Currently, most continuous casting shrouds in domestic steel mills are used directly without preheating. Upon contact with the molten steel at the start of pouring, the inner surface of the shroud instantly rises to the molten steel’s temperature, while the outer surface, exposed to the atmosphere, is much cooler. This generates significant thermal stress within the shroud material, which can easily cause longitudinal cracks. Therefore, the shroud material must be designed to withstand acute thermal shock. Initially, fused quartz shrouds were used; they offer excellent thermal shock resistance, high mechanical strength, resistance to acidic slag attack, and excellent chemical stability, allowing them to be used without baking. However, they offer poor resistance to alkaline slag attack, and SiO₂ readily reacts with Mn and Fe oxides in the molten steel to form low-melting-point compounds. At high temperatures, SiO₂ is easily decomposed and vaporized by carbon. The SiO₂ liquid phase undergoes volume changes during thermal cycling, resulting in a loose shroud structure and reduced strength, making it unsuitable for producing high-purity clean steel and high-manganese steel. With the development of the continuous casting industry and the demands of the metallurgical industry, aluminum-carbon shrouds have been developed. Aluminum-carbon long nozzles are generally baked before use, otherwise accidents can easily occur during pouring. However, they offer excellent thermal shock resistance, strong adaptability to various steel grades, and good corrosion resistance. To prevent oxidation of the carbon in aluminum-carbon nozzles during baking and use, an anti-oxidation coating is applied to the nozzle surface. This coating is primarily composed of materials such as feldspar, quartz, and clay, with a glaze made from graphite. This coating is applied manually or mechanically to the outer or inner surface of the nozzle.
In continuous casting, to improve the quality of the ingot, a submerged nozzle is installed between the tundish and the crystallizer. During continuous casting, the submerged nozzle is positioned below the upper nozzle. The submerged nozzle prevents secondary oxidation and nitridation of the molten steel, as well as splashing; regulates the flow and injection rate of the molten steel; and prevents non-metallic inclusions such as mold slag from being drawn into the molten steel, playing a crucial role in promoting the floating of inclusions. It has a decisive impact on the yield rate and quality of the continuous casting ingot.
The submerged nozzle is installed at the bottom of the tundish and inserted into the crystallizer. The structure and material of the submerged nozzle vary depending on the continuous casting process and the type of continuous casting steel. There are integral stopper rods and submerged nozzle flow control systems, upper nozzle-sliding nozzle-submerged nozzle system flow control, stopper rod-upper nozzle-quick change mechanism-submerged nozzle system flow control, and flat submerged nozzles with special structures for thin slab continuous casting. Among them, the integral submerged nozzle is longer, generally more than 700mm. There are two forms of this nozzle, one is the internal nozzle, which is installed from the inside of the tundish to the outside. The nozzle is an integral structure with good sealing; the other is the external nozzle, which is installed from the bottom of the tundish to the inside. The submerged nozzle in the upper nozzle-sliding nozzle-submerged nozzle system is equivalent to the lower nozzle of the sliding nozzle. Instead of connecting to the tundish nozzle, the submerged nozzle is connected to the lower nozzle of the sliding nozzle or the lower slide plate. The sealing method is the same as that of the combined submerged nozzle.
The submerged nozzle in the upper nozzle-sliding nozzle-submerged nozzle system is shorter, generally less than 400mm. In use, the upper end of the submerged nozzle connects to the lower end of the tundish nozzle, while the lower end is inserted into the molten steel in the crystallizer. The bowl of the submerged nozzle can be spherical or flat.
A fiber pad or cement is used to contact the lower end of the tundish nozzle, and a counterweight in a lever system presses the submerged nozzle against the tundish nozzle, thereby creating a seal and protecting the molten steel.
What are the requirements for submerged nozzles in use?
