U.S. patent application number 11/149320 was filed with the patent office on 2005-10-13 for high strength martensitic stainless steel excellent in carbon dioxide gas corrosion resistance and sulfide stress-corrosion cracking resistance.
Invention is credited to Takabe, Hideki, Ueda, Masakatsu.
Application Number | 20050224143 11/149320 |
Document ID | / |
Family ID | 32677145 |
Filed Date | 2005-10-13 |
United States Patent
Application |
20050224143 |
Kind Code |
A1 |
Takabe, Hideki ; et
al. |
October 13, 2005 |
High strength martensitic stainless steel excellent in carbon
dioxide gas corrosion resistance and sulfide stress-corrosion
cracking resistance
Abstract
The present invention provides a martensitic stainless steel in
which specified elements in a steel composition are limited. The
martensitic stainless steel can have high strength of 0.2% proof
stress of 860 MPa or more and excellent carbon dioxide gas
corrosion resistance and sulfide stress-corrosion cracking
resistance by limiting the steel composition of specified elements
and defining Mo content in the steel by relationships with IM
values as well as by forming microstructure of the steel with main
tempered martensite, carbide precipitated during tempering, and
intermetallic compounds such as a Laves phase, a .sigma. phase and
the like. As a result the martensitic stainless steels of the
present invention can be applied to practical steels, which can be
widely used in oil well tubes and the like under environment
including carbon dioxide gas, hydrogen sulfide, chlorine ions or
two or more of them, in wide fields.
Inventors: |
Takabe, Hideki; (Osaka,
JP) ; Ueda, Masakatsu; (Shiki-gun, JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW
SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
32677145 |
Appl. No.: |
11/149320 |
Filed: |
June 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11149320 |
Jun 10, 2005 |
|
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PCT/JP03/16288 |
Dec 18, 2003 |
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Current U.S.
Class: |
148/325 ; 420/61;
420/67 |
Current CPC
Class: |
C21D 6/004 20130101;
C22C 38/50 20130101; C22C 38/02 20130101; C22C 38/001 20130101;
C22C 38/04 20130101; C21D 1/25 20130101; C22C 38/46 20130101; C22C
38/004 20130101; C21D 2211/008 20130101; C22C 38/44 20130101; C22C
38/42 20130101; C22C 38/06 20130101; C21D 2211/004 20130101 |
Class at
Publication: |
148/325 ;
420/061; 420/067 |
International
Class: |
C22C 038/44; C22C
038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2002 |
JP |
2002-369595 |
Claims
1. A high strength martensitic stainless steel excellent in carbon
dioxide gas corrosion resistance and sulfide stress-corrosion
cracking resistance and having 0.2% proof stress of 860 MPa or
more, which comprises: including, by mass %, C: 0.005-0.04%, Si:
0.5% or less, Mn: 0.1-3.0%, P: 0.04% or less, S: 0.01% or less, Cr:
10-15%, Ni: 4.0-8%, Mo: 2.8-5.0%, Al: 0.001-0.10%, N, 0.07% or
less, Ti: 0-0.25%, V: 0-0.25%, Nb: 0-0.25%, Zr: 0-0.25%, Cu: 0-1%,
Ca: 0-0.005%, Mg: 0-0.005%, La: 0-0.005%, and Ce: 0-005%, and the
balance of Fe and impurities; and satisfying the expression (1)
given below wherein the microstructure mainly comprises tempered
martensite, carbide precipitated during tempering, and
intermetallic compounds such as Laves phase, .sigma. phase and the
like finely precipitated during tempering, Mo.gtoreq.=2.3-0.89
Si+32.2 C (1) where the symbols of the respective elements in the
expression (1) show the content (mass %) of each element.
2. A high strength martensitic stainless steel excellent in carbon
dioxide gas corrosion resistance and sulfide stress-corrosion
cracking resistance and having 0.2% proof stress of 860 MPa or more
according to claim 1, comprising one or more selected from a group
consisting of Ti: 0.005-0.25%, V: 0.005-0.25%, Nb: 0.005-0.25%, and
Zr: 0.005-0.25%.
3. A high strength martensitic stainless steel excellent in carbon
dioxide gas corrosion resistance and sulfide stress-corrosion
cracking resistance and having 0.2% proof stress of 860 MPa or more
according to claim 1, comprising Cu: 0.05-1%.
4. A high strength martensitic stainless steel excellent in carbon
dioxide gas corrosion resistance and sulfide stress-corrosion
cracking resistance and having 0.2% proof stress of 860 MPa or more
according to claim 1, comprising one or more selected from a group
consisting of Ca: 0.0002-0.005%, Mg: 0.0002-0.005%, La:
0.0002-0.005%, and Ce: 0.0002-0.005%.
