Fundamental study to develop process for producing ultra clean steel with confocal scanning laser microscope (In-situ observation and analysis of agglomeration and removal of non-metallic inclusions in metal melts)

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The behavior of various non-metallic inclusion particles on molten steel surface was "in-situ" studied by using a confocal scanning laser microscope combined with an infrared image furnace. A long-range strong attraction between alumina and alumina-rich solid inclusion particles, and detailed sequence of the collision, agglomeration and formation of alumina clusters was observed on molten steel surface. This attraction force was found to be over 10^<-16>N,reach over 10mum for alumina particles larger than 3mum, and play an important role to promote collision between the particles to form intermediate aggregates, the same between the intermediate aggregates to form loose alumina cluster, and to densify the loose clusters into compact ones. This force was not influenced by the existence of surfactant sulfur in the steel melt. The origin of this long-range strong attraction was examined to be caused by capillary effect around alumina particles on molten steel surface. Solid CaO-Al_2O^3 and CaO-Al_2O_3-SiO_2 inclusion particles were subjected to quick agglomeration to form clusters which densified and deformed into massive solid inclusion particles much easier. The attraction was also found to be in the range of 10^<-16>-10^<-13>N for different particles and extended to a distance of up to 100mum. However, such capillary attraction was not at all found between liquid CaO-Al_2O_3-SiO_2 inclusion particles. Even when the two liquid particles came in touch, merger took place only after contacting for a while. The merger of liquid CaO-Al_2O_3 particles was even more difficult for small particles of less than 7mum, but easier for particles larger than 24mum. Furthermore, an interaction of non-metallic inclusions with advancing melt/solid interface of steel crystals was investigated. The engulfment and pushing of inclusions by the melt/solid interface were clearly observed, and criteria for the engulfment were determined for different inclusion species.