The purpose of rolling is to reduce the cross section and increase the length of an ingot. The ingot passes through two rolls which are rotating in opposite directions and at the same peripheral speed. The spacing between the rolls can be adjusted as per the thickness of ingot required.
Types of rolling:
Hot rolling: The hot rolling is accomplished at a temperature above the recrystallization temperature of the metal. The rolling results in improvement of various mechanical properties of metal such as strength, toughness and ductility. The coarse grain structure is converted into a fine grain structure. Various sections sheets, rails and bars are obtained through rolling. The desired shape is not obtained in single pass so metal has to be rolled again and again till the required shape is obtained.
Cold rolling: The purpose of cold rolling is to provide a good surface finish to the previously hot rolled metal. It provides the close tolerances and improves their toughness and hardness. The cold rolling requires much higher pressure than hot rolling.
Types of rolling mills:
Two high rolling mills: It consists of two horizontal rolls rotating in opposite directions. The ingot passes in between the rolls. The spacing between the rolls can be adjusted by lowering or raising the upper roll.
Three high rolling mills: It consists of three horizontal rolls placed one over the other. Two pieces can pass through the mills simultaneously. The first is fed between the upper roll and middle roll, moves in one direction. The second is fed in between the middle roll and lower roll, moves in reverse direction. This arrangement results in higher production rate.
Four high rolling mills: It consists of four horizontal rolls (2 larger diameters, 2 smaller diameters). The smaller diameter rolls are called working rolls while the two having smaller diameter are called back up rolls. The working rolls provide the rolling pressure over the metal. The backs up rolls prevent the deflection of working rolls.
Cluster mills: It consists of two working rolls (smaller diameters) and four back up rolls (larger diameters). One working roll is in contact with the two back up rolls.
Continuous rolling mill: Numbers of two high mills are arranged one after the other. It gives a faster production rate.