Specification
[Title of the Invention] METHOD FOR MANUFACTURING BENT MEMBER,
AND HOT-BENDING APPARATUS FOR STEEL MATERIAL
[Technical Field of the Invention]
[000 I]
The present invention relates to a method for manufacturing a bent member
and a hot-bending apparatus for a steel material.
Priority is claimed on Japanese Patent Application No. 2014-174469, filed
August 28,2014, the content of which is incorporated herein by reference.
[Related Art]
[0002]
Metallic strengthening members, reinforcing members, or structural members
(hereinafter refened to as bent members) having a bent shape are used for automobiles,
various machines, or the like. The bent members are required to be further
strengthened and to be lightweight and small-sized. As related-art methods for
manufacturing the bent members, for example, welding of press working products,
punching of thick plates, and forging are used. However, in the related-art
manufacturing methods, further high-strengthening, weight reduction, and size
reduction of the bent members may be difficult.
[0003]
In recent years, manufacturing a bent member using a tube hydroforming
method has been positively studied (for example, refer to Non-Patent Document 1).
According to the tube hydrofonning method, it is possible to reduce the plate thickness
of a bent member to be manufactured, improve in shape fixability, and improve in
economical efficiency related to manufacture of the bent member are allowed.
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However, there are problems such that materials that can be used for the tube
hydrofonning method are limited, and the degrees of freedom in shape arc insu!Iicicnt
in bending using the tube hydrofonning method.
[0004]
Methods for manufacturing a bent member and a hot-bending apparatus for a
steel material are disclosed in Patent Documents 1 to 3. A method for manufacturing
a bent member and a hot-bending apparatus for a steel material that performs hot
bending on a steel material in a state where the steel material is clamped by movable
roller dies are disclosed in Patent Docnment I. A method for manufacturing a bent
member and a hot -bending apparatus for a steel material that performs hot bending on
a steel material in a state where end patis of a steel material are gripped by chucks are
disclosed in Patent Document 2. A method for manufacturing a bent member and a
hot-bending apparatus for a steel material that performs hot bending on a steel material
in a state where two places of a steel material are gripped by manipulators are
disclosed in Patent Document 3.
[Prior Ali Document]
[Patent Document]
[0005]
[Patent Document l] Japanese Patent No. 4825019
[Patent Document 2] PCT International Publication No. W020 I 0/050460
[Patent Document 3] PCT International Publication No. W02011007810
[Non-Patent Document]
[0006]
[Non-Patent Document I] Automobile Teclmology Vol. 57, No. 6, 2003
Pages 23 to 28
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[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0007]
In the methods for manufacturing a bent member and the hot-bending
apparatus for a steel material disclosed in Patent Documents I to 3, the outside of a
bent portion of a steel material is not appropriately cooled. Therefore, uneven
quenching may occur. Additionally, in a case where bending with a small bending
radius is performed using the methods for manufacturing a bent member and the hotbending
apparatus for a steel material disclosed in Patent Documents I to 3, wrinkle or
cross-sectional distortion may occur.
Moreover, in the methods for manufacturing a bent member and the hotbending
apparatus for a steel material, fmiher improvements in productivity and
economical efficiency are required.
[0008]
The invention has been made in view of the above circumstances, and an
object thereof is to provide a method for manufacturing a bent member and a hotbending
apparatus for a steel material that can reduce occurrence of uneven quenching,
and wrinkle and cross-sectional distmiion and is excellent in productivity and
economical efficiency, even in a case where a bent member having a small bending
radius is manufactured.
[Means for Solving the Problems]
[0009]
The invention adopts the following means in order to solve the above
problems to achieve the relevant object.
[001 0]
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(I) A method for manufacturing a bent member related to an aspect of the
invention includes feeding an elongated steel material in a longitudinal direction with
one end portion of the steel material as a head, performing high-frequency induction
heating to one portion of the steel material in the longitudinal direction by being
supplied high-frequency power to form a high-temperature portion, bending the steel
material by applying a bending moment in an arbitrary direction to the hightemperature
portion to form a bent portion, and injecting a cooling medium to the bent
portion to cool the bent portion.
The bending includes forming the bent portion having a ratio R/W which is
equal to or lower than a predetermined value, where the ratio R/W is a ratio obtained
by dividing a bending radius R [mm) of the bent portion on a centroid line of the steel
material by a dimension W [mm] in a bend direction in a cross-section of the steel
material mihogonal to the centroid line, slowing down a feeding speed of the steel
material less than VI, where the V 1 is the feeding speed of the steel material while
forming the bent portion having the ratio R/W which is more than the predetermined
value, and reducing the high-frequency power supplied while forming the hightemperature
portion less than Q1, where the Q1 is the high-frequency power supplied
while forming the bent pmiion having the ratio R/W which is more than the
predetermined value.
[00 11]
(2) In the method for manufacturing a bent member described in the above (1),
the predetermined value of the ratio R/W may be a value selected from within a range
of3.0 to 8.0.
[0012]
(3) In the method for manufacturing a bent member described in the above (1)
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or (2), the feeding speed of the steel material while forming the bent portion of which
the ratio R/W is equal to or lower than the predetermined value may be lowered to
25% to 75% of the VI in the bending step.
[0013]
(4) In the method for manufacturing a bent member described in any one of
the above (1) to (3), the high-frequency power supplied while forming the bent portion
of which the ratio R/W is equal to or lower than the predetermined value may be
lowered to 25% to 75% of the Q 1 in the bending step.
[0014]
(5) A hot-bending apparatus for a steel material related to another aspect of
the invention ineludes a feeding mechanism that feeds an elongated steel material in a
longitudinal direction with one end pmiion of the steel material in the longitudinal
direction as a head; an induction heating mechanism that performs high-frequency
induction heating on one portion of the steel material in the longitudinal direction by
being supplied high-fiequency power and thereby forming a high-temperature portion;
a bending mechanism that applies a bending moment in an arbitrary direction to the
high-temperature portion and forms a bent p01iion; a cooling mechanism that injects a
cooling medium on the bent poriion and thereby cools the bent portion; and a
controller that controls the feeding mechanism, the induction heating mechanism, the
bending mechanism, and the cooling mechanism such that a feeding speed of the steel
material is slower than VI and the high-frequency power is lower than Ql while
forming the bent portion having a ratio R/W which is equal to or lower than a
predetermined value, where the V 1 is the feeding speed of the steel material while
forming the bent portion having the ratio R/W which is more than the predetermined
value, the Ql is the high-frequency power supplied to the induction heating
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mechanism, and the ratio R/W is a ratio obtained by dividing a bending radius R [nun]
of the bent portion on a centroid line of the steel material by a dimension W [mm] in a
bend direction in a cross-section of the steel material orthogonal to the centroid line.
