[Technical Field of the Invention]
[000 I]
The present invention relates to a hole widening method performed through
press forming particularly with respect to members and the like for automobiles, a
forming tool, and a formed product.
Priority is claimed on Japanese Patent Application No. 2015-173669, filed on
September 3, 2015, aud Japanese Patent Application No. 2016-012360, filed on
January 26,2016, the contents of which are incorporated herein by reference.
[Related Art]
[0002]
Recently, high strength steel sheets are increasingly applied for the purpose of
improving fuel efficiency and collision safety of automobiles. Complicated shapes
are sometimes required for members for automobiles, and excellent working
performance, that is, elongation and hole expansibility are important.
[0003]
In hole widening, a forming tool, which increases in diau1eter from the front
to the rear in a case of being seen in a progressing direction of pushing, is pushed into
a pilot hole in a workpiece in which the pilot hole is provided in advance through
punching or machining. Then, while a circumferential edge portion ofthe pilot hole
is caused to extend in a pushing direction of the forming tool, the pilot hole is radially
widened. Through this working method, a cylindrically protruding stretched flange is
formed with respect to the workpiece.
- I -
[0004]
The thickness of a fonned stretched flange becomes thinner while being close
to a front end portion of the stretched flange. The reason is that the front end portion
con·esponds to the circumferential edge portion of the workpiece, the degree of
working at the time of hole widening increases while being close to the front end
portion, and tbe distortion amount is significant. Therefore, for example, as shown in
FIG. I, in the case offonning a hole 112 and a flange 113 obtained by widening a pilot
hole !II before working through hole widening, a stretch flange crack liS is
sometimes caused in an edge portion 114 which is the front end portion of the stretched
flange.
[0005]
Generally, there is a trade,off relationship between elongation and hole
expansibility of a steel sheet. In a high strength steel sheet having favorable
elongation, hole expansibility generally tends to be degraded. Therefore, there has
been a proposal in which elongation and hole expansibility are balanced by controlling
the composition or the structure of a steel (for example, refer to Patent Document I).
[0006]
On the other hand, as a working technology for avoiding a stretch flange crack
at the time of hole widening, a working method performed through a laser intercept
method, a scraping method, or the like has been proposed (for example, refer to NonPatent
Documents l and 2 below). However, these methods require additional money
and work, and there is a problem in productivity.
[Prior Art Document]
[Patent Document]
[0007]
- 2 -
[Patent Document I] Japanese Unexamined Patent Application, First
Publication No. 2015-086415
[Non-Patent Document]
[0008]
[Non-Patent Document l] Hidenori SHIRASAWA et al: Iron and Steel, VoL
71, No_ 16 (1985), p.1949
[Non-Patent Document 2] Takeo NAKAGAWA et al: Journal of the Japan
Society for Technology of Plasticity, VoL 10, No. 104 (1969), p.665
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0009]
In hole widening, as described above, a crack is sometimes caused in a front
end portion of a stretched flange. Particularly, in a high strength steel sheet having
favorable elongation, hole widening tends to be difficult to be performed. In addition,
although hole widening performed through press forming has an advantage of a short
working time compared to those in the methods disclosed in Non-Patent Documents l
and 2, there are cases where a phenomenon called "spring-back", in which a distorted
material slightly returns to the original state, occurs when a forming tool is released.
[0010]
The present invention has been made in consideration of the foregoing
circumstances, and an object thereof is to provide a hole widening method which is
performed through press forming while causing no crack in a front end portion of a
stretched flange and being able to suppress spring-back after working, a fon11ing tool
which is preferably used in the hole widening method, and a formed product
[Means for Solving the Problem]
- 3 -
[0011]
The gist of the invention is as follows.
(I) According to a first aspect of the present invention, a hole widening
method includes a preparing process of preparing a forming tool which has a diameterincreasing
portion increasing in diameter from a front end side toward a rear end side
and a line-shaped projection formed to protrude outward from a surface of the
diameter-increasing portion, and a workpiece in which a pilot hole is formed; and a
hole widening process of successively widening the pilot hole by pushing the fom1ing
tool into the pilot hole such that the line-shaped projection of the forming tool comes
into point contact with a part of a circumferential edge p01iion of the pilot hole in the
workpiece two times or more, and forming a stretched flange.
[0012]
(2) ln the hole widening method according to ( 1 ), in the hole widening
process, the forming tool may be pushed into the pilot hole while the forming tool
rotates about a central axis thereof in a pushing direction.
[0013]
(3) According to a second aspect of the present invention, there is provided a
forming tool used in the hole widening method according to (l) or (2). The fonning
tool includes a diameter-increasing portion that increases in diameter from a front end
side toward a rear end side; and a line-shaped projection that is formed to protrude
outward from a surface of the diameter-increasing portion. The line-shaped
projection has a spiral shape in a case of being seen from the front end side. In a case
of being seen in a cross section including a central axis of the diameter-increasing
portion, two or more of the line-shaped projections are present on one circumferential
surface of the diameter-increasing portion.
- 4 -
(4) In the forming tool according to (3), the line-shaped projection may extend
over a surface of a body portion.
[0014]
(5) According to a third aspect of the present invention, there is provided a
fanning tool used in the hole widening method according to (2). The forming tool
includes a diameter-increasing portion that increases in diameter from a front end side
toward a rear end side; a line-shaped projection that is formed to protrude outward
from a surface of the diameter-increasing portion; and a rotation mechanism that is
configured to rotate the diameter-increasing portion around a central axis thereof.
(6) In the forming tool according to (5), the line-shaped projection may have a
linear shape in a case of being seen from the front end side.
(7) In the forming too! according to (5), the line-shaped projection may have a
spiral shape in a case of being seen from the front end side.
(8) In the forming tool according to any one of (5) to (7), the line-shaped
projection may extend over a surface of a body portion.
[0015]
(9) According to a fourth aspect of the present invention, a formed product
includes a stretched flange that is fanned through the hole widening method according
to (I) or (2).
[Effects of the Invention]
[00 16]
According to the aspects above, it is possible to prevent occurrence of a
stretch flange crack at the time of hole widening even in a high strength steel sheet
having favorable elongation, and it is possible to improve shape accuracy of a stretched
flange by suppressing spring-back. Therefore, it is possible to apply stretch flange
- 5 -
working or the like for fonning members for automobiles with respect to a wide range
of steel kinds. In addition, there is an advantage in that a fonning tool after hole
widening is easily released.
[0017]
Particularly, in the hole widening method according to (1), the pilot hole is
successively widened by pushing the forming tool into the pilot hole such that the lineshaped
projection of the fonning tool comes into point contact with a part of the
circumferential edge portion of the pilot hole in a workpiece two times or more.
