The only Sigs currently available that have steel frames are the "Sport" series guns. The models available are the 220S, 226S, and 229S. The sport guns are listed under a separate page on their website. I didn't mention the new 220ST because from what I understand it's just the 220S without the muzzle brake.
So if you want a target gun in steel, you can get it in a 229. If you want a true combat gun in steel, you've only got the 220 to choose from. Not that it's a bad option. If it were me, I'd bet my life on the 220.
-Red-