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 similar to 550 degrees C to 700 degrees 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 (sigma(y)similar to 506.0 +/- 3.9 MPa), promising uniform ductility (epsilon(u)similar to 9.6 +/- 0.5%) and static toughness (U-r similar to 108.8 +/- 4.1 MJ/m(3)) is achieved by a specific gradient structure with the grain size distribution from similar to 7.2 mu m at the surface to similar to 16.5 mu m at the core, which is much better than that (sigma(y)similar to 318.2 +/- 1.6 MPa, epsilon(u)similar to 13.4 +/- 0.4% and U-r similar to 102.5 +/- 4.8 MJ/m(3)) of its coarse-grained (CG) counterpart. Related strengthening and toughening mechanisms are also discussed.