Copper or Steel Pipe for a buried steam pipe

I need to replace a long section of 3 inch steam pipe (probably steel or wrought iron, house is from 1929) below a slate sunroom floor. There may have been original insulation beneath the slate and concrete, but now the pipe is sitting on dirt. What are your thoughts on replacing with copper pipe versus steel? Problem becomes having the equipment to thread steel. Copper is an easier material to work with but concerned about the soldered connections with steam heat.

Has anyone used copper in this application?

Any advice would be much appreciated!