Note: Descriptions are shown in the official language in which they were submitted.
CA 02638289 2008-09-12
= DESCRIPTION
Oil Country Tubular Good for Expansion in Well and
Duplex Stainless Steel Used for Oil Country
Tubular Good for Expansion
TECHNICAL FIELD
The present invention relates to an oil country tubular good and
duplex stainless steel, and more specifically, to an oil country tubular good
to be expanded in a well and duplex stainless steel to be used for such an oil
country tubular good for expansion.
BACKGROUND ART
When a well (oil well or gas well) that yields oil or gas is drilled in
general, a plurality of oil country tubular goods called "casings" are
inserted
into a well drilled using a drill pipe in order to prevent the wall of the
well
from being collapsed. A conventional method of constructing a well is as
follows. To start with, when a well is drilled for a prescribed distance, a
first casing is inserted. Then, when the well is further drilled for a
prescribed distance, a second casing having an outer diameter smaller than
the inner diameter of the first casing is inserted. In this way, according to
the conventional construction method, the outer diameters of casings to be
inserted are sequentially reduced as the well is drilled deeper. Therefore,
as the oil well is deeper, the inner diameters of casings used in the upper
part of the well (near the surface of the ground) increase. As a result, the
drilling area increases, which pushes up the drilling cost.
A new technique for reducing the drilling area and thus reducing
the drilling cost is disclosed by JP 7-567610 A and the pamphlet of
International Publication WO 98/00626. The technique disclosed by these
documents is as follows. A casing C3 having a smaller outer diameter than
the inner diameter ID1 of casings Cl and C2 already provided in a well is
inserted into the well. Then, the inserted casing C3 is expanded, so that
its inner diameter is equal to the inner diameter ID1 of the previously
=1=
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provided casings Cl and C2 as shown in Fig. 1. According to the method,
the casing is expanded inside the well and therefore it is not necessary to
increase the drilling area if the oil well to construct is deep. Therefore,
the
drilling area can be reduced. Furthermore, the number of necessary steel
pipes can be reduced because large size casings are not necessary.
In this way, the oil country tubular good expanded in a well must
have a uniformly deforming characteristic when expanded (hereinafter
referred to as "pipe expansion characteristic.") In order to obtain a high
pipe expansion characteristic, the deforming characteristic without local
constriction during working is required, in other words, uniform elongation
that can be evaluated by tensile testing must be high.
As shown in Fig. 1 in particular, in the bell part 10 where casings
vertically placed on each other overlap, the pipe expansion ratio is
maximized. In consideration of the expansion ratio at the bell part, the
uniform elongation of the oil country tubular good for expansion is
preferably more than 20%.
JP 2005-146414 A discloses a seamless oil country tubular good for
expansion. The structure of the disclosed oil country tubular good includes
a ferrite transformation phase and low temperature transformation phases
(such as bainite, martensite, and bainitic ferrite), and has a high pipe
expansion characteristic. However, the uniform elongation of each test
piece in the disclosed embodiment is not more than 20% (see JP 2005-
146414 A, u-El in Tables 2-1 and 2-2). Therefore, the bell part described
above may not deform uniformly.
DISCLOSURE OF THE INVENTION
It is an object of the invention to provide an oil country tubular good
for expansion having a high pipe expansion characteristic. More
specifically, it is to provide an oil country tubular good having a uniform
elongation more than 20%.
In order to achieve the above-described object, the inventors
examined the uniform elongation of various types of steel. As a result, the
inventors have found that duplex stainless steel having prescribed chemical
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components has a uniform elongation significantly higher than those of
carbon steel and martensitic stainless steel.
The inventors have further studied and found that in order to
produce an oil country tubular good having a uniform elongation more than
20%, the following requirements must be fulfilled.
(1) The austenite ratio in the duplex stainless steel is in the range
from 40% to 90%. Herein, the austenite ratio is measured by the following
method. A sample is taken from an arbitrary position of an oil country
tubular good for expansion. The sample is mechanically polished and then
subjected to electrolytic etching in a 30 mol% KOH solution. The etched
surface of the sample is observed using a 400X optical microscope with a 25-
grating ocular lens, and the austenite ratio is measured by a point count
method according to ASTM E562.