Therefore, a force applied by the line-shaped projection is released before distortion
such as elongation, occurrence of necking, and breaking progresses, and the pilot hole
returns to the state before being distorted. Thus, a stretch flange crack can be
suppressed. Furthermore, in a case of focusing on a particular part of the
circumferential edge portion of the pilot hole in a workpiece, the particular part
undergoes a cycle of loading, off-loading, and reloading a plurality of times.
Accordingly, the particular part is in a working state similar to that in which a certain
degree of stress releasing is performed at the time of completion of fonning and
correcting is performed a plurality of times, in addition thereto. Accordingly, springback
of the circumferential edge portion can be suppressed.
Therefore, a stretch flange crack and spring-back can be suppressed.
[0018]
In the hole widening method according to (2), the forming tool is pushed into
the pilot hole while the forming tool rotates. Therefore, it is possible to adjust the
number of times the line-shaped projection is brought into point contact with a
particular part of the pilot hole, through a single press.
Therefore, a stretch flange crack and spring-back in a front end portion ofthe
- 6 -
stretched tlange can be more reliably suppressed.
[0019]
In the fonning tool according to (3), a stretch flange crack and spring-back
can be suppressed by pushing the forming tool into the pilot hole.
In the fom1ing tool according to ( 4 ), the line-shaped projection is also
provided on the surface of the body portion. Therefore, it is possible to enhance
release characteristics of the fanning tool in a case of performing burring.
(0020]
h1 the fanning tool according to (5), a stretch flange crack and spring-back
can be suppressed by pushing the fanning tool into the pilot hole while the rotation
mechanism rotates the fanning tool. In addition, since the rotation mechanism rotates
the fanning tool, it is possible to use a linearly line-shaped projection or a spirally lineshaped
projection of which the number of turns or the number ofthreads is not limited.
Therefore, the manufacturing cost of the forming tool can be reduced.
In the forming tool according to ( 6), the linearly line-shaped projection is used.
Therefore, the manufacturing cost ofthe forming tool can be reduced.
In the fonning tool according to (7), the spirally line-shaped projection of
which the number of turns or the number ofthreads is not limited is used. Therefore,
the manufacturing cost of the forming tool can be reduced.
In the hole widening method according to (8), the line-shaped projection is
also provided on the surface of the body portion. Therefore, it is possible to enhance
release characteristics ofthe forming tool in a case of perfonning burring.
[0021)
the fonned product according to (9), it is possible to obtain a component
having no stretch flange crack and having high dimensional accuracy.
- 7 -
[Brief Description of the Drawings]
[0022]
FIG. l is a perspective view showing a crack of the edge portion of the plate
material caused by a hole widening method in the related art.
FIG. 2A is a view showing the hole widening method in the related art and is a
part of a cross-sectional view showing a state before hole widening.
FIG. 2B is a view showing the hole widening method in the related art and is a
part of a cross-sectional view showing a state at the time of completion of hole
widening.
FIG. 3 relates to the hole widening method in the related art and is a graph in
which a relationship between an angular position of a forming tool and an index ()J1 is
shown in time series.
FIG. 4A is a plan view of the forming toolnsed in a hole widening method
according to an embodiment ofthe present invention.
FIG. 4B is a side view of the same forming tool.
FIG. 4C is a cross-sectional view ofthe same forming tool obtained along line
Al-A! FlG. 4A.
FIG. 5A is a part of a cross-sectional view showing a state before hole
widening in the hole widening method using the same forming tool.
FIG. 5B is a part of a cross-sectional view showing a state at the time of
completion of hole widening in the hole widening method using the same forming tool.
FIG 6A is a side view for showing a change in a relationship between the
same fanning tool and a line-shaped projection.
FIG. 6B is an arrow view along line A-A in FIG. 6A.
FIG. 6C is an arrow view along line B-B in FIG. 6A.
- 8 -
FIG. 6D is an arrow view along line C-C in FIG 6A.
FIG. 6E is an arrow view along lineD-Din FIG 6A.
FIG. 7 relates to the hole widening method according to the same embodiment
and is a graph in which a relationship between the angular position ofthe forming tool
and the index em is shown in time series.
FIG 8A is a plan view of a forming tool according to a first modified example.
FIG. 88 is a side view ofthe same forming tool.
FIG. 8C is a cross-sectional view of the same forming tool obtained along line
Bl-Bl in FIG 8A.
FIG. 9 relates to the hole widening method using the forming tool according to
the first modified example and is a graph in which a relationship between the angular
position of the forming tool and the index em is shown in time series.
FIG. I OA is a plan view of a fanning tool according to a second modified
example.
FIG. lOB is a side view of the same forming tool.
FIG. lOC is a cross-sectional view of the same forming tool obtained along
line Cl-Cl in FIG lOA.
FIG 11 relates to a hole widening method using the forming tool according to
the second modified example and is a graph in which a relationship between the
angular position of the forming tool and the index illl is shown time series.
FIG. 12A is a plan view of the forming tool according to a third modified
exan1ple.
FIG. 12B is a side view of the same fanning tool.
FIG. 12C is a cross-sectional view ofthe same forming tool obtained along
lineDI-Dl FlG.l2A.
- 9 -
FIG. l3 relates to the hole widening method using the forming tool according
to the third modified example and is a graph in which a relationship between the
angular position of the fom1ing tool and the index em is shown in time series.
FIG. l4A is a plan view of a fonning tool according to a fourth modified
example.
FIG. 14B is a side view ofthe same forming tool.
FIG. 14C is a cross-sectional view of the same forming tool obtained along
line E1-El in FIG. l4A.
FIG. 15 relates to a hole widening method using the forming tool according to
the fourth modified example and is a graph in which a relationship between the angular
position of the forming tool and the index em is shown in time series.
FIG. l6A is a perspective view of a forming tool according to a fifth modified
example.
FIG. 16B is a perspective view of a forming tool according to a sixth modified
example.
FIG. 16C is a perspective view of a forming tool according to a seventh
modified example.
FIG 17 A is a plan view of a forming tool according to an eighth modified
example.
FIG. 17B is a side view of the same fonning tool.
FIG 17C is a cross-sectional view of the same fom1ing tool obtained along
lineFl-Fl FIG17A
FIG. 18 relates to a hole widening method using the forming tool according to
the eighth modified example and is a graph in which a relationship between the angular
position of the forming tool and the index em is shown in time series.
- 10 -
FIG. 19A is a plan view of a forming tool according to a ninth modified
example.
FIG. l9B is a side view of the same forming tool.
FIG. 19C is a cross-sectional view of the same forming tool obtained along
line GI-G! in FIG. 19A.
FIG. 20A is a plan view of a forming tool according to a tenth modified
example.
FIG. 20B is a side view of the same forming tool.
FIG. 20C is a cross-sectional view ofthe same forming tool obtained along
line HI-HI in FIG. 20A.
