低硫AISI4340鋼の低破壊じん性とCa処理によるその改善

概要

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A study bas been made to determine a major metallurgical factor controlling low plane-strain fracture toughness (K_<IC>) observed for desulfurized AISI 4340 ultrahigh strength steel and the K_<IC> of the Suggest a potential approach for improving the K_<IC> of the steel. For the desulfurized steel containing a low sulfur content of 0.002 wt%, intergranular failure occurred at the crack tip in the fracture toughness test though the mechanical isotropy was improved. So there was little difference between the a K_<IC> value in the longitudinal orientation for the desulfurized steel and that for the steel with a sulfur content of commercial level (0.016 wt%). Metallographic observations and fractography revealed that the intergranular failure occurs by the mechanism that during slow cooling from hot-rolling temperature, manganese and sulfur resolved in the high-temperature range segregate to the grain boundaries and appear there as fine MnS after reaustenitizing followed by quenching. Calcium addition of 0.0059 wt% (Ca/S≃3) in desulfurized melts just after vacuum-degassing modified the morphology of non-metallic inclusions from fine MnS to spherical CaS, which leads to improved K_<IC> of the desulfurized steel independent of testing orientation. The beneficial effect of calcium treatment on K_<IC> is briefly discussed in terms of crack extension model in fibrous mode.
社団法人日本材料学会の論文
1991-06-15