Method for manufacturing press-molded article

Abstract

There is provided a press-forming method excellent in shape fixability and dimensional accuracy variation capable of stably mass-producing a component close to the target component shape. A method for manufacturing a press-formed article includes manufacturing a product having a curved portion curved convexly or concavely to the side of a top sheet portion in a cross section of a U-shape or the like. The method includes: a first step (2A) of forming an intermediate component (10) using a die with a first radius of curvature (R1) smaller than a top sheet radius of curvature (R0) in a product shape as a top sheet forming radius of curvature; and a second step (2B) of press-forming the intermediate component (10) using a die with a second radius of curvature (R2) equal to or larger than the top sheet radius of curvature (R0) in the product shape as a top sheet forming radius of curvature, in which the first radius of curvature (R1) is set such that a top sheet radius of curvature (R1') after die release of the intermediate component (10) is less than the top sheet radius of curvature (R0), and the first radius of curvature (R1) and the second radius of curvature (R2) are adjusted based on strain caused by the forming 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 article in which a press-formed article is manufactured by press-forming a metal sheet. The press-formed article targeted by the present invention has a U-shaped or L-shaped cross section having vertical wall portions continuing to end portions in the width direction of a top sheet portion. Further, the press-formed article targeted by the present invention has a component shape having one or two or more curved portions curved convexly or concavely to the side of the top sheet portion in side view along the longitudinal direction of the component.Background Art

[0002] In recent years, there has been a demand for both improved collision safety and weight reduction of vehicle bodies. Therefore, as materials for vehicle body structural components, high tensile-strength materials of 590 MPa or more have been applied and further ultrahigh tensile-strength materials of 980 MPa or more have been applied. The high tensile-strength materials and the ultrahigh tensile-strength materials have high yield strength and high tensile strength. This has posed a problem of forming failure due to springback or the like in press-forming of the high tensile-strength materials and the like.

[0003] One of the press-formed articles used for vehicle body structural components includes the following component shape, for example. The component shape is a component shape having a U-shaped cross section having a top sheet portion and vertical wall portions continuous in the width direction and curved with a predetermined radius of curvature along the longitudinal direction in side view. When a component curved convexly to the side of the top sheet portion is manufactured by press-forming, compressive stress generates in the vertical wall portions and tensile stress generates in the top sheet portion at the bottom dead center of forming. Then, these stresses cause springback in the component. This springback is referred to as camber back. When the high tensile-strength materials, particularly the ultrahigh tensile-strength materials, are applied to raw materials of such components, the stresses at the bottom dead center of forming described above tend to increase. As a result, a problem that the springback increases arises.

[0004] Further, the ultrahigh tensile-strength materials have relatively large material strength variations in mass production of the materials. Therefore, the material strength tends to vary among manufactured coils. Therefore, when the ultrahigh tensile-strength materials are used, a problem arises that, even when press-forming is performed in the same manner, the springback amount changes in connection with the variation. The fact that the springback amount changes in connection with the variation in material strength is referred to as a dimensional accuracy variation.

[0005] Examples of technology applicable to such a problem include the press-forming methods described in PTL 1 and PTL 2.

[0006] The press-forming method described in PTL 1 is directed to the following formed article. The formed article is a formed article having a top sheet portion curved in the longitudinal direction and two side wall portions extending from both ends along the longitudinal direction of the top sheet portion toward the inside of the curve. PTL 1 discloses changing the curvature of the top sheet portion and the angle formed by the top sheet portion and the side wall portions in the previous step. PTL 1 describes that this reduces stress generating in the succeeding step and suppresses springback.

[0007] PTL 2 is directed to the following product shape. The product shape is a product shape having a hat-shaped cross section in which a top sheet portion and flange portions are continuous in the width direction via side wall portions and the top sheet portion and the flange portion are curved along the longitudinal direction. In PTL 2, forming is performed such that the radius of curvature is larger than that of the product shape in the succeeding step. PTL 2 describes that this reduces stress generating in the succeeding step and suppresses springback.Citation ListPatent Literatures

[0008] PTL 1: JP 2011-206789 A PTL 2: JP 2019-25533 A Summary of InventionTechnical Problem

[0009] However, in the press-forming method described in PTL 1, the forming is such that vertical wall portions are loosened in the forming of the succeeding step. Therefore, the behavior of the sheet becomes unstable during forming. For such a reason, the method of PTL 1 has a risk that, when applied to actual mass production, the method is difficult to achieve stable dimensional accuracy to the formed article.

[0010] In the press-forming method described in PTL 2, the radii of curvature in the previous step of the top sheet portion and the flange portions are changed. Therefore, the method cannot be applied to components having a U-shaped cross section not having flange portions.

[0011] The present invention has been made focusing on the above-described respects. The present invention is directed to a U-shaped or L-shaped cross-sectional member having a top sheet portion and vertical wall portions curved in the longitudinal direction in side view. It is an object of the present invention to significantly reduce camber back and the dimensional accuracy variation with a simple die structure, even when the high tensile-strength materials or the ultrahigh tensile-strength materials are used for such a cross-sectional member. Further, it is an object of the present invention to provide a press-forming method excellent in shape fixability and dimensional accuracy variation making it possible to stably obtain a high-precision component close to the target component shape (product shape) by mass-production.Solution to Problem

[0012] To solve the problems, one aspect of the present invention is a method for manufacturing a press-formed article in which a press-formed article is manufactured by press-forming a metal sheet into a component shape having a U-shaped or L-shaped cross section in which a top sheet portion and a vertical wall portion are continuous with each other at an end portion in the width direction of the top sheet portion and having one or two or more curved portions curved convexly or concavely to the side of the top sheet portion in side view along the longitudinal direction, the longitudinal direction being a direction intersecting the cross section, the method including: when the radius of curvature along the longitudinal direction of the top sheet portion is defined as a top sheet radius of curvature and the radius of curvature along the longitudinal direction of the top sheet portion of a forming surface forming the top sheet portion in a die is defined as a top sheet forming radius of curvature, a first step of press-forming a metal sheet into an intermediate component having a U-shaped or L-shaped cross section using a die with a first radius of curvature R1 smaller than a top sheet radius of curvature R0 in the target product shape as the top sheet forming radius of curvature in a region where the curved portion is formed; and a second step of press-forming the intermediate component using a die with a second radius of curvature R2 equal to or larger than the top sheet radius of curvature R0 in the target product shape as the top sheet forming radius of curvature in a region where the curved portion is formed, in which, when the first radius of curvature R1 is set such that a top sheet radius of curvature R1' after die release of the intermediate component is less than the top sheet radius of curvature R0, for strain in the longitudinal direction generating at the bottom dead center of press in the press-forming of the first step, the absolute value of the average value of the strain of the top sheet portion is defined as ε1 and the absolute value of the average value of the strain of a vertical wall lower-half of the vertical wall portion is defined as ε1', and, for strain in the longitudinal direction generating at the bottom dead center of press in the press-forming of the second step, the absolute value of the average value of the strain of the top sheet portion is defined as ε2 and the absolute value of the average value of the strain of the vertical wall lower-half of the vertical wall portion is defined as ε2', the first radius of curvature R1 and the second radius of curvature R2 are adjusted to have values satisfying Equations (1) and (2) below in the region to be the curved portion. 0.1 × ε 1 ≤ ε 2 < 0.8 × ε 1 0.3 × ε 1 ′ ≤ ε 2 ′ < 0.85 × ε 1 ′Advantageous Effects of Invention

[0013] According to the aspect of the present invention, even when it is supposed that the high tensile-strength materials are applied to a metal sheet, the following effects can be obtained. More specifically, the aspect of the present invention can significantly reduce the camber back and the dimensional accuracy variation when the material strength varies without complicating a die shape. The camber back is springback in side view.

