Abstract: The oxide and nitride in H13 die steel were studied by scanning electron microscope (SEM) and electron microprobe analysis (EMPA), showing that most oxide inclusions were spherical Al2O3 with a size of 5μm and a few MgO•Al2O3. Besides of oxide, some complex nitrides (Tix,V1-x)N were found, some of which existing cores of 2—3μm MgO•Al2O3 and Al2O3 in the center. The formation possibility of Al2O3, Ti3O5 and SiO2 was calculated based on thermodynamics. It was found that Al2O3 might only be formed in liquid steel when temperature was below 1 823K, while the later two oxides could not be formed. Three nitrides including TiN, VN and AlN could not be formed in liquid and mushy zone under equilibrium. Alloying elements especially Ti could segregate obviously during non-equilibrium solidification process. Theoretical analysis indicated that the binary nitrides of TiN and VN could be generated at critical solid fraction of 0.699 and 0.987 in the solidification front, respectively. Together with the experiment results that Ti and V content having the same trend in nitrides, it was illustrated that the complex nitrides (Tix,V1-x)N might be formed in a way of solid solutions. Experiment showed that the molar fraction x ranged from 0.5 to 0.6, indicating that the complex nitrides could be formed in mushy zone when the solid fraction reached 0.7. The influence of oxide as a precipitation core of nitride was taken into account in the view of forming condition and lattice mismatch. Further research will be done on the influence of oxide on nitride nucleation.

Key words: nitride, segregation, oxide core, critical forming solid fraction, H13 die steel