Understanding the Properties of Oxide Scales on Hot Rolled Steel Strip

Abstract

Surface quality and downstream processability of hot rolled strip depend on tertiary scale thickness and composition at all process stages, from secondary descaling to the cold coil. TGA oxidation experiments provided the basis for quantitative modelling of scale growth during finishing hot rolling of low carbon steels (with or without Si or P). Finishing temperature (FT), gauge, and (particularly at the edges) coiling temperature (CT) are the main influence factors with respect to scale thickness. Based on HTXRD oxidation trials and microscopic analyses, trendlines are presented that roughly predict the final scale composition as a function of CT, for the coil centre and the edges. It appears that a CT of 400‐600°C, in combination with a coil centre position, is favourable for decomposed wustite, which is the preferred component for a good scale ad‐ and cohesion. Retention of wustite, the most preferred phase in the roll bite but very brittle at room temperature, is promoted by relatively high CTs (particularly with a coil quench) and by very low CTs. Hematite is formed at the edges in case of a high CT. This extremely hard phase is unwanted at any stage.