Press molding crack determination method, device, and program, and method for manufacturing press molding

Abstract

A press molding crack determination method according to the present invention: acquires a molding limit (P1) of a metal plate, which is represented by the maximum principal strain and the relationship between the minimum principal strain and the degree of bending deformation of a test piece (100) that is subjected to bulging molding at various degrees of bending deformation; on the basis of the acquired molding limit of the metal plate, the maximum principal strain and minimum principal strain calculated for the press-molded article, and the degree of bending deformation, the presence or absence of cracking in the press-molded article is determined (P3).

Description

Technical Field

[0001] The present invention relates to a press forming crack determination method, device and program for determining whether cracks occur in a press formed part formed by press forming a metal sheet, taking into account the influence of bending deformation. The present invention also relates to a method for manufacturing a press formed part, which adjusts press forming conditions based on the determination result of whether cracks occur to suppress the occurrence of cracks and manufactures the press formed part. Background Art

[0002] Most of the metal sheets used as materials for automobile bodies are processed into body parts by press forming. Although the press formability of body parts varies depending on the shape of the body parts, the material properties of metal sheets such as steel sheets, such as ductility, have a great influence. In addition, due to the urgent demand for weight reduction of automotive bodies in recent years, the high strength of metal sheets such as steel sheets used in body parts has been continuously promoted. However, since the ductility decreases with the high strength of metal sheets, cracks are easily generated during press forming, and the formability of press forming decreases.

[0003] Therefore, in order to avoid such failures as cracking during mass production of body parts by press forming, die design based on advance prediction of press formability using CAE (Computer Aided Engineering) becomes important. In addition, in order to accurately predict the occurrence of cracking during press forming, the importance of technology for accurately determining the forming limit of metal plates such as steel plates becomes high.

[0004] In determining the forming limit of a metal sheet, a forming limit diagram (hereinafter referred to as "FLD") is generally used. FLD is a diagram produced by measuring the forming limits of various deformation modes (equibiaxial deformation, non-equibiaxial deformation, plane strain deformation, uniaxial deformation) in press forming through a laboratory-scale forming test. In addition, in the production of the forming limit diagram, the width of the test piece is changed to several levels and the deformation ratio in the major axis direction and the minor axis direction of the test piece is changed, thereby measuring the strains in the major axis direction and the minor axis direction when the fracture of the test piece occurs.

[0005] Generally speaking, in the press forming of metal plates, following the process of uniform deformation of the metal plate, the process shifts to a process in which strain is concentrated at a specific position. In this process, a thickness reduction called necking occurs at the location where the strain of the metal plate is concentrated, and fracture occurs after the thickness reduction progresses. In press forming, necking will cause product defects, so the FLD used in the advance prediction of product defects in press-formed products needs to be defined by the strain amount before necking occurs. Moreover, in high-strength steel sheets with a tensile strength exceeding 980MPa, necking occurs at a low strain of about 10%, and fracture occurs immediately thereafter. Therefore, a method for determining the forming limit with high precision is proposed.

[0006] In ISO12004, the method for determining the forming limit curve (hereinafter referred to as "FLC") of a metal sheet is standardized (Non-Patent Document 1). This method first measures the strain deformation near the fracture occurrence portion of a test piece that has been bulged until fracture, and the maximum value of the approximation curve of the measured strain distribution is determined as the forming limit strain.

[0007] However, in the method specified in ISO12004, the strain of the fracture occurrence part cannot be sufficiently approximated and the forming limit strain cannot be obtained. Therefore, as a method obtained by improving ISO12004, the following method is proposed in Non-Patent Document 2: the strain generated in the test piece during molding is continuously measured, and the FLC is obtained based on the time change of the strain of the fracture occurrence part.

[0008] Prior art literature

[0009] Non-patent literature

[0010] Non-patent document 1: ISO 12004-2:2008, Metallic materials-Sheet and strip-Determination of forming-limit curves, 2008.

[0011] Non-patent document 2: W Hotz, M Merklein, et al., “Time Dependent FLC Determination Comparison of Different Algorithms to Detect the Onset of Unstable Necking before Fracture”, Key Engineering Materials, Vol. 549, pp. 397-404 (2013). Summary of the invention

[0012] Problems to be solved by the invention

[0013] ISO12004 stipulates two methods, the Nakajima test and the Marciniak test, as test methods for determining the forming limit line. Fig.16 As shown in (a), the Nakajima method is a method for obtaining the molding limit by a bulge test in which a test piece 100 is bulged using a molding die 201 having a hemispherical punch 203, an upper die 205, and a blank holder 207. Fig.16As shown in (b), the Marciniak method uses a molding die 211 having a flat punch 213, an upper die 205, and a wrinkle pressing piece 207. In the Marciniak method, a molding limit is obtained by performing a bulging test in which a driver test 215 is sandwiched between the flat punch 213 and the test piece 100 to bulge the test piece 100.

[0014] The Nakajima method performs bulging molding in a state where the test piece 100 conforms to the shape of the front end portion 203a of the ball punch 203. Therefore, the obtained forming limit (maximum principal strain and minimum principal strain at fracture) is affected by the bending deformation of the test piece 100. In contrast, the Marciniak method performs bulging molding using a flat punch 213 whose front end portion 213a is a flat surface. Therefore, the test piece 100 is not bent and deformed, and the obtained forming limit is not affected by the bending deformation of the test piece 100.

[0015] In addition, generally speaking, if the FLD obtained by the Nakajima method is compared with the FLD obtained by the Marciniak method, Fig.16 As shown in (c), the strain amount before necking of the FLD obtained by the Nakajima method is about 1 to 2% larger. In the press forming of high-strength materials with low ductility (such as high-tensile steel plates, etc.), there are sometimes differences in the occurrence of necking (necking) due to a small difference in strain amount. Therefore, previously, the FLD of the Nakajima method and the Marciniak method were compared based on the strain amount measured from the actual press-formed products and the strain amount obtained from the press forming analysis results using CAE, and the forming limit was evaluated.

[0016] However, in the Nakajima method, even if the press-forming conditions are predicted to be free of cracking based on the strain amount obtained by press-forming analysis, cracking may occur in the actual press-formed products of metal plates, especially in the actual press-formed products of high-strength steel plates of 980 MPa or more. In addition, in the Marciniak method, even if the press-forming conditions are predicted to be prone to cracking, cracking may not occur in the actual press-formed products. In this way, there are many cases where the prediction results of whether cracking occurs based on FLD are greatly different from whether cracking occurs in the actual press-formed products, which is problematic.

[0017] On the other hand, according to the method proposed in Non-Patent Document 2, the forming limit can be obtained with higher accuracy than ISO 12004. However, as with ISO 12004, the forming limit cannot be obtained by taking into account the degree of bending deformation. Therefore, the result of predicting the occurrence of cracks based on the forming limit diagram obtained by the method proposed in Non-Patent Document 2 may not be consistent with the occurrence of cracks in the actual press-molded product.

[0018] Thus, although in actual press molding, there are both parts that are subjected to bending deformation and parts that are not subjected to bending deformation, and the degree of bending deformation varies depending on the parts, the degree of bending deformation is not taken into account in the conventional method of obtaining FLD. Therefore, for press-molded products in which there are both parts that are subjected to bending deformation and parts that are not subjected to bending deformation, it is sometimes not possible to appropriately determine whether cracks have occurred using the conventional FLD. Furthermore, a technology is required for determining in advance whether cracks have occurred in a press-molded product having a part that is subjected to bending deformation, and manufacturing the press-molded product while suppressing the occurrence of cracks based on the determination result.

[0019] The present invention is made to solve the above-mentioned problems, and its purpose is to provide a press-forming crack determination method, device and program for determining the forming limit of a press-formed product based on the forming limit of a metal plate obtained by taking into account the influence of bending deformation. In addition, another purpose of the present invention is to provide a press-formed product manufacturing method for adjusting the press-forming conditions of the press-formed product so that it is determined that no cracks occur using the press-forming crack determination method, and manufacturing the press-formed product using the adjusted press-forming conditions.

[0020] Means for solving problems

[0021] The press-formed crack determination method of the present invention determines whether cracks occur in a press-formed product formed by press-forming a metal plate, wherein the method comprises a forming limit acquisition step and a press-formed product crack determination step, wherein the forming limit acquisition step forms the metal plate with various bending deformation degrees, and obtains the forming limit of the metal plate represented by the relationship between the bending deformation degree of the formed metal plate and the maximum principal strain and the minimum principal strain of the metal plate formed with the bending deformation degree, and in the press-formed product crack determination step, the maximum principal strain and the minimum principal strain in the press-formed product, and the curvature in the maximum principal strain direction as the bending deformation degree in the press-formed product are obtained as crack determination parameters in the press-formed product, and based on the obtained crack determination parameters and the forming limit obtained in the forming limit acquisition step, the presence or absence of cracks in the press-formed product is determined.

