S355ML
1.8834
Thermomechanically rolled weldable fine-grain structural steel · EN 10025-4:2019+A1:2022 · EN 10025-1:2004
S355ML is the low-temperature-toughness quality of the 355 MPa thermomechanically rolled fine-grain structural steel family. It combines thickness-dependent structural strength with a standard longitudinal Charpy requirement of at least 27 J at −50 °C. Its low carbon content and controlled carbon equivalent support welding, but the properties depend on the thermomechanical delivery condition and can be impaired by hot forming or excessive post-weld heat treatment.
- 1.8834Steel number · The steel number and S355ML are alternative designations for the same EN grade.
- 355 MPa minimum yield strengthNominal strength class · Applies only at nominal thickness ≤16 mm; the specified minimum decreases to 320 MPa above 100 mm through 150 mm.
- 27 J minimum at −50 °CLow-temperature quality · Standard verification uses longitudinal Charpy V-notch specimens, subject to the product-size limitations of EN 10025-4.
- Thermomechanically rolled (M)Delivery condition · The condition is integral to the grade and cannot be reproduced by a simple subsequent heat treatment.
- 0.39–0.45%Maximum standard CEV · Heat-analysis limit depends on nominal product thickness.
- Hot forming is not permitted by EN 10025-4Principal limitation · High-temperature processing can destroy the property balance established by thermomechanical rolling.
Overview
- Designation system
- EN 10027-1 steel name; EN 10027-2 steel number
- Product forms
- Plate, Sheet, Wide strip and strip, Wide flats, Hot-rolled sections, Bars, Rod
- Condition
- Thermomechanically rolled condition (M)
- Density
- 7850 kg/m³ (Volumetric mass specified by EN 10025-4 for calculating nominal product mass; not a heat-specific measured density.)
Meaning and scope of the designation
The designation identifies structural steel (S), a minimum yield strength of 355 MPa for nominal thickness up to 16 mm, thermomechanical rolling (M), and the ML toughness quality with specified impact properties down to −50 °C. EN 10025-4 covers hot-rolled flat and long products; S355ML should not be assumed to cover tubes, hollow sections, forgings, castings or pressure-purpose products governed by other product standards.
The current European specification is EN 10025-4:2019+A1:2022. National publications may carry a later publication date, such as DIN EN 10025-4:2023-02, without representing a different grade. EN 10025-4 contains the grade-specific technical delivery requirements; EN 10025-1 additionally applies to products supplied for construction-product applications unless the relevant no-CE-marking option is agreed.
S355ML is classified as an alloy special steel under EN 10020. This classification does not mean it is a highly alloyed steel; it reflects the EN quality classification and the controlled fine-grain composition and properties.
Manufacture and metallurgical condition
S355ML is fully killed and produced with a fine-grain structure. Nitrogen-binding elements must be present in sufficient quantity; the standard cites 0.020% minimum total aluminium as the usual basis when another effective nitrogen-binding system is not used. Microalloying with niobium, vanadium and/or titanium may be used to control recrystallization, precipitation strengthening and grain size.
Thermomechanical rolling controls deformation and cooling through defined temperature ranges to establish the required strength and toughness. The resulting condition is often described as TMCP, although EN 10025-4 permits several thermomechanical process variants. The achieved condition cannot be recreated by normalizing or another simple heat treatment, so S355ML must remain identified and supplied as thermomechanically rolled material.
Chemical composition and weldability control
| Mass fraction (%) | C | Si | Mn | P | S | Nb | V | Ti | Cr | Ni | Mo | Cu | N |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Maximum heat-analysis composition for S355ML flat products | 0.14 | 0.50 | 1.60 | 0.025 | 0.020 | 0.05 | 0.10 | 0.05 | 0.30 | 0.50 | 0.10 | 0.55 | 0.015 |
Total Al is normally at least 0.020%; this minimum does not apply where sufficient alternative nitrogen-binding elements are present. For long products, C may be up to 0.16%, and the P and S limits may each be 0.005 percentage point higher.
| Nominal thickness t | Maximum CEV |
|---|---|
| t ≤ 16 mm | 0.39% |
| 16 < t ≤ 40 mm | 0.39% |
| 40 < t ≤ 63 mm | 0.40% |
| 63 < t ≤ 150 mm | 0.45% |
The applicable value is a specification maximum, not a typical mill value. Special compositions requested for hot-dip galvanizing can increase the permitted CEV as defined by EN 10025-4.
