DESCRIPTION
TITLE OF INVENTION
LOW ALLOY OIL-WELL STEEL PIPE
TECHNICAL FIELD
[0001]
The present invention relates to a steel pipe, more specifically an oil-well steel
pipe.
BACKGROUND ART
[0002]
Deep-well developments of oil wells and gas wells (oil wells and gas wells
are collectively referred to simply as "oil wells", hereinafter) require high strength of
oil-well steel pipes. Conventionally, 80 ksi-grade (yield stress of 80 to 95 ksi, that
is, 551 to 654 MPa) and 95 ksi-grade (yield stress of 95 to 110 ksi, that is, 654to 758
MPa) oil-well steel pipes have been widely used. However, I l0 ksi-grade (yield
stress of 110 to 125 ksi, that is, 758 to 862 MPa) oil-well steelpipes have recently
come into use.
[0003]
Most deep-wells contain hydrogen sulfide having corrosiveness. Hence, oilwell
steel pipes for use in deep wells are required to have not only a high strength but
also a sulfide stress cracking resistance (referred to as a SSC resistance, hereinafter).
ln general, susceptibility to the SSC is increased along with increase in strength of a
steelmaterial.
[0004]
Steel pipes of95 ksi grade or 1 10 ksi grade or less, which are sold as sourresistant
oil-well steel pipes (sour service OCTG), are usually guaranteed to have a
SSC resistance to endure under the HzS environment at I atm in an evaluation by a
test method specified by NACE. Hereafter, the HzS environment at 1 atm is
referred to as a standard condition.
[000s]
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Meanwhile, oil-well steel pipes of 125 ksi grade (yield stress of 862 to 965
MPa) have conventionally been guaranteed only to have a SSC resistance to endure
under an environment in which partial pressure of HzS is much smaller than that
under the standard condition, in many cases. This means that, once the lower limit
of the yield strength becomes more than 110 ksi(758 MPa), it becomes suddenly
difficult to secure an excellent SSC resistance.
[0006]
On this background, there is a need for sour-resistant oil-well steel pipes that
can secures a SSC resistance under the HzS environment at I atrn, and have a lower
limit of the yield strength as great as possible even if the lower lìmit of the yield
strength does not reach 125 ksi(862 MPa).
[0007]
Techniques to enhance the SSC resistance of oil-well steel pipes are disclosed
in Japanese Patent Application Publication No. 62-253720 (Patent Literature 1),
Japanese Patent Application Publication No. 59-232220 (Patent Literature 2),
Japanese Patent Application Publication No. 6-322478 (Patent Literature 3),
Japanese Patent Application Publication No. 8-31 1551 (Patent Literature 4),
Japanese Patent Application Publication No. 2000-256783 (Patent Literature 5),
Japanese Patent Application Publication No. 2000-297344 (Patent Literature 6),
Japanese Patent Application Publication No. 2005-350754 (Patent Literature 7),
National Publication of International Patent Application No. 2012-519238 (Patent
Literature 8), and Japanese Patent Application Publication No.2012-26030 (Patent
Literature 9).
[0008]
Patent Literature I proposes a method of enhancing the SSC resistance of an
oil-well steel pipe by reducing impurities such as Mn and P. Patent Literature 2
proposes a method of enhancing the SSC resistance of steel by performing quenching
twice to refine grains.
[000e]
Patent Literature 3 proposes a method of enhancing the SSC resistance of a
125 ksi-grade steel material by refining steel microstructure through an induction
heat treatment. Patent Literature 4 proposes a method of enhancing the SSC
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resistance ofa steel pipe of 1 10 ksi grade to 140 ksi grade by enhancing
hardenability ofthe steel through direct quenching process, and increasing a
tempering temperature.
[0010]
Each of Patent Literature 5 and Patent Literature 6 proposes a method of
enhancing the SSC resistance of a low alloy oil-well steel pipe of 1 10 ksi grade to
140 ksi grade by controlling the morphology of carbide. Patent Literature 7
proposes a method of enhancing the SSC resistance of an oil-well steel pipe of 125
ksi (862 MPa) grade or more by controlling a dislocation density and a hydrogen
diffusion coefficient to be desired values. Patent Literature 8 proposes a method of
enhancing the SSC resistance of 125 ksi (862 MPa)-grade steel by quenching low
alloy steel containing C of 0.3 to 0.5% several times. Patent Literature 9 proposes a
method of employing a tempering step of two-stage heat treatment to control the
morphology of carbide and the number of carbide particles. More specifically, in
Patent Literature 9, the SSC resistance of 125 ksi (862 MPa)-grade steel is enhanced
by suppressing the number density of large MrC particles or MzC particles.
CITATION LIST
PATENT LITERATURE
[001 l ]
Patent Literature I : Japanese Patent Application Publication No.62-253720
Patent Literature 2: Japanese Patent Application Publication No. 59-232220
Patent Literature 3: Japanese Patent Application Publication No.6-322478
Patent Literature 4: Japanese Patent Application Publication No. 8-31 1551
Patent Literature 5: Japanese Patent Application Publication No. 2000-256783
Patent Literature 6: Japanese Patent Application Publication No.2000-297344
Patent Literature 7: Japanese Patent Application Publication No. 2005-350754
Patent Literature 8: National Publication of International Patent Application No.
2012-519238
Patent Literature 9: Japanese Patent Application Publication No. 2012-26030
NON PATENT LITERATURE
[0012]
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Non Patent Literature 1: TSUCHIYAMA Toshihiro, "Physical Meaning of
Tempering Parameter and Its Application to Continuous Heating or Cooling Heat
Treatment Process", Journal of The Japan Society for Heat Treatment, vol. 42, No. 3,
P. 16s (2002).
[0013]
However, even if applying the techniques disclosed in the above Patent
Literatures 1 to 9, in the case of oil-well steel pipes having a yield strength of I I 5 ksi
(793 MPa) or more, an excellent SSC resistance cannot be stably obtained in some
cases.
SUMMARY OF INVENTION
[0014]
An object of the present invention is to provide a low alloy oil-well steel pipe
having a yield strength of 1 15 ksi grade or more (793 MPa or more) and an excellent
SSC resistance.
[001s]
A low alloy oil-well steel pipe according to the present invention includes a
chemical composition consisting of: in masso/o , C: 0.25 to 0.35%o; Si: 0.05 to 0.500/o;
Mn:0.10 to 1.50%; Cr: 0.40 to 1.50%o; Mo: 0.40 to2.00%o; V: 0.05 to 0.25%;Nb:
0.010 to 0.040%; Ti: 0.002 to 0.050%;sol. Al: 0.005 to 0.10%; N: 0.007% or less; B:
0.0001 to 0.0035%; and Ca: 0 to 0.005%; and a balance being Fe and impurities, the
impurities including: P:0.020Yo or less; S: 0.010% or less; O: 0.006% or less; Ni:
0.10% or less; and Cu: 0J0% or less. In a microstructure, a number of cementite
particles each of which has an equivalent circle diameter of 200 nm or more is 100
particles/l00 ¡rm2 or more. The above low alloy oil-well steel pipe has a yield
strength o1793 MPa or more.
