The gamma-alpha transformation kinetics of low carbon steels under ultra-fast cooling conditions

Current evolutions in the processing techniques of hot rolled steel sheet, like the development of an ultra fast cooling unit which cools the material after the final rolling step, stretch the domain of the material models us ed to control these processes. When employing a so-called ultra fast coolin g unit an accurate description of the transformation kinetics is needed at cooling rates that are significantly higher than the range in which most au stenite to ferrite transformation models have ever been tested. In this pap er a physically based transformation model is applied to a set of dilatomet er experiments involving four different commercial steel grades and cooling rates from 20 K/s to 600 K/s. The model describes the transformation by me ans of moving gamma-alpha interfaces in a single austenite grain. Parameter s that were varied in the modelling procedure are the degree of undercoolin g below the A3-temperature, AT, at which the transformation is assumed to s tart, and the intrinsic interface mobility pre-factor, M-0 By analysing the errors in the fit of the calculated transformation behaviour to the experi mental data, ranges of optimum fit in the DeltaT-M-0 space were determined. In order-to determine a unique combination of DeltaT and M-0 a physically justified value of DeltaT was determined using the classical nucleation the ory. The resulting values of Mo increase with increasing cooling rate and d ecrease with increasing carbon content.