Bending performance evaluation method

Abstract

The purpose of the present invention is to provide a method with which it is possible to efficiently evaluate fine cracks generated in a bending inner surface during bending of a metal sheet. This bending performance evaluation method is for evaluating the bending performance of an evaluation target metal sheet (3) with respect to bending deformation, by performing a bending test for bending and deforming the evaluation target metal sheet (3) by using a punch (2) having a bending radius (R) at the tip (2c), and comprises: a bending-restoration step (11) for restoring bending of the metal sheet (3) that has been bent and deformed in the bending test; and an evaluation step (12) for determining whether or not a crack will emerge in a bending inner surface (3x) of the evaluation target metal sheet (3) due to bending deformation with said bending radius (R), by evaluating the surface shape of the bending inner surface (3x) of the metal sheet (3) after the bending-restoration step (11).

Description

Bending performance evaluation method

[0001] The present invention relates to a technology that enables easy evaluation of bending performance during bending of metal sheets, such as automotive metal sheets. The bending performance refers to, for example, bending deformation. The present invention is particularly applicable to the evaluation of fine cracks that occur on the inner surface of a bent sheet during bending. In this specification, fine cracks refer to cracks that are not easily discernible with the naked eye or cracks that are difficult to discern with the naked eye. It should be noted that the present application does not need to be applied to the evaluation of metal sheets that develop cracks that are easily discernible with the naked eye on the inner surface of a bent sheet during a bending test.

[0002] Many automobile body components are manufactured by roll forming or press forming metal sheets such as steel sheets. Furthermore, efforts are being made to increase the strength of steel sheets, which are used as raw materials, in order to improve automobile crashworthiness and reduce vehicle weight. However, as the strength of metal sheets (materials) increases, there is a problem that cracks are more likely to occur in the metal sheets during forming. In particular, the following problem occurs in the bent portions of formed metal sheets. That is, if cracks or wrinkles occur on the outer bent surface where tensile stress occurs, this can lead to damage to the formed product or a deterioration in the fatigue properties of the formed product. To avoid such problems, it is important to evaluate the optimal bending conditions for each metal sheet material in advance. It is then important to utilize the evaluation results in part design and mold design for part manufacturing.

[0003] For example, the following evaluation method is available as a method for evaluating the bending performance of a metal sheet. Patent Document 1 describes a method of performing bending by pressing a tapered punch into a metal sheet to be evaluated while applying tension to the metal sheet. Patent Document 1 also proposes a method of determining a limiting condition as a relationship between tension and the bending radius at the tip of the punch. The limiting condition is a limiting condition that does not cause minute streaky grooves, cracks, or fractures in the bent portion. Patent Document 1 also describes the minute grooves as "necking."

[0004] Patent Document 2 also describes a method for conducting a bending test while photographing the outer surface of a bent metal plate with a camera. Patent Document 2 also proposes a method for evaluating bending performance based on information such as strain distribution and its changes on the outer side of the bend, and the occurrence of cracks. Patent Document 3 also evaluates a portion that is subjected to unbending after bending. Patent Document 3 also discloses a method for evaluating cracks that occur from the inner side of the bend during bending and unbending.

[0005] JP 2011-235301 A JP 2021-135128 A JP 2015-47605 A

[0006] The evaluation methods described in Patent Documents 1 and 2 are methods for evaluating bending performance on the outer side of a bend. In other words, the evaluation methods described in Patent Documents 1 and 2 do not evaluate bending performance on the inner surface of a bend. Furthermore, the method described in Patent Document 3 is a method for evaluating bending performance on the inner surface of a bend. However, Patent Document 3 evaluates bending performance after bending and unbending. In other words, the method described in Patent Document 3 does not evaluate bending performance after bending. In other words, Patent Document 3 is a method for evaluating cracks that occur during unbending in a portion that undergoes unbending after bending. Furthermore, the method described in Patent Document 3 cannot identify the presence or depth of fine cracks that have already occurred on the inner side of a bend immediately after bending. In other words, the method described in Patent Document 3 has the problem of not being able to evaluate bending performance.

[0007] When a metal plate is bent, cracks may occur not only on the outer surface of the bent plate, which is conventionally considered to be bending cracks, but also on the inner surface of the bent plate. Microcracks on the inner surface of the bent plate are characterized by not being accompanied by necking. Note that necking refers to streaky grooves. Microcracks on the inner surface of the bent plate are also characterized by extremely small crack depths and widths, accompanied by wrinkles. It is extremely difficult or time-consuming to detect cracks with these characteristics through optical observation from the surface of the material.

[0008] Conventionally, when determining whether or not cracks occur due to bending or when identifying the depth of a crack, the following evaluation procedure is required. First, a bent portion of a metal plate is cut out, and the cut-out bent portion is embedded in resin. The cross section of the bent portion is then polished to a mirror finish. The cross section near the inner surface of the bent portion is then observed in detail using a microscope or the like. However, this procedure has the problem of significantly reducing the efficiency of the work required to evaluate cracks on the inner surface of the bent portion due to bending.

