Fig. 1 Schematic diagrams of different heat treatments: a cycle-1; b cycle-2; c cycle-3

Fig. 2 SE images of as-quenched samples: a DQ-820; b DQ-900; c DQ-900-860; d DQ-900-820. Note that the yellow line represents PAGB

Fig. 3 a Martensite transformation behavior of different as-quenched sample; b enlarged image of martensite start region in a. Note that the dots in b indicate MS temperatures of different samples

Fig. 4 Martensite transformation kinetics of different as-quenched samples

Fig. 5 SE images of different Q&P samples: a cycle-1; b cycle-2; c cycle-3. Note that the red lines indicate PAGB

Fig. 6 EBSD results of Q&P samples. The distributions of constituent phases and grain boundaries: a cycle-1 sample; b cycle-3 sample. The distribution of misorientation angle: c cycle-1 sample; d cycle-3 sample. e Comparison of the deviation angle of KS boundary in cycle-1 and cycle-3 samples. Note that dev. indicates deviation and mis. represents misorientation

Fig. 7 XRD results of retained austenite fractions and corresponding austenite carbon contents in different Q&P samples

Fig. 8 Typical TEM micrographs of cycle-3 sample: a lath-type martensite and b corresponding dark filed image of ε carbide, inserted image indicated the diffraction patterns of martensite matrix and ε carbide [25]; c twin martensite with ε carbide precipitation. Note that, the two series of dash lines with parallel directions indicate the habit planes of precipitation

Fig. 9 Element distribution of ε carbide in cycle-3 by STEM + EDX: a bright field image; b HADDF image corresponding to the rectangle in a; c C element; d Mn element; e Si element. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 10 Microstructural analysis of cycle-1 sample by a combination of TEM and PED: a TEM bright field image; b virtual bright field image of PED; c phase distribution; d HAGB (> 10°); e IPF of orientation distribution (ND: Normal direction)

Fig. 11 Microstructural analysis of cycle-3 sample by a combination of TEM and PED: a TEM bright field image; b enlarged image of a; c, d and e PED results corresponding to the rectangle in a; c virtual bright field image + grain boundary (> 10°) + twin boundary (red color); d phase distribution; e IPF of orientation distribution (ND: Normal direction)

Fig. 12 Mechanical properties and work hardening behavior of different Q&P samples: a engineering stress-engineering strain curves; b variations of true stress and work hardening rate as a function of true strain

Fig. 13 Comparison of mechanical properties in different Q&P samples. YS, UTS, UEL, TEL, PSE indicate yield strength, ultimate tensile strength, uniform elongation, total elongation, product of UTS and TEL, respectively