U.S. patent application number 09/985862 was filed with the patent office on 2002-08-08 for high strength steel material for oil well, excellent in sulfide stress cracking resistance, and production method thereof.
Invention is credited to Asahi, Hitoshi, Sakamoto, Shunji.
Application Number | 20020104592 09/985862 |
Document ID | / |
Family ID | 14911579 |
Filed Date | 2002-08-08 |
United States Patent
Application |
20020104592 |
Kind Code |
A1 |
Sakamoto, Shunji ; et
al. |
August 8, 2002 |
High strength steel material for oil well, excellent in sulfide
stress cracking resistance, and production method thereof
Abstract
This invention provides a high-strength steel material for an
oil well, excellent in strength and SSC resistance, and having a
yield strength of at least 120 ksi. The steel material contains C:
0.10 to 0.40%, Si.ltoreq.0.5%, Mn.ltoreq.0.5%, P.ltoreq.0.015%,
S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to
0.1% and at least 3.4 times N, Nb: 0.01 to 0.1%, N.ltoreq.0.01% and
B: 0.0005 to 0.0050%, wherein the yield strength expressed by ksi
and the Mo content satisfy the following relation (1), the balance
of the C, Mn and Mo contents satisfies the following relation (2),
and the steel material may contain, whenever necessary, at least
one of Cr.ltoreq.0.2%, W.ltoreq.0.5%, V: 0.01 to 0.3%, Zr: 0.001 to
0.01%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to
0.01%: .alpha.=Mo-0.15YS .gtoreq.-18.9 (1)
.beta.=2.7C+Mn+2Mo.gtoreq.2.0 (2).
Inventors: |
Sakamoto, Shunji;
(Kitakyushu-shi, JP) ; Asahi, Hitoshi;
(Futtsu-shi, JP) |
Correspondence
Address: |
Platon N. Mandros
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
14911579 |
Appl. No.: |
09/985862 |
Filed: |
November 6, 2001 |
Current U.S.
Class: |
148/320 ;
420/83 |
Current CPC
Class: |
C22C 38/06 20130101;
C21D 1/18 20130101; C22C 38/12 20130101; C22C 38/14 20130101; C22C
38/001 20130101 |
Class at
Publication: |
148/320 ;
420/83 |
International
Class: |
C22C 038/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 2, 2000 |
JP |
PCT/JP00/02917 |
May 6, 1999 |
JP |
11-125497 |
Claims
1. A high-strength steel material, for an oil well, excellent in
sulfide stress cracking resistance, containing, in terms of mass
%:
16 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn.ltoreq.0.5%,
P.ltoreq.0.015%, S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to
0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to
0.1%, N.ltoreq.0.01% and B: 0.0005 to 0.0050%,
wherein the yield strength YS expressed by ksi and the MO content
satisfy the following relation (1) and the balance of the C, MN and
MO contents satisfies the following relation (2):
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1) .beta.=2.7C+Mn+2Mo.gtoreq.2.0
(2).
2. A high-strength steel material, for an oil, excellent in sulfide
stress cracking resistance, containing, in terms of mass %:
17 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn.ltoreq.0.5%,
P.ltoreq.0.015%, S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to
0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to
0.1%, N.ltoreq.0.01% and B: 0.0005 to 0.0050%,
wherein the yield strength YS expressed by ksi and the Mo content
satisfy the following relation (1) and the balance of C, Mn and Mo
contents satisfies the following relation (2);
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1) .beta.=2.7C+Mn+2Mo.gtoreq.2.0
(2), and wherein said steel material contains further at least one
of the following elements:
18 W.ltoreq.0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001
to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%.
3. A high-strength steel material, for an oil well, excellent in
sulfide stress cracking resistance, containing, in terms of mass
%:
19 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn<0.3%, P.ltoreq.0.015%,
S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to
0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N.ltoreq.0.01%
and B: 0.0005 to 0.0050%,
wherein the yield strength YS expressed by ksi and the Mo content
satisfy the following relation (1) and the balance of the C, Mn and
Mo contents satisfies the following relation (2):
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1) .beta.=2.7C+Mn+2Mo.gtoreq.2.0
(2).
4. A high-strength steel material, for an oil well, excellent in
sulfide stress cracking resistance, containing, in terms of mass
%:
20 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn<0.3%, P.ltoreq.0.015%,
S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to
0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N.ltoreq.0.01%
and B: 0.0005 to 0.0050%,
wherein the yield strength YS expressed by ksi and the Mo content
satisfy the following relation (1) and the balance of the C, Mn and
Mo contents satisfies the following relation (2); and Wherein said
steel material contains further at least one of the following
elements:
21 W.ltoreq.0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001
to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%:
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1) .beta.=2.7C+Mn+2Mo.gtoreq.2.0
(2).
5. A high-strength steel material, for an oil well, excellent in
sulfide stress cracking resistance and having a yield strength of
at least 120 ksi, containing, in terms of mass %:
22 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn<0.5%, P.ltoreq.0.015%,
S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to
0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N.ltoreq.0.01%
and B: 0.0005 to 0.0050%,
wherein the yield strength YS expressed by ksi and the Mo content
satisfy the following relation (1) and the balance of the C, Mn and
Mo contents satisfies the following relation (2).
