Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method for
preventing defects such as holes from being formed on
surfaces of blank tubes manufactured by mandrel mill,
wherein the holes are produced during the manufacturing of
the seamless steel tubes, and more particularly during the
manufacturing seamless steel tubes made of a high alloy
steel such as stainless steel.
Description of the Related Art
In one method of manufacturing seamless steel tubes
using a mandrel mill, a heated billet is pierced by a
piercing machine, and a finishing rolling process of the
billet is applied by rolling the inside of the tube. As is
shown in FIG. 4, the mandrel mill employed in this
circumstance normally comprises a plurality of--from five
to eight--roll stands 1 configuring a roll groove having a
plurality of rolls 2 and 2' in alternate pairs arranged
horizontally and vertically.
These plurality of grooved roll stands are disposed
orthogonally about a rolling shaft, and a mandrel bar 3 is
disposed within a roll groove formed by roll stands 1. The
inner surface of the blank tube 4 is rolled by the mandrel
bar 3.
In the manufacturing of seamless steel tubes by
mandrel mill, holes are often formed on surfaces of the
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blank tubes 4 during the rolling process causing
unfavorable defects in the mandrel mill manufacturing.
(Hereinafter, this defect is referred to as the "hole
defect" and reference numeral 6 indicates the portions with
the "hole defect.~)
Conventionally, it was thought that the "hole defect"
was caused by the following reasons. As is shown in FIG.
5, when a blank tube made of a high alloy steel, such as
stainless steel, is rolled by the mandrel mill at a
temperature ranging between 950C and 1050C which is the
normal rolling temperature range for common blank tubes,
the hot-working characteristics of the blank tube
deteriorates.
When a blank tube having an inferior hot-working
characteristic is rolled by the mandrel mill, a
longitudinal tensile force is exerted only on a flange
portion 5 shown in FIG. 2 of the blank tube receiving no
reduction, which eventually causes a rupture or "hole
defect" in the tube. These defects tend to occur with much
greater frequency in steel tubes having a thin wall
thickness.
Various method for preventing the "hole defect" have
been proposed.
One of the common methods for preventing the ~hole
defect" is disclosed in Japanese Patent Laid-Open
Publication No.58-224155, wherein a method of improving the
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hot-working deformability of the rolling tube materials is
proposed.
There is also proposed, in Japanese Laid-Open
Publication No.63-84720, a method of reducing the rolling
reduction of one stand where the "hole defect" occurs in
the mandrel mill and dispersing the reduction load to the
remaining stands, and of reducing the wall thickness of
blank tubes at the entrance of the mandrel mill so as to
reduce the rolling reduction of each stand of the mill.
The method disclosed in Japanese Patent Laid-Open
Publication No.58-22455, however, cannot provide a
sufficient hot-working deformability at rolling
temperatures in the range of 950C to 1050C in the mandrel
mill.
Although the method proposed in Japanese Patent Laid-
Open Publication No.63-84720 can prevent the "hole defect",
it may not be used on blank tubes with a thin wall
thickness for the following reason:
If after reduction loads are dispersed to each stand,
and there remains a stand in which the rolling load exceeds
the reference value, the wall thickness of the blank tube
is reduced at the entrance of the mandrel mill so as to
reduce the rolling reduction of each stand of the mill.
However, rolling reduction on the blank tube by the
piercing machine is limited and therefore the wall
thickness of the blank tube cannot be reduced below a lower
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limit. In the above situation, it is difficult to roll
blank tubes with a thin wall thickness.
Accordingly, an object of the present invention is to
overcome the above described problems of the mandrel mill
and prevent the "hole defect."
SUMMARY OF THE INVENTION
The present invention is directed toward a roll groove
design of a row of stands consecutively disposed in the
mandrel mill to prevent the "hole defect" produced during
the rolling process.
According to the present invention, there is provided
a mandrel mill for rolling seamless steel tubes which
comprises a plurality of roll stands each having a pair of
rolls defining a roll groove therebetween, the rolls being
arranged such that the axis of the rolls of each roll stand
is orthogonal to the axis of the rolls of the adjacent roll
stand, and a mandrel bar disposed in the roll groove
configured by the roll stands wherein the ratio between the
radius of curvature of groove bottom and a distance between
the groove bottom of the roll groove of a first stand
ranges from 0.46 to 0.54.
