The Impact of Long Nozzles, Stopper Rods and Submerged Nozzles on Continuous Casting Efficiency and Billet Quality
Long nozzles, stopper rods, and submerged nozzles are key refractory materials for achieving highly reliable continuous casting, and their performance directly affects casting efficiency and billet quality. With the development of modern high-speed, high-efficiency continuous casting technology and clean-steel smelting technology, it has become necessary to further improve the performance of existing functional refractories for continuous casting and to develop new materials. Functional refractories for continuous casting are evolving toward higher performance, multifunctionality, and longer service life. This paper discusses key properties of functional refractories for continuous casting, measures for extending their service life, and recent development trends.
01 Submerged Nozzle
The performance and service behavior of submerged nozzles directly affect continuous casting efficiency and slab quality. During operation, submerged nozzles must resist erosion by molten steel and mold flux, and must not react with components in the molten steel in a way that causes blockages. Early submerged nozzles were made of fused silica, but their poor erosion resistance could not meet the requirements of multi-heat continuous casting and clean-steel production. Currently, composite submerged nozzles with an Al₂O₃-C body and a ZrO₂-C composite at the slag line are mainly used. While maintaining adequate thermal shock resistance, the slag-line material's resistance to mold-flux erosion and the nozzle interior's resistance to Al₂O₃ clogging are the key factors determining the service life of submerged nozzles.
1. Alumina Clogging and Its Prevention Measures
When casting certain special steels and Al- or Al-Si–killed steels, Al₂O₃-C or Al₂O₃-ZrO₂-C submerged nozzles often experience Al₂O₃ clogging, which leads to unstable molten-steel flow and even complete nozzle blockage. This disrupts normal casting operations, impairs billet quality, and has become the major obstacle preventing Al₂O₃-C or Al₂O₃-ZrO₂-C submerged nozzles from achieving multi-heat continuous casting and improving casting efficiency.
The possible causes of alumina clogging on the inner wall of the nozzle are as follows:
First, deoxidation of molten steel produces Al₂O₃.
Second, the SiO₂ and carbon contained in the refractory materials promote the formation of Al₂O₃, and the reactions are as follows:
SiO₂(s)+C(s)=SiO(g)+CO(g)
3SiO(g)+2Al(l)=Al₂O₃(s)+3Si(l)
3CO(g)+2Al(l)=Al₂O₃(s)+3C(s)
Third, Al₂O₃ particles come into contact with, adhere to, and grow on the inner wall of the submerged nozzle under the influence of factors such as the surface tension between alumina and molten steel.
To mitigate Al₂O₃ clogging, the following measures can be adopted in principle from a mechanistic perspective:
(1)Reduce the melting point of the deoxidation product Al₂O₃, for example by treating molten steel with calcium to convert Al₂O₃ into low-melting-point compounds in the Al₂O₃–CaO system;
(2)Use degassing technology to purify molten steel;
(3)Control the temperature drop of molten steel;
(4)Modify the nozzle lining material so that a specific component within it reacts with Al₂O₃ at high temperatures, forming a low-melting-point phase at the interface between molten steel and the refractory. This phase is then carried away by the molten steel and incorporated into the slag, thus eliminating Al₂O₃ deposition (e.g., incorporating CaO-containing substances on the inner wall of the nozzle);
(5)Adopt silicon-free and carbon-free nozzle lining materials to fundamentally reduce the formation of Al₂O₃.
Currently, research on submerged nozzles for preventing Al₂O₃ clogging is focused primarily on the nozzle lining materials. Materials that have been researched and developed include Sialon-ZrO₂, CaO-MgO-Al₂O₃, ZrO₂-ZrB₂-C, BN-AlN-C, ZrO₂-CaO-C, and others. Recently, silicon-free and carbon-free lining materials have also been developed. Based on the principle that alumina reacts with a specific component in refractory materials to form low-melting-point substances, the developed ZrO₂-CaO-C material has been successfully applied in practical production. However, for specific steel grades and casting conditions, it is difficult to determine the optimal CaO content in the lining material to achieve the best anti-Al₂O₃-clogging effect. Furthermore, as casting proceeds, oxidation of graphite often results in a rough and uneven inner working surface, making it challenging to attain satisfactory anti-alumina-clogging performance.
2. Erosion Resistance of Slag Line Materials
The erosion resistance of the slag-line section of submerged nozzles is another crucial factor affecting their service life. Currently, ZrO₂-C materials are widely used at the slag line, and their erosion resistance has been significantly improved compared with earlier Al₂O₃-C materials. Generally, ZrO₂-C materials with a carbon content of about 15% exhibit good overall performance.
