How Mild Steel Round Bar Is Made: Hot Rolling, Cold Drawing & Mill Finish Explained
2 Jan 2026
Ever wondered why some mild steel round bars are shiny and smooth, while others are darker and rougher? It’s all down to how they’re made.
Whether you’re fabricating, machining or just building something solid, understanding how mild steel round bar is produced helps you choose the right type for the job. Bright or black, precision or strength, workshop or site.
At The Metal Store, we keep things simple. We stock both hot rolled (black) and cold drawn (bright) mild steel round bar, cut to the exact length you need. But before you choose, it’s worth knowing what really goes on behind the scenes.
Article in Brief
Mild steel round bar starts life as a steel billet, heated and shaped into a solid circular section.
The two main production methods are hot rolling and cold drawing, which create “black” and “bright” finishes.
“Mill finish” refers to the natural surface left from the rolling process
Hot rolled bar is strong and cost-effective. Cold drawn bar is smoother and more precise.
1
From billet to bar: how it all begins
Every round bar begins as a steel billet. That’s a solid rectangular block of mild steel that’s been cast and cooled from molten metal.
These billets are reheated to incredibly high temperatures (around 1,200°C) before being fed through heavy rollers that gradually shape them into circular bars.
This early stage sets the foundation for the two main types of mild steel round bar, hot rolled and cold drawn. Both come from the same raw steel, but the way they’re processed afterwards makes all the difference.
All round bars are made to meet strict standards like BS EN 10060, which governs size, roundness and straightness. So, whether it’s for fabrication or precision engineering, you know you’re getting consistent quality.
Related: Furnace to Forge: How Steel Is Made
2
Hot rolled round bar (black bar)
Hot rolled mild steel round bar is exactly what it sounds like. It’s steel that’s been rolled while it’s still red hot.
How it’s made
Once the steel billet is heated, it’s passed through a series of rollers that squeeze it into its final circular shape. After rolling, the bar is cooled naturally in air and cut into standard lengths.
As it cools, a dark, flaky layer known as mill scale forms on the surface. This gives the bar its distinctive black or blue-grey colour, plus the nickname “black bar.”
Properties and uses
Hot rolled bar is:
- Tough and durable.
- Ideal for welding and fabrication.
- Slightly less precise in size than cold drawn bar (but perfectly fine for most structural work).
It’s often used for:
- Structural steelwork.
- Fabrication projects.
- Brackets, frames and reinforcements.
- Jobs where strength matters more than appearance.
Because it’s quicker and cheaper to produce, hot rolled bar is the most cost-effective choice for general steelwork.
2
Cold drawn round bar (bright bar)
Cold drawn mild steel round bar takes things one step further. After being hot rolled, the bar is cooled and then drawn through a series of dies at room temperature.
How it’s made
This cold drawing process pulls the steel through precise dies. It’s like squeezing toothpaste through a smaller nozzle. This reduces its diameter slightly and improves its surface quality.
The result? A smoother, shinier and more accurate bar with tighter tolerances, straighter alignment and increased tensile strength.
Properties and uses
Bright bar is:
- Smooth and visually clean.
- Dimensionally precise, with tight tolerances.
- Slightly stronger due to work hardening during drawing.
It’s the preferred choice for:
- Precision engineering.
- Machined components and turned parts.
- Shafts, pins, axles and tooling.
- Decorative or visible installations.
If your project demands precision or a clean finish, bright bar is the one to go for.
3
Mill finish explained
The term mill finish simply refers to metal that hasn’t been polished, painted or coated after production. It’s the natural surface that comes straight off the rolling mill.
For hot rolled bar, mill finish usually means a darker, textured look with mill scale still intact.
For bright bar, mill finish appears smoother and shinier due to the cold drawing process.
Mill finish is perfectly fine for most fabrication and welding work. But if you need a polished or protective surface, the bar can be further treated by grinding, coating or galvanising.
If you’re after galvanised steel, The Metal Store has that covered too.
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Bright vs black bar: what’s the difference?
Here’s a quick comparison of the two types:
| Feature | Hot Rolled (Black Bar) | Cold Drawn (Bright Bar) |
|---|---|---|
| Process | Rolled at high temperature | Drawn through dies at room temperature |
| Surface finish | Rough, dark (mill scale) | Smooth, bright, clean |
| Tolerance | Standard (per BS EN 10060) | Tight, suitable for precision machining |
| Strength | Good | Slightly higher (work hardened) |
| Cost | Lower | Higher |
| Best for | Fabrication, structural work | Machining, engineering components |
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Why manufacturing method affects size and tolerance
Because cold drawing slightly reduces the bar’s diameter, bright bars are often a touch smaller than their hot rolled equivalents. That’s why precise measurement and tolerance standards matter.
For most fabrication jobs, hot rolled bar’s natural variation is fine. It’s typically within ±0.4mm, depending on size. But for engineering work where exact fits are essential, bright bar’s tighter tolerances make all the difference. They’re sometimes down to ±0.1mm.
That’s also why bright bar tends to cost a little more. You’re paying for the precision.
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Choosing the right type for your project
If you’re not sure which to go for, here’s a quick rule of thumb:
- Go black (hot rolled) for fabrication, welding and heavy structural work.
- Go bright (cold drawn) for machining, turning or applications where exact size and finish are key.
And if you’re working outdoors or in damp environments, consider a galvanised finish to prevent rust.
Stock and supply at The Metal Store
We supply a full range of mild steel round bars in both hot rolled (black) and cold drawn (bright) finishes. Whatever the size, we’ll cut it to your exact length free of charge, pack it carefully and deliver it straight to your door, anywhere in the UK. Browse our full range or call our team on 01274 875 479 for expert advice.
FAQs
What’s the difference between hot rolled and cold drawn mild steel round bar?
Hot rolled bar is shaped at high temperatures and has a darker, rougher surface. Cold drawn (bright) bar is pulled through dies at room temperature, giving it a smoother, more precise finish.
Why is it called “black bar”?
Because of the dark mill scale that forms on the surface during hot rolling. It’s completely normal and can be removed if needed for painting or coating.
What does “mill finish” mean?
It’s the natural surface of the bar straight from the rolling mill. Unpolished, uncoated and untreated. It’s ideal for fabrication and welding jobs where surface appearance isn’t critical.
Is bright bar stronger than black bar?
Yes, slightly. The cold drawing process work-hardens the steel, improving its tensile strength and straightness. But for most applications, both are equally durable.
Why does bright bar cost more than black bar?
Because it goes through extra processing, drawn through dies for tighter tolerances and smoother finish. This adds time and precision to the manufacturing process.
Can I weld both black and bright mild steel round bar?
Absolutely. Both can be welded easily. Just make sure to remove any mill scale from black bar first to get a clean, strong weld.
Which type of bar should I use for machining or turning?
Go for bright bar. Its smooth surface and accurate diameter make it ideal for machining, threading and precision components.
