Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
TITLE OF INVENTION
METHOD FOR PRODUCING SEAMLESS METALLIC TUBE BY COLD ROLLING
TECHNICAL FIELD
[0001]
The present invention relates to a cold rolling method for a seamless metallic
tube, particularly to a method for producing a high-quality seamless metallic
tube by
cold rolling for the purpose of ensuring inside-surface quality of high-grade
specialty
tubes from a viewpoint of suppressing wrinkle imperfections on the inside
surface.
BACKGROUND ART
[0002]
When a seamless metallic tube does not satisfy specific requirements in
quality,
strength, or dimensional accuracy in an as-hot-finished condition, it is
subjected to a
cold working process. Commonly known cold working processes are a cold drawing
method with a die and a plug or a mandrel bar, and a cold rolling method with
a cold
pilger mill.
[0003]
Since available reduction rate for tube material is extremely high in cold
rolling
with a cold pilger mill, the cold rolling has advantages as follows: about ten-
times
elongation is possible by rolling; an excellent effect on correcting eccentric
wall
thickness of tube can be exhibited; a diameter-reducing process is not
required; and no
yield loss is generated.
[0004]
Meanwhile, the cold rolling with a cold pilger mill has a drawback of
extremely low productivity in comparison to the cold drawing method. The cold
rolling with a cold pilger mill is, therefore, mainly suitable for cold
working of
high-grade specialty tube such as stainless steel tube and high-alloy steel
tube that
requires expensive raw materials and costly intermediate treatments,.
[0005]
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Fig. 1 is a view for illustrating a mechanism of the cold rolling with a cold
pilger mill. In a cold rolling method with a cold pilger mill, a hollow shell
1 is
processed between a pair of rolls 2 and a tapered mandrel bar 4 to perform a
diameter-reducing rolling for the hollow shell 1, so as to obtain a rolled
tube 5. Each
roll 2 has a circumferential length-wise tapered groove caliber 3 decreasing
gradually in
diameter along the circumferential length. The tapered mandrel bar 4 decreases
gradually in diameter along a longitudinal direction.
[0006]
That is, the groove caliber 3 is formed along a circumference of each of
paired
rolls 2 of the cold pilger mill, and the groove caliber becomes
narrower/smaller with the
progress of rotation of rolls 2. The rolls 2 repeat forward and backward
strokes along
the tapered mandrel bar while being rotated by driven roll shafts 2s so that
the hollow
shell 1 is rolled between the rolls 2 and the mandrel bar 4 to perform a
diameter-reducing rolling of the hollow shell I (see Non Patent Literature 1,
for
example).
[0007]
Fig. 2 is an explanatory view showing a working principle of cold rolling with
a cold pilger mill. Fig. 2(a) shows a working state at a start point of a
forward stroke,
and Fig. 2(b) shows a working state at a start point of a backward stroke. As
shown in
Fig. 2, in the cold pilger mill, according to an outside diameter and a wall
thickness (do
and to in the figure) of a hollow shell 1 and an outside diameter and a wall
thickness (t
and d in the figure) of a product, selectively adopted are a pair of rolls 2
each having a
tapered groove caliber 3 which decreases gradually in diameter from an
engaging entry
side of the rolls toward a finishing exit side thereof, and a tapered mandrel
bar 4 which
decreases also gradually in diameter from an engaging entry side toward a
finishing exit
side, and forward and backward strokes are repeated to reduce a wall thickness
of the
hollow shell 1 while reducing a diameter thereof.
[0008]
The hollow shell I is turned by about 60 and is given a feed of about 5 to 15
mm at a start point of the forward stroke in reciprocation motion of the cold
pilger mill,
so that a new portion of the hollow shell is rolled, which is repeated.
