Effect of Oxide Composition on the Orientation Relationship and Disregistry in Complex Nucleus of Ti and Nb Stabilized F
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I.
INTRODUCTION
OVER the past 2 decades, there has been a dramatic increase in the demand for Ti and Nb dual-stabilized ferritic stainless steel (FSS) because of its excellent resistance to intergranular corrosion and price advantage.[1–5] However, ridging is a common defect, which has a considerable impact on the surface quality and formability, hampering the widespread application of FSS.[5–12] Many studies have suggested that ridging is strongly related to the original columnar structure of as-cast ingots or slabs.[13–17] Therefore, metallurgical and material researchers consistently try to increase the equiaxed zone ratio (EZR) as much as possible.[18–22] Recent years have witnessed a growing academic interest in improving the EZR through the sufficient oxide-TiN complex nucleus, which is an application of oxide metallurgy technology in stainless steel.[23–29] The study of
YUYANG HOU, GUOGUANG CHENG, and HUIJING CHENG are with State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China. Contact e-mail: [email protected] Manuscript submitted May 16, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fujimura concluded that the composition of oxides in the complex nucleus had a significant influence on the EZR, according to the wetting angle mechanism.[30] The research of Park revealed that the grains of as-cast 11Cr FSS were obviously refined when MgAl2O4–TiN appeared as an inoculant for FSS nucleation.[25] TiN encapsulating MgAl2O4 was also found to be able to promote the hetero-epitaxial nucleation and columnar to equiaxed transition (CET) of 17Cr FSS during gas tungsten arc (GTA) welding.[27] Previous studies by Hou revealed that the EZR was significantly improved when Ti2O3–TiN formed after Ti deoxidization in 18Cr FSS ingots.[26] Although the heterogeneous nucleation of FSS promoted by TiN was verified by a rapid cooling solidification,[31–34] the verification of the nucleation of TiN on these oxides is difficult but of great significance for further application. Hultman,[35,36] Park[37] and Kim[27] concluded that MgAl2O4 could effectively promote the formation of TiN by lattice disregistry calculation after TEM observation of the atomic arrangement at the interface between MgAl2O4 and TiN. However, there was little evidence showing that Ti2O3 could promote the formation of TiN according to existing disregistry calculations.[38]
Although Bramfitt[39] proposed the lattice disregistry theory to evaluate the nucleation efficiency of a substrate, the determination of the low-index plane for the substrate and nucleated phase is critical, which is closely related to the crystal orientation. However, these crystal orientation relationships between TiN and Ti2O3 or d-Fe are seldom confirmed by experiments when calculating the lattice disregistry in FSS. In addition, when Ti and Nb were alloyed to FSS, the composition of the second-phase particles in matrix changed, and new phases such as NbC and NbN appeared, which might also influence the nucleation.[5,40] There
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