(2) The yield strength is adjusted in the range from 276 MPa to 655
MPa. The yield strength herein is 0.2% proof stress according to the
ASTM standard. When an oil country tubular good for expansion is kept
as-solution treated, the yield strength is within the above-described range.
Herein, "as-solution treated" means the state in which after the solution
treatment, no other thermal treatment or no other cold working is carried
out except for cold straightening.
The present invention was made based on the above-described
findings and the invention can be summarized as follows.
An oil country tubular good for expansion according to the invention
is expanded in a well. The oil country tubular good - for expansion
according to the invention is formed of duplex stainless steel having a
composition containing, in percentage by mass, 0.005% to 0.03% C, 0.1% to
1.0% Si, 0.2% to 2.0% Mn, at most 0.04% P, at most 0.015% S, 18.0% to
27.0% Cr, 4.0% to 9.0% Ni, at most 0.040% Al, and 0.05% to 0.40% N, and
the balance consisting of Fe and impurities, and a structure including an
austenite ratio in the range from 40% to 90%. The oil country tubular good
has a yield strength from 256 MPa to 655 MPa, and a uniform elongation
more than 20%.
Herein, the "uniform elongation" means the distortion (%) at the
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maximum load point in a tensile test. The austenite ratio is an austenite
area ratio.
The duplex stainless steel may further contain at most 2.0% Cu.
The duplex stainless steel may further contain one or more selected from
the group consisting of at most 4.0% Mo and at most 5.0% W. The duplex
stainless steel may further contain one or more selected from the group
consisting of at most 0.8% Ti, at most 1.5% V, and at most 1.5% Nb. The
duplex stainless steel may further contain one or more selected from the
group consisting of at most 0.02% B, at most 0.02% Ca and at most 0.02%
Mg.
The duplex stainless steel according to the invention is used for the
above-described oil country tubular good for expansion.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic view for use in illustrating a new method of
constructing a well that yields oil or gas.
BEST MODE FOR CARRYING OUT THE INVENTION
Now, embodiments of the invention will be described in detail.
An oil country tubular good according to an embodiment of the
invention is formed of duplex stainless steel having the following chemical
composition and metal structure. Hereinafter, "%" related to elements
means "% by mass."
1. Chemical Composition
C: 0.005% to 0.03%
Carbon stabilizes the austenite phase. In order to effectively
secure the effect, the C content is not less than 0.005%. Meanwhile, if the
C content exceeds 0.03%, carbide is more easily precipitated, which lowers
the grain boundary corrosion resistance. Therefore, the C content is from
0.005% to 0.03%.
Si: 0.1% to 1.0%
Silicon deoxidizes the steel. In order to secure the effect, the Si
content is not less than 0.1%. Meanwhile, if the Si content exceeds 1.0%,
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intermetallic compounds are acceleratingly generated, which lowers the hot
workability. Therefore, the Si content is from 0.1% to 1.0%.
Mn: 0.2% to 2.0%
Manganese deoxidizes and desulfurizes the steel and improves the
hot workability as a result. Manganese also increases the solid solubility
of N. In order to effectively secure the effect, the Mn content is not less
than 0.2%. Meanwhile, if the Mn content exceeds 2.0%, the corrosion
resistance is lowered. Therefore, the Mn content is from 0.2% to 2.0%.
P= 0.04% or less
Phosphorus is an impurity that promotes central segregation and
degrades the sulfide stress cracking resistance. Therefore, the P content is
preferably as small as possible. Therefore, the P content is not more than
0.04%.
S: 0.015% or less
Sulfur is an impurity and lowers the hot workability. Therefore,
the S content is preferably as small as possible. The S content is therefore
not more than 0.015%.
Cr: 18.0% to 27.0%
Chromium improves the carbon dioxide corrosion resistance. In
order to secure sufficient carbon dioxide corrosion resistance for duplex
stainless steel, the Cr content is not less than 18.0%. Meanwhile, if the Cr
content exceeds 27.0%, intermetallic compounds are acceleratingly
generated, which lowers the hot workability. Therefore, the Cr content is
from 18.0% to 27.0%, preferably from 20.0% to 26.0%.
