Selective Oxidation of Low Alloyed Ferritic Steels
Résumé
Before hot-dip galvanizing, the steel sheets are annealed in an atmosphere of N2 and H2, containing only traces of water (about -30°C dew point). The main purposes of this heat treatment are to recrystallize the steel substrate after cold rolling and to reduce the iron oxides in order to improve the wettability by liquid zinc. At the same time, the less-noble alloying elements (Mn, Si, P, Cr, Al, B) of the steel preferentially oxidize and segregate to the surface. The steel surface is then partly or wholly covered by oxide particles, sometimes resulting in problems of wettability by liquid zinc. The present study focuses on the characterization of the oxide particles formed on the surface of a low alloyed ferritic steel. The steel samples are annealed by means of a laboratory furnace with a temperature profile relevant to galvanizing line practice. The parameters characteristic of the oxide particles (size, surface coverage, chemical composition, depth of oxidation) are then determined using several analysis techniques.