Hot Rolled vs Cold Rolled Steel: Overview and Differences

There are several grades, shapes, specifications, and finishes of steel available. Each of these steel grades has its unique properties. This is why this metal material is commonly used in vehicles, appliances, aerospace parts, electronic parts, and more. The need to understand the various steel types available brings about the hot rolled vs cold rolled steel comparison.

Optimizing the characteristics of your steel for each application is more than changing its chemical composition. Rolling is a process used to improve the shape, uniformity, and mechanical properties of steel materials. There are two main categories of rolled steel — cold rolled steel and hot rolled steel. These rolled steel types have distinct properties, making them suitable for different applications.

It is essential to understand the difference between hot rolled steel and cold rolled steel when choosing them for your project. Such prior knowledge will help you save costs and time while avoiding additional processing. This article describes the difference between hot and cold rolled steel, including their benefits, limitations, and applications.

What is Hot Rolled Steel?

Rolling refers to the specific way the steel material is produced. Hot rolled steel refers to steel produced with extreme heat treatment. That is, the production occurs at extreme temperatures. Manufacturers begin with large, rectangular metals (billets). They then heat the billets before sending them for processing — a stage where they are flattened into large rolls.

The hot-rolling process involves pressing the molten steel at high temperatures of over 926° Celsius (1700 degrees Fahrenheit). Such high temperatures are essential because steel ideally recrystallizes between 750 degrees to 1,300 degrees Fahrenheit, allowing easier forming and reshaping. After rolling the steel through the mill into the desired shape, it is allowed to cool down.

Properties of Hot Rolled Steel

The following properties will help you identify hot rolled steel:

• Scaled surface — cooling from high temperatures leaves remnants on the steel surface to make it look scaly.
• Slight distortions — cooling also produces slightly trapezoidal shapes without perfect angles.
• Corners and edges are slightly rounded — this is a result of shrinkages and less precision in finishing.

Benefits of Hot Rolled Steel

Hot rolled steel offers the following benefits for your applications:

• Lower Cost: The processing of hot rolled steel is much lesser than cold rolled steel, making it cheaper.
• Little to No Internal Stresses: The cooling of hot rolled steel occurs at room temperature, making it essentially normalized. This means that it has little or no internal stresses due to work-hardening or quenching processes.
• Easier Workability: Since the hot rolling process is done at extremely high temperatures, the resulting steel is easier to shape and form. The most common shapes come from hot rolled steel, e.g., UB, UC, RHS, SHS, flats, etc.
• Weldability: Uniform microstructure and ductility of hot rolled steel help to lower residual stresses and improve thermal tolerance, therefore guaranteeing robust, crack-resistant welds. Its low carbon content enhances weldability, although post-weld treatments may still be needed to further optimize performance.
• Applications: It is ideal for applications where tolerance is not the priority.

Drawbacks of Hot Rolled Steel

• Dimensional defects due to expansion during heating and shrinkage/warpage while cooling down.
• It often has a rough texture on the surface that needs to be removed or bugged before any finishing process.
• Slight distortions.

Common Uses of Hot Rolled Steel

As discussed earlier, hot rolled steel slightly shrinks as it cools. This causes manufacturers to have lesser control over the final shape. Therefore, the applications of hot rolled steel are usually those that do not require tight tolerances, including the following:

• Automobile parts, e.g., wheel rims and frames
• Agricultural equipment
• Railway equipment, e.g., tracks and train components
• Construction materials

What is Cold Rolled Steel?

Essentially, cold rolled steel refers to hot rolled steel that has undergone further processing. As mentioned earlier, rolling involves the range of processes involved in forming the steel, including turning, grinding, and polishing. The other operations modify an existing hot rolled steel into a more refined product. The term “cold rolled” essentially applies to steels that have undergone compression.

While making hot rolled steel only involves heating at high temperatures and cooling, cold rolled steel involves an additional process. At the cold reduction mills, the manufacturer cools the steel and re-rolls it at room temperature either by cold roll forming or press-braking. This process helps to achieve desired shape and dimensions.

Properties of Cold Rolled Steel

The following features will help you identify cold rolled steel:

• Smooth surfaces usually have an oily-like touch
• The surface has better finish qualities and tighter tolerances
• Square bars come with well-defined edges
• Tubes often possess better straightness and concentric uniformity

Benefits of Cold Rolled Steel

Here are some of the benefits you can get from using cold rolled steel:

• Better Surface Properties: Parts made with cold rolled steel often have smooth and shiny surfaces void of scale or rust. Thus, making them useful when aesthetics is essential.
• Greater Strength: They are typically stronger and harder (up to 20% greater strength) than hot rolled steel. This makes them useful for high-stress applications.
• Higher Precision: Since cold rolled steel does not shrink while forming, it allows for the fabrication of more precise parts with consistent and accurate shapes.
• Various Surface Finish: It supports an extensive range of surface finishes.