According to the present invention there is provided
a mandrel mill for rolling seamless steel tubes, comprising
a plurality of roll stands each having a pair of rolls
defining a roll groove therebetween, the rolls being
arranged such that the axis of the rolls of each roll stand
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is orthogonal to the axis of the rolls of the adjacent roll
stand, and a mandrel bar disposed in the roll groove
configured by the roll stands wherein the ratio between
the radius of curvature of groove bottom and a distance
between the groove bottom of the roll groove of a second
stand ranges from 0.48 to 0.52.
According to the present invention there is also
provided a mandrel mill for rolling seamless steel tubes,
comprising a plurality of roll stands each having a pair of
rolls defining a roll groove therebetween, the rolls being
arranged such that the axis of the rolls of each roll stand
is orthogonal to the axis of the rolls of the adjacent roll
stands, and a mandrel bar disposed in the roll groove
configured by the roll stands wherein the ratio between the
radius of curvature of groove bottom and a distance between
the groove bottom of the roll groove of the first stand
ranges from 0.46 to 0.54, and that of the second stand
ranges from 0.48 to 0.52.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view of a roll groove
defined by a pair of forming rolls according to the present
invention.
FIG. 2 illustrates a rolling reduction which becomes
smaller at the bottom center of a groove and larger at both
sides of the center in the third stand of a mandrel mill.
FIG. 3 illustrates the relation between a ratio of the
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groove bottom radius of curvature and a distance of the
groove bottom of a pair of rolls and a ratio of defect
occurrence.
FIG. 4 is a schematic drawing of a mandrel mill.
FIG. 5 is a diagram illustrating hot working
characteristics.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As a result of the survey for the cause of the "hole
defect" in a mandrel mill rolling, the present invention
has been brought to discover a new mechanism of hole
occurrence which is not disclosed in the prior art except
as described above.
In a reduced portion of roll groove bottom, when a
rolling reduction of bottom center groove is smaller than
that of both sides of the center groove, the lack of
material on the bottom center groove causes the necking
phenomenon during the process of a rolling. Under this
circumstance, the tube wall thickness becomes thin which,
in an extreme case, will produce a hole.
FIG. 2 illustrates an embodiment of the present
invention in which the rolling reduction of both sides of
the bottom center groove portion is greater than that of
the center groove. In FIG. 2, reference numerals 2,2'
indicate a roll groove defined by a pair of rolls,
reference numeral 3 indicates a mandrel bar, and reference
numeral 4 indicates a blank tube (a portion filled by slant
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bars in the FIG. 2).
Reference numeral 4a indicates the thinnest portion of
the blank tube wall thickness, which is also a roll bottom
center groove portion in the first stand (the first stand
refers to stand No.l in FIG. 2). Reference numeral 4b
indicates both sides of the bottom center groove portion
4a. Reference numerals 11, 12, and 13 indicate roll
grooves of the first, the second, and the third stands,
respectively.
For example, as shown in FIG. 2(a), the shape of the
roll groove 11 of the first stand is normally elliptical
and the mandrel bar 3 substantially round. The wall
thickness of the tube 4 in the circumferential direction at
the exit area of the first stand becomes thinnest in the
lS roll bottom center groove portion 4a, and becomes thicker
as you move away from the bottom center groove. As shown
in FIG. 2(b), in the rolling process at the second stand,
the wall-thickness distribution of the thinnest tube wall
of the bottom center portion 4a and both sides 4b from the
center portion rolled by the first stand can be maintained
even after the tube passes the second stand because the
thinner portion 4a and thicker portions 4b do not suffer
reduction from the roll groove 12 of the second stand.
