Introduction

Refining of hot metal to steel is done under oxidizing atmosphere. During refining oxygen dissolves in steel. Solubility of oxygen in steel is negligibly small. During solidification of molten steel, excess oxygen is rejected by the solidifying steel. This excess oxygen produces defects like blow holes and non- metallic oxide inclusion in solidified casting. Defects have considerable effect on mechanical properties of steel.

Therefore, it is necessary to remove oxygen from steel; removal of oxygen is called deoxidation.

Sources of oxygen in steel

• Rust on steel
• Oxygen blowing
• Steelmaking slag
• Atmospheric oxygen dissolved in steel during teeming
• Oxidizing refractories

At  solubility of oxygen in liquid steel is  which deceases to  in solid steel during solidification. According to the degree of deoxidation, carbon steels may be subdivided into three groups:

i)    Killed steel: Oxygen is removed completely. Solidification of such steels does not give       gas porosity (blow holes).

ii)   Semi –killed steel: Incompletely deoxidized steels containing some amount of oxygen        which froms  during solidification.

iii)   Rimming steel: Partially deoxidized or non- deoxidized low carbon steels evolving               sufficient  during solidification. These steels have good surface finish.