Submerged nozzle and its influence on the flow behavior of slab continuous casting copper mould tube
Slab continuous casting is the primary forming process of various plate and strip production. The quality of the ingot is closely related to the metallurgical effect of the copper mould tube, among which the role of the submerged nozzle is crucial. In view of the current status and development requirements of continuous casting production technology, on the basis of reviewing the development history of submerged nozzles for slab continuous casting, the understanding of the relationship between the submerged nozzle structure, process parameters and its material and the metallurgical behavior of the copper mould tube is summarized and discussed. It is shown that the submerged nozzle directly affects the flow, heat transfer and solidification behavior of molten steel after being injected into the crystallizer, and then affects the metallurgical properties of the crystallizer and the control effect of the quality of the ingot during the continuous casting process, but there is still a lack of quantitative nozzle design parameter criteria. In view of the differences in castability, ingot section and casting speed of various types of steel in actual production, from the perspective of casting machine production organization and electromagnetic flow control, the possibility of achieving ideal casting with a single nozzle under multiple sections and multiple working conditions is discussed.
Keywords: submerged nozzle; slab continuous casting; nozzle structure; nozzle process parameters; nozzle material
As a key process between steelmaking and pressure processing in the modern steel production process, continuous casting has developed rapidly in the past 10 years, and the world’s continuous casting steel production accounts for more than 98% of crude steel production. With the widespread application and continuous improvement of continuous casting technology, as the world’s major steel consumer and producer, China’s plate and strip production has reached about 50%, and the import and export of plate and strip materials account for the largest proportion. From 2008 to 2020, the proportion of plate imports has remained stable at about 85%, while the proportion of exports has increased from 48.6% to 60.9%. Slab continuous casting is the main casting method for the primary forming of plate, strip and large-section pipe products. Due to its complex equipment technology and many factors affecting the quality of the ingot, it is a difficult field in the current development of efficient, high-quality and refined continuous casting technology. The key factors affecting the production efficiency and product quality of slab continuous casting are the metallurgical function of the crystallizer and the secondary cooling roller array technology. Among them, about 80% of the ingot quality problems originate from the continuous casting crystallizer area. This is because the crystallizer is not only a mold for the initial solidification of molten steel, which determines the morphology and surface quality of the billet shell; it is also a heat exchanger with the largest heat flux density during the continuous casting condensation process of molten steel, which has an important influence on the balanced solidification of the billet and the formation of the as-cast structure. Among them, the main factors affecting the metallurgical function of the crystallizer are the flow, heat transfer and solidification behavior of the molten steel injected into it through the submerged entry nozzle (SEN). In the current slab continuous casting production technology, SEN is far from a simple protection of the casting injection pipe. In view of its direct impact on the metallurgical effect and billet quality of the crystallizer, its function has highlighted the important position of “small gate, big effect”. With the continuous development of efficient continuous casting technology, the steel flow of the gate and copper mould tube is increasing, and the growth time of the initial solidification billet shell in the crystallizer is shortened. The flow, heat transfer and thermo-dynamic state of the molten steel and the billet shell formation process inevitably change greatly. Changes in the characteristics or intensity of these physical phenomena often directly affect the stability of the metallurgical function of the continuous casting crystallizer, and then affect the smooth production and billet quality. For example, the increased steel flow and the increased momentum of molten steel injection may cause the molten steel to absorb air and roll slag during the continuous casting process, thereby forming inclusions and bubble defects on the surface and subsurface of the ingot; it may also lead to a decrease in the thickness or uniformity of the protective slag layer, thereby causing the lubrication between the molten steel and the copper plate to deteriorate, increasing the risk of ingot adhesion, leakage or surface longitudinal cracking. In addition, the molten steel at the SEN outlet has a large momentum and high superheat. If the flow into the crystallizer is not effectively controlled, it will directly affect the balanced solidification of the primary solidified shell, and even the solidification mode, segregation morphology and central shrinkage level of the ingot. In view of this, people have paid more and more attention to the design of SEN nozzles (inner cavity structure, bottom shape, side hole morphology and inclination, etc.) and material selection in recent years. Systematically sorting out the technical connotation and development of “small nozzles” has important practical significance for promoting the development of continuous casting technology towards high efficiency and refinement. To this end, this article first briefly reviews the role and development of slab continuous casting submerged nozzles, and then introduces the research and understanding of domestic and foreign counterparts on submerged nozzle structure, process parameters and materials, etc., in an effort to refine consensus, expose objections and deficiencies, and to stimulate discussion. Among them, the focus is on the influence of submerged nozzles on the steel liquid transmission behavior in the crystallizer and the quality of the casting, as well as the requirements of efficient continuous casting development on nozzle design and application technology.
