High drawing speed is a research hotspot in the field of slab continuous casting, which has a direct impact on continuous casting equipment design, production process, product quality, cost control, energy saving and consumption reduction. In comparison, there is still a certain gap between China’s conventional slab continuous casting high-speed indicators and those of foreign countries, but some companies have taken the lead in completing the exploration of drawing speeds higher than 1.8 m/min, and some casting machines have reached or even exceeded the advanced level of 2.0 m/min. Slab high-speed continuous casting is a systematic project, which requires relevant supporting technologies to achieve efficient, safe and stable production. For example, it is necessary to carry out targeted design and optimization of submerged nozzle structure and process parameters, copper mould tube vibration, protective slag, crystallizer cooling, electromagnetic flow control, liquid level fluctuation compensation and other work. A large number of studies have shown that the stable flow and heat transfer of molten steel in the crystallizer and the uniform growth of the slab shell and its good lubrication are key factors affecting the smoothness of slab high-speed continuous casting and product quality. At the current technical level, high-speed continuous casting of slabs is mainly concentrated in the production of low-carbon steel and hyperperitectic steel. Although the drawing speed of subperitectic steel and high-carbon steel has been improved, there is still a certain gap with the former. High-speed continuous casting has different definitions due to differences in steel type, billet type, equipment, operation and technical level, and the high-speed level in different eras is also very different. High-speed crystallizer metallurgical technology is the core guarantee for efficient continuous casting of slabs. The relevant content and conclusions can provide theoretical and technical references for scholars and technicians in the industry to carry out high-speed continuous casting.
High drawing speed; Slab continuous casting; Water nozzle; Vibration; Protective slag; Balanced strong cooling; Electromagnetic flow control; Liquid level fluctuation
In September 2020, China formulated and announced the “dual carbon” goal, namely to achieve “carbon peak” in 2030 and “carbon neutrality” in 2060. At present, energy conservation and carbon reduction work has been comprehensively promoted in many fields. The steel industry is a basic material manufacturing industry and is the second largest carbon emitter in the country after the power industry. As a link between steelmaking and steel rolling, continuous casting itself mainly emits indirect and low-proportion energy and power carbon. However, continuous casting has a direct impact on upstream and downstream energy consumption. When the continuous casting speed increases, the temperature drop of molten steel decreases, the tapping temperature during steelmaking decreases, and the turnover heat loss decreases; at the same time, the billet temperature increases at high drawing speeds, which can save or eliminate heating furnace energy consumption and emissions. Roughly estimated, high-speed continuous casting and direct rolling technology can save 200~300 kg/t of CO2 emissions, which is of great value for China’s steel enterprises to reduce carbon emissions. The concept of high-speed continuous casting has a long history. With the advancement of continuous casting equipment, the continuous casting speed of some thin slab products exceeds 6 m/min, achieving headless rolling; the casting speed of other conventional cross-section varieties of steel is about twice that of 10 years ago. High-speed continuous casting has always been a hot topic in the field of iron and steel metallurgy, and it has outstanding advantages in equipment, technology and product quality. In terms of equipment, high-speed continuous casting can achieve high production capacity by building small-flow and small-section continuous casting machines, while saving space, equipment and manpower requirements; in terms of technology, high-speed continuous casting can achieve efficient production of steel products, which can increase production on the one hand and reduce equipment consumption, energy consumption and emissions on the other hand; in terms of product quality, since high-speed continuous casting production lines are often high-precision systems that integrate multiple technologies such as metallurgy, automation, computers, communications, machinery, and testing, they have stronger anti-interference and anti-fluctuation capabilities, more stable continuous casting thermal state, smaller furnace differences, inter-flow differences and fixed length differences, and better final product quality and performance consistency. High speed is the core of efficient continuous casting and an important development direction of iron and steel metallurgical technology. High-speed continuous casting is a systematic project that requires complete supporting technologies to ensure the surface and internal quality of the continuous casting billet. Scholars and technicians in the industry have carried out a lot of work in this regard and have made breakthrough progress. This paper summarizes the development and current status of high-speed continuous casting of slabs at home and abroad in recent years, focuses on the key technologies of high-speed crystallizer metallurgy, and looks forward to the future development trend of high-speed continuous casting. The results can provide certain reference and reference for other scholars to carry out related research.
Featured charts
Figure 1 Relationship between monthly output of single strand of 1500mm wide slab continuous casting machine and thickness and casting speed
Fig.2 Relationship between negative slip time, vibration frequency and amplitude at different pulling speeds
(a) Slag consumption; (b) Maximum friction force
Figure 3 Variation of slag consumption and maximum friction force with pulling speed
High casting speed is a research hotspot and focus in the field of continuous casting, and has a direct impact on continuous casting equipment design, production process, product quality, cost control, energy conservation and consumption reduction. Some domestic enterprises have taken the lead in completing the exploration of slab continuous casting with a casting speed higher than 1.8 m/min, and some casting strands have even reached or exceeded the advanced level of 2.0 m/min. The relevant technologies have driven and promoted the high casting speed of domestic slabs. The progress of continuous casting technology provides reference and benefit. This paper summarizes the key metallurgical technologies of slab high-speed continuous casting crystallizer. The main conclusions are summarized as follows: 1) High-speed continuous casting has significant advantages in equipment, technology, manpower, production capacity and cost, but it will bring a series of Product quality and process problems, such as surface depressions, inclusions, cracks, and even the proportion of steel leakage accidents will increase; defects such as center segregation, looseness and shrinkage will also be aggravated, requiring supporting relevant control technologies. Crystallizer metallurgy technology is the core guarantee for high-speed continuous casting of slabs. Reasonable design of submerged nozzle, vibration, protective slag and crystallizer structure and material is the basis, while electromagnetic flow control and crystallizer liquid level compensation technology are the key. 2) When casting slabs at high speed, it is suitable to use a concave bottom nozzle and appropriately increase the downward inclination angle and insertion depth of the nozzle outlet; the high-speed casting mold vibration can adopt a low-frequency and large-amplitude reverse control mode, which can not only reduce the equipment The requirements of strength and precision can also expand the control window of slag consumption and vibration mark depth; the high-speed crystallizer protection slag is mainly characterized by low viscosity, high melting rate and low melting temperature, and the heat transfer efficiency of the crystallizer must also be considered. The crystallizer water tank structure, material, thickness and coating parameters significantly affect the thickness and uniformity of the initial solidified billet shell in high-speed continuous casting. In actual production, the water flow rate is not less than 9 m/s. At the same time, the thickness of the copper plate of the crystallizer should be appropriately reduced. 3) FC mold and MM-EMS have a good effect on the actual regulation of the flow behavior of molten steel in the high-speed casting mold of the slab, and their role in improving the surface cleanliness and surface quality of the casting is worthy of recognition; The liquid level fluctuation must be taken seriously, and the feedforward compensation control technology based on the liquid level monitoring system has good application effect. 4) High-speed continuous casting aims at efficient production and takes high-quality control as its core. In the future, the high-speed level of slab continuous casting at home and abroad is still expected to be further broken through.