[0022] In the press-molded product crack determination step, the bending deformation degree in the press-molded product may be calculated using the following formula.

[0023] ρ=(ε 1,outer -ε 1,inner ) / t

[0024] in,

[0025] ρ: Bending deformation

[0026] ε 1,outer : Maximum principal strain on the outside of the bend

[0027] ε 1,inner : Maximum principal strain on the inner side of the bend

[0028] t: Plate thickness

[0029] In the press-molded product crack determination step, the bending deformation degree in the press-molded product may be taken as a maximum principal curvature.

[0030] The metal plate in the forming limit acquisition step may be collected from a metal material used as a sheet material of the press-formed product, the presence or absence of cracking being determined in the press-formed product crack determination step.

[0031] The press-formed crack determination device of the present invention determines whether cracks have occurred in a press-formed product formed by press-forming a metal plate, wherein the device is provided with a forming limit acquisition unit and a press-formed product crack determination unit, wherein the forming limit acquisition unit acquires the forming limit of the metal plate represented by the relationship between the bending deformation degree of the metal plate formed with various bending deformation degrees and the maximum principal strain and the minimum principal strain of the metal plate formed with the bending deformation degree, and the press-formed product crack determination unit calculates the maximum principal strain and the minimum principal strain in the press-formed product and the curvature in the maximum principal strain direction as the bending deformation degree in the press-formed product as crack determination parameters in the press-formed product, and determines whether cracks have occurred in the press-formed product based on the calculated crack determination parameters and the forming limit obtained by the forming limit acquisition unit.

[0032] The press-formed crack determination program of the present invention determines whether cracks have occurred in a press-formed product formed by press-forming a metal plate, wherein a computer is made to function as a forming limit acquisition unit and a press-formed product crack determination unit, the forming limit acquisition unit acquires the forming limit of the metal plate represented by the relationship between the bending deformation degree of the metal plate formed with various bending deformation degrees and the maximum principal strain and the minimum principal strain of the metal plate formed with the bending deformation degree, the press-formed product crack determination unit calculates the maximum principal strain and the minimum principal strain in the press-formed product and the curvature in the direction of the maximum principal strain as the bending deformation degree in the press-formed product as crack determination parameters in the press-formed product, and determines whether cracks have occurred in the press-formed product based on the calculated crack determination parameters and the forming limit obtained by the forming limit acquisition unit.

[0033] The method for manufacturing a pressed product of the present invention utilizes the press-molding crack determination method of the present invention to determine whether cracks occur in a pressed product formed by press-molding a metal plate, and manufactures a pressed product in which cracks are suppressed during press molding based on the result of the determination, wherein the method comprises: a press-molding condition adjustment step, in which, when it is determined that cracks occur in the press-molded product crack determination step of the press-molded crack determination method, the press-molding conditions of the pressed product are adjusted and the press-molded product crack determination step is repeated until it is determined that no cracks occur; and a press-molded product manufacturing step, in which the pressed product is manufactured using the press-molded conditions adjusted in the press-molding condition adjustment step in a manner such that no cracks occur in the press-molded product crack determination step.

[0034] Effects of the Invention

[0035] In the present invention, a metal plate is formed with various bending deformations, and the forming limit of the metal plate is obtained, which is represented by the relationship between the bending deformation of the formed metal plate and the maximum principal strain and the minimum principal strain of the metal plate formed with the bending deformation. In addition, the maximum principal strain and the minimum principal strain in the pressed product formed by press-forming the metal plate, and the curvature in the maximum principal strain direction as the bending deformation in the pressed product are obtained as crack determination parameters in the pressed product. Then, based on the obtained crack determination parameters and the forming limit, it is possible to determine whether cracks occur in the pressed product in consideration of the influence of the bending deformation.

[0036] In addition, in the present invention, for a press-formed product formed by press-forming a metal plate, the presence or absence of cracking is determined by taking into account the influence of the bending deformation, and the press-forming conditions such as the shape change of the metal plate and the modification of the mold are adjusted in a manner to suppress the occurrence of cracking based on the determination result. As a result, the period for determining the press-forming conditions that can suppress the occurrence of cracking in actual press-forming can be greatly shortened. Moreover, in the present invention, by press-forming under the adjusted press-forming conditions, a press-formed product in which the occurrence of cracking is suppressed can be manufactured. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a flowchart showing the flow of processing of the press-molding crack determination method according to the first embodiment of the present invention.

[0038] Figure 2 This is a diagram showing an example of a forming die used for bulging forming of a metal plate test piece in the press forming crack determination method according to the first embodiment of the present invention.

[0039] Figure 3 This is a diagram showing the shape of a punch used for bulging in Embodiment 1 of the present invention.

[0040] Figure 4 This is a diagram showing the shape of a test piece of a metal plate used for bulge forming in Embodiment 1 and Examples of the present invention.

[0041] Figure 5 This is a graph showing the forming limit strain obtained according to the bending deformation degree in the first embodiment of the present invention.

[0042] Figure 6 This is a graph showing the result of plotting the forming limit strains obtained according to the bending deformation in the first embodiment of the present invention and plotting the boundary surface of forming limit representing the relationship between the maximum principal strain, the minimum principal strain and the bending deformation based on the plot.

[0043] Figure 7 This is a block diagram showing the structure of the press forming crack determination device according to the first embodiment of the present invention.

[0044] Figure 8 This is a flowchart showing the flow of processing of the method for producing a press-molded product according to the second embodiment of the present invention.

[0045] Fig. 9 This is a flowchart showing specific processing of the method for producing a press-molded product according to the second embodiment of the present invention.

[0046] Fig.10It is a perspective view showing the shape of a press-molded product as a molding target in the embodiment.

[0047] Fig.11 This is a distribution diagram (contour view) of crack evaluation values ​​calculated based on the forming limit in the press-molded product obtained by the method of the present invention in the examples (Invention Example 1).

[0048] Fig.12 This is a distribution diagram of crack evaluation values ​​calculated based on the forming limit in the press-molded product obtained by the method of the present invention in the examples (Inventive Example 2).

[0049] Fig.13 This is a distribution diagram of crack evaluation values ​​calculated based on the forming limit strain in the press-formed product obtained using the conventional forming limit curve in the examples.

[0050] Fig.14 This is a distribution diagram of crack evaluation values ​​in press-molded products produced under press-molding conditions adjusted to suppress cracking based on the crack determination results of Invention Example 1 in press-molded products in Examples.

[0051] Fig.15 This is a distribution diagram of crack evaluation values ​​in press-molded products produced under press-molding conditions adjusted to suppress cracking based on the crack determination results of Invention Example 2 in press-molded products in Examples.

[0052] Fig.16 The figures show examples of forming dies and forming limit diagrams used in conventional forming limit tests ((a) is the Nakajima method, (b) is the Marciniak method, and (c) is a forming limit diagram obtained using the Nakajima method and the Marciniak method, respectively). DETAILED DESCRIPTION

[0053] [Implementation Method 1]

[0054] <Method for determining cracking during press molding>

[0055] The press-forming crack determination method according to the first embodiment of the present invention is a method for determining whether cracks have occurred in a press-formed product formed by press-forming a metal plate. Figure 1 As shown, the process includes a forming limit obtaining step P1 and a press-molded product crack determination step P3.

[0056] 《Forming limit acquisition process》

[0057] The forming limit obtaining step P1 forms a metal sheet at various bending deformations and obtains the forming limit of the metal sheet represented by the relationship between the bending deformation of the formed metal sheet and the maximum principal strain and the minimum principal strain of the metal sheet formed at the bending deformation.

[0058] In this embodiment 1, if Figure 2 For example, the forming limit obtaining step P1 uses a plurality of punches 113 having different curvatures of the tip portion 113a to bulge the metal sheet test piece 100 at various bending deformations. Then, the forming limit obtaining step P1 obtains the forming limit of the metal sheet represented by the relationship between the bending deformation of the bulged test piece 100 and the maximum principal strain and the minimum principal strain of the bulged test piece 100 at the bending deformation.

[0059] like Figure 3 As shown, the punch 113 has a spherical surface with a curvature radius R, that is, a front end portion 113a with a curvature ρ (=1 / R) greater than 0. Of course, in the present invention, the punch 113 includes Fig.16 (b) shows a flat punch 213 having a front end portion 213a which is a flat surface (radius of curvature R=∞, curvature ρ=0). Thus, the punch 113 of the present invention has a front end portion 113a having a curvature ρ of 0 or more.

