PRECIPITATION STRENGTHENING PRODUCED BY THE FORMATION IN FERRITE OF Nb CARBIDES

Abstract

A Nb microalloyed steel has been thermomechanically processed at laboratory through the use of plane strain
compression sequences followed by simulated coiling. Tensile samples have been machined from the obtained
specimens in order to investigate the effect of different variables: recrystallisation or accumulated strain before
transformation, holding in austenite and coiling temperature on the final mechanical behaviour. Transmission
electron microscopy observation of the precipitates has been carried out after coiling at different temperatures.
It has been shown that when Nb remains in solution in austenite after hot deformation, it can precipitate in
ferrite, leading to an important strengthening effect which is directly related to the concentration of Nb in
solution before transformation and coiling temperature.