Achieving an excellent combination of strength and plasticity in a low carbon steel through dynamic plastic deformation and subsequent annealing

An investigation was conducted to evaluate the effect of dynamic plastic deformation (DPD) and post-DPD annealing on the microstructural and mechanical properties of a tempered low carbon steel. The results showed that ultrafine-grained structures consisting of elongated martensitic laths and sub-grains are achieved after DPD processing. The amounts and sizes of carbides in the steels, identified as (Fe,Cr,Mn,Mo)3C, decreased markedly with DPD straining due to their fragmentation and dissolution but a large number of finer carbides appeared due to re-precipitation during subsequent annealing. A simultaneous improvement in the strength and plasticity was obtained at DPD strains below ∼0.8. This increase in strength by ∼30–60% is mainly attributed to grain boundary strengthening, dislocation strengthening while the good plasticity is due to more active sliding systems, a reduction in the stress concentration during loading because of the amount decreasing of M3C distributed along the interfaces, the increase of crack propagation resistance by more grain boundaries and the energy released through the occurrence of delamination fracture. After post-DPD annealing both the strength and plasticity improved compared with the as-received steel. Strengths higher by ∼20–39% were attributed to a combination of grain boundary strengthening, dislocation strengthening and precipitation strengthening derived from the re-precipitation of fine and dispersed carbides. The dislocation recovery occurring during annealing led to a decrease in strength compared with that before annealing. The incremental increase in plasticity is attributed to a combination of further active slip systems, a decreasing dislocation density and a dispersed distribution of finer carbides.