TECHNICAL FIELD
[0001] The present invention relates to stainless steel and more specifically to stainless
steel used for oil country tubular goods for use in gas wells or oil wells.
BACKGROUND ART
[0002] Oil or natural gas produced from oil wells or gas wells contains associated corroding
gas as such as carbon dioxide gas and hydrogen sulfide. Therefore, oil country tubular
goods used for producing oil or natural gas need high corrosion resistance.
[0003] Carbon steel or low alloy steel has been used as a steel for oil country tubular
goods. As the goods have come to be used in a tougher corroding environment in an
oil well or a gas well, SUS420 martensitic stainless steel (13% Cr-based steel) having
a Cr content of about 13% or stainless steel having high corrosion resistance such
as improved 13% Cr-based steel produced by adding Ni to the 13% Cr-based steel has
been used.
[0004] Recently, deep oil or gas well drilling has created a demand for oil country tubular
goods having higher strength for such deep oil or gas wells. Furthermore, in a deep
oil or gas well, a high temperature chloride aqueous solution environment as high
as 150°C or more including hydrogen sulfide and carbon dioxide gas is created, and
even higher corrosion resistance than the conventional oil country tubular good is
required. In such a high temperature chloride aqueous solution environment including
hydrogen sulfide and carbon dioxide gas, two-phase stainless steel having corrosion
resistance and strength higher than conventional stainless steel may be used. The
two-phase stainless steel however contains a large amount of alloy elements and therefore
the manufacturing cost is high.
[0005] JP 2005-336595 A (hereinafter referred to as "Patent Document 1"),
JP 2006-16637 A (hereinafter referred to as "Patent Document 2"), and
JP 2007-332442 A (hereinafter referred to as "Patent Document 3") propose the use of stainless steel
pipes containing less alloy elements than the two-phase stainless steel and having
high strength and high corrosion resistance in a high temperature chloride aqueous
solution environment including carbon dioxide gas. The stainless steel pipes disclosed
by these patent documents each have a greater Cr content than the conventional 13%
Cr-based steel, such that the corrosion resistance can be improved.
[0006] More specifically, in the disclosure of Patent Document 1, the Cr content of the
stainless steel pipe is from 15.5% to 18% which is greater than that of the conventional
13% Cr-based steel. Furthermore, when Cr+Mo+0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N≥11.5 is
established, the steel has a two-phase structure including a ferrite phase and a martensite
phase, so that the hot workability of the oil country tubular good is improved. The
two-phase structure could lower the corrosion resistance, while when Ni, Mo, and Cu
that improve the corrosion resistance are added such that Cr+0.65Ni+0.6Mo+0.55Cu-20C≥19.5
is established, the reduction in the corrosion resistance of the oil country tubular
good is prevented.
[0007] Similarly, according to the disclosure of Patent Document 2, the Cr content of the
stainless steel is from 15.5% to 18% and Ni that improves the corrosion resistance
is contained. The chemical composition of the stainless steel disclosed by the document
is similar to that in Patent Document 1, but Mo is not an essential element, and therefore
a less costly alloy design is proposed. In addition, Cu is also an optional element.
[0008] The stainless steel disclosed by Patent Document 3 contains 14% to 18% Cr as well
as Ni, Mo, and Cu, so that high corrosion resistance is obtained. Furthermore, the
steel includes a martensite phase and 3% to 15% austenite phase by volume, and therefore
the toughness improves.
[0009] The kinds of stainless steel disclosed by Patent Documents 1 to 3 surely contain
a larger amount of Cr than the conventional 13% Cr-based steel and alloy elements
such as Ni, Mo, and Cu are added, so that the corrosion rate in a high temperature
corroding environment is reduced. For example, in an embodiment in Patent Document
1, using a 20 wt% NaCl aqueous solution at 230°C in a 100 atm CO
2 atmosphere, the corrosion rate (mm/yr) was examined and it was established that the
corrosion rate was reduced (see Table 2 in Patent Document 1).
[0010] However, it was found based on the inventors' investigation that the use of stainless
steel having a high Cr content in a high temperature chloride aqueous solution environment
containing carbon dioxide gas lowers the corrosion rate but SCC (Stress Corrosion
Cracking) is more likely to be caused.
