Method for manufacturing press-formed article

Abstract

A press-formed article manufacturing process according to the present invention comprises: acquiring (P1) a forming limit of a metal plate represented by the relationship between force in the plate thickness direction and a maximum principal strain and a minimum principal strain; determining (P3) presence or absence of occurrence of a crack in a press-formed article by acquiring, as a crack determination parameter, a force in the plate thickness direction and a maximum principal strain and a minimum principal strain generated by press-forming of the press-formed article, and on the basis of the acquired forming limit of the metal plate; when a crack is determined to be present, adjusting to a press-forming condition for increasing the forming limit by increasing the force in the plate thickness direction generated at a crack-prone site in the press-formed article in order to suppress the occurrence of a crack; and press-forming a blank metal plate into the press-formed article under the adjusted press-forming condition.

Description

Manufacturing method for press-molded products

[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.

[0002] Metal sheets (e.g., thin steel sheets) used as materials for automobile bodies are mostly processed into automotive parts by press forming. The press formability of an automotive part varies depending on the shape of the body part and is also greatly affected by the material properties, including the ductility, of the metal sheet used as the material. In recent years, demand for lighter car bodies has led to efforts to increase the strength of metal sheets used in auto 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, to avoid problems such as cracking during the production of auto body parts by press forming, it is important to design dies and adjust press forming conditions based on advance predictions of press formability using computer-aided engineering (CAE). Forming limit diagrams (FLDs) are typically used to predict the press formability of metal sheets using CAE. Forming limit diagrams are created through laboratory-scale forming tests to determine the forming limits for various deformation types of metal sheets in press forming, such as equibiaxial deformation, non-equibiaxial deformation, plane strain deformation, and uniaxial deformation.

[0004] The press formability is predicted in advance by plotting strain in a press-formed product, which is determined by press forming analysis (for example, finite element method analysis) that reproduces the process of actually press-forming a 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 so as to prevent cracks from occurring based on a prior prediction of press formability. This technique determines strain in a press-formed product by press forming analysis and adjusts the press forming conditions so that the plot of strain in all parts of the press-formed product is equal to or less than the forming limit line, thereby suppressing the occurrence of cracks.

[0005] Steel Sheet Forming Technology Research Group, Press Forming Difficulties Handbook, 4th Edition, Nikkan Kogyo Shimbun, (2017)

[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 bulged using a forming tool 200 equipped with a hemispherical punch 201, an upper die 203, and a blank holder 205, as shown in Fig. 12(a). On the other hand, the Marciniak method is a method for determining the forming limit by a forming test in which a test piece 21 is bulged using a forming tool 210 equipped with a flat punch 211, an upper die 203, and a blank holder 205, as shown in Fig. 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 the forming limit 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 was often used to determine whether or not cracks occurred in press-formed products, and the press-forming conditions were 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 deep drawing, when applying a blank-holding force to a 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 its object is 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 cracks in the press-molded product.

[0011] The method for manufacturing a press-formed product according to the present invention is for manufacturing a press-formed product while suppressing the occurrence of cracks during press-forming of a metal plate, and the method is for measuring the force in the plate thickness direction, the maximum principal strain, and the minimum principal strain of a test piece of the metal plate formed using a plurality of tools with different shapes so as to generate various forces in the plate thickness direction. a press-formed 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-formed product as crack determination parameters for the press-formed product, and determines whether or not a crack has occurred in the press-formed product based on the acquired crack determination parameters and the forming limit of the metal plate acquired in the forming limit acquisition process; a press-formed product crack adjustment process that, when it is determined that a crack has occurred in the press-formed product crack determination process, adjusts the press-forming conditions to improve the forming limit by increasing the force in the plate thickness direction generated in a region in the press-formed product where cracking is suspected, in order to suppress crack occurrence; and a press-forming process that press-forms a blank of the metal plate into the press-formed product under the press-forming conditions adjusted in the press-forming condition adjustment process.

[0012] In the press forming condition adjustment process, the force in the plate thickness direction generated in the area at risk of cracking can be increased by sandwiching an elastic body between the metal plate blank and the press forming die and then press forming the metal plate.

[0013] In the press forming condition adjustment process, the force in the thickness direction generated in the crack-prone area may be increased by a pad pressure using a gas cylinder or a die cushion.

[0014] In the press forming condition adjustment process, the force in the thickness direction generated in the area at risk of cracking can be increased by making the forming bottom dead center clearance in the area at risk of cracking less than the thickness of the blank.

[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.