[0009] The present invention has been made in view of the above points, and aims to provide a method for efficiently evaluating microcracks that occur on the inner surface of a bent metal plate during bending.

[0010] To solve the above-mentioned problems, the inventors conducted a bending test on a metal plate and measured the crack depth on the inner surface of the bent plate. The crack depth measurement was performed based on detailed observation of the bent cross section. Additionally, the same type of metal plate was subjected to a bending test under the same bending conditions, followed by a bending-back process. As a result of this, the occurrence of cracks and streak-like minute grooves extending in the direction of the bend ridge was observed on the inner surface of the bent plate after bending back. As described above, streak-like minute grooves are also called streak-like grooves. The inventors then compared the test results from both studies. As a result of this comparison, the inventors found that there is a correlation between the occurrence of cracks and streak-like grooves on the inner surface of the bent plate bent back after the bending test and the depth of the cracks that occurred on the inner surface of the bent plate during the bending test. Based on this, the inventors discovered the following evaluation method for evaluating the bending performance of a metal plate that solves the above-mentioned problems.

[0011] In order to solve the problems, one aspect of the present invention is a bending performance evaluation method for evaluating the bending performance of a metal plate to be evaluated against bending deformation by performing a bending test in which the metal plate to be evaluated is bent using a punch having a bending radius at its tip, the bending performance evaluation method including: a bending back process for bending back the metal plate that has been bent and deformed in the bending test; and an evaluation process for evaluating the surface shape of the inner surface of the bent metal plate after the bending back process, thereby determining whether or not cracks will occur on the inner surface of the bent metal plate to be evaluated due to bending deformation at the bending radius.

[0012] In one embodiment of the present invention, a material that has been bent (bent) is unbent. This allows fine cracks that have developed during bending to propagate, making it easier to observe (evaluate) the presence or absence of cracks during bending. As a result, this embodiment of the present invention makes it possible to easily and efficiently evaluate cracks that develop on the inner surface of a bent metal plate during bending.

[0013] Conventionally, after cutting out a bent portion of a metal plate, the cross section of the bent portion is polished to a mirror finish. Then, it is necessary to closely observe the cross section near the inner surface of the bent portion using a microscope or the like. However, according to the present invention, such cutting and polishing are not required. That is, according to the present invention, it is possible to easily evaluate the presence or absence of cracks and their depths, even if the cracks are extremely small in depth or width and occur on the inner surface of the bent portion when bending a metal plate. Therefore, according to the present invention, it is possible to avoid a significant decrease in work efficiency.

[0014] Furthermore, according to this aspect of the present invention, the range of bending radii at which cracks are likely to occur during bending can be easily estimated by the evaluation in the evaluation process. Therefore, for example, when it is desired to evaluate the bending performance of bending in detail, it is possible to narrow down the materials to those that have undergone bending tests within the limited bending radius range by this aspect of the present invention. Then, it becomes possible to mirror-polish the cross section of the bent portion of only the narrowed down materials, as in the conventional method, and to perform detailed observation of the cross section near the inner surface of the bent portion using a microscope or the like.

[0015] FIG. 1 is a diagram showing an example of the procedure of a bending performance evaluation method according to an embodiment of the present invention. FIG. 2 is a schematic diagram illustrating a bending test according to an embodiment of the present invention. FIG. 3 is a side view showing the shape of a metal plate after a bending test process. FIG. 4 is a schematic diagram showing a bending-back process according to an embodiment of the present invention. FIG. 5 is a side view showing the progress of the bending-back process. FIG. 6 is a side view showing the position of a metal plate and an inner surface of a bent plate after the bending-back process. FIG. 7 is a graph showing the relationship between the bending radius in a bending test and the crack depth of a crack on the inner surface of a bent plate. FIG. 8 is a diagram illustrating the rate of increase in surface roughness before and after a bending-back process when forming a streak-like groove on the inner surface of a bent plate.

[0016] Next, an embodiment of the present invention will be described with reference to the drawings. The evaluation method of the present invention can be applied to any metal sheet. The present invention is suitable for steel sheets, and particularly suitable for hot-rolled steel sheets. The present invention relates to a method for easily evaluating bending performance during bending.

[0017] (Configuration) The procedure for evaluating bending performance in this embodiment is shown in Fig. 1. As shown in Fig. 1, this evaluation method includes a bending test step 10, which is the main test step, a bending undo step 11, an evaluation step 12, and an index value calculation step 13.

[0018] <Bending Test Step 10> First, with reference to FIG. 2 , bending test step 10 will be described. Bending test step 10 is a step of performing a bending test that simulates the actual bending process. In this embodiment, bending test step 10 is performed using a die 1 and a punch 2 as shown in FIG. 2 as molds. As shown in FIG. 2 , the die 1 used in this embodiment has a forming surface with a concave cross-sectional shape. The concave cross-sectional shape is V-shaped. The V-shape is composed of a pair of inclined surfaces 1a and 1b. The intersection of the pair of inclined surfaces 1a and 1b forms the tip of the V. In this embodiment, a recess 1c is formed at the tip of the V. As a result, the forming surface of the die 1 is capable of responding to bending deformations with multiple bending radii R. Note that the forming surface of the die 1 is formed so that the concave cross-sectional shape extends along a direction perpendicular to the plane of FIG. 2 .

