Abstract:

To control the morphology and distribution of sulfide inclusions in steel and to improve the product quality, calcium treatment was replaced by tellurium treatment, yielding high-quality, non-quenched, and tempered tellurium-containing 38MnVS6 steel. However, the production process was hampered by nozzle clogging. In order to determine its cause, X-ray diffraction (XRD) analysis, scanning electron microscopy analysis, and thermodynamic calculations were carried out. The relationship between the main phases of the clogs and the inclusions in the steel was constructed, and the effect of tellurium treatment on nozzle clogging was explored. The results show that the clogs are mainly composed of CaO$\cdot $2Al$_{2}$O$_{3}$ and MgO$\cdot $Al$_{2}$O$_{3}$ and do not contain any tellurium phases, which have a similar composition to oxide inclusions. Therefore, nozzle clogging is not directly caused by tellurium. By treating the steel with tellurium instead of calcium, the calcium content in the steel is insufficiently high for transforming Al$_{2}$O$_{3}$ to low-melting-point 12CaO$\cdot $7Al$_{2}$O$_{3}$. The main calcium aluminate inclusion produced using the specified Al and Ca contents is CaO$\cdot $2Al$_{2}$O$_{3}$. In addition, owing to the small amount of residual Mg ($0.25\times10^{-6}\sim 1.46\times10^{-6}$) in the steel, Al$_{2}$O$_{3}$ is converted into MgO$\cdot $Al$_{2}$O$_{3}$. When the molten steel flows through the nozzle, CaO$\cdot $2Al$_{2}$O$_{3}$ and MgO$\cdot $Al$_{2}$O$_{3}$ sinter and adhere to each other on the inner wall of the nozzle. The inclusions continuously accumulate and gradually thicken, finally resulting in nozzle clogging.

Key words: non-quenched and tempered steel, 38MnVS6, Te, nozzle clogging