Effect of Boron in AHSS on Coatability in Hot-Dip Galvanizing
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Generally, Si and Mn are added to advanced high strength steel (AHSS) sheets. It is well known that these elements can form oxides during annealing and cause surface bare spots in hot-dip galvanizing due to their poor wettability with molten Zn. Boron (B) is also often added to AHSS to improve hardenability. How- ever, few reports have investigated the effect of B on coatability, especially under low dew point condi- tions. To investigate the effect of B, B-free specimens and specimens containing 15 or 30 ppm B were prepared and annealed at low dew points from -60 to -45 ºC. These specimens were dipped in a molten zinc. Their coatability was then evaluated based on the appearance. Results showed that the area of bare spots expanded as the B content increased and the dew point decreased. To understand the mechanism, annealed specimens without galvanizing were also prepared and analysed. XPS analysis showed that boron nitride (BN) was formed on the surface of B-added specimens. BN was considered to deteriorate coat- ability in hot-dip galvanizing because the estimated amount of BN increased as the B content increased and the dew point decreased, which showed the same trend as coatability.
Abstract
Generally, Si and Mn are added to advanced high strength steel (AHSS) sheets. It is well known that these elements can form oxides during annealing and cause surface bare spots in hot-dip galvanizing due to their poor wettability with molten Zn. Boron (B) is also often added to AHSS to improve hardenability. How- ever, few reports have investigated the effect of B on coatability, especially under low dew point condi- tions. To investigate the effect of B, B-free specimens and specimens containing 15 or 30 ppm B were prepared and annealed at low dew points from -60 to -45 ºC. These specimens were dipped in a molten zinc. Their coatability was then evaluated based on the appearance. Results showed that the area of bare spots expanded as the B content increased and the dew point decreased. To understand the mechanism, annealed specimens without galvanizing were also prepared and analysed. XPS analysis showed that boron nitride (BN) was formed on the surface of B-added specimens. BN was considered to deteriorate coat- ability in hot-dip galvanizing because the estimated amount of BN increased as the B content increased and the dew point decreased, which showed the same trend as coatability.
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