Method for predicting shape change of press-formed product

Abstract

The shape change prediction method of the press-formed product of the present invention predicts the shape change caused by stress relaxation accompanying the passage of time after the press-formed product (1) having a punch shoulder (9) and / or a die shoulder (11) is demolded from a mold with springback generated, wherein the method includes: a step (S1) of obtaining the shape and residual stress of the press-formed product (1) immediately after springback is generated by springback analysis of the press-formed product (1); a step (S3) of setting values of residual stress that are smaller than the residual stress of the punch shoulder (9) and / or the die shoulder (11) of the press-formed product (1) immediately after springback is generated; and a step (S5) of obtaining a shape in which moment balance is achieved with respect to the press-formed product (1) in which the values of the residual stress at the punch shoulder (9) and / or the die shoulder (11) are set to be reduced in relaxation.

Description

Technical Field

[0001] This invention relates to a method for predicting the shape change of a press-formed part, and more particularly to a method for predicting the shape change of a press-formed part after it springs back following die release. Background Technology

[0002] Stamping is a manufacturing method that enables the low-cost and short-time production of metal parts, and it is used in the manufacture of many automotive parts. In recent years, in order to balance the improvement of collision safety performance and the weight reduction of automotive bodies, higher-strength metal sheets have been used in the stamping of automotive parts.

[0003] One of the main challenges in stamping high-strength metal sheets is the deterioration of dimensional accuracy in stamped products due to springback. Springback is a phenomenon where residual stress generated during the deformation of the metal sheet using a die during stamping becomes a driving force, causing the stamped product to instantly return to the shape of the metal sheet before stamping, much like a spring.

[0004] The residual stress in stamped products generated by stamping is greater for high-strength metal sheets (e.g., high-tensile steel sheets), resulting in greater shape changes due to springback. Consequently, higher-strength metal sheets are more difficult to shape into the specified dimensions after springback. Therefore, techniques for accurately predicting shape changes in stamped products caused by springback become important.

[0005] In predicting the shape change of stamped parts caused by springback, stamping simulation based on the finite element method is generally used. This stamping simulation is divided into two stages: a first stage (e.g., Patent Document 1), which analyzes the stamping process of stamping a metal sheet to the bottom dead center using a die, predicting the residual stress generated in the stamped part; and a second stage (e.g., Patent Document 2), which analyzes the shape change of the stamped part after it is removed from the die due to springback, predicting the shape of the stamped part when the moment of force and residual stress are balanced.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent No. 5795151

[0009] Patent Document 2: Japanese Patent No. 5866892

[0010] Patent Document 3: Japanese Patent Application Publication No. 2013-113144 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] Up to this point, a stamping simulation integrating the aforementioned first-stage stamping analysis and second-stage springback analysis has been performed to predict the shape of the stamped article immediately after springback following demolding. However, the inventors have noted that when comparing the shape of the stamped article predicted by the stamping simulation with the shape of the actual stamped article, there are stamped articles with lower shape prediction accuracy based on the stamping simulation.

[0013] Therefore, in order to investigate the lower shape prediction accuracy of stamped products based on stamping simulation and the reasons for it, the shape of actual stamped products was measured. The results showed that the shape was different immediately after stamping (immediately after springback from the mold) and several days later.

[0014] As an example of how the shape of a stamped product changes over time, the measurement was performed. Figure 6 The shape change of the stamped product 71 with a U-shaped cross-sectional shape formed by a top portion 73 and a pair of side wall portions 75, as shown, due to the passage of time is illustrated. Figure 7.like Figure 7 As shown, if the opening amount of the stamped product 71 immediately after the longitudinal wall 75 opens and springs back at the moment of demolding from the mold is set as the baseline (0 mm), then the shape change occurs as the opening amount of the longitudinal wall 75 gradually increases over time.

[0015] The shape change of such stamped articles over time seems similar to the phenomenon of a structural member gradually deforming under continuous high external pressure, such as creep (e.g., Patent Document 3). However, the phenomenon of the shape of stamped articles changing over time without being subjected to external pressure has been unknown until now.

[0016] Furthermore, the second stage (springback analysis) in previous stamping simulations predicted the shape of the stamped part immediately after springback occurred upon removal from the die. Therefore, the prediction of shape changes in the stamped part several days later after springback has not been studied at all. Moreover, as mentioned earlier, the shape change of the springback-induced stamped part over time occurs without being subjected to external loads; therefore, analytical methods for handling shape changes caused by creep cannot be used to predict such shape changes over time.

[0017] This invention was made to solve the problems described above, and its purpose is to provide a method for predicting the shape change of a stamped product after a period of time following springback that occurs at the moment the stamped product is demolded from the mold.

