U.S. patent application number 14/490762 was filed with the patent office on 2015-01-08 for austenitic stainless steel.
The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Hiroyuki HIRARA, Yoshitaka NISHIYAMA, Kazuhiro OGAWA, Takahiro OSUKI.
Application Number | 20150010425 14/490762 |
Document ID | / |
Family ID | 52132935 |
Filed Date | 2015-01-08 |
United States Patent
Application |
20150010425 |
Kind Code |
A1 |
OSUKI; Takahiro ; et
al. |
January 8, 2015 |
AUSTENITIC STAINLESS STEEL
Abstract
An austenitic stainless steel, which consists of by mass
percent, C: not more than 0.02%, Si: not more than 1.5%, Mn: not
more than 2%, Cr: 17 to 25%, Ni: 9 to 13%, Cu: more than 0.26% not
more than 4%, N: 0.06 to 0.35%, sol. Al: 0.008 to 0.03%. One or
more elements selected from Nb, Ti, V, TA, Hf, and Zr in controlled
amounts can be included with the balance being Fe and impurities.
P, S, Sn, As, Zn, Pb and Sb among the impurities are controlled as
P: 0.006 to 0.04%, S: 0.0004 to 0.03%, Sn: 0.001 to 0.1%, As: not
more than 0.01%, Zn: not more than 0.01%, Pb: not more than 0.01%
and Sb: not more than 0.01%. The amounts of S, P, Sn, As, Zn, Pb
and Sb and the amounts of Nb, Ta, Zr, Hf, and Ti are further
controlled using formulas.
Inventors: |
OSUKI; Takahiro;
(Nishinomiya-shi, JP) ; OGAWA; Kazuhiro;
(Nishinomiya-shi, JP) ; HIRARA; Hiroyuki; (Osaka,
JP) ; NISHIYAMA; Yoshitaka; (Nishinomiya-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
52132935 |
Appl. No.: |
14/490762 |
Filed: |
September 19, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13368665 |
Feb 8, 2012 |
8865060 |
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14490762 |
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12549639 |
Aug 28, 2009 |
8133431 |
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13368665 |
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PCT/JP2008/067922 |
Oct 2, 2008 |
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12549639 |
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Current U.S.
Class: |
420/54 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/02 20130101; C22C 38/48 20130101; C22C 38/50 20130101; C22C
38/42 20130101; C22C 38/58 20130101; C22C 38/46 20130101; F16L 9/02
20130101; C22C 38/002 20130101; C22C 38/008 20130101; C22C 38/001
20130101; C22C 38/40 20130101; C22C 38/04 20130101 |
Class at
Publication: |
420/54 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/48 20060101 C22C038/48; C22C 38/06 20060101
C22C038/06; C22C 38/00 20060101 C22C038/00; C22C 38/02 20060101
C22C038/02; C22C 38/04 20060101 C22C038/04; C22C 38/46 20060101
C22C038/46; C22C 38/50 20060101 C22C038/50 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 4, 2007 |
JP |
2007-260477 |
Claims
1. An austenitic stainless steel, which consists of by mass
percent, C: not more than 0.02%, Si: not more than 1.5%, Mn: not
more than 2%, Cr: 17 to 25%, Ni: 9 to 13%, Cu: more than 0.26% less
than 2.95%, N: 0.06 to 0.35%, sol. Al: 0.008 to 0.03%, an amount of
Co, the Co amount not more than 1.0%, an amount of Ta, the Ta
amount not more than 0.2%, and further contains one or more
elements selected from Nb: not more than 0.5%, Ti: not more than
0.4%, V: not more than 0.4%, Hf: not more than 0.2% and Zr: not
more than 0.2%, with the balance being Fe and impurities, in which
the contents of P, S, Sn, As, Zn, Pb and Sb among the impurities
are P: 0.006 to 0.04%, S: 0.0004 to 0.03%, Sn: 0.001 to 0.1%, As:
not more than 0.01%, Zn: not more than 0.01%, Pb: not more than
0.01% and Sb: not more than 0.01%, and the values of F1 and F2
defined respectively by the following formula (1) and formula (2)
satisfy the conditions F1.ltoreq.0.075 and
0.05.ltoreq.F2.ltoreq.1.7-9.times.F1;
F1=S+{(P+Sn)/2}+{(As+Zn+Pb+Sb)/5} (1), F2=Nb+Ta+Zr+Hf+2Ti+(V/10)
(2); wherein each element symbol in the formulas (1) and (2)
represents the content by mass percent of the element
concerned.
2. An austenitic stainless steel, which consists of by mass
percent, C: not more than 0.02%, Si: not more than 1.5%, Mn: not
more than 2%, Cr: 17 to 25%, Ni: 9 to 13%, Cu: more than 0.26% less
than 2.95%, N: 0.06 to 0.1%, sol. Al: 0.008 to 0.03%, an amount of
Co, the Co amount not more than 1.0%, an amount of Ta, the Ta
amount not more than 0.2%, and further contains one or more
elements selected from Nb: not more than 0.5%, Ti: not more than
0.4%, V: not more than 0.4%, Hf: not more than 0.2% and Zr: not
more than 0.2%, with the balance being Fe and impurities, in which
the contents of P, S, Sn, As, Zn, Pb and Sb among the impurities
are P: 0.006 to 0.04%, S: 0.0004 to 0.03%, Sn: 0.001 to 0.1%, As:
not more than 0.01%, Zn: not more than 0.01%, Pb: not more than
0.01% and Sb: not more than 0.01%, and the values of F1 and F2
defined respectively by the following formula (1) and formula (2)
satisfy the conditions F1.ltoreq.0.075 and
0.05.ltoreq.F2.ltoreq.1.7-9.times.F1;
F1=S+{(P+Sn)/2}+{(As+Zn+Pb+Sb)/5} (1), F2=Nb+Ta+Zr+Hf+2Ti+(V/10)
(2); wherein each element symbol in the formulas (1) and (2)
represents the content by mass percent of the element
concerned.
3. An austenitic stainless steel, which consists of by mass
percent, C: not more than 0.02%, Si: not more than 1.5%, Mn: not
more than 2%, Cr: 17 to 25%, Ni: 9 to 13%, Cu: more than 0.26% less
than 2.95%, N: 0.06 to 0.35%, sol. Al: 0.008 to 0.03%, an amount of
Co, the Co amount not more than 1.0%, an amount of Ta, the Ta
amount not more than 0.2%, and further contains one or more
elements selected from Nb: not more than 0.5%, Ti: not more than
0.4%, V: not more than 0.4%, Hf: not more than 0.2% and Zr: not
more than 0.2%, with the balance being Fe and impurities, in which
the contents of P, S, Sn, As, Zn, Pb and Sb among the impurities
are P: 0.006 to 0.04%, S: 0.0004 to 0.03%, Sn: 0.001 to 0.1%, As:
not more than 0.01%, Zn: not more than 0.01%, Pb: not more than
0.01% and Sb: not more than 0.01%, and the values of F1 and F2
defined respectively by the following formula (1) and formula (2)
satisfy the conditions F1.ltoreq.0.075 and
0.05.ltoreq.F2.ltoreq.1.7-9.times.F1;
F1=S+{(P+Sn)/2}+{(As+Zn+Pb+Sb)/5} (1), F2=Nb+Ta+Zr+Hf+2Ti+(V/10)
(2); wherein each element symbol in the formulas (1) and (2)
represents the content by mass percent of the element concerned,
wherein the austenitic stainless steel further consists of, by mass
percent, one or more elements of one or more groups selected from
the first to third groups listed below in lieu of a part of Fe:
First group: Mo: not more than 5%; Second group: B: not more than
0.012%; and Third group: Ca: not more than 0.02%, Mg: not more than
0.02% and rare earth element: not more than 0.1%.
4. An austenitic stainless steel, which consists of by mass
percent, C: not more than 0.02%, Si: not more than 1.5%, Mn: not
more than 2%, Cr: 17 to 25%, Ni: 9 to 13%, Cu: more than 0.26% less
than 2.95%, N: 0.06 to 0.1%, sol. Al: 0.008 to 0.03%, an amount of
Co, the Co amount not more than 1.0%, an amount of Ta, the Ta
amount not more than 0.2%, and further contains one or more
elements selected from Nb: not more than 0.5%, Ti: not more than
0.4%, V: not more than 0.4%, Hf: not more than 0.2% and Zr: not
more than 0.2%, with the balance being Fe and impurities, in which
the contents of P, S, Sn, As, Zn, Pb and Sb among the impurities
are P: 0.006 to 0.04%, S: 0.0004 to 0.03%, Sn: 0.001 to 0.1%, As:
not more than 0.01%, Zn: not more than 0.01%, Pb: not more than
0.01% and Sb: not more than 0.01%, and the values of F1 and F2
defined respectively by the following formula (1) and formula (2)
satisfy the conditions F1.ltoreq.0.075 and
0.05.ltoreq.F2.ltoreq.1.7-9.times.F1;
F1=S+{(P+Sn)/2}+{(As+Zn+Pb+Sb)/5} (1), F2=Nb+Ta+Zr+Hf+2Ti+(V/10)
(2); wherein each element symbol in the formulas (1) and (2)
represents the content by mass percent of the element concerned,
wherein the austenitic stainless steel further consists of, by mass
percent, one or more elements of one or more groups selected from
the first to third groups listed below in lieu of a part of Fe:
First group: Mo: not more than 5%; Second group: B: not more than
0.012%; and Third group: Ca: not more than 0.02%, Mg: not more than
0.02% and rare earth element: not more than 0.1%.
