PRESS FORMING TOOL

Abstract

The forming tool 1 according to the present invention includes a punch (3) and a die (5) for press forming a pressure-formed product (11) that includes a punch bottom (13) and a shallow concave portion (15) in the punch bottom (13). The punch (3) includes a punch bottom forming surface (3a) for forming the punch bottom (13), a concave forming portion (3b) formed on the punch bottom forming surface (3a) to form the concave portion (15), and a low Young's modulus member installation region (3d) formed below the punch bottom forming surface (3a) and accommodating a low Young's modulus member (3c) having a lower Young's modulus than that of the punch bottom forming surface (3a).

Description

PRESS FORMING TOOL TECHNICAL FIELD The present invention relates to a press forming tool, in particular, a tool for press forming a workpiece into a press-formed product by pressing, having a punch bottom and a shallow concave portion or a low convex portion at the punch bottom. In the present application, the workpiece is a sheet of metal made of various metals, such as hot-rolled steel sheet, cold-rolled steel sheet, surface-treated steel sheet made by subjecting a steel sheet to surface treatment, including, for example, electrogalvanizing, hot-dip galvanizing, and organic film treatment, stainless steel sheet, aluminum sheet, and magnesium sheet. In the present application, the punch bottom is the bottom of a press-formed product formed with a punch and a die.Specifically, the punch bottom is formed by pushing the punch into the die. Furthermore, in this application, the directional terms “directly below,” “under,” and “above” are used to refer to the vertical direction in which the punch and die are positioned relative to each other in the press-forming direction. For example, if the die is above the punch, the terms “directly below” and “below” the punch indicate the direction opposite the die in the press-forming direction, while “above” the punch indicates the direction of the die in the press-forming direction. BACKGROUND The application of high-strength steel sheet to automotive body construction has increased significantly in response to demands for fuel efficiency and collision safety, along with the need to reduce vehicle body weight. However, when manufacturing automotive body parts from high-strength steel sheet using press forming, formability and dimensional accuracy are likely to be reduced due to the low ductility and high material strength of the steel sheet, which has been a challenge to overcome. Furthermore, increasing the forming load required to shape the parts necessitates changes to the press line and the decomposition of the parts. These are problems that must be addressed when using high-strength steel sheet. To overcome these problems, several technologies have been developed to reduce the forming load applied during press forming of high-strength steel sheet. For example, Patent Literature 1 describes a method for forming a sheet of metal using a die and punch. The method reduces stress on the sheet metal by stopping the punch during the operation, starting with the punch making contact with the sheet metal and ending with the punch reaching the end of the stroke, and resuming the sheet metal forming operation after a predetermined time has elapsed, thus reducing the forming load. Patent Literature 2 describes a method that allows a reduction in the forming load applied during the press forming process on a raw material using a die and punch.In the method, the raw material is formed sequentially by a predetermined section with several parts of the punch operating separately. List of Appointments Patent Literature Patent Literature 1: Japanese Patent Application Open for Public Inspection No. 20(11)-245525. Patent Literature 2: Japanese Patent Application Open for Public Inspection No. 2010207907. BRIEF DESCRIPTION OF THE INVENTION Technical Problem However, the method described in Patent Literature 1 requires a servo press to stop the punch during the forming process, which restricts the installation. With the method in Patent Literature 2, the complex die structure increases production costs. Furthermore, the method can cause fractures and wrinkles in high-strength steel sheets with low formability during press forming. For example, as illustrated in Figure 2, when forming a press-formed product (11) having a concave portion (15) at the bottom of the punch (13) and having a depth close to the plate thickness in the press-forming direction, the forming load increases markedly around the bottom dead center, as is common with plate thinning operations. In such a forming process, reducing the forming load has been an important issue. However, the methods in Patent Literatures 1 and 2 are not intended to produce products like the press-formed product (11), and therefore it has been difficult to sufficiently reduce the forming load applied when forming a concave or convex shape whose depth is close to the plate thickness. Therefore, technologies have been required to achieve a reduction in