JPH101755A - Martensitic stainless steel excellent in corrosion resistance and sulfide stress corrosion cracking resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a martensit stainless steel excellent in CO2 corrosion resistance and sulfide stress corrosion cracking resistance. SOLUTION: This steel is the one having a compsn. contg., by weight, 0.005 to 0.05% C, 0.05 to 0.5% Si, 0.1 to 1% Mn, 10 to 15% Cr, 4.0 to 9.0% Ni, 0.5 to 3% Cu, 1 to 3% Mo, 0.005 to 0.2% Al, 0.005 to 0.1% N, <=0.025% P, <=0.015% S, and the balance Fe with inevitable impurities and satisfying 40C+34N+Ni+0.3Cu-1.1Cr-1.8Mo>=-10 and having a structure composed of tempered martensitic phases, martensitic phases and residual austenitic phases, in which the fractional ratio of the total of the tempered martensitic phases and martensitic phases is regulated to 60 to 90%, and the balance residual austenitic phases.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a martensitic stainless steel for oil country tubular goods and line pipes having excellent resistance to CO ₂ corrosion and sulfide stress corrosion cracking, and a method for producing the same. More specifically, the present invention relates to a martensitic stainless steel having high corrosion resistance in an environment containing wet carbon dioxide gas and wet hydrogen sulfide in an oil or gas well.

[0002]

BACKGROUND OF THE INVENTION In recent years, the environment of wells for extracting oil or natural gas has become more and more harsh. In addition to the increase in mining depth, wet carbon dioxide (CO ₂ ) and hydrogen sulfide (H ₂ S), chlorine ions (Cl ^-) has many well containing corrosive components such. Under these circumstances,
Conventionally, the use of corrosion inhibitors has been attempted. However, corrosion inhibitors often lose their effect at high temperatures (> 150 ° C.). In addition, in terms of the corrosion inhibitor, a large cost is required for the recovery process. Therefore, it has been desired to provide a corrosion-resistant material that does not require the use of a corrosion inhibitor.

In an oil well environment containing a large amount of carbon dioxide gas, martensitic stainless steel containing 0.2% of C and 12 to 13% of Cr, such as AISI420 steel, is widely used as a relatively inexpensive alloy steel. It is used. However, at temperatures above 120 ° C., 420 steel will also corrode. Therefore, in a CO ₂ environment of 120 ° C or more, C
Duplex stainless steel containing 22-25% r is used. However, duplex stainless steel is an expensive material to be used only in a CO ₂ environment.

[0004] In an oil well / gas well environment, not only wet carbon dioxide gas but also wet hydrogen sulfide exists as described above. The presence of wet hydrogen sulfide can cause the steel to undergo sulfide stress corrosion cracking. Therefore, there is wet hydrogen sulfide,
In a high-temperature environment of 120 ° C. or more,
Duplex stainless steel containing 2 to 25% is used. Therefore, it is desired to develop a grade having a middle use performance between 420 steel and duplex stainless steel (a steel that can withstand a high temperature CO ₂ environment of 200 ° C. or less and can be used even in an environment where hydrogen sulfide can exist) and a price. ing.

[0005]

DISCLOSURE OF THE INVENTION The present invention relates to a method in which a temperature of 120.degree.
Excellent in corrosion resistance and sulfide stress corrosion cracking, used as oil country tubular goods or line pipes in an environment containing a large amount of carbon dioxide gas at 00 ° C and in which hydrogen sulfide with a partial pressure of 0.05 MPa or less is present. Another object of the present invention is to provide a martensitic stainless steel having an optimum strength for its use.

