Improvement of the desulphurisation process by slag composition control in the ladle furnace

Abstract: The cleanliness of steel with respect to non-metallic inclusions and the precise alloy compositions in the steel products have always been of great concern in steel making technology. The development of steel making process to meet the compositional requirements for specific mechanical properties
such as ductility, toughness, fatigue and machinability requires dynamic and continuous investigations.
The refining of molten steel in the ladle furnace to meet the required compositional range requires the optimisation of the process parameters. For sulphur removal control, parameters such as argon gas flow rate through the porous plugs, inductive stirring effect, vacuum pressure of the tank degasser, amount & composition of the top slag should be optimised.
In this thesis project an investigation was carried out on the factors that influence the top slag composition before vacuum treatment and also to optimise the top slag composition for precise sulphur removal. 12 heats were followed during the project: slag samples, steel samples, temperature and oxygen activities were taken at eight different process stages at Ovako steel mill. A relatively large variation was observed for all the oxide components of the slag phase before vacuum treatment in all the heats followed. A PLS analysis made shows that topslag composition before degassing is influenced by the amount of slag former added, oxygen potential at tapping, the yield of Al and Si deoxidants into the steel at tapping. The model has a poor predictability because some important parameters such as ladle glaze condition, amount of EAF slag tapped and refractory wear could not be measured.
An alternative solution of extra slag practice was suggested instead of modelling the composition and mass of carry over slag left after slag removal. The extra slag practice involves the addition of lime during tapping so as to aid the removal of all the slag before ladle refining and thus optimisation of the new synthetic slag for precise sulphur removal could be easily achieved. Finally the investigation of the desulphurisation process shows that degassing time, argon gas flow rate through the porous plugs are as well important as the slag mass and composition in order to achieve a precise sulphur removal.