the forming load during the forming of the press-formed product (11) having a shallow concave portion (15) (or a convex portion in a low convex shape in the press-forming direction) at the bottom of the punch (13). From the above points of view, the present invention aims to provide a press forming tool that allows a reduction in the forming load applied during the press forming of a press formed product having a concave portion or a concave or convex portion in the press forming direction at the bottom of the punch, while eliminating the need to use a complex mechanism or structure. zfrccnn / zznz / E / YiAi Solution to the Problem A press forming tool according to the present invention includes: a punch; and a die, wherein the press forming tool is configured to press form a press-formed product that includes a punch bottom and a shallow concave portion or a shallow convex portion in the punch bottom, the punch includes a punch bottom forming surface for forming the punch bottom, a concave forming portion for forming the concave portion or a convex forming portion for forming the convex portion in the punch bottom forming surface, and a low Young's modulus member installation region formed below the punch bottom forming surface and accommodating a low Young's modulus member having a lower Young's modulus than that of the punch bottom forming surface.and the low Young's modulus member installation region includes an area directly below the concave forming portion or the convex forming portion of the punch and has a projected area in a press forming direction that is 110% or greater as large as a projected area of ​​the concave forming portion or the convex forming portion. Advantageous Effects of the Invention The present invention relates to the production of a press-formed product by press forming, which has a punch bottom and a shallow concave or shallow convex portion at the punch bottom. A punch includes a low Young's modulus member installation region in which a low Young's modulus member is installed, below a forming surface of the punch bottom. During the press-forming process, the forming surface of the punch bottom flexes to the low Young's modulus member installation region, facilitating the proximity between a concave or convex forming portion formed on the forming surface of the punch bottom and a workpiece. This configuration is effective in minimizing an increase in the forming load applied around the bottom dead center. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a drawing that illustrates the configuration of a press forming tool according to an embodiment of the present invention; Figure 2 is a drawing that illustrates an example of a press-formed product having a concave portion that is concave in the press-forming direction at the bottom of the punch. Figure 3 is a drawing illustrating the configuration of a conventional press forming tool. Figure 4 is a schematic drawing illustrating the deformation caused in the conventional press forming tool around the bottom dead center. Figure 5 is a schematic drawing illustrating the deformation and bending caused in a press forming tool according to the modality of the present invention, around the zfrccnn / zznz / E / YiAi bottom dead center. Figure 6 is a drawing that illustrates the configuration of another aspect of the press forming tool according to the embodiment of the present invention. Figure 7 is a drawing that indicates the results of press forming analyses that analyze the amount of deformation caused in a press forming tool according to a conventional example of the examples, around the bottom dead center. Figure 8 is a drawing illustrating the results of press forming analyses that analyze the amount of deformation caused in a press forming tool according to the example of the invention, around the bottom dead center. Figures 9A and 9B are graphs that indicate the relationship between the forming stroke and the forming load in the press forming process, analyzed by the press forming analyses in the examples (the graph in Figure 9A indicates the fluctuations from the start of forming to the bottom dead center, Figure 9B is an enlarged view around the bottom dead center). Figure 10 is a graph showing the results of the forming load measured at the bottom dead center in the conventional example and the example of the invention, of the examples. Figure 11 is a graph that indicates the relationship between the proportion of the projected area (A1) of the installation region of the low Young's modulus member with the projected area (A2) of the concave forming portion in the press forming direction and forming load, in the examples. DESCRIPTION OF THE MODALITIES As illustrated in Figure 1 by way of example, a press forming tool (1) according to one embodiment of the present invention includes a punch (3), a die (5), and a blank holder (7). As shown in Figure 2, the press forming tool (1) is used for press forming a press forming product (11) having a punch bottom (13) and a shallow concave portion (15) that is concave at the punch bottom (13). The