Investigation of the hot ductility trough of Nb-containing steel

It is well known that a fine dispersion of precipitates can strongly decrease the hot ductility of low carbon steel at temperatures around 900 to 1000°C. This is attributed to an increase of the recrystallization start temperature during hot forming. In the present investigation, focus is laid on the second ductility minimum and a correlation between the measured fracture characteristics and NbC-precipitation observed in regions at and around the austenite grain boundaries is attempted. In the analysis, several low carbon steel samples were tensile tested to fracture at different temperatures. In order to quantify the size distribution of the NbC precipitates at and around the grain boundaries, carbon extraction replica were produced. The experimentally observed variation of ductility can be explained on the basis of the differences in size and distribution of the NbC precipitates. The high density of small precipitates at the lower test temperatures acts as an efficient inhibitor to recrystallization during deformation and can clearly be made responsible for the significant decrease in hot ductility in the second ductility trough.