JP4577457B2 - Stainless steel used for oil well pipes - Google Patents

Description

  The present invention relates to stainless steel, and more particularly to stainless steel used for oil well pipes used in gas wells and oil wells.

  Oil and natural gas produced from oil and gas wells contain corrosive associated gases such as carbon dioxide and hydrogen sulfide. Therefore, oil well pipes used for the production of oil and natural gas are required to have excellent corrosion resistance.

  Conventionally, carbon steel and low alloy steel have been used as steel materials for oil well pipes. However, as the corrosive environment of oil wells and gas wells used becomes severe, SUS420 martensitic stainless steel having a Cr content of about 13%. Stainless steel having excellent corrosion resistance such as steel (13% Cr steel) and improved 13% Cr steel obtained by adding Ni to 13% Cr steel is used.

  By the way, recently, the development of deep oil wells and gas wells has been promoted, but oil well pipes used in deep oil wells and gas wells are required to have high strength. In addition, deep oil wells and gas wells are required to have higher corrosion resistance than conventional oil well pipes in order to provide a high temperature chloride aqueous solution environment containing hydrogen sulfide and carbon dioxide gas at a high temperature of 150 ° C. or higher. . In such a high-temperature chloride aqueous solution environment containing hydrogen sulfide and carbon dioxide, it is conceivable to use a duplex stainless steel having higher corrosion resistance and higher strength than conventional stainless steel. However, the duplex stainless steel has a problem that the production cost is high because the alloy element content is large.

  Therefore, a stainless steel pipe having a lower alloy element content than duplex stainless steel, high strength, and high corrosion resistance even in a high-temperature chloride aqueous solution environment containing carbon dioxide gas is disclosed in JP-A-2005-336595 ( This is proposed in Japanese Patent Application Laid-Open No. 2006-16637 (hereinafter referred to as Patent Document 2) and Japanese Patent Application Laid-Open No. 2007-332442 (hereinafter referred to as Patent Document 3). All of the stainless steel pipes disclosed in these patent documents can improve the corrosion resistance by making the Cr content higher than that of the conventional 13% Cr steel.

  Specifically, Patent Document 1 sets the Cr content of the stainless steel pipe to 15.5 to 18%, which is higher than the conventional 13% Cr steel. Also, by making Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≧ 11.5, the steel structure becomes a two-phase structure of ferrite phase and martensite phase, and hot working of oil well pipes Improve sexiness. Although the two-phase structure may decrease the corrosion resistance, the oil well pipe can contain Ni, Mo, and Cu to improve the corrosion resistance so that Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5. Prevents the deterioration of corrosion resistance.

  Similarly, Patent Document 2 also includes Ni that improves the corrosion resistance by setting the Cr content of stainless steel to 15.5 to 18%. The chemical composition of the stainless steel disclosed in this document is similar to that of Patent Document 1, but Mo is not an essential element and has an alloy design that suppresses costs. Furthermore, Cu is also an optional element.

  The stainless steel disclosed in Patent Document 3 contains 14 to 18% Cr and contains Ni, Mo, and Cu, and thus has high corrosion resistance. Furthermore, since the steel structure contains a martensite phase and an austenite phase having a volume ratio of 3 to 15%, the toughness is improved.

Certainly, the stainless steel disclosed in the above-mentioned Patent Documents 1 to 3 contains more Cr than the conventional 13% Cr-based steel, and contains alloy elements such as Ni, Mo, Cu, Corrosion rate in high temperature corrosive environment decreases. For example, in the example of Patent Document 1, the corrosion rate (mm / yr) was investigated using a 20 wt% NaCl aqueous solution at 230 ° C. in a 100 atmospheres CO ₂ gas atmosphere, and a decrease in the corrosion rate was proved. (See Table 2 in Patent Document 1).

  However, according to the investigation by the present inventors, if stainless steel with a high Cr content is used in a high-temperature chloride aqueous solution environment containing carbon dioxide gas, the corrosion rate is reduced, but stress corrosion cracking (SCC) is likely to occur. There was found.

  In conventional stainless steel represented by 13% Cr steel and the like, the corrosion rate in a high-temperature chloride aqueous solution environment becomes excessively high. Therefore, although general corrosion occurs, SCC which is a local crack does not occur. On the other hand, when the Cr content is increased as compared with the conventional stainless steel, the corrosion rate decreases as described in Patent Documents 1 to 3 described above. The decrease in corrosion rate is due to the formation of a passive film on the surface of the stainless steel. However, the passive film is locally weakened and destroyed under a high temperature environment. Since dissolution is likely to occur in the destroyed portion, it is considered that SCC is generated by this dissolution.

