Table steel grades 7 Durability. 7 Other Properties 8

 

Table of Contents
Introduction. 3
Steel 3
Source and Sustainability. 3
Iron Ore Exports
by Country. 4
Processing. 5
Smelting. 5
Converting Pig Iron. 5
Hot Rolling. 6
Energy. 6
Properties. 7
Strength. 7
Minimum yield and tensile strengths for common steel grades 7
Durability. 7
Other Properties 8
Speed. 8
Aesthetics 8
Likely Uses 8
Protection Methods 9
End of Life Failure Mechanism.. 9
Recycling. 10
Conclusion. 11
References. 12
Appendix 1. 13
 

 

Introduction

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This report will describe the
properties and use of steel as a construction material, it will look in detail
into the source of the raw materials, and their sustainability, the processing
that is involved in making the raw materials into the finished product, looking
at the energy requirements and the carbon footprints of these processes. It
will also look at the properties of steel, including strength in tension and
compression, long-term durability and likely uses. It will describe methods
that are used in order to protect the steel with regard to life expectancy and
maintenance requirements, describe common end of life failure mechanisms and
recycling methods.

Steel

Steel is what is known as a ferrous
metal, ferrous metals are defined as those in which the element iron
predominates. Steel is a relatively recent material, only being available in quantity
following the development of the Bessemer converter in the later 19th
century (Lyons, 2006). Steel is the most important ferrous metal in
construction; it is an alloy of iron and carbon. According to (Taylor, 2000)
steel’s widespread application for structural uses is account of the following
properties:

·     
Iron is relatively abundant
in the earth’s crust, second only to aluminium.

·     
Its melting point is
sufficiently high to produce high strength, but not so high as to give problems
reaching temperatures in bulk furnaces.

·     
Iron undergoes a change of
crystal structure at red heat, which assists in many production processes and
leads to metallurgical advantages.

Metallurgy is the study of metals
and their properties in bulk and at the atomic level American Heritage
Dictionaries, (2015). 

Source
and Sustainability

Steel is made from iron, this is
mined from areas of the world with high iron densities that is considered
relatively easy to access. There are two distinct categories of iron ore mining,
surface mining such as open cast or strip mining and underground mining.
Some of the big contributors of iron mining are Australia, Venezuela, East
Europe and Russia. Below is a list of the 15 top iron ore exporters of 2014.

 

(Workman,
2015)

 

In terms of sustainability iron is
relatively abundant in the earth’s crust, there is however fear that the raw
material is being consumed faster than predicted. A number of experts have cast
their predictions of the current level of iron available for mining, Geoscience
Australia has calculated that there are 24 billion tonne and the current
production rate of Western Australia is 430 million tonne per year and rising.
Predictions of the amount of time we have left before running out made by some
experts range from 30-56 years with Johnathon Law (CRISO) predicting 56 years
and Gavin Mudd (Monash University) predicting 30 years (Pincock, 2010). However, that is just Australia’s reserves and the study
is ongoing, there are a lot more reserves of iron ore around the world as can
be seen from the table above of the top 15 exporters. To transform iron ore
into steel the ore must go through a process which adds other elements such as
coal and heat, the fossil fuel reserves that are used in order to complete the
process are more likely to deplete before the iron ore reserves do.

 

Processing

 

Smelting

In order to convert the raw iron ore into steel and a
usable product it must go through an intense process, Appendix 1 depicts a flow
chart of the process to create steel from iron ore Tatasteelindia.com, (2017). The
raw element contains many impurities, these need to be removed, the first stage
of the process is called smelting. Smelting is the
process by which a metal is obtained, either as the element or as a simple
compound, from its ore by heating beyond the melting point, ordinarily in the
presence of oxidizing agents, such as air, or reducing agents, such as coke (The
Editors of Encyclopedia Britannica, 2017). The iron ore is put into a blast
furnace, here it is heated to 1200C, coal is then added as a heat source and
limestone as a flux to separate what is known as ‘gangue’ from the molten iron
ore. Gangue is essentially the extra materials that have been mined when mining
the iron, any rock or surrounding material that may be stuck to the ore. At the
bottom of the blast furnace there are two tap-holes where the accumulated slag
and molten iron is collected with the slag going to a disposal area and the
molten iron directed into molds where it solidifies into pig iron and is ready
for the next process.

 

Converting Pig Iron

Following the
smelting process, we are left with what is known as pig iron, pig iron has a
high carbon content which leads to it being brittle. From Pig Iron, we can
produce a number of products including, cast iron, wrought iron or steel.
Wrought iron is produced by continuously beating the material to remove the
impurities added during the smelting process. To create steel the pig iron must
enter another furnace, the two common types of furnace are the electric arc
furnace and the basic oxygen furnace. In these furnaces, the impurities are
completely removed from the pig iron and then carbon is added back in to the
percentage required for the grade of steel, typically 1%. Recycled steel can
also be added into these furnaces to be reused in the new product, this simply
melts down and combines with the molten compound in the furnace, the amount of
recycled material used is controlled depending on the grade of the steel and
the section type it will become.

 

Hot Rolling

Following the second
furnace process the end product that is left behind is steel, however it has
been removed from the furnaces and cut into large slabs, not a useable product.
In order to make this into a usable product the steel must go through the hot
roll process. The rolling of metal is a forming process where metal stock is
passed through one or more pairs of rollers, this will in turn reduce the
thickness of the stock to the desired level and provide a uniform thickness. The steel is sent straight from the furnace to
the hot roll process, as, as its name suggests the process is performed whilst
the material is still hot. At the rolling mill, the slab of steel will be
pushed back and forth through a number of different size and shaped rollers,
thinning the material, lengthening it and reshaping it into the desired product
such as a girder as it cools down. Cold rolling can also be used, but is
generally used for smaller sections such as flats and bars, this is when the
material has cooled down and is below its recrystallization temperature and
makes for a harder material, however it is much harder to reshape and compress
than the hot rolling process.

 

Energy

In order to gain our
end product of steel a tremendous amount of energy is required. The mining of
the ore, the 2 furnace processes and the rolling, all of these processes
require a lot of energy, Appendix 2 shows use the embodied energies of
materials. From this table, we can see that Steel (section – average recycled
content), much like what we would use in construction e.g. a beam, has a value
of 21.50 MJ/kg of energy embodied into it, with 1.42 kg CO2/kg of carbon and a
density of 7800 kg/m3.

 

Properties

 

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, the primary
designation relates to the yield strength, e.g. S355 steel is a structural
steel with a specified minimum yield strength of 355 N/mm². For hot rolled
carbon steels, the number quoted in the designation is the value of yield
strength for material up to 16 mm thick. Yield strength reduces with increasing
plate or section thickness (thinner material is worked more than thick material
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 (Steelconstruction.info,
2014).

 

Minimum yield and
tensile strengths for common steel grades

Grade

Yield strength (N/mm2)

Tensile strength (N/mm2)

 

for nominal thickness t (mm)

for nominal thickness t (mm)

 

t ? 16

16

x

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