Steel material properties
Steel material properties
The properties of structural steel arise from its chemical composition and manufacturing processes, including any treatment during fabrication. Product standards determine the allowances for composition, quality, and performance, which designers utilize. This article reviews essential properties for designers and indicates relevant standards for specific products. Details on specifying steelwork are available in a separate article.
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Schematic stress / strain diagram for steel
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Material properties required for design
Designers must consider various properties when specifying steel construction products.
Mechanical properties for design are derived from minimum values outlined in relevant product standards. Weldability depends on the alloy's chemical composition, which is prescribed in product standards. Durability varies by alloy type—ordinary carbon steel, weathering steel, or stainless steel.
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Factors that influence mechanical properties
Steel's mechanical properties result from its chemical composition, heat treatment, and manufacturing processes. Although iron is the primary component, small amounts of other elements can significantly impact steel's properties. The addition of alloys like manganese, niobium, and vanadium can enhance strength, but may negatively affect ductility, toughness, and weldability.
Lowering sulphur content improves ductility, while adding nickel enhances toughness. Each steel specification's chemical composition is meticulously balanced and tested to achieve the desired properties.
Alloying elements also respond differently to heat treatments involving controlled cooling from high temperatures. The manufacturing process, often combining heat treatment and mechanical working, is critical to steel performance.
As steel is rolled or formed, mechanical working occurs, which strengthens the material. Standards typically show reduced yield strength with increased thickness, as thinner material undergoes more extensive working.
Heat treatment's effect is explained by the various production processes used in steel manufacturing, including:
- As-rolled steel
- Normalized steel
- Normalized-rolled steel
- Thermomechanically rolled (TMR) steel
- Quenched and tempered (Q&T) steel.
As steel cools during rolling (typically finishing around 750°C), the term 'as-rolled' applies to steel allowed to cool naturally. Normalizing involves reheating as-rolled material to approximately 900°C for a specific time before cooling, which refines grain size and enhances toughness. Normalized-rolled processes achieve similar effects without reheating post-rolling. Normalized and normalized-rolled steels are designated with 'N'.
Using high tensile steel minimizes required volume but necessitates toughness at operating temperatures and sufficient ductility to withstand crack propagation. For this, low-carbon clean steels and grain refinement are crucial, effectively achieved through thermomechanical rolling (TMR).
TMR utilizes specific steel chemistry, allowing lower rolling finish temperatures of around 700°C. Rolling at these temperatures requires more force, maintaining properties unless reheated above 650°C, and is designated as 'M'.
The Quenched and Tempered steel process begins with a normalized material at 900°C, which is rapidly cooled ('quenched') to achieve high strength and hardness but reduced toughness. Tempering restores toughness by reheating to 600°C for a specified time before natural cooling. These steels have a 'Q' designation.
Quenching cools products through immersion in water or oil and is often combined with tempering to reduce hardness while increasing toughness and ductility.
Schematic temperature / time graph of rolling processes
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Strength
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Yield strength
Yield strength is a crucial property for designers, forming the basis for most design code rules. In European Standards for structural carbon steels (including weathering steel), designation primarily reflects yield strength; for example, S355 steel has a minimum yield strength of 355 N/mm².
Product standards also define permissible ultimate tensile strength (UTS) ranges, relevant for certain design aspects.
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Hot rolled steels
In hot rolled carbon steels, the designation number indicates yield strength for materials up to 16 mm thick. Designers should note that yield strength declines with increasing thickness (thinner materials are worked more, enhancing strength). The table below illustrates specified minimum yield strengths and minimum tensile strengths for the most common UK steel grades as per BS EN -2[1].
