[Technical Field]
[0001] The present invention relates to a method of forming plates into curved parts (more
specifically, curved frame parts). More particularly, the present invention relates
to a forming method that makes it possible to form high-strength steel sheets having
a tensile strength (TS) that is greater than or equal to 590 MPa into curved parts,
curved parts, and a method for manufacturing the same.
[Background Art]
[0002] Curved parts have hitherto been obtained by press forming single metal plates. In
the press forming, various forming modes including drawing, stretch forming, stretch
flanging, and bending are combined. (The press forming will hereunder be referred
to as "conventional press forming.") Further, a method of bending forming a cylindrical
material (PTL 1), a roll forming technology (PTL 2), and bending forming using a hollow
part (PTL 3 and PTL 4) are proposed. As an example of reinforcing curved parts, a
method of filling with resin foam (PTL 5) is proposed.
[Citation List]
[Patent Literature]
[0003]
PTL 1: Japanese Unexamined Patent Application Publication No. 9-30345
PTL 2: Japanese Unexamined Patent Application Publication No. 11-129045
PTL 3: Japanese Unexamined Patent Application Publication No. 8-174047
PTL 4: Japanese Unexamined Patent Application Publication No. 2005-1490
PTL 5: Japanese Unexamined Patent Application Publication No. 11-348813
[Summary of Invention]
[Technical Problem]
[0004] Increasing the strength of a steel sheet in accordance with the demand for reducing
weight causes at the same time a reduction in drawing ability, stretch forming ability,
and stretch flanging ability on the steel sheet. Therefore, in conventional pressing
forming, defects, such as cracks or wrinkles, occur. In particular, as the shape becomes
complex, there are cases where curved parts cannot obtained. For example, if portions
50A and 50B (which are curved in an X direction and a Y direction in plan view, and
in a Z direction) of a curved part 50 shown in Fig. 11 is formed by performing conventional
press forming on a single high-strength steel sheet having a tensile strength (TS)
that is greater than or equal to 590 MPa, wrinkles occur in a planar section (such
as a wrinkle section in Fig. 11), and cracks occur in a vertical wall at a side surface
or in flanges (such as a crack section in Fig. 11). Here, it is possible to suppress
the occurrence of cracks/wrinkles up to a certain extent by changing the shapes of
parts or optimizing forming conditions of, for example, a blank holder. However, in
such a method, in order to satisfy the need of reducing weight, there is a limit with
regard to achieving a higher tensile strength (TS) that is greater than 980 MPa.
[0005] A method of obtaining high-strength curved parts by performing bending forming or
roll forming on cylindrical materials is disclosed (PTL 1 to PTL 4). From the viewpoints
of formability of the materials and process constraints, it is difficult of obtain
complex curved shapes, and there are serious productivity problems such as an increase
in the number of processes. For example, when low-strength materials are used, complex
shapes can be easily obtained, but parts have insufficient strength. Therefore, there
are, for example, technologies for obtaining reinforcing effects by filling with resin
foam (PTL 5). However, from the viewpoints of costs, production, and recycling, it
is actually not easy to say that such technologies are necessarily useful technologies.
[0006] That is, in conventional forming methods, when single high-strength steel sheets
are used as materials, forming into desired curved parts cannot be performed by one-piece
press forming, or, when single low-strength steel sheets are used as materials, forming
into curved parts can be performed, but the parts have insufficient strength, thereby
making it necessary to, for example, increase the number of reinforcing pats, as a
result of which weight is increased.
[Solution to Problem]
[0007] The present invention for solving the aforementioned problems provides the following:
- (1) A curved-part forming method for obtaining a curved part by performing forming
on a blank formed of a single metal plate. The method includes a bending process in
which the blank having a curved outline corresponding to a curve of the curved part
in a longitudinal direction is bent into a sectional shape corresponding to a division
portion of a sectional shape of the curved part, and a joining process in which two
or more portions obtained by the bending process are joined together.
- (2) The curved-part forming method according to (1), wherein, prior to the bending
process, a folding line is formed in the blank, or a cut is further formed in the
blank.
- (3) The curved part manufactured using the curved-part forming method according to
(1) or (2).
- (4) A curved-part manufacturing method for manufacturing a curved part using the curved-part
forming method according to (1) or (2).
[Advantageous Effects of Invention]
[0008] According to the present invention, since the material is bent and deformed almost
without being variously deformed by drawing, stretch forming, and stretch flanging,
it is possible to perform one-piece pressing forming of a single high-strength steel
sheet into portions of the curved part. In addition, as a result of the shape of the
curved part, which is a target to be formed, being reflected in the outline of the
blank, it is possible to expect easy obtainment of parts having high strength and
having a complex curved shape that could not be hitherto obtained, enlargement of
space due to a reduction in the cross section of the parts, and a large reduction
in weight because, for example, plate thickness is reduced and reinforcing parts are
not used.
