Various types of equipment and systems are used in a rolling mill. The major equipment is obviously the mill stand in which the material is actually rolled between two or three rotating rolls. Apart from the mill stand along with its own different mechanisms and systems for rotating the rolls, positioning the rolls to effect amount of reduction on the stock, to guide the stock in perfect position and orientation with respect to the rolls, etc., various other equipment and systems are necessary based on the rolling process (hot or cold), size and shape of input stock, specifications of product mix, desired quality of the products, and handling of the rolling stock up to finished product stage. These are called auxiliary equipment.

In this section, for each of these equipment and systems the functions, constructional features and specifications and in some cases the basic design features and calculations have been discussed. The design aspects will be limited to mechanical design. Electrical drive, controls and instrumentation plays a very important role in modern rolling mills. A brief section has been devoted to these but their design aspects are beyond the scope of the present treatise.

Mill Stand Components and Mechanisms

Most type of rolling mill stands have certain common basic components, mechanisms and systems. These are:

1. (i) Work rolls between which rolling takes place.

2. (ii) Backup rolls to support work rolls, excepting in case of 2-Hi, 3-Hi mills or cluster mills.

3. (iii) Roll supporting bearing and bearing blocks (chocks).

4. (iv) Roll positioning (or screw-down) mechanism which alters the gap between the work rolls.

5. (v) Roll balancing arrangement which keeps the set of top work and backup rolls in position and does not allow the top work and back-up rolls to come down and touch bottom work roll when there is no stock in roll bite.

6. (vi) Mill housings (generally a pair for normal 2-Hi, 3-Hi, or 4-Hi mills) within whose windows the roll bearing chocks are supported and allowed to be slided up and down during screw down adjustment.

7. (vii) Base Plates (also called shoes) in the form of two girders onto which the housings are fixed. The base plates are mounted on the foundation.

Characteristics of Rolling

In order to understand the rolling process, it is essential to understand some of the terms characterizing the rolling process, which are called characteristics of rolling.

These are also known as rolling process parameters.

While explaining or defining these characteristics, for the sake of simplicity, the following rolling conditions are considered:

1. (i) The case of simple rolling

2. (ii) The rolls are cylindrical with plain surface

3. (iii) The material or stock is rectangular in cross-section

It is to be carefully noted that while discussing the characteristics of rolling or analyzing the mechanics of rolling (i.e., determination of rolling force, etc.) often the stock material will be considered to be rectangular in cross-section meaning the rolls will be in contact with two flat surfaces of the stock, which is known as flat rolling. However, in the field of rolling, different types of rolling operation takes place for rolling products of different cross-sections, and those are achieved by passing different sections of stock through various shaped grooves made by the pair of rolls, several times.

Understanding of rolling characteristics as well as analysis of mechanics of rolling becomes easier with rectangular cross-section material. That is why the above three conditions have been considered. However, at appropriate places, grooved rolling will be taken up and discussed.

Figure 2.1 illustrates the rolling process as considered above.

Initial height and width of the stock are ‘h1’ and ‘w1’, respectively, which enter the roll with a velocity of ‘v1’. The corresponding values of the outgoing rolled stock are ‘h2’, ‘w2’, and ‘v2’. Let lengths of ingoing and outgoing stock are ‘l1’ and ‘l2’, respectively.

The various rolling characteristics or terms are explained below, primarily keeping above simplified rolling condition in view. How these characteristics may get modified under different rolling conditions, have also been mentioned.

Draught and reduction

Draught means the reduction in height in a particular pass. Absolute draught is the difference of entry thickness and exit thickness, i.e., h1-h2. Relative draught is absolute draught expressed as a percentage of initial thickness, i.e., [(h1−h2)/h1] × 100 percent. While the height is reduced during rolling, so does the cross-sectional area of the stock.

Definition of Rolling Process

Rolling is the most important metal forming process. More than 95% of ferrous and non-ferrous metals and alloys are processed to their usable shapes by rolling. Usable shapes of rolled metals are plate, sheet, strip, foil, different sections like rail, beam, channel, angle, bar, rod, and seamless pipe, etc., as shown in Fig. 1.1 and Fig. 1.2.

