JP7151736B2 - Shape change prediction method for press-formed products - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for predicting shape change of a press-formed product, and more particularly, to a press-formed product having a bend-returned portion that has undergone bending-return deformation, and after releasing the press-formed product from the mold and springing back. The present invention relates to a shape change prediction method for a press-formed product for predicting shape change.

Press molding is a manufacturing method capable of manufacturing metal parts at low cost and in a short period of time, and is used in the manufacture of many automobile parts. In recent years, in order to improve the collision safety performance of automobiles and reduce the weight of automobile bodies, higher-strength metal sheets are being used for press molding of automobile parts.

One of the main problems in press-molding a high-strength metal plate is deterioration of the dimensional accuracy of the press-molded product due to springback. Residual stress generated in the press-formed product when the metal plate is deformed using the mold during press-forming becomes the driving force, and the press-formed product released from the mold springs back into the shape of the metal plate before press-forming. The phenomenon of trying to return to the moment is called springback.

The higher the strength of the metal plate (for example, a high-strength steel plate), the greater the residual stress in the press-formed product generated by press-forming. Therefore, the higher the strength of the metal plate, the more difficult it becomes to keep the shape of the press-formed product after springback within the specified dimensions. Therefore, it is important to have a technique for accurately predicting the shape change of the press-formed product due to springback.

To predict the shape change of a press-formed product due to springback, it is common to use a press-forming simulation based on the finite element method. As a procedure in the press forming simulation, first, the press forming analysis of the process of press forming a metal plate to the bottom dead center using a mold is performed, and the first stage ( For example, Patent Document 1) and a springback analysis in which the shape of a press-formed product taken out of a mold changes due to springback is performed, and the shape of the press-formed product that balances the moment of force and the residual stress is predicted. It is divided into stages (for example, Patent Document 2).

Japanese Patent No. 5795151 Japanese Patent No. 5866892 JP 2013-113144 A

Until now, by performing a press forming simulation that integrates the above-mentioned first stage press forming analysis and second stage springback analysis, the shape of the press formed product immediately after releasing from the mold and springing back has been confirmed. has been predicted.
However, when the inventors compared the shape of the press-formed product predicted by the press-forming simulation with the shape of the press-formed product actually press-formed, they found that the shape prediction accuracy by the press-forming simulation was low. I noticed that there is an item.

Therefore, when we investigated the press-formed product whose shape prediction accuracy by press-forming simulation is low and the cause thereof, we found that, like the vertical wall part of the press-formed product with a hat-shaped cross section, the punch used for press-forming the press-formed product For press-formed products with vertical walls formed by bending and unbending deformation of a blank (metal plate, etc.) with a die, immediately after press-forming (immediately after releasing from the mold and springing back) and after a few days have passed. I found that the shapes are different.

Such changes in the shape of press-formed products over time seem to be similar to the phenomenon of gradual deformation of structural members that continue to receive a high external load, such as the creep phenomenon (e.g., Patent Document 3). However, the phenomenon that the shape of a press-formed product changes with the lapse of time in the absence of an external load has not been known so far.

In addition, the second stage (springback analysis) in the conventional press-forming simulation is to predict the shape of the press-formed product immediately after it springs back at the moment it is removed from the mold. No studies have been made so far regarding prediction of shape change after several days have passed.
In addition, since the shape change over time of a spring-back press-formed product occurs without receiving an external load, as described above, it is possible to predict the shape change over time of such a press-formed product. On the other hand, it was not possible to use the analysis method to deal with the shape change due to the creep phenomenon.

The present invention has been made to solve the above-described problems, and after releasing a press-formed product having a bend-returned portion that has undergone bending-return deformation from the mold and springing back, further It is an object of the present invention to propose a shape change prediction method for a press-formed product for predicting shape change of the press-formed product over time.

The present invention specifically consists of the following configurations.