[0014] Thus, the aspect of the present invention makes it possible to stably manufacture a high-precision component having a curved U-shaped or L-shaped cross-sectional shape close to the target product shape. More specifically, the aspect of the present invention can provide a method for manufacturing a press-formed article excellent in shape fixability and dimensional accuracy variation.

[0015] As described above, the aspect of the present invention can provide a component having high dimensional accuracy even when the material strength of a metal sheet varies. As a result, the aspect of the present invention achieves improved yield. Further, when the present invention is applied to vehicle body structural components, for example, the assembling of the components can be facilitated.Brief Description of Drawings

[0016] FIGS. 1A and 1B are schematic views each illustrating an example of a target product shape, in which FIG. 1A is a perspective view and FIG. 1B is a side view; FIG. 2 is a view illustrating steps in this embodiment; FIG. 3 is a side view illustrating the aspect of springback in a common forming method; FIG. 4 is an example of a stress distribution at the bottom dead center of press-forming in the common forming method; FIG. 5 is a schematic view illustrating the mechanism of the generation of dimensional accuracy variation in the common forming method; FIG. 6 is a view illustrating a countermeasure in this embodiment; FIG. 7 is a view illustrating a springback difference among steel grades with respect to (R1' / R0); FIGS. 8A and 8B each are a stress distribution at the bottom dead center of press-forming in this embodiment, in which FIG. 8A illustrates the stress distribution at the bottom dead center of press-forming in a first step and FIG. 8B illustrates the stress distribution at the bottom dead center of press-forming in a second step; FIG. 9 is schematic view illustrating the mechanism of a reduction in dimensional accuracy variation when this embodiment is used; FIGS. 10A and 10B are strain distributions at the bottom dead center of press in this embodiment, in which FIG. 10A illustrates the strain stress distribution at the bottom dead center of press-forming in the first step and FIG. 10B illustrates the strain stress distribution at the bottom dead center of press-forming in the second step; FIG. 11 is a view illustrating a springback difference among steel grades with respect to (ε2 / ε1); FIG. 12 is a view illustrating a springback difference among steel grades with respect to (ε2' / ε1'); and FIG. 13 is a schematic view illustrating angles θ1 and θ2 of a die adjusting the vertical wall angle. Description of Embodiments

[0017] Next, embodiments of the present invention will be described with reference to the drawings.(Target component shape)

[0018] In this embodiment, a target component shape 1 (product shape 1) of a component (press-formed article) to be manufactured is set to the shape illustrated in FIG. 1. More specifically, it is a case where the shape is a U-shaped cross section having a top sheet portion 1A and right and left vertical wall portions 1B continuous to both sides in the width direction of the top sheet portion 1A. More specifically, this embodiment gives an example of a case where the cross-sectional shape is a U-shaped cross section. However, the present invention is also applicable even when the shape is an L-shaped cross section in which the vertical wall portion 1B is present only on one side in the width direction of the top sheet portion 1A.

[0019] Further, the product shape 1 of this embodiment is a component shape having a curved portion in which the top sheet portion 1A and the vertical wall portions 1B are curved convexly or concavely to the side of the top sheet portion 1A in side view along the longitudinal direction. The longitudinal direction is a direction intersecting the above-described U-shaped cross section. Two or more curved portions may be provided. In the example of FIG. 1, a case is exemplified in which the product shape is curved to be convex to the side of the top sheet portion 1A over the longitudinal direction. In this example, a non-curved portion that is not the curved portion is a linear portion. The non-curved portion is, for example, a part where the curvature along the longitudinal direction of the top sheet portion 1A is zero or nearly zero.

[0020] The curved portion may be formed only in a partial region in the longitudinal direction. In the present invention, a product shape having a plurality of curved portions along the longitudinal direction is also a manufacturing target. Further, a component shape may be acceptable in which a convexly curved portion and a concavely curved portion are alternatively formed. The curvature along the longitudinal direction of each curved portion does not need to be a constant curvature.

[0021] In the curved portion, a lower end portion of the vertical wall portion 1B may have a curvature different from the curvature of the top sheet portion 1A along the longitudinal direction. The lower end portion of the vertical wall portion 1B is an end portion on the open end side away from the top sheet portion 1A.(Manufacturing method)

[0022] A method for manufacturing a press-formed article of this embodiment is a method for manufacturing a press-formed article by press-forming a metal sheet (blank material) into the above-described product shape 1. The present invention is an invention suitable when a material of the metal sheet is a high tensile-strength material (steel sheet having tensile strength of 590 MPa or more) or an ultrahigh tensile-strength material. However, the present invention is also applicable even in a case of a metal sheet, such as a mild steel sheet or an aluminum sheet.

[0023] One of the objects of this embodiment is the following reduction when a press-formed article of the product shape is manufactured. More specifically, one of the objects of this embodiment is to reduce camber back, which is springback along the longitudinal direction. Further, one of the objects of this embodiment is to reduce a variation in dimensional accuracy of components among the components in connection with a strength variation (difference in strength) of materials of metal sheets.

[0024] The method for manufacturing a press-formed article of this embodiment includes a first step 2A and a second step 2B as illustrated in FIG. 2.

[0025] The first step 2A is a step of press-forming a metal sheet into the same shape as the target product shape using a die described later. More specifically, the first step 2A is a step of press-forming a metal sheet into an intermediate component having a U-shaped cross section having a top sheet portion 1A. The intermediate component 10 may have a part to be cut off in a trimming step which is a succeeding step. To the press-forming of the first step 2A, drawing or stamping is applied, for example.

[0026] The second step 2B is a step of manufacturing a press-formed article of the target product shape by press-forming the intermediate component 10 using a die described later. To the press-forming of the second step 2B, a restriking step is applied, for example.

[0027] Herein, the radius of curvature along the longitudinal direction of the top sheet portion 1A in the component shape having a U-shaped cross section is defined as a top sheet radius of curvature as described above. The radius of curvature along the longitudinal direction of the top sheet portion 1A of a forming surface forming the top sheet portion 1A in the die is defined as a top sheet forming radius of curvature. The top sheet forming radius of curvature is a top sheet radius of curvature in a die shape. The die shape is the shape of the forming surface of the die.(Die shapes used in first step 2A and second step 2B)

[0028] In this embodiment, the forming surface forming the top sheet portion 1A in each of the die shapes used in the first step 2A and the second step 2B is set as follows.

[0029] In the first step 2A, a die satisfying the following condition is used at least in a region where the curved portion is formed. The die satisfying the condition is a die in which a first radius of curvature R1 smaller than a top sheet radius of curvature R0 in the target product shape is set as a top sheet forming radius of curvature.

[0030] For a die to be used in the first step 2A, the first radius of curvature R1 is set such that a top sheet radius of curvature R1' after die release of the intermediate component 10 is less than the top sheet radius of curvature R0. This setting may be adjusted to satisfy the condition by performing a CAE analysis or the like, for example.

[0031] More specifically, in the die for the first step 2A, the setting is performed to satisfy "R1 < R1' < R0".

[0032] In the second step 2B, a die satisfying the following condition is used at least in the region where the curved portion is formed. The die satisfying the condition is a die with a second radius of curvature R2 equal to or larger than the top sheet radius of curvature R0 in the target product shape as the top sheet forming radius of curvature. More specifically, in the die for the second step 2B, the setting is performed to satisfy "R0 ≤ R2".

[0033] In this embodiment, the first radius of curvature R1 and the second radius of curvature R2 are adjusted to satisfy the following conditions in addition to the above-described conditions.

