THE IMPROVEMENT OF STEEL CLEANLINESS BY SLAG COMPOSITION CONTROL

Background/purpose: At Rautaruukki Oy Raahe Steel Works, a lot of work has been done to optimize the ladle slag composition for different st eel grades to improve the steel cleanliness. In addition, the ladle li ning material has been changed from zirconia silicate to alumina spine l (Al(2)O(3)92%, MgO 5%). Methods: Lime and bauxite are used as slag f orming agents for steels made without steel desulphurization. A liquid calcium aluminate slag is formed which enhances inclusion removal. No fluospar is needed which increases the lining life of steel ladies. S teels with low sulphur specification are produced at the injection sta tion with CaSi/Ca-aluminate injection or with top slag stirring. A lar ge volume of basic slag made of lime, bauxite and fluorspar is used. R esults: The improvement in steel cleanliness is observed especially wi th steels made with top slag stirring. This has affected not only the type and amount of inclusions but also the mechanical properties of fi nal products. Conclusions: The steel cleanliness has been improved by evaluating the optimum composition and amount of ladle slag and by cha nging the ladle lining material. Modified steel desulphurization pract ices have enabled us to achieve better steel cleanliness. The effects of these changes have been studied carefully up to the final products, i.e. plates and coils. (C) Munksgaard, 1996