FIG. 21A is a plan view of a forming tool according to an eleventh modified
example.
FIG. 21B is a side view of the same forming tool.
FIG. 21C is a cross-sectional view of the same forming tool obtained along
line Il-Il in FIG. 21A.
FIG. 22A is a plan view of a forming tool according to a twelfth modified
example.
FIG. 22B is a side view ofthe same forming tool.
FIG. 22C is a cross-sectional view of the same forming tool obtained along
line J 1-Jl in FIG. 22A.
FIG. 23A is a cross-sectional view showing a state before hole widening in the
hole widening method using the same forming tool.
FIG. 23B is a cross-sectional view showing a state at the time of completion
of hole widening in the hole widening method using the same forming tool.
FIG. 24 is a graph having a horizontal axis for the number of threads of the
- ll -
line-shaped projection and a vertical axis for an index G.
FIG 25 is a graph having a horizontal axis for the pitch of the line-shaped
projection and a vertical axis for the index G.
[Embodiment of the Invention]
[0023]
The inventors have intensively examined methods for preventing a stretch
flange crack at the time of hole widening and reducing spring-back, particularly hole
widening methods performed through press forming of a high strength steel sheet As
a result, it has been acknowledged that it is effective to successively perform hole
widening by partially widening a pilot hole instead of concentrically widening the pilot
hole at the time of hole widening.
[0024]
Hereinafter, the present invention which has been made based on the
foregoing knowledge will be described in detail with reference to the drawings.
[0025]
In a hole widening method in the related art, as shown in FIGS. 2A and 2B, in
a state where a fanning tool 100 having a diameter-increasing portion l 01 increasing
in diameter from a front end side toward a rear end side is brought into contact with the
whole circumference of a circumferential edge portion of a circular pilot hole Ill
formed in a steel sheet 110 (workpiece), the pilot hole 111 is pushed using the forming
tool! 00. Accordingly, the pilot hole Ill is concentrically widened, and a hole 112 is
formed.
As the forming tool 100 is inserted into the pilot hole, the pilot hole 1 l I in the
steel sheet 1 10 and the circumferential edge portion thereofare pushed ont toward the
front end side ofthe fanning tool 100 such that a protruding portion is formed. Here,
- 12 -
the front end side of the forming tool I 00 denotes a side which first comes close to the
pilot hole when the fanning tool 100 is inserted into the pilot hole 111.
[0026]
FIG. 3 shows a graph having the horizontal axis for an angnlar position and
the vertical axis for an index em regarding working time points Tl to T4 in the hole
widening method in the related art shown in FIGS. 2A and 2B.
The working time point Tl is a time point immediately after hole widening
starts. The working time point T2 is a time point after the elapse of a time t1 from the
working time point T I. The working time point T3 is a time point after the elapse of
a time t2 from the working time point T2. The working time point T 4 is a time point
after the elapse of a time t3 from the working time point T3. The times tl to t3 are
not necessarily unifonn.
The angular position is an angular position based on a center point (central
axis) in a plan view of the forming tool.
The index em is a size of a load vector acone per unit area pressing a steel
sheet by the forming tool.
[0027]
As shown in FIG. 3, in the hole widening method in the related art, the index
an at each working time point indicates a uniform value at every angular position.
Since the work hardening amount of a steel sheet increases as the working time point
progresses from Tl to T4, the value of the index an increases gradually.
[0028]
As a shape of the diameter-increasing portion 101, the shape only needs to
increase in diameter from the front end side toward the rear end side. Therefore, a
conical shape, a truncated conical shape, a crumon ball shape, or the like is preferably
- 13 -
used. The diameter-increasing portion 101 is not limited to these shapes.
In this specification, the diameter-increasing portion denotes a part in which
the diameter or the equivalent circle diameter of the contour of a cross section
perpendicular to the central axis of the forming tool increases from the front end side
toward the rear end side.
[0029]
In the view showing the hole widening method, only the forming tool and the
steel sheet are shown, and a die, a blank holder, and the like are omitted. General
devices may be used as these omitted devices.
[0030]
In contrast, the hole widening method according to an embodiment of the
present invention includes a preparing process of preparing a forming tool and a steel
sheet, and a hole widening process offorming a stretched flange in the steel sheet. In
the hole widening process, the pilot hole is successively widened by pushing the
forming tool into the pilot hole such that a line-shaped projection of the forming tool
comes into point contact with a part of the circumferential edge portion of the pilot
hole formed in the steel sheet, two or more.
In this specification, "coming into point contact with a part of the
circumferential edge portion" excludes a case of "coming into contact with the whole
circumference of the circumferential edge portion at the same time", and contact with a
limited area is allowed.
Hereinafter, a more detailed description will be given using specific examples.
[0031]
In the hole widening method according to the present embodiment, a forming
tool I 0 shown in FIGS. 4A to 4C can be used. FIG. 4A is a plan view, FIG. 4B is a
- 14 -
side view, and FIG 4C is a cross-sectional view obtained along line Al-Al in FIG. 4A.
As shown in FIGS. 4A to 4C, this fonning tool l 0 includes a diameterincreasing
portion II which has a truncated conical shape, a spirally line-shaped
projection 12 which protrudes outward from a surface of the diameter-increasing
portion 11, a body portion 13 which has a columnar shape and is formed on the rear
end side of the diameter-increasing portion 11, an apex portion 14 which is formed on
the front end side ofthe diameter-increasing portion II, a bottom portion 15 which is
fonned on the rear end side of the body portion 13, and a gripping portion 16 of the
bottom portion 15.
[0032]
According to this forming tool 10, the line-shaped projection 12 is spirally
provided in a case-of-being seen from the front end side. In addition, in regard to the
line-shaped projection 12, in a case of being seen in a cross section including the
central axis of the diameter-increasing portion 11, two or more line-shaped projections
are present on one circumferential surface of the diameter-increasing portion.
Therefore, since a horizontal cross section of the diameter-increasing portion
II does not have a circular shape, in a case where a circular pilot hole S I is pushed
using this forming tool l 0, the whole circumference of the circumferential edge portion
ofthe pilot hole S 1 does not come into contact with the forming tool 10, but a part of
the circumferential edge portion comes into point contact with the fonning toollO.
That is, the line-shaped projection 12 comes into point contact with a part of the
circumferential edge portion of the pilot hole Sl. Then, when the fanning toollO is
pushed, the line-shaped projection can come into point contact with a part of the
circumferential edge portion ofthe pilot hole S l in a workpiece S two times or more.
[0033]
- 15 -
More specifically, as shown in FIGS. SA and 58, in a state where the forming
tool I 0 is brought into contact with a circumferential edge portion of the circular pilot
hole S 1 formed in the steel sheet S (workpiece), the pilot hole S l is widened by
pushing the forming tool10 into the pilot hole Sl, and a formed product is then
obtained.
