Multipass hot deformation behaviour of high Al and Al-Nb steels

The effect of Nb (up to 0.07 wt%) and high Al content (up to 2 wt%) on the multipass deformation behaviour of steels with 0.2% C and 2% Mn was studied with the aid of hot torsion simulations. From the tests, the critical Non-Recrystallisation (Tnr), Recrystallisation Limit and Stop Temperatures (RLT and RST) and the ferrite phase transformation start temperature (Ar3) were determined. It was observed that an increase in Al content from 1% to 2% or a microalloying addition of 0.03% Nb to 1% Al steel both led to a significant increase in the recrystallisation critical temperatures, which is greater than 100 °C in the case of the Tnr. However, the value of the Tnr was not affected when 0.03% or 0.07% Nb was added to the 2% Al steel. Specimens quenched after several deformation passes were examined by optical and TEM means in order to study the interaction between static recrystallisation, strain-induced precipitation and γ→α phase transformation, and determine the mechanisms leading to strain accumulation in the steels investigated. The results suggest that for the 1% Al steels, the Al and Nb solute drag effect is the main mechanism leading to the increase in the critical recrystallisation temperatures, while for the 2% Al steels the occurrence of γ→α phase transformation at temperatures close to the Tnr is the main mechanism involved in softening retardation, with a limited contribution of Nb. However, γ→α phase transformation taking place at temperatures close to the Tnr resulted in a loss of hot ductility, which can limit the industrial applicability of the 2% Al steels.