U.S. patent application number 11/767072 was filed with the patent office on 2008-07-24 for duplex stainless steel and cast article thereof.
This patent application is currently assigned to YAMAHA MARINE KABUSHIKI KAISHA. Invention is credited to Takanobu Suzuki.
Application Number | 20080175742 11/767072 |
Document ID | / |
Family ID | 39167477 |
Filed Date | 2008-07-24 |
United States Patent
Application |
20080175742 |
Kind Code |
A1 |
Suzuki; Takanobu |
July 24, 2008 |
DUPLEX STAINLESS STEEL AND CAST ARTICLE THEREOF
Abstract
A generally Mo-free, ferrite-austenite duplex stainless steel
has a ferrite phase area ratio of about 20 to about 60% and a
composition containing, in mass %, not more than about 0.08% of C,
about 0.5 to about 1.5% of Si, not more than about 1.0% of Mn,
about 4.0 to about 8.0% of Ni, about 23 to about 27% of Cr, about
2.0 to about 6.0% of Cu, about 0.05 to about 0.3% of N, and the
balance being Fe and generally unavoidable impurities.
Inventors: |
Suzuki; Takanobu;
(Shizuoka-ken, JP) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET, FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Assignee: |
YAMAHA MARINE KABUSHIKI
KAISHA
Shizuoka-ken
JP
|
Family ID: |
39167477 |
Appl. No.: |
11/767072 |
Filed: |
June 22, 2007 |
Current U.S.
Class: |
420/49 ; 420/60;
440/49 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/42 20130101; C22C 38/04 20130101; C22C 38/001 20130101 |
Class at
Publication: |
420/49 ; 420/60;
440/49 |
International
Class: |
C22C 38/42 20060101
C22C038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 23, 2007 |
JP |
2007-012845 |
Claims
1. A duplex stainless steel characterized by comprising Fe as a
major component and further containing C, Si, Mn, Ni, Cr, Cu, Ni
and generally unavoidable impurities, and comprising a ferrite
phase and an austenite phase with an area ratio of the ferrite
phase being not less than about 20% but not more than about
60%.
2. A cast article of a duplex stainless steel comprising the duplex
stainless steel according to claim 1.
3. A cast article in the form of a propeller blade for a propulsion
unit of a small watercraft, the cast article comprising the duplex
stainless steel according to claim 2.
4. A duplex stainless steel according to claim 1, characterized by
having a melting point of not higher than about 1,450.degree. C.,
and characterized by being used as a casting material of a cast
article comprising a portion having a thickness of about 3 mm or
less.
5. A cast article of a duplex stainless steel comprising the duplex
stainless steel according to claim 4.
6. A cast article in the form of a propeller blade for a propulsion
unit of a small watercraft, the cast article comprising the duplex
stainless steel according to claim 5.
7. A duplex stainless steel characterized by comprising, in mass %:
up to about 0.08% of C, not less than about 0.5% but not more than
about 1.5% of Si, up to about 1.0% of Mn, not less than about 4.0%
but not more than about 8.0% of Ni, not less than about 23% but not
more than about 27% of Cr, not less than about 2.0% but not more
than about 6.0% of Cu, not less than about 0.05% but not more than
about 0.3% of N, and the balance being Fe and generally unavoidable
impurities, and characterized by comprising a ferrite phase and an
austenite phase with an area ratio of the ferrite phase being not
less than about 20% but not more than about 60%.
8. A cast article of a duplex stainless steel comprising the duplex
stainless steel according to claim 7.
9. A cast article in the form of a propeller blade for a propulsion
unit of a small watercraft, the cast article comprising the duplex
stainless steel according to claim 8.
10. A duplex stainless steel according to claim 7, characterized by
having a melting point of not higher than about 1,450.degree. C.,
and characterized by being used as a casting material of a cast
article comprising a portion having a thickness of about 3 mm or
less.
11. A cast article of a duplex stainless steel comprising the
duplex stainless steel according to claim 10.
12. A cast article in the form of a propeller blade for a
propulsion unit of a small watercraft, the cast article comprising
the duplex stainless steel according to claim 11.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims foreign priority benefits under 35
USC .sctn. 119(a)-(d) from Japanese Patent Application No.
2007-012845, filed on Jan. 23, 2007, which is hereby incorporated
by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to a duplex
stainless steel having a ferrite phase and an austenite phase and
also relates to a cast article of the duplex stainless steel.
[0004] 2. Description of the Related Art
[0005] A two-phase stainless steel having a ferrite phase and an
austenite phase, also referred to as duplex stainless steel, is now
used as a raw material of various members because of its excellent
corrosion resistance.
