ESCRIPTION
STAINLESS STEEL FOR OIL WELLS AND STAINLESS STEEL PIPE
FOR OIL WELLS
Technical Field
[ OOOl]
The present invention relates to a stainless steel
for oil wells and a stainless steel pipe for oil wells,
and more particularly to a stainless steel for oil wells
and a stainless steel pipe for oil wells, which are used
in a high-temperature oi1,well environment and gas well
environment (hereinafter, referred to as a high-
temperature environment).
Background Art
In the present description, an oil well.and a gas
well are collectively referred to simply as "an oil well".
Accordingly, "a stainless steel for oil wells" as used
herein includes a stainless steel for oil wells and a
stainless steel for gas-well. Also "a stainless steel
pipe for oil wells" includes a stainless steel pipe for
oil wells and a stainless steel pipe for gas well.
[0003]
As used herein, the term "a high temperature" means,
unless otherwise stated, a temperature not less than
-_.- -. - .
-
150°C. Also as used herein, the symbol "%" r e l a t i n g to a
chemical element means, unless otherwise stated, "mass%".
A conventional o i l well environment contains carbon
dioxide gas (COz) and/or chlorine ion ( C l - ) . For t h a t
reason, i n a conventional o i l well environment, a
martensitic s t a i n l e s s s t e e l containing 13% of C r
(hereafter, referred t o as a "13% C r s t e e l " ) is commonly
used. The 13%Cr s t e e l is excellent i n carbonic-acid gas
corrosion resistance.
[OOOS]
Recently, the development of deep o i l wells has
advanced. A deep o i l well has a high-temperature
environment. Such high-temperature environment includes.
carbon dioxide gas or carbon dioxide gas and hydrogen
s u l f i d e gas. These gases a r e corrosive gases. Therefore,
s t e e l f o r o i l wells t o be used i n deep o i l wells is
required t o have a higher strength and a higher corrosion
resistance than those of the 13%Cr s t e e l .
[0006]
The C r content of a two-phase s t a i n l e s s s t e e l is
greater than that of the 13%Cr s t e e l . Therefore, a twophase
s t a i n l e s s steel has a higher strength and a higher
corrosion resistance than those of the 13%Cr s t e e l . The
- two-phase s t a i n l e s s s t e e l is, for example, a 22%Cr s t e e l
containing 22% of C r , and a 25%Cr s t e e l containing 25% of
C r . Although the two-phase s t a i n l e s s s t e e l has a high
s t r e n g t h and a high corrosion r e s i s t a n c e , it includes I
many a l l o y elements, and t h e r e f o r e is expensive.
[0007]
JP2002-4009A, JP2005-336595A, JP2006-16637A, JP2007-
332442A, ~02010/050519, and W02010/134498 propose
s t a i n l e s s steels o t h e r than t h e above described two-phase
s t a i n l e s s steel. The s t a i n l e s s steels d i s c l o s e d i n these I
l i t e r a t u r e s c o n t a i n a t the maximum 17 t o 18.5% of C r .
[0008]
JP2002-4009A proposes a m a r t e n s i t i c s t a i n l e s s s t e e l I
f o r o i l w e l l s , which has a y i e l d s t r e n g t h of n o t less 1
than 860 MPa and a carbonic-acid gas corrosion r e s i s t a n c e I
i n a high-temperature environment. The chemical 1
composition of t h e s t a i n l e s s steel d i s c l o s e d i n t h i s
l i t e r a t u r e contains 11.0 t o 17.0% of C r and 2.0 t o 7.0%
of N i , and f u r t h e r s a t i s f i e s : C r + Mo + 0.3Si - 40C - 10N
- N i - 0.3Mn ,< 10. The metal micro-structure of t h i s I
s t a i n l e s s s t e e l is predominantly made up of m a r t e n s i t e , 1
and c o n t a i n s not more than 10% of a r e t a i n e d a u s t e n i t e .
[0009]
JP2005-336595A proposes a s t a i n l e s s steel pipe which I
has a high s t r e n g t h and carbonic-acid gas c o r r o s i o n I
r e s i s t a n c e in a high-temperature environment of 230°C.
The chemical composition of t h e s t a i n l e s s steel pipe
d i s c l o s e d i n t h i s l i t e r a t u r e c o n t a i n s 15.5 t o 18% of C r ,
1.5 t o 5% of N i , and 1 t o 3.5% of Mo, s a t i s f i e s Cr + I
0.65Ni + 0.6Mo + 0.55Cu -20C 2 19.5 and a l s o s a t i s f i e s C r I
The metal micro-structure of this stainless steel .pipe
contains 10 to 60% of a ferrite phase, and not more than
30% of an austenite phase, the balance being a martensite
phase.
[OOlO]
JP 2006-16637A proposes a stainless steel pipe which
has a high strength and carbonic-acid gas corrosion
resistance in a high-temperature environment of more than
170°C. The chemical composition of the stainless steel
pipe disclosed in this literature contains 15.5 to 18.5%
of Cr, and 1.5 to 5% of Ni, satisfies Cr + 0.65Ni + 0.6Mo
+ 0.55Cu -20C 2 18.0 and also satisfies Cr + Mo + 0.3Si -
43.5C - 0.4Mn - Ni -0.3Cu - 9N 2 11.5. The metal microstructure
of this stainless steel pipe may or may not
include an austenite phase.
[OOll]
JP 2007-332442A proposes a stainless steel pipe
which has a high strength of not less than 965 MPa, and a
carbonic-acid gas corrosion resistance in a hightemperature
environment exceeding 170°C. The chemical
composition of the stainless stee-1-pipe disclosed in this
literature contains, by mass%, 14.0 to 18.0% of Cr, 5.0
to 8.0% of Ni, 1.5 to 3.5% of Mo, and 0.5 to 3.5% Cu, and
satisfies Cr + 2Ni + 1.1Mo + 0.7Cu I 32.5. The metal
micro-structure of this stainless steel pipe contains 3
to 15% of an austenite phase, the balance being a
martensite phase.
[0012]
. . . . . . .._ .. . . , ...
-
W02010/050519 proposes a stainless pipe which has a
sufficient corrosion resistance even in a hightemperature
carbon dioxide environment of 200°C, and
further has a sufficient sulfide stress-corrosion
cracking resistance even when the environment temperature
of an oil well or gas well declines due to a temporary
suspension of the collection of crude oil or gas. The
chemical composition of the stain,less steel pipe
disclosed in this literature contains:more than 16% and
not more than 18% of -Cr, more than 2% and not more than
3% of Mo, not less than 1% and not more than 3.5% of Cu,
and not less than 3% and less than 5% ,of Ni, while Mn and
N satisfy [Mn] x ( [N] - 0.0045) I 0.001. The metal
micro-structure of this stainless steel pipe contains 10
to 40% by volume fraction of a ferrit.e phase, and not
more than 10% by volume fraction of a retained y phase
with a martensite phase being as the dominant phase.
