DESCRIPTION
HIGH-STRENGTH STEEL MATERIAL FOR OIL WELL AND OIL WELL PIPES
TECHNICAL FIELD
[OOOl]
The present invention relates to a high-strength steel material for oil well and oil
well pipes, and more particularly, to a high-strength steel material for oil well excellent
in sulfide stress cracking resistance, which is used in oil well and gas well environments
and the like enviromnents containing hydrogen sulfide (H2S) and oil well pipes using the
same
BACKGROUND ART
[0002]
In oil wells and gas wells (hereinafter, collectively referred simply as "oil wells")
of crude oil, natural gas, and the like containing H2S, sulfide stress-corrosion cracking
(hereinafter, referred to as "SSC") of steel in wet hydrogen sulfide environments poses a
problem, and therefore oil well pipes excellent in SSC resistance are needed. In recent
years, the strengthening of low-alloy sour-resistant oil well pipes used in casing
applications has been advanced.
[0003]
The SSC resistance deteriorates sharply with the increase in steel strength.
Therefore, conventionally, steel materials capable of assuring SSC resistance in the
environment of NACE solution A @ACE TM0177-2005) containing 1-bar H2S, which is
the general evaluation condition, have been steel materials of 110 ksi class (yield strength:
758 to 862 MPa) or lower. In many cases, higher-strength steel materials of 125 ksi
class (yield strength: 862 to 965 MPa) and 140 ksi class (yield strength: 965 to 1069 MPa)
can only assure SSC resistance under a limited H2S partial pressure (for example, 0.1 bar
or lower). It is thought that, in the future, the corrosion environment will become more
and more hostile due to larger depth of oil well, so that oil well pipes having higher
strength and higher corrosion resistance must be developed.
[0004]
The SSC is a kind of hydrogen embrittlement in which hydrogen generated on
the surface of steel material in a corrosion environment diffuses in the steel, and
resultantly the steel material is ruptured by the synergetic effect with the stress applied to
the steel material. In the steel material having high SSC susceptibility, cracks are
generated easily by a low load stress as compared with the yield strength of steel material.
[OOOS]
Many studies on the relationship between metal micro-structure and SSC
resistance of low-alloy steel have been conducted so far. Generally, it is said that, in
order to improve SSC resistance, it is most effective to turn the metal micro-strncture into
a tempered martensitic structure, and it is desirable to tum the metal micro-structure into
a fine grain structure.
[0006]
For example, Patent Document 1 proposes a method which refines the crystal
grains by applying rapid heating means such as induction heating when the steel is heated.
Also, PatentDocument 2 proposes a method which refines the crystal grains by quenching
the steel twice. Besides, for example, Patent Document 3 proposes a method which
improve the steel performance by making the structure of steel material bainitic. All of
the object steels in many conventional techniques described above each have a metal
micro-stmcture consisting mainly of tempered martensite, ferrite, or bainite.
[0007]
The tempered martensite or ferrite, which is the main structure of the abovedescribed
low-alloy steel, is of a body-centered cubic system (hereinafter, referred to as
a "BCC"). The BCC structure inherently has high hydrogen embrittlement susceptibility.
Therefore, for the steel whose main structure is tempered martensite or fenite, it is very
difficult to prevent SSC completely. In particular, as described above, SSC
susceptibility becomes higher with the increase in strength. Therefore, it is said that to
obtain a high-strength steel material excellent in SSC resistance is a problem most
difficult to solve for the low-alloy steel.
[0008]
In contrast, if a highly corrosion resistant alloy such as stainless steel or high-Ni
alloy having an austenitic structure of a face-centered cubic system (hereinafter, referred
to as an "FCC"), which inherently has low hydrogen embrittlement susceptibility, is used,
SSC can he prevented. However, the austenitic steel generally has a low strength as is
solid solution treated. Also, in order to obtain a stable austenitic structure, usually, a
large amount of expensive component element such as Ni must be added, so that the
production cost of steel material increases remarkably.
