STEEL FOR OIL WELL PIPE AND METHOD FOR PRODUCING SAME
The present invention relates to steel for oil country tubular goods, and a
method of producing the same, and more particularly, relates to low-alloy steel for oil
country tubular goods which is used for oil country tubular goods such as a casing, a
tubing, and the like for an oil well and 'a gas well, and a method of producing the same.
[Background Art]
[0002]
For oil country tubular goods, high-strengthening is demanded with deepening
wells such as the oil wells and the gas wells (hereinafter, collectively simply referred to
as an "oil well"). Conventionally, oil country tubular goods of 80 ksi grade (yield
stress is 80 to 95 ksi, that is, 551 to 654 MPa) or 95 ksi grade (yield stress is 95 to 110
ksi, that is, 654 to 758 m a ) have been widely used. However, .in recent years, oil
country tubular goods of 110 ksi grade (yield stress is 110 to 125 ksi, that is, 758 to
862 MPa) is starting to be used.
[0003]
Furthermore, most of deep wells,~whicha re developed in recent years, contain
hydrogen sulfide having corrosivity. Therefore, it is necessary for the oil country
tubular goods to have not only high strength but also sulfide stress cracking resistance
.(hereinafter, referred to as SSC resistance).
Methods in which steel is purified or a steel structure is refined are known as
an improvement plan for the SSC resistance of the conventional oil country tubular
goods of 95 to 11 0 ksi grade. For example, Japanese Unexamined Patent Application,
First Publication No. S62-253720 suggests a method of improving the SSC resistance
by reducing impurity elements such as k, P, and the like. Japanese Unexamined
Patent Application, First Publication No. S59-232220 suggests a method of improving
the SSC resistance by canying out a quenching treatment twice to refine crystal grains.
[0005]
In response to the demand for the high-strengthening of the oil country tubular
goods, in recent years, steel for oil country tubular goods of 125 ksi grade (yield stress
is 862 to 965 MPa) has been suggested. However, the sulfide stress cracking (SSC)
tends to occurs with an increase in the strength. Therefore, with regard to steel for oil
1 country tubular goods of 125 ksi grade or higher, a further improvement in the SSC
I I
I resistance is required as compared to the conventional steel for oil country tubular
goods of 95 ksi grade or 11 0 ksi grade.
Japanese Unexamined Patent Application, First Publication No. ,H6-322478,
Japanese Unexamined Patent Application, First Publication No: H8-3 11 55 1, Japanese
Unexamined Patent ~ ~ ~ l i c a t iFoirnst, P ublication No. H11-33573 1, Japanese
Unexamined Patent Application, First Publication No. 2000-1 78682, Japanese
Unexamined Patent Application, First Publication No. 2000-256783, Japanese
Unexamined Patent Application, First Publication No. 2000-297344, Japanese
Unexamined Patent Application, First Publication No. 2000-1 19798, Japanese
Unexamined Patent Application, First Publication No. 2005-350754, Japanese
Unexamined Patent Application, First Publication No. 2006-265657, Japanese
Unexamined Patent Application, First Publication No. 2000-3 13919, and PCT
G ?--."- - - - - - - - . - - -
International Publication No. 20071007678 suggest improvement plans for the SSC
resistance of high-strength steel for oil country tubular goods.
[0007]
Japanese Unexamined Patent Application, First Publication No. H6-322478
suggests a method of improving the SSC resistance of steel materials of 125 ksi grade
by refining the steel structure through an induction-heating heat treatment. Japanese
I Unexamined Patent Application, First Publication No. H8-3 1 155 1 suggests a method
of improving the SSC resistance of steel pipes of 110 ksi grade to 140 ksi grade in a
case of raising hardenability by using a direct quenching method and raising a
tempering temperature. Japanese Unexamined Patent Application, First Publication
No. H11-33573 1 suggests a method of improving the SSC resistance of low-alloy steel
of 110 ksi grade to 140 ksi grade through an adjustment to an optimal alloy component.
I Japanese Unexamined Patent Application, First Publication No. 2000-1 78682,
Japanese Unexamined Patent Application, First Publication No. 2000-256783, and
Japanese Unexamined Patent Application, First Publication No. 2000-297344 suggest a
method of improving the SSC resistance of low-alloy steel for oil country tubular
goods of 110 ksi grade to 140 ksi grade by controlling the morphology of carbides.
Japanese Unexamined Patent Application, First Publication No. 2000- 1 19798 suggests
a method of retarding an SSC occurrence time of steel materials of 110 ksi grade to
125 ksi grade by allowing fine V carbides to sufficiently precipitate. Japanese
Unexamined Patent Application, First Publication No. 2005-350754 suggests a method
of improving the SSC resistance of oil country tubular goods of 125 ksi grade or higher
by controlling dislocation density and a hydrogen diffusion coefficient to desired
values. Japanese Unexamined Patent Application, First Publication No. 2006-265657
suggests a method of improving the SSC resistance of steel for oil country tubular
I_ P-_-~---- - - -. -- - -- - - - -
i
I goods of 125 ksi grade or higher by allowing a large amount of C to be contained,
I
stopping water cooling at 400 to 600°C during the water cooling, and carrying out an
isothermal transformation heat treatment (austempering treatment) at 400 to 600°C to
form a bainite single-phase structure. ~apanesUi nexamined Patent Application, First
J
Publication No. 2000-3 139 19 and PCT International Publication No. 20071007678
discloses a method of improving the SSC resistance of steel pipes by increasing Mo
content as compared to conventional steel pipes for oil well casing.
[Related Art Documents]
[Patent Documents]
[0008]
[Patent Document 11 Japanese Unexamined Patent Application, First
Publication No. S62-253 720
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. S59-232220
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. H6-322478
[Patent Document 41 Japanese Unexamined Patent Application, First
Publication No. H8-3 1 155 1
[Patent Document 51 Japanese Unexamined Patent Application, First
Publication No. H11-3 35 73 1 .
[Patent Document 61 Japanese Unexamined Patent Application, First
Publication No. 2000- 178682
[Patent Document 71 Japanese Unexamined Patent Application, First
Publication No. 2000-256783
[Patent Document 81 Japanese Unexamined Patent Application, First
Publication No. 2000-297344
[Patent Document 91 Japanese Unexamined Patent Application, First
Publication No. 2000- 119798
[Patent Document 101 Japanese Unexamined Patent Application, First
Publication No. 2005-350754
[Patent Document 1 1 1 ~ a ~ a n eUsnee xamined Patent Application, First
Publication No. 2006-265657
[Patent Document 121 Japanese Unexamined Patent Application, First
Publication No. 2000-3 1391 9
[Patent Document 131 PCT International Publication No. 20071007678
[Disclosure of Invention]
[Problems to be Solved by Invention]
[0009]
In recent yeais, a further improvement in SSC resistance of steel for oil
country tubular goods of 110 ksi grade (yield strength is 758 MPa or more) or 125 ksi
grade or more (yield stress is 862 MPa or more) has been required. This.is because
oil wells and gas wells currently in use contain a large amount of hydrogen sulfide.
For example, Japanese Unexamined Patent Application, First Publication No. 2005-
350754 and Japanese Unexamined Patent Application, First Publication No. 2006-
265657 disclose steel for oil country tubular goods which has yield strength of 125 ksi
grade and whch is excellent in the SSC resistance. However, all of test baths used for
an SSC resistance evaluation test are test baths in which hydrogen sulfide of 0.1 atm is
saturated. Therefore, in high-strength steel for oil country tubular goods, the
excellent SSC resistance is required even in a test bath in which hydrogen sulfide of
further high pressure is saturated.
[OO lo]
In addition, the conventional oil country tubular goods having yield strength
of 11 0 ksi grade or higher are not suitable for use in tubing pipes. Oil country tubular
goods having yield strength of 95 ksi grade or lower are used in casing pipes and
tubing pipes. However, in the oil country tubular goods having yield strength of 11 0
ksi grade or higher, the SSC resistance in a case where notch is applied (stress intensity
factor KIssc value in a hydrogen sulfide environment) is lowered. Therefore, in a
case where the conventional oil country tubular goods of 110 ksi grade or higher is
used as a tubing pipe that is directly exposed to a production fluid, SSC may occur
from a latent defect or latent pitting corrosion as a starting point. Accordingly, with
regard to the steel for oil country tubular goods having strength of 110 ksi grade or
higher, it is preferable that the KIssc value be high.
