Method for manufacturing press-molded article

Abstract

To provide a press-molding method excellent in shape fixability and dimensional accuracy fluctuation, with which it is possible to stably mass-produce parts close to a target shape. This method for manufacturing a press-molded article is for manufacturing a product having a cross-section that is C-shaped, etc., and having a curved part that is curved in a concave or convex shape towards the top plate part side. The present invention is provided with: a first step (2A) for molding an intermediate component (10) using a mold having, as a top plate molding curvature radius, a first curvature radius (R1) smaller than the top plate curvature radius (R0) in the product shape; and a second step (2B) for press-molding the intermediate component (10) using a mold having, as a top plate molding curvature radius, a second curvature radius (R2) equal to or greater than the top plate curvature radius (R0) in the product shape, the first curvature radius (R1) being set such that the top plate curvature radius (R1') after release of the intermediate component (10) is smaller than the top plate curvature radius (R0), and the first curvature radius (R1) and the second curvature radius (R2) being further adjusted on the basis of the strain due to molding in the first step (2A) and the second step (2B).

Description

Manufacturing method for press-molded products

[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 be manufactured by the present invention has a U-shaped or L-shaped cross section in which a vertical wall portion is continuous with a widthwise end portion of a top plate portion. Furthermore, the press-formed product to be manufactured by the present invention has a part shape having one or more curved portions that are convex or concave toward the top plate portion in a side view along the longitudinal direction of the part.

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

[0003] One example of a press-formed product used in a vehicle body structural part is the following part shape. This part shape has a top plate portion and a vertical wall portion that are continuous in the width direction, and a U-shaped cross section curved at a predetermined radius of curvature along the longitudinal direction in a side view. When a part that is convexly curved toward the top plate portion is produced 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 then 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 aforementioned stresses tend to increase at the bottom dead center of forming. As a result, problems such as increased springback arise.

[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 using ultra-high tensile steel, even if the same press forming is performed, the amount of springback varies due to this variation, which is an issue. The change in springback due to the variation in material strength is called dimensional accuracy variation.

[0005] Techniques applicable to these problems include press forming methods described in Patent Documents 1 and 2, for example. The press forming 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 of the top plate portion along the longitudinal direction toward the inside of the curve. Patent Document 1 also discloses changing the curvature of the top plate portion and the angle between the top plate portion and the side wall portions in a pre-process. Patent Document 1 states that this reduces stress generated in a post-process and suppresses springback.

[0006] Furthermore, Patent Document 2 targets the following product shape. In this product shape, the top plate portion and flange portion are continuous in the width direction via the side wall portion, and the top plate portion and flange portion have a hat-shaped cross section curved along the longitudinal direction. Patent Document 2 then describes that the product is formed in a subsequent process to have a larger radius of curvature than the product shape. Patent Document 2 states that this reduces stress generated in the subsequent process and suppresses springback.

[0007] JP 2011-206789 A JP 2019-25533 A

[0008] However, in the press-forming method described in Patent Document 1, the vertical wall portion sags during the subsequent forming process. This causes the plate to behave unstably during forming. For these reasons, when the method described in Patent Document 1 is applied to actual mass production, it may be difficult to achieve stable dimensional accuracy for the formed product. Furthermore, in the press-forming method described in Patent Document 2, the curvature radii of the top plate portion and flange portion are changed in the preceding process. This means that the 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 the above points in mind. The present invention is directed to a U-shaped or L-shaped cross-sectional member in which the top plate and vertical wall portions are curved in the longitudinal direction in side view. One of the objects of the present invention is to significantly reduce camber back and dimensional accuracy variation 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 that is excellent in shape preservability and dimensional accuracy variation, and that can stably produce high-precision parts that are close to the target part shape (product shape) through mass production.

[0010] In order to solve the problem, one aspect of the present invention is a method for manufacturing a press-molded product 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 vertical wall portions are continuous at the width direction ends 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 curvature radius, and the radius of curvature along the longitudinal direction of the top plate portion of the mold that forms the top plate portion is defined as the top plate forming radius of curvature, the method comprising: a first step of press-molding the metal plate into an intermediate part having a U-shaped or L-shaped cross section using a mold in which, in a region where the curved portion is formed, a first radius of curvature R1 that is smaller than the top plate curvature radius R0 in the target product shape; and a second step of press-molding the metal plate into an intermediate part having a U-shaped or L-shaped cross section using a mold in which, in a region where the curved portion is formed, a first radius of curvature R2 that is smaller than the top plate curvature radius R0 in the target product shape. a second step of press-molding the intermediate part using a mold having a top plate forming radius of curvature R2 equal to or greater than radius R0, wherein the first radius of curvature R1 is set so that the top plate radius of curvature R1' after the intermediate part is released from the mold is less than the top plate radius of curvature R0, and the absolute value of the average value of the strain in the top plate portion is set to ε1, and the average value of the strain in the lower half of the vertical wall portion is set to ε2, for the strain in the longitudinal direction occurring at the bottom dead center of the press in the first step of press-molding. 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', regarding the longitudinal strain occurring at the bottom dead center of the press forming in the second step, the first radius of curvature R1 and the second radius of curvature R2 are adjusted so that the values ​​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)

