Principles of Metal Rolling

Metal rolling is one of the most important manufacturing processes in the modern world. The large majority of all metal products produced today are subject to metal rolling at one point in their manufacture. Metal rolling is often the first step in creating raw metal forms. The ingot or continuous casting is hot rolled into a bloom or a slab, these are the basic structures for the creation of a wide range of manufactured forms. Blooms typically have a square cross section of greater than 6x6 inches. Slabs are rectangular and are usually greater than 10 inches in width and more than 1.5 inches in thickness. Rolling is most often, (particularly in the case of the conversion of an ingot or continuous casting), performed hot.

Most metal rolling operations are similar in that the work material 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 therefore 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, lubricants can help with this. A basic flat rolling operation is shown in figure:130, this manufacturing process is being used to reduce the thickness of a work piece.

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 here that 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 will be 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 termed 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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