[0009]
Manganese is known as an austenite stabilizing element. Therefore, the use of
austenitic steel containing much Mn as a material for oil well pipes in place of expensive
Ni has been considered. Patent Document 4 discloses a technique in which a steel
containing C: 0.3 to 1.6%, Mn: 4 to 35%, Cr: 0.5 to 20%, V 0.2 to 4%, Nb: 0.2 to 4%,
and the like is used, and the steel is strengthened by precipitating carbides in the cooling
process after solid solution treatment. Also, Patent Document 5 discloses a technique in
which a steel containing C: 0.10 to 1.2%, Mn: 5.0 to 45.0%, V 0.5 to 2.0%, and the like
is subjected to aging treatment after solid solution treatment, and the steel is strengthened
by precipitating V carbides. Further, Patent Document 6 discloses a steel that contains
C: 1.2% or less, Mn: 5 to 45%, and the like and is strengthened by cold working.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[OOlO]
Patent Document 1: JP61-95 19A
Patent Document 2: JP59-232220A
Patent Document 3: JP63-93822A
Patent Document 4: JP60-39150A
Patent Document 5: JP9-249940A
Patent Document 6: JP 10-1 21202A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[OOll]
Since the austenitic steel generally has a low strength, in Patent Documents 4
and 5, the steel is strengthened by the precipitation of carbides. However, to realize high
strength, aging must be performed for a considerably long period of time, and the longterm
aging is not necessarily favorable fiom the viewpoint of productivity.
[0012]
In Patent Document 6, a yield stress a bit larger than 100 kgffnud is attained by
performing cold working of 40% working ratio. However, the result of study conducted
by the present inventors revealed that, in the steel of Patent Document 6, a' martensite is
formed by strain induced transformation due to the increase in degree of cold working,
and the SSC resistance is sometimes deteriorated. Also, for the steel of Patent
Document 6, elongation is decreased sharply with the increase in degree of cold working,
and the workability is decreased, so that there remains room for improvement.
[0013]
An objective of the present invention is to provide a high-strength steel material
for oil well and oil well pipes using the same that is excellent in SSC resistance, has
corrosion resistance as high as that of low-alloy steel from the viewpoint of general
corrosion, and moreover, has a high economic efficiency, and is capable of being
produced without much trouble by using the conventional industrial facility.
MEANS FOR SOLVING THE PROBLEMS
[0014]
As described above, SSC is a kind of hydrogen embrittlement. The present
inventors conducted studies, as in the invention of Patent Document 6, to form austenite
phase by using a relatively large amount of Mn, and to increase the steel strength by
means of cold working. However, as described above, in Patent Document 6, in order
to realize the yield stress of 125 ksi class, the working ratio of about 40% is required,
which is subject to the restriction of facility.
[0015]
The present inventors focused a region containing large amounts of austenite
phase stabilizing elements, that is, a region in which Ni equivalent (Nieq = Ni + 30C +
0.5Mn) defmed in the present invention is high, which region has been unconfirmed
co~~ventionallayn,d examined the practical performance of the region. As the result, the
present inventors came to obtain the following findings.
[OO 161
(A) By increasing mainly the contents of C and Mn for Nieq of 27.5 or higher,
high strength can be realized even at a relatively low working ratio, and the structure ratio
of BCC structure can be restrained even after heavy working, so that the SSC resistance
can be assured.
[0017]
(B) By increasing mainly the contents of C and Mn for Nieq of 27.5 or higher,
large elongation can be maintained even after heavy working, and the occurrence of fine
cracks on the surface can be prevented, so that cold working can be performed reasonably
even at a high working ratio.
[0018]
(C) When the value of Nieq is increased, if the content of Mn is excessive, the
general corrosion resistance is deteriorated.
[0019]
(D) Although Ni contributes to the stabilization of austenite, if Ni is contained
excessively, the SSC resistance deteriorates in a high-strength material.
