JPH06128694A - Martenitic stainless steel excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To produce a stainless steel having superior corrosion resistance in a wet carbon dioxide gas environment by preparing a martensitic stainless steel having a specific composition where respective contents of Cr, C, Ni, and Cu are specified. CONSTITUTION:A martensitic stainless steel having a composition consisting of, by weight, <=0.1% C, <=1% Si, <=2% Mn, <=0.025% P, <=0.015% S, 14-18% Cr, <=6% Ni, 1-5% Cu, 0.005-0.2% Al, <=0.1% N, and the balance Fe with inevitable impurities is prepared. By this method, the stainless steel having superior corrosion resistance in an environment of about 120-180 deg.C, containing large amounts of carbon dioxide gas, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic stainless steel for oil country tubular goods having excellent corrosion resistance. More specifically, it relates to a martensitic stainless steel having high corrosion resistance in an environment containing wet carbon dioxide gas or wet hydrogen sulfide in an oil well or a gas well.

[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. In addition to increasing the depth of mining, wet carbon dioxide (CO ₂ ) and hydrogen sulfide (H 2 _The number of wells containing corrosive components such as ₂ S) and chlorine ions (Cl ⁻ ) is increasing. In such an environment, a corrosion inhibitor has been conventionally used.
However, corrosion inhibitors often lose their effect at high temperatures (150 ° C and above). In addition, the addition and recovery processing of the corrosion inhibitor requires a great deal of cost. Therefore, it has been desired to supply a corrosion-resistant metal material that does not require the application of a corrosion inhibitor.

As an alloy used in an environment containing a large amount of wet carbon dioxide, a relatively inexpensive alloy is AISI420.
There is a martensitic stainless steel containing 0.2% of C and 12 to 13% of Cr such as steel. However, even in such a martensitic stainless steel, corrosion progresses when the temperature of the environment containing a large amount of wet carbon dioxide becomes 120 ° C. or higher. Therefore, in an environment containing a large amount of carbon dioxide at 120 ° C. or higher, a duplex stainless steel containing 22 to 25% of Cr has been used. However, carbon dioxide (CO
₂ ) As materials used in an environment containing a large amount of
Duplex stainless steels containing 25% Cr are too expensive.

Therefore, AISI 420 steel and 22-25%
It has been desired to develop a grade of steel having an intermediate use performance of the Cr-containing duplex stainless steel, that is, a corrosion resistance and a price that can withstand use in an environment containing a large amount of carbon dioxide gas at 120 to 180 ° C. .

[0005]

SUMMARY OF THE INVENTION The present invention provides 120-1
It is an object of the present invention to provide a martensitic stainless steel having excellent corrosion resistance, which is used as an oil well pipe or a line pipe in an environment containing a large amount of carbon dioxide at 80 ° C.

[0006]

In order to achieve the above-mentioned object, the present invention provides C ≤ 0.1% by weight,
Si ≦ 1%, Mn ≦ 2%, P
≤0.025%, S ≤0.015%, C
r: 14-18%, Ni ≦ 6%,
Cu: 1-5%, Al: 0.005-0.2%, N
A martensitic stainless steel having excellent corrosion resistance, characterized by containing ≦ 0.1% and the balance being Fe and unavoidable impurities.

The present invention will be described in detail below. The inventors of the present invention have made various studies on the component system of martensitic stainless steel in order to solve the above problems in the prior art, and as a result, have obtained the following findings. That is, 14-18
% Steel, containing C of less than 0.1% and adding Ni of 6% or less and Cu of 1 to 5%, the corrosion resistance in a carbon dioxide gas-containing saline solution is remarkably improved. , The inventors have found. The present invention has been completed based on this finding.

