Rolling is a bulk deformation technique used to reduce the cross-section of large ingots or plates with the purpose of producing finished and semi-finished components. This is accomplished by passing the work piece between rotating rolls. Examples include structural steel sections, automotive body sheets, tanks sheets for food and beverages etc. Rolling can be either cold or hot; each process has certain pros and cons.
Hot Rolling :As the name indicates, hot rolling is carried out at elevated temperature- typically, a temperature greater than the recrystallization temperature of the microstructure. As metal softens at high temperatures , hot rolling process requires less power compared to cold rolling- rolling at below crystallization temperature. However, it requires lubricants that must resist the combined action of heat and rolling pressure. Thicker gauges, greater than 6 mm, are mostly hot rolled. Hot rolled steel products are often recognised by scaly, round corner finish and are mostly consumed in welding jobs where dimensional tolerances are not of prime importance.
Cold Rolling:Cold rolling, on the other hand is carried out at less than 0.3 of the absolute melting temperature. Since recrystallization does not occur during cold rolling, a strain hardened microstructure is developed, consisting of plastically deformed grains elongated in the rolling direction. The primary advantages of cold rolled steel are superior surface finish, better dimensional accuracy and improved strength and hardness. Due to smooth surface finish and closer tolerances, cold rolled products are preferred for machining jobs.