How’s Steel Manufactured?
K.C. Hari Kumar
Department of Metallurgical & Materials Engineering, IIT Madras
Iron is the most used of all the metals, comprising 95% of all the metal tonnage produced
worldwide. Pure iron is not a good engineering material. It lacks sufficient strength and resistance
to rusting. However, its properties can be significantly altered by alloying, thermal, and mechanical
processing. Steel is the best known alloy (a mixture containing two or more metallic elements
or metallic and nonmetallic elements usually fused together or dissolving into each other when
molten) of iron. Its combination of low cost and high strength make it indispensable, especially
in applications like automobiles, the hulls of large ships, structural components for buildings, and
household appliances. Annual production and consumption of steel is often taken as a reliable
indicator of economic performance of a country.
Raw materials for ironmaking
Iron, like most metals, is not found in the Earth’s crust in a native state but as an ore (a metal
containing mineral valuable enough to be mined). There are many type of iron ores, but the most
common ones are hematite (Fe2 O3 ) and the magnetite (Fe3 O4 ). Iron ore suitable for ironmaking
(note that this is one word) contains about 65 % iron, rest is impurities (known as gangue) such as
alumina (Al2 O3 ) and silica (SiO2 ). In India we have both hematite and magnetite deposits. These
days iron ore is mostly converted to sinter (a porous cake of powdered ore) for more efficient use
in ironmaking.
In order to extract iron from the ore we must remove oxygen from it using a suitable reactant.
Such reactions are known as reduction reactions. In ironmaking carbon is such a reactant that
can combine with oxygen of the ore. That way it acts as a reducing agent. An abundant source
of carbon in the natural form is coal (a fossil fuel consisting of carbonised organic matter). Not
all types of coal are suitable for ironmaking. The type of coal suitable for ironmaking is known as
coking coal or metallurgical coal. Coking coal after washing is heated to about 1000 ◦ C in absence
of air in coke ovens. In the process coal is converted to a porous substance rich in carbon (about
80 %) known as coke. Compared to coal, it has high strength, high porosity, low ash, and low
volatile matter. These qualities are very desirable for ironmaking.
In addition to iron ore and coke, we also need certain amount of flux for ironmaking. Function
of flux is to aid slag (molten impurities, mostly oxides and silicates) formation by combining
with gangue and ash. Limestone (CaCO3 ) and dolomite (MgCa(CO3 )2 ) are common fluxes for
ironmaking.
Other materials required for ironmaking include large quantities of air (to burn the coke) and
water (mainly for cooling).
Ironmaking
Ironmaking is known to man from prehistoric times. Modern ironmaking is about 200 years old. It
is done by a smelting process (extracting metals by heating) in a huge furnace known as the Blast
furnace. These furnaces can be as tall as 70 meters. Blast furnace has a circular cross section and
its diameter varies such that it tapers up like a chimney. Its height and geometry is optimised to
give best performance. Blast furnaces are never stopped except for major repairs. Usually they
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can continue to operate without interruptions up to 15 years. In big steel plants there can be more
than one such furnaces.
Figure 1: Blast Furnace
Iron ore, sinter, coke, and flux are charged through the top of the furnace in a certain sequence.
A hot air blast (900-1200 ◦ C) at high pressure (3 to 4 atmospheres) is send from the bottom of the
furnace. Coke begins to burn when it comes in contact with the hot air. This generates enormous
amount of carbon monoxide and heat. Temperature in the burning zone in the furnace can be
as high as 2000 ◦ C. Hot gases travels upwards through the furnace transferring heat and causing
chemical reactions. As the gas ascends through the furnace it comes in contact with iron oxide
causing reduction reactions. It progressively gets cooled as it moves up and the amount of carbon
monoxide also gets diminished. Some of the most important chemical reactions taking place in
the blast furnace are listed below:
C+O2 =CO2
CO2 +C=2CO
3Fe2 O3 +CO=2Fe3 O4 +CO2
Fe3 O4 +CO=3FeO+CO2
FeO+CO=Fe+CO2
FeO+C=Fe+CO
Iron that is formed by reduction reactions eventually melts and collects at the bottom of the
furnace. Temperature of the molten metal (or hot metal as it popularly known as) can vary
between 1200 and 1500 ◦ C. Besides carbon, hot metal has small amounts of silicon, manganese,
sulfur, and phosphorous dissolved in it. Along with ore reduction, slag formation also takes place.
The slag floats over the hot metal as it is lighter than hot metal. Its temperature is usually about
50 to 100 ◦ C higher than that of the hot metal.
Both hot metal and slag are taken out of the blast furnace through tap holes. Hot metal is send
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to the steelmaking shop, while slag is send to slag pits for granulation and later sold to cement
industry.
Steelmaking Using Oxygen Process
Steelmaking involves precise control of amount of dissolved elements in iron. Hot metal from
blast furnace contains about 4 % carbon. Although carbon makes iron stronger and harder, too
much of it makes it brittle. In steels the amount of carbon is at the most 1.5 % only. Similarly,
amount of sulfur and phosphorous must be as low as possible since they make most steels unusable.