[0060] In order to obtain the forming limit under each deformation mode (equibiaxial deformation, unequal biaxial deformation, plane strain deformation, uniaxial deformation) in press forming, the test piece 100 is used Figure 4 The test pieces 101 and 103 are shaped as shown in the example. Figure 4 As shown in (a), the test piece 101 is a disk-shaped test piece for determining the forming limit of equibiaxial deformation. Figure 4 As shown in (b) and (c), the test piece 103 has a notch 103a formed in a circular arc shape at the position facing each other in the diameter direction of the peripheral portion of the disc. Then, for the test piece 103, the width W of the narrowest part of the central part 103b is changed to various sizes. As the width W is narrowed, the equibiaxial deformation is changed to unequal biaxial deformation and plane strain deformation, gradually approaching uniaxial tension, and the forming limit under each deformation mode is obtained. In addition, the test piece 100 is marked on the surface of the metal plate (a predetermined grid or a pattern for strain measurement).

[0061] The bending deformation degree, which is an index of the forming limit, indicates the degree of bending deformation of the bulged portion of the test piece 100, and the measured value of the curvature radius or curvature of the bulged portion can be used as the bending deformation degree. Of course, in the first embodiment, the curvature radius or curvature of the bulged portion may not be measured, and the bending deformation degree may be expressed by the curvature of the front end portion 113a of the punch 113 used for bulging the test piece 100.

[0062] Furthermore, the method for obtaining the maximum principal strain and the minimum principal strain, which are indicators of the forming limit, is not particularly limited, and the forming limit may be obtained using the same reference for the strains measured using a plurality of punches 113 having different curvatures of the tip portion 113a. For example, the maximum principal strain and the minimum principal strain at the time of fracture may be obtained from the shape of a mark added to the surface of the test piece 100 near the fracture occurrence portion of the test piece 100 that has been bulged until fracture occurs, or may be obtained using the method described below.

[0063] As a specific scheme of the forming limit obtaining step P1 of the first embodiment, Figure 1 As shown, a configuration including a forming test step S11, a forming limit analysis step S13, and a forming limit surface preparation step S15 can be exemplified.

[0064] (Molding test steps)

[0065] Forming test step S11 is to bend the Figure 4 The test piece 100 shown as an example is bulged, and the strain generated in the test piece 100 after bulging is measured to construct a strain database.

[0066] In this embodiment 1, if Figure 3 As shown, the curvature radius of the front end portion 113a is prepared to be R25mm, R50mm, R100mm and R=∞ (equivalent to Fig.16 (b) The four types of punches 113 include the flat punch 213 shown in FIG. The curvatures (unit: 1 / mm) of the tip portion 113a are 0.04, 0.02, 0.01, and 0.00, respectively.

[0067] Next, as a metal plate, a test piece 100 of a high-strength steel plate with a tensile strength of 1470 MPa grade and a plate thickness of 1.4 mm is used as a test material, and the shape of the test piece 100 is determined. Figure 4 As shown, the shapes of the test pieces 100 are set to be a circular test piece 101 with a diameter of φ180 mm and a test piece 103 with a shape cut in the width direction based on the circular test piece 101. In addition, the test piece 103 is set to have five shapes with a width W of the central portion 103b ranging from 25 mm to 160 mm. As an example, Figure 4 (b) shows a test piece 103 with a width W of 60 mm. Figure 4 (c) shows a test piece 103 having a width W of 80 mm. A grid pattern (grid) for strain measurement is transferred onto the surface of the test piece 100 having a shape determined in this manner.

[0068] Then, if Figure 2 As shown, a forming die 111 having a punch 113, an upper die 115, and a pleat holder 117 is set in a hydraulic deep drawing testing machine to perform bulging of the test piece 100. In the first embodiment, in order to suppress the inflow of the test piece 100, a pleat holder pressure of 50 tons is applied to the pleat holder 117, and the punch speed of the punch 113 is set to 5 mm / min. Then, while bulging, an image analysis camera as a photographing device (not shown) provided on the upper part of the forming die 111 is used to photograph the surface of the test piece 100.

[0069] Next, the image of the surface of the test piece 100 captured during the bulging process is analyzed, and the strain (maximum principal strain and minimum principal strain) generated on the surface of the test piece 100 is measured. As a method for measuring the strain, the digital image correlation method (hereinafter referred to as "DIC") can be preferably used. In DIC, the surface of the test piece 100 during the bulging process is photographed at a predetermined time interval. Then, the image captured at each time step is analyzed, and the maximum principal strain and the minimum principal strain are measured according to the deformation of the grid attached to the surface of the test piece 100 as the strain in two directions in the plane (in-plane) generated on the test piece 100. The strain measured in each time step of the bulging process is stored in the strain database in a time series order.

[0070] Next, it is visually determined whether or not a fracture occurs on the test piece 100 during the bulging process. If it is determined that no fracture occurs, the bulging is advanced, and the photographing of the surface of the test piece 100, the measurement of the strain amount by image analysis, and the storage in the strain database are repeated. Thus, in the first embodiment, the surface of the test piece 100 is photographed at a predetermined time interval (e.g., once per second) from the start of molding to the occurrence of fracture, and the measurement of the strain amount is performed for each image photographed, and this is continued until it is determined that a fracture occurs.

[0071] When it is determined that a fracture has occurred, the bulging molding, the measurement of the strain amount, and the storage in the strain database are repeated for the test piece 100 having another shape.

[0072] The above operation is performed for each punch 113 having a different shape of the test piece 100 and a different curvature of the tip portion 113 a , and a strain database is constructed in which strains from the start of forming to fracture are recorded in a time series order according to the degree of bending deformation.

[0073] (Forming limit analysis steps)

[0074] The forming limit analysis step S13 obtains the forming limit strain represented by the maximum principal strain and the minimum principal strain in the fracture occurrence portion of the test piece 100 according to the bending deformation of the test piece 100 based on the strain database constructed in the forming test step S11 .

[0075] In the first embodiment, the forming limit strain is obtained as follows. First, the maximum principal strain and the minimum principal strain near the fracture occurrence part of the test piece 100 from the start of forming to fracture are extracted from the strain database constructed in the forming test step S11 according to the bending deformation degree and at a predetermined time step. Then, time series data of the maximum principal strain and the minimum principal strain extracted according to the bending deformation degree are prepared.

[0076] Next, in the time series data of the maximum principal strain and the minimum principal strain produced according to the bending deformation, the critical point (critical point) at which the test piece 100 changes from uniform deformation to inhomogeneous deformation (inhomogeneous deformation) is determined. Then, the test piece 100 undergoes necking at the critical point of the strain, and the maximum principal strain and the minimum principal strain at this time are obtained as the forming limit strain under the bending deformation. In this way, by obtaining the forming limit strain according to the bending deformation, the forming limit represented by the relationship between the bending deformation, the maximum principal strain, and the minimum principal strain is obtained.

[0077] Table 1 shows the values ​​of the forming limit strain obtained by using the bending deformation as the curvature of the tip end portion 113a of the punch 113 used for bulging the test piece 100 in the first embodiment. Figure 5 2 shows a graph in which the forming limit strain is plotted on a two-dimensional coordinate plane having the minimum principal strain and the maximum principal strain as coordinate axes.

[0078] [Table 1]

[0079]

[0080] In the forming limit analysis step of the present invention, the maximum principal strain and the minimum principal strain at the time when the test piece 100 breaks may be obtained as the forming limit strain.

[0081] Furthermore, regarding the strain measured by the bulging test using a plurality of punches 113 having the tip portions 113a having different curvatures, the method of obtaining the forming limit strain is not limited as long as the forming limit strain is obtained using the same criterion.

[0082] (Steps for making forming limit surfaces)

[0083] The forming limit surface preparation step S15 plots the forming limit strain obtained according to the bending deformation in the forming limit analysis step S13 in a three-dimensional coordinate space with the maximum principal strain, the minimum principal strain and the bending deformation as three axes. Furthermore, the forming limit surface preparation step S15 prepares a forming limit surface represented by the relationship between the maximum principal strain, the minimum principal strain and the bending deformation.

[0084] As a method of preparing the forming limit surface in the forming limit surface preparing step S15, for example, the following method is mentioned.

[0085] In the first method, first, two forming limit strain plots with equal bending deformation are extracted. Then, a forming limit strain plot with the smallest distance from the line segment connecting the two forming limit strain plots is selected from the forming limit strain group with different bending deformation from the two forming limit strains. Then, a triangular plane is formed using the two extracted forming limit strain plots and the selected forming limit strain plot. Such a triangular plane is created for the plots of all forming limit strain groups. Then, a polygonal facet formed by combining the created triangular planes is set as a forming limit surface.

