We use A500 and A512 structural steel tubing for the main rectangular chassis rails. Formed pieces are from 1020 cold-roll sheet. Neither is the pinnacle of steel alloys but nobody's ever bent one except in a major collision (where things bent, absorbing energy, not fractured). Because there are lots of welds in the overall design, I'd rather have weld-friendly alloys rather than something that explodes if something goes wrong.
Round tubing that is used in the suspension and in ancillary chassis pieces is all 1020-1025 DOM tubing. Same reason.
As I explain in the
Chassis Design Page, it's the stiffness, not the strength that you notice on the road.