Method, device and program for predicting change in shape of press-molded article, and method for manufacturing press-molded article

Abstract

A method for predicting shape change of a press-molded article according to the present invention comprises: a step (S1) for acquiring stress and strain of a press-molded article at a molding bottom dead center; a step (S3) for acquiring the residual stress and strain of the press-molded article immediately after springback; a step (S5) for acquiring a stress relaxation amount that reflects a stress-strain change history before and after springback, and setting a residual stress after stress relaxation by adding the stress relaxation amount to the value of the residual stress of the press-molded article immediately after springback; and a step (S7) for determining a torque-balanced shape for the press-molded article for which the residual stress after stress relaxation has been set.

Description

Technical Field

[0001] The present invention relates to a method, an apparatus, and a program for predicting a shape change over time of a press formed part after springback at the moment of die release from a press forming die. Further, the present invention relates to a method for manufacturing a press formed part such that the shape of the press formed part that changes over time after springback at the moment of die release from the die is within a specified range. Background Art

[0002] Press forming is a manufacturing method capable of manufacturing metal parts at low cost and in a short time, and is used for manufacturing many automotive parts. In recent years, in order to achieve both collision safety and weight reduction of an automotive body, higher-strength metal sheets have been used for automotive parts. As one of the main problems in press forming a high-strength metal sheet, there is a decrease in dimensional accuracy due to springback. Springback is referred to as a phenomenon in which residual stress generated in a press formed part when a metal sheet is deformed by press forming becomes a driving force, and the press formed part after die release instantaneously tries to return to the shape of the metal sheet before press forming like a spring. The higher the strength of the metal sheet (for example, a high-tensile steel sheet), the greater the residual stress generated during press forming, and thus the greater the shape change due to springback. Therefore, the higher the strength of the metal sheet, the more difficult it is to control the shape after springback within a specified dimension. Therefore, a technique for accurately predicting the shape change of a press formed part due to springback has become important.

[0003] In the prediction of shape changes caused by springback, press forming simulation based on the finite element method is generally used. The steps in this press forming simulation are divided into two stages. First, in the first stage, a press forming analysis of the process of pressing a metal sheet to the forming bottom dead center is performed to predict the residual stress of the press formed part at the forming bottom dead center (for example, Patent Document 1). In the subsequent second stage, a springback analysis of the process in which the shape of the press formed part changes due to springback after being removed from the mold is performed to predict the shape in which the moment and residual stress of the press formed part after demolding are balanced (for example, Patent Document 2).

[0004] Previously, the shape of the press formed part after springback when removed from the mold was predicted by performing a press forming simulation that integrated the press forming analysis in the first stage and the springback analysis in the second stage. However, when comparing the shape of the press formed part predicted by the press forming simulation with the shape of the actually press formed part, there are press formed parts with low shape prediction accuracy based on the press forming simulation.

[0005] As an example of a press formed part with low shape prediction accuracy, there can be cited Figure 2 a sheet metal press bending formed part 31 in which a metal sheet 21 is press formed (bend formed) using a mold 11 having a punch 13 and a die 15 to form a bent portion 33. The shape of the sheet metal press bending formed part 31 is different due to the deformation of the bent portion 33 immediately after springback when removed from the mold 11 and after several days have passed.

[0006] Such a shape change of the sheet metal press bending formed part 31 accompanying the passage of time units is considered to be similar to a phenomenon in which a structural member that continuously receives a high load from the outside gradually deforms like a creep phenomenon (for example, Patent Document 3). However, the shape change that occurs in the press formed part over time after springback, such as the sheet metal press bending formed part 31, is a phenomenon that occurs without receiving a load from the outside, and an analysis method for dealing with shape changes caused by the creep phenomenon cannot be applied.

[0007] Therefore, for example, Patent Document 4 discloses a method for predicting the shape change of a stamped part caused by the passage of time after springback at the moment of demolding from a mold. In this method, first, the shape and residual stress of the stamped part immediately after springback are obtained by springback analysis of the stamped part. Then, the value of the residual stress obtained by relaxing and reducing the obtained residual stress at a prescribed ratio is set for the stamped part immediately after springback, and the shape of moment balance is obtained, whereby the shape change caused by the passage of time after springback can be predicted.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent No. 5795151

[0011] Patent Document 2: Japanese Patent No. 5866892

[0012] Patent Document 3: Japanese Unexamined Patent Application Publication No. 2013 - 113144

[0013] Patent Document 4: Japanese Patent No. 6888703 Summary of the Invention

[0014] Technical Problem to be Solved by the Invention

[0015] The method disclosed in Patent Document 4 focuses on the stress relaxation phenomenon in which stress gradually decreases with the passage of time when strain is applied to a metal plate and the strain is kept constant. Moreover, in the stamped part after springback, it is also based on the presumption mechanism that the residual stress of the stamped part relaxes and decreases with the passage of time.

[0016] However, in the method disclosed in Patent Document 4, it is necessary to appropriately adjust the ratio of relaxing and reducing the residual stress immediately after springback for each stamped part in a manner consistent with the shape change of the actual stamped part after springback. Therefore, it is desired to accurately predict the shape change of the stamped part after springback without appropriately adjusting the ratio of relaxing and reducing the residual stress in a manner consistent with the shape change of the actual stamped part.

[0017] Moreover, since the ratio of relaxing and reducing the residual stress immediately after springback varies for each stamped part, it is difficult to manufacture the stamped part in a manner that improves the dimensional accuracy of the shape change accompanying the passage of time after springback.

[0018] The present invention is made to solve the above technical problems, and an object thereof is to provide a method, apparatus, and program for predicting a shape change of a stamped part associated with the passage of time after springback with high accuracy. Another object of the present invention is to provide a method for manufacturing a stamped part capable of manufacturing a stamped part with good dimensional accuracy based on a prediction of a shape change of the stamped part after springback caused by stress relaxation associated with the passage of time.

[0019] Technical solutions for solving technical problems

[0020] The method for predicting a shape change of a stamped part according to the present invention predicts a shape change of the stamped part after springback at the moment of demolding from a mold caused by stress relaxation associated with the passage of time, and is characterized by including: a step of obtaining stress and strain at the bottom dead center of forming, performing mechanical calculations of a process of stamping a metal plate into the stamped part using the mold, and obtaining the stress and strain of the stamped part at the bottom dead center of forming; a step of obtaining residual stress and strain immediately after springback, performing mechanical calculations of a process of springback of the stamped part after demolding from the mold, and obtaining the residual stress and strain of the stamped part immediately after springback; a step of setting residual stress after stress relaxation, obtaining a stress relaxation amount reflecting a stress-strain change history before and after springback for all or part of the positions of the stamped part immediately after springback, and adding the obtained stress relaxation amount to the value of the residual stress of each position of all or part of the stamped part immediately after springback to set the residual stress after stress relaxation; a step of analyzing the shape after stress relaxation, for the stamped part with the residual stress after stress relaxation set, obtaining a shape in moment balance by mechanical calculations.

[0021] It may be that, in the step of setting residual stress after stress relaxation, one or a combination of a tension hold test, a tension unloading hold test, a tension unloading compression hold test, a compression hold test, a compression unloading hold test, and a compression unloading tension hold test of the metal plate is performed to conduct a stress relaxation test for reproducing the stress-strain change history of all or part of the positions of the stamped part in the process of springback of the stamped part, measuring the stress change caused by stress relaxation of the metal plate in the stress relaxation test, and obtaining the stress relaxation amount of each position of all or part of the stamped part based on the measured stress change.

[0022] It may be that, in the residual stress setting step after stress relaxation, for all or a part of the stamping part, the difference Δσsb = σp - σq between the stress σp before springback at the forming bottom dead center and the stress σq immediately after springback is calculated as the stress change amount, and the value obtained by multiplying the calculated stress change amount Δσsb by a predetermined specified value a is taken as the stress relaxation amount for each part of all or a part of the stamping part.

[0023] It may be that, in the residual stress setting step after stress relaxation, a stress relaxation test is performed by combining one or more of a tension hold test, a tension unloading hold test, a compression hold test, a compression unloading hold test, a tension unloading compression hold test, and a compression unloading tension hold test of the metal plate, the stress change amount Δσ1 during the tension process or compression process immediately before holding the metal plate and the stress change amount Δσ2 caused by stress relaxation during the holding process of the metal plate are measured, and the ratio Δσ2 / Δσ1 of Δσ2 to Δσ1 is set as the specified value a.

[0024] It may be that the metal plate is a steel sheet, and in the residual stress setting step after stress relaxation, the specified value a is set within the range of 0.01 or more and 0.04 or less.

[0025] The shape change prediction device for a stamping part of the present invention predicts the shape change of the stamping part caused by stress relaxation accompanying the passage of time after springback at the moment of demolding from the mold, and is characterized by comprising: a stress and strain acquisition part at the forming bottom dead center, which performs a mechanical calculation of the process of stamping a metal plate into the stamping part using the mold and acquires the stress and strain of the stamping part at the forming bottom dead center; a residual stress and strain acquisition part immediately after springback, which performs a mechanical calculation of the process of springback when the stamping part is demolded from the mold and acquires the residual stress and strain of the stamping part immediately after springback; a residual stress setting part after stress relaxation, which obtains a stress relaxation amount reflecting the stress-strain change history before and after springback for all or a part of the stamping part immediately after springback, and adds the obtained stress relaxation amount to the value of the residual stress of each part of all or a part of the stamping part immediately after springback to set the residual stress after stress relaxation; and a shape analysis part after stress relaxation, which performs a mechanical calculation to obtain the shape of moment balance for the stamping part with the residual stress after stress relaxation set.

[0026] The shape change prediction program for a stamping formed part of the present invention predicts the shape change of the stamping formed part caused by stress relaxation accompanying the passage of time after springback at the moment of demolding from the mold, and is characterized in that it causes a computer to function as the following parts: a stress and strain acquisition part at the bottom dead center of forming, which performs a mechanical calculation of the process of stamping a metal plate into the stamping formed part using the mold, and acquires the stress and strain of the stamping formed part at the bottom dead center of forming; a residual stress and strain acquisition part immediately after springback, which performs a mechanical calculation of the process of demolding the stamping formed part from the mold and springing back, and acquires the residual stress and strain of the stamping formed part immediately after springback; a residual stress setting part after stress relaxation, which acquires the stress relaxation amount reflecting the stress-strain change history before and after springback for all or a part of the positions of the stamping formed part immediately after springback, and adds the acquired stress relaxation amount to the value of the residual stress of each position of all or a part of the stamping formed part immediately after springback to set the residual stress after stress relaxation; a shape analysis part after stress relaxation, which obtains the shape of moment balance by mechanical calculation for the stamping formed part for which the residual stress after stress relaxation has been set.

