Accordingly, a steel pipe having a high strength and high toughness is desired for use as risers and flow lines. In order to ensure reliability, seamless steel pipe rather than welded steel pipes are used in such applications.

For welded steel pipes, a technique for manufacturing a steel pipe having a strength exceeding X80 grade has already been disclosed. For example, Patent Document 1 (JP H9-41074A) discloses a steel which exceeds X100 grade (a yield strength ofat least 689 MPa) set forth in API standards. A welded steel pipe is manufactured by first producing a steel plate, rolling up the steel plate, and welding the seam to form a steel pipe. In order to impart essential properties such as strength andtoughness at the time of producing the steel plate, control of the microstructure has been employed by subjecting the steel sheet to thermomechanical treatment at the stage of rolling. Also in Patent Document 1, the desired properties of a steel pipeafter welding are secured by performing thermomechanical treatment during hot rolling of a steel sheet in such a manner that the microstructure is controlled so as to include deformed ferrite. Accordingly, the technique disclosed in Patent Document 1can be realized just by a rolling process to form a steel plate in which thermomechanical treatment can be easily applied by controlled rolling, and therefore it can be applied to a welded steel pipe but not to a seamless steel pipe.

In the case of seamless steel pipes, a seamless steel pipe of X80 grade has been developed recently. With seamless steel pipes, since application of the above-described technique including thermomechanical treatment which has been developed forwelded steel pipes is difficult, it is basically necessary to attain the desired properties by heat treatment after pipe formation. For example, a technique for manufacturing a seamless steel pipe of X80 grade (a yield strength of at least 551 MPa) isdisclosed in Patent Document 2 (JP 2001-288532A). However, as disclosed in the examples of that document, the technique is merely demonstrated for a thin-walled steel pipe (with a wall thickness of 11.0 mm) for which hardenability is inherently good. Accordingly, even if the technique disclosed therein is employed, when a seamless steel pipe with a wall thickness of around 40-50 mm which is actually used for risers or flow lines, there is a problem in that an adequate strength and toughness cannot beattained since the cooling speed at the time of hardening is slow particularly in the central portion of such a thick-walled steel pipe.

The present invention aims to solve the above-described problem. Specifically, its object is to provide a seamless steel pipe for line pipe having a high strength and stable toughness and good corrosion resistance particularly in the case of athick-walled seamless steel pipe as well as a process for its manufacture.

A seamless steel pipe according to the present invention can be used for installation in more severe deep seas and particularly as flow lines on the seabed. Accordingly, the present invention greatly contributes to stable supply of energy. When it is used as a riser or a flow line installed in deep seas, it preferably has a wall thickness of at least 30 mm. The upper limit of the wall thickness is not limited, but normally the wall thickness will be at most 60 mm.