Patent Publication Number: US-8980167-B2

Title: Stainless steel pipe having excellent expandability for oil country tubular goods

Description:
This application is the United States national phase application of International Application PCT/JP2006/304032 filed Feb. 24, 2006. 
     TECHNICAL FIELD 
     The present invention relates to steel products for oil country tubular goods used in oil wells for crude oil and gas wells for natural gas. In particular, the present invention relates to a stainless steel pipe having excellent expandability for oil country tubular goods, the stainless steel pipe having high expandability and high corrosion resistance and being suitable for use in extremely severe corrosive wells producing oil and gas containing carbon dioxide (CO 2 ), chlorine ions (Cl − ), and the like. 
     BACKGROUND ART 
     In recent years, deep oil fields (including gas fields) that had not previously received attention have been actively developed on a global scale because of high oil prices and the imminent exhaustion of oil resources predicted in the near future. The depth of such oil fields (or gas fields) is generally very large. Their high-temperature atmospheres containing CO 2 , Cl − , and the like are severe corrosive environments. Thus, oil country tubular goods used for drilling such oil fields and gas fields need to be composed of materials having high strength and corrosion resistance. Oil field development in cold climate areas is also increasing; hence, the materials are often required to have low-temperature toughness as well as high strength. 
     The development of such deep oil wells disadvantageously requires a high drilling cost. A technique for expanding a relatively small pipe in an oil well has recently been brought into practical use (for example, see Patent Documents 1 and 2). The employment of the technique results in a reduction in the cross-sectional area of a drilling hole, thus reducing drilling costs. However, the tubular goods are required to have excellent expandability. 
     Patent Document 1: PCT Japanese Translation Patent Publication No. 7-567010 
     Patent Document 2: WO98/00626 
     DISCLOSURE OF INVENTION 
     In general, 13% Cr martensitic stainless steel pipes having CO 2  corrosion resistance are used under environments containing CO 2 , Cl − , and the like. Disadvantageously, martensitic stainless steel pipes subjected to normal quenching and tempering do not have sufficient expandability. To employ the new technique for expanding a pipe in an oil well, the development of a stainless steel pipe having excellent CO 2  corrosion resistance and excellent expandability for oil country tubular goods is highly desirable. 
     In the above-described situation, it is an object of the present invention to provide a cost-effective stainless steel pipe having excellent expandability for oil country tubular goods, the stainless steel pipe having excellent CO 2  corrosion resistance and excellent expandability under a severe corrosive environment containing CO 2 , Cl − , and the like. 
     To achieve the object, the inventors have focused their attention on a martensitic stainless steel pipe believed to be suitable for oil country tubular goods from the viewpoint of CO 2  corrosion resistance and have planned to improve the expandability thereof by controlling the microstructure thereof. The inventors have conducted intensive studies and experiments to investigate the corrosion resistance of various alloys mainly composed of 13% Cr steel, which is typical martensitic stainless steel, in an environment containing CO 2  and Cl − , in line with this strategy. The inventors have found that in 13% Cr steel having a C content markedly lower than that in the known art, the incorporation of Ni and V, a reduction in contents of S, Si, Al, and O, limitation of contents of elements of alloys to within specific ranges, and preferably the control of a microstructure result in satisfactory hot workability, corrosion resistance and significantly improve expandability. These findings have led to the completion of the present invention. The gist of the present invention will be described below. 
     A high-strength martensitic stainless steel pipe of the present invention for oil country tubular goods can be categorized into one of three groups. 
     Group 1 
     1. A stainless steel pipe having excellent expandability for oil country tubular goods contains, on a percent by mass basis, 0.01% to 0.05% C, 0.50% or less Si, 0.10% to 1.50% Mn, 0.03% or less P, 0.005% or less S, 12.0% to 17.0% Cr, 2.0% to 7.0% Ni, 3.0% or less Mo, 0.05% or less Al, 0.20% or less V, 0.01% to 0.15% N, and the balance being Fe and incidental impurities, wherein a microstructure mainly having a tempered martensitic phase has an austenitic phase content exceeding 20%.
 
2. A stainless steel pipe having excellent expandability for oil country tubular goods contains, on a percent by mass basis, 0.01% to 0.05% C, 0.50% or less Si, 0.30% to 1.50% Mn, 0.03% or less P, 0.005% or less S, 12.0% to 17.0% Cr, 2.0% to 7.0% Ni, 3.0% or less Mo, 0.05% or less Al, 0.20% or less v, 0.01% to 0.15% N, at least one selected from 0.20% or less Nb, 3.5% or less Cu, 0.3% or less Ti, 0.2% or less Zr, 0.0005% to 0.01% Ca, 0.01% or less B, and 3.0% or less W, and the balance being Fe and incidental impurities, wherein a microstructure mainly having a tempered martensitic phase has an austenitic phase content exceeding 20%.
 
Group 2
 
1. A stainless steel pipe having excellent expandability for oil country tubular goods contains a steel composition of, on a percent by mass basis, less than 6.010% C, 0.50% or less Si, 0.10% to 1.50% Mn, 0.03% or less P, 0.005% or less S, 11.0% to 15.0% Cr, 2.0% to 7.0% Ni, 3.0% or less Mo, 0.05% or less Al, 0.20% or less V, less than 0.01% N, 0.008% or less O, and the balance being Fe and incidental impurities, wherein a steel microstructure has tempered martensite as a main phase and an austenite content exceeding 20 percent by volume.
 
2. A stainless steel pipe having excellent expandability for oil country tubular goods contains a steel composition of, on a percent by mass basis, less than 0.010% C, 0.50% or less Si, 0.10% to 1.50% Mn, 0.03% or less P, 0.005% or less S, 11.0% to 15.0% Cr, 2.0% to 7.0% Ni, 3.0% or less Mo, 0.05% or less Al, 0.20% or less V, less than 0.01% N, 0.008% or less O, at least one selected from 0.20% or less Nb, 3.5% or less Cu, 0.3% or less Ti, 0.2% or less Zr, 0.001% to 0.01% Ca, 0.0005% to 0.01% B, and 3.0% or less W, and the balance being Fe and incidental impurities, wherein a steel microstructure has tempered martensite as a main phase and an austenite content exceeding 20 percent by volume.
 
3. The stainless steel pipe having excellent expandability for oil country tubular goods according to claim  1  or  2 , wherein an austenite content exceeding 20 percent by volume is replaced with a quenched martensite content of 3 percent by volume or more and an austenite content of 15 percent by volume or more.
 
Group 3
 
1. A stainless steel pipe having excellent expandability for oil country tubular goods contains a steel composition of, on a percent by mass basis, 0.05% or less C, 0.50% or less Si, 0.10% to 1.50% Mn, 0.03% or less P, 0.005% or less S, 10.5% to 17.0% Cr, 0.5% to 7.0% Ni, 0.05% or less Al, 0.20% or less V, 0.15% or less N, 0.008% or less O, and the balance being Fe and incidental impurities, wherein Cr+0.5Ni−20C&gt;11.3 is satisfied.
 
2. A stainless steel pipe having excellent expandability for oil country tubular goods contains a steel composition of, on a percent by mass basis, 0.05% or less C, 0.50% or less Si, 0.10% to 1.50% Mn, 0.03% or less P, 0.005% or less S, 10.5% to 17.0% Cr, 0.5% to 7.0% Ni, 0.05% or less Al, 0.20% or less V, 0.15% or less N, 0.008% or less O, at least one selected from 0.20% or less Nb, 3.5% or less Cu, 0.3% or less Ti, 0.2% or less Zr, 0.001% to 0.01% Ca, 0.0005% to 0.01% B, and 3.0% or less W, and the balance being Fe and incidental impurities, wherein Cr+0.5Ni−20C+0.45Cu+0.4 W&gt;11.3 is satisfied.
 