Minimum yield and tensile strength for common steel grades| Grade | Yield Strength (N/mm2) for Nominal Thickness t (mm) | Tensile Strength (N/mm2) for Nominal Thickness t (mm) |
|---|---|---|
| S275 | 275 265 255 245 410 400 |
|
| S355 | 355 345 335 325 470 450 |
The UK National Annex to BS EN -1-1[2] permits using the minimum yield value for specific thickness as the nominal (characteristic) yield strength fy and the minimum tensile strength fu as the nominal (characteristic) ultimate strength.
Similar values are available for other grades in other BS EN sections and for hollow sections per BS EN -1[3].
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Cold formed steels
A range of steel grades exists for strip steels suitable for cold forming, with minimum yield strength and tensile strength values as outlined in relevant standards BS EN -1-3[5].
BS EN -1-3[5] presents basic yield strength fyb and ultimate tensile strength fu values for use in design.
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Stainless steels
Stainless steel grades are designated with a numerical 'steel number' (e.g., 1. for typical austenitic steel) instead of 'S' designations for carbon steels. The stress-strain relationship in stainless steel lacks a clear yield point, with 'yield' strengths generally quoted based on a defined proof strength for a specific offset permanent strain (e.g., conventionally 0.2% strain).
The strengths of commonly used structural stainless steels range from 170 to 450 N/mm². Austenitic steels exhibit lower yield strength than carbon steels, while duplex steels display higher yield strength than common carbon steels. For both austenitic and duplex stainless steels, the ratio of ultimate strength to yield strength exceeds that of carbon steels.
BS EN -1-4[6] tabulates nominal (characteristic) values for yield strength fy and minimum tensile strength fu for use in design.
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Toughness
V-notch impact test specimen
All materials inherently possess some imperfections, which in steel can manifest as minute cracks. If steel lacks sufficient toughness, cracks may propagate rapidly without plastic deformation, leading to 'brittle fracture'. Factors such as thickness, tensile stress, stress raisers, and lower temperatures heighten the risk of brittle fracture. The toughness of steel and its resistance to brittle fracture rely on various factors considered during specification. A useful measure of toughness is the Charpy V-notch impact test, which assesses the energy required to fracture a notched specimen at a designated temperature through a single impact.
Product standards establish minimum impact energy values for different steel sub-grades. For non-alloy structural steels, primary subgrade designations include JR, J0, J2, and K2. For fine grain steels and quenched and tempered steels, distinct designations apply. Below is a summary of toughness designations.
Specified minimum impact energy for carbon steel sub-grades| Standard | Subgrade | Impact strength | Test temperature |
|---|---|---|---|
| BS EN -2[1] BS EN -1[3] |
JR | 27J | 20°C |
| J0 | 27J | 0°C | |
| J2 | 27J | -20°C | |
| K2 | 40J | -20°C | |
| BS EN -3[8] | N | 40J | -20°C |
| NL | 27J | -50°C | |
| BS EN -4[9] | M | 40J | -20°C |
| ML | 27J | -50°C | |
| BS EN -5[10] | J0 | 27J | 0°C |
| J2 | 27J | -20°C | |
| K2 | 40J | -20°C | |
| J4 | 27J | -40°C | |
| J5 | 27J | -50°C | |
| BS EN -6[11] | Q | 30J | -20°C |
| QL | 30J | -40°C | |
| QL1 | 30J | -60°C |
Materials less than 6 mm thick for cold forming do not have specified impact energy requirements.
The selection of an appropriate sub-grade to ensure adequate toughness in design circumstances is documented in BS EN -1-10[12] and related UK NA[13]. Guidelines link exposure temperature and stress level to a 'limiting thickness' for each steel sub-grade. PD -1-10[14] contains helpful tables, with ED007 providing guidance on appropriate sub-grade selection.
These design rules were intended for structures vulnerable to fatigue, such as bridges and crane support systems, acknowledging their conservative use in buildings where fatigue's role is minimal.
SCI publication P419 outlines modified steel thickness limits appropriate in buildings without fatigue being a design factor, derived using similar methodologies as Eurocode design rules. The term 'reduce' indicates some fatigue (20,000 cycles) acceptance based on DIN Standard guidance.
