[IROC]

Actually a better term for them is anti-roll bar as they increase the roll stiffness of the car.

Any kind of steel is fine as the modulus of rigidity (the applicable characteristic) is essentially the same for all steels. As long as you don't exceed the elastic limit of the material, you're probably OK. What is really important is the geometry of the bar (diameter) and how it attaches to the car and the resulting moment arm.

Mike