Effect of Nb Addition on Dendrite Growth and Equiaxed Grain Ratio of Fe-20 Pct Cr High-Purity Ferritic Stainless Steel
- PDF / 3,509,632 Bytes
- 13 Pages / 593.972 x 792 pts Page_size
- 51 Downloads / 154 Views
HIGH-PURITY ferritic stainless steel (FSS) is attracting increasing attention, owing to its excellence intergranular corrosion resistance achieved by limiting the content of C + N to below 150 ppm.[1–3] The most important properties of FSS are formability and surface quality, which are badly destroyed by ridging defects. Ridging is the most common defect that results when FSS is subjected to large deformation or rolling processes.[4–7] Numerous studies have shown that an increased equiaxed zone ratio (EZR) can significantly reduce ridging defects and improve the formability and surface quality of FSS.[8–14] It is generally considered that an increased EZR requires extended constitutional undercooling and a sufficient number of nucleation sites.[15] Such undercooling is mainly due to microsegregation at dendritic tips.[16,17] A great deal of research has been conducted to improve the EZR of the as-cast structure of FSS. Most studies focused on heterogeneous nucleation by effective particles. Of these, TiN and oxide-TiN were found to be
YUYANG HOU, SHIJIAN LI, and GUOGUANG CHENG are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China. Contact e-mail: [email protected] Manuscript Submitted April 13, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
effective in promoting heterogeneous nucleation of FSS. Bramfitt[18] found that TiN showed excellent lattice-matching properties with d-Fe, significantly promoting the formation of d-Fe. Itoh[9] pointed out that the EZR of an FSS slab could be increased by addition of Ti, which was independent of the superheating of molten steel. Shi,[19] Fujimura,[20] and Park[21] investigated the formation of equiaxed grain structure during solidification of 17 pct Cr, 16 pct Cr, and 11 pct Cr FSS, respectively. All these studies considered that an increased EZR can be interpreted based on the effectiveness of Ti2O3-TiN and MgAl2O4-TiN complex inclusions as inoculants for the nucleation of d-Fe. In recent years, Nb-bearing high-purity FSS with excellent high-temperature properties has rapidly developed[22–31]; however, little research has been conducted on EZR improvement in Nb-bearing Fe-20 pct Cr high-purity FSS without Ti addition. The nucleation effect of TiN or oxide-TiN is weakened in the absence of Ti.[14] Although a few studies have shown that Nb promotes grain refinement, and attributed this to the nucleation of Nb(C,N), the formation of Nb(C,N) will not take place until the solidification is complete. Shan et al.[32] investigated grain structures and precipitates in the solidification microstructure of 17 wt pct Cr FSS with different Ti and Nb contents. They concluded that the EZR increased as a result of an expanded solidification interval and dendrite arm remelting; however, expansion of the solidification interval was not obvious (within 10 K), and remelting dendrites were not
observed in the samples. Scheller et al.[33] observed the development of microstructure in a simulated strip-casting process of Nb-
Data Loading...