Description
[0001] This is a continuation in part of application Ser. No.
13/368,665, filed on Feb. 8, 2012, which is a continuation in part
of application Ser. No. 12/549,639, filed on Aug. 28, 2009, which
is a continuation of PCT/jp2008/067922, filed on Oct. 2, 2008,
which is incorporated in its entirety herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to an austenitic stainless
steel, particularly to an austenitic stainless steel which contains
C-fixing elements. More particularly, the present invention relates
to an austenitic stainless steel, which contains C-fixing elements
and can be applied in manufacturing heating furnace pipes and the
like which are used in power plant boilers, petroleum refining and
petrochemical plants. Still more particularly, the present
invention relates to an austenitic stainless steel, which contains
C-fixing elements and shows excellent liquation cracking resistance
and embrittling cracking resistance in a weld zone and also has
high corrosion resistance, in particular high polythionic acid
stress corrosion cracking resistance.
BACKGROUND ART
[0003] Due to the recent growing demand for energy, new power plant
boilers, petroleum refining and petrochemical plants have been
built. An austenitic stainless steel to be used in these
manufacturing heating furnace pipes and the like, for use in those
facilities is required to have not only excellent corrosion
resistance but also excellent high temperature strength.
[0004] In such a technological background, for example, the
Non-Patent Document 1 proposes a highly corrosion resistant
austenitic stainless steel, having a reduced content of C together
with N which is set at a level within a specified range, and
containing Nb as a C-fixing element at a level within a specified
range, thereby having excellent stress corrosion cracking
resistance and high temperature strength, and showing no
sensitizing even after a long period of aging without post heat
treatment after welding.
[0005] Concerning the cracking in the Heat Affected Zone
(hereinafter referred to as "HAZ") of the austenitic stainless
steel which contains C-fixing elements after welding, the
Non-Patent Document 2 declares that the carbide dissolution in
welding thermal cycles and reheating to the M.sub.23C.sub.6
precipitation temperature in the subsequent cycles lead to the
formation of a sensitizing region, resulting in an intergranular
corrosion cracking called "knife line attack".
[0006] Further, as a result of detailed examinations using
austenitic stainless steels containing Nb and C at high
concentrations, the Non-Patent Document 3 and the Non-Patent
Document 4 declare that the fusion of low melting point compounds,
such as NbC and/or the Laves phase that has precipitated on the
grain boundaries, causes liquation cracking in the HAZ. Therefore,
they recommend that the precipitation of such low melting point
compounds on the grain boundaries should be suppressed in order to
prevent liquation cracking in the HAZ.
[0007] On the other hand, in the Non-Patent Document 5, it is
pointed out that the weld zone of the 18% Cr-8% Ni type austenitic
stainless heat resistant steels, undergo intergranular cracking in
the HAZ after a long period of heating.
[0008] The Patent Document 1 discloses a stainless steel in which
the C-fixing element is utilized. More concretely, it discloses a
"stainless steel highly resistant to intergranular corrosion and
intergranular stress corrosion cracking" having a specified
chemical composition with Nb/C.gtoreq.4 and N/C.gtoreq.5. In the
description that follows, "stress corrosion cracking" is referred
to as "SCC".
[0009] Further, the Patent Document 2 discloses an "austenitic
stainless steel containing N for use at high temperatures". More
concretely, it discloses an "austenitic stainless steel containing
N, which is excellent in sulfidation resistance and SCC resistance
and is suited for use in a high temperature environment of
350.degree. C. or higher where Cl.sup.- and S coexist" as resulting
from the achievement of the sulfidation resistance under high
temperature and high pressure conditions by an increased Cr
content, improvement in chloride SCC resistance by the combined
effect of increases in Cr content and Ni content and a decrease in
C content and, further, the enhancement of polythionic acid SCC
resistance by a reduction in C content, if necessary together with
incorporation of Nb. [0010] Patent Document 1: JP 50-67215A [0011]
Patent Document 2: JP 60-224764A [0012] Non-Patent Document 1:
Takeo Kudo et al., Sumitomo Metals, 38 (1986), p. 190 [0013]
Non-Patent Document 2: Kazutoshi Nishimoto et al., Sutenresuko no
Yosetsu (Welding of Stainless Steel) (2000), p. 114 [Sanpo
Publications, Inc.] [0014] Non-Patent Document 3: Yoshikuni Nakao
et al., Journal of the JWS, Vol. 51 (1982), No. 1, p. 64 [0015]
Non-Patent Document 4: Yoshikuni Nakao et al., Journal of the JWS,
Vol. 51 (1982), No. 12, p. 989 [0016] Non-Patent Document 5: R. N.
Younger et al.: Journal of the Iron and Steel Institute, October
(1960), p. 188
DISCLOSURE OF INVENTION
Problems to be Solved by the Invention
[0017] The technique disclosed in the above-mentioned Non-Patent
Document 1 is effective in reducing the solidification cracking
susceptibility in the weld metal, since the C content is reduced to
a low level and the content of Nb necessary for the stabilization
of C is also reduced. However, no attention is paid to the
occurrence, in the HAZ, of liquation cracking and of embrittling
cracking during a long period of use. Therefore, the austenitic
stainless steel containing the C-fixing element described in the
Non-Patent Document 1 is indeed excellent in corrosion resistance
and has excellent high temperature strength, but the said
austenitic stainless steel cannot avoid the above-mentioned two
kinds of cracking in the HAZ just after fabrication by the high
heat input TIG welding and during a long period of use at high
temperatures.
[0018] The intergranular corrosion cracking reported in the
Non-Patent Document 2 is quite different from the liquation
cracking on grain boundaries of HAZ which occurs during welding
before exposure to the corrosive environment mentioned above.
[0019] The techniques proposed in the Non-Patent Document 3 and the
Non-Patent Document 4 are effective in reducing cracking
susceptibility in the HAZ when the C content is in a high C range
exceeding 0.1%, and also the Nb is in a high Nb range exceeding 1%.
However, the occurrence of the liquation cracking in the HAZ cannot
be avoided as yet in a region where the C content is reduced to a
level of lower than 0.05% and also the Nb content is reduced to a
level of 0.5% or less in order to improve corrosion resistance. In
addition, when the austenitic stainless steels disclosed in the
Non-Patent Document 3 and the Non-Patent Document 4 are used in the
fields where corrosion resistance is required, the occurrence of
sensitizing corrosion in the HAZ also cannot be avoided, since the
C content is high.
[0020] Although the above-mentioned Non-Patent Document 5 suggests
that such carbides as M.sub.23C.sub.6 and NbC act as factors
influencing the cracking in the HAZ, it does not explain the
mechanisms thereof. Moreover, the technique disclosed in the
Non-Patent Document 5 is nothing but a means for avoiding
embrittling cracking in the HAZ after a long period of heating; it
is not always applicable to cope with the liquation cracking in the
HAZ just after welding.
[0021] Regarding the steel proposed in the Patent Document 1, the
polythionic acid SCC resistance thereof is enhanced by reducing the
C content and increasing the N content. However, such measures
alone cannot suppress polythionic acid SCC under server conditions
as well. Furthermore, the mere C content reduction and N content
increase cannot simultaneously enhance the liquation cracking
resistance and embrittling cracking resistance in the weld
zone.
[0022] The steel proposed in the Patent Document 2 is improved only
in sulfidation resistance and SCC resistance; the liquation
cracking resistance and embrittling cracking resistance thereof
cannot be simultaneously enhanced. Moreover, the steel cannot be
suppressed from undergoing SCC, in particular polythionic acid SCC,
under severer conditions.
[0023] The phenomena of the liquation cracking in the HAZ and the
cracking in the HAZ during a long period of use in highly corrosion
resistant austenitic stainless steels, in which C-fixing elements
are utilized, have been known for long time, as mentioned above. As
for the liquation cracking in the HAZ, however, neither the
mechanisms of occurrence of the liquation cracking in an area in
which the C content is low and the content of the C-fixing element
is also low, nor the measures thereof have yet been established. As
for the cracking in the HAZ during a long period of use as well, no
complete mechanisms have yet been clarified and, further, the
measures thereof, in particular the measures from the material
viewpoint, have not yet been established.
[0024] In view of the above-mentioned state of affairs, it is an
objective of the present invention to provide an austenitic
stainless steel which has C-fixing elements and can be suppressed
from undergoing liquation cracking in the HAZ on the occasion of
welding, and moreover is excellent in embrittling cracking
resistance in the HAZ during a long period of use at high
temperatures and is highly resistant to corrosion, in particular to
polythionic acid SCC.