[0016] FIG. 1 is a flow chart showing a process flow of a manufacturing method for a press-formed product according to an embodiment of the present invention. FIG. 2 is a flow chart showing an example of a specific process of the manufacturing method for a press-formed product according to an embodiment of the present invention. FIG. 3 is a diagram showing a specific example of the shape of (a) a test piece and (b) a forming die used in a forming test to determine the forming limit of a metal sheet in the manufacturing method for a press-formed product according to an embodiment of the present invention. FIG. 4 is a diagram showing a specific example of the shape of the tip of a punch used in a forming test to determine the forming limit of a metal sheet in the manufacturing method for a press-formed product according to an embodiment of the present invention. FIG. 5 is a graph showing (a) the relationship between the forming limit strain of a test piece and the radius of curvature of the punch tip, and (b) the relationship between the contact pressure at the fracture part generated in the test piece and the equivalent plastic strain at the forming limit, obtained by a forming test using punches with variously changed tip curvature radii in Example 1. FIG. 6 is a diagram showing a press-forming die used in V-bending a blank (V-bending) in Example 2 ((a) perspective view, (b) cross-sectional view). FIG. 7 is a diagram showing a V-bending punch used to generate different contact pressures on the blank in Example 2. FIG. 8 is a diagram showing a press-formed product to be formed in Example 3 ((a) perspective view, (b) side view). FIG. 9 is a diagram showing a portion of the press-formed product in Example 3 where cracking is likely to occur and the deformation path at the portion where cracking is likely to occur ((a) portion where cracking is likely to occur, (b) deformation path). FIG. 10 is an explanatory diagram of the press-forming conditions set in Example 3. FIG. 11 is a result showing whether or not cracking occurs in the press-formed product depending on the press-forming conditions in Example 3 ((a) side view of the press die and the press-formed product, (b) top view of the press-formed product). FIG. 12 is a diagram showing examples of the Nakajima method and the Marciniak method, which are conventional methods for determining the forming limit of metal sheets, and forming limit diagrams determined by these methods ((a) Nakajima method, (b) Marciniak method, (c) forming limit diagrams determined by each of the Nakajima method and the Marciniak method).

[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 using various shapes of tools. As a result, they discovered that the force in the thickness direction of a 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] 1 and 2, a method for manufacturing 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. Each process of the method for manufacturing a press-formed product according to this embodiment will be described below.

[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 plate thickness direction generated in a test piece formed using a plurality of tools with different shapes so as to generate various forces in the plate thickness direction, and the maximum principal strain and the 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] <<Forming test step>> The forming test step S11 is a step in which a test piece is formed using a plurality of tools of 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] A plurality of test specimens with different shapes are prepared, and the shape of the test specimen 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 area 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 portion 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 tip 11a with a radius of curvature R, i.e., a spherical head shape with a curvature ρ (= 1 / R) greater than 0. FIG. 4 shows an example of a specific shape of the tip 11a of the punch 11. The punch 11 is not limited to having a spherical head shape with the tip 11a as shown in FIGS. 4(a) to 4(c), but may have a flat tip 11a (with a radius of curvature R = ∞) as shown in FIG. 4(d). In other words, the punch 11 has a tip 11a with a curvature ρ (= 1 / R) greater than or equal to 0.

[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 fractured portion of each molded 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 at the fractured portion can be measured by molding with pressure-sensitive paper (carbonless duplicating paper) 17 placed between the punch 11 and the test piece 21, as shown in FIG. 3(b)(ii). The surface pressure at the fractured portion can also be calculated by finite element method analysis that reproduces the molding of the test piece 21.

[0035] In the forming test step S11, the thickness direction stress generated at the fractured portion of the formed test piece 21 may be calculated as the force in the thickness direction. In this case, the thickness direction stress 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, a sequence of evaluation points is set for each test piece 21 in order to obtain the strain distribution in the vicinity of the fracture portion generated in each test piece 21 formed in the forming test step S11.

[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 stored in chronological order at predetermined time intervals from the start of forming the test piece 21 to the occurrence of fracture, at the times (time steps) before and after the occurrence of fracture in the test piece 21.

[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, but may be any method that determines the forming limit using the same criteria for strain distributions obtained for each combination of test piece 21 and forming die 10 having different shapes, and determines the maximum principal strain and minimum principal strain at the forming limit.

[0044] <Forming Limit Surface Creation Step> 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. The three-dimensional coordinate space has three axes: the force in the thickness direction, the maximum principal strain, and the minimum principal strain. Then, a forming limit surface of the metal plate 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 fracture 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 molded with a molding die 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 using 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 by connecting the two selected plot points with one other plot point. This triangular plane creation is performed 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 a method in which, in the second method described above, 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 is minimized.

