2018 Volume 58 Issue 12 Pages 2298-2307
The effect of Nb on the as-cast structure and compactness degree of ferritic stainless steel (FSS) dual stabilized by Ti and Nb are investigated. In this study, the as-cast structure of experimental ingots were almost composed of equiaxed grains and Ti-Mg-Al-oxide enveloped by Ti(C,N) or (Ti,Nb)x(N,C)y, namely complex nucleus, was found in the interior of FSS grain. When Nb was added to FSS, the as-cast grain size became smaller and the grain boundary precipitates (BP) became smaller and more dispersed. In addition, the composition of the outer layer of the complex nucleus and the BP both converted from Ti(C,N) to (Ti,Nb)x(N,C)y. Thermodynamics analysis showed that, the solidification region will be extended by Nb, which will be beneficial to the nucleation of complex nucleus. Complex nucleus is theoretically possible to form and exhibits good nucleation effect on δ-Fe as well as TiN according to the disregistry calculation. What’s more, (Ti,Nb)x(N,C)y particles in BP will be formed at the end of solidification and will prevent the grain merging by pinning effect.