[0018] Methods for solving problems

[0019] Specifically, the present invention is composed of the following structure.

[0020] The method for predicting the shape change of stamped articles of the present invention predicts the shape change of the stamped article over time after springback occurs at the moment of demolding from the mold. The method includes: a shape / residual stress acquisition process, which obtains the shape and residual stress of the stamped article immediately after springback by analyzing the springback; a residual stress reduction setting process, which sets a residual stress value that is more lenient than the residual stress value for all or part of the curved portions of the stamped article immediately after springback; and a shape analysis process, which calculates the torque-balanced shape of the stamped article with respect to the residual stress value at the curved portions after the lenient stress reduction setting.

[0021] In the above-described shape change prediction method, the curved portion is a punch shoulder in a stamped product having a top plate portion and a longitudinal wall portion, which connects the top plate portion and the longitudinal wall portion.

[0022] In the above-described shape change prediction method, the curved portion is the punch shoulder connecting the top plate portion and the longitudinal wall portion and / or the die shoulder connecting the longitudinal wall portion and the flange portion in a stamped product having a top plate portion, a longitudinal wall portion and a flange portion.

[0023] In the above-described shape change prediction method, the blank supplied to the stamping of the stamped article is a metal sheet with a tensile strength of 150 MPa grade or higher and 2000 MPa grade or lower.

[0024] Invention Effects

[0025] This invention includes a step of obtaining the shape and residual stress of the stamped part immediately after springback by analyzing the springback of the stamped part; a residual stress mitigation and reduction setting step of setting a residual stress value that is more lenient than the residual stress of all or part of the curved portions of the stamped part immediately after springback; and a shape analysis step of determining the shape of the stamped part with torque balance based on the residual stress value that has been mitigated and reduced. This allows for high-precision prediction of shape changes at the curved portions of the stamped part after springback from the mold. As a result, in the manufacturing processes of automotive parts, automotive bodies, etc., stamped parts with further superior dimensional accuracy compared to the past can be obtained, significantly improving manufacturing efficiency. Attached Figure Description

[0026] Figure 1 This is a flowchart illustrating the processing flow of the method for predicting shape changes of stamped articles according to an embodiment of the present invention.

[0027] Figure 2 The figures shown are (a) perspective view and (b) top view, illustrating a stamped article with a top hat-shaped cross-section as the object of the embodiments and embodiment 1 of the present invention.

[0028] Figure 3 The figures shown are (a) perspective view and (b) top view of a stamped article with a Z-shaped cross-sectional shape, which is the object in Embodiment 2.

[0029] Figure 4 This is a diagram showing a stamped article with an L-shaped cross-sectional shape, which is the object in Embodiment 3.

[0030] Figure 5 The figures shown are perspective views ((a) and top views) of the stamped article intended to be the object in Example 4.

[0031] Figure 6 This is an illustration of a stamped product with a U-shaped cross-section, which is used as an example of a measurement object for shape changes caused by the passage of time.

[0032] Figure 7 This is shown as an example of the shape change of a stamped product caused by the passage of time. Figure 6 The graph shows the measurement results of the opening amount of a stamped part with a コ-shaped cross-section after it springs back from the mold and over time.

[0033] Figure 8It is a stress-strain diagram that illustrates the stress relief phenomenon, in which the stress decreases over time when the strain is kept constant.

[0034] Figure 9 These are diagrams illustrating the stress and shape changes at the punch shoulder of a stamped product with a top hat-shaped cross-section ((a) immediately after stamping, (b) immediately after springback, (c) after time).

[0035] Figure 10 This diagram illustrates the change in bending angle at the punch shoulder and die shoulder of a stamped product with a top hat-shaped cross-section, caused by stress reduction. Detailed Implementation

[0036] In order to solve the aforementioned problems, the inventors, regarding stamped articles with bent portions, and in order to establish a method for predicting the shape changes of the stamped article after springback from the mold and subsequent time, as a preliminary stage, will... Figure 2 The stamped product 1 with the example top hat-shaped cross-section was taken as the object, and various studies were conducted on the reasons for the shape change over time.

[0037] As a result, the inventors focused on such Figure 8 As shown, the stress easing phenomenon, in which the strain is constant in the stress-strain diagram, gradually reduces the stress over time, was investigated. It was also found that in the stamped product 1 after springback, the residual stress at the punch shoulder 9 and die shoulder 11, which were bent due to stamping, gradually eased over time, thereby causing the shape change of the stamped product 1 to be in equilibrium with the torque.

[0038] Regarding the shape change caused by the mitigation of residual stress at the curved portion of the stamped part 1, namely the punch shoulder 9 and the die shoulder 11, the following applies: Figure 9 The diagram shown is for illustration. It should be noted that... Figure 9 The illustration shows an example of the cross-sectional shape and residual stress of the punch shoulder 9, but the same residual stress mitigation and shape change also occur at the die shoulder 11.

