Control of inclusions

Inclusions can be controlled either at (a) during liquid state processing stage or (b) during solid state processing.

Liquid state processing

During liquid state processing inclusion control can be exercised at tapping and teeming of steel.

i)

During tapping of molten stream from BOF/EAF carry- over of slag must be minimized if   not prevented. BOF slags are highly oxidizing in nature and contain oxides like         , CaO, MgO etc. These oxides react with Al during ladle treatment and lead to inclusion formation. Consider the reaction

 

Carry over of 1 kg FeO in slag decreases Al by 0.25 kg. which in turn forms 0..47kg . Assuming spherical shape inclusion of 1mm dia (in practice different diameter of inclusions can form) and density of  , number of inclusions are 224/kg carry-over  of slag. Thus carry-over of slag must be minimized by adopting slag free tapping technologies.

Another way for inclusion formations is reoxidation of tapping stream. Tapping stream exposes very large surface area in the atmosphere and hence oxygen pick up leads to oxide inclusion formation.

ii)  Molten steel stream after treatment in the ladle is teemed into tundish and then from     tundish to mould in the continuous casting.

Teeming of steel form ladle to tundish requires shrouding of molten steel stream in order to avoid reoxidation. Here macro-inclusions rich in and  can form. Moreover, dissolved oxygen increases and forms inclusion during solidification. Use of shrouded and submerged nozzles will help control inclusion formation. Different technologies for shrouding molten stream are: refractory tube shroud, circular ring shroud, etc. The interested reader may see the references at the end.