U.S. patent application number 12/292758 was filed with the patent office on 2009-03-26 for austenitic stainless steel.
Invention is credited to Masayuki Sagara, Kiyoko Takeda, Masaaki Terunuma.
Application Number | 20090081069 12/292758 |
Document ID | / |
Family ID | 38778336 |
Filed Date | 2009-03-26 |
United States Patent
Application |
20090081069 |
Kind Code |
A1 |
Takeda; Kiyoko ; et
al. |
March 26, 2009 |
Austenitic stainless steel
Abstract
An austenitic stainless steel, which comprises by mass percent,
C: not more than 0.10%, Si: 0.01 to 1.0%, Mn: 0.01 to 2%, Cr: 16 to
18%, Ni: more than 10% to less than 14%, Mo: more than 2.0% to not
more than 3.0%, N: 0.03 to 0.10%, one or more elements selected
from V, Nb and Ti satisfying the following formulas (1) and (2),
0.0013.ltoreq.(V/51)+(Nb/93)+(Ti/48).ltoreq.0.0025 (1),
{(C/12)+(N/14)}-{(V/51)+(Nb/93)+(Ti/48)}.ltoreq.0.0058 (2), wherein
each element symbol in the formulas (1) and (2) represents the
content (by mass %) of the element concerned, with the balance
being Fe and impurities, wherein the content of P is not more than
0.04% and the content of S is not more than 0.003% among the
impurities, has excellent corrosion resistance, in particular,
excellent intergranular corrosion resistance. The preferable
contents of Nb and Ti are not more than 0.030% and not more than
0.050%, respectively.
Inventors: |
Takeda; Kiyoko;
(Nisinomiya-shi, JP) ; Sagara; Masayuki;
(Nishinomiya-shi, JP) ; Terunuma; Masaaki; (Osaka,
JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW, SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
38778336 |
Appl. No.: |
12/292758 |
Filed: |
November 25, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/JP2007/059094 |
Apr 26, 2007 |
|
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12292758 |
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Current U.S.
Class: |
420/53 |
Current CPC
Class: |
C22C 38/48 20130101;
C22C 38/50 20130101; C22C 38/02 20130101; C22C 38/04 20130101; C22C
38/44 20130101; C22C 38/001 20130101; C22C 38/58 20130101; C22C
38/46 20130101 |
Class at
Publication: |
420/53 |
International
Class: |
C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2006 |
JP |
2006-150122 |
Claims
1. An austenitic stainless steel, which comprises by mass percent,
C: not more than 0.10%, Si: 0.01 to 1.0%, Mn: 0.01 to 2%, Cr: 16 to
18%, Ni: more than 10% to less than 14%, Mo: more than 2.0% to not
more than 3.0%, N: 0.03 to 0.10%, one or more elements selected
from V, Nb and Ti satisfying the following formulas (1) and (2),
with the balance being Fe and impurities, wherein the content of P
is not more than 0.04% and the content of S is not more than 0.003%
among the impurities:
0.0013.ltoreq.(V/51)+(Nb/93)+(Ti/48).ltoreq.0.0025 (1),
{(C/12)+(N/14)}-{(V/51)+(Nb/93)+(Ti/48)}.ltoreq.0.0058 (2), wherein
each element symbol in the formulas (1) and (2) represents the
content (by mass %) of the element concerned.
2. The austenitic stainless steel according to claim 1, wherein the
content of Nb is not more than 0.030% or the content of Ti is not
more than 0.050%, or, the content of Nb is not more than 0.030% and
the content of Ti is not more than 0.050%, by mass percent.
Description
[0001] This application is a continuation of the international
application PCT/JP2007/059094 field on Apr. 26, 2007, the entire
content of which is herein incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to an austenitic stainless
steel, having excellent corrosion resistance, in particular,
excellent intergranular corrosion resistance, which can be used as
structural members for a nuclear power plant, a chemical plant, or
the like.
