Methods for judging gaps in press molding, equipment and programs, and methods for manufacturing pressed parts.

Abstract

Invention details;

Description

Press-molded crack detection method, device, and program, and method for manufacturing press-molded products

[0001] The present invention relates to a press-forming crack detection method, device, and program for determining whether or not a crack has occurred in a press-formed part obtained by press-forming a metal sheet, taking into account the influence of bending deformation. Furthermore, the present invention relates to a press-formed product manufacturing method for manufacturing a press-formed product by adjusting press-forming conditions to suppress crack occurrence based on the result of the crack occurrence determination.

[0002] Most of the metal sheets used as materials for automobile bodies are processed into automotive parts by press forming. The press formability of auto body parts varies depending on the shape of the auto body part, but it is also greatly influenced by the material properties, including the ductility of the metal sheet used, such as steel sheet. In addition, in recent years, the demand for weight reduction of automotive bodies has led to efforts to increase the strength of metal sheets, such as steel sheets, used for auto body parts. However, as metal sheets become stronger, their ductility decreases, making them more susceptible to cracking during press forming, and reducing their press formability.

[0003] Therefore, in order to avoid problems such as cracking during the mass production of auto body parts by press forming, it is becoming increasingly important to design dies based on advance predictions of press formability using computer-aided engineering (CAE).Furthermore, in order to accurately predict the occurrence of cracking during press forming, the technology to accurately determine the forming limits of metal sheets such as steel sheets is becoming increasingly important.

[0004] Forming limit diagrams (hereinafter referred to as "FLDs") are commonly used to determine the forming limits of metal sheets. FLDs are created by measuring the forming limits for various deformation modes in press forming (equibiaxial deformation, non-equibiaxial deformation, plane strain deformation, and uniaxial deformation) through laboratory-scale forming tests. To create a FLD, the width of the test specimen is varied to change the deformation ratio in the major and minor axes of the test specimen, and the strain in both the major and minor axes at the time of fracture is measured.

[0005] Generally, in press forming of metal sheets, the metal sheet undergoes uniform deformation followed by a process in which strain concentrates at specific locations. During this process, thickness reduction, known as necking, occurs at the strain concentration points, and fracture occurs after this thickness reduction. Because necking in press forming results in product defects, FLD, which is used to predict product defects in press-formed products, must be defined as the strain just before necking occurs. Furthermore, in high-strength steel sheets with tensile strengths exceeding 980 MPa, necking occurs at strains as low as 10%, followed by fracture immediately thereafter. Therefore, methods for accurately determining the forming limit have been proposed.

[0006] ISO12004 standardizes a method for determining the forming limit curve (hereinafter referred to as "FLC") of metal sheets (Non-Patent Document 1). This method involves first measuring the strain deformation in the vicinity of the fracture site of a test piece that has been bulge-formed until fracture, and then determining the maximum value of the approximation curve of the measured strain distribution as the forming limit strain.

[0007] However, the method specified in IS12004 cannot adequately approximate the strain at the fracture initiation point, and there are cases where it is not possible to determine the forming limit strain. Therefore, Non-Patent Document 2 proposes an improved method of ISO12004, in which the strain generated in the test piece during forming is continuously measured and the FLC is determined from the time change of the strain at the fracture initiation point.

[0008] ISO 12004-2:2008, Metallic materials-Sheet and strip-Determination of forming-limit curves, 2008.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).

[0009] ISO 12004 specifies two test methods for determining the forming limit line: the Nakajima test and the Marciniak test. The Nakajima test is a method for determining the forming limit by a bulge test in which a test piece 100 is bulged using a molding die 201 equipped with a hemispherical punch 203, an upper die 205, and a blank holder 207, as shown in FIG. 16(a). On the other hand, the Marciniak test is a method for determining the forming limit by a bulge test in which a test piece 100 is bulged using a molding die 201 equipped with a hemispherical punch 203, an upper die 205, and a blank holder 207, as shown in FIG. 16(b). The Marciniak test is a method for determining the forming limit by a bulge test in which a test piece 100 is bulged using a flat punch 213, an upper die 205, and a blank holder 207, as shown in FIG. 16(b). The Marciniak test determines the forming limit by a bulge test in which a driving seat 215 is sandwiched between the flat punch 213 and the test piece 100.

[0010] In the Nakajima method, the test piece 100 is stretch-formed in a state where it conforms to the shape of the tip 203a of the ball-head 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, in the Marciniak method, the test piece 100 is stretch-formed using a flat-head punch 213 whose tip 213a is flat. Therefore, the test piece 100 is not bent, and the obtained forming limit is not affected by the bending deformation of the test piece 100.

[0011] In general, when comparing the FLD calculated by the Nakajima method with the FLD calculated by the Marciniak method, the strain just before necking occurs is 1–2% larger in the Nakajima method, as shown in Figure 16(c). In press forming of high-strength materials with low ductility (e.g., high-tensile steel sheets), even a slight difference in strain can affect the occurrence of necking. Therefore, evaluation of forming limits has been performed by comparing the FLDs calculated by the Nakajima method and the Marciniak method using strain measured on actual press-formed parts or strain obtained from press forming analysis using CAE.

[0012] However, even under press forming conditions where the Nakajima method predicted no cracking based on the strain obtained from press forming analysis, cracking sometimes occurred in actual press-formed metal sheet products, especially in high-strength steel sheets with a strength of 980 MPa or more. Also, even under press forming conditions where the Marciniak method predicted cracking, cracking sometimes did not occur in the actual press-formed products. As such, there were many cases where the prediction of cracking based on FLD differed significantly from the actual occurrence of cracking in the press-formed products, which was problematic.

[0013] On the other hand, the method proposed in Non-Patent Document 2 can determine the forming limit with higher accuracy than ISO 12004. However, as with ISO 12004, it is not possible to determine the forming limit by taking into account the degree of bending deformation. Therefore, the result of predicting whether or not cracks will occur based on the forming limit diagram obtained by the method proposed in Non-Patent Document 2 sometimes does not match the actual occurrence of cracks in press-formed products.

[0014] As such, actual press forming involves a mixture of parts that are subject to bending deformation and parts that are not, and the degree of bending deformation also varies depending on the part. However, conventional methods for calculating FLD did not take the degree of bending deformation into account. As a result, conventional FLD methods were sometimes unable to properly determine whether or not cracks would occur in press-formed parts that have a mixture of parts that are subject to bending deformation and parts that are not. Furthermore, there was a need for a technology that could determine in advance whether or not cracks would occur in press-formed parts that have parts that are subject to bending deformation, and that could use that determination result to suppress cracking in press-formed parts.

[0015] The present invention has been made to solve the above-mentioned problems, and its object is to provide a press-forming crack detection method, an apparatus, and a program for determining the forming limit of a press-formed product based on the forming limit of a metal sheet obtained by taking into account the influence of bending deformation. Furthermore, another object of the present invention is to provide a method for manufacturing a press-formed product by adjusting the press-forming conditions of the press-formed product so that the press-formed product is determined to have no cracks by the press-forming crack detection method, and manufacturing the press-formed product under the adjusted press-forming conditions.

[0016] The press-forming crack determination method according to the present invention determines whether or not cracks have occurred in a press-formed product obtained by press-forming a metal plate, and includes a forming limit acquisition process and a press-formed product crack determination process. The forming limit acquisition process forms the metal plate at various bending deformation degrees and acquires the forming limit of the metal plate expressed by the relationship between the bending deformation degree of the formed metal plate and the maximum principal strain and minimum principal strain of the metal plate formed at the bending deformation degree. The press-formed product crack determination process acquires the maximum principal strain and minimum principal strain in the press-formed product and the curvature in the maximum principal strain direction as the bending deformation degree of the press-formed product as crack determination parameters for the press-formed product, and determines whether or not cracks have occurred in the press-formed product based on the acquired crack determination parameters and the forming limit acquired in the forming limit acquisition process.

