To obtain a steel pipe with high strength and high corrosion resistance by subjecting a steel having a specified componental compsn. to steel pipe forming and direct quenching under specified conditions and refining it to required strength.
A steel billet having a compsn. contg., by weight, 0.2 to 0.35% C, 0.05 to 0.5% Si, 0.1 to 1% Mn, 0.3 to 1.2% Cr, 0.2 to 1% Mo, 0.005 to 0.50% sol.Al, 0.005 to 0.5% Ti, 0.001 to 0.005% B, >0.1 to 0.5% Nb, 0 to 0.5% V, 0 to 1% W, 0 to 0.5% Zr, 0 to 0.01% Ca, and the balance Fe with inevitable impurities, and in which, in the impurities, ≤0.025% P, ≤0.01% S, ≤0.1% Ni, ≤0.01% N and ≤0.01% O are regulated is prepd. At the time of subjecting this steel to hot piercing and rolling, as for the final finish rolling, the temp. is regulated to 1000 to 1150°C, and the reduction of thickness is regulated to ≥40%, and it is directly quenched from ≥1000°C thickness center temp. and <1000°C surface temp. and is tempered. In this way, the seamless steel pipe having 758 to 1068 MPa yield stress and having a softened layer on the surface is obtd.
OMURA TOMOHIKO
KONDO KUNIO
Next Patent: PHOTOSENSITIVE BODY AND IMAGE FORMING METHOD