Means for Solving the Problems
[0025] The present inventors made detailed investigations
concerning the mechanisms of the occurrence of liquation cracking,
embrittling cracking and polythionic acid SCC in order to provide
an austenitic stainless steel which has C-fixing elements and can
be suppressed from undergoing liquation cracking in the HAZ after
welding (hereinafter "liquation cracking in the HAZ after welding"
is also referred to as "liquation cracking" for short) and also can
be suppressed from undergoing embrittling cracking in the HAZ
during a long period of use at high temperatures (hereinafter
"embrittling cracking in the HAZ during a long period of use at
high temperatures" is also referred to as "embrittling cracking"
for short) and is highly resistant to corrosion, in particular to
polythionic acid SCC.
[0026] As a result, the following findings (a) and (b) were first
obtained concerning the occurrence of liquation cracking.
[0027] (a) In a case of austenitic stainless steels which have a C
content lower than 0.05%, in particular lower than 0.04%, and also
have low contents of C-fixing elements, the Cr carbonitrides
precipitate on the grain boundaries, since the carbides resulting
from binding of the said C-fixing elements to C have low
precipitation temperatures. On the other hand, the carbides of the
said C-fixing elements precipitate within grains.
[0028] (b) The above finding (a) indicates that the mechanisms of
occurrence of the liquation cracking are fundamentally different
from those described in the above-mentioned Non-Patent Document 3
and Non-Patent Document 4, that is to say, the mechanisms of the
occurrence involving the fusion of the low melting point compounds
such as NbC and/or the Laves phase that has precipitated on the
grain boundaries.
[0029] Then, further examinations and investigations were made and
the following findings (c) to (h) were obtained.
[0030] (c) When austenitic stainless steels, having a
microstructure in which the Cr carbonitrides precipitate on the
grain boundaries and the carbides of C-fixing elements precipitate
within grains, which have a C content lower than 0.05%, in
particular lower than 0.04%, as mentioned above, and have low
contents of C-fixing elements are heated to high temperatures by
welding thermal cycles, the C-fixing element carbides such as NbC,
which have primarily precipitated within the grains are dissolved.
Consequently, the pinning effect of the precipitates on the crystal
grain growth is lost and the crystal grains in the HAZ, which are
heated to just below the melting point, become very coarse and,
accordingly, the surface area of grain boundaries are markedly
reduced.
[0031] (d) Upon heating at high temperatures, the C-fixing elements
and the C that have dissolved within grains, diffuse within grains
and segregate on the grain boundaries. In addition, in the area
heated to just below the melting point, the surface area of the
grain boundaries becomes markedly reduced as a result of the
coarsening of the crystal grains. Consequently, it is presumed that
the extent of such segregation on the grain boundaries is higher
compared with other areas.
[0032] (e) Therefore, in the HAZ heated to just below the melting
point, the decrease of the surface area on grain boundaries due to
marked coarsening of crystal grains results in a concentration of
the C-fixing elements and/or C on the grain boundaries compared
with other areas heated to lower temperatures, and the very melting
point of the grain boundaries falls.
[0033] (f) Such elements as P and S, being contained in the base
metal, which show a marked tendency toward segregation on grain
boundaries also segregate to the grain boundaries in HAZ.
Therefore, the melting point of grain boundaries in the
coarse-grained HAZ falls markedly.
[0034] (g) The said crystal grain boundaries, which have lower
melting points, are melted upon heating in the welding thermal
cycles in the second pass and thereafter. Then the grain boundaries
are liquefied and the liquation cracking mentioned hereinabove
occurs.
[0035] (h) In order to prevent the above-mentioned liquation
cracking, it is presumably effective to increase the contents of
the C-fixing elements to thereby stabilize the carbides until
higher temperatures. On the other hand, when the content of
C-fixing elements is excessive, it is feared that the corrosion
resistance deteriorates due to the increase in the Cr-sensitizing
region. Therefore, in order to prevent liquation cracking in the
HAZ while maintaining high corrosion resistance, it is effective to
reduce impurity elements such as P and S in the steel and at the
same time optimize the content of C-fixing elements.
[0036] As for the above-mentioned embrittling cracking, the
following findings (i) to (k) were obtained.
[0037] (i) The said embrittling cracking occurs on the crystal
grain boundaries of the so-called "coarse-grained HAZ" which is
exposed to high temperatures during the welding.
[0038] (j) The fractured surface of the said embrittling cracking
is poor in ductility, and concentrations of such elements as P, S,
Sn and so on, which act on grain boundaries as
embrittlement-causing elements, are found on the fractured
surface.
[0039] (k) The microstructure in the vicinity of the said cracking
shows a large amount of carbides and nitrides that have
precipitated within crystal grains.
[0040] Based on the above findings (i) to (k), the present
inventors drew the following conclusions (1) to (n) concerning the
mechanisms of occurrence of the said embrittling cracking.
[0041] (l) During welding thermal cycles and the subsequent use at
high temperatures, such elements as P, S and Sn, which act on grain
boundaries as embrittlement-causing elements, segregate to the
grain boundaries. In particular, these elements segregate markedly
to the coarse-grained HAZ which has a small surface area of grain
boundaries and, therefore, the grain boundaries become markedly
embrittled.
[0042] (m) When external stress is applied during the use at high
temperatures, the intragranular deformation is suppressed by a
large amount of intragranular precipitates of carbonitrides and
nitrides, typically carbide-fixing element carbides such as NbC and
TiC. Therefore, stress concentration occurs on the interface of the
said embrittled grain boundaries and an orifice develops at the
grain boundaries, and this leads a easy occurrence of the said
embrittling cracking. In particular, the said stress concentration
on the grain boundary interface is promoted in areas where the
crystal grain diameter is large, such as in the coarse-grained HAZ,
hence the said embrittling cracking will very readily occur
there.
[0043] (n) Regarding the cracking which shows the similar cracking
mode to the above-mentioned embrittling cracking, for example,
there is the SR cracking in low alloy steels mentioned by Ito et
al. in the Journal of the JWS, Vol. 41 (1972), No. 1, p. 59.
However, the said SR cracking in those low alloy steels is a
cracking which occurs in the step of a short period SR heat
treatment after welding and is quite different in timing from the
above-mentioned embrittling cracking which occurs in the HAZ during
the long period of use at high temperatures. In addition, the base
metal of the said low alloy steels has a ferritic microstructure
and the mechanisms of occurrence of SR cracking therein are quite
different from those in the austenitic microstructure, which is the
intention of the present invention. Therefore, as a matter of
course, the measure for preventing the above-mentioned SR cracking
in low alloy steels as such, cannot be applied as a measure for
preventing the embrittling cracking which occurs in the HAZ during
a long period of use at high temperatures. Consequently, in order
to prevent this kind of embrittling cracking, it is effective to
take the following measures <1> and <2>:
<1> Suppression of intragranular carbide precipitation by
reducing the content of C-fixing elements; <2> Reduction of
the content of such elements as P, S and Sn, which act on grain
boundaries as embrittlement-causing elements, in the steel:
[0044] As mentioned above, it has been revealed that the reduction
in the content of those elements which segregate to grain
boundaries and thus embrittle grain boundaries, such as P, S and
Sn, is effective as a measure for preventing both the liquation
cracking after welding and the embrittling cracking in the HAZ
during a long period of use at high temperatures. However, the
influence of contents of the C-fixing elements on the said
liquation cracking and on the said embrittling cracking is the
contrary.
[0045] Furthermore, the following finding (o) was obtained
concerning the said polythionic acid SCC.
[0046] (o) When the content of impurity elements showing a tendency
toward segregation to grain boundaries, such as P, S, Sn, Sb and
Pb, is high, the polythionic acid SCC resistance, in particular in
the HAZ, deteriorates. Intergranular SCC such as polythionic acid
SCC is a corrosion generally caused by synergistic actions of
intergranular corrosion and stress. Therefore, although the
mechanisms involved have not yet been fully clarified, it is
considered that since the intergranular segregation of impurity
elements facilitate intergranular corrosion and the grain boundary
itself is embrittled, the intergranular SCC in a polythionic acid
environment be promoted by those synergistic actions.
[0047] On the supposition that both the above-mentioned liquation
cracking and embrittling cracking might be prevented, and also the
required level of strength might be secured and the SCC resistance
in a polythionic acid environment might be improved, by optimizing
the amount of carbide precipitates within the grains and at the
same time by reducing the extent of intergranular segregation, the
present inventors made detailed investigations in search of optimum
content levels of Nb, Ti, Ta, Zr, Hf and V, which are C-fixing
elements, and also of S, P, Sn, Sb, Pb, Zn and As, which segregate
in grain boundaries and embrittle grain boundaries. As a result,
the following important findings (p) to (s) were obtained.
[0048] (p) In order to prevent both the above-mentioned liquation
cracking and embrittling cracking and to improve the polythionic
acid SCC resistance, it is important to restrict the contents of P,
S, Sn, Sb, Pb, Zn and As, which segregate to grain boundaries and
embrittle grain boundaries, within respective specific ranges.
[0049] (q) Among the elements mentioned above, S is the most
harmful one, followed by P and Sn. Therefore, in order to prevent
the above-mentioned two kinds of cracking and to improve the
polythionic acid SCC resistance, it becomes essential, in addition
to restricting the contents of the respective elements, that the
value of the parameter F1 defined by the formula (1) given below as
derived by taking into consideration the weights of the influences
of the respective elements should be not more than 0.075; in the
formula, each element symbol represents the content by mass percent
of the element concerned:
F1=S+{(P+Sn)/2}+{(As+Zn+Pb+Sb)/5} (1).
