JPS6043467A - Two-phase stainless steel - Google Patents

Abstract

PURPOSE:To obtain a two-phase stainless steel having superior stress corrosion resistance and corrosion resistance in a corrosive environment at a high temp. by adding specified percentages of C, Si, Mn, P, S, Cr, Ni, Mo and N to Fe and carrying out soln. heat treatment at a specified temp. CONSTITUTION:An alloy consisting of, by weight, <=0.1% C, <=0.6% Si, <=2.0% Mn, <=0.025% P, <=0.005% S, 20.0-27.0% Cr, 3.0-6.0% Ni, 0.1-0.5% Mo, 0.1- 0.4% N and the balance Fe with inevitable impurities is subjected to soln. heat treatment at 1,000-1,100 deg.C. To the alloy may be added <=0.5% W and/or <=2.0% Cu, and/or <=0.008% Ca and/or <=0.05% rare earth element as required.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a two-phase stainless steel used for oil country tubular goods, etc., that is, a stainless steel consisting of an austenite phase and a ferrite phase. This relates to improving stress corrosion resistance under environmental conditions.

In recent years, due to the ongoing depletion of petroleum resources and the soaring price of crude oil, drilling of oil and gas wells under adverse conditions, which had previously been ignored, has become increasingly common. In general, oil and gas wells under such adverse conditions are deep, have a high temperature environment, and are highly corrosive.
Co2. Many come from harsh environments that contain a lot of chlorine ions. Therefore, oil country tubular goods and gas pipes used in such oil and gas wells must not only have high strength and normal corrosion resistance, but also chloride stress corrosion cracking resistance (hereinafter referred to as SCC resistance) and hydrogen sulfide resistance. It is necessary that the cracking property (hereinafter referred to as SSC resistance) is excellent.

By the way, the low-alloy steel that has traditionally been used for ordinary oil country tubular goods has significantly poor corrosion resistance in environments containing CO2, so it cannot be used in the above-mentioned environment. 13Cr steel, duplex stainless steel,
Alternatively, N1-based alloys have been proposed as materials for oil country tubular goods and gas country tubular goods as mentioned above. Of these,
The strength level of 13Cr steel can be significantly changed by heat treatment, and it is inexpensive, so it has a good track record of use in oil wells containing CO2. However, 13 Cr
tlil also has poor corrosion resistance and SSC resistance in an environment containing a trace amount of H2S or a high temperature environment of 200° C. or higher, so it has the disadvantage that the environments in which it can be used are limited to a narrow range.

On the other hand, N1-based alloys have high strength and excellent corrosion resistance, and are also resistant to SC.
Since it has excellent C properties and SSC resistance, it is considered to be a suitable material for oil country tubular goods and gas country tubular goods in harsh environments. However, in addition to being difficult to manufacture, Ni-based alloys are also extremely expensive, making it difficult to actually use them for oil country tubular goods and gas country tubular goods.

On the other hand, duplex stainless steel has good SCC resistance, which is the weak point of austenitic stainless steel, and is also resistant to hydrogen embrittlement, which is the weak point of ferritic stainless steel. Combines the advantages of N-based stainless steel, and also
Since it is cheaper than i-based alloys, it is seen as a promising material for oil country tubular goods and gas country tubular goods as mentioned above. However, even though it is a duplex stainless steel, conventional stainless steel has high temperature H2S-CO2-C1-hp boundary as mentioned above.
The reality is that if the S9 temperature is too high, corrosion cracking will occur, so its use as a material for oil country tubular goods and gas country tubular goods has been discouraged.

This invention was made in view of the above circumstances.
To improve the stress corrosion cracking resistance of duplex stainless steel in a high-temperature H2S-CO2-Cl- environment, thereby providing duplex stainless steel suitable for oil and gas wells in harsh environments as described above. The purpose is to

The present inventors have conducted various experiments and studies in order to achieve the above-mentioned objectives, and as a result, have obtained the following new knowledge.

