Rolling and its types with Terminology

Rolling, Hot Rolling, Cold Rolling, Two high Rolling Mill [Irreversible], Two High Rolling [Reversible], Three High Rolling, Four High Rolling, Cluster Rolling, Planetary Rolling, Terminology for Rolled Products

Principle of Rolling:

           Compressing the metal between two opposite rotating rolls for reducing its thickness - flat strip rolling. High compressive force is exerted on the work piece by the squeezing action of the rolls. The metal is taken into rolls by friction. It is most widely used due to higher productivity at low cost. It is hot working process unless specifically mentioned as cold rolling but it is done both hot and cold.
Rolling operation

Hot Rolling:

          The hot plastic state metal is passed between rolls. Rolling carried out above the recrystallization temperature. The crystals are elongated in the direction of rolling, they start to reform after leaving the stress zone. Best suited for large amount of deformation. Hot rolled metal is generally free of residual stress and its properties are isotropic.
Grain structure in hot rolling process

Cold rolling:

            The rolling process employed to finish bars, rods, sheets and strips at room temperature. It provides good surface finish, better dimensional accuracy and improved physical properties. The crystals which are elongated will retain substantially the shape given to them by the rolling action.
Cold rolling

Types of rolling mills:
1.Two high rolling mill [irreversible]
2.Two high rolling [reversible]
3.Three high rolling
4.Four high rolling
5.Cluster rolling
6.Planetary rolling.

Two high rolling mill:

        It has two rolls which rotates in same direction about the horizontal axes.
Two high rolling mill (Irreversible)
        The stock is returned to the entrance of the rolls for successive reductions. The stock returned by hand or by a platform. This method of reduction slow down the process. Typical two high rolling mill consists of a roller conveyor, rolls with a stand and power drive. The upper roller can be raised or lowered to change the distance between rolls. Alternate method is used to fast the process where the direction of rolls is reversed after the each pass - Two high rolling [reversible].
Two high rolling mill (Reversible)
        The rolls are brought close together for each pass. The rolls in these mills have diameters ranging from 0.6 to 1.5 m. More power consumption than irreversible two high rolling mill. It is often used for first rolling operation. Applicable for both hot and cold rolling.

Three high rolling mill:

        There are three rolls in the vertical column used for two continuous process.
Three high rolling mill
        The direction of rotation of each rolls remains unchanged. Stock is passed through the either side of the rolls. It is raised and lowered after each pass using lifting mechanism. The top and bottom rolls are drive rolls. Intermediate roll rotates by friction. These mills are employed as blooming mills, billet rolling and finish rolling .More complicated due to lifting mechanism.

Four high rolling mill:

        Similar to two high rolling, but additional two rollers are added.
Four high rolling mill
        These extra two rollers are smaller in diameter. Thus the bigger roller will act as backup rollers behind smaller rollers. The arc of contact is minimised by using smaller rollers. The backup rolls are provided to avoid bending of rolls due to their low strength and rigidity.

Terminology for Rolled Products:

        The various rolled products are given names according to the dimensions, but the terminology is fairly loose and sharp limits with respects to dimensions can not be made.
Bloom:
          A bloom is the product of the first break down of ingot. It has square or slightly rectangular section, ranging in size from 150 mm X 150 mm to 250 mm x 300 mm. A bloom is used to make structural shapes, that is, I beams, channels etc.. by hot rolling.
Billet:
         A reduction of bloom by bot rolling results in a billet. The size of a billet ranges from 50 mm X 50 mm to 125 mm x 125 mm. It is rolled to make rounds, wires and bars.
Slab:
      A slab is a product obtained by bot rolling, either from ingot or from bloom. It has a rectangular cross-section, with thickness = 50 to 150 mm and width = 0.6 to 1.5 m. Slabs are further rolled to get plates, sheets, strips, coil  etc.
Plate:
       A plate is a finished or semi-finished product with a minimum thickness of 6.35 mm Its width will be equal to the width of the roll and the length equal to the maximum which can be handled or shipped.
Sheet:
        A sheet is a thin partner of plate with a maximum thickness of 6.35 mm.
Strip:
       A strip is a narrow sheet and has a maximum width of 600 mm with a maximum thickness of 6.35 mm. Since it is normally handled in coil form, its length can be considerable and is limited only by the manufacturing and handling facilities.
Foil:
        It is a thin strip with a maximum width of 300 mm and a maximum thickness of 1.5 mm. It is available in coil form.
Bar:
        It is a long, straight, symmetrical piece of uniform cross section. It may be round, square or of another configuration. A circular bar is called a rod.
Wire:
         A wire is a thin variety of bar, available in coil form and not normally so identified over 9.5 mm cross-section.


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