Search for: All records

As more emphasis is placed on thinner gauges and stronger steel grades for more fuel efficient, and crash-worthy vehicles, corrosion becomes a more important consideration. This demand has led to the development of advanced high strength steel (AHSS) grades such as dual-phase (DP) and transformation-induced plasticity (TRIP) aided steels. To protect these steels from corrosion, a hot-dip galvanizing (HDG) or galvannealing (GA) process is employed. However, alloying with Mn and Si can cause complex surface oxides during annealing, which may result in defects in the zinc coating1-8. In order to increase AHSS galvanizability, selective oxidation of alloying elements in DP and TRIP-aided steel needs to be understood. Depending on annealing atmosphere, the mechanism (internal or external) of oxidation can change, as well as the thermodynamic stability of the oxide. Wagner’s theory of oxidation enables prediction of the mechanism of oxidation for binary oxide systems9. The defects can be present after pickling and fluxing, causing wettability issues during the HDG process. Coating defects may also form or be further exacerbated by the galvannealing operation. For this work, oxidation mechanism predictions were performed using Wagner’s theory of oxidation. In the present study a 0.07C-1.9Mn-0.2Si-0.3Cr galvannealed dual-phase steel which exhibited streaking defects in the coating was analyzed. Light optical metallography (LOM) and scanning electron microscopy (SEM) were performed to characterize defects within the galvannealed coating. Static spectra and 3-D profiling using time of flight secondary ion mass spectrometry (TOF-SIMS) was employed. It was found that Mn rich Si containing oxides are present on the steel-coating interface. It does not appear that these oxides affect wettability, however they may have an influence on intermetallic growth during galvannealing. This, combined with an influence from the micro-grooves of the sink roll in the Zn pot, may lead to the streaking defects observed.