(1) In the press-formed product shape change prediction method according to the present invention, the press-formed product having a bend-returned portion that has undergone bending-return deformation springs back at the moment it is released from the mold, and then the time elapses. It predicts the shape change due to stress relaxation associated with
A shape and residual stress acquisition step immediately after springback for acquiring the shape and residual stress of the press-formed product immediately after springback by springback analysis of the press-formed product;
Remaining bending and bending return portion setting a value of residual stress relaxed and reduced from the residual stress immediately after springback for all or part of the bending and bending return portion in the obtained press-formed product immediately after springback a stress relaxation reduction setting step;
and a shape analysis step of obtaining a shape in which the moment of force is balanced for the press-formed product in which the value of the residual stress in the bending and returning portion is set to be relaxed and reduced.

(2) In the above (1),
The bend-and-return portion is the vertical wall portion of a press-formed product having a hat-shaped cross-section and having a top plate portion, a vertical wall portion, and a flange portion.

(3) In the above (1) or (2).
A blank to be subjected to press forming of the press-formed product is characterized by being a metal plate having a tensile strength of 150 MPa class or more and 2000 MPa class or less.

In the present invention, a step of obtaining a shape and residual stress immediately after springback by springback analysis of a press-formed product having a bend-return portion that has undergone bending-return deformation;
Remaining bending and bending return portion setting a value of residual stress relaxed and reduced from the residual stress immediately after springback for all or part of the bending and bending return portion in the obtained press-formed product immediately after springback a stress relaxation reduction setting step;
and a shape analysis step of obtaining a shape in which the moment of force is balanced for the press-formed product in which the value of the residual stress in the bending and returning portion is set to be relaxed and reduced, so that after releasing the mold from the mold and springing back With the lapse of time, it is possible to accurately predict the shape change in the bent and unbent portion of the press-formed product. As a result, in the manufacturing process of automobile parts, vehicle bodies, etc., it is possible to obtain press-formed products with even better dimensional accuracy than conventional ones, and to greatly improve the manufacturing efficiency.

FIG. 2 is a flow chart showing the flow of processing of a method for predicting shape change of a press-formed product according to an embodiment of the present invention; BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the press-formed article of hat-shaped cross-section which is object by embodiment of this invention, and Example 1. FIG. FIG. 10 is a view showing a press-formed article having a Z-shaped cross section, which is the object of Example 2; FIG. 10 is a diagram showing a press-formed product simulating a floor cross member of an automobile targeted in Example 3; FIG. 4 is a diagram for explaining a stress relaxation phenomenon in which stress decreases over time while strain is kept constant. It is a figure explaining the shape change by stress relaxation in the vertical wall part of the press-formed article of hat-shaped cross section. It is a figure explaining the wall curvature which is a shape change by stress relaxation in the vertical wall part of the press-formed product of hat-shaped cross section.

In order to solve the above-mentioned problems, the inventors predicted the shape change of the press-formed product having a bend-return portion after the release from the mold and the springback of the press-formed product with time. In order to establish a method for doing so, as a preliminary step, as an example in FIG. Various investigations were carried out on the cause of the change in shape of the vertical wall portion 5, which is the bent back portion in 1, with the lapse of time.

As a result, the inventors focused on the stress relaxation phenomenon in which the stress gradually relaxes and decreases with the passage of time while the strain is constant in the stress-strain diagram shown in FIG. In 1, it was found that the residual stress in the vertical wall portion 5 that was bent back by press forming gradually relaxes over time, thereby changing the shape that balances the moment of the force of the press formed product 1. .

A shape change due to relaxation of residual stress in the vertical wall portion 5 of the press-formed product 1 will be described with reference to the schematic diagram shown in FIG.

In the process of press forming (draw forming) a metal plate into a press-formed product 1 using a die comprising a punch, a die, and a blank holder, first, the metal plate is bent at the die shoulder of the die, and the bending is performed. Tensile stress is generated on the outer side of the bend, and compressive stress is generated on the inner side of the bend. Then, when the die is relatively moved toward the punch side, the portion bent by the die shoulder is flattened back by the punch and the die to form the vertical wall portion 5 . Therefore, as shown in FIG. 6( a ), the vertical wall portion 5 at the bottom dead center of the forming is subjected to compressive stress on the side corresponding to the bending outer side of the portion bent by the die shoulder, which corresponds to the bending inner side. There is a tensile stress on the side.

