Steel Deoxidation on Tapping

Deoxidation of steel, also known as killing of steel, occurs toward the end of the steelmaking process, but before desulphurization. After primary steelmaking, for example in a basic oxygen furnace (BOF) or electric arc furnace (EAF), the liquid steel contains a large amount of dissolved oxygen [O] (400 to over 1000 ppm.) This oxygen must be removed from the liquid steel for several reasons, including:

• Desulphurization is not effective with high oxygen content in the steel 
• Continuous casting of un-killed steel is not possible and will result in “boiling” of the steel in the mould and massive porosity, as shown in the figure.

Blowholes formed during casting of a steel billet with insufficient deoxidation.

The two most widely applied methods of deoxidizing or killing the steel are adding Al to the steel (Al-killed steel) and adding Si to the steel (Si-killed steel). In this example we look at how the Process Metallurgy Module in Thermo-Calc can be used to investigate these two widely applied methods of deoxidizing or killing the steel.

This, however, can result in the formation of damaging inclusions that cause problems during further processing, so we go on to look at two simple and very common processes that are used to transform the solid SiO₂ and Al₂O₃ inclusions into liquid oxides so that they are less damaging.