What Is Hot And Cold Rolling? Its Introduction, Production Process, Rolling Mills, Applications

Introduction :-

Rolling is a deformation process in which the thickness of the work is reduced by compressive forces exerted by two opposing rolls. This process is similar to rolling dough with a rolling pin to reduce its thickness. Rolling, which accounts for about 90% of all metals produced by metalworking processes, was first developed in the late 1500s. Modern steelmaking practices and the production of various ferrous and nonferrous metals and alloys now generally involve combining continuous casting with rolling processes. This greatly improves productivity and lowers production costs. Nonmetallic materials also are rolled to reduce their thickness and enhance their properties. Most rolling processes are very capital intensive, requiring massive pieces of equipment, called rolling mills, to perform them. The high investment cost requires the mills to be used fo production in large quantities of standard items such as sheets and plates.
The work starts out as a cast steel ingot that has just solidified. While it is still hot, the ingot is placed in a furnace where it remains for many hours until it has reached a uniform temperature throughout, so that the metal will flow consistently during rolling. For steel, the desired temperature for rolling is around 1200°C. The heating operation is called soaking, and the furnaces in which it is carried out are called soaking pits. From soaking, the ingot is moved to the rolling mill, where it is rolled into one of three intermediate shapes called blooms, billets, or slabs. These intermediate shapes are subsequently rolled into final product shapes. A bloom has a square cross section 150 mm or larger. Blooms are rolled into structural shapes and rails for railroad tracks. A slab is rolled from an ingot or a bloom and has a rectangular cross section of width 250 mm or more and thickness 40 mm or more. Slabs are rolled into plates, sheets, and strips. A billet is rolled from a bloom and is square with dimensions 40 mm on a side or larger. Billets are rolled into bars and rods.

Hot and Cold Rolling :-

Most rolling is carried out by hot working, called hot rolling, owing to the large amount of deformation required. Hot-rolled metal is generally free of residual stresses, and its properties are isotropic. During this phase, the coarse-grained, brittle, and porous structure of the ingot is broken down into a wrought structure having a finer grain size and enhanced properties, such as increased strength and hardness. Disadvantages of hot rolling are that the product cannot be held to close tolerances, and the surface has a characteristic oxide scale.
Further flattening of hot-rolled plates and sheets is often accomplished by cold rolling, in order to prepare them for subsequent sheet metal operations. Cold rolled product has higher strength and hardness and a better surface finish. Cold rolling strengthens the metal and permits a tighter tolerance on thicknes and generally superior to the corresponding hot-rolled product. However, it requires more energy and will result in a product with anisotropic properties.

Applications :-

1. Sheets typically are used for automobile and aircraft bodies, appliances, food and beverage containers, and kitchen and office equipment.
2. Hot-rolled plates are used in shipbuilding, bridges, boilers, welded structures for various heavy machines, tubes and pipes, and many other products.
3. Cold-rolled sheets, strips, and coils used for stampings, exterior panels, and other parts of products ranging from automobiles to appliances and office furniture.
4. Commercial aircraft fuselages and trailer bodies usually are made of a minimum of 1-mm thick aluminum-alloy sheets.
5. Steel sheets used for automobile and appliance bodies are typically about 0.7 mm thick.
6. Aluminum beverage cans are made from sheets 0.28 mm thick.
7. Plates generally have a thickness of more than 6 mm and are used for structural applications, such as ship hulls, boilers, bridges, machinery, and nuclear vessels.
8. Plates can be as thick as 300 mm for large structural supports, 150 mm for reactor vessels, and 100 to 125 mm for machinery frames and warships.

Flat rolling process :-

Flat rolling is a metalworking process that reduces the thickness of a flat metal workpiece by passing it between two rotating rolls. Rotating rolls are powered individually by electric motors.

Various Rolling Processes and Mills :-

Several rolling processes and mills have been developed to produce of product shapes.

