A fast three-dimensional phase transformation model is formulated for the transformation from ferrite to austenite in low-carbon steel. The model addresses the parent microstructure, the nucleation behaviour of the new phase and the growth of the new phase. During the growth, the interface velocity of the ferrite grains is calculated using a mixed-mode growth model. The simulated transformation kinetics is compared with experimental kinetics for an Fe-C-Mn steel for four different cooling rates. In general, the model predicts the kinetics quite well. In addition, the mixed-mode character of the transformation is shown for the different cooling rates.