DESCRIPTION
TITLE OF INVENTION
LOW ALLOY OIL WELL STEEL PIPE AND METHOD FOR
MANUFACTURING SAME
TECHNICAL FIELD
[0001]
The present invention relates to an oil well steel pipe and a method for
manufacturing the same, and more specifically relates to a low alloy oil well steel
pipe used for, e.g., a casing or a tubing for an oil well or a gas well and a method for
manufacturing the same.
BACKGROUND ART
[0002]
With increase in depth of oil wells and gas wells (hereinafter, the oil wells and
the gas wells are collectively and simply referred to as "oil wells"), there is a need for
increase in strength of oil well steel pipes. Conventionally, 80 ksi grade (yield
stress of80 to 95 ksi, that is, 551 to 654 MPa) and 95 ksi grade (yield stress of95 to
110 ksi, that is, 654 to 758 MPa) oil-well steel pipe have widely been used.
However, recently, 110 ksi grade (yield stress of 110 to 125 ksi, that is, 758 to 862
MPa) oil well steel pipes are put into use.
[0003]
Many of the recently-developed deep wells contain hydrogen sulfide, which is
corrosive. Thus, oil well steel pipes are required to have not only high strength but
also sulfide stress cracking resistance (hereinafter referred to as SSC resistance).
[0004]
As a measure to improve the SSC resistance of a conventional 95 to II 0 ksi
grade oil well steel pipe, known methods include cleaning the steel and making the
steel structure finer. For example, Japanese Patent Application Publication No. 62-
253720 proposes the method for improving the sse resistance by reducing impurity
elements such as Mn and P. Japanese Patent Application Publication No. 59-
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232220 proposes the method for improving the sse resistance by performing
quenching twice for grain refining.
[0005]
In response to the need for increase in strength of the oil well steel pipes,
recently, 125 ksi grade (yield stress of 862 to 965 MPa) oil well steel pipes are put
into use. Furthermore, there is an increasing need for oil well steel pipes having
yield strength of 140 ksi (yield stress of 965 MPa) or more.
[0006]
Sulfide stress cracking (SSe) is more liable to occur as the strength is higher.
Therefore, oil well steel pipes of 140 ksi or more need to have further improved sse
resistance, compared to the conventional oil-well steel pipes of the 95 ksi class, the
11 0 ksi grade and the 125 ksi grade.
[0007]
Japanese Patent Application Publication Nos. 6-322478, 8-311551, 11-
335731, 2000-178682, 2000-256783, 2000-297344, 2000-119798, 2005-350754 and
2006-265657 each propose a measure to improve the sse resistance of a steel for a
high-strength oil well pipe.
[0008]
Japanese Patent Application Publication No. 6-322478 proposes the method
for improving the sse resistance of a 125 ksi grade steel product by providing a
finer steel structure by heat treatment using induction heating. Japanese Patent
Application Publication No. 8-311551 proposes the method for improving the SSe
resistance of a steel pipe of the 110 to 140 ksi grades by enhancing the hardenability
using the direct quenching process and increasing the tempering temperature.
Japanese Patent Application Publication No. 11-335731 proposes the method for
improving the sse resistance of a low alloy steel of the 110 to 140 ksi grades by
making adjustment to provide optimum alloy chemical composition. Japanese
Patent Application Publication Nos. 2000-178682,2000-256783 and 2000-297344
each propose the method for improving the sse resistance of a steel for a low alloy
oil well pipe of the II 0 to 140 ksi grades by controlling the shapes of carbides.
Japanese Patent Application Publication No. 2000-119798 proposes the method for
delaying a time of occurrence of sse in a steel product of the II 0 to 125 ksi grades
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by precipitation of a large amount of fine V carbides. Japanese Patent Application
Publication No. 2005-350754 proposes the method for improving the SSC resistance
of an oil well steel pipe of the 125 ksi grade or more by controlling the dislocation
density and the hydrogen diffusion coefficient to desired values. Japanese Patent
Application Publication No. 2006-265657 proposes the method for improving the
SSC resistance of a steel for an oil well pipe of the 125 ksi grade or more by
production of a single-phase bainite structure, which is provided by containing a
large C content in the steel, and during water cooling, stopping the water cooling at
400 to 600°C and performing isothermal transformation heat treatment ( austemper
treatment) at 400 to 600°C.
PRIOR ART DOCUMENTS
PATENT DOCUMENTS
[0009]
Patent Document I: Japanese Patent Application Publication No. 62-253720
Patent Document 2: Japanese Patent Application Publication No. 59-232220
Patent Document 3: Japanese Patent Application Publication No. 6-322478
Patent Document 4: Japanese Patent Application Publication No. 8-31155 I
Patent Document 5: Japanese Patent Application Publication No. 11-33573 I
Patent Document 6: Japanese Patent Application Publication No. 2000-178682
Patent Document 7: Japanese Patent Application Publication No. 2000-256783
Patent Document 8: Japanese Patent Application Publication No. 2000-297344
Patent Document 9: Japanese Patent Application Publication No. 2000-119798
Patent Document 10: Japanese Patent Application Publication No. 2005-350754
Patent Document I I: Japanese Patent Application Publication No. 2006-265657
SUMMARY OF INVENTION
[0010]
However, any of the techniques disclosed in the patent documents may fail to
provide an oil-well steel pipe that has a yield strength of I 40 ksi or more and stably
exhibits excellent sse resistance.
[OOII]
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It is an object of the present invention to provide a low-alloy oil-well steel
pipe that has a yield stress of965 MPa (140 ksi) or more and stably exhibits
excellent sse resistance.
[0012]
The low-alloy oil-well steel pipe according to the present invention includes a
chemical composition consisting, in mass%, of C: 0.40 to 0.65%, Si: 0.05 to 0.50%,
Mn: 0.10 to 1.00%, P: 0.020% or less, S: 0.0020% or less, Cu: 0.15% or less, Cr:
0.40 to 1.50%, Mo: 0.50 to 2.50%, V: 0.05 to 0.25%, Ti: 0 to less than 0.01 %, Nb:
0.01 to 0.2%, sol.Al: 0.010 to 0.100%, N: 0.006% or less, B: 0 to 0.0015%, and Ca: 0
to 0.003%, the balance being Fe and impurities, and a structure consisting of
tempered martensite and 0 to less than 2% in volume ratio of retained austenite. A
grain size number of a prior-austenite grain in the structure is 9.0 or more, and in the
tempered martensite, an equivalent circular diameter of a sub-structure surrounded
by a boundary having a crystal orientation difference of !5° or more from among a
packet boundary, a block boundary and a lath boundary is 3 J.lm or less.
[0013]
The method for manufacturing a low-alloy oil-well steel pipeaccording to the
present invention includes a hot working step of hot-working a starting material
having the above-described chemical composition to form a hollow shell, a final
quenching step of subjecting the hollow shell to quenching in which a cooling rate
when a temperature of the hollow shell is between 500°C and 100°C is set to 1 °C/s to
less than l5°C/s and the temperature of the hollow shell at which cooling is stopped
is set to l00°C or less, and a step of tempering the quenched hollow shell.
