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Products Specification

Martensitic stainless steels for seamless steel pipe

 

Martensitic stainless steels for seamless steel pipe

This invention relates to martensitic stainless steels used in seamless steel pipe such as oil-well pipe and pipeline tubing. Martensitic stainless steels, two representative grades of which are SUS 410 and SUS 420 (Japan Industrial Standard ?JIS! designations), have excellent corrosion resistance in highly corrosive environments containing CO2. These materials are thus regarded as excellent candidates for use in oil-well pipe, geothermal well pipe, and pipeline tubing.


   

The strength of oil-well pipe is normally required to be least equivalent to that of American Petroleum Institute (API) standard L80 grade steel (yield strength��80 ksi). Pipeline tubing should Generally have a strenoth at least equal to that of API standard X60 grade steel (yield strength��60 ksi).


   

Compositions in which the austenite phase (which becomes martensite at room temperature) exists in combination with a ferrite phase comprising 20-30% of the composition have the worst hot workability. When the amount of ferrite is about 40%, the hot workability is about the same as that of austenitic single-phase steels (which become martensitic single-phase steels at room temperature or below the Ms point). The hot workability rises sharply with increasing ferrite content above this point. Thus, because martensitic stainless steels with a ferrite content of 40% or less at 1200�� C. have inferior hot workability, their use in the production of high-strength seamless steel pipe by the processes described below tends to result in defects, complicating pipe manufacture.


   

Seamless stainless steel pipe is generally produced either by an inclined rolling method such as the plug mill or mandrel mill process, or by a hot extrusion method, of which the Ugine-Sejournet and Erhart pushbench processes are typical. However, certain types of martensitic stainless steels (namely, those with a ferrite content of 40% or less at 1200�� C.), have poor hot workability. When seamless steel pipe is manufactured from these steels by a cross rolling process such as the plug mill process or the mandrel mill process, defects arise on both the outside and inside walls of the pipe during piercing of the billet on a piercing mill. For this reason, seamless pipe made of this type of steel is generally produced by a hot extrusion process, such as the Ugine-Sejournet process.


   

However, when a hot extrusion process is employed and the billet directly pierced (a process known as direct piercing), a billet length 5-7 times the diameter results in a greater eccentricity in the wall thickness of the pipe. This makes it difficult to produce long pipe. A partial solution to this problem is provided by a process for producing long pipe that makes use of what is known as the expansion method. This method consists of mechanically opening a guide hole in the center of the billet, then extending the hole. However, even with the use of this expansion method the billet length is still limited to only about 15 billet diameters. Another problem concerns the glass lubricant used in the Ugine-Sejournet hot extrusion process. This must be peeled off following rolling, a process that is both time-consuming and costly.


   

The invention contemplates martensitic stainless steels for use in seamless steel pipe containing not more than 0.30% by weight of C, not more than 1.0% by weight of Si, not more than 2.0% by weight of Mn, 11-14% by weight of Cr, 0.005-0.10% by weight of Al, and not more than 0.10% by weight of N, the remainder being Fe and unavoidable impurities, of which the impurities P and S are held respectively to levels of not more than 0.02% and 0.003% by weight, the ferrite content of these steels being no more than 40% by weight at 1200�� C.


   

This invention also contemplates martensitic stainless steels for use in seamless steel pipe having the contents of C, Si, Mn, Cr, Al, and N noted above, and containing one or more elements selected from the group consisting of up to 3.5% by weight of Ni, up to 2% by weight of Cu, up to 2.5% by weight of Mo, up to 0.10% by weight of Nb, and up to 0.20% by weight of V, the remainder being Fe and unavoidable impurities, of which the impurities P and S are held at the levels cited above, the ferrite content of these steels being no more than 40% by weight at 1200�� C.


   

The present invention furthermore contemplates martensitic stainless steels for use in seamless steel pipe having the above-stated levels of C, Si, Mn, Cr, Al, and N, and containing one or more elements selected from the group consisting of the rare earth elements, Ca, and B, the amount of the rare earth elements ranging from 4��(% of S) to 20��(% of S), that of Ca from 1��(% of S) to 10��(% of S), and that of B from 0.001 to 0.008% by weight, the remainder being Fe and unavoidable impurities, of which the impurities P and S are held at the levels cited above, the ferrite content of these steels being no more than 40% by weight at 1200�� C.


   

Lastly, the invention also contemplates martensitic stainless steels for use in seamless steel pipe having the above-stated levels of C, Si, Mn, Cr, Al, and N, and also containing one or more elements selected from the group consisting of Ni, Cu, Mo, Nb, and V, as well as one or more elements selected from the group consisting of the rare earth elements, Ca, and B, these all being present in the ranges indicated above, the remainder being Fe and unavoidable impurities, of which the impurities P and S are held at the levels cited above, the ferrite content of these steels being no more than 40% by weight at 1200�� C