[OOll]
The present invention is to provide low-alloy steel for oil country tubular
goods excellent in SSC resistance.
[Solution to Problems]
[OO 121
According to an embodiment of the invention, there is provided low-alloy
steel for oil country tubular goods which has a chemical composition containing, in
terms of % by mass, more than 0.35% to 1.00% of C, 0.05% to 0.5% of Si, 0.05% to
1.0% of Mn, 0.025% or less of P, 0.010% or less of S, 0.005% to 0.10% of Al, more
than 1.0% to 10% of Mo, 0.01% or less of 0 , and 0.03% or less of N, the balance
being Fe and unavoidable impurities. A full width at half maximum HW(") of a [2 111
crystal plane, which is obtained by X-ray diffraction, satisfies Expression (1). The
number of hexagonal M2C carbides having a grain size of 1 nrn or more in the steel is 5
or more per 1 square micron. Yield strength is 758 MPa or more.
HW x clJ25 0.38(1)
(here, C in Expression (1) is substituted with a content (% by mass) of
carbon.)
[00 131
The low-alloy steel for oil country tubular goods of the invention has
excellent SSC resistance.
[00 141
The chemical composition of the low-alloy steel for oil country tubular goods
of the invention may satisfy Expression (2).
C x Mo 2 0.6 (2)
(here, element symbols in Expression (2) are substituted with contents (% by
mass) of corresponding elements, respectively.)
[00 151
The low-alloy steel for oil country tubular goods of the invention may contain
2.0% or less of Cr in substitution for a part of Fe. The low-alloy steel for oil country
tubular goods of the invention may contain 0.30% or less of V in substitution for a part
of Fe. The low-alloy steel for oil country tubular goods of the invention may contain
one or more kinds selected from a group consisting of 0.1 % or less of Nb, 0.1% or less
of Ti, and 0.1% or less of Zr in substitution for a part of Fe. The low-alloy steel pipe
for oil well casing of the invention may contain 0.01% or less of Ca in substitution for
a part of Fe. The low-alloy steel for oil country tubular goods of the invention may
contain 0.003% or less of B in substitution for a part of Fe.
[00 161
According to another embodiment of the invention, there is provided a
- - - - - - g2,-+ - Em- -
I method of producing low-alloy steel for oil country tubular goods. The method
includes: a process of hot-working a billet to produce a steel material, the billet having
a chemical composition satisfying Expression (2) and containing, in terms of % by
mass, more than 0.35% to 1.00% of C, 0.05% to 0.5% of Si, 0.05% to 1.0% of Mn,
0.025% or less of P, 0.010% or less of S, 0.005% to 0.10% ofAl, more than 1.0% to
10% of Mo, 0.01% or less of 0 , and 0.03% or less of N, the balance Fe and
unavoidable impurities; a quenching process of continuously cooling the steel material
~ ! at a cooling rate in which a time taken from a quenching temperature to a martensite
transformation start temperature is within 600 seconds to quench the steel; and a
process of tempering the quenched steel material at a tempering temperature of 680°C
to Acl point.
C x Mo 2 0.6 (2)
(here, element symbols in Expression (2) are substituted with contents (% by
mass) of corresponding elements, respectively.)
/ I
[00 1 71
According to still another embodiment of the invention, there is provided a
method of producing low-alloy steel pipe for oil well casing. The method includes: a
process of hot-working a billet to produce a steel material, the billet having a chemical
composition satisfying Expression (1) and containing, in terms of % by mass, more
than 0.35% to 1.00% of C, 0.05% to 0.5% of Si, 0.05% to 1 .O% of Mn, 0.025% or less
of P, 0.010% or less of S, 0.005% to 0.10% ofAl, more than 1.0% to 10% of Mo,
0.01% or less of 0 , and 0.03% or less of N, the balance being Fe and unavoidable
impurities; a process of carrying out quenching including an isothennal treatment with
respect to the steel material; and a process of tempering the quenched steel material at
a tempering temperature of 680°C to Acl point. The quenching process includes an
- -j r* : y-$: I-- - -
- -- - -- - --
initial cooling process of cooling the steel material at a cooling rate of 0.7"C/s or more
from a quenching temperature to a temperature that is higher than 100°C and is equal
to or lower than 300°C, an isothermal treatment process of holding the steel material
after the initial cooling treatment process at a temperature range. of higher than 100°C
to 300°C, and a final cooling process of cooling the steel material after the isothermal;-:.
treatment process.
(here, element symbols in Expression (2) are substituted with contents (% by
mass) of corresponding elements, respectively.)
[Advantageous Effects of Invention]
[OO 1 81
The low-alloy steel for oil country tubular goods according to the aspect of
the invention has excellent SSC resistance.
. .
The low-alloy steel for oil country tubular goods produced by the aspect of
the invention has excellent SSC resistance.
[Brief Description of Drawings]
[00 191
FIG. 1A is a micrograph of a transmission electron microscope of hexagonal
M2C carbides.
FIG. 1 B is an electron diffraction pattern and a view illustrating an
identification result of the hexagonal M2C carbides.
FIG. 2 is a diffraction pattern of X-ray diffraction of carbides which are
residue of electrolytic extraction of a-low-alloy steel pipe for oil well according to the
embodiment.
FIG. 3 is a view illustrating a quenching process including a continuous
cooling treatment and a quenching process including & isothermal treatment.
FIG. 4 is a view illustrating a relationship between a thickness t (mm) of a
steel pipe and a cooling rate Ch.5 ("CIS) in order to suppress quenching crack during
the quenching in the continuous cooling treatment.
[Description of Embodiments]
[0020]
Hereinafter, an' embodiment of the invention will be described in detail
referring to the'drawings. In the drawings, the same reference sign will be given to
the same or corresponding parts in the drawings, and the description thereof will be
referenced. The % relating to an element of a chemical composition represents
The present inventors have made an examination and investigation with
respect to SSC resistance of low-alloy steel for oil country tubular goods, and have
obtained the following findings.
(A) When hexagonal M2C carbides are formed in the low-alloy steel for oil
country tubular goods, the SSC resistance increases. Here, the hexagonal M2C
carbides represent M2C carbides having a hexagonal crystal structure. "My' of M2C
represents Mo, or Mo, and V.
[0023]
No and C promote formation of the hexagonal M2C carbides that are fine
carbides. FIG. 1A shows a TEM micrograph of the low-alloy steel for oil country
tubular goods according to the invention. The hexagonal M2C are the fine carbides
with plate-shaped, and a grain size thereof is approximately 1 nm to 50 nm. The
_ --I------ -
TLL'+~~-J=~:T . - - 2 ? X-c. - yh 5 - - -
hexagonal M2C carbides are different from M2C carbides having a cubic crystal
structure. The hexagonal M2C carbides have the plate-shaped, and thus tend to trap
diffusive hydrogen. Furthermore, the hexagonal M2C carbides are fine, and thus
hardly act as a starting point of SSC. Accordingly, the hexagonal M2C carbides
contribute to an improvement of the SSC resistance. The hexagonal M2C carbides
may be identified by electron microscope observation and electron beam diffraction as
described later. In addition, with regard to presence of the hexagonal M2C carbides,
the presence itself may be also confirmed by X-ray diffraction of an electrolytic
extracted residue as described later.
[0024].
When five pieces or more of the hexagonal M2C carbides having a grain size
of 1 nm or more are present in one square micron (pm2), the SSC resistance of the lowalloy
steel for oil country tubular goods increases. In addition, there is a possibility
that the hexagonal M2C carbides having a grain size of less than 1 nm may be present.
However, the identification of the hexagonal M2C carbides of less than 1 nm by the
electron microscope and electron.beam diffraction is technically difficult. Therefore,
in the specification, the number of the hexagonal M2C carbides having the grain size of
1 nm or more per unit area is provided.
[0025]
(B) The Mo content is to be more than 1% and 10% or less. In the case, not
only the formation of the above-described hexagonal M2C carbides is promoted, but
also penetration of hydrogen into steel under hydrogen sulfide environment is
suppressed. Specifically, Fe sulfide that is a corrosion product is formed on a surface
I of the steel under the hydrogen sulfide environment. Mo concentrates in the Fe
I
I sulfide, and increases a protective performance of the Fe sulfide of the steel surface.
