The Cold, Hot Facts Between Cold Roll Forming and Hot Rolling

The Cold, Hot Facts Between Cold Roll Forming and Hot Rolling

When selecting roll forming for your projects, durability, flexibility, and cost-effectiveness are essential to describe the process. From industrial to commercial applications, roll forming offers remarkable adaptability. 

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But how do you know when to use cold roll forming vs. hot?

Although hot rolled steel shapes comprise the highest volume of rolled items in the United States, did you know that contract roll forming only uses cold rolling? So, to us, cold roll forming is just roll forming. Hot rolling is almost nonexistent in the contracted OEM roll forming world. This process is left up to the steel mills that make standard, commodity-type shapes.

So what differences make the cold process applicable for roll forming and sometimes a better choice than hot forming?

What Is Hot vs. Cold Roll Forming?

While both hot and cold roll forming processes shape metal into desired forms, they are worlds apart regarding temperatures, techniques, and applications. 

Hot Forming

Hot forming combines extruding and rolling using molten steel under extremely high temperatures. Some structural shapes, such as those used in holding up and reinforcing buildings, cannot be made any other way.

A classic example is a large “plunger” filled with hot steel, which extrudes a shape used for an I-beam in structural applications. A series of rollers then fine-tune the shape to the tolerances required for that particular shape.

It doesn't make sense to take a steel bar and cold form it into something like an “I” shape. Hot forming can also produce coiled steel sheets as thin as 0.60”.

The challenge is that this is a very specialized mill process. Using high temperatures and molten material requires huge furnaces, which aren’t abundant. Buying the machinery needed to safely manage molten steel and finding experienced workers to operate the equipment is a huge investment.

Cold Forming

True-ish to its name, cold forming involves making shapes at room temperature or slightly above room temperature.

It only takes a little high-temperature, specialized equipment to produce cold-rolled shapes. Flat and coiled sheets are the two forms of raw material typically fed through roll-forming machines.

It is possible to use hot-rolled steel in sheet form as the raw material in cold roll forming. However, you wouldn’t form it “hot off the presses,” so to speak—it would have been room temperature for days by the time your roll former worked with it.

(Resource: Want to know more about cold roll forming? Click below to read our guide!)

What Is the Difference Between Hot Rolled and Cold Rolled Steel?

Both hot and cold forming have uses. The properties of specific metal grades sometimes dictate whether they should be hot or cold formed.

Differences include:

• Run speed & quality
• Structural uses
• Strength 
• Size limits
  
  

Run Speed & Quality

It might take 50 rolls to thin out a hot steel workpiece with hot roll forming, but that piece may require 100 passes of cold roll forming. Why? It takes more force to manipulate the metal.

However, with more rolls, you can get tighter tolerances on the piece and a higher-quality end product. This process can increase costs because of the additional machinery and labor time required, but it may be worth it to your customer.

Structural Uses

Structural shapes like I-beams are usually hot rolled. The steel used for I beams differs from that used in cold forming. It’s hard and less ductile, which makes it more challenging to bend when cold. Hence, it's used for structural shapes that carry much weight!

Cold roll forming has many commercial and industrial uses. They include:

• Signposts & guard rails
• Solar
• Refrigeration
• Escalators & elevators

Click here for more information on some of the many uses of roll forming.

Strength

For similar grades, cold rolled metal can be stronger than hot rolled metal because of work hardening. When you put a piece through 100 vs. 50 passes, the strain you put on the material hardens it and makes it stronger.

If the roll-formed part requires hot-rolled sheets, you'll still get some added strength. But it would be stronger if you started with standard cold-rolled material.

Size Limits

As we mentioned, steel sheets are only hot rolled up to a certain thickness. You can’t buy 20 gauge hot rolled sheets. At that point, you’ll have to use cold rolled. 

The company is the world’s best cold rolled round bar supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

Since roll-formed parts are usually produced from coiled material, product length is limited only by the amount of material in the coil and the handling of the finished component. Hot-rolled shapes are also limited by equipment capability.

When Roll Forming Makes Perfect Sense

Cold roll forming usually results in better, more attractive finished surfaces with closer tolerances. During the forming process, the material can be quickly galvanized, painted, or powder coated into various shapes.

The point, of course, is to use the best process for the job. Do you need to know whether your application fits with roll forming or an alternative method? Ask a manufacturer before committing to either cold or hot roll forming.

Want to Learn More About Cold Roll Formed Parts?

Our Comprehensive Design Guide to Great Roll Formed Parts gives a good primer for optimizing your design for cold roll forming. Download the guide below:

(Editor's note: This article was originally published in January and was recently updated.)

Steel that has been rolled at a temperature higher than its recrystallization temperature (often F or above) is referred to as “hot rolled”. The treated steel has more formability and workability than unprocessed steel, making it simpler to work with in the following processing steps.

