U.S. patent application number 11/494608 was filed with the patent office on 2006-11-30 for seamless steel pipe for oil wells excellent in sulfide stress cracking resistance and method for producing the same.
Invention is credited to Yuji Arai, Keiichi Nakamura, Tomohiko Omura.
Application Number | 20060266448 11/494608 |
Document ID | / |
Family ID | 34823873 |
Filed Date | 2006-11-30 |
United States Patent
Application |
20060266448 |
Kind Code |
A1 |
Arai; Yuji ; et al. |
November 30, 2006 |
Seamless steel pipe for oil wells excellent in sulfide stress
cracking resistance and method for producing the same
Abstract
A high-strength seamless steel pipe for oil wells excellent in
sulfide stress cracking resistance which comprises, on the percent
by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%,
Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to
0.05% and B: 0.0003 to 0.005%, with a value of equation
"C+(Mn/6)+(Cr/5)+(Mo/3)" of 0.43 or more, with the balance being Fe
and impurities, and in the impurities P: 0.025% or less, S: 0.010%
or less and N: 0.007% or less. The seamless steel pipe may contain
a specified amount of one or more element(s) of V and Nb, and/or a
specified amount of one or more element(s) of Ca, Mg and REM. The
seamless steel pipe can be produced at a low cost by adapting an
in-line tube making and heat treatment process having a high
production efficiency since a reheating treatment for refinement of
grains is not required.
Inventors: |
Arai; Yuji; (Amagasaki-shi,
JP) ; Omura; Tomohiko; (Osaka, JP) ; Nakamura;
Keiichi; (Wakayama-shi, JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW
SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
34823873 |
Appl. No.: |
11/494608 |
Filed: |
July 28, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/JP05/01186 |
Jan 28, 2005 |
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11494608 |
Jul 28, 2006 |
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Current U.S.
Class: |
148/593 ;
148/330 |
Current CPC
Class: |
C22C 38/22 20130101;
C21D 1/18 20130101; C21D 9/085 20130101; C22C 38/02 20130101; C21D
8/105 20130101; C21D 9/08 20130101; C21D 8/10 20130101; C22C 38/04
20130101; C21D 2211/008 20130101 |
Class at
Publication: |
148/593 ;
148/330 |
International
Class: |
C21D 9/08 20060101
C21D009/08; C22C 38/32 20060101 C22C038/32 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 2004 |
JP |
2004-023470 |
Claims
1. A seamless steel pipe for oil wells which comprises, on the
percent by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05
to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti:
0.002 to 0.05% and B: 0.0003 to 0.005%, with a value of A
determined by the following equation (1) of 0.43 or more, with the
balance being Fe and impurities, and in the impurities P: 0.025% or
less, S: 0.010% or less and N: 0.007% or less:
A=C+(Mn/6)+(Cr/5)+(Mo/3) (1), wherein, in the equation (1), C, Mn,
Cr and Mo each represent % by mass of the respective elements.
2. A seamless steel pipe for oil wells which comprises, on the
percent by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05
to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti:
0.002 to 0.05%, B: 0.0003 to 0.005%, and either one or both of V:
0.03 to 0.2% and Nb: 0.002 to 0.04%, with a value of A determined
by the following equation (1) of 0.43 or more, with the balance
being Fe and impurities, and in the impurities P: 0.025% or less,
S: 0.010% or less and N: 0.007% or less: A=C+(Mn/6)+(Cr/5)+(Mo/3)
(1), wherein, in the equation (1), C, Mn, Cr and Mo each represent
% by mass of the respective elements.
3. A seamless steel pipe for oil wells which comprises, on the
percent by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05
to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti:
0.002 to 0.05%, B: 0.0003 to 0.005%, and one or more element(s)
selected from a group of Ca of 0.0003 to 0.005%, Mg of 0.0003 to
0.005% and REM of 0.0003 to 0.005%, with a value of A determined by
the following equation (1) of 0.43 or more, with the balance being
Fe and impurities, and in the impurities P: 0.025% or less, S:
0.010% or less and N: 0.007% or less: A=C+(Mn/6)+(Cr/5)+(Mo/3) (1),
wherein, in the equation (1), C, Mn, Cr, and Mo each represent % by
mass of the respective elements.
4. A seamless steel pipe for oil wells which comprises, on the
percent by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05
to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti:
0.002 to 0.05%, B: 0.0003 to 0.005%, either one or both of V: 0.03
to 0.2% and Nb: 0.002 to 0.04%, and one or more element(s) selected
from a group of Ca of 0.0003 to 0.005%, Mg of 0.0003 to 0.005% and
REM of 0.0003 to 0.005%, with a value of A determined by the
following equation (1) of 0.43 or more, with the balance being Fe
and impurities, and in the impurities P: 0.025% or less, S: 0.010%
or less and N: 0.007% or less: A=C+(Mn/6)+(Cr/5)+(Mo/3) (1),
wherein, in the equation (1), C, Mn, Cr and Mo each represent % by
mass of the respective elements.
5. The seamless steel pipe for oil wells according to claim 1,
wherein the tensile strength is not more than 931 MPa.
6. The seamless steel pipe for oil wells according to claim 2,
wherein the tensile strength is not more than 931 MPa.
7. The seamless steel pipe for oil wells according to claim 3,
wherein the tensile strength is not more than 931 MPa.
8. The seamless steel pipe for oil wells according to claim 4,
wherein the tensile strength is not more than 931 MPa.
9. A method for producing a seamless steel pipe for oil wells,
which comprises the steps of making a pipe by hot-piercing a steel
billet having a chemical composition according to claim 1, with a
value of A determined by the following equation (1) of 0.43 or more
followed by elongating and rolling, and then finally rolling at a
final rolling temperature adjusted to 800 to 1100 degrees
centigrade, assistantly heating the resulting steel pipe in a
temperature range from the Ar.sub.3 transformation point to 1000
degrees centigrade in-line, and then quenching it from a
temperature of the Ar.sub.3 transformation point or higher followed
by tempering at a temperature lower than the Ac.sub.1
transformation point: A=C+(Mn/6)+(Cr/5)+(Mo/3) (1), wherein, in the
equation (1), C, Mn, Cr and Mo each represent % by mass of the
respective elements.
10. The method for producing a seamless steel pipe for oil wells
according to claim 9, wherein the temperature of assistant heating
in-line is the Ac.sub.3 transformation point to 1000 degrees
centigrade.
