Two Designations, Two Different Product Standards
S355N and S355MC both carry a nominal minimum yield strength of 355 MPa, yet they are produced under different European product standards and are not interchangeable on a drawing. S355N is a weldable fine grain structural steel supplied in the normalised or normalised rolled condition under EN 10025-3. S355MC is a thermomechanically rolled, micro-alloyed steel intended for cold forming under EN 10149-2. In the designation, "N" identifies the normalised delivery condition, while "M" identifies thermomechanical rolling and "C" identifies suitability for cold forming.
The practical consequence is that S355N is ordered against plate specifications for welded structures, whereas S355MC is ordered against strip and sheet specifications for parts that will be bent, roll-formed or stamped before assembly.
Delivery Condition and Microstructure
S355N relies on normalising: the plate is reheated above the upper transformation temperature and air cooled, producing a uniform ferrite-pearlite microstructure refined by niobium, vanadium and titanium additions. Grain size is controlled to a fine grain practice, which delivers consistent toughness through the thickness and makes the grade tolerant of hot forming and post-weld heat treatment.
S355MC avoids the reheating step. Controlled rolling in the austenite region with micro-alloying elements produces a fine, largely ferritic grain structure with a low carbon content. The result is higher yield strength with better ductility and surface quality for forming, but the property balance is achieved in the as-rolled condition and is therefore limited to the thickness range covered by the delivery standard.
Mechanical Property Comparison
| Property | S355N (EN 10025-3) | S355MC (EN 10149-2) |
|---|---|---|
| Minimum yield strength, t ≤ 16 mm | 355 MPa | 355 MPa |
| Tensile strength range | 470–630 MPa | 430–550 MPa |
| Delivery condition | Normalised / normalised rolled | Thermomechanically rolled |
| Impact toughness | 27 J at −20 °C for grade N | No mandatory Charpy class in the base grade |
| Typical product form | Plate and heavy plate for welded structures | Hot rolled strip and sheet for cold forming |
The narrower tensile window of S355MC reflects its forming role: a lower tensile strength relative to yield strength keeps the yield-to-tensile ratio favourable for bending. S355N, with its wider tensile range and guaranteed low temperature impact energy, is aimed at welded load-bearing members.
Composition Limits Behind the Difference
The micro-alloyed cold forming route of S355MC is held to a low carbon limit, typically a maximum of 0.12 %, with manganese up to about 1.60 %, silicon up to 0.50 %, phosphorus and sulphur at the low levels required by EN 10149-2, a minimum aluminium content for grain refinement, and a combined niobium, titanium and vanadium addition capped at approximately 0.22 %. This low carbon level is what allows tight bending radii without cracking and keeps weldability straightforward.
S355N permits a higher carbon content, typically up to 0.20 %, with manganese in the range of roughly 0.90–1.65 %, and is micro-alloyed within the limits of EN 10025-3. The slightly higher carbon and nitrogen-bearing composition is acceptable because normalising removes the rolling-induced inhomogeneity.
Forming, Welding and Fabrication Behaviour
S355MC is designed for cold forming. It accepts tight bend radii relative to S355N, shows limited springback variation within a coil, and holds dimensional consistency for roll forming and press braking. It is not normally supplied with a guaranteed Charpy value, so it should not be selected for structures where low temperature impact energy is a design criterion.
S355N is designed for welding. It tolerates typical heat inputs and, because the properties are set by normalising rather than by rolling, it retains its toughness after hot forming and stress relief. Both grades weld readily with low-hydrogen consumables and controlled heat input; S355N is the safer choice when impact requirements or post-weld heat treatment are specified.
Selection Guidance
Choose S355MC when the part is formed from strip or sheet and the governing criteria are bendability, thickness consistency and cost per formed part. Choose S355N when the part is a welded plate component and the governing criteria are guaranteed impact energy, uniform through-thickness properties and tolerance of fabrication heat. Substituting S355MC for S355N in a welded, impact-tested detail is a design change, not a like-for-like material exchange, and should be confirmed against the applicable design code before release to production.
Frequently Asked Questions
Q: Can S355MC be used instead of S355N for a welded structure?
A: Only after verification. S355MC has no mandatory Charpy class and a lower tensile range, so the weld detail and any low temperature toughness requirement must be re-checked against the design code.
Q: Why is the tensile strength of S355MC lower than that of S355N?
A: The thermomechanical route targets a low yield-to-tensile ratio and high elongation so the strip can be cold formed. S355N instead allows a higher tensile range, which suits welded plate members.
Q: Which grade bends to a tighter radius?
A: S355MC. Its maximum carbon content of about 0.12 % and its clean, fine-grained structure allow tighter bend radii than S355N without edge cracking.
Q: Does S355N have a higher carbon content than S355MC?
A: Yes. EN 10025-3 permits carbon up to roughly 0.20 %, while EN 10149-2 holds S355MC to about 0.12 % to preserve formability and weldability.
Q: Is heat treatment needed after forming S355N?
A: Not for the base properties, because normalising is performed at the mill. Where the design requires stress relief after welding, S355N responds predictably, while thermomechanically rolled grades lose part of their strength advantage.