[0034] Herein, for strain in the longitudinal direction generating at the bottom dead center of press in the press-forming of the first step 2A, the absolute value of the average value of the strain in a region where the curved portion is formed of a top sheet portion 10A is defined as ε1. The absolute value of the average value of the strain in a vertical wall lower-half in a region where the curved portion is formed of a vertical wall portion 10B is defined as ε1'. For strain in the longitudinal direction generating at the bottom dead center of press in the press-forming of the second step 2B, the absolute value of the average value of the strain in a region where the curved portion is formed of the top sheet portion 1A is defined as ε2. The absolute value of the average value of the strain in a vertical wall lower-half in a region where the curved portion is formed of the vertical wall portion 1B is defined as ε2'.

[0035] In the present invention, the "strain" is strain in the longitudinal direction. ε2 is the above-described average value at the bottom dead center of press of the second step 2B with strain in the longitudinal direction before the first step 2A (before forming) as the reference value 0. More specifically, ε2 is the cumulative strain which is the total of the strain in the longitudinal direction added by the first step 2A and the strain in the longitudinal direction added by the second step 2B.

[0036] In this case, the first radius of curvature R1 and the second radius of curvature R2 are adjusted to have values satisfying Equations (1) and (2) below at least in the position to be the curved portion. 0.1 × ε 1 ≤ ε 2 < 0.8 × ε 1 0.3 × ε 1 ′ ≤ ε 2 ′ < 0.85 × ε 1 ′

[0037] For this adjustment of the first radius of curvature R1 and the second radius of curvature R2, the first radius of curvature R1 and the second radius of curvature R2 may be adjusted to the optimum values by evaluating the relation between each strain described above and the top sheet forming radius of curvature by a CAE analysis or by performing an experiment of actually carrying out press-forming.

[0038] Further, the first radius of curvature R1 is preferably adjusted to have a value satisfying Equation (3) below at least in the region to be the curved portion in addition to the above-described conditions. 0.45 ≤ R 1 ′ / R 0 ≤ 0.85

[0039] Further, the die shapes used in the first step 2A and the second step 2B may be set as follows in addition to the above-described conditions.

[0040] Herein, the angle between the top sheet portion and the vertical wall portion in a member having a U-shaped cross section is defined as a vertical wall angle.

[0041] Then, a first angle θ1, the first angle θ1 being an angle between a surface forming the top sheet portion 10A and a surface forming the vertical wall portion 10B, in the die used in the first step 2A is set to an angle smaller than a vertical wall angle θ0 in the product shape. A second angle θ2, the second angle θ2 being an angle between a surface forming the top sheet portion 1A and a surface forming the vertical wall portion 1B, in the die used in the second step 2B is set to an angle larger than a vertical wall angle θ1' after die release of the intermediate component 10.

[0042] More specifically, "θ1 < θ0, θ1' < θ2" are set. These settings may be satisfied at least in the region to be the curved portion.

[0043] At this time, the first angle θ1 and the second angle θ2 are preferably set to have values satisfying Equation (4) below. θ 1 < θ 1 ′ < θ 0 ≤ θ 2(Operation and others)

[0044] Next, the action and others of the method for manufacturing a press-formed article according to this embodiment are described.<Common forming method>

[0045] Herein, a case is considered in which the component shape illustrated in FIG. 1 is manufactured by one press-forming by applying padded stamping using a pad as a common press-forming method. The pad presses the entire top sheet surface.

[0046] FIG. 3 is a side view illustrating the component shape before and after die release (before and after springback). As illustrated in FIG. 3, before and after springback (the state at the bottom dead center of forming and the state after die release), the top sheet radius of curvature of the component changes. As a result, it is found that the top sheet radius of curvature increases by the springback.

[0047] FIG. 4 illustrates a stress distribution in the longitudinal direction at the bottom dead center of press. The stress distribution at the center position of the sheet thickness is illustrated. As is understood from FIG. 4, tensile stress generates in a wide range in the top sheet portion 1A and compressive stress generates in a wide range in the vertical wall portion 1B. Therefore, the above-described springback (camber back) is considered to generate due to release of these stresses in die release. Then, with an increase in material strength, this residual stress increases, and the springback amount increases. Further, when the material strength increases, the material strength variation in mass production of the material relatively increases. Therefore, the material strength varies among the above-described coils (among manufacturing lots), which also poses a problem relating to the dimensional accuracy variation in which the springback amount changes.

[0048] This dimensional accuracy variation is described with reference to a schematic view of a stress-strain relation illustrated in FIG. 5. FIG. 5 illustrates the relation between stress and strain of the top sheet portion 1A in the common forming. FIG. 5 illustrates a schematic view of two cases of a high-strength material and a low-strength material, assuming a variation in material strength.

[0049] When a material of the top sheet portion 1A is press-formed into the product shape, the material of the top sheet portion 1A is formed to be elongated. Therefore, tensile stress generates at the bottom dead center of forming. However, as illustrated in FIG. 5, there is a difference in stress values at the bottom dead center of forming (see position of the die shape in FIG. 5) between the high-strength material and the low-strength material.

[0050] When the die is released from this state, camber back generates, and a change in the curvature in the longitudinal direction (a change in the top sheet radius of curvature) generates. At this time, a difference in stress values at the bottom dead center of forming results in a change in the camber back amount in the high-strength material and the low-strength material, and the dimensional accuracy variation generates. Specifically, when the material strength increases, this dimensional accuracy variation increases, and a risk of deviation from the dimensional tolerance of the product becomes high. Therefore, the above-described fact is an important problem to be solved.

[0051] This embodiment reduces this camber back and this dimensional accuracy variation. For that end, as illustrated in FIG. 6, in the press-forming method of this embodiment, forming into the intermediate component 10 is performed in the first step 2A. The intermediate component 10 has a component shape in which the top sheet portion 10A has a U-shaped cross section having the first radius of curvature R1 smaller than the radius of curvature R0 in the product shape for a curvature along the longitudinal direction. In the second step 2B, press-forming is performed such that the top sheet radius of curvature is larger than the top sheet radius of curvature of the intermediate component 10 after springback of the first step 2A.

[0052] Herein, in the first step 2A, the top sheet radius of curvature of the top sheet portion 10A of the intermediate component 10 after the die has been released is slightly larger than the top sheet forming radius of curvature of the die of the first step 2A by the springback. The die is preferably designed such that this top sheet radius of curvature of the intermediate component 10 after springback (after die release) of the first step 2A is smaller than the top sheet radius of curvature of the target product shape. This design is performed by adjusting the top sheet forming radius of curvature of the die of the first step 2A.

[0053] In this embodiment, the top sheet radius of curvature of the intermediate component 10 after die release of the first step 2A is smaller than the top sheet radius of curvature of the product shape. Therefore, in the second step 2B, the top sheet portion 10A of the intermediate component 10 is required to be bent back. Therefore, the top sheet forming radius of curvature of the forming surface forming the top sheet portion 1A of the die of the second step 2B is set to be equal to or larger than the top sheet radius of curvature of the product shape. The die of the second step 2B is preferably designed such that the top sheet forming radius of curvature of the die of the second step 2B is equal to or larger than the top sheet radius of curvature of the product shape.

[0054] From the above, in this embodiment, the values of the top sheet forming radii of curvature of the dies of the first step 2A and the second step 2B are set to values satisfying Equation (5) or (5') below.

