Manufacturing method of press-molded products

Abstract

The present invention provides a press-molding method that is excellent in shape freezing and dimensional accuracy fluctuation, enabling stable mass production of parts close to the target shape. The present invention provides a press-molded product manufacturing method that produces products with a U-shaped or other cross section and a curved portion that is curved convexly or concavely on the top plate side. The method includes a first step (2A) of molding an intermediate part (10) using a die having a first radius of curvature (R1) as the molding radius of curvature of the top plate that is smaller than the radius of curvature (R0) of the top plate in the product shape, and a second step (2B) of press-molding the intermediate part (10) using a die having a second radius of curvature (R2) as the molding radius of curvature of the top plate that is equal to or larger than the radius of curvature (R0) of the top plate in the product shape, in which the first radius of curvature (R1) is set so that the radius of curvature of the top plate (R1') after release of the intermediate part (10) from the die is less than the radius of curvature of the top plate (R0), and the first radius of curvature (R1) and the second radius of curvature (R2) are adjusted based on distortion caused by molding in the first step (2A) and the second step (2B).

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a press-formed product by press-forming a metal plate. The press-formed product to which the present invention is directed has a U-shaped or L-shaped cross section in which a vertical wall portion is continuous with a width direction end portion of a top plate portion. Furthermore, the press-formed product to which the present invention is directed has a part shape having one or more curved portions that are curved convexly or concavely toward the top plate portion side in a side view along the longitudinal direction of the part. [Background technology]

[0002] In recent years, there has been a demand for both improved collision safety and weight reduction of automobile bodies. For this reason, high tensile steel with a strength of 590 MPa or more and even ultra-high tensile steel with a strength of 980 MPa or more are being used as materials for car body structural parts. High tensile steel and ultra-high tensile steel have high yield strength and tensile strength. For this reason, forming defects due to springback and other factors become an issue when press forming high tensile steel.

[0003] One example of a press-formed product used in a body structural part is the following part shape. The part shape is a U-shaped cross section in which the top plate portion and the vertical wall portion are continuous in the width direction and curved with a predetermined radius of curvature along the longitudinal direction in a side view. When a part curved convexly toward the top plate portion is manufactured by press forming, compressive stress occurs in the vertical wall portion and tensile stress occurs in the top plate portion at the bottom dead center of forming. These stresses cause springback in the part. This springback is called camber back. When high tensile steel, especially ultra-high tensile steel, is used as the material for such a part, the above-mentioned stresses at the bottom dead center of forming tend to increase. As a result, a problem occurs in which springback increases.

[0004] Furthermore, ultra-high tensile steel has a relatively large variation in material strength during mass production. This means that the material strength is prone to vary between manufactured coils. Therefore, when ultra-high tensile steel is used, even if the same press forming is performed, the springback amount varies due to the variation, which is an issue. The change in springback amount due to the variation in material strength is called dimensional accuracy variation.

[0005] As techniques applicable to such problems, there are press molding methods described in Patent Documents 1 and 2, for example. The press molding method described in Patent Document 1 targets the following molded product. The molded product has a top plate portion curved in the longitudinal direction and two side wall portions extending from both ends along the longitudinal direction of the top plate portion toward the inside of the curve. Patent Document 1 also discloses that the curvature of the top plate portion and the angle between the top plate portion and the side wall portions are changed in a previous process. Patent Document 1 describes that this reduces the stress generated in a later process and suppresses springback.

[0006] Patent Document 2 also covers the following product shape. The product shape is such that the top plate and flange are continuous in the width direction via the side wall, and the top plate and flange have a hat-shaped cross section curved along the longitudinal direction. Patent Document 2 also describes that the product is formed in a later process with a larger radius of curvature than the product shape. Patent Document 2 describes that this reduces the stress generated in the later process and suppresses springback. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2011-206789 A [Patent Document 2] JP 2019-25533 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, in the press molding method described in Patent Document 1, the vertical wall portion is formed to sag in the forming process in the post-process. Therefore, the behavior of the plate during forming becomes unstable. For this reason, when the method of Patent Document 1 is applied to actual mass production, it may be difficult to achieve stable dimensional accuracy for the molded product. In addition, in the press forming method described in Patent Document 2, the radius of curvature of the top plate portion and the flange portion is changed in the previous process, so this method cannot be applied to parts with a U-shaped cross section that does not have a flange portion.

[0009] The present invention has been made with attention to the above points. The present invention is directed to a U-shaped or L-shaped cross-sectional member in which a top plate portion and a vertical wall portion are curved in the longitudinal direction in a side view. One of the objects of the present invention is to significantly reduce camber back and dimensional accuracy fluctuations with a simple die structure even when high tensile steel or ultra-high tensile steel is used for such cross-sectional members. Another object of the present invention is to provide a press forming method excellent in shape freezing and dimensional accuracy fluctuations, which can stably obtain high-precision parts close to the target part shape (product shape) in mass production. [Means for solving the problem]

[0010] In order to solve the problem, one aspect of the present invention is a manufacturing method for a press-molded product, which is manufactured by press-molding a metal plate into a part shape having a top plate portion and a U-shaped or L-shaped cross section in which a vertical wall portion is continuous with a widthwise end portion of the top plate portion, and having one or more curved portions that are curved convexly or concavely toward the top plate portion in a side view along a longitudinal direction that is a direction intersecting the cross section, wherein the radius of curvature along the longitudinal direction of the top plate portion is defined as the top plate radius of curvature, and the radius of curvature along the longitudinal direction of the top plate portion of the mold that molds the top plate portion in the mold is defined as the top plate molding radius of curvature, the first step is to press-mold the metal plate into an intermediate part having a U-shaped or L-shaped cross section using a mold having a first radius of curvature R1 that is smaller than the top plate radius of curvature R0 in a target product shape in a region where the curved portion is formed, and the second step is to press-molde the metal plate into an intermediate part having a U-shaped or L-shaped cross section using a mold having a top plate radius of curvature R1 that is smaller than the top plate radius of curvature R0 in a target product shape in a region where the curved portion is formed, ... third step is to press-molde the metal plate into an intermediate part having a U-shaped or L-shaped cross section using a mold having a top plate radius of curvature R1 that is smaller than the top plate radius of curvature R0 in a target product shape in a region where the curved portion is formed, the fourth step is to press-molde the metal plate into an intermediate part having a U-shaped or L-shaped cross section using a mold having a top plate radius of cur and a second step of press-molding the intermediate part using a die having a second radius of curvature R2, which is equal to or greater than radius R0, as a radius of curvature for molding the top plate. The first radius of curvature R1 is set so that the radius of curvature R1' of the top plate after the intermediate part is released from the die is less than the radius of curvature R0 of the top plate. Regarding the longitudinal strain occurring at the bottom dead point in the press molding of the first step, the absolute value of the average value of the strain of the top plate portion is ε1, and the average value of the strain of the lower half of the vertical wall portion is ε2. is defined as ε1', and the absolute value of the average value of the strain in the top plate portion is defined as ε2, and the absolute value of the average value of the strain in the lower half of the vertical wall portion is defined as ε2', in the longitudinal direction strain occurring at the bottom dead point in the press forming of the second step, the first radius of curvature R1 and the second radius of curvature R2 are adjusted so that they become values ​​that satisfy the following equations (1) and (2) in the region that becomes the curved portion. 0.1×ε1 ≦ ε2 < 0.8×ε1 (1) 0.3×ε1′ ≦ ε2′ < 0.85×ε1′ (2) Effect of the Invention

