Brittle-ductile transition in low carbon steel deformed by the accumulative roll bonding process

Brittle-ductile transition (BDT) behaviour was investigated in low carbon steel deformed by an accumulative roll-bonding (ARB) process. The temperature dependence of its fracture toughness was measured by conducting four-point bending tests at various temperatures and strain rates. The fracture toughness increased while the BDT temperature decreased in the specimens deformed by the ARB process. Arrhenius plots between the BDT temperatures and the strain rates indicated that the activation energy for the BDT did not change due to the deformation with the ARB process. It was deduced that the decrease in the BDT temperature by grain refining was not due to the increase in the dislocation mobility controlled by short-range obstacles. Molecular dynamics simulations revealed that moving dislocations were impinged against grain boundaries, creating a shielding field, and were reemitted from there with increasing strain. Grain refining led to an increase in the fracture toughness at low temperatures and a decrease in the BDT temperature. In the present paper, the roles of grain boundaries have been discussed in order to explain the enhancement in the fracture toughness of fine-grained materials at low temperatures, and the decrease in the BDT temperature