JP7648021B1 - Bending performance evaluation method - Google Patents

Abstract

The objective of the present invention is to provide a method capable of efficiently evaluating fine cracks that occur on the inner surface of a bent metal sheet during bending processing. The bending performance evaluation method evaluates the bending performance of a metal sheet (3) to be evaluated against bending deformation by performing a bending test in which the metal sheet (3) to be evaluated is bent using a punch (2) having a bending radius (R) at its tip (2c), and includes a bending back process (11) for bending back the metal sheet (3) bent and deformed in the bending test, and an evaluation process (12) for evaluating the surface shape of the inner surface (3x) of the bent metal sheet (3) after the bending back process (11) to determine whether or not cracks occur on the inner surface (3x) of the bent metal sheet (3) to be evaluated due to bending deformation at the bending radius (R).

Description

The present invention relates to a technique for easily evaluating bending performance during bending of metal sheets, such as automotive metal sheets. The bending performance may be, for example, bending deformation. The present invention is particularly applicable to evaluation of fine cracks that occur on the inner surface of a bent sheet during bending. In this specification, the term "fine cracks" refers to cracks that are not easily discernible by visual inspection or that are difficult to discern by visual inspection.
It is not necessary to apply this application to the evaluation of metal plates in which cracks that are easily visible to the naked eye occur on the inner surface of the bent plate during bending tests.

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

The bending performance of a metal sheet can be evaluated, for example, by the following evaluation method.
Patent Document 1 describes a bending process in which a tapered punch is pressed into the metal plate to be evaluated while tension is applied to the metal plate. Patent Document 1 also proposes a method for determining a limit condition as a relationship between tension and the bending radius at the tip of the punch. The limit condition is a limit condition that does not cause minute streaky grooves, cracks, or breaks in the bent portion. Patent Document 1 describes the minute grooves as "necking."

Patent Document 2 also describes a method of performing a bending test while photographing the outer surface of a bent metal plate with a camera. Patent Document 2 also proposes a method of evaluating bending performance based on information such as strain distribution and changes on the outer surface of the bend, and occurrence of cracks.
In addition, in Patent Document 3, a portion that is subjected to bending and then unbending is evaluated. Patent Document 3 also discloses an evaluation method for cracks that occur from the inside of the bend during bending and unbending.

JP 2011-235301 A JP 2021-135128 A JP 2015-47605 A

The evaluation methods described in Patent Documents 1 and 2 are methods for evaluating bending performance on the outer side of a bend, that is, the evaluation methods described in Patent Documents 1 and 2 are not methods for evaluating bending performance on the inner side of a bend.
In addition, the method described in Patent Document 3 is a method for evaluating the bending performance of the inner surface of a bent piece. However, Patent Document 3 is for evaluating bending performance with respect to bending and unbending. In other words, the method described in Patent Document 3 is not for evaluating bending performance with respect to bending. In other words, Patent Document 3 is a method for evaluating cracks that occur during unbending in a portion that is subjected to unbending after bending. And, this method of Patent Document 3 cannot identify the presence or absence of fine cracks that have already occurred on the inner surface of a bent piece immediately after bending, or the depth of the cracks. In other words, the method of Patent Document 3 has a problem in that it cannot evaluate bending performance.

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 streak-like 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 such characteristics by optical observation from the surface of the material.

Conventionally, when determining whether or not cracks have occurred due to bending, or when identifying the depth of a crack, it is necessary to perform an evaluation as follows. That is, first, the bent portion of the metal plate is cut out, and the cut-out bent portion is embedded in resin. After that, the cross section of the bent portion is polished to a mirror finish. Then, the cross section near the inner surface of the bend is observed in detail using a microscope or the like. However, this type of work has the problem that the efficiency of the work for evaluating cracks on the inner surface of the bend due to bending is significantly reduced.

The present invention focuses on the above points. The purpose of the present invention is to provide a method for efficiently evaluating microcracks that occur on the inner surface of a bent metal plate during bending.

