In the previous article, we discussed how to control centerline segregation in high-carbon steel continuous casting slabs. Today, we focus on a topic that is becoming increasingly important with the development of continuous casting technology: mold flux optimization for high-speed continuous casting. As steel enterprises pursue higher production efficiency and lower costs, continuous casting speeds are continuously increasing. Currently, advanced thin slab casting speeds have exceeded 6 m/min, and conventional slab casting speeds are also moving towards 2.0-2.5 m/min. However, the increase in casting speed brings unprecedented challenges to mold flux: insufficient lubrication, abnormal liquid slag consumption, uneven mold heat transfer, and even sticker breakouts. How can you select the right mold flux for high-speed continuous casting to achieve stable, efficient production while ensuring slab quality? Wuxi WeiDa Cored Wire Co.,Ltd provides a comprehensive solution based on low-viscosity, high-crystallization-temperature mold flux and dynamic consumption control.
The Challenges of High-Speed Continuous Casting for Mold Flux: Higher Speed, Higher Requirements
At conventional casting speeds (1.0-1.5 m/min), mold flux has sufficient time to melt and form a stable liquid slag layer. However, under high-speed continuous casting conditions (>1.8 m/min), the residence time of the shell in the mold is significantly shortened, imposing completely different requirements on the mold flux. First, lubrication demand increases dramatically. The higher the casting speed, the greater the relative velocity between the shell and the mold copper plate, and the greater the frictional resistance. If the lubrication capacity of the mold flux is insufficient, the risk of sticker breakouts rises sharply. Second, liquid slag consumption must increase. High-speed continuous casting requires more liquid slag to enter the gap between the shell and the copper plate to maintain a stable lubrication film. However, at excessively high speeds, the liquid slag is easily "flung out" or consumed unevenly. Third, heat transfer control becomes more difficult. At high casting speeds, the shell is thinner, requiring higher uniformity of heat transfer. Non-uniform heat transfer leads to uneven shell thickness, causing cracks.
Limitations of Traditional Mold Flux
Conventional medium-speed casting mold fluxes typically have higher viscosity and lower crystallization temperature to provide adequate lubrication and heat transfer control. However, under high-speed continuous casting conditions, these fluxes often exhibit the following problems: viscosity too high leading to an excessively thin liquid slag layer, unable to provide sufficient lubrication; crystallization temperature too low leading to an unstable crystalline layer, unable to effectively control heat transfer; insufficient consumption leading to a discontinuous slag film, triggering sticking.
Our Solution: Low-Viscosity, High-Crystallization-Temperature Mold Flux
Wuxi WeiDa Cored Wire Co.,Ltd has specially developed low-viscosity, high-crystallization-temperature mold flux series for high-speed continuous casting.
First, reduce mold flux viscosity to improve lubrication capacity. For high-speed continuous casting with casting speeds >1.8 m/min, we recommend controlling the mold flux viscosity (at 1300°C) at 0.6-0.9 poise, significantly lower than the 1.0-1.5 poise for conventional speeds. Low viscosity ensures the formation of a sufficiently thick liquid slag layer in a short time, providing a stable lubrication film. However, excessively low viscosity also risks – the liquid slag becomes too "thin," or be washed away by the steel flow. Therefore, while reducing viscosity, consumption must be controlled through other means.
Second, increase the crystallization temperature of the mold flux to enhance heat transfer control. Contrary to intuition, high-speed continuous casting requires mold flux to have a higher crystallization temperature (1150-1250°C). A higher crystallization temperature means the flux more easily forms a dense crystalline layer on the inner wall of the mold. This crystalline layer has lower thermal conductivity, can effectively "insulate," slow the cooling rate of the shell, and prevent cracks caused by excessively rapid cooling. At the same time, the crystalline layer can "support" the liquid slag film, allowing it to be more uniformly distributed between the shell and the copper plate.
Third, optimize the melting rate and consumption characteristics of the mold flux. High-speed continuous casting requires the mold flux to melt rapidly to keep up with the renewal rate of the steel meniscus. Our mold fluxes control the melting rate at 0.8-1.2 g/cm² per minute (compared to 0.5-0.8 for conventional) by adjusting the basicity (1.0-1.3) and the content of fluxing agents (Na₂O, F, Li₂O) . At the same time, by adjusting the particle size distribution and bulk density of the flux, we ensure stable liquid slag consumption within the range of 0.4-0.7 kg/m².
Fourth, adopt a "dual-layer mold flux" technology to achieve functional separation. For ultra-high casting speeds (>2.5 m/min), we recommend using dual-layer mold flux: the upper layer is an insulating layer, mainly composed of materials with low melting point and low thermal conductivity, used to prevent heat loss from the steel and supercooling of the liquid slag layer; the lower layer is a functional layer, directly contacting the molten steel, responsible for lubrication and heat transfer control. The chemical compositions and physical properties of the two layers are optimized separately, achieving a "1+1>2" effect.
Fifth, optimize the mold flux addition method. High-speed continuous casting requires extremely high uniformity of mold flux addition. We recommend using an automatic flux feeding system with a small, high-frequency addition mode to ensure uniform distribution of the flux across the width of the mold. The addition interval should not exceed 30 seconds, and the single addition amount should be controlled at 0.5-1.0 kg/m².
Effect of Steel Grade on Mold Flux Selection
In addition to casting speed, the steel grade is also an important basis for mold flux selection. For low carbon steel (C<0.08%), the main risk of high-speed continuous casting is sticker breakouts, so lubrication needs priority. We recommend using flux with lower viscosity (0.6-0.8 poise) and medium crystallization temperature. For peritectic steel (C=0.08%-0.15%), the main risk is longitudinal cracks, so heat transfer control needs priority. We recommend using flux with higher crystallization temperature (1150-1250°C) and medium viscosity (0.8-1.0 poise) . For medium and high carbon steel (C>0.30%), the crack risk is relatively lower, so heat transfer control can be relaxed appropriately. We recommend using flux with medium viscosity (0.7-0.9 poise) and lower crystallization temperature.
Synergistic Optimization of Process Parameters
Mold flux optimization must be matched with mold parameters. We recommend:
•Mold cooling water: Control the temperature difference between inlet and outlet at 6-8°C to avoid excessive cooling.
•Mold oscillation: Use a high-frequency, small-amplitude mode (frequency >200 cpm, amplitude 3-5mm), with negative strip time controlled at 0.08-0.12 seconds.
•Submerged entry nozzle: Optimize the port angle (15-25° downward) and immersion depth (120-160mm) to ensure a stable steel flow field.
Quantifiable Benefits
After adopting Wuxi WeiDa's high-speed continuous casting mold flux solution, customers typically achieve: casting speed increased by 20-30% (from 1.5 m/min to 1.8-2.0 m/min), sticker breakout rate reduced by over 50% , incidence of longitudinal cracks on slab surface reduced by over 60% , and mold flux unit consumption reduced by 10-15% .
If you are planning to increase your continuous casting speed, or are experiencing insufficient lubrication, frequent cracks, or sticker breakouts at high speeds, please visit our website https://www.weidamaterials.com/ to obtain the complete solution for high-speed continuous casting mold flux selection and process optimization.