Ni: 4.0% to 9.0%
Nickel stabilizes the austenite phase. If the Ni content is too small,
the amount of ferrite in the steel is excessive, and the characteristic of the
duplex stainless steel does not result. The solid solubility of N in the
ferrite phase is small, and the increase in the ferrite amount causes nitride
to be precipitated, which degrades the corrosion resistance. Meanwhile, an
excessive Ni content reduces the ferrite amount in the steel, and the
characteristic of the duplex stainless steel does not result. In addition, an
excessive Ni content causes a a phase to be precipitated. Therefore, the Ni
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content is from 4.0% to 9.0%, preferably from 5.0% to 8.0%.
Al: 0.040% or less
Aluminum is effective as a deoxidizing agent. However, if the Al
content exceeds 0.040%, inclusions in the steel increase, which degrades the
toughness and the corrosion resistance. Therefore, the Al content is not
more than 0.040%.
N: 0.05% to 0.40%
Nitrogen stabilizes the austenite phase and also improves the
thermal stability and the corrosion resistance of the duplex stainless steel.
In order to achieve an appropriate ratio between the ferrite phase and the
austenite phase in the steel, the N content is not less than 0.05%.
Meanwhile, if the N content exceeds 0.40%, a defect attributable to a
generated blow hole is caused. The toughness and corrosion resistance of
the steel are degraded as well. Therefore, the N content is from 0.05% to
0.40%, preferably from 0.1% to 0.35%.
Note that the balance of the duplex stainless steel according to the
invention consists of Fe and impurities.
The duplex stainless steel for an oil country tubular good for
expansion according to the embodiment further contains Cu in place of part
of Fe if necessary.
Cu: 2.0% or less
Copper is an optional element and improves the corrosion resistance
of the steel. However, an excessive Cu content lowers the hot workability.
Therefore, the Cu content is not more than 2.0%. Note that in order to
effectively secure the above-described effect, the Cu content is preferably
not less than 0.2%. However, if the Cu content is less than 0.2%, the
above-described effect can be obtained to some extent.
The duplex stainless steel for an oil country tubular good for
expansion according to the embodiment further contains one or more
selected from the group consisting of Mo and W in place of part of Fe if
necessary.
Mo: 4.0% or less
W: 5.0% or less
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Molybdenum and tungsten are optional elements. These elements
improve the pitting corrosion resistance and the deposit corrosion
resistance. However, an excessive Mo content and/or an excessive W
content causes a a phase to be more easily precipitated, which embrittles
the steel. Therefore, the Mo content is not more than 4.0% and the W
content is not more than 5.0%. In order to effectively secure the above
described effect, the Mo content is preferably not less than 2.0% and the W
content is preferably not less than 0.1%. However, if the Mo content and
the W content are less than the described lower limits, the above-described
effect can be obtained to some extent.
The duplex stainless steel for an oil country tubular good for
expansion according to the embodiment further contains one or more
selected from the group consisting of Ti, V, and Nb in place of part of Fe if
necessary.
Ti: 0.8% or less
V. 1.5% or less
Nb: 1.5% or less
Titanium, vanadium, and niobium are optional elements. These
elements improve the strength of the steel. However, if the contents of
these elements are excessive, the hot workability is lowered. Therefore,
the Ti content is 0.8% or less, the V content is 1.5% or less, and the Nb
content is 1.5% or less. In order to more effectively secure the above-
described effect, the Ti content is preferably not less than 0.1%, and the V
content is preferably not less than 0.05%. The Nb content is preferably not
less than 0.05%. However, if the Ti, V, and Nb contents are less than the
above-described lower limits, the above effect can be obtained to some
extent.
The duplex stainless steel for an oil country tubular good according
to the embodiment further contains one or more selected from the group
consisting of B, Ca, and Mg in place of part of Fe.
B: 0.02% or less
Ca: 0.02% or less
Mg: 0.02% or less
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Boron, calcium, and magnesium are optional elements. These
elements improve the hot workability. However, if the contents of these
elements are excessive, the corrosion resistance of the steel is lowered.
