Adhesive Scale Formation on Low Silicon Steel Surface; Characterization and Mechanism

Abstract

A systematic study has been conducted to characterize adhesive/non-adhesive scale on low silicon containing hot rolled steel surface and mechanism for formation of such scale. Different characterisation tools like XRD, SEM-EDS, GDOES, Scanning Kelvin probe and XPS were used to identify the chemical composition and characteristics of adhesive and non-adhesive scales. The characterization studies confirmed that the adhesive scale consists of CaFeSi₂O₆ compound along with magnetite and haematite with red in colour whereas non-adhesive scale consists of grey colour hematite-magnetite phase. Humidity in environment promotes oxidation of steel and depending on relative humidity different oxides are formed on the steel surface. Low humidity persists in India during the winter season which lead to lower oxidation potential and steel surface is oxidised accordingly. The wüstite phase is formed under lower oxygen potential at high temperature when slab is released from the mould. Wüstite is reactive to mould powder (tricalcium silicate) at high temperature and forms CaFeSi₂O₆ compound on the steel surface which is very adhesive in nature. Lots of cracks are inheritance attributes of non-adhesive scale whereas eventually no crack exists in adhesive scale. The compound present in the adhesive scale remains on the steel surface even after subsequent hot rolling and acid pickling operation as chloride ions of acid may not able to dissolve or penetrate through such stable compound. The quality of the finished product is deteriorated due to the presence of such adhesive scale as it leads to poor product performance during subsequent processing like cathodic paint deposition.