Influence of grain size on hot ductility of plain C–Mn steels

Abstract

The influence of grain size on the hot ductility of 0·19 and 0·65wt-%C steels of the C–Mn type has been determined. For the low-carbon steel, a gram Size increase from 70 to 180 μm had only a small influence on hot ductility, as measured by tensile reduction in area values. However, increasing the grain size to 290 μm raised the temperature at which ductility started to fall by 50°C. In the finer grained steels it is believed that the ductility trough starts at the Ar3 temperature when films of ferrite form round the stronger austenite grains. Ductility soon recovers as the temperature is lowered because of a thickening of the ferrite and a consequent reduction of strain concentration at the boundaries, so that only a narrow trough is observed. In coarser grained steels it is considered that deformation induced ferrite can have a pronounced influence on hot ductility over a wide range of temperatures leading to a wide ductility trough. Refining the grain size had an even greater influence on the hot ductility of the 0·65wt-%C steel. Intergranular tensile fracture at coarse grain size was by grain boundary sliding in the austenite resulting in a very wide ductility trough. Refining the grain size prevented intergranular failure occurring in the γ down to the lowest temperature examined: 700°C. Although the main influence of grain size is in controlling the width of the trough, the depth also increased with an increase in grain size.MST/420