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DESCRIPTION
STEEL MATERIAL AND EXPANDABLE OIL COUNTRY TUBULAR GOODS
TECHNICAL FIELD
[0001]
The present invention relates to a steel material and expandable oil country
tubular goods, and more particularly, to a steel material excellent in pipe
expandability
and sulfide stress cracking resistance, which is used in oil well and gas well
environments
and the like environments containing hydrogen sulfide (142S) and expandable
oil country
tubular goods using the same.
BACKGROUND ART
[0002]
In drilling of oil wells and gas wells (hereinafter, collectively referred to
simply
as "oil wells"), a general method employed is to insert and bury casings after
a drill hole
reaches a predetermined depth in order to prevent a well wall from collapsing.
Furthermore, the operation of inserting casings having smaller outside
diameter one by
one is repeated while performing the drilling. Therefore, conventionally, in
the case
where it is necessary to perform drilling up to a large depth, a drilling area
of the oil well
in a stratum-near-surface portion becomes larger in an outside-diameter
direction because
of the increase in the number of times a casing is inserted, which increases
drilling cost
and construction period, and is thus economically disadvantageous.
Accordingly, in
recent years, there has been proposed a method of construction in which
casings inserted
in an oil well are expanded in the oil well to reduce a drilling area in a
stratum-near-
surface portion, so that a drilling construction period can be significantly
shortened (for
example, refer to Patent Document 1).
[0003]
In oil wells of crude oil, natural gas, and the like containing H2S, sulfide
stress
cracking (hereinafter, referred to as "SSC") of steel in wet hydrogen sulfide
environments
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poses a problem, and therefore steel pipes for casing excellent in SSC
resistance are
needed. In the above-described method of construction, casings are exposed to
a
corrosive environment after it is subjected to working for expansion without
being
subjected to heat treatment or the like. Therefore, a material used for
casings has to be
excellent in expandability and also in corrosion resistance after cold
working. For
example, Patent Documents 1 to 3 propose materials that are excellent in
expansion
capability and corrosion resistance.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[0004]
Patent Document 1: JP2008-202128A
Patent Document 2: JP2002-266055A
Patent Document 3: JP2006-9078A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0005]
In order to assure the expandability of steel pipes that is indispensable for
use in
the above-described process, a high uniform elongation is required. Patent
Documents
1 and 2 disclose steel pipes that are excellent in SSC resistance but have
room for
improvement because no examination is made about uniform elongation. Also,
Patent
Document 3 discloses the value of uniform elongation. The value, however,
indicates a
result which is 21% or less. In addition, no examination has been made about
SSC
resistance. In order to further increase application opportunities of steel
pipes that are
to be expanded in an oil well, it is necessary to have a uniform elongation
of, for example,
40% or more and assure an SSC resistance after expansion.
[0006]
An objective of the present invention is to provide a steel material that has
a high
expandability, is excellent in SSC resistance after cold working and moreover
has a high
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economic efficiency, and expandable oil country tubular goods using the same.
MEANS FOR SOLVING THE PROBLEMS
[0007]
The present inventors examined the chemical composition of a steel material
that
satisfies the above-described conditions. As the result, the present inventors
came to
obtain the following findings.
[0008]
(A) In order to assure a high SSC resistance and uniform elongation, it is
effective to contain Mn and C, which are austenite stabilizing elements. In
particular, it
is effective to contain a large amount of Mn. An austenitic structure has a
high
resistance to SSC, and if the contents of C and Mn are properly selected, the
austenitic
structure is stable in cold working and difficult to cause strain induced
martensitic
transformation. Therefore, the occurrence of SSC, which is likely to occur in
the
presence of a BCC (body-centered cubic) micro-structure, can be suppressed.
[0009]
(B) Mn has a problem in that it brings about the deterioration in general
corrosion
resistance in wet hydrogen sulfide environments. However, the deterioration of
general
corrosion resistance can be suppressed by containing Cu in a steel material.
[0010]
(C) When a C content is properly managed, in the case where V, which is a
carbide-forming element, is contained, C is consumed to form carbides.
Therefore, it is
necessary to adjust the C content considering the amount of C consumed as
carbides.
[0011]
The present invention has been accomplished on the basis of the above-
described
findings, and involves a steel material and expandable oil country tubular
goods described
below.
[0012]
(1) A steel material having a chemical composition consisting, by mass
percent,
of
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C: 0.6 to 1.8%,
Si: 0.05 to 1.00%,
Mn: more than 25.0% and 45.0% or less,
Al: 0.003 to 0.06%,
P: 0.03% or less,
S: 0.03% or less,
Cu: 0.5 to 3.0%,
N: 0.10% or less,
V: 0 to 2.0%,
Cr: 0 to 3.0%,
Mo: 0 to 3.0%,
Ni: 0 to 1.5%,
Nb: 0 to 0.5%,
Ta: 0 to 0.5%,
Ti: 0 to 0.5%,
Zr: 0 to 0.5%,
Ca: 0 to 0.005%,
Mg: 0 to 0.005%,
REM: 0 to 0.01%,
B: 0 to 0.015%,
the balance: Fe and impurities, and
satisfying the following formula (i),
wherein a metal micro-structure is consisting of an austenite single phase,
a yield strength is 241 MPa or higher, and a uniform elongation is 40% or
higher;
0.6 <C¨ 0.18V< 1.44 (i)
where, the symbol of an element in the formula represents the content (mass%)
of the element contained in the steel material, and is made zero in the case
where the
element is not contained.
[0013]
(2) The steel material according to (1),
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wherein the chemical composition contains, by mass percent,
V: 0.03 to 2.0%.
[0014]
(3) The steel material according to (1) or (2),
wherein the chemical composition contains, by mass percent,
one or more elements selected from
Cr: 0.1 to 3.0%,
Mo: 0.1 to 3.0% and
Ni: 0.1 to 1.5%.