5. A high strength martensitic stainless steel excellent in carbon
dioxide gas corrosion resistance and sulfide stress-corrosion
cracking resistance and having 0.2% proof stress of 860 MPa or
more, which comprises: the compositions defined in claim 1; and
being subjected to tempering in which (20+log t)(T+273) satisfies
13500-17700 when, after quenching the steel at a quenching
temperature of 880.degree. C.-1000.degree. C., a range of a
tempering temperature is set to 450.degree. C.-620.degree. C., a
tempering temperature is set to T (.degree. C.) and tempering time
is set to t (hour).
6. A high strength martensitic stainless steel excellent in carbon
dioxide gas corrosion resistance and sulfide stress-corrosion
cracking resistance and having 0.2% proof stress of 860 MPa or
more, which comprises: the compositions defined in claim 2; and
being subjected to tempering in which (20+log t)(T+273) satisfies
13500-17700 when, after quenching the steel at a quenching
temperature of 880.degree. C.-1000.degree. C., a range of a
tempering temperature is set to 450.degree. C.-620.degree. C., a
tempering temperature is set to T (.degree. C.) and tempering time
is set to t (hour).
7. A high strength martensitic stainless steel excellent in carbon
dioxide gas corrosion resistance and sulfide stress-corrosion
cracking resistance and having 0.2% proof stress of 860 MPa or
more, which comprises: the compositions defined in claim 3; and
being subjected to tempering in which (20+log t)(T+273) satisfies
13500-17700 when, after quenching the steel at a quenching
temperature of 880.degree. C.-1000.degree. C., a range of a
tempering temperature is set to 450.degree. C.-620.degree. C., a
tempering temperature is set to T (.degree. C.) and tempering time
is set to t (hour).
8. A high strength martensitic stainless steel excellent in carbon
dioxide gas corrosion resistance and sulfide stress-corrosion
cracking resistance and having 0.2% proof stress of 860 MPa or
more, which comprises: the compositions defined in claim 4; and
being subjected to tempering in which (20+log t)(T+273) satisfies
13500-17700 when, after quenching the steel at a quenching
temperature of 880.degree. C.-1000.degree. C., a range of a
tempering temperature is set to 450.degree. C.-620.degree. C., a
tempering temperature is set to T (.degree. C.) and tempering time
is set to t (hour).
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a steel material suitable
for its use in severe corrosion environment containing corrosive
materials such as carbon dioxide gas, hydrogen sulfide, chlorine
ions and the like. Specifically, the present invention relates to a
steel material for a seamless steel tube and a seam welded steel
tube such as an electric resistance welding steel tube, a laser
welding steel tube, a spiral welding tube or the like, which is
used in applications for petroleum or natural gas production
facilities, facilities for eliminating carbon dioxide gas, or for
geo-thermal power generation, or for a tank for liquid containing
carbon dioxide gas, especially to a steel material for oil well
tubes for oil wells or gas wells.
[0003] 2. Description of the Related Art
[0004] From the viewpoint of exhaustion of petroleum resources,
which is expected in the near future, development of an oil well
under severe environment that is an oil well in a deeper layer, of
a sour gas field or the like, has often been performed. Thus, high
strength and excellent corrosion resistance and sulfide stress
corrosion cracking resistance are required for oil well steel
tubes, which are used in such uses.
[0005] As a steel material for oil well tubes or the like, carbon
steel or a low-alloy steel has been generally used. However, as the
environment of the well becomes severe, steel which contains
increased amount of alloying elements, has been used. For example,
as oil wells for steel material which contain a large amount of
carbon dioxide gas, 13 Cr series martensitic stainless steel such
as typical SUS 420 and the like have been used.
[0006] However, although the SUS 420 steel has excellent corrosion
resistance to carbon dioxide gas, it has poor corrosion resistance
to hydrogen sulfide. Thus, the SUS 420 steel is liable to generate
sulfide stress-corrosion cracking (SSCC) under the environment
containing carbon dioxide gas and hydrogen sulfide
simultaneously.
[0007] Therefore various steel materials in place of the SUS 420
steel have been proposed.
[0008] Japanese Patent No. 2861024, Japanese Patent Application
Publication No. 05-287455, and Japanese Patent Application
Publication No. 07-62499 disclose steel having improved corrosion
resistance by reducing carbon content of the SUS 420.
[0009] However, such a low carbon-content steel described in these
publications may not have the enough strength required for use in a
deep well, that is proof stress of 860 MPa or more.
