The heterogeneous nucleus distribution of an industrial Ti and Nb-stabilized ferritic stainless steel (FSS) with high-equiaxed zone ratio was investigated in detail. The heterogeneous nucleus is the complex nucleus mainly composed of central Ti-containing oxide and external (Ti,Nb)(CN). A “V” type distribution of complex nucleus was revealed in the thickness direction, indicating that the nucleus density decreases as the slab solidifies. A “W” type distribution of equiaxed crystals was also observed in the thickness direction. A linear relationship between equiaxed crystal density and nucleus density was obtained in the columnar to equiaxed transition (CET) and quarter position of the slab, indicating that the density of equiaxed crystals is significantly influenced by that of the complex nucleus. Theoretical analysis and confocal laser scanning microscopy experiments were carried out to reveal the distribution mechanism of the complex nucleus. The results showed that the cooling rate has a significant influence on the nucleation undercooling of FSS, which indirectly affects the formation of the complex nucleus. Based on these findings, a numerical model was established to investigate the CET of FSS, and the prediction was in good agreement with the experimental results. According to the numerical model, even if the nucleus density decreases during solidification, it is still sufficient for stimulation of CET.
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This work was supported by the National Natural Science Foundation of China [Project Grant No. 51374020]
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Manuscript submitted May 16, 2019.
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Hou, Y., Cheng, G. An Investigation of Columnar to Equiaxed Transition and the Effect of Cooling Rate on Nucleus Density Distribution of an Industrial Ti and Nb-Stabilized Ferritic Stainless Steel. Metall Mater Trans A 50, 4686–4700 (2019). https://doi.org/10.1007/s11661-019-05407-6