Fig. 1. (a) SEM image of as-received DP steel and (b) schematic diagram of the processing routes.

Fig. 2. SEM images of the low carbon steels rolled at 600 °C: (a-1, a-2) with 40 % thickness reduction; (b-1, b-2) with 60 % of thickness reduction.

Fig. 3. SEM images of intercritical annealed DP steels: (a-1, a-2, a-3) the 40 % warm-rolled samples annealed at 780 °C, 800 °C and 840 °C, respectively; (b-1, b-2, b-3) the 60 % warm-rolled samples annealed at 780 °C, 800 °C and 840 °C, respectively.

Fig. 4. Volume fraction of martensite of the warm-rolled (WR) steels after annealing at different temperatures.

Fig. 5. Schematic illustration of microstructural evolution of intercritically annealing: (a) WR40 %; (b) WR60 %.

Fig. 6. TEM images of the 40 % warm-rolled DP steels annealed at (a) 780 °C, (b) 800 °C and (c) 840 °C.

Fig. 7. Effect of rolling strain and annealing temperature on the mechanical behaviors of DP steels. Engineering stress-strain curves of the samples with (a) 40 % rolling reduction and (b) 60 % rolling reduction. (c) Evolution of YS, UTS and UE at different annealing temperatures and (d) strain hardening rate curves from tensile tests.

Fig. 8. LUR tensile tests: (a) the load-unload-reload curves of dual phase steels. (b) Schematic of hysteresis loops for characterizing the Bauschinger effect. (c) Evolution of HDI stress with applied strain. (d) Load-unload curves (hysteresis loops) and the hysteresis areas of HMDP steels varying with true strain. The loops have been shifted in strain for comparison.

Fig. 9. TEM micrographs of 60 %-780 DP and 40 %-840 DP after tensile strain of 6%: (a-1) dislocations in ferrite of 60 %-780 DP; (a-2) enlarged image of the selected area in (a-1); (b-1) dislocations in ferrite of 40 %-840 DP; (b-2) enlarged image of the selected area in (b-1).