BACKGROUND ART
[0003] A SUS 316 stainless steel, which contains Mo, has been used
as structural members for a nuclear power plant, a chemical plant
or the like, because of excellent mechanical properties with good
workability, in addition to excellent resistance to corrosion such
as pitting corrosion and general corrosion, compared to those of a
SUS 304 stainless steel. However, when the said SUS 316 stainless
steel is welded or heated at high temperatures, sometimes a marked
intergranular corrosion occurs in the heat affected zone which is
produced by welding or by high temperature heating. This
intergranular corrosion is called "sensitization", and is caused by
the formation of the Cr depleted zone which is poor in corrosion
resistance. The above-mentioned Cr depleted zone is formed, in the
welding or the heating process, by Cr carbide precipitation at the
grain boundary and a decrease of the Cr concentration there.
Furthermore, depending on the stress condition of the materials,
intergranular stress corrosion crackings may also occur.
[0004] In the conventional measure to suppress the sensitization, a
lowering of the C content and fixing of the C in the Ti and/or Nb
compounds within the grains have been employed in order to prevent
the formation of the Cr depleted zone through Cr carbide
precipitation. However, sufficient results have not been obtained
by the above-described methods.
[0005] Austenitic stainless steels with a lowered C content and/or
with a small additional amount of V, Ti, Nb, and so on are, for
example, disclosed in the following Patent Documents.
[0006] The Patent Document 1 (Japanese Laid-Open Patent Publication
No. 55-89458) discloses an austenitic stainless steel for use in an
environment of high temperature and low chlorine concentration,
containing one or more elements selected from Ti, Nb, Ta, Zr and V
in order to prevent a deterioration of resistance to stress
corrosion cracking due to N. In this Patent Document, stress
corrosion cracking due to N can be prevented by lowering the solute
N within the matrix of the stainless steel by forming nitrides with
Ti, Nb, Ta, Zr or V However, regarding the deterioration of
resistance to stress corrosion cracking caused by the sensitization
due to C, only the lowering the C content is considered.
[0007] The Patent Document 2 (Japanese Laid-Open Patent Publication
No. 2003-213379) discloses an austenitic stainless steel having
excellent corrosion resistance, containing Ti and/or Nb in order to
suppress the Cr nitride precipitation on the grain boundaries. In
this Patent Document, however, it is not considered that Ti and/or
Nb can fix not only N, but also fix C within the grains. Similar
effects of V on the fix of N and C in the grains are also not
considered. Moreover, proper amounts for adding of Ti and Nb, which
should depend on the contents of N and C, are not disclosed.
[0008] The Patent Document 3 (Japanese Laid-Open Patent Publication
No. 5-59494) discloses an austenitic stainless steel excellent in
suppressing irradiation assisted segregation, which contains one or
more elements selected from Ti, Zr, Hf, V, Nb, and Ta. According to
the said Patent Document, a point defect formation by neutron
irradiation is suppressed by Ti, Zr, Hf, V, Nb, or Ta, leading to
the suppression of element migration, that is, Cr from the grain
boundaries, and Ni, Si, P and S to the grain boundaries. However,
the role of the above-described elements on fixing C and/or N as
carbo-nitrides within the grains is not considered. Furthermore,
according to the said Patent Document, a large amount of Ti, Zr,
Hf, V, Nb, and Ta is necessary to achieve the above-described
effects.
[0009] The Patent Document 4 (Japanese Laid-Open Patent Publication
No. 2005-23343) discloses an austenitic stainless steel with a fine
grain structure on the surface layer, containing one or more
elements selected from V, Nb, Ti and Zr. In the said steel, V, Nb,
Ti and Zr are added for grain refinement. However, interaction
between other elements such as C or N and above-described alloying
elements is not considered.
[0010] The Patent Document 5 (Japanese Laid-Open Patent Publication
No. 57-158359) discloses a corrosion resistant austenitic stainless
steel, containing Ti+Nb of 0.05 to 0.10%. The said Patent Document
mentions that grain boundary precipitation of carbides and/or
nitrides is suppressed by the composite addition of Nb and Ta.
