In the steelmaking continuous casting process, especially for square billets, serpentine bending of billets is often encountered, especially in the long-term continuous production process, the equipment cannot be repaired, and the secondary cooling cannot be effectively maintained, the frequency of serpentine bending of billets will be higher.
The so-called serpentine bending refers to the irregular bending phenomenon along the length direction of the billet during the continuous casting process, which seriously affects the quality of the billet and the subsequent rolling process. The following are the reasons for its formation:
1. Equipment factors:
⑴ The crystallizer is not installed vertically: The crystallizer is a key component for billet forming during continuous casting. If the crystallizer is not installed vertically, the billet will be subjected to uneven force at the initial stage of solidification, resulting in billet bending.
⑵ Insufficient installation accuracy of guide rollers: The guide rollers of the continuous casting machine are used to support and guide the billet. If the guide rollers are not installed accurately, such as uneven spacing between the guide rollers and non-parallel axes of the guide rollers, the billet will be subjected to uneven friction and support forces during operation, resulting in serpentine bending.
⑶ Inaccurate arc alignment of the fan-shaped segment: The fan-shaped segment is an important component for cooling and supporting the billet in the continuous casting machine. Inaccurate arc alignment will cause the billet to be subjected to abnormal pressure and bending force when passing through the fan-shaped segment, prompting the formation of serpentine bending.
⑷ Poor maintenance of secondary cooling: If the nozzles in the secondary cooling zone are blocked or the spraying is uneven, the thickness of the solidified shell will be different, and the uneven shrinkage of the billet shell will cause the billet to bend.
⑸ Abnormal leakage of equipment water system: If the water in the continuous casting equipment leaks and a large amount of water is sprayed onto the ingot, it will also cause abnormal bending of the ingot and cause serpentine movement.
2. Process factors:
⑴ Steel temperature is too high: If the temperature of molten steel is too high, the solidification speed of the billet will slow down, the thickness of the billet shell will be thinner, and the billet will be easily deformed under the action of gravity and external force, increasing the possibility of serpentine bending.
⑵ Drawing speed fluctuation: The sudden change of drawing speed will change the friction and tension of the billet, destroy the stress balance during the solidification process of the billet, and cause the billet to bend.
⑶ Uneven cooling: During the cooling process of the billet, if the cooling water flow is unevenly distributed (mainly uneven water distribution in each area, causing a large temperature return), the shrinkage of various parts of the billet will be inconsistent, resulting in thermal stress, thereby causing serpentine bending.
⑷ Steel characteristics: Some high-alloy steels (such as stainless steel) have large solidification shrinkage, and the segregation of low-melting-point elements aggravates the bending tendency.
3. Abnormal operation:
⑴ Continuous casting accidents cause billet flatness. The billet stays in the second cooling chamber for too long and the temperature is too low to be straightened.
⑵ Due to long-term low-speed casting, the billet temperature is too low and the bending will be aggravated.
The following are the prevention and elimination methods:
1. Equipment maintenance and debugging
⑴ Accurately install the crystallizer: When installing the crystallizer, use high-precision measuring instruments, such as laser measuring instruments, to ensure that the verticality and horizontality of the crystallizer are within the specified range, and regularly check and adjust the installation accuracy of the crystallizer.
⑵ Improve the installation accuracy of the guide roller: When installing the guide roller, strictly follow the design requirements to ensure that the spacing between the guide rollers is uniform and the axis is parallel. Use advanced installation technology and testing equipment, such as electronic levels, laser centering instruments, etc., to accurately measure and adjust the installation accuracy of the guide rollers.
⑶ Accurately align the fan-shaped segment: When aligning the fan-shaped segment, use a professional arc template or laser arc alignment system to ensure that the arc of the fan-shaped segment matches the outlet arc of the crystallizer, and regularly check and calibrate the arc accuracy of the fan-shaped segment.
4. Strengthen the maintenance of the secondary cooling chamber: clean up the blocked water nozzles in time and replace them regularly to ensure the spraying effect of the secondary cooling. Strengthen the maintenance of the secondary cooling chamber: clean up the blocked water nozzles in time and replace them regularly to ensure the spraying effect of the secondary cooling.
2. Optimize process parameters
⑴ Control the temperature of molten steel: strictly control the tapping temperature and pouring temperature of molten steel, and formulate a reasonable temperature control range according to the different steel types and ingot sections. Use molten steel refining technology, tundish heating technology and other means to stabilize the temperature of molten steel and reduce temperature fluctuations.
⑵ Stabilize the drawing speed: formulate a suitable drawing speed according to factors such as the temperature, composition and cross-sectional size of the molten steel, and maintain the stability of the drawing speed through an automated control system. Avoid large fluctuations in the drawing speed due to changes in production rhythm, equipment failure and other reasons.
⑶ Uniform cooling: optimize the design and layout of the cooling system to ensure that the cooling water flow is evenly distributed on the surface of the ingot. Use advanced cooling technologies, such as mist cooling and dynamic cooling control, to adjust the cooling water volume in real time according to the solidification state of the ingot, so that all parts of the ingot are evenly cooled.
Through systematic process control and equipment management, serpentine bending can be effectively suppressed, providing high-quality ingots for subsequent hot rolling.
