Rolling of the metals

Rolling of the metals

1. UNIT-03 Manufacturing Science & Technology-I (RME-402) By Akhileshwar Nirala

2. Rolling of Metals

3. Rolling Introduction: Rolling is the plastic deformation of materials caused by compressive force applied through a set of revolving rolls. The thickness of the work piece is reduced by the process with a corresponding increase in the length. The rolls rotate to pull and simultaneously squeeze the work between them.

4. Introduction… It is widely used industrial process to convert steel ingots into blooms, billets, and slabs, and subsequently into plates, sheets and strips. Blooms have square cross section 6” x 6” or larger. They are rolled into structural shapes such as I-sections, channels, rails etc . Billets have square cross section 1.5” x 1.5” or larger. they are rolled into bars and rods . Slabs have rectangular cross section 10” x 1.5” or larger. They are rolled into plates, sheets and strips.

5. Flow diagram of

6. Hot Rolling & Cold Rolling Rolling processes are performed both at high temperature (above the recrystallization temperature), which is referred to as Hot Rolling, as well as at room temperature that is referred to as Cold Rolling. Hot Rolling is usually performed when large amount of deformation is required while Cold Rolling is performed for finished sheet and plate stock.

7. Grain Structure in Hot Rolling Changes in the grain structure of metals during hot rolling is shown in figure. This is an effective method to reduce grain size and refine the microstructure in metals, resulting in improved strength and good ductility .

8. Continue… The initial cast material will possess a non uniform grain structure, typically large columnar grains that grow in the direction of solidification. These structures are usually brittle with weak grain boundaries. Cast structure characteristically contains many defects such as porosity caused by gases, shrinkage cavities, and solid inclusions of foreign material that becomes trapped in the metal, such as metallic oxides . Rolling a metal above its recrystallization temperature breaks apart the old grain structure and reforms a new one. Grain boundaries are destroyed and new tougher ones are formed, along with a more uniform grain structure. Metal rolling pushes material, closing up vacancies and cavities within the metal. In addition, hot rolling breaks up inclusions and distributes their material throughout the work.

9. It should be apparent that the advantages of metal forming are not just in the creation of useful geometric forms but also in the creation of desired material properties as well. Cold rolling processes as discussed earlier, are useful for imparting strength and favorable grain orientation. Since metal rolling affects grain orientation, a part can be rolled in a way as to create grains oriented in a direction such that they give directional strength to a part useful to that part's specific application . An example of this can be the difference in grain structure between the threads of a machined bolt and a rolled bolt. The favorable grain orientation of the cold rolled bolt will give it directional strength beneficial to its application.

10. Advantages and disadvantages of rolling Advantages: hot rolled can damage the cast microstructure of the steel ingot, refinement of the crystal grains of the steel, and eliminate the defects of the microstructure, so that the steel organization compacting, improve the mechanical properties. This improvement is reflected in the rolling direction, so that the steel is no longer to a certain extent isotropic; pouring the formation of bubbles, cracks, and osteoporosis, under high temperature and pressure can also be welded together.

11. Disadvantages: Metallic Inclusion: After the hot rolling, the nonmetallic inclusions in the interior of a steel (mainly sulfides and oxides, as well as the silicate) was pressed into a sheet, stratified (laminated) phenomenon appears. Residual Stresses: Residual stress caused by uneven cooling. The residual stress is the stress of internal self-phase equilibrium in the absence of external force. Usually steel sectional dimension is the greater, the greater the residual stresses. Residual stress is self-phase equilibrium, but the steel members in the performance external force or have a certain influence. Such as deformation, stability, anti-fatigue may adversely. Poor dimensional Control: Hot-rolled steel products, poor control of thickness and edge width. We are familiar with the thermal expansion and contraction,

12. MECHANICS OF ROLLING Rolling process is shown in figure. A strip of thickness. ℎ1 enters the roll gap and is reduced to a thickness of ℎ2 by the rotating rolls. The surface speed of roll is Vr . To keep the volume rate of metal flow constant, the velocity of strip must increase as it moves through the roll gap. At the exit of roll gap, the velocity of the strip is Vf is constant along the roll gap, sliding occurs between the roll and the strip.

13. Continue…… Somewhere in the contact length, the velocities of the metal and roll are same which is designated as neutral plane. This point at which the two velocities are same is known as neutral point or no slip point. To the moves faster than the w/p, and to the right the right the w/p moves faster than the roll. The friction force on the left of the neutral point must be greater than the frictional force on the right. The difference fields a net frictional force to the right, which makes the rolling operation possible by pulling the strip into the roll gap. The velocity of strip at entrance is v 1 since the area of cross section gets decreased the metal leaving the rolls would be at higher velocity than when it entered Vi < Vr < Vf

14. Entry Condition The process of rolling depends upon the frictional forces and roll pressure acting between the surfaces of the roll and the metal. Figure shows the forces acting on the ingot as it contacts the rolls on entry side. The radial pressure of rolls pushes it away from rolls while the frictional force tries to pull it in to the roll gap. The ingot will enter the roll gap if there is net force in the direction of rolling. Therefore, the condition for ingot to enter the roll gap is 2µPcos α ≥ 2Psin α Where, P- roll pressure, α - angle of bite, µ-coefficient of friction

15. Entry Condition………

16. Length of contact

17. Length of contact……

18. Angle of bite The angle subtended by the deformation zone at the roll centre is called angle of bite.

19. Draft Draft: The total reduction taken in rolling is called draft. Δh = (hi− hf )

20. Condition for maximum reduction

21. Condition for maximum reduction………

22. From above equation, it is clear that to increase the reduction we have to either Increase the coefficient of friction Increase the diameter of the roll Increase the coefficient of friction as well as diameter of the roll.