[0011] According to the present invention, even if high-tensile steel is used for the metal sheet, the following effects can be achieved. That is, according to the present invention, camber back and fluctuations in dimensional accuracy due to fluctuations in material strength can be significantly reduced without complicating the mold shape. Note that camber back refers to springback in a side view. As a result, according to the present invention, it is possible to stably manufacture parts with high-precision U-shaped or L-shaped cross-sectional curved shapes that are close to the target product shape. That is, according to the present invention, it is possible to provide a manufacturing method for press-formed products that are excellent in shape fixability and dimensional accuracy fluctuation. As described above, according to the present invention, parts with high dimensional accuracy can be obtained even when the material strength of the metal sheet fluctuates. As a result, according to the present invention, it is possible to improve yield. Furthermore, when the present invention is used for vehicle body structural parts, it is possible to easily assemble the parts, for example.

[0012] 1 is a schematic diagram showing an example of a target product shape. (a) is a perspective view, and (b) is a side view. A diagram explaining the process in this embodiment. (b) is a side view showing the state of springback in a normal forming method. (c) is an example of stress distribution at the bottom dead center of press forming in a normal forming method. (d) is a schematic diagram showing the mechanism of dimensional accuracy fluctuations in a normal forming method. (e) is a diagram showing a countermeasure method in this embodiment. (f) is a diagram showing the difference in springback between steel types with respect to (R1' / R0). (g) is a stress distribution at the bottom dead center of press forming in this embodiment. (a) shows the stress distribution at the bottom dead center of press forming in the first process, and (b) shows the stress distribution at the bottom dead center of press forming in the second process. (g) is a schematic diagram showing the mechanism of reducing dimensional accuracy fluctuations when this embodiment is used. (g) is a strain distribution at the bottom dead center of press forming in this embodiment. (a) shows the strain stress distribution at the bottom dead center of press forming in the first process, and (b) shows the strain stress distribution at the bottom dead center of press forming in the second process. (g) is a diagram showing the difference in springback between steel types with respect to (ε2 / ε1). 1 is a diagram showing the springback difference between steel types with respect to (ε2′ / ε1′) FIG. 2 is a schematic diagram showing angles θ1 and θ2 of a die for adjusting a vertical wall angle.

[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 the shape shown in FIG. 1 . That is, the shape has a U-shaped cross section having a top plate portion 1A and left and right vertical wall portions 1B continuous with both sides of the top plate portion 1A in the width direction. In other words, this embodiment illustrates a case where the cross section has 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 of the top plate portion 1A in the width direction.

[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 convex or concave toward the top plate portion 1A along the longitudinal direction when viewed from the side. The longitudinal direction is the direction that intersects with the U-shaped cross section. The product may have two or more curved portions. The example of Figure 1 illustrates a case in which the product shape is curved so as to be convex toward the top plate portion 1A along 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 approximately zero curvature along the longitudinal direction.

[0015] The curved portion may be formed only in a partial region in the longitudinal direction. Furthermore, the present invention also targets product shapes having multiple curved portions along the longitudinal direction. Furthermore, part shapes in which convex curved portions and concave curved portions are alternately formed may also be manufactured. Furthermore, the curvature of each curved portion along the longitudinal direction does not need to be constant. Furthermore, in the curved portion, the lower end of the vertical wall portion 1B may have a curvature along the longitudinal direction that is different from the curvature of the top plate portion 1A. 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 for a press-formed product of this embodiment is a method for manufacturing a metal plate (blank material) into the above-described product shape 1 by press-forming. The present invention is suitable for use when the metal plate material is a high-tensile steel (steel plate with a tensile strength of 590 MPa or more) or an ultra-high-tensile steel. However, the present invention is also applicable to metal plates such as mild steel plates and aluminum plates. One of the objectives of this embodiment is to reduce the following when manufacturing a press-formed product of a product shape. That is, one of the objectives of this embodiment is to reduce camber back, which is springback along the longitudinal direction. Furthermore, one of the objectives of this embodiment is to reduce the variation in dimensional accuracy between parts due to strength variations (strength differences) in the materials of each metal plate.

