Definition description:
Hanging leak refers to the phenomenon that during the continuous casting process, the local area of the primary billet shell in the crystallizer is connected to the copper plate, corner seam or upper edge of the crystallizer cavity. Under the continuous action of the billet drawing force, the billet shell at the hanging part is gradually thinned and broken, causing the internal molten steel to flow out. From the location of the leak, it often occurs near the outlet of the crystallizer; from the morphology of the leaking billet shell, it can often be found that there are obvious hanging marks and tearing marks on the surface of the billet shell. Hanging leaks can usually be divided into hanging leaks at the corners of the crystallizer and hanging leaks at the top of the crystallizer. Hanging leaks at the corners are often caused by abnormal corner seams of the crystallizer, while hanging leaks at the top are mostly related to the condition of the top of the crystallizer.
Reasons for hanging leaks:
Abnormal corner seams of the crystallizer:
During the long-term use of the crystallizer, due to the high-temperature erosion of molten steel, mechanical vibration and repeated effects of thermal stress, the sealing structure of the corners may be damaged, resulting in larger corner seams. When the corner gap exceeds the normal range, molten steel will drill into it and form cold steel. These cold steels will protrude from the wall of the crystallizer. During the billet drawing process, the billet shell contacts the cold steel, and the resistance increases instantly. The billet shell is very easy to be hung, which leads to hanging steel leakage. When inspecting the continuous casting equipment, a steel plant found that the width of the corner gap of some crystallizers was expanded from the initial 0.2mm to 0.8mm. In subsequent production, the casting machine corresponding to this batch of crystallizers frequently had corner hanging steel leakage accidents.
Surface problems of the crystallizer copper plate (or copper tube):
The quality of the surface of the crystallizer copper plate/copper tube is crucial to the smooth pulling of the billet shell. If the copper plate/copper tube surface has coating shedding, scratches, wear, deformation, etc., the friction between the billet shell and the copper plate will be unevenly distributed. In the area with large friction, the billet shell is easily blocked locally. When the billet drawing force is not enough to overcome this resistance, the billet shell will hang at this part, eventually leading to steel leakage. For example, in one production, a deep scratch appeared on the surface of the copper plate of the crystallizer due to the impact of a foreign hard object. In the subsequent casting process of less than 10 furnaces of steel, a hanging steel leak accident occurred due to the scratch.
Unreasonable crystallizer taper:
The design of the crystallizer taper is to compensate for the shrinkage of the billet during solidification, ensure that the billet shell and the crystallizer wall always maintain good contact, and achieve uniform heat transfer and stable billet drawing. However, when the crystallizer taper is too large, the billet drawing resistance increases significantly, and the billet shell is susceptible to excessive stress during the pulling process, which may cause local damage to the billet shell and increase the risk of hanging steel leaks. On the contrary, if the taper is too small, the air gap between the billet shell and the crystallizer wall will increase, the heat transfer efficiency will be reduced, and the billet shell thickness will be uneven, which is also not conducive to the smooth progress of billet drawing and is prone to steel leaks. When a steel plant was debugging a new crystallizer taper parameter, due to the large taper, the incidence of hanging steel leaks increased by 50% during the trial production stage compared with normal production.
Poor lubrication performance of protective slag:
Protective slag plays an extremely important lubricating role in the continuous casting process. It can form a uniform liquid slag film between the crystallizer wall and the ingot, effectively reducing friction and ensuring that the ingot shell can be smoothly pulled out of the crystallizer. If the lubrication performance of the protective slag is poor, for example, its key performance indicators such as viscosity and melting point do not meet the requirements, the formation of the liquid slag film will be hindered and cannot be evenly covered between the ingot shell and the crystallizer wall. This will increase the friction between the ingot shell and the crystallizer wall, and the ingot shell is prone to local bonding and hanging during the billet drawing process, which will eventually cause hanging steel leakage. After a steel plant replaced a batch of unqualified protective slag, the ingot shell frequently bonded to the crystallizer wall during use, and hanging steel leakage accidents occurred frequently, seriously affecting the normal production.
Fluctuation of billet drawing speed:
The stability of the billet drawing speed is crucial to the smooth progress of continuous casting production. In actual production, if the billet drawing speed suddenly speeds up or slows down, the flow state of molten steel in the crystallizer and the solidification process of the billet shell will change. When the drawing speed increases suddenly, the tensile stress on the billet shell increases in a short period of time, and the billet shell may not be fully solidified at this time, and the strength is insufficient, which may easily cause the billet shell to be pulled apart or hang on the wall of the crystallizer; a sudden decrease in the drawing speed will prolong the residence time of the molten steel in the crystallizer, which may cause the billet shell to over-solidify, increase the friction between the billet shell and the crystallizer wall, and also increase the risk of hanging steel leakage. For example, during the continuous casting production process of a steel plant, due to equipment failure, the billet drawing speed dropped from the normal 1.5m/min to 0.8m/min in a short period of time. Subsequently, during the process of restoring the drawing speed, multiple hanging steel leakage accidents occurred.
Control measures:
Regularly check the copper mould tube:
Establish a complete regular inspection system for the crystallizer, and conduct a comprehensive inspection of the crystallizer during production intervals. Focus on checking the width of the crystallizer corner seam, and use professional measuring tools to ensure that the corner seam is within the specified tolerance range, such as controlling it between 0.2-0.4mm. For crystallizers with excessive corner seams, repair or replace the corner seals in a timely manner. At the same time, carefully check the surface of the crystallizer copper plate. If scratches, wear, deformation, etc. are found, take corresponding measures according to the degree of damage. For example, minor scratches can be polished, and severe deformation requires replacement of the copper plate. Regular inspection and timely treatment can effectively reduce the risk of hanging steel leakage caused by problems with the crystallizer itself.
Reasonably adjust the copper mould tube taper:
According to the characteristics of the steel grade, the cross-sectional dimensions of the ingot, and the actual production situation, accurately calculate and adjust the crystallizer taper. Before production, use simulation software to simulate and analyze the solidification process of the ingot and the force of the billet under different tapers to determine the optimal taper parameters. During the production process, pay close attention to the surface quality of the billet and the change of the billet drawing resistance. If the billet drawing resistance is abnormally increased or abnormal defects appear on the billet surface, fine-tune the taper of the crystallizer in time. For example, for a 150mm×150mm square billet of a certain steel grade, after simulation and actual production verification, when the taper of the crystallizer is adjusted to 0.8%-1.0%, the billet drawing process is smoothest and the hanging steel leakage accident is significantly reduced.
Select high-quality protective slag:
Carefully select protective slag with excellent performance according to factors such as steel grade and continuous casting process. When purchasing protective slag, require suppliers to provide detailed product quality inspection reports, and strictly test the key performance indicators of the protective slag such as viscosity, melting point, basicity, and melting speed to ensure that it meets production requirements. At the same time, establish long-term cooperative relationships with suppliers with good reputation and strong technical strength to ensure the stability and consistency of the protective slag quality, and do not replace it at will due to cost and other issues.
Stabilize the billet drawing speed:
Before production, formulate a reasonable billet drawing speed curve based on the molten steel temperature, steel grade characteristics, and equipment performance of the continuous casting machine. During the production process, closely monitor the changes in parameters such as molten steel temperature and molten steel level in the crystallizer. When these parameters fluctuate, adjust the casting speed in time to maintain the stability of the production process.
