JP2008081793A - High-strength stainless steel pipe with high toughness and excellent corrosion resistance for oil well - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive high-strength stainless steel pipe for use in oil wells which has excellent hot workability and a high strength exceeding 654 MPa of YS, further exhibits excellent CO<SB>2</SB>corrosion resistance under severe high-temperature corrosive environment of ≥170°C containing CO<SB>2</SB>, Cl<SP>-</SP>, etc., and excellent SSC resistance even under H<SB>2</SB>S-containing environments, and has high toughness. <P>SOLUTION: The steel pipe has a composition containing, by mass, ≤0.04% C, ≤0.50% Si, 0.20 to 1.80% Mn, ≤0.03% P, ≤0.005% S, 15.5 to 17.5% Cr, 2.5 to 5.5% Ni, ≤0.20% V, 1.5 to 3.5% Mo, 0.50 to 3.0% W, ≤0.05% Al, ≤0.15% N and ≤0.006% O within the range simultaneously satisfying three correlation expressions. The steel pipe is preferably subjected to quenching-tempering treatment and preferably has a structure containing a martensite phase as a base phase and 10 to 50 vol.% ferrite phase. Moreover, one or more elements among Cu, Nb, Ti, Zr, W, B and Ca can be further contained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to oil wells for crude oil or natural gas, and steel pipes for oil wells used for gas wells. In particular, the present invention includes carbon wells (CO ₂ ), chlorine ions (Cl ⁻ ), etc., and oil wells and gases with extremely severe corrosive environments. The present invention relates to a high-strength stainless steel pipe for oil wells having excellent corrosion resistance suitable for well use. The “high-strength stainless steel pipe” in the present invention refers to a stainless steel pipe having a yield strength exceeding 654 MPa (95 ksi).

In recent years, in order to cope with soaring crude oil prices and the depletion of petroleum resources expected in the near future, deep oil fields that have not been excluded in the past, and highly corrosive once development has been abandoned Development on sour gas fields and the like has become active worldwide. Such oil, gas fields are generally the depth is very deep, and the atmosphere at a high temperature and, CO _2, Cl ^- has a severe corrosive environment and the like. Accordingly, steel pipes for oil wells used for mining such oil fields and gas fields are required to have high strength and excellent corrosion resistance.

Conventionally, 13% Cr martensitic stainless steel pipes excellent in CO ₂ corrosion resistance have been used as oil well steel pipes in oil fields and gas fields in an environment containing CO ₂ , Cl _2- , ^{and the} like. However, 13% Cr martensitic stainless steel pipe generally, Cl ^- at a high temperature environment exceeding comprises large amounts of 100 ° C., there is a problem that becomes intolerable use. Therefore, duplex stainless steel pipes have been used in wells that require corrosion resistance. However, the duplex stainless steel pipe has a problem that it has a large amount of alloy elements, is inferior in hot workability, and can be produced only by a special hot working method, and is expensive. In addition, the conventional 13% Cr martensitic stainless steel pipe has a problem that when the yield strength exceeds 654 MPa, the toughness is significantly lowered and it cannot be used.

In recent years, oil fields have been actively developed in cold regions, and it is often required to have excellent low temperature toughness in addition to high strength.
For this reason, it is based on 13% Cr martensitic stainless steel, which is excellent in hot workability and inexpensive, and has high yield strength exceeding 654 MPa (95 ksi) and excellent CO ₂ corrosion resistance. High strength 13% Cr martensitic stainless steel pipes for oil wells having high properties and high toughness have been strongly desired.

In response to such a request, for example, in Patent Document 1, C is limited to 0.005 to 0.05%, Ni: 2.4 to 6% and Cu: 0.2 to 4% are added in combination, and Mo is further added to 0.5 to 3%. A 13% Cr martensitic stainless steel seamless pipe with excellent corrosion resistance has been proposed having a composition in which Nieq is adjusted to 10.5 or more. This seamless steel pipe is said to be a martensitic stainless steel seamless pipe having high strength of API-C95 grade or higher, corrosion resistance in an environment containing CO _{2 of} 180 ° C or higher, and SCC resistance.

