Ni-Cr ALLOY MATERIAL AND SEAMLESS OIL COUNTRY TUBULAR GOODS
USING THE SAME
TECHNICAL FIELD
[OOOl]
The present invention relates to a Ni-Cr alloy material and seamless oil country
tubular goods using the same material. In particular, the present invention relates to a
high-strength Ni-Cr alloy material that is excellent in hot workability and shock
resistance property, and also excellent in corrosion resistance (especially, stress
corrosion cracking resistance in an environment being at high temperatures more than
200°C and including a large amount of hydrogen sulfide), and seamless oil country
tubular goods using the high-strength Ni-Cr alloy material.
[0002]
The term "high-strength" herein refers to having yield strength (0.2% offset
yield stress) of 965 MPa (140 ksi) or more. Note that the term "oil country tubular
goods" is a general term for casing, tubing, and drill p~pesth at are used for drilling an
oil well or a gas well to collect crude oil, natural gas or the like, as described in the
column of defimtion in the number 3514 " steel pipe for oil well casing, tubing and
drilling" of JIS G 0203 (2009), for example. The term "seamless oil country tubular
goods" is a seamless tube that can be used for, for example, drilling an oil well or a gas
well to collect crude oil, natural gas or the like.
BACKGROUND ART
[0003]
With a rise in the price of crude oil in recent years, the developments of oil and
natural gas wells at greater depths and in more corrosive environments have been
underway. For drilling for oil and natural gas in such harsh environments, oil country
tubular goods have been required to have high-strength and an excellent corrosion
resistance.
[0004]
Oil and natural gas contain corrosive substances such as carbon dioxide (COz),
hydrogen sulfide (H2S), and chloride ions (Cl3. For this reason, materials for oil
country tubular goods used for drilling for oil or natural gas are required to have an
excellent corrosion resistance against these corrosive substances. Especially under
environments being at high temperatures of 150°C or more and including hydrogen
sulfide at 1 atm or more, a main cause of corroding oil country tubular goods is stress
corrosion craclung. Therefore, materials for oil country tubular goods used in such
environments are required to have an excellent stress corrosion cracking resistances.
[0005]
Furthermore, the growing demand of oil and natural gas has brought oil and
gas wells at greater depths and higher temperatures. Materials for oil country tubular
goods used in such wells are required to withstand a corrosion environment containing
carbon dioxide, hydrogen sulfide and chloride ions, and to have high-strength at the
same time. For this reason, in recent years, there has been a greater demand for
high-strength oil country tubular goods having a high yield strength (0.2% offset yield
stress) of 965 MPa or more.
[0006]
The use of Ni-Cr alloy materials such as those disclosed in Patent Document 1
to 3 have been attempted in the above harsh corrosive environments, where low alloyed
steels, such as martensitic stainless steels, or duplex stainless steels cannot be applied.
[0007]
For example, in the alloy for oil country tubular goods in Patent Document 1,
contents of Ni and Cr are adjusted within specific ranges, and a value of Cr (%) +10Mo
(%) +5W (%) and a value of Mo (%) + (112)W (%) are adjusted within specific ranges.
Patent Document 1 discloses that the above alloy can ensure the stress corrosion
cracking resistance in a "H2S-C02-C1-" environment as long as the temperature of the
environment is 150°C or less even if it is a cold worked material. In addition, Patent
Document 1 discloses that it is possible to achieve a 0.2% yield stress of 965 MPa or
more by adjusting N concentration within a range of 0.05 to 0.30% by mass%, and
performing cold working after solution treatment.
[0008]
In the alloy for oil country tubular goods in Patent Document 2, contents of Ni
and Cr are adjusted within specific ranges, and a value of Cr (%) -t 10Mo (%) -t5W (%)
and a value of Mo (%) + (112)W (%) are adjusted within specific ranges. Patent
Document 2 discloses that the above alloy can ensure the stress corrosion cracking
resistance in a "H2S-C02-Cl'" environment having an extremely excellent corrosion
property, in particular in an adverse environment at 150°C or less, even if it is a cold
worked material. In addition, Patent Document 2 discloses that it IS possible to
achieve a 0.2% yleld stress of 965 MPa or more by making N contained within a range
of 0.05 to 0.25% by mass%, and performing both cold worlung and aging treatment
after solution treatment.
[0009]
In the alloy for oil country tubular goods in Patent Document 3, contents of Mn,
Ni, and Cr are adjusted within specific ranges, and a value of (112) Mn (%) +Ni (%), a
value of Cr (%) +Mo (%) + (112)W (%), and a value of Mo (%) + (112)W (%) are
adjusted within specific ranges. Patent Document 3 discloses that the above alloy can
ensure the stress corrosion cracking resistance in a "H2S-C02-C1-" environment having
an extremely strong corrosive properties, in particular, an adverse environment at 150°C
or less, even if it is a cold worked material. In addition, Patent Document 3 discloses
that it is possible to achieve a 0.2% yield stress of about 940 MPa by making N
contained within a range of 0.1 to 0.4% by mass%, and performing cold working after
solution treatment.
[OO 1 01
Note that, strengthening by increasing the N content as in Patent Document 1
to 3 described above is prone to cause a problem of reducing hot workability of the alloy.
For this reason, Patent Document 1 to 3 disclose a technique to improve hot workability
by reducing a content of S to 0.0007% or less by mass%, or by making Ca, Mg, Ti, or
the rare-earth metal elements (hereafter, referred to as "REM") contained
[OOl 11
In the high Cr -high Ni alloy material in Patent Document 4, contents of Cu, Ni,
and Cr are adjusted within specific ranges, and a value of Cu + 0.4(Mo - 1.4)~is
adjusted wihn a specific range. Patent Document 4 discloses that the above alloy can
ensure the good stress corrosion cracking resistance in a corrosive "H~S-COZ-C~-"
environment even when it is subjected to 25% cold working to bring the 0.2% yield
stress of the alloy to a strength level of 861 to 964 MPa (87.75 to 98.28 kgflmm2),
so-called " 125 ksi-class".
[OO 1 21
Note that Patent Document 4 also discloses a technique to improve the hot
workability by reducing a content of S to 0.0007% or less by mass%, or by making Ca,
Mg, REM contained.
