The enhancement of toughness at low temperatures in fine-grained low-C steel was studied, based upon a shielding theory due to dislocations and grain boundaries. Fully-annealed low-C steel was subjected to an accumulative roll bonding process for grain refining. The grain size perpendicular to the normal direction was found to be approximately 200nm following the accumulative roll bonding process. The fracture toughness of low-C steel after accumulative roll bonding was measured at 77K by 4-point bending, and compared with the fracture toughness of samples without accumulative roll bonding. It was found that the value of the fracture toughness at 77K was increased by grain refining due to the accumulative roll bonding; indicating that the accumulative roll bonding process enhanced the toughness at low temperatures, as reported for interstitial-free steel and P-doped interstitial-free steel. It was also deduced that the brittle-ductile transition temperature shifted to a lower temperature. The enhancement of toughness and the decrease in the brittle-ductile transition temperature due to grain refining could not be completely explained by the dislocation pile-up model of dislocations at grain boundaries. Quasi 2-dimensional simulation of dislocation dynamics, taking account of crack-tip shielding due to dislocations, was performed in order to investigate the effect, of dislocation-source spacings along a crack front, upon the brittle-ductile transition. The simulation indicated that the brittle-ductile transition temperature was decreased by decreasing the dislocation-source spacing. A new concept of stress-intensity accommodation at the crack tip, due to grain boundaries, was proposed in order to explain the enhancement of toughness and the decrease in brittle-ductile transition temperature in fine-grained materials.

Fracture Toughness Enhanced by Grain Boundary Shielding in Submicron-Grained Low Carbon Steel. M.Tanaka, N.Fujimoto, K.Higashida: Materials Transactions, 2008, 49[1], 58-63