Note: Descriptions are shown in the official language in which they were submitted.
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Method of Manufacturing a Seamless Steel Tube
Technical Field
This invention relates to a method of manufacturing a seamless steel tube or
pipe (hereinafter merely referred to as tube) containing at least 5% of Cr (in
this
description, unless otherwise specified, "%" means "mass %"). Specifically,
the
present invention relates to a method of manufacturing a seamless steel tube
which
effectively prevents mandrel bar withdrawal troubles after elongation rolling
of a
material being rolled which is made of a high alloy steel containing at least
5% of
Cr.
io Background Art
In the manufacture of a seamless steel tube by the Mannesmann mandrel mill
technique, first, a raw material in the form of a round billet or a square
billet is
introduced into a rotary hearth furnace and heated therein to 1200 - 1260 C.
The
raw material is then pierced with a piercer to form a hollow mother tube. A
mandrel bar is then inserted into the bore of the hollow mother tube until it
extends
beyond the bore, and the mother tube is subjected to elongation rolling using
a
mandrel mill normally including 5 to 8 stands while gripping the outer surface
of
the hollow mother tube with grooved rolls, thereby reducing the wall thickness
of
the mother tube to a predetermined value. After the mandrel bar is withdrawn
from
the mother tube having a reduced wall thickness, the mother tube is subjected
to
rolling for sizing to a predetermined outer diameter using a reducing mill to
manufacture a seamless steel tube.
Mandrel mills for elongation rolling are classified into full floating mandrel
mills which do not constrain the movement of the mandrel bar in the axial
direction,
and retained mandrel mills in which the speed of movement (retained speed) of
the
mandrel bar in the axial direction is maintained constant by holding the rear
end of
the mandrel bar with a bar retainer installed on the inlet side of the mandrel
mill.
When using a full floating mandrel mill, the speed of movement the mandrel bar
unavoidably varies, which tends to result in variations in the dimensions of
the
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resulting mother tube. Therefore, in recent years, retained mandrel mills have
been
widely used in elongation rolling.
Patent Document 1 discloses an invention in which elongation rolling is
performed using a retained mandrel mill for which the speed of movement of the
s mandrel bar is set at a value of at least 0.25 times the rolling speed at
the entrance of
each roll stand and at most 1.5 times the rolling speed at the exit thereof,
thereby
improving the quality of the inner surface of the material being rolled.
Patent Document 1: JP H08-294711 Al
Disclosure of Invention
The present inventors found that if a hollow mother tube of a high alloy steel
containing at least 5% of Cr is subjected to elongation rolling using a
retained
mandrel mill based on the invention disclosed in Patent Document 1, a
phenomenon
in which it becomes difficult to withdraw the mandrel bar from the material
being
rolled after the completion of elongation rolling (in this description, this
phenomenon will be referred to as a "mandrel bar withdrawal trouble") occurs.
Mandrel bar withdrawal troubles are a cause of a marked decrease in
productivity of elongation rolling using a retained mandrel mill. Therefore,
it is an
extremely important technical problem which must be solved in order to mass
produce seamless steel tube of a high alloy steel containing at least 5% of Cr
on an
industrial scale.
The present invention is a method of manufacturing a seamless steel tube
characterized in that a seamless steel tube of a high alloy steel containing
at least
5% of Cr is manufactured by a process comprising subjecting a material being
rolled to elongation rolling using a retained mandrel mill having a constant
speed of
movement of a mandrel bar in the axial direction in such a manner that the
speed
Vb of the mandrel bar, the speed Vi of the material being rolled on the inlet
side of
the retained mandrel mill, and the speed Ve of the material being rolled on
the exit
side of the retained mandrel mill satisfy the relationship:
0.15 s Vb/{(Vi + Ve)/2} < 0.70.
In the manufacturing method of a seamless steel tube according to the
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present invention, the above-described relationship is preferably satisfied by
controlling the value of at least one of the speed Vb of the mandrel bar, the
speed
Vi of the material being rolled on the inlet side of the retained mandrel
mill, and the
speed Ve of the material being rolled on the exit side of the retained mandrel
mill.