[0020]
The present invention has been accomplished on the basis of the above-described
findings, and involves the high-strength steel material for oil well and oil well pipes
described below.
[0021]
(1) A high-strength steel material for oil well having a chemical composition
consisting, by mass percent, of
C: 0.60 to 1.4%,
Si. 0.05 to 1.00%,
Mn: 12 to 25%,
Al: 0.003 to 0.06%,
P. 0.03% or less,
S: 0.03% or less,
N: less than 0.1%,
Cr. 0% or more and less than 5.0%,
Mo: 0% or more and less than 3.0%,
Cu: 0% or more and less than 1.0%,
Ni: 0% or more and less than 1.0%,
V: 0 to 0.5%,
Nb: 0 to 0.5%,
Ta: 0 to 0.5%,
Ti: 0 to 0.5%,
Zr: 0 to 0.5%,
Ca: 0% or more and less than 0.005%,
Mg: 0% or more and less than 0.005%,
B: 0 to 0.015%,
the balance: Fe and impurities,
whereinNieq defined by the following Formula (i) is 27.5 or higher,
a metal micro-structure is a structure consisting mainly of an FCC structure, a
total volume fraction of fenite and a' martensite is less than 0.10%, and
a yield strength is 862 MPa or higher;
Nieq = Ni + 30C + 0.5Mn ... (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.
[OD221
(2) The high-strength steel material for oil well according to (I),
wherein the chemical composition contains, by mass percent,
one or two elements selected from
Cr: 0.1% or more and less than 5.0% and
Mo: 0.1% or more and less than 3.0%.
100231
(3) The high-strength steel material foi oil well according to (1) or (2):
wherein the chemical composition contains, by mass percent,
one or two elements selected from
Cu: 0.1 % or more and less than 1.0% and
Ni: 0.1% or more and less than 1.0%.
[0024]
(4) The high-strength steel material for oil well according to any one of (1) to
(31,
wherein the chemical composition contains, by mass percent,
one or more elements selected from
V: 0.005 to 0.5%,
Nb: 0.005 to 0.5%,
Ta: 0.005 to 0.5%,
Ti: 0.005 to 0.5% and
Zr: 0.005 to 0.5%.
[0025]
(5) The high-strength steel material for oil well according to any one of (1) to
(41,
wherein the chemical composition contains, by mass percent,
one or two elements selected from
Ca: 0.0003% or more and less than 0.005% and
Mg: 0.0003% or more and less than 0.005%.
[0026]
(6) The high-strength steel material for oil well according to any one of (1) to
(51,
wherein the chemical composition contains, by mass percent,
B: 0.0001 to 0.015%.
100271
(7) The high-strength steel material for oil well according to any one of (1) to
(6),
wherein the yield strength is 965 MPa or higher.
[0028]
(8) Oil well pipes, which are comprised of the high-strength steel material for oil
well according to any one of (1) to (7).
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0029]
According to the present invention, a steel material having a high strength and
excellent SSCresistance can be obtained at a low cost by using the conventional industrial
facility. Additionally, because of being also excellent in elongation, the steel material
of the present invention is excellent in workability. Therefore, the high-strength steel
material for oil well according to the present invention can be used suitably for oil weU
pipes in wet hydrogen sulfide environments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030]
[Figure 11 Figure 1 is a graph showing the relationship between degree of cold working
and elongation.
[Figure 21 Figure 2 is a graph showing the relationship between degree of cold working
and total volume fraction of ferrite and a' martensite.
MODE FOR CARRYING OUT THE INVENTION
[003 11
Components of the present invention is described below in detail.
[0032]
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 "% by
mass".
LO0331
C: 0.60 to 1.40%
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, 0.60% 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
remarkably and the hot workability is deteriorated. Therefore, the C content is set to
1.40% or less. In order to obtain the high-strength steel material for oil well excellent
in balance of strength and elongation, the C content is preferably more than 0.80%, further
preferably 0.85% or more. Also, the C content is preferably 1.30% or less, further
preferably 1.25% or less.