[0017] As shown in FIG. 2 , the method for manufacturing a press-formed product according to this embodiment includes a first step 2A and a second step 2B. The first step 2A is a step of press-forming a metal plate into a shape similar to the target product shape using a die described below. That is, it is a step of press-forming 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 forming or form forming is used for the press forming in the first step 2A.

[0018] Furthermore, the second process 2B is a process in which the intermediate part 10 is press-molded using a die described below to produce a press-molded part having the desired product shape. The press molding of the second process 2B is performed, for example, by a restriking process. As described above, the radius of curvature along the longitudinal direction of the top plate portion 1A in a part shape having a U-shaped cross section is defined as the top plate radius of curvature. Furthermore, the radius of curvature along the longitudinal direction of the top plate portion 1A of the molding surface of the die that molds the top plate portion 1A 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 die shape. The die shape is the shape of the molding surface of the die.

[0019] (Regarding the Shapes of the Molds Used in the First Step 2A and the Second Step 2B) In this embodiment, the molding surfaces for molding the top plate portion 1A in the shapes of the molds 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. A mold that satisfies this condition is a mold in which the first radius of curvature R1 is set as the molding radius of curvature of the top plate, and is smaller than the radius of curvature R0 of the top plate in the target product shape. Furthermore, for the mold used in the first step 2A, the first radius of curvature R1 is set so that the radius of curvature R1' of the top plate after release of the intermediate part 10 is less than the radius of curvature R0 of the top plate. This setting can be adjusted to satisfy the condition, for example, by performing CAE analysis. That is, the mold for the first step 2A is set so that "R1 < R1' < R0" holds.

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

[0021] In this embodiment, in addition to the above conditions, the first curvature radius R1 and the second curvature radius R2 are adjusted to satisfy the following conditions. Here, for the longitudinal strain occurring at the bottom dead center of the press during 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 center of the press during 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. 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. That is, 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 that they satisfy the following equations (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 and second curvature radii R1 and R2 can be adjusted to optimal values ​​by evaluating the relationship between the above-mentioned strains and the top plate forming curvature radii through CAE analysis, or by conducting experiments in which actual press forming is performed. Furthermore, in addition to the above conditions, it is preferable to adjust the first curvature radius R1 so that it satisfies the following formula (3) at least in the region that will become the curved portion: 0.45 ≦ (R1' / R0) ≦ 0.85 (3)

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

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

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

[0027] Figure 4 also shows the longitudinal stress distribution at the bottom dead center of the press. The stress distribution is shown at the center of the plate thickness. As can be seen from Figure 4, tensile stress occurs over a wide area in the top plate portion 1A, and compressive stress occurs over a wide area in the vertical wall portion 1B. Therefore, the aforementioned springback (camber back) is thought to occur when these stresses are released when the die is released. As the material strength increases, this residual stress increases, resulting in a larger amount of springback. Furthermore, as the material strength increases, the variation in material strength during mass production also becomes relatively larger. This causes the aforementioned variation in material strength between coils (between production lots), which can lead to issues related to dimensional accuracy fluctuations, resulting in a change in the amount of springback.

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

[0029] In this embodiment, this camber back and dimensional accuracy fluctuations 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 the first step 2A. The intermediate part 10 has a part shape with a U-shaped cross section, with a top plate portion 10A curved along the longitudinal direction with a first radius of curvature R1 smaller than the radius of curvature R0 of the product shape. Furthermore, in the 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 radius of curvature of the top plate portion 10A of the intermediate part 10 after release from the mold is slightly larger than the top plate radius of curvature formed in the mold in the first step 2A due to springback. It is preferable to design the mold so that the top plate radius of curvature of the intermediate part 10 after springback (release from the mold) in the first step 2A is smaller than the top plate radius of curvature of the target product shape. This design is performed by adjusting the radius of curvature of the top plate of the mold 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. For this reason, the radius of curvature of the top plate molding surface of the mold for molding the top plate portion 1A in the second step 2B is set to be equal to or greater than the radius of curvature of the top plate of the product shape. Note that it is preferable to design the mold for the second step 2B so that the radius of curvature of the top plate of the mold for the second step 2B is equal to or greater 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 radius of curvature for each die in the first step 2A and the second step 2B are set so as to satisfy the following formula (5) or (5'). Here, the top plate curvature radius in the product shape is defined as R0, the top plate forming radius of curvature for the die in the first step 2A is defined as R1, the top plate curvature radius after springback of the intermediate part 10 is defined as R1', and the top plate forming radius of curvature for the die in the second step 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 forming die in the first step 2A so that it satisfies 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 center of the press in the second step 2B, which may result in significant spring back 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 center of the press in the second step 2B, which may result in insufficient suppression of springback.

