Rolling: Rolling is the most widely used deformation process. It consists of passing metal

between two rollers, which exert compressive stresses, reducing the metal thickness. (Rolling operations reduce the thickness or change the cross section of a material through compressive forces exerted by rolls.) Where simple shapes are to be made in large quantity, rolling is the most economical process. Rolled products include sheets, structural shapes and rails as well as intermediate shapes for wire drawing or forging. Circular shapes, ‘I’ beams and railway tracks are manufactured using grooved rolls

The gap between the rolls being somewhat less than the thickness of the entering metal. Because the rolls rotate with a surface velocity that exceeds the speed of the incoming metal, friction along the contact interface acts to propel the metal forward.

Practically all metals, which are not used in cast form are reduced to some standard shapes for subsequent processing.

Manufacturing companies producing metals supply metals in form of ingots which are obtained by casting liquid metal into a square cross section.

-Slab (500-1800 mm wide and 50-300 mm thick) [width> 2thickness]

-Billets (40 to 150 sq mm)

-Blooms (150 to 400 sq mm) Sometimes continuous casting methods are also used to cast

the liquid metal into slabs, billets or blooms. These shapes are further processed through hot rolling, forging

or extrusion, to produce materials in standard form such as plates, sheets, rods, tubes and structural sections.

Sequence of operation:

When a piece of metal is rolled in between two rolls, the thickness is reduced as a result of the compressive stresses exerted by the rolls and it can be treated as a two-dimensional deformation in the thickness and length directions neglecting the width direction. This is due to the fact that the length of contact between the rolls and work piece is generally much smaller than the width of the sheet passing through and the undeformed material on both sides of the roll gap is restraining the lateral expansion along the width direction.

The metal piece experiences both vertical and horizontal stresses caused by the compressive load from the rolls and the restrains by the portions of the metal piece before and after the material in contact with the roll respectively.

Basic Principles of Rolling

As the rolls exert a vertical stress on the metal piece, the metal piece exerts the same amount of stress back onto the rolls itself. As such the rolls are subjected to elastic deformation due to this stress induced by the workpiece. As shown in the figure below, the rolls in a 4-high rolling mill are subjected to four kinds of deformation

i. deflection of the back-up rolls,ii. deflection of the work rolls,iii. flattening of the work rolls caused by contact with the back-up

rolls and workpiece andiv. flattening of the back-up rolls caused by contact with the work

rolls 

i. Flat Rolling Metal strip enters the roll gap The strip is reduced in size by the metal rolls The velocity of the strip is increased the metal strip is reduced

in size Factors affecting Rolling Process

Frictional Forces Roll Force and Power Requirement

Rolling is the process of reducing the thickness or changing the cross-section of a long workpiece by applying compressive forces exerted by a set of rolls. Rolling process can be done by

i. Flat rollingii. Shape rollingiii. Production of seamless pipe

FLAT ROLLING PROCESS

Flat-Rolling Practice Hot rolling

The initial break down of an ingot Continuously cast slab Structure may be brittle Converts the cast structure to a wrought structure

Finer grains Enhanced ductility

Reduction in defects Continuous Casting

Is replacing traditional methods Faster & better

Product of the first hot-rolling operation - Bloom or slab Square cross section of 150mm (6in) on one side Processed further by shape rolling

I-beams Railroad rails

Billets – smaller than blooms and rolled into bars and rods Cold rolling

carried out at room temperature Produces sheet and strip metal Better surface finish – less scale

Pack rolling – when two or more layers of metal are rolled together

Grain Structure in Hot Rolling

Rolling Defects' in Sheets & Plates

The elastic deflection of the work rolls results in an uneven widthwise distribution of the workpiece thickness in such a way where the thickness is greater at the center of the width and smaller at the edges. In order to solve the bending of the work rolls, several methods can be adopted.

Smaller work rolls are more prone to greater bending under high roll-separating forces from the vertical stresses induced by the workpiece. As such, back-up rolls are often used to counter this phenomenon.

Another method to reduce or eliminate elastic roll deflection is to use materials of high elastic modulus, such as sintered carbide, for the work rolls.

