Surface prep past heat treat scale to bare scuffed fresh steel and thoroughly degreased.
Depending on thickness and length, could be cooling as a radiator and not enough heat for solder to flow. Resin to steel first for surface wetting, and may end up heating enough to ruin any spring to the spring steel.

I could solder copper wires just fine but when I tried to solder to steel it never worked. My father told me it required acid core solder. I might have been trying to solder to something too thick to heat up properly. In years since I've soldered steel electrical wire to copper electrical wire and it worked just fine.