[0017] In the process of determining cracks in the press-formed product, the bending deformation degree of the press-formed product may be calculated by the following formula: ρ=(ε 1,outer -ε 1,inner ) / t where, ρ: bending deformation degree ε 1,outer : Maximum principal strain on the outer side of bending ε 1,inner : Maximum principal strain on the inner side of bending t: Plate thickness

[0018] In the process of determining cracks in the press-formed product, the bending deformation degree in the press-formed product may be set as a maximum principal curvature.

[0019] The metal plate in the forming limit acquisition process may be taken from a metal material used as a blank for the press-molded product for determining whether or not cracks have occurred in the press-molded product crack determination process.

[0020] The press-molding crack detection device of the present invention determines whether or not cracks have occurred in a press-molded product obtained by press-molding a metal plate, and comprises a forming limit acquisition unit and a press-molded product crack detection unit. The forming limit acquisition unit acquires the forming limit of the metal plate, which is expressed by the relationship between the bending deformation degree of the metal plate formed at various bending deformation degrees and the maximum principal strain and minimum principal strain of the metal plate formed at those bending deformation degrees. The press-molded product crack detection unit calculates the maximum principal strain and 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 detection parameters for the press-molded product, and determines whether or not cracks have occurred in the press-molded product based on the calculated crack detection parameters and the forming limit acquired by the forming limit acquisition unit.

[0021] The press-forming crack determination program of the present invention determines whether or not cracks have occurred in a press-formed product obtained by press-forming a metal plate, and causes a computer 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, which is expressed by the relationship between the bending deformation degrees of the metal plate formed at various bending deformation degrees and the maximum principal strain and minimum principal strain of the metal plate formed at those bending deformation degrees. The press-formed product crack determination unit calculates the maximum principal strain and 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 or not cracks have occurred in the press-formed product based on the calculated crack determination parameters and the forming limit acquired by the forming limit acquisition unit.

[0022] The manufacturing method of the press-formed product of the present invention determines whether or not cracks have occurred in a press-formed product obtained by press-forming a metal plate using the press-forming crack determination method of the present invention, and manufactures a press-formed product in which cracks have been suppressed during press forming based on the results of the determination.The method includes a press-forming condition adjustment process in which, if it is determined that a crack has occurred in the press-formed product crack determination process of the press-formed crack determination method, the press-formed product conditions are adjusted and the press-formed product crack determination process is repeated until it is determined that no cracks have occurred, and a press-formed product manufacturing process in which the press-formed product is manufactured under press-forming conditions adjusted in the press-formed condition adjustment process so that it is determined that no cracks have occurred in the press-formed product crack determination process.

[0023] In the present invention, a metal sheet is formed at various bending deformation degrees, and a forming limit of the metal sheet is obtained, which is expressed by the relationship between the bending deformation degree of the formed metal sheet and the maximum principal strain and minimum principal strain of the metal sheet formed at the bending deformation degree. Furthermore, the maximum principal strain and minimum principal strain of a press-formed product obtained by press-forming the metal sheet, and the curvature in the direction of the maximum principal strain as the bending deformation degree of the press-formed product are obtained as crack determination parameters for the press-formed product. Then, based on the obtained crack determination parameters and the forming limit, it is possible to determine whether or not cracks will occur in the press-formed product, taking into account the influence of the bending deformation degree.

[0024] Furthermore, in the present invention, the presence or absence of cracks in a press-formed product obtained by press-forming a metal plate is determined taking into account the influence of the bending deformation degree, and based on the determination results, press-forming conditions such as changing the shape of the metal plate or modifying the mold are adjusted to suppress the occurrence of cracks. This makes it possible to significantly shorten the time required to determine press-forming conditions that can suppress the occurrence of cracks in actual press-forming. Furthermore, in the present invention, press-forming under the adjusted press-forming conditions makes it possible to produce a press-formed product in which the occurrence of cracks is suppressed.

[0025] FIG. 1 is a flow chart showing the process flow of a press-forming crack detection method according to a first embodiment of the present invention. FIG. 2 is a diagram showing an example of a forming die used for stretch-forming a metal sheet test piece in the press-forming crack detection method according to a first embodiment of the present invention. FIG. 3 is a diagram showing the shape of a punch used for stretch-forming in the first embodiment of the present invention. FIG. 4 is a diagram showing the shape of a metal sheet test piece used for stretch-forming in the first embodiment and examples of the present invention. FIG. 5 is a graph showing the forming limit strain determined for each bending deformation degree in the first embodiment of the present invention. FIG. 6 is a graph showing the results of plotting the forming limit strain determined for each bending deformation degree and creating a forming limit surface (boundary surface of forming limit) represented by the relationship between the maximum principal strain, the minimum principal strain, and the bending deformation degree based on the plot in the first embodiment of the present invention. FIG. 7 is a block diagram showing the configuration of a press-forming crack detection device according to a first embodiment of the present invention. FIG. 8 is a flow chart showing the process flow of a manufacturing method for a press-formed product according to a second embodiment of the present invention. FIG. 9 is a flow chart showing specific processes in the manufacturing method for a press-formed product according to the second embodiment of the present invention. FIG. 10 is a perspective view showing the shape of a press-formed product to be formed in the examples. FIG. 11 is a contour view of a crack evaluation value calculated based on the forming limit of a press-formed product determined by the method of the present invention in the examples (Example 1). FIG. 12 is a contour view of a crack evaluation value calculated based on the forming limit of a press-formed product determined by the method of the present invention in the examples (Example 2). FIG. 13 is a contour view of a crack evaluation value calculated based on the forming limit strain of a press-formed product determined using a conventional forming limit line in the examples. FIG. 14 is a contour view of a crack evaluation value of a press-formed product manufactured under press-forming conditions adjusted to suppress cracking based on the crack determination result of Example 1 in the press-formed product. FIG. 15 is a contour view of a crack evaluation value of a press-formed product manufactured under press-forming conditions adjusted to suppress cracking based on the crack determination result of Example 2 in the examples.FIG. 16 shows examples of forming dies used in conventional forming limit tests and forming limit diagrams ((a) Nakajima method, (b) Marciniak method, and (c) forming limit diagrams obtained by both the Nakajima method and the Marciniak method).

[0026] [Embodiment 1] <Press-forming crack determination method> A press-forming crack determination method according to embodiment 1 of the present invention determines whether or not a crack has occurred in a press-formed product obtained by press-forming a metal plate, and includes a forming limit acquisition process P1 and a press-formed product crack determination process P3, as shown in FIG.

[0027] <<Forming Limit Acquisition Process>> The forming limit acquisition process P1 forms a metal plate at various bending deformation degrees, and acquires the forming limit of the metal plate expressed by the relationship between the bending deformation degree of the formed metal plate and the maximum principal strain and minimum principal strain of the metal plate formed at the bending deformation degree.

[0028] In the first embodiment, the forming limit acquisition process P1 stretches a metal plate test piece 100 at various bending deformation degrees using a plurality of punches 113 having tip portions 113a with different curvatures, as illustrated in Fig. 2. Then, the forming limit acquisition process P1 acquires the forming limit of the metal plate, which is expressed by the relationship between the bending deformation degree of the stretched test piece 100 and the maximum principal strain and the minimum principal strain of the stretched test piece 100 at that bending deformation degree.