3. The stainless steel pipe having excellent expandability for oil country tubular goods according to claim  1  or  2 , wherein a steel microstructure has tempered martensite as a main phase and an austenite content exceeding 5 percent by volume.
 
4. The stainless steel pipe having excellent expandability for oil country tubular goods according to claim  1  or  2 , wherein a steel microstructure has tempered martensite as a main phase and a quenched martensite content of 3 percent by volume or more.
 
5. The stainless steel pipe having excellent expandability for oil country tubular goods according to claim  1  or  2 , wherein a steel microstructure has tempered martensite as a main phase, a quenched martensite content of 3 percent by volume or more, and an austenite content of 5 percent by volume or more.
 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The reason for the limitation of the contents of the components of the stainless steel pipe included in Group 1 of the present invention for oil country tubular goods will be described below. The units of the content of each component in the steel composition are percent by mass and are simply indicated by %. 
     C: 0.01% to 0.05% 
     C relates to the strength of the martensitic stainless steel and is thus an important element. The C content needs to be 0.01% or more. However, the incorporation of Ni described below is liable to cause sensitization during tempering. To prevent sensitization, the C content needs to be 0.05% or less. Thus, the C content is set in the range of 0.01% to 0.05%. A lower C content is desirable also from the viewpoint of corrosion resistance. Thus, the C content is preferably in the range of 0.01% to 0.03%. 
     Si: 0.50% or less 
     Si is an element needed as a deoxidizer in a usual steel-making process. A Si content exceeding 0.50% degrades CO 2  corrosion resistance and hot workability. Thus, the Si content is set to 0.50% or less. 
     Mn: 0.10% to 1.50% 
     The Mn content needs to be 0.10% or more in order to ensure the strength required for martensitic stainless steel for oil country tubular goods. A Mn content exceeding 1.50% adversely affects toughness. Thus, the Mn content is set in the range of 0.10% to 1.50% and preferably 0.30% to 1.00%. 
     P: 0.03% or less 
     P is an element that degrades CO 2  corrosion resistance, resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking. The P content is preferably minimized. However, an extreme reduction in P content increases production costs. In view of providing an allowable range in which the production can be industrially performed at relatively low costs and in which CO 2  corrosion resistance, resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking are not degraded, the P content is set to 0.03% or less. 
     S: 0.005% or less 
     S is an element that significantly degrades hot workability in a process of manufacturing a steel pipe. The S content is preferably minimized. At a S content of 0.005% or less, the steel pipe can be manufactured by a common process. Thus, the upper limit of the S content is set to 0.005%. Preferably, the S content is 0.003% or less. 
     Cr: 12.0% to 17.0% 
     Cr is a main element used to ensure CO 2  corrosion resistance and resistance to CO 2  stress corrosion cracking. From the viewpoint of corrosion resistance, the Cr content needs to be 12.0% or more. However, a Cr content exceeding 17.0% degrades hot workability. Thus, the Cr content is set in the range of 12.0% to 17.0% and preferably 12.0% to 15.0%. 
     Ni: 2.0% to 7.0% 
     Ni is incorporated in order to strengthen a protective film to improve CO 2  corrosion resistance, resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking and in order to increase the strength of 13% Cr steel having a lower C content. At a Ni content of less than 2.0%, the effect is not provided. A Ni content exceeding 7.0% reduces the strength. Thus, the Ni content is set in the range of 2.0% to 7.0%. 
     Mo: 3.0% or less 
     Mo is an element that imparts resistance to pitting corrosion due to Cl − . A Mo content exceeding 3.0% results in the formation of δ ferrite, thereby degrading CO 2  corrosion resistance, resistance to CO 2  stress corrosion cracking, and hot workability. Furthermore, the cost is increased. Thus, the Mo content is set to 3.0% or less. In view of cost, the Mo content is preferably set to 2.2% or less. 
     Al: 0.05% or less 
     Al has a strong deoxidizing effect. An Al content exceeding 0.05% adversely affects toughness. Thus the Al content is set to 0.05% or less. 
     V: 0.20% or less 
     V has effects of increasing strength and improving resistance to stress corrosion cracking. A V content exceeding 0.2% degrades toughness. Thus, the V content is set to 0.20% or less. 
     N: 0.01% to 0.15% 
     N is an element that significantly improves pitting corrosion resistance. At a N content of less than 0.01%, the effect is not sufficient. A N content exceeding 0.5% results in the formation of various nitrides, thereby degrading toughness. Thus, the N content is set in the range of 0.01% to 0.15%. 
     O: 0.008% or less 
     O is a significantly important element for sufficiently exhibiting the performance of the steel of the present invention. A higher 0 content results in the formation of various oxides, thereby significantly degrading hot workability, resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking. Thus, the 0 content is set to 0.008% or less. 
     Nb: 0.20% or less 
     Nb has effects of improving toughness and increasing strength. However, a Nb content exceeding 0.20% reduces toughness. Thus, the Nb content is set to 0.20% or less. 
     Ca: 0.0005% to 0.01% 
     Ca fixes S as CaS and spheroidizes sulfide inclusions, thereby reducing the lattice strain of the matrix around the inclusions to reduce their ability to trap hydrogen. At a Ca content of less than 0.001%, the effect is less marked. A Ca content exceeding 0.01% increases formation of CaO, thereby degrading CO 2  corrosion resistance and pitting corrosion resistance. Thus, the Ca content is set in the range of 0.001% to 0.01%. 
     Cu: 3.5% or less 
     Cu is an element which strengthens the protective film, inhibits the penetration of hydrogen into steel, and improves resistance to sulfide stress corrosion cracking. A Cu content exceeding 3.5% causes the grain boundary precipitation of CuS at a high temperature, thereby degrading hot workability. Thus, the Cu content is set to 3.5% or less. 
     Ti: 0.3% or less, Zr: 0.2% or less, B: 0.0005% to 0.01%, W: 3.0% or less 
     Ti, Zr, B, and W have effects of increasing strength and improving resistance to stress corrosion cracking. Toughness is reduced at a Ti content exceeding 0.3%, a Zr content exceeding 0.2%, or a W content exceeding 3.0%. A B content of less than 0.0005% produces no effect. A B content exceeding 0.01% degrades toughness. Thus, the Ti content is set to 0.3% or less. The Zr content is set to 0.2% or less. The B content is set in the range of 0.0005% to 0.01%. The W content is set to 3.0% or less. 
     A tempered martensitic phase containing an austenitic phase of more than 10% and a quenched martensitic phase of 3% or more exhibits stable expandability. In addition, a ferrite phase of 3% or less may be contained in a microstructure. 
     In the present invention, from the viewpoint of hot workability, significantly low contents of S, Si, Al, and O improve hot workability. Thus, in the case where oil country tubular goods are produced with the steel, a common production process may be employed without any modification. 
     A preferred method for producing a stainless pipe included in Group 1 of the present invention for oil country tubular goods will be described below using a seamless steel pipe by way of example. Preferably, molten steel having the composition described above is formed into an ingot by a known ingot-forming method using a converter, an electric furnace, a vacuum melting furnace, or the like, followed by formation of articles, such as billets, for steel pipes using a known method including a continuous casting method or an ingot-making bloom rolling method. 
     These articles for steel pipes are heated and processed by hot working for making pipes using a production process such as a general Mannesmann-plug mill process or Mannesmann-mandrel mill process, thereby forming seamless steel pipes having desired dimensions. After pipe-making, the seamless steel pipes are preferably cooled to room temperature at a cooling rate higher than that of air cooling. After hot working, the articles may be subjected to rolling and cooling, as described above. Preferably, tempering or quenching and tempering are performed. Preferably, quenching may be performed by reheating the articles to 800° C. or higher, maintaining the articles at the temperature for 5 minutes or more, and cooling the articles to 200° C. or lower and preferably to room temperature at a cooling rate higher than that of air cooling. 
     At a heating temperature of 800° C. or lower, a sufficient martensite microstructure cannot be obtained, thereby reducing strength, in some cases. Tempering is preferably performed by heating the articles to a temperature-exceeding the A C1  temperature. Tempering at a temperature exceeding the A C1  temperature results in the precipitation of austenite or quenched martensite. Alternatively, in place of quenching and tempering described above, only tempering may be performed by heating the articles to a temperature equal to or higher than the A C1  temperature. 
     Although the seamless steel pipe as an example has been described above, the heat-treatment process may be applied to electric resistance welded pipes and welded steel pipes, except for the pipe-making process. 
     The reason for the limitation of the contents of the components of the stainless steel pipe included in Group 2 of the present invention for oil country tubular goods will be described below. 
     C: less than 0.010% 
     C relates to the strength of the martensitic stainless steel and is thus an important element. A higher C content increases the strength thereof. However, from the viewpoint of expandable steel pipes, the strength before expansion is preferably low. Thus, the C content is set to less than 0.010%. 