[0011] According to the aspect of the present invention, even if a high tensile steel is used for the metal plate, the following effect can be obtained. That is, according to the aspect of the present invention, it is possible to greatly reduce camber back and the fluctuation in dimensional accuracy caused by fluctuations in material strength without complicating the shape of the die. Note that camber back is spring back when viewed from the side. As a result, according to the present invention, it is possible to stably manufacture a part having a highly accurate U-shaped or L-shaped curved cross section close to the target product shape. That is, according to the present invention, it is possible to provide a manufacturing method for a press-molded product having excellent shape fixability and dimensional accuracy fluctuation. As described above, according to the aspects of the present invention, even if the material strength of the metal plate varies, parts with high dimensional accuracy can be obtained. As a result, according to the aspects of the present invention, it leads to an improvement in yield. Furthermore, when the present invention is applied to a vehicle body structural part, for example, it becomes possible to easily assemble the parts. [Brief description of the drawings]

[0012] [Figure 1] Schematic diagrams showing examples of target product shapes, (a) being a perspective view and (b) being a side view. [Diagram 2] 1A to 1C are diagrams illustrating steps in the present embodiment. [Diagram 3] FIG. 2 is a side view showing the state of springback in a conventional forming method. [Figure 4] 1 is an example of stress distribution at the bottom dead point of press molding using a conventional molding method. [Diagram 5] 1 is a schematic diagram showing a mechanism by which dimensional accuracy fluctuation occurs in a normal molding method. [Figure 6] FIG. 13 is a diagram showing a countermeasure method according to the present embodiment. [Figure 7] FIG. 1 is a diagram showing the difference in springback between steel types with respect to (R1′ / R0). [Figure 8] 1 shows the stress distribution at the bottom dead point of press molding in this embodiment, where (a) shows the stress distribution at the bottom dead point in the first process and (b) shows the stress distribution at the bottom dead point in the second process. [Figure 9]1A to 1C are schematic diagrams showing a mechanism for reducing fluctuations in dimensional accuracy when the present embodiment is used. [Figure 10] 1 shows the strain stress distribution at the bottom dead point of press forming in this embodiment, where (a) shows the strain stress distribution at the bottom dead point of press forming in the first step, and (b) shows the strain stress distribution at the bottom dead point of press forming in the second step. [Figure 11] FIG. 1 is a diagram showing the difference in springback between steel types with respect to (ε2 / ε1). [Figure 12] FIG. 1 is a diagram showing the difference in springback between steel types with respect to (ε2′ / ε1′). [Figure 13] FIG. 13 is a schematic diagram showing angles θ1 and θ2 of a mold for adjusting a vertical wall angle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Next, an embodiment of the present invention will be described with reference to the drawings. (Target part shape) In this embodiment, the target part shape 1 (product shape 1) of the part (press molded product) to be manufactured is a shape as shown in Fig. 1. That is, the shape is a U-shaped cross section having a top plate portion 1A and left and right vertical wall portions 1B continuing on both sides in the width direction of the top plate portion 1A. That is, in this embodiment, the cross section is illustrated as a U-shaped cross section. However, the present invention is also applicable to an L-shaped cross section in which the vertical wall portion 1B exists only on one side in the width direction of the top plate portion 1A.

[0014] Furthermore, the product shape 1 of this embodiment is a part shape in which the top plate portion 1A and the vertical wall portion 1B have curved portions that are curved convexly or concavely toward the top plate portion 1A side along the longitudinal direction when viewed from the side. The longitudinal direction is the direction that intersects with the above-mentioned U-shaped cross section. The product may have two or more curved portions. The example of FIG. 1 illustrates a case in which the product shape is curved so as to be convex toward the top plate portion 1A side over the entire longitudinal direction. In this example, the non-curved portions that are not curved portions are straight portions. The non-curved portions are, for example, portions of the top plate portion 1A with zero or nearly zero curvature along the longitudinal direction.

[0015] The curved portion may be formed only in a partial region in the longitudinal direction. The present invention also targets a product shape having a plurality of curved portions along the longitudinal direction. The present invention may also target a part shape in which convex curved portions and concave curved portions are alternately formed. The curvature of each curved portion along the longitudinal direction does not need to be a constant curvature. In addition, in the curved portion, the lower end of the vertical wall portion 1B may have a curvature different from the curvature of the top plate portion 1A along the longitudinal direction. The lower end of the vertical wall portion 1B is the end on the open end side away from the top plate portion 1A.

[0016] (Manufacturing method) The manufacturing method of the press-formed product of this embodiment is a method of manufacturing a metal plate (blank material) by press forming into the above-mentioned product shape 1. The present invention is suitable for the case where the material of the metal plate is a high tensile material (steel plate having a tensile strength of 590 MPa or more) or an ultra-high tensile material. However, the present invention is also applicable to metal plates such as mild steel plates and aluminum plates. The present embodiment aims to reduce the following when manufacturing a press-formed product having a product shape. That is, the present embodiment aims to reduce camber back, which is spring back along the longitudinal direction. Furthermore, the present embodiment aims to reduce the variation in dimensional accuracy between parts due to the strength variation (difference in strength) of the material of each metal plate.

[0017] As shown in FIG. 2, the method for manufacturing a press-molded product in this embodiment includes a first step 2A and a second step 2B. The first step 2A is a step of press-molding a metal plate into a shape similar to the target product shape using a die described later. That is, it is a step of press-molding a metal plate into an intermediate part having a U-shaped cross section and a top plate portion 1A. The intermediate part 10 may have a portion to be cut in a subsequent trimming step. For example, draw molding or form molding is applied to the press molding in the first step 2A.

[0018] The second process 2B is a process for producing a press-formed product having a target product shape by press-forming the intermediate part 10 using a die described later. For example, a restriking process is applied to the press forming in the second process 2B. Here, as described above, the radius of curvature along the longitudinal direction of the top plate portion 1A in the part shape with a U-shaped cross section is defined as the top plate radius of curvature. Also, the radius of curvature along the longitudinal direction of the top plate portion 1A of the molding surface that molds the top plate portion 1A in the mold is defined as the top plate molding radius of curvature. The top plate molding radius of curvature is the radius of curvature of the top plate in the mold shape. The mold shape is the shape of the molding surface of the mold.