Since the material of the submerged nozzle itself has a certain porosity and is also breathable, the outside air penetrates into the nozzle under the negative pressure generated by the flow of molten steel, and contacts with the molten steel to oxidize it. Therefore, the surface of the long nozzle and the submerged nozzle must be coated with an anti-oxidation glaze layer. During use, under the action of high temperature, the glaze layer melts and is evenly distributed on the surface of the nozzle, which can prevent the graphite in the nozzle material from oxidizing and also block the infiltration of air. Therefore, the nozzle material is required to meet the following conditions:
(1) Ensure the flow rate of molten steel at normal casting speed; (2) Make the heat flow distribution of the ingot cross section in the crystallizer as uniform as possible; (3) Facilitate the rapid melting of protective slag; (4) Facilitate the floating of inclusions and prevent slag from rolling; (5) Avoid violent agitation of the molten steel liquid level in the crystallizer; (6) Be easy to install and use.
What are the measures to prevent immersion water outlet from clogging?
Nozzle blockage is related to factors such as the composition of the molten steel, the deoxidation method, the pouring temperature and time, and the nozzle material and shape. Blockage is most severe in aluminum-killed steel, steel with a high aluminum content, rare earth steel, and titanium-containing steel. The mineral composition of the blockage is primarily a mixture of α-Al₂O₃ and FeO. Blockage occurs when deoxidation products such as Al₂O₃ in the molten steel, or when dissolved [Al] in the steel reduces SiO₂ in the refractory to produce Al₂O₃, or when the refractory itself undergoes an oxidation reaction, resulting in Al₂O₃ deposited on the nozzle wall. Simultaneously, at high temperatures, Al₂O₃ particles sinter, leading to the growth of a cohesive layer.
There are two main measures to prevent clogging in submerged nozzles: material-based and structural-based.
(1) Anti-clogging type immersion nozzle:
Mainly, a layer of lining with anti-clogging function is compounded on the inner wall of the nozzle, so that the Al₂O₃ in the molten steel and the substances in the nozzle material form low-melting aluminate. As the molten steel flows, the adhesion of aluminum compounds on the inner wall of the nozzle is reduced, preventing the deposition of Al₂O₃.
CaO·ZrO₂ can be used in the lining material. When CaO·ZrO₂ is converted into cubic ZrO₂; when cubic ZrO₂ is converted into baddeleyite, CaO is separated from CaO·ZrO₂ and non-hydrated calcifications and evenly dispersed in the material matrix, accelerating the reaction between CaO and Al₂O₃. For this purpose, the CaO-ZrO₂-C composite immersion nozzle was developed.
Since nitrides are almost not wetted by molten steel, Al₂O₃ in the molten steel is not easy to adhere to the inner wall of the nozzle. At the same time, the interfacial tension between BN and Al₂O₃ is greater than that of graphite. Therefore, BN-based materials can be used in composite immersion nozzles. For this purpose, Al₂O₃-BN-C composite immersion nozzles have been developed.
(2) Structural anti-clogging immersion nozzles:
① Immersion nozzle with annular air plug: A porous air plug is buried inside the zirconia nozzle. Argon is blown in from the outside to form an argon film between the inner surface of the nozzle and the molten steel, preventing the molten steel and Al₂O₃ from adhering to the inner wall of the nozzle. ② Immersion nozzle with embedded air plug: A small air plug is embedded on the wall of the nozzle. Argon is blown into the nozzle through the air plug to form an air film on the inner wall of the nozzle, preventing the deposition of Al₂O₃ in the molten steel. ③ Slit-blown immersion nozzle: A permeable layer is installed inside the nozzle, leaving a gap between the nozzle and the permeable layer. When pouring molten steel, argon gas is blown in from the outside, passing through the permeable layer and forming an air film on the inner wall of the nozzle. This prevents the deposition of Al₂O₃ in the molten steel and prevents nozzle blockage. ④ Stepped immersion nozzle: This nozzle prevents unevenness or turbulence caused by re-agitation of the molten steel flow. It also reduces Al₂O₃ deposition by repeatedly stirring the argon gas and evenly distributing it across the nozzle’s inner surface.
What are the methods to improve erosion of immersed nozzles?
