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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)

A theoretical model for the presence of bainite (B) in ferrite ( ) + pearlite (P) microstructures was validated experimentally for commercial-grade medium-carbon manganese steels. The energies concerning the nucleation and growth of bainitic- were used as the criteria for the B formation during the þ P transformations, which has been first applied to the changing composition of untransformed austenite ( U). Three steels were chosen to study the effect of Mn content on the B fraction at various cooling rates. To account for local variation of austenite grain size (d0), a log-normal distribution was employed. In this model, C enrichment in U proceeds faster in smaller grains, and the shift to P or B transformation occurs in a specific temperature range. Calculations predicted the onset of B at around 603 K, with a fraction that increases with increasing cooling rate. In general, the agreement was good for all steels in terms of both calculated/observed kinetics and final =P=B fractions. Thus, the model provides a practical prediction of the critical cooling rate in order to avoid B in as-forged product, financial support of the Japan Science and Technology Agency (JST). K. Tanaka would like to thank the members of the Phase Transformation Group in CENIM