[0015]
(4) The steel material according to any one of (1) to (3),
wherein the chemical composition contains, by mass percent,
one or more elements selected from
Nb: 0.005 to 0.5%,
Ta: 0.005 to 0.5%,
Ti: 0.005 to 0.5%,
Zr: 0.005 to 0.5%,
Ca: 0.0003 to 0.005%,
Mg: 0.0003 to 0.005%,
REM: 0.001 to 0.01% and
B: 0.0001 to 0.015%.
[0016]
(5) Expandable oil country tubular goods, which are comprised of the steel
material according to any one of (1) to (4).
[0017]
(6) The expandable oil country tubular goods according to (5), which are
seamless oil country tubular goods.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0018]
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According to the present invention, a steel material having a high uniform
elongation and thus a high expandability, and excellent SSC resistance after
cold working
can be obtained. Therefore, the steel material according to the present
invention can be
used suitably for expandable oil country tubular goods in wet hydrogen sulfide
environments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019]
[Figure 1] Figure 1 is a graph showing the relationship between Mn content and
uniform
elongation.
[Figure 2] Figure 2 is a graph showing the relationship between Cu content and
corrosion
rate.
MODE FOR CARRYING OUT THE INVENTION
[0020]
Components of the present invention is described below in detail.
[0021]
1. Chemical composition
The reasons for restricting the elements are as described below. In the
following explanation, the symbol "%" for the content of each element means
"(3/0 by
mass".
[0022]
C: 0.6 to 1.8%
Carbon (C) has an effect of stabilizing austenite phase at a low cost even if
the
content of Mn or Ni is reduced, and also can improve the work hardening
property and
uniform elongation by means of promotion of plastic deformation by twinning,
so that C
is a very important element in the present invention. Therefore, 0.6% or more
of C has
to be contained. On the other hand, if the content of C is too high, cementite
precipitates,
and thereby not only the grain boundary strength is decreased and the stress
corrosion
cracking susceptibility is increased, but also the fusing point of material is
decreased
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remarkably and the hot workability is deteriorated. Therefore, the C content
is set to
1.8% or less. The C content is preferably more than 0.65%, further preferably
0.7% or
more. Also, the C content is preferably 1.6% or less, further preferably 1.4%
or less.
[0023]
Si: 0.05 to 1.00%
Silicon (Si) is an element necessary for deoxidation of steel. If the content
of
Si is less than 0.05%, the deoxidation is insufficient and many nonmetallic
inclusions
remain, and therefore desired SSC resistance cannot be achieved. On the other
hand, if
the content of Si is more than 1.00%, the grain boundary strength is weakened,
and the
SSC resistance is decreased. Therefore, the content of Si is set to 0.05 to
1.00%. The
Si content is preferably 0.10% or more, further preferably 0.20% or more.
Also, the Si
content is preferably 0.80% or less, further preferably 0.60% or less.
[0024]
Mn: more than 25.0% and 45.0% or less
Manganese (Mn) is an element capable of stabilizing austenite phase at a low
cost and important element to assure high uniform elongation. In order to
exert the
effects, more than 25.0% of Mn has to be contained. On the other hand, Mn
dissolves
preferentially in wet hydrogen sulfide environments, and stable corrosion
products are
not formed on the surface of material. As a result, the general corrosion
resistance is
deteriorated with the increase in the Mn content. In the present invention, if
more than
45.0% of Mn is contained, even though a fixed amount or more of Cu is
contained, the
corrosion rate becomes higher than the standard corrosion rate of low-alloy
oil well pipe.
Therefore, the Mn content has to be set to 45.0% or less. The Mn content is
preferably
40.0% or less.
[0025]
In the present invention, the "standard corrosion rate of low-alloy oil well
pipe"
means a corrosion rate converted from the corrosion loss at the time when a
steel is
immersed in solution A (5%NaC1 + 0.5%CH3COOH aqueous solution, 1-bar H2S
saturated) specified in NACE TM0177-2005 for 336 h, being 1.5 g/(m2.11).
[0026]
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Al: 0.003 to 0.06%
Aluminum (Al) is an element necessary for deoxidation of steel, and therefore
0.003% or more of Al has to be contained. However, if the content of Al is
more than
0.06%, oxides are liable to be mixed in as inclusions, and the oxides may
exert an adverse
influence on the toughness and corrosion resistance. Therefore, the Al content
is set to
0.003 to 0.06%. The Al content is preferably 0.008% or more, further
preferably
0.012% or more. Also, the Al content is preferably 0.05% or less, further
preferably
0.04% or less. In the present invention, Al means acid-soluble Al (sol.AI).
[0027]
P: 0.03% or less
Phosphorus (P) is an element existing unavoidably in steel as an impurity.
However, if the content of P is more than 0.03%, P segregates at grain
boundaries, and
deteriorates the SSC resistance. Therefore, the content of P has to be set to
0.03% or
less. The P content is desirably as low as possible, being preferably 0.02% or
less,
further preferably 0.012% or less. However, an excessive decrease in the P
content leads
to a rise in production cost of steel material. Therefore, the lower limit of
the P content
is preferably 0.001%, further preferably 0.005%.
[0028]
S: 0.03% or less
Sulfur (S) exists unavoidably in steel as an impurity like P. If the content
of S
is more than 0.03%, S segregates at grain boundaries and forms sulfide-based
inclusions,
and therefore deteriorates the SSC resistance. Therefore, the content of S has
to be set
to 0.03% or less. The S content is desirably as low as possible, being
preferably 0.015%
or less, further preferably 0.01% or less. However, an excessive decrease in
the S
content leads to a rise in production cost of steel material. Therefore, the
lower limit of
the S content is preferably 0.001%, further preferably 0.002%.
