Effects of niobium and yttrium additions on carbide characteristics, microstructural evolution, and performance enhancement in D2 cold work die steel

To achieve an optimal balance between hardness and impact energy in D2 die steel, different contents of Nb and Y are added. The microstructure and mechanical properties of D2 steel in the as-cast, forged-annealed, and quenched-tempered conditions are systematically studied. The results show that the lattice mismatch between NbC and M7C3 eutectic carbide is only 0.75%, and NbC can reduce the volume fraction and size of M7C3 eutectic carbides. Y preferentially reacts with O and S to form Y2O2S and Y2S3, providing heterogeneous nucleation sites for NbC, thereby refining it. The refined NbC acts as a heterogeneous nucleation core for M7C3 carbides, indirectly improving its morphology and distribution. After forging and annealing, the carbides in 0.8Nb+Y steel exhibit the highest shape factor K. Nb suppresses the growth of secondary carbides by reducing the carbon diffusion coefficient. Y promotes secondary carbide spheroidization. As Nb content increases, the grain size and secondary carbide size in the steel decrease gradually after quenching and tempering. Y further optimizes the morphology and distribution of carbides, enhancing the microstructural uniformity. Finally, the 0.8Nb+Y steel achieves excellent hardness (62.2 HRC) and impact toughness (38.3 J).