As shown in FIG. 2(c), in the third stand, the roll
groove 13 is substantially round so as to provide a uniform
distribution of the tube-wall-thickness in the
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circumferential direction. The distribution of the wall
thickness at the exit area of the first stand shows that
the bottom center groove 4a is thinner and both sides 4b,
from the center groove, are thicker. In the third stand,
the roll reductions of both sides 4b are greater than that
of the center portion 4a in the third stand. Thus, the
present invention has successfully investigated that the
lack of the material along the bottom center groove portion
4a causes the necking phenomenon to reduce the tube-wall
thickness, which eventually causes the "hole defect". The
phenomenon that occurs in the third stand will also occur
in the fourth stand. Due to the elliptical shape of the
roll groove of the second stand, the wall thickness in the
circumferential direction is thinnest in the tube bottom
groove area and as the area goes away from the bottom
center groove, the thickness increases in the second stand.
The groove bottom portion rolled by the second stand does
not suffer reduction in the third stand, in which the wall-
thickness distribution of these portion can be maintained
after the tube passes the third stand. In the fourth
stand, the roll groove is substantially round to provide a
uniform distribution of the tube-wall-thickness in the
circumferential direction. The distribution of wall
thickness at the exit area of the second stand shows that
the wall thickness of the groove bottom is thinner and that
of both sides from the groove bottom is thicker. The
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rolling reduction of both sides from the groove bottom
center portion is greater than that of the groove bottom
portion, which causes the "hole defect" due to the same
reason as described above. To obtain the uniform wall
thickness in the circumferential direction of the finished
tube, the roll groove in the finishing stands of the
mandrel mill is designed such that the groove bottom
portion is substantially round. In normal mandrel mills,
the above described finishing stands are disposed between
the fourth stand and the sixth or eighth stands. In the
first stand and the third stand, when even one roll-groove
configuration has an elliptical shape, the rolling
reduction of both sides from the groove bottom center
portion has to be greater than that of the groove bottom
portion to unify the wall thickness distribution in either
one of the succeeding stands located after the above
described stand having the elliptical-shaped roll groove.
Thus, the following measures have been taken to unify
the rolling-reduction distribution in the circumferential
direction of the groove bottom portions where the rolling
force is applied.
The present invention has proposed a mandrel mill
having a roll groove in which, as is shown in the
representing illustration of FIG. 1, the groove bottom
radius of curvature R1 in the first stand ranges from 0.46
to 0.54 of the groove bottom distance B of the pair of
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rolls, and the groove bottom radius of curvature R1 in the
second stand ranges from 0.48 to 0.52 of the groove bottom
distance B of the pair of rolls.
According to the present invention, "hole defect"
which is caused by the non-uniformity of the groove bottom
draft, a cause which has been overlooked by the prior art,
can be prevented by providing an upper limit and a lower
limit of the groove bottom radius of curvature in the front
stands in the mandrel mill.
Namely, in the mandrel mill which has a tendency to
cause the "hole defect'~ in the groove bottom of the third
stand, the rolling reduction in the circumferential
direction of the groove bottom in the third stand can be
unified by designing the groove bottom radius of curvature
of the roll groove in the first stand to range from 0.46 to
0.54 of the distance between the groove bottom of the pair
of rolls in the first stand. Thus, "hole defect" can be
practically eliminated. Likewise, in the mandrel mill
which has a tendency to cause the "hole defect" in the
groove bottom of the fourth stand, the rolling reduction in
the circumferential direction of the groove bottom in the
fourth stand can be unified by designing the groove bottom
radius of curvature of the roll groove in the second stand
to range from 0.48 to 0.52 of the distance between the
groove bottom of the pair of rolls in the second stand.
Thus, the occurrence of a "hole defect" can be practically
11
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eliminated.
Meanwhile, in the mandrel mill, it depends on the
characteristics of the mill or the reduction distribution
of each stand and the like whether a "hole defect'~ occurs
in either one or both of the third and fourth stands.
Based on the testing results exhibited in FIG. 3, the
ratios of the groove bottom radius of curvature Rl and the
distance B between the groove bottom of the pair of rolls
are determined as ranging from 0.46 to 0.54 in the first
stand and from 0.48 to 0.52 in the second stand.