[0034]
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 LO%, 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.
[0035]
Mn: 12 to 25%
Manganese (Mn) is an element capable of stabilizing austenite phase at a low
cost. In order to exert the effect in the present invention, 12% or more 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. If more than 25% of Mn 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 25% or less.
[0036]
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%NaCI + 0.5%CH3COOH aqueous solution, 1-bar H2S
saturated) specified in NACETM0177-2005 for 336 h, being 1.5 g/(m2.h).
100371
Al: 0.003 to 0.06%
Aluminum (Al) is an element necessary for deoxidation of steel, and therefore
0.003% or more of A1 bas to be contained. However, if the content of A1 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 A1 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, A1 means acid-soluble A1 (sol.Al).
[0038]
P: 0.03% or less
Phosphorus (P) is an element existing unavoidably in steel as an impnrity.
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%.
[0039]
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 fonns 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%.
[0040]
N: less than 0.10%
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.
Therefore, the content of N has to be set to less than 0.10%. 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%.
[0041]
Cr: 0% or more and less than 5.0%
Cbromium (Cr) may be contained as necessary because it is an element for
improving the general corrosion resistance. However, if the content of Cr is 5.0% or
more, Cr segregates at grain boundaries, and thereby the SSC resistance is deteriorated.
Further, the stress corrosion cracking resistance (SCC resistance) may be deteriorated.
Therefore, the content of Cr, if being contained, is set to less than 5.0%. The Cr content
is preferably less than 4.5%, further preferably less than 3.5%. 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.
[DO421
Mo: 0% or more and less than 3.0%
Molybdenum (Mo) may be contained as necessary because it is an element for
stabilizing corrosion products in wet hydrogen sulfide environments and for improving
the general corrosion resistance. However, if the content of Mo is 3% or more, the SSC
resistance and SCC resistance may be deteriorated. Also, since Mo is a very expensive
element, the content ofMo, ifbeing contained, is set to less than 3.0%. 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.
[0043]
Cu: 0% or more and less than 1 .O%
Copper (Cu) may be contained as necessary, if in a small amount, because it is
an element capable of stabilizing austenite phase. However, in the case where the
influence on the corrosion resistance is considered, Cu is an element that promotes local
corrosion, and is liable to form a stress concentrating zone on the surface of steel material.
Therefore, if Cu is contained excessively; the SSC resistance and SCC resistance may be
deteriorated. For this reason, the content of Cu, if being contained, is set to less than
1.0%. In the case where it is desired to achieve the effect of stabilizing austenite, the Cu
content is preferably set to 0.1% or more, further preferably set to 0.2% or more.
[0044]
Ni: 0% or more and less than 1.0%
Nickel (Ni) may be contained as necessary, if in a small amount, because it is an
element capable of stabilizing austenite phase as is the case with Cu. However, in the
case where the influence on the corrosion resistance is considered, 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 and SCC
resistance may be deteriorated. For this reason, the content of Ni, if being contained, is
set to less than 1.0%. In the case where it is desired to achieve the effect of stabilizing
austenite, the Ni content is preferably set to 0.1% or more, further preferably set to 0.2%
or more.
[0045]
V 0 to 0.5%
Nb: 0 to 0.5%
Ta: 0 to 0.5%
Ti: 0 to 0.5%
Zr: 0 to 0.5%
Vanadium (V), 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. The steel
material of the present invention is intended to be strengthened by cold working after
solid solution treatment. 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. 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.
[0046]
Ca: 0% or more and less than 0.005%
Mg: 0% or more and less than 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,
the effect is saturated and the inclusions are liable to be clustered to deteriorate toughness
and corrosion resistance. Therefore, the content of each element is less than 0.005%.
