Fig. 1 a Phase volume fractions diagram of 0.3C-5Al-5Mn-1.5Si steel at equilibrium temperatures. FCC and BCC are considered α-ferrite and γ-austenite. b Heat treatment schematic of the Q&T process. The annealing temperatures are set to be I 980 °C, II 1080 °C, III 1180 °C

Fig. 2 a EBSD band contrast diagram (left) and phase distribution diagram (right) of 1080 °C annealed sample. b-d SEM micrographs of samples annealed at different temperatures: b 980 °C, c 1080 °C, d 1180 °C. BCC phase includes martensite (M) and δ-ferrite (δ-F), while the FCC phase is defined as retained austenite (RA). Block-like RA distributes at the edges of island-like M&RA mixtures while martensite occupies the interior regions

Fig. 3 EPMA line scan results of samples annealed at a 980 °C, b 1080 °C, c 1180 °C. The carbon concentration of δ-F is lowest, and the carbon concentration of M is slightly lower than that of RA due to partition in tempering

Fig. 4 XRD patterns and calculated retained austenite fraction and carbon content of the three groups of specimens a, c before, b, d after tensile test. The (220) γ peak is significantly weakened after tensile test

Fig. 5 Comparisons of Gaussian-fitting diagram of BC value before and after the tensile test: a, d 980 °C, b, e 1080 °C, c, f 1180 °C

Fig. 6 BC maps separated by the Gaussian-fitting diagram before and after the tensile test: a, d 980 °C, b, e 1080 °C, c, f 1180 °C

Fig. 7 EBSD IPF maps before and after the tensile test: a, d 980 °C, b, e 1080 °C, c, f 1180 °C in RD-ND plane. The cracks were circled in e, f

Fig. 8 Average grain size calculated by equivalent diameter, geometrically necessary dislocations (GND) density, and LAGBs and HAGBs percentage distinguished by the software AZtecCrystal [44] before (blue) and after (red) tensile test of different groups

Fig. 9 True stress-strain curves for three sample groups at tensile rates of a 2 mm/min and b 20 mm/min, c, d the corresponding yield strength, tensile strength, and elongation curves, e summary of UTS for medium-Mn steels with different Al contents (Mn < 12 wt%, Al > 3 wt%) [10,25,26,46,47,48,49,50,51]

Fig. 10 Fracture morphologies of the experimental steel at a tensile rate of 20 mm/min: a1, a2 980 °C, b1, b2 1080 °C, and c1, c2 1180 °C. The cleavage plane is highlighted by yellow dashed lines

Fig. 11 a TEM diagram of 1080 °C group before tensile test. Selected area electron diffraction patterns of b δ-F, c film-like RA, d block RA, respectively

Fig. 12 TEM diagram of 1080 °C group after tensile test. a Jagged retained austenite (RA) distributed at the edge of martensite and retained austenite (M&RA) mixture. b The deformation-induced martensite distributed around jagged RA. c, d Dislocation pile-ups and dislocation tangles at grain boundaries (GB) or the interface of δ-ferrite (δ-F) and M&RA mixture

Fig. 13 C-J model and modified C-J model of different groups: a1, a2 980 °C, b1, b2 1080 °C, c1, c2 1180 °C. I: stage one, II: stage two, R2: coefficient of determination

Fig. 14 a Correlation of tensile strength with prior-austenite fraction. b Correlation of total elongation with transformed austenite fraction. (calculated from the data of EBSD in Table 2). Comparation of 1080 °C group and 1180 °C group samples in c tensile strength and d plasticity. Note that the areas of each phase in the pie are consistent with their volume fractions in the steel, and the bigger areas of red regions correspond to higher contribution to strength or plasticity

Fig. 15 OM diagrams processed by Image J a, d 980 °C, b, e 1080 °C and c, f 1180 °C, g statistical results of different groups

Fig. 16 Mechanism during tensile deformation: a1-a3 980 °C, b1-b3 1080 °C, and c1-c3 1180 °C. OM and FM represent original martensite and fresh martensite formed during the TRIP effect