Method for evaluating delayed fracture of metal plate, test piece used in same, and method for manufacturing automobile component

Abstract

Provided is a technique that enables proper delayed fracture evaluation at a shear end surface consisting of a hole-punched end surface. This method for evaluating delayed fracture at a shear end surface of a metal plate made of a high-strength steel plate includes: employing a hole-forming process to form a hole in a metal plate to be evaluated, to prepare a test piece (1) having a hole-punched end surface (2A) of the hole (2) as the shear end surface face to be evaluated; restraining the test piece (1) in a state in which a tensile stress is being applied to the hole (2) in a direction parallel to the plate surface of the test piece (1); and placing the restrained test piece (1) in a hydrogen penetration environment and evaluating delayed fracture at the hole-punched end surface (2A) in accordance with the state of occurrence of cracks at the hole-punched end surface (2A) of the test piece (1).

Description

Delayed fracture evaluation method for metal plate, test piece used therefor, and manufacturing method for automobile parts

[0001] The present invention relates to a method for evaluating delayed fracture of a sheared end surface of a metal sheet during press forming. The present invention also relates to a test piece used for the evaluation. The present invention also relates to a method for manufacturing an automobile part using the delayed fracture evaluation method. Here, by forming one or more holes in the automobile part, it is possible to reduce the weight of the automobile body. Even if the evaluated metal sheet is used for a part other than an automobile part, the weight of the part can be reduced by forming holes in the part. Examples of holes include holes for transportation, assembly, or alignment in a subsequent process. The holes may simply be holes whose main purpose is to reduce the weight of the metal sheet.

[0002] Currently, automobiles are required to improve fuel efficiency and collision safety through weight reduction. Therefore, in order 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 automobile bodies. Particularly in recent years, there has been a trend toward using high-strength steel sheets with a tensile strength of 980 MPa or more for automobile bodies. One of the issues when using such high-strength steel sheets for automobile bodies is the occurrence of delayed fracture. For example, with steel sheets with a tensile strength of 980 MPa or more, delayed fracture occurring from the end surface after shearing has become a significant issue. The end surface after shearing is referred to as a sheared end surface. Therefore, in this specification, the end surface after shearing is also referred to as a sheared end surface.

[0003] It is known that large tensile stress remains at the sheared edge of a press-formed product. For this reason, there is concern about the occurrence of delayed fracture at the sheared edge. It is also known that delayed fracture at the sheared edge is further accelerated by applying external stress. Methods for evaluating delayed fracture at the sheared edge are described, for example, in Patent Documents 1 to 3. Patent Documents 1 and 2 describe a test method in which stress is applied by constant displacement restraint due to bending deformation. Patent Document 3 describes a test method in which stress is applied by applying a constant load due to uniaxial deformation.

[0004] Furthermore, methods proposed in Patent Document 4 and Non-Patent Document 1 are methods for evaluating delayed fracture at notched portions including R portions. However, the method described in Patent Document 4 is an evaluation method for round steel bars. The method described in Non-Patent Document 1 is a method for evaluating delayed fracture in a state where stress is applied to the punched portion by bending. Non-Patent Document 1 reports the results of using stress loading by four-point bending. Incidentally, a method using a jig such as that described in Patent Document 5 has been proposed as a method for applying a tensile load to a test piece.

[0005] JP 2017-142086 A JP 2014-70927 A JP 2016-57163 A JP 2022-14187 A JP 2023-72460 A

[0006] Ji-Young Kim, et.al: Enhanced Hydrogen Delayed Fracture of 1.5 GPa Hot Stamping Steel Sheet with Sheared Surface by Double Punching Method, International Journal of Precision Engineering and Manufacturing.(2023)24:173-186

[0007] In automotive parts, holes such as round holes or elongated holes may be formed in flat portions such as the top surface of the part after molding. Such holes are formed for the purpose of, for example, transportation, assembly, or alignment in a subsequent process. Such holes are not subject to significant deformation due to additional press molding in a subsequent process. However, the holes may be subjected to tensile stress due to residual stress inside the part or deformation during assembly.

[0008] The punched end surface of a hole such as a round hole has a shape in which a circular cross section is continuous in the circumferential direction. For this reason, the methods described in Patent Documents 1 to 3 have difficulty in evaluating delayed fracture of a shear end surface consisting of such a punched end surface. Furthermore, the method described in Patent Document 4 evaluates a round bar. In other words, the method described in Patent Document 4 does not anticipate evaluation of a round hole. The method described in Non-Patent Document 1 evaluates a punched portion. However, the method described in Non-Patent Document 1 evaluates bending of the punched portion. In other words, the method described in Non-Patent Document 1 does not anticipate evaluation of tensile stress applied to a round hole in an automotive part.

