Rolling (metalworking)

• 1. Introduction

  ›

  Rolling, the metalworking process that shapes the modern world's steel, is older and more inventive than most people realize. At its core, it is elegantly simple: pass a piece of metal between two rotating rollers, squeeze it thinner, and watch it emerge transformed. The concept is close enough to rolling out dough that the comparison is often made. Yet rolling mill technology has produced everything from railway rails to aluminum foil, from armor plate to the thin steel strips inside your refrigerator.

  The story begins in Belgium. The earliest rolling mills were slitting mills, introduced from what is now Belgium to England in 1590. From that starting point, the technology would spread and multiply across centuries, driven by patents, steam engines, and the insatiable demand of the Industrial Revolution. How did a process for flattening iron bars become the most tonnage-intensive manufacturing method on earth? What separates hot rolling from cold, and why does that distinction shape the strength of everything from car bodies to kitchen appliances? Those are the questions this documentary will answer.

• 2. From Slitting Mills to Steam

  ›

  The first rolling mills in England did something specific: they passed flat bars between rolls to produce a plate of iron, then ran that plate through grooved rolls called slitters to produce iron rods. The earliest experiments at rolling iron for tinplate took place around 1670. In 1697, Major John Hanbury erected a mill at Pontypool to roll what were called Pontypool plates, or blackplate, which was later rerolled and tinned to make tinplate. Before rolling mills arrived, plate iron in Europe was produced in forges, a far slower method.

  The intellectual history of rolling mills can be traced to the Swedish engineer Christopher Polhem, who described them in his Patriotista Testamente of 1761. He noted that a rolling mill could produce ten to twenty or more bars at the same time, dramatically cutting both time and labor. Patents followed: Thomas Blockley received one in 1759 for polishing and rolling metals, and Richard Ford received one in 1766 for the first tandem mill, designed for hot rolling wire rods.

  Power for these early mills came from water wheels well into the eighteenth century. The shift arrived in 1786 at John Wilkinson's Bradley Works, where a Boulton and Watt steam engine was coupled directly to a slitting and rolling mill. Steam engines expanded what mills could produce, until electric motors displaced steam power shortly after 1900.

• 3. Henry Cort and Modern Practice

  ›

  Henry Cort of Funtley Iron Mills, near Fareham in Hampshire, received a patent in 1783 for using grooved rolls to roll iron bars. The design allowed mills to produce fifteen times more output per day than a hammer could achieve. Cort was not the first person to use grooved rolls, but he was the first to bring together the best features of many ironmaking and shaping processes that existed at the time. For that synthesis, modern writers have called him the father of modern rolling.

  The rail industry pushed the technology further. John Birkenshaw established the first rail rolling mill at Bedlington Ironworks in Northumberland in 1820, producing fish-bellied wrought iron rails in lengths of 15 to 18 feet. As rolling technology advanced, the products grew larger. At the Great Exhibition in London in 1851, the Consett Iron Company exhibited a plate 20 feet long, 3 feet wide, and 7/16 of an inch thick, weighing 1,125 pounds. Three-high mills, introduced in 1853, extended the process to rolling heavy sections that two-high machines could not handle.

• 4. Hot Rolling Versus Cold

  ›

  Hot rolling takes place above the recrystallization temperature of the metal. At that temperature, the grains deformed during rolling recrystallize as the metal is worked, which keeps the internal structure equiaxed and prevents work hardening. The starting material is typically large pieces such as ingots, slabs, blooms, and billets. When these come from a continuous casting operation, they can be fed directly into the rolling mills while still at the proper temperature. Smaller operations must heat the material from room temperature, using a gas- or oil-fired soaking pit for large workpieces or induction heating for smaller ones.

  A finishing temperature is defined as 50 to 100 degrees Celsius above the recrystallization temperature. If the metal drops below that threshold, it must be reheated before rolling continues. Hot-rolled products are generally of good quality but carry a surface oxide called mill scale, which is usually removed through pickling or a smooth clean surface process. Dimensional tolerances run from 2 to 5 percent of the overall dimension.

  Cold rolling, by contrast, takes place below the recrystallization temperature, typically at room temperature. It increases the strength of the metal through strain hardening by up to 20 percent and produces a better surface finish with tighter tolerances. Cold-rolled products come in grades including full-hard, which reduces thickness by 50 percent, as well as half-hard and quarter-hard variants. The lightest treatment, called skin-rolling or a skin-pass, reduces thickness by only 0.5 to 1 percent. Its purpose is not to thin the material but to produce a smooth surface and prevent Luders bands from forming during later processing.

