Method for evaluating the torsion of a press-formed product, apparatus for evaluating the torsion of a press-formed product, program for evaluating the torsion of a press-formed product, and method for manufacturing a press-formed product.

Abstract

This project aims to evaluate the torsion caused by springback in press-formed metal sheet products and to manufacture press-formed products with suppressed torsion. [Solution] The method for evaluating the torsion of a press-formed product according to the present invention evaluates the torsion of a press-formed product 10 due to springback, and involves setting multiple vertical cross-sections at intervals of 1 mm or more along the longitudinal direction of the press-formed product 10 before springback (S1), designating one of the set cross-sections as the evaluation cross-section and the other cross-sections as the action cross-sections, obtaining the stress and stress application position of each action cross-section, calculating the moment in the twisting direction acting from each action cross-section on the evaluation cross-section (S3), summing the moments calculated for each action cross-section to calculate the torsional moment evaluation index at the evaluation cross-section (S5), and determining the torsional moment evaluation index using all or part of the multiple cross-sections as the evaluation cross-section to evaluate the torsion of the press-formed product due to springback (S7).

Description

Technical Field

[0001] The present invention relates to a method for evaluating the twist of a press-formed product, an apparatus for evaluating the twist of a press-formed product, and a program for evaluating the twist of a press-formed product, which evaluate the twist generated in the press-formed product due to springback of a press-formed product made of a thin metal sheet. Furthermore, the present invention relates to a method for manufacturing a press-formed product that suppresses twist due to springback of a press-formed product made of a thin metal sheet.

Background Art

[0002] In recent years, the deterioration of global environmental problems and energy problems have become apparent. In the automotive industry, weight reduction of vehicle bodies, which contributes to improved fuel efficiency of automobiles, is strongly desired. In addition, with the rise of electric vehicles, the number of new manufacturers has increased, and cost reduction is required more than ever to enhance competitiveness. Under these circumstances, many automotive parts manufactured by press forming are progressing in reducing the thickness and increasing the strength of the thin metal sheets used as their materials, and component integration aimed at reducing press dies is also progressing. Here, component integration means press-forming a plurality of automotive parts that were press-formed using different press dies and joined by spot welding or the like into one automotive part using one press die.

[0003] When a press-formed product made of a thin metal sheet is removed from the press die, a shape defect called springback occurs, and the dimensional accuracy decreases. Therefore, various techniques for improving the dimensional accuracy of press-formed products have been proposed so far. For example, Patent Document 1 discloses a technique for reducing the springback (camber back) in a side view of a press-formed product having a hat-shaped cross section in which a top plate portion and a flange portion are convex or concave along the longitudinal direction on the top plate portion side. Patent Document 2 discloses a press forming with excellent dimensional accuracy without causing three-dimensional shape defects such as angular changes, ridge line warping (surface warping ) or twist due to springback after press forming. The technology for obtaining the product has been disclosed. Patent Document 3 discloses a technique for suppressing springback (wall opening) after press forming of press-formed parts having a U-shaped or V-shaped cross-section, without using complex processes. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 6176430 [Patent Document 2] Patent No. 5380890 [Patent Document 3] Patent No. 4992048 [Overview of the project] [Problems that the invention aims to solve]

[0005] The prior art described in Patent Documents 1 to 3 involved measures to suppress springback in press-formed products, such as changing the shape of the stress-generating parts that cause springback. However, press-formed products experience a combination of multiple forms of springback, including wall opening, camber back, warping, and twisting. Therefore, while measures taken at the stress-generating parts that cause springback affect all forms of springback, it was unclear how they would affect individual forms of springback. Among the various forms of springback, torsional springback is particularly problematic because the stress generating site is often far from the site where the torsion manifests, making it difficult to understand the extent of stress's influence on torsion. Furthermore, if countermeasures are taken without clearly identifying the site where the stress causing torsion is occurring, it may be difficult to effectively suppress torsional springback. For example, wall opening may improve, but torsion may worsen.

[0006] Furthermore, press-formed parts that were previously made from multiple components tended to have increased torsional springback at the ends of the parts due to their longer length. Therefore, in order to effectively suppress torsional springback, it was important to appropriately evaluate the torsion by considering the influence of the stress generation site that causes torsion.

[0007] Furthermore, in the manufacturing of press-formed products created by integrating parts, it was desirable to implement measures to suppress torsional springback and improve dimensional accuracy.