Next, when the press-molded product 1 that has been press-molded to the bottom dead center is removed from the mold (released), springback occurs instantaneously with residual stress generated during press-molding as a driving force. At that time, the vertical wall portion 5 is deformed so as to return to the shape bent in the press forming process. However, the rigidity of the shape itself of the metal plate or the press-formed product 1 hinders the force of returning to the bent shape, and the vertical wall portion 5 causes a curved wall warp as shown in FIG. 6(b). , tensile stress on the outside of the bend and compressive stress on the inside of the bend.

After that, as shown in FIG. 6C, the tensile stress and compressive stress in the vertical wall portion 5 relax and decrease with the lapse of time without receiving any force from the outside. As a result, the shape that balances with the moment of force changes, so that the curvature of the vertical wall portion 5 increases even if sufficient countermeasures are taken against springback that occurs instantaneously, causing further wall warpage.

In other words, when springing back from the bottom dead center after press forming, residual stress occurs in the press formed product at that time. With respect to the difference in , the difference between the residual stress on the front side and the residual stress on the back side in the plate thickness direction of the press-formed product is relaxed and reduced with the lapse of time. As a result, the processed portion of the press-formed product has a shape with less residual stress than the shape immediately after springback.

This phenomenon is completely different from the conventional springback behavior due to residual stress reduction.
In the conventional springback behavior, regarding the residual stress generated at the bottom dead center after press molding, the value of the residual stress that is about to be generated is forcibly reduced by a specific means, or the surface side of the press molded product that is about to be generated As a result, the shape of the press-forming bottom dead center is suppressed in springback and is maintained in the state after press-forming.

On the other hand, in the behavior of stress relaxation targeted by the present invention, after springback occurs from the bottom dead center after press forming, the existing residual stress is relaxed without being forced from the outside, so the residual stress is Trying to get back to nothing. As a result, the press-formed product has a shape further away from the target shape, such as an increase in the bending angle and warpage compared to immediately after the springback.

As described above, it was found that, in the press-formed product 1, the residual stress was relaxed with the lapse of time, and as a result, the shape further deviated from the shape at the bottom dead center of the forming, as shown in FIG.

Further, in the above description, the press-formed product 1 is press-formed by drawing, but even if the press-formed product 1 is press-formed by bending (form forming), the vertical wall portion 5 is a portion that has undergone bending and bending deformation during the press molding process, so after releasing the mold from the mold and springing back, the residual stress in the vertical wall portion 5 relaxes with the passage of time, and FIG. 6 ( It was also found that wall warping as shown in c) and FIG. 7 occurs.

Therefore, based on the above new knowledge, the inventors have studied a method of predicting a shape change due to stress relaxation after springback of the press-formed product 1 shown in FIG. 2, for example. As a result, the residual stress of the vertical wall portion 5 of the press-formed product 1 immediately after the springback obtained in the second step (springback analysis) of the press-forming simulation described above is relaxed and reduced, and the moment of force of the press-formed product 1 is reduced. It was discovered that the change in shape of the press-formed product 1 over time as shown in FIG.

Furthermore, the shape prediction method is not limited to the press-formed product 1 having a hat-shaped cross-sectional shape as shown in FIG. It was found that the change in shape over time after backing up could be predicted.

In the press-formed product shape change prediction method according to the embodiment of the present invention, the press-formed product having a bend-returned portion that has undergone bending-return deformation is released from the mold and springs back. As shown in FIG. 1, a shape/residual stress acquisition step S1 immediately after springback, a bending and returning portion residual stress relaxation reduction setting step S3, and a shape analysis step and S5.
In the following, each of the above steps will be described by taking as an example the vertical wall portion 5, which is a portion that has undergone bending and bending deformation, in the press-formed product 1 having a hat-shaped cross section as shown in FIG.
The dimensions and other specific numerical values shown in the specification and drawings of the present application are merely specific examples for explaining the present invention, and do not limit the present invention.