shallow depth of the concave portion (15) means that at least its depth is less than the height of the press forming product (11). For example, the depth of the concave portion (15) in the press forming direction is 10 mm or less and is close to the thickness of the plate. Each component of the press forming tool (1) will now be described according to the modality.In the following description, components that have the same function will be indicated by the same reference number, and their description will be omitted. The punch (3) includes a punch bottom forming surface (3a) for forming the punch bottom (13) of the press-formed product (11), a concave forming portion (3b) for forming the concave portion (15) in the punch bottom (13), and a low Young's modulus member installation region (3d) formed below the punch bottom forming surface (3a) (opposite the die (5) in the press-forming direction) within the punch (3) and accommodating a low Young's modulus member (3c) having a lower Young's modulus than that of the punch bottom forming surface (3a). The low Young's modulus member (3c) is made of a material whose Young's modulus is lower than that of a casting and a steel material that is normally used for the punch (3) and die (5). Examples of the material may include resins, such as a rubber material and a urethane material, and metals, such as aluminum and magnesium. The installation region of the low Young's modulus member (3d) includes the area directly below the concave forming portion (3b), and has a projected area (A1) in the press forming direction that is (11)0% or larger than a projected area (A2) of the concave forming portion (3b). In the press forming tool (1) illustrated in Figure 1, the low Young's modulus member (3c) is installed so that it occupies the entire installation region of the low Young's modulus member (3d). The low Young's modulus member (3c) is installed in the installation region of the low Young's modulus member (3d), for example, by properly inserting the low Young's modulus member (3c) into the punch (3), tightening with bolts, and securing with adhesive. The die (5) is positioned in front of the punch (3) with a die (9) interposed between them. The die (5) is designed to move relative to the punch (3) to form the press-formed product (11) in coordination with the punch (3). The die (5) includes a punch bottom forming surface (5a) for forming the punch bottom (13) of the press-formed product (11) and a concave forming portion 5b for forming the concave portion (15) in the punch bottom (13). The blank holder (7) is positioned in front of the die (5), on the same side as the punch (3) with respect to the blank (9). The blank holder (7) is designed to move relatively toward the punch (3) while holding the ends of the blank (9) with the die (5). The operation of the press forming tool (1) in the above configuration according to the method for producing the press forming product (11) by press forming and the effects of the operation will now be described in comparison with the case of using a conventional press forming tool (21) illustrated in Figure 3. When the press-formed product (11) is produced by press forming using the conventional press-forming tool (21) which includes a punch (23), die (5), and blank holder (7) as illustrated in Figure 3, the blank (9) is placed between the punch (23) and the die (5) with the ends of the blank (9) clamped by the die (5) and blank holder (7). With the blank (9) held by the die (5) and blank holder (7), the die (5) and blank holder (7) are moved relative to the punch (23), thereby forming the bottom of the punch (13). When the press-formed product (11) reaches the bottom around the bottom dead center, the concave forming portion 5b of the die (5) pushes the blank (9) towards a concave forming portion (23b) of the punch (23), thus forming the concave portion (15) at the bottom of the punch (13). Simultaneously, around the bottom dead center, the surfaces of the punch (23) and the die (5) are elastically deformed and pressed downwards in the direction indicated by the arrows in Figure 4, as illustrated in Figure 4. The dashed lines in Figure 4 indicate the surface conditions of the punch (23) and die (5) before deformation. The areas shaded by the diagonal lines are areas where the punch (23) and die (5) are pressed around the bottom dead center. However, since the punch (23) and the die (5) do not deform elastically easily, the amount of deformation is small in each of them. To form the concave portion (15) with the blank (9) that fits sufficiently into the concave forming portion (23b) of the punch (23), the blank (9) must be pressed into the punch (23) by applying a large forming load, so that the thickness of the blank becomes thinner as it is pressed against the die (5), around the bottom dead center. On the other hand, when the press forming tool (1) having the punch (3) and die (5) according to the mode produces the press forming product (11) by press forming, as illustrated in Figure 5, the surfaces of the punch (3) and die (5) are pressed down around the