[0033] Figure 7 shows the results of investigating the difference in springback amount between steel sheets with tensile strengths ranging from 590 MPa to 1180 MPa. Figure 7 shows the results for the case of press-forming the longitudinally curved member shown in Figure 1. The thickness t of the metal sheet was set to 1.4 mm. The target dimensions of the product shape were as follows: the width of the top plate portion 1A was 60 mm, the height of the vertical wall portion 1B was 30 mm, the angle between the top plate portion 1A and the vertical wall portion 1B was 110°, and the longitudinal length of the product was 400 mm.

[0034] In Figure 7, the horizontal axis represents R1' / R0, which is obtained by varying the radius of curvature in the first step 2A under various conditions. In Figure 7, the circles and triangles connected by a line represent steel grades press-formed using dies of the same shape but with different tensile strength levels. The values ​​on the vertical axis represent the difference in springback between the steel grades. As can be seen from Figure 7, when R1' / R0 is in the range of 0.45 to 0.85, the difference in springback between the steel grades is within 3 mm, which is satisfactory. Note that the circles and triangles connected by a line in Figure 7 represent reference data that do not satisfy the conditions of Equations (1) and (2). As described above, by designing the die shapes for the first step 2A and the second step 2B based on the present invention, the stresses that cause camber back are reversed in the second step 2B, reducing camber back and dimensional accuracy variations.

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

[0036] Figure 8(b) shows the longitudinal stress distribution at the bottom dead center of the forming process in the second step 2B. The value at the center of the plate thickness was used as a representative value. The radius of curvature of the top plate formed in the mold for the second step 2B was set to R1600 [mm]. As can be seen from Figure 8(b), at the bottom dead center of the forming process in the second step 2B, tensile stress occurred at the bottom of the vertical wall portion 1B and compressive stress occurred in the top plate portion 1A. Furthermore, compared to the conventional method (see Figure 3), the stress was reversed, and the area where stress occurred was reduced. This indicates that by forming the press-formed product in the second step 2B so that the radius of curvature in the side view (top plate curvature radius) is larger, the press-formed product is bent back as a structure. As a result, the vertical wall portion 1B is formed in the direction of linear extension, resulting in tensile stress. Furthermore, the top plate portion 1A is formed in the direction of linear contraction, resulting in compressive stress.

[0037] The fluctuation in dimensional accuracy when press-molded using the method of this embodiment will be explained using a schematic diagram of the stress-strain relationship. Figure 9 shows the stress-strain relationship of the top plate portion 1A when this embodiment is adopted. Figure 9 shows two schematic diagrams of high-strength material and low-strength material, assuming fluctuations in material strength. In the first process 2A (first forming process), the top plate is molded to a radius of curvature (top plate molding radius of curvature) smaller than the product shape. As a result, as shown in Figure 9, larger strain and tensile stress are imparted than in conventional manufacturing methods. Then, in the second process 2B (second forming process), the top plate is molded to a radius of curvature (top plate molding radius of curvature) larger than the product shape. As a result, the material of the top plate portion 1A is molded in a shrinking direction, and the tensile stress reverses to compressive stress.

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

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

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

[0041] Here, for the longitudinal strain occurring at the bottom dead center 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 portion 1A is defined as ε2. Also, for the longitudinal strain occurring at the bottom dead center 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 portion 10A is defined as ε1. In this case, in this embodiment, the values ​​of the top plate forming curvature radii of the molds in the first process 2A and the second process 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 center 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 center 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 molds 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 FIG. 9 will be too large. As a result, there is a possibility that the difference in springback amount due to the difference in material strength 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 larger than 0.8 × ε1, the amount of strain recovery in the second step 2B will be insufficient. As a result, there is a possibility that the difference in springback amount due to the difference in material strength cannot be reduced to a desired value or less. Therefore, ε2 is preferably 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 will be too large. As a result, it may be impossible to reduce the difference in springback amount due to the difference in material strength below the desired value. 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. As a result, it may be impossible to reduce the difference in springback amount due to the difference in material strength below the desired value. Therefore, ε2' is preferably less than 0.85 × ε1', and more preferably 0.8 × ε1' or less.

[0045] Figure 11 shows the results of investigating the difference in springback amount between steel sheets with tensile strengths ranging from 590 MPa to 1180 MPa. Figure 11 shows the results for the case of press-forming the longitudinally curved member shown in Figure 1. The thickness t of the metal sheet is 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 Figure 11, the horizontal axis represents ε2 / ε1, which is changed by varying the radius of curvature in the first step 2A. In Figure 11, the circles and triangles connected by a line represent steel types with different tensile strength levels that were press-formed using a mold of the same shape. The values ​​on the vertical axis represent the difference in springback between the steel types. As can be seen from Figure 11, when ε2 / ε1 is in the range of 0.1 to 0.8, the difference in springback between the steel types is within 3 mm, which is satisfactory.