[0009]
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There are two types of cold pilger mills: a rolling mill developed by
"MANNESMANN-DEMAG; the rolling mill for reducing wall thickness in both
forward and backward strokes; and a rolling mill developed by BLAWKNOX, the
rolling mill for reducing wall thickness only in a forward stroke. The former
is
commonly used for rolling stainless steel tube, high-alloy metallic tube, or
zirconium
tube, while the latter is used for rolling an aluminum tube, aluminum-alloy
tube, copper
tube, and copper-alloy tube.
CITATION LIST
NON PATENT LITERATURE
[0010]
[NON PATENT LITERATURE I]
The Iron and Steel Institute of Japan, "3rd Edition Iron and Steel Handbook,
Vol. III (2),
Steel Bars/Steel Pipe/Facilities Commonly Used for Rolling", November 20,
1980,
Pages 1183-1189
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0011]
Since characteristic of high-quality is strongly demanded for high-grade
specialty tube subjected to cold rolling with a cold pilger mill, it is
necessary to
suppress generation of inside-surface defects resulting from inside-surface
wrinkle
imperfections on a tube as a product after the cold rolling. There, however,
has
heretofore been no proposal on a method for producing a high-quality seamless
steel
tube, wherein inside-surface defects are inhibited from occurring in the cold
rolling with
a cold pilger mill.
[0012]
The present invention is achieved in view of the above problem, and an object
of the present invention is to propose a method for producing a high-quality
seamless
steel tube by the cold rolling with a cold pilger mill.
[0013]
Although a cold pilger mill performing rolling in both forward and backward
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strokes (MANNESMANN-DEMAG) will be described for the explanation of the
present invention, objects of the present invention are not limited to this
type but can be
applied to a cold pilger mill reducing wall thickness only in a forward stroke
(BLAWKNOX).
SOLUTION TO PROBLEM
[0014]
In order to solve the above problem, the present inventor found from various
examinations the following. That is, in cold rolling of a seamless metallic
tube with a
cold pilger mill, when a reduction rate of an outside diameter becomes
excessive in
comparison to a reduction rate of a wall thickness, circumferential
compressive stress
imposed on a hollow shell becomes excessive, and wrinkle imperfections are,
hence,
easily generated on the tube inside surface.
[0015]
Furthermore, when a hollow shell is produced by the Mannesmann-mandrel
mill process instead of the Ugine-Sejournet extrusion process, inside wrinkle
imperfections may be generated with a sizing mill (a stretch reducer or a
sizer) at a stage
of hollow shell. The inside wrinkle imperfections generated at the stage of
hollow
shell significantly influence quality of a high-grade specialty tube subjected
to the cold
rolling with a cold pilger mill.
[0016]
The present invention is completed based on the above knowledge, and a gist
thereof is methods of the following (1) and (2) each for producing a seamless
metallic
tube by cold rolling.
[0017]
(1) A method for producing a seamless metallic tube by cold rolling with
a cold pilger mill, comprising the steps of. when elongating a hollow shell in
such a
manner that an outside diameter thereof is reduced while reducing a wall
thickness
thereof, according to outside diameters and wall thicknesses of the hollow
shell and a
rolled tube as a product, selectively using a pair of rolls and a tapered
mandrel, the rolls
each having a tapered groove caliber which decreases gradually in diameter
from an
engaging entry side of roll toward a finishing exit side thereof, the tapered
mandrel bar
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decreasing also gradually in diameter from an engaging entry side toward a
finishing
exit side; and setting a reduction rate Rd of the outside diameter to not more
than
one-half of a reduction rate Rt of the wall thickness.
[0018]
The reduction rate Rd of outside diameter and the reduction rate Rt of wall
thickness are calculated by following expressions (a) and (b):
Rd= { 1- (d/do)} x 100 (%) ... (a)
Rt= { 1- (t/to)} x 100 (%) ... (b)
wherein
do: outside diameter of hollow shell, d: finishing outside diameter, to: wall
thickness of hollow shell, and t: finishing wall thickness.