[0010] Alternatively, Japanese Patent Appilcation Publication No.
2000-192196 discloses steel of a martensitic single phase structure
containing Co: 0.5-7% and Mo: 3.1-7% having high strength and
excellent sulfide stress-corrosion cracking resistance. The
invention described in the publication is a steel containing Co in
the above-mentioned range to suppress the generation of retained
austenite during cooling so that the structure is made to be a
martensitic single phase. However, since Co is an expensive
element, it is desirable not to use.
SUMMARY OF THE INVENTION
[0011] The present invention was made in consideration of the
above-mentioned circumstances. The object of the present invention
is to provide a martensitic stainless steel having sufficient
strength to use for oil well tubes for a deep well, that is high
strength of a proof stress of 860 MPa or more, and excellent carbon
dioxide as corrosion resistance and sulfide stress-corrosion
cracking resistance whereby it an be used even under the
environment containing carbon dioxide gas, hydrogen ulfide or
chlorine ions or two or more of them. The symbols of the respective
lements in the following expression show the content (mass %) of
each element.
[0012] Accordingly, the gist of the present invention is high
strength martensitic stainless steels described in the following
(a) and (b).
[0013] (a) A high strength martensitic stainless steel excellent in
carbon dioxide gas corrosion resistance and sulfide
stress-corrosion cracking resistance and having 0.2% proof stress
of 860 MPa or more, which comprises including, by mass %, C:
0.005-0.04%, Si: 0.5% or less, Mn: 0.1-3.0%, P: 0.04% or less, S:
0.01% or less, Cr: 10-15%, Ni: 4.0-8%, Mo: 2.8-5.0%, Al:
0.001-0.10% and N, 0.07% or less, Ti: 0-0.25%, V: 0-0.25%, Nb:
0-0.25%, Zr: 0-0.25%, Cu: 0-1%, Ca: 0-0.005%, Mg: 0-0.005%, La:
0-0.005%, and Ce: 0-005%, and the balance Fe and impurities; and
satisfying the expression (1) given below wherein the
microstructure mainly comprises tempered martensite, carbide
precipitated during tempering, and intermetallic compounds such as
Laves phase, .sigma. phase and the like finely precipitated during
tempering.
Mo.gtoreq.2.3-0.89 Si+32.2 C (1)
[0014] wherein the symbols of the respective elements in the
expression (1) show the content (mass %) of each element.
[0015] Further, the gist of the present invention is martensitic
stainless steels containing at least one of alloying elements
selected from at least one group consisting of the following a
first group, a second group and a third group, in addition to the
components described in the above mentioned (a). In this steel said
expression (1) is also satisfied and the microstructure is the same
as mentioned above.
[0016] First group . . . Ti: 0.005-0.25%, V: 0.005-0.25%, Nb:
0.005-0.25%, and Zr: 0.005-0.25%.
[0017] Second group . . . Cu: 0.05-1%
[0018] Third group . . . Ca: 0.0002-0.005%, Mg: 0.0002-0.005%, La:
0.0002-0.005%, and Ce: 0.0002-0.005%.
[0019] (b) A high strength martensitic stainless steel excellent in
carbon dioxide gas corrosion resistance and sulfide
stress-corrosion cracking resistance and having 0.2% proof stress
of 860 MPa or more, which comprises the compositions defined in any
one of (a); being subjected to tempering in which (20+log t)(T+273)
satisfies 13500-17700 when, after quenching the steel at a
quenching temperature of 880.degree. C.-1000.degree. C., a range of
a tempering temperature is set to 450.degree. C.-620.degree. C., a
tempering temperature is set to T (.degree. C.) and tempering time
is set to t (hour); and satisfying the above mentioned expression
(1) wherein the microstructure of said steel mainly comprises
tempered martensite, carbide precipitated during tempering, and
intermetallic compounds such as a Laves phase, a .sigma. phase and
the like finely precipitated during tempering.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a view showing relationships between Mo contents
of various types of steels tested in examples and the right side in
the expression (1), that is "2.3-0.89 Si+32.2 C" (IM value).
[0021] FIG. 2 is a view for explaining tempering conditions defined
in the present invention, which shows relationships between 0.2%
proof stress obtained by changing values of (20+log t)(T+273) while
changing tempering temperatures in 400-650.degree. C. after
quenching steel at 920.degree. C., and the (20+log t)(T+273).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] The reasons for restrictions of contents of various elements
defined in the present inventors will be described hereinbelow. "%"
of the respective contents means mass %.
