Research On Precipitation Of Nbc On Microstructure And Hardness Of High Nb Microalloyed Steels

Recently, high-Nb X80pipeline steels have been applied in pipeline projects, andmost researches focus on making use of Nb effectively and increase its contribution to thestrength. In this paper, four high-Nb microalloyed steels were taken as research objects. Aquantitative method was designed to study the precipitation behavior of NbC in austeniteand ferrite. Besides, the effects of precipitation of NbC on the microstructure and hardnesswere investigated. In order to further understand the Nb in the steel reinforcement anfrealize the effective utilization of Nb.The hot deformation experiment were carried out on a Gleeble-3500thermo-mechanical simulator to research the strain-induced precipitation of NbC inaustenite and ferrite. A method was designed to determine the content of soluble andundissolved Nb in steel, which was analyzed the effect of the content of Nb and C on theprecipitation. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation is ued fordescribing the kinetics of NbC precipitation. Results showed that increasing Nb contentand C content could effectively promote the NbC precipitation, in which the effect of Nbwas more significant. In ferrite the NbC precipitates nucleated preferentially atdislocations, while in austenite the NbC precipitates nucleated at dislocation and/orrandomly, whcih made the JMAK index, n, slightly greater than the NbC precipitation inferrite.Metallographic microscope, scanning electron microscope (SEM), electronback-scattered diffraction (EBSD) and microhardness test were employed to investigatethe effects of the NbC precipitation in austenite and ferrite on microstructure and strength.The experimental results showed that with the increase of the NbC precipitation inaustenite, the amount of polygonal ferrite and quasi-polygonal ferrite incresed, while theamount of granular bainite decreased, which resulted in the decrease of hardness. With theincrease of the NbC precipitation in ferrite, its effects on microstuctrue was mild and thehardness decreased slightly after increasing to its highest level. The NbC precipitated inthe austenite could prevent the grain growth, but the effect of refining grain was notobvious, even induced grain refinement strengthening, it was hard to compensate for theloss of the phase transformation strengthening. In contranst, the fine NbC precipitated in the ferrite could increase hardness remarkabley because of strong precipitationstrengthening. Therefore, it should make the NbC precipitate in the ferrite so that makeeffective use of Nb in steel.