W02010/134498 proposes a high-strength stainless
steel which has an excellent corrosion resistance in a
high-temperature envir-onment, and has an SSC resistance
(sulfide stress-corrosion cracking resistance) at normal
temperature. The chemical composition of the stainless
steel disclosed in this literature contains more than 16%
and not more than 18% of Cr, not less than 1.6% and' not
more than 4.0% of Mo, not less than 1.5% and not more
than 3.0% of Cu, and more than 4.0% and not more than
5.6% of Ni, and satisfies Cr + Cu + Ni + Mo 2 25.5 and -8
-x-
6
I 3 0 ( C + N) + 0.5Mn + N i + Cu/2 + 8.2 - l . l ( C r + Mo) 5-4.
The metal micro-structure of t b i s s t a i n l e s s s t e e l
contains a martensite phase, 10 t o 40% of a f e r r i t e phase,
and a retained a u s t e n i t e phase, with a f e r r i t e phase
d i s t r i b u t i o n rate being higher than 85%.
Disclosure of t h e Invention
However, i n the s t a i n l e s s steels disclosed i n the
above described patent l i t e r a t u r e s , it is not necessarily
easy t o s t a b l y obtain a desired metal micro-structure,
and t h e r e may be a case where a d e s i r e d yield s t r e n g t h is
not s t a b l y obtained. In the i n d u s t r i a l production of
s t a i n l e s s s t e e l , t i m e spent for a heat treatment process
and a cooling process w i l l be l i m i t e d i n order t o improve
. productivity. Therefore, there may be a case where a
high s t r e n g t h not less than 758 MPa is not stably
obtained.
[OOlS]
It is an object of the present invention t o provide
a s t a i n l e s s -steel f o r o i l wells, which has an e x c e l l e n t
high-temperature corrosion r e s i s t a n c e and can s t a b l y
obtain a strength of not less than 758 MPa.
[0016]
A s t a i n l e s s s t e e l for o i l wells of the present
invention contains, by mass%, C: not more than 0.05%, Si:
not more than 1.0%, Mn: 0.01 to 1.0%, P: not more than
0.05%, S: less than 0.002%, C r : 16 t o 18%, Mo: 1 . 8 t o 3%,
Cu: 1.0 to 3.5%, Ni: 3.0 to 5.5%, Co: 0.01 to 1.0%, Al:
0.001 to 0.1%, 0: not more than 0.05%, and N: not more
than 0.05%, the balance being Fe and impurities, and
satisfies Formulas (1) and (2) :
Cr + 4Ni + 3Mo +. 2Cu 2 44 , (1)
Cr t 3Ni + 4Mo + 2Cu/3 I 46 (2)
where each symbol of element in Formulas (1) and (2)
is substituted by the content (mass%) of a corresponding
element.
[0017]
The above described stainless steel for oil wells
may contain, in place of some of Fe, one or more kinds of
elements selected from the group consisting of V: not
more than 0.3%, Ti: not more than 0.3%, Nb: not more than
0.3%,, and Zr: not more than 0.3%. The above described
stainless steel for oil wells may contain, in place of
some of Fe, one or more kinds of elements selected from .
the group consisting of W: not more than 1.0%, and rare
earth metal (REM) : not more than 0.3%. The above
described stainless steel for oil wells may contain, in
.. place of some of Fe, one or more kinds of elements
selecteh from the group consisting of Ca: not more than
0.01%, and B: not more than 0.01%.
[0018]
The metal micro-structure of the above described
stainless steel preferably contains, by volume ratio, not
less than 10% and less than 60% of a ferrite phase, not
more than 10% of a retained austenite phase, and not less
than 40% of a martensite phase.
[0019]
The.stainless steel pipe for oil wells according to
the present invention is manufactured from the above
described stainless steel for oil wells.
[0020] ,
The stainless steel pipe for oil wells according to
the present invention has a high strength and an
excellent high-temperature corrosion resistance and can
stably obtain high strength.
Best Mode for Carrying Out the Invention
[0021]
Hereafter, embodiments of the present invention will
be described in detail. The present inventors have
conducted a survey and analysis and consequently obtained
the following findings.
[0022]
(A) To obtain a stress corrosion cracking resistance
(SCC resistance) in a high-temperature environment, it is
preferable that Ni, Mo, and Cu besides Cr are contained.
To be more specific, an excellent SCC resistance will be
obtained in a high-temperature environment when the
following Formula (1) is satisfied:
[0023 1
Cr + 4Ni + 3Mo + 2Cu 2 44 (1)
where each symbol of element in Formula (1) is
substituted by the content (mass%) of a corresponding
element.
[0024]
,
(B) When the contents of alloy elements such as Cr,
Ni, Mot and Cu increase, it is not likely that a high
strength is stably obtained. h he variation of strength
will be suppressed, and yield strength of not less than
758 MPa will be stably obtained when the following
Formula (2) is satisfied:
Cr + 3Ni + 4Mo + 2Cu/3 I 46 (2)
where each symbol of element in Formula (2) is
substituted by the content (mass%) of a corresponding
element.
[0025]
(C) Co stabilizes the strength and corrosion
resistance. When Formulas (1) and (2) are satisfied, and
0.01 to 1.0% of Co is contained, a stable metal microstructure
will be obtained, and a stable and high
strength and an excellent corrosion resistance in a hightemperature
environment will be obtained.
[0026]
The present invention has been completed based on
the above described findings. Hereafter, details of the
stainless steel for oil wells of the present invention
will be described.
[0027]
[Chemical composition]
The s t a i n l e s s steel f o r o i l wells according t o the
p r e s e n t i n v e n t i o n has t h e following chemical composition.
[0028]
C: not more than 0.05%
Although carbon (C) c o n t r i b u t e s t o i n c r e a s e strength,
it. produces c a r b i d e a t the t i m e of tempering. Cr carbide
d e t e r i o r a t e s t h e c o r r o s i o n r e s i s t a n c e t o high-temperature
c a r b o n d i o x i d e g a s . Therefore, t h e C content is
p r e f e r a b l y s m a l l e r , The C content is not more than 0.05%. I
P r e f e r a b l y t h e C content is less than 0.05%, more
p r e f e r a b l y not more than 0.03%, and even more preferably
not more than 0.01%.
S i : not more than 1.0%
S i l i c o n ( S i ) deoxidizes steel. However, an
excessive S i c o n t e n t w i l l d e t e r i o r a t e h o t w o r k a b i l i t y .
Moreover, it i n c r e a s e s t h e amount of f e r r i t e to be
produced, thereby reducing y i e l d s t r e n g t h ( y i e l d stress).
Therefore, t h e S i content is not more t h a n 1 . 0 % .
P r e f e r a b l y t h e S i content is not more than 0.8%, more
p r e f e r a b l y not more than 0.5%, and even more p r e f e r a b l y
not more than 0.4%. When t h e Si content is not l e s s than
0.05%, S i a c t s i n a p a r t i c u l a r l y e f f e c t i v e manner as a
d e o x i d i z e r . However, even when t h e S i content is less
than 0.05%, S i deoxidizes steel t o some e x t e n t .
