Role of retained austenite in advanced high-strength steel: ductility and toughness

Enhancing the ductility and toughness of advanced high-strength steels is essential for the wide range of promising applications. The retained austenite (RA) is a key phase due to the austenite-to-martensite transformation and its transformation-induced plasticity effect. It is commonly accepted that slow RA-to-martensite transformation is beneficial to ductility; therefore, the RA fraction and stability should be carefully controlled. The RA stability is related to its morphology, size, carbon content, neighboring phase and orientation. Importantly, these factors are cross-influenced. It is noteworthy that the influence of RA on ductility and fracture toughness is not consistent because of their difference in stress state. There is no clear relationship between fracture toughness and tensile properties. Thus, it is important to understand the role of RA in toughness. The toughness is enhanced during the RA-to-martensite transformation, while the fracture toughness is decreased due to the formation of fresh and brittle martensite. As a result, the findings regarding to the effect of RA on fracture toughness are conflicting. Further investigations should be conducted in order to fully understand the effects of RA on ductility and fracture toughness, which can optimize the combination of ductility and toughness in AHSSs.