[0086] In the second method, first, a boundary flat surface forming limit or a boundary curved surface of forming limit is assumed in a three-dimensional coordinate space, and the vertical distance between the assumed forming limit plane or the forming limit curved surface and the plot of the forming limit strain is calculated. Then, the forming limit plane or the forming limit curved surface is determined so that the square sum of the calculated vertical distances becomes the minimum.

[0087] In the third method, the forming limit plane or forming limit surface is determined in the second method in such a way that the sum of squares of the vertical distances between the assumed forming limit plane or forming limit surface and the plot of the forming limit strain is minimized by weighting. As a weighting method, for example, the weight of the forming limit of the bending deformation degree that is prone to cracking in actual press forming can be increased. In this way, the error between the determined forming limit plane or forming limit surface and the plot of the forming limit strain at the bending deformation degree that is prone to cracking in actual press forming can be reduced.

[0088] For the second method or the third method, a plurality of forming limit planes and / or forming limit surfaces may be combined. For example, a forming limit plane or a forming limit surface may be assumed in each of the region where the minimum principal strain is negative and the region where the minimum principal strain is positive, and the forming limit plane or the forming limit surface may be determined in such a manner that the sum of the squares of the perpendicular distances to the plot of the forming limit strain is minimized for each region. The same is true when the forming limit surface is prepared in such a manner that the sum of the squares of the weighted perpendicular distances to the plot of the forming limit strain is minimized.

[0089] In addition, as a method other than the above, three adjacent points can be selected in the plot of the forming limit strain in the three-dimensional coordinate space, and a triangular plane can be generated by connecting the selected three points with a straight line. The polygonal surface composed of multiple triangular planes generated in this way can be used as the forming limit surface.

[0090] Alternatively, a forming limit line may be prepared based on the forming limit strain for each bending deformation degree, and a forming limit surface including the forming limit lines prepared for all bending deformation degrees may be prepared.

[0091] In the first embodiment, a forming limit surface is produced based on the second method described above. First, a forming limit surface represented by the following formula (1) is assumed for each of a region where the minimum principal strain is negative and a region where the minimum principal strain is positive.

[0092] [Mathematical formula 1]

[0093] ax+by+cz+d=0 …(1)

[0094] Next, the coefficients in equation (1) are determined for the forming limit plane A in the region where the minimum principal strain is negative and the forming limit plane B in the region where the minimum principal strain is positive, respectively, so that the sum of the squares of the perpendicular distances between the assumed forming limit plane and the plot of the forming limit strain group for each bending deformation degree becomes minimum. In this way, the forming limit plane A and the forming limit plane B obtained by determining the coefficients in equation (1) are prepared as forming limit surfaces.

[0095] Table 2 shows the coefficients in equation (1) for the determined forming limit plane A and forming limit plane B. Figure 6 , there are shown plots of the forming limit plane A and the forming limit plane B created based on the determined coefficients and the forming limit strain for each bending deformation degree actually measured.

[0096] [Table 2]

[0097]

[0098] 《Crack determination process for pressed products》

[0099] The press-molded product crack determination step P3 obtains the maximum principal strain and the minimum principal strain in the press-molded product, and the curvature in the maximum principal strain direction as the bending deformation degree in the press-molded product as crack determination parameters in the press-molded product. Furthermore, the press-molded product crack determination step P3 determines whether cracks have occurred in the press-molded product based on the obtained crack determination parameters and the forming limit obtained in the forming limit acquisition step P1.

[0100] As a specific scheme of the press-molded product crack determination step P3, Figure 1 As shown, there is a scheme including a press molding FEM analysis (finite element analysis) step S31, a press molding crack determination parameter calculation step S33, and a press molding crack occurrence determination step S35.

[0101] (Press forming FEM analysis steps)

[0102] The press forming FEM analysis step S31 performs FEM analysis of a process of press forming a metal plate into a press-formed product.

[0103] By the FEM analysis in the press forming FEM analysis step S31 , changes in strain, stress, plate thickness, etc. generated in the press-formed product by press forming of the metal plate can be obtained for each element and node used in the FEM analysis.

[0104] (Press Forming Crack Judgment Parameter Calculation Procedure)

[0105] The press-molding crack determination parameter calculation step S33 calculates the maximum principal strain and the minimum principal strain in the press-molded product and the bending deformation in the press-molded product as crack determination parameters in the press-molded product. The maximum principal strain and the minimum principal strain and the bending deformation in the press-molded product are calculated based on the FEM analysis result in the press-molding FEM analysis step S31.

[0106] Regarding the curvature in the direction of the maximum principal strain as the bending deformation of the pressed product, the maximum principal strain direction of the pressed product can be determined, and the curvature can be obtained based on three points existing in the direction of the maximum principal strain. In this method, the maximum principal strain and the minimum principal strain can be obtained based on the results of FEM analysis of the pressed product as in the past. However, in pressed products with complex shapes, it takes more labor and time to determine the maximum principal strain direction. Therefore, for example, the bending deformation of the pressed product can be easily calculated using any of the methods described below.

[0107] The first method for calculating the bending deformation is a method of calculating based on the maximum principal strain of the surface and back of the press-formed product. The usual deformation state in the press-formed product is not just bending deformation, but a combination of other deformation states. That is, the deformation state of the press-formed product can be called a broadly defined stretch bending state. The strain of the portion subjected to tensile deformation in the tensile bending deformation acts at a constant level in the plate thickness direction, and the strain of the portion subjected to bending deformation has a distribution in the plate thickness direction. Therefore, the bending deformation in the press-formed product can be expressed using the strain difference between the outer side of the bend and the inner side of the bend of the portion subjected to bending deformation.

[0108] Generally speaking, the strain ε of a portion subjected to bending deformation is expressed by the following formula.

[0109] ε=±t / 2r

[0110] t is the plate thickness, r is the radius of curvature, the strain on the outside of the bend is positive, and the strain on the inside of the bend is negative. In addition, since the radius of curvature r is the inverse of the curvature ρ, the following formula is derived from the above formula.

[0111] ε 1,outer -ε 1,inner =t / r

[0112] ρ=(ε 1,outer -ε 1,inner ) / t

[0113] in,

[0114] ρ: Bending deformation

[0115] ε 1,outer : Maximum principal strain on the outside of the bend

[0116] ε 1,inner : Maximum principal strain on the inner side of the bend

[0117] t: Plate thickness

[0118] ε 1,outer and ε1,inner It can be calculated by FEM analysis of the press-molded product.

[0119] The second method for calculating the bending deformation is to calculate the maximum principal curvature, that is, the maximum curvature among the curvatures defined when the cross section of the curved surface is made by a plane including the normal direction at a point on the curved surface. Since the curvature in the maximum principal strain direction is roughly consistent with the maximum maximum curvature among the geometric curvatures defined on the curved surface, the bending deformation can be easily calculated using this method.

[0120] (Steps for determining whether press-molding cracking occurs)

[0121] The press molding crack occurrence determination step S35 determines whether cracks have occurred in the press molded product. The determination is based on the crack determination parameter calculated in the press molding crack determination parameter calculation step S33 and the forming limit acquired in the forming limit acquisition step P1.

[0122] Regarding the case where the forming limit surface is prepared in the forming limit acquisition step P1 as described above, a specific procedure for determining whether cracks have occurred in the press-molded product in the press-molding crack occurrence determination step S35 will be described below.

[0123] First, the crack determination parameter in the press-molded product calculated in the press-molding crack determination parameter calculation step S33 is plotted on the three-dimensional coordinates on which the forming limit surface created in the forming limit surface creation step S15 of the forming limit acquisition step P1 is plotted.

[0124] Then, if the plot of the crack judgment parameter is not below the forming limit surface, that is, the maximum principal strain of the crack judgment parameter is greater than the maximum principal strain of the forming limit surface corresponding to the minimum principal strain of the crack judgment parameter and the bending deformation, it is determined that cracking has occurred.

[0125] In contrast, if the plot of the crack judgment parameter is below the forming limit surface, that is, the maximum principal strain of the crack judgment parameter is less than the minimum principal strain of the crack judgment parameter and the maximum principal strain of the forming limit surface corresponding to the bending deformation, it is judged that no cracking occurs.

[0126] As described above, in the press-forming crack determination method of the first embodiment, the test piece 100 of the metal plate is bulged with various bending deformations, and the forming limit of the metal plate represented by the relationship between the maximum principal strain and the minimum principal strain of the test piece 100 and the bending deformation is obtained. In addition, for the press-formed product formed by press-forming the metal plate, the maximum principal strain and the minimum principal strain, as well as the curvature in the maximum principal strain direction as the bending deformation, are calculated as crack determination parameters. Then, based on the calculated crack determination parameters and the obtained forming limit, the presence or absence of cracks in the press-formed product is determined. In this way, the presence or absence of cracks in the press-formed product can be determined by considering the influence of the bending deformation that has not been considered in the conventional forming limit line.