[0029]
Cu: 0.5 to 3.0%
Copper (Cu) is an element that promotes local corrosion, and is liable to form
a
stress concentrating zone on the surface of steel material, in the case where
the Mn content
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of the steel material is low. However, in the case where a corrosion rate of
parent phase
of the steel material is high, Cu has an effect of suppressing the corrosion
by forming
sulfides on the surface of material in wet hydrogen sulfide environments. In
the present
invention, since the Mn content is high and the increase of a corrosion rate
can be easily
induced 0.5% or more of Cu has to be contained. On the other hand, if Cu is
contained
excessively, the effect is saturated, the local corrosion is promoted and
stress
concentrating zone on the surface of steel material can be formed. Therefore,
the
content of Cu is set to 3.0% or less. The Cu content is preferably 0.6% or
more, further
=
preferably 0.7% or more. Also, the Cu content is preferably 2.5% or less, more
preferably 2.0% or less, further preferably 1.5% or less.
[0030]
V: 0 to 2.0%
Vanadium (V) may be contained as necessary because it is an element that
strengthen the steel material by performing heat treatment at an appropriate
temperature
and time and precipitating fine carbides (V4C3) in the steel. However, if V is
contained
excessively, the effect is saturated and a large amount of C, which stabilize
an austenite
phase is consumed. Therefore, the content of V is set to 2.0% or less. The V
content
is preferably 1.8% or less, more preferably 1.6% or less. In the present
invention,
remarkable increase of strength should be avoided in order to assure high
uniform
elongation. Also, the productivity may be reduced with the increase in the V
content.
Thus, the V content is further preferably less than 0.5%. In the case where it
is desired
to achieve the above-described effect, the V content is preferably set to
0.03% or more.
[0031]
N: 0.10% or less
Nitrogen (N) is usually handled as an impurity element in iron and steel
materials,
and is decreased by denitrification. Since N is an element for stabilizing
austenite phase,
a large amount of N may be contained to stabilize austenite. However, since
the present
invention intends to stabilize austenite by means of C and Mn, N need not be
contained
positively. Also, if N is contained excessively, the high-temperature strength
is raised,
the work stress at high temperatures is increased, and the hot workability is
deteriorated.
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Therefore, the content of N has to be set to 0.10% or less. From the viewpoint
of refining
cost, denitrification need not be accomplished unnecessarily, so that the
lower limit of the
N content is preferably 0.0015%.
[0032]
Cr: 0 to 3.0%
Chromium (Cr) may be contained as necessary because it is an element for
improving the general corrosion resistance. However, if the content of Cr is
more than
3.0%, Cr segregates at grain boundaries, and thereby the SSC resistance is
deteriorated.
Therefore, the content of Cr, if being contained, is set to 3.0% or less. As
described
above, in the present invention, a corrosion is promoted by the increase in
the Mn content
and the corrosion is suppressed by forming Cu sulfides. Therefore, Cr need not
be
contained positively, and the Cr content is preferably less than 1.0%. In the
case where
it is desired to achieve the above-described effect, the Cr content is
preferably set to 0.1%
or more, further preferably set to 0.2% or more, and still further preferably
set to 0.5% or
more.
[0033]
Mo: 0 to 3.0%
Molybdenum (Mo) may be contained as necessary because it is an element
having an effect of suppressing the corrosion by forming sulfides on the
surface of
material in wet hydrogen sulfide environments in the case where a corrosion
rate of parent
phase of the steel material is high as is the case with Cu. However, since the
effect of
Mo is small compared to that of Cu and also Mo is very expensive element, Mo
should
not be contained excessively. If the content of Mo is more than 3.0%, the
effect is
saturated and economic efficiency is deteriorated. Therefore, the content of
Mo, if being
contained, is set to 3.0% or less. In the case where it is desired to achieve
the above-
described effect, the Mo content is preferably set to 0.1% or more, further
preferably set
to 0.2% or more, and still further preferably set to 0.5% or more.
[0034]
Ni: 0 to 1.5%
Nickel (Ni) may be contained as necessary because it is an element capable of
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stabilizing austenite phase as is the case with Cu and having an effect of
suppressing
cracks during hot rolling that sometimes occur in Cu containing steel.
However, Ni is
an element that promotes local corrosion, and is liable to form a stress
concentrating zone
on the surface of steel material. Therefore, if Ni is contained excessively,
the SSC
resistance may be deteriorated. For this reason, the content of Ni, if being
contained, is
set to 1.5% or less. The effect of suppressing the cracks can be obtained even
by a small
amount, and the Ni content is preferably set to 0.1% or more, further
preferably set to
0.2% or more.
[0035]
Nb: 0 to 0.5%
Ta: 0 to 0.5%
Ti: 0 to 0.5%
Zr: 0 to 0.5%
Niobium (Nb), tantalum (Ta), titanium (Ti) and zirconium (Zr) may be contained
as necessary because these are elements that contribute to the strength of the
steel by
combining with C or N to form micro carbides or carbonitrides. In addition the
steel
material can be strengthened by precipitation strengthening during aging heat
treatment
when the elements having abilities to form carbides and carbonitrides are
contained.
However, if these elements are contained excessively, the effect is saturated
and
deterioration of toughness and destabilization of austenite may be caused.
Therefore,
the content of each element is 0.5% or less and preferably 0.35% or less. In
order to
obtain the effect, the content of one or more elements selected from these
elements is
preferably 0.005% or more, further preferably 0.1% or more.
[0036]
Ca: 0 to 0.005%
Mg: 0 to 0.005%
Calcium (Ca) and magnesium (Mg) may be contained as necessary because these
are elements that have effects to improve toughness and corrosion resistance
by
controlling the form of inclusions, and further enhance casting properties by
suppressing
nozzle clogging during casting. However, if these elements are contained
excessively,
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the effects are saturated and the inclusions are liable to be clustered to
deteriorate
toughness and corrosion resistance. Therefore, the content of each element is
0.005%
or less. The content of each element is preferably 0.003% or less. In order to
obtain
the effect, the content of one or two elements from these elements is
preferably 0.0003%
or more, further preferably 0.0005% or more.
