Tinned steel is common, you can solder to bare mild steel
with decent surface prep (steel wool until bright, then get
it done before it re-oxidizes). I have done this many times,
even heavy gauge copper wiring to automotive firewall
sheet metal (with a high powered iron). It's all in the prep,
but the native steel is solderable.
I use it frequently. The steel sheet material I use has a tin coating/finish (tinplate). In the past I recycled tinned metal cans.
When you use old material (no longer shiny), you need to prepare it before soldering (use a sponge, scouring pad, or even steel wool as suggested by Dick). Sometimes I use a stronger water based flux for tinning large parts of thicker steel plate. Clean very well after using water soluble flux.