Modelling of kinetics of isothermal allotriomorphic and idiomorphic ferrite formation in medium carbon vanadium-titanium microalloyed steel

A theoretical model is presented in this work to calculate the evolution of austenite-to-allotriomorphic and idiomorphic ferrite transformation as a function of time in medium carbon vanadium-titanium microalloyed steel. At temperatures above the eutectoid temperature where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modelling. By contrast, at temperatures below the eutectoid temperature, the formation of pearlite avoids the carbon enrichment of austenite due to the previous ferrite formation, and therefore carbon concentration in austenite far from the alpha/gamma interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic and idiomorphic ferrite is considered to grow under a parabolic law. The nature, size and distribution of the inclusions which are responsible of the intragranular nucleation of idiomorphic ferrite have been considered in this study. An excellent agreement has been obtained between experimental and predicted values of volume fraction of ferrite in all the studied range of temperature.