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The end products of the steel industry include, among others, Hot and Cold Rolled Strip, Heavy Plate, Profiles, Bars, Wire Rod, Bright Steel, Open Die Forgings as well as Rolled Disks and Rings. They are produced by forming the input material (slabs, ingots and billets) using high technology plants.
Hot rolling is one of the forming processes that follow casting – both ingot casting and continuous casting. The semi-finished workpieces are reheated to about 1250°C and reduced to the defined thickness through the pressure in the roll gap of the hot rolling mill.
A complete hot rolling mill consists of:
- input material depot
- reheating furnaces
- descaling facilities
- roughing train and finishing train with a varying number of stands and stand types
- cooling equipment
- finishing lines
- finished product depot
- roller tables, connecting the individual areas of a hot rolling mill with one another
- measurement and testing equipment
- roll workshop
Hot Rolling Mill for Production of Hot Strip and Heavy Plate
Slab casting is followed by hot rolling. The semi-finished steel slabs are taken from the depot are then reheated to about 1250°C. Through the pressure in the roll gap of the hot rolling mill the thickness can be reduced as defined. As the volumes of the slabs remain constant, there are changes in the length and width.
Example: A Conventional Wide Hot Strip Mill
Slabs with thicknesses of between 20 and 30 centimetres are collected from the depot and reheated to the necessary rolling temperature in the reheating furnace. Shortly after leaving the furnace, the red-hot slab is descaled.
The slabs are then rolled to rough strip with a thickness of about 25 – 40 mm in the roughing train, using a reversing process. Before the strip is given its final dimensions in the finishing train, both ends of the strip are cut perpendicularly and the newly formed mill scale is removed.
The finishing train generally consists of five to seven consecutive rolling stands. During rolling, the strip is located in all the stands simultaneously and travels onto the run out table at final rolling speeds of about 20 metres per second. The mechanical properties of the hot strip are adjusted by means of regulated cooling using water in the subsequent cooling zone, which is about 100 metres long. The strip is then wound into coils, with diameters of about two metres.
Wide hot strip is produced in widths of from 600 to 2,300 mm and with thicknesses of from 0.8 to 12 mm. Narrow strip with widths of below 600 mm is produced on medium-wide hot strip mills.
Heavy plate is produced in widths of up to 5,000 mm and thicknesses of from 3.5 to 250 mm. Only quarto reversing stands are used here, with two rolls acting as work rolls and two more as backup rolls. The stand entry area is equipped with a system for turning the slabs through 90° so that heavy plate can be produced in widths exceeding the slab width. The mechanical-technical properties are adjusted by means of defined temperatures during the rolling process – in other words, through normalising rolling, thermomechanical rolling and accelerated cooling. Finishing with straightening and cutting, as well as the logistics, are of particular importance in heavy plate rolling mills.