'Quasi-static' covers structures typically experiencing limited load cycling, treated as static in designs. Crack growth under 20,000 cycles is expressed through a formula developed by experts at the University of Aachen, involved in Eurocode development.
Further details are available in an article from the September issue of NSC magazine.
Generally, stainless steels demonstrate greater toughness than carbon steels, with BS EN -4[15] specifying minimum values. BS EN -1-4[6] states that austenitic and duplex steels remain adequately tough and unaffected by brittle fracture down to -40°C.
Xingtai Steel Product Page
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Ductility
Ductility measures how much a material can stretch between yielding and eventual fracture under tensile load, as illustrated below. Designers depend on ductility for various design aspects, including the redistribution of stress at ultimate limits, bolt group design, reduced fatigue crack propagation risks, and during fabrication processes like welding, bending, and straightening. Standards governing steel grades specify minimum ductility values, ensuring valid design assumptions and performance reliability.
Stress ' strain behaviour for steel
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Weldability
Welding stiffeners onto a large fabricated beam
(Image courtesy of Mabey Bridge Ltd)
Structural steels are generally considered weldable. However, the welding process melting the steel results in fast cooling, potentially hardening the heat-affected zone (HAZ) and decreasing toughness, especially in thicker materials.
Alloying elements primarily influence susceptibility to embrittlement, particularly carbon content, expressed as the 'Carbon Equivalent Value' (CEV). Product standards for carbon steels provide methods for calculating this value.
BS EN [1] enforces CEV limit requirements for all structural steel products covered, simplifying the task for welding practitioners to qualify welding procedure specifications by steel grade and CEV.
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Other mechanical properties of steel
Other significant mechanical properties for designers include:
- Modulus of elasticity, E = 210,000 N/mm²
- Shear modulus, G = E/[2(1 + ν)] N/mm², commonly approximated as 81,000 N/mm²
- Poisson's ratio, ν = 0.3
- Coefficient of thermal expansion, α = 12 x 10-6/°C (within ambient temperature ranges).
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Durability
Offsite application of corrosion protection
(Image courtesy of Hempel UK Ltd.)
Corrosion prevention is a crucial property. While specialized corrosion-resistant steels are available, they are generally not employed in building construction, apart from weathering steel.
Common corrosion protection methods for construction steel involve painting or galvanizing, with requirements varying based on exposure, location, and design life. Often, steel in dry internal conditions requires no corrosion protection, aside from fire protection. More detailed information on corrosion protection is accessible.
Weathering steel, a high-strength low-alloy steel, combats corrosion by forming a protective rust 'patina' inhibiting further corrosion. It is commonly used in the UK for bridges and some external buildings, and for architectural features like the Angel of the North.
Angel of the North
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Stainless steel
Typical stress-strain curves for stainless steel and carbon steel in the annealed condition
Stainless steel, known for its corrosion resistance, is suitable for structural use, especially where a high-quality surface is needed. Suitable grades for typical environments include the following.
Stress-strain behavior of stainless steels differs from carbon steels in several aspects. The most notable divergence is the stress-strain curve; while carbon steel exhibits linear elastic behavior until yielding, followed by a plateau before strain hardening, stainless steel demonstrates a more rounded response without a clearly defined yield stress. Consequently, stainless steel 'yield' strengths are typically defined for a specified offset permanent strain (like the 0.2% strain), as depicted in the typical curves for austenitic and duplex stainless steels.
