hot roll
To roll metal in a temperature that is above its recrystallization point.
hot roll: deform metal while it's still hot and soft
Hot rolling is the process of passing heated metal through a series of rotating cylindrical dies to reduce its thickness, change its shape, or improve its surface finish. The metal is heated above its recrystallization temperature, typically 1000 to 1200 degrees Celsius for steel, before and during the rolling operation. At these temperatures, the metal's yield strength drops dramatically, allowing large deformations with modest force.
The recrystallization point is critical. Below it, the metal becomes work-hardened and brittle as its crystal structure deforms. Above it, new crystals form continuously during deformation, so the material stays ductile and can be shaped further without cracking. This is why hot rolling, unlike cold rolling, can achieve major thickness reductions in a single pass or a few passes without intermediate annealing.
Steel mills use hot rolling to produce plate, sheet, coil, and structural shapes like I-beams and rails. Aluminum, copper, and other nonferrous metals are also hot-rolled. The metal typically enters a heating furnace to reach rolling temperature, then moves through a succession of rolls spaced progressively closer together. The metal cools as it travels through the mill, and final rolling often occurs as it cools below the recrystallization point, which strengthens the finished product.
Common problems and design choices
Oxidation is the main material loss in hot rolling. The hot metal surface reacts with oxygen, forming scale, a layer of iron oxide that flakes off or must be removed by pickling in acid. Scale loss typically accounts for 1 to 3 percent of input weight. Heat loss during transport between furnace and mill requires careful scheduling. Uneven cooling creates residual stress and can warp thin sections. Rollers themselves experience thermal fatigue and must be replaced regularly.
Hot rolling produces a rougher surface finish than cold rolling but at lower cost and with less equipment stress. It is the first stage in most steel and aluminum product lines. Cold rolling follows hot rolling to achieve final dimensions, hardness, and surface quality. The term hot roll also appears as a noun, referring to the finished coil or sheet that leaves a hot mill before further processing.