[0039] Firstly, in stamping, if a die consisting of a punch and a die is used to stamp the blank (metal sheet, etc.) to the bottom stop of the forming process, then as follows: Figure 9The punch shoulder 9 is formed as shown in (a). Therefore, tensile stress is generated on the outer side of the bend of the punch shoulder 9, and compressive stress is generated on the inner side of the bend. It should be noted that the outer side of the bend is defined as the side opposite to the center of curvature of the bend relative to the line of the center of the plate thickness in the cross section of the bend, and the inner side of the bend is defined as the side with the same curvature of the bend (the same applies below).

[0040] Next, if the stamped part 1 is removed from the mold (demolded), the residual stress generated during stamping causes the stamped part 1 to spring back instantaneously. At this time, if Figure 9 As shown in (b), the bending angle of the punch shoulder 9 changes, resulting in the return to the flat shape of the blank before stamping. However, the punch shoulder 9 is rigid, thus hindering the force required to return to the shape before stamping, resulting in compressive stress on the outside of the bend and tensile stress on the inside of the bend.

[0041] After that, as Figure 9 As shown in (c), as time passes, the force to restore the shape before stamping gradually weakens without being subjected to external force, and the shape becomes balanced with the torque in the stamped product 1. Therefore, at the punch shoulder 9, a change in the bending angle with further increased bending occurs.

[0042] That is, if the stamped part springs back from the bottom stop point after stamping, residual stress is generated in the stamped part at that point in time. However, regarding this generated residual stress, the difference between the residual stress on the surface and the residual stress on the inside in the thickness direction of the stamped part is mitigated and reduced over time. As a result, it was found that the processed portion of the stamped part becomes a shape with even less residual stress than the shape immediately after springback.

[0043] This phenomenon differs from the springback behavior caused by the reduction of residual stress in the past. In the past, regarding the residual stress generated at the bottom stop of the stamping process, if the value of the residual stress to be generated is forcibly reduced by specific means, or if the difference between the residual stress on the surface and the inside of the stamped part is forcibly reduced, then as a result, the shape of the bottom stop of the stamping process is suppressed from springback and is maintained in the state after stamping.

[0044] On the other hand, in the stress relief behavior that this invention aims to address, after springback occurs from the bottom stop of the forming process after stamping, the existing residual stress is relieved without being forced from the outside, thus restoring the state to one without residual stress. As a result, the stamped product, compared to immediately after springback, exhibits increased bending angle, warpage, etc., and becomes a shape that further deviates from the target shape.

[0045] Furthermore, in this top hat-shaped cross-section stamped product 1, even if measures to counteract the instantaneous springback are fully implemented, it will still be as... Figure 10 As shown, the bending angle changes at both the punch shoulder 9 and the die shoulder 11 due to the stress easing over time, resulting in a deviation in shape from the bottom stop of the forming at the flange 7 of the stamped product 1.

[0046] Therefore, based on the aforementioned new insights, for example, regarding predictions such as... Figure 2 A method for studying the shape change of the stamped product 1 after springback caused by stress relief was conducted. As a result, it was found that by reducing the residual stress in at least one of the punch shoulder 9 or die shoulder 11 of the stamped product 1 immediately after springback, obtained in the second stage (springback analysis) of the aforementioned stamping simulation, and further performing a third stage analysis to determine the shape of the stamped product 1 in torque balance, it is possible to predict the shape change of the stamped product 1 over time.

[0047] Furthermore, the following insights were gained: this shape prediction method is not limited to... Figure 2 The stamped product 1 with the top hat-shaped cross-section shown can predict the shape change over time after springback, as long as it is a stamped product with a curved portion.

[0048] The method for predicting the shape change of stamped articles according to embodiments of the present invention predicts the shape change of the stamped article over time after springback occurs at the moment of demolding from the mold, such as... Figure 1 As shown, it includes a shape / residual stress acquisition process S1 immediately after springback, a residual stress mitigation and reduction setting process S3, and a shape analysis process S5. Hereinafter, as a stamped product, Figure 2Taking a stamped product 1, which has a top-hat shaped cross-section consisting of a top plate portion 3, a longitudinal wall portion 5, and a flange portion 7, and which has a punch shoulder 9 connecting the side of the top plate portion 3 and the upper side of the longitudinal wall portion 5 as a curved portion, and a die shoulder 11 connecting the bottom side of the longitudinal wall portion 5 and the side of the flange portion 7, as an example, the above-described processes will be explained. It should be noted that the dimensions and other specific values ​​shown in the specification and drawings of this application are merely specific examples for illustrating the present invention and do not limit the present invention.

