When working with an angle grinder i'd recommend using good quality discs. Not the ones you find in a pound store, buy some from screwfix or similar. Take the cut nice and slowly, use the weight of the grinder and don't force it down as you will wear down the discs in seconds. It also sounds silly to say but make sure you are using a metal cutting disc (around 3mm thick) and not a grinding disc (about 6mm thick).

I found that using a small 115mm grinder will take a long time with a few mm thick of steel. A 230mm grinder will however go through it like butter. I tend to use my big grinder for cutting and the small one for grinding and those small tricky cuts.

Most importantly, make sure you wear protection! Goggles, gloves and ear defenders are essential.