What S690Q Plate Is
S690Q is a quenched and tempered structural steel plate specified in EN 10025-6, the part of the European standard that covers flat products of high yield strength structural steels in the quenched and tempered condition. The Q suffix identifies the impact quality, and the grade combines very high strength with useful low-temperature toughness. Dual certification with the Chinese system is common: the corresponding grade is Q690 in GB/T 16270, delivered in the quenched and tempered condition.
Specified Properties to EN 10025-6
| Property | Requirement |
|---|---|
| Minimum yield strength, thickness 3-50 mm | 690 MPa |
| Minimum yield strength, thickness above 50 to 100 mm | 650 MPa |
| Minimum yield strength, thickness above 100 to 150 mm | 630 MPa |
| Tensile strength | 770-940 MPa |
| Elongation, minimum | 14 % |
| Charpy V-notch impact, longitudinal | 30 J minimum at minus 20 C |
Strength falls with thickness because the centre of a heavy section cools more slowly during quenching. Designers should use the value for the actual plate thickness and not the headline 690 MPa figure.
Comparison with GB/T 16270 Q690 Grades
GB/T 16270 covers quenched and tempered high strength plate using the Chinese letter system, where Q690 is followed by an impact quality letter. The Q690E grade requires impact testing at minus 40 C, which is lower than the EN quality Q, and the two systems are therefore not interchangeable for every project. When plates are dual certified, the impact temperature that governs must be stated on the order and confirmed on the mill certificate.
Where S690Q Is Used
Mobile crane booms and outriggers, concrete pump placing arms, excavator and mining equipment structures, dump truck bodies, offshore lifting frames, bridge and roof structures with high loads on limited depth, and heavy transport trailer frames. In every case the saving comes from reducing section size and welded volume rather than from a lower material price.
Fabrication: Welding, Forming and Cutting
Strength at this level depends on the quenched and tempered microstructure, so any fabrication step that adds heat has to be controlled. Welding requires a qualified procedure with a low-hydrogen consumable, controlled heat input, a specified interpass temperature and, for heavy joints, preheat matched to the carbon equivalent. Heat input that is too high softens the heat affected zone; too low and it does not cause a problem in the HAZ but may raise hydrogen cracking risk, which is why both limits are written into the procedure. Forming is normally done in the as-delivered condition with generous bend radii; flame cutting leaves a heat affected edge that should be machined or ground back where the edge is fatigue sensitive. Storage and handling should avoid impact on cut edges.
Thickness Range, Tolerances and Testing
Quenched and tempered plate is commonly supplied from 3 mm up to 150 mm or more, in widths suited to welded fabrication and lengths to 12 m. Dimensional tolerances follow EN 10029, and internal soundness can be verified by ultrasonic examination to EN 10160 or ASTM A578/A578M. Surface condition is normally as-rolled with the mill scale intact.
Frequently Asked Questions
Q: What is the minimum yield strength of S690Q?
A: 690 MPa up to 50 mm thickness, 650 MPa from 50 to 100 mm and 630 MPa from 100 to 150 mm.
Q: What does the Q suffix mean?
A: It defines the impact quality class, here a minimum of 30 J longitudinal Charpy energy at minus 20 C.
Q: Is S690Q the same as Q690E?
A: They are comparable grades from EN 10025-6 and GB/T 16270 respectively, but the guaranteed impact temperature differs and the certificate must be read for the governing value.
Q: Can the plate be welded without preheat?
A: Only where the qualified welding procedure allows it for the actual thickness and carbon equivalent; heavy sections normally require preheat and interpass control.
Q: Does the strength survive flame cutting?
A: The base plate does; a narrow heat affected zone at the cut edge has reduced properties and should be removed where that edge carries design stress.
Q: What tensile range should the mill certificate show?
A: 770-940 MPa, with the yield value appropriate to the measured thickness.
