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, 654 to 758
MPa) oil-well steel pipes have been widely used. However, 110 ksi-grade (yield
stress of 110 to 125 ksi, that is, 758 to 862 MPa) oil-well steel pipes 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 sse resistance, hereinafter).
In general, susceptibility to the SSC is increased along with increase in strength of a
steel material.
[0004]
Steel pipes of II 0 ksi grade or less sold as sour-resistant oil-well steel pipes
(sour service OCTO) usually have a guaranteed SSC resistance. The guaranteed
SSC resistance herein denotes durability performance under the H2S environment at
1 atm in an evaluation in a test method specified by NACE. Hereinafter, the H2S
environment at 1 atm is referred to as a standard condition.
[0005]
2
Meanwhile, a SSe resistance guaranteed for oil-well steel pipes of 125 ksi
grade (yield stress of 862 to 965 MPa) is smaller than the above sse resistance. In
these oil-well pipes, only the sse resistance under an environment in which partial
pressure of H2S is much smaller than that under the standard condition is guaranteed
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
sse resistance.
[0006]
On this background, there is a need for sour-resistant oil-well steel pipes that
secures the sse resistance under the H2S environment at 1 atm, and has a yield
strength as great as possible. In this case, even if the lower limit of the yield
strength does not reach 125 ksi (862 MPa), the lower limit ofthe yield strength is
required to be as great as possible.
[0007]
Techniques to enhance the SSe 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. 8-311551 (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), Japanese Patent Application Publication No. 2012-26030 (Patent
Literature 9), and International Application Publication No. W0201 0/150915 (Patent
Literature 1 0).
[0008]
Patent Literature 1 proposes a method of enhancing the sse resistance of an
oil-well steel pipe by reducing impurities such as Mn and P. Patent Literature 2
proposes a method of enhancing the sse resistance of steel by performing quenching
twice to refine grains.
[0009]
3
Patent Literature 3 proposes a method of enhancing the SSe 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 SSe
resistance of a steel pipe of 110 ksi grade to 140 ksi grade by enhancing
hardenability of the steel through direct quenching process, and increasing a
tempering temperature.
[001 0]
Each of Patent Literature 5 and Patent Literature 6 proposes a method of
enhancing the SSe resistance of a low alloy oil-well steel pipe of II 0 ksi grade to
140 ksi grade by controlling the morphology of carbide. Patent Literature 7
proposes a method of enhancing the sse 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 SSe resistance of 125 ksi (862 MPa)-grade steel by quenching low
alloy steel containing e of0.3 to 0.5% several times. Patent Literature 9 proposes a
method of employing a two-stage tempering step to control the morphology of
carbide and the number of carbide particles. More specifically, in Patent Literature
9, the SSe resistance of 125 ksi (862 MPa)-grade steel is enhanced by suppressing
the number density of large M3e particles or M2e particles. Patent Literature 10
proposes a method of coping with both a high strength and the sse resistance by
controlling amount of dissolved Mo, a prior-austenite grain size, and amount of M2e
precipitate to be desired values.
[00 11]
However, even if applying the techniques disclosed in the above Patent
Literatures 1 to 10, in the case of oil-well steel pipes having a yield strength 120 ksi
(827 MPa) or more, an excellent sse resistance cannot be stably obtained in some
cases.
SUMMARY OF INVENTION
[0012]
4
An object of the present invention is to provide a low alloy oil-well steel pipe
having a yield strength of 120 ksi grade or more (827 MPa or more) and an excellent
sse resistance.
[00 13]
A low alloy oil-well steel pipe according to the present invention includes a
chemical composition consisting of: in mass%, C: more than 0.35 to 0.65%; Si: 0.05
to 0.50%; Mn: 0.10 to 1.00%; Cr: 0.40 to 1.50%; Mo: 0.50 to 2.00%; V: 0.05 to
0.25%; Nb: 0.01 to 0.04%; sol.Al: 0.005 to 0.10%; N: 0.007% or less; Ti: 0 to
0.0 12%; Ca: 0 to 0.005%; and a balance being Fe and impurities, the impurities
including: P: 0.020% or less; S: 0.002% or less; 0: 0.006% or less; Ni: 0.10% or
less; Cu: 0.03% or less; and B: 0.0005% or less. In a microstructure, a number of
cementite particles each of which has an equivalent circle diameter of 200 nm or
more is 200 particles/1 00 ).!m2 or more. The above low alloy oil-well steel pipe has
a yield strength of 827 MPa or more;
[0014]
The above chemical composition may contain Ti: 0.003 to 0.012%. The
above chemical composition may contain Ca: 0.0005 to 0.005%.
[0015]
The low alloy oil-well steel pipe according to the present invention has a yield
strength of 120 ksi grade or more (827 MPa or more) and an excellent SSC resistance.
DESCRIPTION OF EMBODIMENT
[0016]
Hereinafter, an embodiment of the present invention will be described in
details.
[0017]
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.
[0018]
If a steel pipe is subjected to tempering at a low temperature, a large amount
of fine cementite is precipitated. The 'precipitated fine cementite has a flat
morphology. Further, if the tempering temperature is low, dislocation density is not
5
decreased. Hydrogen having intruded in the steel is trapped at an interface between
the fine cementite having a flat morphology and a parent phase. The hydrogen
having intruded in the steel is also trapped in dislocation in the steel. sse is likely
to be caused due to the hydrogen trapped at an 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 density is high, the sse resistance becomes
deteriorated.
[0019]
To counter this, 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. In 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, coarse cementite is unlikely to initiate occurrence of SSC. Accordingly,
instead of fine cementite, coarse cementite is formed, thereby enhancing the SSC
resistance.