11. A method for producing a seamless steel pipe for oil wells,
which comprises the steps of making a pipe by hot-piercing a steel
billet having a chemical composition according to claim 2, with a
value of A determined by the following equation (1) of 0.43 or more
followed by elongating and rolling, and then finally rolling at a
final rolling temperature adjusted to 800 to 1100 degrees
centigrade, assistantly heating the resulting steel pipe in a
temperature range from the Ar.sub.3 transformation point to 1000
degrees centigrade in-line, and then quenching it from a
temperature of the Ar.sub.3 transformation point or higher followed
by tempering at a temperature lower than the Ac.sub.1
transformation point: A=C+(Mn/6)+(Cr/5)+(Mo/3) (1), wherein, in the
equation (1), C, Mn, Cr and Mo each represent % by mass of the
respective elements.
12. A method for producing a seamless steel pipe for oil wells,
which comprises the steps of making a pipe by hot-piercing a steel
billet having a chemical composition according to claim 3, with a
value of A determined by the following equation (1) of 0.43 or more
followed by elongating and rolling, and then finally rolling at a
final rolling temperature adjusted to 800 to 1100 degrees
centigrade, assistantly heating the resulting steel pipe in a
temperature range from the Ar.sub.3 transformation point to 1000
degrees centigrade in-line, and then quenching it from a
temperature of the Ar.sub.3 transformation point or higher followed
by tempering at a temperature lower than the Ac.sub.1
transformation point: A=C+(Mn/6)+(Cr/5)+(Mo/3) (1), wherein, in the
equation (1), C, Mn, Cr and Mo each represent % by mass of the
respective elements.
13. A method for producing a seamless steel pipe for oil wells,
which comprises the steps of making a pipe by hot-piercing a steel
billet having a chemical composition according to claim 4, with a
value of A determined by the following equation (1) of 0.43 or more
followed by elongating and rolling, and then finally rolling at a
final rolling temperature adjusted to 800 to 1100 degrees
centigrade, assistantly heating the resulting steel pipe in a
temperature range from the Ar.sub.3 transformation point to 1000
degrees centigrade in-line, and then quenching it from a
temperature of the Ar.sub.3 transformation point or higher followed
by tempering at a temperature lower than the Ac.sub.1
transformation point: A=C+(Mn/6)+(Cr/5)+(Mo/3) (1), wherein, in the
equation (1), C, Mn, Cr and Mo each represent % by mass of the
respective elements.
Description
[0001] This application is a continuation of the international
application PCT/JP2005/001186 filed on Jan. 28, 2005, the entire
content of which is herein incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to a high strength seamless
steel pipe which is excellent in sulfide stress cracking resistance
and a method for producing the same. More specifically, the present
invention relates to a seamless steel pipe for oil wells having a
high yield ratio and also excellent sulfide stress cracking
resistance, which is produced by the method of quenching and
tempering for a specified component-based steel.
[0003] "An oil well" in the present specification includes "a gas
well", and so, the meaning of "for oil wells" is "for oil and/or
gas wells".
BACKGROUND ART
[0004] A seamless steel pipe, which is more reliable than a welded
pipe, is frequently used in a sever oil well environment or
high-temperature environment, and the enhancement of strength,
improvement in toughness and improvement in sour resistance are
therefore consistently required. Particularly, in oil wells to be
developed in future, the enhancement in strength of the steel pipe
is needed more than ever before because a high-depth well will
become the mainstream, and a seamless steel pipe for oil wells also
having stress corrosion cracking resistance is increasingly
required because the pipe is used in a severe corrosive
environment.
[0005] The hardness, namely the dislocation density, of steel
product is raised as the strength is enhanced, and the amount of
hydrogen to be penetrated into the steel product increases to make
the steel product fragile to stress because of the high dislocation
density. Accordingly, the sulfide stress cracking resistance is
generally deteriorated against the enhancement in strength of the
steel product used in a hydrogen sulfide-rich environment.
Particularly, when a member having a desired yield strength is
produced by use of a steel product with a low ratio of "yield
strength/tensile strength" (hereinafter referred to as yield
ratio), the tensile strength and hardness are apt to increase, and
the sulfide stress cracking resistance is remarkably deteriorated.
Therefore, when the strength of the steel product is raised, it is
important to increase the yield ratio for keeping the hardness
low.
[0006] Although it is preferable to make the steel product into a
uniform tempered martensitic microstructure for increasing the
yield ratio of the steel, that alone is insufficient. As a method
for further enhancing the yield ratio in the tempered martensitic
microstructure, refinement of prior-austenite grains is given.
However, the refinement of austenite grains needs quenching in an
off-line heat treatment, which deteriorates the production
efficiency and increases the energy used. Therefore, this method is
disadvantageous in these days where rationalization of cost,
improvement in production efficiency and energy saving are
indispensable to manufacturers.
[0007] It is described in the Patent Documents 1 and 2 that
precipitation of a M.sub.23C.sub.6 type carbide in grain boundary
is inhibited to improve the sulfide stress cracking resistance. An
improvement in sulfide stress cracking resistance by refinement of
grains is also disclosed in the Patent Document 3. However, such
measures have the difficulties as described above.
[0008] Patent Document 1: Japanese Laid-Open Patent Publication No.
2001-73086,
[0009] Patent Document 2: Japanese Laid-Open Patent Publication No.
2000-17389,
[0010] Patent Document 3: Japanese Laid-Open Patent Publication No.
9-111343.
DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE
INVENTION
[0011] From the point of the above-mentioned present situation, the
present invention has an object to provide a high strength seamless
steel pipe for oil wells having a high yield ratio and an excellent
sulfide stress cracking resistance, which can be produced by an
efficient means capable of realizing an energy saving.
MEAN FOR SOLVING THE PROBLEMS
[0012] The gists of the present invention are a seamless steel pipe
for oil wells described in the following (1), and a method for
producing a seamless steel pipe for oil wells described in the
following (2). The percentage for a component content means % based
on mass in the following descriptions.
[0013] (1) A seamless steel pipe for oil wells which comprises C:
0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%,
Mo: 0.05 to 1.0%, Al: 0.1% or less, Ti: 0.002 to 0.05%, B: 0.0003
to 0.005%, further, one or more elements selected from one or both
of the following first group and second group as occasion demands,
with a value of A determined by the following equation (1) of 0.43
or more, with the balance being Fe and impurities, and in the
impurities P: 0.025% or less, S: 0.010% or less and N: 0.007% or
less.
[0014] First Group:
[0015] V: 0.03 to 0.2% and Nb: 0.002 to 0.04%,
[0016] Second Group:
[0017] Ca: 0.0003 to 0.005%, Mg: 0.0003 to 0.005% and REM: 0.0003
to 0.005%, A=C+(Mn/6)+(Cr/5)+(Mo/3) (1), wherein, in the equation
(1), C, Mn, Cr and Mo each represent % by mass of the respective
elements.
[0018] In order to improve the sulfide stress cracking resistance
of the steel pipe for oil wells described in (1), preferably the
tensile strength is not more than 931 MPa (135 ksi).
