Effect of Carbon and Nitrogen on <i>Md</i><sub>30</sub> in Metastable Austenitic Stainless Steel

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. The effect of C concentration on Md30 is known to be identical to that of N, as shown by Nohara's equation. However, we found that Md30 of C-added steel is lower than that of N-added steel, which indicates that the effect of C concentration on the mechanical stability of austenite is more significant than that of N. In addition, the relationship between Md30 and C and N concentration is not linear. The effect of C and N concentration on Md30 is higher at lower C and N concentration (<0.1%). As this effect was not considered in the previous study, the austenite-stabilizing effects of these elements were underestimated. Therefore, in this study, new equations were proposed to accurately estimate Md30 of an Fe–Cr–Ni alloy system. The modified Md30 equation is shown below:Ceq (C equivalent) is a function of C and N concentrations and temperature.These equations show that the difference in austenite-stabilizing effects of C and N increases with increasing temperature due to the difference in stacking fault energy between C- and N-added steels.