[0006] JP-B-3270498 proposes a two-phase stainless steel for a
large-sized article. The proposed stainless steel is composed of up
to 0.02 mass % of C, up to 2.0 mass % of Si, up to 2.0 mass % of
Mn, up to 0.04 mass % of P, up to 0.04 mass % of S, 3 to 7 mass %
of Ni, 17 to 27 mass % of Cr, 0.5 to 6.0 mass % of Mo, 1 to 5 mass
% of Cu, up to 3 mass % of W, 0.05 to 0.3 mass % of N, 0.0005 to
0.0015 mass % of B and the balance being Fe.
[0007] The contents of individual elements of the proposed
stainless steel are optimized to reduce the likelihood of a .sigma.
phase, carbides and nitrides forming during the fabrication of a
thick cast product, such as a propeller blade for a large marine
vessel. The .sigma. phase, carbides and nitrides can form because
of a slow cooling rate due to the large thickness of the cast
product. The optimization adversely affects both corrosion
resistance and toughness of the resulting product.
[0008] JP-A-H09-302446 proposes a two-phase stainless steel having
high mechanical strength and corrosion resistance against seawater.
The disclosed stainless steel is composed of up to 0.06 mass % of
C, 1.5 to 3.5 mass % of Si, 0.1 to 3.0 mass % of Mn, 2 to 8 mass %
of Ni, 18 to 28 mass % of Cr, 0.1 to 0.9 mass % of Mo, 0.03 to 0.2
mass % of N, and the balance being Fe.
SUMMARY OF THE INVENTION
[0009] In the known two-phase stainless steel, the austenite and
ferrite phases are formed by using Cr and various other elements.
Namely, the inclusion of various elements within respective ranges
required is essential in order to form the austenite and ferrite
phases within a desired ratio thereof.
[0010] Of such elements, similar to Cr, Mo is also an essential
element to improve corrosion resistance, especially corrosion
against a reducing environment, of the two-phase stainless steel.
However, Mo is not only expensive but also tends to adversely
affect mechanical properties, such as toughness of the stainless
steel, as the content thereof increases. Although an attempt has
been made to reduce the Mo content in the stainless steel disclosed
in the aforementioned JP-A-H09-302446, Mo continues to be used in
order to obtain the desired corrosion resistance.
[0011] Stainless steel has been hitherto used for a cast article
having a thinner portion thereof, such as a propeller blade of a
propulsion unit for small watercrafts. In producing such a cast
article having a thinner portion, a melt poured into a mold is
liable to be cooled at the section of the mold containing the
thinner portion and the fluidity of the melt is therefore apt to be
reduced there. When the difference between the pouring temperature
and the melting point of the stainless steel is small, therefore,
the melt cannot flow sufficiently into the thinner portion forming
section. This is likely to cause defects, such as incomplete
filling, in the cast product.
[0012] If the pouring temperature is increased to ensure a
sufficient difference from the melting point to reduce the
likelihood of incomplete filling, then the temperature of the melt
introduced into the mold will be also increased correspondingly,
resulting in application of a great thermal load to the mold and
generation of a gas during casting operation. The use of such a
high temperature melt is therefore disadvantageous.
[0013] It is therefore an object of certain features, aspects and
advantages of the present invention to provide a duplex stainless
steel that is substantially free of Mo, that uses a reduced number
of kinds of elements, and that shows satisfactory mechanical
strength and corrosion resistance.
[0014] It is another object of certain features, aspects and
advantages of the present invention to provide a duplex stainless
steel capable of providing a wide temperature range over which the
melt thereof has a suitable fluidity without increasing the pouring
temperature and, therefore, capable of easily affording a cast
article having a thinner portion.
[0015] To solve the problems mentioned above, an embodiment of a
duplex stainless steel can be characterized by having Fe as a major
component and further containing C, Si, Mn, Ni, Cr, Cu, Ni and some
generally unavoidable impurities. The duplex stainless steel also
can comprise a ferrite phase and an austenite phase with an area
ratio of the ferrite phase being not less than about 20% but not
more than about 60%.
[0016] In some configurations, the duplex stainless steel
comprises, in mass up to about 0.08% of C, not less than about 0.5%
but not more than about 1.5% of Si, up to about 1.0% of Mn, not
less than about 4.0% but not more than about 8.0% of Ni, not less
than about 23% but not more than about 27% of Cr, not less than
about 2.0% but not more than about 6.0% of Cu, not less than about
0.05% but not more than about 0.3% of N, and the balance being Fe
and generally unavoidable impurities, and comprising a ferrite
phase and an austenite phase with an area ratio of the ferrite
phase being not less than about 20% but not more than about
60%.