  Therefore, in stainless steel used in a high-temperature chloride aqueous solution environment containing carbon dioxide, it is necessary not only to reduce the corrosion rate but also to improve the SCC resistance.

  An object of the present invention is to provide stainless steel for oil country tubular goods having excellent corrosion resistance in a high temperature chloride aqueous solution environment containing carbon dioxide gas at 150 ° C. or higher. More specifically, it is to provide a stainless steel for oil country tubular goods having a low corrosion rate and excellent SCC resistance in a high-temperature chloride aqueous solution environment containing carbon dioxide gas.

  In order to reduce the corrosion rate in a high-temperature chloride aqueous solution environment containing carbon dioxide gas at 150 ° C. or higher, the present inventors contain 16% or more Cr by mass and a small amount of Mo in the steel. I thought it was necessary. However, since Cr and Mo are ferrite forming elements, if 16% or more of Cr and a slight amount of Mo are contained, most of the steel structure becomes a ferrite phase, and high strength cannot be obtained.

  On the other hand, if Ni, which is an austenite forming element, is contained, the austenite phase at a high temperature is stabilized, so that a martensite phase is formed by quenching and a high strength steel structure is obtained. However, if Ni is contained in a large amount, the martensite transformation start temperature (Ms point) is lowered, so that martensite transformation does not occur even at room temperature, and high strength cannot be obtained. If the Ni content is appropriately adjusted, a structure mainly including a martensite phase and a ferrite phase having a volume ratio of about 10% or more is formed, and high strength can be obtained.

  Moreover, since Cu is effective for strengthening the ferrite phase, a high-strength structure can be obtained if Cu is contained. Cu further reduces the corrosion rate in a high-temperature chloride aqueous solution environment and improves the SCC resistance.

  Based on the above examination, the present inventors have found that 16% to 18% Cr, Mo of more than 2% and 4% or less, 3.5% to 7% Ni, and 1.5% to 4% It was thought that if it contains Cu, a stainless steel having a predetermined strength and a low corrosion rate can be provided.

  Furthermore, the present inventors have found that if a rare earth metal (REM) is contained in the above chemical composition in a predetermined amount or more, excellent SCC resistance can be obtained even in a high-temperature chloride aqueous solution environment containing carbon dioxide gas. It was. Hereinafter, this point will be described in detail.

The present inventors prepared stainless steels having the chemical composition shown in Table 1, and conducted a test for evaluating the SCC resistance of each stainless steel.

  Referring to Table 1, in the stainless steels of numbers A1 to A6, all chemical components other than REM were equivalent. Moreover, REM was made into different content for every number in the range of 0.0001% to 0.03%. Furthermore, each number of stainless steel was quenched and tempered, and the yield stress of each stainless steel was adjusted within the range of 860 to 900 MPa. All stainless steel structures contained 60% martensite phase, 30% ferrite phase and 10% austenite phase in volume fraction.

  A four-point bending test piece having a length of 75 mm, a width of 10 mm, and a thickness of 2 mm was taken from each number of stainless steel. Deflection due to 4-point bending was loaded on each collected specimen. At this time, in accordance with ASTM G39, the amount of deflection of each test piece was determined so that the stress applied to each test piece was equal to the yield stress of each test piece.

Each test piece loaded with deflection was immersed in an aqueous solution of NaCl at 25% by weight for one month in an autoclave at 204 ° C. (400 F) containing 30 atm of CO ₂ under pressure. After immersion for one month, it was investigated whether or not SCC occurred in each test piece. Specifically, the longitudinal section of the test piece was observed with an optical microscope having a 100-fold field of view, and the presence or absence of SCC was judged visually.

  The test results are shown in FIG. The horizontal axis in FIG. 1 indicates the REM content (% by mass), and the vertical axis indicates whether or not SCC is generated. A point “●” on “with SCC” on the vertical axis indicates that an SCC has occurred. Further, when there is a dot “●” in “No SCC”, it indicates that no SCC has occurred. As shown in FIG. 1, it was found that when the REM content is 0.001% or more, SCC does not occur even in a high-temperature chloride aqueous solution environment containing carbon dioxide gas. The reason why REM improves the SCC resistance is not clear, but the following reason is estimated.

