Method for manufacturing press-formed article

Abstract

To provide a method for producing a press-formed product by which the press-formed product is produced by adjusting press-forming conditions so as to suppress the generation of cracks.SOLUTION: Acquiring a forming limit of a sheet metal represented by a relationship between a force in a sheet thickness direction, and a maximum main strain and a minimum main strain (P1), and acquiring the force in the sheet thickness direction, and the maximum main strain and the minimum main strain generated by press forming of the press-formed product as fracture determination parameters; Based on the acquired forming limit of the sheet metal, determine whether or not cracking has occurred in the press-formed product (P3), and in a case where it is determined that cracking has occurred, in order to suppress the occurrence of cracking, adjust the press-forming conditions so as to improve the forming limit by increasing the force in the sheet thickness direction generated in the crack-prone portion of the press-formed product, and press-form the blank of the sheet metal into the press-formed product under the adjusted press-forming conditions.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a press-formed product by adjusting press-forming conditions so as to suppress the occurrence of cracks. [Background technology]

[0002] Metal sheets (e.g., thin steel sheets) used as materials for automobile bodies are mostly processed into body parts by press forming. The press formability of body parts varies depending on the shape of the body part and is also greatly affected by the material properties, such as the ductility of the metal sheet used as the material. In recent years, in response to demands for lighter car bodies, efforts have been made to increase the strength of metal sheets used in body parts. However, as the strength of metal sheets increases, 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 cracks occurring during the production of auto body parts by press forming, it is important to design molds and adjust press forming conditions based on advance predictions of press formability using CAE (Computer Aided Engineering). Forming limit diagrams (FLDs) are usually used to predict the press formability of metal sheets using CAE. Forming limit diagrams are created by laboratory-scale forming tests to determine the forming limits for various deformation modes of metal sheets in press forming, such as equibiaxial deformation, non-equilibrium biaxial deformation, plane strain deformation, and uniaxial deformation.

[0004] The press formability is predicted in advance by plotting the strain in the press-formed product, which is obtained by press forming analysis (for example, finite element analysis) that reproduces the actual press forming process of the press-formed product, on a forming limit diagram and determining whether or not cracks will occur. Furthermore, Non-Patent Document 1 discloses a technique for determining press-forming conditions to prevent cracks from occurring based on a prior prediction of press formability. This technique determines strain in a press-formed product through press-forming analysis, and adjusts the press-forming conditions so that the strain plots in all parts of the press-formed product are equal to or less than the forming limit line, thereby suppressing cracks from occurring. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Steel Sheet Forming Technology Research Group, Press Forming Difficulties Handbook, 4th Edition, Nikkan Kogyo Shimbun, (2017) Summary of the Invention [Problem to be solved by the invention]

[0006] ISO12004, the international standard for forming limit testing to determine the forming limit line of metallic materials, specifies two test methods: the Nakajima method and the Marciniak method.

[0007] The Nakajima method is a method for determining the forming limit by a forming test in which a test piece 21 is stretch-formed using a forming mold 200 equipped with a spherical punch 201, an upper die 203, and a blank holder 205, as shown in Figure 12(a). On the other hand, the Marciniak method is a method for determining the forming limit by a forming test in which a forming die 210 equipped with a flat-head punch 211, an upper die 203, and a blank holder 205 is used, and a driving sheet 213 is sandwiched between the flat-head punch 211 and the test piece 21 to perform bulge forming on the test piece 21, as shown in Figure 12(b).

[0008] However, it is known that the forming limits determined by the Nakajima method and the Marciniak method are different. Generally, as shown in Figure 12(c), the forming limit determined by the Marciniak method is lower than that determined by the Nakajima method. Therefore, when a forming limit diagram determined by the Marciniak method is used to predict press formability (crack evaluation) in mass production of press-formed products, it is often evaluated that excessive cracking will occur compared to actual press-formed products. For this reason, forming limit diagrams created by the Nakajima method are often used to evaluate cracking in mass production of press-formed products.

[0009] Furthermore, in the technology of Non-Patent Document 1, the FLD created by the Nakajima method is often used to determine whether or not cracks occur in press-formed products, and the press-forming conditions are adjusted to suppress cracks. However, the range in which press-forming conditions can be adjusted to suppress both cracks and wrinkles is narrow. For example, in draw forming, when applying a blank-holding force to the blank to suppress wrinkles, the range of blank-holding forces that can suppress both wrinkles and cracks is narrow. Therefore, even when cracks are evaluated using the FLD created by the Nakajima method, it can be difficult to appropriately adjust the press-forming conditions.

[0010] The present invention has been made to solve the above-mentioned problems, and aims to provide a method for manufacturing a press-molded product that can be press-molded by appropriately adjusting the press-molding conditions so as to suppress cracking in the press-molded product. [Means for solving the problem]

[0011] (1) A method for manufacturing a press-formed product according to the present invention is a method for manufacturing a press-formed product by suppressing the occurrence of cracks during press-forming of a metal plate, a forming limit acquisition process for acquiring a forming limit of the metal plate, which is expressed by the relationship between the force in the thickness direction of a test piece of the metal plate formed using a plurality of tools having different shapes so as to generate various forces in the thickness direction, and the maximum principal strain and the minimum principal strain; A press-molded product crack determination process that acquires the force in the plate thickness direction, the maximum principal strain, and the minimum principal strain generated by press forming of the press-molded product as crack determination parameters in the press-molded product, and determines whether or not cracks have occurred in the press-molded product based on the acquired crack determination parameters and the forming limit of the metal plate acquired in the forming limit acquisition process; When it is determined that cracks have occurred in the press-molded product crack determination process, in order to suppress cracks, a press-molding condition adjustment process is performed to adjust the press-molding conditions to improve the forming limit by increasing the force in the plate thickness direction generated in the crack-prone portion in the press-molded product; and a press-forming process in which the metal plate blank is press-formed into the press-formed product under the press-forming conditions adjusted in the press-forming condition adjustment process.