[0019] The punch 2 has a forming surface including a pair of inclined surfaces 2a, 2b and a tip portion 2c connecting the pair of inclined surfaces 2a, 2b. The forming surface has a V-shaped cross section. The pair of inclined surfaces 2a, 2b are shaped to resemble the pair of inclined surfaces 1a, 1b. The tip portion 2c of the punch 2 has an arc-shaped cross section with a bending radius R. In this embodiment, multiple punches 2 are provided. The bending radii R of the tip portions 2c of the punches 2 are different from each other. Here, the angle θ of the V formed by the pair of inclined surfaces 1a, 1b is, for example, 90 degrees. However, the angle θ of the V may be other than 90 degrees. The angle of the V formed by the pair of inclined surfaces 2a, 2b is, for example, equal to the angle θ of the V formed by the pair of inclined surfaces 1a, 1b.

[0020] As shown in FIG. 2 , the bending test is performed by first placing the metal sheet 3 to be evaluated on the die 1. The punch 2 is then pressed toward the die 1 in a direction such that the inclined surfaces 2 a and 2 b of the punch 2 approach the inclined surfaces 1 a and 1 b of the die 1. Through this bending test, the metal sheet 3 is bent into the bent shape shown in FIG. 3 . That is, the metal sheet 3 is clamped between the die 1 and the punch 2. As a result, the metal sheet 3 is bent into a V-shaped cross-sectional shape with a bending radius R at the bottom dead center of the forming. Even when the metal sheet 3 is bent to 90 degrees, springback occurs in the metal sheet 3 upon mold release. Through this bending test, of the two surfaces of the metal sheet 3, the surface 3 a in contact with the punch 2 becomes the inner surface of the bend. Furthermore, the surface 3 b in contact with the die 1 becomes the outer surface of the bend.

[0021] Here, the cracks for which the present invention evaluates bending performance are fine cracks that occur on the inner surface of the bent part, i.e., the surface 3a. The bending test step 10 described above is repeated for a metal plate 3 made of the same material to be evaluated, with the bending radius R of the punch tip 2c being changed. Note that a plurality of metal plates 3 to be evaluated are prepared. The test piece made of metal plate used in the bending test is, for example, in the shape of a strip.

[0022] <Bending Back Step 11> The bending back step 11 is a pre-processing step for the evaluation step 12 and constitutes part of the evaluation process. The bending back step 11 is a step of bending back the metal sheet 3 bent in the bending test step 10. The bending back angle is set to an angle greater than the angle change due to springback caused by demolding in the bending test. The bending back angle is also set to an angle at which minute cracks generated on the inner surface of the bent part can propagate. Such a bending back angle may be determined by computer-based forming analysis, experiments, or the like. This example is an example of bending back to 180 degrees, as described below.

[0023] As shown in FIG. 4 , the unbending process 11 of this embodiment is comprised of upper and lower dies 4 and 5 with opposing forming surfaces. The upper surface 4 a (forming surface) of the lower die 4 and the lower surface 5 a (forming surface) of the upper die 5, which face each other, are both flat. As a result, the forming surfaces have a bending angle of 180 degrees. In the unbending process 11 of this embodiment, the metal sheet 3 (see FIG. 3 ) bent in the bending test process 10 is placed on the upper surface of the lower die 4 as shown in FIG. 4 . Thereafter, in the unbending process 11 of this embodiment, the upper die 5 is moved relatively closer to the lower die 4 (see FIG. 4 ). The unbending process 11 of this embodiment is performed by sandwiching the metal sheet 3 between the lower die 4 and the upper die 5.

[0024] FIG. 5 is a diagram showing the progress of the unbending step 11. As shown in FIG. 5, the V-shaped metal sheet 3 is clamped and pressed between the opposing surfaces 4a, 5b of the lower die 4 and the upper die 5, thereby bending the bent portion back and deforming it into a flat shape. In this example, as shown in FIG. 6, the metal sheet 3 is bent back to 180 degrees at the bottom dead center of forming. However, this is not limited to this. The unbending step 11 may be, for example, a bending back less than 180 degrees or more than 180 degrees. Even if the metal sheet 3 is bent back to 180 degrees, springback occurs upon demolding. The left and right ends of the test piece may be restrained in the bent-back state or when the test piece is flat. The following evaluation step 12 may be performed in this state. Alternatively, the test piece may be observed in a free state.

[0025] <Evaluation step 12> Evaluation step 12 is a step of observing and evaluating the surface shape of the bent inner surface 3x of the bent inner surface 3a of the metal plate 3 that has been bent back as shown in Fig. 6. In this example, the evaluation of the surface shape is performed by one or more methods selected from the following three types of evaluation methods (1) to (3).