[00 1 31
In addition, Patent Document 5 discloses a super austenitic stainless steel
having an excellent corrosion resistance in an acid environment and a seawater
environment, and it is also excellent in the hot workability by adjusting contents of Cr,
Ni, Mo, Mn, and N within specific ranges and making elements such as Mg, Ca, and Ce
contained.
[OO 141
Ln the Cr- Ni alloy material in Patent Document 6, contents of Cu, Ni, Cr, Mo,
N, Al, and REM are adjusted within specific ranges, and a value of N (%) x P (%)/REM
(%) is adjusted within a specific range. Patent Document 6 discloses that the above
alloy material can ensure the good hot workability, and ensure the good stress corrosion
cracking resistance in a corrosive environment including HzS, Cl', and the like at a
temperature of 177°C by performing cold rolling of 40% reduction of area to provide a
high 0.2% yield stress of 941 to 1176 MPa.
[0015]
Patent Document 7 discloses a producing method for a stainless steel in which
contents of Cr, Ni, Si, Mn, C, N, Mo, S, B, P, and 0 are adjusted within specific ranges.
Patent Document 7 describes that the above stainless steel is excellent in strength and
the stress corrosion cracking resistance.
[00 1 61
Patent Document 8 discloses an austenitic alloy in which contents of C, Si, Mn,
Cr, Ni, Mo, La, Al, Ca, 0 , P, and S are adjusted within specific ranges. Patent
Document 8 describes that the above austenitic alloy has a good crack resistance in an
environment rncluding hydrogen sulfide.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[00 171
Patent Document 1 : JP57-203735A
Patent Document 2: JP57-207149A
Patent Document 3: JP58-2 101 55A
Patent Document 4: JPll-302801A
Patent Document 5: JP2005-509751A
Patent Document 6: JP2009-84668A
Patent Document 7: JP 1 -262048A
Patent Document 8: JP63-274743A
NON PATENT DOCUMENT
[OO 1 81
Non-Patent Document 1 : Koichl Nakashima et al.: CAMP-ISIJ, 17 (2004), 396
Non-Patent Document 2: G. K. Williamson and W. H. Hall: Acta Metall., 1
(1 953), 22
Non-Patent Document 3: H. M. Rietveld: J. Appl. Cryst., 2 (1969), 65
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[00 191
By using the alloys disclosed in Patent Documents 1 and 2 described above, it
is possible even with a 0.2% yield stress of 965 MPa or more to ensure the good stress
corrosion cracking resistance in the corrosive environment including hydrogen sulfide
as long as the temperature of the environment is at 150°C or less.
[0020]
In addition, by using the alloy disclosed in Patent Document 3 mentioned
above, it is possible with a 0.2% yield stress of about 940 MPa to ensure the good stress
corrosion cracking resistance even in a corrosive environment including hydrogen
sulfide when the temperature of the environment is 150°C or less.
1002 11
Furthermore, by using the alloy disclosed in Patent Document 4 mentioned
above, it is certainly possible to ensure the good stress corrosion cracking resistance
even in a corrosive environment including hydrogen sulfide when the 0.2% yield stress
of the alloy is at a strength level of 861 to 964 MPa, so-called "125 ksi-class".
Moreover, when a content of Mo is 1.5% or less by mass%, it is possible to
considerably improve the hot workability of the alloy, which enables Mannesmann pipe
making process, when applied, to create a finished pipe without any problems.
[0022]
Similarly, by using the alloy disclosed in Patent Document 6 mentioned above,
it is certainly possible even with a high 0.2% yield stress of 965 MPa or more to ensure
the good stress corrosion cracking resistance even in a corrosive environment including
hydrogen sulfide, as long as the temperature of the enviroiunent is 177°C or less.
[0023]
However, even with the alloy proposed in Patent Documents 1 to 4 and Patent
Document 6, the fact is that when cold working is performed to increase a 0.2% yield
stress to 965 MPa or more, it is still difficult to ensure the good stress corrosion
cracking resistance in a corrosive environment including hydrogen sulfide at a high
temperature region more than 200°C.
[0024]
Furthelmore, even an austenite-based Ni-Cr alloy degrades its toughness
(shock resistance property) inevitably when cold working is performed to increase the
0.2% yield stress to 965 MPa or more. For this reason, there is a possibility that a
product is broken during transportation or in use.
[0025]
Note that in the case of the alloy proposed in Patent Document 3, the content of
P is limited to 0.030% or less by mass%, which is for avoiding co-segregation of Mn
with P. However, since Mn is contained as much as 3.0 to 15.0% by mass%, it may be
difficult to avoid the co-segregation of Mn with P even if the content of P is limited to
0.030% or less. Then, the co-segregation of Mn with P leads to the reduction of
toughness, which may cause, for example, a trouble in transportation of a product as
mentioned above when high cold working is performed to obtain a high strength.
[0026]
In addition, the alloy proposed in Patent Document 5 has a problem that
performing cold working with a high reduction rate to obtain a high strength invokes the
reductions of ductility and toughness. Furthermore, although the above alloy contains
Mn at 1.0 to 6.0%, preferably 2.0 to 6.0%, more preferably 3.0 to 6.0%, and very
preferably 4.0 to 6.0% by mass%, there is no consideration about the content of P at all.
For this reason, it is difficult to avoid a considerable reduction of toughness due to the
co-segregation of Mn with P even by a low level of cold working. Ln addition, the
alloy in Patent Document 5 is not an alloy stably ensuring the good stress corrosion
cracking resistance in a corrosive environment including hydrogen sulfide and in a high
temperature region more than 200°C, in particular when severe cold working is
performed to increase the 0 2% yield stress to 965 MPa or more.
[0027]
In addition, for the stainless steel in Patent Document 7, there is no
consideration about cold working. In other words, there is no description in Patent
Document 7 about an alloy component with which it is possible to stably ensure the
good stress corrosion cracking resistance in a corrosive environment at a high
temperature when severe cold working is perfonned to increase the 0.2% yield stress to
965 MPa or more.
[0028]
In addition, for the austenite alioy in Patent Document 8, it is necessary to
make N contained so as to increase the 0.2% yield stress to 965 MPa or more, but there
is no description in Patent Document 8 about a N content. Ln addition, Patent
Document 8 does not describe an alloy composition that offers a hlgh-strength of 965
MPa or more and exhibits an excellent corrosion resistance in a corrosive environment
at a high temperature.