[0011] In conventional stainless steel such as 13% Cr steel, the corrosion rate is extremely
high in a high temperature chloride aqueous solution environment. Therefore, while
general corrosion is generated, SCC that is local cracking is not generated. On the
other hand, when the Cr content is larger than that in the conventional stainless
steel, the corrosion rate is lowered as disclosed by Patent Documents 1 to 3. The
reduction in the corrosion rate is caused by a passive film that forms on the surface
of the stainless steel. However, the passive film is locally weakened and destroyed
in a high temperature environment. The destroyed part is more likely to dissolve,
and this dissolution is probably the cause for SCC.
[0012] Therefore, in stainless steel used in a high temperature chloride aqueous solution
environment containing carbon dioxide gas, it is necessary not only to reduce the
corrosion rate but also to improve the SCC resistance.
SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to provide stainless steel for an oil country
tubular goods having high corrosion resistance in a carbon dioxide gas contained,
high temperature chloride aqueous solution environment at 150°C or higher. More specifically,
it is to provide stainless steel for an oil country tubular goods having a reduced
corrosion rate and high SCC resistance in a carbon dioxide gas contained, high temperature
chloride aqueous solution environment.
[0014] The inventors considered that it would be necessary to add at least 16 mass % Cr
and a small amount of Mo to steel in order to reduce the corrosion rate in a carbon
dioxide gas contained, high temperature chloride aqueous solution environment at 150°C
or higher. However, Cr and Mo are ferrite forming elements and therefore if at least
16 mass % Cr and a small amount of Mo are contained, a major part of the structure
of the steel becomes a ferrite phase and therefore high strength cannot be obtained.
[0015] On the other hand, an austenite phase at high temperatures is stabilized by adding
Ni that is an austenite forming element, so that a martensite phase is formed by quenching
and a high strength steel structure is obtained. However, if the amount of Ni is too
large, the starting temperature for martensite transformation (Ms point) is lowered
and therefore martensite transformation is not generated even at room temperatures,
so that high strength is not provided. Therefore, when the Ni content is appropriately
adjusted, a structure mainly including a martensite phase and about at least 10% ferrite
phase by volume is formed and high strength can be provided.
[0016] The copper (Cu) effectively enhances a ferrite phase, and therefore a high strength
structure can be provided by adding Cu. In addition, Cu reduces the corrosion rate
in a high temperature chloride aqueous solution environment and improves the SCC resistance.
[0017] Based on the foregoing findings, the inventors concluded that stainless steel having
prescribed strength and reduced corrosion rate can be provided when the steel contains
16% to 18% Cr, more than 2% and not more than 4% Mo, 3.5% to 7% Ni, and 1.5% to 4%
Cu.
[0018] The inventors also found that by adding at least a prescribed amount of an earth
rare metal (REM) in the chemical composition described above, high SCC resistance
results even in a carbon dioxide gas contained, high temperature chloride aqueous
solution environment. Now, this will be described in detail.
[0019] The inventors prepared stainless steel having the chemical compositions in Table
1 and these kinds of stainless steel were evaluated for their SCC resistance.
Table 1
Steel No. |
Chemical composition (unit: mass %, the balance consisting of Fe and impurities) |
C |
Si |
Mn |
P |
S |
Cu |
Cr |
Ni |
Mo |
sol.Al |
Ca |
N |
O |
REM |
A1 |
0.019 |
0.31 |
0.51 |
0.016 |
0.0009 |
1.9 |
17.1 |
3.9 |
2.4 |
0.029 |
0.0010 |
0.020 |
0.003 |
0.0001 |
A2 |
0.018 |
0.30 |
0.55 |
0.015 |
0.0010 |
2.0 |
17.2 |
4.2 |
2.5 |
0.030 |
0.0008 |
0.018 |
0.005 |
0.0002 |
A3 |
0.021 |
0.29 |
0.52 |
0.017 |
0.0010 |
2.1 |
16.9 |
4.1 |
2.5 |
0.029 |
0.0013 |
0.016 |
0.006 |
0.0005 |
A4 |
0.020 |
0.29 |
0.49 |
0.016 |
0.0008 |
2.0 |
17.0 |
4.0 |
2.6 |
0.028 |
0.0016 |
0.019 |
0.003 |
0.0011 |
A5 |
0.019 |
0.31 |
0.51 |
0.015 |
0.0010 |
1.9 |
17.2 |
4.1 |
2.4 |
0.032 |
0.0011 |
0.022 |
0.005 |
0.0028 |
A6 |
0.019 |
0.31 |
0.50 |
0.016 |
0.0009 |
1.9 |
17.1 |
4.1 |
2.4 |
0.029 |
0.0009 |
0.018 |
0.003 |
0.03 |
[0020] Referring to Table 1, in the stainless steel grades with Nos. A1 to A6, the chemical
compositions are the same except for REM. The REM content is different among the numbered
stainless steel grades in the range from 0.0001% to 0.03%. Furthermore, these numbered
stainless steel grades were subjected to quenching-tempering such that the yield stress
of each kind of stainless steel was adjusted in the range from 860 MPa to 900 MPa.
