Development of 1.2 GPa‐Grade Ferrite‐Based Lightweight Steel via Two‐Step Rolling and Low‐Temperature Tempering

The development of ferrite‐based lightweight steels containing a large fraction of coarse δ ‐ferrite has been constrained by their inherently low yield strength and rolling‐induced cracking associated with κ ‐carbides. In this study, an optimized two‐step rolling process consisting of warm rolling followed by light cold rolling, combined with short‐term low‐temperature tempering, is designed for low‐Mn (<3 wt%) ferrite‐based lightweight steel. This approach successfully achieves an excellent property combination, with a yield strength of ≈1.2 GPa and an elongation of 22.8%. Specifically, the optimized warm rolling process suppresses κ ‐carbide formation, thereby eliminating rolling cracks. Subsequent cold rolling introduced a high dislocation density, strong heterodeformation‐induced (HDI) stress, and refined martensite/austenite grains, all of which contributed to the rapid increase in yield strength. After tempering, partial martensite twinning occurred, HDI stress is further enhanced, and strain aging associated with solute atoms and lattice defects is activated. These synergistic effects result in superior matching of yield strength and ductility. The simplicity of this processing route, combined with the excellent mechanical performance, highlights the broad application potential of ferrite‐based lightweight steels.