Grain-refinement simultaneously increases yield strength and toughness while the strengthening without the grain-refinement still deteriorates the toughness. It is worth assessing how the strengthening relates to the toughness to strengthen steels most appropriately. In this study, different mechanisms for strengthening are comparedwith respect to the relationship between yield strength and ductile-brittle fracture transition temperature. Considered are the grain-refinement by solute copper or

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... hermo-mechanical control process (TMCP), the precipitation-hardening by copper, and the bainite-transformation hardening with alloying elements such as boron, molybdenum,and nicke[. The transition temperature rises with strengthening by the precipitation-hardening and the transformation-hardening. In this case, the relationship is approximately linear with the gradient of 0.46'CIMPa. In contrast, the transition temperature lowers in the case of the strengthening by the grain-refinement achieved by the solute copper and the TMCP. This study aims at relating the yield strength to the transition temperature on a dislocation crack model of britt[e fracture. Considering the energy balance for brittle crack initiation and the temperature dependence of the friction stress for the Hail-Petch relationship gives the linear lowering of transition temperature with increasing yield sttength by grain-refinement, of which gradient is estimated at approximately 0.80"C/M Pa. Thecomparison of experimental results with this relationship indicates that the strengthening by solute copper effectively utilizes the grain-refinement,