In the vast field of continuous casting, the mold serves as the core component for rapidly solidifying molten steel; its performance directly affects billet quality and production efficiency. Among the many components of the mold, the wall thickness of the copper tubes is not merely a geometric parameter but a critical process variable that profoundly influences heat transfer, equipment rigidity, and service life. Many frontline operators have likely encountered the troublesome phenomenon of blackening on the cold face of the mold copper tube, which is often an outward manifestation of improper selection of tube wall thickness or an imbalance in the cooling process. Today, drawing on production experience, we will conduct an in-depth analysis of the multifaceted effects of copper tube thickness.
The Complex Interplay Between Copper Tube Thickness and Multidimensional Process Parameters
It is essential to recognize that the thickness of a crystallizer’s copper tubes does not exist in isolation; it is closely linked to process parameters such as tube material, cooling water inlet configuration, water gap dimensions, water flow rate, coating type, and production draw speed. These factors intertwine to collectively determine the crystallizer’s overall cooling capacity and operational stability.
Excessively Thin Wall Thickness: A Double-Edged Sword
From the perspective of heat conduction, assuming a constant cooling intensity in the mold, the thinner the copper tube wall, the lower the thermal resistance at that location. This means that heat can be removed from the molten steel more efficiently, allowing the copper tube itself to maintain a lower temperature—which, in theory, helps extend the service life of the copper tube. Therefore, in high-drawing-speed production, the appropriate use of slightly thinner copper tubes can enhance cooling.
However, excessively thin wall thickness is a double-edged sword.
Thermodynamic Risk: Interstitial Boiling
If the wall thickness is too thin, it will cause the temperature of the cold face of the copper tube to rise. In extreme cases, this can lead to interstitial boiling of the cooling water in the crescent-shaped area of the mold. The occurrence of interstitial boiling severely disrupts the continuity of the water film, abruptly reducing the mold’s heat transfer efficiency, which can result in cracks in the ingot or even steel leakage accidents.
Mechanical Risk: Insufficient Rigidity
Adequate wall thickness is essential for maintaining the rigidity of the copper tube during high-temperature, high-pressure casting. Insufficient wall thickness leads to inadequate rigidity, causing the copper tube to deform unevenly under thermal stress. This results in inconsistent water gap dimensions, which in turn exacerbates uneven cooling.
Therefore, appropriately increasing the copper tube wall thickness ensures both sufficient rigidity and higher thermal resistance, thereby eliminating the risk of boiling in the cooling water gap caused by excessively high temperatures on the cold side of the copper wall at the meniscus.
Excessive Wall Thickness: Cumulative Temperature Rise and Deformation
In contrast to insufficient wall thickness, excessively thick copper tube walls also have negative consequences.
As wall thickness increases, the thermal resistance inside the copper tube rises significantly, causing the temperature of the hot face (i.e., the surface in contact with the molten steel) to rise markedly. This is a very dangerous sign.
If the temperature at the meniscus rises above the recrystallization temperature of the copper (approximately 300°C for deoxidized copper), it will cause localized softening and permanent deformation of the copper tube. This deformation not only directly affects the geometric accuracy and internal quality of the ingot but also significantly shortens the service life of the copper tube and increases production costs.
Practical Case: Overheating-Induced Blackening and Process Improvement
In production practice, theory is always validated through specific cases. During the casting of 170-240 × 425 mm square-section ingots, we encountered a distinct blackening phenomenon on the cold face of the copper tube. This is a typical symptom of overheating, indicating that local temperatures had far exceeded the normal cooling capacity of the water.
Through detailed comparative process analysis, we found that the blackening was primarily concentrated in the meniscus region (i.e., the leading edge where the molten steel meets the copper tube) and gradually diminished with increasing depth. This further confirmed that the meniscus region experiences the highest thermal load and is the most vulnerable to cooling.
To address this challenge, we implemented on-site improvements by appropriately increasing the cooling intensity of the mold. The results demonstrated a significant reduction in the overheating of the copper tubes. This indicates that by optimizing the cooling water flow rate and water gap dimensions to enhance heat removal, the temperature distribution within the copper tubes can be effectively balanced. Of course, adjustments to cooling intensity must not be arbitrary; they require consideration of the crack sensitivity requirements of different steel grades (such as high-carbon steel, low-carbon steel, and low-alloy steel) to achieve optimal process conditions.
Summary
In summary, the wall thickness of the copper tube in the mold not only affects the magnitude of the metal’s thermal resistance but also profoundly influences the stability of the cooling water film (preventing boiling) and the mechanical rigidity of the equipment. A reasonable copper tube wall thickness is the core process foundation for ensuring efficient heat transfer, billet quality, and long-term stable operation. Whether in equipment selection for new projects or process optimization for existing projects, it is necessary to select the most “appropriate” copper tube thickness based on actual material, coating, drawing speed, and steel grade requirements, supplemented by a matching cooling process, thereby achieving safe, efficient, and high-quality continuous casting production.
E-mail： cykc@cykcgroup.com.cn