The properties of structural steel result from both its chemical composition and its method of manufacture, including processing during fabrication. Product standards define the limits for composition, quality and performance and these limits are used or presumed by structural designers.

Schematic stress / strain diagram for steel

Material properties required for design

The properties that need to be considered by designers when specifying steel construction products are:
     Strength
 Toughness
 Ductility
 Weldability
 Durability

For design, the mechanical properties are derived from minimum values specified in the relevant product standard. Weldability is determined by the chemical content of the alloy, which is governed by limits in the product standard. Durability depends on the particular alloy type - 'ordinary' carbon steel, 'weathering steel' or stainless steel.

Factors that influence mechanical properties

Steel derives its mechanical properties from a combination of chemical composition, heat treatment and manufacturing processes. While the major constituent of steel is iron the addition of very small quantities of other elements can have a marked effect upon the properties of the steel. The strength of steel can be increased by the addition of alloys such as manganese, niobium and vanadium. However, these alloy additions can also adversely affect other properties, such as ductility, toughness and weldability .

Minimizing the sulphur level can enhance ductility , and toughness can be improved by the addition of nickel. The chemical composition for each steel specification is therefore carefully balanced and tested during its production to ensure that the appropriate properties are achieved.

The alloying elements also produce a different response when the material is subjected to heat treatments involving cooling at a prescribed rate from a particular peak temperature. The manufacturing process may involve combinations of heat treatment and mechanical working that are of critical importance to the performance of the steel.

Mechanical working takes place as the steel is being rolled or formed. The more steel is rolled, the stronger it becomes. This effect is apparent in the material standards, which tend to specify reducing levels of yield strength with increasing material thickness.

The effect of heat treatment is best explained by reference to the various production process routes that can be used in steel manufacturing, the principal ones being:
 As-rolled steel
 Normalized steel
 Normalized-rolled steel
 Thermomechanically rolled (TMR) steel
 Quenched and tempered (Q&T) steel.

Steel cools as it is rolled, with a typical rolling finish temperature of around 750°C. Steel that is then allowed to cool naturally is termed 'as-rolled' material. Normalizing takes place when as-rolled material is heated back up to approximately 900°C, and held at that temperature for a specific time, before being allowed to cool naturally. This process refines the grain size and improves the mechanical properties, specifically toughness. Normalized-rolled is a process where the temperature is above 900°C after rolling is completed. This has a similar effect on the properties as normalizing, but it eliminates the extra process of reheating the material. Normalized and normalized-rolled steels have an 'N' designation.

The use of high tensile steel can reduce the volume of steel needed but the steel needs to be tough at operating temperatures, and it should also exhibit sufficient ductility to withstand any ductile crack propagation. Therefore, higher strength steels require improved toughness and ductility, which can be achieved only with low carbon clean steels and by maximizing grain refinement. The implementation of the thermomechanical rolling process (TMR) is an efficient way to achieve this.

Thermomechanically rolled steel utilises a particular chemistry of the steel to permit a lower rolling finish temperature of around 700°C. Greater force is required to roll the steel at these lower temperatures, and the properties are retained unless reheated above 650°C. Thermomechanically rolled steel has an 'M' designation.

The process for Quenched and Tempered steel starts with a normalized material at 900°C. It is rapidly cooled or 'quenched' to produce steel with high strength and hardness, but low toughness. The toughness is restored by reheating it to 600°C, maintaining the temperature for a specific time, and then allowing it to cool naturally (Tempering). Quenched and tempered steels have a 'Q' designation.

Quenching involves cooling a product rapidly by immersion directly into water or oil. It is frequently used in conjunction with tempering which is a second stage heat treatment to temperatures below the austenitizing range. The effect of tempering is to soften previously hardened structures and make them tougher and more ductile.

