Metalworking technologies take on many shapes and forms throughout the manufacturing industry. For instance, manufacturers rely on rolled steel products for a multitude of industrial and consumer goods, as well as for various uses on construction sites. Understanding the different types of mechanical deformation used in their manufacture can be valuable when it comes to selecting the best materials for the job at hand.
The Cold Rolling Process
The first step in the production process of cold rolled steel involves cooling hot rolled steel to as close to room temperature as possible. This is so that the steel remains below the recrystallization temperature, where the formation of new grain structures within the metal comes with a corresponding increase in ductility and loss in overall strength.
Next, the material undergoes a series of shaping operations to deform it into the desired shape. As with hot rolling, these shaping operations are carried out through a rolling mill with two or more sets of vertically stacked rollers. There are multiple types of mills in use, all which use heavy amounts of compression to further deform the steel into its final dimensions:
- Reversing mill – Equipped with up to 2 rollers, these mills reverse the direction of the metal strip after each pass until the final thickness is achieved.
- Continuous tandem mills – These mills are equipped with up to 6 rollers, with each one progressively reducing the metal strip's thickness until the desired results are achieved.
- Sendzimir mill – This unique design features two work rollers surrounded by a cluster of intermediate rollers. This allows an extremely high amount of force to be exerted onto the metal strip without damaging the work roller or the product. Sendzimir mills are commonly used on high-carbon and stainless steels.
The resultant deformation effectively changes the underlying structure of the steel, hardening it as it achieves its final dimensions. The steel may undergo occasional heat treatment to prevent stretcher-strain marks and other negative effects for cold rolling from occurring.
How It Differs from Hot Rolling
The cold rolling process is typically performed after the steel has undergone hot rolling. While the two processes are similar in a wide variety of ways, there are plenty of notable differences to consider:
- Cold rolling occurs below the recrystallization temperature. This results in a metal that's less ductile than its hot-rolled counterpart, but offers greater strength and durability. Such metal can be used in applications that require large amounts of deformation without losing inherent strength.
- In comparison, the hot rolling process reworks the metal above its recrystallization temperature, allowing the metal's underlying grain structure to become more uniform. This improves the metal's ductility, although at a cost of overall strength.
- In most cases, the only heat that comes from the cold rolling process, save for the occasional annealing to prevent defects, is the frictional energy given off by the rollers as they reduce the steel material's thickness.
Another benefit of the cold rolling process is that it can be used to form thinner steel from hot rolled strips. As metallurgist Satyendra Kumar Sarna notes, hot rolled steel strips ranging from 1.5 millimeters to 5 millimeters in thick can be rendered as thin as 0.12 millimeters when put through the cold rolling process. The ability to create thinner yet stronger metal sheeting allows for a multitude of applications where durability plays an important role.
Uses for Cold Rolled Steel
Cold rolled steel's unique properties make it highly valuable for a broad range of industrial and consumer uses. Cold rolled steel is commonly used in the manufacture of appliances, shelving, lawn mowers and water heaters, thanks to its high strength. Metal furniture is also commonly made from cold rolled steel.
The cold rolling process is also a boon for industrial and construction applications. Products ranging from hinges, fasteners and lighting fixtures to tubing, sheet metal and steel drums can all trace their genesis from cold rolled steel.