JPH0770700A - High proof stress and high corrosion resistant austenitic stainless cast steel - Google Patents

Abstract

PURPOSE:To provide high proof stress and high corrosion resistant austenitic stainless cast steel in which strength at a high temp. is close to double of that of SCS13 and SCS14, deterioration in toughness hardly occurs and corrosion resistance is equal to that of WCS13 and SCS14 as well. CONSTITUTION:This steel has a componental compsn. contg., by weight, <=0.1% C, <=2.0% Si, 4.0 to 15.0% Mn, 4.0 to 10.0% Ni, 15.0 to 23.0% Cr, 0.2 to 4.0% Mo and 0.1 to 0.4% N, and the balance substantially Fe. This alloy may furthermore contain one or >= two kinds among <=3.0% Cu, <=3.0% W, <=3.0% Co, <=2.0% Nb and <=0.5% Ti.

Description

Detailed Description of the Invention

[0001]

The present invention is used in high temperature and corrosive environments, and is particularly suitable as a constituent material of equipment requiring high strength and high toughness. Specifically, it is specifically used as a pipe material for oil / natural gas oil wells and petroleum. The present invention relates to an austenitic stainless cast steel suitable as a pipe material for heavy oil desulfurization of a refining device, a material for a high temperature rotating body (for example, a centrifuge, a screen, etc.).

[0002]

2. Description of the Related Art As conventional austenitic stainless cast steels, JIS G5121 SCS13 and SCS14 are known. However, the strength of these steel types is remarkably reduced at high temperatures, especially at 300 ° C or higher, and the range of use is extremely limited in these temperature ranges. Also, austenite-ferrite duplex cast stainless steel (SCS10 and SCS11, etc.) in which ferrite is crystallized in austenite has significantly higher strength up to 500 ° C than austenitic stainless cast steel,
In the temperature range from 300 ° C to 500 ° C, the ferrite phase becomes brittle and the toughness is significantly reduced. Furthermore, JIS G4 for forged products
304 SUS201 and 202 are known, but the corrosion resistance is SC
Since it is inferior to S13 and SCS14, it cannot be used in corrosive environments.

Therefore, when high strength and high toughness are required in these temperature ranges, expensive materials such as Ni-based alloys or Ti alloys have been used, and the cost is considerably heavy, Low cost, high strength and high toughness materials are required from all directions.

[0004]

In view of the background as described above, the present invention provides a material which has corrosion resistance equal to or higher than that of SCS13 and SCS14 at 300 ° C. or higher and which has significantly improved high temperature strength and toughness at low cost. It was made to provide in.

[0005]

The present inventors have succeeded in developing the present invention as a result of various studies and experiments for solving the above-mentioned drawbacks of conventional products, and the technical constitution of the present invention is as follows. C: 0.1% or less, Si: 2.0% or less, Mn: 4.0 to 15.
0%, Ni: 4.0 to 10.0%, Cr: 15.0 to 23.0%, Mo: 0.2 to 4.0%,
N: 0.1 to 0.4% is contained, and the balance is such that the balance is substantially Fe. Also, the above alloys have Cu: 3.0% or less, W: 3.0
%, Co: 3.0% or less, Nb: 2.0% or less, Ti: 0.5% or less, and it is possible to include any one or more of them in high temperature strength / high corrosion resistance austenitic stainless cast steel. is there.

With the above-mentioned technical constitution, the austenitic stainless cast steel of the present invention has a strength at high temperature nearly twice that of SCS13 and SCS14, almost no deterioration of toughness occurs, and corrosion resistance of SCS13 or SCS14
And high-corrosion, high-corrosion-resistant austenitic stainless cast steel.

The reasons why the above-mentioned composition components and composition ranges are limited in the present invention will be described below.