[0047] Figure 12 shows the results of investigating the difference in springback amount between steel sheets with tensile strengths ranging from 590 MPa to 1180 MPa. Figure 12 shows the results for the case of press-forming the longitudinally curved member shown in Figure 1. The thickness t of the metal sheet is 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 Figure 12, the horizontal axis represents ε2' / ε1', which is obtained by varying the radius of curvature in the first process under various conditions. In Figure 12, the circles and triangles connected by a line represent steel types with different tensile strength levels that were press-formed using a mold of the same shape. The values ​​on the vertical axis represent the difference in springback between the steel types. As can be seen from Figure 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 satisfactory.

[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 fluctuations in dimensional accuracy simply by adjusting the respective top plate forming curvature radii in the first step 2 A and the second step 2 B. That is, even when a high-tensile steel is used for the metal plate, it is possible to significantly reduce fluctuations in dimensional accuracy when the material strength fluctuates while reducing camber back without complicating the die shape.

[0050] As a result, according to this embodiment, it is possible to stably manufacture parts with a highly accurate U-shaped cross-sectional curved shape that is close to the target product shape. That is, according to this embodiment, it is possible to provide a manufacturing method for press-formed products that are excellent in shape fixability and dimensional accuracy fluctuation. Furthermore, according to this 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, it becomes possible to easily assemble the parts, for example.

[0051] (Variation) In addition to adjusting the radius of curvature of the top plate for 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 even easier, and dimensional change and fluctuations in dimensional accuracy are further reduced. Here, the angle between the top plate and the vertical wall 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 forms the top plate portion 10A and the surface 3B that forms the vertical wall portion 10B. Furthermore, 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' of the intermediate part 10 after it is released from the mold. The second angle θ2 is the angle between the surface 4A that forms the top plate portion 1A and the surface 4B that forms the vertical wall portion 1B. These angles only need to be set in at least the region that will become the curved portion. Furthermore, it is preferable to set the first angle θ1 and the second angle θ2 so that their values ​​satisfy the following formula (4): θ1 < θ1' < θ0 ≦ θ2 (4)

[0053] That is, in the first step 2A, the intermediate part 10 is manufactured by press-forming the intermediate part 10 at the bottom dead center of the forming process 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. In the second step 2B, the intermediate part 10 is press-formed at the bottom dead center of the forming process into a vertical wall angle greater than the vertical wall angle θ1' of the intermediate part 10 after springback. For example, when forming a U-shaped cross-section that is convexly curved toward the top plate portion 1A, the first angle θ1 and the second angle θ2 are set as described above. Then, in the second step 2B, the vertical wall portion 1B is deformed in an opening direction, resulting in a forming process in which the vertical wall portion 1B is stretched in the longitudinal direction. This facilitates reversal of the longitudinal stress in the vertical wall portion 1B, thereby reducing dimensional change and dimensional accuracy fluctuations.

[0054] Here, when designing the mold shapes for the first step 2A and the second step 2B, if you start designing without taking the radius of curvature of the product into consideration, or if you start designing from the mold for the second step 2B, there is a possibility that you will not be able to find an appropriate mold shape, or that it will 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, CAE analysis or actual pressing is performed with the top plate forming radius of curvature in the first step 2A set to a value smaller than the top plate curvature radius of the product. This identifies conditions under 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 radius of curvature larger than the top plate curvature radius of the product shape. This identifies conditions under 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 processes, die shapes can be designed in a short time.

[0056] In this case, it is preferable to make the vertical wall angle in the first step 2A as small as possible compared to the vertical wall angle in the second step 2B. However, there is no problem even if the vertical wall angle is the same as that in the second step 2B. Thus, the present invention can significantly reduce dimensional accuracy fluctuations even when using high-tensile steel with a strength class of 590 PMa or higher or ultra-high-tensile steel with a strength class of 980 MPa or higher. In other words, according to the present invention, a simple mold structure can significantly reduce dimensional accuracy fluctuations when material strength fluctuations occur. Therefore, high-precision parts close to the target part shape can be obtained. While the above description has been given using a U-shaped cross section as an example, the present invention can also be applied to product shapes having an L-shaped cross section.

