In this book, we will investigate phase equilibria at solidification in lightweight steels with Al, in the presence of Sn as an impurity, which have not yet been studied. The purpose of this study is to develop a clear description of equilibrium phase transformations that take place in the Fe-Sn-Al system. The phase diagram of this system will be presented in the form of liquidus and solidus surface projections. The phase diagrams built will correspond to the international standards, and will be included in the current handbooks, and will be used as the basis in the thermodynamic modelling of the systems. For the first the un-investigated regions of the Fe-Sn-Al ternary system were constructed in wide ranges of concentrations and temperatures including melting/crystallization region, or substantially supplemented in areas important in terms of practical use.