Cutting structural steel
Grinder is best tool for this job....maybe even you can use the one that you have just make sure you have several plates in order to complete the job. If your grinder isn't enough 115mm one is big enough for sure....When you cut it don't just go straight down like you are cutting a butter but once you cut through wall on one side rotate your beam (or column, whatever that might be) for 90 degrees and repeat this for all 4 sides. But be really careful grinder is really dangerous tool unless you are skillful. It can be heavy and can slip from the thing you are cutting which may lead to injury. Also be extra cautious about the sparks....they can set things on fire, so make sure that anything flammable isn't around.