[0127] The metal plate in the forming limit acquisition step P1 can be collected from the metal material used as the sheet material of the press-formed product, in which the presence or absence of cracking is determined in the press-formed product crack determination step P3. Deviations in the metal material used as the sheet material of the press-formed product (for example, when the metal material is a steel plate, changes in the metal structure caused by deviations in manufacturing conditions such as hot-rolling conditions and annealing conditions) can affect the occurrence of cracking in the press-formed product. Therefore, for example, before the start of mass production of the press-formed product, the metal plate used in the forming test step S11 can be collected from the metal material actually used as the sheet material of the press-formed product and the forming limit can be obtained to determine the presence or absence of cracking in the press-formed product. As a result, the accuracy of the advance prediction of cracking in press-forming in mass production can be further improved.

[0128] In addition, in the above description, Figure 2 For example, the forming test step S11 is to bulge the metal plate test piece 100 with various bending deformations. Of course, the forming test step is not limited to bulging the metal plate test piece, and a metal plate (blank) with a scribed circle or a grid transferred thereon may be used, for example Fig.10 The press-molded product 120 simulating an automobile part is press-molded. In this case, in the molding test step, the strain generated on the press-molded product is recorded in a time series from the start of molding to fracture, so that a strain database can be constructed. In addition, the strain (maximum principal strain, minimum principal strain) generated on the press-molded product can be obtained from the size change of the ruled circle or grid in the press-molded product. In addition, the bending deformation can be obtained by measuring the curvature in the maximum principal strain direction in the actual press-molded product.

[0129] In addition, in the press-molding crack determination method of the first embodiment, the press-molding crack occurrence determination step S35 performs press-molding analysis of the press-molded product. Furthermore, through the press-molding analysis, the maximum principal strain, the minimum principal strain, and the bending deformation (curvature in the maximum principal strain direction) are calculated as crack determination parameters.

[0130] Of course, in the present invention, the step of determining the occurrence of cracking of the press-molded product can be performed by press-molding the press-molded product using a sheet material to which a ruled circle or a grid is transferred. Then, the maximum principal strain and the minimum principal strain can be obtained from the size change of the ruled circle or the grid in the press-molded product. In addition, the bending deformation can be obtained by measuring the curvature in the maximum principal strain direction in the actual press-molded product.

[0131] Examples of methods for simply obtaining the curvature in the maximum principal strain direction of an actual press-molded product include measurement using a radius gauge and calculation of coordinates of three points in the maximum principal strain direction obtained by measuring the surface profile of the press-molded product.

[0132] In addition, the determination of the occurrence of cracks in the press molding crack occurrence determination step S35 is performed for each part of the press molded product for which the crack determination parameter is calculated in the press molding crack determination parameter calculation step S33. Of course, the crack determination parameter may be calculated only for the part of the press molded product where cracks are concerned, and the occurrence of cracks may be determined for the part. In this case, the crack determination parameter is calculated according to the elements used in the press molding analysis, and the occurrence of cracks may be determined according to the elements of the part of the press molded product where cracks are concerned.

[0133] <Press Forming Crack Judgment Device>

[0134] The above description is related to the method of determining whether cracks have occurred in a press-molded product. Of course, the present invention can also be configured as an apparatus for determining whether cracks have occurred.

[0135] The press-forming crack determination device according to the first embodiment of the present invention determines whether cracks have occurred in a press-formed product formed by press-forming a metal plate. Figure 7 As an example, the press-molding crack determination device 1 includes a forming limit acquisition unit 10 and a press-molded product crack determination unit 20. The press-molding crack determination device 1 can be composed of a CPU (central processing unit) of a computer (PC (Personal Computer) etc.). In this case, the above-mentioned units function by executing a prescribed program by the CPU of the computer.

[0136] 《Forming Limit Acquisition Unit》

[0137] The forming limit acquisition unit 10 acquires the forming limit of the metal sheet represented by the relationship between the bending deformation degree of the metal sheet formed at various bending deformation degrees and the maximum principal strain and the minimum principal strain of the metal sheet formed at the bending deformation degrees.

[0138] In this embodiment 1, if Figure 7 As shown, the forming limit acquisition unit 10 includes a bulging forming test result acquisition unit 11 , a forming limit analysis unit 13 , and a forming limit surface creation unit 15 .

[0139] (Bulging test results entry section)

[0140] The bulging test results are taken into the part 11 and the bending deformation is measured as described above. Figure 2 The bulging test results obtained by measuring the strain generated in the test piece 100 of the metal plate bulged at various bending deformation degrees are shown.

[0141] As an example of a specific structure of the bulging test result taking-in unit 11, there can be mentioned a structure including a forming die, an imaging device, a strain measuring device, and a strain database building device (not shown).

[0142] The molding die is used to bulge the test piece 100 with various bending deformations, such as Figure 2 As shown, a molding die 111 including a plurality of punches 113 having a tip portion 113 a having different curvatures, an upper die 115 , and a wrinkle presser 117 can be exemplified.

[0143] The imaging device captures deformation of the grid or the pattern for strain analysis on the surface of the test piece 100 during the process of expansion molding the test piece 100 with the grid or the pattern for strain analysis by the molding die 111. As an example of the imaging device, a device composed of two cameras and performing stereo photography of the surface of the test piece 100 can be exemplified.

[0144] The strain measuring device analyzes the image of the surface of the test piece 100 photographed by the imaging device, and measures the strain generated on the test piece 100 according to the bending deformation of the bulged test piece 100. As for the strain measuring device, a device that uses a digital image correlation method to measure the maximum principal strain and the minimum principal strain as the strain in two directions in the plane generated on the test piece 100 during the bulging process can be exemplified.

[0145] The strain database construction device constructs a strain database that is created so as to be able to store and extract strains measured by the strain measuring device in a time series from the start of forming to fracture according to the bending deformation degree of the bulge-formed test piece 100 .

[0146] (Forming Limit Analysis Department)

[0147] The forming limit analysis unit 13 obtains the forming limit strain represented by the maximum principal strain and the minimum principal strain in the fracture occurrence part of the test piece 100 according to the bending deformation of the test piece 100 based on the bulge forming test result taken in by the bulge forming test result taking-in unit 11. The forming limit analysis unit 13 can be exemplified by a forming limit analysis unit 13 (not shown) having a strain distribution extraction device and a forming limit acquisition device.

[0148] The strain distribution extraction device extracts the strain distribution near the fracture occurrence portion of the test piece 100 from the strain database constructed by the strain database construction device according to the bending deformation degree of the bulged test piece 100 .

[0149] The forming limit acquisition device obtains the forming limit strain represented by the maximum principal strain and the minimum principal strain in the fracture occurrence portion of the test piece 100 according to the bending deformation of the test piece 100 based on the strain distribution extracted by the strain distribution extraction device.

[0150] The method of obtaining the forming limit strain by the forming limit analysis section 13 can be the same method as the forming limit analysis step of the first embodiment.

[0151] (Forming Limit Surface Production Department)

[0152] The forming limit surface preparation unit 15 plots the forming limit strain obtained by the forming limit analysis unit 13 according to the bending deformation in a three-dimensional coordinate space with the maximum principal strain, the minimum principal strain and the bending deformation as three axes. Furthermore, the forming limit surface preparation unit 15 prepares a forming limit surface represented by the relationship between the maximum principal strain, the minimum principal strain and the bending deformation.

[0153] The forming limit surface preparation by the forming limit surface preparation section 15 can be any of the methods described in the forming limit surface preparation step S15 of the press forming crack determination method according to the first embodiment.

[0154] 《Crack determination unit for pressed products》

[0155] The press-molded product crack determination unit 20 performs press-molding analysis of the press-molded product, obtains the maximum principal strain and the minimum principal strain in the press-molded product, and the curvature in the maximum principal strain direction as the bending deformation degree in the press-molded product as crack determination parameters. Then, the press-molded product crack determination unit 20 determines whether cracks have occurred in the press-molded product based on the obtained crack determination parameters and the forming limit obtained by the forming limit acquisition unit 10.

[0156] As a specific embodiment of the press-molded product crack determination unit 20, Figure 7 As shown, a configuration including a press molding FEM analysis unit 21 , a press molding crack determination parameter calculation unit 23 , and a press molding crack occurrence presence / absence determination unit 25 can be exemplified.

