The main difference is strength: S690QL is a high-strength steel with a minimum yield strength of 690 MPa, achieved through quenching and tempering (Q+L), while S355 is a standard structural steel (like S355J2+N) with a minimum yield strength of 355 MPa, typically delivered normalized. This means S690QL is significantly stronger, allowing for lighter structures, but S355 is more common and cost-effective for general applications, with S690QL requiring careful design, especially for fatigue, due to its enhanced properties and processing.
S355 Steel (e.g., S355J2+N)
Strength: Lower (min. 355 MPa yield).
Processing: Hot-rolled and normalized.
Application: Common, general-purpose structural applications, often used where fatigue loads are a concern due to its predictable behavior and cost.
Chemical composition % of steel S355JR (1.0045): EN 10025-2-2004
| Nominal thickness =< 30(mm) CEV max = 0.45 |
S690QL Steel
Strength: High (min. 690 MPa yield).
Processing: Quenched and Tempered (Q+L) for enhanced strength and toughness.
Composition: Contains alloying elements like Niobium (Nb), Titanium (Ti), Vanadium (V) for grain refinement and strength.
Application: Used for lighter, high-performance structures (e.g., cranes, heavy machinery), but requires careful notch design to manage fatigue crack initiation.
Chemical Composition of S690QL Steel (Typical, Max %)
| Element | % | Element | % |
|---|---|---|---|
| C | 0.20 | Cu | 0.50 |
| Si | 0.80 | Mo | 0.70 |
| Mn | 1.70 | Nb* | 0.06 |
| P | 0.025 | Ni | 2.00 |
| S | 0.015 | Ti* | 0.05 |
| N | 0.015 | V* | 0.12 |
| B | 0.005 | Zr* | 0.15 |
| Cr | 1.50 |
* At least 0.015% of grain-refining elements shall be present. Aluminium may also be used as a grain-refining element.
Key Takeaway
S690QL offers significantly higher strength and performance for lighter designs, but S355 remains the workhorse for general construction due to its balance of cost and adequate properties, with S690QL needing specialized design consideration for fatigue.