[0027] The manufacturing method of the press-formed part of the present invention manufactures the press-formed part in such a way that the shape of the press-formed part that changes due to stress relaxation accompanying the passage of time after springback at the moment of demolding from the mold falls within a specified range. It is characterized by including: a temporary press-forming condition setting step of setting the temporary press-forming condition of the press-formed part; a shape after stress relaxation obtaining step of obtaining, based on the temporary press-forming condition, the shape after stress relaxation of the press-formed part that changes due to stress relaxation accompanying the passage of time after springback at the moment of demolding from the mold by using the shape change prediction method of the press-formed part of the present invention; a shape determination step of determining whether the shape of the obtained press-formed part after stress relaxation is within a pre-set specified range; a temporary press-forming condition changing step of changing the temporary press-forming condition when it is determined in the shape determination step that the shape of the press-formed part after stress relaxation is not within the pre-set specified range; a repeating step of repeatedly executing the temporary press-forming condition changing step, the shape after stress relaxation obtaining step, and the shape determination step until it is determined in the shape determination step that the shape of the press-formed part after stress relaxation is within the pre-set specified range; a press-forming condition determining step of determining the temporary press-forming condition in this case as the press-forming condition of the press-formed part when it is determined in the shape determination step that the shape of the press-formed part after stress relaxation is within the pre-set specified range; a press-forming step of press-forming the metal plate into the press-formed part with the determined press-forming condition.

[0028] Effects of the Invention

[0029] In the present invention, the stress relaxation amount reflecting the stress-strain change history before and after springback is obtained for all or part of the press-formed part immediately after springback. And based on the obtained stress relaxation amount, the residual stress after stress relaxation is set for the press-formed part immediately after springback, and the shape of force balance is obtained by mechanical calculation. Thus, it is possible to accurately predict the shape change of the press-formed part accompanying the passage of time after springback at the moment of demolding from the mold.

[0030] Moreover, in the present invention, based on the shape change of the stamping formed part predicted as described above, the shape of the die for stamping is manufactured and adjusted. And by using the die manufactured and adjusted in this way to manufacture the stamping formed part, it is possible to manufacture a stamping formed part with high dimensional shape accuracy. Moreover, in the present invention, by performing stamping using the actual die adjusted in such a way that the shape of the stamping formed part after the shape change caused by stress relaxation is controlled within a specified dimension, it is possible to manufacture a stamping formed part with good dimensional accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a flowchart showing the process of the method for predicting the shape change of the stamping formed part according to Embodiment 1 of the present invention.

[0032] Figure 2 is a diagram for explaining the shape change after the stamping formed part obtained by stamping a metal plate is demolded and springs back ((a) is the process of bending the metal plate into a sheet metal stamping and bending formed part, (b) is the process of holding the sheet metal stamping and bending formed part in the die, (c) is the process of demolding and unloading the sheet metal stamping and bending formed part, (d) is the process of placing the sheet metal stamping and bending formed part after springback).

[0033] Figure 3 is a diagram showing an example of the stress distribution in the thickness direction of the stamping formed part obtained by stamping a metal plate at (a) the bottom dead center of forming, (b) immediately after springback, (c) after the passage of time, and (d) the stress-strain change history in the stress relaxation test.

[0034] Figure 4 is a graph showing an example of the time-dependent change of (a) the stress-strain change history and (b) the change amount of the true stress caused by stress relaxation in the stress relaxation test of the test piece using the metal plate.

[0035] Figure 5 is a diagram showing the stress relaxation test for measuring the stress change amount caused by stress relaxation in the method for predicting the shape change of the stamping formed part according to Embodiment 1 of the present invention ((a) is the tension and hold test, (b) is the tension unloading and hold test, (c) is the tension unloading and compression and hold test).

[0036] Figure 6It is a diagram showing a stress relaxation test for measuring the stress change amount caused by stress relaxation in the method for predicting the shape change of a stamping formed part according to Embodiment 1 of the present invention ((d) is a compression and hold test, (e) is a compression unloading and hold test, (f) is a compression unloading and tension and hold test).

[0037] Figure 7 It is a graph showing the relationship between the stress change amount Δσ1 during the tensile process or compression process immediately before holding the metal sheet and the stress change amount Δσ2 caused by stress relaxation during the holding process, obtained through a stress relaxation test in the method for predicting the shape change of a stamping formed part according to Embodiment 1 of the present invention.

[0038] Figure 8 It is a flowchart showing one embodiment of the method for calculating the stress relaxation amount of a stamping formed part after springback in the method for predicting the shape change of a stamping formed part according to Embodiment 1 of the present invention.

[0039] Figure 9 It is a block diagram showing the structure of a device for predicting the shape change of a stamping formed part according to Embodiment 1 of the present invention.

[0040] Figure 10 It is a flowchart showing the process flow in the method for manufacturing a stamping formed part according to Embodiment 2 of the present invention.

[0041] Figure 11 It is a diagram explaining the method for measuring the deformation amount caused by the shape change after springback of a sheet metal stamping and bending formed part obtained by bending a metal sheet in Example 1.

[0042] Figure 12 It is a diagram showing a die model and a sheet metal stamping and bending formed part model for predicting the shape change caused by stress relaxation of the bending part of a sheet metal stamping and bending formed part in Example 1.

[0043] Figure 13 It is a graph showing the stress distribution in the thickness direction at the lower dead point of forming and immediately after springback of the bending part of a sheet metal stamping and bending formed part in Example 1.

[0044] Figure 14 It is a graph showing the stress-strain change history at each position in the thickness direction of the bending part of a sheet metal stamping and bending formed part in Example 1.

[0045] Figure 15 It is a graph showing the stress change amount caused by stress relaxation at each position in the thickness direction, obtained through a stress relaxation test for reproducing the stress-strain change history of the bending part of a sheet metal stamping and bending formed part in Example 1.

[0046] Figure 16 It is a graph showing the stress distribution in the thickness direction at the lower dead point of forming, immediately after springback, and after stress relaxation of the bent portion of the sheet metal stamping and bending formed part in Example 1.

[0047] Figure 17 It is a graph showing the measurement results of the amount of deformation caused by stress relaxation in the bent portion of the sheet metal stamping and bending formed part in Example 1.

[0048] Figure 18 It is a graph showing the results of comparing Invention Example 2 and Conventional Examples 1 to 3 of the amount of deformation caused by stress relaxation in the bent portion of the sheet metal stamping and bending formed part in Example 2.

[0049] Figure 19 It is a graph showing the results of Invention Example 2 and Conventional Example 1 of the stress distribution in the thickness direction at the lower dead point of forming, immediately after springback, and after stress relaxation of the bent portion of the sheet metal stamping and bending formed part in Example 2.

[0050] Figure 20 It is a view showing the B-pillar (B-pillar) member 71 of the stamping formed part which is the object in Example 3 and the portion which is the evaluation object of the shape change after springback of the automobile body.

[0051] Figure 21 It is a view showing the results of predicting the shape change caused by stress relaxation accompanying the passage of time using the B-pillar model obtained by modeling the B-pillar member by finite element analysis (finite-element analysis) in Example 3.

[0052] Figure 22 It is a graph showing the measurement results and prediction results (Invention Example 3) of the amount of deformation (corresponding to the shape after stress relaxation 2 days after immediately after springback) caused by the shape change after stress relaxation of a specific portion which is the evaluation object of the shape change accompanying the passage of time. Detailed implementation mode

[0053] <Process of obtaining the invention>

[0054] As described above Figure 2 shown, the inventor conducted various investigations on the cause of the shape change accompanying the passage of time in the sheet metal stamping and bending formed part 31 after springback from the mold 11. In this investigation, as Figure 3 shown, attention was paid to the stress distribution in the thickness direction of the bent portion 33 of the sheet metal stamping and bending formed part 31.

[0055] For the bent portion 33, at the lower dead point of forming (immediately after bending forming) of the metal plate 21, asFigure 3 As shown in (a) of FIG. , tensile stress acts in the region outside the neutral axis towards the bending outer side, and compressive stress acts in the region inside the neutral axis towards the bending inner side. However, immediately after the stamping and bending forming part 31 of the plate is demolded from the mold 11 and the bending moment of the bending part 33 is unloaded and rebounds, as Figure 3 shown in (b) of FIG. , the surface layer part on the bending outer side is reversed to compressive stress, and the surface layer part on the bending inner side is reversed to tensile stress. As a result, tensile stress remains in the region at about 1 / 4 of the plate thickness from the surface on the bending outer side of the bending part 33, and compressive stress remains in the region at about 1 / 4 of the plate thickness from the surface on the bending inner side.

[0056] At this time, the surface layer part on the bending outer side of the bending part 33 (the part shown as point A in (a) of FIG. , hereinafter referred to as "part A") undergoes tensile deformation until the plastic region and becomes a tensile stress state. And during springback, after the tensile stress is unloaded, it undergoes compressive deformation (unloading and reverse compression), and immediately after springback, as Figure 3 shown at point A' in (b) of FIG. , compressive residual stress acts. Moreover, after a period of time from immediately after springback, as Figure 3 shown at point A" in (c) of FIG. , the state where compressive residual stress acts is maintained. Figure 3 shown at point A" in (c) of FIG. , the state where compressive residual stress acts is maintained.

[0057] The part on the bending outer side of the bending part 33, which is closer to the inside than the surface layer part (the part shown as point B in (a) of FIG. , hereinafter referred to as "part B") undergoes tensile deformation until the plastic region during bending forming and becomes a tensile stress state. And immediately after springback, as Figure 3 shown at point B' in (b) of FIG. , the tensile stress is completely unloaded. Moreover, after a period of time from immediately after springback, as Figure 3 shown at point B" in (b) of FIG. , the stress remains in a state of approximately 0. Figure 3 shown at point B" in (b) of FIG. , the stress remains in a state of approximately 0.

