Ultra-purified ferritic stainless steels are the dominate materials in the field of automotive exhaust system.Alloying with niobium(Nb)element can effectively improve the high-temperature performance of ferritic stainless steels.Focusing on the research of high-Nb and ultra-purified ferritic stainless steel 441,the morphology,composi-tion and structure of the banded structure in the cold-rolled annealed sheet were characterized using scanning electron microscopy(SEM),transmission electron microscopy(TEM),and electron probe microanalysis(EPMA).The en-tire process analysis was then performed on the continuous casting slab,hot-rolled plate,and hot-rolled annealed plate,combined with thermodynamic software Thermo-Calc for simulation,to elucidate the segregation behavior,formation mechanism and influencing factors of niobium elements.The results show that niobium segregation occurs during the solidification stage of the slab,and the equiaxed crystal ratio is the main influencing factor.Niobium seg-regation is inherited from the slab and forms a banded structure in the hot-rolled annealed plate and cold-rolled an-nealed sheet.The banded structure is caused by the aggregation and precipitation of Nb element leading to the for-mation of Laves phases and pinning at grain boundaries.The results provide a theoretical basis for the optimal con-trol of the process for high-Nb and ultra-purified ferritic stainless steel.
high-Nb and ultra-purified ferritic stainless steelbanded structuresegregationLaves phaseequiaxed crystal ratio
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