[0020]
Meanwhile, 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
attain a yield strength of 827 MPa or more.
[0021]
In the present embodiment, 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 827 MPa or
more and an excellent SSC resistance. Coarse cementite of which particle has an
equivalent circle diameter of200 nm or more is referred to as "coarse cementite",
hereinafter.
6
[0022]
In order to attain the above described oil-well steel pipe, in the tempering,
low-temperature tempering at 600 to 650°C 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, a large 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
827 MPa or more as well as an excellent SSC resistance.
[0023]
A low alloy oil-well steel pipe according to the present embodiment that has
been accomplished based on the above findings includes a chemical composition
consisting of: in mass %,C: more than 0.35 to 0.65%; Si: 0.05 to 0.50%; Mn: 0.10 to
1.00%; Cr: 0.40 to 1.50%; Mo: 0.50 to 2.00%; V: 0.05 to 0.25%; Nb: 0.01 to 0.04%;
sol.Al: 0.005 to 0.10%; N: 0.007% or less; Ti: 0 to 0.012%; Ca: 0 to 0.005%; and a
balance being Fe and impurities, the impurities containing: P: 0.020% or less; S:
0.002% or less; 0: 0.006% or less; Ni: 0.10% or less; Cu: 0.03% or less; and B:
0.0005% or less. In the microstructure, the number of cementite particles each of
which has an equivalent circle diameter of 200 nm or more is 200 particles/1 00 ).. t m2
or more. The yield strength of the above low alloy oil-well steel pipe is 827 MPa or
more.
[0024]
The low alloy oil-well steel pipe according to the present embodiment will be
described in details, hereinafter.
[0025]
[Chemical Composition]
The chemical composition of the low alloy oil-well steel pipe according to the
present embodiment consisting of the following elements. A sign"%" in the
chemical composition denotes "mass %".
7
[0026]
e: more than 0.35 to 0.65%
A content of carbon (C) in the low alloy oil-well steel pipe according to the
present embodiment is higher than that in a conventional low alloy oil-well steel pipe.
e refines a sub-microstructure of martensite, and enhances strength of the steel. e
also forms carbide to enhance strength of the steel. If thee content is high,
spheroidization ofthe carbide is encouraged, and the sse resistance becomes
enhanced. For example, the carbide may be cementite and alloy carbide (Mo
carbide, V carbide, Nb carbide, Ti carbide, and the like). An excessively lowe
content cannot attain the above effect. For example, the number of precipitated
cementite particles is excessively small, so that strength of the steel becomes
deteriorated. On the other hand, an excessively high e content rather deteriorates
toughness of the steel as quenched, which results in increase in quench cracking
susceptibility. e is an element that stabilizes austenite. Hence, if thee content is
excessively high, the volume ratio of retained austenite becomes excessively high,
which causes variation in strength. Accordingly, the e content is more than 0.35 to
0.65%. A preferable lower limit of thee content is 0.38%, and more preferably
0.45%, and further more preferably 0.50%. A preferable upper limit of thee
content is 0.60%, and more preferably 0.58%.
[0027]
Si: 0.05% to 0.50%
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
SSe resistance. Accordingly, the Si content is 0.05% to 0.50%. A preferable
lower limit of the Si content is 0.1 0%, and more preferably 0.17%. A preferable
upper limit of the Si content is 0.40%, and more preferably 0.35%.
[0028]
Mn: 0.10 to 1.00%
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 impunity elements such as phosphorus (P)
and sulfur (S). In this case, the SSe resistance of the steel becomes deteriorated.
8
Accordingly, the Mn content is 0.10 to 1.00%. A preferable lower limit of the Mn
content is 0.20%, and more preferably 0.25%. A preferable upper limit of the Mn
content is 0.75%, and more preferably 0.50%.
[0029]
Cr: 0.40 to 1.50%
Chromium (Cr) enhances hardenability ofthe 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%. A preferable
lower limit of the Cr content is 0.43%, and more preferably 0.48%. A preferable
upper limit of the Cr content is 0.90%, and more preferably 0.70%.
[0030]
Mo: 0.50 to 2.00%
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 0.50 to 2.00%. A
preferable lower limit of the Mo content is 0.60%, and more preferably 0.65%. A
preferable upper limit of the Mo content is 1.6%, and more preferably 1.3%.
[0031]
V: 0.05 to 0.25%
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%.
[0032]
Nb: 0.01 to 0.04%
9
Niobium (Nb) forms carbide, nitride, or carbonitride in combination with e or
N. These precipitates (carbide, nitride, and carbonitride) refine a sub-
. microstructure of the steel by the pinning effect, and enhances the sse 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 SSe resistance of the steel. Accordingly, the Nb content is 0.01 to 0.04%. A
preferable lower limit ofthe Nb content is 0.012%, and more preferably 0.015%. A
preferable upper limit of the Nb content is 0.035%, and more preferably 0.030%.
[0033]
sol.Al: 0.005 to 0.10%
Aluminum (AI) deoxidizes the steel. An excessively low AI content cannot
attain this effect, and deteriorates the sse resistance of the steel. On the other hand,
an excessively high AI content results in increase of inclusions, which deteriorates
the SSe resistance of the steel. Accordingly, the AI content is 0.005 to 0.1 0%. A
preferable lower limit ofthe AI content is 0.010%, and more preferably 0.020%. A
preferable upper limit of the AI content is 0.07%, and more preferably 0.06%. The
"AI" content referred to in the present specification denotes the content of "acidsoluble
AI", that is, "sol.Al".
