In continuous steel casting, the mold plays a key role in the initial solidification and shaping of molten steel. Through efficient heat exchange, it forms a shell of sufficient thickness from the liquid steel, while also providing a stable foundation for subsequent solidification in the secondary cooling zone. Based on their structural form, billet molds are primarily classified into two categories: copper tube molds (tube-in-tube type) and combined molds. Copper tube molds, with a single copper tube as the core heat transfer element, are widely used in small billet continuous casting machines. Combined molds, constructed from multiple composite panels, dominate the production of large billets and various sizes.
In recent years, as steelmakers have increased their demands for production efficiency and billet quality, the performance differences between the two types of molds have attracted widespread attention. Existing research has primarily focused on optimizing the design of single molds, such as optimizing the copper tube taper and improving the combined panel cooling structure. However, a comprehensive and systematic comparative analysis of the two types is lacking. This paper combines the continuous casting practice data of typical steel grades in some literature and integrates the technical information publicly available on the Internet. Starting from the structural essence, it deeply analyzes the performance advantages and disadvantages and application boundaries of the two types of crystallizers, providing a theoretical basis for industrial selection.
1.Structure and Working Principle of Two Types of copper mould tube
(I) Structure and Principle of the copper mould tube
The copper tube crystallizer is a tube-in-tube structure. Its core components include the mold copper tube, cooling water jacket, vibrating guide device, and foot roller system. Its key component is the integral copper tube, made of copper alloys such as phosphorus-deoxidized copper and chromium-zirconium copper. The wall thickness is typically 6-12 mm. The inner cavity is designed with a single or multiple tapers depending on the shrinkage characteristics of the steel grade (for example, a 150 mm × 150 mm square billet copper tube has an upper taper of 2.02%/m and a lower taper of 0.52%/m). The copper tube is sheathed in a stainless steel water jacket, creating a uniform 5-7 mm water gap. Cooling water flows through the gap at a flow rate of 6-12 m/s for forced cooling.
During operation, high-temperature molten steel comes into direct contact with the inner wall of the copper tube. Heat is rapidly transferred from the copper tube to the cooling water, and the molten steel gradually solidifies within the copper tube along the casting direction, forming a billet shell. The integrated copper tube structure ensures a consistent heat transfer path. Combined with the support provided by the foot roller system for the mold shell, it effectively prevents de-squareness defects.
(II) Structure and Principle of the Modular Crystallizer
The modular crystallizer adopts a modular design, consisting of four independent composite panels (two wide panels and two narrow panels), an outer frame, a clamping mechanism, and an adjustment system. Each composite panel comprises an inner copper plate (20-50mm thick) and an outer steel water tank. Grooves are created between the copper plate and the water tank to form cooling water channels. The four panels are assembled using a clamping mechanism to create the desired billet cross-section. The narrow panels are equipped with upper and lower adjustment devices, enabling online width and taper adjustment to accommodate different cross-sectional specifications.
The operating principle is achieved through the coordinated heat transfer of multiple panels: heat from the molten steel is transferred to the inner copper plates of each panel, where it is then transferred to the cooling water in the cooling channels for removal. The modular structure allows the cooling intensity of each wall panel to be independently designed to meet the heat transfer requirements of different cross-sectional positions, such as differentiated cooling control of the wide and narrow sides of a large square billet.
2. Comparison of the Performance Advantages of the Two Types of Molds
(I) Heat Transfer Advantages
Copper Tube Molds: Uniform Heat Transfer Advantage
The copper tube mold’s monolithic structure eliminates gaps between component joints, ensuring a continuous and consistent heat transfer path and effectively avoiding localized heat transfer bottlenecks. Continuous casting experience with 45 steel at a certain plant has shown that using a three-taper copper tube mold can keep the temperature difference between the corners and the face of the billet within 15°C, a reduction of over 60% compared to a modular mold. This uniform heat transfer characteristic is crucial for suppressing corner cracking defects. Production data from a certain steel plant shows that the internal corner crack rate of 170mm×170mm square billets produced using a copper tube mold is only 1.42%, significantly lower than the comparable figure for modular molds. Furthermore, the copper tube has a relatively thin wall thickness (6-12mm), resulting in low thermal resistance and uniform thermal distribution. Combined with high-velocity cooling water at a speed of 8-12m/s, the average heat flux can reach 2680-3350W/m², ensuring a stable shell thickness of 12mm or above after exiting the mold, paving the way for high-speed casting.