[0019] (Regarding the shapes of the molds used in the first process 2A and the second process 2B) In this embodiment, the molding surfaces for molding the top plate portion 1A in the respective mold shapes used in the first step 2A and the second step 2B are set as follows. In the first step 2A, a mold that satisfies the following condition is used at least in the region where the curved portion is formed. The mold that satisfies the condition is a mold that sets a first radius of curvature R1, which is smaller than the radius of curvature R0 of the top plate in the target product shape, as the forming radius of curvature of the top plate. Furthermore, for the mold used in the first step 2A, the first radius of curvature R1 is set so as to satisfy the requirement that the radius of curvature R1' of the top plate after the intermediate part 10 is released from the mold is less than the radius of curvature R0 of the top plate. This setting may be adjusted, for example, by performing CAE analysis or the like so as to satisfy the conditions. That is, the mold for the first process 2A is set so that "R1 < R1' < R0".

[0020] In the second process 2B, a mold is used that satisfies the following condition at least in the region where the curved portion is formed. A mold that satisfies this condition is a mold that sets a second radius of curvature R2, which is equal to or greater than the radius of curvature R0 of the top plate in the target product shape, as the radius of curvature for forming the top plate. In other words, the mold for the second process 2B is set so that "R0 ≦ R2" holds.

[0021] In this embodiment, in addition to the above conditions, the first radius of curvature R1 and the second radius of curvature R2 are adjusted so as to satisfy the following condition. Here, for the longitudinal strain occurring at the bottom dead point in the press forming of the first step 2A, the absolute value of the average value of the strain in the region forming the curved portion of the top plate portion 10A is defined as ε1. Also, the absolute value of the average value of the strain in the lower half of the vertical wall in the region forming the curved portion of the vertical wall portion 10B is defined as ε1'. Also, for the longitudinal strain occurring at the bottom dead point in the press forming of the second step 2B, the absolute value of the average value of the strain in the region forming the curved portion of the top plate portion 1A is defined as ε2. Also, the absolute value of the average value of the strain in the lower half of the vertical wall in the region forming the curved portion of the vertical wall portion 1B is defined as ε2'.

[0022] In the present invention, "strain" refers to strain in the longitudinal direction. Also, ε2 is the average value at the bottom dead center of pressing in the second step 2B, with the longitudinal strain before the first step 2A (before forming) set as a reference value of 0. In other words, ε2 is the cumulative strain, which is the sum of the longitudinal strain applied in the first step 2A and the longitudinal strain applied in the second step 2B. In this case, the first curvature radius R1 and the second curvature radius R2 are adjusted so as to satisfy the following expressions (1) and (2) at least at the position that will become the curved portion. 0.1×ε1 ≦ ε2 < 0.8×ε1 (1) 0.3×ε1′ ≦ ε2′ < 0.85×ε1′ (2)

[0023] The first radius of curvature R1 and the second radius of curvature R2 can be adjusted to optimal values ​​by evaluating the relationship between each of the above distortions and the top plate forming radius of curvature through CAE analysis, or by conducting experiments to actually perform press forming. Furthermore, in addition to the above conditions, it is preferable to adjust the first radius of curvature R1 so that the value satisfies the following formula (3) at least in the region that becomes the curved portion. 0.45 ≦ (R1′ / R0) ≦ 0.85 ····(3)

[0024] In addition to the above conditions, the shapes of the dies used in the first step 2A and the second step 2B may be set as follows. Here, the angle between the top plate portion and the vertical wall portion in a member having a U-shaped cross section is defined as the vertical wall angle. In the mold used in the first step 2A, a first angle θ1 between the surface for molding the top plate portion 10A and the surface for molding the vertical wall portion 10B is set to an angle smaller than the vertical wall angle θ0 of the product shape. In the mold used in the second step 2B, a second angle θ2 between the surface for molding the top plate portion 1A and the surface for molding the vertical wall portion 1B is set to an angle larger than the vertical wall angle θ1' after the intermediate part 10 is released from the mold.

[0025] That is, the settings are "θ1 < θ0, θ1' < θ2." These settings should be satisfied at least in the region that will become the curved portion. At this time, it is preferable to set the first angle θ1 and the second angle θ2 so as to have values ​​that satisfy the following formula (4). θ1 < θ1′ < θ0 ≦ θ2 (4)

[0026] (Operation etc.) Next, the operation and other aspects of the method for producing a press-molded product according to this embodiment will be described. <Normal molding method> Here, we consider a case where a padded form molding is applied as a normal press molding method to manufacture the part shape shown in Fig. 1 in one press molding. The pad presses the entire top plate surface. Figure 3 is a side view showing the part shape before and after mold release (before and after springback). As shown in Figure 3, the radius of curvature of the top plate of the part changes before and after springback (at the bottom dead center of molding and after mold release). As a result, it can be seen that the radius of curvature of the top plate increases due to springback.

[0027] FIG. 4 shows the longitudinal stress distribution at the bottom dead point of the press. The stress distribution at the center of the plate thickness is shown. As can be seen from FIG. 4, tensile stress occurs over a wide range in the top plate portion 1A, and compressive stress occurs over a wide range in the vertical wall portion 1B. Therefore, the above-mentioned springback (camber back) is considered to occur when these stresses are released when the die is released. As the material strength increases, the residual stress increases and the amount of springback increases. Furthermore, as the material strength increases, the material strength fluctuation during mass production of the material also becomes relatively large. For this reason, the material strength varies between the coils (between production lots) as mentioned above, which causes a problem of dimensional accuracy fluctuation that changes the amount of springback.

[0028] This variation in dimensional accuracy will be explained with reference to the stress-strain relationship diagram shown in Figure 5. Figure 5 shows the relationship between stress and strain in the top plate portion 1A during normal molding. Figure 5 shows schematic diagrams for two cases, one for high-strength material and one for low-strength material, assuming variation in material strength. When press-molded into the product shape, the material of the top plate 1A is stretched. This causes tensile stress to occur at the bottom dead point of molding. However, as shown in Figure 5, there is a difference in the stress value at the bottom dead point of molding (see the position of the mold shape in Figure 5) between materials with high and low strength. When the mold is released from this state, camber back occurs, causing a change in curvature in the longitudinal direction (a change in the radius of curvature of the top plate). At this time, the difference in stress values ​​at the bottom dead center of molding causes the amount of camber back to vary between high-strength and low-strength materials, resulting in fluctuations in dimensional accuracy. Specifically, as the material strength increases, this fluctuation in dimensional accuracy becomes greater, increasing the risk of the product deviating from its dimensional tolerances. For this reason, the above is considered an important issue.

[0029] In this embodiment, the camber back and the variation in dimensional accuracy are reduced. To this end, as shown in Fig. 6, in the press forming method of this embodiment, an intermediate part 10 is formed in a first step 2A. The intermediate part 10 has a top plate portion 10A that is curved in the longitudinal direction and has a U-shaped cross section with a first radius of curvature R1 smaller than the radius of curvature R0 in the product shape. Furthermore, in a second step 2B, the intermediate part 10 is press formed to have a top plate radius of curvature larger than the top plate radius of curvature of the intermediate part 10 after springback in the first step 2A. Here, in the first step 2A, the top plate curvature radius of the top plate portion 10A of the intermediate part 10 after the die is released is slightly larger than the top plate forming curvature radius of the die in the first step 2A due to springback. It is preferable to design the die so that the top plate curvature radius of the intermediate part 10 after the springback (release) in the first step 2A is smaller than the top plate curvature radius of the target product shape. This design is performed by adjusting the top plate forming curvature radius of the die in the first step 2A.