[0030]
Mn: 0.01 t o 1.0%
~ a n g a n e s e (Mn) deoxidizes and desulfurizes s t e e l ,
thereby improving hot workability. However, an excessive
Mn content is l i k e l y t o cause segregations in s t e e l ,
thereby d e t e r i o r a t i n g t h e toughness and t h e SCC
r e s i s t a n c e i n a high-temperature chloride aqueous
solution. Moreover, Mn is an a u s t e n i t e forming element.
Therefore, when s t e e l contains N i and Cu which are
a u s t e n i t e forming elements, an excessive Mn content w i l l
lead t o an increase of r e t a i n e d a u s t e n i t e , thereby
reducing the yield s t r e n g t h ( y i e l d stress). Therefore,
the Mn content is 0.01 t o 1.0%. The lower l i m i t of Mn
content is preferably 0.03%, more preferably 0.05%, and
even more preferably 0.07%. The upper l i m i t of Mn
content is preferably 0.5%, more preferably l e s s than
0.2%, and even more preferably 0.14%.
[0031]
P: not more than 0.05%
Phosphor ( P ) is an impurity. P d e t e r i o r a t e s the
s u l f i d e s t r e s s - c o r r o s i o n cracking r e s i s t a n c e (SSC
r e s i s t a n c e ) and the SCC r e s i s t a n c e i n a high-temperature
chloride aqueous solution environment of s t e e l .
Therefore, t h e P content is preferably as low as possible.
The P content is not more than 0.05%. Preferably the P
content is less than 0.05%, more preferably not more than
0.025%, and even more preferably not more than 0.015%.
[0032]
S: less than 0.002%
. . .-. . .. . .
Sulfur (S) is an impurity. S deteriorates hot
workability of s t e e l . The metal micro-structure of a
s t a i n l e s s s t e e l of the present invention becomes a twophase
micro-structure including a f e r r i t e phase and an
a u s t e n i t e phase during 'hot working. S deteriorates hot '
workability of such two-phase micro-structure. Further,
S combines with Mn e t c . t o form inclusions. The
inclusions formed a c t as a s t a r t i n g point of p i t t i n g and I
SCC, thereby d e t e r i o r a t i n g t h e corrosion resistance of * 1
s t e e l . Therefore, the S content is preferably as low as
possible. The S content is l e s s than 0.002%. Preferably
the S content is not more than 0.0015%, and more
preferably not more than 0.001%.
[0033]
Cr: 16 to 18%
Chromium (Cs) improves the SCC resistance i n a hightemperature
chloride aqueous solution environment.
However, since Cr is a f e r r i t e forming element, an
excessive C r content w i l l lead t o an excessive increase
i n t h e amount of f e r r i t e in s t e e l , thereby deteriorating
the y i e l d strength of s t e e l . Therefore, C r content is 16
t o 18%. The lower l i m i t of C r content is ~ r e f e r a b ml ~or e
than 16%, more preferably 16.3%, and even more preferably
16.5%. he upper l i m i t of C r content is preferably l e s s
than la%, more preferably 17.8%, and even more preferably .
17.5%.
Mo: 1.8 to.3%
. . . .. . . .
When the production of f l u i d is temporarily stopped
i n an o i l w e l l , t h e temperature of the f l u i d i n an o i l
w e l l pipe w i l l decline. A t t h i s moment, t h e s u l f i d e
s t r e s s - c o r r o s i o n cracking s u s c e p t i b i l i t y of a highs
t r e n g t h m a t e r i a l generally increases. Molybdenum (Mo)
improves the s u l f i d e stress-corrosion cracking
s u s c e p t i b i l i t y . Further, Mo improves the SCC r e s i s t a n c e
of steel under coexistence with Cr. However, since Mo. is
a f e r r i t e forming element, an excessive Mo content. w i l l
lead t o an increase of the amount of f e r r i t e i n s t e e l ,
thereby reducing t h e strength of s t e e l . Therefore, the
Mo content is 1.8 t o 3%. The lower l i m i t of Mo content
is preferably more than 1.8%, more preferably 2.0%, and
even more p r e f e r a b l y 2.1%. The upper l i m i t of Mo content
is preferably less thari 3%, more preferably 2.7%, ..and
even more p r e f e r a b l y 2.6%.
Cupper (Cu) strengthens a f e r r i t e phase by age
p r e c i p i t a t i o n , t h e r e b y i n c r e a s i n g the strength of s t e e l .
Further, Cu reduces t h e ' d i s s o l u t i o n r a t e of steel i n a
high-temperature chloride aqueous solution environment,
thereby improving t h e corrosion r e s i s t a n c e of s t e e l .
However, an excessive Cu content w i l l lead t o a
d e t e r i o r a t i o n of t h e hot workability of s t e e l , thereby
d e t e r i o r a t i n g t h e toughness of steel: Therefore, the Cu
content is 1.0 t o 3.5%. The lower l i m i t of Cu content is
preferably more than 1.0%, more preferably 1.5%, and even
more preferably 2.2%. The upper l i m i t of Cu content is
less than 3.5%, more preferably 3.2%, and even more
preferably 3.0%.
[0036]
N i : 3.0 t o 5.5%
Since nickel ( N i ) is an austenite forming element,
it s t a b i l i z e s austenite a t high temperature and increases .
the amount of martensite at normal temperature.
Therefore, N i increases t h e strength o f ' s t e e l . Further, .
N i improves the corrosion resistance i n a hightemperature
chloride aqueous solution environment.
However, an excessive N i content tends t o lead to an
increase of retained y phase, and it becomes d i f f i c u l t to
stably obtain a high strength especially a t the t i m e of
i n d u s t r i a l production. Therefore, the N i content is 3.0
to 5.5%: The lower l i m i t of N i content is preferably
more than 3.0%, more preferably 3.5%, even more
preferably 4.0%,' and even.more preferably 4.2%. The
upper l i m i t of N i content is preferably l e s s than 5.5%,
more preferably 5.2%, and even more preferably 4.9%.
COO371
Co: 0.01 t o 1.0% I
Cobalt (Co) .improves the hardenability of s t e e l , and I
ensures a s t a b l e and high strength especially a t the t i m e
of i n d u s t r i a l production. To be more s p e c i f i c , .Co
suppresses retained .austenite, thereby suppressing the
v a r i a t i o n o f strength. However, an excessive Co content
w i l l lead t o a d e t e r i o r a t i o n of the toughness of s t e e l .
. . .I '
Therefore, the Co content is 0.01 t o 1.0%. The lower
l i m i t of Co content is p r e f e r a b l y more than 0.01%, more .
p r e f e r a b l y 0.02%, even more p r e f e r a b l y 0.1%, and even
more p r e f e r a b l y 0.25%. The upper, l i m i t of Co content is
p r e f e r a b l y less than 1.0%, more p r e f e r a b l y 0.95%, and
even m o r e p r e f e r a b l y 0.75%.