[0009] The present invention has been made with a focus on delayed fracture occurring when a tensile stress parallel to the sheet surface is applied to a punched edge formed in a flat portion of an automobile part made of high-strength steel sheet. An object of the present invention is to provide a technique that enables appropriate delayed fracture evaluation of a shear edge formed by a punched edge.

[0010] The inventors have discovered the following: In the method described in Non-Patent Document 1, bending deformation is applied to the punched portion. However, situations in which the hole undergoes bending deformation are rare in actual automotive parts. Furthermore, the method described in Non-Patent Document 1 results in stress concentration at the tip of the bend. This makes it difficult to grasp the actual stress applied to the hole. Based on these findings, the inventors have discovered that a new testing method for punched sheared edges is needed to solve the above-mentioned problems. The inventors have also found that the following is necessary to enable delayed fracture evaluation of the sheared edges of punched portions in automotive parts. Specifically, the inventors have found that evaluation requires applying a tensile stress parallel to the sheet surface to the sheared hole in a flat metal plate. Furthermore, they have found that it is preferable to consider the effects of punching clearance, wear, and other factors depending on the actual shearing conditions. They have also found that it is preferable to evaluate shearing conditions such as the punch cutting edge radius as an evaluation variable. The present invention is based on these findings.

[0011] To solve the problem, one aspect of the present invention is a method for evaluating delayed fracture at the shear edge of a metal plate made of high-strength steel plate, in which a hole is formed in the metal plate to be evaluated by punching, a test specimen is prepared with the punched edge of the hole as the shear edge to be evaluated, the test specimen is restrained while a tensile stress is applied to the end of the hole in a direction parallel to the plate surface of the test specimen, and the restrained test specimen is placed in a hydrogen penetration environment, and delayed fracture at the punched edge of the test specimen is evaluated based on the occurrence of cracks at the punched edge.

[0012] In this case, it is advisable to carry out the evaluation by performing the punching process while changing shear conditions such as the clearance and the die cutting edge R. Also, it is advisable to estimate the load stress on the punched end face by, for example, comparing strain measurements using a highly accurate strain gauge with the results of CAE analysis of a test piece carried out in advance. Also, for example, it is advisable to evaluate delayed fracture by evaluating the stress level at or just before the occurrence of delayed fracture according to the shear conditions.

[0013] According to an aspect of the present invention, it is possible to more appropriately evaluate delayed fracture of a sheared edge formed by a punched edge. According to an aspect of the present invention, for example, it is possible to evaluate delayed fracture by applying a tensile stress parallel to the sheet surface to the punched edge of an automobile part. In this case, it becomes easy to predict the occurrence of delayed fracture when high-strength steel sheets are used in various parts such as panel parts, structural / framework parts, etc. of an automobile. Furthermore, according to an aspect of the present invention, it is possible to reduce the weight of an automobile body by using high-strength steel sheets.

[0014] 2(a) is a diagram showing an example of the procedure of an evaluation method according to an embodiment of the present invention; FIG. 2(b) is a plan view showing an example of a test piece according to an embodiment of the present invention; FIG. 2(a) is an example in which a hole is formed in the center in the plate width direction; FIG. 2(b) is an example in which holes are formed at both ends in the plate width direction of the test piece; FIG. 2(a) is a diagram showing an example of CAE analysis of tensile deformation in the test piece of FIG. 2(a). FIG. 2(b) is a diagram showing the relationship between tensile direction strain and first principal stress (tensile direction stress) by CAE analysis of tensile deformation in the test piece of FIG. 2(a). FIG. 2(b) is a plan view of the test piece showing the setup state before applying load to the test piece; FIG. 2(a) is an explanatory diagram of an evaluation test piece in an example, which is machined by mechanical grinding, leaving a punched hole of Φ10 mm; FIG. 2(b) is a diagram showing the relationship between tensile direction strain and first principal stress (tensile direction stress) by CAE analysis of tensile deformation (FEM analysis) in an example; FIG. 2(b) is a diagram showing an example of the relationship between shear clinic / plate thickness and critical stress for delayed fracture initiation; FIG. 2(b) is a diagram showing an example of evaluation of holes to be evaluated with different hole diameters; FIG. 2(b) is a diagram showing an example of evaluation of holes to be evaluated with different hole shapes.