[0029] As shown in Fig. 3, the punch 113 has a tip 113a that is spherical with a curvature radius R, i.e., a curvature ρ (= 1 / R) that is greater than 0. However, in the present invention, the punch 113 includes a flat-head punch 213 that has a flat tip 213a (curvature radius R = ∞, curvature ρ = 0) as shown in Fig. 16(b). As a result, the punch 113 according to the present invention has a tip 113a with a curvature ρ that is greater than or equal to 0.

[0030] Test pieces 101 and 103, each having the shape shown in FIG. 4, were used to determine the forming limit for each deformation mode (equibiaxial deformation, non-uniform biaxial deformation, plane strain deformation, and uniaxial deformation) in press forming. Test piece 101, as shown in FIG. 4(a), is disc-shaped and is used to determine the forming limit for equibiaxial deformation. On the other hand, test piece 103, as shown in FIGS. 4(b) and 4(c), has arc-shaped notches 103a formed at diametrically opposed positions on the periphery of the disc. The width W of the narrowest part of the central portion 103b of test piece 103 was varied. As the width W was narrowed, the deformation changed from equibiaxial deformation to non-uniform biaxial deformation, plane strain deformation, and gradually approached uniaxial tension, thereby determining the forming limit for each deformation mode. Furthermore, the test piece 100 is a metal plate having markings (predetermined grids or strain measurement patterns) on the surface thereof.

[0031] The bending deformation degree, which is an index of the forming limit, represents the degree of bending deformation in the bulge-molded portion of the test piece 100, and the actual measured value of the radius of curvature or curvature of the bulge-molded portion can be used as the bending deformation degree. However, in the present embodiment 1, the bending deformation degree may be represented by the curvature of the tip 113a of the punch 113 used in bulge-molding the test piece 100, without actually measuring the radius of curvature or curvature of the bulge-molded portion.

[0032] Furthermore, the method for determining the maximum principal strain and minimum principal strain, which are indicators of the forming limit, is not particularly limited as long as the forming limit is determined using the same criteria for strains measured using a plurality of punches 113 with different curvatures at the tip 113 a. For example, the maximum principal strain and minimum principal strain at the time of fracture may be determined from the shape of a marking made on the surface of the test piece 100 in the vicinity of the fracture occurrence portion of the test piece 100 that has been stretch-formed until fracture occurs, or they may be determined by the method described below.

[0033] A specific example of the forming limit acquisition process P1 according to the first embodiment includes a forming test step S11, a forming limit analysis step S13, and a forming limit surface creation step S15, as shown in FIG. 1 .

[0034] (Forming Test Step) In the forming test step S11, a test piece 100 as exemplified in FIG. 4 is stretch-formed at various bending deformation degrees, the strain generated in the stretch-formed test piece 100 is measured, and a strain database is constructed.

[0035] In the first embodiment, four types of punches 113 were prepared, each having a tip 113a with a radius of curvature of R25 mm, R50 mm, R100 mm, and R=∞ (corresponding to the flat-head punch 213 shown in FIG. 16(b)), as shown in FIG. 3. The curvatures (unit: 1 / mm) of the tip 113a were 0.04, 0.02, 0.01, and 0.00, respectively.

[0036] Next, a test piece 100 made of a high-strength steel plate having a tensile strength of 1470 MPa (MPa-grade) and a thickness of 1.4 mm was used as a metal plate, and the shape of the test piece 100 was determined. In this embodiment, as shown in FIG. 4, the shape of the test piece 100 was a circular test piece 101 having a diameter of 180 mm and a test piece 103 having a shape cut out in the width direction based on the circular test piece 101. Furthermore, five types of shapes were prepared for the test piece 103, with the width W of the central portion 103b ranging from 25 mm to 160 mm. As an example, FIG. 4(b) shows a test piece 103 having a width W of 60 mm, and FIG. 4(c) shows a test piece 103 having a width W of 80 mm. A lattice pattern (grid) for strain measurement was transferred onto the surface of the test piece 100 whose shape was determined in this manner.

[0037] Next, as shown in Figure 2, a forming die 111 equipped with a punch 113, an upper die 115, and a blank holder 117 was set in a hydraulic deep drawing testing machine, and bulge-forming of the test piece 100 was performed. In this embodiment, in order to prevent the test piece 100 from flowing in, a blank holder pressure of 50 tons was applied by the blank holder 117, and the punch speed of the punch 113 was set to 5 mm / min. Then, during bulge-forming, the surface of the test piece 100 was photographed using an image analysis camera, which is a photographing device (not shown) installed above the forming die 111.

[0038] Next, images of the surface of the test piece 100 taken during the stretch-forming process were analyzed to measure the amount of strain (maximum principal strain and minimum principal strain) occurring on the surface of the test piece 100. Digital image correlation (hereinafter referred to as "DIC") can be suitably used as a method for measuring the amount of strain. In DIC, the surface of the test piece 100 during the stretch-forming process is imaged at predetermined time intervals. The images taken at each time step are then analyzed to measure the maximum principal strain and minimum principal strain as the in-plane strain in two directions occurring on the test piece 100 based on the deformation of the lattice attached to the surface of the test piece 100. The strain amounts measured at each time step of the stretch-forming process in this way were stored in a strain database in chronological order.

[0039] Next, it was visually determined whether or not a fracture occurred in the test piece 100 during the bulge-forming process. If it was determined that no fracture occurred, the bulge-forming was continued, and the surface of the test piece 100 was photographed, the amount of strain was measured by image analysis, and the results were stored in a strain database. In this way, in the first embodiment, the surface of the test piece 100 was photographed at predetermined time intervals (e.g., once per second) from the start of forming until fracture occurred, and the amount of strain was measured for each photographed image, and this was continued until it was determined that fracture occurred.

[0040] If it was determined that fracture had occurred, the bulging process, measurement of the strain amount, and storage of the strain amount in the strain database were repeated for a test piece 100 of a different shape.

[0041] The above-mentioned work was performed for each punch 113 having a different shape of the test piece 100 and a different curvature of the tip 113a, and a strain database was constructed in which the strain from the start of forming to fracture was recorded in chronological order for each degree of bending deformation.

[0042] (Forming limit analysis step) In the forming limit analysis step S13, based on the strain database constructed in the forming test step S11, a forming limit strain represented by the maximum principal strain and the minimum principal strain at the fracture occurrence portion of the test piece 100 is calculated for each bending deformation degree of the test piece 100.

[0043] In the present embodiment 1, the forming limit strain was determined as follows. First, from the strain database constructed in the forming test step S11, the maximum principal strain and the minimum principal strain near the fracture occurrence portion of the test piece 100 from the start of forming to fracture were extracted for each bending deformation degree at predetermined time steps. Then, time series data for each of the maximum principal strain and the minimum principal strain extracted for each bending deformation degree was created.

[0044] Next, a critical point was determined for the time series data of the maximum principal strain and the minimum principal strain created for each bending deformation, at which the test piece 100 transitioned from uniform deformation to inhomogeneous deformation. Then, it was assumed that necking occurred in the test piece 100 at this critical point, and the maximum principal strain and the minimum principal strain at that point were determined as the forming limit strain for that bending deformation. Thus, by determining the forming limit strain for each bending deformation, the forming limit expressed by the relationship between the bending deformation, the maximum principal strain, and the minimum principal strain was determined.

