Fig. 1 The principle schematic diagram of ECAP [24]

Fig. 2 TEM micrographs of low carbon steel subjected to ECAP and annealing: a ferrite phase of as-ECAP; b ferrite phase annealed for 1 h at 450 °C; c ferrite phase annealed for 1 h at 510 °C [17]

Fig. 3 The principle schematic diagram of ARB [24]

Fig. 4 TEM morphology of nano-structured steel produced by 50% cold rolling and annealing at 500 °C for 30 min [19, 20]

Fig. 5 Microstructural hierarchy of the lath martensite [32]

Fig. 6 Nominal stress–nominal strain curves of specimens corresponding to 50% cold rolling and annealing at various temperatures [34]

Fig. 7 UFG microstructure and Mn distribution across retained austenite [37]

Fig. 8 TEM images of cold-rolled AISI 301LN SS a, SADP of lath-type martensite b, SADP of dislocation-cell type martensite c

Fig. 9 TEM images of cold-rolled AISI 301LN SS samples annealed at 800 °C for 1 s a and 10 s b

Fig. 10 Microstructures of the samples with ultrafine-grained austenite after cooling from 900 °C to room temperature (RT) with different ways: a cooling rate 10 °C/s, ferrite + pearlite; b cooling to 700 °C, 75% deformation and air-cooling to RT

Fig. 11 TEM image showing the microstructure a and corresponding selected area diffraction pattern b of the sample deformed 75% at 700 °C and air-cooling to RT after cycle-quenching for three times

Fig. 12 Photo of nano-grained bulk experimental steel plates with the dimensions of 5.5 mm in thickness and 65 mm in width

Fig. 13 SEM micrograph of the warm-rolled and air-cooled experimental steel, showing the polygonal ferrite with dispersive precipitates a and EDS analysis of precipitates b

Fig. 14 SEM micrographs of the warm-rolled and water-quenched experimental steel: a fully transformation and completely recrystallization; b partly transformation and inadequately recrystallization

Fig. 15 Ultrafine austenite obtained after four-time repetitive treatment of reheating and quenching

Fig. 16 True strain versus stress curves of samples deformed at 900 °C a and 950 °C b with different austenite grain size (\( \dot{\varepsilon } \) = 10 s-1)

Fig. 17 Morphology of austenite grains evolving with deformation at 900 °C: aε = 0.2; bε = 0.4

Fig. 18 Logarithmic plot for ultra-fine austenite grains growth behaviors

Fig. 19 Reliability of isothermal grain growth model

Fig. 20 Dependence of austenite grain size on deformation temperature, strain value and strain rate: a austenite grain size versus deformation temperature; b austenite grain size versus strain and strain rate

Fig. 21 Austenite grains obtained in sample without reheating after deforming with various strains at 800 °C: aε = 0.8, \( \dot{\varepsilon } \) = 0.1 s-1; bε = 0.9,\( \dot{\varepsilon } \) = 0.1 s-1; cε = 1.1,\( \dot{\varepsilon } \) = 0.1 s-1