In order to solve the above problems, the inventors conducted a bending test on a metal plate and measured the crack depth on the bent inner surface. The crack depth was measured based on detailed observation of the bent cross section. In addition, the same type of metal plate was subjected to a bending test under the same bending conditions, and then a bending-back process was carried out. As a result of this, the occurrence of cracks and streak-like minute grooves extending in the direction of the bent ridgeline was observed on the bent inner surface after bending back. As described above, streak-like minute grooves are also called streak-like grooves.
The inventors compared the test results of both methods, and as a result, the inventors found that there is a correlation between the state of cracks and streaky grooves on the inner surface of the bent metal plate that was bent back after the bending test and the depth of the cracks that occurred on the inner surface of the bent metal plate in the bending test.
In light of this, the following evaluation method was found to be a method for evaluating the bending performance of a metal sheet that solves the above problems.

In order to solve the problem, 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 a punch having a bending radius at its tip is used to bend the metal plate to be evaluated, the bending performance evaluation method including a bending back process for bending back the metal plate 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 to determine 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.

In one embodiment of the present invention, a material that has been bent (bent) is bent back. This allows fine cracks that occur 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 occur on the inner surface of a bent metal plate when it is bent.

Conventionally, after cutting out the 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 bend using a microscope or the like. However, according to the present invention, such cutting out and polishing are not necessary. That is, according to the present invention, it is possible to easily evaluate the presence or absence of cracks and their depths for cracks that occur on the inner surface of the bend when bending a metal plate and have extremely minute depths and widths. Therefore, according to the present invention, it is possible to avoid a significant decrease in work efficiency.

In addition, according to the present invention, the range of bending radii in which cracks can be estimated to occur during bending can be easily estimated by the evaluation in the evaluation process. Therefore, for example, if one wishes to evaluate the bending performance of bending in detail, one can narrow down the materials to those that have undergone bending tests within the range of bending radii limited by the present invention. Then, for only the narrowed down materials, it becomes possible to mirror-finish polish the cross section of the bent part as in the conventional method, and to perform detailed observation of the cross section near the inner surface of the bend using a microscope or the like.

1 is a diagram showing an example of a 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. 4 is a side view showing the shape of the metal plate after a bending test process. FIG. 4 is a schematic diagram illustrating an unbending process according to an embodiment of the present invention. FIG. 11 is a side view showing a state in the middle of a bending back process. FIG. 13 is a side view showing the metal plate and the position of the bent inner surface after the unbending process. 1 is a graph showing the relationship between the bending radius in a bending test and the crack depth of a crack on the inside of the bend. FIG. 13 is a diagram illustrating an example of an increase rate of surface roughness before and after an unbending process when a stripe-shaped groove is formed on the bent inner surface.

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 plate. The present invention is suitable for steel plates, and particularly suitable for hot-rolled steel plates. The present invention relates to a method for easily evaluating bending performance during bending.

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

<Bending test step 10>
First, the bending test step 10 will be described with reference to Fig. 2. The bending test step 10 is a step of performing a bending test simulating bending processing, which is an actual test.
In the bending test step 10 of this embodiment, a die 1 and a punch 2 as shown in FIG. 2 are used as metal molds.
The die 1 used in this embodiment has a forming surface with a concave cross-sectional shape as shown in FIG. 2. The concave cross-sectional shape is V-shaped. The V-shape is composed of a pair of inclined surfaces 1a, 1b. The intersection of the pair of inclined surfaces 1a, 1b is 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 a plurality of bending radii R. The forming surface of the die 1 is formed so that the concave cross-sectional shape extends along a direction perpendicular to the paper surface of FIG. 2.

The punch 2 has a forming surface having 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 have a shape following 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, a plurality of punches 2 are provided. The bending radii R of the tips 2c of the punches 2 are different from each other.
Here, the angle θ of the V shape formed by the pair of inclined surfaces 1a and 1b is, for example, 90 degrees. However, the angle θ of the V shape may be other than 90 degrees. In addition, the angle of the V shape formed by the pair of inclined surfaces 2a and 2b is, for example, equal to the angle θ of the V shape formed by the pair of inclined surfaces 1a and 1b.