[0057] (Other) The present disclosure may also have the following configurations: (1) A method for manufacturing a press-formed product by press-forming a metal plate into a part shape having a top plate portion and a U-shaped or L-shaped cross section with vertical wall portions continuing at widthwise ends 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 longitudinal direction that intersects with the cross section, in a side view, 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 a mold that forms the top plate portion is defined as the top plate forming radius of curvature, the method comprising: a first step of press-forming the metal plate into an intermediate part having a U-shaped or L-shaped cross section using a mold that has, at least in a region where the curved portion is formed, a first radius of curvature R1 as the top plate forming radius of curvature that is smaller than the top plate radius of curvature R0 of a target product shape; a second step of press-molding the intermediate part using a mold in which 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, is set as the radius of curvature of the top plate after the intermediate part is released from the mold, the first radius of curvature R1 is set so that the radius of curvature R1' of the top plate after release from the mold is less than the radius of curvature R0 of the top plate; and further, regarding the longitudinal strain occurring at the bottom dead center in the press molding 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 of the vertical wall portion is defined as ε1', and regarding the longitudinal strain occurring at the bottom dead center in the press molding 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 of the vertical wall portion is defined as ε2', A method for manufacturing a press-formed product, comprising adjusting the first radius of curvature R1 and the second radius of curvature R2 so as to have values ​​that satisfy the following formulas (1) and (2) at least in a region that will become the curved portion: 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 a region that will become the curved portion.0.45≦(R1′ / R0)≦0.85 (3) (3) When the angle between the top plate portion and the vertical wall portion in the U-shaped cross section or the 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, 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 first step, is set to an angle smaller than the vertical wall angle θ0 of the product shape, and at least in the region that will become the curved portion, a 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 step, is set to an angle larger than the vertical wall angle θ1′ of the intermediate part after release from the mold. (4) When the vertical wall angle of the intermediate part molded in the first step after release from the mold is defined as θ1′, the first angle θ1 and the second angle θ2 are set to values ​​that satisfy the following formula (4). θ1 < θ1' < θ0 ≦ θ2 (4) (5) Draw forming or form forming is applied to the press forming of the first step, and a restriking process is applied to the press forming of the second step. (6) The metal plate is a steel plate having a material strength of 590 MPa or more.

[0058] In this example, press forming analysis and springback analysis were performed using the finite element method (FEM) to confirm the camber-back reduction effect of the press forming according to this embodiment. This example focused on the press forming of a longitudinally curved component as shown in FIG. 1 . The metal plate used for press forming had a thickness t = 1.4 mm. To compare dimensional accuracy variations, 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 portion 1A, a height of 30 mm for the vertical wall portion 1B, an angle of 110° between the top plate portion 1A and the vertical wall portion 1B, and a longitudinal length of 400 mm.

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

[0060] The conditions and evaluation results are shown in Table 1. In Table 1, die R refers to the radius of curvature of the top plate during forming, and top plate R refers to the radius of curvature of the top plate. The amount of camber back was evaluated as the amount of deviation in the Z direction (height direction) from the bottom dead center of press forming in the second step 2B. A product was deemed to have passed when the difference between the steel types in the amount of deviation in the height direction after springback in the second step 2B was 3.0 mm or less.

[0061]

[0062] <No. 1-3> No. 1-3 are the results of the curvature radius of the top plate portion 1A of the 590 MPa material, 980 MPa material, and 1180 MPa material when the top plate forming curvature radius R1 of the mold in the first step 2A is R1600 [mm]. The top plate curvature radius R0 of the target product shape in No. 1-3 is R1600 [mm]. In this case, the top plate curvature radius of each material after springback in the first step 2A significantly exceeds R1600 [mm], which is the top plate forming curvature radius of the mold in the second step 2B. Looking at the 1180 MPa material, the curvature radius of the top plate portion 1A after springback in the first step 2A was large at R2390 [mm]. The radius of curvature of the top plate 1A after springback in the second step 2B for the 1180 MPa steel was as large as R3056 [mm]. Furthermore, the difference in the height deviation between the steel types after springback in the second step 2B was 3.49 [mm]. This value exceeded the target of 3.0 [mm].

[0063] <Nos. 4-6> Nos. 4-6 show the results of the curvature radius of the top plate portion 1A of the 590 MPa material, 980 MPa material, and 1180 MPa material when the top plate forming curvature radius R1 of the mold in the first step 2A is R1040 [mm]. The top plate curvature radius R0 of the target product shape in Nos. 4-6 is R1600 [mm]. In this case, the top plate curvature radius after springback in the first step 2A of each material is smaller than the top plate forming curvature radius of the mold in the second step 2B, R1600 [mm]. Looking at the 1180 MPa material, the curvature radius of the top plate portion 1A after springback in the first step 2A was R1360 [mm]. Furthermore, the curvature radius of the top plate portion 1A of the 1180 MPa material after springback in the second step 2B was R1823 [mm]. The difference between the steel grades in the amount of deviation in the height direction after springback in the second process 2B was 2.65 [mm], which was the target difference between the steel grades of 3.0 [mm] or less.