[0034]
N: 0.007% or less
Nitrogen (N) is inevitably contained. N forms coarse nitride, and
deteriorates the sse resistance of the steel. Accordingly, theN content is 0.007%
or less. A preferable N content is 0.005% or less, and more preferably 0.0045% or
less.
[0035]
If Ti described below is contained in the steel, N forms TiN to refine grains.
In this case, a preferable lower limit of theN content is 0.002%.
[0036]
Ti: 0 to 0.012%
Titanium (Ti) is an optional element, and may not be contained. If contained,
Ti forms nitride, and refines grains by the pinning effect. However, an excessively
high Ti content coarsens Ti nitride, which deteriorates the sse resistance of the steel.
10
Accordingly, the Ti content is 0 to 0.012%. A preferable lower limit of the Ti
content is 0.003%, and more preferably 0.005%. A preferable upper limit of the Ti
content is 0.008%.
[0037]
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, theCa content is 0 to 0.005%. A preferable
lower limit of theCa content is 0.0005%, and more preferably 0.001%. A
preferable upper limit of theCa content is 0.003%, and more preferably 0.002%.
[0038]
The balance of the chemical composition ofthe low alloy oil-well steel pipe
according to the present embodiment 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 embodiment, each content of P, S, 0, Ni, and Cu in the impurities is
specified as follows.
[0039]
P: 0.020% or less
Phosphorus (P) is an impurity. P segregates at grain boundaries, and
deteriorates the SSC resistance of the steel. Accordingly, the P content is 0.020%
or less. A preferable P content is 0.015% or less, and more preferably 0.010% or
less. It is preferable to set the P content to be as small as possible.
[0040]
S: 0.002% 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 0.002% or less. A
preferable S content is 0.0015% or less, and more preferably 0.001% or less. It is
preferable to set the S content to be as small as possible.
[0041]
11
0: 0.006% or less
Oxygen (0) is an impurity. 0 forms coarse oxide, and deteriorates a
corrosion resistance of the steel. Accordingly, the 0 content is 0.006% or less. A
preferable 0 content is 0.004% or less, and more preferably 0.0015% or less. It is
preferable to set the 0 content to be as small as possible.
[0042]
Ni: 0.10% or less
Nickel (Ni) is an impurity. Ni deteriorates the SSC resistance ofthe steel.
If the Ni content is more than 0.1 0%, the SSC resistance becomes significantly
deteriorated. Accordingly, the content ofNi as an impurity element is 0.10% or less.
[0043]
Cu: 0.03% or less
Copper (Cu) is an impurity. Copper embrittles the steel, and deteriorates the
SSC resistance of the steel. Accordingly, the Cu content is 0.03% or less. A
preferable Cu content is 0.02% or less.
[0044]
B: 0.0005% or less
Boron (B) is an impurity. B forms M23(CB)6 at grain boundaries, and
deteriorates the SSC resistance of the steel. A slight amount of effective B (B
uncombined with N) is effective to enhance hardenability, but it is relatively difficult
to stably secure a sight amount of effective B within the range of the Ti content of
the present embodiment. Accordingly, the B content is 0.0005% or less. A
preferable B content is 0.0003% or less.
[0045]
[Microstructure]
The microstructure of the low alloy oil-well steel pipe including the
aforementioned chemical composition is formed oftempered martensite and retained
austenite of 0 to less than 2% in terms of a volume fraction.
[0046]
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
12
strength of the low alloy oil-well steel pipe of the present embodiment is 827 MPa or
more (120 ksi grade or more). The yield strength referred to in the present
specification is defined by the 0.7% total elongation method.
[0047]
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
2% in the present embodiment. 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
ofthe retained austenite is 0% (i.e., microstructure formed oftempered martensite).
[0048]
By controlling the carbon (C) content in the low alloy oil-well steel pipe and
the cooling stop ,temperature at the time of quenching, it is possible to suppress the
volume ratio of the retained austenite to be less than 2%. Specifically, the C
content of the low alloy oil-well steel pipe is set to be 0.65% or less. In addition,
the cooling stop temperature at the time of quenching is set at 50°C or less.
Through this configuration, it is possible to suppress the volume ratio of the retained
austenite to be less than 2%.
[0049]
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
is carried out on each chemically polished surface by using a CoKa ray as an
incident X ray. Specifically, using each sample, respective surface integrated
intensities of a (200) plane and a (211) plane in a ferrite phase (a phase), and
respective surface integrated intensities of a (200) plane, a (220) plane, and (311)
plane in the retained austenite phase (y phase) are respectively found. Subsequently,
the volume ratio Vy(%) is calculated by using Formula (1) for each combination
between each plane in the a phase and each plane in they phase (6 sets in total).
13
An average value of the volume ratios Vy(%) of the 6 sets is defined as the volume
ratio (%) of the retained austenite.
Vy = I 00/(1 + (Ia x Ry)/(Iy x Ra)) (I),
where "Ia" and "Iy" are respective integrated intensities of the a phase and
they phase. "Ra" and "Ry" are respective scale factors of the a phase and they
phase, and these values are obtained through a crystallographic logical calculation
based on the types of the substances and the plane directions.
[0050]
The aforementioned microstructure can be obtained by carrying out the
following producing method.
[0051]
[Prior-austenite Grain Size No.]
In the present embodiment, it is preferable that the grain size No. based on
ASTM E 112 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
827 MPa or more. A preferable grain size No. of the prior-y grains is 9.5 or more.
[0052]
The grain size No. of the prior-y grains may be measured by using a steel
material after being quenched and before being tempered (so-called material as
quenched), or by using a tempered steel material (referred to as a tempered material).
The size of the prior-y grains is never 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 grain size No. of the prior-y grains becomes 9.0 or more through wellknown
quenching described later.