<Shape and residual stress acquisition process immediately after springback>
The shape and residual stress acquisition step S1 immediately after springback is a step of acquiring the shape and residual stress of the press-formed product 1 immediately after springback by springback analysis of the press-formed product 1 .

As an example of specific processing for acquiring the shape and residual stress of the press-formed product 1 immediately after springback, using a mold model that models the mold used for press-molding the actual press-formed product 1, A first step in which the press forming analysis of the process of press forming a metal plate to the forming bottom dead point is performed to determine the press formed product 1 at the forming bottom dead center, and the press formed product 1 at the forming bottom dead center obtained is put into a mold. and a second stage of springback analysis to determine the shape and residual stress that balances the moment of force of the press-formed product 1 after it is released from the model.

<Bending and returning part residual stress relaxation reduction setting process>
The bending and returning portion residual stress relaxation reduction setting step S3 is performed to relax the vertical wall portion 5 of the press-formed product 1 immediately after springback acquired in the shape/residual stress acquisition step S1 immediately after springback more than the residual stress. This is the step of setting the value of the reduced residual stress.

Here, the vertical wall portion 5 refers to the entire range or part of the vertical wall portion 5, and when there are two or more vertical wall portions 5 like the press-formed product 1 shown in FIG. A value obtained by relaxing and decreasing the residual stress is set for all or one of the vertical wall portions 5 .

Residual stress refers to tensile stress and compressive stress remaining in the press-formed product 1 immediately after springback. value) and compressive stress (negative value).

<Shape analysis process>
The shape analysis step S5 is a step of performing an analysis to obtain a shape in which the moment of force is balanced for the press-formed product 1 for which the residual stress is set to be relaxed and reduced in the bending and returning portion residual stress relaxation and reduction setting step S3.

According to the method for predicting shape change of a press-formed product according to the present embodiment, the vertical wall portion 5, which is the bend and return portion of the press-formed product 1 immediately after springback, acquired by the springback analysis. On the other hand, by setting the value of the residual stress that is relaxed and decreased from the residual stress, and by analyzing the shape that balances the moment of force for the press-formed product 1 in which the value of the residual stress is set to be relaxed and decreased, the actual press-forming Simulating the stress relaxation and shape change over time in the product 1, and predicting the shape change (wall warpage) over time of the vertical wall portion 5 of the press-formed product 1 after it is released from the mold and springs back. be able to.

In the above description, the bending and bending portion residual stress relaxation reduction setting step S3 applies to all or part of the vertical wall portion 5, which is the bending and bending portion of the press-formed product 1, to relax and reduce the residual stress. It is something that makes

Here, the part of the bent and bent back portion may be any one of the vertical wall portions 5 when there are two or more vertical wall portions 5 like the press-formed product 1 shown in FIG. , it may be a part of the range in the bend-bend-back portion in which a value obtained by relaxing and reducing the residual stress is set.

Furthermore, for example, in the press-formed product 1 shown in FIG. 2, the vicinity of the die shoulder portion 11 connected to the flange portion 7 in the vertical wall portion 5 is a portion that has received a large amount of bending and unbending deformation, and stress relaxation with the passage of time. It can be said that it is a part that has a large influence on the shape change due to Therefore, in the present invention, the residual stress relaxation reduction setting step of the bending and bending return portion relaxes and reduces the residual stress of a portion of the portion of the bending and bending return portion that has a large effect on the shape change due to stress relaxation. You may set the value of the residual stress.

In the press-formed product shape change prediction method according to the present invention, there is no particular limitation on the shape and type of the metal plate or press-formed product used as a blank for press-forming the press-formed product, but the residual stress of the press-formed product It is more effective for press-molded automobile parts using a metal plate with a high tensile strength.

Specifically, the blank is preferably a metal plate having a tensile strength of 150 MPa class or more and 2000 MPa class or less and a plate thickness of 0.5 mm or more and 4.0 mm or more.

Since metal sheets having a tensile strength of less than 150 MPa are rarely used for press-formed products, there is little advantage in using the press-formed product shape change prediction method according to the present invention. For parts with low rigidity such as automobile outer panel parts that use metal plates with a tensile strength of 150 MPa or more, they are susceptible to shape changes due to changes in residual stress. It can be applied suitably.

