Preparation of heat-treated steel pipe with excellent low temperature toughness

PURPOSE: To obtain a heat-treated steel pipe with excellent low temperature toughness by a method wherein a steel pipe material comprising a specific component composition such as C, Si or the like is subjected to a molding process and, thereafter, hardening is twice repeated under a specific temperature condition and, moreover, tempering is carried out at a specific temperature. CONSTITUTION: After a steel plate material containing C; below 0.3%, Si; 0.02W 1%, Mn; 0.3W2%, P; below 0.03%, S; below 0.01% and, according to necessity, containing one kind or two or more kinds of Al; below 0.9%, Mo; below 0.6%, Cr; below 1%, Nb; below 0.15%, V; below 0.15%, Ti; below 0.15% or B; below 0.01% is subjected to a molding process, the obtained molded steel material is heated to above AC 3 -30°C by an induction heating system and cooled by water, thereafter heated again betweeen AC 1 and AC 3 at a speed of below 180sec stay time and cooled by water. Moreover, tempering is carried out at above 550°C and below AC 1 temperature to obtain a normalized steel plate of which ductility- brittleness transition temperature is remarkably low suitable for pipe arrangement in a cold area. COPYRIGHT: (C)1980,JPO&Japio