Methods for predicting shape changes in stamped products

Abstract

The method for predicting the shape change of stamped products of the present invention predicts the shape change of a stamped product (1) after springback from a mold over a time unit, comprising: a shape / residual stress acquisition step (S1) after springback, which obtains the shape and residual stress of the stamped product (1) after springback by analyzing the springback of the stamped product (1); a residual stress mitigation and reduction setting step (S3) which sets a residual stress value that is more mitigating and reducing than the residual stress of the stamped product (1) for the stamped product (1) after springback; and a shape analysis step (S5) which calculates the shape of the force torque balance for the stamped product (1) with the residual stress value mitigated and reduced.

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 from a die release. Background Technology

[0002] Stamping is a manufacturing method that can produce metal parts at low cost and in a short time, and it is used in the manufacture of many automotive parts. In recent years, in order to simultaneously improve the collision safety performance of automobiles and reduce the weight of the automotive body, higher-strength metal sheets are being 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 caused by springback. Springback is the phenomenon where the residual stress generated in the stamped product during the deformation of the metal sheet becomes a driving force, causing the stamped product, after being ejected from the die, to instantly return to the shape of the metal sheet before stamping, much like a spring.

[0004] The higher the strength of the metal sheet (e.g., high-tensile steel sheet), the greater the residual stress generated during stamping, and therefore the greater the shape change caused by springback. Consequently, the higher the strength of the metal sheet, the more difficult it is to bring the springback shape back to within the specified dimensions. Therefore, techniques for accurately predicting the shape changes of stamped parts caused by springback are crucial.

[0005] In predicting shape changes caused by springback, stamping simulation based on the finite element method is typically used. This stamping simulation is divided into two steps: first, performing a stamping analysis of the process of stamping a metal sheet to the bottom dead center, predicting the residual stress generated in the stamped part (e.g., Patent Document 1); and second, performing a springback analysis of the shape change of the stamped part after it is removed from the die, predicting the shape to achieve a balance between the moment of force and the residual stress (e.g., Patent Document 2).

[0006] Prior art literature

[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] Summary of the invention

[0012] The problem that the invention aims to solve

[0013] Up to now, by performing the aforementioned stamping simulation that combines the stamping analysis of the first step with the springback analysis of the second step, it is possible to predict the shape of the stamped part that springs back from the mold. However, when the inventors compared the shape of the stamped part predicted by the stamping simulation with the shape of the actual stamped part, they discovered that there were cases where the shape prediction accuracy based on the stamping simulation was reduced.

[0014] Therefore, when studying the reasons for the decrease in shape prediction accuracy based on stamping simulation, it was found that even the same stamped product can have different shapes immediately after stamping (immediately after demolding and springback) and several days later.

[0015] As an example, Figure 5 The measurement was shown Figure 4 The shape change of the stamped product 21, which has a U-shaped cross-sectional shape formed by a top portion 23 and a pair of side wall portions 25, is the result of time-related changes. For example... Figure 5As shown, taking the opening amount of the stamped product 21 immediately after springback when the longitudinal wall portion 25 opens instantly after demolding from the mold as a reference, it can be seen that the shape changes as the opening amount of the longitudinal wall portion 25 of the stamped product 21 gradually increases, and each increase occurs over time units such as 30 minutes or 10 hours.

[0016] The shape change of such stamped articles over time can be considered similar to the gradual deformation of a structural member subjected to a sustained high press load, such as creep (e.g., patent document 3), but in Figure 5 The shape changes that occur in stamped parts that are not subjected to external loads, as shown, are still unknown.

[0017] Furthermore, the second step (springback analysis) in conventional stamping simulations predicts the shape of the stamped part immediately after springback from the die. Therefore, the prediction of shape changes in springbacked stamped parts, for example, after several days, as is the objective of this application, has not been discussed to date. Moreover, the shape change caused by the elapsed time of the springbacked stamped part, as mentioned above, is a shape change not caused by external loads. Therefore, even if attempts were made to predict the shape change caused by the elapsed time of the stamped part, analytical methods for handling shape changes based on creep phenomena are not applicable.

[0018] The present invention was made to solve the aforementioned problems, and its object is to provide a method for predicting the shape change of a stamped article based on a time unit after the instantaneous springback from demolding.

[0019] Solution for solving the problem

[0020] Specifically, the present invention is composed of the following structure.