[0029]
An objective of the present invention, which has been made in a view of the
above circumstances, is to provide a Ni-Cr alloy material that enables the prevention of
reducing hot workability, corrosion resistance, and toughness with an increase of high
strength, and seamless oil country tubular goods using the Ni-Cr alloy material. In
particular, an objective of the present invention is to provide a high-strength Ni-Cr alloy
material that is excellent in hot workability and toughness, also excellent in corrosion
resistance (more specifically, the stress corrosion cracking resistance in an environment
being at high temperatures more than 200°C and including hydrogen sulfide), and has a
yield strength (0.2% yield stress) of 965 MPa or more, and to provide seamless oil
country tubular goods using the high-strength Ni-Cr alloy material.
MEANS FOR SOLVING THE PROBLEMS
[0030]
To solve the above problems, the present inventors first conducted basic studies
about the improvement of yield strength (0.2% y~elds tress) using Ni-Cr alloy materials,
based on the Ni-Cr alloy materials having been proposed so far, with the chemical
composition thereof adjusted in various manners. As a result, the following points (a)
to (e) were found.
[003 11
(a) Effective means for increasing a yield strength of a Ni-Cr alloy material are
to raise a cold working rate to increase a dislocation density of the alloy material, or to
increase a N content of the alloy material, especially the N content in a solid solution
state.
[0032]
(b) In contrast, making N contained in a large amount results in the reduction
of not only toughness but also hot workability. For this reason, for example, it is
difficult to avoid the occurrence of a lot of defects in working a product such as
pipe-malung. Ln addition, making N contained in a large amount may cause a Cr
nitride to remain in a micro-structure even after solution treatment. In this case,
corrosion resistance at a high temperature and hlgh-pressure hydrogen sulfide
environment is considerably reduced.
[0033]
(c) To inhibit the reduction of hot workability, it is effective to prevent
segregation of S in grain boundaries, which occurs at about 900°C.
COO341
(d) Toughness of an alloy material is reduced also by an increase in dislocation
density through cold working.
[0035]
(e) In the case of a Ni-Cr alloy material not containing nitrogen, which has
been proposed, increasing a cold working rate to bring the 0.2% yield stress to 965 MPa
or more disables ensuring the stress corrosion cracking resistance in an environment
including hydrogen sulfide at 200°C.
[0036]
Next, the present inventors conducted various researches to inhibit in particular
the reduction in hot workability within a low-temperature region in a nitrogen-contained
material, and the degradation in toughness when the cold working rate is increased for a
high strength. As a result, the present inventors obtained the following findings (f) to
(h).
[0037]
(f) Only by conventional desulfurization using Ca andlor Mg, it is difficult to
stably inhibit the reduction of hot workability in a low-temperature region. In contrast,
REM has a desulfurizing effect equivalent to or more than Ca or Mg, but prone to be
oxidized. Therefore, REM alone contained as an element for desulfurization does not
result in a sufficient desulfurizing effect, and it is dificult to stably inhibit the reduction
of hot workability in a low-temperature region.
[003 81
(g) By malung REM contained in combination with Ca and/or Mg to perform
desulhrization, it is possible to stably inhibit the reduction of hot workability in a
low-temperature region.
[0039]
(h) Note that addition of REM increases the amount of inclusions, which
makes the toughness degradation of the alloy material conspicuous when the cold
working rate is increased for a high strength.
[0040]
Thus, the present inventors conducted researches in further details. As a
result, an important fact described in the following (i) was discovered.
[004 11
(i) When the N content is adjusted within specific ranges, and REM is
contained in combination with Ca and/or Mg, it is possible to stably inhibit the
reduction of hot workability in a low-temperature region, and moreover to provide a
Ni-Cr alloy that has a high strength, a good toughness and the excellent stress corrosion
cracking resistance in a high temperature environment including hydrogen sulfide.
Specifically, there is a Ni-Cr alloy that has a 0.2% yield stress of 965 MPa or more, a
good toughness (shock resistance property) at a low temperature, which is an impact
value of more than 63 ~ I c r nat~ - lO°C, and the excellent stress corrosion cracking
resistance in an environment at high temperatures more than 200°C and including
hydrogen sulfide. Such an alloy satisfies specified conditions about chemical
composition as well as dislocation density.
[0042]
The present invention has been completed based on the above facts, and the
gist thereof is a Ni-Cr alloy material described below and seamless oil country tubular
goods using the same.
[0043]
(1) A Ni-Cr alloy material having a chemical composition that consists of, by
mass%:
Si: 0.0 1 to 0.5%, Mn: 0.0 1 % or more and less than 1.0%, Cu: 0.0 1 % or more
and less than 1.0%, Ni: 48% or more and less than 55%, Cr: 22 to 28%, Mo: 5 6% or
more and less than 7.0%, N: 0.04 to 0.16%, sol.Al: 0.03 to 0.20%, REM: 0.01 to
0.074%, W: 0% or more and less than 8.0%, Co: 0 to 2.0%, one or more of Ca and Mg:
0.0003 to 0.01% in total, and one or more of Ti, Nb, Zr, and V: 0 to 0.5% in total; and
the balance being Fe and impurities, wherein
C, P, S, and 0 in the impurities are C: 0.03% or less, P: 0.03% or less, S:
0.00 1 % or less, and 0: 0.0 1 % or less, and furthermore
the dislocation density satisfies a following formula:
7.0 x 10" 5 p 5 2.7 x 1016 - 2.67 x lOI7 x [EM(%)]
where p denotes dislocation density expressed in m-2, and [REM(%)] denotes
REM content expressed by mass%.
[0044]
(2) The Ni-Cr alloy material according to the above (I), containing 0.1% or
more and less than 8.0% of W, by mass%.
[0045]
(3) The Ni-Cr alloy material according to the above (1) or (2), containing 0.01
to 0.5% of one or more of Ti, Nb, Zr, and V in total, by mass%.
100461
(4) The Ni-Cr alloy material according to any one of the above (1) to (3),
containing 0.01 to 2.0% of Co, by mass%.
[0047]
(5) Seamless oil country tubular goods comprising the Ni-Cr alloy material
according to any one of the above (1) to (4).
