S420ML
1.8836
Thermomechanically rolled weldable fine-grain structural steel · EN 10025-4:2019+A1:2022 · EN 10025-1
S420ML is a high-strength, weldable fine-grain structural steel supplied exclusively in the thermomechanically rolled condition. It combines a minimum yield strength of 420 MPa at thicknesses up to 16 mm with specified longitudinal Charpy V-notch toughness down to −50 °C. Its properties are thickness-dependent and depend on retaining the TMCP condition, so product form, thickness, test orientation, inspection requirements and any subsequent thermal processing must be controlled when ordering and fabricating it.
- S = structural; 420 = minimum yield strength at t ≤ 16 mm; M = thermomechanically rolled; L = low-temperature impact qualityGrade meaning · The 420 MPa value does not apply throughout the full thickness range.
- 1.8836Steel number · Alternative numerical designation under EN 10027-2.
- 27 J minimum average at −50 °CStandard toughness requirement · Longitudinal Charpy V-notch specimens; applicability and specimen size depend on product dimensions.
- 150 mmMaximum standard thickness · EN 10025-4 scope limit; commercial availability can be substantially narrower for a particular product form or mill.
- Thermomechanically rolled onlyDelivery condition · The defining properties cannot necessarily be reproduced by conventional heat treatment after delivery.
- 0.43–0.47%Maximum CEV · IIW carbon equivalent limit increases with nominal product thickness.
Overview
- Designation system
- EN 10027-1 steel name; EN 10027-2 steel number
- Product forms
- Plate, Sheet, Hot-rolled strip, Wide flats, Structural sections, Bars, Rod
- Condition
- Thermomechanically rolled condition (M/TMCP)
- Density
- 7850 kg/m³ (Volumetric mass prescribed by EN 10025-4 for calculating nominal mass; not a heat-specific measured density.)
Designation, scope and metallurgical basis
EN 10025-4 covers hot-rolled flat and long products of weldable fine-grain structural steel in the thermomechanically rolled condition. S420ML is intended particularly for highly loaded welded structures operating at ambient or low temperatures. Typical fields include bridges, hydraulic structures, storage tanks and other weight-sensitive fabricated structures, but suitability for a particular structure remains governed by the applicable design, execution and service standards.
| Symbol | Meaning | Practical significance |
|---|---|---|
| S | Structural steel | The designation is based primarily on mechanical properties rather than a fixed alloy recipe. |
| 420 | Minimum specified yield strength of 420 MPa for nominal thickness ≤ 16 mm | Lower minimum yield strengths apply as thickness increases. |
| M | Thermomechanically rolled delivery condition | Strength and toughness result from controlled rolling, potentially with accelerated cooling; this is not the normalized or quenched-and-tempered condition. |
| L | Low-temperature quality | Standard impact verification is at −50 °C rather than the −20 °C used for the corresponding M quality. |
The steel is fully killed and has a fine-grain structure. Microalloying and controlled rolling refine the microstructure and permit the required strength to be obtained with relatively restrained carbon and carbon-equivalent levels. EN 10025-4 classifies S420 grades as alloy special steels under EN 10020, even though they are commonly described commercially as HSLA structural steels.
Chemical requirements and weldability control
| Element | Requirement, mass % | Comment |
|---|---|---|
| C | ≤ 0.16 | For long products, ≤ 0.18% applies. |
| Si | ≤ 0.50 | A special composition should be ordered when coating-reactive silicon control is needed for hot-dip galvanizing. |
| Mn | ≤ 1.70 | Maximum, not a target composition. |
| P | ≤ 0.025 | For long products, the limit may be 0.005% higher. |
| S | ≤ 0.020 | For long products, the limit may be 0.005% higher; a tighter sulfur option is available for applications such as railways. |
| Nb | ≤ 0.05 | Permitted microalloying element. |
| V | ≤ 0.12 | Permitted microalloying element. |
| Al total | ≥ 0.020 | Minimum does not apply if sufficient other nitrogen-binding elements are present. |
| Ti | ≤ 0.05 | Permitted grain-refining/nitrogen-binding element. |
| Cr | ≤ 0.30 | Residual or intentional alloying allowance. |
| Ni | ≤ 0.80 | Supports hardenability and low-temperature toughness where used. |
| Mo | ≤ 0.20 | Maximum allowance. |
| Cu | ≤ 0.55 | This limit does not make S420ML a weathering-steel grade. |
| N | ≤ 0.025 | Nitrogen must be adequately bound by aluminium or other suitable elements. |
| Nominal product thickness (CEV) | ≤ 16 mm | > 16 to 40 mm | > 40 to 63 mm | > 63 to 150 mm |
|---|---|---|---|---|
| Maximum IIW carbon equivalent from heat analysis | 0.43% | 0.45% | 0.46% | 0.47% |
These are specification maxima. The actual CEV on the inspection certificate is more useful for welding procedure assessment than the grade maximum alone.
