Hot rolling

Hot rolling is a metalworking process where large pieces of metal, such as slabs or billets, are heated above their recrystallization temperature and then deformed between rollers to form smaller cross sections. Hot rolling produces thinner cross sections than cold rolling processes with the same number of stages. Hot rolling, due to recrystallization, will reduce the average grain size of a metal while maintaining an equiaxed microstructure where as cold rolling will produce a hardened microstructure.

Process

A slab or billet 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. As the rolling process breaks up the grains, they recrystallize maintaining an equiaxed structure and preventing the metal from hardening. Hot rolled material typically does not require annealing and the high temperature will prevent residual stress from accumulating in the material resulting better dimensional stability than cold worked materials.

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.

Applications

Hot rolling is used mainly to produce sheet metal or simple cross sections such as rail road bars from billets.

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 convert|200|mm|in thick 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 plate mills 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 re-rolling, 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 convert|16|mm|in in diameter are more efficiently rolled from billet in a rod mill.

ee also

* Cold rolling
* Crystallization
* Foil rolling
* Smooth clean surface
* Sheet metal
* Steel

References