What the +N Symbol Means
S355J2+N is the grade S355J2 supplied in the normalized delivery condition. Breaking the designation down, S identifies a structural steel, 355 is the minimum yield strength in megapascals at the reference thickness, J2 states that the steel is impact tested at minus 20 degrees Celsius with a minimum longitudinal Charpy V-notch energy of 27 J, and +N records that the plate has been normalized or normalized rolled before delivery. Under EN 10025-2 the grade carries the steel number 1.0577, and the same steel supplied as rolled is written S355J2+AR. Normalizing refines the grain structure, improves toughness and relieves internal stress from rolling, which is the reason it is specified for heavy plate, for low service temperatures and for components that will be formed or welded under restraint.
Mechanical Properties and Composition
| Thickness | Minimum yield strength | Tensile strength |
|---|---|---|
| Up to 16 mm | 355 MPa | 470 to 630 MPa |
| 16 to 40 mm | 345 MPa | 470 to 630 MPa |
| 40 to 63 mm | 335 MPa | 470 to 630 MPa |
| 63 to 100 mm | 325 MPa and 315 MPa | 470 to 630 MPa |
| 100 to 150 mm | 295 MPa | 450 to 650 MPa |
| 150 to 250 mm | 285 MPa and 275 MPa | 450 to 650 MPa |
| Over 250 mm | 265 MPa | 450 to 650 MPa |
Minimum elongation is 20 % in plate up to 40 mm and 18 % in heavier sections, density is 7850 kg per cubic metre, and typical hardness for plate up to 16 mm lies in the range of 150 to 190 HB. The chemical composition is a plain carbon-manganese balance with carbon capped at 0.20 % for thickness up to 30 mm, silicon at 0.55 % maximum, manganese at 1.60 % maximum, and phosphorus and sulphur held to the tighter limits that apply to the J2 impact class.
Equivalent Grades
| System | Designation | Minimum yield | Tensile range |
|---|---|---|---|
| EN 10025-2 | S355J2+N, steel number 1.0577 | 355 MPa | 470 to 630 MPa |
| DIN 17100, former | St52-3N | 355 MPa | 490 to 630 MPa |
| GB/T 1591 | Q355D, formerly Q345D | 355 MPa | 470 to 630 MPa |
| ASTM | A572 Grade 50 | 345 MPa | 450 MPa minimum |
| ASTM | A992/A992M, shapes only | 345 MPa | 450 MPa minimum |
| JIS G3106 | SM490YB | 355 MPa | 490 to 610 MPa |
The equivalents cluster around the same yield level but differ in the impact test condition, the tensile band and the thickness range for which the grade is approved. Q355D in GB/T 1591 is impact tested at minus 20 degrees Celsius in common with S355J2+N, which makes it the closest match of the group. A572 Grade 50 has a slightly lower minimum yield of 345 MPa, and SM490YB has a narrower tensile band, so both require a check before being accepted as substitutes for a design written around S355J2+N.
Applications
Because it combines a 355 MPa yield with assured toughness at minus 20 degrees Celsius, S355J2+N is used for steel frame buildings, crane and conveyor structures, heavy machinery bases, tanks and pressure equipment supports, bridge components, earthmoving and forklift structures and general heavy fabrication. It is also common in cold-climate construction where as-rolled material does not provide enough toughness margin, and in components that will be bent, rolled or welded under restraint where the uniform microstructure produced by normalizing gives more predictable forming behaviour. Plate thickness is available up to about 250 mm from heavy plate mills, with the yield requirement stepping down as thickness rises.
Fabrication and Welding
The carbon content of the grade keeps the carbon equivalent low, so it is welded by shielded metal arc, gas metal arc, submerged arc and flux-cored processes with matching consumables and without preheat for moderate thickness under normal shop conditions. Preheat and interpass control are applied where thickness, joint restraint or low ambient temperature raises the risk of hydrogen cracking, and the welding procedure is qualified for the grade and thickness actually used. Cold forming is straightforward within the bend radii given in the relevant fabrication standard, hot forming requires care to avoid losing the strength achieved by the composition, and post-weld heat treatment is applied only where the design code requires it, because unnecessary thermal cycles reduce the benefit of normalizing. Thermal cutting edges are dressed where fatigue or corrosion performance matters.
Inspection, Documentation and Ordering
Acceptance testing covers ladle and product chemical analysis, tensile testing in the direction required by the order, Charpy V-notch testing at minus 20 degrees Celsius, and hardness where specified. Ultrasonic examination to EN 10160 is ordered where internal soundness is a project requirement, dimensional tolerances follow the plate tolerance standard, and plates are marked with the grade, delivery condition and heat number. The documentation package comprises a mill test certificate at EN 10204 3.1, with third-party witness at 3.2 available. An order should state the grade with the delivery condition, the steel number where the buyer works in the numeric system, the thickness and quantity, the impact class, the tolerance standard and any ultrasonic or third-party requirement, so that the material delivered matches the design assumption without dispute.
Frequently Asked Questions
Q: What is the equivalent grade of S355J2+N?
A: The closest equivalents are DIN 17100 St52-3N, GB/T 1591 Q355D, ASTM A572 Grade 50 or A992 for shapes, and JIS G3106 SM490YB. Q355D matches most closely because it carries the same minus 20 degree Celsius impact requirement.
Q: What is the difference between S355J2+N and S355J2+AR?
A: The two are the same steel grade supplied in different conditions. S355J2+AR is delivered as rolled, while S355J2+N is normalized, which gives better toughness and a more uniform structure at a slightly higher cost.
Q: What does the J2 in S355J2 mean?
A: It specifies a longitudinal Charpy V-notch impact energy of 27 J at minus 20 degrees Celsius, which is the property that qualifies the steel for cold-climate service and for dynamically loaded structures.
Q: What is the hardness of S355J2+N?
A: Typical hardness for normalized plate up to 16 mm thickness falls in the range of 150 to 190 HB. Hardness is not a specified property of the grade and is reported for information rather than acceptance.
Q: What is the density of S355J2+N?
A: 7850 kg per cubic metre, equivalent to 7.85 g per cubic centimetre. A plate of 20 mm thickness therefore weighs approximately 157 kg per square metre.
Q: Does S355J2+N need post-weld heat treatment?
A: Only where the design code or the service condition requires it. Post-weld heat treatment is applied when it is specified, and the temperature and holding time are chosen so that the properties developed by normalizing are not degraded.
