JP2020040111A - Deformation limit evaluation method, crack prediction method and press metal mold design method - Google Patents

Abstract

To provide a technique for predicting bending crack limit of a sheared surface of a metal plate and determining press molding condition in order to prevent occurrence of bending crack of the sheared surface.SOLUTION: A deformation limit evaluation method for evaluating deformation limit in a sheared surface 10 of a metal plate 1 when performing press molding including bending deformation for the sheared metal plate 1. A relationship between a bending radius R of a bending part due to bending deformation, which is given to a part including the sheared surface 10 of the metal plate 1, and a crack length L generated on the sheared surface 10 is determined. From the determined relationship between the bending radius R and the crack length L, bending deformation limit of the metal plate 1 at the part including the sheared surface 10 is evaluated.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a technology for evaluating and predicting cracks on a sheared surface (shear end surface) when a metal plate (material) after being subjected to shearing is formed by press forming including bending deformation and processed. This is a technique relating to the design of a mold shape capable of suppressing cracks in a metal plate in advance based on the technique.

  Currently, automobiles are required to have both performance of improving fuel efficiency and improving collision safety by reducing weight. For the purpose of balancing these two performances, there is a tendency to use high-strength steel sheets for structural parts of automobiles. Cracks are one of the forming defects during press forming of high-strength steel sheets. One of the important issues is the prevention of cracking by press molding, particularly the prevention of cracking at the end face (sheared surface) after shearing.

  Cracks on the sheared surface are roughly classified into cracks due to stretch flange deformation and cracks due to bending deformation. For prediction of stretch flange cracks, there are methods described in Patent Documents 1 to 3, for example. Patent Literature 1 describes a prediction method that considers a strain gradient in a plate surface direction and a prediction method that considers a stress gradient in a plate surface. Patent Literature 2 describes a method that uses the relationship between strain gradient, strain concentration, and breaking strain in stretch flange deformation. Patent Literature 3 describes a crack prediction method using a relationship between a forming limit strain and strain gradients in a sheet surface direction and a sheet thickness direction.

JP 2010-69533 A JP 2011-140046 A JP 2014-115269 A

However, no suitable method has been developed for predicting cracks related to bending cracks on the sheared surface, and a method for predicting cracks on the sheared surface due to this bending deformation has been required.
In particular, when a high-strength steel plate having a tensile strength of 590 MPa or more is used as a metal plate, bending cracks on a sheared surface have become apparent during press forming.
The present invention focuses on the above problems, and proposes a technique for predicting a bending crack limit of a sheared surface of a metal plate in order to prevent occurrence of bending cracks on a sheared surface and determining press forming conditions. The purpose is to provide.

The inventor obtained a bending condition on the sheared surface by bending test to determine a forming condition at which a bending crack on the sheared surface occurs, and further formed a deformation limit strain εlimit of bending crack by a forming simulation under the same forming condition as the bending test. I asked for it. The inventor has obtained a finding that the occurrence of cracks in the sheared surface can be predicted by comparing the obtained deformation limit strain εlimit of bending crack with the strain of the sheared surface during press forming.
The present invention relates to a method for predicting the bending property required for a metal plate and a method for predicting cracks generated in the metal plate using the same. Further, the present invention provides a method of evaluating and predicting a deformation limit and a method of designing a mold shape for press-forming without causing cracks in a target metal plate.

  That is, one embodiment of the present invention is a method for evaluating a deformation limit for evaluating a deformation limit on a sheared surface of the metal plate when performing press forming including bending deformation on the sheared metal plate, The relationship between the bending radius of the bent part due to bending deformation and the crack length generated on the sheared surface by applying bending deformation to the part including the sheared surface of the metal plate was determined, and the relationship between the calculated bending radius and the crack length was calculated. From the relationship, the gist is to evaluate the limit of the bending deformation of the metal plate on the sheared surface.

  Another aspect of the present invention is a method of evaluating a deformation limit for evaluating a deformation limit of a sheared surface of the metal plate when performing press forming including bending deformation on the sheared metal plate, By giving bending deformation to the part including the sheared surface of the metal plate, the relationship between the bending radius of the bent part due to bending deformation and the crack length generated on the sheared surface was obtained, and from the obtained relationship, The gist is to determine the deformation limit strain of a bending crack in which a crack occurs, and to evaluate the limit of bending deformation of the metal plate on the sheared surface using the obtained deformation limit strain.

