Acicular ferrite formation and Mn depletion behavior around the inclusions in Ti-Mg oxide metallurgy steel with different holding times

In this study, the effect of different holding times at 1250 °C on acicular ferrite formation, the inclusion characteristics, and Mn depletion behavior at the inclusion-matrix interface in Ti-Mg oxide metallurgy steel was systematically investigated. The results suggested that the volume fraction of grain boundary products decreased with increased holding time from 60 s to 3600 s, with the volume fraction of acicular ferrite initially increasing and then decreasing. Regarding the inclusion characteristics, different holding times showed no obvious effect on the inclusion type, but they affected the size and area number density of the inclusions. The domain inclusions at different holding times all consisted of MgTi2O4+MnS, and these inclusions were found to be effective for the nucleation of acicular ferrite. The average diameter of the inclusions increased, and the area number density of the effective inclusions significantly decreased when the holding time was increased from 900 s to 1800 s and 3600 s. Mn-depleted zones (MDZs) with larger widths and higher Mn concentration gradients formed at the inclusion-matrix interfaces at holding times of 60 s and 900 s, and MDZ formation was weakened by the continuous diffusion of Mn when the holding time was increased to 1800 s and 3600 s. At a holding time of 900 s for the Ti-Mg oxide metallurgy steel, the volume fraction and relative nucleation potential of acicular ferrite reached their maximum values, and MDZ formation with the lowest Mn concentration (~0.8 wt%) was observed around the MgTi2O4+MnS inclusions. Therefore, 900 s was the optimal holding time for acicular ferrite formation.