Enhancing high-cycle fatigue properties of cold-drawn Fe-Mn-C TWIP steels

High-cycle fatigue properties of cold-drawn twinning-induced plasticity (TWIP) steel, a favored candidate for replacing fully pearlitic (FP) steels in wire applications, were investigated. The high-cycle fatigue tests were conducted on cold-drawn TWIP and FP steels that had comparable ultimate tensile strength for comparison. Fatigue strength of both TWIP and FP steels increased with the tensile strength, but the TWIP steel cold-drawn to a tensile strength of 1.5 GPa exhibited a very low fatigue ratio (a ratio of fatigue strength to tensile strength) which deviated far from the predicted linear relationship. Fracture surface analysis showed that crack initiation mainly occurred at the ferrite matrix in FP steels, while either at grain or twin boundaries in TWIP steels where a large density of dislocations piled up during cold drawing. In the case of TWIP steels, the presence of inclusions at grain boundaries led to high local stress concentration and caused early intergranular fatigue cracking as notch sensitivity increased with tensile strength. Subsequent annealing after cold-drawing effectively increased fatigue strength of TWIP steels. It was suggested that TWIP steel revealing both high tensile strength and excellent high cycle fatigue strength could be a promising alternative for replacing conventional FP steels.