Note: Descriptions are shown in the official language in which they were submitted.
SO
The present invention relates to a duplex
stainless steel seamless pipe and a method for producing
the same, and more particularly, relates to a method
capable of producing inexpensively a duplex stainless
05 steel seamless pipe in high productivity and yield.
Duplex stainless steel represented by
JIS SUS 329Jl has high resistances against corrosion,
stress corrosion cracking and grooving corrosion, and a
high weldability, and has been noticed as a material
lo for piping in various chemical plants, for pipe for oil
well, for pipe for subterranean, for line pipe and the
like.
Stainless steel seamless pipe is generally
produced by plug mill process, mandrel process, pilgar
mill process, Ugine Séjournet process, Ehrhardt-Verfahren
mill process and the like.
Duplex stainless steel has a ferrite-austenite
duplex texture, and is poor in hot workability.
Therefore, hot extrusion methods such as Ugine-Séjournet
process and the like have hitherto been used in the
production of seamless pipe from stainless steel having
a poor hot workability.
However, in the direct piercing method by a
hot extrusion method, when the length of a billet
becomes as large as 5-7 times the diameter thereof
during the piercing, the deviation of the wall thickness
of the billet becomes large, and hence it is difficult
to produce a continuous length pipe. In order to solve
,., . ~ ~ .. j . ~
o
this problem, a continuous length pipe is produced by a
so-called expansion method, wherein a hole has previously
been bored through a billet at its center by the machin-
ing, and the bore is expanded. ~owever, even in this
05 expansion method, the length of a billet is limited to
not more than about 15 times the diameter of the billet.
In the Ugine Séjournet process, a vitreous
lubricant is used, and therefore a step for peeling the
vitreous lubricant from a rolled hollow piece is
necessary. This step is a troublesome step.
The object of the present invention is to
solve the above described drawbacks of the conventional
technics and to provide a duplex stainless steel seamless
pipe having a continuous length, and a method for
producing the pipe in a high productivity.
The above described object of the present
invention can be attained by the following three aspects
of this invention.
The first aspect of the present invention
lies in a duplex stainless steel seamless pipe produced
by a plug-mill process, and havlng a composition
consisting of~ in % by weight, C: not more than 0.03%,
Si: not more than 2.00%, Mn: not more than 2.00%,
Cr: 20.0-30.0%, Ni: 1.0-9.0%, Cu: not more than 3.0%,
Mo: 0.5-5.0%, N: 0.05-0.30%, Al: 0.01-0.10%, S: not
more than 0.004%, P: not more than 0.030%, at least one
of Ca: (l-lO)x[%S] and B: 0.0005-0.010%; and the
remainder being Fe and incidental impurities.
5C~
The second aspect o~ the present invention
lies in a method for producing a duplex stainless steel
seamless pipe, which has the same composition as that
defined in the first aspect of the present invention,
05 by a plug mill process comprising a piercing step by
means of a piercing mill, a cross rolling step by means
of a cross rolling mill, a rolling step by means of a
plug mill, a reeling step by means of a reeler and an
outer diameter reducing step by means of a sizer,
lo wherein a billet is pierced in the piercing step under
a condition that the billet is kept at a temperature of
1,200-1,350C in its center portion and at a -temperature
of 1,100-1,350C in its outer surface portion; the
cross rolling of the resulting hollow piece is finished
at a temperature not lower than 1,100C in the cross
rolling step; and the cross rolled hollow piece is
worked in the plug mill rolling step, in the reeling
step and in the outer diameter reducing step under
conditions that the equivalent strains which will be
caused in the hollow piece in the plug mill rolling
step, in the reeling step and in the outer diameter
reducing step are keep to not higher than 0.5, not
higher than 0.4 and not higher than 0.2 respectively,
said equivalent strains being calculated by the following
formula (1) and that the total amount of the equivalent
strain which will be caused in the hollow piece in the
above described plug mill rolling step and in the
reeling step is kept to not higher than 0.6, said total
amount of the equivalent strain being calculated by the
following formula (2):
- 2 ~~~-~- -- ............................... (1)
wherein ~t = Qn (t/to)
~B Qn ~D/Do)o, Do : average wall thickness and average outer
diameter of the hollow piece before rolling
in each step, respectively, D : average wall thickness and average outer
diameter of the hollow piece after rolling in
each step, respectively; and
~ ( r t2 + ~ I t 'B + ~1B2) ...... (2)
wherein t Qn(tp/tp ~ + Qn(tR/tR )