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1) .beta.=2.7C+Mn+2Mo.gtoreq.2.0
(2).
6. A high-strength steel material, for an oil well, excellent in
sulfide stress cracking resistance and having a yield strength of
at least 120 ksi, containing, in terms of mass %:
23 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn.ltoreq.0.5%,
P.ltoreq.0.015%, S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to
0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to
0.1%, N.ltoreq.0.01% and B: 0.0005 to 0.0050%,
wherein the yield strength YS expressed by ksi and the Mo content
satisfy the following relation (1) and the balance of the C, Mn and
Mo contents satisfies the following relation (2); and wherein said
steel material further contains at least one of the following
elements:
24 W .ltoreq. 0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001
to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%:
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1) .beta.=2.7C+Mn+2Mo.gtoreq.2.0
(2).
7. A high-strength steel material for an oil well, excellent in
sulfide stress cracking resistance and having a yield strength of
at least 120 ksi, containing, in terms of mass %:
25 C: 0.10 to 0.40%, Si .ltoreq. 0.5%, Mn < 0.3%, P .ltoreq.
0.015%, S .ltoreq. 0.0050%, Mo: 1.0 to 2.5%, Al: 0.0050 to 0.1%,
Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N
.ltoreq. 0.01% and B: 0.0005 to 0.0050%,
wherein the yield strength YS expressed by ksi and the Mo content
satisfy the following relation (1) and the balance of the C, Mn and
Mo contents satisfies the following relation (2):
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1) .beta.=2.7C+Mn+2Mo.gtoreq.2.0
(2).
8. A high-strength steel material for an oil well, excellent in
sulfide stress cracking resistance and having a yield strength of
at least 120 ksi, containing, in terms of mass %:
26 C: 0.10 to 0.40%, Si .ltoreq. 0.5%, Mn < 0.3%, P .ltoreq.
0.015%, S .ltoreq. 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti:
0.0050 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N
.ltoreq. 0.01% and B: 0.0005 to 0.0050%,
wherein the yield strength YS expressed by ksi and the Mo content
satisfy the following relation (1) and the balance of the C, Mn and
Mo contents satisfies the following relation (2); and wherein said
steel material further contains at least one of the following
elements:
27 W .ltoreq. 0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001
to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%:
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1) .beta.=2.7C+Mn+2Mo.gtoreq.2.0
(2).
9. A method of producing a high-strength steel material, for an oil
well, excellent in sulfide stress cracking resistance, said method
comprising the steps of: hot processing said steel containing said
components according to any of claims 1 through 8 and having the
balance of the C, Mn and Mo contents satisfying the following
relation (2); heating for austenitizing said steel material to a
temperature within the range of [Ac.sub.3 point+20.degree. C.] to
1,000.degree. C.; quenching said steel material to obtain a
microstructure in which the hardness as-quenched at the remotest
position of said steel material from the cooled surface thereof is
at least 95% of the hardness of the cooled surface; and tempering
said steel material at a temperature of 620 to 720.degree. C.:
.beta.=2.7C+Mn+2Mo.gtoreq.2.0 (2).
10. A high-strength steel material for an oil well excellent in
sulfide stress cracking, according to any of claims 1 through 9,
wherein a threshold stress for crack occurrence determined by a
constant load type sulfide stress cracking test according to NACE
TM0177-A is at least 80% of the yield strength.
Description
TECHNICAL FIELD
[0001] This invention relates to a high-strength steel material
such as a steel pipe, for example, casing, tubing, drill pipe, for
an oil well, excellent in both strength and sulfide stress cracking
(hereinafter referred to as "SSC") resistance, that is used in oil
and gas wells while it is exposed to a sour gas environment.
BACKGROUND ART
[0002] A steel material used for oil and gas wells containing
hydrogen sulfide is required to possess SSC resistance. The true
nature of this SSC is hydrogen embrittlement. Hydrogen
embrittlement is more likely to occur as the strength of the steel
material becomes higher. It has been therefore difficult to
simultaneously satisfy the high strength and the high SSC
resistance requirements.
[0003] With such a background, technical development has been done
to obtain steel materials excellent in the SSC resistance and
having high yield strength. For example, Japanese Examined Patent
Publication (Kokoku) No.6-104849 describes a technology that
achieves high strength, while SSC resistance is secured, by
optimizing the steel components and the microstructure. However,
the upper limit of the strength level that can be achieved by this
technology presumably remains at a yield strength level of 120 ksi.
Japanese Unexamined Patent Publication (Kokai) No.4-66645 discloses
a technology that particularly addresses the interaction between Cr
and Ni, and secures SSC performance and toughness with a higher
strength by controlling the contents of these components.
Nonetheless, the strength level that can be achieved by this
technology is also, presumably, at the yield strength level of 120
ksi.
[0004] Market needs for oil well pipes have required, in recent
years, a new type of steel material that has a yield strength of
higher than about 125 ksi while it provides a sufficient SSC
resistance. Demands for such a steel material are expected to grow
in future.
[0005] Therefore, the yield strength of 120 ksi, that is, the level
that has been achieved so for, is not sufficient to cope with the
future needs, and development of a new type of steel is
necessary.
