Hot rolling vs. cold rolling

Before moving on to cold rolling, the metal is ‘pickled’, which means the scale formed by hot rolling is removed from the surface of the metal, otherwise it would interfere with the process.

Cold rolling takes the hot rolled product and processes it further. After hot rolling, the steel is cooled to room temperature, then passed through cold rollers at a temperature lower than its recrystallisation temperature. This rolling process is called annealing and relieves stress and leads to a higher yield strength and greater hardness. This is due to reorientation of the grain and the creation of flaws in the crystal structure, leading to microstructure hardening.

The cold rolling mill is usually fitted with thickness gauges that check the steel as it comes out of the rollers. Reversing mills are designed so that the steel can be reversed and pushed back through the rollers which are brought closer together each time until the desired thickness is achieved. Multi-stand mills have three to six pairs of rollers in a series, each pre-set to reduce the thickness by a certain percentage until the final thickness is reached.

Usually each pass will reduce the thickness between 50 to 90%. Due to the high compression forces and friction, the temperature of each pass has the potential to reach as high as 250 °C. Therefore a cooling agent must be used to keep the rollers and metal cold and lubricated. Oil or water are usually used for this purpose.

The resulting metal typically has a thickness of between 0.12 to 2.5 mm. Due to the thinness of cold-rolled steel, it is used for applications such as drinks cans or lightweight vehicle panels, light aircraft etc.

Modern, efficient cold rolling mills can roll pickled sheets or strips at speeds of up to 4 m/s and tandem mills that are continuously working can produce up to 2.5 million tons of cold rolled steel annually.