S420NL

1.8912

Weldable fine-grain structural steel for low-temperature service · EN 10025-3:2019 · EN 10025-1 for applications under the Construction Products Regulation

S420NL is the low-temperature-toughness quality of a 420 MPa-class, normalized or normalizing-rolled weldable fine-grain structural steel. EN 10025-3 covers hot-rolled flat and long products through 250 mm thickness, but the 420 MPa minimum yield strength applies only at thicknesses up to 16 mm and decreases progressively with thickness. The NL quality is distinguished by mandatory verification of longitudinal Charpy V-notch impact energy at −50 °C under the standard testing regime.

420 MPa at t ≤ 16 mmMinimum yield strength · The specified minimum falls to 320 MPa in the >200–250 mm thickness range.
27 J minimum longitudinal KV2 at −50 °CLow-temperature toughness · This is the defining NL-quality verification temperature; transverse impact testing is an ordering option with different requirements.
Normalized or normalizing rolledDelivery condition · These routes are intended to provide equivalent specified properties after normalizing.
Up to 250 mmStandard thickness scope · Actual mill availability is normally narrower and depends on product form and dimensions.
0.48–0.52%Maximum CEV · IIW carbon equivalent limit increases with product thickness.
Alloy special steelSteel classification · Classification under EN 10020; the grade remains a structural steel rather than a general-purpose heat-treatable alloy steel.

Overview

Designation system: Steel name to EN 10027-1; steel number to EN 10027-2
Product forms: Hot-rolled plate, sheet and strip, Wide flats, Hot-rolled structural sections, Hot-rolled bars and rod
Condition: Normalized, Normalizing rolled
Density: 7850 kg/m³ (Volumetric mass specified by EN 10025-3 for calculating nominal product mass; not a heat-specific measured density.)

What the designation means

The symbol S identifies structural steel. The number 420 is the minimum specified yield strength in MPa for nominal thickness up to and including 16 mm; it must not be treated as a thickness-independent property. N identifies the normalized or normalizing-rolled delivery condition. L identifies the quality having specified minimum impact-energy values down to −50 °C.

EN 10025-3:2019 applies to hot-rolled flat and long products of weldable fine-grain structural steel in thicknesses up to 250 mm. It is particularly directed at heavily loaded welded structures operating at ambient or low temperatures. It is not, by itself, a pressure-vessel, line-pipe, offshore or hollow-section product specification; those applications can impose different manufacturing, testing and certification requirements.

Chemical control and weldability indicator

S420NL heat-analysis limits under EN 10025-3:2019

Element	Requirement, mass %
C	≤ 0.20
Si	≤ 0.60
Mn	1.00–1.70
P	≤ 0.025
S	≤ 0.020
Nb	≤ 0.05
V	≤ 0.20
Al total	≥ 0.020
Ti	≤ 0.05
Cr	≤ 0.30
Ni	≤ 0.80
Mo	≤ 0.10
Cu	≤ 0.55
N	≤ 0.025

For long products, the P and S limits may each be 0.005 percentage point higher. The minimum total aluminium requirement does not apply when sufficient alternative nitrogen-binding elements are present. These are heat-analysis limits; product-analysis tolerances are separately defined and product analysis is ordered as an option.

IIW carbon equivalent used by EN 10025-3

CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

All element contents are entered as mass percentages. CEV is a composition-based welding indicator, not a complete welding procedure criterion.

Maximum CEV based on heat analysis

Nominal product thickness	Maximum CEV
t ≤ 63 mm	0.48%
63 < t ≤ 100 mm	0.50%
100 < t ≤ 250 mm	0.52%

Special composition control for hot-dip galvanizing can permit the tabulated CEV limit to be increased as specified by the standard. This must be recognized when reviewing certificates or planning welding.

Microalloying with niobium, vanadium, titanium and nitrogen-binding additions supports grain refinement and strength. The specification permits considerable compositional latitude, so two compliant heats can have different welding behavior. Welding assessment should use the actual certificate chemistry, section thickness, hydrogen level, restraint, heat input and applicable fabrication standard rather than the grade name alone.