C: 0.1% or less C is an austenite-forming element, and when it forms a solid solution in the matrix, it has the effect of remarkably increasing the strength. However, the solid solution amount of C is extremely small, and when added in excess, it forms carbides with Cr, Nb, Ti, etc., leading to deterioration of corrosion resistance and deterioration of toughness.
Therefore, the content of C is 0.1% which is the maximum solid solution amount of the alloy of the present invention.
Below.

Si: 2.0% or less Si is used as a deoxidizing agent during molten steel refining and has the effect of improving castability. However, if the content exceeds 2.0%, the formation of a ferrite phase is promoted, and the toughness and weldability are impaired. Therefore, the Si content is set to 2.0% or less.

Mn: 4.0-15.0% Mn is a characteristic element of the alloy of the present invention. In the case of alloys of this type, Mn is generally used as a deoxidizer during molten steel refining, similar to Si. However, Mn, like Ni, is an austenite-forming element and has the function of increasing the amount of solid solution of N. In the alloy of the present invention, by setting Mn to 4.0% or more, Ni can be reduced and a large amount of N can be added, which makes it possible to reduce the cost and significantly improve the strength. However, if it exceeds 15.0%, the work hardenability also increases and the workability decreases. Therefore, Mn is set to 4.0 to 15.0%.

Ni: 4.0 to 10.0% Ni has the effect of stabilizing austenite and improving corrosion resistance, and is an essential element in the alloy of the present invention. In order to obtain corrosion resistance equal to or higher than SCS13 and SCS14 in the alloy of the present invention, at least 4.0% or more is required. Further, the more Ni, the more the corrosion resistance is improved, but it is an expensive element, and the addition of Ni more than necessary causes an increase in cost. Therefore, Ni is set to 4.0 to 10.0%.

Cr: 15.0 to 23.0% Cr, like Ni, is also an essential element for obtaining corrosion resistance, and the higher the content, the higher the corrosion resistance. However, if contained in a large amount, a ferrite phase is likely to be generated, which causes embrittlement at high temperature. In the alloy of the present invention, its content is Ni or Mn.
Although it is determined by the balance of N and N, and the usage environment, if it is less than 15.0%, corrosion resistance equivalent to that of SCS13 or SCS14 cannot be obtained, and if it exceeds 23.0%, a large amount of ferrite phase is formed. Therefore, Cr is set to 15.0 to 23.0%.

Mo: 0.2 to 4.0% Mo is an element that not only significantly improves pitting corrosion resistance and crevice corrosion resistance, but also improves general corrosion resistance.
Contains 2.0 to 3.0%. M also in the alloy of the present invention
Although o is an element effective for improving the local corrosion resistance, its effect becomes remarkable at 0.2% or more due to the synergistic action with N.
However, if it exceeds 4.0%, a ferrite phase is likely to be generated, which causes embrittlement at high temperature. Therefore, Mo is set to 0.2 to 4.0%.

N: 0.1 to 0.4% N is an austenite-forming element and is an element that significantly improves local corrosion resistance. Further, it has a remarkable effect on strength improvement and is an essential element in the alloy of the present invention. However, 0.1% or less is insufficient for obtaining the characteristics of the alloy of the present invention in terms of structure, corrosion resistance and strength. Further, if it exceeds 0.4%, bubbles are generated as N gas in the alloy, and a sound alloy cannot be obtained. Therefore, N is 0.1 to 0.4%.

Cu: 3.0% or less Cu is an austenite-forming element and is also effective in corrosion resistance to non-oxidizing acids. However, if added in excess of 3.0%, it segregates at the grain boundaries, causing embrittlement at the grain boundaries, and significantly lowering the weldability. Therefore, Cu is 3.0% or less.

W: 3.0% or less W improves the corrosion resistance and strength when it forms a solid solution in the matrix. However, W is an expensive element, and even if added in excess of 3%, the effect corresponding to the cost increase cannot be obtained, and machinability is also deteriorated. Therefore, W should be 3.0% or less.

