Evaluating the Effect of the Competition between NbC Precipitation and Grain Size Evolution on the Hot Ductility of Nb Containing Steels

The hot ductility of steels containing 0-0.06 wt.%Nb has been evaluated through gamma grain growth experiments and hot stage tensile tests of the alpha + gamma two phase region in order to clarify the roles of NbC precipitation and gamma grain size evolution resulting from Nb-initiated solute drag on hot ductility in this important material property. The experimental results show that (1) a decrease in gamma grain size as a result of Nb-initiated solute drag improves hot ductility, (2) for a given gamma grain size, hot ductility decreases with increasing Nb content because the corresponding increase in NbC precipitation fraction increases strength, and (3) the variation in ductility with Nb content is smaller when the gamma grain size is smaller. These competing effects of gamma grain size and NbC precipitation affect the strain incompatibility between the alpha and gamma phases, leading to the onset of surface cracking during continuous casting when the incompatibility is high. The underlying mechanisms controlling ductility in Nb-containing steels are demonstrated using a model that partitions strain between the alpha and gamma phases.