EP1956109B1

EP1956109B1 - Two-phase stainless steel

Info

Publication number: EP1956109B1
Application number: EP08001223.0A
Authority: EP
Inventors: Takanobu c/o Yamaha Marine Kabushiki Kaisha Suzuki
Original assignee: Yamaha Motor Co Ltd
Current assignee: Yamaha Motor Co Ltd
Priority date: 2007-01-23
Filing date: 2008-01-23
Publication date: 2014-05-07
Anticipated expiration: 2028-01-23
Also published as: CA2592890A1; US20080175742A1; CN101230435A; JP2008179844A; EP1956109A1; CN101230435B

Description

The present invention relates to a method to manufacture a cast article of a two-phase stainless steel having a ferrite phase and an austenite phase and also relates to a cast article of the two-phase stainless steel produced by the method.
The document EP-A-0 261 346 discloses a highly pitting residual duplex stainless steel alloy, which comprises, In weight percentage, C: 0.10% and below; Si: 1.5% and below; Mn: 2.0% and below; Cr: 25.0% to 27.0%; Ni: 5.0% to 7.5%; Cu: 1.5% to 3.5%; N: 0.15% and below; Mo: 0.5% and below; and the remaining portion being substantially Fe. According to EP-A-0 261 345 a preferred composition of the highly pitting residual duplex stainless steel alloy comprises, In weight percentage, C: 0.02%; Si: 0.5%; Mn: 0.8%; Cr: 25.7%; Ni: 6.0%; Cu: 2.8%; N: 0.07%; Mo: 0.5% max; and the remaining portion being substantially Fe.
A two-phase stainless steel having a ferrite phase and an austenite phase is now used as a raw material of various members because of its excellent corrosion resistance.
Patent Document 1 as specified later 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 prevent a σ phase, carbides and nitrides from being formed during the fabrication of a thick cast product such as a propeller blade for a large marine vessel because of slow cooling rate, to adversely affect both corrosion resistance and toughness of the product.
Patent Document 2, as specified later, 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.

Patent Document 1: JP-B-3270498
Patent Document 2: JP-A-H09-302446

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 the mechanical property 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 aforementioned JP-A-H09-302446 , Mo must be inevitably used in order to obtain the desired corrosion resistance.
A 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 thinner potion forming section of the mold and its fluidity 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 for prevention of occurrence of the 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 the present invention to provide a cast article comprising a two-phase stainless steel which is substantially free of Mo, which uses a reduced number of kinds of elements, and which shows satisfactory mechanical strength and corrosion resistance and a method for manufacturing such cast article of a two-phase stainless steel.
According to the present invention said object is solved by a cast article of a two-phase stainless steel having the features of independent claim 1. Moreover said object is also solved by a method to manufacture a cast article cast article according to claim 3.
Accordingly, it is provided a cast article in the form of a propeller blade for a propulsion unit of a small watercraft, wherein the cast article comprises a two-phase stainless steel having a ferrite phase and an austenite phase containing specific contents of C, Si, Mn, Ni, Cr, Cu, N and Fe, and unavoidable impurities, and having an area ratio of the ferrite phase in a range between 20% and 60%.
With the present teaching, there is provided a cast article of a two-phase 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.
The specific contents in mass % are: up to 0.08 % of C, not less than 0.5 % but not more than 1.5 % of Si, up to 1.0 % of Mn, not less than 4.0 % but not more than 8.0 % of Ni, not less than 23 % but not more than 27 % of Cr, not less than 2.0 % but not more than 6.0 % of Cu, not less than 0.05 % but not more than 0.3 % of N, and the balance being Fe and unavoidable impurities.
Further, preferably the two-phase stainless steel has a melting point of not higher than 1,450°C, and being used as a casting material of a cast article comprising a portion having a thickness of 3mm or less.
Accordingly, it is also provided a method to manufacture a cast article in the form of a propeller blade for a propulsion unit of a small watercraft, wherein the cast article comprises a two-phase stainless steel having a ferrite phase and an austenite phase, by preparing a melt including specific contents of C, Si, Mn, Ni, Cr, Cu, N and Fe, and substantially free of Mo addition, and adjusting an area ratio of the ferrite phase in a range between 20% and 60%.
The melt is prepared with the specific contents in mass %: up to 0.08 % of C, not less than 0.5 % but not more than 1.5 % of Si, up to 1.0 % of Mn, not less than 4.0 % but not more than 8.0 % of Ni, not less than 23 % but not more than 27 % of Cr, not less than 2.0 % but not more than 6.0 % of Cu, not less than 0.05 % but not more than 0.3 % of N, and the balance being Fe and unavoidable impurities.
In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with the accompanying drawings, wherein:

FIG. 1: is a plan view, schematically illustrating a propeller of a small watercraft, and
FIG. 2: is a phase diagram, schematically showing phase constitution as a function of the Ni equivalent vs. Cr equivalent.

Description of Reference Numerals:

10: propeller
11: central base section
12: blade (thin portion)

An embodiment of the present teaching will be hereinafter described.
FIG. 1 shows a propeller, as a cast article of an embodiment, for a propulsion unit of a small watercraft.
Reference numeral 10 denotes a propeller formed of a certain two-phase stainless steel. The propeller has a central base section 11 and a pair of blades 12 extending in radial, opposite directions from the base section 11, which are formed in one body. Each blade 12 includes its wide surface portion preferably having a thickness of 3 mm or less, more preferably 2 mm or less.
The propeller 10 is formed using a casting mold including two hollow potions, one for forming the central base portion 11 and the others for forming casting blades 12. The two hollow portions 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 two-phase stainless steel of the propeller formed. The propeller 10 is formed by pouring the melt into the casting mold at a pouring temperature of for example 1,550 to 1,650°C and then leaving as it is for heat release.
The two-phase stainless steel as a material configuring such propeller 10 comprises C, Si, Mn, Ni, Cr, Cu, N and the balance. The balance includes Fe and unavoidable impurities. Thus, the stainless steel is substantially free of expensive Mo. Yet, the two-phase stainless steel has a ferrite phase and an austenite phase and a reduced melting point.
The reason for selecting the contents of constitutional elements are as follows:

Carbon (C) is highly effective to form a stable austenite phase and to improve the strength of the two-phase stainless steel. If the C content is excessively high, however, chromium carbide is apt to be formed so that the corrosion resistance of the steel is deteriorated and, further, the steel becomes brittle. Additionally, as the C content increases, the melting point of the steel decreases but the strength of the steel is adversely affected. Accordingly, the C content is preferable to be low, i.e. up to 0.08 mass % in this case.
Silicon (Si) is a deoxidizer and is somewhat effective to stabilize the ferrite phase. Further, the melting point of the steel decreases as an increase of the Si content. Since Mo which is a ferrite-stabilizing element is not used in the present teaching, the use of Si in a relatively large amount is desired. Accordingly, the Si content is preferable to be high, i.e. not less than 0.5 mass % but not more than 1.5 mass %.
Manganese (Mn) is a deoxidizer and, as well as Ni, contributes to an increase of solid solution of N in the two-phase stainless steel. Mn is also less effective to stabilize the austenite phase. The melting point of the stainless steel decreases with an increase of the Mn content. The use of Mn in an excessive amount, however, adversely affects corrosion resistance such as pitching corrosion resistance. Accordingly, the Mn content is preferably up to 1.0 mass %.
Nickel (Ni) improves mechanical properties and moldability, contributes to maintain corrosion resistance and stabilize an austenite phase. Ni also has a small influence upon the melting point of the stainless steel although depending upon the amount thereof. Because Mo which is a ferrite-stabilizing element is not used in the present teaching, Ni which is an austenite stabilizing element and has small influence upon the melting point is not desired to be used in a large amount. Accordingly, the Ni content is preferable to be low, i.e. not less than 4.0 mass % but not more than 8.0 mass %.
Chromium (Cr) a main component which contributes to impart corrosion resistance to the two-phase stainless steel and stabilizes a ferrite phase. The higher the Cr content, the better is the corrosion resistance due to an improved stability of a passive film. Cr also has a small influence upon the melting point of the steel although depending upon the amount thereof. Because Mo which is a ferrite-stabilizing element and contributes to corrosion resistance is not used in the present teaching, Cr which is a ferrite stabilizing element and contributes to improve corrosion resistance is desired to be used in a large amount. However, too large an amount of Cr adversely affects the mechanical properties and moldability. Accordingly, the Cr content is preferably not less than 23 mass % but not more than 27 mass %.
Copper (Cu) imparts corrosion resistance to the two-phase stainless steel. The melting point of the steel can be reduced by increasing the Cu content. Since Mo which contributes to improve corrosion resistance is not used in the present teaching, the Cu content is desired to be high. However, too high a Cu content causes brittleness of the steel. Accordingly, the Cu content is preferably not less than 2.0 mass % but not more than 6.0 mass %.
Nitrogen (N) can increase the strength of the two-phase stainless steel even in a small amount and is highly effective to stabilize an austenite phase. The N content has little influence upon the melting point of the steel. Too large an amount of N is undesirable because of the precipitation of nitrides. Accordingly, the N content is preferably not less than 0.05 mass % but not more than 0.3 mass %.

