.. create a wall and slap a steel fence texture but it turned black on the transparent areas..

Which is a good idea. Only, transparent textures do not work on walls, but would on a thin box (model instead of wall).

ok

PS Model suggested by bdfd looks like a good fit, but there are also some simpler iron fences and gates in the SH3D repositories that would.
http://sweethome3d.com/searchModels.jsp?model=fence
https://sourceforge.net/p/sweethome3d/d ... h/?q=fence