[0021] The method for predicting the shape change of stamped articles of the present invention predicts the shape change of a stamped article after springback at the instant of demolding from a mold over a subsequent time unit. The method includes: a shape / residual stress acquisition process, which acquires the shape and residual stress of the stamped article immediately after springback through springback analysis; a residual stress reduction setting process, which sets a residual stress value that is more moderately reduced than the residual stress of the stamped article for the acquired stamped article immediately after springback; and a shape analysis process, which calculates the shape of force torque balance for the stamped article with respect to the residual stress value that has been moderately reduced.

[0022] In the above-mentioned shape change prediction method, the residual stress mitigation and reduction setting process reduces the residual stress of the stamped product as a whole by a specified ratio after springback.

[0023] In the above-mentioned shape change prediction method, the residual stress mitigation and reduction setting step calculates the ratio of the residual stress of the stamped product after springback to the tensile strength of the blank supplied for the stamping of the stamped product, and reduces the residual stress only in the parts where the calculated ratio is above a specified value by a specified proportion.

[0024] In the above-described shape change prediction method, the residual stress mitigation and reduction setting step calculates the ratio of the residual stress to the tensile strength of the blank supplied for the stamping of the stamped product after springback, and reduces the residual stress in each part at different ratios for parts where the calculated ratio is above a specified value and parts where it is below the specified value.

[0025] In the above-mentioned shape change prediction method, the blank for stamping the stamped article is a metal sheet with a tensile strength of 150 MPa or higher and 2000 MPa or lower.

[0026] Invention Effects

[0027] The present invention includes: a step for obtaining the shape / residual stress immediately after springback, which obtains the shape and residual stress of the stamped product immediately after springback by analyzing the springback of the stamped product; a residual stress mitigation and reduction setting step, which sets a residual stress value that is more mitigated and reduced than the residual stress of the obtained stamped product immediately after springback; and a shape analysis step, which calculates the shape of the force torque balance for the stamped product with the residual stress value mitigated and reduced, thereby enabling the simulation of stress mitigation over time units of the springback stamped product and the high-precision prediction of the shape change of the stamped product. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating the processing flow of the method for predicting shape changes in stamped products according to an embodiment of the present invention.

[0029] Figure 2 This is a diagram showing a stamped article that is an example of a forming object in the embodiments and examples of the present invention.

[0030] Figure 3 This is a graph showing the shape change of the stamped part after springback, as determined by springback analysis, resulting in reduced residual stress, in the embodiment.

[0031] Figure 4 This is a diagram illustrating an example of a stamped product with a U-shaped cross-sectional shape, representing a measurement object whose shape change is determined based on a unit of time.

[0032] Figure 5 This is a graph showing the result of measuring the opening amount after springback from a die for a stamped product with a U-shaped cross-section, representing an example of shape change over time.

[0033] Figure 6 It is a diagram illustrating the stress relief phenomenon, where stress decreases over time when the strain is kept constant. Detailed Implementation

[0034] In order to solve the above problems, as a preliminary step in establishing a method for predicting the shape change of a stamped article over a time unit after it springs back from the mold, the inventors conducted various discussions on the main causes of the shape change that occurs as a time unit passes over the stamped article.

[0035] In this seminar, the inventors focused on Figure 6As shown, stress easing is a phenomenon where strain is applied to a metal sheet while maintaining a constant strain, and the stress gradually decreases over time. This phenomenon also occurs in stamped products after springback. Figure 6 Similarly, the stress relief phenomenon shown is presumed to be due to the reduction of residual stress in the stamped product over time. Furthermore, it was found that the residual stress in the stamped product is relieved as time decreases, and thus the shape of the stamped product changes to balance the moment of stress relief.

[0036] That is, when the stamped part springs back from the bottom dead center after stamping, residual stress is generated 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 back side in the thickness direction is mitigated and reduced over time. As a result, it is found that the part of the stamped part that has undergone processing further transforms from its shape immediately after springback into a shape without residual stress.

[0037] This phenomenon is distinctly different 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 dead center after stamping, when the value of the residual stress to be generated was forcibly reduced by a specific method, or when the difference between the residual stress on the front and back sides of the stamped part was forcibly reduced, the shape of the bottom dead center of the stamping was suppressed from springback and remained in the state after stamping.