[0038]
Al: 0.001 t o 0.1%
Aluminum ( A l ) deoxidizes steel; However, an
excessive A1 content w i l l lead t o a n i n c r e a s e of t h e
amount of ferrite i n steel, thereby d e t e r i o r a t i n g t h e
s t r e n g t h of steel. Further, a l a r g e amount of aluminabased
i n c l u s i o n s a r e produced i n steel, thereby
d e t e r i o r a t i n g the toughness of steel. Therefore, t h e A1
content is 0.001 t o 0.1%. The lower l i m i t of A1 content
is p r e f e r a b l y more than 0.001%, and more p r e f e r a b l y 0.01%:
The upper l i m i t of A1 content is p r e f e r a b l y less than
0. I%,. and more p r e f e r a b l y 0.06%.
[00391 .
As used herein, t h e term "A1 content" means t h e
content of acid-soluble A1 ( s o l . A l ) .
[0040]
0 (Oxygen): not more than 0.05%
Oxygen (0) d e t e r i o r a t e s t h e toughness and corrosion
r e s i s t a n c e of steel. Therefore, t h e 0 content is
p r e f e r a b l y lower. The 0 content is not more than 0.05%.
P r e f e r a b l y t h e 0 content is l e s s than 0.05%; more
. .
. . -
preferably not more than 0.01%, and even more preferably
not more than 0.005%.
[0041]
N: not more than 0.05%
Nitrogen (N) increases t h e s t r e n g t h o f steel.
Further, N s t a b i l i z e s a u s t e n i t e , thereby improving
p i t t i n g .resistance. When even a small amount of N is
contained, the above described e f f e c t s can be obtained t o
some extent. On the .other hand, an excessive N content
w i l l lead t o a production of a . l a r g e amount of n i t r i d e s
i n steel, thereby d e t e r i o r a t i n g the toughness of s t e e l .
Further, a u s t e n i t e becomes more l i k e l y t o be retained,
thereby reducing the strength of s t e e l . Therefore, the N
content is not more than 0.05%. The lower l i m i t of N
content is preferably 0.002%, and more preferably 0.005%.
The upper l i m i t of N content is preferably 0.03%, more
preferably 0.02%, even more preferably 0.015%, and even
more preferably 0.010%.
[0042]
The balance of the chemical composition of a
s t a i n l e s s s t e e l f o r o i l wells is made up of impurities.
The t e r m "an impurity" as used herein r e f e r s t o an
element which is mixed from ores and scraps which are
used as the s t a r t i n g material of s t e e l , or t h e
environments i n the manufacturing process, e t c .
[0043]
[Regarding s e l e c t i v e elements]
An s t a i n l e s s s t e e l f o r o i l wells may further contain,
i n place of some of Fe, one or more kinds of elements
selected from the group consisting of V: not more than
0.3%, T i : not more than 0.3%, Nb: not more than 0.3%, and
Z r : not more than 0.3%.
100441
V: not more than 0.3%,
Nb: not more than 0.3%,
T i : not more than 0.3%, and
Z r : not more than 0.3%.
Vanadium (V) , niobium (Nb) , titanium ( T i ) , and
zirconium ( Z r ) are a l l s e l e c t i v e elements. Any of these
elements forms carbide and increases t h e strength and
toughness of s t e e l . F u r t h e r , t h e s e elements immobilize C
and thereby suppress C r carbide from being produced. For
t h a t reason, the p i t t i n g r e s i s t a n c e of s t e e l is improved,
and the SCC s u s c e p t i b i l i t y is reduced. When these
elements a r e contained even i n a small amount, the above
described e f f e c t s are obtained to some extent. On the
other hand, when the contents of these elements a r e
excessively l a r g e , carbides a r e coarsened and thereby the
toughness and the corrosion resistance of s t e e l
d e t e r i o r a t e . Therefore, the V content, Nb content, T i
content, and Z r content are not more than 0.3%,
respectively. The lower l i m i t s of V, Nb, T i , and Z r a r e
preferably 0.005%, respectively. The upper l i m i t s of V,
Nb, T i , and Z r are preferably l e s s than 0.3%,
r e s p e c t i v e l y .
A stainless steel for oil wells may contain, in
place of some of Fe, one or more kinds of elements
selected from the group consisting of W: not more than
1.0% and rare earth metal (REM) : not more than 0.3%.
[0046]
W: not more than 1.0%
REM: not more than 0.3%
Tungsten (W) and rare earth metal (REM) are both
selective elements. Herein, the term "REM" refers to one
or more kinds of elements selected'from the group
consisting of yttrium (Y) of atomic number 39, lanthanum
(La) of atomic number 57 to lutetium (Lu) of atomic
number 71 which are lanthanoid elements, and actinium
(Ac) of atomic number 89 to lawrencium (Lr) of atomic
number 103, which are actinoid elements.
[0047]
W and REM both improve the SCC resistance in a hightemperature
environment. When these elements are
contained even in a small amount, the above described
effect will be ac-hieved to some extent. On the other
hand, when the contents of these elements are excessively
large, the effects thereof will be saturated. Therefore,
the W content is not more than 1.0% and the REM content
is not more than 0.3%. When REM includes a plurality of
elements selected from the above described group, the REM
content means a total content of those elements. The
. -
, . . .
lower l i m i t of W content is preferably 0.01%. The lower
l i m i t of REM content is preferably 0.001%.
[0048]
A s t a i n l e s s s t e e l f o r o i l wells may contain, i n
place of some of Fe, one or more kinds of elements
s e l e c t e d from the group consisting of Ca: not more than
0.01% and B: not more than 0.01%.
100491 '
Ca: not more than .0.01%
B: not more than 0.01%
Calcium (Ca) and boron (B) a r e both s e l e c t i v e
elements. A s t a i n l e s s steel for o i l w e l l s during hot
working has a two-phase micro-structure of f e r r i t e and
a u s t e n i t e . For t h a t reason, flaws and defects may be
produced i n the s t a i n l e s s s t e e l due t o hot working. Ca
and B suppress 'flaws and defects from been produced
during hot working. When these elements are contained
even i n a small amount, the above described e f f e c t w i l l
be obtained t o some e x t e n t .
[OOSO]
On t h e other hand, an excessive Ca content w i l l lead
to an i n c r e a s e of i n c l u s i o n s in steel, thereby
d e t e r i o r a t i n g the toughness and c o r r o s i o n r e s i s t a n c e of
s t e e l . Further, an excessive B content w i l l lead t o a
p r e c i p i t a t i o n of carbo-boride a t g r a i n boundaries,
thereby d e t e r i o r a t i n g t h e toughness of s t e e l . Therefore,
the Ca content and B content are both not more than 0.01%.
[0051]
The lower limits of Ca content and B content are
both preferably 0.0002%. .In this case, the above
described effect will be remarkably obtained. The-upper
limits of Ca content and B content are both preferably
less than 0.01%, and are both more preferably 0.005%.
[0052]
[Regarding Formulas (1) and (2) 1
The chemical composition of the stainless steel for
oil wells further satisfies Formulas (1) and (2) :
Cr + 4Ni + 3Mo + 2Cu 2 44 (1)
Cr + 3Ni + 4Mo + 2Cu/3 I 46 (2)
where each symbol of element in Formulas (1) and (2)
is substituted by the content (%) of a corresponding
element.
