface 1B of lower die 1 is in contact with metal sheet 10 in a shear standby state (see metal sheet 10 indicated by a dashed line in Figure 4). C: When surface 2B of sheet holder 2 is in contact with metal sheet 10 in a shear standby state, but surface 1B of lower die 1 is not in contact with metal sheet 10 near lower blade 1A (see Figure 6).

[0016] To simulate these defects (plate restraint states) of contact A, B, and C, the inventors used a spacer 4 interposed between the surface 1B of the lower die 1 and the surface 2B of the plate clamp 2 (see Figure 3, etc.). The inventors then considered reproducing these defects (plate restraint states) by adjusting the position and thickness of the spacer 4. Furthermore, the thickness H (height H) of the spacer 4 and the distance L from the lower blade 1A in a plan view were adjusted to quantitatively characterize the degree of defect as a shearing condition. In this specification, "plan view" refers to a view from the shearing direction. In the case of shearing to prepare test specimens, the shearing direction and the opposing direction of the surface 1B of the lower die 1 and the surface 2B of the plate clamp 2 are the same.

[0017] Then, a flat metal plate 10 was sheared under various shear conditions simulating the above-mentioned defective state to prepare test specimens. The test specimens thus prepared were then placed in a hydrogen penetration environment for a predetermined time while an external stress was applied to the test specimens by four-point bending or the like. In this embodiment, the occurrence of delayed fracture was evaluated based on the placement time.

[0018] Next, referring to the above evaluation, the range of defective conditions for the sheet clamp that do not cause delayed fracture for a specific stress load was determined for each of categories A, B, and C. The specific stress load is, for example, the load applied to the sheared edge after shearing. Next, the post-trim process of the press-formed product is carried out under the determined shear conditions that do not cause delayed fracture. This enabled the production of press-formed products with excellent delayed fracture resistance at the sheared edge. Note that for processing conditions other than the shear conditions, the delayed fracture resistance at the sheared edge can be evaluated separately.

[0019] Next, an example of this embodiment will be described with reference to the drawings. (Delayed fracture property evaluation method) This embodiment is a delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared end surface of a metal plate 10. The present invention is particularly effective when the metal plate 10 is a high-strength steel plate. In this embodiment, the delayed fracture property evaluation process for the metal plate 10 to be evaluated comprises a testing step involving actual experiments and an evaluation step. As shown in FIG. 1 , the delayed fracture property evaluation method of this embodiment comprises a first step 50A to a fourth step 50D. The first step 50A to the third step 50C correspond to the testing step. The fourth step 50D corresponds to the evaluation step.

[0020] <First Step 50A> The first step 50A is a step of shearing a flat metal plate 10 to prepare a test specimen having a sheared end surface. In this embodiment, various test specimens are prepared by adjusting the shearing conditions of the shearing. In this embodiment, the plate constraint conditions during shearing are exemplified as the adjusted shear conditions. Evaluation of delayed fracture properties for shear conditions may be performed using other shear conditions, such as the amount of clearance, as variables. In this embodiment, the shear conditions are quantified (variableized) using a spacer 4 for each classification of plate constraint failure conditions that are likely to occur during post-trim, and test specimens are prepared. The test specimens are, for example, rectangular in shape to facilitate evaluation. Furthermore, the plate constraint failure conditions are classified into three categories, A, B, and C, as described above.

[0021] [Preparation of test pieces for shear condition A (classification A)] Under the poor restraint condition of shear condition A, as shown in Figure 2, the opposing surface 2B of the sheet holder 2 and surface 1B of the lower die 1 come into contact with the metal sheet 10, firmly restraining the metal sheet 10. However, in plan view, the sheet holder 2 is separated from the position of the lower blade 1A. Note that surface 2B of the sheet holder 2 and surface 1B of the lower die 1 are surfaces facing the metal sheet 10 to be restrained.

[0022] 2, in the actual production process of press-molded products, such as mass production, the metal sheet 10 is firmly restrained by the surface 1B of the lower die 1 and the surface 2B of the plate holder 2. However, in plan view, there is a separation distance L' between the plate holder 2 and the lower blade 1A. Note that in this specification, the separation direction refers to the direction in which the upper blade 3A and the lower blade 1A face each other in plan view. The same applies hereinafter.

[0023] 2 simulates a state in which, in a shear standby state, there is an area in the metal sheet 10 near the contact point of the lower blade 1A where the metal sheet 10 and the surface 2B of the sheet holder 2 are not in contact in a plan view. In this state, the distance L' between the sheet holder 2 and the lower blade 1A indicates, in a plan view, the distance from the lower blade 1A at which the surface 2B of the sheet holder 2 is not in contact with the metal sheet 10. This distance L' can be a variable for the degree of defect (constraint condition).

[0024] To simulate the imperfect contact conditions (constraint conditions) of this type, spacers 4 were used to reproduce these imperfect conditions (see Figure 3). The thickness H and position of the spacers 4 were adjusted to provide quantitative measurement of the degree of imperfection. The position was adjusted by adjusting the separation distance L. Similarly, for shear conditions B and C described below, the use of spacers 4 also provided quantitative measurement of the degree of imperfection. The material used for the spacers 4 is preferably a metal-based material, which allows negligible deformation of the spacers 4. However, other materials, such as hard rubber, can also be used as the spacer 4 material. The spacers 4 are also flat, with a constant thickness along the separation direction.

[0025] FIG. 3 is a schematic diagram of a shearing apparatus for reproducing the shearing conditions under shearing condition A during shearing to prepare test pieces. The shearing apparatus shown in FIG. 3 includes a lower die 1 having a lower blade 1A at its shoulder, an upper die 3 having an upper blade 3A at its shoulder, and a sheet clamp 2. The lower die 1 is, for example, a die, and the upper die 3 is, for example, a punch. The shearing apparatus is configured so that the surface 1B of the lower die 1 and the surface 2B of the sheet clamp 2 can restrain the metal sheet 10 during shearing. With the metal sheet 10 restrained, the upper blade 3A is moved relative to the lower blade 1A in the shearing direction to shear the metal sheet 10. At this time, the clearance ΔC between the upper blade 3A and the lower blade 1A is set to a predetermined value, and shearing is performed.

