The chemical compositions (wt.%) of the phases of the steel were analyzed by EDS which is standard equipment in TEM. Because of carbon contamination on the surface of the TEM specimen, all the carbon contents of the constituent phases measured by EDS are neglected. The chemical compositions of the phases at 1023 K are as follows. The grain boundary M3C carbide is Fe–17.3 Mn and the austenite matrix is Fe–10.6 Mn. Note that cementite has high carbon content as indicated in the EDS analysis. Thus, the cementite contains a high concentration of Mn and C. The Mn content of the light austenite layer in Fig. 3(a) is Fe–7.2 Mn, which is lower than that of the austenite matrix. This can be explained by the following. M3C precipitates at the austenite grain boundaries. During the growth of carbide, Mn and C solute atoms must be transported from the matrix to the grain boundaries. The diffusion of Mn and C solute atoms from the austenite to M3C carbide results in the depletion of the Mn and C atoms in the austenite adjacent to carbide. The austenite layer near the cementite grains shows distinguishing characteristics of lower Mn content and a lighter color than the austenite matrix.