[0030] In this embodiment, the radius of curvature of the top plate of the intermediate part 10 after demolding in the first step 2A is smaller than the radius of curvature of the top plate of the product shape. Therefore, in the second step 2B, it is necessary to bend back the top plate portion 10A of the intermediate part 10. Therefore, the radius of curvature of the top plate molding of the molding surface for molding the top plate portion 1A of the mold in the second step 2B is set to be equal to or larger than the radius of curvature of the top plate of the product shape. It is preferable to design the mold in the second step 2B so that the radius of curvature of the top plate of the mold in the second step 2B is equal to or larger than the radius of curvature of the top plate of the product shape.

[0031] From the above, in this embodiment, the values ​​of the top plate forming curvature radius of each mold in the first step 2A and the second step 2B are set so as to satisfy the following formula (5) or (5'). Here, the radius of curvature of the top plate in the product shape is defined as R0, the radius of curvature of the top plate formed in the die in the first process 2A is defined as R1, the radius of curvature of the top plate after springback of the intermediate part 10 is defined as R1', and the radius of curvature of the top plate formed in the die in the second process 2B is defined as R2. R1 < R1′ < R0 ≦ R2 (5) Or, R1 < R1′ < R0 < R2 ···(5′)

[0032] Furthermore, it is desirable to set the value of the radius of curvature of the top plate in the mold in the first step 2A so as to satisfy the following formula (3). 0.45 ≦ (R1′ / R0) ≦ 0.85 ····(3) Here, if (R1' / R0) is smaller than 0.45, the stress in the top plate portion 1A and the vertical wall portion 1B may be excessively reversed at the bottom dead point of the press in the second step 2B, and a large spring back may occur in the press-formed product. Conversely, if (R1' / R0) is larger than 0.85, the stress in the top plate portion 1A and the vertical wall portion 1B may not be reversed at the bottom dead point of the press in the second step 2B, and spring back may not be sufficiently suppressed.

[0033] FIG. 7 shows the results of an investigation into the difference in springback amount between steel sheets with tensile strengths ranging from 590 MPa to 1180 MPa. Figure 7 shows the results when the member curved in the longitudinal direction shown in Figure 1 was press-formed. The thickness of the metal plate was t = 1.4 [mm]. The target dimensions of the product shape are as follows: the width of the top plate 1A is 60 [mm], the height of the vertical wall 1B is 30 [mm], the angle between the top plate 1A and the vertical wall 1B is 110°, and the longitudinal length of the product is 400 [mm].

[0034] In Fig. 7, the horizontal axis represents R1' / R0, which is the radius of curvature changed under various conditions in the first process 2A. In Fig. 7, the circles and triangles connected by a line represent steel types with different tensile strength levels that are press-formed using a die of the same shape. The values ​​on the vertical axis represent the difference in springback between the steel types. As can be seen from Fig. 7, when R1' / R0 is in the range of 0.45 to 0.85, the difference in springback between steel types is within 3 mm, which is favorable. In Fig. 7, the ● and ▲ connected by a straight line represent reference data that do not satisfy the conditions of formula (1) and formula (2). As described above, by designing the mold shape for the first process 2A and the mold shape for the second process 2B based on the present invention, the stress causing camber back is reversed in the second process 2B, thereby reducing camber back and dimensional accuracy fluctuations.

[0035] FIG. 8 shows the longitudinal stress distribution at the center position of the plate thickness at the bottom dead center when press-forming is performed using the manufacturing method for press-formed products according to the present invention as described above. Fig. 8(a) shows the longitudinal stress distribution (at the center of the plate thickness) at the bottom dead point in the first step 2A. The radius of curvature of the top plate in the die in the first step 2A was set to R800 [mm]. As can be seen from Fig. 8(a), at the bottom dead point in the first step 2A, compressive stress occurs over a wide range in the longitudinal direction in the vertical wall portion 10B, and tensile stress occurs over a wide range in the longitudinal direction in the top plate portion 10A.

[0036] FIG. 8(b) shows the longitudinal stress distribution at the bottom dead point in the second process 2B. The value at the center of the plate thickness was used as a representative value. The radius of curvature of the top plate in the mold in the second process 2B was set to R1600 [mm]. As can be seen from FIG. 8(b), at the bottom dead point in the second process 2B, tensile stress occurred at the bottom of the vertical wall portion 1B and compressive stress occurred in the top plate portion 1A. Compared to the normal method (see FIG. 3), the stress was reversed, and the area where stress occurred was reduced. This shows that the press-formed product is bent back as a structure by forming the side view radius of curvature (top plate radius of curvature) to be larger in the second process 2B. As a result, the vertical wall portion 1B is formed in the direction in which the line length extends, so that it is subjected to tensile stress. The top plate portion 1A is formed in the direction in which the line length shortens, so that it is subjected to compressive stress.

[0037] The fluctuation in dimensional accuracy when press molding is performed using the method of this embodiment will be described with reference to a schematic diagram of the stress-strain relationship. Fig. 9 shows the relationship between stress and strain in the top plate portion 1A when this embodiment is adopted. In Fig. 9, two schematic diagrams of a high-strength material and a low-strength material are shown assuming variation in material strength. In the first process 2A (first forming process), it is formed into a radius of curvature (the top plate forming radius of curvature) smaller than the product shape. This imparts greater strain and tensile stress than in normal manufacturing methods, as shown in Figure 9. After that, in the second process 2B (second forming process), it is formed into a radius of curvature (the top plate forming radius of curvature) larger than the product shape. Then, because the material of the top plate portion 1A is formed in a shrinking direction, the tensile stress is reversed to compressive stress.

[0038] When the stress is reversed, a point appears where the curves of the high strength material and the low strength material intersect in the low stress region (symbol X in Fig. 9). By targeting this intersection point and forming the material with the first step 2A by reducing the radius of curvature (the radius of curvature for forming the top plate), the stress and stress difference at the bottom dead center of the material with high strength and the material with low strength are reduced. This reduces the camber back and the dimensional accuracy fluctuation. If the curvature of the curve in the longitudinal direction is not constant, it is sufficient to reduce the radius of curvature of only a part of the curved portion in the first step 2 A. Also, for a part shape consisting of multiple curved portions along the longitudinal direction, it is sufficient to reduce the radius of curvature of at least one or more curved portions in the first step 2 A.

[0039] FIG. 10 shows the distribution of longitudinal strain at the center of the plate thickness at the bottom dead point when a press-formed product is formed in this embodiment. FIG. 10(a) shows the distribution of longitudinal strain at the center of the plate thickness at the bottom dead point in the first process 2A. FIG. 10(b) shows the distribution of longitudinal strain at the center of the plate thickness at the bottom dead point in the second process 2B. Note that the radius of curvature of the top plate formed in the first process 2A was set to 800 [mm]. The radius of curvature of the top plate formed in the second process 2B was set to 1600 [mm].