[0034]
FIGS. 6A to 6E schematically show a change in a relationship between the
forming tool 10 and the line-shaped projection 12. FIG. 6A is a side view ofthe
forming toollO. FIGS. 6B to 6E are an arrow view along line A-A of the forming
tool I 0 shmvn in FIG. 6A, an arrow view along line B-8, an arrow view along line C-C,
an arrow view along line D-D, and an arrow view along line E-E. In cross-sectional
views shown in FIGS, 6B to 6E, oblique line regions indicate cross sections ofthe
forming tool 10, and outer shape curve lines thereof become parts coming into contact
with the steel sheetS shown in FIGS. 5A and 58.
In the hole widening method using the forming tool 100 in the related art as
shown in FIGS. 2A and 28, the pilot hole 111 is widened while maintaining the
circular shape. However, in the hole widening method according to the present
embodiment, since the line-shaped projection 12 in each cross section comes into
contact with the steel sheet S priority, the hole shape in the middle of forming is a
non-circular shape.
At the time of hole widening, the spirally line-shaped projection 12 comes
into point contact with a part ofthe steel sheetS. Theretore, the part of the steel sheet
S is pushed by the forming tool l 0, and the pilot hole Ill is partially widened. As the
fanning tool l 0 progresses, the state successively shifts from that in FIG. 6B to that
F!G. 6E. The contact position between the fonning tool 10 and the steel sheetS
- 16 -
changes, and the pilot hole Ill is successively widened. As a result, the stretched
flange can be fonned without causing a stretch flange crack at the time of hole
widening.
[0035]
FIG. 68 is an initial stage of hole widening. The left side in the view ofthe
circumferential edge portion ofthe pilot hole S I is in contact with the spirally lineshaped
projection 12 provided in the fanning tool I 0. However, in the pilot hole S 1,
a part adjacent to the part coming into contact with the line-shaped projection 12 does
not come into contact with the forming toollO. Therefore, a pushing/widening force
of the fanning tool 10 is intensively applied to the left side in the view of the pilot hole.
Thereafter, the forming tool 10 moves relatively with respect to the steel sheet S. In
the state of FIG 6C, since the right side in the view of the pilot hole comes into contact
with the spirally line-shaped projection 12 provided in the fanning tool 10, a
pushing/widening force of the forming toollO is intensively applied to the right side in
the view. Between the states of FIGS. 6B and 6C, the contact position between the
circumferential edge portion ofthe pilot hole lll and the fanning tool 10 changes
continuously in accordance with movement of the fanning toollO. Accordingly, the
location in the circumferential edge portion of the pilot hole Ill intensively receiving a
pushing/widening force of the forming tool 10 also changes continuously. Thereafter,
hole widening progresses similarly in FIGS. 60 and 6E as well.
[0036]
FIG. 7 shows a graph having the horizontal axis for the angular position and
the vertical axis for the index em regarding the working time points Tl to T4 in the
hole widening method according to the present embodiment
As shown in FIG. 7, at the working time point Tl, a peak ofthe index em is
- 17 -
generated at the 90-degree position, and as working progresses to the working time
points T2 to T4, the peak ofthe index em moves to positions of 180 degrees, 270
degrees, and 360 degrees. The peak gradually increases as working progresses from
the working time point Tl to the working time point T 4 due to an influence of work
hardening of a workpiece plate.
[0037]
The reason that no stretch flange crack is caused at the time of hole widening
in the hole widening method according to the present embodiment is assumed as
follows. That is, according to the hole widening method in the related art, as shown
in FIG. 3, during working, since tensile stress is continuously applied to the whole
circumference ofthe circumferential edge portion of the pilot hole 111 in the steel
sheet 110 at all times, the circumferential edge portion is uniformly elongated. When
tensile stress is continuously applied furthermore, necking is caused in a part of the
circumferential edge portion, and a stretch flange crack is finally caused.
Meanwhile, according to the working method of the present invention, as
shown in FIG. 7, at a certain time during working, the location to which a force is
applied in the circumferential edge portion ofthe pilot hole S l in the steel sheet S is a
part of the circumferential edge portion, and the location to which a force is applied
changes in accordance with a change in time. That is, the location to which tensile
stress is applied becomes a part of the circumferential edge portion. Furthermore, in
the location, tensile stress is released before breaking due to necking is caused, and
tensile stress is applied to a different location. Therefore, even if a force is applied,
the force is released before distortion such as elongation, occurrence of necking, and
breaking progresses, and the pilot hole returns to the state before being distorted.
Thus, a stretch flange crack can be suppressed.
- 18 -
[0038]
Furthermore, in the hole widening method according to the present
embodiment, a force is applied to only a part of the circumferential edge portion of the
pilot hole S 1 in the steel sheet S during working and the part moves as forming
progresses. Therefore, in a case of focusing on a particular part of the circumferential
edge portion to be worked, the particular part undergoes a cycle of loading, off-loading,
and reloading a plurality oftimes. Accordingly, the particular part is in a working
state similar to that in which a certain degree of stress releasing is performed at the
time of completion offonning and correcting is performed a plurality oftimes, in
addition thereto. Accordingly, spring-back of the circumferential edge portion can be
suppressed. Thus, shape accuracy of the stretched flange is improved.
In addition, in a case where the forming tool 10 is in contact with only a part
of the circumferential edge portion ofthe pilot hole S I when working ends, the
forming toollO is easily released.
[0039]
In the hole widening method according to the present embodiment, without
being limited to the forming tools l 0 having the shapes described above, it is possible
to use forming tools according to various modified examples. Hereinafter, for
simplification of description, the same reference signs are used for the configurations
which have already been described in the forming tool l 0.
[0040]
In a forming tool 1 OA according to a first modified example, as shown
FIGS. 8A to 8C, two line-shaped projections 12a and l2b are spirally tormed on a
surface of the diameter-increasing portion II in the same directions as each other.
FIG. 8A is a plan view, FIG. 8B is a side view, and FIG. 8C is a cross-sectional view
- 19 -
obtained along line Bl-Bl in FIG 8A
[0041]
FIG. 9 shows a graph having the horizontal axis for the angular position and
the vertical axis for the index em regarding the working time points Tl to T4 in the
hole widening method in a case of using the fanning tool I OA according to the first
modified example. As shown in this graph, in a case of using the forming tool I OA
according to the first modified example, the number of peaks of the index 011 can be
two within the same cross section. Therefore, it is possible to further enhance the
effect of preventing a stretch flange crack at the time of hole widening and the effect of
reducing spring-back.