According to one embodiment of the present invention, when press-forming a metal plate after a shearing process, it is possible to accurately evaluate a deformation limit of the metal plate on a sheared surface due to bending deformation. As a result, for example, it is possible to design a mold shape capable of accurately predicting the occurrence of cracks from the end face and suppressing the occurrence of cracks.
According to one aspect of the present invention, for example, when press-forming various parts such as panel parts of automobiles, structural / framework parts, etc., it is possible to accurately predict whether a metal plate to be used is appropriate or not. Press molding can be performed stably. Further, it can greatly contribute to the reduction of the defective rate of the press-formed product. Further, according to one aspect of the present invention, for example, the shape of the press die can be accurately predicted at the design stage, which can contribute to shortening the manufacturing period of the press die.

It is a figure which shows the example of the press part shape which pressed-formed the metal plate, (a) is the metal plate before press work, (b) is the part shape after press work, (c) is FIG. 1 (b) FIG. FIG. 4 is a diagram illustrating an example of a processing procedure according to the first embodiment based on the present invention. It is the schematic of a V bending test. It is a figure explaining the example of a definition of a crack length. It is a figure explaining the example of other definitions of crack length. It is a figure which shows the relationship between a bending radius and a crack length. It is a figure showing a critical bending radius. It is a figure showing change of a rate of change. It is a figure showing the shape of the test piece in an example. It is a figure in an example which shows the relationship between a bending radius and a crack length. It is a figure in an example which shows the relationship between a bending radius and the maximum principal strain in a crack generation part. It is a figure in an example showing the example of a bending crack evaluation part.

Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 1B shows an example of a pressed part 2 in which the metal plate 1 is formed by press forming including bending. FIG. 1 shows an example in which a metal plate 1 is bent into a U-shaped cross section.
This example is an example in which a metal plate 1 that has been sheared into the shape shown in FIG. 1A is press-formed into the shape of a pressed part 2 shown in FIGS. 1B and 1C. Is a bent portion generated by performing a bending deformation on a portion including the shearing surface 10. In FIG. 1A, reference numeral 1A indicates a position where bending deformation can be applied.

In FIG. 1, the position indicated by the symbol X is a portion on the end face where it is estimated that the largest strain occurs during bending deformation and a crack is generated. The position where cracks due to bending are likely to occur is the position of the shearing surface 10 at or near the bent portion 2A.
The present embodiment is an evaluation method for evaluating a deformation limit on the sheared surface 10 of the metal plate 1 when performing press forming including bending deformation on such a sheared metal plate 1.
The application of the present invention is not limited to bending into a U-shaped cross section, and there is no particular limitation on the shape of the pressed part as long as it has press forming including bending deformation in a sheared metal plate. .

Next, a method of evaluating the deformation limit in the present embodiment will be described.
FIG. 2 shows an example of a procedure for evaluating the deformation limit on the sheared surface 10 of the metal plate 1 in the present embodiment. As shown in FIG. 2, the evaluation of the deformation limit in the present embodiment includes, for example, a bending test 3A, obtaining the relationship between the bending radius and the crack length 3B, determining the limit bending radius 3C, obtaining the deformation limit strain 3D, and shearing. It is performed in the order of the crack determination 3E on the processing surface.

<Bending test 3A>
In the evaluation method of the present embodiment, a plurality of test pieces made of the metal plate 1 subjected to shearing are prepared.
Here, the material of the test piece is the same as that of the metal plate 1 to be evaluated.
Further, the radius of curvature of the end face shape (contour shape) of the shearing surface 10 as viewed in a plan view at the position where the bending deformation is performed on the test piece is determined by the contour shape when shearing the metal plate 1 to be evaluated. Is preferably equal to or less than the radius of curvature R0. Further, the thickness of the metal plate 1 is also preferably equal to or close to the thickness of the metal plate 1 to be evaluated.

Then, a bending test is performed on each test piece so that a bent portion due to bending deformation passes through the shearing surface 10. At this time, a plurality of bending tests are performed with different bending radii at the bending portions. Specifically, this is realized by sequentially using punches having different bending radii as punches used for the bending test.
Here, there are various types of bending tests such as a U bending test and a V bending test. A V-bending test as shown in FIG. 3 is preferable for evaluating a vehicle body structural member. However, any bending test method may be used as long as bending deformation can be applied to the sheared surface 10 of the test piece 20 and the bending test can be reproduced by a forming simulation described later. In FIG. 3, reference numeral 21 denotes a die, and reference numeral 22 denotes a punch 22.

