Grain Boundary Segregation of Phosphorus and Boron in Extra-Low Carbon Steels

Abstract

Grain boundary segregation of P and B in extra-low carbon cold-rolled steel sheet containing from 0.01 and 0.15 mass% P and up to 38 mass ppm B after annealing at 1123 K was investigated by Auger electron spectroscopy (AES). AES measurements on the intergranular fracture surfaces of the specimen indicate that the Auger peak intensity ratio (PIR) of P increased and that of B decreased with increasing bulk concentration of P, and that PIR of P markedly decreased, while that of B increased with a B addition of up to 10 ppm. In the bulk concentration range of B of more than 10 ppm, however, these ratios hardly changed. The segragation behavior was nearly the same between specimens annealed for 20 s and 500 s, indicating that the segregation of P and B was in an equilibrium state. Calculations of the grain boundary coverages of P and B indicate that (1) the segregation free energy of B was about twice that of P, (2) the coverage of B was larger than that of P in the specimen with B concentration of more than 10 mass ppm, and (3) the total coverage of P and B was nearly constant in the specimen with B concentration of more than 10 mass ppm. It is concluded that the improvement in secondary workability of P-added steel sheet by also adding B is attributable to the segregation of less P and more B on the grain boundary, resulting in enhanced grain boundary cohesion.