  Patent Document 2 includes C: 0.005 to 0.05%, N: 0.005 to 0.1%, Ni: 3.0 to 6.0%, Cu: 0.5 to 3%, and Mo: 0.5 to 3%. A martensitic stainless steel excellent in sulfide stress corrosion cracking resistance having a martensitic stainless steel composition and a structure in which tempered martensite and a γ phase of 20% by volume or more are mixed has been proposed. This martensitic stainless steel has a tempered martensite structure containing 20% by volume or more of the γ phase, and has markedly improved sulfide stress corrosion cracking resistance.

  In Patent Document 3, the composition of martensitic stainless steel containing 10 to 15% Cr, C is limited to 0.005 to 0.05%, Ni: 4.0% or more, and Cu: 0.5 to 3% are added in combination. Furthermore, it is composed of a composition in which Mo is added in an amount of 1.0 to 3.0% and Nieq is adjusted to -10 or more, and a tempered martensite phase, a martensite phase, and a retained austenite phase. A martensitic stainless steel having a structure with a fraction of 60 to 90% and excellent corrosion resistance and sulfide stress corrosion cracking resistance has been proposed. This martensitic stainless steel is said to be excellent in corrosion resistance and sulfide stress corrosion cracking resistance in a wet carbon dioxide environment and a wet hydrogen sulfide environment.

  Patent Document 4 contains more than 15% and 19% or less of Cr, C: 0.05% or less, N: 0.1% or less, Ni: 3.5-8.0%, and Mo: 0.1-4.0% In addition, a martensitic stainless steel material for oil wells having a steel composition satisfying 30Cr + 36Mo + 14Si−28Ni ≦ 455 (%) and 21Cr + 25Mo + 17Si + 35Ni ≦ 731 (%) and excellent in sulfide stress cracking has been proposed. This martensitic stainless steel material is said to have excellent corrosion resistance even in a harsh oil well environment in which chloride ions, carbon dioxide gas and a small amount of hydrogen sulfide gas exist.

In Patent Document 5, C: 0.005 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5 to 18%, Ni: 1.5 to 5%, Mo: 1 to 3.5%, V: 0.02 to 0.2%, N: 0.01 to 0.15%, O: 0.006% or less, and Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5 and Cr + Mo + 0. Composition satisfying 3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5, martensite phase as base phase, ferrite phase 10-60% by volume, or austenite phase 30% by volume or less A high-strength stainless steel pipe for oil wells having excellent corrosion resistance has been proposed.
JP-A-8-120345 JP-A-9-268349 Japanese Patent Laid-Open No. 10-1755 Japanese Patent No. 2814528 JP 2005-336595 A

Produced by the technique described in Patent Documents 1 5, the improved martensitic stainless steel tube, as compared with the conventional 13% Cr martensitic stainless steel pipes far, in particular CO _2, Cl ^- and It is said that the corrosion resistance under the high temperature corrosive environment is significantly improved. However, these improved martensitic stainless steel pipes have a problem that they tend to cause sulfide stress corrosion cracking (SSC) in a corrosive environment where H ₂ S exists. Such a problem becomes remarkable particularly in a high-strength martensitic stainless steel pipe having a yield strength of 792 MPa (110 ksi) or more.

The present invention has been made in view of the problems of the prior art described above is excellent in hot workability, YS: it has a high strength of more than 654MPa, CO _2, Cl ^-, etc. than 170 ° C. containing Excellent oil resistance for CO ₂ in the severe hot corrosion environment, and excellent SSC resistance and high toughness even in the presence of H ₂ S. It aims at providing a strength martensitic stainless steel pipe.

  The “high-strength martensitic stainless steel pipe” in the present invention refers to a martensitic stainless steel pipe having a yield strength exceeding 654 MPa (95 ksi). Further, “high toughness” in the present invention refers to a case where the absorbed energy at −20 ° C. of the Charpy impact test shows 50 J or more.