That is, high temperature H2S-CO2-C of duplex stainless steel
As a result of investigating stress corrosion cracking at the 6-JJ boundary,
Mo and Cr, which are normally contained as essential components in duplex stainless steel, and Si, which is contained as a deoxidizing element, have an adverse effect on stress corrosion cracking in the above environment. It has been found that the 1-2% content contained in duplex stainless steel has a very negative effect on stress corrosion cracking. This means that the Mo content can be lowered than before, and at the same time, C
It has been found that limiting r and Si to a certain level of F is effective in improving stress corrosion cracking properties in the above environment. On the other hand, MO is known to have good pitting corrosion resistance in duplex stainless steel, and therefore M.

There is a risk that the pitting corrosion resistance will decrease due to the decrease in Mo content, but as a result of experiments, the decrease in pitting corrosion resistance due to the decrease in Mo content is due to the N content in the steel, or due to N and W or C.
It has been found that this can be compensated for by increasing the content of u. Furthermore, the stress corrosion cracking resistance of such steels is greatly affected by the solution treatment temperature, and in order to exhibit excellent stress corrosion cracking resistance, solution treatment must be performed within a certain limited temperature range. It turned out that it wasn't.

This invention was completed based on the above knowledge, and the gist is that the duplex stainless steel of the first invention is
0.1% or less, Si 0.6% or less, Mn 2.0% or less,
PO,025 cm or less, SO,005 cm or less, Cr
20.0-27.0%, Ni 3.0-6.0%, M.

0.1-0.5%, NO, 1-0.4%, balance free from Fe and unavoidable impurities, and solution treatment: 100%
It is characterized in that it was carried out within a temperature range of 0 to 1100°C. In addition to the components of the first invention, the duplex stainless steel of the second invention further contains W 0.5 inch, Cu
2. It contains 0% or less of one or more kinds, and has been subjected to the same solution treatment as described above. Further, the duplex stainless steel of the third invention further contains C in addition to the components of the first invention.
a O,008% or less, rare earth elements (REM) 0.
0.05% or less, and has been subjected to the same solution treatment as described above.

Further, the duplex stainless steel of the fourth invention contains, in addition to the components of the first invention, one or more of W 0.5'1 or less or Cu 2.0% or less, and Ca 0.008%.
or REV O, 05eI) and one or more of the following, and has been subjected to the same solution treatment as described above.

This invention will be explained in more detail below.

First, the reason for limiting the components in the duplex stainless steel of the present invention will be explained.

C: C has Cr in the grain boundaries due to welding or inappropriate heat treatment.
The lower the value, the better since it causes carbide precipitation and intergranular corrosion, but in the case of oil country tubular goods, most of the time they are used as seamless pipes and no welding is performed, so the 01- The following was made.

Si: Si is added as a deoxidizing agent in ordinary steelmaking, but it significantly deteriorates stress corrosion cracking resistance on a high-temperature H2S-CO2-C11-g plate, so the lower the Si content, the better. The upper limit was set.

Mn: Mn is used as a deoxidizing and desulfurizing agent during normal steel manufacturing, and has little effect on stress corrosion cracking resistance, so it is set at the upper limit of 2.0 F, which is the normally used upper limit. . Note that the lower limit is not particularly specified, but as a deoxidizing and desulfurizing agent, it is usually sufficient as long as it is about 0.4% or more.

P: P has almost no effect on stress corrosion cracking resistance, but degrades hot workability, so the upper limit was set at 0.025%.

S: S significantly deteriorates the hot workability of duplex stainless steel in the pipe manufacturing process, so its content should be reduced to 0.
．． 0.005% or less.

Cr: Cr is an important element that determines the phase ratio in duplex stainless steel, that is, the ratio of austenite phase to ferrite phase, and also controls its corrosion resistance. At least 20 qb of Cr is required to form a two-phase structure of austenite ferrite, but conversely, if it exceeds 27 qb, the stress corrosion cracking resistance will be significantly degraded.
．． 0-27. The range was 0%.

Ni: Ni is also an important element in determining the phase ratio of duplex stainless steel, and at least 3 or more is required to form a duplex structure, but if it exceeds 6, the precipitation of the σ phase may occur. The content is determined to be in the range of 30 to 60% because it accelerates stress corrosion cracking and deteriorates stress corrosion cracking resistance.

Mo: If Mo exceeds 0.5%, high temperature H2S-C0
2-(J-) Because it significantly deteriorates the stress corrosion cracking resistance of duplex stainless steel in an environment, the upper limit was set at 0.5% or less, which is lower than that of conventional duplex stainless steel.On the other hand, Mo
has the effect of improving pitting corrosion resistance, and the effect is 0.1
It is recognized that Mo is O, less than 1% (Ii (#
Since it would be difficult to compensate for the lack of pitting corrosion resistance due to N, W, and Cu, which will be described later, the lower limit was set at 0.1%.

