Continuous cooling transformation behaviour of high strength microalloyed steels for linepipe applications

Continuous cooling transformation (CCT) behaviour of high strength microalloyed steels containing two different levels of Mn+Si additions is investigated in undeformed and thermomechanically processed conditions using quench and deformation dilatometry respectively. The deformation schedule used in the dilatometer is designed to simulate the industrial controlled rolling procedures for the production of plates as closely as possible in laboratory. CCT diagrams for the undeformed and thermomechanically processed steels are constructed. Effects of thermomechanical processing (TMP), accelerated cooling and composition (Mn+Si levels) on gamma transformation start temperature (Ar-3), phase transformation kinetics, CCT diagrams and microhardness are investigated. The results show that TMP accelerates the onset of gamma/alpha transformation (Ar-3 is raised), but the progress of gamma/alpha transformation is retarded considerably in deformed samples. Significant retardation is observed during the final 30% of the phase transformation reaction. Increase in cooling rate lowers the Ar, significantly and accelerates the progress of transformation. The steel with a higher level of Mn+Si addition (1.96%) exhibits lower Ar-3, sluggish transformation kinetics and higher hardnesses in undeformed and thermomechanically processed conditions as compared with the steel with a lower level of Mn+Si addition (1.17%). These effects are explained in terms of the effects of Mn and Si contents on the carbon partitioning and the subsequent phase transformation behaviour of these steels during continuous cooling. Increase in cooling rate increases the microhardnesses of both steels while TMP lowers them.