[0012] (2) In the above (1), In the press molding condition adjustment process, the force in the plate thickness direction generated in the area at risk of cracking is increased by pressing the metal plate with an elastic body sandwiched between the blank and the press molding die.

[0013] (3) In the above (1), In the press molding condition adjustment process, the force in the plate thickness direction generated in the portion at risk of cracking is increased by a pad mechanism using a gas cylinder or a die cushion.

[0014] (4) In the above (1), In the press molding condition adjustment process, the force in the thickness direction generated in the area at risk of cracking is increased by making the forming bottom dead center clearance in the area at risk of cracking less than the thickness of the blank. [Effects of the Invention]

[0015] In the present invention, the presence or absence of cracking in a press-formed product is determined based on a forming limit that takes into account the force in the plate thickness direction, and press-forming is performed by adjusting the press-forming conditions so as to increase the force in the plate thickness direction at a portion at which cracking is suspected to occur and thereby improve the forming limit. This makes it possible to manufacture a press-formed product while suppressing cracking at the portion at which cracking is suspected. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 2 is a flowchart showing a process flow of a method for manufacturing a press-formed product according to an embodiment of the present invention. [Figure 2] FIG. 2 is a flowchart showing an example of specific steps in a method for manufacturing a press-formed product according to an embodiment of the present invention. [Figure 3] 1A and 1B are diagrams showing specific examples of the shape of a test piece and a forming die used in a forming test for determining the forming limit of a metal plate in a method for manufacturing a press-formed product according to an embodiment of the present invention. [Figure 4] 1A to 1C are diagrams showing examples of specific shapes of the tip of a punch used in a forming test for determining the forming limit of a metal plate in a manufacturing method of a press-formed product according to an embodiment of the present invention. [Figure 5] 1A and 1B are graphs showing the relationship between the forming limit strain of a test piece and the radius of curvature of the punch tip, and the relationship between the surface pressure at the fracture part of the test piece and the equivalent plastic strain at the forming limit, obtained in Example 1 by a forming test using punches with various radiuses of curvature of the tip. [Figure 6] 10A and 10B are diagrams showing a press molding die used for V-bending a blank in Example 2 ((a) is a perspective view, (b) is a cross-sectional view). [Figure 7] FIG. 10 is a diagram showing a V-bending punch used in Example 2 to generate different surface pressures on the blank. [Figure 8] 1A and 1B are diagrams showing a press-molded product that was the object of molding in Example 3 ((a) is a perspective view, (b) is a side view). [Figure 9]10A and 10B are diagrams showing areas where cracking is likely to occur in a press-formed product in Example 3, and deformation paths at the areas where cracking is likely to occur ((a) area where cracking is likely to occur, (b) deformation paths). [Figure 10] FIG. 10 is an explanatory diagram of press molding conditions set in Example 3. [Figure 11] 1 shows the results of Example 3 showing whether or not cracks occur in press-molded products depending on the press-molding conditions ((a) side view of the press mold and press-molded product, (b) top view of the press-molded product). [Figure 12] FIG. 1 shows the Nakajima method and the Marciniak method, which are conventional methods for determining the forming limit of metal sheets, and examples of forming limit diagrams determined by these methods ((a) Nakajima method, (b) Marciniak method, (c) forming limit diagrams determined by both the Nakajima method and the Marciniak method). DETAILED DESCRIPTION OF THE INVENTION

[0017] [Background to the invention] The inventors focused on the differences in the tools used in the Nakajima method and the Marciniak method for determining forming limit diagrams, and conducted forming tests with variously changed tool shapes. As a result, they discovered that the force in the thickness direction generated in the test piece differs depending on the shape of the tool, and that the greater the force in the thickness direction that the test piece receives from the tool, the greater the forming limit strain.

[0018] Based on the knowledge gained, the inventors came up with the idea that it might be possible to suppress cracking in press-formed products by locally increasing the force in the plate thickness direction in areas where cracking is a concern during press forming so as to improve the forming limit.

[0019] The present invention has been made based on the above findings and ideas, and its specific configuration is as described below.

[0020] [Embodiment Mode] As shown in Figures 1 and 2, the manufacturing method of a press-formed product according to this 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-forming process P7. Hereinafter, each process of the method for manufacturing a press-formed product according to this embodiment will be described.

[0021] <Forming limit acquisition process> The forming limit acquisition process P1 is a process for acquiring the forming limit of a metal plate, which is expressed by the relationship between the force in the thickness direction generated in a test piece formed using multiple tools of different shapes so as to generate various forces in the thickness direction, and the maximum principal strain and minimum principal strain. In this embodiment, the forming limit acquisition process P1 includes a forming test step S11, a forming limit analysis step S13, and a forming limit surface creation step S15.

