JPH08239735A - Cast austnitic stainless steel - Google Patents

Abstract

PURPOSE: To improve mechanical properties and corrosion resistance by specifying respective contents of Cr, Ni, Mo, Co, Cu, N, W, C, Si, and Mn in a cast stainless steel. CONSTITUTION: This austenitic stainless steel has a composition consisting of, by weight, 20-35% Cr, 15-35% Ni, 0.1-10% Mo, 0.01-0.1% CO, 0.5-3% Cu, 0.01-0.5% N, 1-8% W, <=0.12% C, <=3% Si, <=2% Mn, and the balance Fe with inevitable impurities. It is desirable to produce this steel by means of centrifugal casting. The resultant cast steel has mechanical properties of 40kgf/mm<2> of 0.2% proof stress, >=75kgf/mm<2> tensile strength, and <=60% elongation and also has excellent corrosion resistance to local corrosion, such as pitting corrosion and crevice corrosion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless cast steel suitable as a structural material for an apparatus member which requires local corrosion resistance and strength such as a rotating body of a seawater pump.

[0002]

2. Description of the Related Art Seawater pumps are used in various fields such as pumps for cooling water of power plants, steel mills, petrochemical complexes, large desalination plants, pumps for Blyle recirculation, etc. The reliability and durability are strongly demanded. Conventionally, most commonly used for seawater pumps is austenitic stainless steel, SCS16, SUS304L, SUS316.
L is often used.

However, these austenitic stainless steels are susceptible to pitting and crevice corrosion in the stagnation and crevices in the pump. Further, the seawater pump structural material itself is required to have strength with the increase in size and speed of the seawater pump, but at present, these austenitic stainless steels are not sufficient.

[0004]

The present invention is a cast steel having a mechanical property of 0.2% proof stress 40 kgf / mm ² , tensile strength 75.
It is an object of the present invention to provide an austenitic stainless steel having a corrosion resistance of not less than kgf / mm ² and an elongation of 60% or more, which is excellent in local corrosion such as pitting and crevice corrosion.

[0005]

As a result of various studies to achieve the above object, the present inventors have found that it is effective to limit the amounts of Co and W added. That is, the first austenitic stainless cast steel of the present invention has a composition of wt%, Cr: 20 to 35%, Ni:
15-35%, Mo: 0.1-10%, Co: 0.01
To 0.1%, Cu: 0.5 to 3%, N: 0.01 to 0.
5%, W: 1-8%, C: 0.12% or less, Si: 3%
Hereinafter, Mn: 2% or less, the balance consisting of Fe and unavoidable impurities, the second is more preferable than the first, the composition is wt%, Cr: 23-27%, Ni:
20-30%, Mo: 5-8%, Co: 0.01-0.
1%, Cu: 0.5 to 1.5%, N: 0.15 to 0.2
5%, W: 1 to 5%, C: 0.03% or less, Si: 0.
5 to 1% or less, Mn: 1.5% or less, and the balance Fe and unavoidable impurities. The third is the first
And, the second austenitic stainless cast steel is melted and then cast by a centrifugal casting method.

[0006]

The reason for limiting the composition of the stainless cast steel of the present invention will be described below. Cr: 20 to 35% Cr is an essential element for forming stainless steel, and is a basic element for improving corrosion resistance. If it is less than 20%, the corrosion resistance is lowered, and if it exceeds 35%, the toughness is lowered. The range of 23 to 27% is preferable.

Ni: 15 to 35% Ni improves the toughness and local corrosion resistance of steel. It is an austenite forming element and is an element essential as a stabilizing element of the austenite phase. Therefore, Ni is 15 ~
35%. The range of 20 to 30% is preferable.

Mo: 0.1-10% Mo improves local corrosion resistance. If it is less than 0.1%, the local corrosion resistance deteriorates. If it exceeds 10%, the toughness decreases and the cost increases. Preferably, the range is 5 to 8%.

Cu: 0.5-3% Cu improves the corrosion resistance in a non-oxidizing environment. 0.5%
If it is less than 3%, the effect is small, and if it exceeds 3%, segregation of casting occurs and strength is lowered. Preferably 0.5-1.5
% Range is good.

N: 0.01 to 0.5% N significantly improves the local corrosion resistance. If it is less than 0.01%, the effect is not obtained. If it exceeds 0.5%, it bonds with Cr, which has a strong bonding force with nitrogen, to form chromium nitride, and the corrosion resistance decreases. In addition, bubbles may occur in the steel, resulting in defects. The range of 0.15-0.25% is preferable.

Co: 0.01 to 0.1% Co is essential for improving the local corrosion resistance and contributes to the improvement of elongation. If it is less than 0.01%, the local corrosion resistance is not sufficient, and if it exceeds 0.1%, the yield strength decreases.

