The results of theoretical and experimental studies of the physicochemical properties of the СаО – SiO₂ – B₂O₃ slag system which contains 15 - 25% Al₂O₃ and 8% MgO were used in the development of the composition of ladle slags and technological methods for their formation ina ladle - furnace (LF) unit. The technology for forming ladle slags of the developed composition inaLFwas adopted in a converter shop of JSC "ArcelorMittal Temirtau".It provided process ofsimultaneous desulfurization and direct microalloying of structural steels with boron as well as decrease in the environmental pressure due to the elimination of the silicate decompositionprocessin solid slags and a refuse to use fluorspar. It also reduced amanganese ferroalloy consumption by 0.3 - 0.6 kg/(t steel), provided high strength properties while preserving ductility of hot-rolled metalof the investigated structural steels grade composition, and production of pipe steel ofX80strength class without heat treatment.