Microstructure Transformation Behavior of Hot‐Rolled Steel Oxide Scale during the Preheating Process

The microstructure transformation of oxide scale in hot‐rolled steel strips during the preheating process is studied by thermal simulation at temperatures ranging from 530 to 800 °C. The results show that the original structure of the oxide scale comprises an outer Fe 3 O 4 layer, an inner Fe 3 O 4 /Fe eutectoid, and some precipitated Fe 3 O 4 . The Fe 3 O 4 /Fe eutectoid structure is first transformed into wüstite. After the eutectoid structure is completely transformed into wüstite, Fe 3 O 4 precipitates are transformed into wüstite with aging. Then, white Fe is dissolved and diffused into wüstite to give a homogenous wüstite layer. The thickness of outer Fe 3 O 4 layer decreases as the temperature increases, indicating that the outer Fe 3 O 4 also transforms into wüstite. The heating rate and temperature are important parameters influencing the phase transformation of the oxide scale. More wüstite is formed at a slower heating rate and higher temperature; a larger number of pores are found inside the wüstite layer with increasing temperature; pores caused by transformation of the oxide scale are created by the density difference between wüstite and Fe 3 O 4 .