[0017] In some configurations, the duplex stainless steel comprises
a melting point of not higher than about 1,450.degree. C., and is
used as a casting material of a cast article comprising a portion
having a thickness of about 3 mm or less. Thus, in some
configurations, the cast article is formed of duplex stainless
steel as described herein. In one embodiment, the cast article can
comprise a propeller blade for a propulsion unit of a small
watercraft.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] These and other features, aspects and advantages of
embodiments of the present invention will be described below with
reference to the attached drawings. The drawings comprise two
figures.
[0019] FIG. 1 is a plan view that schematically illustrates a
propeller of a small watercraft.
[0020] FIG. 2 is a phase diagram that schematically illustrates
phase constitution as a function of the Ni equivalent vs. Cr
equivalent.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0021] With reference now to FIG. 1, a propeller 10 is formed of a
certain duplex stainless steel. The propeller comprises a central
base section 11 and blades 12 that extend in radial, opposing
directions from the base section 11. The blades 12 are formed in
one body with the central base section 11. Each blade 12 preferably
comprises a wide surface portion that has a thickness of about 3 mm
or less, and more preferably about 2 mm or less.
[0022] The propeller 10 can be formed using a casting mold that
includes two hollow potions, one for forming the central base
portion 11 and the other for forming the blades 12. The two hollow
portions are positioned such that they are in communication with
each other. The melt to be poured into the casting mold includes
constituent elements whose amounts are selected to obtain a
specified duplex stainless steel of which the propeller is formed.
The propeller 10 is formed by pouring the melt into the casting
mold at a pouring temperature of, for example, about 1,550 to about
1,650.degree. C. and then leaving as it is for heat release.
[0023] In one preferred embodiment, the duplex stainless steel used
in casting the propeller 10 comprises C, Si, Mn, Ni, Cr, Cu, Ni and
the balance includes Fe and some generally unavoidable impurities.
Thus, the stainless steel is substantially free of expensive Mo.
Yet, the duplex stainless steel has a ferrite phase and an
austenite phase and a reduced melting point.
[0024] Carbon (C) has been selected because it is highly effective
in forming a generally stable austenite phase. Carbon also improves
the strength of the duplex stainless steel. If the carbon content
is excessively high, however, chromium carbide is apt to be formed,
which reduces the corrosion resistance of the steel and, further,
the steel becomes brittle. Additionally, as the carbon content
increases, the melting point of the steel decreases but the
strength of the steel is adversely affected. Accordingly, the
carbon content preferably is low, i.e. up to about 0.08 mass % in
one preferred configuration.
[0025] Silicon (Si) is a deoxidizer and is somewhat effective in
stabilizing the ferrite phase. Further, the melting point of the
steel decreases with an increase of the silicon content. Since Mo,
which is a ferrite-stabilizing element, is not used in the
preferred embodiment, the use of a relatively large amount of
silicon is desired. Accordingly, the silicon content is preferred
to be high, i.e. not less than about 0.5 mass % but not more than
about 1.5 mass %.
[0026] Manganese (Mn) is a deoxidizer and, as well as nickel,
contributes to an increase of solid solution of nitrogen in the
duplex stainless steel. Manganese is also less effective to
stabilize the austenite phase. The melting point of the stainless
steel decreases with an increase in the manganese content. The use
of manganese in an excessive amount, however, adversely affects
corrosion resistance, such as pitching corrosion resistance.
Accordingly, the manganese content is preferably up to about 1.0
mass %.
[0027] Nickel (Ni) improves mechanical properties and moldability,
helps to maintain corrosion resistance and helps to stabilize an
austenite phase. Nickel also has a small influence upon the melting
point of the stainless steel depending upon the amount of nickel
used. Because Mo, which is a ferrite-stabilizing element,
preferably is not used in the preferred embodiment, nickel, which
is an austenite stabilizing element and which has a small influence
upon the melting point, preferably is not used in a large amount.
Accordingly, the nickel content preferably is low, i.e. not less
than about 4.0 mass % but not more than about 8.0 mass %.
[0028] Chromium (Cr) a main component that contributes to impart
corrosion resistance to the duplex stainless steel and stabilizes
the ferrite phase. Higher chromium content results in a better
corrosion resistance due to an improved stability of a passive
film. Chromium also has a small influence upon the melting point of
steel depending upon the amount of chromium. Because Mo, which is a
ferrite-stabilizing element and which contributes to corrosion
resistance, preferably is not used, chromium, which is a ferrite
stabilizing element and which contributes to improved corrosion
resistance, preferably is used in a large amount. However, too
large an amount of chromium adversely affects the mechanical
properties and moldability. Accordingly, the chromium content
preferably is not less than about 23 mass % but not more than about
27 mass %.