[00 15]
(6) In the hot-bending apparatus for a steel material described in the above (5),
the predetermined value of the ratio R!W may be a value selected from within a range
of 3.0 to 8.0.
[0016]
(7) The hot-bending apparatus for a steel material described in the above (5)
or (6), the controller may control the feeding mechanism such that the feeding speed of
the steel material while forming the bent portion of which the ratio R!W is equal to or
lower than the predetermined value is lowered to 25% to 75% of the VI.
[0017]
(8) The hot-bending apparatus for a steel material described in any one of the
above (5) to (7), the controller may control the induction heating mechanism such that
the high-frequency power supplied while fonning the bent portion of which the ratio
R!W is equal to or lower than the predetermined value is· lowered to 25% to 75% of the
Ql.
[Effects of the Invention]
[0018]
According to the above respective aspects, it is possible to provide a method
for manufacturing a bent member and a hot-bending apparatus for a steel material that
can suppress occurrence of uneven quenching, and wrinkle and cross-sectional
distortion and is excellent in productivity and economical efficiency, even in a case
where a bent member having a small bending radius is manufactured.
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[Brief Description of the Drawings]
[00 19]
FIG. 1 is a plan view showing a bending apparatus related to the present
embodiment.
FIG. 2 is an explanatory view showing a heating method and a cooling method
for a steel material related to the present embodiment as seen along a feed direction of
the steel material.
FIG. 3 is a front view showing a cooling device related to the present
embodiment.
FIG. 4 is a graph showing a relationship between the feed position of a steel
pipe and the surface temperature of the steel pipe in a case where only heating and
cooling are performed to the steel pipe without performing bending, using an induction
heating device and the cooling device.
FIG. 5 is an explanatory view showing the shape of a bent member
manufactured in a bending test.
FIG. 6A is a plan view showing the state of cooling of the steel pipe by the
cooling device when bending is not performed to the steel pipe.
FIG. 6B is a plan view showing the state of cooling of the steel pipe by the
cooling device in a case where bending with a bending radius R is performed to the
steel pipe.
FIG. 6C is a plan view showing the state of cooling of the steel pipe by the
cooling device in a case where the bending with the bending radius R is performed to
the steel pipe.
FIG. 60 is a plan view showing the state of cooling of the steel pipe by the
cooling device in a case where the bending with the bending radius R is performed to
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the steel pipe.
fiG. 6E is a plan view showing the state of cooling of the steel pipe by the
cooling device in a case where the bending with the bending radius R is performed to
the steel pipe.
(a) of FIG. 7 A is a schematic view showing a centroid 0 and a width
dimension W in a case where a tip portion of a bent member with a circular sectional
shape is seen from an opposed sight line, and (b) of FIG. 7 A is a view of the bent
portion of the bent member with the circular sectional shape as looked down
perpendicularly to a bending plane.
(a) of FIG. 7B is a schematic view showing the centroid 0 and the width
dimension Win a case where the tip portion of the bent member with a rectanglar
sectional shape is seen from the opposed sight line, and (b) of FIG. 7B is a view of the
bent pmiion of the bent member with the rectanglar sectional shape as looked down
perpendicularly to the bending plane.
(a) of FIG. 7C is a schematic view showing the centroid 0 and the width
dimension Win a case where the tip portion of the bent member with an elliptical
sectional shape is seen from the opposed sight line, and (b) of FIG. 7C is a view ofthe
bent portion of the bent member with the elliptical sectional shape as looked down
perpendicularly to a bending plane.
(a) of FIG. 7D is a schematic view showing the centroid 0 and the width
dimension W in a case where the tip portion of the bent member with a
parallelogrammic sectional shape is seen from the opposed sight line, and (b) of FIG.
7D is a view of the bent portion of the bent member with the parallelogrammic
sectional shape as looked down perpendicularly to the bending plane.
(a) of FIG. 7E is a schematic view showing the centroid 0 and the width
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dimension Win a case where the tip portion of the bent member with a pentagonal
sectional shape is seen from the opposed sight line, and (b) of FIG. 7E is a view of the
bent portion of the bent member with the pentagonal sectional shape as looked down
perpendicularly to the bending plane.
(a) ofF! G. 7F is a schematic view showing the centroid 0 and the width
dimension Win a case where the tip portion of the bent member with a triangular
sectional shape is seen from an opposed sight line, and (b) of FIG. 7F is a view of the
bent pmiion of the bent member with the triangular sectional shape as looked down
perpendicularly to the bending plane.
FIG. 8 shows measurement results of the surface temperature of the outside of
the bent portion of the steel pipe in the bending shown in FIGS. 6B to 6E.
FIG. 9 shows measurement results of the surface temperature of the inside of
the bent pmiion of the steel pipe in the bending shown in FIGS. 6B to 6E.
FIG. I 0 is a graph showing a relationship between the surface temperature of a
certain point and the feed position of a steel pipe in a case where only quenching is
perfmmed to the steel pipe without performing bending.
FIG. I lA is a graph showing a pattern ofthe feeding speed of a steel pipe in
Comparative Example 2-1.
FIG. liB is a graph showing a pattern of high-frequency power supplied to the
induction heating device in Comparative Example 2-1.
FIG. 12A is a graph showing a pattern of the feeding speed of a steel pipe in
Comparative Example 2-2.
FIG. 12B is a graph showing a pattern of high-frequency power supplied to
the induction heating device in Comparative Example 2-2.
FIG. 13 is a schematic view showing the shape of the bent members
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manufactured in Example 2-1, Comparative Example 2-1, and Comparative Example
2-2.
FIG. 14A is a graph showing a pattern of the feeding speed of a steel pipe in
Example 2-l.
FIG. 14B is a graph showing a pattern of high-frequency power supplied to
the induction heating device in Example 2- I.
[Embodiments of the Invention]
[0020]
Hereinafter, a method for manufacturing a bent member and a hot-bending
apparatus for a steel material related to an embodiment of the present invention will be
described with reference to the drawings.
[0021]
(Hot-Bending Apparatus for Steel Material)
A hot-bending apparatus 0 for a steel material shown in FIG. 1 includes a
gripping device (gripping mechanism) 7, an induction heating device (induction
heating mechanism) 5, a cooling device (cooling mechanism) 6, a feeding device
(feeding mechanism) 3, a bending device (bending mechanism), and a control device
(not shown), and performs hot bending to a steel pipe (steel material) I.
