The influence of a high aluminium addition on the strength and impact behaviour of hot-rolled Nb containing steels

An addition of 0.3 wt-% Al has been made to a Nb containing 0.1 wt-% C, 1.4 wt-% Mn steel in an attempt to improve its impact behaviour on hot rolling to 15 mm thick plate; the intention being to replace some of the specifications that currently can only be achieved by the more expensive methods of control rolling or normalising. Previous work had shown that a 0.2 wt-% Al addition to a hot-rolled plain C-Mn steel resulted in the impact transition temperature decreasing by 40 degrees C with no significant change in strength making such a proposition possible. Unfortunately, the coarse austenite grain size of the hot-rolled Nb containing steel resulted on transformation in colonies of lower transformation products and Widmanstatten ferrite forming in the ferrite/pearlite microstructure resulting in poor impact behaviour. Reducing the cooling rate by working with thicker plate, 30 mm, led to little change in the mechanical behaviour on hot rolling. In contrast, on control rolling, the finer-grained steels which were virtually free of these lower transformation products showed a small benefit from the high Al addition even when Nb was present, particularly for the slower cooling rate, the 27J impact transition temperature decreasing by similar to 20 degrees C together with a small increase in yield strength. Further work is suggested to decrease the hardenability so that the original aim might be achieved.