Fig. 1. Tensile properties in all the specimens to subjected to various annealing processes (a) and the work hardening behaviors during the tensile deformation of UFH-300 and IA-1 h specimens (b).

Fig. 2. EBSD results on HRA (after hot rolling and annealing 700 °C for 2 h) (a-c) and CR (after cold rolling) specimens (d-i). (a, d, g, i) Image quality (IQ) figures overlapped with the grain boundary (GB), phase mapping; (b, e, h) The corresponding normal direction inverse pole figures (ND-IPF) on RD⊥ND section, (c) line scanning results of Mn intensity by Auger electron spectroscopy, and (f) Kernel average misorientation (KAM) map, and (i) the CR samples along the TD-RD section. The thick yellow lines in (a) and (c) represent K-S OR with deviation less than 5°; the dark and red lines high-angle (15-180°) and low-angle (2-15°) grain boundaries respectively; the green is γ phase and the rest bcc phase. High-angle grain boundaries in fcc are revealed by dark lines in (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 3. EBSD results on the specimens after ultrafast heating and the long intercritical annealing. (a, b, b1) UFH-10; (c, d, d1) UFH-100; (e, f, f1) UFH-300; (g, h, h1): IA-1 h. (a-h) Image quality (IQ) figures overlapped with the grain boundary (GB), phase mapping. (b1, d1, f1, h1) Kernel average misorientation (KAM) map overlapped with high-angle grain boundaries (dark lines) in fcc phase. (a, c, e, g) RD⊥TD section; (b-b1, d-d1, f-f1, h-h1) RD⊥ND section. The thick yellow lines in (a-h) represent K-S OR with deviation less than 5°; the dark and red lines high-angle (15-180°) and low-angle (2-15°) grain boundaries respectively; the green is γ phase and the rest bcc phase. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. SEM results on the microstructures in UFH-10 (a), UFH-100 (b)

Fig. 5. (a) Comparison of RA fractions in all the specimens before and after the tensile deformation, and the corresponding lattice parameters. Note that all the data were calculated from XRD; (b) The measured grain size distribution of RA grains in all the specimens.

Fig. 6. TEM results on the microstructures in the UFH samples. The compositions were measured by EDS. (a-c) UFH-10 sample; (d) UFH-100; and (e, f)) UFH-300.

Fig. 7. Numerical simulation results on the reverse transformation during the heating to 730 °C at 300 °C/s. (a) Configuration for γ nuclei to grow into the neighboring ferrite (α) and SIM (α′) with the assumed initial compositions (in weight percentage); (b) the calculated positions of moving γ/α and γ/α′ interfaces during the heating; and the calculated C (c), Mn (d) and Al (e) concentration profiles developed during the heating.

Fig. 8. Numerical simulation results on the growth kinetics of austenitic nuclei into martensite with the various compositions during the heating at the different rates. (a) The calculated movement of γ/α′ interface during the heating at either 10 or 300 °C/s for the diffusion couple having the assumed compositions and sizes; (b) Dependence of calculated A3 temperature on Mn and C contents; the (c) C and (d) Mn concentration profiles developed during the heating to 730 °C under the condition ‘4′ in (a); (e) C concentration profiles developed during the heating to 730 °C under the condition ‘4′ and ‘5′ in (a).

Fig. 9. Schematic illustration on the microstructural changes when the cold rolled 7Mn steel was subjected to the UFHs at 10, 100/300 °C/s to 730 °C or IA at 730 °C for 1 h.

Fig. 10. The reverse transformation temperature, ${{T}_{0}}$, is calculated for the compositional ranges of 0.6-0.8% C, 4-12% Mn and 2% Al, and the additional driving force of 500 J and 1000 J/mol are assumed for calculating $T_{0}^{\prime }$. The austenitization temperatures (As) during the heating at the rates of 10, 100 and 300 °C/s were measured using dilatometer and included for comparison.