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.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority benefits under 35 USC § 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

1. Field of the Invention

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.

2. Description of the Related Art

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.

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.

The contents of individual elements of the proposed stainless steel are optimized to reduce the likelihood of a σ phase, carbides and nitrides forming during the fabrication of a thick cast product, such as a propeller blade for a large marine vessel. The σ 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.

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

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.

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.

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.

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.

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.

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.

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%.

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%.

In some configurations, the duplex stainless steel comprises a melting point of not higher than about 1,450° 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

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.

FIG. 1 is a plan view that schematically illustrates a propeller of a small watercraft.

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

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.

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° C. and then leaving as it is for heat release.

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.

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.

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 %.

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 %.

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 %.

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 %.

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 %.

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 %.

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.

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.

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×% Si+0.5×% Nb  (1)

Ni equivalent=% Ni+30×% C+0.5×% Mn+30×% 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.

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.

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° C. or lower, more preferably about 1,430° 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.

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.

The above-described duplex stainless steel advantageously has a melting point of about 1450° 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.

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.

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 (α phase) and austenite phase (γ phase) were evaluated by actual measurement and by simulation. The results are shown in Table 1.

	TABLE 1
									Liquid
									phase line
									temperature	α	γ
	C	Si	Mn	Ni	Cr	Cu	Mo	N	(° 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

Test pieces formed of Example 6 and Comparative Example 1 were tested for tensile strength and impact resistance in the following manners.

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 2
	Tensile		Elongation
	strength	0.2% Proof	after fracture	Impact strength
	(MPa)	strength (MPa)	(%)	(J/cm2)
Comparative	654	430	12.7	66.4
Example 1
Example 6	734	450	11.3	105

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.

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.

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° 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.

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.

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° 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° 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.