DISCLOSURE OF THE INVENTION
[0006] In view of the background described above, the present
invention aims at providing a steel material having an excellent
SSC resistance with a high strength of at least 120 ksi in terms of
the yield strength, that has been difficult to achieve in the
past.
[0007] The inventor of the present invention conducted a series of
studies to solve the problems described above. As a result, the
present inventor has acquired the knowledge necessary and
sufficient to constitute the present invention.
[0008] When the strength of the steel material becomes high, SSC
occurs from the grain boundary. To suppress such a rupture, the
microstructure must first be uniform. To acquire a high strength of
at least 120 ksi and a high SSC resistance, there is no other
selection but to use tempered martensite as the microstructure, and
this structure, must be as uniform as possible.
[0009] If any heterogeneous phase having different characteristics
is contained in the microstructure, the boundary of this
heterogeneous phase or the heterogeneous phase itself functions as
a breaking start point, and a sufficient SSC resistance cannot be
obtained. Uniformity of this structure is substantially determined
by the condition of quenching. In other words whether or not the
heterogeneous phase develops depends on whether or not a sufficient
and uniform quenched martensite structure is obtained throughout
the steel material. Needless to say, a complete full martensitic
microstructure is preferred. In consideration of thick steel
materials or the limitation of the contents of elements capable of
contributing to hardenability, that will be described later, the
present inventor has examined the condition that is essential to
the high strength level to which the present invention is directed.
FIG. 1 shows the result.
[0010] In FIG. 1, a 25 mm-thick sheet material is subjected to
quenching with water cooling from the austenite temperature of 900
to 930.degree. C. Next, the hardness at the center portion of the
thickness, that is, the most distant from the quenched end of the
steel material, and the hardness of the portion just below the
material surface that is just below the quenched end are measured.
Hardenability is evaluated in terms of the ratio of the hardness of
the former to that of the latter. At the same chance, a test
specimen is machined from the center portion of the thickness to
evaluate the SSC resistance.
[0011] The present inventor has acquired from FIG. 1 the knowledge
that, when the ratio of the hardness of the center portion of the
thickness to the hardness just below the plate surface is at least
95%, sufficient SSC resistance can be obtained even at a high
strength of 120 ksi or more. The present inventor has also acquired
the knowledge that, in order to sufficiently harden a steel
material having a thickness of about 25 mm, an index .beta.
(2.7C+Mn+2Mo) calculated from the C, Mn and Mo contents must be at
least 2.0.
[0012] The present inventor has examined the relationship between
the strength that governs SSC performance, and the contents of the
alloy elements, on the premise of the uniform microstructure
without the heterogeneous phase, and has discovered the results
shown in FIG. 2, wherein the relationship between Mo content and
yield strength YS of the steel material after tempering is plotted.
The contents of Mn and P affecting SSC performance was controlled
to the lowest level that can be accomplished industrially. The
relationship can be shown extremely clearly. The present inventor
has found that to secure sufficient SSC resistance in a high
strength material of YS.gtoreq.120 ksi, the Mo content must be at
least 0.5%, and the .alpha. value expressed by the relation with
the yield strength YS, that is, .alpha.=Mo (wt %)-0.15YS (ksi),
must be at least -18.9.
[0013] Furthermore, the present inventor has examined in detail the
influences of Mn and Mo after P is set to a low level. As a result,
the present inventor has found that both components interact as
shown in FIG. 4. In the region where the Mn content is 0.3% or
below, the SSC resistance depends solely on the Mo content. Within
the range of the Mn content of 0.3 to 0.5%, however, the Mo content
must be increased so as to correspond to the increase of the Mn
content. When the Mn content exceeds 0.5%, the SSC resistance
cannot be improved and further even when the Mo content is
increased. Therefore, in order to make the most of the utility of
Mo, the Mn content must be limited to 0.5% or below, and preferably
to less than 0.3%.
[0014] The present invention has been completed on the basis of the
observations described above. The gist of the present invention
resides in the following points.
[0015] (1) A high-strength steel material for an oil well,
excellent in sulfide stress cracking resistance, containing, in
terms of mass %:
1 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn.ltoreq.0.5%,
P.ltoreq.0.015%, S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to
0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to
0.1%, N.ltoreq.0.01% and B: 0.0005 to 0.0050%,
[0016] wherein the yield strength YS expressed by ksi and the Mo
content satisfy the following relation (1) and the balance of the
C, Mn and Mo contents satisfies the following relation (2).
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1)
.beta.=2.7C+Mn+2Mo.gtoreq.2.0 (2)
[0017] (2) A high-strength steel material for an oil, excellent in
sulfide stress cracking resistance, containing, in terms of mass
%:
2 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn.ltoreq.0.5%,
P.ltoreq.0.015%, S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to
0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to
0.1%, N.ltoreq.0.01% and B: 0.0005 to 0.0050%,
[0018] wherein the yield strength YS expressed by ksi and the Mo
content satisfy the following relation (1) and the balance of C, Mn
and Mo contents satisfies the following relation (2); and
[0019] wherein the steel material contains further at least one of
the following elements:
3 W.ltoreq.0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001 to
0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%.