[0026] (1) In the first evaluation method, whether or not a visually discernible crack has occurred on the bent inner surface 3x after unbending is observed. If it is determined that a visually discernible crack has occurred on the bent inner surface 3x, the evaluation is performed as follows. That is, it is determined that a crack has occurred on the bent inner surface of the metal sheet 3 being evaluated due to bending deformation at the bending radius R used in the bending test in which it was determined that the crack occurred. Furthermore, if a visually discernible crack has occurred on the bent inner surface 3x after unbending, the evaluation is performed as follows. That is, the bending radius R used in the bending test in which it was determined that the crack has occurred is determined. When the metal sheet 3 being evaluated is bent at that bending radius R, fine cracks occur on the bent inner surface of the metal sheet 3. Then, the depth of the crack that occurs on the bent inner surface during bending using that bending radius R is evaluated to be 10 μm or more. That is, if a visually discernible crack has occurred on the bent inner surface 3x after unbending, it is evaluated that a crack with a depth of 10 μm or more has occurred during bending.

[0027] (2) The second evaluation method involves observing whether or not visually discernible, minute streak-like grooves extending in the direction of the bend ridge are present on the bent inner surface 3x after unbending. As described above, these grooves are also referred to as streak-like grooves. If it is determined that visually discernible streak-like grooves are present on the bent inner surface 3x, the evaluation is performed as follows. That is, it is determined that minute cracks are generated on the bent inner surface of the metal sheet 3 being evaluated due to bending deformation at the bending radius R used in the bending test. Furthermore, if visually discernible streak-like grooves are present on the bent inner surface 3x after unbending, the evaluation is performed as follows. That is, the bending radius R used in the bending test in which it is determined that the cracks are present is determined. When the metal sheet 3 being evaluated is bent at that bending radius R, minute cracks are generated on the bent inner surface of the metal sheet 3. The depth of the cracks generated on the bent inner surface during bending using that bending radius R is evaluated to be 5 μm or more. In other words, if a visually discernible streak-like groove is formed on the inner surface 3x of the bent piece after unbending, it is evaluated that a crack having a depth of 5 μm or more has occurred during the bending process.

[0028] (3) In the third evaluation method, whether or not the surface roughness of the bent inner surface 3x after unbending is equal to or greater than a predetermined threshold value is observed. If it is determined that the surface roughness of the inner surface is equal to or greater than the predetermined threshold value, the following evaluation is made. That is, it is determined that microcracks occur on the bent inner surface 3x of the metal plate 3 to be evaluated due to the bending deformation with the bending radius R used in the bending test. The observation direction of the surface roughness is, for example, a direction intersecting the direction of the bend ridgeline, e.g., a direction perpendicular to the direction of the bend ridgeline. The predetermined threshold value may be determined, for example, as follows. That is, the surface roughness of a surface that has been previously evaluated as having easily visible cracks or streak-like grooves on the surface 3x is measured. The above threshold value is then determined based on that surface roughness.

[0029] Here, the observation of cracks and streaky grooves using the first and second evaluation methods can be performed as follows. That is, the presence or absence of cracks and streaky grooves can be visually observed at the position of the surface 3X (see FIG. 6 ) on the inner bent surface 3a, which was the bend apex. A visually discernible crack is a crack with a width of 0.1 mm or more, and a visually discernible streaky groove is a groove with a width of 0.05 mm or more. However, the method for determining the surface shape of the surface condition at the position of the inner bent surface 3X is not limited to visual determination. For example, the determination may be performed by roughness measurement using a stylus roughness meter. Alternatively, the roughness and crack depth derived from the surface shape obtained by irradiating a laser beam and measuring the distance to the target may be utilized. Here, the third evaluation method is a method for evaluating the observations made using the first and second evaluation methods as surface roughness. That is, the third evaluation method is a method for observing and evaluating the surface shape using a measuring device such as a surface roughness meter.

[0030] Table 1 shows the correlation between the state of the bent inner surface 3x after the unbending step 11 in the evaluation method of this embodiment and the crack depth of the crack on the bent inner surface after the bending test. The contents of Table 1 were confirmed by experiments.

[0031]

[0032] As shown in Table 1, when the state of the bent inner surface 3x after the unbending step 11 remained unchanged, the crack depth of the crack on the bent inner surface after the bending test was less than 5 μm. When the state of the bent inner surface after the unbending step 11 showed linear grooves extending in the direction of the bend ridge, the crack depth of the crack on the bent inner surface after the bending test was 5 μm or more and less than 10 μm. When the state of the bent inner surface after the unbending step 11 showed cracks, the crack depth of the crack on the bent inner surface after the bending test was 10 μm or more.

[0033] The threshold value in the third evaluation method may be set as follows. That is, a surface shape in which cracks or streaky grooves that can be easily visually confirmed occur on the bent inner surface after the unbending process 11 is identified. The surface roughness corresponding to the identified surface shape is determined, and the threshold value may be set from that surface roughness. Furthermore, as can be seen from Table 1, by using the first evaluation method and the second evaluation method in combination, the depth of cracks that occur during bending can be easily classified.

