準安定オーステナイト系ステンレス鋼のMd30に及ぼす炭素および窒素の影響

増村 拓朗, 藤野 昂平, 土山 聡宏, 高木 節雄, 木村 謙

doi:10.2355/tetsutohagane.tetsu-2019-062

Md₃₀ is defined as the temperature at which 50 vol.% of α’-martensite is formed at a true tensile strain of 0.3 in metastable austenitic steels. It has been generally believed that the effect of carbon content on Md₃₀ was estimated to be identical to that of nitrogen as shown by Nohara’s equation. However, we found in this study that Md₃₀ in carbon-added steel is lower than that in nitrogen-added steel, which indicates that the effect of carbon content on the mechanical stability of austenite is more significant than that of nitrogen. In addition, the relationship between Md₃₀ and carbon and nitrogen content is not linear. The effect of carbon and nitrogen content on Md₃₀ is higher at lower carbon and nitrogen content region (<0.1%). As this effect was not considered in the previous study, the austenite-stabilizing effects of both the elements were underestimated. Therefore, in this study, new equations are proposed to accurately estimate Md₃₀ of a Fe-Cr-Ni alloy system. As a result, modified Md₃₀ equation is suggested as below:Md₃₀(K) = 800 – 333 C eq – 10.3Si – 12.5Mn – 10.5Cr – 24.0Ni – 5.6MoCarbon equivalent, C_eq is a function of carbon and nitrogen concentrations and temperature.C_eq = C + aNa = 0.931 – 0.000281exp (0.0219T)Above equations show that the difference in austenite-stabilizing effects of carbon and nitrogen increases with rising temperature, owing to the difference in stacking fault energy between carbon-added and nitrogen-added steels.

準安定オーステナイト系ステンレス鋼の<i>Md</i><sub>30</sub>に及ぼす炭素および窒素の影響

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