Laboratory investigation on quantitative effect of ladle filler sands on the cleanliness of a bearing steel

Abstract

Laboratory experiments were performed using two kinds of heating furnaces to investigate the quantitative effect of ladle filler sands (LFS) on the cleanliness of a bearing steel at the casting start of the first heat and non-first heats of a casting sequence. After the direct addition of LFS into the molten steel, which was the condition of the first heat, inclusions in steel changed from MgO · Al2O3 to Al2O3. As the LFS/steel ratio increased from 0 to 1:600, the number density of inclusions significantly increased from 5.3 #/mm2 to 14.5 #/mm2 and the area fraction of inclusions sharply increased from 7.8 ppm to 60.6 ppm, inducing the sharp increase of T.O to 16.1 ppm. After the reaction between the LFS contained tundish covering powder (TCP) and the molten steel, inclusions in steel were separated into a MgO-Al2O3 system and a CaO-Al2O3(-MgO) system. With the increase of LFS/TCP ratio to 1:12, the average MgO content in inclusions decreased to 19.1 wt% while the Al2O3 content increased to 77.0 wt%, meanwhile, the T.O content increased to 6.0 ppm and the [Al] content decreased to 50 ppm. Based on experimental results and the thermodynamic analysis, the detrimental effect of LFS on the steel cleanliness of non-first heats was much slighter than that of the first heat owing to the use of high-basicity TCP. Accordingly, more efforts should be made to prevent the LFS charging into the molten steel at the casting start of the first heat of a casting sequence during the industrial production.