Other elements such as silicon and manganese must also be controlled and extra elements such
as chromium, nickel, vanadium, etc. may be added depending up on the type of the steel being
made. For example, stainless steel contains substantial amounts of chromium and nickel. There
are about 3000 types of steel!
Main chemical process of steelmaking involves oxidation of carbon and other elements. This
is quite a contrast to the ironmaking process where the main reactions are reducing. For this hot
metal from blast furnace is first poured into a bucket-shaped vessel known as LinzDonawitz (LD)
converter situated in the steelmaking shop. Usually some amount of steel scrap is placed in the
converter prior to the pouring of hot metal to protect the basic refractory of the converter. Like
in ironmaking, here also we need a flux for slag formation. For this purpose powdered lime (CaO)
is added to the converter. Oxygen is then blown into the converter at supersonic velocity. Violent
oxidation reactions take place in the converter during the oxygen blowing. Carbon is oxidised to
form carbon monoxide which burns at the mouth of converter. Silicon and phosphorous are also
oxidised to become a part of the slag. During the process composition and temperature of the
metal is monitored and blowing is stopped at an appropriate time.
Figure 2: Steelmaking Shop
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The converter is turned and the molten steel is tapped out through the taphole into a ladle.
Once the steel has been drained, the furnace is turned upside down and the slag that is left inside
is transferred into another ladle. The solidified slag may be send back to the blast furnace for
recycling as it contain lots of lime and iron oxides.
Sulfur control, removal dissolved gases, addition of other alloying elements, etc. are done outside
the converter in the ladle. Finally molten steel is solidified into various shapes in continuous casting
machines. Some products may be shaped to sheets or rods by rolling in special mills.
Figure 3: L-D Converter
Steelmaking Using Electric Arc Furnace
Nearly 50 % of the steel poduced these days is by electric arc funace (EAF) route, which uses
high-current electric arcs to melt steel scrap and convert it into liquid steel of a specified chemical
composition and temperature.
Figure 4: Electric Arc Furnace
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An electric arc furnace used for steelmaking consists of a refractory-lined vessel, usually watercooled, covered with a retractable roof, and through which one (DC) or three (AC) graphite
electrodes enter the furnace. It has a dish-shaped (looks like halved egg in modern furnaces)
refractory hearth. The bottom i.e., the hearthhas the shape of a halved egg and it IS lined with
tar-bonded magnesite refractory bricks. It has on one side a vertical taphole. The furnace sits on
a hydraulically operated rocker that tilts it for steel and slag removal. In modern meltshops, the
furnace is often raised off the ground floor, so that ladles and slag pots can easily be maneuvered
under either end of the furnace. Separate from the furnace structure is the electrode support and
electrical system, and the tilting platform on which the furnace rests. Before charging, the taphole
is closed from the outside by a movable bottom plate and is filled with refractory sand.
The major charge material of electric-arc steelmaking is scrap steel, and its availability at
low cost and proper quality is essential. The importance of scrap quality becomes apparent when
making steels of high ductility, which must have a total maximum content of residuals (i.e., copper,
chromium, nickel, molybdenum, and tin) of 0.2 percent. Most of these residuals are present in scrap
and, instead of oxidizing during steelmaking, they accumulate and increase in recycled scrap. In
such cases some shops augment their scrap charges with direct-reduced iron or cold blast-furnace
iron, which do not contain residuals. Generally, the higher contents of carbon, nitrogen, and
residuals make the electric-arc process less attractive for producing low-carbon, ductile steels.
Most scrap yards keep various grades of scrap separated. High-alloy shops, such as stainlesssteel producers, accumulate, purchase, and charge scrap of similar composition to the steel they
make in order to minimize expensive alloying additions.
An overhead crane charges the furnace with scrap from a cylindrical bucket that is open on
the top for loading and fitted with a drop bottom for quick charging. Scrap buckets are loaded
in such a manner as to assure a cushioning of heavy scrap when the load drops onto the hearth
in order to obtain good electrical conductivity in the charge, low risk of electrode breakage, and
good furnace wall protection during meltdown. Carbon and slag formers are sometimes added to
the charge to prevent overoxidation of the steel and to quicken slag formation. After charging one
bucket, the roof is moved back to the furnace, and the electrodes are lowered. Meltdown begins
with a low power setting until the electrodes have burned themselves into the light scrap on top
of the charge, protecting the sidewalls from overheating during higher-power meltdown. Leaving
some scrap unmelted at the furnace wall for its protection, a second bucket is charged and the
same meltdown procedure is followed. Melting very light scrap sometimes requires the charging
of a third or even fourth bucket.
