Technical guide
Q235B and Q355B are the two grades that cover most square hollow section orders. The difference is yield strength (235 vs 355 MPa), but the practical decision is about weight, weld procedure and price — not the number on the certificate.
Q235B guarantees a minimum yield of 235 MPa; Q355B guarantees 355 MPa — about 50 % more. For the same load, Q355B lets you drop one wall thickness or one section size, which usually saves 8–15 % on steel weight.
The suffix letter is impact toughness: B = +20 °C, C = 0 °C, D = −20 °C, E = −40 °C. A cold-store or Nordic project needs D or E even if Q355B would otherwise be strong enough.
| Property | Q235B | Q355B |
|---|---|---|
| Yield strength (min) | 235 MPa | 355 MPa |
| Tensile strength | 370–500 MPa | 470–630 MPa |
| Elongation | ≥ 26 % | ≥ 22 % |
| Typical carbon | 0.17–0.20 % | 0.18–0.20 % + micro-alloy |
| Weldability | Very easy | Easy, needs matching consumable |
Use Q235B when the design is governed by stiffness or deflection rather than stress: guards, covers, signage frames, light shelving, handrails. It welds with anything, it is the cheapest grade in stock, and inspectors accept it without discussion.
It is also the safer choice for fabricators without a written WPS — Q355B needs a matching electrode (E50 class) and slightly higher heat input control.
Q355B wins as soon as the structure is strength-governed and weight matters:
Rule of thumb from our order book: below 100×100 mm, Q235B is usually specified; from 150×150 mm up, Q355B dominates because the weight saving is large enough to notice on the invoice.
Q355B carries a premium of roughly 3–6 % per tonne over Q235B at mill level, but it is the grade we hold 8,000 t of in stock, so lead time is identical. If your drawing says "S355J2H" or "ASTM A500 Gr.C", the equivalent we roll is Q355B/C/D — the mill certificate states the equivalence.
Related sizes: 100, 150, 200, 250.
Yes for strength — Q355B is stronger and welds similarly. Check impact requirements: Q355B is only +20 °C rated, so cold-climate work needs Q355D or E.
Slightly. Use E50 electrodes, keep heat input in the range of your WPS and preheat above 20 mm wall in cold weather. Standard MIG/MAG with matching wire is fine.
Q235B per tonne, but Q355B often wins per metre of finished structure because you can use a thinner wall.