The structures of these numbered stainless steel grades include, in volume percentage,
60% martensite phase, 30% ferrite phase, and 10% austenite phase.
[0021] A specimen for four-point bending test having a length of 75 mm, a width of 10 mm,
and a thickness of 2 mm was sampled from each of the numbered stainless steel grades.
The sampled specimens were subjected to a bending load by four-point bending. At the
time, the bending amount of each specimen was determined according to ASTM G39 such
that stress applied on each specimen was equal to the yield stress of each specimen.
[0022] Each bent specimen was immersed for one month in a 25 wt% NaCl aqueous solution in
an autoclave at 204°C (400F) having CO
2 enclosed therein under a pressure of 30 atm. After the immersion for one month, each
specimen was examined for the presence of SCC. More specifically, a longitudinal section
of each specimen was observed with a 100x magnification optical microscope and determined
for the presence/absence of SCC by visual inspection.
[0023] The test result is given in Fig. 1. In Fig. 1, the abscissa represents the REM content
(% by mass) and the ordinate represents the presence/absence of SCC. In Fig. 1, "●"
in the "SCC present" on the ordinate indicates the presence of SCC, while "●" in the
"no SCC" indicates the absence of SCC. As can be clearly seen from Fig. 1, when the
REM content was not less than 0.001%, no SCC was generated even in a carbon dioxide
contained, high temperature chloride aqueous solution environment. While how REM improves
the SCC resistance is not clearly known, this may be for the following reason.
[0024] As a result of microscopic observation of stainless steel specimens that had SCC
in the above-described tests, it was found that SCC was originated from a pit and
propagated along an prior austenite grain boundary in a martensite predominant structure.
This may suggest that the accumulation behaviors of dislocations toward the prior
austenite boundary under stress and crack propagation are somehow correlated. Then,
REM probably has some effect on the accumulation behaviors of dislocations toward
the prior austenite boundary and the SCC resistance of the stainless steel containing
at least 0.001% REM may be improved. Note that the stainless steel grades with Nos.
A1 to A3 contained 0.0008% to 0.0013% Ca but their REM contents were less than 0.001%
and therefore SCC was generated. Therefore, at least 0.001% REM content contributed
to the improvement of the SCC resistance more than Ca did.
[0025] The inventors have completed the following invention based on the foregoing findings.
[0026] Stainless steel used for an oil country tubular goods according to the invention
includes, in percent by mass, 0.001% to 0.05% C, 0.05% to 1% Si, at most 2% Mn, at
most 0.03% P, less than 0.002% S, 16% to 18% Cr, 3.5% to 7% Ni, more than 2% and at
most 4% Mo, 1.5% to 4% Cu, 0.001% to 0.3% rare earth metal, 0.001% to 0.1% sol. Al,
0.0001% to 0.01% Ca, at most 0.05% O, and at most 0.05% N, and the balance consists
of Fe and impurities.
[0027] The stainless steel according to the invention preferably further includes, in place
of a part of Fe, at least one selected from the group consisting of at most 0.5% Ti,
at most 0.5% Zr, at most 0.5% Hf, at most 0.5% V, and at most 0.5% Nb.
[0028] In this way, the generation of a pit attributable from a Cr depleted layer can be
reduced.
[0029] The stainless steel described above preferably has a structure including, in volume
percentage, 10% to 60% ferrite phase and 2% to 10% residual austenite phase.
[0030] The stainless steel according to the invention preferably has a yield stress of at
least 654 MPa.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Fig. 1 is a graph showing the relation between the contents of rare earth metals
in stainless steel and SCC.
