JPH08176742A - Duplex stainless steel excellent in corrosion resistance in hydrogen sulfide atmosphere - Google Patents

Abstract

PURPOSE: To obtain a two-phase stainless steel excellent in hydrogen sulfide corrosion resistance under atmosphere containing high concn. of the hydrogen sulfide. CONSTITUTION: The duplex stainless steel containing, by wt., 0.01-1.0% Si, 0.1-1.5% Mn, 20.0-30.0% Cr, 5.0-10.0% Ni, 0.01-0.2% sol-Al, >0.2 to 0.4% N and 2.0-5.0% Cu, and 2.5-5.0% Mo and/or 2.0-3.0% W, and further, one or more kinds among 0-0.10% REM, 0-0.20% Y, 0-0.10% Mg and 0-0.10% Ca, and the balance Fe with inevitable impurities, in which C, P and S are <=0.030%, <=0.03% and <=0.01%, respectively is used. By this constitution, the two layer structural films composed of the corrosion resistant film of mainly Ni sulfide containing copper sulfide at the outer layer on the steel surface by adding Cu and the passive film composed of densified Cr oxide at the inner layer are produced and formed.

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, and more particularly to a duplex stainless steel having excellent corrosion resistance in a hydrogen sulfide environment.

[0002]

2. Description of the Related Art Duplex stainless steel is disclosed, for example, in Japanese Patent Laid-Open No.
As disclosed in Japanese Patent Publication No. 0-91516 and the like, alloy design is performed in consideration of seawater corrosion resistance, and since it is excellent in seawater corrosion resistance, it is often used as a material for seawater resistance.

However, generally, the duplex sulfide stainless steel is not good in hydrogen sulfide resistance, and is a material for an oil well pipe or an oil well flow line used when producing crude oil or natural gas containing hydrogen sulfide of about 0.3 atm. , Or when handling substances containing hydrogen sulfide at the same level, for example, when used as piping material for desulfurization equipment or piping material for geothermal power generation equipment, stress corrosion cracking susceptibility is high. It has the disadvantage that it becomes the highest around ℃. That is, in the corrosion of the duplex stainless steel in a hydrogen sulfide environment, the selective corrosion is remarkably generated in the ferrite phase and the austenite phase constituting the duplex stainless steel, which causes stress corrosion cracking in the ferrite phase. Occurs, and the selective corrosion of the ferrite phase is most likely to occur in the temperature range around 80 ° C, and thus the stress corrosion cracking susceptibility becomes highest near 80 ° C (Performance of
high corrosion resistant duplex stainless steel i
n ch-loride and sour environments, Corrosion / '93,
Paper No.125, NACE Interna-tional, Houston (1993)
reference).

Therefore, there is a strong demand for a duplex stainless steel having excellent hydrogen sulfide corrosion resistance in an environment containing a large amount of hydrogen sulfide.

[0005]

DISCLOSURE OF THE INVENTION The object of the present invention was made in view of the above circumstances, and is a duplex stainless steel excellent in hydrogen sulfide corrosion resistance in an environment containing about 0.5 atm of hydrogen sulfide. To provide.

[0006]

The gist of the present invention resides in a duplex stainless steel having excellent corrosion resistance in the following hydrogen sulfide environment.

% By weight, Si: 0.01-1.0%, M
n: 0.1 to 1.5%, Cr: 20.0 to 30.0%,
Ni: 5.0-10.0%, sol-Al: 0.01-
0.2%, N: more than 0.1%, 0.4% or less and C
u: 2.0 to 5.0%, and Mo: 2.5 to 5.0%
And W: 2.0 to 3.0% of 1 type or 2 types, and further REM: 0 to 0.10%, Y: 0 to 0.20.
%, Mg: 0 to 0.10% and Ca: 0 to 0.10%
1 or 2 or more of the above, the balance is Fe and unavoidable impurities, and C, P, and S in the impurities are 0.03% or less, 0.03% or less, and 0.01% or less, respectively. A duplex stainless steel with excellent corrosion resistance in a hydrogen sulfide environment, which is characterized by being present.