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1)
.beta.=2.7C+Mn+2Mo.gtoreq.2.0 (2)
[0020] (3) A high-strength steel material for an oil well,
excellent in sulfide stress cracking resistance, containing, in
terms of mass %:
4 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn<0.3%, P.ltoreq.0.015%,
S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to
0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N.ltoreq.0.01%
and B: 0.0005 to 0.0050%,
[0021] wherein the yield strength YS expressed by ksi and the Mo
content satisfy the following relation (1) and the balance of the
C, Mn and Mo contents satisfies the following relation (2).
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1)
.beta.=2.7C+Mn+2Mo.gtoreq.2.0 (2)
[0022] (4) A high-strength steel material for an oil well,
excellent in sulfide stress cracking resistance, containing, in
terms of mass %:
5 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn<0.3%, P.ltoreq.0.015%,
S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to
0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N.ltoreq.0.01%
and B: 0.0005 to 0.0050%,
[0023] wherein the yield strength YS expressed by ksi and the Mo
content satisfy the following relation (1) and the balance of the
C, Mn and Mo contents satisfies the following relation (2); and
[0024] wherein said steel material further contains at least-one of
the following elements:
6 W.ltoreq.0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001 to
0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%.
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1)
.beta.=2.7C+Mn+2Mo.gtoreq.2.0 (2)
[0025] (5) A high-strength steel material for an oil well,
excellent in sulfide stress cracking resistance and having a yield
strength of at least 120 ksi, containing, in terms of mass %:
7 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn<0.5%, P.ltoreq.0.015%,
S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to
0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N.ltoreq.0.01%
and B: 0.0005 to 0.0050%,
[0026] wherein the yield strength YS expressed by ksi and the Mo
content satisfy the following relation (1) and the balance of the
C, Mn and Mo contents satisfies the following relation (2).
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1)
.beta.=2.7C+Mn+2Mo.gtoreq.2.0 (2)
[0027] (6) A high-strength steel material for an oil well,
excellent in sulfide stress cracking resistance and having a yield
strength of at least 120 ksi, containing, in terms of mass %:
8 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn<0.5%, P.ltoreq.0.015%,
S.ltoreq.0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to
0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N.ltoreq.0.01%
and B: 0.0005 to 0.0050%,
[0028] wherein the yield strength YS expressed by ksi and the Mo
content satisfy the following relation (1) and the balance of the
C, Mn and Mo contents satisfies the following relation (2); and
[0029] wherein said steel material further contains at least one of
the following elements:
9 W.ltoreq.0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001 to
0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%.
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1)
.beta.=2.7C+Mn+2Mo.gtoreq.2.0 (2)
[0030] (7) A high-strength steel material for an oil well,
excellent in sulfide stress cracking resistance and having a yield
strength of at least 120 ksi, containing, in terms of mass %:
10 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn<0.3%, P.ltoreq.0.015%,
S.ltoreq.0.0050%, Mo: 1.0 to 2.5%, Al: 0.0050 to 0.1%, Ti: 0.005 to
0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N.ltoreq.0.01%
and B: 0.0005 to 0.0050%,
[0031] wherein the yield strength YS expressed by ksi and the Mo
content satisfy the following relation (1) and the balance of the
C, Mn and Mo contents satisfies the following relation (2).
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1)
.beta.=2.7C+Mn+2Mo.gtoreq.2.0 (2)
[0032] (8) A high-strength steel material for an oil well,
excellent in sulfide stress cracking resistance and having a yield
strength of at least 120 ksi, containing, in terms of mass %:
11 C: 0.10 to 0.40%, Si.ltoreq.0.5%, Mn<0.3%, P.ltoreq.0.015%,
S.ltoreq.0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.0050 to
0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N.ltoreq.0.01%
and B: 0.0005 to 0.0050%,
[0033] wherein the yield strength YS expressed by ksi and the Mo
content satisfy the following relation (1) and the balance of the
C, Mn and Mo contents satisfies the following relation (2); and
[0034] wherein said steel material further contains at least one of
the following elements:
12 W.ltoreq.0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001
to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%.
.alpha.=Mo-0.15YS.gtoreq.-18.9 (1)
.beta.=2.7C+Mn+2Mo.gtoreq.2.0 (2)
[0035] (9) A method of producing a high-strength steel material for
an oil well, excellent in sulfide stress cracking resistance, said
method comprising the steps of hot processing the steel containing
the components according to any of (1) through (8) and having the
balance of the C, Mn and Mo contents satisfying the following
relation (2);
[0036] heating for austenitizing the steel material to a
temperature within the range of [Ac.sub.3 point+20.degree. C.] to
1,000.degree. C.;
[0037] quenching the steel material to obtain a microstructure in
which the hardness as-quenched at the remotest position in the
steel from the cooled surface thereof is at least 95% of the
hardness of the cooled surface; and
[0038] tempering the steel material at a temperature of 620 to
720.degree. C.
.beta.=2.7C+Mn+2Mo.gtoreq.2.0 (2)
[0039] (10) A high-strength steel material for an oil well,
excellent in sulfide stress cracking, according to any of (1)
through (9), wherein a threshold stress for crack occurrence
determined by a constant load type sulfide stress cracking test
according to NACE TM0177-A is at least 80% of the yield
strength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is a graph showing a ratio of the hardness at the
center portion of the thickness in the as-quenched state to the
hardness of the portion immediately below the surface layer, by a
hardenability index .beta., and showing also the relationship
between the SSC resistance of the steel material after quenching
and .beta..