BEST MODE TO CARRY OUT THE INVENTION
[0032] Now, embodiments of the invention will be described in detail. Stainless steel according
to the invention is applicable to oil country tuber goods for use in a carbon dioxide
gas contained, high temperature chloride aqueous solution environment at 150°C or
more. Hereinafter, this carbon dioxide gas contained, high temperature chloride aqueous
solution environment at 150°C or more will be simply referred to as "high temperature
chloride aqueous solution environment."
1. Chemical Composition
[0033] The stainless steel according to the invention has the following chemical composition.
Hereinafter, "%" related to elements means "% by mass."
C: 0.001% to 0.05%
[0034] Carbon (C) forms carbide with Cr and lowers the corrosion resistance of steel in
a high temperature chloride aqueous solution environment. Therefore, the C content
is preferably as small as possible. Therefore, the upper limit for the C content is
0.05%. Note that the lower limit for the C content that can substantially be controlled
is 0.001%.
Si: 0.05% to 1%.
[0035] Silicon (Si) deoxidizes steel in refining process. To obtain the effect, the lower
limit for the Si content is 0.05%. On the other hand, an excessive Si content not
only saturates the deoxidizing effect but also lowers the hot workability of the steel.
Therefore, the upper limit for the Si content is 1%.
Mn: 2% or less
[0036] Manganese (Mn) improves the strength of the steel. However, an excessive Mn content
is more likely to cause segregation in the steel. The segregation in the steel lowers
the toughness of the steel and also lowers the SCC resistance in a high temperature
chloride aqueous solution environment. Therefore, the Mn content is not more than
2%. The Mn content is preferably not less than 0.2% in order to improve the strength.
However, if the Mn content is less than 0.2%, the strength of the steel is improved
to some extent.
P: 0.03% or less
[0037] Phosphorus (P) is an impurity and lowers the SSC (sulfide stress cracking) resistance
and the SCC resistance in a high temperature chloride aqueous solution environment.
Therefore, the P content is preferably as small as possible. The P content is therefore
not more than 0.03%.
S: less than 0.002%
[0038] Sulfur (S) combines with Mn or the like and forms an inclusion. The formed inclusion
becomes an origin for a pit or SCC and lowers the corrosion resistance of the steel.
In addition, S lowers the hot workability of the steel. Therefore, the S content is
preferably as small as possible. Therefore, the S content is less than 0.002%.
Cr: 16% to 18%
[0039] Chromium (Cr) is an essential element that improves the corrosion resistance in a
high temperature chloride aqueous solution environment. In order to achieve high SCC
resistance in the high temperature chloride aqueous solution environment, the lower
limit for the Cr content is 16%. On the other hand, since Cr is a ferrite forming
element, an excessive Cr content increases the ratio of a ferrite phase in the steel
structure and lowers the strength of the steel. Furthermore, it lowers the ratio of
a residual austenite phase, which lowers the toughness of the steel. Therefore, the
upper limit for the Cr content is 18%. The Cr content is preferably from 16.5% to
17.5%.
Ni: 3.5% to 7%
[0040] Nickel (Ni) improves the corrosion resistance in a high temperature chloride aqueous
solution environment and also improves the toughness of the steel. In order to obtain
these effects, the lower limit for the Ni content is 3.5%. On the other hand, Ni is
an austenite forming element and an excessive Ni content increases the ratio of a
residual austenite phase in the structure of the steel, which lowers the strength
of the steel. Therefore, the upper limit for the Ni content is 7%. The Ni content
is preferably from 3.5% to 6.5%, more preferably from 3.8% to 5.8%.
Cu: 1.5% to 4%
[0041] Copper (Cu) lowers the dissolution rate of the steel in a high temperature chloride
aqueous solution environment and also improves the SCC resistance of the steel. In
addition, Cu strengthens a ferrite phase in the structure of the steel. In order to
obtain these effects, the lower limit for the Cu content is 1.5%. On the other hand,
an excessive Cu content lowers the hot workability of the steel. Therefore, the upper
limit for the Cu content is 4%. The Cu content is preferably from 1.5% to 3.0%, more
preferably from 1.5% to 2.5%.
Mo: more than 2% and not more than 4%
[0042] Molybdenum (Mo) improves the pitting corrosion resistance and the SCC resistance
of the steel when it coexists with Cr. In order to obtain the effects, the Mo content
is more than 2%. On the other hand, Mo is a ferrite forming element and therefore
an excessive Mo content increases the ratio of a ferrite phase in the structure of
the steel, which lowers the strength. Therefore, the Mo content is not more than 4%.
