Note: Descriptions are shown in the official language in which they were submitted.
CA 02830586 2013-09-18
AUSTENITIC STAINLESS CAST STEEL
Technical Field
[0001]
The present invention relates to an austenitic stainless cast steel.
Background Art
[0002]
An austenitic stainless cast steel exhibits excellent properties especially in
corrosion
resistance, strength, weldability and the like, and has been widely used for
piping, valves and
the like in chemical plants and power plants. The austenitic stainless cast
steel is formed of,
for example from metallurgical viewpoint, two phases including approximately
10 - 20% of
an alpha phase and approximately 90 - 80% of a gamma phase (austenitic phase).
[0003]
As for steel castings of the austenitic stainless steel, CF8C has been known.
For
example, a CF8C austenitic stainless steel casting includes: up to 0.08
percent by mass of C
(carbon); up to 2.0 percent by mass of Si (silicon); up to 1.5 percent by mass
of Mn
(manganese); 18.0 - 21.0 percent by mass of Cr (chromium); 9.0 - 12.0 percent
by mass of Ni
(nickel); and up to 1.0 percent by mass of Nb (niobium).
[0004]
CF8C includes approximately 12.0% of a ferrite phase. The ferrite phase can
be,
for example, measured as ferrite content in the austenitic stainless steel
with a known ferrite
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scope, or calculated using a Schaeffler diagram based on component elements,
and is
indicated with volume fraction (percent (%)).
[0005]
The ferrite phase is considered effective for preventing weld cracking and
reducing
stress corrosion cracking. However, if a ferrite phase content is large, for
example, exposure
of CF8C to high temperature for a long period of time may transform the
ferrite phase into a
sigma phase (a phase), which is a compound of iron and chromium. This may lead
to
embrittlement of the steel casting.
[0006]
Patent Document 1 discloses CF8C-Plus, which is an alloy modified from CF8C,
and
describes that CF8C-Plus does not contain ferrite phase. Patent Document 1
also describes
that CF8C-Plus includes: 0.05 - 0.15 percent by mass of C; 0.2 - 1.0 percent
by mass of Si;
0.5 - 10.0 percent by mass of Mn; 18.0 - 25.0 percent by mass of Cr; 10.0-
15.0 percent by
mass of Ni; 0.1 - 1.5 percent by mass of Nb; and 0.05 -0.5 percent by mass of
N.
[0007]
In Patent Document 1, an absence of the ferrite phase from CF8C-Plus is
considered
important for retaining the properties imparted at casting of materials during
a life of the
component part produced from the materials.
[0008]
When CF8C is exposed to high temperature for a long period of time under usage
environment, the sigma phase is precipitated to cause aging embrittlement, and
thus aging
ductility may become poor. Also in the case of CF8C-Plus described in Patent
Document 1,
further improvement has been demanded in oxidation resistance.
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[0009]
Therefore, it has been desired to provide an austenitic stainless cast steel
exhibiting
excellent aging ductility and oxidation resistance.
Citation List
Patent Literature
[0010]
Patent Document 1: Japanese translation of a PCT application Kohyo No. 2009-
545675
Summary of Invention
[0011]
In order to provide such an austenitic stainless cast steel, the inventions of
the
following items (1) - (6) are provided.
(1) An austenitic stainless cast steel having a volume fraction of a
ferrite phase of 0.1 -
5.0%.
(2) The austenitic stainless cast steel according to item (1), including:
0.01 - 0.10 percent
by mass of C; 0.6 - 1.0 percent by mass of Si; 2.0- 2.8 percent by mass of Mn;
and 0.1 -0.4
percent by mass of N.
(3) The austenitic stainless cast steel according to item (1) or (2),
including: 18.0 - 24.0
percent by mass of Cr; 8.0 - 15.0 percent by mass of Ni; and 0.2 - 0.7 percent
by mass of Nb.
(4) An austenitic stainless cast steel, wherein a volume fraction of the
ferrite phase is 0.1
- 5.0%, and the cast steel includes: 0.01 - 0.10 percent by mass of C; 0.6 -
1.0 percent by mass
of Si; 2.0 - 2.8 percent by mass of Mn; 0.1 -0.4 percent by mass of N; 18.0 -
24.0 percent by
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mass of Cr; 8.0 - 15.0 percent by mass of Ni; 0.2 - 0.7 percent by mass of Nb;
and the balance
is Fe and inevitable impurities.
