Mechanical Adhesion and Pickling Behaviour of Thermal Oxide Scales on Hot-Rolled Low Carbon Steel Strips Produced by Different Finishing Temperatures

Abstract

Hot-rolled low carbon steel strips were produced using two different finishing temperatures at 910 and 820 °C in an industrial hot-rolling line. Mechanical adhesion of scale on the steel substrate at 40 mm from the edge was investigated by tensile test. It was found that the strain initiating the first spallation of scale produced at higher finishing temperature was lower. Spallation ratio which is a spalled area of scale divided by the total area of scale examined under an optical microscope was steeper when the scale was produced at higher finishing temperature. The lower values of strain initiating the first spallation and the higher values of spallation ratio of scale formed both at higher finishing temperature were due to higher thickness of that scale. Pickling behaviour of the hot-rolled steels was investigated by immersing the studied steels in a 10%v/v HCl solution at 80 °C. X-ray diffraction (XRD) peak of hematite relative to that of iron decreased with pickling time and approached zero during pickling periods from 3 to 10 s, while magnetite-and-iron ratio gradually decreased and tended to be zero at longer pickling time. This might indicate the existence of hematite as the outermost layer of scale and subscale containing magnetite as the inner part.