The rolls used in rolling mills are in different sizes and shapes and of different materials. The basic requirements of roll material are strength and hardness (ie. resistance to wear) Rolls are made of cast iron, cast steel or forged steel. For smaller rolls in a cluster mill, tungsten carbide may be used. Forged steel rolls are better in strength, stiffness and toughness Rolls for cold rolling are ground or polished to have a better finish. Lubricants are also used during rolling Hot rolling of ferrous metals is usually done without lubricants, although graphite is sometimes used for lubrication. Cooling of the rolls is done with water-based solutions which also help in breaking up the scales formed on the surface of metal. Hot rolling of aluminium alloys is done using various types of compounded oils, emulsions and fatty acids, Cold rolling is, however, carried out using water soluble oils or low viscosity lubricants including mineral oils, emulsions, paraffin and fatty oils.
The rollers used in rolling mills essentially comprise body, neck and wabblers The body portion of the roll comes in contact with the metal during rolling The body has different shapes, plain or grooved and may have diameter up to 1.5 metres. The neck is where a roll is supported in bearings. The wabbler is the star-shaped end of the roll and engages with the hollow coupling, Rolls are supported in bearings in heavy frames.
The power drive equipment for rolling mills comprise a motor which gives power to a breaking piece having star-shaped construction at its both ends and the hollow couplings which are loose fit on both the splined shaft of breaking piece and the wabbler of the roll. The hollow coupling keeps rotating always with the breaking piece. By moving the hollow coupling axially, it can be engaged with the wabbler of the roll such that the breaking piece. hollow coupling and the roll are made to rotate together as one assembly. The rolls may be of two types, (a) plain roll and (b) grooved rolls, which may be of open groove type or closed groove type. Plain rolls are used for rolling sheets and plates whereas rolls with open grooves are used for rolling round and square sections. The rollers with closed grooves
are used for rolling flats, bars, joists, rails, etc. The designer of the rolls should keep in mind that the number of passes (for rolling) should be minimum possible since it is necessary to roll out the given section from the billet to the finished size in one heat. A typical set of rolls for rolling flats.
Defects in Rolled Plates and Sheets
Defects in hot rolled plates and sheets may occur on the surface or internally in the rolled plates and sheets. Surface defects include: scale, rust, scratches, pits, cracks, etc. These defects are caused by inclusions and impurities present in the ingots or by other conditions related to material preparation and rolling operation. Other defects
These are wavy edges on sheets due to roll bending; strip is thinner along edges than its centre. Other defects, namely, zipper cracks in the centre of strip and edge crack are due to poor material ductility at the rolling temperature. Alligatoring defect is a complex phenomenon and may be because of nonuniform deformation during rolling or defects already present in the cast ingot.
See More: Types of Rolling Mills
See More: Hot Rolling Process
See More: Hot Working Process