Modeling of Transformation Behavior and Compositional Partitioning in TRIP Steel

Abstract

Transformation behavior and compositional partitioning in TRIP (Transformation Induced Plasticity) steel was investigated by means of microstructural observation and computer modeling. Studies were made on each of three stages of the continuous annealing process applied to TRIP steel. Ortho-equilibrium partitioning of alloying elements of Si and Mn was attained even in short intercritical annealing time. A transformation model, in which transformation is controlled by carbon diffusion, well described the volume fractional change of ferrite and pearlite during the cooling to austempering temperature. Slower cooling rates significantly increased carbon concentration enriched in untransformed austenite and caused pearlite transformation. Ultimate bainite volume fraction obtained by austempering increased with austempering temperature. Analysis with computer modeling revealed that transformation kinetics above 350°C followed a model based on the diffusional mechanism, while it complied with a model based on the displacive mechanism below 350°C.