[0008] The present invention was made to solve the above-mentioned problems, and aims to provide a method for evaluating the torsion of a press-formed product, a device for evaluating the torsion of a press-formed product, and a program for evaluating the torsion of a press-formed product, taking into account the influence of the stress generation site that causes torsion due to springback of the press-formed product. Furthermore, the present invention aims to apply a method for manufacturing press-formed articles that suppresses twisting due to springback. [Means for solving the problem]

[0009] (1) The method for evaluating the torsion of a press-formed product according to the present invention evaluates the torsion of a press-formed product of a thin metal sheet due to springback, A cross-section setting step of setting multiple cross-sections perpendicular to the longitudinal direction at intervals of 1 mm or more along the longitudinal direction of the press-formed product before springback, A moment calculation step is performed by selecting one of the multiple cross-sections as the evaluation cross-section and the other cross-sections as the action cross-sections, obtaining the stress generated in the action cross-section and the stress action location in the action cross-section for each action cross-section, transforming the obtained stress and stress action location into the coordinate system of the evaluation cross-section, and then multiplying the stress in the action cross-section by the distance between the centroid of the evaluation cross-section and the stress action location to calculate the moment in the twisting direction of the evaluation cross-section. A torsional moment evaluation index calculation step involves calculating a torsional moment evaluation index at the evaluation cross-section by summing the moments calculated for each of the aforementioned cross-sections of action, The method is characterized by including a torsional evaluation step in which all or part of the multiple cross-sections are designated as evaluation cross-sections, and for each evaluation cross-section, the torsional moment evaluation index is determined by the moment calculation step and the torsional moment evaluation index calculation step, and the torsional evaluation step is evaluated for the torsion of the press-formed product due to springback.

[0010] (2) In the items described in (1) above, The torsion evaluation step is characterized by selecting a reference cross section from a plurality of cross sections, and evaluating the torsion of the press-formed product based on the difference between the torsion moment evaluation index obtained for each of the evaluation cross sections and the torsion moment evaluation index of the selected reference cross section.

[0011] (3) In the case of the items described in (1) or (2) above, The twist evaluation step is characterized in that the twist of the press-formed product is evaluated by dividing the torsional moment evaluation index by the polar moment of inertia of the cross section of each evaluation cross section.

[0012] (4) The torsion evaluation device for press-formed products according to the present invention evaluates the torsion of a press-formed product of a thin metal sheet due to springback, A cross-section setting section that sets multiple cross-sections perpendicular to the longitudinal direction at intervals of 1 mm or more along the longitudinal direction of the press-formed product before springback, A moment calculation unit calculates a moment in the twisting direction of the evaluation cross-section by multiplying the stress in the evaluation cross-section by the distance between the centroid of the evaluation cross-section and the stress application position in the evaluation cross-section, after converting the acquired stress and stress application position to the coordinate system of the evaluation cross-section, and then multiplying the stress in the evaluation cross-section by the distance between the centroid of the evaluation cross-section and the stress application position. A torsional moment evaluation index calculation unit calculates a torsional moment evaluation index at the evaluation cross-section by summing the moments calculated for each of the aforementioned cross-sections of action, The invention is characterized by comprising: a torsion evaluation unit that uses all or part of the multiple cross-sections as evaluation cross-sections, and for each evaluation cross-section, uses the moment calculation unit and the torsion moment evaluation index calculation unit to determine the torsion moment evaluation index and evaluates the torsion of the press-formed product due to springback.

[0013] (5) The torsion evaluation program for press-formed products according to the present invention evaluates the torsion of a press-formed product of a thin metal sheet due to springback, Computers, A cross-section setting section that sets multiple cross-sections perpendicular to the longitudinal direction at intervals of 1 mm or more along the longitudinal direction of the press-formed product before springback, A moment calculation unit calculates a moment in the twisting direction of the evaluation cross-section by multiplying the stress in the evaluation cross-section by the distance between the centroid of the evaluation cross-section and the stress application position in the evaluation cross-section, after converting the acquired stress and stress application position to the coordinate system of the evaluation cross-section, and then multiplying the stress in the evaluation cross-section by the distance between the centroid of the evaluation cross-section and the stress application position. A torsional moment evaluation index calculation unit calculates a torsional moment evaluation index at the evaluation cross-section by summing the moments calculated for each of the aforementioned cross-sections of action, The invention is characterized by having a function that allows all or part of the multiple cross-sections to be used as evaluation cross-sections, and for each evaluation cross-section, the torsional moment evaluation index is determined by the moment calculation unit and the torsional moment evaluation index calculation unit, and the torsional evaluation unit is executed to evaluate the torsion of the press-formed product due to springback.

[0014] (6) The manufacturing method of the press-formed product according to the present invention manufactures a press-formed product of a metal thin plate while suppressing the twist due to springback, evaluates the twist due to springback of the press-formed product by the twist evaluation method of the press-formed product described in the above (1) or (2), and determines the presence or absence of twist in the press-formed product, a press-formed product twist determination process; when it is determined that there is twist in the press-formed product twist determination process, until it is determined that there is no twist, a press-forming condition adjustment process that adjusts the press-forming conditions of the press-formed product and repeats the press-formed product twist determination process; and a press-formed product manufacturing process that manufactures the press-formed product under the press-forming conditions adjusted in the press-forming condition adjustment process so as to determine that there is no twist in the press-formed product twist determination process.

Effects of the Invention

[0015] According to the present invention, by calculating the moment in the direction of twisting the evaluation cross-section using the stress of the working cross-section other than the evaluation cross-section that evaluates the twist due to springback of the press-formed product, the twist can be evaluated in consideration of the influence of the stress at the site that causes the twist. Further, according to the present invention, the site that causes the twist can be specified, and it becomes possible to efficiently take measures to suppress the twist.