The hot rolling process begins with a billet, which is a huge, rectangular piece of metal. The billet is first heated and rolled into a big roll. It travels through a sequence of spinning rollers while still hot to attain the correct size. The rolled steel is subsequently twisted into coiled rolls and allowed to cool in sheet metal manufacturing processes. The processed material is chopped into the appropriate units and packed into manufacturing activities involving additional forms.

The Benefits of Hot Rolled Steel

Hot rolled steel has various benefits in the production process, including:

• Affordability: Because it requires less processing, hot rolled steel is often less costly than cold rolled steel.
• Minimal internal tension: Hot rolling requires cooling steel over time, allowing it to normalize its microstructure and eliminate internal tensions.
• Improved usability: The hot rolling technique results in a material that is easier to mold and shape because of the material’s increased workability.

Hot Rolled Round Bar

The lower cost and low carbon content of hot rolled round bar (also known as long products, re-rolled bar, and merchant bar) make it an excellent choice for a wide range of applications. Forging, fabricating, and machining operations may be performed with no need for specific equipment or heat treatments because of the steel’s low carbon content (0.22 percent max) and low yield point (275N/mm2 min).

Pickling and oiling (P&O) or a variety of abrasives, including shot blasting, may remove the surface scale off the bar’s exterior while it is in the AR (as rolled) state. Due to a fast temperature shift during shot blasting the scale offers a limited level of protection against mild corrosion. A rust-like tint may emerge due to humidity and the steel’s mineral composition.

Hot rolled steel is no different from the rest of the metals when it comes to manufacturing tolerances. A billet or bloom is heated to roughly C and passed through a sequence of rollers comprising roughing, intermediate, and finishing. The bloom length or billet is increased by using these rolls, which lower the cross-sectional area. At this point, the section is also generated in terms of its form and specified measurements. The pace at which the steel cools after exiting the furnace is essential to its qualities and appropriateness for use, as the steel cools from C to roughly 900C.

The temperature drop from hot rolling to cold and no additional polishing before end usage makes it impossible to regulate section tolerances properly. Tolerances range from 0.4 mm to millimeters on bigger sections, depending on the size of the piece. Greater precision of 0.2 mm or less is necessary for bright drawn and peeled and ground components.

Hot Rolled Steel Bars Grades

For general-purpose applications, hot-rolled steel bars are employed. This steel has a low carbon content and high mechanical qualities in general. The standard structural procedures, such as moderate cold and hot shaping and welding, are simple to implement.

Below is a detailed breakdown of the grades, applications, and sizes available round shaped bars.

GRADE ASTM Sizes Length A576 Round from 1/2″ to 12″ 20′ RL

is a manganese-rich, low-carbon steel that differs from mild steel as well as other low-carbon steel. This is superior steel for carburized components since it’s higher in manganese, which results in a tougher, more uniform casing. It also has improved machining qualities and mechanical properties, such as Brinell hardness. Chemical composition, heating, rolling, surface preparation, and other manufacturing controls are all employed in its manufacture. As a consequence, a high-quality product suited for forging, heat treatment, cold drawing, machining, and other applications is produced.

Application: Because is an excellent carburizing steel, it’s ideal for pinions, gears, kingpins, worms, chain pins, liners, oil tool slips, and ratchets that need a high surface hardness with a relatively soft core. Tie rods, anchor pins, studs, special bolts, and other items are often specified in .

GRADE ASTM Size Length A576 Round from 1/2″ to 12″ 20′ RL

In the production of , special controls for heating, rolling, chemical composition, surface preparation, and other aspects of medium-carbon steel manufacture are utilized. These bars may be used for forging, heat treatment (including flame hardening), cold drawing, machining, and other applications as a consequence of their meticulous preparation.

Application: When lower carbon steels aren’t strong enough, this steel should be used instead. It becomes even stronger when heat treated. Alloy steel is often used in the construction of automobile and heavy equipment components, as well as in stud bolts and other types of fasteners.

GRADE ASTM Size Length A576 Round 2″ to 12″ 20′ RL

is manganese steel with modest carbon content. It’s melted to meet unique bar grade and fine-grain standards while maintaining excellent consistency and homogeneity. Strength properties are vital in the as-rolled form, and heat treatment may increase hardness and strength. Because of the inclusion of sulphur, the machinability is great.

Application: This grade is advantageous in applications requiring superior machinability and increased strength, such as shafts, studs, axles, tie rods, bolts, and so on.

GRADE ASTM Size Length A36 A36 Round from 1/2″ to 3″ 20′ RL

This steel has a low carbon content and excellent overall mechanical characteristics. It is simple to make using standard structural procedures such as moderate cold and hot shaping and welding. It is simple to make using standard structural procedures such as moderate cold and hot shaping and welding.

Application: This material is utilized for non-critical and general purpose structural applications involving moderate cold bending, mild hot forming, punching, and welding. General machine components, agricultural tools, transportation equipment, and so on are examples of this kind of use for the material It’s employed in situations when seams and other minor flaws in the surface are acceptable.

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