Surface Modification of Various Fe–si and Fe–mn Alloys by Oxidation/Reduction Treatments

Abstract

Oxidation of segregated elements (Si, Mn, Al, Cr) during annealing of steel sheets in a 5%H₂ –N₂ atmosphere at 820°C and dew point -30°C makes the steel surface unsuitable for subsequent galvanising, as the oxides formed are poorly wetted by the Zn. One way to modify these surfaces is by selective oxidation achieved by changing the partial pressure of O₂ in the annealing atmosphere, and another way is by preoxidation of the sheets before annealing. During preoxidation, the formation of Fe oxides is primarily expected. These oxides can be reduced in the annealing cycle, leaving metallic Fe on the surface, which is well wetted in a Zn bath. Various Fe–Si (Fe with 1%, 1.5%, 3% and 5% Si) and Fe–Mn (Fe with 0.3%, 1%, 3% and 5%Mn) model alloys were chosen and investigated after preoxidation in air at 700°C and subsequent annealing. X-ray photoelectron spectroscopy and field emission scanning electron microscopy measurements were used to characterise the surfaces.