• 5. Mill Designs and Configurations

  ›

  The simplest mill configuration is the two-high non-reversing, in which two rolls turn in one direction only. The two-high reversing mill adds the ability to rotate in both directions, but the rolls must stop, reverse, and return to rolling speed between each pass. The three-high mill resolves that inefficiency by using three rolls that all turn in one direction; the metal passes through two rolls in one direction and returns through another pair, though an elevator is needed to lift and lower the workpiece. Roll diameters in these primary rolling mills typically range from 60 to 140 centimeters.

  Reducing roll diameter brings advantages: less roll surface contacts the metal, which lowers the force and power required. The trade-off is reduced stiffness, which is addressed by adding larger backup rolls behind the working rolls. A four-high mill uses two small working rolls and two larger backup rolls. A cluster mill uses more than four rolls, usually arranged in three tiers. These configurations handle cold rolling and foil production, where the material is thin and requires precise control.

  For very thin sheet metal under 200 micrometers thick, cluster mills are necessary because the small thickness demands small-diameter rolls. Pack rolling is used to reduce the number of passes: multiple sheets are rolled together to increase the effective starting thickness. Aluminum foil is the most commonly produced product through pack rolling, a fact visible in its two different surface finishes. The shiny side comes from contact with the roll; the dull side presses against the adjacent sheet of foil.

• 6. Specialized Processes

  ›

  Ring rolling is a specialized form of hot rolling that increases the diameter of a ring rather than flattening a bar. A thick-walled ring is placed between an inner idler roll and a driven roll that presses from outside. As the wall thins, the diameter grows. The resulting grain structure is circumferential, which gives the finished ring better mechanical properties than other forming methods. Diameters can reach as large as 8 meters with face heights up to 2 meters. Applications span railway tyres, bearings, gears, aircraft components, rockets, turbines, pipes, and pressure vessels.

  Forge rolling takes a different approach. It reduces the cross-sectional area of heated bars or billets by passing them between two contrary-rotating roll segments. The goal is not a finished product but an optimized material distribution for subsequent die forging. Roll systems can process blanks up to approximately 127 millimeters in thickness and over one meter in length. Parts commonly preformed by forge rolling include crankshafts, connection rods, steering knuckles, and vehicle axles.

  Controlled rolling integrates deformation and heat treatment in a single operation. The heat that raises the workpiece above its recrystallization temperature is also used to perform heat treatments, eliminating subsequent steps. Variables under continuous monitoring include starting material composition, deformation levels, temperatures at each stage, and cool-down conditions. Outcomes include fine grain structures, controlled distributions of transformation products such as ferrite, austenite, pearlite, bainite, and martensite, and precipitation hardening.

• 7. Defects and Surface Quality

  ›

  Six recognized categories of surface defects can afflict rolled products. Laps appear when a corner or fin folds over and is rolled but not welded into the metal, leaving seams across the surface. Mill-shearing defects resemble feather-like laps. Rolled-in scale occurs when mill scale is pressed into the metal surface. Scabs are long patches of loose metal rolled into the surface. Seams are open, broken lines running along the length of the metal, caused by scale or pass roughness in the roughing mill. Slivers are prominent surface ruptures.

  Thickness variation along the length of a strip creates another category of problem. In hot rolling, uneven temperature causes the material to flow more in warmer regions and less in cooler ones; a large enough temperature difference produces cracking. In cold rolling, most thickness variation traces to eccentricity and out-of-roundness in the backup rolls. The eccentricity can reach up to 100 micrometers per stack. At the 5 Stand Cold Mill at Bluescope Steel in Port Kembla, a modified Fourier analysis tracked this variation from 1986 until that mill ceased production in 2009.

  Flatness defects include symmetrical edge wave, where both edges are longer than the center; asymmetrical edge wave, where one edge is longer than the other; center buckle, where the center strip is longer than the edges; and the rare quarter buckle, where fibers in the region between the center and the edge are elongated. Compensating for roll deflection, which causes the workpiece to come out thinner at the edges and thicker in the middle, has driven the development of continuously variable crown, pair cross rolling, and work roll bending techniques. CVC, developed by SMS-Siemag AG, uses a third-order polynomial curve ground into the work rolls, which can be shifted laterally to vary the crown dynamically during rolling.