Process Description :
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 re-crystallization 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 re-crystallize 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.








Equipment :
Prior to continuous casting technology, ingots were rolled to approximately 200 millimeters (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.






Combustion Technology:
Undesirable mechanical properties and stresses are induced in the formed steel as a result of the changes in shape and cooling from the manufacturing processes. Heat treating encompasses a variety of processes by which carefully controlled changes in the temperature of the metal are used to improve the materials mechanical properties and stress conditions.



Solution Heat Treating - Cast, extruded, or forged pieces are heated to 850°F to 1050°F so that soluble alloying elements can diffuse evenly throughout the metal in solid solution. Precise temperature control and timing are important. Larger castings may require up to twenty hours of soaking while thin sheet may require only a minute.

Quenching - After solution treatment, it is important to cool the work-piece quickly to preserve the proper crystalline microstructure. This cooling is usually achieved by water immersion or water sprays. Quenching operations must be carefully integrated with heat treatment furnaces to avoid uncontrolled slow cooling.

Precipitation Heat Treatment - Also called artificial aging, this treatment promotes the precipitation of solute atoms of alloying elements for enhanced strength characteristics. Aging ovens operate in the range of 250° to 350°F. While the process is often integrated with solution heat treatment, a separate furnace is typically used because the heating temperatures and capacity required are much lower.

Annealing - Large work-pieces and those that have been subjected to cold working operations may require annealing, or stress relieving, to soften the metal and relieve the stresses of working. Annealing temperatures range between 300° and 600° with soak times of up to five hours.