During the pouring process of continuous casting machine, billet warping accidents often occur, which has many negative impacts on the smooth progress of production, the safe operation of equipment and the quality of billets. This article deeply explores the hazards and causes of billet warping accidents, and formulates effective countermeasures to ensure the stable and efficient operation of continuous casting production.
Hazards of billet warping accidents
Equipment damage
Sector and straightening machine: When the billet warps too high, it cannot pass through the sector or straightening machine roll gap at the straight section normally. In severe cases, the billet will press against the upper roller of the sector or straightening machine. The strong force may cause the roller to deform, the bearing to be damaged, and even the structural components of the entire sector or straightening machine to be damaged.
Flame cutting machine: Billets with too large warps may collide with some equipment of the flame cutting machine during movement. This can easily damage the cutting gun of the flame cutting machine, causing the nozzle of the cutting gun to deform and damage, affecting the cutting accuracy and efficiency. At the same time, the collision may also cause impact on the transmission mechanism, control system, etc. of the cutting machine, causing failures, increasing equipment maintenance costs and downtime.
Production interruption
Affecting casting continuity: Once the billet warps, it is often necessary to take immediate measures to deal with it, such as reducing the pulling speed, stopping the machine for adjustment, and manually cutting the head billet. This will disrupt the originally stable casting rhythm, making continuous casting production unable to proceed continuously, and in severe cases, causing production interruption.
Reduced quality of ingots
Internal defects: During the process of billet warping, uneven stress distribution will be generated inside the billet shell. This stress may cause defects such as cracks and looseness inside the billet, reducing the internal quality of the billet. When these billets with internal defects enter the steel rolling process, the defects may further expand during the rolling process, affecting the performance and quality of the steel.
Surface quality problems: When the billet with warping collides or rubs with equipment parts, scratches, dents and other defects will occur on the surface of the billet, seriously affecting the surface quality of the billet. Billets with unqualified surface quality require additional processing in the subsequent processing, which increases production costs and may even become scrap due to failure to meet quality requirements.
Analysis of the causes of billet warping
Uneven cooling
Cooling differences in the second cooling zone: The arc structure of the continuous casting machine causes differences in cooling conditions between the inner arc and the outer arc in the second cooling zone. After the cooling water of the inner arc is sprayed onto the surface of the billet, it will flow down along the surface of the billet, making the cooling intensity of the inner arc side relatively large; while the cooling water of the outer arc will fall directly to the ground after being sprayed onto the surface of the billet, and the cooling intensity of the outer arc side is relatively small. This uneven cooling intensity can easily induce the tendency of the billet head to warp upward. For example, in some actual production, by measuring the temperature of the inner and outer arc sides of the billet, it is found that the temperature of the inner arc side is significantly lower than that of the outer arc side, which leads to inconsistent contraction of the inner and outer arcs of the billet, causing the warping phenomenon.
Abnormal cooling at the contact point between the crystallizer and the dummy rod: The billet has a greater cooling intensity at the contact point between the billet head and the dummy rod head due to factors such as heat conduction of the dummy rod. This will cause the billet in this part to shrink faster, resulting in a difference in contraction with other parts, which will lead to the billet head warping. In addition, if the solidification rate of the molten steel in the crystallizer in this part is uneven, this shrinkage difference will also be aggravated, increasing the possibility of billet warping.
The drawing speed does not match the cooling speed
The drawing speed is too low: When the drawing speed is low, the billet stays in the crystallizer and the secondary cooling zone for a long time, and the billet is overcooled, resulting in excessive shrinkage of the billet. Especially in the early stage of pouring, the head of the billet has not yet completely solidified. At this time, if the drawing speed is too low, the head is more likely to curl up due to excessive shrinkage.
Improper adjustment of the drawing speed: During the continuous casting process, the drawing speed needs to be reasonably adjusted according to factors such as the molten steel temperature and the cross-section of the billet. If the drawing speed is not adjusted to the target drawing speed in a timely and stable manner after pouring, and the time for the starting drawing speed to be adjusted to the target drawing speed is too long, the billet will also shrink unevenly and cause warping.
