Position-dependent shear-induced austenite–martensite transformation in double-notched TRIP and dual-phase steel samples

摘要

While earlier studies on transformation-induced-plasticity (TRIP) steels focusedon the determination of the austenite-to-martensite decomposition in uniformdeformation or thermal fields, the current research focuses on the determinationof the local retained austenite-to-martensite transformation behaviour in aninhomogeneous yet carefully controlled shear-loaded region of double-notchedTRIP and dual-phase (DP) steel samples. A detailed powder analysis has beenperformed to simultaneously monitor the evolution of the phase fraction and thechanges in average carbon concentration of metastable austenite together withthe local strain components in the constituent phases as a function of themacroscopic stress and location with respect to the shear band. The metastableretained austenite shows a mechanically induced martensitic transformation inthe localized shear zone, which is accompanied by an increase in average carbonconcentration of the remaining austenite due to a preferred transformation ofthe austenite grains with the lowest carbon concentration. At the laterdeformation stages the geometry of the shear test samples results in thedevelopment of an additional tensile component. The experimental strain fieldwithin the probed sample area is in good agreement with finite elementcalculations. The strain development observed in the low-alloyed TRIP steelwith metastable austenite is compared with that of steels with the same chemicalcomposition containing either no austenite (a DP grade) or stable retainedaustenite (a TRIP grade produced at a long bainitic holding time). Thetransformation of metastable austenite under shear is a complex interplaybetween the local microstructure and the evolving strain fields.