Therefore, the B content, the Ca content, and the Mg content are each not
more than 0.02%. In order to more effectively secure the above-described
effect, the B content, the Ca content, and the Mg content are each
preferably not less than 0.0002%. However, if the B, Ca, and Mg contents
are less than the lower limits, the above-described effect can be obtained to
some extent.
2. Metal Structure
The duplex stainless steel that forms an oil country tubular good for
expansion according to the invention has a metal structure including a
ferrite phase and an austenite phase. It is considered that the austenite
phase as a soft phase contributes to improvement of the uniform elongation.
The austenite ratio in the steel is from 40% to 90%. Herein, the
austenite ratio is an area ratio measured by the following method. A
sample is taken from an arbitrary position of an oil country tubular good for
expansion and mechanically polished, and then the polished sample is
subjected to electrolytic etching in a 30 mol% KOH solution. The etched
surface of the sample is observed using a 400X optical microscope with a 25
grating ocular lens, and the austenite ratio is measured by the point count
method according to ASTM E562.
If the austenite ratio is less than 40%, the uniform elongation is
reduced to 20% or less. Meanwhile, if the austenite ratio exceeds 90%, the
corrosion resistance of the steel is degraded. Therefore, the austenite ratio
is from 40% to 90%. The austenite ratio is preferably from 40% to 70%,
more preferably from 45% to 65%.
3. Manufacturing Method
The oil country tubular good for expansion according to the
invention is produced by the following method.
Molten steel having the above-described composition is cast and
then formed into billets. The produced billet is subjected to hot working
and made into an oil country tubular good for expansion. As the hot
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working, for example, the Mannesmann method is carried out. As the hot
working, hot extrusion may be carried out, or hot forging may be carried out.
The produced oil country tubular good for expansion may be a seamless
pipe or a welded pipe.
The oil country tubular good for expansion after the hot working is
subjected to solution treatment. The solution treatment temperature at
the time is from 1000 C to 1200 C. If the solution treatment temperature
is less than 1000 C, a a phase is precipitated, which embrittles the steel.
The yield strength is raised and exceeds 655 MPa because of the
precipitation of the a phase, and therefore the uniform elongation is 20% or
less. On the other hand, if the solution treatment temperature exceeds
1200 C, the austenite ratio is significantly lowered and becomes less than
40%. The solution treatment temperature is preferably from 1000 C to
1175 C, more preferably from 1000 C to 1150 C.
The oil country tubular good for expansion according to the
invention is in an as-solution-treated state (so-called as-solution-treated
material). More specifically, the tubular good is used as a product right
after the solution treatment without being subjected to other heat
treatment and cold working (such as cold reduction or pilger rolling) except
for cold straightening. In this way, since the oil country tubular good for
expansion according to the invention is in an as-solution-treated state, and
therefore the yield strength may be in the range from 276 MPa to 655 MPa
(40 ksi to 95 ksi). It is considered that in this way, the uniform elongation
exceeds 20% and a high expansion characteristic is obtained even in a well.
Note that if the yield strength exceeds 655 MPa, the uniform elongation is
20% or less. The oil country tubular good for expansion needs strength in
a certain level, and the yield strength is 276 MPa or more.
Note that if cold working is carried out after the solution treatment,
the yield strength exceeds 655 MPa. Therefore, the uniform elongation is
less than 20%.
Example
A plurality of steel products having the chemical compositions in
Table 1 were cast and formed into billets. The produced billets were
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CA 02638289 2008-09-12
subjected to hot forging and hot rolling and a plurality of steel plates for
testing having a thickness of 30 mm, a width of 120 mm, and a length of
300 mm were formed.
-10=
CA 02638289 2008-09-12
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inventive steel comparative steel
CA 02638289 2008-09-12
In Table 1, steel types for test numbers are given in the "structure"
column. In the table, "D" represents duplex stainless steel, "C" represents
carbon steel, and "M" represents martensitic stainless steel. With
reference to Table 1, test Nos. 1 to 11 and 21 to 23 were duplex stainless
steel. Test Nos. 12 to 14 were carbon steel and test Nos. 15 to 20 were
martensitic stainless steel.