[0037]
REM: 0 to 0.01%
Rare earth metal (REM) may be contained as necessary because these are
elements that have effects to improve toughness and corrosion resistance by
controlling
the form of inclusions as is the case with Ca and Mg. However, if REM is
contained
excessively, the effect is saturated and the inclusions are liable to be
clustered to
deteriorate toughness and corrosion resistance. Therefore, the content of REM
is 0.01%
or less. The REM content is preferably 0.005% or less. In order to obtain the
effect,
the REM content is preferably 0.001% or more, further preferably 0.002% or
more.
[0038]
REM is the general term of a total of 17 elements consisting of Sc (scandium),
Y (yttrium), and lanthanoids, and the REM content means the total content of
one or more
elements from the 17 elements.
[0039]
When two or more elements selected from Ca, Mg and REM are contained
complexly the total content of these elements is preferable 0.008% or less.
[0040]
B: 0 to 0.015%
Boron (B) may be contained as necessary because this is an element that has
effects to refine the precipitates and the austenite grain size. However, if B
is contained
excessively, low-melting-point compounds may be formed to deteriorate hot
workability.
Especially, if the B content is more than 0.015%, the hot workability may be
deteriorated
remarkably. Therefore, the B content is 0.015% or less. In order to obtain the
effect,
the B content is preferably 0.0001% or more.
[0041]
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The steel material of the present invention has the chemical composition
consisting of the above-described elements ranging from C to B, the balance
being Fe and
impurities.
[0042]
The term "impurities" means components that are mixed in on account of various
factors in the production process including raw materials such as ore and
scrap when the
steel is produced on an industrial basis, which components are allowed in the
range in
which the components does not exert an adverse influence on the present
invention.
[0043]
0.6 < C-0.18V < 1.44 (i)
where the symbols of elements in the formula each represent the content of
each
element (mass%) contained in the steel material and is each made zero in the
case where
the element is not contained.
In the present invention, although the C content is regulated within the above-
described range in order to stabilize an austenite phase, in the case where a
steel material
is strengthened by precipitating V carbides, there is a risk that part of C is
consumed,
austenite stability is decreased, and thereby uniform elongation is decreased.
Assuming
that V carbides are all V4C3, an effective amount of C that contributes to the
stabilization
of austenite is expressed by C-0.18V as shown in the formula (i), and it is
necessary to
adjust the contents of C and V such that the effective amount of C exceeds
0.6. On the
other hand, an effective amount of C of 1.44 or more poses problems of the
inhomogeneity of a micro-structure and the deterioration in hot workability
with the
formation of cementite, and it is necessary to adjust the contents of C and V
such that the
effective amount of C is less than 1.44. The effective amount of C is
preferably 0.65 or
more, more preferably, 0.7 or more. Also, the effective amount of C is
preferably 1.4 or
less, more preferably, 1.3 or less.
[0044]
2. Metal micro-structure
As described above, if an a' martensite and a ferrite, which have BCC
structures,
are intermixed in a metal micro-structure, there is a risk of not only
decreasing a uniform
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elongation but also causing the decrease in an SSC resistance. Therefore, in
the present
invention, the metal micro-structure is made an austenite single phase, which
has an FCC
(face-centered cubic) structure.
[0045]
Even if the mixing amounts an a' martensite and a ferrite having BCC
structures
are such small that they cannot be detected by X-ray diffraction (XRD), there
is a risk of
the deterioration in a uniform elongation and an SSC resistance. Therefore, in
the
present invention, the volume amounts of the ferrite and the a' martensite
having BCC
structures are measured and evaluated using a ferrite meter made by Helmut
Fischer
(model number: FE8e3).
[0046]
3. Mechanical properties
The steel material according to the present invention has a yield strength of
241
MPa or higher. On the other hand, in order to assure expandability, it is
desirable that
the yield strength of a steel material is lower than 862 MPa. In particular,
in the case of
using the steel material according to the present invention as expandable oil
country
tubular goods, it is desirable that the yield strength of the steel material
is lower than 758
MPa, and more desirably, lower than 654 MPa.
[0047]
Also, the steel material according to the present invention has to have a high
uniform elongation in order to assure a good expandability. In an expanding
method for
normal oil wells, a pipe expansion rate is about 25%, but it is practically
desirable that
the material shows a sufficient elongation after being subjected to cold
working of 25%.
Therefore, the steel material of the present invention has a uniform
elongation of 40% or
higher.
[0048]
The uniform elongation of a steel material generally tends to be in inverse
proportion to the yield strength thereof. Therefore, for a steel material
having a low
yield strength, it is desirable to have a higher uniform elongation
corresponding to the
yield strength. Therefore, the steel material according to the present
invention desirably
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satisfies the following formula (ii).
uEl (%) > 70-0.06xYS (MPa) (ii)
where, in the formula, uEl means the uniform elongation (%) of the steel
material,
and YS means the yield strength (MPa) thereof
[0049]
In particular, if the yield strength is less than 500 MPa, it is also supposed
that
steel pipes having been subjected to solid solution heat treatment are
strengthened by cold
working in advance before shipment, and it is therefore desirable to satisfy
the formula
(ii).
[0050]
4. Application
As described above, the steel material according to the present invention is
excellent in expandability and, in addition, has a feature that the corrosion
resistance
thereof does not deteriorate after expansion even without being subjected to
heat
treatment. Therefore, the steel material according to the present invention is
suitable to
be used as expandable oil country tubular goods. The kind of the tubular goods
is not
specifically limited, and a seamless steel pipe, an electric resistance welded
steel tube, an
arc welded steel pipe, or the like can be used.
[0051]
Typically, in expansion, it is desirable to use steel pipes that are produced
by
processing steel strips or steel plates having uniform thicknesses into
tubular shapes and
thereafter joining them, rather than seamless steel pipes that have some
variations in
thickness. However, the steel material according to the present invention
has
characteristics of being considerably hardened by working. Therefore, in the
case of
expanding a steel pipe having variations in thickness, a thin portion is first
expanded to
be hardened, and the further elongation thereof is restricted. A thick portion
is then
expanded, and the steel pipe is uniformly expanded as a consequence.