[0055] Herein, the top sheet radius of curvature in the product shape is defined as R0, the top sheet forming radius of curvature in the die of the first step 2A is defined as R1, the top sheet radius of curvature after springback of the intermediate component 10 is defined as R1', and the top sheet forming radius of curvature in the die of the second step 2B is defined as R2. R 1 < R 1 ′ < R 0 ≤ R 2 or R 1 < R 1 ′ < R 0 < R 2

[0056] The value of the top sheet forming radius of curvature in the die of the first step 2A is set to a value satisfying Equation (3) below. 0.45 ≤ R 1 ′ / R 0 ≤ 0.85

[0057] Herein, when (R1' / R0) is smaller than 0.45, there is a risk that stresses of the top sheet portion 1A and the vertical wall portions 1B are excessively reversed at the bottom dead center of press in the second step 2B, and large springback in the opposite direction generates in a press-formed article. Conversely, when (R1' / R0) is larger than 0.85, there is a possibility that stresses of the top sheet portion 1A and the vertical wall portion 1B are not reversed at the bottom dead center of press in the second step 2B, and the springback is not sufficiently suppressed.

[0058] FIG. 7 illustrates the results of investigating a springback amount difference among steel sheets of 590 MPa-grade to 1180 MPa-grade tensile strength.

[0059] FIG. 7 illustrates the results of a case where a member curved in the longitudinal direction illustrated in FIG. 1 is press-formed. At that time, the sheet thickness t of the metal sheet was set to 1.4 [mm]. The dimensions of the target product shape are as follows. More specifically, the width of the top sheet portion 1A is 60 [mm], the height of the vertical wall portion 1B is 30 [mm], the angle formed by the top sheet portion 1A and the vertical wall portion 1B is 110°, and the length in the longitudinal direction of the product is 400 [mm].

[0060] In FIG. 7, R1' / R0 changed by setting the radius of curvature in the first step 2A to various conditions is plotted on the horizontal axis. In FIG. 7, the circles and the triangles connected by straight lines indicate steel grades press-formed with dies having the same shape and different in tensile strength level. The value on the vertical axis represents the springback amount difference among the steel grades.

[0061] As is understood from FIG. 7, the springback amount difference among the steel grades is within 3 mm, which is good, when R1' / R0 ranges from 0.45 to 0.85. In FIG. 7, black circles and black triangles connected by straight lines are reference data not satisfying the conditions of Equations (1) and (2).

[0062] As described above, by designing the die shape of the first step 2A and the die shape of the second step 2B based on the present invention, factor stress of the camber back is reversed in the second step 2B, and the camber back and the dimensional accuracy variation decrease.

[0063] FIGS. 8A and 8B each illustrate a stress distribution in the longitudinal direction at the center position of the sheet thickness at the bottom dead center of forming when press-forming is performed adopting the method for manufacturing a press-formed article in the invention of this application described above.

[0064] FIG. 8A illustrates the stress distribution in the longitudinal direction at the bottom dead center of forming (at the center position of the sheet thickness) in the first step 2A. The top sheet forming radius of curvature in the die of the first step 2A was set to R800 [mm]. As is understood from FIG. 8A, at the bottom dead center of forming of the first step 2A, compressive stress generates in a wide range over the longitudinal direction in the vertical wall portion 10B and tensile stress generates in a wide range over the longitudinal direction in the top sheet portion 10A.

[0065] FIG. 8B illustrates the stress distribution in the longitudinal direction at the bottom dead center of forming in the second step 2B. As the representative value, a value at the center position of the sheet thickness was used. The top sheet forming radius of curvature in the die of the second step 2B was set to R1600 [mm]. As is understood from FIG. 8B, at the bottom dead center of forming of the second step 2B, tensile stress generates in a lower part of the vertical wall portion 1B and compressive stress generates in the top sheet portion 1A. Then, the stress was reversed as compared with that of the common method (see FIG. 3), and further a region where the stress generates decreased. This shows that the press-formed article is bent back as a structure by performing forming such that the radius of curvature (top sheet radius of curvature) in side view increases in the second step 2B. As a result, the vertical wall portions 1B are formed in a direction in which the line length extends, and therefore tensile stress generates. The top sheet portion 1A is formed in a direction in which the line length decreases, and therefore compressive stress generates.

[0066] Such a dimensional accuracy variation when press-forming is performed by the method of this embodiment is described using a schematic view of a stress-strain relation.

[0067] FIG. 9 illustrates the relation between stress and strain of the top sheet portion 1A when this embodiment is adopted. FIG. 9 illustrates, assuming a variation in material strength, two schematic views of a high-strength material and a low-strength material.

[0068] In the first step 2A (first step of forming), forming is performed such that the radius of curvature (top sheet forming radius of curvature) is smaller than that of the product shape. Then, strain and tensile stress, which are larger than those of the common method, are imparted as illustrated in FIG. 9. Thereafter, in the second step 2B (second step of forming), forming is performed such that the radius of curvature (top sheet forming radius of curvature) is larger than that of the product shape. Then, the material of the top sheet portion 1A is formed in a shrinking direction, and therefore tensile stress is reversed to compressive stress.

[0069] Herein, when stress is reversed, a point (reference sign X in FIG. 9) appears where the curves of the high-strength material and the low-strength material intersect with each other in a low stress region. Forming is performed to reduce the radius of curvature (top sheet forming radius of curvature) of the first step 2A aiming at this intersection point, the stresses of the high-strength material and the low-strength material at the bottom dead center of forming and the stress difference decrease. Then, the camber back can be reduced and the dimensional accuracy variation can be reduced.

[0070] When the curvature of the curve in the longitudinal direction is not constant, only the radius of curvature of part of the curved portion may be reduced at least in the first step 2A. In the component shape containing a plurality of curved portions along the longitudinal direction, the radius of curvature of at least one or more curved portions may be reduced in the first step 2A.

[0071] FIGS. 10A and 10B illustrate distributions for strain in the longitudinal direction at the center of the sheet thickness at the bottom dead center of forming when the press-formed article is formed in this embodiment. FIG. 10A illustrates the distribution for the strain in the longitudinal direction at the center of the sheet thickness at the bottom dead center of forming in the first step 2A. FIG. 10B illustrates the distribution for the strain in the longitudinal direction at the center of the sheet thickness at the bottom dead center of forming in the second step 2B. The top sheet forming radius of curvature in the first step 2A was set to 800 [mm]. The top sheet forming radius of curvature in the second step 2B was set to 1600 [mm].

[0072] As is understood from FIG. 10A, large compressive strain generates along the longitudinal direction in the vertical wall portion 10B of the intermediate component 10 in the first step 2A. Further, tensile strain generates along the longitudinal direction in the top sheet portion 10A. However, the compressive strain of the vertical wall portion 1B decreases and the tensile strain of the top sheet portion 1A decreases as illustrated in FIG. 10B in the second step 2B. Thus, it is found that the strain behavior is as illustrated in FIG. 9.

[0073] Herein, for the strain in the longitudinal direction generating at the bottom dead center of press of the second step 2B, the absolute value of the average value of the strain in the curved region of the top sheet portion 1A is defined as ε2. For the strain in the longitudinal direction generating at the bottom dead center of press of the first step 2A, the absolute value of the average value of the strain in the curved region of the top sheet portion 10A is defined as ε1.

[0074] In this case, the values of the top sheet forming radii of curvature of the dies of the first step 2A and the second step 2B are set to values satisfying Equation (1) below in this embodiment. 0.1 × ε 1 ≤ ε 2 < 0.8 × ε 1

[0075] Further, for the strain in the longitudinal direction generating at the bottom dead center of press of the second step 2B, the absolute value of the average value of the strain in a curved region of a lower half of the vertical wall portion 1B is defined as ε2'. For the strain in the longitudinal direction generating at the bottom dead center of press of the first step 2A, the absolute value of the average value of the strain in a curved region of a lower half of the vertical wall portion 10B is defined as ε1'.