[0022] <Molding test step> The forming test step S11 is a step in which a test piece is formed using a plurality of tools with different shapes so that various forces in the plate thickness direction are generated, and the forces in the plate thickness direction and strain generated in the test piece are measured.

[0023] In the forming test step S11, first, a plurality of test pieces having different shapes are prepared by applying a predetermined grid or strain analysis pattern to the surface of a metal plate.

[0024] The strain analysis pattern applied to the surface of the test piece can be, for example, an arrangement of circles or dots. The grid or strain analysis pattern can be a regular repeating pattern or an irregular random pattern. To apply the grid or strain analysis pattern, for example, a sample grid can be transferred onto the surface of the test piece.

[0025] In addition, multiple test pieces with different shapes should be prepared. The shape of the test piece should be determined appropriately so that the forming limit can be determined for each deformation mode (equiaxial deformation, non-uniform biaxial deformation, plane strain deformation, and uniaxial deformation).

[0026] 3(a) shows a specific example of the shape of the test piece 21. The test piece 21 has a notch 21a formed in the outer edge of a circular shape, and it is preferable to prepare test pieces with the width W of the central part 21b changed at multiple levels.

[0027] Next, as shown in FIG. 3(b), a plurality of molding dies 10 having different shapes are used to mold each test piece 21 while photographing the surface of each test piece 21.

[0028] The molding die 10 is an example of a tool for molding the test piece 21, and includes a punch 11, an upper die 13, and a blank holder 15.

[0029] The punch 11 has a spherical surface with a radius of curvature R, that is, a spherical head-shaped tip portion 11a with a curvature ρ (=1 / R) greater than zero. Figure 4 shows an example of a specific shape of the tip 11a of the punch 11. The punch 11 is not limited to one with a spherical tip 11a as shown in Figures 4(a) to 4(c), but may have a flat tip 11a (with a radius of curvature R = ∞) as shown in Figure 4(d). In other words, the punch 11 has a tip 11a with a curvature ρ (= 1 / R) of 0 or more.

[0030] The test piece 21 is formed for each combination of one of a plurality of forming dies 10 having different shapes of the tip portion 11a of the punch 11 and one of a plurality of test pieces 21 having different shapes.

[0031] Furthermore, the surface of the test piece 21 may be photographed by placing a camera (photographing device) above the molding die 10 and photographing the surface of the test piece 21 during the molding process at predetermined time intervals.

[0032] Next, the images of the surface of each test piece 21 taken during the forming process of the test piece 21 are analyzed to measure the strain occurring in each test piece 21. Then, the strain measured for each test piece 21 is stored in chronological order from the start of forming to the occurrence of fracture, and a strain database is constructed.

[0033] The strain on the surface of the test piece 21 may be measured using digital image correlation (DIC). DIC is a method for measuring the strain in two in-plane directions on the surface of the test piece 21 from the deformation of a lattice or strain analysis pattern attached to the surface of the test piece 21. In DIC, it is preferable to measure the maximum principal strain and the minimum principal strain as the strain in two in-plane directions on the surface of the test piece 21.

[0034] Next, the force in the thickness direction at the fracture portion of each formed test piece 21 is determined. In this embodiment, the force in the thickness direction is determined as the surface pressure at the fractured portion of the molded test piece 21. The surface pressure generated at the fractured portion can be measured by molding with pressure-sensitive paper 17 placed between punch 11 and test piece 21, as shown in Figure 3(b)(ii). The surface pressure generated at the fractured portion can also be calculated by finite element method analysis that reproduces the molding of test piece 21.

[0035] In the forming test step S11, the force in the thickness direction may be calculated as the stress in the thickness direction generated at the fractured portion of the formed test piece 21. In this case, the stress in the thickness direction can be calculated by a finite element method analysis that reproduces the forming of the test piece 21. In such a finite element method analysis, it is preferable to divide the test piece into elements using solid elements.

[0036] <Forming limit analysis step> The forming limit analysis step S13 is a step of determining the maximum principal strain and the minimum principal strain at the forming limit based on the strain measured for each test piece 21 formed in the forming test step S11.

[0037] In this embodiment, the maximum principal strain and the minimum principal strain at the forming limit are determined by the following procedure. First, in the forming test step S11, a series of evaluation points is set for each test piece 21 in order to obtain the strain distribution in the vicinity of the fracture portion that occurs in each formed test piece 21.

[0038] The evaluation point sequence may be set at predetermined intervals along the direction perpendicular to the fractured portion that occurs in the test piece 21, for example, so as to straddle the fractured portion.

[0039] Next, the strains at the evaluation point sequence set on each test piece 21 are extracted from the strain database, and the strain distribution in the vicinity of the fractured portion is obtained.

[0040] The strain distribution can be obtained, for example, by extracting the strains at the times (time steps) before and after the occurrence of a fracture in the test piece 21 from the strains stored in chronological order at predetermined time intervals from the start of forming the test piece 21 to the occurrence of fracture.

[0041] Next, for each of the test pieces 21 formed using the forming dies 10 having different shapes, the forming limit is determined based on the acquired strain distribution, and the maximum principal strain and the minimum principal strain at the forming limit are found.

[0042] The forming limit can be determined, for example, by the method described in the publicly known document "JP 2023-35533 A." The maximum principal strain and minimum principal strain at the forming limit can be obtained, for example, by measuring the maximum principal strain and minimum principal strain at a predetermined time interval and then obtaining the maximum principal strain and minimum principal strain at the time step determined to be the forming limit or the time step immediately before that.