In order to achieve the above-mentioned object, the present inventors based on the composition of an improved 13% Cr martensitic stainless steel pipe as a base composition, while maintaining a high strength exceeding yield strength: 654 MPa (95 ksi), CO _2, Cl ^- and anti CO ₂ corrosion resistance under high temperature environment and the like, made extensive studied the effect of alloying elements on the SSC resistance in an environment that there are more H ₂ S. As a result, 13% Cr martensitic stainless steel with C significantly reduced compared to the prior art contains Cr in an amount of 15.5 to 17.5%, and includes appropriate amounts of Ni and Cu, and Mo + 0.8W is 2.0 to 2.0 In addition to adjusting the Mo and W contents to be within the range of 4.5, the following formula (1)
Cr + 3.2Mo + 2.6W-10C ≧ 23.4 (1)
(Here, Cr, Mo, W, C: content of each element (mass%))
It was found that the SSC resistance in an environment where H ₂ S is present is remarkably improved by adjusting the Cr, Mo, W, and C contents so as to satisfy the above. Furthermore, the following equation (2)
Cr + Mo + 0.5W + 0.3Si-43.5C-0.4Mn-0.3Cu-Ni-9N ≧ 11.5 (2)
(Here, Cr, Mo, W, Si, C, Mn, Cu, Ni, N: content of each element (mass%))
By adjusting the content of Cr, Mo, W, Si, C, Mn, Cu, Ni, N so as to satisfy the requirements, excellent hot workability sufficient for seamless steel pipe production, and It has been found that the desired high strength can be ensured.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.04% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5 to 17.5%, Ni: 2.5 to 5.5 %, V: 0.20% or less, Mo: 1.5 to 3.5%, W: 0.50 to 3.0%, Al: 0.05% or less, N: 0.15% or less, O: 0.006% or less, the following formulas (1) to (3)
Cr + 3.2Mo + 2.6W-10C ≧ 23.4 (1)
Cr + Mo + 0.5W + 0.3Si-43.5C-0.4Mn-0.3Cu-Ni-9N ≧ 11.5 (2)
2.2 ≦ Mo + 0.8W ≦ 4.5 (3)
(Here, Cr, Mo, W, Si, C, Mn, Cu, Ni, N: content of each element (mass%))
A high-strength stainless steel pipe for oil wells with high toughness and excellent corrosion resistance, characterized in that it has a composition comprising the balance Fe and inevitable impurities.

(2) A high-strength stainless steel pipe for oil wells characterized in that, in addition to the above composition, the composition further includes mass: Cu: 0.5 to 3.5%.
(3) In (1) or (2), in addition to the above composition, it is further selected in mass% from Nb: 0.20% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less A high-strength stainless steel pipe for oil wells, characterized in that the composition contains one or more kinds.

(4) In any one of (1) to (3), a high-strength stainless steel pipe for oil wells characterized in that, in addition to the above-mentioned composition, the composition further contains Ca: 0.0005 to 0.01% in mass%.
(5) The high-strength stainless steel for oil wells according to any one of (1) to (4), characterized in that it has a structure containing a martensite phase as a base phase and a ferrite phase in a volume ratio of 10 to 50%. Steel pipe.

According to the present invention, CO _2, Cl ^- wherein the further even in severe corrosive environment of high temperature containing H ₂ S, can provide a high strength stainless steel exhibits sufficient corrosion resistance easily, the durability of the oil well pipe Is remarkably improved and has a remarkable industrial effect.

First, the reasons for limiting the composition of the high-strength stainless steel pipe of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply written as%.
C: 0.04% or less C is an important element related to the strength of martensitic stainless steel. To ensure the desired strength, 0.005% or more is desirable, but more than 0.04% is contained. Then, the sensitization at the time of tempering by Ni containing tends to increase. Therefore, C is limited to 0.04% or less for the purpose of preventing sensitization during tempering. In addition, Preferably, it is 0.005 to 0.03% of range from a viewpoint of corrosion resistance.

Si: 0.50% or less
Si is an element that acts as a deoxidizer in the normal steelmaking process. In this invention, it is desirable to contain 0.05% or more, but if it exceeds 0.50%, the CO ₂ corrosion resistance decreases, Hot workability also decreases. For this reason, Si was limited to 0.50% or less. In addition, Preferably it is 0.10 to 0.35%.