The bending test is performed using, for example, a plurality of punches 22 having a bending radius R in a range of 0.5 mm to 10 mm and different bending radii R from each other. As the bending radius R of the punch 22, it is preferable to use two or more punches 22 different from each other by 6 mm or more and 4 mm or less.
The pitch of the bending radius R of each punch 22 (difference between adjacent bending radii R) is relatively larger when the bending radius R of the punch 22 is 5 mm or more than when the bending radius R of the punch 22 is less than 5 mm. It is good to set a large pitch. The pitch does not need to be equal.

  For example, when the bending radius R of the punch 22 is 5 mm or more, the bending test may be performed by changing the bending radius R to 6.0 mm, 7.0 mm, 8.0 mm. However, the maximum value of the pitch is, for example, 3 mm or less. The accuracy of the test increases as the bending radius R is changed at a smaller pitch. However, when the bending radius R is larger than 5 mm, the accuracy hardly changes even though the number of tests increases. This is, as can be seen from the strain associated with bending deformation calculated from the bending theory based on the Kirchhoff-Love hypothesis, as the bending radius R is large at the strain on the outer bending surface, which is the position where the maximum strain occurs in bending deformation. This is because the amount of change in the surface strain outside the bend due to the change in the bending radius R is small.

  On the other hand, if the bending radius R of the punch 22 is less than 5 mm, the accuracy of the test decreases as the pitch increases, so that it is preferable to change the bending radius R at a pitch of less than 1 mm. The finer the pitch, the higher the accuracy. However, even if the pitch is too small, the accuracy cannot be improved. Therefore, the lower limit of the pitch is set to, for example, 0.2 mm. For example, when the bending radius R of the punch 22 is less than 5 mm, the bending radius R of the punch 22 may be changed to 4.5 mm, 4.0 mm, 3.5 mm,.

<Acquisition of relationship between bending radius R and crack length 3B>
Next, after performing the bending test, the length of the crack generated in the sheared surface 10 in each test piece 20 is measured. Note that the crack does not necessarily extend in a direction perpendicular to the shearing surface 10.
The method of measuring the crack length L is preferably defined by the minimum distance from the crack tip k1 to the outer bending surface of the shearing surface 10 as shown in FIG. 4, for example. In addition, as shown in FIG. 5, as the crack length L, a method of measuring the distance between the tip k1 of the crack and the midpoint N of the opening of the crack on the bending outer surface, or measuring the length along the crack There are ways to do that. The method for measuring the crack length L may be evaluated under the same conditions for measuring the crack length L, and any method for measuring the crack length L may be used.

As a result, a plurality of pieces of evaluation data including a set of (bending radius R of punch 22 and crack length L) can be obtained. Note that the bending radius R of the punch 22 has a value equivalent to the bending radius of the bent portion.
The relationship between the bending radius R of the bent portion due to bending deformation and the crack length L generated on the shearing surface 10 is expressed by the evaluation data composed of a plurality of sets (bending radius R of the punch 22 and crack length L). it can. The relationship is, for example, as shown in FIG. As can be seen from FIG. 6, it can be seen that the crack has suddenly extended from the predetermined bending radius R.

<Determination of critical bending radius 3C>
Statistical processing such as regression analysis is performed on the evaluation data composed of a plurality of sets (the bending radius R of the punch 22 and the crack length L), and a change in the bending radius R of the punch 22 as shown in FIG. A change rate L / R consisting of a change in the crack length L is determined. The bending radius R corresponding to the change position CH where the change rate L / R changes abruptly is defined as a critical bending radius RX which is a forming limit (deformation limit) of the bending deformation on the evaluated shearing surface 10. Here, the molding limit represents a molding limit.

  More specifically, as shown in FIG. 8, the rate of change L / R, which is the ratio of the change in the crack length L to the change in the bending radius R of the punch 22, is 0.03 or more, preferably 0.05 or more. It is preferable to set the boundary position of the bending radius to be the critical bending radius RX. The critical bending radius RX is the maximum bending radius R among the bending radii at which the rate of change L / R changes rapidly. The reason for adopting this value is that if the rate of change L / R is less than 0.03, the crack length L sharply changes due to the change in bending strain of the sheared surface 10 due to the change in the bending radius R of the punch 22, which is a forming condition. This is because there is a possibility that the influence on the change cannot be confirmed. Note that a small crack is generated on the sheared surface 10 by the shearing. Then, the crack rapidly expands at the change position where the change rate L / R is 0.03 or more, and the maximum value of the bending radius R at that time becomes the critical bending radius RX.

