Asymmetric rolling (ASR) is an efficient processing for fabricating ultrafine-grained (UFG) materials. In the present investigation, interstitial-free (IF) steels with the grain size ranged from 500 nm to 500 μm were obtained by asymmetric rolling and subsequent annealing. The evolution of microstructures and mechanical properties of IF steel were studied. Accordingly, the Hall-Petch relationship of IF steel with a wide grain size range was established. It was found that ultimate tensile strength (UTS) corresponds well to the Hall-Petch relationship over the whole grain size range. However, the yield strength (YS) and hardness deviated from the Hall-Petch relationship as the grain size is larger than 100 μm, which is mainly attributed to the slight effect of grain boundary as obstacle on the dislocation movement and/or pile-up under small deformation in coarse grain (CG).

A dual-phase (12 vol.% delta-ferrite + 78 vol.% austenite) high manganese twinning-induced plasticity (TWIP) steel was produced by hot rolled and annealing treatment. In comparison with the fully austenitic TWIP steel, both the yield and ultimate tensile strength of the dual-phase TWIP steel reinforced by hard delta-ferrite are significantly increased. It was found that the delta-ferrite in dual-phase steel exhibits a high hardness owing to the formed DO₃ structured intermetallic phase within ferrite. The presence of delta-ferrite dramatically improves the strain-hardening ability of TWIP steel. This is principally attributed to the effects of strain partitioning between hard delta-ferrite and softer austenite on the kinetics of deformation twinning and/or additional geometrical necessary dislocation (GND) during the deformation process.

The present study investigates the microstructure and mechanical properties of ultra-low carbon bainitic steel (UCBS) under different cold rolling reductions. When the rolling reduction ratios were increased to 80%, the microstructure was refined, and the lath width of the bainite decreased from 601 nm to 252 nm. The ultimate tensile strength and yield strength increased from 812 MPa and 683 MPa to 1195 MPa and 1150 MPa, respectively, whereas the elongation decreased from 15.9% to 7.9%. In addition, the dislocation density increased from 8.3 × 10¹³ m⁻² to 4.87 × 10¹⁴ m⁻² and a stronger γ-fiber texture was obtained at the 80% cold rolling reduction ratio. The local stress distribution and kernel average misorientation were not uniform and became more severe with increased rolling reduction ratios. The strength increment of UCBS was primarily due to boundary strengthening and dislocation strengthening. The theoretical strength increment agreed well with the experimental measurements, which can be helpful for the design and production of UCBS for broad engineering applications.