[0045] Table 1 shows the values ​​of the forming limit strain obtained for each degree of bending deformation in this embodiment 1, where the degree of bending deformation is defined as the curvature of the tip 113a of the punch 113 used in bulging the test piece 100. Furthermore, Fig. 5 shows a graph in which the forming limit strain is plotted on a two-dimensional coordinate plane with the minimum principal strain and the maximum principal strain as the coordinate axes.

[0046]

[0047] The forming limit analysis step according to the present invention may determine the maximum principal strain and the minimum principal strain at the time when fracture occurs in the test piece 100 as the forming limit strain.

[0048] The forming limit strain is measured in an extension test using a plurality of punches 113 with different curvatures at the tip end 113a, and the method of determining the strain does not matter as long as it is determined using the same criteria.

[0049] (Forming limit surface creation step) In the forming limit surface creation step S15, the forming limit strains determined for each bending deformation degree in the forming limit analysis step S13 are plotted in a three-dimensional coordinate space having the maximum principal strain, the minimum principal strain, and the bending deformation degree as three axes. Furthermore, in the forming limit surface creation step S15, a forming limit surface is created which is expressed by the relationship between the maximum principal strain, the minimum principal strain, and the bending deformation degree.

[0050] The forming limit surface can be created in the forming limit surface creating step S15 by, for example, the following method.

[0051] In the first method, first, two plots of forming limit strains with the same bending deformation are extracted. Next, from among a group of forming limit strains with different bending deformations from the two forming limit strains, the plot of the forming limit strain with the smallest distance to the line segment connecting the plots of the two forming limit strains is selected. Next, a triangular plane is formed using the two extracted plots of forming limit strains and the selected plot of forming limit strain. This triangular plane creation is performed for all plots of the group of forming limit strains. The polygonal facet created by combining the created triangular planes is then used as the forming limit surface.

[0052] In the second method, a forming limit plane (boundary flat surface forming limit) or a forming limit curved surface (boundary curved surface of forming limit) is first assumed in a three-dimensional coordinate space, and the perpendicular distance between this assumed forming limit plane or forming limit curved surface and the plot of the forming limit strain is calculated. Then, the forming limit plane or forming limit curved surface is determined so that the square sum of the calculated perpendicular distances is minimized.

[0053] In the third method, the forming limit plane or forming limit curved surface is determined so as to minimize the sum of squares of the weighted perpendicular distance between the assumed forming limit plane or forming limit curved surface and the plot of the forming limit strain in the second method. For example, the weighting can be increased for forming limits at bending deformations at which cracks are likely to occur in actual press forming. This can reduce the error between the determined forming limit plane or forming limit curved surface and the plot of the forming limit strain at bending deformations at which cracks are likely to occur in actual press forming.

[0054] The second and third methods may be a combination of a plurality of forming limit planes and / or forming limit surfaces. For example, forming limit planes or forming limit surfaces may be assumed for the negative and positive regions of the minimum principal strain, and the forming limit planes or forming limit surfaces may be determined so that the sum of squares of the perpendicular distances to the plot of the forming limit strain for each region is minimized. The same applies to the case where a forming limit surface is created so that the sum of squares of the weighted perpendicular distances to the plot of the forming limit strain is minimized.

[0055] As a method other than the above, three adjacent points may be selected from a plot of the forming limit strain in a three-dimensional coordinate space, and a triangular plane may be generated by connecting the three selected points with a straight line, and the polygonal surface consisting of the multiple triangular planes thus generated may be used as the forming limit surface.

[0056] Alternatively, a forming limit line may be created based on the forming limit strain for each bending deformation degree, and a forming limit surface may be created that includes the forming limit lines created for all bending deformation degrees.

[0057] In this embodiment, the forming limit surface was created based on the second method. First, the forming limit planes expressed by the following formula (1) were assumed for the negative and positive minimum principal strain regions.

[0058]

[0059] Next, the coefficients in formula (1) were determined for forming limit plane A in the negative region of the minimum principal strain and forming limit plane B in the positive region so that the sum of 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 was minimized. In this way, the forming limit plane A and forming limit plane B whose coefficients in formula (1) were determined were created as forming limit surfaces.

[0060] Table 2 shows the coefficients in formula (1) for the determined forming limit plane A and forming limit plane B. Furthermore, Fig. 6 shows the forming limit plane A and forming limit plane B created based on the determined coefficients, and a plot of the forming limit strain for each measured bending deformation degree.

[0061]

[0062] <Press-formed product crack determination process> The press-formed product crack determination process P3 acquires the maximum principal strain and 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. Furthermore, the press-formed product crack determination process P3 determines whether or not a crack has occurred in the press-formed product based on the acquired crack determination parameters and the forming limit acquired in the forming limit acquisition process P1.

[0063] A specific example of the press-molded product crack determination process P3 includes a press-molded FEM analysis (finite element analysis) step S31, a press-molded crack determination parameter calculation step S33, and a press-molded crack occurrence determination step S35, as shown in FIG. 1.

[0064] (Press-forming FEM analysis step) In the press-forming FEM analysis step S31, an FEM analysis is performed on the process of press-forming a metal plate into a press-formed product.

[0065] By the FEM analysis in the press forming FEM analysis step S31, the changes in strain, stress, plate thickness, etc. that occur in the press-formed product due to press forming of the metal plate can be obtained for each element and node used in the FEM analysis.

[0066] (Press-forming crack determination parameter calculation step) In the press-forming crack determination parameter calculation step S33, the maximum principal strain and minimum principal strain in the press-formed product and the bending deformation degree in the press-formed product are calculated as crack determination parameters in the press-formed product. The maximum principal strain and minimum principal strain and the bending deformation degree in the press-formed product are calculated based on the FEM analysis results in the press-forming FEM analysis step S31.

[0067] The curvature in the maximum principal strain direction, which is the bending deformation of a press-formed product, can be determined by identifying the maximum principal strain direction of the press-formed product and calculating the curvature from three points in the maximum principal strain direction. In this method, the maximum principal strain and the minimum principal strain can be determined from the results of FEM analysis of the press-formed product, as in the past. However, for press-formed products with complex shapes, it is time-consuming to identify the maximum principal strain direction. Therefore, for example, it is preferable to simply calculate the bending deformation of a press-formed product using one of the methods described below.

[0068] The first method for calculating the degree of bending deformation is to calculate it from the maximum principal strain on the front and back surfaces of a press-formed product. The normal deformation state of a press-formed product is not just bending deformation, but a combination of other deformation states. In other words, the deformation state of a press-formed product can be broadly described as a stretch bending state. In tensile bending deformation, the strain in the area subjected to tensile deformation acts uniformly in the thickness direction, while the strain in the area subjected to bending deformation has a distribution in the thickness direction. Therefore, the degree of bending deformation in a press-formed product can be expressed using the difference in strain between the outside and inside of the area subjected to bending deformation.

[0069] Generally, the strain ε at a portion subjected to bending deformation is expressed by the following equation:

[0070] ε=±t / 2r

[0071] Where t is the plate thickness and 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. And because the radius of curvature r is the reciprocal of the curvature ρ, the following equation can be derived from the above equation.

[0072] ε 1,outer -ε 1,inner =t / r ρ=(ε 1,outer -ε 1,inner ) / t where, ρ: bending deformation degree ε 1,outer : Maximum principal strain on the outer side of bending ε 1,inner : Maximum principal strain on the inner side of bending t: Plate thickness

[0073] ε 1,outer and ε 1,inner can be calculated by FEM analysis of the press-molded product.

[0074] The second method for calculating the bending deformation is to calculate the maximum principal curvature, i.e., the maximum curvature among the curvatures defined when a cross section of the curved surface is created on a plane including the normal direction at a point on the curved surface. Because the curvature in the maximum principal strain direction and the maximum curvature, which is the largest among the geometric curvatures defined on the curved surface, generally coincide with each other, this method may be used to simply calculate the bending deformation.