[0058] The part on the bending outer side of the bending part 33, which is further closer to the inside (the part shown as point C in (a) of FIG. , hereinafter referred to as "part C") undergoes tensile deformation until the plastic region during bending forming and becomes a tensile stress state in the same way as the aforementioned part B. However, part C immediately after springback, as Figure 3 shown at point C' in (b) of FIG. , becomes a state where the tensile stress is unloaded halfway. And after a period of time from immediately after springback, as Figure 3 shown at point C' in (b) of FIG. , becomes a state where the tensile stress is unloaded halfway. And after a period of time from immediately after springback, asFigure 3 As shown by point C in (c), it remains in a state where tensile residual stress acts thereon.

[0059] In this way, it can be understood that each part in the thickness direction of the bent portion 33 of the sheet metal stamping bent member 31 is maintained after undergoing various stress-strain change histories (deformation histories) during the process of springback after demolding from the die 11.

[0060] Therefore, the inventor conducted an experiment in which the stress-strain change histories of each part in the thickness direction of the bent portion 33 from immediately after the start of bending forming ( Figure 3 (a)) to immediately after springback ( Figure 3 (b)) were reproduced on a test piece of a metal sheet. In this experiment, after applying a uniaxial tensile load or a uniaxial compressed load to the test piece of the metal sheet and maintaining it for a certain period of time, the stress-strain change history associated with the passage of time from the start of maintenance was investigated. Hereinafter, this experiment will be referred to as the "stress relaxation test".

[0061] Figure 4 (a) shows the results of simulating the stress-strain change histories of part A, part B, and part C in the thickness direction of the bent portion 33 of the sheet metal stamping bent member 31 through a stress relaxation test on a test piece of a steel sheet with a tensile strength of 1180 MPa grade and a plate thickness of 1.2 mm.

[0062] As the conditions for simulating the stress-strain change history at part A (refer to Figure 3 (a)), which is the surface layer part on the outer side of the bend of the bent portion 33, as shown in Figure 4 (a), first, tensile deformation (point A) was imparted to the test piece until the strain ε = 0.056. Thereafter, the test piece was unloaded, reversed and compressed until the compressive stress σ = -336 MPa (point A'), and the test piece was maintained in this state for a certain period of time (2 days) (point A").

[0063] As the conditions for simulating the stress-strain change history at part B (refer to Figure 3 (a)), which is the part on the inner side of the surface layer part on the outer side of the bend of the bent portion 33, as shown in Figure 4 (a), first, tensile deformation (point B) was imparted to the test piece until the strain ε = 0.042. Thereafter, the test piece was unloaded until the tensile stress σ = 69 MPa and stopped halfway (point B'), and the test piece was maintained in this state for a certain period of time (2 days) (point B").

[0064] As the conditions of the stress-strain change history at the portion C (refer to Figure 3 (a)) which is further inside than the outer side of the simulated bent portion 33, as shown in Figure 4 (a), first, tensile deformation (point C) was applied until the strain ε = 0.023. After that, unloading was stopped halfway to a tensile stress σ = 508 MPa (point C'), and the test piece was held in this state for a certain period of time (2 days) (point C").

[0065] Figure 4 (b) shows the relationship between the change amount of the true stress from the start of holding during the process of holding the test piece and the holding time. As shown in Figure 4 (a) and (b), it was found that: at any of the portions A to C of the bent portion 33, stress changes occurred in the direction of approaching the state where the test piece was stretched (equivalent to immediately after bending forming) from the state immediately after unloading (equivalent to immediately after springback) over time.

[0066] That is, the surface layer portion (portion A) on the outer side of the bent portion 33 is in a tensile stress (positive side) state immediately after bending forming ( Figure 4 point A in (a)), but becomes a compressive residual stress (negative side) state immediately after springback ( Figure 4 point A' in (a)). And after springback, stress changes occur in the direction of relaxing the compressive residual stress over time ( Figure 4 point A" in (a)).

[0067] In addition, the portions B and C which are inside the surface layer portion on the outer side of the bend in the thickness direction are in a tensile stress (positive side) immediately after bending forming ( Figure 4 points B and C in (a)), and tensile residual stress (positive side) also remains immediately after springback ( Figure 4 points B' and C' in (a)). And after springback, stress changes occur in the direction of approaching the tensile stress state immediately after the original bending forming over time ( Figure 4 points B" and C" in (a)).

[0068] Moreover, as shown in Figure 4 (b), the stress change amount of the bent portion 33 of the sheet metal stamping bent part 31 after springback is different in the thickness direction, and the result is that it is larger the closer to the surface layer portion on the outer side of the bend and smaller the closer to the inner side of the bend (portion A > portion B > portion C).

[0069] Thus, it can be understood that the residual stress inside the bending portion 33 gradually and unevenly changes (stress relaxation) in the thickness direction, resulting in a shape change of the sheet metal stamping bent member 31 after springback. And, according to Figure 4 the results of the stress-strain change history at each part in the thickness direction of the bending portion 33 shown in

[0070] it was found that the stress change amounts caused by stress relaxation accompanying the passage of time are different.

[0071] Moreover, the inventors believe that the stress change amounts over time based on stress relaxation at each part in the thickness direction of the bending portion 33 can be obtained by conducting a stress relaxation test using a test piece of a metal sheet and simulating the stress-strain change history of each part of the bending portion 33. Figure 5 As stress relaxation tests, there are Figure 6 the (a) holding test after tension shown in

[0072] Figure 5 the (b) holding test after tension unloading shown in

[0073] Figure 5 the (c) holding test after tension unloading compression shown in

[0074] Figure 5 The (a) holding test after tension shown in

[0073] Figure 5 is a test as follows: A uniaxial tensile load is applied to the test piece 41, and the test piece 41 is held in the state where the tensile load is applied, and the stress-strain change history accompanying the passage of time is measured. In this holding test after tension, the testing machine is stopped midway during the uniaxial tensile test in which the uniaxial tensile load is applied to the test piece 41, and the holding portion 43 of the test piece 41 is fixed, and the test piece 41 is held in the state where the tensile load is applied.

[0074] Figure 5 The (b) holding test after tension unloading shown in

[0074] Figure 5 is a test as follows: After a uniaxial tensile load is applied to the test piece 41, the test piece 41 is held in the state where the tensile load is unloaded midway, and the stress-strain change history accompanying the passage of time is measured. In this holding test after tension unloading, the testing machine is switched to unloading midway during the application of the uniaxial tensile load to the test piece 41, and the testing machine is stopped at the midway stage of unloading the tensile stress or at the stage of complete unloading, and the holding portion 43 of the test piece 41 is fixed. And, the test piece 41 is held in the state where the tensile load is applied or in the state where the tensile load is unloaded to zero.The tension-unloading compression-after-holding test shown in (c) is as follows: After applying a uniaxial tensile load to the test piece 41, the tensile load is unloaded, and then the test piece 41 is held in a state where a uniaxial compressive load is applied to the test piece, and the stress-strain change history associated with the passage of time is measured. In this tension-unloading compression-after-holding test, first, during the application of the uniaxial tensile load, the testing machine is switched to unloading to completely unload the tensile load. After unloading the tensile load, a uniaxial compressive load (reverse compression) is applied to the test piece 41. And, during the application of the uniaxial compressive load, the testing machine is stopped to fix the holding portion 43 of the test piece 41, and the test piece 41 is held in a state where the compressive load is applied.

[0075] Figure 6 The compression-after-holding test shown in (d) is as follows: A uniaxial compressive load is applied to the test piece, and the test piece 41 is held in a state where the compressive load is applied, and the stress-strain change history associated with the passage of time is measured. In this compression-after-holding test, during the uniaxial compression test in which a uniaxial compressive load is applied to the test piece 41, the testing machine is stopped to fix the holding portion 43 of the test piece 41, and the test piece 41 is held in a state where the compressive load is applied.

[0076] Figure 6 The compression-unloading-after-holding test shown in (e) is as follows: After applying a uniaxial compressive load to the test piece 41, the test piece 41 is held in a state where the compressive load is unloaded, and the stress-strain change history associated with the passage of time is measured. In this compression-unloading-after-holding test, during the application of the uniaxial compressive load to the test piece 41, the testing machine is switched to unloading, and the testing machine is stopped to fix the holding portion 43 of the test piece 41 at an intermediate stage of unloading the compressive stress or at the stage of complete unloading. And, the test piece 41 is held in a state where the compressive load is applied or in a state where the compressive load is unloaded to zero.

[0077] Figure 6 The compression-unloading tension-after-holding test shown in (f) is as follows: After applying a uniaxial compressive load to the test piece 41, the compressive load is unloaded, and then the test piece 41 is held in a state where a uniaxial tensile load is applied to the test piece, and the stress-strain change history associated with the passage of time is measured. In this compression-unloading tension-after-holding test, first, during the application of the uniaxial compressive load, the testing machine is switched to unloading to completely unload the compressive load. After unloading the compressive load, a uniaxial tensile load (reverse tension) is applied to the test piece 41. And, during the application of the uniaxial tensile load, the testing machine is stopped to fix the holding portion 43 of the test piece 41, and the test piece 41 is held in a state where the tensile load is applied.

[0078] In the case where buckling may occur in the test piece 41 when a uniaxial compressive load is applied in each of the above tests, a test device equipped with a jig disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2019-035603 can be used to suppress the buckling of the test piece 41.

[0079] The inventors conducted Figure 5 and Figure 6 the stress relaxation tests shown in, and repeatedly studied the relationship between the stress-strain change history and the stress change amount caused by stress relaxation accompanying the passage of time. From the results of the stress relaxation tests shown in Figure 4 it was found that the stress relaxation phenomenon is a phenomenon in which a stress change occurs in the direction of recovering from the stress state equivalent to immediately after springback to the original stress state equivalent to immediately after the bending forming before springback. Therefore, the inventors believe that the stress change amount before and after springback becomes the driving force for the stress relaxation phenomenon after springback and determines the stress change amount caused by stress relaxation.

[0080] The inventors conducted Figure 5 and Figure 6 the (b) tension unloading and holding test, (c) tension unloading and compression holding test, and (f) compression unloading and tension holding test in the stress relaxation tests shown in, and investigated the relationship between the stress change amount before and after springback and the stress change amount caused by stress relaxation. For the test piece 41, steel plates with a tensile strength of 1180 MPa grade and 590 MPa grade and a plate thickness of 1.2 mm were used as the metal plates.

