2 February 2016

What is rolling process? | Working principle | Types

The rolling process is a very economical process for producing large volumes of material with a constant cross-section. It is one of the most important and widely used industrial metal forming operations providing the final product with high production and close control.

Working principle of rolling

Rolling is a process where the metal is compressed to reduce its cross-sectional area between two rotating rolls. This is one of the most widely used of all the metalworking process, because of its higher productivity and low cost. Rolling would be able to produce components having a constant cross-section throughout its length. Many shapes such as L, I, T are possible, but not very complex shapes. Special sections such as railway wagon wheels can also be produced by rolling individual pieces.

Working principle of rolling
Working principle of rolling 

Roll passes 

The final rolled products such as plates, flats, sheets, rounds and sections are obtained in a number of passes starting from billets or slabs. The roll passes sequence can be broadly classified into three types. 
  • Break down pass :
There are used for reducing the cross-sectional area nearer to what is desired. This is the first pass of the sequence. 
  • Roughing pass :
In this pass also, the cross-section gets reduced, but along with it, the shape of the rolled material comes nearer to the final shape. 
  • Finishing pass :
This is the final pass, which gives the required shape of the rolled section. Generally, the finishing pass follows a leader pass. 

Rolling arrangement 

The arrangement of rolls in the rolling operation is also called a rolling stand. It varies depending on the application. The different possible configuration is presented below. 

Rolling arrangement
Rolling Arrangement

The first one is the two high non-reversing rolling stand arrangement, is the most common arrangement. In this, the rolls always move in only one direction. Another 2-high rolling stand arrangement is the same but the direction of roll rotation can be reversed. 

The three high rolling mill is used for rolling of two continuous passes in a rolling sequence without reversing the drives. 

A four-high rolling mill is essentially a two-high rolling mill, but with small-sized rolls. The other two rolls are backup rolls for providing the necessary rigidity to the small rolls. A better backup with a cluster arrangement of rolls can be provided to the small rolls. 

Generally, the rolls are made of chilled cast iron, carbon steel and alloy steel. The smaller rolls may be made of hard materials like tungsten carbide.

Types of the rolling process 

The rolling process mainly divided into two types 

  • Hot Rolling 
  • Cold Rolling 

Hot Rolling 

Hot rolling is a process of milling that involves rolling the steel at a high temperature (usually above 1700°F) above the recrystallization temperature of the steel. It can be easily shaped and formed when steel is above the recrystallization temperature, and the steel can be made in much larger sizes. 

At the time of scaling, the major problem occurred. During the hot rolling process, no dimensional precision is maintained for hot rolled steel. 

Hot rolling can improve the processing performance of metals and alloys because coarse grains during foundry are broken, the cracks are healed, casting defects are reduced and eliminated hence, cast microstructure is transformed into a deformed structure to improve processing properties is the main advantage. The hot-rolled steel of various sections has residual stress caused by uneven cooling that has some influence on the performance of steel member under external force. Such as stability, deformation, fatigue and other aspects may have adverse effects is the main disadvantage. 

Cold Rolling 

Cold rolling is a process that introduces the sheet metal or strip stock between rollers and then compresses and squeezes it. The cold-rolled sheet can be manufactured under different conditions such as skin-rolled, quarter hard, half-hard, full hard depending on how much cold work has been performed.

In a cold rolling metal passes through rollers at temperatures below its recrystallization temperature so that increases the yield strength and hardness of the metal.

Commonly cold-rolled products include strips, sheets, bars and rods that are usually smaller than their counterparts that are hot rolled.

This process actually increases strength by up to 20 per cent through strain hardening and good dimensional accuracy and surface finish are the main advantages of cold rolling. There will be residual stress on the cross-section, which will affect the buckling resistance of the steel is the main disadvantages.