Fatigue strength and crack initiation-propagation behavior of TRIP-aided bainitic sheet steels which are associated with the transformationinduced plasticity (TRIP) of retained austenite were investigated for the automotive applications. The steels composing of bainitic ferrite lath matrix and the retained austenite films completed the highest fatigue limit of several high-strength dual-phase type and bainitic type of steels. The threshold value of the stress intensity factor range (ΔKth) was lower than those of the other dual-phase type of steels, although the crack propagation rate at a high stress intensity factor range agreed well with those of the other steels. It was concluded that the retained austenite films suppressed a micro-crack initiation and propagation in the matrix and a long crack propagation due to "stress relaxation" and "block effect" resulting from the strain-induced transformation, because the mean interfilm space of retained austenite is smaller than a plastic zone of crack tip.
