A calculation model for predicting the progress of transformations of low carbon steels during continuous cooling and the microstructure after cooling has been developed. In this model, the progress of tranformations is assumed as follows. The transformation from austenite to ferrite starts at the equilibrium temperature, Ae3. The ferrite transformation follows 'nucleation and growth' model at the early stage and then follows 'site saturation' model at the later stage. The carbon content of the untransformed austenite increases with the progress of ferrite transformation. The pearlite transformation starts when the carbon content of the austenite reaches the extrapolated Acm line in the equilibrium diagram. The bainite transformation starts when the temperature reaches the bainite-start temperature, Bs, which is experimentally determined. Besides, the effect of chemical composition on the transformation kinetics are taken into consideration by using the theoretically calculated nucleation rate and growth rate.
Excellent agreements are obtained between the calculated and experimental results for the effect of compositions (C, Mn, Si) on the transformation kinetics, the continuous cooling transformation diagrams and the microstructure after cooling.
