What are the other materials which can be used in DLP Printing, other than polymers? I know that melted polymers are used for DLP (digital light processing) printing, by displaying the model onto the liquid material, which is the melted polymer.

But, can I use materials like steel, carbon fiber, etc?

If yes, then what precautions and arrangement should I make to my apparatus for coping with their high melting point?
DLP doesn't use "melted polymer". It uses a photosenstive resin which is at room temperature and polymerizes when exposed to (UV) light. At no point is any material melted.

As such, steel or carbon fiber can not be used as these materials are not photosensitive.