A four-roll cold rolling mill is a type of rolling mill used in metalworking to reduce the thickness of metal strips or sheets at room temperature. It's called "four-roll" because of its specific work roll and backup roll configuration.
Applications of Four-Roll Cold Rolling Mill
The four-roll cold rolling mill is suitable for cold rolling of carbon steel, alloy steel, copper, aluminum, and stainless steel strips below 3mm thickness. It can be used for both rough rolling and finish rolling. This machine features high rolling force, high precision, and strong stability.
Composition and Structural Features of the Four-Roll Cold Rolling Mill
The machine comprises a main motor, reducer, bevel gear housing, universal joint shaft, working frame, and electrical control cabinet. The roll system utilizes specialized rolling mill bearings.
Backup Rolls: These are the two larger rolls located above and below the work rolls. Their primary function is to support the work rolls and prevent them from bending or deflecting under the immense rolling forces. Without backup rolls, the smaller work rolls would flex, leading to an inconsistent thickness across the width of the strip (thicker in the middle, thinner at the edges).
Mill Housing/Frame: This is the robust structure that contains and supports all the rolls, bearings, and force application mechanisms. It must be extremely rigid to withstand the high rolling forces.
Screw-down Mechanism (or Hydraulic Cylinders): This system applies the vertical force to the rolls, pressing them against the metal strip. Historically, screws were used, but modern mills often use hydraulic cylinders for more precise and rapid control of the roll gap and rolling force.
Entry and Exit Guides: These mechanisms help to properly feed the metal strip into the roll gap and guide it out smoothly after rolling.
Tension Reels (Coilers/Uncoilers): For continuous rolling of long strips, there will be uncoilers at the entry side to feed the strip into the mill and coilers at the exit side to wind up the thinned strip. These also apply tension to the strip, which can aid in the rolling process and improve strip shape.
Cooling and Lubrication System: Cold rolling generates heat due to friction and deformation. A cooling system (often using rolling oil) is essential to maintain roll temperature and prevent the strip from overheating. The rolling oil also acts as a lubricant to reduce friction between the rolls and the strip, which lowers rolling forces and improves surface finish.
How it Works (General Process):
Preparation: A coil of metal strip (e.g., steel, aluminum, copper) is loaded onto the uncoiler.
Feeding: The strip is fed into the gap between the upper and lower work rolls.
Rolling: As the strip passes through the work rolls, the intense pressure deforms the metal, reducing its thickness and increasing its length. The backup rolls prevent the work rolls from bending, ensuring uniform thickness.
Force Application: The screw-down mechanism or hydraulic cylinders adjust the gap between the work rolls and apply the necessary force to achieve the desired reduction.
Cooling/Lubrication: Rolling oil is continuously applied to the rolls and the strip to cool them and reduce friction.
Coiling: The thinned strip exits the mill and is wound onto a coiler.
Multiple Passes: Often, multiple passes through the mill (or a series of mills in a tandem setup) are required to achieve the final desired thickness. Between passes, the material may be annealed (heat-treated) to restore ductility if it becomes too hard due to work hardening.
Key Features and Advantages
High Precision and Tight Tolerances: The backup rolls effectively counteract the deflection of the work rolls, allowing for the production of extremely thin sheets with very precise dimensions and consistent thickness.
Superior Surface Quality: By rolling the metal at room temperature (below its recrystallization temperature), the mill produces a smoother, more refined surface finish with fewer imperfections compared to hot rolling.
Enhanced Material Properties: Cold rolling increases the hardness and strength of the metal through work hardening. It also improves the material's dimensional accuracy.
Versatility: Four-roll mills are used to roll a wide range of metals, including steel, stainless steel, aluminum, copper, and various alloys.
Reduced Rolling Force: The smaller diameter of the work rolls in a 4-high mill reduces the overall force required for thickness reduction, which in turn lowers energy consumption.
Four-roll mills are widely used in industries producing flat-rolled products that require precise thickness and good surface quality, such as automotive, appliance, and packaging industries.
