Display omitted •The crystallization and magnetic properties of SmCo4B-based ribbons are sensitive to melt-spinning speeds.•The critical cooling rate for forming an amorphous matrix in SmCo4B-based ribbons is 35–40 m/s.•A low melt-spinning speed tends to obtain uniform microstructure, while a high speed improves the magnetization.•The high content of amorphous phase in the 40 m/s ribbons forms three sizes of platelet-shaped grains during annealing.•The Mr and Ms increased by 38.5 % and 28.5 % by melt-spinning at 40 m/s. The SmCo4B-based ribbons were melt-spun at 35 m/s and 40 m/s, respectively, and annealed at 800 °C for 30 min. The dependence of the annealed ribbons’ microstructure and magnetic properties on melt-spinning speed sensitivity was studied. The results show that the as-spun ribbons comprise an amorphous matrix and Sm-Co-B nanograins. For SmCo4B-based ribbons, the critical cooling rate to obtain an amorphous matrix is 35–40 m/s. Compared to the annealed ribbons melt-spun at 40 m/s, the formation of the uniform platelet-shaped grains in the annealed ribbons prepared at 35 m/s helps to improve coercivity but decreases the remanence and saturation magnetization. These results provide a reference for a comprehensive understanding of the relationship between the microstructure and magnetic properties of SmCo4B-based magnets.