What Does Hot Rolling Mean?
Hot rolling is a metalworking process in which metal is heated above the recrystallization temperature to plastically deform it in the working or rolling operation.
This process is used to create shapes with the desired geometrical dimensions and material properties while maintaining the same volume of metal. The hot metal is passed between two rolls to flatten it, lengthen it, reduce the cross-sectional area and obtain a uniform thickness. Hot-rolled steel is the most common product of the hot rolling process, and is widely used in the metal industry either as an end product or as raw material for subsequent operations.
Corrosionpedia Explains Hot Rolling
The non-uniform initial grain structure of metal consists of large columnar grains growing in the direction of solidification. This is usually brittle with weak grain boundaries and may contain defects such as shrinkage cavities, porosity caused by gases, and foreign material such as metallic oxides. Hot rolling breaks the grain structures and destroys the boundaries, giving rise to the formation of new structures with strong boundaries having uniform grain structures.
Starting materials such as blooms or slabs at an elevated temperature flow from the continuous casting process directly into the rolling mills. In smaller operations the materials start at room temperature and must be heated either in a soaking pit, or by induction heating before being fed into the mill.
Hot rolling improves:
- Toughness and strength
- Resistance to vibration and shock
Hot-rolled steel products are classified into four groups:
Typical applications of hot-rolled steel are:
- Automotive structural parts such as frames
- Tabular products such as pipe and gas cylinders
- Machine structures such as saws and springs
- Agriculture equipment
- Metal buildings
- Guard rails
Surface defects may occur due to scale, rust or impurities. Scale is produced when the hot surface is oxidized by air. This mill scale can reduce corrosion if steel is exposed to air over short periods, however, the corrosion increases if exposed for long periods. Large amounts of mill scale also cause severe pitting corrosion if the surface is in contact with water.
Scale on the surface is usually removed mechanically, using water under high pressure or acid pickling and then oiling to reduce rusting.