Patent Grant
11998968
The present invention relates to a press forming method, and in particular to a press forming method for a press-formed product that has a hat-shaped cross section with a web portion, side wall portions, and flange portions, and that is concavely curved in the height direction along the longitudinal direction in side view.
Press forming is a method of processing a metal sheet, such as a steel sheet, by clamping the metal sheet with a die of press forming and transferring the shape of the die. In particular, many automotive parts are manufactured by press forming. Nowadays, there has been a strong tendency to use high-strength steel sheets for body parts in view of weight reduction of automotive bodies. However, as the strength as a property of steel sheets and other metal materials increases, elongation tends to decrease, and forming defects such as fractures and wrinkles often occur in the press forming of high-strength steel sheets, causing problems.
Among the frame parts of an automotive body, curved parts having a steeply curved shape, such as front side members and rear side members, tend to have fractures and wrinkles when manufactured by press forming. These members are thus considered to be parts difficult to form. Recently, automobile and parts manufactures have been studying the application of high-strength steel sheets in the manufacture of such curved parts in order to further reduce the weight of automotive bodies, and an issue is how to perform press forming while preventing fractures and wrinkles.
Several techniques have been developed for press forming of curved parts while suppressing fractures and wrinkles. For example, Patent Literature 1 discloses a technique for avoiding wrinkles in a punch bottom and fractures in a flange in press forming of an L-shaped part that is curved in top view, by using a forming load to form the flange and a side wall and sliding the material at the punch bottom.
Patent Literature 2 provides a method of preventing out-of-plane deformation and suppressing wrinkles of parts that are curved in the vertical direction by performing drawing forming while applying pressure to the punch bottom of a blank in the thickness direction with a pad.
Patent Literature 3 discloses a technique for suppressing the occurrence of wrinkles in a flange portion in press forming of a curved press part with a hat-shaped cross section curved in the longitudinal direction, by preforming a folding portion at an end portion of a blank material in the width direction and then press-forming the curved press part while leaving the folding portion. It is described that, with this technique, the stiffness of the end portion of the blank material in the width direction increases due to the folding portion added to the end portion of the blank material in the preforming process, and the resistance against force of shrinking the blank in the longitudinal direction increases, and thus the occurrence of wrinkles in the flange portion can be suppressed even when the force of shrinking the blank in the longitudinal direction is applied due to excess metal resulted from the curved shape.
In addition, several techniques have been developed to press-form a curved part by adding a bead, with the aim of suppressing the occurrence of fractures and wrinkles. Patent Literature 4 discloses a technique for suppressing the occurrence of wrinkles in a material formed section when press-forming, in one process, a material into a shape that has a curvature when an end portion of the material is viewed in plan view and that has a flange surface below a side wall surface in side view, by adding a convex bead to the side wall surface and a concave bead to the flange surface directly below the concave bead.
Patent Literature 1: Japanese Patent No. 5168429
Patent Literature 2: Japanese Patent No. 5733475
Patent Literature 3: Japanese Patent No. 5965159
Patent Literature 4: Japanese Patent Application Laid-open No. 2010-115674
However, the technique disclosed in Patent Literature 1 has limited applicability to parts that have a shape such as a mounted surface at the punch bottom or that have a closed shape such as a bag shape, because the material cannot be moved significantly.
In the technique disclosed in Patent Literature 2, a blank holder and pad are used at the same time for forming, and when a formed product is removed from a die, if the blank holder or pad remains under pressure, it will crush the formed product. Thus, a locking structure is necessary to stop movement. However, since press machines equipped with this mechanism are not common, the technique lacks versatility.
In the technique disclosed in Patent Literature 3, the bending shape of the flange portion needs to be flattened in the next process, but there is a risk that curling may remain. Particularly in the case of automotive parts, a flange is often a surface for joining with another part, and high surface accuracy is required. Thus, care needs to be taken in applying this forming method.
The technique disclosed in Patent Literature 4 is press forming in one process, and has the problem that the bead added to prevent wrinkle occurrence and fracturing remains as it is.
The present invention has been made in order to solve the above problems, and an object of the present invention is to provide a press forming method that can press-form, into a favorable shape, a press-formed product that has a hat-shaped cross section with a web portion, side wall portions, and flange portions, and that is concavely curved in the height direction along the longitudinal direction, while suppressing fractures and wrinkles.