[0015] Next, an embodiment based on the present invention will be described with reference to the drawings. This embodiment is a method for evaluating delayed fracture at a sheared edge of a metal plate made of a high-strength steel plate. As described above, the high-strength steel plate is a steel plate having a tensile strength of 980 MPa or more. In this embodiment, the sheared edge to be evaluated is a pierced edge of a hole formed by piercing.

[0016] (Configuration) As shown in FIG. 1, the evaluation method of this embodiment includes a test piece preparation step 10, a stress application step 11, a hydrogen penetration environment placement step 12, and an evaluation step 13.

[0017] <Test Piece Preparation Step 10> The test piece preparation step 10 is a step of forming a hole in a metal plate to be evaluated by punching to prepare a test piece having a sheared end surface to be evaluated. In the present invention, the punched end surface of the hole formed by punching is the sheared end surface to be evaluated. FIG. 2 shows an example of a test piece having an evaluation hole 2 prepared in this embodiment. In the test piece preparation step 10 of this embodiment, the metal plate to be evaluated is cut into a strip shape to prepare a test piece body. This cutting may be performed, for example, by machining. Further, a hole is punched in the longitudinal center of the test piece body to form the evaluation hole 2. Furthermore, pin holes 3 are opened at both left and right ends of the test piece body in the longitudinal direction. The left and right pin holes 3 are holes for applying stress. Note that the order of the processing steps of cutting into strips, opening the pin holes 3, and the evaluation hole 2 described above is not limited to the above order.

[0018] The evaluation hole 2 is positioned between two paired pin holes 3. As a result, when a tensile force (tensile deformation) parallel to the plate surface of the test piece 1 is applied through the paired pin holes 3, a stress load corresponding to the tensile force is applied to the end of the evaluation hole 2. Here, the thickness of the metal plate is desirably 3 mm or less, taking into account the plate thickness of a realistic high-strength steel thin steel plate. Furthermore, the piercing method for opening the evaluation hole 2 is desirably the same shearing method as that used for actual automobile parts. The cross-sectional shape of the evaluation hole 2 may be a round hole, an elongated hole, or an ellipse. The dimension of the minor axis of the hole 2 is desirably in the range of 3 mm or more and 50 mm or less. The dimension of the minor axis of the hole 2 described above takes into account the actual dimensions of holes opened in automobile parts. Furthermore, the dimension of the major axis of the hole 2 is desirably 50 mm or less. Note that the evaluation hole 2 may be a shape other than a round hole, an elongated hole, or an ellipse. However, it is preferable that the outline (contour shape) of the hole does not have any steep portions where the curvature is sharp along the circumferential direction. A steep portion is a portion that becomes a corner.

[0019] FIG. 2 shows two examples of the opening position of the evaluation hole 2 in the plate width direction of the test specimen 1. The example shown in FIG. 2(a) is an example in which the center of the sheared evaluation hole 2 is set at the center of the plate width direction of the test specimen 1. The example shown in FIG. 2(b) is an example in which the sheared end surfaces of the sheared evaluation hole 2 are semicircular and provided at two locations, one on the left and one on the right in the plate width direction of the test specimen 1. That is, the example shown in FIG. 2(b) is an example in which the evaluation hole 2 is located at the end of the plate width direction of the test specimen 1. Of these two examples, the opening position of the evaluation hole 2 is preferably the former case (see FIG. 2(a)). That is, it is preferable that there is one evaluation hole 2 and that the outline of the hole 2 is closed, as in an actual part. Furthermore, it is preferable that the evaluation holes 2 are arranged symmetrically in the plate width direction of the test specimen 1.

[0020] The shear conditions for punching the evaluation holes 2 are set taking into account variations in hole formation during actual automobile production processes. For example, multiple types of test specimens 1 are prepared by varying the clearance between the punch and die used in the punching process, the punch cutting edge R, or the die cutting edge R. Evaluation is then performed on the punched end surface 2A. In this case, the effectiveness of the evaluation can be further improved. Here, the application of tensile force applies higher stress to the edge of the stress-loading pin hole 3 than to the punched end surface 2A of the evaluation hole 2. To alleviate this, it is preferable to machine-ground the end surface 3A of the pin hole 3. This makes the end surface 3A of the pin hole 3 less susceptible to delayed fracture than the punched end surface 2A of the evaluation hole 2. It is also preferable to protect (seal) the end surface 3A of the pin hole 3 with polymer tape to suppress delayed fracture at the end surface 3A of the pin hole 3. It is also preferable to make the end surface forming the outer periphery of the test specimen 1 less susceptible to delayed fracture than the punched end surface 2A of the evaluation hole 2. The end faces forming the outer peripheral surface of the test piece 1 are an end face 1a extending along the longitudinal direction and an end face 1b extending in the width direction.

