1. Yield Strength and Temperature Range
| Property | Q235 | Q345 |
|---|---|---|
| Yield Strength (lower limit) | 235 MPa | 345 MPa |
| Temperature Range (B grade) | 0–350°C | -20–425°C |
| Material Type | Low-carbon structural steel | Low-alloy structural steel |
| Yield Strength Notes | Yield decreases with increasing thickness | Yield decreases with increasing thickness |
| Tensile Strength | ≥370 MPa | ≥510 MPa |
Explanation:
The letter "Q" in both Q235 and Q345 stands for "Qu" (yield), and the number represents the minimum yield strength in MPa.
Yielding occurs when stress exceeds the elastic limit; the lower yield point is the stable value used as the material's yield strength indicator.
2. Grades Based on Impact Temperature
Q235 Grades:
Q235A: No impact test
Q235B: 20°C impact test
Q235C: 0°C impact test
Q235D: -20°C impact test
Q235E: -40°C impact test
Q345 Grades:
Q345A: No impact test
Q345B: 20°C impact test
Q345C: 0°C impact test
Q345D: -20°C impact test
Q345E: -40°C impact test
Notes:
Q235 is typically used in general steel plates (old name: A3).
Q345 is part of low-alloy steel series (old name: 16Mn).
3. Mechanical Properties
| Property | Q235 | Q345 |
|---|---|---|
| Yield Strength | ≥235 MPa | ≥345 MPa |
| Tensile Strength | ≥370 MPa | ≥510 MPa |
| Strength Grade | I | II |
| Weldability | Good | Slightly poorer |
| Elongation | Large | Slightly smaller |
Notes:
Q235 is easier to weld and has higher ductility.
Q345 has higher strength, better low-temperature performance, but slightly less ductile and harder to weld.
4. Alloy Content
Q235: Ordinary carbon steel
Q345: Low-alloy steel
Implication: The higher alloy content in Q345 provides better mechanical properties, particularly for medium/low-pressure vessels, structural parts, and low-temperature applications.
5. Typical Applications
Q235:
General metal structural parts
Light-duty components not requiring high core strength
Levers, connecting rods, hooks, couplers, bolts, nuts, sleeves, shafts, welded parts
Q345:
Medium/low-pressure vessels, oil tanks
Vehicle parts, cranes, mining machinery
Power station, bridges, building structures
General metal structural parts requiring higher strength and low-temperature performance
Can be used in cold areas down to -40°C
6. Practical Considerations
Selection depends on structural design:
Strength-controlled designs: Q345 preferred
Stiffness-controlled designs: Q235 sufficient
Cost difference: Q345 is slightly more expensive, but high strength allows material savings.
Q235: Low-carbon, good weldability, moderate strength, cost-effective.
Q345: Low-alloy, higher strength, better low-temperature performance, slightly higher cost.
Cannot be used interchangeably without approval from the design engineer.