[0023] C: 0.005-0.04%
[0024] Although C (carbon) is an effective alloying element to
enhance strength of steel, from a viewpoint of corrosion resistance
small C content is preferable. However, if the content of C is less
than 0.005%, proof stress does not reach 860 Mpa or more. Thus, the
lower limit of the C content was set to 0.005%. On the other hand,
if the C content exceeds 0.04%, the hardness of the tempered steel
becomes hard excessively, the steel has high sulfide
stress-corrosion cracking sensibility. Accordingly, the C content
was set to 0.005-0.04%.
[0025] Si: 0.5% or Less
[0026] Si (Silicon) is an alloying element necessary as a
deoxidizer. An amount of Si retained in the steel may be a level of
impurities. However, to obtain a large deoxidation effect it is
preferred that the Si content is set to 0.01% or more. On the other
hand, if the Si content exceeds 0.5%, the toughness of the steel is
decreased and the workability of the steel is also decreased.
Accordingly, the Si content was set to 0.5% or less.
[0027] Mn: 0.1-3.0%
[0028] Mn (Manganese) is an effective alloying element to enhance
the hot workability. To obtain this effect Mn content of 0.1% or
more is needed. On the other hand, if the Mn content exceeds 3.0%,
the effect is saturated resulting in an increase in cost.
Accordingly, the Mn content was set to 0.1-3.0%.
[0029] P: 0.04% or Less
[0030] P (Phosphorus) is an impurity element contained in the steel
and the P content is better as low as possible. Particularly, if
the P content exceeds 0.04%, the sulfide stress-corrosion cracking
resistance is remarkably decreased. Accordingly, the P content was
set to 0.04% or less.
[0031] S: 0.01% or Less
[0032] S (Sulfur) is an impurity element contained in the steel and
the S content is better as low as possible. Particularly, if the S
content exceeds 0.01%, the hot workability, corrosion resistance
and toughness are remarkably decreased. Accordingly, the S content
was set to 0.01% or less.
[0033] Cr: 10-15%
[0034] Cr (Chromium) is an effective alloying element to enhance
the carbon dioxide gas corrosion resistance. To obtain this effect
Cr content of 10% or more is needed. On the other hand, if the Cr
content exceeds 15%, it becomes difficult to make the
microstructure of tempered steel a martensite phase mainly.
Accordingly, the Cr content was set to 10-15%.
[0035] Ni: 4.0-8%
[0036] Ni (Nickel) is an alloying element, which is necessary for
making the microstructure of tempered steel a martensite phase
mainly. However, if the Ni content is 4.0% or less, a number of
ferrite phases were precipitated in the microstructure of tempered
steel and the microstructure of tempered steel does not become a
martensite phase mainly. On the other hand, if the Ni content
exceeds 8%, the microstructure of tempered steel becomes an
austenite phase mainly. Accordingly, the Ni content was set to
4.0-8%. More preferably the Ni content was set to 4-7%.
[0037] Mo: 2.8-5.0%
[0038] Mo (Molybdenum) is an effective alloying element to enhance
the sulfide stress-corrosion cracking resistance for a high
strength material. To obtain this effect Mo content of 2.8% or more
is needed. However, if the Mo content exceeds 5.0%, this effect is
saturated, resulting in an increase in cost. Accordingly, the Mo
content was set to 2.8-5.0%.
[0039] Al: 0.001-0.10%
[0040] Al (Aluminum) is an alloying element, which is used as a
deoxidizer in a melting process. To obtain this effect Al content
of 0.001% or more is needed. However, if the Al content exceeds
0.10%, many inclusions are formed in the steel so that the
corrosion resistance is lost. Accordingly, the Al content was set
to 0.001-0.10%.
[0041] N, 0.07% or Less
[0042] N (Nitrogen) is an impurity element contained in the steel
and the N content is better as low as possible. Particularly, if
the N content exceeds 0.07%, many inclusions are formed so that the
corrosion resistance is lost. Accordingly, the N content was set to
0.07% or less.
[0043] One of martensitic stainless steels according to the present
invention consists the above-mentioned chemical composition as well
as the balance Fe and indispensable impurities. Another martensitic
stainless steel according to the present invention further
contains, in addition to the above-mentioned components, at least
one alloying element selected from at least one group consisting of
a first group, a second group and a third group shown as follows.
The components (elements) of the respective groups will be
described below.
