Seamless steel pipe and method of manufacturing the same

Abstract

A seamless steel pipe contains, (mass %), C: 0.02 to 0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, where carbon equivalent is not less than 0.430% and less than 0.500%, the microstructure main phase from the surface to an in-the-wall portion is tempered martensite or tempered bainite, prior austenite grain size is lower than 6.0, a portion between 1 mm from the inner surface and 1 mm from the outer surface has Vickers hardness of 250 Hv or lower, and yield strength is 555 MPa or higher.

Description

TECHNICAL FIELD[0001]The present invention relates to a seamless steel pipe and a method of manufacturing the same, and, more particularly, to a seamless steel pipe suitable for line pipe and a method of manufacturing the same.BACKGROUND ART[0002]Oil and gas resources from oil wells located on land and in shallow seas are drying up and, to address this, increasing numbers of offshore oil fields in deep seas are being developed. In an offshore oil field, crude oil or gas must be transported from the pithead of an oil well or gas well installed on the seabed to the platform above the sea using a flow line or a riser. A flow line is a line pipe laid along the topography of the surface of the earth or the seabed. A riser is a line pipe disposed to rise from the seabed toward the platform (i.e. upward).[0003]The inner side of a steel pipe forming part of a flow line laid in the deep sea is subject to a high interior fluid pressure having a pressure from deep strata added thereto and, whe...

AI Technical Summary

Benefits of technology

[0013]To improve the strength and toughness of a seamless steel pipe produced by quenching-and-tempering, the content of alloy elements such as carbon may be increased to increase hardenability. However, if the content of alloy elements such as carbon is increased, the strength (i.e. hardness) of the surface of the steel pipe increases. In a seamless steel pipe produced by quenching-and-tempering, the surface layer is cooled at a high rate during quenching and can easily be hardened, increasing the hardness, while the in-the-wall portions have low hardness. This tendency may remain after tempering. As such, in a seamless steel pipe having a strength of ×80 grade or higher, a surface layer hardness may exceed 250 Hv, which is the higher limit required in the sour resistance grade according to the API 5 L standards.

[0015]An object of the present invention is to provide a seamless steel pipe that can be manufactured by a relatively reasonable manufacturing process and that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner.

[0020]The present invention provides a seamless steel pipe that can be manufactured by a relatively reasonable manufacturing process and that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner.

Problems solved by technology

Even when one or more of the above conventional techniques are used, a seamless steel pipe having a strength of ×80 grade or higher as defined by the American Petroleum Institute (API) standards (i.e. a lower limit yield strength of 555 MPa or higher) may not have good SSC resistance in a reliable manner.

Although the techniques of Patent Document 1 are effective in achieving high strength and high toughness, they do not sufficiently consider reducing the hardness of the surface layer or thus improving SSC resistance.

Patent Document 4 provides some considerations about the hardness of the surface layer of a steel pipe and HIC resistance; however, a reheating-and-quenching step is necessary and, after hot forming, direct quenching or in-line quenching is used as necessary, which means manufacturing costs that are not very reasonable.

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