The content of each element is preferably 0.003% or less. When both Ca and Mg are
contained the total content of these elements is preferable less than 0.005%. In order to
obtain the effect, the content of one or two elements fiom these elements is preferably
0.0003% or more, further preferably 0.0005% or more.
[0047]
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 1s 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.
[0048]
The high-strength steel material for oil well of the present invention has the
chemical composition consisting of the elements ranging from C to B, the balance being
Fe and impurities.
[0049]
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.
[0050]
Nieq: 27.5 or higher
Nieq means Ni equivalent, and is defined by the following Formula (i). In the
present invention, the high strength of steel material can be attained by cold working.
However, in the case where austenite phase is not stable, strain induced a' martensite is
formed, and thereby the SSC resistance is deteriorated remarkably. Even in the case
where the steel material has the above-described chemical composition, if both of the
contents of C and Mn are low, the austenite phase becomes unstable. Therefore, for the
steel material of the present invention, to stabilize the austenite phase sufficiently, the
chemical composition must be regulated so that the Nieq represented by Formula (i) is
27.5 or higher. The Nieq is preferably set to 29 or higher, hrther preferably set to 32 or
higher.
Nieq = Ni + 30C + O.SMn ... (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.
[0051]
2. Metal micro-structure
As described above, if a' martensite and ferrite each having a BCC structure are
intermixed in the metal micro-structure, the SSC resistance is deteriorated. In particular,
if the total volume kaction of the a' martensite and ferrite is 0.1% or more, the SSC
resistance is deteriorated remarkably. Considering this point, in the present invention,
the metal micro-structure is made a structure consisting mainly of an FCC structure, and
the total volume fraction of the a' martensite and ferrite is defined as less than 0.1%.
[0052]
In the present invention, as a structure consisting mainly of an FCC structure,
the intermixing of E martensite of an HCP structure besides an FCC structure serving as
a matrix of steel is allowed. The volume fraction of E martensite is preferably 10% or
less.
[0053]
Since the a' martensite and femte exist in the metal micro-structure as fine
crystals, it is difficult to measure the volume fraction thereof by means of X-ray
diffraction, microscope observation or the like. Therefore, in the present invention, the
total volume fraction of the structure having a BCC structure is measured by using a
ferrite meter.
[0054]
Since Nieq defmed by Formula (i) is made 27.5 01- higher, the steel material
according to the present invention has a metal micro-structure consisting mainly of
austenite in the state after solid solution heat treatment. To realize a yield strength of
862 MPa or higher, the steel material according to the present invention is strengthened
by cold working. h the case where an austenitic steel is cold-worked, a part of austenite
is sometimes transformed by strain induced transformation.
[0055]
The steel material according to the present invention has a possibility of being
subjected to E martensitic transformation by strain induced transformation; however, even
if a' martensite is formed, the formation is suppressed to a very small amount. Also,
since the E martensite has an HCP structure, even if & martensite is formed, hydrogen
embrittlement does not occur, and the SSC resistance is not adversely affected. That is
to say, for the steel material of the present invention, even if strain induced transformation
occurs, a' martensite is scarcely formed, so that the SSC resistance is less liable to be
deteriorated.
100561
3. Mechanical properties
The steel material according to the present invention is a high-strength steel
material for oil well having a yield strength of 862 MPa or higher. As described above,
the SSC resistance deteriorates rapidly with the rise in the strength of steel; however, in
the steel material according to the present invention, a yield strength as high as 862 MPa
and excellent SSC resistance can be compatible with each other. Also, when the yield
strength is 965 MPa or higher, the high-strength steel material for oil well according to
the present invention further achieves the effects thereof.
[0057]
The high-strength steel material for oil well according to the present invention
has a feature of having a large elongation even when being cold-worked at a high working
ratio. The steel material according to the present invention exhibits an elongation
(elongation after fracture) of preferably 15% or more, further preferably 20% or more.
[0058l
4. Production method
Tbe method for producing the steel material according to the present invention
is not subject to any special restriction as far as the above-described strength can be given
by the method. For example, the method described below can be employed.