[0019]
(2) In the method of (1) for producing a seamless metallic tube by cold
rolling, it is preferable to use a hollow shell subjected to a hot reducing
mill process
with a stretch reducer under the condition that a reduction rate of outside
diameter is not
more than 77%. Or it is preferable to use a hollow shell subjected to a hot
reducing
mill process with a sizer under the condition that a reduction rate of outside
diameter is
not more than 33%.
ADVANTAGEOUS EFFECTS OF INVENTION
[0020]
According to the method of the present invention for producing a seamless
metallic tube by cold rolling, it is possible to suppress generation of inside-
surface
defects resulting from inside-surface wrinkle imperfections by improving a
working
balance between a reduction rate Rd of outside diameter and a reduction rate
Rt of wall
thickness at the time of the elongation-rolling which accompanies the
reduction of the
diameter while reducing the wall thickness. It is, therefore, possible to
ensure
high-quality for the product after cold rolling.
[0021]
Furthermore, when a hollow shell is produced by the Mannesmann-mandrel
mill process, it is possible to further improve product quality after cold
rolling by
limiting a reduction rate of outside diameter in a sizing mill (a stretch
reducer or a
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sizer).
BRIEF DESCRIPTION OF THE DRAWINGS
[0022]
[Fig. I] A view for illustrating a mechanism of the cold rolling with a cold
pilger mill.
[Fig. 2] An explanatory view showing a working principle of the cold rolling
with a
cold pilger mill, Fig. 2(a) shows a working state at a start point of a
forward stroke, and
Fig. 2(b) shows a working state at a start point of a backward stroke.
[Fig. 3] A view showing a divided model of a cross-section of a tube rolled
with a cold
pilger mill.
[Fig. 4] A view showing deformation behaviors of a cross-section of a tube
rolled with
a cold pilger mill.
[Fig. 5] A view for illustrating one example of production steps in the
Mannesmann-mandrel mill process for hot-producing seamless steel tube.
DESCRIPTION OF EMBODIMENTS
[0023]
Fig. 3 is a view showing a segmentation model of a cross-section of an
in-process tube rolled during rolling with a cold pilger mill. The cross-
section of tube
can be segmented into groove bottom regions 11, 14 and flange regions 12, 13
based on
whether or not the inner surface of tube 1 is in contact with a mandrel bar 4.
The
groove bottom regions 11, 14 are elongated by being subjected to a wall
thickness
reduction work by means of the rolls and the mandrel bar 4, and the flange
regions 12,
13 are deformed by being pulled by the elongation of the groove bottom
regions. That
is, metals of the groove bottom regions 11, 14 are deformed under external
pressure,
internal pressure, and axial compression force, and metals of the flange
regions 12, 13
are deformed under external force and axial force. of tension
[0024]
Fig. 4 is a view showing deformation behaviors with respect to the
cross-section of tube during rolling with a cold pilger mill. Fig. 4(a) shows
a
deformation behavior during rolling in a forward stroke (forward rolling), and
Fig. 4(b)
shows a deformation behavior during rolling in a backward stroke (backward
rolling).
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The deformation behaviors shown in Fig. 4 are based on a working pattern in
which the
tube 1 is turned only in the forward stroke rolling and not turned in the
backward stroke
rolling. That is, when seen from tube side, the rolls are turned relative to
the
in-process tube to be repositioned only during the forward stroke but not
relatively
turned during the backward stroke.
[0025]
As shown in Figs. 4(a) and 4(b), in a type of cold pilger mill performing
rolling
in both forward and backward strokes (MANNESMANN-DEMAG), A turn of 60 is
basically adopted. Deformation of the cross-section of tube is, thus, not
symmetrical
but asymmetrical to a horizontal axis and a vertical axis of a groove caliber.
In
deformation behaviors shown in Fig. 4, segments 11 and 14 indicate the groove
bottom
regions, and segments 12 and 13 indicate the flange regions in the i-th
forward stroke
rolling.