  As a result of micro observation of the stainless steel in which SCC was generated in the above test, the SCC propagated mainly along the prior austenite grain boundaries in the martensite structure, starting from pits (pitting corrosion). From this, it is considered that there is some correlation between the accumulation behavior of dislocations in the prior austenite grain boundaries during stress loading and crack propagation. And REM has some influence on the accumulation behavior of dislocations in the prior austenite grain boundaries, and as a result, it is estimated that the SCC resistance of stainless steel containing 0.001% or more of REM is improved. In addition, although the stainless steel of number A1-A3 contains 0.0008-0.0013% of Ca, since REM content is less than 0.001%, SCC generate | occur | produced. From this, it was found that REM having a content of 0.001% or more than Ca contributes to the improvement of SCC resistance.

  Based on the above findings, the present inventors have completed the following invention.

  The stainless steel according to the present invention is used for oil country tubular goods. The stainless steel of the present invention is in mass%, C: 0.001 to 0.05%, Si: 0.05 to 1%, Mn: 2% or less, P: 0.03% or less, S: 0.002 %: Cr: 16-18%, Ni: 3.5-7%, Mo: more than 2% and 4% or less, Cu: 1.5-4%, rare earth metal: 0.001-0.3%, sol. Al: 0.001 to 0.1%, Ca: 0.0001 to 0.01%, O: 0.05% or less and N: 0.05% or less, with the balance being Fe and impurities.

  Preferably, in the stainless steel according to the present invention, Ti: 0.5% or less, Zr: 0.5% or less, Hf: 0.5% or less, V: 0.5% or less, instead of a part of Fe And Nb: one or more selected from the group consisting of 0.5% or less.

  In this case, the occurrence of pitting corrosion due to the Cr-deficient layer is suppressed.

  Preferably, the above-mentioned stainless steel has a structure containing a ferrite phase of 10 to 60% and a residual austenite phase of 2 to 10% by volume fraction.

  Preferably, the stainless steel according to the present invention has a yield stress of 654 MPa or more.

It is a figure which shows the relationship between rare earth metal content of stainless steel, and SCC.

  Hereinafter, embodiments of the present invention will be described in detail. The stainless steel according to the present invention can be used for oil well pipes used in a high temperature chloride aqueous solution environment containing carbon dioxide gas at 150 ° C. or higher. Hereinafter, a high-temperature chloride aqueous solution environment containing carbon dioxide and having a temperature of 150 ° C. or higher is simply referred to as a high-temperature chloride aqueous solution environment.

  1. Chemical composition

  The stainless steel according to the present invention has the following chemical composition. Hereinafter, “%” related to elements means “% by mass”.

  C: 0.001 to 0.05%

  Carbon (C) forms carbides with Cr and lowers the corrosion resistance of steel in a high temperature chloride aqueous solution environment. For this reason, the C content is preferably as low as possible. Therefore, the upper limit of the C content is 0.05%. The lower limit of the C content that can be substantially controlled is 0.001%.

  Si: 0.05 to 1%

  Silicon (Si) deoxidizes steel in the refining process. In order to obtain this effect, the lower limit of the Si content is set to 0.05%. On the other hand, if Si is contained excessively, not only the deoxidation effect is saturated, but also the hot workability of the steel is lowered. Therefore, the upper limit of Si content is 1%.

  Mn: 2% or less

  Manganese (Mn) improves the strength of the steel. However, if Mn is contained excessively, segregation is likely to occur in the steel. Segregation in steel reduces the toughness of the steel and further reduces the SCC resistance in a high-temperature chloride aqueous solution environment. Therefore, the Mn content is 2% or less. In order to obtain the effect of improving the strength, the preferable Mn content is 0.2% or more. However, even if the Mn content is less than 0.2%, the strength of the steel is improved to some extent.

  P: 0.03% or less

  Phosphorus (P) is an impurity. P decreases SSC (sulfide stress cracking) resistance and SCC resistance in a high-temperature chloride aqueous solution environment. For this reason, the P content is preferably as low as possible. Therefore, the P content is set to 0.03% or less.

  S: Less than 0.002%

  Sulfur (S) combines with Mn and the like to form inclusions. The formed inclusion becomes a starting point of pitting corrosion and SCC, and reduces the corrosion resistance of steel. Moreover, S reduces the hot workability of steel. Therefore, the S content is preferably as low as possible. Therefore, the S content is less than 0.002%.

