Soft magnetic materials are critical for bulk/large cores in transformers and motors,and yet achieving uniform properties in industrial-scale devices remains challenging.This study develops a one-step,low heating rate annealing process to achieve uniform fine α-Fe nanocrystallization(grain size 20 nm) across the entire volume of Fe_(81.5)Si_(0.5)B_(4.5)P_(11)Cu_(0.5)C_2 bulk toroidal cores.The proposed process effectively eliminates microstructural heterogeneity induced by crystallization dynamics and temperature gradients.At the same time,it achieves a high magnetic flux density(B_(80)) of～169 emu/g,low coercivity of～5.4 A/m,and low core loss of W_(1.0/50)=0.12 W/kg and W_(1.5/50)=0.31 W/kg.The permeability remains constant at a value greater than 4.5×10~3 up to a frequency of 4×10~4 Hz.This research provides a scalable solution for the industrial-scale production of high-performance nanocrystalline magnetic cores,advancing the development of next-generation power electronic devices.