For hot rolling of metals they are first cast into ingot moulds of suitable form (rectangular, square or round). The size of ingot depends upon the metal to be cast and the end-product desired. For hot rolling, the ingot is removed from the mould after solidification when it is hot and placed in the gas fired furnace called soaking pits.
Here the uniform working temperature is attained throughout the ingot and it is then taken to rolling mills. Hot rolling consists in compressing and lengthening the metal as it is fed between two rolls rotating in opposite directions.
Because of the large variety of finished shapes to be made, ingots are first rolled into such intermediate shapes as blooms (square 15 cm x 15 cm), billets (4 cm x 4 cm square cross-section approximately) or slabs (rectangular is cross- sections), with reheating, if required.
The flow of metal during rolling is continuous and almost entirely in longitudinal direction. Most primary rolling is done in either a two high reversing mill or a three-high continuous rolling mill. Final products from hot rolling include plates, strips and a wide variety of bars and sections. Plates and strips are normally rolled between two plain cylindrical rolls, or on four-high mills.
a. Two-High Reversing Mill:
It comprises of two rollers rotating in opposite direction. The direction of rotation of these rollers can be reversed. Two high reversing mills are often used for the first rolling of ingot. Generally the metal to be rolled passes through the rolls, which are then stopped and reversed in direction and operation is repeated.
It is desirable to turn (rotate) the metal by 90° at frequent intervals in order to have the uniform section and refine the metal throughout. To reduce a big ingot into a bloom by two- high reversible mill, several passes are needed. This mill is quite versatile, as it has a wide range of adjustment for size of produce and rate of reduction.
Its use is limited by the length that can be rolled and also by the inertia forces which must be overcome at each time when a reversal is made. The various reductions in cross-sectional area are achieved by providing different sizes of grooves on both the upper and the lower rolls.
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b. Three-High Mill:
The disadvantages of two-high mill can be eliminated by the use of three-high mill. It consists of three rolls mounted one above the other as shown in Fig. 5.6. This arrangement eliminates the need of reversing the rolls. In this arrangement the top and bottom rollers revolve in the same direction, and middle one in opposite direction.
This requires less costly motive power but is however not as flexible as a two-high mill. This requires the use of an elevating mechanism for lifting lower bar to upper position. Although there occurs some difficulty due to lack of correct speed for all passes, the three-high mill is less expensive and has a higher output than two high reversing mill. These mills are very common for rolling structural steel shapes, such as I-beam, channels and angles.
c. Continuous Mill:
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This consists of several stands of two- high mills arranged one after the other. As the metal comes out of one set of rolls, it enters second, third and so on and finally comes out in required size and shape.
The speed of every stage goes on increasing in comparison to preceding roll in order to accommodate the increasing length of the metal and thereby making it a continuous process. The operation is thus very fast and space requirement for mill is less. The production is economical and at same time mass production is possible.
d. Four-High Mill (Refer Fig. 5.7):
It consists of 4 rolls, two smaller in size and two bigger. The bigger rolls back up the rolling operation by two smaller rolls. These are commonly used for both hot and cold rolling of plates and sheets. The operation of rolling after slab production is generally carried out in four-high mills.
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This arrangement minimises the tendency of the rolls to bow and produce stock which is thicker at the centre. For rolling of special sections, the rolls are designed so that cross-section is brought, pass by pass, to its final shape. When large quantities of similar sized strip and rod are required, several mills are often arranged for continuous rolling.
Metal is passed from one mill to the next, until it emerges at the correct size. The speed of each rolling mill is adjusted carefully so that the length of the material between mills is kept constant.
e. Special Rolling Machine:
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It takes intermediately rolled products and fabricates them into such finished articles as rails, structural shapes, plates and bars. The design of forms and grooves in rolls is a very complicated job.
Generally round or square cross-sections are obtained by the corresponding grooves in rolls, whereas flat shapes like rectangular flat bars can be obtained by giving proper spacing between two flat rolls. In designing the grooves, the allowance for shrinkage must be taken into consideration and also the attempt should be made for minimum number of passes so that the complete operation may be possible in one heating.
f. Planetary Rolling Mills:
It has been observed that rolling force is conveyed more effectively in deforming the metal by small diameter rolls than by bigger diameter rolls because of the reduced area of contact of smaller rolls which produces higher rolling pressures. Planetary rolls consist of two heavy backing rolls which are surrounded by small diameter rolls mounted in cages. The strip is forced forward by serrated feed rolls and fed to planetary rolls.
The upper and lower cages of the planetary rolls are geared together so that corresponding small supper and lower rolls are brought into contact with the metal in synchronism, so that each pair of rolls bites into the red hot metal along the arcs of contact and successively extends the strip. While thickness reduction with conventional rolling is possible only upto 2 : 1, in the case of planetary rolling, thickness reduction ratio of 25 : 1 is possible.
The materials commonly hot rolled are aluminium, copper, magnesium, their alloys and many grades of steel. The typical parts produced by hot rolling are automobile rear axles, gear shift levers, connecting rods, leaf springs and blanks for eyebolts and aluminium propellers.
How working leads to grain refinement by recrystallization which takes place as shown in Fig. 5.9. The coarse grain structure is elongated and then small refined grains are formed when recrystallization temperature after rolls is reached.
If temperature is very high then rapid grain growth will be there. Therefore, finishing temperature is usually scheduled close to the upper limit of cold working, or finished product may be cold rolled for close tolerances and better surface finish.
Problem:
In a four high mill rolling machine, 20 mm thick (t1) plate is rolled to 16 mm (t2) thickness using a roll of 400 mm diameter (D). Determine the coefficient of friction, position of neutral plane, maximum pressure and values of backward and forward slip.
Solution:
Roll Bending:
In order to take care of unavoidable roll bending, which results in production of flat with convex surfaces (Refer Fig. 5.10 a), it is usual to provide non-cylindrical rolls with convex camber (Refer Fig. 5.10 b). Under roll separating force, these rolls bend and provide a uniform gap between the roll.
The rolls can be considered as thick and treated as short beams simply supported at the ends.
Then deflection at the centre is:
