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  Oxygen control     5 of 11

The steel which is tapped from a steelmaking converter has a major impact on the ladle inclusion population; at tap the steel has a very high dissolved oxygen content. This oxygen must be removed before the steel is cast, otherwise carbon monoxide gas would be formed during solidification causing excessive porosity. To remove this oxygen sacrificial alloys are added to the molten steel during converter tapping; these additions react with the oxygen in the steel to form oxides, acting to reduce the 'free' oxygen content in the steel, these generally form solid/liquid oxides which remain in the steel as inclusions or float to the surface of the steel to form a slag layer (oxide inclusions are less dense than steel).

The most appropriate choice of alloying additions are those which will satisfy the final steel aim chemistry, and also react strongly with dissolved oxygen. In order to determine which additions are appropriate to de-oxidise the steel we need to examine the thermodynamic equations for the reaction between oxygen and the addition. For example, for the reaction between dissolved carbon and dissolved oxygen the equation is:

[O]dissolved + [C]dissolved

[CO]dissolved  DG = -550 kJ mol-1
[CO]dissolved CO

where DG is the Gibbs free energy of the reaction. The greater the negative value of DG the more favourable the reaction. Therefore an effective de-oxidation addition will have a DG value more negative than that of the carbon and oxygen reaction.

Below we have a slightly simplified Ellingham diagram which shows the Gibbs free energy for various oxidation reactions over a range of temperatures. The steel making process occurs at approximately 1600C, this point is marked on the diagram.

Have a go!Investigate the value of the Gibbs free energy for reaction with oxygen through the position of the reaction line for various possible additions that could be made to molten steel to provide de-oxidation.


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