On the other hand, a metal plate having a tensile strength exceeding 2000 MPa class has poor ductility. In some cases, cracks are likely to occur, and press molding may not be possible.

The shape of the press-formed product is not limited to the press-formed product 1 having a hat-shaped cross section as shown in FIG. 21 and a press-formed product 41 having a shape simulating an automobile cross member as shown in FIG. The results of applying the present invention to press-formed products having these shapes and predicting the shape change of the press-formed products over time after springback will be described in Examples 1 to 3 below.

Furthermore, the press-formed product targeted by the present invention may be one that undergoes bending and unbending deformation in the press-forming process, has a high residual stress, and causes wall warpage after releasing from the mold and springing back. . For example, by applying the present invention to a press-formed product having a portion (flange portion, etc.) that is subjected to ironing in the press-forming process, stress relaxation over time after springback can be simulated, and the press-formed product can be It is possible to predict the shape change (wall warpage) of the molded product over time.

In addition, the types of press-formed products include outer panel parts such as doors, roofs, and hoods with low rigidity, A-pillars, B-pillars, roof rails, side rails, front side members, rear side members, crosses, etc., which use high-strength metal plates. It is preferable to apply the present invention to automobile parts such as frame parts such as members.

The present invention can be applied to press-formed products that have been press-formed by bending and unbending deformation, regardless of the press method (bending, form forming, or draw forming) of the press-formed product.

<Press molded product with hat-shaped cross section>
In Example 1, first, a metal plate A having mechanical properties shown in Table 1 below was used to press-form a press-formed product 1 having a hat-shaped cross-sectional shape shown in FIG. ) was performed. The bottom dead center shape of the press-formed product 1 was such that the vertical wall height of the vertical wall portion 5 in the press-forming direction was 80 mm.

Then, the press-molded product 1 press-molded to the bottom dead center was released from the mold, and the change in shape of the press-molded product 1 over time was measured.

Next, an analysis for predicting shape change of the press-formed product 1 was performed.
In the analysis, first, using a mold model that modeled the mold used for press forming, press forming analysis of the process of press forming metal plate A to the bottom dead point of forming was performed. 1 was obtained.

Subsequently, springback analysis was performed to determine the shape and residual stress of the press-formed product 1 immediately after the press-formed product 1 was released from the mold model at the bottom dead center of the molding.

Furthermore, a residual stress value was set by relaxing and reducing a predetermined percentage of the residual stress of the vertical wall portion 5 of the press-formed product 1 immediately after springback, which was obtained by the springback analysis.
Then, an analysis was performed to obtain a shape in which the moment of force is balanced for the press-formed product 1 in which the residual stress has been relaxed and reduced.

In Example 1, for all of the vertical wall portions 5 of the press-formed product 1 obtained by springback analysis, the residual stress immediately after springback was relaxed and reduced at a predetermined rate (residual stress relaxation reduction rate). were set as Invention Examples 1 to 4.

In addition, as a comparison object, press forming analysis and springback analysis of the press formed product 1 were performed in the same manner as in Invention Examples 1 to 4, and Comparative Example 1 and Comparative Example 1, which did not perform the analysis for determining the shape in which the moment of force is balanced. Alternatively, Comparative Example 2 was obtained by conducting springback analysis and then conducting analysis to obtain a shape in which the moment of force is balanced without relaxing or reducing the residual stress of the vertical wall portion 5 of the press-formed product 1 .

For each of Invention Examples 1 to 4 and Comparative Examples 1 and 2, from the shape of the press-formed product 1 at the bottom dead center of the molding at the longitudinal end (evaluation point a) of the flange portion 7 of the press-formed product 1 was calculated.
Table 2 summarizes the results of the relaxation reduction rate of the residual stress and the deviation amount of the evaluation point a in Examples 1 to 4 and Comparative Examples 1 and 2.

