Pilot plant study of nozzle clogging mechanisms during casting of REM treated stainless steels

A pilot scale bottom teemed high frequency furnace with a nominal capacity of 600 kg and an adjustable nozzle temperature was used to study clogging of rare earth metal (REM) treated stainless steels. The influence of the following variables on the clogging was studied: amount of REM, fraction of oxide clusters, total oxygen content in the steel, reoxidation, aluminium additions before REM additions and silicon additions during casting. Overall, the results show that, during the present experimental conditions, mainly two typical clogging rates could be identified, fast and slow clogging. Two main differences could be detected between the fast and slow clogging rates. Steel containing mainly small single inclusions clogged faster than steel containing mainly large inclusion clusters. The reason was believed to be that the small inclusions could stick to the nozzle wall at narrow passages where the steel flow velocity was high while the larger ones could not. The source of the small inclusions was believed to be reoxidation. It was also found that the main part of the nozzle accretion was caused by agglomeration of inclusions and inclusion clusters, while only a thin oxide film along the nozzle wall was caused by oxide precipitation at the nozzle wall.