Ferrous metals:

Iron:

Iron is made from its original ore (known as magnetite) this in result leaves impure iron, known as slag. This is removed from the furnace leaving a soft grey metal when it's cooled. Iron is most likely to be usually combined with carbon to give it a better strength, forming steel. The carbon levels can be monitored by adding oxygen to it.

Steel:

Steel is made from a combination of both Iron & Carbon, carbon is used in making this because it provides strength and as said previously, the carbon levels are monitored by adding oxygen while the iron is being heated. Steel may have a variation of carbon content so it can be used for separate purposes. Be that it may, however. The more carbon content there is, the more brittle the steel could become. Steel is more oftenly used though because it is more resistant to corrosion.

Carbon:

Carbon is one of the most common elements found in a wide variation of materials and objects, it can make up materials like coal or diamonds using carbon sheet layers in complicated forms. Carbon can affect steel's strength and hardness but also may cause brittleness.

Iron & Carbon:

Iron is generically soft and ductile which makes it less useful, however it is commonly used because it's extremely cheap and can be combined with other metals to form alloys with certain properties. Carbon is added in with iron to change its properties, and this is what creates steel itself.

Carbon may increase hardness, but may also reduce the toughness of the material. However, with certain metals you may anneal them which means to treat the material by heating them, increasing the strength and hardness.


Extra notes:
Annealing works by heating the material until it is very hot, and then letting it cool slowly to change the shape of the molecules within the material.

Ductility (tensile strength) is testing the tensile strength by having the material pulled to see how much tensile strength it holds as it stretches until its breaking point where it breaks apart.

Quenching is when a material is heated to a high temperature, then quickly cooled by being placed in water causing it to increase toughness. This puts the material in its hardest state.

Tempering works by heating the material in order to reduce the hardness of it, but it does in this process decrease the brittleness of the material. It is usually done after the process of quenching.

 Hot dip galvanising: the material is coated in a thin layer of zinc to prevent corrosion.
Chrome plating (like galvanising) the material is coated in chrome to prevent corrosion.