In addition, in the hot-bending apparatus 0 for a steel material shown in FIG. 1,
a suppmt device 2 and a movable roller dies 4 constitute the bending device.
[0022]
Specifically, the steel pipe 1 is rapidly heated in a temperature zone where it is
possible to perform partial quenching, by an annular induction heating device 5 that
surrounds an outer periphery of the steel pipe 1 downstream of the support device 2.
Accordingly, a high-temperature portion (red heat portion) 1 a that moves in the
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longitudinal direction of the steel pipe I is formed in the steel pipe I.
Thereafter, the position of the movable roller dies 4 that has at least one set of
roll pair capable of supporting the steel pipe I while feeding the steel pipe is moved in
an arbitrary direction, and a bending moment is applied to the high-temperature portion
Ia.
Thereafter, a cooling medium, such as cooling water, is injected from the
cooling device 6 disposed downstream of the induction heating device 5 to the steel
pipe I to rapidly cool the heated steel pipe I. Accordingly, bending is performed to
the steel pipe I, and the bent member 8 is manufactured.
[0023]
When bending is performed to the steel pipe I, the steel pipe I can be
quenched by controlling the heating temperature and the cooling rate of the steel pipe I.
For this reason, according to the method for manufacturing a bent member 8 using the
hot-bending apparatus 0 for a steel material, it is possible to achieve high strength,
weight reduction, and size reduction of the bent member 8.
In addition, in the present embodiment, the method for manufacturing the bent
member 8 using the hot-bending apparatus 0 for a steel material is referred to as 3DQ
(abbreviation of "3 Dimensional Hot Bending and Quench").
[0024]
[Steel Pipe (Steel Material)]
The elongated steel pipe I that is a target for bending is not particularly
limited. As an example of the material of the steel pipe I, carbon steel that contains
0.15 mass% to 0.25 mass% of C is preferable, and pmiicularly carbon-steel that
contains 0.2 mass% ofC is preferable. An example of the plate thickness of the steel
pipe I is 0.8 nun to 4 mm.
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In addition, the sectional shape of the steel pipe I is not limited to the circular
shape, and may be other sectional shapes.
[0025)
FIGS. 7 A to 7F are schematic views showing a centroid 0 and a width
dimension Win a case where a tip portion of the bent member 8 seen from an opposed
sight line and views of the bent portion of the bent member 8 as looked down
perpendicularly to a bending plane, according to the sectional shapes of the bent
member 8. In addition, FIG. 7A shows a case where the sectional shape of the steel
pipe I is a circular shape, FIG. 7B shows a case where the sectional shape of the steel
pipe I is a rectanglar shape, FIG. 7C shows a case where the sectional shape of the
steel pipe I is an elliptical shape, FIG. 7D shows a case where the sectional shape of
the steel pipe I is a parallelogrammie shape, FIG. 7E shows a case where the sectional
shape of the steel pipe I is a pentagonal shape, and FIG. 7F shows a case where the
sectional shape of the steel pipe I is a triangular shape.
As shown in FIGS. 7 A to 7F, a dimension in a bend direction in a crosssection
of the steel pipe I orthogonal to a centroid line is referred to W in the present
embodiment. In addition, the dimension in the bend direction in the cross-section of
the steel pipe I orthogonal to the centroid line means the width dimension of the steel
pipe I when the bent portion is seen from a sight line along a centerline of curvature of
bending thereof. Additionally, the centerline of curvature of bending means the
centerline of a circular arc in a case where the bending is approximated as a pmiion of
the circular arc.
The above-described width dimension W is I 0 nun to 100 nun, for example.
[0026)
[Gripping Device (Gripping Mechanism)]
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The gripping device 7 grips at least one of one end portion (lip portion) and
the other end portion (rear end portion) of the steel pipe !. An example of the
gripping device 7 is a chuck.
[0027]
[Induction Heating Device (Induction Heating Mechanism)]
The induction heating device 5 has an annular outer shape, and is disposed so
as to surround the steel pipe I from a position apart at a predetermined distance from
an outer peripheral surface of the steel pipe !. The induction heating device 5 is
supplied high-frequency power from a high-frequency power generating device (not
shown), thereby rapidly heating one portion of the steel pipe I to a desired temperature
equal to or higher than an Ac3 point for a short time (about 2 seconds), and forming the
high-temperature portion (red heat pmtion) I a in the steel pipe I.
In addition, since the heating amount of the steel pipe I can be adjusted by
adjusting the high-frequency power supplied to the induction heating device 5, it is
possible to adjust the highest arrival temperature of the steel pipe I. In the present
embodiment, the high-frequency power supplied to the induction heating device 5 is
adjusted such that the highest ani val temperature of the steel pipe I is 900°C to
1050°C.
[0028]
[Cooling Device (Cooling Mechanism)]
As shown in FIGS. I and 2, the cooling device 6 is disposed at the
downstream side in the feed direction of the steel pipe I than the induction heating
device 5, and injects a cooling medium 62. The cooling medium 62 is preferably
liquid, and cooling water is an example of the cooling medium 62.
As shown in FIGS. 2 and 3, eight rows of injecting holes 61 are concentrically
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provided from the inside in the cooling device 6. As shown in FIG. 3, the rows of the
injecting holes 61 are respectively referred to as row A, row B, row C, row D, rowE,
row F, row G, and row H in order from the inside row thereof.
(0029]
The cooling device 6 injects the cooling medium 62 to an outer surface of the
steel pipe I, which is heated by the induction heating device 5, from the respective
injecting holes 61 obliquely to the downstream side in the feed direction of the steel
pipe I.
Although the temperature of the cooling medium 62 injected from the cooling
device 6 is not particularly limited, in order to appropriately cool the steel pipe I after
heating, 5°C to 25°C is preferable as the temperature of the cooling medium 62.
Although the hole diameter of the injecting holes 6I in the cooling device 6 is
not particularly limited, 1.5 mm to 3.0 mm is preferable and 1.8 mm is particularly
preferable.
[0030]
Although the injecting velocity of the cooling medium 62 injected from the
injecting holes 6 I is not particularly limited, in order to appropriately cool the steel
pipe 1, 3 m/sec to 12m/sec is preferable, and 4 m/sec to 6 m/sec is particularly
preferable.
Although the injecting angle (the collision angle of the cooling medium 62 to
the steel pipe I) of the cooling medium 62 in the feed direction of the steel pipe l is
not particularly limited, 15° to 70° is preferable, and 30° is particularly preferable.