[0006]

In order to achieve the above-mentioned object, the present invention relates to a method of the present invention, wherein: C: 0.005 to 0.05%; Si: 0.05 to 0.5% or less; 0.1 to 1.0%, P: 0.025% or less, S: 0.015% or less, Cr: 10 to 15%, Ni: 4.0 to 9.0%, Cu: 0.5 to 3% , Mo: 1.0-3.0%, Al: 0.005-0.2%, N: 0.005% -0.1%, and if necessary, one of Ca, Mg, and REM Alternatively, 0.001 to 0.3% of each of two or more kinds is contained, and the balance is composed of Fe and inevitable impurities, and 40C + 34N + Ni + 0.3Cu-1.1Cr-1.
8Mo ≧ −10, and is composed of a tempered martensite phase, a martensite phase, and a retained austenite phase, and the total fraction of the tempered martensite phase and the martensite phase is 60% or more and 90% or less, and the remainder is retained austenite. This is a martensitic stainless steel characterized by being composed of a phase and having excellent corrosion resistance and sulfide stress corrosion cracking resistance. First, a thermal expansion measurement was performed on the steel described in the preceding section to determine the relationship between the austenite fraction and the temperature. The steel having the above-described composition was hot-worked, allowed to cool naturally to room temperature, and then subjected to Ac ₁ The heat treatment is performed at a temperature not lower than the point and not higher than the temperature at which the austenite fraction is 80% on the curve obtained from the relationship between the austenite fraction and the temperature.
A method for producing a martensitic stainless steel having excellent corrosion resistance and sulfide stress corrosion cracking resistance, characterized by performing heat treatment at a temperature not higher than.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
In the following description, “%” means “% by weight”. The present inventors have made various studies on the component system of martensitic stainless steel in order to solve the above-mentioned problems in the conventional technology, and as a result, have obtained the following knowledge. That is, 10 to 15
% Of steel containing less than 0.05% C and less than 4% Ni and 0.5-3% Cu.
The present inventors have found that the addition of the composite significantly improves the corrosion resistance in the carbon dioxide-containing salt.

Further, for steel containing 10 to 15% of Cr, C is reduced to less than 0.05% and Ni is reduced to 4.0%.
As described above, Mo was added to steel containing 0.5 to 3% of Cu by 1.0%.
The inventors have found that the addition of 〜3% significantly improves sulfide stress corrosion cracking resistance. In a manufacturing process, Ni equivalent = 40 to improve hot workability.
C + 34N + Ni + 0.3Cu-1.1Cr-1.8M
The inventors have also found that it is necessary for o to be greater than -10.

Further, in order to satisfy the strength as an oil country tubular good, the yield stress is 80 ksi (551MP) according to the standard of the oil country tubular goods.
It is necessary to refine in the range from a) to 110 ksi (861 MPa). Then, the present inventors measured the austenite and ferrite fractions at each temperature by the thermal expansion curve using the thermal expansion measurement, and as a result, the following changed. That Ac ₁ point or more, and after the austenite fraction on the thermal expansion curve was subjected to heat treatment at a temperature below the temperature at which 80%, followed by the austenite fraction in the thermal expansion curve is a heat treatment below a temperature at which 60% When done, yield stress is 551MPa
From 861 MPa.

FIG. 1 is a graph showing the austenite fraction at each temperature based on the thermal expansion curve obtained by measuring the thermal expansion at that time. The steel used was 0.02C-13Cr-6Ni-2M.
o-1.5Cu-0.02N steel. The structure at that time is composed of a tempered martensite phase, a martensite phase, and a retained austenite phase. The total fraction of the tempered martensite phase and the martensite phase is 60% or more and 90% or less, and the remainder is a retained austenite phase. I knew it was there.

[0011] The present invention is to determine the appropriate structure distribution of conventional martensitic stainless steel having excellent corrosion resistance and sulfide stress corrosion cracking resistance to make it suitable for oil country tubular goods and line pipes. It was completed on the basis of.