B Qn(Dp/Dp ) + Qn(DR/DR )
p, tp : average wall thicknesses of the hollow piece
after and before rolling by the plug mill,
respectivelyR~ tR : average wall thicknesses of the hollow piece
after and before rolling by the reeler,
respectively , D : average outer diameters of the hollow piece
after and before rolling by the plug mill,
respectively
-- 5 --
s~
R, DR : average outer diameters of the hollow piece
after and before rolling by the reeler,
respectively
The third aspect of the present invention
lies in a method for producing a duplex stainless steel
seamless pipe, which has the same composition as that
defined in the first aspect of the present invention,
by a plug mill process, wherein a billet is pierced in
the piercing step under a eondition that the billet is
kept at a temperature of l,200-1,350C in its center
portion and at a temperature of 1,100-1,350C in its
outer surface portion; the cross rolling of the
resulting hollow piece is finished at a temperature not
lower than l,100C in the cross rolling step; the cross
rolled hollow piece is worked in the plug mill rolling
step and in the reeling step under a condition that the
equivalent strains which will be caused in the hollow
piece in the plug mill rolling step and in the reeling
step and are calculated by the above described
formula (1) are kept to not higher than 0.5 and not
higher than 0.4, respectively; the reeled hollow piece
is reheated at a temperature of 850-1,200C for a
period of not longer than 10 minutes; and the reheated
hollow piece is worked in the outer diameter reducing
step under a condition that the equivalent strain which
will be caused in the hollow piece in the step and is
calculated by the above described formula (1) is kept
s~
to not higher than 0.4.
Fig. 1 is a diagram iLlustrating a working
schedule of a test piece in a hot tensile test in the
present invention; and
05 Fig. 2 is a graph illustrating a relation
between the stretching temperature and the hot work-
ability of a test piece measured by means of a hot
tensile tester in a case where a prestrain is applied
to the test piece at a given temperature and then the
lo test piece is stretched at a temperature lower than the
prestrained temperature.
The inventors have investigated the chemical
composition and rolling condition of a duplex stainless
steel in order to produce a duplex stainless steel
seampless pipe through the plug mill process having
various merits, and fo~md out that the pipe can be
produced without forming surface defect and failure of
the top and bottom ends by a proper combination of
proper chemical composition and rolling condition.
The plug mill process is a process for
producing a seamless steel pipe through the following
steps: a piercing mill (which may be called as the
first piercer) - a cross rolling mill ~which may be
called as the second piercer) - a plug mill - a reeler
(which may be called as a reeling mill) - a sizer.
The plug mill process is higher in productivity and
yield and is less expensive in the production of seamless
steel pipes than the Ugine Séjournet process.
~ 7
so
An explanation will be made with respect to
the reason of the limitation of the chemical composition
of the duplex stainless steel of the present invention.
C:
05 C is an element incidentally contained in
steel. When the C content in a steel exceeds 0.03%,
the steel is poor in corrosion resistance and in
resistance against grain boundary corrosion. Therefore,
the C content is limited to not more than 0.03%.
Si: ,
Si is an element used as a deoxidizer at the
melting of steel. However, when the Si content in a
steel exceeds 2.0%, the ~ phase is developed, and the
steel is very poor in cold workability. Therefore, the
Si content is limited to not more than 2.0%.
Mn:
Mn is added to steel in order to improve its
strength. However~ a steel containing more than 2.0%
of Mn is poor in hot workability. Therefore, the
Mn content is limited to not more than 2.0%.
Cr:
Cr is an indispensable element in order to
improve the corrosion resistance of steel and to form a
duplex texture consisting of austenite and ferrite in
steel. When the Cr content in a steel is less than
20%, the effect of Cr is poor in giving to the steel
resistances against pitting corrosion and crevice
corrosion. The pitting corrosion resistance increases
s~
corresponding to the increase of the Cr content.
However, when the Cr content in a steel exceeds 30.0%,
the u phase is apt to be easily precipitated in the
steel and the steel is poor in toughness. Therefore,
05 the Cr content is limited to 20.0-30.0%.
Ni:
Ni is an indispensable element for improving
the resistance against general corrosion of steel and
for forming a duplex texture in steel. However, less
than 1.0% of Ni content in a steel can not give to the
steel a satisfactorily high resistance against corrosion.