[0038] On the other hand, in the stress relief behavior that this invention aims to address, after springback occurs from the bottom stop point after stamping, the existing residual stress is not relieved by external force, and therefore returns to a state without residual stress. As a result, the stamped product has increased bending angle and warping compared to immediately after springback, becoming a shape further away from the target shape.

[0039] Next, the inventors further explored methods for predicting shape changes in stamped parts caused by stress relief. The result was the following insight: by reducing the residual stress relief of the stamped part immediately after springback, obtained from the second step (springback analysis) of the aforementioned stamping simulation, and then performing a third step to determine the shape balanced with the torque of the forces on the stamped part, it is possible to predict the shape changes in the stamped part caused by stress relief over time.

[0040] The method for predicting the shape change of stamped articles according to embodiments of the present invention is a method for predicting the shape change of a stamped article after a springback from the mold in a given time unit, such as... Figure 1 As shown, the process includes a shape / residual stress acquisition step S1 after springback, a residual stress mitigation and reduction setting step S3, and a shape analysis step S5. It should be noted that in this embodiment, as... Figure 2 As an example, the above-described processes will be explained using a stamped product 1 with a hat-shaped cross section having a top plate portion 3, a longitudinal wall portion 5, and a flange portion 7 as an example.

[0041] <Process for obtaining the shape / residual stress immediately after springback>

[0042] 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 product 1 immediately after springback by analyzing the springback of the stamped product 1.

[0043] As an example of the specific processing of obtaining the shape and residual stress of the stamped product 1 after springback, a stamping simulation based on the finite element method can be listed, including: using a mold model that models the mold used in the actual stamping of the stamped product 1, performing a stamping analysis of the process of stamping the metal sheet to the forming bottom stop, and obtaining the shape and residual stress of the stamped product 1 at the forming bottom stop; and performing a springback analysis, which obtains the shape of the stamped product 1 after it has just been demolded from the mold model at the forming bottom stop, whereby the force and torque of the stamped product 1 at the forming bottom stop are balanced.

[0044] <Standardization and Reduction of Residual Stress Setting Process>

[0045] The residual stress mitigation and reduction setting process S3 is a process for setting a residual stress value that is more than the residual stress mitigation and reduction value for the stamped product 1 obtained in the shape / residual stress acquisition process S1 immediately after springback.

[0046] As a specific method for setting a value that reduces the residual stress of the stamped product 1 after springback, for example, there are the following (1) to (3).

[0047] (1) Reduce the residual stress of the stamped part 1 as a whole by a specified ratio.

[0048] (2) For the stamped product 1, calculate the ratio of its residual stress to the tensile strength of the blank supplied for stamping the stamped product 1, and reduce the residual stress in the part of the stamped product 1 where the calculated ratio is above a specified value by a specified proportion.

[0049] (3) For the stamped product 1, the ratio of its residual stress to the tensile strength of the blank supplied for stamping the product 1 is calculated. The residual stress is reduced at different ratios in areas where the calculated ratio is above a specified value and in areas where it is below the specified value. In this case, the areas in the stamped product 1 where the residual stress is reduced are not limited to two areas. The residual stress can be reduced at different ratios in three or more areas where the residual stress values ​​are different.

[0050] The specified proportions for reducing residual stress in (1) to (3) above can all be set appropriately. For example, in the case of (1) above, it is preferable to reduce the residual stress of the stamped product 1 as a whole by more than 10% after springback.

[0051] Furthermore, in (2) or (3) above, the reason for specifying the location for reducing residual stress based on the ratio of the value of residual stress to the tensile strength of the blank supplied for stamping is as follows.

[0052] exist Figure 2 The punch shoulder 9, die shoulder 11, and other areas where the blank (metal sheet) undergoes significant deformation during the stamping process, as shown in the stamped product 1, tend to exhibit high residual stress not only within the die at the bottom stop of the stamping process but also immediately after springback from the die (after the aforementioned second step). Furthermore, if the residual stress is high, its value tends to change significantly over time, thus increasing its impact on the shape change of the stamped product 1.

[0053] Furthermore, if the material strength of the blank increases, the residual stress generated during the stamping process also increases. Therefore, the material strength of the blank needs to be considered when determining the location where residual stress is reduced. Therefore, when the method described in (2) or (3) above is applied in the residual stress mitigation and reduction setting process S3, the location where residual stress is reduced can be determined using the ratio R shown below.