2, the bending test is performed by first placing the metal plate 3 to be evaluated on the die 1. The bending test is performed by pressing the punch 2 toward the die 1 in a direction in which the inclined surfaces 2a and 2b of the punch 2 approach the inclined surfaces 1a and 1b of the die 1.
In 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 at the bottom dead center of forming into a V-shaped cross-sectional shape in which the bent portion has a bending radius R. Even if the metal sheet 3 is bent to 90 degrees, springback occurs in the metal sheet 3 when it is released from the mold.
In this bending test, of the two surfaces of the metal sheet 3, the surface 3a in contact with the punch 2 becomes the bent inner surface, and the surface 3b in contact with the die 1 becomes the bent outer surface.

Here, the cracks for which bending performance is evaluated in the present invention are fine cracks that occur on the inner surface of the bent portion, i.e., on surface 3a.
The bending test step 10 described above is repeatedly performed on the metal plate 3 made of the same material to be evaluated, while changing the bending radius R of the punch tip 2c.
A plurality of evaluation target metal plates 3 are prepared in advance. The test pieces made of metal plates used in the bending test are, for example, rectangular in shape.

<Bending back process 11>
The bending back process 11 is a pre-processing process for the evaluation process 12 and constitutes a part of the process for evaluation.
The bending back process 11 is a process for bending back the metal sheet 3 bent in the bending test process 10. The bending back angle is set to an angle larger than the angle change caused by springback due to demolding in the bending test. The bending back angle is 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 forming analysis using a computer, experiments, or the like. This example is an example of bending back up to 180 degrees, as described later.

In the bending-back process 11 of this embodiment, as shown in Fig. 4, the die 4 and 5 are configured with their forming surfaces facing each other. The upper surface 4a (forming surface) of the lower die 4 and the lower surface 5a (forming surface) of the upper die 5, which face each other, are both flat surfaces. As a result, the forming surfaces have a bending angle of 180 degrees.
In the bending back 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. Then, in the bending back process 11 of this embodiment, the upper die 5 is moved relatively closer to the lower die 4 (see FIG. 4). Then, the bending back process 11 of this embodiment is performed by sandwiching the metal sheet 3 between the lower die 4 and the upper die 5.

Fig. 5 is a diagram showing an intermediate state in the bending-back process 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, whereby the bent portion is bent back and deformed into a flat shape.
In this embodiment, as shown in Fig. 6, the metal sheet 3 is bent back to 180 degrees at the bottom dead center of forming. However, the present invention is not limited to this. The bending back process 11 may be performed by 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 due to demolding.
The left and right ends of the test piece may be restrained in a bent-back state or in a flattened state. In this state, the observation in the evaluation step 12 described below may be performed. Also, the test piece may be observed in a free state.

<Evaluation step 12>
The 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 bent back as shown in FIG.
In this example, the surface shape is evaluated by one or more methods selected from the following three types of evaluation methods (1) to (3).

(1) In the first evaluation method, after bending back, the bent inner surface 3x is observed to see whether or not a visually noticeable crack has occurred. If it is determined that a visually noticeable 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 plate 3 to be evaluated due to the bending deformation caused by the bending radius R used in the bending test in which it was determined that the crack has occurred.
Furthermore, if a visually noticeable crack occurs 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 occurred is determined. When the metal plate 3 to be evaluated is bent at that bending radius R, fine cracks occur on the bent inner surface of the metal plate 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 noticeable crack occurs on the bent inner surface 3x after unbending, it is evaluated that a crack with a depth of 10 μm or more occurs during bending.

(2) The second evaluation method is to observe whether or not minute streak-like grooves extending in the direction of the bend ridgeline that can be visually identified are generated on the bent inner surface 3x after unbending. As described above, this groove is also called a streak-like groove. If it is determined that a streak-like groove that can be visually identified is generated 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 plate 3 to be evaluated due to bending deformation at the bending radius R used in the bending test.
Furthermore, if a streak-like groove that can be visually identified occurs on the bent inner surface 3x after unbending, it is evaluated as follows. That is, the bending radius R used in the bending test in which it was determined that the crack occurred is determined. When the metal plate 3 to be evaluated is bent and deformed with that bending radius R, fine cracks occur on the bent inner surface of the metal plate 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 5 μm or more. That is, if a streak-like groove that can be visually identified occurs on the bent inner surface 3x after unbending, it is evaluated that a crack with a depth of 5 μm or more has occurred during bending.

