Structural steel is a type of carbon steel, and in composition most of it is what the trades call mild steel: low-carbon steel that is strong, ductile and easy to weld. What makes it "structural" is that it is produced to a recognized grade, such as CSA G40.21 350W or ASTM A992, with a guaranteed minimum strength and a mill certificate. Stainless steel is a separate family of alloys, defined by at least 10.5% chromium, and it is not what beams and columns are normally made of.
The terms overlap, which is why "structural steel vs mild steel" gets asked so often. Here's how they relate and what to put on an order.
The labels at a glance
| Term | What it means | Guaranteed properties? | Typical uses |
|---|---|---|---|
| Carbon steel | Steel whose main alloying element is carbon | Only if a grade is specified | Almost everything: structures, pipe, machinery, tools |
| Mild steel | Shop name for low-carbon steel | No: it is not a grade | General fabrication, brackets, gates, railings |
| Structural steel | Steel made to a structural specification and grade | Yes: minimum yield, tensile and chemistry | Beams, columns, angles, channels, HSS, plate |
| Stainless steel | Alloy steel with at least 10.5% chromium | Yes, by grade (for example 304, 316) | Railings, food and pharma plants, marine and architectural work |
So these aren't four separate materials. Structural steel sits inside carbon steel, mostly at the low-carbon end that people call mild steel. Stainless sits outside that group.
Carbon steel: low, medium and high carbon
Carbon is what turns iron into steel, and the amount changes the steel's character a great deal:
- Low-carbon (mild) steel. The least carbon. Soft enough to bend and form, tough, and easy to weld. It is the most common steel because it is inexpensive and does most jobs well.
- Medium-carbon steel. More carbon, giving more strength and hardness but less ductility. Used for shafts, axles, gears and rails, often heat treated.
- High-carbon steel. Hard and strong but brittle by comparison and difficult to weld. Used for springs, cutting tools and high-strength wire.
Between plain carbon steel and true alloy steel sits a group often called high-strength low-alloy (HSLA) steel. These grades add small, controlled amounts of elements to raise strength while keeping the steel weldable. Several common structural grades, including 350W and ASTM A572 Grade 50, are made this way, which is why a 350W beam is stronger than an older 300W or A36 section without being any harder to weld under the right procedure.
The published boundaries between these classes vary from one reference to another, so it is better to think of them as a spectrum than as fixed cut-offs.
Building steel lives at the low-carbon end for good reasons. Structures need steel that stretches and yields visibly before it fails, and that can be welded in the shop and on site without cracking. Higher-strength structural grades get their strength mostly from controlled chemistry and small alloy additions rather than from more carbon.
Is structural steel mild steel?
In everyday shop talk, yes. A W-beam, an angle or a base plate is low-carbon steel, and a welder will call it mild steel. The difference is what's guaranteed:
- A structural grade comes with numbers. CSA G40.21 300W guarantees a minimum yield of 300 MPa, 350W guarantees 350 MPa, and ASTM A36 guarantees 250 MPa. Engineers design to those minimums.
- "Mild steel" comes with none. It describes the type of steel, not its strength. Two pieces of "mild steel" can differ, and neither has to meet a design value.
- Traceability. Structural steel is marked with heat numbers that link it to a mill certificate showing its chemistry and test results. Steel with no paperwork can't simply be assumed to meet a grade.
That's why a structural drawing never says "mild steel." It names a grade. Our guides to structural steel grades in Canada and steel plate grades explain the grade names and what each one guarantees.
Where "mild steel" is fine: non-structural brackets, trims, gates, guards designed by a fabricator, and other pieces where the drawings or the designer don't call for a specific grade. Even there, most stock is already certified to a structural grade, because that's what mills produce.
Structural steel vs stainless steel
Stainless steel gets its corrosion resistance from chromium, which forms a thin, self-healing oxide film on the surface. The most common grades in construction are austenitic stainless steels such as 304 and 316, with 316 used where chlorides (salt, de-icers, pools, coastal air) are a concern.
How stainless compares with structural carbon steel:
- Corrosion. Stainless resists rust without coatings. Structural steel needs primer, paint or galvanizing wherever moisture reaches it.
- Cost. Stainless costs considerably more as material and takes more care to fabricate.
- Fabrication. It needs its own welding procedures and should be kept away from carbon steel tools and grinding dust, which can embed in the surface and cause rust spots.
- Design. It behaves differently under load and heat, and it is designed with its own rules. The Ontario Building Code's own notes acknowledge there is currently no Canadian standard for designing cold-formed stainless steel structural members, and point designers to an American standard in the interim.
- Stiffness. Being stainless does not make a member stiffer. Deflection depends mainly on the section size and span.
For outdoor structural steel, the usual answer in Ontario isn't stainless; it's carbon steel that is galvanized or properly painted. See galvanized vs painted structural steel for how those finishes compare.
Which is better: mild steel or stainless steel?
Neither is better in general. The question is what the piece has to do and where it lives.
Choose structural carbon (mild) steel when:
- The piece carries load in a beam, column, frame or connection.
- It will be indoors, encased in concrete, or protected by a coating.
- Budget and ease of fabrication matter.
Choose stainless steel when:
- The piece is exposed to salt, chemicals, pools or wash-downs and can't be easily recoated.
- Hygiene matters, as in food and pharmaceutical plants.
- The finish is on show and must stay bright without paint.
- The specification calls for it, for example for certain anchors and attachments on building exteriors.
For railings, where this choice comes up most often, see stainless vs mild steel railings.
What to put on the order
Avoid generic labels on anything structural:
- Name the grade, for example CSA G40.21 350W for HSS and plate, or ASTM A992 for W-shapes, as the drawings require.
- Ask for mill certificates if the project specification requires them.
- State the finish: bare, primed, painted or galvanized.
- For stainless, name the grade (304, 316) and the surface finish, and confirm the shop handles stainless.
If your drawings say only "steel" or "mild steel" for a structural piece, ask your designer to confirm the grade before fabrication.
Structural steel supplied to your drawings
We fabricate beams, columns, posts, lintels, plates and channels in structural grades to your shop drawings, with CWB certified welding, and we can prime or galvanize to suit. We deliver across the GTA and Ontario and supply and install structural steel on residential, commercial and industrial jobs.
Send your drawings for a free quote, or see our steel products.
Frequently asked questions
Is structural steel the same as mild steel?
In composition, mostly yes: structural steel is a low-carbon steel, which is what people mean by mild steel. The difference is the guarantee. Structural steel is made to a grade such as CSA G40.21 350W or ASTM A992, with a minimum yield strength and a mill certificate. Mild steel is a shop name with no guaranteed properties.
Is structural steel carbon steel?
Yes. Ordinary structural steel is a carbon steel, usually low in carbon for ductility and weldability, with small additions of other elements in the higher-strength grades. It is not stainless and it rusts unless it is protected.
Is structural steel stainless?
No. Stainless steel contains at least 10.5% chromium, which forms a thin protective film on the surface. Structural steel does not, so exterior structural steel is primed, painted or galvanized instead.
Which is better, mild steel or stainless steel?
It depends on the job. Mild and structural carbon steel is less expensive, easy to cut and weld, and ideal for beams, columns and plates that are protected from corrosion. Stainless steel is chosen where corrosion resistance, hygiene or appearance justify its higher cost and more careful fabrication.