[0053]
[Size of Coarse Cementite]
The above mentioned low alloy oil-well steel pipe includes cementite
particles each of which is 200 nm or more in terms of the equivalent circle diameter.
As aforementioned, hydrogen having intruded in the steel is trapped at the interface
between the cementite and the parent phase. Cementite whose particle is 200 nm or
14
more in terms of the equivalent circle diameter (coarse cementite) has a smaller
specific surface area compared with that of refine cementite. Hence, if cementite is
coarsened, the interfaces between the cementite and the parent phase become
reduced. Reduction of the interfaces decreases trap sites of hydrogen, thereby
enhancing the SSC resistance of the low alloy oil-well steel pipe. Meanwhile, fine
cementite has a greater specific surface area compared with that of coarse cementite.
In addition, fine cementite has a needle-like morphology or a flat morphology. In
this case, the specific surface area of the cementite becomes further increased.
Hence, fine cementite is likely to become an initiator of occurrence of the SSC.
Accordingly, the size of the cementite is 200 nm or more in terms of the equivalent
circle diameter. The upper limit of the size of the cementite is not limited to
specific one, but 350 nm for example.
[0054]
By appropriately selecting a heat treatment condition in the high-temperature
tempering step described later, it is possible to coarsen cementite.
[0055]
[Number of Coarse Cementite Particles]
In the aforementioned substructure, the number of coarse cementite particles
CN is 200 particles/! 00 1-1m2 or more.
[0056]
Cementite enhances the yield strength of the steel pipe. Hence, as the
number of cementite particles becomes increased, the yield strength of the steel pipe
becomes enhanced. Specifically, ifthere are cementite particles of200
particles/1 00 ~-tm2 or more, the yield strength of the steel pipe becomes enhanced.
[0057]
By appropriately selecting the chemical composition and a heat treatment
condition in the tempering step described later, it is possible to coarsen fine
cementite. If cementite is coarsened, the number of fine cementite particles
becomes decreased. As a result, the SSC resistance becomes improved.
Specifically, if the number of cementite particles CN each of which has an equivalent
circle diameter of 200 nm or more is 200 particles/1 00 ~-tm2 or more, it is possible to
15
attain an excellent sse resistance even if the steel pipe has a yield strength of 827
MPaormore.
[0058]
A preferable lower limit of the number of coarse cementite particles CN is
220 particles/1 00 f!m2
• The upper limit of the number of coarse cementite particles
CN is not limited to specific one, but in the case ofthe aforementioned chemical
composition, a preferable upper limit of the number of coarse cementite particles CN
is 500 particles/1 00 f!m2
•
[0059]
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. The number of coarse
cementite particles CN is measured by the following method.
[0060]
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 of the steel pipe) of each steel
pipe (referred to as an observation surface, hereinafter) is polished. Each
observation surface after being polished is etched using a nita! etching reagent.
Specifically, each observation surface is immersed into the nita! etching reagent (a
mixture of 3% of nitric acid and 97% of ethyl alcohol) for 10 seconds at ordinary
temperature and is etched.
[0061]
Using a scanning electron microscope, any 10 visual fields in each etched
observation surface are observed. Each visual field has an area of 10 f!m x 10 f!m.
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 J1.47v), 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.
16
[0062]
In 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/1 0 (2)
The number of coarse cementite particles can be measured in the above
manner.
[0063]
[Producing Method]
An example of a producing method of the low alloy oil-well steel pipe
according to the present embodiment 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
step, a quenching step, and a tempering step.
[0064]
[Pipe Making Step]
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.
[0065]
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 raw pipes. First, the billets are heated in a
heating furnace. The billets extracted from the heating furnace are subjected to hot
working into raw pipes (seamless steel pipes). For example, the Mannesmann
process is carried out as the hot working so as to produce the raw pipes. In this case,
17
round billets are piercing-rolled by a piercing machine. The piercing-rolled round
billets are further hot-rolled by a mandrel mill, a reducer, a sizing mill, or the like
into the raw pipes. The raw pipes may be produced from billets with other hot
working methods.
[0067]
[Quenching Step]
The raw pipes after the hot working are subjected to quenching and tempering.
A quenching temperature in the quenching is the Ac3 point or more. A preferable
upper limit of the quenching temperature is 930°C. If the quenching temperature is
high, austenite particles become coarsened. In this case, the grain size No. of the
prior-y grains becomes less than 9.0, and thus the SSC resistance is deteriorated. A
preferable quenching temperature is 91 0°C or less.
[0068]
At the time of quenching, a preferable cooling rate in a temperature range of
500 to 1000°C ofthe raw pipe is 1 to 15°C/second. Ifthe cooling rate in the above
temperature range is excessively great, quenching crack may be caused in some cases.
On the other hand, if the cooling rate in the above temperature range is excessively
small, a large amount of bainite is contained in the microstructure, and thus
martensite in the microstructure becomes decreased. A cooling stop temperature at
the time of quenching is 50°C or less. Thereby the volume ratio of the retained
austenite is possible to be suppressed to less than 2%.
[0069]
The grain size No. of the prior-y grains of the raw pipe after the above
quenching step becomes 9.0 or more. The grain size No. of the prior-y grains is
never changed even after the tempering described below.
[0070]
[Tempering Step]
The tempering step includes a low-temperature tempering step and a hightemperature
tempering step.
[0071]
[Low-temperature Tempering Step]
18
First, the low-temperature tempering step is carried out. The tempering
temperature T L in the low-temperature tempering step is 600 to 650°C. A LarsonMiller
parameter LMPL in the low-temperature tempering step is 17700 to 18750.
(hr).