[0043] The method for determining the forming limit is not limited to the above-mentioned method, as long as the forming limit is determined using the same criteria for the strain distributions obtained for each combination of test piece 21 and forming die 10 having different shapes, and the maximum principal strain and minimum principal strain at the forming limit are found.

[0044] <Steps for creating the forming limit surface> The forming limit surface creation step S15 is a step of creating a forming limit surface expressed by the relationship between the force in the plate thickness direction and the maximum principal strain and the minimum principal strain. The force in the plate thickness direction is the force in the plate thickness direction at the fracture part of the test piece 21 measured in the forming test step S11. The maximum principal strain and the minimum principal strain are the maximum principal strain and the minimum principal strain at the forming limit of the test piece 21 obtained in the forming limit analysis step S13.

[0045] In the forming limit surface creation step S15, first, the force in the thickness direction at the fractured portion and the maximum principal strain and minimum principal strain at the forming limit, which are obtained for each formed test piece 21, are plotted in a three-dimensional coordinate space. Here, the three-dimensional coordinate space has three axes: the force in the thickness direction, the maximum principal strain, and the minimum principal strain. Then, the forming limit surface of the metal sheet is created based on the group of plot points of the force in the thickness direction at the fractured portion and the maximum principal strain and minimum principal strain at the forming limit, which are plotted in the three-dimensional coordinate space.

[0046] There are three methods for creating the forming limit surface, for example: In the following description, the force in the thickness direction at the fractured part and the maximum principal strain and minimum principal strain at the forming limit are referred to as forming limit data.

[0047] In the first method, first, in a three-dimensional coordinate space in which the forming limit data is plotted, two adjacent plot points are selected from the group of plot points of the forming limit data obtained for each of a plurality of test pieces 21 formed using a forming mold 10 of the same shape.

[0048] Next, from the group of plot points of the forming limit data obtained for each of a plurality of test pieces 21 molded with a molding die 10 having a different shape from the two plot points, one plot point is selected that is closest to the line segment connecting the two selected plot points.

[0049] Then, a triangular plane is created connecting the two selected plot points with one other plot point. This process of creating triangular planes is carried out for all plot points of the forming limit data plotted in the three-dimensional coordinate space. The polygonal surface created by combining the created triangular planes is then used as the forming limit surface.

[0050] The second method for creating a forming limit surface is to first assume a forming limit plane or a forming limit curved surface in a three-dimensional coordinate space, calculate the perpendicular distance between this assumed forming limit plane or curved surface and each plot point of the plot point group of forming limit data plotted in the three-dimensional coordinate space, and then determine the forming limit plane or curved surface so that the sum of squares of the calculated perpendicular distances is minimized.

[0051] The third method for creating the forming limit surface is to minimize the sum of squares weighted on the perpendicular distance between the assumed forming limit plane or forming limit curved surface and each plot point of the plot point group of the forming limit data in the second method described above.

[0052] One way of weighting is to increase the weight of the vertical distance between the plot points of the forming limit data for the shape or forming path of the forming mold 10 for which you particularly want to reduce the error and the forming limit plane or forming limit curved surface.

[0053] The second or third method may be a combination of a plurality of forming limit planes and / or forming limit curved surfaces. For example, a forming limit plane or a forming limit curved surface may be assumed for each of the regions where the minimum principal strain is negative and positive, and the forming limit plane or the forming limit curved surface may be determined so that the sum of squares of the perpendicular distances between each plot point of the plot point group of forming limit data for each region is minimized. The same applies when creating a forming limit surface so that the sum of squares of weighted perpendicular distances between each plot point of the forming limit data is minimized.

[0054] As another method, three adjacent plot points are selected from the group of plot points of the forming limit data in the three-dimensional coordinate space, and a triangular plane is generated by connecting the selected three plot points with a straight line. Then, a polygonal surface consisting of a plurality of triangular planes generated by repeating this process may be used as the forming limit surface.

[0055] <Press-molded product crack detection process> The press-formed product crack determination process P3 acquires the force in the plate thickness direction of the press-formed product, the maximum principal strain, and the minimum principal strain as crack determination parameters for the press-formed product. Then, 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. A specific example of the press-molded product crack determination process P3 includes a press-molded FEM analysis step S31, a press-molded crack determination parameter calculation step S33, and a press-molded crack occurrence determination step S35, as shown in Figure 2.

[0056] <Press forming FEM analysis step> The press-forming FEM analysis step S31 is a step of performing an FEM analysis (finite element method analysis) of the process of press-forming a metal plate into a press-formed product.

[0057] In the press-forming FEM analysis step S31, first, provisional press-forming conditions are set. Then, FEM analysis of the press-forming process of the press-formed product is performed under the provisional press-forming conditions. As a result, changes in strain, stress, plate thickness, etc. that occur in the press-formed product are obtained for each element and node used in the FEM analysis.

[0058] In addition, when calculating the thickness direction stress as the force in the thickness direction in the press forming crack judgment parameter calculation step S33, it is recommended to perform FEM analysis using a blank (metal plate) divided into elements using solid elements in the press forming FEM analysis step S31.

