Optimizing mechanical properties of gradient-structured low-carbon steel by manipulating grain size distribution

In this paper, various gradient structures (i.e., the ferrite grain size increases continuously with the depth) with different grain size distributions are produced in low-carbon steel by using pre-torsion deformation and thermal annealing at ~ 550 °C to 700 °C for 0.5 h. Results indicate that the grain size distribution possesses a crucial influence on mechanical properties of gradient-structured samples. The yield strength decreases steady accompanied by a gradual increase in uniform ductility and static toughness with increasing the ferrite grain size. The optimal synergy of high yield strength (σy~ 506.0 ± 3.9 MPa), promising uniform ductility (εu~ 9.6 ± 0.5%) and static toughness (Ur~ 108.8 ± 4.1 MJ/m3) is achieved by a specific gradient structure with the grain size distribution from ~ 7.2 µm at the surface to ~ 16.5 µm at the core, which is much better than that (σy~ 318.2 ± 1.6 MPa, εu~ 13.4 ± 0.4% and Ur~ 102.5 ± 4.8 MJ/m3) of its coarse-grained (CG) counterpart. Related strengthening and toughening mechanisms are also discussed.