[0026]
In the deformation behaviors shown in Fig. 4, when a reduction rate Rd of
outside diameter is excessively large relative to a reduction rate Rt of wall
thickness,
compressive strain cpO in a circumferential direction on the flange regions is
increased.
As a result, compressive stress 60 in a circumferential direction (not shown)
becomes
excessive, so that inside-surface wrinkle imperfections are generated and
folded on the
groove bottom regions. This process is repeated to be developed into inside-
surface
defects, resulting in deterioration of inside-surface quality.
[0027]
In the production of a specialty tube which requires a high level of quality
characteristic, a ratio of the reduction rate Rd of outside diameter to the
reduction rate
Rt of wall thickness determines quality of product. Furthermore, when a hollow
shell
to be processed with a cold pilger mill is produced by the Mannesmann-mandrel
mill
process instead of the hot extrusion (Ugine-Sejournet extrusion) process, the
hollow
shell includes inside-surface wrinkle imperfections generated in a hot
reducing mill
process. The inside-surface wrinkle imperfections further encourages the
development
thereof and affects the cold rolling process.
[0028]
Fig. 5 is a view for illustrating one example of production steps in the
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Mannesmann-mandrel mill process for hot-producing seamless steel tube. In this
process, a solid round billet 21 heated to a predetermined temperature serves
as a
starting material to be rolled. This round billet 21 is fed to a piercing mill
23, and a
central portion thereof is pierced so as to produce a hollow piece (hollow
shell) 22.
Next, the produced hollow piece 22 is directly fed to a successive elongating
device,
which is a mandrel mill 24, to be elongated, so as to obtain a hollow shell
22.
[0029]
At the time of the elongation rolling with the mandrel mill 24, a material
temperature of the hollow shell 22 is lowered with a mandrel bar 24b inserted
into the
inside of the hollow shell and rolling rolls 24r for constraining outer
surface of the
hollow shell. Therefore, the hollow shell 22 rolled in the mandrel mill 24 is
then
placed into a re-heating furnace 25 to be re-heated. After that, the hollow
shell goes
through a sizing mill such as a stretch reducer 26 or a sizer (not shown) and
becomes a
hot-rolled seamless steel tube. When a temperature drop in the mandrel mill is
small,
the re-heating furnace is not required.
[0030]
However, in the stretch reducer or the sizer for performing the sizing mill
process in the above Mannesmann-mandrel mill process, the hollow shell 22 goes
through rolling rolls26r to be finished by a reducing mill process for an
outside diameter
without using the inside surface constraining tool such as a mandrel bar.
Wrinkle
imperfections are, thus, easily generated on the inner surface of the hot-
finished steel
tube.
[0031]
The present inventor, therefore, performed rolling tests in which, as test
specimens, not only hollow shells hot-extruded but also hollow shells
subjected to a
reducing mill process with a stretch reducer and a sizer are used. The rolling
tests
varying in reduction rate of outside diameter in a reducing mill process and
varying in
reduction rates of outside diameter along with wall thickness in cold rolling
are
performed. Macroscopic structure observations for the specimens are conducted
to
investigate conditions for suppressing wrinkle imperfections.
[0032]
As described above, in the cold rolling of a seamless metallic tube with a
cold
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pilger mill, when a reduction rate of outside diameter becomes excessive in
comparison
to a reduction rate of wall thickness, strain in a circumferential direction
on the flange
regions becomes excessive. As a result, compressive stress in a
circumferential
direction becomes excessive, so that wrinkle imperfections are generated on
the inside
surface of tube and folded on the groove bottom regions to become folded
imperfections.
This process is repeated to be developed into detrimental inside surface
defects.