[0021] That is, it is desirable to use cut end faces 3A, 1a, and 1b other than the punched end face 2A of the evaluation hole 2 as cut end faces or seal them. This prevents delayed fracture from occurring in areas other than the shear hole portion being evaluated. Sealing methods include applying a polymer adhesive to the end face. Another sealing method is applying acid-resistant polyimide tape to the end face. Sealing the end face in this manner prevents the end face from reacting with the chemical solution used for delayed fracture evaluation, thereby suppressing delayed fracture. The chemical solution is a solution used in a hydrogen penetration environment. In this embodiment, as shown in FIG. 2( a), a constricted portion is formed in the longitudinal center of the test specimen 1, narrowing the plate width. The evaluation hole 2 is then provided in this constricted portion. In this embodiment, this makes it easier for tensile deformation to occur in the evaluation hole 2.

[0022] <Stress Application Step 11> The stress application step 11 is a step of applying a tensile stress in a direction parallel to the plate surface to the pierced end surface 2A of the evaluation hole 2 and restraining the test piece 1 while the tensile stress is applied. In this embodiment, pins are passed through the pin holes 3 at both ends of the test piece 1, and the test piece 1 is pulled in the left-right direction by the pins passed through the pin holes 3. This applies a tensile displacement along the longitudinal direction of the test piece 1 to the entire test piece 1. Then, in this state, the displacement generated in the test piece 1 is restrained. As a result, a tensile load is applied to the pierced end surface 2A of the evaluation hole 2 in a direction parallel to the plate surface. A convenient jig for applying a tensile force and restraining the test piece 1 using a pin is described in Patent Document 5. However, other known methods, such as clamping with a chuck, may also be used to apply the tensile force.

[0023] The load stress on the piercing end face 2A of the evaluation hole 2 may be determined, for example, as follows. First, the correlation between the strain at the hole end of the evaluation hole 2 and the stress generated at the piercing end face 2A in response to the tensile stress applied to the test displacement is determined in advance. This correlation is determined in advance by CAE analysis such as FEM analysis. It is preferable to use the maximum value of the generated stress as the stress in the above correlation. Then, by referring to the determined correlation, the load stress generated at the piercing end face 2A is determined from the strain measurement value measured at the hole end of the evaluation hole 2.

[0024] FIG. 3 shows an example of stress distribution with respect to tensile deformation, determined by CAE analysis. FIG. 4 illustrates the relationship determined by CAE analysis with respect to tensile deformation. Specifically, FIG. 4 illustrates the relationship between "tensile strain and first principal stress (stress in the axial direction)" in the element of the hole end face to be evaluated. In the analysis shown in FIGS. 3 and 4, the shape of the test specimen was as shown in FIG. 2(a), with the center of the evaluation hole 2 located at the center of the test specimen 1.

[0025] At the punched end face 2A of the evaluation hole 2, it is expected that simple uniaxial tension will not occur due to the influence of stress concentration on the R portion. For this reason, it is desirable to determine the relationship between "tensile direction strain and first principal stress" in advance using CAE analysis, as illustrated in Figure 4. This CAE analysis confirmed that the tensile stress is the same on the front and back surfaces of the sheet, and further confirmed that the tensile stress load is parallel to the sheet surface. The attachment position of the strain gauge 4 used to measure strain at the hole edge is set, for example, as shown in Figure 5. That is, the attachment position is preferably the edge of the hole near the punched end face 2A, which is perpendicular to the tensile direction relative to the center of the evaluation hole 2. Figure 4 shows the strain amount calculated at that position. Strain is also measured at that position.