A press forming method according to the present invention forms a press-formed product having a hat-shaped cross section and including: a web portion; a side wall portion continuous from the web portion; a flange portion continuous from the side wall portion; and a concave curved portion in which the web portion and/or the flange portion is concavely curved in a height direction along a longitudinal direction in side view, and includes: a first forming process of press-forming a preformed part in which a portion corresponding to web corresponding to the web portion, and a portion corresponding to side wall corresponding to the side wall portion and including a twisted side wall portion of a twisted shape along the longitudinal direction, are formed, the preformed part including a portion corresponding to concave curve corresponding to the concave curved portion; and a second forming process of press-forming the preformed part into the press-formed product, wherein the twisted side wall portion at the first forming process is twisted such that an angle between the twisted side wall portion and the portion corresponding to web is smaller on an end portion side than at a longitudinal center of the portion corresponding to concave curve.
The twisted side wall portion at the first forming process may have a torsion amount T given by a following equation, the torsion amount T being set to be in a range of 10° or larger and 20° or smaller:
T=Δθ×(H/L), where
Δθ: an angle difference (=θ1−θ2),
θ1: an angle (°) between the twisted side wall portion and the portion corresponding to web at the longitudinal center of the portion corresponding to concave curve,
θ2: an angle (°) between the twisted side wall portion and the portion corresponding to web at an longitudinal end portion of the twisted side wall portion,
H: a side wall height (mm) of the twisted side wall portion, and
L: a longitudinal length (mm) of the twisted side wall portion.
According to the present invention, shear deformation can be generated in the twisted side wall portion, and the occurrence of fractures and wrinkles is suppressed, enabling the press-formed product to be press-formed into a favorable shape.
Before explaining a press forming method according to an embodiment of the present invention, the following explains a press-formed product to be formed in the present invention, the reason that fractures and wrinkles occur when the press-formed product is press-formed, and the background that led to the present invention. In the present embodiment, the height direction of the press-formed product coincides with the press-forming direction of the press-formed product.
<Press-Formed Product>
As illustrated in
Thus, in the concave curved portion 11, the longitudinal length of each of the flange portions 7 becomes longer while the longitudinal length of the web portion 3 becomes shorter due to the concentration of the material, resulting in a line length difference in the longitudinal direction between the web portion 3 and the flange portion 7. As a result, tensile deformation acts on the flange portion 7, causing it to fracture easily, and compressive deformation acts on the web portion 3, causing it to wrinkle easily.
Therefore, in order to suppress the occurrence of fractures and wrinkles during press forming of the press-formed product 1, it is considered important to change the movement of the material during the press forming process so that tensile deformations do not occur in the flange portion 7 and compressive deformations do not occur in the web portion 3 in the concave curved portion 11, thereby reducing the line length difference in the longitudinal direction between the web portion 3 and the flange portion 7.
Thus, consider the ideal state of press forming that does not cause a line length difference in the longitudinal direction between the web portion 3 and the flange portion 7 in the concave curved portion 11.
In order to avoid a line length difference in the longitudinal direction between the web portion 3 and the flange portion 7, as illustrated in
The inventors have studied a method to induce in-plane shear deformation in the material. As a result, it has been found that in-plane shear deformation can be generated in the portion corresponding to side wall of the blank by press-forming it into a curved surface shape with an out-of-plane twist along the longitudinal direction. The present invention has been made based on such studies, and the press forming method according to the embodiment of the present invention is described below.
<Press Forming Method>
The press forming method according to the present embodiment is to press-form the press-formed product 1 illustrated in
<<First Forming Process>>
The first forming process is a process to preform the blank into the preformed part 21 (
As illustrated in
In the preformed part 21, the twisted side wall portions 25a are formed over the entire length of the portions 25 corresponding to side wall in the longitudinal direction.
Each of the twisted side wall portions 25a is twisted such that an angle θ2 at the longitudinal end portion is smaller than an angle θ1 at the concave curve center where an angle between the twisted side wall portion 25a and the portion 23 corresponding to web is θ, as illustrated in
In the present embodiment, the portion 23 corresponding to web of the preformed part 21 has the same shape as that of the web portion 3 (
The angle between the portion 23 corresponding to web and the twisted side wall portion 25a of the preformed part 21 is smaller at the longitudinal end portion (θ2) than at the concave curve center (θ1) (refer to
Furthermore, the ridgeline length of a punch corner portion 24 (
For example, if the portion 23 corresponding to web is formed into the same shape as that of the web portion 3 of the press-formed product 1 having the target shape, the ridgeline length of the punch corner portion 24 is the same as that of the press-formed product 1, but the ridgeline length of the die corner portion 26 is different from that of the press-formed product 1. If the portion 27 corresponding to flange is formed into the same shape as that of the flange portion 7 of the press-formed product 1 having the target shape, the ridgeline length of the die corner portion 26 is the same as that of the press-formed product 1, but the ridgeline length of the punch corner portion 24 is different from that of the press-formed product 1.