[0059] <Steps for calculating parameters for determining press forming cracks> The press-forming crack determination parameter calculation step S33 is a step of calculating the force in the thickness direction of the press-formed product, and the maximum principal strain and minimum principal strain as crack determination parameters for the press-formed product. Here, the force in the thickness direction of the press-formed product, the maximum principal strain and the minimum principal strain are calculated based on the FEM analysis results in the press-forming FEM analysis step S31.

[0060] The force in the plate thickness direction of the press-formed product is obtained according to the force in the plate thickness direction that is used as an index of the forming limit of the metal plate, which is acquired in the forming limit acquisition process P1. That is, when the surface pressure is used as the force in the thickness direction as an index of the forming limit, the surface pressure generated during the press forming process of the press-formed product is calculated. On the other hand, when the thickness direction stress is used as the force in the thickness direction as an index of the forming limit, the thickness direction stress generated during the press forming process of the press-formed product is calculated. The surface pressure or stress in the thickness direction generated in the press-formed product should be calculated for each element used in the FEM analysis.

[0061] <Steps for determining whether or not cracks have occurred during press molding> The press-molding crack occurrence determination step S35 is a step for determining whether or not a crack has occurred in the press-molded product. The presence or absence of a crack in the press-molded product is determined 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 process P1.

[0062] The specific procedure for determining whether or not cracks have occurred in the press-molded product in the press-molding crack occurrence determination step S35 is as follows.

[0063] First, the crack determination parameters for the press-formed product calculated in the press-forming crack determination parameter calculation step S33 are plotted on the three-dimensional coordinate space in which the forming limit surface created in the forming limit surface creation step S15 of the forming limit acquisition process P1 is drawn.

[0064] Then, if the plot of the crack determination parameter is not located below the forming limit surface, for example, if the maximum principal strain of the crack determination parameter is greater than or equal to the minimum principal strain of the crack determination parameter and the maximum principal strain of the forming limit surface corresponding to the force in the plate thickness direction, it is determined that a crack has occurred.

[0065] In contrast, if the plot of the crack determination parameter is located below the forming limit surface, for example, 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 force in the plate thickness direction, it is determined that no cracks have occurred.

[0066] <Press molding condition adjustment process> The press-forming condition adjustment process P5 is a process for adjusting the press-forming conditions to suppress cracking when it is determined that cracking has occurred in the press-formed product crack determination process P3. Here, the press-forming conditions are adjusted to increase the force in the thickness direction generated in the press-formed product at the crack-prone portion, thereby improving the forming limit.

[0067] The specific processing of the press-molding condition adjustment process P5 is as follows. First, if it is determined that a crack has occurred in the press-molded product crack determination process P3, the provisional press-molding conditions of the press-molded product are changed so as to increase the force in the thickness direction generated in the area at which cracking is suspected, thereby improving the forming limit (S51).

[0068] Specific examples of changing the provisional press-forming conditions to increase the force in the thickness direction include inserting an elastic body between the metal blank and the press-forming die. Other examples include using a pad mechanism that utilizes a gas cylinder or die cushion, or setting the forming bottom dead center clearance at the area where cracking is likely to occur to less than the blank's thickness. The forming bottom dead center clearance refers to the gap height in the press-forming die at the forming bottom dead center (e.g., the gap height between the punch and the upper die). Several of the above-mentioned methods for increasing the force in the thickness direction may be combined.

[0069] To increase the force in the thickness direction at the area where cracking is likely to occur by changing the elastic body, pad mechanism, or bottom dead center clearance of the forming, it is advisable to determine the force in the thickness direction to be generated at the area where cracking is likely to occur using the following method and adjust the press forming conditions. First, based on the relationship between the maximum principal strain and minimum principal strain representing the forming limit surface and the force in the thickness direction, and the maximum principal strain and minimum principal strain in the crack-prone area, the minimum value of the force in the thickness direction required to make the strain in the crack-prone area below the forming limit surface is determined. For example, if the forming limit surface is expressed by the formula (1) described below, the minimum principal strain (x) and maximum principal strain (z) in the crack-prone area are substituted into the formula (1) to determine the minimum value of the contact pressure (y) required to make the strain in the crack-prone area below the forming limit surface. Then, the press forming conditions, such as the material (elastic modulus) and thickness of the elastic body, pad pressure, or bottom dead center clearance value, are adjusted so that a force (surface pressure) in the plate thickness direction greater than or equal to the minimum value determined in this way is generated in the area where cracking is likely to occur.

[0070] Then, after the provisional press-forming conditions are changed (S51), in the press-forming crack determination process P3, a press-forming analysis is performed under the changed provisional press-forming conditions (S31). For example, if the provisional press forming conditions are changed so that an elastic body is sandwiched between the blank and the press forming die, a press forming analysis is performed in which the blank is press formed with an elastic body sandwiched between the area in the blank where cracking is suspected and the press forming die. Furthermore, if the provisional press forming conditions are changed to use a pad mechanism utilizing a gas cylinder or die cushion, the load applied by the pad mechanism to press the areas of the blank at risk of cracking is increased and press forming analysis is performed. Alternatively, if the provisional press forming conditions are changed so that the forming bottom dead center clearance at the area where cracking is suspected is less than the thickness of the blank, press forming analysis is performed using a press forming mold in which the forming bottom dead center clearance at the area where cracking is suspected has been changed to be less than the thickness of the blank.