In the above duplex stainless steel, RE
None of M, Y, Mg and Ca may be added. In the case of positively adding these, one or more kinds can be selected from REM, Y, Mg and Ca and added, but the content range is 0.0 in REM.
01 to 0.10%, Y 0.001 to 0.20%, Mg
0.001-0.10%, Ca 0.001-0.1%
It is desirable to set it to 0%.

As a result of various experimental studies, the present inventor has made the present invention based on the findings described in and.

In order to improve the stress corrosion cracking resistance of the duplex stainless steel in a low hydrogen sulfide environment containing about 0.5 atm of hydrogen sulfide, the effect of various alloy components was examined, and as a result, an appropriate amount was found. While adding and containing Cu, 2
Cr, Ni, which are the main alloying components of duplex stainless steel,
We have found a new fact that hydrogen sulfide corrosion resistance is dramatically improved when the content of N added is set in an appropriate range. The mechanism for improving the hydrogen sulfide corrosion resistance by an appropriate amount of Cu is not clear in detail, but it is presumed as follows.

That is, the corrosion resistance of duplex stainless steel is
Usually, it is secured by a single passivation film made of Cr oxide. However, when an appropriate amount of Cu is added and contained, since Cu is an element which easily forms sulfide, Cu sulfide is a corrosion product. And a corrosion resistant film mainly composed of Ni sulfide containing copper sulfide is formed and formed on the passivation film composed of the Cr oxide formed on the steel surface.
While this corrosion-resistant film acts as a barrier layer for preventing the invasion of hydrogen sulfide, the above Cu does not change to C in the same manner as Mo and / or W in an acid environment, particularly in an acid environment containing hydrogen sulfide.
Since it is an element that easily forms u oxide, the above Cr
It is considered that hydrogen sulfide corrosion resistance is dramatically improved as a result of accelerating the formation of the lower passivation film made of an oxide and densifying the passivation film to enhance its corrosion resistance.

It is possible to further improve the hot workability by selecting one or more of REM, Y, Mg and Ca as an alloy component and adding an appropriate amount thereof.

[0013]

The function and effect of the constituents of the duplex stainless steel of the present invention and the reason why the proper content is determined as described above will be described below. In the following,% is% by weight
Means

Si: 0.01 to 1.0% Si is a necessary component as a deoxidizing component, but its effect can be obtained at a content of 0.01% or more. However, if the content exceeds 1.0%, the hot workability deteriorates. Therefore, the Si content is set to 0.01 to 1.0%. It is preferably 0.2 to 0.5%.

Mn: 0.1 to 1.5% Mn, like Si, is a necessary component as a deoxidizing component, but its effect can be obtained at a content of 0.1% or more. However, if the content exceeds 1.5%, the hot workability deteriorates. Therefore, the Mn content is set to 0.1 to 1.5%. Preferably, it is 0.5 to 0.75%.

Cr: 20.0 to 30.0% Cr is resistant to seawater and hydrogen sulfide corrosion (mainly resistance to corrosion in the presence of other main components, Ni, N and Mo or / and W components). Although it is a component that improves the stress corrosion cracking property), if its content is less than 20.0%, its effect cannot be obtained. Further, even if it is reduced to less than 20.0%, hot workability is not improved at all. On the other hand, its content is 30.0%
If it exceeds, the effect is saturated, and it is inevitable that the hot workability is deteriorated no matter how much the S content is reduced. Therefore, the Cr content is determined to be 20.0 to 30.0%. It is preferably 22.0 to 27.0%.

Ni: 5.0-10.0% Ni has the function of improving the hydrogen sulfide corrosion resistance,
If the content is less than 5.0%, the desired effect cannot be obtained. On the other hand, even if the content exceeds 10.0%, the effect is saturated, the cost is increased due to the expensive component, and the economical efficiency is impaired. Therefore, the Ni content is set to 5.0 to 10.0%. It is preferably 6.0 to 8.0%.