[0042]
In a forming tooll OB according to a second modified example, as shown in
FIGS. lOA to lOC, two line-shaped projections 12c and 12d are spirally formed on a
surface of the diameter-increasing portion 11 in directions opposite to each other. FIG
lOA is a plan view, FIG I OBis a side view, and FIG. lOC is a cross-sectional view
obtained along line Cl-Cl in FIG. lOA
[0043]
FIG. II shows a graph having the horizontal axis for the angular position and
the vertical axis for the index an regarding the working time points Tl to T4 in the
hole widening method in a case of using the forming tool I OC according to the second
modified example. As shown in this graph, even in a case of using the forming tool
1 OC according to the second modified example, similar to the forming tool I OB
according to the first modified example, the number of peaks of the index 011 within
the same cross section can be increased. Therefore, it is possible to further enhance
the effect of preventing a stretch flange crack at the time of hole widening and the
- 20 -
effect of reducing spring-back.
[0044]
In a fonning tool I OC according to a third modified example, as shown in
FIGS. 12A to 12C, three line-shaped projections 12e, 12f, and 12g are spirally fonned
on a surface of the diameter-increasing portion II in the same directions as each other.
FIG. 12A is a plan view, FIG. 12B is a side view, and FIG. 12C is a cross-sectional view
obtained along line Dl-Dl in FIG. 12A.
[0045]
FIG. 13 shows a graph having the horizontal axis for the angular position and
the vertical axis for the index an regarding the working time points Tl to T4 in the
hole widening method in a case of using the forming tool l OC according to the third
modified example. A;; shown in this grap~ in a case of using the fonning tool 1 OC
according to the third modified example, the number of peaks of the index Gl1 can be
three within the same cross section. Therefore, it is possible to further enhance the
effect of preventing a stretch flange crack at the time of hole widening and the effect of
reducing spring-back.
[0046]
In a fonning tool l OD according to a fourth modified example, as shown in
FIGS. 14A to 14C, four line-shaped projections 12h, 12i, 12j, and 12k are spirally
fonned on a surface ofthe diameter-increasing portion ll in directions by two opposite
to each other. FIG. 14A is a plan view, FIG. 148 is a side view, and FIG. !4C is a
cross-sectional view obtained along line El-El in FIG. 14A.
[0047]
FIG. 15 shows a graph having the horizontal axis for the angular position and
the vertical axis for the index on regarding the working time points T1 to T4 in the
- 21 -
hole widening method in a case of using the forming tool 1 OD according to the fourth
modified example. As shown in this graph, in a case of using the forming tool I OD
according to the fourth modified example, the number of peaks of the index on can be
four within the same cross section. Therefore, it is possible to further enhance the
effect of preventing a stretch flange crack at the time of hole widening and the effect of
reducing spring-back.
[0048]
All of the forming tools I 0 and I OA to 1 OD has a configuration in which a
single or a plurality of the spirally line-shaped projections 12 are provided in the
conical diameter-increasing portion 11. However, the essence of the present
invention is that the pilot hole is successively pushed and widened due to a change in
part of the circumferential edge portion of the pilot hole S 1 in the steel sheet S with
which the forming tool comes into contact, in accordance with relative movement of
the forming tool with respect to the steel sheet S. That is, as long as the fom1ing tool
can realize this configuration, the forming tool is not particularly limited to a forming
tool having a spirally line-shaped projection.
In a plan view ofthe forming tool seen from the front end side, if the forming
tool has a shape such that the line-shaped projection is present in an arbitrary direction
seen from the center, a part ofthe circumferential edge portion ofthe pilot hole S 1 in
the steel sheetS with which the fom1ing tool comes into contact changes in accordance
with movement of the forming tool, so that the pilot hole Sl can be successively
pushed and widened. The shape of the stretched flange to be formed can change
depending on the shape of the line-shaped projection provided in the forming tool.
Therefore, the shape ofthe line-shaped projection may be suitably adjusted in
accordance with the shape of the desired stretched flange. Therefore, it is possible to
-22-
use forming tools lOE to lOG according to modified examples as shown in FIGS. 16A
to 16C
ln the modified examples shown in FIGS. l6A to l6C, a diameter-increasing
portion 11' having a truncated square pyramid shape is used as the diameter-increasing
portion 11, a quadrangular prism-shaped body portion 13' provided in the rear end of
the diameter-increasing portion 11' is used as the body portion 13, and a square apex
portion 14' formed on the front end side ofthe diameter-increasing portion II' is used
as the apex portion 14.
[0049]
In the forming tool 1 OE according to a fifth modified example, as shown in
FIG. 161\ a plurality of disconnected line-shaped projections 121 are formed on
surfaces of the diameter~increasing portion 11' and the body portion l3' such that the
line-shaped projections 121 are inclined with respect to the axial direction of the
forming tool l OE.
In the forming tool I OF according to a sixth modified example, as shown in
FIG. 16B, a plurality ofline-shaped projections 12m are formed parallel to one another
on surfaces of the diameter-increasing portion ll' and the body portion !3' such that
that line-shaped projections 12m are inclined with respect to the axial direction of the
forming tool lOF. In this modified example, since the line-shaped projection 12m
formed.on the comer portion is inclined with respect to the axial direction of the
fom1ing.tooi!OF, the effect of the present invention can be achieved.
the forming tool lOG according to a seventh modified example, as shown
in FIG. 16C, a single line-shaped projection !2n is spirally provided on surfaces ofthe
diameter-increasing portion ! 1' and the body portion 13'.
[0050]
- 23 -
In the forming tools JOE, !OF, and lOG according to the fifth to seventh
modified examples shown in FIGS. 16A to 16C as well, similar to the forming tool l 0,
the pilot hole Sl is successively pushed and widened due to a change in a part of the
circumferential edge portion ofthe pilot hole S 1 in the steel sheet S with which the
line-shaped projections 121, 12m, and 12n come into contact, in accordance with
relative movement ofthe forming tools l OE to I OG with respect to a metal material.
Accordingly, the location to which tensile stress is applied becomes a part of the
circumferential edge portion. Furthermore, in the location, tensile stress is released
before necking is caused, and tensile stress is applied to a different location.
Therefore, even if a force is applied, the force is released before distortion such as
elongation, occurrence of necking, and breaking progresses, and the pilot hole returns
to the state bef<:lre being distorted. Thus, a stretch flange crack at the time of hole
widening can be suppressed.
[0051]
In the hole widening method according to the present embodiment, the pilot
hole may be widened by pushing the forming tool into the pilot hole while the forming
tool is rotating about the central axis in a pushing direction. In such a case, it is
preferable in that the number oftimes the line-shaped projection 12 abuts the pilot hole
can be adjusted throngh a single press. That is, as shown in FIGS. SA and 58, in a
case where the fanning tool is pushed without rotating, the number of times of contact
of the line-shaped projection in a predetermined angular position of the pilot hole is
approximately four. However, in a case where the forming tool 10 is pushed while
the forming tool I 0 rotates, the number oftimes of contact thereof can be increased or
reduced in accordance with the rotation frequency.