[0034] <Index Value Calculation Step 13> The index value calculation step 13 is a step of calculating an evaluation value. Here, the thickness of the metal plate 3 evaluated as having cracks in the evaluation step 12 is defined as t [mm]. Furthermore, the bending radius of the punch tip 2c used in the bending test in which the cracks were evaluated as having occurred is defined as R [mm]. In this case, the index value calculation step 13 calculates the evaluation value by dividing R [mm] by the plate thickness t [mm] (R / t). The evaluation value is a value for evaluating bending performance according to the bending radius R at which fine cracks occur during bending. Furthermore, in the index value calculation step 13, the maximum value of the calculated multiple evaluation values ​​is calculated as the limit value of bending performance. This maximum evaluation value is the limit value at which cracks occur on the inner surface 3x of the bent portion when the metal plate 3 to be evaluated is bent. The maximum evaluation value is then used when determining (designing) the processing conditions, for example, by setting the bending radius R so that the evaluation value is smaller than the maximum evaluation value when bending the evaluation object.

[0035] (Effects) According to this embodiment, by unbending the metal plate 3 that has been bent (bent), fine cracks that have occurred during the bending process are propagated. This makes it easier to observe whether fine cracks have occurred during the bending process. As a result, this embodiment makes it possible to easily and efficiently evaluate cracks that occur on the inner surface of the bent metal plate 3 during the bending process.

[0036] Conventionally, the following procedure is required. That is, after cutting out the bent portion after the bending test, the cross section of the cut-out bent portion is polished to a mirror finish. Then, the cross section near the bent inner surface is observed in detail using a microscope or the like. On the other hand, in this embodiment, such processes as cutting out and polishing are not necessary. Therefore, in this embodiment, the presence or absence of microcracks and the depth of cracks that occur on the bent inner surface when bending the metal plate 3 can be easily evaluated. Here, microcracks are cracks with extremely small depths and widths. As a result, this embodiment has the effect of avoiding a significant decrease in work efficiency.

[0037] Furthermore, according to this aspect of the present invention, the range of the bending radius R at which cracks occur during bending can be easily estimated. Furthermore, if it is desired to evaluate bending performance in detail, it is sufficient to narrow the bending test to within the range of the bending radius R limited by this aspect of the present invention. That is, it is sufficient to separately and in detail observe only the bent portion in the bending test limited by the evaluation method of this embodiment using a conventional method. A conventional method is, for example, a method in which the cross section of the bent portion is mirror-polished and the cross section near the inner surface of the bent portion is observed in detail using a microscope or the like. By narrowing the range of the metal plate to be observed in detail, the work efficiency of the detailed observation is improved compared to conventional methods. Note that if clear, visible cracks occur on the inner surface of the bent portion after the bending test, there is no need to perform the simple evaluation according to this embodiment. The present disclosure is a technology suitable for evaluating bending performance when minute cracks that are not easily visible occur during bending.

[0038] (Other) The present disclosure may also have the following configurations. (1) Disclosure 1 discloses a bending performance evaluation method for evaluating the bending performance of a metal sheet to be evaluated against bending deformation by performing a bending test in which the metal sheet to be evaluated is bent using a punch having a bending radius at its tip, the bending performance evaluation method including: a bending back step of bending back the metal sheet bent in the bending test; and an evaluation step of evaluating the surface shape of the inner surface of the bent metal sheet after the bending back step to determine whether cracks will occur on the inner surface of the bent metal sheet to be evaluated due to bending deformation at the bending radius. (2) Disclosure 2 discloses that, in the evaluation step, if the inner surface of the bent metal sheet is evaluated to have a surface shape with visually discernible cracks, it is determined that cracks will occur on the inner surface of the bent metal sheet to be evaluated when bending deformation is performed at the bending radius. (3) Disclosure 3 discloses that, in the evaluation step, if the inner surface of the bent metal sheet is evaluated to have a surface shape with visually discernible cracks, it is determined that cracks with a depth of 10 μm or more will occur on the inner surface of the bent metal sheet to be evaluated when bending deformation is performed at the bending radius. (4) Disclosure 4 states that, in the evaluation step, if the bent inner surface is evaluated as having a surface shape with visually discernible streak-like grooves extending in the bend ridge line direction, it is determined that a crack will occur on the bent inner surface of the metal plate being evaluated when bending is performed at the above bend radius. (5) Disclosure 5 states that, in the evaluation step, if the bent inner surface is evaluated as having a surface shape with visually discernible streak-like grooves extending in the bend ridge line direction, it is determined that a crack with a depth of 5 μm or more will occur on the bent inner surface when bending is performed at the above bend radius. (6) Disclosure 6 evaluates that in the evaluation process, if the inner surface of the bend is evaluated as having a surface shape with visually discernible cracks, when the metal plate to be evaluated is bent at the bending radius, cracks with a depth of 10 μm or more will occur on the inner surface of the bend; and if the inner surface of the bend is evaluated as having a surface shape with visually discernible streak-like grooves extending in the direction of the bending ridge, when the metal plate to be evaluated is bent at the bending radius, cracks with a depth of 5 μm or more will occur on the inner surface of the bend.(7) Disclosure 7 describes a method for evaluating a bending performance of a metal plate to be evaluated by changing the bending radius of a punch tip, dividing the bending radius of a punch tip at a bending test when the evaluation step determines that a crack has occurred by the thickness of the metal plate, and evaluating the maximum value of the evaluation value as the limit value of bending performance at which a crack will occur on the inner side of the bend when the metal plate is bent. (9) Disclosure 9 changes the bending radius of the punch tip and performs a bending test at each bending radius, determines the range of bending radii in which cracks are estimated to occur on the inner surface of the bend in the bending test from the bending radius in the bending test when it is determined that a crack has occurred in the evaluation process, performs a bending test in which a metal plate to be evaluated is bent using a punch with a bending radius that includes the determined range of bending radii, and evaluates the presence or absence of cracks on the inner surface of the bend and the depth of the cracks if they occur by observing the inner surface bent and deformed in the bending test. (10) Disclosure 10 uses a 90-degree V-shaped punch as the punch for the bending test, and uses two punches with opposing flat pressing surfaces as punches for performing the unbending.

