Characterization of Y-bearing particles in CNS-I-ODS steel fabricated by vacuum induction melting & casting technique

Vacuum induction melting (VIM) & casting is not a common but a promising approach to produce ODS steel with large volume and high throughput. Meanwhile, the Y-bearing particles in the cast ODS steel presented various chemical compositions and wide size distribution. Thus it is important and necessary to clarify the Y-bearing particle characteristics in the cast ODS steel. To determine the distribution characteristics, chemical compositions, crystal structure and size distribution of the Y-bearing particles, CNS-I-ODS was fabricated by adding 1.0 wt.% metallic yttrium into the molten China Nuclear Steel (CNS) during VIM. The results indicated that some banding structures comprised of yttrium oxides and Y-bearing intermetallic compounds formed along rolling direction. XRD, SEM, TEM and EDS demonstrated that yttrium oxides with the size of 550–900 nm were consisted of Y2O3 and Y(O, C) phases while Y-bearing intermetallic compounds with the size of 26–500 nm were consisted of Fe-Y and Y-W-Ta phases. Besides, Y addition also facilitated the formation of δ-ferrite, refinement of prior austenite grains and precipitation of carbides. Furthermore, it is acceptable that the CNS-I-ODS exhibited similar elongation of 22.7–23.5%, slightly lower strength of 622 MPa and higher ductile-brittle transition temperature of −42 °C compared to the Y-free steel, although some large yttrium oxides were distributed in the former matrix. Keywords: Ferrite/martensitic steel, ODS, Yttrium, Vacuum induction melting, Casting, Microstructure characteristics