[0040] As can be seen from FIG. 10(a), in the first step 2A, a large compressive strain occurs in the vertical wall portion 10B of the intermediate part 10 along the longitudinal direction. Also, a tensile strain occurs in the top plate portion 10A along the longitudinal direction. However, in the second step 2B, as shown in FIG. 10(b), the compressive strain in the vertical wall portion 1B is reduced, and the tensile strain in the top plate portion 1A is reduced. Therefore, it can be seen that the strain behavior is as shown in FIG. 9.

[0041] Here, for the longitudinal strain occurring at the bottom dead point of pressing in the second process 2B, the absolute value of the average value of the strain in the curved region of the top plate 1A is defined as ε2. Also, for the longitudinal strain occurring at the bottom dead point of pressing in the first process 2A, the absolute value of the average value of the strain in the curved region of the top plate 10A is defined as ε1. In this case, in this embodiment, the values ​​of the top plate forming curvature radii of the dies in the first step 2A and the second step 2B are set so as to satisfy the following formula (1). 0.1×ε1 ≦ ε2 < 0.8×ε1 (1)

[0042] Furthermore, for the longitudinal strain occurring at the bottom dead point of pressing in the second step 2B, the absolute value of the average value of the strain in the curved region of the lower half of the vertical wall portion 1B is defined as ε2'. Also, for the longitudinal strain occurring at the bottom dead point of pressing in the first step 2A, the absolute value of the average value of the strain in the curved region of the lower half of the vertical wall portion 10B is defined as ε1'. In this case, in this embodiment, the values ​​of the top plate forming curvature radii of the dies in the first step 2A and the second step 2B are set so as to satisfy the following formula (2). 0.3×ε1′ ≦ ε2′ < 0.85×ε1′ (2)

[0043] Here, if ε2 is smaller than 0.1 × ε1, the amount of strain recovery in the second step 2B shown in Figure 9 becomes too large. Therefore, there is a possibility that the difference in the amount of springback due to the difference in strength of the material cannot be reduced to a desired value or less. Therefore, ε2 is preferably 0.1 × ε1 or more, and more preferably 0.2 × ε1 or more. On the other hand, if ε2 is greater than 0.8 × ε1, the amount of strain recovery in the second step 2B will be insufficient. Therefore, there is a possibility that the difference in springback amount due to the difference in strength of the material cannot be reduced to a desired value or less. Therefore, it is preferable that ε2 is less than 0.8 × ε1, and more preferably 0.7 × ε1 or less.

[0044] Furthermore, if ε2' is smaller than 0.3 × ε1', the amount of strain recovery in the second step 2B shown in Figure 9 becomes too large. For this reason, it may not be possible to reduce the difference in springback amount due to the difference in strength of the material to a desired value or less. Therefore, ε2' is preferably 0.3 × ε1' or more, and more preferably 0.4 × ε1' or more. On the other hand, if ε2' is larger than 0.85 × ε1', the amount of strain recovery in the second step 2B will be insufficient. Therefore, there is a possibility that the difference in springback amount due to the difference in strength of the material cannot be reduced to a desired value or less. Therefore, ε2' is preferably less than 0.85 × ε1', and more preferably less than 0.8 × ε1'.

[0045] FIG. 11 shows the results of an investigation into the difference in springback amount between steel sheets with tensile strengths ranging from 590 MPa to 1180 MPa. Fig. 11 shows the results when the member curved in the longitudinal direction shown in Fig. 1 is press-formed. The thickness of the metal plate is t = 1.4 [mm]. The dimensions of the target product shape are as follows: the width of the top plate portion 1A is 60 [mm], the height of the vertical wall portion 1B is 30 [mm], the angle between the top plate portion 1A and the vertical wall portion 1B is 110°, and the longitudinal length of the product is 400 [mm].

[0046] In Fig. 11, the horizontal axis represents ε2 / ε1, which is changed by varying the radius of curvature in the first step 2A. In Fig. 11, circles and triangles connected by a line represent steel types with different tensile strength levels that were press-formed using a die of the same shape. The value on the vertical axis represents the difference in springback between the steel types. As can be seen from FIG. 11, when ε2 / ε1 is in the range of 0.1 to 0.8, the difference in the amount of springback between the steel types is within 3 mm, which is favorable.

[0047] FIG. 12 shows the results of an investigation into the difference in springback amount between steel sheets with tensile strengths ranging from 590 MPa to 1180 MPa. Figure 12 shows the results when press forming the member curved in the longitudinal direction shown in Figure 1. The thickness of the metal plate is t = 1.4 [mm]. The dimensions of the target product shape are as follows: the width of the top plate portion 1A is 60 [mm], the height of the vertical wall portion 1B is 30 [mm], the angle between the top plate portion 1A and the vertical wall portion 1B is 110°, and the longitudinal length of the product is 400 [mm].

[0048] In Fig. 12, the horizontal axis shows ε2' / ε1', which is obtained by changing the radius of curvature in the first process under various conditions. In Fig. 12, the circles and triangles connected by a line represent steel types with different tensile strength levels that were press-formed using a die of the same shape. The value on the vertical axis is the difference in springback between the steel types. As can be seen from FIG. 12, when ε2′ / ε1′ is in the range of 0.3 to 0.85, the difference in springback between the steel types is within 3 mm, which is favorable.

[0049] For the above reasons, according to the manufacturing method of the press-formed product of this embodiment, it is possible to significantly reduce camber back and dimensional accuracy fluctuations by simply adjusting the top plate forming curvature radius in the first step 2A and the second step 2B. That is, even when a high tensile material is used for the metal plate, it is possible to significantly reduce camber back and dimensional accuracy fluctuations when the material strength varies without complicating the die shape.

[0050] As a result, according to this embodiment, it is possible to stably manufacture a part having a highly accurate U-shaped curved cross section close to the target product shape. That is, according to this embodiment, it is possible to provide a manufacturing method for a press-formed product having excellent shape fixability and dimensional accuracy fluctuation. In the present embodiment, even if the material strength of the metal plate varies, parts with high dimensional accuracy can be obtained, leading to improved yield. Furthermore, when the present invention is applied to vehicle body structural parts, for example, it becomes possible to easily assemble the parts.

[0051] (Modification) In addition to adjusting the radius of curvature of the top plate in each mold shape used in the first step 2A and the second step 2B described above, the vertical wall angle between the top plate and the vertical wall may also be adjusted as described below. In this case, stress reversal becomes easier, and dimensional changes and fluctuations in dimensional accuracy become easier to reduce. Here, the angle between the top plate portion and the vertical wall portion in the U-shaped cross section of the part is defined as the vertical wall angle.

