Mild steel is low-carbon steel: plain iron-carbon steel with only a small amount of carbon, roughly 0.05 to 0.25 percent, and no significant alloying additions. That low carbon content is what makes it soft enough to bend, easy to weld and cheap to produce, which is why it is the most common steel you will buy as plate, bar, angle, channel, sheet and tube.
The term is loose. “Mild steel” is a description, not a standard you can order against, and that matters the moment the steel is going into something that carries load. This guide explains what the name actually covers, which real grades sit underneath it, and when you need to stop saying “mild steel” and name a specification instead.
What Makes Steel “Mild”
All steel is mostly iron with some carbon. Carbon is the main thing that controls how strong and hard plain steel can get. More carbon allows more strength and hardness, especially after heat treatment, but the steel becomes less ductile and harder to weld. Mild steel sits at the low end of that scale.
A useful working definition comes from Oak Ridge National Laboratory’s report Advanced High-Strength Steel: Basics and Applications in the Automotive Industry (ORNL/TM-2021/2047, read October 2026). It describes mild or low-carbon steels as having a tensile strength around 400 MPa and a carbon content of 0.05 to 0.25 percent, with a microstructure that is normally a single phase, ferrite, which is why the material is relatively ductile and easy to form.
You will find other sources that put the upper limit a little higher, at 0.30 percent. There is no single official cut-off, which is one more reason the name is fine for conversation but not for a purchase order.
Mild Steel Properties
Because “mild steel” covers many grades, exact numbers depend on which one you have. The general behavior is consistent, though.
Strength
Moderate. The most common structural mild steel in the US, ASTM A36, has a specified minimum yield strength of 36 ksi and a tensile strength of 58 to 80 ksi. The 58 ksi minimum is about 400 MPa, which lines up with the ORNL description above. Our A36 steel guide covers those numbers and the mill test data behind them.
Ductility and formability
High. Mild steel stretches a long way before it breaks, so it bends, rolls and presses without cracking. That same ductility is a safety feature in structures: an overloaded mild steel member usually deforms visibly before it fails.
Weldability
Excellent. Low carbon means the area next to a weld is much less likely to turn hard and brittle as it cools, so mild steel can be welded with every common process and rarely needs preheat in thin sections. Thick sections and restrained joints still need a qualified procedure.
Hardening
Poor by quenching. There is not enough carbon for a quench to make it much harder, so mild steel is not used for blades, springs or wear parts. Surface hardening processes such as case hardening can give it a hard skin over a soft core when that is needed.
Corrosion and magnetism
Mild steel rusts readily when bare and wet, and it is strongly magnetic. Both follow from it being almost pure iron. If it lives outdoors it needs paint, galvanizing or another rust prevention coating. Galvanizing does not change the magnetism, as our piece on whether galvanized steel is magnetic explains.
Weight
Grade makes almost no difference to weight. The standard figure for carbon steel is 490 lb per cubic foot, so 1 inch thick plate weighs about 40.8 lb per square foot. The steel beam weight calculator uses the same density for shapes and bars.
Real Grades That Count as Mild Steel
When a supplier hands you “mild steel”, it was made to an actual specification. These are the ones you are most likely to meet in the US:
| Grade | What it is | Typical products |
|---|---|---|
| ASTM A36 | Carbon structural steel with guaranteed minimum strength | Plate, flat bar, angle, channel, S-shapes |
| SAE/AISI 1018, 1020 | Plain carbon steels named by chemistry; the last two digits give the nominal carbon in hundredths of a percent | Cold-finished round, square and flat bar for machining |
| Commercial and structural sheet | Low-carbon hot rolled or cold rolled sheet and coil | Brackets, panels, enclosures, light formed parts |
| Welded mechanical tube | Low-carbon tube made from strip | Frames, furniture, handrail, non-structural tubing |
The ASTM A36/A36M scope (checked October 2026) describes A36 as carbon steel shapes, plates and bars of structural quality for riveted, bolted or welded construction of bridges and buildings and for general structural purposes. That “structural quality” with guaranteed tensile properties is the difference between A36 and a generic mild steel bar. A 1018 bar, for example, is ordered by chemistry and is not certified for structural strength unless the order asks for it.