[00r 6]
The above chemical composition may contain Ca: 0.0005 to 0.005%.
[00 r 7]
The low alloy oil-well steel pipe according to the present invention has a yield
strength of 1 l5 ksi grade or more (793 MPa or more) and an excellent SSC resistance.
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BRIEF DESCRIPTION OF DRAWING
[00r 8]
[FIG. 1l FIG. I is a diagram to show the relationship between yield strength YS and
Klssc.
DESCRIPTION OF EMBODIMENT
[001e]
Hereinafter, an embodiment of the present invention will be described in
details.
[0020]
The present inventors have studied on a SSC resistance of a low alloy oil-well
steel pipe. As a result, the present inventors have found the following findings.
[0021]
If a steel pipe is subjected to tempering at a low temperature , a large amount
of fine cementite is precipitated. The precipitated cementite has a flat morphology.
Such fine cementite initiates occurrence of SSC. Further, if the tempering
temperature is low, dislocation density is not decreased. Hydrogen having intruded
in the steel is not only trapped at an interface between a fine cementite having a flat
morphology and a parent phase, but also trapped in dislocation. SSC is likely to be
caused due to the hydrogen trapped at the interface between the fine cementite and
the parent phase and in the dislocation. Hence, if a large amount of fine cementite
is formed, and the dislocation clensity is high, the SSC resistance becomes
deteriorated.
10022)
Therefore, Mo and V that are alloy elements to enhance a temper softening
resistance are contained in the steel pipe, and this steel pipe is subjected to tempering
at a high temperature. In this case, the dislocation density becomes decreased.
Hence, the SSC resistance becomes enhanced. ln addition, in the case of
performing tempering at a high temperature, cementite grows into coarse cementite.
Fine cementite is flat, as aforementioned, and SSC is likely to be induced in its
surface. To the contrary, coarse cementite grows into a spherical form so that its
specific surface area becomes reduced. Hence, compared with fine cementite,
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coarse cementite is unlikely to initiate occurrence of SSC. Accordingly, instead of
fine cementite, coarse cementite is formed, thereby enhancing the SSC resistance.
[0023]
However, cementite enhances strength of a steel pipe through precipitation
strengthening. As aforementioned, if tempering is performed at a high temperature,
coarse cementite is formed, but only a small amount of coarse cementite is formed.
In this case, although an excellent SSC resistance can be attained, it is difficult to
aÍfain a yield strength of 793 MPa or more.
100241
In the present invention, it is configured to increase the number of coarse
cementite particles each of which has an equivalent circle diameter of 200 nm or
more, thereby obtaining an oil-well steel pipe having a high strength of 793 MPa or
more and an excellent SSC resistance. Coarse cementite of which particle has an
equivalent circle diameter of 200 nm or more is referred to as "coarse cementite",
hereinafter.
[002s]
In order to attain the above described oil-well steel pipe, in the tempering,
low-temperature tempering at 600 to 650oC is carried out, and thereafter, hightemperature
tempering at 670 to 720"C is carried out. In this case, a large number
of fine cementite particles are formed in the low-temperature tempering. Fine
cementite particles serve as nucleuses of coarse cementite particles. By
precipitating a large number of fine cementite particles in the low-temperature
tempering, alarge number of fine cementite particles grow in the high-temperature
tempering, and consequently, a large number of coarse cementite particles are
formed. Hence, the number density of coarse cementite becomes enhanced.
Accordingly, it is possible to attain an oil-well steel pipe having a high strength of
793 .}y4Pa or more as well as an excellent SSC resistance.
[0026]
A low alloy oil-well steel pipe according to the present invention that has
been accomplished based on the above findings includes a chemical composition
consisting of: in masso/o , C: 0.25 to 0.35%o; Si: 0.05 to 0.50%; Mn: 0.10 to 1.50o/o; Cr:
0.40 to 150%; Mo: 0.40 to 2.00%o; V: 0.05 to 0.25Yo; Nb: 0.010 to 0.040%o; Ti: 0.002
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to 0,050%; sol. Al: 0.005 to 0.10%: N: 0.007% or less; B: 0.0001 to 0.0035%; and
Ca: 0 to 0.005%: and a balance being Fe and impurities, the impurities including: P:
0.020% or less; S: 0.010/o or less; O: 0.006% or less;Ni: 0.10% or less;and Cu:
0)0% or less. In a microstructure, a number of cementite particles each of which
has an equivalent circle diameter of 200 nm or more is 100 particles/l00 ¡rm2 or
more. The above low alloy oil-well steel pipe has a yield strength of 793 MPa or
more.
100271
The low alloy oil-well steel pipe according to the present invention will be
described in details, hereinafter.
[0028]
[Chemical Composition]
The chemical composition of the low alloy oil-well steel pipe according to the
present invention contains the following elements.
[002e1
C:0.25 to 0.35Yo
The C content in the low alloy oil-well steel pipe according to the present
invention is somewhat higher. C refines a sub-microstructure of martensite, and
enhances strength of the steel. C also forms carbide to enhance strength of the steel.
For example, the carbide may be cementite and alloy carbide (Mo carbide, V carbide,
Nb carbide, Ti carbide, and the like). If the C content is high, spheroidization of the
carbide is encouraged further, and a large number of coarse cementite particles are
likely to be formed through the heat treatment to be described below, thereby
enabling to attain both strength and SSC resistance. If the C context is less than
0.25yo, those effects will be insufficient. On the other hand, if the C content
becomes more than 0.35yo, the susceptibilify to quench cracking increases, so that
the risk of occurrence of quench cracking increases in normal quenching treatment.
Accordingly, the C content is 0.25 to 0.35%o. A preferable lower limit of the C
content is 0.260/o. A preferable upper limit of the C content is 0.32Yo, and more
preferably 0.30%.
[0030]
Si: 0.05% to 0.50%o
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Silicon (Si) deoxidizes the steel. An excessively low Si content cannot attain
this effect. On the other hand, an excessively high Si content rather deteriorates the
SSC resistance. Accordingly, the Si content is 0.05% to 0.50%o. A preferable
lower limit of the Si content is 0.10%, and more preferably 0J7%. A preferable
upper limit of the Si content is 0.40%o, and more preferably 0.35%.
[0031]
Mn: 0.10 to 1.50%o
Manganese (Mn) deoxidizes the steel. An excessively low Mn content
cannot attain this effect. On the other hand, an excessively high Mn content causes
segregation at grain boundaries along with impurity elements such as phosphorus (P)
and sulfur (S). In this case, the SSC resistance of the steel becomes deteriorated.
Accordingly, the Mn content is 0.1 0 to I .50%. A preferable lower timit of the Mn
content is 0.20Vo, and more preferably 0.25%. A preferable upper limit of the Mn
content is 1.00%, and more preferably 0.75%.
