What Is S690 Fine Grain Structural Steel Plate?
S690 is a high yield strength, fine grain structural steel supplied in the quenched and tempered condition. The designation states a minimum specified yield strength of 690 MPa, reached through micro-alloying, controlled rolling and a water quench followed by tempering. Because the microstructure is refined and free from coarse grains, the plate combines very high strength with the toughness required by dynamically loaded structures.
Against conventional S355 structural steel, S690 lets designers reduce section thickness substantially for the same design load. The result is lower self-weight, less weld volume, smaller foundations and cheaper transport and erection. The 690 MPa family is standardised in EN 10025-6, which covers flat products of high yield strength structural steels in the quenched and tempered condition.
Grades and Delivery Conditions under EN 10025-6
EN 10025-6 divides the 690 MPa yield level into three toughness classes. All are delivered quenched and tempered; the classes differ in the minimum Charpy V-notch test temperature, which is selected according to the lowest service temperature of the structure.
| Grade | Impact test temperature | Minimum impact energy, longitudinal |
|---|---|---|
| S690Q | -20 °C | 30 J |
| S690QL | -40 °C | 30 J |
| S690QL1 | -60 °C | 30 J |
Where a project is designed to an American specification, ASTM A514 and ASTM A517 quenched and tempered alloy steel plates cover a comparable minimum yield strength of 690 MPa and are often quoted as the nearest equivalent for cross-reference. Equivalence is never automatic: impact class, thickness limits and chemistry must be compared item by item.
Chemical Composition and Mechanical Properties
Grain refining elements such as aluminium are added, nitrogen is held low, and the alloy content is balanced against a controlled carbon level so that the plate can be welded with conventional low-hydrogen consumables. The table below lists the maximum ladle limits that apply to the quenched and tempered 690 MPa group.
| Element | Maximum, % |
|---|---|
| C | 0.20 |
| Si | 0.80 |
| Mn | 1.70 |
| P | 0.025 |
| S | 0.015 |
| Cr | 1.50 |
| Cu | 0.50 |
| Ni | 2.00 |
| Mo | 0.70 |
| N | 0.015 |
| B | 0.005 |
Yield strength falls as thickness grows, because the centre of a heavy plate cools more slowly and the microstructure coarsens. The mechanical property table therefore has to be read together with the ordered thickness.
| Property | Condition | Value |
|---|---|---|
| Minimum yield strength, thickness ≤ 50 mm | Quenched and tempered | 690 MPa |
| Minimum yield strength, 50 < t ≤ 100 mm | Quenched and tempered | 650 MPa |
| Minimum yield strength, 100 < t ≤ 150 mm | Quenched and tempered | 630 MPa |
| Tensile strength | Quenched and tempered | 770 - 940 MPa |
| Elongation, minimum | Quenched and tempered | 14 % |
| Hardness, typical | Quenched and tempered | 200 - 300 HBW |
| Modulus of elasticity | Room temperature | 210 GPa |
Physical Properties
| Property | Reference condition | Typical value |
|---|---|---|
| Density | Room temperature | 7850 kg/m³ |
| Melting range | - | 1425 - 1540 °C |
| Thermal conductivity | 20 °C | 50 W/m·K |
| Specific heat capacity | 20 °C | 460 J/kg·K |
| Coefficient of thermal expansion | 20 - 100 °C | 12 × 10-6/K |
Thermal expansion and conductivity are close to those of ordinary carbon steel, so welding distortion and heat input calculations follow the same logic as for lower strength grades. The higher strength simply means that a given restraint force acts on a smaller section.
Fabrication, Welding and Machining
Plates are usually cut by plasma, laser or abrasive waterjet. Thermal cutting leaves a thin hardened layer on quenched and tempered steel, so cut faces that will carry fatigue load or be welded should be lightly ground back to bright metal. Flame cutting is practical in heavier gauges but needs preheat on thick sections.
Cold forming is possible within limits. As a working rule, the minimum inside bend radius is about three times the plate thickness when bending transverse to the rolling direction, and noticeably larger when bending parallel to it, because the rolling direction has lower ductility. Bending should be carried out at ambient temperature and never with a radius so tight that surface cracking appears.
Welding follows normal practice for high strength quenched and tempered plate: matching strength class, low-hydrogen consumables kept dry, controlled heat input and interpass temperature, and preheat for thick or highly restrained joints. Heat input should be kept inside the range recommended for the consumable, because excessive heat coarsens the heat-affected zone and destroys the toughness that the quench and temper treatment created. Machining is straightforward with carbide tooling at moderate speeds and feeds.
Applications and Design Benefits
Mobile crane booms, outriggers and chassis components
Excavator buckets, arms and mining equipment wear structures
Bridge girders, launching gantries and heavy transport frames
High-rise and long-span structural frames where depth is limited
Offshore platform structures and wind turbine tower transition pieces
Heavy machinery bases, press frames and formwork systems
The design benefit is not only weight saving. Thinner plate reduces weld cross-section, which cuts filler metal consumption, heat input and inspection cost. Lighter members can be handled with smaller cranes, and a reduced structural envelope can lower foundation loads. The price to pay is stricter fabrication discipline: distortion control, edge preparation quality and weld procedure qualification all matter more than they do for S355.
Frequently Asked Questions
Q: What does the 690 in S690 mean?
It states the minimum specified yield strength of 690 MPa, measured for the thinnest thickness range as the stress at 0.2 % permanent strain.
Q: What is the difference between S690Q, S690QL and S690QL1?
Only the guaranteed impact toughness temperature differs: the grades are tested at -20 °C, -40 °C and -60 °C respectively, with a minimum Charpy energy of 30 J.
Q: Is S690 the same as a 100 ksi yield steel?
690 MPa is very close to 100 ksi, and ASTM A514 or A517 plates occupy the same strength class, but chemistry, thickness limits and toughness rules still have to be compared before substitution.
Q: Can S690 be welded without preheat?
Thin, unrestrained joints can often be welded with suitable low-hydrogen consumables at ambient temperature, while thick sections and restrained joints require preheat and a qualified welding procedure.
Q: How much weight does S690 save compared with S355?
Because the permitted stress is roughly doubled, section thickness can often be reduced by 40 to 50 % for a strength-governed member, though deflection, buckling and fatigue usually set the real limit.
Q: What thicknesses are available?
Quenched and tempered fine grain plate is commonly rolled from about 4 mm up to 150 mm, with the minimum yield strength stepping down above 50 mm and again above 100 mm.