JPH05247592A - Dual phase stainless steel excellent in cracking resistance and corrosion resistance - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance and cracking resistance of a dual phase stainless steel. CONSTITUTION:The steel has a composition consisting of, by weight, <=0.02% C, <=2.0% Si, <=2.0% Mn, <=0.04% P, <=0.04% S, 3-7% Ni, 17-27% Cr, 0.5-6.0% Mo, 1-5% Cu, <=3% W, 0.05-0.3% N, and the balance essentially Fe. Further, 0.1-1.0% Co and/or 0.0005-0.0015% B can be incorporated, if necessary. Moreover, respective contents of Cr, Mo, N, and delta-ferrite are regulated so that they satisfy the relations in Cr(%)+3.3Mo(%)+16N(%)>=30 and 8.09-0.26Cr(%)-0.62Mo(%)+0.028.(delta-ferrite)(%)>=1.7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in duplex stainless steel, and more specifically, it is used as a material for thick-walled centrifugally cast pipes such as sleeves for papermaking machines and thick-walled cast products such as large propellers for seawater. To improve duplex stainless steel.

[0002]

2. Description of the Related Art For example, a sleeve for a papermaking machine,
Heavy-walled large-sized castings such as large-scale propellers for seawater are required to have corrosion resistance and high strength in a corrosive environment. Conventionally, various duplex stainless steels having two phases of austenite and ferrite have been used for this kind of application. However, large stainless steel products have a large mass effect and the cooling rate after casting or forging is extremely slow. Therefore, there is a problem that sigma phase, carbonitride, etc. are deposited during the cooling to cause cracks. Further, these precipitates are difficult to dissolve in the matrix even if solution treatment is performed at a high temperature, and there is a problem that the corrosion resistance and toughness are adversely affected. Therefore, for large-sized stainless steel products, the emergence of a material having high crack resistance and having desired corrosion resistance and strength is required.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a duplex stainless steel which is excellent in both crack resistance and corrosion resistance and is suitable as a material for large stainless steel products.

[0004]

In order to achieve the above object, the duplex stainless steel of the present invention has a C: 0.
02% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.
04% or less, S: 0.04% or less, Ni: 3 to 7%, Cr: 17 to
27%, Mo: 0.5 to 6.0%, Cu: 1 to 5%, W: 3% or less, N: 0.05 to 0.3%, the balance consisting essentially of Fe, and the amounts of Cr, Mo, N and δ ferrite are (a) Cr (%) + 3.3Mo (%) + 16N (%) ≧ 30, and (b) 8.09−0.26Cr (%) − 0.62Mo (%) + 0.028 · δ
Ferrite (%) ≧ 1.7.

The duplex stainless steel of the present invention may optionally contain Co: 0.1 to 1.0% and / or B: 0.0005 to 0.0015%.
Can be included.

[0006]

[Explanation of reasons for limiting ingredients]

C: 0.02% or less C is an austenite forming element and contributes to the improvement of strength, but when the content is large, carbides are precipitated and corrosion resistance is impaired. Therefore, the upper limit is 0.02%.

Si: 2% or less Si is effective for deoxidizing molten steel, but if the content is high, the toughness is impaired. Therefore, the upper limit is 2%.

Mn: 2% or less Mn is effective for desulfurization of molten steel, but the corrosion resistance deteriorates when the content increases. Therefore, the upper limit is 2%.

P and S: 0.04% or less P and S are impurities, but there is no particular problem as long as both elements contain 0.04% or less.

Ni: 3 to 7% Ni stabilizes the austenite phase and contributes to the improvement of toughness. On the other hand, if the content is too large, the quantitative balance with the ferrite phase is lost. Therefore, specify 3 to 7%.

Cr: 17-27% Cr contributes to the improvement of corrosion resistance and also enhances the strength by forming a ferrite phase. Therefore, it is necessary to contain at least 17%. On the other hand, if the content is too large, the toughness of the steel is lowered, and the sigma phase precipitates, causing embrittlement during casting. Therefore, the upper limit is 27%.

Mo: 0.5-6% Mo contributes to the improvement of corrosion resistance, so at least 0.5%
The above is contained. However, when the content is large, embrittlement is caused due to precipitation of carbides, sigma phase and the like. Therefore, the upper limit is 6%.

Cu: 1-5% An austenite forming element, which contributes to solid solution strengthening of the austenite phase and improvement of corrosion resistance. However, if the content is too large, the toughness is lowered due to the formation of the intermetallic compound. Therefore, the content is specified as 1 to 5%.

W: 3% or less A ferrite-forming element that contributes to the improvement of corrosion resistance.
On the other hand, if the content exceeds 3%, the corresponding effect cannot be obtained. Therefore, the upper limit is 3%.

N: 0.05 to 0.3% N delays the precipitation of the sigma phase and greatly contributes to the improvement of the yield strength, so at least 0.05% is contained. on the other hand,
If it exceeds 0.3%, nitrides are likely to precipitate, resulting in deterioration of toughness and deterioration of corrosion resistance. Therefore, the upper limit is specified as 0.3%.

The above duplex stainless steel contains each of the above components, and the balance is unavoidably mixed impurity elements and Fe.
Consists of.

