Deoxidation of Al- and Si-killed steel

Abstract: To keep up with the tough competition today and to be able to offer the
customers competitive prices, it is desirable to shorten the ladle treatment
and still keep a high quality of the product. To be able to shorten the
ladle treatment under these restrictions, faster deoxidation process is
demanded. This master’s thesis investigates what influences the reaction
coefficient, which describes the separation rate of oxygen. A higher
reaction coefficient indicates a fast separation rate. Special attention has
been taken to the factor reoxidation, with regard to the nitrogen pick up
after tapping and an increase of the oxygen content after degassing.

The reaction coefficient has been calculated for four steel groups, two
silicon deoxidised groups, one silicon and aluminium deoxidised group and
one aluminium deoxidised steel group. The apparent deoxidation is in the
reality the sum of two phenomena: deoxidation and reoxidation. An increase
of the deoxidation rate or a decrease of the reoxidation rate would result
in a higher k-value. A small increase of the reaction coefficient could gain
minutes in the process of oxygen removal, which would result in economical
benefits.

The deoxidisers have naturally a strong effect on the oxygen content and
thus on the reaction coefficient. The fastest separation – highest reaction
coefficient – after tapping is found for steel groups deoxidised with the
strongest deoxidiser: aluminium.

Reoxidation occurs after tapping/before degassing due to the breakthrough
eye caused by the strong gas injection. By using nitrogen as a tracer for
reoxdiation from the atmosphere it was possible to determine if reoxidation
had an effect on the deoxidation rate, which it had since the reaction
coefficient decreases as the nitrogen content increases. Stirring makes
deoxidation quicker, but on the other hand, too intensive stirring retards
the deoxidation rate somewhat due to reoxidation. An optimisation of the gas
flow rate might be useful to get an optimum relation between deoxidation and
reoxdiation, where the fastest separation of oxides is achieved.

The deoxidisers have an effect on the oxygen content after degassing and in
the tundish. Reoxidation can occur during these time steps too, but most
probably not due to reactions with the atmosphere, since the nitrogen
content is stable. The oxygen content increases for the silicon deoxidised
heats after degassing due to a higher iron oxide content in the top slag.
Thus, the FeO content in the slag is too high to keep the low oxygen content
in the steel stable. The heats with higher amount of deoxidisers in the
steel tend to be able to stay at a lower oxygen concentration too. Though,
in the later parts of the process additional alumina formation should be
avoided for the best separation.