[0039] Examples based on this embodiment will be described below. In this example, two types of steel plates with different metal microstructures were used as the metal plate 3. Both of these types of steel plates had a tensile strength of 780 MPa (SPH780) and a plate thickness of 2.6 mm. Furthermore, a steel plate with a tensile strength of 980 MPa (SPH980) and a plate thickness of 2.9 mm was used as the metal plate 3. Each of these steel plates was processed into a rectangular shape to prepare respective test specimens. The dimensions of each test specimen were 100 mm long x 30 mm short.

[0040] In the following description, the above-mentioned test specimen having a tensile strength of 980 MPa will be referred to as test specimen A. Furthermore, the above-mentioned test specimens made of two types of steel plates having a tensile strength of 780 MPa will be referred to as test specimen B and test specimen C, respectively. In this example, each test specimen was subjected to a bending test step 10 using the mold described in the above embodiment. Thereafter, each test specimen was subjected to a bending-back step 11, and the inner surface of each test specimen after the bending-back step 11 was observed.

[0041] <Step 10 of bending test> The die 1 and punch 2 used in the bending test were made of SKD11 (alloy tool steel). The cross-sectional shapes of the die 1 and punch 2 were the same as those shown in the embodiment (see FIG. 2). Furthermore, several types of punches with different bending radii R of the tip 2c were used as the punch 2. Note that bending radius is synonymous with radius of curvature. The bending radii R of the tip 2c of the punches used were selected from the range of 1.0 mm to 10 mm. Specifically, four types of bending radii R of the punch tip 2c were used: 1.0 mm, 3.0 mm, 6.0 mm, and 10 mm.

[0042] The bending test was performed by setting the bending direction so that the tip of the punch 2 contacted the line connecting the center points of the long sides of the test piece. That is, the bending test was performed by setting the direction so that the short sides of the test piece were parallel to the bending ridge line. A universal testing machine capable of outputting a maximum load of 250 kN in the vertical direction was used as the testing machine for pressing the punch 2 into the die 1. Step 10 of the bending test was performed by fixing the punch 2 to the movable part of the testing machine. The vertical direction is the plate thickness direction. At this time, the moving speed of the punch 2 was set to a constant speed of 30 mm / min. Furthermore, when the punch 2 was pressed into the die 1, the maximum load received by the punch 2 was 150 kN.

[0043] <Bending-back Process 11> SKD11 (alloy tool steel) was used as the material for the lower die 4 and the upper die 5 used in the bending-back process 11. The shapes of the forming surfaces of the lower die 4 and the upper die 5 were the same as those shown in the embodiment (see FIG. 4). The bending-back process 11 was performed by placing the test piece on the flat upper surface 4a of the lower die 4 so that the test piece after the bending test would be in an inverted V shape (see FIG. 4). The upper die 5 was then lowered so that the flat lower surface 5a of the upper die 5 contacted the apex of the bend of the test piece. A universal testing machine capable of outputting a maximum load of 500 kN in the vertical direction was used as the testing machine for lowering the upper die 5. The bending-back process 11 was performed by fixing the upper die 5 to the movable part of the testing machine. The moving speed of the upper die 5 was set to 30 mm / min. The maximum load received by the upper die 5 during the test was 375 kN.

[0044] <Evaluation> The test piece bent back in the bending back step 11 was removed from the testing machine, and the surface that had been the inner surface during the bending test was visually observed. The presence or absence of cracks and minute grooves (striated grooves) extending in the direction of the bend ridge was evaluated. Table 2 shows the evaluation results of the surface condition after bending back based on this embodiment.

