Role of dew points and Fe pre-coats on the galvanizing and galvannealing of dual phase steel

Abstract

In this study, the galvanizing and galvannealing ability of dual phase steel sheet (DP 590), with and without Fe pre-coatings, is investigated with varying dew points using a hot dip process simulator (HDPS). Good quality defect free adherent galvanized coating on the substrate surface is obtained by appropriate selection of the inter-critical annealing atmosphere consists of N2-5%H2 gas mixture with +10 °C dew point without substrate pre-coating. However, the production of defect-free coating on DP 590 substrates is highly challenging when the substrates are without any pre-coating. It is observed that the selective oxidation of manganese on the substrate surface during intercritical annealing is primarily responsible for the appearance of bare spots on the galvanized surface in case of the coated specimen produced without pre-coating. A significant improvement in the reactive wetting behaviour between the liquid zinc alloy and pure Fe results in defect free adherent coatings (both galvanized and galvannealed) with electrodeposited Fe pre-coating on the substrate surface irrespective of dew points. Both the galvannealed specimens prepared with and without iron pre-coatings, exhibit partial passive-active metal characteristics in potentiodynamic polarization corrosion test. Iron pre-coating on the substrate surface is observed to improve the quality of galvannealed coatings by reducing defects and enhancing the formation of more compact and dense delta (δ) phase compared to galvannealed coatings produced without pre-coating under identical conditions. This leads to the improvement in corrosion resistance of galvannealed coating produced with pre-coating by lowering the corrosion potential and corrosion current density compared to that of galvannealed specimens produced without iron pre-coating.