INFLUENCE OF COOLING RATE AND MNS INCLUSIONS ON HOT DUCTILITY OF STEELS

The influence of cooling late on the hot ductility of plain C-Mn steel s containing 0.1-0.7%C has been examined for tensile specimens tested in the temperature range 1000-550 degrees C. Increasing the cooling ra te from 10 to 60 K min(-1) in all cases caused the ductility to deteri orate. This deterioration was found to be associated with the presence of a finer sulphide distribution in the ferrite surrounding the auste nite grains, as well as a reduction in the thickness of the ferrite co vering. Decreasing the strain rate from 3 x 10(-3) to 5.3 x 10(-4) s(- 1) led to a smaller influence of cooling rate. However, ductility was always poorer at the lower stain rate as grain boundary sliding was en hanced. Evidence is also submitted to indicate that MnS inclusions can encourage the production of deformation induced ferrite, leading to a n earlier recovery in ductility at the low temperature end of the trou gh in the hot ductility curve.