[0045]

[0046] As can be seen from Table 2, when test piece A having a tensile strength of 980 MPa and a thickness of 2.9 mm was used, the following was found. That is, when the bending radius R in the bending test was 3.0 mm, cracks occurred on the inner surface of the bent piece after the unbending step 11. Furthermore, when the bending radius R was 6.0 mm, small streak-like grooves extending in the direction of the bend ridgeline occurred on the inner surface of the bent piece. Furthermore, when the bending radius R was 10 mm, the inner surface of the bent piece remained unchanged. Similarly, from Table 2, the following was found when test pieces B and C having a tensile strength of 780 MPa and a thickness of 2.6 mm were used. That is, when the bending radius R was 1.0 mm, cracks occurred on the inner surface of the bent piece. Furthermore, when the bending radius R was 3.0 mm, small streak-like grooves extending in the direction of the bend ridgeline occurred on the inner surface of the bent piece. Furthermore, when the bending radius R was 10 mm, the inner surface of the bent piece remained unchanged.

[0047] <Verification of Evaluation Method Based on This Embodiment> The following experiment verified that the bending performance table method of this embodiment can evaluate the crack depth of cracks that occur on the bent inner surface during actual bending of a metal plate 3. Test pieces A, B, and C made of the same material as the metal plate 3 used in the above experiment were subjected to a bending test under the same conditions as in this example, and then bent portions were cut out from each test piece after the bending test. Then, the cross section of the cut-out bent portion was polished to a mirror finish as in the conventional method, and the cross section near the bent inner surface was observed in detail using a microscope or the like, and the crack depth of cracks that occurred on the bent inner surface during the bending test was measured.

[0048] Figure 7 shows a graph plotting the relationship between the bending radius R and the crack depth of cracks on the inner side of the bend in the bending test. As can be seen from Figure 7, in the case of test piece A, which had a tensile strength of 980 MPa and a plate thickness of 2.9 mm, the following was found. That is, when the bending radius R was 3.0 mm, cracks with a depth exceeding 30 μm occurred on the inner side of the bend. Furthermore, when the bending radius R was 6.0 mm, cracks with a depth of 5 μm or more but less than 10 μm occurred. Furthermore, when the bending radius R was 10 mm, cracks with a depth less than 5 μm occurred.

[0049] Similarly, in the case of test pieces B and C with a tensile strength of 780 MPa and a plate thickness of 2.6 mm, the following was found. That is, when the bending radius R was 1.0 mm, cracks with a depth exceeding 10 μm occurred on the inside of the bend. Furthermore, when the bending radius R was 3.0 mm, cracks with a depth of 5 μm or more and less than 10 μm occurred. Furthermore, when the bending radius R was 10 mm, cracks with a depth of less than 5 μm occurred. It was found that the results in FIG. 7 correlate with the results in Table 2.

[0050] As a result, the bending performance of test piece A can be evaluated by bending using a punch 2 with a bending radius R of 3.0 mm or more but less than 6.0 mm, which is the limit value at which cracks of 10 μm or more in depth occur on the inner surface of the bent piece. For example, the bending radius R corresponding to this limit value is assumed to be 3.0 mm. In this case, the evaluation value for the limit value at which cracks of 10 μm or more in depth occur is 3.0 / 2.9, which is approximately 1.0. Furthermore, the evaluation value for the limit value at which cracks of 5 μm or more in depth occur can be evaluated by bending using a punch 2 with a bending radius R of 6.0 mm or more but less than 10.0 mm. For example, the bending radius R corresponding to this limit value is assumed to be 6.0 mm. In this case, the evaluation value for this limit value is 6.0 / 2.9, which is approximately 2.1.

[0051] Similarly, the bending performance of test pieces B and C can be evaluated by bending using a punch 2 with a bending radius R in the range of 1.0 mm or more and less than 3.0 mm, which is the limit value at which cracks of 10 μm or more in depth occur on the bent inner surface. For example, the bending radius R corresponding to this limit value corresponds to 1.0 mm. In this case, the evaluation value for this limit value is 1.0 / 2.6, which is approximately 0.4. Furthermore, the limit value at which cracks of 5 μm or more in depth occur can be evaluated by bending using a punch 2 with a bending radius R in the range of 3.0 mm or more and less than 10.0 mm. For example, the bending radius R corresponding to this limit value corresponds to 3.0 mm. In this case, the evaluation value for this limit value is 3.0 / 2.6, which is approximately 1.1.

[0052] <Verification of streak-like grooves> Fig. 8 shows the increase in surface roughness due to the unbending process when streak-like grooves are formed on the inner surface of the bent piece according to this example. The increase in surface roughness is the increase in surface roughness (%) due to the unbending process 11, with the surface roughness after the bending test process 10 and before the unbending process 11 as the reference. Test pieces B and C were used here as test pieces for verification.

[0053] The surface roughness of each bent inner surface was measured before and after the unbending process 11 using a laser microscope (Keyence VK-X3000). The measurements showed that the surface roughness increase rate when minute streak-like grooves (streak-like grooves) extending in the direction of the bend ridgeline occurred was approximately 5.3 times or more in Ra and approximately 4.6 times or more in Rz, as shown in Figure 8. Thus, it was found that the surface roughness after the unbending process 11 can also be used to determine the presence or absence of cracks or streak-like grooves that can be easily identified by visual inspection after the unbending process 11.