[0052] One possible 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 and third methods may be a combination of a plurality of forming limit planes and / or forming limit curved surfaces. For example, forming limit planes or forming limit curved surfaces may be assumed for the negative and positive minimum principal strain regions, respectively, and the forming limit planes or forming limit curved surfaces 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 to the case where a forming limit surface is created 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 other than the above, 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, this process is repeated to generate a polygonal surface consisting of a plurality of triangular planes, which may be used as the forming limit surface.

[0055] <Press-formed product crack determination 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. The press-formed product crack determination process P3 then 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 embodiment of the press-formed product crack determination process P3 includes a press-forming FEM analysis step S31, a press-forming crack determination parameter calculation step S33, and a press-forming crack occurrence determination step S35, as shown in FIG.

[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 process of press-forming a press-formed product is performed under the provisional press-forming conditions. As a result, changes in strain, stress, plate thickness, etc. occurring in the press-formed product are obtained for each element and node used in the FEM analysis.

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

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

[0060] The force in the thickness direction of the press-formed product is determined according to the force in the thickness direction used as an index of the forming limit of the metal plate acquired in the forming limit acquisition process P1. That is, when 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 thickness direction stress is used as the force in the thickness direction used 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 thickness direction stress generated in the press-formed product may be calculated for each element used in the FEM analysis.

[0061] <Press-forming crack occurrence determination step> The press-forming crack occurrence determination step S35 is a step for determining 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 the press-forming 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-molded product calculated in the press-molding 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-forming 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. The press-forming conditions are adjusted to increase the force in the plate 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-forming condition adjustment process P5 is as follows: First, if it is determined that a crack has occurred in the press-formed product crack determination process P3, the provisional press-forming conditions of the press-formed product are changed so as to increase the force in the plate thickness direction generated in the portion 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 plate thickness direction include inserting an elastic body between the metal plate blank and the press forming die. Other examples include using a pad mechanism using a gas cylinder or die cushion, or setting the forming bottom dead center clearance in the area where cracking is likely to occur to less than the plate thickness of the blank. 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 aspects of increasing the force in the plate thickness direction may be combined.

[0069] To increase the force in the thickness direction at the crack-prone area by changing the elastic body, pad mechanism, or bottom dead center clearance, the force in the thickness direction to be generated at the crack-prone area can be calculated using the following method, and the press-forming conditions can be adjusted accordingly. First, the minimum value of the force in the thickness direction required to keep the strain at the crack-prone area below the forming limit surface is calculated 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 at the crack-prone area. For example, if the forming limit surface is expressed by Equation (1) described below, the minimum principal strain (x) and maximum principal strain (z) at the crack-prone area are substituted into Equation (1) to calculate the minimum value of the surface pressure (y) required to keep the strain at 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, the pad pressure, or the bottom dead center clearance, are adjusted so that a force in the thickness direction (surface pressure) greater than or equal to the calculated minimum value is generated at the crack-prone area.

[0070] Then, after changing the provisional press-forming conditions (S51), in the press-molded product crack determination process P3, press-molding analysis is performed under the changed provisional press-molding conditions (S31). For example, if the provisional press-molding conditions are changed so that an elastic body is sandwiched between the blank and the press-molding die, press-molding analysis is performed in which the blank is press-molded with the elastic body sandwiched between the portion of the blank at risk of cracking and the press-molding die. Also, if the provisional press-molding conditions are changed to use a pad mechanism utilizing a gas cylinder or die cushion, press-molding analysis is performed by increasing the load applied by the pad mechanism to press the portion of the blank at risk of cracking. Alternatively, if the provisional press-molding conditions are changed so that the forming bottom dead center clearance at the portion at risk of cracking is less than the plate thickness of the blank, press-molding analysis is performed using a press-molding die in which the forming bottom dead center clearance at the portion at risk of cracking is changed to be less than the plate 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 crack has 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 crack has 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-forming Process> The press-forming process P7 is a process of 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 aspect in which the force in the plate thickness direction is increased in the press molding condition adjustment process P5.

[0076] That is, when the press-forming conditions are adjusted so that an elastic body is sandwiched between the blank and the press-forming die, the blank is press-formed with the elastic body sandwiched between the portion of the blank at risk of cracking and the press-forming die. When the press-forming conditions are adjusted so that a pad mechanism utilizing a gas cylinder or die cushion is used, the load applied by the pad mechanism to the portion of the blank at risk of cracking is increased during press-forming. Alternatively, when the press-forming conditions are adjusted so that the forming bottom dead center clearance at the portion of the blank at risk of cracking is less than the thickness of the blank, the shape of the press-forming die is changed so that the forming bottom dead center clearance at the portion of the blank at risk of cracking is less than the thickness of the blank, and press-forming is performed.

