Process Encyclopaedia

Rolling

Other names / variants: tandem mill, reversing mill

Overview

Rolling was first used in the 1500s. The basic operation is a bit like flattening dough with a rolling pin.
Rolling is unusual in that it is primarily used for making stock items rather than making finished components. Over 90% of worked metals are processed at some point by rolling.

Reversing mill

Process details

Reversing mill

In a reversing mill, a hot ingot in moved back and forth through a set of connected die rolls.

Each roll gets closer the final shape, the last pass will finish the rolled shape.

Reversing mills are used for making thick sections such as slabs or large I-beams. In practice, there do not need to be many separate ‘dies’ (as is shown here) if the operator can move the rolls closer together between passes.

Tandem mill

In a tandem mill, a hot slab is passed through a series of flat rolls.

Each of the rolls reduces the thickness slightly, until the desired thickness is reached. If the final sheet is not too thick it can be ‘coiled-up’ while it is still hot.

Tandem mills are mainly used for producing plate and sheet. In practice, 5 or more rolls in series can be used – in which case the material coming out the end can be going very fast!

Materials and shapes

For flat sections, ingots over 1m wide are reduced to plates (usually 6mm-300mm), sheet (0.1mm - 6mm) or foil (about 0.008mm).
Shaped sections (such as rails and I beams) up to 300 mm across are made using a series of shaped rolls.
Specialised forms of rolling can be used to make large rings.
Hot rolling has poor dimensional tolerance and leaves a poor surface finish.
Cold rolling can improve these and also improve mechanical properties, but only for small reductions in thickness.

Economics

For making stock items, rolling has few competitors.For this reason, it is usually performed by the foundries before passing on to customers for further processing.
For long shaped sections, rolling is the only viable option for larger cross sections - for smaller cross section extrusion may be more economic.
Machines can cost millions of pounds.

Typical products

I-beams, rails, sheets, plates, foil