Prediction on the spatial distribution of the composition of inclusions in a heavy rail steel continuous casting bloom

Abstract

Industrial trials were performed to investigate the evolution of inclusions in the molten steel of the continuous casing tundish and the bloom of a heavy rail steel, and the spatial distribution of inclusion composition along the bloom thickness was studied. An integrated model was established to predict the spatial distribution of the inclusion composition on the entire cross-section of a heavy rail steel continuous casting, which coupled the heat transfer and solidification of steel during continuous casting, the thermodynamic transformation of inclusions with temperature and the kinetic diffusion of dissolved elements in the steel. Both industrial trials and model predictions achieved similar conclusions.Inclusions within the 30mm subsurface layer had a similar composition and size to those of the molten steel and were more than other locations of the bloom and at 1/4 thickness of the bloom inclusions were larger and contained the biggest concentration of CaS. Along the thickness of the bloom, an opposite fluctuation of CaO to that of CaS in inclusions existed so that the main reaction between the inclusion and the steel matrix was (CaO)+[S]=(CaS)+[O]. When the cooling rate was larger than 150K/s, the newly precipitated CaO–Al2O3–MgO–SiO2–(<5 pct CaS) inclusions averaged <1.5μm in diameter. If the cooling rate was smaller than 150K/s, inclusions were transformed to larger CaO–Al2O3–MgO–SiO2–(>10 pct CaS) ones. In order to achieve an optimized composition of inclusions, the total oxygen and aluminum in the steel should be <10ppm and <40ppm.