In the previous article, we discussed how to achieve ultra-low sulfur steelmaking without extending refining time. Today, we focus on one of the most challenging steel families for continuous casting: peritectic steel. Peritectic steels (carbon content between 0.08% and 0.15%) undergo a δ→γ peritectic phase transformation during solidification, accompanied by approximately 0.38% volume shrinkage. This drastic volume change leads to uneven initial shell shrinkage, making the slab surface highly susceptible to longitudinal cracks, depressions, and even breakouts. For products like automotive high-strength steel, line pipe steel, and electrical steel, surface quality control of peritectic steel is key to ensuring smooth production and high product yield. How can you fundamentally prevent surface cracks during peritectic steel continuous casting to achieve stable, efficient production? Wuxi WeiDa Cored Wire Co.,Ltd provides a comprehensive solution based on mold flux optimization, mold metallurgy control, and microalloying.
The Peritectic Steel Challenge: Phase Transformation Shrinkage and Stress Concentration
During the solidification of peritectic steel, the transformation from δ-ferrite to γ-austenite results in a linear shrinkage of approximately 0.38%, far exceeding that of other carbon ranges. This drastic shrinkage causes the initial shell to lose contact with the mold copper plate, forming an air gap. The air gap leads to uneven heat transfer, locally thinning the shell. Under the combined action of thermal and mechanical stresses, these weak spots develop longitudinal cracks. The crack sensitivity of peritectic steel is closely related to its carbon content. Research shows that the crack risk is highest when the carbon content is in the range of 0.09%-0.12% ; the risk is relatively lower when the carbon content approaches 0.08% or 0.15%.
Limitations of Traditional Methods
Traditional methods for solving peritectic steel cracks – such as reducing casting speed and slowing cooling intensity – while effective, sacrifice production efficiency. Increasing mold flux basicity can reduce heat transfer, but excessively high basicity leads to poor lubrication and increases the risk of sticker breakouts. Using high-viscosity mold flux can also reduce heat transfer but may result in a liquid slag layer that is too thin, leading to insufficient lubrication. The key is finding the optimal balance between lubrication and heat transfer.
Our Solution: High-Performance Mold Flux and Mold Metallurgy Optimization
Wuxi WeiDa Cored Wire Co.,Ltd provides specially designed high-performance mold fluxes for casting peritectic steel. Unlike ordinary mold fluxes, peritectic steel fluxes need to possess the following characteristics.
First, relatively high crystallization temperature. The flux forms a dense crystalline layer on the inner wall of the mold. This crystalline layer can "support" the liquid slag film, allowing it to more uniformly fill the air gap between the shell and the copper plate, thereby improving heat transfer uniformity. Our peritectic steel mold flux crystallization temperature is controlled between 1150-1250°C, which is 50-100°C higher than ordinary fluxes.
Second, appropriate viscosity and basicity. The basicity of peritectic steel mold flux is typically controlled between 1.0-1.3, and the viscosity at 1300°C between 0.8-1.2 poise, to ensure effective lubrication while providing sufficient heat transfer resistance. Basicity that is too low leads to excessively fast heat transfer; basicity that is too high leads to poor lubrication.
Third, controlled crystallization rate. The crystallization rate of the flux needs to match the casting speed. Too fast leads to poor lubrication; too slow fails to provide effective support for the shell. We customize the crystallization kinetics of the flux based on your casting speed range (typically 1.0-1.6 m/min).
Key Parameters for Mold Metallurgy
Beyond the mold flux itself, the metallurgical conditions within the mold are equally critical. Wuxi WeiDa's technical team can assist you in optimizing the following parameters.
Fourth, mold level control. Keep level fluctuations within ±3mm to avoid slag entrapment and abnormal shell growth at the meniscus. Level fluctuation is a major cause of cracks in peritectic steel.
Fifth, mold taper. For peritectic steel characteristics, parabolic taper or multi-stage taper is recommended to better match the shell's shrinkage curve. Traditional single-stage taper is often ineffective for casting peritectic steel.
Sixth, cooling water intensity. Appropriately reduce the intensity and flow rate of mold cooling water to slow the cooling rate of the initial shell and reduce phase transformation stress. We recommend controlling the temperature difference between inlet and outlet cooling water at 6-8°C, rather than 8-10°C for ordinary steel grades.
Seventh, high-frequency, small-amplitude oscillation. Adopt a high-frequency, small-amplitude mold oscillation mode to improve oscillation mark quality and reduce surface defects. Recommended parameters: frequency 180-300 cycles per minute, amplitude 3-6mm.
(Image placement suggestion: Diagram of peritectic steel mold flux forming a crystalline layer to support the shell in the mold)
Microalloying Assistance: Reducing Crack Susceptibility Through Composition Design
Beyond casting parameters, the steel composition itself significantly influences crack susceptibility. Wuxi WeiDa's cored wire technology can assist in reducing peritectic cracks through composition design.
Eighth, titanium microalloying. By feeding ferro titanium cored wire, fine TiN and TiC particles are formed, refining the solidification structure, reducing segregation, and improving the high-temperature ductility of the shell. Recommended titanium content: 0.01%-0.03% .
Ninth, rare earth treatment. As discussed previously, rare earths can modify sulfide and oxide inclusions, improving the high-temperature ductility of the steel. Adding 20-50 grams of mischmetal per ton of steel can achieve significant results.
Tenth, boron microalloying. Appropriate amounts of boron (0.001%-0.003% ) can improve grain boundary strength, enhancing crack resistance. However, boron addition must be with titanium or aluminum for protection to prevent deactivation by reaction with nitrogen.
Systematic Solution: Saying Goodbye to Peritectic Cracks
Surface crack control for peritectic steel cannot be solved by a single measure. Wuxi WeiDa's strength lies in providing a systematic solution: from the selection and supply of specialized mold fluxes, to optimization recommendations for casting parameters, to compositional fine-tuning via cored wire. Our technical team possesses extensive experience in peritectic steel production and can develop a customized peritectic steel continuous casting process package tailored to your specific steel grades and equipment conditions.
After implementing the systematic solution, you will see the following results: longitudinal crack incidence reduced by 60%-80% , slab surface depression depth controlled within acceptable limits, sticker breakout risk significantly reduced, and surface conditioning requirements cut by more than 50% .
If you are producing peritectic steel and struggling with surface cracks and depressions, wishing to achieve defect-free continuous casting, please visit our website https://www.weidamaterials.com/ to obtain the complete solution for peritectic steel continuous casting technology.