While, when the Ni content in a steel exceeds 9.0%, the
effect of Ni is saturated. Moreover, Ni is an expensive
element. Accordingly, the Ni content is limited within
the range of 1.0-9.0%.
Cu:
Cu improves the corrosion resistance of steel
against non-oxidizing acid, but a steel containing more
than 3.0% is poor in hot workability. Therefore, the
Cu content is limited to not more than 3.0%.
Mo:
Mo is an element, which improves the resistance
of steel against the local corrosion in a corrosion
environment containing chlorine ion. However, a steel
containing less -than 0.5% of Mo has not a satisfactorily
high corrosion resistance; while even when more than
5.0% of Mo is contained in a steel, the effect of Mo
for improving the corrosion resistance of the steel
S~
does not so increase. Moreover, Mo is very expensive.
Therefore, the ~o content is limited within the range
of 0.5-5.0%-
N:
05 N is an important element in order to form a
duplex texture in steel, and further serves to increase
the corrosion resistance of steel. However, a steel
containing less than 0.05% of N has not a satisfactorily
high resistance ag~inst pitting corrosion; while a
steel containing more than 0.30% of N has a very poor
hot workability. Therefore, the N content is limited
within the range of 0.05-0.30%.
Al:
Al is an effective element for decreasing the
lS amount of oxygen contained in a steel, which oxygen
deteriorates the hot workability of the steel of the
present invention. However, an Al content of less than
0.01% in a steel can not decrease the oxygen content in
the steel in order to improve its hot workability.
While, when an Al content in a steel is more than
0.10%, the surface defects of the steel due to alumina
cluster is increased. Therefore, the Al content is
limited within the range of 0.01-0.10%.
S:
S is contained in a s-teel as an incidental
impurity, and deteriorates the hot workability of the
steel of the present invention. The adverse affect of
S appears particularly noticeably at the production of
- 10 -
3S~:~
pipe through the plug mill process. When S is contained
in a steel in an amount of more than 0.004%, it is
difficult to produce pipes witho~Lt causing defects even
in the addition of rare earth metal (hereinafter,
05 referred to a REM), Ca and the like, which metals are
effective for fixing sulfides. Therefore, the S content
is limited to not more than 0.004%.
P:
P is contained in steel as an incidental
lo impurity. When P is contained in a steel in an amount
of more than 0.030%, it is difficult to produce pipes
without causing defects in the pipes by the plug mill
process. Therefore, the P content is limited to not
more than 0.030%.
The duplex s-tainless steel of the present
invention contains the above described C, Si, Mn Cr,
Ni, Cu, Mo, N, Al, S and P as basic components in the
above limited amounts. Further, it is necessary that
the duplex stainless steel of the present invention
contain at least one of Ca and B in the following
amounts concurrently with the above described basic
components in order to improve the hot workability.
The reason of the limitation of the amounts of these
elements is as follows.
Ca:
Ca is a powderful sulfide-forming element,
and is effective for decreasing the amount of S solid-
solved in a steel and for improving the hot workability
3S~
of the steel by forming its sulfide. However, when the
amount of Ca is within the range of Ca<[%S], the effect
is low; while, when the amount oE Ca is within the
range of Ca>lOx[%S], the effect is saturated, and
05 further there is a risk of increasing the surface
defects due to the formation of oxide or sulfide of Ca.
Therefore, the Ca content is limited within the range
of Ca ~ lO)x[%S].
B:
B is effective for improving the hot work-
ability of a steel by adding a slight amount of B to
the steel. However, when the amount of B is less than
0.0005%, the effect does not appear; while, when the
amount exceeds 0.010%, the hot workability of the steel
is lowered. Therefore, the B content is limited within
the range of 0.0005-0.010%.
Hereinafter, an explanation will be made with
respect to the reason of the limitation of the production
condition of a duplex stainless steel seamless pipe
having the above described composition by the plug mill
process.
In the piercing step, the temperature of the
center portion and outer surface portion of a billet
are limited within the ranges of 1,200-1,350C and
1,100-1,350C respectively based on the following
reason. Defects are apt to be formed in the center
portion of a billet due to the Mannesmann effect even
in the case where the billet has a chemical composition
- 12 -
. ~
8SO
defined in the present invention, and hence the center
portion of a billet must be kept to a temperature not
lower than 1,200C in order to pierce the billet without
forming defects in the center portion. However, even
05 when the temperature of the center portion exceeds
1,350C, defects are formed in the center portion.
