Bending property evaluation method

Abstract

The present invention aims to provide a method capable of efficiently evaluating fine cracks generated on the inside of a bend in bending processing of a metal sheet. The above-described bend performance evaluation method uses a punch (2) having a bend radius (R) at a tip end portion (2c) to perform a bend test on an evaluation target metal sheet (3) to bend and deform the metal sheet (3), thereby evaluating the bend performance of the evaluation target metal sheet (3) against the bend and deformation. The bend performance evaluation method has a return bend process (11) of returning the metal sheet (3) that has been bent and deformed in the bend test, and an evaluation process (12) of determining whether cracks will be generated on the inside of a bend of the evaluation target metal sheet (3) due to the bend and deformation at the bend radius (R) by evaluating the surface shape of the inside of the bend of the metal sheet (3) after the return bend process (11).

Description

Technical Field

[0001] This invention relates to a technique for easily evaluating the bending performance of sheet metal, such as automotive sheet metal, during bending processes. Bending processes refer to bending deformation, for example. This invention is particularly suitable for evaluating micro-cracks that occur on the inner surface of a bent surface due to bending processes. In this specification, micro-cracks refer to cracks that are not easily visible to the naked eye, or cracks that are difficult to detect visually.

[0002] Furthermore, this application is not required when evaluating metal plates with easily identifiable cracks on the inner surface of the bend during bending tests. Background Technology

[0003] Automobile body components are mostly manufactured by roll forming or stamping of sheet metal such as steel. Furthermore, to improve vehicle crash safety and reduce vehicle weight, the strength of steel sheets as raw materials is continuously being increased. However, with the increasing strength of metal sheets (materials), issues arise such as the tendency for cracking to occur in the sheet metal during the forming process.

[0004] In particular, the bending process of formed metal sheets presents the following challenges: when cracks and wrinkles occur on the outer surface of the bent sheet where tensile stress is generated, it can lead to breakage of the formed product and a reduction in its fatigue properties. To avoid such problems, it is crucial to evaluate the optimal bending conditions for each type of metal sheet used as raw material in advance. Furthermore, it is essential to apply the evaluation results to component design and mold design during component manufacturing.

[0005] As an evaluation method for the bending performance of metal sheets, there are, for example, the following evaluation methods.

[0006] Patent Document 1 describes a bending process performed by pressing in a punch with a tapered tip while the metal sheet to be evaluated is under tension. Furthermore, Patent Document 1 proposes a method for determining the limiting condition based on the relationship between tension and the bending radius of the punch tip. This limiting condition is the condition that prevents the formation of fine, strip-shaped grooves, cracks, or fractures in the bent portion. Moreover, Patent Document 1 describes these fine grooves as necking.

[0007] Furthermore, Patent Document 2 describes a bending test performed while photographing the bent outer surface of a metal plate. Also, Patent Document 2 proposes a method for evaluating bending performance based on information such as strain distribution and changes on the bent outer surface, and crack formation.

[0008] Furthermore, Patent Document 3 considers the portion undergoing a rebending process after bending as the evaluation object. Also, Patent Document 3 discloses a method for evaluating cracks generated from the inside of the bend during bending and rebending processes.

[0009] Patent Document 1: Japanese Patent Application Publication No. 2011-235301

[0010] Patent Document 2: Japanese Patent Application Publication No. 2021-135128

[0011] Patent Document 3: Japanese Patent Application Publication No. 2015-47605

[0012] The evaluation methods described in Patent Documents 1 and 2 are methods for evaluating the bending performance of the outer side of a bend. That is, the evaluation methods described in Patent Documents 1 and 2 are not methods for evaluating the bending performance of the inner surface of a bend.

[0013] Furthermore, the method described in Patent Document 3 is a method for evaluating the bending performance of the inner surface of a bend. However, Patent Document 3 is a method for evaluating the bending performance for bending and back-bending processes. That is, the method described in Patent Document 3 is not a method for evaluating the bending performance for bending processes. In other words, Patent Document 3 is a method for evaluating cracks generated during back-bending at a part that has undergone bending and then back-bending. Moreover, in the method of Patent Document 3, it is impossible to determine the presence or absence of fine cracks generated on the inner surface of the bend immediately after bending, the depth of the cracks, etc. In other words, the method of Patent Document 3 has the problem of being unable to evaluate bending performance.

[0014] Here, during the bending process of metal sheets, cracks sometimes form not only on the outer surface of the bend, which is traditionally considered a bending crack, but also on the inner surface. These fine cracks on the inner surface are characterized by the absence of necking. Furthermore, necking refers to grooves or strips. Additionally, these fine cracks on the inner surface are characterized by extremely small crack depth and width, often accompanied by wrinkling. Detecting cracks with these characteristics through optical observation of the material surface is extremely difficult or time-consuming.

[0015] Previously, determining the presence or depth of cracks caused by bending processes required an evaluation process as follows: First, the bent portion of the metal sheet was cut out and then embedded in resin. Next, the cross-section of the bent portion was mirror-polished. Then, the cross-section near the inner surface of the bend was examined in detail using a microscope or similar instrument. However, this process significantly reduced efficiency in evaluating cracks on the inner surface of the bend caused by bending processes. Summary of the Invention

[0016] This invention addresses the problems described above. The object of this invention is to provide a method for efficiently evaluating micro-cracks that occur on the inner surface of a bent sheet during bending processes.

