In the previous article, we discussed mold flux selection guides for different steel grades, mentioning the special requirements of high-aluminum steels. Today, we focus on a rapidly growing family of advanced high-strength steels: high-aluminum TRIP steel and TWIP steel. These steels contain 1-3% aluminum, offering excellent strength-ductility combinations, making them key materials for automotive lightweighting. However, the high aluminum content continuous casting challenges: aluminum reacts violently with SiO₂ in traditional mold fluxes, flux properties, leading to flux thickening, excessive slag rim formation, sticker breakouts, and even complete inability to cast continuously. How can you overcome the continuous casting challenges of high-aluminum TRIP/TWIP steels to achieve stable, efficient production? Wuxi WeiDa Cored Wire Co.,Ltd provides specialized solutions based on novel silica-free mold fluxes and process optimization.
The Continuous Casting Challenge for High-Aluminum Steels: A "Chemical Reaction" Disaster
The aluminum content in TRIP/TWIP steels is typically 1-3%, 10-30 times that of conventional steel grades. During continuous casting, the active aluminum in the molten steel undergoes a displacement reaction with the SiO₂ in traditional silicate-based mold fluxes: 4Al + 3SiO₂ → 2Al₂O₃ + 3Si. This reaction a series of disastrous consequences:
•Drastic change in flux composition: SiO₂ is consumed, Al₂O₃ is generated in large quantities, and the flux's basicity rises sharply.
•Increased melting point: The generated Al₂O₃-rich slag has a greatly increased melting point, causing the flux to "freeze."
•Increased viscosity: The flux thickens, loses fluidity, and cannot lubricate the shell.
•Excessive slag rim formation: Thickened flux accumulates at the meniscus, forming a thick slag rim that hinders shell growth.
•Risk of sticker breakouts: Lubrication failure leads to sticking between the shell and the mold copper plate, potentially causing breakouts in severe cases.
The Dilemma with Traditional Fluxes: Unsustainable "Consumption"
When casting high-aluminum steels with traditional silicate-based fluxes, the flux consumption rate is extremely high. Sometimes within 30 minutes of casting, the flux properties completely fail, forcing an interruption of casting. Even with strategies of frequent new flux addition, the property degradation caused by the reaction cannot be fundamentally solved.
Our Solution: The Silica-Free Mold Flux Revolution
Wuxi WeiDa has developed CaO-Al₂O₃ system silica-free mold fluxes specifically for casting high-aluminum steels, fundamentally the source of the Al-SiO₂ reaction.
First, silica-free design. The SiO₂ content in our flux is controlled below 1% , completely eliminating the material basis for the aluminum-silicon reaction. The main components of the flux are CaO, Al₂O₃, Na₂O, F, Li₂O, etc.
Second, low-melting-point formulation. The eutectic temperature of the CaO-Al₂O₃ binary system is approximately 1400°C, which can meet the melting point requirements for mold flux. By adding fluxing agents like Na₂O, F, Li₂O, we further the flux's melting temperature to 1050-1150°C, ensuring rapid formation of a stable liquid slag layer in the mold.
Third, high absorption capacity. The small amount of Al₂O₃ generated by the reaction is absorbed by the flux without causing changes in flux properties. Our silica-free flux has an extremely high Al₂O₃ absorption capacity (up to 25-30%), ensuring stable flux properties during long casting sequences.
Fourth, optimized physical properties. Tailored to the solidification characteristics of high-aluminum steels, we adjust the flux's viscosity and crystallization behavior:
•Viscosity: Controlled at 0.8-1.2 poise at 1300°C to ensure good lubrication.
•Crystallization temperature: Appropriately increased to form a stable crystalline layer, improving heat transfer uniformity.
•Consumption: Target liquid slag consumption controlled at 0.4-0.7 kg/m².
Synergistic Optimization of Process Parameters
In addition to using specialized flux, optimizing process parameters is equally important:
•Reduce steel superheat: Control superheat within 20-30°C to reduce the thermal on the flux.
•Optimize casting speed: Control casting speed at 1.0-1.4 m/min to avoid excessively high speeds exacerbating the reaction.
•Enhance argon sealing: Prevent secondary oxidation of the steel, reducing the amount of newly generated Al₂O₃ entering the flux.
•Increase flux addition frequency: Use automatic flux feeding systems for small, frequent additions to maintain stable liquid slag layer thickness.
(Image placement suggestion: Photo of mold flux condition in the mold during casting of high-aluminum steel with silica-free flux)
From "Impossible" to "Stable Production"
In the past, continuous casting of high-aluminum TRIP/TWIP steels was considered an "impossible mission." Wuxi WeiDa's silica-free flux technology has turned this "impossible" into "possible." After adopting our solution, several steel companies have achieved stable multi-heat continuous casting of high-aluminum steels, increasing sequence casting heats from 1-2 to 8-12 heats, with slab surface quality meeting cold rolling feed requirements.
If you are developing or producing high-aluminum TRIP/TWIP steels and are troubled by inability to cast continuously or poor surface quality, please visit https://www.weidamaterials.com/ to obtain our specialized solution for continuous casting technology of high-aluminum steels.