Empirical microstructure prediction method for combined intercritical annealing and bainitic transformation of TRIP steel

Abstract

AbstractAbstractThe processing of cold rolled intercritically annealed steels which exhibit transformation induced plasticity (TRIP) requires the precise determination of equilibrium parameters, the kinetics of the intercritical γ phase formation and the kinetics of the isothermal bainitic transformation. In addition, the aging phenomena associated with the bainitic transformation must be taken into account. A detailed analysis of the kinetics of the combination of both transformations carried out in succession has hitherto not been reported. The present contribution proposes an empirical method to study these transformations in low C intercritically annealed TRIP steels: a standard CMnSi TRIP steel and a CMnAlSi TRIP steel in which part of the Si is replaced by Al. The latter TRIP steel is more likely to be used in continuous galvanising lines. Dilatometry was used to determine the soaking time necessary to obtain the equilibrium phase distribution during the intercritical annealing. Furthermore, the transformation kinetics and the evolution of the C content in the retained austenite and the bainitic ferrite were evaluated. The results show clearly that the kinetics of the intercritical austenite decomposition during the bainitic transformation cannot be fitted to a single transformation mechanism owing to the formation of carbides. It is shown that during intercritical annealing local equilibrium conditions are achieved at the phase boundaries for the substitutional solutes. This results in an inhomogeneous γ phase composition, which was observed in the Al alloyed TRIP steels. In addition, it was found that whereas equilibrium thermodynamic calculations can be used to predict phase boundaries reliably, the C content of the retained austenite was much larger than the calculated C content based on the free energy of ferrite=free energy of austenite condition.