Fatigue behavior of a multiphase medium carbon V-bearing microalloyed steel processed through two thermomechanical routes

In view of their capacity to develop high strength following limited alloying and ease of processing, medium carbon microalloyed (MA) steels are very cost effective compared with quenched and tempered (Q&T) steels for the production of automotive components. Recently, a two-step cooling (TSC) procedure with an additional anneal following low temperature forging/rolling was developed to obtain multiphase (ferrite–bainite–martensite), microstructures in a microalloyed steel. The microstructure predominantly contained granular/lower bainite, lath martensite and polygonal ferrite, with inter-lath films as well as blocks of retained austenite. Vanadium carbide precipitates were observed only in the polygonal ferrite and the ferrite in the bainite region. The F–B–M (R) exhibited (rolling route) significantly higher yield and tensile strength values than the F–B–M (F) microstructure. Under low cycle fatigue (LCF) loading, the F–B–M (R) microstructure exhibited a cyclically stable response at total strain amplitudes ≤0.6% and continuous cyclic softening till failure. However, the F–B–M (R) microstructure exhibited a greater fatigue life than the F–B–M (F) microstructure when the comparison was based on the total strain amplitude, (Δɛt/2). Fatigue tested microstructure of F–B–M (F) at a total strain amplitude of 0.4% and the microstructure of F–B–M (R) at a total strain amplitude of 0.55% were stable. The inter-lath retained austenite strips/films played a significant role in preventing the softening during fatigue loading.