JPH11310855A - Martensitic stainless steel for oil well, excellent in corrosion resistance, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a martensitic stainless steel capable of exhibiting sufficient corrosion resistance (sulfide stress corrosion cracking resistance) under the environment where chloride ions, carbon dioxide gas, and hydrogen sulfide gas coexist together. SOLUTION: The steel has a composition which contains, by weight, <=0.05% C and 7-15% Cr and in which the content of Cu in a state of solid solution is 0.25-5%. This steel can contain, as substitute for a part of Fe, one or >=2 elements selected from proper amounts of Si, Mn, Al, Ni, Mo, W, Ca, Mg, La, Ce, Ti, Zr, and Nb, and it is desirable to control the contents of P, S, N, and O among impurities to specific values or below, respectively. Further, this steel can be obtained by merely performing air cooling or rapid cooling (water cooling) after the finishing stage of plastic working without delay or after temporary reheating to a temperature in the austenite region.

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 used for an oil well or a gas well (hereinafter simply referred to as an "oil well").
The present invention relates to a martensitic stainless steel for oil wells having excellent corrosion resistance suitable for use in a severe corrosive environment containing a large amount of corrosive impurities such as chlorine ions and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the environment of wells for extracting oil or natural gas has become increasingly severe, and in addition to the increase in depth, the number of oil wells containing carbon dioxide and hydrogen sulfide has increased. Embrittlement of materials due to corrosion or the like is a major problem.

[0003] Conventionally, carbon steel or low alloy steel is usually used for oil well pipes, which are one of the general steel materials for oil wells. However, as the environment of the oil well used becomes severer, alloys are increasingly used. Increasing steel is being used.

For example, in an oil well containing a large amount of carbon dioxide gas, it is known that the addition of Cr significantly improves the corrosion resistance, and 9% Cr-1% M containing about 9% Cr is known.
o steel and martensitic stainless steel typified by SUS420 containing about 13% of Cr are often used.

However, steel containing a large amount of Cr has a poor corrosion resistance to hydrogen sulfide in spite of the fact that it is a stainless steel. Stress corrosion cracking (S
SC) is likely to occur, and its use is actually limited.

In such an oil well environment containing carbon dioxide gas and hydrogen sulfide, at present, a duplex stainless steel or an austenitic stainless steel in which the alloy elements are further increased must be used, but the addition of alloy elements increases. Therefore, the production cost of the product increases remarkably.

Therefore, Japanese Patent Application Laid-Open No. 7-62499 discloses that the above-mentioned SUS420 steel is used as a base, and a specific amount of Ni and Mo is added thereto while the C content is 0.02 to 0.02.
A martensitic stainless steel is disclosed in which by limiting the content to 0.05%, the overall corrosion resistance and the stress corrosion cracking resistance in a high-temperature corrosion environment of 100 to 150 ° C. are ensured.

Japanese Patent Application Laid-Open No. Hei 8-246107 discloses that, similarly to the above, based on SUS420 steel, the C content is limited to 0.005 to 0.05%,
% Cu and 2 to 3% Mo, the carbon dioxide corrosion resistance and the sulfide stress corrosion cracking resistance (SS resistance)
(C property) is shown. The publication states that the effects of Cu and Ni are effective in reducing the corrosion rate in a high-temperature carbon dioxide gas environment, and that the addition of Mo or W is effective in the resistance to sulfide stress corrosion cracking. .

Further, Japanese Patent Application Laid-Open No. Hei 8-260041 discloses a method for producing such a low C martensitic stainless steel, in which a quenching and tempering heat treatment is performed after hot pipe making.

[0010] That is, C disclosed in these publications
In order to control the strength of u-added martensitic stainless steel, it is premised that tempering treatment is always performed after quenching, so that Cu is precipitated in the product.

However, in the recent development of oil wells, development in a severe environment containing a large amount of corrosive gas such as carbon dioxide gas and hydrogen sulfide has been promoted, and high corrosion resistance in such a severe environment has been promoted. There is a strong demand for oil country tubular goods.

Conventionally, it has been known that in carbon steel, a reduction in tensile strength, that is, a reduction in hardness, is effective in reducing susceptibility to sulfide stress corrosion cracking. Therefore,
The above-mentioned JP-A-07-62499 and JP-A-8-24
In JP-A-6107 and JP-A-8-260041, tempering is performed after quenching to soften the steel and control the strength to a low level.

