Freckle defects were found in electro-slag remelting (ESR) ingots, which severely limitthe development of the larger-diameter ingots. The macrostructure and the composition ofthe freckles in a low-alloy structure steel ESR ingot were investigated to clarify thefreckle formation mechanism. The results shows that the freckles in the ingot were causedby the upward solute-rich liquid flow in the mushy zone, and the composition of thefreckles corresponds to that of the liquid, with a liquid fraction of 0.36 to 0.43. Therelative Rayleigh number (Ra), a freckle criterion considering the effect of atilted solidification front, was adopted to evaluate the freckle formation tendency in theindustrial-scale ESR ingots based on the results of the experiments, kinetic simulationand thermophysical property calculation. The calculated results of Ra are in good agreementwith the actual distribution of the freckles in the ingot.

A problem of thermal instability of a compressible micropolar fluid layer heated from below in the presence of suspended particles has been investigated. Dispersion relation is derived and Rayleigh number curve is then plotted against the wavenumber at different values of compressibility parameter for a model example. Compressibility is found to be responsible to destabilize the system in the presence and absence of suspended particles for both stationary and over stationary convection.