[0054] R = (Residual stress) ÷ (Tensile strength of billet) × 100

[0055] Furthermore, in cases such as stamping using blanks with similar tensile strength, the residual stress in the stamped product 1 can be reduced by a predetermined proportion only in the parts where the residual stress value is above the predetermined value, or the residual stress can be reduced by different proportions in the parts where the residual stress value is above the predetermined value and in the parts where the residual stress value is below the predetermined value. Additionally, the residual stress can be reduced by different proportions in multiple parts with different residual stress values.

[0056] <Shape Analysis Process>

[0057] The shape analysis process S5 is a process for analyzing the shape of the stamped product 1, whose residual stress has been reduced through the residual stress mitigation and reduction setting process S3, to determine the force torque balance.

[0058] Thus, according to the shape change prediction method of the stamped product of this embodiment, for the stamped product 1 immediately after springback obtained by springback analysis, a value of residual stress that is reduced by a certain amount is set for the residual stress. For the stamped product 1 for which the value of residual stress is reduced by a certain amount, the shape that is balanced with the torque of the force is obtained by analysis. By simulating the stress relief and shape change caused by the passage of time in the actual stamped product 1, the shape change of the stamped product 1 after springback from the mold can be predicted with the passage of time.

[0059] It should be noted that in the residual stress mitigation and reduction setting step S3 of this embodiment, the effect of the aforementioned (1) "reducing the residual stress of the stamped product 1 as a whole by a predetermined ratio after springback" was discussed in the embodiments described later.

[0060] In contrast, in the residual stress mitigation and reduction setting step S3, steps (2) and (3) above determine the location for mitigating and reducing residual stress based on the ratio of the residual stress of the stamped product 1 immediately after springback to the tensile strength of the blank supplied for stamping. This is because, as mentioned earlier, when the residual stress is large, its value tends to change significantly over time, thus increasing the impact on the shape change of the stamped product 1. Furthermore, when the material strength of the blank increases, the residual stress generated during the stamping process also increases.

[0061] Therefore, when the method of (2) or (3) is applied in the residual stress relief and reduction setting step S3, it is expected that the shape change caused by stress relief of the stamped article 1 can be well predicted. It should be noted that in the method of (2) or (3), the specified values ​​for determining the location where residual stress is reduced and the proportion of residual stress reduction are discussed in the embodiments described later.

[0062] However, when the tensile strength of the blank has little effect on the shape change of the stamped part after springback over a unit of time, the residual stress reduction setting process can also determine the part of the stamped part that reduces residual stress immediately after springback based on the absolute value or relative value of the residual stress immediately after springback (e.g., the ratio to the maximum value of the residual stress of the stamped part immediately after springback).

[0063] Furthermore, in this invention, the residual stress mitigation and reduction setting process is not limited to reducing the residual stress of the stamped product 1 immediately after springback by a predetermined ratio. Alternatively, the residual stress value can be reduced by subtracting any certain value (>0) from the absolute value of the residual stress of the entire or part of the stamped product 1 immediately after springback.

[0064] Regarding the specific method of subtracting a certain value from the residual stress of the stamped product 1, for example, it is possible to appropriately select a method that subtracts a certain value from the residual stress of the stamped product 1 as a whole, a method that subtracts a certain value only from the parts where the residual stress is above a specified value, or a method that changes the value of subtracting the residual stress from each part in parts where the residual stress is above a specified value and parts where the residual stress is below a specified value.

[0065] Furthermore, in the method for predicting the shape change of stamped articles of the present invention, there are no particular limitations on the blank (metal sheet) for stamping the stamped articles, the shape of the stamped articles, or the type of stamped articles. However, using blanks with increased residual stress in stamped articles is more effective for stamped automotive parts.

[0066] Specifically, regarding the billet, metal plates with a tensile strength of 150MPa or higher and 2000MPa or lower, and a thickness of 0.5mm or higher and 4.0mm or lower are preferred.

[0067] Blanks (metal sheets) with tensile strength less than 150 MPa are rarely used in stamped products, thus limiting the advantages of the shape change prediction method for stamped products according to the present invention. However, for low-rigidity components such as automotive outer panels that use blanks with tensile strength of 150 MPa or higher, shape changes caused by variations in residual stress are more likely to occur, thus increasing the advantages of applying the present invention, making it a preferred method.

[0068] On the other hand, billets with tensile strengths exceeding 2000 MPa lack elongation, thus exhibiting, for example, in... Figure 2In cases where the punch shoulder 9 and die shoulder 11 of a stamped product 1 with a hat-shaped cross-section as shown break during the stamping process and cannot be stamped, the situation arises where stamping is impossible.

