Effect of cold rolling and subsequent annealing treatments on fatigue behaviour and microstructure evolution of a high-mn TWIP steel

A high-Mn TWIP steel represents excellent combination of ultimate tensile strength and ductility, but it have low yield strength. In particular, high yield strength is crucial for majority of the regularly encountered service conditions, like vehicle’s suspension and chassis. On the other hand, majority of the structural components are served at cyclic loading condition and low cycle fatigue performance is regarded to be significant in automobile durability analysis. Therefore, finding strategies to increase yield strength accompanied by great low cycle fatigue performance will go a long way towards upscaling TWIP steels to many industrial applications. In this study the influence of cold rolling and subsequent annealing treatment on tensile and low cycle fatigue behavior of Fe–17Mn–2Al–0.6C TWIP steel were comprehensively investigated focusing on the effects of the imposed strain amplitude and fraction of recrystallized area or grain size. The amount of cold rolling reduction (60 and 80%) and annealing temperature (550, 575, 610, 650, 750 and 1100 ºC) were varied, which resulted in the variation of recrystallized fraction area and grain size. Tensile tests and fully-reversed strain-controlled fatigue tests were conducted on plate- type specimens at strain amplitudes of 0.25×10−² ≤ εt/2≤ 0.8×10−². Increasing cold rolling reduction percentage and annealing at lower part of partially recrystallization region presents high yield strength and good combination of strength-ductility. Decreasing grain size to 1.8 µm results in higher yield strength. However, it bring about decrease strength-ductility combination compare to medium or coarse grained specimens, i.e. 7.4 and 87 µm respectively. The low cycle fatigue behavior of specimens at a fixed amount of cold rolling reduction, i.e. 80 %, were analyzed in the context of the fractions of recrystallized area and grain size. The presence of a small amount of recrystallized area, i.e. 13 %, results in high yield stress and fatigue performance, whereas a high fraction of recrystallized area degraded yield strength and low cycle fatigue performance. In the other hand, the medium grained specimen, i.e. 7.4 µm, enhanced both tensile properties and fatigue performance under the conditions used in this work. Results suggest that increasing cold rolling reduction percentage and annealing at lower part of partially recrystallization region can provide superior tensile properties and fatigue performance. Also, medium grained TWIP steel shows better low cycle fatigue performance than fine or coarse grained one, both at strain amplitudes εt/2≥ 0.4×10−². High strength is the determinant factor for longer low cycle fatigue life at strain amplitude of 0.25×10−² in all conditions investigated.

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