[Brief Description of Drawings]
[0009]
[Fig. 1] Fig. 1 is a schematic view of an embodiment of the present invention.
[Fig. 2] Fig. 2 is a schematic view of an embodiment of the present invention (differing
from the already mentioned embodiment).
[Fig. 3] Fig. 3 is a schematic view of an embodiment of the present invention (differing
from the already mentioned embodiments).
[Fig. 4] Fig. 4 is a schematic view of an embodiment of the present invention (differing
from the already mentioned embodiments).
[Fig. 5] Fig. 5 is a schematic view of an embodiment of the present invention (differing
from the already mentioned embodiments).
[Fig. 6] Fig. 6 is a schematic view of an embodiment of the present invention (differing
from the already mentioned embodiments).
[Fig. 7] Fig. 7 is a schematic view of an embodiment of the present invention (differing
from the already mentioned embodiments).
[Fig. 8] Fig. 8 is a schematic view of an embodiment of the present invention (differing
from the already mentioned embodiments).
[Fig. 9] Fig. 9 is a sectional view of various exemplary sectional shapes of curved
parts.
[Fig. 10] Fig. 10 is a schematic view of examples of how folding lines are formed.
[Fig. 11] Fig. 11 is a schematic view of an exemplary curved part formed by conventional
press forming.
[Description of Embodiments]
[0010] Figs. 1 to 8 are schematic views of different embodiments of the present invention.
Figs. 1 and 2 each show an exemplary case in which a curve of a curved part 30 in
a longitudinal direction is along folding lines in only one of two opposite directions.
Further, in Fig. 1, the sectional size is constant in the longitudinal direction of
the part, and, in Fig. 2, the sectional size changes in the longitudinal direction
of the part. Figs. 3 and 4 each show an exemplary case in which a curve of a curved
part 30 in the longitudinal direction along folding lines changes from either one
of two opposite directions to the other one of the two opposite directions. Further,
in Fig. 3, the sectional size is constant in the longitudinal direction of the part,
and, in Fig. 4, the sectional size changes in the longitudinal direction of the part.
Figs. 5, 6, 7, and 8 each show an exemplary case in which a curve of a curved part
30 in the longitudinal direction is such that the curved part 30 is continuously curved
in only one of two opposite directions (Figs. 7 and 8 each show an exemplary case
in which the curved part has a warped sectional shape in the longitudinal direction).
Further, in Fig. 5, the sectional size is constant in the longitudinal direction of
the part, and, in Figs. 6, 7, and 8, the sectional size changes in the longitudinal
direction of the part.
[0011] In these embodiments, two blanks 1 and 2 have the same planar shape, and the planar
shape thereof has a side-bend outline corresponding to the curve of the curved part
30, which is a target to be formed, in the longitudinal direction of the curved part
30. It goes without saying that the blanks 1 and 2 may be previously provided with
working holes or beads, etc. In a bending process, the blanks 1 and 2 are each bent
into a sectional shape corresponding to a division portion of a sectional shape of
the curved part 30, so that portions 10 and 20 constituting the curved part 30 are
formed. Reference numerals 1F and 2F denote portions corresponding to flanges of the
blanks 1 and 2, or denote the flanges of the portions 10 and 20. In Figs. 1 to 8,
broken lines and dotted lines that are formed in regions of the shapes of the blanks
1 and 2 represent mountain folding and valley folding, respectively, and indicate
places corresponding to bend portions (protrusion edges and recess edges) formed by
bending in the bending process. In the bending process according to the present invention,
using a die, the blanks are press bended so that forming portions of the blanks become
bend portions that are in correspondence with target parts. By the press bending,
forming materials primarily undergo deformation of bending forming, and are formed
into target shapes.
[0012] Next, in a joining process, the portions 10 and 20 are joined together, to obtain
the curved part 30. Joining methods may be any one of, for example, welding, caulking,
riveting, and adhesion using an adhesive.
Although the embodiments shown in Figs. 1 to 6 are those in which the blanks are formed
into a part sectional shape shown in Fig. 9(a), the present invention is not limited
thereto. It is obvious that the present invention is applicable to cases in which,
for example, as shown in Fig. 9(b), the blanks are formed into a part sectional shape
that is the reverse of that in Fig. 9(a) at the left and right sides; or, as shown
in Fig. 9(c), the blanks are formed into a part sectional shape so that the flanges
2F of only the structural portion 20 are bent. The embodiments shown in Figs. 7 and
8 are those in which the blanks are formed into a part sectional shape shown in Fig.
9(d).
