S420MLH Hollow Sections
S420MLH Hollow Sections: High-Strength Fine-Grain Steel for Arctic Structural Integrity
Explore the properties and applications of S420MLH cold-formed welded hollow sections according to EN 10219, a thermomechanically rolled steel with minimum 420 MPa yield strength and guaranteed impact toughness at -50°C.
Thermomechanical rolling, cold forming, high-frequency induction or submerged arc welding, post-weld normalizing may be applied. Suitable for cutting, machining, bending, and galvanizing.
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S420MLH Hollow Sections Introduction
S420MLH is a thermomechanically rolled fine-grain structural steel grade defined in EN 10219 for cold-formed welded structural hollow sections of circular, square, or rectangular form. It offers a minimum yield strength of 420 MPa with excellent toughness at low temperatures down to -50°C, designated by the 'ML' suffix. The steel is fully killed and contains grain-refining elements such as niobium, vanadium, and titanium, ensuring a fine ferritic microstructure that delivers high strength, good weldability, and consistent ductility. Typical products are SHS (square hollow sections) and RHS (rectangular hollow sections), widely used in demanding structural applications like offshore platforms, bridges, crane booms, and high-rise buildings where reduced weight and enhanced impact resistance are critical.
S420MLH Hollow Sections Chemical Composition in % by Mass per EN 10219-1
The S420MLH grade is a fully killed fine-grain steel with controlled additions of microalloying elements. The chemical composition ensures adequate hardenability for consistent mechanical properties across wall thicknesses. Maximum limits are given unless a range is specified. Nitrogen-binding elements such as Al, Nb, V, and Ti are present to guarantee fine grain and toughness.
| Element | Standard Value (max.%) | Remarks |
|---|---|---|
| Carbon (C) | 0.16 | Max. for product thickness ≤ 40 mm |
| Silicon (Si) | 0.50 | Max. |
| Manganese (Mn) | 1.70 | Max. |
| Phosphorus (P) | 0.025 | Max. |
| Sulfur (S) | 0.020 | Max. |
| Nitrogen (N) | 0.020 | Max.; for nitrogen-binding, Al is also specified |
| Aluminium (Alₜₒₜₐₗ) | ≥ 0.020 | Minimum total Al; acid-soluble Al permissible |
| Niobium (Nb) | 0.05 | Max. |
| Vanadium (V) | 0.12 | Max. |
| Titanium (Ti) | 0.05 | Max. |
| Molybdenum (Mo) | 0.20 | Max. |
| Nickel (Ni) | 0.30 | Max. optional; may be present but not required |
| Chromium (Cr) | 0.30 | Max. optional |
| Copper (Cu) | 0.35 | Max. optional |
| Ceq (CEV) | ≤ 0.45 (typ.) | Carbon equivalent for weldability; calculated as CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 |
S420MLH Hollow Sections Thermal and Electrical Physical Properties
The following physical properties are typical for fine-grain structural steel at room temperature unless a temperature range is stated. These values are not mandatory per the product standard but are representative for design calculations. They are based on published data for similar S420 thermomechanically rolled steels.
| Property | Typical Value | Unit | Test Condition / Remarks |
|---|---|---|---|
| Density (ρ) | 7850 | kg/m³ | At 20°C |
| Elastic modulus (E) | 210 | GPa | At 20°C; dynamic modulus may be slightly higher |
| Shear modulus (G) | 81 | GPa | Calculated from E and Poisson's ratio |
| Poisson's ratio (ν) | 0.3 | dimensionless | In elastic range |
| Thermal expansion coefficient (α) | 12.0 × 10⁻⁶ | K⁻¹ | Mean value between 20°C and 100°C |
| Thermal expansion coefficient (α) | 12.5 × 10⁻⁶ | K⁻¹ | Mean value between 20°C and 200°C |
| Thermal expansion coefficient (α) | 13.0 × 10⁻⁶ | K⁻¹ | Mean value between 20°C and 300°C |
| Thermal conductivity (λ) | 50 | W/(m·K) | At 20°C; decreases with rising temperature |
| Thermal conductivity (λ) | 45 | W/(m·K) | At 200°C |
| Thermal conductivity (λ) | 38 | W/(m·K) | At 400°C |
| Specific heat capacity (cₚ) | 460 | J/(kg·K) | At 20°C; increases with temperature |
| Specific heat capacity (cₚ) | 520 | J/(kg·K) | At 200°C |
| Specific heat capacity (cₚ) | 600 | J/(kg·K) | At 400°C |
| Electrical resistivity (ρₑ) | 0.23 × 10⁻⁶ | Ω·m | At 20°C, typical for low-alloy steel |
S420MLH Hollow Sections Mechanical Properties
Mechanical properties are valid for wall thickness up to 40 mm. Values are minimum unless otherwise indicated. The test pieces are taken transverse to the weld. Yield strength decreases with increasing thickness. Impact properties are guaranteed for the ML grade at -50°C on longitudinal Charpy-V specimens.