In accordance with the present invention, mandrel bar withdrawal troubles
after elongation rolling of a material being rolled made of a high alloy steel
containing at least 5% of Cr are effectively prevented, and the service life
of a
mandrel bar is greatly lengthened. Thus, according to the present invention,
seamless steel tube of a high alloy steel containing at least 5% of Cr can be
reliably
io mass produced on an industrial scale.
Brief Description of the Drawings
Figures 1(a) - 1(d) are explanatory views showing the behavior over time of
a mandrel bar and a material being rolled in a retained mandrel mill having 5
stands.
Figure 2 is a graph showing the relationship between the rate of overlap K
with the rate of occurrence of rolling troubles {(number of occurrences of
rolling
troubles/number of members being rolled) x 100} or with the surface roughness
Ra
of a mandrel bar after it has been used for rolling of 50 members when a
seamless
steel tube was manufactured by carrying out elongation rolling of a material
being
rolled made of 13% Cr steel while varying the rate of overlap K in the range
of 0.1
to 0.8.
List of Reference Numerals
1: retained mandrel mill
2: mandrel bar
2a: rear end
3: material being rolled
Best Mode for Carrying Out the Invention
The best mode for carrying out a method of manufacturing a seamless steel
tube according to the present invention will be explained in detail while
referring to
the accompanying drawings.
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In this embodiment, first, a raw material in the form of a round billet or a
square billet made of a high alloy steel containing at least 5% of Cr is
introduced
into a rotary hearth furnace and heated therein to 1200 - 1260 C. The raw
material
is then removed from the heating furnace, and it is subjected to piercing with
a
piercer to form a hollow mother tube.
The formation of the hollow mother tube can be performed in a conventional
manner. Such a method is known to those skilled in the art, and a further
explanation of this step will be omitted.
Next, a mandrel bar is inserted into the bore of the hollow mother tube until
io it extends beyond the bore, and the wall thickness of the hollow mother
tube is
reduced to a predetermined value by subjecting the hollow mother tube to
elongation rolling using a retained mandrel mill normally comprising 5 to 8
stands
(5 stands in this embodiment) while gripping the outer surface of the hollow
mother
tube with grooved rolls in each stand.
is A bar retainer (not shown) for holding the rear end of the mandrel bar is
installed on the inlet side of the retained mandrel mill for elongation
rolling.
During elongation rolling, the rear end of the mandrel bar is held by this bar
retainer. As a result, the speed of movement Vb of the mandrel bar in the
axial
direction is maintained constant during elongation rolling.
20 In this embodiment, elongation rolling is carried out on a material being
rolled such that the speed of movement Vb of the mandrel bar, the speed Vi of
the
material being rolled on the inlet side of the retained mandrel mill, and the
speed Ve
of the material being rolled on the exit side of the retained mandrel mill
during
elongation rolling satisfy the following relationship:
25 0.15 <_ Vb/{(Vi + Ve)/2} < 0.70.
The reason why elongation rolling is carried out while satisfying this
relationship
will be explained below.
The present inventors found that if the retained speed of movement Vb of the
mandrel bar, the speed Vi of the material being rolled on the inlet side of
the
3o retained mandrel mill, and the speed Ve of the material being rolled on the
exit side
of the retained mandrel mill satisfy certain conditions during elongation
rolling, the
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length of the region of the material being rolled after the completion of
elongation
rolling (the material being rolled at this stage also being referred to as a
"shell" in
this description) contacting the mandrel bar (this region referred to in this
description as the "overlapping region") can be set at a suitable dimension,
thereby
5 making it possible to effectively prevent mandrel bar withdrawal troubles
and
minimize a decrease in the service life of the mandrel bar.
Namely, in the overlapping region of the material being rolled, a greater
decrease in the temperature occurs due to extraction of heat through the
contacting
mandrel bar. As a result, the overlapping region undergoes thermal contraction
so
io that its outer diameter decreases. This causes mandrel bar withdrawal
troubles.
