Abstract: In order to reveal formation mechanism of slab corner transverse crack of the third generation automobile steel, analogy analysis on the thermal expansion coefficient, thermoplasticity, phase change feature and the second phase precipitation behavior in the scope of the third brittle zone temperature had been carried out by using methods such as mathematical calculation, physical test, metallographic examination etc. The results show that precipitation of ferrite film and AlN on the austenitic grain boundary should be the internal reason for reducing steel high temperature ductility. Supercooling imposed on slab edge should be the external reason for promoting corner transverse crack. Besides, multiple phase change and precipitation of (Mn，Fe)3C in low temperature would increase the crack sensitivity of this steel at further. Based on this research, the controlling measures for corner transverse crack had been put forward and the whole process production for about 250t steel had been realized.

Key words: the third generation automobile steel, corner transverse crack, AlN, supercooling, ferrite film, the third brittle zone