  Cr: 16-18%

  Chromium (Cr) is an essential element that improves the corrosion resistance in a high-temperature chloride aqueous solution environment. In order to obtain high SCC resistance in a high-temperature chloride aqueous solution environment, the lower limit of the Cr content is 16%. On the other hand, since Cr is a ferrite-forming element, if it is contained excessively, the proportion of the ferrite phase increases in the steel structure, and the strength of the steel decreases. Furthermore, since the ratio of a retained austenite phase falls, the toughness of steel falls. Therefore, the upper limit of the Cr content is 18%. A preferable Cr content is 16.5 to 17.5%.

  Ni: 3.5-7%

  Nickel (Ni) improves the corrosion resistance in a high-temperature chloride aqueous solution environment. Ni further improves the toughness of the steel. In order to obtain these effects, the lower limit of the Ni content is 3.5%. On the other hand, since Ni is an austenite forming element, if it is excessively contained, the proportion of the retained austenite phase is excessively increased in the steel structure, and the strength of the steel is lowered. Therefore, the upper limit of the Ni content is 7%. A preferable Ni content is 3.5 to 6.5%, and a more preferable Ni content is 3.8 to 5.8%.

  Cu: 1.5 to 4%

  Copper (Cu) reduces the elution rate of steel in a high temperature chloride aqueous environment. Cu further improves the SCC resistance of the steel. Cu also strengthens the ferrite phase in the steel structure. In order to obtain these effects, the lower limit of the Cu content is 1.5%. On the other hand, if Cu is contained excessively, the hot workability of the steel is lowered. Therefore, the upper limit of the Cu content is 4%. A preferable Cu content is 1.5 to 3.0%, and a more preferable Cu content is 1.5 to 2.5%.

  Mo: more than 2% and less than 4%

  Molybdenum (Mo) improves the pitting corrosion resistance and SCC resistance of steel in the presence of Cr. In order to obtain this effect, the Mo content is more than 2%. On the other hand, since Mo is a ferrite-forming element, if Mo is contained excessively, the proportion of the ferrite phase in the steel structure increases and the strength decreases. Therefore, the Mo content is 4% or less. A preferable Mo content is 2.1 to 3.3%, and a more preferable Mo content is 2.3 to 3.0%.

  Sol. Al: 0.001 to 0.1%

  Aluminum (Al) deoxidizes steel in the refining process. In order to obtain this effect, the lower limit of the Al content is 0.001%. On the other hand, if Al is contained excessively, alumina inclusions are produced in a large amount in the steel and the toughness of the steel is lowered. Therefore, the upper limit of the Al content is 0.1%. In addition, Al content said by this specification means content of acid-soluble Al (Sol.Al:Soluble Aluminum).

  Ca: 0.0001 to 0.01%

  Calcium (Ca) deoxidizes steel in the refining process. Ca further improves hot workability. In order to obtain these effects, the lower limit of the Ca content is 0.0001%. On the other hand, if Ca is contained excessively, inclusions such as CaO are produced in a large amount in the steel, and the toughness of the steel is lowered. Furthermore, inclusions such as CaO also serve as starting points for pitting corrosion. Therefore, the upper limit of the Ca content is 0.01%.

  N: 0.05% or less

  Nitrogen (N) stabilizes the austenite phase. N further improves pitting corrosion resistance. On the other hand, if N is contained excessively, various nitrides are formed in the steel and the toughness of the steel is lowered. Therefore, the N content is 0.05% or less. The minimum with preferable N content for acquiring the above-mentioned effect effectively is 0.005%.

  O: 0.05% or less

  Oxygen (O) is an impurity. O combines with other elements to form oxides, reducing the toughness and corrosion resistance of the steel. Therefore, it is preferable that the O content is as small as possible. Therefore, the O content is 0.05% or less.

  Rare earth metal: 0.001-0.3%

  Rare Earth Metals (REM) is an important element in the present invention. As described above, REM improves the SCC resistance in a high-temperature chloride aqueous solution environment. In order to obtain this effect, the lower limit of the REM content is 0.001%. On the other hand, even if REM is excessively contained, the effect is saturated. Therefore, the upper limit of the REM content is 0.3%. A preferable REM content is 0.001 to 0.1%, and a more preferable REM content is 0.001 to 0.01%.

  In addition, REM as used in the field of this invention is lanthanoid from yttrium (Y) of atomic number 39 and lanthanum (La) of atomic number 57 to lutetium (Lu) of 71.

  The stainless steel according to the present invention contains one or more of the above REMs. Accordingly, the REM content is a total content of one or more selected from the above-described plurality of REMs.

  The balance of the chemical composition is Fe and impurities.

  The stainless steel of the present invention further contains one or more selected from the group consisting of Ti, Zr, Hf, V and Nb in place of part of Fe as required.

