U.S. patent application number 16/328755 was filed with the patent office on 2019-06-27 for austenitic stainless steel.
The applicant listed for this patent is Nippon Steel & Sumitomo Metal Corporation. Invention is credited to Etsuo Dan, Shinnosuke Kurihara, Hirokazu Okada, Takahiro Osuki, Masahiro Seto.
Application Number | 20190194787 16/328755 |
Document ID | / |
Family ID | 61300959 |
Filed Date | 2019-06-27 |
United States Patent
Application |
20190194787 |
Kind Code |
A1 |
Okada; Hirokazu ; et
al. |
June 27, 2019 |
Austenitic Stainless Steel
Abstract
An objective of the present invention is to provide an
austenitic stainless steel that is excellent in polythionic acid
SCC resistance and also excellent in creep ductility. An austenitic
stainless steel according to the present invention includes a
chemical composition consisting of, in mass %, C: 0.030% or less,
Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040% or less, S: 0.010%
or less, Cr: 16.0 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.1 to 5.0%, Nb:
0.20 to 1.00%, N: 0.050 to 0.300%, sol.Al: 0.0005 to 0.100%, and B:
0.0010 to 0.0080%, with the balance being Fe and impurities, and
satisfying Formula (1): B+0.004-0.9C+0.017Mo.sup.2.gtoreq.0 (1)
where symbols of elements in Formula (1) are to be substituted by
contents of corresponding elements (mass %).
Inventors: |
Okada; Hirokazu;
(Chiyoda-ku, Tokyo, JP) ; Kurihara; Shinnosuke;
(Chiyoda-ku, Tokyo, JP) ; Dan; Etsuo; (Chiyoda-ku,
Tokyo, JP) ; Seto; Masahiro; (Chiyoda-ku, Tokyo,
JP) ; Osuki; Takahiro; (Chiyoda-ku, Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Steel & Sumitomo Metal Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
61300959 |
Appl. No.: |
16/328755 |
Filed: |
August 30, 2017 |
PCT Filed: |
August 30, 2017 |
PCT NO: |
PCT/JP2017/031157 |
371 Date: |
February 27, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/46 20130101;
C22C 38/02 20130101; C22C 38/42 20130101; C22C 38/44 20130101; C22C
38/54 20130101; C22C 38/52 20130101; C21D 8/0236 20130101; C22C
38/00 20130101; C22C 38/004 20130101; C22C 38/001 20130101; C22C
38/48 20130101; C21D 6/004 20130101; C21D 8/0205 20130101; C21D
9/46 20130101; C22C 38/04 20130101; C22C 38/06 20130101; C22C 38/58
20130101; C22C 38/002 20130101; C22C 38/005 20130101; C21D 8/0226
20130101; C21D 6/02 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/06 20060101 C22C038/06; C22C 38/00 20060101
C22C038/00; C22C 38/42 20060101 C22C038/42; C22C 38/44 20060101
C22C038/44; C22C 38/48 20060101 C22C038/48; C22C 38/54 20060101
C22C038/54; C22C 38/46 20060101 C22C038/46; C22C 38/52 20060101
C22C038/52 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2016 |
JP |
2016-168596 |
Claims
1. An austenitic stainless steel comprising a chemical composition
consisting of, in mass %: C: 0.030% or less; Si: 0.10 to 1.00%; Mn:
0.20 to 2.00%; P: 0.040% or less; S: 0.010% or less; Cr: 16.0 to
25.0%; Ni: 10.0 to 30.0%; Mo: 0.1 to 5.0%; Nb: 0.20 to 1.00%; N:
0.050 to 0.300%; sol.Al: 0.0005 to 0.100%; B: 0.0010 to 0.0080%;
Cu: 0 to 5.0%; W: 0 to 5.0%; Co: 0 to 1.0%; V: 0 to 1.00%; Ta: 0 to
0.2%; Hf: 0 to 0.20%; Ca: 0 to 0.010%; Mg: 0 to 0.010%; and rare
earth metals: 0 to 0.10%, with the balance being Fe and impurities,
and satisfying Formula (1): B+0.004-0.9C+0.017Mo.sup.2.gtoreq.0 (1)
where symbols of elements in Formula (1) are to be substituted by
contents of corresponding elements (mass %).
2. The austenitic stainless steel according to claim 1, wherein the
chemical composition contains one or more elements selected from
the group consisting of: Cu: 0.1 to 5.0%; W: 0.1 to 5.0%; and Co:
0.1 to 1.0%.
3. The austenitic stainless steel according to claim 1, wherein the
chemical composition contains one or more elements selected from
the group consisting of: V: 0.1 to 1.00%; Ta: 0.01 to 0.2%; and Hf:
0.01 to 0.20%.
4. The austenitic stainless steel according claim 1, wherein the
chemical composition contains one or more elements selected from
the group consisting of: Ca: 0.0005 to 0.010%; Mg: 0.0005 to
0.010%; and rare earth metals: 0.001 to 0.10%.
5. The austenitic stainless steel according to claim 1, wherein the
chemical composition contains Cu: 0 to 1.9%.
6. The austenitic stainless steel according to claim 2, wherein the
chemical composition contains one or more elements selected from
the group consisting of: V: 0.1 to 1.00%; Ta: 0.01 to 0.2%; and Hf:
0.01 to 0.20%.
7. The austenitic stainless steel according to claim 2, wherein the
chemical composition contains one or more elements selected from
the group consisting of: Ca: 0.0005 to 0.010%; Mg: 0.0005 to
0.010%; and rare earth metals: 0.001 to 0.10%.
8. The austenitic stainless steel according to claim 3, wherein the
chemical composition contains one or more elements selected from
the group consisting of: Ca: 0.0005 to 0.010%; Mg: 0.0005 to
0.010%; and rare earth metals: 0.001 to 0.10%.
9. The austenitic stainless steel according to claim 6, wherein the
chemical composition contains one or more elements selected from
the group consisting of: Ca: 0.0005 to 0.010%; Mg: 0.0005 to
0.010%; and rare earth metals: 0.001 to 0.10%.
Description
TECHNICAL FIELD
[0001] The present invention relates to a stainless steel, more
specifically to an austenitic stainless steel.
BACKGROUND ART
[0002] Some components for plant facilities, such as a heating
furnace pipe of a thermal boiler, an oil-refining and petrochemical
plant, or other facilities, are used under a high-temperature
corrosive environment, the temperature of which is as high as 600
to 700.degree. C., and a corrosive fluid containing sulfide and/or
chloride is contained in the environment. When such a plant
facility is not operated due to a regular inspection or other
reasons, air, moisture, and sulfide scale react to form polythionic
acid on a surface of a component. The polythionic acid induces
stress corrosion cracking in a grain boundary (hereafter, referred
to as polythionic acid SCC). Accordingly, components used in the
high-temperature corrosive environment described above are required
to have an excellent polythionic acid SCC resistance.
[0003] A steel with an increased polythionic acid SCC resistance is
proposed in Japanese Patent Application Publication No. 2003-166039
(Patent Literature 1) and International Application Publication No.