Product analysis is not the default compositional basis: it is performed when ordered and has separate permissible limits. Do not reject a product analysis by comparing it directly with the heat-analysis table.
Room-temperature tensile requirements
| Nominal thickness t | Minimum ReH or Rp0.2 | Tensile strength Rm | Minimum elongation A |
|---|---|---|---|
| t ≤ 16 mm | 355 MPa | 470–630 MPa | 22% |
| 16 < t ≤ 40 mm | 345 MPa | 470–630 MPa | 22% |
| 40 < t ≤ 63 mm | 335 MPa | 450–610 MPa | 22% |
| 63 < t ≤ 80 mm | 325 MPa | 440–600 MPa | 22% |
| 80 < t ≤ 100 mm | 325 MPa | 440–600 MPa | 22% |
| 100 < t ≤ 150 mm | 320 MPa | 430–590 MPa | 22% |
Values apply in the thermomechanically rolled delivery condition. ReH is used where a yield phenomenon occurs; otherwise Rp0.2 is determined. Elongation is for proportional specimens with L0 = 5.65√S0. Values for products below 3 mm using an 80 mm gauge length must be agreed when ordering.
For plate, strip and wide flats at least 600 mm wide, the tensile requirements apply to specimens transverse to the rolling direction. For other products, they apply to longitudinal specimens. For long products of irregular section, the relevant thickness is the thickness of the part from which the sample is taken, not necessarily the overall section dimension.
Impact toughness and test interpretation
| KV2 impact energy (J) | +20 °C | 0 °C | −10 °C | −20 °C | −30 °C | −40 °C | −50 °C |
|---|---|---|---|---|---|---|---|
| Minimum longitudinal Charpy V-notch energy | 63 | 55 | 51 | 47 | 40 | 31 | 27 |
The standard verification temperature for quality ML is −50 °C. Another listed temperature may be agreed at the time of order.
| KV2 impact energy (J) | +20 °C | 0 °C | −10 °C | −20 °C | −30 °C | −40 °C | −50 °C |
|---|---|---|---|---|---|---|---|
| Optional transverse Charpy V-notch requirements | 40 | 34 | 30 | 27 | 23 | 20 | 16 |
These lower transverse values apply only when transverse verification is specifically ordered under the relevant EN 10025-4 option.
The designation's 27 J at −50 °C requirement is an average Charpy-energy requirement under the standard test and acceptance rules; it is not a direct fracture-toughness value or an automatic minimum service-temperature rating. Structural detail, stress state, section thickness, strain rate, weld quality and applicable design rules remain necessary for brittle-fracture assessment.
Impact testing is not required for flat products or sections below 6 mm nominal thickness. Size-dependent exemptions also apply to small bars and rod. Where reduced-width Charpy specimens are used, the minimum energy is reduced in proportion to specimen cross-sectional area. For flat products at least 40 mm thick, impact specimens are taken at the quarter-thickness position.
Welding, forming and thermal processing
EN 10025-4 requires S355ML to be suitable for welding and refers to EN 1011-2 for general arc-welding recommendations. Its low carbon content and controlled CEV normally provide better weldability than normalized steel at the same yield-strength level. Nevertheless, required preheat, heat input, interpass temperature and hydrogen control must be established from the actual inspection-certificate composition, thickness, joint restraint, welding process and consumable hydrogen level. The ML base-metal designation does not by itself guarantee −50 °C toughness in the weld metal or heat-affected zone.