  Ti: 0.5% or less

  Zr: 0.5% or less

  Hf: 0.5% or less

  V: 0.5% or less

  Nb: 0.5% or less

  Titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), and niobium (Nb) are not essential elements but are elements that are arbitrarily added. All of these elements fix C and suppress the formation of Cr carbides. Therefore, the occurrence of pitting caused by the Cr-deficient layer formed around the Cr carbide is suppressed, and the SCC sensitivity is reduced. However, if these elements are contained excessively, the toughness of the steel is lowered. Accordingly, the upper limit of the content of these elements is 0.5%. In order to obtain the above-mentioned effects remarkably, the lower limit of the content of these elements is preferably 0.005% for all elements. However, even if the content of these elements is less than the preferable lower limit, the above-described effects can be obtained to some extent.

  2. Production method

  The stainless steel according to the present invention can have the structure described later by performing a heat treatment of quenching and tempering, and can obtain the desired corrosion resistance and the strength required for oil well pipe use. Hereinafter, the manufacturing method of the stainless steel pipe of this invention is demonstrated as an example.

  A billet is produced by melting steel having the above chemical composition. The manufactured billet is hot-worked into a stainless steel pipe. As hot working, for example, the Mannesmann method is performed to manufacture a seamless steel pipe. In addition, you may implement hot extrusion as hot processing, and you may implement hot forging.

  Quenching and tempering treatment is performed on the manufactured stainless steel pipe. At this time, a preferable quenching temperature is 900-1200 degreeC, and a preferable tempering temperature is 450-650 degreeC.

  3. Organization

  The structure of stainless steel manufactured by the above-described manufacturing method includes a ferrite phase of 10 to 60% and a residual austenite phase of 2 to 10% by volume fraction.

  Here, the volume fraction of the ferrite phase is obtained by the following method. The test piece whose surface has been polished is etched using a mixed solution of aqua regia and glycerin. Using the etched specimen, the area ratio of the ferrite phase on the specimen surface is measured by a point calculation method based on JISG0555. Let the measured area ratio be the volume fraction. The volume fraction of the retained austenite phase is measured by an X-ray diffraction method.

  In the stainless steel structure, the part other than the ferrite phase and the retained austenite phase is mainly a tempered martensite phase. In addition to the martensite phase, carbides, nitrides, borides, and Cu phases may be included.

  Since the stainless steel according to the present invention has the above-described structure, the yield stress becomes 654 MPa (corresponding to 95 ksi) or more. Furthermore, it can be adjusted to 758 MPa (corresponding to 110 ksi) or more, and further to 862 MPa (corresponding to 125 ksi) or more. The yield stress here is 0.2% offset proof stress based on the ASTM standard.

  In addition, the stainless steel of the present invention has high toughness because it contains the retained austenite phase having the above volume fraction in the structure.

  A plurality of stainless steels having various chemical compositions were produced, and the SCC resistance in a high temperature chloride aqueous solution environment was investigated.

[Manufacture of test materials]
A plurality of stainless steels having the chemical composition shown in Table 2 were melted.

  Each numerical value in Table 2 indicates the content (% by mass) of the corresponding element. Moreover, among the chemical composition of each steel, the remainder other than the elements described in Table 1 is Fe and impurities. Symbols a) to c) attached to the side of the numerical value in the “REM” column indicate the type of REM contained in each steel. Specifically, a) indicates that the contained REM is neodymium (Nd). b) shows that the contained REM is yttrium (Y). c) shows that the contained REM is misch metal. Misch metal is composed of 51.0% cerium (Ce), 25.5% lanthanum (La), 18.6% neodymium (Nd), and 4.8% praseodymium (Pr). 0.1% samarium (Sm).

  Referring to Table 2, regarding the steels of Nos. 1 to 12, the chemical compositions were all within the range defined in the present invention. On the other hand, regarding the steel of No. 13, the Mo content was less than the lower limit of the range defined in the present invention. Regarding the steel of No. 14, the Cr content was less than the lower limit of the range defined in the present invention. Regarding the steel of No. 15, the Cu content was less than the lower limit of the range defined in the present invention. Regarding the steel of No. 16, REM was not contained. Regarding the steel of No. 17, the Ni content was less than the lower limit of the range defined in the present invention.

