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 8x10 6 to 26x10 6. It shows that not only the MgO Al2O3 MnS and MgO Al2O3 P2O5 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 Al2O3 MnS, which is believed to be one of the possible mechanisms to explain the IAF nucleation. The MDZ around the MgO Al2O3 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.