Sourcing structural steel from China but not sure what Q355 or Q460 really means? You’re not alone—understanding GB/T 1591 can save your project.
GB/T 1591-2018 defines China’s high-strength low-alloy structural steels like Q355 and Q460, covering mechanical strength, chemistry, delivery conditions, and weldability.
Whether you’re managing bridge projects or industrial steel structures, decoding this standard helps ensure safety, compliance, and quality.
1. What is GB/T 1591-2018 standard?
Think all structural steel is the same? This national standard proves otherwise.
GB/T 1591-2018 specifies technical requirements for structural steels with high yield strength, including plates, bars, H-beams, and more.
Dive deeper:
Originally released in 1979 and revised multiple times, the 2018 version includes major upgrades:
- Introduced Q355 grade to replace Q345
- Updated chemical composition and mechanical property tables
- Includes options for hot rolled, normalized, and TMCP deliveries
- Adds carbon equivalent (CEV) and Pcm weldability index
These steels are widely used in buildings, bridges, offshore platforms, and pressure-bearing equipment, thanks to their superior strength-to-weight ratio and good toughness.
2.How are Q-series grades defined?
What does Q355ND mean? It’s not random.
Q-series grades use a naming rule: Q + yield strength (MPa) + delivery condition + quality level.
Naming Breakdown:
| Segment | Meaning | Example |
|---|---|---|
| Q | “Qu” for Yield strength | Q355 |
| 355 | Minimum yield strength (MPa) | 355 MPa |
| N | Delivery condition (Normalized) | N |
| D | Quality level (impact toughness) | D (tested at -20°C) |
Common grades include:
- Q355B – basic quality, hot rolled
- Q355NHD – normalized delivery, tested at low temp
- Q460C/D/E – higher strength, better cold performance
This coding helps engineers quickly identify the mechanical profile and intended use.
3. Typical chemical composition of Q355/Q420/Q460
Curious about what elements make these steels “high strength”?
GB/T 1591 sets clear limits on C, Mn, Si, and microalloys like V, Nb, and Ti.
Dive deeper:
Example: Chemical Composition (% by mass)
| Grade | C (max) | Mn | Si | Nb+V+Ti (max) | P/S (max) |
|---|---|---|---|---|---|
| Q355B | 0.20 | 1.0–1.6 | ≤0.50 | ≤0.20 | ≤0.030 |
| Q420C | 0.22 | ≤1.70 | ≤0.50 | ≤0.30 | ≤0.025 |
| Q460E | 0.22 | ≤1.80 | ≤0.55 | ≤0.30 | ≤0.020 |
These controlled elements provide higher strength and good weldability while keeping brittle phases low. For normalized or TMCP grades, elements are carefully refined.
Yige Steel provides material certificates listing these compositions for every heat batch.
4. Mechanical properties explained
Numbers matter—especially when your structure carries tons of weight.
Key properties include yield strength, tensile strength, elongation, and impact energy.
Example: Q355 Mechanical Properties
| Property | Requirement |
|---|---|
| Yield Strength | ≥ 355 MPa (≤16 mm thick) |
| Tensile Strength | 470–630 MPa |
| Elongation | ≥ 21% |
| Impact Energy | 27J @ 0°C (Grade C) / -20°C (Grade D) / -40°C (Grade E) |
High strength doesn’t mean brittleness—these steels can stretch and absorb impact without failing suddenly.
5. Delivery conditions: AR, N, +N, TMCP
Delivery condition impacts toughness, weldability, and surface finish.
GB/T 1591 allows four main delivery states: AR, N, +N, and TMCP.
| Code | Description | Application |
|---|---|---|
| AR | As Rolled | General use |
| N | Normalized | Improved ductility |
| +N | Normalized Rolling | Better shape and toughness |
| TMCP | Thermo-Mechanical Control Process | Best strength/toughness combo |
⚠️ Not all suppliers offer TMCP—Yige Steel supports N/+N delivery for all Q-series grades.
6. Weldability of GB/T 1591 steels
Strong steel must still be weldable—or it fails in assembly.
GB/T 1591 introduces CEV and Pcm indexes to guide welding suitability.
- CEV formula:
C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 - Pcm formula (for low-C steels):
C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B
Guidelines:
| Grade | Max CEV | Max Pcm |
|---|---|---|
| Q355 | 0.39 | 0.20 |
| Q460 | 0.45 | 0.22 |
| Q690 | 0.49 | 0.25 |
Lower CEV = better weldability. Always preheat above 0.40 CEV, and ask your welding team to refer to EN ISO 1011-2 or AWS D1.1 procedures.
Continuing the article…
7. CEV vs Pcm – What’s the difference and why it matters?
Both measure weldability, but which one should you use?