Accordingly, the temperature of the center portion of a
billet is limited within the range of 1,200-1,350C.
The outer surface portion of a billet is not subjected
lo to a working under so severe condition as that applied
to the center portion of the billet, but when the
temperature of the outer surface portion of a bllet is
lower than l,100C, defects are formed in the outer
surface portion; while even when the temperature is
higher -than 1,350C, defects are also formed.
Accordingly, the temperature of the outer surface
portion of a billet is limited within the range of
1,100-1,350C.
The reason why the cross rolling in the cross
rolling step is finished at a temperature not lower
than l,100C will be explained hereinafter. In the
beginning stage of the investigation of the present
invention, when a hollow piece was cross rolled at a
temperature of 1,000C, the hollow piece cracked. When
a hollow piece was cross rolled at a temperature of
1,080C, although the hollow piece did not crack in the
cross rolling step, the piece cracked in the next step
of rolling by means of a plug mill in spite of the fact
- 13 -
that the pl~lg mill rolling was carried out at a lowreduction rate. It can be seen from this fact that the
strain caused in a hollow piece by plastic working
during the cross rolling has a high influence upon the
05 rolling by the plug mill. When the cross rolled hollow
piece is reheated to eliminate the strain by plastic
working, the adverse affect of the strain can be
obviated. However, such reheating is not advantageous
in view of the energy saving demanded at present.
The inventors have made experiments and
investigations in order to find out a method for rolling
a cross rolled hollow piece without carrying out the
reheating. That is, a simulation experiment was effected
in a laboratory scale by means of a hot tensile tester.
In the cold working, there is commonly carried
out a method, wherein an equivalent stress and an
equivalent strain are defined, and a stressed state of
a steel during various workings is calculated from the
uniaxial strain-hardening curve of the steel. However,
in the hot working, recrystallization occurs during the
hot working, and therefore the conception of equivalent
strain is used in order to evaluate quantitatively the
working state of a steel in the hot working.
The experimental method will be explained in
detail hereinafter. In general, an equivalent strain
caused in a steel at the cross rolling and calculated
by the formula (1) is about 0.7. Accordingly, the
value of prestrain¦ln (A/Ao)l in the hot tensile test
. ~
was assumed to be 0.7, wherein Ao is the cross-sectional
area of a test piece before the test, and A is the
cross-sectional area thereof after applied with a
prestrain.
That is, according to the working schedule
shown by a diagram in Fig. 1, after a test piece was
heated and kept at 1,250C, the test piece was cooled
to a given temperature Tl, kept at this temperature T
for tl seconds to give a prestrain (Wl) to the test
piece. The test piece was then cooled to a given
temperature T2, kept at this temperature T2 for
t2 seconds, and then subjected to a tensile test W2.
Fig. 2 illustrates the results of the test.
That is, a test piece produced from a steel having a
chemical composition shown in the following Table 1 was
used and the test piece was treated under conditions
shown in the following Table 2.
Table 1
Chemical composition (wt.%)
C ¦ Si ¦ Mn ¦ P ¦ S ¦ Cr ¦ Ni¦ Cu¦ N ¦A1 Ca
0.014~0.5111.0210.02510.003122.015.511.210.1510.0 ~
- 15 -
~.~lZ8'jV
Table 2
_ Retention time
Prestrain (seconds)
PrestrainedlTemp(raturell 125
l I1,050 ---o--- __~
Not prestrained O
It can be seen from Fig. 2 tha-t, when a
prestrain is given to the steel at a temperature not
higher than 1,050C, and the prestrained steel is
stretched at a temperature lower than the prestraining
temperature, the hot workability of the steel represented
by its elongation at break of ¦ln (A/A')¦, wherein A'
is the cross-sectional area of the prestrained test
piece, and A is the cross-sectional area of the broken
test piece, is remarkably lower than the hot workability
of the case where the steel is not prestrained. Even
when the steel is prestrained at a temperature not
lower than l,100C and the prestrained steel is stretched
at a temperature lower than the prestraining temperature,
the elongation at break of the steel is substantially
the same as the case where the steel is not prestrained.