[0032]
Cr: 0.40 to 1.50Yo
Chromium (Cr) enhances hardenability of the steel, and enhances strength of
the steel. An excessively low Cr content cannot attain the above effect. On the
other hand, an excessively high Cr content rather deteriorates toughness and the SSC
resistance of the steel. Accordingly, the Cr content is 0.40 to 1.50%o. A preferable
lower limit of the Cr content is 0.43Yo,and more preferably 0.48%. A preferable
upper limit of the Cr content is 1.20%o, and more preferably 1'10%.
[0033]
Mo: 0.40 to2.00Yo
Molybdenum (Mo) forms carbide, and enhances the temper softening
resistance of the steel. As a result, Mo contributes to enhancement of the SSC
resistance by the high-temperature tempering. An excessively low Mo content
cannot attain this effect. On the other hand, an excessively high Mo content rather
saturates the above effect. Accordingly, the Mo content is 0.40 to 2.00Yo. A
preferable lower limit of the Mo content is 0.50%, and more preferably 0.65%. A
preferable upper limit of the Mo content is 1.50%, and more preferably 090%.
[0034]
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V: 0.05 to 0.25Yo
Vanadium (V) forms carbide, and enhances the temper softening resistance of
the steel, as similar to Mo. As a result, V contributes to enhancement of the SSC
resistance by the high-temperature tempering. An excessively low V content
cannot attain the above effect. On the other hand, an excessively high V content
rather deteriorates toughness of the steel. Accordingly, the V content is 0.05 to
0.25%. A preferable lower limit of the V content is 0.07%. A preferable upper
limit of the V content is 0.15%, and more preferably 0.12%.
[003s]
Nb: 0.010 to 0.040o/o
Niobium (Nb) forms carbide, nitride, or carbonitride in combination with C or
N. These precipitates (carbide, nitride, and carbonitride) refine a submicrostructure
of the steel by the pinning effect, and enhances the SSC resistance of
the steel. An excessively low Nb content cannot attain this effect. On the other
hand, an excessively high Nb content forms excessive precipitates, and destabilizes
the SSC resistance of the steel. Accordingly, the Nb content is 0.010 to 0.040Yo.
A preferable lower limit of the Nb content is 0.012o/o, and more preferably 0.015%.
A preferable upper limit of the Nb content is 0.035%, and more preferably 0.030%.
[0036]
Ti: 0.002 to 0.050%
Titanium (Ti) is an effective element to prevent cast cracking. Ti forms
nitride, thereby contributing to prevent the coarsening of crystal grains. For that
reason, at least 0.002% of Ti is contained in the present embodiment. On the other
hand, if the Ti content becomes more than 0.050%, it forms large-size nitride,
destabilizing the SSC resistance of the steel. Accordingly, the Ti content is 0.002 to
0.050%. A preferable lower limit of the Ti content is 0.004%, and a preferable
upper limit of the Ti content is 0.035%, more preferably 0.020%, and further
preferably 0.015%.
[0037]
sol.Al: 0.005 to 0.10%
Aluminum (Al) deoxidizes the steel. An excessively low Al content cannot
attain this effect, and deteriorates the SSC resistance of the steel. On the other hand,
to
,l
an excessively high Al content results in increase of inclusions, which deteriorates
the SSC resistance of the steel. Accordingly, the Al content is 0.005 to 0. l0%. A
preferable lower limit of the Al content is 0.01%, and more preferably 0.02%. A
preferable upper limit of the Al content is 0.07%o, and more preferably 0.06%. The
"Al" content referred to in the present specification denotes the content of "acidsoluble
Al", that is, "sol.Al".
[0038]
N: 0.007% or less
Nitrogen (N) is inevitably contained. Nicombines with Tito form fine TiN,
thereby refining crystal grains. On the other hand, if the N content is excessively
high, coarse nitride is formed, thereby deteriorating the SSC resistance of the steel.
Accordingly, the N content is 0.007% or less. A preferable N content is 0.005% or
less, and more preferably 0.00450/o or less. In the viewpoint of forming fine TiN,
thereby refining crystal grains, a preferable lower limit of the N content is 0.002%.
[003e]
B: 0.0001 to 0.0035%
Boron (B) enhances the hardenability of the steel. When B is contained
0.0001% (1 ppm) or more, the aforementioned effect is attained. On the other hand,
B tends to form Mz:(CB)o at grain boundaries, and if the B content becomes more
than 0.0035o/o, the SSC resistance of the steel deteriorates. Accordingly, the B
content is 0.0001 to 0.0035%. A preferable lower limit of the B content is 0.0003%
(3 pprn), and more preferably 0.0005% (5 ppm). The B content is preferably
0.0030% or less, and more preferably 0.0025% or less. Note that to utilize the
effects of B, it is preferable to suppress the N content or to immobilize N with Ti
such that B which does not combine with N can exist.
[0040]
Ca: 0 to 0.005%
Calcium (Ca) is an optional element, and may not be contained. If contained,
Ca forms sulfide in combination with S in the steel, and improves morphology of
inclusions. In this case, toughness of the steel becomes enhanced. However, an
excessively high Ca content increases inclusions, which deteriorates the SSC
resistance of the steel. Accordingly, the Ca content is 0 to 0.005%. A preferable
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lower limit of the Ca content is 0.0005%, and more preferably 0.001%. A
preferable upper limit of the Ca content is 0.003%, and more preferably 0.002%.
[0041]
The balance of the chemical composition of the low alloy oil-well steel pipe
according to the present invention includes Fe and impurities. Impurities referred to
herein denote elements which come from ores and scraps for use as row materials of
the steel, or environments of manufacturing processes, and others. In the present
invention, each content of P, S, O, Ni, and Cu in the impurities is specified as follows.
100421
P:0.020% or less
Phosphorus (P) is an impurity. P segregates aÍ. grain boundaries, and
deteriorates the SSC resistance of the steel. Accordingly, the P content is 0.020%o
or less. A preferable P content is 0.01 5%o or less, and more preferably 0.010% or
less. The content of P is preferably as low as possible.
[0043]
S: 0.010% or less
Sulfur (S) is an impurity. S segregates at grain boundaries, and deteriorates
the SSC resistance of the steel. Accordingly, the S content is 0.010% or less. A
preferable S content is 0.005% or less, and more preferably 0.002% or less. The
content of S is preferably as low as possible.
[0044]
O: 0.006% or less
Oxygen (O) is an impurity. O forms coarse oxide, and deteriorates a
corrosion resistance of the steel. Accordingly, the O content is 0.006% or less. A
preferable O content is 0.004% or less, and more preferably 0.0015% or less. The
content of O is preferably as low as possible.
[004s]
Ni: 0.1 0olo or less
Nickel Qrli) is an impurity. Ni deteriorates the SSC resistance of the steel.
