Q345B (now updated to Q355B under the new GB/T 1591-2018 standard) is a low-alloy high-strength structural steel grade from China. With a minimum yield strength of 345/355 MPa, it offers better mechanical performance than Q235B while maintaining excellent weldability. It is equivalent to ASTM A572 Gr.50 (USA), JIS SM490 (Japan), and EN S355JR (1.0045, Europe). Widely used in bridges, buildings, pressure vessels, and heavy machinery.
Chemical Composition
Maximum allowable content (% by mass) per GB/T 1591.
| Grade | C (%) | Si (%) | Mn (%) | P (%) | S (%) |
|---|---|---|---|---|---|
| Q345B | ≤0.20 | ≤0.50 | ≤1.70 | ≤0.035 | ≤0.035 |
| Q355B | ≤0.20 | ≤0.50 | ≤1.70 | ≤0.035 | ≤0.035 |
Note: Q345B and Q355B are essentially the same material. Q355B is the updated designation per GB/T 1591-2018, replacing Q345B (GB/T 1591-2008).
Mechanical Properties
| Grade | Yield (MPa) | Tensile (MPa) | Elongation (%) | Impact (J) |
|---|---|---|---|---|
| Q345B | ≥345 | 470–630 | ≥21 | 27 @ 20°C |
| Q355B | ≥355 | 470–630 | ≥22 | 27 @ 20°C |
Available Sizes & Delivery
| Thickness | 0.3–200 mm |
| Width | 1000–3000 mm |
| Length | 2500–12000 mm or as required |
| Delivery Condition | Hot rolled as-rolled / Normalized / TMCP |
| Standard | GB/T 1591 (Q345B: 2008 / Q355B: 2018) |
| Mill Test Certificate | EN 10204 3.1 with every shipment |
| Surface | Mill finish, pickled, or shot-blasted primed |
| MOQ | 3 Tons |
| Delivery Time | Within 7 days (≤500 tons) |
Equivalent Grades
| Standard | Grade | Country |
|---|---|---|
| GB/T 1591 | Q345B / Q355B | China |
| ASTM | A572 Gr.50 | USA |
| JIS G3106 | SM490 | Japan |
| EN 10025-2 | S355JR (1.0045) | Europe |
| DIN | St 52-3 | Germany |
Applications
- Bridge construction and heavy structural steelwork
- High-rise building columns and beams
- Pressure vessels and boilers
- Crane booms and heavy machinery frames
- Shipbuilding and offshore platforms
- Transmission towers and wind power structures
Values are typical reference ranges per GB/T 1591. Actual properties may vary by mill and production route. Mill test certificates (EN 10204 3.1) are supplied with every shipment — contact us for exact data sheets.