[0026] The reason for measuring strain at this position is as follows. The position at which delayed fracture is likely to occur in the circumferential direction of the pierced end face 2A is the position where the extension direction of the pierced end face 2A is along the tensile direction. In other words, delayed fracture is likely to occur at or near the position of the pierced end face 2A that is perpendicular to the tensile direction with respect to the center of the evaluation hole 2. Furthermore, as can be seen from FIG. 3 , when a tensile load is applied due to tensile deformation, a stress distribution exists near the hole end face that is perpendicular to the tensile direction with respect to the center of the hole 2. For this reason, it is desirable to attach the strain gauge 4 near the hole end face. It is preferable to attach the strain gauge 4 in an area as close as possible to the edge of the hole. For example, it is desirable to attach the strain gauge 4 in an area at least within 3 mm from the pierced end face 2A.

[0027] In this example, as shown in FIG. 5 , strain gauges 4 are attached to both the left and right locations perpendicular to the tensile direction. Alternatively, strain gauges 4 may be attached to only one of the two locations perpendicular to the tensile direction. A correlation, such as that shown in FIG. 4 , is determined in advance by CAE analysis. That is, the correlation between the stress load occurring at the position on the punched end face 2A where delayed fracture is likely to occur and the strain in the tensile direction at the hole end near that end face is determined in advance. As shown in FIG. 4 , there is a positive correlation between the strain near the hole end face and the tensile stress at that end face. It can be seen that the tensile stress applied to the punched end face 2A can be determined from the strain near the end face by referring to this correlation. The load stress applied to the punched end face 2A can then be determined from this correlation and the measured strain amount.

[0028] FIG. 5 shows an example of the setup state before a load is applied to the test piece 1. In the state shown in FIG. 5, the outer peripheral end faces 1a and 1b of the test piece 1 and the end face 3A of the pin hole 3 are sealed. In FIG. 5, the sealing area 5 is indicated by a dashed line. As shown in FIG. 5, strain gauges 4 are attached to the tensile deformation area located at both ends of the evaluation hole 2 in the sheet width direction, thereby measuring strain during tensile deformation. There is a positional accuracy error in the punching position of the evaluation hole 2. For this reason, as shown in FIG. 5, it is desirable to attach strain gauges 4 in two locations and use the higher strain value of the two strain gauges 4. In other words, the strain amount at the circumferential position of the punched end face 2A where the tensile stress is maximum is used.

[0029] Then, the amount of displacement of each pin inserted into the left and right pin holes 3 is changed to change the amount of strain at the hole end. At this time, the amount of displacement is changed based on the measurement value of the strain gauge 4. At this time, the load stress at the punched end face 2A is estimated from the amount of strain measured by the strain gauge 4 based on the correlation between the amount of strain and tensile stress obtained by the CAE analysis. Then, when the target stress is reached, the position of the pin is fixed and the test piece 1 is held. In other words, the test piece 1 is constrained in displacement. "Constraining displacement" refers to holding the test piece 1 in a constrained state. As described above, in this embodiment, the load stress at the hole end face is defined as the load stress at the circumferential position of the punched end face located in the plate width direction of the test piece 1 relative to the center position of the hole. In other words, the value at the position where the maximum stress load occurs along the circumferential direction of the punched end face 2A is adopted.

[0030] <Hydrogen entry environment placement step 12> In the hydrogen entry environment placement step 12, the test piece 1, which has been subjected to a predetermined stress load and restrained from displacement, is placed under predetermined hydrogen entry conditions for a predetermined time. This determines whether or not delayed fracture occurs. Specifically, the delayed fracture properties of the metal plate are evaluated based on the occurrence of cracks on the punched end surface 2A of the evaluation hole 2. The occurrence of cracks is evaluated, for example, by the time until occurrence. The test piece 1 is placed in a hydrogen entry environment, for example, by using hydrochloric acid or NH ４ This is done by immersing the test piece 1 in a bath containing an acid solution such as an SCN aqueous solution.

[0031] <Evaluation Step 13> In the evaluation step 13 of this embodiment, for example, the delayed fracture property is evaluated by evaluating the delayed fracture property at the evaluation hole 2 in the metal plate to be evaluated. The series of steps, including the test piece preparation step 10, stress application step 11, and hydrogen penetration environment placement step 12, described above, are performed multiple times while changing the tensile stress applied to each test piece 1. This, for example, determines the limit load stress that can be applied to the evaluation hole 2, at which delayed fracture does not occur for the diameter and shape of the evaluation hole 2. Furthermore, for example, the shear conditions for forming the evaluation hole are changed, and for each shear condition, the limit load stress at which cracks do not occur at the sheared end face of the evaluation hole 2 is determined. This determines the limit load stress for each shear condition of the piercing process.