[0075] (Press-forming crack occurrence determination step) In press-forming crack occurrence determination step S35, it is determined whether or not a crack has occurred in the press-formed product. The presence or absence of a crack in the press-formed product is determined based on the crack determination parameter calculated in press-forming crack determination parameter calculation step S33 and the forming limit acquired in the forming limit acquisition process P1.

[0076] As described above, when the forming limit surface is created in the forming limit acquisition process P1, the specific procedure for determining whether or not cracks have occurred in the press-formed product in the press-forming crack occurrence determination step S35 will be described below.

[0077] First, the crack determination parameters for the press-molded product calculated in the press-molding crack determination parameter calculation step S33 are plotted on the three-dimensional coordinate system on which the forming limit surface created in the forming limit surface creation step S15 of the forming limit acquisition process P1 is drawn.

[0078] Then, if the plot of the crack determination parameter is not located below the forming limit surface, i.e., if the maximum principal strain of the crack determination parameter is equal to or greater than the minimum principal strain of the crack determination parameter and the maximum principal strain of the forming limit surface corresponding to the bending deformation degree, it is determined that a crack has occurred.

[0079] In contrast, if the plot of the crack determination parameter is located below the forming limit surface, i.e., if the maximum principal strain of the crack determination parameter is less than the minimum principal strain of the crack determination parameter and the maximum principal strain of the forming limit surface corresponding to the bending deformation degree, it is determined that no cracks have occurred.

[0080] As described above, in the press-forming crack detection method according to the first embodiment, a metal sheet test piece 100 is stretch-formed at various bending deformations, and the forming limit of the metal sheet, which is expressed as the relationship between the bending deformation and the maximum principal strain and minimum principal strain of the test piece 100, is obtained. Furthermore, for a press-formed product obtained by press-forming the metal sheet, the maximum principal strain and minimum principal strain, and the curvature in the maximum principal strain direction as the bending deformation, are calculated as crack detection parameters. Then, the presence or absence of cracks in the press-formed product is determined based on the calculated crack detection parameters and the obtained forming limit. This makes it possible to determine the presence or absence of cracks in the press-formed product by taking into account the influence of the bending deformation, which was not considered in conventional forming limit curves.

[0081] The metal plate in the forming limit acquisition process P1 may be sampled from a metal material used as a blank for a press-formed product for which crack occurrence is determined in the press-formed product crack determination process P3. Variations in the metal material used as a blank for a press-formed product (e.g., if the metal material is a steel plate, changes in the metal structure due to variations in manufacturing conditions such as hot-rolling conditions and annealing conditions) affect the occurrence of cracks in the press-formed product. Therefore, for example, before the start of mass production of press-formed products, a metal plate to be used in the forming test step S11 may be sampled from the metal material actually used as a blank for the press-formed product to obtain the forming limit and determine the occurrence of cracks in the press-formed product. This can further improve the accuracy of advance prediction of crack occurrence during press forming during mass production.

[0082] In the above description, the forming test step S11 involves stretch-forming a metal plate test piece 100 at various bending deformation degrees, as illustrated in FIG. 2 . However, the forming test step is not limited to stretch-forming a metal plate test piece. For example, the forming test step may involve press-forming a press-formed product 120 simulating an automobile part, as shown in FIG. 10 , using a metal plate (blank) onto which a scribed circle or grid has been transferred. In this case, in the forming test step, a strain database can be constructed by chronologically recording the strains occurring in the press-formed product from the start of forming to fracture. The strains occurring in the press-formed product (maximum principal strain, minimum principal strain) can be obtained from the dimensional changes of the scribed circle or grid in the press-formed product. Furthermore, the bending deformation degree can be obtained by measuring the curvature in the maximum principal strain direction of an actual press-formed product.

[0083] In the press-forming crack detection method according to the first embodiment, the press-forming crack occurrence determination step S35 performs a press-forming analysis of the press-formed product, and calculates the maximum principal strain, minimum principal strain, and bending deformation (curvature in the maximum principal strain direction) as crack detection parameters through the press-forming analysis.

[0084] However, in the present invention, the step of determining whether a crack has occurred in a press-formed product may involve press-forming a press-formed product using a blank to which a scribed circle or a grid has been transferred. The maximum principal strain and the minimum principal strain can then be obtained from the dimensional changes of the scribed circle or the grid in the press-formed product. Furthermore, the degree of bending deformation can be obtained by measuring the curvature in the maximum principal strain direction of the actual press-formed product.

[0085] Examples of simple methods for obtaining the curvature in the maximum principal strain direction of an actual press-formed product include measurement using an R gauge (radius gauge) and calculation from the coordinates of three points in the maximum principal strain direction obtained by measuring the surface profile of the press-formed product.

[0086] Furthermore, the determination of the presence or absence of cracks in the press-molding crack occurrence determination step S35 is performed for each portion of the press-molded product for which the crack determination parameters were calculated in the press-molding crack determination parameter calculation step S33. However, the crack determination parameters may be calculated only for portions of the press-molded product where cracks are likely to occur, and the presence or absence of cracks may be determined for those portions. In this case, the crack determination parameters may be calculated for each element used in the press-molding analysis, and the presence or absence of cracks may be determined for each element of the portions of the press-molded product where cracks are likely to occur.

[0087] <Press-molded crack detection device> The above description has been directed to a method for detecting the presence or absence of cracks in a press-molded product. However, the present invention can also be configured as a device for detecting the presence or absence of cracks.

[0088] The press-molding crack detection device according to the first embodiment of the present invention determines whether or not a crack has occurred in a press-formed product obtained by press-forming a metal plate. As shown in FIG. 7 as an example, the press-molding crack detection device 1 includes a forming limit acquisition unit 10 and a press-formed product crack detection unit 20. The press-molding crack detection device 1 may be configured by a CPU (Central Processing Unit) of a computer (e.g., a PC (Personal Computer)). In this case, each of the above units functions when the CPU of the computer executes a predetermined program.

[0089] <Forming limit acquisition unit> The forming limit acquisition unit 10 acquires the forming limit of a metal plate expressed by the relationship between the bending deformation degree of a metal plate formed at various bending deformation degrees and the maximum principal strain and minimum principal strain of the metal plate formed at the bending deformation degree.

[0090] In the first embodiment, the forming limit acquisition unit 10 includes a bulge forming test result import unit 11, a forming limit analysis unit 13, and a forming limit surface creation unit 15, as shown in FIG.

[0091] (Stretch forming test result import unit) The stretch forming test result import unit 11 imports the stretch forming test results in which the strain generated in the metal plate test piece 100 stretched at various bending deformation degrees is measured for each bending deformation degree as shown in Figure 2 described above.

[0092] An example of a specific configuration of the bulge forming test result import unit 11 includes a forming mold, a photographing device, a strain measuring device, and a strain database creation device (not shown).

[0093] The forming mold is used to bulge-form the test piece 100 with various degrees of bending deformation, and an example of such a forming mold 111 includes a plurality of punches 113 with different curvatures at the tip portions 113a, an upper die 115, and a blank holder 117, as shown in Figure 2.

[0094] The photographing device photographs the deformation of the lattice or strain analysis pattern on the surface of the test piece 100 during the process of bulging the test piece 100, to which the lattice or strain analysis pattern is attached, using the molding die 111. An example of the photographing device is one that is configured with two cameras and that takes stereo photography of the surface of the test piece 100.

