Hot rolling

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Hot rolling is a hot working 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 thinner 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 whereas cold rolling will produce a hardened microstructure.

Process

A slab, billet, or ingot is passed or deformed between a set of work rolls revolving at the same speed, but in opposite directions. The distance between the work rolls is slightly less than that of the passing metal which allows for thinning. 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 in 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. The degree of change to the metal is directly related to the heat of the metal, high heats allowing for greater thinning.

Applications

Hot rolling is used mainly to produce sheet metal or simple cross sections such as railroad rails 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 200 millimetres (7.9 in) thick in a slab- or bloom- mill. Blooms have a nominally 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 16 millimetres (0.63 in) in diameter are more efficiently rolled from billet in a rod mill.

History

In 1779 a rolling mill was created in Fontley, Hampshire where Henry Cort developed ideas for rolling processes. In 1783 Cort received a patent for his groove rolling process and in 1784 a patent for pudding furnace. Much of Cort's work was based on the previous ideas of Thomas and George Cranege who developed the process of the reverberatory furnace for the production of wrought iron from cast iron in 1784.

See also

• Crystallization
• Flame cleaning
• Foil rolling
• Mill scale
• Sheet metal
• Smooth clean surface
• Steel

References

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