Effect of BN surface segregation on coatability in hot-dip galvanizing of B-added steel

Abstract

Boron (B) is frequently used as an additive to improve the hardenability of advanced high strength steel. Based on thermodynamical calculations, it has been reported that B in steel reacts with atmospheric N₂ during annealing at a low oxygen potential (low dew point) to form boron nitride (BN). In this study, the effect of BN formation on the steel surface on coatability during hot-dip galvanizing was investigated experimentally. B-free specimens and specimens containing 15 or 30 ppm B were annealed at various temperatures and dew points and then hot-dip galvanized. Annealed specimens were also prepared and analyzed by GD-OES, XPS, SEM-EDX and TEM-EELS to investigate oxide and nitride formation on the steel surface during annealing. As results, coatability was deteriorated as the B content in the steel and the annealing temperature increased and as the dew point decreased. These trends were not correlated with the amount of oxides, but rather, with the amount of BN formation, indicating that BN formation deteriorates coatability. Surface and cross-sectional analyses revealed that BN formed around oxides, covering the steel surface. It was suggested that this leads to deterioration of coatability because most of the steel surface is covered by BN and oxides, which are both known to have low wettability with molten Zn.