[0076] In this case, the values of the top sheet forming radii of curvature of the dies of the first step 2A and the second step 2B are set to values satisfying Equation (2) below in this embodiment. 0.3 × ε 1 ′ ≤ ε 2 ′ < 0.85 × ε 1 ′

[0077] Herein, when ε2 is smaller than 0.1 × ε1, the return amount of the strain in the second step 2B illustrated in FIG. 9 becomes excessively large. Therefore, there is a possibility that the springback amount difference due to the material strength difference cannot be reduced to a desired value or less. Accordingly, ε2 is preferably 0.1 × ε1 or more and more preferably 0.2 × ε1 or more.

[0078] When ε2 is larger than 0.8 × ε1, the return amount of the strain of the second step 2B is insufficient. Therefore, there is a possibility that the springback amount difference due to the material strength difference cannot be reduced to a desired value or less. Accordingly, ε2 is preferably less than 0.8 × ε1 and more preferably 0.7 × ε1 or less.

[0079] When ε2' is smaller than 0.3 × ε1', the return amount of the strain of the second step 2B illustrated in FIG. 9 becomes excessively large. Therefore, there is a possibility that the springback amount difference due to the material strength difference cannot be reduced to a desired value or less. Accordingly, ε2' is preferably 0.3 × ε1' or more and more preferably 0.4 × ε1' or more.

[0080] When ε2' is larger than 0.85 × ε1', the return amount of the strain in the second step 2B is insufficient. Therefore, there is a possibility that the springback amount difference due to the material strength difference cannot be reduced to a desired value or less. Accordingly, ε2' is preferably less than 0.85 × ε1' and more preferably 0.8 × ε1' or less.

[0081] FIG. 11 illustrates the results of investigating a springback amount difference among steel sheets of 590 MPa-grade to 1180 MPa-grade tensile strength.

[0082] FIG. 11 illustrates the results for a case where a member curved in the longitudinal direction illustrated in FIG. 1 is press-formed. The sheet thickness t of the metal sheet is 1.4 [mm]. The dimensions of the target product shape are as follows. More specifically, the width of the top sheet portion 1A is 60 [mm], the height of the vertical wall portion 1B is 30 [mm], the angle formed by the top sheet portion 1A and the vertical wall portion 1B is 110°, and the length in the longitudinal direction of the product is 400 [mm].

[0083] In FIG. 11, ε2 / ε1 changed by setting the radius of curvature in the first step 2A to various conditions is plotted on the horizontal axis. In FIG. 11, the circles and the triangles connected by straight lines indicate steel grades press-formed with dies having the same shape and different in tensile strength level. The value on the vertical axis represents the springback amount difference among the steel grades.

[0084] As is understood from FIG. 11, the springback amount difference among the steel grades is within 3 mm, which is good, when ε2 / ε1 ranges from 0.1 to 0.8.

[0085] FIG. 12 illustrates the results of investigating a springback amount difference among steel sheets of 590 MPa-grade to 1180 MPa-grade tensile strength.

[0086] FIG. 12 illustrates the results for a case where a member curved in the longitudinal direction illustrated in FIG. 1 is press-formed. The sheet 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 sheet portion 1A is 60 [mm], the height of the vertical wall portion 1B is 30 [mm], the angle formed by the top sheet portion 1A and the vertical wall portion 1B is 110°, and the length in the longitudinal direction of the product is 400 [mm].

[0087] In FIG. 12, ε2' / ε1' changed by setting the radius of curvature in the first step to various conditions is plotted on the horizontal axis. In FIG. 12, the circles and the triangles connected by straight lines indicate steel grades press-formed with dies having the same shape and different in tensile strength level. The value on the vertical axis represents the springback amount difference among the steel grades.

[0088] As is understood from FIG. 12, the springback amount difference among the steel grades is within 3 mm, which is good, when ε2' / ε1' ranges from 0.3 to 0.85.

[0089] From the above-described reasons, according to the method for manufacturing a press-formed article of this embodiment, the camber back and the dimensional accuracy variation can be significantly reduced simply by adjusting the top sheet forming radii of curvature of the first step 2A and the second step 2B. More specifically, even when the high tensile-strength materials are applied to the metal sheet, the dimensional accuracy variation when the material strength varies can be significantly reduced while the camber back is reduced without complicating the die shape.

[0090] Thus, this embodiment makes it possible to stably manufacture a high-precision component having a curved U-shaped cross-sectional shape close to the target product shape. More specifically, this embodiment can provide a method for manufacturing a press-formed article excellent in shape fixability and dimensional accuracy variation.

[0091] In this embodiment, even when the material strength of the metal sheet varies, a component having high dimensional accuracy can be obtained, which leads to an improved yield. Further, when the present invention is applied to vehicle body structural components, for example, the assembling of the component can be facilitated.(Modifications)

[0092] In addition to the adjustment of the top sheet forming radii of curvature in the die shapes used in the first step 2A and the second step 2B described above, the vertical wall angle between the top sheet portion and the vertical wall portion may also be adjusted as described below. In this case, the stress is more easily reversed, and the dimensional change and the dimensional accuracy variation are more easily reduced.

[0093] Herein, the angle between the top sheet portion and the vertical wall portion in the U-shaped cross section of the component is defined as the vertical wall angle.

[0094] A first angle θ1 (see FIG. 13A) in the die 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 a surface 3A forming the top sheet portion 10A and a surface 3B forming the vertical wall portion 10B. A second angle θ2 (see FIG. 13B) in the die used in the second step 2B is set to an angle larger than the vertical wall angle θ1' after die release of the intermediate component 10. The second angle θ2 is the angle between a surface 4A forming the top sheet portion 1A and a surface 4B forming the vertical wall portion 1B. The setting of these angles may be carried out at least in the region to be the curved portion.

[0095] The above-described first angle θ1 and the second angle θ2 are preferably set to have values satisfying Equation (4) below. θ 1 < θ 1 ′ < θ 0 ≤ θ 2

[0096] More specifically, in the first step 2A, the intermediate component 10 is manufactured by performing press-forming into a component shape having a U-shaped cross-sectional shape in which the first angle θ1 is smaller than the vertical wall angle θ0 of the product shape at the bottom dead center of forming. In the second step 2B, press-forming is performed such that the vertical wall angle is larger than the vertical wall angle θ1' after springback of the intermediate component 10 at the bottom dead center of forming.

[0097] For example, when a U-shaped cross section convexly curved to the side of the top sheet portion 1A, the first angle θ1 and the second angle θ2 are set as described above. Then, the vertical wall portion 1B is deformed in the opening direction in the second step 2B, resulting in forming in which the vertical wall portion 1B is pulled in the longitudinal direction. Thus, the stress in the longitudinal direction of the vertical wall portion 1B is easily reversed, and the dimensional change and the dimensional accuracy variation are easily reduced.

[0098] Herein, in a case where the die shapes of the first step 2A and the second step 2B are designed, when the design is started without any consideration for the radius of curvature of the product or the design is performed from the die of the second step 2B, there is a possibility that an appropriate die shape is not determined. Alternatively, there is a possibility that it takes a lot of time to determine an appropriate die shape.

[0099] Thus, in designing the die shapes of the first step 2A and the second step 2B described above, a CAE analysis or actual pressing is first performed with the top sheet forming radius of curvature of the first step 2A as a value smaller than the top sheet radius of curvature of the product. Thus, a condition is found under which the top sheet radius of curvature of the intermediate component 10 after springback of the first step 2A is smaller than the top sheet radius of curvature of the product. Thereafter, a CAE analysis or actual press is performed with a die shape in which the top sheet forming radius of curvature is larger than the top sheet radius of curvature of the product shape in the second step 2B. Thus, a condition is found under which the top sheet radius of curvature after springback of the second step 2B matches the top sheet radius of curvature in the product shape and the dimensional accuracy variation is suppressed. By performing the above-described processing, the die shape can be designed in a short time.

