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.