A rolling mill is a machine used in metalworking to shape materials, typically metals, by passing them through one or more pairs of rotating rolls. The fundamental purpose of this process is to reduce the thickness of the material, flatten it, or change its cross-sectional shape to create a uniform product. This mechanical process transforms large metal ingots or slabs into usable forms like sheets, plates, rods, or structural shapes. Rolling mills achieve this transformation without sacrificing any material, instead displacing the volume to achieve the desired dimensions.
The Mechanics of Metal Deformation
The process begins as the metal stock is drawn into the gap between two counter-rotating work rolls. This drawing action is governed by friction between the roll surfaces and the material, which must be sufficient to overcome the metal’s resistance to deformation. The gap between the rolls, known as the roll gap, is set smaller than the initial thickness of the workpiece, applying immense compressive stress.
The compressive forces exerted by the rolls exceed the yield strength of the metal, causing the material to undergo plastic deformation. This involves the movement and multiplication of dislocations within the metal’s crystal lattice structure. The reduction in thickness, or draft, is compensated for by an increase in the material’s length, called elongation, and a slight increase in width, known as spreading.
The rolls rotate at a constant speed, but the material’s speed increases as it thins out to maintain a constant volume flow rate. There is a specific point in the contact zone, known as the neutral point, where the roll surface speed exactly matches the material speed. Before this point, the rolls pull the material faster than it is moving, and afterward, the material moves faster than the roll surface. The pressure and strain applied in this zone convert the initial cast microstructure into a wrought structure, enhancing the final product’s mechanical properties.
Hot Versus Cold Rolling
Rolling processes are categorized based on the temperature of the metal relative to its recrystallization temperature. Hot rolling is performed above this temperature, typically over 1,700°F (926°C) for steel, making the metal highly malleable. Working above the recrystallization point prevents work hardening, allowing for significant reductions in a single pass and promoting a uniform, fine-grained microstructure.
The high-temperature environment of hot rolling causes surface oxidation, resulting in a rough, scaly finish called mill scale. Products from this process have less precise dimensional tolerances because the metal shrinks unevenly as it cools. Hot rolled products are often used for structural components such as I-beams and railway tracks, where strength and toughness are more important than a smooth surface or tight precision.
Cold rolling is conducted below the metal’s recrystallization temperature, often at or near ambient room temperature. This process is typically applied to metals that have already been hot rolled and pickled to remove scale. Since the metal is less pliable, the process requires greater force but results in superior dimensional accuracy and a smooth surface finish.
The deformation below the recrystallization temperature causes strain hardening, which increases the material’s yield strength and hardness by up to 20%. This increase in strength makes cold-rolled products suitable for applications requiring precision and a high-quality surface, such as automotive body panels, household appliances, and electronics. However, this work hardening can introduce internal stresses, sometimes requiring a subsequent annealing heat treatment to restore ductility.
Common Rolling Mill Configurations
Rolling mills utilize varied configurations to accommodate different materials and production requirements. The most basic setup is the two-high mill, which consists of two opposing work rolls stacked vertically. This configuration is versatile and can be used for both hot and cold breakdown rolling, though it is limited in the amount of reduction achievable for very hard metals.
To achieve greater reduction and improve dimensional control, the four-high mill configuration is frequently employed. This setup uses two smaller work rolls that contact the metal, supported by two larger backup rolls positioned above and below them. The backup rolls prevent the smaller work rolls from bowing or deflecting under the separating force, which is crucial for producing wide, thin sheets with uniform thickness.
When extremely thin gauges and high precision are necessary, specialized arrangements like the cluster mill are used. A cluster mill, such as a Sendzimir mill, surrounds the small working rolls with many support rolls in a complex radial pattern. This design allows for the use of small diameter work rolls to minimize contact area and rolling force, enabling the production of thin foil materials with exceptional accuracy.
A different approach to increasing efficiency is the tandem mill, which is a production line setup rather than a roll arrangement. It consists of multiple rolling stands arranged sequentially in a continuous line. The material passes through each stand in turn, undergoing progressive reduction without interruption, which is essential for high-volume manufacturing of strip and sheet products.