Effect of NbMo Addition on the Precipitation Behaviour of V Microalloyed Steel during Intercritical Annealing

This paper reports on the precipitation behaviour and microstructural evolution in ~20% cold rolled NbMoV and V single microalloyed low carbon steels during intercritical annealing. The microstructure and precipitation behaviour were studied by optical and scanning/transmission electron microscopy and microanalysis, X-ray diffraction technique and Vickers hardness testing. After intercritical annealing, the resulting microstructure in both steels comprised acicular/bainitic ferrite matrix with an uneven proportion of allotriomorphic ferrite and retained austenite and martensite. The average density of microalloying precipitates increased in both steels during intercritical annealing. NbV, NbMoV and V carbides were observed in the NbMoV steel, while V-carbide extensively appeared in the V steel. The results also showed a much greater precipitation strengthening in the NbMoV steel after intercritical annealing compared to the V steel. The overall findings suggested that NbMo addition could retard the growth/coarsening of microalloy precipitates with a size of lower than 15nm during the intercritical annealing.