____ ._ - ------
ii- R i -*a--~wa ,A, -I s : $ K - - -
I
Accordingly, the penetration of hydrogen into the steel under the sulfide environment
is suppressed, and thus the SSC resistance incieases.
[0026]
(C) In a low-alloy steel pipe for oil well, various carbides in addition to the
hexagonal M2C carbides are further formed during quenching and tempering. Among
the carbides, M3C carbides and M23C6 carbides that are mainly formed at grain
boundaries or lath interfaces of martensite structure are defined as "grain boundary
carbides" in this specification. Here, " M of the M3C carbides and the M23C6
carbides represents Fe, Cr, or Mo.
[0027]
The grain boundary carbides are greatly larger than the hexagonal M2C
carbides and have a size of several 100 nm. Since the grain boundary carbides are
large, when the shape of the grain boundary carbides is flat, the sulfide stress cracking
(SSC) tends to occur at the grain boundary carbides as the starting point. On the
other hand, when the shape of the grain boundary carbides is to be spherical, the SSC
is hard to occur at the grain boundary carbides, and thus the SSC resistance is
improved. Accordingly, to improve the SSC resistance, it is preferable to spheroidize
the grain boundary carbides.
The grain boundary carbides may be spheroidized to some extent by an
increase in a tempering temperature. However, the spheroidizing of the grain
boundary carbides by the increase in the tempering temperature has a limit.
Therefore, it is preferable to further spheroidize the grain boundary carbides by a .
method other than the method of the increase in the tempering temperature.
When increasing the amount of C, specifically, when the amount of C is
increased to be more than 0.35%, the grain boundary carbides in the steel may be
further spheroidized. Accordingly, the SSC resistance further increases. The reason
why the grain boundary carbides is spheroidized when increasing the amount of C is
assumed as follows. When the amount of C increases, the total of the grain boundary
carbides increases. Accordingly, the concentration of Cr and Mo in each grain
boundary carbides decreases, and thus the grain boundary carbides is spheroidized.
[0030]
@) When dislocation density in the steel is high, ,the SSC resistance decreases.
This is because the dislocation acts as a hydrogen trap site. Accordingly, it is
preferable that the dislocation density be low.
[003 11
A full width at half maximum of a crystal plane in X-ray diffraction is
affected by the dislocation density. Specifically, the full width at half maximum is
broadened with an increase in the dislocation density. Accordingly, in the invention,
the full width at half maximum HW(") of a difTraction peak of [211] crystal plane,
which is obtained by X-ray diffraction, is considered as a parameter indicating lattice
strain. The lattice strain increases with the increase in the dislocation density. In a
case where the amount of C is more than 0.35% and the amount of Mo is more than
I%, when the full width at half maximum HW satisfies an Expression (I), the
dislocation density in the steel is sufficiently low, and the excellent SSC resistance may
be obtained.
HW x c1I25 0.38 (1)
Here, the amout of carbon (mass%) is substituted for C in the Expression (1).
[0032]
___-_-----
As described above, in a range of chemical composition that is specified in the
invention, when five pieces or more of the hexagonal M2C carbides having the grain
size of 1 nrn or more precipitate in one square micron (pm2), and the full width at half
maximum HW satisfies the~x~ressio(In), the excellent SSC resistancemay be
obtained.
[0033]
The present inventors have accomplished the invention based on the abovedescribed
findings. Hereinafter, the low-alloy steel for oil country tubular goods
according to the invention will be described.
[003 41
[Chemical Composition]
The low-alloy steel for oil country tubular goods according to the invention
has the following chemical composition.
[003 51
C: more than 0.35% to 1.00%
In the low-alloy steel for oil country tubular goods according to the invention,
the amount of carbon (C) is more than that of conventional low-alloy steel for oil
country tubular goods. When a large amount of C is contained, the spheroidizing of
the grain boundary carbides is promoted, and thus the SSC resistance of the steel is
improved. On the other hand, when C is excessively contained, the effect is saturated.
Accordingly, the amount of C is to be more than 0.35% and 1.00% or less. The lower
limit of the amount of C is preferably 0.45%, and more preferably 0.56%. The upper
limit of the arnount of C is less than 1.00%, more preferably 0.80%, and still more
preferably 0.70%.
[0036]
& a, r* : qg .__ - -----
.- , -- - - - -
Si: 0.05% to 0.5%
Silicon (Si) deoxidizes the steel. On the other hand, when Si is excessively
contained, the effect is saturated. Accordingly, the amount of Si is to be 0.05% to
0.5%. The lower limit of the amount of Si is preferably more than 0.05%, more
preferably 0.1 %, and still more preferably 0.13%. The upper limit of the amount of
Si is preferably less than 0.5%, more preferably 0.40%, and still more preferably
0.30%.
[0037]
Mn: 0.05% to 1.0%
Manganese (Mn) increases hardenability of the steel. On the other hand,
when Mn is excessively contained, manganese segregates at the grain boundaries
together with impurity elements such as phosphorous (P), sulfur (S), and the like. As
a result, the SSC resistance of the steel decreases. Accordingly, the amount of Mn is
to be 0.05% to 1.0%. The lower limit of the amount of Mn is preferably more than
0.05%, more preferably 0.10%, and still more preferably 0.35%. The upper limit of
the amount of Mn is preferably less than 1.0%, more preferably 0.70%, and still more
preferably 0.50%.
. .
[003 81
P: 0.025% or less
Phosphorous (P) is an impurity. P segregates at the grain boundaries, and
decreases the SSC resistance of the steel. Therefore, it is preferable that the amount
of P be small. Accordingly, the amount of P is to be 0.025% or less. The amount of
P is preferably less than 0.025%, more preferably 0.020% or less, and still more
preferably 0.0 1 5% or less.
[0039]
S: 0.0 10% or less
Sulfur (S) is an impurity in common with P. S segregates at the grain
boundaries, and decreases the SSC resistance of the steel. Therefore, it is preferable
that the amount of S be small. Accordingly, the amount of S is to be 0.01 0% or less.
The amount of S is preferably less than 0.010%, more preferably 0.005% or less, and
still more preferably 0.003% or less.
[0040]
. .
Al: 0.005% to 0:10%
Aluminum (Al) deoxidizes the steel. On the other hand, when A1 is
excessively contained, the effect is saturated, and inclusions increase. Accordingly,
the amount of A1 is to be 0.005% to 0.10%: The lower limit of the amount ofAl is
preferably more than 0.005%, more preferably 0.010%, and still more preferably
0.020%. The upper limit of the amount of A1 is preferably less than 0.10%, more
preferably 0.06%, and still more preferably 0.05%. In the specification, the amount
of "Al" represents "acid-soluble Al", that is, the amount of "sol. Al".
[0041]
Mo: more than 1.0% to 10%
Molybdenum (Mo) increases the hardenability, and increases a fraction of
martensite in a structure. Accordingly, Mo increases the strength of the steel.
Furthermore, Mo concentrates in the Fe sulfide (corrosion product).that is formed on
the surface of the steel under the hydrogen sulfide environment, and increases the
protective performance of the Fe sulfide of the steel surface. Accordingly, the
penetration of hydrogen into the steel is suppressed, and thus the SSC resistance of the
steel increases. Furthermore, Mo forms the hexagonal Mo2C carbides that are the
fine carbides. The hexagonal Mo2C carbides fix (trap) the diffusive hydrogen, and
4 P . q - C91F4+):.p q 7 , ? 2-> . _ _ - I----- - , --
1- n . J ? - n
thus occurrence of the SSC due to hydrogen is suppressed. On the other hand, when
Mo is excessively contained, the effect is saturated. Accordingly, the amount of Mo
is to be more than 1 .O% and 10% or less. The lower limit of the amount of Mo is
preferably 1.20%, and more preferably 1.30%. The upper limit of the amount of MO
is preferably less than lo%, more preferably 4.0%, and still more preferably 3.0%.
0 : 0.01% or less
Oxygen'(0) is an impurity. When 0 is excessively contained, coarse oxides
are formed, and thus toughness and the SSC resistance of the steel decrease.
Therefore, it is preferable that the amount of 0 be small. Accordingly, the amount of
0 is to be 0.01% or less, and more preferably 0.005% or less.