CEV is used for general weldability evaluation, while Pcm is more accurate for low carbon steels and high-performance welding.
Dive deeper:
- CEV (Carbon Equivalent Value) is a traditional metric. It works well for predicting cracking tendencies in medium carbon and general-use steels.
- Pcm (Welding Crack Susceptibility Index) is better for low carbon, high-strength steels where microstructure control is crucial.
If your steel has C ≤ 0.12%, then Pcm is preferred. It better predicts the risk of cold cracking and hydrogen-induced issues during welding.
At Yige Steel, we provide both values on request so your welding engineers can make informed decisions.
8. Surface quality and tolerance requirements
What surface defects are acceptable? How straight is “straight enough”?
Surface finish and tolerances must meet GB/T 702, 709, and 11263, depending on the product type.
8.1 Surface Quality:
- No cracks, folds, delamination
- Minor scratches, rust allowed if not harmful
- Sharp edges must be removed
- Shot blasting or pickling optional
8.2 Dimensional Tolerance Examples:
| Product | Tolerance (Typical) |
|---|---|
| Plates | ±0.3–1.5 mm (thickness) |
| Bars | ±0.5 mm (diameter) |
| Beams | ±3 mm (height) |
Yige Steel can also provide ultrasonic testing and Z-direction (thickness) performance for projects needing strict NDT compliance.
9 Comparison with ASTM A572 and EN 10025
How does Q355 stack up against international standards?
Q-series steels are comparable to ASTM A572 Gr.50 and EN 10025 S355J2, with small variations.
| Property | Q355B | S355J2 | A572 Gr.50 |
|---|---|---|---|
| Yield (MPa) | ≥355 | ≥355 | ≥345 |
| Tensile (MPa) | 470–630 | 470–630 | 450–620 |
| Impact Temp | 0°C (B), -20°C (D) | -20°C | 0°C |
| CEV (max) | 0.39 | 0.45 | 0.47 |
💡 Tip: If your specification allows “equivalent or better,” Q355 can often be used instead of S355J2 or A572—especially in Asian projects.
10. Applications of Q355, Q420, Q460 steels
These aren’t just for buildings—think big.
GB/T 1591 steels are used in major infrastructure and mechanical projects due to their high yield strength and good formability.
Common Use Cases:
- Bridges and overpasses
- Storage tanks and pressure equipment
- High-rise building frames
- Marine and offshore structures
- Construction machinery and heavy equipment
- Railway infrastructure
Depending on the grade and quality class (B to E), you can choose steel suitable for cold climates, seismic zones, or corrosive environments.
How to source GB/T 1591 steel internationally
Buying from China? Ask smart questions.
Checklist when sourcing Q-series structural steel:
- Ask for GB/T 1591:2018 edition
- Confirm CEV/Pcm values if welding
- Verify impact test temperature (esp. for D/E grade)
- Request mill test certificates (MTCs) with full chemical & mechanical data
- Choose reliable suppliers who offer quality control and traceability
At Yige Steel, we offer export-ready Q-series steel plates and profiles with small MOQ (5 tons) and shipping support to Europe, Southeast Asia, and the Middle East.
Yige Steel’s offering: From Q355 to Q690M
Not just Q355—we stock the full series.
Yige Steel supplies structural steel plates and profiles from Q355 to Q690M with customizable delivery conditions (AR, N, +N).
- Q355B, C, D (stock items)
- Q460C/D/E (for load-bearing & impact-resistant structures)
- Q690 (high-strength heavy machinery)
We offer:
- Cold and hot rolled stock
- MTCs, 3rd party inspection, SGS/BV reports
- Support for CIF/DDP delivery to port or jobsite
FAQs about GB/T 1591 standard
- Is Q355 equal to S355J2?
Largely equivalent, but check CEV and impact temperature. Q355D ≈ S355J2. - What is the difference between Q355B and Q355D?
Mainly impact testing. B is tested at room temp, D is tested at -20°C. - Can Q460 be welded easily?
Yes, with proper preheat and using low-hydrogen electrodes. Mind the CEV/Pcm. - Does Q690M require preheating?
Yes. For high strength grades, preheat and controlled cooling are critical. - Is Q420M used in shipbuilding?
Sometimes, but only if approved by classification societies. Better to use CCS or ABS certified plates. - Is impact testing mandatory?
For grades D and E—yes. For B, not required unless specified.
Conclusion
GB/T 1591-2018 is a comprehensive and reliable standard for high-strength low-alloy structural steels. With grades like Q355, Q420, and Q460, it provides flexible choices for modern construction, machinery, and transport applications.
By understanding the grade coding, mechanical properties, and delivery options, buyers can confidently select the right steel for their project.