[0014]
The low-alloy oil-well steel pipe according to the present invention has yield
stress of965 MPa (140 ksi) or more and stably exhibits excellent SSC resistance.
BRIEF DESCRIPTION OF DRAWINGS
[0015]
[FIG. I] FIG. 1 illustrates a prior-austenite grain boundary map of a structure whose
sub-structures have a grain diameter of2.6 J.lm.
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[FIG. 2] FIG. 2 illustrates a high-angle grain boundary map of a structure whose substructures
have a grain diameter of 2.6 11m.
[FIG. 3] FIG. 3 illustrates a prior-austenite grain boundary map of a structure whose
sub-structures have a grain diameter of 4.1 11m.
[FIG. 4] FIG. 4 is a high-angle grain boundary map of a structure whose substructures
have a grain diameter of 4.1 llill·
DESCRIPTION OF EMBODIMENTS
[0016]
An embodiment of the present invention will be described in detail below.
In the present description, "%" means "mass%" unless otherwise noted.
[0017]
The present inventors researched and studied the SSC resistances of low-alloy
oil-well steel pipes having yield strength of 965 MPa or more and obtained the
following findings.
[0018]
(I) In order to provide a high-strength low-alloy oil-well steel pipe that stably
exhibits excellent SSC resistance, it is effective to use a steel having a high C content
of0.40% or more. Reasons of this are indicated below.
[0019]
Generally, a low-alloy oil-well steel pipe is adjusted to have a metal structure
that mainly includes tempered martensite, by quenching and tempering after hot
rolling. Conventionally, it is believed that as the carbides are more spheroidized in
the tempering process, the SSC resistance is more improved. The precipitating
carbides mainly include cementite, and the rest of the carbides include alloy carbides
(Mo carbides, Vcarbides and Nb carbides, and Ti carbides ifTi is contained). If
carbides precipitate on a grain boundary, as the carbides are flatter, SSC is more
liable to occur with these carbides as the starting point. In other words, as the
carbides are closer to a spherical shape, the carbides on the grain boundary become
less likely to cause SSC, and thus the SSC resistance is enhanced. Therefore, in
order to improve the sse resistance, spheroidizing of carbides, in particular,
cementite is desirable.
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[0020]
It is known that carbides can be spheroidized by tempering at a maximum
possible temperature. However, high-temperature tempering decreases dislocation
density of the inside of the steel pipe. The dislocation density decrease itself is
effective for SSC resistance enhancement. However, in general, it may cause a
decrease in strength. A dislocation serves as a trap site for hydrogen, and thus, if
the dislocation density is high, it is difficult to stably provide excellent sse
resistance. Containing alloy elements that enhance the temper softening resistance
such as Mo and Vis effectively for suppression of the strength decrease caused by
high-temperature tempering. However, such effect has a limit.
[0021]
If 0.40% or more of C is contained, spheroidizing of carbides, in particular,
cementite, is accelerated, and furthermore, many carbides precipitate in the steel.
Thus, the steel can be reinforced by precipitation strengthening, independent of the
dislocation density. In other words, tempering a low-alloy steel having a high C
content under proper tempering conditions enables expectation of ensuring of
strength by precipitation strengthening resulting from precipitation of many carbides
and ensuring of SSC resistance resulting from spheroidizing of the carbides.
[0022]
According to the above, if a C content is 0.40% or more, a large amount of
carbides precipitate and disperse, and thus, the strength of the steel can further be
enhanced by precipitation strengthening, independent of the dislocation density.
Furthermore, if a C content is 0.40% or more, the concentration of the alloy elements
in the cementite decreases and the cementite is spheroidized. Thus, the SSC
resistance is stabilized while a high strength is achieved. Furthermore, if a C
content is 0.40% or more, the volume ratio of martensite in the structure increases.
As the volume ratio of martensite is higher, the dislocation density after tempering
more decreases, and thus the sse resistance is also stabilized.
[0023]
In order to achieve yield strength of 965 MPa or more, it is preferable that the
structure substantially consist of single-phase tempered martensite; and the volume
ratio of retained austenite to the entire structure (hereinafter referred to as the volume
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ratio of retained austenite)(%) be as low as possible. If the structure contains any
phase other than tempered martensite (e.g., bainite), the aforementioned high
strength cannot be achieved. Also, if the volume ratio of retained austenite is high,
the strength varies. Accordingly, the structure of a low-alloy oil-well steel pipe
needs to be a structure consisting of tempered martensite and 0 to less than 2% in
volume ratio of retained austenite.
[0024]
(2) In order to provide a low-alloy oil-well steel pipe that has a high strength of965
MPa or more and stably exhibits excellent sse resistance, the tempered martensite
structure is made finer. Tempered martensite contains a plurality of prior-austenite
grains, a plurality of packets, a plurality of blocks and a plurality oflaths. More
specifically, tempered martensite contains a plurality of prior-austenite grains, and
each prior-austenite grain includes a plurality of packets. Each packet includes a
plurality of plate-like blocks, and each block includes a plurality oflaths.
[0025]
From among the regions of tempered martensite defined by the boundaries
such as the packet boundaries, the block boundaries and the lath boundaries, a region
surrounded by a high-angle grain boundary is defined as "sub-structure."
Furthermore, from among the aforementioned respective boundaries (the packet
boundaries, the block boundaries and the lath boundaries), boundaries having a
crystal orientation difference of 15° or more are defined as "high-angle grain
boundaries."
[0026]
As the prior-austenite grains and the sub-structures of a low-alloy oil-well
steel pipe having yield strength of965 MPa or more are finer, excellent sse
resistance can stably be provided. More specifically, if the grain size number of the
prior-austenite grains that conforms to ASTM E112 is 9.0 or more and the equivalent
circular diameter of the sub-structures is 3 !!ill or less, a low-alloy oil-well steel pipe
that has high strength of965 MPa or more and stably exhibits excellent sse
resistance can be provided.
[0027]
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(3) In order to make the equivalent circular diameter of the sub-structures be 3 J-tm or
less, preferably, tbe final quenching conditions during the manufacturing process are
controlled. More specifically, in tbe final quenching, tbe cooling rate for a hollow
shell temperature range of 500 to I 00°C is set to I °C/s or more, and the hollow shell
temperature at which the cooling is stopped (hereinafter referred to as cooling stop
temperature) is set to 100°C or less.
[0028]
( 4) Before the final quenching, intermediate heat treatment may be performed.
More specifically, the hollow shell subjected to hot rolling is soaked at a point tbat is
an A1 point or more (an Aci point or an Ari point). In this case, since austenite is
produced in the structure, the prior-austenite grains are further refined, providing
excellent sse resistance.
[0029]
Based on the above findings, the present inventors have completed tbe
following invention.