[0026] The shearing apparatus shown in Fig. 3 is a shearing apparatus for producing test pieces. Therefore, as shown in Fig. 3, the surface 1B of the lower die 1 that contacts the flat metal plate 10 and the surface 2B of the plate holder 2 are both flat surfaces. Furthermore, the shearing apparatus shown in Fig. 3 is configured such that a spacer 4 is interposed between the metal plate 10 and the surface 2B of the plate holder 2. The spacer 4 is a flat member. The spacer 4 can move toward and away from the lower blade 1A along the surface 2B of the plate holder 2.

[0027] In the shearing apparatus shown in Figure 3, at least one of the distance L of the spacer 4 from the lower blade 1A in a plan view and the height H of the spacer 4 is set as a variable (adjustment amount) in shearing condition A. In other words, the distance L and height H of the spacer 4 set the distance by which the sheet holder 2 is separated from the metal sheet 10 near the contact point of the lower blade 1A with the metal sheet. By adjusting the distance L and height H of the spacer 4 as variables in shearing condition A, it is possible to produce test pieces for each variable value in shearing condition A. The units of the distance L and height H are [mm].

[0028] Here, in the shearing process under shearing condition A, the thickness H of the spacer 4 is preferably set to, for example, 0.5 mm or more, taking into consideration the influence of defective sheet holding. There is no particular upper limit to the thickness H, but it is set to, for example, 10 mm. Furthermore, in order to determine the defective state, it is preferable that the separation distance L of the spacer 4 is set to, for example, 1 mm or more.

[0029] [Preparation of Test Pieces for Shear Condition B (Classification B)] The poor restraint condition for shear condition B is when, as shown in FIG. 4 , surface 2B of the sheet clamp 2 does not contact the metal sheet 10 in the shear standby state, but surface 1B of the lower die 1 contacts the metal sheet 10. The metal sheet 10 shown by the dashed-dotted line is in the shear standby state. The metal sheet 10A shown by the solid line is in the sheared state. The restraint state shown in FIG. 4 occurs in actual production processes for press-molded products, such as mass production, due to factors such as insufficient sheet clamp load. In other words, the restraint state shown in FIG. 4 simulates a state in which the sheet clamp 2 floats above the metal sheet 10 in the standby position. FIG. 4 simulates a state in which the metal sheet 10 is sheared between the lower blade 1A and the upper blade 3A from that state. In the restrained state of FIG. 4, the gap (opposing distance) between the surface 2B of the sheet holder 2 and the metal sheet 10 during shear standby can be a variable for the degree of defect corresponding to the floating of the sheet holder 2.

[0030] FIG. 5 shows a schematic diagram of a shearing apparatus for reproducing the shearing condition B in shearing for preparing test pieces. In FIG. 5, the metal plate 10 shown by the dashed line is in a shear-ready state, while the metal plate 10A shown by the solid line is in a shearing state. The shearing apparatus shown in FIG. 5 includes a lower die 1 having a lower blade 1A at its shoulder, an upper die 3 having an upper blade 3A at its shoulder, and a sheet clamp 2. The lower die 1 is, for example, a die, and the upper die 3 is, for example, a punch. The shearing apparatus is configured so that the surface 1B of the lower die 1 and the surface 2B of the sheet clamp 2 can restrain the metal plate 10 during shearing. Then, with the plate restrained, the upper blade 3A is moved relative to the lower blade 1A in the shearing direction. This shears the metal plate 10. At this time, the clearance ΔC between the upper blade 3A and the lower blade 1A is set to a predetermined value, and shearing is performed.

[0031] The shearing apparatus shown in Figure 5 is a shearing apparatus for producing test pieces. Therefore, as shown in Figure 5, surface 1B of lower mold 1, which contacts flat metal plate 10, and surface 2B of plate holder 2 are flat. Furthermore, in order to reproduce the shearing condition B, the shearing apparatus shown in Figure 5 has a spacer 4, having a thickness H greater than the thickness of metal plate 10, inserted between surface 2B of plate holder 2 and surface 1B of lower mold 1. The spacer 4 is positioned so as not to contact (interfere with) metal plate 10.

[0032] The shearing device shown in Figure 5 can simulate the degree to which the metal sheet 10 bounces up during shearing by adjusting the height H of the spacer 4. In other words, the thickness H of the spacer 4 is adjusted as a variable that indicates the degree of defect, i.e., the degree to which the sheet clamp 2 is raised. In shearing under shearing condition B, it is desirable that the thickness H of the spacer 4 be at least 0.2 mm thicker than the thickness of the metal sheet 10, taking into account the influence of defects in the sheet clamp 2. However, the evaluation of a normal restraint state may also include a case in which the thickness H of the spacer 4 is equal to the thickness of the metal sheet.

[0033] [Preparation of Test Pieces for Shear Condition C (Classification C)] As shown in FIG. 6 , the poor restraint condition for shear condition C occurs when the surface 2B of the sheet clamp 2 contacts the metal sheet 10, but the surface 1B of the lower die 1 does not contact the metal sheet 10 near the lower blade 1A. In FIG. 6 , the symbol H′ indicates the gap between the surface 1B of the lower die 1 and the metal sheet 10 near the lower blade 1A. The symbol S in FIG. 6 also indicates the portions where the surface 1B of the lower die 1 and the surface 2B of the sheet clamp 2 contact the metal sheet 10 in the shear standby state. In FIG. 6 , the surfaces of the surface 1B of the lower die 1 and the surface 2B of the sheet clamp 2 that restrain the metal sheet 10 are not flat. The reason for this is as follows. That is, FIG. 6 simulates the state in which the metal sheet 10 is restrained after press forming. That is, Figure 6 is a schematic diagram showing that the surface 1B of the lower mold 1 and the surface 2B of the plate holder 2 are shaped to conform to the shape of the metal plate at the restraint position of the press-molded product that is restrained during shearing.

