Slag Infiltration, Lubrication and Frictional Forces

Abstract

It is essential to lubricate the shell since inadequate lubrication leads to defects in the steel product (e.g. longitudinal cracks, sticker breakouts and star cracks). The liquid layer of the slag film, formed between the shell and the mould, lubricates the newly formed shell; the lubrication increases with increasing liquid slag thickness (dl). Lubrication is usually represented by the powder consumption (Qs in units of kg slag (or powder) m−2) which is related to liquid film thickness (dl). However, there are several terms used for powder consumption and these terms are interrelated (e.g. Qs, Qt and QMR). The frictional forces acting on the shell are highest in the centre of slabs and thus slabs need more lubrication. The required powder consumption, Qs increases with increasing distance from the corner and thus Q s slab > Q s bloom > Q s billet . The required powder consumption can be calculated from the relation, Q s req = 2/(R* – 5) where R* = {2(w + t)/w · t} = (surface area/volume) of the mould. However, the powder consumption, Qs, is also affected by other parameters, namely, the casting speed (Vc), slag viscosity (η), the break temperature of the slag and the oscillation frequency (f) and stroke (s). There is general agreement that Qs decreases with increasing casting speed and viscosity (e.g. empirical rules, Q sreq slag = 0.55/ η0.5 · Vc). There is some dispute with regard to the effect of f, s and Tbr but most plant studies indicate that Q s req slag decreases as f, s and Tbr increase. The required values of powder consumption and viscosity can be calculated for the given casting conditions using empirical rules. The predictions of a mathematical model indicate that slag infiltration into the model/ strand channel occurs when the mould and slag rim are descending but little powder consumption occurs when the mould is ascending. The changes in mould direction are accompanied by periods of confused flow in the mouth of the channel and little slag infiltration occurs in these periods. Frictional forces and the factors affecting them are also discussed; it was found that liquid friction increased with increasing mould dimensions, slag viscosity, casting speed and (Vm − Vc). Plots of liquid friction (Fl) versus casting speed exhibit a minimum since Fl increases with increasing Vc but decreases with decreasing (Vm − Vc).