(3) In the third evaluation method, it is observed whether the surface roughness of the inner surface 3x of the bent part after bending back is equal to or greater than a preset threshold value. If it is determined that the surface roughness of the inner surface is equal to or greater than the preset threshold value, the following judgment is made. That is, it is determined that the bending deformation of the bending radius R used in the bending test causes fine cracks to occur on the inner surface 3x of the bent metal plate 3 to be evaluated. The observation direction of the surface roughness is, for example, a direction intersecting the bending ridge direction, for example, a direction perpendicular to the bending ridge direction. The preset 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.

Here, the observation of cracks and streaky grooves by the above-mentioned first evaluation method and second evaluation method may be carried out as follows. That is, it is sufficient to visually observe whether cracks and streaky grooves have occurred at the position of the surface 3X (see FIG. 6) that was the bend apex on the bent inner surface 3a. A crack that can be visually identified is a crack with a width of 0.1 mm or more, and a streaky groove that can be visually identified is a groove with a width of 0.05 mm or more.
However, the method of judging the surface shape of the surface condition at the position of the bent inner surface 3X is not limited to visual judgment. For example, it may be judged by roughness measurement using a stylus roughness meter. In addition, roughness and crack depth derived from the surface shape obtained by irradiating a laser beam and measuring the distance to the target may be used.
Here, the third evaluation method is a method of evaluating the observations made by the first and second evaluation methods as surface roughness. That is, the third evaluation method is a method of observing and evaluating the surface shape using a measuring device such as a surface roughness meter.

Table 1 shows the correlation between the state of the inner bend surface 3x after the bending back process 11 in the evaluation method of this embodiment and the crack depth of the crack on the inner bend surface after the bending test. The contents of Table 1 were confirmed by experiments.

As shown in Table 1, when the state of the bent inner surface 3x after the unbending step 11 was unchanged, the crack depth of the cracks on the bent inner surface after the bending test was less than 5 μm.
Furthermore, when the state of the bent inner surface after the unbending step 11 was such that a streak-like groove extending in the direction of the bent ridge line occurred, 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.
Furthermore, when cracks were found on the bent inner surface after the unbending step 11, the depth of the cracks on the bent inner surface after the bending test was 10 μm or more.

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 confirmed by visual inspection are generated on the bent inner surface after the unbending process 11 is identified. The surface roughness corresponding to the identified surface shape is obtained, and the threshold value may be set from the surface roughness.
Furthermore, as can be seen from Table 1, by using the first evaluation method and the second evaluation method in combination, it becomes possible to easily classify the depth of cracks that occur during bending.

<Index value calculation step 13>
The index value calculation step 13 is a step of determining an evaluation value.
Here, the sheet thickness of the metal sheet 3 evaluated as having cracks in the evaluation step 12 is t [mm]. The bending radius of the punch tip 2c used in the bending test in which the cracks were evaluated as having occurred is R [mm]. In this case, the index value calculation step 13 obtains the value (R/t) obtained by dividing R [mm] by the sheet thickness t [mm] as the evaluation value. The evaluation value is a value for evaluating bending performance according to the bending radius R at which fine cracks occur during bending.
In addition, in the index value calculation process 13, the maximum value of the obtained evaluation values is obtained 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.
Then, for example, when bending the evaluation object, the maximum evaluation value is used when determining (designing) the processing conditions, such as by setting the bending radius R so that the evaluation value is smaller than the maximum evaluation value.

(effect)
According to this embodiment, the metal plate 3 that has been bent (bent) is unbent to allow microcracks that have occurred during bending to propagate. This makes it easier to observe whether or not microcracks have occurred during bending. As a result, this embodiment makes it possible to easily and efficiently evaluate cracks that have occurred on the inner surface of the bent metal plate 3 during bending.

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 mirror-finished and polished. Then, the cross section near the inner surface of the bend is observed in detail using a microscope or the like. On the other hand, in this embodiment, such cutting out and polishing processes are not required. Therefore, in this embodiment, the presence or absence of fine cracks and the depth of the cracks that occur on the inner surface of the bend when the metal plate 3 is bent can be easily evaluated. Here, fine cracks are cracks with extremely small crack depths and crack widths. As a result, this embodiment has the effect of avoiding a significant decrease in work efficiency.

