Using confocal scanning laser microscopy to characterise as-cast microstructures using cooling rates representative of thin slab direct cast steels

To meet the increasing demand for better energy efficiency and higher production rates, a range of accelerated cooled continuous casting methods are being used. Replicating the solidification conditions in the laboratory is challenging but essential if issues such as segregation and microstructure development are to be understood. In this paper a method for analysing an as-cast structure both in- and ex-situ at cooling rates of 1 and 50 °C/s, using a high temperature confocal scanning laser microscope, has been detailed. The method has been developed to obtain directional dendritic solidification in an orientation that allows for solute rejection 360° around the dendrite arms, replicating that seen in bulk casting. Experiments using an Fe-Al-0.1Ti alloy at rates of 1 and 50 °C/s from 1540°C are discussed. A significant retarding effect, due to segregation, on the liquid-solid transformation kinetics was observed for the 50 °C/s sample. Thermodynamic predictions, based on the Lever-Rule (ThermoCalc) and back diffusion via the Clyne-Kurz model, have been used to predict the levels of segregation. The higher segregation expected at the faster cooling rate results in the formation of TiC in the liquid after approximately 80% solidification, whilst at the slower cooling rate TiC is only predicted to form in the liquid after approximately 93% solidification. The predictions have been supported with optical analysis of the resultant TiC precipitate shape, distribution and morpholog