The Mo content is preferably from 2.1% to 3.3%, more preferably from 2.3% to 3.0%.
Sol. Al: 0.001% to 0.1%
[0043] Aluminum (Al) deoxidizes steel in refining process. In order to obtain the effect,
the lower limit for the Al content is 0.001%. On the other hand, an excessive Al content
causes a large amount of an alumina inclusion to be generated in the steel, which
lowers the toughness of the steel. Therefore, the upper limit for the Al content is
0.1%. Note that the Al content in the specification means the content of acid soluble
aluminum (sol. Al).
Ca: 0.0001% to 0.01%
[0044] Calcium (Ca) deoxidizes steel in refining process. In addition, Ca improves the hot
workability. In order to obtain these effects, the lower limit for the Ca content
is 0.0001%. On the other hand, an excessive Ca content causes a large amount of an
inclusion such as CaO to be generated in the steel, which lowers the toughness of
the steel. Furthermore, the inclusion such as CaO forms an origin for a pit. Therefore,
the upper limit for the Ca content is 0.01%.
N: 0.05% or less
[0045] Nitrogen (N) stabilizes an austenite phase and also improves the pitting corrosion
resistance. On the other hand, an excessive N content causes various nitrides to be
formed in the steel, which lowers the toughness of the steel. Therefore, the N content
is not more than 0.05%. In order to effectively obtain the effect, the lower limit
for the N content is preferably 0.005%.
O: 0.05% or less
[0046] Oxygen (O) is an impurity and combines with another element to form oxide, which
lowers the toughness and the corrosion resistance of the steel. Therefore, the O content
is preferably as small as possible. Therefore, the O content is not more than 0.05%.
Rare Earth Metals: 0.001% to 0.3%
[0047] Rare earth metals (REM) are important elements according to the invention. The REM
improve the SCC resistance in a high temperature chloride aqueous solution environment
as described above. In order to obtain the effect, the lower limit for the REM content
is 0.001%. On the other hand, an excessive REM content saturates the effect. Therefore,
the upper limit for the REM content is 0.3%. The REM content is preferably from 0.001%
to 0.1%, more preferably from 0.001% to 0.01%.
[0048] Note that the REM according to the invention refer to yttrium (Y) with atomic number
39 and lanthanoids from lanthanum (La) with atomic number 57 to lutetium (Lu) with
atomic number 71.
[0049] The stainless steel according to the invention contains at least one of the above
REM. Therefore, the REM content means the total content of at least one selected from
the plurality of REM described above.
[0050] The balance of the chemical composition includes Fe and impurities.
[0051] The stainless steel according to the invention contains at least one selected from
the group consisting of Ti, Zr, Hf, V, and Nb in place of a part of Fe if necessary.
Ti: 0.5% or less
Zr: 0.5% or less
Hf: 0.5% or less
V: 0.5% or less
Nb: 0.5% or less
[0052] Titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), and niobium (Nb) are not
essential elements and added as optional elements. These elements each fix C and reduce
the generation of Cr carbide. Therefore, the generation of a pit attributable to a
Cr depleted layer formed around Cr carbide is reduced and the SCC sensitivity is reduced.
However, an excessive content of any of these elements lowers the toughness of the
steel. Therefore, the upper limits for the contents of these elements are each 0.5%.
In order to effectively obtain the above-described effect, the lower limits for the
contents of these elements are each preferably 0.005%. Note however that if the contents
of these elements are less than the preferable lower limit, the above-described effect
is obtained to some extent.
2. Manufacturing Method
[0053] The stainless steel according to the invention can have the following structure by
carrying out quenching-tempering as heat treatment, so that the corrosion resistance
as intended and strength necessary when it is used as oil country tubular goods can
be provided. Now, a method of manufacturing a stainless steel pipe according to the
invention will be described by way of example.
[0054] Steel having the above-described chemical composition is melted and made into a billet.
The produced billet is subjected to hot working and made into a stainless steel pipe.
A Mannesmann method for example is employed as the hot working to make a seamless
steel pipe. Note that the hot working may be hot extruding or hot forging.
[0055] The produced stainless steel pipe is subjected to quenching and tempering. At the
time, the preferable quenching temperature is from 900°C to 1200°C, and the preferable
tempering temperature is from 450°C to 650°C.
