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 steel grades to improve the steel cleanliness. In addition, the ladle lining material has been changed from zirconia silicate to alumina spinel (Al 2 O 3 92%, MgO 5%). Methods : Lime and bauxite are used as slag forming 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 ladles. Steels with low sulphur specification are produced at the injection station with CaSi/Ca-aluminate injection or with top slag stirring. A large volume of basic slag made of lime, bauxite and fluorspar is used. Results : The improvement in steel cleanliness is observed especially with steels made with top slag stirring. This has affected not only the type and amount of inclusions but also the mechanical properties of final products. Conclusions : The steel cleanliness has been improved by evaluating the optimum composition and amount of ladle slag and by changing the ladle lining material. Modified steel desulphurization practices 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.