Steel plates with test Nos. 1 to 23 were subjected to heat treatment
as described in the "heat treatment" column and cold working in Table 1.
More specifically, the steel plates with test Nos. 1 to 11 were subjected to
solution treatment in the temperature range from 1050 C to 1150 C ("ST"
in the "heat treatment" column in Table 1). The solution treatment
temperature for each of the steel plates is shown in the "ST temperature" in
Table 1. The steel plates with test Nos. 1 to 11 were each a so-called as-
solution-treated material without being subjected to other heat treatment
or cold working such as cold reduction after the solution treatment.
The steel plates with test Nos. 12 to 20 were quenched at 920 C and
then tempered in the temperature range from 550 C to 730 C ("QT" in the
"heat treatment" column in Table 1). The steel plate with test No. 21 was
subjected to solution treatment at a temperature less than 1000 C, and the
steel plate with test No. 22 was subjected to solution treatment at a
temperature higher than 1200 C. The steel plates with test Nos. 21 and
22 are as-solution-treated materials. The steel plate with test No. 23 was
subjected to solution treatment at 1085 C followed by cold drawing.
Measurement of Austenite Ratio
For the steel plates of duplex stainless steel with test Nos. 1 to 11
and 21 to 23, the austenite ratio was obtained after the heat treatment.
More specifically, a test piece was taken from each of these steel plates.
The sampled test pieces were mechanically polished and the polished test
pieces were subjected to electrolytic etching in a 30 mol% KOH solution.
The etched surfaces of the samples were observed using a 400X optical
microscope with 25 grating ocular lens in 16 fields. The austenite ratio (%)
was obtained for each of the observed fields. The austenite ratios were
obtained by the point count method according to ASTM E562. The average
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CA 02638289 2008-09-12
of the austenite ratios (%) obtained for each of the fields is given in the
"Y"
column in Table 1.
Tensile Testing
A round bar specimen having an outer diameter of 6.35 mm, and a
parallel part length of 25.4 mm was taken from each of the steel plates 1 to
23 in the lengthwise direction and subjected to a tensile test at room
temperature. The yield strengths (MPa) obtained by the tensile tests are
given in the "YS" column in Table 1, the tensile strengths (MPa) are given
in the "TS" column in Table 1, and the uniform elongations (%) are given in
the "UE" column in Table 1. The 0.2% proof stress according to the ASTM
standard was defined as the yield strength (YS). The distortion of a
specimen at the maximum load point was defined as the uniform elongation
(%)-
Test Result
With reference to Table 1, the steel plates with test Nos. 1 to 11 each
had a chemical composition, a metal structure and a yield strength within
the ranges defined by the invention, and therefore their uniform
elongations all exceeded 20%.
Meanwhile, the steel plates with test Nos. 12 to 20 were not made of
duplex stainless steel and therefore their uniform elongations were not
more than 20%.
The steel plate with test No. 21 is made of duplex stainless steel and
has a chemical composition within the range defined by the invention, but
its solution-treatment temperature was less than 1000 C. Therefore, the
yield strength exceeded the upper limit by the invention and the uniform
elongation was not more than 20%. It was probably because the solution-
treatment temperature was low and therefore a 6 phase was precipitated,
which raised the yield strength.
Since the steel plate with test No. 22 exceeded 1200 C, the
austenite ratio was less than 40% and the uniform elongation was not more
than 20%. The steel plate with test No. 23 was not an as-solution-treated
material, but subjected to cold working after the solution-treatment.
Therefore, the yield strength exceeded the upper limit of the range defined
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by the invention and the uniform elongation was not more than 20%.
Although the embodiments of the present invention have been
described and illustrated in detail, it is clearly understood that the same is
by way of illustration and example only of how to carry out the invention
and is not to be taken by way of limitation. The invention may be
embodied in various modified forms without departing from the spirit and
scope of the invention.
INDUSTRIAL APPLICABILITY
The oil country tubular good for expansion and duplex stainless
steel according to the invention are applicable to an oil country tubular
good and particularly applicable as an oil country tubular good for
expansion in a well.
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