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STEEL AS Engineering MATERIAL
Steel is
the backbone of all Industries and the basic ingredient for growth
and development of a country. Traditionally, the fortunes of the
steel industry have been linked to the economic cycle of the
country. Per capita consumption of steel speaks volumes about the
relative position of the country on the development frontier. In
India the per capita consumption of steel stands very low (38 kg) in
compare to world average per capita consumption of steel (170 kg),
where total production of steel is about 1200 MT. Moreover, steel is
completely recyclable and thus environment friendly. Hence a large
potential exists in furthering the usage of steel in various
segments of industry.
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If anyone metal is to be
named, which has maximum impact on mankind, it is steel. It is
the most important metal today, which enjoys about 95% of total
metal output globally. A compound basically made of iron and carbon
and some other elements is called Steel. Thus steels are
alloys of iron (Fe) and carbon (C). Its mechanical properties
(and the electrical or magnetic properties) are influenced by
the addition of various elements (Cr,Mn,Ni,Si etc). Carbon is
the element that modify the allotropic (crystal structure)
changes that iron displays during heating and cooling. These
structural changes occur during processing or manufacture of the
steel. Different types of steel are produced for particular
applications and are manufactured within precise composition
limits and processing conditions in order to provide the
required microstructure, properties and functionality.Steel as
an Engineering Material is of great interest to Architects, Civil and
Structural engineers, Designers, Consultants, Metallurgists
and Project owners. |
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Like all metals iron (Pig Iron) is made from iron
ore, coke and limestone in blast furnace or by direct reduction
processes. Production of steel from pig iron involves processes like
steel making, primary and secondary steelmaking, casting and hot
rolling. The carbon content of steels is up to 2.0%. The
microstructure and atomic structure varies with carbon content and
temperature. A wide range of strength levels, from 120 to over 3000
mpa can be obtained with the help of carbon and alloying elements by
mechanical working and heat treatment.
Steel
enjoys the widest range of applications among the materials known to
mankind due to different mechanical properties and product forms. It
touches everybody’s life everyday everywhere.
Segments
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Typical
Applications |
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Construction |
Industrial, Hospital, Institutional Buildings,
Exhibition Halls, Stadiums, Railway and Bus stations, offshore
structures |
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Agriculture & Rural areas |
Machinery, Storage tanks, Grain bins, Bullock
Carts, Meeting Halls |
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House Hold |
Buckets, Scissors, Kitchen cabins, Ovens, white
goods, utensils, furniture |
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Infrastructures |
Bridges, Flyovers, Foot Bridges, Culverts and RCC
Structures |
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Oil, Gas & Power |
Wells, Platforms and items related to
power |
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Transport |
Railways, Buses, Trucks, Luxury Coaches, Cars,
two-wheelers, bicycles |
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Electrical & Electronics |
Transformer core, Motors, Transmission
Towers |
Grades of Steel:
Mild Steel is the commonest grade of steel
containing less than 0.25% Carbon. Medium Carbon Steel has carbon
content 0.25% to 0.45% and used as input material in the engineering
industries. High Carbon Steel refers to the harder steel with carbon
content 0.45% to 0.90% and is used to make precision tools and
instruments. Alloy Steels/Special Steels including Stainless Steel
are produced by adding alloying elements like Mn,W,Ni etc. Stainless
Steel contains Cr and Ni with proper proportions. Weathering steels
are used for atomoshpheric corrosion resistance
purpose.
Finished Steel is divided into two categories like
Non Flat products (bars, rods, angles, joist, channels &
railways materials) and Flat (plates, hot rolled coils, cold rolled
coils/sheets, galvanized plated, galvanized corrugated coils/sheets,
tin plates and electrical sheets.)
Hot metal is processed
further to get better quality, strength and specific application
through different routes like:
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Basic Oxygen Furnace Steel making
(BOF) : The purpose is to refine the hot metal
produced in the blast furnace, which may be subsequently
refined in the secondary steelmaking shop. The main functions
are to remove carbon & phosphorus from the hot metal, and
to optimize the steel temperature so that any further
treatments prior to casting can be performed with minimal
reheating or cooling of the steel.
Electric
Arc Furnace (EAF) : Recycled steel scrap is melted
and converted into high quality steel by using high-power
electric arcs. The main task is to convert the solid raw
materials to liquid crude steel and then refine further in
subsequent secondary steelmaking processes.