[0016] Furthermore, according to the present invention, it is possible to manufacture a press-formed product with the twist due to springback suppressed, and the period of determining the press-forming conditions of the press-formed product by trial and error can be significantly shortened.

Brief Description of the Drawings

[0017] [Figure 1] It is a flowchart showing the flow of processing in the twist evaluation method of the press-formed product according to Embodiment 1 of the present invention. [Figure 2]This figure shows the press-formed product that was subjected to torsion evaluation in Embodiments 1 and 2 and the Examples of the present invention. [Figure 3] This diagram illustrates the coordinate system of the cross-section set for a press-formed product in Embodiment 1 of the present invention. [Figure 4] This diagram shows the configuration of a torsion evaluation device for press-formed products according to Embodiment 1 of the present invention. [Figure 5] This is a flowchart showing the process flow in the manufacturing method of a press-formed product according to Embodiment 2 of the present invention. [Figure 6] This figure shows the press-molded product that was molded in the example. [Figure 7] This figure shows an intermediate molded product of a press-formed product that is press-formed in two steps in the example. [Figure 8] This graph shows the torsional moment evaluation index of the evaluation cross-section set for the press-formed product in the example. [Figure 9] This figure shows the cross-sectional shape of the longitudinal end of a press-formed product before and after springback, without any countermeasures against twisting, in the example. [Figure 10] This figure shows the cross-sectional shape of the longitudinal end of a press-formed product with torsional stiffness countermeasures in place, before and after springback, in the embodiment. [Modes for carrying out the invention]

[0018] [Embodiment 1] <Method for evaluating the torsion of press-formed products> The torsion evaluation method for press-formed products according to Embodiment 1 of the present invention (hereinafter also simply referred to as the "torsion evaluation method") evaluates the torsion due to springback of a press-formed product 10 of a thin metal sheet, as shown in Figure 2, as an example. The torsion evaluation method according to Embodiment 1 includes, as shown in Figure 1, a cross-section setting step S1, a moment calculation step S3, a torsional moment evaluation index calculation step S5, and a torsion evaluation step S7.

[0019] The press-formed product 10 has a hat-shaped cross section comprising a top plate portion 11, a pair of vertical wall portions 13 that are continuous from both ends of the top plate portion 11 in the width direction, and flange portions 15 that are continuous from the lower ends of each vertical wall portion 13. Furthermore, the press-formed product 10 includes a curved portion 10A that curves when viewed from above, and straight portions 10B that extend linearly from both ends of the curved portion in the longitudinal direction. The following describes each of the above steps when evaluating the torsion of the press-formed product 10 due to springback.

[0020] ≪Cross section setting process≫ The cross-section setting process S1 involves setting multiple vertical cross-sections at intervals of 1 mm or more along the longitudinal direction of the press-formed product 10 before springback. The cross-sections should be set at equal intervals of 20 mm or less. Here, "along the longitudinal direction" means, for example, along the curved shape in the case of a shape that is curved along the longitudinal direction, such as the press-formed product 10.

[0021] ≪Moment Calculation Process≫ In the moment calculation step S3, first, one of the multiple cross-sections set on the press-formed product 10 in the cross-section setting step S1 is designated as the evaluation cross-section, and the other cross-sections are designated as the action cross-sections. Next, the stress at each action cross-section and the stress application location at each action cross-section are obtained. Then, the obtained stress and stress application location are transformed into the coordinate system of the evaluation cross-section. The coordinate transformation of stress and stress application location will be described later.

[0022] Furthermore, for each working cross-section, after performing a coordinate transformation on the acquired stress and stress application location, the moment in the twisting direction of the working cross-section is calculated by multiplying the stress in the working cross-section by the distance between the centroid of the evaluation cross-section and the stress application location of the working cross-section. Here, the direction of twisting the evaluation cross-section is defined as the direction of rotation around the longitudinal axis centered on the centroid of the evaluation cross-section.

[0023] In the moment calculation process S3, the stress on the acting cross-section and the stress application location can be obtained based on the stress distribution of the press-formed product 10 at the bottom dead center, which is determined by FEM analysis (finite element method analysis) during the process of press-forming the thin metal sheet to the bottom dead center.

[0024] To obtain the stress on the working cross-section and the stress application location from the stress distribution of the press-formed product 10 obtained by FEM analysis, for example, the nodes that are on or near the working cross-section among the nodes used in the FEM analysis can be used as the stress application locations, and the stress at the nodes can be used as the stress at the stress application locations. The moment in the twisting direction of the evaluation cross-section due to the action cross-section is calculated by multiplying the stress at each stress application point of the action cross-section by the distance between the centroid of the evaluation cross-section and the stress at that point, and then summing these values.