[0032] Examples of shear conditions for punching include the clearance between the punch and die used in the punching process, and the cutting edge R of the punch or die used in the punching process. Then, for example, a model for determining whether or not cracks will occur can be prepared, using the shear conditions and the limit value of the load stress as variables, and delayed fracture evaluation can be performed on the end surface of the hole. For example, if the designed press-formed product has a hole, the delayed fracture evaluation can be performed on a metal plate made of the same material and a hole with the same shape and diameter as the hole. Then, for example, a CAE analysis of assembly to the press-formed product can be performed to determine the load stress occurring on the end surface of the hole in the press-formed product.

[0033] Then, by referring to the above model, it is determined whether delayed fracture will occur at the end face of the hole in the press-formed product under the machining conditions for hole 2 currently set in the design and the load stress determined by CAE. If it is determined that delayed fracture will occur, measures are taken, such as changing the machining conditions for the hole or changing the hole formation position. In this way, it is possible to evaluate delayed fracture at the punched end face 2A according to the shape of hole 2, the shear conditions for hole 2, and the load stress applied to hole 2 in the sheet surface direction.

[0034] That is, according to this embodiment, it is possible to more appropriately evaluate delayed fracture of the shear end surface consisting of the punched end surface 2A. For example, it is possible to evaluate delayed fracture by applying a tensile stress parallel to the sheet surface to the punched end surface 2A on the plane of an automobile part. In this case, it becomes easy to predict the occurrence of delayed fracture when high-strength steel sheets are used in various parts such as panel parts, structural / framework parts, etc. of automobiles. Furthermore, the use of high-strength steel sheets makes it possible to reduce the weight of automobile bodies.

[0035] Here, an automobile part is manufactured, for example, as follows. First, a metal plate is selected that satisfies the material conditions evaluated as not causing delayed fracture at the punched end face by the method for evaluating delayed fracture of a metal plate of this embodiment. Next, the selected metal plate is subjected to punching and press forming to manufacture an automobile part of the desired shape. Here, the order of the punching and press forming steps is not particularly limited. This manufactures an automobile part that does not cause delayed fracture at the opening. It also enables the weight of an automobile body using this automobile part. Furthermore, in this embodiment, for example, if the opening is a hole for assembling another part, it is possible to suppress delayed fracture at the assembly position.

[0036] (Other) The present disclosure may also have the following configurations: (1) Disclosure 1 is a method for evaluating delayed fracture at a shear edge of a metal plate made of a high-strength steel plate, comprising: forming a hole in the metal plate to be evaluated by punching, preparing a test specimen with the punched edge of the hole as the shear edge to be evaluated; restraining the test specimen while applying a tensile stress to the end of the hole in a direction parallel to the plate surface of the test specimen; and placing the constrained test specimen in a hydrogen penetration environment to evaluate delayed fracture at the punched edge of the test specimen based on the occurrence of cracks at the punched edge. (2) Disclosure 2 involves determining in advance by CAE analysis the correlation between the strain at the end of the hole and the stress generated at the punched end face of the hole in response to the application of tensile stress in a direction parallel to the plate surface of the test piece, measuring the strain generated at the end of the hole due to the application of the tensile stress to the test piece, and estimating the load stress applied to the punched end face from the correlation and the measured strain value. (3) Disclosure 3 involves evaluating delayed fracture at the punched end face under multiple shear conditions by changing the shear conditions of the punching process. (4) Disclosure 4 involves one of the shear conditions being the clearance between the punch and die used in the punching process. (5) Disclosure 5 involves one of the shear conditions being at least one of the cutting edge R of the punch or the cutting edge R of the die used in the punching process. (6) Disclosure 6 involves placing the test piece in a hydrogen penetration environment with the end faces of the test piece sealed, except for the punched end face where the delayed fracture is evaluated. (7) Disclosure 7 is a manufacturing method of an automobile part, comprising: selecting a metal plate that has been evaluated as not causing delayed fracture at a punched end face by the method for evaluating delayed fracture of a metal plate described in the present disclosure; and manufacturing the automobile part by performing punching and press forming on the selected metal plate. (8) Disclosure 8 is a test piece used for evaluating delayed fracture at a sheared end face of a metal plate, wherein a hole is formed in the test piece by punching, and the punched end face of the hole is the sheared end face to be evaluated, and the end faces of the test piece other than the punched end face to be evaluated for delayed fracture are sealed to delay delayed fracture at the end face.(9) Disclosure 9 is the test piece according to Disclosure 7, wherein the diameter of the hole is in the range of 3 mm or more and 50 mm or less.