[0050] (r) When, in particular, the contents of Nb, Ti, Ta, Zr, Hf
and V, which are the C-fixing elements, are adjusted within
respective specific ranges according to the contents of the
above-mentioned elements P, S, Sn, Sb, Pb, Zn and As, which
segregate to grain boundaries and embrittle grain boundaries, it
becomes possible to secure the required level of strength and
improve the SCC resistance in a polythionic acid environment and,
in addition, prevent both the above-mentioned liquation cracking
and embrittling cracking.
[0051] (s) Ti, in particular, among the above-mentioned C-fixing
elements exerts the greatest influence, followed by Ta, Nb, Zr and
Hf. Therefore, in order to secure the required strength and to
improve the SCC resistance in a polythionic acid environment and at
the same time to prevent the above-mentioned two kinds of cracking,
it is essential, in addition to restricting the contents of the
respective elements, that the value of the parameter F2 defined by
the formula (2) given below as derived by taking into consideration
the weights of the influences of the respective elements should be
not less than 0.05 and the upper limit thereto should be set at
[1.7-9.times.F1]: in the formula, each element symbol represents
the content by mass percent of the element concerned:
F2=Nb+Ta+Zr+Hf+2Ti+(V/10) (2).
[0052] The present invention has been accomplished on the basis of
the above-described findings. The main points of the present
invention are austenitic stainless steels shown in the following
(1) to (3).
[0053] (1) An austenitic stainless steel, which comprises by mass
percent, C: less than 0.04%, Si: not more than 1.5%, Mn: not more
than 2%, Cr: 15 to 25%, Ni: 6 to 30%, N: 0.02 to 0.35%, sol. Al:
not more than 0.03% and further contains one or more elements
selected from Nb: not more than 0.5%, Ti: not more than 0.4%, V:
not more than 0.4%, Ta: not more than 0.2%, Hf: not more than 0.2%
and Zr: not more than 0.2%, with the balance being Fe and
impurities, in which the contents of P, S, Sn, As, Zn, Pb and Sb
among the impurities are P: not more than 0.04%, S: not more than
0.03%, Sn: not more than 0.1%, As not more than 0.01%, Zn: not more
than 0.01%, Pb: not more than 0.01% and Sb: not more than 0.01%,
and the values of F1 and F2 defined respectively by the following
formula (1) and formula (2) satisfy the conditions F1.ltoreq.0.075
and 0.05.ltoreq.F2<1.7-9.times.F1;
F1=S+{(P+Sn)/2}+{(As+Zn+Pb+Sb)/5} (1),
F2=Nb+Ta+Zr+Hf+2Ti+(V/10) (2);
[0054] In the formulas (1) and (2), each element symbol represents
the content by mass percent of the element concerned.
[0055] (2) An austenitic stainless steel, which comprises by mass
percent, C: less than 0.05%, Si: not more than 1.5%, Mn: not more
than 2%, Cr: 15 to 25%, Ni: 6 to 13%, N: 0.02 to 0.1%, sol. Al: not
more than 0.03% and further contains one or more elements selected
from Nb: not more than 0.5%, Ti: not more than 0.4%, V: not more
than 0.4%, Ta: not more than 0.2%, Hf: not more than 0.2% and Zr:
not more than 0.2%, with the balance being Fe and impurities, in
which the contents of P, S, Sn, As, Zn, Pb and Sb among the
impurities are P: not more than 0.04%, S: not more than 0.03%, Sn:
not more than 0.1%, As: not more than 0.01%, Zn: not more than
0.01%, Pb: not more than 0.01% and Sb: not more than 0.01%, and the
values of F1 and F2 defined respectively by the following formula
(1) and formula (2) satisfy the conditions F1.ltoreq.0.075 and
0.05.ltoreq.F2<1.7-9.times.F1;
F1=S+{(P+Sn)/2}+{(As+Zn+Pb+Sb)/5} (1),
F2=Nb+Ta+Zr+Hf+2Ti+(V/10) (2);
[0056] In the formulas (1) and (2), each element symbol represents
the content by mass percent of the element concerned.
[0057] (3) The austenitic stainless steel according to the above
(1) or (2), which further contains, by mass percent, one or more
elements of one or more groups selected from the first to third
groups listed below in lieu of a part of Fe:
[0058] First group: Cu: not more than 4%, Mo: not more than 5%, W:
not more than 5% and Co: not more than 1%;
[0059] Second group: B: not more than 0.012%; and
[0060] Third group: Ca: not more than 0.02%, Mg: not more than
0.02% and rare earth element: not more than 0.1%.
[0061] The term "rare earth element" (hereinafter referred to as
"REM") refers to a total of 17 elements including Sc, Y and
lanthanoid collectively, and the REM content mentioned above means
the content of one or the total content of two or more of the
REM.
[0062] Hereinafter, the above-mentioned inventions (1) to (3)
related to the austenitic stainless steels are referred to as "the
present invention (1)" to "the present invention (3)",
respectively. They are sometimes collectively referred to as "the
present invention".
Effects of the Invention
[0063] The austenitic stainless steels of the present invention
have excellent liquation cracking resistance and embrittling
cracking resistance in a weld zone, and moreover they have
excellent polythionic acid SCC resistance and high temperature
strength. Consequently, they can be used as raw materials for
various apparatuses which are used in a sulfide-containing
environment at high temperatures for a long period of time; for
example in power plant boilers, petroleum refining and
petrochemical plants and so on.
BEST MODES FOR CARRYING OUT THE INVENTION
[0064] In the following, the reasons for restricting the contents
of the component elements of the austenitic stainless steels in the
present invention are described in detail. In the following
description, the symbol "%" for the content of each element means
"% by mass".
[0065] C: less than 0.05%
[0066] From the viewpoint of securing corrosion resistance, in
particular polythionic acid SCC resistance, the content of C is
desirably as low as possible so that the sensitizing due to
precipitation of Cr carbides formed by its binding to Cr may be
suppressed. On the other hand, C is an element having an
austenite-forming effect and at the same time forming fine carbides
within the grains thereby contributing to improvements in high
temperature strength. Therefore, from the viewpoint of securing
high temperature strength, a content of C corresponding to the
content of carbide-forming elements is preferable for the purpose
of strengthening by carbides which precipitate within the grains.
However, when the C content is excessive, in particular at a
content level of 0.05% or more, C causes an increase in
susceptibility to weld solidification cracking and, in addition,
causes marked deterioration in corrosion resistance. Therefore, the
C content of the present invention (2) is set to less than 0.05%.
The content of C is more preferably less than 0.04%. Therefore the
C content of the present invention (1) is set to less than 0.04%.
The content of C is still more preferably less than 0.03% and most
preferably not more than 0.02%.
[0067] Si: not more than 1.5%
[0068] Si is an element which has a deoxidizing effect during the
step of melting the austenitic stainless steels. It is also
effective in increasing the oxidation resistance, steam oxidation
resistance and so on. However, when the content thereof is
excessive, in particular at a content level exceeding 1.5%, it
causes a marked increase in weld cracking susceptibility and, since
Si is a ferrite-forming element, it deteriorates the stability of
the austenite phase. Therefore, the content of Si is set to not
more than 1.5%. The content of Si is preferably not more than 1%,
more preferably not more than 0.75%. On the other hand, in order to
ensure the above-mentioned effects of Si, the lower limit of the Si
content is preferably set to 0.02%. The lower limit of the Si
content is more preferably 0.1%.
[0069] Mn: not more than 2%
[0070] Mn is an austenite-forming element and, at the same time, it
is an element effective in preventing the hot working brittleness
due to S and in deoxidation during the step of melting. However, if
the content of Mn exceeds 2%, Mn promotes the precipitation of such
intermetallic compound phases as the .sigma. phase and also causes
a decrease in toughness and ductility due to the deterioration in
microstructural stability at high temperatures in case of use in a
high temperature environment. Therefore, the content of Mn is set
to not more than 2%. The content of Mn is preferably not more than
1.5%. The lower limit of the Mn content is preferably set to 0.02%
and the lower limit of the Mn content is more preferably 0.1%.
[0071] Cr: 15 to 25%
[0072] Cr is an essential element for ensuring the oxidation
resistance and corrosion resistance at high temperatures and, in
order to obtain the said effects, it is necessary that the Cr
content be not less than 15%. However, when the content thereof is
excessive, in particular at a content level exceeding 25%, it
deteriorates the stability of the austenite phase at high
temperatures and thus causes a decrease in creep strength.
Therefore, the content of Cr is set to 15 to 25%. The preferable
lower limit of the Cr content is 17% and the preferable upper limit
thereof is 20%.
[0073] Ni: 6 to 30%
[0074] Ni is an essential element for ensuring a stable austenitic
microstructure and is also an essential element for ensuring the
microstructural stability during a long period of use and thus
obtaining the desired level of creep strength. However, in order to
obtain the said effects, the balance with the Cr content mentioned
above is important and a Ni content of not less than 6% is required
relative to the lower limit of the Cr content in the present
invention. On the other hand, the addition of the expensive element
Ni in an amount exceeding 30% results in an increase in cost.
