Fe-Mn-Al-C steels, previously developed in the 1950s for replacing Fe-Cr-Ni steels as oxidation or corrosion resistance steels or cryogenic temperature steels, are currently revisited due to two reasons. From an engineering point of view, the specific weight of the steels is reduced when a large amount of light element Al is added, resulting in the so-called low-density or lightweight steels. From an academic point of view, the metallurgical theories of the steels are not well established. The low-density or lightweight steels are expected to have potential applications for structural parts in the automotive industry. This chapter discusses the basic metallurgy, processing strategies, strengthening mechanisms and mechanical properties of these steels from the published literature over a period of many years and suggests avenues for future applications of these alloys in the automotive sector.

The effects of hot deformation conditions in the stable austenite state on the transformation kinetics and morphology of bainite were examined using dilatometry, electron back scatter diffraction (EBSD) and X-ray diffraction (XRD) measurements in a carbide-free bainitic steel with a composition of Fe-0.34C-2Mn-1.5Si-1Cr (in wt.%). Both uniaxial tensile and compression tests were applied to study the effect of the deformation mode. The temperature, strain and strain rate of deformation were varied in the ranges of 820-1000 °C, 0.1-0.6 and 0.001-0.1/s respectively. It has been revealed that hot tensile deformation retards the austenite transformation to lower bainite. The overall transformation kinetics slows down and the final attained amount of bainite decreases after completion of the isothermal transformation at 350 °C. However, hot compression deformation accelerates the bainite transformation, increasing both the bainite transformation rate and the final amount of bainite formed. The total amount of bainite increases with decreasing the strain rate irrespective of the mode of deformation. The effect of the deformation temperature and strain on the bainite transformation is in a complicated manner depending on the deformation mode.