The erosion mechanism of ZrO₂-C materials includes the oxidation and dissolution of carbon by molten steel, as well as reactions between CaO (the stabilizer of ZrO₂) and erosive components in the slag-such as SiO₂ and Na₂O-resulting in the cracking of partially stabilized zirconia and its transformation into fine monoclinic zirconia. The former leads to embrittlement of the microstructure of ZrO₂-C materials, while the latter produces m-ZrO₂ and glassy phases. These materials are then carried into the molten slag by the oscillatory movement of the mold. When casting certain special steels or under particular casting conditions, the erosion resistance of ZrO₂-C materials faces more severe challenges. For example, during the casting of high-oxygen steel, in processes where tundish-lifting operations are restricted, or in thin-slab continuous casting, efficient multi-heat continuous casting cannot be achieved.
To further improve the erosion resistance of slag-line materials, both nozzle structure and material composition can be considered. Based on the erosion mechanism of ZrO₂-C materials, reducing their carbon content may enhance erosion resistance, but this inevitably compromises thermal shock resistance.
02 Submerged Nozzles for Thin Slab Continuous Casting
Submerged nozzles are one of the three key materials for enabling the thin slab continuous casting process. Due to the extremely thin thickness of slabs produced in thin slab continuous casting (only 50–70 mm), the distance between the two side walls of the mold must be short. Consequently, the external dimensions of submerged nozzles are strictly constrained by the mold's shape and size. To ensure the same productivity as conventional slab continuous casting, the inner cavity of the nozzle should be as large as possible. These requirements necessitate designing the nozzle with thin walls, typically ranging from 10 mm to 17.5 mm in thickness. However, the erosion resistance of the slag line-a critical factor determining nozzle service life-is directly proportional to the nozzle wall thickness. This means that, when using mold flux of the same composition, the service life of nozzles for thin slab continuous casting is shorter than that of submerged nozzles for conventional casting. Additionally, submerged nozzles face more severe challenges in thin slab casting due to the high mold oscillation frequency, fast molten steel flow inside the nozzle, and mold flux with low viscosity and stronger corrosiveness.
Therefore, to extend the service life of submerged nozzles for thin slab continuous casting, the nozzle material must have excellent thermal shock resistance, high erosion resistance, high hot strength, and superior oxidation resistance. Furthermore, as the external dimensions of these nozzles are constrained by the mold shape and their structure is relatively complex, structural stress must be minimized during design and fabrication.
Owing to their thin walls (approximately 15 mm), the service life of such submerged nozzles is generally only three heats, which severely limits continuous casting and reduces casting efficiency. Thus, there is an urgent need to develop new nozzle materials with outstanding performance. With the rapid development of China's iron and steel industry toward high-efficiency continuous casting, thin slab continuous casting has grown rapidly in recent years. However, all submerged nozzles currently used are fully dependent on imports. Consequently, research on submerged nozzles for thin slab continuous casting is an important topic for continuous casting refractories both domestically and internationally.
At present, the Luoyang Institute of Refractories Research has conducted extensive studies on materials and heat treatment processes (such as heat treatment atmosphere), achieving promising results. For example, adding non-oxide additives with high thermal conductivity and resistance to molten steel erosion has been verified through high-temperature simulated erosion tests to improve the material's erosion resistance. Adopting nitrogen-protected heat treatment has expanded the selection of antioxidants, resulting in excellent overall performance-particularly high-temperature strength, erosion resistance, and thermal shock resistance. These characteristics are highly beneficial for meeting the service requirements of thin-walled nozzles.
03 Long Nozzle
The material design of long nozzles is mainly determined by the steel grade, casting duration, and type of tundish covering flux. Currently, Al₂O₃-C materials are predominantly used because they exhibit strong adaptability to various steel grades, are particularly suitable for casting special steels, and cause minimal contamination of molten steel.
To meet the requirements of high-efficiency continuous casting, optimal composition design can be implemented according to the specific service conditions of each part of the long nozzle to maximize its service life. The wrist section is prone to wear and air aspiration; therefore, Al₂O₃-C materials with low or no SiO₂ content, or with appropriately reduced graphite content, are typically used in this area. Due to erosion by tundish covering flux and molten steel, the slag line often becomes the key factor limiting the service life of long nozzles. To enhance the erosion resistance of the slag line, ZrO₂-C or MgO-C materials can be selected based on the specific steel grade and type of tundish covering flux.
