Prevention of longitudinal cracks in a continuously cast slab of Fe-Cr-Ni superalloy containing Al and Ti

The continuous casting of UNS N08811 containing Al and Ti has been established by improving the start flux. In initial trials serious longitudinal cracking appeared during the earlier stages of casting. Investigation revealed that the reaction of Al in the alloy with SiO2 in the flux during casting, raised the Al2O3 content in the film after casting to 28 wt-% which is substantially higher than the 12 wt-% measured in the steady state. In the film, the major portion contained 28 wt-% Al2O3 and a thin layer with even lower Al2O3 content existed at the shell-side. The higher Al2O3 portion could correspond to the initial infiltration into the gap between the solidified shell and the mould. On the other hand, the layer with lower Al2O3 content could correspond to steady state infiltration. Initial infiltration, therefore, was estimated to be obstructed, resulting in an uneven film, because of its more viscous and glassier properties. A start flux designed with lower viscosity and higher heat generation helped to ensure even infiltration and this was shown to prevent the cracking.