[0044] First Group (Ti, V, Nb, Zr: 0.005-0.25% Respectively)
[0045] Since Ti, V, Nb and Zr have effect to fix C so as to reduce
variations of strength, one or more selected from these elements
may be optionally contained. However, if any one of the elements is
less than 0.005%, the above-mentioned effect cannot be obtained. On
the other hand, if any one of the elements exceeds 0.25%, the
microstructure of the steel cannot become a martensite phase mainly
so that highly strengthening of the steel with a proof stress of
860 MPa or more cannot be attained. Accordingly, the respective
contents in selectively containing these elements were set to
0.005-0.25%.
[0046] Second Group (Cu: 0.05-1%)
[0047] Cu is an effective element to make the microstructure of
tempered steel a martensite phase mainly like Ni. To obtain the
effect by the addition of Cu the Cu content may be 0.05% or more.
However, if the Cu content exceeds 1%, the hot workability of the
steel is lowered. Accordingly, when Cu is contained in the steel
the Cu content was set to 0.05-1%.
[0048] Third Group (Ca, Mg, La, Ce: 0.0002-0.005% Respectively)
[0049] Since Ca, Mg, La and Ce are effective elements to enhance
the hot workability of the steel, one or more selected from these
elements may be optionally contained. However, if any one of the
elements is less than 0.0002%, the above-mentioned effect cannot be
obtained. On the other hand, if any one of the elements exceeds
0.005%, coarse oxide is formed in the steel whereby the corrosion
resistance of the steel is decreased. Accordingly, the respective
contents in selectively containing these elements were set to
0.0002-0.005%. Particularly, it is preferred to contain Ca and/or
La in the steel.
[0050] The steel according to the present invention should have the
above-mentioned chemical composition and satisfy the following
expression (1). This is because, if the steel satisfies the
expression (1), strength of the steel can be enhanced to proof
stress of 860 Mpa or more without deteriorating sulfide
stress-corrosion cracking resistance.
Mo.gtoreq.2.3-0.89 Si+32.2 C (1)
[0051] wherein the symbols of the respective elements in the
expression (1) show the content (mass %) of each element.
[0052] FIG. 1 is a view showing relationships between Mo contents
of various types of steels tested in examples, which will be
described later, and the right side in the expression (1), that is
"2.3-0.89 Si+32.2 C" (IM value). Specifically, the results shown in
FIG. 1 are based on steels of the present invention and comparative
steels (test Nos. 18-21). The mark ".smallcircle." shows an example
that did not generate rupture in a sulfide stress-corrosion
cracking test, and the mark "x" shows an example that generated
rupture therein. Even if the Mo content exceeds 2.8%, if the Mo
content does not satisfy the expression (1), the steel has a poor
sulfide stress-corrosion cracking resistance.
[0053] When Mo content is out of a range (that is less than 2.8%)
defined in the present invention, the 0.2% proof stress of the
steel is less than 860 Mpa. Further, even if Mo content is in a
range (that is 2.8-5%) defined in the present invention, if the Mo
content does not satisfy the above-mentioned expression (1), the
0.2% proof stress of the steel is less than 860 Mpa.
[0054] However, if steel satisfies the above-mentioned expression
(1), the 0.2% proof stress of the steel reaches 860 Mpa or more and
the steel can endure the use of an oil well steel material due to
its sufficient strength. Accordingly, the steel according to the
present invention should be in a range of said chemical composition
and satisfy the above-mentioned expression (1).
[0055] Further, the present inventors have checked the influences
of microstructure. As a result the present inventors have found
that if the microstructure is a structure mainly comprising
tempered martensite, carbide precipitated during tempering, and
intermetallic compounds such as Laves phase, .sigma. phase and the
like finely precipitated during tempering, the strength of the
steel can be enhanced without deteriorating sulfide
stress-corrosion cracking resistance.
[0056] It is noted that "mainly comprising tempered martensite"
means that a 70 vol % or more of the microstructure of the steel is
a tempered martensitic structure, and a retained austenitic
structure and/or a ferritic structure other than a tempered
martensitic structure may be present.
[0057] Further, the "intermetallic compounds such as Laves phase,
.sigma. phase and the like" may contain intermetallic compounds
such as .mu. phase and .chi. phase other than Laves phase such as
Fe.sub.2Mo and the like and .sigma. phase.
[0058] The microstructure of the steel according to the present
invention contains carbide precipitated during tempering. Although
carbide is an effective microstructure to ensure the strength of
the steel, high strength of proof stress of 860 Mpa or more cannot
be realized by only carbide contained in the steel. Accordingly, in
the present invention precipitation of carbide as well as fine
precipitation of intermetallic compounds such as the
above-mentioned Laves phase, .sigma. phase and the like are
needed.