Specified mechanical properties of common stainless steels to EN -4[15]| Description | Grade | Minimum 0.2% proof strength (N/mm2) | Ultimate tensile strength (N/mm2) | Elongation at fracture (%) |
|---|---|---|---|---|
| Basic chromium-nickel austenitic steels | 1. | 210 | 520 ' 720 | 45 |
| 1. | 200 | 500 ' 700 | 45 | |
| Molybdenum-chromiumnickel austenitic steels | 1. | 220 | 520 ' 670 | 45 |
| 1. | 220 | 520 ' 670 | 45 | |
| Duplex steels | 1. | 450 | 650 ' 850 | 30 |
| 1. | 460 | 640 ' 840 | 25 |
These mechanical properties apply to hot-rolled plates. Cold-rolled and hot-rolled strip strengths are generally 10-17% higher.
Guidelines for stainless steel selection BS EN ISO [16] Atmospheric Corrosion ClassTypical outdoor environment
Suitable stainless steel
| Atmospheric Corrosion Class | Typical outdoor environment | Suitable stainless steel |
|---|---|---|
| C1 (Very low) | Deserts and arctic areas (very low humidity) | 1./1., 1. |
| C2 (Low) | Arid or low pollution (rural) | 1./1., 1. |
| C3 (Medium) | Coastal areas with low salt deposits Urban or industrialised areas with moderate pollution |
1./1., 1. (1./1.) |
| C4 (High) | Polluted urban and industrial atmosphere Coastal areas with moderate salt deposits Road environments with de-icing salts |
1., (1./1.), other more highly alloyed duplex or austenitics |
| C5 (Very high) | Severely polluted industrial atmospheres with high humidity Marine atmospheres with significant salt deposits or splashes |
1., other highly alloyed duplex or austenitics |
Materials suitable for elevated classes may be appropriate for lower ones but are not always cost-effective. Materials in brackets may be considered where slight corrosion is tolerable. Accumulation of corrosive pollutants and chlorides increases in sheltered areas, necessitating grade selection from the next higher corrosion class.
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References
- 1.0 1.1 1.2
BS EN -2: Hot rolled products of structural steels. Technical delivery conditions for non-alloy structural steels, BSI.
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NA+A1: to BS EN -1-1:+A1:, UK National Annex to Eurocode 3: Design of steel structures General rules and rules for buildings, BSI
- 3.0 3.1
BS EN -1: Hot finished structural hollow sections of non-alloy and fine grain steels. Technical delivery requirements, BSI.
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BS EN : Continuously hot-dip coated steel flat products for cold forming. Technical delivery conditions. BSI
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BS EN -1-3: Eurocode 3: Design of steel structures. General rules - Supplementary rules for cold-formed members and sheeting, BSI.
- 6.0 6.1
BS EN -1-4:+A1: Eurocode 3. Design of steel structures. General rules. Supplementary rules for stainless steels, BSI
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BS EN -1: Stainless steels. List of stainless steels, BSI
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BS EN -3: , Hot rolled products of structural steels, Part 3: Technical delivery conditions for normalized / normalized rolled weldable fine grain structural steels, BSI
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BS EN -4: +A1:, Hot rolled products of structural steels, Part 4: Technical delivery conditions for thermomechanical rolled weldable fine grain structural steels, BSI
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BS EN -5: , Hot rolled products of structural steels, Part 5: Technical delivery conditions for structural steels with improved atmospheric corrosion resistance, BSI
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BS EN -6: +A1:, Hot rolled products of structural steels, Part 6: Technical delivery conditions for flat products of high yield strength structural steels in the quenched and tempered condition, BSI
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BS EN -1-10: Eurocode 3. Design of steel structures. Material toughness and through-thickness properties, BSI.
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NA to BS EN -1-10: , UK National Annex to Eurocode 3: Design of steel structures. Material toughness and through-thickness properties. BSI
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PD -1-10: Recommendations for the design of structures to BS EN -1-10. BSI
- 15.0 15.1
BS EN -4: Stainless steels. Technical delivery conditions for sheet/plate and strip of corrosion resisting steels for construction purposes, BSI.
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BS EN ISO : Corrosion of metals and alloys, Corrosivity of atmospheres, Classification, determination and estimation. BSI
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Resources
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See also
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