NbとBの複合添加による18%Niマルエージ鋼の高靭性化

抄録

Effects of Nb content and solution treatment temperature on the tensile strength and fracture toughness were examined in 18% Ni maraging steel containing 0.00120.0013% B. When Nb was added as a boride former element, the recovery process of reverse-transformed austenite was retarded. This retardation of recovery led to the suppression of recrystallization and further to the rising in recrystallization temperature of reverse-transformed austenite. The recrystallization temperature was raised with increasing the Nb contents. When the addition of Nb was more than 0.16mass%, however, the fracture toughness was reduced markedly. This decrease of fracture toughness was ascribed to the thermal embrittlement which was caused by growth of NbB₂ type borides. In conclusion, when Nb was added by an amount of 0.05 to 0.07 mass%, both strengthening and toughening can be realized without any thermal embrittlement, by means of the solution treatment in a wide temperature range where the steels are in a state of unrecrystallized austenite.