[0081] Tables 1, 2, and 3 show the conditions and results of the stress relaxation tests. The pre-reversal strain εa and the pre-reversal stress σa are the strain and stress immediately before changing from tension or compression to unloading in the stress relaxation test, and are equivalent to the strain and stress of the stamping part before the start of springback. In addition, the post-reversal stress σb is the stress at the time point when the testing machine is stopped and the holding part 43 of the test piece 41 is fixed and holding starts in the stress relaxation test, and is equivalent to the residual stress of the stamping part immediately after springback. Moreover, the stress change amount Δσ1 before and after reversal is the stress change amount in the tensile process or compressive process immediately before holding, and is the difference between the pre-reversal stress σa and the post-reversal stress σb. And the stress change amount Δσ1 before and after reversal is equivalent to the stress change amount of the stamping part before and after springback. In addition, the stress change amount Δσ2 during the holding process is the stress relaxation amount during the holding for a certain period (2 days) in the stress relaxation test, and is equivalent to the stress change amount of the stamping part after springback and demolding.

[0082] [Table 1]

[0083] (Table 1)

[0084]

[0085] [Table 2]

[0086] (Table 2)

[0087]

[0088] [Table 3]

[0089] (Table 3)

[0090]

[0091] Figure 7 It shows the result of representing as a graph the relationship between the stress change amount Δσ1 during the tensile process or compression process immediately before holding the metal plate, which is obtained through the stress relaxation test, and the stress change amount Δσ2 caused by stress relaxation during the holding process. According to Figure 7 the results shown, it was found that regardless of the difference in the material strength (tensile strength of 590 MPa grade and 1180 MPa grade) of the metal plate used as the test piece 41, the stress change amount Δσ2 caused by stress relaxation after holding is proportional to the stress change amount Δσ1 before and after inversion.

[0092] Therefore, based on Figure 7 the results shown, the inventors obtained the slope (a = Δσ2 / Δσ1) of the proportional relationship according to the stress change amount Δσ1 before and after inversion and the stress change amount Δσ2 caused by stress relaxation during the holding process. As a result, as Figure 7 shown, the value of a obtained by linearly approximating each plotted point of the stress change amounts Δσ1 and Δσ2 obtained through the stress relaxation test was calculated to be 0.025. Moreover, from Figure 7 the results shown, the range in which a can be taken is 0.01 or more and 0.04 or less.

[0093] Based on the above results, the inventors found that the stress change amount caused by stress relaxation accompanying the passage of time after springback can be calculated by multiplying the stress change amount before and after springback by a specified value (= a). Moreover, the inventors conceived of adding the stress relaxation amount calculated in this way to the residual stress immediately after springback to predict the shape change of the press-formed part accompanying the passage of time after springback.

[0094] The present invention has been completed based on the above research results. Hereinafter, a method, apparatus, and program for predicting the shape change of a press-formed part according to Embodiment 1 of the present invention, and a method for manufacturing a press-formed part according to Embodiment 2 of the present invention will be described.

[0095] [Embodiment 1]

[0096] <Method for Predicting Shape Change of Stamped Part>

[0097] The method for predicting the shape change of the stamped part according to Embodiment 1 of the present invention predicts the shape change of the stamped part caused by stress relaxation accompanying the passage of time after springback at the moment of demolding from the mold. And, as Figure 1 shown, the method for predicting the shape change of the stamped part according to Embodiment 1 includes a stress and strain acquisition step S1 at the bottom dead center of forming and a residual stress and strain acquisition step S3 immediately after springback. Moreover, as Figure 1 shown, the method for predicting the shape change of the stamped part according to Embodiment 1 includes a residual stress setting step S5 after stress relaxation and a shape analysis step S7 after stress relaxation. Hereinafter, each of these steps will be described.

[0098] <Stress and Strain Acquisition Step at the Bottom Dead Center of Forming>

[0099] The stress and strain acquisition step S1 at the bottom dead center of forming includes a step (S1a) of performing a mechanical calculation of the process of stamping a metal plate into a stamped part using a mold. And, the stress and strain acquisition step S1 at the bottom dead center of forming includes a step (S1b) of obtaining the stress and strain of the stamped part at the bottom dead center of forming through the mechanical calculation of the stamping process.

[0100] The mechanical calculation of the process of stamping a metal plate can utilize stamping analysis based on the finite element method. In the stamping analysis, a mechanical calculation of the process of stamping a metal plate to the bottom dead center of forming using a mold model obtained by modeling the mold is performed. Thereby, the stress distribution and strain distribution of the stamped part at the bottom dead center of forming can be obtained. And, through the stamping analysis, the shape of the stamped part at the bottom dead center of forming can be obtained. Hereinafter, the stress distribution of the stamped part at the bottom dead center of forming is represented as "stress distribution (A)", and the strain distribution of the stamped part at the bottom dead center of forming is represented as "strain distribution (A)".

[0101] <Residual Stress and Strain Acquisition Step Immediately after Springback>

[0102] The residual stress and strain acquisition step S3 immediately after springback includes a step (S3a) of performing a mechanical calculation of the process of demolding the stamped part from the mold and springing back. And, the residual stress and strain acquisition step S3 immediately after springback includes a step (S3b) of obtaining the residual stress and strain of the stamped part immediately after springback through the mechanical calculation of the springback process.

[0103] The mechanical calculation of the springback process of a stamping formed part can utilize springback analysis based on the finite element method. In the springback analysis, through mechanical calculation, the shape that achieves moment balance immediately after the stamping formed part that has been stamped to the bottom dead center of forming is demolded from the die model is obtained for the stress and strain at the bottom dead center of forming. Thus, the residual stress distribution and strain distribution of the stamping formed part immediately after springback can be obtained. Moreover, through springback analysis, the shape of the stamping formed part immediately after springback can be obtained. Hereinafter, the residual stress distribution of the stamping formed part immediately after springback will be expressed as "residual stress distribution (B)", and the strain distribution of the stamping formed part immediately after springback will be expressed as "strain distribution (B)".

[0104] 《Residual Stress Setting Step after Stress Relaxation》

[0105] The residual stress setting step S5 after stress relaxation includes a step (S5a) of obtaining a stress relaxation amount reflecting the stress-strain change history before and after springback for all or part of the parts of the stamping formed part immediately after springback. Moreover, the residual stress setting step S5 after stress relaxation includes a step (S5b) of setting the residual stress after stress relaxation by adding the obtained stress relaxation amount to the residual stress value of each part of all or part of the stamping formed part immediately after springback. Hereinafter, the stress relaxation amount reflecting the stress-strain change history before and after springback will be expressed as "stress relaxation amount (C)". Moreover, the residual stress distribution after stress relaxation set by adding the stress relaxation amount (C) to the residual stress value of each part of all or part of the stamping formed part immediately after springback will be expressed as "residual stress distribution (D)".

[0106] (Calculation Method of Stress Relaxation Amount)

[0107] The following describes a scheme of the specific steps for obtaining the stress relaxation amount in the residual stress setting step S5 after stress relaxation.

[0108] First, a stress relaxation test is performed to reproduce the stress-strain change history of each part of the stamping formed part during the process of changing from the stress distribution (A) and strain distribution (A) at the bottom dead center of forming to the residual stress distribution (B) and strain distribution (B) immediately after springback on a test piece of a metal sheet. The stress relaxation test is set as a test composed of one or more combinations of the stress relaxation tests shown in (a) to (c) of the foregoing Figure 5 and Figure 6 (d) to (f) of the foregoing. Then, the stress change amount is measured through the stress relaxation test, and the measured stress change amount is obtained as the stress relaxation amount (C) of each part of the stamping formed part.

[0109] The stress relaxation test does not need to be performed in a manner that reproduces the stress-strain change history of all parts (such as finite element meshes) of the stamped part. In this case, first, determine the parts (for example, the parts A, B, and C corresponding to points A, B, and C of the bending part 33 shown in Figure 3 ) that are characteristic of the stress-strain change history before and after springback. Then, directly measure the stress relaxation amount of each determined part through the stress relaxation test. Moreover, for the stress relaxation amount outside the determined parts, as shown in Figure 7 , utilize the fact that the stress change amount Δσ1 before and after inversion and the stress change amount Δσ2 caused by stress relaxation during the holding process are in a proportional relationship, and interpolate (interpolation) or extrapolate (extrapolation) the stress relaxation amount obtained for each part through the stress relaxation test to obtain it.

[0110] 《Steps for Shape Analysis after Stress Relaxation》

[0111] The shape analysis step S7 after stress relaxation is a step of obtaining the shape of force balance through mechanical calculation for the stamped part for which the residual stress after stress relaxation is set in the residual stress setting step S5 after stress relaxation.

[0112] In the present Embodiment 1, for the stamped part for which the residual stress distribution (D) after stress relaxation is set, the shape of force balance is obtained through mechanical calculation.

[0113] Thus, in the method for predicting the shape change of the stamped part in the present Embodiment 1, the stress relaxation amount reflecting the stress-strain change history before and after springback is obtained for all or a part of the stamped part immediately after springback. Then, based on the obtained stress relaxation amount, the residual stress after stress relaxation is set for the stamped part immediately after springback, and the shape of force balance is obtained through mechanical calculation. Thereby, it is possible to accurately predict the shape change of the stamped part over time after springback with high precision without appropriately adjusting the ratio of reducing the residual stress relaxation immediately after springback in a manner consistent with the actual shape change of the stamped part after springback.

[0114] In the above description, the residual stress setting step S5 after stress relaxation obtains the stress relaxation amount of each part of all or a part of the stamped part through the stress relaxation test. However, in the present invention, the residual stress setting step S5 after stress relaxation may also calculate the stress relaxation amount by multiplying the stress change amount before and after springback at each part of the stamped part by a specified value a without performing the stress relaxation test that reproduces the stress-strain history of each part in the stamped part.

[0115] In this case, the stress change amount Δσsb before and after springback can be calculated based on the stress σp before springback and the stress σq immediately after springback at the bottom dead center of forming for each part of the stamping part (Δσsb = σp - σq). And the stress relaxation amount can be obtained by multiplying the stress change amount Δσsb by a specified value a.

