Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
[Title of Invention] CURVED PARTS AND METHOD FOR
MANUFACTURING THE SAME
[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]
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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
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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
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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.
According to another aspect of the present invention,
there is provided a curved-part forming method for obtaining a
curved part having a closed sectional shape and a curve in a
longitudinal direction, the method comprising:
preparing two or more planar blanks each formed of a
single metal plate, each of the two or more planar blanks
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having a curved outline corresponding to the curve of the
curved part;
bending each of the two or more planar blanks into a
sectional shape corresponding to a division portion of the
sectional shape of the curved part to form a division part;
and
joining two or more of the division parts obtained by the
bending together to form the curved part having the closed
sectional shape.
(2) The curved-part forming method as described above,
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 as described above.
(4) A curved-part manufacturing method for manufacturing
a curved part using the curved-part forming method as
described above.
[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
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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
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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
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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
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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
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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
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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
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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.
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[0016]
[Table 1]
MATERIAL PLATE YS (MPa) TS (MPa) El (
%)
SYMBOL THICKNESS
(mm)
A 1.6 710 990 17
B 1.6 810 1190 13
C 1.6 1300 1500 9
[0017]
[Table 2]
No. MATERIAL FORMING METHOD RESULT OF FORMING REMARKS
SYMBOL
1 A
CONVENTIONAL NO GOOD CRACKS/WRINKLES COMPARATIVE
PRESS FORMING PRODUCED EXAMPLE
2 A METHOD GOOD NO
EXAMPLE OF
ILLUSTRATED IN CRACKS/WRINKLES PRESENT
FIG. I PRODUCED INVENTION
3 A METHOD GOOD NO
EXAMPLE OF
ILLUSTRATED IN CRACKS/WRINKLES PRESENT
FIG. 4 PRODUCED INVENTION
4 A METHOD GOOD NO
EXAMPLE OF
ILLUSTRATED IN CRACKS/WRINKLES PRESENT
FIG. 7 PRODUCED INVENTION ,
B CONVENTIONAL NO
GOOD CRACKS/WRINKLES COMPARATIVE
PRESS FORMING PRODUCED EXAMPLE ¨
6 B METHOD GOOD NO
EXAMPLE OF
ILLUSTRATED IN CRACKS/WRINKLES PRESENT
FIG. 3 PRODUCED INVENTION
7 B METHOD GOOD NO
EXAMPLE OF
ILLUSTRATED IN CRACKS/WRINKLES PRESENT
FIG. 6 PRODUCED INVENTION
E3 B METHOD GOOD NO
EXAMPLE OF
ILLUSTRATED IN CRACKS/WRINKLES PRESENT
FIG. 8 PRODUCED INVENTION
9 C
CONVENTIONAL NO GOOD CRACKS/WRINKLES COMPARATIVE
PRESS FORMING PRODUCED EXAMPLE
C METHOD GOOD NO EXAMPLE OF
ILLUSTRATED IN CRACKS/WRINKLES PRESENT
FIG. 5 PRODUCED INVENTION
11 c METHOD GOOD NO
EXAMPLE OF
ILLUSTRATED IN CRACKS/WRINKLES PRESENT
FIG. 2 PRODUCED INVENTION
12 c METHOD GOOD NO
EXAMPLE OF
ILLUSTRATED IN CRACKS/WRINKLES PRESENT
FIG. 7 PRODUCED INVENTION
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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.
