JPH0813094A - Duplex stainless cast steel and production thereof - Google Patents

Abstract

PURPOSE:To produce a duple,t stainless cast steel excellent in 0.2% proof stress and elongation and in which the causing of scales is not observed at all even if being used for a rotary ring of a centrifugal separator for a long time. CONSTITUTION:This duplex stainless cast steel is a one having a compsn. contg., by weight, 20 to 35% Cr, 3 to 12% Ni, 0.5 to 6% Mo, 0.01 to 0.27% Co, 0.1 to 3% Cu, 0.05 to 3% N, 1 to 3% W, <=0.12% C, <=3% Si, <=5% Mn, and the balance iron with inevitable impurities, and the duplex stainless cast steel is produced by executing casting using a centrifugal casting method after melting.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a duplex stainless cast steel suitable for use as a material for an apparatus member such as a rotating body of a centrifuge that requires high strength and high corrosion resistance.

[0002]

2. Description of the Related Art For example, centrifuges used in the food industry and sewage treatment are becoming more and more demanding for the strength and corrosion resistance of their materials due to the large size of the equipment, the increase in the number of revolutions and the deterioration of the corrosive environment. ing. At present, 0.2% proof stress is required to be 55 kgf / mm ² or more and elongation is required to be 25% or more. Further, more excellent corrosion resistance is required.

Hitherto, as such a high-strength, high-corrosion resistant material, a high Cr low Ni type duplex stainless cast steel has been proposed (Japanese Patent Laid-Open Nos. 52-153821 and 64-641).
No. 31953).

However, although some of the conventional duplex stainless cast steels described above satisfy the above requirements in strength, they do not have sufficient corrosion resistance. That is, according to the present inventor, the above-mentioned conventional duplex stainless steel cast steel is proposed to have excellent crevice corrosion resistance,
When actually used for a rotating ring that engages with a fixed member of a centrifuge, there were quite a few steel grades in which scale generation was observed at one or more places of the rotating ring after 1000 hours of operation for sludge treatment. , This is called the centrifugation test).

[0005]

In view of the above circumstances, the object of the present invention is to provide a 0.2% proof stress of 55 kgf / mm ² or more,
Elongation is 25% or more, and it can be used for centrifuge rotating ring 1
It is to provide a duplex stainless cast steel in which no scale generation is observed even after 000 hours of use.

[0006]

In order to achieve the above object, the present inventor first determines a pitting corrosion potential from a conventional polarization curve in a chloride ion-containing aqueous solution as a method for evaluating crevice corrosion resistance. As a substitute for the measuring method, a method showing a strong correlation with the result of the centrifugation test was earnestly studied. As a result, the pitting corrosion potential of the duplex stainless cast steel measured by the pitting potential measuring method of JIS G0577 using a sample provided with a gap using silicon rubber shows a strong correlation with the result of the centrifugal separation test, and Its pitting potential is 800
It has been found that if it is mV or more, it exhibits excellent crevice corrosion resistance. Therefore, next, the effects of various elements on the pitting potential according to JIS G 0577 were examined in detail. As a result, they have found that the above objects can be achieved by limiting the addition amounts of Co and W in particular.

That is, the first of the duplex stainless cast steel of the present invention
Of Cr: 20-35% by weight, Ni: 3-12% by weight, Mo: 0.5-6% by weight, Co: 0.01-0.
27% by weight, Cu: 0.1 to 3% by weight, N: 0.05 to
3% by weight, W: 1-3% by weight, C: 0.12% by weight or less, Si: 3% by weight or less, Mn: 5% by weight or less,
The balance consists of iron and unavoidable impurities, the second is
More preferable one of the above-mentioned first ones, Cr: 25
~ 27 wt%, Ni: 5.5-7 wt%, Mo: 3-
4.5% by weight, Co: 0.01 to 0.27% by weight, C
u: 0.5 to 1.5% by weight, N: 0.15 to 0.3% by weight, W: 1 to 1.5% by weight, C: 0.05% by weight or less,
Si: 1% by weight or less, Mn: 1% by weight or less, the balance consisting of iron and unavoidable impurities. The method for producing the duplex stainless cast steel of the present invention is one in which the first and second duplex stainless cast steels are melted and then cast by a centrifugal casting method.

[0008]

FUNCTION The reasons for limiting the components and composition of the duplex stainless cast steel of the present invention will be described.

Cr is a basic element for forming duplex stainless steel and improving corrosion resistance, and the addition amount thereof is 20 to 35% by weight (hereinafter,% means% by weight unless otherwise specified), It is preferably 25 to 27%. If it is less than 20%, the corrosion resistance is lowered, and if it exceeds 35%, the toughness is lowered.

Ni is an element that improves the toughness and pitting corrosion resistance of steel, and its addition amount is 3 to 12%, preferably 5.
It is 5 to 7%. If it is less than 3%, the austenite phase disappears, and if it exceeds 12%, the amount of the austenite phase increases and the strength decreases.

