The effect of Nb on the high temperature proof strength of 0.46 % Nb-added ferritic stainless steel was studied from the viewpoints, of solid solution strengthening, precipitation strengthening, and precipitation strengthening during tensile test and/or preceding hold time. The results obtained were as follows. (1) The increase in high temperature proof strength in Nb-added ferritic stainless steel is particularly remarkable at around 700°C in comparison with that in Ti-added steels. (2) After aging treatment for 2 h at 700°C, in addition to the approximately 0.14 % content of coarse Nb carbides and nitrides which had precipitated before this aging treatment, fine Fe2Nb Laves phase particles with a size of 0.02 μm precipitated out in an amount of approximately 0.3 %, and virtually no solid solution Nb existed in the steel. The 700°C proof strength in this condition showed a value near the 700°C proof strength before the aging treatment, that is, when approximately 0.3 % solid solution Nb was present. (3) When aging treatment is performed at 700°C for 2 h or more, virtually all the added Nb precipitates out. The coarsening of the Fe2Nb laves phase particles within the range of 0.02-0.05 μm during aging at 700°C greatly decreases the high temperature proof strength at 700°C. (4) Cold rolled steel sheets of Nb-added ferritic stainless steel are normally manufactured by annealing at a temperature of 900°C or higher in order to cause recrystallization. Accordingly, before measurement of the proof strength at 700°C, the steel contains a large quantity of solid solution Nb. The phenomenon of particularly high strength at 700°C is considered to be attributable to the precipitation of this solid solution Nb as a fine Fe2Nb Laves phase during the tensile test and/or preceding hold time.
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