A procedure of calculating the ratio of the length of the overlapping region
to the length of the shell (referred to in this description as the "rate of
overlap") will
be explained.
Figures 1(a) - 1(d) are explanatory views showing the behavior over time of
a mandrel bar 2 and a material being rolled 3 in a retained mandrel mill 1
which
includes 5 stands (lstd - 5std). In order to simplify Figure 1 and for ease of
explanation, the bar retainer which is provided for holding the rear end 2a of
the
mandrel bar 2 has been omitted from Figure 1.
As shown in Figure 1(a), the material being rolled 3 prior to elongation
z.o rolling using the retained mandrel mill 1(the material being rolled at
this stage also
referred to below as the "mother tube") has a length Si. As shown in Figure
1(d),
the material being rolled 3 after elongation rolling using the retained
mandrel mill 1
(the shell) has a length Se. In Figures 1(a) - 1(d), the rate of elongation
achieved by
the retained mandrel mill is EL (EL = Se/Si), the speed of the material being
rolled
3 on the inlet side of the retained mandrel mill 1(the speed of the mother
pipe) is
Vi, the speed of the material being rolled 3 on the exit side of the retained
mandrel
mill 1(the speed of the shell) is Ve (Ve = Vi x EL), and the speed of movement
of
the mandrel bar 2 is Vb.
The average rolling speed of the retained mandrel mill 1 in the time from the
state shown in Figure 1(c) in which the leading front end of the material
being
rolled 3 has just been gripped by the grooved rolls of the fifth stand 5std to
the state
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shown in Figure 1(d) in which the rear end of the material being rolled 3 has
passed
through the grooved rolls of the fifth stand 5std is (Vi + Ve)/2. In addition,
the
length of rolling time from the state shown in Figure 1(c) to the state shown
in
Figure 1(d) is Se/{(Vi + Ve)/2}.
Accordingly, the length AL by which the mandrel bar 2 advances during the
time of elongation rolling from the state shown in Figure 1(c) to the state
shown in
Figure 1(d) is expressed by AL = Vb x Se/{(Vi + Ve)/2}.
In Figure 1(c), if the length by which the mandrel bar 2 projects beyond the
front end of the material being rolled 3 (the projecting length) is L5, then
the length
io of the overlapping region L in Figure 1(d) is expressed by L = L5 + AL. If
that
projecting length L5 is empirically approximated as L5 z 0, then Lz AL = Vb x
Se/{(Vi + Ve)/2}. Accordingly, the rate of overlap K is expressed by the
following
Equation 2.
K = L/Se z Vb x Se/{(Vi + Ve)/2}/Se
= Vb/{(Vi + Ve)/2} ..... (2)
The present inventors found that if the rate of overlap K given by the
approximate equation K = Vb/{(Vi + Ve)/2} becomes large, the temperature in
the
overlapping region decreases at an early stage, and to this extent, withdrawal
troubles of the mandrel bar 2 more easily occur, while if the rate of overlap
K
2o becomes small, an early decrease in temperature of the overlapping region
is
suppressed and withdrawal troubles of the mandrel bar 2 are prevented.
However, if the rate of overlap K is too small, the mandrel bar 2 tends to
suffer from wear, surface roughening, or even cracking or galling to the shell
3, and
the service life of the mandrel bar 2 decreases.
Namely, if the rate of overlap K is too small, elongation rolling takes place
in
a narrow region of the mandrel bar 2, and the amount of work per unit length
(or
unit area) of the mandrel bar 2 locally increases. Therefore, wear, surface
roughening, or the like of the mandrel bar 2 easily takes place.
As stated below, the rate of overlap K is inversely proportional to the
3o average rolling speed in the retained mandrel mill 1. Therefore, if the
rate of
overlap K becomes too small due to making the difference in speed between the
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average rolling speed and the speed of movement of the mandrel bar 2 large,
friction between the mandrel bar 2 and the material being rolled 3 becomes
large.
