Bill deviation is a common problem in continuous casting production. Billet deviation not only directly reduces product quality, but also indirectly increases production costs and production risks by affecting subsequent processing and equipment stability. Therefore, controlling billet deviation is a key link to ensure continuous casting billet quality and production efficiency.
Impact of billet deviation:
Surface quality defects
Scratches: After the billet deviates, it will squeeze and rub the roller edge of the straightening machine, the guide roller (guide plate) or the roller bearing seat, causing scratches on the surface. For example, in the roller at the rear of the continuous casting machine, the billet deviates and squeezes the bearing seats on both sides of the roller, causing surface wear; when the dummy rod deviates, it may also scratch the billet surface during the delivery of the dummy rod.
Indentation and scratches: When the convex roller straightening machine deviates, the lower surface of the billet will be scratched by the squeezed wheel edge; when the upper roller of the straightening machine deviates and lightly presses down, it will cause the indentation on the upper surface of the billet to be of different depths, affecting the surface flatness.
Excessive cross-sectional dimensions: When the straightening machine deviates or presses down unevenly, the cross-sectional dimensions of the billet will seriously deviate from the design requirements. For example, when the convex roller straightening machine deviates, the billet is pressed away, which cannot meet the requirements of steel rolling.
Internal quality problems
Segregation and cracks: The uneven cooling associated with the deviation of the billet will lead to thermal stress concentration, which may cause internal cracks; at the same time, the deviation during light pressure will cause the pressure position to deviate from the two-phase zone, aggravating the central looseness and segregation. For example, when the straight arc continuous casting machine is not accurate enough, the internal cracks and segregation of the billet are prominent.
Uneven organization: The uneven secondary cooling spray leads to different cooling speeds in different parts of the billet, and the grain size of the internal organization is uneven, which affects the uniformity of mechanical properties.
Impact of subsequent processing
Rolling defects: The deviated billet is prone to plate defects such as edge wave shape and sickle bend after entering the rolling mill, and even causes accidents such as steel piling and tail swinging. For example, the deviation of thin slabs will shift to one side during rolling, making it difficult to thread the belt during high-speed rolling, and the difficulty of finishing rolling adjustment will increase.
Reduced yield rate: Severely deviated billets may be scrapped due to size tolerance, surface defects or internal quality problems, and the number of unplanned materials will increase, resulting in a decrease in yield rate.
Equipment and production impact
Intensified equipment wear: The deviation of the billet will increase the wear of the rollers, guide devices and other equipment of the straightening machine. For example, the deviation of the horizontality of the support rollers of the fan-shaped segment will increase the bearing damage rate and increase the maintenance cost.
Factors affecting the deviation of the billet
The factors affecting the deviation of the billet are relatively complex, mainly involving equipment accuracy, uneven cooling, mechanical force and process operation, as follows:
Equipment accuracy problem: The arc and opening of the sector rollers do not meet the standards, or the supporting structure beams are deformed, which will cause the billet to be unevenly stressed during operation, resulting in deviation. For example, the arc accuracy of the sector rollers is insufficient, and the opening deviation of adjacent rollers is too large, resulting in different support forces on both sides of the billet. ◦
Defects of the straightening machine: Deviations in the horizontality and parallelism of the straightening machine roller surface, or unreasonable design of the convex rollers, will apply uneven force to the billet during the straightening process, causing the billet to deviate. If the upper and lower rollers of the straightening machine are not parallel, the billet will be offset by lateral force.
Guide device problem: The foot roller at the bottom of the crystallizer, the side guide roller of the fan-shaped segment and other guide devices are not installed in place or worn, and the billet cannot be effectively guided, causing the billet to deviate from the center line. For example, the gap between the side guide roller and the billet increases after wear, and the guiding effect is weakened.
