In the continuous casting process, the submerged nozzle is located below the ladle and above the mold, serving as a passageway for molten steel and regulating its flow. The submerged nozzle directly influences the flow velocity and flow field distribution of molten steel within the mold. When producing high-quality steel grades such as aluminum-killed steel, rare earth steel, and titanium-containing steel, the submerged nozzle is prone to issues such as nodule formation and blockage. Scaling on the submerged nozzle affects the flow velocity of molten steel and reduces production efficiency; in severe cases, it can cause the nozzle to become blocked. It also disrupts the flow pattern within the mold, hindering the rise of inclusions and leading to slag inclusion defects in the ingot. Scaling on the nozzle is the result of the combined and mutually influencing effects of various metallurgical, physical, and chemical processes.
Location of the nodule at the water inlet and structure of the nodule
The formation of nodules in the nozzle is a common issue encountered during the production of aluminum-deoxidized steel. Analysis of the distribution of nodules reveals that they adhere relatively uniformly to the inner wall, with the most significant nodule formation occurring at the outlet, where the structure is loose. Changes in cross-section and flow direction at the outlet alter the molten steel flow field, potentially creating vortices or stagnant zones that facilitate the deposition of inclusions. Inclusions in the molten steel react with the inner surface of the nozzle to form new inclusions, resulting in nodule formation on the nozzle wall. In the straight section of the nozzle, the flow field is stable and the flow velocity is high, making it difficult for inclusions to aggregate and form nodules.
Analysis of the composition and structure of the nodules blocking the nozzle revealed that the nodules consist entirely of powdery inclusions and generally exhibit a layered structure. The first layer consists of a metallic layer formed by molten steel adhering to the inner wall of the nozzle. The second layer is a dense, pure Al₂O₃ deposit formed by the reaction between the refractory material and inclusions; it accumulates in a network-like pattern with a certain degree of surface roughness and serves as the foundation for nodule formation. The third layer consists of loosely aggregated block-shaped or flake-shaped Al₂O₃ particles.
Mechanisms of Inclusion Agglomeration
During the deoxidation of molten steel, aluminum is widely used due to its low cost and strong deoxidizing capacity. The resulting Al₂O₃ inclusions remain suspended in the molten steel and readily adhere to the inner walls of the nozzle, forming nodules. Generally, nozzle nodule formation caused by Al₂O₃ inclusions is divided into three stages: inclusion formation, inclusion movement, and inclusion adhesion.
Formation of Al₂O₃ inclusions. When molten steel reacts with graphite-containing nozzle refractories, a series of reactions occur at the refractory interface, resulting in the formation of Al₂O₃. Under the combined effects of the decarburization reaction and molten steel erosion, the inner wall of the nozzle gradually becomes roughened. The high-melting-point, high-viscosity inclusions produced by these reactions come into contact with the nozzle’s inner wall and adhere to the roughened surface, forming a dense deposit layer. Air penetrates through cracks and joints in the nozzle, and the molten steel undergoes secondary oxidation, generating a small amount of Al₂O₃ inclusions. The temperature at the lower outlet of the nozzle is low; as the molten steel passes through and cools, this also promotes the precipitation of Al₂O₃ inclusions, which adhere to the inner wall of the nozzle and form nodules.
Movement of Al₂O₃ inclusions. Studies on the relationship between the flow field inside the nozzle and the rate of inclusion adhesion have revealed a molten steel swirl zone at the upper edge of the outlet and a distinct vortex region at the bottom of the slide plate. These areas of turbulent flow promote the deposition and adhesion of inclusion particles to the nozzle walls. Higher turbulent energy and surface flow velocities carry more and smaller inclusions into the steel. These nodules exacerbate the turbulence within the nozzle, causing asymmetric jets and leading to the adhesion of inclusions at the nozzle outlet and beneath the slide plate.
Using the Lagrangian trajectory tracking method, it was also found that molten steel flows through the nozzle in an asymmetric manner, with inclusion particles tending to accumulate in the molten steel recirculation zone and adhere to the inlet and outlet of the nozzle. Smaller inclusion particles, rougher contact surfaces, and slower molten steel flow rates all promote the formation and growth of nodules. In the stagnation zone within the nozzle, where the molten steel flow velocity is slower, inclusions are more prone to forming nodules.
In molten steel at temperatures above 1500°C, the nozzle refractory and Al₂O₃ inclusion particles become electrically charged. Under the influence of electrostatic forces, Al₂O₃ inclusion particles migrate toward the negatively charged nozzle wall and deposit on the alumina-carbon material. The charge on the nozzle wall refractory is directly proportional to the billet pulling speed, and the deposition rate of inclusion particles increases with the strength of the electric field.
Adhesion of Al₂O₃ inclusions. When molten steel flows at a velocity of 1 m/s, two Al₂O₃ inclusion particles with a radius of 2 μm fuse together in less than 0.03 seconds. At the nozzle neck, they can grow to 0.1 μm, and the strength of the aggregate is sufficient to withstand the impact stress caused by the flow of molten steel. Whether Al₂O₃ particles adhere is determined by the relative magnitudes of the bonding and desorption forces. The binding forces of Al₂O₃ particles consist primarily of van der Waals forces, surface tension, and liquid bridge forces, with the adhesive force generated by liquid bridge forces far exceeding that of surface tension and van der Waals forces. The desorption forces acting on Al₂O₃ particle aggregates include buoyancy and stresses generated by molten steel flow. Since the binding forces between Al₂O₃ particles are far greater than the desorption forces, once Al₂O₃ particles aggregate to form clusters, they are very difficult to dissociate.
