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The kinetics of austenite-to-ferrite diffusional transformation in a medium carbon Fe-C-Mn steel was calculated based on classical nucleation and growth theory coupled with CALPHAD multi-component thermodynamics. The description of the growth rate of proeutectoid ferrite includes a time-dependence due to the carbon enrichment in the remaining austenite. The experimental slower kinetics, especially a stagnating behavior at the later stage, has been successfully reproduced when a transition from initial paraequilibrium (PE) to local equilibrium negligible partition (LENP) conditions at austenite:ferrite (γ/α) interface was assumed. This transition is allowed when the velocity of moving γ/α interface is slow enough to be compared with Mn diffusivity, which leads to built up a Mn spike in the interface. This assumption is consistent with a series of scanning transmission electron microscopy (STEM) analyses for Mn and C which indicates that initial unpartitioned Mn ferrite growth is replaced by partitioned growth, financial support of Japan Science and Technology Agency (JST). The authors gratefully acknowledge the support of the Spanish Ministry of Science and Innovation for funding this research under the contract MAT2007 – 63873. J. Cornide acknowledges the Spanish Ministerio de Ciencia e Inovación for financial support in the form of PhD research grant (FPI)

The successive ferrite ( ) + pearlite (P) transformations from austenite ( ) were modeled to predict the presence of bainite in as-forged medium-carbon manganese steels. The kinetics of diffusional transformations were calculated based on classical nucleation and growth theory coupled with CALPHAD multi-component thermodynamics. The description of the growth rate of proeutectoid- includes a time dependence due to the carbon enrichment in the remaining . The / interface was assumed to be in negligible-partitioned local equilibrium (NPLE). The kinetics calculation of P nucleating on the surface was integrated into the model. Given the transformation temperature range in continuous cooling, the growth rate of P was also expressed in the NPLE constraint for /cementite. The concentration of untransformed ( U) can be monitored and should be dependent on the extent of the preceding transformations. Thus, the energies available for the nucleation and diffusionless growth of bainitic- were evaluated from the thermodynamics of the U single-phase system, which is proposed as a method to predict the inclusion of bainite in the final þ P microstructure, financial support of Japan Science and Technology Agency (JST)