Mild Steel Fastener
The reason is that Mild Steel Fastener [up to 0.3%carbon] is strong and tough.The thumb rule is that if more carbon included in the alloy, the steel becomes stronger and harder but the toughness is diminished, making it more brittle. The general run of the bolts needs to be cheap, strong enough for the job and exhibit a degree of flexibility, which mild steel does. Higher grades can have more tensile strength, and be stronger, but they lose elasticity and are more expensive like the stainless steel bolts.
The reason carbon steel is used is because of the following considerations:
Functional requirements – it meets all of the requirements for the intended use.
Material properties – it can be tailored to suit the application thru control of chemistry, heat treatment(s), forming, etc.
Manufacturing process – it has a cheap, easy manufacturing process
Fabricability – like above, it sis easy to form into the desired shape with the required mechanical properties.
Availability – it is the most commonly used material for construction used. widely available.
Thermal stability – with the proper heat treatment and chemistry, it is very stable within limits,, normally matching what it is bolting together in the first place.
Cost – it is cheap to make.
All of the above considerations can be applied to other materials as well. However, carbon steel is the most commonly used material and cheapest compared to other material. Stainless Steel and Nickel alloy bolting does exist for applications that carbon steel is not suitable for (i.e. corrosive or elevated temperatures).