ADVANTAGEOUS EFFECTS OF THE INVENTION
100481
The Ni-Cr alloy material according to the present invention is excellent in hot
workability and toughness (shock resistance property). In addition, even with a
high-strength, a yield strength (0.2% yleld stress) of 965 MPa or more, the Ni-Cr alloy
material according to the present invention is still excellent in corrosion resistance
represented by the stress corrosion cracking resistance, in an environment being at high
temperatures more than 200°C and including hydrogen sulfide. For this reason, the
Ni-Cr alloy material according to the present invention can be suitably used for
high-strength seamless oil country tubular goods.
BRIEF DESCRLPTION OF THE DRAWINGS
[0049]
[Figure 11 Figure 1 is a diagram illustrating for alloys used in Example how yeld
strength and toughness vary as E M content and dislocation density vary, summarizing
the results of Table 2. Note that Figure 1 excludes the results on the alloys Nos. 1 to 6
and Nos. 8 to 13 because, as to alloys Nos. 1 to 5 and Nos. 8 to 13, at least one of
elements of these alloys other than REM is out of a range specified in the present
invention, and as to an alloy No. 6, evaluation of toughness and the like was not
performed due to its deteriorated hot workability in itself. The line in the figure
expresses the relationship: p = 2.7 x 1016 - 2.67 x 10'' x [EM(%)].
MODE FOR CARRYING OUT THE INVENTION
[0050]
Hereinafter, each requirement of the invention will be described in detail.
Note that "%" of the content of each element represents "mass%".
1005 l]
(A) Chemical Composition
Si: 0.01 to 0.5%
Si (Silicon) is an element necessaly for deoxidation, and the Si content is
0.01% or more. The Si content more than 0.5% tends to result in degradation in hot
workability. Therefore, the Si content is set at 0.01 to 0.5%. A preferable lower limit
of the Si content is 0.05%, and a more preferable lower limit thereof is 0.07%. In
addition, a preferable upper limit of the Si content is 0.40%, and a more preferable
upper limit thereof is 0.33%.
[0052]
Mn: 0.01% or more and less than 1 .O%
Mn (manganese) is an element necessary for deoxidizing andlor desulfurizing,
but the effects thereof do not exhibit sufficiently when the Mn content is less than
O.OlOh. On the other hand, the Mn content of 1 .O% or more results in degradation of
hot workability. Therefore, the Mn content is set at 0.01% or more and less than 1 0%.
A preferable lower limit of the Mn content is 0.10%, a more preferable lower limit
thereof is 0.20, and a still more preferable lower limit is 0.24%. In addition, a
preferable upper limit of the Mn content is 0.80%, a more preferable upper limit thereof
is 0.70%, and a still more preferable upper limit thereof is 0.66%.
100531
Cu: 0.01 % or more and less than 1 .O%
Cu (copper) has an effect of stabilizing a passivation film formed on a surface
of a Ni-Cr alloy material and is thus necessary to enhance pitting resistance and general
corrosion resistance. However, the Cu content less than 0.01% makes the effect
insufficient, and the Cu content of 1.0% or more results in degradation of hot
workability. For this reason, the Cu content is set at 0.01% or more and less than 1.0%.
A preferable lower limit of the Cu content is 0.20%, and a more preferable lower limit
thereof is 0.55%. In addition, a preferable upper limit of the Cu content is 0.85%, and
a more preferable upper limit thereof is 0.8%.
[0054]
Ni: 48% or more and less than 55%
Ni (nickel) is contained as an austeilite stabilizing element. The Ni content is
48% or more from the viewpoint of corrosion res~stanceb, ut the Ni content of 55% or
more leads to an increase in costs and degradation of hydrogen cracking resistance.
For thls reason, the Ni content is set at 48% or more and less than 55%. A preferable
lower limit of the Ni content is 49%, and a more preferable lower limit thereof is 49.2%.
In addition, a preferable upper limit of the Ni content is 52%, and a more preferable
upper limit thereof is 5 1.1 %.
[0055]
Cr: 22 to 28%
Cr (chromium) is a component that considerably improves the stress corrosion
cracking resistance, but the Cr content less than 22% makes the effect thereof
insufficient. On the other hand, the Cr content more than 28% leads to degradation in
hot workability, and facilitates formation of TCP (topologically closed packed phases)
represented by Sigma phase, leading to degradation of the stress corrosion cracking
resistance. Therefore, the Cr content is set at 22 to 28%. A preferable lower limit of
the Cr content is 23%, and a more preferable lower limit thereof is 23.5%. In addition,
a preferable upper limit of the Cr content is 26%, and a more preferable upper limit
thereof is 25.7%.
100561
Mo: 5.6% or more and less than 7.0%
Mo (molybdenum) has an effect of, as with Cu, stabilizing a passivation film
formed on a surface of a Ni-Cr alloy material and an effect of improving pitting
resistance and the stress corrosion cracking resistance. However, the Mo content less
than 5.6% makes the effect insufficient. On the other hand, the Mo content of 7.0% or
more results in an increase in high temperature strength of austenite and promotes
formation of a deleterious phase such as Sigma phase and mu phase during casting the
alloy. This degrades hot workability. Furthermore, an excessive Mo content leads to
an increase in alloy cost. Therefore, the Mo content is set at 5.6% or more and less
than 7.0%. A preferable lower limit of the Mo content is 5.7%, and a more preferable
lower limit thereof is 5.8%. In addition, a preferable upper limit of the Mo content is
6.8%, and a more preferable upper liinit thereof is 6.7%.
100571
N: 0.04 to 0.16%
N (nitrogen) is an important element in the invention. N functions to increase
a strength of a Ni-Cr alloy, but the N content less than 0.04% does not allow for
ensuring a desired high-strength and easily leads to an abrupt reduction of the stress
corrosion cracking resistance due to an increase in dislocation density. On the other
hand, the N content more than 0.16% leads to decrease in the maximum workable
temperature in hot worlung and deterioration of the stress corrosion cracking resistance
with precipitation of a chromium nitride. For this reason, the N content is set at 0.04
to 0.16%. A preferable lower limit of the N content is 0.06%, a more preferable lower
limit thereof is 0 08%, and a still more preferable lower limit thereof is 0.098%. In
addition, a preferable upper limit of the N content is 0 14%, and a more preferable upper
limit thereof is 0.125%.
[0058]
sol.Al: 0.03 to 0.20%
A1 (aluminum) has not only an effect of deoxidation to improve hot workability
but also an effect of preventing the effect of REM from decreasing due to oxidation.