(5) The austenitic stainless cast steel according to any one of items (1) -
(4), obtained by
performing cooling from a temperature range of 1,150 - 1,350 C to a
temperature range of
600 - 800 C at a cooling rate of 30 C/min or more.
(6) A valve formed of austenitic stainless cast steel according to any one
of items (1) -
(5).
[0012]
The austenitic stainless cast steel of the present invention is excellent in,
for example,
aging ductility, tensile strength and oxidation resistance, as will be
described in Examples.
Especially, the aging ductility in Examples of the present invention was
approximately 2.4
times as high as that in Comparative Examples. Likewise, oxidation resistance
in Examples
of the present invention was approximately 9.5 times as high as that in
Comparative
Examples.
[0013]
The reason that the austenitic stainless cast steel exhibits such excellent
properties
seems to be that the volume fraction of the ferrite phase is 0.1 - 5.0%, and
the contents of the
components C, Si, Mn, Cr, Ni, Nb and N seem to play important roles.
Hereinbelow, each
component will be described in detail.
[0014]
By setting the volume fraction of the ferrite phase to 0.1 - 5.0%, even when
the cast
steel is exposed to high temperature for a long period of time, a
precipitation amount of the
sigma phase can be suppressed low. Since the precipitation amount of the sigma
phase is
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low, the austenitic stainless cast steel is unlikely to be embrittled, and
exhibits excellent aging
ductility.
[0015]
C has an effect of lowering a melting point and improving fluidity, i.e.
castability of
molten metal. In addition, it is preferable that the amount of C is low from
the viewpoint of
corrosion resistance, and if a large amount is added, the corrosion resistance
of the base metal
is reduced. In view of these, in order to improve high-temperature ductility,
an additive
amount of C in the present invention is set to 0.01 - 0.10 percent by mass.
[0016]
Si serves as deoxidizing agent for molten metal, and is effective for
improving
fluidity, oxidation resistance, and weldability. However, an excessive
addition will make the
austenitic structure unstable, leading to deterioration of castability, hinder
workability and
weldability, and promotion of weld cracking. Therefore, an additive amount of
Si in the
present invention is set to 0.6 - 1.0 percent by mass.
[0017]
Mn is effective as deoxidizing agent for molten metal, and enhances fluidity
during
the casting to thereby improve productivity. In addition, it is also effective
for reducing weld
cracking. Since an excessive addition will deteriorate oxidation resistance,
an additive
amount of Mn in the present invention is set to 2.0 - 2.8 percent by mass.
When Mn is in
this range, the austenitic stainless cast steel exhibiting excellent oxidation
resistance can be
obtained, as will be described in Examples.
[0018]
N improves high-temperature strength and thermal fatigue resistance, and is a
strong
' CA 02830586 2013-09-18
austenite forming element which stabilizes an austenitic matrix. In addition,
N is an element
effective for grain refining. With this grain refining, ductility of the
material which is
important as structure can be secured, and in addition, a drawback of poor
machinability,
which is specific in the austenitic stainless cast steel, can be improved.
Especially, N renders
excellent perforation machinability to a member to be perforated for
connecting parts. When
N is added in a large amount, embrittlement is promoted, while an effective Cr
amount is
reduced and thus oxidation resistance is deteriorated. Therefore, an additive
amount of N in
the present invention is set to 0.1 - 0.4 percent by mass.
[0019]
Cr improves oxidation resistance and stabilizes the ferrite structure. In
order to
reliably attain this effect, the amount of Cr is set to 18.0 percent by mass
or more. On the
other hand, an excessive addition will lower the aging ductility of the steel
due to excessive
precipitation of Cr carbide when the case steel is used at high temperature,
and thus the upper
limit of the Cr amount is set to 24.0 percent by mass.
[0020]
Ni facilitates the formation of the stable austenitic matrix, stabilizes the
austenitic
phase, and enhances high-temperature strength and oxidation resistance of the
steel. Taking
excellent castability, corrosion resistance and weldability into
consideration, an additive
amount of Ni in the present invention is set to 8.0 - 15.0 percent by mass.
[0021]
Nb binds with C to form fine carbide, and improves high-temperature strength.
In
addition, the formation of Cr carbide is suppressed, and thus oxidation
resistance can be
improved. In order to effectively exert these effects, the content of 0.2% or
more is required.
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However, when Nb is added in an excessive amount, heat cracking susceptibility
is notably
enhanced, and inner quality will be deteriorated. Therefore, an additive
amount of Nb in the
present invention is set to 0.2 - 0.7 percent by mass.
