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, 0.1-1.5% Si, <0.1% Mn, ≤0.05% P, ≤0.01% S, 0.1-1.5% 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, rolled at ≥40% reduction of area at 800-1050°C finish rolling temp., reheated at temp. T between 850 and 1100°C for time (t), and, after the value of fn2 represented by equation II is regulated to 23500-26000, subjected, without delay, to direct hardening and then to tempering at a temp. not higher than the Ac1 point. By this method, the development cost for an oil well can be reduced.
KUSHIDA TAKAHIRO
KONDO KUNIO