That is, a prestrain applied to a steel at a temperature
not lower than 1,100C does not adversely influence the
working of the steel in the subsequent steps. That is,
when a cross rolling of a hollow piece is carried out
- 16 -
l~Z~S~
at a temperature lower than l,100C, the low temperature
cross rolling has an adverse influence upon the sub-
sequent rollings by a plug mill, a reeler and a sizer,
and hence the rolling of the hollow piece in these
05 steps, which in itself is difficult due to the lowering
of the temperature, becomes more difficult. Due to the
reason, the temperature in the cross rolling is limited
to not lower than 1,100C. Further, it can be seen
from the result of this experiment that, when a billet
lQ is pierced at a temperature not lower than lj100C, the
strain caused in the resulting hollow piece by plastic
working during the pierclng has not an adverse influence
upon the cross rolling of the hollow piece.
The reason why the equivalent strain which
will be caused in a hollow piece in the plug mill
rolling step is limited to not higher than 0.5 is as
follows. When the equivalent strain exceeds 0.5, the
hollow piece cracks during the plug mill rolling; or
even when the hollow piece does not crack during the
plug mill rolling, defects are formed in the hollow
piece in the sebsequent reeling step even in the case
where the hollow piece is rolled in a samll amount in
the reeling step. Based on this reason, the equivalent
strain, which will be caused in the hollow piece in the
plug mill rolling step, is limited to not higher than 0.5.
When a hollow piece does not crack during the plug mill
rolling, but defects are formed in the hollow piece
during the subsequent reeling step, the formation of
- 17 -
~L~ S~I
the defects can be prevented by reheating the hollow
piece after the plug mill rolling. However, some
energy are required for the reheating, resulting in a
high production cost of pipe. Accordingly, when the
05 equivalent strain which will be caused in a hollow
piece in the plug mill rolling step is limited to not
higher than 0.5, the reheating is not necessary, and a
pipe can be prodused inexpensively.
The reason why the equivalent strain which
lo will be caused in a hollow piece in the reeling step is
limited to not higher than 0.4 will be explained herein-
after. In order to produce a pipe having a dimension
which agrees with the demand of users, it is impossible
to omit the plug mill rolling step. In the present
invention, the cross rolling is carried out at a
temperature not lower than l,100C, and the plug mill
rolling must be carried out at a temperature lower than
the cross rolling temperature. As clearly understood
from Fig. 2, a working at a temperature lower than
l,100C has a high influence upon the working in the
subsequent step. The worked amount of a hollow piece
in the plug mill rolling step has a high influence upon
the rolling of the hollow piece in the reeling step,
and therefore the amount of a hollow piece to be worked
in the reeling step must be determined by taking into
consideration the worked amount of the hollow piece in
the plug mill rolling step. When a hollow piece is
worked in the plug mill rolling step in an amount
- 18 -
~ 2 ~ 0
necessary for producing a pipe having a dimension which
agrees with the demand by the users, there is a risk of
formation of defects in the hollow piece in the reeling
step. As the result o~ experiments and investigations,
05 the inventors have found out that, when a hollow piece
is rolled in the reeling step under a condition that
the equivalent strain which will be caused in the
hollow piece exceeds 0.4, the hollow piece cracks in
the reeling step, and therefore the amount of a hollow
lo piece to be worked in the reeling step is limited to
such an amount that will cause an equivalent strain of
not higher than 0.4 in the hollow piece.
An explanation will be made hereinafter the
reason why the total amount of equivalent strain which
will be caused in a hollow piece in the plug mill
rolling step and in the reeling step is limited to not
higher than 0.6, and further the equivalent strain
which will be caused in the hollow piece in the outer
diameter reducing step is limited to not higher than 0.2
in the second aspect of the present invention. When
the total amount of the equivalent strain caused in a
hollow piece in the plug mill rolling step and in the
reeling step exceeds 0.6, defects are formed in the
hollow piece during its working in the outer diameter
reducing step by the sizer even in the case where
defects are not formed in the hollow piece in the
reeling step. While, even when the total amount of the
equivalent strain is not higher than 0.6, if the hollow
- 19 -
piece is rolled by a sizer uncler a condition that the
equivalent strain caused in the hollow piece exceeds 0.2,
defects are formed in the hollow piece during the
rolling by the sizer. Therefore, in the second aspect
05 of the present invention, the total amount of the
equivalent strain which will be caused in the hollow
piece in the plug mill rolling step and in the reeling
step is limited to not higher than 0.6, and the
equivalent strain which will be caused in the hollow
piece in the outer diameter reducing step by the sizer
is limited to not higher than 0.2.