[0041] FIG. 2 is a graph showing the SSC resistance of a steel
having high hardenability by the relationship between the Mo
content and YS.
[0042] FIG. 4 is a graph showing the SSC resistance of a steel
having an equivalent YS in association with the Mn and Mo
contents.
[0043] FIG. 5 is a graph showing the SSC resistance in terms of the
ratio of a threshold stress for crack ocurrence to YS as an index
of the SSC resistance when YS changes.
BEST MODE FOR CARRYING OUT THE INVENTION
[0044] Hereinafter, the present invention will be explained in
detail.
[0045] First, the reasons for limitation of the alloy components in
the present invention will be explained. The content of each
component is expressed in percent by weight.
[0046] C: Carbon is an essential element for securing
simultaneously the intended high strength and the SSC resistance.
The strength and the SSC resistance depend on hardenability. When
the C content is less than 0.10%, quenching becomes incomplete and
the strength drops. Even if required strength can be secured by
adjusting the tempering condition, a sufficient SSC resistance
cannot be acquired. When the C content exceeds 0.40%, on the other
hand, the SSC resistance gets into saturation and susceptibility to
quench crack and delayed fracture increases. Therefore, an
appropriate range of the C content is set to the range of 0.10 to
0.40%.
[0047] Si: Silicon exists as a residue of a deoxidizer used in the
steel making process. When the Si content exceeds 0.5%, the steel
becomes brittle and the SSC resistance deteriorates. Therefore, the
upper limit is set to 0.5%.
[0048] Mn: Manganese is an element detrimental to the SSC
resistance and should not be added. However, since Mn has the
function of improving hardenability, up to 0.5% as the upper limit
of Mn may be contained in case that the C and Mo contents for
improving hardenability are small and hardenability is not
sufficient. When Mn is contained in the amount exceeding 0.5%, a
satisfactory SSC resistance cannot be obtained even when complete
quenching is conducted. Therefore the upper limit is set to 0.5%.
Incidentally the preferred content of Mn is less than 0.3%.
[0049] P: Phosphorus is an impurity element which deteriorates the
SSC resistance as it segregates in the grain boundary. Therefore,
the P content must be restricted to as low as possible. The upper
limit of the P content is set to 0.015% as an allowable level that
can be attained stably according to existing refining technologies
in view of the cost.
[0050] S: Sulfur is also an element that segregates in the grain
boundary and deteriorates the SSC resistance. It is a basic
requirement in the present invention to inhibit the Mn content that
fixes S. Therefore, the S content should be reduced as low as
possible. Since remarkable deterioration of the SSC resistance is
not observed at a S content of less than 0.0050%, the upper limit
is set to 0.0050%.
[0051] Mo: Molybdenum is one of the essential elements in the
present invention. It is the element that restricts the grain
boundary segregation of P and improves the temper softening
resistance. Therefore, Mo is a suitable element for obtaining the
high strength. At least 0.5% of Mo must be contained in order to
secure a sufficient SSC resistance in the high strength region of
YS.gtoreq.120 ksi as shown in FIG. 2. The higher YS, the greater
the Mo amount that is to be contained. The preferred range is at
least 1.0%. When Mo is contained in a greater amount, the effect
gets into saturation, and freedom of strength adjustment becomes
small. Therefore, the upper limit is set to 2.5%.
[0052] Al: Aluminum is necessary for sufficiently deoxidizing the
steel during the steel making process, and at least 0.005% of Al
should be contained. When a greater amount of Al is contained,
however, the amounts of alumina type inclusions increase with the
result that the susceptibility to SSC is likely to increase.
Therefore, the upper limit is set to 0.1%.
[0053] Ti: Titanium is contained in order to let B, that will be
described later, sufficiently exhibit its hardenability enhancing
function. In other words, N must be fixed as TiN in advance in
order to prevent precipitation of BN. For this purpose, at least
0.005%, and moreover, at least 3.4 times the N content, of Ti must
be added. However, a greater Ti content promotes precipitation of
TiN and enhances SSC susceptibility. Therefore, the upper limit is
set to 0.1%.
[0054] Nb: Niobium is the element effective for improving the SSC
resistance because it reduces the grain boundary segregation of P
through its grain refining effect. Therefore, at least 0.01% of Nb
is contained. However, even when a greater amount of Nb is
contained, the grain refining effect gets into saturation. On the
contrary, the SSC resistance drops due to the drop of the grain
boundary strength resulting from coarsening of carbides. Hence, the
upper limit is set to 0.1%.
[0055] N: Nitrogen is an impurity element that hinders the
hardenability improving effect of B and should be restricted to as
low as possible. The upper limit is set to 0.01% as the allowable
level that can be attained industrially and stably by the existing
refining technologies in view of the cost.
