Abstract
The effect of the manganese content of 7% nickel-added steel on cryogenic toughness was investigated. When Charpy absorbed energy was investigated in the steels with wide range manganese content from 0.05% to 2.16%, the absorbed energy at −196°C increased with a decrease of manganese content. The two steels with manganese contents of 0.2% and 0.8% were selected for use in a more detailed investigation regarding the dependence of toughness at −196°C on the intermediate heat treatment temperature, fracture behaviors and the retained austenite. Although the 0.2% manganese steel stably exhibited a high absorbed energy at −196°C at all intermediate heat treatment temperatures examined in this work, it was necessary to select an appropriate intermediate heat treatment temperature for the 0.8% manganese steel in order to achieve high absorbed energy at −196°C. The total amount of the retained austenite and the size and the nickel content locally segregated into an identical retained austenite were quantified by using XRD and SEM/EDS, and the characteristics of the retained austenite in the steels with low manganese alloy designs were investigated. The cleavage-type brittle fracture that appeared in some specimens was discussed from the viewpoint of the stability of retained austenite.
