Do you have to use C1018? It takes a lot of speed to make that come out shiny. Dark sulfer bearing cutting oil will also assist in smoothness of the thread, it still may not come out bright and shiny.
The partial profile insert may also be pointier than ideal, forcing you to cut deeper than necessary, and the sharp tip is easily damaged, leading to surface galling. You could perhaps hone or grind a bit of a flat on the insert. A coated insert may resist chip built up edge too, but you really need more rpms to take advantage of that. If you cannot get above 350 to 400 SFM, then you won't get a bright surface.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)