Loading Direction Dependence of Yield-Point Phenomenon and Bauschinger Effect in API X70 Steel Sheet

In this study, variations in the tensile behavior and Bauschinger effect with the applied loading direction in an American Petroleum Institute (API) X70 steel sheet are investigated by conducting tensile and strain-reversal tests along the rolling direction (RD), the transverse direction (TD), and a direction inclined at an angle of 45° from the RD to the TD (45D). The microstructure of the rolled steel sheet consists of polygonal ferrite and acicular ferrite, and it has a rolling texture that is typical of body-centered cubic metal materials. During tensile deformation, the yield-point phenomenon, which involves occurrence of discontinuous yielding and development of Lüders strain, occurs in all the loading directions. However, the material shows a lower yield strength and larger Lüders strain under tension along the 45D than under tension along the RD and TD. The Bauschinger effect, which occurs under subsequent reversal tension after compression, is also more pronounced under loading along the 45D. These results are attributed to the fact that dislocation slips are activated more easily under deformation along the 45D, which is evidenced by the lower Taylor factor and larger number of active slip systems under loading along the 45D. Furthermore, results of viscoplastic self-consistent simulation reveal that along all the directions, the {112} < 111 > slip system is vigorously activated during tensile and compressive deformations and that its activation plays an important role in the deformation behavior and yield strength of the material.