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The heat transfer in mold has a vital effect on the quality of strands. As a necessary channel of heat from solidifying strands to water-cooled mold, the heat transfer of mold flux is a hottest topic in the field of mold flux at present, and it is also one important function of mold flux. Controlling the heat transfer of mold flux in mold is beneficial to the casting techniques and defect free strands.In the preivious studies, the heat transfer of mold flux is reflected by crystallization property, thermal conductivity coefficient, thermal diffusive coefficient, and extinction coefficient etc, which can’t be aquired the heat flux density of mold flux quantitatively and directly. The paper constructed a new set-up and method to measure the heat transfer of mold flux. The water-cooled sensor simulates real mold, and it is inserted into the molten slag at 1400℃. The temperature difference of water in and out is collected by computer, and then the heat flux density is calculated. What measured heat flux density includes conductivity and radiative heat transfer, and it reflects the all processes which heat is transferred from liquid slag film to solid slag film, to the Cu-slag interface, and to the mold. So the apparatus can simulate the practical heat transfer of slag film in mold more realistically. The industrial mold fluxes were studied in laboratory, and it was founded that the heat flux density, crystals morphology, mineral compositions, crystals size had a good corresponding relation between simulation and practice. The quantitative equation of transient average heat flux density was attained by lots of experiments, which was q=a-b t . In addition, the relation between average heat flux density of slag film and comprehensive heat flux density of mold was established in the conditions which was common slab at different casting speed. Thus, the measured heat flux density of mold slag in laboratory can be corresponding to the on-line detection in practice. The paper also studied the effect of chemical compositons （basicity, F-, Na2O, Al2O3, MgO, B2O3 and Li2O） and physical properties（melting temperature, viscosity, thickness of solid slag film and crystallization ratio） of mold fluxes on heat flux density, and the influential rules and quantitative equations were achieved. And thus, the index of heat transfer of mold flux had a reliable theory basis for collocating slag scientifically.At last, aiming at the problem of the surface longitudinal crack of peritectic steel for wide and thick slab at Baosteel, the paper utilized the simulator and the results of basic slag systems to optimize the original mold fluxes which could not control the heat transfer well. The optimized processes included the investigation in plant, study in laboratory and the trial in plant. The practice showed the surface longitudinal crack of peritectic steel for wide and thick slab of 3# caster at Baosteel was controlled with the developed mold fluxes effectively.
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