Thread Rolling :-

Thread rolling is a cold-forming process by which straight or tapered threads are formed on round rods or wire by rolling them between two dies. It is the most important commercial process for mass producing external threaded components like bolts and screws. Most thread rolling operations are performed by cold working in thread rolling machines. These machines are equipped with special dies that determine the size and form of the thread. The dies are of two types: (1) flat dies, which reciprocate relative to each other and (2) round dies, which rotate relative to each other to accomplish the rolling action.
Production rates in thread rolling can be high, ranging up to eight parts per second for small bolts and screws.
Thread rolling makes stronger threads due to work hardening. It gets smoother surface, and better fatigue resistance due to compressive stresses introduced by rolling. The thread-rolling process is capable of generating other shapes as well, such as grooves and various gear forms.

Ring Rolling :-

In ring rolling, a thick ring is expanded into a large-diameter thinner one. The ring is placed between two rolls, one of which is driven while the other is idle. Its thickness is reduced by bringing the rolls closer together as they rotate. Since the volume of the ring material remains constant during plastic deformation, the reduction in ring thickness results in an increase in its diameter. There are several advantages of ring rolling over alternative methods of making the same parts: raw material savings, ideal grain orientation for the application, and strengthening through cold working. however, dimensional control and surface finish will not be as good as in ring rolling.

Applications of ring rolling include ball and roller bearing races, steel tires for railroad wheels, and rings for pipes, pressure vessels, and rotating machinery. The ring walls are not limited to rectangular cross sections; the process permits rolling of more complex shapes.

Gear Rolling :-

Gear rolling is a cold working process to produce certain gears. The setup in gear rolling is similar to thread rolling, except that the deformed features of the cylindrical blank or disk are oriented parallel to its axis rather than spiraled as in thread rolling. Advantages of gear rolling compared to machining are similar to those of thread rolling: higher production rates, better strength and fatigue resistance, and less material waste.

Rotary Tube Piercing :-

Roll piercing is a specialized hot working process for making seamless thick-walled tubes. Developed in the 1880s, this process is based on the principle that when a round bar is subjected to radial compressive forces, tensile stresses develop at the center of the bar. If compression is high enough, an internal crack is formed. When it is subjected continuously to these cyclic compressive stresses, the bar begins to develop a small cavity at its center, which then begins to grow.
Rotary tube piercing is carried out using an arrangement of rotating rolls. Compressive stresses on a solid cylindrical billet are applied by two rolls, whose axes are oriented at slight angles from the axis of the billet, so that their rotation tends to pull the billet through the rolls. An internal mandrel assists the operation by expanding the hole and sizing the inside diameter of the tube. The terms rotary tube piercing and Mannes mann process are also used for this tube-making operation. Because of the severe deformation that the bar undergoes, the material must be high in quality and free from defects.

Shape Rolling :-

Straight and long structural shapes such as channels, I-beams, railroad rails, and solid bars are formed at elevated temperatures by shape rolling, in which the stock goes through a set of specially designed rolls. Cold shape rolling also can be done with the starting materials in the shape of wire with various cross sections.

Skew Rolling :-

A process similar to roll forging is skew rolling, typically used for making ball bearings. Round wire or rod is fed into the roll gap, and roughly spherical blanks are formed continuously by the action of the rotating rolls. Another method is to shear pieces from a round bar and then upset them in headers between two dies with hemispherical cavities. The balls subsequently are ground and polished in special machinery.

Tube Rolling :-

The diameter and thickness of pipes and tubing can be reduced by tube rolling, which utilizes shaped rolls. Some of these operations can be carried out either with or without an internal mandrel. During the gap cycle on the roll, the tube is advanced and rotated, starting another cycle of tube reduction. As a result, the tube undergoes a reduction in both diameter and Wall thickness. Steel tubing of 265 mm in diameter have been produced by this process.

Roll Forging :-

In this operation, the cross section of a round bar is shaped by passing it through a pair of rolls With profiled grooves. Roll forging typically is used to produce tapered shafts and leaf springs, table knives, and hand tools; it also may be used as a preliminary forming operation, to be followed by other forging processes.