[0031]
[Feeding Device (Feeding Mechanism)]
The feeding device 3 is a device that feeds the steel pipe 1 in the longitudinal
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direction relatively to the induction heating device 5 and the cooling device 6. As the
feeding device 3, a device having the function of feeding the steel pipe I in the
longitudinal direction may be used, or a device having the function of feeding the
induction heating device 5 and the cooling device 6 in the longitudinal direction of the
steel pipe I may be used.
[0032]
An example of the device having the function of feeding the steel pipe I in the
longitudinal direction includes a device that feeds the steel pipe I in the longitudinal
direction using a ball screw, and an industrial robot that feeds the steel pipe I in the
longitudinal direction in a state where the steel pipe I is gripped.
An example of the device having the function of feeding the induction heating
device 5 and the cooling device 6 in the longitudinal direction of the steel pipe I
includes an industrial robot that feeds the induction heating device 5 and the cooling
device 6 in the longitudinal direction of the steel pipe I in a state where the induction
heating device 5 and the cooling device 6 are suppmted.
[0033]
[Bending Device (Bending Mechanism)]
The bending device is a device that applies a bending moment in an arbitrary
direction to the high-temperature portion I a. When the bending device applies the
bending moment in an arbitrary direction to the high-temperature portion Ia, a bent
portion that is bent in two-dimensional directions (for example, S-shaped bending) or
tluee-dimensional directions is formed on the steel pipe I.
As shown in FIG. 6B, the bending device bends the steel pipe I in a bending
direction D with a bending radius R. In the present embodiment, the bending radius
R represents a bending radius on the centroid line of the steel pipe I.
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[0034]
Next, the results ofthe study that has led to the knowledge of the present
invention will be described.
[0035]
FIG. 4 shows a relationship between the feed position of the steel pipe 1 and
the surface temperature of the steel pipe I in a case where only heating and cooling are
performed to the steel pipe I without performing bending, using the induction heating
device 5 and the cooling device 6. A to H shown on the horizontal axis of FIG. 4
represent points where the cooling medium 62 il*cted from the injecting holes 6 I in
the rows A to H collides against the surface of the steel pipe I. The vertical axis of
FIG. 4 represents surface temperature at respective feed positions when a cetiain point
located on the surface of the steel pipe 1 are fed in the longitudinal direction with the
tip portion of the steel pipe 1 as a head.
As shown in FIG. 4, the surface temperature of the steel pipe I is rapidly
heated to about 1 000°C by the induction heating device 5, and a highest arrival
temperature is shown in the vicinity of point A. Thereafter, the steel pipe I is cooled
by the cooling medium 62 injected from the injecting holes 6 I in the rows B to H
together with the feed of the steel pipe 1. Under the conditions of FIG. 4, the
temperature of the steel pipe I falls substantially to room temperature in the vicinity of
point H.
[0036]
Next, bending is performed to the steel pipe I with various bending radii R,
using the hot -bending apparatus 0 for a steel material, and the bent member 8 is
manufactured.
FIG. 6A is a plan view showing the state of cooling of the steel pipe 1 by the
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cooling device 6 when bending is not performed to the steel pipe I. FIGS. 68 to 6E
are plan views showing the state of cooling of the steel pipe I by the cooling device 6
in a case where bending with the bending radius R is performed to the steel pipe 1, and
the bending radius R becomes smaller as proceeding from FIG. 68 to FIG. 6E.
As shown in FIGS. 6A to 6E, not only in a case where bending is not
performed to the steel pipe I, but also in a case where bending is performed to the steel
pipe I with the bending radius R, it is possible to cool the steel pipe I with the cooling
medium 62 injected from the injecting holes 61 provided in the cooling device 6.
[0037]
Measurement results of the surface temperature of the outside of the bent
portion in the steel pipe 1 in bending shown in FIGS. 68 to 6E are shown in FIG. 8,
and the measurement results of the surface temperature of the inside of the bent pm1ion
are shown in FIG. 9.
In addition, bending conditions I to 4 in FIGS. 8 and 9 respectively
correspond to the bending conditions shown in FIGS. 68 to 6E. Additionally, an
example of the shape of the bent member 8 manufactured according to the bending
conditions of FIGS. 8 and 9 is shown in FIG. 5.
[0038]
As shown in FIG. 8, as a measurement result of the surface temperature of the
outside of the bent portion of the steel pipe 1 under a bending condition 1, the same
result as the measurement result of the surface temperature in a case where bending is
not performed to the steel pipe 1 shown in FIG. 4 was obtained.
Meanwhile, the surface temperature of the outside of the bent portion ofthe
steel pipe 1 in the case of the bending conditions 2 to 4 was different from that under
the bending condition 1, as shown in FIG. 8. Specifically, the surface temperatures at
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points D to H under the bending conditions 2 to 4 were higher than that under the
bending condition I at the outside of the bent portion.
[0039]
Meanwhile, as shown in FIG. 9, there are no big differences in the surface
temperature of the inside of the bent portion of the steel pipe I due to the bending
conditions.
It is considered as a reason why the surface temperature at the outside of the
bent portion of the steel pipe I varies due to the bending conditions, whereas a large
difference is not caused in the surface temperature at the inside of the bent pmiion of
the steel pipe I due to the bending conditions, the angles of collision of the cooling
medium62 injected from respective itljecting holes 61 against the surface of the steel
pipe I are difterent limn each other between inside and outside of the bent portion of
the steel pipe 1.
[0040]
Specifically, the angle of collision of the cooling medium62 against the
surface of the steel pipe I is large at the inside of the bent portion. Therefore, the
pressure of collision of the cooling medium 62 against the surface of the steel pipe I is
large, and the water amount density of the cooling medium 62 becomes high.
On the other hand, the angle of collision of the cooling medium 62 against the
surface of the steel pipe I is small at the outside of the bent portion. Therefore, the
pressure of collision of the cooling medium 62 against the surface of the steel pipe I is
small, and the injected water density of the cooling medium 62 becomes low.
From the above-described reason, the cooling rate~ofthe inside of the bent
pmiion is larger compared to that of the outside of the bent portion in the steel pipe 1.
[0041]
- 18 -
Bending (bending condition 2) shown in FIG. 6C is described as an example.
The angle of collision of the cooling medium 62 injected from the injecting holes 61 in
the row F against the outside of the bent portion of the steel pipe I is extremely small.
Moreover, the cooling medium 62 injected from the injecting holes 61 in the rows of G
and H do not hit the outside of the bent portion of the steel pipe I.
