What we need is a new item, Scrap Steel, or just Steel, just like we already have scrap iron (i.e. Iron). It should be obtainable from logical sources in balanced and semi-realistic amounts. The main variables would be how much damage you do per hit and what tool you are using to harvest.
Vehicles: 5% chance per X damage done, 20% chance for double harvest if using wrench
Safes: 5% chance per X damage done, 20% chance for double harvest if using pick
Steel doors: 5% chance per X damage done, 20% chance for double harvest if using pick
Steel beams: 10% chance per X damage done
Misc steel construction: 10% chance per X damage done
Rebar: 10% chance per X damage done
..and so on. I'm sure the devs can think of at least a few more sources. I also leave it to them to decide the damage amounts needed to trigger a harvest drop, and yes, if you aren't doing a minimum amount of damage to trigger one then you will get nothing. There should be a warning that tells you this after the first ineffective hit...strength and skill will effectively gate steel harvesting. I actually wish everything worked this way in 7D - as in real life, the strongest and most resilient tend to be the best at manual labor so pure strength would be tops for mining, woodcutting and salvaging. I also wish they would add another tool like a Metal Shear or something similar to better deal with iron and steel (particularly molded stuff like beams). Using a pick is not really that logical and shouldn't be very effective vs steel even if its a steel pick because humans simply don't have the strength to pull it off. Wrenches are obviously steel as well but they can at least be thought of as disassembling a steel item, so wrenches should be less effective vs steel beams and other things that aren't really mechanically assembled.