[0013] Although, the embodiments shown in Figs. 1 to 6 and Fig. 8 use two blanks having
the same planar shape for one curved part, the present invention is not limited thereto.
It is obvious that the present invention is applicable to a case in which three or
more blanks are used for one curved part, with at least one of the blanks having a
planar shape that differs from the planar shapes of the remaining blanks.
Further, in the present invention, in order to increase position precision of the
bend portions during the bending, it is desirable to previously provide folding lines
in portions of the blanks where the mountain folding and the valley folding are performed.
The present invention is not only limited to (continuously) forming the folding lines
along an entire bending processing portion. The folding lines may be (intermittently)
formed in only portions of the bending processing portion according to the circumstances.
As a method of forming the folding lines, it is desirable to use, for example, coining.
Another example thereof is a method of continuously transferring the unevenness of
a roller surface to surfaces of the materials. Suitable forms of folding lines may
be provided by forming V grooves, such as that shown in Fig. 10(d), in a linear form
(10(a)), a broken-line form (10(b)), or a dotted-line form (10(c)), or in a combination
of any of these forms. Here, it is desirable that the depth of the V grooves be less
than or equal to 20% of the thickness of a metal plate (abbreviated as "plate thickness").
If the depth of the V grooves exceeds 20% of the plate thickness, the strength of
the parts required for, for example, the frame of an automobile may be reduced, or
cracks may be formed in the bend portions; and, in a high-strength metal material,
it is not easy to form the grooves deeply, thereby causing serious production and
cost problems.
[0014] The shape of the grooves is not limited to a V shape (the grooves are not limited
to the V groove shown in Fig. 10(d)), so that the grooves may have various recessed
shapes such as U shapes. When the curvature radius of the bend portions is large,
a plurality of long and narrow grooves may be formed parallel to each other.
When there are localized portions where wrinkles or cracks are very likely to be formed
due to localized excessive stretching or compression during bending (for example,
when there are a plurality of localized portions at portions of the blanks corresponding
to the flanges that are likely to be subjected to excessive stretch flanging or shrink
flanging), previously forming cuts in such localized portions makes it possible to
more reliably prevent the formation of cracks and wrinkles, which is desirable.
[First Examples]
[0015] Blanks formed of thin steel sheets (material symbols A, B, and C) having plate thicknesses
and tensile properties (yield strength YS, tensile strength TS, elongation El) shown
in Table 1 were formed into curved parts by forming methods based on Table 2, and
the shapes of the obtained curved parts were visually observed, to evaluate the forming
methods. The results are as shown in Table 2. In conventional press forming according
to a comparative example, wrinkles are formed in the wrinkle section and cracks are
formed in the crack section shown in Fig. 11, whereas in the examples of the present
invention, curved parts substantially having target shapes and without having cracks
or wrinkles were obtained.
[0016]
[Table 1]
MATERIAL SYMBOL |
PLATE THICKNESS (mm) |
YS (MPa) |
TS (MPa) |
E1 (%) |
A |
1.6 |
710 |
990 |
17 |
B |
1.6 |
810 |
1190 |
13 |
C |
1.6 |
1300 |
1500 |
9 |
[0017]
[Table 2]
No. |
MATERIAL SYMBOL |
FORMING METHOD |
RESULT OF FORMING |
REMARKS |
1 |
A |
CONVENTIONAL PRESS FORMING |
NO GOOD |
CRACKS/WRINKLES PRODUCED |
COMPARATIVE EXAMPLE |
2 |
A |
METHOD ILLUSTRATED IN FIG. 1 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
EXAMPLE OF PRESENT INVENTION |
3 |
A |
METHOD ILLUSTRATED IN FIG. 4 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
EXAMPLE OF PRESENT INVENTION |
4 |
A |
METHOD ILLUSTRATED IN FIG. 7 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
EXAMPLE OF PRESENT INVENTION |
5 |
B |
CONVENTIONAL PRESS FORMING |
NO GOOD |
CRACKS/WRINKLES PRODUCED |
COMPARATIVE EXAMPLE |
6 |
B |
METHOD ILLUSTRATED IN FIG. 3 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
EXAMPLE OF PRESENT INVENTION |
7 |
B |
METHOD ILLUSTRATED IN FIG. 6 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
EXAMPLE OF PRESENT INVENTION |
8 |
B |
METHOD ILLUSTRATED IN FIG. 8 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
EXAMPLE OF PRESENT INVENTION |
9 |
C |
CONVENTIONAL PRESS FORMING |
NO GOOD |
CRACKS/WRINKLES PRODUCED |
COMPARATIVE EXAMPLE |
10 |
C |
METHOD ILLUSTRATED IN FIG. 5 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
EXAMPLE OF PRESENT INVENTION |
11 |
C |
METHOD ILLUSTRATED IN FIG. 2 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
EXAMPLE OF PRESENT INVENTION |
12 |
C |
METHOD ILLUSTRATED IN FIG. 7 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
EXAMPLE OF PRESENT INVENTION |
EXAMPLES 2
[0018] Folding lines provided by V grooves (whose depths are shown in Table 3) in a linear
form, a broken-line form, or a dotted-line form, such as those shown in Fig. 10, were
previously formed in blanks formed of thin steel sheets (material symbols A, B, and
C) having plate thicknesses and tensile properties (yield strength YS, tensile strength
TS, extension El) shown in Table 1. Then, the blanks were formed into curved parts
using forming methods based on Table 3, and the shapes of the obtained curved parts
were visually observed, to evaluate the forming methods. The results are as shown
in Table 3. In the examples of the present invention, cracks or wrinkles were not
produced, and curved parts whose shapes more closely matched the target shapes compared
to the curved parts in the first examples of the present invention (that is, curved
parts whose dimensional precisions were good) were obtained.
