Deoxidation of Al- and Si-killed steel

In this project report we will talk about ladle treatment, deoxidation procedure, reoxidation, etc. To compete in today’s cut throat competition and to be capable of providing the customers competitive prices, it is desirable to shorten the ladle treatment and still maintain a superior quality of the product. In order to reduce the ladle treatment under these restrictions, quicker deoxidation procedure is demanded. This dissertation looks into exactly what impacts the reaction coefficient, which describes the separation rate of oxygen. A greater reaction coefficient implies a fast separation rate. Special attention has been taken to the factor reoxidation, with respect to the nitrogen pick up after tapping and a rise of the oxygen content after degassing.

The reaction coefficient is computed for 4 steel groups, 2 silicon deoxidised groups, 1 silicon and aluminium deoxidised group and 1 aluminium deoxidised steel group. The apparent deoxidation is in the reality the sum of 2 phenomena: deoxidation and reoxidation. A rise of the deoxidation rate or a reduction in the reoxidation rate would lead to a higher k-value. A little rise of the reaction coefficient can gain minutes in the procedure of oxygen removal that will bring about economical advantages.

The deoxidisers have normally a substantial influence on the oxygen content and hence on the reaction coefficient. The quickest separation – highest reaction coefficient – after tapping is found for steel groups deoxidised with the strongest deoxidiser: aluminium.

Reoxidation takes place after tapping/before degassing because of the discovery eye attributable to the strong gas injection. Through the use of nitrogen as a tracer for reoxdiation from the atmosphere it had been possible to assess if reoxidation had an impact on the deoxidation rate, which it had considering that the reaction coefficient decreases as the nitrogen content increases. Stirring makes deoxidation faster, but in contrast, too intensive stirring retards the deoxidation rate slightly due to reoxidation. An optimization of the gas flow rate could be helpful to get an optimum relation between deoxidation and reoxdiation, where the fastest separation of oxides is accomplished.

The deoxidisers have an impact on the oxygen content after degassing and in the tundish. Reoxidation can happen during these time steps too, but most likely not because of reactions with the atmosphere, because the nitrogen content is steady. The oxygen content increases for the silicon deoxidised heats after degassing as a result of higher iron oxide content in the top slag. Therefore, the FeO content in the slag is too high to maintain the low oxygen content in the steel stable. The heats with higher amount of deoxidisers in the steel tend to be able to remain at a lower oxygen concentration too. Though, in the later parts of the procedure additional alumina formation needs to be avoided for the best separation.

Source: Luleå University of Technology

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