Advantages of Controllable Heat Transfer in the Modular Mold
The modular mold utilizes independent wall panels to achieve zoned heat transfer control, optimizing cooling intensity based on the solidification requirements of different parts of the bloom. For example, in the production of 280mm×380mm blooms, increasing the cooling water flow to 5000L/min on the wide wall panels while controlling the water flow to 800L/min on the narrow side panels ensures uniform shell thickness growth along the cross-section, avoiding central bulging defects on the wide side.
For crack-sensitive steel grades, the modular mold can optimize temperature gradients through localized cooling. A document describing heavy rail steel bloom production practices demonstrates that, when using a modular mold, adjusting the cooling water circuits in the corner wall panels can control temperature fluctuations in the hot zone (25-30 mm from the corner) to within ±5°C, reducing the risk of longitudinal cracking.
(2) Equipment Lifespan and Maintenance Economy Advantages
Copper Tube Mold: Ease of Maintenance
The copper tube mold has a simple structure, and routine maintenance primarily focuses on replacing the copper tubes. Replacing a single tube takes ≤30 minutes, and no specialized tools are required. Thanks to its monolithic structure, it has fewer points of failure, and the maintenance workload is only 30% of that of a modular mold. Statistics show that the average monthly maintenance time for a copper tube mold is approximately 8 hours, while the average monthly maintenance time for a modular mold is over 25 hours due to the need to inspect the wall panel seals and adjustment mechanisms.
For small steel mills, the copper tube mold offers a lower maintenance threshold. Manually repairing minor wear on the copper tube inner wall can extend the mold’s service life to over 400 hours, reducing maintenance requirements.
Advantages of the Modular Crystallizer’s Lifespan and Consumables Economy
The modular crystallizer uses thick-walled copper plates (20-50 mm) as heat transfer elements, coupled with a chrome-plated surface. A single plate has a lifespan of 150-400 hours, and after wear, it can be re-grinded and reused 4-5 times, for a total lifespan of 1000-2000 hours. References to improving the lifespan of copper tubes in the mold indicate that ordinary copper tubes have a total lifespan of only approximately 800 hours, requiring complete scrapping and being irreparable.
In terms of consumables costs, the modular crystallizer’s partially scrappable nature significantly reduces consumption—replacing a narrow plate costs only one-fifth of the cost of replacing a copper tube mold. Data from bloom production at a steel plant shows that the modular crystallizer’s annual consumables costs are 35% lower than those of a copper tube mold.
(3) Advantages of Compatibility with Electromagnetic Stirring
The thin-walled nature of the copper tube mold (6-12 mm) has minimal impact on the penetration of the electromagnetic stirring (EMS) magnetic field, reducing the power requirements of the EMS equipment. Industrial practice shows that in the production of 170mm×170mm square billets, the current required for a copper tube mold equipped with an EMS is only 200-240A. However, due to the severe magnetic field attenuation caused by the copper plate thickness of 20-50mm in a combined mold, the EMS current must be increased to over 300A, resulting in a 30%-40% increase in equipment investment and a 25% increase in operating energy consumption.
For the production of high-carbon steel billets, the synergistic effect of a copper tube mold and an EMS can increase the equiaxed grain rate by 10% and reduce the central segregation index to below 1.1. However, due to the insufficient magnetic field penetration of a combined mold, the equiaxed grain rate improvement under the same conditions is only 5%-7%.