[0029] Copper (Cu) imparts corrosion resistance to the duplex
stainless steel. The melting point of the steel can be reduced by
increasing the copper content. Since Mo, which improves corrosion
resistance, preferably is not used, the copper content preferably
is high. However, too high a copper content can cause the steel to
become brittle. Accordingly, the copper content preferably is not
less than about 2.0 mass % but not more than about 6.0 mass %.
[0030] Nitrogen (N) can increase the strength of the duplex
stainless steel even in a small amount and is highly effective in
stabilizing an austenite phase. The nitrogen content has little
influence upon the melting point of the steel. Too large an amount
of nitrogen is undesirable because of the precipitation of
nitrides. Accordingly, the nitrogen content is preferably not less
than about 0.05 mass % but not more than about 0.3 mass %.
[0031] The balance of the duplex stainless steel is Fe and some
generally unavoidable impurities such as phosphorus (P) and sulfur
(S). Such impurities may possibly include Mo. As long as the amount
of impurity Mo is not more than about 0.3 mass %, it may be further
removed, or may be used as it is without removal, because the Mo
content, which is an unavoidable impurity, is very small.
[0032] To obtain the duplex stainless of the present invention, the
contents of the components described above are selected within the
respective ranges thereof to adjust the area ratio of the ferrite
phase and the austenite phase. The area ratio of the ferrite phase
should be not less than about 20% but not more than about 60% in
order for the stainless steel to exhibit well balanced corrosion
resistance, particularly pitting corrosion resistance and
mechanical strength.
[0033] The area ratio of the ferrite phase varies depending upon
the cooling rate and other production conditions but the area ratio
may be suitably controlled by adjustment of mixing proportions of
the elements of the duplex stainless steel in terms of the chromium
equivalent (ferrite forming elements) and nickel equivalent
(austenite forming elements), which may be expressed by the
following formulas (1) and (2), for example:
Cr equivalent=% Cr+% Mo+1.5.times.% Si+0.5.times.% Nb (1)
Ni equivalent=% Ni+30.times.% C+0.5.times.% Mn+30.times.% N (2)
wherein individual % elements show the contents of these elements
in terms of mass %, and % Nb (Niobium) is taken into account only
when it is present.
[0034] FIG. 2 shows a schematic phase diagram that gives the area
ratio of the ferrite phase as a function of the nickel equivalent
compared to the chromium equivalent. In accordance with certain
features, aspects and advantages of the present invention, the
proportion of the constituent elements is adjusted so that the
chromium equivalent and nickel equivalent fall within the region S
where the area ratio of the ferrite phase is not less than about
20% but not more than about 60%. Because the strength is apt to be
reduced due to an excessively small area ratio of the ferrite phase
while the corrosion resistance is apt to deteriorate due to
excessive large area ratio of the ferrite phase.
[0035] Further, the area ratios of the ferrite phase and the
austenite phase are adjusted in such a manner described above and
the contents of the component elements also described above are
adjusted so that the melting point of the duplex stainless steel of
the embodiment is preferably to about 1,450.degree. C. or lower,
more preferably about 1,430.degree. C. or lower. The melting point
preferably is as low as possible. If the melting point is
determined to be excessively high, then the temperature of the melt
must be increased. Otherwise the fluidity of the melt would be
reduced, which would cause difficulties when forming the thin
portion of the casting.
[0036] The duplex stainless steel described above contains specific
content of C, Si, Mn, Ni, Cr, Cu, N and Fe, and unavoidable
impurities. The described stainless steel also has the ferrite
phase and the austenite phase. Moreover, the described stainless
steel has an area ratio of the ferrite phase in a range between
about 20% and about 60%. Therefore, the duplex stainless steel
described above has well-balanced mechanical strength and corrosion
resistance even though Mo is not added thereto. Because the
propeller 10 is designed to be brought in contact with water or
seawater, a duplex stainless steel is desired that, while being
inexpensive, is durable in practical use with fewer kinds of
component elements.
[0037] The above-described duplex stainless steel advantageously
has a melting point of about 1450.degree. C. or less and a wider
range of temperature in which the melt can flow can be easily
secured without increasing the pouring temperature. The thermal
load applied to the casting mold therefore is not necessarily
increased and the fluidity of the melt can be improved.