bottom dead center. The dashed lines in Figure 5 represent the surface conditions of the punch (3) and die (5) before deformation. Specifically, as with the conventional press forming tool (21), the die surface (5) is elastically deformed and pressed down around the bottom dead center (areas on the die (5) shaded with diagonal lines in Figure 5). Since the installation region of the low Young's modulus member (3d) is disposed in the region that includes the area directly below the concave forming portion (3b), below the forming surface of the punch bottom (3a), the forming surface of the punch bottom (3a) around the bottom dead center flexes into the installation region of the low Young's modulus member (3d) while deformation of the punch surface (3) is induced (the shaded areas on the punch (3) in Figure 5). Because the amount of deformation of the punch (3) caused by bending of the forming surface of the punch bottom (3a) is greater than the amount of elastic deformation of the punch (23) of the conventional press forming tool (21), the proximity of the concave forming portion (3b) of the punch (3) to the workpiece (9) is easily achieved. Consequently, the press forming tool (1), depending on the modality, can produce the press-formed product (11) having the concave portion (15) formed by press forming while minimizing an increase in the forming load around the bottom dead center. The present invention can further minimize an increase in forming load around the bottom dead center, when the blank (9) to be pressed is a high tensile strength steel sheet of 590 MPa or more in the MPa class. The press forming tool (1) according to the present invention accommodates the low Young's modulus member (3c) within the punch (3). This structure can reduce the forming load with the punch (3) flexed during the forming process while ensuring structural strength, even when the tool structure does not allow the punch (3) to be hollow (e.g., when the press forming tool (1) is too small and therefore does not allow the punch (3) to be hollow to ensure sufficient strength). As described above, in the present invention, the projected area of ​​the installation region of the low Young's modulus member (3d) of the punch (3) in the press-forming direction is 110% or greater than the projected area of ​​the concave forming portion (3b). This structure allows the forming surface of the punch bottom (3a) to flex during the press-forming process. The thickness of the forming surface of the punch bottom (3a) in the press-forming direction can be appropriately set within the range that permits flexing of the forming surface of the punch bottom (3a). The projected area of ​​the concave forming portion (3b) is the area projected from the bottom of the concave forming portion (3b) of the punch bottom forming surface (3a). In the preceding description, the punch (3) has the low Young's modulus member (3c) installed in the low Young's modulus member installation region (3d) in a manner that completely occupies the region. In another configuration of the present invention, as illustrated in Figure 6, a press forming tool 31 may have low Young's modulus members (33c) partially installed in a low Young's modulus member installation region (33d) of a punch (33). With respect to the location and area where the low Young's modulus members (33c) are installed, no restriction is provided if: the low Young's modulus members (33c) have a lower Young's modulus than that of a punch bottom forming surface (33a) of the punch (33), the installation region of the low Young's modulus member (33d) includes the area directly below a concave forming portion (33b), and the projected area (A1) in the press forming direction is 110% or greater than the projected area (A2) of the concave forming portion (33b). The method of forming the press-formed product (11) having the concave portion (15) concave at the bottom of the punch (13) in the press-forming direction has been described above. Alternatively, the press-forming tools (1) and (31) according to the present invention can be designed to press-form a (non-lustered) press-formed product having a convex portion that is convex at the bottom of the punch in the press-forming direction and is low. The low convex portion means that at least the height of the convex portion is less than the forming height of the press-formed product. The present invention is preferably applicable to such a press-formed product having a convex portion whose height is 10 mm or less and nearly equal to the thickness of the plate. As described above, such a press-formed product having a convex portion at the bottom of the punch can be produced by press forming using the press-forming tool according to the present invention. In the process of forming the convex portion around the bottom dead center, the die is pushed toward the punch, causing the forming surface of the punch bottom to flex toward the installation region of the low Young's modulus member located underneath. This configuration can easily achieve close proximity between the workpiece and the convex