Simultaneous formation of inter- and intragranular ferrite in microalloyed steels studied by modeling and experiment

In the present study, the simultaneous formation of inter-and intragranular ferrite in microalloyed steels under continuous casting condition has been studied by modeling and experiment. Through adapting the classic Johnson-Mehl-Avrami theory, a model for predicting the volume fraction of ferrite nucleated from different sites has been developed, in which the transition of kinetic regimes in ferrite growth is introduced. Concerning the ferrite volume fraction and the half-thickness of film-like ferrite, a series of thermal expansion tests by Gleeble-3500 and in-situ observation by confocal laser scanning high-temperature microscope are carried out to validate the accuracy of model. In both two microalloyed steels, for which six cooling processes have been investigated, the calculation agrees with the measurement excellently when cooling rate is below 1 degrees C s(-1). Hence, the model is manifested to be appropriate for predicting the ferrite formation behavior in microalloyed steels during continuous casting process.