Steel

Steel is one of the most widely used materials, particularly in

construction and engineering and in the manufacture of cars. It is

estimated that there are over 20 billion tonnes of steel in use,

equivalent to well over 2 tonnes for every person on Earth.

Steels are alloys of iron, carbon and other metals and non-metals. The

composition of the steel is adjusted so that it has the precise

properties needed.

The term alloy steel is confined to steels containing some combination

of one or more of the following elements: nickel, chromium, tungsten,

molybdenum, vanadium, manganese, cobalt, copper, niobium,

zirconium, selenium and lead.

Steels can be repeatedly recycled without any loss of performance.

Uses of steel

The construction industry is a main user of steel, from small buildings to huge

bridges, and uses it in multiple ways, even within a single construction. A

bridge, for example, might use steel in the huge suspension ropes, the steel

plate flooring for the road, the beams for the columns, and for the safety

barriers and lighting columns.

Much steel is also used to reinforce concrete.

Figure 1 Uses of steel in the UK.

Chromium increases the corrosion resistance of steel, and a minimum of 12%

chromium is necessary to produce a stainless steel. The best known of the

stainless steels contains about 74% iron, 18% chromium and 8% nickel (known

as 18-8 stainless). Stainless steel is perhaps most familiar as kitchenware

(sinks, kettles and cutlery).

Figures 2 and 3 Both these structures used about 45 000 tonnes

of steel in their construction.

Figure 2 is the barrier across the

River Thames, to protect London I

from flooding. It is a system of

stainless steel plated hollow flood

gates.

By kind permission of Peter Trimming.

Figure 3 is the interwoven structure

of the Olympic Stadium in Beijing

made of steel plate. Unwrapped, the

I strands of the 'Bird's Nest' would

stretch for 36 km.

By kind permission of World Steel Association.

Steels containing molybdenum, vanadium, chromium and tungsten in various

combinations produce very hard, if brittle, steels. These are used, for example,

in drill bits which need to retain a cutting edge. Steels are used widely in the

manufacture of electrical motors, power generators (nuclear, conventional fuels

and wind), gears and engines, which have to be very tough and withstand high

temperatures.

Figure 4 A wind turbine constructed from steel.

By kind permission of World Steel Association.

Steels with cobalt are used as magnets and those with nickel are used in the

construction of nuclear reactors.

There is a group of steels known as Advanced High Strength Steels, AHSS,

which are specially treated steels that can be rolled very thin without losing the

element of strength needed for the specific purpose. They are particularly

useful in the manufacture of cars, helping to reduce the overall mass and thus

decrease fuel consumption.

Steels with a thin coating of tin are used to make cans for beverages and food.

Steels coated in various ways with zinc are used in roofing, for example, and in

cars as the zinc gives protection against rusting.

Figure 5 The container

ship and the containers

are both constructed from

steel plate.

By kind permission of World Steel

Association.

Annual production of steel

World 1548 million tonnes

Europe 169 million tonnes

North America 122 million tonnes

China 717 million tonnes

Rest of Asia 298 million tonnes

Data from:

World Steel Association, 2012

Manufacture of steel

There are two main processes used to make steel. The Basic Oxygen

Steelmaking Process, which is used for the majority of steel production, uses

iron freshly produced from the blast furnace together with some scrap steel.

The Electric Arc Furnace Process uses scrap steel only.

The Basic Oxygen Steelmaking Process

Figure 6 IIllustrating the Basic

Oxygen Steelmaking (BOS)

Process. The process uses

modern furnaces lined with

special bricks containing 90%

magnesium oxide and 10%

carbon. These can take up to

350 tonnes of reactants and

convert them to steel in less

than 40 minutes.

The furnace (also known as a converter or vessel) is charged with steel scrap

(up to about 30%) and molten iron from a ladle. An oxygen lance, cooled by

circulating water, is lowered into the furnace and high purity oxygen is injected

into the vessel at twice the speed of sound which ensures that all the impurities

are converted into their oxides. The main chemical reactions are:

With the exception of the carbon monoxide, the products react with lime,

added during the oxygen blow, to form a slag.

The above reactions are all exothermic and controlled quantities of scrap are

added as a coolant to maintain the desired temperature.

The steel at this stage contains ca 0.04% carbon.

The Electric Arc Furnace Process

Steel scrap is first tipped from an overhead crane into a furnace. The scrap

comes from three sources:

Home scrap: excess material from steel works and foundries.

Industrial scrap: from processes using steel (such as excess steel from making a

car).

Obsolete scrap: discarded used products (for example, used cans).

Figure 7 Illustrating the Electric Arc Process which uses scrap steel to produce pure steel very

efficiently.

The furnace is a circular bath with a movable roof through which three graphite

electrodes are raised or lowered. These electrodes are massive, often 6 m

high and 4 m wide, and the furnace can hold over 100 tonnes of liquid steel.

