In the continuous casting process, the copper mould tube is a critical piece of equipment for molten steel solidification and molding. Mold level protection is crucial for ensuring a smooth continuous casting process and improving ingot quality. Good mold level protection effectively isolates the air, prevents secondary oxidation of the molten steel, reduces the formation of inclusions, and stabilizes the solidification process, profoundly impacting the internal and surface quality of the ingot. The following details common methods for mold level protection.
Optimizing Mold Equipment and Operation
Optimizing Submerged Nozzles
(1) Structural Optimization: The structure of the submerged nozzle has a significant impact on the flow field and liquid level stability within the mold. Using side-port submerged nozzles, such as the Shinagawa mushroom type and the step-down type, can redirect the steel flow and reduce the impact of the steel flow on the mold surface, thereby minimizing liquid level fluctuations and reducing the risk of slag entrainment and secondary oxidation. By optimizing parameters such as the angle, number, and diameter of the side holes, the steel flow can be more evenly distributed within the mold, creating a stable flow field that facilitates the flotation and removal of inclusions, thereby improving ingot quality. Research has shown that compared to straight-hole nozzles, side-hole nozzles can reduce mold level fluctuations from ±15mm to ±5mm, reducing the risk of slag entrainment by approximately 70%.
(2) Insertion Depth Control: The insertion depth of submerged nozzles requires precise control. If the insertion depth is too shallow, the impact of the steel flow on the liquid surface can be excessive, leading to turbulence and slag entrainment, increasing the chance of the molten steel coming into contact with air. However, if the insertion depth is too deep, it can hinder the flotation of inclusions, compromising the purification of the molten steel. Different billet sizes and steel grades require different submerged nozzle insertion depths. Generally speaking, for continuous casting of Φ250mm round billets, a nozzle insertion depth of 130-155mm results in a liquid level fluctuation of less than 9mm, and a significantly lower hot-rolled plate defect index than at insertion depths of 125mm and 160mm. In actual production, the optimal submerged nozzle insertion depth must be determined through testing and simulation analysis based on specific process parameters and billet quality requirements, and its stability must be strictly controlled during production.
Mold Level Detection and Automatic Control
(1) Molten Level Detection Technology: Accurately detecting the mold liquid level is a prerequisite for achieving stable mold level control. Currently, commonly used mold level detection methods include radioisotope, eddy current, and laser methods. The radioisotope method, such as the radioisotope Co60 liquid level gauge, uses the absorption of gamma rays by the molten steel to indicate the liquid level. This method offers advantages such as stable measurement and high accuracy, but carries certain radioactive safety risks. The eddy current method, based on the principle of battery induction, measures the liquid level by detecting changes in the magnetic field above the mold. It features high accuracy, fast response, and immunity to mold slag, making it suitable for all types of continuous casting machines. The laser method, based on laser ranging technology, utilizes the time delay between laser pulse emission and reception to measure the molten steel level. It offers advantages such as a wide measurement range, fast response, and high accuracy. However, the thickness of the mold slag may affect the measurement value, and the reflector in the measuring tube requires regular replacement. These detection technologies provide reliable means for real-time monitoring of the mold liquid level.
(2) Automatic control system: Automatic control of the molten steel level in the crystallizer can reduce the operator’s labor intensity, improve the accuracy and stability of liquid level control, and thus improve the surface quality of the ingot. Automatic control of the crystallizer liquid level is divided into flow type, speed type, and hybrid type according to the control type. The flow type controls the flow of molten steel entering the crystallizer, that is, controls the opening of the stopper rod or sliding gate to maintain a stable liquid level; the speed type controls the casting speed to maintain a stable liquid level. This method is widely used in the continuous casting of small square billets and has less splashing; the hybrid type controls the casting speed and the molten steel flow rate to control the liquid level. In terms of control algorithms, in addition to conventional PID control and improved algorithms based on PID, algorithms based on modern control theory are also used, such as liquid level control strategies based on zero-pole configuration, self-correcting controllers, predictive control, adaptive control, etc., as well as intelligent control methods for the crystallizer liquid level, such as fuzzy control and expert systems. These advanced control technologies comprehensively account for multiple interfering factors, including variations in mold vibration frequency and amplitude, mold width, molten steel weight in the tundish, and casting speed. They precisely control the mold liquid level, ensuring that liquid level fluctuations are kept within a narrow range, generally requiring a liquid level deviation of less than ±3mm, thereby ensuring high-quality casting.
Standardized Operational Procedures
(1)Start-Pouring and Steady-State Pouring Operations: During the start-pouring phase, the molten steel level in the mold is unstable, making exposed steel a concern. This requires special attention to protection. Start-pouring slag should be added promptly. It should have a low melting point (1150-1200°C) and fast melting properties to quickly form a protective slag layer and prevent exposed steel from absorbing nitrogen during start-pouring. During steady-state pouring, the casting speed must be strictly controlled, maintaining fluctuations within ±0.1 m/min. A stable casting speed helps maintain a stable flow of molten steel and a stable liquid level in the mold. At the same time, ensure the proper addition and melting of mold slag. Slag must be removed promptly and replenished immediately after removal. The amount of slag replenished can be calculated based on the exposed area of the mold surface, generally calculated as “exposed area x 0.5 kg/m²” to prevent the molten steel from being exposed for extended periods after removal.
(2) Abnormal Situation Handling and Operation During Unsteady Phases: When abnormal conditions such as mold surface fluctuation exceeding 10 mm or mold slag crusting occur within the mold, the operator must take immediate action. First, reduce the casting speed to slow the flow of molten steel and minimize surface fluctuations. Then, manually replenish fine-grained mold slag to enhance the protective effect. Enhanced protection is also necessary during unsteady phases such as water inlet and ladle changes. When changing the water inlet, reduce the casting speed in advance (to approximately 70% of the normal casting speed) and increase the mold slag layer thickness to approximately 15 mm. Replenishing slag to the normal thickness within 3 minutes after the water inlet change can effectively reduce problems such as nitrogen accumulation. When changing ladles, the tundish liquid level must be maintained high (>700mm) to prevent the tundish liquid level from falling too low and forming slag vortices, thereby reducing the nitrogen content of the tail billet. By standardizing these operating procedures, the mold level can be effectively protected under various operating conditions, ensuring smooth continuous casting and consistent billet quality.
Mold level protection is an essential component of continuous casting. Through various common methods, such as the appropriate application of mold slag, the use of gas shielding, and optimized mold equipment and operation, it effectively isolates the atmosphere, prevents secondary oxidation of molten steel, stabilizes the solidification process, and improves billet quality and production efficiency, laying a solid foundation for steel companies to produce high-quality steel. In actual production, steel companies should comprehensively utilize these mold level protection methods based on their own process characteristics and product requirements, and continuously optimize and innovate them to meet increasingly stringent market demands and quality standards.
