Influence of Al on strength and impact behaviour of hot rolled plain C-Mn steels

The influence of Al on the strength and impact properties of hot rolled plain C-Mn low S steels has been examined at two C levels, 0˙02 and 0˙1% and two N levels, 0˙001 and 0˙004-0˙005% (where percentage is to be taken as wt-%). It has been found that Al is an excellent element to add to hot rolled steels, an addition of 0˙2% to a plain 0˙1C-1˙4Mn steel with 0˙004-0˙005%N resulting in an impact transition temperature of −95°C at a strength level of ∼300 MPa. At this Al level, Al removes the N from solution but also importantly refines the grain boundary carbides and to a lesser degree the grain size resulting in a 40°C lowering of the impact transition temperature in a coarse grained steel (15-20 μm; d− 1/2, 7-8 mm−1/2). Strength is not affected by this Al addition. The decrease in strength due to the removal of nitrogen from solution as AlN is balanced by the solid solution hardening from the Al in solution in the iron lattice, (a 1%Al addition increasing the strength by ∼70 MPa), and the small grain refinement that occurs from AlN precipitation. Increasing the Al level to 1% in the 0˙1%C steels, results in the formation of martensite with consequent deterioration in impact behaviour. When the N level is reduced to very low levels, 0˙001%N, no grain refinement in the γ occurs in the 0˙02% and 0˙1%C steels at the 0˙2%Al level. The solid solution hardening effect of Al now dominates the properties with the impact transition temperature remaining constant due to Al refining the grain boundary carbides