Basic of Rollings and Rolling Mills
Metal rolling is an important production process. It has industrial importance among various production processes because of its several advantages such as cost effectiveness, enhanced mechanical properties, flexible operations, higher productivity, and considerable material saving. Rolling is the most widely used forming process, since it provides high production rates and close control of the final product.
Rolling is the process of plastically deforming metal by passing it between rolls. It is the process of reducing the thickness or changing the cross section of a long work-piece by compressive forces applied through a set of rolls rotating in the opposite direction. The rolling operation takes a solid piece of metal and breaks it down successively in several steps into different shapes such as flats, rounds, and sections etc. During rolling, metal is subjected to high compressive stresses as a result of the friction between the roll and the metal surface.
Rolling has existed for hundreds of years. The first rolls were small, hand driven and they were used to flatten gold and silver in the manufacture of jewelry and art. In 1480, Leonardo da Vinci sketched a machine for the rolling of lead for stained glass windows. This was the world’s first available record of a rolling mill, but no evidence exists which shows that the machine was built. By the 1600s, rolling machines, rather than small, hand-driven rolls, were known to have been in operation and iron was just being introduced as a metal capable of rolling. By the late 1700s, the first hot rolling mills appeared, allowing iron to become a more popularly rolled material.
Modern rolling practice is attributed to Henry Cort, who got a patent for use of grooved rolls for rolling iron bars. Henry Cort is also called ‘father of modern rolling’. The first rail mill was established in 1820 while the first plate mill was exhibited in 1851. Three high mills for rolling heavy sections were introduced in 1853. Hot strip mill were developed in America in the first half of the twentieth century.
Basic concept of the rolling
Rolling is a process used to shape metal into a thin long layer by passing it through a gap of two rollers rotating in different directions (clockwise and anti-clockwise). The gap between the two rollers is supposed to be smaller than the thickness of the work piece of the material to be formed. When the metal piece is put between the rollers, it experiences forces of friction and compression from the rollers compressing it to become thin and elongated, or longer than its original length (Fig 1). When the piece completes its way through the gap between rollers, it has lesser thickness than the original one with an increased length and width. This decrease in thickness is referred to draft and the increase in length and width is called an absolute elongation and spread respectively.
The initial materials for the primary rolling mills are ingots or continuous cast slabs, blooms or billets. 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 possesses a non-uniform grain structure, typically consisting of large columnar grains which grow in the direction of solidification. These structures are normally brittle with weak grain boundaries. Cast structure characteristically contains many defects such as porosity caused by gases, shrinkage cavities, and solid inclusions of foreign materials which are trapped in the metal. Rolling a metal above its recrystallization temperature breaks apart the old grain structure and forms a new one. Grain boundaries are destroyed and new tougher ones are formed, along with a more uniform grain structure. The rolling process also closes the vacancies and cavities within the metal. In addition, hot rolling process breaks up the inclusions and distributes the material throughout the work piece.
The process of rolling is a specialized form of metal forming for shaping large bulk material into thin sheets, plates, or different types of cross-sections such as rounds, flats, squares, angles, channels, T-bars, rails, and beams etc. of large lengths. The rolling operation is to ensure the final shape geometry of the work piece being rolled, the uniformity of the material, and the change in property due to the deformation process. Fig 1 shows the concept of rolling of metals.
Most metal rolling operations are similar in that the work piece 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 the draft. 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 hence 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, use of lubricants can help with this.
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 the place where the 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 is 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 point is known as 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.
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