Therefore, the
steel material according to the present invention can be suitably used for
seamless steel
pipes. In addition, it is more desirable that seamless steel pipes include no
weld zone to
stably exhibit a good SSC resistance.
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[0052]
4. Production method
The steel material according to the present invention can be manufactured, for
example, by the method described below, but the method is not subject to any
special
restriction.
[0053]
<Melting and casting>
Concerning melting and casting, a method carried out in the method for
producing general austenitic steel materials can be employed, and either ingot
casting or
continuous casting can be used. In the case where seamless steel pipes are
produced, a
steel may be cast into a round billet form for pipe making by round continuous
casting.
[0054]
<Hot working (forging, piercing, rolling)>
After casting, hot working such as forging, piercing, and rolling is
performed.
In the production of seamless steel pipes, in the case where a circular billet
is cast by the
round continuous casting, processes of forging, blooming, and the like for
forming the
circular billet are unnecessary. In the case where the steel material is a
seamless steel
pipe, after the piercing process, rolling is performed by using a mandrel mill
or a plug
mill. Also, in the case where the steel material is a plate material, the
process is such
that, after a slab has been rough-rolled, finish rolling is performed. The
desirable
conditions of hot working such as piercing and rolling are as described below.
[0055]
The heating of billet may be performed to a degree such that hot piercing can
be
performed on a piercing-rolling mill; however, the desirable temperature range
is 1000 to
1250 C. The piercing-rolling and the rolling using a mill such as a mandrel
mill or a
plug mill are also not subject to any special restriction. However, from the
viewpoint of
hot workability, specifically, to prevent surface defects, it is desirable to
set the finishing
temperature at 900 C or higher. The upper limit of finishing temperature is
also not
subject to any special restriction; however, the finishing temperature is
preferably lower
than 1100 C.
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[0056]
In the case where a steel plate is produced, the heating temperature of a slab
or
the like is enough to be in a temperature range in which hot rolling can be
performed, for
example, in the temperature range of 1000 to 1250 C. The pass schedule of hot
rolling
is optional. However, considering the hot workability for reducing the
occurrence of
surface defects, edge cracks, and the like of the product, it is desirable to
set the finishing
temperature at 900 C or higher. The finishing temperature is preferably lower
than
1100 C as in the case of seamless steel pipe.
[0057]
<Solid solution heat treatment>
The steel material having been hot-worked is heated to a temperature enough
for
carbides and the like to be dissolved completely, and thereafter is rapidly
cooled. In this
case, it is preferable that the steel material be rapidly cooled after being
held in the
temperature range of 1000 to 1200 C for 10 min or longer. That is, if the
heating
temperature is lower than 1000 C, carbides, especially Cr-Mo based carbides in
the case
where Cr and Mo are contained, cannot be dissolved completely. Therefore, a Cr
and
Mo deficient layer is formed around the Cr-Mo based carbide, and stress
corrosion
cracking caused by the occurrence of pitting occurs, so that in some cases,
desired SSC
resistance cannot be achieved. On the other hand, if the heating temperature
is higher
than 1200 C, a heterogeneous phase of ferrite and the like is precipitated, so
that in some
cases, desired SSC resistance cannot be achieved. Also, if the holding time is
shorter
than 10 min, the effect of solutionizing is insufficient, and thereby carbides
cannot be
dissolved completely. Therefore, in some cases, desired SSC resistance cannot
be
achieved for the same reason as that in the case where the heating temperature
is lower
than 1000 C.
[0058]
The upper limit of the holding time depends on the size and shape of steel
material, and cannot be determined unconditionally. Anyway, the time for
soaking the
whole of steel material is necessary. From the viewpoint of reducing the
production cost,
too long time is undesirable, and it is proper to usually set the time within
1 h. Also,
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concerning cooling, to prevent carbides (cementite or Cr-Mo based carbides)
during
cooling, other intermetallic compounds, and the like from precipitating, the
steel material
is desirably cooled at a cooling rate higher than the oil cooling rate.
[0059]
The above-described lower limit value of the holding time is holding time in
the
case where the steel material is reheated to the temperature range of 1000 to
1200 C after
the steel material having been hot-worked has been cooled once to a
temperature lower
than 1000 C. However, in the case where the finish temperature of hot working
(finishing temperature) is made in the range of 1000 to 1200 C, if
supplemental heating
is performed at that temperature for 5 mm or longer, the same effect as that
of solid
solution heat treatment performed under the above-described conditions can be
achieved,
so that rapid cooling can be performed as it is without reheating. Therefore,
the lower
limit value of the holding time in the present invention includes the case
where the finish
temperature of hot working (finishing temperature) is made in the range of
1000 to
1200 C, and supplemental heating is performed at that temperature for 5 mm or
longer.
[0060]
<Aging heat treatment>
For the present steel material, aging heat treatment can be performed with the
purpose of precipitation strengthening by mainly precipitating carbides and
carbonitrides.
In particular, it is effective in the case where one or more elements selected
from V, Nb,
Ta, Ti and Zr is contained. However, exceeding aging heat treatment induces
formation
of excess carbides and reduce C concentration in parent phase to lead
destabilization of
austenite. As a heating condition, it is preferable to heat the steel material
about several
ten min to several h at the temperature range of 600 to 800 C.
[0061]
<Cold working>
Cold working may be performed as necessary for the steel material having been
subjected to solid solution heat treatment or further aging heat treatment. A
working
ratio (reduction of area) is not subject to any special restriction but, in
particular, in order
to obtain a yield strength of 400 MPa or higher and lower than 862 MPa, it is
preferable
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to make the working ratio about 10%. In the case where the steel material of
the present
invention is used as expandable oil country tubular goods, it is not
preferable to perform
cold working excessively and a working ratio is preferably set to 25% or less,
in order to
assure high expandability. Excessively high working ratio makes it difficult
to expand
the tubular goods uniformly in the oil wells because a uniform elongation is
reduced and
a strength is enhanced.