For steels containing 10 to 15% Cr, the C is reduced to less than 0.05% and the Ni is reduced to 4.0%.
As described above, when Cu is added in an amount of 0.5 to 3%, carbon dioxide (C
The inventors presume as follows about the reason why the corrosion resistance in the O ₂ ) -containing saline solution is remarkably improved.
In general, it is known that the carbon dioxide corrosion resistance of an alloy is improved in proportion to the amount of Cr in a base material. Currently 120
0.2% C-1 in an environment containing a large amount of carbon dioxide below ℃
3% Cr steel is used. Furthermore, C in the matrix
When the amount of C is reduced to further increase the amount of r and the amount of Cr carbide is reduced, 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 gas reaches 150 ° C., and the corrosion rate also becomes 0.1 mm / y or less.
The corrosion temperature does not fall below 0.1 mm / y unless a significant amount of Cr (about 20%) is added only by increasing the amount of r. Further, when a considerable amount of Cr is added to low C, a ferrite phase is observed. The presence of a ferrite phase lowers the strength and significantly reduces hot workability.
It becomes difficult in manufacturing.

Therefore, it has been found that when Cu and Ni are simultaneously added to steel having a low C and a high Cr base material, the corrosion rate becomes 0.1 mm / y or less even at a temperature of 150 ° C. or more. The present inventors speculate as follows as to why the carbon dioxide gas corrosiveness was improved by adding Cu and Ni to the low C 10-15% Cr steel in combination. 10-15% with low C
When Cu is added to Cr steel, the corrosion film shows a behavior as if it shows a passivity in the anodic polarization curve. It was found that when Ni and Cu were further added to the steel in combination, the current density exhibiting the passivity of the anodic polarization curve decreased by one order, and the corrosion film became more and more stable. When this corrosion film is analyzed, the corrosion film when Cu and Ni are added in combination is a phase composed of fine particles, and when observed by an electron microscope, the diffraction pattern becomes ring-shaped, and the diffraction pattern becomes amorphous. become. When only Ni was added, the diffraction pattern of the electron microscope was observed as a crystalline diffraction pattern, indicating that the corrosion was progressing. Also C
Even when only u was added, the diffraction pattern of the electron microscope was an amorphous and crystalline diffraction pattern. It can be seen that the corrosion was progressing, though not so much as Ni. That is, N
When i and Cu are added in combination, the corrosion film does not crystallize, and the corrosion does not progress. Therefore, the corrosion film is covered with a very dense corrosion film, and it is considered that the CO ₂ corrosion resistance is improved.

It has been found that when Mo is further added by 1% or more to 10-15% Cr steel to which Cu and Ni are added in combination at a low C, sulfide stress corrosion cracking resistance is remarkably increased. This low C was added with a composite addition of Cu and Ni.
The inventors speculate as follows about the reason why the addition of Mo to 15% Cr steel improved the resistance to sulfide stress corrosion cracking. In general, the starting point of sulfide stress corrosion cracking of stainless steel is that Cl ions break through the passive film and enter.
It is known that hydrogen penetrates from that location and cracks develop. In general, Mo is known to improve pitting resistance. Mo dissolves as ions and adheres to the steel surface, increasing the film resistance. By adding Mo, the number of ions adhering to the steel surface is increased by several tens of times, so that the resistance of the passive film in the H ₂ S environment is remarkably improved, and the resistance to sulfide stress corrosion cracking is greatly improved. It is presumed to have improved.

In order to improve hot workability in the manufacturing process, that is, to prevent scratches, heating,
The structure in the rolling region must be an austenitic single phase. Therefore, Ni equivalent = 40C + 34N + Ni + which is an index to become an austenite phase when the steel is heated to a high temperature.
When 0.3Cu-1.1Cr-1.8Mo was larger than -10, it was found that ferrite was suppressed in the rolled region and became an austenite single phase.

