摘要
In order to improve the performances of high-strength steel sheets, considerable amounts of alloying elements, such as P, Mn, and Si, are often added into the steels. However, the presence of alloying elements often deteriorates the surface properties of steels, and further affects the appearance and properties of the galvannealed (GA) coatings. In this study, the effect of alloying elements on the microstructure of GA coatings has been investigated by OM of chemically etched cross sections, SEM/EDS, XRD and galvanostatic electrochemical stripping technique. The GA coatings of all investigated specimen mainly consist of the randomly columnar δ phase, as revealed by SEM surface morphology and XRD. Continuous phase appears at the substrate/coating interface in interstitial-free (IF) steels and high-strength (HS) steel, while only Γ1 phase is observed in interstitial-free rephosphorized (IFP) steel. The Γ1 phase appearing in a light-blue color on the etched cross section displays a columnar structure observed using SEM. Surface defects have been identified on HS steel, which is due to the poor wettabiltiy with molten Zn caused by surface oxides of alloying elements in the HS steel. The galvanostatic electrochemical stripping curve oscillates as the attack approaches the steel substrate, especially for the galvannealed IFP and HS steels. As a result, this oscillation is likely associated with the surface segregation of P and the presence of surface oxides. Copyright © 2010 MS&T'10®.