With the appropriate metal cutting blade (same material that your grinder and cutoff wheels are made of) from your home improvement center, you can cut steel with a circular saw. You just may not want to. It is a real PITA. The carbide blades are much better than those, although they may be designed to spin slower.
I use a number of 4.5 inch cutoff wheels and grinding wheels spinning at 10000 + RPM on metal all the time. You know how messy those are, the one for the circular saw is even worse.
Best solution? borrow a torch and get ready to be grinding for a while.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
"If the women don't find you handsome, they should at least find you handy"
-RedGreen show.