In the rolling process, permanent deformation is achieved by subjecting the material to high compressive stress by allowing the material to pass through the gap between two rotating cylindrical rolls.

The rolls may be flat or grooved, and are kept at a fixed distance apart from each other. The rolls are rotated in opposite direction by means of electrical drive system (motor, gearbox, spindle and couplings).

Depending on the direction of rotation of the rolls, the input material enters the gap between the rolls from one end and comes out from the other end with a reduced cross-section, the roll gap area being kept less than the cross-sectional area of the input material (rolling stock). For obtaining the desired final shape of rolled material, it is generally necessary to pass the material through the rotating rolls several times. During each of the passes, the roll gap is adjusted by bringing the two rolls closer to each other, or by allowing the material to pass through different set of roll gaps with diminishing cross sectional area.

The entire assembly of the rolls mounted on bearings is held in bearing blocks (called chocks), which in turn are held between the gaps of two cast frames (called housings), complete with roll gap adjustment facilities and roll driving arrangement. The entire set up is called a rolling mill stand. One or more number of rolling stands in combination with other necessary and related equipment to obtain finished rolled products from one or similar group of input materials is called a rolling mill or rolling plant.

Rolling process can be classified based on various conditions/methods employed in rolling.

Organization of this book is somewhat different from a normal textbook involving machines and equipment, in the field of mechanical engineering. Generally, stress is given to the theories and principles involved and the processes are explained to a great detail. There is no doubt that these are essential for an engineer. But it is equally important for an engineer to know the basic design, working principles and operations of the various machines and equipment which are used in the practical field for conversion of the raw materials into desired products. Baring the subject of ‘Machine Tools’, in most other fields of manufacturing processes, available textbooks seem to be rather miserly in thorough discussions on the description, design, working principles of various machines and systems involved and practices followed in actual operation.

An attempt has been made to bridge this gap by introducing laboratory exercises and workshops along with industry visits, in the engineering curricula. More often than not these prove to be inadequate. It is next to impossible that an equipment or machine like a turbine, an extrusion press or a rolling mill can be installed in an academic institution. By observing the operation of a machine or system during a visit to an industrial plant, definitely a lot can be learned about the manufacturing process, but seldom can it give an idea about the working principles of the various mechanisms, their design details or about the intricacies of operational practices.

While teaching ‘Manufacturing Technology’ to MTech students at the National Institute of Technical Teachers’ Training and Research (NITTTR), Kolkata, I observed this shortcoming in my students and cherished the desire to bridge the gap as soon as possible. With 30 years’ experience in the industry in design, development and commissioning to operation plant and machinery (out of which more than half the period was in the field of Rolling Mills), I decided to write this book titled Principles and Applications of Metal Rolling.

In chapter 1, the process of rolling has been discussed along with the definitions and meaning of different terms used in the field of rolling technology. The different types of rolling stands that have evolved over time with their specific design features and uses have also been discussed.

In chapter 2, the mechanics of the rolling process has been discussed. Theoretical analysis of the rolling process in roll bite has been carried out to estimate the mean specific pressure necessary to be exerted on the metal in order to have the rolling with desired reduction possible. The role of the static yield stress of the metal and the effect of various other rolling conditions on this yield stress have also been discussed so that the actual rolling force involved can be worked out. Estimation of the rolling force along with torque and power necessary for rolling are the basic data for selection of the rolling stand to do a particular rolling operation.

In this chapter, the rolling practices generally followed by rollers have been discussed. For producing one or a group of finished rolled products (of different metals/alloys, shapes, sizes, quantities, etc.) the choice of mill stand design, its layout, rolling methods and techniques, pass sequence and schedule, etc., followed by a roller, is generally the meaning of the term rolling practice.

It is important to understand that the various rolling practices have been developed based on practical experiences of many rolling practitioners over ages. Still there exists a lot of variation between practices followed by different rollers and rolling plants. For producing same or similar product mix, different plants may be having different rolling equipment and / or different equipment layout. To suit the shape and size of the input materials and the type and layout of equipment, the plants are following different alternative rolling methods and pass schedules.

In this chapter effort has been made to explain some of the widely accepted practices for rolling of common products and provide background information on the principles and applications of roll pass design for the products.