     Si: 0.50% or less 
     Si is an element needed as a deoxidizer in a usual steel-making process. A Si content exceeding 0.50% degrades CO 2  corrosion resistance and hot workability. Thus, the Si content is set to 0.50% or less. 
     Mn: 0.10% to 1.50% 
     The Mn content needs to be 0.10% or more in order to ensure the strength required for martensitic stainless steel for oil country tubular goods. A Mn content exceeding 1.50% adversely affects toughness. Thus, the Mn content is set in the range of 0.10% to 1.50% and preferably 0.30% to 1.00%. 
     P: 0.03% or less 
     P is an element that degrades CO 2  corrosion resistance, resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking. The P content is preferably minimized. However, an extreme reduction in P content increases production costs. In view of providing an allowable range in which the production can be industrially performed at relatively low costs and in which resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking are not degraded, the P content is set to 0.03% or less. 
     S: 0.005% or less 
     S is an element that significantly degrades hot workability in a process of manufacturing a pipe. The S content is preferably minimized. At a S content of 0.005% or less, the steel pipe can be manufactured by a common process. Thus, the upper limit of the S content is set to 0.005%. Preferably, the S content is 0.003% or less. 
     Cr: 11.0% to 15.0% 
     Cr is a main element used to ensure CO 2  corrosion resistance and resistance to CO 2  stress corrosion cracking. From the viewpoint of corrosion resistance, the Cr content needs to be 11.0% or more. However, a Cr content exceeding 15.0% degrades hot workability. Thus, the Cr content is set in the range of 11.0% to 15.0% and preferably 11.5% to 14.0%. 
     Ni: 2.0% to 7.0% 
     Ni is incorporated in order to strengthen a protective film to improve CO 2  corrosion resistance, resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking and in order to increase the strength of 13% Cr steel having a lower C content. At a Ni content of less than 2.0%, the effect is not provided. A Ni content exceeding 7.0% reduces the strength. Thus, the Ni content is set in the range of 2.0% to 7.0%. 
     Mo: 3.0% or less 
     Mo is an element that imparts resistance to pitting corrosion due to Cl − . A Mo content exceeding 3.0% results in the formation of δ ferrite, thereby degrading CO 2  corrosion resistance, resistance to CO 2  stress corrosion cracking, and hot workability. Furthermore, the cost is increased. Thus, the Mo content is set to 3.0% or less. In view of cost, the Mo content is preferably set in the range of 0.1% to 2.2%. 
     Al: 0.05% or less 
     Al has a strong deoxidizing effect. An Al content exceeding 0.05% adversely affects toughness. Thus the Al content is set to 0.05% or less. 
     V: 0.20% or less 
     V has effects of increasing strength and improving resistance to stress corrosion cracking. A V content exceeding 0.2% degrades toughness. Thus, the V content is set to 0.20% or less. 
     N: less than 0.01% 
     N is an element that significantly improves pitting corrosion resistance. N is an important element that relates to the strength of martensitic stainless steel. A higher N content increases the strength thereof. However, for expandable stainless steel pipes, the strength before expansion is preferably low. Thus, the N content is set to less than 0.01%. 
     O: 0.008% or less 
     O is a significantly important element for sufficiently exhibiting the performance of the steel pipe of the present invention. In particular, the O content needs to be controlled. A higher O content results in the formation of various oxides, thereby significantly degrading hot workability, resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking. Thus, the O content is set to 0.008% or less. 
     The steel composition according to the present invention may contain at least one selected from 0.2% or less Nb, 3.5% or less Cu, 0.3% or less Ti, 0.2% or less Zr, 0.001% to 0.01% Ca, 0.0005% to 0.01% B, and 3.0% or less W as an additional element. 
     Nb: 0.20% or less 
     Nb has effects of improving toughness and increasing strength. However, a Nb content exceeding 0.20% reduces toughness. Thus, the Nb content is set to 0.20% or less. 
     Ca: 0.001% to 0.01% 
     Ca fixes S as CaS and spheroidizes sulfide inclusions, thereby reducing the lattice strain of the matrix around the inclusions to reduce their ability to trap hydrogen. At a Ca content of less than 0.001%, the effect is less marked. A Ca content exceeding 0.01% increases formation of CaO, thereby degrading CO 2  corrosion resistance and pitting corrosion resistance. Thus, the Ca content is set in the range of 0.001% to 0.01%. 
     Cu: 3.5% or less 
     Cu is an element which strengthens the protective film, inhibits the penetration of hydrogen into steel, and improves resistance to sulfide stress corrosion cracking. A Cu content exceeding 3.5% causes the grain boundary precipitation of CuS at a high temperature, thereby degrading hot workability. Thus, the Cu content is set to 3.5% or less. 
     Ti: 0.3% or less, Zr: 0.2% or less, B: 0.0005% to 0.01%, W: 3.0% or less 
     Ti, Zr, B, and W have effects of increasing strength and improving resistance to stress corrosion cracking. Toughness is reduced at a Ti content exceeding 0.3%, a Zr content exceeding 0.2%, or a W content exceeding 3.0%. A B content of less than 0.0005% produces no effect. A B content exceeding 0.01% degrades toughness. Thus, the Ti content is set to 0.3% or less. The Zr content is set to 0.2% or less. The B content is set in the range of 0.0005% to 0.01%. The W content is set to 3.0% or less. 
     The reason for the limitation of the microstructure will be described. To obtain stable expandability, the microstructure of the steel pipe of the present invention has tempered martensite as a main phase (phase of 50 percent by volume or more) and an austenite content exceeding 20 percent by volume. In the case of a quenched martensite content of 3 percent by volume or more and an austenite content of 15 percent by volume or more in place of an austenite content exceeding 20 percent by volume, the same effect is provided. 
     A preferred method for producing a stainless pipe included in Group 2 of the present invention for oil country tubular goods will be described below using a seamless steel pipe by way of example. Preferably, molten steel having the composition described above is formed into an ingot by a known ingot-forming method using a converter, an electric furnace, a vacuum melting furnace, or the like, followed by formation of articles, such as billets, for steel pipes using a known method including a continuous casting method or an ingot-making bloom rolling method. These articles for steel pipes are heated and processed by hot working for making pipes using a production process such as a general Mannesmann-plug mill process or Mannesmann-mandrel mill process, thereby forming seamless steel pipes having desired dimensions. After pipe-making, the seamless steel pipes are preferably cooled to room temperature at a cooling rate higher than that of air cooling. 
     The steel pipes cooled after pipe-making may be used as steel pipes of the present invention. Preferably, the steel pipes cooled after pipe-making are subjected to tempering or quenching and tempering. 
     Preferably, quenching may be performed by reheating the articles to 800° C. or higher, maintaining the articles at the temperature for 5 minutes or more, and cooling the articles to 200° C. or lower and preferably to room temperature at a cooling rate higher than that of air cooling. At a heating temperature of 800° C. or lower, a sufficient martensite microstructure cannot be obtained, thereby reducing strength, in some cases. 
     Tempering after quenching is preferably performed by heating the articles to a temperature exceeding the A C1  temperature. Tempering at a temperature exceeding the A C1  temperature results in the precipitation of austenite or quenched martensite. 
     In the case where the steel pipes cooled after pipe-making are subjected to tempering alone, the steel pipes are preferably heated to a temperature between the A C1  temperature and 700° C. 
     In the present invention, from the viewpoint of hot workability, significantly low contents of S, Si, Al, and O improve hot workability of the steel. Thus, in the case where steel pipes are produced with the steel, a common production process may be employed without any modification. The steel of the present invention may be applied to electric resistance welded pipes and UOE steel pipes as well as seamless steel pipes. 
     The reason for the limitation of the contents of the components of the stainless steel pipe included in Group 3 of the present invention for oil country tubular goods will be described below. 
     C: 0.05% or less 
     C relates to the strength of the martensitic stainless steel and is thus an important element. To sufficiently ensure expandability, the C content needs to be 0.05% or less. During tempering, C causes precipitation of chromium carbides, thereby degrading corrosion resistance. To prevent the degradation of corrosion resistance, the C content needs to be 0.05% or less. Thus, the C content is set to 0.05% or less. Preferably, the C content is 0.03% or less. 
     Si: 0.50% or less 
     Si is an element needed as a deoxidizer in a usual steel-making process. A Si content exceeding 0.50% degrades CO 2  corrosion resistance and hot workability. Thus, the Si content is set to 0.50% or less. 
     Mn: 0.10% to 1.50% 
     The Mn content needs to be 0.10% or more in order to ensure the strength required for martensitic stainless steel for oil country tubular goods. A Mn content exceeding 1.50% adversely affects toughness. Thus, the Mn content is set in the range of 0.10% to 1.50% and preferably 0.30% to 1.00%. 
     P: 0.03% or less 