[0043]
N: 0.03% or less
Nitrogen (N) bonds to Al, Nb, Ti, and Zr to form nitrides or carbonitrides,
which results in refinement of the structure of the steel by a pinning effect. A lower
limit of N to obtain the effect is preferably 0.003%, and more preferably 0.004%. On
the other hand, when N is excessively contained, coarse nitrides are formed. The
coarse nitrides act as the starting point of the pitting corrosion, and thus the SSC
resistance may decrease. Accordingly, the amount of N is to be 0.03% or less. The
upper limit of the amount of N is preferably less than 0.03%, more preferably 0.025%,
and still more preferably 0.02%. '
In addition, N is an impurity for production of the steel. In a case where the
above-described effects of the nitrides or carbonitrides are not positively required, N
may be less than 0.003% as the impurity.
[0045]
In the chemical composition of the low-alloy steel for oil country tubular
goods, the balance consists of Fe and impurities. The mentioned impurities represent
elements which are contaminated fiom ores and scarp that are used as a raw material of
the steel, or fiom environment of a production process.
[0046]
[Optional Elements]
Furthermore, the low-alloy steel for oil country tubular goods may contain Cr
in substitution for a part of Fe.
LO0471
Cr: 2% or less
Chromium (Cr) is an optional element. Cr increases the hardenability of the
steel. Even when a little amount of Cr is contained, the effect may be obtained. On
the other hand, when Cr is excessively contained, the effect is saturated. Accordingly,
the amount of Cr is to be 2% or less. The lower limit of the amount of Cr is
preferably 0.1%, more'preferably 0.2%, and still more preferably 0.5%. The upper
limit of the amount of Cr is preferably less than 2%, more preferably 1.5%, and still
more preferably 1.0%.
[0048]
Furthermore, the low-alloy steel for oil country tubular goods may contain V
in substitution for a part of Fe.
[0049]
V: 0.30% or less
Vanadium (V) forms the hexagonal M2C carbides, which are the fine carbides,
in combination with Mo, and thus the SSC resistance increases. Here, " M of the - ___- -- ----
- - . - . --
hexagonal M2C carbides represents Mo and V. Furthermore, V forms MC (M
represents Mo and V), and thus increases the tempering temperature of the steel for
obtaining high yield strength. Even when a little amount of V is contained, the
above-described effect may be obtained. On the other hand, V is excessively
! contained, the amount of solid-soluted V at the quenching is saturated, and thus the
effect of the increase in the tempering temperature is also saturated. Accordingly, the
amount of V is to be 0.30% or less. The lower limit of V is.preferably 0.05%, more
preferably 0.07%, and still more preferably 0.1%. The upper limit of the amount of v
is preferably less than 0.30%, more preferably 0.25%, and still more preferably 0.20%.
[OOSO]
Furthermore, the low-alloy steel for oil country tubular goods may contain at
least one selected from a group consisting of Nb, Ti, and Zr in substitution for a part of
Fe.
[005 11
Nb: 0.1 % or less
Ti: 0.1 % or less
Zr: 0.1 % or less
Niobium (Nb), titanium (Ti), and zirconium (Zr) are optional elements. The
elements bond to C or N to form carbides, nitrides, or carbonitrides. The precipitates
(the carbides, the nitrides, and the carbonitrides) refine the structure of the steel by the
pinning effect. Even when at least one selected from the group consisting of Nb, Ti,
and Zr is contained in a little amount, the above-described effect may be obtained.
On the other hand, when Nb, Ti, and Zr are excessively contained, the effect is
saturated. Accordingly, the amoui~ot f Nb is to be O.l%'or less, the amount of Ti is to
be 0.1% or less, and the amount of Zr is to be 0.1% or less. When the amount of Nb
&---%$+% $ - - - - - - -- -
- -
- - - - - . -
is 0.002% or more, the amount of Ti is 0.002% or more, or the amount of Zr is 0.002%
or more, the above-described effect may be significantly obtained. he lower limits
of the amount of Nb, the amount of Ti, and the amount of Zr are more preferably
0.005%. The upper limits of the amount of Nb, the amount of Ti, and the amount of
Zr are preferably 0.05%.
[0052]
Furthermore, the low-alloy steel for oil country tubular goods may contain Ca
in substitution for a part of Fe.
[0053]
Ca: 0.01% or less
Calcium (Ca) is an optional element. Ca bonds to S in the steel to form
sulfides, and improves a shape of inclusions, thereby the SSC resistance increases.
Even when a little amount of Ca is contained, the above-described effect may be
obtained. On the other hand, when Ca is excessively contained, the effect is saturated.
Accordingly, the amount of Ca is to be 0.01% or less. The lower limit of the amount
of Ca is preferably 0.0003%, and more preferably 0.0005%. The upper limit of the
amount of Ca is preferably 0.0030%, and'more preferably 0.002%.
[0054]
Furthermore, the low-alloy steel for oil country tubular goods may contain B
in substitution for a part of Fe.
[0055]
B: 0.003% or less
Boron (B) is an optional element. B increases the hardenability of the steel.
Even when a little amount of B is contained, the above-described effect may be
obtained. On the other hand, when B is excessively contained, the effect is saturated.
.-u, mf-n * $2 T&q----- 2 , 2 >--%G+K-c --- -
. - - - - - - --
Accordingly, the amount of B is to be 0.003% or less. The lower limit of the amount
of B is preferably 0.0003%, and more preferably 0.0005%. The upper limit of the
amount of B is preferably 0.0015%; and more preferably 0.0012%.
[0056]
[Hexagonal M2C carbides]
The low-alloy steel for oil country tubular goods contains five pieces or more
of the hexagonal M2C carbides per one square.micron (that is, 5 pieces/prn2 or more).
Here, the hexagonal M2C carbides are the carbides having the hexagonal crystal
structure, and are different from the M2C carbides having the cubic crystal structure.
" M of the hexagonal M2C carbides is Mo, or Mo and V.
[0057]
The number of the hexagonal M2C carbides is measured by the following
method. A sample for TEM (transmission electron microscope) is.collected from an
arbitrary part of the low-alloy steel for oil country tubular goods. As a method
collecting the sample, methods such as a thin film method and an extraction replica
method are used. 10 visual fields in the collected sample are observed by the TEM to
obtain TEM micrographs of the respective visual fields. An area of each of the visual
fields is to be 1 pm2. An electron beam diffraction pattern of the carbides in each
visual field is confirmed to identify a type of the carbides. FIG. 1B shows a typical
pattern of the diffraction pattern of the hexagonal M2C carbides by electron
microscope observation. In addition, the hexagonal M2C carbides may also be clearly
distinguished from other carbides including the cubic M2C carbides by X-ray
diffraction. Accordingly, the confirmation about whether the M2C precipitates or not
may also be possible by carrying out X-ray diffraction of the extracted residue after
electrolytically extracting the carbides in a steel material. FIG. 2 shows a diffraction
pattern of the X-ray diffraction. In FIG. 2, the horizontal axis represents 28 (") of Xrays
(providing that, 8 is an incidence angle), and the vertical axis represents
diffraction intensity. "Mo2C" and "MC" in FIG. 2 represent the type of the carbides.
The "Mo2C" represents the hexagonal M2C carbides. The "MC" represents cubic MC
carbides (M is Mo, Cr, or Fe) or the cubic M2C carbides. In FIG. 2, "(021)", "(1 12)",
and the like represent crystal planes (miller indices). As shown in FIG. 2, the
hexagonal M2C carbides may be clearly distinguished from other carbides including
the cubic M2C carbides by the X-ray diffraction.
[0058]
I The number of the hexagonal M2C carbides, which are identified at each
visual field of the electron microscope observation and have the grain size of 1 nm or
more, is counted to calculate an average number per 1 pm2. The calculated average
number is defined as the number of the hexagonal M2C carbides per 1 pm2
(pieceslPm2). When 5 pieces/pm2 or more of the hexagonal M2C are present, the
excellent SSC resistance may be obtained.
[0059]
The grain size of the hexagonal M2C carbides is approximately 1 nm to 50 nm.
Here, the grain size of the hexagonal M2C carbides is measured by the following
method. An area of each of the hexagonal M2C carbides is obtained by image
analysis. An equivalent circle diameter of the obtained area is defined as the grain
size of the hexagonal M2C carbides. As described above, in the invention, the
number of the hexagonal M2C carbides having the grain size of 1 nm or more is
counted.