[0049] <Form / Residual Stress Acquisition Process Immediately After Springback>

[0050] The process S1 for obtaining the shape / residual stress immediately after springback is a process of obtaining the shape and residual stress of the stamped part 1 immediately after springback by analyzing the springback of the stamped part 1.

[0051] As an example of the specific processing to obtain the shape and residual stress of the stamped product 1 immediately after springback, a stamping simulation based on the finite element method can be cited. This stamping simulation has the following steps: First, using a mold model modeled from the mold used in the actual stamping of the stamped product 1, a stamping analysis is performed on the process of stamping a metal sheet to the lower limit of forming, and the stamped product 1 at the lower limit of forming is obtained; and Second, a springback analysis is performed to obtain the shape and residual stress of the stamped product 1 after it is demolded from the mold model at the obtained lower limit of forming, based on the equilibrium of the torque.

[0052] <Standardization and Reduction of Residual Stress Setting Procedure>

[0053] The residual stress mitigation and reduction setting process S3 is a process of setting the residual stress value of the punch shoulder 9 and die shoulder 11 of the stamped product 1 after springback, which is obtained in the shape / residual stress acquisition process S1 after springback, to a value that mitigates and reduces their residual stress.

[0054] <Shape Analysis Process>

[0055] The shape analysis process S5 is a process for analyzing the shape of the stamped part 1, which has had its residual stress reduced in the residual stress mitigation and reduction setting process S3, to determine the torque balance.

[0056] According to the shape change prediction method of the stamped product of this embodiment, a residual stress value that is reduced more gently than the residual stress is set for the bent portion of the stamped product 1, namely the punch shoulder 9 and the die shoulder 11, which are obtained by springback analysis immediately after springback occurs. The shape that is balanced with the torque is obtained by analysis for the stamped product 1 with the residual stress value reduced more gently. Thus, the stress relief and shape change caused by time in the actual stamped product 1 can be simulated. After springback occurs at the moment of demolding from the mold, the shape change of the stamped product 1 that occurs over time is predicted.

[0057] In the above description, a residual stress value that is milder than their residual stress is set for all the bent portions, namely punch shoulder 9 and die shoulder 11, in the stamped product 1 immediately after springback obtained by springback analysis. However, the present invention may also set a value that is milder than their respective residual stress for a portion of the bent portions, namely punch shoulder 9 and die shoulder 11, in the stamped product 1, or for a portion of the components in which there are multiple punch shoulders 9 or die shoulders 11.

[0058] However, the present invention is not limited to using such Figure 2 Let the stamped product 1 with the top hat-shaped cross-section shown be the object, for example, as shown in the figure. Figure 3 The stamped product 21 with the Z-shaped cross-section shown is an example. Figure 4 The L-shaped cross-section stamped product 41 shown is as follows: Figure 5 The stamped article 51, which simulates the shape of a car beam, is also considered as an object. The results of predicting the shape changes of the stamped article over time after springback by applying the present invention to these shapes will be described in Examples 1 to 4 below.

[0059] Furthermore, the stamped product targeted in this invention only needs to have a bent portion where tensile stress is generated on the outside of the bend and compressive stress is generated on the inside of the bend at the bottom stop of the stamping process when the metal sheet is bent. This allows for the simulation of stress relief over time after springback and the prediction of the shape change of the stamped product.

[0060] Furthermore, in the method for predicting the shape change of stamped articles of the present invention, there are no particular restrictions on the metal sheet used as blank in the stamping of the stamped articles, the shape and type of the stamped articles, etc., but it is more effective for automotive parts that are stamped using metal sheets with high residual stress in the stamped articles.

[0061] Specifically, regarding the blank, it is preferably a metal plate with a tensile strength of 150MPa or higher and 2000MPa or lower, and a plate thickness of 0.5mm or higher and 4.0mm or lower.

[0062] Since blanks with tensile strength less than 150 MPa are rarely used in stamped articles, the advantages of the shape change prediction method for stamped articles of the present invention are limited. However, for low-rigidity structures such as automotive exterior panel parts that use blanks with tensile strength of 150 MPa or higher, which are susceptible to shape changes caused by variations in residual stress, the advantages of applying the present invention increase, making the present invention suitable for application.

[0063] On the other hand, metal sheets with tensile strengths exceeding 2000 MPa lack ductility, therefore, for example, in... Figure 2 In the stamped product 1 with the top hat-shaped cross-section shown, cracks occur at the punch shoulder 9 and the die shoulder 11 during the stamping process, sometimes making it impossible to stamp.

[0064] Furthermore, this invention is preferably applied to stamped products with shapes that have high residual stress at the bending portion, such as stamped products with a U-shaped cross-section or a top hat-shaped cross-section.

