By the sounds of it, you want a less springy boring bar.
Use as large a boring bar as possible, and if you really do have to use a relatively long overhang from the tool post, a carbide bar will help as it's not as springy.

As mekanik says, HSS should help, as it's a sharper edge, and not relying on a blunt edge with brute force to cut the metal. Although one thing you could try if you have an insert bar, is use an aluminium insert, as they typically have sharper edges than inserts for steel. You won't be able to take as big a cut, and the insert won't be as durable, but should produce a smoother finish.
And a key thing with inserts, is keep them cutting. Don't let them rub.