[0062]
The cold working method is not subject to any special restriction as far as
the
steel material can be worked evenly by the method. However, in the case where
the
steel material is a steel pipe, it is advantageous on an industrial basis to
use a so-called
cold draw bench using a holed die and a plug, a cold rolling mill called a
cold Pilger
rolling mill, or the like. Also, in the case where the steel material is a
plate material, it
is advantageous on an industrial basis to use a rolling mill that has been
used to produce
the ordinary cold rolled plate.
[0063]
<Annealing>
After the cold working, annealing can be performed. In particular, annealing
can be applied with a view to reducing a strength when the excess strength is
obtained by
the cold working, and recovering an elongation. As an annealing condition, it
is
preferable to heat the steel material about several min to 1 h at the
temperature range of
300 to 500 C.
[0064]
Hereunder, the present invention is explained more specifically with reference
to examples; however, the present invention is not limited to these examples.
EXAMPLE 1
[0065]
Twenty-three kinds of steels of A to P and AA to AG having the chemical
compositions given in Table 1 were melted in a 50kg vacuum furnace to produce
ingots.
Each of the ingots was heated at 1180 C for 3 h, and thereafter was forged and
cut by
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electrical discharge cutting-off. Thereafter, the cut ingots were further
soaked at 1150 C
for 1 h, and were hot-rolled into plate materials having a thickness of 20 mm.
Subsequently, the plate materials were subjected to solid solution heat
treatment at
1100 C for 1 h to obtain test materials (test Nos. 1 to 23). Additionally,
test materials
produced in the same manner as test Nos. 1 to 23 are further cold-rolled at a
working ratio
of 10% to obtain strengthened test materials (test Nos. 24 to 46).
[0066]
[Table 1]
Table 1
Chemical composition (in mass%, balance: Fe and impurities)
Steel
C Si Mn Al P S Cu N V Cr Mo Ni Nb Ta Ti Zr Ca Mg REM B C-0.18V
A 1.21 0.29 26.12 0.033 0.012 0.006 1.48 0.013 - - - - -
- - - - - - - 1.21
B 1.19 0.30 36.00 0.036
0.010 0.006 1.51 0.012 - - - - - - - - - - - -
1.19
C 1.16 0.27 25.95 0.028 0.014 0.004 0.91 0.013 - - - - -
_ - - - - _ - 1.16
D 1.21 0.28 26.13 0.031
0.013 0.004 2.13 0.012 - - _ - - - - - - - . -
1.21
E 0.68 0.19 32.11 0.032
0.012 0.006 0.82 0.016 - 0.78 - - - - - - - - -
- 0.68
F 0.69 0.23 31.82 0.020 0.012 0.006 0.81 0.011 - - 0.81 - -
- - - - - - - 0.69
_
G 0.69 0.22 32.31 0.033
0.010 0.004 0.81 0.011 - - - 0.80 - - - - - 0.003 - -
0.69
H 0.99 0.31 36.12 0.021 0.013 0.006
1.19 0.013 - - - - 0.18 , - 0.10 - 0.002 - - -
0.99
I 1.03 0.33 36.38 0.026 0.011 0.006 1.17 0.011 - - - - -
0.11 - 0.12 0.002 - - - 1.03
J 0.62 0.31 35.87 0.019 0.013 0.005 1.21 0.012 - 0.31 0.30 -
- . - - - - - 0.001 0.62
K 0.90 0.20 29.69 0.028 , 0.011 0.005 1.02
0.011 1.49 - - - - - - - - - - - 0.63
P
L 0.88 0.19 30.08 0.020
0.012 0.005 0.62 0.015 0.77 - - - - - - - -
- . - 0.74 o
IV
M 1.02 020 27.82 0.029 0.011 0.005 0.60 0.013 - - - 0.57 -
- - - - - - _ 1.02 '
..,
n,
ND N 0.99 0.22 27.91 0.034 0.013 0.007 0.61 0.011 - - - 1.22
- - - - - - - - 0.99 n,
1-
O 1.22 0.15 26.02 0.025
0.012 0.006 0.78 0.011 0.29 - - - - - - - -
- - - 1.17 n,
o
P 0.90 0.25 25.48 0.041
0.011 0.006 0.55 0.012 - - .. - - - - - - -
0.003 - 0.90 1-
....1
1
AA 0.41 * 0.31 30.24 0.028 0.010 0.005 0.68 0.013 -
- - - . - - - - - - - 0.41 * 0
L.
_
AB 1.18 0.28 8.12 * 0.019 0.012 0.006 1.45 0.013
- - - - - - - - - - - - 1.18
n,
AC 1.18 0.28 26.28 0.021 0.012 0.005 0.29 * 0.011 - -
- - - - - - - - - - 1.18
AD 0.91 0.20 28.12 0.026 0.013 0.006 1.02 0.011 -
3.94 * - - - - - - - - . - 0.91
AE 0.87 0.18 28.22 0.029 0.012 0.005 0.95 0.011 -
- - 1.52 * - - - - - - - - 0.87
AF 0.70 0.32 31.94 _ 0.031 0.012 0.007 0.71 0.012
1.03 - - - - - - - - - - - 0.51 '
AG 0.64 0.21 IS. : * 0.018 0.011 0.008 0.58 0.011
- - - - - - - - - - - 0.64
* indicates that conditions do not satisfy those defined by the present
invention.
eD
.
eD
"r7
IN
CT
Cn
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[0067]
With use of the above-described test materials, mechanical properties and a
metal micro-structure were examined. Thereafter, the test materials were
subjected to
cold working at working ratio of 25% simulating the expansion. And, mechanical
properties, a metal micro-structure, SSC resistance and a corrosion rate were
examined
with use of the cold-worked test materials. Concerning the mechanical
properties, yield
strength and uniform elongation were measured. From each of the steels, a
round-bar
tensile test specimen having a parallel part measuring 6 mm in outside
diameter and 40
mm in length was sampled. A tension test was conducted at normal temperature
(25 C),
whereby the yield strength YS (0.2% yield stress) (MPa) and the elongation (%)
were
determined.
