Revealing the mechanisms behind precursor process-driven microstructural and property variations in ferrite-based lightweight steels

Abstract

For ferrite-based lightweight steels containing more than 5 wt% Al, the major challenges are the formation of κ-carbides, which induce rolling-related cracking, and the limited strength–ductility synergy attainable through conventional processing routes. In this study, a novel carbide-precursor processing strategy was developed to effectively suppress κ-carbide formation. As a result, no significant centerline or edge cracking was detected after rolling. Compared with conventional approaches based on alloying adjustments and hot-rolling modifications, this strategy provides a simpler and more efficient process with improved control over both composition and microstructure. Importantly, it also facilitated the formation of a previously less studied heterogeneous structure composed of granular retained austenite alternately distributed with α-ferrite. This architecture increased the density of heterogeneous interfaces, thereby significantly enhancing hetero-deformation-induced hardening. Moreover, the granular retained austenite, enriched by elements from the carbide precursor, exhibited improved stability and delayed transformation during deformation. In addition, the reduced driving force for reverse transformation strengthened the role of dislocation hardening, while residual carbides and chain-like VC co-precipitates further contributed to precipitation strengthening. Consequently, the ferrite-based lightweight steel achieved an ultimate tensile strength of nearly 1 GPa together with a uniform elongation of 38.5 %.