[0059] Heat treatment for the steel of the present invention is
typical quenching-tempering. To precipitate fine intermetallic
compounds during tempering it is necessary to sufficiently dissolve
the intermetallic compounds during quenching. The quenching
temperature is preferably 880-1000.degree. C.
[0060] Further, conditions in which intermetallic compounds such as
a fine Laves phase, .sigma. phase and the like are precipitated and
0.2% proof stress of 860 Mpa or more can be obtained resides in a
case where when a temperature range for tempering is
450-620.degree. C., as well as the tempering temperature is set to
T(.degree. C.) and the tempering time is set to t (hour), (20+log
t)(T+273) can satisfy 13500-17700.
[0061] FIG. 2 is a view for explaining tempering conditions defined
in the present invention. FIG. 2 shows relationships between 0.2%
proof stress obtained by changing values of (20+log t)(T+273) while
changing tempering temperatures in 400-650.degree. C. after
quenching steel at 920.degree. C., and the (20+log t)(T+273).
[0062] As shown in FIG. 2, when (20+log t)(T+273) is in a range of
13500-17700, 0.2% proof stress reaches 860 Mpa or more.
[0063] When tempering is performed at a condition that (20+log
t)(T+273) exceeds 17700, dislocation density is reduced or
imtermetallic compounds are dissolved in microstructure of the
steel, whereby high strengthening of 0.2% proof stress of 860 Mpa
or more cannot be attained. On the other hand, when the steel is
tempered at a condition of less than 13500, intermetallic compounds
and carbide are not precipitated. Accordingly, 0.2% proof stress of
860 Mpa or more cannot be attained.
[0064] From the above-mentioned principal, the steel of the present
invention should have the above-mentioned chemical compositions and
satisfy the expression (1) and the microstructure of the steel
should be mainly comprising tempered martensite, carbide
precipitated during tempering, and intermetallic compounds such as
a Laves phase, .sigma. phase and the like finely precipitated
during tempering.
EXAMPLES
[0065] Steels having chemical compositions shown in Tables 1 (1)
and 1 (2) were melted and cast, and the obtained cast ingots were
forged and hot rolled to prepare steel plates each having a
thickness of 15 mm, a width of 120 mm and a length pf 1,000 mm.
These steel plates were subjected to quenching (water cooling at
920.degree. C.) and tempering [air cooling after soaking at
550.degree. C. for 30 min. ((20+log t)(T+273)=16212), and the
obtained steel plates were provided in various tests as testing
steel plates.
1 TABLE 1 Test Chemical composition (mass %) No. C Si Mn P S Cu Cr
Ni Mo Al N Steels of the intention 1 0.014 0.17 0.43 0.015 0.0010
11.81 6.85 2.93 0.030 0.0055 2 0.016 0.17 0.46 0.015 0.0010 12.08
6.90 2.93 0.030 0.0055 3 0.026 0.18 0.87 0.016 0.0011 0.08 12.02
7.67 4.50 0.028 0.0050 4 0.034 0.04 0.44 0.015 0.0010 0.04 12.01
7.39 3.88 0.034 0.0062 5 0.008 0.48 0.41 0.011 0.0010 0.98 11.98
7.87 3.98 0.024 0.0050 6 0.015 0.17 0.98 0.015 0.0010 10.11 4.21
2.98 0.030 0.0055 7 0.017 0.17 1.02 0.015 0.0010 14.10 7.92 2.88
0.030 0.0066 8 0.016 0.15 0.23 0.013 0.0009 12.10 6.87 2.86 0.025
0.0065 9 0.015 0.15 1.44 0.012 0.0008 12.07 6.85 2.91 0.025 0.0066
10 0.014 0.18 0.44 0.015 0.0008 0.28 12.01 6.91 2.96 0.034 0.0062
11 0.014 0.21 0.44 0.015 0.0010 0.85 12.01 6.55 2.97 0.034 0.0062
12 0.014 0.20 0.44 0.015 0.0010 1.54 12.01 6.25 2.97 0.034 0.0062
