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Iron oxide scale can be divided into primary iron oxide scale, secondary iron oxide scale, tertiary iron oxide scale, and red iron oxide scale. Primary iron oxide scale: Before hot rolling, steel is often heated and held at temperatures between 1100 and 1300°C. At this temperature, an oxidation reaction occurs on the steel surface upon contact with high-temperature furnace gases, resulting in the formation of primary scale that is 1–3 mm thick; this is caused by insufficient side compression during rough rolling and inadequate descaling. This primary scale is also known as primary iron oxide scale. Inside the scale, there are large voids; the primary iron oxide scale is a grayish-black scale layer that covers the surface of the steel plate in flaky form. The main component of the scale layer is magnetite (Fe3O4). Secondary scale: After the hot-rolled steel billet comes out of the heating furnace and the primary scale is removed using high-pressure water, the surface oxide scale falls off, followed by rough rolling. During the short rough rolling process, the surface of the billet comes into contact with water and air, resulting in the formation of secondary scale on its surface, also known as primary scale. The thickness of the secondary scale is thin due to horizontal rolling, resulting in low interfacial stress between the billet and the scale, hence poor peelability. If high-pressure water jetting cannot completely remove the secondary scale, and finishing rolling is carried out with scale remaining on the steel sheet surface, defects will appear on the product’s surface. Secondary iron oxide scale appears as red scales that are distinctly elongated and depressed in shape, distributed in bands along the rolling direction; the main components of these scales consist of particles such as magnetite (FeO) and hematite (Fe2O3). Three layers of scale: During the hot rolling and finish rolling processes, a surface layer of iron oxide forms each time the strip enters a rolling mill. After rolling, scale is formed again through final descaling or while passing between each rolling mill. Therefore, the surface condition of the steel strip under the action of the rolls will depend on the amount and properties of the iron oxide scale formed before entering each set of rolling mills. The scale at this point is called tertiary scale, as it forms after descaling. It is formed before entering the finish rolling mill. The defects in the three-layer iron oxide scale are visible to the naked eye: dark brown in color and boat-shaped. They are distributed relatively densely, in a fine, sandy pattern on the surface of the defective steel strip; they can be felt to the touch. After pickling, these defects leave pinhole-like small pits of varying depths on the surface of the steel strip, pits that are not visible on the surface of normally rolled steel strips. Red iron oxide scale: Red iron oxide scale occurs only in certain steel grades with high silicon content, primarily due to the strong interaction between the surface oxides and the base metal during the heating of the steel billet. It has no distinct depth and appears as irregular flakes. Red iron oxide scale comes in two types: one is unevenly distributed across the width of the plate, mainly found in the middle area and leaning towards the side where operations are carried out; there are distinct markings in red and blue areas. It is also uneven along the length of the steel plate, with some areas having a lighter degree of this scale. This type of red iron oxide scale is relatively thick; it can break off during straightening and can be blown away with high-pressure air. The remaining red color can be easily wiped off, so calling this red iron oxide scale \"red rust\" is a more appropriate term. The other type of red iron oxide scale is distributed more evenly across the width of the sheet; it is generally slightly thicker within 100 mm of the edges, and the outer part of the coil is thicker than the inner part. This red oxide scale is relatively thin and difficult to wipe off. The thicker the steel plate, the deeper the red color. This type of red iron oxide scale is also present in other steel grades, showing a certain degree of prevalence.