[0017] To address the aforementioned issues, the inventors conducted bending tests on metal sheets and measured the crack depth on the inner surface of the bend. Furthermore, this crack depth measurement was performed based on detailed observation of the bending cross-section. After conducting bending tests on the same type of metal sheet under identical bending conditions, a back-bending process was performed. Through this process, it was observed that cracks and tiny, strip-shaped grooves extending along the bending ridge direction were generated on the inner surface of the bend after the back-bending. As described above, these tiny, strip-shaped grooves are also referred to as strip grooves.

[0018] The inventors then compared the test results of the two experiments. The result of this comparison was that the inventors obtained the following insights: the cracks on the inner surface of the bent metal plate after the bending test, the formation state of the grooves, and the depth of the cracks formed on the inner surface of the bent plate during the bending test are related.

[0019] Based on this, the following evaluation method was discovered as a way to evaluate the bending performance of metal plates in order to solve the above problems.

[0020] To address this issue, one aspect of the present invention is a bending performance evaluation method. This method uses a punch with a bending radius at its front end to perform a bending test on a metal plate of the evaluation object, thereby evaluating the bending performance of the metal plate of the evaluation object in response to the bending deformation. The bending performance evaluation method includes the following steps: a re-bending step, in which the metal plate deformed in the bending test is re-bent; and an evaluation step, in which the surface shape of the inner bending surface of the metal plate after the re-bending step is evaluated to determine whether cracks will occur on the inner bending surface of the metal plate of the evaluation object due to the bending deformation at the bending radius.

[0021] In this invention, the material after bending (bending) is bent back. This allows the fine cracks generated during bending to propagate, making it easier to observe (evaluate) the presence or absence of cracks during bending. As a result, according to the invention, cracks generated on the inner surface of a bent metal sheet during bending can be evaluated easily and efficiently.

[0022] Traditionally, after cutting out the bending section of a metal sheet, the cross-section of the bending section needs to be mirror-polished and ground. Then, the cross-section near the inner surface of the bend is observed in detail using a microscope or similar instrument. However, according to the present invention, such cutting and grinding are unnecessary. That is, according to the present invention, the presence and depth of cracks that occur on the inner surface of the bend during the bending process of the metal sheet, with extremely small depth and width, can be easily evaluated. Therefore, according to the present invention, a significant reduction in work efficiency can be avoided.

[0023] Furthermore, according to the method of the present invention, the range of bending radii from which cracks are likely to occur during bending can be easily inferred through evaluation in the evaluation process. Therefore, for example, when it is desirable to evaluate the bending performance of bending in detail, it is possible to limit the bending test to materials that have undergone bending tests within the range of bending radii defined by the method of the present invention. Moreover, it is also possible to perform mirror polishing and grinding on the cross-section of the bent portion only on the limited material, as in the conventional method, and to observe the cross-section near the inner surface of the bend in detail using a microscope or the like. Attached Figure Description

[0024] Figure 1 This is a diagram illustrating an example of the steps in a bending performance evaluation method based on an embodiment of the present invention.

[0025] Figure 2 This is a schematic diagram illustrating a bending test based on an embodiment of the present invention.

[0026] Figure 3 It is a side view showing the shape of the metal plate after the bending test.

[0027] Figure 4 This is a schematic diagram illustrating the bending process according to an embodiment of the present invention.

[0028] Figure 5 This is a side view showing the situation during the backbend process.

[0029] Figure 6 It is a side view showing the position of the metal sheet and the inner surface of the bend after the back bending process.

[0030] Figure 7 It is a graph showing the relationship between the bending radius and the crack depth on the inside of the bend in a bending test.

[0031] Figure 8 This is a graph illustrating the rate of increase in surface roughness before and after the back bending process when forming a strip-shaped groove on the inner surface of a bend. Detailed Implementation

[0032] Next, embodiments of the present invention will be described with reference to the accompanying drawings.

[0033] The evaluation method of this invention can be applied to any metal sheet. This invention is applicable to steel sheets, and particularly to hot-rolled steel sheets. This invention relates to a simple method for evaluating bending performance during bending processes.

[0034] (structure)

[0035] exist Figure 1 The steps for evaluating the bending performance during bending processes in this embodiment are shown. For example... Figure 1 As shown, this evaluation method includes a bending test step 10, a back bending step 11, an evaluation step 12, and an index value calculation step 13, which is the main test step.

[0036] <Step 10 of the bending test>

[0037] First, refer to Figure 2 The bending test procedure 10 will be explained. The bending test procedure 10 is a procedure that simulates the bending process as the main test.

[0038] In step 10 of the bending test in this embodiment, a mold is used. Figure 2 The die 1 and punch 2 are used to perform the operation as shown.

[0039] like Figure 2 As shown, 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 formed by a pair of inclined surfaces 1a and 1b. The intersection of the pair of inclined surfaces 1a and 1b is the front end of the V. In this embodiment, a recess 1c is formed at the front end of the V. As a result, the forming surface of the die 1 can cope with bending deformation under multiple bending radii R. Furthermore, the forming surface of the die 1 is formed with the aforementioned concave cross-sectional shape along... Figure 2 Extend in orthogonal directions on the paper.