[0033]
As a result of the above investigation, when a hollow shell is produced by the
Mannesmann-mandrel mill process instead of the hot extrusion process, inside-
surface
wrinkle imperfections may be generated with a sizing mill (a stretch reducer
or a sizer)
at a stage of hollow shell. And when these inside-surface wrinkle
imperfections are
present, the inside wrinkle imperfections further encourage the development
thereof in
cold rolling, to which attention shall be paid..
[0034]
With respect to the method of the present invention for producing a seamless
metallic tube by cold rolling, taking into consideration that not only hot-
extruded
hollow shell but also hollow shell made by a hot sizing mill process are to be
used, it is
necessary to set a reduction rate of outside diameter to not more than one-
half of a
reduction rate of wall thickness in a cold pilger mill.
[0035]
In the method of the present invention for producing a seamless metallic tube
by cold rolling, when a sizing mill process is performed with a stretch
reducer, it is
preferable to use a hollow shell made by a hot reducing mill process under the
condition
that a reduction rate of outside diameter is not more than 77%. Or when a
sizing mill
process is performed with a sizer, it is preferable to use a hollow shell made
by a hot
reducing mill process under the condition that a reduction rate of outside
diameter is not
more than 33%.
EXAMPLES
[0036]
As test specimens, hollow shells produced by the hot extrusion
(Ugine-Sejournet extrusion) process and hollow shells produced by the
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Mannesmann-mandrel mill process (finished with a stretch reducer and a sizer)
are used.
Inside-surface quality of product was evaluated for samples that underwent
cold
working with a cold pilger mill for diameter-reducing rolling..
[0037]
(Example 1)
A 25Cr-3ONi-3Mo high-alloy steel tube having an outside diameter of 50.8 mm
and a wall thickness of 5.5 mm produced by the hot extrusion process was used
as a
hollow shell for a test specimen. The hollow shell was rolled with a cold
pilger mill to
reduce to 38.1 mm in outside diameter and to 2.4 mm in wall thickness. The
hollow
shell was fed and turned at the start point of each forward stroke. Test
conditions are
summarized below.
[0038]
Diameter of tapered mandrel bar: dm varying from 39.6 to 33.1 mm (tapered)
Feed in forward stroke: f=8.0 mm
Turn angle in forward stroke: 0=60
Hollow shell dimension: do x to=50.8 mm x 5.5 mm
Finishing dimension: d x t=38.1 mm x 2.4 mm
Ratio between diameters before and after reduction: d/do=0.75
Elongation ratio: to (do-to)/t (d-t)=2.91
Wall thickness/Outside diameter: t/d=0.063
Reduction rate of outside diameter / Reduction rate of wall thickness:
Rd/Rt=0.46<1/2
wherein
Reduction rate of outside diameter: Rd= { 1- (d/do)} x 100 (%)
Reduction rate of wall thickness: Rt= { 1- (t/to)} x 100 (%)
[0039]
Since no wrinkle imperfections were generated on the hollow shell produced
by the extrusion, generation of inside-surface defects resulting from the
wrinkle
imperfections was extremely few on a product after cold rolling, and
satisfactory
inside-surface quality was obtained.
[0040]
(Example 2)
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A 25Cr-3ONi-3Mo high-alloy steel tube having an outside diameter of 48.6 mm
and a wall thickness of 6.0 mm produced by the Mannesmann-mandrel mill process
with an inclined roll type piercing mill, a mandrel mill, and a stretch
reducer was used
as a hollow shell for a test specimen. The hollow shell was rolled with a cold
pilger
mill to reduce to 41.0 mm in outside diameter and to 2.2 mm.in wall thickness
The
reduction rate of outside diameter in the stretch reducer was not more than
77%. Test
conditions are summarized below.
