Influence of microalloying precipitates on Bauschinger effect during UOE forming of line pipe steels

Large diameter steel line pipes are generally produced by cold deforming hot rolled or thermomechanically controlled rolled (TMCR) plate by the three-stage 'UOE' process. Pipe strength has been found to increase or decrease relative to the plate, depending on the steel grade and plate processing history. The strength increase is due to work hardening whereas any decrease in strength arises from the Bauschinger effect due to the reverse cold deformation of the UOE process. The steel chemistry, through the presence of strengthening microalloy precipitates, and prior processing, through the size and distribution of microalloy precipitates and the presence of retained work hardening, affect the magnitude of the Bauschinger effect. In the present work, the microstructures of two microalloyed (Nb and Nb+V) steel plates have been examined in terms of their grain size distributions, microalloy precipitate type, size and volume fraction to determine the major contributions to the plate strength levels. It has been found that both steels are primarily strengthened by a combination of grain size refinement and precipitate strengthening. Compression-tension testing has been carried out to determine the Bauschinger parameters for the two steels. The Bauschinger stress parameter, defined for the yield point (0˙1% of reverse deformation), was higher for the Nb+V steel (greater microalloying content) than the Nb steel. Transmission electron microscopy (TEM) investigations are being used to determine dislocation-dislocation and precipitate-dislocation interactions and how the spatial inhomogeneity in precipitate distribution affects the Bauschinger parameters.