【正文】 ting the gap, due to the pression rollers reduce the material cross section, the length of processing methods to increase the pressure. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is termed as cold rolling. There are many types of rolling processes, including flat rolling, foil rolling, ring rolling, roll bending, roll forming, profile rolling, and controlled rolling.1. temperature Hot rollingHot rolling is a metalworking process that occurs above the recrystallization temperature of the material. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. The starting material is usually large pieces of metal, like semifinished casting products, such as slabs, blooms, and billets. If these products came from a continuous casting operation the products are usually fed directly into the rolling mills at the proper temperature. In smaller operations the material starts are room temperature and must be heated. This is done in a gas or oilfired soaking pit for larger workpieces and for smaller workpieces induction heating is used. As the material is worked the temperature must be monitored to make sure it remains above the recrystallization temperature. To maintain a safety factor a finishing temperature is defined above the recrystallization temperature。 this is usually 50 to 100 176。C (122to 212176。F) above the recrystallization temperature. If the temperature does drop below this temperature the material must be reheated before more hot rolling. Hot rolling is used mainly to produce sheet metal or simple cross sections, such as rail tracks. Cold rollingCold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the strength via strain hardening up to 20%. It also improves the surface finish and holds tighter tolerances. Commonly coldrolled products include sheets, strips, bars, and rods。 these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the workpieces and their greater strength, as pared to hot rolled stock, fourhigh or cluster mills are used. Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass.Coldrolled sheets and strips e in various conditions: fullhard, halfhard, quarterhard, and skinrolled. Fullhard rolling reduces the thickness by 50%, while the others involve less of a reduction. Quarterhard is defined by its ability to be bent back onto itself along the grain boundary without breaking. Halfhard can be bent 90176。, while fullhard can only be bent 45176。, with the bend radius approximately equal to the material thickness. Skinrolling, also known as a skinpass, involves the least amount of reduction: %. It is used to produce a smooth surface, a uniform thickness, and reduce the yieldpoint phenomenon (by preventing Luder bands from forming in later processing). It is also used to breakup the spangles in galvanized steel. Skinrolled stock is usually used in subsequent coldworking processes where good ductility is required. 2. Processes Flat rollingFlat rolling is the most basic form of rolling with the starting and ending material having a rectangular crosssection. The material is fed in between two rollers, called working rolls, that rotate in opposite directions. The gap between the two rolls is less than the thickness of the starting material, which causes it to deform. The decrease in material thickness causes the material to elongate. The friction at the interface between the material and the rolls causes the material to be pushed through. The amount of deformation possible in a single pass is limited by the friction between the rolls。 if the change in thickness is too great the rolls just slip over the material and do not draw it in. The final product is either sheet or plate, with the former being less than 6mm (in) thick and the latter greater than。 however, heavy plates tend to be formed using a press, which is termed forming, rather than rolling.Oftentimes the rolls are heated to assist in the workability of the metal. Lubrication is often used to keep the workpiece from sticking to the rolls. To fine tune the process the speed of the rolls, the roller resistance, and the temperature of the rollers are adjusted. Foil rollingFoil rolling is a specialized type of flat rolling, specifically used to produce foil, which is sheet metal with a thickness less than 200181。m (in). The rolling is done in a cluster mill because the small thickness requires a small diameter rolls. To reduce the need for small rolls pack rolling is used, which rolls multiple sheets together to increase the effective starting thickness. As the foil sheets e through the rollers, they are trimmed and slitted with circular or razorlike knives. Trimming refers to the edges of the foil, while slitting involves cutting it into several sheets. Aluminum foil is the most monly produced product via pack rolling. This is evident from the two different surface finishes。 the shiny side is on the roll side and the dull side is against the other sheet of foil. Controlled rollingControlled rolling is a type of thermomechanical processing which integrates controlled deformation and heat treating. The heat which brings the workpiece above the recrystallization temperature is also used to perform the heat treatments so that any subsequent heat treating is unnecessary. Types of heat treatments include the production of a fine grain structure。 controlling the nature, size, and distribution of various transformation products (such as ferrite, austenite, pearlite, baini