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 1600°C, this point is marked on the
diagram.
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.