[0034] That is, the state shown in Figure 6 is a state in which the metal sheet 10 is firmly constrained by the surface 1B of the lower die 1 and the surface 2B of the sheet clamp 2 in an actual production process such as mass production. However, Figure 6 simulates the assumption that the constrained parts have a three-dimensional shape. Furthermore, Figure 6 simulates a state in which a gap exists between the surface 2B of the sheet clamp 2 and the metal sheet 10, or between the surface 1B of the lower die 1 and the metal sheet 10. In other words, the state shown in Figure 6 simulates a state in which the metal sheet 10 and the surface 1B of the lower die 1 do not come into contact with each other near the lower blade 1A during shear standby due to errors during molding or the like. In this state shown in Figure 6, the degree of defect can be determined by the distance L' between the contact portion S of the lower die 1 and the contact portion of the lower blade 1A at the contact portion S of the lower die 1 and the metal sheet 10 near the contact portion S of the upper blade 3A in a plan view.

[0035] FIG. 7 is a schematic diagram of a shearing apparatus for reproducing the shearing conditions under shearing condition C during shearing to prepare test pieces. The shearing apparatus shown in FIG. 7 includes a lower die 1 having a lower blade 1A at its shoulder, an upper die 3 having an upper blade 3A at its shoulder, and a sheet clamp 2. The lower die 1 is, for example, a die, and the upper die 3 is, for example, a punch. The shearing apparatus is configured such that, during shearing, the surface 1B of the lower die 1 and the surface 2B of the sheet clamp 2 can restrain the metal sheet 10 in the thickness direction. Then, with the metal sheet 10 restrained, the upper blade 3A is moved in the shearing direction relative to the lower blade 1A. This shears the metal sheet 10. At this time, the clearance ΔC between the upper blade 3A and the lower blade 1A is set to a predetermined value, and shearing is performed.

[0036] The shearing apparatus shown in Figure 7 is a shearing apparatus for producing test pieces. In shearing for producing test pieces, the metal plate 10 has a flat shape. Therefore, the surface 1B of the lower mold 1 and the surface 2B of the plate holder 2 shown in Figure 7 are flat surfaces. In the shearing apparatus shown in Figure 7, a spacer 4 is inserted at least partially between the surface 1B of the lower mold 1 and the metal plate 10 to reproduce the shearing condition C.

[0037] In this state, the distance L of the spacer 4 from the lower blade 1A and the thickness H of the spacer 4 are adjusted in plan view. This expresses how far the lower die 1 is separated from the metal sheet 10 near the contact point of the lower blade 1A. In other words, the distance L and the thickness H can be adjusted as variables for the degree of defect under shearing condition C. Here, in shearing under shearing condition C, the thickness H of the spacer 4 is preferably 0.5 mm or more, taking into account the influence of sheet clamping defects. Furthermore, the distance L of the spacer 4 is preferably 1 mm or more, taking into account the influence of sheet clamping defects. Furthermore, in the first step 50A, multiple test pieces are prepared individually for each of the shearing conditions A, B, and C.

[0038] <Second Step 50B> The second step 50B is a step of applying a predetermined external load stress to the shear end face of each of the various test specimens prepared in the first step 50A and restraining the test specimens in this 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. This loading method may be any known method. For example, stress is applied to a test specimen having a shear end face by four-point bending. Stress adjustment is preferably performed using a strain gauge previously attached to the metal plate 10. Alternatively, the load may be axial tension load, etc. Here, it is preferable to vary the applied stress in sufficiently small increments, such as 100 MPa, and prepare multiple test specimens for each variable (adjustment amount) of the shear condition.

[0039] <Third step 50C> In the third step 50C, the test specimen, which has been subjected to an external load stress in the second step 50B and restrained in that state, is placed in a predetermined hydrogen entry environment for a predetermined time. Then, in the third step 50C, the test specimen in that state is evaluated for crack occurrence. The hydrogen entry environment and the exposure 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 under evaluation in the actual environment in which it will be used.

[0040] The test piece is placed in a hydrogen penetration environment, for example, using hydrochloric acid or NH ４This is done by immersing the test specimen 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 penetrates the test specimen. Then, for each test specimen prepared in the first step 50A and the second step 50B, the above-mentioned third step 50C to fourth step 50D are repeatedly performed while changing the conditions of the applied load stress. In this way, by setting multiple load levels of the external load stress, it is possible to determine the limit value of the load stress at which delayed fracture does not occur for each variable of the shear condition. This limit value of the load stress is also referred to as the delayed fracture limit stress.

[0041] <Fourth step 50D> In the fourth step 50D, multiple combinations of data are obtained between the variables of the shear conditions in the first step 50A and the limit values ​​of the load stress at which delayed fracture does not occur (delayed fracture limit stress) obtained in the third step 50C. The multiple combinations of data are obtained individually for each of the shear conditions A, B, and C. By referring to the multiple combinations of data, information on the correlation between the variables of the shear conditions and the corresponding limit values ​​of the load stress is obtained for each of the shear conditions A, B, and C.

[0042] The correlation information may be obtained by a known calculation method. For example, the correlation information may be a multiple regression model or a model formula based on machine learning. A plurality of combinations of data may be used as the correlation information. By obtaining the correlation information in advance in this way, it is possible to predict whether delayed fracture will occur in an actual press-formed product or in a shape of a press-formed product to be formed, for each shear condition.

[0043] Specifically, by referring to the correlation information, it is possible to predict whether delayed fracture will occur based on the shear conditions during trimming and the load stress applied after trimming. The load stress is, for example, the residual stress when the metal sheet 10 is press-formed after shearing. The load stress may also be the load stress after trimming, which is the residual stress when the metal sheet 10 is press-formed plus the assembly stress of the press-formed parts. This load stress after trimming is also called forming stress. Conversely, if the upper limit of the load stress after forming the sheared edge (after post-trim) is determined, it is possible to address this by determining the range of shear conditions that can be applied to post-trim.

[0044] As described above, it is advisable to design the part shape and the shearing conditions for the shearing of the rear trim so as to prevent delayed fracture by determining the risk of delayed fracture in advance. In this case, it is possible to manufacture a press-formed product having a sheared edge with excellent delayed fracture resistance. When a single press-formed product contains both a rear trim and a front trim, the front trim may be evaluated separately from the rear trim. Here, residual stresses and other characteristics of the press-formed product after trimming can be calculated using conventional CAE forming analysis. When the shearing conditions are different at multiple locations on the sheared edge, the occurrence of delayed fracture can be evaluated individually for each of the multiple locations or for a representative location, using the shearing conditions of each location as a variable.