[0022]
In addition, the austenitic stainless cast steel of the present invention can
be produced
by performing cooling from a temperature range of 1,150 - 1,350 C to a
temperature range of
600 - 800 C at a cooling rate of 30 C/min or more. By producing the austenitic
stainless
cast steel of the present invention under the above-described conditions, even
when the cast
steel is left as-cast, excellent strength property can be obtained, and thus
solution heat
treatment can be omitted.
The produced austenitic stainless cast steel is used as, for example,
materials for
piping, valves and the like in chemical plants and power plants.
Brief Description of Drawings
[0023]
Fig. 1 is a graph showing results of oxidation resistance (mm/year) examined
with
respect to the austenitic stainless cast steel.
[0024]
Description of Embodiments
Hereinbelow, embodiments of the present invention will be described with
reference
to the drawings.
The austenitic stainless cast steel of the present invention is formed in such
a manner
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that the volume fraction of the ferrite phase becomes 0.1 - 5.0%, preferably
0.5 - 3.0%. The
austenitic stainless cast steel of the present invention includes C, Si, Mn,
Cr, Ni, Nb, N and
the like as components thereof
The contents are as follows:
C: 0.01 - 0.10 percent by mass, preferably 0.02 - 0.04 percent by mass;
Si: 0.6 - 1.0 percent by mass, preferably 0.7 - 0.9 percent by mass;
Mn: 2.0 - 2.8 percent by mass, preferably 2.2 - 2.4 percent by mass;
N: 0.1 - 0.4 percent by mass, preferably 0.15 - 0.25 percent by mass;
Cr: 18.0 -24.0 percent by mass, preferably 19.5 -21.5 percent by mass;
Ni: 8.0 - 15.0 percent by mass, preferably 10.5 - 12.5 percent by mass; and
Nb: 0.2 - 0.7 percent by mass, preferably 0.2 - 0.4 percent by mass.
The compositions (percent by mass) of the austenitic stainless cast steel of
the
present invention, and of CF8C and CF8C-Plus for comparison, are shown in
Table 1.
[0025]
[Table 1]
Austenitic stainless cast CF8C CF8C-Plus
steel of the present invention
Ferrite (volume fraction 0.1 - 5.0 12.0
(%))
C (percent by mass) 0.01 -0.10 Up to 0.08 0.05 - 0.15
Si (percent by mass) 0.6- 1.0 Up to 2.0 0.2- 1.0
Mn (percent by mass) 2.0 - 2.8 Up to 1.5 0.5- 10.0
Cr (percent by mass) 18.0 - 24.0 18.0 - 21.0 18.0 - 25.0
Ni (percent by mass) 8.0- 15.0 9.0- 12.0 10.0- 15.0
Nb (percent by mass) 0.2 - 0.7 Up to 1.0 0.1 - 1.5
N (percent by mass) 0.1 - 0.4 0.05 - 0.5
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[0026]
In the austenitic stainless cast steel of the present invention, by setting
the volume
fraction of the ferrite phase to 0.1 - 5.0%, even when the cast steel is
exposed to high
temperature for a long period of time, the precipitation amount of the sigma
phase can be
suppressed low. Therefore, the austenitic stainless cast steel of the present
invention is
unlikely to be embrittled, and exhibits excellent aging ductility.
In addition, the austenitic stainless cast steel of the present invention has
a higher Mn
content and a lower C content than those of CF8C. With this configuration, the
strength and
oxidation resistance at high temperature can be improved.
[0027]
In addition to the components described above, the austenitic stainless cast
steel of
the present invention may further include W, B, Al, Mo, Co, Ti, Zr, Cu, rare-
earth element (La,
Ce, Y, Pd, Nd and the like) or the like, and the balance is Fe and inevitable
impurities.
[0028]
The austenitic stainless cast steel of the present invention can be produced
by melting
the above-described metal components in a melting furnace and performing
cooling from a
temperature range of 1,150- 1,350 C to a temperature range of 600 - 800 C at a
cooling rate
of 30 C/min or more. By producing the austenitic stainless cast steel of the
present
invention under the above-described conditions, even when the cast steel is
left as-cast,
excellent strength property can be obtained, and thus solution heat treatment
can be omitted.
[0029]
The produced austenitic stainless cast steel is used, for example, for piping,
valves
and the like in chemical plants and power plants.
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Example 1
[0030]
Example of the present invention will be described.
