Abstract: Wear failure is the most common failure mode for D2 cold working die steels. The wear rate of the steel decreases by reducing the number of the inclusions and by improving the morphology and distribution of them. The granular magnesium was used to generate magnesium vapor at high temperatures， and the vapor was carried into the molten steel by high purity argon gas. The effects of magnesium on nonmetallic inclusions modification were discussed. The results showed that the size of nonmetallic inclusions in the steel were obviously finer with the magnesium vapor flowing into molten steel than that without it. The composition of inclusions changed from pure alumina inclusions or compound of Al2O3 and the oxide of element Cr，Mo and V to aluminamagnesia complex inclusions. The observation of ascast microstructure showed that small inclusions remained in the steel were benefit for grain refining in the process of solidification and phase transitions.

Key words: cold working die steels, magnesium vapor, inclusion modification, grain refining