CONTINUOUS COOLING TRANSFORMATION BEHAVIOR OF HIGH-STRENGTH MICROALLOYED STEELS FOR LINEPIPE APPLICATIONS

Continuous cooling transformation (CCT) behaviour of high strength mic roalloyed steels containing two different levels of Mn+Si additions is investigated in undeformed and thermomechanically processed condition s using quench and deformation dilatometry respectively. The deformati on schedule used in the dilatometer is designed to simulate the indust rial controlled rolling procedures for the production of plates as clo sely as possible in laboratory. CCT diagrams for the undeformed and th ermomechanically processed steels are constructed. Effects of thermome chanical processing (TMP), accelerated cooling and composition (Mn+Si levels) on gamma transformation start temperature (Ar-3), phase transf ormation kinetics, CCT diagrams and microhardness are investigated. Th e results show that TMP accelerates the onset of gamma/alpha transform ation (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 accelerate s the progress of transformation. The steel with a higher level of MnSi addition (1.96%) exhibits lower Ar-3, sluggish transformation kinet ics and higher hardnesses in undeformed and thermomechanically process ed conditions as compared with the steel with a lower level of Mn+Si a ddition (1.17%). These effects are explained in terms of the effects o f Mn and Si contents on the carbon partitioning and the subsequent pha se transformation behaviour of these steels during continuous cooling. Increase in cooling rate increases the microhardnesses of both steels while TMP lowers them.