SPLIT FRACTURE PHENOMENON AND MECHANISM IN TENSILE TESTS OF HIGH STRENGTH LOW CARBON BAINITIC STEEL

Split fracture phenomenon in tensile tests of a high strength low carbon bainitic steel plate was investigated. Regular tensile tests in the longitudinal direction and short bar tensile tests in three principal axis directions were performed. The results showed that split fracture always occurred when the tensile direction was longitudinal or transverse. The splitting surface was almost parallel to the direction of thickness. According to SEM observation of splitting crack, it was found that the split had features of cleavage fracture. Meanwhile, the original steel plate was experimentally proved to have similar strength and ductility in the longitudinal direction, the transverse direction and the direction of thickness. Based on a finite element analysis simulation, the lateral tensile stress within the necked zone was calculated to be much lower than the principle tensile stress. Thus, a mechanism transition from ductile to brittle during the tension process was proposed. Moreover, it was revealed that the split fracture is not definitely the result of lower performance along the direction of thickness, but it is caused by the comprehensive influence of texture evolution, the redistribution of grain boundaries and the state of three dimensional stresses with large tensile plastic deformation, which can be considered as the characteristic of mechanical property of bainitic steel.