     P is an element that degrades CO 2  corrosion resistance, resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking. The P content is preferably minimized. However, an extreme reduction in P content increases production costs. Also from the viewpoint of hot workability, a lower P content is preferred. In view of providing an allowable range in which the production can be industrially performed at relatively low costs and in which CO 2  corrosion resistance, resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking are not degraded, the P content is set to 0.03% or less. 
     S: 0.005% or less 
     S is an element that significantly degrades hot workability in a process of manufacturing a pipe. The S content is preferably minimized. At a S content of 0.005% or less, the steel pipe can be manufactured by a common process. Thus, the upper limit of the S content is set to 0.005%. Preferably, the S content is 0.003% or less. 
     Cr: 10.5% to 17.0% 
     Cr is a main element used to ensure CO 2  corrosion resistance and resistance to CO 2  stress corrosion cracking. From the viewpoint of corrosion resistance, the Cr content needs to be 10.5% or more. However, a Cr content exceeding 17.0% degrades hot workability. Thus, the Cr content is set in the range of 10.5% to 17.0% and preferably 10.5% to 13.5%. 
     Ni: 0.5% to 7.0% 
     Ni is incorporated in order to strengthen a protective film to improve CO 2  corrosion resistance, resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking and in order to increase the strength of 13% Cr steel having a lower C content. At a Ni content of less than 0.5%, the effect is not provided. A Ni content exceeding 7.0% reduces the strength. Thus, the Ni content is set in the range of 0.5% to 7.0%. Preferably, the Ni content is set in the range of 1.0% to 3.0%. 
     Al: 0.05% or less 
     Al has a strong deoxidizing effect. An Al content exceeding 0.05% adversely affects toughness. Thus the Al content is set to 0.05% or less. 
     V: 0.20% or less 
     V has effects of increasing strength and improving resistance to stress corrosion cracking. A V content exceeding 0.2% degrades toughness. Thus, the V content is set to 0.20% or less. 
     N: 0.15% or less 
     N is an element that significantly improves pitting corrosion resistance. A N content exceeding 0.15% results in the formation of various nitrides, thereby degrading toughness. Thus, the N content is set to 0.15% or less. 
     O: 0.008% or less 
     O is a significantly important element for sufficiently exhibiting the performance of the steel of the present invention. A higher O content results in the formation of various oxides, thereby significantly degrading hot workability, resistance to CO 2  stress corrosion cracking, pitting corrosion resistance, and resistance to sulfide stress corrosion cracking. Thus, the O content is set to 0.008% or less. 
     The steel composition according to the present invention may contain at least one selected from 0.20% or less Nb, 3.5% or less Cu, 0.3% or less Ti, 0.2% or less Zr, 0.001% to 0.01% Ca, 0.0005% to 0.01% B, and 3.0% or less W as an additional element. 
     Nb: 0.20% or less 
     Nb has effects of improving toughness and increasing strength. However, a Nb content exceeding 0.20% reduces toughness. Thus, the Nb content is set to 0.20% or less. 
     Ca: 0.001% to 0.01% 
     Ca fixes S as CaS and spheroidizes sulfide inclusions, thereby reducing the lattice strain of the matrix around the inclusions to reduce their ability to trap hydrogen. At a Ca content of less than 0.001%, the effect is less marked. A Ca content exceeding 0.01% increases formation of CaO, thereby degrading CO 2  corrosion resistance and pitting corrosion resistance. Thus, the Ca content is set in the range of 0.001% to 0.01%. 
     Cu: 3.5% or less 
     Cu is an element which strengthens the protective film, inhibits the penetration of hydrogen into steel, and improves resistance to sulfide stress corrosion cracking. A Cu content exceeding 3.5% causes the grain boundary precipitation of CuS at a high temperature, thereby degrading hot workability. Thus, the Cu content is set to 3.5% or less. 
     Ti: 0.3% or less, Zr: 0.2% or less, B: 0.0005% to 0.01%, W: 3.0% or less. 
     Ti, Zr, B, and W have effects of increasing strength and improving resistance to stress corrosion cracking. Toughness is reduced at a Ti content exceeding 0.3%, a Zr content exceeding 0.2%, or a W content exceeding 3.0%. A B content of less than 0.0005% produces no effect. A B content exceeding 0.01% degrades toughness. Thus, the Ti content is set to 0.3% or less. The Zr content is set to 0.2% or less. The B content is set in the range of 0.0005% to 0.01%. The W content is set to 3.0% or less. 
     Cr+0.5Ni−20C+0.45Cu+0.4 W&gt;11.3 (where the symbols of the elements represent contents (percent by mass) of the elements in steel, and a term of element that is not contained is ignored) 
     To obtain sufficient corrosion resistance in a high-temperature carbon-dioxide-gas environment in which a steel pipe of the present invention is used, it is necessary to sufficiently incorporate alloying elements required for corrosion resistance and to reduce the content of C that degrades corrosion resistance. Thus, the relationship Cr+0.5Ni−20C+0.45Cu+0.4 W&gt;11.3 is determined. 
     With respect to a steel microstructure, from the viewpoint of providing a stable expandability, preferably, the steel microstructure has tempered martensite as a main phase and one selected from: 
     an austenite content exceeding 5 percent by volume; 
     a quenched martensite content of 3 percent by volume or more; and 
     a quenched martensite content of 3 percent by volume or more and an austenite content of 5 percent by volume or more. 
     A preferred method for producing a stainless pipe included in Group 2 of the present invention for oil country tubular goods will be described below using a seamless steel pipe by way of example. Preferably, molten steel having the composition described above is formed into an ingot by a known ingot-forming method using a converter, an electric furnace, a vacuum melting furnace, or the like, followed by formation of articles, such as billets, for steel pipes using a known method including a continuous casting method or an ingot-making bloom rolling method. These articles for steel pipes are heated and processed by hot working for making pipes using a production process such as a general Mannesmann-plug mill process or Mannesmann-mandrel mill process, thereby forming seamless steel pipes having desired dimensions. After pipe-making, the seamless steel pipes are preferably cooled to room temperature at a cooling rate higher than that of air cooling. 
     The steel pipes cooled after pipe-making may be used as steel pipes of the present invention. Preferably, the steel pipes cooled after pipe-making are subjected to tempering or quenching and tempering. 
     Preferably, quenching may be performed by reheating the articles to 800° C. or higher, maintaining the articles at the temperature for 5 minutes or more, and cooling the articles to 200° C. or lower and preferably to room temperature at a cooling rate higher than that of air cooling. At a heating temperature of 800° C. or lower, a sufficient martensite microstructure cannot be obtained, thereby reducing strength, in some cases. 
     Tempering after quenching is preferably performed by heating the articles to a temperature exceeding the A C1  temperature. Tempering at a temperature exceeding the A C1  temperature results in the precipitation of austenite or quenched martensite. 
     In the case where the steel pipes cooled after pipe-making are subjected to tempering alone, the steel pipes are preferably heated to a temperature between the A C1  temperature and 700° C. 
     In the present invention, from the viewpoint of hot workability, significantly low contents of S, Si, Al, and O improve hot workability of the steel. Thus, in the case where steel pipes are produced with the steel, a common production process may be employed without any modification. The steel of the present invention may be applied to electric resistance welded pipes and UOE steel pipes as well as seamless steel pipes. 
     EXAMPLES 
     Example 1 of Group 1 of the Invention 
     Table 1 shows sample symbols and compositions of steels in inventive examples and comparative examples. These molten steels having the chemical compositions were sufficiently degassed and were each formed into a 100-kg steel ingot. Steel pipes each having an outer diameter of 3.3 inches and a thickness of 0.5 inches were formed with a research model seamless rolling mill. Specimens were cut out from the steel pipes and were subjected to quenching and tempering. Furthermore, expandability and corrosion resistance of the steel pipes were tested. Table 2 shows the results of the expandability test. Expandability was evaluated by a method in which a limit of the expansion ratio is determined by insertion of plugs. The evaluation was performed using the plugs such that the expansion ratio in 5% increments was determined. A target expansion ratio is 35% or more. 
     Furthermore, corrosion test pieces each having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm were formed from 15%-expanded steel pipes by mechanical processing. A corrosion test was performed under conditions described below. 
     Corrosion Test Conditions 
     NaCl: 20% aqueous solution, CO 2 : 30 atoms, temperature: 150° C., test period: 2 weeks. 
     In the corrosion test, evaluation was based on the corrosion rate obtained by calculation from the reduction in weight of each test piece and observation of the presence or absence of pitting corrosion with a 10-power loupe. Table 2 shows the results. 
     When the Cr content is 12% or less (type of steel: J), the corrosion rate is increased (No. 15). The allowable limit of the corrosion rate is 0.127 mm/y. 
     The results demonstrate that the steels of the present invention have high expandability and excellent carbon-dioxide-gas corrosion resistance. 
     Therefore, the steels of the present invention can be sufficiently used as expandable oil country tubular goods. 
     In each of Nos. 16 to 19 according to comparative examples, the austenite (γ) content is less than 20%, and the expansion ratio is low. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Type 
                   
