Abstract: A process for producing a seamless steel pipe wherein pipe manufacturing steps and the heat treatment steps are carried out in one production line. The properties of the pipe are comparative or superior to those of the pipe manufactured in the conventional reheating, quenching and tempering process. The process is characterized by using the billet of a low alloy steel containing C: 0.15-0.50%, Cr: 0.1-1.5%, Mo: 0.1-1.5%, Al: 0.005-0.50%, Ti: 0.005-0.50% and Nb: 0.003-0.50%, and comprising the following steps (1) to (5).(1) hot rolling with 40% or more of cross sectional reduction ratio,(2) finishing the hot rolling in a temperature range of 800-1100.degree. C.,(3) putting the manufactured steel pipe promptly in a complementary heating apparatus after the finish rolling, and complementarily heating at the temperature and time satisfying the following formula (a).

Abstract: A tandem mill has a plurality of stands and has at each stand one mill suitably selected from a common four-high mill, a sleeve sliding mill, an intermediate back-up roll shifting mill, and a sleeve expanding mill.