WO2009/044802 (Patent Literature 2). The polythionic acid SCC
occurs due to Cr precipitating in a form of an M.sub.23C.sub.6
carbide in a grain boundary and a resultant Cr depleted zone formed
in the proximity of the grain boundary. Therefore, according to
Patent Literature 1 and Patent Literature 2, the polythionic acid
SCC resistance is increased by reducing an amount of C to inhibit
the formation of the M.sub.23C.sub.6 carbide.
[0004] Specifically, an heat resistant austenitic steel disclosed
in Patent Literature 1 contains, in mass %, C: 0.005 to less than
0.03%, Si: 0.05 to 0.4%, Mn: 0.5 to 2%, P: 0.01 to 0.04%, S: 0.0005
to 0.005%, Cr: 18 to 20%, Ni: 7 to 11%, Nb: 0.2 to 0.5%, V: 0.2 to
0.5%, Cu: 2 to 4%, N: 0.10 to 0.30%, and B: 0.0005 to 0.0080%, with
the balance being Fe and unavoidable impurities. A total of
contents of Nb and V is 0.6% or more, and a solubility of Nb in the
steel is 0.15% or more. In addition, N/14.gtoreq.Nb/93+V/51 and
Cr-16C-0.5Nb-V.gtoreq.17.5 are satisfied. In Patent Literature 1,
the polythionic acid SCC resistance is increased by reducing the
amount of C and regulating a relation between Cr, and C, Nb, and
V.
[0005] An austenitic stainless steel disclosed in Patent Literature
2 contains in mass %, C: less than 0.04%, Si: 1.5% or less, Mn: 2%
or less, Cr: 15 to 25%, Ni: 6 to 30%, N: 0.02 to 0.35%, and Sol.Al:
0.03% or less, and further contains one or more elements selected
from the group consisting of Nb: 0.5% or less, Ti: 0.4% or less, V:
0.4% or less, Ta: 0.2% or less, Hf: 0.2% or less, and Zr: 0.2% or
less, with the balance being Fe and impurities. The impurities
include P: 0.04% or less, S: 0.03% or less, Sn: 0.1% or less, As:
0.01% or less, Zn: 0.01% or less, Pb: 0.01% or less, and Sb: 0.01%
or less. In addition,
F1=S+{(P+Sn)/2)+((As+Zn+Pb+Sb)/5}.ltoreq.0.075 and
0.05.ltoreq.Nb+Ta+Zr+Hf+2Ti+(V/10).ltoreq.1.7-9.times.F1 are
satisfied. In Patent Literature 2, the polythionic acid SCC
resistance is increased by setting the amount of C at less than
0.05%. In addition, grain boundary embrittling elements in the
steel such as P, S, and Sn are reduced by reducing C immobilizing
elements such as Nb and Ti, thereby enhancing embrittlement
cracking resistance in a weld heat affected zone (HAZ).
CITATION LIST
Patent Literature
[0006] Patent Literature 1: Japanese Patent Application Publication
No. 2003-166039 [0007] Patent Literature 2: International
Application Publication No. WO2009/044802
SUMMARY OF INVENTION
Technical Problem
[0008] Components used in the high-temperature corrosive
environment described above have recently been required to have
high creep ductilities. As described above, a plant facility may
undergo a regular inspection with its equipment deactivated. The
regular inspection involves examination of what components are in
need of replacement. A high creep ductility allows checking how
much a component deforms to be used as a criterion for replacing
the component in the regular inspection.
[0009] Patent Literature 1 and Patent Literature 2 aim at improving
the polythionic acid SCC resistance but do not aim at enhancing the
creep ductility. The steels proposed in these Patent Literatures
each have a reduced amount of C in order to increase the
polythionic acid SCC resistance. In this instance, there are cases
where a high creep ductility cannot be obtained.
[0010] An objective of the present invention is to provide an
austenitic stainless steel excellent in the polythionic acid SCC
resistance and excellent in the creep ductility.
Solution to Problem
[0011] An austenitic stainless steel according to the present
invention includes a chemical composition consisting of, in mass %,
C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040%
or less, S: 0.010% or less, Cr: 16.0 to 25.0%, Ni: 10.0 to 30.0%,
Mo: 0.1 to 5.0%, Nb: 0.20 to 1.00%, N: 0.050 to 0.300%, sol.Al:
0.0005 to 0.100%, B: 0.0010 to 0.0080%, Cu: 0 to 5.0%, W: 0 to
5.0%, Co: 0 to 1.0%, V: 0 to 1.00%, Ta: 0 to 0.2%, Hf: 0 to 0.20%,
Ca: 0 to 0.010%, Mg: 0 to 0.010%, and rare earth metals: 0 to
0.10%, with the balance being Fe and impurities, and satisfying
Formula (1):
B+0.004-0.9C+0.017Mo.sup.2.gtoreq.0 (1)
where symbols of elements in Formula (1) are to be substituted by
contents of corresponding elements (mass %).
Advantageous Effects of Invention
[0012] The austenitic stainless steel according to the present
invention is excellent in the polythionic acid SCC resistance and
excellent in the creep ductility.
DESCRIPTION OF EMBODIMENTS
[0013] The present inventors conducted investigations and studies
on steels that are excellent not only in the polythionic acid SCC
resistance but also in the creep ductility.
[0014] When a content of C is reduced to 0.030% or less, the
formation of M.sub.23C.sub.6 carbide in use under a
high-temperature corrosive environment is inhibited, and the
formation of a Cr depleted zone in the proximity of a grain
boundary is inhibited. Furthermore, in the present invention, 0.20
to 1.00% of Nb is contained to immobilize C with Nb, so as to
further reduce an amount of dissolved C, which causes the formation
of M.sub.23C.sub.6 carbide. In the present invention, Mo is further
contained at 0.1 to 5.0%. Mo inhibits formation of the
M.sub.23C.sub.6 carbide. Therefore, the formation of the Cr
depleted zone is reduced. With the measures described above, the
polythionic acid SCC resistance can be increased.
[0015] However, investigations conducted by the present inventors
showed that reducing the content of C to 0.030% or less leads to a
decrease in the creep ductility. The reason is considered as
follows. Precipitates produced in grain boundaries increase grain
boundary strength. With an increase in the grain boundary strength,
the creep ductility is increased. However, if the content of C is
reduced to 0.030% or less, the precipitates (carbide, or the like)
produced in the grain boundaries are also reduced. As a result, the
grain boundary strength is less likely to be obtained, which
results in the decrease in the creep ductility.
[0016] Hence, the present inventors conducted further studies about
an austenitic stainless steel which can establish compatibility
between an excellent polythionic acid SCC resistance and an
excellent creep ductility. B (boron) is considered to be able to
increase the grain boundary strength through segregating in crystal
grain boundaries under the high-temperature corrosive environment
at 600 to 700.degree. C. described above.
[0017] The present inventors thus considered that the compatibility
between the excellent polythionic acid SCC resistance and the
excellent creep ductility can be established with an austenitic
stainless steel consisting of, in mass %, C: 0.030% or less, Si:
0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040% or less, S: 0.010% or
less, Cr: 16.0 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.1 to 5.0%, Nb:
0.20 to 1.00%, N: 0.050 to 0.300%/a, sol.Al: 0.0005 to 0.100%, B:
0.0010 to 0.0080%, Cu: 0 to 5.0%, W: 0 to 5.0%, Co: 0 to 1.0%, V: 0
to 1.00%, Ta: 0 to 0.2%, Hf: 0 to 0.20%, Ca: 0 to 0.010%, Mg: 0 to
0.010%, and rare earth metals: 0 to 0.10%, with the balance being
Fe and impurities.
