An Investigation of Columnar to Equiaxed Transition and the Effect of Cooling Rate on Nucleus Density Distribution of an
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less steel (FSS) with rapid development for its outstanding properties and price advantages is a type of stainless steel that the matrix maintains ferrite phase from solidification to room temperature.[1–4] With the advancement of metallurgical technology, specialized equipment such as VOD,[5] SSVOD,[6,7] REDA,[8,9] and SSRF[10,11] have been gradually applied to the refining of FSS. As a result, the carbon and nitrogen content could be reduced significantly, which made the corrosion resistance of FSS gradually improved.[12–16] However, the reduction of carbon and nitrogen facilitates the development of columnar crystals in the as-solidified structure of FSS,[17–19] often leading to the appearance of ridging defect on the surface of the final product.[20–25] TiN was
YUYANG HOU and GUOGUANG CHENG are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted May 16, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
considered to be an effective substance for promoting heterogeneous nucleation of FSS.[26–29] It is of considerable significance to increase the nucleation efficiency of TiN as much as possible. Therefore, increasing the equiaxed zone ratio (EZR) of FSS by controlling the heterogeneous nucleation of the oxide-TiN complex nucleus has become a research hotspot.[8,22–24,30–35] Suito studied the effect of deoxidation on the EZR of FSS. The results showed that the multi-phase particles of the ingots with higher EZR were mainly composed of (Ti,Mg,Al)Ox-TiN.[22,23] Nippon Steel successfully improved the EZR of high-purity FSS with 30 mass ppm carbon content manufactured by REDA technology by controlling the formation of (Ti,Mg,Al)Ox-TiN complex nucleus, and effectively eliminated the ridging defect.[8] Recently, in the casting experiments of Hou, Ti2O3 was also found to be an effective substance to promote the formation of TiN, and the ingots with improved EZR were obtained with a large amount of Ti2O3-TiN complex nuclei.[35] Moreover, the bulk density of the complex nucleus also has a significant influence on EZR.[34,35] However, in the industrial production of FSS, the EZR had not always been stably improved even when Ti was added.[17,36,37] It is of great importance for the steady improvement of EZR to
reveal the density distribution of heterogeneous nuclei during solidification. In the present work, the density distribution and characteristic of heterogeneous complex nuclei were investigated in detail along the thickness direction of an industrial continuous casting slab with high EZR. The distribution regulation of complex nuclei varied with the solidification parameters was revealed through the combination of experimental statistics and solidification calculation. Furthermore, the effects of cooling rate on the nucleation undercooling of FSS and the distribution of complex nuclei were revealed by the application of confocal laser scanning microscopy (CLSM). Through this study, the forma
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