An explanation will be made with respect to
the reheating of a reeled hollow piece, and the amount
of the reheated hollow piece to be worked in the outer
diameter reducing step by the sizer in the third aspect
of the present invention. When a reeled hollow piece
is rolled at a temperature not lower than 850C by a
sizer in the outer diameter reducing step, the ~ phase
is apt to form in the reeled hollow piece and the
working of the hollow piece by the sizer becomes
difficult. Even when the rolling temperature of a
reeled hollow piece by the sizer is not lower than 8~0C,
if the reele~ hollow piece is kept to this temperature
for a period longer than lO minutes, the ~ phase is apt
to be formed, and the working of the reeled hollow
piece by the sizer is difficult. It is not necessary
to heat a reeled hollow piece at a temperature higher
than l,200C, and a reheating at a lower temperature is
- 20 -
desirable in view of energy saving. Further, when a
reheated hollow piece is rolled under a condition that
the equivalent strain which will be caused in the
hollow piece, is higher than 0.4, the hollow piece
05 buckles along its peripheral direction. Based on the
above described reason, the treating condition of a
reeled hollow piece is limited such that -the reeled
hollow piece is heated at a temperature of 850-l,200C
for a period of not longer than lO minutes, and then
lo the reheated hollow piece is worked by the sizer under
a condition that the equivalent strain which will be
caused in the hollow piece is kept to not higher
than 0.4.
The following example is given for the purpose
of illustration of this invention and is not intended
as a limitation thereof.
Example
Pipes having a chemical composition shown in
the following Table 3 were produced under a production
condition shown in Table 3. The obtained results are
shown in Table 3 together with the composition and
production condition. In Table 3, items which do not
satisfy the requirements defined in the present invention
are indicated by underline. In Sample No. l of compara-
tive example, the steel did not contain Ca and B, andhence the hollow piece cracked during the cross rolling.
In Sample Nos. 2, 3 and 4 of comparative example, the
content of Ca or B in the steel was lower than the
- 21 -
SO
lower limit or was higher than the upper limit defined
in the present invention, and hence the hollow piece
cracked during the working by the reeler. In Sample
Nos. 5 and 6 of comparative example, the S content
o5 exceeded the upper limit defined in the present
invention, and hence the hollow piece cracked during
the cross rolling. In Sample Nos. 7, 8 and 9 of compar-
ative example, the temperature of the outer surface
portion of the billet in the piercing step was lower or
higher than the lower limit or higher limit defined in
the present invention, and hence the hollow piece
cracked at its outer surface during the piercing.
In Sample Nos. 10 and 11 of comparative example, the
temperature of the hollow piece in the cross rolling
step is lower ~han the lower limit defined in the
present invention, the hollow piece cracked during the
cross rolling. In Sample Nos. 12, 13, 14 and 15 of
comparative example, the equivalent strain, which was
caused in a hollow piece in the plug mill rolling step,
reeling step or outer diameter reducing step, is higher
than the upper limit defined in the present invention,
and hence the hollow piece cracked during the working
in respective steps. In Sample No. 16 of comparative
example, the reheating temperature of the reeled hollow
piece was lower than the lower limit defined in the
present invention; and in Sample No. 17 of comparative
example, the reheating temperature of the reeled hollow
piece was higher than the upper limit defined in the
v - 22 -
~2~
present invention; and thereEore the reeled hollow
pieces of Sample Nos. 16 and 17 cracked during the
working by means of a sizer. In Sample No. 18 of
comparative e~ample, the equivalent strain caused in
05 the reeled hollow piece during the working by the sizer
was higher than the upper limited defined in the present
invention, and therefore the reeled hollow piece was
buckled during the working by the sizer.
On the contrary, in Sample Nos. 19-25 of the
present invention, all the requirements defined in the
present invention were satisfied, and therefore pipes
were able to be produced without causing cracks in
respective steps.
I5
- 23 -
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135(~
As clearly understood from the above described
example, according to the present invention, the composi-
tion of a duplex stainless steel was limited, the
billet temperatures in the piercing step and cross
05 rolling step were limited, the equivalent strains which
would be caused in the hollow piece in the plug mill
rolling step, reeling step and outer diameter reducing
step were limited, and occasionally the reheating
temperature was limited, whereby a duplex stainless
o steel seamless pipe was able to be produced in high
productivity and yield.
26 -