[0018] However, results of investigations into polythionic acid SCC
resistance and creep ductility of the austenitic stainless steel
having the chemical composition described above showed that the
excellent creep ductility could not always be obtained, although
the excellent polythionic acid SCC resistance could be obtained.
The present inventors thus conducted further studies. As a result,
it was found that a possible mechanism of the creep ductility is as
follows.
[0019] As described above, the present embodiment involves both
setting the content of C at 0.030% or less to increase the
polythionic acid SCC resistance, and making 0.20 to 1.00% of Nb
contained to immobilize C on Nb, so as to reduce the dissolved C.
Specifically, Nb combines with C through solution treatment or
short-time aging, precipitating in a form of MX carbo-nitride.
However, in an environment in which the steel material according to
the present embodiment is supposed to be used (a high-temperature
corrosive environment at 600 to 700.degree. C.), the MX
carbo-nitride is of a metastable phase. Therefore, when a steel
material having the chemical composition described above is used in
the high-temperature corrosive environment at 600 to 700.degree. C.
for a long-time, an MX carbo-nitride of Nb transforms into a Z
phase (CrNbN), a stable phase, and an M.sub.23C.sub.6 carbide. B
segregating in grain boundaries is replaced with C being part of
the M.sub.23C.sub.6 carbide, so as to be absorbed into the
M.sub.23C.sub.6 carbide. Therefore, an amount of B segregating in
the grain boundaries is reduced, resulting in a decrease in the
grain boundary strength. Consequently, obtaining a sufficient creep
ductility fails.
[0020] Thus, the present inventors conducted further studies on a
method for restricting the reduction in the amount of segregating B
in grain boundaries in use under the high-temperature corrosive
environment at 600 to 700.degree. C. As a result, it was found that
the following mechanism can be conceived.
[0021] As described above, Mo restricts the formation of the
M.sub.23C.sub.6 carbide itself. In addition, Mo may be replaced
with M being part of M.sub.23C.sub.6 carbide, being dissolved into
the M.sub.23C.sub.6 carbide. The M.sub.23C.sub.6 carbide with Mo
dissolved therein is defined herein as "Mo-dissolved
M.sub.23C.sub.6 carbide". The Mo-dissolved M.sub.23C.sub.6 carbide
resists allowing B to be dissolved therein. Therefore, even when
the MX carbo-nitride containing Nb transforms into the Z phase and
the M.sub.23C.sub.6 carbide in use under the high-temperature
corrosive environment, it is possible to restrict the dissolution
of B into the M.sub.23C.sub.6 carbide and restrict the reduction in
the amount of segregating B in grain boundaries, as long as the
M.sub.23C.sub.6 carbide is an Mo-dissolved M.sub.23C.sub.6 carbide.
It is considered that compatibility between an excellent
polythionic acid SCC resistance and an excellent creep ductility
can be consequently established.
[0022] Hence, for the austenitic stainless steel having the
chemical composition described above, the present inventors
conducted further studies on a chemical composition that can form
Mo-dissolved M.sub.23C.sub.6 carbide to restrict reduction in an
amount of segregating B in grain boundaries even when MX
carbo-nitride containing Nb transforms into a Z phase and an
M.sub.23C.sub.6 carbide in use under a high-temperature corrosive
environment at 600 to 700.degree. C. As a result, it was found that
restricting the reduction in the amount of segregating B by the
formation of the Mo-dissolved M.sub.23C.sub.6 carbide has a close
relation with B, C, and Mo in the chemical composition described
above. It was then found that compatibility between an excellent
polythionic acid SCC resistance and an excellent creep ductility
can be established when B, C, and Mo in the chemical composition
described above satisfy Formula (1) even in use under the
high-temperature corrosive environment at 600 to 700.degree.
C.:
B+0.004-0.9C+0.017Mo.sup.2.gtoreq.0 (1)
where symbols of elements in Formula (1) are to be substituted by
contents of corresponding elements (mass %).
[0023] The present inventors further conducted studies, and it was
found as a result that, in a case where the above austenitic
stainless steel contains Cu, an optional element, containing Cu at
5.0% or less makes it possible to obtain an excellent creep
strength as well as to keep a creep ductility, but setting an upper
limit of a content of Cu at 1.9% or less makes it possible to
further enhance the creep strength as well as to keep a higher
creep ductility. The reason is considered as follows. In use under
a high-temperature corrosive environment, Cu precipitates in
grains, forming Cu phases. The Cu phases enhance creep strength but
can degrade creep ductility. Accordingly, for an austenitic
stainless steel including the above chemical composition and
satisfying Formula (1), it is more preferable that the content of
Cu is 1.9% or less. When the content of Cu is 1.9% or less, it is
possible to keep an excellent creep ductility more effectively.
[0024] The present inventors further conducted studies, and it was
found as a result that the creep ductility is further enhanced when
a content of Mo is set at 0.5% or more. The reason for this is
unclear, but the following idea is conceivable. When the content of
Mo is additionally set at 0.5% or more in the above chemical
composition (satisfying Formula (1)), Mo further segregates in
grain boundaries and forms its intermetallic compounds in use under
a high-temperature corrosive environment at 600 to 700.degree. C.
This grain-boundary segregation and intermetallic compounds further
enhance the grain boundary strength. As a result, the creep
ductility is further enhanced. Accordingly, a lower limit of the
content of Mo is preferably 0.5%. In order to further enhance the
creep ductility, a lower limit of the content of Mo is preferably
0.8%, more preferably 1.0%, more preferably 2.0%.
[0025] An austenitic stainless steel according to the present
invention that is made based on the findings described above
includes a chemical composition consisting of, in mass %, C: 0.030%
or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040% or less,
S: 0.010% or less, Cr: 16.0 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.1 to
5.0%, Nb: 0.20 to 1.00%, N: 0.050 to 0.300%, sol.Al: 0.0005 to
0.1000%, B: 0.0010 to 0.0080%, Cu: 0 to 5.0%, W: 0 to 5.0%, Co: 0
to 1.0%, V: 0 to 1.00%, Ta: 0 to 0.2%, Hf: 0 to 0.20%, Ca: 0 to
0.010%, Mg: 0 to 0.010%, and rare earth metals: 0 to 0.10%, with
the balance being Fe and impurities, and satisfying Formula
(1):
B+0.004-0.9C+0.017Mo.sup.2.gtoreq.0 (1)
where symbols of elements in Formula (1) are to be substituted by
contents of corresponding elements (mass %).
[0026] The chemical composition may contain one or more elements
selected from the group consisting of, in mass %, Cu: 0.1 to 5.0%,
W: 0.1 to 5.0%, and Co: 0.1 to 1.0%.
[0027] The chemical composition may contain one or more elements
selected from the group consisting of, in mass %, V: 0.1 to 1.00%,
Ta: 0.01 to 0.2%, and Hf: 0.01 to 0.20%.