[0052]
- 24 -
In this manner, in a case where the pilot hole S l in the steel sheet S is
widened while the forming tool I 0 rotates, the position of the circumferential edge
portion ofthe pilot hole S I in the steel sheet S to be in contact with the forming tool I 0
successively changes due to the rotation. Therefore, there is no need to spirally
provide the line-shaped projection 12 or to provide a plurality of line-shaped
projections 12 at equal intervals in the circumferential direction of the forming toollO.
Therefore, for example, it is possible to use a forming tool l OH according to
an eighth modified example as shown in FIGS. 17 A to 17C.
In this forming tool 1 OH, six line-shaped projections 12o are linearly formed
in the diameter-increasing portion 11, and a rotation mechanism R for rotating the
forming toollOH is provided in the gripping portion 16. This rotation mechanism R
rotates the formingtoollOH in accordance with relative movement of the fonning tool
I OH with respect to the steel sheetS. The rotation mechanism R only needs to be
able to rotate the line-shaped projection 12o and is not limited to the form of being
provided in the gripping portion 16.
[0053]
FIG. 18 shows a graph having the horizontal axis for the angular position and
the vertical axis for the index em regarding the working time points T1 to T4 in a
working method of widening the pilot hole while the forming tool lOH according to
the eighth modified example rotates. As shown in this graph, in the working method
of widening the pilot hole while the fom1ing tool lOH according to the eighth modified
example rotates, the linearly line-shaped projection 12o is provided the diameterincreasing
portion 11 such that the forming tool I OH comes into contact with a part of
the pilot hole Sl in the steel sheetS. Thereafter, the location of the circumferential
edge portion of the pilot hole S I to be in contact with the forming tool moves in
- 25 -
accordance with hole widening by rotating the forming tool I OH in accordance with
relative movement of the forming tool I OH with respect to the steel sheetS.
That is, the location to which tensile stress is applied becomes a part of the
circumferential edge portion. Furthermore, in the location, tensile stress is released
before necking is caused, and tensile stress is applied to a different location.
Therefore, even if a force is applied, the force is released before distortion such as
elongation, occurrence of necking, and breaking progresses, and the pilot hole returns
to the state before being distorted. Thus, a stretch flange crack at the time of hole
widening can be suppressed.
[0054]
In a case or rotating the forming toollOH, the moving speed of the peak of
the index em within the same cross section can be adjusted by controlling the rotation
frequency. Therefore, it is possible to employ an appropriate rotation speed in
accordance with material characteristics of the steel sheet S by nsing a single forming
tool 1 OH, so that it is possible to reliably enhance an effect of preventing a stretch
flange crack at the time of hole widening and an effect of reducing spring-back.
Furthermore, since punch and stroke of the forming tool l OH can be shortened, there is
an advantage in that a large-sized press machine no longer needs to be used.
[0055]
The forming tool 10 used in the hole widening method according to tl1e
present embodiment has the body portion 13. However, the body portion 13 is not
essential, and the gripping portion 16 may be directly provided on the bottom surface
of the diameter-increasing portion II.
However, in a case of having the body portion 13, it is preferable in that
particularly the front end section of the stretched flange during working is pushed and
- 26 -
widened and burring of uniformly straightening the inner diameter of the stretched
flange can be performed.
[0056]
In a case where the forming tool I 0 has the body portion 13, the line-shaped
projection 12 may be continuously provided to the body portion 13 lead from the
diameter-increasing portion 11. That is, it is possible to use a forrning tool I OJ
according to a ninth modified example shown in FIGS. 19A to 19C, a forming tool 1 OJ
according to an eleventh modified example shown in FIGS. 20A to 20C, and a forming
toollOK according to a twelfth modified example shown in FIGS. 21A to 21C.
[0057]
In the fonning tool 1 OI according to the ninth modified example, as shown in
FIGS. 19A to 19C,the line-shaped projection 12 is continuously formed in a spiral
state even on a surface of the body portion 13.
FIG l9A is a plan view, FIG 198 is a side view, and FIG l9C is a crosssectional
view obtained along line Gl-Gl in FIG 19A
[0058]
In the forrning tool l OJ according to the tenth modified example, as shown in
FIGS. 20A to 20C, the line-shaped projection 12 is continuously formed in a linear
state parallel to the axial direction of the forming tool 1 OJ on a surface of the body
portion 13.
FIG 20A is a plan view, FIG 208 is a side view, and FIG 20C is a crosssectional
view obtained along line Hl-Hl in FIG 20A
[0059]
In the forming tool IOK according to the eleventh modified example, as
shown in FIGS. 21A to 21C, the line-shaped projection l2linearly fanned in the
- 27 -
diameter-increasing portion 11 is formed to extend to the body portion 13.
FIG. 21 A is a plan view, FIG. 21 B is a side view, and FIG. 21 Cis a crosssectional
view obtained along line Il-l! in FIG. 21 A.
[0060]
As shown in the ninth modified example to the eleventh modified example, in
a case where the line-shaped projection 12 is formed to the body portion, the contact
area between the pilot hole S I after working ends and the forming tool I OI, I 0.1, or
l OK is reduced. Therefore, in addition to an effect of facilitating release due to
reduction of spring-back, it is possible to achieve an effect of further facilitating
release.
[0061]
The hole.widening.method according to the present embodiment has been
described with reference to a case where hole widening is performed by pushing the
gripping portion 16 using the forming tool 10 in which the gripping portion 16 is
provided on the rear end side, that is, the bottom portion 15. However, as described
in a twelfth modified example shown in FIGS. 22A to 22C, hole widening may be
performed by drawing the gripping portion 16' toward the pilot hole using a fonning
tool 1 OL in which a gripping portion I 6' is provided in the apex portion 14.
[0062]
The time required for hole widening performed through press forming is
approximately one second. Although it is a short time from a viewpoint of
productivity, the time is not so short in consideration from a viewpoint of a distortion
speed of a material. That is, it is assumed that the working such as one second is
a time sufficient for changes such as applying tensile stress to the steel sheet S during
working, releasing the force before necking is caused, and returning to the state before
- 28 -
being distorted.
[0063]
In addition, if the number of times the line-shaped projection 12 comes into
contact with the same location in the pilot hole S I is two times or more, loading and
releasing tensile stress in the location can be repeated a plurality of times. Therefore,
it is possible to achieve an effect of preventing a stretch flange crack at the time of hole
widening and an effect of reducing spring-back.
However, in a case where the number oftimes the line-shaped projection 12
comes into contact with the same location in the pilot hole S 1 exceeds I 0 times, the
interval of repeating loading and releasing tensile stress becomes short, and it is
difficult to achieve the effect described above. Therefore, it is preferable that the
number of times the line-shaped projection 12 comes into contact with the same
location in the pilot hole S I is l 0 times or less.