23. Roll pressure distribution Assumption:- The rolls are straight and rigid cylinders The width of strip is must longer then its thickness and no significant widening takes place The coefficient of friction remains console throughout the process. Neutral point is within the arc of contact Deformation is homogeneous

24. Continue… Figure shows a typical rolling operation of strip with initial thickness hi rolled to final thickness hf. It both rolls are of equal radius R and rotate with the same circumferential velocity of the strip are Viand Vf respectively and in actual practice. 𝑉f > 𝑉 > 𝑉𝑖. Hence at velocity of strip will be equal to V and this point is referred as neutral point. Now considering the element of length 𝑑𝑥 the various forces acting are as shown in figure. Considering the thickness of the element perpendicular to the plane of paper to be unity. Considering the equilibrium of forces in x-direction, we get the following equation:

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34. Roll pressure distribution Assumption:- The rolls are straight and rigid cylinders The width of strip is must longer then its thickness and no significant widening takes place The coefficient of friction remains console throughout the process. Neutral point is within the arc of contact Deformation is homogeneous

35. Continue… Figure shows a typical rolling operation of strip with initial thickness hi rolled to final thickness hf. It both rolls are of equal radius R and rotate with the same circumferential velocity of the strip are Viand Vf respectively and in actual practice. 𝑉f > 𝑉 > 𝑉𝑖. Hence at velocity of strip will be equal to V and this point is referred as neutral point. Now considering the element of length 𝑑𝑥 the various forces acting are as shown in figure. Considering the thickness of the element perpendicular to the plane of paper to be unity. Considering the equilibrium of forces in x-direction, we get the following equation:

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45. Neutral plane in rolling In the process of rolling, the speed of the metal is continuously changes between the entry and exit of roll whereas roll speed remains constant. Since cross section area is decreases, metal enters between the rolls with a speed less than the speed of rolls and exit from the rolls travelling at a higher speed than it enters. At a point between contact length the metal speed is same as the roll speed. This is designated as neutral plane.

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47. Rolling Mill Rolling mill is a machine used for shaping the metal by passing it through rollers. A rolling mill basically consist of: > Rolls > Bearings and gears >A housing for containing these parts > A drive (motor) for applying power to the rolls and controlling the speed.

48. Types of rolling mills Two high rolling mill Two high reversing rolling mill Three high rolling mill Four high rolling mill Cluster mill Continuous rolling mill or Tandem mill Planetary mill

49. Various rolling mill or rolling stand

50. Continue.. Two high reversing rolling mill: In the two high reversing mill the direction of spin of the rolls can be reversed. This enables the work to travel through in one direction, then back through in the other direction. A series of reductions can be made using the same set of rolls, by passing the work back and forth. Disadvantages of the two high reversing mill include the mechanical requirements and power to constantly overcome and reverse the angular momentum of the rolls . Three high rolling mill: The three high rolling mill utilizes the principle of passing the work back and forth to achieve a series of reductions. Unlike the two high reversing mill, the three high mill has three rolls that always spin in the same direction. An elevator mechanism lifts and lowers the work so that it can be passed back and forth through the rolls.

51. Continue.. Four high rolling mill: It is known, in metal rolling practice, that the amount of roll force is reduced with a smaller radius of the rolls. Smaller radius rolls, however, deflect easier and must be supported by other rolls. The four high mill uses this principle with two smaller work rolls each supported by a larger backing roll . Cluster Mill: The cluster mill, uses a small work roll backed up by many other rolls. This extremely rigid setup is often used for cold rolling high strength material to a very thin width.

52. Continuous rolling mill or Tandem mill The tandem rolling mill consists of several stands, that the work material constantly passes through. At each stand the thickness of the work strip is reduced a certain amount. The total reduction between the first and last stand can be significant. The major problems associated with tandem rolling is the speed of the work material, which is different at each stand (because speed of the work material increases as it passes through each stand).   The speed of each set of rolls is synchronized so that the input speed of each stand is equal to the output speed of preceding stand .

53. Planetary mill Planetary mill has a pair of large heavy rolls, surrounded by a number of smaller rolls around their circumference. Each planetary roll gives an almost constant reduction to the slab as it sweeps out a circular path between the backing rolls and the slab . In this mill, a slab can be reduced to strip directly in one pass.

54. Roll Deflections (or Roll B ending) Flatness and uniform thickness: Strength and rigidity are important characteristics of the rolls used to form product in metal rolling. The roll gap must be perfectly parallel to produce sheets/plates with equal thickness at both ends. Flatness of rolled sheets depends on the roll deflection. The thickness of the sheet becomes uneven as roll deflection occurs.

55. Roll Deflections (or Roll Bending) Roll Deflection: During the rolling process, rolls will be subject to some degrees of deflection. The rolls initially start out flat. During the rolling process great forces act upon the rolls. It is observed that the work material exert greater force on the rolls towards the center of the material than at its edges. This will cause the rolls to deflect at the center.

56. Roll deflection results in uneven sheet thickness across the length. If rolls are elastically deflected, the rolled sheets become thin along the edge, whereas at centre, the thickness is large. Thicker centre means the edges would be plastically elongated more than the centre, Due to continuity of the sheet, we could say that the centre is subjected to tension, while edges are subjected to compression. This leads to waviness along edges.

57. Cambering ---Solutions to flatness problems Cambered rolls are used to compensate for roll deflection. The cambered rolls are grounded with extra thickness at the centre in such a way as to exactly offset the deflection that will occur during the process. This extra thickness is called camber.