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

(others)
The present disclosure may also have the following configuration.
(1) Disclosure 1 provides a bending performance evaluation method for evaluating bending performance of a metal plate to be evaluated against bending deformation by performing a bending test in which a punch having a bending radius at a tip end is used to bend the metal plate to be evaluated,
a bending back process for bending back the metal plate bent and deformed in the bending test;
an evaluation step of evaluating a surface shape of the inner surface of the bent metal plate after the unbending step, thereby determining whether or not a crack occurs on the inner surface of the bent metal plate due to bending deformation at the bending radius;
The bending performance evaluation method has the following characteristics.
(2) Disclosure 2 states that in the above evaluation process, if the inner surface of the bend is evaluated to have a surface shape having a crack that is visible to the naked eye, it is determined that a crack will occur on the inner surface of the bent metal plate being evaluated when bending deformation is performed at the above bending radius.
(3) Disclosure 3 evaluates that, if the evaluation process evaluates that the inner surface of the bend has a surface shape having a crack that is visible to the naked eye, when the metal plate to be evaluated is bent and deformed at the bending radius, a crack having a depth of 10 μm or more will occur on the inner surface of the bend.
(4) Disclosure 4 states that in the above evaluation process, if the inner surface of the bend is evaluated to have a surface shape having a streak-like groove extending in the direction of the bend ridge line that is discernible by visual inspection, it is determined that a crack will occur on the inner surface of the bent metal plate being evaluated when bending deformation is performed with the above bending radius.
(5) Disclosure 5 evaluates, in the above evaluation process, that the inner surface of the bend has a surface shape having a streak-like groove extending in the direction of the bend ridge line that is discernible by visual inspection, and evaluates that when the metal plate to be evaluated is bent and deformed at the above bending radius, a crack having a depth of 5 μm or more will occur on the inner surface of the bend.
(6) Disclosure 6 relates to the above-mentioned evaluation step,
When the bent inner surface is evaluated as having a surface shape having a crack that can be visually identified, it is evaluated that when the metal plate to be evaluated is bent and deformed at the bending radius, a crack having a depth of 10 μm or more is generated on the bent inner surface,
When the inner surface of the bend is evaluated as having a surface shape having a streak-like groove extending in the direction of the bend ridge line that is visible to the naked eye, it is evaluated that when the metal plate to be evaluated is bent and deformed at the above bending radius, a crack having a depth of 5 μm or more will occur on the inner surface of the bend.
(7) Disclosure 7 discloses that, in the evaluation process, if the inner surface of the bend is evaluated to have a surface shape in which the surface roughness of the surface is equal to or greater than a preset threshold value, it is determined that a crack will occur on the inner surface of the bent metal plate being evaluated when bending deformation is performed at the bending radius.
(8) Disclosure 8 conducts bending tests 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 above evaluation step is divided by the plate thickness of the metal plate to be evaluated to obtain an evaluation value;
The maximum value of the evaluation values is evaluated as the limit value of bending performance at which cracks occur on the inner side of the bent portion during bending deformation in the metal plate being evaluated.
(9) Disclosure 9 changes the bending radius of the punch tip and performs bending tests at each bending radius.
From the bending radius in the bending test when it is determined that a crack has occurred in the evaluation step, the range of the bending radius in which a crack is estimated to occur on the inner surface of the bend in the bending test is determined;
A bending test is performed on the metal plate to be evaluated using a punch with a bending radius that includes the determined range of bending radii, and the presence or absence of cracks on the inner bent surface and the depth of the cracks, if any, are evaluated by observing the inner bent surface that has been bent and deformed in the bending test.
(10) Disclosure 10 uses a 90-degree V-shaped punch as the punch for the bending test,
As the punches for bending back the above-mentioned, two punches having opposing pressing surfaces each having a flat surface are used.

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

In the following description, the above test piece having a tensile strength of 980 MPa will be referred to as test piece A. Moreover, the above test pieces made of the two types of steel plates having a tensile strength of 780 MPa will be referred to as test piece B and test piece C, respectively.
In this example, the mold described in the above embodiment was used to carry out the bending test step 10 on each test piece. After that, the test pieces were subjected to the unbending step 11, and the inner surface of each test piece after the unbending step 11 was observed.