[0056] B: Boron is the element that remarkably improves
hardenability, and is one of the essential elements in the present
invention for securing hardenability. If the B content is less than
0.0005%, sufficient hardenability cannot be secured. Therefore, the
lower limit is set to 0.0005%. If the B content exceeds 0.0050%, on
the other hand, the hardenability improving effect gets into
saturation, and precipitation of carbo-borides becomes remarkable
and the SSC resistance gets deteriorated, on the contrary.
Therefore, the upper limit is set to 0.0050%.
.alpha.=Mo-0.15YS:
[0057] When the value .alpha., that is calculated as the function
of the yield strength YS (ksi) and the Mo content (wt %), exceeds
-18.9 as shown in FIG. 2, an excellent SSC resistance can be
obtained. When it is below -18.9, a satisfactory SSC resistance
cannot be obtained even when the individual components satisfy the
respective conditions. For this reason, the present invention
stipulates .alpha..gtoreq.-18.9 as the essential requirement of the
invention in addition to the component conditions described
above.
.beta.=2.7C+Mn+2Mo:
[0058] In addition to the component conditions described above, the
index .beta., that is calculated from the contents (wt %) of the
alloy elements contributing to hardenability, such as C, Mn and Mo,
is set to at least 2.0, as shown in FIG. 1, in order to secure
sufficient hardenability. The upper limit is calculated as 6.11
from each of the upper limit contents of C, Mn and Mo.
[0059] In addition to the elements described above, the steel
material of the present invention may contain at least one element
selected from the following elements, whenever necessary.
[0060] W: Tungsten has the function of improving both hardenability
and temper softening resistance. However, its effect is not
sufficient if its content is less than 0.01%. When the content
exceeds 0.5%, the effect gets into saturation. Therefore the
suitable range of the W content is set to 0.01 to 0.5%.
[0061] V: Vanadium has the function of improving temper softening
resistance. When its content is greater than 0.01%, it is effective
for improving the strength. When an excessive amount of V is
contained, however, the SSC resistance gets deteriorated.
Therefore, the upper limit is set to 0.3%.
[0062] Zr: Zirconium has an effect of restricting grain boundary
segregation of P. At least 0.001% of Zr must be contained in order
to obtain the effect. Zr is an expensive element and, if contained
too much, Zr increases the amounts of oxides and possibly increases
the SSC susceptibility. Therefore, the upper limit is set to
0.010%.
[0063] Ca, Mg and REM: These elements mitigate the stress
concentration by spheroidizing the morphology of inclusions, at the
same time, mitigate the grain boundary segregation of S by fixing
it. If the content of these elements is less than 0.001%, the
effect is small. On the other hand, at an excessive content, the
amounts of oxides increase and the SSC susceptibility is possibly
enhanced. Therefore, the upper limit of these element is set to
0.010%.
[0064] In the present invention, the steel having the alloy
composition described above is molten in a converter or an electric
furnace and cast. It is then shaped by an ordinary hot rolling
method into a desired shape such as a pipe, a sheet or rod, is
subjected to quenching and tempering, and is refined to a desired
strength.
[0065] The heat-treating condition in the present invention will be
explained as follows.
[0066] The austenitizing temperature for quenching is at least
[Ac.sub.3 point+20.degree. C.] and not higher than 1,000.degree. C.
If it is lower than Ac.sub.3 point+20 .degree. C., austenitizing of
the steel material is not sufficient and a uniform martensite
structure cannot be obtained easily. When the austenitizing
temperature exceeds 1,000.degree. C., on the other hand, the grain
growth becomes remarkable and the area of the grain boundary
decreases. In consequence, the SSC resistance deteriorating effect
of the segregation elements such as P becomes remarkable.
Therefore, the suitable austenitizing temperature is within the
range of [Ac.sub.3+20.degree. C.] to 1,000.degree. C. Incidentally,
the typical Ac.sub.3 point of the steel material according to the
present invention is 830.degree. C.
[0067] As described above, further, the steel material quenched
from this austenitizing temperature must have a uniform structure
in order to acquire a satisfactory SSC resistance. It is necessary
that the hardness of the portion that is most distant from the
quenched end has a value of at least 95% in terms of the ratio to
the hardness of the portion immediately below the quenched end. In
the chemical composition of the present invention, expected
hardenability can be secured even in a thick material having a
thickness of about 25 mm.
[0068] A suitable range of the tempering temperature is 620 to
720.degree. C. Under the low temperature condition of lower than
620.degree. C., YS becomes excessively high and the value a
described above becomes low. In consequence, the satisfactory SSC
resistance cannot be obtained. When the tempering temperature
exceeds 720.degree. C., on the other hand, the temperature is in
the dual phase region, and uniformity of the microstructure is lost
with the result of an increase of the SSC susceptibility.
Therefore, the range of 630 to 720.degree. C. is set as the
suitable condition for a tempering temperature.
[0069] The YS value can be secured within the predetermined range
due to the heat treatment temperature range described above, for
example, by controlling the quenching temperature, and the relation
.alpha.=Mo-0.15YS.gtoreq.-18.9 can be satisfied.