If the Ni content is more than 0.1 0%o,the SSC resistance becomes significantly
deteriorated. Accordingly, the content of Nias an impurity element is 0.10% or less.
The Ni content is preferably 0.05% or less, and more preferably 0.03% or less.
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[0046] I
Cu:O.10% or less
Copper (Cu) is an impurity. Copper embrittles the steel, and deteriorates the
SSC resistance of the steel. Accordingly, the Cu content is 0.10% or less. The Cu
content is preferably 0.05% or less, and more preferably 0.03% or less.
[0047]
IMicrostructure]
The microstructure of the low alloy oil-well steel pipe having the
aforementioned chemical composition is formed of tempered martensite and retained
austenite of 0 to less than 2%o in terms of a volume fraction.
[0048]
The microstructure of the low alloy oil-well steel pipe according to the
present invention is substantially a tempered martensite microstructure. Hence, the
yield strength of the low alloy oil-well steel pipe is high. Specifically, the yield
strength of the low alloy oil-well steel pipe of the present invention is 793 MPa or
more (l l5 ksi grade or more). The yield strength referred to in the present
specification is defined by the 0.7%o total elongation method.
[004e]
In the aforementioned low alloy oil-well steel pipe, retained austenite still
remains after the quenching in some cases. The retained austenite causes variation
in strength. Accordingly, the volume ratio (%) of the retained austenite is less than
2Yo in the present invention. The volume ratio of the retained austenite is preferably
as small as possible. Accordingly, it is preferable that in the microstructure of the
aforementioned low alloy oil-well steel pipe, the volume ratio of the retained
austenite is 0% (i.e., microstructure formed of tempered martensite). If the cooling
stop temperature in the quenching process is sufficiently low, preferably 5OoC or less,
the volume ratio (%o) of the retained austenite is suppressed less than 2%.
l00s0l
The volume ratio of the retained austenite is found by using X-ray diffraction
analysis by the following process. Samples including central portions of wall
thickness of produced low alloy oil-well steel pipes are collected. A surface of each
collected sample is subjected to chemical polishing. The X-ray diffraction analysis
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is carried out on each chemically polished surface by using a CoKcr ray as an
incident X ray. Specifically, using each sample, respective surface integrated
intensities of a (200) plane and a(2ll) plane in a ferrite phase (cr phase), and
respective surface integrated intensities of a (200) plane, a (220) plane, and a (3 1 1)
plane in the retained austenite phase (y phase) are respectively found. Subsequently,
the volume ratio Vy(%) is calculated by using Formula (l) for each combination
between each plane in the ct phase and each plane in the y phase (6 sets in total).
An average value of the volume ratios Vy(%) of the 6 sets is defTned as the volume
ratio (%) of the retained austenite.
Vy: 100/(l + (lcr x Ry)/(ly x Rcr)) (l),
where "Icr" and "Iy" are respective integrated intensities of the c¿ phase and
the y phase. "Rcx," and "Ry" are respective scale factors of the c¿ phase and the y
phase, and these values are obtained through a crystallographic logical calculation
based on the fypes ofthe substances and the plane directions.
[00s 1]
The aforementioned microstructure can be obtained by carrying out the
following producing method.
[00s2]
[Prior-austenite Grain Size No.]
ln the present invention, it is preferable that the grain size No. based on
ASTM El l2 of prior-austenite grains (also referred to as prior-y grains, hereinafter)
in the aforementioned microstructure is 9.0 or more. If the grain size No. is 9.0 or
more, it is possible to attain an excellent SSC resistance even if the yield strength is
793 MPaor more. A preferable grain size No. of the prior-y grains (also referred to
as prior-y grain size No., hereafter) is 9,5 or more.
[00s3]
The prior-y grain size No. may be measured by using a steel material after
being quenched and before being tempered (so-called as-quenched material), or by
using a tempered steel material (referred to as a tempered material). The size of the
prior-y grains is not changed in the tempering. Accordingly, the size of the prior-y
grains stays the same using any one of a material as quenched and a tempered
material. If steel including the aforementioned chemical composition is used, the
[+
prior-y grain size No. becomes 9.0 or more through well-known quenching described
later.
[0054]
[Number of Coarse Cementite Particles]
In the present invention, further, in the aforementioned substructure, the
number of coarse cementite particles CN each of which has an equivalent circle
diameter of 200 nm or more is I 00 particles/ I 00 pm2 or more.
[00ss]
Cementite enhances the yield strength of the steel pipe. Hence, if the
number of cementite particles is excessively small, the yield strength of the steel pipe
decreases. On the other hand, if the cementite is fine, the cementite has a needlelike
morphology. In this case, the cementite is more likely to be an initiator of
occurrence of the SSC, resulting in deterioration of SSC resistance.
[00s6]
If fine cementite is grown to be coarsened by appropriately selecting a steel
composition and a heat treatment condition, the number of fine cementite becomes
decreased. As a result, the SSC resistance becomes improved.
[00s7]
It is difficult to directly measure the number of fine cementite particles. For
this reason, this is substituted by measurement of the number of coarse cementite
particles. The total amount of cementite is determined by the carbon content in the
steel. Consequently, if the number of coarse cementite particles is greater, the
number of fine cementite particles becomes smaller. If the number of coarse
cementite particles CN is 100 particles/100 pm2, it is possible to attain an excellent
SSC resistance even if the steel pipe has a yield strength of 793 MPa or more. The
number of coarse cementite particles CN is measured by the following method.
[00s8]
Samples including central portions of wall thickness of steel pipes are
collected. Of a surface of each sample, a surface equivalent to a cross sectional
surface (sectional surface vertical to an axial direction ofthe steel pipe) ofeach steel
pipe (referred to as an observation surface, hereinafter) is polished. Each
observation surface after being polished is etched using a nital etching reagent.
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[00se]
Using a scanning electron microscope, any l0 visual fields in each etched
observation surface are observed. Each visual flield has an area of 10 ¡rm x 10 pm.
In each visual field, each area of plural cementite particles is found. The area of
each cementite particle may be found using image processing software (brand name:
Image J.|.47v), for example. A diameter of a circle having the same area as that of
the obtained area is defined as an equivalent circle diameter of the cementite particle
of interest.
[0060]
ln each visual field, cementite particles each of which has an equivalent circle
diameter of 200 nm or more (i.e., coarse cementite particles) are identified. A total
number of coarse cementite particles TN in all the 10 visual fields are found. Using
the total number TN, the number of coarse cementite particles CN is found based on
Formula (2).
CN = TN/Total area of 10 visual fields x 100 (2)
[0061]
With the above chemical composition, and a number of coarse cementite
particles CN of 100 particles/100 pm2 or more, a low alloy oil-well steel pipe has a
yield strength of 793 MPa and more, and an excellent SSC resistance.
[0062]
A preferable lower limit of the number of coarse cementite particles CN is
120 particles/I00 pm2. Although the upper limit of the number of coarse cementite
particles CN is not particularly limited, in the case of the aforementioned chemical
composition, a preferable upper limit of the number of coarse cementite particles CN
is 250 particles/l00 ¡rm2.
