Optimization Of Alloy Compositions And Rolling Procedure Of Non-qenched&Tempered OCTG Of N80-1Class With Nb Addition

In this paper, in order to optimize alloy compositions and rolling procedure ofnon-quenched&tempered OCTG of N80-1class containing Nb, the hot compressionbehaviors, microstrucutures and mechanical properties of experimental steels withdifferent levels of carbon content and niobium content were investigated using theGleeble-3500thermo-mechanical simulator, observations under optical, scanning electronand transmission electron microscopes, as well as tensile and impact testing. The effect ofNb on dynamic/static recrystallization, and the effect of carbon and Nb contents onmicrostructure and mechanical properties were estimated.The results indicate that with the increasing content of Nb, both the flow stress andthe activation energy for hot deformation increase. The hot-deformed microstructures at1075-1100oC are refined. Moreover, the critical amount of deformation and intervaltime for static recrystallization with volume fraction higher than75%are also increased.In addition, the amount of bainite increases with the increasing content of Nb and coolingrate at the final stage after hot-stretch in reduction.It is also shown that with the hot-rolling temperature raised from1050oC to1100oC or the hot-stretching temperature from850oC to910oC, the amount of pearlite andbainite increase at the dispense of ferrite in the final microstructure.The tensile and impact properties of0.24-0.02Nb steel can meet the technicalrequirements specified by the API5CT standard due the formation of high fraction offine-grained ferrite, pearlite and the homogeneously distributed fine precipitates. Thecorresponding procedure is the hot-rolling at1080oC followed by the hot-stretching at850-890oC. A poor impact property may occur due to the formation of high-fractionbainite in other experimental steel.