[0116] The specified value a to be multiplied by the stress change amount Δσsb can be determined through, for example, Figure 8 each of the steps S5a1 to S5a4 shown below. First, for a test piece 41 of a metal plate that is the work material of the stamping part, a stress relaxation test (S5a1) is performed by combining one or more of (a) to (c) of Figure 5 and (d) to (f) of Figure 6 shown below. Next, the stress change amount Δσ1 during the tensile process or the compressive process immediately before holding the test piece 41 in the stress relaxation test is measured (S5a2). Next, the stress change amount Δσ2 caused by stress relaxation during the process of holding the test piece 41 is measured (S5a3). Then, the measured stress change amounts Δσ1 and Δσ2 are plotted, and as shown in Figure 7 below, the ratio Δσ2 / Δσ1 of Δσ1 to σ is calculated, and the calculated Δσ2 / Δσ1 is used as the slope a (S5a4).

[0117] When the metal plate to be stamped is a steel plate, even if the stress relaxation test using the metal plate as the test piece 41 is not performed, the specified value a can be set within the range of 0.01 or more and 0.04 or less based on the results shown in Figure 7 below.

[0118] <Shape change prediction device for stamping part>

[0119] The shape change prediction device for a stamping part according to Embodiment 1 of the present invention (hereinafter referred to as "shape change prediction device") predicts the shape change of the stamping part caused by stress relaxation accompanying the passage of time after springback at the moment of demolding from the mold. And as shown in Figure 9 below, the shape change prediction device 1 includes a stress and strain acquisition unit 3 at the bottom dead center of forming, a residual stress and strain acquisition unit 5 immediately after springback, a residual stress setting unit 7 after stress relaxation, and a shape analysis unit 9 after stress relaxation. The shape change prediction device 1 can be constituted by a CPU (central processing unit) of a computer (such as a PC (personal computer)). In this case, the above-mentioned units function by the CPU of the computer executing a specified program.

[0120] Stress and Strain Acquisition Unit at the Forming Bottom Dead Center

[0121] The stress and strain acquisition unit 3 at the forming bottom dead center performs mechanical calculations for the process of stamping a metal sheet into a stamped part using a mold, and acquires the stress and strain of the stamped part at the forming bottom dead center.

[0122] In the present Embodiment 1, the stress and strain acquisition unit 3 at the forming bottom dead center executes the stress and strain acquisition step S1 at the forming bottom dead center of the above-described shape change prediction method for the stamped part of the present Embodiment 1.

[0123] Residual Stress and Strain Acquisition Unit Immediately after Springback

[0124] The residual stress and strain acquisition unit 5 immediately after springback performs mechanical calculations for the process of demolding the stamped part from the mold and springing back. Moreover, the residual stress and strain acquisition unit 5 immediately after springback acquires the residual stress and strain of the stamped part immediately after springback through the mechanical calculations of the springback process.

[0125] In the present Embodiment 1, the residual stress and strain acquisition unit 5 immediately after springback executes the residual stress and strain acquisition step S3 immediately after springback of the above-described shape change prediction method for the stamped part of the present Embodiment 1.

[0126] Residual Stress Setting Unit after Stress Relaxation

[0127] The residual stress setting unit 7 after stress relaxation acquires the stress relaxation amount reflecting the stress-strain change history before and after springback for all or part of the parts of the stamped part immediately after springback. Moreover, the residual stress setting unit 7 after stress relaxation adds the acquired stress relaxation amount to the value of the residual stress of each part of all or part of the stamped part immediately after springback to set the residual stress after stress relaxation.

[0128] In the present Embodiment 1, the residual stress setting unit 7 after stress relaxation executes the residual stress setting step S5 after stress relaxation of the above-described shape change prediction method for the stamped part of the present Embodiment 1.

[0129] The residual stress setting unit 7 after stress relaxation may acquire the stress relaxation amount for all parts of the stamped part, or may acquire the stress relaxation amount for part of the parts of the stamped part. In the case of acquiring the stress relaxation amount for part of the parts of the stamped part, it is only necessary to determine the parts having a characteristic stress-strain change history before and after springback and acquire the stress relaxation amount of the determined parts. Moreover, for the stress relaxation amounts other than the determined parts, it is only necessary to interpolate or extrapolate the stress relaxation amount obtained for the determined parts based on the stress change before and after springback.

[0130] When obtaining the stress relaxation amount, the residual stress setting unit 7 after stress relaxation can directly obtain the measurement results of the stress relaxation test that reproduces the stress-strain change history of each part of the stamped part. Alternatively, the residual stress setting unit 7 after stress relaxation can also obtain the stress relaxation amount as the value calculated by multiplying the stress change amount before and after springback of each part of the stamped part by a specified value a. Moreover, when the metal plate to be stamped is a steel plate, as described above Figure 7 As shown, the specified value a may be set within the range of 0.01 or more and 0.04 or less.

[0131] "Shape Analysis Unit after Stress Relaxation"

[0132] The shape analysis unit 9 after stress relaxation performs a mechanical calculation to find the shape of moment balance for the stamped part for which the residual stress after stress relaxation has been set by the residual stress setting unit 7 after stress relaxation.

[0133] In the first embodiment, the shape analysis unit 9 after stress relaxation executes the shape analysis step S7 of the shape change prediction method for the stamped part of the first embodiment described above.

[0134] <Shape Change Prediction Program for Stamped Parts>

[0135] The first embodiment of the present invention can be configured as a shape change prediction program for a stamped part. That is, the shape change prediction program for the stamped part of the first embodiment of the present invention predicts the shape change of the stamped part over time after springback at the moment of demolding from the mold. And, as Figure 9 shown, the shape change prediction program of the first embodiment causes the computer to function as the stress and strain acquisition unit 3 at the forming bottom dead center and the residual stress and strain acquisition unit 5 immediately after springback. Moreover, as Figure 9 shown, the shape change prediction program of the first embodiment causes the computer to function as the residual stress setting unit 7 after stress relaxation and the shape analysis unit 9 after stress relaxation.

[0136] As described above, in the shape change prediction device and program for the stamped part of the first embodiment, the stress relaxation amount reflecting the stress-strain change history before and after springback is obtained for all or part of the stamped part immediately after springback. And, based on the obtained stress relaxation amount, the residual stress after stress relaxation is set for the stamped part immediately after springback, and the shape of force balance is obtained by mechanical calculation. Thus, it is possible to accurately predict the shape change of the stamped part over time after springback without appropriately adjusting the ratio of reducing the residual stress relaxation immediately after springback in a manner consistent with the actual shape change of the stamped part after springback.

[0137] [Embodiment 2]

[0138] <Method for manufacturing a stamped part>

[0139] The method for manufacturing a stamped part according to Embodiment 2 of the present invention manufactures a stamped part in such a manner that the shape of the stamped part that changes due to stress relaxation accompanying the passage of time after springback at the moment of demolding from the mold falls within a specified range. And, as Figure 10 shown, the method for manufacturing a stamped part according to Embodiment 2 includes a temporary stamping condition setting step S11, a shape acquisition step S13 after stress relaxation, and a shape determination step S15. Moreover, as Figure 10 shown, the method for manufacturing a stamped part according to Embodiment 2 includes a temporary stamping condition change step S17, a repetition step S19, a stamping condition determination step S21, and a stamping step S23. Hereinafter, each of these steps will be described.

[0140] <Temporary stamping condition setting step>

[0141] The temporary stamping condition setting step S11 is a step of setting temporary stamping conditions for a stamped part.

[0142] As the temporary stamping conditions set in the temporary stamping condition setting step S11, for example, in the case of taking as an object a sheet metal stamped and bent part 31 having a bent portion 33 as shown in the foregoing Figure 2 shown, the bending R (bending radius) (radius of die shoulder part) of the bent portion 33 and the bending angle can be cited.

[0143] As other temporary stamping conditions, for example, the interval between the punch and the die of the die for stamping the stamped part can be cited. In addition, in the case of stamping a stamped part by the draw-bending method (stretch bending, bending and stretching), the die shoulder radius of the die, the interval between the side wall portions (longitudinal wall forming portions) of the die and the punch, the blank holder pressure between the die and the blank holder, etc. can also be cited.

[0144] <Shape acquisition step after stress relaxation>

[0145] The stress relaxation shape acquisition step S13 is a step of obtaining the shape after stress relaxation that changes due to stress relaxation accompanying the passage of time for a press-formed part after springback at the moment of demolding from the mold. In the stress relaxation shape acquisition step S13, based on the temporary press-forming conditions set in the temporary press-forming condition setting step S11, the shape of the press-formed part after stress relaxation is obtained by using the shape change prediction method of the press-formed part of the aforementioned Embodiment 1.

[0146] 《Shape determination step》

[0147] The shape determination step S15 is a step of determining whether the shape of the press-formed part after stress relaxation obtained in the stress relaxation shape acquisition step S13 is within a preset specified range.

[0148] The specified range related to the shape of the press-formed part can be appropriately set based on, for example, the dimensional error allowed by the actual part.

[0149] 《Temporary press-forming condition change step》

[0150] The temporary press-forming condition change step S17 is a step of changing the temporary press-forming conditions when it is determined in the shape determination step S15 that the shape of the press-formed part after stress relaxation is not within the preset specified range.

[0151] As described above, for the shape change of the press-formed part after springback, the stress change amount before and after springback is the driving force of the stress relaxation phenomenon, and the stress change amount caused by stress relaxation is imparted. Therefore, in order to reduce the shape change caused by stress relaxation, the temporary press-forming conditions are changed in such a way that the degree of springback generated at the moment of demolding from the mold becomes smaller. As the temporary press-forming conditions changed in the temporary press-forming condition setting process S17, conditions related to the shape of the mold and conditions related to the settings of the press forming machine can be cited.

[0152] For example, in the case of bending a metal plate (refer to Figure 2) In the case of a stamping formed part based on a drawing bending method (stretch bending, bend stretching), by reducing the radius of the die shoulder (a condition related to the shape of the die) and the clearance between the side wall parts of the die and the punch (a condition related to the setting of the stamping machine), the stretch bending part can be stretched to offset the strain of the bending part. By changing the temporary stamping conditions in this way, it is possible to expect a reduction in the degree of springback. In addition, the shape change caused by stress relaxation can also be estimated, and the temporary stamping conditions can be changed by changing the shape of the die such as correcting the bending angle of the bending part. In addition to this, the setting of the control panel of the stamping machine can also be changed.

