Fig. 1 a Schematic illustration showing the two-step IA process. b, c SEM images and b1, c1 corresponding XRD patterns of the PA and P-ART, samples. d, e Dilatation curves of the annealing samples as a function of temperature treated by the PA and P-ART processes. f1-f3, g1-g3 EPMA maps revealing microstructures and alloying distributions of C and Mn of the PA and P-ART samples: f1, g1 BSE images, f2, g2 C distribution maps and f3, g3 Mn distribution maps. (αF: ferrite, αM: fresh martensite, γ: retained austenite)

Fig. 2 STEM images and relevant Mn distributions in the PA a-d and P-ART e-h samples: a, e BF images, b, f HAADF images, c, g Mn distribution maps, and d, h Mn line scan profiles along the white dashed arrows in c, g. (αF: ferrite, αM: fresh martensite, γ: retained austenite)

Fig. 3 a Scheme of the austenite reversion path during the two-step IA process. b-e SEM images showing microstructures at various stages during the two-step IA process: b the initial martensitic microstructure, c the duplex microstructure after the PA process, d the microstructure at the beginning of the P-ART process, e the resultant microstructure after the P-ART process. (αF: ferrite, αM: fresh martensite, γ: retained austenite, θ: cementite)

Fig. 4 Resultant microstructures of the C-ART sample a-d and the P-ART sample e-h: a, e SEM-BSE images, b, f the magnified views of the microstructure in the white rectangles in a, e, c, g corresponding EBSD phase distribution map overlapped with band contrast images, d, h Mn distribution maps. (αF: ferrite, γ: retained austenite, θ: cementite)

Fig. 5 a Engineering stress-strain curves and b corresponding strain hardening and true stress-strain curves of the P-ART and C-ART samples