Effect of Sulfur Content on Inclusion and Microstructure Characteristics in Steels with Ti2O3 and TiO2 Additions

The effect of the sulfur content on the inclusion and microstructure characteristics of steels where Ti2O3 and TiO2 have been added was studied. Based on the microscopic examinations, it is found in the steel samples with Ti2O3 additions that the area fraction of intragranular ferrite decreases from 52.68% to 39.09% as the sulfur content increases from 0.009 mass.% to 0.030 mass.%. In the steel samples with TiO2 additions, this value also decreases from 49.05% to 36.26% as the sulfur content increases. The nucleant inclusion was identified as a TiOx+MnS phase based on SEM-EDS measurements as well as on equilibrium calculations with thermodynamic calculation software, Thermo-Calc. Also, TiOx was found to be the nucleation site for an intragranular ferrite formation. Moreover, the nucleation probability increases with an increased inclusion size. It is also noted that the nucleation probability decreases slightly with an increased sulfur content. The minimum size of TiOx+MnS inclusions for an IGF nucleation is about 0.85 mu m in the present samples. Furthermore, this minimum size of TiOx inclusions is shifted to a size of about 0.5 mu m by excluding the depth of a MnS layer. In addition, the effective nucleation size range of TiOx inclusions in the steels, where Ti2O3 and TiO2 had been added, is smaller than that of TiN+Mn-Al-Si-Ti-O inclusions in steel samples where TiN had been added.