What Is Continuous Casting Mold Flux?
In steel production, continuous casting mold flux is the powder or granular slag applied to the mold surface that forms a protective slag film between the copper mold wall and the solidifying steel shell. Although mold flux represents only a tiny fraction of total tonnage cost, it plays a critical role in determining strand quality. As automotive, marine, power, and other sectors demand higher-grade steel products, mold flux performance matching and dynamic control have become core competitive factors for steel producers.
Continuous Casting Mold Flux Composition
Mold flux for continuous casting consists primarily of SiO2, CaO, Al2O3, Na2O, and K2O oxides. Each component contributes to the flux melting point, viscosity, crystallization properties, and heat transfer characteristics.
SiO2 in Mold Flux
SiO2 is the primary network-forming oxide in continuous casting mold flux, constituting the glassy skeleton of the slag structure. SiO2 content directly determines the basicity ratio (CaO/SiO2), the master control variable for viscosity, melting point, and crystallization behavior. In high-aluminum steel grades, SiO2 in the mold flux can be reduced by aluminum in the steel bath, causing composition drift.
CaO in Mold Flux
CaO is the most important basic oxide in continuous casting mold flux. Paired with SiO2, CaO defines the basicity ratio. In the high-basicity range (CaO/SiO2 > 1.0), increasing CaO raises slag viscosity and crystallization temperature, improving lubrication at high casting speeds. The CaO-SiO2-Al2O3 ternary system is the foundation of most commercial mold flux formulations.
Al2O3 in Mold Flux
Al2O3 is an amphoteric oxide in continuous casting mold flux. In standard fluxes, added Al2O3 raises melting temperature and viscosity while suppressing crystallization. In high-Al steels, aluminum from the steel reduces SiO2, causing Al2O3 accumulation and fundamental slag structure changes.
Na2O and K2O in Mold Flux
Na2O and K2O act as network modifiers that lower slag viscosity and melting temperature. They improve slag fluidity and infiltration into the mold-strand air gap. In ultra-high basicity slags, Na2O promotes nepheline (NaAlSiO4) formation while suppressing high-melting-point cuspidine, significantly altering heat transfer.
Fluoride (F-) in Mold Flux
F- plays dual roles as flux and performance regulator in continuous casting mold flux. It breaks the silica-oxygen network structure, significantly reducing viscosity, melting temperature, and break temperature while stabilizing high-temperature fluidity. F- also promotes high-melting-point cuspidine formation and suppresses wollastonite, increasing slag film crystallization rate and reducing thermal conductivity.
Physical Properties of Continuous Casting Mold Flux
The key physical properties of mold flux include melting point, viscosity, melting rate, and heat transfer control capability. Melting point determines the slag film formation temperature window. Viscosity directly affects lubrication efficiency and slag film uniformity. Melting rate determines the initial slag layer coverage capability. These parameters are interconnected and must be precisely matched to specific continuous casting conditions.
Mold Flux Selection by Steel Grade
Different steel grades require different mold flux properties for continuous casting.
Low Carbon Steel Mold Flux
Low carbon steel has small solidification shrinkage and narrow two-phase zone. Requirements: low viscosity at 1300C (typically 0.1-0.3 Pa-s), moderate-low basicity (R=CaO/SiO2 = 0.9-1.1) to ensure adequate liquid slag film thickness and reduce friction. Fast melting rate with liquid slag layer 8-15mm to meet high-speed casting consumption.
Medium Carbon Steel Mold Flux
Medium carbon steel undergoes peritectic phase transformation with large volume shrinkage and crack sensitivity. Requirements: high basicity (1.2-1.5), high viscosity (0.5-0.8 Pa-s) to increase slag film thickness and buffer thermal contraction mismatch between shell and copper wall. Requires ‘high melting point, low melting rate’ to prevent liquid slag from flowing away too early.
High Carbon Steel Mold Flux
High carbon steel has poor steel flow and serious carbon segregation. Requirements: low basicity (0.5-0.8 Pa-s), high viscosity (0.6-0.8 Pa-s). Mold flux cannot directly change segregation but must ensure uniform slag film heat transfer to avoid local undercooling that induces interdendritic bridging.
How Mold Flux Affects Strand Quality
Surface cracks are directly related to mold flux heat transfer and lubrication performance. If heat transfer is too strong, strand surface cooling rate is too fast, causing thermal stress concentration and crack initiation. If heat transfer is insufficient, surface temperature is too high, also increasing crack risk. Poor lubrication increases friction during drawing, increasing tensile stress on the slab.
Mold Flux and Surface Cracks
Control heat transfer coefficient and friction coefficient within optimal range by adjusting mold flux basicity, viscosity, and slag film thickness. Surface inclusions control depends on mold flux adsorption and assimilation capacity. High-viscosity slag has strong adsorption but limited fluidity; low-viscosity slag has good fluidity but poor adsorption. High-basicity mold flux is more favorable for extracting acidic inclusions.
Mold Flux and Internal Cracks
Internal cracks (such as subsurface cracks, corner cracks) are the most common internal defects in continuous casting. The core cause is stress concentration from uneven solidifying shell. Poor lubrication from high viscosity or insufficient flux inflow increases friction between shell and mold wall, causing tensile stress to exceed material strength limit. Uneven heat transfer from non-uniform slag film causes inconsistent shell growth and stress concentration at weak shell areas.
Mold Flux FAQ for Continuous Casting
Q: What is the primary function of continuous casting mold flux? A: Mold flux serves two simultaneous functions: lubrication of the strand as it descends through the oscillating mold, and regulation of heat transfer between the solidifying steel shell and the water-cooled copper mold wall. These functions are delivered through the slag film that fills the air gap.
Q: How does mold flux viscosity affect casting quality? A: Mold flux viscosity controls slag infiltration rate into the mold-strand gap. Too high viscosity causes insufficient lubrication, sticker breakouts, and sliver defects. Too low viscosity suppresses heat transfer too much, causing irregular shell growth.
Q: Why does high-aluminum steel require special mold flux? A: Aluminum in the steel bath reduces SiO2 from the mold flux, causing composition drift that raises viscosity and reduces lubrication performance. Specialized low-SiO2 or CaO-Al2O3-based fluxes are required for high-Al steel grades.
Conclusion
For mold flux products and continuous casting equipment, consult LMMROLL’s comprehensive product catalog. Continuous casting mold flux chemical composition, physical properties, and process conditions form a highly coupled multi-variable system. Its effects on strand surface cracks, inclusions, internal cracks, and segregation are not isolated but intertwine through lubrication, heat transfer, adsorption, and feeding mechanisms. As high-strength steel and corrosion-resistant steel continue developing, mold flux design must further evolve toward ‘steel grade-process-slagsystem’ collaborative matching, integrating online monitoring and big data analysis to achieve real-time mold flux performance optimization.