[0037] Next, an example based on this embodiment will be described. In this example, a metal plate made of 1470 MPa-class steel with a plate thickness of 1.4 mm was used as a test material to be evaluated. However, the present invention is not limited to such a metal plate, and can be applied to metal materials such as high-tensile steels with a tensile strength of 980 MPa or more, which are prone to delayed fracture at the shear end surface.

[0038] First, a rectangular plate material measuring 30 mm x 90 mm was prepared as the test material. Then, a punching process (hole punching) with a hole diameter of Φ10 was performed with the center of the hole being the intersection of the diagonal lines of the four corners of the test material. Thus, a hole 2 for evaluation was formed. Three types of punches A, B, and C, each with a punch diameter of 10 mm, were prepared as punches for the hole punching process. The cutting edges of the three types of punches were: Punch A: Cutting edge R0 mm, Punch B: Cutting edge R0.2 mm, and Punch C: R0.5 mm. Four types of dies A, B, C, and D with different die diameters were prepared for the hole punching process. The die diameters of the four types of dies were: Die A: Die diameter 10.28 mm, Die B: Die diameter 10.42 mm, Die C: Die diameter 10.60 mm, and Die D: Die diameter 10.90 mm. In addition, the cutting edge R of each of the dies A to D was set to 0 mm.

[0039] By combining these punches and dies, evaluation holes 2 were formed under a total of 12 different shear conditions. In this way, multiple test materials for each test piece 1 were prepared. For die A, the one-sided clearance between the punch and die relative to the plate thickness was 10%. Furthermore, the clearance ratios were 15% for die B, 20% for die C, and 30% for die D. In this example, as described above, the diameter of evaluation hole 2 was set to 10 mm. However, the hole diameter can be selected as desired depending on the actual part. Furthermore, the shape of hole 2 is not limited to a circular hole, and any desired shape can be selected.

[0040] Next, as shown in Figure 6, each test material was machined into a rectangular shape, leaving a Φ10 punched hole, by mechanical grinding. In this way, a test specimen 1 for evaluation was prepared from each test material. Furthermore, a 12 mmΦ pin hole 3 was formed at each longitudinal end of the test specimen 1 by mechanical grinding. The pin hole 3 was opened by mechanical grinding. The end face formed by mechanical grinding is relatively less susceptible to delayed fracture compared to a sheared end face. This prevented delayed fracture from occurring at end faces other than the evaluation area. Furthermore, for end faces other than the evaluation area, methods such as cutting by laser cutting or wire cutting, or applying polymer tape for protection can also be applied. The dimensions of the formed test specimen 1 are as shown in Figure 6. A strain gauge 4 was attached to the test specimen 1. Specifically, strain gauges 4 were attached to two locations at the end of the evaluation hole 2, which are locations that will be subjected to tensile deformation (see Figure 5). Then, while checking the amount of strain measured by the strain gauges 4, the Φ10 pins inserted through the pin holes 3 at both ends were moved in a direction that separated them from each other. This applied tensile deformation along the plate surface to the test piece 1. In this example, the jig described in Patent Document 5 was used to apply tensile deformation with high precision.

[0041] At this time, the correlation between the tensile strain near the hole end and the first principal stress at the punched end surface 2A was determined in advance by FEM analysis (CAE analysis) of the elements at the hole end, as shown in Figure 7. Then, based on the determined correlation, the tension using the jig was stopped when the strain amount that applied the target load stress was reached. The test piece 1 was then fixed, i.e., the displacement was constrained. Using the above process, multiple test pieces 1 were prepared with load stresses varied in 100 MPa increments. In addition, the above CAE analysis confirmed that the tensile stress along the sheet surface as described above was the same on the front and back surfaces of the sheet surface, and that the tensile stress was parallel to the sheet surface.

[0042] Next, each test piece 1 to which the target load stress had been applied was immersed in an ammonium thiocyanate aqueous solution of pH 7 for 96 hours. The maximum load stress at which delayed fracture did not occur was determined as the limit stress. Table 1 shows the limit stress at which delayed fracture did not occur for each punch and die combination (shear condition).

