What is Hot Rolling? What is Cold Rolling? What’s the Difference?

Steel comes in many grades, sizes, shapes and finishes – the World Steel Association lists more than 3,500 different grades of steel, each with unique properties. The wide variety of steels means that steel can be used in a wide range of applications including infrastructure, appliances, vehicles, wind turbines and many more.

However, optimizing the properties of steel for each application involves more than just changing the chemical composition. The manufacturing process of steel can also have a significant impact on the steel product – even if the grade and specification are the same. One of the main differences between prefabricated steel products is the difference between hot-rolled steel and cold-rolled steel.

The main difference between hot rolled steel and cold rolled steel is the process. “Hot rolling” refers to heating processing, and “cold rolling” refers to a process performed at or near room temperature. Although these techniques can affect overall performance and application, they should not be confused with the formal specifications and grades of steel related to metallurgical composition and performance levels. Different grades and specifications of steel can be hot or cold rolled – including basic carbon steel and other alloy steels. Knowing which ones to use can help avoid spending too much on raw materials. It also saves time and money on additional processing. Understanding the difference between hot and cold steel is essential in choosing one.

Hot rolled steel

Hot rolled steel is steel that has been rolled at extremely high temperatures above 1700˚F, which is higher than the recrystallization temperature of most steels. This makes the steel easier to shape, resulting in a product that is easier to process.

To process hot-rolled steel, manufacturers start with a large rectangular piece of metal called a billet. The billet is heated and then sent for preprocessing, where it is flattened into a large roll. It is kept at high temperatures and passed through a series of rollers to reach its final size. White-hot steel strands are pushed through rollers at high speed. For sheet metal, rolled steel is spun into coils and cooled. For other forms, such as rods or sheets, the material is packed in sections.

Steel shrinks slightly as it cools. Because hot-rolled steel cools after processing, there is less control over its final shape, making it less suitable for precision applications. Hot rolled steel is often used in applications where detailed dimensional requirements are less precise. Hot-rolled steel is commonly used in rail tracks and construction projects.

Hot rolled steel can usually be identified by the following characteristics:

Scaled surface – cooling residue from extreme temperatures

Rod and sheet products have slightly rounded edges and corners (due to shrinkage and less precise finishing)

Slight distortion, cooling may result in a slightly trapezoidal shape rather than perfectly squared corners

Hot-rolled steel generally requires less processing than cold-rolled steel, which makes it much cheaper. Because hot-rolled steel can be cooled at room temperature, it is essentially normalized—meaning it has no internal stresses that may arise from the quenching or work-hardening process.

Hot rolled steel is ideal where dimensional tolerances are less important than overall material strength and surface finish is not a critical issue. If surface finish is a problem, scaling can be removed by grinding, sandblasting or pickling.

Cold rolled steel

Cold rolled steel is essentially hot rolled steel that has been further processed. After the hot-rolled steel cools, it is re-rolled at room temperature to obtain more precise dimensions and better surface quality.

Cold rolled steel can usually be identified by the following characteristics:

Better, more finished surfaces with tighter tolerances

Smooth surface, often greasy to the touch

Bars usually have well-defined edges and corners

Tubing has better concentric uniformity and straightness

Cold-rolled steel has better surface properties than hot-rolled steel, so it is not surprising that cold-rolled steel is used in applications that technically require more precision or where beauty is important. But it is more expensive due to the additional processing of the cold finished product.

In terms of physical properties, cold rolled steel is generally harder and stronger than standard hot rolled steel. Because the metal is formed at a lower temperature, the steel’s hardness, ability to break in tension, and resistance to deformation are all increased by work hardening.

However, these additional treatments also create internal stresses within the material. If the steel is not stress relieved before cutting, grinding or welding, unpredictable warping may result.