forming portion of the punch. In this way, the convex portion is formed while minimizing an increase in the press-forming load applied around the bottom dead center. The press forming tool (1) according to the embodiment has the die (5) above the punch (3) as illustrated in Figure 1, with the direction in which the die (5) descends to the punch (3) defined as the press forming direction. In the present invention, the press forming direction can be the direction in which the punch (3) rises to the die (5). Furthermore, in the press forming tool according to the present invention, the die can be positioned below the punch with the punch configured to descend to the die or the die configured to rise to the punch (not illustrated). The present invention does not purport to limit the position of the punch with respect to the die or the press forming direction. In the preceding description, the press forming tool (1) illustrated in Figure 1 includes the blank holder (7) and produces the press forming product (11) by draw forming with both ends of the blank (9) pinched by the die (5) and the blank holder (7). The present invention can be applied to a press forming tool that does not have a blank holder and can be configured to produce the press forming product (11) by impact forming with the punch (3) and the die (5). EXAMPLES Several experiments were carried out to validate the effects of the press forming tool according to the present invention, and the details of these will now be described. In the experiments, the press forming process to produce the press forming product (11) of Figure 2 using the press forming tool (1) according to the present invention illustrated in Figure 1 was analyzed to evaluate the deformation of the punch (3) and the forming load applied during the press forming process. In the press forming analyses, a high-tensile steel sheet with a tensile strength of 1180 MPa and a plate thickness of 1.2 mm was used as the blank (9). The press forming product (11) has a hat-shaped cross-section with the bottom of the punch (13) and the concave portion (15) concave at the bottom of the punch (13) in the press forming direction, as illustrated in Figure 2. The dimensions of each part are indicated in Figure 2. The concave portion (15) to be formed at the bottom of the punch (13) has a depth of 3 mm, which is smaller than the forming height (20 mm) of the press forming product (11) and close to the plate thickness. In the experiments, the press-formed product (11) was produced using the press-forming tool (1) comprising the punch (3), die (5), and blank holder (7) as illustrated in Figure 1. The press-forming process analyses are described as an example of the invention. In the example of the invention, the punch (3) of the press-forming tool (1) comprises the punch bottom forming surface (3a), the concave forming portion (3b), and the low Young's modulus member installation region (3d) accommodating the low Young's modulus member (3c) having a lower Young's modulus than that of the punch bottom forming surface (3a), located below the punch bottom forming surface (3a). In the press forming analyses, the punch (3) made of FC250 (gray cast iron) with a Young's modulus of 115 GPa was used. The Young's modulus of the low Young's modulus member (3c) was set at 0.1 GPa, which was lower than the Young's modulus of the punch bottom forming surface (3a) of the punch (3). The installation region of the low Young's modulus member (3d) includes the area directly below the concave forming portion (3b) and has a projected area (A1) in the press forming direction that is 110% as large as the projected area (A2) of the concave forming portion (3b). For comparison, the press-formed product (11) was produced using the conventional press-forming tool (21) which includes the punch (23), die (5), and blank holder (7) as illustrated in Figure 3. The press-forming process analyses are described as a conventional example. Figure 7 and Figure 8 illustrate the results of the deformation of the punch (3), punch (23), and die (5) in the press-forming direction measured around the bottom dead center in the conventional example and the example of the invention. In the example illustrated in Figure 7, the concave forming portion (23b) of the punch (23) and the concave forming portion (5b) of the die (5) deformed in the press-forming direction in areas near the edges. The greatest amount of deformation was 0.02 mm. In the example of the invention illustrated in Figure 8, deformation in the press-forming direction was observed in extensive areas below the concave forming portion (3b) of the punch (3) and above the concave forming portion (5b) of the die (5). The greatest amount of deformation was 0.05 mm, which was a significant increase from the conventional example. Figures 9A and 9B are graphs showing the relationship between the forming stroke and the forming load in the press forming process. As is evident from Figures 9A and 9B, in both the conventional and the example of the invention, the forming load increases gradually after the start of