<<Second Forming Process>>
The second forming process is a process to press-form the preformed part 21 (
<<Reason that Fractures and Wrinkles can be Suppressed>>
Next, the following explains the reason that the press forming method according to the present embodiment can press-form a press-formed product that is concavely curved in the height direction along the longitudinal direction in side view, while suppressing fractures and wrinkles.
At the first forming process, as illustrated in
This configuration suppresses the movement of the material toward the center of the portion 23 corresponding to web in the longitudinal direction in the portion 31 corresponding to concave curve, and also suppresses the movement of the material toward the end portion side in the longitudinal direction in the portion 27 corresponding to flange. Thus, in the twisted side wall portion 25a, the line length difference between the line length of the portion 27 corresponding to flange in the longitudinal direction and the line length of the portion 23 corresponding to web in the longitudinal direction is reduced, as illustrated in
<<Suitable Range of Torsion Amount of Twisted Side Wall Portion>>
As illustrated in
Here, the magnitude of in-plane shear deformation in the twisted side wall portion 25a depends on the degree of torsion of the twisted side wall portion 25a. In the present invention, the degree of torsion of the twisted side wall portion 25a can be expressed using an angle change and an aspect ratio of the twisted side wall portion 25a.
The angle change of the twisted side wall portion 25a is given by the angle difference Δθ (θ1−θ2) between the angle θ1 at the concave curve center (the center of the portion 31 corresponding to concave curve in the longitudinal direction) and the angle θ2 at the longitudinal end portion (the end portion of the twisted side wall portion 25a in the longitudinal direction), which are the angles between the twisted side wall portion 25a and the portion 23 corresponding to web (refer to
The aspect ratio of the twisted side wall portion 25a is given by a ratio H/L, which is the ratio of a side wall height H to a longitudinal length L of the twisted side wall portion 25a, as illustrated in
The torsion amount T (°) is then given by the following Equation (1).
T=Δθ×(H/L)=(θ1−θ2)×(H/L) (1)
It is understood from Equation (1) that the torsion amount T can be changed by changing (1) the angle θ1 between the twisted side wall portion 25a and the portion 23 corresponding to web at the concave curve center, (2) the angle θ2 between the twisted side wall portion 25a and the portion 23 corresponding to web at the longitudinal end portion, (3) the side wall height H of the twisted side wall portion 25a, and (4) the longitudinal length L of the twisted side wall portion 25a.
The preformed part 21 illustrated in
Regarding the angle between the twisted side wall portion 25a and the portion 23 corresponding to web of the preformed part 21 press-formed at the first forming process, the angle θ2 at the longitudinal end portion needs to be smaller than the angle θ1 at the concave curve center, as described above. For example, if the angle θ2 at the longitudinal end portion is larger than the angle θ1 at the concave curve center as illustrated in
In addition, the torsion amount T suitable for suppressing fractures and wrinkles was investigated by finite element method (FEM) simulation. As a result, it was found that setting the torsion amount T to be in the range of 10° or larger and 20° or smaller is desirable to suppress both fractures and wrinkles. When the torsion amount T is smaller than 10°, the in-plane shear deformation of the twisted side wall portion 25a may be insufficient. When the torsion amount T is larger than 20°, the twisted side wall portion 25a may undergo excessive shear deformation at the first forming process, resulting in wrinkles by shear deformation in the portion corresponding to side wall.
The aspect ratio H/L of the twisted side wall portion 25a may be given by using the side wall height H at the center of the longitudinal length (a middle position between the concave curve center and the longitudinal end portion) and the longitudinal length L at the center in the side wall height direction, of the twisted side wall portion 25a.