[0069] Furthermore, regarding the shape of the stamped product, it is preferable to apply the present invention to stamped products with shapes having increased residual stress in the bent portion and longitudinal wall portion, such as stamped products with cross-sectional shapes like the shape of a ko or a hat.

[0070] Furthermore, as a type of stamped product, the present invention is preferably applied to automotive parts such as low-rigidity exterior panel components like doors, roofs, and hoods, and frame parts such as A-pillars, B-pillars, roof rails, longitudinal beams, front longitudinal beams, rear longitudinal beams, and cross beams that use high-strength metal sheets.

[0071] It should be noted that the present invention is applicable to stamped articles formed by bending, crash forming or deep drawing, and the stamping method of the stamped articles is arbitrary.

[0072] Example

[0073] The effectiveness of the method for predicting shape changes in stamped articles according to the present invention was verified, and the results are described below.

[0074] Here, the shape change of the stamped article over time was measured, and the shape change of the stamped article was predicted based on the shape change prediction method of the present invention.

[0075] <Determination of shape change of stamped parts caused by the elapsed time unit>

[0076] Using metal plate A with the mechanical properties shown in Table 1 below as the blank, the following process was carried out. Figure 2 The stamping of the cap-shaped cross-section of the stamped article 1 is shown. Then, the stamped article 1, stamped to the bottom stop point, is demolded from the mold, and the shape change of the stamped article 1 over time is measured. It should be noted that the metal plate A has a thickness of 1.6 mm, a yield strength of 880 MPa, a tensile strength of 1210 MPa, and an elongation of 13%.

[0077] [Table 1]

[0078] (Table 1)

[0079] Plate thickness / mm Yield strength / MPa Tensile strength / MPa Strength / % Metal Plate A 1.6 880 1210 13

[0080] Figure 3 The results show the shape measurements of metal sheet (steel sheet) A immediately after stamping (just after springback from the mold) and after 2 days. Figure 3 The values ​​for the stamped part 1 shown are the deviations from the shape of the stamped part 1 at the bottom dead center of the forming process (hereinafter referred to as the "bottom dead center shape"). The values ​​of 10 mm to 50 mm shown in the figure represent the values ​​of the contour lines of the deviation of the stamped part 1 when the deviation at the center of the top plate portion 3 in the length direction is set to 0 mm.

[0081] Using a blank with a fine mesh printed on its surface, the blank is stamped and formed. The intersections (nodes) of each mesh are aligned with the top plate 3 at the center of the length direction of the stamped part 1. The distance from the bottom stop of the forming to the point immediately after springback and the distance from the bottom stop of the forming to two days later are measured to determine the deviation amount.

[0082] like Figure 3 As shown in (a), the stamped part 1 springs back after being demolded from the mold. Furthermore, the part that deviates most from the shape of the bottom stop of the forming process is the edge portion of the stamped part 1 (the front end in the longitudinal direction of the flange portion 7, hereinafter referred to as "evaluation point a"), with a deviation of 50.4 mm. And, as... Figure 3 As shown in (b), the shape of stamped part 1 changed after 2 days compared to immediately after springback. Specifically, the deviation at evaluation point a became 52.8 mm, which is significantly different from the shape immediately after springback. Figure 3 Compared to (a), it changed by 2.4 mm.

[0083] <Example 1: Prediction of Shape Changes in Stamped Parts>

[0084] As Example 1, a stamped product of the present invention was subjected to drawing forming, and the shape change prediction method of the present invention was used to perform the forming process. Figure 2 Prediction of shape change of the stamped part 1 shown.

[0085] First, using a mold model that was modeled using the mold used in the stamping of stamped product 1, the stamping process of stamping metal plate A to the lower limit of forming was analyzed, and the residual stress of stamped product 1 at the lower limit of forming was obtained.

[0086] Next, springback analysis was performed to determine the shape and residual stress of the stamped part 1 after it was just demolded from the mold at the bottom stop of the forming process.

[0087] Furthermore, for the stamped product 1 immediately after springback, a value for residual stress is set that reduces the overall residual stress of the stamped product 1 immediately after springback, as determined by springback analysis, by a specified proportion. Moreover, for the stamped product 1 that reduces residual stress, an analysis of the shape to determine the force torque balance is performed.