[0100] At this time, the vertical wall angle of the first step 2A is preferably made as small as possible as compared with the vertical wall angle of the second step 2B. However, the same vertical wall angle as that of the second step 2B is acceptable.

[0101] Thus, the invention of this application can significantly reduce the dimensional accuracy variation even when the high tensile-strength materials with a strength class of 590 PMa or more or the ultrahigh tensile-strength materials with a strength class of 980 MPa or more are used. More specifically, in accordance with the invention of this application, the dimensional accuracy variation when the material strength variation generates can be significantly reduced with a simple die structure. Thus, a high-precision component close to the target component shape can be obtained.

[0102] Herein, although the description above gives the description taking the case of the U-shaped cross section as an example, the present invention is also applicable to a product shape having an L-shaped cross-section as the target.(Others)

[0103] The present disclosure can also take the following configurations. (1) A method for manufacturing a press-formed article in which a press-formed article is manufactured by press-forming a metal sheet into a component shape having a U-shaped or L-shaped cross section in which a top sheet portion and a vertical wall portion are continuous with each other at an end portion in the width direction of the top sheet portion and having one or two or more curved portions curved convexly or concavely to the side of the top sheet portion in side view along the longitudinal direction, the longitudinal direction being a direction intersecting the cross section, the method including: when the radius of curvature along the longitudinal direction of the top sheet portion is defined as a top sheet radius of curvature and the radius of curvature along the longitudinal direction of the top sheet portion of a forming surface forming the top sheet portion in a die is defined as a top sheet forming radius of curvature, a first step of press-forming a metal sheet into an intermediate component having a U-shaped or L-shaped cross section using a die with a first radius of curvature R1 smaller than a top sheet radius of curvature R0 in the target product shape as the top sheet forming radius of curvature at least in a region where the curved portion is formed; and a second step of press-forming the intermediate component using a die with a second radius of curvature R2 equal to or larger than the top sheet radius of curvature R0 in the target product shape as the top sheet forming radius of curvature at least in a region where the curved portion is formed, in which when the first radius of curvature R1 is set such that a top sheet radius of curvature R1' after die release of the intermediate component is less than the top sheet radius of curvature R0, for strain in the longitudinal direction generating at the bottom dead center of press in the press-forming of the first step, the absolute value of the average value of the strain of the top sheet portion is defined as ε1 and the absolute value of the average value of the strain of a vertical wall lower-half of the vertical wall portion is defined as ε1', and for strain in the longitudinal direction generating at the bottom dead center of press in the press-forming of the second step, the absolute value of the average value of the strain of the top sheet portion is defined as ε2 and the absolute value of the average value of the strain of the vertical wall lower-half of the vertical wall portion is defined as ε2', the first radius of curvature R1 and the second radius of curvature R2 are adjusted to have values satisfying Equations (1) and (2) below at least in the region to be the curved portion. 0.1 × ε 1 ≤ ε 2 < 0.8 × ε 1 0.3 × ε 1 ′ ≤ ε 2 ′ < 0.85 × ε 1 ′ (2) At least in the region to be the curved portion, the first radius of curvature R1 is adjusted to have a value satisfying Equation (3) below. 0.45 ≤ R 1 ′ / R 0 ≤ 0.85 (3) When the angle between the top sheet portion and the vertical wall portion in the U-shaped cross section or the L-shaped cross section is defined as a vertical wall angle, at least in the region to be the curved portion, a first angle θ1, the first angle θ1 being an angle between a surface forming the top sheet portion and a surface forming the vertical wall portion, in the die used in the first step is set to an angle smaller than a vertical wall angle θ0 of the product shape, and at least in the region to be the curved portion, a second angle θ2, the second angle θ2 being an angle between a surface forming the top sheet portion and a surface forming the vertical wall portion, in the die used in the second step is set to an angle larger than a vertical wall angle θ1' after die release of the intermediate component. (4) When the vertical wall angle after die release of the intermediate component formed in the first step is defined as θ1', the first angle θ1 and the second angle θ2 are set to have values satisfying Equation (4) below. θ 1 < θ 1 ′ < θ 0 ≤ θ 2 (5) To the press-forming of the first step, drawing or stamping is applied, and to the press-forming of the second step, a restriking step is applied. (6) The metal sheet is a steel sheet having material strength of 590 MPa or more. EXAMPLES

[0104] In this example, a press-forming analysis and a springback analysis by a finite element method (FEM) were performed to confirm the camber back reduction effect by the press-forming according to this embodiment.

[0105] This example was directed to a case where a member curved in the longitudinal direction illustrated in FIG. 1 is press-formed.

[0106] Herein, in a metal sheet used for the press-forming, the sheet thickness t was set to 1.4 [mm]. To also compare the dimensional accuracy variations, three types of steel sheets of 590 MPa-grade tensile strength, 980 MPa-grade tensile strength, and 1180 MPa-grade tensile strength were used. The target product shape was set such that the width of the top sheet portion 1A was 60 [mm], the height of the vertical wall portion 1B was 30 [mm], the angle formed by the top sheet portion 1A and the vertical wall portion 1B was 110°, and the length in the longitudinal direction of the product was 400 [mm].(Example 1)

[0107] First, a press-forming analysis and a springback analysis were conducted in a case where only the radius of curvature of the curve in side view in the first step 2A was reduced. Then, the radii of curvature before and after springback were measured.

[0108] In this example, the top sheet forming radius of curvature R2 of the die used in the second step 2B was set to R1600 [mm]. Then, an FEM analysis was conducted while the top sheet forming radius of curvature R1 of the die of the first step 2A was changed in the range of R700 [mm] to R1600 [mm].

[0109] Table 1 shows the conditions and the evaluation results.

[0110] In Table 1, the die R indicates the top sheet forming radius of curvature and the top sheet R indicates the top sheet radius of curvature.

[0111] The camber back amount was evaluated by the deviation amount in the Z-direction (height direction) from the bottom dead center of press-forming in the second step 2B. A case where a difference among the steel grades in the deviation amount in the height direction after springback of the second step 2B was 3.0 [mm] or less was evaluated as "Pass". [Table 1]No.MaterialDie R of first stepθ1Die R of second stepθ2Average strain ε1 (absolute value) of top sheet at bottom dead center of first stepAverage strain ε1' (absolute value) of vertical wall lower-half at bottom dead center of first stepTop sheet R after SB of first stepAverage strain ε2 (absolute value) of top sheet at bottom dead center of second stepAverage strain ε2' (absolute value) of vertical wall lower-half at bottom dead center of second stepTop sheet R after SB of second stepε2 / ε1ε2' / ε1'Maximum variation amount of top sheet R after SB of second stepHeight variation amount of top sheet portion after SB of second stepDifference among steel grades in height differenceRemarks15901600110°1600110°0.00130.006619330.00130.006720020.9921.01510542.493.49Comparative Example2980↑↑↑↑0.00220.006122520.00230.006027161.0410.9795.3231180↑↑↑↑0.00240.005923900.00240.005830561.0080.9915.9845901040↑↑↑0.00170.012711720.00090.006914380.5200.543385-1.102.65Present Invention5980↑↑↑↑0.00310.009512910.00180.006117640.5730.6391.1561180↑↑↑↑0.00280.008413600.00220.007218230.8000.8571.5575901000↑↑↑0.00160.013211110.00050.006814240.3050.517318-1.602.45Present Invention8980↑↑↑↑0.00270.010012160.00140.006116710.5040.61305491180↑↑↑↑0.00310.010512370.00120.005917420.4070.5620.8510590800↑↑↑0.00230.01658690.00040.007213610.1750.435107-2.181.00Present Invention11980↑↑↑↑0.00390.0123930000070.006213000.1860.507-2.68121180↑↑↑↑0.00390.01189560.00080.006114070.2030.519-1.6813590700↑↑↑0.00260.01937190.00020.006013400.0960.311164-2.401.60Present Invention14980↑↑↑↑0.00450.01507930.00030.006011760.0690.400-4.00151180↑↑↑↑0.00450.01448200.00030.006312200.0680.438-3.70 <Nos. 1 to 3>

[0112] Nos. 1 to 3 are the results of the radius of curvature of the top sheet portion 1A of each of the 590 MP material, the 980 MPa material, and the 1180 MPa material when the top sheet forming radius of curvature R1 of the die of the first step 2A was set to R1600 [mm]. The top sheet radius of curvature R0 in the target product shape in Nos. 1 to 3 is R1600 [mm].

