Effects of annealing temperature on microstructure and mechanical properties of a low density δ-TRIP steel plate via intercritical quenching-annealing process

Abstract

In this study, the effects of annealing temperature on microstructure and mechanical properties of Fe–9Mn–5Al-0.3C (wt.%) low density steel plates via intercritical quenching-annealing process were investigated through field-emission scanning electron microscopy, electron backscatter diffraction, X-ray diffraction, and tensile tests. The results showed that the experimental steel maintained micro-laminated microstructure after intercritical quenching-annealing treatment, consisting of δ-ferrite band and tempered martensite lamellar. After annealing at the range of 600–800 °C following quenching of 1050 °C, tempered martensite lamellar mainly changed, and affected the mechanical properties of the steel. The annealed experimental steels exhibited wide yield strength grades (460–620 MPa) with excellent ductility (40–63 %) and low yield-tensile ratio (0.65–0.75). The proportion of austenite, the density of geometrically necessary dislocations, and high-angle grain boundary density of tempered martensite region, as well as the presence or absence of carbides, were considered to contributed to the mechanical properties. Austenitic mechanical ability was analyzed. Various annealed temperature led to the different levels of austenitic stability through changing the enrichment of alloyed elements in austenite, further causing different degrees of TRIP effect. TWIP effect existed during deformation according to estimation of SFE. The steel annealed at 740 °C had maximum uniform elongation (62.4 %) and the highest product of strength and elongation (45.2 GPa%), depending on the maximum proportion of austenite transformed into martensite during deformation. However, the 680 °C annealed steel with the highest austenitic stability exhibited deficient TRIP effect, leading to the occurrence of necking. Moreover, work hardening behavior and fracture morphology of the experimental steel were analyzed.