However, when the content of Nb is more than 0.5%, pitting
corrosions or macro-streak-flaws may occur.
[0011] Patent Document 1: Japanese Laid-Open Patent Publication No.
55-89458,
[0012] Patent Document 2: Japanese Laid-Open Patent Publication No.
2003-213379,
[0013] Patent Document 3: Japanese Laid-Open Patent Publication No.
5-59494,
[0014] Patent Document 4: Japanese Laid-Open Patent Publication No.
2005-23343,
[0015] Patent Document 5: Japanese Laid-Open Patent Publication No.
57-158359.
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0016] The objective of the present invention is to provide an
austenitic stainless steel having excellent corrosion resistance,
in particular, excellent intergranular corrosion resistance.
Means for Solving the Problems
[0017] The basic technical idea of the present invention is to
prevent intergranular corrosion due to sensitization in austenitic
stainless steels, by suppressing the Cr carbo-nitrides
precipitation on the grain boundaries by means of fixing C as Cr
carbo-nitrides within the grains.
[0018] In order to suppress the precipitation of the Cr base
carbo-nitrides on the grain boundaries in the SUS 316 type
stainless steels, it is desirable that the carbo-nitride formation
elements of C and N should be fixed within the grains as
carbo-nitrides with V, Nb and Ti, whose affinities with C and N are
much greater than with Cr. It can be seen from the aspect of the
standard formation free energy that it is easier for V, Nb and Ti
to form a nitride than to form a carbide.
[0019] SUS 316 type stainless steels used for structural members,
usually, contain 0.03 to 0.10% N, in order to ensure the strength.
However, in the case when V, Nb or Ti is added to the SUS 316 type
stainless steels with high N content, nitrides of these elements
are preferentially formed. Therefore, according to the conventional
idea, in order to fix C as carbides within the grains for
suppressing the intergranular corrosion, a large amount of V, Nb or
Ti which can fix C as carbides should be added in addition to the
amount to fix N as nitrides.
[0020] Or in another way, the increase of intragranular
precipitates (carbo-nitrides) owing to the excessive contents of V,
Nb and/or Ti may cause a deterioration of pitting corrosion
resistance or may encourage production defects such as
macro-streak-flaws. From these points of view, the excessive
addition of V, Nb or Ti is not desirable.
[0021] Thus, the present inventors have studied the proper amount
of V, Nb or Ti addition to achieve enough suppression effect on the
sensitization without the above-described negative effects, and
following new findings were obtained.
[0022] (a) The present inventors investigated the precipitation
behaviors of the carbo-nitrides for practical steel products, and
found that, even in the case when just enough of V, Nb and/or Ti
for forming nitrides with N were added, the said elements also
combined with C, and therefore, carbo-nitrides were also formed.
This can be ascribed to the kinetic behavior in addition to the
above-described equilibrium thermodynamics. That is to say, V, Nb
and Ti combine not only with N by equilibrium thermodynamics but
also with C.
[0023] (b) Next the present inventors investigated the resistance
to the intergranular corrosion of the SUS 316 type stainless steels
with various amounts of V, Nb and Ti, and found that stainless
steels having excellent intergranular corrosion resistance were
obtained by containing the proper amounts of V, Nb and Ti so that
the following formula (1) was satisfied, and moreover, by
controlling the relationship between the C and N contents and the
contents of V, Nb and Ti so that the following formula (2) was
satisfied.
0.0013.ltoreq.(V/51)+(Nb/93)+(Ti/48).ltoreq.0.0025 (1),
{(C/12)+(N/14)}-{(V/51)+(Nb/93)+(Ti/48)}.ltoreq.0.0058 (2),
wherein each element symbol in the formulas (1) and (2) represents
the content (by mass %) of the element concerned.