Embodiment 1
Rolling conditions and results of a mandrel mill using
a tube material of a plain carbon steel according to the
present invention are exhibited in Tables 1 and 2
respectively. In the rolling conditions of the present
invention, the ratios between the groove bottom radius of
curvature of the roll groove and the distance between the
groove bottom formed by the pair of rolls in the first and
second stands are set as 0.54 and 0.52 respectively. On
the other hand, in the rolling conditions of the prior art,
the ratios between the groove bottom radius of curvature of
the roll groove and the distance between the groove bottom
formed by the pair of rolls in the first and second stands
are set as 0.6 and 0.55 respectively.
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Table 1
The Present Invention The Prior Art
stand No- G.B.R.C.*l D.G.B.*2 G.B.R.C. D.G.B.
Rl Rl B
1 99.1 183.5110.1 183.5
2 93.1 179.098.5 179.0
3 89.2 176.889.2 176.8
4 87.8 175.687.8 175.6
87.3 174.587.3 174.5
6 87.3 174.587.3 174.5
7 87.3 174.587.3 174.5
8 90.0 180.090.0 180.0
(*l G.B.R.C.: Groove Bottom Radius of Curvature)
( *2 D.G.B.: Distance between the Groove Bottom)
Diameter of employed mandrel bar: 166.5 mm
Rolling material: Plain carbon steel
Dimension at the mill exit: Outer diameter 180 mm,
Wall thickness 4 mm,
Length 24 m
Table 2
Rolling by the Rolling by the
Present Invention Prior Art
Number of Tubes None out of 200 44 out of 200
Having "Hole tubes tubes
Defect"
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According to the present invention, it is understood
that plain carbon steel with dimension of the outer
diameter of 180 mm and wall thickness of 4 mm at the exit
of the mandrel mill can be manufactured without "hole
defect."
Embodiment 2
Rolling conditions and results of a mandrel mill using
a tube material of 13 % Cr-steel according to the present
invention are exhibited in Tables 3 and 4, respectively.
In the rolling conditions of the present invention, the
ratios between the groove bottom radius of curvature of the
roll groove and the distance between the groove bottom
formed by the pair of rolls in the first and second stands
are set as 0.54 and 0.52, respectively. On the other hand,
in the rolling conditions of the prior art, the ratios
between the groove bottom radius of curvature of the roll
groove and the distance between the groove bottom formed by
the pair of rolls in the first and second stands are set as
0.6 and 0.55, respectively.
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Table 3
The Present Invention The Prior Art
stand No- G.B.R.C-*1 D.G.B.*2 G.B.R.C. D.G.B.
R1 R1 B
1 99.1 183.5110.1 183.5
2 93.1 179.098.5 179.0
3 89.2 176.889.2 176.8
4 87.8 175.687.8 175.6
87.3 174.587.3 174.5
6 87.3 174.587.3 174.5
7 87.3 174.587.3 174.5
8 90.0 180.090.0 180.0
(*l G.B.R.C.: Groove Bottom Radius of Curvature)
( *2 D.G.B.: Distance between the Groove Bottom)
Diameter of employed mandrel bar: 164.5 mm
Rolling material: 13% Cr-steel
Dimension at the mill exit: Outer diameter 180 mm,
Wall thickness 4 mm,
Length 24 m
Table 4
Rolling by the Rolling by the
Present Invention Prior Art
Number of Tubes None out of 200 30 out of 200
Having "Hole tubes tubes
Defect"
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According to the present invention, it is understood
that a 13 % Cr-steel with dimension of the outer diameter
of 180 mm and wall thickness of 4 mm at the exit of the
mandrel mill can be manufactured without a "hole defect."
Therefore, to carry out the present invention, it is
not necessary to provide a new device for an existing
mandrel mill.
According to the present invention, a "hole defect",
which conventionally has occurred at the groove bottom
center portion in a roll groove, can be successfully
prevented by designing the groove bottom radius of
curvature of the roll groove at the first and second stands
in the mandrel mill. Thus, a remarkable effect is obtained
for preventing a "hole defect~ in mandrel mill rolling
especially for tubes with thin wall-thickness and for a
high alloy steel having an inferior deformability.
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