The as cast hot ductility of low C -V, low C -V -Nb, and niobium microalloyed steels has been investigated using in situ melted tensile specimens, which were subjected to cooling rates and strain rates found typically in thin slab casting. Stress relaxation tests were performed on in situ melted specimens to monitor the kinetics of strain induced precipitation in the above steels. Although the addition of niobium to low C -V and low C -V -N steels increased the temperature at which ductility began to deteriorate, ductility was improved in the low temperature region of the austenite. This was attributed to a delay in NbV(C,N) precipitation in V -Nb steels. Increasing the nitrogen content did not influence the temperature at the onset of ductility loss in low C -V and low C -V -Nb steels, but did increase the temperature marking the onset of the ductility trough. Faster cooling rates led to a deterioration of ductility in low C -V -N and low C -V -Nb -N steels. The as cast ductility of low C -V and low C -V -Nb steels was superior to that as of cast peritectic C -Nb steel as well as reheated peritectic C -V and peritectic C -V -Nb steels. MST/4988Dr Banks