Casting machine equipment factors
Continuous casting machine arc accuracy deviation: The continuous casting process requires the continuous casting machine to have a high arc accuracy, and generally requires the gap between the arc template and the outer arc surface of the support roller to be less than 0.3mm. When the arc accuracy of the continuous casting machine exceeds the allowable range, the billet cannot move in the designed arc route when the machine is turned on, and the billet drawing resistance increases. This will affect the connection between the billet head and the ingot starter head, and the billet will be subjected to uneven force during movement, which may easily lead to the billet warping.
Ingot starter system problem: If the ingot starter rod is deformed or worn during use, it will also affect the connection effect with the billet and increase the possibility of billet warping.
Influence of steel grade characteristics
Different steel grades have different physical properties, such as yield strength, thermal expansion coefficient, etc. For some high-alloy steels, high-nitrogen stainless steels and other steel grades, the yield strength and hardness are relatively high. During the continuous casting process, the billet is more likely to warp due to changes in internal stress.
Measures to prevent billet warping
Optimize cooling process
Reasonably control the cooling water volume and sequence of the second cooling zone: By controlling the opening sequence of the water spray valves of each cooling circuit in the second cooling zone, adjust the cooling water volume on the inner and outer arc sides of the continuous casting billet, so as to ensure uniform cooling of the inner and outer arcs of the billet, reduce the billet’s deformation resistance, and inhibit the occurrence of billet warping. At the same time, according to factors such as steel type and billet cross-section, accurately calculate and adjust the water spray volume of each cooling circuit to ensure appropriate cooling intensity.
Optimize billet drawing process
Reasonably set the starting drawing speed and drawing speed adjustment strategy: Determine the appropriate starting drawing speed according to factors such as steel type characteristics and billet cross-section. For steel types that are prone to warping, appropriately increase the starting drawing speed, shorten the time for adjusting the starting drawing speed to the target drawing speed, and reduce the possibility of billet head warping caused by excessive billet shrinkage. At the same time, during the drawing speed adjustment process, adopt a smooth and continuous adjustment method to avoid frequent acceleration and deceleration.
Achieve matching of drawing speed and cooling speed: Establish dynamic control between drawing speed and cooling water volume, and adjust the cooling water volume in the secondary cooling zone in real time according to the change of drawing speed. When the drawing speed increases, increase the cooling water volume accordingly to ensure that the billet can be fully cooled; when the drawing speed decreases, reduce the cooling water volume appropriately to prevent the billet from being overcooled. In this way, a good match between the drawing speed and the cooling speed is achieved, reducing the risk of warping of the billet due to uneven shrinkage.
Strengthen equipment maintenance and management
Ensure the arc accuracy of the continuous casting machine: Regularly inspect and adjust the arc accuracy of the continuous casting machine to ensure that it meets the process requirements. When shutting down for maintenance, use professional inspection tools, such as arc templates, feeler gauges, etc., to inspect the outer arc surface of the support roller of the continuous casting machine, and promptly discover and correct the arc accuracy deviation. At the same time, strengthen the daily maintenance of the continuous casting machine equipment to prevent the equipment from changing the arc accuracy due to vibration, deformation and other reasons.
Optimize the ingot dummy system: Regularly inspect and maintain the ingot dummy rod, and promptly discover and deal with problems such as deformation and wear of the ingot dummy rod.
Adjust the process according to the characteristics of the steel grade
For special steel grades such as high alloy steel: In the continuous casting process, in addition to the above-mentioned general preventive measures, it is also necessary to further optimize the process parameters according to their special physical properties. For example, high pressure is used in the process of drawing the ingot, and then it is turned to low pressure after the ingot rod drives the head billet through the first straightening machine to reduce the bonding resistance between the head billet and the straightening machine. At the same time, in the process of drawing the billet, the water flow rate of the crystallizer and the starting drawing speed are further increased to better adapt to the solidification characteristics of special steel grades and reduce the occurrence of billet warping.
