On the interrelationship between oscillation marks, austenite grain size and transverse surface cracks in continuously cast steel slabs

Transverse surface cracks cause severe problems in the continuous casting of steel slabs. They seem to initiate. at the base of oscillation marks, they are inter-granular and run along prior austenite grain boundaries. The formation of oscillation marks, in turn, is associated with the vertical oscillation of the water-cooled copper mould that causes significant changes in the local heat transfer,fluid flow and solidification in the meniscus region. Laboratory studies on the hot-ductility of steels have shown that at strain rates similar to those in the caster,most steels exhibit a ductility trough with a minimum in ductility somewhere between 750 degrees C and 850 degrees C. The temperature at which the minimum occurs. generally coincides with the formation of weaker ferrite along austenite grain boundaries but precipitates that form on the austenite grain boundaries can exacerbate the loss in ductility. Very little work seems to have been done in assessing the influence on hot-ductility of the very large grains typically observed along transverse surface cracks and moreover,the influence of precipitation on hotductility has not been clearly distinguished from the grain size effect. For this reason,we have determined the hot-ductility of selected pure Fe-C alloys as a function of prior austenite grain size. In addition we have assessed the hot-ductility of the same Fe-C alloys melted in-situ in a Gleeble 3500 thermo-mechanical simulator. These results will be discussed and compared to hot-ductility tests conducted on a variety of commercially produced low-carbon and micro-alloyed steels. In all cases,an increase in prior austenite grain size leads to not only a reduction in hot-ductility but also to a broadening of the ductility trough.