[0059]

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.
[0060]

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.
[0061]
The heating of billet inay 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.
[0062]
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.
[0063]

The steel material having been subjected to solid solution heat treatment or
further aging heat treatment is cold-worked to realize the target yield strength, a strength
of 862 MPa (125 ksi) or higher. In this case, it is preferable to perform cold working at
a working ratio (reduction of area) of 20% or higher. In order to obtain a high strength
of 965 MPa or higher, it is preferable to make the working ratio 30% or higher. Since
the steel material according to the present invention holds a high ductility even after being
heavily worked, even if the working ratio is increased to 40%, cold working can be
performed without the occurrence of fine cracks and the like on the surface.
[0068]
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.
[0069]

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 50OoC.
[0070]
Hereunder, the present invention is explained more specifically with reference
to examples; however, the present invention is not limited to these examples.
EXAMPLE 1
[0071]
Thirty-five kinds of steels of A to V and AA to AM 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
electrical discharge cutting-off. Thereafter, the cut ingot was further soaked at 1150°C
for 1 h, and was hot-rolled into a plate material having a thickness of 20 mm.
Subsequently, the plate material was subjected to solid solution heat treatment at 1100°C
for 1 h. Finally, the plate material was cold-rolled up to 50% reduction in thickness
("reduction of thickness" is substantially equal to "reduction of area" in this case) to
obtain a test material.
[0072]
[Table 11
[0073]
On the obtained test material, first, the total volume ratio of fenite and a'
martensite was measured by using a ferrite meter (model number: FE8e3) manufactured
by Helmut Fischer. On the obtained test specimen, a' martensite and 6 martensite were
confirmed by X-ray diffraction. However, on all of the test specimens, the existence of
these kinds of martensite could not be detected with the X-ray diffraction.
[0074]
By using the above-described test materials, the SSC resistance, the SCC
resistance, and the mechanical properties were examined. The SSC resistance and SCC
resistance were evaluated by using a round-bar type tensile test specimen (parallel part:
6.35 mm in diameter x 25.4 mm in length) sampled from the L direction (rolling direction)
of the test material. The load stress was made 90% of the measured value of the yield
strength ofbasemetal. Thereason why the SCC resistance was evaluated is as described
below.
[0075]
As one kind of environment cracks of an oil well pipe occurring in the oil well,
inherently, attention must be paid to SCC (stsess corrosion cracking). The SCC is a
phenomenon in which cracks are propagated by local corrosion, and is caused by partial
fracture of the protection film on the surface of material, grain-boundary segregation of
alloying element, and the like. Conventionally, SCC has scarcely been studied ffom the
view point of the SCC resistance because corrosion advances wholly in a low-alloy oil
well pipe having tempered martensite, and the excessive adding of alloying element that
brings about grain-boundary segregation leads to the deterioration in SCC resistance.
Further, sufficient findings have not necessarily been obtained concerning the SCC
susceptibility of a steel equivalent or similar to the steel material of the present invention,
which has a component system vastly different from that of low-alloy steel, and has
austenitic structure. Therefore, an influence of component on the SCC susceptibility
and the like must be clarified.
[0076]
The SSC resistance was evaluated as described below. Aplate-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 (S%NaCl + O.S%CH3COOH
aqueous solution, 1-bar HzS 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 "NF" in Table 2), and a ruptured steel material was evaluated so that the
SSC resistance is poor (referred to as "F" in Table 2).
[0077]
Concerning the SCC resistance as well, aplate-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, the same solution A as described above, and was held
in a test environment of 60°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
SCC resistance is good (referred to as "NF" in Table 2), and aruptured steel material was
evaluated so that the SCC resistance is poor (referred to as "F" in Table 2). This test
solution is a test environment less liable to produce SSC because the temperature thereof
is 60°C and thereby the saturated concentration of H2S in the solution is decreased
compared with that at normal temperature. Concerning the test specimen in which
cracking occurred in this test, whether this cracking is SCC or SSC was judged by
o b s e ~ ntgh e propagation mode of crack under an optical microscope. Concerning the
specimen of this test, it was confirmed that, for all of the test specimens in which cracking
occurred in the above-described test environment, SCC had occurred.