[0040] Furthermore, the punch 2 has a forming surface having a pair of inclined surfaces 2a, 2b and a front end portion 2c connecting the pair of inclined surfaces 2a, 2b to each other. This forming surface has a V-shaped cross-section. The pair of inclined surfaces 2a, 2b mimics the shape of a pair of inclined surfaces 1a, 1b. The front end 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 radius R of the front end portion 2c of each punch 2 is different.

[0041] Here, the angle θ of the V-shape formed by a pair of inclined surfaces 1a and 1b is, for example, 90 degrees. However, the angle θ of the V-shape can also be other than 90 degrees. In addition, the angle of the V-shape formed by a pair of inclined surfaces 2a and 2b can be, for example, equal to the angle θ of the V-shape formed by a pair of inclined surfaces 1a and 1b.

[0042] Then, as Figure 2 As shown, in the bending test, firstly, the metal plate 3 of the evaluation object is placed on the die 1. Then, the bending test is carried out by pressing the punch 2 toward the die 1 in a direction where the inclined surfaces 2a, 2b of the punch 2 are close to the inclined surfaces 1a, 1b of the die 1.

[0043] Through this bending test, the metal plate 3 was bent into shape. Figure 3 The bending shape shown is such that the metal plate 3 is clamped by the die 1 and the punch 2. As a result, the metal plate 3 is bent at the bottom of the forming process into a V-shaped cross-section with a bending radius R. Furthermore, even if the metal plate 3 is bent to 90 degrees, it will spring back due to demolding.

[0044] Through this bending test, the surface 3a that contacts the punch 2 on the two surfaces of the metal plate 3 becomes the inner bending surface. The surface 3b that contacts the die 1 becomes the outer bending surface.

[0045] Here, for the cracks used to evaluate the bending performance of this invention, the fine cracks generated on the inner surface of the bend, i.e., surface 3a, are the target.

[0046] For a metal plate 3 made of the same material as the evaluation object, the bending radius R of the punch tip 2c is changed to repeat the bending test procedure 10 above.

[0047] In addition, prepare multiple metal plates 3 for the evaluation objects. The test pieces made of the metal plates used in the bending test are, for example, long strips.

[0048] <Backbend Process 11>

[0049] The back-bending process 11 is a pretreatment process for the evaluation process 12 and constitutes part of the treatment used for evaluation.

[0050] The back-bending process 11 is a process of back-bending the metal sheet 3 that was bent in the bending test process 10. The back-bending angle is set to be greater than the angle change caused by springback due to demolding in the bending test. In addition, the back-bending angle is set to the angle at which micro-cracks generated on the inner surface of the bend can propagate. Such a back-bending angle can be determined by using computer forming analysis, experiments, etc. This example, described later, is an example of back-bending to 180 degrees.

[0051] In the bending process 11 of this embodiment, as follows Figure 4 As shown, the device consists of upper and lower dies 4 and 5 facing each other with the forming surfaces opposite each other. Furthermore, the upper surface 4a (forming surface) of the lower die 4 and the lower surface 5a (forming surface) of the upper die 5 are both flat surfaces. Thus, the forming surface becomes a surface with a bending angle of 180 degrees.

[0052] In the bending process 11 of this embodiment, the metal plate 3 (referring to the bending test process 10) that has been bent is used as a reference. Figure 3 )like Figure 4 The upper die 5 is placed on the upper surface of the lower die 4 as shown. Then, in the bending process 11 of this embodiment, the upper die 5 is moved relative to the lower die 4 in a manner close to it (see reference). Figure 4 Furthermore, in this embodiment, the bending process 11 is performed by clamping the metal plate 3 with the lower die 4 and the upper die 5.

[0053] Figure 5 This diagram illustrates the intermediate steps during the bend-back process 11. Furthermore, as shown... Figure 5 As shown, the V-shaped metal plate 3 is clamped and processed by the opposing surfaces 4a and 5b of the lower die 4 and the upper die 5, thereby bending the curved part back towards a flat shape.

[0054] Here, in this example, as Figure 6 As shown, at the bottom stop of the forming process, the metal plate 3 is bent back to 180 degrees. However, this is not a limitation. In the bending process 11, the bending angle can be less than 180 degrees or more than 180 degrees. Furthermore, even if the metal plate 3 is bent back to 180 degrees, springback will occur due to demolding.

[0055] Furthermore, the left and right ends of the test piece can be constrained after bending and when the test piece is flat. The observation in evaluation step 12 described below can also be performed in this state. Alternatively, the test piece can be observed in a free state.

[0056] <Evaluation Process 12>

[0057] Evaluation step 12 is to observe and evaluate. Figure 6 The process of shaping the inner curved surface 3x of the bent inner surface 3a of the metal plate 3 as shown.

[0058] In this example, the evaluation of the surface shape is performed by selecting one or more of the following three evaluation methods (1) to (3).

