cutting hole for steel chimney

Unless there is something that you are not telling me, then you should be able to do this with a sawzall. Get a long blade (12"), hold the sawzall body above the center of the hole you want to cut and saw away. Simply rotate the saw in the center of the circle.

Jig saw as much of the circle as you can, cut "relief cuts" (if possible) to drawn circle

finish the cut:

by hand (key hole saw or Milwaukee job saw) Multimaster Dremel right angle drill with 1" to 2" diameter hole saw or paddle bits air die grinder with 1/4" diameter rotary file

You need to be able to get some horsepower to bear on the work. Depending on access, any one of these (or a combination) will get the job done.

cheers Bob