Drawbacks of Cold Rolled Steel

• More expensive due to the additional processing involved.
• Internal stresses occur in the material due to additional treatments leading to unpredictable warping in some cases.
• Fewer shapes are available, e.g., sheets and box section shapes.

Common Uses of Cold Rolled Steel

The ideal applications for cold rolled steel are those requiring better metal surface finishing and tighter tolerances. Examples of such components include the following:

• Aerospace parts
• Mechanical components
• Home appliances
• Rods, bars, strips, and sheets
• Metal furniture structures

Cold Rolled Steel vs Hot Rolled Steel: Main Differences 

After examining their forming process and various properties, you probably have an idea of the difference when considering cold rolled steel versus hot rolled steel. The differences in their mechanical properties are summarized in the table below:

Asides from the mechanical properties, the primary difference between hot rolled and cold rolled steel is their processing temperature. While the rolling of hot rolled steel occurs above the recrystallization temperature of steel, cold rolled steel is processed at room temperature.

The following details will help us further compare cold rolled steel vs hot rolled steel better:

1. Appearance and Surface Quality

The edges and surfaces of hot rolled steel are usually rough. This is because cooling from extreme temperatures leaves remnants on the steel surface, making it look scaly. Thus, such surfaces may require decarburization or other surface treatments to prepare the steel for subsequent operations.

On the other hand, cold rolled steel has a smooth and shiny surface since it does not involve using very high temperatures. Thus, cold rolled steel is ideal for use in production operations without any surface treatments.

2. Recrystallization Point

When comparing hot rolled vs cold rolled steel, one of the major points to consider is the recrystallization point. Cold rolling occurs when the metal material is able to form “new grains.” The rolling and bending of the metal often destroy old grains. Cold work on steel may reduce its strength, so manufacturers add a final step of annealing. This process involves heating the steel to 1,333 – 1,400 degrees Fahrenheit (just above the recrystallization point of steel).

Cooling of the steel occurs very slowly without bringing it to room temperature too quickly. This way, the metal can create a uniform microstructure to reset its grains. In contrast, the processing of hot rolled steel occurs at above 1700 degrees Fahrenheit, which is higher than the recrystallization point of steel. This condition is necessary for the easy forming and shaping of the metal.

3. Steel Strength and Hardness

Material strength and hardness also help mentioned earlier, the process of forming cold rolled steel gives a uniform microstructure. This process helps to create metal with higher strength and hardness. It is not too hard, however, making it easily malleable.

The hot rolling process, on the other hand, involves extreme heat and rapid cooling, preventing the stress from regaining its grains. Thus, the resulting metal has more liver strength and hardness than cold rolled steel.

4. Internal Stresses

Steel strength and hardness significantly impart internal stresses on the material. Therefore, cold rolled steel with greater strength and hardness has greater internal stresses than hot rolled steel. It is crucial to relieve such stresses before processing the material to prevent the warping of the final product.

5. Distortions

Hot rolled steel experiences slight distortions (e.g. through sheet metal bending) because the cooling process gives slightly trapezoidal shapes and forms. Cold rolled steel has perfectly squared angles with well-defined corners and edges. The tubes formed also have excellent concentric uniformity and better straightness.

This is a video about hot rolled vs cold rolled steel:

Hot Rolled vs Cold Rolled Steel: Which is Cheaper?

Hot rolled steel is typically less expensive than cold rolled steel. The reason for this is not far-fetched. The manufacturing of this steel type does not involve any delay or further processing. There is no need for re-heating and reforming, as is the case with cold rolling. Cold finishing often consists of other processes like cold drawing, turning, grinding, and polishing. The additional production operations make cold rolled steel more expensive than hot rolled steel.

Should I Choose Hot Rolled Steel or Cold Rolled Steel?

Your choice of materials for sheet metal fabrication will depend on several factors. This comparison table outlines the major factors to consider when deciding between cold rolled steel vs hot rolled steel.

RapidDirect — Quality Steel Parts Manufacturer Company

Regardless of your intended steel choice, RapidDirect has the capacity to make custom metal parts for various industries, including aerospace, automotive, home appliances, and many others. We use high-quality cold rolled steel or hot rolled steel, depending on what steel part you’re looking to make.

Our technicians have an adequate understanding of steel parts manufacturing, and you can be sure of getting the best results. Once you upload your CAD files, our system carries out automated DfM analysis and gives you a quote within 12 hours.

Conclusion

This article covers the fundamental comparison of hot vs cold rolled steel, highlighting the major differences. Each steel type is best suited for some applications than others. Therefore, it is essential to know their properties, benefits, and weaknesses. This knowledge will help designers and project contractors to ensure efficient and effective completion of projects. There’s no better way to get the best results than working with a top-quality manufacturing company. Contact our team of experts at RapidDirect today! We look forward to working with you.

FAQs About Cold Versus Hot Rolled Steel