Converter carryover slag and synthetic slag additions are two elements of the ladle steelmaking system that can be considered together, as they are the principal components which go to make up the 'Ladle Top Slag'. The ladle top slag floats on the surface of the steel in the ladle and has a profound impact on the inclusion population in the steel.

Converter carryover slag - during primary steel refining in the BOS / EAF converter, slag-making additions are made to form a slag which absorbs impurities from the steel (such as sulphur and phosphorus). During converter tapping, in addition to the crude steel tapped into the ladle, some converter slag is also carried over into the steelmaking ladle, hence the name 'carryover slag'. Since slag contains impurities from primary refining, it would be very desirable to have zero slag carryover, as these impurities can revert to the steel in the ladle. However, some carryover is inevitable. Variable levels of carryover slag also cause undesirable variation in the ladle top slag composition, and so it is important to control both the consistency and level of slag carryover.

Synthetic Slag Additions - form the bulk of the ladle top slag (normally about 1 tonne). These additions are made to achieve an aim slag chemistry appropriate to the steel type being made and its manufacturing process route. Most synthetic slags are lime (CaO) based, but a wide range of compositions are employed to satisfy different metallurgical and operational needs. Roles of synthetic slag include inclusion engineering, desulphurisation and heat transfer during ladle re-heating.

The ladle top slag chemically interacts and is in equilibrium with the steel. In certain steels, the ladle top slag closely reflects steel inclusion population composition; thus by manipulating the ladle top slag composition, the inclusion composition can be controlled to a desirable chemistry. The slag, which is usually liquid also acts as a capture surface for inclusions which float out of the steel, and therefore promotes steel cleaning.

Poor slag control can also lead to poor steel cleanness. For example, in a steel alloyed with aluminium (Al 'killed'), if the ladle top slag is highly oxidised (high levels of FeO and MnO), then aluminium in the steel can react with 'oxygen' in the slag to form hard alumina inclusions in the steel which are detrimental to product properties.