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| Stainless Steel Pipe Having Excellent Expandability for Oil Country Tubular Goods |
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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 (CO2), chlorine ions (Cl-), and the like. 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 CO2, 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. In general, 13% Cr martensitic stainless steel pipes having CO2 corrosion resistance are used under environments containing CO2, 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 CO2 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 CO2 corrosion resistance and excellent expandability under a severe corrosive environment containing CO2, 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 CO2 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 CO2 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. |