In order to satisfy the strength as an oil country tubular good,
After performing heat treatment at a temperature of not less than _one point Ac and a temperature not higher than the temperature at which the austenite fraction is 80% on the thermal expansion curve, the austenite fraction is then 60% in the thermal expansion curve.
% Yield stress is 551M
It turned out to be in the range from Pa to 861 MPa. The present inventors presume the reason for this as follows. These tissues were identified by optical microscopic observation and electron microscopic observation. As a result, the structure is composed of a tempered martensite phase, a martensite phase, and a retained austenite phase, and the total fraction of the tempered martensite phase and the martensite phase becomes 60% or more and 90% or less, and the remainder becomes a retained austenite phase. I understood that. Therefore, it was found that when the structure fractions of the tempered martensite phase, martensite phase, and retained austenite phase were defined as described above, the strength as an oil country tubular good was satisfied.

Next, the limited ranges of the components will be described below. C: An element necessary for producing martensitic stainless steel. If it is less than 0.005%, it becomes difficult to make the structure into a martensitic single phase, and if it exceeds 0.05%, C becomes an element.
Since a large amount of r carbides is present and the CO ₂ corrosion resistance deteriorates, the content range is set to 0.005 to 0.05%.

Si: an element necessary for deoxidation,
If the content is less than 0.05%, the effect is not sufficient, and if the content exceeds 0.5%, the impact toughness is reduced, so the content range is set to 0.05 to 0.5%. Mn: an element effective for deoxidation and securing strength,
If the content is less than 0.1%, the effect is not sufficient, and even if added over 1%, the effect is saturated.
To 1%.

Cr: Cr is the most basic and indispensable element constituting martensitic stainless steel.
Although it is an element necessary for imparting O ₂ corrosion, if the content is less than 10%, the corrosion resistance is not sufficient. On the other hand, if the content exceeds 15%, it becomes difficult to form a martensite single phase, so the upper limit content is 15%. Should be%. It is desirable that the content be 10% or more and 14% or less in order to obtain a martensite single phase.

Al: An element necessary for deoxidation, and if its content is less than 0.005%, its effect is not sufficient,
If added in excess of 0.2%, coarse oxide-based inclusions remain in the steel and lower the toughness, so the content range is 0.
005 to 0.2%. N: N is essential since it is an austenite-forming element, but if it is less than 0.005%, it becomes difficult to form a martensite single phase at room temperature, and if it exceeds 0.1%, the impact toughness of the base material is reduced. 0.005 to 0.1
Should be%.

P: Since the element lowers the toughness, the upper limit content is set to 0.025%. S: Since S is an element that lowers toughness like P, the upper limit content is set to 0.015%.

Ni: Stabilizes the martensite phase with an austenite-forming element. Also, by adding complex with Cu,
Although the CO ₂ corrosion resistance is improved, the effect is not sufficient if it is less than 4.0%, and if it exceeds 9%, the Ac ₁ transformation point becomes too low, and it becomes difficult to obtain a stable strength. The range was 4-9%. Desirably, 4% to 6% is appropriate. Cu: Cu is also an austenite-forming element like Ni, and the effect of improving CO ₂ corrosion resistance by adding Ni in combination with Ni is less than 0.5%, the effect is not sufficient. Since it becomes difficult, the content range is set to 0.5 to 3%. Desirably, it is 0.5 to 2%.

Mo: Mo is resistant to CO ₂ corrosion or S
An effective element for improving the SC property, but sufficient S
If it is less than 1%, the effect is not enough to obtain SC property,
If it exceeds 3%, ferrite is likely to be formed, and hot workability is reduced. Therefore, the content range is set to 1 to 3%.

Ca, Mg, REM: These are all S
To suppress the reduction in hot workability due to
If it is less than 01%, its effect is not sufficient, and if it exceeds 0.3%, its effect is saturated.
%.

From the viewpoint of improving hot workability, Ni equivalent = 40C + 34N + Ni + 0.3Cu-1.1Cr-
If 1.8 Mo is smaller than -10, ferrite is generated in the heating / rolling region, so the Ni equivalent was set to -10 or more.

