The manufacture of steel

Steel is composed of iron and carbon. The iron is extracted from iron ores, mined and then refined to remove the oxygen content.

Part of the refining process is to remove oxygen from these ores by heating them with coke and limestone to a temperature of about 1600° C in a blast furnace

After reducing the iron oxide the resultant material is pig or cast iron, which is brittle. Some carbon remains in addition to other unwanted elements and these must be reduced by further refinement before the material becomes steel. The carbon content of steel is crucial to its strength and is usually lower than 2.11%. A controlled system of oxidisation is used to manipulate the carbon content. Other impurities must be removed from the melt in order to achieve the correct composition. Plain carbon steels are those in which the only other element remaining is manganese.

Iron ore furnace

Iron ore furnace

 

After the iron has been extracted from iron oxide it is known as 'pig' or 'cast' iron.

The iron oxide is reduced either directly by the oxide combining with carbon to form iron and carbon monoxide, or indirectly by combining with this newly formed carbon monoxide to form iron and carbon dioxide. In this state the material is known as pig or cast iron.

The pig iron must be further refined to reduce the carbon content and the other unwanted elements before the material can be categorised as a particular steel.

Iron ore

Iron ore

 

Refining iron into steel requires the re-melting of the iron in a steelmaking furnace with a large oxygen input.After this process there is always some carbon left in the solid iron formed amounting to about 4% by weight. The high carbon content of cast iron results in a material which is brittle and cannot take large tensile stresses. In addition there is a total of about 6% of other unwanted elements, including silicon, manganese, sulphur and phosphorous in unmeasured quantities.

In steelmaking the impurities in the melt have to be removed before the correct composition can be achieved. These include phosphorous and silicon (which make steel hard and induce brittleness), and sulphur (which can cause cracking in poured castings and welds).

A fundamental distinguishing property of steel is its great strength which depends partly upon the carbon content.

Most steels have carbon contents far lower than 2.11%. This figure marks the critical maximum where in the processing of the material, at 900° C, a complete phase change can occur. Above this level of carbon content, an iron-carbon alloy becomes more brittle taking on the type of performance associated with cast irons. The manipulation of the carbon content is carried out by controlled removal by oxidation.

This range of iron-carbon steels are referred to as plain carbon steels as the only other element present is manganese, present in up to 1.6% by weight and left in the metal from de-oxidation and de-sulphurisation processes. The addition of any other elements will bring the metal into the metallurgical category of alloy steels. (The general term alloy steels in the industry refers to those steels which have contributions from other elements amounting to 5% by weight and over.)

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