Based on a series of related investigations,a mechanism for the formation of longitudinal surface cracks on a continuous casting slab of high-carbon steel was investigated. High-temperature tensile tests performed on slab samples,metallographic and scanning electron microscopy studies,as well as heat flux and shell thickness in continuous casting predicted on the basis of a mathematical model were conducted. The results showed that highcarbon steel exhibited a much lower liquidus temperature and a wider brittle temperature range immediately after solidification compared with those of low-carbon steel. Concentrations of elements K and Na,which are contained in the mold fluxes,were not observed in the cracks during this study. The calculated results showed that the heat flux and the shell thickness were uneven along the mold width and that the shell was thinner and close to the center line of the slab surface. The longitudinal cracks were observed in situ using confocal laser scanning microscopy,to first occur close to the solidification front,where the ductility was extremely low;in addition,the shell growth was slower than in other regions,which led to a thinner shell and depressed shrinkage owing to a lack of lubrication by the mold fluxes below the meniscus. Furthermore,the pouring temperature of high-carbon steel is ~ 100 ℃ lower than that of low-carbon steel,so the formation of a stable liquid flux near the meniscus within a short time at the beginning of casting is difficult. The amounts of liquid slag film and crystalline slag film were insufficient to provide adequate lubrication between the shell and the mold,which resulted in greater friction force that induced or aggravated cracks. Therefore,the homogeneity of mold fluxes and initial solidification in the mold should be improved to reduce the concentration of slab surface defects. Based on a series of related investigations, a mechanism for the formation of longitudinal surface cracks on a continuous casting slab of high-carbon steel was investigated. High-temperature tensile tests performed on slab samples, metallographic and scanning electron microscopy studies, as well as heat flux and shell thickness in continuous process predicted on the basis of a mathematical model were conducted. The results showed that high carbon steel exhibited a much lower liquidus temperature and a wider brittle temperature range immediately after solidification compared with those of low-carbon steel. of elements K and Na, which are contained in the mold fluxes, were not observed in the cracks during this study. The calculated results showed that heat heat and the shell thickness were uneven along the mold width and that the shell was thinner and close to the center line of the slab surface. The longitudinal cracks were observed in situ using confocal laser scanning mi croscopy, to first occur close to the solidification front, where the ductility was extremely low; in addition, the shell growth was slower than in other regions, which led to a thinner shell and depressed shrinkage due to a lack of lubrication by the mold fluxes below the meniscus. The pouring temperature of high-carbon steel is ~ 100 ° C lower than that of low-carbon steel, so the formation of a stable liquid flux near the meniscus within a short time at the beginning of casting is difficult. The amounts of liquid slag film and crystalline slag film were insufficient to provide adequate lubrication between the shell and the mold, which resulted in greater friction force that induced or aggravated cracks. Thus, the homogeneity of mold fluxes and initial solidification in the mold should be improved to reduce the concentration of slab surface defects.