  Each number of steel was hot forged and hot rolled to produce a 12 mm thick steel plate. Quenching and tempering were performed on each number of steel plates. In the quenching treatment, each number of steel plates was heated at a quenching temperature of 980 to 1200 ° C. for 15 minutes and then cooled with water. In the tempering treatment, the tempering temperature was set to 500 to 650 ° C. By the above process, it adjusted so that the yield strength of each steel plate might be in the range of 800-950 MPa.

[Structural observation and tensile test]
The volume fractions (%) of the ferrite phase and the retained austenite phase of the steel plates of the respective numbers are described in the above 3. It was determined by the measurement method described in 1.

  Furthermore, a round bar tensile test piece was collected from each number of steel plates, and a tensile test was performed. The longitudinal direction of the round bar tensile test piece was the rolling direction of the steel sheet, the diameter of the parallel part of the round bar tensile test piece was 14 mm, and the length was 20 mm. The tensile test was performed at room temperature.

[SCC evaluation test]
Four-point bending test pieces having a length of 75 mm, a width of 10 mm, and a thickness of 2 mm were collected from each number of steel plates. Deflection due to 4-point bending was loaded on each collected specimen. At this time, in accordance with ASTM G39, the amount of deflection of each test piece was determined so that the stress applied to each test piece was equal to the yield stress of each test piece.

Each test piece loaded with deflection was immersed in an aqueous solution of NaCl at 25% by weight for one month in an autoclave at 204 ° C. (400 F) containing 30 atm of CO ₂ under pressure. After immersion for one month, it was investigated whether or not SCC occurred in each test piece. Specifically, the longitudinal section of the test piece was observed with an optical microscope having a 100-fold field of view, and the presence or absence of SCC was judged visually. Moreover, the weight of each test piece before and after the test was measured. The corrosion weight loss of each test piece was obtained from the change in the measured weight, and the corrosion rate was obtained based on the corrosion weight loss.

[Test results]
Table 3 shows the test results.

The “YS” column in Table 3 indicates the yield stress (MPa) of each numbered steel plate obtained by a tensile test. In the “ferrite phase” and “residual austenite phase” columns, the volume fractions (%) of the ferrite phase and the retained austenite phase of the steel plates of the respective numbers are shown. “No SCC” in the “SCC evaluation result” column indicates that no SCC occurred in the four-point bending test piece, and “With SCC” indicates that SCC occurred. “<0.1” in the “Corrosion Rate” column indicates that the corrosion rate was less than 0.1 g / (m ² · hr), and “≧ 0.1” indicates that the corrosion rate was 0.1 g. / (M ² · hr) or more.

Referring to Table 3, none of the steels of Nos. 1 to 12 had SCC and the corrosion rate was less than 0.1 g / (m ² · hr). Moreover, all yield stress was 654 MPa or more.

On the other hand, in the steels of Nos. 13, 15, and 17, SCC occurred because the contents of Mo, Cu, and Ni were small. Moreover, in the steel of No. 14, since there was little Cr content, SCC generate | occur | produced and also the corrosion rate was 0.1 g / (m < ² > * hr) or more. Furthermore, in the steel of No. 16, since REM was not contained, SCC occurred.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

  The stainless steel according to the present invention can be used as an oil well pipe, and is particularly suitable for use in an oil well pipe used in a high-temperature chloride aqueous solution environment containing carbon dioxide gas at 150 ° C. or higher.

Claims (5)

Stainless steel used for oil well pipes,
In mass%, C: 0.001 to 0.05%, Si: 0.05 to 1%, Mn: 2% or less, P: 0.03% or less, S: less than 0.002%, Cr: 16 to 18%, Ni: 3.5-7%, Mo: more than 2% and 4% or less, Cu: 1.5-4%, rare earth metal: 0.001-0.3%, sol. Al: 0.001 to 0.1%, Ca: 0.0001 to 0.01%, O: 0.05% or less and N: 0.05% or less, with the balance being Fe and impurities Features stainless steel. The stainless steel according to claim 1,
In place of a part of Fe, Ti: 0.5% or less, Zr: 0.5% or less, Hf: 0.5% or less, V: 0.5% or less, and Nb: 0.5% or less Stainless steel characterized by containing one or more selected from the group consisting of: The stainless steel according to claim 1,
A stainless steel having a structure containing a ferrite phase of 10 to 60% by volume fraction and a residual austenite phase of 2 to 10%. The stainless steel according to claim 2,
A stainless steel having a structure containing a ferrite phase of 10 to 60% by volume fraction and a residual austenite phase of 2 to 10%. The stainless steel according to any one of claims 1 to 4,
Stainless steel characterized by having a yield stress of 654 MPa or more.

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