  Here, the evaluation data composed of a set of (the bending radius R of the punch 22 and the crack length L) by the bending test and the crack measurement described above may be sequentially registered in a database using a steel type or the like as a key. Then, if evaluation data that is the same or similar to the condition of the metal plate 1 to be evaluated already exists in the database, the evaluation data in the database may be used without performing the bending test. Even if the evaluation data of the same steel material or the like is not present in the database, the approximate evaluation data in the database is subjected to an interpolation process to form a plurality of sets (bending radius R of punch 22 and crack length L). Evaluation data may be obtained.

<Calculation (acquisition) 3D of deformation limit strain>
Next, a forming simulation that reproduces the test method of the bending test in which the critical bending radius RX is determined is performed. In this forming simulation, the surface strain of the crack generating portion on the sheared surface 10 with respect to the bending radius R is estimated. As the surface strain, for example, the maximum principal strain is adopted.
As a method of the forming simulation, an FEM analysis widely used as a press forming simulation for determining a bending crack of the sheared surface 10 at the time of press forming is preferable. From the analysis result, the maximum principal strain at the crack generating portion of the shearing surface 10 is obtained, and the relationship between the maximum principal strain of the crack generating portion of the shearing surface 10 and the bending radius R of the punch 22 as the forming condition is obtained. In the FEM analysis, the maximum principal strain is obtained by a finite element.

Then, the maximum principal strain under the forming conditions of the above-mentioned limit bending radius RX of the crack limit is determined, and this is defined as the deformation limit strain εlimit. In this way, the deformation limit strain is calculated.
Also, a forming simulation under the forming conditions at the crack limit may be performed, and the strain at the crack limit may be measured and used as the deformation limit strain. The strain evaluation position may be simply obtained from the outermost finite element on the outer side of the bend.
In addition, a marking such as a scribed circle is marked on the surface of the metal plate 1 during the bending test, and the maximum principal strain in the vicinity of the shearing surface 10 is experimentally obtained from a change in a marking position before and after the bending to reduce a deformation limit strain. May be acquired. That is, a minute mark may be formed on the surface of the test piece 20 and the strain may be obtained from the deformation of the mark.

The shape of the mark may be any shape such as a circle pattern, a dot pattern, a grid pattern, a concentric pattern, or the like, which can measure the strain after molding. The mark method includes electrolytic etching, photoetching, transfer with ink (stamp printing), and the like, and any method may be used. However, scribing is not preferable because it induces cracks. In the case of the forming simulation, it is not necessary to reproduce the shearing processing, and a model reproducing the end face shape of the test piece 20 subjected to the shearing processing or a model having a simple flat end face shape may be used.
If the finite element method using a three-dimensional solid element is used, the crack strain can be calculated with high accuracy.

<Determination of crack on sheared surface during press molding 3E>
The FEM analysis of the press forming is performed to calculate the maximum principal strain εedge of the portion where the crack is likely to occur on the sheared surface 10 at the portion where the crack is to be determined. Then, the presence or absence of a crack is determined by comparing the maximum principal strain εedge of the sheared surface 10 of the portion where the crack is to be determined with the above-described deformation limit strain εlimit obtained above.
Here, the position of the sheared surface where the crack is to be determined and the evaluation position of the sheared surface where the crack occurs at the critical bending radius RX need not coincide.
Specifically, when the following condition (1) is satisfied, it is determined that a crack has occurred.
εedge ≧ εlimit (1)
Here, the cracking of the sheared surface 10 may be determined based on whether or not the bending radius R of the portion where the crack is to be determined in the portion where the cracking is to be determined, at which the crack is likely to occur, is greater than or equal to the critical bending radius RX. .

<Press die design>
The design or change of the design of the press die used in the press forming is performed so that the bending radius R in the shape of the die to be used is larger than the critical bending radius RX evaluated and predicted as described above.