[0052] The first angle θ1 (see FIG. 13(a)) in the mold used in the first step 2A is set to an angle smaller than the vertical wall angle θ0 of the target product shape. The first angle θ1 is the angle between the surface 3A that molds the top plate portion 10A and the surface 3B that molds the vertical wall portion 10B. The second angle θ2 (see FIG. 13(b)) in the mold used in the second step 2B is set to an angle larger than the vertical wall angle θ1′ after the intermediate part 10 is released from the mold. The second angle θ2 is the angle between the surface 4A that molds the top plate portion 1A and the surface 4B that molds the vertical wall portion 1B. These angles only need to be set in at least the area that will become the curved portion. Furthermore, it is preferable to set the first angle θ1 and the second angle θ2 so as to have values ​​that satisfy the following formula (4). θ1 < θ1′ < θ0 ≦ θ2 (4)

[0053] That is, in the first process 2A, at the bottom dead point of forming, the intermediate part 10 is press-formed into a part shape having a U-shaped cross-sectional shape with a first angle θ1 smaller than the vertical wall angle θ0 of the product shape, to manufacture the intermediate part 10. Also, in the second process 2B, the intermediate part 10 is press-formed at the bottom dead point of forming into a vertical wall angle larger than the vertical wall angle θ1' after springback. For example, when forming a U-shaped cross section that is curved convexly toward the top plate portion 1A, the first angle θ1 and the second angle θ2 are set as described above. Then, in the second process 2B, the vertical wall portion 1B is deformed in the opening direction, and the vertical wall portion 1B is stretched in the longitudinal direction. This makes it easier to reverse the longitudinal stress of the vertical wall portion 1B, and makes it easier to reduce dimensional changes and fluctuations in dimensional accuracy.

[0054] Here, when designing the mold shapes for the first process 2A and the second process 2B, if you start designing without considering the radius of curvature of the product, or if you start designing from the mold for the second process 2B, you may not be able to find an appropriate mold shape. Or, it may take a very long time to find it.

[0055] Therefore, when designing the die shapes for the first step 2A and the second step 2B, first, the top plate forming curvature radius in the first step 2A is set to a value smaller than the top plate curvature radius of the product, and CAE analysis or actual pressing is performed. As a result, a condition is found in which the top plate curvature radius of the intermediate part 10 after springback in the first step 2A is smaller than the top plate curvature radius of the product. Then, in the second step 2B, CAE analysis or actual pressing is performed with a die shape having a top plate forming curvature radius larger than the top plate curvature radius of the product shape. As a result, a condition is found in which the top plate curvature radius after springback in the second step 2B matches the top plate curvature radius of the product shape and dimensional accuracy fluctuations are suppressed. By performing the above processing, it is possible to design the die shape in a short time.

[0056] At this time, it is preferable that the vertical wall angle in the first step 2A is as small as possible compared to the vertical wall angle in the second step 2B, but there is no problem if the vertical wall angle is the same as that in the second step 2B. In this way, the present invention can significantly reduce the variation in dimensional accuracy even when using high tensile steel with a strength class of 590 PMa or more, or ultra-high tensile steel with a strength class of 980 MPa or more. In other words, according to the present invention, it is possible to significantly reduce the variation in dimensional accuracy when a variation in material strength occurs, with a simple die structure. Therefore, it is possible to obtain a highly accurate part that is close to the target part shape. Here, in the above description, a U-shaped cross section has been used as an example, but the present invention is also applicable to a product shape having an L-shaped cross section.

[0057] (others) The present disclosure may also have the following configuration. (1) A method for manufacturing a press-molded product, which is manufactured by press-molding a metal plate into a part shape having a top plate portion and a U-shaped or L-shaped cross section in which a vertical wall portion is continuous with a widthwise end portion of the top plate portion, and having one or more curved portions that are curved convexly or concavely toward the top plate portion in a side view along a longitudinal direction that intersects with the cross section, When the radius of curvature along the longitudinal direction of the top plate portion is defined as the top plate radius of curvature, and the radius of curvature along the longitudinal direction of the top plate portion of the molding surface that molds the top plate portion in the mold is defined as the top plate molding radius of curvature, A first step of press-molding a metal plate into an intermediate part having a U-shaped or L-shaped cross section using a die having a first radius of curvature R1 smaller than the radius of curvature R0 of the top plate in the target product shape at least in the region where the curved portion is formed; A second step of press-molding the intermediate part using a die having a second radius of curvature R2, which is equal to or greater than the radius of curvature R0 of the top plate in the target product shape, at least in the region in which the curved portion is formed; Equipped with The first radius of curvature R1 is set so that the radius of curvature R1′ of the top plate after the intermediate part is released from the mold is less than the radius of curvature R0 of the top plate; Furthermore, Regarding the longitudinal strain occurring at the bottom dead point of the press in the press forming of the first step, the absolute value of the average value of the strain of the top plate portion is defined as ε1, and the absolute value of the average value of the strain of the lower half of the vertical wall portion is defined as ε1', Regarding the longitudinal strain occurring at the bottom dead point of the press in the press forming of the second step, the absolute value of the average value of the strain of the top plate portion is defined as ε2, and the absolute value of the average value of the strain of the lower half of the vertical wall portion is defined as ε2'. The first curvature radius R1 and the second curvature radius R2 are adjusted so as to satisfy the following formulas (1) and (2) at least in the region that becomes the curved portion. A manufacturing method for press-molded products. 0.1×ε1 ≦ ε2 < 0.8×ε1 ···(1) 0.3×ε1′ ≦ ε2′ < 0.85×ε1′ (2) (2) Furthermore, the first radius of curvature R1 is adjusted so as to have a value that satisfies the following formula (3) at least in the region that becomes the curved portion. 0.45 ≦ (R1′ / R0) ≦ 0.85 ····(3) (3) When the angle between the top plate and the vertical wall in a U-shaped or L-shaped cross section is defined as the vertical wall angle, At least in the region that will become the curved portion, a first angle θ1 between a surface that forms the top plate portion and a surface that forms the vertical wall portion in the mold used in the first step is set to an angle smaller than the vertical wall angle θ0 of the product shape; At least in the region that will become the curved portion, the second angle θ2, which is the angle between the surface that molds the top plate portion and the surface that molds the vertical wall portion in the mold used in the second process, is set to an angle larger than the vertical wall angle θ1' after the intermediate part is released from the mold. (4) If the vertical wall angle of the intermediate part molded in the first process after release from the mold is defined as θ1', the first angle θ1 and the second angle θ2 are set so as to have values ​​that satisfy the following equation (4). θ1 < θ1′ < θ0 ≦ θ2 (4) (5) Applying draw molding or form molding to the press molding of the first step, A restriking process is applied to the press forming in the second process. (6) The metal plate is a steel plate having a material strength of 590 MPa or more. EXAMPLES

[0058] In this example, in order to confirm the effect of reducing camber back by the press forming according to this embodiment, a press forming analysis and a spring back analysis were performed by the finite element method (FEM). In this embodiment, the case where a member curved in the longitudinal direction shown in FIG. 1 is press-formed is targeted. Here, the metal plate used for press forming had a plate thickness t = 1.4 [mm]. In order to compare the dimensional accuracy fluctuation, three types of steel plate with tensile strengths of 590 MPa, 980 MPa, and 1180 MPa were used. The target product shape was a width of 60 [mm] for the top plate part 1A, a height of 30 [mm] for the vertical wall part 1B, an angle of 110° between the top plate part 1A and the vertical wall part 1B, and a longitudinal length of 400 [mm] for the product.