[0064] <Nos. 7-9> In contrast, the results for the radius of curvature of the top plate portion 1A of the 590 MPa material, 980 MPa material, and 1180 MPa material when the radius of curvature of the top plate formed by the mold in the first step 2A is R1000 [mm] are shown in Nos. 7-9. The radius of curvature R0 of the top plate in the target product shape in Nos. 7-9 is R1600 [mm]. The radius of curvature of the top plate after springback in the first step 2A for each material is smaller than the radius of curvature of the top plate formed by the mold in the second step 2B, R1600 [mm]. For the 1180 MPa material, the radius of curvature of the top plate after springback in the first step 2A was R1237 [mm]. Furthermore, the radius of curvature of the top plate after springback in the second step 2B for the 1180 MPa material was R1742. The difference between the steel grades in the amount of deviation in the height direction after springback in the second process 2B was 2.45 [mm], which was the target difference between the steel grades of 3.0 [mm] or less.

[0065] <Nos. 10-12> Next, the results of the curvature radius of the top plate portion 1A of the 590 MPa material, 980 MPa material, and 1180 MPa material when the top plate forming radius of the die in the first step 2A is R800 [mm] are shown in Nos. 10-12. The top plate curvature radius R0 for the target product shape in Nos. 10-12 is R1400 [mm]. The curvature radius of the top plate portion 1A after springback in the first step 2A for each material is smaller than the curvature radius of the top plate forming radius of the die in the second step 2B, R1600 [mm]. Looking at the 1180 MPa material, the curvature radius of the top plate portion 1A after springback in the first step 2A was R956 [mm]. Furthermore, the curvature radius of the top plate portion 1A of the 1180 MPa material after springback in the second step 2B was R1407 [mm]. The difference between the steel grades in the amount of deviation in the height direction after springback in the second process 2B was 1.00 [mm], which was the target difference between the steel grades of 3.0 [mm] or less.

[0066] <Nos. 13-15> Next, the results of the curvature radius of the top plate portion 1A of the 590 MPa material, 980 MPa material, and 1180 MPa material when the top plate forming radius of curvature of the die in the first step 2A is R700 [mm] are shown in Nos. 13-15. The top plate curvature radius R0 for the target product shape in Nos. 13-15 is R1300 [mm]. The curvature radius of the top plate portion 1A after springback in the first step 2A of each material is smaller than the curvature radius of the top plate forming radius of the die in the second step 2B, R1600 [mm]. Looking at the 1180 MPa material, the curvature radius of the top plate portion 1A after springback in the first step 2A was R820 [mm]. Furthermore, the curvature radius of the top plate portion 1A of the 1180 MPa material after springback in the second step 2B was R1220 [mm]. The difference between the steel grades in the amount of deviation in the height direction after springback in the second process 2B was 1.60 [mm], which was the target difference between the steel grades of 3.0 [mm] or less.

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

[0068] Example 2: For the case of press-forming a longitudinally curved member as shown in FIG. 1 , press forming analysis and springback analysis were performed when the radius of curvature of the side view curve and the vertical wall angle in the first step 2A were reduced. The radius of curvature before and after springback were measured. The metal plate used for press-forming was the same as in Example 1. In this example, the radius of curvature R2 of the top plate of the mold 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 top plate of the mold 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, die R refers to the radius of curvature of the top plate during forming, 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 step 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 springback in the second step 2B was 3.0 mm or less.

[0070] <Nos. 16-18> Nos. 16-18 show the results of the radius of curvature of the top plate portion 1A of the 590 MPa, 980 MPa, and 1180 MPa materials when the radius of curvature of the top plate formed by the mold in the first step 2A was set to R1600 [mm] and the vertical wall angle was changed to 90°. The radius of curvature R0 of the top plate in the target product shape for Nos. 16-18 is R1600 [mm]. The radius of curvature of the top plate portion 1A after springback in the first step 2A for each material significantly exceeds R1600 [mm], which is the radius of curvature of the top plate formed by the mold in the second step 2B. Looking at the 1180 MPa material, the radius of curvature of the top plate portion 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 was less than 110°, which was the vertical wall angle after springback in the second step 2B. The radius of curvature of the top plate 1A after springback in the second step 2B of the 1180 MPa steel was large at R2643 [mm]. Furthermore, the difference in the height deviation between the steel grades after springback in the second step 2B was 3.45 [mm].