Regarding steel containing 14 to 18% of Cr, if C is reduced to less than 0.1% and Ni is added at 6% or less and Cu is added to 1 to 5%, carbon dioxide (CO ₂ ) -containing saline solution is added. The present inventors presume as follows for the reason that the corrosion resistance in the above is significantly improved. In general,
It is known that the carbon dioxide corrosion resistance of the alloy becomes good in proportion to the amount of Cr in the base material. Currently, 0.2 to 13% Cr steel is used in an environment containing a large amount of carbon dioxide below 120 ° C. Then, increase the amount of Cr,
When the C content is reduced to reduce the Cr carbide, the carbon dioxide corrosion resistance of the alloy is further improved. This tendency continues until the temperature of the environment containing a large amount of carbon dioxide reaches 150 ° C, but above 150 ° C, the carbon dioxide corrosion resistance of the alloy is not proportional to the amount of Cr in the base metal, and the phase stability The rate of dependence on is increased. That is, in the case of a mixed phase including the δ phase, the amount of melting due to corrosion from one phase becomes extremely large, and uniform corrosion does not occur. Therefore, when Ni, Cu or the like which is an austenite forming element is added to form an austenite single phase at a high temperature, the structure after cooling is a martensite single phase, so that the corrosion resistance becomes good. Furthermore, C
The present inventors presume that the addition of u stabilizes the corrosion film by precipitating Cu, so that the simultaneous addition of Ni and Cu further improves the corrosion resistance.

Next, the limited range of the components will be described below. C: When the C content is large, a large amount of Cr carbide is present and the corrosion resistance deteriorates. Therefore, the upper limit content should be 0.1%. Si: an element necessary for deoxidation, but if added in excess of 1%, the impact toughness will decrease, so the upper limit content is made 1%. Mn: an element effective for deoxidation and ensuring strength, but if added in excess of 2%, the effect will be saturated, so
The upper limit content is 2%.

Cr: Cr is the most basic and essential element that constitutes martensitic stainless steel, and is an element necessary for imparting corrosion resistance, but it is an oil well pipe in a CO ₂ environment of 150 ° C or higher. If it is added in an amount of less than 14%, the corrosion resistance is not sufficient, while if it is added in an amount of more than 19%, it becomes difficult to form a martensite single phase. Therefore, the upper limit content should be 18%.

Al: an element necessary for deoxidation, and if the content is less than 0.005%, its effect is not sufficient,
If added in excess of 0.2%, coarse oxide inclusions remain in the steel and reduce the toughness, so the content range is 0.
It was set to 005 to 0.2%. N: If N exceeds 0.1%, the impact toughness of the base material decreases, so the upper limit content should be 0.1%. P: Since it is an element that reduces toughness, the lower limit content is 0.
It was set to 025%. S: S is an element that lowers the toughness like P, so the lower limit content is made 0.015%.

Ni: Stabilizes martensite with an austenite forming element to improve corrosion resistance, but if 7% or more is added, the effect is saturated, and if Ni is increased, the cost is increased. Therefore, the upper limit content is 4%. Cu: Cu also improves corrosion resistance by an austenite forming element like Ni, but if it exceeds 5%, Cu precipitates in the matrix, and if Cu is increased, the cost increases, so the upper limit content was made 5%.

[0013]

[Example] As a test material, C0.02% by weight, Si
0.03%, Mn 0.45%, P 0.013%, S0.
0013%, Cr15.00%, Ni4.0%, Cu
A steel containing 3.0%, Al 0.028% and N 0.01% and the balance being essentially Fe was used. FIG. 1 shows the amounts of corrosion when the above test materials were subjected to various treatments and the temperature was changed in a carbon dioxide gas environment of 4 MPa. As is clear from the drawing, the component steel of the test material, that is, low C-15% Cr-4
It was found that the Ni-3Cu steel has a very good corrosion resistance with a corrosion rate of 0.1 mm / y or less at 200 ° C. or less in a CO ₂ environment of 4 MPa.

[0014]

INDUSTRIAL APPLICABILITY The steel of the present invention constructed as described above can be provided as a high Cr steel for oil country tubular goods having excellent corrosion resistance in a wet carbon dioxide environment.

[Brief description of drawings]

[Figure 1] Temperature when the temperature is changed in a CO ₂ environment of 4 MPa-
Corrosion rate curve.

Claims (1)

[Claims] 1. By weight, C ≤ 0.1%, Si ≤ 1%, Mn ≤ 2%, P ≤ 0.025%, S ≤ 0.015%, Cr: 14-18%, Ni ≤ 6%. , Cu: 1 to 5%, Al: 0.005 to 0.2%, N ≤ 0.1%, balance: Fe and inevitable impurities, martensite excellent in corrosion resistance Series stainless steel.