3. Structure
[0056] The structure of the stainless steel produced by the above-described method includes,
in percent by volume, 10% to 60% ferrite phase and 2% to 10% residual austenite phase.
[0057] Now, the volume percentage of the ferrite phase is obtained by the following method.
A specimen having its surface polished is etched using a mixture solution of aqua
regia and glycerin. Using the etched specimen, the area ratio of the ferrite phase
at the specimen surface is measured by a point-counting method according to JISG0555.
The measured area ratio is used as a volume percentage. The volume percentage of the
residual austenite phase is measured by X-ray diffraction.
[0058] Note that in the structure of the stainless steel, the portion other than the ferrite
phase and the residual austenite phase is mainly a tempered martensite phase. Carbide,
nitride, boride and a Cu phase may be included other than the martensite phase.
[0059] The stainless steel according to the invention has the above-described structure,
so that the yield stress is not less than 654 MPa (that corresponds to 95 ksi). The
yield stress may be adjusted to 758 MPa (that corresponds to 110 ksi) or more and
further to 862 MPa (that corresponds to 125 ksi) or more. The yield stress in this
specification refers to 0.2% offset yield stress based on the ASTM standard.
[0060] The stainless steel according to the invention has high toughness since it contains
the residual austenite phase as much as the above-described volume percentage in the
structure.
Examples
[0061] A plurality of kinds of stainless steel having various chemical compositions were
produced and examined for their SCC resistance in a high temperature chloride aqueous
solution environment.
Manufacture of Specimens
[0062] A plurality of kinds of stainless steel having the chemical compositions in Table
2 were melted.
Table 2
Steel No. |
Chemical composition (unit: mass %, the balance consisting of Fe and impurities) |
C |
Si |
Mn |
P |
S |
Cu |
Cr |
Ni |
Mo |
sol.Al |
Ca |
N |
O |
REM |
Ti |
Zr |
Hf |
V |
Nb |
1 |
0.022 |
0.36 |
0.51 |
0.017 |
0.001 |
2.4 |
16.3 |
3.6 |
2.4 |
0.014 |
0.0003 |
0.019 |
0.005 |
0.03 a) |
- |
- |
- |
- |
- |
2 |
0.011 |
0.39 |
0.65 |
0.018 |
0.0008 |
1.6 |
17.6 |
6.3 |
2.1 |
0.039 |
0.0005 |
0.044 |
0.018 |
0.002 a) |
- |
- |
- |
- |
- |
3 |
0.018 |
0.38 |
1.28 |
0.019 |
0.0009 |
3.3 |
17.2 |
3.9 |
2.2 |
0.029 |
0.001 |
0.011 |
0.003 |
0.001 a) |
- |
- |
- |
- |
- |
4 |
0.032 |
0.84 |
1.79 |
0.017 |
0.001 |
1.7 |
16.3 |
4.5 |
3.5 |
0.021 |
0.0015 |
0.016 |
0.004 |
0.008 c) |
- |
- |
- |
- |
- |
5 |
0.042 |
0.29 |
0.23 |
0.011 |
0.0015 |
3.5 |
17.8 |
6.2 |
3.6 |
0.033 |
0.0005 |
0.008 |
0.004 |
0.019 b) |
- |
- |
- |
- |
- |
6 |
0.039 |
0.11 |
0.59 |
0.019 |
0.0018 |
1.8 |
16.2 |
4.2 |
2.3 |
0.018 |
0.0008 |
0.015 |
0.005 |
0.22 b) |
- |
- |
- |
- |
- |
7 |
0.022 |
0.36 |
0.55 |
0.022 |
0.0009 |
2.3 |
16.4 |
4.5 |
2.6 |
0.015 |
0.0012 |
0.026 |
0.004 |
0.015 c) |
- |
- |
- |
0.05 |
- |
8 |
0.025 |
0.52 |
0.68 |
0.028 |
0.0008 |
1.6 |
16.5 |
5.6 |
2.5 |
0.029 |
0.0029 |
0.022 |
0.008 |
0.011 c) |
0.04 |
- |
- |
- |
- |
9 |
0.019 |
0.48 |
0.66 |
0.019 |
0.0009 |
1.9 |
17.2 |
6.1 |
2.5 |
0.036 |
0.0005 |
0.018 |
0.005 |
0.007 a) |
- |
0.02 |
- |
0.09 |
- |
10 |
0.018 |
0.33 |
0.55 |
0.017 |
0.001 |
2.1 |
16.6 |
5.9 |
2.8 |
0.055 |
0.0002 |
0.019 |
0.003 |
0.002 a) |
- |
- |
0.01 |
0.08 |
- |
11 |
0.023 |
0.22 |
0.49 |