| Property | Standard Requirement | Unit | Test Condition / Remarks |
|---|---|---|---|
| Upper yield strength (ReH) | 420 (t ≤ 16 mm) | MPa | Wall thickness ≤ 16 mm |
| Upper yield strength (ReH) | 400 (16 < t ≤ 40 mm) | MPa | Wall thickness > 16 mm, ≤ 40 mm |
| Tensile strength (Rm) | 500 – 660 | MPa | All wall thicknesses |
| Elongation at fracture (A) | ≥ 19 | % | Gauge length L₀ = 5.65√S₀; for wall thickness ≤ 40 mm, longitudinal test |
| Impact energy (KV₂) at -50°C | ≥ 27 (longitudinal) | J | Charpy V-notch, average of 3 specimens; single min. 70% of average |
| Bend test (mandrel diameter) | d = 3a (for a ≤ 16 mm) | - | 180° bend; a = wall thickness; no cracks |
| Bend test (mandrel diameter) | d = 4a (for a > 16 mm) | - | Applicable for thickness 16 – 40 mm |
S420MLH Hollow Sections Fully Equivalent Material Standards and Substitutable Grades
| Country / Region | Standard | Grade | Remarks |
|---|---|---|---|
| European Union | EN 10219-1 | S420MLH | Exact standard and grade for cold-formed welded hollow sections |
| European Union | EN 10210-1* | S420MLH | Hot-finished hollow sections; technically equivalent chemistry and strength but different production route. |
| International | ISO 4951-3 | S420ML | For hot-finished hollow sections; S420ML may be comparable, but the standard does not cover cold-formed hollow sections directly. |
S420MLH Hollow Sections Application Introduction
S420MLH hollow sections are designed for welded, bolted, and riveted structures requiring high strength-to-weight ratio and reliable low-temperature toughness. The combination of fine grain and thermomechanical rolling ensures good formability and excellent weldability, making the material suitable for automated fabrication processes.
Product Applications: Structural hollow sections – square (SHS), rectangular (RHS), and circular (CHS), Welded beams and columns fabricated from hollow sections, Truss nodes and cast steel connectors, Pre-fabricated steel towers and pylons, Pile foundations and retaining wall soldier piles, Crane and excavator booms, Communication and lighting mast structures
Processed into products: Tubular chords and braces for offshore jackets, Rectangular hollow section columns in seismic-resistant frames, Crane lattice boom lacings and main chords, Wind turbine tower door frames and internal platforms, Bridge deck cross girders and arch ribs, Heavy-duty conveyor galleries, Junction cans for fast tubular connections
Application industries: Offshore and marine engineering (jacket legs, flare booms, modules), Bridge construction (truss chords, arch ribs, bracing), Wind energy (tower internals, transition pieces, tower flanges), Building construction (high-rise columns, space frames, long-span roofs), Mining and heavy lifting equipment (dragline booms, crane telescopic arms), Railway and transportation (wagon frames, rail bridges), Shipbuilding (hull stiffeners, crane pedestals)
S420MLH Hollow Sections Similar or Alternative Grades with Comparable Performance
| Country / Region | Standard | Grade | Remarks |
|---|---|---|---|
| European Union | EN 10025-4 | S420ML | Flat and long products, thermomechanically rolled, similar strength and toughness, but not hollow section geometry; basis for hollow section chemistry. |
| European Union | EN 10219-1 | S460MLH | Next higher strength hollow section (460 MPa yield) with identical toughness class; may be used if weight reduction needed. |
| European Union | EN 10219-1 | S420MH | Same yield strength, but impact tested at -20°C only (MH); suitable where lower toughness is acceptable. |
| China | GB/T 1591 | Q420D | Similar yield strength with impact at -20°C; not identical in fine-grain practice but can meet many structural requirements with caution. |
| USA | ASTM A500 / A500M | Grade C (rect. shapes) | Yield 345 MPa (50 ksi) – lower strength; not directly comparable, but commonly available hollow section. |
Notes:
Dimensional tolerances and shape specifications are per EN 10219-2. Welding consumables should be matched to the base material, typically using low-hydrogen processes and filler metals with a minimum yield strength of 420 MPa and good low-temperature toughness. Preheating is generally not required for thicknesses up to 20 mm under normal conditions. Post-weld heat treatment (PWHT) may be avoided if the steel is used in the as-welded condition, but stress relief at 530-580°C is possible with controlled cooling.
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