Basic oxygen furnace based steelmaking processes and cleanliness control at Baosteel

Abstract

Optical microscopy, total oxygen measurements and slime tests have been conducted to quantify the size distribution and amount of inclusions at various processing steps during basic oxygen furnace (BOF) based steel production at Baosteel. The effects on steel cleanliness of specific operational improvements during steel refining and continuous casting have been investigated. Such improvements to these processes and the resulting level of steel cleanliness at Baosteel are summarised in the present paper. Ladle slag reduction lowers FeO+MnO in the slag to below 5%, decreasing steel reoxidation by the slag. Calcium treatment by CaSi wire injection during ladle furnace (LF) refining is used to modify inclusions and improve submerged entry nozzle (SEN) clogging. Slag detection is employed at the ladle bottom during continuous casting. Flow control devices, CaO containing filters and high CaO based basic powder with CaO/SiO₂>4 are used in the tundish to remove more inclusions. Several improvements to the castability and in the attainment of clean steel at mould operations have also been made. With this BOF based steelmaking process, impurity levels can be controlled to achieve total oxygen (TO)<16 ppm, [S]<5 ppm, [P]<35 ppm, [N]<29 ppm, [H]<1 ppm in line pipe steels, and [C]<16 ppm, TO<19 ppm, [N]<15 ppm in interstitial free (IF) steels.