Hot rolling is a milling process whereby steel is rolled at very high temperatures (usually above 17000F) which is above the steel’s recrystallization temperature. The manufacture of hot rolled profiles is fast because it does not involve the reheating of steel and, therefore, makes it cheaper than the cold rolled profiles.
Hot rolled profiles can be produced using either the conventional method or the endless rolling method. The conventional method involves first heating slabs of a thickness of up to 250mm in a furnace and then passing them through a roughing mill where they are reduced to a thickness of between 30 to 60mm. They are further reduced to a smaller thickness of between 0.8mm to 25.4mm by passing through a finishing mill. When the head and the tail ends of the profiles are finished-rolled, they both run out to the cooler without any tension between the finishing mill and the coiler. Hence both ends of the profiles have a tendency of passing through the roller table unstably thus the size, profile and properties of the head and tail profiles are different from those at the middle due to the instability of the finished rolling. Therefore, the production of hot rolled profiles with a thickness of less than 0.8mm is difficult using this method. To improve the productivity and yield, the endless rolling process of hot rolling is used.
The process consists of three reheating furnaces, a sizing press that reduces the width of the slab, a coil box, sheet bar welder, three roughing mills, a 7 stand finishing mill, a strip shear and two down coilers. This method entails a continuous supply of profiles, and this is achieved by binding the tail end of the prevenient sheet to the head end of the succeeding sheet at the linking device. The next step is the cooling of the metal sheet after which the sheet is divided by the high-speed strip shear before the down coilers. It is important to stop the tail end of the preceding sheet immediately and guide the head of the succeeding sheet going at high speed into the next coiler during the dividing and coiling process. This hot rolling process is efficient as it improves the profile, microstructure and dimension of the whole length of the hot rolled profiles and can also produce those of a thickness under 0.8mm.
Products of hot rolling include concrete reinforcing bars used in the construction of buildings and bridges, rails used in the construction of railway lines, wire rods, structurals and sheets, plates and strips which are employed in the manufacture of tubes, pipes, body panels, stampings and construction materials used in the appliance and automotive industries.