There are a variety of steel grades, shapes, specifications, and surface finishes available. Each of these steel types has its own unique properties. That is why this metallic material is commonly used in vehicles, appliances, aerospace parts, electronic parts, and more. The need to understand the various types of steel available brings up the comparison of hot rolled steel vs cold rolled steel.
Optimizing the properties of steel for each application is more than just 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 steels have different properties and are suitable for different applications.
When choosing rolled steel for your project, it is crucial to understand the differences between rolled steel types. This prior knowledge will help you save costs and time while avoiding additional processing. This article explains the differences between hot rolled steel and cold rolled steel, including their advantages, limitations, and applications.
What is hot rolled steel?
Rolling refers to the specific way in which steel is produced. Hot rolled steel refers to steel produced using a process that uses extreme heat treatment. That is, the production occurs at extreme temperatures. Manufacturers start with large rectangular pieces of metal (billets). They then heat the billets before sending them off for processing – at this stage they are flattened into large coils.
The hot rolling process involves pressing molten steel at temperatures exceeding 926 degrees Celsius (1,700 degrees Fahrenheit). Such high temperatures are necessary because steel ideally recrystallizes between 750 degrees Fahrenheit and 1,300 degrees Fahrenheit, making it easier to form and reshape. After the steel has been run through the rolling mill into the desired shape, it is allowed to cool.
Properties of Hot Rolled Steel
The following properties will help you identify hot rolled steel:
Scaly surface – Cooling from high temperatures can leave residue on the surface of the steel, making it appear scaly.
Slight distortion – Cooling can also produce a slightly trapezoidal shape without perfect angles.
Corners and edges are slightly rounded – this is a result of shrinkage and less precision in finishing.
Benefits of Hot Rolled Steel
Hot rolled steel can provide the following benefits to your application:
Lower costs: Hot rolled steel requires much less machining than cold rolled steel, making it less expensive.
Virtually no internal stresses: Cooling of hot rolled steel occurs at room temperature, making it essentially normalized. This means it has little to no internal stresses due to the work hardening or quenching process.
Easier to Process: Since the hot rolling process is done at extremely high temperatures, the resulting steel is easier to form. The most common shapes that come from hot rolled steel are UB, UC, RHS, SHS, flat bar, etc.
Applications: It is well suited for applications where tolerances are not a priority.
Disadvantages of Hot Rolled Steel
Dimensional defects due to expansion during heating and contraction/warping during cooling.
Its surface often has a rough texture that needs to be removed or stained before any finishing operations can be performed.
Slight distortion.
Common Uses of Hot Rolled Steel
As mentioned earlier, hot rolled steel shrinks slightly as it cools. This results in manufacturers having less control over the final shape. Therefore, applications for hot rolled steel are typically those that do not require tight tolerances, including:
Automotive parts, such as rims and frames
Agricultural equipment
Railway equipment, such as tracks and train components
Construction materials
What is Cold Rolled Steel?
Essentially, cold rolled steel is hot rolled steel that has been further processed. As mentioned earlier, rolling involves forming the steel into sheets, including turning, grinding, and polishing. Other operations transform the existing hot rolled steel into a more refined product. The term "cold rolled" basically applies to steel that has been compressed.
While manufacturing hot rolled steel involves only high temperature heating and cooling, cold rolled steel involves additional processes. On the cold rolling mill, the manufacturer cools the steel and re-rolls it at room temperature through a cold roll forming or bending machine. This process helps achieve the desired shape and size.
Properties of Cold Rolled Steel
The following characteristics will help you identify cold rolled steel:
Smooth surface usually has a greasy touch
Surface has a better finish and tighter tolerances
Square bars have well-defined edges
Tubes usually have better straightness and concentric uniformity
Benefits of Cold Rolled Steel
Here are some of the benefits you can get by using cold rolled steel:
Better surface properties: Parts made with cold rolled steel usually have a smooth and shiny surface without scale or rust. Thus, making them useful when aesthetics are essential.
More Strength: They are usually stronger and harder (up to 20% stronger) than hot rolled steel. This allows them to be used in high stress applications.
More Precision: Since cold rolled steel does not shrink when formed, more precise parts with consistent and accurate shapes can be manufactured.
Various Surface Finishes: It supports a wide range of surface finishes.
Disadvantages of Cold-Rolled Steel
More expensive due to the extra processing involved.