[0153] 《Repeated Steps》

[0154] The repeated step S19 is a step of repeatedly executing the shape acquisition step S13 and the shape determination step S15 after stress relaxation under the temporary stamping conditions changed in the temporary stamping condition change step S17. This repetition is performed until it is determined in the shape determination step S15 that the shape of the stamped part after stress relaxation is within a specified range. Therefore, even when the temporary stamping conditions are only changed once and it is not determined that the shape of the stamped part after stress relaxation is within the specified range, the temporary stamping condition change step S17 will be repeated.

[0155] 《Stamping Condition Determination Step》

[0156] The stamping forming condition determination step S21 is a step of determining the temporary stamping forming conditions in this case as the stamping forming conditions of the stamping formed part when the shape of the stamping formed part after stress relaxation is within a preset specified range. For example, when the shape of the die is changed as the temporary stamping forming condition, the shape data of the die model for stamping forming analysis is input into a CAD / CAM program that cooperates with a numerically-controlled machine tool, and is transformed into numerical control data (numerical control program) for numerical control machining. The numerically-controlled machine tool is a device for machining a polystyrene foam casting die model or a steel die based on the full mold casting method. Using the numerical control data (numerical control program), a polystyrene foam casting die model or a steel die is manufactured using the numerically-controlled machine tool. Thus, the shape of the die for actual stamping forming can be determined as the stamping forming conditions of the stamping formed part. In addition, when the gap between the female die and the male die of the die is changed as the temporary stamping forming condition, the height of the distance block can be determined in accordance with this gap. Moreover, when the setting of the control panel of the stamping forming machine is changed as the temporary stamping forming condition, the changed setting of the control panel of the stamping forming machine is determined as the stamping forming conditions of the stamping formed part.

[0157] "Stamping Forming Step"

[0158] The stamping forming step S23 is a step of stamping a metal plate into a stamping formed part under the stamping forming conditions determined in the stamping forming condition determination step S21.

[0159] As described above, in the manufacturing method of the stamping formed part of the second embodiment, the shape of the die for stamping forming is manufactured and adjusted based on the shape change of the stamping formed part predicted by the shape change prediction method of the stamping formed part of the first embodiment. And by manufacturing the stamping formed part using the die manufactured and adjusted in this way, a stamping formed part with high dimensional shape accuracy can be manufactured. Moreover, in the manufacturing method of the stamping formed part of the second embodiment, by performing stamping forming using the actual die adjusted in such a way that the shape of the stamping formed part after shape change caused by stress relaxation is controlled within a specified dimension, a stamping formed part with good dimensional accuracy can be manufactured.

[0160] Example 1

[0161] Since experiments were conducted to confirm the effects of the present invention, the following description is provided. In Example 1, as shown in Figure 2 , the L-bending test of the metal plate 21 using the die 11 equipped with the punch 13 and the die 15 was taken as the object, and the shape change caused by stress relaxation of the sheet metal stamping bent part 31 after springback from the die 11 was predicted. Moreover, the shape change after springback of the sheet metal stamping bent part 31 formed by actual L-bending test was measured and compared with the prediction result for verification.

[0162] In the L-bending test in Example 1, as the metal plate 21, a rectangular shape with a width of 90 mm, a length of 120 mm, and a plate thickness of 1.2 mm, an ultra-high-tensile steel sheet with a tensile strength of 1180 MPa grade having the mechanical properties shown in Table 4 and a relatively large shape change after springback was used.

[0163] [Table 4]

[0164] (Table 4)

[0165]

[0166] In the L-bending test, first, as shown in Figure 2 , (a) the metal plate 21 was placed on the die 15 having a die shoulder part 15a with a specified die shoulder radius (8 mm), and (b) the punch 13 was pressed toward the die 15 side. Thereby, the metal plate 21 was bent at 90° along the die shoulder part 15a, and the sheet metal stamping bent part 31 having a bent part 33 and a side wall portion 35 was formed by bending. In the L-bending test, the interval between the forming bottom dead center of the punch 13 and the die 15 was 25% (0.3 mm) of the plate thickness of the metal plate 21.

[0167] Next, (c) after bending and forming, the punch 13 was raised to unload (demold). Thereby, springback occurred in the bent part 33 of the sheet metal stamping bent part 31. After that, (d) after springback, the sheet metal stamping bent part 31 was placed for 2 days in a state where the top portion 37 of the sheet metal stamping bent part 31 was pressed by the pressure pad 17 (refer to Figure 11 ).

[0168] During the process of placing the sheet metal stamping and bending formed part 31, the time-dependent change in the shape change caused by stress relaxation of the sheet metal stamping and bending formed part 31 from immediately after springback was measured. As the shape change of the sheet metal stamping and bending formed part 31, as Figure 11 shown, a laser displacement meter 51 was used to irradiate the longitudinal wall portion 35 of the sheet metal stamping and bending formed part 31 with laser light, and the amount of deformation of the longitudinal wall portion 35 was measured. In Figure 11 , (A) schematically shows the shape of the sheet metal stamping and bending formed part 31 immediately after springback, and (B) schematically shows the shape of the sheet metal stamping and bending formed part 31 after a lapse of time after springback.

[0169] Next, an analysis of predicting the shape change of the sheet metal stamping and bending formed part 31 was performed using the method for predicting the shape change of the stamping formed part of the present invention (Inventive Example 1). In the analysis, as Figure 12 shown, a die model 61 and a sheet metal stamping and bending formed part model 63 were used. The die model 61 was obtained by modeling the die 11 (punch 13 and die 15) used for the Figure 2 shown L-bending test. On the other hand, the sheet metal stamping and bending formed part model 63 was obtained by modeling using a two-dimensional solid element with plane strain having 7 layers equally divided in the thickness direction, and an isotropic hardening model was applied to the hardening model.

[0170] In the analysis, first, a stamping forming analysis of bending the metal sheet to the forming bottom dead center was performed. Then, using the stamping forming analysis, the shape, stress, and strain of the sheet metal stamping and bending formed part model 63 at the forming bottom dead center were obtained. Figure 13 represents the stress ( Figure 13 the △ notation in Figure 13 ) of each part in the thickness direction of the bending portion 65 in the sheet metal stamping and bending formed part model 63 at the forming bottom dead center. In

[0171] Next, a springback analysis was performed to obtain the shape, residual stress, and strain of the sheet metal stamping and bending formed part model 63 immediately after the sheet metal stamping and bending formed part model 63 at the forming bottom dead center was demolded from the die model 61. Figure 13It represents the stress at each position in the thickness direction of the bent portion 65 in the sheet metal stamping and bending formed part model 63 immediately after springback ( Figure 13 the ○ marks in).

[0172] Moreover, for each position in the thickness direction of the 1st to 7th layers of the bent portion 65 of the sheet metal stamping and bending formed part model 63, a stress relaxation test for reproducing the stress-strain change history before and after springback was carried out, and the stress change caused by stress relaxation was measured.

[0173] Figure 14 It represents the measurement result of the stress-strain change history measured by the stress relaxation test for each position in the thickness direction of the 1st to 7th layers of the bent portion 65 of the sheet metal stamping and bending formed part model 63. In addition, Figure 15 It represents the measurement result of the time-dependent change of the true stress of the 1st to 7th layers of the bent portion 65 of the sheet metal stamping and bending formed part model 63 as an example of the stress change caused by stress relaxation. And based on Figure 14 and Figure 15 the results shown, the stress relaxation amount of each position (1st to 7th layers) of the bent portion 65 of the sheet metal stamping and bending formed part model 63 was obtained.

[0174] Next, the stress relaxation amount obtained for each position (1st to 7th layers) of the bent portion 65 of the sheet metal stamping and bending formed part model 63 was added to the value of the residual stress immediately after springback, and the residual stress after stress relaxation was set.

[0175] Table 5 represents the stress at each position in the thickness direction of the bent portion 65 of the sheet metal stamping and bending formed part model 63 (bottom dead center of forming, immediately after springback). Moreover, Table 5 represents the stress relaxation amount obtained by the stress relaxation test and the residual stress after stress relaxation set by adding the stress relaxation amount to the residual stress immediately after springback. In addition, Figure 16 It represents the stress distribution obtained by adding the stress relaxation amount obtained by the stress relaxation test to Figure 13 the residual stress at each position in the thickness direction of the bent portion 65 in the sheet metal stamping and bending formed part model 63 immediately after springback shown ( Figure 16 the ● marks in). In Figure 16 , the vertical axis normalizes the positions of each layer in the thickness direction in the same way as the vertical axis of the aforementioned Figure 13 .

[0176] [Table 5]

[0177] (Table 5)

[0178]

[0179] As shown in Table 5, for example, the residual stress immediately after springback of the first layer on the outer side of the bend is -354 MPa, and the stress relaxation amount is +45 MPa. Therefore, the residual stress after stress relaxation becomes -309 MPa obtained by adding +45 MPa and -354 MPa.

[0180] Moreover, for the sheet metal stamping and bending formed part model 63 in which the values of the residual stress after stress relaxation are set for each part in the thickness direction of the first layer to the seventh layer, a mechanical calculation for obtaining the shape of moment balance was performed. Further, based on the shape of the sheet metal stamping and bending formed part model 63 obtained by the mechanical calculation, the shape change of the longitudinal wall portion 35 of the sheet metal stamping and bending formed part 31 was predicted.

[0181] Figure 17 It shows the prediction result of the shape change of the longitudinal wall portion 35 of the sheet metal stamping and bending formed part 31 caused by stress relaxation immediately after springback and the measurement result of the deformation amount of the longitudinal wall portion 35 of the actually stamped and formed sheet metal stamping and bending formed part 31.

[0182] As Figure 17 shown, the tendency of the time-dependent change of the prediction result and the measurement result of the deformation amount caused by stress relaxation is in good agreement. In addition, for the deformation amount x of stress relaxation, the predicted value is 0.71 mm (prediction accuracy 95%) with respect to the measured value of 0.75 mm, and the difference of 0.04 mm between the two is smaller than the dimensional accuracy (within ±0.5 mm) generally required for stamped parts.

[0183] The above results indicate that the shape change of the stamped part predicted by the shape change prediction method of the stamped part of the present invention has sufficient accuracy in practical use.