Therefore, it is necessary to take two or more heat treatment steps of quenching and tempering as a process.
In terms of manufacturing, the process is complicated, and there is a need to develop steel that can be manufactured by a simpler process.

[0014]

SUMMARY OF THE INVENTION An object of the present invention is to exhibit excellent corrosion resistance in a harsh environment containing a large amount of both hydrogen sulfide and carbon dioxide gas, specifically, sulfide stress corrosion cracking resistance.
Another object of the present invention is to provide a martensitic stainless steel which can be manufactured by a simple process when applied to actual industrial production, and a method for manufacturing the same.

[0015]

The gist of the present invention is to produce the following martensitic stainless steels for oil wells excellent in corrosion resistance (1) and the following martensitic stainless steels for oil wells excellent in corrosion resistance (2). In the way.

(1) By weight%, C: 0.05% or less, C
r: Martensitic stainless steel for oil wells containing 7 to 15% and having a solid solution Cu content of 0.25 to 5% and excellent in corrosion resistance.

(2) By weight%, C: 0.05% or less, C
After heating and holding a martensitic stainless steel containing r: 7 to 15% and Cu: 0.25 to 5% in an austenite temperature range, a solution heat treatment of air cooling or quenching is performed to produce a product as it is (1). A) a method for producing a martensitic stainless steel for oil wells having excellent corrosion resistance as described in (1).

The steel (1) according to the present invention comprises a specific amount of S instead of a part of Fe in addition to the above chemical components.
i, Mn, Al, Ni, Mo, W, Ca, Mg, La,
It may contain one or more selected from Ce, Ti, Zr and Nb. Further, it is desirable that P, S, N, and O in the impurities are suppressed to a specific value or less. Further, in the above method (2), a heat treatment for adjusting the strength may be performed in a temperature range equal to or lower than the Cu deposition temperature after the solution heat treatment.

The present invention has been completed based on the following findings. That is, the present inventors have found that as a result of intensive experimental research, the following has been found in a low C-Cr-Fe-based martensitic stainless steel in a predetermined amount, specifically 0.25 to 5% by weight. When Cu in the solid solution state exists, the resistance to sulfide stress corrosion cracking in an environment containing hydrogen sulfide is remarkably improved.

Although the addition of Cu deteriorates hot workability, particularly when improvement of hot workability is required, 8%
By adding and adding Ni in an amount satisfying the following expression within the following range, hot workability is improved. Ni ≧ 0.64 × Cu−0.15 Here, the element symbols represent the contents (% by weight) of the respective elements in the steel.

Cu in the solid solution state effective for improving the resistance to sulfide stress corrosion cracking is, for example, a solution which is air-cooled or quenched immediately after finish rolling in a hot pipe forming process or once after reheating to an austenite region. By performing the chemical heat treatment (quenching), the total amount of Cu contained in the steel is dissolved and can be easily secured.

In this case, since a tempering process for controlling the strength is not required, the production can be performed by a simple process, and the production cost of the product can be reduced. Further, the yield ratio (YR) can be stabilized by performing a heat treatment at a temperature not higher than the precipitation temperature of Cu (450 ° C.) after the solution heat treatment, and there is no problem.

The steel of the present invention is basically used as it is as a solution heat treatment.
A yield strength of 0 to 860 MPa can be ensured.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting each component are described below. In the following, “%” means “% by weight”.

C: 0.05% or less C is an element inevitably contained in steel. In general, if the content is large, the strength becomes too high, and the sulfide stress corrosion cracking resistance is lowered. to degrade. In particular, since the steel of the present invention is not tempered or used under low-temperature tempering, the upper limit is set to 0. 0 so that the strength does not become too high in the solution treatment in the austenite region. 05%.
From the viewpoint of corrosion resistance, the smaller the C content, the better, preferably 0.025% or less, more preferably 0.1% or less.
01% or less.

Cr: 7 to 15% Cr is a main essential component for ensuring the corrosion resistance of the martensitic stainless steel according to the present invention, and is a viewpoint for ensuring corrosion resistance that is practically acceptable in an oil well environment. Requires a content of 7% or more. However, if the content exceeds 15%, the cost increases more than the improvement in corrosion resistance, and it is not economical. In addition, the M
The synergistic effect with o causes the formation of a δ ferrite phase,
Not only does the corrosion resistance decrease, but also the hot workability. Therefore, the Cr content is set to 7 to 15%. The preferred range is 9-14%.

