Importance

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Abstract

Provided is a steel pipe designed to have a further increased high strength and a further improved fatigue life with suppressing the material cost and the production cost, especially such a steel pipe suitable for machine structure members that are required to be lightweight. The steel pipe is a high fatigue life steel pipe produced according to a process of using a steel plate having a composition that comprises, % by mass, C: 0.1 to 0.4%, Si: 0.5 to 1.5%, Mn: 0.3 to 2%, P: at most 0.02%, S: at most 0.01%, Cr: 0.1 to 2%, Ti: 0.01 to 0.1%, Nb: 0.01 to 0.1%, Al: at most 0.1%, B: 0.0005 to 0.01%, N: at most 0.01%, and optionally at least one of Ni: at most 0.5%, Ca: at most 0.02%, Mo: at most 0.5%, and V: at most 0.5%, with a balance of Fe and inevitable impurities, and having, on at least one side thereof, a smoothed surface of which the surface roughness Ra in the direction (C direction) perpendicular to the direction to be the longitudinal direction of the steel pipe is at most 0.5 μm, welding it into a pipe in such a manner that the smoothed surface thereof could be the inner surface of the steel pipe, and then quenching/tempering it.

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