Crucial Mechanism to the Eutectoid Transformation of Wüstite Scale on Low Carbon Steel

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Abstract
It is important to realize the transformation behavior of wüstite, because it greatly affects the final structure of the oxide layer and the surface quality of steel products. In the present study, the transformation behaviors of the wüstite layer are examined under nearly‐oxygen‐free conditions in order to simulate the cooling processes after the hot rolled strip is coiled. As the single phase wüstite is prepared at 950 °C, the 460 °C transformed oxide layer is composed of a mixture of iron and magnetite formed through eutectoid reactions. For the 750 °C‐fabricated wüstite, only magnetite is observed after transformation, without iron precipitates, and residual wüstite. It is speculated that the unusual transformation behavior of the low‐temperature‐made wüstite results from the pseudo‐structural intermediate phase transformation between wüstite and magnetite. This pseudo‐structure is a pre‐transformed wüstite and contains various concentrations of ferrous ion, which is determined by the fabrication conditions. During the hot strip mill process, wüstite is produced continuously from finish mill to laminar flow sections and culminated at 570 °C. Consequently, the final eutectoid transformation below 570 °C is dominantly controlled by the surface temperature ranging from 750 to 950 °C for low carbon steel.

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