(Action and others)
In the present embodiment, when the target metal plate 1 is press-formed after the shearing, the deformation limit of the sheared surface 10 of the metal plate 1 can be accurately evaluated. As a result, it is possible to design a mold shape capable of accurately predicting the occurrence of cracks from the end face and suppressing the occurrence of cracks.
For example, it is possible to accurately predict whether or not the selection of the metal plate 1 to be used when press-molding various parts such as a panel part, a structure and a skeleton part of an automobile is appropriate. In addition, it is possible to stably perform press molding, and to greatly contribute to a reduction in a defective rate of a press-formed product. Furthermore, according to the present embodiment, for example, the shape of the press die can be accurately predicted at the design stage, which can contribute to shortening the manufacturing period of the press die.

Next, an example based on the present embodiment will be described.
An example will be described for a metal plate 1 made of three types of test materials A, B and C shown in Table 1. Table 1 shows the material properties of each test material.

  After making a punched hole having a diameter of 10 mm for each test material, each test material was cut so as to pass through the center position of the punched hole, and a test piece 20 as shown in FIG. 9 was manufactured. The punching was performed using a high-speed steel tool having a diameter of the punch 22 of 10 mm and a die diameter of 10.3 mm using a crank press to form a punched hole. This punched hole forms the shearing surface 10.

  In the present embodiment, the sheared surface 10 is a round hole having a diameter of 10 mm. However, a punched hole having any end surface shape may be used as long as the sheared surface 10 is used. However, the radius of curvature of the shearing surface 10 is preferably 1 mm or more. If the radius of curvature is less than 1 mm, stress concentration is likely to occur during bending deformation, so that effects other than bending deformation may be applied to the sheared surface 10. Furthermore, if the radius of curvature is less than 1 mm, the rigidity of the tool during the shearing process is low, so that the sheared surface 10 may not be formed in a predetermined shape. Therefore, it is preferable that the radius of curvature of the shearing surface 10 is set to 1 mm or more. However, it is preferable to set the radius to be equal to or less than the radius of curvature of the shearing surface 10 at the position where the bending is applied in the target product shape. The position of the shearing surface 10 to which bending is applied and its vicinity are positions where cracks are presumed to be easily generated.

In this example, a V-bending test was performed on each test piece 20.
In the V-bending test, the test piece 20 was set so that the center line of the test piece 20 (the position passing through the center of the punched hole) and the ridge line of the V-bending punch 22 coincided with each other, and the moving speed of the punch 22 was 3 mm. The bending test was carried out with a predetermined pressing load of 100 kN / s. At this time, the bending was performed using the V-bending punches 22 having different bending radii R.
Then, of the cracks generated on the sheared surface 10 of each test piece 20 bent by the punch 22, the maximum length of the crack L was measured as the crack length L on the sheared surface 10. .

As a method of measuring the crack length L of the sheared surface 10, the minimum distance from the crack tip k1 to the outer bending surface of the sheared surface 10 was measured as the crack length L as described above (see FIG. 4). The shear length L was measured by observing the sheared surface 10 at an observation magnification of 100 times using a digital microscope VHX-6000 manufactured by Keyence Corporation for the measurement of the crack length L. .
FIG. 10 shows the relationship between the crack length L of the sheared surface 10 and the bending radius R of the punch 22 in the V bending test.
As can be seen from FIG. 10, it can be seen that the crack length L changes abruptly at a predetermined bending radius R, and the change position differs depending on each test material.
In the present example, a test result in which the crack length L rapidly changed was determined to be a crack. The bending radius R at the crack limit was determined from the crack determination. For example, in the test material A, 5 mm was determined as the bending radius R at the crack limit.

Here, at a bending radius R larger than the bending radius R at the crack limit, the crack lengths L are substantially equal. Before the bending process, a small crack is present on the sheared surface 10 by the shearing process. For example, in the test material A made of a high-strength steel plate, a crack of about 50 μm is generated on the sheared surface 10 by shearing. The lower the tensile strength of the material, the smaller the cracks due to shearing are usually small.
Next, the relationship between the surface strain consisting of the maximum principal strain and the bending radius R at the crack limit was obtained by FEM analysis reproducing the above-mentioned V bending test. The result is shown in FIG.