[0095] The strain measuring device analyzes an image of the surface of the test piece 100 taken by the photographing device, and measures the strain occurring in the test piece 100 for each degree of bending deformation of the stretch-formed test piece 100. An example of a strain measuring device is one that uses a digital image correlation method to measure the maximum principal strain and the minimum principal strain as the strain in two in-plane directions occurring in the test piece 100 during the stretch-forming process.

[0096] The strain database construction device constructs a strain database in which the strains measured by the strain measuring device can be stored and extracted in chronological order from the start of forming to fracture for each bending deformation degree of the stretch-formed test piece 100.

[0097] (Forming Limit Analysis Unit) The forming limit analysis unit 13 determines a forming limit strain represented by a maximum principal strain and a minimum principal strain at a fracture occurrence portion of the test piece 100 for each bending deformation degree of the test piece 100 based on the bulge forming test results imported by the bulge forming test result import unit 11. The forming limit analysis unit 13 can be exemplified by one having a strain distribution extraction device and a forming limit acquisition device (not shown).

[0098] The strain distribution extraction device extracts the strain distribution in the vicinity of the fracture occurrence portion of the test piece 100 for each bending deformation degree of the bulged test piece 100 from the strain database constructed by the strain database construction device.

[0099] The forming limit acquisition device obtains the forming limit strain, which is represented by the maximum principal strain and the minimum principal strain at the fracture occurrence portion of the test piece 100, for each bending deformation degree of the test piece 100 from the strain distribution extracted by the strain distribution extraction device.

[0100] The forming limit analysis unit 13 can obtain the forming limit strain in the same manner as in the forming limit analysis step according to the first embodiment described above.

[0101] (Forming limit surface creation unit) The forming limit surface creation unit 15 plots the forming limit strains determined for each bending deformation degree by the forming limit analysis unit 13 in a three-dimensional coordinate space having the maximum principal strain, the minimum principal strain, and the bending deformation degree as three axes. Furthermore, the forming limit surface creation unit 15 creates a forming limit surface expressed by the relationship between the maximum principal strain, the minimum principal strain, and the bending deformation degree.

[0102] The creation of the forming limit surface by the forming limit surface creation unit 15 can be by any of the methods described in the forming limit surface creation step S15 of the press forming crack determination method according to the first embodiment described above.

[0103] <Press-formed product crack determination unit> The press-formed product crack determination unit 20 performs press forming analysis of the press-formed product and acquires the maximum principal strain and 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. Then, the press-formed product crack determination unit 20 determines whether or not a crack has occurred in the press-formed product based on the acquired crack determination parameters and the forming limit acquired by the forming limit acquisition unit 10.

[0104] A specific example of the press-molded product crack determination unit 20 is one having a press-molded FEM analysis unit 21, a press-molded crack determination parameter calculation unit 23, and a press-molded crack occurrence determination unit 25, as shown in Figure 7.

[0105] (Press-forming FEM analysis unit) The press-forming FEM analysis unit 21 performs FEM analysis of the process of press-forming a metal plate into a press-formed product.

[0106] By the FEM analysis by the press forming FEM analysis unit 21, changes in strain, stress, plate thickness, etc. that occur in a press-formed product due to press forming of a metal plate can be found for each element and node used in the FEM analysis.

[0107] (Press-forming crack determination parameter calculation unit) The press-forming crack determination parameter calculation unit 23 calculates the maximum principal strain and minimum principal strain in the press-formed product, and the bending deformation degree, as crack determination parameters in the press-formed product.

[0108] The maximum principal strain, minimum principal strain, and bending deformation degree of the press-formed product are calculated based on the FEM analysis results obtained by the press-forming FEM analysis unit 21. The bending deformation degree may be calculated in the same manner as in the press-forming crack determination parameter calculation step S33 of the press-forming crack determination method according to the first embodiment described above.

[0109] (Press-molding crack occurrence determination unit) The press-molding crack occurrence determination unit 25 determines whether or not a crack has occurred in a 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.

[0110] The determination of whether or not cracks have occurred 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 determination method according to the first embodiment described above.

[0111] <Press-forming crack determination program> The first embodiment of the present invention can be configured as a press-forming crack determination program that causes a press-forming crack determination device configured by a computer to function. That is, the press-forming crack determination program according to the first embodiment determines whether or not a crack has occurred in a press-formed product obtained by press-forming a metal plate. The press-forming crack determination program causes the computer to function as a forming limit acquisition unit 10 and a press-formed product crack determination unit 20, as shown as an example in FIG. 7.

[0112] The press-forming crack determination program according to the first embodiment causes the forming limit acquisition unit 10 to function as a stretch-forming test result import unit 11, a forming limit analysis unit 13, and a forming limit surface creation unit 15. In this case, the stretch-forming test result import unit 11 imports, as an example, a strain database in a strain database creation device that is constructed by a stretch-forming test in which a metal plate test piece is stretch-formed at various bending deformation degrees.

[0113] Furthermore, the press molding crack judgment program according to this embodiment 1 causes the press molding product crack judgment unit 20 to function as a press molding FEM analysis unit 21, a press molding crack judgment parameter calculation unit 23, and a press molding crack occurrence judgment unit 25.

[0114] As described above, the press-forming crack detection device and press-forming crack detection program according to the first embodiment acquire a forming limit for a metal sheet, which is expressed by the relationship between the maximum principal strain, the minimum principal strain, and the bending deformation. Furthermore, for a press-formed product obtained by press-forming a metal sheet, the maximum principal strain, the minimum principal strain, and the curvature in the direction of the maximum principal strain as the bending deformation are calculated as crack detection parameters for the press-formed product. Then, based on the calculated crack detection parameters and the acquired forming limit, it is possible to determine whether or not a crack has occurred in the press-formed product, taking into account the influence of the bending deformation.

[0115] The stretch forming test result import unit may have a strain database import unit (not shown) that imports a pre-constructed strain database, instead of having the forming mold, the imaging device, the strain measuring device, and the strain database construction device as described above. In this case, the forming limit surface may be created by the above-mentioned forming limit analysis unit and the forming limit surface creation unit based on the strain database imported by the strain database import unit.

[0116] Alternatively, if a forming limit surface expressed by the relationship between the bending deformation degree and the maximum principal strain and the minimum principal strain is created in advance, in the press crack determination device and program according to the present invention, the forming limit acquisition unit may acquire the formed limit surface created in advance.

[0117] In the above description, the forming limit acquisition unit acquires forming limits obtained by a stretch-forming test in which a metal plate test piece is stretch-formed at various bending deformation degrees. However, in the present invention, the forming limit acquisition unit may also acquire forming limits obtained by press-forming a metal plate (blank) onto which a scribed circle or a grid has been transferred into a press-formed product.

[0118] [Embodiment 2] <Method for manufacturing press-formed product> A method for manufacturing a press-formed product according to embodiment 2 of the present invention determines whether or not a crack has occurred in a press-formed product obtained by press-forming a metal plate, and manufactures a press-formed product in which the occurrence of cracks during press-forming is suppressed based on the determination result. In the method for manufacturing a press-formed product according to embodiment 2, the determination of whether or not a crack has occurred in the press-formed product obtained by press-forming a metal plate is made by the press-forming crack determination method according to embodiment 1 of the present invention described above.

[0119] The method for manufacturing a press-formed product according to the second embodiment includes a forming limit acquisition process P1, a press-formed product crack determination process P3, a press-forming condition adjustment process P5, and a press-formed product manufacturing process P7, as shown in Fig. 8. Specific processes in the method for manufacturing a press-formed product according to the second embodiment will be described below with reference to Fig. 9.

