What is a good way to cleanly cut through steel?
A Nibbler, is a tool that is used for cutting thin metal. A heavy duty nibbler is usually capable of cutting up to 14 gauge steel. I'm not sure what gauge metal is used for mini kegs, so this tool may not be appropriate in your situation.  

Some nibblers work similar to a punch and die, nibbling small sections of the metal at a time (think hole punch). Other versions make two parallel cuts, and roll the scrap material in front of the tool as they cut.

Nibblers come in both manual, and powered variations. Powered version can be powered by both air, or electricity.





There are even some available that attach to a drill, converting the drills rotary action to a reciprocating motion.



Nibblers will give you a clean cut, and will not create sparks.