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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 Fe 2 Nb Laves phase particles with a size of 0.02 mm 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 Fe 2 Nb laves phase particles within the range of 0.02-0.05 mm 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 Fe 2 Nb Laves phase during the tensile test and/or preceding hold time.

The effect of Si was investigated on the precipitation behavior and the amount of Nb in solid solution at temperature ranging from 1073 to 1173 K in high purity 17Cr-0.5Nb steels. Adding Si promoted the precipitation of the Nb Laves phase and then decreased the solubility of Nb in steels. The Nb Laves phase which was composed of Fe, Cr and Nb could be expressed as (Fe, Cr) 2 Nb in a 0.002 mass% Si steel. On the other hand, in a 0.5 mass% Si steel the Nb Laves phase which was composed of Fe, Si, Cr and Nb could be expressed as (Fe, Si, Cr) 2 Nb. Based on calculations from the experimental results assuming the Laves phase is Fe 2 Nb, the standard free energy change of the Fe 2 Nb precipitation reaction was about À61 k J/mol for a 0.002 mass% Si steel.

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