What EN 10025-6 S690QL1 Designates
S690QL1 is a quenched and tempered structural steel plate covered by EN 10025-6, the European standard for technical delivery conditions of flat products of high yield strength structural steels in the quenched and tempered condition. The designation is read in three parts: S for structural steel, 690 for the minimum yield strength in MPa, and QL1 for the impact toughness class. Within the standard, Q is tested at -20 °C, QL at -40 °C and QL1 at -60 °C, so QL1 is the toughest class. Tensile strength for the usual supply thicknesses falls between 770 and 940 MPa.
The combination of a 690 MPa minimum yield strength with verified toughness at -60 °C makes the grade suitable for structures that must be light and safe at the same time in cold service conditions, where a lower strength steel would require heavier sections.
Quenching and Tempering Control
Plates are supplied in the quenched and tempered condition. Quenching cools the plate faster than still air and produces a hard martensitic structure; tempering is then applied to bring strength, ductility and toughness into the required balance. Both steps are controlled over the full plate length so that properties stay uniform from head to tail and from surface to core.
Process points that follow from the delivery condition:
Roller pressure quenching is used for thin and medium plate to limit distortion.
Tempering temperature is selected so that yield strength stays above 690 MPa while impact energy at -60 °C is preserved.
Plates are levelled and, where required, ultrasonically tested after heat treatment.
Test coupons are taken in the final delivery condition, so the reported results represent the steel as supplied.
Chemical Composition
Maximum ladle analysis values in percent by mass:
| Element | C | Si | Mn | P | S | N | B |
|---|---|---|---|---|---|---|---|
| Max % | 0.20 | 0.80 | 1.70 | 0.025 | 0.015 | 0.015 | 0.005 |
| Element | Cr | Cu | Mo | Nb | Ni | Ti | V |
| Max % | 1.50 | 0.50 | 0.70 | 0.06 | 2.00 | 0.05 | 0.12 |
Carbon is held at or below 0.20 % to protect weldability and low temperature toughness. Niobium, titanium and vanadium act as micro-alloying elements, while nickel, chromium and molybdenum support hardenability through the thickness. Boron is limited to 0.005 % so that the quenched structure remains predictable, and sulphur and phosphorus are kept low to avoid inclusions that would reduce notch toughness at -60 °C.
Mechanical Properties by Thickness
| Thickness range (mm) | Yield strength min (MPa) | Tensile strength (MPa) | Elongation min (%) | Impact at -60 °C |
|---|---|---|---|---|
| 3 < t ≤ 50 | 690 | 770-940 | 14 | 30 J |
| 50 < t ≤ 100 | 650 | 760-930 | 14 | 30 J |
| 100 < t ≤ 150 | 630 | 710-900 | 14 | 30 J |
Yield strength decreases in steps as thickness grows, reflecting the slower cooling rate and the coarser microstructure in the core of heavy plate. Impact testing uses Charpy V-notch specimens at -60 °C, and the minimum average energy of 30 J applies across the whole thickness range listed above.
Dimensional Range and Supply Options
Thickness: 3 to 300 mm
Width: 1500 to 4050 mm
Length: 3000 to 27000 mm
Delivery condition: quenched and tempered, with mill test certificates to EN 10204 3.1
Edges: sheared or gas cut, with machined edges available for critical components
Third party inspection: ABS, DNV, GL, CCS, LR, RINA, KR, TUV and CE marking arrangements
Plates are supplied with the marking, plate identity and test documents needed for traceability from the heat number to the finished component.
Applications and Fabrication Notes
The grade is used for mobile crane booms and outriggers, excavator arms, mining truck bodies, concrete pump booms, bridge girder components, offshore structures and heavy transport frames, where a lower steel weight translates into a higher payload or a lighter supporting structure.
Welding consumables matched to the 690 MPa strength class should be used, with preheat applied as thickness and restraint increase.
Cutting is normally done by plasma, laser or waterjet for thin plate and by gas cutting for heavy plate; cut edges are ground before welding.
Forming should respect a minimum bend radius that increases with thickness, and cold forming is best carried out above 5 °C.
Drilling and machining use carbide tooling at reduced cutting speed because of the hardness of the quenched and tempered plate.
Frequently Asked Questions
Q: What is the difference between S690Q, S690QL and S690QL1?
All three share the same 690 MPa minimum yield strength and differ only in the guaranteed impact test temperature: Q is tested at -20 °C, QL at -40 °C and QL1 at -60 °C.
Q: What is the minimum yield strength of S690QL1?
690 MPa for plate up to 50 mm thick, 650 MPa between 50 and 100 mm, and 630 MPa between 100 and 150 mm.
Q: Can S690QL1 plate be welded?
Yes. It is welded with consumables matched to the 690 MPa class, controlled heat input and preheat that increases with plate thickness and joint restraint.
Q: Why is the plate quenched and tempered?
Quenching creates the high strength level, and tempering restores the ductility and the 30 J impact energy at -60 °C that the grade guarantees.
Q: What sizes can be supplied?
Thickness from 3 to 300 mm, width from 1500 to 4050 mm and length from 3000 to 27000 mm; other combinations depend on the rolling programme.
Q: Which documents accompany the steel?
A mill test certificate to EN 10204 3.1 is standard, and inspection by classification societies such as ABS, DNV or LR can be arranged before delivery.