Significance of Reoxiation in Steelmaking

Abstract

In recent years, reoxidation has clearly emerged as a major source of non-metallic inclusion in steel, and the main contributory causes have been clarified. These are direct oxidation of the steel stream as it falls through the air, chemical erosion of siliceous refrectories and oxidation by aie entrained during casting, the last being the most important. In spite of this the factors governing entrainment are not well understood. Consequently, in this work, falling water jets were used to carry out through investigation of the effects of nozzle diameter, height of fall, discharge rate and turbulence level of the stream an jet disintegration, the rate at which air is entrained and the volume of air retained in the receiving pool during-pouring. Of the variable employed, the turbulence level emerges as the most important. Four mechanisms of entra-inment were distinguished and their relative contributions to the overall rates elucidated. The way in which entra-ined air is distributed as bubbles, their residence times and their effect on flow patterns within the receiving pool are mentioned, since these factors influence the gas-metal area and time available for oxygen pick-up. Exper-iments have also been conducted to study gas absorption by molten steel droplets falling through air. The pract-ical implications of the results are discussed with refe-rence to the reactions which occur as steel is trans-ferred from the furnace to the ladle and then to the ingot mould. The influence of reoxidation on alloy loss, slag formation, refractory interactions, sulphide shape cont-rol and steel cleanliness are described and the paper concludes wiht a discussion of the critical importance of controlling stream characteristics and eliminating air entrainment.