[0025] In the moment calculation process S3, in order to determine the moment that rotates the evaluation cross section due to the stress at each stress application location on the action cross section, the distance between the centroid of the evaluation cross section and the stress application location on the action cross section is used as the arm length of the moment. Note that actual automobile parts, such as the press-formed product 10 shown in Figure 2, generally have a curved shape rather than a uniform straight line in the longitudinal direction. Therefore, the orientation of the coordinate system differs depending on the longitudinal position of the evaluation cross section and the action cross section set on the press-formed product 10.

[0026] In a press-formed product 10 with a curved shape along its longitudinal direction, to calculate the moment acting from the working cross-section to the evaluation cross-section, it is necessary to use the stress obtained by coordinate transformation of the stress at the working cross-section to match the coordinate system of the evaluation cross-section. Furthermore, since the moment is the product of force and arm length, the distance between the centroid of the evaluation cross-section and the stress application position at the working cross-section requires coordinate transformation of the stress application position at the working cross-section to match the coordinate system of the evaluation cross-section.

[0027] The coordinate transformation of the stress application location is expressed as shown in equation (1) below.

number

[0028] Similarly, the coordinate transformation of stress can be expressed as shown in equation (2) below.

number

[0029] Thus, in the moment calculation process S3, the stress application locations on the working cross-section are transformed based on equation (1), and the stress on the working cross-section is transformed based on equation (2). Then, using the stress and stress application locations transformed to the coordinate system of the evaluation cross-section, the moment in the twisting direction of the evaluation cross-section is calculated based on the stress at each stress application location on the working cross-section.

[0030] <<Process for calculating the torsional moment evaluation index>> In the torsional moment evaluation index calculation step S5, the moments calculated for each acting cross-section are added together to calculate the torsional moment evaluation index at the evaluation cross-section.

[0031] <<Twist evaluation process>> In the torsion evaluation step S7, all or part of the multiple cross-sections are used as evaluation cross-sections, and the torsion moment evaluation index is determined by the moment calculation step S3 and the torsion moment evaluation index calculation step S5, and the torsion of the press-formed product 10 due to springback is evaluated.

[0032] In the torsion evaluation process S7, the torsion of the press-formed product 10 is evaluated as follows. First, each of the multiple cross-sections set on the press-formed product 10 is used as an evaluation cross-section, and the moment calculation process S3 and the torsional moment evaluation index calculation process S5 are performed to determine the torsional moment evaluation index. Then, the torsional moment evaluation indices obtained for all the cross-sections designated for evaluation are compared, and the cross-sections with larger torsional moment evaluation indices are evaluated as having greater torsion. Furthermore, for cross-sections with a large torsional moment evaluation index, the moments of each acting cross-section used to calculate the torsional moment evaluation index for that cross-section are compared, and the acting cross-section with the largest moment is evaluated as the part that has a significant influence on the torsion of that cross-section.

[0033] In the torsion evaluation method for press-formed products according to this embodiment 1, when evaluating torsion in the longitudinal direction, the moment in the direction of torsion of the evaluation cross-section is calculated using the stress of the action cross-section other than the evaluation cross-section being evaluated. This makes it possible to evaluate torsion while considering the influence of stress in the parts that cause torsion. In particular, the torsion evaluation method for press-formed products according to this embodiment 1 can appropriately evaluate torsion in press-formed products where the length of the part has been increased by integrating multiple parts, by considering the influence of the stress generation site that causes torsion due to springback at the end of the part.

[0034] Furthermore, according to the torsion evaluation method for press-formed products of this embodiment 1, a cross-section with a large moment relative to a cross-section with a large torsional moment evaluation index can be evaluated as a part that has a large influence on the torsion of that cross-section. This makes it possible to identify the part that is causing the torsion and to efficiently implement measures to suppress the torsion.

[0035] The above explanation involved directly comparing the values ​​of the torsional moment evaluation index calculated for each of the multiple cross-sections in the torsional evaluation process S7. However, in the present invention, it is preferable to select a reference cross-section from among the cross-sections for which the torsional moment evaluation index has been calculated, and to evaluate the torsion occurring in the press-formed product 10 by calculating the difference between the torsional moment evaluation index obtained for each cross-section and the torsional moment evaluation index of the reference cross-section. This makes it possible to evaluate the effect of torsion on a reference point when evaluating the torsion that occurs in the actual press-formed part 10.

[0036] There are no particular restrictions on the reference cross-section to be selected, but for example, it is advisable to select a reference part (such as the central part or end in the longitudinal direction, or the part where the press-formed parts 10 before and after springback are superimposed) as the reference cross-section for evaluating the twist angle that occurs in the press-formed product 10.

[0037] Furthermore, in the torsional evaluation process S7, the torsional moment evaluation index for each cross section should be the value obtained by dividing it by the polar moment of inertia of the cross section. Polar moment of inertia I p This can be calculated using the following formula (3).

number

[0038] The polar moment of inertia is an index that represents the torsional stiffness caused by the shape of the cross-section. Therefore, by dividing the torsional moment evaluation index by the polar moment of inertia, it is possible to evaluate the effect of springback on the twisting of the press-formed product 10, taking into account the difference in torsional stiffness caused by the cross-sectional shape of the press-formed product 10.