<Bending test step 10>
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).
In addition, a plurality of types of punches having different bending radii R of the tip portions 2c were used as the punch 2. The bending radius is synonymous with the radius of curvature.
The bending radius R of the punch tip 2c used was selected from the range of 1.0 mm to 10 mm. Specifically, four types of bending radius R of the punch tip 2c, 1.0 mm, 3.0 mm, 6.0 mm, and 10 mm, were used.

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. In other words, 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 a testing machine for pressing the punch 2 into the die 1. The bending test step 10 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. The maximum load received by the punch 2 when pressing the punch 2 into the die 1 was set to 150 kN.

<Bending back process 11>
SKD11 (alloy tool steel) was used as the material of the lower die 4 and upper die 5 for the bending back process 11. The shapes of the forming surfaces of the lower die 4 and upper die 5 were the same as those shown in the embodiment (see FIG. 4).
In the bending back process 11, the test piece after the bending test was placed on the flat upper surface 4a of the lower die 4 so as to be in an inverted V shape (see FIG. 4 ).Then, the upper die 5 was lowered so that the flat lower surface 5a of the upper die 5 was in contact with the bent apex 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. At this time, 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 set to 375 kN.

<Evaluation>
The test piece bent back in the bending back step 11 was taken out of 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 streak-like minute grooves (streak-like grooves) extending in the direction of the bend ridgeline was evaluated.
Table 2 shows the evaluation results of the surface condition after bending back according to this embodiment.

As can be seen from Table 2, when test piece A having a tensile strength of 980 MPa and a plate 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 bent inner surface after the unbending step 11. When the bending radius R was 6.0 mm, minute streaky grooves extending in the direction of the bent ridgeline occurred on the bent inner surface. Furthermore, when the bending radius R was 10 mm, there was no change in the bent inner surface.
Similarly, from Table 2, when test pieces B and C with a tensile strength of 780 MPa and a plate thickness of 2.6 mm were used, the following was found. That is, when the bending radius R was 1.0 mm, cracks occurred on the inner surface of the bent piece. When the bending radius R was 3.0 mm, minute grooves in the form of stripes extending in the direction of the bent ridgeline occurred on the inner surface of the bent piece. Furthermore, when the bending radius R was 10 mm, there was no change in the inner surface of the bent piece.

<Verification of the 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 inner surface of a bent metal plate 3 during actual bending processing.
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 a bent portion was cut out from each test piece after the bending test. 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 inner surface of the bend was observed in detail using a microscope or the like to measure the crack depth of the crack that occurred on the inner side of the bend during the bending test.

FIG. 7 shows a graph plotting the relationship between the bending radius R in the bending test and the crack depth on the inside of the bend.
As can be seen from Fig. 7, in the case of test piece A with 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 inside of the bend. Also, when the bending radius R was 6.0 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 less than 5 µm occurred.

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. Also, 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 less than 5 μm occurred.
It was also found that the results in FIG. 7 correlated with the results in Table 2.

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

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, at which cracks with a depth of 10 μm or more occur on the inner surface of the bent piece. For example, the bending radius R corresponding to the limit value is assumed to be 1.0 mm. In this case, the evaluation value of the limit value is 1.0/2.6, which is approximately 0.4. Furthermore, the limit value at which cracks with a depth of 5 μm or more 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 the limit value is assumed to be 3.0 mm. In this case, the evaluation value of the limit value is 3.0/2.6, which is approximately 1.1.

<Verification of striated grooves>
8 shows the increase in surface roughness due to the unbending process when stripe-like grooves are formed on the inner surface of the bent piece according to this embodiment. The increase in surface roughness is the increase in surface roughness (%) due to the unbending process 11, based on the surface roughness after the bending test process 10 and before the unbending process 11.
Here, test pieces B and C were used as test pieces for verification.

In addition, the surface roughness of each bent inner surface before and after the unbending step 11 was measured by a laser microscope (Keyence Corporation, VK-X3000).
According to the measurements, when minute streak-like grooves (streak-like grooves) extending in the bending ridge direction were generated, the surface roughness increase rate was about 5.3 times or more in Ra and about 4.6 times or more in Rz, as shown in FIG. 8.
In this way, it was found that the presence or absence of cracks or streak-like grooves that can be easily identified by visual inspection after the bending back process 11 can also be determined using the surface roughness after the bending back process 11.