[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, it is difficult to appropriately adjust the press-forming conditions based on the determination of whether or not cracking occurs using a forming limit diagram created by the Nakajima method. In contrast, the method for manufacturing a press-formed product according to this embodiment adjusts the press-forming conditions to improve the forming limit by increasing the force in the plate thickness direction generated in the region at risk of cracking. Therefore, it is possible to widen the region in which the press-forming conditions can be adjusted to suppress both cracking and wrinkling, and it is possible to appropriately adjust the press-forming conditions to suppress cracking.

[0079] In the manufacturing method of the press-formed product according to the present embodiment, in the forming limit acquisition process P1, a forming test of a metal plate is performed using a plurality of tools having 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 above forming test, and may also be a process for acquiring a forming limit surface created in advance.

[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] Figure 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 fracture point of the test specimen 21 and the equivalent plastic strain at the forming limit, for test specimens 21 formed with various radii of curvature of the tip 11a of the punch 11. The equivalent plastic strain at the forming limit was calculated using strain measured in a forming test of the test specimen 21. In Figure 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 Figures 4(a) to 4(c). Furthermore, "flat bottom" represents the case where the radius of curvature of the tip 11a is infinite, i.e., the tip of the punch 11 has a flat bottom shape, as shown in Figure 12, as shown in Figure 4(d).

[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] 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. Thus, it was suggested that by forming the test piece 21 so as to increase the force (contact pressure) in the thickness direction, the forming limit of the metal plate is increased and the occurrence of cracks is suppressed.

[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 (the V-bent blank 101) when the force in the plate thickness direction generated in the blank 101 was changed.

[0087] A high-strength steel plate having a tensile strength of 1470 MPa and a thickness of 1.4 mm was used as the test material for the blank 101. As shown in Fig. 6, the press-forming die 110 was equipped with 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 had a diameter of φ100 mm, and the tip 111a had a radius of curvature of R50 mm, and the upper die 113 had a die shoulder 113a with a radius of curvature of R5 mm.

[0088] Furthermore, in Example 2, in order to change the force in the plate thickness direction generated on the blank 101, as shown in Fig. 7(b) , a V-bending punch 111A was used, which had a gap forming portion (non-contact area) 111b at the apex of the tip portion 111a so as to form a gap between the blank 101. By preventing the apex of the V-bending punch 111A from contacting the blank 101 during press forming, the force (surface pressure) in the plate thickness direction generated on the blank 101 was made to be 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 as the force in the thickness direction at the fractured portion using pressure-sensitive paper placed between the V-bending punch 111 or 111A and the blank 101.

[0091] When a V-bending punch 111A having a gap forming portion 111b at its tip 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.

[0095]

[0096] In formula (1), x, y, and z are the minimum principal strain, the surface pressure, and the maximum principal strain. Table 1 shows the values ​​of the coefficients a, b, c, and d in formula (1).

[0097]

[0098] 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 a plane in which the coefficients of Equation (1) are determined for a region in which the minimum principal strain is negative, and the forming limit plane B is a plane in which the coefficients of Equation (1) are determined for a region in which the minimum principal strain is positive.

[0099] 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.

[0100] 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 blanks made using a V-bending punch 111 without a gap forming portion.

[0101] 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.

[0102] Therefore, it can be seen that by press-forming the blank 101 so as to increase the surface pressure, i.e., the force in the plate thickness direction, the forming limit is increased and the occurrence of cracks is suppressed. Conversely, the above results show that when press-forming is performed so as to reduce the force generated in the plate thickness direction of the blank, the forming limit is lowered and cracks are more likely to occur.

[0103] 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 occurrence of cracks was verified.

[0104] The press-formed product 121 was produced by press-forming a high-strength steel plate having a tensile strength of 1470 MPa and a thickness of 1.4 mm using a press-forming die 130 including a lower die 131, an upper die 133, and a holder 135, as shown in Fig. 9(a) . As shown in Fig. 9(a) , the press-formed product 121 has regions A and B, which are regions where the blank 123 deforms without coming into contact with the press-forming die 130 (the lower die 131 and the upper die 133) during press-forming until just before the bottom dead center of forming.