Thickness-dependent tensile properties

Specified room-temperature tensile properties

Nominal thickness t	Minimum ReH	Rm	Minimum elongation A
t ≤ 16 mm	420 MPa	520–680 MPa	19%
16 < t ≤ 40 mm	400 MPa	520–680 MPa	19%
40 < t ≤ 63 mm	390 MPa	520–680 MPa	19%
63 < t ≤ 80 mm	370 MPa	520–680 MPa	18%
80 < t ≤ 100 mm	360 MPa	520–680 MPa	18%
100 < t ≤ 150 mm	340 MPa	500–650 MPa	18%
150 < t ≤ 200 mm	330 MPa	500–650 MPa	18%
200 < t ≤ 250 mm	320 MPa	500–650 MPa	18%

Elongation applies to proportional test pieces with L0 = 5.65√S0. If a yield phenomenon is absent, Rp0.2 is determined. For plate, strip and wide flats at least 600 mm wide, tensile values apply to transverse test pieces; for other products they apply parallel to the rolling direction.

The thickness reduction in yield strength is central to design and purchasing. A certificate showing S420NL does not establish a 420 MPa yield minimum for every supplied thickness. The nominal thickness of a flat product governs; for an irregular long-product section, the thickness of the part from which the sample is taken governs.

Impact toughness and test orientation

Minimum longitudinal Charpy V-notch impact energy KV2

Test temperature (J)	+20 °C	0 °C	−10 °C	−20 °C	−30 °C	−40 °C	−50 °C
Minimum longitudinal Charpy V-notch impact energy KV2	63	55	51	47	40	31	27

The standard verification temperature for NL quality is −50 °C. Another listed temperature may be agreed at ordering.

Minimum transverse Charpy V-notch impact energy KV2 — Option 30

Test temperature (J)	+20 °C	0 °C	−10 °C	−20 °C	−30 °C	−40 °C	−50 °C
Minimum transverse Charpy V-notch impact energy KV2 — Option 30	40	34	30	27	23	20	16

Transverse verification is not the default. It must be specified at the time of order and replaces longitudinal verification under Option 30.

Impact values are averages from three specimens tested using a 2 mm striker. One result may fall below the specified average provided it is at least 70% of that value; the standard defines additional-test acceptance rules when the initial set does not meet all criteria. Requirements for subsize specimens are reduced in direct proportion to specimen cross-sectional area.

Impact testing is generally not required for flat products or sections below 6 mm thickness. For bars and rod, dimensional limits also determine whether a standard Charpy specimen can be taken. The designation therefore does not prove that every small-section product was physically impact tested at −50 °C; the inspection certificate and applicable specimen rules must be checked.

Delivery condition and subsequent heating

Products are supplied normalized or in an equivalent condition obtained by normalizing rolling. Normalizing rolling finishes deformation in a controlled temperature range so that the resulting condition is equivalent to normalizing and the specified properties are retained after a subsequent normalizing treatment. For grades with a minimum yield strength of at least 420 MPa, the manufacturer may apply delayed cooling or additional tempering after normalizing or normalizing rolling; this treatment must be reported in the inspection document.

Stress relieving above 580 °C or for longer than one hour can deteriorate properties. For flat products, EN 10025-3 advises a maximum stress-relief temperature of 560 °C. If higher temperatures or longer times are intended, the required post-treatment mechanical properties should be agreed when ordering rather than assumed from the original certificate.

Welding, forming and fabrication implications

S420NL is specified as suitable for welding, with general arc-welding recommendations referenced to EN 1011-2. This does not remove the need for a qualified welding procedure. At this strength level, especially in thicker or highly restrained joints, hydrogen-assisted cold cracking must be controlled through consumable selection and storage, joint preparation, preheat and interpass control, heat input, restraint management and inspection.

Cold-forming capability is not an unconditional consequence of the grade designation. When Option 11 is specified, flat products up to 16 mm must be suitable for flanging without cracking using minimum inside bend radii of four times nominal thickness where the bend axis is transverse to rolling and five times thickness where the bend axis is longitudinal. Option 12 addresses roll forming of plate, sheet and strip up to 8 mm. Cold work reduces ductility and can affect subsequent galvanizing behavior.

