Hot Rolled Steel vs Cold Rolled Steel

The main difference between hot rolled steel and cold rolled steel is in the production process and the resulting properties of the finished products. In both processes (hot and cold), steel is rolled between two or more rollers, which compress the metal to the desired thickness.

Hot Rolled Steel

Hot rolling, the process used to make hot-rolled steel, involves rolling steel at high temperatures (usually over 926°C (1700°F)). These high temperatures are above the recrystallization temperature of steel, allowing the steel to be easily formed and made into larger sizes. As it cools, hot-rolled steel tends not to shrink evenly, giving less control over the size and shape of the final product compared to cold-rolled steel.

The hot rolling process begins with a large, rectangular sheet of metal called a billet. First, the billet is heated and compressed into a large roll. While still hot, it passes through a series of rotating rollers to obtain the desired dimensions. In sheet steel production, the rolled steel is then coiled and cooled. In manufacturing processes with other shapes, the processed material is cut into the specified units and packaged.

Advantages of hot rolled steel

Hot rolled steel offers many advantages..

Cost-effective: In general, hot rolled steel is cheaper than cold rolled steel. No additional steps are required to process hot rolled steel, making it cheaper.
Ductile: Because hot-rolled steel is processed at a high temperature, it can be formed into any shape. Because it is also cooled at room temperature, hot-rolled steel is normalized, meaning there are no internal stresses that can damage the steel during quenching or hardening.
Less time: Ease of use has its advantages. Working with hot-rolled steel often takes less time because fabricators can easily manipulate it.

Disadvantages of hot rolled steel

Less Accurate Sizes: Hot-rolled steel is less accurate in terms of dimensions than cold-rolled steel.
Inferior Surface Finish: Hot-rolled steel has a scaled surface finish that is not as smooth as cold-rolled steel.
More Brittle: Hot-rolled steel is more brittle than cold-rolled steel and is not ideal for projects requiring high strength.

Applications of hot rolled steel

Because hot-rolled steel shrinks slightly during cooling, there is less control over its final shape. Therefore, it is usually used in applications that do not require extremely tight tolerances, such as..

Agricultural machinery
Building materials (e.g., I-beams)
Automotive parts (such as frames and rims)
Railroad equipment (e.g., rails and wagon parts)

Cold rolled steel

What about cold-rolled steel is different from hot-rolled steel? It starts with how cold rolled steel is made. Cold-rolled steel is hot-rolled steel that has undergone additional processing. This type of steel goes through additional steps that increase strength and improve surface finish.

Cold rolling does not require the heat to be increased. As the name implies, cold rolling is done at or near room temperature. Once hot-rolled steel has cooled, it is rolled again at room temperature to achieve a specific thickness. The metal goes through rolling several times, creating a flat sheet.

Cold-rolled steel has different properties than hot-rolled steel. The cold rolling process increases tensile strength and makes the metal harder.

Advantages of cold rolled steel

Compared to hot rolled steel, cold rolled steel offers several advantages, including..

Greater strength: Cold-rolled steel can be up to 20% stronger than hot-rolled steel, making it more suitable for use in high-pressure applications.
Better surface finish: Parts and products made from cold-rolled steel generally have a smooth and shiny surface that is free of rust and scale.
Higher precision: Unlike hot-rolled steel, cold-rolled steel does not shrink after the forming process. This quality makes it possible to make highly accurate parts that require little to no secondary machining.

Disadvantages of cold rolled steel

More Expensive: Cold-rolled steel is more expensive because of the additional processing required to make it.
Residual stress: There is no thermal plastic compression during the deformation process, but there is still residual stress in the cross section, which will inevitably affect the global and local buckling characteristics of the steel.
Machining: Cold-rolled steel is more difficult to machine, so it may take longer to complete a project. Additional metalworking can cause cold-rolled steel to warp. The steel must be stress-relieved before cutting or welding.
Less Flexible: Cold-rolled steel is not as easy to form as hot-rolled steel. The cold-rolled profile is usually an open profile, so the free torsional stiffness of the profile is low. Torsion occurs easily upon bending, and torsional bending occurs easily upon compression, and the torsional resistance is poor.

Uses for cold rolled steel

Metal furniture, household appliances and automotive parts are usually made from cold rolled steel. It is the best option for technically precise projects or projects where aesthetics are a priority. Cold-rolled steel provides a superior finish and precise dimensions. Sheets, bars or tubes are uniformly sized with well-defined edges.

Properties of Hot rolled steel & cold rolled steel

After studying the forming process and different properties, the relationship between hot and cold-rolled steels is probably clear. The mechanical properties are summarized in the table below.

Properties	Hot Rolled Steel	Cold Rolled Steel
Tensile Strength	67,000 psi	85,000 psi
Yield Strength	45,000 psi	70,000 psi
Elongation in 2”	36	28
Reduction of Area	58	55
Brinell Hardness	137	167

Weldability

The weldability of hot- and cold-rolled steel varies considerably due to several factors, mainly the type and quality of steel, the welding process and conditions.

Hot-rolled steel is considered more weldable than cold-rolled steel because it usually has a lower carbon content and flows more easily during welding.

However, hot-rolled steel can also have internal stresses and irregularities that can affect weldability and may require preheating or other measures to reduce the risk of cracking or deformation.

Cold-rolled steel often has a higher carbon content and a harder, more brittle surface that makes it more difficult to weld than hot-rolled steel. But the uniformity of composition and structure of cold-rolled steel makes it easier to predict and control the welding process.

In general, the weldability of both hot- and cold-rolled steels depends more on the specific type and grade of steel than on the initial forming process.

Proper selection of welding materials and techniques, including preheating, post-weld heat treatment and use of proper filler materials, can improve the weldability and overall quality of both steels.

Note that cold-rolled steel loses its work hardening at the weld points, which can significantly affect overall structural properties.

In summary, hot-rolled steel is best suited for applications where strength and cost effectiveness are the top priorities, while cold-rolled steel is ideal for applications requiring precise dimensions and a smooth surface.