To produce a seamless steel tube having high strength and excellent in toughness and corrosion resistance by successively subjecting a steel billet of specific composition to piercing, to rolling at respectively specified reduction of area and finishing temp., to reheating at specific temp. for specific time, to hardening, and then to tempering.
The steel billet has a composition which consists of, by mass, 0.15-0.50% C, <0.1% Si, 0-1.5% Mn, ≤0.05% P, <0.01% S, 1-1.5% Cr, ≤0.1% Ni, 0.1-1.5% Mo, 0.005-0.5% Al, 0.005-0.5% Ti, 0.003-0.5% Nb, 0-0.5% V, 0-0.5% Zr, 0.0001-0.01% B, 0-0.01% Ca, ≤0.01% N, ≤0.01% O, and the balance Fe and in which the value of fn1 represented by equation I is regulated to >0. This steel billet is pierced, finish-rolled at ≥40% reduction of area at 800-1050°C, reheated at temp. T between 850 and 1100°C for time (t), while regulating the value of fn2 represented by equation II to 23500 to 26000, and then subjected to direct hardening and to tempering at a temp. not higher than the Ac1 point. By this method, development of an oil well of large depth is made possible.
KUSHIDA TAKAHIRO
KONDO KUNIO