From the above-described reason, since the cooling of the steel pipe I by the
cooling medium 62 injected from the rows F to H is insufficient, reheat occurs, and as
shown in the bending condition 2 of FIG. 8, the surface temperature at the downstream
side of the point F rises, as seen along the feed direction.
Meanwhile, as shown in FIG. 6C, the angle of collision of the cooling medium
62 injected from the injecting holes 61 in the rows F to H against the inside of the bent
pmtion of the steel pipe I are large. For that reason, as shown in the bending
condition 2 of FIG. 9, the inside of the bent portion the steel pipe 1 is sufficiently
cooled by the cooling medium 62.
[0042]
Under the bending condition 4 in which the bending radius R is smaller than
that under the bending condition 2, as shown in FIG. 6E, the cooling medium 62
injected from the rows A to Chit the outside of the bent portion of the steel pipe 1, but
the cooling medium 62 injected from the rows D to H do not hit the outside of the bent
pottion of the steel pipe I. For that reason, since the cooling of the steel pipe 1 is
insufficient, reheat occurs, and as shown in the bending condition 4 of FIG. 8, the
surface temperature on the downstream side of the point D rises, as seen along the feed
direction.
Meanwhile, as shown in FIG. 6E, the angle of collision of the cooling medium
62 injected from the injecting holes 61 in the rows D to I-l against an inside surface of
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the bent portion ofthe steel pipe I are large. for that reason, as shown in the bending
condition 4 off! G. 9, the inside of the bent portion the steel pipe I is sufficiently
cooled by the cooling medium 62.
[0043]
As described above, in a case where bending with a small bending radius R is
performed, the outside of the bent portion of the steel pipe I is cooled insufficiently.
Therefore, a microstructure that has first been subjected to martensitic transformation
as a result of quenching is tempered and softened at the outside of the bent portion of
the steel pipe I. Additionally, since the cooling of the outside of the bent portion of
the steel pipe I is insufficient, a non-uniform structure is formed in a part of the
outside of the bent portion.
Hence, in a case where bending with a small bending radius R is performed,
the bent member 8 manufactured by the 3DQ is not cured because not only the
hardness of the inside and the outside of the bent portion are non-uniform but also
quenching that is one of the purposes of heating and cooling is not appropriately
performed. Additionally, since a relatively high residual stress is generated in the
bent member 8 due to the cooling of the inside and the outside of the bent potiion
being non-uniform, desired product performance may not be obtained when high
fatigue strength is required to the bent member 8.
[0044]
In addition, in the above description, a case where the sectional shape of the
steel pipe I is circular has been described as an example. However, the problem that
the cooling of the inside and the outside of the bent pmiion is non-uniform is similarly
caused irrespective of the sectional shape of the steel pipe I, for example, even in a
case where the steel pipe I has a rectangular sectional shape, a flat sectional shape, a
- 20 -
!I
polygon sectional shape, or more complicated sectional shapes.
[0045]
As one of methods for reducing the above-described non-uniformity of
cooling, it is considered that not only the above-described cooling device 6 but also a
cooling device capable of injecting the cooling medium 62 in correspondence with
various bending shapes is used. However, in this method, there is a possibility that a
injecting region for the cooling medium 62 may come into contact with the steel pipe 1.
In addition, this method is not preferable from a viewpoint of economical efficiency.
[0046]
As another method for reducing the above-described non-uniformity of
cooling, a method for slowing down the feeding speed of the steel pipe I is considered.
Since long time is required to pass the points A to H by slowing down the feeding
speed of the steel pipe I, a larger amount of the cooling medium62 is ir~ected to the
surface of the steel pipe I. For that reason, since the cooling medium 62 is also
sufficiently injected to the outside of the bent portion of the steel pipe I, the nonunifonnity
of cooling between the outside and the inside of the bent pmtion does not
easily occur.
However, since the productivity of bending falls by slowing down the feeding
speed of the steel pipe I, tllis method is not preferable.
[0047]
Additionally, in a case where bending with a small bending radius is
performed, occurrence of wrinkle and cross-sectional distortion is a problem.
When the bent member 8 is manufactured with the steel pipe I as a material
by a cold draw bender, it is general to insert a mandrel into an inner surface of the steel
pipe I to perform bending, in order to suppress wrinkle and cross-sectional distortion
- 21 -
!I
!1
(flattening) in the bent member 8.
[0048]
On the other hand, in the 3DQ, generally, it is possible to suppress wrinkle
and cross-sectional distortion more than the cold draw bender without constraining the
inner surface of the steel pipe I by a mandrel or the like. In the 3DQ, the length of
the high-temperature portion I a in the longitudinal direction, which is formed in the
steel pipe I, is extremely short. Accordingly, since the high-temperature portion I a is
constrained due to a low-temperature portion that is present on both sides in the
longitudinal direction of the high-temperature portion Ia, wrinkle and cross-sectional
distortion resulting from bending are suppressed.
However, when the bending radius of the steel pipe I is small, wrinkle and
cross-sectional distortion occurs remarkably. For that reason, in a case where the
bending radius of the steel pipe 1 is small, it is necessary to suppress wrinkle and
cross-sectional distortion even in a case where bending is performed to the steel pipe 1
using the 3DQ.
[0049]
[Control Device (Controller)]
According to the above-described results of study, the control device (not
shown) related to the present embodiment performs a control such that the feeding
speed of the steel pipe 1 is set to be slower than VI and the high-frequency power is
set to be lower than Q I in the bending step while forming the bent portion of which a
ratio R/W is equal to or lower than a predetermined value, in a case where V 1 is the
feeding speed of the steel pipe I while forming the bent portion of which the ratio RJW
is more than the predetermined value, Ql is the high-frequency power supplied to the
induction heating mechanism 5 while forming the high-temperature portion 1 a on the
- 22 -
!I
!!
steel pipe l, and RJW is the ratio obtained by dividing the bending radius R [mm] of
the bent portion on the centroid line of the steel pipe 1 by the dimension W [mm] in the
bend direction in the cross-section of the steel pipe 1 orthogonal to the centroid line.
In addition, the dimension in the bend direction in the cross-section of the
steel pipe 1 orthogonal to the centroid line means the width dimension of the steel pipe
1 when the bent portion is seen from the sight line along the centerline of curvature of
bending thereof.
In addition, although a case where the dimension W of the steel pipe 1 does
not vary in the longitudinal direction but has the same width dimension W is shown in
FIGS. 7A to 7F, in a case where the dimension W of the steel pipe 1 varies in the
longitudinal direction, the dimension W of the steel pipe I is determined for each bent
pmtion of which RJW is determined.
