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Hot rolling is a metallurgical process in which the metal is passed through a pair of rolls and the temperature of the metal is above its recrystallization temperature, as opposed to cold rolling, which takes place below it. This permits large deformations to be achieved with a low number of rolling cycles. Because the metal is worked before crystal structures have formed, this process does not itself affect its microstructural properties. Hot rolling is mainly used to produce sheet metal, or simple cross sections from billets. Hot rolling is primarily concerned with manipulating material shape and geometry rather than mechanical properties; This is achieved by heating a component or material to its upper critical temperature and then applying controlled load which forms the material to a desired specification or size.
History of Hot Rolling
The first hot rolling mills were single stand mills, two rolls positioned one above the other, and each sheet was rolled between the rolls one at a time. The sheets started as plates, usually 3/4 to one inch thick 12 to 18 inches long and 24 to 80 inches wide. When finished the sheets could be as wide as 80 inches, and up to 16 feet long and weighing 300 plus pounds each. Workers used tongs to get the plates out of a furnace and it was passed through the rolls. The rolls were adjusted closer together after each pass untill the desired thickness (gage) was obtained. The mills were not reversable so the workers had to use their tongs to lift each red hot sheet high enough to feed the sheet over the top roll and carry the sheet back over to the feeder side of the mill after each pass, while the rolls were adjusted closer together. Then the sheet was run back through the mill. If the desired sheet was eight feet or less in length, the sheet was folded together at some point and the folded sheet was worked untill the correct gage was obtained. Therefore the workers did not have any easy orders to fill. Usually, during this process, the sheets had to be reheated, which ment the workers had to use their tongs and manhandle the sheets back in the furnace for reheating. The work was brutally hard, and the process was expensive. Over the years, many attempts were made to line up mills so that a plate could be passed through a series of mills (known as a continuous hot strip mill) and be the correct gage when it passed through the last mill (set of rolls). In order for this process to work each mill had to run at the correct speed and the proper amount of tension between each mill had to be applied to the strip or the strip would tear or buckle and you would end up with a cobble. This would disable the mills untill the strip (now scrap) was pulled out of the way. This continuous process was finally accomplished in 1929 in Ashland Kentucky at an Armco Steel plant under the guidence of John Tytus, who was from Middletown Ohio, where Armco was founded. This discovery enabled slabs of steel, usually six to twelve inches thick to be passed through a series of bar mills, usually five or six mills in a straight line, the slab being reduced in thickness ,usually from 6 to 12 inches, and as it passed through each mill it was reduced to a thickness of about one inch. The strip would then enter the hot strip mill, usually five to seven mills spaced about ten feet apart.The strip would enter the first mill at a speed of a slow walk and enter the second mill faster, the third mill faster, fourth mill faster, fifth mill faster, and exit the sixth mill with a speed sometimes in excess of thirty miles per hour. Keep in mind that the first mill in the series was still being fed slowley. From here it was cooled by a series of water sprays and then fed to the coiliers. The end product was a coil of steel. This process made the production of sheet steel much less expensive. Today some hotstrip mills turn slabs that are 12 inches thick, up to 90 inches wide and 24 feet long into coils weighing up to 70,000 pounds each. The entire process, slab to coil, takes about 120 seconds. Some of the larger mills, have the capacity to roll over 800 tons per hour. Many people credit the Bessimer furnace and Open Hearth furnace processes for making steel production economical. However, if it were not for the continuous hot strip mill the production of sheet steel would not be economically possible, and our standard of living, would not be as we know it. The techonology gained also made possible the continuous cold strip mill, usually the next step in sheet steel manufacturing. It is after the cold strip rolling process that the coils can be cut into sheets.
See also
"Hot rolling"