Microstructure, mechanical properties and strengthening mechanism of high strength hot-rolled ferrite-martensite dual phase steel

Abstract

This study is focused on investigating the microstructure and mechanical properties of high strength ferrite-martensite (F-M) dual phase steels manufactured through a simple hot rolling and step cooling. Additionally, it seeks to elucidate the strengthening mechanism of the ‘unexpected microstructures’, namely lower bainite and auto-tempered martensite, within F-M dual phase steels. During the step cooling process, the morphology of ferrite is determined by the air-cooling temperature, while the ferrite content is affected by the air-cooling temperature and time. Since the morphology of continuously cooled bainite is determined by the cooling rate, granular bainite is formed in the air-cooling stage and lower bainite is formed in the laminar water-cooling stage. Combined with the reasonable microstructure construction and the strengthening effect of ‘unexpected microstructures’, the sample air-cooled at 700 °C for 10s achieves the best comprehensive mechanical properties (yield strength: 1155 MPa, tensile strength: 1511 MPa, total elongation: 11.5%, impact toughness in −20 °C: 16.6 J). The strengthening effect of ‘unexpected microstructure’ is mainly to improve strain strengthening and impact toughness. The presence of lower bainite and auto-tempered martensite both promotes the incompatible strain during the deformation, resulting in higher strain hardening. This effect is further enhanced by the segmentation of the prior austenite grains by lower bainite and the plastic constraints imposed by surrounding martensite during deformation. Auto-tempering contributes to an increase in impact toughness by reducing areas with high dislocation density. The improvement of impact toughness by lower bainite is mainly attributed to the increase of high angle grain boundaries when partitioning the prior austenite.