Influence of Ni and Cu on Oxide-Scale Structure of Steel

Abstract

In steel products, high surface quality is required in addition to characteristics corresponding to intended use. For improving surface quality of steel products, it is important to control structure of oxide scales. In hot rolling process, heterogeneous structure of oxide scales, such as uneven scale/steel interface, causes surface defects. For steels containing Ni, it has been reported that structure of oxide scales at the scale/steel interface is characteristic and complicated. Consequently, in this report, Ni and Cu, which are nobler than Fe, are focused on, and the effects of these elements on structure of oxide scales of steels are investigated. Fe-10mass%Ni and Fe-10mass%Cu alloys were oxidized at 1000°C or 1200°C in an atmosphere of 10%O2+20%H2O+70%N2 for 1800s. In all cases, uneven scale/steel interfaces are observed (see Fig.1). In case of Fe-Ni alloy, Ni-enriched metal phase of complicated form is observed in oxide scales. Concentration of Ni in metal phase increases as it approaches the surface. On the other hand, in case of Fe-Cu alloy, precipitation of Cu phase is observed. Cu precipitates as particles in oxide scale at 1000°C, while it precipitates as a layer at the scale/steel interface under oxide scale at 1200°C. With regard to the uneven scale/steel interface, it should be caused by the property that both Ni and Cu are nobler than Fe. The mechanism can be explained as follows: first, selective oxidation of Fe proceeds, and Ni or Cu enriches at the scale/steel interface. Then, internal oxides of Fe precipitate and Ni or Cu enriches around them. Finally, with proceeding of such heterogeneous oxidation, internal oxides of Fe and those locally Ni- or Cu-enriched area are simultaneously occluded into the scale/steel interface, and therefore uneven scale/steel interface forms. With regard to the different structure of oxide scales at the scale/steel interface in Fe-Ni and Fe-Cu alloys, that is, enrichment of Ni and precipitation of Cu, it should be caused by the difference of the solid solubility limit of Ni and Cu to austenite. According to the phase diagram of Fe-Ni alloy, Ni dissolves in austenite completely at both 1000°C and 1200°C. On the other hand, according to the phase diagram of Fe-Cu alloy, Cu dissolves in austenite partly at both 1000°C and 1200°C. Because the solid solubility limit of Cu to austenite is just about 10% at 1200°C, Cu which exceeds its solid solubility limit with proceeding of oxidation of Fe should precipitate as liquid phase at the scale/steel interface. Although both Ni and Cu are nobler than Fe, different structure of oxide scales forms at the scale/steel interface in Fe-Ni and Fe-Cu alloys corresponding to the solid solubility limit to austenite. Figure 1