Retardation of Austenite Recrystallization by the Strain Induced Segregation of Boron

Abstract

A physical model is proposed to account for the recrystallization behaviour of hot worked austenite in ultra low carbon bainitic steels containing 0.055% Nb and 30 ppm of B. Previous work had shown that: (i) Nb and B can act synergistically to retard the process; and (ii) the magnitude of the retardation is not constant, but depends on the deformation conditions. Carbon extraction replicas were prepared and particle tracking autoradiography was employed to provide experimental evidence regarding the state of solution of the B during deformation or cooling and subsequent holding. It was found that B acts as a solute (for test temperatures higher than 950°C) and segregates at the recrystallizing grain boundaries. The results are consistent with the hypothesis that B decreases the grain boundary diffusivity (D^GB) of the Nb atoms close to the grain boundaries. According to the solute drag theory, such a change increases the drag force acting on the boundary; the theory also predicts that the order of effectiveness of the substitutional alloying elements commonly found in austenite in retarding recrystallization decreases in the order: Nb > Ti > Mo > V > Mn > Cr > Ni.