[0065] Therefore, as a type of stamped product, this invention is preferably applied to automotive parts such as outer panel components with low rigidity, such as doors, roofs, and hoods, and frame parts such as A-pillars, B-pillars, roof rails, side beams, front longitudinal beams, rear longitudinal beams, and cross beams that use high-strength metal sheets.

[0066] It should be noted that this invention can be applied to stamped products formed by extrusion forming, bending forming, or deep drawing, regardless of the pressing method used.

[0067] Example 1

[0068] <Stamped products with a top hat-shaped cross-section>

[0069] In Example 1, firstly, a metal plate A having mechanical properties shown as an example in Table 1 below was used for... Figure 2The stamped product 1 with a top hat-shaped cross-section shown is a stamped product based on bending forming. In the bottom stop shape of the stamped product 1, the radius of curvature and bending angle of the punch shoulder 9 are set to 5mm and 95°, respectively, and the radius of curvature and bending angle of the die shoulder 11 are set to 5mm and 95°, respectively. It should be noted that the thickness of the metal plate A is 1.6mm, the yield strength is 880MPa, the tensile strength is 1210MPa, and the elongation is 13%.

[0070] [Table 1]

[0071] (Table 1)

[0072] Plate thickness / mm Yield strength / MPa Tensile strength / MPa Elongation / % Metal Plate A 1.6 880 1210 13

[0073] Then, after the stamped part 1, which was stamped to the lower limit of the forming process, was demolded from the mold, the change in the shape of the stamped part 1 over time was measured.

[0074] Next, an analysis was performed to predict the shape change of the stamped product 1. In the analysis, firstly, using a mold model modeled from the mold to be used in the stamping process, the stamping process of the metal sheet A was performed to the bottom stop of the forming process, and the residual stress of the stamped product 1 at the bottom stop of the forming process was calculated.

[0075] Next, springback analysis was performed to determine the shape and residual stress of the stamped part 1 immediately after it was demolded from the mold at the bottom stop of the forming process.

[0076] Furthermore, for the stamped product 1 immediately after springback, determined through springback analysis, values ​​are set for the residual stress of the punch shoulder 9 and / or die shoulder 11 such that their residual stress is reduced by a predetermined ratio. Then, an analysis is performed on the stamped product 1 with reduced residual stress to determine the shape of torque balance.

[0077] In Example 1, the following is defined as Examples 1 to 4 of the Invention: the punch shoulder 9 and / or die shoulder 11 of the stamped product 1 obtained by springback analysis are set such that the residual stress immediately after springback occurs is reduced by a predetermined ratio (the rate of reduction of residual stress).

[0078] In addition, as a comparison, the case in which the stamping analysis and springback analysis of the stamped product 1 were performed in the same manner as in Examples 1 to 4 of the Invention, but the analysis of the shape for which torque balance was not determined was defined as Comparative Example 1; or, the case in which the analysis of the shape for which torque balance was determined without reducing the residual stress of the punch shoulder 9 and the die shoulder 11 in the stamped product 1 after the springback analysis was performed was defined as Comparative Example 2.

[0079] For each of Invention Examples 1 to 4 and Comparative Examples 1 and 2, the deviation amount from the shape of the stamped article 1 at the front end (evaluation point a) of the flange portion 7 in the length direction of the stamped article 1, from the bottom stop of forming, was calculated. Table 2 summarizes the results of the locations where residual stress was reduced and eased in Invention Examples 1 to 4 and Comparative Examples 1 and 2, the reduction and easement rates, and the deviation amount at evaluation point a.

[0080] [Table 2]

[0081] (Table 2)

[0082]

[0083] Following Table 2, assuming the center of the top plate portion 3 in the length direction of the stamped product 1 is aligned, the predicted value Dc is the deviation of evaluation point a in Invention Examples 1 to 4 and Comparative Examples 1 to 2, and the experimental value De is the deviation of evaluation point a (=16mm) after 2 days in a cross-section parallel to the top plate portion 3 of the actual stamped product 1. Furthermore, the difference between the predicted value and the experimental value, and the error rate, are calculated using the following formulas.

[0084] The difference between predicted values ​​(mm) = De - Dc···(1)

[0085] The error of the predicted value (%) = (De - Dc) ÷ Dc × 100···(2)

[0086] The deviations of evaluation point a in Comparative Example 1 and Comparative Example 2 are equal, with a difference of 1.5 mm from the experimental value and an error of 10.3% in the predicted value.