[0070] As described above, it is possible to obtain a high
strength, SSC-resistant steel material satisfying YS 120 ksi, which
has not been achieved in the past, by optimizing the microstructure
and the components. When the present invention is applied to the
strength region of YS of lower than 120 ksi, the SSC resistance ca
also be improved. FIG. 5 shows the measurement result of the
threshold stress for crack occurrence (.sigma.th) of a steel
material, that satisfies the microstructure and the chemical
compositions of the present invention and is refined to YS of 117
to 120 ksi, while the applied stress is varied by a NACE TM0177-A
constant load SSC test. It can be thus said that the structural
factors of the present invention improve not only the SSC
resistance of the steel material having YS.gtoreq.120 ksi but also
that of the steel material having YS of less than 120 ksi due to
the improvement of .sigma.th.
EXAMPLE
[0071] Each steel material having a chemical composition tabulated
in Table 1 was molten in a vacuum melting furnace, and the
resulting ingot was hot rolled to a plate having a thickness of 25
mm. The plate was heated to 900 to 1,025.degree. C. for
austenitizing, and was then subjected to quenching treatment by
water. The hardness at the center of the thickness and the hardness
of a portion just below the surface were measured. Hardenability
was evaluated by calculating their ratio. Subsequently, tempering
was conducted at 630 to 730.degree. C. A round rod tensile test
specimen was machined from the center of the thickness of this
plate, and the tensile test was carried out.
[0072] A test specimen for a sulfide stress cracking test, which
had a round rod shape having a length of 25 mm at parallel portions
and a diameter of 6.2 mm and was stipulated in NACE-TM0177-A, was
also machined from the center of the thickness. This test was
carried out in a corrosive solution containing 0.5% acetic acid+5%
NaCl and containing H.sub.2S saturated at a partial pressure of 1
atm at 25.degree. C., for 720 hours while a constant stress
corresponding to 80% of the yield strength was loaded to the
specimen. Additional tests were carried out by decreasing the
applied stress step-wise by 5% YS, whenever necessary, for those
specimens which underwent rupture at the applied stress of 80% YS,
and by increasing the applied stress step-wise by 5% YS for those
specimens which did not undergo rupture. In this way, a threshold
stress for crack occurrence (.sigma.th) was determined.
[0073] Hardenability was evaluated as "good" when the hardness of
the center portion of the thickness was greater than 95% of the
hardness of the portion immediately below the surface. The SSC
resistance was evaluated as "good" for those specimens which did
not undergo rupture during the test for 720 hours. The test result
was tabulated in Tables 2 and 3.
[0074] Nos. 1 to 11 and Nos. 101 to 104 represent the test results
that fell within the range of the present invention. These
specimens satisfied both high strength of YS 120 ksi and excellent
SSC resistance.
[0075] On the other hand, Nos. 18 to 30 and Nos. 105 to 107 of
Comparative Examples could not provide excellent SSC resistance
because their components were outside the range of the present
invention. The components of Nos. 12, 13, 14, 16 and 17 of
Comparative Examples were within the range of the present invention
but the .alpha. values were outside the range of the present
invention. Nos. 15 and 31 of Comparative Examples could not provide
sufficient SSC resistance because the tempering temperature and the
austenitizing temperature were outside the range of the present
invention, respectively.
[0076] In Nos. 108 and 109 tabulated in Table 3, YS was less than
120 ksi but .sigma.th was higher than that of Comparative Examples
Nos. 113 and 114 having an equivalent YS value. Therefore, the SSC
resistance was improved. Nos. 110 and 111 had the YS values higher
than 120 ksi and their .sigma.th values, were as high as 85% of YS.
On the other hand, in Comparative Example No. 112, YS remained at
85% of .sigma.th though YS was low. No. 115 had insufficient SSC
resistance.
13TABLE 1 Chemical Compositions (wt %; * ppm) Item Symbol C Si Mn
P* S* Mo Al Ti Nb N* B* Ti/N W V Zr* Ca* Mg* REM* .beta. This X-1
