What Is S355JR Steel Tube?
S355JR is a low-alloy, high-strength structural steel defined in EN 10025-2. In the grade name, S means structural steel, 355 is the minimum yield strength in megapascals, and JR requires a minimum Charpy impact energy of 27 J at 20 deg C. When the steel is formed into tube, it is supplied as square or rectangular hollow sections per EN 10210-1 (hot finished) or EN 10219-1 (cold formed), or as round tube for structural and mechanical use.
S355JR occupies the strength tier most often chosen for medium and heavy structures: roughly 30 % stronger than S235JR and 15 % stronger than S275JR, while remaining a weldable, formable and economical non-alloy steel. The regular cross-section of square tube simplifies connection design, and the hollow section gives a high strength-to-weight ratio that reduces dead load in frames and trusses.
Chemical Composition and Material Characteristics
The composition of S355JR is formulated for stable strength and good processing behavior:
| Element | Requirement, % (t <= 40 mm) |
|---|---|
| Carbon (C), max | 0.24 |
| Silicon (Si), max | 0.55 |
| Manganese (Mn), max | 1.60 |
| Phosphorus (P), max | 0.035 |
| Sulfur (S), max | 0.035 |
| Nitrogen (N), max | 0.012 |
Carbon and manganese are the main strengthening elements; silicon improves strength and aids deoxidation; phosphorus, sulfur and nitrogen are tightly limited to preserve toughness and weld quality. Trace copper may be present. The steel is produced by converter or electric-arc furnace steelmaking as a fully killed steel, which gives a dense, homogeneous microstructure and stable mechanical values from heat to heat.
Mechanical Properties
| Property | Thickness <= 16 mm | Thickness 16 - 40 mm | Thickness 40 - 63 mm |
|---|---|---|---|
| Minimum yield strength | 355 MPa | 345 MPa | 335 MPa |
| Tensile strength | 470 - 630 MPa | 470 - 630 MPa | 470 - 630 MPa |
| Minimum elongation | 22 % | 21 % | 20 % |
| Minimum impact energy at 20 deg C | 27 J | 27 J | 27 J |
The high yield strength keeps sections and weights down under load, while the 470 - 630 MPa tensile range and the elongation values provide ductility and fracture resistance. The grade also shows a certain degree of abrasion resistance in abrasive environments, although it is not an abrasion-resistant steel and should not be specified as one.
Characteristics of S355JR Hollow Sections
Balance of strength and toughness: the combination of 355 MPa yield with good elongation makes the tube suitable for dynamic loads and impact-prone structures such as crane supports and machine frames. Good weldability: the low carbon equivalent of the grade keeps crack sensitivity low, so S355JR can be welded by standard processes in most sections without special measures. Weather resistance: compared with plain low-carbon steel its atmospheric corrosion behavior is marginally better, but it is not a weathering steel; outdoor use still requires painting or galvanizing. Dimensional stability: the hollow section resists torsional and lateral buckling better than open profiles of the same weight, which is why it is preferred for compression members and space frames.
The grade is not a premium material, however. In extreme low-temperature service, or in strongly corrosive environments, a higher grade (S355J2 for -20 deg C impact, or a stainless or coated product) or additional protective measures may be necessary.
Applications
Building structures: factory columns and beams, stair rails, canopy frames, exhibition halls and stadium space frames, where load capacity and clean geometry matter. Mechanical manufacturing: equipment frames, conveyor supports, gantry structures and machine bases that must carry dynamic loads. Bridges and infrastructure: secondary members, gantries and towers, selected on the basis of the governing design specification. Mining and metallurgical equipment: chutes, supports and platforms subject to impact and abrasion. Shipbuilding: deck structures and stiffeners where certified material is required.
Selection and Design Points
Different applications place very different demands on size and wall thickness, so the section must be chosen from the project design, not guessed: a lightly loaded railing uses a thin wall while a heavily loaded column uses a thick wall. Confirm the impact-test temperature: JR guarantees 27 J at 20 deg C only, so for cold environments specify J0 (0 deg C) or J2 (-20 deg C). For welded connections in thick sections, use a qualified welding procedure with controlled heat input and, where required, preheat. Verify tolerances against EN 10219-1 or EN 10210-1 and request the mill certificate for certified structures.
Frequently Asked Questions
What is the yield strength of S355JR tube?
The minimum yield strength is 355 MPa for thickness up to 16 mm, 345 MPa for 16 - 40 mm and 335 MPa for 40 - 63 mm. The grade name refers to the 355 MPa value.
What is the tensile strength of S355JR?
The tensile strength range is 470 - 630 MPa across common thicknesses, with a minimum elongation of 22 % for thin sections. This is noticeably higher than S275JR (410 - 560 MPa), which is why S355JR is used for heavier loads.
Is S355JR suitable for welding?
Yes. The carbon equivalent is low enough for normal welding without preheat in most sections, and the grade is one of the most welded structural steels in Europe. For thick walls, low ambient temperature or high restraint, a qualified procedure with moderate preheat is recommended.
Can S355JR be used at low temperatures?
The JR suffix guarantees impact energy only at 20 deg C. For service below that, specify S355J0 (tested at 0 deg C) or S355J2 (tested at -20 deg C). Design codes also impose their own minimum service temperature rules on the JR grades.
Is S355JR a weathering steel?
No. It is a structural carbon-manganese steel. It has no deliberate corrosion-resisting additions beyond trace copper, and it must be coated for outdoor exposure. For unpainted atmospheric-corrosion service, a weathering steel grade is required.
What is the difference between S355JR square tube and round tube?
Square tube (SHS) has equal bending resistance in two perpendicular axes and flat faces that simplify joints, making it ideal for columns, frames and architectural work. Round tube offers uniform properties in all radial directions and is preferred for truss chords, conveyors and rotating structures. Both can be produced from S355JR to the same EN standards.