Hot forming and flame straightening should follow CEN/TR 10347. Where hot forming is used, the thermal cycle must preserve or restore the normalized-equivalent condition. Uncontrolled heating can alter grain size, strength and toughness, particularly where local heating is severe or cooling is rapid.

Ordering and inspection points that matter

Items to define or verify

Item	Practical requirement
Full designation	Specify EN 10025-3 — S420NL or EN 10025-3 — 1.8912, including the applicable edition.
Product and dimensions	State product form, nominal dimensions and the dimensional/tolerance standard. Hot-rolled plate defaults to EN 10029 thickness tolerance Class A unless otherwise agreed.
Inspection document	Specify the required EN 10204 document type. Do not assume a particular certificate type solely from the grade designation.
Impact testing	Confirm test temperature, specimen orientation and whether standard-size specimens are possible. Longitudinal testing at −50 °C is the normal NL verification.
Chemical verification	Heat analysis is standard. Product analysis must be ordered if independently required.
Through-thickness properties	Specify an EN 10164 quality where lamellar-tearing resistance or through-thickness ductility is required.
Internal soundness	Ultrasonic examination is optional and must be ordered with the applicable acceptance standard and class.
Surface quality	Check the applicable surface standard and class. Plates and wide flats default to EN 10163-2 Class A, subclass 1; sections default to EN 10163-3 Class C, subclass 1.
Forming requirements	Call up the appropriate flangeability or roll-forming option rather than relying on an unstated expectation of cold formability.
Post-fabrication heat treatment	Disclose intended stress-relief, normalizing or hot-forming cycles and agree retained properties where the standard's recommended limits will be exceeded.
Galvanizing	If zinc-coating behavior is important, specify the relevant silicon/phosphorus composition option and account for its permitted effect on CEV.

Standard mechanical testing is organized by cast and test unit, normally 60 tonnes or part thereof, with special 80-tonne provisions for certain heavy sections. If properties must be demonstrated for each parent plate or coil, the corresponding optional testing must be stated in the order. Review certificates for grade, steel number, delivery condition, actual thickness range, heat chemistry and CEV, tensile orientation, impact orientation and temperature, and any reported delayed cooling or tempering.

Substitution boundaries

Substitution should be based on the complete product specification, not nominal yield strength alone. S420N has the same tensile-property basis but a less demanding toughness class. S420ML has similar nominal strength and low-temperature toughness but is thermomechanically rolled under EN 10025-4 and has different chemistry, heat-treatment behavior and dimensional scope. Quenched-and-tempered, offshore, pressure-purpose and hollow-section grades likewise cannot be treated as automatic equivalents without checking their governing specifications and project requirements.

Sources

BS EN 10025-3:2019 — Hot rolled products of structural steels: Technical delivery conditions for normalized/normalized rolled weldable fine grain structural steelsBritish Standards Institution
DIN EN 10025-3:2019-10 — Hot rolled products of structural steels, Part 3DIN Media
BS EN 10025-3:2019 — Standards text reproductionBritish Standards Institution reproduction hosted by Sanon Pipe
S420 structural high-strength steel gradesArcelorMittal Europe

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Discussing: S420NL / 1.8912

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S420N / 1.8902	Same EN 10025-3 strength grade and delivery-condition family, with N toughness quality	Normally verified longitudinally at −20 °C rather than −50 °C; not an equivalent replacement where NL toughness is required.
S420ML / 1.8836	Related 420 MPa-class low-temperature fine-grain structural steel	Governed by EN 10025-4 and delivered thermomechanically rolled. Differences in chemistry and response to subsequent heat treatment prevent automatic substitution.
S355NL / 1.0546	Lower-strength NL-quality grade in EN 10025-3	Shares the normalized/normalizing-rolled family and low-temperature toughness class but has lower specified yield strength.
S460NL / 1.8903	Higher-strength NL-quality grade in EN 10025-3	Higher strength does not automatically make it interchangeable; design, welding, forming and availability must be reassessed.