Spheroidizing annealing is a critical heat treatment process for high-carbon steels to balance hardness and machinability. This study develops a rapid spheroidizing annealing process by employing low-temperature pretreatment followed by subcritical heating. The key is to utilize carbide precipitates from non-equilibrium phases (e.g., martensite/lower bainite) as nucleation sites, thereby accelerating spheroidization. At an optimized pretreatment temperature of 400 °C, the process achieves a homogeneous spheroidized microstructure with a hardness of 206.7 HV, comparable to that obtained via conventional prolonged annealing. This method significantly reduces processing time and energy consumption.