[0024] When the value of (V/51)+(Nb/93)+(Ti/48) in the above
formula (1) is less than 0.0013, the excellent intergranular
corrosion resistance is not obtained. However, the increase of
intergranular precipitates owing to the excessive contents of V, Nb
and/or Ti is not desirable, since a large amount of intragranular
precipitates may cause a deterioration of pitting corrosion or may
encourage production defects such as macro-streak-flaws. Therefore,
the upper limit of the value of (V/51)+(Nb/93)+(Ti/48) is set to
0.0025.
[0025] In the relationship between the C and N contents and the
contents of V, Nb and Ti, when the value of the left hand member in
the formula (2) exceeds 0.0058, the intergranular corrosion
resistance deteriorates due to the increase of intergranular
carbo-nitride precipitates.
[0026] Nb and Ti easily form carbides, because of the strong
affinity with C, compared to V, and grow into intragranular
precipitates, leading to a deterioration in pitting corrosion
resistance. Therefore, the excessive addition of Nb and Ti should
be avoided and preferable contents are less than 0.030% for Nb and
0.050% for Ti.
[0027] The present invention has been accomplished on the basis of
the above-described findings. The gists of the present invention
are the following austenitic stainless steels.
[0028] (1) An austenitic stainless steel, which comprises by mass
percent, C: not more than 0.10%, Si: 0.01 to 1.0%, Mn: 0.01 to 2%,
Cr: 16 to 18%, Ni: more than 10% to less than 14%, Mo: more than
2.0% to not more than 3.0%, N: 0.03 to 0.10%, one or more elements
selected from V, Nb and Ti satisfying the following formulas (1)
and (2), with the balance being Fe and impurities, wherein the
content of P is not more than 0.04% and the content of S is not
more than 0.003% among the impurities.
0.0013.ltoreq.(V/51)+(Nb/93)+(Ti/48).ltoreq.0.0025 (1),
{(C/12)+(N/14)}-{(V/51)+(Nb/93)+(Ti/48)}.ltoreq.0.0058 (2),
wherein each element symbol in the formulas (1) and (2) represents
the content (by mass %) of the element concerned.
[0029] (2) The austenitic stainless steel according to above (1),
wherein the content of Nb is not more than 0.030% or the content of
Ti is not more than 0.050%, or, the content of Nb is not more than
0.030% and the content of Ti is not more than 0.050%, by mass
percent.
EFFECT OF THE INVENTION
[0030] An austenitic stainless steel in the present invention has
excellent corrosion resistance, in particular, excellent
intergranular corrosion resistance. Therefore, it is very suitable
to be used as a structural member where an intergranular corrosion
may occur.
BEST MODE FOR CARRYING OUT THE INVENTION
[0031] The reasons for restricting the chemical compositions of the
austenitic stainless steel in the present invention will next be
explained. In the following description, the symbol "%" for the
content of each component represents "% by mass".
[0032] C: not more than 0.10%
[0033] C is used for deoxidation and ensuring the strength of
steels. However, in order to suppress the carbide precipitation,
from the view point of ensuring the corrosion resistance, it is
preferable to make the C content as low as possible. Therefore, the
upper limit of the C content is set to 0.10%. A more preferable
content of C is not more than 0.05%. On the other hand, in order to
ensure the strength as structural members, the C content is
preferably not less than 0.01%. More preferably, the content of C
is not less than 0.015%.
[0034] Si: 0.01 to 1.0%
[0035] Si is used for deoxidation of steels. In the steel of the
present invention, the content of Si is set to not less than 0.01%.
Since excessive Si content encourages forming inclusions, it is
preferable to make the Si content as low as possible. Therefore,
the content of Si is set to 0.01 to 1.0%.
[0036] Mn: 0.01 to 2%
[0037] Mn is effective in deoxidizing the steel and stabilizing the
austenitic phase. The said effects are obtained if the content of
Mn is not less than 0.01%. On the other hand, Mn forms sulfides
with S, and the said sulfides exist as non-metallic inclusions in
the steel. Moreover, when the steel products are welded, Mn
preferentially condenses the surface of the welds and therefore
brings out a deterioration of corrosion resistance in the steel
products. Therefore, the proper Mn content is set to 0.01 to
2%.