[0053]
[Regarding Formula (1) 1
Definition is made as El = Cr + 4Ni + 3Mo + 2Cu. As
F1 increases, the SCC resistance in a high-temperature
oil well environment will be improved. When the value of
F1 is not less than 44, an excellent SCC resistance will
be obtained in a high-temperature oil well environment of
150°C to 200°C. The value of F1 is preferably not less
than 45, and more preferably not less than 48. A
sufficient SCC resistance at room temperature is also
ensured if the value of El is not less than 44.
[0054]
The upper limit of the value of F1 will not be
particularly limited. However, when the value of F1
exceeds 52, it becomes d i f f i c u l t t o s a t i s f y Formula ( 2 ) ,
and thereby the' s t a b i l i t y of y i e l d s t r e n g t h d e t e r i o r a t e s .
[OOSS]
[Regarding Formula (2) 1 I
A d e f i n i t i o n is made a s F2 = C r + 3Ni + 4Mo t 2 ~ u / 3 . I
I n t h e s t a i n l e s s s t e e l pipe f o r o i l wells of t h e present
invention, t h e above described Co is contained and t h e ~
value of F2 is made not more than 46 t o s t a b l y secure t h e
s t r e n g t h . When t h e value of F2 exceeds 46, a retained
a u s t e n i t e is excessively formed, and it becomes d i f f i c u l t 1
t o s t a b l y s e c u r e t h e y i e l d s t r e n g t h .
[0056]
The value of F2 is p r e f e r a b l y not more than 44, more
p r e f e r a b l y n o t more than 43, and even more p r e f e r a b l y not
more than 42. The lower l i m i t of the value of F2 is not I
p a r t i c u l a r l y l i m i t e d . However, when t h e value of F2 is
not more than 36, there w i l l be a case where t h e value of
F1 is not l i k e l y t o become not less than 44.
[0057]
[Relation between C and N] 1
Th.e chemical composition of a s t a i n 1 , e s s s t e e l f o r
o i l w e l l s p r e f e r a b l y s a t i s f i e s or mu la (3) :
where C and N i n Formula (3) a r e s u b s t i t u t e d by t h e
C content (%) and N content (%) , r e s p e c t i v e l y .
[0058]
A d e f i n i t i o n is made a s F3 = 2.7C + N. When the I
value of F3 is not more than 0.060, a r e t a i n e d a u s t e n i t e ~
. . . .
1 ,
... .
is further suppressed from being produced. Therefore,
combined with the effect of Formula (2), it is possible
to secure the strength more stably. The value of F3 is
preferably not more than 0.050, and more preferably not
more than 0.045.
[0059]
[Metal micro-structure]
The metal micro-structure of a stainless steel for
oil wells preferably contains, by volume ratio, less than
10 to 60% of a ferrite phase, not more than 10% of a
retained austenite phase, and a martensite phase.
[0060]
Ferrite phase: not less than 10% and less than 60%
.by volume ratio
The stainless steel for oil wells of the present ,
invention has large contents of Cr and Mo which are
ferrite forming elements. On the other hand, 'although Ni
is contained in the view point of stabilizing austenite
at high temperature and securing martensite at normal
temperature, the content of Ni which is an austenite
forming element, is suppressed to a level at which the
amount of retained austenite is not excessive. Therefore,
the stainless steel of the present invention will not be
a martensite single-phase micro-structure at normal
temperature, and will be a mixed micro-structure
including at least a martensite phase and a ferrite phase
at normal temperature. While a martensite phase in the
metal micro-structure contributes to an increase in
strength, an excessive volume r a t i o of f e r r i t e phase w i l l
. d e t e r i o r a t e . the strength of s t e e l . heref fore, the volume
r a t i o of f e r r i t e phase is preferably not l e s s than 10%
and less than 60%. The lower l i m i t of the volume r a t i o
of f e r r i t e phase is preferably more than lo%, more
preferably 12%, and even more preferably 14%. The upper
l i m i t of the volume r a t i o of f e r r i t e phase is preferably
48%, .more preferably 45%, and even more preferably 40%.
[0061]
The volume r a t i o of f e r r i t e phase is determined by
the following method. A sample is taken from an
a r b i t r a r y location of a s t a i n l e s s s t e e l . In the sample
taken, a sample surface which corresponds t o a cross
section of the s t a i n l e s s s t e e l is ground. After grinding,
the ground sample surface is etched by using a mixed
solution of aqua regia and glycerin. The a r e a f r a c t i o n
of f e r r i t e phase on the etched surface is measured by a
point counting method conforming t o JIS GO555 by using an
o p t i c a l microscope (observation magnifications of 100).
The measured area fraction is defined as a volume r a t i o
of f e r r i t e phase.
[0062]
Retained a u s t e n i t e phase: not more than 10% by
volume r a t i o
A small am0un.t of retained austenite w i l l not cause
a remarkable decline of strength, and w i l l remarkably
improve the toughness of s t e e l . However, an excessive
volume r a t i o of r e t a i n e d a u s t e n i t e w i l l lead t o a
-z.
.
,- .
remarkable decline of t h e strength of s t e e l . Therefore,
the volume r a t i o of r e t a i n e d austenite-phase is not more
than 10%. From view point of securing s t r e n g t h , a more
p r e f e r a b l e volume r a t i o of retained a u s t e n i t e phase is
not more than 8%.
[00631
When the volume r a t i o of r e t a i n e d a u s t e n i t e phase is
not less than 0.5%, t h e above described e f f e c t of
improving toughness w i l l be obtained e f f e c t i v e l y .
However., even i f the,volume r a t i o of retained a u s t e n i t e
phase is l e s s than 0.5%, the above described efEect w i l l
be obtained t o some e x t e n t .
[0064]
The volume r a t i o o f r e t a i n e d a u s t e n i t e phase is
determined by an X-ray d i f f r a c t i o n method. To be
s p e c i f i c , a sample is taken from an a r b i t r a r y l o c a t i o n of
a s t a i n l e s s s t e e l . The s i z e '6f the sample is 15 mfn x 15
mm x 2 rnm. Respective X ray i n t e n s i t i e s of the (200) and
(211) planes of f e r r i t e phase (a phase), and (200), (220),
and (311) planes of retained a u s t e n i t e phase (y phase)
are measured by usin-g- a sample. Then, the i n t e g r a t e d
i n t e n s i t y of each plane is calculated. After t h e
c a l c u l a t i o n , a volume r a t i o of retained a u s t e n i t e phase
Vy(%) is calculated f o r each of combinations (a t o t a l of
6 combinations) of each plane of t h e a phase and each
plane of the y phase by using Formula (1). Then, an
average value of.volume r a t i o s Vy of 6 combinations is
defined as the volume r a t i o (%) of retained a u s t e n i t e .
Vy = 1 0 0 / ( 1 + (Ia x Ry) /(Iy x Ra)) (1)
Where "Iau is t h e i n t e g r a t e d i n t e n s i t y of a phase.
"Raw i s a c r y s t a l l o g r a p h i c t h e o r e t i c a l c a l c u l a t i o n value
of a phase. "Iy" is t h e i n t e g r a t e d i n t e n s i t y of j phase.
