Fig. 1 Schematic heat treatment diagrams of two-step annealing (a) and interrupted cooling (b)

Fig. 2 Dimension of the tensile specimen

Fig. 3 Microstructures of dual-phase carbon steel intercritically annealed at 775 °C: a fibrous martensite, b bulky martensite

Fig. 4 Microstructures of dual-phase carbon steel intercritically annealed at 820 °C: a fibrous martensite, b bulky martensite

Fig. 5 Stress–strain diagrams of the steels with fibrous (a) and bulky (b) martensites intercritically annealed at 775 °C with the martensite volume fractions of (63 ± 2)% and then bake hardened at 180 °C for different times of 10–160 min

Fig. 6 Stress–strain diagrams of the steels with fibrous (a) and bulky (b) martensite intercritically annealed at 820 °C with the martensite volume fractions of (86 ± 2)% and then bake hardened at 180 °C for different times of 10–160 min

Fig. 7 SEM fractographs of the as-annealed and bake-hardened dual-phase carbon steel with the fibrous martensite intercritically heat treated at 775 °C (a); then bake hardened at 180 °C for 20 (b); 160 min (c)

Fig. 8 SEM fractographs of the as-annealed and bake-hardened dual-phase carbon steel with bulky martensite intercritically heat treated at 775 °C (a); then bake hardened at 180 °C for 20 (b); 160 min (c)

Fig. 9 SEM fractographs of the as-annealed and bake-hardened dual-phase carbon steel with fibrous martensite intercritically heat treated at 820 °C (a); then bake hardened at 180 °C for 20 (b); 160 min (c)

Fig. 10 SEM fractographs of the as-annealed and bake-hardened dual-phase carbon steel with bulky martensite intercritically heat treated at 820 °C (a); then bake hardened at 180 °C for 20 (b); 160 min (c)