JPH05302150A - Duplex stainless steel excellent in hydrogen sulfide corrosion resistance - Google Patents

Abstract

PURPOSE:To provide duplex stainless steel in which H2S corrosion resistance is improved and hot workability is scarcely deteriorated comparted to that of the conventional steel SUS 320J2L. CONSTITUTION:The amounts of S, O and Ca to be added are limited as well as the amounts of Cr, Ni, Mo and N to be added contributing to the phase balance affecting its hot workability are limited, by which, while hot workability equal to that of the conventional steel is maintained, the amounts of Cr, Ni, Mo and N controlling its H2S corrosion resistance are optimized within the limiting range. The compsn. of the steel is constituted of <=0.03% C, <=1.0% Si, <=1.5% Mn, <=0.03% P, <=0.0015% S, 24 to 26% Cr, 9 to 13% Ni, 4 to 5% Mo, 0.03 to 0.20% N, 0.01 to 0.04% Al, <=0.005% O and 0.001 to 0.005% Ca.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a duplex stainless steel having corrosion resistance suitable for oil well pipes, line pipes and the like used in a wet H ₂ S environment.

[0002]

2. Description of the Related Art S is a typical composition of duplex stainless steel.
US329J ₂ L (C ≦ 0.03%, Si ≦ 1.0%,
Mn ≦ 1.5%, P ≦ 0.04%, S ≦ 0.03%, C
r: 22-26%, Ni: 4.5-7.5%, Mo:
2.5-4.0%, N: 0.08-0.3%). Steel having such a composition is composed of a mixed structure of ferrite (α) and austenite (γ), and is known to exhibit excellent mechanical properties and corrosion resistance that are difficult to obtain with each single-phase stainless steel because of the mixed structure. There is.

Various applications have been developed that take advantage of this feature. For example, it has a track record of being widely used in seawater heat exchangers and the like because of its excellent resistance to stress corrosion cracking in a chloride environment. Recently, evaluation studies have been conducted to apply it to highly corrosive oil and gas wells, and duplex stainless steel pipes have been applied to some oil and gas wells and pipelines. Highly corrosive oil-and-gas well here means CO ₂ in addition to Cl ⁻ ,
It is an environment containing an acidic gas such as H ₂ S.

However, SUS329J having the above composition
_{With 2} L, there is a limit to application to oil and gas wells containing acid gas from the viewpoint of corrosion resistance. For example, J. Sakai et al .: CO
22Cr-6Ni-3Mo-0.15, as described in RROSION '87 No. 292 (1987).
It is known that N-type DIN 1.4462 steel cannot be used at a H ₂ S partial pressure of 0.02 atm or more, regardless of temperature, because corrosion resistance deteriorates. The limit of corrosion resistance of duplex stainless steel is said to be determined by the H ₂ S partial pressure.

[0005]

High alloying is effective for improving the corrosion resistance of stainless steel. High Ni under H ₂ S environment
The alloy is used because Ni addition is effective for H ₂ S corrosion resistance. However, if Ni is simply applied to the duplex stainless steel, the α-γ phase balance deviates from the optimum range in use performance, and in order to compensate for this, high Cr, M
If it is also combined with o, the hot workability is significantly reduced.
As described above, it is extremely difficult to satisfy both the conditions of use performance and the conditions of production, and the upper limit of the component system that satisfies both of these conditions is SUS329J ₂ L.

The present invention has been made in view of the above circumstances, and improves the H ₂ S corrosion resistance as well as the conventional steel SU.
It is an object of the present invention to provide a duplex stainless steel that does not significantly deteriorate hot workability as compared with S329J ₂ L.

[0007]

Means for Solving the Problems The inventors of the present invention have, after studying, concluded that the above-mentioned objects can be achieved by the following component systems. That is, in weight%, C ≦ 0.03% and Si ≦ 1.
0%, Mn ≦ 1.5%, P ≦ 0.03%, S ≦ 0.00
15%, Cr: 24.0 to 26.0%, Ni: 9.0 to
13.0%, Mo: 4.0-5.0%, N: 0.03-
0.20%, Al: 0.01 to 0.04%, O ≦ 0.0
In a component system containing 05%, Ca: 0.001 to 0.005%, and the balance being substantially Fe, a duplex stainless steel having excellent H ₂ S corrosion resistance and good hot workability is obtained. can get.

In the present invention, the grounds for setting the component system of such steel will be described. C: C forms a Cr deficient layer along with the formation of Cr carbides and reduces the corrosion resistance, so the upper limit of the addition amount was made 0.03%. Si: added as a deoxidizer and remained, and an excessive content lowers the hot workability, so the upper limit was 1%. Like Mn: Si, it is added as a deoxidizing agent and remains, and an excessive content reduces corrosion resistance, so the upper limit is 1.
It was set to 5%. If P: 0.03% or more is contained, the stress corrosion cracking resistance decreases, so the upper limit was made 0.03%. S: S is an element that is extremely harmful to hot workability, so it should be reduced as much as possible. Considering the balance between O and Ca, which is related to hot workability, like S, the upper limit is 0.001.
It was set to 5%.