[0060]
[Full width at half maximum HW]
EROL-DEE_J&I-B-U~~- - ._!&1-LB,E-k.X5 -
In the low-alloy steel for oil country tubular goods according to the invention,
the full width at half maximum HW(") of the difiaction peak of [2 111 crystal plane,
which is obtained by the X-ray diffraction method, satisfies the Expression (1).
Here, the amout of C (mass%) is substituted for the element symbol C.
[006 11
The dislocation acts as the hydrogen trap site, and the SSC resistance
decreases. Accordingly, it is preferable that the dislocation density be low. When
the full width at half maximum satisfies the Expression (I), it is considered that the
dislocation density is suppressed, and thus hydrogen is hard to be accumulated in the
steel. Accordingly, the excellent SSC resistance may be obtained. On the other
hand, when the full width at half maximum does not satisfy the Expression (I), it is
considered that the suppression of the dislocation density is insufficient, and thus the
SSC resistance is insufficient.
[0062]
[Relationship between amount of C and amount of Mo]
Preferably, the chemical composition of the low-alloy steei for oil country
tubular goods further satisfies an Expression (2).
C x Mo 2 0.6 (2)
Here, in the Expression (2), the amout of C (mass%) is substituted for the
element symbol C, and the amout of Mo (mass%) is substituted for the element symbol
Mo.
[0063]
When the amount of C and the amount of Mo satisfy the Expression (2) and
production is carried out under production conditions as described later, 5 pieces/pm2 - . - . - .-- -- - .
or more of the hexagonal M2C are obtained. Accordingly, the excellent SSC
resistance may be obtained. More specifically, the sufEcient SSC resistance may be
obtained for the 125 ksi grade or higher, and the high KIssc value may be obtained for
the 1 10 ksi or higher.
[0064]'
[Structure]
The structure of the low-alloy steel for oil country tubular goods according to
the invention includes a mixed structure of tempered martensite and tempered bainite.
More specifically, the structure of the low-alloy steel for oil country tubular goods
mainly includes the tempered martensite and the tempered bainite, and may include
precipitates such as carbides, nitrides, carbonitrides, and the like, inclusions, or
residual austenite. However, a fraction of the residual austenite (a volume fraction of
the residual austenite to the entire structure, in unit of %) may be 5% or less. This is
because the residual austenite causes unevenness of strength. In addition, in a case
where the thickness of the oil country tubular goods is thin and possibility of
occurrence of quenching crack is very low, the structure of the low-alloy steel for oil
country tubular goods may be a single phase of the tempered martensite.
The fraction of the residual austenite is measured by the'X-ray diffraction
method as follows. Specifically, a sample, which includes the thickness central
portion of the produced steel plate or the produced steel pipe, is collected. A surface
of the collected sample is chemically polished. X-ray diffraction is carried out in the
chemically polished surface by using CoKa-ray (Kal) as an incident X-ray. The
fraction of the residual austenite is quantitatively analyzed from the plane integrated
intensity of (21 1) plane, (200) plane, and (1 10) plane of ferrite and the plane integrated
-- - - -- ---
- - - - - - - - - -
- -
intensity of (220) plane, (200) plane, and (1 11) plane of austenite.
[0066]
When the amount of C is high as the low-alloy steel for oil country tubular
goods according to the invention, the quenching crack due to martensite transformation
tends to occur. As a method of suppressing the quenching crack, there is a method in
which water cooling during quenching is temporarily stopped in order for the structure
of the steel for oil country tubular goods to have a structure mainly composed of
bainite. However, in a case where the structure is mainly composed of the bainite, a
considerable amount of carbides are formed during the quenching. The carbides
retard the recovery of the dislocation during the tempering. Therefore, in the case of
the structure mainly composed of the bainite, the dislocation density increases, and
thus the Expression (1) is not satisfied.
[0067]
When a fraction of the martensite in the structure after the quenching is high,
the dislocation density decreases. It is dificult to quantitatively measure the volume
fraction of the martensite and the volume fraction of the bainite in the steel after the
quenching. However, the hardness of the steel after the quenching (that is, a
quenched material) increases with an increase in the fraction of the martensite in the
steel. Accordingly, when Rockwell hmess (HRC) of the low-alloy steel for oil
country tubular goods after the quenching and before the tempering (that is, the
quenched material) preferably satisfies a following Expression (3), the martensite is
formed to a sufficient amount for decreasing the dislocation density.
Rockwell hardness (HRC) 2 50 x C + 26 . (3)
For example, in the steel in which the amount of C is 0.6%, when the
*r?m " y & w - 4 t - K r - b & - - >Be .- PA : . . . . -. - ------- .. . - . - -- - 4 p " IpQ-
- . - - + - - . - - -- -
Rockwell hardness (HRC) is 56 or more, the dislocation is sufficiently recovered after
the tempering (that is, Expression (1) is satisfied), and the SSC resistance increases.
[0069]
[Production method]
An embodiment of a method of producing the low-alloy steel for oil country
tubular goods will be described. In the embodiment, a method of producing seamless
steel pipes (low-alloy oil country tubular goods) will be described.
[0070]
The steel'having the above-described chemical composition is melted, and is
refmed by a conventional method. Subsequently, the molten steel is formed into a
continuous casting material by a continuous casting method. The continuous casting
material is, for example, a slab, a bloom, or a billet. In addition, the molten steel may
be formed into an ingot by an ingot-making method.
[007 11
The slab, bloom, or ingot is hot-worked to form a billet. The billet may be
formed by hot rolling or hot forging.
The billet obtained by the continuous casting or the hot working is hot-worked
to produce a steel material. In the embodiment, the steel material is a material pipe.
For example, a Mannesmann process is carried out as the hot working to produce the
material pipe. The material pipe may be produced by other hot working methods.
Quenching is carried out for the hot-worked steel material (material pipe).
As the quenching, for example, either quenching C10 by a continuous cooling
treatment or quenching C11 including an isothermal treatment as shown in FIG. 3 may
a "- -- B'A1 4 7 . - --- - -
be adapted. In the specification, both the quenching C 10 by the continuous cooling
treatment and the quenching C 11 including the isothermal treatment are defined as
"quenching".
[0074]
Even in any quenching (the quenching,by the continuous cooling treatment, or
the quenching including the isothermal treatment), it is preferable that a quenching
temperature of the steel material (a surface temperature of the steel material at the
quenching) be to be 850 to 920°C.
[0075]
[Quenching by continuous cooling treatment]
In the case of the quenching by the continuous cooling treatment, as shown in
a curve C 10 of FIG. 3, the steel material is continuously cooled from the quenching
temperature, and the surface temperature of the steel material continuously is
decreased. As the continuous cooling treatment, for example, a method of cooling the
steel material by immersing it in a water bath or an oil.-bath, or a method of cooling the
steel material by shower water cooling.
[0076] .
In the continuous cooling treatment, a time (Ms point passing time), for which
the surface temperature of the steel material reaches a martensite transformation start
point (Ms point) from the quenching temperature, is preferably 600 seconds or less.
When the passing time is more than 600 seconds, it is difficult to obtain the hardness
satisfying the Expression (3), and thus the fraction of the martensite in the steel
structure decreases. Therefore, the Expression (1) is not satisfied, and thus the
excellent SSC resistance may not be obtained.
[0077]
In a case where the steel material is the material pipe (steel pipe) and the
quenching by the continuous cooling treatment is carried out, a cooling rate in a range
where a temperature of an outer surface of the steel pipe reaches 500°C from 800°C is
defined as C&-5 ("CIS). In a case where the amount of C of the material pipe is
approximately 0.6%, the cooling rate CR8-5 preferably satisfies a following Expression
(4).
CRs-5 1 ~ 8 3 7 t - ~ . ~(4 )
Here, t represents the thickness (in unit of mm) of the steel pipe.
[0078]
When the cooling rate C&-5 satisfies the Expression (4), occurrence of the
quenching crack is suppressed in the steel pipe to which the quenching by the
continuous cooling treatment has been carried out. During the quenching, there is a
time difference in occurrence of the martensite transformation between an outer
surface side and an inner surface side of the steel pipe. Therefore, it is considered
that residual stress which causes the quenching crack generates in the steel pipe. The
residual stress derived from the quenching may be obtained by stress-strain distribution
.. .. .
analysis of a finite element method (FEM). By comparison between the residual
stress value obtained from the FEM analysis result and quenching crack behavior of
the actual steel pipe, when a tensile residual stress is 200 MPa or less, it could be
confirmed that the quenching crack of the steel pipe of the embodiment is suppressed.
