Has to be sprung temepred steel, but steel content has to be precise as it needs to have other ingredients in it when made, that is why a lot of springs are snapping now, due to certain compounds being used less due to emissions.
I personally have always found KONI's too soft for my liking, but very good for the road.
If you are serious about handling the first thing I would do is to change to coilovers like AVO.
tabetha