What is Metal Rolling? Metal rolling or more commonly "rolling" is the process of shaping metal by passing it through "rollers" or "rolls" held at a controlled distance or separation. Rolling is an efficient process, and in terms of tonnage is the dominant method of working most metals into items that a person is likely to encounter.
Rolling is a mechanical process that refines the grain, the micro structure, and the finish of the metal that is rolled. Rolling is an incremental process. Each time the metal is passed through a set of rollers or rolls, which is referred to as a pass, only a small amount of deformation occurs. To achieve the reduction from an ingot to a strip or foil, it is not uncommon for a rolling operation to consist of 10, 20 or more passes to complete each of the many transformations or rolling cycles. From start to finish metal might be rolled 5, or 10, or more times, it all depends on the finished product. The incremental nature of the process is a key advantage of the rolling operation. Since each pass handles only a fraction of the work, the passes can be placed very close together, and operate at high speeds allowing the rolling operation to be very productive.
The basic design of a roll pass or stand consists of two rolls made of hardened and polished cast steel with parallel axes of rotation and a mechanism to set or change the separation between the rolls. In reality a roll stand would consist of 'work rolls' that contact the metal, and backing tolls that provide additional rigidity and stiffness to the work rolls. In operation the work rolls are set a distance apart that is slightly closer than the thickness of the starting material. When the material is brought into contact with the rotating rolls friction grabs the metal and drags it into the gap. The metal exerts a separating force on the rolls which is resisted be the strong frame of the roll stand, and the backing rolls. The pressure causes the metal to deform and elongate into the gap between the rolls. This deformation increases the length and decreases the thickness of the metal. The width of material also increases somewhat during rolling, but to a much lesser extent.
The dynamics of the rolling operation can be explained with the help of a velocity diagram. Although the peripheral velocity of the roll is constant the material being rolled has two different velocity profiles, the entry and the exit. It can be shown that the roll and the work are in a state of slipping contact in the direction of rolling, but that essentially no delta-V exists in the direction of increasing width.
Rolling is divided into two major categories with the topmost division being 1) hot rolling, and the other being 2) cold rolling. The definition of hot or cold is relative. The cold rolling of steel could occur at a temperature higher than the hot rolling of an exotic metal like lithium or a common metal like aluminum or tin or lead
1) Hot rolling occurs when the metal is heated sufficiently to cause it to lose an appreciable portion of its inherent strength. This will typically require a temperature above a given "transition temperature" specific to the metal being rolled. Hot rolling is significant in that more total tonnage of intermediate and finished products are produced by hot rolling than any other metal forming process. The benefits of hot rolling include the increased amount of deformation of the metal in each pass, and reduced power required to deform the metal. The disadvantages of hot rolling are that furnaces are required to heat the metal, and oxidation or scaling of the metal can reduce the quality of the finished surface.
2) Cold rolling is the process when the metal is rolled at a temperature below a specific transition temperature (contrast to hot rolling) that is specific to the metal being rolled. But cold rolling can and is performed at temperatures well above ambient temperatures. Cold rolling is often performed as the final operation to a previously hot rolled metal intermediate, because all of the benefits of the cold rolling can be imparted to the metal in the last few percent of the total working operations. This results in great savings. As you might expect, when metal is cold rolled, it is mush stronger than when it is hot rolled, and so cold rolling machined must employ stronger rolls, motors, bearings, and frames than hot rolling machines designed to handle metal of the same size. Cold rolling is one of the best ways to impart a smooth and lustrous finish on a metal product. Anyone that has seen a fresh sheet of aluminum foil knows how shiny it is.
Rolling starts with a relatively short and thick starting material, known as an ingot. Ingots are metal castings that are the first step of the metal refining or recycling process. Ingots are invariably rolled hot, either directly following the casting process or after a reheating operation.
