Kinetics and Microstructure Transformations of Oxide Scale on Hot Rolled Steel Strip for Hot-Dip Galvanizing during Reduction Annealing in 30% H2-N2 Atmosphere

The understanding of oxide scale reduction mechanisms after microstructure transformation under different preheating conditions is incomplete. Herein, the reduction kinetics and microstructure transformations of hot rolled steel strip oxide scale during reduction (30% H-2-N-2) annealing are studied in detail using thermogravimetric and microstructure analyses, and mathematical modeling based on the gas-solid reaction. The results show that the rate-determining steps of the reduction reaction are nucleation and growth of new phase between 500 and 600 degrees C, and gas-solid interface reactions between 700 and 800 degrees C. Microstructure transformation of the oxide scale occurs during the preheating process: at 500 degrees C, the eutectoid iron preferentially dissolves, and short-range diffusion of Fe ions into adjacent Fe3O4 increases their content in the metal oxide; at 600 degrees C, nucleation and growth of Fe1 - yO occur, and some Fe3O4 precipitates and white alpha-Fe are found in the Fe1 - yO layer, whereas the majority of the Fe3O4/iron eutectoid structure is retained; between 700 and 800 degrees C, the oxide scale forms outer and inner layers of Fe3O4 and Fe1 - yO, respectively. After preheating and reduction, the reduced products are porous iron over 500-600 degrees C, while porous and dense iron occur at 700 degrees C; dense iron forms as whiskers or granules between 700 and 800 degrees C. The surface quality of hot rolled steel strip after reduction annealing is subsequently improved as surface cracks are infilled by the newly formed dense iron.