[0050]
It is preferable that the predetermined value ofRJ\V is a value selected from
within a range of3.0 to 8.0. By setting the predetermined value ofRJW to the value
selected fi·mn within the range of3.0 to 8.0 to control manufacture of the bent member
8 with the control device (not shown), productivity can be suitably improved while
suitably suppressing uneven quenching, wrinkle, and cross-sectional distortion. It is
more preferable that the above-described predetermined value of R!W is a value
selected from within a range of 4.0 to 7.0.
In addition, a case where RJW is more than the predetermined value includes
a case where a bent portion of which RIW is more than the predetermined value is
formed and a case where a region in which bending is not performed is formed. In
addition, in the present embodiment, a region in which bending is not performed is
referred to as a straight pipe pmtion, and RJW while forming this straight pipe portion
- 23 -
II
is supposed to be infinite.
[0051]
In the control device (not shown) of the present embodiment, it is preferable
to lower the feeding speed of the steel pipe I to 25% to 75% of the above-described VI
in a case where R/W is equal to or lower than the predetermined value.
By lowering the feeding speed of the steel pipe I to 25% to 75% ofVl, the
cooling medium 62 can be sufficiently injected to the outside of the bent portion even
in a case where the bending radius is small. Thus, the outside of the bent pmtion can
be appropriately cooled.
[0052]
Additionally, by lowering the feeding speed of the steel pipe 1 to 25% to 75%
ofV1, the steel pipe 1 is uniformly cooled in the circumferential direction thereof, and
a deformation zone becomes unifonn in the circumferential direction. As a result,
occunence of wrinkle and cross-sectional distortion is suppressed.
[0053]
In the control device (not shown) of the present embodiment, it is preferable
to lower the high-fi'equency power supplied to the induction heating device 5 to 25% to
75% of the above-described Ql in a case where R/W is equal to or lower than the
predetermined value.
In the present embodiment, as described above, the high-frequency power
supplied to the induction heating device 5 is controlled such that the highest arrival
temperature of the steel pipe 1 becomes 900°C to 1 050°C. However, by lowering the
feeding speed of the steel pipe 1, there is a case where the steel pipe I is superfluously
heated and the steel material melts, or a case where grain coarsening of the steel
material proceeds and a decrease in the touglmess of the steel material occurs. By
- 24 -
ll
lowering the high-frequency power supplied to the induction heating device 5 to 25%
to 75% of Q I, the steel pipe I can be prevented Jl'Om being superfluously heated.
(0054]
The method for changing the feeding speed of the steel pipe I and the highfrequency
power supplied to the induction heating device 5 on the basis of the abovedescribed
RIW when bending of the steel pipe 1 is performed is the knowledge that has
been first found out by the present invention.
Additionally, the control device (not shown) just has to be control devices that
can perfonn the above-described control, and is not particularly limited.
[0055]
(Method for-Manufacturing Bent Member)
Next, the method for manufacturing the bent member 8 using the hot-bending
apparatus 0 for a steel material related to the present embodiment will be described.
The method for manufacturing the bent member 8 related to the present
embodiment has a gripping step, a feeding step, a heating step, a bending step, and a
cooling step.
(0056]
In the gripping step, at least one of the one end portion (tip pmiion) and the
other end portion (rear end portion) of the steel pipe 1 is gripped by the gripping device
7.
In the feeding step, the steel pipe 1 after the gripping step is relatively fed in
the longitudinal direction with respect to the induction heating device 5 and the cooling
device 6. That is, in the feeding step, the steel pipe 1 may be fed in the longitudinal
direction with respect to the induction heating device 5 and the cooling device 6, or the
induction heating device 5 and the cooling device 6 may be fed in the longitudinal
- 25 -
II !i
direction of the steel pipe I.
In the heating step, the high-temperature portion 1 a is formed by performing
high-frequency induction heating on one portion of the steel pipe I in the longitudinal
direction. In the heating step, the highest arrival temperature of the steel pipe 1 is
controlled by controlling the high-frequency power supplied to the induction heating
device 5.
[0057]
In the bending step, a bending moment in an arbitrary direction is applied to
the high-temperature portion !a. Accordingly, a bent portion is formed on the steel
pipe I.
In the cooling step, the bent portion is cooled by injecting the cooling medium
62 to the bent portion.
[0058]
The control device (not shown) for manufacturing the bent member 8 related
to the present embodiment performs a control such that the feeding speed of the steel
pipe 1 is set to be slower than VI and the high-frequency power is set to be lower than
Q 1 while forming the bent portion of which the ratio RIW is equal to or lower than the
predetermined value, in a case where VI is the feeding speed of the steel pipe 1 while
forming the bent portion of which the ratio RIW is more than the predetermined value,
Ql is the high-frequency power supplied to the induction heating device 5 while
forming the high-temperature portion I a on the steel pipe I, and RIW is the ratio
obtained by dividing the bending radius R [mm] of the bent pmtion on the centroid line
of the steel pipe 1 by the dimension W [mm] in the bend direction in the cross-section
of the steel pipe I orthogonal to the centroid line.
In order to improve the productivity preferably and to suppress uneven
- 26 -
'tI..
quenching, wrinkle, and cross-sectional distortion, it is preferable that the abovedescribed
predetermined value ofR/W is a value selected from within the range of3.0
to 8.0. It is more preferable that the above-described predetermined value of R/W is a
value selected from within a range of 4.0 to 7.0.
[0059]
As described above, according to the present embodiment, even in a case
where the bent member 8 having a small bending radius R is manufactured, it is
possible to suppress occurrence of uneven quenching, wrinkle, and cross-sectional
distortion, and to manufacture the bent member 8 with excellent productivity.
Additionally, according to the present embodiment, it is possible to
manufacture the bent member 8 using the related-art cooling device 6 that is used in
the 3DQ without using an exclusive cooling device 6. For that reason, this is suitable
from a viewpoint of economical efficiency.
[0060]
In addition, the present invention is not limited only to the above-described
embodiment.
For example, in the above-described embodiment, the method for
manufacturing the bent member 8 having the bent portion of which R/W is equal to or
lower than the predetermined value was described. However, in cases where R1W of
all bent portions included in the bent member 8 are more than the predetermined value,
even if the related art method for manufacturing the bent member 8, it is possible to
suppress occurrence of uneven quenching, wrinkle, and cross-sectional distortion, and
a decrease in productivity does not occur, either. For that reason, in cases where R/W
of all bent portions included in the bent member 8 are more than the predetermined
value, it is not necessary to lower the feeding speed of the steel pipe 1 relative to the
- 27 -
u
!!
cooling device 6, and the high-frequency power supplied to the induction heating
device 5.
