[ainarssems]

For the brake discs I find that cheap discs are usually made out of steel instead of cast iron. I am not talking about the process of making them as steel discs are still made by casting and then machining but about classification where iron with less than 2% carbon is called steel and cast iron is over 2% carbon.

Simple cast iron is cheap to make but extra carbon make it brittle and not suitable for brake discs, to make stronger cast iron more advanced , more expensive manufacturing techniques and additives are needed so many cheap manufacturers just choose steel. Cast iron has better thermal properties, better corrosion resistance and is less likely to squeak. It also wears slower so cast iron discs last longer. On the other hand steel is stronger and has lower density, discs can be made thinner reducing weight or you can let the wear thinner an still maintain required strength. On the track car it might be beneficial to get steel discs if you can save weight but for everyday driver cast iron is more suitable. The issue is that manufacturers are not very upfront about material or carbon conten. It's also not necessarily easy to tell when you have them in your hand. Easiest indicator is probably when they are on the car when it's raining to compare how quickly and how rusted friction surface gets when parked in rain.