Cu: 0.25 to 5% in a solid solution state Cu is the most important element for ensuring corrosion resistance in an environment containing carbon dioxide and hydrogen sulfide of steel in the present invention. It is essential that it exists in a solid solution state in steel. The reason is unknown, but when placed in an environment where hydrogen sulfide is slightly present, Cu
It is presumed that a Cu sulfide film is quickly formed on the surface of C-Cr-Fe-based martensitic stainless steel in which is present in a solid solution state, and this film acts as an anticorrosion film.

However, the Cu content in the solid solution state is 0.25
%, The effect of improving corrosion resistance as described above cannot be obtained. On the other hand, if it exceeds 5%, not only the effect is saturated, but also the hot workability is significantly deteriorated. Therefore, the content of Cu is set to 0.25 to 5% in a solid solution state.

Si: 1% or less Si need not always be added, but is usually an element used as a deoxidizing agent in the steel making process, and can be added for this purpose. However, since the toughness whose content exceeds 1% is significantly reduced, the content is preferably set to 1%. A preferred upper limit is 0.75%.

Mn: 5% or less Mn is an element which is not necessarily added but is effective as a deoxidizing agent in the steel making process, but has an effect of improving hot workability. It can be added if desired. However, if the content exceeds 5%, retained austenite is likely to be generated and it becomes difficult to secure a martensite single phase. Therefore, the content is desirably 5% or less. The preferred upper limit is 1%, in which case the pitting resistance is improved.

Al: 0.001 to 0.1% Al is an element usually used as a deoxidizing agent in the steel making process, like Al, although it is not always necessary to add it. Can be added. But,
If the content is less than 0.001%, the desired deoxidizing effect cannot be obtained, and if it exceeds 0.1%, non-metallic inclusions increase to deteriorate the corrosion resistance. Therefore, when Al is added, the Al content is desirably 0.001 to 0.1%. A preferred range is 0.003-0.05%.

Ni: not more than 8% and (0.64 × [%
Cu] -0.15) or more Ni is not necessarily added, but is an element effective for preventing deterioration of hot workability due to addition of Cu, improving strength and corrosion resistance, and securing a martensite single phase. And
They can be added when these effects are desired. However, if the content exceeds 8%, the retained austenite increases and the corrosion resistance decreases. If the content is less than the value determined by the following formula, hot workability is not improved. Therefore, it is desirable that the Ni content when added is not more than 8% and not less than (0.64 × [% Cu] −0.15). A preferred upper limit is 7%.

P: not more than 0.04% P is an impurity element inevitably contained in steel, and if its content exceeds 0.04%, the sulfide stress corrosion cracking resistance is remarkably deteriorated. The upper limit is desirably 0.04%. A preferred upper limit is 0.02%, and a more preferred upper limit is 0.01%.

S: 0.005% or less S, like P, is an impurity unavoidably contained in steel, and is an element that degrades sulfide stress corrosion cracking resistance and hot workability. Therefore, from the viewpoint of corrosion resistance and hot workability, the smaller the better, the better. However, excessive reduction increases the cost of desulfurization and is not economical. Since the reduction effect saturates at 0.005%, the upper limit is 0.005%.
% Is desirable. The preferred upper limit is 0.0008
%, And a more preferable upper limit is 0.0003%. In this case, the formation of sulfide-based nonmetallic inclusions can be suppressed, and the flaws generated from the sulfide-based nonmetallic inclusions at the time of pipe production start. Can be prevented from occurring.

N: 0.05% or less N, like P and S, is an unavoidable impurity contained in steel and is an element which increases the strength to increase the susceptibility to sulfide stress corrosion cracking. If the content exceeds 0.05%, the strength is excessively increased and the sulfide stress corrosion cracking resistance is significantly deteriorated. Therefore, the upper limit is desirably set to 0.05%. A preferred upper limit is 0.02%.

O (oxygen): 0.005% or less O, like P, S and N, is an impurity element unavoidably contained in steel and degrades hot workability, so that it is low. The lower the better. However, excessive reduction increases the cost of deoxidation, is not economical, and has a content of 0.0
Since the desired hot workability can be obtained if the content is not more than 05%, the upper limit is desirably 0.005%. A preferred upper limit is 0.004%.

