2007 Volume 47 Issue 1 Pages 95-104
During continuous casting mould powder forms a pool of liquid flux which infiltrates into the solidifying shell/mould gap and forms a flux film containing liquid and/or solid layers. Subsequent crystallisation of the film results in the formation of an air gap at the mould wall. An air gap can also be formed by the shrinkage of the solidified shell. In practise, no air gap is formed by shell shrinkage in the upper mould since molten flux will flow immediately into and fill any gap formed. However, in the lower mould, if the shell temperature falls below the break temperature of the flux there is no liquid to fill any gap formed by shell shrinkage and hence an air gap can form. A model was developed which determines the effect of the various layers (including the air gap) on the horizontal heat transfer between shell and mould. A fully coupled, heat transfer/stress analysis was used to calculate: (i) the thickness of the various layers of the flux film; (ii) horizontal heat flux; and (iii) the resultant stress in the solidified shell.
The model predicted that: (a) the hoop stress increased as the interfacial thermal resistance decreases (i.e. the heat flux increases); (b) the maximum hoop stress at the exit decreased with increasing flux film thickness; (c) the ferro-static pressure tended to hinder the formation of an air gap at mid-face positions whereas in the corners the thicker shell tended to counteract the ferro-static pressure resulting in air gap formation; (d) the thickness of the liquid film at various positions in the mould was computed for fluxes with optimum break temperatures to determine if they provided liquid lubrication throughout the mould; and (e) the maximum stress occurred in the corner when casting with mould fluxes but at the mid-face when using oil.