[0028] The chemical composition may contain one or more elements
selected from the group consisting of, in mass %, Ca: 0.0005 to
0.010%, Mg: 0.0005 to 0.010%, and rare earth metals: 0.001 to
0.10%.
[0029] The chemical composition may contain, in mass %, Cu: 0 to
1.9%.
[0030] The chemical composition may contain, in mass %, Mo: 0.5 to
5.0%.
[0031] Hereafter, the austenitic stainless steel according to the
present embodiment will be described in detail. The sign "%"
following each element means mass percent unless otherwise
noted.
[Chemical Composition]
[0032] The austenitic stainless steel according to the present
embodiment has a chemical composition containing the following
elements.
[0033] C: 0.030% or Less
[0034] Carbon (C) is contained unavoidably. When the austenitic
stainless steel according to the present embodiment is in use under
the high-temperature corrosive environment at 600 to 700.degree.
C., C produces M.sub.23C.sub.6 carbide in grain boundaries,
degrading polythionic acid SCC resistance. Accordingly, a content
of C is 0.030% or less. An upper limit of the content of C is
preferably 0.020%, more preferably 0.015%. The content of C is
preferably as low as possible. However, since C is contained
unavoidably as described above, at least 0.0001% of C can be
contained in industrial production. Accordingly, a lower limit
value of the content of C is preferably 0.0001%.
[0035] Si: 0.10 to 1.00%
[0036] Silicon (Si) deoxidizes steel. In addition, Si enhances
oxidation resistance and steam oxidation resistance of steel. An
excessively low content of Si fails to provide the effects
described above. Meanwhile, an excessively high content of Si
causes a sigma phase (a phase) to precipitate in steel, degrading
toughness of the steel. Accordingly, a content of Si is 0.10 to
1.00%. An upper limit of the content of Si is preferably 0.75%,
more preferably 0.50%.
[0037] Mn: 0.20 to 2.00%
[0038] Manganese (Mn) deoxidizes steel. In addition, Mn stabilizes
austenite, enhancing the creep strength. An excessively low content
of Mn fails to provide the effects described above. Meanwhile, an
excessively high content of Mn degrades creep strength of steel.
Accordingly, a content of Mn is 0.20 to 2.00%. A lower limit of the
content of Mn is preferably 0.40%, more preferably 0.50%. An upper
limit of the content of Mn is preferably 1.70%, more preferably
1.50%.
[0039] P: 0.040% or Less
[0040] Phosphorus (P) is an impurity. P decreases hot workability
and toughness of steel. Accordingly, a content of P is 0.040% or
less. An upper limit of the content of P is preferably 0.035%, more
preferably 0.032%. The content of P is preferably as low as
possible. However, since P is contained unavoidably, at least
0.0001% of P can be contained in industrial production.
Accordingly, a lower limit value of the content of P is preferably
0.0001%.
[0041] S: 0.010% or Less
[0042] Sulfur (S) is an impurity. S degrades hot workability and
creep ductility of steel. Accordingly, a content of S is 0.010% or
less. An upper limit of the content of S is preferably 0.005%. The
content of S is preferably as low as possible. However, since S is
contained unavoidably, at least 0.0001% of S can be contained in
industrial production. Accordingly, a lower limit value of the
content of S is preferably 0.0001%.
[0043] Cr: 16.0 to 25.0%
[0044] Chromium (Cr) enhances polythionic acid SCC resistance of
steel. In addition, Cr enhances oxidation resistance, steam
oxidation resistance, high-temperature corrosion resistance, and
the like of steel. An excessively low content of Cr fails to
provide the effects described above. In contrast, an excessively
high Cr content degrades creep strength and toughness of steel.
Accordingly, a content of Cr is 16.0 to 25.0%. A lower limit of the
content of Cr is preferably 16.5%, more preferably 17.0%. An upper
limit of the content of Cr is preferably 24.0%, more preferably
23.0%.
[0045] Ni: 10.0 to 30.0%
[0046] Nickel (Ni) stabilizes austenite, enhancing creep strength.
An excessively low content of Ni fails to provide the effect
described above. In contrast, an excessively high content of Ni
results in saturation of the effect described above and in
addition, increases production costs. Accordingly, a content of Ni
is 10.0 to 30.0%. A lower limit of the content of Ni is preferably
11.0%, more preferably 13.0%. An upper limit of the content of Ni
is preferably 25.0%, more preferably 22.0%0/.
[0047] Mo: 0.1 to 5.0%
[0048] Molybdenum (Mo) restricts formation of M.sub.23C.sub.6
carbide in grain boundaries in use under a high-temperature
corrosive environment at 600 to 700.degree. C. In addition, in use
under the high-temperature corrosive environment at 600 to
700.degree. C., Mo restricts dissolution of B into M.sub.23C.sub.6
carbide when MX carbo-nitride of Nb transforms into the
M.sub.23C.sub.6 carbide, restricting reduction of an amount of
segregating B in grain boundaries under the high-temperature
corrosive environment. This allows a sufficient creep ductility to
be obtained in the high-temperature corrosive environment. An
excessively low content of Mo fails to provide the effects
described above. In contrast, an excessively high content of Mo
degrades stability of austenite. Accordingly, a content of Mo is
0.1 to 5.0%. A lower limit of the content of Mo is preferably 0.2%,
more preferably 0.3%.
[0049] When the content of Mo is 0.5% or more, Mo segregates in
grain boundaries and forms intermetallic compounds, further
enhancing grain boundary strength. In this case, a further
excellent creep strength can be obtained under the high-temperature
corrosive environment. Accordingly, a lower limit of the content of
Mo is more preferably 0.5%, still more preferably 0.8%, still more
preferably 1.0%, still more preferably 1.5%, still more preferably
2.0%. A content of Mo of 1.5% or more also enhances creep strength.
An upper limit of the content of Mo is preferably 4.5%, more
preferably 4.0%. A content of Mo of 1.5% or more also enhances
creep strength.
[0050] Nb: 0.20 to 1.00%
[0051] Niobium (Nb) combines with C in use under a high-temperature
corrosive environment at 600 to 700.degree. C. to form MX
carbo-nitride, reducing an amount of dissolved C in steel. This
enhances polythionic acid SCC resistance of the steel. The formed
MX carbo-nitride of Nb also enhances creep strength. An excessively
low content of Nb fails to provide the effects described above. In
contrast, an excessively high content of Nb causes 8 ferrite to be
produced, degrading long-term creep strength, toughness, and
weldability of steel. Accordingly, a content of Nb is 0.20 to
1.00%. A lower limit of the content of Nb is preferably 0.25%. An
upper limit of the content of Nb is preferably 0.90%, more
preferably 0.80%.
[0052] N: 0.050 to 0.300%
[0053] Nitrogen (N) is dissolved in a matrix (parent phase) to
stabilize austenite, enhancing creep strength. In addition, N forms
its fine carbo-nitride in grains, enhancing creep strength of
steel. That is, N contributes to the creep strength through both
solid-solution strengthening and precipitation strengthening. An
excessively low content of N fails to provide the effects described
above. In contrast, an excessively high content of N causes Cr
nitride to be formed in grain boundaries, degrading polythionic
acid SCC resistance in a welding heat affected zone (HAZ). In
addition, an excessively high content of N also degrades
workability of steel. Accordingly, a content of N is 0.050 to
0.300%/a. A lower limit of the content of N is preferably 0.070%/a.