[0019] (2) A method for producing a seamless steel pipe for oil
wells, which comprises the steps of making a pipe by hot-piercing a
steel billet having a chemical composition described in the above
(1) and a value of A determined by the above equation (1) of 0.43
or more followed by elongating and rolling, and finally rolling at
a final rolling temperature adjusted to 800 to 1100 degrees
centigrade, assistantly heating the resulting steel pipe in a
temperature range from the Ar.sub.3 transformation point to 1000
degrees centigrade in-line, and then quenching it from a
temperature of the Ar.sub.3 transformation point or higher followed
by tempering at a temperature lower than the Ac.sub.1
transformation point.
[0020] In order to obtain more uniform microstructure, in the
method for producing a seamless steel pipe for oil well described
in (2), preferably the temperature of the assist heating of the
steel pipe in-line is between the AC.sub.3 transformation point and
1000 degrees centigrade.
BRIEF DESCRIPTION OF THE DRAWING
[0021] FIG. 1 is a graphic representation of the influence of the
content of C on the relationship between yield strength (YS) and
yield ratio (YR) in a quenched and tempered steel plate.
BEST MODE FOR CARRYING OUT THE INVENTION
[0022] The present invention has been accomplished on the basis of
the following findings.
[0023] The yield ratio of a steel product having a quenched and
tempered microstructure is most significantly influenced by the
content of C. The yield ratio generally increases when the C
content is reduced. However, even if the C content is simply
reduced, a uniform quenched microstructure cannot be obtained since
the hardenability is deteriorated, and the yield ratio cannot be
sufficiently raised. Therefore, it is important for the
hardenability deteriorated by reducing the C content to be improved
by adding Mn, Cr and Mo.
[0024] When the A-value of the above-mentioned equation (1) is set
to 0.43 or more, a uniform quenched microstructure can be obtained
in a general steel pipe quenching facility. The present inventors
confirmed that when the A-value of the equation (1) is 0.43 or
more, the hardness in a position 10 mm from a quenched end
(hereinafter referred to as "Jominy end") in a Jominy test exceeds
the hardness corresponding to a martensite ratio of 90% and
satisfactory hardenability can be ensured. The A-value is
preferably set to 0.45 or more, and more preferably 0.47 or
more.
[0025] The present inventors further examined the influence of
alloy elements on the yield ratio and sulfide stress cracking
resistance of a steel product having a quenched and tempered
microstructure. The examination results are as follows:
[0026] Each of steels having chemical components shown in Table 1
was melted by use of a 150 kg vacuum melting furnace. The obtained
steel ingot was hot forged to form a block with 50 mm thickness, 80
mm width and 160 mm length. A Jominy test piece was taken from the
remaining ingot austenitized at 1100 degrees centigrade, and
submitted to a Jominy test to examine the hardenability of each
steel. The prior-austenite grain size of each steel A to G of Table
1 was about No. 5 and relatively coarse.
[0027] The Rockwell C hardness in the position 10 mm from the
Jominy end in the Jominy test (JHRC.sub.10) of each steel A to G
and the Rockwell C hardness predicted value at 90%-martensite ratio
corresponding to the C content of each steel A to G are shown in
Table 1. The position 10 mm from the Jominy end in the Jominy test
corresponds to a cooling rate of 20 degrees centigrade/second. The
predicted value of the Rockwell C hardness at 90%-martensite ratio
based on the content C is given by "58C%+27" as shown in the
following Non-Patent Document 1.
[0028] Non-Patent Document 1: "Relationship between hardenability
and percentage martensite in some low alloy steels" by J. M. Hodge
and M. A. Orehoski, Trans. AIME, 167, 1946, pp. 627-642.
[0029] [Table 1] TABLE-US-00001 TABLE 1 Chemical composition (mass
%) The balance: Fe and impurities A- Ac.sub.1 Ac.sub.3 58 C. Steel
C Si Mn P S Cr Mo V Ti B Ca sol. Al N value point point JHRC.sub.10
% + 27 A 0.10 0.21 0.61 0.012 0.002 0.70 0.30 0.05 0.019 0.0010
0.0025 0.042 0.0040 0.442 758 897 35.4 32.8 B 0.15 0.18 0.59 0.010
0.002 0.58 0.29 0.05 0.019 0.0010 0.0025 0.042 0.0040 0.461 754 872
38.5 35.7 C 0.20 0.18 0.60 0.011 0.001 0.61 0.30 0.05 0.025 0.0012
0.0028 0.043 0.0041 0.522 753 848 41.0 38.6 D 0.27 0.18 0.58 0.010
0.002 0.59 0.30 0.05 0.010 0.0015 0.0025 0.033 0.0037 0.585 752 816
45.8 42.7 E 0.35 0.19 0.60 0.011 0.002 0.60 0.30 0.05 0.016 0.0013
0.0032 0.035 0.0048 0.670 750 778 52.5 47.3 F 0.16 0.18 0.95 0.010
0.002 0.30 0.12 0.05 0.015 0.0010 0.0025 0.042 0.0040 0.418 739 855
34.1 36.3 G 0.20 0.38 0.79 0.011 0.001 0.59 0.68 0.05 0.008 --
0.0028 0.031 0.0041 0.676 765 870 36.5 38.6 A = C + (Mn/6) + (Cr/5)
+ (Mo/3). In the columns both "Ac.sub.1 point" and "Ac.sub.3
point", the temperature unit is "degrees centigrade". JHRC.sub.10
means the Rockwell C hardness in the position 10 mm from the
quenched end in the Jominy test.
[0030] In the steels A to E with A-values of 0.43 or more of the
said equation (1), JHRC.sub.10 exceeds the Rockwell C hardness
corresponding to 90%-martensite ratio, and satisfactory
hardenability can be ensured.
[0031] On the other hand, the steel F with an A-value smaller than
0.43 of the equation (1) and the steel G containing no B (boron)
are short of hardenability, since JHRC.sub.10 is below the Rockwell
C hardness corresponding to the 90%-martensite ratio.
[0032] Next, above-mentioned each block was subjected to a heating
treatment of soaking at 1250 degrees centigrade for 2 hours,
immediately carried to a hot rolling machine, and hot-rolled to a
thickness of 16 mm at a finish rolling temperature of 950 degrees
centigrade or higher. Each hot-rolled material was then carried to
a heating furnace before the surface temperature becomes lower than
the Ar.sub.3 transformation point, allowed to stand therein at 950
degrees centigrade for 10 minutes, and then inserted and
water-quenched in an agitating water tank.
[0033] Each water-quenched plate was divided to a proper length,
and a tempering treatment of soaking for 30 minutes was carried out
at various temperatures to obtain quenched and tempered plates.
