Characteristics and Generating Mechanism of Large Precipitates in Nb-Ti-microalloyed H13 Tool Steel

The characteristics of precipitates larger than 1 mu m in Nb-Ti microalloyed H13 tool steel were studied. Four types of large phases exist in the as-cast ingot according to the compositional characteristics, that is, (Ti,Nb,V)(C,N), V-rich carbide, Mo-Cr-rich carbide and sulfide. (Ti,Nb,V)(C,N) could be further classified as the Ti-rich one, the Nb-rich one and the Ti/Nb transforming one. V-rich carbide normally has a strip shape, while Mo-Cr-rich carbide presents a eutectic appearance. The compositional characteristics of large precipitates have little relation with the sample positions. V-rich and Mo-Cr-rich carbide have the tendency to dissolve at 1 000 degrees C and 1 100 degrees C, while the (Ti,Nb,V)(C,N) is more stable and there are no apparent changes on the morphology after holding 6 h at 1 000 C and 3 h at 1 100 degrees C. These precipitates are generated during solidification. The precipitating process of (Ti,Nb,V)(C,N) could be well speculated through Thermo-Calc software. Ti-rich carbonitride precipitates first. Nb-rich carbonitride appears later, singly or on the Ti-rich carbonitride. V-rich carbide precipitates at the end of solidification. Eutectics consisting of iron matrix and Mo-Cr-rich carbide will be generated when the solutes in liquid steel reach the eutectic point.