[0030]
The low-alloy oil-well steel pipe according to the present invention includes a
chemical composition consisting, in mass%, of C: 0.40 to 0.65%, Si: 0.05 to 0.50%,
Mn: 0.10 to 1.00%, P: 0.020% or less, S: 0.0020% or less, Cu: 0.15% or less, Cr:
0.40 to 1.50%, Mo: 0.50 to 2.50%, V: 0.05 to 0.25%, Ti: 0 to less than 0.0 I%, Nb:
0.01 to 0.2%, soLA!: 0.010 to 0.100%, N: 0.006% or less, B: 0 to 0.0015%, and Ca: 0
to 0.003%, the balance being Fe and impurities, and a structure consisting of
tempered martensite and 0 to less than 2% in volume ratio of retained austenite, and
a grain size number of a prior-austenite grain in the structure is 9.0 or more, and in
the tempered martensite, an equivalent circular diameter of a sub-structure
surrounded by a boundary having a crystal orientation difference of 15° or more from
among a packet boundary, a block boundary and a lath boundary is 3 J-tm or less.
[0031]
The method for manufacturing the low-alloy oil-well steel pipe according to
the present invention includes a hot working step of hot-working a starting material
having the above-described chemical composition to form a hollow shell, a final
quenching step of subjecting tbe hollow shell to quenching in which a cooling rate
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for the hollow shell between 500°C and 1 oooc is set to 1 °C/s to less than l5°C/s and
the temperature of the hollow shell at which cooling is stopped is set to 1 00°C or
less; and a step of tempering the quenched hollow shell.
[0032]
The above manufacturing method may further include an intermediate heat
treatment step of soaking the hollow shell at a temperature of an A1 point or more
after the hot working step and before the final quenching step.
[0033]
The low-alloy oil-well steel pipe according to the present invention and the
method for manufacturing the same will be described in detail below.
[0034]
[Chemical composition]
The low-alloy oil-well steel pipe according to the present invention has the
following chemical composition.
[0035]
C: 0.40 to 0.65%
The low-alloy oil-well steel pipe according to the present invention has a
carbon (C) content that is larger than those of the conventional low-alloy oil-well
steel pipes. As a result of a large amount of C being contained, a large amount of
fine carbides disperse in the steel and the strength of the steel is thereby enhanced.
Examples of carbides include cementite and alloy carbides (e.g., Mo carbides, V
carbides, Nb carbides and Ti carbides). Furthermore, the sub-structures are made
finer and the SSC resistance thereby increases. If the C content is too low, the
aforementioned effect cannot be obtained. On the other hand, if the C content is too
high, the toughness of the steel in as-quenched condition decreases, resulting in an
increase in quench cracking susceptibility. Therefore, the C content is 0.40 to
0.65%. The lower limit of the C content is preferably 0.50%, more preferably more
than 0.50%, still more preferably 0.55%. The upper limit of the C content is
preferably 0.62%, more preferably 0.60%.
[0036]
Si: 0.05 to 0.50%
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Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot
be achieved. On the other hand, if the Si content is too high, the SSC resistance
decreases. Therefore, the Si content is 0.05 to 0.50%. The lower limit of the Si
content is preferably 0.1 0%, more preferably 0.20%. The upper limit of the Si
content is preferably 0.40%, more preferably 0.35%.
[0037]
Mn: 0.10 to 1.00%
Manganese (Mn) deoxidizes steel. If the Mn content is too low, this effect
cannot be achieved. On the other hand, if the Mn content is too high, Manganese is
segregated on the grain boundaries together with impurity elements such as
phosphorus (P) and sulfur (S). As a result, the SSC resistance of the steel decreases.
Therefore, the Mn content is 0.10 to 1.00%. The lower limit of the Mn content is
preferably 0.20%, more preferably 0.28%. The upper limit of the Mn content is
preferably 0.80%, more preferably 0.50%.
[0038]
P: 0.020% or less
Phosphorus (P) is an impurity. Phosphorus is segregated on the grain
boundaries and thereby decreases the SSC resistance of the steel. Thus, the P
content is preferably as low as possible. Therefore, the P content is 0.020% or less.
The P content is preferably 0.015% or less, more preferably 0.012% or less.
[0039]
S: 0.0020% or less
Sulfur (S) is an impurity as well as phosphorus. Sulfur is segregated on the
grain boundaries and thereby decreases the SSC resistance of the steel. Thus, the S
content is preferably as low as possible. Therefore, the S content is 0.0020% or less.
The S content is preferably 0.0015% or less, more preferably 0.00 I 0% or less.
[0040]
Cu: 0.15% or less
Copper (Cu) is an impurity. Copper embrittles steel and thereby decreases
the SSC resistance of the steel. Thus, the Cu content is preferably as low as
possible. Therefore, the Cu content is 0.15% or less. The upper limit of the Cu
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content is preferably less than 0.03%, more preferably 0.02%, still more preferably
O.oi%.
[0041]
Cr: 0.40 to 1.50%
Chromium (Cr) enhances the hardenability of steel and the strength of the
steel. On the other hand, if an excessive amount of Cr is contained, the toughness
of the steel decreases and the SSC resistance of the steel decreases. Therefore, the
Cr content is 0.40 to 1.50%. The lower limit of the Cr content is preferably 0.45%.
The upper limit of the Cr content is preferably 1.30%, more preferably 1.00%.
[0042]
Mo: 0.50 to 2.50%
Molybdenum (Mo), as described above, forms carbides and thereby enhances
the temper softening resistance, and consequently contributes to enhancement of the
SSC resistance by high-temperature tempering. If the Mo content is too low, this
effect cannot be achieved. On the other hand, the Mo content is too high, the above
effect is saturated. Therefore, the Mo content is 0.50 to 2.50%. The lower limit of
the Mo content is preferably 0.60%, more preferably 0.65%. The upper limit of the
Mo content is 2.0%, more preferably 1.6%.
[0043]
V: 0.05 to 0.25%
Vanadium (V), as described above, forms carbides and thereby enhances the
temper softening resistance, and consequently contributes to enhancement of the SSC
resistance by high-temperature tempering. If the V content is too low, this effect
cannot be achieved. On the other hand, if the V content is too high, the toughness
of the steel decreases. Th~refore, the V content is 0.05 to 0.25%. The lower limit
of the V content is preferably 0.07%. The upper limit of the V content is preferably
0.15%, more preferably 0.12%.
[0044]
Nb: O.oi to 0.2%
Niobium (Nb) combines with C and/or N to form carbides, nitrides or a carbonitrides.
These precipitates (the carbides, the nitrides and the carbo-nitrides) subject
the sub-structures of a steel to grain refinement by the pinning effect and thereby
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enhance the SSe resistance of the steeL If the Nb content is too low, this effect
cannot be achieved. On the other hand, if the Nb content is too high, excess nitrides
are produced and thereby destabilize the sse resistance of the steel. Therefore, the
Nb content is 0.01 to 0.2%. The lower limit of the Nb content is preferably 0.012%,
more preferably 0.015%. The upper limit of the Nb content is preferably 0.10%,
more preferably 0.05%.
[0045]
sol.Al: 0.010 to 0.100%
Aluminum (AI) deoxidizes steel. If the AI content is too low, deoxidation of
the steel is insufficient, resulting in a decrease in sse resistance of the steel. On
the other hand, if the AI content is too high, oxides are produced, resulting in a
decrease in SSe resistance of the steel. Therefore, the AI content is 0.010 to
0.100%. The lower limit of the AI content is preferably 0.015%, more preferably
0.020%. The upper limit of the AI content is preferably 0.080%, more preferably
0.050%. In the present description, the content of" AI" means the content of "acidsoluble
AI," that is, "soLA!."
