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What is steel? Why is it important?
Steel is an alloy of iron and carbon. An alloy is an admixture of metals, or a metal combined with one or more other elements. In steel, the carbon content ranges up to 2%. (with a higher carbon content, the material is defined as cast iron).
It is by far the most widely used material for building the world’s infrastructure and industries. It is used to fabricate everything from sewing needles to oil tankers.
In addition, the tools required to build and manufacture such articles are also made of steel.
As an indication of the relative importance of this material, in 2013 the world’s raw steel production was about 1.6 billion tons, while production of the next most important engineering metal, aluminium, was about 47 million tons.
Global steel production has tripled over the past 50 years, with China’s steel production eclipsing the rest of the world.
Steel is popular because of its relatively low cost of making, forming, and processing it, the abundance of its two raw materials (iron ore and scrap), and its unparalleled range of mechanical properties.
IRON
The major component of steel is iron, a metal that in its pure state is not much harder than copper.
In its pure form, iron is soft and generally not useful as an engineering material; the principal method of strengthening it and converting it into steel is by adding small amounts of carbon.
EFFECTS OF HEAT-TREATING
Adjusting the carbon content is the simplest way to change the mechanical properties of steel. Additional changes are made possible by heat-treating.
EFFECTS OF ALLOYING
A third way to change the properties of steel is by adding alloying elements other than carbon that produce characteristics not achievable in plain carbon steel.
Each of the approximately 20 elements used for alloying steel has a distinct influence on microstructure and on the temperature, holding time, and cooling rates at which microstructures change.
A good example of the effects of alloying is the making of a high-strength steel with good weldability. This cannot be done by using only carbon as a strengthener, because carbon creates brittle zones around the weld, but it can be done by keeping carbon low and adding small amounts of other strengthening elements, such as nickel or manganese.
Improved hardenability is achieved by adding such elements as manganese, molybdenum, chromium, nickel, and boron. These alloying agents also permit tempering at higher temperatures, which generates better ductility at the same hardness and strength.
Whatever way you look at it, steel is the world's most important engineering and construction material. It is used in every aspect of our lives; in cars and construction products, refrigerators and washing machines, cargo ships and surgical scalpels. It can be recycled over and over again without loss of property.
