Hot rolling

The metallurgical process of Hot rolling, used mainly to produce sheet metal or simple cross sections from billets describes the method of when industrial metal is passed or deformed between a set of work rolls and the temperature of the metal is generally above its recrystallization temperature, as opposed to cold rolling, which takes place below this temperature. Hot rolling permits large deformations of the metal to be achieved with a low number of rolling cycles.

Additional recommended knowledge

Essential Laboratory Skills Guide

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Weighing the right way

Because the metal is worked before crystal structures have formed, this process does not itself affect its microstructural properties. Hot rolling is primarily concerned with manipulating material shape and geometry rather than mechanical properties. This is achieved by heating a component or material to its upper critical temperature and then applying controlled load which forms the material to a desired specification or size.

Mechanical properties of the material in its final 'as-rolled' form are a function of:

• material chemistry,
• reheat temperature,
• rate of temperature decrease during deformation,
• rate of deformation,
• heat of deformation,
• total reduction,
• recovery time,
• recrystallisation time, and
• subsequent rate of cooling after deformation.

Types of Rolling Mills

Prior to continuous casting technology, ingots were rolled to approximately 200mm in thickness in a slab or bloom mill. Blooms have a nominal square cross section, whereas slabs are rectangular in cross section.

Slabs are the feed material for Hot Strip Mills or a Plate Mill and blooms are rolled to billets in a Billet Mill or large sections in a Structural Mill.

The output from a Strip Mill is coiled and, subsequently, used as the feed for a Cold Rolling Mill or used directly by fabricators. Billets, for rerolling, are subsequently rolled in either a Merchant, Bar or Rod Mill.

Merchant or Bar Mills produce a variety of shaped products such as angles, channels, beams, rounds (long or coiled) and hexagons. Rounds less than 16mm in diameter are more efficiently rolled from billet in a Rod Mill.

See also

• Sheet metal
• Crystallization
• Steel