The use of mild steel is perfectly ok for this use, typically having 0.05% - 0.15% carbon, a low carbon steel could be used also slightly softer but more ductile, has 0.16% - 0.29% carbon content as it's main alloying constituent, mild can also be carburized where needed for extra hardness, low carbon steels can also develop luder bands, these are like a elastic stretch it a certain amount and it will stretch, too much and it will break, shoudln't be stretched at all as weakens them, leading to deformation called luder bands.
So in short mild is better, anything over 2% carbon by the way is considered as cast!!
I am referring to everday obtainable metals as ther eis a bewildering array of treatments both hot and cold as well as gas teatments that can produce massively different properties compared to the same untreated materials.
Mild is a safe bolt for this application.
tabetha