Measures to Reduce Gate Scaling
Taking into account both the sources of scale deposits and the causes of their formation, current optimization measures to reduce scale formation in nozzles primarily include optimizing the nozzle structure, changing the material, improving the purity of the molten steel, and applying an external electric field.
Optimizing the nozzle structure. Research has shown that the use of a stepped annular nozzle, under the bundling effect of the stepped annular flow, prevents the inner wall of the stepped annular nozzle from being directly eroded by molten steel, thereby reducing the adhesion of Al₂O₃ inclusions to some extent. In actual production, a modified double-annular stepped nozzle based on this design has proven to be even more effective.
Optimizing the argon blowing structure and increasing argon blowing treatment are among the most widely used methods for reducing nozzle scaling. Research indicates that a dual-blowing configuration, in which both the stopper and the nozzle are simultaneously blown with argon, significantly outperforms single-blowing configurations in terms of gas flow distribution and stability, thereby providing superior protection against nodule formation.
Optimizing nozzle materials. Due to the harsh operating environment, submerged nozzles must possess sufficient mechanical strength, as well as excellent thermal shock resistance, erosion resistance, and wear resistance. Optimizing nozzle materials is an effective method for reducing nodule formation and blockages. Aluminum-carbon-zirconium-carbon composite submerged nozzles possess erosion resistance and are capable of meeting the demands of high-speed continuous casting. When casting aluminum-killed steel, spinel-based anti-clogging submerged nozzles can suppress Al₂O₃ deposition, and have achieved good results in the casting of deep-drawing steel, silicon steel, ultra-low-carbon steel, and stainless steel.
The material for the nozzle should be selected based on its poor wettability with molten steel, low reactivity, and low tendency to bind with inclusion particles, as well as its ability to form substances with weak bonding strength or low melting points. When optimizing aluminum-carbon materials, β-Al₂O₃ is used to replace α-Al₂O₃; during use, it forms a glassy film that inhibits the adhesion of Al₂O₃ inclusion particles. Currently, efforts to optimize nozzle materials are hindered by drawbacks such as high costs or reduced nozzle durability, making widespread adoption in actual production difficult. Furthermore, a single material cannot be developed for all steel grades; instead, anti-scaling performance must be considered in conjunction with other properties. Research should focus on diverse and composite materials to comprehensively improve the overall performance of nozzles.
Improve molten steel cleanliness. Reducing the content of Al₂O₃ inclusions in molten steel can alleviate nozzle nodulation and optimize steel flowability. Adding an appropriate amount of Ca-Si wire to the molten steel allows Ca to react with Al₂O₃ inclusions to form low-melting-point 12CaO·7Al₂O₃. However, the calcium treatment process requires strict control of the calcium dosage to prevent the formation of (Ca,Mn)S, which is even more difficult to address and exacerbates nozzle nodulation.
Application of an external electric field. By using an electromagnetic field to control the rotation of the molten steel within the nozzle, the movement of inclusions is regulated, thereby reducing their deposition and adhesion. By connecting the immersion nozzle and the stopper rod to the negative and positive terminals, respectively, and simultaneously applying a low-density pulsed current, a stable flow field is maintained within the nozzle. This controls inclusion adhesion, effectively extending the nozzle’s service life and enhancing its anti-scaling performance. Applying an alternating magnetic field with a flux density of 0.08 T to the nozzle causes the molten steel flow to be vigorously agitated under the influence of the electromagnetic field, effectively reducing the adhesion of high-melting-point inclusions such as Al₂O₃ and CaS. Analysis of the relationship between the flow field, temperature field, and swirl intensity reveals that the swirl velocity is proportional to the strength of the rotating magnetic field, with a maximum swirl velocity of 3 m/s achievable, effectively mitigating nozzle scaling and blockage. During the continuous casting process at the steel mill, an external electric field is applied with the positive terminal of the power source connected to the nozzle and the negative terminal connected to the integral stopper rod, maintaining power throughout the entire casting process. The nozzle subjected to the external electric field exhibited virtually no scaling, performing significantly better than the nozzle without the electric field.
The submerged nozzle is the core flow control component in the continuous casting process and has a significant impact on the smooth operation of continuous casting and the final steel quality. The scaling of the submerged nozzle is caused by the coupled effects of multiple factors. To prevent nodule formation in the nozzle, it is necessary to improve the purity of the molten steel, optimize the nozzle structure and material, and reduce the reaction and adhesion of inclusions to the inner wall of the nozzle. At the same time, an external electric field and argon purging of the nozzle should be employed to control the migration of inclusions. With advancements in technology and equipment, research and development should focus on multi-layer composite nozzle materials and structures to fully leverage the advantages of various materials and configurations; further study of the triboelectric charging theory for different steel grades and refractory materials is needed to refine the control of inclusion movement in molten steel through external electric fields.
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