Search on this forum Mech Engr? there was a good discussion a week or so ago--and at MUCH MUCH higher tensile strength. Amazing how much strength has gone up in last few years.
Must it be steel? there are a number of superalloys that are available in wire form for use as springs that exceed 230ksi yield. Elgilloy, MP35N, Carpenter's Aeromet 300, are a few that come to mind.