If you are sourcing structural steel from China, you have almost certainly encountered the shift from GBT 1591-2008 Q345 vs GBT 1591-2018 Q355. This change, which officially took effect on February 1, 2019, has caused confusion for engineers, buyers, and fabricators worldwide. Is Q355 simply a renamed Q345? Does the 10 MPa yield strength difference actually matter in design? Which standard should your project specify?
This guide draws on our metallurgical expertise and daily procurement reality to explain exactly what changed, what stayed the same, and how to make smart sourcing decisions for your structural projects
Understanding GBT 1591-2008 Q345 Steel Plate - The Old Workhorse
What Was GBT 1591-2008 Q345 Material?
GBT 1591-2008 Q345 was the Chinese national standard for high-strength low-alloy structural steels that served as the backbone of construction, bridge building, and machinery manufacturing for over a decade. Under this standard, Q345 designated a steel with a minimum yield strength of 345 MPa, with quality grades ranging from Q345A to Q345E, differentiated primarily by their impact toughness requirements.
Q345B, the most commonly used grade, required Charpy V-notch impact testing at +20°C, while Q345D tested at -20°C and Q345E at -40°C.
The chemical composition of Q345B under the 2008 standard for plates up to 16 mm thickness limited carbon to 0.20% maximum, manganese to 1.00–1.60%, silicon to 0.50% maximum, phosphorus to 0.035% maximum, and sulfur to 0.035% maximum. These controlled limits ensured good weldability while providing the strength necessary for structural frames, bridges, and heavy equipment. The standard served the industry well, but as Chinese steel production became increasingly precise and global projects demanded higher performance margins, the need for an updated standard became clear.
Understanding GBT 1591-2018 Q355 Steel Plate - The Modern Replacement
What Is GBT 1591-2018 Q355 Material?
GBT 1591-2018 Q355 is the direct successor to Q345 under the revised national standard. The most visible change is the yield strength designation: Q355 guarantees a minimum yield strength of 355 MPa, a 10 MPa increase over Q345. This was not merely a paper adjustment - in practice, most Q345 steel produced in China already exceeded 345 MPa, commonly reaching 355 MPa or higher. The 2018 standard simply aligned the grade designation with the actual typical performance of the steel, while formally raising the minimum threshold.
Quality grades under the 2018 standard follow the same pattern - Q355B, Q355C, Q355D, Q355E - and now also include Q355F, a new grade tested for impact toughness at -60°C, which did not exist under the 2008 standard. This addition expands the usable temperature range for Chinese structural steels in extreme-cold applications. The chemical composition limits for Q355B were adjusted slightly: carbon is allowed up to 0.24% maximum (a relaxation from the 0.20% limit in Q345B for thin plates), while manganese is set at 1.60% maximum (without a minimum), phosphorus and sulfur limits remain at 0.035% each.
Key Differences Between GBT 1591-2008 Q345 and GBT 1591-2018 Q355
Chemical Composition Differences Between Q345 and Q355
The chemical composition adjustments between the two standards are subtle but significant for welding procedure qualification and material traceability. The following table compares Q345B and Q355B as the most commonly specified grades.
| Element | Q345B (GB/T 1591-2008, ≤16 mm) | Q355B (GB/T 1591-2018, ≤16 mm) | Change |
|---|---|---|---|
| Carbon (C), max % | 0.20 | 0.24 | Relaxed |
| Silicon (Si), max % | 0.50 | 0.55 | Relaxed |
| Manganese (Mn), % | 1.00–1.60 | ≤ 1.60 | Lower minimum removed |
| Phosphorus (P), max % | 0.035 | 0.035 | Unchanged |
| Sulfur (S), max % | 0.035 | 0.035 | Unchanged |
| Niobium (Nb), max % | 0.07 | 0.05 | Tightened |
| Vanadium (V), max % | 0.15 | 0.15 | Unchanged |
| Titanium (Ti), max % | 0.20 | 0.20 | Unchanged |
Mechanical Property Differences Between Q345 and Q355
The most practical difference between the two standards lies in the guaranteed minimum yield strength values, which vary by plate thickness. The following table presents a direct comparison.