Therefore, the Ni content of the preset invention (1) is set to 6
to 30%. The upper limit of the Ni content is preferably set to 20%
and the upper limit of the Ni content is more preferably 13%.
Therefore, the Ni content of the present invention (2) is set to 6
to 13%. The upper limit of the Ni content is most preferably set to
12%. The lower limit of the Ni content is preferably set to 7% and
the lower limit of the Ni content is more preferably 9%.
[0075] N: 0.02 to 0.35%
[0076] N is an austenite-forming element and is an element soluble
in the matrix and precipitates as the fine carbonitrides within the
grains and thus effective in improving the creep strength. In order
to obtain these effects sufficiently, the content of N is required
to be not less than 0.02%. However, when the N content is
excessive, and at a content level of more than 0.35%, Cr nitrides
are formed on the grain boundaries and, therefore, the polythionic
acid SCC resistance in the HAZ deteriorates due to the resulting
sensitization. Therefore, the content of N is set to 0.02 to 0.35%.
The lower limit of the N content is preferably set to 0.04% and the
lower limit of the N content is more preferably 0.06%. The upper
limit of the N content is preferably set to 0.3% and the upper
limit of the N content is more preferably 0.1%.
[0077] Sol. Al: not more than 0.03%
[0078] Al has a deoxidizing effect but, at high additional levels,
it markedly impairs the cleanliness and deteriorates the
workability and ductility; in particular, when the Al content
exceeds 0.03% as sol. Al ("acid-soluble Al"), it causes a marked
decrease in workability and ductility. Therefore, the content of
sol. Al is set to not more than 0.03%. The lower limit of the
sol.Al content is not particularly restricted, however the lower
limit of the sol.Al content is preferably 0.0005%.
[0079] One or more elements selected from Nb: not more than 0.5%,
Ti: not more than 0.4%, V: not more than 0.4%, Ta: not more than
0.2%, Hf: not more than 0.2% and Zr: not more than 0.2%
[0080] Nb, Ti, V, Ta, Hf and Zr, which are the C-fixing elements,
constitute an important group of elements which form the basis of
the present invention. That is to say, when these elements bind to
C to form carbides and the carbides precipitate within grains, the
precipitation of the Cr carbides on the grain boundaries is
suppressed and the sensitizing is prevented, and hence high levels
of corrosion resistance can be ensured. Furthermore, the
above-mentioned carbides that have precipitated within grains also
contribute to improvement in creep strength. However, when the
content of the above-mentioned elements is excessive, the
dissolution temperature of the said carbides in the welding thermal
cycles rises. Therefore, the segregation of the above-mentioned
elements, caused by the dissolution of the carbides on the grain
boundaries in a coarse-grained HAZ is reduced. Consequently, the
liquation cracking on the grain boundaries, due to exposure to
thermal cycles in the next layer welding can be prevented. However,
on the other hand, the carbides precipitate excessively within
grains and the intragranular deformation is hindered thereby,
causing further stress concentration on the grain boundary
interface that has become fragile due to the segregation of the
impurity elements to be mentioned later herein, the result of the
embrittling cracking in the coarse-grained HAZ during a long period
of use at high temperatures is promoted. Furthermore, the
Cr-sensitized region is enlarged, such as in the so-called "knife
line attack", resulting in marked deterioration of the corrosion
resistance. In particular, when the content of Nb exceeds 0.5% or
when the content of each of Ti and V exceeds 0.4% and, further,
when the content of each of Ta, Hf and Zr exceeds 0.2%, the
above-mentioned harmful influences become significant. Therefore,
in order to ensure a high level of corrosion resistance and to
suppress both the liquation cracking after welding and the
embrittling cracking during a long period of use, the content of
each of Nb, Ti, V, Ta, Hf and Zr is set to as follows: Nb: not more
than 0.5%, Ti: not more than 0.4%, V: not more than 0.4%, Ta: not
more than 0.2%, Hf: not more than 0.2% and Zr: not more than
0.2%.
[0081] The upper limit of each of the contents of the
above-mentioned elements is preferably as follows: 0.4% for Nb,
0.3% for Ti, 0.2% for V, 0.15% for Ta, 0.15% for Hf and 0.1% for
Zr.
[0082] The steels of the present invention can contain only one or
a combination of two or more of the above-mentioned elements
selected from Nb, Ti, \) Ta, Hf and Zr. However, in order to secure
excellent polythionic acid SCC resistance, it is necessary that the
value of the said parameter F2 mentioned hereinabove should be set
to not less than 0.05 and, in order to reduce the cracking
susceptibility in the HAZ just after welding and during a long
period of use, it is necessary that the upper limit of the value of
the said parameter F2 should be set to [1.7-9.times.F1], as
described later herein.
[0083] In the present invention, it is necessary to restrict the
contents of P, S, Sn, As, Zn, Pb and Sb among the impurities to not
more than the specified levels.
[0084] That is to say, all of the above-mentioned elements
segregate on the grain boundaries in the coarse-grained HAZ during
welding thermal cycles or during the subsequent use at high
temperatures, and lower the melting point of the grain boundaries
together with the binding force of the grain boundaries, and thus,
cause liquation cracking due to fusion of the grain boundaries in
the coarse-grained HAZ upon exposure to thermal cycles in the next
layer welding step or embrittling cracking during use at high
temperatures. In addition, these elements promote intergranular
corrosion and lower the strength of grain boundaries, and therefore
lead to the deterioration in polythionic acid SCC resistance.
Therefore, first, it is necessary to restrict the contents thereof
as follows: P: not more than 0.04%, S: not more than 0.03%, Sn: not
more than 0.1%, As: not more than 0.01%, Zn: not more than 0.01%,
Pb: not more than 0.01% and Sb: not more than 0.01%.
[0085] Among the elements mentioned above, S exerts the most
harmful influence on the liquation cracking in the coarse-grained
HAZ after welding and on the embrittling cracking and polythionic
acid SCC resistance during a long period of use, followed by the
harmful influences of P and Sn. In order to prevent both the
above-mentioned liquation cracking and embrittling cracking and
also to improve the polythionic acid SCC resistance as well, it is
necessary that the value of the parameter F1 mentioned hereinabove
should be not more than 0.075 and that this parameter F1, in
relation to the parameter F2, should satisfy the condition
[F2<1.7-9.times.F1]. These requirements will be explained
below.
[0086] The value of the parameter F1: not more than 0.075
[0087] When the value of F1 defined by the said formula (1), that
is to say, by [S+{(P+Sn)/2}+{(As+Zn+Pb+Sb)/5}], exceeds 0.075, it
becomes impossible to prevent the decrease in grain boundary
binding force and, therefore, the occurrence of liquation cracking
in the coarse-grained HAZ after welding, and of embrittling
cracking during a long period of use. Further, the deterioration in
polythionic acid SCC resistance cannot be avoided. Therefore, it is
necessary that the value of the parameter F1 should be set to not
more than 0.075. It is preferable that the value of the parameter
F1 is reduced as low as possible.
[0088] The value of the parameter F2: not less than 0.05 to not
more than [1.7-9.times.F1]
[0089] When the value of F2 defined by the said formula (2), that
is to say, by [Nb+Ta+Zr+Hf+2Ti+(V/10)], is not less than 0.05,
excellent polythionic acid SCC resistance can be ensured. And, when
the value of F2 satisfies the condition of not more than
[1.7-9.times.F1] in relation to the above-mentioned parameter F1,
it becomes possible to prevent the liquation cracking in the
coarse-grained HAZ after welding and the embrittling cracking
during a long period of use.
[0090] From the reasons mentioned above, the austenitic stainless
steels according to the present inventions (1) and (2) are defined
as the ones which contain the above-mentioned elements C to sol. Al
within their respective content ranges and further contain one or
more elements selected from Nb, Ti, V, Ta, Hf and Zr within their
respective content ranges, with the balance being Fe and
impurities, in which the contents of P, S, Sn, As, Zn, Pb and Sb
among the impurities are within their respective content ranges,
and the values of F1 and F2 defined respectively by the said
formulas (1) and (2) satisfy the conditions F1.ltoreq.0.075 and
0.05.ltoreq.F2.ltoreq.1.7-9.times.F1.
[0091] The austenitic stainless steels according to the present
invention (1) or the present invention (2) can further selectively
contain, according to need, one or more elements of each of the
following groups of elements in lieu of a part of Fe:
[0092] First group: Cu: not more than 4%, Mo: not more than 5%, W:
not more than 5% and Co: not more than 1%;
[0093] Second group: B: not more than 0.012%; and
[0094] Third group: Ca: not more than 0.02%, Mg: not more than
0.02% and REM: not more than 0.1%.
[0095] That is to say, one or more of the first to third groups of
elements may be added, as optional element(s), to the
above-mentioned steels and thereby contained therein.
[0096] The above-mentioned optional elements will be explained
below.