forming (the 0 mm forming stroke) and increases sharply around the bottom dead center (the approximately 21 to 23 mm forming stroke). As illustrated in Figures 9A and 9B, the forming load applied from the start of forming to around the bottom dead center (the 22.7 mm forming stroke) was almost the same in both the conventional and the example of the invention. On the other hand, as illustrated in Figure 9B, the forming load around the bottom dead center was small in the example of the invention compared to the conventional example. Figure 10 is a graph showing the forming load of the conventional example and the example of the invention around the bottom dead center (23 mm forming stroke). As shown in Figure 10, the forming load of the example of the invention was approximately 2.0 tonf lower than the conventional example. zfrccnn / zznz / E / YiAi In the next step, fluctuations in the forming load applied at the bottom dead center were evaluated by changing the dimensions of the concave forming portion (3b) of the punch (3), thereby changing the ratio of the projected area (A1) of the installation region of the low Young's modulus member (3d) that includes the area directly below the concave forming portion (3b) with respect to the projected area (A2) of the concave forming portion (3b). The results are shown in Figure 11. As is evident from Figure 11, when the ratio of the projected area (A1) of the installation region of the low Young's modulus member (3d) to the projected area (A2) of the concave forming portion (3b) is 110% or greater, which is within the scope of the present invention, the forming load decreases drastically. When the installation region of the low Young's modulus member (3d) was displaced from the area directly below the concave forming portion (3b), the forming load was similar to the results obtained when the ratio of the projected area (A1) to the projected area (A2) in Figure 11 was less than 100%. Therefore, the forming load could not be effectively reduced. Although the examples fit a press-formed product having a concave portion that is concave in the press-forming direction at the bottom of the punch, the effect of reducing the forming load around the bottom dead center is also exerted on such a press-formed product having a convex portion that is convex in the press-forming direction at the bottom of the punch. As has been shown, the press forming tool (1) according to the present invention allows a reduction in the forming load applied around the bottom dead center, by flexing the forming surface of the punch bottom (3a) of the punch (3) towards the installation region of the low Young's modulus member (3d) formed under the forming surface of the punch bottom (3a) during the press forming of the press forming product (11) having the concave portion (15) or the convex portion that is concave or convex at the punch bottom (13) in the press forming direction. INDUSTRIAL APPLICABILITY According to the present invention, a press forming tool is provided that allows a reduction in the forming load applied in the press forming process to produce a press-formed product having a concave portion or a convex portion that is concave or convex in the press forming direction at the bottom of the punch, without using any complex mechanism or structure. List of Reference Signs (1) PRESS FORMING TOOL (3) PUNCH (3a) PUNCH BOTTOM FORMING SURFACE zfrccnn / zznz / E / YiAi (3b) CONCAVE SHAPE FORMING PORTION (3c) LOW YOUNG'S MODULE MEMBER (3d) LOW YOUNG'S MODULE MEMBER INSTALLATION REGION (5) DIE (5a) PUNCH BOTTOM FORMING SURFACE (5b) CONCAVE SHAPE FORMING PORTION (7) BOOKHOLDER (9) BOOK (11) PRESS FORMING PRODUCT (13) PUNCH BOTTOM (15) CONCAVE PORTION (21) PRESS FORMING TOOL (CONVENTIONAL STRUCTURE) (23) PUNCH (23a) PUNCH BOTTOM FORMING SURFACE (23b) CONCAVE FORMING PORTION (31) PRESS FORMING TOOL (33) PUNCH (33a) PUNCH BOTTOM FORMING SURFACE (33b) CONCAVE FORMING PORTION (33c) LOWER YOUNG'S MODULE MEMBER (33d) LOWER YOUNG'S MODULE MEMBER INSTALLATION REGION

Claims

1. A press forming tool comprising: a punch; and a die, wherein the press-forming tool is configured to press-form a press-formed product that includes a punch bottom and a shallow concave portion or a shallow convex portion in the punch bottom, the punch includes a punch bottom forming surface for forming the punch bottom, a concave forming portion for forming the concave portion or a convex forming portion for forming the convex portion in the punch bottom forming surface, and a low Young's modulus member installation region formed below the punch bottom forming surface and accommodating a low Young's modulus member having a lower Young's modulus than that of the punch bottom forming surface,and the Young's module member installation region includes an area directly below the concave forming portion or the convex forming portion of the punch and has a projected area in a press forming direction that is 110% or larger than a projected area of ​​the concave forming portion or the convex forming portion.

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