[0043]

[0044] As can be seen from Table 1, it is now possible to evaluate the influence of the threshold stress for delayed fracture at the punched end surface 2A on the shear conditions of the piercing process, such as the punch cutting edge R and the clearance between the punch and the die. This delayed fracture evaluation can be performed for each target hole diameter. The example in Table 1 is for a case where the diameter of the target evaluation hole 2 is 10 mm. For example, the results shown in Table 1 can be organized as shown in Figure 8, with the horizontal axis representing the clearance % relative to the plate thickness and the vertical axis representing the threshold stress.

[0045] Furthermore, more advanced evaluations may be possible where the diameter of the hole 2 to be evaluated is different, as in the schematic diagram of Figure 9, or where the shape of the hole 2 is not a round hole but an elongated hole, as in the schematic diagram of Figure 10. Here, the hatched areas in Figures 9 and 10 indicate the hole end portions where tensile stress and tensile strain become large due to tensile deformation. Even in such cases, evaluations can be made in the same way as described above. For example, even when the diameter of the hole 2 is different, as in Figure 9, it is possible to measure the strain at the point where tensile stress concentrates in response to a tensile load, as described above, and measure the critical stress for delayed fracture.

[0046] Furthermore, as shown in Figure 10, evaluation is also possible in the case where the hole shape of hole 2 is an elongated hole. In this case, the direction in which the tensile load is applied may be changed relative to the shape of the elongated hole. In this case, evaluation is possible under different conditions, when tensile stress is applied to the straight portion of the elongated hole and when tensile stress is applied to the curved portion.

[0047] The entire contents of Japanese Patent Application No. 2024-014457 (filed February 1, 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 would be obvious to one skilled in the art.

[0048] REFERENCE SIGNS LIST 1 Test piece 1a, 1b End face 2 Evaluation hole 2A Punched end face 3 Pin hole 3A End face 4 Strain gauge 10 Test piece preparation process 11 Stress application process 12 Installation process under hydrogen penetration environment 13 Evaluation process

Claims

1. A method for evaluating delayed fracture of a sheared edge of a metal plate made of a high-strength steel plate, comprising: A hole is formed in a metal plate to be evaluated by punching, and a test piece is prepared in which the punched end surface of the hole is the shear end surface to be evaluated. The test piece is restrained in a state where a tensile stress is applied to the end of the hole in a direction parallel to the plate surface of the test piece, The restrained test piece is placed in a hydrogen penetration environment, and the delayed fracture at the punched end surface is evaluated based on the occurrence of cracks at the punched end surface of the test piece. Delayed fracture evaluation method for metal plates.

2. A correlation between strain at the end of the hole and stress generated at the punched end face of the hole in response to a tensile stress applied in a direction parallel to the plate surface of the test piece is determined in advance by CAE analysis; measuring the strain generated at the end of the hole due to the application of the tensile stress to the test piece; The load stress applied to the punched end surface is estimated from the correlation and the strain measurement value. A method for evaluating delayed fracture of a metal plate according to claim 1.

3. By changing the shear conditions of the above punching process, delayed fracture at the punched end face can be evaluated under multiple shear conditions. A method for evaluating delayed fracture of a metal plate according to claim 1.

4. One of the shear conditions is the clearance between the punch and die used in the hole punching process. The method for evaluating delayed fracture of a metal plate according to claim 3.

5. One of the shearing conditions is at least one of the cutting edge R of the punch or the cutting edge R of the die used in the piercing process. The method for evaluating delayed fracture of a metal plate according to claim 3.

6. the end faces of the test specimen other than the punched end face for evaluating the delayed fracture are sealed, and the test specimen is placed in a hydrogen penetration environment. A method for evaluating delayed fracture of a metal plate according to any one of claims 1 to 5.

7. A method for manufacturing an automobile part, comprising: A metal plate that is evaluated as not causing delayed fracture at the punched end surface by the method for evaluating delayed fracture of a metal plate according to any one of claims 1 to 5 is selected, The selected metal plate is subjected to punching and press forming to manufacture an automobile part. Manufacturing methods for automotive parts.

8. A test piece used to evaluate delayed fracture of a sheared edge of a metal plate, A hole is formed in the test piece by punching, and the punched end surface of the hole is used as the shear end surface to be evaluated. the end faces of the test specimen other than the punched end faces for evaluating the delayed fracture are sealed to delay the delayed fracture at the end faces; Test piece.

9. The diameter of the hole is in the range of 3 mm or more and 50 mm or less. The test piece according to claim 8.