In a Ni-Cr alloy containing REM but containing no Al, a large part of the REM is
consumed to be an oxide thereof. This results in an increase of solute S significantly
reducing hot workability. Therefore, when containing REM, it is necessary to contain
A1 together. Note that the Al content less than 0.03% as sol.Al makes the effect
insufficient. On the other hand, the A1 content more than 0.20% as sol.Al rather leads
to degradation in hot workability. Therefore, the A1 content is set at 0.03 to 0.20% as
sol.Al. A preferable lower limit of the Al content is 0.05% as sol.Al, a more preferable
lower limit thereof is 0.07%, and a still more preferable lower limit thereof is 0.076%.
In addition, a preferable upper limit of the A1 content is 0.18% as sol.Al, a more
preferable upper limit thereof is 0.14%, and a still more preferable upper limit thereof is
0.135%. Note that the term "sol.Al" refers to so-called "acid-soluble Al".
[0059]
RElM: 0.01 to 0.074%
REM is an important element in the present invention. REM is contained
because of its effect of improving hot workability and the stress corrosion cracking
resistance. Note that it is necessary to contain A1 together because REM is easy to
oxidize. Making an alloy containing REM in combination with Ca andfor Mg stably
inhibits degradation in hot workability in a low-temperature region and provides the
alloy with high-strength, good toughness at a low temperature range, and the stress
corrosion cracking resistance in a high temperature environment that includes hydrogen
sulfide. However, the REM content less than 0.01% makes the above effects
insufficient, and result in degradation in stress corrosion cracking resistance due to the
high-strength. In contrast, the REM content more than 0.074% rather results in
degradation in hot workability and toughness even when the REM is contained in
combination with Ca andlor Mg. Therefore, the REM content is set at 0.01 to 0.074%.
A preferable lower limit of the REM content is 0.015%, and a more preferable lower
limit thereof is 0.019%. In addition, a preferable upper limit of the REM content is
0.06%, and amore preferable upper limit thereof is 0.058%.
[0060]
In the present invention, the term "REM" refers to 17 elements in total, Sc
(scandium), Y (yttrium), and Lanthanoid elements, and the term "REM content" refers
to a content of a REM if the number of REMs is one, or to a total content of REMs if
the number of REMs is two or more. In addition, REMs are supplied in general as a
misch metal, which is a mixture of REMs. For this reason, one, two or more of
individual elements may be added such that the amount of REMs falls within the above
range, or for example, the elements may be added in the form of the misch metal such
that the amount of REMs falls within the above range.
[006 11
One or more of Ca and Mg: 0.0003 to 0.01% in total
Making an alloy containing Ca (calcium) andlor Mg (magnesium) in
combination with REM stably inhibit degradation in hot workability in a
low-temperature region and provides the alloy with hlgh-strength, good toughness at a
low temperature range, and the stress corrosion cracking resistance in an elevated
temperature environment that includes hydrogen sulfide. The above effects are
obtained by making the alloy contain 0.0003% or more of one or more of Ca and Mg in
total. However, making the alloy containing more than 0.01% of one or more of Ca
and Mg in total rather causes degradation of hot workability even when the one or more
of Ca and Mg is contained in combination with REM. Therefore, the content of one or
more of Ca and Mg is set at 0.0003 to 0.01% in total. A preferable lower limit of the
total content of one or more of Ca and Mg is 0.0005%, and a more preferable lower
limit thereof is 0.0007%. In addition, a preferable upper limit of the total content of
one of more of Ca and Mg is 0.005%, and a more preferable upper limit thereof is
0.0042%.
[0062]
Note that it is not necessary to make the alloy containing Ca and Mg in
combination. In the case of making the alloy contain only Ca, the content of Ca may
be set at 0.0003 to 0.01%, and in the case of making the alloy contain only Mg, the
content of Mg may be set at 0.0003 to 0.01%.
[0063]
W: 0% or more and less than 8.0%
W (tungsten) functions to increase the stress corrosion cracking resistance.
For this reason, W may be contained as necessary. However, the W content of 8.0% or
more deteriorates hot workability and economic efficiency. Therefore, the amount of W
is set at less than 8.0% when W is to be contained. The amount of W is preferably less
than 7.0% when W is to be contained.
[0064]
On the other hand, to stably give rise to an advantageous effect of improving
the stress corrosion cracking resistance brought by W described above, the amount of W
is preferably 0.1% or more when W is to be contained.
[0065]
Ti, Nb, Zr, and V: 0 to 0.5% of one or more thereof in total
Ti (titanium), Nb (niobium), Zr (zirconium), and V (vanadium) all function to
refine grains so as to increase strength and ductility. For this reason, Ti, Nb, Zr, and V
may be contained alone or two or more thereof may be contained in combination as
necessity. However, making an alloy contain more than 0.5% of one or more of the
above elements in total deteriorates hot workability and generates a large number of
inclusions, rather causing a phenomenon of reducing ductility. Therefore, the amount
of the one or more these elements is set at 0.5% or less in total when these elements are
contained. The amount of the one or more of these elements is preferably 0.3% or less
in total when these elements are to be contained.
[0066]
On the other hand, to stably give rise to an advantageous effect of improving
strength and ductility brought by Ti, Nb, Zr, or V described above, the amount of the
one or more of Ti, Nb, Zr, and V is preferably 0.01% or more in total, and more
preferably 0.05% or more.
[0067]
Co: 0 to 2.0%
Co (cobalt) contributes to stabilization of the austenite phase and functions to
improve the stress corrosion cracking resistance at high temperatures. For this reason,
Co may be contained as necessary. However, an excessive Co content leads to a rise
of alloy price, significantly impairing economic efficiency. Therefore, the amount of
Co with a specified upper limit is set at 2.0% or less when Co is to be contained. The
amount of Co is preferably 1.0% or less when Co is to be contained.
100681
On the other hand, to stably develop give rise to an advantageous effect of
Improving stress corrosion cracking resistance at high temperatures brought by Co
described above, the amount of Co is preferably 0.01% or more when Co is to be
contained.
[0069]
A Ni-Cr alloy material according to the present invention has a chemical
composition that consists of the elements described above, the balance thereof being Fe
and impurities, and C, P, S, and 0 in the impurities are C: 0.03% or less, P: 0.03% or
less, S: 0.001% or less, and 0: 0.01% or less.
