Application of solute drag theory to model ferrite formation in multiphase steels

In novel multiphase steels for automotive applications, alloying elements are usually employed to control the austenite-ferrite transformation, in order to produce microstructures with an excellent combination of strength and formability. A revised austenite-to-ferrite transformation model for low-carbon steels is proposed which is applicable to industrial heat-treatment conditions of commercial steels. In the model, the effect of alloying elements on the transformation kinetics is described from a fundamental point of view. In the framework of the mixed-mode model in which carbon diffusion in the remaining austenite is coupled to the interface reaction, the partitioning and drag effect of the solute elements are explicitly accounted for. The thermodynamic driving pressure is calculated assuming paraequilibrium conditions, and the solute drag theory of Purdy and Brechet has been modified to remove the artifact of residual solute drag at zero interface velocity. This rather complex model employs, similarly to the semiempirical Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach, four adjustable parameters. However, these parameters are now clearly defined in terms of their physics; i.e., they are pertinent to the interface mobility and solute-interface interaction. The model has been validated with experimental data for a C-Mn steel and two multiphase steels containing either Mo or Si as an additional alloying element. The physical relevance of the resulting solute drag parameters and the inherent challenges regarding their selection are discussed.