Abstract: Transverse corner cracks in boron steel were studied through TEM,SEM and optical microscope analyses,and the formation reasons of the cracks were analyzed.The results indicates that BN and AlN precipitate around austenitic grain boundaries due to undercooling at the corner,which aggravates the cracks during continuous casting.Tiny cracks firstly appear along vibration marks at higher temperature zone of the continuously cast slab,then extend to transverse corner cracks during bending and straightening process.Moreover,high-temperature mechanical properties of SS400B was investigated,and based on the results,"intensified cooling" was put forward to prevent the formation of transverse corner cracks in slabs.