Daniel Blain

Jennifer is of course referring to case hardening, the hotter/longer the piece is in the case the deeper the carbon migrates. However in use the higher carbon layer will wear through eventually.

The temperature needs to be at least low red to be effective. The chemical in the leather is carbon.

It may be like the procedure to make charcoal but it's not like the process! When you make charcoal you are removing all the volatiles leaving only carbon. When you case harden you are migrating carbon into the steel by exposure to high temperature carbon monoxide.

It is possible to make mild steel strikers. The best way is to use oak tanned leather.
Take a coffee can, and pack it with pieces of oak tanned leather and a striker or two.
Cover the top of the can with foil
Lay it in a good fire in your forge and let it sit.
(you don't need to force any air - just let it burn)
Its very similar to the process used to make charcoal -
Anyways, when it cools, the chemicals from the leather will have given the steel enough hardness to spark.
def want to make sure the flint is sharp) I have made a few of these simple trade strikers and they work well enough.

The files/high carbon steel that the others have mentioned do make the best strikers, but this maybe a way for you to salvage the strikers you already made.