Abstract: The hot ductility of low carbon high strength Q345B was determined using thermal analogue machine Gleeble-3500. The precipitates, fracture surface and microstructure of tested samples were observed by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and optical microscopy (OM). The fracture mechanisms were analyzed. The results show that there are three regions between 1 350 ℃ and 650 ℃. The temperature range of regionI is from melting point to 1 307 ℃. The fracture mode is intergranular fracture caused by the segregation of S and O. The temperature range of region Ⅱ is from 1 307 ℃ to 920 ℃. The occurrence of dynamic recrystallization leads to the intragranular fracture. The temperature range of region Ⅲ is from 920 ℃ to 650 ℃. The fracture mode is intergranular fracture caused by the pinning effect of precipitates and the precipitation of pro-eutectoid ferrite along the austenite grain boundaries. The results provide the theoretical basis for the production of continuous casting.