Metal Rolling

Ain Shams University

Faculty of Engineering

Design & Production department

Production Engineering

3rd Year Mechanical

Manufacturing Technology

Report (1)

“ Rolling“

Presented to : Dr. Hala Abd Al Hakim . : Eng. Mahmoud Zakareia . Prepared by : Mohamed Sayed Saber Allamy . Sec: 2 . Year : 3rd

year Mechanical Department .

Index :

1. Principles Of Metal Rolling .

2. Front & Back tensions effect .

3. Spreading In Metal Rolling .

4. Grain Structure In Metal Rolling .

5. Rolls For Metal Rolling .

6. Roll Deflections .

7. Defects In Metal Rolling .

8. Rolling Mills Types .

Metal rolling : is one of the most important manufacturing processes in the modern

world. The large majority of all metal products produced today are subject to metal

rolling at one point in their manufacture. Metal rolling is often the first step in creating

raw metal forms. The ingot or continuous casting is hot rolled into a bloom or a slab,

these are the basic structures for the creation of a wide range of manufactured forms.

Blooms typically have a square cross section of greater than 6x6 inches. Slabs are

rectangular and are usually greater than 10 inches in width and more than 1.5 inches

in thickness. Rolling is most often, (particularly in the case of the conversion of an

ingot or continuous casting), performed hot.

At a rolling mill, blooms and slabs are further rolled down to intermediate parts such

as plate, sheet, strip, coil, billets, bars and rods. Many of these products will be the

starting material for subsequent manufacturing operations such as forging, sheet metal

working, wire drawing, extrusion, and machining. Blooms are often rolled directly

into I beams, H beams, channel beams, and T sections for structural applications.

Rolled bar, of various shapes and special cross sections, is used in the machine

building industry, as well as for construction. Rails, for the production of railroad

track, are rolled directly from blooms. Plates and sheets are rolled from slabs, and are

extremely important in the production of a wide range of manufactured items. Plates

are generally considered to be over 1/4", (6mm), in thickness. Plates are used in heavy

applications like boilers, bridges, nuclear vessels, large machines, tanks, and ships.

Sheet is used for the production of car bodies, buses, train cars, airplane fuselages,

refrigerators, washers, dryers, other household appliances, office equipment .

Principles Of Metal Rolling

Most metal rolling operations are similar in that the work material is plastically

deformed by compressive forces between two constantly spinning rolls. These forces

act to reduce the thickness of the metal and affect its grain structure. The reduction in

thickness can be measured by the difference in thickness before and after the

reduction, this value is called thedraft. In addition to reducing the thickness of the

work, the rolls also act to feed the material as they spin in opposite directions to each

other. Friction is therefore a necessary part of the rolling operation, but too much

friction can be detrimental for a variety of reasons. It is essential that in a metal rolling

process the level of friction between the rolls and work material is controlled,

lubricants can help with this. A basic flat rolling operation is shown in figure:130, this

manufacturing process is being used to reduce the thickness of a work piece

During a metal rolling operation, the

geometric shape of the work is

changed but its volume remains

essentially the same. The roll zone is

the area over which the rolls act on the

material, it is here that plastic

deformation of the work occurs. An

important factor in metal rolling is that

due to the conservation of the volume

of the material with the reduction in

thickness, the metal exiting the roll

zone will be moving faster than the

metal entering the roll zone. The rolls

themselves rotate at a constant speed,

hence at some point in the roll zone

the surface velocity of the rolls and

that of the material are exactly the

same. This is termed the no slip point. Before this point the rolls are moving faster

than the material, after this point the material is moving faster than the rolls.

Front & Back tensions :

Sometimes in metal rolling practice,

tension, (force), is applied to a work

piece as it is being rolled. This

tension may be applied to the front,

(front tension), the back, (back

tension), or both sides. This

technique will assist the forces

necessary to form the work, and is

usually used on hard to roll

materials.

Spreading In Metal Rolling

In metal rolling operations, the plastic deformation causing a reduction in thickness

will also cause an increase in the width of the part, this is called spreading.

When the work being processed has

a high width to thickness ratio, the

increase in width is relatively small

and usually of no concern in

industrial manufacturing practice.

