No problem soldering steel to copper, you can use normal plumbers solder & flux or if it is a small piece of steel then flux cored solder as used for electronics will do. Just make sure both surfaces are clean and free from grease, wire wool is the most common way to clean the surface.
Depending on the loads involves the soft solder will be the weakest link so make the solder area as large as possible. Silver solder would be stronger but the process is a little more involved and the temps are getting closer to the melting point of copper.
not wishing to sound pedantic but i seem to remember reading that the joint should be mechanicaly sound before its soldered hence wrap it to form an eye
If you are wraping it around the bar then there is little point in soldering it as the wire will likely be stronger than the solder. The only reasons I can think of it needing to be mechanically sound is if the solder is only to act as a seal (as in plumbing fittings) or for continuity and reduced resistance ( as in electrical joints)
If youmean forning an eye and then soldering this to one face of the bar, then I agree that it will increase the solder area as I said in my previous post
Having said all that is copper the right material to be using as it has quite a low tensile strengh, brass or even piano wire would be better.
If you are attempting to affix copper to steel, solder is not the way to go. You should be brazing it using a flux and a brass filler rod. TIG welding would also do the job, but I would re-iterate that copper is very poor in the area of tensile strength. I can't imagine why you would select copper for this job. Perhaps if you could tell us what you are attempting to achieve, we could offer some more suitable alternatives. Maybe, the use of steel throughout and then copper plate the parts you want to be.. er.. copper?
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