[0072]
The Larson-Miller parameter is defined by Formula (3).
LMP = (T + 273) x (20 + log(t)) (3)
In Formula (3), T denotes a tempering temperature (0 C), and t denotes a time
The tempering step includes a heating process and a soaking process. The
Larson-Miller parameter taking account of the heating process can be found by
calculating an integrated tempering parameter in accordance with Non-Patent
Literature 1 (TSUCHIYAMA, Toshihiro. 2002. "Physical Meaning of Tempering
Parameter and Its Application for Continuous Heating or Cooling Heat Treatment
Process". "Heat Treatment" Vol. 42(3): pp.163-166).
[0073]
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 L'1t of total number N. Herein, an average temperature in the (n-1)-th
section is defined as T n-I and an average temperature in the n-th section is defined as
Tn. An LMP (1) corresponding to the first micro time (the section when n = 1) can
be obtained by following formula.
LMP (1) = (T1 + 273) x (20 + log(L'1t))
The LMP (1) can be described as a value equivalent to an LMP calculated based on a
temperature T2 and a heating time 12 by following formula.
(T1 + 273) x (20 + log(L'1t)) = (T2 + 273) x (20 + log(12))
The time 12 is a time required (an equivalent time) to obtain an LMP at temperature
T2 equivalent to an integrated value of LMP calculated based on a heating at a
section before the second section. The heating time at second section (temperature
T2) is a time obtained by adding an actual heating time L'1t to the time 12.
Accordingly, an LMP (2) which is an integrated value ofLMP when the heating of
the second section is completed can be obtained by following formula.
LMP (2) = (T2 + 273) x (20 + log(12 +i1t))
19
By generalizing this formula, following formula can be obtained.
LMP (n) = (Tn + 273) x (20 + log(tn + ~t))
The LMP (n) is the integrated value ofLMP when the heating of n-th section is
completed. The time tn is an equivalent time to obtain an LMP at temperature Tn
equivalent to an integrated value ofLMP when the heating ofthe (n-1)-th section is
completed. The time tn can be obtained by Formula (4).
log(tn) = ((Tn-1 + 273) I (Tn + 273)) X (20 + log(tn-1))- 20 (4)
[0074]
In the low-temperature tempering step, as described above, a large amount of
e (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 T Lor an excessively low LMPL results in a small amount of precipitated
cementite. On the other hand, an excessively high temperature of the lowtemperature
tempering T Lor an excessively high LMPL causes growth of coarse
cementite, but results in a small amount of precipitated cementite.
[0075]
If the temperature of the low-temperature tempering T Lis 600 to 650°e, and
the LMPL is 17700 to 18750, a large amount of fine cementite serving as a nucleus of
coarse cementite is precipitated in the low-temperature tempering step.
[0076]
[High-temperature Tempering Step]
The high-temperature tempering step is carried out after the low-temperature
tempering step. In the high-temperature tempering step, the fine cementite
precipitated in the low-temperature tempering step is coarsened, thereby forming
coarse cementite. Accordingly, it is possible to prevent the cementite from
becoming an initiator of sse, as well as to enhance strength of the steel with the
coarse cementite.
[0077]
In the high-temperature tempering step, dislocation density in the steel is
reduced. Hydrogen having intruded in the steel is trapped in the dislocation, and
becomes an initiator of sse. Hence, if the dislocation density is lower, the sse
20
resistance becomes enhanced. The dislocation density in the steel becomes reduced
by carrying out the high-temperature tempering step. Accordingly, the SSC
resistance becomes enhanced.
[0078]
For the purpose of attaining the above effect, the tempering temperature TH in
the high-temperature tempering step is 670 to 720°C, and the Larson-Miller
parameter LMPH defined by Formula (3) and Formula (4) is 18500 to 20500.
[0079]
If the tempering temperature THis excessively low, or the LMPH is
excessively low, the cementite is not coarsened, and the number of the coarse
cementite particles becomes less than 200 particles/1 00 J.tm2
• Furthermore, the
dislocation density is not sufficiently reduced. Consequently, the SSC resistance is
deteriorated.
[0080]
On the other hand, if the tempering temperature THis excessively high, or the
LMPH is excessively high, the dislocation density is excessively reduced. In this
case, the yield strength of the steel pipe including the aforementioned chemical
composition becomes less than 827 MPa.
[0081]
In the tempering step of the present embodiment, the two-stage tempering
including the low-temperature tempering step and the high-temperature tempering
step may be carried out, as aforementioned. Specifically, the steel pipe is cooled
down to a normal temperature after the low-temperature tempering step is carried out.
Subsequently, the high-temperature tempering step is carried out by heating the steel
pipe having the normal temperature. Alternatively, immediately after the lowtemperature
tempering step is carried out, the high-temperature tempering step may
be carried out by heating the steel pipe up to the temperature of the high-temperature
tempering TH without cooling the steel pipe.
[0082]
Alternatively, the low-temperature tempering step and the high-temperature
tempering step 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
21
rate so as to increase the retaining time in a temperature range of 600 to 650°C
(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°C/minute or less in a temperature range of 500°C to
700°C, and the steel pipe is soaked at 710°C for a predetermined time (e.g., for 60
minutes). In this case, it is only required that an integrated value of the LarsonMiller
parameter LMPL in the temperature range of the low-temperature tempering
TL (i.e., 600 to 650°C range) is 17700 to 18750, and an integrated value of the
Larson-Miller parameter LMPH in the temperature range of the high-temperature
tempering TH (i.e., 670 to 720°C range) is 18500 to 20500. In other words, in the
tempering step, as far as the LMPL in the temperature range of the low-temperature
tempering TL satisfies the above condition, and the LMPH in the temperature range of
the high-temperature tempering T H satisfies the above condition, the tempering
method is not limited to specific one.
