The quantitative analysis of inclusion and microstructure characteristics in the steels with TiN additions has been studied. The typical inclusion was detected to be a TiN+Mn-Al-Si-Ti-O+MnS phase. This identification was based on the measurements of scanning electron microscopy with energy dispersive X-ray spectrometer (SEM-EDS), electron probe microanalysis (EPMA) which equipped wavelength dispersive X-Ray spectroscopy (WDS) and equilibrium calculations by using the commercial software Thermo-Calc.. TiN was found to be the effective nucleation site for the formation of intragranular ferrite (IGF). Furthermore, the increased inclusion size led to the increased probability of IGF nucleation, and this probability of IGF nucleation was slightly decreased with the increased sulfur content. This tendency is in agreement with the tendency of the area fraction of IGF in the steels containing different sulfur contents. 

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.

The present work provides a method to calculate the critical diameters of TiO, TiN and VN inclusions for IGF nucleation. It is noted that the critical diameters of TiO, TiN and VN inclusions for IGF nucleation were calculated to be 0.192, 0.355 and 0.810 μm. The calculation result was in agreement with the experiment data in the steels with TiN and Ti-oxide additions. Moreover, it is the first attempt to predict the critical diameters of inclusions in the steels containing different contents of Mn and S. The critical diameters were increased with the increase of Mn content. However, the S content does not have a direct effect on the critical diameters for IGF nucleation.

The effect of the carbon content on the probability of the intragranular ferrite (IGF) formation was investigated in the present work. The TiN inclusion was detected to be the effective nucleation site for the IGF formation in the Fe-0.2 mass.% C alloy and the Fe-0.4 mass.% C alloy. It is noted that the probability of the IGF formation for each inclusion size is decreased with the increase of carbon content. Moreover, the critical diameters of the TiN, TiO and VN inclusions in the steels with different carbon contents were calculated based on the classical nucleation theory. The calculated critical diameter was also found to be decreased with the increase of carbon content. This is in agreement with the experiment results. Finally, the decrease of the probability of IGF formation for each inclusion size is due to a larger amount of pearlite formation in the steel contains a higher carbon content, which was detected by differential scanning calorimetry (DSC) measurements.

The dynamics of intragranular ferrite (IGF) formation in inclusion engineered steels with Ti₂O₃ and TiN additions were investigated using in-situ high temperature confocal laser scanning microscopy (CLSM). Furthermore, the chemical composition of the inclusions and the final microstructures after continuous cooling was investigated using electron probe microanalysis (EPMA) and electron backscatter diffraction (EBSD), respectively. The results show that there is a significant effect of the chemical composition of the inclusions, the cooling rate and the prior austenite grain size on the phase fractions and the starting temperatures of IGF and grain boundary ferrite (GBF) formation. The fraction of IGF is larger in the steel with Ti₂O₃ addition compared to the steel with TiN addition after the same thermal cycle has been imposed. This is because the TiO_x phase provides more potent nucleation sites for IGF than the TiN phase does. The fraction of IGF in the steels was highest after at an intermediate cooling rate of 70 ºC/min since competing phase transformations were avoided, however, the IGF was refined with increasing cooling rate. In addition, the IGF fraction increases and the starting temperature of GBF decreases with the increasing prior austenite grain size, however, the starting temperature of IGF keeps almost the value when the grain size changes.

In situ high-temperature confocal laser scanning microscopy and differential scanning calorimetry studies of ferrite formation in inclusion engineered (Ti₂O₃ and TiN) steels have been performed. The applied methodology allows distinction between intragranular ferrite, grain boundary ferrite, and pearlite. The effect of the inclusions and cooling rates on the initiation of phase transformation and the final microstructure is discussed. It is concluded that the applied hybrid methodology could provide vital details of solid-state phase transformations within the field of inclusion engineering.