PURPOSE: To produce a high strength steel pipe low in hardness and yield ratio by subjecting a steel pipe having specified components to controlled cooling and executing low temp. tempering.
CONSTITUTION: A steel pipe having a compsn. essentially consisting of, by mass, 0.03% C and 0.3 to 1.8% Mn so as to satisfy 0.13<C+(11/90)Mn<0.25 and Si, Al or the like and contg., as selective elements, Mo, Cr, Nb or the like is rolled. Immediately after that, or after being reheated to a γ range, it is subjected to controlled cooling in such a manner that the cooling rate of a point of 1mm below the surface is regulated to ≤150°C/sec and the minimum cooling rate is regulated to ≥10°C/sec. Subsequently, it is subjected to low temp. tempering at 300 to 500°C. In this way, the cooling method is devised and the steel components are limited, by which the steel pipe having a high strength and a low hardness is obtd. as hardened, and its low yield ratio is embodied by the low temp. tempering.
FUJII HIROMI
UENO MASAKATSU