[0038] Cr: 16 to 18%
[0039] Cr is an indispensable element in order to ensure the
corrosion resistance of steels. A sufficient corrosion resistance
is not obtained, when the content of Cr is less than 16%. In the
present invention, a Cr content of not more than 18% is sufficient.
A Cr content exceeding 18%, leads to the deterioration of
workability and also increases the cost of steels for practical
use. Moreover, it makes difficult to keep the austenitic phase
stable. Therefore, the upper limit of the Cr content is set to 18%.
The content of Cr is more preferably not more than 17.5%.
[0040] Ni: more than 10% to less than 14%
[0041] Ni is an important element for the stabilization of the
austenitic phase and maintains the corrosion resistance. From the
view point of corrosion resistance, the Ni content of more than 10%
is necessary. From a view point of weldability, the upper limit of
the Ni content has a relationship with the content of Cr, and is
set to less than 14%. The lower limit of the Ni content is more
preferably 10.5%, and the upper limit thereof is more preferably
13%.
[0042] Mo: more than 2.0% to less than 3.0%
[0043] Mo has an effect for stabilizing the passive film, and is an
indispensable element to ensure the pitting corrosion resistance
and/or general corrosion resistance. However, when Mo precipitates
as intermetallic compounds with Fe, Ni, Cr and the like on the
grain boundaries, the intergranular corrosion resistance is
deteriorated. Therefore, the content of Mo, which ensures the
general corrosion resistance without deteriorating the
intergranular corrosion resistance, is set to more than 2.0% to
less than 3.0%. The more preferable upper limit content of Mo is
2.5%.
[0044] N: 0.03 to 0.10%
[0045] The content of N is set to not less than 0.03%, in order to
ensure the strength of steels. However, N forms nitrides with Cr in
the steel, and the said nitrides cause a deterioration of
intergranular corrosion resistance. Thus, the content of N is set
to not more than 0.10%. The lower limit of the N content is more
preferably 0.04%, and the upper limit thereof is more preferably
0.08%.
[0046] V, Ti and Nb: one or more elements selected from these,
within a range satisfying the above-mentioned formulas (1) and
(2)
[0047] By the reasons described in paragraphs [0021] and [0022],
the contents of V, Ti and Nb are set within the range satisfying
the formulas (1) and (2). Also, by the reason described in
paragraph [0023], the preferable contents of Nb and Ti are set to
not more than 0.030% and not more than 0.050%, respectively.
[0048] The stainless steel according to the present invention
comprises the components described above with the balance being Fe
and impurities. It is necessary, however, to suppress the contents
of P and S among the impurities in the following manner.
[0049] P: not more than 0.04%
[0050] Since the increase of P content deteriorates the corrosion
resistance, the content of P is preferable as low as possible.
Therefore, the upper limit of the P content is set to 0.04%.
[0051] S: not more than 0.003%
[0052] forms sulfides, which are non-metallic inclusions. Also, S
is an element which deteriorates the hot workability. Therefore,
the content of S is preferable as low as possible. Consequently,
the upper limit of the S content is set to 0.003%.
EXAMPLE
[0053] Stainless steels having chemical compositions shown in Table
1 were melted and cast to form ingots. The ingots were hot-forged
and hot-rolled into 6 mm thick plates, and then the said hot rolled
plates with 6 mm thickness were cold rolled into 4 mm thick plates.
The cold rolled plates were subjected to solution treatment, namely
maintained at 1060.degree. C. for 15 minutes and water cooled. Then
they were subjected to a sensitizing treatment, that is to say,
they were heated at 650.degree. C. for 2 hours and cooled in air.
After the said sensitizing treatment, the corrosion rate was
measured according to the Method of Ferric Sulfate-Sulfuric Acid
Test (JIS G 0572), which is a typical test method for evaluating
the intergranular corrosion resistance. The value of
"(V/51)+(Nb/93)+(Ti/48)" in the formula (1) and the value of left
side member of the formula (2) are also shown in Table 1.