[0071] After the press forming analysis is performed, as described above, the crack determination parameters are calculated (S33), the crack determination parameters are plotted on the three-dimensional coordinate system on which the forming limit surface is drawn, and the presence or absence of cracks is determined (S35).

[0072] In the press-molded crack occurrence determination step S35 of the press-molded crack determination process P3, if the crack determination parameter is located below the forming limit surface, it is determined that no cracks have occurred. 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 confirmed as the press-molding conditions (S53), and the adjustment of the press-molding conditions is terminated (S55).

[0073] 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.

[0074] <Press molding process> The press-forming process P7 is a process for press-forming a metal plate blank into a press-formed product under the press-forming conditions adjusted in the press-forming condition adjustment process P5.

[0075] To perform press molding under the press molding conditions adjusted in the press molding condition adjustment process P5, it is preferable to perform press molding so as to increase the force in the plate thickness direction in accordance with the embodiment in which the force in the plate thickness direction is increased in the press molding condition adjustment process P5.

[0076] In other words, when the press molding conditions are adjusted so that an elastic body is sandwiched between the blank and the press molding die, the blank is press molded with the elastic body sandwiched between the area in the blank where cracking is likely to occur and the press molding die. Furthermore, when the press forming conditions are adjusted to use a pad mechanism utilizing a gas cylinder or a die cushion, the load that presses the portion of the blank at risk of cracking using the pad mechanism is increased during press forming. Alternatively, if the press molding conditions are adjusted so that the forming bottom dead center clearance at the area where cracking is likely to occur is less than the thickness of the blank, the shape of the press molding die is changed and press molding is performed so that the forming bottom dead center clearance at the area where cracking is likely to occur is less than the thickness of the blank.

[0077] As described above, in the method for manufacturing a press-formed product according to the present embodiment, the presence or absence of cracking during press-forming of a metal plate is determined based on a forming limit that takes into account the force in the plate thickness direction. Then, the press-forming conditions are adjusted so as to increase the force in the plate thickness direction in the area where it is determined that cracking will occur, thereby improving the forming limit. This makes it possible to manufacture a press-formed product while suppressing cracking in areas where cracking is a concern.

[0078] As described above, when the region in which the press-forming conditions can be adjusted to suppress both cracking and wrinkling is narrow, there is a problem in that it is difficult to appropriately adjust the press-forming conditions based on the determination of the occurrence of cracking using the forming limit diagram created by the Nakajima method. In contrast, the method for manufacturing a press-formed product according to the present embodiment adjusts the press-forming conditions so as to increase the force in the thickness direction generated in the region at risk of cracking, thereby improving the forming limit. This widens the range in which the press-forming conditions can be adjusted to suppress both cracking and wrinkling, and allows the press-forming conditions to be appropriately adjusted to suppress cracking.

[0079] In the manufacturing method of the press-formed product according to this embodiment, in the forming limit acquisition process P1, a forming test of a metal plate is performed using a plurality of tools with different shapes, and a forming limit surface expressed by the relationship between the force in the plate thickness direction and the maximum principal strain and the minimum principal strain is created. However, the forming limit acquisition process is not limited to the process of performing the forming test described above, and may be a process of acquiring a forming limit surface that has been created in advance. [Example]

[0080] An experiment was conducted to demonstrate the effects of the method for manufacturing a press-formed product according to the present invention, and the results will be described below.

[0081] Example 1 In Example 1, in a forming test using a metal plate test piece, it was verified that the forming limit increases by increasing the force in the plate thickness direction.

[0082] In the forming test, test pieces 21 were formed by changing the curvature of the tip of the punch 11 of the forming die 10 shown in Fig. 3(b) described above, and the strain (maximum principal strain and minimum principal strain) generated in the test piece 21 and the force in the thickness direction at the fracture part of the test piece were measured. Regarding the force in the thickness direction, pressure-sensitive paper 17 was placed between the forming die 10 and the test piece 21, and the surface pressure was measured.

[0083] 5 shows (a) the relationship between the maximum principal strain and the minimum principal strain at the forming limit, and (b) the relationship between the surface pressure at the fractured part of the test piece 21 and the equivalent plastic strain at the forming limit, for test piece 21 formed by changing the radius of curvature of the tip 11a of punch 11. The equivalent plastic strain at the forming limit was calculated using the strain measured in a forming test of test piece 21. In Fig. 5, R25, R50, and R100 represent the cases where the radius of curvature R of the tip 11a of the punch 11 is 25 mm, 50 mm, and 100 mm, respectively, as shown in Fig. 4(a) to (c). Also, a flat bottom represents the case where the radius of curvature of the tip 11a is infinite, that is, the tip of the punch 11 has a flat bottom shape, as shown in Fig. 12 (described above).

[0084] As shown in FIG. 5(a), it can be seen that the smaller the radius of curvature of the tip 11a of the punch 11, the larger the forming limit.

[0085] Furthermore, as shown in FIG. 5(b), it can be seen that the smaller the radius of curvature of the tip 11a of the punch 11, the higher the contact pressure, and accordingly the larger the equivalent plastic strain at the forming limit. In this way, it was suggested that forming the test piece 21 so as to increase the force (surface pressure) generated in the thickness direction of the sheet increases the forming limit of the metal sheet, thereby suppressing the occurrence of cracks.

