JPS5912737B2 - Duplex stainless steel for oil country tubular goods with excellent corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a duplex stainless steel for oil country tubular goods that has excellent corrosion resistance, particularly stress corrosion cracking resistance.

In recent years, there has been a marked trend toward deeper oil and natural gas wells, and there have been many cases where the produced gas contains large amounts of corrosive substances such as wet hydrogen sulfide and chlorine ions. As the usage conditions for oil country tubular goods become more severe along with this trend, corrosion protection of oil country tubular goods becomes even more important for stable operation. Examples of 2 to 3 include measures against corrosion of oil country tubular goods and the use of corrosion inhibitors called inhibitors to prevent corrosion by forming a film on the pipe surface, but all of these methods are used under the harsh operating conditions mentioned above. However, we cannot expect a satisfactory effect. Due to these circumstances, there has been a recent trend toward using high-grade corrosion-resistant materials for oil country tubular goods, and consideration has begun to be given to the use of various types of stainless steel, as well as high-alloy steels such as Incoloy and Bass Alloy. However, to date, the details of the corrosion behavior in the HS-CO2-Cl oil well environment have not been fully elucidated, and the relationship between corrosion and steel composition in such an environment has not been fully elucidated.
Only a few existing high-alloy steels have been tested for suitability.

The present invention is directed to this highly corrosive, H2S-CO
The object of the present invention is to provide steel for oil country tubular goods that exhibits excellent durability even under the oil well environment of 2-CI.

That is, the gist of the present invention is that C: A☆0.
1% or less, Sil. 0% or less, Mn2. 0% or less, PO
．． O3% or less, 80.005% or less, NO. 3% or less, MO4. Contains 0% or less, Ni. Contains Cr within a range that satisfies the following formula and formula, 24%≦Cr(%)-1-
MO (%)≦29% ...and in some cases C
The duplex stainless steel for oil country tubular goods has excellent corrosion resistance and is characterized by containing ul% or less and the remainder being substantially Fe.

According to the experiments and research of the present inventors, H2S-CO2-C
The main type of corrosion in the Z-environment is stress corrosion cracking, and the corrosion behavior of stress corrosion cracking in this case is completely different from that of general stainless steel, and is different from general stress corrosion cracking. is closely related to the presence of CI-, whereas in the oil well environment, it has become clear that the influence of H2S is greater than that of Cl-.

On the other hand, steel pipes used for practical use as oil country tubular goods are generally cold-worked prior to use in order to obtain the strength required for oil country tubular goods, but it is said that cold working has a significant negative effect on stress corrosion cracking. This was also found as a result of research by the present inventors. Duplex stainless/stainless steel is no exception, and generally shows fairly high resistance to stress corrosion cracking in an H2S-CO2-CA environment, but this steel is also used as oil country tubular goods due to solid solution treatment. Cold working is essential because the strength is insufficient if heat treated, and when this cold working is applied, stress corrosion cracking resistance is improved, especially when tensile stress is applied perpendicular to the rolling direction (T direction). There is a strong tendency to decline. The present inventors have discovered that the inherent H2S of duplex stainless steel even after cold working.
-CO2-CI- We are conducting systematic experiments and research on steel compositions that maintain stress corrosion cracking resistance in the environment, and as a result, we have found that stress corrosion cracking resistance in the T direction does not deteriorate even after cold working. This led to the development of a small duplex stainless steel. Figure 1 shows stress corrosion cracking resistance and Cr (%) + MO (%) (hereinafter indicated by X) under the above oil well environment.
, Ni (support)) −+−30 {C (%) ten NφB (hereinafter,
(denoted by y). This shows the results of one test, but that test was for Cr, Ni, MO*C.
．． After melting Cr-Ni-MO steel with various N contents, forging and hot rolling to a thickness of C7mJX,
After holding at 1050°C for 30 minutes and cooling with water, the plate was subjected to solid solution treatment, and then the strength was increased with 30% cold treatment, and a test piece of 2T1j/t thickness x 107LIn width x 75 lengths was made from this board in the T direction. A stress corrosion cracking test was conducted. For the stress corrosion cracking test, a tensile stress equivalent to 70% of the 0.2% proof stress was applied to the test piece using the three-point beam jig shown in Figure 2, and the test piece was heated to room temperature with ``'' C1 atm H2S, lO
20 saturated H2S and CO2 with atmospheric CO2
% NaCl solution (temperature = 200°C) for 1000 hours, and the presence or absence of stress corrosion cracking was examined.
In the figure, ○2 indicates no cracking, and × indicates cracking occurred.
What is clear from the figure is that the value of X is 24% or more and
Good stress corrosion cracking resistance is maintained even after cold working when the values of It is.

