Flat Iron (Straightener) – When choosing a flat iron* there are number of things to consider. Cheap irons may work just as well as the expensive kind but the damage they will do to your hair will be irreparable.
Black hair, whether natural or relaxed does not agree with a lot of heat so a temperature control on the tool is an absolute must. A ceramic (not ceramic coated) or tourmaline flat iron* will be best.
The ceramic ensures that heat is distributed evenly on the hair and tourmaline is a natural source of negative ions so will leave the hair silky and moisturized.
I think a must-have for really curly, and thick, hair is the karmin g3 straightener. I have 3B hair and it leaves it pin-straight and shiny!