Example I
[0061]
As shown in FIG. 6A, only quenching was performed without performing
bending to a steel pipe, and a feeding speed V0 at which suitable hardness ( 420 Hv or
more) and suitable surface residual stress (surface residual stress measured by an the
X-ray diffraction method is equal to or lower than 80 MPa at tensile residual stress)
were obtained was determined using the hot-bending apparatus for a steel material of
the present embodiment. The feeding speed Vo determined as described above was
used as a reference feeding speed.
Bending was performed to the steel pipe while feeding the steel pipe at the
reference feeding speed V0• In that case, the bending radius R was changed, and a
relationship between the bending radius Rand the acceptance rate of quality was
investigated.
[0062]
As for evaluation of the quality, a case where the suitable hardness ( 420 Hv or
more) and the suitable surface residual stress (the surface residual stress measured by
the X-ray diffraction method is equal to or lower than 80 MPa at tensile residual stress)
were obtained was considered to be acceptable. Then, bending tests were performed
20 times for each bending radius R, the hardness and the surface residual stress of
obtained bent members were measured, and the acceptance rate of quality was
determined. In addition, all the tests were performed such that wrinkle did not occur.
Test results are shown in Table I.
[0063]
- 28 -
[Table I]
(Bending Radius R/Width Dimension IV) Acceptance Rate of Quality
R/\V>I5.0 100%
IS.O<:RIW> I 0.0 100%
I O.O<:R/\V>8.0 98%
8.0<:R/\V>5.5 92%
5.5<:R/W>3.0 88%
3.0<:R/IV>2.0 61%
2.0<:R/W> 1.5 47%
[0064]
As shown in Table 1, in a case where R/W was equal to or lower than 8.0, the
acceptance rate of quality decreased compared to a case where R/W is more than 8.0.
Particularly in a case where R/W was equal to or lower than 3.0, the acceptance rate of
quality decreased compared to a case where R/W is more than 3.0.
[0065]
The acceptance rate of quality with respect to R/W in a case where the steel
pipe was fed at the reference feeding speed Yo is shown in Table 1. The acceptance
rate of quality with respect to R/W in a case where the steel pipe was fed at a speed
slower than the reference feeding speed Y 0 is shown shown in Table 2. As shown in
Table 2, as the feeding speed, feeding speeds of75%, 50%, and 25% of the reference
feeding speed Yo were used.
[0066]
[Table 2]
Acceptance Rate of Quality
(Bending Radius R/\Vidth Dimension IV) 0.75xV0 1 0.50xV0 I 0.25xVo
- 29 -
II n
R/W>15.0 - - -
15.0<:R/IV> I 0.0 - - -
I 0.02:R/IV>8.0 100% - -
8.0<:R/W>5.5 97% 100% -
5.52:R/W>3.0 96% 98% 100%
3.0<:R/W>2.0 88% 94% 100%
2.02:Rf\V> 1.5 73% 85% 98%
[0067]
As shown in Table 2, the acceptance rate of quality was improved by lowering
the feeding speed of the steel pipe 1.
Example 2
[0068]
A bent member having a shape shown in FIG. 13 was manufactured by the
3DQ, using a carbon steel pipe (C content 0.2 mass%) with a width dimension of25.4
mm and a thickness of 1. 8 nnn. The feeding speed of a steel pipe and the highfrequency
power supplied to the induction heating device when manufacturing a bent
member were changed, and the presence/absence of occurrence of wrinkle and
working time were investigated. Results relating to Example 2-1, Comparative
Example 2-1, and Comparative Example 2-2 are shown in Table 3.
In addition, in Example 2-1, Comparative Example 2-1, and Comparative
Example 2-2, the high-frequency power supplied to the induction heating device was
adjusted such that the highest arrival temperature of the steel pipes was I 000°C.
[0069]
[Table 3]
Bending
Radius R
Yo
[mm/s]
Ys
[mm/s]
- 30 -
Eo Es Occurrence \Vorking
[k\V] [k\V] of Wrinkle Time [ s]
u n
..
[mm]
Comparative
90 80 - 128.8 - Yes 27
Example 2-1
Comparative
90 - 30 - 48.3 No 73
Example 2-2
Example 2-1 90 80 30 128.8 48.3 No 33
[0070]
(Comparative Example 2-1)
Comparative Example 2-1 in Table 3 represents a related-art example, and
bending was performed to a steel pipe according to the feeding speed of the steel pipe
shown in FIG. IIA and the supply of high-frequency power to the induction heating
device shown in FIG. liB. Specifically, a feeding speed Vo of the steel pipe was set
to 80 mm/sec, and high-frequency power Eo supplied to the induction heating device
was set to 128.8 kW.
In a bent member manufactured by Comparative Example 2-1, a crease of
about 0.6 mm was generated on an inside surface of a bent potiion. Moreover, it was
understood that, according to the observation of the outside surface of the bent portion,
a non-uniform tempered structure was generated in part. The hardness of the abovedescribed
tempered structure was about 350 Hv, and was softened as compared to a
hardness of about 450 Hv of a straight pipe portion. Additionally, when the residual
stress on an outer peripheral side of the bent portion was measured with X rays, the
residual stress was tensile residual stress more than 80 MPa.
[0071]
(Comparative Example2-2)
Comparative Example 2-2 shown in Table 3 represents a related-mi example,
and bending was performed to a steel pipe using the feeding speed of the steel pipe
- 31 -
shown in FIG. 12A and the supply of high-frequency power to the induction heating
device shown in FIG. 12B. Specifically, a feeding speed Va of the steel pipe was set
to 30 mm/sec, and high-frequency power EB supplied to the induction heating device
was set to 48.3 kW.
In a bent member manufactured by Comparative Example 2-2, a crease or a
non-uniform tempered stmcture was not generated on the inside of the bent portion.
Additionally, suitable hardness of about 450 Hv was obtained in the overall
longitudinal direction of the steel pipe including the bent pmtion. Additionally, when
the residual stress of the outside of the bending was measured with X rays, similar to
the straight pipe pmtion, suitable compressive residual stress that was about -50 MPa
was obtained in the overall longitudinal direction.
However, in Comparative Example 2-2, the tin1e required for bending was 73
seconds which was about 2. 7 times of that of Comparative Example 1, and a decrease
in productivity was remarkable.
[0072]
(Example 2-1)
Example 2-1 shown in Table 3 represents an example of the present invention,
and bending was performed to a steel pipe using the feeding speed of the steel pipe
shown in FIG. 14A and the supply ofhigh-fl-equency power to the induction heaiing
device shown in FIG. 14B.