[0113] In this case, the top sheet radius of curvature after springback of the first step 2A of each material greatly exceeds R1600 [mm], which is the top sheet forming radius of curvature of the die in the second step 2B. In terms of the 1180 MPa material, the radius of curvature of the top sheet portion 1A after springback of the first step 2A was as large as R2390 [mm]. The radius of curvature of the top sheet portion 1A after springback of the second step 2B of the 1180 MPa material was as large as R3056 [mm]. A difference among the steel grades in the deviation amount in the height direction after springback of the second step 2B was 3.49 [mm]. This is a value exceeding 3.0 [mm], which is the target.<Nos. 4 to 6>

[0114] Nos. 4 to 6 are the results of the radius of curvature of the top sheet portion 1A of each of the 590 MP material, the 980 MPa material, and the 1180 MPa material when the top sheet forming radius of curvature R1 of the die of the first step 2A was set to R1040 [mm]. The top sheet radius of curvature R0 in the target product shape in Nos. 4 to 6 is R1600 [mm].

[0115] In this case, the top sheet radius of curvature after springback of the first step 2A of each material is smaller than R1600 [mm], which is the top sheet forming radius of curvature of the die in the second step 2B. In terms of the 1180 MPa material, the radius of curvature of the top sheet portion 1A after springback of the first step 2A was R1360 [mm]. The radius of curvature of the top sheet portion 1A after springback of the second step 2B of the 1180 MPa material was R1823 [mm]. A difference among the steel grades in the deviation amount in the height direction after springback of the second step 2B was 2.65 [mm], and the difference among the steel grades was 3.0 [mm], which is the target, or less.<Nos. 7 to 9>

[0116] In contrast thereto, the results of the radius of curvature of the top sheet portion 1A of each of the 590 MP material, the 980 MPa material, and the 1180 MPa material when the top sheet forming radius of curvature of the die of the first step 2A was set to R1000 [mm] are shown in Nos. 7 to 9. The top sheet radius of curvature R0 in the target product shape in Nos. 7 to 9 is R1600 [mm] .

[0117] The top sheet radius of curvature after springback of the first step 2A of each material is smaller than R1600 [mm], which is the top sheet forming radius of curvature of the die in the second step 2B. In terms of the 1180 MPa material, the top sheet radius of curvature after springback of the first step 2A was R1237 [mm]. The top sheet radius of curvature after springback of the second step 2B of the 1180 MPa material was R1742 [mm]. A difference among the steel grades in the deviation amount in the height direction after springback of the second step 2B was 2.45 [mm], and the difference among the steel grades was 3.0 [mm], which is the target, or less.<Nos. 10 to 12>

[0118] Next, the results of the radius of curvature of the top sheet portion 1A of each of the 590 MP material, the 980 MPa material, and the 1180 MPa material when the top sheet forming radius of curvature R1 of the die of the first step 2A was set to R800 [mm] are shown in Nos. 10 to 12. The top sheet radius of curvature R0 in the target product shape in Nos. 10 to 12 is R1400 [mm].

[0119] The radius of curvature of the top sheet portion 1A after springback of the first step 2A of each material is smaller than R1600 [mm], which is the top sheet forming radius of curvature of the die in the second step 2B. In terms of the 1180 MPa material, the radius of curvature of the top sheet portion 1A after springback of the first step 2A was R956 [mm]. The radius of curvature of the top sheet portion 1A after springback of the second step 2B of the 1180 MPa material was R1407 [mm]. A difference among the steel grades in the deviation amount in the height direction after springback of the second step 2B was 1.00 [mm], and the difference among the steel grades was 3.0 [mm], which is the target, or less.<Nos. 13 to 15>

[0120] Next, the results of the radius of curvature of the top sheet portion 1A of each of the 590 MP material, the 980 MPa material, and the 1180 MPa material when the top sheet forming radius of curvature R1 of the die of the first step 2A was set to R700 [mm] are shown in Nos. 13 to 15. The top sheet radius of curvature R0 in the target product shape in Nos. 13 to 15 is R1300 [mm].

[0121] The radius of curvature of the top sheet portion 1A after springback of the first step 2A of each material is smaller than R1600 [mm], which is the top sheet forming radius of curvature of the die in the second step 2B. In terms of the 1180 MPa material, the radius of curvature of the top sheet portion 1A after springback of the first step 2A was R820 [mm]. The radius of curvature of the top sheet portion 1A after springback of the second step 2B of the 1180 MPa material was R1220 [mm]. A difference among the steel grades in the deviation amount in the height direction after springback of the second step 2B was 1.60 [mm], and the difference among the steel grades was 3.0 [mm], which is the target, or less.

[0122] From the above, the camber back amount and the dimensional accuracy variation significantly decreased by forming under the conditions in which the top sheet forming radius of curvature of the die of the first step 2A is set to that of the examples the present invention. However, the relation between the springback and the top sheet forming radii of curvature of the dies of the first and second steps 2A, 2B above changes depending on the steel grade or the shape. Therefore, it is necessary to design the shape suitable for each steel grade or each shape.(Example 2)

[0123] A press-forming analysis and a springback analysis were conducted when the radius of curvature and the vertical wall angle of the curve in side view of the first step 2A were reduced with a case where a member curved in the longitudinal direction illustrated in FIG. 1 was press-formed as the target. Then, the radii of curvature before and after springback were measured.

[0124] As metal sheets used for press-forming, the same conditions as those in Example 1 were used.

[0125] The top sheet forming radius of curvature R2 of the die of the second step 2B in this example was set to R1600 [mm]. The second angle θ2 was set to 110°. The top sheet forming radius of curvature R1 of the die in the first step 2A was changed from R1000 [mm] to R1600 [mm]. Further, the first angle θ1 was changed to 90°, and an FEM analysis was conducted.

[0126] Table 2 shows the conditions and the evaluation results.

[0127] In Table 2, the die R indicates the top sheet forming radius of curvature, and the top sheet R indicates the top sheet radius of curvature.

[0128] The camber back amount was evaluated by the deviation amount in the Z-direction (height direction) from the bottom dead center of press-forming of the second step 2B. A case where a difference among the steel grades in the deviation amount in the height direction after springback of the second step 2B was 3.0 [mm] or less was evaluated as "Pass". [Table 2]No.MaterialDie R of first stepθ1Die R of second stepθ2Average strain ε1 (absolute value) of top sheet at bottom dead center of first stepAverage strain ε1' (absolute value) of vertical wall lower-half at bottom dead center of first stepTop sheet R after SB of first stepθ1'Average strain ε2 (absolute value) of top sheet at bottom dead center of second stepAverage strain ε2' (absolute value) of vertical wall lower-half at bottom dead center of second stepTop sheet R after SB of second stepε2 / ε1ε2' / ε1'Maximum variation amount of top sheet R after SB of second stepHeight variation amount of top sheet portion after SB of second stepDifference among steel grades of height differenceRemarks16590160090°1600110°0.00100.0083184395.9°0.00100.007017991.0200.8448441.383.45Comparative Example17980↑↑↑↑0.00190.00802233107.6°0.00190.006823811.0110.8504.25Comparative Example181180↑↑↑↑0.00200.00792418109.4°0.00200.007226430.9950.9144.83Comparative Example195901000↑↑↑0.00120.0139107294.4°0.00030.009114100.2060.65332-1.670.18Present Invention20980↑↑↑↑0.00190.01411156106.4°0.00040.009413940.2150.668-1.77Present Invention211180↑↑↑↑0.00210.01381191109.1°0.00040.008114260.1910.583-1.59Present Invention <Nos. 16 to 18>

[0129] Nos. 16 to 18 are the results of the radius of curvature of the top sheet portion 1A of each of the 590 MP material, the 980 MPa material, and the 1180 MPa material when the top sheet forming radius of curvature R1 of the die of the first step 2A was set to R1600 [mm], and the vertical wall angle was changed to 90°. The top sheet radius of curvature R0 in the target product shape in Nos. 16 to 18 is R1600 [mm].

