1987 Volume 73 Issue 10 Pages 1373-1380
Effect of thermo-mechanical history during cooling from solution treatment temperature of 1 300°C on the ductility at 800°C at a strain rate of 10-3 s-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 evaluated by reduction in area 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 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 1 100°C, the ductility is significantly improved and the fracture mode changes into transgranular ductile, because the dynamic precipitation is much reduced by the depletion of preciptating elements due to 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 1 050°C at strain rates larger than 10-2 s-1. The ductility of the specimens predeformed at lower temperatures can also be improved by employing the subsequent reheating process.
