Abstract: Precipitation behavior of carbides in Nb-V HSLA steel during isothermal quenching process and its strengthening effect were studied using TEM and nanoindentation tests. The results indicated that interphase precipitations and dispersed precipitations could occur simultaneously in the specimen treated at 700℃ for 60min， while in other specimens， only random precipitations were observed. This phenomenon can be explained by mass balance criterion and ledge mechanism during diffusional phase transformation. It was also found that the average hardness of the specimen held at 600℃ for 20min was 3.87GPa and was 4.10GPa at 650℃ for 20min. Moreover， the TEM results illustrated that carbide precipitations in the specimen holding at 650℃ for 20min were dispersed more uniformly and the volume fraction of the precipitations was greater than that treated at 600℃ for 20min. The strengthening contributed by the precipitations was 110MPa in the specimen treated at 650℃ for 20min， evaluated by the Ashby-Orowan mechanism.

Key words: HSLA steel, interphase precipitation, dispersed precipitation, nanoindentation