In modern industries, gears function as pivotal transmission elements whose operational performance is directly dependent on the microstructural characteristics of gear steels. While high-temperature carburizing (950–1050 °C) substantially improves process efficiency through accelerated carbon diffusion, it inevitably promotes austenite grain coarsening. This study investigates the effect of Nb microalloying on grain stability in SAE8620H gear steel during high-temperature carburizing. Experimental steels with varying Nb contents were prepared via vacuum induction suspension melting, followed by hot rolling, solution treatment, and pseudo-carburizing. Thermodynamic calculations, optical microscopy, transmission electron microscopy, and energy-dispersive spectroscopy were employed to analyze the mechanisms. Thermodynamic results revealed that higher Nb content retains more Nb(C, N) phases at elevated temperatures, effectively suppressing grain coarsening. Without preheating, increased Nb content refined grains but exhibited limited inhibition at high temperatures. Preheating (1330 °C × 10 min + water quenching) promoted uniform and fine Nb(C, N) precipitates, significantly enhancing grain refinement. When Nb content exceeded 0.053 wt.%, grain coarsening was fully inhibited under 1050 °C × 2 h carburizing. This study establishes the optimal Nb content range, elucidates the micro-mechanisms, and proposes a preheating process to improve high-temperature carburizing performance in gear steels.

High-temperature carburization (HTC, 950–1050 °C) has emerged as a pivotal low-carbon, energy-efficient manufacturing technology for gear steels, accelerating carbon diffusion for reducing processing cycles by over 60% while achieving significant energy savings and emission reductions. However, the inherent contradiction between HTC efficiency and microstructural stability, specifically austenite grain coarsening, severely degrades mechanical properties (e.g., strength, toughness, fatigue resistance) and limits widespread application. This review systematically synthesizes recent advances in austenite grain size regulation during HTC of gear steels, focusing on the core scientific framework of “grain coarsening mechanism—regulation strategy—performance enhancement”. It elaborates on thermodynamic and kinetic mechanisms of austenite grain growth, ripening behavior of microalloying precipitates (Nb(C,N), Ti(C,N), AlN, etc.), and their synergistic grain-refining effects. Comprehensive coverage of regulatory strategies (microalloying design, pretreatment technologies, process optimization, and integrated regulation) and characterization techniques is provided, along with a quantitative correlation between grain size, microstructure, and surface performance (wear resistance, corrosion resistance, and fatigue life). Numerical simulation and predictive models (empirical, theoretical, multiphysics coupling, machine learning-based) are critically analyzed, and current challenges (temperature-grain stability trade-off, multifactor synergy understanding, industrial scalability) and future research directions (advanced microalloying systems, intelligent process optimization, cross-scale modeling, green technology integration) are proposed. This review aims to provide theoretical guidance and technical support for optimizing the HTC performance of gear steels, catering to the demands of high-power-density transmission systems in automotive, aerospace, and heavy machinery industries.