               
               
                 of 
                 Chemical composition (mass %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 steel 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Al 
                 Cr 
                 Ni 
                 Mo 
                 V 
                 N 
                 O 
                 Cu 
                 Other 
               
               
                   
               
               
                 A 
                 0.012 
                 0.26 
                 0.49 
                 0.01 
                 0.002 
                 0.02 
                 13.3 
                 5.7 
                 2.5 
                 0.047 
                 0.049 
                 0.0031 
                 — 
                   
               
               
                 B 
                 0.011 
                 0.28 
                 0.45 
                 0.02 
                 0.002 
                 0.01 
                 13.3 
                 4.3 
                 1.2 
                 0.057 
                 0.053 
                 0.0023 
                 — 
                 Nb: 0.068 
               
               
                 C 
                 0.014 
                 0.22 
                 0.42 
                 0.01 
                 0.002 
                 0.01 
                 12.7 
                 4.2 
                 1.1 
                 0.059 
                 0.057 
                 0.0027 
                 — 
                 Ti: 0.036 
               
               
                 D 
                 0.018 
                 0.24 
                 0.49 
                 0.02 
                 0.001 
                 0.01 
                 12.6 
                 5.2 
                 2.2 
                 0.049 
                 0.062 
                 0.0035 
                 0.80 
                 Zr: 0.025 
               
               
                 E 
                 0.017 
                 0.27 
                 0.41 
                 0.01 
                 0.002 
                 0.02 
                 13.6 
                 5.0 
                 1.7 
                 0.038 
                 0.044 
                 0.0028 
                 1.24 
                 Ti: 0.021, B: 0.001 
               
               
                 F 
                 0.025 
                 0.20 
                 0.44 
                 0.01 
                 0.001 
                 0.01 
                 12.8 
                 5.1 
                 2.1 
                 0.051 
                 0.039 
                 0.0025 
                 — 
                 Ca: 0.002 
               
               
                 G 
                 0.021 
                 0.24 
                 0.49 
                 0.02 
                 0.001 
                 0.01 
                 12.9 
                 4.9 
                 1.6 
                 0.046 
                 0.050 
                 0.0019 
                 0.75 
                 Nb: 0.044, Ca: 0.001 
               
               
                 H 
                 0.027 
                 0.29 
                 0.44 
                 0.02 
                 0.002 
                 0.02 
                 13.4 
                 5.1 
                 1.9 
                 0.055 
                 0.063 
                 0.0016 
                 — 
                 W: 0.26 
               
               
                 I 
                 0.017 
                 0.27 
                 0.44 
                 0.02 
                 0.001 
                 0.01 
                 13.5 
                 3.2 
                 1.1 
                 0.046 
                 0.056 
                 0.0028 
                 — 
                   
               
               
                 J 
                 0.026 
                 0.23 
                 0.42 
                 0.01 
                 0.002 
                 0.02 
                 11.7 
                 4.8 
                 1.7 
                 0.055 
                 0.106 
                 0.0017 
                 — 
                   
               
               
                 K 
                 0.014 
                 0.27 
                 0.41 
                 0.02 
                 0.001 
                 0.02 
                 12.7 
                 3.3 
                 0.4 
                 0.065 
                 0.058 
                 0.0034 
                 1.16 
                 Nb: 0.061 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Quenching 
                 Tempering 
                   
                   
                 γ 
                 Quenched 
                 Tempered 
                 Limit of 
                 Corrosion 
                   
               
               
                   
                   
                 Type of 
                 temperature 
                 temperature 
                 YS 
                 TS 
                 content 
                 martensite 
                 martensite 
                 expansion 
                 rate 
                 Pitting 
               
               
                 Category 
                 No 
                 steel 
                 (° C.) 
                 (° C.) 
                 (MPa) 
                 (MPa) 
                 (%) 
                 (vol %) 
                 (vol %) 
                 ratio (%) 
                 (mm/y) 
                 corrosion 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Inventive 
                 1 
                 A 
                 890 
                 640 
                 740 
                 945 
                 27.7 
                 0 
                 72.3 
                 55 
                 0.075 
                 None 
               
               
                 example 
                 2 
                 B 
                 890 
                 640 
                 766 
                 939 
                 24.8 
                 0 
                 75.2 
                 45 
                 0.087 
                 None 
               
               
                   