Roller abnormality: The rollers at the rear of the continuous casting machine are unevenly worn and installed tilted, which will cause the billet to have different linear speeds on both sides during transportation, resulting in deviation. For example, the diameter difference on both sides of the roller is large, resulting in inconsistent movement speeds on both sides of the billet.
Uneven cooling: The blockage of the secondary cooling water nozzle, the deviation of the water spray angle or the uneven flow rate will cause different cooling intensities on both sides of the billet, causing the billet to deviate due to thermal stress. For example, if the nozzle on one side is blocked, the cooling intensity on that side is reduced, and the billet bends to the side with strong cooling.
Dummy rod problem: The unreasonable design of the dummy rod structure, installation deviation or wear will cause the initial position of the billet to shift during the delivery of the dummy rod, thereby causing deviation. If the connection gap between the sections of the dummy rod is too large, the accumulated deviation will cause the dummy rod to deviate.
Preventive measures for billet deviation
In view of the above-mentioned influencing factors, the following preventive measures can be taken to reduce the occurrence of billet deviation:
- Equipment precision control: Regularly perform offline centering and online arc adjustment on the fan-shaped segments to ensure that the roller curvature and opening meet the standards, and strengthen the measurement and adjustment of the supporting structure beam, and repair or replace deformed parts in time. For example, optimize the arc measurement error value of the fan-shaped segment, control the arc measurement error of the new roller and the worn roller within ±0.1mm, and the absolute value of the arc error of adjacent rollers is not greater than 0.15mm, ensuring that the opening error is within ±0.2mm.
- Optimization of the straightening machine: adjust the horizontality and parallelism of the straightening machine roller surface, install anti-deviation devices, such as adding a guide bracket at the end of the reduction motor to eliminate the horizontality deviation of the straightening roller. At the same time, reasonably design the convex roller structure to avoid uneven force on the billet due to convex roller design problems.
- Improvement of guide device: Set effective side guide rollers in the secondary cooling guide section, select appropriate side guide device types according to the width of the dummy rod and the cross-sectional size of the billet, such as dynamically adjustable side guide rollers, to ensure the guiding accuracy during the operation of the billet. For example, in the case where the width of the dummy rod is designed according to the maximum billet cross-section, a device with dynamically adjustable side guide roller opening is used to meet the guiding requirements of billets with different cross-sections.
- Roller maintenance: Replace the rollers of the rollers with severe wear in time, adjust the horizontality of the roller installation, ensure that the rollers are arranged in parallel, and reduce the deviation of the billet caused by roller problems. For example, add guide devices and positioning devices to the rollers between the first and second cuts to prevent the deviation of the billet.
- Cooling system optimization: Clean the secondary cooling water nozzles regularly to prevent blockage, optimize the nozzle selection and layout, and ensure uniform water spray angle and flow. At the same time, strengthen water quality control, regularly clean the secondary cooling water network, and ensure cooling effect. For example, the water nozzles in the secondary cooling zone are replaced with models with good anti-pollution and atomization performance to reduce the probability of blockage and improve cooling uniformity. At the same time, water is regularly drained at the blind end of the cooling water network in the fan-shaped section to remove debris and improve the cleanliness of the cooling water.
- Adjustment of the dummy rod and process parameters: Improve the structural design of the dummy rod, such as using a guide sleeve, a dummy cap with a guide ring, or an upper floating “V”-shaped roller guide device to improve the accuracy of the dummy rod in feeding the dummy rod. At the same time, according to the characteristics of the steel grade, the process parameters such as drawing speed and light pressure are optimized to reduce the risk of deviation. For example, for high carbon steel, the light pressure parameters are reasonably set to reduce the deviation of the billet caused by uneven solidification shrinkage.
Through the above analysis of the factors affecting the deviation of the billet and the implementation of preventive measures, the incidence of billet deviation can be effectively reduced, and the quality of the billet and the continuous casting production efficiency can be improved. In actual production, it is necessary to combine specific equipment and process conditions and use a variety of measures in combination to achieve precise control of the deviation of the billet.
