Equiaxed crystal cast nickel-based superalloys are widely used in the low-temperature hot-end components ofaeroengines. K492M superalloys, owing to their wide solidification range and high pouring temperature, are prone to coarsegrains and uneven microstructures. The effects of Cr-Fe-Nb/Co-Fe-Nb composite grain refiners on the microstructure andmechanical properties of K492M cast superalloys were studied, revealing the fine-grain strengthening mechanism andproviding a basis for controlling the microstructure of this alloy for engineering applications. The results show that thegrain refiners can significantly refine the grains of the K492M alloy, reducing the average grain size from 3 128.25 μm to297.59 μm and increasing the proportion of equiaxed grains in the cross-section from 60.28% to 94.06%. Moreover, thesize, content, and eutectic content of the carbides decrease. The reason is that the addition of the refiner increases thenumber of nucleation sites, reduces the grain size, and limits the growth space for eutectic structures and carbides,thereby hindering their growth. In addition, the second dendritic arm space remains at the same level with the addition ofgrain refiners. Further studies indicate that the addition of the refiner can increase the yield strength of the K492M alloy under 760 ℃ high-temperature tensile conditions from 690.38 MPa to 978.76 MPa, the tensile strength from 941.25 MPa to1 147.95 MPa, and the elongation from 4.88% to 12.50%; under 760 ℃/662 MPa creep conditions, the life increases from35.88 h to 110.99 h. The analysis shows that the fracture surfaces of samples without the refiner exhibit brittle fracturecharacteristics, with significant stress concentration at the grain boundaries, whereas samples with the refiner show mixedductile-brittle fracture characteristics, and the dislocation density in the grain boundary regions is reduced by an order ofmagnitude compared to that of the original samples. Therefore, after grain refinement, the grain boundary area per unitvolume of the K492M alloy increases, enhancing the ability of the superalloy to coordinate deformation and therebypromoting a synergistic improvement in both strength and toughness.