0.015 |
0.001 |
2.8 |
17.5 |
4.8 |
2.7 |
0.057 |
0.0009 |
0.017 |
0.004 |
0.009 c) |
- |
- |
- |
0.11 |
0.08 |
12 |
0.026 |
0.28 |
0.48 |
0.011 |
0.0009 |
2.4 |
16.8 |
4.2 |
3.2 |
0.022 |
0.001 |
0.021 |
0.006 |
0.026 b) |
0.15 |
- |
- |
0.04 |
0.13 |
13 |
0.021 |
0.35 |
0.44 |
0.018 |
0.0008 |
2.3 |
16.9 |
3.7 |
1.9 |
0.028 |
0.0012 |
0.019 |
0.005 |
0.011 a) |
- |
- |
- |
- |
- |
14 |
0.018 |
0.39 |
0.41 |
0.017 |
0.0008 |
2.3 |
15.1 |
3.8 |
2.3 |
0.019 |
0.0011 |
0.009 |
0.005 |
0.008 a) |
- |
- |
- |
- |
- |
15 |
0.023 |
0.32 |
0.49 |
0.015 |
0.001 |
1.3 |
17.2 |
4.5 |
2.5 |
0.033 |
0.0009 |
0.018 |
0.011 |
0.015 a) |
- |
- |
- |
- |
- |
16 |
0.025 |
0.33 |
0.51 |
0.014 |
0.001 |
2.3 |
17.5 |
4.4 |
2.3 |
0.036 |
0.0009 |
0.015 |
0.004 |
- |
- |
- |
- |
- |
- |
17 |
0.021 |
0.35 |
0.55 |
0.014 |
0.0009 |
2.2 |
16.9 |
3.1 |
2.8 |
0.039 |
0.0012 |
0.014 |
0.004 |
0.014 a) |
- |
- |
- |
- |
- |
An underlined numerical value is outside the range defined by the invention. |
[0063] The numerical values in Table 2 refer to the contents of corresponding elements (%
by mass). Among these chemical compositions, the balance other than the elements described
in Table 1 includes Fe and impurities. The symbols a) to c) attached to the numerical
values in the "REM" column each represent the kind of REM included in the steel. More
specifically, a) means that the contained REM is neodymium (Nd). Similarly, b) means
that the contained REM is yttrium (Y) and c) means that the contained REM is misch
metal. The misch metal contains, in percent by mass, 51.0% cerium (Ce), 25.5% lanthanum
(La), 18.6% neodymium (Nd), 4.8% praseodymium (Pr) and 0.1% samarium (Sm).
[0064] With reference to Table 2, the steel kinds with Nos. 1 to 12 each had a chemical
composition within the range defined by the invention. Regarding the steel with No.
13, its Mo content was less than the lower limit defined by the invention. Regarding
the steel with No. 14, its Cr content was less than the lower limit defined by the
invention. Regarding the steel with No. 15, its Cu content was less than the lower
limit defined by the invention. The steel with No. 16 contained no REM. Regarding
the steel with No. 17, its Ni content was less than the lower limit defined by the
invention.
[0065] These numbered steels were each subjected to hot forging and hot rolling, and a steal
plate having a thickness of 12 mm was produced. The numbered steel plates were subjected
to quenching and tempering. In the quenching processing, the steel plates were each
heated for 15 minutes at a quenching temperature from 980°C to 1200°C and then cooled
with water. In the tempering processing, the tempering temperature was from 500°C
to 650°C. Through these steps, the yield stress of each steel plate was adjusted to
be in the range from 800 MPa to 950 MPa.
Structure Observation and Tensile Tests
[0066] The volume percentage (%) of the ferrite phase and the residual austenite phase of
each steel plate was obtained by the measuring method described in 3.
[0067] Then, a round rod tensile test specimen was sampled from each of the steel plates
and subjected to a tensile test. The length-wise direction of the round rod tensile
test specimen was arranged in the direction of rolling the steel plate and the parallel
part of the round rod tensile test specimen had a diameter of 14 mm, and a length
of 20 mm. The tensile tests were carried out at room temperatures.
