The role of slag in steelmaking:

• It acts as a sink for impurities during refining of steel
• It controls oxidizing and reducing potential during refining through FeO content. Higher FeO makes the slag oxidizing and lower FeO reducing
• It prevents passage of nitrogen and hydrogen from atmosphere to the molten steel
• It absorbs oxide/sulphide inclusions
• It acts as a thermal barrier to prevent heat transfer from molten steel to the surrounding.
• It protects steel against re-oxidation
• It emulsifies hot metal and promotes carbon oxidation.
• In electric steelmaking slag prevents the radiation of heat of arc to the walls of the furnace and roof

The above functions require that slag should possess certain physical (density, melting point, viscosity) and chemical properties (basicity, oxidation potential). Both physical and chemical properties are controlled by composition and structure of slag. In steelmaking slag is predominantly a mixture of oxides with small amounts of sulphides and phosphides. The oxides are either acidic or basic in nature. We will first consider the structure of pure oxides and then we discuss what happens on addition of one type of oxide to the other.