Q235B vs Q345B Steel: Difference and International Equivalents
Quick Summary
Q235B is a 235 MPa minimum-yield mild structural steel (GB/T 700) used for general construction and light framing; Q345B is a 345 MPa minimum-yield high-strength low-alloy steel (GB/T 1591) used for bridges, cranes and heavy structures where ~47% more yield strength justifies its modest premium.
Q235B vs Q345B Steel: Difference and International Equivalents
Q235B is a 235 MPa minimum-yield mild structural steel (GB/T 700) used for general construction and light framing; Q345B is a 345 MPa minimum-yield high-strength low-alloy steel (GB/T 1591) used for bridges, cranes and heavy structures where ~47% more yield strength justifies its modest premium. Their nearest international equivalents: Q235B ≈ ASTM A36 / EN S235JR / JIS SS400, and Q345B ≈ ASTM A572 Gr.50 / EN S355JR. Choose Q235B for standard loads, Q345B when weight, span or section size drives the design.
Chinese structural steel dominates export supply chains for construction and machinery, and nearly every buyer's first order involves choosing between these two grades. The difference is simpler than most comparison pages make it: yield strength and alloying — and one substitution note that surprises buyers after 2019.
What the Grade Names Actually Encode
Chinese carbon steel grades start with "Q" (from 屈服, qufu — yield) followed by the minimum yield strength in MPa. The letter after the number is the impact-test temperature class: A (no impact test), B (+20 °C), C (0 °C), D (−20 °C), E (−40 °C). So:
- Q235B: 235 MPa minimum yield, impact-tested at +20 °C — mild structural steel to GB/T 700.
- Q345B: 345 MPa minimum yield, impact-tested at +20 °C — high-strength low-alloy (HSLA) steel to GB/T 1591.
Q345 achieves its higher yield not through heavy alloying but through small additions (manganese up to ~1.6%, plus micro-alloying with vanadium, niobium or titanium in some variants) refined by controlled rolling — which is also why its premium over Q235B is modest, typically 5–10% on plate.
Side-by-Side Comparison
| Property | Q235B | Q345B |
|---|---|---|
| Standard | GB/T 700 | GB/T 1591 |
| Min yield (≤16 mm) | 235 MPa | 345 MPa |
| Tensile strength | 370–500 MPa | 470–630 MPa |
| Carbon | ≤0.22% | ≤0.20% |
| Manganese | ≤1.40% | ≤1.60% |
| Alloying | Plain carbon | Micro-alloyed (V/Nb/Ti variants) |
| Elongation | ≥26% | ≥20% |
| Weldability | Excellent, no preheat needed in most sections | Very good; preheat for thick sections |
| Relative price | Baseline | +5–10% |
International Equivalents
The nearest cross-standard matches — "nearest," because chemistry windows and testing rules differ slightly; critical designs should verify against the original standard:
| Chinese | ASTM (USA) | EN (Europe) | JIS (Japan) |
|---|---|---|---|
| Q235B | A36 | S235JR | SS400 |
| Q345B | A572 Gr.50 | S355JR | SM490 |
Two verification notes buyers regularly need:
- Q235B vs SS400: SS400 has no yield-strength minimum in older JIS editions (it is a tensile-based specification), so "SS400 ≈ Q235B" is accepted in trade but the certificate should still show the 235 MPa yield when a structural design depends on it.
- Q345B is being superseded: China's 2018/2019 standard revision replaced Q345 with Q355 (GB/T 1591-2018), aligning chemistry with EN S355. Mill stock and drawings may show either designation; Q355 B-temper is the like-for-like successor. Export buyers seeing "Q345B" on offers from Chinese mills should confirm which edition the mill certifies against — both remain in circulation in the trade.
When Each Grade Is the Right Choice
Q235B — general construction, light frames, warehouse racking, fences and gates, non-load-bearing equipment structures, furniture tube. Anywhere the load is modest and the design is not weight-critical. Its ductility (elongation ≥26%) also makes it forgiving in cold forming and bending.
Q345B — bridge components, crane structures and booms, heavy equipment frames, high-rise structural members, long spans and anywhere a designer wants either more load per section or less weight per load. The strength step-up lets a design use thinner sections for the same load, which sometimes recovers the price premium in material weight alone — before accounting for welding and handling.
A practical boundary: if switching from Q235B to Q345B does not change section sizes in the design, the upgrade bought nothing — the designer should confirm the strength gain is actually exploited.
Buying Q235B/Q345B Plate from Chinese Mills
Both grades are staple export products. Yuze Metal supplies Q235B round bar 8–200 mm, hot-rolled plate 1250–2500 mm wide and 2000–12000 mm long, angle steel and seamless pipe, plus Q345B plate 1500–4000 mm, H-beam, welded pipe and seamless pipe for bridge and heavy-structure programs.
Order-confirmation checks that matter on export plate:
- Standard edition — GB/T 1591-2018 (Q355) vs the legacy Q345B designation, matched to the design drawings.
- Impact-test temperature class — B (+20 °C) is standard; C/D classes for cold-climate sites cost more and should be called out explicitly.
- Mill test certificate with yield, tensile and elongation heat-matched to the shipment — the certificate lines that structural approval actually checks.
- Z-direction (through-thickness) testing where welded T-joints risk lamellar tearing — a specified extra, not a default.
For a deeper dive on reading the certificates behind these grades, see the guide to mill test certificates; for choosing between Chinese suppliers generally, the metals supplier selection guide covers the verification sequence.
Talk to Yuze Metal About Structural Grades
Yuze Metal Limited supplies Q235B round bar 8–200 mm, hot-rolled plate 1250–2500 mm wide and up to 12000 mm long, angle steel and seamless pipe, plus Q345B plate to 4000 mm, H-beam and welded pipe for bridge and heavy-structure programs — with certificates that state the standard edition (Q345B vs Q355), impact-test class and full mechanicals per heat. Structural approvals are certificate-driven; ours are issued to match your design drawings.
Write to us at info@yuzemetal.com or message us on WhatsApp; we reply fast.