[0079]
With an increase in the thickness t (mm) of the steel pipe, the time difference
; in the occurrence of the martensite transformation between the inner surface and outer
surface of the steel pipe increases, and thus the tensile residual stress increases. With
a decrease in the cooling rate, the time difference in the above-described martensite
- F P F I - - + F ~ - ~ . -C - - - !. '7% -? T. 1 d - ' U- - . - . -
transformation decreases. Accordingly, the tensile residual stress decreases, and thus
the occurrence of the quenching crack is also suppressed.
[0080]
FIG. 4 is a view illustrating a relationship between the thickness t (mm) of the
steel pipe and the cooling rate C&-5 ("CIS) in order to suppress the quenching crack
during the quenching in the continuous cooling treatment. A curve C4 in FIG. 4
represents the right-hand side (=2837t-2.2o) f the Expression (4). The curve C4
represents a relationship between the cooling rate CR8-5 ("CIS) and the thickness t
(mm) of the steel pipe with which the tensile residual stress becomes 200 MPa.
[008 11
Referring to FIG. 4, the quenching crack is suppressed at a lower side of the
curve C4. On the other hand, the quenching crack tends to occur at an upper side of
the curve C4. Accordingly, the steel pipe is preferably cooled during the cooling so
that the cooling rate CR8-5 satisfies the Expression (4). In the case, particularly, it is
possible to produce the seamless steel pipe, which does not have the quenching crack
defect or in which occurrence of the quenching crack is suppressed, in the seamless
steel pipe having an outer diameter of 100 to 400 mm and a thickness of 5 to 100 rnm.
In addition, the right-hand side value (=2~37t-~of. ~th)e Expression (4) corresponds to
a case in which the amount of C in the steel is approximately 0.6%. With an increase
in the amount of C increases, the upper limit cooling rate for suppressing the
quenching crack shifts to a cooling rate which is smaller than that calculated by the
right-hand side of the Expression (4). With a decrease in the amount of C increases,
the upper limit cooling rate for suppressing the quenching crack shifts to a cooling rate
which is larger than that calculated by the right-hand side of the Expression (4).
[0082]
r = re, - -"$L--&-&--JL-+ .->*T .c" 7. " " ._. __ __ ___-_- ------- , - . , -- - --..- . ,---
[Quenching including isothermal treatment]
The quenching (curve C11) with the isothermal treatment in FIG. 3 includes
an initial cooling process, an isothermal treatment process, and a final cooling process.
In the initial cooling process, the steel material (material pipe) after the hot working is
cooled from the quenching temperature to a temperature range of higher than 100°C to
300°C, and the cooling is stopped at the temperature of higher than 100°C to 300°C.
When the cooling stop temperature is higher than 300°C, a fraction of the bainite in the
steel structure excessively increases, and thus a large amount of carbides are formed.
Therefore, the dislocation is hard to be recovered during the tempering treatment, and
the dislocation density is hard to decrease. As a result, the hardness of the steel after
the cooling does not satisfy the Expression (3), and thus the Expression (1) is not
satisfied. Therefore, the excellent SSC resistance may not be obtained.
[0083]
In the isothermal treatment process, the steel material after the initial cooling
is held for a predetermined time in the temperature range of higher than 100°C to
300°C. It is sufficient that the steel material is held within the above-described
temperature range in the isothermal treatment, and it is not limited for the steel
material to be held at a constant temperature. A preferable holding time atthe
isothermal treatment is 5 to 60 minutes.
In the final cooling process, the steel material after the isothermal treatment
process is cooled. The final cooling may be water cooling or air cooling. In other
words, the cooling rate during the final cooling process is not particularly limited.
[0085]
In the quenching process including the isothermal treatment, the temperature
---
(higher than 100°C to 300°C) of the isothermal treatment is lower than a temperature
range where bainite transformation tends to occur. Therefore, the quenching process
including the isothermal treatment is different from austempering disclosed in Japanese
Unexamined Patent Application, First Publication No. 2006-265657.
[0086]
From the viewpoint of controlling the quenching crack, the isothermal
treatment is preferably carried out at a temperature of higher than Ms point and 300°C
or lower. In the case, the cooling rate of the initial cooling may be controlled to be
sufficiently large. Although a detailed mechanism is not clear, in the case, it is
assumed that a slight amount of the bainite that precipitates during the isothermal
treatment suppresses the occurrence of the quenching crack during the final cooling.
[0087]
The isothermal treatment may be conducted at a temperature range of 100°C
to Ms point. In the case, the cooling rate in the initial cooling process is suppressed.
However, when the cooling rate is excessively slow, the hardness of the steel after the
quenching is excessively low. It should be to avoid the cooling rate with which ferrite
and perlite or a large amount of the bainite is formed during the initial cooling process
at least. Accordingly, a preferable cooling rate in the initial cooling process is 0.7
"CIS or more.
[OOSS]
[Tempering]
After carrying out the quenching by the continuous cooling treatment or the
quenching including the isothermal treatment, the tempering is carried out for the steel
material. A tempering temperature is appropriately adjusted according to the
chemical composition of the steel materia, and the intended yield strength. In other
- A - u I_I __ ---. - -
.words, the yield strength may be controlled to 758 MPa or more, and more preferably
to 862 MPa or more by adjusting the tempering temperature.
[0089]
The tempering temperature is preferably 680°C to Acl point. The lbwer
limit of the tempering temperature is more preferably 690°C, and the upper limit of the
tempering temperature is more preferably 720°C. A preferable tempering time is 10
to 90 minutes in soaking.
[0090]
In a case where the chemical composition of the steel material satisfies the
Expression (2) and the tempering is carried out at the above-described preferable
tempering temperature, 5 pieces/pm2 or more of the hexagonal M2C carbides having
the grain size of 1 nm or more precipitate in the steel, and thus the SSC resistance
increases.
[009 11
From the above-described processes, the low-alloy oil country tubular goods
(seamless steel pipes) having the excellent SSC resistance are produced.
[0092]
In the above-described production method according to the embodiment, the
quenching is carried out after the hot working. However, a normalizing treatment
may be carried out between the hot working and the quenching. Specifically, the steel
material (material pipe) after the hot working is held for a predetermined time at a high
temperature of A3 point or higher, and thereafter, the steel material is cooling. The
holding temperature is preferably 900°C to 920°C. A holding time is, for example, 5
to 60 minutes.
[0093]
-- . - - - - - -
Commonly, in the normalizing treatment, the steel material after the hot
working is cooled to a room temperature, and thereafter, the steel material is heated to
AC3p oint or higher. However, the normalizing treatment may be carried out by
directly holding the steel material after the hot working at the temperature of An point
or higher.
[0094]
When the normalizing treatment is carried out, the crystal grain of the steel is
rebed. Specifically, after the quenching in which the normalizing treatment is
carried out, (that is, in the as-quenched material), a grain size number of prior-austenite
-grain becomes 10 or more which is defined in ASTM El 12. Through the refinement
of the crystal grain, the SSC resistance is further improved.
[0095]
In the above production method, the method of producing the seamless steel
pipe is explained regarding the steel material as the material pipe or the steel pipe.
However, the shape of the steel material is not particularly limited. The steel material
may be a plate material, a steel bar, or a wire rod.
[0096]
Furthermore, in the above-described production method, the steel material
having the chemical composition satisfying theExpression (2) is used and the
tempering temperature is specified in order to form 5 pieces/pm2 or more of the
hexagonal M2C carbides having the grain size of 1 nm or more in the steel. However,
5 pieces/pm2 or more of the hexagonal M2C having the grain size of 1 nm or more may
be precipitated in the steel by different production conditions.
[Example 11
[0097]
-- . - -
Ingots of steel A to steel Z and steel AA to steel AG that have chemical
compositions shown in Table 1 were produced.
[0098]
[Table 11
11 Numerical values to which * is attached are numerical values .out of range of the invention
1 i
[0099]
.. A value that is obtained by a following expression is shown in a column of
"F2" in Table 1.