Mn: 0.20 to 1.80%
Mn is an element that increases the strength, and in this invention, it is necessary to contain 0.20% or more in order to ensure the strength required as a martensitic stainless steel pipe for oil wells, but if it exceeds 1.80%, Adversely affect. For this reason, Mn was limited to the range of 0.20 to 1.80%. In addition, Preferably it is 0.25 to 0.60%.

P: 0.03% or less P is an element that degrades CO ₂ corrosion resistance, CO ₂ stress corrosion cracking resistance, pitting corrosion resistance and sulfide stress corrosion cracking resistance, and should be reduced as much as possible in this invention. However, extreme reduction leads to increased manufacturing costs. P is limited to 0.03% or less as a range that can be implemented relatively inexpensively industrially and does not deteriorate CO ₂ corrosion resistance, CO ₂ stress corrosion cracking resistance, pitting corrosion resistance and sulfide stress corrosion cracking resistance. . In addition, Preferably it is 0.02% or less.

S: 0.005% or less S is an element that significantly deteriorates hot workability in the pipe manufacturing process, and it is desirable that it be as small as possible. However, if it is reduced to 0.005% or less, pipes can be manufactured in the normal process. Therefore, S is limited to 0.005% or less. In addition, Preferably it is 0.003% or less.
Cr: 15.5-17.5%
Cr is an element improving the corrosion resistance by forming a protective coating, in particular resistance to CO ₂ corrosion, as well as effectively contribute to the improvement of resistance to CO ₂ stress corrosion cracking resistance, resistance to sulfide stress corrosion cracking (SSC resistance ). In the present invention, the content of 15.5% or more is particularly required from the viewpoint of improving the corrosion resistance at high temperatures. On the other hand, the content exceeding 17.5% decreases the strength. For this reason, in this invention, Cr was limited to the range of 15.5 to 17.5%. In addition, Preferably it is 16.0 to 17.0%.

Ni: 2.5-5.5%
Ni has an effect of strengthening the protective film, and is an element that enhances CO ₂ corrosion resistance, CO ₂ stress corrosion cracking resistance, pitting corrosion resistance, and sulfide stress corrosion cracking resistance. In order to secure such an effect in the severe corrosive environment targeted by the present invention, Ni content of 2.5% or more is required. On the other hand, if the content exceeds 5.5%, the stability of the martensite structure decreases and the strength decreases. For this reason, Ni was limited to the range of 2.5 to 5.5%. In addition, Preferably it is 3.0 to 5.0%.

V: 0.20% or less V is an element that has the effect of increasing the strength and improving the SSC resistance. To obtain such an effect, it is desirable to contain 0.01% or more, but 0.20% If it is contained in excess, the toughness decreases. For this reason, V was limited to 0.20% or less. In addition, Preferably it is 0.05 to 0.08%.

Mo: 1.5-3.5%
Mo is, Cl ^- by an element having the effect of increasing the resistance to pitting, further effectively acts to improve the SSC resistance. In order to ensure such an effect under the severe corrosive environment targeted by the present invention, the content of 1.5% or more is required. On the other hand, if the content exceeds 3.5%, the strength is lowered and the material cost is increased. For this reason, Mo was limited to the range of 1.5 to 3.5%. In addition, Preferably it is 1.8 to 3.0%.

W: 0.50 ~ 3.0%
W, like Mo, is an element effective for improving the SSC resistance, and such an effect becomes remarkable when the content is 0.50% or more. On the other hand, the content exceeding 3.0% deteriorates toughness. For this reason, Mo was limited to the range of 0.50 to 3.0%. In addition, Preferably it is 0.7 to 2.0%.
In the present invention, the contents of Mo and W are within the above-mentioned range and the following formula (3)
2.2 ≦ Mo + 0.8W ≦ 4.5 (3)
(Where, Mo, W: content of each element (mass%))
It is adjusted to satisfy. By containing Mo and W in combination, the SSC resistance is significantly improved. With a Mo + 0.8 W 2.2 or more, CO _2, Cl which the invention is applied ^- wherein the further even under severe corrosive environment of the hot containing H ₂ S, can ensure excellent SSC resistance. On the other hand, if Mo + 0.8W exceeds 4.5, toughness and hot workability are reduced. Therefore, in the present invention, the Mo and W contents are adjusted so that Mo and W satisfy the formula (3), and Mo + 0.8W is in the range of 2.2 to 4.5.