<<Shapes of Preformed Part and Press-Formed Product>>
In the above explanation, the preformed part 21 (
However, the present invention may form portions 107 corresponding to flange having the same shape as that of the flange portions 7 of the target shape (
In the preformed part 101 as described above, when the angle θ2 between each of twisted side wall portions 105a and the portion 103 corresponding to web at the longitudinal end portion is smaller than the angle θ1 between the twisted side wall portion 105a and the portion 103 corresponding to web at the concave curve center, the twisted side wall portion 105a formed in each of portions 105 corresponding to side wall is press-formed while undergoing in-plane shear deformation as illustrated in
However, as illustrated in
Furthermore, the press-formed product 1 to be formed as described above includes the web portion 3 and the flange portion 7 both being concavely curved in the height direction along the longitudinal direction. However, as illustrated in
The above explanation is for forming a press-formed product such as the press-formed product 1 illustrated in
In such a case, for each concave curved portion with a constant radius of curvature, the twisted side wall portion in the concave curved portion may have a curved surface shape twisted from the center toward the end portion of the concave curved portion in the longitudinal direction. Then, for each concave curved portion, the angle between the portion corresponding to web and the twisted side wall portion at the longitudinal end portion of the twisted side wall portion of the concave curved portion may be smaller than that at the center of the concave curved portion in the longitudinal direction.
In addition, the press-formed product 1 to be formed in the present embodiment includes the straight portions 13 on both sides of the concave curved portion 11 in the longitudinal direction. However, the present invention may be used to form a press-formed product including a straight portion on one side of the concave curved portion in the longitudinal direction or a press-formed product including the concave curved portion alone.
Furthermore, in the press-formed product 1 illustrated in
In such a case, the angle difference between the angle at the end portion of the twisted side wall portion in the longitudinal direction and the angle at the center of the portion corresponding to concave curve in the longitudinal direction in the preformed part may be made larger than the angle difference between the angle at the center of the concave curved portion in the longitudinal direction (concave curve center) and the angle at the end portion of the side wall portion in the longitudinal direction (longitudinal end portion) in the press-formed product of the target shape, so that the twisted side wall portion of the preformed part may have a curved surface shape more twisted along the longitudinal direction than that of the side wall portion of the target shape.
For example, the angle between the portion corresponding to web and the twisted side wall portion at the concave curve center of the preformed part may be the angle between the web portion and the side wall portion at the concave curve center of the target shape, and the angle between the portion corresponding to web and the twisted side wall portion at the longitudinal end portion of the preformed part may be smaller than the angle between the web portion and the side wall portion at the longitudinal end of the target shape.
<<Regarding Pressing Technique>>
Even when the twisted side wall portion 25a is formed at the first forming process to produce in-plane shear deformation as described above, the portion 23 corresponding to web may undergo compressive deformation and wrinkles may occur. In such a case, drawing forming is desirable for the first forming process, in which a blank is press-formed with its end portions being clamped with blank holders and a die.
On the other hand, for the second forming process, crash forming that performs forming by sandwiching a preformed part between a die and a punch may be employed, or crash forming with a pad may be employed if wrinkle occurrence is a concern for a web portion.
In the pad-applied crash forming at the first forming process (
Specific press forming experiments were conducted on the operation and effect of the press forming method according to the present invention, and are described below.
In the press forming experiments, as illustrated in
The dimensions of the press-formed product 1 were as illustrated in
The pressing technique used at the first forming process was crash forming with a pad (refer to
At the first forming process, as illustrated in
For the evaluation of fractures, “x” indicates that fractures are present, “Δ” indicates that fractures are not present but necking due to thickness reduction is present, and “∘” indicates that no fracture or necking is present at all. For the evaluation of wrinkles, “x” indicates that remarkable wrinkles are present, and “Δ” indicates that minute wrinkles are present, and “∘” indicates that no wrinkle is present at all.
In the present examples, conventional examples were set to include an example in which the press-formed product 1 is press-formed in one process of crash forming or drawing forming, and an example in which the press-formed product 1 is press-formed in two processes of the first forming process and the second forming process, and the portion 25 corresponding to side wall of the preformed part 21 press-formed at the first forming process does not have a curved surface shape twisted along the longitudinal direction.
Furthermore, comparative examples were set to include an example in which the press-formed product 1 is press-formed in two processes of the first forming process and the second forming process, and the twisted side wall portion 25a of the preformed part 21 press formed at the first forming process is twisted such that an angle between the twisted side wall portion 25a and the portion 23 corresponding to web is larger on an end portion side than at a longitudinal center of the portion 31 corresponding to concave curve, that is, the angle difference Δθ (=θ1−θ2) between the angle θ1 at the concave curve center and the angle θ2 at the longitudinal end portion is a negative value.