[0059] (1) In the first evaluation method, it is observed whether a crack that can be identified by visual inspection has occurred on the inner surface 3x of the bend after the bend. Furthermore, if it is determined that a crack that can be identified by visual inspection has occurred on the inner surface 3x of the bend, the evaluation is performed as follows. That is, it is determined that a crack has occurred on the inner surface of the metal plate 3 being evaluated due to bending deformation at the bending radius R used in the bending test in which the crack was determined to have occurred.

[0060] Furthermore, when a crack that is visually identifiable appears on the inner surface 3x after the bend, an evaluation is conducted as follows: The bending radius R used in the bending test in which the crack was determined to have occurred is determined. If the metal plate 3 under evaluation is bent with this bending radius R, a fine crack appears on the inner surface of the bent metal plate 3. Furthermore, when the bending process using this bending radius R is performed, the depth of the crack appearing on the inner surface is evaluated as 10 μm or more. In other words, when a crack that is visually identifiable appears on the inner surface 3x after the bend, it is evaluated as a crack with a depth of 10 μm or more that occurred during the bending process.

[0061] (2) In the second evaluation method, it is observed whether a small, strip-shaped groove extending along the bending edge direction, which can be visually identified, is produced on the inner surface 3x after the bend. Furthermore, as mentioned above, this groove is also referred to as a strip-shaped groove. And, if it is determined that a strip-shaped groove, which can be visually identified, has been produced on the inner surface 3x after the bend, the evaluation is performed as follows: That is, it is determined that a fine crack has been produced on the inner surface of the metal plate 3 being evaluated due to bending deformation at the bending radius R used in the bending test.

[0062] Furthermore, when a visually perceptible groove appears on the inner surface 3x of the bend after a back bend, an evaluation is conducted as follows: The bending radius R used in the bending test in which the crack was determined to have occurred is determined. If the metal plate 3 under evaluation is bent with this bending radius R, a fine crack appears on the inner surface of the bent metal plate 3. Furthermore, the depth of the crack appearing on the inner surface during the bending process using this bending radius R is evaluated as 5 μm or more. In other words, when a visually perceptible groove appears on the inner surface 3x of the bend after a back bend, it is evaluated as a crack with a depth of 5 μm or more that occurred during the bending process.

[0063] (3) In the third evaluation method, it is observed whether the surface roughness of the inner surface 3x after the bend is above a predetermined threshold. Furthermore, if it is determined that the surface roughness of the inner surface is above the predetermined threshold, the determination is made as follows: That is, it is determined that due to the bending deformation of the bending radius R used in the bending test, fine cracks are generated on the inner surface 3x of the metal plate 3 being evaluated. The observation direction of the surface roughness is, for example, set to a direction intersecting the bending ridge direction, or, for example, a direction orthogonal to the bending ridge direction. In addition, the predetermined threshold can be determined, for example, as follows: That is, the surface roughness of the surface that is pre-evaluated as being easily visually observable on the surface 3x is measured. And, the aforementioned threshold is determined based on this surface roughness.

[0064] Here, the observation of cracks and grooves in the first and second evaluation methods described above can be performed as follows: That is, visually observe the position of the face 3X, which was once the apex of the bend, on the inner surface 3a of the bend (refer to...). Figure 6 The test is to check for cracks or grooves. Cracks that can be identified by visual inspection are those with a width of 0.1 mm or more, and grooves that can be identified by visual inspection are those with a width of 0.05 mm or more.

[0065] However, the method for determining the surface shape at position 3X on the inner side of the bend is not limited to visual determination. For example, it can also be determined by roughness measurement using a stylus-type roughness meter. Alternatively, the roughness and crack depth derived from the surface shape obtained by measuring the distance to the object by irradiating a laser can also be utilized.

[0066] Here, the third evaluation method uses the observations from the first and second evaluation methods as a method to evaluate surface roughness. In other words, the third evaluation method is a method of observing and evaluating surface shape using measuring devices such as surface roughness meters.

[0067] Table 1 shows the state of the inner surface 3x after the back-bending process 11 in the evaluation method of this embodiment, and the correlation between the crack depth of the inner surface after the bending test. The contents of Table 1 are confirmed through experiments.

[0068]

[0069] As shown in Table 1, when the state of the inner surface 3x after the bending process 11 remains unchanged, the crack depth of the inner surface after the bending test is less than 5 μm.

[0070] In addition, if the inner surface of the bend after the bending process 11 has a strip-shaped groove extending along the bending ridge direction, the crack depth of the inner surface of the bend after the bending test is 5 μm or more and less than 10 μm.

[0071] In addition, if the inner surface of the bend after the bending process 11 is cracked, the crack depth of the inner surface of the bend after the bending test is 10 μm or more.

[0072] Furthermore, the threshold in the third evaluation method can be set as follows: That is, determine the surface shape of cracks or grooves that can be easily visually identified on the inner surface of the bend after the back-bending process 11. Calculate the surface roughness corresponding to this determined surface shape and set the threshold accordingly.

[0073] Furthermore, as shown in Table 1, the depth of cracks during bending can be easily classified by using both the first and second evaluation methods.

[0074] <Indicator Value Calculation Procedure 13>

[0075] Step 13, which calculates the index value, is the step of obtaining the evaluation value.