Since the duplex stainless steel of the present invention has desired corrosion resistance, Cr, Mo and N have a relationship of Cr (%) + 3.3Mo (%) + 16N (%) ≧ 30. is doing. Particularly effective elements for corrosion resistance are Cr, Mo and N. It is considered that the more these elements are contained, the higher the corrosion resistance is. Therefore, the coefficient is determined in consideration of the contents and efficacy of these elements, and the present inventor obtains desired corrosion resistance when Cr (%) + 3.3Mo (%) + 16N (%) is 30 or more. I found that I can be. These will be clarified by Examples described later.

Further, the duplex stainless steel of the present invention has desired crack resistance, so that Cr, Mo and δ ferrite content (area ratio) is 8.09-0.26Cr (%)-0.62Mo (% ) + 0.028 · δ ferrite (%) ≧ 1.7. Cr and Mo are effective for corrosion resistance, but if they are contained too much, precipitates are generated and cracking is likely to occur. If the amount of δ-ferrite in the metal structure is too small, the chromium-forming element is concentrated in the ferrite, which promotes precipitation centered on the sigma phase. The present inventor determined the coefficient in consideration of the above, and determined the ease of harmful precipitates to be 8.09-0.26Cr (%)-0.62Mo.
It was expressed by the parameter (%) + 0.028 · δ ferrite (%), and it was found that the crack resistance becomes high when this parameter value is 1.7 or more. This will be clarified also in the examples described later.

In the stainless steel of the present invention,
If necessary, a part of the above component elements may be replaced with one or two of the component elements described below.

Co: 0.1 to 1.0% Co has the effect of improving strength and corrosion resistance in the coexistence of Cr, Mo, Cu and W. Therefore, it is desirable to contain at least 0.1%. However, even if the content exceeds 1.05, the corresponding effect cannot be obtained.
Therefore, the upper limit is 1.0.

B: 0.0005 to 0.0015% Since B has the effect of improving the corrosion resistance, it is desirable to contain 0.0005% or more. On the other hand, if the content of Ti exceeds 0.0015%, the cleanliness of the material is lowered and the material becomes brittle. Therefore, the upper limit is 0.0015%.

[0022]

Example: Stainless steel of each component composition was melted and cast
(Outer diameter φ400 mm, inner diameter φ300 mm, length 600 mm). Next, after solution treatment (1100 ℃ x 4 hours, water cooling), annealing (600 ℃
× 1 hour). In addition, sand casting casting was performed with the same molten metal, and a crackability test was performed. A pitting corrosion resistance test was performed on each test material to examine the pitting corrosion resistance. Table 1 shows the component composition of each test material and the test results. In Table 1, sample materials No. 1 to 11 are examples of the present invention, and sample materials No. 12 to
Reference numeral 17 is a comparative example in which the numerical value of the formula (a) or the formula (b) deviates from the scope of the present invention. Of the comparative examples, sample materials No. 13 and 14
Formula (a) deviates from the scope of the present invention, and test materials Nos. 12 and 15
(17) deviates from the scope of the present invention.

[0023]

[Table 1]

In Table 1, "corrosion weight loss" is the result of the pitting corrosion test, and the 3% NaCl-0.08N HCl pitting corrosion test (3
It shows the amount of corrosion when immersed in a boiling solution of% NaCl-HCl for 24 hours. The smaller the number, the smaller the amount of corrosion and the better the corrosion resistance.

In the cracking test, a casting casting product was produced by sand casting according to the method shown in FIG. 1, and the presence or absence of cracking in the casting product was evaluated. Penetrant flaw detection test (JIS Z 2343)
It was evaluated by.

As is clear from the results shown in Table 1, the specimens whose formula (a) deviates from the scope of the present invention have poor corrosion resistance, and the specimens whose formula (b) deviates from the scope of the present invention. In contrast to the fact that cracking is observed, the test material of the present invention has a small amount of corrosion loss, no cracking is observed, and is excellent in pitting corrosion resistance and cracking resistance.

[0027]

INDUSTRIAL APPLICABILITY The duplex stainless steel of the present invention has high crack resistance and excellent corrosion resistance, and therefore is very suitable as a material for large-sized stainless steel products.

[Brief description of drawings]

FIG. 1 is a schematic view of a cast-in-place cast product manufactured to check for cracks.

Claims (2)

[Claims] 1. C: 0.02% or less and Si: 2. in% by weight.
0% or less, Mn: 2.0% or less, P: 0.04% or less, S: 0.04
% Or less, Ni: 3 to 7%, Cr: 17 to 27%, Mo: 0.5 to
6.0%, Cu: 1-5%, W: 3% or less, N: 0.05-0.3
%, The balance consisting essentially of Fe, and the amounts of Cr, Mo, N and δ ferrite (area ratio) are in the ranges of (a) and (b) below. Excellent duplex stainless steel. (a) Cr (%) + 3.3Mo (%) + 16N (%) ≧ 30 (b) 8.09−0.26Cr (%) − 0.62Mo (%) + 0.028 ・ δ
Ferrite (%) ≧ 1.7 2. Duplex stainless steel according to claim 1, containing Co: 0.1-1.0% and / or B: 0.0005-0.0015% by weight.