SummaryA technique was developed to study in situ the early stages of the reaction between iron and air. Using a high-temperature microscope, we observed at temperatures between 1000• C and 1050• C and within the first 30 s of reaction, the formation of iron-oxide layers on the surface of low-carbon steel. We observed the nucleation and growth of a first layer of iron oxide and the consecutive formation in sequence, of higher iron oxides sweeping over the surface of the former oxide. The grain boundaries of the steel substrate remain visible for quite some time following exposure to an oxidizing atmosphere indicating that diffusion through steel grain boundaries may have a determining influence on the formation of oxides. These findings emphasize the importance of conducting further studies to better understand the kinetics and mechanisms by which iron-oxide layers form in the early stages of oxidation.

Nickel and silicon are attractive alloying elements for high-strength low-alloyed (HSLA) steel production. However, it is well known that the presence of Ni and Si in the steel can impair the surface quality, making it unsuitable for certain markets. The combined effects of Ni+Si on the oxide scale formation are still relatively unknown. Literature is dealing mostly with steels containing combinations of Ni and Si, with either traces of nickel (0.1%) or very high (8-16%) nickel levels. At Tata Steel we explored the effect of an optimum composition selected to achieve steel properties (0.15%Si and 1%Ni) on the formation of oxide in the reheating furnace and its descalability. Pilot hydraulic descaling trials were performed on blocks of three steel grades, applying reheating and hydraulic descaling in conditions closely resembling the industrial practice.The oxidation experiments show that synergistic effects occurring during the oxidation of alloys containing Ni (1.1%) are already obvious at relatively low levels of Si of 0.05%. This effect is even enhanced at higher Si levels of 0.15% and consists of increasing the adherence of oxide scale to the steel substrate by forming an entangled layer with oxidic pegs.In order to maximize descalability of (Ni,Si)-alloyed steels slabs, the metal/scale entanglement has to be minimised. In this respect, it was found that the slab surface temperature is the most important parameter. A gentle, smooth reheating process is required in which slab surface temperatures exceeding 1 300°C should be avoided.

The influence of the carbon concentration of directly reduced iron (DRI) powders on the compressibility and fracture strength of hot briquetted iron (HBI) has been studied. Industrially produced DRI, pure iron powder and Fe-C alloy powders (synthetic DRI) were used in the study. It was found that the mechanism of compaction could be attributed to pure yielding. The pressure required to attain a given density increased proportionally with the carbon content. The morphology and phases present in DRI powder had a significant influence on the compressibility. The fracture strength of the compacts increased with increasing carbon content of the DRI powder. These observations are discussed with reference to the current understanding of the mechanisms of compaction and fracture of compacted particulate materials.