Round bar tensile test pieces were cut off from the longitudinal
direction of the thus-obtained hot-rolled and heat-treated plates,
and a tensile test was carried out.
[0034] FIG. 1 is a graphic representation of the relationship
between yield strength (YS) and yield ratio (YR, the unit is
represented by %) of plates changed in strength by variously
changing the tempering temperature of the steels A to E. The unit
of YS is represented by ksi, wherein 1 MPa=0.145 ksi. The concrete
data of tempering temperature and tensile properties are shown in
Table 2.
[0035] [Table 2] TABLE-US-00002 TABLE 2 Tensile Properties Steel
Mark Tempering Temperature YS (ksi) TS (ksi) YR (%) A 1 640 118 123
96.1 2 660 112 117 95.8 3 680 107 112 95.4 4 700 102 107 94.5 5 720
92 99 92.4 B 1 640 124 131 94.9 2 660 119 126 94.6 3 680 112 119
94.1 4 700 98 107 92.0 5 720 85 96 88.9 C 1 640 135 144 93.5 2 660
127 136 93.1 3 680 120 129 92.8 4 700 109 119 91.4 5 720 97 109
89.2 D 1 640 131 143 91.4 2 660 120 132 91.2 3 680 113 125 90.3 4
700 103 117 88.6 5 720 93 108 86.8 E 1 640 136 149 90.9 2 660 126
140 89.7 3 680 115 129 88.9 4 700 102 118 86.6 5 720 90 106 84.8 F
1 640 120 137 88.0 2 660 114 131 87.0 3 680 104 125 85.8 4 700 92
115 84.3 5 720 81 104 81.0 G 1 640 130 137 88.0 2 660 122 131 87.2
3 680 114 125 85.4 4 700 95 105 82.0 5 720 87 104 78.0 In the
columns "Tempering Temperature", the temperature unit is "degrees
centigrade".
[0036] As is apparent from FIG. 1 and Table 2, in spite of the
prior-austenite grain sizes are about No. 5, which are relatively
coarse, the steels A to C with 0.20% or less of C have yield ratios
larger than the steels D to E with 0.25% or more of C by 2% or
more. Thus, this clearly shows that a material with high yield
ratio can be obtained over a wide strength range by reducing the C
content in a quenched and tempered steel while ensuring the
hardenability to make the steel into a uniform quenched
microstructure. It is apparent that the effect of raising the yield
ratio cannot be obtained in the steels F to G even with 0.20% or
less of C but insufficient hardenability.
[0037] The reason for specifying the chemical composition of the
steel of a seamless steel pipe for oil wells in the present
invention will be now described in detail.
[0038] C:
[0039] C is an element effective for inexpensively enhancing the
strength of steel. However, with the C content of less than 0.1%, a
low-temperature tempering must be performed to obtain a desired
strength, which causes a deterioration in sulfide stress cracking
resistance, or the necessity of addition of a large amount of
expensive elements to ensure the hardenability. With the C content
exceeding 0.20%, the yield ratio is reduced, and when a desired
yield strength is obtained, a rise of hardness is caused to
deteriorate the sulfide stress cracking resistance. Accordingly,
the C content is set to 0.1 to 0.20%. The preferable range of the C
content is 0.12 to 0.18%, and the more preferable range is 0.14 to
0.18%.
[0040] Si:
[0041] Si is an element, which enhances the hardenability of steel
to improve the strength in addition to deoxidation effect, and a
content of 0.05% or more is required. However, when the Si content
exceeds 1.0%, the sulfide stress cracking resistance is
deteriorated. Accordingly, the proper content of Si is 0.05 to
1.0%. The preferable range of the Si content is 0.1 to 0.6%.
[0042] Mn:
[0043] Mn is an element, which enhances the hardenability of steel
to improve the strength in addition to deoxidation effect, and a
content of 0.05% or more is required. However, when the Mn content
exceeds 1.0%, the sulfide stress cracking resistance is
deteriorated. Accordingly, the content of Mn is set to 0.05 to
1.0%
[0044] P:
[0045] P is an impurity of steel, which causes a deterioration in
toughness resulted from grain boundary segregation. Particularly
when the P content exceeds 0.025%, the sulfide stress cracking
resistance is remarkably deteriorated. Accordingly, it is necessary
to control the content of P to 0.025% or less. The P content is
preferably set to 0.020% or less and, more preferably, to 0.015% or
less.
[0046] S:
[0047] S is also an impurity of steel, and when the S content
exceeds 0.010%, the sulfide stress cracking resistance is seriously
deteriorated. Accordingly, the content of S is set to 0.010% or
less. The S content is preferably 0.005% or less.
[0048] Cr:
[0049] Cr is an element effective for enhancing the hardenability
of steel, and a content of 0.05% or more is required in order to
exhibit this effect. However, when the Cr content exceeds 1.5%, the
sulfide stress cracking resistance is deteriorated. Therefore, the
content of Cr is set to 0.05 to 1.5%. The preferable range of the
Cr content is 0.2 to 1.0%, and the more preferable range is 0.4 to
0.8%.
[0050] Mo:
[0051] Mo is an element effective for enhancing the hardenability
of steel to ensure a high strength and for enhancing the sulfide
stress cracking resistance. In order to obtain these effects, it is
necessary to control the content of Mo to 0.05% or more. However,
when the Mo content exceeds 1.0%, coarse carbides are formed in the
prior-austenite grain boundaries to deteriorate the sulfide stress
cracking resistance. Therefore, the content of Mo is set to 0.05 to
1.0%. The preferable range of the Mo content is 0.1 to 0.8%.
[0052] Al:
[0053] Al is an element having a deoxidation effect and effective
for enhancing the toughness and workability of steel. However, when
the content of Al exceeds 0.10%, streak flaws are remarkably
caused. Accordingly, the content of Al is set to 0.10% or less.
Although the lower limit of the Al content is not particularly set
because the content may be in an impurity level, the Al content is
preferably set to 0.005% or more. The preferable range of the Al
content is 0.005 to 0.05%. The Al content referred herein means the
content of acid-soluble Al (what we called the "sol.Al").
[0054] B:
[0055] Although the hardenability improving effect of B can be
obtained with a content of impurity level, the B content is
preferably set to 0.0003% or more in order to obtain the effect
more remarkably. However, when the content of B exceeds 0.005%, the
toughness is deteriorated. Therefore, the content of B is set to
0.0003 to 0.005%. The preferable range of the B content is 0.0003
to 0.003%.
[0056] Ti:
[0057] Ti fixes N in steel as a nitride and makes B present in a
dissolved state in the matrix at the time of quenching to make it
exhibit the hardenability improving effect. In order to obtain such
an effect of Ti, the content of Ti is preferably set to 0.002% or
more. However, when the content of Ti is 0.05% or more, it is
present as a coarse nitride, resulting in the deterioration of the
sulfide stress cracking resistance. Accordingly, the content of Ti
is set to 0.002 to 0.05%. The preferable range of Ti content is
0.005 to 0.025%.