[0045] (Method for Manufacturing Press-Molded Product) When manufacturing a press-molded product that includes a shearing process (post-trim process) in which a press-molded part is sheared, it is preferable to manufacture the press-molded product as follows. That is, the delayed fracture property evaluation method of this embodiment is used to determine shear conditions in the post-trim process that evaluate the press-molded product as not causing delayed fracture. Then, the shearing process for the post-trim is performed under the determined shear conditions to manufacture the press-molded product.

[0046] Alternatively, the delayed fracture property evaluation method of this embodiment is used to determine limit values ​​(ranges) of variables of shear conditions that are permissible when manufacturing a target press-formed product. Then, the variables of the shear conditions for shearing for post-trim are adjusted so that they are equal to or less than the determined limit values ​​(within the range). Here, when shearing (post-trim) is performed after press forming, the external load stress is, for example, residual stress due to further press forming after the shearing for post-trim. Furthermore, the external load stress is, for example, assembly stress that occurs when the press-formed part is assembled to another structure.

[0047] (Program) An example of a program used in the delayed fracture property evaluation method described above is shown below. The program of this example stores, in a storage unit 40 (see FIG. 8 ), information 40A relating to the correlation between variables of shear conditions during shearing and the limit value of the external load stress that can be applied to the sheared end face (delayed fracture limit stress). The variables of the shear conditions are the height H and the separation distance L of the spacer 4 for each shear condition. That is, the correlation information is calculated and stored individually for each of shear conditions A, B, and C. The correlation information 40A can be calculated by the processing of the delayed fracture property evaluation method described above. The program of this example is a program for executing a process that refers to the correlation information 40A and calculates the limit value of the load stress that can be applied after shearing, corresponding to the input variables of the shear conditions during shearing.

[0048] Another example of the program is a program for causing a computer to execute a first processing step and a second processing step. The first processing step executes a process of determining a limit value of the load stress corresponding to the input variables of the shearing conditions during shearing by referring to the correlation information 40A. The second processing step executes a process of determining whether or not delayed fracture has occurred based on the forming stress, which is the load stress when press-forming the metal sheet 10 into the shape of the press-formed product, and the limit value of the load stress determined in the first processing step.

[0049] Next, an example of processing by a program of the evaluation method described above will be described with reference to Fig. 8. If evaluation is performed using the processing shown in Fig. 8, delayed fracture can be evaluated more efficiently. The example shown in Fig. 8 includes a correlation information calculation unit 20, an evaluation main unit 30, and a storage unit 40. The programs that perform the processing of the correlation information calculation unit 20 and the evaluation main unit 30 are stored in the storage unit 40, such as RAM or ROM, of a computer and executed by the computer.

[0050] <Storage Unit 40> The storage unit 40 is composed of a recording medium such as a database. The processing of the first step 50A to the fourth step 50D is repeated while variously changing the material conditions of the metal plate 10, the hydrogen environment conditions, the shear conditions, and the amount of forming strain. As a result, data on the load stress d calculated for the variables of the shear conditions is stored in the storage unit 40, using the test conditions as variables for each of the material conditions of the metal plate 10, the hydrogen environment conditions, and the shear conditions during shearing. The storage unit 40 also stores information 40A on the calculated correlation. The correlation information 40A may be stored in the same medium as the program storage location.

[0051] <Correlation Information Calculation Unit 20> First, in step S10, the correlation information calculation unit 20 prompts the operator to input the basic conditions for evaluation, such as the type of material (steel type and thickness H) and the hydrogen environment conditions (acidity and installation time), which are conditions for delayed fracture. The correlation information calculation unit 20 then acquires the above inputs through the operator's input operation. Next, in step S20, the operator is prompted to input variables for the shearing conditions during shearing, and acquires the above inputs through the operator's input operation. The variables for the shearing conditions are, for example, the thickness H and separation distance L of the spacer 4.

[0052] Next, in step S30, a data group of load stresses for the variables of each shear condition that matches the conditions input in steps S10 and S20 is acquired from the storage unit 40. The data group is a collection of data (variables of shear conditions, load stresses). This data group is collected separately for each shear condition A, B, and C. Alternatively, the operator is prompted to input a data group of load stresses for the variables of each shear condition determined by testing, and the input information is acquired through the operator's input operation. The acquired data is stored in the storage unit 40.

[0053] Next, in step S40, the data group of load stresses relative to the shear condition variables acquired in step S30 is referenced, and correlation detection processing is performed based on a known processing method. Specifically, in step S40, a calculation process is performed to determine the load stress d as a function f(x) with the shear condition variable x as a variable. Next, in step S50, the function of load stress d determined in step S40 is changed to the following equation, which takes into account a safety factor s (: 0<s≦1): d = s f(x). Then, information on the determined function of load stress d (correlation information 40A) is stored in memory unit 40 using the test conditions as a key.

[0054] <Evaluation Main Unit 30> First, in step S100, the evaluation main unit 30 prompts the operator to input the conditions for the type of material (steel type and thickness H) to be evaluated and the conditions for the hydrogen environment (acidity and installation time), which are the conditions for delayed fracture. The evaluation main unit 30 then acquires the above inputs through the operator's input operation. Next, in step S110, the operator is prompted to input the value of the variable x, which is a shearing condition during shearing, and the load stress g, which is applied to the metal plate 10 after the sheared end face is formed. The evaluation main unit 30 then acquires the above inputs through the operator's input operation. These pieces of information may be acquired from analysis information from a CAE analysis.

[0055] In step S120, correlation information 40A consisting of the function "s·f(x)" of the load stress d that matches the conditions input in step S100 is obtained from the storage unit 40. Then, the limit value d of the load stress corresponding to the value of the variable x of the input shear condition is calculated. A process of outputting the limit value d of the load stress may be executed.

[0056] In step S130, the limit value d (= s·f(x)) of the load stress calculated in step S120 is compared with the load stress g applied to the metal plate 10 after the sheared edge has been formed. This comparison is then used to determine whether there is a risk of delayed fracture. The load stress g is, for example, a forming stress. Here, in step S130 of FIG. 8, it is determined whether there is a risk of delayed fracture, but the margin of the load stress up to delayed fracture (= d−g) may also be output.