[0087] In Invention Example 1, only the residual stress value after reducing the residual stress of the punch shoulder 9 by 5% was set, the difference in the predicted value became 0.9 mm, and the error in the predicted value became 6.0%, which was improved compared with Comparative Example 1 and Comparative Example 2. In Invention Example 2, the residual stress values after reducing the residual stresses of both the punch shoulder 9 and the die shoulder 11 by 10% were set respectively, the difference in the predicted value became 0.5 mm, and the error in the predicted value became 3.2%, which was improved compared with Comparative Example 1 and Comparative Example 2, and was a better result than Invention Example 1. In Invention Example 3, the residual stress values after reducing the residual stresses of both the punch shoulder 9 and the die shoulder 11 by 20% were set respectively, the difference in the predicted value became 0.2 mm, and the error in the predicted value became 1.3%, which was improved compared with Comparative Example 1 and Comparative Example 2, and was a further better result than Invention Example 2. In Invention Example 4, the residual stress values after reducing the residual stresses of the punch shoulder 9 and the die shoulder 11 by 30% and 20% respectively were set, the difference in the predicted value became -0.2 mm, and the error in the predicted value became -1.2%, both of which were negative values. However, when compared in absolute value, it was improved compared with Comparative Example 1 and Comparative Example 2, and was a result equivalent to Invention Example 3.

[0088] Example 2

[0089] <Stamped product with Z-shaped cross-sectional shape>

[0090] In Example 2, first, similar to the aforementioned Example 1, a metal plate A having the mechanical properties shown in Table 1 was used, and Figure 3 the stamping of the Z-shaped cross-sectional shaped stamped product 21 based on bending forming as shown was carried out. In the forming bottom dead center shape of the stamped product 21, the radius of curvature and bending angle of the punch shoulder 29 were set to 7 mm and 100°, and the radius of curvature and bending angle of the die shoulder 31 were set to 7 mm and 100°.

[0091] Then, the stamped product 21 stamped to the forming bottom dead center was removed from the mold, and the temporal change in the shape of the stamped product 21 was measured.

[0092] Next, the following was set as Invention Example 5: performing stamping analysis of the stamped product 21 and springback analysis following it, and for each of the punch shoulder 29 and die shoulder 31 in the stamped product 21 immediately after springback, setting the residual stress values after reducing their residual stress by 20% to relieve it, and performing analysis of the shape for calculating moment balance with respect to the stamped product 21. Additionally, as a comparison object, the following was set as Comparative Example 3: after performing stamping analysis and springback analysis of the stamped product 21 in the same manner as in Invention Example 5, performing analysis of the shape for calculating moment balance without setting the reduction of the residual stress value.

[0093] Then, for each of Invention Example 5 and Comparative Example 3, the deviation amount from the shape of the stamped product 21 at the bottom dead center of forming was calculated at the front end in the longitudinal direction of the flange portion 27 of the stamped product 21 (evaluation point b). It should be noted that regarding the deviation amount, the center in the longitudinal direction of the top plate portion 23 of the stamped product 21 was made to coincide in the same manner as in Example 1, and the distance within the cross-section in the width direction parallel to the top plate portion 23 was used. In Table 3, the results of the portions and reduction rates for reducing residual stress and the deviation amount at evaluation point b in Invention Example 5 and Comparative Example 3 are summarized and shown.

[0094] [Table 3]

[0095] (Table 3)

[0096]

[0097] In Table 3, the predicted value Dc is the deviation amount at evaluation point b in Invention Example 5 and Comparative Example 3, and the experimental value De is the deviation amount at evaluation point b after 2 days of the actually stamped product 21 ( = 14.5 mm). Additionally, the difference and error between the predicted value and the experimental value were calculated by the aforementioned formulas (1) and (2) respectively.

[0098] In Comparative Example 3, the difference between the predicted value and the experimental value is 1.2 mm, and the error of the predicted value is 9.0%. In Invention Example 5, with respect to both the punch shoulder 29 and the die shoulder 31, their residual stresses were reduced by 20%, the difference from the experimental value is 0.4 mm, the error of the predicted value is 2.8%, which is an improvement compared to Comparative Example 3.

[0099] Example 3

[0100] <Stamped product with L-shaped cross-sectional shape>

[0101] In Example 3, first, using the metal plate A having the mechanical properties shown in Table 1 above, Figure 4The L-shaped cross-section stamped article 41 shown is a stamped article based on bending forming. In the bottom dead center shape of the stamped article 41, the radius of curvature and bending angle of the punch shoulder 47 are set to 6 mm and 120°, respectively. Then, after the stamped article 41 is demolded from the mold at the bottom dead center, the change in the shape of the stamped article 41 over time is measured.

