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
Grade |
Yield
strength (N/mm2) for nominal thickness t (mm) |
Tensile
strength (N/mm2) for nominal thickness t (mm) |
t ≤ 16 |
16 < t ≤ 40 |
40 < t ≤ 63 |
63 < t ≤ 80 |
3 < t ≤ 100 |
100 < t ≤ 150 |
S275 |
275 |
265 |
255 |
245 |
410 |
400 |
S355 |
355 |
345 |
335 |
325 |
470 |
450 |
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
Standard |
Subgrade |
Impact strength |
Test temperature |
BS EN 10025-2
BS EN 10025-5
BS EN 10210-1 |
JR |
27J |
20℃ |
J0 |
27J |
0℃ |
J2 |
27J |
-20℃ |
K2 |
40J |
-20℃ |
BS EN 10025-3 |
N |
40J |
-20℃ |
NL |
27J |
-50℃ |
BS EN 10025-4 |
M |
40J |
-20℃ |
ML |
27J |
-50℃ |
BS EN 10025-6 |
Q |
30J |
-20℃ |
QL |
30J |
-40℃ |
QL1 |
30J |
-60℃ |
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
Description |
Grade |
Minimum 0.2% proof strength
(N/mm2) |
Ultimate tensile strength (N/mm2) |
Elongation at fracture (%) |
Basic chromium-nickel austenitic
steels |
1.4301 |
210 |
520 – 720 |
45 |
1.4307 |
200 |
500 – 700 |
45 |
Molybdenum-chromiumnickel austenitic
steels |
1.4401 |
220 |
520 – 670 |
45 |
1.4404 |
220 |
520 – 670 |
45 |
Duplex steels |
1.4162 |
450 |
650 – 850 |
30 |
1.4462 |
460 |
640 – 840 |
25 |
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
ISO 9223 Atmospheric Corrosion Class |
Typical outdoor environment |
Suitable stainless steel |
C1 (Very low) |
Deserts and arctic areas (very low
humidity) |
1.4301/1.4307, 1.4162 |
C2 (Low) |
Arid or low pollution (rural) |
1.4301/1.4307, 1.4162 |
C3 (Medium) |
Coastal areas with low deposits of
salt
Urban or industrialised areas with moderate pollution |
1.4401/1.4404, 1.4162
(1.4301/1.4307) |
C4 (High) |
Polluted urban and industrialised
atmosphere
Coastal areas with moderate salt deposits
Road environments with de-icing salts |
1.4462, (1.4401/1.4404), other more
highly alloyed duplexes or austenitics |
C5 (Very high) |
Severely polluted industrial
atmospheres with high humidity
Marine atmospheres with high degree of salt deposits and
splashes |
1.4462, other more highly alloyed
duplexes or austenitics |
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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