W: 1 to 8% W significantly improves the yield strength. It also improves local corrosion resistance. If the effect is less than 1%, the effect is not sufficient, and if it exceeds 8%, the toughness decreases and the cost also increases. The range of 1 to 5% is preferable.

C: 0.12% or less When the content of C is large, Cr carbides are precipitated and the ductility,
It causes deterioration of toughness and corrosion resistance. Therefore, the upper limit is 0.12
%. It is preferable that the range is 0.03% or less.

Si: 3% or less Si is an element that must be included in order to ensure deoxidation and castability of molten steel, and also improves corrosion resistance. If it exceeds 3%, the σ phase precipitates and the strength and corrosion resistance decrease. Preferably 0.5-1
% Range is good.

Mn: 2% or less Mn is added for deoxidation and desulfurization of molten steel. Mn has the effect of increasing the solubility of N, but if its content is large, it causes corrosion resistance deterioration and embrittlement, so the content is made 2% or less. Preferably, the range is 1.5% or less.

As a method for producing the austenitic stainless cast steel of the present invention, a usual melting casting method can be adopted. In the centrifugal casting method, a sound product which is dense and has a small amount of non-metallic inclusions by the action of centrifugal force is usually used. It is a more preferable casting method because it is easier to manufacture than the melt casting method.

[0017]

EXAMPLES Next, the features of the present invention will be described by examples. A molten metal melted to a predetermined composition in a high-frequency induction air melting furnace was cast into a cylindrical mold with a diameter of 50 cm rotating at a rotation speed of 750 rpm of a vertical centrifugal casting device, and a wall thickness of 15 cm.
To obtain a cylindrical ingot. This ingot was subjected to heat treatment at 1150 ° C. for 2 hours, and a tensile test piece and a test piece for evaluating local corrosion resistance were collected from the ingot after the heat treatment. JIS for tensile test
The test was performed at room temperature using a Z2201 No. 14 test piece. The local corrosion resistance was judged by evaluating crevice corrosion, which is more problematic in operation than pitting corrosion. In the crevice corrosion test, a test piece cut out into a width of 15 mm, a thickness of 5 mm, and a length of 40 mm was pressed from both sides with acrylic jigs to create an artificial gap, and the test piece was cut into 3 pieces.
The corrosion weight loss was measured after 72 hours of immersion in a 10% FeCl ₃ .6H ₂ 0 solution kept at 5 ° C. The composition (% by weight) of the obtained cast steel is shown in Table 1, and the results of the tensile test and crevice corrosion test are shown in Table 2.

[0018]

[Table 1]

[Table 2] The following can be seen from Tables 1 and 2. That is, W is 1 to 8
Examples 1 to 4 in which wt% and Co are added in an amount of 0.01 to 0.1 wt%, and the amounts of W and Co added are constant, and the amounts of addition of other elements listed in the claims are within the scope of the claims. In each of Examples 5 to 13 which are changed within 0.2% yield strength ≧ 40
kgf / mm ^2, the tensile strength ≧ 75kgf / mm ^2, elongation ≧
We are satisfied with 60%. On the other hand, Comparative Example 1 in which the added amount of W is less than 1 wt%, Comparative Example 2 in which the added amount of W exceeds 8 wt%, Comparative Example 3 in which the added amount of Co exceeds 0.1 wt%, and Conventional Examples 1 to 4
Then, none of them satisfies at least one of the above three conditions. Further, Examples 1 to 13 show local corrosion resistance superior to that of the conventional example.

[0019]

According to the present invention, 0.2% proof stress 40 kgf
/ Mm ² or more, tensile strength 75 kgf / mm ² , elongation 60%
It is possible to provide an austenitic stainless cast steel having the above characteristics and exhibiting excellent local corrosion resistance.

Claims (3)

[Claims] 1. The composition is wt%, Cr: 20-35%,
Ni: 15-35%, Mo: 0.1-10%, Co:
0.01-0.1%, Cu: 0.5-3%, N: 0.0
1-0.5%, W: 1-8%, C: 0.12% or less, S
i: 3% or less, Mn: 2% or less, the balance being Fe and unavoidable impurities, and a high-strength and highly corrosion-resistant austenitic stainless cast steel. 2. The composition is wt%, Cr: 23-27%,
Ni: 20-30%, Mo: 5-8%, Co: 0.01
~ 0.1%, Cu: 0.5-1.5%, N: 0.15-
0.25%, W: 1-5%, C: 0.03% or less, S
i: 0.5-1% or less, Mn: 1.5% or less, the balance is F
High strength and corrosion resistant austenitic stainless cast steel consisting of e and inevitable impurities. 3. The high-strength, high-corrosion-resistant austenitic stainless cast steel according to claim 1, which is manufactured by a centrifugal casting method.