Studies on softening kinetics of low manganese steel microalloyed with niobium for high-strength sour service ERW pipe

The aim of the paper is to develop a quantitative understanding of the recrystallization behavior of low manganese - 0.10 percent niobium line pipe steel for sour service application. Experimental results on recrystallization behavior of low manganese - 0.10 percent niobium steels from WUMSI and stress relaxation tests are analyzed using a physically based model for time evolution of strain induced precipitation of NbC and its interaction with recovery and recrystallization. The effect of accelerated kinetics of strain induced precipitation of NbC in low manganese steel is shown to retard the recovery, which, in turn, increases the driving force for recrystallization. As a consequence, the incubation time for recrystallization nucleation is decreased, which enhances the refinement of austenite grain size in low manganese steel prior to pancaking. This is shown to be beneficial for increasing the density and uniform dispersion of high angle boundaries upon transformation of pancaked austenite grain. This is the target microstructure to obtain high-strength and high Charpy toughness at low temperatures in hot rolled low manganese 0.10 percent niobium skelp for sour service application. Details of several full scale industrial trials are presented.