Steel - the most precious metal

An ordinary material you can find all around. In fact, your family probably owns several tons of it and still you may not feel especially wealthy. But this fact is actually what makes steel so precious. It is so useful and in the same time so cheap – simply precious to our civilization. What would or civilization be like without it?

Normally, the steel is an alloy of iron and carbon - less than 2% of carbon (by weight). Even the slightest amount of carbon affects iron heavily making it harder and stronger.

As it looks to me, making steel is some sort of magic. Certainly it looks that way (all the heat and sparks) and for sure there is some mystery in this procedure.

In first phase they produce so called “pig iron” in blast furnaces. The pig iron is no good for anything because it contains high concentration of carbon (4%-5%) making it too brittle and too hard. That is why, in second phase, the pig iron must be refined – lowering its carbon concentration and removing impurities.

The blast furnace

To make the pig iron you need: iron ore, coke, limestone and, as we already said, a blast furnace.

The iron ore is mined. The iron ore has chemical formula Fe2O3 – the iron-oxide. Almost all the iron in nature comes in this form. There is no metallic iron around (except, interestingly, if you find some iron-nickel meteorite).

The coke is mostly carbon. It is made from high-quality coal by heating it in an airless container to a high temperature (say 1200K). This drives out volatile components of coal and creates almost pure carbon. In the blast furnace the coke burns at higher temperature than the coal would and, of course, introduces fewer impurities.

The limestone is a sedimentary rock and it is mined. The composition of the limestone is mostly CaCO3 (calcium-carbonate).

The blast furnace is supplied with iron ore, coke, limestone (from above) and hot air (from below). The coke burns in the supplied air producing heat (say 1700K), carbon-monoxide (CO) and carbon-dioxide (CO2). The released carbon-monoxide and heat reduce the iron ore to iron and carbon-dioxide. So, as you can see, the coke has dual role in this process.

Fe2O3 + 3 CO -> 2 Fe + 3 CO2

At the same time the limestone is, because of heat, decomposed into calcium-oxide (CaO) and also carbon-dioxide. The calcium-oxide is helpful because it reacts with various impurities in the iron and drives them out -> forming the “slag”. Of course, if we would have a perfect iron ore and a perfect coke, we would not need the limestone.

The generated molten iron, being heavy, drops to the bottom of the furnace where it can be tapped out of the furnace. The slag is somewhat lighter and so it floats on the iron – it is also regularly tapped out.

Now, the molten iron has the property that it can dissolve a large quantity of carbon. As the carbon is readily present in the blast furnace (from the coke) the resulting iron is highly saturated with the carbon – the pig iron (the one that is good for nothing).

Making steel

In the second phase the pig iron has to be refined – the carbon portion has to be lowered. This is done by blowing oxygen through the molten pig iron (for, say, 20 minutes). The carbon dissolved in the pig-iron burns in the oxygen and the carbon-dioxide gas is created and expelled. When the carbon concentration is low enough, the process is stopped.

Not only carbon, but also some other impurities react with the oxygen in this process and are expelled (either in gaseous form or in form of slag). Other impurities are also handled at some point in this phase using various techniques. Also, various additives can be added to the steel in order to further improve its properties.

The funny fact is that blowing oxygen into the pig iron actually generates heat – sure, because the excessive carbon burns. This makes the process quite efficient.

(What I newer understood is: how come that the iron itself doesn’t get oxidized in this process making the iron-oxide again? Strange.)

Now we have the steel – the most precious metal in our possession.

It has to be noted that a large portion of steel is recycled. Actually, the steel is the most recycled material we use.

One personal note at the end… I got somewhat disappointed about the steel when I saw the WTC towers collapsed. The steel has its own limitations. However, you can look at this from the other perspective – the steel lost this battle, but it lost it to the worthy opponent. Because the gasoline is just as miraculous stuff as the steel is.

Danijel Gorupec, 2006

See also the Why titanum submarine dives deeper than steel submarine article