Hot rolling is a process where steel is rolled at an extremely high temperature usually exceeding 1700° F. The amount of heat has to exceed the recrystallization of the steel and is performed in a mill. Sometimes the rolling and steel making are done at the same site but many of the mills in the United Kingdom are for rolling only.
The steel products used for hot rolling have two separate classifications depending on the shape. Long products and flat products. The flat products are hot rolled with slabs and the billets and blooms are used for the hot rolling of the long products. Extremely long projects such as beams require a slab.
The Semi-Finished Product
The semi-finished hot rolling products require reheating in a furnace until they reach the point of becoming red hot. This occurs at around 1200o C. In all mills these hot rolling products will then go to a roughing stand. This stand consists of numerous steel roller. Pressure is used on the hot steel as it passes through. This is how the shape of the steel is formed. This is also the first stage of the hot rolling mill. As the product goes through this hot rolling numerous times the dimension and shape of the steel changes and becomes much closer to what is required of the final product.
The Plate Mill
Plate is made with the use of slabs. Once the hot rolling product has passed through the roughing stand it is sent to the finishing stand. This reversing mill takes the steel forwards and backwards then it is turned 900. The steel is now rolled sideways during a specific stage in the hot rolling. The plate is a flat, large piece of metal. This can be used to make the decks and hulls of a ship or rolled and then welded into a large steel tube. These tubes are used for gas and oil pipelines.
The Tata Steel
Steel strip is made with the use of slabs. These are called hot rolled coil. The first step once finished in the roughing stand is the finishing stand. This squeezes the steel until it is much thinner. The hot rolling also lengthens the steel. Close control is required of the speed because different parts of the piece are moving at different speeds. This part of the hot rolling is controlled with the use of a computer. By the time the steel gets to the end of the mill its speed is forty miles per hour. The steel is now in the form of a long strip and is cooled. This strip is flat and is coiled so it is easier to store and move. It is much thinner than plate.
The Cooling Process
The cooling of the steel in all hot rolling is critical. The mechanical properties of the steel are directly affected by the speed used to cool the steel. The speed of the cooling is controlled with water sprayed onto the steel as it goes through or is leaving the mill. Sometimes large fans are used to cool the steel with the use of air. Montanstahl is one the larger producers of rolled steel made by hot rolling.