0.20 0.15 0.06 80 15 1.30 0.025 0.021 0.026 39 12 5.38 3.20 Inven-
X-2 0.20 0.16 0.46 81 15 1.30 0.026 0.022 0.031 41 13 5.37 3.60
tion X-3 0.27 0.15 0.15 70 25 0.58 0.029 0.031 0.019 55 13 5.64
2.04 X-4 0.29 0.15 0.04 60 10 1.51 0.025 0.026 0.026 39 12 6.67
0.21 3.84 A 0.29 0.15 0.45 81 10 0.90 0.040 0.035 0.029 70 15 5.00
0.25 0.02 21 3.03 B 0.20 0.15 0.33 70 25 0.62 0.029 0.031 0.019 55
13 5.64 2.11 C 0.20 0.15 0.33 70 15 0.89 0.029 0.031 0.019 55 13
5.64 2.65 D 0.20 0.20 0.13 81 25 1.40 0.036 0.075 0.021 39 7 19.23
3.47 E 0.19 0.25 0.08 86 16 1.80 0.038 0.021 0.031 41 38 5.12 15 15
4.19 F 0.12 0.15 0.23 57 9 2.21 0.047 0.029 0.041 51 11 5.69 4.97 G
0.19 0.20 0.05 75 10 1.50 0.035 0.041 0.031 45 21 9.11 21 12 3.56
Comp- Y-1 0.23 0.16 0.30 80 16 0.78 0.024 0.023 0.025 44 11 5.23
2.48 arative Y-2 0.25 0.15 0.13 80 16 0.42 0.025 0.025 0.025 38 12
6.58 1.65 Ex- Y-3 0.21 0.16 0.55 80 16 1.88 0.026 0.024 0.029 41 12
5.85 4.88 ample H 0.15 0.21 0.30 91 25 1.55 0.026 0.008 0.018 65 13
1.90 19 3.81 I 0.28 0.15 0.46 135 21 0.48 0.032 0.040 0.036 39 20
10.30 2.18 J 0.17 0.14 0.21 70 23 0.61 0.036 0.025 0.038 46 13 5.43
1.89 K 0.12 0.15 0.20 75 28 0.60 0.046 0.036 0.030 55 12 6.55 1.72
L 0.25 0.16 0.33 51 18 1.40 0.040 0.034 0.034 51 56 6.67 3.81 M
0.19 0.17 0.45 165 14 0.75 0.032 0.033 0.036 55 15 6.00 2.46 N 0.25
0.15 0.33 71 16 0.91 0.031 0.031 0.039 56 13 5.54 2.83 O 0.18 0.15
0.60 136 13 0.71 0.033 0.035 0.029 60 12 5.83 2.51 P 0.19 0.14 0.36
129 55 0.72 0.026 0.029 0.034 78 9 3.71 2.31 Q 0.23 0.15 0.33 71 16
0.91 0.031 0.031 0.039 56 13 5.54 2.77 R 0.21 0.18 0.31 120 20 1.19
0.034 0.021 0.111 40 13 5.25 3.26 The underlined values are out of
the scope of the invention.
[0077]
14TABLE 2 Heat Treatment Conditions, Strength and SSC Test Results
SSC Test Results under Quenching Tempering conditions of
Temperature Temperature YS load stress = Item No. Composition
(.degree. C.) Hardenability (.degree. C.) (ksi) .alpha. 80% of YS
This 101 X-1 950 Good 670 126 -17.90 Good Invention 102 X-2 930
Good 670 125 -17.45 Good 103 X-3 930 Good 670 123 -17.87 Good 104
X-4 930 Good 710 131 -18.14 Good 1 A 900 Good 680 123 -17.85 Good 2
B 900 Good 670 124 -17.98 Good 3 C 900 Good 660 125 -17.86 Good 4 C
930 Good 650 129 -18.46 Good 5 D 900 Good 670 129 -17.95 Good 6 E
900 Good 700 131 -17.85 Good 7 E 930 Good 680 136 -18.60 Good 8 E
900 Good 720 121 -16.35 Good 9 F 900 Good 690 135 -18.04 Good 10 F
930 Good 680 138 -18.49 Good 11 G 930 Good 670 131 -18.15 Good
Comparative 105 Y-1 930 Good 650 124 -17.82 Poor Example 106 Y-2
930 Poor 650 123 -18.03 Poor 107 Y-3 930 Good 690 124 -16.72 Poor
12 C 900 Good 640 134 -19.21 Poor 13 C 900 Good 630 140 -20.11 Poor
14 E 900 Good 680 141 -19.35 Poor 15 E 900 Good 730 103 -15.27 Poor
16 G 930 Good 660 145 -20.25 Poor 17 G 900 Good 660 137 -19.05 Poor
18 H 900 Poor 650 129 -17.80 Poor 19 I 930 Good 690 125 -18.27 Poor
20 J 900 Poor 650 119 -17.24 Poor 21 K 900 Poor 650 115 -16.65 Poor
22 L 930 Good 700 131 -18.25 Poor 23 M 900 Good 660 128 -18.45 Poor
24 N 900 Good 650 129 -18.44 Poor 25 N 900 Good 670 118 -16.79 Good
26 O 930 Good 660 126 -18.19 Poor 27 P 900 Good 660 128 -18.19 Poor
28 Q 900 Good 650 125 -17.84 Poor 29 Q 900 Good 660 118 -16.79 Good
30 R 900 Good 670 130 -18.31 Poor 31 C 1025 Good 660 130 -18.61
Poor The underlined data are out of the scope of the invention.
[0078]
15TABLE 3 Heat Treatment Conditions, Strength and SSC Test Results
Quenching Tempering Temperature Temperature YS .sigma.th Item No.
Composition (.degree. C.) Hardenability (.degree. C.) (ksi) .alpha.
(.times. YS) This 108 X-1 950 Good 690 111 -15.35 0.95 Invention
109 X-1 950 Good 680 119 -16.55 0.90 110 X-1 950 Good 670 126
-17.90 0.85 111 X-1 950 Good 660 132 -18.50 0.85 Comparative 112
Y-1 950 Good 690 106 -15.12 0.85 Example 113 Y-1 950 Good 680 112
-16.02 0.85 114 Y-1 950 Good 670 118 -16.92 0.80 115 Y-1 950 Good
660 121 -18.42 0.55 The underlined data are out of the scope of the
invention.
[0079] INDUSTRIAL APPLICABILITY
[0080] According to the present invention, as described above, a
steel material for an oil well that satisfies both a high strength
of at least 120 ksi in terms of a yield strength and excellent SSC
resistance can be produced.
* * * * *