[0054] The above verification experiment results showed that there is a correlation between the state of cracks and streak-like grooves on the bent inner surface of the metal sheet 3 after the bending test and the unbending process 11, and the depth of the cracks that occurred on the bent inner surface during the bending test. In other words, the effectiveness of the bending performance evaluation results based on this embodiment was verified. In this example, three test pieces of the same steel type were prepared, and the same bending evaluation was performed three times each to evaluate the reproducibility of the invented evaluation method. Since the same evaluation results were obtained all three times, it can be said that the reproducibility of the evaluation method of the present invention is high.

[0055] The entire contents of Japanese Patent Application No. 2024-042963 (filed March 19, 2024), from which this application claims priority, are incorporated herein by reference. While the present application has described 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.

[0056] REFERENCE SIGNS LIST 1 Die 2 Punch 2c Tip 3 Metal plate 3X Observation position of bent inner surface 4 Lower die 5 Upper die 10 Bending test process 11 Unbending process 12 Evaluation process 13 Index value calculation process R Bending radius

Claims

1. A bending performance evaluation method for evaluating the bending performance of a metal plate to be evaluated against bending deformation by performing a bending test in which the metal plate to be evaluated is bent using a punch with a bending radius at its tip, the bending performance evaluation method comprising: a bending back process for bending back the metal plate that has been bent and deformed in the bending test; and an evaluation process for evaluating the surface shape of the inner surface of the bent metal plate after the bending back process, thereby determining whether or not cracks will occur on the inner surface of the bent metal plate to be evaluated due to bending deformation at the bending radius.

2. A bending performance evaluation method as described in claim 1, wherein, in the evaluation step, if the inner surface of the bend is evaluated as having a surface shape with visually discernible cracks, it is determined that cracks will occur on the inner surface of the bent metal plate when bent at the bending radius.

3. A bending performance evaluation method according to claim 2, wherein, if the evaluation step evaluates that the inner surface of the bend has a surface shape with cracks that are visible to the naked eye, it is evaluated that when the metal plate to be evaluated is bent at the bending radius, a crack with a depth of 10 μm or more will occur on the inner surface of the bend.

4. A bending performance evaluation method according to any one of claims 1 to 3, wherein in the evaluation step, if the inner surface of the bend is evaluated as having a surface shape with visually discernible linear grooves extending in the direction of the bend ridgeline, it is determined that cracks will occur on the inner surface of the bent metal plate when bending is performed at the bending radius.

5. A bending performance evaluation method as set forth in claim 4, wherein, in the evaluation step, if the inner surface of the bend is evaluated as having a surface shape with visually discernible linear grooves extending in the direction of the bend ridgeline, it is evaluated that when the metal plate to be evaluated is bent at the bending radius, a crack with a depth of 5 μm or more will occur on the inner surface of the bend.

6. A bending performance evaluation method according to claim 1, wherein in the evaluation step, if the inner bend surface is evaluated to have a surface shape with visually discernible cracks, it is evaluated that cracks 10 μm or more deep will occur on the inner bend surface when the metal plate to be evaluated is bent at the bending radius, and if the inner bend surface is evaluated to have a surface shape with visually discernible streak-like grooves extending in the direction of the bend ridge, it is evaluated that cracks 5 μm or more deep will occur on the inner bend surface when the metal plate to be evaluated is bent at the bending radius.

7. A bending performance evaluation method according to any one of claims 1 to 6, wherein in the evaluation step, if the surface roughness of the inner bend surface is evaluated to be equal to or greater than a predetermined threshold, it is determined that cracks will occur on the inner bend surface of the metal plate being evaluated if bending deformation is performed at the bending radius.

8. A bending performance evaluation method according to any one of claims 1 to 7, wherein a bending test is carried out by changing the bending radius of the punch tip, the bending radius in the bending test when it is determined that a crack has occurred in the evaluation process is divided by the thickness of the metal plate being evaluated to determine an evaluation value, and the maximum of these evaluation values ​​is evaluated as the limit value of bending performance at which a crack will occur on the inside of the bend when the metal plate being evaluated is bent.

9. A bending performance evaluation method according to any one of claims 1 to 7, comprising: varying the bending radius of the punch tip and performing a bending test at each bending radius; determining a range of bending radii within which cracks are estimated to occur on the inner surface of the bend in the bending test from the bending radius in the bending test when it is determined that a crack has occurred in the evaluation process; performing a bending test in which the metal plate to be evaluated is bent using a punch having a bending radius that includes the determined range of bending radii; and evaluating the presence or absence of cracks on the inner surface of the bend and the depth of the cracks if they have occurred by observing the inner surface bent and deformed in the bending test.

10. A bending performance evaluation method according to any one of claims 1 to 7, wherein a 90-degree V-shaped punch is used as the punch for the bending test, and two punches with opposing flat pressing surfaces are used as the punches for performing the unbending.

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