Co: 3.0% or less Co, like W, forms a solid solution in the matrix to improve corrosion resistance and strength. Co is also an austenite forming element. However, even if added over 3%, the effect corresponding to the cost increase cannot be obtained. Therefore, Co is 3.0% or less.

Nb: 2.0% or less Nb is a strong carbide-forming element and combines with C in the alloy to form Nb carbide. The formation of Nb carbide improves the intergranular corrosion resistance of the alloy and is effective in preventing localized corrosion such as welds. However, if over 2.0% is added, Nb carbides are excessively formed, leading to a decrease in toughness. Therefore, Nb should be 2.0% or less.

Ti: 0.5% or less Ti, like Nb, is a strong carbide-forming element, and combines with C in the alloy to form Ti carbide. However, Ti easily bonds with N, and if added in excess of 0.5%, Ti precipitates as Ti nitride, leading to not only corrosion resistance but also strength reduction. Therefore, Ti is 0.5% or less. The alloy of the present invention contains the above-mentioned constituent elements, and the balance consists essentially of Fe. However, P, S, and other impurities that are inevitably mixed in in the melting technology are acceptable within the range normally allowed for this type of alloy. Next, the features of the present invention will be clarified with reference to examples.

[0020]

[Example] The alloys shown in Table 1 were melted in a high frequency induction melting furnace and cast in a JIS G 0307 No. A test piece mold (sand mold). Each alloy was subjected to solution treatment by holding it at 1100-1150 ° C. for 4 hours and then cooling with water, and subjected to each test. Table 1 also shows the results of measuring the amount of ferrite after solution heat treatment with a ferrite scope.
Are also shown.

[0021] Table 2 shows the tensile properties of each alloy at 450 ° C, the solution heat treated material, and the result of the Charpy impact test at room temperature after aging for 1000 hours at 475 ° C after the solution heat treatment. The alloys of the present invention have significantly higher strength than conventional austenitic stainless cast steels (SCS13, SCS14, etc.) and are comparable to austenitic-ferrite duplex stainless cast steels. Further, the toughness after aging of the alloy of the present invention hardly deteriorates.

[0022] Table 3 shows the results of pitting potential measurement in a 3.5% NaCl aqueous solution at 30 ° C. It is clear that the alloy of the present invention exhibits pitting corrosion resistance equal to or higher than that of conventional austenitic stainless cast steel, and is superior in pitting corrosion resistance to the forged product SUS202.

[0023]

[0024]

As shown in Tables 2 and 3, the alloy of the present invention has a strength at high temperature which is almost twice as high as that of SCS13 and SCS14, and deterioration of toughness hardly occurs. Furthermore, the corrosion resistance is equal to or higher than that of SCS13 and SCS14. This is a Ni-based alloy
The characteristics are comparable to those of Ti alloy, and the cost is SCS1.
There is almost no difference from 3 and SCS14.

Therefore, the alloy of the present invention is suitable as a substitute material for high-temperature structural members which have conventionally used Ni-based alloys and Ti alloys.

Claims (2)

[Claims] 1. C: 0.1% or less by weight%, Si: 2.0% or less, M
n: 4.0-15.0%, Ni: 4.0-10.0%, Cr: 15.0-23.0%, Mo: 0.
High yield strength and high corrosion resistance austenitic stainless cast steel consisting of 2 to 0.4%, balance Fe and inevitable impurities. 2. C: 0.1% or less, Si: 2.0% or less, Mn:
4.0 to 15.0%, Ni: 4.0 to 10.0%, Cr: 15.0 to 23.0%, Mo: 0.2
~ 4.0%, N: 0.1 to 0.4%, further Cu: 3.0% or less, W: 3.0% or less, Co: 3.0% or less, Nb: 2.0% or less, Ti: 0.5% or less, any one kind or two kinds. A high-strength, high-corrosion-resistant austenitic stainless cast steel containing the above and the balance being Fe and inevitable impurities.