[0076] Here, the thickness of the metal plate 3, which was evaluated as having developed a crack in evaluation step 12, is set as t [mm]. Furthermore, the bending radius of the punch tip 2c used in the bending test, which was evaluated as having developed this crack, is set as R [mm]. Then, in the index value calculation step 13, the value obtained by dividing R [mm] by the plate thickness t [mm] (R / t) is used as the evaluation value. The evaluation value is used to evaluate bending performance based on the bending radius R that caused the fine crack during bending.

[0077] Furthermore, in the index value calculation step 13, the maximum value among the multiple calculated evaluation values ​​is taken as the limit value of bending performance. The maximum value of this evaluation value is the limit value at which a crack will occur on the inner surface 3x of the bend when the metal plate 3 of the evaluation object is bent.

[0078] Then, for example, when bending the object being evaluated, the bending radius R is set to be smaller than the maximum value of the evaluation value, and the maximum value of the evaluation value is used when determining (designing) the processing conditions.

[0079] (Effect)

[0080] According to this embodiment, the metal sheet 3, after bending (bending), is bent back, thereby allowing the fine cracks generated during the bending process to propagate. Therefore, according to this embodiment, the presence or absence of fine cracks during the bending process is easily observed. As a result, according to this embodiment, cracks generated on the inner surface of the bent metal sheet 3 during the bending process can be evaluated easily and efficiently.

[0081] Previously, this process involved mirror polishing and grinding the cross-section of the bent portion after the bending test. Then, the cross-section near the inner surface of the bend was observed in detail using a microscope or similar instrument. However, in this embodiment, such cutting and grinding are unnecessary. Therefore, in this embodiment, the presence and depth of fine cracks generated on the inner surface of the bend during the bending process of the metal sheet 3 can be easily evaluated. Here, fine cracks refer to cracks with extremely small depth and width. As a result, this embodiment avoids a significant reduction in work efficiency.

[0082] Furthermore, according to the present invention, the range of the bending radius R in which cracks occur during bending can be easily deduced. Moreover, when a detailed evaluation of bending performance for bending is desired, bending tests limited to the range of bending radius R defined by the present invention can be conducted. That is, only the bent portion in the bending test defined by the evaluation method of this embodiment can be observed in detail using existing methods. Existing methods include, for example, mirror polishing and grinding the cross-section of the bent portion and observing the cross-section near the inner surface of the bend in detail using a microscope. By limiting the range of the metal sheet to be observed in detail, the efficiency of detailed observation is improved compared to the past.

[0083] Furthermore, when obvious cracks that can be visually observed appear on the inner surface of the bend after a bending test, the simplified evaluation method of this embodiment is unnecessary. This disclosure is a technique applicable to evaluating bending performance when minute cracks that cannot be easily observed visually occur during bending processing.

[0084] (other)

[0085] This disclosure may also take the following structure.

[0086] (1) Disclosure 1 is a bending performance evaluation method, which uses a punch with a bending radius at the front end to perform a bending test on a metal plate of the evaluation object to cause it to bend and deform, thereby evaluating the bending performance of the metal plate of the evaluation object under bending deformation. The bending performance evaluation method described above is characterized by having the following steps:

[0087] The bending process involves bending the metal plate that deformed during the bending test back into its original shape; and

[0088] The evaluation process involves assessing the shape of the inner surface of the bent metal sheet after the aforementioned bending process to determine whether cracks will occur on the inner surface of the bent metal sheet due to bending deformation at the aforementioned bending radius.

[0089] (2) In disclosure 2, in the above evaluation process, if the evaluation determines that the inner surface of the bend has a surface shape with cracks that can be identified by visual inspection, it is determined that if bending deformation is performed at the above bending radius, cracks will be generated on the inner surface of the metal plate of the evaluation object.

[0090] (3) In disclosure 3, in the above evaluation process, if the evaluation is that the inner surface of the bend has a surface shape with cracks that can be identified by visual inspection, it is evaluated that if the metal plate of the evaluation object is bent and deformed at the above bending radius, cracks with a depth of 10 μm or more are generated on the inner surface of the bend.

[0091] (4) In disclosure 4, in the above evaluation process, if the surface of the inner side of the bend is evaluated as having a surface shape with a strip-shaped groove extending along the bending ridge direction that can be identified by visual inspection, it is determined that if bending deformation is performed at the bending radius, a crack will be generated on the inner side of the bend of the metal plate of the evaluation object.

[0092] (5) In disclosure 5, in the above evaluation process, if the surface of the inner side of the bend is evaluated as having a surface shape with a strip-shaped groove extending along the bending ridge direction that can be identified by visual inspection, it is evaluated that if the metal plate of the evaluated object is bent and deformed at the bending radius, a crack with a depth of 5 μm or more is generated on the surface of the inner side of the bend.

[0093] (6) In disclosure 6, in the above evaluation process,

[0094] If the evaluation determines that the inner surface of the bend has a surface shape with cracks that can be visually identified, and if the metal plate of the evaluated object is bent and deformed at the aforementioned bending radius, cracks with a depth of 10 μm or more will be generated on the inner surface of the bend.

[0095] If the surface of the aforementioned bend is evaluated as having a surface shape with a strip-shaped groove extending along the bend ridge direction that can be visually identified, it is evaluated that if the metal plate of the evaluated object is bent and deformed at the aforementioned bend radius, a crack with a depth of 5 μm or more will be generated on the surface of the bend.