A more common method to counter the effects of roll bending is the usage of cambered rolls. The degree of cambering depends on the width of the metal piece, flow stress of the material and the reduction per pass. However certain problems arise with improper work rolls cambering.

Rolling Defects' in Sheets & Plates(contd.)

Lack of camber or insufficient cambering of the work rolls results in producing a workpiece that has a thicker center than the edge. The thicker center implies that the edges are plastically elongated more than the center. This induces a residual stress pattern of compression at the edges and tension along the centerline of the workpiece (Figure a). The consequences of this uneven distribution of stress within the workpiece can be centerline cracking (Figure b), warping (Figure c) or edge wrinkling (Figure d) of the final metal sheet.

Rolling Defects' in Sheets & Plates(contd.)

In the case where the work rolls are over-cambered, the edges of the workpiece will be thicker than the center and the residual stress pattern is exactly the opposite of that of insufficient cambering, i.e. tension at the edges and compression along the centerline (Figure e). Possible undesirable results of the workpiece being produced in such a manner are edge cracking (Figure f), splitting (Figure g) or centerline wrinkling (Figure h).

Schematic Illustration of Various Roll arrangements

ii. Shape-Rolling Operations

Various shapes can be produced by shape rolling Bars Channels I-beams Railroad rails

Roll-pass design requires considerable experience in order to avoid external and internal defects

Stages in Shape Rolling of an H-section part. Various other structural sections such as channels and I-beams, are rolled by this kind of process

Ring Rolling

A thick ring is expanded into a large diameter ring The ring is placed between the two rolls One of which is driven The thickness is reduced by bringing the rolls together

The ring shaped blank may be produced by: Cutting from plate Piercing Cutting from a thick walled pipe

Various shapes can be produced by shaped rolls Typical applications of ring rolling:

Large rings for rockets Gearwheel rims Ball-bearing and roller-bearing races

Can be carried out at room temperature Has short production time Close dimensional tolerances

RING ROLLING

(a) Schematic illustration of Ring-rolling operation. Thickness reduction results in an increase in the part diameter.

(b) Examples of cross-sections that can be formed by ring-rolling

Thread Rolling

Cold-forming process Straight or tapered threads are formed on round rods by passing

the pipe though dies Typical products include

Screws Bolts

Thread Rolling (Contd.)

Threads are rolled in the soft condition Threads may then be heat treated, and subjected to final

machining or grinding Uncommon or special-purpose threads are machined

iii. Production of Seamless Pipe & Tubing Rotary tube piercing (Mannesmann process)

Hot-working process Produces long thick-walled seamless pipe Carried out by using an arrangement of rotating rolls

Tensile stresses develop at the center of the bar when it is subjected to compressive forces

Continuous Casting & Integrated Mills & Minimills Continuous casting

Advantages Highly automated Reduces product cost Companies are converting

over to this type of casting

Continuous Casting & Integrated Mills & Minimills (Contd.)

Integrated Mills utilize everything from the production of hot metal to the casting and rolling of the finished product

Minimills Scrap metal is melted Cast continuously Rolled directly into specific lines of products Each minimill produces one kind of rolled product

Rod Bar Structural steel

Salient points about rolling

Rolling is the most extensively used metal forming process and its share is roughly 90%

The material to be rolled is drawn by means of friction into the two revolving roll gap

The compressive forces applied by the rolls reduce the thickness of the material or changes its cross sectional area.

The geometry of the product depend on the contour of the roll gap Roll materials are cast iron, cast steel and forged steel because of

high strength and wear resistance requirements. Hot rolls are generally rough so that they can bite the work, and cold

rolls are ground and polished for good finish In rolling the crystals get elongated in the rolling direction. In cold

rolling crystal more or less retain the elongated shape but in hot rolling they start reforming after coming out from the deformation zone.

Salient points about rolling (contd.)

The peripheral velocity of rolls at entry exceeds that of the strip, which is dragged in if the interface friction is high enough.

In the deformation zone the thickness of the strip gets reduced and it elongates. This increases the linear speed of the at the exit

Thus there exist a neutral point where roll speed and strip speeds are equal. At this point the direction of the friction reverse.

When the angle of contact α exceeds the friction angle λ ,rolls cannot draw fresh strip

Roll torque, power etc. increase with increase in roll work contact length or roll radius