Mo, W, (Mo + 0.5 W): 0.2 to
5% Mo and W need not be added, but are elements having an effect of improving the local corrosion resistance in a carbon dioxide gas environment in the presence of Cr. Or both can be added. However, if the content is (Mo + 0.5W) and less than 0.2%, the above effects cannot be obtained. If it exceeds 5%, not only the effect is saturated, but also
The synergistic action with Cr leads to the formation of a δ ferrite phase, which lowers the corrosion resistance and hot workability. Therefore, the content of Mo and / or W when added is (Mo + 0.
5 W) and preferably 0.2 to 5%. A preferred range is 0.5-3%.

Ca, Mg, La, and Ce: 0.001 to 0.05% in each case These elements do not necessarily need to be added and contained, but any of these elements improves the hot workability of steel. In order to obtain the effect, one or more selected from these can be added. However, if the content of any of the elements is less than 0.001%, the above effects cannot be obtained. If the content exceeds 0.05%, a coarse oxide is generated, and the corrosion resistance is reduced. Therefore, the content when added is 0.001 to 0.05 for each element.
% Is desirable. A preferable range is 0.005 to 0.03% for each element.

Ti, Zr and Nb: all 0.01
These elements are not necessarily required to be added, but any of these elements is effective for reducing the strength of the solution treatment in the austenite region as it is, and as a result, sulfide stress corrosion cracking resistance is obtained. Since it has the effect of improving the properties, one or two or more selected from these can be added when this effect is desired. However, if the content of any of the elements is less than 0.01%, the above effects cannot be obtained, and even if added over 0.5%, the effects are saturated.
Therefore, when added, the content of each element is 0.1.
It is desirably set to 01 to 0.05%. A preferable range is 0.02 to 0.35% for each element.

The martensitic stainless steel of the present invention having the above-mentioned chemical composition is made from a slab obtained by melting and casting according to a conventional method, and is subjected to hot plastic working according to a conventional method. In addition, it is formed into a predetermined product shape, and is manufactured by performing a solution heat treatment (quenching) immediately or immediately after the finishing step of the hot plastic working or once reheating to the austenite region and then air cooling or quenching.

More specifically, when the product is a seamless steel pipe manufactured by, for example, the Mannesmann-mandrel mill method, immediately after the finish rolling by a finish rolling machine such as a stretch reducer or a sizer, or once again to the austenite region. It is manufactured by applying a solution heat treatment (quenching) of air cooling or quenching after heating.

Further, if the heat treatment is performed in a temperature range not higher than the Cu precipitation temperature (450 ° C.) after the solution heat treatment, the strength of the final product can be adjusted without depositing Cu.

[0043]

EXAMPLES Ten kinds of steels having the chemical compositions shown in Table 1 were melted to produce ingots having a rectangular section of 50 mm on a side. After heating these ingots to 1200 ° C., they were subjected to hot forging and hot rolling to form a 10 mm-thick plate.

[0044]

[Table 1]

Thereafter, each of the obtained plate members was heated at 1000 ° C. for 1 hour.
After heating and holding for 5 minutes, a solution treatment (quenching) of cooling with water was performed to obtain a test material. Among them, a plate material made of steel of No. 11 was subjected to a tempering treatment of heating and holding at 600 ° C. for 30 minutes and then air cooling after the above solution treatment (quenching) for comparison.

Then, the sulfide stress corrosion cracking resistance of the test materials made of these steels was examined by the following method. In addition, the Cu content of the steel of No. 11 in the solid solution state was determined from a calculation phase diagram determined by thermocalc.

<< Sulfide Stress Corrosion Cracking Resistance Test >> Two test pieces 1 each having the shape and dimensions shown in FIG. 1 were sampled from each of the above-mentioned test pieces, and the test pieces 1 were prepared as shown in FIG. In a state where a predetermined bending amount y is provided by being set on the bending application jig 3 shown in FIG.
H ₂ S-30 atm CO ₂ ”for 336 hours in a 5% NaCl corrosion aqueous solution.

After that, the test piece 1 was taken out, and the appearance of the test piece and the cross section of the test piece were examined by optical microscope for the presence or absence of cracks.
Those with cracks were evaluated as "x".

The test piece 1 has a bending stress σ expressed by the following equation based on the dimensions of each part shown in FIG.
The bending amount y was given so that the 0.2% proof stress of each test steel was obtained.

[0050] σ = Ety {(2/3) L 1 2 + L 1 × L 2 +
(1/4) L ₂ ^{2 -1 -1} where, E: Young's modulus (kg / mm ² ), y: Bending amount (mm), L ₁ , L ₂ : Distance between bending fulcrum (mm), t: Test Piece thickness (mm) The above results are also shown in Table 1.

