Nano-structured pearlite obtained by slow cooling in Fe–5Mn–5Al–1C light-weight steel

To meet the demand for simultaneous light-weighting and high strength in bridge-cable steels, 5 wt% Mn and 5 wt% Al were added to a Fe–0.97C alloy to develop a lightweight pearlitic steel with a theoretical density of 6.81 g/cm 3 , representing a 13.2 % reduction compared with conventional steels. At a cooling rate of 0.1 °C/s, κ-carbide ((Fe,Mn) 3 AlC) precipitates preferentially, acting as heterogeneous nucleation sites and, by expanding the austenite phase region and regulating carbon distribution, refines the interlamellar spacing to 63.7 ± 4.6 nm–54.7 % finer than that of the base steel cooled at 1 °C/s (140.7 ± 13.9 nm). The κ-carbide exhibits a lower formation energy than cementite and a reduced interfacial energy between its (111) plane and the ferrite (112) plane, thereby enhancing the transformation driving force. Consequently, the lightweight steel develops a fully pearlitic microstructure with a micro-hardness of 456 ± 6 HV, which is 25 % higher than that of the base steel (364 ± 3 HV), and the yield strength is improved from 699 MPa to 983 MPa. These findings demonstrate that Mn–Al alloying effectively regulates carbide precipitation and interfacial energy, enabling the formation of nano-structured pearlite under slow cooling, and providing a practical and cost-effective pathway for producing high-performance lightweight steels.