[0068]
In the present example, the test material that had a uniform elongation being
40%
or higher and satisfying the following formula (ii) in relation to a yield
strength was
evaluated so that the uniform elongation property is good. In the following
Table 2 is
indicated required elongation (%) which is higher value of 40% and 70 - 0.06 x
YS.
uEl (%) > 70-0.06xYS (MPa) (ii)
where, in the formula, uEl means the uniform elongation (%) of the steel
material,
and YS means the yield strength (MPa) thereof.
[0069]
The SSC resistance was evaluated as described below. A plate-shaped smooth
test specimen was sampled, and a stress corresponding to 90% of yield stress
was applied
to one surface of the test specimen by four-point bending method. Thereafter,
the test
specimen was immersed in a test solution, that is, solution A (5%NaC1+
0.5%CH3COOH
aqueous solution, 1-bar H2S saturated) specified in NACE TM0177-2005, and was
held
at 24 C for 336 h. Subsequently, it was judged whether or not rupture
occurred. As
the result, a not-ruptured steel material was evaluated so that the SSC
resistance is good
(referred to as "o" in Table 2), and a ruptured steel material was evaluated
so that the SSC
resistance is poor (referred to as "x" in Table 2).
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[0070]
Also, to evaluate the general corrosion resistance, the corrosion rate was
determined by the method described below. The above-described test material
was
immersed in the solution A at normal temperature for 336 h, the corrosion loss
was
determined, and the corrosion loss was converted into the average corrosion
rate. In the
present invention, the test material that showed the corrosion rate of lower
than 1.5
g/(m2.11) was evaluated so that the general corrosion resistance is good.
[0071]
On the obtained test materials of test Nos. 1 to 46 before and after the cold
working at the working ratio of 25%, the total volume amounts of ferrite and
a' martensite
having BCC structures were measured by using the ferrite meter. For all of the
test
materials before the cold working, the phases having BCC structures could not
be
detected and the metal micro-structures were austenite single phases.
Therefore, the
volume amounts of the phases having BCC structures for the test materials only
after the
cold working are shown as a BCC ratio by volume % in tables. The results are
given in
Tables 2 and 3.
[0072]
[Table 2]
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Table 2
After solid solution heat treatment After simulated expansion (25% cold
working)
Test Yield Required Uniform Yield Corrosion
Steel BCC ratio SSC
No. strength elongation elongation strength (%) resis e rate
tanc
(MPa) (%) (%) (MPa)
1 A 391 47 76 942 _ # o 0.9
_ _
2 B 378 _ 47 69 921 _ a o 1.0
3 C 385 47 78 945 _ # 0 0.8
4 D 391 , 47 80 956 _ a o 0.9
E 298 52 72 838 _ a o 1.0
_
6 F 307 52 74 842 _ # o 0.9
7 G 301 52 70 830 _ a o 0.7
_ _
8 H 358 49 69 901 _ # o 0.8 Inventive
9 I 362 48 65 915 _ a o 1.0 example
_
3 295 52 67 863 _ a o 0.9 .
11 K 342 49 62 899 _ # o 0.8
_
12 L 340 50 60 905 . # o 1.2
_
13 M 352 49 71 910 _ # o 1.1
_
14 N 363 48 66 932 _ # o , 1.3
0 380 47 70 921 - 4 0 1.2
16 P 344 49 69 889 _ # o 1.0
_
17 AA * 267 54 38* 618 0.04* o 1.1
-
18 AB * 325 51 28 * 813 _ # o 0.8
19 AC * 386 49 68 928 _ # o 1.5
- ..
AD * 343 49 76 903 _ # o 1.6 Comparative
_ _ exminple
21 AE * 321 51 77 862 _ # x 0.9
_ ..
22 AF * 308 52 42 782 0.03 * o 1.1
- ..
23 AG * 313 51- 49 811 _ # o 1.1
*indicates that conditions do not satisfy those defined by the present
invention.
# indicates that measured value is below the detection limit (0.01%).
[0073]
[Table 3]
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Table 3
After 10% cold wodcing After simulated
expansion (25% cold woilcing)
Test Yield Required Uniform yield Corrosion
Steel BCC mtio SSC
No. strength elongation elongation strength rate
(MPa) (%) (%) (MPa) (%) resistance(g/m2/11)
24 A 622 ao 67 1154 _ # 0 09
25 B 604 40 , 58 1142 _ # o Li
26 C 609 40 68 1148 _ # o 0.9
27 D 601 ao 66 1170 _ # o 1.0
28 E 520 ao 62 1080 - II 0 0.9
29 F 531 ao 60 1083 _ # 0 0.8
30 G 524 40 64 1071 # o 0.8
31 II 578 ao 58 1120 _ # o 0.8 Inventive
32 1 582 40 , 56 1135 _ # o 1.0 example
33 J 519 40 _ 59 1070 _ # o 0.9
34 K 552 40 50 1120 _ # o 0.9
35 L 546 40 47 1083 _ # o 1.1
36 M 564 40 60 1100 _ 6 o 1.2
37 N 598 ao 56 1146 - # o 1.3
38 0 601 40 58 1124 _ # o 1.2
39 P 588 40 61 1089 _ # o 0.9
40 AA * 480 40 26 * 930 0.21 * o 1.0
41 AB * 542 40 19 * 1042 _ # o 0.8
42 AC * 607 ao 64 1193 _ # o 1.6
43 AD * 562 40 59 1104 _ a o 1.8 Comparative
example
44 AE * 545 40 61 1095 _ # x 0.9
45 AF * 528 ao 29 * , 978 0.14 * o 1.0
46 AG * 514 40 36 * 958 _ # o 1.1
* indicates that conditions do not satisfy those defmed by the present
invention.
