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Hot Rolled vs Cold Rolled Sheel: What’s the Difference

The diversity and unique properties of steel make it a crucial material in industries such as automotive, electrical appliances, and aerospace. Hot-rolled steel and cold-rolled steel, two widely used types of steel in the industry, differ significantly in the temperature conditions during their production processes. Hot-rolled steel is processed at high temperatures, while cold-rolled steel is processed at lower temperatures.

When deciding which type of steel to use, it is essential to first understand the distinct characteristics of these two types of steel. Below, we will delve into the differences, advantages, limitations, and applications of hot-rolled steel and cold-rolled steel.

What is Hot Rolled Steel?

hot-rolled steel

Hot rolling is a steel processing method that occurs under conditions above the alloy’s recrystallization temperature (typically 1700°F or higher). In this process, the material is heated to a softened state and then pressed into sheets or changes the cross-sectional shape of the steel ingot through rolling. However, this deformation does not alter the material’s physical properties; it merely changes its shape.

In essence, hot rolling is the process whereby a steel slab, heated to high temperatures, undergoes several rolling stages, followed by edge trimming, straightening, and other treatments, ultimately resulting in steel plates. The final temperature of hot rolling is generally 800 to 960°C, which is equivalent to a normalizing treatment for metal materials.

Hot rolling is the most common method in steel processing, with the majority of steel being manufactured through hot rolling. Metal materials delivered in the hot-rolled condition are covered with an oxide scale on the surface, which provides some corrosion resistance, allowing for outdoor storage.

Properties of Hot Rolled Steel

Typically, hot-rolled steel can be identified by the following characteristics:

  1. A scaled surface – residues from extreme temperature cooling.
  2. Slightly rounded edges and corners on bar and sheet products (due to shrinkage and imprecise finishing).
  3. Slight distortions – cooling may result in angles that are slightly trapezoidal rather than perfectly square.
steel mills transport hot-rolled steel

What Are The Benefits Of Hot Rolled Steel?

The production process of hot-rolled steel is usually more efficient than that of cold-rolled steel because it requires fewer processing steps. This efficient production method makes the cost of hot-rolled steel relatively low, thus having a significant price advantage.

Hot rolled steel is cooled at room temperature, which maintains a relatively uniform microstructure without the internal stress generated during quenching or work hardening. This stress free state makes hot-rolled steel more stable and less prone to deformation or cracking during subsequent processing and use.

Hot rolled steel has fast forming speed, high yield, and can be made into various cross-sectional forms to adapt to different usage conditions. This flexibility has made hot-rolled steel widely used in fields such as building structures, bridges, and mechanical manufacturing.

Although the surface smoothness of hot-rolled steel may not be as good as that of cold-rolled steel, the oxide film (scaling) on its surface can be easily removed by grinding, sandblasting, or acid bath pickling. After removing scaling, the surface of hot-rolled steel can be further processed to achieve the desired decorative effect. In addition, descaling steel provides a better foundation for painting and other surface coatings, making the coating more uniform and durable.

Steel

Disadvantages Of Hot Rolled Steel

However, hot-rolled steel also has some drawbacks. For example, during the hot rolling process, non-metallic inclusions (such as sulfides and oxides) inside the steel may be pressed into thin sheets, leading to delamination. This layering phenomenon significantly reduces the tensile performance of steel in the thickness direction and may cause interlayer tearing during welding.

In addition, the thickness dimensions of hot-rolled products are difficult to control, which may lead to product inconsistency. In addition, the surface of hot-rolled products is relatively rough, with Ra values generally ranging from 0.5 to 1.5 μ Between m, this may affect its use in some applications that require high surface smoothness.

What is Cold Rolled Steel?

cold-rolled steel

Cold rolling is the process of further rolling materials at temperatures lower than the alloy’s recrystallization temperature, after they have undergone hot rolling, descaling, and deoxidation processes. Due to the effects of pickling, cold-rolled steel exhibits higher precision and surface quality. When touched by hand, the surface of cold-rolled steel feels relatively smooth.

To be more specific, cold rolling is a process carried out on the basis of hot-rolled coils or sheets, typically involving the sequence of hot rolling, pickling, and then cold rolling.

As cold-rolled steel hasn’t undergone annealing, its hardness is very high (HRB greater than 90). This high hardness results in relatively poor machinability, typically limited to simple, directional bending operations of less than 90 degrees (perpendicular to the direction of coiling).

Properties of Cold Rolled Steel

Cold-rolled steel can typically be identified by the following characteristics:

  1. Improved and more precisely processed surface with smaller tolerances.
  2. Smooth surface, often feels greasy to the touch.
  3. Rectangular bars usually have distinct sharp edges.
  4. Pipes exhibit better concentricity and straightness.
cold rolled steel strip

What Are The Benefits Of Cold Rolled Steel?

