Bill shaking is a common problem in continuous casting production, which will affect the quality of the billet and cause steel leakage and shutdown in severe cases. Its causes are complex and closely related to equipment, process parameters and steel grade characteristics. High carbon steel and high alloy steel occur more frequently. The following is a systematic explanation from the two aspects of cause analysis and improvement measures:
Core reasons
Steel strength differences:
High carbon steel and high alloy steel (such as GCr15, 28GrMo40) have higher carbon and alloying elements (Cr, Mo, etc.), and the billet shell shrinkage rate is greater during solidification, and the requirements for the crystallizer taper are more stringent. If the crystallizer taper is too large, it will significantly increase the billet drawing resistance (such as when the taper is too large, the billet drawing resistance can increase by 20%-30%), making it difficult to demold the billet shell and causing shaking. The high-alloy steel billet shell has higher strength and poorer plasticity at high temperatures. When the friction in the crystallizer exceeds the weight of the billet shell itself, it is easy to produce periodic vibration. Alloy elements increase the solidification temperature range of molten steel, and the growth of the billet shell is uneven, resulting in uneven heat transfer in the crystallizer, local overheating and deformation of the water seam, and further increasing the friction resistance. High-carbon steel liquid has high viscosity, and the protective slag is difficult to penetrate. The liquid slag layer is thin and the lubrication effect is poor, which increases the proportion of solid friction at the meniscus and significantly increases the friction force (for example, when the lubrication is poor, the friction force can increase by more than 50%).
Matching of protective slag performance and steel type:
The lubrication effect is significantly different when using different protective slags. High-carbon steel and high-alloy steel are more sensitive to the viscosity and melting point of protective slag. For example, when using high-viscosity protective slag (such as viscosity > 0.6Pa・s), the liquid slag is difficult to penetrate and the lubrication effect is poor, resulting in an increase of 20%-30% in the billet drawing resistance and causing shaking. Key parameters: protective slag viscosity (η) and liquid slag layer thickness (dL) directly affect the friction resistance formula fL=η(Vm−Vc)/dL. The higher the viscosity and the thinner the liquid slag layer, the greater the friction force.
When the consumption of protective slag is lower than 0.3kg/t, a complete liquid slag film cannot be formed, and the billet shell directly rubs against the crystallizer. Especially during the continuous casting of high-alloy steel (such as 28GrMo40), due to the high strength of the billet shell, the friction resistance increases sharply and easily causes shaking.
Crystallizer taper is too large:
When the crystallizer taper is too large, when the molten steel solidifies and shrinks, the excessive taper increases the extrusion stress of the inner wall of the crystallizer on the billet shell, resulting in an increase in demoulding resistance, which will significantly increase the billet drawing resistance. When the friction force exceeds the weight of the billet in the arc section, it will cause shaking. For example, when the taper is too large, the billet drawing resistance can be increased by 20%-30%. High-alloy steel (such as 28GrMo40) is more likely to break the weight balance due to the high strength of the billet shell. In “A Brief Discussion on Practical Experience in Solving the Problem of Continuous Casting Jitter”, when a steel plant produced Φ150 section GCr15 steel, the crystallizer taper was too large (the reverse taper exceeded 5.2%・m), resulting in a surge in friction resistance 300mm below the meniscus, causing high-frequency shaking.
Deviation in arc alignment and support system
High-alloy steel billets have stronger rigidity, and higher arc alignment accuracy requirements are required for billet support rollers and closely spaced rollers. If the gap between the billet support rollers exceeds 10-30mm (such as when producing 28GrMo40), or the arc deviation of the closely spaced rollers leads to uneven support of the billet, and the imbalance between gravity and friction in the second cooling section causes shaking.
Equipment wear (such as failure to adjust the billet support rollers in time after wear) has a more significant impact on high-alloy steel, because of its high billet shell strength, a small support deviation can cause periodic shaking.
Preventive measures
Crystallizer selection:
Different series of varieties use different taper crystallizer copper tubes. Many companies select the same taper copper mould tubes for different steel grades during production, which causes shaking when producing some high-carbon steel and high-alloy steel, especially in the early stage of copper tube use.
| Steel type | Representative steel grades | Shrinkage characteristics | Recommended back taper range | Document case support |
| High carbon bearing steel | GCr15 | High shrinkage (1.0%-1.1%) | 3.8%·m-4.1%·m | 《Brief discussion on practical experience in solving the problem of continuous casting jitter.pdf》: When a steel plant produced Φ150mmGCr15, the taper dropped from 5.2%·m to 4.0%·m, and the jitter rate dropped from 85% to 5%. |
| High alloy structural steel | 28GrMo40、 26CrMo | Extremely high shrinkage(1.2%) | 3.9%·m-4.0%·m | 《Analysis and improvement measures of jitter problem of small square billet continuous casting_Zheng Xueran.pdf》: Through mathematical model calculation, when the pulling speed of 28GrMo40 steel is 1.8m/min, the taper should be 3.9%·m to avoid jitter caused by excessive resistance. |
| Low carbon steel | 20、16Mn | Low shrinkage (0.8%) | 4.5%·m-4.8%·m | 《Brief discussion on practical experience in solving the problem of continuous casting jitter.pdf》: When producing 150 square low-carbon steel, the billet pulling resistance was moderate when the taper was 4.6%·m, and no jitter problem occurred.
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Use of protective slag:
Use protective slag with low viscosity and low melting point to ensure that the thickness of the liquid slag layer is 8-15mm to improve the lubrication effect. Improve the automatic liquid level control system to reduce liquid level fluctuations (controlled within ±5mm) to ensure uniform spreading of the protective slag; at the same time, use an automatic adding device to ensure stable consumption of protective slag. Special protective slag is required for high-carbon steel and high-alloy steel, such as CaO-SiO2-Al2O3-based slag system. By adjusting the alkalinity and viscosity, it can adapt to the shrinkage characteristics of steel grades and reduce friction resistance.
Improve arc calibration accuracy:
Use the double benchmark of arc template and ingot guide rod to adjust the arc of the support roller and the close-packed roller to ensure that the gap is ≤0.1mm and improve the accuracy of the equipment.