Al: 0.05% or less
Al is an element having a strong deoxidizing action, and in order to obtain such an effect, it is desirable to contain 0.002% or more, but inclusion exceeding 0.05% adversely affects toughness. For this reason, Al was limited to 0.05% or less. In addition, Preferably it is 0.03% or less.
N: 0.15% or less N is an element that remarkably improves pitting corrosion resistance, and such an effect becomes remarkable when the content is 0.01% or more. On the other hand, the content exceeding 0.15% forms various nitrides and lowers the toughness. For this reason, N was limited to 0.15% or less. In addition, Preferably it is 0.02 to 0.08%.

O: 0.006% or less O is present as an oxide in steel and adversely affects various properties, so it is desirable to reduce it as much as possible. In particular, when the O content exceeds 0.006%, the hot workability, the CO ₂ stress corrosion cracking resistance, the pitting corrosion resistance, the sulfide stress corrosion cracking resistance, and the toughness are significantly reduced. For this reason, O was limited to 0.006% or less.

Although the above-described components have a basic composition, in the present invention, in addition to the above-described composition, Cu: 0.5 to 3.5% and / or Nb: 0.20% or less, Ti: 0.3% or less, Zr: 0.2% Hereinafter, one or more of B: 0.01% or less and / or Ca: 0.0005 to 0.01% may be selected and contained as necessary.
Cu: 0.5-3.5%
Cu is an element having an action of strengthening the protective film and suppressing the penetration of hydrogen into the steel and improving the resistance to sulfide stress corrosion cracking, and can be contained as required. In order to acquire such an effect, it is preferable to contain 0.5% or more. On the other hand, if the content exceeds 3.5%, grain boundary precipitation of CuS occurs at a high temperature, and the hot workability decreases. For this reason, when it contains, it is preferable to limit Cu to 0.5 to 3.5% of range. In addition, More preferably, it is 0.5 to 2.5%, More preferably, it is 0.8 to 1.5%.

One or more selected from Nb: 0.20% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less
Each of Nb, Ti, Zr, and B is an element that increases the strength, and can be selected as required and contained in one or more kinds. Nb has an effect of further improving toughness in addition to the above-described effects. Ti, Zr, and B have an effect of further improving the stress corrosion cracking resistance in addition to the above-described effects. In order to obtain such an effect, it is desirable to contain Nb: 0.01% or more, Ti: 0.01% or more, Zr: 0.01% or more, B: 0.0005% or more. On the other hand, if Nb: 0.20%, Ti: 0.3%, Zr: 0.2%, and B: more than 0.01%, the toughness decreases. For this reason, when it contains, it is preferable to limit to Nb: 0.20% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less. More preferably, Nb: 0.02 to 0.10%, Ti: 0.02 to 0.12%, Zr: 0.02 to 0.08%, B: 0.0005 to 0.003%.

Ca: 0.0005 to 0.01%
Ca is an element having an action of fixing S as CaS and spheroidizing sulfide inclusions. This has the effect of reducing the lattice strain of the matrix around the inclusions and reducing the hydrogen trapping ability of the inclusions. Such an effect becomes prominent when the content is 0.0005% or more. However, if the content exceeds 0.01%, CaO increases, and the resistance to CO ₂ corrosion and pitting corrosion decreases. For this reason, when it contains Ca, it is preferable to limit to 0.0005 to 0.01% of range.

In the present invention, each of the above components is included within the above range, and the following formula (1)
Cr + 3.2Mo + 2.6W-10C ≧ 23.4 (1)
And the following equation (2)
Cr + Mo + 0.5W + 0.3Si-43.5C-0.4Mn-0.3Cu-Ni-9N ≧ 11.5 (2)
(Here, Cr, Mo, W, Si, C, Mn, Cu, Ni, N: content of each element (mass%))
The content of each component is adjusted so as to satisfy the above.

