We employed Gleeble thermal-simulation experiments combined with color metallography, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to systematically investigate the microstructural evolution and variation of retained austenite in U20Mn bainitic rail steel under different step-controlled cooling-quenching-tempering processes. Mechanical property tests, including tensile, impact, and reduction-of-area measurements, were performed to determine the optimal heat treatment parameters. The results indicate that austenitizing at 900 °C for 15 min, followed by step-controlled cooling at rates of 3 - 0.5–0.3 °C/s, air cooling to room temperature, and tempering at 300 °C for 4 h provides the best overall performance. Under these conditions, the steel exhibits a microstructure consisting of approximately 70.09% fine lath bainite and 29.91% martensite-film-like retained austenite, with an average bainitic lath width of about 147 nm. The optimized microstructure yields a tensile strength of 1 400 MPa, an elongation exceeding 20%, a reduction of area of about 55%, and a significantly enhanced impact toughness, with the impact energy increasing from about 60 J before tempering to above 85 J after tempering. By precisely controlling phase-transformation kinetics and refining the microstructure, the process achieves a balanced improvement in strength and toughness, ensuring better service performance of bainitic rail steel.
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