# Study on the Causes and Control Measures of Mg-Al Spinel Inclusions in U75V Heavy Rail Steel
## 3 Results and Discussion
### Formation Analysis of CaO-SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\)-MgO Inclusions  
Heavy rail steel adopts aluminum-free deoxidation process. In the steelmaking process, MnSi, FeSi and other alloys are added for deoxidation, and BaCaSi is added for deoxidation in the LF refining process. The smelting process of U75V heavy rail steel is studied here. The composition distribution of inclusions in converter outbound steel is shown in Figure 5, and the average composition of inclusions in the steel is shown in Table 3. In the figure, the position of the black circle represents the proportion of each component of the inclusions, and the size of the black circle represents the size of the inclusions. The main components of the inclusion were SiO\({}_{2}\) and MnO, the content of Al\({}_{2}\)O\({}_{3}\) was low and there was basically no MgO. From the inclusion composition, it can be concluded that it was composed of deoxidized products produced during the process of steel production. The transformation of the liquid steel composition and inclusions in the U75V heavy rail steel smelting process are shown in Figures 6 and 7. Due to the addition of FeSi and other alloy exciepients in the refining process, the mass fraction of impurity elements such as Ca and Al in steel increased, and the MnO-SiO\({}_{2}\) inclusions gradually transformed into CaO-SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\)-MgO inclusions. The composition of alloy auxiliary materials is shown in Table 4. The composition of the U75V heavy rail steel refined slag is shown in Table 5. With the smelting process, the mass fraction of Al\({}_{2}\)O\({}_{3}\) and MgO in the refining slag increased continuously. The MgO content increased from 4.61% to 6.69%, indicating that the refractories were eroded into the slag during smelting; this phenomenon was more obvious during the VD vacuum stage. The materials and erosion of the U75V heavy rail steel ladle are shown in Table 6. The slag line was composed of MgO-C, and the Al\({}_{2}\)O\({}_{3}\) content in the furnace lining was very high. Field data show that the weight loss of each ladle after smelting was about 160 kg, and the erosion of the refractory materials was serious. In addition, the results show that VD vacuum refining has a great influence on the composition of liquid steel and the composition of inclusions. The reaction between steel, slag and inclusions-refractories was promoted by the long stirring time of the steel slag under vacuum conditions. The mass fraction of Al and Ca in steel decreased clearly, while the mass fraction of CaO in the inclusions increased. VD refining ended with CaO inclusions exceeding 40%, and finally, the inclusion was completely transformed into a spherical liquid CaO-SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\)-MgO composite inclusion [27]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Area** & **CaO** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **MgO** & **MnO** & **C** & **Fe** \\ \hline
1 & 31.86 & 33.24 & 24.60 & 5.10 & 0.00 & 5.21 & 0.00 \\
2 & 0.00 & 0.70 & 67.64 & 28.84 & 0.85 & 1.97 & 0.00 \\
3 & 31.27 & 30.82 & 24.79 & 0.00 & 0.00 & 13.12 & 0.00 \\
4 & 3.18 & 3.10 & 64.93 & 24.26 & 0.00 & 4.54 & 0.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Components of nodules in each position. Figure 5: Composition distribution of typical inclusions outside converter: SiO\({}_{2}\)–CaO–Al\({}_{2}\)O\({}_{3}\) (**a**) and SiO\({}_{2}\)–MnO-Al\({}_{2}\)O\({}_{3}\) (**b**). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Al (wt\%)** & **Mg (wt\%)** & **Ca (wt\%)** & **Ti (wt\%)** \\ \hline FeSi & 1.37 & 0.036 & 0.87 & 0.13 \\ FeV & 0.096 & \textless{}0.005 & 0.02 & \textgreater{} \\ BaCaSi & 0.20 & 0.036 & 12.68 & 0.11 \\ MnSi & \textless{}0.02 & \textless{}0.005 & 0.025 & 0.16 \\ CaC\({}_{2}\) & 0.52 & 0.024 & 55.22 & \textgreater{} \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Alloy impurity element content. Figure 6: Changes in the composition of the molten steel.