Abstract: The grain structures and the precipitates in the solidification microstructure of the ultra pure 17 wt% Cr ferritic stainless steels with different Ti and/or Nb micro-alloying were investigated both experimentally and theoretically. It was found by the grain structure observation that the addition of Ti or Nb to the steel
reduced the grain size (D) and elongation factor (E), and improved the equiaxed grain proportion (P) and globularity factor (ζ). Among the four steels studied, the minimum grain size and maximum equiaxed grain proportion were obtained by jointly adding both Ti and Nb to the steel. The SEM observation indicated that several kinds of precipitations, such as TiN, MC (rich in Nb), Laves phase (Fe₂Nb) and so on, formed in the corresponding steels. In addition, the results calculated using the Thermo-Calc software illustrated that TiN precipitates in the liquid at proper Ti and N contents. Meanwhile, the solidification interval (?T) was enlarged by the addition of Ti or Nb, and the effectiveness of Nb was stronger than Ti. Based on the experimental and calculation results, the mechanisms of grain refinement and increment in equiaxed grain proportion were
discussed.

Key words: Ultra pure ferritic stainless steel, Micro-alloying, Solidification structure, Titanium, Niobium