Fig. 1 Transition temperatures during rapid continuous cooling (100 °C/s) a, schematic diagram showing the heat treatment process b

Fig. 2 SEM micrographs and band contrast (BC) maps depicting boundary distribution: a, e quenching, b, f 400 °C, c, g 450 °C, d, h 500 °C (white line: 15°>θ>5°, black line: 45°>θ>15°, yellow line: θ>45°)

Fig. 3 Dilatation versus temperature during the whole transformation process of isothermal process: a 400 °C, b 450 °C, c 500 °C

Fig. 4 Length fraction of inter-variant boundaries

Fig. 5 Inverse pole figure (IPF) showing coinciding variants a, band contrast map b, corresponding experimental and theoretical pole figure c of Quenching sample (white line: 15°>θ>5°, black line: θ>15°)

Fig. 6 IPF figure showing coinciding variants a, band contrast map b and corresponding experimental and theoretical pole figure c of 450 °C isothermal transformation sample (white line: 15°>θ>5°, black line: θ>15°)

Fig. 7 IPF figure showing coinciding variants a, band contrast map b and corresponding experimental and theoretical pole figure c of 500 °C isothermal transformation sample (white line: 15°>θ>5°, black line: θ>15°)

Fig. 8 Band contrast map a showing coinciding variants and grain boundary distribution and corresponding experimental b and theoretical pole figure c of 400 °C isothermal process (white line: 15°>θ>5°, black line: θ>15°)

Fig. 9 Schematic diagram a, experimental observation b of grain boundary nucleation for bainite transformation and schematic diagram of nucleation at the austenite Σ3 boundary for 450 c and 500 °C d isothermal transformation