The control of carbide formation is crucial for the development of advanced steels. One of the more stable carbides occurring at room temperature ageing of martensite is η-Fe2C. In this work, we study structural transformations which occur during low-temperature ageing in Fe-C martensite using computer modelling based on the atomic density field method. It is shown that homogeneous bct solid solution with 2at.% of C undergoes bct →η-Fe2C transformation. An important result is that the ordering Fe2C structure may be considered a deformed rutile structure. In addition, the link between η-carbide and other meta-stable carbides is discussed based on the Fe2n+1Cn (n=1,2,3,…) Magnéli phases. The Magnéli phases comprise a sequence of carbon-rich 110α′ layers with different periods to accommodate the loss in stoichiometry. The simulated microstructure is compared to experimental data found in the literature.