Compared to hot-rolled steel, cold-rolled steel can achieve thinner thicknesses, typically down to 0.1mm, meeting the requirements of high precision tolerances.

Cold-rolled steel is generally harder and more robust than standard hot-rolled steel. Because the metal is formed at lower temperatures, the hardness, tensile strength, and resistance to deformation of the steel increase due to work hardening.

Disadvantages Of Cold Rolled Steel

Cold-rolled steel sections typically have open cross-sections, resulting in lower torsional rigidity. This makes them prone to torsional twisting under bending and susceptible to local buckling under compression, leading to poor torsional performance.

Moreover, the thinner wall thickness of cold-rolled steel, combined with the absence of thickening at the corners of plate connections, results in a weaker ability to withstand localized concentrated loads.

Hot Rolled Steel vs Cold Rolled Steel: Main Differences 

hot rolled steel vs cold rolled steel

The main difference between cold rolling and hot rolling lies in the temperature of the rolling process. “Cold” refers to room temperature, while “hot” refers to high temperatures.

From a metallurgical perspective, the distinction between cold rolling and hot rolling should be based on the recrystallization temperature. That is, rolling below the recrystallization temperature is cold rolling, while rolling above the recrystallization temperature is hot rolling. The recrystallization temperature of steel typically ranges from 450°C to 600°C.

Strength and Hardness

Strength and hardness determine the suitability of steel for different applications. The differences in strength and hardness between hot-rolled steel and cold-rolled steel mainly stem from their processing methods.

Hot-rolled steel is processed at high temperatures, allowing for plastic deformation during processing. However, this processing method often results in a relatively coarse grain structure and potentially uneven microstructure. Therefore, while hot-rolled steel exhibits good toughness and ductility, its strength and hardness are typically lower.

In contrast, cold-rolled steel is processed at room temperature or below the recrystallization temperature. Due to the lower processing temperature, cold-rolled steel undergoes continuous cold deformation during processing, resulting in grain refinement and a more uniform microstructure. This processing method significantly enhances the strength and hardness of cold-rolled steel, providing higher tensile and yield strengths.

Production Process

Hot-rolled steel is rolled through mills at high temperatures. At high temperatures, the steel’s plasticity is enhanced, making it easier to form into desired shapes.

Cold-rolled steel, on the other hand, is rolled at room temperature. Because it is rolled at lower temperatures, cold-rolled steel exhibits higher precision and surface quality. However, due to the reduced plasticity of the steel during cold rolling, it often requires initial hot rolling processing before being cooled and cold-rolled.

Surface Finish and Appearance

Hot-rolled steel is rolled at high temperatures, which can result in a relatively rough surface and the formation of scale. This is because during the hot rolling process, the surface of the steel comes into direct contact with oxygen in the air, leading to a reaction between iron atoms and oxygen atoms, forming an oxide layer, known as scale. This scale not only affects the appearance of the steel but may also impact its subsequent processing and performance.

In contrast, cold-rolled steel is rolled at room temperature. Due to the lower processing temperature, it exhibits higher surface quality, appearing smoother and more polished. During the cold rolling process, the steel undergoes compression and shaping by a series of rollers, resulting in a more uniform surface finish. This smooth surface makes cold-rolled steel more suitable for applications where aesthetics are important, such as household appliances and automobiles.

Tolerances and Dimensional Accuracy

Due to the relatively simple production process of hot-rolled steel, there is a certain amount of thermal expansion and deformation during rolling. Therefore, hot-rolled steel typically has larger tolerances and lower costs, making it suitable for applications where high dimensional accuracy is not required.

In contrast, cold-rolled steel offers higher dimensional accuracy and stricter tolerances, making it suitable for applications requiring precision and tight fit. This is because, during the cold rolling process, the steel undergoes multiple rolling and stretching stages to improve its surface finish and dimensional accuracy.

Price Differences

The production process of hot-rolled steel is relatively simple and typically does not involve further processing or heat treatment, resulting in lower costs. In contrast, the production of cold-rolled steel requires additional steps, including rolling at room temperature, cold drawing, turning, grinding, and polishing. These additional processing steps increase production costs, making cold-rolled steel generally more expensive.

Application Industry

Hot-rolled steel, due to its good plasticity and toughness, finds wide applications in sectors such as construction, agricultural equipment, railway equipment, and petrochemical industries. Particularly in construction and railway equipment, hot-rolled steel is extensively used in parts like rails and train components.

