Various factors affecting the fluctuation of the crystallizer liquid level were analyzed, and corresponding measures were taken to effectively control the fluctuation of the crystallizer liquid level and provide guarantee for stable casting.
Keywords: continuous casting; mold liquid level; fluctuation; measures
The crystallizer is the heart of continuous casting equipment. The molten steel is cooled in the crystallizer to form a billet shell of a certain thickness; the initially formed billet continues to cool in the secondary cooling zone, and the billet shell becomes thicker; the billet is further cooled during the drawing process until the interior is completely solidified. The liquid level fluctuation of molten steel in the mold is an important cause of surface quality problems of cast slabs. Crystallizer liquid level fluctuation means that the molten steel liquid level in the mold is more than 5mm higher or lower than the specified value under normal production conditions, and the duration is greater than 30 seconds. During the casting process, excessive fluctuations in the molten steel level can easily cause slag inclusions on the surface, causing bubbles to form under the surface of the cast slab and subcutaneous slag inclusions on the surface. In the production of ultra-low carbon steel, more than 60% of defects are caused by slag entrapment; occasional liquid level fluctuations will cause lateral depression of the cast slab; in addition, when the liquid level fluctuation is greater than 10mm, the incidence of longitudinal cracks on the slab surface will be reaches 30%. Therefore, the state of molten steel in the mold determines the quality of the final product. The fluctuation of the liquid level in the mold not only affects the stability of continuous casting production, but also has a great impact on the quality of the cast slab.
Reasons for crystallizer liquid level fluctuations
Chemical composition of molten steel
The chemical composition of molten steel has a great influence on the fluctuation of the crystallizer liquid level, because some chemical components in the molten steel will react under specific conditions, affecting the stability of the molten steel liquid level. For example, peritectic steel is very prone to liquid level fluctuations, which is caused by the inhomogeneity of the primary green shell caused by peritectic phase transformation. In addition, if the content of sulfur and aluminum in the steel water is too high, it will cause Al2O3 and sulfide nodules in the nozzle, causing liquid level fluctuations.
The liquid level fluctuation in the crystallizer is closely related to the performance and dryness of the mold powder. Among them, 70-80% of the molten steel level fluctuations caused by poor protection of the water tank. Poor performance of the mold slag and insufficient drying will result in uneven shell formation and insufficient strength, causing the liquid core of the billet to jump back and forth when the billet is pressed by rollers after leaving the mold, causing liquid level fluctuations.
The cooling during the continuous casting process includes the cooling of the molten steel in the mold and the cooling of the cast slab in the secondary cooling zone. The cooling of the molten steel in the crystallizer must be as uniform as possible, so that the shell grows evenly and the molten steel level is stable with small fluctuations. The cooling of the slab in the secondary cooling zone also affects the liquid level fluctuations. Traditionally, secondary cooling adopts a weak cooling system, which will cause the growth of the billet shell in the secondary cooling area to slow down and the surface temperature of the billet shell to be too high, exacerbating the tendency of the billet to bulge, resulting in liquid level fluctuations.
The greater the change in the pulling speed, the more obvious the change in the flow rate of the molten steel in the entire mold, the greater the impact on the liquid level, the greater the fluctuation, and the easier it is to cause quality defects such as slag entrapment on the slab surface and subcutaneous bubbles.
Equipment failure can also cause crystallizer liquid level fluctuations. The reasons for the fluctuation of the liquid level are: the failure of the tension leveling machine, the tension leveling machine reducer or the potentiometer; the foreign matter in the guide section, which blocks the slab; the failure of the conveying roller, which causes the slab to get stuck. Fluctuations of the molten steel level back and forth or left and right are mostly caused by abnormal spring plates, wear of individual connecting pins, and foreign matter in the mold hindering the movement of the slab.
The flow field of the immersed nozzle of the crystallizer can affect the fluctuation of the steel liquid level. Due to the influence of the pouring molten steel jet, the molten steel near the wall is more active, and the fluctuations at the meniscus are larger. However, at a certain immersion depth (about 3mm), the flow field of the nozzle affects the liquid level fluctuations of the bloom mold. Not big.