[0039] For example, even if the values ​​of the torsional moment evaluation index are the same, a cross-section with low torsional stiffness will have a relatively larger torsional moment evaluation index when divided by the polar moment of inertia of the area. Therefore, such a cross-section can be evaluated as a part that is greatly affected by torsion.

[0040] <Twist evaluation device for press-formed products> The torsion evaluation device for press-formed products according to this embodiment 1 (hereinafter simply referred to as the "torsion evaluation device") evaluates the torsion of a press-formed product made of thin metal sheet due to springback. As shown in Figure 4, the torsion evaluation device 1 comprises a cross-section setting unit 3, a moment calculation unit 5, a torsional moment evaluation index calculation unit 7, and a torsion evaluation unit 9. The torsion evaluation device 1 may be composed of a CPU (Central Processing Unit) of a computer (PC, etc.). In this case, each of the above parts functions when the computer's CPU executes a predetermined program.

[0041] ≪Cross section setting section≫ The cross-section setting unit 3 sets multiple cross-sections perpendicular to the longitudinal direction at intervals of 1 mm or more along the longitudinal direction of the press-formed product 10 before springback. The aforementioned cross-sections should be set at equal intervals of 20 mm or less.

[0042] ≪Moment Calculation Section≫ The moment calculation unit 5 first sets one of the multiple cross-sections set on the press-formed product 10 by the cross-section setting unit 3 as the evaluation cross-section, and the other cross-sections as the working cross-sections. Next, the moment calculation unit 5 obtains the stress of each working cross-section and the distance between the stress application position of the working cross-section and the centroid position of the evaluation cross-section. Furthermore, for each working cross-section, the moment calculation unit 5 transforms the obtained stress and stress application position into the coordinate system of the evaluation cross-section, and then calculates the moment in the twisting direction of the evaluation cross-section by multiplying the stress of the working cross-section by the distance between the centroid position of the evaluation cross-section and the stress application position.

[0043] The moment calculation unit 5 may obtain the stress on the action cross-section and the stress action location, similar to the moment calculation step S3 described above. Furthermore, as shown in Figure 2, the moment calculation unit 5 also performs a coordinate transformation on the stress and stress application location of the working cross section in the same way as the moment calculation step S3 described above, even for press-formed products 10 with a curved shape along the longitudinal direction, and calculates the moment in the twisting direction of the evaluation cross section. The coordinate transformation of the stress and stress application location of the working cross section can be performed in the same way as the moment calculation step of the torsion evaluation method for press-formed products described above. In addition,

[0044] <Twist moment evaluation index calculation section> The torsional moment evaluation index calculation unit 7 and the moment calculation unit 5 calculate the moments for each acting cross-section, and the sum of these moments is obtained as the torsional moment evaluation index for the cross-section being evaluated.

[0045] <Twist Evaluation Section> The torsion evaluation unit 9 uses all or part of the multiple cross-sections as evaluation cross-sections, and uses the moment calculation unit 5 and the torsion moment evaluation index calculation unit 7 to determine the torsion moment evaluation index and evaluate the torsion of the press-formed product 10 due to springback.

[0046] The torsion evaluation by the torsion evaluation unit 9 should be performed in the same manner as the torsion evaluation process S7 described above. That is, all of the multiple cross-sections set on the press-formed product 10 are used as evaluation cross-sections, and the torsion moment evaluation index for each evaluation cross-section is determined by the moment calculation unit 5 and the torsion moment evaluation index calculation unit 7.

[0047] Then, the torsional moment evaluation indices obtained for all the cross-sections designated for evaluation are compared, and the cross-sections with larger torsional moment evaluation indices are evaluated as having greater torsion. Furthermore, by comparing the moments acting from each working cross-section to a cross-section with a large torsional moment evaluation index, it is possible to evaluate that the working cross-section with the largest moment is the part that has a significant influence on the torsion of that cross-section.

[0048] <Program for evaluating the torsion of press-formed products> Embodiment 1 of the present invention can be configured as a torsion evaluation program for press-formed products (hereinafter simply referred to as the "torsion evaluation program"). In other words, the torsion evaluation program evaluates the torsion of a metal sheet press-formed product 10 due to springback. The torsion evaluation program has the function of running a computer as, for example, a cross-section setting unit 3, a moment calculation unit 5, a torsional moment evaluation index calculation unit 7, and a torsion evaluation unit 9, as shown in Figure 4.

[0049] As described above, according to the torsion evaluation device and torsion evaluation program for press-formed products of this embodiment 1, when evaluating torsion in the longitudinal direction, the moment in the direction of torsion of the evaluation cross-section is calculated using the stress of the action cross-section other than the evaluation cross-section being evaluated, thereby enabling the evaluation of torsion while considering the influence of stress in the parts that cause torsion. In particular, for press-formed products where multiple parts are integrated to increase the part length, it is possible to appropriately evaluate torsion by considering the influence of the stress generation site that causes torsion due to springback at the end of the part.