Currently, long nozzles are developing along two main directions. The first is the development of SiO₂-free long nozzles. Traditional Al₂O₃-C long nozzles usually contain a certain amount of fused silica to improve thermal shock resistance. However, SiO₂ reacts with MnO or FeO to form low-melting-point compounds, which reduces the material's erosion resistance. Therefore, SiO₂-free Al₂O₃-C materials have been developed for casting high-manganese or high-oxygen steel. Compared with traditional Al₂O₃-C materials, these SiO₂-free materials have a relatively higher thermal expansion coefficient, so preheating conditions must be carefully controlled.
In addition, the thermal shock resistance of the material can be improved by rationally designing the particle size distribution of alumina, adding ZrO₂-mullite, or incorporating an appropriate amount of low-melting-point substances. At Chiba Works of Kawasaki Steel Corporation in Japan, the content of C and Al₂O₃ in Al₂O₃-C long nozzles was increased while SiO₂ and metal powders were eliminated, resulting in an average nozzle service life of 10 heats and a maximum of 22 heats.
The second development direction is unbaked long nozzles for multi-tundish casting. These nozzles usually contain a relatively high amount of graphite and a certain quantity of fused silica, resulting in lower oxidation and erosion resistance, but they save time and energy. For multi-tundish casting, the preheating temperature of the nozzle must be at least 800℃ to extend service life. Domestically, long nozzles are mainly Al₂O₃-C and fused silica-based, with a typical service life of 4–6 heats. The unbaked long nozzles jointly developed by the No. 2 Steelmaking Plant of Wuhan Iron and Steel Co., Ltd. and the Luoyang Institute of Refractories Research can achieve continuous casting for more than 7 heats.
04 Tundish Stopper Rod
Integral stopper rods are mainly used in tundishes. The use of stopper rods can reduce the accident rate and improve billet quality. Argon-blowing holes can also be incorporated into the integral stopper rod to inject argon into the submerged nozzle, helping to prevent nozzle clogging. Currently, the main body material of integral stopper rods is Al₂O₃-C, which also contains a certain amount of fused silica. The head of the stopper rod is severely eroded by molten steel, and its erosion and scouring resistance are the key factors determining its service life. To extend the service life of the stopper rod, the rod head can be made of Al₂O₃-C, MgO-C, or ZrO₂-C materials depending on the steel grade. For example, MgO-C is used when casting calcium-treated steel, while ZrO₂-C is used when casting high-manganese or high-oxygen steel. Additionally, when designing the rod head material, it is important to match it with the material of the wrist section of the submerged nozzle to prevent adhesion and similar issues. To improve molten steel cleanliness and reduce Al₂O₃ clogging in the submerged nozzle, stopper rods with argon-blowing channels-featuring porous plugs at the stopper head-have also been developed, mainly in single-hole and multi-hole designs.
The slag-line section of the stopper rod is another factor affecting its service life. Depending on the steel grade and type of tundish covering flux, its material can be Al₂O₃-C, MgO-C, or ZrO₂-C.
In the thin slab continuous casting process, the integral stopper rod remains the primary and effective molten steel throttling device. In the future, to meet the requirements of high-speed, high-efficiency continuous casting and clean steel production, it will still be necessary to develop high-performance rod head materials suitable for different steel grades.
05 Conclusion
On the premise of maintaining thermal shock resistance, the two key factors affecting the service life of submerged nozzles are their ability to prevent alumina clogging and the erosion resistance of the slag line against mold flux and molten steel. Silicon-free and carbon-free nozzle lining materials are not only effective for preventing alumina clogging but also serve as excellent functional refractories for the continuous casting of clean steel. Improving the composition and structure of slag-line materials is essential for enhancing their erosion resistance.
In addition, increasing the oxidation resistance of materials further contributes to improving their erosion resistance. To extend the service life of long nozzles, the material composition of each part can be tailored to the specific working conditions, such as steel grade, casting duration, and type of tundish covering flux, in order to achieve optimal performance. Moreover, balancing thermal shock resistance and erosion resistance through careful adjustment of material composition helps maximize the service life of long nozzles. Similarly, the erosion resistance of the stopper rod head material is crucial for the overall service behavior and lifespan of the stopper rod. Selecting appropriate materials based on steel grade and casting duration can significantly prolong its service life.