Fig. 1. EBSD inverse pole figure (IPF) maps of CG (a) and CFT760 (b) steels. TEM micrographs of a RA zone (c) and coexistence zone of RA and PRA (d) in the CFT760 steel. (e) STEM image showing Cr23C6 morphology in the CFT760 steel with corresponding EDS elemental maps of C, Cr, and Nb in the dotted box region of (e) and the SADP of Cr23C6. (f) TEM image showing Nb(C, N) morphology of CFT760 steel with the EDS elemental maps of C, N, and Nb in the dotted box region of (f). The atomic arrangement of Nb(C, N) is detected by the high-resolution transmission electron microscope (HRTEM). (Note: Because Fig. 1(f) was obtained from carbon extraction replicas, the red color at the location of the precipitates is lighter than the surrounding area in the EDS elemental maps of carbon.)

Fig. 2. Williamson-Hall plots based on XRD profiles (inset) of the CFT760 and CG steels. The internal strain (ε) of CFT760 and CG steels are 0.417% and 0.0304%, respectively. The dislocation density of CFT760 and CG steels is 1.09 × 1015 m-2 and 5.77 × 1012 m-2, respectively.

Fig. 3. (a) Engineering stress-strain curves of the CFT750, CFT760, CFT770, CFT780 and CG steels. YS, UTS, UE, and TE are yield strength, ultimate tensile strength, uniform elongation, and total elongation, respectively. (b) WHR curves of CG and CFT760 steels. (c) Comparison of the YS versus UE of CFT steels with other representative AHSS with different matrix phases [[1], [2], [3], [5], [6], [7], [8], [9], 11, 14, [20], [21], [22], [23], [24], [25], [26], [27]].

Fig. 4. TEM micrographs of PRA at the true strains of 7% (a) and 10% (c), and RA at the true strains of 4% (b) and 7% (d). EBSD image quality and phase maps of CFT760 steel at the true strains of (e) 7%, (f) 14%, and (g) 24%. The red area indicates α’-martensite. TEM micrographs of RA at the true strains of 14% (h) and 35% (i). DDCs and M indicate the destroyed dislocation cells and α’-martensite, respectively. The XRD results of CFT760 (j) and CG (k) steels at different true stains (ε). (l) The volume fraction of α’-martensite in CFT760 and CG steels.