Further, the reasons for limiting the metallographic structure will be described. When the total fraction of the tempered martensite phase and the martensite phase is less than 60%, the yield stress is 80 ksi.
(556 MPa), and the strength cannot be satisfied. On the other hand, when the sum of the tempered martensite phase and the fraction of the martensite phase exceeds 90%, the yield stress exceeds the upper limit of 110 ksi, so that it is set to 60% or more and 90% or less.

The reasons for limiting the heat treatment will also be described.
If the first heat treatment is performed at a temperature lower than the Ac ₁ point, the tempered martensite phase is not sufficiently recovered, and Cu
The upper limit of the strength of 110 ksi (86
Exceeds 1 MPa). Also, measure the thermal expansion in advance,
When the heat treatment is performed at a temperature exceeding the temperature at which the amount of austenite becomes 80% on the curve obtained from the relationship between the austenite fraction and the temperature, a new martensite phase is precipitated, which also has a strength of 11%.
Exceeds the upper limit of 0 ksi. Therefore, the temperature of the _first heat treatment was set to be equal to or higher than Ac ₁ and equal to or lower than the temperature at which the austenite amount becomes 80% on the thermal expansion curve.

When the second heat treatment temperature exceeds the temperature at which the austenite phase becomes 80% on the above-mentioned curve obtained from the relationship between the austenite fraction and the temperature, the martensite phase is newly precipitated and yielded. Since the stress exceeds the upper limit of 110 ksi, the temperature was set to be equal to or lower than the temperature at which the austenite phase becomes 60%.

[0029]

EXAMPLE A stainless steel having a chemical composition shown in Table 1 (inventive steel) and Table 2 (comparative steel) was melted and hot-rolled to form a steel sheet having a thickness of 12 mm. gave. Using a specimen taken from this steel sheet, the mechanical properties (yield stress, tensile stress) of the steel sheet, the structure identification and measurement of the structure fraction of the steel by electron microscopy, thermal expansion test, corrosion test in a wet carbon dioxide gas environment SSC test was performed in a wet hydrogen sulfide environment.

The test piece for measuring mechanical properties (yield stress, tensile stress) has a parallel portion of 6.0 mm and a parallel portion length of 24 mm. In order to observe with an electron microscope, a thin film sample mechanically polished and chemically polished to 50 μm is punched out into a 4 mmφ circular shape.
A hole was made in the center of the circle by electrolytic polishing, and observation was made with an electron microscope.

Measurement of thermal expansion (test piece 3 mmφ × length 10 mm)
Prepared a thermal expansion curve from Ac ₁ to Ac ₃ at a very low heating rate of 2.5 ° C./min, and measured the austenite and ferrite fractions at each temperature.

As a corrosion test in a wet carbon dioxide gas environment, a test piece having a thickness of 3 mm, a width of 15 mm and a length of 20 mm was sampled and placed in a 200 ° C. autoclave at a carbon dioxide partial pressure of 4 MPa.
It was immersed in artificial seawater for 4 days under the condition of a, and the corrosion rate was calculated from the change in weight before and after the test. The unit of the corrosion rate is mm / y
In general, when it is 0.1 mm / y or less, it can be said that the corrosion resistance is good.

An SSC test in a wet hydrogen sulfide environment was also performed. For the SSC test, a smooth round bar tensile test piece (parallel part 6.4 mm, parallel part length 25 mm) was collected and 5% Na
A constant load test was carried out in a solution prepared by mixing 1 M acetic acid and 1 M sodium acetate with a Cl solution to adjust the pH to 3.5, and then saturating 0.5 atm H ₂ S gas with the solution. 90% stress
A yield stress was applied.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

No. 1 to 19 steels are steels of the present invention,
o. 20 to 29 are comparative climates. No. In any of the steel types 1 to 19, the corrosion rate in a CO ₂ environment at 200 ° C. is 0.1 mm / y or less, the CO ₂ corrosion resistance is good, and the yield stress falls within the range of 551 MPa to 861 MPa. You can see that. Naatsu No. None of the steel types 1 to 19 did not break at a hydrogen sulfide partial pressure of 0.5 atm.

