The primary purpose of a hot strip mill is to reheat thick steel slabs into thin sheets with varying thickness. The thick steel slab passes through several rolling mill stands that are driven by powerful motors. The rolled sheets then pass through coilers, thereafter these coils move on to the next process in the plant. From the startup to the end, the steel material undergoes several treatments through each stage that are the main features of a hot strip mill. Though traditional strip mills had limited facilities, modernization has brought many changes in order to meet out the market demands.
Older hot strip mills had pusher style furnaces. As pusher style furnaces were inefficient to meet the growing demands of production, they are replaced with modern walking beam furnaces. They are automated and are capable to spread the heat uniformly. It produces a smooth texture free from bumps and can receive both hot and cold steel slabs of various grades. They are capable to increase the production rate up to 25% than usual.
The steel slab picks up oxidized iron particles as a result of the heat treatment inside the reheat furnace. This slab passes through the next phase of descaling where the iron layer is removed. The descaler is a closed unit with four spray headers. These headers spray high pressurized water on the slab to remove the oxidized iron layer. These headers have advanced nozzles to spray water effectively.
The steel slab enters into the process of roughing where there are six separate rolling mill stands. Unlike conventional edger, this slab sizing press is efficient and accurate in producing steel sheets with a good surface finish. The thickness is reduced and the steel tends to spread width wise. Edgers are used to avoid this. Edgers provide a uniform width to the rolled steel sheet.
During the roughing operation, iron scales from back on the surface of the steel. Another descaling process removes the excess formation of oxidized iron particles from the steel surface.
Two steel drums with shear blades along their body are used to trim the edges of the steel bars. Unfinished edges may cause threading problems during the process. The transferred steel bars are passed in the cropping system to smooth out the rough edges.
The finishing phase has six separate finishing mills that are automated. The transferred steel bar is rolled again in all the six finishing mills simultaneously in order to reduce the thickness as per the customer’s requirements.
A target temperature is fixed and the steel bar passes through the finishing mill. The automation system adjusts the speed of the finishing mill based on the temperature at which the bar will cool down before passing on to the next stage.
The rolled hot steel is cooled down in the laminar cooling network which is automated. It has several spray headers that spray cool water on the steel bar. The automated system estimates the temperature and the amount of water required and the number of sprays required for cooling.
Coilers are located between the rolling mill and the finishing mill. They roll and form coils of the transferred steel bar. The temperature is also reduced when the steel bar passes through these coils.
These are the main features of a modern hot strip mill that increases quality and productivity. This also allows flexibility of processing and facilitates low maintenance and reduced energy consumption.