@DavidEF, steel is super hard to solder to.
The opposite is true with copper, nickel, silver, and a thick layer of gold: it’s so easy to solder it’s not even funny.
There are 2 reasons for this, and a bonus one:
1. Steel has chromium, and forms a chromium layer that can penetrate a layer of thin gold, which is why something like gold plated steel springs solder poorly, even when gold plated.
2. Steel has poor thermal conductivity compared to nickel, and especially copper and silver. For this reason, the soldering iron can transfer the heat to the solderable area very quickly, making soldering a breeze.
Bonus reason: Nickel and silver don’t form oxides, and even silver tarnish isn’t an oxide that prevents soldering, meaning nickel and silver platings make the soldering even easier.
That’s the reason why soldering pure copper that is oxidized is so hard: copper oxide has very poor thermal conductivity.