In-situ Observation of Non-metallic Inclusions in Steel using Confocal Scanning Laser Microscopy: A Review

The characteristics of non-metallic inclusions evolving during the entire production process of steel had a significant influence on steel performance. It was an effective method to in-situ observe the behavior of inclusions on the surface of the molten steel using the confocal scanning laser microscopy (CSLM). In the current paper, investigations on inclusions using CSLM were reviewed and summarized, especially the collision of various inclusions, the dissolution of inclusions in the liquid slag, and the reaction between inclusions and the steel. Solid inclusions exhibited a higher collision tendency while pure liquid ones collided little due to the small attraction force induced by their < 90o contact angle with the molten steel. It was suggested to study the collision of complex inclusions in the future due to the formation of complex inclusions in the real molten steel. Higher CaO/Al2O3 and CaO/SiO2 ratios of the slag promoted the dissolution of Al2O3-based inclusions in the liquid slag. The formation of solid phases in slag should be avoided to improve the dissolution of inclusions. To precisely simulate the dissolution of inclusions into the liquid slag, it was necessary to in-situ observe the dissolution of inclusions into slag at the steel/slag interface. There were several methods to in-situ observe the modification of inclusions by the dissolved element in the steel. With the combination of CSLM and scanning electron microscopy- energy dispersive spectroscopy, the composition and morphology evolution of the inclusion during the modification process were observed and analyzed. It was useful to develop the addition technique combining with CSLM experiments to better understand the modification process of inclusions. The in-situ observation of the precipitation of MnS and TiN was widely studied, while the in-situ observation of the evolution of oxide inclusions in the steel during solidification and heating processes was rarely reported. The effects of temperature, heating or cooling rate, and inclusion characteristics on the formation of acicular ferrites was widely studied. With the cooling rate of 3-5 K/s, the sequence of the growth rate of acicular ferrites induced by different inclusions reported in literature was Ti-O < Ti-Ca-Zr-Al-O < Mg-O < Ti-Zr-Al-O < Mn-Ti-Al-O < Ti-Al-O < Zr-Ti-Al-O. More comprehensive experiments are necessary to be performed to investigate the quantitative relationship between the formation of acicular ferrites and inclusions.