JPH0813084A - Martensitic stainless steel with excellent resistance to sulfide stress corrosion cracking - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a martensitic steel excellent in sulfide stress corrosion cracking resistance. [Structure] C: 0.005-0.1%, Si: 0.05
~ 1%, Mn: 0.1-2%, Cr: 8-11%, M
o: 0.1 to 3%, Cr + Mo ≦ 12%, Al:
0.005-0.2%, P: 0.025% or less, S:
0.010% or less, N: 0.005 to 0.15%, the balance consisting of Fe and unavoidable impurities, martensitic stainless steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Cr steel for oil well pipes having excellent resistance to sulfide stress corrosion cracking, and more specifically, an environment containing wet carbon dioxide and wet hydrogen sulfide in oil wells and gas wells. Among them, it relates to martensitic stainless steel having high sulfide stress corrosion cracking resistance.

[0002]

2. Description of the Related Art The environment of wells for collecting oil or natural gas has become more and more severe in recent years, and in addition to the increase of the mining depth, wet carbon dioxide (CO ₂ ) and hydrogen sulfide ( The number of wells containing corrosive components such as H ₂ S) and chlorine ions (Cl ⁻ ) is also increasing.

In such an environment, since carbon steel and low alloy steel are significantly corroded, a corrosion inhibitor has been conventionally added as a countermeasure. However, the corrosion inhibitor often loses its effect at high temperatures, and the cost required for the addition / recovery treatment of the corrosion inhibitor becomes enormous and it may not be applicable. Therefore, there is a great need recently for corrosion resistant materials that do not require the addition of corrosion inhibitors.

As an alloy steel used in an oil well environment containing a large amount of carbon dioxide gas, AISI 42 is a relatively inexpensive alloy steel.
Like steel No. 0, it contains 0.2% of C and contains 12 to 13% of C.
There is a martensitic stainless steel containing r. However, even such a martensitic stainless steel may cause sulfide stress corrosion cracking in an environment where hydrogen sulfide is present.

Therefore, it is the current situation that duplex stainless steel is used in a CO ₂ environment containing H ₂ S.
However, duplex stainless steel is AISI in terms of price.
It is 2.5 times the price of 420 steel, and is expensive. Therefore, AISI 420 steel is equivalent in CO ₂ corrosiveness (withstands use in a CO ₂ environment of 120 ° C. or less) and is A in a CO ₂ environment in which hydrogen sulfide is present.
It is desired to develop a grade of steel that has better resistance to sulfide stress corrosion cracking than ISI 420 steel.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a martensitic stainless steel excellent in sulfide stress corrosion cracking resistance for use in oil country tubular goods.

[0007]

Means for Solving the Problems As a result of various studies on the components of martensitic stainless steel in order to achieve the above object, the present inventors finally found the following findings. First, the content of Cr is 8 to 11% and the amount of Mo is 3%.
When the C content of steel having a total Cr + Mo content of 12% or less is reduced to 0.1% or less and the N content is 0.15% or less,
5% saline and 0.5% saturated with hydrogen sulfide at 1 atm
The resistance to sulfide stress corrosion cracking in acetic acid was remarkably improved, and the corrosion resistance in a CO ₂ environment of 120 ° C or lower was AIS.
It was found to be equivalent to I420 steel.

The present invention, in% by weight, C: 0.005-
0.1%, Si: 0.05 to 1%, Mn: 0.1 to 2
%, P: 0.025% or less, S: 0.015% or less, C
r: 8-11%, Mo: 0.1-3% and Cr + Mo ≦
12%, Al: 0.005-0.2%, N: 0.005
The gist is a martensitic stainless steel excellent in sulfide stress corrosion cracking resistance, which is characterized by containing 0.15% by weight and the balance Fe and unavoidable impurities.

[0009]

The reason why the sulfide stress corrosion cracking resistance of the steel of the present invention having the above-mentioned constitution is significantly improved is as follows.
The present inventors presume as follows. It is generally known that the addition of Ni and Cu increases the susceptibility to sulfide stress corrosion cracking. Therefore, firstly Ni and Cu
Was not added. C is further reduced, and C in the matrix is reduced.
The amount of r was made larger than that of AISI 420 steel. Further, it is known that the addition of Mo has the property of improving pitting corrosion resistance. Therefore, the element Ni, which is harmful to hydrogen sulfide,
The present inventors presume that the corrosion resistance and further the pitting corrosion resistance were increased and the sulfide stress corrosion cracking resistance was improved by reducing C without adding Cu and adding Mo.