[0083]
Through the above producing method, the low alloy seamless steel pipe
according to the present embodiment is produced. The microstructure of the
produced seamless steel pipe is formed of the tempered martensite and the retained
austenite of 0 to less than 2%. In addition, the grain size No. of the prior-y grains is
9.0 or more. Through the above described tempering step, the number of coarse
cementite particles CN in the microstructure becomes 200 particles/1 00 l-lm2 or more.
[0084]
[Heat Treatment Other Than Quenching and Tempering]
In the producing method of the present embodiment, other heat treatment
(intermediate heat treatment) may be carried out additionally after the pipe making
step and before the quenching step. For example, the raw pipe after the hot working
may be subjected to normalizing treatment. Specifically, the raw pipe after the hot
working is retained at a temperature higher than the A3 point (e.g., 850 to 930°C) for
a predetermined time, and subsequently the raw pipe is subjected to allowing cooling.
The retaining time is 15 to I30 minutes, for example.
[0085]
22
In the normalizing treatment, the raw pipe after the hot working is usually
cooled down to a normal temperature, and thereafter, is heated up to the Ac3 point or
more. However, the normalizing treatment in the present embodiment may be
carried out such that the raw pipe after the hot working is retained at a temperature of
the Ac3 point or more after the hot working.
[0086]
By carrying out the normalizing treatment, the prior-y grains are further
refined. Specifically, if the raw pipe subjected to the normalizing treatment is
quenched, the grain size No. ofthe prior-y grains of the material as quenched
becomes 9.5 or more.
[0087]
Instead of the above normalizing treatment, quenching may be carried out.
In this case, the quenching is carried out plural times. The above intermediate
treatment may be heat treatment at a two-phase region temperature of ferrite +
austenite (referred to as a "two-phase region heating", hereinafter). In the
intermediate heat treatment, it is only required that at least part of the microstructure
of the steel is transformed to austenite. In this case, it is possible to attain a
preferable effect due to grain refinement. Accordingly, in the intermediate heat
treatment, it is sufficient to soak the raw pipe at least at a temperature of the Ac1
point or more.
EXAMPLE
[0088]
There was produced molten steel including each chemical composition as
shown in Table 1 A and Table I B.
[0089]
[Table lA]
TABLEIA
Steel c
A 0.53
B 0.50
c 0.60
Chemical Composition (Unit: mass%, Balance: Fe and Impurities)
Si Mn Cr Mo v Nb sol.Al N
0.27 0.43 0.52 0.68 0.088 0.031 0.029 0.0038
0.26 0.43 0.51 1.57 0.090 0.033 0.033 0.0051
0.29 0.43 0.52 0.71 0.090 0.030 0.039 0.0034
23
[0090]
[Table 1B]
TABLE 1B (Continued from TABLE 1A)
Steel
Chemical Composition (Unit: mass%, Balance: Fe and Impurities)
Ti Ca p s 0 Ni Cu B
A 0.006 - 0.007 0.0010 0.0009 0.01 0.01 0.0002
B 0.005 - 0.006 0.0005 0.0009 0.02 0.03 0.0001
c 0.005 - 0.007 0.0005 0.0008 0.04 0.01 0.0001
D 0.009 0.0018 0.012 0.0014 0.0007 0.03 0.01 0.0001
E 0.008 0.0020 0.005 0.0015 0.0010 0.01 0.01 0.0012
[0091]
With reference to Table 1A and Table 1B, all the chemical compositions of
Steel A to Steel D were within the range ofthe present invention. The C content of
Steel E was excessively low, further, the B content of SteelE was excessively high.
[0092]
Molten steel was continuously casted into blooms. The blooms 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 of244.48 mm and a wall thickness of 13.84 mm through the Mannesmannmandrel
process.
[0093]
Each seamless steel pipe was subjected to the normalizing treatment. The
normalizing temperature for each pipe was 920°C, and the soaking time at the
normalizing temperature for each pipe was 15 minutes. Each of the seamless steel
pipes after the normalizing treatment was cooled down to a room temperature (24°C).
[0094]
Each of the seamless steel pipes cooled down to the room temperature was
subjected to the quenching. The quenching temperature for each pipe was 900°C.
Each of the seamless steel pipes was soaked at the quenching temperature for 15
minutes. After the soaking, each seamless steel pipe was subjected to mist cooling.
24
During the mist cooling, an average cooling rate in a temperature range of 500 to
1 00°C of each seamless steel pipe was 5°C/second. The cooling stop temperature at
the time of quenching was 50°C or less.
[0095]
Each of the seamless steel pipes after being quenched was subjected to the
tempering as shown in Table 2.