[0019]
[Table 3]
No. |
MATERIAL SYMBOL |
V GROOVE |
V GROOVE DEPTH (%) |
FORMING METHOD |
RESULT OF FORMING |
DIMENSIONAL PRECISION |
REMARKS |
1 |
A |
LINEAR FORM |
7 |
METHOD ILLUSTRATED IN FIG. 1 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
2 |
A |
LINEAR FORM |
6 |
METHOD ILLUSTRATED IN FIG. 2 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
3 |
A |
BROKEN-LINE FORM |
12 |
METHOD ILLUSTRATED IN FIG. 3 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
4 |
A |
BROKEN- LINE FORM |
19 |
METHOD ILLUSTRATED IN FIG. 4 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
5 |
A |
DOTTED- LINE FORM |
10 |
METHOD ILLUSTRATED IN FIG. 5 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
6 |
A |
DOTTED- LINE FORM |
16 |
METHOD ILLUSTRATED IN FIG. 6 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
7 |
A |
LINEAR FORM |
12 |
METHOD ILLUSTRATED IN FIG. 7 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
8 |
A |
LINEAR FORM |
5 |
METHOD ILLUSTRATED IN FIG. 8 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
9 |
B |
LINEAR FORM |
10 |
METHOD ILLUSTRATED IN FIG. 5 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
10 |
B |
LINEAR FORM |
8 |
METHOD ILLUSTRATED IN FIG. 6 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
11 |
B |
DOTTED- LINE FORM |
4 |
METHOD ILLUSTRATED IN FIG. 1 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
12 |
B |
DOTTED-LINE FORM |
15 |
METHOD ILLUSTRATED IN FIG. 2 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
13 |
B |
BROKEN- LINE FORM |
6 |
METHOD ILLUSTRATED IN FIG. 3 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
14 |
B |
BROKEN-LINE FORM |
13 |
METHOD ILLUSTRATED IN FIG. 4 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
15 |
B |
DOTTED- LINE FORM |
16 |
METHOD ILLUSTRATED IN FIG. 7 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
16 |
B |
DOTTED-LINE FORM |
6 |
METHOD ILLUSTRATED IN FIG. 8 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
17 |
C |
BROKEN- LINE FORM |
8 |
METHOD ILLUSTRATED IN FIG. 3 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
18 |
C |
BROKEN- LINE FORM |
12 |
METHOD ILLUSTRATED IN FIG. 4 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
19 |
C |
DOTTED-LINE FORM |
4 |
METHOD ILLUSTRATED IN FIG. 5 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
20 |
C |
DOTTED- LINE FORM |
9 |
METHOD ILLUSTRATED IN FIG. 6 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
21 |
C |
LINEAR FORM |
3 |
METHOD ILLUSTRATED IN FIG. 1 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
22 |
C |
LINEAR FORM |
5 |
METHOD ILLUSTRATED IN FIG. 2 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
23 |
C |
BROKEN- LINE FORM |
5 |
METHOD ILLUSTRATED IN FIG. 7 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
24 |
C |
BROKEN- LINE FORM |
10 |
METHOD ILLUSTRATED IN FIG. 8 |
GOOD |
NO CRACKS/WRINKLES PRODUCED |
GOOD |
EXAMPLE OF PRESENT INVENTION |
Reference Signs List
[0020]
- 1, 2
- Blanks
- 1F, 2F
- Flanges, Portions corresponding to flanges
- 10, 20
- Portions constituting curved parts according to present invention
- 30
- Curved part according to present invention (target to be formed)
- 50
- Curved part formed by conventional press forming (50A and 50B denote portions constituting
curved part 50)