Further, the resistance to CO ₂ corrosion is due to the presence of Cr in the matrix,
It is known that it depends on the amount of Mo. Therefore, in the steel of the present invention, as described above, the C content of the 0.2% C-13% Cr stainless steel was halved or less to be equal to the Cr content in the matrix of AISI 420 steel. For this reason, the present inventors presume that the AISI 420 steel and the CO ₂ environment have the same corrosion resistance. However, when the carbon content is lowered, the austenite phase and the ferrite phase become two phases at high temperature, making it difficult to roll. Therefore, N, which is an austenite forming element, was added.

Next, the limited ranges of the components will be described below. C: C is an essential element for producing martensitic stainless steel. If C is less than 0.005%, it becomes difficult to make the structure a single phase of martensite, and 0.1
If it exceeds 0.1%, a large amount of Cr carbide is present and the CO ₂ corrosion resistance deteriorates, so the content range was made 0.005-0.1%.

Si: Si is an element necessary for deoxidation. If Si is less than 0.05%, the effect is not sufficient,
On the other hand, when the content exceeds 1%, the impact toughness decreases, so the content range was made 0.05 to 1%. Mn: Mn is an element effective for deoxidation and securing strength. If Mn is less than 0.1%, the effect is not sufficient,
On the other hand, if added over 2%, the effect will be saturated, so
The content range was 0.1 to 2%.

Cr: Cr is the most basic and essential element constituting the martensitic stainless steel, and is an element necessary for imparting corrosion resistance. In order to use as an oil country tubular good in a CO ₂ environment at 120 ° C or lower, if the Cr content is less than 8%, the corrosion resistance is not sufficient, and if it is added in excess of 11%, rolling becomes difficult, and the above environment Below 11% addition is enough to withstand use, and Cr
Is expensive, the upper limit should be 11%.

Al: Al is an element necessary for deoxidation. If the Al content is less than 0.005%, the effect is not sufficient, while if added over 0.2%, coarse oxide-based inclusions remain in the steel and reduce the toughness. Was 0.005 to 0.2%. N: N is an austenite forming element, but 0.005
If it is less than%, ferrite is formed. On the other hand, N is 0.15
%, The impact toughness of the base material decreases, so the content range was made 0.005 to 0.15%.

P: P is an element that reduces toughness, so the upper limit content is made 0.025%. S: S is an element that reduces toughness like P, so
The upper limit content was 0.015%. Mo: Mo is an element that improves corrosion resistance and pitting corrosion resistance. If Mo is less than 0.1%, the effect is not sufficient,
On the other hand, if added in excess of 3%, it is difficult to form a martensite single phase at room temperature because it is a ferrite-forming element, so the content range was made 0.1-3%.

The reason why Cr + Mo ≦ 12% is set is that the upper limit amount of Cr + Mo is set to 12% because the addition of Cr + Mo ≦ 12% under the above environment can sufficiently withstand use and Cr and Mo are expensive.

[0017]

[Examples] Table 1 shows the chemical composition of the sample steel and 100 ° C.
Results of investigation on corrosion rate in 120 ° C. CO ₂ environment and whether or not it fractured within 720 hours under 85% yield stress in 1 atm hydrogen sulfide saturated 5% salt + 0.5% acetic acid solution Indicates. Of the steels shown in Table 1, No. Steel Nos. 1 to 6 are steels of the present invention, and No. 7 is a comparative steel. The steel of the present invention is
Existing AISI 420 steel, namely No. 1 in Table 1. The corrosion rates at 100 ° C and 120 ° C are almost the same as those of No. 7 steel. Further, the comparative steels ruptured within 720 hours under a yielding stress of 85% in a solution of 5% saturated sodium sulfide + 0.5% acetic acid solution at 1 atm, while all the steels of the present invention in Table 1 Does not break until 720 hours.

From the above, the steel of the present invention has good resistance to sulfide stress corrosion cracking in an environment where hydrogen sulfide is present,
Moreover, the corrosion rate in the CO ₂ environment is the same as the existing AISI 42.
It can be seen that it is equivalent to 0 steel.

[0019]

[Table 1]

[0020]

EFFECTS OF THE INVENTION Since the present invention is configured as described above, an oil well having excellent sulfide stress corrosion cracking resistance in a wet hydrogen sulfide and wet carbon dioxide gas environment, and at a low manufacturing cost. It is possible to provide Cr steel for pipes.

Claims (1)

[Claims] 1. C: 0.005 to 0.1% by weight,
Si: 0.05-1%, Mn: 0.1-2%, P: 0.
025% or less, S: 0.015% or less, Cr: 8 to 11
%, Mo: 0.1 to 3% and Cr + Mo ≦ 12%, A
1: 0.005-0.2%, N: 0.005-0.15
%, And the balance is Fe and inevitable impurities, the martensitic stainless steel having excellent resistance to sulfide stress corrosion cracking.