[0046]
N: 0.006% or less
Nitrogen (N) is an impurity. Nitrogen forms a nitride and thereby
destabilizes the SSe resistance of the steeL Thus, theN content is preferably as low
as possible. Therefore, theN content is 0.006% or less. TheN content is
preferably 0.005% or less, more preferably 0.004% or less.
[0047]
The balance of the chemical composition of the low-alloy oil-well steel pipe
is Fe and impurities. The impurities referred to herein are elements that are mixed
in from ore and scrap used as steel raw materials or from, e.g., the environment in the
manufacturing process.
[0048]
[Regarding optional elements]
The low-alloy oil-well steel pipe may further contain Ti instead of a part ofF e.
[0049]
Ti: 0 to less than 0.01%
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Titanium (Ti) is an optional element. Ti forms nitrides and thereby subjects
the steel to grain refinement. Ti further suppresses surface cracking of a cast piece
that are occasionally generated during continuous casting. During continuous
casting, dissolved N combines with AI in a cast piece, AI nitrides precipitate due to
strain induction. In this case, the surface of the cast piece becomes liable to crack.
Ti preferentially combines with dissolved N to form Ti nitrides, and thereby suppress
precipitation of AI nitrides. Thus, the surface cracking of the cast piece during the
continuous casting is suppressed. Even if only a small amount ofTi is contained,
these effects can be achieved. However, if the Ti content is too high, TiN coarsens
and thereby destabilizes the SSC resistance of the steel. Therefore, the Ti content is
0 to less than 0.0 I%. The lower limit of the Ti content is preferably 0.00 I%, more
preferably 0.003%. The upper limit of the Ti content is preferably 0.008%, more
preferably 0.006%.
[0050)
The low-alloy oil-well steel pipe may further contain B instead of a part of Fe.
[0051]
B: 0 to 0.0015%
Boron (B) is an optional element. Boron enhances the hardenability, and
enhances the strength of steel. Even if only a small amount ofB is contained, the
above effect can be achieved. However, if the B content is too high, M23CB6 is
formed on the grain boundaries, resulting in a decrease in sse resistance of the steel.
Thus, a low B content is preferable even if B is contained. Therefore, the B
content is 0 to 0.0015% or less. The lower limit of the B content is preferably
0.0003%, more preferably 0.0005%. The upper limit of the B content is preferably
0.0012%, more preferably 0.0010%.
[0052)
The low-alloy oil-well steel pipe may further contain Ca instead of a part of
Fe.
[0053)
Ca: 0 to 0.003%
Calcium (Ca) is an optional element. Calcium combines with S in a steel to
forin sulfides, and thereby improves the shapes of inclusions and enhances the
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toughness of the steel. Even if only a small amount ofCa is contained, the above
effect can be achieved. On the other hand, if the Ca content is too high, this effect
is saturated. Therefore, the Ca content is 0 to 0.003%. The lower limit of theCa
content is preferably 0.0005%, more preferably 0.0010%. The upper limit of theCa
content is preferably 0.0025%, more preferably 0.0020%.
[0054]
[Structure (microstructure)]
The structure of the low-alloy oil-well steel pipe according to the present
invention mainly includes tempered martensite. More specifically, the matrix in the
structure consists of tempered martensite and 0 to less than 2% in volume ratio of
retained austenite.
[0055]
For the low-alloy oil-well steel pipe according to the present invention, the
cooling rate in quenching is restricted from the perspective of quench cracking
prevention because of the relatively-high C content in the steel. Thus, retained
austenite may remain in the steel pipe after quenching. In the end product (state
after tempering), retained austenite causes large variation of strength. Therefore,
the volume ratio of retained austenite is preferably as low as possible. The volume
ratio of retained austenite is preferably less than 1%. The low-alloy oil-well steel
pipe according to the present invention has more preferably a structure consisting of
tempered martensite, the volume ratio of retained austenite being 0%.
[0056]
The volume ratio of retained austenite is measured as follows using X-ray
diffractometry. A sample including a center part in the wall thickness of a produced
oil-well steel pipe is collected. The surface of the collected sample is chemically
polished. The chemically-polished surface is subjected to X-ray diffraction using a
CoKa ray as an incident X ray. From the surface integrated intensities of the (211)
surface, the (200) surface and the (II 0) surface of ferrite and the surface integrated
intensities of the (220) surface, the (200) surface and the (Ill) surface of austenite,
the volume ratio of retained austenite is determined and obtained.
[0057]
[Grain size of prior-austenite grains J
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The grain size number of the prior-austenite grains in the above structure is
9.0 or more. The grain size number of the prior-austenite grains referred to in the
present description is measured in conformity of ASTM E112. If the grain size
number of the prior-austenite grains is 9.0 or more, the steel exhibits excellent sse
resistance even if the steel has a yield strength of965 MPa (140 ksi) or more. If the
grain size number of the prior-austenite grains is less than 9.0, the steel has low sse
resistance where the steel has the aforementioned strength. The grain size number
of the prior-austenite grains is preferably more than 9.0, more preferably 10.0 or
more.
[0058]
The grain size number of the prior-austenite grains may be measured using a
steel product after quenching and before tempering (what is called as-quenched
product) or may also be measured using a tempered steel product. Use of either of
these steel products provides no change in size of the prior-austenite grains.
[0059]
[Size of sub-structures]
As described above, tempered martensite contains a plurality of prioraustenite
grains, a plurality of packets, a plurality of blocks and a plurality of laths.
From among the regions of tempered martensite that are defined by the boundaries
that are packet boundaries, block boundaries and lath boundaries, the equivalent
circular diameter of a sub-structure surrounded by a boundary with a crystal
orientation difference of 15° or more (high-angle grain boundary) is 3 ~-tm or less.
[0060]
The sse resistance of a steel having a high strength of 965 MPa or more
depends not only on the prior-austenite grain size but also on the sub-structure size.
If the grain size number of the prior-austenite grains is 9.0 or more and the
equivalent circular diameter of the sub-structures is 3 ~-tm or less, a low-alloy oil-well
steel pipe that has a high strength of 965 MPa or more and stably exhibits excellent
sse resistance can be provided. The equivalent circular diameter of the substructures
is preferably 2.5 1-tffi or less, more preferably 2.0 ~-tm or less.
[0061]
16
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i
i
! ·:
The equivalent circular diameter of the sub-structures is measured by the
following method. A sample including an observation surface of 25 J.llil x 25 J.lm
with the center in the wall thickness as the center is collected from an arbitrary
transverse surface of a low-alloy oil-well steel pipe (a surface perpendicular to the
axial direction of the low-alloy oil-well steel pipe).
[0062]
The observation surface is subjected to crystal orientation analysis by electron
backscatter diffraction imaging (EBSP). Then, based on the results of the analysis,
boundaries having a crystal orientation difference of 15° or more are drawn on the
observation surface to identifY a sub-structure.