[0057] In the above description, the correlation information calculation unit 20 calculates correlation information 40A, such as d = f(x) or d = s f(x) for a variable x of the shear condition. Alternatively, correlation information 40A may be calculated as a variable x = h(d) of the limit shear condition, using the load stress d as a variable. In this case, in step S120, the correlation information 40A is referenced to calculate the limit value of the shear condition corresponding to the load stress d after the planned sheared edge formation. Then, in step S130, the value of the variable of the shear condition during shearing is compared with the calculated limit plastic strain x. Whether or not there is a risk of delayed fracture is determined based on this comparison.

[0058] (Effects) In this embodiment, it is possible to set the critical stress for delayed fracture or to determine the occurrence of delayed fracture by taking into account the shear conditions in the rear trim of a press-formed product. As a result, in this embodiment, it is possible to improve delayed fracture resistance when applying high-strength steel sheets to various parts such as panel parts, structural / frame parts, etc. of automobiles.

[0059] (Other) The present disclosure may also have the following configurations: (1) Disclosure 1 is a delayed fracture property evaluation method for evaluating the delayed fracture property of a sheared edge of a metal plate made of a high-strength steel plate, comprising: subjecting the metal plate to a shearing process to prepare a metal plate test piece having the sheared edge; and placing the prepared test piece in a hydrogen penetration environment while applying a load stress to the sheared edge of the test piece, thereby evaluating the delayed fracture property of the sheared edge under the shearing conditions of the shearing process.

[0060] (2) Disclosure 2 prepares a plurality of test pieces with different shear conditions for the shearing process as the test piece, and evaluates the delayed fracture properties of the metal plate for each shear condition. (3) Disclosure 3 determines the limit value of the load stress at the sheared edge after shearing under the shear conditions, at which delayed fracture does not occur in a hydrogen penetration environment, and evaluates the delayed fracture properties of the sheared edge based on the determined limit value of the load stress.

[0061] (4) Disclosure 4, based on the delayed fracture property evaluation method of the present disclosure, determines in advance the correlation between the shearing conditions and the limit value of the load stress, and evaluates the allowable load stress for the shearing conditions based on the correlation. (5) Disclosure 5, the shearing conditions are constraint conditions for the metal plate in the shearing process. (6) Disclosure 6, the shearing process is performed by shearing the metal plate with the lower blade and upper blade in a state where the metal plate is constrained by a lower mold having a lower blade at a shoulder and a plate holder, and inserting a spacer between the constrained metal plate and the plate holder, and changing the shearing conditions by changing at least one of the distance from the lower blade to the spacer and the thickness of the spacer.

[0062] (7) Disclosure 7 discloses that the shearing process is performed by shearing the metal plate with the lower blade and upper blade while the metal plate is restrained by a lower mold having a lower blade at a shoulder portion and a plate holder, and by inserting a spacer thicker than the thickness of the metal plate between the lower mold and the plate holder at a position where the spacer does not come into contact with the metal plate, the opposing distance between the lower mold and the plate holder that restrain the metal plate is specified, and by changing the thickness of the spacer, the shearing conditions are changed.

[0063] (8) In Disclosure 8, the shearing process is performed by restraining the metal plate with a lower mold having a lower blade at a shoulder portion and a plate holder, and shearing the metal plate with the lower blade and upper blade, and by inserting a spacer between the restrained metal plate and the lower mold, and by changing at least one of the conditions of the distance from the lower blade to the spacer and the thickness of the spacer, the shearing conditions are changed.

[0064] (9) Disclosure 9 provides a method for predicting whether or not delayed fracture will occur due to shearing in a press-formed product manufactured by press-forming a metal plate, the press-formed product being manufactured by a step of shearing the press-formed product, the method comprising: determining in advance a correlation between the shearing conditions of the shearing and a limit value of the load stress based on the delayed fracture property evaluation method of the present disclosure; and predicting whether or not delayed fracture will occur in the press-formed product due to the shearing of the press-formed product based on the correlation. (10) Disclosure 10 provides a method for predicting whether or not delayed fracture will occur in the press-formed product due to the shearing of the press-formed product.

[0065] (11) Disclosure 11 provides a program for causing a computer to execute a process of determining the limit value of the applicable load stress corresponding to the input shear condition by referring to the stored correlation information, wherein the information on the correlation between the shear condition of the shear processing and the limit value of the applied load stress, which is determined based on the delayed fracture property evaluation method of the present disclosure, is stored in a storage unit.

[0066] (12) Disclosure 12 is a program for predicting whether or not delayed fracture will occur due to shearing in a press-formed product manufactured by a shearing process on a press part manufactured by press-forming a metal plate, wherein information on the correlation between the shearing conditions of the shearing process and the limit value of the load stress determined based on the delayed fracture property evaluation method of the present disclosure is stored in a storage unit, and the program causes a computer to execute: a first processing step of determining the limit value of the load stress corresponding to the input shearing conditions by referring to the stored correlation information, and a second processing step of determining whether or not delayed fracture will occur based on a forming stress, which is the load stress applied to a sheared end face formed by the shearing process of the press part after the shearing process, and the limit value of the load stress determined in the first processing step. (13) Disclosure 13 is a program for predicting whether or not delayed fracture will occur in a press-formed product manufactured by a shearing process on a metal plate, the program causing a computer to execute: a first processing step of determining the limit value of the load stress corresponding to the input shearing conditions by referring to the stored correlation information, and a second processing step of determining the limit value of the load stress determined in the first processing step.

[0067] (14) Disclosure 14 is a manufacturing method of a press-formed product produced by a process of shearing a press part manufactured by press-forming a metal plate, the manufacturing method of a press-formed product comprising: determining shear conditions under which the press-formed product is evaluated not to undergo delayed fracture by the delayed fracture property evaluation method of the present disclosure; and performing the shearing under the determined shearing conditions. (15) Disclosure 15 is a manufacturing method of a press-formed product, the shearing conditions being constraint conditions for the metal plate in the shearing.

[0068] (16) Disclosure 16 is a shearing device that shears a metal plate to manufacture a test piece for evaluating delayed fracture properties, the shearing device comprising a lower die having a lower blade on a shoulder portion, a plate holder, and an upper die having an upper blade on a shoulder portion, and configured to shear the metal plate with the lower blade and the upper blade while the metal plate is restrained by the lower die and the plate holder, and having a spacer interposed between the metal plate and the plate holder.

