Steel can be found everywhere. But what makes it so versatile?

http://i.imgsafe.org/52c4e98a4d.jpg
*Steel production in Iran. [Source](http://www.iran-daily.com/News/140205.html)*

# Why steel
http://i.imgsafe.org/52d146730a.jpg
*Smelting simplified. [Source](http://www.voltzwiki.com/wiki/Steel_Ingot)*

Steel is an alloy of iron and carbon. It is used already for centuries, starting with the first smiths forging daggers and nails in their coal foundries. This process lead to increased robustness of the rather soft iron.

## Iron + Carbon
Carbon steel accounts for around 90% of steel production. The rest is made up by alloy steels, which is known as stainless steel, but has a high variety of different mixtures of indigents.

https://upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Steel_pd.svg/420px-Steel_pd.svg.png
*The Iron-Carbon diagram shows the different mixtures of steel depending on carbon content. [Wikimedia](https://en.wikipedia.org/wiki/Allotropes_of_iron)*

Iron can have two different structures: alpha (α) and gamma (γ) iron. At room temperature, α-iron assembles a cubic structure, with 8 atoms forming the cube, and a 9th atom being in the middle. This is called cubic room centric. 
At higher temperatures, above 1394 °C, it changes to γ-iron, a face-centered cubic structure. It has one atom at every corner and one on each face (think at a dice).

https://upload.wikimedia.org/wikipedia/commons/thumb/a/aa/IronAlfa%26IronGamma.svg/640px-IronAlfa%26IronGamma.svg.png
*Alpha (α) and gamma (γ) iron structure. [Wikimedia](https://en.wikipedia.org/wiki/Allotropes_of_iron)*

The magic now happens when iron is heated while carbon is around: The γ-iron has more atoms, but also a bigger spacing between them. This makes it possible for carbon atoms to slip between them.
If the mixture is then cooled down and the iron switches back to the α-iron, cubic room centric structure, the carbon atom becomes locked between the iron atoms. The speed of cooling can increase the hardness of the steel, as more carbon is trapped. 
If the metal is cooled down too fast and it took in much carbon, it can become so hard that it isn't elastic anymore, and will shatter like glass instead of bending.

https://media.giphy.com/media/Q0cLfJaa2GkcU/giphy.gif
*Big pieces would need a long time to cool down; water increases the cooling and leads to a more homogenous material. [Source](https://giphy.com/gifs/satisfying-quench-Q0cLfJaa2GkcU)*

The main focuses on producing carbon steel are:
- Getting as much of the unwanted components, other than iron and carbon, out of the material.
- Having the metal cooled down in the right way to reach the desired percentage of carbon.

Other elements like sulfur or oxygen can make the steel brittle, susceptible to corrosion and harder to machine. A manufacturer strives to drive these amounts as low as possible, without increasing the costs too much.

## Alloys

Being more expensive than carbon steel, alloys with components additional to carbon bring a lot more abilities to this material.

### Low Alloy steel

Adding molybdenum, manganese, chromium, or nickel, in amounts of up to 10% by weight, the hardness of the metal is improved. While it is vulnerable to rust, it is often cheap enough to add a surface treatment instead of choosing a more expensive alloy. This can be done my painting or galvanic treatment, which covers the surface with a thin layer of non corrosive metal.
### High strength low alloy steel
High strength low alloy steel has additions that are usually < 2%. Adding 1.5% manganese can increase the strength for a relatively low price increase. They combine a lower price with a higher strength, and can so offer something in between carbon and stainless steel.
### Stainless steels
Stainless steels contain at least 11% chromium, often combined with nickel. This gives stainless steel its famous resistance to corrosion. By adding other components the steel can also become non magnetic, harder, have a different color, stronger, more elastic, lower density and more. With adding more components, the material also gets a lot more expensive. That explains while normal carbon steel is so popular.

>It is often possible to find out if the steel is stainless by checking it with a magnet. If the magnet doesn't stick, it is stainless steel. If it does stick, you might have a polished piece of carbon steel.

## Benefits of Steel

The greatest benefits of steel are:
- **Formable**: Steel can be formed in almost every imaginable form. This can be done in many different ways: Casting, 3D printing, forging and so on.
- **Variability**: A multitude of other metals and elements can be added to iron to get different properties like resistance to corrosion, high temperature resistance, higher flexibility, color, magnetism, higher strength.
- **High tensile strength**: Steel is a very strong material, but can still be formed easily if it is heated above a certain point.
- **Elasticity**: Most steel alloys have the ability to deform elastically and go back to their original form. This makes it possible to calculate in deformation without sacrificing structural integrity. 
- **Recyclable**: Steel can be completely recycled.

After characteristics, there will be a closer look on the ways steel is made, formed and used in the industry.

Thank you for reading!

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