The effect of microstructure and processing variables on the yield to ultimate tensile strength ratio in a Nb–Ti and a Nb–Ti–Mo line pipe steel

In a hot rolled Nb–Ti and a Nb–Ti–0.09%Mo micro-alloyed steel, the ratio of yield strength to tensile strength (YS/UTS) was found to be a function of the microstructure and cooling rate in those tests where no coiling simulation and no prior deformation. The coarse bainite or acicular ferrite, which was formed at high cooling rates, raised the YS/UTS ratio under these process conditions. With coiling simulation, the ratio was not sensitive to the cooling rate or the microstructure as coiling allows the recovery of dislocations, thereby decreasing the difference in dislocation density that had arisen between a low and a high cooling rate. Deformation with a 33% reduction below the nil-recrystallisation temperature (Tnr) prior to the transformation, led to a high YS/UTS ratio that ranged from 0.81 to 0.86. The prior deformation, therefore, had a stronger effect on the YS/UTS ratio than microstructural changes through cooling rate variations.