Fig. 1. (a) Equilibrium phase diagram of the experimental steel; (b) schematic illustration of the thermomechanical process used in the present study. α: ferrite, γ: austenite, θ: cementite.

Fig. 2. (a) SEM micrograph and prior austenite grain boundary of the HR sample; (b) TEM micrograph of the HRA sample; (c) XRD patterns and (d) engineering stress-strain curves of the HR and HRA samples. The inserted image in (b) shows the diffraction pattern of the selected area as circled in white.

Fig. 3. SEM micrographs of the as cold-rolled and simulated continuous annealed samples: (a) 0CR; (b) 0CRA; (c) 20CR; (d) 20CRA; (e) 40CR; (f) 40CRA. RD: rolling direction, ND: the direction normal to the rolling plane.

Fig. 4. XRD patterns of the (a) as cold-rolled and (b) simulated continuous annealed samples; (c) the austenite component in the simulated continuous annealed samples.

Fig. 5. Bright-field TEM micrographs of the simulated continuous annealed samples: (a) 0CRA; (b) 20CRA; (c) 40CRA. The inserted images in (a) and (b) show diffraction patterns of selected areas as circled in white.

Fig. 6. Orientation maps of (a, c, e) austenite and (b, d, f) ferrite in the simulated continuous annealed samples: (a, b) 0CRA; (c, d) 20CRA; (e, f) 40CRA. The black lines represent phase boundary and high-angle boundary (>15°), and blue lines represent low-angle boundary (2°-15°).

Fig. 7. (a) Engineering stress-strain curves and (b) true stress-strain curves and strain hardening rate curves of the simulated continuous annealed samples.

Fig. 8. Schematic illustration of the microstructure evolution of the hot-rolled sample during the simulated batch annealing, cold-rolling, and the simulated continuous annealing.

Fig. 9. (a) The calculated yield strength of austenite and ferrite of the simulated continuous annealed samples; (b) the calculated yield strength of the simulated continuous annealed samples by “rule of mixtures” and the experimental results for comparison.

Fig. 10. (a) The uniform elongation (UEL) of the present medium-Mn steel and a ferritic steel as a function of grain size; (b) The change in austenite volume fraction as a function of true strain in simulated continuous annealed samples; (c) C and Mn contents in austenite and (d) the k value and UEL of the simulated continuous annealed samples as a function of cold-rolled reduction.

Fig. 11. The contributions of γ→α′ transformation and dislocation strengthening on the strain hardening of the simulated continuous annealed samples.