| Thickness Range (mm) | Q345 Yield Strength, min (MPa) | Q355 Yield Strength, min (MPa) | Increase (MPa) |
|---|---|---|---|
| ≤ 16 | 345 | 355 | +10 |
| 16–40 | 335 | 345 | +10 |
| 40–63 | 325 | 335 | +10 |
| 63–80 | 315 | 325 | +10 |
| 80–100 | 305 | 315 | +10 |
| Property | Q345 (all thicknesses) | Q355 (all thicknesses) |
|---|---|---|
| Tensile Strength | 470–630 MPa | 470–630 MPa |
| Elongation, min | 20–22% (thickness-dependent) | 20–22% (thickness-dependent) |
| Impact Test Temperature (Grade B) | +20°C | +20°C |
| Impact Test Temperature (Grade C) | 0°C | 0°C |
| Impact Test Temperature (Grade D) | -20°C | -20°C |
| Impact Test Temperature (Grade E) | -40°C | -40°C |
| Impact Test Temperature (Grade F) | Not available | -60°C |
The consistent 10 MPa increase in minimum yield strength across all thickness ranges means that Q355 plates can carry approximately 3% more load than Q345 plates of the same cross-section. In many practical designs, this small difference may be absorbed by existing safety factors, but for optimized designs or strict compliance with updated Chinese codes, Q355 is now the mandatory specification. Tensile strength ranges and elongation requirements remain unchanged, as do the impact test temperatures for the common B, C, D, and E grades.
Quality Grade System and New Q355F Grade
One noteworthy addition under the 2018 standard is the introduction of Q355F, which requires Charpy V-notch impact testing at -60°C with minimum absorbed energy values. This grade did not exist under the 2008 standard and provides a Chinese-standard high-strength structural steel option for projects in extreme arctic environments, cryogenic storage facilities, and polar infrastructure - applications that previously required European EN 10025-6 S690QL1 or proprietary grades.
International Equivalents and Cross-Reference
When integrating Chinese Q345 or Q355 steel into global projects, understanding international equivalents is essential. Both Q345 and Q355 are functionally comparable to EN 10025-2 S355JR, S355J0, and S355J2 grades, as well as ASTM A572 Grade 50. The following cross-reference table provides a quick mapping:
| Chinese Standard | Grade | EN Equivalent | ASTM Equivalent |
|---|---|---|---|
| GB/T 1591-2008 | Q345B | S355JR | A572 Gr. 50 |
| GB/T 1591-2018 | Q355B | S355JR | A572 Gr. 50 |
| GB/T 1591-2008 | Q345D | S355J2 | A572 Gr. 50 + impact |
| GB/T 1591-2018 | Q355D | S355J2 | A572 Gr. 50 + impact |
| GB/T 1591-2018 | Q355F | S355NL | Not directly equivalent |
Because Q355 is now the current standard, specifying the updated grade facilitates smoother cross-border project acceptance, particularly when European engineering firms and certifying authorities review the material documentation.
Send us your requirement today - whether you need Q345B for a maintenance project or Q355E for a new arctic-grade structure. Tell us the grade, thickness, quantity, and destination, and receive a free quotation with full compliance certificates within 24 hours. Let's build stronger together.
Q1: Is Q355 simply a renamed Q345?
Not exactly. While the change appears small, Q355 has a formally raised minimum yield strength of 355 MPa, adjusted chemical composition limits, and an additional Q355F grade for -60°C service. Most Q345 material already met Q355 requirements, but the new standard makes it mandatory.
Q2: Can I use Q345 mill certificates for a project specifying Q355?
No. If your project specification calls for GB/T 1591-2018 Q355, the mill certificate must reference the 2018 standard. Accepting a Q345 certificate on the basis of "it probably meets Q355" is a compliance risk. GNEE Steel supplies material with the correct certificate for your required standard.
Q3: Does the Q345 to Q355 change affect welding procedures?
It may. Due to the slight relaxation of carbon and silicon limits, the carbon equivalent (CEV) of some Q355 heats may differ from Q345 heats. Existing welding procedure specifications (WPS) qualified for Q345 should be reviewed and re-qualified if necessary.
Q4: Are Q345 and Q355 the same price?
For practical purposes, yes. The cost difference is minimal, typically absorbed within standard pricing bands for S355-equivalent structural steel. GNEE Steel offers competitive factory-direct quotes for both.
Q5: Is Q355B the same as S355JR?
Q355B and S355JR are functionally equivalent with comparable yield strength, tensile range, and +20°C impact toughness. For formal project acceptance, confirm equivalency with the design engineer.
