Control measures to reduce or minimize nodules and blockages in submerged nozzles

The continuous casting submerged nozzle is a key refractory material connecting the tundish and the crystallizer. Molten steel enters the crystallizer through the submerged nozzle. By adjusting the relative position of the upper portion of the submerged nozzle and the stopper rod in real time and changing the gap between the two, the amount of steel passing through the submerged nozzle cross-section and the production efficiency of the continuous casting machine are controlled, which determines the state of the crystallizer liquid level and thus affects the quality of the continuous casting billet. During the transmission process, the molten steel inevitably comes into contact with the inner wall of the submerged nozzle. Inclusions contained in the molten steel adhere and aggregate on the inner wall of the nozzle, forming nodules over time. On the one hand, these nodules are easily washed into the crystallizer by the molten steel, remaining as larger inclusions in the continuous casting billet. On the other hand, the irregular shape of the nodules can cause instability in the stopper rod control and large fluctuations in the crystallizer liquid level. In severe cases, this can cause nozzle blockage and even lead to production accidents such as the suspension of continuous casting.

Based on the sources of nodules and blockages and the causes of their formation (see the article on this site: Nozzle Nodules, Blockage Structure and Cause Analysis for details), current control measures to reduce or minimize submerged nozzle nodules and blockages include: improving nozzle material and structure, reducing or modifying inclusions, optimizing local molten steel flow conditions, and applying an external field.

1. Improving Nozzle Material and Structure

Since inclusions adhere to the inner wall of the nozzle, leading to nodules and blockage, the surface condition of the nozzle inner wall affects the effectiveness of adhesion. Using carbon-free materials for the nozzle inner wall can avoid surface defects caused by baking and decarburization, improve the inner wall smoothness, and reduce nodules. Applying a dense Al2O3 coating on the nozzle inner wall can also improve the performance. Changing the nozzle base material, especially using materials with good temperature stability under molten steel conditions, can inhibit nodules. Adding YSZ material to the zirconium layer can reduce nozzle blockage for some steel grades. Calcium titanate nozzles are significantly more effective in preventing blockage than aluminum carbon nozzles, while calcium zirconate nozzles are slightly better than aluminum carbon nozzles. Furthermore, improving the nozzle shape and reducing sudden changes in molten steel flow velocity or temperature, with a focus on reducing the stagnation time of molten steel on the inner wall surface, can prevent nodules and blockage.

2. Reduce inclusions or modify inclusions

Inclusions are the material basis for nozzle nodules and blockages. Reducing inclusions or modifying inclusions will affect the adhesion behavior of inclusions in the nozzle. Under the premise of meeting the composition requirements, high-melting-point inclusions are converted into low-melting-point inclusions. For example, calcium treatment can convert Al2O3 inclusions with a high melting point into low-melting-point calcium aluminate substances. T. Botelho[32] and R.E. Lino et al. have detailed the composition range of liquid inclusions that can reduce nozzle blockage.

3. Optimize the local molten steel flow state

Blowing argon around the submerged nozzle, including the plug rod head, nozzle seat brick, nozzle top or nozzle middle, can reduce the negative pressure formed by air flow, reduce the oxygen inhaled from the nozzle, reduce the formation of inclusions, optimize the local molten steel flow state, promote the floating of inclusions, prevent the contact between molten steel and the inner wall, and can specifically reduce nodules or blockages in certain parts. Under the influence of an external electromagnetic field, the molten steel inside the nozzle can rotate, significantly reducing the occurrence of biased flow, flushing inclusions adhering to the nozzle wall, and, to a certain extent, preventing nozzle blockage. Alternatively, installing an insulation or heating device on the outside of the submerged nozzle and, drawing on the method used for electromagnetic ladle tapping, inserting an induction coil within the nozzle base brick can help prevent nozzle blockage in the upper area of the submerged nozzle. Regarding submerged nozzle insulation, literature has proposed measures such as using ceramic fiber insulation on the nozzle outer layer, increasing the thickness of the nozzle inner layer, and applying thermal insulation materials.

4. Applying an External Field

With the advancement of manufacturing and equipment levels, more and more advanced technologies are being applied to steel production. For certain steel grades, applying an external field may have beneficial effects. Literature has reported that electromagnetic stirring can reduce the stagnation time of molten steel in the upper part of the submerged nozzle and reduce the adhesion of inclusions to the nozzle wall. Electromagnetic stirring technology is also used in the lower area of the submerged nozzle. Yang Ying and others used an electromagnetic device to make the molten steel form a vortex inside the nozzle, and inclusions such as Al2O3 moved toward the center of the vortex, which suppressed the clogging of the nozzle. On the basis of the theory based on the double-layer charge model, the external power supply and the continuous casting components formed a loop, the plug rod was connected to the positive electrode and the nozzle was connected to the negative electrode (Figure 2c), which can reduce the clogging of Al2O3 on the submerged nozzle. Zhang Xinfang and others systematically summarized the technology of pulse current regulation of non-metallic inclusions in molten metal, compared the effects of external fields such as direct current, alternating current, and pulse current on the behavior of inclusions, and pointed out that the pulse current inclusion removal technology has excellent application prospects in suppressing the clogging of nozzles of rare earth steel and other steel grades.