F2=CxMo
In short, F2 is the left-hand side of the Expression (2).
[O 1 001
All of the chemical compositions of the steel A to steel U were within the
range of the invention, and F2 satisfied the Expression (2). On the other hand, in the
steel V to the steel Z and the steel AB to the steel AE, at least one of the amounts of the
elements was out of the range of the invention. The chemical compositions of the
steel AA, the steel AF, and the steel AG were within the range of the invention, but did
not satisfy the Expression (2).
[OlOl]
The weight of the respective ingots was 30 kg to 150 kg. Blocks were taken
from the respective ingots. The blocks were heated to 1 ,250°C. The heated blocks
were hot-forged and hot-rolled to produce steel materials (plate materials) having a
thickness of 15 mm to 25 mm.
Quenching and tempering treatments, or quenching and tempering treatments
after normalizing treatment were carried out by using the produced plate materials.
The yield strength of the plate materials was controlled to 11 0 ksi grade (758 MPa or
more) and 125 ksi grade (862 MPa or more).
[0 1 031
In the normalizing treatment, soaking was carried out for 10 minutes at a
temperatwe (920°C) of A3 point or higher, and then cooling was conducted in air. On
__ ...._ _ . - . -. .- -. - . . 1.. .. -.- -- - -. -- - .- -- . - - - -- - -...
. . . . _.. . ~- _ . . .. - - - - - --. ~ - - -
/
the other hand, the quenching and the tempering were carried out as follows. ,
[0 1 041
[Quenching]
The quenching temperature during the quenching was controlled to a range of
850°C to 920°C.
[0 1051
[Quenching by Continuous Cooling Treatment]
In a case of carrying out the quenching by the continuous cooling treatment,
after each of the plate materials was heated to the quenching temperature, the passing
time at the Ms point (time taken from the quenching start temperature to the martensite
transformation start temperature (Ms point)) Tcc (second) was controlled by shower
cooling, mist cooling, ,or air cooling.
[0 1061
[Quenching including Isothermal Treatment]
In a case of carrying out the quenching including the isothermal treatment,
initial cooling was carried out at a cooling rate of 0.7"CIs or more by salt bath cooling
or water cooling. The plate material was pulled up midway through the cooling, and
a cooling stopping temperature ATIc ("C) during the initial cooling was made to vary.
Holding was carried out at the cooling stopping temperature ATIc for 25 to 40 minutes
(isothermal treatment), and then water cooling was carried out to an ordinary
temperature (final cooling).
[0 1 071
[Test for Quenched Material]
The following tests were carried out for the plate material after the quenching
(hereinafter, referred to as a quenched material). - s - - - - -- ~ k k p 1&-I -: ----- p " T q - -
[0108]
[Hardness Test of Quenched Material]
The hardness of the quenched material was measured by the following
method. The quenched material was cut in a plate thickness direction. Then, the
Rockwell hardness (HRC) of the central portion of a cross-section in the plate
thickness direction was obtained based on JIS G0202. Specifically, the Rockwell
hardness HRC was obtained at arbitrary three points of the central portion of the crosssection
in the plate thickness direction. An average of the Rockwell hardness (HRC)
obtained at the three points was defined as hardness of a corresponding mark.
[0 1 091
[Prior Austenite Grain Size Test]
Furthermore, a prior-austenite grain size test was carried out using the
quenched material. Specifically, the quenched material was cut along the plate
thickness direction. In addition, the cut plate material was embedded in resin, and
then a cross-section was etched by picric acid. The etched cross-section was
observed, and the grain size number of the prior-austenite grain was determined based
on the ASTM El 12.
[OllO]
[Tempering]
The tempering was carried out for the plate materials after the quenching.
The yield strength of the respective plate materials was controlled to 11 0 ksi grade and
125 ksi grade by controlling a tempering temperature (OC) and a tempering time
(minutes).
[Olll]
[Evaluation Test for Plate Material After Tempering]
k4'L. C,-------- -- ---- --------------- - _ _ - ___-_ _ _ --
The following evaluation tests were carried out using the plate materials that
were subjected to the quenching and the tempering.
[0112]
[Full width at half maximum Measuring Test and Fraction of Residual
Austenite Measuring Test]
Test specimens were taken from the plate materials after the tempering. The
surface of each of the test specimens was polished with emery paper. The finer size
of the emery paper was used with proceeding the polishing. After the surface of the
test specimen was polished with emery paper of No. 1200, a work-hardened layer that
was formed by polishing on the surface of the test specimen was removed by
immersing the test specimen in hydrogen peroxide that contained a small amount of
hydrofluoric acid and was held at an ordinary temperature. X-ray diffraction test was
carried out for the test specimen from which the work-hardened layer was removed
under conditions of 30 kV and 100 mA using CoKa-ray (Kal having a wavelength of
1.7889 A) to obtain the full width at half maximum (") of a diffraction peak of a [21 l]
crystal plane of the test specimen.
[0113]
Specifically, Kal and Ka2 in the CoKa-ray were separated by fitting to
extract only the Kal, and the full width at half maximum (") diffracted by the Kal-ray
of an a-Fe [221] plane of the test specimens was obtained. In addition, the full width
at half maximum was measured at a height which was a half height of the peak height
(peak top method). In addition, with regard to a full width at half maximum derived
from a device, the full width at half maximum derived from the device was measured
by using a single crystal (ideal single crystal which does not have a full width at half
maximum) of LaB6 (lanthanum hexaboride), and correction was carried out by
3== --E+-J++'$---~-B;- --a-~----*F.fl ft re '.: -- -- B;; % L -- --
subtracting the full width at half maximum derived from the device from the actually
measured value.
[0114]
Furthermore, the fraction of the residual austenite (a volume fraction (%) of
the residual austenite to the entirety) was measured by the above-described X-ray
method. Specifically, a sample, which includes the central portion of the steel
material in a thickness direction, was taken. A surface of the taken test specimen was
chemically polished. X-ray diffraction was carried out to the chemically polished
surface using CoKa-ray (Kal having a wavelength of 1.7889 A) as an incident ray.
The fraction of the residual austenite was quantitatively analyzed from the plane
integrated intensity of (2 11) plane, (200) plane, and (1 10) plane of femte, and the plane
integrated intensity of (220) plane, (200) plane, and (1 11) plane of austenite.
[0115]
[Yield Strength Test]
Round-bar tensile test specimens having a parallel portion with an outer
diameter of 6 mm and a length of 40 mm were taken from the respective plate
materials after the tempering. A tensile test was carried out using the taken round-bar
tensile test specimens at an ordinary temperature (25OC) to obtain the yield strength
(0.2% proof stress, in unit of MPa).
[0116]
[SSC Resistance Test]
In the SSC resistance test, a constant load tensile test and an autoclave test
were carried out by using the plate material having strength of 125 ksi (862 MPa) or
more.
[Constant Load Tensile Test]
Round-bar tensile test specimens having a parallel portion extending in a
rolling direction were taken from the respective plate materials. The outer diameter
of the parallel portion was 6.35 rnm, and the length thereof was 25.4 mm. The
constant load tensile test was carried out in test bath at the ordinary temperature (25OC)
based on NACE TMO177 Method A. As the test bath, A bath was used. The A bath
was an aqueous solution of 5% of NaCl and 0.5% of CH3COOH, which was held at the
ordinary temperature and in which hydrogen sulfide gas of 1 atrn was saturated.
[0118]
Each of the test specimens was immersed in the A bath. A constant load that
was 90% of an actual yield stress was applied to the test specimen in the A bath.
Occurrence of the cracking was confirmed in the test specimen after 720 hours. The
plate materials in which the cracking did not occur were judged as the plate material
having excellent SSC resistance.
[0119]
[Autoclave Test]
Assuming that the steel material would be used under a well environment
where pressure of hydrogen sulfide would be 1 atrn or higher, which would be recently
required, the autocalve test was canied out by using B bath. The B bath was an
aqueous solution of 5% of NaCl in which hydrogen sulfide of 10 atm was saturated.
A specific test method was as follows.
[O 1201
Four-point bending test specimens having 2 mm x 10 nun x 75 rnm were
taken from the respective plate materials. A stress of 90% of an actual yield stress
(yield stress of the respective marks) was applied to the taken foui-point bending test
specimens by using four-point bending jig based on ASTM G39. The four-point
bending test specimen to which the stress was applied was placed in an autoclave.