After the steel scrap is placed in the furnace, the roof is put into position and

the electrodes lowered into the furnace. An arc is struck by passing an electric

current through the metal. The heat generated melts the scrap metal. Lime (as

calcium oxide or calcium carbonate), fluorspar (which helps to keep the hot

slag as a fluid) and iron ore are added and these combine with impurities to

form a slag. When the steel has reached the correct composition the slag is

poured off and the steel tapped from the furnace.

Figure 8 The liquid steel is

tapped (poured) into a ladle

and the slag is tapped into a

separate 'slag pot'. This photo

shows a later stage when the

molten slag is poured from the

slag pot. The slag is treated so

that any iron left is recovered

and the residue is then used

as an aggregate.

By kind permission of World Steel

Association.

Secondary steelmaking

The term secondary production is often used when referring to recycling.

However, in steelmaking the term secondary steelmaking refers to the

production of steels which are needed for specific purposes and which require

the addition of very carefully controlled quantities of other elements.

Molten steel from either process is transferred to a ladle where the alloying

elements are added.

The process provides precise control of harmful impurities (particularly sulfur,

phosphorus and, in some cases, trace metals and hydrogen) by adding

materials via ladle injection. For example, aluminium and silicon are added to

reduce any oxidized material.

Other techniques used to help to improve the quality of the steel include

stirring (ladle stirring) and applying a vacuum to the steel to remove gases

(vacuum degassing).

Casting

Steel is produced in three forms, the form chosen being dependent on its

ultimate use:

as a slab, a long thick piece of metal with a rectangular cross-section

as a bloom, a long piece of metal with a square cross-section

as a billet, similar to a bloom but with a smaller cross-section.

Most steel is continuously cast to the desired shape, but a small quantity (ca

10-20%) is first cast into ingots which are cooled and then worked on to

produce the shape required.

The casting is a very precise set of processes. The following descriptions are

an outline.

Continuous casting

In continuous casting, the steel, still molten from the furnace, is poured into a

water-cooled mould (teeming) from which it emerges as a strand which is

solidifying at the surface. The strand passes through a series of rollers which

are water sprayed to produce a solid (a slab, bloom or billet) which is then sent

to be hot rolled.

Figure 9 Steel tube is being produced in a continuous casting process.

By kind permission of World Steel Association.

Ingot casting

Molten steel is poured into a cast iron mould to solidify as an ingot. This

generally weighs less than 20 tonnes but rotor forgings can weigh up to 500

tonnes.

When the ingot has solidified, the mould is removed. Each ingot is of carefully

pre-arranged dimensions and mass from which articles of the required size can

be rolled.

Rolling

Steel products are classified into flat products and long products. Slabs of steel

are rolled to produce flat products, for example steel sheet for the construction

of ships. The sheet is rolled further to produce thinner sheet, used for example

in the manufacture of cars.

Blooms and billets are used to roll long bars of steel for construction and for

drawing into wires.

Often there are three stages to this part of the process, hot rolling, cold rolling

and drawing.

Hot rolling occurs when the slabs, blooms and billets are heated in a furnace

until they are red hot (ca 1400 K) and then rolled until they have acquired the

desired shape.

The speed at which the hot steel is subsequently cooled is a crucial factor,

affecting the strength and other properties of the steel. Cooling is done by

spraying water as the steel passes through the rollers.

During this rolling, oxygen in the air has reacted with the hot iron to form a very

thin layer of iron(III) oxide on the surface. It is blue/grey in colour (only when it

is thicker does it appear red). This must be stripped from the surface prior to

the next stage, otherwise the final product will be susceptible to rusting and

unsuitable for galvanizing with zinc and other surface treatments.

The stripping process is known as pickling. The steel is passed through several

baths of hydrochloric acid (sometimes sulfuric acid) which dissolves the oxide

without attacking the metal. The spent acid is recycled.

The 'pickled' steel is then subjected to cold rolling. As the name implies, the

steel, following hot rolling, is rolled cold and gradually compressed to the

required thickness. This improves the quality of the surface and also hardens

the steel. On annealing (heating the strip very carefully), it can be pressed into

shapes without cracking. Such sheet is used, for example, to press out car

bodies. Steel cans are pressed out with sides and bottoms as a single entity,

needing only the top to be fitted after filling.

Very strong wires are produced by cold drawing.

Recycling

The recovery of scrap steel probably constitutes the world's largest scale

recycling process. The scrap is either part of the charge for the Basic Oxygen

Process or is the complete charge for the Electric Arc Furnace Process.

About 40% of the iron-containing materials used in steel production are now

from recycled sources. It is estimated that recycling one tonne of steel saves

1.1 tonnes of iron ore, 0.6 tonnes of coal and 0.5 tonnes of limestone, with an

overall energy saving of 60-75%.

Date last amended: 3rd May 2013