Influence of B on hot ductility of high Al,TWIP steels

Abstract

The influence of B on the hot ductility of high Al, Ti containing twinning induced plasticity (TWIP) steels has been examined. It was established that provided the B was fully protected by adding sufficient Ti to combine with all the N, then B could segregate to the austenite grain boundaries and improve ductility. This improvement was particularly marked for the temperature range of 700–900°C, the range in which the straightening operation often takes place in continuous casting. Of most importance in the present work has been the detection of B at the boundaries using a secondary ion mass spectrometry technique. The cooling rate from the reheating temperature of 1250°C to the tensile testing temperature range of 700–1200°C was 60 K min⁻¹, but it is likely that slower cooling rates ≤25 K min⁻¹, more in keeping with the secondary cooling rate on continuous casting, will give even better ductility. Ti additions in themselves are beneficial to the hot ductility of these steels as precipitation of AlN at the austenite boundaries is avoided, but only if the cooling rate is sufficiently slow to allow the TiN particles to coarsen. However, to ensure freedom from cracking, an addition of B is also required.