The balance of the two-phase stainless steel is Fe and unavoidable impurities such as P and S. Such impurities may possibly include Mo. As long as the amount of impurity Mo is not more than 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 two-phase stainless steel of the present teaching, 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 20 % but not more than 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 on cooling rate and the like production conditions, but may be suitably controlled by adjustment of mixing proportions of the elements of the two-phase stainless steel in terms of the Cr equivalent (ferrite forming elements) and Ni equivalent (austenite forming elements) which may be expressed by the following formulas (1) and (2), for example:

Formula 1

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 is taken into account only when it is present.
FIG. 2 shows a schematic phase diagram which gives the area ratio of the ferrite phase as a function of the nickel equivalent vs. the chromium equivalent. In the present teaching, the proportion of constituent elements is adjusted so that the Cr equivalent and Ni equivalent fall within the region S where the area ratio of the ferrite phase is not less than 20 % but not more than 60 %. Because the strength is apt to be reduced due to excessive small area ratio of the ferrite phase, wile the corrosion resistance is apt to be deteriorated due to excessive large area ratio of the ferrite phase.
Further, the area ratios of the ferrite phase and the austenite ratios 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 two-phase stainless steel of the embodiment is adjusted preferably to a 1,450°C or lower, more preferably 1,430°C or lower. The melting point is preferable to be low as far as possible. Because, if it is determined to be excessively high then the temperature of the melt must be determined to be higher. The fluidity of the melt would be otherwise apt to be reduced, causing difficulty of forming the thin portion.
As the two-phase stainless steel described above: contains specific content of C, Si, Mn, Ni, Cr, Cu, N and Fe, and unavoidable impurities; has the ferrite phase and the austenite phase; and has an area ratio of the ferrite phase in a range between 20% and 60%. Therefore, the two-phase stainless steel of the present teaching can show well-balanced mechanical strength and corrosion resistance even though Mo is not added thereto. For the propeller 10 brought in contact with water or seawater, a two-phase stainless steel can be provided which is inexpensive and durable in practical use with a less number of kinds of component elements.
Also, as this two-phase stainless steel has a melting point of 1450°C or less, a wider range of the temperature in which the melt can flow is easily secured without a higher pouring temperature being determined. The thermal load applied to the casting mold is therefore not increased and the fluidity of the melt is improved. As a result, molding defects hardly occur in the blades 12 even though the blades 12 have a thin portion of 3mm or less thick.
The propeller 10 of such two-phase stainless steel can be fabricated inexpensively because of less kinds of component elements and has a well fluidity of the melt thereof for avoiding casting defects of the blades 12, resulting in easy production thereof. A mechanical strength and a corrosion resistance sufficient to resist against stresses generated and applied corresponding to a propulsive force in normal temperature water and seawater can be secured.

Example

The following examples and comparative examples will be described and illustrated.
Stainless steels containing components shown in Table 1 and the balance containing Fe and unavoidable impurities were prepared and determined for their liquid phase line temperatures (as melting points) and area ratios of the ferrite phase (α phase) and austenite phase (γ phase) by actual measurement and by simulation.

The results are shown in Table 1.