As is clear from the results shown in Table 1, the steels of the present invention (Nos. 1 to 7) had good sulfide stress corrosion cracking resistance.

On the other hand, among the steels of the comparative examples,
Steel (No. 8) having a small amount and a solid solution Cu amount as small as 0.18% and out of the range specified in the present invention caused sulfide stress corrosion cracking.

Although the amount of solid solution Cu is within the range specified in the present invention, the steel (No. 9) having a small Cr content of 6.8% and out of the range specified in the present invention has insufficient corrosion resistance. Therefore, the steel (No. 10) having a C content as high as 0.061% and out of the range specified in the present invention has too high strength, so that sulfide stress corrosion cracking occurred in each case.

Furthermore, although the amount of added Cu is large, almost all of the added Cu is precipitated due to the tempering treatment at 600 ° C., and the amount of solid solution Cu is 0.1%, which is within the range specified in the present invention. The steel (No. 11) that came off exhibited sulfide stress corrosion cracking.

[0055]

According to the present invention, a martensitic stainless steel exhibiting sufficient corrosion resistance, particularly sulfide stress corrosion cracking resistance, in a harsh environment where chloride ions, carbon dioxide gas and hydrogen sulfide gas coexist. Can be provided. Further, the martensitic stainless steel can be manufactured only by solution heat treatment, that is, quenching, and the heat treatment process can be simplified, so that the production cost of the product can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic view showing the shape and dimensions of a notched four-point bending test piece used in Examples, wherein FIG. 1A is a side view, FIG. 1B is a plan view, and FIG. FIG. 4 is an enlarged side view of a notch.

FIGS. 2A and 2B are diagrams showing the relationship between a bending jig and a test piece, wherein FIG. 2A is a front view showing a set state, and FIG. 2B is a view showing a bending state.

[Explanation of symbols]

 1: Test piece, 2: Notch, 3: Bending jig.

Claims (8)

[Claims] (1) C: 0.05% or less, Cr: 7% by weight
-15%, and the Cu content in the solid solution state is 0.2%
A martensitic stainless steel for oil wells having excellent corrosion resistance, characterized in that it is 5 to 5%. 2. The composition according to claim 1, further comprising: Si: 1% or less;
n: 5% or less, Al: 0.001 to 0.1% and 8%
The martensitic stainless steel for oil wells excellent in corrosion resistance according to claim 1, wherein the steel contains an amount of Ni satisfying the following expression within the following range. Ni ≧ 0.64 × Cu−0.15 Here, the element symbols represent the contents (% by weight) of the respective elements in the steel. 3. P, S, N and O in impurities in steel are
0.04% or less, 0.005% or less, 0.05 respectively
The martensitic stainless steel for oil wells having excellent corrosion resistance according to claim 1 or 2, wherein the content is 0.005% or less and 0.005% or less. 4. The composition according to claim 1, further comprising Mo + 0.5W: 0.
The martensitic stainless steel for oil wells according to any one of claims 1 to 3, which contains Mo and / or W satisfying 2 to 5%. 5. The method according to claim 1, further comprising:
0.05%, Mg: 0.001 to 0.05%, La:
0.001 to 0.05% and Ce: 0.001 to 0.
The martensitic stainless steel for oil wells excellent in corrosion resistance according to any one of claims 1 to 4, wherein the stainless steel contains one or more kinds selected from 05%. 6. Ti: 0.01 to 0.1% by weight.
5%, Zr: 0.01-0.5% and Nb: 0.01
The martensitic stainless steel for oil wells excellent in corrosion resistance according to any one of claims 1 to 5, comprising one or more selected from among 0.5% or less. 7. C: 0.05% or less, Cr: 7% by weight
After heating and holding a martensitic stainless steel containing up to 15% and Cu: 0.25 to 5% in an austenitic temperature range, a solution heat treatment for air cooling or rapid cooling is performed to produce a product as it is. Item 7. A method for producing a martensitic stainless steel for oil wells having excellent corrosion resistance according to any one of Items 1 to 6. 8. A martensitic stainless steel for oil wells having excellent corrosion resistance according to claim 7, wherein a heat treatment for adjusting the strength is performed in a temperature range equal to or lower than a precipitation temperature of Cu after said solution heat treatment. Steel production method.