S500QL1 Within the EN 10025-6 Family
EN 10025-6 covers flat products of high yield strength structural steels in the quenched and tempered condition. The family runs from S460Q up to S960Q, and every strength level is offered in three toughness classes: Q impact tested at -20 °C, QL tested at -40 °C and QL1 tested at -60 °C, all with a minimum Charpy V-notch energy of 30 J. S500QL1 therefore sits in the middle of the strength range while carrying the toughest of the three low-temperature classes, combining a 500 MPa minimum yield strength with impact verification at -60 °C.
Because the plate is quenched and tempered rather than normalized or thermomechanically rolled, the properties come from a martensitic or bainitic microstructure that is tempered back to a controlled strength and ductility balance. That route gives a much higher strength-to-weight ratio than a normalized grade, which is why the grade is used where dead weight and section size both matter.
Chemical Composition
The chemistry is a low-carbon, micro-alloyed, alloy steel. Carbon and manganese are balanced to keep weldability acceptable, while chromium, nickel, molybdenum and boron support the hardenability needed for through-thickness properties in heavy plate.
| Element (max, %) | S500QL1 |
|---|---|
| C | 0.20 |
| Si | 0.80 |
| Mn | 1.70 |
| P | 0.025 |
| S | 0.015 |
| N | 0.015 |
| B | 0.005 |
| Cr | 1.50 |
| Cu | 0.50 |
| Mo | 0.70 |
| Nb | 0.06 |
| Ni | 2.00 |
| Ti | 0.05 |
| V | 0.12 |
| Zr | 0.15 |
Carbon equivalent is calculated from the analysis as CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. Because alloying elements are present at meaningful levels, the carbon equivalent of S500QL1 is higher than that of a normalized S355 grade, and welding procedures must be built around a controlled preheat and heat input rather than around the base metal carbon content alone.
Mechanical Properties by Thickness
Quenched and tempered plate loses strength as it becomes thicker, so the standard sets separate minimum yield values and tensile ranges for each thickness band.
| Thickness range (mm) | Minimum yield strength (MPa) | Tensile strength (MPa) | Minimum elongation (%) | Charpy at -60 °C |
|---|---|---|---|---|
| over 3 to 50 | 500 | 590-770 | 17 | 30 J |
| over 50 to 100 | 480 | 590-770 | 17 | 30 J |
| over 100 to 150 | 440 | 540-720 | 17 | 30 J |
Impact testing at -60 °C with a 30 J minimum on longitudinal specimens is the defining feature of the QL1 class and the reason the grade is chosen for cold-climate and low-temperature equipment. As with all thickness-dependent tables, the value for the thickness actually ordered is the one that must be used in design calculations.
Toughness Classes, Dimensions and Applications
The three toughness classes share the same strength targets and differ only in the guaranteed impact test temperature: Q at -20 °C, QL at -40 °C and QL1 at -60 °C. Selecting a colder class adds cost, so the class should be matched to the minimum design metal temperature of the component rather than chosen as a general safety margin.
Plates are commonly supplied in thicknesses from 3 mm up to 300 mm, with widths in the range of 1,500 to 4,050 mm and lengths up to about 27,000 mm depending on the mill and the order. Typical applications include:
Bridge girders and highway construction structures
Crane booms, mobile crane structures and lifting equipment
Mining and earthmoving machinery frames
Pressure vessels, storage tanks and penstocks
Wind tower sections and heavy transport equipment
Offshore and cold-climate structural components
Fabrication follows the usual rules for high-strength quenched and tempered steel: preheat according to thickness and carbon equivalent, use low-hydrogen consumables, keep heat input within the qualified window, avoid excessive cold forming, and restore any thermally affected zone by controlled machining or re-heat-treatment where drawings require it. Third-party inspection certificates from recognised classification societies can be supplied with the plates for project documentation.
FAQ
Q: What does S500QL1 stand for?
S is structural steel, 500 is the minimum yield strength in MPa for the thinner thickness bands, Q indicates quenched and tempered delivery, and L1 denotes the coldest toughness class, impact tested at -60 °C.
Q: What is the minimum yield strength of S500QL1?
500 MPa for plate over 3 mm up to 50 mm thick, 480 MPa for over 50 mm up to 100 mm, and 440 MPa for over 100 mm up to 150 mm.
Q: How much impact energy does S500QL1 guarantee?
A minimum of 30 J Charpy V-notch at -60 °C. The Q and QL classes of the same strength level guarantee the same energy at -20 °C and -40 °C respectively.
Q: Is S500QL1 difficult to weld?
It needs a qualified procedure rather than a difficult one. Preheat, interpass temperature and heat input must be controlled because the alloy content raises the carbon equivalent well above that of normalized structural steels.
Q: Where is S500QL1 normally used?
Cold-climate bridges, cranes, mining and earthmoving equipment, pressure equipment, wind towers and heavy transport structures where a 500 MPa yield strength has to be combined with toughness at -60 °C.