  By adjusting the Cr, Mo, W, and C contents so as to satisfy the above-described expression (1), the SSC resistance is remarkably improved. If the formula (1) cannot be satisfied, sufficient SSC resistance cannot be secured. Further, in addition to reducing P, S, and O to the above ranges, Cr, Mo, W, Si, C, Mn, Cu, Ni, N, so as to satisfy the above-described formula (2). By adjusting the content, it is possible to ensure the necessary and sufficient hot workability for forming a martensitic stainless steel seamless pipe, and to maintain a desired strength. Only by reducing P, S, and O within the above ranges, it is not possible to ensure the hot workability necessary and sufficient for forming a martensitic stainless steel seamless pipe. Moreover, when the formula (2) cannot be satisfied, the hot workability for the production of martensitic stainless steel seamless pipes is insufficient.

The balance other than the above components is Fe and inevitable impurities.
The high-strength stainless steel pipe according to the present invention has the above-described composition, and preferably has a structure containing a martensite phase as a base phase and further containing a ferrite phase in a volume ratio of 10 to 50%. In the high-strength stainless steel pipe according to the present invention, the structure is a structure having a martensite phase as a base phase in order to maintain high strength. And in this invention, it is preferable to set it as the structure | tissue which contains a ferrite phase 10% or more by volume for the purpose of improving corrosion resistance, without reducing intensity | strength. On the other hand, when the ferrite phase exceeds 50% by volume, the strength is significantly reduced. For this reason, in this invention steel pipe, it is preferable to limit a ferrite phase fraction to the range of 10 to 50% by volume ratio. More preferably, it is 12 to 30%. In addition, there is no problem even if the second phase other than the ferrite phase contains an austenite phase of 20% by volume or less.

Next, the preferable manufacturing method of the high strength stainless steel pipe which becomes this invention is demonstrated taking a seamless steel pipe as an example.
First, molten steel having the above composition is melted by a generally known melting method such as a converter, an electric furnace, a vacuum melting furnace, etc., and billet is obtained by a conventional method such as a continuous casting method or an ingot-bundling rolling method. It is preferable to use a steel pipe material such as. Subsequently, these steel pipe materials are heated and hot-worked and formed using a normal Mannesmann-plug mill system or Mannesmann-Mandrel mill process to obtain seamless steel pipes of desired dimensions.

After the pipe making, the seamless steel pipe is preferably cooled to room temperature at a cooling rate equal to or higher than air cooling. Thereby, the structure | tissue of a steel pipe can be made into the structure | tissue which makes a martensite phase a base phase. In addition, you may manufacture a seamless steel pipe by the hot extrusion by a press system.
In addition, after pipe making, following cooling at a cooling rate of air cooling or higher, after reheating to a temperature of 800 ° C. or higher, a quenching treatment is performed at a cooling rate of air cooling or higher of 100 ° C. or lower, preferably to room temperature. Is preferred. Thereby, it is possible to obtain a fine and high toughness martensite structure preferably containing an appropriate amount of ferrite phase. If the quenching heating temperature is less than 800 ° C., the desired strength cannot be secured. For this reason, the heating temperature in the quenching treatment is preferably set to a temperature of 800 ° C. or higher, preferably 1100 ° C. or lower.

  The seamless steel pipe that has been subjected to the quenching treatment is then preferably tempered by being heated to a temperature of 650 ° C. or lower and cooled at a cooling rate of air cooling or higher. By heating to 650 ° C or less, preferably 500 ° C or more and tempering, the structure becomes a structure composed of a tempered martensite phase and a small amount of ferrite phase, and the desired high strength and further desired high toughness, desired It becomes a seamless steel pipe having excellent corrosion resistance. In addition, you may give only the above-mentioned tempering process without quenching process.

