Design of cooling route for carbide-free bainitic rail steels and resultant microstructures and properties

In the present work, two new low-alloy, carbide-free bainitic rail steels (steel A: 0.29C + 2.49MnCr & steel B: 0.19C + 3.18MnCr wt.%) were designed to match the controlled cooling production process that combines forced air-cooling and nature air-cooling. The effect of alloy design and continuous cooling rate on the bainite production processing window and resultant microstructure was investigated, followed by the establishment of controlled cooling routes. The microstructure, transformation kinetics, and crystallographic characteristics of the specimens under the process were studied, and cuboid specimens were employed to validate the processing feasibility and investigate the correlation between related microstructures and mechanical properties. The results showed that both steels exhibited ultrahigh strength levels, with a tensile strength approaching 1400 MPa and elongation exceeding 16 %. However, the impact toughness of Steel A deteriorated due to the formation of granular bainite. Due to the homogeneous lath bainite morphology and refined bainitic ferrite substructure, Steel B exhibits excellent strength and toughness, essentially meeting the Chinese technical specifications for the procurement of 43 kg/m ∼75 kg/m rail.