Effect of Grain Size on Low Cycle Fatigue Behavior and Microstructure after Fatigue of IF Steel

IF steels with average grain size of 40 μm and 210 μm were prepared by annealing at 850 ℃ and 950 ℃, respectively. The effects of the grain size on the low-cycle fatigue behavior and the microstructure after fatigue were investigated. The results show that during fatigue cycle, the initial average peak stress of the fine-grained test steel was higher than that of the coarse-grained test steel. The average peak stress of the two test steels tended to be equal with increasing number of cycles. The fine-grained test steel always showed cyclic work hardening during fatigue, while the coarse-grained test steel showed initial cycle hardening, cycle saturation and secondary cycle hardening. After 400 fatigue cycles, the microstructure of the fine-grained test steel was composed of similarly sized and uniformly distributed dislocation cells, while the microstructure of the coarse-grained test steel was mainly composed of macro-persistent slip band (Macro-PSB) and dislocation cells. The Macro-PSB contained relatively small dislocation cells. The coarse-grained test steel had higher dislocation density and relatively significant structural heterogeneity.