[0064]
Hereinabove, specific examples of the present invention have been described
based on the embodiment and the moditied examples of the present invention.
However, the present invention is not limited to these examples. The present
invention includes various modifications and changes of the specific examples
described above.
[0065]
The workpiece plate is not limited to a steel sheet. It is possible to use a
metal plate such as an aluminum plate and a titanium plate, a glass-fiber reinforced
resin plate such as FRP and FRTP, and a composite plate thereof
In addition, a hollow tube member such as a steel tube may be adopted as a
workpiece plate.
- 29 -
[0066]
As a cross-sectional shape of the line-shaped projection 12, shapes other than
a semicircle can be employed. However, since the line-shaped projection 12 is an
element for forming a stretched flange through hole widening, it is preferable that the
location which comes into contact with the circumferential edge portion of the pilot
hole does not have an acute angle portion.
As the cross-sectional shape ofthe line-shaped projection 12, it is preferable
that at least a location which comes into contact with the circumferential edge portion
ofthe pilot hole has an arc shape of which the radius of curvature is 0.1 mm or greater.
The protrusion height of the line-shaped projection 12 does not vary due to
the relationship with respect to the dimensions ofthe pilot hole. However, the
protrusion height may be fanned to be gradually reduced from the front end side
toward the rear end side.
[0067]
The inclination of the diameter-increasing portion 11 does not have to be
uniform from the front end section to the rear end section, and the inclination may vary
in the middle. The fonning tool may have a shape in which the diameter gently
varies between the diameter-increasing portion ll and the body portion 13.
[0068]
The apex portion 14 fonned on the front end side of the diameter-increasing
portion II is not necessarily a flat surface. The apex portion 14 may be a curved
surface.
[0069]
The shape of the pilot hole Sl is not limited to a circle or a square. The
shape thereof may be an elliptical shape or a different polygonal shape.
- 30 -
In addition, a projected shape of the forming tool I 0 in a plan view is not
limited to a circle or a square. The projected shape thereof may also be an elliptical
shape or a different polygonal shape.
[0070]
(Example A)
An experiment was performed in order to check for an effect of preventing a
stretch flange crack at the time of hole widening and an effect of reducing spring-back
according to the present invention. As the steel sheet S (workpiece), a high strength
hot rolled steel sheet of 780 MPa having the sheet thickness of2.4 mm was prepared.
[0071]
Pilot holes of various sizes and shapes were provided in the steel sheetS in
advance through pllllching, Hole widening was performed by pushing various
forming tools against the pilot holes at the speed of l 0 mm/sec.
[0072]
As the evaluation for a stretch flange crack at the time of hole widening, the
sizes of the pilot holes were reduced in the unit of l mm with respect to each of
Examples of the invention having the line-shaped projection and Comparative
Examples having no line-shaped projection, and evaluation was conducted based on
the smallest size of the pilot hole in which no stretch flange crack was caused.
[0073]
In regard to spring-back, since it was unfair if the sizes ofthe pilot holes
not match each other between Examples of the present invention and Comparative
Examples, and since spring-back could not be evaluated in a case where a stretch
flange crack was caused, hole widening was performed with respect to each of
Examples of the invention and Comparative Examples for the size of the pilot hole at
- 31 -
which a stretch flange crack was caused in Comparative Example, and the ratio of the
cross-sectional area of the forming tool and the hole area was evaluated as a K-value
(K-value= hole area after release/ projected area offorming tool in plan view).
[0074]
Tables I to 3 show the shapes of the forming tools used in various
experimental examples, the dimensions of the fonning tools, the dimensions of the
pilot holes, the rotation speeds, the dimensions of the pilot holes in which a stretch
flange crack was caused, the K-values, and the evaluation results of release
characteristics.
[0075]
[Table l]
Dimensions o
I· Shape Of Dimensions
Dimensions Rotation
pilot holes in Evaluation of
forming tool
of forming
of pilot hole speed
which stretch K-value release
tool flange crack characteristics
caused
Circle having
Example 1-1 of
FIG. 4B
diameter of 3 5
95% Good
invention lllill
Circle
Reduced in
Circle having
unit of
Example 1-2 of
FIG. 12B
having
I mmfrom
0 diameter of 31
99%
Very
invention diameter of
diameter of
times/sec mm Good
60mm
Line-shaped
60mm
Circle having
Comparative projection in Diameter of
88% Bad
Example 1 FIGS. 4B and 50mm
12B removed ·-
- 32 -
[0076]
[Table 2]
Dimensions o
Shape of
Dimensions
Dimensions o Rotation
pilot holes in Evaluation of
fanning tool
of forming
pilot hole speed
which stretch K-value release
tool flange crack characteristics
f-------- caused
Square having
Example 2-l
FIG. l6A one side of 94% Good
of invention
Square
Reduced in -- 22 rnm unit of
Example 2-2
having one
1 nm1 from
Square having
FIG. l6B side of one side of 93% Good
of invention
30mm
·one side of30
r-- 23 rnrn (radius of
mm 0
Square having
Example 2-3 (radius of times/sec
of invention
FIG l6C curvature of
cLITVature of
one side of 95% Good
comer
comer portion
21 mm
Line-shaped portion is
lS Square having
Comparative projection -in 5mm)
5mm) one side of 85% Bad
Example 2 FIGS. 16A to
28mm
16C removed
[0077]
[Table 3]
Dimensions o
Shape of
Dimensions
Dimensions Rotation
pilot holes in Evaluation of
of forming which stretch K-value release
forming tool
tool
of pilot hole speed
flange crack characteristics
caused
Example 3-1
Circle havlng
of invention
FIG. 17B diameter of 30 97% Good
---m-m-
Example 3-2
Circle having
Very
of invention
FIG. 2\B
Reduced in
diameter of 30 98%
Good
Circle
unit of
mm
~---
Line-shaped having
1 mm from 8 times/sec Circle having
Comparative projection in diameter of
diameter of diameter of 48 88% Bad
Example 3-1 FIG 17B 60mm
removed
55mrn mm
Linecshaped Circle having
Comparative projection in
diameter of 48 88% Bad
Example 3-2 FIG 2\B
removed
mm
[0078]
In Example 1-1 ofthe invention, a fom1ing tool having one line-shaped
projection shown in FIG 4B was used. In Example I ~2 of the invention, a fonning
tool having three line-shaped projections shown in FIG 12B was used.
Comparative Example 1, a forming tool, that is, the forming tool shown in
- 33 -
FIG 4B or 12B, from which the line-shaped projection was removed, was used.