[0088] In Example 1, an example in which the shape change was determined by setting a value of residual stress that reduces the residual stress of the stamped article 1 obtained by springback analysis is designated as an invention example. The prescribed proportions for reducing residual stress are set to 10% (Invention Example 1), 20% (Invention Example 2), and 40% (Invention Example 3).

[0089] In addition, as a comparison, the stamping forming analysis and springback analysis were performed in the same way as the invention example. The example that was still after springback was designated as Comparative Example 1, and the example that was analyzed to determine the shape without reducing the residual stress after springback analysis was designated as Comparative Example 2.

[0090] Table 2 also shows the proportion of residual stress reduction (reduction rate) and the amount of deviation of the stamped part 1 from the bottom stop of forming at evaluation point a as a result of the shape change of the stamped part 1 that reduces residual stress.

[0091] [Table 2]

[0092] (Table 2)

[0093]

[0094] Following Table 2, the predicted value Dc is the deviation of evaluation point a in Invention Examples 1 to 3 and Comparative Examples 1 to 2 when the top plate portion 3 at the center of the stamped product 1 along its length is aligned. The experimental value De is the deviation of evaluation point a (=52.8mm) after 2 days for the stamped product 1 that was actually stamped. Furthermore, the difference between the predicted value and the experimental value, and the error rate, are calculated using the following formulas.

[0095] Difference between predicted values ​​(mm) = De - Dc

[0096] Predicted error (%) = (De - Dc) ÷ Dc × 100

[0097] The deviations of Comparative Example 1 and Comparative Example 2 were equal, with a difference of 3.3 mm from the experimental value and an error of 6.3% in the predicted value.

[0098] In Invention Example 1, reducing the residual stress by 10% resulted in a difference in predicted values ​​of 1.5 mm and an error in predicted values ​​of 2.8%, which is an improvement compared to Comparative Examples 1 and 2. In Invention Example 2, reducing the residual stress by 20% resulted in a difference in predicted values ​​of 0.6 mm and an error in predicted values ​​of 1.1%, which is an improvement compared to Comparative Examples 1 and 2, and is a better result than Invention Example 1. In Invention Example 3, reducing the residual stress by 40% resulted in a difference in predicted values ​​of -0.6 mm and an error in predicted values ​​of -1.1%, both negative values. However, when comparing absolute values, this is an improvement compared to Comparative Examples 1 and 2, and is a better result than Invention Example 1.

[0099] <Example 2: Prediction of Shape Changes in Stamped Parts>

[0100] As an example 2, for the stamped product 1 (obtained through springback analysis after springback) Figure 2 The residual stress value was set to reduce the residual stress in a part of the stamped part 1, and the shape of the force torque balance was analyzed.

[0101] The shape and residual stress of the stamped product 1 immediately after springback are the same as in Example 1 described above. The analysis of the stamping process, from stamping the metal sheet A (Table 1) to the bottom stop using a die, and the analysis of the springback of the stamped product 1 immediately after demolding at the bottom stop, are performed. Next, by reducing the residual stress in a portion of the stamped product 1, an analysis of the shape to determine the force-moment balance of the stamped product 1 is conducted.

[0102] In Example 2, based on the ratio R of the residual stress of the stamped product 1 obtained through springback analysis to the tensile strength of the metal plate A, it is determined that the residual stress of the stamped product 1 should be reduced at the determined location by a specified ratio.

[0103] Table 3 also shows the specified values ​​(judgment criteria in Table 3) for determining the respective locations where residual stress is reduced in Examples 4 to 11 of the Invention, the proportion of residual stress reduction (reduction rate in Table 3), and the deviation of evaluation point a as a result of the shape change of the stamped article 1 with reduced residual stress.

[0104] [Table 3]

[0105] (Table 3)

[0106]

[0107] In Invention Examples 4 to 8, the prescribed values ​​for determining the location where residual stress is reduced, i.e., the criterion, are set to 5% (Invention Example 4), 10% (Invention Example 5), 20% (Invention Example 6), 30% (Invention Example 7), and 40% (Invention Example 8). Only the residual stress of the portion where the ratio R of the residual stress of the stamped product 1 to the tensile strength of the metal plate A is above each criterion is reduced by a moderate reduction rate of 20%.