[0157] (Press Forming FEM Analysis Department)

[0158] The press forming FEM analysis unit 21 performs FEM analysis of a process of press forming a metal plate into a press-formed product.

[0159] By performing FEM analysis by the press forming FEM analysis unit 21 , changes in strain, stress, plate thickness, etc. generated in a press-formed product by press forming of a metal plate can be obtained for each element and node used in the FEM analysis.

[0160] (Press Forming Crack Judgment Parameter Calculation Section)

[0161] The press-molding crack determination parameter calculation unit 23 calculates the maximum principal strain and the minimum principal strain and the bending deformation degree in the press-molded product as crack determination parameters in the press-molded product.

[0162] The maximum principal strain, the minimum principal strain, and the bending deformation in the press-formed product are calculated based on the FEM analysis result obtained by the press-forming FEM analysis unit 21. The bending deformation may be calculated by the same method as the press-forming crack determination parameter calculation step S33 of the press-forming crack determination method of the first embodiment.

[0163] (Pressure molding cracking occurrence determination section)

[0164] The press-molding crack occurrence determination unit 25 determines the occurrence of cracks in the press-molded product based on the crack determination parameter calculated by the press-molding crack determination parameter calculation unit 23 and the forming limit acquired by the forming limit acquisition unit 10 .

[0165] The determination of the occurrence of cracks in the press-molded product by the press-molding crack occurrence determination unit 25 may be performed in the same manner as the press-molding crack occurrence determination step S35 of the press-molding crack occurrence determination method of the first embodiment.

[0166] <Press Forming Crack Judgment Procedure>

[0167] The first embodiment of the present invention can be configured as a press forming crack determination program that enables a press forming crack determination device configured by a computer to function. That is, the press forming crack determination program of the first embodiment determines whether cracks have occurred in a press-formed product formed by press forming a metal plate. Furthermore, the press forming crack determination program enables the computer to function as a press forming crack determination device. Figure 7 The forming limit acquisition unit 10 and the press-molded product crack determination unit 20 shown as an example function.

[0168] Furthermore, the press forming crack determination program of the present embodiment 1 causes the forming limit acquisition unit 10 to function as a bulging test result taking-in unit 11, a forming limit analysis unit 13, and a forming limit surface preparation unit 15. In this case, as an example, the bulging test result taking-in unit 11 takes in a strain database in a strain database construction device constructed by a bulging test in which a test piece of a metal plate is bulged with various bending deformation degrees.

[0169] Furthermore, the press-molding crack determination program of the first embodiment causes the press-molded product crack determination unit 20 to function as a press-molding FEM analysis unit 21 , a press-molding crack determination parameter calculation unit 23 , and a press-molding crack occurrence presence determination unit 25 .

[0170] As described above, the press-forming crack determination device and press-forming crack determination program of the first embodiment obtain the forming limit of the metal sheet represented by the relationship between the maximum principal strain, the minimum principal strain, and the bending deformation. In addition, for the press-formed product formed by press-forming the metal sheet, the maximum principal strain, the minimum principal strain, and the curvature in the maximum principal strain direction as the bending deformation are calculated as crack determination parameters in the press-formed product. Then, based on the calculated crack determination parameters and the obtained forming limit, the presence or absence of cracks in the press-formed product can be determined by considering the influence of the bending deformation.

[0171] The bulging forming test result taking-in unit may include a strain database taking-in unit (not shown) for taking in the strain database constructed in advance, instead of the forming die, the imaging device, the strain measuring device, and the strain database constructing device as described above. In this case, the forming limit analysis unit and the forming limit surface creating unit may create the forming limit surface based on the strain database imported by the strain database taking-in unit.

[0172] Alternatively, when a forming limit surface represented by the relationship between the bending deformation degree and the maximum principal strain and the minimum principal strain is prepared in advance, in the press cracking determination device and program of the present invention, the forming limit acquisition unit may acquire the pre-prepared forming limit surface.

[0173] In the above description, the forming limit acquisition unit acquires the forming limit obtained by the bulging forming test of bulging a metal plate test piece with various bending deformation degrees. Of course, in the present invention, the forming limit acquisition unit may also acquire the forming limit obtained by press-forming a metal plate (sheet material) to which a ruled circle or a grid is transferred into a press-formed product.

[0174] [Implementation Method 2]

[0175] <Method for Manufacturing Pressed Product>

[0176] The method for manufacturing a press-formed product according to the second embodiment of the present invention determines whether cracks have occurred in a press-formed product formed by press-forming a metal plate, and manufactures a press-formed product in which cracks during press-forming are suppressed based on the determination result. In addition, in the method for manufacturing a press-formed product according to the second embodiment, the determination of whether cracks have occurred in a press-formed product formed by press-forming a metal plate is performed by the press-forming crack determination method according to the first embodiment of the present invention.

[0177] like Figure 8 As shown, the method for manufacturing a press-molded product according to the second embodiment includes a forming limit obtaining step P1, a press-molded product crack determination step P3, a press-molding condition adjustment step P5, and a press-molded product manufacturing step P7. Fig. 9 Specific processes in the method for producing a press-molded product according to the second embodiment will be described.

[0178] 《Forming limit acquisition process》

[0179] Similar to the first embodiment, the forming limit obtaining step P1 bulges a metal plate test piece at various bending deformations and obtains the forming limit of the metal plate represented by the relationship between the bending deformation of the test piece and the maximum principal strain and the minimum principal strain generated in the test piece.

[0180] In the forming limit obtaining step P1, first, the Figure 4 The test piece 100 shown is subjected to inflation molding, and the strain generated in the test piece 100 after the inflation molding is measured, and a strain database is constructed (S11).

[0181] Next, based on the constructed strain database, the forming limit strain represented by the maximum principal strain and the minimum principal strain in the fracture occurrence portion of the test piece 100 is extracted according to the bending deformation degree of the test piece 100 ( S13 ).

[0182] Next, the forming limit strain obtained by the bending deformation is plotted in a three-dimensional coordinate space with the maximum principal strain, the minimum principal strain and the bending deformation as three axes, and a forming limit surface represented by the relationship between the maximum principal strain, the minimum principal strain and the bending deformation is approximated (S15). Regarding the forming limit surface, for example, a forming limit plane represented by the above formula (1) can be assumed, and the forming limit plane with the minimum sum of squares of perpendicular distances from the plot of the forming limit strain obtained by the bending deformation is determined.

[0183] The metal plate test piece 100 can be collected from a metal material used as a sheet material for a press-formed product to be manufactured. By obtaining the forming limit of the metal material used as a sheet material for a press-formed product to be manufactured and determining the forming limit surface, the accuracy of the pre-prediction of cracking in the press-formed product can be further improved.

[0184] 《Crack determination process for pressed products》

[0185] Similar to the first embodiment, the press-molded product crack determination step P3 determines the presence or absence of cracks in the press-molded product based on the crack determination parameter calculated for the press-molded product and the forming limit acquired in the forming limit acquisition step.

[0186] In the press-molded product crack determination step P3, if Fig. 9 As shown, first, provisional press-molding conditions for press-molding the press-molded product are set (S31a).

[0187] Next, a press forming analysis is performed on the process of press forming the press-formed product under the set provisional press forming conditions (S31b). Then, the maximum principal strain, minimum principal strain and bending deformation in the press-formed product are calculated as crack determination parameters in the press-formed product (S33).

[0188] Next, the calculated crack judgment parameter is plotted on the three-dimensional coordinates depicting the forming limit surface using the maximum principal strain, the minimum principal strain and the bending deformation as coordinate axes (S35a). Then, it is determined whether the plotted crack judgment parameter is located in the area below the forming limit surface (S35b).

[0189] If it is determined that the crack determination parameter is below the forming limit surface, it is determined that no crack has occurred (S35c). On the other hand, if it is determined that the crack determination parameter is not below the forming limit surface, it is determined that crack has occurred (S35d).

[0190] 《Pressing molding condition adjustment process》

[0191] In the press molding condition adjustment step P5, if it is determined in the press molded product crack determination step P3 that cracks have occurred, the press molding conditions of the press molded product are adjusted and the press molded product crack determination step P3 is repeated until it is determined that no cracks have occurred.

[0192] First, in the press-molded product crack determination step P3, when the crack determination parameter is not below the forming limit surface (S35b), it is determined that cracking has occurred (S35d). In this case, the provisional press-molding conditions of the press-molded product are changed in the press-molding condition adjustment step P5 (S51).

