Continuous Cooling Transformation Behavior and Microstructural Hardness of LZ50 Axle Steel

To elucidate the microstructural evolution of LZ50 axle steel during continuous cooling and its implications for service performance, this study systematically investigates the microstructural evolution and hardness variation characteristics of LZ50 axle steel under diverse continuous cooling conditions. Employing JMatPro 7.0 thermodynamic software, a Gleeble‐3800 thermal simulator, optical microscopy (OM), and Vickers hardness testing, the investigation integrates dilatometric and metallographic hardness methodologies, thereby elucidating the regulatory mechanism of cooling rate on microstructure and performance. The results reveal that, at 0.1–5 °C/s, the microstructure is predominantly composed of pearlite and ferrite; as the cooling rate increases, ferrite content diminishes, pearlite refines, and hardness rises gradually. In the 10–15 °C/s range, a pearlite‐to‐bainite transformation occurs, accompanied by a rapid increase in hardness. At 20–40 °C/s, the microstructure is dominated by bainite and martensite, with hardness stabilizing at a peak value of approximately 690 HV. Bainitic transformation exhibits an inverted S‐shaped kinetic profile, adhering to a nucleation‐and‐growth mechanism, with the transformation rate peaking at a 50% transformed fraction and shifting to lower temperatures as the cooling rate escalates. These findings provide a theoretical basis for optimizing heat treatment, tailoring properties, and applying CCT curves for LZ50 axle steel.