Needless to say, (x-y is equivalent to the intermediate equation in the above equation.The reasons for limiting each component in the steel of the present invention are as follows.

C: An element that contributes to improving strength and austenite phase stability, and is effective in reducing the amount of Ni used, but if it exceeds 0.1%, it becomes a starting point for corrosion during cooling after solution treatment. Formation of chromium carbide is observed, resulting in deterioration of corrosion resistance.

N: Has the same effect as C, but if it exceeds 0.3%, it will have an adverse effect on corrosion resistance.

Both Cr and MO2 are effective elements for increasing stress corrosion cracking resistance, and MO in particular has a great effect even in small amounts.

However, M
If O exceeds 4%, the σ phase tends to precipitate during cooling after solution treatment, and there is a risk of loss of toughness.
%) + MO (%)≦29%, MO≦4%. Cu: Addition of up to 1% is advantageous in some cases since it has the effect of increasing corrosion resistance, especially in an acidic environment.

If it exceeds 1%, hot workability deteriorates.

Regarding other steel components, Si: necessary as a deoxidizing agent, but if it exceeds 1.0%, hot workability deteriorates.

Mn: Necessary as a deoxidizer and does not affect stress corrosion cracking, so Mn was allowed up to 2.0%.

P: Since it is harmful to stress corrosion cracking properties, the content should be 0.0a% or less.

S: 0.00 as S significantly deteriorates hot workability.
5% or less.

Next, the effects of the present invention will be explained based on examples.

Steel having each component (1}-(17) shown in Table 1 is melted,
A steel pipe with an outer diameter of 607njI1 and a wall thickness of 4u was manufactured using a normal pipe manufacturing method, and 20% cold stress was added to the pipe to increase its strength, thereby making it into an oil country tubular product. From this oil country pipe, a 20 mm long tubular body with a central angle corresponding to 60 mm of oil cut out was taken as a test piece, set as shown in Fig. 3, and bolted and nutted to 0.0 mm on the outer surface of the tube. A tensile stress equivalent to 70% of the 2% proof stress is applied in the T direction, that is, a direction where stress corrosion cracking is likely to occur, and 20% is saturated with 1 atm H2S and 10 atm CO2.
The specimens were immersed in NaCit solution (temperature: 2200°C) for 1000 hours, and the presence or absence of stress corrosion cracking was investigated. The results are shown in Table 1. 應(n) is a duplex stainless steel equivalent to SUS329Jl, but this and other comparative examples (11H47) in which any of the steel components are outside the scope of the present invention are all In contrast to the occurrence of stress corrosion cracking under the H2S-CO?-CllJj condition, no stress corrosion cracking was observed in the inventive examples of (1) to Oki, demonstrating the usefulness of the inventive steel. .

As is clear from the above description, the duplex stainless steel of the present invention maintains high resistance to stress corrosion cracking in an H2S-CO2-CI environment even after cold working. It exhibits excellent durability when used in oil country tubular goods that are used after partial processing.

[Brief explanation of the drawing]

Figure 1 shows Cr (a) + MO1 and Ni (a) 130 in steel.
Figure 2 shows the influence of {C(A) 10N (%)} on stress corrosion cracking resistance. Figure 2 shows the stress corrosion cracking tester for plate specimens. Figure 3 shows the stress corrosion cracking tester for tubular specimens. FIG.

Claims (1)

[Claims] 1 C 0.1% or less, Si 1.0% or less, Mn 2.0%
Below, P0.03% or less, S0.005% or less, N0.
3% or less, Mo4.0% or less, Ni and Cr within a range that satisfies the following formulas (1) and (2), and 24%≦Cr(
%)+Mo(%)≦29%...(1)10%≦Cr(
%)+Mo(%)-Ni(%)-30{C(%)+N(
%)}≦16%...(2) A duplex stainless steel for oil country tubular goods having excellent corrosion resistance, characterized in that the remainder is substantially made of Fe. 2 C0.1% or less, Si1.0% or less, Mn2.0%
Below, P0.03% or less, S0.005% or less, N0.
3% or less, Mo4.0% or less, Ni and Cr within a range that satisfies the following formulas (1) and (2), and 24%≦Cr
(%)+Mo(%)≦29%...(1)10%≦Cr
(%) + Mo (%) - Ni (%) - 30 {C (%) + N
(%)}≦16% (2) A duplex stainless steel for oil country tubular goods having excellent corrosion resistance, which further contains 1% or less of Cu, and the remainder is substantially composed of Fe.