[0071] <Nos. 19-21> In contrast, the results for the radius of curvature of the top plate portion 1A of the 590 MPa material, 980 MPa material, and 1180 MPa material when the radius of curvature of the top plate formed by the mold in the first step 2A is R1000 [mm] are shown in Nos. 19-21. The radius of curvature R0 of the top plate in the target product shape in Nos. 19-21 is R1600 [mm]. The radius of curvature of the top plate portion 1A after springback in the first step 2A of each material is smaller than the radius of curvature R1600 [mm] of the top plate formed by the mold in the second step 2B. In the case of the 1180 MPa material, the radius of curvature of the top plate portion 1A after springback in the first step 2A was R1191 [mm]. In addition, the radius of curvature of the top plate portion 1A after springback in the second process 2B for the 1180 MPa steel was R1426 [mm]. The difference in height deviation between steel grades after springback in the second process 2B was 0.18 [mm], below the target of 3.0 [mm]. This shows that the difference between steel grades was reduced compared to Nos. 7 to 9 in Table 1. Nos. 7 to 9 in Table 1 represent cases where the top plate forming radius of curvature of the mold in the first process 2A was reduced to R1000 [mm], and the vertical wall angle remained the same as in the second process 2B at 110°.

[0072] From the above, by reducing the angle of the die that adjusts the radius of curvature of the side view curve (the radius of curvature of the top plate forming) and the vertical wall angle in the first process 2A, 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 of the top plate forming die in the first, 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 August 23, 2023), from which this application claims priority, are incorporated herein by reference. While the present invention has been described with reference to a limited number of embodiments, the scope of the invention is not limited thereto, and modifications of each embodiment based on the above disclosure would be obvious to one skilled in the art.

[0074] 1 Product shape 1A Top plate portion 1B Vertical wall portion 2A First process 2B Second process 10 Intermediate part 10A Top plate portion 10B Vertical wall portion

Claims

1. A method for manufacturing a press-molded product by press-molding a metal plate into a part shape having a top plate portion and a U-shaped or L-shaped cross section with a vertical wall portion continuing at the width direction end 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, the method comprising: 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 curvature radius R1 in an area where the curved portion is formed, the first step being defined as a top plate curvature radius along the longitudinal direction of the top plate portion, and a curvature radius of the molding surface that molds the top plate portion along the longitudinal direction of the top plate portion being defined as a top plate molding curvature radius in the die; a second step of press-molding the intermediate part using a die having a top plate forming radius of curvature R2 equal to or greater than the top plate radius of curvature R0 in a target product shape in a region where the curved portion is formed, wherein the first radius of curvature R1 is set so that the top plate radius of curvature R1' after the intermediate part is released from the die is less than the top plate radius of curvature R0, and the absolute value of the average value of the strain in the top plate portion is defined as ε1 and the absolute value of the average value of the strain in the lower half of the vertical wall portion for the longitudinal strain occurring at the bottom dead point in the press molding of the first step is defined as ε1 and ε1', respectively, 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 for the longitudinal strain occurring at the bottom dead point in the press molding of the second step is defined as A method for manufacturing a press-formed product, comprising adjusting the first radius of curvature R1 and the second radius of curvature R2 in a region that becomes the curved portion so as to satisfy the following expressions (1) and (2): 0.1×ε1≦ε2<0.8×ε1 (1) 0.3×ε1′≦ε2′<0.85×ε1′ (2) 2. The method for producing a press-formed product according to claim 1, wherein the first radius of curvature R1 is adjusted so as to satisfy the following formula (3) in the region that becomes the curved portion: 0.45≦(R1′ / R0)≦0.85 (3) 3. A manufacturing method for a press-molded product as described in claim 1 or claim 2, wherein, when the angle between the top plate portion and the vertical wall portion in the cross section is defined as the vertical wall angle, in the region that will become the curved portion, a first angle θ1, 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 first step, is set to an angle smaller than the vertical wall angle θ0 of the product shape, and in the region that will become the curved portion, a 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 step, is set to an angle larger than the vertical wall angle θ1' after the intermediate part is released from the mold.

4. The method for producing a press-molded product according to claim 3, wherein, when the vertical wall angle of the intermediate part molded in the first step after demolding is defined as θ1', the first angle θ1 and the second angle θ2 are set so as to satisfy the following formula (4): θ1 < θ1' < θ0 ≦ θ2 (4) 5. A method for manufacturing a press-molded product according to any one of claims 1 to 4, wherein draw molding or form molding is applied to the press molding in the first step, and a restriking step is applied to the press molding in the second step.

6. A method for manufacturing a press-molded product according to any one of claims 1 to 4, wherein the metal plate is a steel plate having a material strength of 590 MPa or more.

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