Morphology of carbonitrides and hot ductility of low carbon low alloy steels.

Abstract

Effect of thermo-mechanical history during cooling from solution treatment temperature of 1300°C on the ductility at 800°C at a strain rate of -10-3s-1 of low carbon low alloy steels has been investigated by means of hot tensile test in relation to surface cracking of continuously cast (CC) slabs. The ductility of the specimens directly cooled to the deformation temperature is largely reduced by the slow strain rate deformation especially in Nb steels, since ductile intergranular fracture of austenite easily occurs because of the dynamic precipitation of carbonitrides such as NbC and/or AlN within the matrix as well as on the grain boundaries. By decreasing the cooling rate or by isothermal holding at around 1100°C, the ductility is significantly improved and the fracture mode changes into transgranular ductile, because the dynamic precipitation is much reduced by nucleation and coarsening of the precipitates in these processes. This, however, takes extremely long time to achieve the coarsening of the precipitates in conventional CC process, and is not practical. In order to accelerate this phenomenon, the effect of the prior deformation was examined, and the optimum conditions suitable for CC process were determined. In the case of continuous cooling, the predeformation larger than 5% should be applied at temperatures around 1050°C at strain rates larger than -10-2s-1. The ductility of the specimens predeformed at lower temperatures can also be improved by employing the subsequent reheating process. These results can be explained in terms of carbonitride precipitation behavior. It is quite difficult to recrystallize coarse austenite such as that in as-cast steels, though the ductility improvement can be obtained by grain refinement due to recrystallization.