Schematic temperature / time graph of rolling processes

Strength

Yield strength

Yield strength is the most common property that the designer will need as it is the basis used for most of the rules given in design codes. In European Standards for structural carbon steels (including weathering steel ), the primary designation relates to the yield strength, e.g. S275 steel is a structural steel with a specified minimum yield strength of 275 N/mm².

The product standards also specify the permitted range of values for the ultimate tensile strength (UTS). The minimum UTS is relevant to some aspects of design.

Hot rolled steels

For hot rolled carbon steels, the number quoted in the designation is the value of yield strength for material up to 16 mm thick. Designers should note that yield strength reduces with increasing plate or section thickness (thinner plate is worked more than thick plate and working increases the strength). For the two most common grades of steel used in UK, the specified minimum yield strengths and the minimum tensile strength are shown in table below for steels to BS EN 10025-2

Minimum yield and tensile strength for common steel grades

The UK National Annex to BS EN 1993-1-1 allows the minimum yield value for the particular thickness to be used as the nominal (characteristic) yield strength fy and the minimum tensile strength fu to be used as the nominal (characteristic) ultimate strength.

Similar values are given for other grades in other parts of BS EN 10025 and for hollow sections to BS EN 10210-1

Cold formed steels

There is a wide range of steel grades for steels suitable for cold forming. Minimum values of yield strength and tensile strength are specified in the relevant product standard BS EN 10346:2009.
BS EN 1993-1-3 tabulates values of basic yield strength fyb and ultimate tensile strength fu that are to be used as characteristic values in design.

Stainless steels

Grades of stainless steel are designated by a numerical 'steel number' (such as 1.4401 for a typical austenitic steel) rather than the 'S' designation system for carbon steels. The stress-strain relationship does not have the clear distinction of a yield point and stainless steel 'yield' strengths for stainless steel are generally quoted in terms of a proof strength defined for a particular offset permanent strain (conventionally the 0.2% strain).

The strengths of commonly used structural stainless steels range from 170 to 450 N/mm². Austenitic steels have a lower yield strength than commonly used carbon steels; duplex steels have a higher yield strength than common carbon steels. For both austenitic and duplex stainless steels, the ratio of ultimate strength to yield strength is greater than for carbon steels.

BS EN 1993-1-4 tabulates nominal (characteristic) values of yield strength fy and ultimate minimum tensile strength fu for steels to BS EN 10088-1 for use in design.

Toughness

It is in the nature of all materials to contain some imperfections. In steel these imperfections take the form of very small cracks. If the steel is insufficiently tough, the 'crack' can propagate rapidly, without plastic deformation and result in a 'brittle fracture'. The risk of brittle fracture increases with thickness, tensile stress, stress raisers and at colder temperatures. The toughness of steel and its ability to resist brittle fracture are dependent on a number of factors that should be considered at the specification stage. A convenient measure of toughness is the Charpy V-notch impact test - see image on the right. This test measures the impact energy required to break a small notched specimen, at a specified temperature, by a single impact blow from a pendulum.

The various product standards specify minimum values of impact energy for different sub-grades of each strength grade. For non-alloy structural steels the designations of the subgrades are JR, J0, J2 and K2. For fine grain steels and quenched and tempered steels (which are generally tougher, with higher impact energy) different designations are used. A summary of the toughness designations is given in the table below.

Specified minimum impact energy for carbon steel sub-grades

For thin gauge steels for cold forming, no impact energy requirements are specified for material less than 6 mm thick.

The selection of an appropriate sub-grade, to provide adequate toughness in design situations is given in BS EN 1993‑1‑10. The rules relate the exposure temperature, stress level etc, to a 'limiting thickness' for each sub-grade of steel. Guidance on selection of an appropriate sub-grade is given in ED007 Selection of steel subgrades in accordance with the Eurocodes.

Stainless steels are generally much tougher than carbon steels; minimum values are specified in BS EN 10088-4. BS EN 1993-1-4 states that austenitic and duplex steels are adequately tough and not susceptible to brittle fracture for service temperatures down to -40°C.