[0102] Next, the following case is designated as Invention Example 6: After performing the stamping analysis and subsequent springback analysis of the stamped product 41 in the same manner as in Embodiment 1, the residual stress value of the punch shoulder 47 in the stamped product 41 immediately after springback is further set to reduce the residual stress, and an analysis is performed on the stamped product 41 to determine the shape of torque balance. Additionally, as a comparison, the following case is designated as Comparative Example 4: After performing the stamping analysis and springback analysis of the stamped product 41 in the same manner as in Invention Example 6, no analysis was performed to reduce the residual stress value and determine the shape of torque balance.

[0103] Furthermore, regarding Invention Example 6 and Comparative Example 4 respectively, the center of the lower side of the longitudinal wall portion 45 in the length direction (evaluation point c, refer to) was calculated. Figure 4 The deviation amount from the bottom stop shape at point c is calculated. It should be noted that, regarding the deviation amount, similar to Example 1, the center of the top plate portion 43 of the stamped article 41 is aligned along its length, and the distance within a cross-section in the width direction parallel to the top plate portion 43 is used. Table 4 summarizes the results of the residual stress mitigation rate and the deviation amount at evaluation point c in Invention Example 6 and Comparative Example 4.

[0104] [Table 4]

[0105] (Table 4)

[0106]

[0107] In Table 4, the predicted value Dc is the deviation of evaluation point c in Invention Example 6 and Comparative Example 4, and the experimental value De is the deviation of evaluation point c (=19.5mm) of the stamped product 41 after 2 days. In addition, the difference and error between the predicted value and the experimental value are calculated by the aforementioned equations (1) and (2), respectively.

[0108] In Comparative Example 4, the difference between the predicted value and the experimental value was 3.8 mm, and the error of the predicted value was 24.2%. In Invention Example 6, the residual stress of the punch shoulder 47 was reduced by 20%, the difference from the experimental value was 1.1 mm, and the error of the predicted value was 6.0%, which is an improvement compared to Comparative Example 4.

[0109] Example 4

[0110] Floor beams

[0111] In Example 4, firstly, a metal plate A having the mechanical properties shown in Table 1 above was used, and the following was performed: Figure 5 The stamped article 51 shown is a stamped article based on bending forming, simulating the shape of a car floor beam. The stamped article 51 is provided with a mounting flange portion 59 extending outward from the longitudinal end edges of the top plate portion 53, the longitudinal wall portion 55, and the flange portion 57. As a bending portion, it has: a punch shoulder 61 connecting the top plate portion 53 and the longitudinal wall portion 55; and a die shoulder 63 connecting the lower edge of the longitudinal wall portion 55 and the side edge of the flange portion 57, and connecting the longitudinal end edges of the top plate portion 53, the longitudinal wall portion 55, and the flange portion 57 and the mounting flange portion 59.

[0112] In the bottom stop shape of the stamped product 51, the radius of curvature and bending angle of the punch shoulder 61 are set to 5mm and 90°, respectively, and the radius of curvature and bending angle of the die shoulder 63 are set to 4mm and 90°, respectively.

[0113] Then, the stamped part 51, which was stamped to the bottom stop point, was demolded from the mold, and the change in the shape of the stamped part 51 over time was measured.

[0114] Next, the following situations are set as Invention Examples 7 and 8: the stamping analysis of the stamped product 51 and the subsequent springback analysis are performed, and the residual stress values ​​of the punch shoulder 61 and / or die shoulder 63 in the stamped product 51 immediately after springback are set to reduce their residual stress, and the shape of the stamped product 51 with torque balance is analyzed.

[0115] It should be noted that in the stamped product 51, the mounting flange 59 is formed continuously along the three end edges of the top plate portion 53, the longitudinal wall portion 55, and the flange portion 57 in the length direction. The punch shoulder 61 connects the side edge of the top plate portion 53 and the upper edge of the longitudinal wall portion 55. The die shoulder 63 has a portion that connects the lower edge of the longitudinal wall portion 55 and the side edge of the flange portion 57. The length direction end mounting portion 65 has a portion that connects the length direction end edge of the top plate portion 53, the longitudinal wall portion 55, and the flange portion 57 and the mounting flange portion 59.

[0116] In addition, as a comparison object, the following case is designated as Comparative Example 5: After performing the stamping analysis and springback analysis of the stamped product 51 in the same manner as in Invention Example 7 and Invention Example 8, the punch shoulder 61 and the die shoulder 63 were not analyzed to obtain the shape of torque balance by reducing the value of residual stress.

[0117] Then, regarding each of Invention Example 7, Invention Example 8, and Comparative Example 5, the upper end (evaluation point d, refer to) of the top plate side mounting flange 59a, which bends upward continuously from the longitudinal end edge of the top plate portion 53, was calculated. Figure 5 The deviation amount from the bottom stop shape at point d is calculated. It should be noted that, regarding the deviation amount, similar to Example 1, the center of the top plate portion 53 of the stamped article 51 is aligned along its length, and the distance within a cross-section in the width direction parallel to the top plate portion 53 is used. Table 5 summarizes the results of the deviation amount at the locations where residual stress was reduced and eased in Examples 7, 8, and Comparative Example 5, along with the reduction rate and evaluation point d.

