Effect of surface-modified MgO nanoparticles on intragranular ferrite nucleated on inclusions in low-alloy steel

Despite extensive research into steel refining with nanoparticle addition, some aspects remain unclear, such as the low utilization ratio of the nanoparticles. In this study, application of surface-modified nanoparticles in the steel refining process was investigated. The effects of their addition amount on inclusion modifications and microstructure refinements were investigated and the mechanism of intragranular ferrite formation was clarified. Initial experiments were performed to determine the appropriate steelmaking holding time, and the utilization ratio of MgO@C nanoparticle was 65% from the calculated total oxygen content. After nanoparticle addition, the inclusions were mainly composed of MgAl2O4 spinel or MgAl2O4-TiN-based composite inclusions. The inclusion size sharply decreased with nanoparticle addition. The number of sub-micrometer inclusions accounted for 77.2% of all inclusions in S2 steel sample, which contributed to inhibit the migration of prior austenite grain boundaries, and effectively induced acicular ferrite nucleation. Based on the theory that inclusions act as heterogeneous nuclei to induce ferrite nucleation, the results from thermodynamic calculations showed that the critical nucleation diameter for the TiN inclusions is 0.346 mu m, which is consistent with experimental data of the minimum inclusion size for ferrite nucleation in actual steel samples. (C) 2019 Published by Elsevier Ltd.