低炭素鋼へのNi添加による高靭化機構

抄録

It is well known that nickel is one of the most effective alloy elements to improve the fracture toughness of low carbon steels. Although several mechanisms of toughening due to nickel addition were proposed, influential and persuasive mechanisms have not been established yet. In the present work, the fracture toughness of steels with nickel addition from 0 to 4 mass % were investigated for two series of steels; one for the steels subjected to the same heat treatment and the other for the steels with the same bainitic microstructure. Improvement of the fracture toughness with nickel addition was observed for the steels subjected to the same heat treatment. For the steels with same bainitic microstructure, however, the fracture toughness did not improve with nickel addition.
The authors have previously formulated fracture toughness of low carbon steels in terms of tensile properties on the basis of the statistical local fracture criterion. They showed in the previous work that fracture toughness of the steels is expressed as a function of yield stress and cleavage fracture stress. The effect of nickel addition on fracture toughness was analyzed through the change of yield stress and cleavage fracture stress. Decrease of the temperature dependency on yield stress with nickel addition was observed for the both series of steels, indicating the tendency of toughening. Whether the fracture toughness was improved or not by nickel addition depends on the variation of the cleavage fracture stress with nickel addition.