Fatigue behavior of plain C–Mn steel plates with fine grained ferrite in surface layers

Highlights: ► Grain refinement can improve effectively the fatigue properties of C–Mn steel plates. ► The area of fatigue striations in plate with fine grained ferrite is less than that with coarse grained ferrite. ► The ferrite grain refinement in the surface layers of the steel plates can hold back or postpone the formation of surface fatigue cracks. ► The banding pearlite can promote the formation and extension of the secondary cracks. - Abstract: The effect of fine-grained surface layers on the fatigue behavior of plain C–Mn steel plates is investigated. The plain C–Mn steel plates have been manufactured by a special thermo-mechanical controlled process (TMCP). For plates rolled by the special TMCP (designated special plates), the ferrite grain size approaches 5.5 μm in the surface layers and reaches 6.5 μm on average in the whole thickness of the plates, while for usually rolled plates (designated usual plates), the grain size is 15 μm on average in the whole thickness of the plates, without obvious difference between surface and central layers. Significant improvements of fatigue properties have been achieved by the ferrite grain refinement. Under the similar stress condition, the fatigue lifetime of the special plate is more than 10 times as long as that of the usual plate, and the first stage of fatigue crack propagation can be prolonged. With a similar lifetime of the usual plate under a load ratio R (σmin/σmax) approaching zero, the special plate can sustain a load 40 MPa higher than that of the usual plate. Furthermore, fatigue fractographs have been observed and analyzed by a scanning electron microscope (SEM).