So far, the seamless steel pipe has been described as an example, but the steel pipe of the present invention is not limited to this. Using a steel pipe material having a composition within the scope of the present invention as described above, it is possible to produce an electric-welded steel pipe and a UOE steel pipe in accordance with a normal process to obtain a steel pipe for an oil well.
Using steel pipe material having a composition within the scope of the present invention described above, steel pipes other than seamless steel pipes obtained in accordance with a normal manufacturing process, such as ERW steel pipes and UOE steel pipes, are described above in steel pipes after pipe making. It is a quenching-tempering treatment, which is re-heated to a temperature of 800 ° C. or higher and then cooled to a temperature of 100 ° C. or lower, preferably room temperature, at a cooling rate of air cooling or higher, then 650 ° C. or lower, preferably 500 ° C. or higher. It is preferable to perform a tempering treatment that heats and cools at a cooling rate equal to or higher than air cooling.

After degassing the molten steel with the composition shown in Table 1, it is cast into a 100kg steel ingot (steel pipe material), piped by hot working with a model seamless rolling mill, air-cooled or water-cooled after pipe making, outer diameter 83.8mm x meat It was a seamless steel pipe with a thickness of 12.7mm (3.3in x thickness 0.5in).
About the obtained seamless steel pipe, the presence or absence of the crack generation | occurrence | production of the inner and outer surface was visually examined with cooling after pipe making, and hot workability was evaluated. In addition, the case where the crack of 1 mm or more exists in the front-and-rear end surface of a steel pipe was made into a crack (x), and the other was made into no crack ((circle)).

In addition, a specimen material was cut out from the obtained seamless steel pipe, and after quenching under the conditions shown in Table 2, ie, heating at 920 ° C. for 1 h, water cooling (average cooling rate from 800 to 500 ° C .: 10 ° C./s) Treated. Furthermore, the tempering process which hold | maintained for 30 minutes on the conditions shown in Table 2, ie, the temperature of the range of 500-650 degreeC, and air-cooled was performed. Some steel pipes were kept cooled after pipe making.
In this way, the specimen for tissue observation is collected from the specimen material which has been cooled after pipe formation or has been further quenched and tempered, and the specimen for tissue observation is corroded with aqua regia and scanned. The structure | tissue was imaged with the electron microscope (400 time), and the structure fraction (volume%) of the ferrite phase was computed using the image-analysis apparatus.

Further, the retained austenite phase structure fraction was measured using an X-ray diffraction method. Test specimens are taken from the quenched and tempered test specimen material, and the X-ray diffraction intensity of γ (220) plane and α (211) plane is measured by X-ray diffraction. Formula γ (volume ratio) = 100 / (1+ (IαRγ / IγRγ))
Where Iα: Integral intensity of α
Rγ: Theoretical calculation value of α
Iγ: Integral intensity of γ
Rγ: Conversion was performed using a crystallographic theoretical calculation value of γ. The fraction of the martensite phase was calculated as the remainder other than these phases.

In addition, API arc-shaped tensile test specimens are collected from specimen materials that have been cooled after pipe forming or that have been further quenched and tempered, and subjected to tensile tests (yield strength YS, tensile strength). TS).
In addition, a V-notch test piece (5 mm thick) is collected from a test piece material that has been cooled after pipe making or that has been further quenched and tempered in accordance with the provisions of JIS Z 2242, and subjected to a Charpy impact test. And the absorbed energy vE _-40 at -40 ° C was determined to evaluate toughness.

Furthermore, a corrosion test piece with a thickness of 3 mm x width 30 mm x length 40 mm is made by machining from a test piece material that has been cooled after pipe making or that has been further quenched and tempered, and subjected to a corrosion test. did.
The corrosion test was carried out by immersing the corrosion test piece in a test solution held in an autoclave: 20% NaCl aqueous solution (liquid temperature: 220 ° C., 100 atm CO ₂ gas atmosphere), and the immersion period was 2 weeks. . The test piece after the corrosion test was weighed, and the corrosion rate calculated from the weight loss before and after the corrosion test was obtained. Moreover, about the corrosion test piece after a test, the presence or absence of pitting corrosion on the test piece surface was observed using a magnifying glass with a magnification of 10 times. In addition, when the pitting corrosion was 0.3 mm or more in diameter, pitting corrosion was assumed to be generated, and otherwise, pitting corrosion was not detected.

