Influence of soaking duration on the selective oxidation and galvanizability of a high‐strength dual phase steel

Abstract

High‐strength dual phase steels readily exhibit bad galvanizability and coating defects because of selective oxides formed on steel surface during the annealing process prior to galvanizing. To investigate selective oxidation of alloying elements and their effects on glavanizability, a high‐strength dual phase steel was annealed with soaking duration for 45, 90, and 120 s, respectively, and then galvanized using a hot‐dip simulator. Field‐emission scanning electron microscopy characterization revealed that when dual phase steel was soaked for 45 s, selective oxides mainly precipitated along grain boundaries, while only a few of the oxides formed on grains. With soaking duration increased, oxides were so dense that nearly all steel surface was covered, leaving little bare area of the steel surface. Further XPS analysis showed that selective oxides mainly consisted of MnO and Cr2O3. In addition, the chemical nature of oxides did not change at all although soaking duration prolonged. Scanning Auger microprobe depth profiles presented that Mn had a much higher tendency to segregate than Cr and Mo. Oxygen penetration depth to subsurface was promoted as soaking duration increased. The formation of interfacial inhibition layer was founded to be greatly influenced by the density and size of surface oxides. The widely spaced small oxides had virtually no adverse effect on wettability because of aluminothermic reduction of oxides by the bath dissolved Al. As the oxides became dense and considerably big, the grains of the inhibition layer in some certain zones became coarse and the galvanizability tended to deteriorate. Copyright © 2011 John Wiley & Sons, Ltd.