[0078]
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 nonnal temperature for 336 h, the corrosion loss was
determined, and the corrosion loss was converted into the average corrosion rate.
[0079]
Concerning the mechanical properties, yield strength and 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 rnm in length was sampled. A tension
test was conducted at normal temperature (25"C), whereby the yield strength YS (0.2%
yield s.tress) (MPa) and the elongation (%) were determined.
[OOSO]
These results are collectively given in Table 2. For the examination results of
the total volume ratio of ferrite and a' martensite, the SSC resistance, the SCC resistance,
and the corrosion rate, Table 2 gives the values of a test material having been subjected
to 40% cold working. This is because, since these measurement results tend to be
deteriorated with the increase in degree of cold working, evaluation is performed under
severer condition.
[0081]
Furthermore, concerning the yield strength and elongation, the values of a test
material having been subjected to 30% cold working are given. This is because, if the
degree of cold working is 30%, the yield strength and elongation can be provided without
much trouble by using the general cold working facility, so that the obtained values can
be judged to be realistic values.
[0082]
[Table 21
[0083]
From Table 2, it can be seen that for Test Nos. 1 to 22, which are example
embodiments of the present invention, a yield strength of 862 MPa or higher can be
provided by cold working at a working ratio of 30%, which can be performed without
much trouble by using the conventional industrial facility. Also, even in the case where
heavy working is performed at a working ratio of 40%, which is a severer condition, the
SSC resistance and SCC resistance are excellent, and also the corrosion rate can be kept
at 1.5 g/(m2.h), which is the target value, or lower.
[0084]
On the other hand, for Test Nos. 23 to 27 in which the C content or the Mn
content were lower than the lower limits defined in the present invention, the test result
was such that the total volume fraction of BCC structure was 0.1% or more, and the SSC
resistance was poor. Likewise, for Test No. 28, in which, although the contents of C and
Mn were witlun the range defined in the present mnvention, the value of Nieq was lower
than the lower limit defined in the present invention, the test result was such that the SSC
resistance was poor.
[OOSS]
Also, for Test Nos. 29 to 3 1 in which the Mn content was higher than the upper
limit defined in the present invention, the test result was such that, although the SSC
resistance was good, the corrosion rate was high, and the general corrosion resistance was
poor. Besides, for Test No. 32 in which the Cr content was out of the defined range, and
Test No. 34 in which the Cu content was out of the defined range, the test result was such
that the SCC resistance was poor. For Test No. 33 in which the Mo content was out of
the defined range, and Test No. 35 in which the Ni content was out of the defined range,
the test result was such that the SSC resistance and SCC resistance were poor.
[0086]
Figures 1 and 2 are graphs showing the elongation and the total volume fraction
of ferrite and a' martensite, respectively, at the degree of cold working of 0 to 50% for
steel A satisfying the definition of the present invention and steels AA and AD out of the
defined range. As is also apparent from Figures 1 and 2, the steel material according to
the present invention is excellent in elongation, and can keep the volume ffaction of BCC
structure low even in the case of being cold-worked at a high working ratio.
EXAMPLE 2
[0087]
Effects of aging heat treatment after solid solution treatment and before cold
working, and annealing after cold working, respectively, were investigated using steels C,
F and M after hot rolling which were prepared in EXAMPLE 1. The condition of solid
solutioll heat treatment is same as EXAMPLE 1. Additionally the aging heat treatment
is performed under the condition of 600°C and 30 min, and the annealing is performed
under the condition of 500°C and 30 min. For Test Nos. 36 to 38, steels C, F and M
were subjected to the aging heat treatment before cold working. On the other hand, for
Test Nos. 39 to 41, similarly steels C, F and M were subjected to the annealing after cold
working. The methods for cold working and evaluation test were same as EXAMPLE
1. Table 3 shows these results.
[0088]
[Table 31
[0089]
Table 3 illustrates that it is effective to contain V andNb because for Test No. 38
higher yield strength is achieved by performing aging heat treatment before cold working
as compared to that of Test No. 13 for which steel M is used. In contrast, for Test Nos.