# indicates that measured value is below the detection limit (0.01%).
[0074]
Table 2 shows that for Test Nos. 1 to 16, which are example embodiments of the
present invention, a uniform elongation of 60% or higher can be provided and
even in the
case where the cold working is performed at the working ratio of 25%
simulating the
expansion, the SSC resistance is excellent, and also the corrosion rate can be
kept at lower
than 1.5 g/(m2.11). Also, Table 3 shows that for Test Nos. 24 to 39, which are
example
embodiments of the present invention, a uniform elongation of 47% or higher
can be
provided in spite of the yield strength of 519 MPa or higher by performing the
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working at the working ratio of 10%, demonstrating that the present steel
materials have
excellent balance of strength and expandability. Even in the case where the
cold
working is performed at the working ratio of 25% simulating the expansion, the
SSC
resistance is excellent, and also the corrosion rate can be kept at lower than
1.5 g/(m2.11).
[0075]
On the other hand, for Test Nos. 17, 18, 22, 23, 40, 41, 45 and 46 in which
the C
content, the Mn content or the effective amount of C were less than the lower
limits
defined in the present invention, the test result was such that the uniform
elongation was
low and the expandability was poor. It is to be noted that for Test Nos. 22
and 23,
although the uniform elongation were 42% and 49%, respectively and tentatively
satisfied
the claimed definition, formula (ii) was not satisfied and the expandability
was not enough
considering the yield strength were as low as 308 MPa and 313 MPa.
[0076]
For Test Nos. 17, 22,40 and 45 it is thought that small amount of micro-
structure
having BCC structure was detected because the effective amount of C was out of
the
defined range and austenite stability was deteriorated, and consequently the
uniform
elongation was decreased. On the other hand, since mixed amount of micro-
structure
having BCC structure was small and strength was not so high, in the present
example,
deterioration of the SSC resistance was not observed.
[0077]
For Test Nos. 21 and 44 in which the Ni content was more than the upper limit
defined in the present invention, the test result was such that the SSC
resistance was poor.
Also, for Test Nos. 19 and 42 in which the Cu content was less than the
claimed lower
limit and Test Nos. 20 and 43 in which the Cr content was more than the
claimed upper
limit, the test result was such that, although the SSC resistance was good,
the corrosion
rate was high, and the general corrosion resistance was poor.
[0078]
Figure 1 is a graph showing the relationships between Mn content and uniform
elongation of steels after solid solution heat treatment and after cold
working at working
ratio of 10%, respectively, for steels A and B satisfying the definition of
the present
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invention and steels AB and AG out of the defined range. These steels have
similar
chemical composition except for the Mn content. As is apparent from Figure 1,
the steel
material according to the present invention in which the Mn content is more
than 25%
has high uniform elongation and excellent expandability.
[0079]
Figure 2 is a graph showing the relationships between Cu content and corrosion
rate of steels after solid solution heat treatment and after cold working at
working ratio of
10%, respectively, for steels A, C and D satisfying the definition of the
present invention
and steel AC out of the defined range. These steels have similar chemical
composition
except for the Cu content. As is apparent from Figure 2, for the steel
material according
to the present invention in which the Cu content is 0.5% or more, the
corrosion rate is
decreased and the general corrosion resistance is improved.
EXAMPLE 2
[0080]
Effects of aging heat treatment after solid solution treatment were
investigated
using steels K, L, 0 and AF which were prepared in EXAMPLE 1. The condition of
solid solution heat treatment is same as EXAMPLE 1. Additionally the aging
heat
treatment is performed under the condition of 800 C and 1 hour. The method for
evaluation test was same as EXAMPLE 1.
[0081]
Metal micro-structures of the aging heat treated test materials before and
after
the cold working at the working ratio of 25% were investigated by using the
ferrite meter
as is the case with EXAMPLE 1. For all of the test materials before the cold
working,
the phases having BCC structures could not be detected and the metal micro-
structures
were austenite single phases. Therefore, the volume amounts of the phases
having BCC
structures for the test materials only after the cold working are shown as a
BCC ratio by
volume % in tables. The results are given in Table 4.
[0082]
[Table 4]
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Table 4
After aging heat treatment After simulated expansion (25% cold working)
Test Steel BCC ratio SSC Yield Required Uniform Ykld
Concision
No. strength elongation elongation strength rate
(0/0) resis ta re (MPa) (%) (%) (MPa)(g/m2no
47 K 544 4() 42 1092 _ # 0.9
48 L 508 40 49 1053 # 1.1 Inventive
example
49 0 510 ao 46 1014 _ # o 1.3
50 AF * 503 40 34 * 998 0.12 * a 1.1 Comp. ex.
* indicates that conditions do not satisfy those defined by the present
invention.
# indicates that measured value is below the detection limit (0.01%).
[0083]
Table 4 demonstrates that for Test Nos. 47 to 49, which are example
embodiments of the present invention, a uniform elongation of 40% or higher
can be
assured while strengthening the steels such that the yield strength is 500 MPa
or higher
by performing the aging heat treatment for steels that contain V. On the other
hand, for
Test No. 50, which.is comparative example, small amount of micro-structure
having BCC
structure was detected because the effective amount of C was out of the
defined range,
although the yield strength was 500 MPa or higher due to the aging heat
treatment.
Consequently the uniform elongation was 34% and the result was such that
expandability
was poor.
INDUSTRIAL APPLICABILITY
[0084]
According to the present invention, a steel material having a high uniform
elongation and thus a high expandability, and excellent SSC resistance after
cold working
can be obtained. Therefore, the steel material according to the present
invention can be
used suitably for expandable oil country tubular goods in wet hydrogen sulfide
environments.
28
