Study on Precipitation of Effective Inclusions in 20MnSi and Their Influence Mechanism on Formation of Acicular Ferrite

Based on the concept of "oxide metallurgy", the "oxide metallurgy production technology" was proposed.Using the technology to control the oxygen content after steelmaking, 20MnSi steel was produced. Utilizing metallographic microscope image analyzers and scanning electron microscopy（SEM）, the effects of different oxygen contents on the formation of inclusions and effective inclusions were analyzed. Using metallographic microscopes, scanning electron microscopy, energy dispersive spectroscopy（EDS） and ImageJ software, the impact of inclusion size, quantity, composition,shape and austenite grain size on the formation of primary acicular ferrite was analyzed. The results show that when the inclusion sizes are less than 4 μm, it is most conducive to inducing the formation of primary acicular ferrite. Within this range,the larger the effective inclusion size, the more acicular ferrite forms. Effective inclusions O-Ca-F-Mn-S, O-Si-Ca-V-Mn-S and O-Si-Ca-Ti-Mn-S have a strong ability to induce the formation of primary acicular ferrite. Coarse austenite grains facilitate the nucleation and growth of acicular ferrite within the grain on effective inclusions. Ultimately, the mechanism of preferentially precipitation of carbon in austenite on the surface of effective inclusions is revealed, forming C grain boundaries extending outward, with primary acicular ferrite preferentially nucleating and growing in the interstices of these C grain boundary of effective inclusions.