The main components (percent by mass) of the austenitic stainless cast steel
of the
present invention (Examples 1-1 - 1-6) and CF8C (Comparative Examples 1-1 - 1-
5) are
shown in Tables 2 and 3, respectively.
[0031]
[Table 2]
Example
1-1 1-2 1-3 1-4 1-5 1-6
Ferrite (volume 0.2 0.2 0.2 0.2 0.2 0.2
fraction (%))
0.04 0.03 0.04 0.03 0.08 0.06
(percent by mass)
Si 0.76 0.86 0.76 0.86 0.89 0.86
(percent by mass)
Mn 2.07 2.15 2.07 2.15 2.07 2.12
(percent by mass)
Cr 20.55 19.90 20.55 19.90 22.35 22.10
(percent by mass)
Ni 11.38 11.12 11.38 11.12 10.50 10.34
(percent by mass)
Nb 0.27 0.26 0.27 0.26 0.29 0.32
(percent by mass)
0.21 0.20 0.21 0.20 0.19 0.21
(percent by mass)
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[0032]
[Table 3]
Comparative Example
1-1 1-2 1-3 1-4 1-5
Ferrite (volume fraction (%)) 12.0 9.0 0 0 0
C (percent by mass) 0.03 0.03 0.017 0.08 0.06
Si (percent by mass) 0.47 0.63 0.37 0.48 0.57
Mn (percent by mass) 1.04 4.48 1.83 1.02 2.02
Cr (percent by mass) 19.98 19.93 19.93 19.35 19.60
Ni (percent by mass) 9.92 9.45 11.63 11.49 11.55
Nb (percent by mass) 0.59 0.42 0.43 0.69 0.71
N (percent by mass) 0.03 0.10 0.24 0.25 0.24
[0033]
In these Examples and Comparative Examples, aging ductility (700 C - 620
hours),
tensile strength (900 C), 0.2% proof stress (900 C) and oxidation resistance
(1,000 C) were
examined, and further a high-temperature low-cycle fatigue test (alternate
triangular waves,
strain rate of 0.1%/sec, 700 C, total strain of 0.5%) was performed.
[0034]
It should be noted that both in Examples and Comparative Examples, casting was
performed using normal static casting method. In Examples 1 and 2, the cast
steel was left
as-cast, while in the other Examples and Comparative Examples, the cast steel
was subjected
to SHT (solution heat treatment). Aging ductility, tensile strength, 0.2%
proof stress, and
oxidation resistance were examined and the results are shown in Table 4.
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[0035]
[Table 4]
ductility strength stress resistance low-cycle
fatigue
(%) (Mpa) (Mpa) (mm/year) test (times)
Example 1-1 24.4 120 90 0.300
Example 1-2 28.8 125 87 0.370 6200
Example 1-3 24.0 113 91 0.066 3400
Example 1-4 29.2 134 89 0.122 2420
Example 1-5 20.4 131 91 0.489
Example 1-6 22.1 129 88 0.394
Comparative 17.2 93 70 1.278 2388
Example 1-1
Comparative 6.8 101 75 3.494
Example 1-2
Comparative 8.6 127 84 1.854
Example 1-3
Comparative 11.2 98 73 4.101
Example 1-4
Comparative 8.2 104 77 3.124
Example 1-5
[0036]
As a result, regarding aging ductility, Examples exhibited 20.4% or more,
while
Comparative Examples exhibited 17.2% or less.
Regarding tensile strength, Examples exhibited 113 - 134 Mpa, while
Comparative
Examples exhibited 93 - 127 Mpa.
Regarding 0.2% proof stress, Examples exhibited 87 - 91 Mpa, while Comparative
Examples exhibited 70 - 84 Mpa.
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Regarding oxidation resistance, Examples exhibited 0.489 mm/year or less,
while
Comparative Examples exhibited 1.278 mm/year or more.
[0037]
To sum up, though Examples and Comparative Examples were not notably
distinguishable in the 0.2% proof stress, it was found that Example exhibited
excellent result
in aging ductility, tensile strength and oxidation resistance. Especially, an
average value of
the aging ductility in Examples was 24.8%, while an average value in
Comparative Examples
was 10.4%, and thus the value in Example was approximately 2.4 times as high
as that in
Comparative Example. Likewise, an average value of oxidation resistance in
Examples was
0.290mm/year, while an average value in Comparative Examples was 2.770
mm/year, and
thus the value in Example was improved approximately 9.5 times as much as that
in
Comparative Example.
