Abstract
By the heat treatment of a ultra low carbon hot-rolled steel containing 0.31mass%Si-1.02mass%Mn, not only readily oxidizable elements such as Si and Mn but also Fe were internally oxidized at the steel surface region. The scale of the steel acted as the source of oxygen for internal oxidation. The composition of the grain-boundary oxides differed from those in the grain, presumably due to the difference in oxygen potential ; oxides at the grain-boundary consisted mainly of Fe-Si-Mn-O, while oxides in the grain consisted mainly of Si-Mn-O. The internal oxidation layer remained after scale pickling and cold rolling. Fe containing grain-boundary oxides, arranged in layers by the cold rolling process, were considered to act as the oxidant of Si and Mn, which diffuse toward the steel surface in recrystallization annealing. By not only the low Si and Mn activity due to the depletion layer formed during the heat treatment, but also the redox reaction between the grain-boundary oxides and solute Si and Mn, the amounts of Si and Mn selective surface oxidation were remarkably decreased. The resultant suppression of the selective surface oxidation brought about improvement in the wettability by molten Zn of the annealed sheet steel surface.
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