Recently, the surface quality of hot rolled steel strip has been considered as an important qualification to determine the product quality with the general increasing requirement of commercial steel strip. The fine control of oxide scale is essential to maintain the high surface quality of products during the hot rolled process.1) The formation temperature of oxide scale is above 560°C, and it is comprised of a thin outer hematite (Fe2O3) layer, an intermediate magnetite (Fe3O4) layer, and a thick inner wüstite (FeO) layer. After hot rolled process, as shown in Fig. 1, the steel strips are coiled and stacked, and slowly cooled down to room temperature. Phase Transformation Behavior of Oxide Scale on Plain Carbon Steel Containing 0.4 wt.% Cr during Continuous Cooling