Accordingly, molding defects are less likely to occur in the blades
12 or other reduced thickness regions even though the blades 12
have a portion that is less than about 3 mm thick.
[0038] The duplex stainless steel propeller 10 can be fabricated
inexpensively because of less elements that make up the stainless
steel mixture and the duplex stainless steel has a good melt
fluidity, which reduces the likelihood of casting defects in the
blades 12. Mechanical strength and corrosion resistance can be
secured that are sufficient to resist stresses generated that
correspond to a propulsive force in normal temperature water and
seawater.
[0039] Stainless steels containing components shown in Table 1 and
the balance containing Fe and generally unavoidable impurities were
prepared and their liquid phase line temperatures (as melting
points) and area ratios of the ferrite phase (.alpha. phase) and
austenite phase (.gamma. phase) were evaluated by actual
measurement and by simulation. The results are shown in Table
1.
TABLE-US-00001 TABLE 1 Liquid phase line temperature .alpha.
.gamma. C Si Mn Ni Cr Cu Mo N (.degree. C.) phase phase Comparative
0.054 0.7 0.58 8.87 22.98 0.15 3.31 0.13 1453 -- -- Example 1
Comparative 0.05 0.8 0.8 7 25.00 2.5 3.00 0.15 1425 41.8 57 Example
2 Example 1 0.05 0.8 0.8 7 25.00 2.5 0 0.15 1425 24.4 74.6 Example
2 0.05 0.8 0.8 7 25.00 4 0 0.15 1404 30.2 65.2 Example 3 0.05 1.2
0.8 6 25.00 4 0 0.15 1409 40.4 54.7 Example 4 0.05 1.2 0.8 6 25.00
4 0 0.2 1405 34.4 60.4 Example 5 0.05 1.2 0.8 6 25.00 4 0 0.3 1399
28.8 65.8 Example 6 0.050 1.17 0.82 5.59 25.10 4.00 0 0.15 -- 50 50
Note 1: % mass for element
Note 2: measurement value as to Comparative Example 1 and Example
1
[0040] Test pieces formed of Example 6 and Comparative Example 1
were tested for tensile strength and impact resistance in the
following manners.
[0041] Using the similarly shaped test pieces, a tensile test was
carried out using the metallic material tensile test in accordance
with JIS Z2371. In addition, using similarly shaped test pieces, an
impact test was carried out by metallic material impact test in
accordance with JIS Z2371. The test results are summarized in Table
2.
TABLE-US-00002 TABLE 2 Tensile Elongation strength 0.2% Proof after
fracture Impact strength (MPa) strength (MPa) (%) (J/cm.sup.2)
Comparative 654 430 12.7 66.4 Example 1 Example 6 734 450 11.3
105
[0042] As is evident from the results shown in Table 2, the
generally Mo-free duplex stainless steel of Example 6 has an equal
or greater tensile strength and impact strength as compared with
the Mo-containing stainless steel of Comparative Example 1.
[0043] Using the stainless steels of Example 6 and Comparative
Example 1, propellers as shown in FIG. 1 were prepared by casting.
The minimum thickness of the blades 12 of the propeller 10 was 1.6
mm. As a result, a good cast propeller could be obtained using the
stainless steel of Example 6. On the other hand, due to a high
melting point, the blades 12 made of the stainless steel of
Comparative Example 1 had casting defects when the same pouring
temperature was used to thereby reduce the temperature range. Thus,
it was revealed that the stainless steel of Example 6 was able to
give a thin cast article more easily than that of Comparative
Example 1.
[0044] The propellers 10 made of the stainless steels of Example 6
and Comparative Example 1 were each subjected to a corrosion test.
An aqueous brine solution spray test as the corrosion test was
carried out under conditions in accordance with JIS Z 2371. Thus, a
5% by weight aqueous brine solution having a temperature of
35.degree. C. was sprayed over the test piece. The test piece was
then allowed to stand for 4 days to check rust formation with
unaided eyes. No rust was observed on surfaces of the test pieces
of Example 6 and Comparative Example 1. Thus, it was revealed that
the stainless steel of Example 6 has corrosion resistance similar
to the stainless steel Comparative Example 1.
[0045] Although the present invention has been described in terms
of certain embodiments, other embodiments apparent to those of
ordinary skill in the art also are within the scope of this
invention. Thus, various changes and modifications may be made
without departing from the spirit and scope of the invention. For
instance, various components may be repositioned as desired.
Moreover, not all of the features, aspects and advantages are
necessarily required to practice the present invention.
Accordingly, the scope of the present invention is intended to be
defined only by the claims that follow.
* * * * *