Generating Mechanism of Large Heterogeneous Carbonitrides with Multiple Layers in H13+Nb Bar

The characteristics and generating mechanism of heterogeneous carbonitride with micron size in H13+Nb bar are studied. Three types of precipitates are observed according to the number of layers, that is, two-layer ones, three-layer ones, and four-layer ones. Two-layer precipitate contains the oxide core and the outer Ti-V-rich carbonitride, while three-layer precipitate has one more Nb-rich carbonitride at the outside. Four-layer precipitate has a V-rich carbonitride at the outmost layer besides the inner three types of phases similar to that in the three-layer precipitate. The size of these large precipitates can be even up to tens of microns. The oxide cores are mainly stoichiometric spinel MgAl2O4 and generated in the liquid steel before solidification. The compositional characteristics and precipitating sequence of carbonitride can be well explained through calculation by Scheil-Gulliver model in Thermo-Calc. Ti-V-rich phase precipitates first during solidification, after which Nb-rich phase and V-rich phase precipitates successively. Some dissolution occurs between the different layers during the following temperature reducing and heating treatment process.