                 3 
                 C 
                 890 
                 640 
                 773 
                 942 
                 24.1 
                 0 
                 75.9 
                 45 
                 0.092 
                 None 
               
               
                   
                 4 
                 D 
                 890 
                 640 
                 769 
                 945 
                 29.2 
                 0 
                 70.8 
                 55 
                 0.094 
                 None 
               
               
                   
                 5 
                 E 
                 890 
                 640 
                 751 
                 933 
                 26.2 
                 0 
                 73.8 
                 55 
                 0.070 
                 None 
               
               
                   
                 6 
                 F 
                 890 
                 640 
                 747 
                 938 
                 26.8 
                 0 
                 73.2 
                 55 
                 0.090 
                 None 
               
               
                   
                 7 
                 G 
                 890 
                 640 
                 759 
                 934 
                 25.6 
                 0 
                 74.4 
                 50 
                 0.089 
                 None 
               
               
                   
                 8 
                 H 
                 890 
                 640 
                 749 
                 941 
                 26.7 
                 0 
                 73.3 
                 55 
                 0.084 
                 None 
               
               
                   
                 9 
                 I 
                 890 
                 640 
                 755 
                 949 
                 25.9 
                 0 
                 71.5 
                 50 
                 0.083 
                 None 
               
               
                   
                 10 
                 A 
                 890 
                 650 
                 651 
                 976 
                 29.1 
                 0 
                 70.9 
                 55 
                 0.074 
                 None 
               
               
                   
                 11 
                 A 
                 680 
                 630 
                 767 
                 975 
                 32.4 
                 0 
                 67.6 
                 60 
                 0.071 
                 None 
               
               
                   
                 12 
                 A 
                 890 
                 670 
                 720 
                 1031 
                 20.2 
                 6.9 
                 72.9 
                 50 
                 0.070 
                 None 
               
               
                   
                 13 
                 B 
                 890 
                 670 
                 725 
                 1069 
                 21.5 
                 8.3 
                 70.2 
                 50 
                 0.082 
                 None 
               
               
                   
                 14 
                 F 
                 680 
                 630 
                 759 
                 970 
                 30.8 
                 0 
                 69.2 
                 60 
                 0.089 
                 None 
               
               
                 Comparative 
                 15 
                 J 
                 890 
                 640 
                 761 
                 936 
                 25.5 
                 0 
                 74.5 
                 45 
                 0.189 
                 None 
               
               
                 example 
                 16 
                 K 
                 890 
                 640 
                 841 
                 944 
                 19.1 
                 0 
                 80.9 
                 30 
                 0.097 
                 Observed 
               
               
                   
                 17 
                 B 
                 890 
                 550 
                 953 
                 1019 
                 2.4 
                 0 
                 97.6 
                 25 
                 0.091 
                 None 
               
               
                   
                 18 
                 B 
                 890 
                 590 
                 911 
                 995 
                 10.2 
                 0 
                 89.8 
                 25 
                 0.089 
                 None 
               
               
                   
                 19 
                 H 
                 890 
                 550 
                 961 
                 1055 
                 3.9 
                 0 
                 96.1 
                 25 
                 0.095 
                 None 
               
               
                   
               
            
           
         
       
     
     Example of Group 2 of the Invention 
     Molten steels having compositions shown in Table 3 were formed in a vacuum melting furnace, sufficiently degassed, and were each formed into a 100-kg steel ingot. The resulting ingots were subjected to hot piercing rolling with a research model seamless roll mill and were air-cooled to make pipes each having an outer diameter of 3.3 inches and a thickness of 0.5 inches. Specimens were cut out from the steel pipes and were subjected to quenching and tempering under the conditions shown in Table 4. 
     The specimens after the treatment were tested as follows.
     Test for tensile properties: a tensile test according to ASTM A370 was performed in the longitudinal direction of each pipe to measure yield strength (YS) and tensile strength (TS).   Investigation of microstructure: A microstructure in the central portion in the thickness direction was exposed by etching. Tempered martensite, austenite, and quenched martensite phases were identified by image processing to determine the proportion (percent by volume) of each phase.   Expandability test: Each pipe was expanded by insertion of plugs, the diameters of the plugs being increased in such a manner that the expansion ratio ((plug diameter−initial inner diameter of pipe)/initial inner diameter of pipe×100 (%)) was increased in increments of 5%. Evaluation of expandability was performed on the basis of the expansion ratio (limit of expansion ratio) when the pipe during expanding was cracked. A target expansion ratio is 25% or more.   Corrosion test: Corrosion test pieces each having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm were formed from 15%-expanded steel pipes by mechanical processing. A corrosion test was performed (conditions: the test pieces were immersed in an aqueous solution of 20% NaCl at 140° C. for two weeks, the solution being in equilibrium with a CO 2  atmosphere under a pressure of 30 atm). Evaluation of corrosion resistance was performed on the basis of the corrosion rate obtained by calculation from the reduction in weight of each test piece after the test and observation of the presence or absence of pitting corrosion with a 10-power loupe.   

     Table 4 shows the results. When the Cr content is less than 11.0%, the corrosion rate is increased. The allowable limit of the corrosion rate is 0.127 mm/y. When Mo is not contained, pitting corrosion occurs. The results clearly demonstrate that the steels according to the inventive examples have high expandability and excellent CO 2  corrosion resistance. Therefore, the steel pipes of the present invention can be sufficiently used as expandable oil country tubular goods. 
     
       
         
           
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Type 
                   
               
               
                 of 
                 Chemical composition (mass %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 steel 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Al 
                 Cr 
                 Ni 
                 Mo 
                 V 
                 N 
                 O 
                 Cu 
                 Other 
               
               
                   
               
               
                 A1 
                 0.007 
                 0.29 
                 0.46 
                 0.02 
                 0.001 
                 0.02 
                 12.4 
                 5.3 
                 1.9 
                 0.050 
                 0.007 
                 0.0029 
                 — 
                 — 
               
               
                 B1 
                 0.008 
                 0.30 
                 0.47 
                 0.01 
                 0.002 
                 0.02 
                 12.1 
                 4.9 
                 4.8 
                 0.047 
                 0.008 
                 0.0056 
                 — 
                 Nb: 0.050 
               
               
                 C1 
                 0.004 
                 0.24 
                 0.50 
                 0.01 
                 0.002 
                 0.02 
                 12.2 
                 4.9 
                 2.5 
                 0.051 
                 0.009 
                 0.0051 
                 — 
                 Ti: 0.081 
               
               
                 D1 
                 0.008 
                 0.27 
                 0.47 
                 0.02 
                 0.002 
                 0.01 
                 12.9 
                 5.3 
                 2.5 
                 0.051 
                 0.009 
                 0.0045 
                 1.23 
                 Zr: 0.014 
               
               
                 E1 
                 0.005 
                 0.20 
                 0.41 
                 0.02 
                 0.002 
                 0.01 
                 12.1 
                 5.0 
                 2.1 
                 0.049 
                 0.004 
                 0.0036 
                 0.69 
                 Ti: 0.037, B: 0.001 
               
               
                 F1 
                 0.009 
                 0.25 
                 0.44 
                 0.02 
                 0.002 
                 0.02 
                 12.8 
                 4.6 
                 2.4 
                 0.049 
                 0.006 
                 0.0023 
                 — 
                 Ca: 0.001 
               
               
                 G1 
                 0.007 
                 0.25 
                 0.42 
                 0.02 
                 0.001 
                 0.01 
                 12.2 
                 5.0 
                 2.5 
                 0.051 
                 0.008 
                 0.0049 
                 0.92 
                 Nb: 0.061, Ca: 0.001 
               
               
                 H1 
                 0.005 
                 0.22 
                 0.42 
                 0.02 
                 0.002 
                 0.02 
                 12.6 
                 5.4 
                 1.6 
                 0.054 
                 0.008 
                 0.0054 
                 — 
                 W: 0.72 
               