[0096]
[Table 2]
25
TABLE2
Test
Steel
First-stage Tempering Second-stage Tempering
No. TL(OC) k(min) LMPL TH(0C) tH(min) LMPH
I A 600 120 17732 695 60 19382
2 A 600 120 17732 700 60 19483
3 A 600 120 17732 705 60 19585
4 A 600 120 17732 710 60 19687
5 B 600 120 17732 700 80 19599
6 B 600 120 17732 700 45 19369
7 B 600 120 17732 710 45 19573
8 c Low Heating Rate 17743 710 45 19633
9 c 600 120 17732 700 60 19483
10 c 600 120 17732 700 80 19599
11 c 600 120 17732 700 45 19369
12 D 600 180 17916 715 90 19954
13 0 Low Heating Rate 17743 710 45 19633
14 A 690 60 19282 - - -
15 A 695 60 19382 - - -
16 A 700 60 19483 - - -
17 A 705 60 19585 - - -
18 B 700 45 19369 - - -
19 c 700 45 19369 - - -
20 c 700 30 19213 - - -
21 0 705 40 19425 - - -
22 E 600 120 17732 700 60 19483
[0097]
With reference to Table 2, in Test No. 1 to 7 Test No. 9 to 12, and Test 22
two-stage tempering was carried out. Specifically, in each Test No., first, the low-
26
Note
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Inventive
Example
Comparative
Example
Comparative
Example
Comparative
Example
Comparative
Example
Comparative
Example
Comparative
Example
Comparative
Example
Comparative
Example
Comparative
Example
temperature tempering was carried out under tempering conditions (T L, tL, LMPL) as
shown in Table 2. Reference Numeral tL in Table 2 denotes an actual soaking time
(minutes) at the tempering temperature TL. 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 (25°C). Using the seamless steel pipe after the
allowing cooling, the high-temperature tempering was carried out under tempering
conditions (TH, tH, LMPH) as shown in Table 2. Reference Numeral tHin Table 2
denotes an actual soaking time (minutes) at the tempering temperature TH. In each
Test No., the heating rate in the heating process was 8°C/minute, and the temperature
of each seamless steel pipe was continuously increased. Taking account of the
heating process for each Test No., the LMPL and the LMPH were respectively
calculated in the above manner. In calculation of the LMPL and the LMPH, L1t was
set to 1160 hour (1 minute). Except Test No.8 and Test No.13, T 1 (the average
temperature of the first section) was set to the temperature 1 oooc lower than the
soaking temperature. The results are shown in Table 2.
[0098]
In Test No.8 and Test No. 13, the temperature of each seamless steel pipe was
continuously increased at a heating rate of2°C/minute until the tempering
temperature reached 71 0°C, and after the tempering temperature reached 71 0°C,
each steel pipe was soaked at 71 ooc for the corresponding time tH as shown in Table
2. Specifically, in Test No. 8 and Test No. 13, tempering at a low heating rate was
carried out. In the tempering at a low heating rate, each LMPL in a tempering
temperature range of 600 to 650°C was as shown in Table 2. Each total LMPH of
the LMP where the tempering temperature was increased from 670 to 71 ooc and the
LMP where each pipe was soaked at 71 ooc fortH minutes was as shown in Table 2.
[0099]
Each LMPL and each LMPH in the continuous temperature increasing of Test
No. 8 and Test No. 13 were calculated by calculating respective integrated tempering
parameters in accordance with Non-Patent Literature 1 in the same manner as the
above.
[01 00]
27
In each of Test No. 14 to Test No. 21, only one-stage tempering (hightemperature
tempering) was carried out.
[0101]
[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 112E was found. Each obtained prior-y
grain size No. is shown in Table 3. Each prior-y grain size No. was 9.0 or more.
[0 I 02]
[Microstructure Observation Test]
A sample including a central portion of wall thickness 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 using natal. Specifically each sample surface was immersed into the nita!
etching reagent (a mixture of 3% of nitric acid and 97% of ethyl alcohol) for 10
seconds at ordinary temperature and was etched. 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 than 2%.
[0103]
[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 (particles/1 00 J-tm2) was found in the above
described manner. Each obtained number of coarse cementite particles CN was
shown in Table 3.
[01 04]
[Yield Strength Test]
A No. 12test specimen (width: 25mm, gage length: 50mm) specified in JIS
Z2241 (2011) was 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
28
with the longitudinal direction of each seamless steel pipe. Using each collected
test specimen, a tensile test conforming to JIS Z2241 (2011) was carried out in the
atmosphere at a normal temperature (24°C) so as to find a yield stress (YS). The
yield stress was found by the 0.7% total elongation method. Each obtained yield
stress (MPa) was shown in Table 3. In each Test No., the yield stress of the
seamless steel pipe was 827 MPa or more. In addition, the steel pipes each having a
yield strength of 125 ksi grade (862 to 925 MPa) were obtained.
[01 05]
[DCB Test]
The seamless steel pipe of each Test No. was subjected to a DCB (double
cantilever beam) test so as to evaluate the sse resistance.
[01 06]
Specifically, three DCB test specimens each of which had a thickness of I 0
mm, a width of 25 mm, and a length of I 00 mm were collected from each seamless
steel pipe. A wedge having a thickness of 2.89 mm was driven into a central
portion of wall thickness of each collected DCB test specimen, and this was defined
as an initial crack. A length from a load point to a front end of the initial crack was
approximately 33.75 mm. Using these test specimens, the DCB test was carried out
in compliance with NACE (National Association of Corrosion Engineers) TM0177-
2005 Method D. A 5% salt+ 0.5% 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.
[0107]
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 K1ssc(ksi.Yin) was found
based on the following Formula (5).
K1ssc = Pa((2(.Y3) + 2.38 x (h/a)) x (B/Bn) 11<"3l)f(B x h312) (5)
[01 08]
In Formula (5), "h" denotes a height of each arm of each DCB test specimen,
"B" denotes a thickness of each DCB test specimen, and "Bn" denotes a web
29
thickness of each DCB test specimen. These are specified in the above NACE
TM0177-2005 Method D.
[0109]
An average value of the stress intensity factors of the three DCB test
specimens in each Test No. was defined as a stress intensity factor K1ssc of Test No.
of interest. Furthermore, a standard deviation of the stress intensity factors of the
three DCB test specimens was also found.
[0 11 0]
[Test Results]
[0111]
[Table 3]
30
TABLE 3
Test
No.