[0058] N:
[0059] N is unavoidably present in steel, and binds to Al, Ti or Nb
to form a nitride. The presence of a large amount of N not only
leads to the coarsening of AlN or TiN but also remarkably
deteriorates the hardenability by also forming a nitride with B.
Accordingly, the content of N as an impurity element is set to
0.007% or less. The preferable range of N is less than 0.005%.
[0060] Limitation of the A-value Calculated by the Equation
(1):
[0061] The A-value is defined by the following equation (1) as
described above, wherein C, Mn, Cr, and Mo in the equation (1) mean
the percentage of the mass of the respective elements.
A=C+(Mn/6)+(Cr/5)+(Mo/3) (1).
[0062] The present invention is intended to raise the yield ratio
by limiting C to improve the sulfide stress cracking resistance.
Accordingly, if the contents of Mn, Cr, and Mo are not adjusted
according to the adjustment of the C content, the hardenability is
impaired to rather deteriorate the sulfide stress cracking
resistance. Therefore, in order to ensure the hardenability, the
contents of C, Mn, Cr and Mo must be set so that the said A-value
of the equation (1) is 0.43 or more. The said A-value is preferably
set to 0.45 or more, and more preferably to 0.47 or more.
[0063] The optional components of the first group and the second
group which are included as occasion demands will be then
described.
[0064] The first group consists of V and Nb. V precipitates as a
fine carbide at the time of tempering, and so it has an effect to
enhance the strength. Although such effect is exhibited by
including 0.03% or more of V, the toughness is deteriorated with
the content exceeding 0.2%. Accordingly, the content of added V is
preferably set to 0.03 to 0.2%. The more preferable range of the V
content is 0.05 to 0.15%.
[0065] Nb forms a carbonitride in a high temperature range to
prevent the coarsening of grains to effectively improve the sulfide
stress cracking resistance. When the content of Nb is 0.002% or
more, this effect can be exhibited. However, when the content of Nb
exceeds 0.04%, the carbonitride is excessively coarsened to rather
deteriorate the sulfide stress cracking resistance. Accordingly,
the content of added Nb is preferably set to 0.002 to 0.04%. The
more preferable range of the Nb content is 0002 to 0.02%.
[0066] The second group consists of Ca, Mg and REM. These elements
are not necessarily added. However, since they react with S in
steel when added, to form sulfides to thereby improve the form of
an inclusion, the sulfide stress cracking resistance of the steel
can be improved as an effect. This effect can be obtained, when one
or two or more selected from the group of Ca, Mg and REM (rare
earth elements, namely Ce, Ra, Y and so on) is added. When the
content of each element is less than 0.0003%, the effect cannot be
obtained. When the content of every element exceeds 0.005%, the
amount of inclusions in steel is increased, and the cleanliness of
the steel is deteriorated to reduce the sulfide stress cracking
resistance. Accordingly, the content of added each element is
preferably set to 0.0003 to 0.005%. In the present invention, the
content of REM means the sum of the contents of rare earth
elements.
[0067] Previously described, in general, the higher the strength of
a steel becomes, the worse the sulfide stress cracking resistance
becomes in the circumstance containing much hydrogen sulfide. But
the seamless steel pipe for oil wells comprising the chemical
compositions described above retains the good sulfide stress
cracking resistance if the tensile strength is not more than 931
MPa. Therefore the tensile strength of the seamless steel pipe for
oil well is preferably not more than 931 MPa (135 ksi). More
preferably the upper limit of the tensile strength is 897 MPa (130
ksi).
[0068] Next, the method for producing a seamless steel pipe for oil
wells of the present invention will be described.
[0069] The seamless steel pipe for oil wells of the present
invention is excellent in sulfide stress cracking resistance with a
high yield ratio even if it has a relatively coarse microstructure
such that the microstructure is mainly composed of tempered
martensite with an prior-austenite grain of No. 7 or less by a
grain size number regulated in JIS G 0551 (1998). Accordingly, when
a steel ingot having the above-mentioned chemical composition is
used as a material, the freedom of selection for the method for
producing a steel pipe can be increased.
[0070] For example, the said seamless steel pipe can be produced by
supplying a steel pipe formed by piercing and elongating by the
Mannesmann-mandrel mill tube-making method to a heat treatment
facility provided in the latter stage of a finish rolling machine
while keeping it at a temperature of the Ar.sub.3 transformation
point or higher to quench it followed by tempering at 600 to 750
degrees centigrade. Even if an energy-saving type in-line tube
making and heat treatment process such as the above-mentioned
process is selected, a steel pipe with a high yield ratio can be
produced, and a seamless steel pipe for oil wells having a desired
high strength and high sulfide stress cracking resistance can be
obtained.
[0071] The said seamless steel pipe can be also produced by cooling
a hot-finish formed steel pipe once down to room temperature,
reheating it in a quenching furnace to soak in a temperature range
of 900 to 1000 degrees centigrade followed by quenching in water,
and then tempering at 600 to 750 degrees centigrade. If an off-line
tube making and heat treatment process such as the above-mentioned
process is selected, a steel pipe having a higher yield ratio can
be produced by the refinement effect of prior-austenite grain, and
a seamless steel pipe for oil wells with higher strength and higher
sulfide stress cracking resistance can be obtained.
[0072] However, the production method described below is most
desirable. The reason is that since the pipe is held at a high
temperature from the tube-making to the quenching, an element such
as V or Mo can be easily kept in a dissolved state in the matrix,
and such elements precipitates in a high-temperature tempering
which is advantageous for improving the sulfide stress cracking
resistance, and contribute to the increase in strength of the steel
pipe.
[0073] The method for producing a seamless steel pipe for oil wells
of the present invention is characterized in the final rolling
temperature of elongating and rolling, and the heat treatment after
the end of rolling. Each will be described below.
[0074] (1) Final Rolling Temperature of Elongating and Rolling
[0075] This temperature is set to 800 to 1100 degrees centigrade.
At a temperature lower than 800 degrees centigrade, the deformation
resistance of the steel pipe is excessively increased to cause a
problem of tool abrasion. At a temperature higher than 1100 degrees
centigrade, the grains are excessively coarsened to deteriorate the
sulfide stress cracking resistance. The piercing process before the
elongating and rolling may be carried out by a general method, such
as Mannesmann piercing method.