[0050] Furthermore, in the torsion evaluation device and torsion evaluation program for press-formed products according to this embodiment 1, the cross-section with a large moment relative to the evaluation cross-section with a large torsional moment evaluation index is evaluated as a part that has a large influence on torsion. This makes it possible to identify the part that causes torsion and to efficiently implement measures to suppress torsion.

[0051] Furthermore, the torsion evaluation unit of the press-formed product torsion evaluation device and press-formed product torsion evaluation program according to the present invention may, similar to the torsion evaluation step of the press-formed product torsion evaluation method described above, calculate the difference between the torsion moment evaluation index of each evaluation cross section and the torsion moment evaluation index of a reference cross section, or calculate the value obtained by dividing it by the polar moment of inertia of the area. This makes it possible to evaluate the torsion of the press-formed product 10 due to springback more appropriately.

[0052] [Embodiment 2] <Method for manufacturing press-formed products> The method for manufacturing a press-formed product according to Embodiment 2 of the present invention manufactures a press-formed product of a thin metal sheet while suppressing twisting due to springback. The method for manufacturing a press-formed product according to Embodiment 2 includes a press-formed product twisting determination process P1, a press-forming condition adjustment process P3, and a press-formed product manufacturing process P5, as shown in Figure 5. The above processes will now be explained using the press-formed product 10 shown in Figure 2 as an example.

[0053] <<Process for determining twist in press-formed products>> The press-formed product twist determination process P1 evaluates the twist of the press-formed product 10 due to springback using the twist evaluation method according to this embodiment 1, and determines whether or not there is twist in the press-formed product 10.

[0054] The presence or absence of twisting in the press-formed product 10 can be determined by comparing the torsional moment evaluation index obtained for each evaluation cross-section using the torsional evaluation method according to this embodiment 1 with a preset threshold. For example, if the torsional moment evaluation index of all evaluation cross-sections set for the press-formed product 10 is smaller than a preset threshold, it is determined that there is no torsion due to springback in the press-formed product 10. "No torsion" means that the torsion is within an acceptable range and there is no problem. In contrast, if the torsional moment evaluation index of any of the evaluation cross-sections is greater than a preset threshold, the press-formed product 10 is determined to have torsion due to springback. The presence of torsion means that the torsion is outside the acceptable range and countermeasures are necessary. The threshold used to determine whether or not twisting occurs should be set appropriately based on, for example, the dimensional accuracy required for the press-formed product 10.

[0055] <Press forming condition adjustment process> The press forming condition adjustment process P3, if the press forming product twist determination process P1 determines that there is twist, adjusts the press forming conditions of the press forming product 10 and repeats the press forming product twist determination process P1 until it determines that there is no twist.

[0056] Specific methods for adjusting press forming conditions To reduce the stress on the vertical wall portion in the cross-section where twisting occurs, it is advisable to modify the shape of the press-formed product 10, etc.

[0057] ≪Press-molded product manufacturing process≫ The press-formed product manufacturing process P5 manufactures the press-formed product 10 using press-formed conditions adjusted in the press-formed condition adjustment process P3 so that the press-formed product twist determination process P1 determines that there is no twist.

[0058] In the method for manufacturing press-formed products according to this second embodiment, the press forming conditions are adjusted to suppress twisting based on the results of determining whether or not twisting due to springback occurs in the press-formed product of a thin metal sheet. This significantly shortens the period required to determine the actual press forming conditions for the press-formed product 10 through trial and error. Furthermore, by using the press forming conditions adjusted in this way, it is possible to manufacture a press-formed product 10 in which twisting due to springback is suppressed. [Examples]

[0059] We have verified the effects and advantages of the torsion evaluation method for press-formed products according to the present invention, and will describe them below.

[0060] In this embodiment, the torsion due to springback was evaluated for the press-formed product 10 shown in Figure 6 using the torsion evaluation method for press-formed products according to Embodiment 1 described above. The press-formed product 10 was made by press-forming a high-strength steel plate with a tensile strength of 1470 MPa and a thickness of 1.4 mm as a blank using draw forming.

[0061] The press-formed product 10 had a longitudinal length of 1000 mm, a radius of curvature R of the curved section 10A of 600 mm, a width of the top plate section 11 of 80 mm (=W), a forming height of 80 mm, and a bending angle of 19.2° for the curved section 10A. Furthermore, the press-formed product 10 had a radius of curvature R of 10 mm and a bending angle of 120° for the bent ridge line 17 connecting the top plate section 11 and the vertical wall section 13, a radius of curvature R of 10 mm and a bending angle of 120° for the die shoulder section 19 connecting the vertical wall section 13 and the flange section 15, and a width of 40 mm for the flange section 15. In addition, the coordinate system was established by defining the longitudinal direction of the press-formed product 10 as the x-direction, the direction perpendicular to the top plate section 11 as the z-direction, and determining the y-direction by the vector product of the x-direction and the z-direction.

[0062] To evaluate the torsion of the press-formed product 10, we first performed a FEM analysis of the press-formed product 10 up to the bottom dead center and obtained the stress distribution of the press-formed product 10 at the bottom dead center.

