The Difference Between Hot Rolling & Cold Rolling

difference between hot and cold rolling process

Did you know there are over 3,500 different steel grades, each with their own unique physical chemical, and environmental properties? It’s true, according to the World Steel Association, and steel production is only getting better. In fact, the WSA reports that if the Eiffel Tower were to be rebuilt today, engineers would only need around a third of the steel that was originally used. Here’s a much simpler fact: there are two main manufacturing processes for the production of steel. These are: (1) hot rolling and (2) cold rolling. Interested in learning more about the production processed involved in the creation of steel products? Read on to learn about the unique differences between hot and cold rolling from the experts at Lechler.

How Does Hot Rolling Work?

Hot rolling is a process by which steel is pressed at very high temperatures These high temperatures must be above steel’s re-crystallization temperature for the steel to be shaped and formed. The re-crystallization temperature of stainless steel is an impressively high 650 to 700 degrees Celsius – that’s a range of approximately 1,200 to 1,300 degrees Fahrenheit. Typically, however, hot rolling occurs at much higher temperatures, generally over 1700 degrees Fahrenheit.

Manufacturers begins with a “billet,” or large rectangular piece of metal. The billet is heated up, flattened, and spun into a large roll.  During the entire process, it’s important to maintain the material at a very high temperature while it’s being run through multiple rollers. The metal is pushed through rollers at high speeds. Afterwards, the metal is cooled into its desired shape, such as coils, bars, or plates.  After this process, as the steel cools, it shrinks somewhat. Hot rolling is therefore not the ideal process for use in applications where precision is required. t.

When is Hot Rolling Used?

Hot rolling steels and other metals is the preferred processing method when dimensional tolerances and surface finishes are less important than the material’s strength. Of course, uneven scaling is a concern that can be addressed through various processes, including grinding, sanding, and pickling. Finishes like brush and mirror finishes can be applied after descaling. Afterwards, the material can be painted or coated.

Hot rolled steel is commonly used as a structural component, including in railroad tracks, agricultural equipment, and car and truck frames. It’s also frequently used for construction purposes. 

What are the Benefits of Hot Rolling?  

  • Hot rolling is less expensive than cold rolling
  • Hot rolled metal is normalized and free from the internal stresses that may be produced through work-hardening processes

How does Cold Rolling Work?

In brief, cold rolling is a process by which steel is strengthened without the use of heat (as opposing to hot rolling, where heat is used). But this doesn’t mean stainless steel is subjected to freezing temperatures; rather, the process generally occurs at a normal room temperature. In fact, unlike hot rolling, cold rolling can only occur when a metal is below its re-crystallization temperature. Instead of heat, cold rolling replies on sheer mechanical stress to change the structure of stainless steel.

The cold rolling process begins with either a sheet or a strip coil of stainless steel, or whatever metal is to be rolled. The metal is placed in a large roller that compresses the material under very high pressure, a rate of right under its ultimate tensile strength. As a result, the processed steel is made up to 80 percent thinner, depending on requirements for the end product. After steel has passed through the cold rolls, it will be brittle. The next step in the process is annealing the metal at a higher temperature. This serves to soften the steel, making it more malleable and ready to be turned into a variety of products.

Not all cold rolling is equal, and you can cold roll for different results depending on your industry’s specific needs. With cold rolling, you’re able to achieve various levels of temper in your processed metals, including quarter-hard, half -hard, and full-hard steel. The more a metal is processed, the harder it becomes. To specify, quarter-hard steel has the ability to be bent completely in half without breaking; half-hard processed steel can bend to a 90 degree angle; and finally, full-hard stainless steel can bend to a 45 degree angle. In summary, when cold rolling stainless steel, there is a tradeoff between hardness and malleability.

When is Cold Rolling Used?

Cold rolling produces stainless steel that known for its malleability, durability, and strength. The process of cold rolling is often used in various industrial applications, spanning from the medical industry, to consumer goods, to aerospace and automotive engineering.

Cold rolled steel is utilized in a wide range of applications throughout various industries, ranging from industrial buildings to kitchen appliances to filing cabinets. Overarchingly, cold rolled metal is popular whenever a manufacturer needs metal to bend without breaking.

Most commonly, the following stainless grades are used in cold rolling: austenitic stainless steel

ferritic stainless steel, martensitic stainless steel, and precipitation hardening stainless steel. Of course, other metals and alloys can be cold rolled as well. Titanium, nickel alloy, and aluminum are often cold rolled too. When processed, the strength and hardness of these metals are also improved.

What are the Benefits of Cold Rolling?

  • Strain hardening increases processed metal’s strength by up to 20 percent
  • Improved finish of stainless steel and other metals
  • Permanent changes to the crystalline structure of the metal caused by cold rolling improves the corrosion resistance of processed metals
  • Cold rolling produces better dimensional accuracy
  • Cold rolled stainless steel can be precision manufactured
  • Processed metal has extremely high tolerance when bent
  • Better concentric uniformity and straightness

Cold Rolling Solutions: The Lechler SELECTOSPRAY® Roll Cooling System.

The Lechler SELECTOSPRAY® roll cooling system is the industry standard for selective roll cooling in the rolling of flat steel, aluminium and other non-ferrous strip. SELECTOSPRAY is an indispensable actuator for shape control. It corrects reliably asymetrical strip shape defects and supports work roll bending.

To date, more than 300 Lechler SELECTOSPRAY roll cooling systems have been installed globally in cold rolling mills for steel, aluminium and non-ferrous metals, as well as in aluminium hot rolling mills and foil mills.

Hot Rolling Solutions: Lechler SCALEMASTER Descaling Nozzles.

In hot rolling mills around the world, Lechler SCALEMASTER descaling nozzles ensure the best surface qualities and low energy consumption. Our most popular descaling nozzles include:

  • SCALEMASTER HPS, which provides a remarkable increase of impact for better descaling, reduction of descaling water flow, and is extremely durable, with high mechanical strength due to its unique window design. There’s also a potential for energy savings, thanks to the HPS’ reduced slab/strip colling.
  • SCALEMASTER HP, which has the highest impact and longest lifetime. It has a max pressure of up to 6,526 psi.
  • MiniSCALEMASTER HP, for the highest impact and longest life time at low spray heights, also with a max  pressure of up to 6,526 psi.
  • MicroSCALEMASTER, for the smallest dimesions for space saving mountings. The nozzle insert on the MicroSCALEMASTER is made from tungsten carbine or hardened steel.
  • MiniSCALEMASTER, our standard nozzle for small spray heights.
  • SCALEMASTER, our standard nozzle.
  • Water Stop Valves for SCALEMASTER nozzles, which stop the water flow at low pressure. These are perfect for all SCALEMASTER series except the MicroSCALEMASTER.

Need Advice from the Hot & Cold Rolling Experts? Lechler Has You Covered.

Interested in partnering with Lechler? Just a few of the benefits we provide include:

  • Advanced design and engineering capabilities with more than 400 systems and 1000 headers designed and manufactured.
  • In depth application experience and process knowledge.
  • Comprehensive range of high-performance electrical, pneumatic and electro/pneumatic valves.
  • In-house design and build of dedicated control cabinets.
  • Tailored engineered systems as well as standard configurations.

Learn more about Lechler SCALEMASTER Descaling Nozzles, the SELECTOSPRAY Nozzle System, or contact Lechler today for general information. Whether hot rolling or cold rolling, Lechler offers nozzles and systems to increase quality and efficiency. Call us at (800) 777-2926 or reach out via email at info(at)lechlerusa.com.