1. What Is S690 Steel?
S690 is a high-strength low-alloy (HSLA) structural steel belonging to the family of quenched and tempered steels. It is manufactured in flat products with a minimum yield strength of 690 MPa, and the QL quality group additionally guarantees impact toughness of 30 J at -40 °C per EN 10025-6. The high strength is achieved through a quench and temper heat treatment that refines the microstructure and balances strength with toughness and weldability. Because S690 carries roughly twice the yield strength of conventional structural steel S355, it allows engineers to reduce section thickness and component weight while maintaining load capacity.
2. Key Properties of S690
| Property | Typical value |
|---|---|
| Minimum yield strength | 690 MPa |
| Tensile strength | 770–940 MPa |
| Impact toughness (S690QL) | 30 J at -40 °C |
| Delivery condition | quenched and tempered (QT) |
| Governing standard | EN 10025-6 |
The combination of high yield strength, good toughness under dynamic loads and reduced weight makes S690 one of the most reliable options when balancing performance, safety and lightweight design in demanding applications.
3. S690 Compared with Conventional Steel S355
Compared with S355, S690 offers more than twice the yield strength and a higher tensile range (770–940 MPa versus 470–630 MPa). This allows lighter structures and higher load-bearing capacity. The trade-offs are higher material cost, a requirement for more controlled processing and slightly lower elongation. Impact behaviour is different but not inferior: S690QL guarantees 30 J at -40 °C, while S355J2 guarantees 27 J at -20 °C. The two grades are therefore selected for different design philosophies: S355 for conventional fabricated structures, S690 where weight saving justifies the extra processing care and cost.
4. Applications of S690 in the Automotive Sector
S690 is used where the strength-to-weight ratio directly improves vehicle performance. Body-in-white components use S690 to increase crash performance while reducing weight. Safety systems, including side impact bars, bumper reinforcements, door reinforcements and front-end structures, benefit from the energy absorption characteristics of the high-strength material. Suspension and chassis parts take advantage of lightweight design to improve handling and dynamics. Beyond cars, the same material family is used in cranes, lifting equipment, mobile machinery and heavily loaded fixed structures such as bridges and buildings.
5. Forming and Welding Challenges
Processing S690 requires specialised equipment and expertise. Forming forces are higher than for mild steel, and springback is more pronounced. After manufacturing processes such as hot forming, correct heat treatment or controlled cooling is essential to avoid degradation of the mechanical properties. Welding S690 demands precise control of heat input and interpass temperature to limit the size of the heat-affected zone and preserve the parent plate properties. Consumables are selected with matching or slightly under-matching strength, and a qualified welding procedure specification is mandatory. Advanced methods such as laser welding can help achieve optimal results where conventional welding is too critical.
Frequently Asked Questions
What is the yield strength of S690? The minimum yield strength is 690 MPa, which is the basis of the grade designation. Higher-strength siblings such as S890 and S960 also exist in EN 10025-6.
What standard governs S690QL? S690QL is defined in EN 10025-6 (hot rolled products of structural steels, part 6: high yield strength structural steels in the quenched and tempered condition). The L suffix guarantees impact toughness of 30 J at -40 °C.
What thicknesses are available for S690 plate? Plates are commonly available from 8 mm to 100 mm for the Q and QL quality groups, and from 8 mm to 50 mm for the QL1 group. Widths reach about 3200 mm and lengths about 12200 mm, with length restrictions for very thick and wide combinations.
Is S690 difficult to weld? S690 welds successfully with controlled procedures. Heat input, preheat and interpass temperature must be managed carefully, and consumables must be selected for the strength level. The weld procedure must be qualified before production welding.
What are the typical uses of S690QL? S690QL is used for mobile applications such as cranes and lifting equipment, and for fixed steel structures that carry very heavy loads, including bridges, offshore structures and heavy construction equipment, where weight saving matters.
Is S690 cheaper to use than S355 despite the higher material price? In many designs yes, because the reduced section weight lowers transport, handling and fabrication quantities, and the longer service life reduces life-cycle cost. The decision depends on the specific design and fabrication setup.
