(W100)Beneficial effects of molten silicates on the oxidation of Type 304 stainless steel under slab reheating conditions

摘要

Type 304 (austenitic) stainless steel tends to form a subscale during reheating before hot rolling. The subscale is difficult to remove, is typically several hundred microns thick, and may cause surface defects during hot rolling; the subscale mainly consists of chromite (spinel) and metallic remnants (containing mainly iron and nickel). Mold flux residues, which are present on the slab before reheating, have been found to suppress formation of the subscale. The effect has been found not to be specific to mold flux, and occurs with other molten silicates too. The beneficial effect of the molten silicates on surface quality is the opposite of what would be expected from a simple hot corrosion mechanism. This paper gives an overview of the likely mechanisms of the beneficial effect; these are related to the low solubility of chromium oxide in molten silicates, and suppression of volatilization of chromium oxide. The low solubility of chromium oxide in molten silicate, compared with the high solubility of iron and nickel oxides, may cause iron and nickel oxides to be preferentially removed from the scale/metal interface, giving enrichment in chromium oxide which may promote formation of a protective chromium oxide layer. Chromium oxide is known to volatilize readily from the oxidized steel surface at reheating temperatures, with increased volatilization rates in atmospheres which contain water vapor. Molten silicates (and borates), with their low solubility for chromium oxide, may simply form a physical barrier to chromium loss by volatilization, and so promote formation of a more protective oxide.