[0069] (17) Disclosure 17 is a shearing device that shears a metal plate to manufacture a test piece for evaluating delayed fracture properties, the shearing device comprising a lower die having a lower blade on a shoulder portion, a plate holder, and an upper die having an upper blade on a shoulder portion, and configured to shear the metal plate with the lower blade and the upper blade while the metal plate is restrained by the lower die and the plate holder, and a spacer is interposed between the lower die and the plate holder at a position where it does not come into contact with the metal plate, and the thickness of the spacer is greater than the thickness of the metal plate.

[0070] (18) Disclosure 18 is a shearing device that shears a metal plate to manufacture a test piece for evaluating delayed fracture properties, the shearing device comprising a lower die having a lower blade on a shoulder portion, a plate holder, and an upper die having an upper blade on a shoulder portion, and configured to shear the metal plate with the lower blade and the upper blade while the metal plate is restrained by the lower die and the plate holder, and having a spacer interposed between the metal plate and the lower die.

[0071] Next, an example based on this embodiment will be described. In this example, test material A made of 1470 MPa-class steel with a plate thickness of 1.4 mm will be described as the metal plate to be evaluated. However, the present invention is not limited to such a metal plate. The present invention can be suitably applied to metal materials, including ultra-high tensile strength steels with a tensile strength of 980 MPa or more, which are likely to cause delayed fracture at the shear edge.

[0072] In this example, a test piece having a length of 120 mm and a straight sheared end surface was prepared for evaluation by shearing using the following die setup. The clearance between the upper blade 3A and the lower blade 1A during shearing was 12% of the thickness of the test piece. However, for comparison, normal shearing conditions without the spacer 4 were also performed. The normal shearing conditions are conditions in which the lower die and the sheet clamp both restrain the metal sheet in surface contact.

[0073] (Preparation of Test Piece for Example A) In order to reproduce the situation under shearing condition A, shearing was performed using a shearing apparatus in which a spacer 4 was inserted between the surface 2B of the sheet holder 2 and the metal sheet 10, as shown in FIG. 3 . In this state, the distance L of the spacer 4 from the lower blade 1A and the thickness H of the spacer 4 were adjusted as variables. A 590 MPa-class steel plate material was used as the material for the spacer 4. In this Example A, the height H of the spacer 4 was fixed at 2 mm. Furthermore, the distance L of the spacer 4 from the lower blade 1A was varied as a variable under shearing condition A, to 2 mm, 4 mm, and 6 mm. In this manner, a test piece for Example A was prepared.

[0074] (Preparation of Test Pieces for Example B) In order to reproduce the shearing conditions B, an apparatus was used in which a spacer 4 having a thickness greater than the thickness of the metal sheet 10 was provided between the surface 2B of the sheet holder 2 and the surface 1B of the lower mold 1, as shown in FIG. 5 . The spacer 4 was provided in a position where it did not contact the metal sheet 10. Then, in the shearing standby state, the shearing process was performed in a state where the metal sheet 10 did not contact the surface 2B of the sheet holder 2. In Example B, the thickness H of the spacer 4 was adjusted to adjust the shearing conditions. In Example B, the thickness H of the spacer 4 was adjusted to 1.9 mm, 2.4 mm, and 3.4 mm. This corresponds to a state where gaps of 0.5 mm, 1.0 mm, and 2.0 mm, respectively, were formed between the metal sheet 10 and the surface 2B of the sheet holder 2.

[0075] (Preparation of Test Piece for Example C) In order to reproduce the situation under shearing condition C, shearing was carried out using an apparatus in which a spacer 4 was inserted at least partially between the surface 1B of the lower die 1 and the metal plate 10, as shown in Fig. 7. In this state, the distance L of the spacer 4 from the lower blade 1A and the thickness H of the spacer 4 were adjusted. In Example C, the thickness H of the spacer 4 was set to 2 mm, and the distance L of the spacer 4 from the lower blade 1A was changed to 2 mm, 4 mm, and 6 mm.

[0076] (Evaluation of the sheared edge of the test piece itself) The evaluation here was performed by visually inspecting the sheared surface. Under each condition of shear condition A, the sheared edge was in a state almost identical to that under normal shear conditions, and the spacer 4 had almost no effect. Furthermore, under each condition of shear condition B, due to the metal sheet 10 being released from the sheet holder 2, the metal sheet 10 was sheared while jumping up toward the sheet holder 2. As a result, the sheared surface was not perpendicular to the sheet thickness but oblique. Furthermore, the sheared edge was also in an unstable state, with secondary shear surfaces partially occurring in the extension direction. This tendency was stronger as the height H of the spacer 4 increased.

[0077] Furthermore, under shearing condition C, the metal sheet 10 was sheared while being subjected to bending deformation toward the lower die 1. As a result, the sheared end surface had an end surface property elongated toward the lower die 1. However, when the spacer 4 was 6 mm or more away, the metal sheet 10 underwent plastic deformation due to bending toward the lower die 1, which was presumed to be too severe a shearing condition.

[0078] (Application of Load) Next, stress was applied to each of the prepared test specimens by four-point bending. The stress was measured using a strain gauge previously attached to the metal plate 10. The stress was then measured and adjusted by multiplying the strain amount by 205 GPa, which is the Young's modulus. Here, multiple test specimens were prepared by changing the applied stress in 100 MPa increments for each test specimen under the same shear conditions.

[0079] (Placement in a hydrogen penetration environment) Next, each test piece to which a load was applied was placed in a hydrogen penetration environment and the occurrence of delayed fracture was investigated. That is, the test piece in a stress-loaded state was immersed in hydrochloric acid of pH 3.5 for 96 hours, and if cracks occurred on the sheared edge after immersion, it was determined that delayed fracture had occurred. (Evaluation of delayed fracture properties) Here, under normal shear conditions without the spacer 4, the critical stress for delayed fracture was 1200 MPa. Here, this was used as the evaluation standard.

[0080] Example A Table 1 shows the results of Example A when the height H of the spacer 4 was 2 mm.