Internal stresses are created in the material due to the extra processing which can cause unpredictable warping in some cases.
Fewer shapes are available, such as sheet and box section shapes.
Common Uses of Cold-Rolled Steel
Ideal applications for cold-rolled steel are those that require better metal finish and tighter tolerances. Examples of such components include:
Aerospace parts
Machinery components
Appliances
Bars, rods, strips and sheets
Metal furniture structures
Cold Rolled Steel vs. Hot Rolled Steel: Main Differences
After examining their forming processes and various properties, you may have an idea of the difference between hot rolled steel and cold rolled steel. The following table summarizes the differences in their mechanical properties:
Hot rolled steel | Cold rolled steel | |
Tensile strength | 67,000psi | 85,000psi |
Yield strength | 45,000psi | 70,000psi |
Elongation 2” | 36 | 28 |
Reduced area | 58 | 55 |
Brinell hardness | 137 | 167 |
Besides mechanical properties, the main difference between hot rolled steel and cold rolled steel is the temperature at which they are processed. Hot rolled steel rolling occurs above the recrystallization temperature of the steel whereas cold rolled steel is processed at room temperature.
The following details will help us further better compare cold rolled steel vs hot rolled steel:
1. Appearance and Surface Quality
Hot rolled steel often has rough edges and surfaces. This is because the cooling from extreme temperatures leaves residue on the surface of the steel, making it look scaly. Therefore, these surfaces may require decarburization or other surface treatments to prepare the steel for subsequent operations.
Cold rolled steel, on the other hand, has a smooth and shiny surface because it does not involve the use of very high temperatures. Therefore, cold rolled steel is well suited for production operations that do not require any surface treatment.
2. Recrystallization Point
One of the main things to consider when comparing hot rolled steel vs cold rolled steel 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 breaks up the old grains. Cold working the steel can 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 the steel).
The steel is cooled very slowly, not too quickly to bring it to room temperature. This allows the metal to develop a uniform microstructure to reset its grains. In contrast, hot rolled steel is processed at temperatures above 1700 degrees Fahrenheit, above the steel's recrystallization point. This condition is necessary for the metal to be easily formed and shaped.
3. Strength and Hardness of Steel
The strength and hardness of the material also contribute to the previously mentioned, forming process of cold rolled steel imparts a uniform microstructure. This process helps create a metal with higher strength and hardness. However, it is not too hard, so it is easily ductile.
On the other hand, the hot rolling process involves extreme heat and rapid cooling, which prevents stress from restoring its grains. Therefore, the resulting metal has higher liver strength and hardness than cold rolled steel.
4. Internal Stress
The strength and hardness of steel significantly impose internal stresses on the material. Therefore, cold rolled steel, which is greater in strength and hardness, has greater internal stresses than hot rolled steel. It is crucial to eliminate this stress before processing the material to prevent the final product from warping.
5. Distortion
Hot rolled steel undergoes slight deformation (such as through sheet metal bending) as the cooling process gives it a slightly trapezoidal shape and form. Cold rolled steel has perfectly square corners with sharp edges. The formed tubes also have excellent concentric uniformity and better straightness.
Hot Rolled vs Cold Rolled Steel: Which is Cheaper?
Hot rolled steel is generally cheaper than cold rolled steel. The reasons for this are not far-fetched. The manufacturing of this steel does not involve any delays or further processing. There is no need to reheat and reshape as with cold rolling. Cold finishing usually includes other processes such as 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?
The material you choose for sheet metal fabrication will depend on several factors. One of the main factors to consider when deciding between cold rolled steel vs hot rolled steel is the application of the final product. Hot rolled steel is better suited for large structural components that do not have tight tolerances and aesthetic requirements.
However, if you need smaller parts that require more durability and precision qualities, cold rolled steel will be the best choice. Cold rolled steel is also stronger and harder than hot rolled steel. Therefore, it is better suited for components that are used in high stress conditions. If budget is an issue, then you should choose hot rolled steel because it is cheaper than cold rolled steel.
Summary
This article presents a basic comparison of hot rolled steel vs cold rolled steel, highlighting the main differences. Each steel type is better suited for certain applications than the others. Therefore, it is important to understand their characteristics, strengths, and weaknesses. This knowledge will help designers and project contractors ensure that the project is completed efficiently and effectively. There is no better way to achieve the best results than working with a top manufacturing company. Contact our team of experts at JWB! We look forward to working with you.
