Assessment of Tundish Process Performance at Industrial Scale

Abstract

Air ingression through ladle shroud and consequent re-oxidation of dissolved aluminium in steelmaking tundish systems have been investigated at industrial scale. To this end, methods and measurements, reproducibility in measurements, relationship between rates of dissolution of nitrogen and oxygen, etc. have been analysed and discussed. Embodying measured soluble and total (= soluble + insoluble) aluminium content of LCAK (low carbon aluminium killed) steel in a material balance frame work, a formalism has been developed to quantify metallurgical performance of steelmaking tundish systems. Two dimensionless tundish process performance parameters namely, generation index, GI and floatation index, FI have been derived and represented as: $$G_{\text{I}} = \frac{{ - \Delta {\text{wt}}\% {\text{Al}}_{\text{sol}} }}{{\left( {{\text{wt}}\% {\text{Al}}_{{{\text{insol}},{\text{VD}}}} + [ - \Delta {\text{wt}}\% {\text{Al}}_{\text{sol}} ]} \right)}}$$ $$F_{\text{I}} = \frac{{ - \Delta {\text{wt}}\% {\text{Al}}_{\text{sol}} - \Delta {\text{wt}}\% {\text{Al}}_{\text{insol}} }}{{\left( {{\text{wt}}\% {\text{Al}}_{{{\text{insol}},{\text{VD}}}} + [ - \Delta {\text{wt}}\% {\text{Al}}_{\text{sol}} ]} \right)}}$$ in which, Δ represents composition differential between tundish exit and entry, respectively. To demonstrate the present work, operating data from six different domestic steels plants, including integrated and special steel mills, have been applied. It is shown, that while floatation of inclusions from tundishes is barely satisfactory, aluminium fading due to re-oxidation, and consequent generation of inclusions, are significant. In addition to such, on the basis of relevant theory and measurements, nitrogen and oxygen absorption from air during ladle to tundish transfer have been investigated at length and possible routes of dissolved aluminium re-oxidation in tundish have been examined further. It is shown that chemical reactions between slag and/or gunning material (viz., SiO2) and dissolved aluminium, rather than air ingression through shrouds, dominate [Al] re-oxidation in the tundish systems. The formalism developed in this study can also be conveniently extended to investigate equivalent phenomena in calcium treated LCAK steels, containing magnesium.