The general-purpose finite element method software LS-DYNA ver. 800, and a dynamic explicit solution was used for the solution. The element of the test piece 20 was a first-order complete integral 8-node hexahedral solid element, the minimum mesh size in the in-plane direction was 0.16 mm, and the mesh was divided into ten in the plate thickness direction. The material model was an isotropic hardening model, and the true stress-true strain relationship obtained from the uniaxial tensile test was approximated by the Swift equation as the stress-strain relationship of the material, and this was given as the equivalent stress-equivalent plastic strain relationship. . The punch 22 and the die were rigid bodies, and the elements used were four-node first-order reduced integral shell elements.
From the results shown in FIG. 11, the deformation limit strain εlimit under the crack determination conditions in the V-bending test was as shown in Table 2. From this result, if the other forming conditions are reproduced by FEM analysis and the strain of the sheared surface 10 is obtained, it is possible to predict whether or not cracking of the sheared surface 10 occurs.

As an example, an example using a press-formed product using the material of the test material C in Table 1 will be described.
The critical bending radius RX of the test material C was 6.0 mm.
FIG. 12 shows a bending crack evaluation section of the sheared surface 10 in the press-formed product.
The bending radius R of the ridge line of the part including the evaluation part in FIG. 12 was changed to 4 mm, 6 mm, and 8 mm, and verification was performed by FEM analysis and actual part trial production. Table 3 shows the results of crack prediction and crack determination based on trial production of actual parts.

From these results, it was confirmed that the crack prediction based on the bending radius R and the crack length L was also effective in actual part trial production. This indicates that the crack limit can be predicted with high accuracy by this method.
Here, the present invention is not limited to the contents described above. For example, in the above embodiment, an example is shown in which the present invention is applied to a steel plate having a tensile strength of 980 MPa class or more (a steel plate of 1180 MPa class). The present invention is preferably applied to press forming of such a high-strength steel sheet, but can also be applied to a steel sheet having a tensile strength of less than 980 MPa or a metal sheet 1 other than a steel sheet.

DESCRIPTION OF SYMBOLS 1 Metal plate 2 Pressed part 2A Bending part 3A Bending test 3B Acquisition of relation between bending radius and crack length 3C Determination of critical bending radius 3D Acquisition of deformation critical strain 3E Crack judgment on sheared surface 10 Sheared surface 20 Test piece 22 Punch CH Change position L Crack length L / R Change rate R Punch bending radius RX Limit bending radius

Claims (8)

When performing press forming including bending deformation on the sheared metal plate, a deformation limit evaluation method for evaluating the deformation limit on the sheared surface of the metal plate,
The relationship between the bending radius of the bent portion due to bending deformation and the crack length generated on the sheared surface by applying bending deformation to the portion including the sheared surface of the metal plate was determined, and the relationship between the calculated bending radius and the crack length was calculated. A method of evaluating a deformation limit, comprising evaluating a limit of bending deformation of a metal plate on a sheared surface from a relationship.   A bending radius corresponding to a change position at which a change rate, which is a change in the crack length with respect to a change in the bending radius, changes more than a preset change rate, is determined as a forming limit of bending deformation on the evaluated sheared surface. The method for evaluating a deformation limit according to claim 1, wherein:   The method according to claim 2, wherein the set change rate is 0.03.   The method for evaluating a deformation limit according to any one of claims 1 to 3, wherein the position of the evaluation is a position on a sheared surface where a crack is presumed to be generated during bending deformation. When performing press forming including bending deformation on the sheared metal plate, a deformation limit evaluation method for evaluating the deformation limit on the sheared surface of the metal plate,
By giving bending deformation to the part including the sheared surface of the metal plate, the relationship between the bending radius of the bent part due to bending deformation and the crack length generated on the sheared surface was obtained, and from the obtained relationship, A method for evaluating a deformation limit, comprising: determining a deformation limit strain of a bending crack in which a crack occurs, and evaluating a bending deformation limit of a metal sheet on a sheared surface using the obtained deformation limit strain.   From the relationship between the bending radius and the crack length, the bending radius of the forming limit at which cracking occurs on the sheared surface is determined, and the forming simulation is performed under the bending forming conditions at the time when the bending radius of the forming limit is obtained, and the shearing surface is determined. The method for evaluating a deformation limit according to claim 5, wherein the deformation limit strain of bending crack is determined by calculating the surface strain at (1).   A method for predicting cracks due to bending deformation on a sheared surface of a metal plate, using the method for evaluating a deformation limit according to any one of claims 1 to 6. Using one of the evaluation method according to any one of claims 1 to 6 and the prediction method according to claim 7, a press die that suppresses the occurrence of cracks at the end face of the metal plate is designed. A method for designing a press die, characterized in that:

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