After the four-point bending test specimen was placed, the degassed aqueous solution
of 5% of NaCl was filled into the autoclave. Thereafter, hydrogen sulfide of 10 atm
was filled therein. By the above-described procedure, the B bath was prepared in the
autoclave, and the four-point bending test specimen was immersed in the B bath.
Occurrence of the cracking was visually confirmed in the test specimen after 720 hours
from filling the hydrogen sulfide of 10 atm. The plate materials in which the cracking
did not occur were judged as the plate material having excellent SSC resistance. In
addition, a pressure inside the autoclave during the test was constantly controlled to be
10 atm.
[DCB Test]
DCB test specimens having a thickness of 10 mm, a width of 25 mm, and a
length of 100 rnm were taken from the plate materials having the strength of 11 0 ksi
(758 MPa) or more. The,DCB test was carried out by using the taken DCB test
specimens based on NACE (National Association of Corrosion Engineers) TMO 177
Method D. As test bath, the B bath was used. Each of the DCB test specimens was
immersed in the B bath for 336 hours. After 336 hours, a length of crack propagation
occurred in the DCB test specimen was measured. A stress intensity factor Krssc
(ksidin) was obtained based on the measured length of the crack propagation.
[O 1 221
[Result of SSC Resistance Test]
Production conditions and results of SSC resistance test are shown in Tables 2
and 3.
. - . - ---- -- - - - - - . - - - -
.- _ .. -. . .. . . . . . . _ .. a - . . . .
gsa
p [Table 31 z
6-1
Numerical values to which * is attached are numerical values out of range of the invention
I
[Table 31
Numerical values to which * is attached are numerical values out of range of the invention
[0125]
Table 3 continues from Table 2. "Yes" in a "normalizing" column in Tables
2 and 3 indicates that the normalizing treatment was carried out for the steel material
of the corresponding mark. "CC" in a "cooling method" column indicates that the
quenching by the continuous cooling treatment was carried out for the steel of the
corresponding mark. "IC" indicates that the quenching including the isothermal
treatment was carried out for the steel of the corresponding mark. In a "Tccm column,
the passing time at the Ms point Tcc (s) in the continuous cooling treatment is shown.
In an "ATIc" column, the cooling stopping temperature ATIc ("C) of the initial cooling
in the quenching including the isothermal treatment is shown. In a "hardness HRC"
column, Rockwell hardness (HRC) of the corresponding mark is shown. In a "grain
size number" column, prior-austenite grain size number of the corresponding mark is
shown. In an "F3" column, F3 (= 50C + 26) which is a value of the right-hand side of
the Expression (3) is shown. In a "temperature" column and a "time" column of a
"tempering" column, tempering temperature ("C) and tempering time (minute) of the
corresponding mark are shown, respectively. In an "HW column, the full width at
half maximum (") of the corresponding mark is shown. In an "Fl" column, F1 (=
HW x c'") which is a value of a left-hand side of the Expression (1) is shown. In an
"M2C" column, the number (pieces/pm2) of hexagonal M2C is shown. In a "YS"
column, yield strength (MPa) of the corresponding mark is shown.
[0126]
In an "SSC test" column, results of SSC resistance test in the A bath and the B
bath are shown. "No" indicates that the cracking did not occur. "Yes" indicates that
the cracking occurred.
In addition, fraction of the residual austenite of all marks 1 to 68 were 0%.
[0128]
Referring to Tables 2 and 3, it could be seen that all of the chemical
compositions of the steel materials of marks 1 to 5 1 were within the range of the
chemical composition of the low-alloy steel for oil country tubular goods of the
invention. Furthermore, in the steel materials of the marks 1 to 5 1, the F 1 value was
0.38 or less, and satisfied the Expression (1). Furthermore, 5 pieces/pm2 or more of
the hexagonal M2C carbide having a grain size of 1 nm or more were present in the
steel materials of the marks 1 to 5 1. Therefore, in the steel materials of the marks 1 to
5 1, cracking was not observed in the SSC resistance test in both of the A bath and the
B bath.
[0 1 291
In addition, the grain size number of the prior-austenite grain of marks (3, 4, 7,
8, 11, 12, 15, 16, 19,20,23,24,26-28,32,33, 35, 36,38,39,41,and42)inwhichthe
normalizing treatment was carried out was 10 or more, and the grains were refined as
compared to marks in which the normalizing treatment was not carried out by using the
same type of steel (for example, the mark 1 or the like in comparison with the mark 4).
[0130]'
On the other hand, in marks 52,53,55, and 56, the chemical composition was
within the range of the invention, and the Expression (2) was satisfied, but the passing
time at the Ms point Tcc in the quenching by the continuous cooling treatment
exceeded 600 seconds. Furthermore, the tempering temperature was lower than
680°C. Therefore, the Rockwell hardness was lower than the F3 value and did not
satisfy the Expression (3) in the quenched material, and the F1 value exceeded 0.38
and did not satisfy the Expression (1). In addition, the hexagonal M2C carbides
$--a,p&-+ S- F- - - - -
L - - - . - - - - -- -
having the grain size of 1 nm or more were less than 5 pieces/pm2. Accordingly, in
the marks 52, 53, 55, and 56, cracking was observed in the SSC resistance test of both
of the A bath and the B bath.
[0131]
In a mark 54, the chernical~compositionw as within the range of the invention
and the Expression (2) was satisfied. However, the cooling stopping temperature
ATIc in the quenching including the isothermal treatment was higher than 300°C.
Furthermore, the tempering temperature was lower than 680°C. Therefore, the
Rockwell hardness did not satisfy the Expression (3) in the quenched material, and the "
F1 value exceeded 0.38 and did not satisfy the Expression (1). In addition, the
hexagonal M2C carbides having the grain size of 1 nrn or more were less than 5
pieces/pm2. Accordingly, in the mark 54, the cracking was observed in the SSC
resistance test of both of the A bath and the B bath.
In a mark 57, C content was less than the lower limit of the C content of the
invention. Therefore, the cracking was observed in the SSC resistance test of both of
the A bath and the B bath. It.is assumed that the C content is insufficient, and thus
grain boundary carbides are not sufficiently spheroidized.
[0133]
In a mark 58, Mn content exceeded the upper limit of the invention. In a
mark 59, P content exceeded the upper limit of the invention. In a mark 60, S content
exceeded the upper limit of the invention. Therefore, the cracking was observed in
the SSC resistance test in both of the A bath and the B bath. ' It is assumed that
excessive Mn, P, and S segregated at grain boundaries, and thus the SSC resistance
decreased.
I
[0134]
In marks 61, 63, and 64, Mo content was less than the lower limit of the
invention. Therefore, the number of the hexagonal M2C carbides having the grain
size of 1 nm or more was less than 5 pieces/pm2. Furthermore, in the marks 61 and
63, the haidenability was deficient, and the fraction of the martensite decreased, and
thus a structure mainly composed of the bainite was formed. Therefore, the full width
at half maximum HW of [211] crystal, plane also increased. Therefore, in the marks
61,63, and 64, the cracking was observed in the' SSC resistance test in both of the A
bath and the B bath.
[0135]
In marks 62,67, and 68, the chemical composition was within the range of the
invention, but the Expression (2) was not satisfied. In addition, the number of the
hexagonal M2C carbides having the grain size of 1 nm or more was less than 5
pieces/pm2. Furthermore, in the mark 62, the hardenability was deficient, and the
fraction of the martensite decreased, and thus a structure mainly composed of the
bainite was formed. Therefore, the full width at half maximum HW of [211] crystal
plane also increased. Therefore, in the marks 62,67, and.68, the cracking was
observed in the SSC resistance test in both of the A bath and the B bath.
[0136]
In a mark 65, C content was less than the lower limit of the invention and the
Expression (2) was not satisfied. Therefore, the number of the hexagonal M2C
carbides having the grain size of 1 nm or more was less than 5 pieces/pm2. Therefore,
the cracking was observed in the SSC resistance test in both of the A bath and the B
bath.
In a mark 66, A1 was not included in the steel material, and 0 content
exceeded the upper limit of the invention. Therefore, the cracking was observed in
the SSC resistance test in both of the A bath and the B bath.
[0138]
[Result of DCB Test]
Production conditions and results of DCB test are shown in Table 4.
[0139]