36 and 37 which used steels C and F containing neither V nor Nb, yield strengths are not
enhanced as compared to those of Test Nos. 3 and 6 for which same steels are used.
Additionally, for Test Nos. 39, 40 and 41 annealing is performed after cold working,
resulting in decrease of the yield strengths of about 20 to 100 MPa and enhancement of
the elongation of up to 4%.
INDUSTRIAL APPLICABILITY
[0090]
According to the present invention, a steel material having a high strength and
excellent SSC resistance can be obtained at a tow cost by using the conventional industrial
facility. Additionally, because of being also excellent in elongation, the steel material
of the present invention is excellent in workability. Therefore, the high-strength steel
material for oil well according to the present invention can be used suitably for oil well
pipes in wet hydrogen sulfide environments.

We claim:
1. A high-strength steel material for oil well having a chemical composition
consisting, by mass percent, of
C: 0.60 to 1.4%,
Si: 0.05 to 1.00%,
Mn: 12 to 25%,
Al: 0.003 to 0.06%,
P: 0.03% or less,
S: 0.03% or less,
N: less than 0.1%,
Cr: 0% or more and less than 5.0%,
Mo: 0% or more and less than 3.0%,
Cu: 0% or more and less than 1.0%,
Ni: 0% or more and less than 1.0%,
V: 0 to 0.5%,
Nb: 0 to 0.5%,
Ta: 0 to 0.5%,
Ti: 0 to 0.5%,
Zr: 0 to 0.5%,
Ca: 0% or more and less than 0.005%,
Mg: 0% or more and less than 0.005%,
B: 0 to 0.015%,
the balance: Fe and impurities,
wherein Nieq defined by the following Formula (i) is 27.5 or higher,
a metal micro-structure is a structure consisting mainly of an FCC structure, a
total volume fraction of ferrite and a' martensite is less than 0.10%, and
a yield strength is 862 MPa or higher;
Nieq = Ni + 30C + 0.5Mn . . . (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.
2. The high-strength steel material for oil well according to claim 1,
wherein the chemical composition contains, by mass percent,
one or two elements selected from
Cr: 0.1% or more and less than 5.0% and
Mo: 0.1% or more and less than 3.0%. .
3. The high-strength steel material for oil well according to claim 1 or 2,
wherein the chemical composition contains, by mass percent,
one or two elements selected fiom
Cu: 0.1% or more and less than 1.0% and
Ni: 0.1% or more and less than 1.0%.
4. The high-strength steel material for oil well according to any one of claims
1 to 3,
wherein the chemical composition contains, by mass percent,
one or more elements selected from
V: 0.005 to 0.5%,
Nb: 0.005 to 0.5%,
Ta: 0.005 to 0.5%,
Ti: 0.005 to 0.5% and
Zr: 0.005 to 0.5%.
5. The high-strength steel material for oil well according to any one of claims
1 to 4,
wherein the chemical composition contains, by mass percent,
one or two elements selected from
Ca: 0.0003% or more and less than 0.005% and
Mg: 0.0003% or more and less than 0.005%.
6. The high-strength steel material for oil well according to any one of claims
1 to 5,
wherein the chemical composition contains, by mass percent,
B: 0.0001 to 0.015%.
7. The high-strength steel material for oil well accordiiig to any one of claims
1 to 6,
wherein the yield strength is 965 MPa or higher.
8. Oil well pipes, which are comprised of the high-strength steel material for
oil well according to any one of claims 1 to 7.