[0096] (7) In disclosure 7, in the above evaluation process, if the surface of the inner side of the bend is evaluated as a surface shape with a surface roughness of more than a predetermined threshold, it is determined that if bending deformation is performed at the bending radius, a crack will be generated on the inner side of the bend of the metal plate of the evaluation object.

[0097] (8) In disclosure 8, the bending radius of the punch tip is changed to conduct a bending test.

[0098] The evaluation value is obtained by dividing the bending radius (used in the bending test where cracks were determined to have occurred in the above evaluation process) by the thickness of the metal plate being evaluated.

[0099] The maximum value of this evaluation value is used as the limit value of the bending performance of the metal plate being evaluated, which causes cracks to form on the inside of the bend during bending deformation.

[0100] (9) In disclosure 9, the bending radius of the punch tip is changed, and bending tests are performed at each bending radius.

[0101] Based on the bending radius in the bending test that indicates crack formation during the aforementioned evaluation process, the range of bending radii from which cracks are inferred to have formed on the inner side of the bending surface during the bending test is determined.

[0102] Using a punch with a bending radius that includes the range of the calculated bending radius, a bending test is performed on the metal plate being evaluated to deform it. By observing the inner surface of the bent surface during the bending test, the presence or absence of cracks on the inner surface of the bent surface, and the depth of cracks if they occur, are evaluated.

[0103] (10) In disclosure 10, a 90-degree V-shaped punch is used as the punch for the aforementioned bending test.

[0104] As the punches used to perform the aforementioned bend, two punches with opposing pressing surfaces having flat surfaces are used.

[0105] Example

[0106] The following describes an embodiment based on this implementation.

[0107] In this example, two steel plates with different microstructures were used as metal plate 3. Both steel plates have a tensile strength of 780 MPa (SPH780) and a thickness of 2.6 mm. Furthermore, a steel plate with a tensile strength of 980 MPa (SPH980) and a thickness of 2.9 mm was used as metal plate 3.

[0108] The steel plates were processed into long strips to serve as their respective test pieces. Furthermore, each test piece had dimensions of 100mm on the long side and 30mm on the short side.

[0109] In the following description, the test piece with a tensile strength of 980 MPa will be referred to as Test Piece A. The test pieces made of two types of steel plates with a tensile strength of 780 MPa will be referred to as Test Piece B and Test Piece C, respectively.

[0110] Furthermore, in this embodiment, the bending test step 10 was performed on each test piece using the mold described in the above embodiments. Afterwards, a back-bending step 11 was performed on each test piece, and the inner surface of the bent test piece after the back-bending step 11 was observed.

[0111] <Step 10 of the bending test>

[0112] 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 those shown in the embodiment (see reference). Figure 2 ).

[0113] Furthermore, several types of punches with different bending radii R at the front end 2c were used as punch 2. Also, bending radius is synonymous with radius of curvature.

[0114] The bending radius R of the punch tip 2c is selected from the range of 1.0mm to 10mm. Specifically, four bending radii R of 1.0mm, 3.0mm, 6.0mm and 10mm are used as the punch tip 2c.

[0115] The bending test is performed with the bending direction set such that the front end of the punch 2 is in line contact with the center point of the long side of the test piece. That is, the bending test is performed with the short side of the test piece parallel to the bending edge.

[0116] 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. The bending test step 10 was performed by fixing the punch 2 to the movable part of the testing machine. Furthermore, the vertical direction was the plate thickness direction. At this time, the moving speed of the punch 2 was set to a constant speed of 30 mm / min. Additionally, the maximum load that the punch 2 could withstand when pressing it into the die 1 was set to 150 kN.

[0117] <Backbend Process 11>

[0118] SKD11 (alloy tool steel) was used as the material for the lower die 4 and upper die 5 used in the bending process 11. The shapes of the forming surfaces of the lower die 4 and upper die 5 were those shown in the embodiment (see reference). Figure 4 ).

[0119] In the bending process 11, the test piece after the bending test is placed in an inverted V-shape on the flat upper surface 4a of the lower die 4 (see reference). Figure 4 Then, the process is carried out by lowering the upper die 5 so that the flat lower surface 5a of the upper die 5 contacts the curved apex of the test piece.

[0120] 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. Furthermore, in the bending process 11, the upper die 5 was fixed 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. Additionally, the maximum load borne by the upper die 5 during the test was set to 375 kN.

[0121] <Evaluation>

[0122] The test piece, after being bent through the bending process 11, is removed from the testing machine, and the surface that was the inner side of the bending test is observed visually. In this observation, the presence or absence of cracks and tiny strip-shaped grooves (strip grooves) extending along the bending edge direction are evaluated.

[0123] Table 2 shows the evaluation results of the surface condition after bending based on this embodiment.

[0124]

[0125] According to Table 2, the following results were obtained when using test piece A with a tensile strength of 980 MPa and a thickness of 2.9 mm: Specifically, when the bending radius R in the bending test was 3.0 mm, cracks appeared on the inner surface of the bend after the back bending process 11. Furthermore, when the bending radius R was set to 6.0 mm, tiny strip-shaped grooves extending along the bending edge direction appeared on the inner surface of the bend. However, when the bending radius R was set to 10 mm, the aforementioned inner surface of the bend remained unchanged.

