What is Hot Rolled Steel?

Metal rolling replaced the traditional hammer-and-anvil approach of pre-industrial metalworkers and blacksmiths, allowing them to create metal alloy sheets with significantly reduced labor requirements. As the technology enhanced, this processing method became the benchmark for mass production of high-quality steel for almost any application. The end-product from rolling heated steel alloys is known as hot rolled steel.

Outlining Hot Rolled Steel

Hot rolled steels are defined by their processing method as opposed to their qualities, characteristics, or composition. They are manufactured by heating semi-finished casting products such as ingots or billets to temperatures above 1700°C (3092°F), which exceed the recrystallization points of most alloys. These casting products are usually heated through electromagnetic induction or in a combustion-fuelled soaking pit. When the alloy exceeds its recrystallization temperature, its grains deform into a novel crystallographic structure that displays reduced strength and increased ductility.

These elevated temperatures must be maintained as the billet is fed through the rolling mill or casting apparatus. Hot rolled steel is kept at refractory temperatures until the product is considered finished. If the temperature is allowed to drop below the alloy’s recrystallization point, it can harden and cause significant mechanical defects in the end-product. Billets that have been allowed to cool between rolling or forming must be reheated and re-rolled to ensure the end-product hot rolled steel is suitably defect free with the requisite mechanical characteristics.

Advantages of Hot Rolling

Hot rolling is one of the preferred methods for cost-effective steel forming due to its simplicity. Once the hot rolled steel is considered finished, it is cooled at ambient temperatures as opposed to quenching in a liquid bath. This negates the generation of internal stress from artificial hardening, enabling the manufacture of high-yield steels in fewer stages.

The converse of this is that hot rolled steels exhibit lower dimensional tolerances due to uncontrolled shrinking during the cooling process. This is not usually an issue as hot rolled steel products are typically applied in areas where dimensional precision is not critical.

Applications of Hot Rolled Steel

Hot rolled steel is ubiquitous in the steel industry. It is one of the most common forming methodologies and can be used to fabricate sheet metal or hot rolled steel angles for a variety of construction applications.

Difference Between Hot Rolled Steel and Cold Rolled Steel

Customers often ask us about the differences between hot rolled steel and cold rolled steel. There are some fundamental differences between these two types of metal. The differences between hot rolled steel and cold rolled steel relates to the way these metals are processed at the mill, and not the product specification or grade. Hot rolled steel involves rolling the steel at high temperatures, where cold rolled steel is processed further in cold reduction mills where the material is cooled followed by annealing and/or tempers rolling.

Hot Rolled Steel

Hot rolling is a mill process which involves rolling the steel at a high temperature (typically at a temperature over 1700° F), which is above the steel’s recrystallization temperature. When steel is above the recrystallization temperature, it can be shaped and formed easily, and the steel can be made in much larger sizes. Hot rolled steel is typically cheaper than cold rolled steel due to the fact that it is often manufactured without any delays in the process, and therefore the reheating of the steel is not required (as it is with cold rolled). When the steel cools off it will shrink slightly thus giving less control on the size and shape of the finished product when compared to cold rolled.

Uses: Hot rolled products like hot rolled steel bars are used in the welding and construction trades to make railroad tracks and I-beams, for example. Hot rolled steel is used in situations where precise shapes and tolerances are not required.

Cold Rolled Steel

Cold rolled steel is essentially hot rolled steel that has had further processing. The steel is processed further in cold reduction mills, where the material is cooled (at room temperature) followed by annealing and/or tempers rolling. This process will produce steel with closer dimensional tolerances and a wider range of surface finishes. The term Cold Rolled is mistakenly used on all products, when actually the product name refers to the rolling of flat rolled sheet and coil products.

When referring to bar products, the term used is “cold finishing”, which usually consists of cold drawing and/or turning, grinding and polishing. This process results in higher yield points and has four main advantages:

• Cold drawing increases the yield and tensile strengths, often eliminating further costly thermal treatments.
• Turning gets rid of surface imperfections.
• Grinding narrows the original size tolerance range.
• Polishing improves surface finish.

All cold products provide a superior surface finish, and are superior in tolerance, concentricity, and straightness when compared to hot rolled.

Cold finished bars are typically harder to work with than hot rolled due to the increased carbon content. However, this cannot be said about cold rolled sheet and hot rolled sheet. With these two products, the cold rolled product has low carbon content and it is typically annealed, making it softer than hot rolled sheet.