The most common situation where you would need to solder copper to mild steel is in the home. Copper and steel are used to make pipes for plumbing systems, and the two types of pipes are connected by a dielectric union. This is a joint union that is threaded in the middle, with one side copper and the other steel, and a rubber gasket in between. The copper side of the union is soldered into place on the steel pipe. Soldering this union is done the same way you would solder copper to mild steel in any situation.
Clean debris off your copper and metal with a wire brush. If you are soldering a dielectric union, unthread the union connection and wrap Teflon tape around the thread. Screw the steel part of the union onto the pipe by hand, then tighten it with a wrench.
Sand the area of the copper you will be soldering with an emery cloth. Apply soldering paste, or flux, to the sanded area. Join the copper to the steel as the two metal pieces or pipes will be connected.
Unroll 6 to 10 inches of solder. If you are soldering a dielectric union, bend the last few inches to a 90-degree angle. Heat the areas you will solder with a propane torch. Wait for the flux to sizzle. Touch the seam with the tip of the solder. When it melts, apply 1 inch of solder to the area or around the seam. Wipe off any extra solder with a rag and wait for the metal to cool.