[0126] Similarly, according to Table 2, the following results were obtained when using test pieces B and C with a tensile strength of 780 MPa and a plate thickness of 2.6 mm: When the bending radius R was set to 1.0 mm, cracks appeared on the inner surface of the bend. Furthermore, when the bending radius R was set to 3.0 mm, tiny, strip-shaped grooves extending along the bending ridge direction appeared on the inner surface of the bend. However, when the bending radius R was set to 10 mm, the aforementioned inner surface of the bend remained unchanged.

[0127] <Verification of the evaluation method based on this implementation>

[0128] The following experiments verified that the bending performance evaluation method of this embodiment can evaluate the crack depth of the cracks generated on the inner surface of the bending process of the actual metal plate 3.

[0129] Test pieces A, B, and C, made of the same material as the metal plate 3 used in the above experiment, were subjected to bending tests under the same conditions as in this example. After the bending tests, bending portions were cut out from each test piece. Then, as before, the cross-sections of the cut bending portions were mirror-polished and ground, and the cross-sections near the inner surface of the bend were observed in detail using a microscope or the like. The crack depth of the cracks generated on the inner side of the bend during the bending test was measured.

[0130] Figure 7 A graph showing the relationship between the bending radius R and the crack depth on the inside of the bend in a bending test is shown.

[0131] according to Figure 7 It can be seen that, in the case of test piece A with a tensile strength of 980 MPa and a thickness of 2.9 mm, the following results were obtained: Specifically, with a bending radius R of 3.0 mm, cracks with a depth exceeding 30 μm were generated on the inner side of the bend. Furthermore, with a bending radius R of 6.0 mm, cracks with a depth of 5 μm or more but less than 10 μm were generated. Moreover, if the bending radius R is set to 10 mm, cracks with a depth less than 5 μm were generated.

[0132] Similarly, for test pieces B and C with a tensile strength of 780 MPa and a plate thickness of 2.6 mm, the following results were obtained: With a bending radius R of 1.0 mm, cracks with a depth exceeding 10 μm were generated on the inner side of the bend. Furthermore, with a bending radius R of 3.0 mm, cracks with a depth of 5 μm or more but less than 10 μm were generated. Moreover, when the bending radius R was set to 10 mm, cracks with a depth less than 5 μm were generated.

[0133] Furthermore, it is known that Figure 7 The results are correlated with those in Table 2.

[0134] As a result, for the bending performance of test piece A under bending processing, the limit value for the generation of cracks with a depth of 10 μm or more on the inner surface of the bend can be evaluated by bending processing using a punch 2 with a bending radius R of 3.0 mm or more and less than 6.0 mm. For example, the bending radius R corresponding to this limit value is set to 3.0 mm. In this case, the evaluation value for the limit value of generating cracks with a depth of 10 μm or more is 3.0 / 2.9, which is approximately 1.0. Furthermore, the limit value for generating cracks with a depth of 5 μm or more can be evaluated by bending processing using a punch 2 with a bending radius R of 6.0 mm or more and less than 10.0 mm. For example, the bending radius R corresponding to this limit value is set to 6.0 mm. In this case, the evaluation value for this limit value is 6.0 / 2.9, which is approximately 2.1.

[0135] Similarly, for the bending performance of test pieces B and C, the limit value for cracks with a depth of 10 μm or more on the inner surface of the bend can be evaluated by bending using a punch 2 with a bending radius R of 1.0 mm or more and less than 3.0 mm. For example, the bending radius R corresponding to this limit value is set to 1.0 mm. In this case, the evaluation value of this limit value is 1.0 / 2.6, which is approximately 0.4. Furthermore, the limit value for cracks with a depth of 5 μm or more can be evaluated by bending using a punch 2 with a bending radius R of 3.0 mm or more and less than 10.0 mm. For example, the bending radius R corresponding to this limit value is set to 3.0 mm. In this case, the evaluation value of this limit value is 3.0 / 2.6, which is approximately 1.1.

[0136] <Verification of the strip groove>

[0137] in addition, Figure 8 This diagram illustrates the increase in surface roughness caused by the back-bending process when a strip groove is formed on the inner surface of the bend, based on this embodiment. The increase in surface roughness is the percentage increase caused by the back-bending process 11, based on the surface roughness after process 10 of the bending test and before process 11.

[0138] Here, test pieces B and C were used as verification test pieces.

[0139] In addition, the surface roughness of each inner surface of the bend before and after the back-bending process 11 was measured using a laser microscope (KEYENCE, VK-X3000).

[0140] According to this measurement, the rate of increase in surface roughness when producing tiny, strip-shaped grooves (strip grooves) extending along the direction of the curved edge is as follows: Figure 8As shown, Ra is approximately 5.3 times greater and Rz is approximately 4.6 times greater.

[0141] Thus, the surface roughness after using the bending process 11 can be determined, and the presence or absence of cracks and grooves that can be easily identified by visual inspection after the bending process 11 can also be determined.