[0063]
The equivalent circular diameter of each of the specified sub-structures is
measured. The equivalent circular diameter means a diameter of a circle where the
area of the sub-structure is converted into a circle having the same area. The
equivalent circular diameter can be measured by, for example, employing method of
measuring the mean intercept length of crystal grains defined in liS G 0551 to obtain
mean grain size. The average of the obtained equivalent circular diameters of the
respective sub-structures is defined as the equivalent circular diameter of the substructures
referred to in the present description.
[0064]
FIGS. 1 and 2 each illustrate an example structure whose equivalent circular
diameter of the sub-structures is 2.6 J.lm. FIG. 1 is a prior-austenite grain boundary
map, and FIG. 2 is a high-angle grain boundary map. The structure is one obtained
from a steel having a prior-austenite grain size number of 10.5 and containing C:
0.51 %, Si: 0.31 %, Mn: 0.47%, P: 0.012%, S: 0.0014%, Cu: 0.02%, Cr: 1.06%, Mo:
0.67%, V: 0.098%, Ti: 0.008%, Nb: 0.012%, Ca: 0.0018%, B: 0.0001%, sol.AI:
0.029% and N: 0.0034%, the balance being Fe and impurities.
[0065]
FIGS. 3 and 4 each illustrate an example structure whose grain diameter of
the sub-structures is 4.1 J.lm. FIG. 3 is a prior-austenite grain boundary map, and
FIG. 4 is a high-angle grain boundary map. The structure is one obtained from a
steel having a prior-austenite grain size number of 11.5 and containing C: 0.26%, Si:
17
I
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1
0.19%, Mn: 0.82%, P: 0.013%, S: 0.0008%, eu: 0.01%, er: 0.52%, Mo: 0.70%, V:
0.11 %, Ti: 0.018%, Nb: 0.013%, ea: 0.0001%, B: 0.0001%, soLA!: 0.040% and N:
0.0041%, the balance being Fe and impurities.
[0066]
[Yield strength]
The low-alloy oil-well steel pipe according to the present invention has yield
strength of965 MPa or more (140 ksi or more). The yield strength is defined based
on the 0.2% yield stress. Although the upper limit of the yield strength is not
specifically limited, the upper limit of the yield strength is, for example, 1150 MPa.
[Yield ratio]
The low-alloy oil-well steel pipe according to the present invention, which
has the above-described chemical composition and structure, has yield ratio YR ( =
yield strength YS/tensile strength TS) of 0.90 or more. If the yield ratio YR is less
than 0.90, a phase other than tempered martensite (for example, bainite) is contained
in the structure. In this case, the SSe resistance decreases. The yield ratio YR of
the low-alloy oil-well steel pipe according to the present invention is 0.90 or more,
and the structure consists of the single phase of tempered martensite, or tempered
martensite and 0 to less than 2% of retained austenite. Thus, the low-alloy oil-well
steel pipe according to the present invention has high yield strength (965 MPa or
more) as well as excellent sse resistance.
[0067]
[Manufacturing method]
An example of the low-alloy oil-well steel pipe manufacturing method
according to the present invention will be described. In this example, a seamless
steel pipe (low-alloy oil-well steel pipe) manufacturing method will be described.
[0068]
A steel having the above-described chemical composition is melted and
refined by a well-known method. Subsequently, the molten steel is made into a
continuously-casted material by the continuous casting process. The continuouslycasted
material may be, for example, a slab, a bloom or a billet. Also, the molten
steel may be made into an ingot by the ingot-making process.
[0069]
18
The slab, the bloom or the ingot is hot-worked into a billet. The billet may
be formed by hot rolling or hot forging.
[0070]
The billet is hot-worked to produce a hollow shell. First, the billet is heated
in a heating furnace. The billet extracted from the heating furnace is subjected to
hot working to produce a hollow shell (seamless steel pipe). For example, the
Mannesmann process is performed as the hot working to produce a hollow shell. In
this case, a round billet is piercing-rolled using a piercing machine. The piercingrolled
round billet is further hot-rolled into a hollow shell using, e.g., a mandrel mill,
a reducer or a sizing mill. The hollow shell may be produced from the billet by
another hot working process.
[0071]
The hot-worked hollow shell is subjected to quenching by at least one
reheating, and tempering.
[0072]
A quenching temperature in quenching is a well-known temperature
(temperature of an Ac3 point or more). The upper limit of the quenching
temperature is preferably 900°C or less. In this case, the prior-austenite grains are
further made finer.
[0073]
Quenching may be performed once or a plurality oftimes. A hollow shell
temperature at which cooling during final quenching (that is, cooling stop
temperature) is stopped is 100°C or less. If the cooling stop temperature is higher
than I 00°C, the equivalent circular diameter of the sub-structures becomes larger
than 3 f.!m.
[0074]
Furthermore, in the final quenching, the cooling rate when the hollow shell
temperature is between 500°C and 100°C is set to I °C/s to less than I5°C/s. If the
cooling rate for the aforementioned temperature range is less than I °C/s, the
equivalent circular diameter of the sub-structures becomes larger than 3 f.!m.
Furthermore, in the structure, not only martensite but also bainite is produced. On
the other hand, if the cooling rate is I5°C/s or more, quench cracking is liable to
19
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I I I
I I
I 1 I I
occur. If the cooling rate when the hollow shell temperature is between 500°C and
l00°C is I °C/s to less than l5°C/s, the equivalent circular diameter of the substructures
becomes 3.0 J..Lm or less and quench cracking is less liable to occur. The
lower limit of the cooling rate is preferably 2°C/s, more preferably 3°C/s.
[0075]
After the final quenching is performed, tempering is performed at a tempering
temperature of less than the Ac1 point. The tempering temperature is arbitrarily
adjusted according to the chemical composition of the hollow shell and the intended
yield strength. The tempering temperature is preferably 650°C to 700°C, and the
soaking period for the tempering temperature is preferably 15 to 120 minutes. The
volume ratio of retained austenite can also be reduced by adjusting the tempering
temperature. As a result of the tempering, the yield strength of the hollow shell
becomes 965 MPa or more. Quenching and tempering may be performed a
plurality of times.
[0076]
The low-alloy oil-well steel pipe according to the present invention is
produced by the above-described manufacturing process.
[0077]
[Intermediate heat treatment process]
In the above-described manufacturing method, quenching is performed after
hot working. However, another heat treatment (intermediate heat treatment) may be
performed after hot working.
[0078]
For example, the hot-worked hollow shell may be subjected to normalizing.
More specifically, the hot-worked hollow shell is held at a temperature that is higher
than an A3 point (for example, 850°C to 950°C) for a given period of time and then
allowed to cool naturally. The holding period is, for example, 15 to 120 minutes.
For normalizing, generally, after hot working, the hollow shell is cooled to normal
temperature and then heated to the Ac3 point or more. However, in the present
invention, normalizing may be performed by holding the hollow shell as it is at a
temperature of the Ac3 point or more after hot working.
[0079]
20
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I l
! ' I -1 l
I
i
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I
If normalizing is performed, the grains (prior-austenite grains) of the steel are
further refined. More specifically, if the normalized hollow shell is quenched, the
grain size number of the prior-austenite grains of the as-quenched material becomes
I 0.0 or more.