In Example 2-1, the feeding speed V0 of the steel pipe when a portion that is
scheduled to be a straight pipe portion passed through the induction heating device and
the cooling device was set to 80 nun/sec. Additionally, the high-frequency power Eo
supplied to the induction heating device when heating the portion that is scheduled to
be the straight pipe pottion was set to 128.8 kW.
- 32 -
H
!I
Meanwhile, the feeding speed V 8 of the steel pipe when a portion that is
scheduled to be a bent portion passed through the induction heating device and the
cooling device was set to 30 nun/sec. Additionally, the high-frequency power En
supplied to the induction heating device when heating the portion that is scheduled to
be the bent portion was set to 48.3 kW.
[0073] '
In addition, in Example 2-1, the high-frequency power supplied to the
induction heating device when heating a region where the feeding speed shifts fi·om V0
to V11 and a region where the feeding speed shifts from V8 to V0 was controlled such
that the highest arrival temperature of the steel pipe was 1000°C, on the basis of
preliminmy experimental results using a thermocouple.
[0074]
In a bent member manufactured by Example 2-1, a crease or a non-uniform
tempered structure was not generated in the bent portion, Additionally, excellent
hardness of about 450 Hv was obtained in the overall longitudinal direction of the steel
pipe including the bent portion. Additionally, suitable residual stress was obtained.
Moreover, in Example 2-1, the time required for working was 33 seconds, and this was
about 1.2 times even if it is compared with Comparative Example 2-1.
From the above results, suitable hardness, residual stress, and productivity
could be obtained in Example 2-1, without generating a crease or a non-uniform
tempered structure.
[Industrial Applicability]
[0075]
According to the above embodiments, it is possible to provide a method for
manufacturing a bent member and a hot-bending apparatus for a steel material that can
- 33 -
reduce occurrence of uneven quenching and occurrence of wrinkle and cross-sectional
distortion and is excellent in productivity and economical efficiency, even in a case
where a bent member having a small bending radius is manufactured.
[Brief Description of the Reference Symbols]
[0076]
0: BENDING DEVICE (HOT-BENDING APPARATUS FOR STEEL
MATERIAL)
I: STEEL PIPE (STEEL MATERIAL)
Ia: HIGH-TEMPERATURE PORTION (RED HEAT PORTION)
2: SUPPORTING DEVICE
3: FEEDING DEVICE (FEEDING MECHANISM)
4: MOVABLE ROLLER DIE
5: INDUCTION HEATING DEVICE (INDUCTION HEATING
MECHANISM)
6: COOLING DEVICE (COOLING MECHANISM)
7: GRIPPING DEVICE (GRIPPING MECHANISM)
8: BENTMEMBER
61: INJECTING HOLE
62: COOLING MEDIUM

[Document Type]
What is claimed is:
CLAIMS
1. A method for manufacturing a bent member, the method comprising:
feeding an elongated steel material in a longitudinal direction with one end
portion of the steel material as a head;
performing high-frequency induction heating to one portion of the steel
material in the longitudinal direction by being supplied high-frequency power to form
a high-temperature portion;
bending the steel material by applying a bending moment in an arbitrary
direction to the high-temperature pmtion to form a bent portion, the bending
compnsmg:
forming the bent portion having a ratio R/W which is equal to or
lower than a predetermined value, where the ratio R/W is a ratio obtained by dividing a
bending radius R [mrn] of the bent portion on a centroid line of the steel material by a
dimension W [mm] in a bend direction in a cross-section of the steel material
orthogonal to the centroid line;
slowing down a feeding speed of the steel material less than V 1,
where the V 1 is the feeding speed of the steel material while forming the bent portion
having the ratio R/W which is more than the predetermined value;
reducing the high-fi·equency power supplied while forming the hightemperature
portion less than Q1, where the Q1 is the high-frequency power supplied
while forming the bent portion having the ratio R/W which is more than the
predetermined value; and
injecting a cooling medium to the bent portion to cool the bent portion.
2. The method for manufacturing a bent member according to Claim I,
- 35 -
wherein the predetermined value of the ratio R/W is within a range of3.0 to
8.0.
3. The method for manufacturing a bent member according to Claim I or 2,
wherein the feeding speed of the steel material while forming the bent portion
having the ratio R/W which is equal to or lower than the predetermined value is
lowered to 25% to 75% of the Vl during bending.
4. The method for manufacturing a bent member according to any one of
Claims I to 3,
wherein the high-fi·equency power supplied while forming the bent portion
having the ratio R/W which is equal to or lower than the predetermined value is
lowered to 25% to 75% of the Ql during bending.
5. A hot-bending apparatus for a steel material comprising:
a feeding mechanism that feeds an elongated steel material in a longitudinal
direction with one end portion of the steel material in the longitudinal direction as a
head;
an induction heating mechanism that performs high-frequency induction
heating on one p01tion of the steel material in the longitudinal direction by being
supplied high-frequency power to form a high-temperature portion;
a bending mechanism that applies a bending moment in an arbitrary direction
to the high-temperature portion to form a bent portion;
a cooling mechanism that injects a cooling medium on the bent portion to cool
the bent portion; and
a controller that controls the feeding mechanism, the induction heating
mechanism, the bending mechanism, and the cooling mechanism such that a feeding
speed of the steel material is slower than Vl and the high-frequency power is lower
- 36 -
than Q 1 while forming the bent portion having a ratio R/W which is equal to or lower
than a predetermined value, where the Vl is the feeding speed of the steel material
while forming the bent portion having the ratio R/W which is more than the
predetermined value, the Q 1 is the high-frequency power supplied to the induction
heating mechanism, and the ratio R/W is a ratio obtained by dividing a bending radius
R [mm] of the bent portion on a centroid line of the steel material by a dimension W
[mm] in abend direction in a cross-section of the steel material orthogonal to the
centroid line.
6. The hot-bending apparatus for a steel material according to Claim 5,
wherein.the predetermined value of the ratio R/\V is within a range of3.0 to
8.0.
7. The hot-bending apparatus for a steel material according to ClaimS or 6,
wherein the feeding speed of the steel material while forming the bent portion
having the ratio R/W which is equal to or lower than the predetermined value is
lowered to 25% to 75% of the Vl.
8. The hot-bending apparatus for a steel material according to any one of
Claims 5 to 7,
wherein the high-fi·equency power supplied while forming the bent portion
having the ratio R/W which isequal to or lower than the predetermined value is
lowered to 25% to 75% of the Ql.