For consistent results with base metals such as brass or ferrous
metals (iron, steel, stainless steel) I use either EasyFlo or
I’ve done this sort of work a few times - certainly no expert. But
here’s another tip that can help, but it depends on what results you
need in the end. If you’re soldering a single gold ball onto steel,
this is not useful.
Otherwise, “Tin” the steel first. Clean, use whatever flux works, put
the solder down but not your design, and use some tool to drag the
solder all over the steel, and scrub it as needed, until the surface
is covered - really covered means the solder won’t “break” when you
get it molten, showing steel underneath. Once you do that, the steel
is shielded from the atmosphere, and the rest is relatively easy.
Usually I’ll cool it, clean it, push fresh flux and THEN do the
soldering. Again, it’s not a method for every job, just