[0080]
Also, instead of the above-described normalizing, quenching may be
performed. In this case, as described above, quenching is performed a plurality of
times.
[0081]
Also, the intermediate heat treatment provides an effect similar to the above
even if it is a heat treatment in a temperature range of two phases, ferrite+ austenite
(hereinafter referred to as "two-phase range heating"). In the intermediate heat
treatment, if at least a part of the structure of the steel is transformed to austenite, an
effect that is favorable for grain refining can be achieved. Accordingly, in the
intermediate heat treatment, it is preferable that at least the hollow shell be soaked at
a temperature of the Ac1 point or more.
[0082]
If the intermediate heat treatment is performed before the final quenching, the
prior-austenite grains are further refined and the sse resistance is further enhanced.
[0083]
As described above, the cooling·stop temperature in the final quenching is set
to 1 oooe or less and the cooling rate when the hollow shell temperature is between
soooe and 1 oooe is set to 1 oe/s to less than l5°e/s, whereby the grain size number
of the prior-austenite grains become 9.0 or more and the equivalent circular diameter
of the sub-structures become 3.0 f.Lm or less. Thus, a low-alloy oil-well steel pipe
that has yield strength of965 MPa or more and stably exhibits excellent sse
resistance can be provided.
[Example]
[0084]
Ingots of steels A to K having the respective chemical compositions indicated
in Table 1 were produced.
[0085]
21
[Table I]
TABLE 1
Steel c
A 0.51
B 0.47
c 0.51
D 0.59
E 0.50
F 0.62
G 0.51
H 0.55
I 0. 28'
J 0.51
K 0.53
Si
0.25
0.35
0.31
0.20
0.10
0.23
0.31
0.28
0.19
0.25
0.25
-~,.-.-·~.;...:.:.:.:_. _____ ~~---~-~~~--~·- ---=~--·-~---~-'-'- ~----
Chemical composition (unit: mass%, balance being Fe and impurities)
sol-
Mn p s Cu Cr Mo v Ti Nb
Al B N Ca
0.43 0. 013 0.0015 0.01 0.53 1.10 0.09 - 0.032 0.035 - 0. 0027 -
0.40 0.012 0.0014 0.02 0.61 1. 20 0.08 - 0.031 0.041 - 0.0035 0.0008
0.47 0.012 0.0014 0.02 1. 06 0. 67 0.10 0.004 0.012 0.029 0.0001 0.0034 0.0018
0.45 0. 011 0.0010 0.01 1. 00 0.70 0.10 0.005 0.029 0.038 0.0001 0.0039 -
0.40 0.010 0.0010 0.01 1. 28 0.74 0.24 0.003 0.038 0.035 - 0.0026 0.0015
0.35 0.009 0.0008 0.02 0.51 0.73 0.08 0.003 0.034 0.041 - 0.0032 -
0.47 0.012 0.0014 0.02 0.57 1. 51 0.10 0.003 0.028 0.029 0.0002 0.0034 -
0.43 0.006 0.0010 0.02 0.50 0.73 0.09 0.005 0.029 0.032 0.0001 0.0038 0.0013
0.45 0. 013 0.0008 0.01 0.52 0.70 0.11 0.006 0.032 0.040 0.0001 0.0041 0.0001
0.50 0.012 0. 0013 0.02 0. 67 1.10 0.10 o.ol4· 0.031 0.051 - 0.0039 -
1. 1o· 0.012 0.0007 0.02 0.90 0.69 0.08 - 0.031 0.032 - 0.0044 0.0012
* indicates that the relevant content falls out of the scope of the claims of the present application.
22
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[0086]
The symbol "-"in Table I indicates that the content is substantially "0"%.
Referring to Table I, the chemical compositions of steels A to H were ones falling
within the scope of the present invention. On the other hand, the C content in steel I
was less than the lower limit of the C content in the present invention. The Ti
content in the steel J was more than the upper limit of the Ti content in the present
invention. The Mn content in steel K was more than the upper limit of the Mn
content in the present invention.
[0087]
After the respective ingots were heated, seamless steel pipes each having an
outer diameter of244.5 mm and a wall thickness of !3.8 mm were produced by hot
working (piercing-rolling). The seamless steel pipes were subjected to the
intermediate heat treatment and the final quenching by reheating the pipes to the
temperatures indicated in Table 2.
[0088]
23
I.
!
[Table 2]
TABLE 2
Nwnber
1
2
3
4
5
6
7
8
-•--•--·~··•-•T;
Steel
A
A
A
A
A
B
B
c
Intermediate heat treatment Final quenching
Heat Quenching Cooling
treatment Cooling Cooling rate
Type
temperature method temperature method
(OC) (oC) (°C/S)
- - - 890 sMpriasty 5
Normalizing 920
natural
890
Mist
cooling spray 5
Quenching 920
Mist 890 Mist 5
spray spray
Two-phase natural Mist
range 800 cooling
890 5
heating
spray
Quenching 920 Mist 890
Mist
spray spray 5
Quenching 920
Mist 890
Mist
spray spray 5
Oil tank Oil tank
Quenching 920 irnmersio 890 immersion 3
n
Normalizing 920 natural 890 Mist
cooling spray 5
24
-•. ,.,,·,:•.•:<:'"·'-' '• ., ,.,-, '"·''-"·'-' ~---~·-,.-:;------~:-···:;~"f~''?''~J.-->,_.~-.,>;.;,>~9-',"."'i'&'T;{~q,;'i:';.S-5'K7,.<.;_>-;,;.•;:{.c+;1-;:{k');':'}~-\r;;i;'&~>:':~<;ce]",•.'.•P.v,:-,o~'"'""""'"""~"~"·~-·-=c-;~7::··7-:--··-
I
-----•••-·-~-·-••·--·-- ,.,_,_,,,,,;,.;.,,,~.~--·••~m--~•-·~.o-,.-·_;__ __ _' _, _ _: _ __ ___;...l.:.:..:._,~_,.,.,_,.,,~-~~-·--·-"'·-·~"""""'===....-... -=~-'-~---·---::..:..~--
22
I
c I Normalizing I 920 I natu~al I 890
I
Mist
I
o. s·
I
65°C
coollng spray I
* indicates that the relevant steel or value falls out of the scope of the claims of the present
application.
26
•;-~-"·' ·~• '· "•'-·"·"''•'•'•'<'£>,-,-,<,• ,.,,,-•"•' '<''""'"'''"-" N "•'-""-''r < •--~-~• • • ·---·-;-.------~·~:=;•;'"'"'""·"•"C>,~C•'"'·'>'•'•~.-'XC,';.K"'·''-'7;'->;>;.•"E"1':_.:_.;,;,-?_;;_:;:>,9.c;?.~.'C.V0', ',.<.0,0,0-"-''•'7---'>0•<·''-'«'"•,.·'~'=··•··-e--:--:-;:-:;:- --;-· '
10
[0089]
The "Intermediate heat treatment" column in Table 2 indicates the contents of
intermediate heat treatments performed for hot-worked seamless steel pipes of the
respective test numbers. More specifically, the "Type" column in the "Intermediate
heat treatment" column indicates the type of the heat treatment performed
(normalizing, two-phase region heating or quenching). For example, in the case of
number 2, it is indicated that normalizing was performed at a heat treatment
temperature of 920°C as the intermediate heat treatment and the cooling method was
natural cooling. In the case of number 3, it is indicated that quenching was
performed at a quenching temperature of 920°C and forced-cooling was performed
using mist spray. In the case of number 4, it is indicated that the relevant seamless
steel pipe was heated to a temperature that is the Aci point or more and then allowed
to cool naturally, in the case of number 7, it is indicated that the hollow shell is
immersed in an oil tank in quenching for reinforced-cooling, and in the case of
number I 8, it is indicated that the hollow shell is immersed in a water tank for
reinforced -coo ling.