[0076] (2) Assistant Heating Treatment
[0077] The elongated and rolled steel pipe is charged in line,
namely in a assistant heating furnace provided within a series of
steel pipe production lines, and assistantly heated in a
temperature range from the Ar.sub.3 transformation point to 1000
degrees centigrade. The purpose of the assistant heating is to
eliminate the dispersion in the longitudinal temperature of the
steel pipe to make the microstructure uniform.
[0078] When the temperature of the assistant heating is lower than
the Ar.sub.3 transformation point, a ferrite starts to generate,
and the uniform quenched microstructure cannot be obtained. When it
is higher than 1000 degrees centigrade, the grain growth is
promoted to cause the deterioration of the sulfide stress cracking
resistance by grain coarsening. The time of the assistant heating
is set to a time necessary for making the temperature of the whole
thickness of the pipe to a uniform temperature, that is about 5 to
10 minutes. Although, the assistant heating process may be omitted
when the final rolling temperature of elongating and rolling is
within a temperature range from the Ar.sub.3 transformation point
to 1000 degrees centigrade, the assistant heating is desirably
carried out in order to minimize the longitudinal and
thickness-directional dispersion in temperature of the pipe.
[0079] The more uniform microstructure is obtained when the
temperature of the assist heating of a steel pipe in-line is
between the Ac.sub.3 transformation point and 1000 degrees
centigrade. Therefore, the temperature of the assist heating of a
steel pipe in-line is preferably between the Ac.sub.3
transformation point and 1000 degrees centigrade.
[0080] (3) Quenching and Tempering
[0081] The steel pipe laid in a temperature range from the Ar.sub.3
transformation point to 1000 degrees centigrade through the above
processes is quenched. The quenching is carried out at a cooling
rate sufficient for making the whole thickness of the pipe into a
martensitic microstructure. Water cooling can be generally adapted.
The tempering is carried out at a temperature lower than the
Ac.sub.1 transformation point, desirably, at 600 to 700 degrees
centigrade. The tempering time may be about 20 to 60 minutes
although it depends on the thickness of the pipe.
[0082] According to the above processes, a seamless steel pipe for
oil wells with excellent properties formed of tempered martensite
can be obtained.
PREFERRED EMBODIMENT
[0083] The present invention will be described in more detail in
reference to preferred embodiments.
EXAMPLE 1
[0084] Billets with an outer diameter of 225 mm formed of 28 kinds
of steels shown in Table 3 were produced. These billets were heated
to 1250 degrees centigrade, and formed into seamless steel pipes
with 244.5 mm outer diameter and 13.8 mm thickness by the
Mannesmann-mandrel tube-making method.
[0085] [Table 3] TABLE-US-00003 TABLE 3 Chemical composition (mass
%) The balance: Fe and impurities Steel C Si Mn P S Cr Mo B sol. Al
N 1 0.12 0.26 0.91 0.010 0.002 0.43 0.35 0.0012 0.024 0.0039 2 0.11
0.33 0.61 0.010 0.004 0.61 0.51 0.0021 0.026 0.0038 3 0.15 0.22
0.61 0.010 0.004 0.30 0.50 0.0012 0.025 0.0041 4 0.20 0.25 0.60
0.010 0.004 0.31 0.50 0.0013 0.029 0.0040 5 0.17 0.30 0.60 0.010
0.004 0.61 0.45 0.0012 0.032 0.0036 6 0.13 0.23 0.63 0.010 0.004
0.60 0.61 0.0003 0.031 0.0018 7 0.13 0.40 0.75 0.011 0.004 0.36
0.58 0.0012 0.028 0.0037 8 0.16 0.30 0.80 0.011 0.004 0.30 0.51
0.0011 0.028 0.0043 9 0.15 0.19 0.82 0.010 0.004 0.25 0.40 0.0010
0.030 0.0047 10 0.15 0.63 0.40 0.010 0.004 0.60 0.30 0.0015 0.029
0.0041 11 0.16 0.19 0.62 0.010 0.004 0.89 0.16 0.0019 0.031 0.0043
12 0.14 0.22 0.44 0.008 0.004 0.88 0.36 0.0010 0.030 0.0035 13 0.14
0.19 0.60 0.008 0.004 0.61 0.48 0.0013 0.028 0.0044 14 0.16 0.22
0.63 0.009 0.004 0.30 0.51 0.0011 0.026 0.0024 15 0.15 0.17 0.79
0.008 0.004 0.30 0.50 0.0013 0.024 0.0027 16 0.15 0.17 0.99 0.009
0.004 0.61 0.31 0.0026 0.026 0.0024 17 0.15 0.18 0.87 0.009 0.004
0.21 0.72 0.0022 0.028 0.0040 18 0.18 0.17 0.50 0.008 0.004 0.51
0.72 0.0012 0.029 0.0035 19 0.16 0.18 0.81 0.009 0.004 0.51 0.73
0.0012 0.030 0.0038 20 0.13 0.20 0.57 0.006 0.003 0.57 0.32 0.0017
0.036 0.0049 21 0.14 0.46 0.81 0.015 0.003 0.36 0.26 0.0008 0.031
0.0018 22 0.17 0.33 0.68 0.011 0.003 0.87 0.16 0.0019 0.033 0.0022
23 0.16 0.31 0.48 0.008 0.002 0.36 0.45 0.0011 0.034 0.0038 24 0.16
0.41 0.48 0.012 0.003 0.10 *0.01 0.0010 0.019 0.0010 25 0.14 0.22
0.81 0.012 0.002 0.16 0.08 0.0011 0.031 0.0052 26 0.12 0.33 0.61
0.008 0.003 *1.63 0.77 0.0015 0.025 0.0038 27 0.17 0.28 0.56 0.011
0.003 0.92 *0.01 0.0012 0.031 0.0041 28 *0.26 0.27 0.51 0.012 0.004
0.60 0.30 0.0010 0.031 0.0045 Chemical composition (mass %) The
balance: Fe and impurities A- Ac.sub.1 Ac.sub.3 Steel Ti Nb V Ca Mg
REM value point point 1 0.018 -- -- -- -- -- 0.474 755 888 2 0.007
-- -- -- -- -- 0.504 767 907 3 0.013 -- -- -- -- -- 0.478 757 883 4
0.020 -- -- -- -- -- 0.529 756 861 5 0.011 -- -- -- -- -- 0.542 763
875 6 0.007 -- -- -- -- -- 0.558 767 896 7 0.013 -- -- -- -- --
0.520 762 903 8 0.013 -- -- -- -- -- 0.523 756 880 9 0.014 -- -- --
-- -- 0.470 750 874 10 0.016 -- -- 0.0012 -- -- 0.437 768 901 11
0.008 -- -- 0.0031 -- -- 0.495 761 861 12 0.008 -- -- -- 0.0010 --
0.509 769 883 13 0.013 0.006 -- -- -- -- 0.522 765 884 14 0.006 --
0.18 -- -- -- 0.495 749 879 15 0.013 0.005 -- -- -- -- 0.508 755
877 16 0.003 0.008 0.05 -- -- -- 0.540 753 864 17 0.007 0.011 0.08
-- -- -- 0.577 754 885 18 0.011 -- -- 0.0021 -- -- 0.605 766 876 19
0.014 -- 0.15 0.0019 -- -- 0.640 757 880 20 0.012 0.002 0.13 0.0020
-- -- 0.446 753 884 21 0.018 -- -- 0.0010 0.0005 -- 0.434 754 888
22 0.002 -- -- 0.0008 0.0001 0.001 0.511 762 863 23 0.011 0.003
0.08 0.0010 0.0010 -- 0.462 756 884 24 0.012 -- -- -- -- -- *0.263
747 874 25 0.014 -- -- -- -- -- *0.334 741 869 26 0.012 -- --
0.0018 -- -- 0.804 798 908 27 0.015 -- -- -- -- -- 0.451 761 857 28
0.013 0.003 0.06 -- -- -- 0.565 756 827 A = C + (Mn/6) + (Cr/5) +
(Mo/3). In the columns both "Ac.sub.1 point" and "Ac.sub.3 point",
the temperature unit is "degrees centigrade". The symbol "*" means
that the content fails to satisfy the conditions specified in the
invention.