[0063] Next, 200 cross-sections were set at equal intervals of 5 mm in the longitudinal direction of the press-formed product 10. Of the defined cross-sections, one was designated as the evaluation cross-section, and the other cross-sections were designated as the action cross-sections. The moment acting from each action cross-section in a twisting direction towards the evaluation cross-section was then calculated.

[0064] To calculate the moment, first, the stress and stress application location of each working cross section were obtained based on the stress distribution of the press-formed product 10 at the bottom dead center of forming, which was determined by FEM analysis. Then, for each working cross section, the moment in the twisting direction of the evaluation cross section was calculated by multiplying the stress at each node by the distance between the coordinate of that node and the centroid of the torsion evaluation cross section. Furthermore, in calculating the moment, the stress application location and stress on the action cross-section were transformed using the aforementioned equations (1) and (2).

[0065] Next, the moments calculated for each working cross-section were added together to calculate the torsional moment evaluation index for the evaluation cross-section. The torsional moment evaluation index was calculated using the method described above, with each of the cross-sections set on the press-formed product 10 being used as the evaluation cross-section. Furthermore, the calculated torsional moment evaluation index was divided by the polar moment of inertia of the area obtained by equation (3) described above.

[0066] In this example, the torsional moment evaluation index was calculated for both a press-formed product 10 that was press-formed without any measures to suppress twisting, and a press-formed product 10 that was press-formed with such measures, and the difference in twisting with and without the torsional measures was evaluated.

[0067] The press-formed product 10 without any countermeasures against twisting was press-formed in a single process by draw forming. In contrast, the press-formed product 10 with torsional protection was manufactured by press-forming an intermediate molded product 20 shown in Figure 7 in the first step, and then press-forming the intermediate molded product 20 into the press-formed product 10 shown in Figure 6 in the second step, thereby reducing the stress on the vertical wall portion 13.

[0068] The intermediate molded product 20, press-formed in the first step, has a hat-shaped cross section, as shown in Figure 7, comprising an intermediate top plate portion 21, an intermediate vertical wall portion 23, and an intermediate flange portion 25. Furthermore, the intermediate molded product 20 has an intermediate curved portion 20A corresponding to the curved portion 10A of the target shape, and a gradually changing section 20B1 and an intermediate straight portion 20B2 corresponding to the straight portion 10B of the target shape.

[0069] The curved ridge 27a in the intermediate curved section 20A is located inward in the width direction compared to the curved ridge 17 of the curved section 10A, and the curved ridge 27b2 in the intermediate straight section 20B2 is located outward in the width direction compared to the curved ridge 17 of the straight section 10B, via the curved ridge 27b1 of the gradually changing section 20B1. Here, the amount of inward displacement of the curved ridge 27a in the intermediate curved section 20A is 20 mm, the amount of outward displacement of the curved ridge 27b2 in the intermediate straight section 20B2 is 20 mm, and the length of the gradually changing section 20B1 is 100 mm.

[0070] Figure 8 shows the results of the torsional moment evaluation index obtained for press-formed products 10 with and without torsional suppression measures. In Figure 8, the horizontal axis represents the section number (section No.) set from one end to the other in the longitudinal direction, and the vertical axis represents the torsional evaluation value obtained by dividing the torsional moment evaluation index by the polar moment of inertia of the area of ​​each section.

[0071] As shown in Figure 8, both with and without countermeasures, the torsional evaluation value in the cross-section at the longitudinal ends is larger than that in the central part, indicating that torsion occurs at the longitudinal ends. Furthermore, comparing the product with and without countermeasures, the torsional evaluation value is generally smaller for the product with countermeasures, suggesting that the press-formed product 10 with countermeasures exhibits less torsion due to springback.

[0072] Figure 9 shows the cross-sectional shape of the longitudinal end of the press-formed product 10 without torsional countermeasures before and after springback, and Figure 10 shows the cross-sectional shape of the longitudinal end of the press-formed product 10 with the above-mentioned torsional countermeasures before and after springback. In Figures 9 and 10, the press-formed product 10 before springback (forming bottom dead center) and the press-formed product 10 after springback are arranged to overlap at the center in the longitudinal direction.

[0073] Comparing the cross-sectional shapes of the press-formed product 10 in Figures 9 and 10, the twist is smaller in the case with the countermeasure, which is consistent with the results of the twist evaluation value shown in Figure 8. These results confirm the validity of the torsion due to springback in the press-formed product evaluated according to the present invention. [Explanation of Symbols]

[0074] 1. Torsion evaluation device 3 Cross section setting section 5 Moment Calculation Unit 7. Torsional moment evaluation index calculation unit 9. Torsion evaluation unit 10 Press-formed products 10A Curved section 10B Straight section 11 Top panel 13 Vertical wall section 15 Flange section 17. Curved Ridge 19. Die shoulder section 20 Intermediate molded products 20A Intermediate curved section 20B1 Slow change section 20B2 Intermediate straight section 21 Intermediate top plate section 23 Intermediate vertical wall section 25 Intermediate flange section 27a Bent ridge 27b1 Curved ridge 27b2 Bent ridge

