Oxygen is introduced into the molten metal, for example via a lance, to rid steel of excess carbon and other impurities. It forms oxides (oxygen compounds) with the impurities which are released as gases or remain in the slag as solids. After cleaning (blowing), however, the steel contains too much oxygen, which makes it brittle. The oxygen content now needs to be reduced, using a process called "deoxidation". The most effective and most widely used "deoxidising agent" is aluminium thanks to its ability to oxidise readily at high temperatures to form aluminium oxide by removing oxygen from the other substances (this property is also made use of in aluminothermics).
In practice, deoxidation is usually performed using aluminium pellets (see powder). They are introduced into the steel melt in various ways, for example by stirring them in. For each tonne of steel, 0.4 to 1.0 kilograms of aluminium is required (thus the steel industry uses thousands of tonnes a year). The oxidation product, aluminium oxide, can block openings during casting if the melt is not stirred correctly; in solidified steel, aluminium oxide is distributed in the form of extremely fine inclusions.
Other applications of aluminium in steel production:
- Alloying with 0.5 to 2.5 per cent of aluminium makes steel particularly resistant to heat and non-scaling (important for furnace construction).
- Coating the steel with aluminium (for example hot-dip aluminising) improves corrosion resistance.
- Nickel-cobalt steel with five to 15 per cent aluminium produces Alnico steels, which are very strong permanent magnets.