Sol-Al: 0.01-0.2% Al is a necessary component as a deoxidizing component, like Si and Mn. The effect is sol-Al content of 0.
It is obtained at 01% or more. However, if the sol-Al content exceeds 0.2%, coarse oxides are produced and hot workability deteriorates.

Therefore, the sol-Al content is 0.01 to
It was set at 0.2%. Preferably, it is 0.05 to 0.1%.

N: Exceeding 0.1% and 0.4% or Less N has the effect of significantly improving seawater resistance and increasing the strength of steel by solid solution strengthening, but its content is 0.1%. The desired effect cannot be obtained below. On the other hand, 0.
If it is contained in excess of 4%, nitrogen does not form a solid solution sufficiently to form a nitride and deteriorate the corrosion resistance of steel. Therefore, the N content is determined to be more than 0.1% and 0.4% or less. Preferably, it is 0.25 to 0.35%.

Cu: 2.0 to 5.0% Cu is the most important component in constituting the duplex stainless steel of the present invention, and the hydrogen sulfide corrosion resistance of the steel in a low hydrogen sulfide environment. However, in order to obtain the effect, a content of 2.0% or more is required. However, even if the content exceeds 5.0%, the effect is saturated, and conversely the hot workability deteriorates. Therefore, the Cu content is set to 2.0 to 5.0%. Preferably 2.0-
It is 3.0%.

Mo and W: Mo is 2.5 to 5.0%,
W is 2.0 to 3.0% Mo and W are originally components that significantly improve seawater corrosion resistance, but also have an effect of improving hydrogen sulfide corrosion resistance. Among these components, W has an action of improving seawater corrosion resistance as well as Mo, and also has an action of suppressing precipitation of an embrittlement phase to further improve hydrogen sulfide corrosion resistance. However, if the Mo content is less than 2.5% and the W content is less than 2.0%, the effect cannot be obtained.
On the other hand, even if Mo is contained in an amount of 5.0% and W is contained in an amount of more than 3.0%, the effect is saturated, and hydrogen sulfide corrosion resistance is not significantly improved. Therefore, the Mo content is 2.5 to 5.
0% and W content were set to 2.0 to 3.0%. A preferable Mo content is 3.0 to 4.0%, and a W content is 2.0.
~ 3.0%.

These Mo and W may be contained alone or in a combination of two kinds.

C: Upper limit 0.03% When the content of C exceeds 0.03%, stress corrosion cracking easily occurs at the grain boundary, so the upper limit is 0.03%.
I decided.

P: upper limit 0.03% P is contained as an unavoidable impurity, but if its content exceeds 0.03%, it increases the sensitivity to stress corrosion cracking in a hydrogen sulfide environment. Therefore, the upper limit is set to 0.03% or less. It is preferably 0.02% or less.

S: upper limit 0.01% S is contained as an unavoidable impurity like P, but if the content exceeds 0.01%, hot workability is significantly deteriorated. Therefore, the upper limit is set to 0.01%.
As described above, when the content of S component is large, the hot workability is remarkably deteriorated, but the content thereof is decreased to 0.
On the contrary, if it is reduced to 0007% or less, the hot workability is further improved. Therefore, when hot workability under severe conditions is required, the S content is 0.0007%.
The following is preferable.

In addition to the above components, the duplex stainless steel of the present invention has the following REM (rare earth element), Y, Mg and Ca.
One or two or more of them can be contained.

REM, Y, Mg and Ca: The upper limit is R
EM, Mg and Ca 0.10%, Y 0.20% Since these components have the effect of improving hot workability, hot working under harsh conditions is necessary. When it is necessary to improve workability, REM,
One or more of Y, Mg and Ca can be selected and contained. However, if the content of any of the above components is less than 0.001%, the desired effect due to the above action cannot be obtained. On the other hand, REM, Mg and C
In the case of a, the content of each component is 0.10%.
, And in the case of Y, the content is 0.20%
If it exceeds, coarse oxides are produced, which rather deteriorates hot workability. Therefore, when these components are contained, the content of REM is 0.001 to 0.10%, Y is 0.001 to 0.20%, and Mg is 0.001 to 0.10.
%, And 0.001 to 0.10% in Ca, respectively.

