Based on a series of related investigation,a mechanism for the formation of longitudinal surface cracks on continuous casting slab of high carbon steel has been studied.The high-temperature tensile test of slab samples,metallographic and scanning electron microscopy studies,as well as mathematic model of prediction of heat flux and shell thickness in continuous casting,have been carried out.The results showed that high carbon steel was found to have much lower liquidus temperature and wider brittle temperature range immediately after solidification than that of low carbon steel.Concentrations of the element K and Na,which are contained in the mold fluxes,were not found in cracks of this study.The longitudinal cracks occur first close to the solidification front observed in situ using confocal laser scanning microscope,where the ductility is extremely low,and the shell growth was slower than others leading to the thinner shell and depressed shrinkage owing to the entrapment of molten flux slightly below the meniscus.Furthermore,pouring temperature of high carbon steel is about 100 °C lower than low carbon steel,so it is hard to form stable liquid flux near the meniscus in short time at the beginning of casting.There is not enough liquid slag film,as well as crystalline slag film,providing enough lubrication between the shell and mold resulting in higher friction force that induce or aggravate cracks.Therefore,the homogeneity of mold fluxes and initial solidification in mold should be improved to solve the slab surface defects.