Fig. 1 Micrographs showing degenerate pearlite and quasi-polygonal ferrite in the steel No. 1 at a cooling rate of 0.03 °C/s. a SEM image indicating quasi-polygonal ferrite and degenerate pearlite, b magnified image of degenerate pearlite observed by TEM

Fig. 2 Micrographs showing quasi-polygonal ferrite and granular bainite in the steel No. 1 at a cooling rate of 0.5 °C/s: a OM and b SEM images

Fig. 3 Microstructure and alloying elements distribution of granular bainite obtained at cooling rate of 1.5 °C/s. a OM, c, d alloying elements distribution plots from b line scanning by EPMA

Fig. 4 Micrographs showing two types of GB. Typical microstructures of GB1 a, b and GB2 c, d obtained at cooling rate of 2 and 5 °C/s in the steel No. 1, respectively. a, c SEM and b, d TEM images

Fig. 5 Results of XRD analysis for steel No. 1 at cooling rates of 2 and 5 °C/s

Fig. 6 Micrographs showing lath bainite and martensite in steel No. 1 at a cooling rate of 50 °C/s. a SEM image revealing the mixture with lath bainite and martensite; b TEM micrograph of lath bainite; c TEM image showing lath bainite ferrite associated with intralath carbides; d TEM image showing martensite with high density of dislocations and without second constituents or carbides

Fig. 7 Optical micrographs of steels No. 1 and No. 2 at different cooling rates. a-d in steel No. 1 and e-h in steel No. 2 when cooling rates are 0.075, 0.25, 1 and 1.5 °C/s, respectively

Fig. 8 SEM micrographs of steels No. 1 and No. 2 at different cooling rates. a, c, e, g, i in steel No. 1; b, d, f, h, j in steel No. 2 when cooling rates are 2, 7.5, 15, 30 and 150 °C/s, respectively

Fig. 9 Prior austenite grain boundaries of steels No. 1 a and No. 2 b

Fig. 10 Dilatometric curves of steels Nos. 1 and 2 for measuring austenitizing critical points. Dark and red curves represent steels Nos. 1 and 2, respectively

Fig. 11 Plots showing the temperature derivative of change in length versus temperature at cooling rates of a 0.075 °C/s, b 1.5 °C/s, c 5 oC/s, d 150 °C/s. Dark and red curves represent steels Nos. 1 and 2, respectively (Fs, the start temperature of ferrite; Bs, the start temperature of bainite: Ff, finish temperature of bainite; Bf, finish temperature of bainite)

Fig. 12 CCT curves of low alloy Cr-Mo-V steels Nos. 1 and 2. Dark and red curves represent steels Nos. 1 and 2, respectively. DP degradation pearlite, QPF quasi-polygonal ferrite, LB lath bainite, M martensite