Über Eisen und Stahl

 

Iron in nature and its properties

Iron is considered to be the most important metal and is almost omnipresent in our daily life. Its annual production exceeds by far that of all other metals combined.

Iron is an Anglo-Saxon word, the symbol Fe comes from Latin ferrum. Its main characteristics are as follows:

  • number in the Periodic Table: 26
  • atomic mass: 55.847
  • bulk density: 7.87 g/cm³ (at 20°C)
  • melting point: 1536 ± 2°C
  • boiling point: 2750°C
  • allothrophic modifications:
    • until 910°C it exists as α-iron; it is ferromagnetic, but above 768°C magnetic properties disappear
    • from 910 until 1400°C: γ-iron
    • above 1400°C: δ-iron

Iron is believed to be the sixth most abundant element in the universe. It is found in the sun and many stars in considerable quantity. Iron is found native as a principle component of a class of meteorites known as siderites. The core of the earth is believed to be largely composed of iron (probably greater than 80 %, perhaps even a nearly pure iron crystal at the inner core). The metal is the forth most abundant element in the earth's outer crust: about 4.5 % is iron (oxygen 49.7 %, silicon 26 %, and aluminium 7.5 %, for comparison gold 0.5*10-7 %). Iron is the most abundant element by mass of our entire planet, making up 34.6 % of the mass of the Earth as a whole. Iron is a vital constituent of plant and animal life, and appears in haemoglobin and mioglobin (muscles).

The historical development of the use of iron is influenced by the fact that metallic iron is not stable under thermodynamical conditions on earth. Without particular processing techniques iron was only available in form of meteor-iron.

 

What are pig iron and steel?

Pig iron or hot metal and steel are ferrous alloys of iron with carbon (see Iron-Carbon Phase Diagram). Point E corresponds to maximum dissolubility of carbon in γ-iron (2.11 %). Carbon content in steel is up to 2.0 % (usually less than 1.5 %). Carbon content in pig iron or hot metal makes up more than 2.5 %. Alloys with carbon content from 2 to 2.5 % have no technical application. The properties of pig iron and steel depend significantly on their carbon content. Up to certain limits, the more carbon they contain, the stronger and harder they will be.

Beside iron and carbon, pig iron and steel contain small amounts of other elements (silicon, manganese, sulphur, phosphorus etc.). Actually, the term steel is a generic name representing thousands of products encompassing a great variety of compositions, shapes and qualities. There are over 3,000 catalogued grades (chemical compositions), not counting those created to meet custom demand.

 

How steel is made?

First, the raw materials - either iron ore or scrap iron, depending on the process - are converted into molten steel. The ore-based process uses a blast furnace or smelter (e.g. Corex) and the scrap-based process uses an electric arc furnace (sponge iron produced in direct reduction processes can also be used in an electric arc furnace). Next, the molten steel is poured and solidified in a continuous caster. This produces what are known as semi-finished products. These can be either slabs, which have a rectangular cross-section, or blooms or billets, which have a square cross-section. They are the blanks which are used to form the finished product. Lastly, these semi-finished products are transformed or "rolled" into finished products. Some of these undergo a heat treatment, known as "hot rolling". More than half the hot-rolled sheet is subsequently rolled again at ambient temperatures (known as "cold rolling"). It can then be coated with an anti-corrosion protective material.

 

Is steel an advanced material?

Steel is commonly used in modern society and is probably the most important construction materials of today.

Virtually everything contains steel: machines, trains, tracks, buildings, aircraft and spacecraft. Below the ground, miles upon miles of steel tubes and pipes carry water and contain and protect telephone and electric power cables. In our homes, iron and steel is ever present.

 

Is steel a green material?

The global warming is the main challenge for the steel industry. Steel industry is responsible for approx. 5% of the world-wide energy consumption. The demand of society on steel is in conflict with effect of steel industry on environment. How these contradictory demands can be met to provide a sustainable development?

A "green material" can be defined as one which is being produced in an environmentally benign manner (during its extraction, refining and manufacturing), which is environmentally benign during its use, and one which may be readily and environmentally recycled (J.Szekely, 1996). In this course, we shall discuss to which extent ironmaking contributes to steel meeting these criteria.
Environmental aspects of ironmaking are one of the most important issues because blast furnace together with coking plant and sinter plant consume 65-75 % of the entire energy at an integrated steelworks.