Types of steel

 

 

   When carbon is added to iron, it manages to change its properties greatly, forming steel. The following table and graph show the impurities in pig iron straight from the blast furnace, and in steel

  

 

Impurity	% Impurity in pig iron	% Impurity in mild steel
Carbon	3 – 5	0.15
Silicon	1 – 2	0.03
Sulphur	0.05 - 0.10	0.05
Phosphorous	0.05 - 1.5	0.05
Manganese	0.5 - 1.0	0.5

 

 

 

   Steel exhibits great strength and hardness which iron does not have.  For this reason, the low manufacturing cost, and abundant source of raw materials, steel is the most widely used structural metal.

 

   In total, there are several thousand different types of steels, all with different chemical compositions.  This means that their physical properties are different, so useful for different purposes.

 

   Steels can be grouped into three major classes: carbon steels; low-alloy steels, and high alloy steels.  All these steels will contain a small percentage of elements left over from steel making.  These include manganese, silicon, aluminium, phosphorous, sulphur and copper.  However, as all of these elements combine to give 1% of the total composition, they are not considered as alloys.

 

Carbon Steels

 

Carbon steels account for nearly 90% of the worlds steel production.  All carbon steels are defined as having less than 1.65% manganese, 0.6% silicon, and 0.6% copper.  Carbon steels are usually grouped into 5 classes, all with different percentages of carbon.

 

 

Type of carbon steel	Percentage carbon content
High-carbon steel	above 0.5%
Medium-carbon steel	0.2 - 0.49%
Low-carbon steel	0.05 - 0.19%
Extra-low-carbon steel	0.015 - 0.05%
Ultralow-carbon steel	below 0.015%

 

 

Low-alloy steels

 

Low-alloy steels will contain up to 8% alloying elements. There are over 20 different elements that may be used as alloys, the main ones being manganese and chromium.  In many cases several of these elements are added simultaneously to achieve the desired properties.  Even though low-alloy steels are very useful, the demand is usually met by the less expensive and easier to produce carbon steels.  These steels are only used when the addition of alloys is required when very specific properties are required.

 

High-alloy steels

 

High-alloy steels contain over 8% alloying elements.  These steels have extremely unusual properties, and are only used for specific tasks.  An example of a high-alloy steel is stainless steels, which are very resistant to corrosion, oxidation and heat.  These will often contain 16 to 26% chromium and up to 35% nickel. Their properties make them suitable to be used for jet-engine parts or table cutlery.