The old method was angle irons as main support, about 50mm x 50mm x 2mm, it was however hardened in some way...same as a spring is, otherwise it would have bent.
.....
Are you going to remake the bed from scratch or utilise parts of the existing design?
If you put up some pics of your original bed (clean one's please... ) I'm sure there will be plenty more ideas and suggestions from everyone...
I've made all my own beds (and other furniture) for the past 20 years, but I prefer wood, for beds, because its warmer to the touch and has (imo) better characteristics for "the job" than metal. My first hand-made bed had sections trimmed with full grain hide (leather) and this might be a good idea if you want to add some contrast & warmth (covering bare metal surfaces), to any areas, that the body comes into contact with. Leather's also quite aesthetically appealing...
It's surprising the amount of stress, and movements, a bed has to take. I prefer futon styles and this can cut-down on the; stresses, construction, costs & materials used. Futons also allow one to use the floor too, if the bed... gets... too... small...
I'll get my coat....
Could have been cast iron, I recall the phrase "cast iron bedstead" from my youth