Hot Rolling :
Rolling is the most rapid method of forming metal into desired shapes by plastic deformation in between rolls. The crystals are elongated in the direction of rolling, and they start to reform after leaving the zone of stress, but in cold rolling they retain substantially the shape given to them by the action of paths.
In deforming metal between rolls, the work is subjected too high compressive stresses from the squeezing action of the rolls and to surface shear stresses as a result of a friction between the rolls and the metal. The frictional forces are also responsible for drawing the metal into the rolls.
In the hot rolling process, metal in a hot plastic state is passed between two rolls revolving at the same speed but in opposite direction.
As the metal passes through the rolls, it is reduced in thickness and increased in length. The forming of bars, plates, sheets, rails, and other structural sections requires many passes through plain or grooved rolls.
Working Process :
The process starts with the reduction of ingots which have been heated in soaking pits to the desired rolling temperature. This ingot will be about 1.5m long, slightly tapering and approximately square in cross-section, sizes varying from about 300mm to 500mm square. The ingots are rolled on blooming mills between two grooved rollers, where these are reduced to blooms. A bloom is a piece of steel usually 200mm to 300mm square and about 3.5 m to 5.5 m in length. The space between rolls can be adjusted and the direction of rolling can be reversed for alternate trips. On each side of the mill are motor-driver roller-conveyors on which the hot ingot is moved to and from the rolls. During rolling at blooming mill, the ingot is frequently turned 90* so that all surfaces of the ingot are in contact with the rolls, and devices known as "manipulators" are provided to turn the ingot. Blooming mill is often called the "mother mill" because all ingots pass through it.
A set of rolls and the housings in which they are mounted are known as a stand. When stands are placed one in front of another, this arrangement is known as continuous mill. When there are only top and bottom rolls the mill is called a "two-high" mill. If three rolls are mounted so that rolling may be done between the top, or bottom roll, and the center one, it is called a "Three-high" mill. Likewise, "a four - high mill" has four rolls. A cluster mills consists of two working rolls of smaller diameter and four or more back-up rolls of large diameter. Rolls are made of cast steel with or without alloys. Finishing rolls are usually made of chilled cast iron.
When the metal is to be used for bars, blooms are reduced in size to billets by passing through a continuous series of rolls known as billet mill. The billet, which has dimensions between 50 mm to 150 mm square, is then cut into shorter lengths which are reheated for further rolling in the bar mill.
The blooming mill also feeds the rail mill with blooms and the plate mill with slabs. Slabs are flat rectangular pieces of steel 50 to 100mm thick. In the rail mill, the heavier structural sections and rails are made. So it is from this mill that girders, channels, angle irons, and tee-irons are obtained, and the plate mill rolls slabs into plates.
The materials , commonly hot rolled are aluminum , copper, magnesium, their alloys and many grades of steel .
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