13 0.015 0.18 0.43 0.017 0.0090 2.70 12.08 5.85 2.96 0.030 0.0642
14 0.014 0.17 0.47 0.014 0.0012 0.48 12.08 6.90 2.96 0.027 0.0074
15 0.016 0.15 0.67 0.015 0.0010 12.01 6.78 2.94 0.030 0.0059 16
0.014 0.02 0.46 0.012 0.0009 11.99 6.89 2.91 0.025 0.0065 17 0.015
0.17 0.44 0.014 0.0010 0.43 12.01 6.88 2.88 0.027 0.0067 Chemical
composition (mass %) Test IM Mo - IM No. Nb V Ti Zr Ca Mg La Ce
value value Steels of the intention 1 -- -- -- -- -- -- 2.60 0.33 2
0.060 -- -- -- -- -- -- 2.66 0.27 3 0.040 0.003 -- -- -- -- -- 2.98
1.52 4 0.040 0.091 -- -- -- -- -- 3.36 0.52 5 0.05 0.105 0.040 --
-- 0.0009 0.0010 2.13 1.85 6 0.004 -- -- -- 0.0005 -- -- 2.63 0.35
7 0.060 -- -- -- -- -- -- 2.70 0.18 8 0.004 -- -- -- -- -- -- 2.68
0.28 9 0.060 -- -- -- -- -- -- 2.65 0.26 10 0.040 0.091 -- -- -- --
-- 2.59 0.37 11 0.040 0.092 -- -- -- -- -- 2.56 0.41 12 0.040 0.091
-- -- -- -- -- 2.57 0.40 13 0.060 0.088 -- -- -- -- -- 2.62 0.34 14
-- -- -- -- -- -- -- 2.60 0.36 15 -- -- -- 0.0005 -- -- -- 2.68
0.26 16 -- 0.098 -- 0.0004 -- -- -- 2.74 0.17 17 -- -- -- -- -- --
0.0007 2.63 0.23 Note 1) Mark * shows out of range defined in the
present invention. Note 2) IM value shows (2.3 - 0.89 Si + 32.2C)
Note 3) Mo - IM value shows a calculated value of (Mo content - IM
value), and if this value is 0 or more, it satisfied the expression
(1) defined in the present invention.
[0066]
2 TABLE 1 (2) Test Chemical composition (mass %) No. C Si Mn P S Cu
Cr Ni Mo Al Comparative Steels 18 0.018 0.17 0.45 0.012 0.0012 0.04
10.50 6.11 2.44 0.048 19 0.012 0.11 0.44 0.011 0.0010 0.03 12.08
6.15 2.46 0.043 20 0.028 0.05 0.45 0.015 0.0011 0.03 12.70 6.45
2.61 0.058 21 0.020 0.21 0.45 0.015 0.0011 0.03 12.70 6.45 2.65
0.058 22 0.018 0.15 0.46 0.015 0.0009 0.03 12.30 6.34 *1.51 0.032
23 0.017 0.16 0.45 0.015 0.0011 0.03 11.98 6.38 *0.98 0.035 24
0.023 0.17 0.43 0.014 0.0011 0.03 11.97 6.48 *2.04 0.034 25 0.015
0.35 0.44 0.013 0.0011 0.03 *9.50 6.01 2.46 0.011 Chemical
composition (mass %) Test IM Mo - IM No. N Nb V Ti Zr Ca Mg La Ce
value value Comparative Steels 18 0.0064 0.030 0.101 -- -- -- -- --
2.73 *-0.29 19 0.0062 0.080 0.098 -- -- -- -- -- 2.59 *-0.13 20
0.0055 0.040 0.067 -- -- -- -- -- 3.16 *-0.55 21 0.0055 0.040 0.067
-- -- -- -- -- 2.76 *-0.11 22 0.0060 0.040 0.003 -- -- -- -- --
2.75 *-1.24 23 0.0061 0.050 0.100 0.098 -- -- -- -- -- 2.71 *-1.73
24 0.0058 0.040 0.097 -- -- -- -- -- 2.89 *-0.85 25 0.0053 0.050
0.088 -- -- -- -- -- 2.47 *-0.01 Note 1) Mark * shows out of range
defined in the present invention. Note 2) IM value shows (2.3 -
0.89 Si + 32.2C) Note 3) Mo - IM value shows a calculated value of
(Mo content - IM value), and if this value is 0 or more, it
satisfied the expression (1) defined in the present invention.
[0067] First, round bar test pieces each having a diameter of 6.35
mm and a length of the parallel portion of 25.4 mm were taken from
the respective testing steel plates and subjected to tensile tests
at normal temperatures. The obtained 0.2% proof stresses are shown
in Table 2.
[0068] Then, test pieces each having a thickness of 3 mm, a width
of 20 mm and a length of 50 mm were taken from the respective
testing steel plates and these testing pieces were polished with a
No. 600 emery paper and degreased and dried. Then the obtained
testing pieces were immersed into 25% NaCl water solution saturated
with 0.973 Mpa CO.sub.2 gas and 0.0014 Mpa H.sub.2S gas
(temperature: 165.degree. C.) for 720 hours.