The above verification experiment results show 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, it was verified that the bending performance evaluation results based on this embodiment are effective.
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.

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. Although the present application has been described with reference to a limited number of embodiments, the scope of the rights is not limited thereto, and modifications of each embodiment based on the above disclosure would be obvious to one skilled in the art.

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 Bending back process 12 Evaluation process 13 Index value calculation process R Bending radius

Claims (10)

A bending performance evaluation method for evaluating bending performance of a metal plate to be evaluated against bending deformation by performing a bending test in which a metal plate to be evaluated is bent using a punch having a bending radius at a tip portion,
a bending back process for bending back the metal plate bent and deformed in the bending test;
an evaluation step of evaluating a surface shape of the inner surface of the bent metal plate after the unbending step, thereby determining whether or not a crack occurs on the inner surface of the bent metal plate due to bending deformation at the bending radius;
The bending performance evaluation method has the following characteristics. In the evaluation step, if the bent inner surface is evaluated as having a surface shape having a visually identifiable crack, it is determined that a crack will occur on the bent inner surface of the metal plate to be evaluated when bending deformation is performed at the bending radius.
The bending performance evaluation method according to claim 1. In the evaluation step, if the bent inner surface is evaluated to have a surface shape having a visually discernible crack, it is evaluated that when the metal plate to be evaluated is bent and deformed at the bending radius, a crack having a depth of 10 μm or more is generated on the bent inner surface.
The bending performance evaluation method according to claim 2. In the evaluation step, when the bent inner surface is evaluated to have a surface shape having a stripe-like groove extending in the direction of the bend ridge line that can be visually identified, it is determined that a crack will occur on the bent inner surface of the metal plate to be evaluated when bending deformation is performed at the bending radius.
The bending performance evaluation method according to claim 1 . In the above evaluation step, when the surface on the inner side of the bend is evaluated to have a surface shape having a streak-like groove extending in the direction of the bend ridge line that can be visually identified, it is evaluated that when the metal plate to be evaluated is bent and deformed at the above bending radius, a crack having a depth of 5 μm or more is generated on the surface on the inner side of the bend.
The bending performance evaluation method according to claim 4. In the above evaluation process,
When the bent inner surface is evaluated as having a surface shape having a crack that can be visually identified, it is evaluated that when the metal plate to be evaluated is bent and deformed at the bending radius, a crack having a depth of 10 μm or more is generated on the bent inner surface,
When the inner surface of the bend is evaluated as having a surface shape having a streak-like groove extending in the direction of the bend ridge line that can be visually identified, it is evaluated that when the metal plate to be evaluated is bent at the bending radius, a crack having a depth of 5 μm or more is generated on the inner surface of the bend.
The bending performance evaluation method according to claim 1. In the evaluation step, when the surface roughness of the bent inner surface is evaluated to be equal to or greater than a preset threshold value, it is determined that a crack will occur on the bent inner surface of the metal plate to be evaluated when the metal plate is bent at the bending radius.
The bending performance evaluation method according to claim 1 . A bending test was 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 above evaluation step is divided by the plate thickness of the metal plate to be evaluated to obtain an evaluation value;
The maximum value of the evaluation value is evaluated as the limit value of bending performance at which cracks occur on the inner side of the bent metal plate during bending deformation.
The bending performance evaluation method according to any one of claims 1 to 7. The bending radius of the punch tip was changed and bending tests were performed at each bending radius.
From the bending radius in the bending test when it is determined that a crack has occurred in the evaluation step, the range of the bending radius in which a crack is estimated to occur on the inner surface of the bend in the bending test is determined;
A bending test is performed on the metal plate to be evaluated using a punch having a bending radius that includes the determined range of bending radii, and the presence or absence of cracks on the inner surface of the bent plate that has been bent and deformed in the bending test, and the depth of the cracks, if any, are evaluated based on the observation of the inner surface of the bent plate that has been bent and deformed in the bending test.
The bending performance evaluation method according to any one of claims 1 to 7. A 90 degree V-shaped punch was used as the punch for the bending test.
As the punch for performing the bending back, two punches having opposing pressing surfaces each having a flat surface are used.
The bending performance evaluation method according to any one of claims 1 to 7.

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