N: N is effective in improving pitting corrosion resistance, and at least 0.1 inch or more is required to compensate for the decrease in pitting corrosion resistance due to the reduction of MO. However, if N exceeds 04%, hot workability will be significantly impaired, so the upper limit was set as 04%.

W: Like N, W is also effective in improving pitting corrosion resistance, but adding more than 05% will not increase the effect any further and will only increase the economic cost, so the upper limit has been set to 05%. And so. Note that since the effect of W on improving the pitting corrosion resistance is recognized at 0.1% or more, the amount of W added is preferably 0.1% or more.

Cu: Cu is effective in improving the corrosion resistance and crevice corrosion resistance of duplex stainless steel, but if it is added in excess of 2.0%, it will deteriorate the pitting corrosion resistance, so the upper limit should be set at 2.0%. did. Note that the effect of adding Cu is observed at 0.5% or more, so when adding Cu, it is preferable to add 0.5% or more.

Ca*RED/i: These contain Mn, which adversely affects hot workability by forming a compound with S in the steel.
It is effective in preventing the formation of S and improving hot workability, but if added in excess, it will generate oxides and adversely affect mechanical properties, so the upper limit for adding these should be set. Ca was set to o, oos%, and REM was set to 0.05%. Note that the hot workability improvement effect of Ca and REM is
It is recognized at O,O'-02% or more and REM 0.01% or more, and therefore, when added, it is preferable that the amount added is greater than this.

The 2L stainless steel of the present invention not only contains the components defined above, but also needs to be subjected to solution treatment (solution heat treatment) particularly within the range of 10006C to 1100C. That is, the present inventors satisfy the above component ranges of 25.51Cr -5.8% Ni -0,
For steel with a composition of 3496Mo-0.32%N,
By varying the solution treatment temperature, 10% FeCl5.
6 When a corrosion resistance test was conducted in H20 solution (308C), as shown in Figure 1, the solution treatment temperature was 1000℃.
If the temperature is below or above 1100°C, the corrosion resistance will be significantly deteriorated. Therefore, in order to obtain excellent corrosion resistance,
It has been found that the solution treatment must be carried out within the range of 1000 to 1100°C. The deterioration in corrosion resistance at temperatures below 1000°C is due to the surrounding Cr-deficient phase due to the precipitation of the σ phase, and the deterioration in corrosion resistance at temperatures above 1100°C is due to the decreased austenite phase forming a film at the ferrite grain boundaries. This is thought to be due to the small amount of Cr in the remaining austenite phase.

Examples and comparative examples of this invention are described below.

Steel ingots of 15 kinds of duplex stainless steels (sample numbers 1 to 15) having different compositions as shown in Table 1 were melted, and each ingot was hot rolled to a thickness of 4 mm. 1 from 900℃ to each hot-rolled plate
After solution treatment at various temperatures up to 100°C, the U-bending test piece (thickness 211I + 111 width 15
I12111 (length: 80mm, surface W finish) was manufactured, and after U-bending with a bending radius of 7mm at Amsler, stress was applied and the corrosion test was conducted. The test was carried out in an autoclave.
The conditions were a temperature of 250°C, a 201 NaCl solution, and a H2
The S partial pressure was 20 atm, and the CO2 partial pressure was 60 atm. After one week, the autoclave was removed, the rust was removed, and the presence or absence of cracks (stress corrosion cracking) was examined.Then, the weight was measured, and the corrosion rate (bee'year) was determined from the corrosion loss (weight before test - mold weight after test). ). The results are shown in Table 1.