               
                 I1 
                 0.009 
                 0.28 
                 0.48 
                 0.02 
                 0.001 
                 0.01 
                 12.2 
                 5.2 
                 1.7 
                 0.044 
                 0.006 
                 0.0037 
                 — 
                 — 
               
               
                 J1 
                 0.008 
                 0.29 
                 0.47 
                 0.01 
                 0.002 
                 0.02 
                 10.6 
                 4.8 
                 2.0 
                 0.051 
                 0.006 
                 0.0085 
                 — 
                 — 
               
               
                 K1 
                 0.006 
                 0.24 
                 0.45 
                 0.01 
                 0.001 
                 0.01 
                 12.0 
                 4.7 
                 — 
                 0.045 
                 0.008 
                 0.0057 
                 0.85 
                 Nb: 0.061 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Limit of 
                   
                   
                   
               
               
                   
                 Type 
                 Quenching 
                 Tempering 
                   
                   
                   
                 Quenched 
                 Tempered 
                 expansion 
                 Corrosion 
                   
                   
               
               
                   
                 of 
                 temperature 
                 temperature 
                 YS 
                 TS 
                 Austenite 
                 martensite 
                 martensite 
                 ratio 
                 rate 
                 Pitting 
                   
               
               
                 No 
                 steel 
                 (° C.) 
                 (° C.) 
                 (MPa) 
                 (MPa) 
                 (vol %) 
                 (vol %) 
                 (vol %) 
                 (%) 
                 (mm/y) 
                 corrosion 
                 Remarks 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 101 
                 A1 
                 890 
                 650 
                 596 
                 795 
                 25.7 
                 0 
                 74.3 
                 55 
                 0.079 
                 None 
                 Inventive 
               
               
                 102 
                 B1 
                 890 
                 650 
                 653 
                 846 
                 25.5 
                 0 
                 74.5 
                 55 
                 0.094 
                 None 
                 example 
               
               
                 103 
                 C1 
                 890 
                 650 
                 597 
                 802 
                 25.7 
                 0 
                 74.3 
                 55 
                 0.079 
                 None 
                   
               
               
                 104 
                 D1 
                 890 
                 650 
                 629 
                 837 
                 27.7 
                 0 
                 72.3 
                 55 
                 0.072 
                 None 
                   
               
               
                 105 
                 E1 
                 890 
                 650 
                 598 
                 807 
                 25.9 
                 0 
                 74.1 
                 55 
                 0.087 
                 None 
                   
               
               
                 106 
                 F1 
                 890 
                 650 
                 625 
                 826 
                 24.1 
                 0 
                 75.9 
                 55 
                 0.075 
                 None 
                   
               
               
                 107 
                 G1 
                 890 
                 650 
                 642 
                 836 
                 26.3 
                 0 
                 73.7 
                 55 
                 0.085 
                 None 
                   
               
               
                 108 
                 H1 
                 890 
                 650 
                 620 
                 818 
                 26.8 
                 0 
                 73.2 
                 55 
                 0.076 
                 None 
                   
               
               
                 109 
                 I1 
                 890 
                 650 
                 628 
                 825 
                 26.5 
                 0 
                 73.5 
                 55 
                 0.087 
                 None 
                   
               
               
                 110 
                 A1 
                 890 
                 670 
                 564 
                 792 
                 28.9 
                 0 
                 71.1 
                 60 
                 0.076 
                 None 
                   
               
               
                 111 
                 A1 
                 680 
                 640 
                 604 
                 781 
                 32.4 
                 0 
                 67.6 
                 65 
                 0.074 
                 None 
                   
               
               
                 112 
                 A1 
                 890 
                 690 
                 534 
                 897 
                 20.7 
                 7.9 
                 71.4 
                 50 
                 0.081 
                 None 
                   
               
               
                 113 
                 B1 
                 890 
                 690 
                 538 
                 904 
                 20.4 
                 6.1 
                 73.5 
                 50 
                 0.098 
                 None 
                   
               
               
                 114 
                 F1 
                 690 
                 640 
                 545 
                 837 
                 29.1 
                 0 
                 70.9 
                 60 
                 0.073 
                 None 
                   
               
               
                 115 
                 J1 
                 890 
                 650 
                 607 
                 828 
                 26.7 
                 0 
                 73.3 
                 55 
                 0.176 
                 None 
                 Comparative 
               
               
                 116 
                 K1 
                 890 
                 640 
                 582 
                 836 
                 27.5 
                 0 
                 72.5 
                 55 
                 0.103 
                 Observed 
                 example 
               
               
                 117 
                 B1 
                 890 
                 540 
                 762 
                 899 
                 3.7 
                 0 
                 96.3 
                 25 
                 0.102 
                 None 
                   
               
               
                 118 
                 B1 
                 890 
                 580 
                 705 
                 876 
                 12.1 
                 0 
                 87.9 
                 30 
                 0.096 
                 None 
                   
               
               
                 119 
                 H1 
                 890 
                 540 
                 741 
                 892 
                 3.8 
                 0 
                 96.2 
                 25 
                 0.078 
                 None 
               
               
                   
               
            
           
         
       
     
     Example of Group 3 of the Invention 
     Molten steels having compositions shown in Table 5 were formed in a vacuum melting furnace, sufficiently degassed, and were each formed into a 100-kg steel ingot. The resulting ingots were subjected to hot piercing rolling with a research model seamless roll mill and were air-cooled to make pipes each having an outer diameter of 3.3 inches and a thickness of 0.5 inches. Specimens were cut out from the steel pipes and were subjected to quenching and tempering under the conditions shown in Table 6. 
     The specimens after the treatment were tested as follows.
     Test for tensile properties: a tensile test according to ASTM A370 was performed in the longitudinal direction of each pipe to measure yield strength (YS) and tensile strength (TS).   Investigation of microstructure: A microstructure in the central portion in the thickness direction was exposed by etching. Tempered martensite, austenite, and quenched martensite phases were identified by image processing to determine the proportion (percent by volume) of each phase.   Expandability test: Each pipe was expanded by insertion of plugs, the diameters of the plugs being increased in such a manner that the expansion ratio ((plug diameter−initial inner diameter of pipe)/initial inner diameter of pipe×100 (%)) was increased. Evaluation of expandability was performed on the basis of the expansion ratio (limit of expansion ratio) when the pipe during expanding was cracked.   Corrosion test: Corrosion test pieces each having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm were formed from tempered pipes by mechanical processing. A corrosion test was performed (conditions: the test pieces were immersed in an aqueous solution of 10% NaCl at 100° C. for two weeks, the solution being in equilibrium with a CO 2  atmosphere under a pressure of 30 atm). Evaluation of corrosion resistance was performed on the basis of the corrosion rate obtained by calculation from the reduction in weight of each test piece after the test and observation of the presence or absence of pitting corrosion with a 10-power loupe.   

     Table 6 shows the results. When the C content is 0.05% or less, a limit of expansion ratio of 40% or more was ensured. When Cr+0.5Ni−20C+0.45Cu+0.4W is 11.3 or less, the corrosion rate is increased. The results clearly demonstrate that the steels according to the inventive examples have high expandability and excellent CO 2  corrosion resistance. Therefore, the steel pipes of the present invention can be sufficiently used as expandable oil country tubular goods in oil well environments containing carbon dioxide gas. 
     
