Effect of microalloying additions on steel plate to pipe property variations during UOE linepipe processing

Abstract

Composition variations have been used to generate three steels, developing plate strengths in the range 350-530 MPa for the same rolling schedules. The tensile properties of the plate have been determined and compared with those of first, pipe formed by the UOE process and second, specimens from the same pipe after flattening. It has been shown that the difference between plate and pipe strength levels for the specific pipe geometry investigated is related to the steel grade. In particular, changes have been related to the Bauschinger stress parameter, optical microstructure, fine particle distribution (from TEM replicas) and stored energy (measured using differential scanning calorimetry) differences between the steels. The trends in property variation result from the balance of work hardening behaviour and Bauschinger effect, and have been related to the presence of fine microalloying element precipitates in higher strength grades, increasing the Bauschinger effect and forming a more highly ordered dislocation structure during the various deformation stages.