Mo improves local corrosion resistance and pitting corrosion resistance, and the addition amount thereof is 0.5 to 6%, preferably 3 to 4.5.
%. If it is less than 0.5%, the local corrosion resistance and pitting corrosion resistance deteriorate, and if it exceeds 6%, the toughness decreases.

Co improves the crevice corrosion resistance and concentrates in the austenite phase to improve the elongation, and the addition amount thereof is 0.01 to 0.27%. If it is less than 0.01%, the crevice corrosion resistance is not sufficient, and if it exceeds 0.27%, the yield strength decreases.

Cu improves the corrosion resistance in a non-oxidizing environment, especially the pitting corrosion resistance, and the addition amount thereof is 0.1 to 3%, preferably 0.5 to 1.5%. If it is less than 0.1%, its effect is small, and if it exceeds 3%, segregation of casting occurs and strength is lowered.

N significantly improves pitting corrosion resistance and crevice corrosion resistance, and the addition amount thereof is 0.05 to 3%, preferably 0.1.
It is 15 to 0.3%. If it is less than 0.05%, its effect is small, and if it exceeds 3%, the amount of austenite phase becomes excessive, or bubbles occur in the steel, resulting in defects.

W strengthens the ferrite phase to remarkably improve the yield strength and also improves the crevice corrosion resistance, and the addition amount thereof is 1 to 3%, preferably 1 to 1.5%. If it is less than 1%, the effect is not sufficient, and if it exceeds 3%, the toughness decreases.

C is an austenite phase forming element, and the addition amount thereof is 0.12% or less, preferably 0.05.
% Or less. If it exceeds 0.12%, Cr carbides are precipitated and ductility, toughness, and corrosion resistance are reduced.

Si is necessary for deoxidizing molten steel and ensuring castability during melting of steel, and improves corrosion resistance.
The amount added is 3% or less, preferably 1% or less. Three
%, The σ phase precipitates and the strength and corrosion resistance decrease.

Mn is necessary for deoxidation and desulfurization of molten steel during melting of steel, and its addition amount is 5% or less, preferably 1%.
% Or less. If it exceeds 5%, the amount of austenite phase increases and mechanical properties deteriorate.

As the method for producing the duplex stainless cast steel of the present invention, an ordinary melting and casting method can be adopted. In the centrifugal casting method, however, non-metallic inclusions and gas in the molten steel are centrifugally separated by the action of centrifugal force so as to be dense This is a preferable casting method because a sound product can be easily obtained.

[0020]

EXAMPLE A molten metal melted to a predetermined composition by high-frequency atmospheric melting was cast into a cylindrical mold having a diameter of 50 cm rotating at a rotation speed of 750 rpm of a vertical centrifugal casting device to obtain a ring-shaped ingot having a thickness of 15 cm. . Then, the ingot was heat-treated at 1150 ° C. for 2 hours. A tensile test piece, a pitting potential measurement test piece and a microscopic test piece were cut out from the obtained heat-treated ingot, and the ingot was subjected to a centrifugal separation test.

The tensile test is conducted according to JIS Z 220.
According to 1, the JIS No. 4 test piece was used. From the results, 0.2% proof stress and elongation were measured. Further, the measurement of the pitting potential was performed in accordance with JIS G0577 using a test piece in which a silicone rubber was pressed to form a gap. Furthermore, after the surface of the test piece was polished, the number of inclusions was calculated by counting the inclusions having a particle size of 2 μm or more in a 4 mm square with a cross section of 1 mm ² with an optical microscope. Further, the surface of the test piece was subjected to etching treatment, and its structure was observed with an optical microscope.

The composition (%) of the cast steel test piece of each test is shown in Table 1.
Table 2 shows the test results obtained. In the observation of the above structure, a clear two-phase structure composed of a ferrite phase and an austenite phase was observed in all tests.

[0023]

