Scaling Behavior and Surface Quality of High-Strength Low-Alloy Steels with Higher Levels on Cu, Sn, and Ni Under Slab Reheating Conditions

Nowadays the production of steel from scrap in electric arc furnaces is the most common bridging technology to reduce CO₂ emissions. Depending on scrap quality, a non-negligible content of tramp elements such as Cu, Sn, or Ni is introduced into the steel. As their affinity to oxygen is lower than that of iron, they typically enrich at the steel/scale interface area and along grain boundaries during oxidation, which may result in quality problems. Oxidation processes are unavoidable in solid steel processing, and therefore, a deeper understanding of the occurring phenomena, such as intergranular oxidation and liquid metal infiltration of grain boundaries, is essential to continuously improve the product quality. In this study, oxidation experiments for slab reheating were performed by simultaneous thermal analysis under near-process conditions. For a clear statement on the role of tramp elements during oxidation, steel grades with and without tramp elements were investigated. The addition of the expected future contents of Cu and Sn does not affect external oxidation, but at the interface the presence of Cu and Sn leads to the formation of liquid Cu phases and infiltration of grain boundaries. The additional presence of Ni counteracts this formation, but due to its huge impact on iron activity it favors the formation of a rough steel/scale interface. In contrast with Ni, Cu and Sn hardly have any influence on iron activity. Numerical calculations based on a diffusion model and results of the well-known thermochemical software FactSage confirm these effects.