The features of inclusion and microstructure for carbon structural steel containing Mg-Al-O inclusions were studied through the scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS). It can be seen that, in Mg-Al-O inclusions, the elements of Mn, Si, and S coexist, and their central mole ratio of Mg/Al varies in a wide range. This value for most inclusions is larger than 0.5, which suggests the formation of solid solution between MgAl₂O₄and MgO. After etching, the typical microstructure of intragranular acicular ferrites is observed, which is due to the nucleation effect induced by Mg-Al-O inclusions. From the SEM-EDS mapping images, it is found that the element of sulfur accumulates on the periphery of nucleation inclusion. Moreover, line EDS analysis hints that Mn-depletion zone (MDZ) exists in steel matrix, which is adjacent to the complex inclusion. Combined with the theoretical analysis, this phenomenon can be explained by the absorption of Mn due to the magnesium vacancy in MgAl₂O₄, and this MDZ promotes the nucleation of intragranular acicular ferrite. Through statistical analysis of SEM images for microstructure, the probabilistic nature of inducing nucleation effect is revealed. These results may be helpful to clarify the nature of oxide metallurgy.

TiO₂ nanoparticles and Mg alloy were added to molten steel in sequence to investigate the inclusion and microstructure characteristics. Compared with a non-treated sample, these additives resulted in the formation of Ti–Mg-bearing inclusions, which proves that the additives were valid. The size evolution from nanometer-scale TiO₂ to micrometer-scale oxides hints at the agglomeration and growth of the TiO₂ nanoparticles, which is due to the possible formation of a liquid-capillary force, the decomposition reaction of TiO₂, and the higher Gibbs free energy of the nanoparticle. Furthermore, the statistical analysis of the oxides indicated that with the addition of the TiO₂ nanoparticles and Mg alloy, the oxides were refined and their density was higher. Few pure MnS were observed in the treated sample. This is due to the fact that most oxides separated out in the liquid region at 1873 K based on the oxide composition and the calculated Al₂O₃–Ti₃O₅–MgO phase diagram. Thus, MnS preferred to segregate on them during solidification. After etching, it was found that the Ti–Mg-bearing oxide can induce the nucleation of intragranular acicular ferrites. The appearance of these acicular ferrites was not observed in the non-treated sample. This comparison indicates the effectiveness of the external adding method in oxide metallurgy.

The role of Ti-Al-O inclusions on nucleation of intragranular acicular ferrite (IAF) was investigated by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). From SEM-EDS mapping images, two kinds of titanium containing oxides are observed. Type I is a Ti-Al-O phase, in which the elements of Al and Ti coexist. Type II is the titanium containing oxide, which precipitates on Al₂O₃. By statistical analysis, it can be seen that Type I is dominating in number, and its molar ratio of Ti/Al varied in a wide range, which reflects its composition complexity. After etching, microstructure of IAF is observed. It can be seen that sulfur accumulates on the periphery of nucleation Ti-Al-O inclusion, which is attributed to the precipitation of MnS. Moreover, this accumulation accompanies increasing in content ratio of Al/Ti, which could be explained by titanium vacancies. These vacancies can absorb Mn ions from the steel matrix around inclusions, which leads to a Mn depletion zone (MDZ). Its existence is confirmed by EDS line scanning, and considered to facilitate the formation of IAF since Mn is an austenite stabilizer.

The features of inclusion and microstructure for Ti-added carbon structural steel were studied through the scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS). From the SEM-EDS mapping images, two kinds of titanium-contained oxides are observed. The type I is the Al-Ti-O phase, in which the elements of Al and Ti coexist. The type II is the titanium-contained oxides, which precipitate on the Al ₂ O ₃ . Through the statistical analysis, it can be seen that the former type is dominating in number and smaller in size. After etching, the microstructure of intragranular acicular ferrites (IAF) is observed, which is totally different from that in the comparison sample without Ti addition. Furthermore, it’s found that the type I inclusion can act as nucleation sites for the IAF. However, the similar nucleation phenomenon is complex for the type II inclusion. This complexity is attributed to the diversity of precipitation of Ti and Mn.

The relationship between MnS precipitation and the induced nucleation effect of Ti-bearing inclusions has been analyzed using SEM and EDS. In comparison with a non-treated sample, it can be seen that the Ti treatment leads to the oxide evolution from Al-Mn-Si-O to Ti-Al-Si-Mn-O. Moreover, it was found that sulfur congregates around both types of oxide, which is attributed to the MnS precipitation. As for pure MnS, its proportion in the total inclusion decreases when a Ti treatment is performed. EDS line analysis suggested that a Mn depletion zone (MDZ) occurs in the Fe matrix surrounding Ti-Al-Si-Mn-O. It does not occur surrounding Al-Mn-Si-O in the comparison sample. This difference leads to different microstructures. The intragranular acicular ferrites (IAFs) are only observed in the Ti-treated sample. Further analysis indicated that this microstructure is induced by Ti-Al-Si-Mn-O inclusions. In the same sample, the similar induced nucleation effect and a MDZ could not be found surrounding pure MnS. This comparison suggests that the formation of the MDZ may be independent of a MnS precipitation.