SCC Evaluation Tests
[0068] A four-point bending specimen having a length of 75 mm, a width of 10 mm, and a thickness
of 2 mm was sampled from each of the steel plates. Each of the sampled specimens was
bent by four-point bending. At the time, according to ASTM G39, the amount of bending
of each of the specimens was determined so that stress applied on each of the specimens
was equal to the yield stress of each of the specimens.
[0069] The bent specimens were each immersed for one month in a 25 wt% NaCl aqueous solution
in an autoclave at 204°C (400F) having CO
2 enclosed therein under a pressure of 30 atm. After the immersion for one month, the
specimens were examined for the presence of SCC. More specifically, a longitudinal
section of each specimen was observed with a 100x magnification optical microscope
and examined for the presence/absence of SCC by visual inspection. The weight of each
specimen was measured before and after the test. From the difference between the measured
weights, the weight loss of each specimen caused by corrosion was obtained and the
corrosion rate was calculated based on the weight loss.
Test Result
[0070] The test result is given in Table 3.
Table 3
Steel |
YS No. (MPa) |
Ferrite phase in volume percentage (%) |
Austenite phase in volume percentage (%) |
SCC evaluation result |
Corrosion rate (g/(m2·hr)) |
1 |
924 |
25 |
2.1 |
NO SCC |
<0.1 |
2 |
915 |
26 |
5.2 |
NO SCC |
<0.1 |
3 |
901 |
28 |
5.6 |
NO SCC |
<0.1 |
4 |
893 |
35 |
3.2 |
NO SCC |
<0.1 |
5 |
940 |
15 |
4.2 |
NO SCC |
<0.1 |
6 |
886 |
38 |
2.2 |
NO SCC |
<0.1 |
7 |
922 |
20 |
4.1 |
NO SCC |
<0.1 |
8 |
928 |
21 |
4.5 |
NO SCC |
<0.1 |
9 |
926 |
24 |
3.6 |
NO SCC |
<0.1 |
10 |
933 |
19 |
6.8 |
NO SCC |
<0.1 |
11 |
911 |
28 |
5.2 |
NO SCC |
<0.1 |
12 |
903 |
33 |
3.3 |
NO SCC |
<0.1 |
13 |
880 |
38 |
2.3 |
SCC PRESENT |
<0.1 |
14 |
932 |
20 |
4.9 |
SCC PRESENT |
≥0.1 |
15 |
873 |
42 |
2.0 |
SCC PRESENT |
<0.1 |
16 |
899 |
30 |
3.5 |
SCC PRESENT |
<0.1 |
17 |
880 |
35 |
2.8 |
SCC PRESENT |
<0.1 |
[0071] The "YS" column in Table 3 represents the yield stress (MPa) of each of the numbered
steel plates obtained by the tensile tests. The "ferrite phase" and "residual austenite
phase" columns represent the volume percentages (%) of the ferrite phase and the residual
austenite phase in each of the steel plates. In the "SCC evaluation result" column,
the "NO SCC" indicates that there was no SCC generated at the four-point bending test
specimen, and the "SCC PRESENT" indicates that there was SCC. In the "corrosion rate"
column, the "<0.1" indicates that the corrosion rate was less than 0.1 g/(m
2·hr), while the "≥ 0.1" indicates that the corrosion rate was not less than 0.1 g/(m
2·hr).
[0072] With reference to Table 3, the steels with Nos. 1 to 12 did not have any SCC and
their corrosion rates were all less than 0.1 g/(m
2·hr). Their yield stress values were all 654 MPa or more.
[0073] On the other hand, the steels with Nos. 13, 15 and 17 had SCC because their Mo, Cu,
and Ni contents were small. The steel with No. 14 had SCC because it contained only
a small amount of Cr, and its corrosion rate was not less than 0.1 g/(m
2·hr). Furthermore, the steel with No. 16 had SCC because it did not contain REM.
[0074] While preferred embodiments of the present invention have been described above, it
is to be understood that variations and modifications will be apparent to those skilled
in the art without departing the scope and spirit of the present invention. The scope
of the present invention, therefore, is to be determined solely by the following claims.
Applicable Field in the Industry
[0075] The stainless steel according to the invention can be applied as oil country tubular
goods and particularly suitably applied to an oil country tubular good for use in
a carbon dioxide gas contained, high temperature chloride aqueous solution environment
at 150°C or higher.