An Exceptional Synergy of High Strength, Ductility and Toughness in a Gradient-Structured Low-Carbon Steel

Generally, the improvement in the strength comes at the cost of the ductility and toughness for most metallic materials. Here, an exceptional synergy of high strength (σy ~ 453.8 MPa), ductility (εf ~ 24.5%) and static toughness (Ur ~ 115.0 MJ/m3) is achieved in a low-carbon steel subjected to torsion deformation and annealing treatments, compared with that (σy ~ 282.4 MPa, εf ~ 27.3% and Ur ~ 102.9 MJ/m3) of its coarse-grained counterpart. The enhancement of mechanical properties is attributed to the formation of a specific gradient structure with a thickness of ~ 3 mm, that is, the ferrite size increases while the volume fraction of the pearlite decreases continuously with the depth from the sample surface to the core. The strengthening and toughening mechanisms of the gradient-structured low-carbon steel are also discussed.