What characterizes hot-rolled steel in terms of its surface condition?

Hot-rolled steel is characterized by the presence of a hard black scale on its surface. This scale forms during the hot rolling process due to the oxidation of the steel when it is subjected to high temperatures. As the steel is heated and passed through rollers to achieve the desired shape and size, the high temperatures facilitate the oxidation of its surface, resulting in this distinctive scale.

This characteristic hard black scale not only acts as a protective layer against further corrosion during handling and temporary storage but also indicates that the steel has been processed at high temperatures, which affects its mechanical properties compared to cold-rolled steel. In contrast, cold-rolled steel is processed at room temperature and does not exhibit this surface condition. Similarly, alloy steel and tubing refer to different categories or forms of steel, and they do not specifically describe the surface condition associated with hot-rolled steel.