Sheet products are also usually low-carbon steel. Whether they arrive hot rolled with mill scale or cold rolled with a bright finish is a separate question from the carbon content, and our hot rolled vs cold rolled steel guide explains why.
Mild Steel vs Other Common Steels
| Mild steel | High-strength structural steel | Stainless steel | |
|---|---|---|---|
| Example | A36, 1018 | A992, A572 Grade 50 | 304, 316 |
| Strength | Moderate | Higher (50 ksi minimum yield) | Varies by grade and condition |
| Rusts when bare | Yes | Yes | Resists rust because of its chromium content |
| Weldability | Excellent | Very good with the right procedure | Good, but needs stainless filler and technique |
One point surprises people: a modern wide flange beam is usually not mild steel in the A36 sense. W shapes in the US are normally rolled to ASTM A992, which has a 50 ksi minimum yield, compared with 36 ksi for A36. If you are matching an existing beam or replacing one, the grade on the drawings matters as much as the size.
What Mild Steel Is Used For
- Structural plate and small shapes: base plates, gussets, angles, channels and flat bar in buildings, usually as A36.
- Fabrication and repair: trailers, gates, racks, brackets and frames, because it cuts, drills, bends and welds easily.
- Vehicles: the ORNL report notes mild steels are commonly used in vehicle body structure and trunk closures.
- Sheet metal work: ducts, panels, cabinets and enclosures, usually galvanized or painted.
- Machined parts: 1018 and similar bar for shafts, pins, spacers and fixtures that do not need high hardness.
Buying Mild Steel: When the Name Is Not Enough
For a shelf bracket or a garden gate, “mild steel” is a perfectly good thing to ask for. For anything structural, it is not, because it does not guarantee a strength. Use this rule of thumb:
- Name the specification. Write “ASTM A36 plate” or “A992 W8x31”, not “mild steel plate” or “steel beam”.
- Ask for the mill test report on structural material. It ties the piece to a heat number with real chemistry and tensile results.
- Do not substitute by eye. A36, A572 Grade 50 and generic low-carbon bar can look identical. Only the paperwork and markings tell them apart.
- Get the sizing from an engineer. Any beam, column, post or connection that carries load has to be sized and checked by a licensed structural engineer, whatever the steel is called.
Mild Steel FAQ
Is mild steel the same as carbon steel?
Mild steel is one kind of carbon steel, the low-carbon kind. “Carbon steel” also includes medium and high carbon steels used for tools, springs and blades, which are much harder and much less weldable.
Is A36 mild steel?
Yes, A36 is generally treated as mild steel. The difference is that A36 is a specification with guaranteed minimum strength and chemistry limits, while “mild steel” on its own guarantees nothing.
Does mild steel rust?
Yes, quickly when bare and wet. Indoors in a dry space it can last indefinitely with a basic coat of paint. Outdoors it needs a proper coating system or galvanizing.
Can you harden mild steel?
Not much by quenching, because it has too little carbon. Case hardening can add a hard surface layer. If a part needs real hardness or wear resistance, start with a higher carbon or alloy steel instead.
Is mild steel strong enough for a beam?
Mild steel shapes were the standard choice for beams for decades, and A36 is still common for channels and angles. Whether a particular member is strong enough depends on its size, span and load, and that has to be calculated by a licensed structural engineer.
The Short Version
Mild steel is low-carbon steel, about 0.05 to 0.25 percent carbon, which makes it ductile, very weldable, magnetic, prone to rust and inexpensive. It is a family name rather than a grade. For anything structural, order a named specification such as ASTM A36, keep the mill test report, and have an engineer size the part.
Reference information only, not engineering advice. Disclaimer.