Your acid core solder is suitable for joining things like steel wire. You'll want to clean the steel well, perhaps with an acid flux, and use a bit higher wattage solder gun or iron. I think cleaning, and the use of flux, will handle your problem, but if not, get a more powerful soldering gun.

Don't use this solder on electrical wiring, you would want resin core solder of a small diameter for that.

Good luck,
Dave Olson