[0118] [Table 5]

[0119] (Table 5)

[0120]

[0121] In Table 5, the predicted value Dc is the deviation of evaluation point d in Invention Example 7, Invention Example 8 and Comparative Example 5, and the experimental value De is the deviation of evaluation point d (=4.4mm) of the actual stamped product 51 after 2 days. In addition, the difference and error between the predicted value and the experimental value are calculated by the aforementioned equations (1) and (2).

[0122] In Comparative Example 5, the difference between the predicted value and the experimental value was 0.9 mm, and the error of the predicted value was 25.7%. In Invention Example 7, the residual stress of the punch shoulder 61 was reduced by 5%, the difference from the experimental value was 0.3 mm, and the error of the predicted value was 7.3%, which is an improvement compared to Comparative Example 5.

[0123] In Invention Example 8, the residual stress of the punch shoulder 61 and the die shoulder 63 was reduced by 10%, the difference from the experimental value was 0.2 mm, and the error of the predicted value was 4.8%, which is a further improvement compared with Comparative Example 5.

[0124] It should be noted that in Example 7, the shape change was achieved only by reducing the residual stress in the punch shoulder 61. Therefore, the angular change in the punch shoulder 61 caused by stress reduction does not directly affect the shape change of the mounting flange 59a on the top plate side. However, even when the residual stress in the punch shoulder 61 is reduced, the shape becomes a torque-balanced shape for the entire stamped product 51, thus affecting the shape change of the mounting flange 59a on the top plate side, resulting in an improvement in the deviation at evaluation point d.

[0125] Industrial availability

[0126] According to the present invention, a method for predicting the shape change of a stamped article is provided, which can predict the shape change of the stamped article after a period of time following springback that occurs at the moment of demolding the stamped article from the mold.

[0127] Explanation of reference numerals in the attached figures

[0128] 1. Stamped products

[0129] 3. Top plate section

[0130] 5. Longitudinal wall section

[0131] 7. Flange portion

[0132] 9. Slam the head and shoulders

[0133] 11. Shoulder of the stamping die

[0134] 21 Stamped Products

[0135] 23 Top Plate Section

[0136] 25. Longitudinal wall section

[0137] 27. Flange portion

[0138] 29. Head and shoulders

[0139] 31. Shoulder of the stamping die

[0140] 41 Stamped products

[0141] 43 Top Plate Section

[0142] 45 Longitudinal wall section

[0143] 47. Head and shoulders

[0144] 51 Stamped Products

[0145] 53 Top Plate Section

[0146] 55 Longitudinal wall section

[0147] 57 Flange portion

[0148] 59 Install flange part

[0149] 59a Top plate side mounting flange

[0150] 61. Head and shoulders

[0151] 63. Stamping die shoulder

[0152] 65. End mounting part in the length direction

[0153] 71 Stamped Products

[0154] 73 Top Plate Section

[0155] 75. Longitudinal wall section.

Claims

1. A method for predicting the shape change of a stamped article, predicting the shape change of the stamped article over time after springback occurs at the moment of demolding from a mold, and simulating stress relief of the stamped article over a unit of time to predict the dimensional accuracy of the stamped article, wherein... include: The process of obtaining the shape / residual stress immediately after springback involves analyzing the springback of the stamped product to obtain the shape and residual stress of the stamped product immediately after springback. The residual stress mitigation and reduction setting process sets a residual stress value for all or part of the bent portions in the stamped article immediately after springback, such that the residual stress is reduced by a predetermined proportion; and In the shape analysis process, for the stamped product for which the residual stress value at the bending portion has been set to be mitigated and reduced, the shape of torque balance is determined.

2. The method for predicting shape changes of stamped products according to claim 1, The curved portion is a punch shoulder in a stamped product having a top plate portion and a longitudinal wall portion, connecting the top plate portion and the longitudinal wall portion.

3. The method for predicting the shape change of stamped products according to claim 1, The curved portion is a punch shoulder connecting the top plate portion and the longitudinal wall portion and / or a die shoulder connecting the longitudinal wall portion and the flange portion in a stamped product having a top plate portion, a longitudinal wall portion and a flange portion.

4. The method for predicting the shape change of stamped articles according to any one of claims 1 to 3, The blank supplied to the stamping of the stamped article is a metal sheet with a tensile strength of 150 MPa or higher and 2000 MPa or lower.

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