As shown in Tahle 1, in a case of Comparative Example 1 having no lineshaped
projection, a stretch flange crack was caused in a case where the dimensions of
the pilot hole were 50 mm. Meanwhile, in Example 1-1 of the invention and
Example 1-2 ofthe invention having a line-shaped projection, a stretch flange crack
was caused in a case where the dimensions of the pilot holes were 35 mm and 31 mm
respectively. That is, it could be checked that an excellent effect of suppressing a
crack could be achieved by providing a line-shaped projection.
Furthermore, in Example l-1 of the invention and Example 1-2 ofthe
invention, high K-values could be obtained compared to Comparative Example 1.
That is, it could be checked that an excellent effect of suppressing spring-back could be
achieved by providing a line-shaped projection.
Furthermore, in cases of Example 1-1 of the invention and Example 1-2 of the
invention, since spring-back was reduced, when the forming tool was pulled out, there
was no occurrence of a situation in which a hole edge portion of the steel sheetS was
stuck on the forming tool and was unlikely to be separated. That is, improvement in
release characteristics was also recognized.
[0079]
In Example 2-1 of the invention, the forming tool shown in FIG. 16A was
used. In Example 2-2 of the invention, the fom1ing tool shown in FIG 16B was used.
In Example 2-3 of the invention, the fanning tool shown in FIG 16C.
In Comparative Example 2, a forming tool, that is, the forming tool shown in
FIG. 16A, 16B, or !6C, from which the line-shaped projection was removed, was used.
As shown in Table 2, even in a case of using a forming tool which had a
diameter-increasing portion having a truncated square pyramid shape, it could be
- 34 -
checked that an excellent effect of suppressing a crack and an effect of reducing
spring-back could be exhibited by having a line-shaped projection.
Furthermore, in cases of Example 2-1 ofthe invention, Example 2-2 ofthe
invention, and Example 2-3 of the invention, since spring-back was reduced, when the
fanning tool was pulled out, there was no occurrence of a situation in which a hole
edge portion of the steel sheetS was stuck on the fonning tool and was unlikely to be
separated. That is, improvement in release characteristics was also recognized.
[0080]
In Example 3-1 of the invention, the forming tool shown in FIG 17B was
used. In Example 3-2 of the invention, the fonning tool shown in FIG 21B was used.
Hole widening was performed while the forming tool was rotating, by transmitting a
drive force of a motor embedded in the fanning tool to the gripping portion of the
fanning tool by means of a gear transmission mechanism.
In Comparative Example 3-1 and Comparative Example 3-2, a fonning tool,
that is, the fonning tool shown in FIG. 17B or FIG. 2IB, from which the line-shaped
projection was removed, was used. Hole widening was perfonned while the fonning
tool was rotating, by transmitting a drive force of a motor embedded in the fanning
tool to the gripping portion of the forming tool by means of a gear transmission
mechanism.
As shown in Table 3, even in a case of using a forming tool which had a
linearly line-shaped projection, it could be checked that an excellent effect of
suppressing a crack and an effect of reducing spring-back could be exhibited by
performing hole widening while the fanning tool was rotating.
Furthennore, in cases of Example 3-l ofthe invention and Example 3-2 of the
invention, since spring-back was reduced, when the fanning tool was pulled out, there
- 35 -
was no occurrence of a situation in which a hole edge portion of the steel sheet S was
stuck on the forming tool and was unlikely to be separated. That is, improvement in
release characteristics was also recognized. Particularly, in Example 3-2 of the
invention, since a line-shaped projection was provided in the body portion as well,
more excellent release characteristics could be achieved.
[0081]
(Example B)
An experiment was performed in order to check for an influence of the
number of threads and the pitch of the line-shaped projection of the forming tool on an
effect of preventing a stretch flange cracks at the time of hole widening and an effect
of reducing spring-back.
[0082]
Hole widening was performed based on the forming tool ofthe examples of
the present invention shown in FIGS. 4A to 4C, while the spiral angle was fixed to 45
degrees and the number ofthreads of the line-shaped projection was varied.
[0083]
Here, a numerical value index o at which successive forming can be
appropriately performed with the line-shaped projection is defined as follows. When
the index em has an absolute maximum value mnax and an absolute minimum value
amin in a case where distribution ofthe index an is observed at a certain time, the
index em is defined as follows.
o=j am ax -aminj/ mnax
As the factor o described above, it is possible to employ a value within a
range of 0.0<5<1.0. When o=O.O, an1ax=0min is established. Therefore, since there
is no occurrence of difference between the ridge and the valley of the index 0n, there is
- 36 -
no occurrence of partial contact between the forming tool and the steel sheet S, so that
successive forming is not executed. When o=l.O, cnnin=O.O MPa is established,
thereby indicating that partial contact is conducted in a location where the index
cm=amax is established. From the above, as o is closer to l.O, the partial contact
occurs and successive forming is appropriately performed. In addition, as o is closer
to 0.0, it indicates that continuous contact occurs in a wide range and working falls
short of successive forming.
[0084]
FIG 24 shows a change in the index o when hole widening is performed using
the forming tool of which the number ofthreads ranges zero to 12. In a case of
performing burring forming by means ofthe forming tool of which the number of
threads was zero, that is, the forming tool according to Comparative Example, the
equal index cn1 was effectuated throughout the entire region of the hole edge.
Therefore, o=O. 0 was established.
In the successive burring forming tool, when the line-shaped projection was
provided even by one thread, a high value of 8>0.70 or higher was employed.
However, at this shape level of the forming tool, there were cases where contact
occurred even on the base surface (conical surface) other than the line-shaped
projection in the cases of one spiral thread and two spiral threads, and the value
remained lower than o=l.O.
When the number of spiral threads was greater than three, multiple point
contact was ideally realized and successive forming was performed, thereby being
close to o=l.O. When the number of threads was increased, o decreased. It denotes
that when the contact point increases, the cnnin which is a non-zero value comes close
to the value of Gmax and sufficient valleys are not formed in the distribution of the
- 37 -
index crn so that successive forming is not sufficiently exhibited.
[0085]
From the above, when the number of spiral threads, that is, the number of
contact points becomes excessively great, successive forming cannot be sufficiently
realized. In addition, when the number of spiral threads becomes excessively small,
divergence is recognized from the postulated condition of successive forming causing
contact other than the line-shaped projection. That is, the number of contact parts
which can perform successive forming is limited to a certain range.
[0086]
In addition, FIG 25 shows the evaluation result of the influence ofthe index o
on tbe spiral pitch. The pitch was varied while maintaining the number of spiral
threads as three threads. The shape oftbe forming tool having tbe spiral pitch=O.O
coincides with the shape of a conical punch having no line-shaped projection.
Therefore, o=O.O is established. In the range in which the spiral pitch is small, since
the line-shaped projection becomes dense and a valley having tbe suftlcient index