“Quartus scale” of hot-rolled strip steels and its formation mechanism

The types and growth of various oxide scales formed during the different phases of the production of hot- rolled strip steel products are reviewed.Similarities and differences between the"tertiary scale"on the surface of carbon steels at high temperatures and the oxide scale on pure iron are compared.The micro-structural features of the"final oxide scale"on the surface of strip steels at room temperature as well as the relationship between these features and the position of the steel coil(plate)and the subsequent processes of recoiling,temper rolling and trimming,etc, are summarized.The actual oxide scales retained on the commercial hot-rolled strip steels at room temperature have been proposed to define as"quartus scale"for the first time.The micro-structural development and phase transformation of the initial"tertiary scale"during and after cooling and coiling are described.The reasons for the"tertiary scale"on carbon steels differing from the oxide scale formed on pure iron,and the major influencing factors in the formation of various types of"quartus scales"are analyzed from both thermodynamic and dynamic viewpoints.The development mechanism of"quartus scales"is discussed and the potential effects of the"quartus scale"state (thickness,constitution,structure and defects),on the rusting and pickling properties of commercial hot-rolled strip steel,as well as on the mechanical properties of oxide scales are analyzed.