[0070]
The term "impurities" refers to substances which are mixed in from ores as raw
material, scraps, or a producing environment in producing an alloy material industrially.
[007 11
C: 0.03% or less
C (carbon) is contained as an impurity, and C content more than 0.03%
facilitates the stress corrosion cracking involving an intergranular fracture due to
precipitation of M23C6 carbide ("Mu refers to an element such as Cr, Mo, andlor Fe.).
Therefore, the C content is set at 0.03% or less. A preferable upper limit of the C
content in the impurity is 0.02%, a more preferable upper limit thereof is 0.01 5%, and a
still more preferable upper limit thereof is 0.012%.
[0072]
P: 0.03% or less
P (phosphorus) is an impurity contained in an alloy and significantly reduces
hot workability and the stress corrosion cracking resistance. Therefore, the content of
P is set at 0.03% or less. A preferable upper limit of the P content in the impurity is
0.025%, a more preferable upper limit thereof is 0.020%, and a still more preferable
upper limit thereof is 0.019%.
[0073]
S: 0.001 % or less
S (sulfur) is also an impurity that significantly reduces hot workability, as with
P. From the viewpoint of preventing the reduction of hot workability, it is desirable to
set S content as low as possible, and thus an upper limit was specified on S content and
the S content is set at 0.001% or less. A preferable upper limit of S content in the
impurity is 0.0009%, a more preferable upper limit thereof is 0.0008%, and a still more
preferable upper limit thereof is 0.0006%.
[0074]
0 (oxygen): 0.01 % or less
0 (oxygen) is an impurity contained in an alloy, significantly reducing hot
workability. Therefore, the content of 0 is set at 0.01% or less. A preferable upper
limit of the 0 content in the impurity is 0.009%, and a more preferable upper limit
thereof is 0.005%.
[0075]
(B) Dislocation Density
In a micro-structure of Ni-Cr alloy materials according to the present invention
having chemical compositions described in the above sectlon (A), the dislocation
density thereof have to satisfy the following formula:
7.0 x 1015 I p 52.7 x 10l6 - 2 67 x 1017 x [EM(%)]
where, p denotes the dislocation density expressed in m-2, and [REM(%)]
denotes the REM content by mass%.
LO0761
In a Ni-Cr alloy material having the chemical composition described above, the
dislocation density p of a micro-structure less than 7.0 x lo'* m-2 results in failure of
obtaining a 0.2% yield stress of 965 MPa or more. On the other hand, the dislocation
density p of the micro-structure more than 12.7 x 1016 - 2.67 x 1017 x [EM(%)]] m-*
results in a reduction of toughness, as well as reduction of stress corrosion cracking
resistance in an environment being at high temperatures more than 200°C and including
hydrogen sulfide. Therefore, jt is determined that the micro-structure has a dislocation
density satisfying the above formula.
[0077]
Note that the dislocation density p is preferably 2.0 x 10'6m.2 or less.
The Ni-Cr alloy matenal according to the present invention can be produced in
the following manner, for example.
[0079]
First, an alloy is melt using an electric fumace, an AOD fumace, or a VOD
fumace to adjust a chemical composition. In a desulfurizing process using REM in
combination with Ca andlor Mg, it is desirable to add REM and Ca and/or Mg, after a
sufficient deoxidation with A1 or the like.
[0080]
A molten metal having the adjusted chemical composition is next cast into an
ingot and may be afterward subjected to hot working such as forging to be worked into
a so-called "alloy object" such as a slab, bloom, and billet. Alternatively, the above
molten metal may be subjected to continuous casting and directly made into a so-called
"alloy object" such as a slab, bloom, and billet.
LO08 11
Furthermore, the above "alloy object" is subjected to hot working to be formed
into a desired shape such as a plate material or a tubular blank. For example, when the
"alloy object" is worked into a plate matenal, the "alloy object" can be subjected to hot
working into a plate shape or coil shape by a hot rolling. In addition, for example,
when the "alloy object" is worked into a tubular blank, the "alloy object" can be
subjected to hot working to be formed into a tubular shape by a hot-extrusion
tube-malung process or Mannesmann pipe making process.
[0082]
Next, to provide the micro-structure with the dislocation density p satisfying
the above formula, in the case of a plate material, a solution treatment is performed on
the hot rolled material, and cold working is thereafter performed by cold rolling. In
the case of a tubular blank, a solution treatment is performed on a material pipe having
been subjected to hot working, and cold working is thereafter performed by cold
drawing or cold rolling such as Pilger rolling.
[0083]
Depending on a chemical composition, the alloy is cold deformed for 31 to
50% in area reduction. This deformation could be at a time or divided into multiple
times. Sim~larly, if intermediate heat treatment is performed after cold working,
further cold working for 31 to 50% in area reduction depending on the chemical
composition is performed at a time or multiple times to a predetermined size.
[0084]
When the REM content is high, it is necessary to control the reduction of area
in cold working such that the dislocation density p does not exceed a value determined
by the above formula. In addition, when the N content is high, the reduction of area in
cold working should be suppressed. On the other hand, when the REM content or the
N content is low, it is preferable to select a high reduction of area in cold working.
[0085]
For example, to make the dislocation density 7.0 x 10" m-2 or more, it is
preferable to set the reduction of area in cold working described above to be 42% or
more when the N content is about 0.04%. It is preferable to set the reduction of area to
be 3 1 % or more when the N content is about 0.16%. Note that the N content to make
the dislocation density 7.0 x 101Sm-2or more may vary with relations with contents of
elements other than N.
[0086]
An upper limit of the reduction of area described above in cold working
depends on the REM content and the N content, but when the reduction of area (%) is
substantially determined to be equal to or less than the value expressed by the formula
[{(I .78 - 17.78 x [REM(%)])'.' - 2 x [N(%)])/0.02] (wherein, [REM(%)] and IN(%)]
refer to the contents of REM and N by mass%, respectively), it is possible to control the
dislocation density p within a range that does not exceed the value specified in the
present invention. Note that an alloy material having a REM content of 0.05% or more,
and N content of 0.14% or more makes the range of reduction of area allowed in cold
working narrow.
[0087]
Hereafter, the present invention will be described in detail by way of examples.
EXAMPLE
[0088]
Alloys having chemical compositions shown in Table 1 were melted in a
vacuum induction furnace and cast into 50 kg ingots.
