What Is S690Q Steel?
S690Q is a quenched and tempered high-strength structural steel plate produced under the European standard EN 10025-6, which covers flat products of high yield strength structural steels in the quenched and tempered condition. The designation is a compact description of the material: S denotes structural steel, 690 is the minimum yield strength in MPa for the thinner thickness range, and Q confirms that the plate is quenched and tempered. A related grade, S690QL, carries the same strength level but is impact tested at minus 40 °C instead of minus 20 °C.
The grade bridges the gap between ordinary structural steel and ultra-high-strength plate. It allows designers to reduce section thickness while maintaining the same load-bearing capacity, a decisive advantage in mobile machinery, long-span structures and equipment where dead weight directly reduces payload, reach or energy efficiency.
Core Characteristics of S690Q
Strength and toughness balance. The minimum yield strength is 690 MPa for plate up to 50 mm thick, with tensile strength in the range of 770 to 940 MPa for the same thickness band. Ordinary carbon structural steel reaches roughly one third of that yield level.
Low-temperature toughness. The Q condition requires a minimum Charpy V-notch impact energy of 30 J at minus 20 °C, tested on longitudinal specimens, which protects the material against brittle fracture in cold climates and outdoor installations.
Weldability. The chemistry is balanced to keep the carbon equivalent low for a 690 MPa steel. With matching consumables and a qualified preheat and interpass control, welded joints can reach a strength close to that of the base plate without sacrificing notch toughness.
Dimensional stability after quenching and tempering. Because the plate is tempered after quenching, internal stresses are relieved and the structure is uniform through the thickness, which limits distortion and cracking during cutting, bending and machining.
Weight saving. Design studies for load-bearing components commonly report thickness and weight reductions in the order of 30 % to 40 % compared with ordinary structural steel at the same duty, which improves payload and reduces fuel or energy consumption in mobile equipment.
Forming, cutting, bending and stamping can be carried out with conventional workshop methods, although the high strength requires more powerful equipment, carbide tooling and a suitable bend radius. Where corrosion or wear resistance is the limiting factor, surface protection such as hot-dip galvanizing or a protective coating system is applied, and because the grade retains stable properties at moderate service temperatures up to about 200 °C, it also suits continuously loaded components in industrial plant.
Chemical Composition
The table below gives the maximum values specified for S690Q under EN 10025-6.
| Element | C | Si | Mn | P | S | N | B | Cr |
|---|---|---|---|---|---|---|---|---|
| Max (%) | 0.20 | 0.80 | 1.70 | 0.025 | 0.015 | 0.015 | 0.005 | 1.50 |
| Element | Cu | Mo | Nb | Ni | Ti | V | Zr | |
| Max (%) | 0.50 | 0.70 | 0.06 | 2.00 | 0.05 | 0.12 | 0.15 |
The alloy concept behind these limits is straightforward. Nickel, chromium and molybdenum raise hardenability so that the full thickness transforms to a strong, tough microstructure during quenching, while niobium, vanadium and titanium form fine carbonitrides that restrict grain growth during rolling and welding. Boron is present only in trace amounts, but it makes an important contribution to hardenability, which is why it is capped at 0.005 %. Carbon is kept at or below 0.20 % and sulphur at 0.015 % to protect weldability and through-thickness toughness.
Mechanical Properties by Thickness
EN 10025-6 grades are specified in thickness bands, because the achievable strength falls as the section becomes thicker and the cooling rate during quenching slows down.
| Thickness | Min yield (MPa) | Tensile (MPa) | Min elongation (%) | Impact test temp. | Min impact energy |
|---|---|---|---|---|---|
| 8 - 50 mm | 690 | 770 - 940 | 14 | minus 20 °C | 30 J |
| 51 - 100 mm | 650 | 760 - 930 | 14 | minus 20 °C | 30 J |
| 101 - 150 mm | 630 | 710 - 900 | 14 | minus 20 °C | 30 J |
Elongation is relatively modest, which is a normal feature of quenched and tempered high-strength plate. In design practice this is compensated by keeping stress concentrations low, by avoiding abrupt section changes, and by verifying fatigue behaviour at welded details rather than by relying on plastic redistribution. The impact energy requirement is applied to the finished product regardless of the thickness band, so a heavy plate must be produced with a chemistry and a heat treatment that deliver toughness through the full section.
Typical Applications
Heavy machinery: structural parts of cranes, excavators, loaders and drilling rigs, including booms and arms where every kilogram saved increases working reach.
Construction: beams, columns and connections in high-rise buildings, stadiums and exhibition halls that combine long spans with heavy loads.
Bridge engineering: main girders, piers and connection plates designed for high load capacity and seismic performance.
Marine and offshore engineering: offshore platform components and subsea structures exposed to severe sea states.
Shipbuilding: hull, deck and bulkhead components where strength and durability are critical.
Petrochemical and energy plant: pressure vessels, storage tanks and support structures subject to pressure, heat and corrosive media.
In all of these applications the choice of S690Q is a structural decision rather than a material preference: it is adopted when the finished component must carry a defined load at the lowest possible weight, and when the fabrication route, welding procedure and inspection plan have been arranged around a high-strength quenched and tempered plate.
FAQ
Q: What does the designation S690Q stand for?
S indicates structural steel, 690 is the minimum yield strength in MPa for plate up to 50 mm thick, and Q indicates that the plate is supplied in the quenched and tempered condition under EN 10025-6.
Q: What is the difference between S690Q and S690QL?
Both have the same strength requirements. The difference lies in the impact test temperature: S690Q is tested at minus 20 °C, while the L variant, S690QL, is tested at minus 40 °C for colder service conditions.
Q: Why does the minimum yield strength change with thickness?
Because thicker plate cools more slowly during quenching, the resulting microstructure is slightly coarser. EN 10025-6 therefore specifies lower minimum yield values as thickness increases, from 690 MPa up to 50 mm to 630 MPa in the 101 to 150 mm band.
Q: Is S690Q difficult to weld?
It requires more control than ordinary structural steel. A low carbon equivalent, matching consumables, a qualified preheat and a limited heat input are used to keep the heat-affected zone tough, and welding procedures are normally qualified by test.
Q: How much weight can be saved by using S690Q?
For components designed by strength rather than by stiffness, published design studies report weight and thickness reductions in the order of 30 % to 40 % compared with ordinary structural steel carrying the same load.
Q: Can S690Q be used at elevated temperatures?
The grade retains stable creep behaviour at moderate service temperatures up to about 200 °C. Above that range the design temperature limit of the relevant application standard governs the material selection.