[0086] <Example 2> In Example 2, press forming was performed to V-bend the blank 101 using the press forming die 110 shown in Figure 6, and it was verified whether or not cracks occurred in the press-formed product (V-bent blank 101) when the force in the plate thickness direction generated in the blank 101 was changed.

[0087] The blank 101 was made of a high-strength steel plate having a tensile strength of 1470 MPa and a thickness of 1.4 mm. 6, the press molding die 110 includes a V-bending punch 111 having a tip 111a with a mountain-shaped cross section, an upper die 113, and a holder 115. The V-bending punch 111 has a diameter of φ100 mm, a radius of curvature of the tip 111a of R50 mm, and a radius of curvature of a die shoulder 113a of the upper die 113 of R5 mm.

[0088] Furthermore, in Example 2, in order to change the force in the thickness direction generated on the blank 101, a V-bending punch 111A was used, which had a gap forming portion 111b at the apex of its tip portion 111a so as to form a gap between the blank 101 and the punch, as shown in Fig. 7(b). By preventing the apex of the V-bending punch 111A from coming into contact with the blank 101 during press forming, the force (surface pressure) generated on the blank 101 in the thickness direction was made zero.

[0089] During the V-bending process, a blank holding force of 50 tonf was applied to the blank 101 to suppress the inflow of material, and the punch speed was set to 5 mm / min. An image analysis camera was installed above the press-forming die 110, and images of the surface of the blank 101 were taken at time intervals of once per second from the start of the V-bending process until fracture occurred. Furthermore, pressure-sensitive paper was installed between the press-forming die 110 (V-bending punch 111) and the blank 101 to measure the surface pressure generated on the blank 101.

[0090] After the V-bending process was completed, the maximum principal strain and the minimum principal strain were determined in the formed blank 101. Furthermore, the surface pressure was measured using pressure-sensitive paper placed between the V-bending punch 111 or 111A and the blank 101 as the force in the plate thickness direction at the fractured portion.

[0091] When a V-bending punch 111A having a gap forming portion 111b at a tip portion 111a was used, the maximum principal strain of the V-bent blank 101 was 0.12, the minimum principal strain was −0.04, and the surface pressure was 0 MPa.

[0092] On the other hand, when a V-bending punch 111 without a gap forming portion at the tip was used, the maximum principal strain of the V-bent blank 101 was 0.155, the minimum principal strain was -0.06, and the surface pressure was 70 MPa.

[0093] Next, the maximum principal strain, the minimum principal strain and the surface pressure in the blank 101 that was V-bent using each of the V-bending punches 111 and 111A were used as crack determination parameters to determine whether or not cracks occurred in the blank.

[0094] The forming limit plane expressed by the following formula (1) was used to determine whether or not cracks occurred.

number

[0095] The coefficients shown in Table 1 were determined so as to minimize the least squares sum of the perpendicular distance between the forming limit plane expressed by Equation (1) and each plot of the forming limit data obtained by performing the forming test step S11 described in the embodiment. In Table 1, the forming limit plane A is determined by determining the coefficients of Equation (1) for the region where the minimum principal strain is negative, and the forming limit plane B is determined by determining the coefficients of Equation (1) for the region where the minimum principal strain is positive.

[0096] The occurrence of cracks in the V-bent blank was determined by plotting the surface pressure and the maximum and minimum principal strains at the forming limit in a three-dimensional coordinate space in which the forming limit surface, which is a combination of forming limit plane A and forming limit plane B, was depicted.

[0097] When a V-bending punch 111 without a gap forming portion at the tip 121a was used, the plot point of the crack determination parameter was located below the forming limit surface. From this, it was determined that no cracks would occur in the blank made using a V-bending punch 111 without a gap forming portion.

[0098] In contrast, when the V-bending punch 111A having the gap forming portion 121b at the tip 121a was used, the plot point of the crack determination parameter was located above the forming limit surface. From this, it was determined that cracks would occur in the blank made using the V-bending punch 111A having the gap forming portion 121b.

[0099] Therefore, it can be seen that by press-forming the blank 101 so as to increase the surface pressure, that is, the force in the plate thickness direction, the forming limit is increased and the occurrence of cracks is suppressed. Conversely, the above results indicate that if press forming is performed so as to reduce the force acting on the blank in the thickness direction, the forming limit will be lowered and cracks will be more likely to occur.

[0100] Example 3 In Example 3, when press-forming a press-formed product 121 having the shape shown in FIG. 8, the force acting in the plate thickness direction on the press-formed product 121 was changed, and the presence or absence of cracks was verified.

[0101] The press-formed product 121 was prepared by press-forming a high-strength steel plate having a tensile strength of 1470 MPa and a thickness of 1.4 mm as the test material using a press-forming die 130 equipped with a lower die 131, an upper die 133, and a holder 135, as shown in FIG. 9(a). As shown in Figure 9(a), the press-molded product 121 has regions A and B, which are areas where the blank 123 deforms without coming into contact with the press molding die 130 (lower die 131 and upper die 133) during press molding until just before the bottom dead center of molding.

[0102] FIG. 9(b) is a graph showing the relationship between the minimum principal strain and the maximum principal strain in the forming process of the portions A and B, and shows the deformation paths of the portions A and B. Region A is formed by plane strain tensile deformation as shown in Figure 9(b), whereas region B is formed by uniaxial tensile deformation as shown in Figure 9(b). Generally, areas formed by plane strain tensile deformation have a small forming limit strain, so there is a greater risk of cracking in area A than in area B.