Secondary Steelmaking: It is a
critical step in the steel production process between the
primary processes (BOF or EAF) and casting. Some elements are
added and some have to be removed during secondary steelmaking
in order to fine-tune the composition of the steel to meet the
specification. . After secondary steelmaking, the ladle of
liquid steel is taken, at the required composition, quality,
cleanness, time, temperature and at least cost to the casting
process.
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Continuous Casting: The molten
steel is continuously cast through a tundish into a
water-cooled copper mould causing a thin shell to solidify.
The solidified shell continues to thicken until the strand is
fully solidified. Finally, the strand is cut into desired
lengths. Depending on final application cast dimensions may be
slab for flat products (plate, strip etc), blooms for
structural sections (beams, channels etc) and billets for long
products (wire).
Hot Rolling: It is
the most efficient process of primary forming used for the
mass production of steel. The principal effects of hot rolling
are the elimination of the cast ingot structure defects and
obtaining the required shape, dimensions and surface quality
of a product. This makes both semi-finished and finished
products. Semi-finished hot-rolled steel products are the
starting materials for further hot metal forming processes
(flat, long products, seamless tubes, wheels, rings, bars etc)
and cold rolling.
Cold Forming:
Hot-rolled products are often subjected to further processing
like cold rolling, forming, machining and joining, in order to
get desired strength and properties for specific
applications. |
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A Airport in China Made of Steel Tubes |
Mechanical Properties:
Like all metallic materials specific steels are
having specific mechanical properties. Steel properties can be
divided into two
groups:
a.
Structure insensitive
b. Structure
sensitive
Principal structure insensitive properties are elastic
modulus, density and some chemical, electrical and thermal
characteristics. The structure sensitive properties are wholly
dependent upon the past history – whether hot rolled or cold rolled,
whether heat treated and if so how. The most important structure
sensitive properties are yield strength, tensile strength, ductility
fractures toughness, fatigue characteristics, etc.
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Elastic Moduli : These
are commonly defined in terms of the relationship between
stress s, and strain €, in that region where the curve
is linear.
The most frequently used is the modulus in
tension :
Young’s Modulus ‘E’ = Tension
Stress / Tension Strain
Also used is the Shear Modulus ‘G’
= Shear Stress / Shear Strain
The units of the modulus are
the same as those of stress i.e. N/mm2 (ksi). Since the
modulus is a structure insensitive property, the normal design
value E = 205 kN/mm2 may be used for all steels, regardless of
composition, origin, prior history etc.
Yield
Strength: Yield point is the transition between
reversible and permanent deformation when the material is
stressed till yield point stress -strain curve is linear and
at the yield point some non-uniform deformation take place as
shown in figure. It is in fact that transition from elastic to
plastic behavior yield strength is affected by composition and
grain size. Some steels do not exhibit a clearly defined and
the stress strain as a smooth continuous curve. In such cases,
a stress which corresponds to a definite amount of permanent
extension (equivalent 2%) is taken as yield stress and is
called proof stress.
Tensile Strength:
This occurs when material starts to deform locally, a
waist or neck is produced at which facture is eventually
occur. This is a maximum stress subject to the material before
the necking starts. In figure the stress level is shown as
(E).
Ductility: The reduction in area
and the total elongation at fracture are used as measure of
ductility.
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Fracture Toughness:
Fracture toughness is
a measurement of ductile to brittle transition of material.
Brittle fracture always starts at a discontinuity such as a
notch an incompletely fused weld or a design discontinuity
such as a hole or a corner where stress is locally increased.
For a steel of given position, the onset of brittle rather
than ductile, behavior is affected both by temperature and
rate of loading. The chemical composition of steel and the
grain size affect the ductile brittle transition. Practically
notch impact test is done in case of steel to assess its
relative toughness. The fracture toughness is generally
measure in terms of ductile-brittle transition
temperature.
Fatigue: Failure by
fatigue occurs as a result of reversing or fluctuating stress
like brittle fracture, always starts from a stress raiser /
discontinuity. Fatigue property has to be taken into
consideration while designing a machine, bridge, industrial
structure etc.
Specifications:
Different classes of steels are available meeting
desired requirement of end applications, which are clearly
defined in respective specifications. Carbon Steel Fy=240 to
250 MPa is widely used in construction Industry, Micro Alloy
Steel with Fy 350MPA and above is also being used for all type
of construction and all type of equipment manufacturing, where
Fy is yield strength of steel
Joining
of Steel: Mostly can be done easily by welding,rivetting and
bolting.
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