[0193] For example, when a press-forming product is deep-drawn using a press-forming die composed of a die, a punch, and a wrinkle pressing piece (blank material holder), the provisional press-forming conditions are changed by changing the sheet size or sheet shape, modifying the die shape (radius of die shoulder part, radius of punch shoulder part), wrinkle pressing force, and lubricating the sheet in contact with the die and the wrinkle pressing piece. The sheet size change or sheet shape change can be performed, for example, as follows: the provisional press-forming conditions are adjusted in a manner that reduces the wrinkle pressing force to optimize the tension acting on the sheet during press-forming, and the inflow resistance of the material flow of the sheet is adjusted. In addition, since the shoulder of a die and the shoulder of a punch, which are subjected to bending and unbending deformation while the sheet metal is subjected to tension, rapidly promote the reduction of sheet thickness and easily reach cracking, and affect the flow of material, the shape of the press forming die can be changed and the radius of the die shoulder and the radius of the punch shoulder can be adjusted. Moreover, the deep drawing limit can be adjusted by lubricating the sheet metal in contact with the die and the wrinkle pressing piece.

[0194] Then, after the provisional press molding conditions are changed (S51), in the press molded product crack determination step P3, press molding analysis is performed using the provisional press molding conditions after the change (S31b). Furthermore, as described above, calculation of crack determination parameters (S33), plotting of crack determination parameters on three-dimensional coordinates depicting the forming limit surface (S35a), and determination of whether cracks have occurred (S35b) are performed.

[0195] On the other hand, in the press-molded product crack determination step P3, when the crack determination parameter is below the forming limit surface (S35b), it is determined that no crack occurs (S35c). In this case, in the press-molding condition adjustment step P5, the provisional press-molding condition determined to be no crack occurs is determined as the press-molding condition (S53), and the adjustment of the press-molding condition is terminated (S55).

[0196] The adjustment of the press molding conditions (changing of the tentative press molding conditions) in the press molding condition adjustment step P5 is performed until it is determined in the press molded product crack determination step P3 that no cracks have occurred in all regions of the press molded product.

[0197] 《Press-molded product manufacturing process》

[0198] In the press-molded product manufacturing step P7, a press-molded product is manufactured using the press-molding conditions adjusted in the press-molding condition adjustment step P5 so that no cracks are determined to have occurred in the press-molded product crack determination step P3.

[0199] In the press molded product manufacturing process P7, Fig. 9 As shown, the press molding conditions that determine that no cracks will occur in the press-molded product crack determination step P3 are determined (S53), and after the adjustment of the press molding conditions is completed (S55), the press-molded product is manufactured.

[0200] In order to actually manufacture the pressed product using the adjusted pressing conditions, the settings of the pressing machine can be changed as follows. For example, when adjusting the wrinkle pressing force as a pressing condition, the setting value on the control panel of the air pressure of the air pressure die cushion (auxiliary pressure device) of the pressing machine can be changed. In addition, when adjusting the shape of the pressing die (die shoulder radius, punch shoulder radius), the shape data of the molding die (model) used in the pressing analysis (S31b) is input into the CAD / CAM program that cooperates with the NC machine tool (numerically-controlled machine) and converted into NC data (numerical control data) (NC program) for NC machining (NC machining). Then, using the converted NC data (NC program), the NC machine tool can be used to machine a steel mold or a casting mold model made of expanded polystyrene based on the full mold casting method. Moreover, when adjusting the lubrication of the sheet material in contact with the die and the pleat press, it is sufficient to change the type of lubricating oil (increase the viscosity, add extreme pressure additives), insert polymer films such as polyethylene films, etc.

[0201] As described above, in the method for manufacturing a press-formed product of the second embodiment, in press-forming of a metal plate, the press-forming conditions are adjusted in a manner to suppress the occurrence of cracks based on the result of determining the occurrence of cracks in consideration of bending deformation. As a result, the period of determining the press-forming conditions of the actual press-formed product through repeated experiments can be greatly shortened. Furthermore, the press-formed product can be manufactured by suppressing the occurrence of cracks using the press-forming conditions adjusted in this manner.

[0202] Example

[0203] Experiments and analyses were conducted to verify the effects of the press-molding crack determination method and press-molded product manufacturing method of the present invention, and these will be described below.

[0204] In this embodiment, the presence or absence of cracking in a press-formed product formed by deep drawing is determined based on a forming limit surface produced by a forming test in which a test piece of a metal plate is bulged with various bending deformations. In addition, in this embodiment, the press-forming conditions are adjusted in a manner to suppress the occurrence of cracking based on the result of the determination of the presence or absence of cracking, and the crack suppression effect when a press-formed product is manufactured under the adjusted press-forming conditions is verified.

[0205] First, as described in the first embodiment, a metal plate test piece 100 is bulged with various bending deformations using a forming die 111 having a plurality of punches 113 with different curvatures of the tip end 113a (see Figure 2 ). Then, the forming limit of the metal sheet is expressed by the relationship between the bending deformation of the bulged test piece 100 and the maximum principal strain and the minimum principal strain of the bulged test piece 100 at the bending deformation. For the test piece 100, a steel plate with a tensile strength of 1470 MPa and a plate thickness of 1.6 mm is used as the test material.

[0206] In this example, the forming limit surface represented by the formula (1) described in Embodiment 1 is obtained as the forming limit. The values ​​of the coefficients a to d in the formula (1) are the values ​​shown in the above Table 2.

[0207] Next, proceed Fig.10 The FEM analysis shown simulates the process of press-molding a press-molded product 120 of an automobile part, and determines whether cracks occur in the press-molded product 120 .

[0208] In the press forming of the press-formed product 120, a steel plate with a tensile strength of 1470 MPa and a thickness of 1.6 mm is used as the sheet material, similarly to the forming test for obtaining the forming limit described in the above-mentioned embodiment 1. In addition, the forming process of the press-formed product 120 is stretch forming, and the wrinkle pressing force is set within the range of 1 to 10 tons.

[0209] Then, by FEM analysis, the maximum principal strain, the minimum principal strain and the bending deformation are calculated as crack judgment parameters according to the elements used in the FEM analysis of the press-formed product 120. In this embodiment, the case where the difference between the maximum principal strain on the outside of the bend and the maximum principal strain on the inside of the bend is divided by the plate thickness is set as the bending deformation, and the case where the maximum principal curvature is used as the bending deformation is set as Invention Example 2. For each of Invention Example 1 and Invention Example 2, the evaluation value of crack judgment is calculated for each part of the press-formed product 120. The limit value of the maximum principal strain is calculated based on the minimum principal strain and the bending deformation of each part of the press-formed product 120 press-formed with a wrinkle pressing force of 10 tons, and the evaluation value of crack judgment is set to the value obtained by dividing the maximum principal strain obtained from the FEM analysis by the limit maximum principal strain. Here, the area where the evaluation value of crack judgment is less than 1 corresponds to the case where the judgment parameter obtained from the FEM analysis is below the forming limit surface, and it is judged that no crack occurs. The region where the evaluation value for crack determination is 1 or more corresponds to a case where the determination parameter obtained from the FEM analysis is not below the forming limit surface, and it is determined that cracking has occurred.

[0210] Furthermore, as a comparison object, the presence or absence of cracking is determined based on the maximum principal strain and the minimum principal strain without considering the bending deformation (conventional example). Then, the evaluation value of cracking determination is obtained for each part of the press-formed product 120 of the conventional example. The limit value of the maximum principal strain is calculated based on the minimum principal strain of each part of the press-formed product obtained by press-forming with a wrinkle pressing force of 10 tons, and the evaluation value of cracking determination of the conventional example is set to the value obtained by dividing the maximum principal strain obtained from the FEM analysis by the limit maximum principal strain.

[0211] exist Fig.11 The distribution diagram of the evaluation value of crack determination in Invention Example 1 is shown in FIG. Fig.12 , a distribution diagram of the evaluation values ​​of crack determination in Invention Example 2 is shown. Fig.13 2 is a distribution diagram of crack determination results in a conventional example. Fig.13 This is a distribution diagram showing that the area with an evaluation value of crack determination of 1 or more is an area where cracks have occurred, and the area with an evaluation value of less than 1 is an area where cracks have not occurred.

[0212] like Fig.11 and Fig.12 As shown in the figure, in Invention Examples 1 and 2, the evaluation values ​​at the portion α of the flat portion 121a of the punch bottom portion 121 of the press-molded product 120 are 1.04 and 1.06. Therefore, since the evaluation value is greater than 1, it is determined that cracking has occurred at the portion α. ​​In addition, since the evaluation value is less than 1 at other portions (such as portion β) of the press-molded product 120, it is determined that no cracking has occurred. In contrast, Fig.13 As shown, in the conventional example, since the evaluation value is less than 1 in the entire press-molded product 120, it is determined that no cracks have occurred, which is a determination result different from that of Invention Examples 1 and 2.