[0063]
[Producing Method]
An example of a producing method of the low alloy oil-well steel pipe
according to the present invention will be explained. In this example, the producing
method of a seamless steel pipe (low alloy oil-well steel pipe) will be described.
The producing method of the seamless steel pipe includes a pipe making process, a
quenching process, and a tempering process.
IL
i
i::
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:i:-
':., f
[0064]
[Pipe Making Process]
Steel including the aforementioned chemical composition is melted, and
smelted by using a well-known method. Subsequently, the molten steel is formed
into a continuous casted material through a continuous casting process, for example.
The continuous casted material is slabs, blooms, or billets, for example.
Alternatively, the molten steel may be formed into ingots through an ingot-making
process.
[006s]
Slabs, blooms, or ingots are subjected to hot working into billets. The billets
may be formed by hot-rolling or hot-forging the steel.
[0066]
The billets are hot-worked into hotlow shells. First, the billets are heated in
a heating furnace. The billets extracted from the heating furnace are subjected to
hot working into hollow shells (seamless steel pipes). For example, the
Mannesmann process is carried out as the hot working so as to produce the hollow
shells. In this case, round billets are piercing-rolled by a piercing mill
. The piercing-rolled round billets are further hot-rolled by a mandrel mill, a
reducer, a sizing mill, or the like into the hollow shells. The hollow shells may be
produced from billets with other hot working methods.
[0067]
[Quenching Process]
The hollow shells after the hot working are subjected to quenching and
tempering. A quenching temperature in the quenching is the Ac¡ point or more. A
preferable upper limit of the quenching temperature is 93OoC.
[0068]
In the present invention, the prior-y grain size No. of a steel pipe is 9.0 or
more. In order to realize this grain size, it is preferable that at least one
hansformation from a BCC (Body-Centered Cubic) phase to an FCC (Face-Centered
Cubic) phase is performed, and it is preferable to perform off-line quenching. It is
difficult to realize fine grains of a prior-y grain size No. of 9.0 or more by direct
\+
i
a
.l
l:
,)
/
quenching or in-line quenching (quenching after soaking at Ar: point or more
without significant temperature drop after hot pipe-making).
[006e]
To attain fine grains of a prior-y grain size No. of 9.0 or more, it is preferable
to perform normalizing (normalizing as an intetmediate heat treatment) by heating
the steel pipe to Aø point or more before performing off-line quenching. Moreover,
in place of normalizing, ofÊline quenching (quenching as an intermediate heat
treatment) may be carried out.
[0070]
Moreover, in place of the aforementioned norrnalizing and quenching as
intermediate heat treatments, heat treatment at a temperature in a two phase range
from more than the Acr point to less than the Ac: point (a two phase range heat
treatment as an intermediate heat treatment) may be carried out. Also in this case,
there is remarkable effect in refining the prior-y grains.
[0071]
It is possible to refine the prior-y grains of the hollow shells which has been
quenched once by a direct quenching or an inline quenching by further performing
off-line quenching. In such a case, by subjecting the hollow shell, which has been
subjected to a direct quenching or an inline quenching, to a heat treatment at a
temperature of 500oC to 580'C for about 10 to 30 minutes, it is possible to suppress
season cracking and impact cracking which may occur during storage before off-line
quenching or during transportation.
l0o7zl
The quenching is carried out by a rapid cooling from a temperature of the Ac¡
point or more to the martensite transformation-start temperature. The rapid cooling
includes, for example, water cooling, mist spray quenching, etc.
[0073]
The prior-y grain size No. of the hollow shell after the aforementioned
quenching step becomes 9.0 or more. Note that, the grains size of prior-y grains is
not changed even after the tempering to be described later.
[0074]
[Tempering Process]
;:
i,
ii.
ij:
l.'S'
/
The tempering step includes a low-temperature tempering process and a hightemperature
tempering process.
[007s]
[Low-temperature Tempering Process]
First, the low-temperature tempering process is carried out. The tempering
temperature TL in the low-temperature tempering process is 600 to 650'C. A
Larson-Miller parameter LMPI in the low-temperature tempering process is 17500 to
l 8750.
When the tempering temperature is constant, the Larson-Miller parameter is
defined by following Formula (3).
LMP = (T + 273) x (20 + log(t)) (3)
In Formula (3), T denotes a tempering temperature (oC), and t denotes a time
(hÐ.
[0076]
When the tempering temperature is not constant, in other word, the tempering
process includes a heating process in which temperature increases and a soaking
process in which temperature is constant, the Larson-Miller parameter taking account
of the heating process can be found by calculating it as an integrated tempering
parameter in accordance with Non-Patent Literature I (TSUCHIYAMA, Toshihiro.
2002. "Physical Meaning of Tempering Parameter and lts Application for
Continuous Heating or Cooling Heat Treatment Process", "Heat Treatment" Yol. 42,
No. 3, pp.l63-1 66 (2002)).
[0077]
In the method of calculating the abovementioned integrated tempering
parameter, a time from start of the heating until end of the heating is divided by
micro times
^t
of total number N. Herein, an average temperature in the (n-1)-th
section is defined as Tn-r(oC) and an average temperature in the n-th section is
defìned as Tn('C). An LMP (1) conesponding to the first micro time (the section
when n = I ) can be obtained by the following formula.
LMP (l): (Tr + 273) x (20 + log(Ât))
[0078]
1,1
tl,
':
l
t:
The LMP (l ) can be described as a value equivalent to an LMP calculated
based on a temperature Tz and a heating time tz by the following formula.
(Tt + 273) x (20 + log(At)) : (Tz + 273) x (20 + log(tz))
[007e]
The time tz is a time required (an equivalent time) to obtain an LMP at
temperature Tz equivalent to an integrated value of LMP calculated based on a
heating at a section before the second section (that is, the first section). The heating
time at the second section (temperature Tz) is a time obtained by adding an actual
heating time At to the time tz. Accordingly, an LMP (2) which is an integrated
value of LMP when the heating of the second section is completed can be obtained
by the following formula.
LMP (2): (Tz + 273) x (20 + log(tz +
^Ð)
toosòl
By generalizing this formula, the following formula can be obtained.
LMP (n) = (Tn * 273) x (20 + log(tn + AÐ) (4)
The LMP (n) is the integrated value of LMP when the heating of n-th section
is completed. The time tn is an equivalent time to obtain an LMP equivalent to an
integrated value of LMP when the heating of the (n-l)-th section is completed, at
temperature Tn. The time tn can be obtained by Formula (5).
[0081]
log(tn) : ((Tn-r + 273)/ (T" + 273)) x(20 + log(tn-r)) - 20 (5)
As so far described, when heating process needs to be taken into account,
Formula (4) in place of Formula (3) is applied.