Then, the press-formed products according to the conventional examples and the comparative examples were evaluated for the presence of fractures and wrinkles in the same manner as the inventive examples. Table 1 presents the press forming conditions and the evaluation results of press formability.
In Table 1, the concave curve center angle θ1 is the angle between the portion 23 corresponding to web and the twisted side wall portion 25a at the center of the portion 31 corresponding to concave curve of the preformed part 21 in the longitudinal direction (
In Table 1, Conventional Examples 2 to 4, Comparative Examples 1 to 3, and Inventive Examples 1 to 10 are presented by being grouped by condition with the same angle difference Δθ. In Conventional Example 1, the press-formed product 1 was press-formed in one process of crash forming. The angle between the web portion 3 and the side wall portion 5 is 100°, which is the target shape.
In Conventional Example 1, occurrence of wrinkles was not observed in the web portion 3, but fractures occurred in the flange portion 7.
In Conventional Examples 2 to 4, the angle difference Δθ between the concave curve center angle θ1 and the longitudinal end portion angle θ2 is zero, and thus press-forming of the preformed part 21 by giving in-plane shear deformation to the portion 25 corresponding to side wall was not possible. Thus, in the press-formed product 1 obtained by press-forming the preformed part 21 into the target shape, a line length difference in the longitudinal direction was generated between the web portion 3 and the flange portion 7, and both wrinkles in the web portion 3 and fractures in the flange portion 7 could not be suppressed at the same time.
In each of Comparative Examples 1 to 3, the longitudinal end portion angle θ2 is larger than the concave curve center angle θ1. Thus, the angle difference Δθ between the concave curve center angle θ1 and the longitudinal end portion angle θ2 is not zero, and the twisted side wall portion 25a was formed under in-plane shear deformation at the first forming process. However, the direction of the in-plane shear deformation is opposite to the direction of the shear deformation in the twisted side wall portion according to the present invention (refer to
In each of Inventive Examples 1 to 13, the longitudinal end portion angle θ2 is smaller than the concave curve center angle θ1, and the concave curve center angle θ1, the longitudinal end portion angle θ2, and the longitudinal length L and the side wall height H of the twisted side wall portion 25a are changed.
Table 1 presents that a press-formed product was able to be press-formed while suppressing both fractures and wrinkles at the same time in all of Inventive Examples 1 to 4 (Δθ=20°), Inventive Examples 5 to 7 (Δθ=40°), Inventive Examples 8 and 9 (Δθ=60°), Inventive Example 10 (Δθ=80°), Inventive Example 11 (θ2=95°) in which the longitudinal end portion angle θ2 is smaller than the angle (=100°) between the web portion 3 and the side wall portion 5 of the target shape, and Inventive Example 12 (L=125 mm) and Inventive Example 13 (H=35 mm) in which the longitudinal length L and the side wall height H of the twisted side wall portion 25a are changed.
It is considered that these results were obtained because, in the preformed part 21 according to each of Inventive Examples 1 to 13, the twisted side wall portion 25a was formed under in-plane shear deformation as illustrated in
Furthermore, in each of Inventive Examples 5 to 7 (T=11.2°), Inventive Examples 8 and 9 (T=16.8°), and Inventive Example 11 (T=18.2°) in which the torsion amount T is within the suitable range of the present invention (10° or larger and 20° or smaller), no occurrence of fractures or wrinkles was observed at all in the press-formed product 1 and favorable results were obtained.
In summary, it has been demonstrated that the press forming method according to the present invention enables press forming of a press-formed product having a hat-shaped cross section, the press-formed product being concavely curved in the height direction along the longitudinal direction in side view, while suppressing both fractures and wrinkles.
According to the present invention, a press forming method can be provided that can press-form, into a favorable shape, a press-formed product having a hat-shaped cross section with a web portion, a side wall portion, and a flange portion, the press-formed product being concavely curved in the height direction along the longitudinal direction, while suppressing fractures and wrinkles.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-080727 | Apr 2019 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/051363 | 12/27/2019 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2020/217594 | 10/29/2020 | WO | A |
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| Number | Date | Country |
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| 107073547 | Aug 2017 | CN |
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| 112334244 | Feb 2021 | CN |
| 3 437 751 | Feb 2019 | EP |
| 3 819 037 | May 2021 | EP |
| H05-168429 | Jul 1993 | JP |
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| 2010-188393 | Sep 2010 | JP |
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| Number | Date | Country | |
|---|---|---|---|
| 20220176435 A1 | Jun 2022 | US |
