Further insights on compositional control and heat treatment for improving the impact behaviour of hot-rolled steels containing Nb and Al

Abstract

The influence of Al and Nb on the strength and impact behaviour of hot rolled steels containing 0.06 wt% of C, 1.4 wt% of Mn has been determined after hot rolling to 15 mm and 30 mm thick plate. When 0.16 wt% of Al was added to the plain C-Mn steel, the impact behaviour significantly improved even though Widmanstätten ferrite (WF) was present. This improvement was due to refinement of the grain boundary carbides and removing the N from solution as AlN. The hot rolled steels all contained WF but when Nb was added more WF formed as well as martensite/retained austenite (MA) giving poor impact behaviour. Reducing the hardenability from that shown in previous work by decreasing C from 0.1 to 0.06 wt%, Nb from 0.03 to 0.02 wt%, and cooling rate from 33 to 17 K/min had no effect in improving the impact performance of hot rolled Nb steels. To ensure optimum properties not only is it necessary to reduce the hardenability, but WF formation must be discouraged by having a high Ar3. This can only be presently achieved by refining the austenite grain size via control rolling the Nb containing steels; the benefit of adding Al can then, readily be seen. Suggestions are made as to how this might be achieved for hot rolling. Keywords: Hot rolling, High Al additions, Nb, Steel, MA, Widmanstätten ferrite, Grain boundary carbides, Impact and strength