Characterization of hot rolled scales-root cause identification and remedial action

Abstract

Hot rolled steel strips are produced by rolling of steel slabs of 210mm thickness at 1150–1200°C through roughing and finishing mills of a hot strip mill to a thickness of 1.6–6mm. During hot rolling, some adherent oxides, known as scales, is formed on the strip surface in the reheating furnace prior to hot rolling due to high temperature oxidation. But these scales should be removed away by the usual hydraulic descaling method during hot rolling operation. Primary scale that is not removed by de-scaler in hot strip mill and subsequently gets rolled into the surface is known as rolled in scale (RIS).This RIS creates serious surface quality problem in the finished product. So scales are removed by acid treatment, known as Pickling operation. Here HR steel strip is passed through the acid bath. But the efficiency of the picking process depends on the micro-structural characteristics of scale, e.g. presence of different phases in scale, interfacial morphology. Thus the microstructural characteristics have to be understood for efficient removal process.There are several methods of determining the nature of the oxide scales present on the hot rolled sheets. Amongst the conventional techniques, Scanning Electron Microscopy, EDS analysis, X-ray characterization and electron X-ray Diffraction techniques (XRD) are presently being used for scale characterization.In current study different types of rolled in scales have been characterized depending on the following parameters: appearance, prone grades, photographs of defect, metallographic characterization, present information, knowledge and experience at hot strip mill. Finally based on the analysis the root cause and remedial action of different types of scale have been identified.