[0130] The radius of curvature of the top sheet portion 1A after springback of the first step 2A of each material greatly exceeds R1600 [mm], which is the top sheet forming radius of curvature of the die in the second step 2B. In terms of the 1180 MPa material, the radius of curvature of the top sheet portion 1A after springback of the first step 2A was as large as R2418 [mm]. The vertical wall angle after springback of the first step 2A is 110° or less of the vertical wall angle of the second step 2B. The radius of curvature of the top sheet portion 1A after springback of the second step 2B of the 1180 MPa material was as large as R2643 [mm]. A difference among the steel grades in the deviation amount in the height direction after springback of the second step 2B was 3.45 [mm].<Nos. 19 to 21>

[0131] In contrast thereto, the results of the radius of curvature of the top sheet portion 1A of each of the 590 MP material, the 980 MPa material, and the 1180 MPa material when the top sheet forming radius of curvature R1 of the die of the first step 2A was set to R1000 [mm] are shown in Nos. 19 to 21. The top sheet radius of curvature R0 in the target product shape in Nos. 19 to 21 is R1600 [mm].

[0132] The radius of curvature of the top sheet portion 1A after springback of the first step 2A of each material is smaller than R1600 [mm], which is the top sheet forming radius of curvature of the die in the second step 2B. In terms of the 1180 MPa material, the radius of curvature of the top sheet portion 1A after springback of the first step 2A was R1191 [mm]. The radius of curvature of the top sheet portion 1A after springback of the second step 2B of the 1180 MPa material was R1426 [mm]. A difference among the steel grades in the deviation amount in the height direction after springback of the second step 2B was 0.18 [mm], and the difference among the steel grades was 3.0 [mm], which is the target, or less.

[0133] This shows that the difference among the steel grades decreased as compared with Nos. 7 to 9 in Table 1. Nos. 7 to 9 in Table 1 are the cases where the top sheet forming radius of curvature of the die of the first step 2A was reduced to R1000 [mm], and the vertical wall angle was kept at 110° which was the same as that of the second step 2B.

[0134] From the above, the camber back amount and the dimensional accuracy variation significantly decreased by reducing the angle of the die adjusting the radius of curvature (top sheet forming radius of curvature) and the vertical wall angle of the curve in side view of the first step 2A. The relation among the springback, the vertical wall angle of the die of the first step 2A, and the top sheet forming radii of curvature of the dies of the first and second steps 2A, 2B above changes depending on the steel grade or the shape. Therefore, it is necessary to design the shape suitable for each steel grade or each shape.

[0135] Herein, the entire contents of JP 2023-135449 A (filed August 23, 2023), for which this application claims priority, form part of the present disclosure by reference. Herein, the description is given with reference to a limited number of embodiments, but the scope of the invention is not limited thereto and modifications of each embodiment based on the disclosure above are obvious to those skilled in the art.Reference Signs List

[0136] 1:product shape 1A:top sheet portion 1B:vertical wall portion 2A:first step 2B:second step 10:intermediate component 10A:top sheet portion 10B:vertical wall portion

Claims

1. A method for manufacturing a press-formed article in which a press-formed article is manufactured by press-forming a metal sheet into a component shape having a U-shaped or L-shaped cross section in which a top sheet portion and a vertical wall portion are continuous with each other at an end portion in the width direction of the top sheet portion and having one or two or more curved portions curved convexly or concavely to a side of the top sheet portion in side view along a longitudinal direction, the longitudinal direction being a direction intersecting the cross section, the method comprising: when a radius of curvature along the longitudinal direction of the top sheet portion is defined as a top sheet radius of curvature and a radius of curvature along the longitudinal direction of the top sheet portion of a forming surface forming the top sheet portion in a die is defined as a top sheet forming radius of curvature, a first step of press-forming a metal sheet into an intermediate component having a U-shaped or L-shaped cross section using a die with a first radius of curvature R1 smaller than a top sheet radius of curvature R0 in a target product shape as the top sheet forming radius of curvature in a region where the curved portion is formed; and a second step of press-forming the intermediate component using a die with a second radius of curvature R2 equal to or larger than the top sheet radius of curvature R0 in the target product shape as the top sheet forming radius of curvature in a region where the curved portion is formed, wherein when the first radius of curvature R1 is set such that a top sheet radius of curvature R1' after die release of the intermediate component is less than the top sheet radius of curvature R0, for strain in the longitudinal direction generating at a bottom dead center of press in the press-forming of the first step, an absolute value of an average value of the strain of the top sheet portion is defined as ε1 and an absolute value of an average value of the strain of a vertical wall lower-half of the vertical wall portion is defined as ε1', and for strain in the longitudinal direction generating at the bottom dead center of press in the press-forming of the second step, an absolute value of an average value of the strain of the top sheet portion is defined as ε2 and an absolute value of an average value of the strain of the vertical wall lower-half of the vertical wall portion is defined as ε2', the first radius of curvature R1 and the second radius of curvature R2 are adjusted to have values satisfying Equations (1) and (2) below in the region to be the curved portion. 0.1 × ε 1 ≤ ε 2 < 0.8 × ε 1 0.3 × ε 1 ′ ≤ ε 2 ′ < 0.85 × ε 1 ′2. The method for manufacturing a press-formed article according to claim 1, wherein, in the region to be the curved portion, the first radius of curvature R1 is adjusted to have a value satisfying Equation (3) below. 0.45 ≤ R 1 ′ / R 0 ≤ 0.

853. The method for manufacturing a press-formed article according to claim 1 or 2, wherein when an angle between the top sheet portion and the vertical wall portion in the cross section is defined as a vertical wall angle, in the region to be the curved portion, a first angle θ1, the first angle θ1 being an angle between a surface forming the top sheet portion and a surface forming the vertical wall portion, in the die used in the first step is set to an angle smaller than a vertical wall angle θ0 of the product shape, and in the region to be the curved portion, a second angle θ2, the second angle θ2 being an angle between a surface forming the top sheet portion and a surface forming the vertical wall portion, in the die used in the second step is set to an angle larger than a vertical wall angle θ1' after die release of the intermediate component.

4. The method for manufacturing a press-formed article according to claim 3, wherein when the vertical wall angle after die release of the intermediate component formed in the first step is defined as θ1', the first angle θ1 and the second angle θ2 are set to have values satisfying Equation (4) below. θ 1 < θ 1 ′ < θ 0 ≤ θ 25. The method for manufacturing a press-formed article according to any one of claims 1 to 4, wherein to the press-forming of the first step, drawing or stamping is applied, and to the press-forming of the second step, a restriking step is applied.

6. The method for manufacturing a press-formed article according to any one of claims 1 to 4, wherein the metal sheet is a steel sheet having material strength of 590 MPa or more.

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