In addition, from the specimen material that has been cooled after pipe making or that has been further quenched and tempered, in accordance with the provisions of NACE-TM0177 Method A, round bar specimens (diameter of parallel part: 6.4 mm) ) Was fabricated by machining, and an SSC resistance test was performed.
The SSC resistance test was conducted in a test solution held in a test vessel: 20% NaCl aqueous solution (liquid temperature: 25 ° C., H ₂ S: 0.1 atm, CO ₂ : 0.9 atm atmosphere, solution pH: 3.5 (0.5% CH ₃ The specimen is immersed in COOH + CH ₃ COONa), the immersion period is 30 days (720h), 100% SMYS (Specified Minimum Yield Strength) stress is applied to the specimen, and the specimen breaks. The presence or absence of was investigated. The case where it broke was evaluated as “poor” as being inferior in SCC resistance, and the case where it was not broken was evaluated as “O”, as being excellent in SCC resistance.
The obtained results are shown in Table 3.

In all of the examples of the present invention, the occurrence of cracks on the surface of the steel pipe is not recognized, and it is understood that the hot workability is excellent. In addition, all of the examples of the present invention have high strength of yield strength YS: 654 MPa or more, have a low corrosion rate in a severe corrosive environment at a high temperature of 220 ° C. including CO ₂ , and have excellent CO ₂ corrosion resistance. In addition, there is no occurrence of pitting corrosion, and the steel pipe has excellent corrosion resistance in a severe corrosive environment at a high temperature of 220 ° C containing CO ₂ . In addition, it can be seen that all of the examples of the present invention do not generate SSC even in an atmosphere containing H ₂ S, and the steel pipe has excellent SSC resistance.

  On the other hand, the comparative example out of the scope of the present invention has cracks on the surface and the hot workability is reduced, or the corrosion rate is large, pitting corrosion occurs and the corrosion resistance is reduced, or SSC. Occurs and the SSC resistance is reduced. In particular, SSC occurs in the comparative example having a composition not satisfying the formula (1) and / or the formula (3). In particular, in the comparative example having a composition not satisfying (2), the hot workability is lowered. In addition, when the ferrite content is outside the preferred range of the present invention, the strength is lowered, and the high strength of yield strength YS: 654 MPa or more cannot be satisfied.

Claims (5)

mass%
C: 0.04% or less, Si: 0.50% or less,
Mn: 0.20 to 1.80%, P: 0.03% or less,
S: 0.005% or less, Cr: 15.5-17.5%,
Ni: 2.5-5.5%, V: 0.20% or less,
Mo: 1.5-3.5%, W: 0.50-3.0%
Al: 0.05% or less, N: 0.15% or less,
O: 0.001% or less containing so as to satisfy the following formulas (1) to (3), and having a composition comprising the balance Fe and inevitable impurities, an oil well having high toughness and excellent corrosion resistance For high strength stainless steel pipe.
Record
Cr + 3.2Mo + 2.6W-10C ≧ 23.4 (1)
Cr + Mo + 0.5W + 0.3Si-43.5C-0.4Mn-0.3Cu-Ni-9N ≧ 11.5 (2)
2.2 ≦ Mo + 0.8W ≦ 4.5 (3)
Here, Cr, Mo, W, Si, C, Mn, Cu, Ni, N: Content of each element (mass%)   The high-strength stainless steel pipe for oil wells according to claim 1, wherein the composition further includes, in addition to the composition, mass% and Cu: 0.5 to 3.5%.   In addition to the above composition, the composition further contains one or more selected from mass%, Nb: 0.20% or less, Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less The high-strength stainless steel pipe for oil wells according to claim 1 or 2.   The high-strength stainless steel pipe for oil wells according to any one of claims 1 to 3, wherein in addition to the composition, the composition further includes Ca: 0.0005 to 0.01% in mass%.   The high-strength stainless steel pipe for oil wells according to any one of claims 1 to 4, which has a structure containing a martensite phase as a base phase and further containing a ferrite phase in a volume ratio of 10 to 50%.