A heterostructured bainitic steel produced by two-step austempering and low-temperature ausforming

Heterogeneous microstructure has been become a very effective scheme for evading the yield strength (YS) ductility trade-off in metallic materials, however, strategies of fabricating heterostructured bainite (HeteroB) in carbide-free bainite have not yet been explored. Herein, we successfully produced a HeteroB composed of submicron-scale primary bainite and nano-scale low-temperature bainite by combining two-step austempering (AT) and low-temperature ausforming (AF). Compared with the homogeneous bainite (HomoB) in the conventional AT sample, the HeteroB in the AT&AF sample exhibits a wider lath size distribution due to its bimodal size variation. The difference in average nano-hardness between the primary bainite and nanobainite reaches 0.78 GPa. The HeteroB renders an impressive enhancement in strength (-310 MPa of YS) and ductility (-26.1 % of uniform elongation) in the AT&AF sample compared to the conventional AT sample. This synergistic improvement in YS and ductility relies on the sustained transformation-induced plasticity and heterogeneous deformation-induced strengthening effect.