Can I apply for a patent? There is a mechanical part, which was originally cast. Now I want to make it out of steel plate.

As you said, the quality is good and the production efficiency is high. Of course you can apply for a patent. Patented technology is not only for the cost, but also for the low rigidity of pan-cast parts (especially where the stress or vibration force is large, it is easier to be damaged). If the steel plate has good rigidity, the quality will be better after heat treatment. This belongs to the patent of process method, and others have no right to deny it.