On the other hand, no. 22,23,24,2
6 steel has a corrosion rate of 0.1mm / y at 200 ℃ in CO ₂ environment.
From the above, it can be seen that the corrosion resistance is very poor. No. 22,
25 steel is SS resistant at 0.5 atm H ₂ S partial pressure, pH 3.5
It turns out that C property is bad.

No. In the case of No. 20, the austenite fraction exceeds 60% due to the relationship between the austenite fraction and the temperature determined by the thermal expansion measurement in the second heat treatment temperature, and the total fraction of the tempered martensite phase and the martensite phase is 90%. Therefore, the yield stress exceeds 861 MPa.
No. 21 first heat treatment temperature tempered martensite phase because it is Ac ₁ or less and the fraction of total martensite phase is above 90%, the yield stress is 861MPa
Is over.

From the above, it can be seen that the steel of the present invention has much better CO ₂ corrosivity than the existing AISI420 steel, and has improved sulfide stress corrosion cracking resistance even in an environment where hydrogen sulfide is present. Also, the strength could be made suitable for oil country tubular goods and line pipes. As a result, it was found that the test component steel had good resistance to CO ₂ corrosion and resistance to sulfide stress corrosion cracking, and furthermore had optimum strength for oil country tubular goods and line pipes.

[0042]

Since the present invention is configured as described above, it has the following effects. To be provided as a high Cr steel for oil country tubular goods and line pipes that has excellent corrosion resistance and sulfide stress corrosion cracking resistance in wet carbon dioxide gas environment and wet hydrogen sulfide environment and has strength as oil country tubular goods and line pipes. Enabled.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between austenite fractions at respective temperatures according to a thermal expansion curve using thermal expansion measurement.

Claims (4)

[Claims] C: 0.005 to 0.05%; Si: 0.05 to 0.5%; Mn: 0.1 to 1.0%; P: 0.025% or less; S: 0.015% or less, Cr: 10 to 15%, Ni: 4.0 to 9.0%, Cu: 0.5 to 3%, Mo: 1.0 to 3%, Al: 0.005 to 0.2%, N: 0.005% to 0.1%, the balance being Fe and inevitable impurities, 40C + 34N + Ni + 0.3Cu-1.1Cr-1.
8Mo ≧ −10 and a tempered martensite phase, a martensite phase and a retained austenite phase, and the total fraction of the tempered martensite phase and the martensite phase is 60%.
A martensitic stainless steel excellent in corrosion resistance and sulfide stress corrosion cracking resistance characterized in that at least 90% or less and the remainder is a retained austenite phase. 2. The steel according to claim 1, further comprising at least one of Ca, Mg, and REM in an amount of 0.0% by weight.
A martensitic stainless steel excellent in corrosion resistance and sulfide stress corrosion cracking resistance characterized by containing 01 to 0.3%. 3. A steel containing the component described in claim 1 is first subjected to thermal expansion measurement to determine the relationship between the austenite fraction and the temperature, and then hot-worked the steel having the component composition. Then, after natural cooling to room temperature, heat treatment is performed at _one point or more of Ac and below a temperature at which the austenite fraction becomes 80% on the curve obtained from the relationship between the austenite fraction and the temperature. A martensitic stainless steel excellent in corrosion resistance and sulfide stress corrosion cracking resistance, wherein the heat treatment is performed at a temperature not higher than 60%. 4. A steel containing the component according to claim 2 is first subjected to a thermal expansion measurement to determine a relationship between an austenite fraction and a temperature, and hot-worked the steel having the component composition. After natural cooling to room temperature, heat treatment is performed at a temperature of not less than _one point Ac and not more than a temperature at which the austenite fraction becomes 80% on a curve obtained from the relationship between the austenite fraction and the temperature, and further, the austenite fraction is reduced. A method for producing a martensitic stainless steel having excellent corrosion resistance and sulfide stress corrosion cracking resistance, wherein heat treatment is performed at a temperature of 60% or less.