[0059] Example 1 First, press forming analysis and springback analysis were carried out for the case where only the radius of curvature of the side view curve in the first process 2A was reduced. Then, the radius of curvature before and after springback was measured. In this embodiment, the radius of curvature R2 of the top plate of the mold used in the second process 2B was set to R1600 [mm]. The radius of curvature R1 of the top plate of the mold used in the first process 2A was changed between R700 [mm] and R1600 [mm] to perform FEM analysis.

[0060] The conditions and evaluation results are shown in Table 1. In Table 1, mold R refers to the radius of curvature of the top plate molding, and top plate R refers to the radius of curvature of the top plate. The camber back amount was evaluated as the deviation in the Z direction (height direction) from the bottom dead center of the press forming in the second process 2B. A steel grade with a difference in the deviation in the height direction after spring back in the second process 2B of 3.0 mm or less was deemed to be acceptable.

[0061] [Table 1]

[0062] <No.1~3> No. 1 to 3 are the results of the radius of curvature of the top plate 1A of the 590 MPa, 980 MPa and 1180 MPa materials when the top plate forming radius of curvature R1 of the mold in the first process 2A is R1600 [mm]. The top plate radius of curvature R0 of the target product shape in No. 1 to 3 is R1600 [mm]. In this case, the radius of curvature of the top plate after springback in the first process 2A of each material greatly exceeds R1600 [mm], which is the radius of curvature of the top plate formed by the die in the second process 2B. Looking at the 1180MPa material, the radius of curvature of the top plate part 1A after springback in the first process 2A was large at R2390 [mm]. In addition, the radius of curvature of the top plate part 1A after springback in the second process 2B of the 1180MPa material was large at R3056 [mm]. Furthermore, the difference between the steel types in the amount of deviation in the height direction after springback in the second process 2B was 3.49 [mm]. This value exceeded the target of 3.0 [mm].

[0063] <No.4~6> Nos. 4 to 6 are the results of the radius of curvature of the top plate 1A of the 590 MPa, 980 MPa and 1180 MPa materials when the top plate forming radius of curvature R1 of the mold in the first process 2A is R1040 [mm]. The top plate radius of curvature R0 of the target product shape in Nos. 4 to 6 is R1600 [mm]. In this case, the radius of curvature of the top plate after springback in the first process 2A for each material is smaller than the radius of curvature of the top plate formed by the die in the second process 2B, R1600 [mm]. Looking at the 1180MPa material, the radius of curvature of the top plate part 1A after springback in the first process 2A was R1360 [mm]. In addition, the radius of curvature of the top plate part 1A after springback in the second process 2B for the 1180MPa material was R1823 [mm]. The difference between the steel types in the amount of deviation in the height direction after springback in the second process 2B was 2.65 [mm], which was less than the target difference between the steel types of 3.0 [mm].

[0064] <No.7~9> In contrast, the results of the radius of curvature of the top plate 1A of the 590 MPa, 980 MPa and 1180 MPa materials when the top plate forming radius of curvature of the mold in the first process 2A is R1000 [mm] are shown in Nos. 7 to 9. The top plate radius of curvature R0 for the target product shape in Nos. 7 to 9 is R1600 [mm]. The radius of curvature of the top plate after springback in the first process 2A for each material is smaller than the radius of curvature of the top plate formed by the die in the second process 2B, R1600 [mm]. Looking at the 1180MPa material, the radius of curvature of the top plate after springback in the first process 2A was R1237 [mm]. The radius of curvature of the top plate after springback in the second process 2B for the 1180MPa material was R1742. The difference between the steel types in the amount of deviation in the height direction after springback in the second process 2B was 2.45 [mm], which was less than the target of 3.0 [mm].

[0065] <No.10~12> Next, the results of the radius of curvature of the top plate 1A of the 590MPa material, 980MPa material, and 1180MPa material when the top plate forming radius of curvature of the mold in the first process 2A is R800 [mm] are shown in Nos. 10 to 12. The top plate radius of curvature R0 for the target product shape in Nos. 10 to 12 is R1400 [mm]. The radius of curvature of the top plate 1A after springback in the first process 2A for each material is smaller than the radius of curvature R1600 [mm] of the top plate forming mold radius of curvature in the second process 2B. Looking at the 1180MPa material, the radius of curvature of the top plate 1A after springback in the first process 2A was R956 [mm]. In addition, the radius of curvature of the top plate 1A after springback in the second process 2B for the 1180MPa material was R1407 [mm]. The difference between the steel types in the amount of deviation in the height direction after springback in the second process 2B was 1.00 [mm], and the difference between the steel types was less than the target of 3.0 [mm].

[0066] <No.13~15> Next, the results of the radius of curvature of the top plate 1A of the 590MPa material, 980MPa material and 1180MPa material when the top plate forming radius of curvature of the die in the first process 2A is R700 [mm] are shown in Nos. 13 to 15. The top plate radius of curvature R0 for the target product shape in Nos. 13 to 15 is R1300 [mm]. The radius of curvature of the top plate portion 1A after springback in the first process 2A for each material is smaller than the radius of curvature of the top plate formed by the die in the second process 2B, R1600 [mm]. Looking at the 1180MPa material, the radius of curvature of the top plate portion 1A after springback in the first process 2A was R820 [mm]. In addition, the radius of curvature of the top plate portion 1A after springback in the second process 2B for the 1180MPa material was R1220 [mm]. The difference between the steel types in the amount of deviation in the height direction after springback in the second process 2B was 1.60 [mm], and the difference between the steel types was less than the target of 3.0 [mm].

[0067] From the above, by forming the top plate forming radius of curvature of the die in the first process 2A under the conditions of the invention example, the amount of camber back and the variation in dimensional accuracy were significantly reduced. However, the relationship between the above spring back and the top plate forming radius of curvature of the die in the first and second processes 2A and 2B varies depending on the steel type and shape. For this reason, it is necessary to design a shape that suits each case.

[0068] Example 2 We carried out press forming analysis and springback analysis for the case where the radius of curvature of the side view curvature and the vertical wall angle of the first process 2A were reduced when press forming the longitudinally curved member shown in Figure 1. We then measured the radius of curvature before and after springback. The metal plates used in the press molding were the same as those in Example 1. In this embodiment, the radius of curvature R2 of the mold for forming the top plate in the second step 2B was set to R1600 [mm]. The second angle θ2 was set to 110°. The radius of curvature R1 of the mold for forming the top plate in the first step 2A was changed between R1000 [mm] and R1600 [mm]. Furthermore, the first angle θ1 was changed to 90°, and an FEM analysis was performed.

