Steel gets rolled like pastry dough, and that shapes more metal than anything else
Squeeze a slab of metal between spinning rollers and it thins and lengthens, much as dough does under a rolling pin. This humble idea handles more tonnage than any other manufacturing process on Earth, turning glowing ingots into rails, I-beams, sheet for car bodies and the tin cans in your cupboard.
Temperature decides the character of the product. Hot rolling happens above the point where a metal's crystal grains reform, so the grains keep healing as they are squashed and the metal stays workable instead of hardening. Mills keep the stock 50 to 100 degrees Celsius above that threshold and reheat it if it cools. Hot rolling produces sheet and simple shapes such as rails. Cold rolling, usually at room temperature, strain-hardens the metal, making it up to 20 percent stronger, with a smoother surface and tighter tolerances; full-hard sheet has been squeezed to half its thickness.
The earliest mills were slitting mills, brought to England from what is now Belgium in 1590 to press iron into plate and cut it into rods for nail makers. In 1697 John Hanbury's mill at Pontypool rolled iron plate that was later tinned to make tinplate. The Swedish engineer Christopher Polhem noted in 1761 that a mill could turn out 10 to 20 bars at once, saving labour. Water wheels powered them until 1786, when John Wilkinson coupled a Boulton and Watt steam engine directly to a mill.
Henry Cort, working near Fareham in Hampshire, is called the father of modern rolling. His 1783 patent for grooved rolls let mills make 15 times more iron bar per day than hammering could. He was not the first to use grooves, but he combined the best existing techniques into one process.
Scale followed. John Birkenshaw opened the first rail mill at Bedlington in Northumberland in 1820, producing wrought-iron rails 15 to 18 feet long, and at the Great Exhibition of 1851 the Consett Iron Company showed off a single plate 20 feet long weighing 1,125 pounds. Today continuous mills pass metal through stand after stand without stopping, and engineers use computer models to cut the number of passes needed.
Source: Rolling (metalworking)
