What S690 Means in EN 10025-6
S690 is the European designation for a quenched and tempered high yield strength structural steel plate governed by EN 10025-6. The letter S marks structural steel, 690 is the minimum yield strength in MPa over the thinner thickness bands, and the Q shows that the plate is delivered in the quenched and tempered condition rather than normalized or thermomechanically rolled. The grade is supplied in three toughness classes: S690Q impact tested at -20 °C, S690QL at -40 °C and S690QL1 at -60 °C, each with a minimum Charpy V-notch energy of 30 J on longitudinal specimens.
The high strength comes from a low-carbon, micro-alloyed composition that is water quenched to a martensitic or bainitic structure and then tempered to a controlled strength and toughness balance. The result is a steel with roughly double the yield strength of a conventional S355 structural grade, which allows lighter sections, lower dead weight and reduced welding volume in heavily loaded structures.
Chemical Composition
The chemistry is common to the S690Q, S690QL and S690QL1 classes; only the impact test temperature changes between them.
| Element (max, %) | S690Q / S690QL / S690QL1 |
|---|---|
| C | 0.20 |
| Si | 0.80 |
| Mn | 1.70 |
| P | 0.025 |
| S | 0.015 |
| N | 0.015 |
| B | 0.005 |
| Cr | 1.50 |
| Cu | 0.50 |
| Mo | 0.70 |
| Nb | 0.06 |
| Ni | 2.00 |
| Ti | 0.05 |
| V | 0.12 |
| Zr | 0.15 |
Values are maximum figures for the product analysis. Carbon equivalent, calculated as CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, is noticeably higher than for normalized structural steels, so welding procedures must be qualified for the actual joint rather than transferred from a lower strength grade.
Mechanical Properties by Thickness
| Thickness range (mm) | Minimum yield strength (MPa) | Tensile strength (MPa) | Minimum elongation (%) | Charpy V-notch |
|---|---|---|---|---|
| 3 to 50 | 690 | 770-940 | 14 | 30 J longitudinal, 27 J transverse |
| over 50 to 100 | 650 | 760-930 | 14 | 30 J longitudinal, 27 J transverse |
| over 100 to 150 | 630 | 710-900 | 14 | 30 J longitudinal, 27 J transverse |
Strength falls by roughly 60 MPa between the thinnest and the thickest band in this table, and ductility is lower than for normalized grades, which is normal for quenched and tempered high strength steel. Design should use the yield value for the thickness actually ordered, and forming and machining allowances should be based on the measured hardness and yield level of the delivered plate.
Toughness Classes and the Q, QL, QL1 Difference
The three classes are chemically identical and differ only in the guaranteed impact test temperature. S690Q is tested at -20 °C, S690QL at -40 °C and S690QL1 at -60 °C, in every case at 30 J minimum on longitudinal specimens and 27 J on transverse specimens. Matching the class to the minimum design metal temperature avoids paying for toughness that the application does not need, and conversely prevents an under-specified plate being used in a cold-service structure.
All three classes belong to the same EN 10025-6 delivery condition, so the choice of class does not change the strength tables or the chemical limits shown above; it only changes the low-temperature verification that accompanies the plate.
Manufacturing Route, Fabrication and Applications
Plates are produced through a controlled steelmaking route: selection of scrap, pig iron and molten iron, melting in an electric arc furnace or converter, ladle furnace refining, vacuum degassing, continuous casting or ingot casting, reheating in a walking beam or soaking furnace, rolling on wide plate mills, then quenching and tempering, followed by cutting and final inspection. Vacuum treatment and tight sulfur control are what keep hydrogen and inclusion levels low enough for the toughness requirement.
Fabrication is where high strength steel earns or loses its advantage. Preheat and interpass temperature must be qualified for the thickness and restraint of the joint, low-hydrogen consumables are essential, heat input must stay within the tested range to protect the heat-affected zone, and cold forming should respect generous radii with consideration of springback. Thermal cutting leaves a hardened edge on quenched and tempered plate, so edges that will carry load are usually machined or ground back.
Typical applications include:
Heavy road vehicles: chassis, dumper and tipper bodies
Mobile crane booms and lifting equipment
Bridge components and offshore structural parts
Pressure vessels, fixed and transportable storage tanks
Power plant penstocks and spiral cases
Mining roof supports, earthmoving equipment and agricultural trailers
FAQ
Q: What does S690 stand for?
S denotes structural steel, 690 is the minimum yield strength in MPa for the thinner plate bands, and Q indicates quenched and tempered delivery under EN 10025-6.
Q: Where should S690 be used instead of S355?
Where weight, section size or welding volume drives cost. S690 offers roughly double the yield strength, so lighter structures can carry the same load, but the savings have to be weighed against more demanding fabrication controls.
Q: What is the difference between S690Q, S690QL and S690QL1?
Only the guaranteed impact test temperature: -20 °C, -40 °C and -60 °C respectively, all at a minimum of 30 J on longitudinal Charpy specimens.
Q: What is the minimum yield strength of S690 plate?
690 MPa for 3 mm to 50 mm plate, 650 MPa for over 50 mm up to 100 mm, and 630 MPa for over 100 mm up to 150 mm.
Q: Can S690 be welded with standard procedures?
No. The higher carbon equivalent means welding procedures must be qualified for S690 specifically, with controlled preheat, low-hydrogen consumables and a tested heat input window.
Q: Does S690 need heat treatment after welding?
Normally it is not stress relieved, because post-weld heat treatment can reduce the strength of quenched and tempered plate. Where heat treatment is required by the design, the temperatures must be agreed with the plate producer to avoid losing the specified properties.