The slag line of a submerged nozzle, located at the interface between the molten steel and the mold slag, is subject to severe corrosion and is the most vulnerable point during nozzle operation. Severe localized corrosion can lead to necking and even nozzle fracture. Improving the corrosion resistance of the ZrO₂-C material in the slag line is key to extending the service life of submerged nozzles. The oxidation and dissolution of graphite in molten steel, and the erosion of ZrO₂ by the slag, are the two primary corrosion processes in the ZrO₂-C material of submerged nozzles during operation. When in contact with molten steel, graphite oxidation and dissolution are the primary processes. When in contact with slag, graphite is not wetted by the slag, and ZrO₂ dissolution is the primary process. Slowing the corrosion rates of both processes can help extend the service life of the slag line. The interaction between slag and zirconia primarily occurs through the interaction of the slag with impurities and CaO stabilizers in the ZrO₂ particles. The extent of this interaction is closely related to the composition and structure of the zirconia particles. Materials with low density and relatively high impurity content have poor corrosion resistance.
The main improvement measures are: using the up and down floating of the tundish during the pouring process to change the contact area between the protective slag and the nozzle, which is a commonly used method in production; in addition, appropriately thickening the size of the slag line can also increase the service life of the submerged nozzle; in terms of raw material, the erosion resistance of the submerged nozzle slag line can be improved by controlling the quality of the fused zirconia raw material (density, stabilization rate, purity, etc.), the amount of zirconia added, the particle size composition of zirconia, the quality of flake graphite, suitable additives, etc.; however, too high a ZrO₂ content will lead to poor thermal shock resistance. The ratio that takes both into account is a carbon content between 15% and 20%. In addition, the erosion resistance of the submerged nozzle slag line can also be improved by spraying erosion-resistant materials and ZrB₂ and other materials as a protective ring.
Immersed nozzle anti-nodulation and large-size inclusion removal technology and application based on high-temperature charging discovery
Nodule blockage in the submerged nozzle during the continuous casting process is a major factor leading to the formation of large-sized non-metallic inclusions such as aluminum oxide. Continuous casting is an important process in steel production, and the submerged nozzle (hereinafter referred to as the nozzle) is the most important functional refractory component to ensure efficient continuous casting. It plays an important role in controlling the flow and flow field of molten steel and preventing secondary oxidation of molten steel. Its service stability and life play an important role in the quality of steel billets, continuous casting efficiency, and the continuous coordination of the manufacturing process. However, in the continuous casting of steel grades such as automotive sheet steel and bearing steel, because the molten steel is not suitable for calcium treatment, Al2O3 nodule blockage in the nozzle not only deteriorates its service function, but also causes instability of the molten steel flow field in the crystallizer, detachment of nodules, and slag curling, resulting in an increase in large-sized inclusions in the ingot, affecting the quality of the ingot. Nodule blockage can even cause nozzle service failure, resulting in interruption of continuous casting and serious impact on efficiency. To address this issue, in addition to efforts to cleanse molten steel, domestic and international researchers have conducted extensive research on optimizing nozzle materials, innovating nozzle and stopper structures, improving insulation, protecting pouring, argon blowing, and electromagnetic field treatment. These efforts have achieved some success. However, with the development of high-end and high-quality steel products, nozzle blockage has become an increasingly prominent problem, becoming a common key technical challenge hindering the high-quality and efficient manufacturing of automotive sheet steel, bearing steel, and other products, drawing significant attention from the steel industry.
In response to the international problem of submerged nozzle nodules forming and causing large inclusions that seriously affect continuous casting efficiency and the quality and performance of steel, we focused on the driving force of the migration of Al2O3, the main inclusion in molten steel, to the nozzle wall. Based on the cross-integration of multiple disciplines such as physics, electrochemistry, defect chemistry, materials science, and metallurgy, we carried out research on the charging characteristics of alumina inclusions in the serving nozzles and molten steel during high-temperature continuous casting. Based on the scientific discovery of high-temperature charging characteristics, we explained the behaviors of inclusion migration and deposition to the nozzle from the perspective of electrostatic force, developed and improved the nozzle nodule blockage mechanism, innovated new technologies to inhibit nozzle nodule blockage and remove large-sized inclusions, and carried out research on industrial application technologies.