3. Analysis of the Performance Disadvantages of the Two Types of Molds
(I) Inherent Disadvantages of Copper Tube Molds
Limited Cross-Sectional Adaptability
The monolithic structure of copper tube molds dictates a fixed cross-section. Changing specifications requires the complete replacement of the copper tube, water jacket, and other components, resulting in high equipment investment costs. Due to limitations in copper tube manufacturing processes, the largest cross-section currently offered by domestic suppliers is only 300mm × 400mm. Furthermore, large-section copper tubes are susceptible to deformation due to thermal stress, leading to increased air gaps, uneven heat transfer, and a higher risk of breakout. Literature indicates that when a copper tube mold is used for 280mm × 380mm square billets, the corner air gap thickness reaches 0.4mm, more than five times greater than that of a modular mold.
Equipment Lifespan and Consumables Cost Issues
Copper tubes have a relatively thin wall thickness (6-12mm), resulting in insufficient wear resistance at high temperatures. The throughput of a single copper tube is typically only around 5,000 tons. For high-speed production, replacements can average 8-10 times per month. Moreover, copper tubes must be scrapped entirely after wear and cannot be repaired or reused. The long-term operating consumables cost is significantly higher than that of modular molds. Data from a small billet steel mill shows that the annual copper tube consumption cost reaches 1.2 million yuan, 60% higher than that of modular molds.
Inspection and Maintenance Limitations
The closed structure of the copper tube mold makes it difficult to install thermocouples, making it difficult to monitor the internal temperature distribution in real time. Heat transfer status can only be indirectly determined by cooling water temperature, resulting in a significant lag in fault warnings. Furthermore, wear and cracks on the inner wall of the copper tube are difficult to detect online, requiring shutdown and disassembly for inspection, impacting production continuity.
(II) Inherent Disadvantages of the Modular Mold
Insufficient Heat Transfer Uniformity
The modular mold’s wall panel splicing inevitably creates heat transfer gaps, resulting in a 20%-30% reduction in heat flux at corners and joints. Continuous casting of 40Cr steel at a modular mold has shown that the shell thickness at the corners of billets produced is 1-2 mm thinner than at the surface, which can easily lead to corner cracks and requires additional optimization of the secondary cooling system to compensate. Furthermore, the cooling systems for the wide and narrow panels are independently controlled. Improper parameter matching can easily lead to unbalanced temperature gradients, increasing the risk of longitudinal cracking.
Structural Complexity and Installation Difficulty
The modular mold consists of multiple panels, clamping mechanisms, and adjustment devices. Its structure is complex and requires high installation precision—arc deviation must be controlled within 0.3mm, otherwise uneven stress on the billet shell can occur. Its footprint is over 40% larger than that of a copper tube mold, and installation requires specialized personnel. The commissioning cycle can take up to 1-2 days, significantly longer than the 2-3 hours of a copper tube mold.
Initial Investment and Vibration Inertia
The modular design and precise adjustment system of the modular mold result in higher initial equipment investment, increasing by 50%-80% compared to a copper tube mold of the same cross-section. Furthermore, its overall mass (typically exceeding 2 tons) creates significant inertial forces during vibration, requiring a high-power vibration drive system. This not only increases energy consumption but also can lead to increased vibration trajectory deviation, compromising billet surface quality.
4. Conclusion
The core advantages of the copper tube mold lie in its simple structure, uniform heat transfer, good EMS compatibility, and easy maintenance. Its disadvantages include poor cross-sectional adaptability, short equipment life, and high consumables costs. It is suitable for the efficient production of small-section, single-size billets.
The modular mold offers advantages such as high cross-sectional flexibility, long equipment life, and economical partial replacement. However, it has disadvantages such as insufficient heat transfer uniformity, complex structure, and high initial investment. It is suitable for the production of large-section and multi-size billets.
The performance difference between the two mold types stems from their structural design concepts: the copper tube mold focuses on “overall heat transfer consistency,” while the modular mold is guided by “modular flexibility.” Selection requires a comprehensive consideration of the billet cross-section, production specifications, steel grade characteristics, and maintenance capabilities.
