If I may just add a few thoughts about soldering steel. I would
recommend Easy Flo flux to use with Easy flo solder. Use the
round wire solder. Make sure that the steel is really clean
before soldering. Make sure the joints are really close. Put flux
on to the joints before bringing the joints together. I wouldn’t
get the steel too hot, perhaps a very dull red when the flux
melts and goes runny. Easy flo flows very quickly hence the name.
It should go through the joints very quickly, just a matter of
seconds. In the UK both the solder and flux are available from
Johnson Matthey, I can’t say for the US.