Claims

1. A method for evaluating the torsion of a press-formed product made of thin metal sheet due to springback, A cross-section setting step of setting multiple cross-sections perpendicular to the longitudinal direction at intervals of 1 mm or more along the longitudinal direction of the press-formed product before springback, A moment calculation step is performed by selecting one of the multiple cross-sections as the evaluation cross-section and the other cross-sections as the action cross-sections, obtaining the stress generated in the action cross-section and the stress action location in the action cross-section for each action cross-section, transforming the obtained stress and stress action location into the coordinate system of the evaluation cross-section, and then multiplying the stress in the action cross-section by the distance between the centroid of the evaluation cross-section and the stress action location to calculate the moment in the twisting direction of the evaluation cross-section. A torsional moment evaluation index calculation step involves calculating a torsional moment evaluation index at the evaluation cross-section by summing the moments calculated for each of the aforementioned cross-sections of action, A method for evaluating the torsion of a press-formed product, characterized in that all or part of the plurality of cross-sections are designated as evaluation cross-sections, and for each evaluation cross-section, the torsion moment evaluation index is determined by the moment calculation step and the torsion moment evaluation index calculation step, and the torsion of the press-formed product due to springback is evaluated.

2. The method for evaluating the torsion of a press-formed product according to claim 1, characterized in that, in the torsion evaluation step, a reference cross section is selected from a plurality of cross sections, and the torsion of the press-formed product is evaluated by the difference between the torsion moment evaluation index obtained for each of the evaluation cross sections and the torsion moment evaluation index of the selected reference cross section.

3. The method for evaluating the torsion of a press-formed product according to claim 1 or 2, characterized in that, in the torsion evaluation step, the torsion of the press-formed product is evaluated by the value obtained by dividing the torsion moment evaluation index by the polar moment of inertia of the area of ​​each evaluation cross section.

4. A torsion evaluation device for press-formed products that evaluates the torsion due to springback of press-formed products made of thin metal sheets, A cross-section setting section that sets multiple cross-sections perpendicular to the longitudinal direction at intervals of 1 mm or more along the longitudinal direction of the press-formed product before springback, A moment calculation unit calculates a moment in the twisting direction of the evaluation cross-section by multiplying the stress in the evaluation cross-section by the distance between the centroid of the evaluation cross-section and the stress application position in the evaluation cross-section, after converting the acquired stress and stress application position to the coordinate system of the evaluation cross-section, and then multiplying the stress in the evaluation cross-section by the distance between the centroid of the evaluation cross-section and the stress application position. A torsional moment evaluation index calculation unit calculates a torsional moment evaluation index at the evaluation cross-section by summing the moments calculated for each of the aforementioned cross-sections of action, A torsion evaluation device for a press-formed product, characterized in that all or part of the multiple cross-sections are designated as evaluation cross-sections, and for each evaluation cross-section, the torsion moment evaluation index is determined by the moment calculation unit and the torsion moment evaluation index calculation unit, and the torsion of the press-formed product due to springback is evaluated by the torsion evaluation unit.

5. A torsion evaluation program for press-formed products that evaluates the torsion due to springback of press-formed products made of thin metal sheets, Computers, A cross-section setting section that sets multiple cross-sections perpendicular to the longitudinal direction at intervals of 1 mm or more along the longitudinal direction of the press-formed product before springback, A moment calculation unit calculates a moment in the twisting direction of the evaluation cross-section by multiplying the stress in the evaluation cross-section by the distance between the centroid of the evaluation cross-section and the stress application position in the evaluation cross-section, after converting the acquired stress and stress application position to the coordinate system of the evaluation cross-section, and then multiplying the stress in the evaluation cross-section by the distance between the centroid of the evaluation cross-section and the stress application position. A torsional moment evaluation index calculation unit calculates a torsional moment evaluation index at the evaluation cross-section by summing the moments calculated for each of the aforementioned cross-sections of action, A torsion evaluation program for a press-formed product, characterized in that it has a function to execute a torsion evaluation unit that uses all or part of the multiple cross-sections as evaluation cross-sections, and for each evaluation cross-section, uses the moment calculation unit and the torsion moment evaluation index calculation unit to determine the torsion moment evaluation index and evaluates the torsion of the press-formed product due to springback.

6. A method for manufacturing press-formed products of thin metal sheets, which involves suppressing twisting due to springback, A press-formed product torsion determination process that evaluates the torsion due to springback of a press-formed product using the torsion evaluation method for press-formed products described in claim 1 or 2, and determines whether or not there is torsion in the press-formed product, If twisting is determined to be present in the press-formed product twist determination process, the press-formed product twist determination process is repeated by adjusting the press-formed product's press-formed product's press-formed product's press-formed product's press-formed product's press-formed product twist determination process until twisting is determined to be absent. A method for manufacturing a press-formed product, comprising: a press-formed product manufacturing process, in which the press-formed product is manufactured under press-formed conditions adjusted in the press-formed condition adjustment process so that the press-formed product is determined to be free of twist in the press-formed product twist determination process.

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