"Ry" is a c r y s t a l l o g r a p h i c t h e o r e t i c a l c a l c u l a t i o n value
of y phase.
[0065]
Martensite phase: Balance
I n t h e metal micro-structure of a s t a i n l e s s s t e e l of
t h e p r e s e n t i n v e n t i o n , t h e p o r t i o n s o t h e r t h a n the above ,
described f e r r i t e phase and' t h e r e t a i n e d a u s t e n i t e phase
a r e predominantly a tempered martensite phase. To be
m o r e s p e c i f i c , t h e metal micro-structure of t h e s t a i n l e s s
steel of t h e p r e s e n t i n v e n t i o n p r e f e r a b l y c o n t a i n s not
less than 40% by volume r a t i o of a m a r t e n s i t e phase. The
lower l i m i t of t h e volume r a t i o of m a r t e n s i t e is more
' p r e f e r a b l y 48%, and even more preferably 52%. The volume
r a t i o af m a r t e n s i t e phase is determined by s u b t r a c t i n g
t h e volume r a t i o s of f e r r i t e phase and r e t a i n e d a u s t e n i t e
phase, which are determined by the above described method,
from 100%. .
[0066]
The m e t a l m i c r o - s t r u c t u r e of a s t a i n l e s s s t e e l f o r
o i l w e l l s may c o n t a i n p r e c i p i t a t e s and/or i n c l u s i o n s such
a s carbides, n i t r i d e s , borides, and a Cu phase b e s i d e s a
f e r r i t e phase, a r e t a i n e d a u s t e n i t e phase, and a
m a r t e n s i t e phase.
[0067]
- H -
26
. .
[Manufacturing method]
A method for manufacturing a seamless steel pipe
will be described as one example of a method for
manufacturing a stainless steel for oil wells.
[0068]
A starting material having the above described
chemical composition is prepared. The starting material
may be a cast piece manufactured by a continuous casting
method (including a round CC) . Moreover, it may be a
billet manufactured by hot working an ingot manufactured
by an ingot-making process. It may also be a billet
manufactured from the cast piece.
[0069]
The prepared starting material is charged into a
reheating furnace or a soaking pit to be heated. Next,
the heated starting material is subjected to hot working
to manufacture a hollow shell. For example, a Mannesmann
process is performed as hot working. To be specific, the
starting material is piercing-rolled by a piercing
machine to be formed into a hollow shell. Next, the
hollow shell is further rolled, for example, by a mandrel
mill and a sizing mill. As hot working, hot extrusion
may be performed, or hot forging may be performed.
[0070]
It is preferable that the reduction of area of a
starting material while the temperature of the starting
material is 850 to 1250°C is not less than 50% during hot
working. In the range of the chemical composition of the
-F-
22
steel of t h e p r e s e n t invention, performing hot working
such t h a t the reduction of area of the s t a r t i n g - m a t e r i a l
while t h e temperature of the s t a r t i n g material is 850 to
1250°C is not l e s s than 50% w i l l r e s u l t in t h a t a micros
t r u c t u r e including a martensite phase and a f e r r i t e
phase which is long-stretched (for example, about 50 to
200 p) i n the r o l l i n g d i r e c t i o n is formed i n t h e nearsurface
portion of steel. Since a f e r r i t e phase is more
l i k e l y t o contain C r etc. than a martensite, it
e f f e c t i v e l y c o n t r i b u t e s t o the prevention of t h e
propagation of SCC at high temperature. As so f a r
described, when t h e f e r r i t e phase is long-stretched in
the r o l l i n g d i r e c t i o n , even i f SCC occurs on t h e surface
a t high temperature, it becomes more l i k e l y t o reach the
f e r r i t e phase during the course of the propagation of
crack. For t h i s reason, SCC r e s i s t a n c e a t high
temperature improves.
[0071]
The hollow s h e l l a f t e r hot working is cooled t o
normal temperature. 'The cooling method may be e i t h e r a i r
cooling or water cooling. Since i n a s t a i n l e s s steel of
the present invention, martensite transformation w i l l
occur when it is cooled t o or lower than a M s p o i n t even
by a i r cooling, it is possible t o obtain a mixed micros
t r u c t u r e including martensite and f e r r i t e . However,
when attempting t o s t a b l y secure a high strength of not
l e s s than 758 MPa, p a r t i c u l a r l y a high strength of not
l e s s than 862 MPa, it is p r e f e r a b l e t h a t t h e h o t r o l l e d
hollow s h e l l is a i r cooled, t h e r e a f t e r reheated t o not I
lower t h a n a n Ac3 transformation point, and is quenched I
by performing water cooling suc,h as a dipping method and
a spray method.
[0072]
Although decreasing t h e value of F2 or increasing
the'Co content may make it possible t o obtain a high
strength even by a i r cooling, there may be a lack of
s t a b i l i t y i n strength. To stably obtain a high strength,
the steel is cooled by water cooling till the surface
temperature of the hollow s h e l l becomes not more than
60°C. That is, the hollow s h e l l a f t e r hot working is
preferably water coo1,ed and a water-cooling stopping
temperature is made not more than 60°C. The watercooling
stopping temperature is more preferably n o t more
than 45OC, and even more preferably not more than 30°C,.
[0073]
The quenched hollow s h e l l is tempered a t not more
than an AC1 point so t h a t the y i e l d s t r e n g t h is adjusted
t o be not l e s s than 758 MPa. When t h e tempering
temperature exceeds the AC1 point, t h e volume r a t i o of
retained a u s t e n i t e sharply increases, and the strength
d e t e r i o r a t e s .
[0074]
The high-strength s t a i n l e s s steel f o r o i l wells
manufactured by the above described processes has a yield
s t r e s s of not less than 758 MPa, and has an excellent
corrosion r e s i s t a n c e even i n a high-temperature o i l w e l l
~ environment of 200°C owing to the effects of Cr, Mo, Ni,
1 ' and Cu-contained therein.
Examples
[0075]
Steels of marks 1 to 28 having chemical compositions
shown in Table 1 were melted, and cast pieces were
manufactured by a continuous casting.
[007 61
[Table 11
TAR1 F 1
Nmerak marked MI "' mean th3t W v a k are out of the range of the present hventbn
-9'-
. . .-- -
R
44.4
44.0
~l
51.0
48.8
Mark
1
2
B
0.033
0.051
Ulemlel Composition (In mass%, the balance Wng Fe and ImpurIUes)
C
0.008
0.013
51
0.20
0.26
G
16.28
17.41
Mn
0.11
0.20
Mo
2.82
234
P
0.012
0.010
Cu
3.44
132
S
0.0012
0.0012
NI
4.85
5.44
,Co
0.230
0.202
A1
0.040
0.045
0 REM
0.0017
0.0020
Ca 8 -
-
N
0.0122
0.0151
V -
-
TI
-
-
Zr -
-
Nb -
-
W -
-
COO77 1
Referring t o Table 1, the s t e e l s of marks 1 ta 20
f e l l into the range of the present invention. On the
other hand, the chemical compositions of marks 21 t o 28
.were out of the range of the present invention.
