Some grades of steel lend themselves very well to soldering. You do not have to use acid for this operation. Ordinary fluxes, which are also used for brazing copper, are suitable: rosin or LTI-120.
Make sure the product you want to solder is made of a steel grade that can be soldered. Staples and nails are very well soldered. With great difficulty, if not at all, the shafts of microelectromotors do not lend themselves to this process.
Take a soldering iron of such power that it can warm up the product. Small nails and paper clips can be soldered with the same device that you are used to soldering regular radio components (25 - 30 W). For larger products, you will have to take a soldering iron with a power of 40 to 200 W, depending on their dimensions.
Make sure that heating the steel part to a temperature that can be brazed will not damage other parts that are in thermal contact with it. For example, a steel furniture nail might have a polypropylene overlay that melts at just 165 degrees Celsius. If you cannot remove the parts at risk, use pliers as a heat sink and solder quickly.
Tinning the steel product using regular neutral flux (even rosin will do). Try tinning it without stripping it first - if the steel can be soldered, this is usually quick. The only thing that has to be done is to warm up the object well. Since it is more massive than, for example, the output of a radio component, it will take longer to warm up. Of course, it is necessary to hold it not with your fingers, but with pliers or a similar tool, in order to avoid burns.
If it is not possible to tin the surface of the product, despite the fact that it is made of brazeable steel, try cleaning the surface of the product and repeating the operation.
Tin the second part in the same way. Solder the tinned parts together.
Finally, if steel cannot be soldered with conventional flux, do so. Tin it using an active flux. Be extremely careful as these fluxes are acidic. You can also use a domestic aspirin tablet (not effervescent). After that, immediately tin the part again, this time using rosin or another neutral flux. The durability of such a connection is not guaranteed.
Do not subject soldered joints to mechanical stress. Unlike welded ones (and even then not any), they are not designed for this.