Bolts produced on an automatic screw machine are normally made of 12L14.
Same thing as used for most muzzle loading barrels, by the way.
In modern times the word 'steel" is used not only for rather soft stuff, like nails, but also for hardenable metal, such as used in hammers, knives, springs, files, etc.
If you want bolts that won't shear off like a piece of sharp cheese (read that 12L14) they need to be made of some higher carbon material, and/or heat treated.
Personally I do not know what is commonly available in strong bolts suitable for lock bolts.