Conclusion and Prospect
The submerged nozzle is a seemingly simple channel-type metallurgical reactor invented in the development of continuous casting technology, which is rich in technical connotations. It plays a very important role in molten steel protection casting, multi-furnace continuous casting and ingot quality control. The core function of the submerged nozzle is to improve the metallurgical properties of the continuous casting crystallizer, improve the surface quality of the ingot, and affect the development of the as-cast structure; its mechanism of action is to directly affect the flow, heat transfer and solidification behavior of the molten steel after it is injected into the crystallizer. Among them, the reasonable design of the nozzle channel and the structural stability of its service process are important factors that determine its application effect. From the perspective of ingot quality control, people have conducted a lot of research on the geometric structure, use process and its influence on the flow behavior of molten steel in the crystallizer based on physical simulation and numerical simulation, but most of them are limited to the cognition under specific working conditions or process ranges, and there are even differences; from the perspective of nozzle life and service effect, the nozzle material and structure have also been continuously improved, but the combination with the actual production process, steel grade and even fluid dynamics behavior analysis needs to be strengthened. Due to the complexity of actual production conditions, service conditions, and even the differences in casting performance and product quality requirements of different steel grades, the understanding of the metallurgical properties of submerged nozzles is still at the level of quantitative research but can only give qualitative evaluation. The understanding and prospects are briefly summarized as follows: 1) The design and use of submerged nozzles should ensure that there is a reasonable flow pattern and flow intensity in the crystallizer. The current general understanding is that the double circulation mode is conducive to the relative stability of the steel slag interface in the crystallizer, and can reasonably control the impact depth and intensity of the superheated molten steel, which is also conducive to protecting the slag slag, stabilizing the liquid slag layer, and floating and removing inclusions in the molten steel. 2) The flow pattern of molten steel is not only related to the geometric parameters related to the nozzle channel (including the side hole area ratio) and the immersion depth, but also depends on the cross-section of the billet and the casting speed. It can be seen that under the common multi-section, multi-steel and corresponding different casting speeds in production, it is almost impossible to guarantee a certain ideal flow pattern by using a specific nozzle. The use of special casting machine operation modes for different steel types or sections will be conducive to the stability of product quality; research and development of electromagnetic flow conversion and electromagnetic flow control technology for crystallizers under specified submerged nozzles will have great application prospects. 3) The thermal stability and chemical stability of nozzle materials and channel structures are crucial to their long-term service performance and steel adaptability. Functional long-life nozzles with small slag line erosion on the outer wall of the nozzle, stable chemical properties of the inner wall material, no wetting and nodulation with high-temperature melt, and suitable technical and economic performance will also be an important development direction in the future. 4) The study of crystallizer flow behavior based on fluid dynamics analysis and quantitative characterization and its correlation with metallurgical function and billet quality still lacks reliable parameter basis or nozzle design criteria. This may be related to the fact that the existing large number of laboratory studies and actual problems in the production process are not closely combined. It also shows that the development of refined continuous casting technology also urgently needs a refined and reliable research foundation support.