[0213] In the actual press molding of the press molded product 120, cracks occurred in the flat portion 121a of the punch bottom 121, and no cracks were observed in other portions. This shows that the presence or absence of cracks can be well determined in Inventive Examples 1 and 2.

[0214] As described above, it has been demonstrated that according to the press-molding crack determination method of the present invention, the presence or absence of cracking in a press-molded product can be determined with high accuracy by taking into account the influence of the bending deformation degree that is not considered in the conventional forming limit line.

[0215] Then, based on Fig.11 and Fig.12 The cracking parameters determined as shown are obtained by changing the wrinkle pressing force generated by the wrinkle pressing member from 10 tonf to 1 tonf in such a way that cracking does not occur in the manufacture of the pressed product 120 by stretching. Then, the FEM analysis of the press molding conditions after the wrinkle pressing force is changed is performed to calculate the cracking determination parameters for the pressed product 120. Fig.14 and Fig.15 , a distribution diagram is shown in which the evaluation value of crack determination obtained using the calculated crack determination parameter is displayed on the press-molded product 120. As in the above-mentioned Invention Example 1, Fig.14 The difference between the maximum principal strain on the outside of the bend and the maximum principal strain on the inside of the bend in the press-formed product 120 is divided by the plate thickness to obtain a value obtained as the bending deformation degree and the crack determination parameter is calculated. Fig.15 The maximum principal curvature of the press-molded product 120 is used as the bending deformation degree to calculate the crack determination parameter.

[0216] like Fig.14 and Fig.15 As shown in the figure, by changing the wrinkle pressing force to 1 tonf, the maximum value of the evaluation value at the flat portion 121a of the pressed product 120 is reduced to less than 1. The evaluation values ​​at other locations are also not greater than 1. Thus, it is shown that the occurrence of cracks in the pressed product 120 is suppressed. In addition, it was confirmed that when the pressed product 120 was actually manufactured under the press molding conditions in which the wrinkle pressing force was set to 1 tonf as described above, no cracks occurred in the pressed product 120.

[0217] Thus, the following is shown: according to the manufacturing method of the press-molded product of the present invention, by adjusting the press-molding conditions in a manner that suppresses the occurrence of cracks based on the result of determining whether cracks have occurred in consideration of the influence of the bending deformation degree, it is possible to suppress the occurrence of cracks and manufacture press-molded products.

[0218] Industrial Applicability

[0219] According to the present invention, a press-forming crack determination method, device, and program can be provided for determining the forming limit of a press-formed product based on the forming limit of a metal plate obtained by taking into account the influence of bending deformation. Furthermore, according to the present invention, a press-forming product manufacturing method can be provided for adjusting the press-forming conditions of the press-formed product so that no cracking occurs as determined by the press-forming crack determination method, and manufacturing the press-formed product using the adjusted press-forming conditions.

[0220] Description of Reference Numerals

[0221] 1 Press forming crack determination device

[0222] 10 Forming limit acquisition unit

[0223] 11. Bulging test result collection section

[0224] 13 Forming Limit Analysis Department

[0225] 15 Forming limit surface production department

[0226] 20 Pressed product cracking determination unit

[0227] 21 Press Forming FEM Analysis Department

[0228] 23 Press forming cracking judgment parameter calculation section

[0229] 25 Press forming cracking occurrence determination section

[0230] 100 test pieces

[0231] 101 Test piece

[0232] 103 Test piece

[0233] 103a Notch

[0234] 103b Central

[0235] 111 Forming mold

[0236] 113 Punch

[0237] 113a Front end

[0238] 115 upper mold

[0239] 117 Pleated Press

[0240] 120 Pressed products

[0241] 201 Forming mold

[0242] 203 Ball Punch

[0243] 203a Front end

[0244] 205 upper mold

[0245] 207 Pleated Press

[0246] 211 Forming mold

[0247] 213 Flat Punch

[0248] 213a Front end

[0249] 215 drive plate

Claims

1. A press-molding crack determination method for determining whether cracks have occurred in a press-molded product formed by press-molding a metal plate, wherein: The press-molding crack determination method comprises a forming limit acquisition step and a press-molded product crack determination step. In the forming limit obtaining step, the metal plate is formed at various bending deformations, and the forming limit of the metal plate is obtained by the relationship between the bending deformation of the formed metal plate and the maximum principal strain and the minimum principal strain of the metal plate formed at the bending deformation. In the press-molded product crack judgment step, the maximum principal strain and the minimum principal strain in the press-molded product, and the curvature in the maximum principal strain direction as the bending deformation degree in the press-molded product are obtained as crack judgment parameters in the press-molded product, and based on the obtained crack judgment parameters and the forming limit obtained in the forming limit acquisition step, it is determined whether cracks have occurred in the press-molded product.

2. The press forming cracking determination method according to claim 1, wherein: In the press-molded product crack determination step, the bending deformation degree in the press-molded product is calculated using the following formula: p=(e 1,outer -e 1,inner ) / t in, ρ: Bending deformation ε 1,outer : Maximum principal strain on the outside of the bend ε 1,inner : Maximum principal strain on the inner side of the bend t: plate thickness.

3. The press forming cracking determination method according to claim 1, wherein: In the press-molded product crack determination step, the bending deformation degree in the press-molded product is taken as a maximum principal curvature.

4. The press forming cracking determination method according to claim 1, wherein: The metal plate in the forming limit acquisition step is collected from the metal material used as the sheet material of the press-formed product, the presence or absence of cracking being determined in the press-formed product crack determination step.

5. A press-molding crack determination device for determining whether cracks have occurred in a press-molded product formed by press-molding a metal plate, wherein: The press-molding crack determination device comprises a forming limit acquisition unit and a press-molded product crack determination unit. The forming limit acquisition unit acquires the forming limit of the metal plate represented by the relationship between the bending deformation degree of the metal plate formed at various bending deformation degrees and the maximum principal strain and the minimum principal strain of the metal plate formed at the bending deformation degrees. The press-molded product crack judgment unit calculates the maximum principal strain and the minimum principal strain in the press-molded product and the curvature in the maximum principal strain direction as the bending deformation degree in the press-molded product as crack judgment parameters in the press-molded product, and judges whether cracks have occurred in the press-molded product based on the calculated crack judgment parameters and the forming limit obtained by the forming limit acquisition unit.

6. A press-forming crack determination procedure for determining whether cracks have occurred in a press-formed product formed by press-forming a metal plate, wherein: The press-molding crack determination program enables the computer to function as a forming limit acquisition unit and a press-molded product crack determination unit. The forming limit acquisition unit acquires the forming limit of the metal plate represented by the relationship between the bending deformation degree of the metal plate formed at various bending deformation degrees and the maximum principal strain and the minimum principal strain of the metal plate formed at the bending deformation degrees. The press-molded product crack judgment unit calculates the maximum principal strain and the minimum principal strain in the press-molded product and the curvature in the maximum principal strain direction as the bending deformation degree in the press-molded product as crack judgment parameters in the press-molded product, and judges whether cracks have occurred in the press-molded product based on the calculated crack judgment parameters and the forming limit obtained by the forming limit acquisition unit.

7. A method for producing a press-molded product, comprising determining the presence or absence of cracking in a press-molded product obtained by press-molding a metal plate using the press-molding cracking determination method according to any one of claims 1 to 3, and producing a press-molded product in which cracking during press-molding is suppressed based on the determination result, wherein: The method for manufacturing the pressed molded product comprises: a press-molding condition adjustment step, in which, when it is determined that cracking has occurred in the press-molded product cracking determination step of the press-molded product cracking determination method, the press-molding conditions of the press-molded product are adjusted and the press-molded product cracking determination step is repeated until it is determined that no cracking has occurred; and The press-molded product manufacturing step manufactures the press-molded product using the press-molding conditions adjusted in the press-molding condition adjustment step so that no cracks are determined to occur in the press-molded product crack determination step.

8. A method for producing a press-molded product, comprising determining the presence or absence of cracking in a press-molded product obtained by press-molding a metal plate using the press-molding cracking determination method according to claim 4, and producing a press-molded product in which cracking during press-molding is suppressed based on the determination result, wherein: The method for manufacturing the pressed molded product comprises: a press-molding condition adjustment step, in which, when it is determined that cracking has occurred in the press-molded product cracking determination step of the press-molded product cracking determination method, the press-molding conditions of the press-molded product are adjusted and the press-molded product cracking determination step is repeated until it is determined that no cracking has occurred; and The press-molded product manufacturing step manufactures the press-molded product using the press-molding conditions adjusted in the press-molding condition adjustment step so that no cracks are determined to occur in the press-molded product crack determination step.

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