Hot forming must not be undertaken. Stress relieving above 580 °C or for longer than one hour can reduce mechanical properties; where such treatment is intended, the post-treatment minimum properties should be agreed with the steel producer at the time of order. Unqualified normalizing or prolonged high-temperature flame straightening can similarly alter the thermomechanically established microstructure.
Cold forming reduces ductility and is not an unconditional grade requirement. If the appropriate option is ordered, S355ML plate, sheet, strip and wide flats up to 12 mm can be supplied for flanging without cracking, using minimum bend radii of 2t when the bend axis is transverse and 2.5t when it is longitudinal. A separate option covers roll forming of flat products up to 8 mm. These capabilities should not be assumed unless stated in the order and inspection documentation.
If galvanizing is intended, the required silicon and phosphorus composition category should be agreed under the EN 10025-4 galvanizing option. Coating behavior cannot be predicted from the designation alone. Cold-worked regions also require consideration because the standard warns of reduced ductility and possible brittle-fracture risk in connection with hot-dip zinc coating.
Specification, ordering and inspection
| Item | Why it matters |
|---|---|
| Complete designation and edition | Specify EN 10025-4 — S355ML or 1.8834 and the applicable standard edition. |
| Product form and dimensions | Mechanical limits, specimen orientation and testing provisions depend on form and thickness. |
| Dimensional standard and tolerance class | For plate, EN 10029 Class A is the basic thickness tolerance unless another class is agreed; plate cut from strip normally follows EN 10051. |
| Inspection document | State the required EN 10204 document type; project or execution standards commonly determine whether 3.1 or another type is needed. |
| Charpy verification | State longitudinal or optional transverse orientation, test temperature and whether per-parent-plate or per-coil testing is required. |
| Through-thickness properties | Specify an EN 10164 Z-quality where lamellar-tearing resistance or through-thickness ductility is required. |
| Internal soundness | Ultrasonic examination is optional and must be ordered with the appropriate acceptance standard and class. |
| Surface condition | Default surface classes differ among plate, sections, bars and rod; specify any more stringent class. |
| Forming or galvanizing options | Flangeability, roll-forming suitability and galvanizing chemistry are optional requirements, not automatic consequences of the grade name. |
Standard mechanical verification is performed by test units within a cast, generally 60 tonnes or part thereof, with special 80-tonne provisions for certain sections. This does not mean every individual plate or section is mechanically tested. If component criticality requires traceability to each parent plate or coil, the corresponding per-parent-product test option must be included in the order.
Availability can be narrower than the full dimensional scope of the standard, especially for sections, bars and very thick plate. Confirm the producer's qualified manufacturing route, available dimensions and certified options before design or procurement is finalized.
Use and substitution boundaries
S355ML is selected where welded structural components require the S355 strength class together with verified low-temperature base-metal toughness. Typical application areas include bridges, hydraulic structures, tanks, offshore or heavy welded fabrications and other dynamically or heavily loaded structures, but suitability remains governed by the applicable design, execution and service requirements.
A proposed substitute should be checked against product standard, delivery condition, thickness-dependent strength, Charpy temperature and orientation, chemistry and CEV, dimensions, tolerances, inspection frequency, certification and any optional requirements. Matching a nominal 355 MPa yield class is insufficient. Normalized, quenched-and-tempered, hollow-section and pressure-vessel grades can have materially different processing and certification requirements even where their strength appears similar.
Sources
- BS EN 10025-4:2019+A1:2022 — Hot rolled products of structural steels: Technical delivery conditions for thermomechanical rolled weldable fine grain structural steelsBritish Standards Institution
- DIN EN 10025-4:2023-02DIN Media
- EN 10025-4:2019 — Standard previewiTeh Standards
- S355ML fine-grain structural steel data sheetSalzgitter Flachstahl GmbH
- Professional Guide on Selection of Equivalent Steel Materials to European Steel Materials SpecificationsThe Hong Kong Polytechnic University, Chinese National Engineering Research Centre for Steel Construction
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