Fig. 1 CCT curves of the experimental steel. Cooling rates are labeled beside cooling curves, and Vickers hardness values of the samples are labeled in circles at the ends of cooling curves

Fig. 2 Microstructures of GB-steel, BF-steel, and PF-steel: a optical metallograph of GB-steel; b secondary electron image of GB-steel; c optical metallograph of BF-steel; d secondary electron image of BF-steel

Fig. 3 Force-displacement curves and energy-displacement curves of the GB-steel and the BF-steel measured through instrumented Charpy V-notch pendulum impact tests at room temperature

Fig. 4 Optical metallograph of PF-steel

Fig. 5 Variations in the strength of experimental steels with temperature and holding time: a proof strength Rp0.2 with temperature; b ratio of Rp0.2 at high temperature to Rp0.2 at room temperature with temperature; c remained strengthening effect of bainitic transformation with temperature; dRp0.2 with holding time during isothermal holding at 600 °C

Fig. 6 TEM images showing granular bainite a bainitic ferrite b

Fig. 7 Evolution of dislocation configuration during isothermal holding at 600 °C by TEM: a before reheating; b after holding for 1 h; c, d after holding for 12 h

Fig. 8 Partial disappearance of lath boundary in the steel after isothermal holding at 600 °C for 6 h by TEM

Fig. 9 Optical metallographic observation of granular bainite a bainitic ferrite b in the steel after holding at 600 °C for 48 h

Fig. 10 Hardness variation of BF-steel during isothermal holding at 600 and 650 °C

Fig. 11 Optical metallographic observation of BF-steel after isothermal holding at 600 °C for different times of 15 min a, 16 h b, 64 h c, 192 h d