[0090]
The "Final quenching" column in Table 2 indicates the contents of final
quenching performed for the seamless steel pipe of the respective test numbers,
which have been subjected to the respective intermediate heat treatments. More
specifically, the "Cooling rate" column indicates the cooling rates (°C/s) when the
temperatures of the respective seamless steel pipes were between 500°C and 100°C.
[0091]
In Table 2, the symbol "-"in the "Intermediate heat treatment" column
indicates that no intermediate heat treatment was performed for the relevant number.
[0092]
[Prior-austenite grain size test]
Using the seamless steel pipes (as-quenched products) subjected to the final
quenching, a prior-austenite grain size test was conducted. More specifically,
samples were collected by cutting the as-quenched products in the wall thickness
direction. Then, the samples were embedded in resin, and a surface of each sample
that corresponds to the cut surface perpendicular to the axial direction of the
27
seamless steel pipe (hereinafter referred to as observation surface) was etched using
picric acid. The etched observation surface was observed and the grain size number
of the prior-austenite grains was determined in conformity of ASTM Ell2.
[0093]
(Tempering]
The seamless steel pipes subjected to the final quenching were subjected to
tempering at the respective tempering temperatures (0 C) for the soak period
(minutes) indicated in Table 3 to adjust the seamless steel pipes of the respective
numbers to have a yield strength of965 MPa (140 ksi) or more.
[0094]
28
---------.--------~·--'·'•-"·-·-·--· , .. __________ -'--' -~---------~-~
[Table 3]
TABLE 3
Number Steel
1 A
2 A
3 A
4 A
5 A
6 B
7 B
8 c
9 D
10 D
Retained
Structure y grain
ratio
(%)
M 0.2
M 0.1
M 0.1
M 0.1
M 1.0
M 0
M 0
M 0
M 0.1
M 0.1
Tempering
Prior-y
Soaking
Temperature
grain
size period
number (oC) (min)
9.0 670 60
10 670 60
11 670 60
11 670 60
11 660 60
11 670 60
11 670 60
10 670 60
9.5 670 60
10 670 60
29
Metal Tensile
structure characteristics sse resistance test
Substructure
Equivalent YS TS YR
circular
First Second Third
diameter
(mm) (MPa) (MPa)
2.8 1005 1100 0.91 NF NF NF
2.4 1040 1120 0.93 NF NF NF
.
2.2 1050 1115 0.94 NF NF NF
2.2 1045 1120 0.93 NF NF NF
3. 5' 1020 1150 0.89 F F F
1.8 1100 1150 0. 96 NF NF NF
2.0 1110 1145 0.97 NF NF NF
2.5 1035 1125 0.92 NF NF NF
2.7 1000 1100 0.91 NF NF NF
2.2 1050 1160 0. 91 NF NF NF
11
12
13
14
15
16
17
18
19
20
21
22
'~~----~--•-~--"·--r---•-·•...>.>~-"'·~-"·'·•~·~-•- ""'"--·---~~~-- ---·~..:....~~~---~~~----~---·-------··~--·-·-·-- ------·----·---·--··-·· :.:_ ___________ ······----
D M 0.5 10 675 60 3. 6' 1035 1165 0.89 F F
E M 0.1 11 700 60 2. 9 1045 1160 0.90 NF NF
F M 0.3 10.5 670 60 1.7 1105 1160 0.95 NF NF
G M 0.1 10.5 670 60 2.2 1075 1170 0.92 NF NF
H M 0.2 9.5 670 60 2.8 1015 1130 0.90 NF NF
H M 0 11.5 670 60 1.7 1120 1190 0.94 NF NF
H M 0.1 10.5 670 60 1.9 1110 1190 0.93 NF NF
r• M 0.1 10.5 650 60 4. o· 1000 1085 0.92 F F
J" M 0 10 680 60 2.7 1020 1090 0.94 NF F
K" M 0.1 11.5 670 60 2.5 1035 1125 0.92 NF F
c M 10
No subsequent processes performed because of occurrence of quench -
cracking
c B 0.2 10 670 60 4.2' 1005 1150 0.87 F
* indicates that the relevant steel or value falls out of the scope of the claims of the present
application.
30
F
•••w·o Ao •-~ < - ··-··:···~~----~~·-·· ,.,,,,,,,~ , ' --,0,•,-, •,-,·;ooY;C,o:•:~-~;<;~:'.';'•"•' i, , .. , ' ··----;---:-·-;-----~':~'•'7•"•'<•'<•'•""· :,•,•C•;V',"'.;~~·;,;:i:e;>;>,;';:''">f~'\;:v;-<}•)f,Xofo/'J;f<;"Q:;~;??.0t'g',';~ZY.";,"'i::.::i:.Y~/.'"d"~~'Wo'.>"-'• "o>N"M-.- .• - -· • ••-- •••••••••••---~---~·~---~--~-•n-~---•••-·•••---••
F
NF
NF
NF
NF
NF
NF
F
F
F
F
[0095]
[Evaluation test for seamless steel pipes subjected to tempering]
[Microstructure observation test and retained austenite volume ratio measurement
test]
The following microstructure observation test was conducted for the seamless
steel pipe of each number, which has been subjected to tempering. A center part in
the wall thickness of a transverse cross-section of each seamless steel pipe (surface
perpendicular to the axial direction of the seamless steel pipe) was etched using nita!.
Three arbitrary visual fields (each having an area of I 00 flm x 100 f-tm) in the etched
center part in the wall thickness were observed. For the observation, a scanning
electronic microscope (SEM) was used. As a result of the etching, tempered
martensite was confirmed.
[0096]
The results of the microstructure observation are indicated in Table 3. The
symbol "M" in the Table means that the microstructure of the relevant number
consists of tempered martensite or tempered martensite and retained austenite. The
symbol "B" in the Table means that the microstructure of the relevant number is a
bainite structure.
[0097]
In each of numbers 1 to 21, the microstructure was a structure consisting of
tempered martensite or tempered martensite and retained austenite. In number 22, a
bainite structure was observed.
[0098]
By means of the aforementioned X-ray diffractometry, the volume ratio(%)
of the retained austenite in each steel was measured. More specifically, a sample
including a center part in the wall thickness of each seamless steel pipe was collected.