[0054] [Table 1]
TABLE-US-00001 TABLE 1 Test Chemical Composition (mass %, Balance:
Fe) Value of Value of Division No. C Si Mn P S Cr Ni Mo V Ti Nb N
formula (1) formula (2) Examples of the 1 0.037 0.35 1.39 0.025
0.0003 16.21 11.22 2.19 0.091 0.002 0.021 0.0555 0.0021 0.0050
present 2 0.042 0.36 1.46 0.028 0.0007 16.92 11.20 2.09 0.082 0.004
0.012 0.0516 0.0018 0.0054 invention 3 0.036 0.34 1.59 0.027 0.0005
16.07 11.33 2.03 0.065 0.003 0.011 0.0509 0.0015 0.0052 4 0.045
0.41 1.51 0.026 0.0006 16.73 11.16 2.07 0.077 0.038 0.006 0.0452
0.0024 0.0046 5 0.046 0.47 1.47 0.025 0.0004 17.16 11.89 2.21 0.063
0.012 0.007 0.0428 0.0016 0.0053 Comparative 6 0.040 0.36 1.37
0.021 0.0005 16.36 11.23 2.05 0.060 0.003 0.002 0.0542 0.0013
0.0059 examples 7 0.039 0.39 1.39 0.025 0.0008 16.42 11.16 2.11
0.050 0.002 0.003 0.0489 0.0011 0.0057 Note 1: Value of formula
(1): (V/51) + (Nb/93) + (Ti/48) Note 2: Value of left hand member
of formula (2): {(C/12) + (N/14)} - {(V/51) + (Nb/93) + (Ti/48)}
Note 3: Underlined numbers mean out of the range regulated by the
present invention.
[0055] The test and evaluation results of intergranular corrosion
resistance are shown in Table 2. In the tests of intergranular
corrosion resistance, the scatter of the two specimens was
negligibly small in the examples of the present invention. On the
other hand, in the comparative examples, since the scatter of the
two specimens was very large, an additional 4 specimens were tested
and so a total of 6 specimens were evaluated. The large scatter of
the data in the comparative examples originated from falling-off of
surface grains during testing due to the poor intergranular
corrosion resistance. The intergranular corrosion resistance was
evaluated by ".largecircle.", when corrosion rates of all the
specimens were less than 3 g/m.sup.2h, and was evaluated by "x",
when at least one result of the plural specimens exceeded 3
g/m.sup.2h.
[0056] [Table 2]
TABLE-US-00002 TABLE 2 Test Division No. Corrosion rate
(g/m.sup.2h) Evaluation Examples of the 1 0.81, 0.93 .smallcircle.
present 2 0.93, 0.98 .smallcircle. invention 3 0.79, 0.80
.smallcircle. 4 0.78, 0.77 .smallcircle. 5 1.05, 1.20 .smallcircle.
Comparative 6 1.42, 1.65, 2.32, 2.87, 3.72, 5.14 x examples 7 3.05,
3.64, 5.75, 6.35, 6.84, 8.00 x
[0057] As is apparent from Table 2, the corrosion rates of Nos. 1
to 5 in the examples of the present invention were low, that is,
they had excellent intergranular corrosion resistance. On the other
hand, in the comparative examples No. 6 and No. 7, the steel had a
chemical composition outside the value of the formula (1) or (2)
specified in accordance with the present invention. Consequently,
their intergranular corrosion resistance was poor.
[0058] Although only some exemplary embodiments of the present
invention have been described in detail above, those skilled in the
art will readily appreciated that many modifications are possible
in the exemplary embodiments without materially departing from the
novel teachings and advantages of the present invention.
Accordingly, all such modifications are intended to be included
within the scope of the present invention.
INDUSTRIAL APPLICABILITY
[0059] According to the present invention, an austenitic stainless
steel having excellent intergranular corrosion resistance, and so
having both excellent pitting corrosion and general corrosion
resistance can be provided. This stainless steel can show excellent
effects, when they are used as structural members for a nuclear
power plant, a chemical plant, or the like.
* * * * *