[0108] According to Table 3, the predicted deviation of evaluation point a in Invention Example 4 is equivalent to that in Invention Example 2 (refer to Table 2 above), which reduces the residual stress of the stamped product 1 by 20%. Furthermore, when comparing the predicted deviation values ​​Dc of evaluation point a in Invention Examples 4 to 8, the larger the value of the criterion for determining the location of residual stress reduction—the ratio R (%) of residual stress to tensile strength—the larger the deviation from the experimental value De, and the lower the prediction accuracy. As a result, the value of the criterion for determining the location of residual stress reduction is preferably smaller within the range of this embodiment.

[0109] In Table 3, Examples 9 to 11 of the Invention show that the ratio R of the residual stress of the stamped product 1 to the tensile strength of the metal plate A is set to different residual stress mitigation and reduction rates (not 0) for parts above and below the judgment standard, thereby reducing the residual stress.

[0110] In Invention Examples 9 and 10, the judgment criteria are set to 5% (Invention Example 9) and 10% (Invention Example 10), respectively. The rate of reduction in residual stress in the portion above the judgment criteria is set to 20%, and the rate of reduction in residual stress in the portion below the judgment criteria is set to 10%. In Invention Examples 4 and 5, which only reduce the residual stress in the portion above the judgment criteria, this is equivalent to reducing the residual stress in the portion below the judgment criteria by setting the rate of reduction in residual stress in that portion to 10%. As shown in Table 3, the difference and error of the predicted value of the deviation amount of evaluation point a in Invention Example 9 (compared to Invention Example 4) and Invention Example 10 (compared to Invention Example 5) are both good results.

[0111] Example 11 is an example of Example 10 where the rate of reduction of residual stress in the portion above the judgment benchmark is set to 30%. As shown in Table 3, the difference and error of the predicted value of the deviation at evaluation point a in Example 11 are better results than those in Example 10.

[0112] The above-mentioned method for predicting the shape change of stamped products according to the present invention verifies that it can well predict the shape change of stamped products that spring back from the mold after demolding due to stress relief over a unit of time.

[0113] Industrial availability

[0114] According to the present invention, a method for predicting the shape change of a stamped article is provided, which predicts the shape change of the stamped article caused by the passage of a unit of time after the instant of springback from the mold.

[0115] Label Explanation

[0116] 1. Stamped parts

[0117] 3. Top plate section

[0118] 5. Longitudinal wall section

[0119] 7. Flange portion

[0120] 9. Slam the head and shoulders

[0121] 11. Shoulder of the stamping die

[0122] 21 Stamped Formed Products

[0123] 23 Top Plate Section

[0124] 25. Longitudinal wall section.

Claims

1. A method of predicting a shape change of a press-formed product, which predicts a shape change of a press-formed product over a time unit after an instant of demolding from a mold, in which, including: a shape / residual stress after springback acquisition step of acquiring a shape and a residual stress of the press-formed product after springback by analyzing springback of the press-formed product; a residual stress relaxation reduction setting step of setting, for the press-formed product after springback whose shape and residual stress are acquired, a value of the residual stress which is relaxed and reduced by a prescribed ratio from the residual stress of the press-formed product; and a shape analysis step of finding a shape of a force moment balance of a force in a state where no load is received from the outside with respect to the press-formed product whose value of the residual stress is set to be relaxed and reduced in the residual stress relaxation reduction setting step.

2. The press-formed product shape change prediction method according to claim 1, wherein the residual stress relaxation reduction setting step reduces the residual stress of the press-formed product after springback by a prescribed ratio for the entire press-formed product.

3. The press-formed product shape change prediction method according to claim 1, wherein the residual stress relaxation reduction setting step calculates, for the press-formed product after springback, a ratio of the residual stress to a tensile strength of a blank for press-forming of the press-formed product, and reduces the residual stress by a prescribed ratio only for a portion where the calculated ratio is a prescribed value or more.

4. The press-formed product shape change prediction method according to claim 1, wherein the residual stress relaxation reduction setting step calculates, for the press-formed product after springback, a ratio of the residual stress to a tensile strength of a blank for press-forming of the press-formed product, and reduces the residual stress by different ratios for a portion where the calculated ratio is a prescribed value or more and for a portion where the calculated ratio is less than the prescribed value.

5. The press-formed product shape change prediction method according to any one of claims 1 to 4, wherein the blank for press-forming of the press-formed product is a metal sheet having a tensile strength of 150 MPa or more and 2000 MPa or less.

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