Display omitted •Increasing the melt-spinning speed to 50 m/s refines the grain size.•Changing the crystallization process is the mechanism of grain refinement.•The size of equiaxed grains in the annealed ribbons is reduced to 50–200 nm.•The intense exchange coupling effect contributes to the higher remanence.•The microstructure evolution models are proposed. Given the uneven grain size distribution in the ribbons melt-spun at 25 m/s, the strategy of improving the microstructure and magnetic properties of the ribbons by increasing the melt-spinning speed is discussed. The experimental results show that the grain size of Nd13.65Fe73.55B5.6Co6.6Ga0.6 ribbons annealed at 700 °C is reduced to 50–200 nm by melt-spinning at 50 m/s. The change in the crystallization process is the primary mechanism for grain refinement. At high quenching speed, the amorphous matrix is formed first in the as-spun ribbons, and then uniform crystallization can be achieved through annealing. Compared to the annealed ribbons melt-spun at 25 m/s, the Mr and M2T increased by ∼8 % due to the intense exchange coupling effect and the refined grains.