[0082]
In the low-temperafure tempering process, as described above, a large amount
of C (carbon) supersaturatedly dissolved in the martensite is precipitated as cementite.
The precipitated cementite at this stage is fine cementite, and serves as a nucleus of
coarse cementite. An excessively low temperature of the low-temperature
tempering Tl or an excessively low LMPI results in a small amount of precipitated
cementite. On the other hand, an excessively high temperature of the lowtemperature
tempering Tl or an excessively high LMPr- causes growth of coarse
cementite, but results in a small amount of precipitated cementite.
í,
ir
,. I
90
ffi
[0083]
If the temperature of the low-temperature tempering Tr- is 600 to 65OoC, and
the LMP¡ is 17500 to 18750, a large amount of fine cementite serving as a nucleus of
coarse cementite is precipitated in the low-temperature tempering process.
[0084]
IHigh-temperature Tempering Process]
The high-temperature tempering process is carried out after the lowtemperature
tempering process. In the high-temperature tempering process, the fine
cementite precipitated in the low-temperature tempering process is coarsened,
thereby forming coarse cementite. Accordingly, it is possible to prevent the
cementite from becoming an initiator of SSC, as well as to enhance strength of the
steel with the coarse cementite.
[008s]
In the high-temperature tempering process, dislocation density in the steel is
reduced. Hydrogen having intruded in the steel is trapped in the dislocation, and
becomes an initiator of SSC. Hence, if the dislocation density is higher, the SSC
resistance becomes enhanced. The dislocation density in the steel becomes reduced
by carrying out the high-temperature tempering process. Accordingly, the SSC
resistance becomes improved.
[0086]
For the purpose of attaining the above effect, the tempering temperature TH in
the high-temperature tempering process is 670 to 720"C, and the Larson-Miller
parameter LMPu defined by Formula (3) and Formula (a) is 1 .85x 10a to 2.05x 104.
[0087]
If the tempering temperature TH is excessively low, or the LMPH is
excessively low, the cementite is not coarsened, and the number of the coarse
cementite particles CN becomes less than 100 particles/100 ¡rm2. Furthermore, the
dislocation density is not sufflrciently reduced. Consequently, the SSC resistance is
low.
[0088]
On the other hand, if the tempering temperature TH is excessively high, or the
LMPu is excessively high, the dislocation density is excessively reduced. ln this
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It
[,
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t:
case, the yield strength of the steel pipe including the aforementioned chemical
composition becomes less than 793 MPa.
[008e]
In the tempering process of the present invention, the two-stage tempering
including the low-temperature tempering process and the high-temperature
tempering process may be carried out, as aforementioned. Specifically, the steel
pipe is cooled down to a normal temperature after the low-temperature tempering
process is carried out. Subsequently, the high-temperature tempering process is
carried out by heating the steel pipe having the normal temperature. Alternatively,
immediately after the low-temperature tempering process is carried out, the hightemperature
tempering process may be carried out by heating the steel pipe up to the
temperature of the high-temperature tempering TH without cooling the steel pipe.
Iooe0]
Alternatively, the low-temperature tempering process and the hightemperature
tempering process may be continuously carried out in such a manner
that the temperature of the steel pipe is brought to a high-temperature range at a low
heating rate so as to increase the retaining time in a temperature range of 600 to
65OoC (tempering with slow temperature increase). For example, at the time of
tempering the steel pipe after being quenched, the steel pipe is continuously heated
up to 71 OoC at an average heating rate of 3"Clminute or less in a temperature range
of 500oC to 700oC, and the steelpipe is soaked at710"C for a predetermined time
(e.g., for 60 minutes). In this case, it is only required that an integrated value of the
Larson-Miller parameter LMPI in the temperature range of the low-temperature
tempering Tl (i.e.,600 to 650'C range) is 1.75x104 to 1.88x104, and an integrated
value of the Larson-Miller parameter LMPH in the temperature range of the hightemperature
tempering TH (i.e., 670to720"C range) is 1.85x104 to 2.05x104. In
other words, in the tempering process, as far as the LMPI in the temperature range of
the low-temperature tempering Tr- satisfies the above condition, and the LMPu in the
temperature range of the high-temperature tempering TH satisfies the above condition,
the tempering method is not limited to specific one.
[00e 1]
u
t¡'
Ë:
Ë
[:
,1_L
Through the above producing method, the low alloy seamless steel pipe
according to the present invention is produced. The microstructure of the produced
seamless steel pipe is formed of the tempered martensite and the retained austenite of
0 to less than2Yo. In addition, the prior-y grain size No. is 9.0 or more. Through
the above described tempering process, the number of coarse cementite particles CN
in the microstructure becomes 100 particles/l00 ¡rm2 or more.
EXAMPLE
[00e2]
There were produced molten steels having each chemical composition as
shown in Table I A and Table 18.
[00e3]
[Table lA]
TABLE 1A
[00e4]
[Table 1B]
TABLE 1B (Continued from TABLE 1A)
[00e5]
V/ith reference to Table 1A and Table lB, the chemical compositions of Steel
A and Steel B were within the range of the present invention. The C (carbon)
content of Steel C was excessively low. Steel D contained excessively high C
(carbon) and no B.
Steel
Chemical Comnosition lUnit: mass%. Balance: Fe and Impurities)
C Si Mn Cr Mo V Nb Ti sol.Al N
A 0.26 0.30 0.44 0.49 0.70 0.090 0.012 0.010 0.047 0.0030
B 0.26 0.30 0.44 1.00 0.70 0.090 0.030 0.011 0.040 0.0045
C 0.20 0.20 0.60 0.59 0.69 0.060 0.012 0.008 0.035 0.0036
D 0.45 0.31 0.47 1l04 0.70 0.1 00 0.013 0.009 0.030 0.0026
Steel
Chemical Composition (Unit: masso/0. Balance: Fe and Impurities)
B Ca P S o Ni Cu
A 0.0013 0.0018 0.007 0.00 0 0.0012 0.03 0.03
B 0.0012 0.007 0.00 0 0.0011 0.02 0.02
C 0.00r 2 0.0020 0.005 0.00 5 0.00r 0 0.01 0.01
D 0.0018 0.012 0.00 4 0.0007 0.03 0.01
2_3
,¿
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ii
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[00e6]
The above molten steels were used to produce slabs by continuous casting.
The slabs were bloomed into round billets each having a diameter of 310 mm. The
round billets were piercing-rolled and drawing-rolled into seamless steel pipes each
having a diameter of 244.48 mm and a wall thickness of 13.84 mm through the
Mannesmann-mandrel process.
[00e7]
Regarding the case where steels A and B were used, quenching (inline
quenching) was carried out after soaking at920oC without lowering the temperature
of the steel pipe to the Ar: point or less after completion of hot rolling. In the case
where steels C and D were used, the steel pipe was subjected to allowing cooling
after hot pipe rnaking.
[00e8]
Each seamless steel pipe was subjected to quenching in which each steel pipe
was reheated to 900'C and soaked for 15 minutes, thereafter being water cooled.