[Table 1]

	C	Si	Mn	Ni	Cr	Cu	Mo	N	Liquid phase line temperature (°C)	α phase	γ phase
Comparative Example 1	0.054	0.7	0.58	8.87	22.98	0.15	3.31	0.13	1453	-	-
Comparative Example 2	0.05	0.8	0.8	7	25.00	2.5	3.00	0.15	1425	41.8	57
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

[Strength test]

Example 6 and Comparative Example 1 were tested for tensile test and impact test for test pieces thereof as follows.
Using the test pieces having the same shape, a tensile test was carried out by the metallic material tensile test in accordance with JIS Z2371.
Using the test pieces having the same shape, 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 strength (MPa) 0.2 % Proof strength (MPa) Elongation after fracture (%) Impact strength (J/cm²)

Comparative Example 1 654 430 12.7 66.4

Example 6 734 450 11.3 105
As is evident from the results shown in Table 2, the substantially Mo-free two-phase stainless steel of Example 6 has equal or higher tensile strength and impact strength as compared with the Mo-containing stainless steel of Comparative Example 1.

Casting test

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. 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.

Corrosion resistance test

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 its 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 naked 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.
The description above discloses, amongst others, an embodiment of a two-phase stainless steel comprising Fe as a major component and further containing C, Si, Mn, Ni, Cr, Cu, N and unavoidable impurities as indicated in claim 1, and comprising a ferrite phase and an austenite phase with an area ratio of the ferrite phase being not less than 20 % but not more than 60 % (first aspect).
There is disclosed an embodiment of a two-phase stainless steel comprising, in mass %: up to 0.08 % of C, not less than 0.5 % but not more than 1.5 % of Si, up to 1.0 % of Mn, not less than 4.0 % but not more than 8.0 % of Ni, not less than 23 % but not more than 27 % of Cr, not less than 2.0 % but not more than 6.0 % of Cu, not less than 0.05 % but not more than 0.3 % of N, and the balance being Fe and unavoidable impurities, and comprising a ferrite phase and an austenite phase with an area ratio of the ferrite phase being not less than 20 % but not more than 60 % (second aspect).
Further, preferably there is disclosed a two-phase stainless steel having a melting point of not higher than 1,450°C, and being used as a casting material of a cast article comprising a portion having a thickness of 3mm or less (third aspect).
Further, preferably there is disclosed a cast article of a two-phase stainless steel, comprising the two-phase stainless steel according to any one of the first to third aspects (fourth aspect).
Further, there is disclosed an embodiment of a cast article in the form of a propeller blade for a propulsion unit of a small watercraft, comprising the two-phase stainless steel according to the third aspect (fifth aspect).
To solve the problems mentioned above, the description above discloses a two-phase stainless steel of the first aspect comprising Fe as a major component and further containing C, Si, Mn, Ni, Cr, Cu, N and unavoidable impurities, and comprising a ferrite phase and an austenite phase with an area ratio of the ferrite phase being not less than 20 % but not more than 60 %.
Further, it discloses a two-phase stainless steel of the second aspect comprising, in mass %: up to 0.08 % of C, not less than 0.5 % but not more than 1.5 % of Si, up to 1.0 % of Mn, not less than 4.0 % but not more than 8.0 % of Ni, not less than 23 % but not more than 27 % of Cr, not less than 2.0 % but not more than 6.0 % of Cu, not less than 0.05 % but not more than 0.3 % of N, and the balance being Fe and unavoidable impurities, and comprising a ferrite phase and an austenite phase with an area ratio of the ferrite phase being not less than 20 % but not more than 60 %.
Further, it discloses a two-phase stainless steel of the third aspect having a melting point of not higher than 1,450°C, and being used as a casting material of a cast article comprising a portion having a thickness of 3mm or less.
Further, it discloses a cast article of a two-phase stainless steel of the fourth aspect comprising the two-phase stainless steel according to any one of the first to third aspects.
Further, it discloses a cast article in the form of a propeller blade for a propulsion unit of a small watercraft of the fifth aspect comprising the two-phase stainless steel according to the third aspect.