The above-described results shows the case where the volume fraction of the
ferrite
phase of the austenitic stainless cast steel of the present invention was
0.2%, and it is
considered that similar results will be obtained when a lower limit of the
volume fraction of
the ferrite phase is set to 0.1%.
Example 2
[0038]
In Example 1, the volume fraction of the ferrite phase of the austenitic
stainless cast
steel of the present invention was 0.2% (Examples 1-1 - 1-6). In addition,
also for a case in
which the volume fraction of the ferrite phase is 1 - 3%, aging ductility,
tensile strength, 0.2%
proof stress and oxidation resistance were examined (Examples 2-1 - 2-4) under
the same
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condition for Example 1. The components of Examples 2-1 - 2-4 are shown in
Table 5, and
the results are shown in Table 6.
[0039]
[Table 5]
Example
2-1 2-2 2-3 2-4
Ferrite (volume fraction (%)) 2 1 3 1
C (percent by mass) 0.014 0.013 0.020 0.013
Si (percent by mass) 0.67 0.72 0.62 0.72
Mn (percent by mass) 2.26 2.37 2.00 2.22
Cr (percent by mass) 21.10 21.10 21.70 22.22
Ni (percent by mass) 11.29 11.38 12.09 11.54
Nb (percent by mass) 0.29 0.29 0.27 0.27
N (percent by mass) 0.22 0.23 0.16 0.23
[0040]
[Table 6]
Aging ductility Tensile 0.2% proof Oxidation
(%) strength (Mpa) stress (Mpa) resistance
(mm/year)
Example 2-1 27.0 128 89 0.006
Example 2-2 24.0 123 88 0.058
Example 2-3 27.0 95 63 0.558
Example 2-4 20.4 137 88 0.015
[0041]
As a result, an average value of aging ductility in Examples 2-1 - 2-4 was
24.6%, and
an average value of oxidation resistance was 0.159 mm/year. Like in Example 1,
these
values are recognized as being excellent over the values in Comparative
Example. It is
considered that similar results will be obtained when an upper limit of the
volume fraction of
the ferrite phase of the austenitic stainless cast steel of the present
invention is set to 5%.
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Example 3
[0042]
With respect to the austenitic stainless cast steel whose Mn content was
approximately 1.0 - 4.5 percent by mass, oxidation resistance (mm/year) was
examined. As
the austenitic stainless cast steel of the present invention, those with the
Mn content of 2.26
percent by mass (Example 3-1) and 2.33 percent by mass (Example 3-2) were
used. As the
austenitic stainless cast steel of Comparative Example, those with the Mn
content of 1.04
percent by mass (Comparative Example 3-1), 1.17 percent by mass (Comparative
Example
3-2), 1.81 percent by mass (Comparative Example 3-3), 4.37 percent by mass
(Comparative
Example 3-4), and 4.48 percent by mass (Comparative Example 3-5) were used.
The
components for these Examples and Comparative Examples are shown in Table 7.
The
results are shown in Table 8 and Fig. 1.
[0043]
[Table 7]
Example Comparative
Example
3-1 3-2 3-1 3-2 3-3 3-4 3-5
Ferrite (volume fraction 2 3 12 8 0.2 10 9
(%))
C (percent by mass) 0.03 0.03 0.03 0.03 0.017 0.03
0.03
Si (percent by mass) 0.65 0.64 0.47 0.61 0.36 0.62 0.63
Mn (percent by mass) 2.26 2.33 1.04 1.17 1.81 4.37 4.48
Cr (percent by mass) 20.45 20.47 19.98 20.09 19.87 19.87
19.93
Ni (percent by mass) 11.35 11.33 9.92 9.92 12.49 9.35
9.45
Nb (percent by mass) 0.65 0.62 0.59 0.62 0.29 0.66 0.42
N (percent by mass) 0.14 0.12 0.03 0.12 0.20 0.10 0.10
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[0044]
[Table 8]
Oxidation resistance (mm/year)
Example 3-1 0.5062
Example 3-2 0.4521
Comparative Example 3-1 1.2782
Comparative Example 3-2 2.6405
Comparative Example 3-3 1.7060
Comparative Example 3-4 3.6345
Comparative Example 3-5 3.4943
[0045]
As can be seen in Fig. 1, in the austenitic stainless cast steel of the
present invention
where the Mn content is 2.0 - 2.8 percent by mass, oxidation resistance can be
reduced to 1
mm/year or less.
Industrial applicability
[0046]
The present invention is applicable to the production of the austenitic
stainless cast
steel.
16
