Q345BQ345D are two grades of low-alloy structural steel under the Chinese standard GB/T 1591, commonly used in welded structures, bridges, cranes, and cold-climate equipment.
Both are H-beam compatible, providing excellent load-bearing capacity and structural stability, but they differ primarily in low-temperature toughness.
GB/T 1591 - Hot-rolled low-alloy structural steel (China)
ASTM A572, EN S355 families (similar HSLA steels, but not identical in impact or chemistry)
| Q345B | 345 | 0 ℃ | |
| Q345D | 345 | −20 ℃ | Enhanced low-temperature fracture toughness for cold environments |
| Q345B | Q345D | ||
|---|---|---|---|
| C | <=0.20 | <=0.20 | Slightly tighter C control for Q345D improves toughness and weldability |
| 0.4–1.6 | 0.4–1.6 | ||
| <=0.50 | <=0.50 | ||
| P | <=0.035 | <=0.035 | |
| S | <=0.035 | <=0.035 | |
| Microalloying and controlled processing improve low-temperature toughness for Q345D |
| Q345B | Q345D | ||
|---|---|---|---|
| ~345 MPa | ~345 MPa | ||
| 470–630 MPa | 470–630 MPa | ||
| >=20% | >=20% | ||
| Charpy V-Notch | 27 J at 0 ℃ | 27 J at −20 ℃ | Q345D designed for better low-temperature impact resistance |
Both grades have a ferrite–pearlite microstructure in the as-rolled condition.
Q345D achieves better low-temperature toughness via:
Fine-grain thermo-mechanical controlled processing (TMCP)
Applications of Q345B & Q345D H-Beams
Cold-climate structures: Q345D preferred for sub-zero temperature operation
Q345B H-Beams: General structural use at ambient conditions.
Q345D H-Beams: Same nominal strength but optimized for low-temperature impact resistance, reducing brittle fracture risk in cold environments.
