MATHEMATICAL-MODELING OF TRANSFORMATION IN NB MICROALLOYED STEELS

Thermodynamic and kinetic modelling was performed to describe the decomposition of austenite and the precipitation of alloy carbides in Nb microalloyed steels. Phase analysis of the Fe-C-Mn and the Fe-C-Nb systems was carried out considering local equilibrium and paraequilibrium, their nucleation behavior (matrix/grain boundary) and the shear energy during transformation. From the analysis, equilibrium transformation temperatures of each phase, the Gibbs energy changes for nucleation and compositions of elements at the phase boundary were derived. The transformation kinetics were formulated using classical nucleation and growth theories. Both local equilibrium and para-equilibrium conditions were considered to evaluate the partitioning of Mn during growth. The calculated TTT, CCT diagrams and the volume fraction of each phase were compared with the experimental results, and the most probable behaviors of gamma/alpha transformation and NbC precipitation in Nb steels were discussed.