[0069] Table 2 shows the conditions and evaluation results. In Table 2, mold R refers to the top plate forming radius of curvature, and top plate R refers to the top plate radius of curvature. The camber back amount was evaluated as the deviation in the Z direction (height direction) from the bottom dead center of the press forming in the second process 2B. In addition, a product was deemed to have passed when the difference between the steel types in the deviation in the height direction after the spring back in the second process 2B was 3.0 mm or less. [Table 2]

[0070] <No.16~18> Nos. 16 to 18 are the results of the radius of curvature of the top plate 1A of the 590 MPa, 980 MPa and 1180 MPa materials when the top plate forming radius of curvature of the mold in the first process 2A is R1600 [mm] and the vertical wall angle is changed to 90°. The top plate radius of curvature R0 for the target product shape in Nos. 16 to 18 is R1600 [mm]. The radius of curvature of the top plate 1A after springback in the first step 2A of each material is much larger than the radius of curvature of the top plate formed by the die in the second step 2B, R1600 [mm]. Looking at the 1180MPa material, the radius of curvature of the top plate 1A after springback in the first step 2A was large at R2418 [mm]. On the other hand, the vertical wall angle after springback in the first step 2A is less than the vertical wall angle of the second step 2B, which is 110°. In addition, the radius of curvature of the top plate 1A after springback in the second step 2B of the 1180MPa material was large at R2643 [mm]. Furthermore, the difference between the steel types in the amount of deviation in the height direction after springback in the second step 2B was 3.45 [mm].

[0071] <No.19~21> In contrast, the results of the radius of curvature of the top plate 1A of the 590 MPa material, 980 MPa material, and 1180 MPa material when the top plate forming radius of curvature of the mold in the first process 2A is R1000 [mm] are shown in Nos. 19 to 21. The top plate radius of curvature R0 for the target product shape in Nos. 19 to 21 is R1600 [mm]. The radius of curvature of the top plate 1A after springback in the first process 2A for each material is smaller than the radius of curvature of the top plate formed by the die in the second process 2B, R1600 [mm]. Looking at the 1180MPa material, the radius of curvature of the top plate 1A after springback in the first process 2A was R1191 [mm]. In addition, the radius of curvature of the top plate 1A after springback in the second process 2B for the 1180MPa material was R1426 [mm]. The difference between the steel types in the amount of deviation in the height direction after springback in the second process 2B was 0.18 [mm], and the difference between the steel types was less than the target of 3.0 [mm]. This shows that the difference between steel types has been reduced compared to No. 7 to 9 in Table 1. No. 7 to 9 in Table 1 are cases where the top plate forming curvature radius of the die in the first process 2A was reduced to R1000 [mm], and the vertical wall angle was kept at 110°, the same as in the second process 2B.

[0072] As a result of the above, by reducing the angle of the die that adjusts the radius of curvature of the side view curve in the first process 2A (the radius of curvature for forming the top plate) and the vertical wall angle, the amount of camber back and the variation in dimensional accuracy were significantly reduced. Note that the relationship between the above spring back and the vertical wall angle of the die in the first process 2A, and the radius of curvature for forming the top plate of the die in the first and second processes 2A and 2B varies depending on the steel type and shape, so it is necessary to design a shape that suits each case.

[0073] The entire contents of Japanese Patent Application No. 2023-135449 (filed on August 23, 2023), from which this application claims priority, are incorporated herein by reference. Although the present application has been described with reference to a limited number of embodiments, the scope of the rights is not limited thereto, and modifications of each embodiment based on the above disclosure would be obvious to one skilled in the art. [Explanation of symbols]

[0074] 1 Product shape 1A Top plate 1B Vertical wall section 2A First Step 2B Second step 10 Intermediate parts 10A Top plate 10B Hemisphere

Claims

1. A method for manufacturing a press-molded product manufactured by press-molding a metal plate into a part shape having a top plate portion and a U-shaped or L-shaped cross section in which a vertical wall portion is continuous with a widthwise end portion of the top plate portion, and having one or more curved portions that are curved convexly or concavely toward the top plate portion in a side view along a longitudinal direction that intersects with the cross section, When the radius of curvature along the longitudinal direction of the top plate portion is defined as the top plate radius of curvature, and the radius of curvature along the longitudinal direction of the top plate portion of the molding surface that molds the top plate portion in the mold is defined as the top plate molding radius of curvature, A first step of press-molding a metal plate into an intermediate part having a U-shaped or L-shaped cross section using a die having a first radius of curvature R1 smaller than the radius of curvature R0 of the top plate in the target product shape in the region where the curved portion is to be formed; A second step of press-molding the intermediate part using a die having a second radius of curvature R2, which is equal to or greater than the radius of curvature R0 of the top plate in the target product shape, in a region in which the curved portion is to be formed; Equipped with The first radius of curvature R1 is set so that a radius of curvature R1' of the top plate after the intermediate part is released from the mold is less than a radius of curvature R0 of the top plate; Regarding the longitudinal strain occurring at the bottom dead point in the press forming of the first step, the absolute value of the average value of the strain of the top plate portion is defined as ε1, and the absolute value of the average value of the strain of the lower half of the vertical wall portion is defined as ε1'. Regarding the longitudinal strain occurring at the bottom dead point of the press in the press forming of the second step, if the absolute value of the average value of the strain of the top plate portion is defined as ε2 and the absolute value of the average value of the strain of the lower half of the vertical wall portion is defined as ε2', In the region that becomes the curved portion, the first curvature radius R1 and the second curvature radius R2 are adjusted so as to have values ​​that satisfy the following formulas (1) and (2). A manufacturing method for press-molded products. 0.1×ε1 ≦ ε2 < 0.8×ε1 (1) 0.3×ε1′ ≦ ε2′ < 0.85×ε1′ ・・・(2)

2. In the region that becomes the curved portion, the first radius of curvature R1 is adjusted so as to satisfy the following formula (3): A method for producing a press-molded product according to claim 1. 0.45 ≦ (R1' / R0) ≦ 0.85 (3)

3. When the angle between the top plate portion and the vertical wall portion in the above cross section is defined as the vertical wall angle, In the region that will become the curved portion, a first angle θ1, which is an angle between a surface that forms the top plate portion and a surface that forms the vertical wall portion in the mold used in the first step, is set to an angle smaller than the vertical wall angle θ0 of the product shape; In the region that will become the curved portion, a second angle θ2, which is an angle between a surface that molds the top plate portion and a surface that molds the vertical wall portion in the mold used in the second step, is set to an angle larger than the vertical wall angle θ1′ after the intermediate part is released from the mold. A method for producing a press-molded product according to claim 1 or 2.

4. When the vertical wall angle after the intermediate part molded in the first step is defined as θ1′, The first angle θ1 and the second angle θ2 are set so as to satisfy the following formula (4): A method for producing a press-molded product according to claim 3. θ1 < θ1' < θ0 ≦ θ2 (4)

5. Applying draw molding or form molding to the press molding of the first step, A restriking process is applied to the press forming of the second process. A method for producing a press-molded product according to claim 1 or 2.

6. The metal plate is a steel plate having a material strength of 590 MPa or more. A method for producing a press-molded product according to claim 1 or 2.

Images