An upper limit of the content of N is preferably 0.250%/a, more
preferably 0.200%.
[0054] Sol.Al: 0.0005 to 0.100%
[0055] Aluminum (Al) deoxidizes steel. An excessively low content
of Al fails to provide the above effect. In contrast, an
excessively high content of Al degrades cleanliness of steel,
degrading workability and ductility of the steel. Accordingly, a
content of Al is 0.0005 to 0.100%. A lower limit of the content of
Al is preferably 0.001%, more preferably 0.002%. An upper limit of
the content of Al is preferably 0.050%/a, more preferably 0.030%.
In the present embodiment, the content of Al means a content of
acid-soluble Al (sol.Al).
[0056] B: 0.0010 to 0.0080%
[0057] Boron (B) segregates in grain boundaries in use under a
high-temperature corrosive environment at 600 to 700.degree. C.,
enhancing grain boundary strength. As a result, creep ductility can
be enhanced. An excessively low content of B fails to provide the
effects described above. In contrast, an excessively high content
of B degrades weldability and hot workability at high temperature.
Accordingly, a content of B is 0.0010 to 0.0080%/a. A lower limit
of the content of B is preferably 0.0015%, more preferably
0.0020%/a. An upper limit of the content of B is preferably less
than 0.0060%, more preferably 0.0050%.
[0058] The balance of the chemical composition of the austenitic
stainless steel according to the present embodiment is Fe and
impurities. Here, the impurities mean elements that are mixed from
ores and scraps used as raw material, a producing environment, or
the like when the austenitic stainless steel is produced in an
industrial manner, and are allowed to be mixed within ranges within
which the impurities have no adverse effect on the austenitic
stainless steel of the present embodiment.
[Optional Elements]
[0059] The austenitic stainless steel according to the present
embodiment may further contain, in lieu of a part of Fe, one or
more elements selected from the group consisting of Cu, W, and Co.
These elements all enhance creep strength of steel.
[0060] Cu: 0 to 5.0%
[0061] Copper (Cu) is an optional element and need not be
contained. When contained, Cu precipitates in use under a
high-temperature corrosive environment at 600 to 700.degree. C. in
a form of Cu phases in grains, exerting precipitation strengthening
to enhance creep strength of steel. However, an excessively high
content of Cu degrades hot workability and weldability of steel.
Accordingly, a content of Cu is 0 to 5.0%. In order to enhance the
creep strength more effectively, a lower limit of the content of Cu
is preferably 0.1%, more preferably 2.0%, more preferably 2.5%. An
upper limit of the content of Cu is preferably 4.5%, more
preferably 4.0%. Meanwhile, in order to keep a more excellent creep
ductility, the content of Cu is preferably 0 to 1.9%, and a more
preferable upper limit of the content of Cu is 1.8%.
[0062] W: 0 to 5.0%
[0063] Tungsten (W) is an optional element and may not be
contained. When contained, W is dissolved in a matrix (parent
phase), enhancing creep strength of steel. However, an excessively
high content of W degrades stability of austenite, degrading creep
strength and toughness of steel. Accordingly, a content of W is 0
to 5.0%. A lower limit of the content of W is preferably 0.1%, more
preferably 0.2%. An upper limit of the content of W is preferably
4.5%, more preferably 4.0%.
[0064] Co: 0 to 1.0%
[0065] Cobalt (Co) is an optional element and need not be
contained. When contained, Co stabilizes austenite, enhancing creep
strength. However, an excessively high content of Co increases a
raw-material cost. Accordingly, a content of Co is 0 to 1.0%. A
lower limit of the content of Co is preferably 0.1%, more
preferably 0.2%.
[0066] The austenitic stainless steel according to the present
embodiment may further contain, in lieu of a part of Fe, one or
more elements selected from the group consisting of V, Ta, and Hf.
These elements all enhance polythionic acid SCC resistance and
creep strength of steel.
[0067] V: 0 to 1.00%
[0068] Vanadium (V) is an optional element and need not be
contained. When contained, V combines with C to form its
carbo-nitride in use under a high-temperature corrosive environment
at 600 to 700.degree. C., so as to reduce dissolved C, enhancing
polythionic acid SCC resistance of steel. The formed V
carbo-nitride also enhances creep strength. However, an excessively
high content of V causes 8 ferrite to be produced, degrading creep
strength, toughness, and weldability of steel. Accordingly, a
content of V is 0 to 1.00%. In order to enhance the polythionic
acid SCC resistance and the creep strength more effectively, a
lower limit of the content of V is preferably 0.10%. An upper limit
of the content of V is preferably 0.90%, more preferably 0.80%.
[0069] Ta: 0 to 0.2%
[0070] Tantalum (Ta) is an optional element and need not be
contained. When contained, Ta combines with C to form its
carbo-nitride in use under a high-temperature corrosive environment
at 600 to 700.degree. C., so as to reduce dissolved C, enhancing
polythionic acid SCC resistance of steel. The formed Ta
carbo-nitride also enhances creep strength. However, an excessively
high content of Ta causes 5 ferrite to be produced, degrading creep
strength, toughness, and weldability of steel. Accordingly, a
content of Ta is 0 to 0.2%. In order to enhance the polythionic
acid SCC resistance and the creep strength more effectively, a
lower limit of the content of Ta is preferably 0.01%, more
preferably 0.02%.
[0071] Hf: 0 to 0.20%
[0072] Hafnium (Hf) is an optional element and need not be
contained. When contained, Hf combines with C to form its
carbo-nitride in use under a high-temperature corrosive environment
at 600 to 700.degree. C., so as to reduce dissolved C, enhancing
polythionic acid SCC resistance of steel. The formed Hf
carbo-nitride also enhances creep strength. However, an excessively
high content of Hf causes 6 ferrite to be produced, degrading creep
strength, toughness, and weldability of steel. Accordingly, a
content of Hf is 0 to 0.20%. A lower limit of the content of Hf is
preferably 0.01%, more preferably 0.02%.
[0073] The austenitic stainless steel according to the present
embodiment may further contain, in lieu of a part of Fe, one or
more elements selected from the group consisting of Ca, Mg, and
rare earth metals. These elements all enhance hot workability and
creep ductility of steel.
[0074] Ca: 0 to 0.010%
[0075] Calcium (Ca) is an optional element and need not be
contained. When contained, Ca immobilizes O (oxygen) and S (sulfur)
in forms of its inclusions, enhancing hot workability and creep
ductility of steel. However, an excessively high content of Ca
degrades hot workability and creep ductility of steel. Accordingly,
a content of Ca is 0 to 0.010%. A lower limit of the content of Ca
is preferably 0.0005%, more preferably 0.001%. An upper limit of
the content of Ca is preferably 0.008%, more preferably 0.006%.
[0076] Mg: 0 to 0.010%
[0077] Magnesium (Mg) is an optional element and need not be
contained. When contained, Mg immobilizes O (oxygen) and S (sulfur)
in forms of its inclusions, enhancing hot workability and creep
ductility of steel. However, an excessively high content of Mg
degrades hot workability and long-term creep ductility of steel.
