I cut up a bed spring frame for the rim steel just the other week.
I figured it to be low carbon, the part I used was the 3/8 diameter rod that formed the rim around the top edge.
It was one of those spring sets that go under the mattress.
I made a coat rack from it, it forged fairly easy, but I did find it was a high carbon steel once I got into it.
The bends won't lay flat and it's really springy.
I guess I'll have to anneal it to be able to straighten it out so it fits flat on the wall.
I would say any part that would be expected to flex and still support weight might be med to high carbon.
Thinking about it, I did chisel cut some of the angle iron and it broke more like high carbon than cold rolled.
Hummmmmm,
Perhaps there is more good steel in my trash pile than I thought.
Jim