As a result, it becomes easy for wear, surface roughening, and similar
problems to
occur in the mandrel bar 2.
Thus, regarding the rate of overlap K, there exist a preferred upper limit in
order to achieve ease of withdrawal of the mandrel bar 2 and a preferred lower
limit
in order to suppress a decrease in the service life of the mandrel bar 2.
Figure 2 is a graph showing the relationship between the rate of overlap K
and the rate of occurrence of rolling troubles {= number of occurrences of
mandrel
io bar withdrawal troubles/number of members being rolled) x 100} or the
surface
roughness Ra of the mandrel bar after rolling 50 members being rolled using
the
same mandrel bar. In this description, "rolling troubles" means troubles in
withdrawing a mandrel bar from a material being rolled.
As shown in the graph of Figure 2, if the rate of overlap K becomes larger
than 0.70, the ease of withdrawal of the mandrel bar worsens. On the other
hand, if
the rate of overlap K becomes smaller than 0.15, the surface roughness of the
mandrel bar after elongation rolling increases and the service life of the
mandrel bar
decreases.
As shown in the graph of Figure 2, when the rate of overlap K is at most
0.70, withdrawal troubles of the mandrel bar 2 occurring after elongation
rolling of
the material being rolled 3 made of a high alloy steel containing at least 5%
of Cr
can be effectively suppressed, and when the rate of overlap K is at least
0.15, a
decrease in the service life of the mandrel bar 2 can be suppressed.
From the same standpoint, the upper limit on the rate of overlap K is
preferably 0.60 and more preferably 0.50. The lower limit on the rate of
overlap K
is preferably 0.2 and more preferably 0.3.
The rate of overlap K can be made to be in the range of at least 0.15 to at
most 0.70 by controlling the value of at least one of the speed of movement Vb
of
the mandrel bar 2 which is held at its rear end by the bar retainer of the
retained
mandrel mill 1, the speed Vi of the material being rolled 3 on the inlet side
of the
retained mandrel mill 1, and the speed Ve of the material being rolled 3 on
the exit
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side of the retained mandrel mill 1. The simplest way to control the rate of
overlap
K to be in the range of at least 0.15 to at most 0.70 is to maintain the
values of the
speed Vi of the material being rolled 3, the rate of elongation, and the speed
Ve of
the material being rolled 3 constant and to suitably vary the speed of
movement Vb
s of the mandrel bar 2, as described in Examples 1 and 2 given below.
In this embodiment, a mandrel bar 2 is withdrawn from a material being
rolled 3 which has a decreased wall thickness by elongation rolling performed
in
this manner. According to this embodiment, since elongation rolling is carried
out
with the rate of overlap K made at least 0.15 to at most 0.70 by controlling
the value
io of at least one of the speed of movement Vb of the mandrel bar 2, the speed
Vi of
the material being rolled 3, and the speed Ve of the material being rolled 3,
withdrawal troubles of the mandrel bar 2 occurring after elongation rolling of
the
material being rolled 3 can be effectively suppressed, and the occurrence of
damage
to the mandrel bar can be suppressed, thereby achieving an extended service
life of
15 the mandrel bar.
In this embodiment, the material being rolled 3 from which the mandrel bar 2
has been withdrawn without occurrence of withdrawal troubles is then subjected
to
sizing into a predetermined outer diameter using a reducing mill. In this
manner,
according to this embodiment, a seamless steel tube of a high alloy steel
containing
2o at least 5% of Cr can be reliably mass produced on an industrial scale.
Example 1
The present invention will be explained more specifically while referring to
examples.
A mother tube measuring 350 mm in outer diameter, 27.55 mm in wall
25 thickness, and 9849 mm in length and having a steel composition containing
13%
Cr and 6% Ni was subjected to elongation rolling using a retained mandrel mill
to
obtain a shell having an outer diameter of 295 mm, a wall thickness of 12.55
mm,
and a length of 24685 mm.
