The oxidation and copper enrichment behaviors of several copper-containing mild steels under isothermal and step-isothermal conditions at 980-1220 • C in ambient air, and the effects of nickel additions, are examined. The oxidation kinetics for all steels does not obey the parabolic law because of the formation of blisters in the scale. At 980 • C, decreased parabolic oxidation kinetics is observed, whereas at 1120 and 1220 • C, the oxidation kinetics exhibits an undulating pattern. High nickel content and high oxidation temperature are two prerequisites for the occlusion mechanism to operate during steel oxidation. For the low nickel steel, a planar scale-steel interface develops at all temperatures, and a copper phase is always seen to form and spread along the scale-steel interface. For the high nickel steels, a planar scale-steel interface develops at 980 and 1120 • C, but at 1220 • C, the scalesteel interface becomes rugged and the copper-rich phase is occluded into the scale. Introduction of a 980 • C and/or 1220 • C oxidation step significantly affects the copper enrichment behaviors of all steels normally exhibited at 1120 • C.

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In this study, the oxidation kinetics of low-carbon, low-silicon steel in flowing air at 850-1,180°C within 30 or 60 s were examined. The parabolic kinetics were established from the very early stage at 850°and 1,000°C, whereas the oxidation kinetics at 1,100-1,180°C appeared to obey a linear law initially and a more-parabolic one at a later stage. When the oxidation kinetics followed the linear law, ''rough''-scale with an undulating, saw-teeth like microstructure developed, whereas when the parabolic law was followed, smooth scale developed. It appeared that a critical scale thickness existed, at which the scale-growth mechanism changed from linear to parabolic. This thickness was less than 7 lm at 850°C, about 10 lm at 1,000°C, about 50 lm at 1,100°C and in the range of 60-80 lm at 1,180°C under the conditions examined. Blister formation at 900°C prevented clear observation of the linear-to-parabolic transition.

The copper enrichment behaviours of several copper-containing steels under conditions similar to those in the thin-slab casting processes are examined. Formation of a molten copper phase at the scale-steel interface can be avoided when the substrate phase is occluded in the scale during steel oxidation. Significant enrichment of nickel in the surface layer of the substrate is a necessary condition for the occlusion of the substrate phase into the scale before the precipitation of a molten copper phase. The critical nickel content above which the occlusion mechanism would be operative is between 0.02 % and 0.07 %. The critical temperature above which the occlusion mechanism becomes operative in the high nickel steels differs when the isothermal oxidation step, following continuous cooling in ambient air, is conducted in different atmospheres. When conducted in ambient air, this temperature is between 1 150°C and 1 230°C, whereas when conducted in moist air or in simulated furnace atmospheres, it is lowered to between 1 100°C and 1 150°C. Preferential oxide growth along the grain boundaries of the substrate is another important occlusion mechanism, however, the deep oxide penetrations in the substrate may lead to descaling difficulty.

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