[0078]
The cast piece of each mark was r o l l e d by a blooming
m i l l to manufacture a round b i l l e t . The round b i l l e t of
each s t e e l had a diameter' of 232 mrn. Then, the outer
surface of each round b i l l e t was cut such t h a t the
diameter of the round b i l l e t was 225 nun.
[0079]
. . Each round b i l l e t was heated to 1150 t o 1200°C in a
reheating furnace.. After heating, each round b i l l e t was
hot rolled. To be specific, the round b i l l e t was
piercing-rolled by a piercing machine t o manufacture a
hollow s h e l l . The hollow s h e l l was drawn and rolled by a
mandrel m i l l , and was further reduced i n diameter such
t h a t t h e o u t e r diameter of the hollow s h e l l was 196.9 to
200 rnm and the wall thickness was 15 t o 40 mrn. A11 the
cooling of the hollow s h e l l a f t e r hot r o l l i n g was
performed by spontaneous cooling.
[0080]
Quenching was performed on the hollow s h e l l a f t e r it
was.allowed t o cool. To be specific, the hollow s h e l l
was charged i n t o a heat treatment furnace to be soaked a t
980°C for 20 minutes. The hollow shell a f t e r soaking was
water cooled by a spray method to be quenched. The
. . . ..
hollow s h e l l a f t e r quenching w a s soaked a t a tempering'
temperature of 550 OC f o r 30 minutes t o be tempered.
[0081] ~ Through t h e above described processes, a p l u r a l i t y
of seamless s t e e l pipes of p l u r a l s i z e s were manufactured
a t each mark.
100821
The manufactured seamless steel pipes w e r e used t o
perform t h e following evaluation . t e s t s .
[0083]
[Tensile t e s t ]
Round bar specimens ( d i a . 6.35 mm x GL 25.4 mrn)
conforming t o API s p e c i f i c a t i o n were taken from a
p l u r a l i t y of seamless s t e e l pipes of each mark. The
t e n s i l e d i r e c t i o n of the round bar specimen was s e t t o a
pipe axis d i r e c t i o n of the seamless s t e e l pipe. By using
the prepared round bar specimens, t e n s i i e tests were
conducted a t normal temperature (25OC) conforming to API
s p e c i f i c a t i o n .
[0084]'
After the t e n s i l e t e s t , among the p l u r a l i t y of
seamless s t e e l pipes of each mark, the seamless s t e e l
pipe having a maximum yield s t r e s s a t each mark
( h e r e a f t e r , r e f e r r e d t o as a high YS m a t e r i a l ) and the
seamless s t e e l pipe having a minimum yield s t r e s s
( h e r e a f t e r , r e f e r r e d to as a low YS material) were
s e l e c t e d . The high YS material and the low YS material
of each mark were used t o perform the following
evaluati-on t e s t .
[Metal micro-structure observation]
Samples f o r micro-structure observation were taken
from a r b i t r a r y l o c a t i o n s of t h e high YS m a t e r i a l and the
low YS m a t e r i a l o f each mark. In a sample taken, a
sample surface of a cross s e c t i o n normal t o t h e a x i a l
d i r e c t i o n of. the seamless steel pipe was ground. After
grinding, the ground sample s u r f a c e was etched by using a
mixed solution of aqua regia and glycerin. The area
r a t i o of f e r r i t e phase on the etched surface w a s measured
by t h e point counting method conforming t o JIS G0555.
The measured area r a t i o was defined as the volume r a t i o
of f e r r i t e phase.
[0086]
Further, the volume r a t i o o f r e t a i n e d a u s t e n i t e
phase w a s determined by the above described X-ray
d i f f r a c t i o n method. Furthermore, based on t h e determined
volume r a t i o s of f e r r i t e phase and retained a u s t e n i t e
phase, the volume r a t i o of martensite phase was
determined by the above described method.
[0087]
[Toughness t e s t ]
Full s i z e specimens (L . d i r e c t i o n ) conforming t o ASTM
E23 were taken from a high YS m a t e r i a l and low YS
material of each mark. The Charpy impact test was
performed by using the f u l l s i z e specimen t o determine an
absorbed energy a t -lO°C.
[0088]
[High-temperature corrosion r e s i s t a n c e t e s t ]
Four-point bending test specimens were taken from a
high YS material and low YS material of each mark. The
specimen had a length of 75mm, a width of 10 mm, and a
thickness of 2 mrn. Each specimen w a s given a d e f l e c t i o n
by four-point bending. In t h i s occasion, t h e d e f l e c t i o n
amount of each specimen w a s d.etermined conforming t o ASTM
G39 such t h a t the stress given t o t h e specimen 2s equal
t o the y i e l d s t r e s s of' t h e specimen.
[008 91
An autoclave of 200°C in which C02 of 30 bar and H2S
of 0.01 bar were sealed under pressure was prepared.
Each specimen subjected t o a d e f l e c t i o n was s t o r e d i n
each autoclave. Each specimen was immersed in an aqueous
solution containing 25 w t % NaCl + 0.41g/L CH3COONa (pH =
4.5 i n CH3COONa + CH3COOH buffer system) in each
autoclave f o r one month. .
[oogo j
After 720h immersion, the occurrence or
nonoccurrence of s t r e s s corrosion cracking (SCC) was
i n v e s t i g a t e d on each specimen. To be s p e c i f i c , the cross
section of a portion of each specimen t o which t e n s i l e
s t r e s s is applied was observed by an o p t i c a l microscope
having a visual f i e l d of 100 magnifications t o determine
t h e presence or absence of a crack.
-M-
35-
Further, the weight of the specimen before and after
the test was measured. A corrosion loss of each specimen
was determined based on the amount of change in the
measured weight. From the corrosion loss, an annual
corrosion loss (mm/y) was calculated.
[SSC resistance test at normal temperature]
Round bar specimens for NACE TM0177 METHOD A were
taken from a high YS material and low YS material of each
mark. The sizes of the specimen were 6.35 mm in diameter
and 25.4 mm in GL. A tensile stress was applied to each specimen in its axial direction. At this moment, in
conformity to NACE TM0177-2005, the deflection amount of
each specimen was determined such that the stress given
to each specimen was 90% of the yield stress (actual
measurement) of each specimen.
The test bath was a 25 wt% aqueous solution of NaCl
in which 0.01 bar of H2S and 0.99 bar of C02 were
saturated. The pH of the test bath was regulated to be
4.0 by a cH~COON~/CH~CObOuHf fer solution containing 0.41
g/L of CH3COONa. The temperature of the test bath was
[0094]
A round bar specimen was immersed in the above
described test bath for 720 hours. After immersion,
determination was made on whether or not cracking (SSC)
. . . - . . ' ;
occurred in each specimen by the same method as in the .
high-temperature corrosion resistance test.
[0095]
[Investigation results]
Table 2 shows the test results.