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[0019]
[Table 3]
No. MATERIAL V V FORMING RESULT OF FORMING DIMENSIONAL REMARKS
SYMBOL GROOVE GROOVE METHOD PRECISION
DEPTH
(%)
1 A LINEAR 7 METHOD GOOD NO GOOD
EXAMPLE OF
FORM ILLUSTRATED CRACKS/WRINKLES PRESENT
IN FIG. 1 PRODUCED INVENTION
2 A LINEAR 6 METHOD GOOD NO GOOD
EXAMPLE OF
FORM ILLUSTRATED CRACKS/WRINKLES PRESENT
IN FIG. 2 PRODUCED INVENTION
3 A BROKEN-12 METHOD GOOD NO GOOD
EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES PRESENT
FORM IN FIG. 3 PRODUCED INVENTION
4 A BROKEN-19 METHOD GOOD NO GOOD
EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES PRESENT
FORM IN FIG. 4 PRODUCED INVENTION
A DOTTED-10 METHOD GOOD NO GOOD EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES PRESENT
FORM IN FIG. 5 PRODUCED INVENTION
6 A DOTTED-16 METHOD 'GOOD NO GOOD EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES PRESENT
FORM IN FIG. 6 PRODUCED INVENTION
7 A LINEAR 12 METHOD GOOD NO GOOD EXAMPLE OF
FORM ILLUSTRATED CRACKS/WRINKLES PRESENT
IN FIG. 7 PRODUCED INVENTION
8 A LINEAR 5 METHOD GOOD NO GOOD EXAMPLE OF
FORM ILLUSTRATED CRACKS/WRINKLES PRESENT
INVENTION
IN FIG. 8 , PRODUCED
,
9 B LINEAR 10 METHOD GOOD NO GOOD EXAMPLE OF
FORM ILLUSTRATED CRACKS/WRINKLES PRESENT
IN FIG. 5 PRODUCED INVENTION
B LINEAR 8 METHOD GOOD NO GOOD EXAMPLE OF
FORM ILLUSTRATED CRACKS/WRINKLES PRESENT
IN FIG. 6 PRODUCED INVENTION
1
11 B 'DOTTED-4 METHOD GOOD NO GOOD EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES PRESENT
FORM IN FIG. 1 PRODUCED INVENTION
,
12 B DOTTED-15 METHOD GOOD NO GOOD EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES PRESENT
FORM IN FIG. 2 PRODUCED INVENTION
13 B BROKEN-6 METHOD GOOD NO GOOD EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES PRESENT
FORM IN FIG. 3 PRODUCED INVENTION
14 B BROKEN-13 METHOD GOOD NO GOOD EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES PRESENT
FORM IN FIG. 4 PRODUCED INVENTION
B DOTTED-16 METHOD GOOD NO GOOD EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES PRESENT
FORM IN FIG. 7 PRODUCED INVENTION
16 B DOTTED-6 METHOD GOOD NO GOOD EXAMPLE OF
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Table 3 continued
LINE ILLUSTRATED CRACKS/WRINKLES
PRESENT
FORM IN FIG. 8 PRODUCED
INVENTION
17 C BROKEN-8 METHOD GOODNO GOOD
EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES
PRESENT
FORM IN FIG. 3 PRODUCED
INVENTION
18 C BROKEN-12 METHOD GOOD NO GOOD
EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES
PRESENT
FORM IN FIG. 4 PRODUCED
INVENTION
19 C DOTTED-4 METHOD GOOD NO GOOD
EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES
PRESENT
FORM IN FIG. 5 PRODUCED
INVENTION
20 C DOTTED-9 METHOD GOOD NO GOOD
EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES
PRESENT
FORM IN FIG. 6 PRODUCED
INVENTION
21 C LINEAR 3 METHOD GOOD NO GOOD
EXAMPLE OF
FORM ILLUSTRATED CRACKS/WRINKLES
PRESENT
IN FIG. 1 PRODUCED
INVENTION
22 C LINEAR 5 METHOD GOOD NO GOOD
EXAMPLE OF
FORM ILLUSTRATED CRACKS/WRINKLES
PRESENT
IN FIG. 2 PRODUCED
INVENTION
23 C BROKEN-5 METHOD GOOD NO GOOD
EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES
PRESENT
FORM IN FIG. 7 PRODUCED
INVENTION
24 C BROKEN-10 METHOD GOOD NO GOOD
EXAMPLE OF
LINE ILLUSTRATED CRACKS/WRINKLES
PRESENT
FORM IN FIG. 8 PRODUCED
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)