The composition table defines permissible limits, not a mandatory alloy recipe. Individual producers may use substantially lower carbon and CEV values. Product analysis is not automatic and must be ordered as an option if required.
Strength and ductility requirements
| Nominal thickness t | Minimum yield strength ReH | Tensile strength Rm | Minimum elongation A |
|---|---|---|---|
| t ≤ 16 mm | 420 MPa | 520–680 MPa | 19% |
| > 16 to 40 mm | 400 MPa | 520–680 MPa | 19% |
| > 40 to 63 mm | 390 MPa | 500–660 MPa | 19% |
| > 63 to 80 mm | 380 MPa | 480–640 MPa | 19% |
| > 80 to 100 mm | 370 MPa | 470–630 MPa | 19% |
| > 100 to 150 mm | 365 MPa | 460–620 MPa | 19% |
Elongation is based on proportional gauge length L0 = 5.65√S0. Where no yield phenomenon is present, Rp0.2 is determined instead of ReH.
For plate, strip and wide flats at least 600 mm wide, the tensile values apply to specimens transverse to the rolling direction. For other products they apply parallel to the rolling direction. For irregular long-product sections, the relevant thickness is the thickness of the part from which the test sample is taken—not necessarily the largest overall section dimension.
A purchase order or design calculation should not use “420 MPa” without checking the actual product thickness. At the upper end of the standard's range, the specified minimum yield strength is 365 MPa.
Low-temperature toughness and impact testing
| Test temperature (J) | +20 °C | 0 °C | −10 °C | −20 °C | −30 °C | −40 °C | −50 °C |
|---|---|---|---|---|---|---|---|
| Minimum longitudinal Charpy V-notch impact energy | 63 J | 55 J | 51 J | 47 J | 40 J | 31 J | 27 J |
The defining verification temperature for ML quality is −50 °C. Values at another listed temperature may be specified by agreement.
| Test temperature (J) | +20 °C | 0 °C | −10 °C | −20 °C | −30 °C | −40 °C | −50 °C |
|---|---|---|---|---|---|---|---|
| Optional transverse Charpy requirements | 40 J | 34 J | 30 J | 27 J | 23 J | 20 J | 16 J |
Transverse impact testing is not the default. It must be ordered using the applicable EN 10025-4 option.
The listed impact energy is the required average of three specimens. One individual result may fall below the specified average provided it is at least 70% of that value; the standard defines additional testing and acceptance rules when the initial set does not meet these conditions. Reduced-width specimens use proportionally reduced energy requirements.
For flat products and sections below 6 mm nominal thickness, impact testing is not required. Reduced-width specimens are used where practicable at intermediate thicknesses. For plates at least 40 mm thick, impact specimens are taken at the quarter-thickness position, making the result more representative of through-thickness material than a near-surface test.
Charpy qualification at −50 °C is not, by itself, a universal authorization for a structure to operate at −50 °C. Brittle-fracture assessment also depends on design stress, detail category, plate thickness, strain rate, welding effects, flaw assumptions and the applicable structural design standard.
Welding and thermal processing
S420ML is specified as suitable for welding, with general arc-welding provisions taken from EN 1011-2. Its low-carbon TMCP metallurgy generally provides better weldability than a normalized steel at the same strength level. Nevertheless, hydrogen-assisted cold cracking remains possible, particularly with increasing thickness, joint restraint, weld-metal hydrogen and strength level. Welding procedure qualification should therefore use the actual certificate chemistry and CEV, joint geometry, consumable hydrogen class, heat input and anticipated restraint rather than relying only on the grade name.
Welding heat input and interpass temperature require control because excessive thermal exposure can coarsen or soften the heat-affected zone, while inadequate thermal control can increase hardness and cracking risk. Required weld-metal strength and toughness should be selected for the joint design and service temperature; overmatching is not automatically necessary or beneficial in every application.
Stress relieving above 580 °C or for longer than one hour may deteriorate mechanical properties. If such treatment is intended, guaranteed post-treatment properties should be agreed with the steelmaker when ordering.
Hot forming shall not be undertaken. The thermomechanically rolled condition and its property balance cannot be assumed to survive or be restored by subsequent conventional heat treatment.