[0142] The above verification experiment results show that there is a correlation between the crack and groove formation on the inner surface of the metal plate 3 after the bending test and the depth of the cracks generated on the inner surface during the bending test. That is, the bending performance evaluation results based on this embodiment are verified to be effective.

[0143] Furthermore, in this example, three test pieces of the same steel grade were prepared, and the same bending evaluation was performed three times for each to evaluate the reproducibility of the invented evaluation method. All three evaluations yielded the same results, therefore it can be said that the evaluation method of the present invention has high reproducibility.

[0144] The entire contents of Japanese Patent Application No. 2024-042963 (filed March 19, 2024), which claims priority in this application, constitute a part of this disclosure by reference. Although a limited number of embodiments have been described herein, the scope of the claims is not limited to these, and modifications based on the embodiments disclosed above will be apparent to those skilled in the art.

[0145] Explanation of reference numerals in the attached figures

[0146] 1... Die; 2... Punch; 2c... Front end; 3... Metal plate; 3X... Observation position of the inner surface of the bend; 4... Lower die; 5... Upper die; 10... Bending test procedure; 11... Bending back procedure; 12... Evaluation procedure; 13... Index value calculation procedure; R... Bending radius.

Claims

1. A method for evaluating bending performance, comprising using a punch with a bending radius at its front end to perform a bending test on a metal plate of the evaluation object to deform it, thereby evaluating the bending performance of the metal plate of the evaluation object in response to the bending deformation, the method being characterized by comprising the following steps: The bending process involves bending the metal plate that has deformed during the bending test back into its original shape; and The evaluation process involves assessing the shape of the inner surface of the bent metal sheet after the bending process to determine whether cracks will form on the inner surface of the bent metal sheet due to bending deformation at the bending radius.

2. The bending performance evaluation method according to claim 1, characterized in that, In the evaluation process, if the evaluation determines that the inner surface of the bend has a surface shape with cracks that can be visually identified, it is determined that if bending deformation is performed at the bending radius, cracks will be generated on the inner surface of the metal plate being evaluated.

3. The bending performance evaluation method according to claim 2, characterized in that, In the evaluation process, if the evaluation indicates that the inner surface of the bend has a surface shape with cracks that can be visually identified, it is further evaluated that if the metal plate of the evaluated object is bent and deformed at the bending radius, cracks with a depth of 10 μm or more are generated on the inner surface of the bend.

4. The method for evaluating bending performance according to any one of claims 1 to 3, characterized in that, In the evaluation process, if the evaluation determines that the inner surface of the bend has a surface shape with a strip-shaped groove extending along the bending ridge direction that can be visually identified, it is determined that if bending deformation is performed at the bending radius, a crack will occur on the inner surface of the metal plate being evaluated.

5. The bending performance evaluation method according to claim 4, characterized in that, In the evaluation process, if the evaluation indicates that the inner surface of the bend has a surface shape with a strip-shaped groove extending along the bend ridge direction that can be visually identified, it is evaluated that if the metal plate of the evaluation object is bent and deformed at the bending radius, a crack with a depth of 5 μm or more will be generated on the inner surface of the bend.

6. The bending performance evaluation method according to claim 1, characterized in that, In the evaluation process, If the evaluation indicates that the inner surface of the bend has a surface shape with cracks that can be visually identified, then if the metal plate of the evaluated object is bent and deformed at the stated bending radius, cracks with a depth of 10 μm or more will be generated on the inner surface of the bend. If the evaluation indicates that the inner surface of the bend has a surface shape with a strip-shaped groove extending along the bend ridge direction that can be visually identified, then if the metal plate of the evaluated object is bent and deformed at the bending radius, a crack with a depth of 5 μm or more will be generated on the inner surface of the bend.

7. The method for evaluating bending performance according to any one of claims 1 to 6, characterized in that, In the evaluation process, if the surface of the inner side of the bend is evaluated as having a surface roughness that is above a predetermined threshold, it is determined that if bending deformation is performed at the bending radius, a crack will occur on the inner side of the bend of the metal plate being evaluated.

8. The method for evaluating bending performance according to any one of claims 1 to 7, characterized in that, The bending radius of the punch tip is changed to conduct a bending test. The evaluation value is obtained by dividing the bending radius (used in the bending test at which cracks are determined to have occurred) by the thickness of the metal plate being evaluated. The maximum value of this evaluation value is used as the limit value of the bending performance of the metal plate being evaluated, which causes cracks to form on the inside of the bend during bending deformation.

9. The method for evaluating bending performance according to any one of claims 1 to 7, characterized in that, By changing the bending radius of the punch tip, bending tests were conducted at each bending radius. Based on the bending radius in the bending test at which cracks were determined to have occurred during the evaluation process, the range of bending radii from which cracks were inferred to have occurred on the inner side of the bending test is determined. Using a punch with a bending radius that includes the range of the calculated bending radius, a bending test is performed on the metal plate being evaluated to deform it. By observing the inner surface of the bent surface during the bending test, the presence or absence of cracks on the inner surface of the bent surface, and the depth of cracks if they occur, are evaluated.

10. The method for evaluating bending performance according to any one of claims 1 to 7, characterized in that, The punch used for the bending test is a 90-degree V-shaped punch. As the punches used to perform the bending, two punches with opposing pressing surfaces having flat surfaces are used.

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