[0041]
Diameter of mandrel bar: dm=36.4 mm (without taper)
Feed in forward stroke: f=8.0 mm
Turn angle in forward stroke: 0=60
Hollow shell dimension: do x to=48.6 mm x 6.0 mm
Finishing dimension: d x t=41.0 mm x 2.2 mm
Ratio between diameters before and after reduction: d/do=0.84
Elongation ratio: to (do-to)/t (d-t)=3.0
Wall thickness/Outside diameter: t/d=0.054
Reduction rate of outside diameter / Reduction rate of wall thickness:
Rd/Rt=0.25<1/2
wherein
Reduction rate of outside diameter: Rd= { 1- (d/do)} x 100 (%)
Reduction rate of wall thickness: Rt= { 1- (t/to)} x 100 (%)
[0042]
While the reduction rate of outside diameter in the stretch reducer was not
more than 77%, generation of inside wrinkle imperfections was extremely
suppressed
since a reducing mill process was performed while imparting maximum inter-
stand
tensional force by full-stretch setup. Also generation of inside-surface
defects
resulting from the wrinkle imperfections was, thus, mild on a product after
cold rolling,
and satisfactory inside-surface quality was obtained.
[0043]
(Example 3)
A 25Cr-3ONi-3Mo high-alloy steel tube having an outside diameter of 101.6
mm and a wall thickness of 7.0 mm produced by the Mannesmann-mandrel mill
process
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with an inclined roll type piercing mill, a mandrel mill, and a sizer was used
as a hollow
shell for a test specimen. The hollow shell was rolled with a cold pilger mill
to reduce
to 88.9 mm in outside diameter and to 2.8 mm in wall thickness. A reduction
rate of
outside diameter in the sizer was not more than 33%. Test conditions are
summarized
below.
[0044]
Diameter of mandrel bar: dm=83.8 mm (without taper)
Feed in forward stroke: f=10.0 mm
Turn angle in forward stroke: 0=60
Hollow shell dimension: do x to=101.6 mm x 7.0 mm
Finishing dimension: d x t=88.9 mm x 2.8 mm
Ratio between diameters before and after reduction: d/do=0.88
Elongation ratio: to (do-to)/t (d-t)=2.8
Wall thickness/Outside diameter: t/d=0.032
Reduction rate of outside diameter / Reduction rate of wall thickness:
Rd/Rt=0.21 <1 /2
wherein
Reduction rate of outside diameter: Rd= { 1- (d/do)} x 100 (%)
Reduction rate of wall thickness: Rt= { 1- (t/to)} x 100 (%)
[0045]
Since the reduction rate of outside diameter in the sizer was not more than
33%,
which was considerably small in comparison to the reduction rate of outside
diameter in
case of the stretch reducer, generation of inside-surface wrinkle
imperfections was
extremely suppressed. Generation of inside-surface defects resulting from the
wrinkle
imperfections was, thus, mile on a product after cold rolling, and
satisfactory
inside-surface quality was obtained.
INDUSTRIAL APPLICABILITY
[0046]
According to the method of the present invention for producing a seamless
metallic tube by cold rolling, it is possible to suppress generation of inside-
surface
defects resulting from inside wrinkle imperfections by improving a working
balance
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between a reduction rate Rd of outside diameter and a reduction rate Rt of
wall
thickness at the time of elongation rolling accompanying diameter reduction
working
while reducing wall thickness. It is, therefore, possible to obtain a high-
quality tube as
a product after cold rolling.
[0047]
Furthermore, when a hollow shell is produced by the Mannesmann-mandrel
mill process, it is possible to further improve the product quality after cold
rolling by
limiting a reduction rate of outside diameter in a sizing mill (a stretch
reducer or a sizer).
The present invention, thus, can be widely applied as a method for producing a
high-quality seamless metallic tube by cold rolling.
REFERENCE SIGNS LIST
[0048]
1: Hollow shell
2: Groove caliber roll
3: Tapered groove caliber
4: Tapered mandrel bar
5: Rolled tube
11, 14: Segment on groove bottom side
12, 13: Segment on flange side
21: Round billet
22: Hollow piece, hollow shell
24: Mandrel mill
25: Re-heating furnace
26: Sizing mill, stretch reducer
