Hot rolled steels (also known as hot worked steels) are formed when raw steel is rolled through a compressive force at a temperature of above 926°C (the known recrystallization temperature for most types of metal) to form a new shape. The hot rolling process alters the thickness of the steel and/or changes the cross-sectional area and shape. Both hot and cold rolled steel go through the initial hot rolling process. The development of the modern rolling mill process is attributed to the British ironmaster Henry Cort in the late 18th century. Today, there are several hot rolling processed used for steel, among them controlled, ring and profile rolling.
When determining whether steel is hot rolled or cold rolled, there are three characteristics to observe. Hot rolled steel typically have a scaled surface, as opposed to cold rolled steel, which is smoother. This scaled appearance is the result of the cooling process from a temperature over 926°C. The edges and corners of hot worked steel bars are rounded and not precise. Finally, the surface of the steel product is never oily. Hot rolled steel is typically formed in four ways: flat, long, seamless or specialty products.
Once the desired metals for the alloy have been melted together in a furnace, they are formed into a billet – a large, rectangular piece of metal. This is then pre-processed and flattened at high speeds using one of several hot rolling processes. The desired temperature depends on the alloy, but it must be hot enough to facilitate recrystallization while being cool enough to prevent liquifying. The temperature is controlled constantly throughout the process. The metal is then cooled and prepared for distribution. Hot rolling processes most often result in sheets, cross sections or tubular steel.
Hot rolled steel products are perfect for applications in which the shape and tolerance precision is not important enough to warrant the use of cold rolled steel, which requires extra processing and is therefore more expensive. Typical structural applications using hot worked steels include railroad tracks, vehicle frameworks and wheel rims, machine structures, pipes, cylinders, guardrails and springs.