Ductility

Ductility is a measure of the degree to which a material can strain or elongate between the onset of yield and eventual fracture under tensile loading as demonstrated in the figure below. The designer relies on ductility for a number of aspects of design, including redistribution of stress at the ultimate limit state, bolt group design, reduced risk of fatigue crack propagation and in the fabrication processes of welding, bending and straightening. The various standards for the grades of steel in the above table insist on a minimum value for ductility so the design assumptions are valid and if these are specified correctly the designer can be assured of their adequate performance.

Stress – strain behaviour for steel

Weldability

All structural steels are essentially weldable. However, welding involves locally melting the steel, which subsequently cools. The cooling can be quite fast because the surrounding material, e.g. the beam, offers a large 'heat sink' and the weld (and the heat introduced) is usually relatively small. This can lead to hardening of the 'heat affected zone' (HAZ) and to reduced toughness. The greater the thickness of material, the greater the reduction of toughness.

The susceptibility to embrittlement also depends on the alloying elements principally, but not exclusively, the carbon content. This susceptibility can be expressed as the 'Carbon Equivalent Value' (CEV), and the various product standards for carbon steels standard give expressions for determining this value.

BS EN 10025 sets mandatory limits for CEV for all structural steel products covered, and it is a simple task for those controlling welding to ensure that welding procedure specifications used are qualified for the appropriate steel grade, and CEV.

Welding stiffeners onto a large fabricated beam

Other mechanical properties of steel

Other mechanical properties of structural steel that are important to the designer include:
 Modulus of elasticity, E = 210,000 N/mm²
 Shear modulus, G = E/[2(1 + ν)] N/mm², often taken as 81,000 N/mm²
 Poisson's ratio, ν = 0.3
 Coefficient of thermal expansion, α = 12 x 10-6/°C (in the ambient temperature range).

Durability

A further important property is that of corrosion prevention. Although special corrosion resistant steels are available these are not normally used in building construction. The exception to this is weathering steel .

The most common means of providing corrosion protection to construction steel is by painting or galvanizing. The type and degree of coating protection required depends on the degree of exposure, location, design life, etc. In many cases, under internal dry situations no corrosion protection coatings are required other than appropriate fire protection. Detailed information on the corrosion protection of structural steel is available.

Weathering steel

Weathering steel is a high strength low alloy steel that resists corrosion by forming an adherent protective rust 'patina', that inhibits further corrosion. No protective coating is needed. It is extensively used in the UK for bridges and has been used externally on some buildings. It is also used for architectural features and sculptural structures such as the Angel of the North.

Stainless steel

Stainless steel is a highly corrosion-resistant material that can be used structurally, particularly where a high-quality surface finish is required. Suitable grades for exposure in typical environments are given below.

The stress-strain behaviour of stainless steels differs from that of carbon steels in a number of respects. The most important difference is in the shape of the stress-strain curve. While carbon steel typically exhibits linear elastic behaviour up to the yield stress and a plateau before strain hardening is encountered, stainless steel has a more rounded response with no well-defined yield stress. Therefore, stainless steel 'yield' strengths are generally defined for a particular offset permanent strain (conventionally the 0.2% strain), as indicated in the figure on the right which shows typical experimental stress-strain curves for common austenitic and duplex stainless steels. The curves shown are representative of the range of material likely to be supplied and should not be used in design.

Specified mechanical properties of common stainless steels to EN 10088-4

The mechanical properties apply to hot rolled plate. For cold rolled and hot rolled strip, the specified strengths are 10-17% higher.

Guidelines for stainless steel selection

Materials suitable for a higher class may be used for lower classes but might not be cost effective. Materials within brackets might be considered if some moderate corrosion is acceptable. Accumulation of corrosive pollutants and chlorides will be higher in sheltered locations; hence it might be necessary to choose a recommended grade from the next higher corrosion class.

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