In Table 1, the steels of sample numbers 1 to 3 all have compositions outside the scope of the present invention, that is, sample number 1 is S
i is in excess, sample number 2 is in excess of Cr, sample number 3 is in Mo
were excessive, and stress corrosion cracking occurred in all of these. Steel sample number 4 is a comparative steel with a N content of less than 0.1%, and although no stress corrosion cracking occurred in this case, the corrosion rate was high. On the other hand, sample numbers 5 to 7 have the same composition and are within the composition range of the present invention, but sample numbers 5 and 6 have a solution treatment temperature of 100%.
In contrast, in sample number 7, the solution treatment temperature was outside the range of 105°C, which was outside the range of 0 to 1100°C, and stress corrosion cracking occurred and the corrosion rate was high.
0°C, in which case no stress corrosion cracking occurs and the corrosion rate is usually 0, which is considered to have extremely good corrosion resistance.
It was less than 13 m/year. Also sample number 8-1
0 is also within the composition range of the present invention, and the solution treatment temperature conditions also satisfy the present invention, and these also do not cause stress corrosion cracking and the corrosion rate is significantly lower than 0.13 mm/year. , Therefore, the steel of the present invention is high temperature H2S-C
It is clear that the sample numbers 11 to 15 are within the scope of the present invention.
However, no deterioration in corrosion resistance or stress corrosion cracking resistance was observed due to these additions, and no edge cracking occurred during the hot rolling stage.
It was confirmed that the hot workability improvement effect of EM was achieved.

As described above, the duplex stainless steel of the present invention is co2-
The amount of Mo, which has an adverse effect on stress corrosion cracking properties on the H2s-cl-i stage, is regulated to a much smaller amount than before, and Cr and Si, which also have an adverse effect on stress corrosion cracking properties, are
On the other hand, the decrease in pitting corrosion resistance due to the reduction of MO is not compensated for by increasing the amount of N, or by increasing the amount of N and W or Cu, and the solution treatment temperature is adjusted to 1000 to 1100°C, where the best corrosion resistance is exhibited. Therefore, the duplex stainless steel of this invention is regulated within the range of H2
It has extremely good stress corrosion cracking resistance in S-Co2-Cl- environment, and also has excellent corrosion resistance such as pitting corrosion resistance.
Therefore, it can be safely used for oil country tubular goods, gas country tubular goods, etc. under harsh conditions such as those exposed to 112S-CO2- (J- environments).

[Brief explanation of the drawing]

FIG. 1 is a correlation diagram showing the relationship between solution treatment temperature and corrosion rate when steel having components within the composition range of the present invention is subjected to solution treatment at various temperatures. Applicant Kawasaki Steel Co., Ltd. Agent Patent attorney Takehito Toyota (and 1 other person) Figure 1 Condolence meal test scissors thread making

Claims (4)

[Claims] (1) C0.1% (weight%, same below) or less, 5iO9
6 pieces, Mn 2.0% or less, P 0.025% or less, S 0.005% or less, Cr 20.0-27.0
%, Ni30~6.0chi, M001~0.5%, N
A duplex stainless steel characterized by having a temperature of O.1 to 0.4. (2) C0, 1% or less, SiO, 6% or less, Mn 2.
0% or less, P 0.025% or less, S 0.005
% or less, Cr200-270chi, Ni3. Contains O~6.0chi, MO01~0.5chi, N001~0.4chi,
Furthermore, it must contain at least one of W O, 5% or less, and Cu2.0% or less, with the balance consisting of Fe and unavoidable impurities, and the solution treatment must have been performed within a temperature range of 1000 to 1100°C. A duplex stainless steel characterized by: (3) C0.1% or less, Si 0.6% or less, Mn 2.
0% or less, Po, 025% or less, So, 005% or less, Cr20, 0-27, Ochi, Ni3.0-6.0%
, Mo 0.1-0.5%, NO, 1-0.4%, and further contains CaO, 0.08% or less and rare earth elements 0.
．． 05% or less, with the remainder being Fe.
and unavoidable impurities, and after solution treatment for 10
A duplex stainless steel characterized by being produced within a temperature range of 00 to 1100°C. (4) C0.1% or less, Si 0.6% or less, Mn 2.
0 qb or less, Po, 025% or less, So, 005% or less, Cr20, 0-27.0%, N13゜0-60%,
Contains Mo0.1 to 0.5 thi, NO, 1 to 0.4 thi,
Furthermore, it contains one or more of W 0.5% or less and Cu 2.0% or less, one or more of Ca 0.008% or less and rare earth element 0.05% or less, and the balance is Fe. and a duplex stainless steel which is free from unavoidable impurities and which has been subjected to solution treatment at a temperature within the range of 1000 to 1100°C.