[0081]

[0082] As can be seen from Table 1, when the height H of the spacer 4 was 2 mm and the distance L from the lower blade 1A was 0.5 mm, there was no difference from the normal shearing conditions without the spacer 4. However, when the distance L was 1 mm or more, it was found that the delayed fracture critical stress tended to decrease.

[0083] Table 2 shows the results of Example A when the separation distance L of the spacer 4 is 6 mm.

[0084]

[0085] As can be seen from Table 2, when the separation distance L of the spacer 4 was 6 mm and the height H of the spacer 4 was 0.5 mm, there was no difference from the normal shear conditions without the spacer 4. However, when the height H of the spacer 4 was 1 mm or more, the delayed fracture critical stress decreased.

[0086] Example B Table 3 shows the evaluation results of Example B. In Table 3, D is the gap between the surface 2B of the sheet holder 2 and the metal sheet 10.

[0087]

[0088] Table 3 shows the delayed fracture critical stress when the height H of the spacer 4 was changed to 1.5 mm, 1.7 mm, 1.9 mm, 2.4 mm, and 3.4 mm. As can be seen from Table 3, the delayed fracture critical stress decreased when the height H was 1.7 mm or greater. The larger the height H of the spacer 4, the lower the delayed fracture critical stress. In other words, the larger the gap between the metal plate 10 and the surface 2B of the sheet holder 2, the lower the delayed fracture critical stress. In this example, the sheet thickness of the test specimen was 1.4 mm. Therefore, the gap between the surface 2B of the sheet holder 2 and the metal plate 10 was calculated by subtracting 1.4 mm from the height H of the spacer 4.

[0089] [Example C] Table 4 shows the evaluation results of Example C. Table 4 shows an example in which the height H of the spacer 4 was kept constant (2 mm) and the separation distance L of the spacer 4 was changed.

[0090]

[0091] Table 4 shows the results when the height H of the spacer 4 was set to 2 mm and the distance L of the spacer 4 from the lower blade 1A was changed to 0 mm, 0.5 mm, 1 mm, 2 mm, 4 mm, and 6 mm. When the distance L was 0, the spacer 4 itself functioned as the lower die 1, so the critical stress did not change. However, when the distance L was 0.5 mm or more, the delayed fracture critical stress decreased as the distance L increased.

[0092] Table 5 shows the results when the separation distance L of the spacer 4 is kept constant (2 mm) and the height H is changed.

[0093]

[0094] Table 5 shows the delayed fracture critical stress when the separation distance L of the spacer 4 is 2 mm and the height H is changed. As can be seen from Table 5, a decrease in the delayed fracture critical stress is observed when the height H is 0.1 mm or more, and it was found that the delayed fracture critical stress decreases as the height H increases.

[0095] [Overall Evaluation] From the results in Tables 1 to 5 above, when considering conditions under which delayed fracture does not occur based on a maximum load stress of 1000 MPa, the following was found. In this example, shearing condition A indicated that shearing was sufficient as long as the sheet holder 2 was within 6 mm of the contact point of the lower blade 1A and the distance from the metal sheet 10 was within 2 mm. Shearing condition B indicated that shearing was sufficient as long as the sheet holder 2 was within 0.5 mm of the metal sheet 10 (i.e., the height H of the spacer 4 was 1.9 mm or less). Shearing condition C indicated that shearing was sufficient as long as the gap between the contact point of the lower blade 1A and the metal sheet 10 and the surface 1B of the lower die 1 was within 2 mm and the distance from the metal sheet 10 was within 2 mm.

[0096] FIG. 9 shows an example of a rear trim part. In FIG. 9 , the position indicated by the reference numeral 12 is the shearing position. This part was sheared at the position indicated by the reference numeral 12. Shearing was performed under the following conditions D and E. Under condition D, a gap of 2 mm was provided between the surface 2B of the sheet clamp 2 and the metal sheet 10. Under condition E, the gap between the surface 2B of the sheet clamp 2 and the metal sheet 10 was set to 0.5 mm or less, and the gap between the metal sheet 10 and the surface 1B of the lower mold 1 was also set to 1 mm or less, resulting in almost complete contact. Condition D corresponds to the case where the height H of the spacer 4 in Table 3 is 3.4 mm. Condition E corresponds to the case where the height H of the spacer 4 in Table 3 is less than 1.9 mm at the locations where the metal sheet 10 is not in contact with the upper and lower molds 1 and 3. Furthermore, even when the metal sheet 10 is not in contact with only the lower mold 1, the separation distance L in Table 4 is less than 1 mm. Therefore, it is considered that condition E satisfies the condition for preventing delayed fracture when the maximum load stress is 1000 MPa.

[0097] However, in the post-trimming section, no strain is introduced after the sheared edge is generated. Therefore, the effect of stress relaxation at the sheared edge due to plastic strain can be ignored. Therefore, the delayed fracture threshold stresses in the sheared state shown in Tables 1, 2, and 3 can be applied. On the other hand, when processing strain is applied after shearing, there is an effect of relaxation of the shear residual stress due to plastic strain at the sheared edge. Therefore, depending on the amount of processing strain, the present invention may not necessarily be applicable.

[0098] Table 6 shows the residual stresses determined by CAE analysis of representative locations A to J (not shown) in the rear trim portion of the part in Figure 9, and the delayed fracture test results under the above conditions D and E.

[0099]

[0100] As can be seen from Table 6, there were some locations where delayed fracture occurred under condition D. On the other hand, under condition E, delayed fracture did not occur because shearing was performed under appropriate conditions using the delayed fracture evaluation method described above. In other words, under condition E, a press-formed product having a rear trim edge surface with excellent delayed fracture resistance was obtained. This demonstrates the validity of the evaluation of delayed fracture properties based on the present invention.

[0101] The entire contents of Japanese Patent Application No. 2023-072154 (filed April 26, 2023), 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 would be obvious to those skilled in the art.