The surfaces of the collected sample were chemically polished. The chemicallypolished
surface was subjected to X-ray diffraction using a CoKa ray as an incident
X ray. Form the surface integrated intensities of the (211) surface, the (200) surface
and the (110) surface of ferrite and the surface integrated intensities of the (220)
surface, the (200) surface and the (111) surface of austenite, the volume ratio(%) of
retained austenite was determined.
31
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1
[0099]
The results of the determination are indicated in Table 3. In each of the
numbers, the volume ratio of retained austenite was less than 2%.
[01 00]
[Test for measurement of equivalent circular diameter of sub-structures]
The seamless steel pipe of each number, which has been subjected to
tempering, was subjected to crystal orientation analysis using EBSP by the abovedescribed
method to obtain the equivalent circular diameter of the sub-structures.
[0101]
[Yield stress test]
From each of the seamless steel pipes subjected to tempering, a round-bar
tension test specimen having a parallel part measuring 6 mm in outer diameter and
40 mm in length was collected. The parallel part was parallel to the axial direction
of the seamless steel pipe. Using each of the collected round-bar tension test
specimens, a tension test was conducted at normal temperature (25°C) to obtain the
yield strength YS (0.2% proof stress) (MPa) and the tensile strength TS (MPa). As
a result, as indicated in Table 3, the yield strength was 965 MPa (140 ksi) or more in
each of the numbers.
[01 02]
[SSC resistance test]
The following constant-load tension test was performed for the seamless steel
pipe of each number. From the seamless steel pipe of each number, three round-bar
tension test specimens each including a parallel part extending in the axial direction
were collected. The parallel part of each round-bar tension test specimen measured
6.35 mm in outer diameter and 25.4 mm in length. A constant load tension test was
conducted at normal temperature (24°C) in a test bath based on the NACE TM0177
A method. For the test bath, an aqueous solution of 5% ofNaCl + 0.5% of
CHJCOOH charged with a hydrogen sulfide gas of 0.1 bar (the balance being C02
gas) was used. Under the condition ofpH3.5, a constant load that is 90% of the
yield strength measured in the tension test was imposed on each ofthe test specimens
(three in total) in the test bath. It was determined that no sse occurred if the test
specimen did not break off even after the passage of 720 hours, and it was
32
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determined that sse occurred if the test specimen broken off during the test (that is,
within 720 hours).
[0103]
[Test results]
Table 3 indicates the test results. The symbol "NF" in the "SSe resistance
test" column in Table 3 indicates that the relevant test specimen did not break off
even after the passage of 720 hours (that is, no SSe occurred). The symbol "F"
indicates that the relevant test specimen broke off during the test (that is, sse
occurred).
[01 04]
In each of numbers I to 4, 6 to 10 and 12 to 17, the chemical composition of
the starting material was within the scope of the present invention and the production
conditions (the cooling rate and the cooling stop temperature in the final quenching)
were proper. Thus, the structure consisted of tempered martensite and 0 to less than
2% of retained austenite, and the grain size number of the prior-austenite grains was
9.0 or more. Furthermore, the equivalent circular diameter of the sub-structures
was 3.0 )..lm or less. Furthermore, the yield ratio YR was 0.90 or more. Thus, in
the sse resistance test, none of the three test specimens broke off and excellent sse
resistance was stably obtained.
[0105]
Inparticular,ineachofnumbers2to4,6to8, 10, 12to 14, 16and 17,
normalizing, quenching or two-phase range heating was performed as the
intermediate heat treatment. Thus, the grain size number of the prior-austenite
grains of the seamless steel pipe of each of these numbers was 10.0 or more, which is
higher than those of numbers I, 9 and 15 in which no intermediate heat treatment
was performed.
[0106]
On the other hand, in each of numbers 5 and II, although the chemical
composition of the starting material and the cooling rate in the final quenching were
proper, the cooling stop temperature was more than I 00°e. Thus, the equivalent
circular diameter of the sub-structures was more than 3.0 ).liD, and in the sse
resistance test, all of the three test specimens broke off.
33
I
I
!
:.[
[0107]
In number 18, the e content in the starting material was too low.
Furthermore, the cooling rate in the final quenching was more than l5°e/s. Since
the e content was low, no quench cracking occurred, but the equivalent circular
diameter of the sub-structures was more than 3.0 f!m, and in the sse resistance test,
all of the three test specimens broke off.
[0108]
In number 19, the Ti content in the starting material was too high. Thus, in
the sse resistance test, two of the three test specimens broke off and the sse
resistance was unstable.
[0109]
In number 20, the Mn content in the starting material was too high. Thus, in
the sse resistance test, two of the three test specimens broke off and the sse
resistance was unstable.
[0110]
In number 21, although the chemical composition of the starting material was
within the scope of the present invention, the cooling rate in the final quenching was
too high. Thus, quench cracking occurred.
[0111]
In number 22, although the chemical composition of the starting material was
within the scope of the present invention, the cooling rate in the final quenching was
too low. Thus, the structure becomes a bainite structure, and the yield ratio YR was
less than 0.90. Thus, in the SSe resistance test, all of the three test specimens broke
off.
[0112]
Although the embodiment of the present invention has been described above,
the above-described embodiment is a mere illustration for carrying out the present
invention. Therefore, the present invention is not limited to the embodiment, and
the present invention can be carried out with the embodiment arbitrarily modified
without departing from the spirit of the embodiment.

We claim:
1. A low-alloy oil-well steel pipe characterized by comprising:
a chemical composition consisting, in mass%, of
C: 0.40 to 0.65%,
Si: 0.05 to 0.50%,
Mn: 0.10 to 1.00%,
P: 0.020% or less,
S: 0.0020% or less,
Cu: 0.15% or less,
Cr: 0.40 to 1.5 0%,
Mo: 0.50 to 2.50%,
V: 0.05 to 0.25%,
Ti: 0 to less than 0.0 I%,
Nb: 0.01 to 0.2%,
soLA!: 0.010 to 0.100%,
N: 0.006% or less,
B: 0 to 0.0015%, and
Ca: 0 to 0.003%,
the balance being Fe and impurities; and
a structure consisting of tempered martensite and 0 to less than 2% in volume
ratio of retained austenite,
wherein the steel pipe has yield strength of 965 MPa or more;
wherein a grain size number of a prior-austenite grain in the structure is 9.0 or
more; and
wherein in the tempered martensite, a equivalent circular diameter of a substructure
surrounded by a boundary having a crystal orientation difference of ]5° or
more from among a packet boundary, a block boundary and a lath boundary is 3 f.tm
or less.
2. A method for manufacturing a low-alloy oil-well steel pipe, characterized by
compnng:
35
a hot working step of hot-working a starting material having the chemical
composition according to claim I to form a hollow shell;
a final quenching step of subjecting the hollow shell to quenching in which a
cooling rate when a temperature of the hollow shell is between 500°C and 100°C is
set to I °C/s to less than I5°C/s and the temperature of the hollow shell at which
cooling is stopped is set to I 00°C or less; and
a step of tempering the quenched hollow shell.
3. The method according to claim 2, characterized in that
the method further comprises an intermediate heat treatment step of soaking
the hollow shell at a temperature of an A1 point or more after the hot working step
and before the final quenching step.