Effect of BN Surface Segregation on Coatability in Hot-dip Galvanizing of B-added Steel

Abstract

Boron (B) is frequently used as additives to improve the hardenability of advanced high strength steel. It has been reported that B in steel reacts with atmospheric N₂ during annealing at low oxygen potential (low dew point) to form boron nitride (BN) by the thermodynamical calculation. In this study, the effect of BN formation on the steel surface on the coatability during hot-dip galvanizing was investigated, experimentally. B-free specimens and specimens containing 15 or 30 ppm B were annealed at various temperature and dew point, and then hot-dip galvanized. The annealed specimens were also prepared and analyzed with GD-OES, XPS, SEM-EDX and TEM-EELS to investigate the oxide and nitride formation on the steel surface during annealing. As results, coatability deteriorated as the amount of B in steel and the annealing temperature increase, and as the dew point decrease. These trends were not correlated with the amount of oxide but the amount of BN formation, suggesting that BN formation deteriorated the coatability. The surface and cross-sectional analysis revealed that BN formed around the oxide to cover the steel surface. This would lead the deterioration of the coatability because most of the steel surface was covered with BN as well as oxide, which are known to have low wettability with molten Zn.