A coil of hot-rolled steel
See also: Hot working
Hot rolling is a metalworking
process that occurs above the recrystallization temperature of the material. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure
and prevents the metal from work hardening. The starting material is usually large pieces of metal, like semi-finished casting products
, such as slabs, blooms, and billets. If these products came from a continuous casting
operation the products are usually fed directly into the rolling mills at the proper temperature. In smaller operations the material starts at room temperature and must be heated. This is done in a gas- or oil-fired soaking pit
for larger workpieces and for smaller workpieces induction heating
is used. As the material is worked the temperature must be monitored to make sure it remains above the recrystallization temperature. To maintain a safety factor
a finishing temperature
is defined above the recrystallization temperature; this is usually 50 to 100 °C (90 to 180 °F) above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated before more hot rolling.
Hot rolled metals generally have little directionality in their mechanical properties and deformation induced residual stresses
. However, in certain instances non-metallic inclusions
will impart some directionality and workpieces less than 20 mm (0.79 in) thick often have some directional properties. Also, non-uniformed cooling will induce a lot of residual stresses, which usually occurs in shapes that have a non-uniform cross-section, such as I-beams
. While the finished product is of good quality, the surface is covered in mill scale
, which is an oxide
that forms at high-temperatures. It is usually removed via pickling
or the smooth clean surface
process, which reveals a smooth surface.
Dimensional tolerances are usually 2 to 5% of the overall dimension.
Hot rolled mild steel seems to have a wider tolerance for amount of included carbon than cold rolled, making it a bit more problematic to use as a blacksmith. Also for similar metals, hot rolled seems to typically be more costly.
Hot rolling is used mainly to produce sheet metal
or simple cross sections, such as rail tracks
Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the strength
via strain hardening
up to 20%. It also improves the surface finish
and holds tighter tolerances
. Commonly cold-rolled products include sheets, strips, bars, and rods; these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the workpieces and their greater strength, as compared to hot rolled stock, four-high or cluster mills are used.
Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass.
Cold-rolled sheets and strips come in various conditions: full-hard
, and skin-rolled
. Full-hard rolling reduces the thickness by 50%, while the others involve less of a reduction.Skin-rolling, also known as a skin-pass
, involves the least amount of reduction: 0.5-1%. It is used to produce a smooth surface, a uniform thickness, and reduce the yield point
phenomenon (by preventing Lüders bands
from forming in later processing). It locks dislocations at the surface and thereby reduces the possibility of formation of Lüders bands. To avoid the formation of Lüders bands it is necessary to create substantial density of unpinned dislocations in ferrite matrix. It is also used to breakup the spangles in galvanized steel. Skin-rolled stock is usually used in subsequent cold-working processes where good ductility is required.
Other shapes can be cold-rolled if the cross-section is relatively uniform and the transverse dimension is relatively small. Cold rolling shapes requires a series of shaping operations, usually along the lines of sizing, breakdown, roughing, semi-roughing, semi-finishing, and finishing.
If processed by a blacksmith, the smoother, more consistent, and lower levels of carbon encapsulated in the steel makes it easier to process, but at the cost of being more expensive.