[0069] After the immersion weight reductions of the test pieces by
corrosion [(mass before testing)-(mass after testing)] were
measured and the presence and absence of local corrosion on
surfaces of the testing pieces were confirmed by a visual test. As
a result the corrosion rate of the steel according to the present
invention is 0.5 mm/year or less, and no local corrosion on its
surface could be found.
[0070] Subsequently, examples in which 0.2% proof stresses were 860
Mpa or more in the tensile tests were subjected to fixed load tests
by use of a spring type (proof ring type) testing machine in
accordance with TM0177-96 Method A of NACE. Specifically, round bar
test pieces each having a diameter of 6.3 mm and a length of the
parallel portion of 25.4 mm were taken from the respective testing
steel plates and subjected to 0.2% proof stress-85% (test stress)
fixed load tests at a test temperature of 25.degree. C., for 720
hours by use of 0.003 Mpa H.sub.2S gas (CO.sub.2 bal.) saturated
25% NaCl water solution (pH 4.0). As a result all test pieces were
not ruptured.
[0071] The Microstructures of the test pieces were observed by an
optical microscope and an extraction replica. These results are
also shown in Table 2.
3TABLE 2 0.2% Carbon SSC poof dioxide Corro- Test stress gas
corrosion sion No. (Mpa) test test Microstructure Steels of the 1
951 .smallcircle. .smallcircle. M + IM + C invention 2 944
.smallcircle. .smallcircle. M + IM + C 3 1,007 .smallcircle.
.smallcircle. M + IM + C 4 1,027 .smallcircle. .smallcircle. M + IM
+ C 5 1,020 .smallcircle. .smallcircle. M + IM + C 6 910
.smallcircle. .smallcircle. M + IM + C 7 882 .smallcircle.
.smallcircle. M + F + IM + C 8 944 .smallcircle. .smallcircle. M +
IM + C 9 965 .smallcircle. .smallcircle. M + IM + C 10 972
.smallcircle. .smallcircle. M + IM + C 11 958 .smallcircle.
.smallcircle. M + IM + C 12 951 .smallcircle. .smallcircle. M + IM
+ C 13 965 .smallcircle. .smallcircle. M + IM + C 14 958
.smallcircle. .smallcircle. M + IM + C 15 972 .smallcircle.
.smallcircle. M + IM + C 16 882 .smallcircle. .smallcircle. M + IM
+ C 17 979 .smallcircle. .smallcircle. M + IM + C Comparative 18
841 .smallcircle. x M + C steels 19 843 .smallcircle. x M + C 20
858 .smallcircle. x M + C 21 840 .smallcircle. x M + C 22 829
.smallcircle. x M + C 23 832 .smallcircle. x M + C 24 849
.smallcircle. x M + C 25 841 x x M + C Note 1) In carbon dioxide
gas corrosion test a steel, whose corrosion rate is 0.5 mm/y or
less, and which did not generate local corrosion, is shown by
".smallcircle." and otherwise "x". Note 2) In SSC test, a steel,
which did not generate rupture, is shown by ".smallcircle." and a
steel, which generated rupture, is shown by "x". Note 3) In
microstructure, tempered martensite is shown by "M", ferrite is
shown by "F", intermetallic compounds are shown by "IM" and carbide
is shown by "C".
[0072] A shown in Table 2, examples Nos. 1 to 17 of the present
invention each have 0.2% proof stress of 860 Mpa or more and
excellent carbon dioxide gas corrosion resistance and sulfide
stress-corrosion cracking resistance. On the other hand,
comparative examples Nos. 22 to 25, which have Cr and/or Mo
contents out of range defined in the present invention, and
comparative examples Nos. 18 to 21, which have the content ranges
of the respective components are in the range defined in the
present invention but the expression (1) previously described was
not satisfied, were not sufficient in carbon dioxide gas resistance
and/or stress cracking resistance.
INDUSTRIAL APPLICABILITY
[0073] The martensitic stainless steel according to the present
invention can have high strength of 0.2% proof stress of 860 Mpa or
more and excellent carbon dioxide gas corrosion resistance and
sulfide stress-corrosion cracking resistance by limiting the steel
composition of specified elements and defining Mo content in the
steel by relationships with IM values as well as by forming
microstructure of the steel with tempered martensite mainly,
carbide precipitated during tempering, and intermetallic compounds
such as a Laves phase, a .sigma. phase and the like. As a result
the martensitic stainless steels of the present invention can be
applied to practical steels, which can be widely used in oil well
tubes and the like under environment including carbon dioxide gas,
hydrogen sulfide, chlorine ions or two or more of them, in wide
fields.
* * * * *