Table 1 shows the speed Vi (mm/second) of the mother tube on the inlet side
30 of the retained mandrel mill 1 in the elongation rolling, the rate of
elongation EL
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(EL = Se/Si) in the retained mandrel mill, the speed Ve (Ve = Vi x EL;
mm/second)
of the shell on the exit side of the retained mandrel mill, and the speed of
movement
Vb (mm/second) of the mandrel bar together with the rate of overlap K = Vb/{Vi
+
Ve)/2}, the ease of withdrawal of the mandrel bar, and the state of damage to
the
bar.
In this example, the speed Vi (mm/second) of the mother tube, the rate of
elongation EL, and the speed Ve (mm/second) of the shell were set to constant
values for each condition in Table 1, and the speed of movement Vb of the
mandrel
bar was varied in the range of 500 - 2600 mm/second. Those runs having a rate
of
io overlap K of at least 0.15 to at most 0.70 were examples of this invention,
and those
outside of this range were comparative examples.
The "ease of withdrawal" in Table 1 is indicated by "0" when the rate of
occurrence of rolling troubles was less than 0.1 %, it is indicated by "A"
when the
rate of occurrence of rolling troubles was at least 0.1 % to at most 1.0%, and
it is
indicated by "X" when the rate of occurrence of rolling troubles exceeded 1%.
The "bar damage" in Table 1 is indicated by "0" when the surface roughness
Ra of the mandrel bar after elongation rolling of 50 members using the same
mandrel bar was less than 3 m, it is indicated by "A" when it was at least
3ym to
at most 7ym, and it is indicated by X when it exceeded 7 /tm.
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Table 1
Vi EL Ve Vb Vb/((Vi+Ve)/2) Ease of Bar Remarks
withdrawal damage
2000 2.51 5020 2600 0.74 X 0 Comparative
Example
2000 2.51 5020 2400 0.68 A 0 This
invention
5 2000 2.51 5020 2000 0.57 0 0 This
invention
2000 2.51 5020 1000 0.28 0 0 This
invention
2000 2.51 5020 800 0.23 0 A This
invention
2000 2.51 5020 500 0.14 0 X Comparative
Exam le
As shown in Table 1, if the rate of overlap K is in the range of at least 0.15
to
io at most 0.70, good results were obtained with respect to both ease of
withdrawal
and bar damage whereas when the rate of overlap K did not satisfy this range,
the
results were unsatisfactory for either the ease of withdrawal or bar damage.
Example 2
A mother tube measuring 337 mm in outer diameter, 41.03 mm in wall
thickness, and 5473 mm in length and having a steel composition containing 18%
Cr and 8% Ni was subjected to elongation rolling using a retained mandrel mill
to
obtain a shell with an outer diameter of 295 mm, a wall thickness of 31.03 mm,
and
a length of 8115 mm.
Table 2 shows the values of the same parameters as those set forth in above-
2o described Table 1. In this example as well, the speed Vi (mm./second) of
the mother
tube, the rate of elongation EL, and the speed Ve of the shell (mm/second)
were set
to the constant values as shown in Table 2 for each condition, but in contrast
to
Example 1, the speed of movement Vb of the mandrel bar was varied in the range
of 250 - 1400 mm. In the same manner as in Example 1, the test results are
compiled in Table 2.
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Table 2
Vi EL Ve Vb Vb/((Vi+Ve)/2) Ease of Bar Remarks
withdrawal damage
1500 1.48 2220 1400 0.75 X 0 Comparative
Example
1500 1.48 2220 1250 0.67 A 0 This
invention
1500 1.48 2220 1000 0.54 0 0 This
invention
1500 1.48 2220 800 0.43 0 0 This
invention
1500 1.48 2220 400 0.22 0 A This
invention
1500 1.48 2220 250 0.13 0 X Comparative
Exam le
As shown in Table 2, when the rate of overlap K is in the range of at least
io 0.15 to at most 0.70, good results are obtained with respect to the ease of
withdrawal and bar damage, whereas if the rate of overlap does not satisfy
this
range, the results for either the ease of withdrawal or bar damage are
unsatisfactory.