In cases of low width to thickness

ratios, such as a bar with a square

cross section, spreading can be an

issue. Vertical rolls can be

employed to edge the work and

maintain a constant width.

Grain Structure In Metal Rolling

In common industrial manufacturing industry, the ingot or continuous casting is hot

rolled into a bloom or slab. In addition to producing a useful shape for further

processing, the hot rolling process converts the cast grain structure into a wrought

grain structure. 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.

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.

Rolls For Metal Rolling

Metal rolling manufacturing can produce a wide range of different products. The

width of rolled work can be as much as several meters, or narrower than a thousandth

of an inch. Metal rolling manufacture also creates rolled work over a wide range of

thicknesses. Metal plates for some boilers may be rolled to a thickness of 12 inches,

while foil for wrapping cigarettes and candy can be .0003 inches thick. Rolls used in

metal rolling are of various sizes and geometries. In flat rolling processes, during

industrial manufacture, the rolls may typically be 24 to 54 inches in diameter. In some

metal rolling operations, in the forming of very thin work, the rolls can be as small as

1/4 inch.

Rolls are subject to extreme operating conditions during the metal rolling process.

Conditions include, tremendous forces, bending moments, thermal stresses, and wear.

Roll materials are selected for strength, rigidity, and wear resistance. Roll materials

vary dependent upon the specific metal rolling process.

Common roll materials are cast iron, cast steel, and forged steel. Forged rolls

are stronger and more rigid than cast rolls but are more difficult to manufacture.

In industrial metal manufacturing processes, rolls are commonly made from

nickel steel or molybdenum steel alloys. With metal rolling operations of certain

materials, rolls made of tungsten carbide can provide extreme resistance to

deflection.

Roll Deflections

Strength and rigidity are important characteristics of the rolls used to form product in

metal rolling manufacture. The particular attributes of the rolls will affect dimensional

accuracy as well as other factors in the operation. During the rolling process great

forces act upon the rolls. Rolls will be subject to different degrees of deflection. In

any particular metal rolling process, it is important to understand how these

deflections will affect the rolls and hence the work being rolled. The rolls initially

start out flat. During a basic flat rolling operation, it can be observed that the work

material will exert greater force on the rolls towards the center of the material than at

its edges. This will cause the rolls to deflect more at the center, and hence gives the

work a greater thickness in the middle.

To solve this problem in industrial metal rolling manufacture, the rolls are often

ground so that they are thicker towards the center in such a way as to exactly offset

the deflection that will occur during the process. This extra thickness is called camber.

Figure:138

The camber that must be ground into a roll is very specific to a particular work width,

material, and force load. A roll must usually be manufactured for only one metal

rolling process. In some industrial metal rolling processes, rolls are given temporary

camber by applying forces through their bearings. Another way that rolls deflect is by

the shortening of their radius along the contact of the work. In other words, they

flatten like a tire on a car might. This type of deflection is important to consider in

manufacturing practice, as it will affect roll radius calculations and friction.

Defects In Metal Rolling

A wide variety of defects are possible in metal rolling

manufacture. Surface defects commonly occur due to

impurities in the material, scale, rust, or dirt.

Adequate surface preparation prior to the metal

rolling operation can help avoid these. Most serious

internal defects are caused by improper material

distribution in the final product. Defects such as edge

cracks, center cracks, and wavy edges, are all

common with this method of metal manufacturing.

Often times a sheet is not defective, it is just not flat enough. In

sheet metal industrial practice, a sheet may be passed through a

series of leveling rolls that flex the sheet in opposite directions to

flatten it. Another interesting defect that can occur in flat rolling is

alligatoring, where the work being rolled actually splits in two

during the process. The two parts of the work material travel in

opposite directions relative to their respective rolls.

In shape rolling manufacture, a work piece will often experience

different amounts of reduction in different areas of its cross

section. One of the goals of roll pass design is to properly design

a series of reductions in such a way as to mitigate the relative

differences in shape change between areas, in order to avoid

material defects. Improper reductions of the product can cause

warping or cracking of the material. Metal rolling practice is not

always the cause of warping or cracking, sometimes defects in

the metal being rolled may be the reason.

Rolling Mills Types

Three High Mills