Effect of Mg Addition on Nucleation of Intra-granular Acicular Ferrite in Al-killed Low Carbon Steel

To verify the formation behaviors and mechanisms of intra-granular acicular ferrite (IAF) grains nucleated by Mg-Al-O in low carbon steel, the steels containing different Mg contents were refined in a vacuum induction furnace. The effect of Mg addition on the formation of IAF structure in Al-killed low carbon steel was investigated by optical microscope (OM) and scanning electron microscope with energy dispersive X-ray spectroscope (SEM-EDX). It reveals that the IAFs are only detected in Mg-added steels, and the volume fraction of IAF increases with the Mg concentration from 8×10−6 to 26×10−6. It shows that not only the MgO-Al2O3-MnS and MgO-Al2O3-P2O3 particles are the effective nucleation sites for IAF, but also the pure MgO · Al2O3 phase can promote the ferrite nucleation. A Mn-depletion zone (MDZ) is characterized adjacent to the MgO-Al2 O3-MnS, which is believed to be one of the possible mechanisms to explain the IAF nucleation. The MDZ around the MgO-Al2 O3-MnS inclusion would be induced by the MnS precipitation on the inclusion. It seems that the ability of Mg-containing inclusions to induce the nucleation of ferrite might be attributed to a new mechanism, i. e., the P-rich zone formed on a few Mg-Al-O inclusions might be another factor for promoting the IAF formation.