Accordingly, a content of Mg is 0 to 0.010%. A lower limit of the
content of Mg is preferably 0.0005%, more preferably 0.001%. An
upper limit of the content of Mg is preferably 0.008%, more
preferably 0.006%.
[0078] Rare Earth Metals: 0 to 0.10%
[0079] Rare earth metals (REMs) are optional elements and need not
be contained. When contained, REMs immobilize O (oxygen) and S
(sulfur) in forms of its inclusions, enhancing hot workability and
creep ductility of steel. However, an excessively high content of
REMs degrades hot workability and long-term creep ductility of
steel. Accordingly, a content of REMs is 0 to 0.01%. A lower limit
of the content of REMs is preferably 0.001%, more preferably
0.002%. An upper limit of the content of REMs is preferably 0.08%,
more preferably 0.06%.
[0080] REMs herein contain at least one element of Sc, Y, and
lanthanoid (La, with atomic number 57, to Lu, with atomic number
71), and the content of REMs means a total content of these
elements.
[Formula (1)]
[0081] The above chemical composition further satisfies Formula
(1).
B+0.004-0.9C+0.017Mo.sup.2.gtoreq.0 (1)
Symbols of elements in Formula (1) are to be substituted by
contents of corresponding elements (in mass %).
[0082] As described above, the present embodiment involves both
setting the content of C at 0.030% or less to increase the
polythionic acid SCC resistance, and making 0.20 to 1.00% of Nb
contained to produce MX carbo-nitride of Nb in use under a
high-temperature corrosive environment at 600 to 700.degree. C.,
reducing an amount of dissolved C. However, the MX carbo-nitride of
Nb transforms into a Z phase and an M.sub.23C.sub.6 carbide in use
under the above high-temperature use environment because the MX
carbo-nitride of Nb is a metastable phase. B segregating in grain
boundaries is dissolved in the M.sub.23C.sub.6 carbide, and an
amount of segregating B in the grain boundaries is reduced. As a
result, the creep ductility deteriorates.
[0083] However, when Mo is dissolved in the M.sub.23C.sub.6 carbide
to form a "Mo-dissolved M.sub.23C.sub.6 carbide", B is hard to be
dissolved in the Mo-dissolved M.sub.23C.sub.6 carbide. Therefore,
the amount of segregating B in the grain boundaries is kept, which
enables obtaining an excellent polythionic acid SCC resistance as
well as an excellent creep ductility.
[0084] Let F1 be defined as F1=B+0.004-0.9C+0.017Mo.sup.2. F1 is an
index indicating a ratio of an Mo-dissolved M.sub.23C.sub.6 carbide
to a plurality of kinds of M.sub.23C.sub.6 carbides formed in steel
in use under a high-temperature corrosive environment. If F1 is
zero or more, the ratio of the Mo-dissolved M.sub.23C.sub.6 carbide
is high even when the plurality of kinds of M.sub.23C.sub.6
carbides are formed in the steel in use under the high-temperature
corrosive environment. Therefore, B segregating in grain boundaries
is hard to be dissolved in the M.sub.23C.sub.6 carbides, and
therefore an amount of B segregating in the grain boundaries is
kept. As a result, it is possible to establish compatibility
between an excellent polythionic acid SCC resistance and an
excellent creep ductility. Accordingly, F1 is zero (0.00000) or
more. F1 is preferably 0.00100 or more, more preferably 0.00200 or
more, more preferably 0.00400 or more, more preferably 0.00500 or
more, more preferably 0.00800 or more, most preferably 0.01000 or
more.
[0085] When the above chemical composition of the austenitic
stainless steel contains Cu, it is preferable that the upper limit
of the content of Cu is 1.9% or less as described above.
Considering enhancing a creep strength as well as obtaining an
excellent creep ductility, the content of Cu is preferably 0% to
1.9%. When the content of Cu is 1.9% or less, a Cu phase is
subjected to precipitation strengthening, which makes it possible
to keep the excellent creep ductility with the excellent creep
strength obtained.
[0086] In the above chemical composition of the austenitic
stainless steel, a lower limit of the content of Mo is preferably
0.5%. In the case, in use under a high-temperature corrosive
environment at 600 to 700.degree. C., Mo additionally segregates in
grain boundaries and forms intermetallic compounds. This
grain-boundary segregation and intermetallic compounds further
enhance the grain boundary strength. As a result, the creep
ductility is further enhanced. Accordingly, the lower limit of the
content of Mo is preferably 1.0%. Note that, when the lower limit
of the content of Mo is 1.0% or more, an F1 value is preferably
0.00500 or more, more preferably 0.00800 or more, more preferably
0.01000 or more.
[Producing Method]
[0087] An example of a producing method of the austenitic stainless
steel according to the present invention will be described. The
present producing method includes a preparation process of
preparing a starting material, a hot working process of performing
hot working on the starting material to produce a steel material, a
cold working process of, as necessary, performing cold working on
the steel material subjected to the hot working, and a solution
treatment process of, as necessary, performing solution treatment
on the steel material. The producing method will be described
below.
[Preparation Process]
[0088] A molten steel having the above chemical composition and
satisfying Formula (1) is produced. The molten steel is produced
using, for example, an electric furnace, an AOD (Argon Oxygen
Decarburization) furnace, or a VOD (Vacuum Oxygen Decarburization)
furnace. As necessary, the produced molten steel is subjected to a
well-known degassing treatment. From the molten steel subjected to
the degassing treatment, a starting material is produced. Examples
of the producing method for the starting material include a
continuous casting process. By the continuous casting process, a
continuous casting material (the starting material) is produced.
The continuous casting material is, for example, a slab, a bloom, a
billet, and the like. The molten steel may be subjected to an
ingot-making process into an ingot.
[Hot Working Process]
[0089] The prepared starting material (a continuous casting
material or an ingot) is subjected to hot working to be produced
into an austenitic stainless steel material. For example, the
starting material is subjected to the hot rolling to be produced
into a steel plate, a steel bar, or a wire rod. Alternatively, the
starting material is subjected to hot-extrusion process, hot
piercing-rolling, or the like to be produced into an austenitic
stainless steel pipe. A specific method of the hot working is not
specially limited, and performing hot working conforming to a shape
of a finished product will suffice. A finish working temperature of
the hot working is, for example, 1050.degree. C. or more. The
finish working temperature used herein means a temperature of the
steel material immediately after completion of final hot
working.
[Cold Working Process]
[0090] Cold working may be performed, as necessary, on the
austenitic stainless steel material subjected to the hot working.
When the austenitic stainless steel material is a steel bar, a wire
rod, or a steel pipe, the cold working is, for example, cold
drawing or cold rolling. When the austenitic stainless steel
material is a steel plate, the cold working is cold rolling or the
like.
[Solution Treatment Process]
[0091] After the hot working or the cold working, solution
treatment may be performed as necessary. A solution treatment step
involves uniformizing a structure and dissolving a carbo-nitride. A
preferable solution treatment temperature is as follows.
[0092] Preferable solution treatment temperature: 1000 to
1250.degree. C.