[0086] Each formed seamless steel pipe was charged in a assistant
heating furnace of a furnace temperature of 950 degrees centigrade
constituting a heat treatment facility provided in the latter stage
of a finish rolling machine (namely elongating and rolling
machine), allowed to stand therein to uniformly and assistantly
heated for 5 minutes, and then quenched in water.
[0087] The water-quenched seamless steel pipe was charged in a
tempering furnace, and subjected to a tempering treatment of
uniformly soaking at a temperature between 650 and 720 degrees
centigrade for 30 minutes, and the strength was adjusted to about
110 ksi (758 MPa) in terms of yield strength to produce a product
steel pipe, namely a seamless steel pipe for oil wells. The grain
size of the said water-quenched steel pipe was No. 7 or less by the
grain size number regulated in JIS G 0551 (1998) in all the steels
Nos. 1 to 28.
[0088] Various test pieces were taken from the product steel pipe,
and the following tests were carried out to examine the properties
of the steel pipe. The hardenability of each steel was also
examined.
[0089] 1. Hardenability
[0090] A Jominy test piece was taken from each billet before
tube-making rolling, austenitized at 1100 degrees centigrade, and
subjected to a Jominy test. The hardenability was evaluated by
comparing the Rockwell C hardness in a position 10 mm from a Jominy
end (JHRC.sub.10) with the value of 58C%+27, which is a predicted
value of the Rockwell C hardness corresponding to 90%-martensite
ratio of each steel, and determining one having a JHRC.sub.10
higher than the value of 58C%+27 to have "excellent hardenability",
and one having a JHRC.sub.10 not higher than the value of 58C%+27
to have "inferior hardenability".
[0091] 2. Tensile Test
[0092] A circular tensile test piece regulated in 5CT of the API
standard was cut off from the longitudinal direction of each steel
pipe, and a tensile test was carried out to measure the yield
strength YS (ksi), tensile strength TS (ksi) and yield ratio YR
(%).
[0093] 3. Corrosion Test
[0094] An A-method test piece regulated in NACE TM0177-96 was cut
off from the longitudinal direction of each steel pipe, and an NACE
A-method test was carried out in the circumstance of 0.5% acetic
acid and 5% sodium chloride aqueous solution saturated with
hydrogen sulfide of the partial pressure of 101325 Pa (1 atm) to
measure a limit applied stress (that is maximum stress causing no
rupture in a test time of 720 hours, shown by the ratio to the
actual yield strength of each steel pipe). The sulfide stress
cracking resistance was determined to be excellent when the limit
applied stress was 90% or more of YS.
[0095] The examination results are shown in Table 4. The column of
hardenability of Table 4 is shown by "excellent" or "inferior" by
comparison between JHRC.sub.10 and the value of 58C%+27.
[0096] [Table 4] TABLE-US-00004 TABLE 4 Tensile Properties Limit
Applied Steel Hardenability YS (ksi) TS (ksi) YR (%) Stress 1
Excellent 108 113 95.6 90% YS 2 Excellent 107 112 95.5 90% YS 3
Excellent 110 117 94.0 90% YS 4 Excellent 109 119 91.6 90% YS 5
Excellent 109 117 93.2 90% YS 6 Excellent 106 111 95.5 90% YS 7
Excellent 108 113 95.6 90% YS 8 Excellent 105 113 92.9 90% YS 9
Excellent 108 115 93.9 90% YS 10 Excellent 105 113 92.9 95% YS 11
Excellent 110 117 94.0 95% YS 12 Excellent 107 112 95.5 95% YS 13
Excellent 105 112 93.8 90% YS 14 Excellent 110 117 94.0 95% YS 15
Excellent 110 118 93.2 90% YS 16 Excellent 109 117 93.2 90% YS 17
Excellent 108 116 93.1 90% YS 18 Excellent 108 114 94.7 90% YS 19
Excellent 110 118 93.2 90% YS 20 Excellent 109 117 93.2 90% YS 21
Excellent 106 111 95.5 90% YS 22 Excellent 108 114 94.7 90% YS 23
Excellent 110 116 94.8 95% YS 24 Inferior 110 124 88.7 80% YS 25
Inferior 100 121 82.6 70% YS 26 Excellent 110 116 94.8 75% YS 27
Excellent 108 117 92.3 75% YS 28 Excellent 110 125 88.0 80% YS
[0097] As is apparent from Table 4, the steels Nos. 1 to 23, having
chemical compositions regulated in the present invention, have
excellent hardenability, high yield ratio, and excellent sulfide
stress cracking resistance.
[0098] On the other hand, all the steels Nos. 24 to 38, out of the
component range regulated in the present invention, are inferior in
sulfide stress crack resistance.
[0099] The steel No. 24 is too short of hardenability to obtain the
uniform quenched and tempered microstructure, namely the uniform
tempered martensitic microstructure, and also poor in sulfide
stress cracking resistance with a low yield ratio, since the
content of Mo is out of the range regulated in the present
invention.
[0100] The steel No. 25 is too short of hardenability to obtain the
uniform quenched and tempered microstructure, namely the uniform
tempered martensitic microstructure, and also poor in sulfide
stress cracking resistance with a low yield ratio, since the
conditions regulated in the present invention are not satisfied
with an A-value of the said equation (1) lower than 0.43 although
the independent contents of C, Mn, Cr and Mo are within the ranges
regulated in the present invention.
