S690QL Steel Plate: Properties, Composition and Applications

S690QL Steel Plate at a Glance

S690QL is a quenched and tempered fine grain structural steel plate specified in EN 10025-6. The designation is easy to read once it is broken down: S stands for structural steel, 690 is the minimum yield strength in MPa, Q means the plate is delivered in the quenched and tempered condition, and L indicates guaranteed impact toughness tested at -40 °C. The related grade S690Q is tested at -20 °C and S690QL1 at -60 °C, so the final letter of the designation is the fastest way to confirm whether a plate suits a cold climate project.

Because the plate is quenched to a martensitic or bainitic microstructure and then tempered, it develops a very high strength to weight ratio while remaining tough enough for dynamically loaded structures. That combination is why S690QL is selected for mobile cranes, long span bridges, mining and earth moving equipment, heavy trailers and stadium roof structures, where trimming dead weight pays for itself through lower steel consumption, lighter transport loads and smaller foundations.

Chemical Composition to EN 10025-6

The grade is a low alloy steel with tightly controlled residuals. The values below are maximum contents for the ladle analysis.

Micro additions of niobium, titanium and vanadium refine the grain and support toughness, boron is restricted because it can harm weldability, and the low phosphorus and sulphur limits reduce the risk of brittle fracture and lamellar tearing in restrained welded joints.

Mechanical Properties by Thickness

Yield and tensile levels fall slightly as the section thickens, because the core of a heavy plate cools more slowly during quenching. The impact requirement stays at -40 °C across all three thickness bands, which is what the L suffix guarantees.

Equivalent and Comparable Grades

The American entry is an approximate comparison rather than a true equivalent, so a project governed by a particular design code should verify the acceptance criteria of that code before any substitution is approved.

Why Designers Specify S690QL

Strength to weight efficiency: because the minimum yield strength is close to twice that of S355, section thickness can often be reduced by roughly 40 % for the same design resistance, which cuts steel tonnage and handling cost.

Guaranteed low temperature toughness: a 30 J minimum at -40 °C keeps the material suitable for outdoor and cold region structures.

Fatigue performance: the tempered microstructure resists crack initiation under repeated loading, which matters for crane booms and transport frames.

Abrasion resistance: high surface hardness slows material loss in chutes, liners and buckets that handle abrasive bulk material.

Availability in heavy sections: plates from 3 mm to about 150 mm allow both thin fabricated panels and thick node plates to be sourced from one grade.

Typical Applications

Mobile, tower and gantry cranes, including boom sections and outrigger components

Bridges and long span structures where self weight governs the design

Earth moving, mining and quarry machinery such as buckets, arms and wear liners

Heavy duty truck chassis, trailers and tipper bodies

Stadium roofs, arenas and high rise building nodes

Offshore and cold climate steel structures requiring toughness below freezing

Fabrication, Welding and Inspection

Quenched and tempered plate rewards disciplined workshop practice. Thermal cutting by plasma, laser or water jet keeps the heat affected zone narrow, and any hard cut edge should be dressed before the part is loaded. Forming is normally carried out with generous bend radii and, where the radius is tight, with the rolling direction taken into account so that the bend line runs across the plate rather than along it.

Welding is performed to a qualified procedure using low hydrogen consumables, with preheat, interpass temperature and heat input all held inside the qualified window. Excessive heat input softens the tempered structure in the heat affected zone, while excessive restraint combined with high hardness can promote cracking, so joint design, fit up and, where required, post weld heat treatment should be agreed before production starts. Thick plates and safety critical nodes are commonly supplied with ultrasonic examination, and the mechanical properties are verified with tensile and Charpy impact tests taken from the plate in the delivery condition.

Frequently Asked Questions

Q: What does the QL suffix mean in S690QL?
Q means the plate is quenched and tempered, and L means a minimum Charpy impact energy of 30 J is guaranteed at -40 °C. The related grades S690Q and S690QL1 are guaranteed at -20 °C and -60 °C respectively.

Q: Can S690QL be welded without losing its strength?
Yes, provided a qualified welding procedure is used. Heat input, preheat and interpass temperature must be controlled so that the heat affected zone does not soften beyond the design allowance and does not harden to a level that risks cracking.

Q: Why does the minimum yield strength drop in thick plates?
Heavy sections cool more slowly in the core during quenching, so the achievable strength is lower. EN 10025-6 therefore sets 690 MPa for 3 mm to 50 mm, 650 MPa for 50 mm to 100 mm and 630 MPa for 100 mm to 150 mm.

Q: Is S690QL the same as S690Q?
No. Both are quenched and tempered structural plates with the same strength levels, but S690Q is impact tested at -20 °C while S690QL is tested at -40 °C, and S690QL1 at -60 °C.

Q: Is S690QL suitable for pressure vessels or boilers?
It is a structural grade rather than a pressure vessel grade. Vessels and boilers are normally built from dedicated plate specifications such as ASTM A516 or ASTM A537, so the applicable design code should always be checked.

Q: Which thickness range is normally stocked?
Quenched and tempered high strength plate is commonly produced from about 3 mm up to 150 mm, with width and length cut to order. Heavier sections are usually quoted against a specific rolling programme.