DEVELOPMENT OF HIGH STRENGTH AND HIGH TOUGHNESS SEAMLESS STEEL LlNE PIPES

The paper summarizes the development of the X60 and X65 seamless steel pipes for pipeline purpose,describes the determination of the steel chemical composition and the processes of on-line normalization and controlled-cooling for the pipe and discusess toughening mechanism of the steels. Relevant pilot commercial production has proven that with steel containing O. 16%C,l. 45%Mn, 0. 05%Nb and 0. 0l NV and the on-line normalization treatment, the X60 seamless steel tube can be obtained. Based on the said chemical complsition,the X65 pipe can also be made by means of addition of Cr in a small ammount,properly increasing Si content,prlperly controlling of N content and adopting on-line normalization treatment and controlled-cooling processes. Relevant operation practice has proven that the chemical composition selected and the processes em-ployed are adequate.