[0097] First group: Cu: not more than 4%, Mo: not more than 5%, W:
not more than 5% and Co: not more than 1%
[0098] Each of Cu, Mo, W and Co being elements of the first group,
if added, has the effect of enhancing the high temperature
strength. In order to obtain this effect, the said elements may be
added to the steels and thereby contained therein. The elements,
which are in the first group, are now described in detail.
[0099] Cu: not more than 4%
[0100] Cu precipitates finely at high temperatures. Therefore, Cu
is an effective element which enhances high temperature strength.
Cu is also effective in stabilizing the austenite phase. However,
when the content of Cu is increased, the Cu phase precipitation
becomes excessive and the susceptibility to embrittling cracking in
the coarse-grained HAZ increases; in particular when the content of
Cu exceeds 4%, the susceptibility to embrittling cracking in the
coarse-grained HAZ becomes markedly higher. Therefore, if Cu is
added, the content of Cu is set to not more than 4%. The content of
Cu is preferably set to not more than 3% and the content of Cu is
more preferably not more than 2%. On the other hand, in order to
ensure the above-mentioned effects, the lower limit of the Cu
content is preferably set to 0.02% and the lower limit of the Cu
content is more preferably 0.05%.
[0101] Mo: not more than 5%
[0102] Mo dissolves in the matrix and is an element which makes a
contribution to the enhancement of high temperature strength, in
particular to the enhancement of creep strength at high
temperatures. Mo is also effective in suppressing the precipitation
of Cr carbides on the grain boundaries. However, when the content
of Mo is increased, the stability of the austenite phase
deteriorates; hence the creep strength is rather low, and moreover,
the susceptibility to embrittling cracking in the coarse-grained
HAZ increases. In particular, when the content of Mo exceeds 5%,
the creep strength markedly deteriorates and, at the same time, the
susceptibility to embrittling cracking in the coarse-grained HAZ
becomes markedly higher. Therefore, if Mo is added, the content of
Mo is set to not more than 5%. The content of Mo is preferably not
more than 1.5%. On the other hand, in order to ensure the
above-mentioned effects, the lower limit of the Mo content is
preferably set to 0.05%.
[0103] W: not more than 5%
[0104] W also dissolves in the matrix and is an element which makes
a contribution to the enhancement of high temperature strength, in
particular to the enhancement of creep strength at high
temperatures. However, when the content of W is increased, the
stability of the austenite phase deteriorates; hence the creep
strength is rather low, and moreover, the susceptibility to
embrittling cracking in the coarse-grained HAZ increases. In
particular, when the content of W exceeds 5%, the creep strength
markedly deteriorates and, at the same time, the susceptibility to
embrittling cracking in the coarse-grained HAZ becomes markedly
higher. Therefore, if W is added, the content of W is set to not
more than 5%. The content of W is preferably set to not more than
3% and the content of W is more preferably not more than 1.5%. On
the other hand, in order to ensure the above-mentioned effects, the
lower limit of the W content is preferably set to 0.05%.
[0105] Co: not more than 1%
[0106] Like Ni, Co increases the stability of the austenite phase
and makes a contribution to the enhancement of high temperature
strength. However, Co is a very expensive element and, therefore,
an increased content thereof results in an increase in cost. In
particular, when the content of Co exceeds 1%, the cost markedly
increases. Therefore, if Co is added, the content of Co is set to
not more than 1%. The content of Co is preferably set to not more
than 0.8% and the content of Co is more preferably not more than
0.5%. The Co content is further preferably not more that 0.28%. On
the other hand, in order to ensure the above-mentioned effects, the
lower limit of the Co content is preferably set to 0.03%.
[0107] The steels of the present invention can contain only one or
a combination of two or more of the above-mentioned Cu, Mo, W and
Co.
[0108] Second group: B: not more than 0.012%
[0109] B, which is the element of the second group, if added, has
the effect of strengthening the grain boundaries. In order to
obtain this effect, B may be added to the steels and thereby
contained therein. B, which is in the second group, is now
explained in detail.
[0110] B: not more than 0.012%
[0111] B segregates on the grain boundaries and also disperses
carbides precipitating on the grain boundaries finely, and is an
element which makes a contribution to strengthening the grain
boundaries. However, an excessive addition of B lowers the melting
point of the grain boundaries; in particular, when the content of B
exceeds 0.012%, the decrease of the grain boundary melting point
becomes remarkable, and therefore, in the step of welding, the
liquation cracking on the grain boundaries in the HAZ vicinity to
the fusion line occurs. Therefore, if B is added, the content of B
is set to not more than 0.012%. The content of B is preferably not
more than 0.005% and more preferably not more than 0.0045%. On the
other hand, in order to ensure the above-mentioned effect, the
lower limit of the B content is preferably set to 0.0001%. The
lower limit of the B content is more preferably 0.001%.
[0112] Third group: one or more elements selected from Ca: not more
than 0.02%, Mg: not more than 0.02% and REM: not more than
0.1%.
[0113] Each of Ca, Mg and REM being elements of the third group, if
added, has the effect of increasing the hot workability. In order
to obtain this effect, the said elements may be added to the steels
and thereby contained therein. The elements, which are in the third
group, are now described in detail.
[0114] Ca: not more than 0.02%
[0115] Ca has a high affinity for S and so, it has an effect of
improving the hot workability. Ca is also effective, although to a
slight extent, in reducing the possibility of the embrittling
cracking in the coarse-grained HAZ which is caused by the
segregation of S on the grain boundaries. However, an excessive
addition of Ca causes deterioration of cleanliness, in other words,
an increase of the index of cleanliness, due to the binding thereof
to oxygen; in particular, when the content of Ca exceeds 0.02%, the
deterioration of the cleanliness markedly increases and the hot
workability rather deteriorates. Therefore, if Ca is added, the
content of Ca is set to not more than 0.02%. The content of Ca is
preferably not more than 0.01%. On the other hand, in order to
ensure the above-mentioned effects, the lower limit of the Ca
content is preferably set to 0.0001% and the lower limit of the Ca
content is more preferably 0.0005%.
[0116] Mg: not more than 0.02%
[0117] Mg also has a high affinity for S and so, it has an effect
of improving the hot workability. Mg is also effective, although to
a slight extent, in reducing the possibility of the embrittling
cracking in the coarse-grained HAZ which is caused by the
segregation of S on the grain boundaries. However, an excessive
addition of Mg causes deterioration of cleanliness due to the
binding thereof to oxygen; in particular, when the content of Mg
exceeds 0.02%, the deterioration of the cleanliness markedly
increases and the hot workability rather deteriorates. Therefore,
if Mg is added, the content of Mg is set to not more than 0.02%.
The content of Mg is preferably not more than 0.01%. On the other
hand, in order to ensure the above-mentioned effects, the lower
limit of the Mg content is preferably set to 0.0001%.
[0118] REM: not more than 0.1%
[0119] REM has a high affinity for S and so, it has an effect of
improving the hot workability. REM is also effective in reducing
the possibility of the embrittling cracking in the coarse-grained
HAZ which is caused by the segregation of S on the grain
boundaries. However, an excessive addition of REM causes
deterioration of cleanliness due to the binding thereof to oxygen;
in particular, when the content of REM exceeds 0.1%, the
deterioration of the cleanliness markedly increases and the hot
workability rather deteriorates. Therefore, if REM is added, the
content of REM is set to not more than 0.1%. The content of REM is
preferably not more than 0.05%. On the other hand, in order to
ensure the above-mentioned effects, the lower limit of the REM
content is preferably set to 0.001%.
[0120] As already mentioned hereinabove, the term "REM" refers to a
total of 17 elements including Sc, Y and lanthanoid collectively,
and the REM content means the content of one or the total content
of two or more of the REM.
[0121] The steels of the present invention can contain only one or
a combination of two or more of the above-mentioned Ca, Mg and
REM.
[0122] From the reasons mentioned above, the austenitic stainless
steel according to the present invention (3) is defined as the one
which contains one or more elements of one or more groups selected
from the first to third groups listed below in lieu of a part of Fe
in the austenitic stainless steel according to the present
invention (1) or (2):
[0123] first group: Cu: not more than 4%, Mo: not more than 5%, W:
not more than 5% and Co: not more than 1%;
[0124] second group: B: not more than 0.012%; and
[0125] third group: Ca: not more than 0.02%, Mg: not more than
0.02% and REM: not more than 0.1%.
[0126] The austenitic stainless steels, according to the present
inventions (1) to (3), can be produced by selecting the raw
materials to be used in the melting step based on the results of
careful and detailed analyses so that, in particular, the contents
of Sn, As, Zn, Pb and Sb among the impurities may fall within the
above-mentioned respective ranges, namely Sn: not more than 0.1%,
As not more than 0.01%, Zn: not more than 0.01%, Pb: not more than
0.01% and Sb: not more than 0.01% and the values of F1 and F2
respectively defined by the said formula (1) and formula (2)
satisfy the conditions F1.ltoreq.0.075 and
0.05.ltoreq.F2.ltoreq.1.7-9.times.F1, respectively and then melting
the materials using an electric furnace, an AOD furnace or a VOD
furnace.