[008 91
Alloys A to I in Table 1 are alloys having chemical compositions that all within
a range specified in the present invention. In contrast, alloys 1 to 13 are alloys having
chemical compositions that deviate from the conditions specified in the present
invention.
[0090]
[Table 11

COO9 11
Each ingot was subjected to a soalung process at 1200°C for 3 h and thereafter
subjected to hot forging to be worked into a square bar having a cross section of 50 rnrn
x 50 rnm.
[0092]
The square bar obtained in such a manner was hrther heated at 1200°C for 1 h
and thereafter subjected to hot rolling to be made into a plate material having a
thickness of 14.2 mm.
[0093]
Next, from a part of the plate material described above having a thickness of
14.2 mrn obtained by the hot rolling, a test specimen of which diameter and length are
10 mm and 130 mm, respectively, was taken in a direction parallel to a rolling direction
of the plate material (hereafter, referred to as an "L direction"), and a hot workability of
the test specimen was evaluated by Gleeble machine.
[0094]
In a hot workability test, the above test specimen having been heated at 1200°C
for 3 min was cooled at a rate of 100°C/min, and was subjected to tensile rupture at time
points when a temperature thereof dropped to llOO°C, 1000°C, 900°C, 800°C, and
700°C.
[0095]
The evaluatio~lw as made to be good in hot workability when a reduction of
area after the tensile rupture was 60% or more at all the above test temperatures. This
is based on the experience that when the above reduction of area falls below 60%, a
large number of surface defects and cracks developed during hot tube-making process
using an alloy billet as a starting material.
[0096]
The test results on the above hot workability are also shown Table 1 . Note
that the mark "0" in the hot workability column indicates that the reduction of area
after the tensile rupture was 60% or more at all the test temperatures, meaning a good
hot workability. On the other hand, the mark "x" indicates that the reduction of area
after the tensile rupture fell below 60% at any one of the test temperatures.
[0097]
The remaining hot rolled plate material of 14.2 mrn in thickness of the alloys A
to I, and 1 to 5, 7, 8, 10, and 13, which were evaluated to be good in hot workability by
the Gleeble test mentioned above, was solution treated at 1 100°C for 1 h followed by
water-cooling to obtain an austenite single-phase structure having an FCC crystalline
structure.
[0098]
Subsequently, cold rolling was performed on the plate material havlng been
subjected to the water cooling to make three kinds of plate materials having thicknesses
of 9.8 mm, 8.4 mm, and 7 mrn, respectively. Note that the plate materials having
thicknesses of 9.8 mm and 7 mm were not made from the alloy 10.
[0099]
Next, test specimens of which size is 20 mm by 20 mm and 2 mm in thickness
were cut from the center portion in thickness of the above three kinds of cold rolled
materials having thicknesses of 9.8 mn, 8.4 mm, and 7 mm. Then, the surface of the
test specimen was subjected to electrolytic polishing at 10°C using 10% perchloric acid
- acetic acid mixture solution as electrolyte. Dislocation density was measured using
the test specimens after the electrolytic polishing.
[Ol 001
Note that the measurement of dislocation density was performed using an
evaluating method based on the Williamson-Hall method described in Non-Patent
Document 2 and proposed by Nakajima et. a1 in Non-Patent Document 1.
[OlOl]
Specifically, to measure an X-ray diffraction profile, a Co tube was used as a
cathode tube, and the profile was measured using the 8-20 diffraction method in a range
from 40" to 130" in terms of 28. Then, strain E was determined from each of
diffractions on (1 11 ), (220) and (3 11 ) planes of an FCC crystalline structure using a
half-value width by performing fitting using the Rietveld method, which is described in
Non-Patent Document 3. Furthermore, the dislocation density p in me* was determined
by calculating the formula expressed using the above strain E and a Burgers vector b:
p = 14.4c2/b2
[O 1 021
Note that, to measure a profile derived from a measurement apparatus, a
material solution treated at llOO°C followed by water cooling, which is considered to
have a very low dislocation density was used for each alloy. In addition, a value used
as the above Burgers vector b was 0.2545 x m.
[0 1 031
Furthermore, test specimens were cut from the three kinds of cold rolled
materials having thicknesses of 9.8 mm, 8.4 mm, and 7 mm obtained in the above
manner, and tensile property and toughness (shock resistance property) were examined.
[0 1 041
In the examination of tensile property, two round-bar tensile test specimens
having a parallel portion of 4mrn in diameter and 34mm in gauge length were taken
from each cold rolled material. Specifically, for each cold rolled material, a test
specimen having a parallel portion parallel to the above L direction, and a test specimen
having a parallel portion perpendicular to the above L direction were taken. On these
two test specimens, a tensile test was conducted at room temperature to determine a
yield strength (0.2% yield stress). Yield strengths shown in Table 2 are each an
average value of yield strengths of the two test specimens. A tensile speed in the test
was set at 1.0 mmlmin, which corresponds to strain rate of 4.9 x lo4 Is. In the
examination of tensile property, an alloy was determined to have a sufficiently high
strength when an average value of 0.2% yield stresses of the two test specimens was
965 MPa or more. In this example, a target yield strength was set at 965 MPa.
[0 1 051
In the examination of toughness, two sub-size V-notched Charpy impact test
specimens of which width are 5rnm were taken from each cold rolled material.
Specifically, for each cold rolled material, a test specimen having a longitudinal
direction parallel to the above L direction, and a test specimen having a longitudinal
direction perpendicular to the above L direction were taken. On these test specimens,
Charpy impact test was conducted at -lO°C, which can be considered to be a usage
environment of oil country tubular goods. Then, a value obtained by dividing an
absorbed energy by an original sectional area of the notch portion (hereafter, referred as
an "impact value") was determined. Impact values shown in Table 2 are each an
average value of impact values of the two test specimens. In the examination of
toughness, an alloy was determined to have sufficiently high shock resistance property
when an average value of the impact values of the two test specimens exceeds 63 ~Icm*.
[0 1 061
Furthermore, test specimens were cut from the above three kinds of cold rolled
materials having thicknesses of 9.8 mrn, 8.4 mm, and 7 rnrn, and conosion resistance
was examined.
[0 1071
In the examination of stress corrosion cracking resistance, slow-strain rate
tensile test specimen were taken from each cold rolled material. The slow-strain rate
tensile test specimen having a parallel portion that measured 3.81 mrn in diameter and
25.4 mrn in length, in conformity with the slow-strain rate tensile test method specified
in NACE TM0198. Specifically, from each cold rolled material, the test specimen was
taken in such a manner that made the parallel portion parallel to the above L direction.
