Abstract: By solution treated at 1250 ℃ and salt bath isothermally treated at 700 ℃, the effect of rare earth on isothermal precipitation behavior of NbC in Nb-microalloyed steels with adding La of 0, 0.0006% and 0.0048%, respectively, in the ferrite region for different time was studied. The results show that the addition of rare earth La into the steel can promote the dissolving of niobium at 1250 ℃ and reduce the number of undissolved second phases. When isothermally treated at 700 ℃, the promotion effect of rare earth is relatively weak at the beginning of precipitation, and gradually becomes significant with the increase of isothermal time. Rare earth La makes the NbC precipitation kinetic curve shift to the left. The addition of 0.0048% rare earth reduces the precipitation start time from 199 s to 125 s, and the end time from 35 183 s to 29 128 s. When isothermally treated at 700 ℃ for 10^4.25 s, the size of the second phase NbC precipitated in the ferrite area of the steel containing rare earth is smaller compared with that in the steel without rare earth. The size of the precipitated NbC in the steel without La is mainly in the range of 10-20 nm, accounting for 42.8%, while the size of the precipitated NbC in the steel containing 0.0006%La and 0.0048%La is mainly in the range of 1-10 nm, accounting for 53.6% and 64.3%, respectively, which indicates that the addition of rare earth La can refine the size of NbC significantly.

Key words: La, microalloyed steel, NbC, precipitation kinetics