In-situ observation of nonmetallic inclusions in steel using confocal scanning laser microscopy: A review

The characteristics of nonmetallic inclusions formed during steel production have a significant influence on steel performance. In this paper, studies on inclusions using confocal scanning laser microscopy (CSLM) are reviewed and summarized, particularly the collision of various inclusions, dissolution of inclusions in liquid slag, and reactions between inclusions and steel. Solid inclusions exhibited a high collision tendency, whereas pure liquid inclusions exhibited minimal collisions because of the small attraction force induced by their < 90° contact angle with molten steel. The collision of complex inclusions in molten steel was not included in the scope of this study and should be evaluated in future studies. Higher CaO/Al₂O₃ and CaO/SiO₂ ratios in liquid slag promoted the dissolution of Al₂O₃-based inclusions. The formation of solid phases in the slag should be prevented to improve dissolution of inclusions. To accurately simulate the dissolution of inclusions in liquid slag, in-situ observation of the dissolution of inclusions at the steel–slag interface is necessary. Using a combination of CSLM and scanning electron microscopy–energy dispersive spectroscopy, the composition and morphological evolution of the inclusions during their modification by the dissolved elements in steel were observed and analyzed. Although the in-situ observation of MnS and TiN precipitations has been widely studied, the in-situ observation of the evolution of oxide inclusions in steel during solidification and heating processes has rarely been reported. The effects of temperature, heating and cooling rates, and inclusion characteristics on the formation of acicular ferrites (AFs) have been widely studied. At a cooling rate of 3–5 K/s, the order of AF growth rate induced by different inclusions, as reported in literature, is Ti–O < Ti–Ca–Zr–Al–O < Mg–O < Ti–Zr–Al–O < Mn–Ti–Al–O < Ti–Al–O < Zr–Ti–Al–O. Further comprehensive experiments are required to investigate the quantitative relationship between the formation of AFs and inclusions.