After meltdown, the carbon level in the steel is about 0.25 percent above the final tap level,
which prevents overoxidation of the melt. By this time a basic slag has formed, typically consisting
of 55 percent lime, 15 percent silica, and 15 to 20 percent iron oxide. Slag foaming is often
generated by injecting carbon or a lime-carbon mixture, which reacts with the iron oxide in the
slag to produce carbon monoxide gas. This foam shields the sidewall and permits a higher power
setting. As required, the carbon content of the steel is either decreased by oxygen blowing or
increased by carbon injection. Samples are taken, the temperature is checked, additions are made,
and, when all conditions are right, the furnace is tapped by rotating it forward so that the steel
flows through the vertical taphole into a ladle. When slag appears, a quick back tilt is applied
and the slag is poured through the rear door of the furnace into a slag pot. Some shops leave 15
percent of the liquid steel in the furnace. This hot heel practice permits complete slag separation.
A typical EAF is the source of steel for a mini-mill. EAF plants are smaller and less expensive
to build than integrated steelmaking plants, which, in addition to basic oxygen furnaces, contain
blast furnaces, sinter plants, and coke batteries for the making of iron. EAFs are also cost-efficient
at low production rates -e.g., 150,000 tons per yearwhile basic oxygen furnaces and their associated
blast furnaces can pay for themselves only if they produce more than 2,000,000 tons of liquid steel
per year. Moreover, EAFs can be operated intermittently, while a blast furnace is best operated
at very constant rates. The electric power used in EAF operation, however, is high, at 360 to
600 kilowatt-hours per ton of steel, and the installed power system is substantial. A 100-ton EAF
often has a 70-megavolt-ampere transformer.
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EAF process can operate with 100 % scrap, thus saving valuable natural resources. External
arc heating permits better thermal control than does the oxygen process, in which heating is
accomplished by the exothermic oxidation of elements contained in the charge. This allows larger
alloy additions to be made than are possible in basic oxygen steelmaking. However, electric-arc
steelmaking is not as oxidizing, and slag-metal mixing is not as intense; therefore, electric-arc
steels normally have carbon contents higher than 0.05 percent. In addition, they usually have a
higher nitrogen content of 40 to 120 parts per million, compared with 30 to 50 parts per million
in basic-oxygen steels. Nitrogen, which renders steel brittle, is absorbed by liquid steel from air
in the high-temperature zone of the arc. The nitrogen content can be lowered by blowing other
gases into the furnace, by heating with a short arc, and by applying a vigorous carbon monoxide
boil or argon stir to the melt.
Very clean steeli.e., with low oxygen and sulfur contentcan be produced in the EAF by a twoslag practice. After removal of slag from the first oxidizing meltdown, new slag formers are added.
The new reducing slag may consist of 65 percent lime, 20 percent silica, calcium carbide or alumina
(or all three), and practically no iron oxide. Alloys, which oxidize easily, are added at this time to
minimize losses and to improve metallurgical control. Refining continues under the reducing slag
until the heat is ready for tapping. Total heat time is one to four hours, depending on the type
of steel madethat is, on the amount of refining applied and auxiliary heating used. Many shops
do not apply a two-slag practice but treat the steel, after scrap meltdown and tapping, in ladle
treatment stations. These secondary metallurgical plants allow the EAF to run only as a highly
efficient scrap melter.
From time to time, as the arc erodes their tips and the high-temperature furnace atmosphere
oxidizes their bodies, new electrodes are added to the top of the electrode strings at the furnace.
Electrodes are consumed at the rate of three to six kilograms per ton of steel, depending on the
type of operation.
Steelmaking Using Induction Furnace
Used by many specialty steelmaking shops and foundries, induction furnaces are cylindrical, opentopped, tiltable refractory crucibles with a water-cooled induction coil installed on the outside,
around the side wall. The coil is powered by alternating current, which induces eddy currents in
the metallic charge that generate heat. The refractory wall of the crucible is usually thin enough
to achieve good penetration of the electromagnetic field into the charge.
Induction furnaces are used typically for remelting and alloying and have very limited refining
capabilities; in other words, they are not used for carbon, phosphorus, or sulfur removal. The slag
is cold and not very active, and often there is no slag at all. However, the electromagnetic field
stirs the melt well, and this is beneficial for alloying. Most furnaces’ coils are powered by line
frequency (i.e., 50 or 60 hertz), but there are also furnaces powered by medium frequency (e.g., up
to 4,500 hertz), utilizing solid-state frequency converters. The electrical system always includes
capacitor banks to compensate for the high inductance of the furnace coil. Efficiency of converting
electric power into heat is about 75 percent, and power consumption is around 550 kilowatt-hours
per ton of steel.
In commercial operation, a hot heel is often left in the furnace after tapping in order to
decrease the thermal shock on the lining generated by the water-cooled coil. Smaller furnaces
use prefabricated crucibles, but larger furnaces have a compacted and dried refractory mass as
lining. Computer control is well utilized in this system, monitoring, for instance, the crucible
lining thickness by the electrical performance of the furnace coil. The capacity of the furnace
varies from a few kilograms to 50 tons.
Many induction furnaces are installed and operated in vacuum chambers. This is called vacuum
induction melting, or VIM. When liquid steel is placed in a vacuum, removal of carbon, oxygen,
and hydrogen takes place, generating a boil in the crucible. In many cases, the liquid steel is cast
directly from the furnace into ingot molds that are placed inside the vacuum chamber.
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