Metal Rolling Process: Hot vs. Cold Techniques and Deformation Mechanics

Fundamentals of Metal Rolling (Laminating)

General Principles of Rolling

Rolling (Laminating) is the process of deforming a metal slab by passing it between two overlapping cylinders rotating in opposite directions. This operation can be performed hot or cold.

Hot Rolling

• It can be considered continuous forging.
• It is performed at temperatures between recrystallization and melting.
• As in forging, the material does not acquire strain hardening (work hardening).

Cold Rolling

• This is done at room temperature.
• Materials acquire strain hardening due to deformation.
• Annealing is required to finish the operation, or even during the process if the deformation is very deep.

Deformations Produced by Smooth Rolling Cylinders

1. Spread (Widening)

   The forces acting on the material at the entrance of the cylinders cause stress, resulting in a widening of the section immediately behind the contact area of the cylinders.

2. Maximum Strain

   This takes place in the plane passing through the axes of the two cylinders, which is called the rolling plane.

3. Expansion at the Output (Springback)

   When the material leaves the cylinders and the pressure ceases, its thickness or height increases slightly, making it greater than it was in the rolling plane. This is due to the elastic nature of the material, which tends to partially recover its primitive form.

4. Flanging (Width Increase)

   The width of the material increases very little during rolling, unlike forging, where the material expands equally in all directions. This is because the rotation of the cylinders causes a forward flow of metal.

5. Elongation and Acceleration

   The decrease in material thickness and the minimal increase in width result in a section reduction, causing a significant increase in the length (elongation) of the rolled part. Since the rate of entry of the billet into the rolling cylinders must account for this length increase per unit time, the speed of the material at the outlet is higher than the input speed. This phenomenon is sometimes incorrectly designated as acceleration.

Rolling Mill Characteristics and Components

The simplest rolling unit consists of two cylinders whose carrier bearings are supported on two stands (housings), each consisting of a base, two columns, and a crossbar connecting them. Both stands are joined by other elements that maintain their proper position and provide overall strength.

This elementary mill may comprise more than two cylinders (horizontal axis and vertical axis), forming what is called a rolling stand (or rolling box).

Types of Rolling Stands

• Closed Stands: If the supporting stands are one piece.
• Open Stands: If the housing rail is removable.

Cylinder Adjustment Mechanism

At least one of the cylinders can slide up and down to adjust the distance between them.

This adjustment can be done using screw-down mechanisms. If the adjustment must be made after each pass, the operation of the screws is performed by a remote-controlled motor, which is part of the gauge setting mechanism group.