However, as shown in Table 2, Test Nos. 4 to 6, and Test Nos. 1l to 13 were
subjected to quenching in which each steel pipe was reheated to 920oC and soaked
for l5 minutes, thereafter being water cooled. Moreover, Test No. 15 used steel D.
Although, Test No. l5 was planned to be subjected to quenching twice, since quench
cracking occurred in the first quenching operation, the following procesq was
cancelled, excluding it from evaluation.
[00ee]
Each of the seamless steel pipes after being quenched was subjected to the
tempering as shown in Table 2.
[0100]
[Table 2]
TABLE 2
¡
Test
No.
Steel
Intermediate
heat
treatment
Low-Tmeperature
Temoerins
High-Temperature
Temnerins Note
Tr-(oC) tr(min) LMPr Tn('C) t¡1(min) LMPH
i
[0101]
With reference to Table 2, in Test Nos. 3, 6, 14, and Test No. 16, two-stage
tempering was carried out. Specifically, in each Test No., first, the low-temperature
tempering was carried out under tempering conditions (Tr-, tr-, LMPI) as shown in
Table 2. Reference Numeral û- in Table 2 denotes a soaking time (minutes) at the
tempering temperature Tr-. After the low-temperature tempering was carried out,
each seamless steel pipe was subjected to allowing cooling to be cooled down to a
room temperature (25oC). Using the seamless steel pipe after the allowing cooling,
the high-temperature ternpering was carried out under tempering conditions (Tu, tH,
LMPu) as shown in Table 2. Reference Numeral tH in Table 2 denotes a soaking
time (minutes) at the tempering temperature Tu. In each Test No., the heating rate
in the heating process was 8oC/minute, and the temperature of each seamless steel
pipe was continuously increased. Taking account of each heating process, the
LMPI and the LMPn were calculated by using Formulae (3) and (4), as in the above
manner. In calculating an integrated value of the LMPr- and the LMPH, At was set
to be li60 hour (1 minute). As for TestNos. 3,6,7 tol4 and 16, Tr (average
A Low Heating Rate 17743 700 60 19518 Inventive Example
2 A Low Heatins Rate I 7583 680 155 19462 lnventive Example
3 A 600 t20 17732 700 60 r 9483 lnventive Example
4 B Water
cooling after
soaking at
920'C for l5
minutes
Low Heating Rate 17743 700 60 195t8 Inventive Example
5 B Low Hea ng Rate I 7583 680 t55 19462 Inventive Example
6 B 600 t20 17732 700 60 r 9483 Inventive Example
7 A 7t0 45 19567 Comparative Example
8 A 710 60 r 9683 Comparative Example
9 A 700 30 19210 Comoarative Examole
l0 A 705 45 I 9468 Comoarative Examole
lt B Water
cooling after
soaking at
920"C for 15
minutes
700 60 t9482 Comoarative Examole
t2 B 710 45 19567 Comoarative Examole
l3 B 695 60 19382 Comparative Example
t4 C 600 t20 17732 700 60 I 9483 Compalative Example
l5 D Water
cooling after
soaking at
920"C for 15
minutes
Comparative Example
r6 B ó00 120 t7732 720 300 20560 Comparative Example
Lç
I
temperature of the first section) was set to a temperature 100oC lower than the
tempering temperature of each Test No. The results are shown in Table 2.
[0 r 02]
On the other hand, tempering was carried out after: each steel pipe was
continuously heated at a heating rate of 2"C/min until the temperature reaches 700'C
in Test Nos. I and 4; each steel pipe was continuously heated at a heating rate of
3oClmin until the tempering temperature reaches 680'C in Test Nos. 2 and 5; and
each steel pipe was soaked at 700'C for 60 minutes in Test Nos. I and 4, and each
steel pipe was soaked at 680oC for 155 minutes in Test Nos. 2 and 5. That is, in
Test Nos. \,2,4, and 5, ternpering at a low heating rate was carried out. In the
tempering at a low heating rate, the LMPI (calculated by Formula (a)) in a tempering
temperature range of 600 to 650"C was as shown in Table 2. Moreover, the total
LMPu of the LMP (calculated based on Formula (4)) while the tempering
temperature was increased from 670'C to the tempering ternperature (Tu), and the
LMP (calculated based on Formula (3)) when soaking was carried out at the
tempering temperature (TH) for tH minutes was as shown in Table 2. In Test Nos. l,
2,4, and 5, the equivalent tirne at the tempering temperature Tr¡ of the hightemperarute
tempering was calculated based on an integrated value of LMP in the
heating process from 670'C to the tempering temperature TH. The LMPH was
calculated by Formula (4) using the sum of a soaking time at the tempering
temperature TH and the equivalent time.
[0103]
In Test Nos. 7 to 13, only one stage tempering (high temperature tempering)
was carried out. In this case, each steel pipe was continuously heated at a heating
rate of 8'C/min.
[0104]
[Prior-y Grain Size No. Measurement Test]
Using the seamless steel pipe after being quenched of each Test No., the
prior-y grain size No. conforming to ASTM 1l2E was found. Each obtained prior-y
grain size No. is shown in Table 3. Each prior-1 grain size No. was 9.0 or more.
[0105]
[Microstructure Observation Test]
l:
:
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Il
l,
t-.
f,
LL
A sample including a central portion of wallthickness of the seamless steel
pipe after being tempered in each Test No. was collected. Of each collected sample,
a sample surface of a cross section vertical to the axial direction of each seamless
steel pipe was polished. After being polished, each polished sample surface was
etched usingnital. Each etched surface was observed with a microscope, and as a
result, in each Test No., the sample had a microstructure formed of the tempered
martensite. The volume ratio of the retained austenite was measured in the above
described manner, and as a result, in each Test No., the volume ratio of the retained
austenite was less than2%o.
[0106]
[Number of Coarse Cementite Particles CN]
Using the seamless steel pipe after being tempered of each Test No., the
number of coarse cementite particles CN (parlicles/l00 pm2) was found in the above
described manner. Each obtained number of coarse cementite particles CN was
shown in Table 3.
[0107]
[Yield Strength Test]
A No. l2 test specimen (width: 25mm, gage length: 5Omm) specified in JIS
22201was collected from a central portion of wall thickness of the seamless steel
pipe of each Test No. A central axis of each test specimen was located at the
central position of the wall thickness of each seamless steel pipe, and was parallel
with the longitudinal direction of each seamless steel pipe. Using each collected
test specimen, a tensile test conforming to JIS 22241was carried out in the
atmosphere at a normal temperature (24'C) so as to find a yield strength (YS). The
yield strength was found by the 0.7% total elongation method. Each obtained yield
strength (MPa) was shown in Table 3. In examples of the present invention, every
seamless steel pipe has a yield strength of I l5 ksi (793 MPa) or more.
