Removal of solid inclusions from steel melts

The removal of non-metallic inclusions from steel melts prior to casting has much merit both financially and technologically. Laboratory prepared steel melts have been successfully filtered through an alumina filter bed to remove carefully prepared alumina inclusions at 1600(DEGREES)C. Three distinct types of filters have been used in this investigation: (i) tabular alumina (0.2-0.5 cm size) packed bed (ii) extruded monolithic alumina (400 square cells/sq. in.) and (iii) open pores alumina foam (10 and 30 pores per linear inch). Filter heights of 5, 10 and 15 cm and melt velocity range of 0.08 to 0.58 cm/s have been used in this investigation. The steel cleanliness has been characterized both quantitatively and qualitatively. Chemical and quantitative metallographic analyses were performed for the filtered and unfiltered steel. In chemical analysis, total oxygen, total aluminum, soluble aluminum and insoluble aluminum contents were determined, while in quantitative metallographic analysis, volume fractions of alumina inclusions and their size distributions were determined. The kinetics of the filtration process have been modeled and inclusion removal efficiencies up to 96% have been achieved. The inclusion removal efficiency is indluened by the melt velocity, the type of filter, the length of the filter and the total surface area of the filter. Out of the above variables, melt velocity and total surface area of the filter play a dominant role in melt filtration process. The inclusion removal efficiency has been found to decrease with an increase in melt velocity and to increase with an increase in the total surface area of the filter. For a given filter height and melt velocity, the monolithic alumina filter yields a cleaner steel than the tabular alumina filter. The alumina inclusions present in unfiltered steel melts are < 5 microns in size and in most of the filtration experiments inclusions greater than 2.5 microns are totally removed from the melt. A good correlation has been found between the filtration performance results obtained from chemical analysis and quantitative metallographic techniques. The inclusion capture kinetics and filtration characteristics of the filter media tested have been discussed.