Precipitation behaviour of Nb in X70 pipeline steel

Based on the composition of X70 pipeline steel produced in Shougang Group,the phase structure,the precipitation temperature of phases and the precipitation behaviour of Nb at different temperatures were analyzed by calculation with Thermo-Calc software.The effect rules of Nb and C contents on the precipitation behaviour of Nb were studied.The precipitation temperature of Nb in the steel was analyzed by thermal simulation and scanning electron microscopy(SEM).It is indicated that precipitation phases in the steel are the carbonitrides of Ti and Nb,alloying cementite,Ti₄C₂S₂,MnS,AlN,M₇C₃ and Mo carbides.The Nb precipitation phase mainly consists of the elements of Nb and C,and in the Nb precipitation phase,Ti and N are dissolved.With increasing Nb and C contents,the precipitation temperature of the Nb alloy phase rises and the precipitated Nb content at the same temperature increases.