Abstract: Effects of different processes and the function of Nb on the mechanical properties and microstructure of V140 steel for OCTG (oil country tubular goods) were studied through hot rolling and post-rolling heat treatment experiments. The results show that Nb(C, N) precipitated at high temperature can inhibit the recrystallization of austenite, make the finishing rolling process possible in the non-recrystallization zone and refine grains, thus improving the strength and toughness of the steel. Under three kinds of processes including post-rolling air cooling+offline quenching and tempering, post-rolling fast cooling+tempering, and post-rolling fast cooling+offline quenching and tempering, the microstructures of the steel are all uniform ones of tempered sorbite with excellent mechanical properties. Moreover, the post-rolling fast cooling+tempering process can meet the requirements of mechanical properties, shorten the production process and save cost, with the best tempering temperature and time being 660 ℃ and 30 min, respectively.

Key words: V140 steel for OCTG, niobium, heat treatment, microstructure, mechanical properties