[0101] The steel No. 26 is excellent in hardenability and has a
high yield ratio, but it is poor in sulfide stress cracking
resistance since the content of Cr is higher than the regulation in
the present invention.
[0102] The steel No. 27 is short of hardenability, and also poor in
sulfide stress cracking resistance with a low yield ratio, since
the content of Mo is lower than the lower limit value regulated in
the present invention although the A-value of the said equation (1)
satisfies the condition regulated in the present invention.
[0103] The steel No. 28 is excellent in hardenability, but it is
inferior in sulfide stress cracking resistance with a low yield
ratio, since the content of C is higher than the regulation of the
present invention.
EXAMPLE 2
[0104] Billets with an outer diameter of 225 mm formed of 3 kinds
of steels shown in Table 5 were produced. These billets were heated
to 1250 degrees centigrade, and formed into seamless steel pipes
with 244.5 mm outer diameter and 13.8 mm thickness by the
Mannesmann-mandrel tube-making method. The steels Nos. 29 to 31 in
Table 5 satisfied the chemical composition defined by the present
invention.
[0105] [Table 5] TABLE-US-00005 TABLE 5 Chemical composition (mass
%) The balance: Fe and impurities Steel C Si Mn P S Cr Mo B sol. Al
N 29 0.15 0.15 0.76 0.010 0.002 0.35 0.40 0.0013 0.025 0.0032 30
0.19 0.21 0.61 0.010 0.002 0.45 0.30 0.0009 0.021 0.0038 31 0.14
0.32 0.66 0.008 0.001 0.41 0.71 0.0012 0.025 0.0041 Chemical
composition (mass %) The balance: Fe and impurities A- Ac.sub.1
Ac.sub.3 Steel Ti Nb V Ca Mg REM value point point 29 0.016 -- 0.07
0.0018 -- -- 0.480 750 872 30 0.013 -- 0.10 -- 0.0008 -- 0.482 752
855 31 0.013 -- 0.12 0.0020 -- 0.0005 0.569 761 900 A = C + (Mn/6)
+ (Cr/5) + (Mo/3). In the columns both "Ac.sub.1 point" and
"Ac.sub.3 point", the temperature unit is "degrees centigrade".
[0106] Each formed seamless steel pipe was charged in a assistant
heating furnace of a furnace temperature of 950 degrees centigrade
constituting a heat treatment facility provided in the latter stage
of a finish rolling machine (namely elongating and rolling
machine), allowed to stand therein to uniformly and assistantly
heated for 5 minutes, and then quenched in water.
[0107] The water-quenched seamless steel pipe was divided in two
pieces and charged in a tempering furnace, and subjected to a
tempering treatment of uniformly soaking for each piece at a
temperature between 650 and 720 degrees centigrade for 30 minutes,
and the strength was adjusted to about 125 ksi (862 MPa) to 135 ksi
(931 MPa) in terms of tensile strength to produce a product steel
pipe, namely a seamless steel pipe for oil wells. The grain size of
the said water-quenched steel pipe was No. 7 or less by the grain
size number regulated in JIS G 0551 (1998) in all the steels Nos.
29 to 31.
[0108] Various test pieces were taken from the product steel pipe,
and the following tests were carried out to examine the properties
of the steel pipe. The hardenability of each steel was also
examined.
[0109] 1. Hardenability
[0110] A Jominy test piece was taken from each billet before
tube-making rolling, austenitized at 1100 degrees centigrade, and
subjected to a Jominy test. The hardenability was evaluated by
comparing the Rockwell C hardness in a position 10 mm from a Jominy
end (JHRC.sub.10) with the value of 58C%+27, which is a predicted
value of the Rockwell C hardness corresponding to 90%-martensite
ratio of each steel, and determining one having a JHRC.sub.10
higher than the value of 58C%+27 to have "excellent hardenability",
and one having a JHRC.sub.10 not higher than the value of 58C%+27
to have "inferior hardenability".
[0111] 2. Tensile Test
[0112] A circular tensile test piece regulated in 5CT of the API
standard was cut off from the longitudinal direction of each steel
pipe, and a tensile test was carried out to measure the yield
strength YS (ksi), tensile strength TS (ksi) and yield ratio YR
(%).
[0113] 3. Corrosion Test
[0114] An A-method test piece regulated in NACE TM0177-96 was cut
off from the longitudinal direction of each steel pipe, and an NACE
A-method test was carried out in the circumstance of 0.5% acetic
acid and 5% sodium chloride aqueous solution saturated with
hydrogen sulfide of the partial pressure of 101325 Pa (1 atm) to
measure a limit applied stress (that is maximum stress causing no
rupture in a test time of 720 hours, shown by the ratio to the
actual yield strength of each steel pipe). The sulfide stress
cracking resistance was determined to be excellent when the limit
applied stress was 90% or more of YS.
[0115] The examination results are shown in Table 6. The column of
hardenability of Table 6 is shown by "excellent" or "inferior" by
comparison between JHRC.sub.10 and the value of 58C%+27.
[0116] [Table 6] TABLE-US-00006 TABLE 6 Limit Tensile Properties
Applied Mark Steel Hardenability YS (ksi) TS (ksi) YR (%) Stress
29-1 29 Excellent 125 132 94.7 90% YS 29-2 29 Excellent 120 127
94.5 95% YS 30-1 30 Excellent 125 135 92.6 90% YS 30-2 30 Excellent
121 130 93.1 95% YS 31-1 31 Excellent 125 130 96.2 95% YS 31-2 31
Excellent 120 125 96.0 95% YS
[0117] As is apparent from Table 6, the steels Nos. 29 to 31,
having chemical compositions regulated in the present invention,
have excellent hardenability, high yield ratio, and excellent
sulfide stress cracking resistance. In particular, the marks 29-2,
30-2, 31-1 and 31-2, whose tensile strengths are not more than 130
ksi (897 MPa), have better sulfide stress cracking resistance.
[0118] Although only some exemplary embodiments of the present
invention have been described in detail above, those skilled in the
art will readily appreciated that many modifications are possible
in the exemplary embodiments without materially departing from the
novel teachings and advantages of the present invention.
Accordingly, all such modifications are intended to be included
within the scope of the present invention.
INDUSTRIAL APPLICABILITY
[0119] The seamless steel pipe for oil wells of the present
invention is highly strong and excellent in sulfide stress cracking
resistance because it has a high yield ratio even with a quenched
and tempered microstructure, namely a tempered martensitic
microstructure, in which the prior-austenite grains are relatively
coarse gains of No. 7 or less by the grain size number regulated in
JIS G 0551 (1998).
[0120] The seamless steel pipe for oil wells of the present
invention can be produced at a low cost by adapting an in-line tube
making and heat treatment process having a high production
efficiency since a reheating treatment for refinement of grains is
not required.
* * * * *