High-performance steels containing niobium have widespread use in high-end manufacturing sectors such as aerospace, automotive, energy, and construction engineering. Due to its capacity as a strong carbide-forming element, Nb favors the formation of NbC. These carbides, dispersed within the matrix, significantly contribute to precipitation strengthening, precipitation hardening, and grain refinement. In addition, Nb serves as a positive segregation element. When steel solidifies from its molten state, Nb segregates in the liquid phase and forms primary NbC carbides. These carbides significantly affect the mechanical properties of steel. Herein, to investigate the effect of Nb content on primary carbides in medium-carbon Nb-alloyed steel, the microstructure including the morphology, size, and distribution of niobium and carbon was characterized through SEM and TEM. To further understand the role of lattice vibrations and electrons at different temperatures, the first principle calculations with quasi-harmonic Debye model were combined to study the evolution of thermodynamic parameters. Primary carbides form because of solute segregation at the solid-liquid interface. For a more detailed investigation, a solute microsegregation model coupled with solidification phase transition was developed. This model was adopted to quantitatively analyze the effects of solidification phase transition and Nb content on solute microsegregation. The experiments yielded the following results: with increasing Nb content, the morphology of primary carbides shifted from spherical to polyhedral geometry. Furthermore, a distinct zonal distribution of primary carbides was observed. The laws of thermodynamics indicate that the increase in free energy change due to electrons at different temperatures is compensated by the decrease in free energy change arising from lattice vibrations, indicating the key role of lattice vibrations in maintaining the stability of NbC. The Gibbs free energy change at different temperatures was negative, indicating the thermostatic stability of NbC. Furthermore, the absence of imaginary frequency in the phonon spectrum indicates the dynamic stability of NbC. From a microsegregation viewpoint, the mass fraction of solute carbon decreases while that of the solute Nb increases at the solidification front during the phase transition from L + δ to L + γ. As the Nb content increases, the solid fraction of the solidification phase transition increases. Increased Nb content promotes the precipitation of primary carbides at low solid fractions.