       
         
           
               
               
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Type 
                   
                   
               
               
                 of 
                 Chemical composition (mass %) 
                 Formula 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 steel 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Al 
                 Cr 
                 Ni 
                 V 
                 N 
                 O 
                 Cu 
                 Other 
                 (1) 
               
               
                   
               
               
                 A2 
                 0.008 
                 0.33 
                 0.81 
                 0.01 
                 0.001 
                 0.02 
                 11.1 
                 2.4 
                 0.054 
                 0.015 
                 0.0035 
                 — 
                 — 
                 12.14 
               
               
                 B2 
                 0.013 
                 0.32 
                 0.84 
                 0.02 
                 0.002 
                 0.02 
                 12.0 
                 2.0 
                 0.052 
                 0.022 
                 0.0039 
                 — 
                 Nb: 0.036 
                 12.74 
               
               
                 C2 
                 0.012 
                 0.33 
                 0.86 
                 0.02 
                 0.002 
                 0.01 
                 11.4 
                 1.8 
                 0.048 
                 0.040 
                 0.0066 
                 — 
                 T1: 0.078 
                 12.06 
               
               
                 D2 
                 0.007 
                 0.34 
                 0.89 
                 0.01 
                 0.001 
                 0.01 
                 11.3 
                 1.5 
                 0.045 
                 0.007 
                 0.0037 
                 0.62 
                 Zr: 0.019 
                 12.19 
               
               
                 E2 
                 0.018 
                 0.30 
                 0.88 
                 0.02 
                 0.001 
                 0.01 
                 10.9 
                 2.3 
                 0.051 
                 0.031 
                 0.0071 
                 0.88 
                 Ti: 0.045, B: 0.001 
                 12.09 
               
               
                 F2 
                 0.028 
                 0.33 
                 0.85 
                 0.02 
                 0.001 
                 0.01 
                 11.2 
                 1.8 
                 0.046 
                 0.024 
                 0.0030 
                 — 
                 Ca: 0.001 
                 11.54 
               
               
                 G2 
                 0.019 
                 0.32 
                 0.86 
                 0.01 
                 0.002 
                 0.01 
                 10.9 
                 1.7 
                 0.047 
                 0.027 
                 0.0035 
                 1.31 
                 Nb: 0.069, Ca: 0.001 
                 11.96 
               
               
                 H2 
                 0.029 
                 0.25 
                 0.88 
                 0.02 
                 0.001 
                 0.01 
                 11.2 
                 1.7 
                 0.051 
                 0.011 
                 0.0047 
                 — 
                 W: 0.95 
                 11.85 
               
               
                 I2 
                 0.026 
                 0.29 
                 0.86 
                 0.01 
                 0.001 
                 0.02 
                 11.3 
                 1.9 
                 0.051 
                 0.020 
                 0.0058 
                 — 
                 — 
                 11.73 
               
               
                 J2 
                 0.019 
                 0.34 
                 0.84 
                 0.01 
                 0.001 
                 0.02 
                 10.3 
                 1.6 
                 0.051 
                 0.017 
                 0.0094 
                 — 
                 — 
                 10.72 
               
               
                 K2 
                 0.055 
                 0.31 
                 0.95 
                 0.01 
                 0.001 
                 0.01 
                 11.1 
                 1.5 
                 0.054 
                 0.028 
                 0.0055 
                 0.62 
                 Nb: 0.032 
                 11.03 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Limit of 
                   
                   
                   
               
               
                   
                 Type 
                 Quenching 
                 Tempering 
                   
                   
                   
                 Quenched 
                 Tempered 
                 expansion 
                 Corrosion 
                   
                   
               
               
                   
                 of 
                 temperature 
                 temperature 
                 YS 
                 TS 
                 Austenite 
                 martensite 
                 martensite 
                 ratio 
                 rate 
                 Pitting 
                   
               
               
                 No 
                 steel 
                 (° C.) 
                 (° C.) 
                 (MPa) 
                 (MPa) 
                 (vol %) 
                 (vol %) 
                 (vol %) 
                 (%) 
                 (mm/y) 
                 corrosion 
                 Remarks 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 201 
                 A2 
                 890 
                 700 
                 537 
                 695 
                 9.7 
                 0 
                 90.3 
                 50 
                 0.081 
                 None 
                 Inventive 
               
               
                 202 
                 B2 
                 890 
                 700 
                 641 
                 696 
                 7.9 
                 0 
                 92.1 
                 50 
                 0.078 
                 None 
                 example 
               
               
                 203 
                 C2 
                 890 
                 700 
                 547 
                 708 
                 8.8 
                 0 
                 91.2 
                 50 
                 0.089 
                 None 
                   
               
               
                 204 
                 D2 
                 890 
                 700 
                 634 
                 686 
                 6.5 
                 0 
                 93.5 
                 50 
                 0.082 
                 None 
                   
               
               
                 205 
                 E2 
                 890 
                 700 
                 565 
                 712 
                 9.4 
                 0 
                 90.6 
                 50 
                 0.084 
                 None 
                   
               
               
                 206 
                 F2 
                 890 
                 700 
                 607 
                 752 
                 8.5 
                 0 
                 91.5 
                 50 
                 0.108 
                 None 
                   
               
               
                 207 
                 G2 
                 890 
                 700 
                 564 
                 719 
                 8.0 
                 0 
                 92.0 
                 50 
                 0.091 
                 None 
                   
               
               
                 208 
                 H2 
                 890 
                 700 
                 612 
                 766 
                 8.4 
                 0 
                 91.6 
                 50 
                 0.094 
                 None 
                   
               
               
                 209 
                 I2 
                 890 
                 700 
                 583 
                 735 
                 8.6 
                 0 
                 91.4 
                 50 
                 0.098 
                 None 
                   
               
               
                 210 
                 A2 
                 890 
                 720 
                 564 
                 667 
                 14.6 
                 0 
                 85.4 
                 55 
                 0.076 
                 None 
                   
               
               
                 211 
                 A2 
                 680 
                 650 
                 674 
                 732 
                 0 
                 0 
                 100 
                 40 
                 0.082 
                 None 
                   
               
               
                 212 
                 A2 
                 890 
                 760 
                 509 
                 755 
                 13.7 
                 8.7 
                 77.6 
                 55 
                 0.084 
                 None 
                   
               
               
                 213 
                 B2 
                 890 
                 740 
                 513 
                 767 
                 11.9 
                 5.9 
                 82.2 
                 55 
                 0.077 
                 None 
                   
               
               
                 214 
                 F2 
                 890 
                 650 
                 604 
                 805 
                 0 
                 0 
                 100 
                 40 
                 0.103 
                 None 
                   
               
               
                 215 
                 J2 
                 890 
                 700 
                 565 
                 719 
                 8.9 
                 0 
                 91.1 
                 40 
                 0.155 
                 Observed 
                 Comparative 
               
               
                 216 
                 K2 
                 890 
                 700 
                 655 
                 793 
                 6.4 
                 0 
                 93.6 
                 35 
                 0.135 
                 None 
                 example 
               
               
                 217 
                 J2 
                 890 
                 650 
                 595 
                 769 
                 0 
                 0 
                 100 
                 35 
                 0.158 
                 Observed 
               
               
                   
               
            
           
         
       
     
     INDUSTRIAL APPLICABILITY 
     The stainless steel pipe of the present invention for oil country tubular goods has sufficient corrosion resistance and high workability in which the steel pipe can be expanded at a high expansion ratio even in high-temperature severe corrosion environments containing CO 2  and Cl − . The stainless steel pipe is obtained by in 13% Cr steel having a C content markedly lower than that in the known art, limitation of contents of C, Si, Mn, Cr, Mo, Ni, N, and O, the formation of a microstructure mainly having a tempered martensitic phase with an austenite content exceeding 20 percent by volume or with a quenched martensite content of 3 percent by volume or more, and an austenite content of 15 percent by volume or more, optional limitation of contents of Cu, W, and the like, and the control of a microstructure. Therefore, the steel pipe of the present invention is suitable as oil country tubular goods used in the above-described severe corrosion environments. The steel of the present invention has excellent corrosion resistance and workability and thus can be applied to electric resistance welded pipes and UOE steel pipes.