Q345B and Q345D 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.
Standards and Equivalents
Primary Standard: GB/T 1591 - Hot-rolled low-alloy structural steel (China)
International Comparisons: ASTM A572, EN S355 families (similar HSLA steels, but not identical in impact or chemistry)
Grade Designations
| Grade | Yield Strength (MPa) | Impact Temperature | Notes |
|---|---|---|---|
| Q345B | 345 | 0 °C | General structural applications at ambient conditions |
| Q345D | 345 | −20 °C | Enhanced low-temperature fracture toughness for cold environments |
Chemical Composition (Typical Limits, wt%)
| Element | Q345B | Q345D | Notes |
|---|---|---|---|
| C | ≤0.20 | ≤0.20 | Slightly tighter C control for Q345D improves toughness and weldability |
| Mn | 0.4–1.6 | 0.4–1.6 | Main strength contributor |
| Si | ≤0.50 | ≤0.50 | Deoxidation and modest strengthening |
| P | ≤0.035 | ≤0.035 | Minimize embrittlement |
| S | ≤0.035 | ≤0.035 | Minimize inclusion and brittleness |
| Cr, Ni, Mo, V, Nb, Ti, B, N | Trace amounts | Trace amounts | Microalloying and controlled processing improve low-temperature toughness for Q345D |
Mechanical Properties
| Property | Q345B | Q345D | Notes |
|---|---|---|---|
| Yield Strength | ~345 MPa | ~345 MPa | Same nominal yield |
| Tensile Strength | 470–630 MPa | 470–630 MPa | Overlapping range; depends on thickness |
| Elongation | ≥20% | ≥20% | Comparable ductility |
| Charpy V-Notch | 27 J at 0 °C | 27 J at −20 °C | Q345D designed for better low-temperature impact resistance |
| Hardness (HB) | Moderate | Moderate | Not primary spec control; compatible with welding and forming |
Microstructure and Processing
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)
Microalloying with Nb, V, Ti
Controlled rolling schedules to refine grain size and improve Charpy impact performance
Applications of Q345B & Q345D H-Beams
General construction: buildings, industrial plants, bridges
Cold-climate structures: Q345D preferred for sub-zero temperature operation
Cranes, heavy machinery, and transportation infrastructure
Welded frames and structural support where toughness and ductility are essential
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.
