关键词: COST-FB2, 氮的合金化, 氮分压, Factsage 方法, 氮化硼(BN)

Abstract: The gas phase nitriding process of COST-FB2 steel was studied by conducting the gas phase nitriding experiment and thermodynamic calculation under different nitrogen partial pressure and nitriding time in a 30kg vacuum induction furnace. The optical emission spectrometer ARL 4460， gas analyzer of TC-500 and infrared analyzer HIR-944B were used to analyze the content in the middle part of ingot sample. The results showed that the calculated data is in a good agreement with the experimental one when the nitriding pressure is ≤20kPa. Since gas phase nitriding is a first-order reaction， the nitrogen content in the tested steel can reach the equilibrium value with the nitriding time of 15min. When the partial pressure of nitrogen is 1.5~6kPa，the mass fraction of nitrogen in COST-FB2 steel can be controlled between 0.015%~0.03%. In order to avoid the formation of porosity defects inside the ingot during casting， pressure casting should be preferred to use with a pressure of at least 20kPa. Moreover， the mass fraction of nitrogen in steel should be controlled between 0.015%~0.02% and the nitrogen partial pressure should be 1.5~3kPa during smelting so that the formation of large particle BN in the tested steel can be avoided.

Key words: COST-FB2, nitrogen alloying, partial pressure of N₂, Factsage method, boron nitride(BN)