[0127] Next, a slab, a bloom or a billet is produced by casting the
molten metal, which is prepared by a melting process, into an ingot
by the so-called "ingot making method" and subjecting the ingot to
hot working, or by continuous casting. Then, in the case of plate
manufacturing, for example, the said raw material is subjected to
hot rolling into a plate or a coil shaped sheet. Or, in the case of
pipe manufacturing, for instance, any of such raw materials is
subjected to hot working into a tubular product by the hot
extrusion pipe manufacturing process or Mannesmann pipe
manufacturing process.
[0128] That is to say, the hot working may use any hot working
process. For example, in a case where the final product is a steel
pipe or tube, the hot working may include the hot extrusion pipe
manufacturing process represented by the Ugine-Sejournet process,
the hot pushing pipe manufacturing process, and/or the rolling pipe
manufacturing process (Mannesmann pipe manufacturing process)
represented by the Mannesmann-Plug Mill rolling process or the
Mannesmann-Mandrel Mill rolling process or the like. In a case
where the final product is a steel plate or sheet, the hot working
may include the typical process of manufacturing a steel plate or a
hot rolled steel sheet in coil.
[0129] The end temperature of the hot working is not particularly
defined, but may be preferably set to not less than 1000.degree. C.
This is because if the end temperature of the hot working is less
than 1000.degree. C., the dissolution of the carbonitrides of Nb,
Ti and V becomes insufficient, and therefore the creep strength and
ductility may be impaired.
[0130] The cold working can be carried out after the hot working.
For instance, in a case where the final product is a steel pipe or
tube, the cold working may include the cold drawing pipe
manufacturing process in which the raw pipe produced by the
above-mentioned hot working is subjected to drawing and/or the cold
rolling pipe manufacturing process. In a case where the final
product is a steel plate or sheet, the cold working may include the
typical process of manufacturing a cold rolled steel sheet in coil.
Furthermore, in order to homogenize the microstructure and to
further stabilize the strength, it is preferable to apply strains
on the materials and then to perform a heat treatment for obtaining
the recrystallization and uniform grains. In order to apply
strains, it is recommended that the final working in the case of
cold working be carried out at a rate of reduction in area of not
less than 10%.
[0131] The final heat treatment after the above-mentioned hot
working or the final heat treatment after a further cold working
following the hot working may be carried out at a heating
temperature of not less than 1000.degree. C. The upper limit of the
said heating temperature is not particularly defined, but a
temperature exceeding 1350.degree. C. may cause not only high
temperature intergranular cracking or a deterioration of ductility
but also very coarse crystal grains. Moreover, the said temperature
may cause a marked deterioration of workability. Therefore, the
upper limit of the heating temperature is preferably set to
1350.degree. C.
[0132] The following examples illustrate the present invention more
specifically. These examples are, however, by no means limited to
the scope of the present invention.
Examples
[0133] Austenitic stainless steels A1 to A10, B1 to B5, C1 and C2
and D1 to D5 having the chemical compositions shown in Tables 1 and
2 were melted using an electric furnace and cast to form ingots.
Each ingot was hot worked by a hot forging and a hot rolling, and
then, was subjected to a heat treatment comprising heating at
1100.degree. C., followed by water cooling and, thereafter
subjected to machining to produce steel plates having a thickness
of 20 mm, a width of 50 mm and a length of 100 mm.
[0134] The steels D1 to D5 shown in Tables 1 and 2 are steels
having chemical compositions which fall within the range regulated
by the present invention. On the other hand, the steels B1 to B5
are steels of comparative examples in which one or more of the
contents of the component elements and the values of the parameters
F1 and F2 are out of the ranges regulated by the present invention.
The steels A1 to A10 and C1 and C2 are steels of reference
examples.
TABLE-US-00001 TABLE 1 Chemical composition (% by mass) The
balance: Fe and impurities Steel C Si Mn P S Cr Ni sol. Al N Nb Ta
Hf Ti V Sn A1 0.010 0.39 1.43 0.028 0.0010 17.76 10.65 0.002 0.088
0.31 -- -- 0.004 0.068 0.004 A2 0.009 0.42 1.50 0.022 0.0005 17.17
9.91 0.017 0.081 0.30 0.002 -- 0.002 0.020 0.004 A3 0.007 0.36 1.48
0.028 0.0005 17.16 9.95 0.029 0.081 0.31 0.002 -- 0.003 0.040 0.002
A4 0.008 0.37 1.48 0.022 0.0005 17.25 9.93 0.026 0.083 0.30 0.002
-- 0.004 0.040 0.001 A5 0.012 0.38 1.48 0.019 0.0005 17.17 9.88
0.018 0.076 0.29 0.002 -- 0.002 0.020 0.003 A6 0.014 0.46 1.74
0.028 0.0011 17.73 10.21 0.002 0.090 0.31 0.010 -- 0.006 0.068
0.004 A7 0.013 0.48 1.53 0.027 0.0004 17.24 9.86 0.015 0.082 0.32
0.010 -- 0.005 0.060 0.003 A8 0.012 0.29 1.47 0.027 0.0007 17.39
9.70 0.008 0.088 0.35 0.010 -- 0.003 0.057 0.003 A9 0.012 0.36 1.53
0.027 0.0005 17.30 10.02 0.023 0.076 0.31 0.010 -- 0.003 0.063
0.004 A10 0.011 0.25 1.19 0.006 0.0004 24.98 19.76 0.020 0.250 0.29
-- -- 0.002 0.012 0.001 B1 0.008 0.48 1.38 0.034 0.0230 17.42 9.96
0.002 0.080 0.42 -- 0.01 0.080 0.050 0.092 B2 0.009 0.33 1.41 0.028
0.0010 17.26 9.89 0.002 0.082 0.48 0.080 0.14 0.350 0.280 0.048 B3
0.042 0.34 1.42 0.031 0.0020 17.25 9.94 0.004 0.079 *1.02 -- --
0.005 0.055 0.006 B4 *0.250 0.34 1.45 0.024 0.0010 18.17 9.93 0.002
0.086 0.48 0.005 -- 0.003 0.021 0.004 B5 0.010 0.32 1.44 0.036
0.0060 17.80 9.95 0.002 *0.007 0.45 0.010 -- 0.003 0.035 0.003 C1
0.014 0.38 1.48 0.016 0.0005 17.30 11.00 0.012 0.072 0.28 -- --
0.003 0.012 0.002 C2 0.008 0.36 1.52 0.015 0.0004 17.40 9.71 0.016
0.064 0.29 -- -- 0.003 0.025 0.003 D1 0.008 0.37 0.85 0.025 0.0006
17.44 11.23 0.011 0.084 0.30 0.005 -- 0.004 0.046 0.002 D2 0.009
0.46 0.90 0.026 0.0007 17.48 11.78 0.023 0.088 0.34 0.010 -- 0.003
0.036 0.004 D3 0.011 0.36 0.94 0.025 0.0007 17.40 11.85 0.016 0.084
0.38 0.010 -- 0.002 0.028 0.002 D4 0.010 0.36 0.99 0.025 0.0008
17.52 11.99 0.018 0.086 0.31 0.006 -- 0.002 0.040 0.002 D5 0.008
0.40 0.90 0.024 0.0006 17.45 11.83 0.022 0.092 0.33 0.010 -- 0.005
0.033 0.001
TABLE-US-00002 TABLE 2 (continued from Table 1) Chemical
composition (% by mass) The balance: Fe and impurities Value Value
of Value Steel As Zn Pb Sb Others of F1 [1.7-9 .times. F1] of F2 A1
-- -- -- -- *-- 0.017 1.547 0.325 A2 0.001 -- -- -- *B: 0.0015
0.0137 1.5767 0.308 A3 0.001 -- -- -- *Ca: 0.001 0.0157 1.5587
0.322 A4 0.001 -- -- -- *Mo: 0.37 0.0122 1.5902 0.314 A5 0.004 --
-- -- *Cu: 0.08 0.0123 1.5893 0.298 A6 -- -- -- -- *Co: 0.21 0.0171
1.5461 0.339 A7 -- 0.002 -- 0.002 *Cu: 0.2, Mo: 0.37 0.0162 1.5542
0.346 A8 -- -- 0.001 -- *Cu: 0.21, B: 0.0015, Co: 0.44 0.0159
1.5569 0.372 A9 -- -- -- -- *Cu: 0.26, Mo: 0.46, Co: 0.12, B:
0.0019 0.016 1.556 0.332 A10 -- -- -- -- *Zr: 0.02, Nd: 0.015
0.0039 1.6649 0.295 B1 0.008 0.007 -- -- Cu: 0.27, Co: 0.14 *0.089
0.899 0.595 B2 0.005 -- 0.006 -- *Mo: 0.37 0.0412 1.3292 *1.428 B3
0.002 -- -- -- *B: 0.0016 0.0209 1.5119 1.036 B4 0.002 0.001 -- --
*B: 0.0015, Co: 0.17 0.0156 1.5596 0.493 B5 -- -- -- -- *Cu: 0.18,
B: 0.0016 0.0255 1.4705 0.470 C1 -- -- -- -- Cu: 2.95, B: 0.003
0.0093 1.6163 0.287 C2 -- -- -- -- Cu: 2.98 0.0094 1.6154 0.299 D1
-- -- -- -- Cu: 2.44, Co: 0.36 0.0141 1.5731 0.318 D2 -- -- -- --
Cu: 0.77, Co0.28, B: 0.0020 0.0157 1.5587 0.360 D3 -- -- -- -- Cu:
1.55, Mo: 0.85, Co: 0.20 0.0142 1.5722 0.397 D4 -- -- -- -- Cu:
0.96, Co: 0.22 0.0143 1.5713 0.324 D5 -- -- -- -- Cu: 0.45, Mo:
0.41, Co: 0.14, B: 0.0020 0.0131 1.5821 0.353 F1 = S + {(P + Sn)/2}
+ {(As + Zn + Pb + Sb)/5} F2 = Nb + Ta + Zr + Hf + 2Ti + (V/10) The
mark * indicates falling outside the conditions regulated by the
present invention.