In Table 2 onwards, when the longitudinal center of the top plate portion 3 of the press-formed product 1 is aligned, the predicted value Dc is the deviation of the evaluation point a in Invention Examples 1 to 4 and Comparative Examples 1 to 2. The amount and the experimental value De are deviation amounts (=16 mm) of the evaluation point a after two days in the cross section in the width direction parallel to the top plate portion 3 of the actually press-molded press-molded product 1 . Moreover, the difference and the error of the predicted value with respect to the experimental value are calculated by the following formulas.
Difference in predicted value (mm) = De - Dc (1)
Error in predicted value (%) = (De - Dc) ÷ Dc x 100 (2)

The difference between the evaluation point a in Comparative Example 1 and Comparative Example 2 was the same, the difference from the experimental value was 1.5 mm, and the error in the predicted value was 10.3%.

In invention example 1, the value of the residual stress is set by reducing the residual stress of the vertical wall portion 5 by 5%. Compared with Example 1 and Comparative Example 2, it was improved.
In Invention Example 2, the residual stress values of the vertical wall portion 5 are set by reducing the residual stress by 10%. It was improved as compared with Comparative Examples 1 and 2, and was a better result than Inventive Example 1.
In Invention Example 3, the residual stress values of the vertical wall portion 5 are set by reducing the residual stress by 20%. Compared with Comparative Examples 1 and 2, the results were improved, and the results were even better than Inventive Example 2.
In Invention Example 4, the residual stress value of the vertical wall portion 5 was reduced by 30%. , Both are negative values, but when compared in terms of absolute values, they were improved compared to Comparative Examples 1 and 2, and the results were equivalent to Inventive Example 3.

<Press molded product with Z-shaped cross section>
In Example 2, first, a metal plate A having mechanical properties shown in Table 1 was used in the same manner as in Example 1 described above, and a press-formed product 21 having a Z-shaped cross-sectional shape shown in FIG. Forming including back deformation) was performed. The press-formed product 21 has a bottom dead center shape in which the vertical wall height of the vertical wall portion 25 in the press-forming direction is set to 100 mm.

Then, the press-molded product 21 press-molded to the bottom dead center was released from the mold, and the change in the shape of the press-molded product 21 over time was measured.

Next, a press forming analysis of the press formed product 21 and a subsequent springback analysis were performed. Inventive Example 5 was obtained by setting a stress value and analyzing the press-formed product 21 to obtain a shape in which the moment of force is balanced.
In addition, as a comparison target, press forming analysis and springback analysis of the press formed product 21 were performed in the same manner as in Invention Example 5, but analysis was performed to find a shape in which the moment of force is balanced by setting the value of residual stress to relax and decrease. Comparative Example 3 was taken as Comparative Example 3.

Then, for each of Inventive Example 5 and Comparative Example 3, the amount of deviation from the shape of the press-formed product 21 at the bottom dead center of the forming at the longitudinal end (evaluation point b) of the flange portion 27 of the press-formed product 21 was calculated. . As in Example 1, the center of the top plate portion 23 of the press-formed product 21 in the longitudinal direction was matched, and the distance in the cross section in the width direction parallel to the top plate portion 23 was used as the deviation amount.
Table 3 summarizes the results of the relaxation reduction rate of the residual stress and the amount of divergence between the evaluation point b in Inventive Example 5 and Comparative Example 3.

In Table 3, the predicted value Dc is the deviation amount of the evaluation point b in Inventive Example 5 and Comparative Example 3, and the experimental value De is the deviation of the evaluation point b after two days of the actually press-molded press-molded product 21. amount (= 14.5mm). Also, the difference and the error of the predicted value with respect to the experimental value are calculated by the above-described formulas (1) and (2), respectively.

In Comparative Example 3, the difference between the predicted value and the experimental value was 1.2 mm, and the error of the predicted value was 9.0%.
In Invention Example 5, the residual stress of the vertical wall portion 25 is reduced by 20%. Improved.

<Floor cross member>
In Example 3, first, a metal plate A having the mechanical properties shown in Table 1 was used to press-form (bend) a press-formed product 41 having a shape simulating a floor cross member of an automobile as shown in FIG. Forming including bending back deformation) was performed.

