In view of these circumstances, an object of the present invention is to provide a method forproducing a two-phase stainless steel pipe which has not only a corrosion resistance required for the oil well pipes used in deep wells or in severe corrosive environments but at the same time has a targeted strength.For the purpose of solving the above-described problems, the present inventors produced two-phase.
stainless steel pipes by using two-phase stainless steel materials having various chemical compositions under the conditions that the extent of the final cold drawing was varied in different ways, and performed an experiment to determine the tensile strengths of these pipes.
Consequently, the present inventors obtained the following findings (a) to (g).
(a) The two-phase stainless steel pipes used in deep oil wells or in oil wells in severe corrosive environments are required to have corrosion resistance. However, when the content of C is larger,the precipitation of the carbides tends to be excessive due to the thermal effects at the time of a heat treatment, welding or the like, and hence, from the viewpoints of the corrosion resistance and the workability of the steel, in particular, from the viewpoint of the corrosion resistance, it is necessary to reduce the content of C.
(b) When the content of C is reduced, the strength comes to be insufficient without applying any other operation; however, a material pipe produced by hot working of the two-phase stainless steel material or further by solid-solution heat treatment of the two-phase stainless steel material can be improved in strength by subsequently applying cold drawing. Here, it is to be noted that when the working ratio exceeds 35% in terms of the reduction of area, the high strength is maintained but the work hardening occurs, and hence the ductility or the toughness is deteriorated. On the other hand, when the working ratio is less than 5% in terms of the reduction of area, no intended high strength can be attained. Consequently, it is necessary to set the working ratio of the cold drawing at 5 to 35% in terms of the reduction of area.
(c) Also, it has been found that when the working ratio Rd at the time of cold drawing is in a range from 5 to 35% in terms of the reduction of area, the larger is the working ratio Rd of the final cold drawing in the two-phase stainless steel pipe, the higher is the yield strength YS obtained for the two-phase stainless steel pipe, and the relation between the working ratio Rd and the yield strength YS is represented as a linear relationship.It has also been found that the strength of the two-phase stainless steel pipe is significantly affected by the content of Cr, and the higher is the content of Cr in the steel material, the higher-strength two-phase stainless steel pipe can be obtained. Further, it has also been found that the strength of the two-phase stainless steel pipe is also significantly affected by the content of Mo and the content of W, and the content of Mo or W enables to produce a high- strength two-phase stainless steel pipe.FIG. 1 is a plot of the yield strength YS (MPa) values obtained in a tensile test against the working ratio Rd (%) values in terms of the reduction of area, for the two-phase stainless steel pipes having the various chemical compositions used in Example described below. FIG. 1 shows that there occurs a linear relationship between the working ratio Rd in terms of the reduction of area and the yield strength YS.
(d) Next, the present inventors have thought up that the yield strength of the two-phase stainless steel pipe is dependent on the working ratio Rd at the time of performing the cold drawing and the chemical composition of the two-phase stainless steel pipe, and accordingly it comes to be possible to establish an appropriate component design technique to be associated with the pipe working conditions, for the purpose of attaining the yield strength targeted for the two-phase stainless steel pipe. In other words, for the purpose of attaining the yield strength targeted for
the two-phase stainless steel pipe, not the line regulation based on the chemical composition of the two-phase stainless steel pipe, but the fine regulation based on the working ratio Rd at the time of performing the cold drawing conies to be realizable. Also, it comes to be unnecessary to perform the melting of a large number of types of two-phase stainless steels prepared by varying the alloy composition according to the demanded strength level, and consequently, the overstock of the material billets can be suppressed.As described above, when the appropriate component design technique associated with the pipe working conditions can be established, it is only required to perform the cold drawing, for the purpose of obtaining a two-phase stainless steel pipe having a targeted strength, under the cold
drawing conditions targeted by taking account of the alloy composition of the stock, namely, with the targeted working ratio Rd or the higher working ratio than the targeted working ratio, without being required to vary the alloy composition of the stock on a case-by-case basis.