N, NL, M and ML: Normalised and Thermomechanically Rolled Grades

The designations S355N, S355NL, S355M and S355ML describe fine-grain structural steels produced through different controlled delivery routes and with different low-temperature toughness qualities.

The most important distinction is:

• N / NL → normalized or normalized-rolled fine-grain steels under EN 10025-3.
• M / ML → thermomechanically rolled (TMCP) fine-grain steels under EN 10025-4.

The additional L identifies the lower-temperature toughness quality within each family.

The family map

*Shown as a useful S355-family orientation. Exact requirements, thickness ranges, test direction and specimen rules must be checked in the governing standard and order.

N and NL: normalized fine-grain structural steels

EN 10025-3 covers hot-rolled weldable fine-grain structural steels in the normalized or normalized-rolled delivery condition.

Normalizing involves heating into the appropriate austenitizing range followed by controlled cooling, normally in air, to obtain the required microstructure and properties. Normalizing rolling reaches an equivalent target condition through controlled finishing of the rolling process rather than a separate full-furnace normalizing treatment.

The important procurement point is that normalized rolling is an accepted production route in this family. That does not mean that the mill can simply look at chemistry and label an arbitrary as-rolled product “N.” The declared delivery condition is part of the product requirement and has to be supported by the manufacturer's controlled production route and conformity to the product standard.

M and ML: thermomechanically rolled steels

EN 10025-4 covers hot-rolled weldable fine-grain structural steels in the thermomechanically rolled condition.

TMCP combines controlled deformation, temperature control and, depending on the route, controlled/accelerated cooling to develop the required strength and toughness with relatively lean chemistry.

This is not simply a heat-treatment label that can be added later. The properties are developed through the rolling process itself. A thermomechanically rolled grade should therefore not be treated as automatically interchangeable with a normalized grade just because both have “355” in the name.

What does the L mean?

Within these families, L identifies the lower-temperature toughness quality.

For the S355 grades, published EN/JRC material-selection tables show the familiar pattern:

• S355N / S355M: impact-quality level associated with -20 °C.
• S355NL / S355ML: impact-quality level extending to -50 °C.

The L should therefore be considered part of the required grade, not an optional marketing suffix.

S355N is not the same thing as S355J2+N

This is one of the most common nomenclature traps.

S355J2+N

• Product family: typically EN 10025-2 non-alloy structural steel.
• J2: impact toughness quality.
• +N: delivery condition — normalized or normalizing rolled.

S355N

• Product family: EN 10025-3 weldable fine-grain structural steel.
• N is part of the grade designation/family itself.
• Chemistry, mechanical-property tables and technical delivery requirements come from EN 10025-3, not simply EN 10025-2 plus a heat-treatment suffix.

So the expression “S355N = S355J2+N” is not a reliable equivalence statement.

Can a mill certify +N without furnace normalizing?

Yes if the permitted route is normalizing rolling and the product is actually manufactured in accordance with that route and the applicable standard.

No if the claim means that chemistry alone replaces the specified delivery condition.

This distinction matters when reviewing a mill's statement such as: “we do not normalize the beam; we certify +N because chemistry and test values are acceptable.” The buyer should ask what the actual manufacturing route is. If the mill means normalizing rolling, that can be entirely legitimate. If it means ordinary as-rolled production with no qualifying delivery route, the claim requires much closer scrutiny against the product standard and mill's conformity documentation.

Are N and M grades interchangeable?

Not automatically.

Before substituting S355N/NL and S355M/ML, compare at least:

• the project specification and permitted standard;
• thickness range;
• chemistry and carbon-equivalent limits;
• yield/tensile requirements at the ordered thickness;
• impact toughness and test temperature;
• welding procedure and heat-input restrictions;
• heat treatment after forming/welding;
• NDT and supplementary requirements.

TMCP steels often achieve strength/toughness with lower alloying than conventionally normalized steels, which can be favorable for weldability. At the same time, excessive subsequent heat treatment can alter properties produced by the thermomechanical route. Fabrication procedures should therefore follow the material and project requirements.

MetalMate takeaway

N/NL and M/ML are different fine-grain structural-steel families, not simply different spellings of S355. N/NL belongs to the normalized/normalized-rolled route; M/ML belongs to the TMCP route; L identifies the lower-temperature toughness quality. Keep these separate from the “+N” delivery-condition suffix in grades such as S355J2+N.

References

• NEN, EN 10025-3:2019 — Normalized/normalized rolled weldable fine-grain structural steels: https://www.nen.nl/en/nen-en-10025-3-2019-en-262498
• NEN, EN 10025-4:2019+A1:2022 — Thermomechanically rolled weldable fine-grain structural steels: https://www.nen.nl/en/nen-en-10025-4-2019-a1-2022-en-304866
• NEN, EN 10025-2:2019 — Non-alloy structural steels: https://www.nen.nl/en/nen-en-10025-2-2019-en-262497
• NEN, EN 10027-1:2016 — Steel names: https://www.nen.nl/nen-en-10027-1-2016-en-226697
• European Commission JRC, Choice of material for hollow section structures / EN 1993-1-10 extended tables: https://eurocodes.jrc.ec.europa.eu/sites/default/files/2021-12/JRC72702_Choice%20of%20material%20for%20hollow%20section%20structures_2016ed.pdf