[0184] Example 2

[0185] In Example 2, in the shape change prediction method of the stamped part of the present invention, the residual stress after stress relaxation was set by a method different from that in Example 1, and the shape change of the stamped part after springback was predicted.

[0186] In Example 2, as the stress relaxation amount for each part in the thickness direction of the bending portion 65 of the sheet metal stamping and bending formed part model 63 which was the object in Example 1, a value obtained by multiplying the stress change amount before and after springback for each part in the thickness direction by a prescribed value a was set. Since the sheet metal stamping and bending formed part 31 is obtained by stamping a steel plate, the prescribed value a for calculating the stress relaxation amount was set to a = 0.025 (Inventive Example 2) according to the Figure 7 results shown.

[0187] In addition, as a comparison object, in accordance with the method disclosed in the aforementioned Patent Document 4, the values of the residual stress obtained by relaxing and reducing the residual stress after springback at a specified ratio (uniformly 15%) were compared and studied with those of Invention Example 2 (conventional example).

[0188] In the conventional examples, the positions in the thickness direction of the bent portion 65 that reflects the stress relaxation amount in the sheet metal stamping bent part model 63 were changed. In Conventional Example 1, the residual stress was relaxed and reduced for all the positions from the first layer to the seventh layer in the thickness direction of the bent portion 65. In Conventional Example 2, the residual stress was relaxed and reduced for the three layers (the first layer to the third layer) on the outer side of the bend. In Conventional Example 3, the residual stress was relaxed and reduced only for the outermost layer (the first layer) on the outer side of the bend.

[0189] Table 6 shows the stress (at the lower dead point of forming, immediately after springback) at each position in the thickness direction in Invention Example 2, the stress change (= (stress at the lower dead point of forming) - (residual stress immediately after springback)), the stress relaxation amount (= 0.025 × stress change), and the residual stress after stress relaxation.

[0190] [Table 6]

[0191] (Table 6)

[0192]

[0193] Figure 18 It shows the measured values and predicted results (Invention Example 2 and conventional examples) of the deformation amount x (corresponding to the shape after stress relaxation 2 days after immediately after springback) caused by the shape change after stress relaxation. As described above, the deformation amount of the longitudinal wall portion 35 caused by stress relaxation in the actually bent sheet metal stamping bent part 31 is 0.75 mm. The deformation amount in Invention Example 2 is 0.74 mm, and the difference from the actually measured deformation amount is 0.01 mm (1.3%). In contrast, the deformation amounts in Conventional Example 1, Conventional Example 2, and Conventional Example 3 are 0.11 mm, 0.13 mm, and 0.48 mm respectively, and compared with Invention Example 2, the differences from the actually measured deformation amount are larger. Based on this result, the method of the present invention has been improved compared with the conventional examples in terms of presenting good prediction accuracy for the shape change after springback.

[0194] Figure 19 It shows the distribution in the thickness direction of the residual stress after stress relaxation in Invention Example 2 and Conventional Example 1. In Figure 19 the vertical axis normalizes the positions of the respective layers in the thickness direction in the same way as the vertical axis of the aforementioned Figure 13 .

[0195] In Invention Example 2 as well, as shown in Table 5, the stress relaxation amounts of the outermost layers (the first layer and the seventh layer) on the outer side and the inner side of the bend are large, and the stress relaxation amount becomes smaller as it goes toward the center in the thickness direction. From this, it can be seen that stress relaxation occurs in the direction of making the residual stress immediately after springback close to the residual stress at the forming bottom dead center.

[0196] In contrast, in Comparative Example 1, the stress relaxation amounts of the third layer and the fifth layer (the portions approximately 1 / 4 of the plate thickness away from the outermost layer) are large, and the direction of stress relaxation is also opposite to that of Invention Example 2, and is also different from the results of the stress relaxation test shown in Figure 3 above. Therefore, it can be seen that it is necessary to adjust the ratio of stress relaxation reduction for each part in the stamped part in a manner consistent with the shape change immediately after springback of the actual stamped part.

[0197] The above shows that by the method for predicting the shape change of the stamped part of the present invention, it is possible to accurately predict the shape change of the stamped part caused by stress relaxation accompanying the passage of time after springback.

[0198] Example 3

[0199] In Example 3, a stamped part different from those in Example 1 and Example 2 was used as an object, and similar to Example 2, the residual stress after stress relaxation was set, and the shape change of the stamped part after springback was predicted.

[0200] As Figure 20 shown, the stamped part used as an object in Example 3 is a B-pillar member 71 of an automobile body having a bulge forming portion 73 that is substantially T-shaped in plan view. The B-pillar member 71 has a hat-shaped cross section (refer to Figure 20 (c)), and warps convexly upward along the length direction (refer to Figure 20 (b)). In Figure 20 , the X direction is the length direction of the B-pillar member 71, the Y direction is the width direction of the B-pillar member 71, and the Z direction is the vertical direction of the B-pillar member 71 with respect to the horizontal plane (the same applies in Figure 21 described later).

[0201] In Example 3, a stamping test (deep drawing) of the B-pillar member 71 was performed using a steel sheet of 1180 MPa grade, and then shape measurement was continuously performed at regular intervals using a three-dimensional shape measurement instrument.

[0202] The deformation of the B-pillar component 71 caused by rebound can be roughly classified into the opening deformation (wall opening deformation) of the cross section, the camber deformation (camber deformation) and the floating of the flange portion of the cross section. Therefore, the shape change accompanying the passage of time after rebound is also targeted. Figure 20 In each of the specific locations shown, opening, warping, and lifting are evaluated.

[0203] For the opening deformation of the section, such as Figure 20 As shown in (c), the opening deformation in the BB' section (opening B) and the opening deformation in the CC' section (opening C) are set as evaluation objects. Figure 20 As shown in (a), the warpage deformation (warpage D) at the intersection D1 of the BB' section and the DD' section of the top plate 75 is set as the evaluation object. Figure 20 As shown in (c), the floating deformation (warping C) of the flange portion 77 in the CC' section was set as the evaluation object. And, 4 minutes after the B-pillar component 71 was released from the mold, the relationship between the time after demolding and the opening B, opening C, warping D and warping C was measured.

[0204] Next, the shape change of the B-pillar member 71 after rebounding was predicted by the following procedure (Invention Example 3).

[0205] First, Figure 20 The stamping process of the B-pillar member 71 is analyzed by using the finite element method, and the stress and strain at the bottom dead point of the stamping are obtained by modeling the B-pillar member 71 using the finite element method.

[0206] Next, a springback analysis using the finite element method was performed on the process of springback of the B-pillar model 81 after being released from the mold model, and the residual stress and strain of the B-pillar model 81 immediately after the springback were obtained.

[0207] Next, as the stress relaxation amount at each portion in the thickness direction of the B-pillar model 81, a value obtained by multiplying the stress change amount before and after the springback at each portion in the thickness direction by a predetermined value a is set. The B-pillar model 81 is obtained by press-forming a steel plate, and therefore the predetermined value a used for calculating the stress relaxation amount is calculated based on the above-mentioned Figure 7 The results shown are set to a=0.025.

[0208] Then, a mechanical calculation for obtaining the shape that satisfies the moment balance was performed on the B-pillar model 81 with the stress relaxation amount set, and the shape change caused by the stress relaxation accompanying the passage of time was predicted.

[0209] Regarding the prediction result of the shape change of the B-pillar model 81 caused by the stress relaxation accompanying the passage of time, the openings B and C, and the warps D and C were also set as the evaluation objects, similarly to the actual stamping-formed B-pillar member 71.

[0210] Figure 21 The figure shows the result of predicting the shape change of the B-pillar model 81 caused by the stress relaxation accompanying the passage of time. Figure 21 (a) of the figure shows the deformation amount in the Z direction (the vertical direction of the B-pillar model 81 with respect to the horizontal plane), Figure 21 (b) of the figure shows the deformation amount in the Y direction (the width direction of the B-pillar model 81), Figure 21 (c) of the figure shows the moving distance (displacement amount) of each element of the B-pillar model 81 from immediately after springback.

[0211] The amount of change in the B-pillar model 81 caused by the stress relaxation accompanying the passage of time is such that the long side (between B - B' and C - C') of the roughly T-shaped view from above is larger. This is because the top plate portion 83 in the corresponding area ( Figure 21 within the dashed circle in (a)) sinks in the Z direction, and a shape change that expands in the Y direction occurs from the top plate portion 83 to the flange portion ( Figure 21 within the dashed circle in (b) of the figure).

[0212] Figure 22 The figure shows the measurement results and prediction results (Inventive Example 3) of the deformation amounts (corresponding to the shape after stress relaxation 2 days after immediately after springback) of the specific parts (opening B, opening C, warp D, warp C) set as the evaluation objects caused by the shape change after stress relaxation. The errors of the prediction results of the opening B, opening C, warp D, and warp C with respect to the measurement results are all within ±0.05 mm, and they are in good agreement with the measurement results.

[0213] The above indicates that, by the method for predicting the shape change of the stamping-formed part of the present invention, even when the B-pillar part of the automobile body is a stamping-formed part, it is possible to accurately predict the shape change of the stamping-formed part caused by the stress relaxation accompanying the passage of time after springback.

[0214] Example 4

[0215] In Example 4, by the manufacturing method of the stamping-formed part of the present invention, Figure 2The shown sheet metal stamping and bending formed part 31 has verified its dimensional accuracy. In Example 4, similar to the aforementioned Example 1 and Example 2, as the metal sheet, a rectangular shape with a width of 90 mm, a length of 130 mm, and a plate thickness of 1.2 mm, having the mechanical properties in Table 4 and a tensile strength of 1180 MPa grade with a large shape change after springback, an ultra-high tensile steel plate was used.

[0216] And, as Figure 2 shown, first, (a) the metal sheet 21 is placed on the die 15 having a die shoulder radius (6 mm) of a specified value at the die shoulder 15a. Then, (b) by pressing down the punch 13, the bending angle of the bending part 33 is targeted at 75° (the angle on the acute side with respect to the horizontal direction) along the die shoulder 15a and the length of the vertical wall part is made 100 mm, and the metal sheet 21 is bent and formed into the sheet metal stamping and bending formed part 31.

[0217] The target dimensional accuracy (a specified range set in advance) of the sheet metal stamping and bending formed part 31 is set within ±0.50 mm.

