Study of the thermomechanical behavior of API 5L X80 steel micro-alloyed with Nb-Ti through hot torsion tests

Abstract

The controlled rolling of high strength low alloy steel has been widely used in the production of large diameter pipes for the oil and gas industry. The thermo-mechanical control process (TMCP) is used to maximize grain refinement and achieve both higher strength and toughness in the steels. In this sense, API 5L X80 micro-alloyed Nb-Ti steel was submitted to hot torsion tests to evaluate its hot deformation behavior. The critical temperatures (Tnr, Ar3 and Ar1) were obtained by testing with multiple strains in continuous cooling. From the flow stress curves generated in the isothermal tests, it was possible to obtain a critical strain that starts dynamic recrystallization, the peak strain for temperatures ranging from 1150oC to 850oC, and the strain rates at 0.2; 0.4 and 0.8s-1. The results showed that peak and critical stress values increase as the temperature decreases and strain rate increases.