Abstract: Features and causes of corner transverse cracks for Q345B slab were analyzed based on metallographic microscopy,scanning electron microscopy and energy dispersion scanner.The results shows that the causes of forming cracks are the chain-like manganese sulfide compound and the film-like proeutectoid ferrite precipitated surrounding the austenite grain boundaries.The fine MnS precipitated along the austenite grains boundaries during slab solidification,then further accelerated the formation of thin films proeutectoid ferrite when the temperature of the slab surface droped to the austenite-ferrite transformation temperature range.When the slab was subject to straightening stress,all the strain mainly concentrates in these thin films,encouraging the formation of micropore around the MnS boundaries and these micropore polymerization gradually results in the formation of the crakes.