Then, the slow-strain rate tensile test conforming to NACE TM0198 was conducted to
evaluate corrosion resistances.
[O 1081
The test environment for the above slow-strain rate tensile test was specified as
two conditions: in the atmosphere; and in an environment imitating a hostile oil-well
environment (H2S partial pressure: 0.689 MPa (100 psi), 25%NaC1 + O.5%CH3COOHY
pH: 2.8, temperature: 204°C). In both of the environments, the strain speed in the
tensile test was set at 4.0 x 10 1s.
[0 1091
Note that in the examination of corrosion resistance, specifically, four
slow-strain rate tensile test specimens were taken from each cold rolled material. On
one of the above four test specimens, values of fracture ductility and reduction of area at
fracture were determined by the tensile test in the atmosphere (hereafter, referred to as
"reference value of fracture ductility" and "reference value of reduction of area at
fracture"). On the other three test specimens, values of fracture ductility and reduction
of area at fracture were determined by the tensile test in the environment imitating a
hostile oil-well environment (hereafter, referred to as "comparative value of fracture
ductility" and "con~parative value of reduction of area at fiacture"). That is, in this
example, for each cold rolled material, one reference value of fracture ductility, three
comparative values of fracture ductility, one reference value of reduction of area at
fracture, and three comparative values of reduction of area at fracture were determined.
[Ol lo]
Then, for each cold rolled material, differences between the one reference
value of fracture ductility and the three comparative values of fracture ductility were
determined (hereafter, each difference will be referred to as a "difference in fracture
ductility"). Similarly, differences between the one reference value of reduction of area
at fracture and the three comparative values of reduction of area at fracture were
determined (hereafter, each difference will be referred to as a "difference in reduction of
area at fracture"). In this examination, a coirosion resistance target was set as follows:
all the "differences in fracture ductility" are 20% or less of the "reference value of
fracture ductility"; and all the "differences in reduction of area at fracture" are 20% or
less of the "reference value of reduction of area at fracture". Then, an alloy was
determined to be good in corrosion resistance when the above target is reached.
[Olll]
Table 2 shows the results of the above examinations. For each alloy material,
reference numerals 1 to 3 denotes the examination results on the cold rolled materials
having thicknesses of 9.8 mm, 8.4 mm, and 7 mm. The mark "0" in the column
llcorrosion resistance" indicates that the alloy reached the above corrosion resistance
target, and the mark "x" indicates that the alloy fell short of the corrosion resistance
target.
[0112]
h addition, the results shown in Table 2 are summarized in Figure 1 that
illustrates how the yield strength and the toughness vary as the REM content and the
dislocation density vary. Note that Figure 1 excludes the results on the alloy 1 to 6 and
8 to 13 because, as to the alloy 1 to 5 and 8 to 13, a content of at least one of elements
other than REMs fell out of the range specified in the present invention, and as to the
alloy 6, the evaluation of toughness and the like was not performed due to its
deteriorated hot workability in itself. The line in the drawing is expressed as follows.
p = 2.7 x 1016 - 2.67 x lOI7 x [REM(%)]
[0113]
[Table 21
/ U9 I ~ ~ f ~ , ~ ~ ~ ~ I Value of I Dislocation dasW
f-wmeral formula [a] below M
(ma)
The mark "" indicates that the alloy has chemical cornpositin that deviated from the conditions specified in the
present invention. I The mark '3' indicates that the alloy fell short of the target.
It is evident from Table 2 that alloy materials satisfying the conditions
specified in the present invention are excellent in hot workability and in toughness
(shock resistance property) at -lO°C. It is further evident that, even with a
high-strength, a yleld strength (0.2% yleld stress) of 965 MPa or more, the alloy
materials satisfying the conditions specified in the present invention are excellent in
corrosion resistance (in particular, stress corrosion cracking resistance in an
environment being at hgh temperatures more than 200°C and including hydrogen
sulfide).
INDUSTRIAL APPLICABILITY
[0115]
The Ni-Cr alloy material according to the present invention is excellent in hot
workability and toughness (shock resistance property), and in corrosion resistance (in
particular, stress corrosion cracking resistance in an environment being at high
temperatures more than 200°C and including hydrogen sulfide) even with a
high-strength, a yield strength (0.2% yield stress) of 965 MPa or more. For this reason,
the Ni-Cr alloy material according to the present invention is suitable as a starting
material for high-strength seamless oil country tubular goods.

We claim:
[Claim 11
A Ni-Cr alloy material having a chemical composition that consists of, by
mass%:
Si: 0.01 to 0.5%, Mn: 0.01% or more and less than 1.0%, Cu: 0.01% or more
and less than 1.0%, Ni: 48% or more and less than 55%, Cr: 22 to 28%, Mo: 5.6% or
more and less than 7.0%, N: 0.04 to 0.16%, sol.Al: 0.03 to 0.20%, REM: 0.01 to
0.074%, W: 0% or more and less than 8.0%, Co: 0 to 2.0%, one or more of Ca and Mg:
0.0003 to 0.01% in total, and one or more of Ti, Nb, Zr, and V: 0 to 0.5% in total, and
the balance being Fe and impurities, wherein
C, P, S, and 0 in the impurities are C: 0.03% or less, P: 0.03% or less, S:
0.001% or less, and 0: 0.01% or less, and furthermore
the dislocation density satisfies a following formula:
7.0 x 1015 I p 5 2.7 x 1016 - 2.67 x lOI7 x [REM(%)]
where p denotes dislocation density expressed in m-2, and [REM(%)] denotes
REM content expressed by mass%.
[Claim 21
The Ni-Cr alloy material according to claim 1, containing 0.1 % or more and
less than 8.0% of W, by mass%.
[Claim 31
The Ni-Cr alloy material according to claim 1 or 2, containing 0.01 to 0.5% of
one or more of Ti, Nb, Zr, and V in total, by mass%.
[Claim 41
The Ni-Cr alloy material according to any one of claims 1 to 3, containing 0.01
to 2.0% of Co, by mass%.
[Claim 51
Seamless oil country tubular goods comprising the Ni-Cr alloy material
according to any one of claims 1 to 4.