Abstract
Galvannealing rate becomes fast by heat treatment in N2 of the hot rolled sheet with scale. During the heat treatment, Fe inner oxides which contain the easily oxidizable metals are formed just under the scale by internal oxygen diffusion, and this inner oxides remain at the surface layer after pickling and cold rolling. The cause that of galvannealing rate becomes fast is shown in the following.
(1) Amount of surface segregated oxide which is formed by reduction annealing extremely decrease, because the oxidation-reduction occurs inside the sheet, in other words, Fe-containing oxides produced during heat treatment changes to oxides of easily oxidizable metals during the annealing.
(2) The concentration of easily oxidizable metals at surface layer decreases by the formation of this inner oxide.
