Abstract: In the context of the global "dual carbon" strategy,ultra-high speed thin slab continuous casting has become a key technology for the green transformation of the iron and steel industry,owing to its significant advantages in energy saving,emission reduction,and product quality.However,high casting speeds（6-8m/min）lead to issues such as unstable molten steel flow and inadequate lubrication by mold flux,which hinder further development.Although electromagnetic braking（EMBr）technology can suppress molten steel turbulence through applied magnetic fields,its specific effects on mold flux properties remain unclear.This study systematically investigates the evolution of physicochemical properties of ultra-high speed thin slab mold flux under magnetic fields.The results indicate that a mold flux with a basicity（R）of 1.67,viscosity of 0.22 Pa·s,melting point of 1 042℃,and a Na₂ O-Li₂ OF flux system exhibits excellent process adaptability,meeting the requirements of rapid melting and uniform lubrication at high casting speeds.Under a magnetic field intensity ranging from 0 to 90 mT,the crystallization behavior of the mold flux is significantly influenced:increasing magnetic field strength advances nucleation time,prolongs the crystallization interval,inhibits crystal growth,and increases solidification volume expansion by 23%.Microstructural analysis reveals that the magnetic field promotes preferential growth of the Ca_0.87Mn_0.19Mg_0.94Si₂O₆ phase,increasing its proportion by 5.2%,and induces a transition in the silicate network from a disordered to an ordered structure.Water quenching experiments combined with XRD and Raman spectroscopy confirm that the magnetic field alters solute transport by suppressing melt flow,thereby regulating the solidification shrinkage behavior and microstructure of the mold flux.This study reveals,for the first time,the performance evolution mechanism of ultra-high speed casting mold flux under an electromagnetic field,providing a theoretical basis for developing mold fluxes compatible with EMBr technology and contributing to the industrial application of ultra-high speed continuous casting.