[0218] As the temporary stamping forming conditions for the sheet metal stamping and bending formed part 31, for the bending angle of the bending part 33, considering the estimated correction of the springback amount of 18 mm, the die 11 (the die 15 and the punch 13) is used, and the bending angle of the bending part 33 is made 85.5°. And the die shoulder radius is made 6 mm, and the interval (gap) between the punch 13 and the die 15 is made 1.2 mm, the same as the plate thickness.

[0219] Next, similar to Example 1, using the shape change prediction method for the stamping formed part of this Embodiment 1, the shape change of the sheet metal stamping and bending formed part 31 caused by stress relaxation after springback at the moment of demolding from the die was predicted. The prediction result of the deformation amount of the vertical wall part 35 of the sheet metal stamping and bending formed part 31 was +0.64 mm.

[0220] It was determined that this prediction result deviated from the range of the target dimensional accuracy of ±0.50 mm.

[0221] Therefore, in such a way that the shape of the stamping formed part after stress relaxation satisfies the target dimensional accuracy, the bending angle of the bending part 33 as the temporary stamping forming condition was also estimated for the shape change (+0.64 mm) caused by stress relaxation and changed to 85.8°.

[0222] Then, the temporary stamping forming conditions were changed to make the bending angle of the bending part 33 85.8° and keep the other stamping forming conditions (die shoulder radius, interval between punch and die) the same.

[0223] Next, under the changed temporary stamping conditions, an 1180 MPa grade steel sheet with a width of 90 mm, a length of 130 mm, and a thickness of 1.2 mm was used as a blank, and an L-bending test was actually carried out with the bending angle of the bending portion 33 being 85.8°, and a sheet metal stamping bent part 31 was stamped (Inventive Example 4).

[0224] Moreover, as a comparison object, an L-bending test using a die with a bending angle of 85.3° for the bending portion 33 was also carried out, and a sheet metal stamping bent part 31 was stamped (Comparative Example). And, in Inventive Example 4 and the Comparative Example, after springback after demolding from the die, it was left for 2 days, and the deformation amount of the longitudinal wall portion 35 was measured.

[0225] In Inventive Example 4, the deviation of the longitudinal wall portion 35 from the target shape was 0.02 mm, which was within the target dimensional accuracy (±0.50 mm). In contrast, in the Comparative Example, the deviation of the longitudinal wall portion 35 from the target shape was +0.62 mm, which was outside the target dimensional accuracy (±0.50 mm).

[0226] This shows that by the manufacturing method of the stamping part according to Embodiment 2 of the present invention, a stamping part with good dimensional accuracy can be manufactured.

[0227] Industrial Applicability

[0228] According to the present invention, there can be provided a method, device, and program for predicting the shape change of a stamping part accompanying the passage of time after springback with high accuracy. Moreover, according to the present invention, there can be provided a manufacturing method of a stamping part capable of manufacturing a stamping part with good dimensional accuracy based on the prediction of the shape change of the stamping part after springback caused by stress relaxation accompanying the passage of time.

[0229] Description of Reference Numerals

[0230] 1 Shape change prediction device;

[0231] 3 Stress and strain acquisition unit at the forming bottom dead center;

[0232] 5 Residual stress and strain acquisition unit immediately after springback;

[0233] 7 Residual stress setting unit after stress relaxation;

[0234] 9 Shape analysis unit after stress relaxation;

[0235] 11 Die;

[0236] 13 Punch;

[0237] 15 Die;

[0238] 15a female die shoulder;

[0239] 17 pressure pad;

[0240] 21 metal plate;

[0241] 31 sheet metal stamping and bending formed part;

[0242] 33 bending part;

[0243] 35 longitudinal wall part;

[0244] 37 top plate part;

[0245] 41 test piece;

[0246] 43 holding part;

[0247] 51 laser displacement meter;

[0248] 61 die model;

[0249] 63 sheet metal stamping and bending formed part model;

[0250] 65 bending part;

[0251] 71 B-pillar component;

[0252] 73 protruding part;

[0253] 75 top plate part;

[0254] 77 flange part;

[0255] 81 B-pillar model.

Claims

1. A method for predicting the shape change of a stamped part, which predicts the shape change of the stamped part caused by stress relaxation accompanying the passage of time after springback at the moment of demolding from the mold, characterized in that Comprising: A step of obtaining stress and strain at the forming bottom dead center, performing a mechanical calculation of the process of stamping a metal sheet into the stamped part using the mold, and obtaining the stress and strain of the stamped part at the forming bottom dead center; A step of obtaining residual stress and strain immediately after springback, performing a mechanical calculation of the process of springback of the stamped part after demolding from the mold, and obtaining the residual stress and strain of the stamped part immediately after springback; A step of setting residual stress after stress relaxation, obtaining a stress relaxation amount reflecting the stress-strain change history before and after springback for all or part of the parts of the stamped part immediately after springback, and adding the obtained stress relaxation amount to the value of the residual stress of each part of all or part of the stamped part immediately after springback to set the residual stress after stress relaxation; A step of analyzing the shape after stress relaxation, for the stamped part with the residual stress after stress relaxation set, obtaining the shape of moment balance through mechanical calculation.

2. The method for predicting the shape change of a stamped part according to claim 1, In the step of setting the residual stress after stress relaxation, One or more combinations of a tension holding test, a tension unloading holding test, a tension unloading compression holding test, a compression holding test, a compression unloading holding test, and a compression unloading tension holding test of the metal sheet are performed to conduct a stress relaxation test for reproducing the stress-strain change history of all or part of the parts of the stamped part during the springback process of the stamped part, measuring the stress change caused by stress relaxation of the metal sheet in the stress relaxation test, and obtaining the stress relaxation amount of each part of all or part of the stamped part based on the measured stress change.

3. The method for predicting the shape change of a stamped part according to claim 1, In the step of setting the residual stress after stress relaxation, For all or part of the parts of the stamped part, the difference Δσsb = σp - σq between the stress σp before springback and the stress σq immediately after springback at the forming bottom dead center is calculated as the stress change amount, and the value obtained by multiplying the calculated stress change amount Δσsb by a predetermined specified value a is obtained as the stress relaxation amount of each part of all or part of the stamped part.

4. The method for predicting the shape change of a stamped part according to claim 3, In the step of setting the residual stress after stress relaxation, A stress relaxation test composed of one or more combinations of a tension holding test, a tension unloading holding test, a compression holding test, a compression unloading holding test, a tension unloading compression holding test, and a compression unloading tension holding test of the metal sheet is performed, measuring the stress change amount Δσ1 during the tensile process or compression process immediately before holding the metal sheet and the stress change amount Δσ2 caused by stress relaxation during the holding process of the metal sheet, and setting the ratio Δσ2 / Δσ1 as the specified value a.

5. The method for predicting the shape change of a stamping part according to claim 3, wherein the metal plate is a steel plate, and in the step of setting the residual stress after stress relaxation, the specified value a is set within a range of 0.01 or more and 0.04 or less.

6. A shape change prediction device for a stamping formed part, which predicts the shape change of the stamping formed part caused by stress relaxation accompanying the passage of time after springback at the moment of demolding from a mold, characterized in that comprises: a stress and strain acquisition part at the forming bottom dead center, which performs a mechanical calculation of the process of stamping a metal plate into the stamping part using the die, and acquires the stress and strain of the stamping part at the forming bottom dead center; a residual stress and strain acquisition part immediately after springback, which performs a mechanical calculation of the process of springing back the stamping part by demolding it from the die, and acquires the residual stress and strain of the stamping part immediately after springback; a residual stress setting part after stress relaxation, which acquires a stress relaxation amount reflecting the stress-strain change history before and after springback for all or part of the stamping part immediately after springback, and adds the acquired stress relaxation amount to the residual stress value of each part of all or part of the stamping part immediately after springback to set the residual stress after stress relaxation; a shape analysis part after stress relaxation, which performs a mechanical calculation to obtain the shape of moment balance for the stamping part with the residual stress after stress relaxation set.

7. A program for predicting the shape change of a stamping part, which predicts the shape change caused by stress relaxation over time after springback at the moment of demolding from a die, characterized in that it causes a computer to function as the following parts: a stress and strain acquisition part at the forming bottom dead center, which performs a mechanical calculation of the process of stamping a metal plate into the stamping part using the die, and acquires the stress and strain of the stamping part at the forming bottom dead center; a residual stress and strain acquisition part immediately after springback, which performs a mechanical calculation of the process of springing back the stamping part by demolding it from the die, and acquires the residual stress and strain of the stamping part immediately after springback; a residual stress setting part after stress relaxation, which acquires a stress relaxation amount reflecting the stress-strain change history before and after springback for all or part of the stamping part immediately after springback, and adds the acquired stress relaxation amount to the residual stress value of each part of all or part of the stamping part immediately after springback to set the residual stress after stress relaxation; a shape analysis part after stress relaxation, which obtains the shape of moment balance by mechanical calculation for the stamping part with the residual stress after stress relaxation set.

8. A method for manufacturing a stamped part, wherein the stamped part is manufactured in such a manner that the shape of the stamped part, which changes due to stress relaxation accompanying the passage of time after springback at the moment of demolding from the mold, falls within a specified range. It is characterized in that, includes: a step of setting temporary stamping conditions for the stamping part, which sets the temporary stamping conditions for the stamping part; a step of obtaining the shape after stress relaxation, which, based on the temporary stamping conditions, uses the method for predicting the shape change of a stamping part according to any one of claims 1 to 5 to obtain the shape after stress relaxation that changes due to stress relaxation over time for the stamping part after springback at the moment of demolding from a die; Shape determination step, determining whether the shape of the obtained stress-relieved stamped part is within a preset specified range; Temporary stamping condition change step, in the case where it is determined in the shape determination step that the shape of the stress-relieved stamped part is not within the preset specified range, changing the temporary stamping conditions; Repeating step, until it is determined in the shape determination step that the shape of the stress-relieved stamped part is within the preset specified range, repeatedly executing the temporary stamping condition change step, the stress-relieved shape acquisition step, and the shape determination step; Stamping condition determination step, in the case where it is determined in the shape determination step that the shape of the stress-relieved stamped part is within the preset specified range, determining the temporary stamping conditions in this case as the stamping conditions of the stamped part; Stamping step, stamping the metal plate into the stamped part with the determined stamping conditions.

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