Cold-rolled steel, prized for its high strength and excellent surface quality, is primarily used in manufacturing products requiring high precision and surface demands, such as automobile bodies, household appliance casings, and precision instruments. Additionally, cold-rolled steel is widely employed in the production of steel pipes, plates, strips, and more.

Mechanical Properties

Due to the high temperature and plastic deformation experienced during the hot rolling process, hot-rolled steel exhibits a relatively loose internal structure and organization. Consequently, hot-rolled steel has lower strength and hardness but higher elongation and toughness. This makes hot-rolled steel more suitable for applications requiring higher ductility and toughness.

Cold-rolled steel, rolled at lower temperatures, features a tighter internal structure and organization. As a result, it possesses higher strength and hardness but lower elongation and toughness. This makes cold-rolled steel more suitable for applications requiring higher strength and rigidity, such as automotive manufacturing and mechanical engineering.

Should I Choose Hot Rolled Steel or Cold Rolled Steel?

Choosing between hot rolled steel and cold rolled steel depends on your specific application and requirements. Here are some factors to consider:

Budget Considerations

For projects with limited budgets, hot-rolled steel may be a more cost-effective choice as it typically comes at a lower price. However, for projects requiring high precision and high-quality surface finishes, despite the higher cost, cold-rolled steel’s performance enhancements may justify the investment.

When assessing steel costs, don’t forget to consider other factors such as transportation costs, processing fees, and potential secondary operations (such as painting or machining). These additional expenses can significantly impact the total project cost.

Material Requirements

Material requirements vary depending on the project. Some applications may require steel with specific properties such as corrosion resistance, strength, or flexibility. When selecting steel, it’s essential to consider the intended use and environmental conditions of the project to ensure the chosen steel can meet performance requirements.

Additionally, consider the size and tolerances of the steel. Hot-rolled steel and cold-rolled steel undergo different size variations and tolerance controls during production. Depending on project requirements, choose the type of steel that can provide the best size accuracy and tolerance control.

Project Timeline

Hot-rolled steel, due to its widespread availability and faster supply speed, is typically more suitable for projects requiring quick completion. Cold-rolled steel may require a longer manufacturing cycle as it needs additional processing steps to improve size accuracy and surface quality.

Due to the higher strength and hardness of cold-rolled steel, more machining time and effort may be required to achieve the desired shapes and sizes. This may further extend the overall project timeline.

Finding Reliable Suppliers and Manufacturers

When searching for reliable suppliers and manufacturers for your steel project, look for companies with good reputations and widespread recognition in the industry. These companies typically provide high-quality products and excellent services.

As your trusted partner, BOYI – Premium steel parts manufacturer, is committed to providing you with outstanding steel parts manufacturing services. Whether you choose cold-rolled steel or hot-rolled steel, we have the capability to manufacture custom metal parts for various industries. Our technical personnel have extensive experience in steel parts manufacturing, ensuring you receive the best product quality and performance.

To better meet your needs, we offer fast-response quoting services. Simply upload your CAD files, and we’ll provide you with a detailed quote and manufacturing plan within 12 hours.

Conclusion

This article aims to help readers better understand the differences between hot-rolled steel and cold-rolled steel, including their basic characteristics, advantages, and disadvantages, to assist in determining their suitability for various applications. Each type of steel has its unique physical and mechanical properties, making them excel in certain specific applications.

Representing premium steel parts manufacturers, BOYI is dedicated to providing customers with excellent steel parts manufacturing services. If you are looking for reliable steel suppliers or manufacturers, please contact BOYI. We look forward to working with you to create high-quality, cost-effective projects.

FAQ

What is better, hot rolled or cold rolled steel?

While hot rolling results in steel with good ductility, the cold rolling process enhances the material’s strength and hardness to a significantly higher level. However, this increased strength and hardness in cold rolled steel also brings with it a greater accumulation of internal stresses.

What is the main advantage of hot rolling over cold rolling?

A primary advantage of hot rolling over cold rolling lies in its ability to effectively eliminate defects such as welding pores and coarse grains. As the metal undergoes recrystallization during the hot rolling process, the coarse grains transform into finer grains, resulting in favorable alterations to the metal’s properties.

What is the difference between hot rolled and cold-rolled forging?

The key difference between hot-rolled and cold-rolled forging lies in the temperatures and outcomes of the respective processes. Cold forging involves compression of a metal workpiece at room temperature, which hardens and strengthens the metal. Conversely, hot forging occurs at extremely high temperatures, where the metal is softened and subjected to compression, resulting in optimal yield strength, lower hardness, and enhanced ductility.


Tagged: Materials Guide

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