Influence of pre-strain on microstructural characteristics and tensile deformation behaviour of a cold-rolled Al-containing medium Mn steel

Abstract

Automobile steel sheet parts often experience unavoidable plastic deformation during cold stamping forming and service life. In this study, we explored the microstructure variation and tensile deformation behaviour of a cold-rolled Al-containing medium Mn steel (0.25C–8.67Mn–0.54Si–2.69Al, wt%) subjected to different degrees of pre-straining by tensile testing, scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction and X-ray diffraction. The cold-rolled steel sheet processed by intercritical annealing at 725 °C for 60 min exhibited a superior combination of strength and ductility [the product of ultimate tensile strength (UTS) and total elongation (TEL), UTS × TEL] as high as 69 GPa% due to the TWIP (twinning-induced plasticity) and TRIP (transformation-induced plasticity) plasticity-enhancing mechanism during tensile deformation. The tensile behaviour, which originally exhibited notable Luders plus Portevin–Le Châtelier (PLC) banding phenomenon, was significantly changed by pre-straining. The strength increased, while the TEL and UTS × TEL decreased linearly with an increase in the pre-strain. A pre-strain equalling to the Luders strain, i.e. 3%, could cause an elimination of the Luders bands, while it had almost no influence on the PLC bands. The increase in the dislocation density, the enhanced stability of retained austenite and the strain-induced martensite and mechanical twins caused the reduction and elimination of the Luders bands with increasing amounts of pre-strain.