[0120] <Forming Limit Acquisition Process> In the forming limit acquisition process P1, similar to the above-described first embodiment, a metal plate test piece is stretch-formed at various bending deformation degrees, and the forming limit of the metal plate expressed by the relationship between the bending deformation degree of the test piece and the maximum principal strain and minimum principal strain generated in the test piece is acquired.

[0121] In the forming limit acquisition process P1, first, the test piece 100 shown in FIG. 4 is stretch-formed at various bending deformation degrees, the strain generated in the stretch-formed test piece 100 is measured, and a strain database is constructed (S11).

[0122] Next, based on the constructed strain database, a forming limit strain represented by the maximum principal strain and the minimum principal strain at the fracture occurrence portion of the test piece 100 is extracted for each bending deformation degree of the test piece 100 (S13).

[0123] Next, the forming limit strains obtained for each bending deformation are plotted in a three-dimensional coordinate space having the maximum principal strain, the minimum principal strain, and the bending deformation as three axes, and a forming limit surface expressed by the relationship between the maximum principal strain, the minimum principal strain, and the bending deformation is approximated (S15). For example, the forming limit surface may be assumed to be the forming limit plane expressed by the above-mentioned formula (1), and the forming limit plane that minimizes the sum of squares of the perpendicular distances to the plot of the forming limit strains obtained for each bending deformation may be determined.

[0124] The metal plate test piece 100 may be taken from a metal material to be used as a blank for a press-formed product to be manufactured. By obtaining the forming limit of the metal material to be used as the blank for the press-formed product to be manufactured and determining the forming limit surface, it is possible to further improve the accuracy of predicting crack occurrence in the press-formed product.

[0125] <Press-molded product crack determination process> As in the first embodiment described above, the press-molded product crack determination process P3 determines whether or not a crack has occurred in the press-molded product based on the crack determination parameters calculated for the press-molded product and the forming limit acquired in the forming limit acquisition process.

[0126] In the press-formed product crack determination process P3, as shown in FIG. 9, first, provisional press-forming conditions for press-forming the press-formed product are set (S31a).

[0127] 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, the minimum principal strain, and the bending deformation degree in the press-formed product are calculated as crack determination parameters in the press-formed product (S33).

[0128] Next, the calculated crack determination parameter is plotted on a three-dimensional coordinate system on which the forming limit surface is drawn, with the maximum principal strain, minimum principal strain, and bending deformation degree as coordinate axes (S35a). Then, it is determined whether the plotted crack determination parameter is located in a region below the forming limit surface (S35b).

[0129] 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 a crack has occurred (S35d).

[0130] <Press-molding condition adjustment process> When it is determined that cracks have occurred in the press-molded product crack determination process P3, the press-molding condition adjustment process P5 adjusts the press-molding conditions of the press-molded product and repeats the press-molded product crack determination process P3 until it is determined that no cracks have occurred.

[0131] First, in the press-formed product crack determination process P3, if the crack determination parameter is not located below the forming limit surface (S35b), it is determined that a crack has occurred (S35d). In this case, in the press-formed condition adjustment process P5, the provisional press-formed conditions for the press-formed product are changed (S51).

[0132] For example, when deep drawing a press-formed product using a press-forming die consisting of a die, punch, and blank holder, changes to the interim press-forming conditions include changes to the blank size and shape, modifications to the die shape (radius of the die shoulder part, radius of the punch shoulder part), blank holder force, and lubrication of the blank in contact with the die and blank holder. Changes to the blank size and shape can be made, for example, by adjusting the interim press-forming conditions to reduce the blank holder force, thereby optimizing the tension acting on the blank during press-forming and adjusting the inflow resistance of the blank's material flow. In addition, at the die shoulder and punch shoulder, where the blank is subjected to bending and unbending deformation under tension, the thickness reduction is accelerated rapidly, leading to cracks and affecting material flow. Therefore, it is advisable to adjust the die shoulder radius and punch shoulder radius by changing the shape of the press forming die. Furthermore, the deep drawing limit can be adjusted by lubricating the blank that contacts the die and blank holder.

[0133] Then, after changing the provisional press-forming conditions (S51), in the press-formed product crack determination process P3, press-forming analysis is performed under the changed provisional press-forming conditions (S31b). Furthermore, as described above, the calculation of crack determination parameters (S33), plotting of the crack determination parameters on the three-dimensional coordinate system on which the forming limit surface is drawn (S35a), and determination of the presence or absence of cracks (S35b) are performed.

[0134] In contrast, in the press-molded product crack determination process P3, if the crack determination parameter is below the forming limit surface (S35b), it is determined that no cracks have occurred (S35c). In this case, in the press-molding condition adjustment process P5, the provisional press-molding conditions for which it has been determined that no cracks have occurred are established as the press-molding conditions (S53), and the adjustment of the press-molding conditions is terminated (S55).

[0135] The adjustment of the press-forming conditions (change of the provisional press-forming conditions) in the press-forming condition adjustment process P5 is performed until it is determined in the press-formed product crack determination process P3 that no cracks have occurred in the entire region of the press-formed product.

[0136] <Press-molded product manufacturing process> The press-molded product manufacturing process P7 manufactures a press-molded product under press-molding conditions adjusted in the press-molding condition adjustment process P5 so that it is determined that no cracks occur in the press-molded product crack determination process P3.

[0137] In the press-molded product manufacturing process P7, as shown in FIG. 9, the press-molding conditions that were determined to result in no cracks in the press-molded product crack determination process P3 are confirmed (S53), and after adjustment of the press-molding conditions is completed (S55), the press-molded product is manufactured.

[0138] To actually manufacture a press-formed product under the adjusted press-forming conditions, the settings of the press molding machine can be changed as follows. For example, to adjust the blank holder force as a press-forming condition, the settings on the air pressure control panel of the pneumatic die cushion (auxiliary pressure device) of the press molding machine can be changed. Furthermore, to adjust the press mold shape (die shoulder radius, punch shoulder radius), the shape data of the molding mold (model) used in the press molding analysis (S31b) is input into a CAD / CAM program linked to an NC machine tool and converted into NC data (numerical control data) (NC program) for NC machining. The converted NC data (NC program) can then be used to machine a steel mold or a casting mold model made of expanded polystyrene by the full mold casting method using the NC machine tool. Furthermore, when adjusting the lubrication of the blank that contacts the die and the blank holder, the type of lubricating oil may be changed (by increasing the coefficient of viscosity, adding an extreme pressure additive), or a polymer film such as a polyethylene film may be inserted.

[0139] As described above, in the method for manufacturing a press-formed product according to the second embodiment, the press-forming conditions are adjusted to suppress cracking based on the results of determining whether or not cracking occurs during press-forming of a metal plate, taking bending deformation into consideration. This significantly reduces the time required to determine the press-forming conditions for an actual press-formed product through trial and error. Furthermore, the press-formed products can be manufactured with cracking suppressed using the press-forming conditions adjusted in this manner.

[0140] Experiments and analyses were conducted to verify the effects of the press-molding crack detection method and the press-molded product manufacturing method according to the present invention, and these will be described below.

[0141] In this example, the presence or absence of cracking in a press-formed product press-formed by deep drawing was determined based on forming limit surfaces created by forming tests in which metal plate test pieces were stretch-formed at various bending deformation degrees. Furthermore, in this example, the press-forming conditions were adjusted to suppress cracking based on the results of determining whether or not cracking occurred, and the crack suppression effect when a press-formed product was produced under the adjusted press-forming conditions was verified.

