Oh, what do I know! Have you ever watched the contractor who puts gutters on a house? He has a big roll of aluminum, or sometimes copper, in the back of his pickup truck. Out near the tail gate is a machine about the size of a case of beer. The machine consists of a bunch of rollers. He feeds the flat sheet metal into one end of the roller machine, and when it comes out the other end it's bent into the shape of a gutter. Perfect, uniform, and very flimsy since it's made of aluminum or copper.

To make a steel rim you'd basically do the same thing. Feed a steel tube into one end of the machine and a series of rollers reshapes it into a rim section and then curls it around a mandrel of some kind. I presume a lot of heat and pressure are involved. Somehow the cut ends have to be joined, but I'm sure that isn't a problem even if it involves someone brazing them by hand. It can probably be done by machine. The whole process can probably be done by machine.