[0102] REFERENCE SIGNS LIST 1 Lower die 1A Lower blade 1B Surface for restraining metal plate 2 Plate holder 2B Surface for restraining metal plate 3 Upper die 3A Upper blade 4 Spacer 10 Metal plate 20 Correlation information calculation unit 30 Evaluation main body unit 40 Storage unit 40A Correlation information 50A First step 50B Second step 50C Third step 50D Fourth step H Thickness (height) of spacer L Spacer separation distance

Claims

1. A delayed fracture property evaluation method for evaluating the delayed fracture property of a shear end surface of a metal plate made of a high-strength steel plate, comprising: The metal plate is subjected to a shearing process to prepare a metal plate test piece having the sheared end surface; A load stress is applied to the sheared end surface of the prepared test piece, and the test piece is placed in a hydrogen penetration environment. Evaluating the delayed fracture characteristics of the sheared end surface with respect to the shearing conditions of the shearing process; Delayed fracture property evaluation method.

2. As the test piece, a plurality of test pieces having different shear conditions for the shear processing are prepared, Evaluate the delayed fracture properties of the above metal plates for each shear condition. A delayed fracture property evaluation method according to claim 1.

3. The limit value of the load stress at which delayed fracture does not occur in a hydrogen intrusion environment at the sheared end surface after shearing is obtained for the above shearing conditions, Based on the obtained limit value of the load stress, the delayed fracture property of the shear end surface is evaluated.

3. A method for evaluating delayed fracture properties according to claim 2.

4. a correlation between the shear condition and the limit value of the load stress is obtained in advance based on the delayed fracture property evaluation method according to claim 3; Using this correlation, the allowable load stress for the above shear conditions is evaluated. Delayed fracture property evaluation method.

5. The shearing conditions are constraint conditions for the metal plate during the shearing process. A delayed fracture property evaluation method according to any one of claims 1 to 4.

6. The shearing process is performed by shearing the metal plate with the lower blade and the upper blade while the metal plate is restrained by a lower die having a lower blade at a shoulder portion and a plate holder, A spacer is inserted between the metal plate to be restrained and the plate holder, The shearing condition is changed by changing at least one of the conditions of a distance between the lower blade and the spacer and a thickness of the spacer. A delayed fracture property evaluation method according to claim 5.

7. The shearing process is performed by shearing the metal plate with the lower blade and the upper blade while the metal plate is restrained by a lower die having a lower blade at a shoulder portion and a plate holder, a spacer having a thickness greater than the thickness of the metal plate is inserted between the lower die and the plate holder at a position where the spacer does not come into contact with the metal plate, thereby defining a facing distance between the lower die and the plate holder that restrains the metal plate; By changing the thickness of the spacer, the shear condition is changed. A delayed fracture property evaluation method according to claim 5.

8. The shearing process is performed by shearing the metal plate with the lower blade and the upper blade while the metal plate is restrained by a lower die having a lower blade at a shoulder portion and a plate holder, A spacer is inserted between the restrained metal plate and the lower die; The shearing condition is changed by changing at least one of the conditions of the distance between the lower blade and the spacer and the thickness of the spacer. A delayed fracture property evaluation method according to claim 5.

9. A method for predicting the occurrence of delayed fracture due to shear processing in a press-formed product produced by press-forming a metal plate, the method comprising: a correlation between the shearing conditions of the shearing process and the limit value of the applied stress is obtained in advance based on the delayed fracture property evaluation method according to claim 3; By using the correlation, it is possible to predict whether or not delayed fracture of the press-formed product will occur due to shear processing of the press part. A method for predicting delayed fracture.

10. The shearing conditions are constraint conditions for the metal plate during the shearing process. The method for predicting delayed fracture according to claim 9.

11. information on the correlation between the shearing conditions of the shearing process and the limit value of the load stress, which is obtained based on the delayed fracture property evaluation method according to claim 3, is stored in a storage unit; A program for causing a computer to execute a process for determining the limit value of the load stress that can be applied corresponding to the input shear conditions by referring to the stored correlation information.

12. A program for predicting the occurrence of delayed fracture due to shear processing in a press-formed product produced by press-forming a metal plate, the program comprising: information on the correlation between the shearing conditions of the shearing process and the limit value of the load stress, which is obtained based on the delayed fracture property evaluation method according to claim 3, is stored in a storage unit; On the computer, a first processing step of determining a limit value of the load stress corresponding to the input shear condition by referring to the stored correlation information; a second processing step of determining whether or not delayed fracture occurs based on a forming stress, which is a load stress applied to a sheared end surface formed by the shearing process of the press part after the shearing process, and the limit value of the load stress obtained in the first processing step; A program for executing the above.

13. The shearing conditions are constraint conditions for the metal plate during the shearing process.

13. The program according to claim 11 or 12.

14. A method for manufacturing a press-molded product, which includes a step of shearing a press part manufactured by press-molding a metal plate, determining shear conditions under which the press-formed product is evaluated as not undergoing delayed fracture by the delayed fracture property evaluation method according to any one of claims 1 to 4; The shearing process is carried out under the determined shearing conditions. A manufacturing method for press-molded products.

15. The shearing conditions are constraint conditions for the metal plate during the shearing process. A method for producing a press-molded product according to claim 14.

16. A shear processing apparatus for shearing a metal plate in order to produce a test piece for evaluating delayed fracture properties, comprising: The cutting machine is configured to include a lower die having a lower blade on a shoulder portion, a plate holder, and an upper die having an upper blade on a shoulder portion, and the metal plate is sheared by the lower blade and the upper blade while the metal plate is restrained by the lower die and the plate holder, A spacer is interposed between the metal plate and the plate holder. Shear processing equipment.

17. A shear processing apparatus for shearing a metal plate in order to produce a test piece for evaluating delayed fracture properties, comprising: The cutting machine is configured to include a lower die having a lower blade on a shoulder portion, a plate holder, and an upper die having an upper blade on a shoulder portion, and the metal plate is sheared by the lower blade and the upper blade while the metal plate is restrained by the lower die and the plate holder, a spacer is interposed between the lower die and the plate holder at a position not in contact with the metal plate, the thickness of the spacer being greater than the thickness of the metal plate; Shear processing equipment.

18. A shear processing apparatus for shearing a metal plate in order to produce a test piece for evaluating delayed fracture properties, comprising: The cutting machine is configured to include a lower die having a lower blade on a shoulder portion, a plate holder, and an upper die having an upper blade on a shoulder portion, and the metal plate is sheared by the lower blade and the upper blade while the metal plate is restrained by the lower die and the plate holder, A spacer is provided between the metal plate and the lower die. Shear processing equipment.