Operational data of BOF and the slag samples for different starting conditions of phosphorus (0 . 06-0 . 26%P) and silicon content (0 . 3-1 . 2%Si) of hot metal have been analysed. The contribution of parameters which are well known to affect phosphorus distribution at tap, such as basicity, temperature, FeO content of slag, slag mass etc., is investigated through models of the ionic theory of slag, optical basicity, regular solution approach, and molecular theory of slag. The best overall results are obtained by the model based on the molecular theory of slag in which several operational parameters are also incorporated. Investigations of different slag samples, based on optical, SEM, EPMA and X-ray studies, reveal the effect of MgO and Al 2 O 3 on slag morphology and phosphorus distribution in different phases. It is important to consider the phosphorus distribution ratio in the solid and liquid part of the slag. The solid part of the slag, which is mostly dicalciumsilicate, can contain up to 5% phosphorus. The phosphorus content of the liquid part of the slag may depend upon the phosphorus content of the hot metal or the phosphorus load of the slag. It is found that incorporation of the effect of dicalciumsilicate in the model improves the accuracy of prediction. For better process control the addition of iron ore towards the end of the blow must be avoided while treating high phosphorus hot metal or during the production of ultralow phosphorus steels.

The morphology of alumina was studied in laboratory experiments and in samples from the steel plant. Varying morphologies of alumina were observed after deoxidation and reoxidation but a clear discrimination between alumina formed by deoxidation or reoxidation was not observed. Local concentration gradients during the addition and dissolution of aluminium can explain different growth mechanisms which lead to the observed varying morphologies of alumina.

A unique in blow sampling system has been applied to a blowing converter to retrieve simultaneously representative bulk metal bath and slag/metal emulsion samples from seven specified positions and every 2 min from start of blow. Full sample datasets from 20 heats have been grouped according to differences in the bulk bath phosphorus removal profiles and analysed with respect to relative refining ability of the slag/metal emulsion and the bulk metal bath. The complexity of the thermokinetic relationships behind the removal of carbon and the transfer of silicon, phosphorus, manganese and sulphur between the metal and slag is highlighted and the metal circulation rate in the emulsion is derived.

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