Post-mortem Ladle Shroud Analysis from the Casting of Al-Killed Steel: Microstructures and Origin of Alumina Clogging Deposits

This work presents post-mortem microstructural analysis on the inner surfaces of single-use ladle shrouds of a ten-heat casting series of Ti-alloyed, Al-killed IF steel, in continuation of earlier work on the origin of alumina clogging formation in the casting system. It is found that ladle shrouds can be used as a representative sampler of materials exiting the ladle during the casting, which can be used to extract useful information about processes taking place in the shrouds and lower ladle during casting. All shrouds showed no ladle glaze emission but significant clog-like deposition of alumina. The alumina particles occur in two main forms (NMI embedded in the steel and alumina powder) and show the same two PDF populations (small Population I and coarse Population II) with the cross over at 5 µm, as the alumina particles found in the tundish clog of the same cast series. Detailed characterisation of inclusions in the shrouds revealed metal sub-inclusions containing Cr and Si, also similar to the tundish clogs. Thus, the hypothesis that the coarser population originates from the local oxidation of the steel in ladle bottom interface and ladle well region, including the well filler sand, is evidenced in this study. The fine Population I (often associated with dendritic and planar shapes) seen in the tundish clog, appears to be related to local carbothermic reaction in the ladle shrouds. Microstructures found on the surfaces on the shrouds are extensively shown and characterised.