Effect

As the two-phase stainless steel according to the first or second aspect: contains Fe as a major component, and C, Si, Mn, Ni, Cr, Cu, N and unavoidable impurities; has the ferrite phase and the austenite phase; and has an area ratio of the ferrite phase in a range between 20% and 60% that is obtained by adjustment of contents of the component elements. Therefore, the two-phase stainless steel of the present teaching can show well-balanced mechanical strength and corrosion resistance even though Mo is not added thereto. For the propeller 10 brought in contact with water or seawater, a two-phase stainless steel can be provided which is inexpensive and durable in practical use with a less number of kinds of component elements.
Also, as the two-phase stainless steel according to the third aspect having a melting point of 1450°C or less is used as a material of cast article including a thin portion having a thickness of 3 mm or less, a wider range of the temperature in which the melt can flow is easily secured without a higher pouring temperature being determined. The thermal load applied to the casting mold is therefore not increased and the fluidity of the melt is improved. As a result, molding defects hardly occur in the blades 12 even though the blades 12 have a thin portion of 3mm or less thick.
As the two-phase stainless cast article of the fourth aspect comprises the substantially Mo-free, 2-phase stainless steel according to claim 1 or 2, it is given corrosion resistance and mechanical strength enough to be used in contact with water and seawater.
The two-phase stainless steel cast article of the fifth aspect is directed to a propeller having blades and made of two-phase stainless steel according to the first or second aspect, for a propulsion unit of a small watercraft, the blades having widely extending thin portions. The propeller 10 of such two-phase stainless steel can be fabricated with well fluidity of the melt thereof for avoiding casting defects of the blades 12, resulting in easy production. A mechanical strength and a corrosion resistance enough to resist against stresses generated and applied corresponding to a propulsive force in water and seawater of normal temperature can be secured. Accordingly, a propeller can be provided, which is easily fabricated and given corrosion resistance and mechanical strength enough to be used in practical use.
The description, in particular discloses, in order to provide an inexpensive, substantially Mo-free, ferrite-austenite two-phase stainless steel having satisfactory mechanical strength and corrosion resistance in practical use, an embodiment of the stainless steel which has a ferrite phase area ratio of 20 to 60 % and a composition containing, in mass %, not more than 0.08 % of C, 0. 5 to 1.5 % of Si, not more than 1.0 % of Mn, 4.0 to 8.0 % of Ni, 23 to 27 % of Cr, 2.0 to 6.0 % of Cu, 0.05 to 0.3 % of N, and the balance being Fe and unavoidable impurities.

Claims

Cast article in the form of a propeller blade for a propulsion unit of a small watercraft, wherein the cast article comprises a two-phase stainless steel having a ferrite phase and an austenite phase, containing specific contents of C, Si, Mn, Ni, Cr, Cu, N and Fe, and unavoidable impurities, and having an area ratio of the ferrite phase in a range between 20% and 60%, wherein the specific contents of the two-phase stainless steel in mass % are:
up to 0.08 % of C,

not less than 0.5 % but not more than 1.5 % of Si,

up to 1.0 % of Mn,

not less than 4.0 % but not more than 8.0 % of Ni,

not less than 23 % but not more than 27 % of Cr,

not less than 2.0 % but not more than 6.0 % of Cu,

not less than 0.05 % but not more than 0.3 % of N, and

the balance being Fe and unavoidable impurities.
Cast article according to claim 1, characterized in that the two-phase stainless steel has a melting point of not higher than 1,450°C, and characterized by comprising a portion having a thickness of 3 mm or less.
Method to manufacture a cast article in the form of a propeller blade for a propulsion unit of a small watercraft, wherein the cast article comprises a two-phase stainless steel having a ferrite phase and an austenite phase, by preparing a melt including specific contents of C, Si, Mn, Ni, Cr, Cu, N and Fe, and substantially free of Mo addition, and adjusting an area ratio of the ferrite phase in a range between 20% and 60%, wherein the melt is prepared with the specific contents in mass %:
up to 0.08 % of C,

not less than 0.5 % but not more than 1.5 % of Si,

up to 1.0%ofMn,

not less than 4.0 % but not more than 8.0 % of Ni,

not less than 23 % but not more than 27 % of Cr,

not less than 2.0 % but not more than 6.0 % of Cu,

not less than 0.05 % but not more than 0.3 % of N, and

the balance being Fe and unavoidable impurities.