I
2
3
4
5
6
7
8
9
10
II
12
13
14
15
16
17
18
19
20
21
22
Prior-y CN YS K1ssc Average Value
K1ssc Standard
Deviation
Steel Grain (grains/
(MPa
Note
Size No. 100 ~tm2 ) )
(ksi) (MPa,Jm) (ksi'-'inch) (MPa,Jm) (ksi,Jinch)
A 9.5 205 917 133 24.0 21.8 0.3 0.3
Inventive
Example
A 9.5 220 883 128.1 24.5 22.3 0.8 0.7
Inventive
Example
A 9.7 225 862 125 25.6 23.3 1.6 1.5
Inventive
Example
A 9.6 240 843 122.2 27.4 24.9 1.1 1.0
Inventive
Example
B 10 210 852 123.6 26.9 24.5 0.8 0.8
Inventive
Example
B 10 250 877 127.2 25.8 23.4 0.6 0.5
Inventive
Example
B 10 300 896 130 24.8 22.5 0.7 0.6
Inventive
Example
c II. I 260 838 121.5 26.3 24.0 0.5 0.5
Inventive
Example
c II. I 245 844 122.4 25.5 23.2 0.5 0.5
Inventive
Example
c 11.1 320 856 124.2 25.5 23.2 0.2 0.2
Inventive
Example
c 11.1 230 876 127.1 24.9 22.7 1.2 1.1
Inventive
Example
D 10.5 230 831 120.5 26.0 23.7 1.1 1.0
Inventive
Example
D 10.5 205 874 126.8 24.8 22.6 0.8 0.8
Inventive
Example
A 9.5 120 925 134.2 20.0 18.2 2.2 2.0
Comparative
Example
A 9.8 130 896 130 19.6 17.8 2.2 2.0
Comparative
Example
A 9.7 140 872 126.5 21.0 19.1 2.3 2.1
Comparative
Example
A 9.5 160 862 125 22.6 20.5 2.2 2.0
Comparative
Example
B 10 160 896 130 21.4 19.4 2.5 2.3
Comparative
Example
c 11.1 190 872 126.5 21.1 19.2 2.4 2.1
Comparative
Example
c II. I 175 896 130 20.7 18.9 2.3 2.1
Comparative
Example
D I 0.5 140 878 127.3 19.8 18.0 2.3 2.1
Comparative
Example
E 9.6 35 793 115 22.5 20.5 - - Comparative
Example
[0 112]
With reference to Table 3, each of Test No. I to Test No. 7 and Test No. 9 to
Test No. 12 had an appropriate chemical composition. In the tempering, the two-
31
stage tempering (the low-temperature tempering and the high-temperature
tempering) was carried out, and each tempering condition was appropriate. Each
seamless steel pipe had a grain size No. of the prior-y grains of 9.0 or more, and the
number of coarse cementite particles CN of 200 particles/1 00 ).!m2 or more. Hence,
each K1 sse was greater than 22.6 MPam0
·
5
, and an excellent SSC resistance was
obtained. In addition, the standard deviation of each K1ssc was 2.0 MPam05 or less,
so that a stable sse resistance could be attained.
[0113]
Each of Test No.8 and Test No. 13 had an appropriate chemical composition.
The low-heating rate tempering was carried out, and each condition thereof was
appropriate. Each seamless steel pipe had a grain size No. of the prior-y grains of
9.0 or more, and the number of coarse cementite particles CN of 200 particles/1 00
).!m2 or more. Each K1ssc was greater than 22.6 MPam05, and an excellent SSC
resistance was obtained. In addition, the standard deviation of each K1ssc was 0.8
MPam0
·
5 or less, so that a stable SSC resistance could be attained.
[0 114]
Meanwhile, in each of Test No. 14 to Test No. 21, no low-temperature
tempering was carried out. Consequently, in each Test No., the number of coarse
cementite particles CN was less than 200 particles/1 00 ).!m2• As a result, each K1ssc
was 22.6 MPam0
·
5 or less, so that the SSC resistance was small. The standard
deviation of each KJSSC was greater than 2.0 MPam05, so that no stable sse
resistance could be attained.
[0 115]
A chemical composition of Test No. 22 had an excessively low C content and
an excessively high B content. Therefore, although the condition of tempering was
appropriate, the number of coarse cementite particles CN was less than 200
particles/1 00 ).!m2
• As a result, K1ssc was 22.6 MPam05 or less, so that the SSC
resistance was small.
[0 116]
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
limited 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:
1. A low alloy oil-well steel pipe comprising a chemical composition consisting
of:
in mass%,
C: more than 0.35 to 0.65%;
Si: 0.05 to 0.50%;
Mn: 0.10 to 1.00%;
Cr: 0.40 to 1.50%;
Mo: 0.50 to 2.00%;
V: 0.05 to 0.25%;
Nb: 0.01 to 0.04%;
sol.Al: 0.005 to 0.1 0%;
N: 0.007% or less;
Ti: 0 to 0.012%;
Ca: 0 to 0.005%; and
a balance being Fe and impurities,
the impurities including:
P: 0.020% or less;
S: 0.002% or less;
0: 0.006% or less;
Ni: 0.10% or less;
Cu: 0.03% or less; and
B: 0.0005% or less,
wherein
in a microstructure, a number of cementite particles each of which has an
equivalent circle diameter of 200 nm or more is 200 particles/1 00 ~-tm2 or more
and
a yield strength is 827 MPa or more.
2. The low alloy oil-well steel pipe according to claim 1, wherein
the chemical composition contains Ti: 0.003 to 0.012%.
34
3. The low alloy oil-well steel pipe according to claim 1 or claim 2, wherein
the chemical composition contains Ca: 0.0005 to 0.005%.