eTheses Repository

# Bauschinger effect in Nb and V microalloyed line pipe steels

Chemical composition of structural steels with a ferrite-pearlite microstructure has been developing towards decreasing carbon content, to increase weldability, with increased microalloying element content, to provide grain refinement, solid solution and precipitation strengthening. During the UOE forming of large diameter (more than 400 mm) welded pipes the strength drop from plate to pipe, as a result of reverse deformation (the Bauschinger effect), depends on steel grade, namely microalloying element content, and processing. In this project the microstructure of two Nb- and V-microalloyed steels has been studied with optical, scanning and transmission electron microscopy. The dislocation density and (Ti,Nb,V,Cu)-rich particle diameter, volume fraction and number density were measured for as-rolled and annealed (30 min. at 400 $$^0$$C and 550 $$^0$$C) steels. The Bauschinger effect was measured during compression-tension testing for the same steel conditions. The yield stress drop during reverse deformation has been found to increase with an increase in forward pre-strain, dislocation density and particle number density within the effective particle diameter range of 12-50 nm. On the basis of dislocation-particle interaction analysis, a quantitative model of work-hardening behaviour dependence on particle number density and dislocation density has been derived for the reverse deformation of studied steel grades.