[0108]
[DCB Test]
The seamless steel pipe of each Test No. was subjected to a DCB (double
cantilever beam) test so as to evaluate the SSC resistance.
[010e]
:1
:
L+
is.r"r.ffint @
Specifically, three DCB test specimens each of which had a thickness of 10
mm, a width of 25 mm, and a length of 100 mm were collected from each seamless
steel pipe. Using the collected DCB test specimens, the DCB test was carried out in
compliance with NACE (National Association of Corrosion Engineers) TMO177-
2005 Method D. A 5o/o salt + 05% acetic acid aqueous solution having a normal
temperature (24'C) in which hydrogen sulfide gas at I atm was saturated was used
for a test bath. The DCB test was carried out in such a manner that each DCB test
specimen was immersed in the test bath for 336 hours. Each test specimen was put
under tension by using a wedge which gives the two arms of the DCB test specimen
a displacement of 0.51 mm (+0.03 mm/-0.05 mm) and exposed in a test liquid for 14
days.
t01 r 0l
After the test, a length of crack propagation "a" generated in each DCB test
specimen was measured. Using the measured length of the crack propagation "a"
and a wedge-release stress P, each stress intensity factor Krssc(ksi{in) was found
based on the following Formula (6).
Krssc: Pa((2(i/3) +2.38 x (h/a)) x (B/B¡)r/({:¡4n " h3/2) (6)
[0r r r]
'Where, "h" in Formula (6) denotes a height of each arm of each DCB test
specimen, "8" denotes a thickness of each DCB test specimen, and "Bn" denotes a
web thickness of each DCB test specimen. These are specified in the above NACE
Tlyf0l77-2005 Method D.
[01 12]
An average value of the stress intensity factors obtained in the three DCB test
specimens in each Test No. was defined as a stress intensity factor Klssc of that Test
No.
[0] r3]
[Test Results]
[0114]
[Table 3]
TABLE3
f
L
t,'
!i
Lg
t;
ìl i
¡i
a:
Test
No.
Steel
Prior'-y
Grain
Size No,
CN
(grains/
100 um2)
YS Krssc Average Value
Note
(MPa) (ksi) (MPa{m) (ksi{inch)
I A 9.2 t45 796 I 15.4 27.9 2s.4
Inventive
Example
2 A 9.0 192 814 il8 27.1 24.7
Inventive
Examnle
3 A 9.1 t38 835 t2t.l 26.4 24.0
Inventive
Example
4 B r0. r 124 845 122.5 25.3 23.0
Inventive
Example
5 B 10.0 t79 795 l 15.3 28.5 25.9
Inventive
Examole
6 B l0.l t50 829 120.2 26.7 24.3
Inventive
Example
7 A 8.8 76 819 I 18.8 23.3 21.2
Comparative
Example
8 A 9.0 85 803 I16.5 25.9 23.6
Comparative
Examole
I A 9.0 46 834 t2t 23.s 21.4
Compæative
Example
r0 A 9.9 35 807 117 22.6 20.6
Comparative
Example
il B 10.3 59 824 r 19.5 24.9 22.7
Comparative
Example
t2 B 10.3 60 794 1t5.2 26.5 24.1
Comparative
Examnle
l3 B 10.3 50 8s0 123.3 23.4 21.3
Comparative
Example
t4 C 9.6 35 793 lls 22.s 20.s
Comparative
Example
l5 D
Comparative
Example
l6 B 10.0 659 95.5
Comparative
Example
[011s]
With reference to Table 3, each of Test Nos. 3 and 6 had an appropriate
chemical composition. Also, in the tempering, the two-stage tempering (the lowtemperature
tempering and the high-temperature tempering) was carried out, and
each tempering condition was appropriate. As a result, each seamless steel pipe had
a prior-y grain size No. of 9.0 or more, and a number of coarse cementite particles
CN of 100 particles/l00 ¡-rm2 or more. Further, each seamless steel pipe had a Krssc
greater than those of Comparative Examples having the same level of yield strength
YS, and had an excellent SSC resistance.
[0r l6]
L1
F
Each of Test Nos. I and 2, and Test Nos. 4 and 5 had an appropriate chemical
composition. Further, the low-heating rate tempering was carried out, and each
condition thereof was appropriate. As a result, each seamless steel pipe had a priory
grain size No. of 9.0 or more, and a number of coarse cementite particles CN of
100 particles/l00 pm2 or more. Further, each seamless steel pipe had a Krssc
greater than those of Comparative Examples having the same level of yield shength
YS, and had an excellent SSC resistance.
t0l17l
Meanwhile, in each of Test Nos. 7 to 13, the low-temperature tempering and
the tempering corresponding to the low-heating rate tempering were not carried out.
As a result, in each of these Test Nos., the number of coarse cementite particles CN
was less than 100 particles/100 pm'.
[01 r 8]
Test No. l4 was subjected to the two-stage tempering; since the C content
was 0.20%o which was less than the lower limit of the present invention, the number
of coarse cementite particles CN was less than 100 particles/l 00 ¡rm2. Test No. 16
was also subjected to the two-stage tempering; since the LMPH of the hightemperature
tempering was too high, the yield strength YS was too low.
[0r le]
FIG. I is a diagram to show the result of Table 3 as a relationship befween
yield strength YS and Ktssc. ln general, it is well known that in a low alloy steel,
Krssc tends to decrease as yield strength YS increases. However, in FIG. l, it was
made clear that the steel pipe of the present invention showed a higher Krssc at a
same yield strength.
[0120]
As aforementioned, the embodiment of the present invention has been
explained. However, the aforementioned embodiment is merely an exemplification
for carrying out the present invention. Accordingly, the present invention is not
Iimited to the aforementioned embodiment, and the aforementioned embodiment can
be appropriately modified and carried out without departing from the scope of the
present invention.

We claim:
l. A low alloy oil-well steel pipe characterized by cornprising a chemical
composition consisting of:
in masso/0,
C: 0.25 to 0.35%:
Si: 0.05 ro0.50Vo;
Mn: 0.l0 to I .50%;
Cr: 0.40 to 1.50Yo;
Mo: 0.40 to 2.00%;
V: 0.05 to 0.25o/o;
Nb: 0.010 to 0.040%;
Ti: 0.002 to 0.050%;
sol.Al:0.005 to 0.10%;
N: 0.007% or less;
B: 0.0001 to 0.0035%;
Ca: 0 to 0.005%; and
a balance being Fe and irnpurities,
the impurities including:
P:0.020% or less;
S: 0.010% or less;
O: 0.006% or less;
Ni: 0.10% or less; and
Cu:0.10% or less,
wherein
in a micro_structure of the low alloy oil-well steelpipe, a number of cementite
particles each of which has an equivalent circle diameter of 200 dm or more is I 00
particles/100 pm2 or more, and
a yield strength is 793 MPa or more.
2. The low alloy oil-well steel pipe according to claim l, characterized in that
the chemical composition contains Ca: 0.0005 to 0.005%.