In the duplex stainless steel of the present invention, the unavoidable impurities other than C, P and S are:
Even if B, Sn, As, Sb, Bi, Pb and Zn are contained in the range of 0.10% or less, the characteristics of the steel of the present invention are not impaired.

[0030]

EXAMPLES 16 kinds of steels having the composition shown in Table 1 were prepared in a conventional electric furnace and an Ar-oxygen decarburizing furnace (AO) for desulfurization purpose.
After being melted using a D furnace), an ingot having a diameter of 500 mmφ was prepared for casting. In Table 1, No. 1-10
Is the steel of the present invention, No. 11 to 16 are comparative steels.

[0031]

[Table 1]

The obtained ingots were heated at 1250 ° C.
150m in diameter after hot forging at 1200 ℃
A round billet of mφ was formed, and the obtained billet was used to form a tube having a diameter of 60 mmφ and a wall thickness of 5 mm by the hot extrusion tube forming method, and the obtained tube was held at 1100 ° C. for 0.5 hours →
After water-cooled solution treatment, predetermined test pieces were sampled and subjected to the following tests. The hot workability was evaluated by examining the presence or absence of flaws on the inner surface of the pipe after the hot extrusion pipe manufacturing.

(Tensile test) Test temperature: normal temperature Test piece: 4.0 mmφ, parallel part length 20 mm (hydrogen sulfide corrosion test) Test solution: 0.5 atmH ₂ S-30 atmCO ₂ -2
0% NaCl aqueous solution Test temperature: 80 ° C. Immersion time: 720 hours Additional stress: 0.2% 100% of actual yield strength Test piece: 10 mm width x 2 mm thickness x 75 mm length of 0.25
mmU Notch These test results are also shown in Table 1. In addition,
In the hydrogen sulfide corrosion test, those that did not cause cracking and pitting corrosion are indicated by "○" and those that did occur are indicated by "x".

As is clear from Table 1, none of the steels of the present invention showed cracking and pitting corrosion in the hydrogen sulfide corrosion test, and were excellent in hydrogen sulfide corrosion resistance. In addition, no flaws are formed on the inner surface of the pipe after hot extrusion, and the hot workability is excellent.

On the other hand, all the comparative steels having a composition outside the range of the present invention had cracks and pitting corrosion in the hydrogen sulfide corrosion test, and were inferior in hydrogen sulfide corrosion resistance.

Further, some of the steels have flaws on the inner surface of the pipe after the hot extrusion pipe making, and the hot workability is poor.

[0037]

EFFECTS OF THE INVENTION According to the present invention, a duplex stainless steel having a high corrosion resistance can be provided which is used for an oil well pipe which is in contact with a corrosive fluid of an oil well containing hydrogen sulfide, and which greatly contributes to the field.

[0038]

Claims (1)

[Claims] 1. Si: 0.01 to 1.0% by weight, M
n: 0.1 to 1.5%, Cr: 20.0 to 30.0%,
Ni: 5.0-10.0%, sol-Al: 0.01-
0.2%, N: more than 0.1%, 0.4% or less and C
u: 2.0 to 5.0%, and Mo: 2.5 to 5.0%
And W: 2.0 to 3.0% of 1 type or 2 types, and further REM: 0 to 0.10%, Y: 0 to 0.20.
%, Mg: 0 to 0.10% and Ca: 0 to 0.10%
1 or 2 or more of the above, the balance is Fe and unavoidable impurities, and C, P, and S in the impurities are 0.03% or less, 0.03% or less, and 0.01% or less, respectively. A duplex stainless steel with excellent corrosion resistance in a hydrogen sulfide environment, which is characterized by being present.