Q345 is a widely used Chinese-designation high-strength low-alloy (HSLA) structural steel series, applied globally in bridges, pressure vessels, heavy machinery, and structural components. Within this family, suffix letters denote variants tested at different impact temperatures:
Q345B – General structural grade, tested at 0 °C
Q345C – Colder-grade variant, tested at −40 °C
These grades share similar chemical composition and yield strength (~345 MPa), but their low-temperature toughness differs, guiding material selection for ambient or cold-service applications.
1. Standards and Equivalents
Primary Standard: GB/T 1591 - Hot-rolled steel for welded structures (China)
International Comparisons: ASTM A572 Grade 50, EN S355 (similar HSLA steels), JIS G3106 (different classification approach)
2. Chemical Composition (Typical Limits, wt%)
| Element | Q345B | Q345C | Notes |
|---|---|---|---|
| C | ≤0.20 | ≤0.20 | Low carbon for weldability; tighter control for Q345C may improve low-temp toughness |
| Mn | 0.50–1.60 | 0.50–1.60 | Strength and hardenability |
| Si | 0.10–0.50 | 0.10–0.50 | Deoxidation; slight strength gain |
| P | ≤0.035 | ≤0.035 | Impurity control for toughness |
| S | ≤0.035 | ≤0.035 | Same as above |
| Cr, Ni, Mo, V, Nb, Ti, B, N | Trace amounts | Trace amounts | Microalloying and TMCP for Q345C improves low-temp toughness |
Notes:
Microalloying elements (Nb, V, Ti) refine grain structure and improve precipitation strengthening, enhancing low-temperature fracture resistance.
Both grades maintain low carbon equivalents for good weldability.
3. Microstructure and Heat Treatment
As-Rolled Microstructure: Predominantly ferrite–pearlite with microalloy precipitates.
Q345C vs Q345B: Achieves lower-temperature toughness via tighter control of impurities, rolling schedules, and thermo-mechanical processing (TMCP).
Heat Treatments:
Normalizing: improves toughness modestly.
Quenching & tempering: not typical; these steels are delivered hot-rolled/controlled-rolled.
4. Mechanical Properties
| Property | Q345B | Q345C | Notes |
|---|---|---|---|
| Yield Strength | ≥345 MPa | ≥345 MPa | Same nominal yield |
| Tensile Strength | 470–630 MPa | 470–630 MPa | Thickness dependent |
| Elongation | ≥20% | ≥20% | Comparable ductility |
| Impact Toughness | 27 J at −20 °C | 27 J at −40 °C | Key differentiator; Q345C suited for colder environments |
| Hardness (HB) | 120–190 | 120–190 | Similar, process-dependent |
5. Weldability
Both grades exhibit good weldability due to low carbon and controlled alloying.
Q345C's tighter low-temperature requirements may slightly affect carbon equivalent, but proper welding procedures ensure performance.
Heavy sections require controlled preheat/interpass temperatures.
6. Corrosion and Surface Protection
Both are non-stainless structural steels with modest inherent corrosion resistance.
Typical protection:
Hot-dip galvanizing
Primer + topcoat painting (epoxy, polyurethane)
7. Fabrication and Machinability
Cutting: Plasma, laser, oxy-fuel
Bending/Forming: Readily formable; tighter bend radii for Q345C at ultra-cold service need attention
Machinability: Slightly reduced with microalloying; standard tooling adjustments suffice
8. Typical Applications
| Q345B | Q345C |
|---|---|
| General structural sections: beams, H-beams, channels, plates for buildings, cranes | Cold-climate structural components: offshore platforms, refrigerated storage supports |
| Bridges, civil engineering (−20 °C toughness sufficient) | Pressure vessels, low-temperature frames (−40 °C) |
| Machinery bases and welded frames | Petrochemical piping supports, cold-region infrastructure |
| Cost-sensitive projects where standard performance suffices | Safety-critical applications with low-temp brittle fracture risk |
Selection Guideline:
Use Q345B for ambient conditions where −20 °C toughness is adequate.
Use Q345C for colder environments or where higher safety margins against brittle fracture are required.