[0093] When the solution treatment temperature is 1000.degree. C.
or more, a carbo-nitride of Nb is dissolved sufficiently, further
increasing the creep strength. When the solution treatment
temperature is 1250.degree. C. or less, excessive dissolution of C
can be restricted, further increasing the polythionic acid SCC
resistance.
[0094] A retention duration in the solution treatment at the above
solution treatment temperature is, for example but not specially
limited to, 2 minutes to 60 minutes.
[0095] In place of the solution treatment, rapid cooling may be
performed immediately after the hot working on the steel material
produced through the hot working step. In this case, a finish
working temperature of the hot working is preferably set at
1000.degree. C. or more. When the finish hot working temperature is
1000.degree. C. or more, the carbo-nitride of Nb is dissolved
sufficiently, which makes it possible to establish compatibility
between an excellent polythionic acid SCC resistance and an
excellent creep ductility in use under a high-temperature corrosive
environment at 600 to 700.degree. C., and the carbo-nitride of Nb
is formed in use under a high temperature environment, which allows
a sufficient creep strength to be obtained.
[0096] A shape of the austenitic stainless steel of the present
embodiment is not specially limited. The austenitic stainless steel
of the present embodiment may be a steel plate, a steel pipe, a
steel bar or a wire rod, or a shape steel.
EXAMPLES
[0097] Molten steels having chemical compositions shown in Table 1
were produced.
TABLE-US-00001 TABLE 1 Test Chemical composition (in mass %,
balance being Fe and impurities) number C Si Mn P S Cr Ni Mo Nb N
sol. Al B Cu Other F1 1 0.010 0.25 0.68 0.019 0.001 17.1 14.2 0.8
0.43 0.099 0.005 0.0033 -- -- 0.00918 2 0.012 0.23 0.85 0.024 0.001
18.5 13.1 0.5 0.35 0.086 0.008 0.0045 3.2 -- 0.00195 3 0.010 0.25
0.98 0.008 0.001 17.8 14.5 0.6 0.34 0.091 0.004 0.0041 1.5 --
0.00522 4 0.010 0.33 0.56 0.018 0.001 18.1 14.2 0.5 0.38 0.100
0.009 0.0029 0.8 -- 0.00215 5 0.017 0.18 0.77 0.026 0.001 17.6 14.2
1.2 0.32 0.085 0.003 0.0023 -- -- 0.01548 6 0.015 0.23 0.65 0.029
0.001 17.4 15.2 1.6 0.31 0.098 0.008 0.0031 -- -- 0.03712 7 0.015
0.39 1.52 0.018 0.001 17.5 14.9 2.3 0.41 0.120 0.015 0.0035 --
0.15V, 0.002Ca 0.08393 8 0.008 0.19 1.32 0.031 0.002 17.6 13.2 0.4
0.25 0.110 0.004 0.0025 2.8 1.0W, 0.1Ta 0.00202 9 0.006 0.42 0.63
0.021 0.001 18.2 14.3 0.3 0.28 0.092 0.030 0.0020 3.4 0.5Co 0.00213
10 0.014 0.33 0.94 0.019 0.001 22.7 16.2 0.9 0.62 0.180 0.021
0.0031 2.5 0.002Mg 0.00827 11 0.020 0.23 1.23 0.013 0.001 20.3 21.3
3.8 0.33 0.110 0.007 0.0023 -- 0.08Hf, 0.03Nd 0.23378 12 0.012 0.43
0.51 0.031 0.002 16.8 13.4 3.1 0.27 0.110 0.026 0.0026 -- 0.5W
0.15917 13 0.008 0.26 0.85 0.025 0.001 18.2 15.3 0.7 0.43 0.120
0.010 0.0028 -- 0.1V 0.00793 14 0.007 0.25 1.05 0.028 0.001 17.9
14.8 0.8 0.35 0.092 0.009 0.0029 -- 0.004Ca 0.01148 15 0.009 0.18
0.65 0.015 0.001 17.5 11.2 0.4 0.29 0.078 0.008 0.0045 1.8 --
0.00312 16 0.008 0.25 0.69 0.012 0.001 17.6 11.3 0.3 0.35 0.082
0.005 0.0046 -- -- 0.00293 17 0.020 0.24 1.11 0.028 0.001 18.0 13.9
0.3 0.35 0.086 0.010 0.0025 -- -- -0.00997 18 0.016 0.23 0.62 0.003
0.001 17.8 14.2 0.4 0.32 0.086 0.012 0.0012 -- -- -0.00648 19 0.040
0.20 0.85 0.027 0.001 17.5 12.8 1.3 0.42 0.110 0.007 0.0055 -- --
0.00223 20 0.012 0.24 1.05 0.002 0.001 18.2 12.9 0.4 0.34 0.095
0.012 0.0025 2.5 -- -0.00158 21 0.012 0.26 1.03 0.022 0.002 17.2
13.2 -- 0.33 0.130 0.023 0.0019 -- 2.8W -0.00490 22 0.016 0.35 0.80
0.018 0.001 17.8 14.7 2.4 0.28 0.110 0.080 0.0002 -- -- 0.08772 23
0.013 0.23 0.78 0.023 0.001 18.6 13.8 0.5 -- 0.130 0.015 0.0041 --
0.4Co, 0.003Ca 0.00065
[0098] In a column "F1" of Table 1, a value of F1 of a steel of
each test number is written. A symbol of an element in a column
"OTHER" of a column "CHEMICAL COMPOSITION" and a numerical value
preceding the symbol of the element means an optional element and
its content (in mass %). Of the chemical composition of each test
number, the balance, all but elements shown in Table 1 was Fe and
impurities.
[0099] The molten steels were used to produce ingots each having an
outer diameter of 120 mm and weighing 30 kg. The ingots were
subjected to hot forging to be formed into steel plates each having
a thickness of 40 mm. The steel plates were further subjected to
the hot rolling into steel plates each having a thickness of 15 mm.
Final working temperatures of the hot rolling was 1050.degree. C.
or more for all test numbers. The steel plates subjected to the hot
rolling were further subjected to the cold rolling to be produced
into steel plates each having a thickness of 10.5 mm, a width of 50
mm, and a length of 100 mm. The steel plates subjected to the cold
rolling were each subjected to the solution treatment. For all of
the steel plates having the respective test numbers, the solution
treatment temperature was 1150.degree. C., and a solution treatment
duration was 10 minutes. The steel plates subjected to the solution
treatment were subjected to water cooling. Through the above steps,
austenitic stainless steel materials were produced.
[0100] With a thickness of a produced austenitic stainless steel
plate defined as t (mm), a sample taken from a position at t/4
depth from a surface of the steel plate was used to perform
well-known component analysis methods (the infrared absorptiometric
method after combustion for C and S, the thermal desorption
spectroscopy for N, and the ICP spectrometry for other alloying
elements). As a result, chemical compositions of the austenitic
stainless steel plates having the respective test numbers matched
those shown in Table 1.