[Table 1] Cr Ni Mo Co Cu NWVC Si Mn Fe Example 1 26.9 6.68 3.34 0.01 0.90 0.16 1.2 ― 0.03 0.61 0.72 Remaining 〃 2 〃 〃 〃 〃 〃 ― 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 0.27 undefined undefined undefined - undefined undefined undefined undefined Comparative example 1 undefined undefined undefined 0.33 undefined undefined undefined - undefined undefined undefined undefined undefined 2 undefined undefined undefined 0.08 undefined undefined 0.8 - undefined undefined undefined undefined example 4 undefined undefined undefined 0.08 undefined undefined 1.1 - 〃 undefined undefined undefined undefined 5 undefined undefined undefined 0.08 undefined undefined 1.5 - undefined undefined undefined undefined undefined 6 undefined undefined undefined 0.08 undefined undefined 2.0 - undefined undefined undefined undefined undefined 7 undefined undefined undefined 0.08 undefined undefined 2.9 - undefined undefined undefined undefined undefined 8 20.6 11.8 〃 undefined undefined undefined 1.2 - undefined undefined undefined undefined undefined 9 34.2 3.42 undefined undefined undefined undefined undefined - undefined undefined undefined undefined 〃10 26.9 6.68 0.63 undefined undefined undefined undefined - undefined undefined undefined undefined undefined undefined 〃11 5.87 undefined undefined undefined undefined - undefined undefined undefined 〃 〃 12 〃 〃 3.34 〃 0.15 0.06 〃 ― 〃 〃 〃 〃 〃 〃 〃 〃 2.87 2.93 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 0.90 0.16 〃 ― 0.12 2.96 4.98 〃 Conventional example 1 19.2 13.0 3.51 ― ― ― ― ― 0.02 0.50 0.80 〃 〃 2 25.1 4.52 2.25 ― ― ― ― 0.05 0.60 0.70 〃 〃 3 24.8 5.87 3.03 ― 0.79 0.13 1.04 0.89 0.02 0.5 1.04 〃 〃 4 26.0 6.65 3.43 ― 0.58 0.12 0.15 0.08 0.01 0.61 0.55 〃 〃 5 25.0 4.11 1.05 ― 1.17 0.19 0.57 ― 0.05 2.07 0.58 〃 〃 6 25.2 7.07 3.42 0.48 1.87 0.17 0.99 ― 0.03 0.96 0.86 〃

[0024]

[Table 2] [Tensile test] 0.2% yield strength Elongation Pitting corrosion potential Number of inclusions Centrifuge test (kgf / mm ² ) (%) (mV) (pieces / mm ² ) (presence or absence of scale generation) Example 1 61.2 29.5> 1000 36 None 〃 2 63.1 25.1 〃 31 〃 〃 3 56.0 32.0 〃 42 〃 Comparative Example 1 23.6 39.8 〃 41 〃 〃 2 56.4 25.5 780 43 Yes Example 4 55.7 28.6> 1000 32 None 〃 5 ッ 58.1 〃 6 59.4 27.7 〃 41 〃 〃 7 61.2 27.2 〃 44 〃 〃 8 55.9 25.4 〃 36 〃 〃 9 63.1 30.8 950 38 〃 〃 10 58.2 28.3 880 39 〃 〃 10 〃 11 59.3 28.3 〃 13 56.3 29.3> 1000 32 〃 〃 14 59.8 26.0 〃 48 〃 Conventional example 1 22.0 43.0 610 47 Yes 〃 2 41.0 22.0 700 42 〃 〃 3 58.6 22.4 810 40 None 〃 4 50.1 31.0> 1000 42 〃 5 〃 5 〃 5 43 Yes 〃 6 54.3 35.1> 1000 39 No

From Table 2, the following can be seen: (1)
In each of Examples 1 to 14, 0.2% proof stress ≧ 55 kg
f / mm ² , elongation ≧ 25%, and all three conditions without scale generation in the centrifugal separation test are satisfied, whereas Comparative Examples 1 and 2 and Conventional Examples 1 to 6 all have the above three conditions. In all of the cases where (2) the pitting potential was 800 mV or more, the scale did not occur at all in the centrifugation test, whereas the pitting potential was 8
In all the examples of less than 00 mV, the generation of scale was observed at one or more places in the centrifugation test.

[0026]

According to the present invention, the 0.2% proof stress is 55 kg.
It is possible to provide a duplex stainless cast steel having f / mm ² or more and an elongation of 25% or more and excellent in crevice corrosion resistance.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryoichi Mima 17-1, Higashi Kibogaoka, Asahi-ku, Yokohama, A-203

Claims (4)

[Claims] 1. Cr: 20 to 35% by weight, Ni: 3-1
2% by weight, Mo: 0.5 to 6% by weight, Co: 0.01 to
0.27% by weight, Cu: 0.1 to 3% by weight, N: 0.0
5 to 3% by weight, W: 1 to 3% by weight, C: 0.12% by weight
Hereinafter, a duplex stainless cast steel containing Si: 3 wt% or less and Mn: 5 wt% or less, with the balance being iron and inevitable impurities. 2. Cr: 25 to 27% by weight, Ni: 5.5
˜7% by weight, Mo: 3 to 4.5% by weight, Co: 0.01
~ 0.27 wt%, Cu: 0.5-1.5 wt%, N:
0.15-0.3 wt%, W: 1-1.5 wt%, C:
Duplex stainless cast steel containing 0.05 wt% or less, Si: 1 wt% or less, Mn: 1 wt% or less, and the balance being iron and inevitable impurities. 3. The duplex stainless cast steel according to claim 1, which is obtained by a centrifugal casting method. 4. The method for producing a duplex stainless cast steel according to claim 1, wherein the casting after melting is performed using a centrifugal casting method.