[0135] First, the steel plates made of the steels A1 to A10, B1 to
B5, C1 and C2 and D1 to D5 were machined for providing each of them
with a shape of V-groove with an angle of 30.degree. in the
longitudinal direction and a root thickness of 1 mm. Then each of
them was subjected to four side-restrained welding onto a
commercial SM400C steel plate, 25 mm in thickness, 200 mm in width
and 200 mm in length, as standardized in JIS G 3106 (2004) using
"DNiCrFe-3" defined in JIS Z 3224 (1999) as a covered
electrode.
[0136] Thereafter, each steel plate was subjected to multilayer
welding in the groove using a welding wire having the chemical
compositions shown in Table 3 by the TIG welding method under the
heat input condition of 20 kJ/cm.
TABLE-US-00003 TABLE 3 Chemical composition (% by mass) The
balance: Fe and impurities C Si Mn P S Ni Cr Nb N 0.032 0.32 1.5
0.015 0.003 6.95 19.37 0.38 0.19
[0137] After the above welding procedure, 10 test specimens for
microstructure observations of the joint section were taken from
each test object and were subjected to sectional mirror-like
polishing and then to etching and observed for the occurrence of
liquation cracking in the coarse-grained HAZ using an optical
microscope at a magnification of 500.
[0138] The results of the above-mentioned liquation cracking
investigation are shown in Table 4. The symbol "o" in the column
"liquation cracking" in Table 4 indicates that no liquation
cracking was observed in all the 10 test specimens for the relevant
steels and the symbol ".DELTA." indicates that cracking was
observed in one or two of the test specimens.
TABLE-US-00004 TABLE 4 Liquation Embrittling SSC Creep Test No.
Steel cracking cracking resistance characteristics Note 1 *A1
.smallcircle. .smallcircle. .smallcircle. .smallcircle. Reference 2
*A2 .smallcircle. .smallcircle. .smallcircle. .smallcircle. example
3 *A3 .smallcircle. .smallcircle. .smallcircle. .smallcircle. 4 *A4
.smallcircle. .smallcircle. .smallcircle. .smallcircle. 5 *A5
.smallcircle. .smallcircle. .smallcircle. .smallcircle. 6 *A6
.smallcircle. .smallcircle. .smallcircle. .smallcircle. 7 *A7
.smallcircle. .smallcircle. .smallcircle. .smallcircle. 8 *A8
.smallcircle. .smallcircle. .smallcircle. .smallcircle. 9 *A9
.smallcircle. .smallcircle. .smallcircle. .smallcircle. 10 *A10
.smallcircle. .smallcircle. .smallcircle. .smallcircle. 11 *B1
.DELTA. x .DELTA. .smallcircle. Comparative 12 *B2 .DELTA. .DELTA.
.DELTA. .smallcircle. example 13 *B3 .DELTA. .DELTA. x
.smallcircle. 14 *B4 .DELTA. .DELTA. x .smallcircle. 15 *B5
.smallcircle. .smallcircle. .smallcircle. x 16 *C1 .smallcircle.
.smallcircle. .smallcircle. .smallcircle. Reference 17 *C2
.smallcircle. .smallcircle. .smallcircle. .smallcircle. example 18
D1 .smallcircle. .smallcircle. .smallcircle. .smallcircle.
Inventive 19 D2 .smallcircle. .smallcircle. .smallcircle.
.smallcircle. example 20 D3 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. 21 D4 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. 22 D5 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. The mark * indicates falling outside
the conditions regulated by the present invention.
[0139] From Table 4, it is evident that no liquation cracking
occurred in Test Nos. 18 to 22 which are taken as inventive
examples and in which the steels D1 to D5 according to the present
invention were used.
[0140] The restraint-welded joint specimens obtained from the
steels A1 to A10, B1 to B5, C1 and C2 and D1 to D5 in the manner
mentioned above were subjected to aging heat treatment at
550.degree. C. for 10000 hours. In order to observe the
microstructure of the joint section, 4 test specimens were taken
from each test object. The section of each specimen was mirror-like
polished, then etched and observed for the occurrence of
embrittling cracking in the coarse-grained HAZ using an optical
microscope at a magnification of 500.
[0141] The results of the above-mentioned embrittling cracking
investigation are also shown in Table 4. The symbol "o" in the
column "embrittling cracking" indicates that no embrittling
cracking was observed in all the 4 test specimens for the relevant
steels. The symbol ".DELTA." indicates that cracking was observed
in one or two test specimens and the symbol "x" indicates that
cracking was observed in 3 or more test specimens.
[0142] From Table 4, it is evident that no embrittling cracking
also occurred in Test Nos. 18 to 22 which are taken as inventive
examples and in which the steels D1 to D5 according to the present
invention were used.
[0143] From the data given above, it is evident that, in order to
ensure the excellent liquation cracking resistance and the
excellent embrittling cracking resistance during a long period of
use in the HAZ, the conditions concerning not only the contents of
the respective component elements, but also the parameters F1 and
F2 should be satisfied.
[0144] Furthermore, welded joints were prepared from the steels A1
to A10, B1 to B5, C1 and C2 and D1 to D5 using the same welding
material under the same welding conditions as the above-mentioned
restraint-welded joints except that no restraint was applied. The
following test specimens were taken from each test object and
evaluated for corrosion resistance and the high temperature
strength characteristics (i.e. the "creep characteristics").
[0145] In order to investigate corrosion resistance, the so-called
"U-bend test specimens", namely rectangular shaped specimens, 2 mm
in thickness, 10 mm in width and 75 mm in length and restrained at
a radius of 5 mm with the site of welding as the center, were used.
They were immersed in the Wackenroder's solution (solution prepared
by blowing a large amount of H.sub.2S gas into a saturated aqueous
solution of H.sub.2SO.sub.3 prepared by blowing SO.sub.2 gas into
distilled water) at 700.degree. C. for 1000, 3000 or 5000 hours and
then observed under an optical microscope at a magnification of 500
for the occurrence of cracking to evaluate the polythionic acid SCC
resistance of each welded joint.
[0146] In order to investigate high temperature strength
characteristics, round bar creep test specimens having a parallel
portion, 6 mm in diameter and 60 mm in length, with the weld metal
as the center were used, and a creep rupture test was carried out
under conditions of 600.degree. C. and 200 MPa. When the fracture
time was not less than 5000 hours, the test specimen was judged
"acceptable" as capable of accomplishing the objective of the
present invention.
[0147] The results of the above-mentioned investigations of
polythionic acid SCC resistance and high temperature strength
characteristics (i.e. creep characteristics) are also shown in
Table 4. The column "SCC resistance" in Table 4 means the
above-mentioned polythionic acid SCC resistance, in which the
symbol ".smallcircle." means that no cracking occurred during 5000
hours of immersion. The symbol ".DELTA." means that cracking was
observed during 3000 hours of immersion and the symbol "x" means
that cracking was observed during 1000 hours of immersion. Further,
in the column "Creep characteristics", the symbol ".smallcircle."
means that the rupture time was not less than 5000 hours and the
symbol "x" means that the rupture time was less than 5000
hours.
[0148] As for the corrosion resistance, it was found from Table 4
that cracking occurred during 1000 hours of immersion in Test Nos.
13 and 14 which are taken as comparative examples and in which the
steels B3 and B4, having the contents of Nb and C exceed the upper
limits regulated by the present invention respectively, were used.
It was also found that, cracking occurred during 3000 hours of
immersion in Test Nos. 11 and 12 which are taken as comparative
examples and in which the steels B1 and B2, having the values of
parameter F1 and parameter F2 fall outside the range regulated by
the present invention respectively, were used. Therefore, it is
clear that these steels are inferior in corrosion resistance
(polythionic acid SCC resistance). As for the high temperature
strength characteristics, the rupture time was less than 5000 hours
in Test No. 15 which is taken as a comparative example and in which
the steel B5, having the N content less than the value regulated by
the present investigation, was used. Consequently, it is clear that
this steel is inferior in high temperature characteristics.
INDUSTRIAL APPLICABILITY
[0149] The austenitic stainless steels of the present invention
have excellent liquation cracking resistance and embrittling
cracking resistance in a weld zone, and moreover they have
excellent polythionic acid SCC resistance and high temperature
strength. Consequently, they can be used as raw materials for
various apparatuses which are used in a sulfide-containing
environment at high temperatures for a long period of time; for
example in power plant boilers, petroleum refining and
petrochemical plants and so on.
* * * * *