Cold forming, galvanizing and surface condition
Cold forming is possible subject to the product dimensions, bend direction and ordered requirements, but it reduces local ductility and can influence brittle-fracture performance. Suitability for flange forming or roll forming is an order option rather than an unconditional property of every S420ML product. Forming procedures should account for rolling direction, edge quality, thickness and the actual producer's recommendations.
For hot-dip galvanizing, coating appearance and thickness are sensitive to silicon and phosphorus. EN 10025-4 provides an option to agree a chemistry suitable for zinc coating. This should be invoked at the order stage; standard S420ML chemistry alone does not guarantee a particular galvanizing response. Cold-worked regions also require attention because galvanizing can aggravate brittle-fracture concerns.
| Product | Basic provision | Possible purchaser action |
|---|---|---|
| Plate and wide flat | EN 10163-2, class A, subclass 1 | Order a different surface class or repair subclass if the application requires it. |
| Sections | EN 10163-3, class C, subclass 1 | Specify another class where critical surfaces or repairs require tighter control. |
| Bars and rod | EN ISO 9443, class A | Agree another surface class if needed. |
| Hot-rolled plate dimensions | EN 10029; thickness tolerance class A unless otherwise agreed | State the required dimensional standard and tolerance class in the order. |
| Plate cut from continuously rolled strip | Normally EN 10051 thickness tolerances | Use the available option if EN 10029 tolerances are required. |
Ordering and certification
A complete order should identify the quantity, product form, dimensional and tolerance standard, nominal dimensions, full steel designation, required options and the EN 10204 inspection document. Writing only “S420ML plate” leaves important matters unresolved, including thickness tolerance, surface class, internal soundness, inspection scope and certificate type.
| Requirement | Why it may matter |
|---|---|
| Product analysis | Verifies the chemistry of the delivered product rather than only the cast heat analysis. |
| EN 10164 Z-quality | Controls through-thickness ductility where welded details impose significant shrinkage strain and lamellar-tearing risk. |
| Ultrasonic testing | Addresses internal soundness for critical plates, sections or bars; acceptance class must be specified. |
| Transverse Charpy testing | May better represent the critical crack orientation in some plate designs than the default longitudinal test. |
| Testing by parent plate or coil | Provides finer traceability than normal test-unit sampling for critical flat products. |
| Galvanizing chemistry | Controls silicon and phosphorus to obtain a more predictable zinc-coating response. |
| Alternative surface or thickness-tolerance class | Aligns the delivered product with machining, fit-up, fatigue or appearance requirements. |
| Alternative impact-test temperature | Useful when the governing design specification requires verification at a listed temperature other than −50 °C. |
Standard verification includes heat analysis, tensile testing and impact testing where product dimensions permit. Mechanical test units are organized by cast, product form, grade, quality and yield-strength thickness range. The inspection certificate should be checked for grade and condition, dimensions, heat identity, actual chemistry and CEV, tensile results, Charpy specimen orientation and temperature, individual and average impact values, and any ordered optional tests.
The standard's 150 mm scope limit does not guarantee that every mill offers every product form throughout that range. Availability, dimensional range and producer-specific fabrication limits must be confirmed before design or substitution is finalized.
Use and substitution boundaries
S420ML is most valuable where higher strength can reduce section mass while low-temperature toughness and weldability remain important. The economic benefit is not automatic: stability, fatigue, deflection, connection design, local buckling, fabrication tolerances and weld requirements may govern before yield strength does.
Substitution should not be based solely on nominal yield strength. S420M has the same strength class but a less demanding standard impact test temperature. Normalized S420 grades, quenched-and-tempered steels, pressure-vessel steels and similarly named ISO or national grades can differ in delivery condition, permitted composition, thickness range, toughness, testing frequency and certification. Any substitution must be checked against the complete product specification and project requirements.
Sources
- BS EN 10025-4:2019+A1:2022 — Hot rolled products of structural steels, Part 4British Standards Institution
- DIN EN 10025-4:2023-02DIN Media
- EN 10025-4:2019+A1:2022 — Technical delivery conditions for thermomechanically rolled weldable fine-grain structural steelsCEN reproduction via iTeh Standards
- EN 10025-4:2019 technical tables and requirementsWugang Steel reference library
- S420ML / 1.8836 material designation recordApplus Laboratories Materials Portal
Ask MetalMate about S420ML
Discussing: S420ML / 1.8836
AI assistant · Responses are generated and can contain errors; verify specifications before acting.