After the first set of passes of the ingot through the rolls it is termed either a bloom or a slab. Blooms and slabs are the heaviest sections that are produced from the initial reduction of an ingot by rolling. Blooms are made by rolling the ingot and rotating it 90 degrees and rolling again along the same long axis. Slabs are the result when the rotation is 90 degrees and the metal is rolled on the same flat surfaces with the long axis perpendicular to the initial travel through the mill. Blooms and slabs are commonly in excess of a foot or two thick and/or a meter or two wide, they may be allowed to cool or may be conveyed hot to the next stage of rolling.
When rolling is used to produce intermediate or semi-finished metal goods, there are 4 general divisions that we see regarding how the rolled metal is delivered.
a) When steel is destined to produce plates, the intermediate form is the slab. Since each pass of rolling only serves to increase the length, slabs are repeatedly turned and rolled until the form is correct to produce a plate or the required finished width. Plates are categorized by low length to width ratio. The width of a plate is limited by the length of the rollers that the plate passes through.
The longer the rolls the larger in diameter they must be to avoid bending, and the more expensive the mill becomes, and that is what usually limits the width of plate products. The width of a rolled plate is controlled by turning the stock perpendicular to the first pass by 90 degrees along the long axis and rolling the same flat, unlike a bloom or billet where the long axis is maintained. Once the width is determined the length of the plate is limited by the mass of the ingot the mill started out rolling.
b) When straight items are the finished product, like rails, beams, bars, etc, the bloom is passed through the rolls and then repeatedly rotated 90 degrees always keeping the long axis the same. This rolling and rotating causes the length of the bloom to increase. After sufficient rolling and turning. as required to refine it structure and to achieve the correct width for the finishing operation, the bloom is known as a Billet. This distinguishes it as being a more refined product than the bloom, just as the bloom is more refined than the ingot.
Lengths of special cross section such as railroad track, I-beams, angle iron, and a slew of structural shapes are also rolled using the hot-rolling technique. Since the cross sections of these shapes would be damaged by bending the section, these rolled metal products are delivered in uniform lengths from the mill. Each finished piece is cut off from a longer starting shape or master length which the rolling machine processes. US railroad track has a traditional length of 39 feet.
c) When the thickness, or thinness, or the finished product warrants, the bloom is directed to a strip mill. In the strip mill the bloom or billet is reduced sufficiently so that the material may be conveniently wound into a compact coil. Coils can be enormous, containing tens of thousands of feet of material and weighing several tens of tons. Strip is further reduced to sheet and foil, the distinction being arbitrarily based on the final thickness an on the metal being considered.
d) Roll forming is a process where a flat coil that may be known as sheet or strip, is passed through rollers whose purpose is to bend and deform the metal rather than reduce its thickness or change its length. An example familiar to most people is residential rain gutters. These are generally U shaped channels with a few ridges and flats to improve their appearance and conceal the sharp edges. Gutters are roll formed from coils of flat strip by passing the metal through a series of roller pairs each with a slightly different shape. as the metal advances through the rollers it is forced to bend in very controlled locations by the rollers and it takes on a permanent set. after 8-12 pairs of rolls or passes, the gutter emerges from the machine and is cut to the desired length. Since this type of rolling is considered a finishing process and requires very little power, it is almost invariably a cold rolling operation. Heavier parts like equipment rack rails with features such as periodic holes are also produced by roll forming.
To summarize, blooming and slabbing operations are always performed as hot rolling operations. Billets and plates too are started by hot rolling, and may add a final pass in a cold reduction mill to impart desirable physical characteristics and superior surface finish. Strip mills also begin with hot reductions, however strip is commonly finished as a cold rolling operation. And roll forming is typically conducted stone cold.
Metal Rolling is both an art and a science with centuries of tradition, and decades of scientific analysis. While is process requires significant capital infrastructure to perform, the process is so efficient that rolled metal products are produced for pennies per pound. When contemplating a metal forming operation it is safe to say that with regards to metal rolling...
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