[0096]

-
[0097]
The "low YS materialr1 column' in Table 2 shows -
evaluation test r e s u l t s . u s i n g t h e low YS m a t e r i a l of each
mark, and the "high YS material" column shows the r e s u l t s
u s i n g t h e high YS material. "F" (%) in Table 2 shows the
volume r a t i o (%) of f e r r i t e phase i n the m e t a l micros
t r u c t u r e of a corresponding mark, "M" shows the volume
r a t i o (%) of martensite phase, and "A" shows the volume
r a t i o (%) of r e t a i n e d a u s t e n i t e phase, respectively.
"NF" i n the "SCC" and "SSC" columns of "Corrosion
resistance" column shows t h a t SCC or SSC was not observed
i n a corresponding mark. "F" shows t h a t SCC or SSC was
observed in a corresponding mark.
[0098]
[Regarding m e t a l micro-structure and yield strength]
Referring t o Table 2, the chemical compositions of
the seamless s t e e l pipes of marks 1 t o 20 were within t h e .
range of the present invention and s a t i s f i e d Formulas (1)
and ( 2 ) , and t h e metal micro-structures were a l s o within
the range of t h e present invention. For t h a t reason, the
yield strength of any of the seamless s t e e l pipes of each
mark was not l e s s than 758 MPa (110 k s i ) even i n low YS,
and thus a y i e l d strength of not l e s s than 110 ksi was
s t a b l y obtained.
[0099]
Further, t h e r e was a tendency observed t h a t a yield
strength of a 125 k s i level was obtained even i n low YS
materials f o r marks 1, 3, 4 , 11, 16, and 19 f o r which the
left hand side value of Formula (3), that is, the value
of F3 was not more than 0.045 among the seamless steel
pipes of marks 1 to 20. Moreover, in marks 5, 6, ,8, 10,
12, 13, and 17 in which the value of F3 exceeded 0.060,
.it was recognized in low YS materials that although a
yield strength of 110 ksi level was satisfied, there was
a tendency observed that the yield strength at the same
level of F2 was somewhat lower compared with the case
where the value of F3 was not' more than 0.0045 .at a value
of F2 of. the same level.
[OlOO]
Further, in the seamless steel pipes.of marks 1 to
20, the absorption energy at -lO°C was not less than 150
J, exhibiting high toughness. Further, no SCC was
observed at the high-temperature corrosion resistance
test, and also no SSC was observed in the SSC resistance
test at normal temperature.
[OlOl]
Note that the corrosion rate was less than 0.10 mm/y
in any of marks 1 to 28.
[0102]
On the other hand, in marks 21 and 22, the Co
content was less than the lower limit of Co content of
the present invention. For that reason, the yield stress
of low YS material became less than 758 MPa, and the
volume ratio of retained austenite phase exceeded 10% as
well. Therefore, it was not possible to stably obtain a
strength not less than 110 ksi.
In mark 23, t h e Co content exceeded the upper l i m i t
of Co content of t h e p r e s e n t i n v e n t i o n . For t h a t reason,
both t h e high YS material and the low YS material had an
adsorption energy a t -lO°C l e s s t h a n 150 J (83 J i n the
high YS material and 86 J in the low YS m a t e r i a l ) ,
e x h i b i t i n g a low toughness.
[0104]
Although the co'ntent of each element of mark 24 was
within the range of t h e present invention, it did not
s a t i s f y Formula (1). For that reason, SSC was observed
in t h e SSC r e s i s t a n c e t e s t , e x h i b i t i n g a low SSC
r e s i s t a n c e . Moreover, SCC was observed i n the hightemperature
corrosion resistance test, exhibiting a low I
high-temperature corrosion r e s i s t a n c e .
Although t h e c o n t e n t of each element of marks 25 to I
28 was within the range of the present invention, it did
not s a t i s f y Formula ( 2 ) . For t h a t reason, in a l l of the
low YS materials, t h e volume r a t i o of retained a u s t e n i t e ,
phase exceeded l o % , and the yield s t r e n g t h was l e s s than
758 MPa (110 k s i ) . Although there w a s a case where the
yield s t r e n g t h was not l e s s than 758 MPa as in the high
YS m a t e r i a l of mark 27, it was c l e a r t h a t when the value
of F2 did not s a t i s f y Formula ( 2 ) , a high strength s t e e l
pipe could not be s t a b l y manufactured.
[0106]
Although so far embodiments of the present invention
have been described, the above described embodiments are
merely examples for carrying out the present invention.
Therefore, the present invention will not be limited to
the above described embodiments, and can be carried out
by appropriately modifying the above described
embodiments within a range not departing from the spirit
of the invention.
Industrial Applicability
[0107]
The stainless steel for oil wells according to the
present invention can be utilized in oil wells and gas
wells. Particularly, it can be used in a deep oil well
having a high-temperature environment.
We claim:
1. A stainless steel for oil wells comprising, by mass%,
C: not more than 0.05%,
Si: not more than 1.0%,
Mn: 0.01 to 1.0%,
P: not more than 0.05%,
S: less than 0.002%,
Cr: 16 to 18%,
Mo: 1.8 to 3%,
Cu: 1.0 to 3.5%,
Ni: 3.0 to 5.5%,
Co: 0.01 to 1.0%,
Al: 0.001 to 0.1%,
0: not more than 0.05%, and
N: not more than 0.05%, the balance being Fe and
impurities, and satisfying Formulas (1) and (2) :
Cr + 4Ni + 3Mo + 2Cu 2 44 (1
Cr + 3Ni + 4Mo + 2Cu/3 S 46 (2)
where each symbol of element in Formulas (1) and (2) is
substituted by a content , in mass%, of a corresponding
element.
2. The stainless steel for oil wells according to claim
1, wherein
the stainless steel for oil wells contains, in place
of some of Fe, one or more kinds of elements selected
from the group consisting of
V: not more than 0.3%,
T i : not more than 0.3%,
Nb: not more than- 0.3%, and
Z r : not more than 0.3%.
3. The s t a i n l e s s s t e e l ior o i l w e l l s according t o claim
1 or 2, wherein
the s t a i n l e s s s t e e l for o i l w e l l s contains, i n place
of some of Fe, one or more kinds of elements selected
from the group consisting of
W:.not more than 1.0%, and
r a r e e a r t h metal (REM): not more than 0.3%.
4 . The s t a i n l e s s s t e e l for o i l wells according to any
one. of claims 1 t o 3, wherein
the s t a i n l e s s s t e e l for o i l wells contains, in place
of some of Fe, one or more kinds of elements selected
from the group consisting of
Ca: not more than 0.01%, and
B: not more than 0.01%.
5. The s t a i n l e s s s t e e l for o i l wells according t o any
one of claims 1 t o 4, wherein
a metal micro-structure of the s t a i n l e s s s t e e l for
o i l wells contains, by volume r a t i o , not l e s s than 10%
and l e s s than 60% of f e r r i t e phase, not more than 10% of
retained a u s t e n i t e phase, and not less than 40% of
martensite phase.
6. The s t a i n l e s s s t e e l for o i l wells according to any
one of claims 1 t o 5, wherein
the s t a i n l e s s s t e e l for o i l wells has a yield
strength of not l e s s than 862 MPa.
An oil well pipe manufactured from the stainless
steel for oil wells according to any one of claims 1 to 6.
Dated this 8" day of July, 2014.