Precipitates in Nb and Nb-V microalloyed X80 pipeline steel.

ABSTRACT

Precipitates in two X80 pipeline steels were studied by transmission electron microscopy equipped with an energy filtering system. The steels are microalloyed with niobium and niobium-vanadium (Nb-V), respectively, and produced by continuous hot rolling. Besides the precipitates TiN and (Ti, Nb) (C, N), which were 10-100 nm in size, a large number of precipitates smaller than 10 nm distributed in the two steels have been observed. In the Nb-V microalloyed steel, only a few titanium nitrides covered by vanadium compounds on the surface have been observed. It is inferred that the vanadium exists mainly in the matrix as a solid solution element. The fact has been accepted that there was no contribution to the precipitation strengthening of the X80 steel by adding 0.04-0.06% vanadium under the present production process. By contrast, the toughness of the Nb-V steel is deteriorated. Therefore, a better toughness property of the Nb microalloyed X80 results from the optimum microalloying composition design and the suitable accelerating cooling after hot rolling.