Cr: An essential element from the viewpoint of corrosion resistance, but the addition of a large amount not only impairs the α-γ phase balance but also promotes precipitation of the σ phase, leading to deterioration of hot workability. Similar to Cr, it is set to 24 to 26% in consideration of the balance with Ni, Mo and N related to corrosion resistance and hot workability. Ni: An essential element from the viewpoint of corrosion resistance, but if added in a large amount, it destroys the α-γ phase balance and deteriorates the function as a duplex stainless steel. 9 to 1 considering the balance with Cr, Mo and N related to phase balance, corrosion resistance and hot workability.
It was set to 3%. Mo: Cr is an element more effective in corrosion resistance than Cr. However, like Cr, addition of a large amount thereof impairs the α-γ balance and promotes precipitation of the σ phase, leading to deterioration in hot workability. Considering the balance with Ni, Cr, N related to corrosion resistance and hot workability,
It was set to 5%.

N: N is an element effective for corrosion resistance. However, if added in a large amount, the phase balance is disturbed or casting defects are caused to hinder the hot workability. Therefore, it is related to the phase balance, the corrosion resistance and the hot workability. In consideration of the balance with Cr, Ni, and Mo, the content was set to 0.03 to 0.20%. Al: Al is added as a deoxidizer and remains, and if it is contained too much, the toughness decreases, so its content was made 0.04% or less. O: O is an element that is extremely harmful to hot workability, so it should be reduced as much as possible. S and Ca related to hot workability
In consideration of the balance with the above, the upper limit was made 0.005%. Ca: Ca is an element that improves hot workability, but if it is added in a large amount, the hot workability is rather deteriorated, so the addition amount was made 0.001 to 0.005%.

[0011]

EXAMPLES Next, examples of the present invention will be described. Table 1 shows the chemical composition of the test steel. The sample steel was melted in a vacuum melting furnace to form a steel ingot. Then, hot rolling was performed at a reduction rate of 80% to evaluate the presence or absence of cracks. For the types that did not crack, the rolled material was subjected to solution heat treatment and the thickness was 1 mm x
A 4-point bending test piece with a width of 10 mm and a length of 65 mm was sampled, and stress of 100% of the yield strength was applied to it to obtain 1 atm H ₂ S.
A 20% NaCl solution containing 80 ° C. was subjected to a dip test for 336 hours to evaluate the presence or absence of pitting corrosion or SCC. Occurrence of rolling cracks (○: Small cracks or no cracks,
Table 2 shows the occurrence of pitting or stress corrosion cracking (x: large cracking) and pitting or stress corrosion cracking (○: no corrosion, ●: pitting or stress corrosion cracking).

From Table 2, it is clear that the steel of the present invention has excellent corrosion resistance under H ₂ S environment. Further, the steel of the present invention has a comparative example No. 1 (SUS329J ₂ L) does not cause large rolling cracks. On the other hand, Comparative Example No. 1 (SUS32
9J ₂ L) has little Ni and Mo, so pitting corrosion and stress corrosion cracking occur. Comparative Example No. Nos. 3, 4, 6 and 8 contained a small amount of Cr, Ni, Mo and N, so that pitting corrosion or stress corrosion cracking occurred. Furthermore, Comparative Example No. 2, 5,
In Nos. 7 and 9, since Cr, Ni, Mo, and N were contained in excess of the appropriate ranges, severe rolling cracking occurred due to σ phase precipitation, improper phase balance, and casting defects. Also, Comparative Example N
o. In Nos. 10, 11, and 12, Ca, O, and S exceeded the proper range, respectively, and thus severe rolling cracks occurred.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

According to the present invention, excellent corrosion resistance is exhibited in an H ₂ S environment and hot working can be performed relatively easily. 2
A duplex stainless steel was obtained. In other words, the steel of the present invention is said to be unusable in conventional duplex stainless steels.
_It can withstand environmental conditions of ₂ S partial pressure of about 1 atm, and has a function as a substitute steel for the high-cost high Ni alloy that has been used for such environment. Therefore, when the present invention is applied to oil well pipes, line pipes, etc., it has great effects in reducing the construction cost of oil wells and pipeline equipment and improving the durability of equipment. In addition, conventional steel (SUS329J
It has the same level of hot workability as ₂ L) and can be manufactured by ordinary processes such as rolling and extrusion.

Claims (1)

[Claims] 1. By weight%, C ≦ 0.03%, Si ≦ 1.0%, Mn ≦ 1.5%, P ≦ 0.03%, S ≦ 0.0015%, Cr: 24.0. 26.0%, Ni: 9.0 to 13.0%, Mo: 4.0 to 5.0%, N: 0.03 to 0.20%, Al: 0.01 to 0.04%, O ≦ 0.005%, Ca: 0.001 to 0.005%, a duplex stainless steel excellent in hydrogen sulfide (H ₂ S) corrosion resistance, the balance being substantially Fe.