[0142] First, as described in the first embodiment, a forming die 111 equipped with a plurality of punches 113 having different curvatures at their tip portions 113a was used to stretch-form metal sheet test pieces 100 at various bending deformation degrees (see FIG. 2). Then, the forming limit of the metal sheet, expressed as the relationship between the bending deformation degree of the stretch-formed test piece 100 and the maximum principal strain and minimum principal strain of the stretch-formed test piece 100 at the corresponding bending deformation degree, was obtained. The test material for the test piece 100 was a steel sheet having a tensile strength of 1470 MPa and a thickness of 1.6 mm.

[0143] In this example, the forming limit surface was obtained as the forming limit, as expressed by the formula (1) explained in embodiment 1. The values ​​of coefficients a to d in formula (1) were the values ​​shown in Table 2 above.

[0144] Next, an FEM analysis was performed on the process of press-forming a press-formed product 120 simulating an automobile part shown in FIG. 10, and the presence or absence of cracks in the press-formed product 120 was determined.

[0145] For the press-forming of the press-formed product 120, a steel plate having a tensile strength of 1470 MPa and a plate thickness of 1.6 mm was used as a blank, as in the forming test for obtaining the forming limit described in the above-mentioned embodiment 1. Furthermore, the forming process of the press-formed product 120 was draw forming, and the blank holding force was set in the range of 1 to 10 tons.

[0146] Then, using FEM analysis, the maximum principal strain, minimum principal strain, and bending deformation were calculated as crack determination parameters for each element used in the FEM analysis of the press-formed product 120. In this example, Example 1 was the case where the bending deformation was calculated by dividing the difference between the maximum principal strain on the outer side of the bend and the maximum principal strain on the inner side of the bend by the plate thickness, and Example 2 was the case where the bending deformation was calculated as the maximum principal curvature. For each of Examples 1 and 2, a crack determination evaluation value was calculated for each portion of the press-formed product 120. The evaluation value for crack determination was calculated by calculating the limit value of the maximum principal strain from the minimum principal strain and bending deformation for each portion of the press-formed product 120 press-formed with a blank holder force of 10 ton, and dividing the maximum principal strain obtained from the FEM analysis by the limit maximum principal strain. Here, a region where the evaluation value for crack determination is less than 1 corresponds to a case where the judgment parameter obtained from the FEM analysis is located below the forming limit surface, and it is determined that no cracks have occurred. In addition, the region where the evaluation value for crack judgment is 1 or more corresponds to the case where the judgment parameter obtained from the FEM analysis is not located below the forming limit surface, and it is judged that cracks have occurred.

[0147] Furthermore, for comparison, the occurrence of cracks was judged based on the maximum principal strain and the minimum principal strain without considering the degree of bending deformation (conventional example). Evaluation values ​​for crack judgment were then obtained for each portion of the press-formed product 120 of the conventional example. The evaluation values ​​for crack judgment for the conventional example were calculated by calculating the limit value of the maximum principal strain from the minimum principal strain for each portion of the press-formed product press-formed with a blank holder force of 10 ton, and dividing the maximum principal strain obtained from the FEM analysis by the limit maximum principal strain.

[0148] Fig. 11 shows a contour diagram of the evaluation value of the crack determination in Example 1, and Fig. 12 shows a contour diagram of the evaluation value of the crack determination in Example 2. Fig. 13 shows a contour diagram of the crack determination results in the conventional example. Fig. 13 is a contour diagram in which the region where the evaluation value of the crack determination is 1 or more is represented as a region where a crack has occurred, and the region where the evaluation value is less than 1 is represented as a region where a crack has not occurred.

[0149] As shown in Figures 11 and 12, in Examples 1 and 2, the evaluation values ​​were 1.04 and 1.06 in region α, which is the flat portion 121a of the punch bottom portion 121 of the press-formed product 120. Therefore, since the evaluation value was 1 or more, it was determined that a crack would occur in region α. ​​Furthermore, in other regions of the press-formed product 120 (region β, etc.), the evaluation value was less than 1, so it was determined that no crack would occur. In contrast, as shown in Figure 13, in the conventional example, the evaluation value was less than 1 throughout the entire press-formed product 120, so it was determined that no crack would occur, resulting in a different evaluation result from Examples 1 and 2.

[0150] In the actual press-forming of the press-formed product 120, cracks occurred in the flat portion 121a of the punch bottom 121, but no cracks were observed in other areas. This demonstrates that the presence or absence of cracks can be accurately determined in invention examples 1 and 2.

[0151] As described above, it has been shown that the press-forming crack determination method according to the present invention makes it possible to accurately determine whether or not cracks have occurred in a press-formed product by taking into account the influence of the bending deformation degree, which was not taken into account in the conventional forming limit line.

[0152] Next, based on the crack parameters determined as shown in Figures 11 and 12, the blank holder force applied by the blank holder was changed from 10 tonf to 1 tonf to prevent cracks from occurring during the production of the press-formed product 120 by draw forming. Then, FEM analysis was performed under the press forming conditions with the changed blank holder force, and crack determination parameters were calculated for the press-formed product 120. Figures 14 and 15 show contour diagrams in which the evaluation values ​​for crack determination obtained using the calculated crack determination parameters are displayed on the press-formed product 120. As in Example 1 described above, Figure 14 shows a diagram in which the crack determination parameter was calculated as the bending deformation degree, which is the difference between the maximum principal strain on the outer side of the bend and the maximum principal strain on the inner side of the bend, divided by the plate thickness. In contrast, Figure 15 shows a diagram in which the crack determination parameter was calculated as the bending deformation degree, which is the maximum principal curvature of the press-formed product 120, as in Example 2 described above.

[0153] As shown in Figures 14 and 15, by changing the blank holder force to 1 tonf, the maximum evaluation value in the flat portion 121a of the press-formed product 120 fell to less than 1. The evaluation value did not exceed 1 in other areas either. This indicates that the occurrence of cracks was suppressed in the press-formed product 120. Furthermore, when the press-formed product 120 was actually manufactured under the press-forming conditions in which the blank holder force was 1 tonf as described above, it was confirmed that no cracks occurred in the press-formed product 120.

[0154] As described above, according to the manufacturing method of the press-molded product of the present invention, it has been shown that it is possible to manufacture press-molded products while suppressing the occurrence of cracks by adjusting the press molding conditions to suppress the occurrence of cracks based on the results of determining whether or not cracks will occur, taking into account the influence of the degree of bending deformation.

[0155] According to the present invention, it is possible to provide a press-forming crack determination method, an apparatus, and a program for determining the forming limit of a press-formed product based on the forming limit of a metal sheet determined in consideration of the influence of bending deformation. Furthermore, according to the present invention, it is possible to provide a method for manufacturing a press-formed product, in which press-forming conditions for the press-formed product are adjusted so that the press-formed product is determined to be free of cracks by the press-forming crack determination method, and the press-formed product is manufactured under the adjusted press-forming conditions.

[0156] 1 Press forming crack determination device 10 Forming limit acquisition unit 11 Stretch forming test result import unit 13 Forming limit analysis unit 15 Forming limit surface creation unit 20 Press formed product crack determination unit 21 Press forming FEM analysis unit 23 Press forming crack determination parameter calculation unit 25 Press forming crack occurrence determination unit 100 Test piece 101 Test piece 103 Test piece 103a Notch portion 103b Center portion 111 Forming die 113 Punch 113a Tip portion 115 Upper die 117 Blank holder 120 Press formed product 201 Forming die 203 Ball head punch 203a Tip portion 205 Upper die 207 Blank holder 211 Forming die 213 Flat head punch 213a Tip portion 215 Driving seat

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