[0105] Figure 9(b) is a graph showing the relationship between the minimum principal strain and the maximum principal strain during the forming process of parts A and B, and shows the deformation paths of parts A and B. As shown in Figure 9(b), part A is formed by undergoing plane strain tensile deformation. In contrast, part B is formed by undergoing uniaxial tensile deformation, as shown in Figure 9(b). In general, parts formed by undergoing plane strain tensile deformation have a smaller forming limit strain, so there is a greater risk of cracking in part A than in part B.

[0106] In Example 3, as shown in FIG. 10, a press-formed product 121 was press-formed under each of the press-forming conditions (1) to (4).

[0107] Condition (1) is a standard press forming condition, and the press forming die 130 shown in FIG. 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.

[0108] 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.

[0109] Condition (4) is the same as condition (3), in that the blank 123 is pressed by the pad with a pad load of 10 tonf, and in addition, the draw molding is performed with the elastic body 139 sandwiched between the lower mold 131 and the blank 123. The elastic body 139 is a sheet of polyvinyl chloride (PVC) and has a thickness of 3 mm.

[0110] Comparing conditions (1) to (4), under condition (1), part A does not come into contact with the press molding die during press molding, so the surface pressure is zero. Under conditions (2), (3), and (4), the pad load increases in that order, and the surface pressure is increased by further sandwiching an elastic body.

[0111] In Example 3, press molding was performed under each of the conditions (1) to (4) with the blank holder force varied within the range of 5 to 25 tonf, and the occurrence of cracks in the press-molded product 121 was compared.

[0112] 11 shows, as an example of the results of a press-forming experiment, a press-formed product 121 press-formed under condition (1) with a blank holder force of 10 tonf, and a press-formed product 121 press-formed under condition (4) with a blank holder force of 20 tonf. In the press-formed product 121 press-formed with a surface pressure of 10 tonf, a crack occurred in portion A, as shown in FIG. 11(b)(i). In contrast, in the press-formed product 121 press-formed with a surface pressure increased to 20 tonf, no cracks were observed, as shown in FIG. 11(b)(ii).

[0113] 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.

[0114]

[0115] As shown in Table 2, it can be seen that the region of blank holding force that can be formed without generating cracks expands in the order from condition (1) to condition (4). As mentioned above, the surface pressure, i.e., the force in the plate thickness direction, increases in the order from condition (1) to condition (4) at the crack-prone portion of the blank 123. Therefore, the results shown in Table 2 suggest that by increasing the force in the plate thickness direction generated in the blank 123, the region that can be formed without generating cracks expands, and press forming conditions that suppress cracking can be appropriately determined.

[0116] According to the present invention, it is possible to provide a method for manufacturing a press-formed product that can be press-formed by appropriately adjusting the press-forming conditions so as to suppress cracks in the press-formed product.

[0117] 10 molding die 11 punch 11a tip portion 13 upper die 15 blank holder 17 pressure-sensitive paper 21 test piece 21a notch portion 21b center portion 101 blank 110 press molding die 111 V-bending punch 111A V-bending punch 111a tip portion 111b gap forming portion 113 upper die 113a die shoulder portion 115 holder 121 press-molded product 123 blank 130 press molding die 131 lower die 133 upper die 135 holder 137 pad 139 elastic body 200 molding die 201 ball-head punch 203 upper die 205 blank holder 210 molding die 211 flat-head punch 213 driving sheet

Claims

1. A manufacturing method of a press-formed product for manufacturing a press-formed product while suppressing cracking during press forming of a metal plate, comprising: a forming limit acquisition process for acquiring the forming limit of the metal plate, which is expressed by the relationship between the force in the thickness direction and the maximum principal strain and the minimum principal strain of a test piece of the metal plate formed using a plurality of tools with different shapes so as to generate various forces in the thickness direction; a press-formed product crack determination process for acquiring the force in the thickness direction and the maximum principal strain and the minimum principal strain generated by press forming of the press-formed product as crack determination parameters for the press-formed product, and determining whether or not cracking has occurred in the press-formed product based on the acquired crack determination parameters and the forming limit of the metal plate acquired in the forming limit acquisition process; and a press-formed condition adjustment process for adjusting press-formed conditions to improve the forming limit by increasing the force in the thickness direction generated in the part of the press-formed product where cracking is suspected, in order to suppress cracking, if it is determined that cracking has occurred in the press-formed product crack determination process. 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. A method for manufacturing a press-formed product as described in claim 1, wherein in the press-forming condition adjustment process, the force in the 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-forming die.

3. A method for manufacturing a press-molded product as described in claim 1, wherein, in the press-molding condition adjustment process, the force in the 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, wherein, in the press-forming 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.

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