AbstractHot ductility, measured by reduction in area, has been determined over the temperature range 550–950°C for a series of plain C–Mn steels having the same base composition except for the carbon content, which was in the range 0·04–0·65 wt-%. A ductility trough was obtained for all the steels and minimum ductility values were similar. Raising the carbon content from 0·04 to 0·28 wt–% caused the ductility trough to move to lower temperatures and this was in agreement with the observed changes in transformation temperature. Tensile fracture at the minimum ductility temperature was along thin films of ferrite which formed round the austenite grains–generally by deformation–induced transformation. The softer ferrite allowed strain concentration to cause ductile voiding at the MnS inclusions, and the voids eventually linked up to give intergranular failure. Raising the carbon content above the 0·28% level caused a change in the fracture mode. Instead of the ductility troughs moving to lower temperatures, a shift of over 100 K to higher temperatures was observed. Intergranular failure now occurred in the austenite as a result of grain boundary sliding. It is suggested that this change in fracture mode is caused by carbon increasing the activation energy, and hence the critical strain required for dynamic recrystallization, so favouring the linking of cracks formed by grain boundary sliding.MST/366