[0103] In Example 3, as shown in FIG. 10, a press-molded product 121 was press-molded under each of the press-molding conditions (1) to (4).

[0104] Condition (1) is the standard press forming condition, and the press forming die 130 shown in Figure 9(a) is used, and draw forming is performed by applying a wrinkle suppression force to the blank 123 using the upper die 133 and holder 135.

[0105] Conditions (2) and (3) involve draw forming using a press-forming die 130A equipped with a lower die 131, an upper die 133, a holder 135, and a pad 137 that presses an area including portions A and B in the blank 123. Condition (2) involves a pad load of 5 tonf, and condition (3) involves a pad load of 10 tonf.

[0106] Condition (4) is the same as condition (3), in that the blank 123 is pressed by a pad with a pad load of 10 tonf, and in addition, draw molding is performed with an elastic body 139 sandwiched between the lower die 131 and the blank 123. Here, the elastic body 139 is a sheet-like polyvinyl chloride (PVC) with a thickness of 3 mm.

[0107] Comparing conditions (1) to (4), in condition (1), part A does not come into contact with the press molding die during press molding, so the surface pressure is zero. In conditions (2), (3), and (4), the pad load increases, and by sandwiching an elastic body, the surface pressure increases in that order.

[0108] In Example 3, press-forming was performed under each of conditions (1) to (4) with the blank holder force varied within a range of 5 to 25 tonf, and the occurrence of cracks in the press-formed product 121 was compared.

[0109] FIG. 11 shows, as an example of the results of a press molding experiment, a press-molded product 121 press-molded under condition (1) with a blank holding force of 10 tonf, and a press-molded product 121 press-molded under condition (4) with a blank holding force of 20 tonf. In the press-molded product 121 press-molded at a surface pressure of 10 tonf, a crack occurs in part A, as shown in Fig. 11(b)(i). In contrast, in the press-molded product 121 press-molded at a surface pressure of 20 tonf, no crack occurs, as shown in Fig. 11(b)(ii).

[0110] Table 2 shows the results of summarizing the blank holder forces set under each of the conditions (1) to (4) and the occurrence of cracks in the press-formed product 121. In Table 2, ◯ indicates that no cracks occurred, and × indicates that cracks occurred. [Table 2]

[0111] As shown in Table 2, the range of blank holding force that can be used to form the sheet without cracking increases in the order of conditions (1) to (4). As mentioned above, the surface pressure, i.e., the force in the thickness direction, increases in the order of conditions (1) to (4) at the crack-prone areas of the blank 123. Therefore, the results shown in Table 2 suggest that by increasing the force in the thickness direction applied to the blank 123, the area that can be formed without generating cracks expands, and it is possible to appropriately determine press forming conditions that suppress crack generation. [Explanation of symbols]

[0112] 10. Molding mold 11 Punch 11a Tip 13 Upper die 15 Wrinkle holder 17 Pressure-sensitive paper 21 Test specimen 21a Notch 21b Central part 101 Blank 110 Press molding die 111 V-bending punch 111A Bending Punch 111a Tip 111b Gap forming portion 113 Upper die 113a Die shoulder 115 Holder 121 Press-molded products 123 Blank 130 Press molding dies 131 Lower mold 133 Upper mold 135 Holder 137 Pad 139 Elastic Body 200 molding dies 201 Ball Head Punch 203 Upper die 205 Wrinkle holder 210 Molding mold 211 Flat Head Punch 213 Driving Seat

Claims

1. A method for manufacturing a press-formed product by suppressing cracking during press forming of a metal plate, a forming limit acquisition process for acquiring a forming limit of the metal plate, which is expressed by the relationship between the force in the thickness direction of a test piece of the metal plate formed using a plurality of tools having different shapes so as to generate various forces in the thickness direction, and the maximum principal strain and the minimum principal strain; A press-molded product crack determination process that acquires the force in the plate thickness direction, the maximum principal strain, and the minimum principal strain generated by press forming of the press-molded product as crack determination parameters in the press-molded product, and determines whether or not cracks have occurred in the press-molded product based on the acquired crack determination parameters and the forming limit of the metal plate acquired in the forming limit acquisition process; When it is determined that cracks have occurred in the press-molded product crack determination process, in order to suppress cracks, a press-molding condition adjustment process is performed to adjust the press-molding conditions to improve the forming limit by increasing the force in the plate thickness direction generated in the crack-prone portion in the press-molded product; a press-forming process in which the metal plate blank is press-formed into the press-formed product under press-forming conditions adjusted in the press-forming condition adjustment process.

2. The method for manufacturing a press-formed product described in claim 1, characterized in that in the press-forming condition adjustment process, the force in the plate thickness direction generated in the area at risk of cracking is increased by press-forming with an elastic body sandwiched between the metal plate blank and the press-forming die.

3. The method for manufacturing a press-molded product described in claim 1, characterized in that in the press-molding condition adjustment process, the force in the plate thickness direction generated in the area at risk of cracking is increased by a pad mechanism using a gas cylinder or a die cushion.

4. A method for manufacturing a press-formed product as described in claim 1, characterized in that in the press-forming condition adjustment process, the force in the plate thickness direction generated in the area at risk of cracking is increased by making the forming bottom dead center clearance in the area at risk of cracking less than the plate thickness of the blank.

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