Abstract: A Q235Bsteel plate with thickness of 8 mm was bent more than 90°during room temperatureplastic deformation process,and cracks initiated in the inboard and propagated to the outboard of the steel plate atthe bending corner when bent back.In order to find the reason for cracks occurring,fracture surface observation,mechanical properties test,metallographic examination,micro-hardness test and EBSD analysis had been done.Theresults indicated that the over length inclusions in the steel plate,bad ductility reflected by the impacting value,theworking hardening and crystal change induced by the normal bending,worsened the mechanical properties of thebending corner of the steel plate and caused it to crack with the formation of cleavage during the contrary flexure.