[Evaluation Test for Polythionic Acid SCC Resistance]
[0101] The steel plates of the respective test numbers were
subjected to a 5000-hour aging treatment at 600.degree. C. on the
assumption that they are used under the high temperature
environment. From these aging-treated materials, plate-shaped test
specimens were taken, the test specimens each having a thickness of
2 mm, a width of 10 mm, and a length of 75 mm. An evaluation test
for polythionic acid SCC resistance was conducted conforming to
"Stress corrosion cracking test in chloride solution for stainless
steels" in JIS G 0576(2001). Specifically, each test specimen was
bended around a punch having an inside radius of 5 mm to have a
U-bend shape. The test specimen with the U-bend shape was immersed
in Wackenroder solution (solution made by blowing a large quantity
of H.sub.2S gas into H.sub.2SO.sub.3 saturated aqueous solution
that is made by blowing SO.sub.2 gas into distilled water) at
normal temperature for 100 hours. The immersed test specimen was
subjected to microscopic observation at 500.times. magnification to
check for a crack.
[0102] When no crack was found in a test specimen, the test
specimen was determined to be excellent in polythionic acid SCC
resistance (marked as "E" (Excellent) in a column "POLYTHIONIC ACID
SCC RESISTANCE" in Table 2). When any crack was found in a test
specimen, the test specimen was determined to be low in polythionic
acid SCC resistance (marked as "NA" (Not Accepted) in the column
"POLYTHIONIC ACID SCC RESISTANCE" in Table 2).
[Evaluation Test for Creep Ductility and Creep Strength]
[0103] For each test number, a creep rupture test specimen
conforming to JIS Z2271(2010) was fabricated from the steel plate.
A cross section of the creep rupture test specimen perpendicular to
its axial direction was in a round shape, and the creep rupture
test specimen had an outer diameter of 6 mm and a parallel portion
measuring 30 mm. The parallel portion was parallel to a rolling
direction of the steel plate. The fabricated creep rupture test
specimen was used to conduct a creep rupture test conforming to JIS
Z2271(2010). Specifically, the creep rupture test specimen was
heated at 750.degree. C. and then subjected to the creep rupture
test. A test stress was set at 45 MPa, and a creep rupture time
(hour) and a percentage reduction of area after creep rupture (%)
were determined.
[0104] As to the creep strength, when a creep rupture time of a
test specimen was 5000 to 10000 h or less, the test specimen was
determined to be excellent in creep strength (marked as "G" (Good)
in a column "CREEP STRENGTH" in Table 2). When a creep rupture time
of a test specimen was more than 10000 hours, the test specimen was
determined to be markedly excellent in creep strength (marked as
"E" (Excellent) in the column "CREEP STRENGTH" in Table 2). When a
creep rupture time of a test specimen was less than 5000 hours, the
test specimen was determined to be low in creep strength (marked as
"NA" (Not Accepted) in the column "CREEP STRENGTH" in Table 2).
When a test specimen was marked as "G" or "E" in creep rupture
time, it was determined that a sufficient creep strength was
obtained with the test specimen.
[0105] As to the creep ductility, when a percentage reduction of
area after creep rupture of a test specimen was 20.0% to 30.0% or
less, the test specimen was determined to be good in creep
ductility (marked as "P" (Passing) in a column "CREEP DUCTILITY" in
Table 2). When a percentage reduction of area after creep rupture
of a test specimen was more than 30.0% to 50.0% or less, the test
specimen was determined to be excellent in creep ductility (marked
as "G" (Good) in the column "CREEP DUCTILITY" in Table 2). When a
percentage reduction of area after creep rupture of a test specimen
was more than 50.0%, the test specimen was determined to be
markedly excellent in creep ductility (marked as "E" (Excellent) in
the column "CREEP DUCTILITY" in Table 2). When a percentage
reduction of area after creep rupture of a test specimen was less
than 20.0%, the test specimen was determined to be low in creep
ductility (marked as "NA" (Not Accepted) in the column "CREEP
DUCTILITY" in Table 2). When a test specimen was marked as "P",
"G", or "E" in percentage reduction of area after creep rupture, it
was determined that a sufficient creep ductility was obtained with
the test specimen.
[Test Results]
[0106] Table 2 shows test results.
TABLE-US-00002 TABLE 2 Test Polythionic acid Creep Creep number SCC
resistance ductility strength 1 E E G 2 E G E 3 E E E 4 E E E 5 E E
G 6 E E E 7 E E E 8 E P E 9 E P E 10 E G E 11 E E E 12 E E E 13 E G
G 14 E G G 15 E G E 16 E G G 17 E NA NA 18 E NA NA 19 NA E E 20 E
NA E 21 E NA NA 22 E NA NA 23 NA G NA
[0107] Referring to Table 1 and Table 2, the contents of elements
in the chemical compositions of the steels of the test numbers 1 to
16 were appropriate, and F1 of the steels satisfied Formula (1).
Therefore, the steel plates of these test numbers provided
excellent polythionic acid SCC resistances. In addition, the
rupture times of the steel plates were 5000 hours or more, and
excellent creep strengths were obtained. Furthermore, their
percentage reductions of area after creep rupture were 20.0% or
more, and excellent creep ductilities were obtained. Moreover, as
to test numbers 2 to 4, 6 to 12, and 15, since they contained Cu or
contained Mo in a large quantity, their rupture times in the creep
rupture test was longer than those of test numbers 1, 5, 13, 14,
and 16, 10000 hours or more, and superior creep strengths were
obtained.
[0108] In addition, as to test numbers 3 and 4, which contained Cu
at 1.9% or less and contained Mo at 0.5% or more, and as to test
numbers 5 to 7, 11, and 12, which did not contain Cu but contained
Mo at 1.0% or more, sufficient creep strengths were obtained, and
at the same time, superior creep ductilities were obtained.
[0109] In contrast, as to test numbers 17 and 18, their F1 failed
to satisfy Formula (1). As a result, their percentage reductions of
area after creep rupture were less than 20%, and creep ductilities
of their steels were low. This is considered to be due to a failure
to obtain a grain-boundary strengthening effect by grain-boundary
segregation of B. In addition, their creep strengths were also
low.
[0110] As to a test number 19, its content of C was excessively
high. As a result, its polythionic acid SCC resistance was low.
[0111] As to a test number 20, it contained Cu, and therefore its
creep strength was high whereas its F1 did not satisfy Formula (1).
As a result, its percentage reduction of area after creep rupture
was less than 20.0%, and a creep ductility of its steel was
low.
[0112] As to a test number 21, it contained no Mo. In addition, its
F1 was less than the lower limit of Formula (1). As a result, its
percentage reduction of area after rupture was less than 20.0%, and
a creep ductility of its steel was low. In addition, its creep
strength was also low.
[0113] As to the test number 22, its content of B was low. As a
result, its percentage reduction of area after creep rupture was
less than 20.0%, and a creep ductility of its steel was low. In
addition, its creep strength was also low.
[0114] As to a test number 23, it contained no Nb. As a result, its
polythionic acid SCC resistance was low. In addition, its rupture
time was less than 5000 hours, and a creep strength of its steel
was low.
[0115] The embodiment according to the present invention has been
described above. However, the aforementioned embodiment is merely
an example for practicing the present invention. Therefore, the
present invention is not limited to the aforementioned embodiment,
and the aforementioned embodiment can be modified and implemented
as appropriate without departing from the scope of the present
invention.
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