The press-formed product 41 has a mounting flange portion 49 that bends and extends outward from the longitudinal edges of the top plate portion 43 , the vertical wall portion 45 , and the flange portion 47 .

As for the shape of the press-formed product 41 at the bottom dead center, the vertical wall height of the vertical wall portion 45 in the press-forming direction was set to 130 mm.

Then, the press-molded product 41 press-molded to the bottom dead center was released from the mold, and the change in the shape of the press-molded product 41 with time was measured.

Next, a press forming analysis of the press formed product 41 and a subsequent springback analysis are performed. Inventive Example 6 was obtained by setting the values and analyzing the press-formed product 41 to obtain a shape in which the moment of force is balanced.
In addition, in the press-formed product 41, the mounting flange portion 49 is formed so as to be continuous along three edges of the top plate portion 43, the vertical wall portion 45, and the flange portion 47 in the longitudinal direction.

For comparison, press forming analysis and springback analysis of the press formed product 41 were performed in the same manner as in Invention Example 6, but the residual stress value of the vertical wall portion 45 was relaxed and decreased, and the moment of force was reduced. Comparative Example 4 was obtained without analysis for obtaining a balanced shape.

Then, for each of Invention Example 6 and Comparative Example 4, the amount of deviation from the forming bottom dead center at the longitudinal center position (evaluation point c, see FIG. 4) of the side edge of the flange portion 47 was calculated. As in Example 1, the center of the top plate portion 43 of the press-formed product 41 in the longitudinal direction was matched, and the distance in the cross section in the width direction parallel to the top plate portion 43 was used as the deviation amount.
Table 4 summarizes the results of the relaxation reduction rate of the residual stress of the vertical wall portion and the deviation amount of the evaluation point c in Inventive Example 6 and Comparative Example 4.

In Table 4, the predicted value Dc is the amount of deviation of the evaluation point c in Invention Example 6 and Comparative Example 4, and the experimental value De is the deviation of the evaluation point c of the actually press-molded press-formed product 41 after two days. amount (= 13.1 mm). Also, the difference and the error of the predicted value with respect to the experimental value are calculated by the above-described formulas (1) and (2), respectively.

In Comparative Example 4, the difference between the predicted value and the experimental value was 1.0 mm, and the error of the predicted value was 8.3%.
In Invention Example 6, the residual stress of the vertical wall portion 45 is reduced by 5%, the difference from the experimental value is 0.2 mm, and the error in the predicted value is 1.6%, which is an improvement over Comparative Example 4.

1 press-formed product 3 top plate portion 5 vertical wall portion 7 flange portion 9 punch shoulder portion 11 die shoulder portion 21 press-formed product 23 top plate portion 25 vertical wall portion 27 flange portion 41 press-formed product 43 top plate portion 45 vertical wall portion 47 flange portion 49 mounting flange portion

Claims (3)

Predicting the shape change of a press-formed product that has a bent-back part that has undergone bending-back deformation, and predicts the shape change due to stress relaxation over time after the springback occurs at the moment the mold is released from the mold. a method,
By springback analysis of the press-formed product released from the mold , the shape of the press-formed product that balances the moment of force and the residual stress immediately after springback and the shape immediately after springback that obtains the residual stress. - a residual stress acquisition step;
Remaining bending and bending return portion setting a value of residual stress relaxed and reduced from the residual stress immediately after springback for all or part of the bending and bending return portion in the obtained press-formed product immediately after springback a stress relaxation reduction setting step;
and a shape analysis step of obtaining a shape in which the moment of force is balanced for the press-formed product in which the value of the residual stress in the bend-bending return portion is set to be relaxed or decreased. 2. The press molding according to claim 1, wherein the bent and bent back portion is the vertical wall portion of a press-molded product having a hat-shaped cross-section and having a top plate portion, a vertical wall portion, and a flange portion. A method for predicting shape change of products. 3. The press-formed product shape change prediction method according to claim 1 or 2, wherein the blank to be subjected to press-forming of the press-formed product is a metal plate having a tensile strength of 150 MPa class or more and 2000 MPa class or less.

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