Hot rolling is a metalworking process that occurs above the recrystallization temperature. 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 work pieces and for smaller work pieces 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 (122 to 212 °F) above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated before more hot
Hot rolled metals generally have little directionality in their mechanical properties and deformation induced residual. However, in certain instances non-metallic inclusions will impart some directionality and work pieces 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 and H-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
Hot rolling is used mainly to produce sheet metal or simple cross sections, such as rail
Cold Rolling
Cold-rolled sheets and strips come in various conditions: full-hard, half-hard, quarter-hard, and skin- rolled. Full-hard rolling reduces the thickness by 50%, while the others involve less of a reduction. Quarter-hard is defined by its ability to be bent back onto itself along the grain boundary without Half-hard can be bent 90°. While full-hard can only be bent 45°, with the bend radius approximately equal to the material thickness. 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 Luder bands from forming in later processing). 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; approximately less than 50 mm (2.0 in). This may be a cost-effective alternative to extruding or machining the profile. Cold rolling shapes requires a series of shaping operations, usually along the lines of: sizing, breakdown, roughing, semi- roughing, semi-finishing, and
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Hello there!!! It was wonderful to go through your blog. It is full of information and adds great values to the readers. This kind of blog helps readers like us in many ways and increases our interests. Thank you for sharing and looking forward to reading more of your blogs.