Hot Ductility of Nb- and Ti-Bearing Microalloyed Steels and the Influence of Thermal History

The hot ductility of Nb, Ti, and Nb-Ti containing steels has been studied under direct-cast conditions. A Gleeble 3500 thermomechanical simulator was used to determine hot ductility over the temperature range 1100 A degrees C to 700 A degrees C at a low strain rate of 7.5 x 10(-4) s(-1). Tensile samples were cooled at two different cooling rates, 100 A degrees C/min and 200 A degrees C/min, simulating, respectively, thick and thin slab casting processes. Complex thermal patterns designed to simulate the cooling conditions experienced near the surface of a slab during continuous casting were carried out for the Nb-Ti steel. The Nb-Ti steel had lower ductility than both the Nb and Ti steels. Increasing the cooling rate generally deteriorated ductility. The low recovery of ductility at higher temperatures is explained in terms of a low strain rate and fine precipitation delaying the onset of dynamic recrystallization. This can promote intergranular cracking as a result of grain boundary sliding in the austenite. At lower temperatures, ductility was further reduced due to the formation of thin ferrite films at the prior austenite grain boundaries. Simulating the thermal history experienced near the surface of thin (90 mm) cast slab improved ductility of the Nb-Ti steel by promoting coarser NbTi(C,N). This exposes a potential flaw in a simplified hot-ductility test: a failure to accurately represent the influence of the thermomechanical schedule on precipitation and, hence, hot ductility.