In addition, the liquid level fluctuation is caused by the periodic compression of the slab bulging deformation due to the equidistant arrangement of the roller rows. However, by modifying the sector section, the crystallizer liquid level fluctuation value was reduced, but the liquid level fluctuation phenomenon was not eradicated. In fact, relative to the arrangement of the roller rows (equal or non-equidistant), the continuous casting process has a greater impact on the fluctuation of the crystallizer liquid level.
Measures to control crystallizer liquid level fluctuations
In summary, the following measures should be taken to control crystallizer liquid level fluctuations:
(1) Appropriately increase the Mn content or reduce the S content in the steel to increase the Mn/S ratio in the steel, reduce the accumulation of sulfides at the vibration marks, reduce the hot brittleness tendency of the steel, alleviate liquid level fluctuations, and at the same time improve the steel’s performance.
(2) Improve mold mold powder
Use special mold mold powder. Theoretical research shows that when the mold slag is alkaline, the mold slag film close to the wall of the crystallizer precipitates crystals during solidification, which can control the heat transfer from the shell to the crystallizer. By changing the composition of the mold flux to increase the alkalinity, increase the melting temperature and viscosity of the mold flux, increase the crystalline phase in the slag film in the mold copper plate and the primary shell, and stabilize the shell, thereby avoiding the mold liquid level caused by unevenness. fluctuation. In addition, improving the dryness of the mold powder can also improve the liquid level fluctuations.
(3) Cooling control
By reducing the amount of cooling water in the cooling zone of the crystallizer, the cast shell can grow evenly and suppress fluctuations in the crystallizer liquid level. For the secondary cooling zone, reduce the target temperature in the upper part of the secondary cooling zone, increase the cooling intensity in the upper area of the secondary cooling zone (mainly sections 1, 2, and 3), reduce the tendency of the billet to bulge, and alleviate the liquid level fluctuation phenomenon.
(4) Pulling speed control
Although increasing the drawing speed can increase the output, the product quality will decrease, so the appropriate and optimal drawing speed should be selected according to the different steel products produced.
(5) Control the flow rate of molten steel injected into the crystallizer to keep the amount of molten steel in the crystallizer constant to reduce liquid level fluctuations. At the same time, the superheat of the molten steel entering the mold is reduced, thereby reducing the temperature difference between the molten steel and the copper plate of the mold, reducing the thermal stress and shrinkage stress of the shell of the cast slab, increasing the solidification speed, and alleviating liquid level fluctuations.
(6) Maintenance equipment
Eliminate equipment failures and reduce fluctuations in the molten steel level in the crystallizer. In addition, at this stage, some domestic and foreign companies use modern automatic control methods to accurately control the liquid level of the mold to ensure the relative stability of the steel liquid level in the mold. By optimizing and adjusting the crystallizer control system, occasional crystallizer periodic liquid level fluctuations can be reduced. For example: SiemensVAI’s recently launched SIMETALLevCon system uses advanced control algorithms and fully considers factors such as system response lag, tundish plug rod or sliding nozzle blockage, and mechanical losses. It can significantly improve the dynamic behavior of the system and control the effects within the crystallizer at any time. The factors of liquid level fluctuation are controlled to control the liquid level fluctuation within the allowable range to ensure the best quality of cast slab.
If the surface of the molten steel fluctuates too violently, it will cause some defects in the steel. However, if the liquid level is too calm, the slag cannot easily absorb slag and lubricate, and it will also affect the quality of the steel. Therefore, it is important to study the phenomenon of crystallizer liquid level fluctuations. Liquid level fluctuations should be controlled within a certain range rather than completely eliminated. In addition, during the continuous casting process, the state of the molten steel in the mold changes all the time, and the liquid level should be controlled dynamically. Various factors that cause the fluctuation of the mold liquid level, and corresponding measures are taken to effectively control the fluctuation of the mold liquid level, stabilize the constant drawing rate, and at the same time reduce the inclusion rate of the cold-rolled base steel grade, providing guarantee for stable casting.