I bought this to stamp a VIN on a trailer. I placed the VIN on the steel angle stock that makes up the frame. Here are my observations.
It worked. It did leave an imprint in the steel, even though it is a fairly hard material. Not as deep and clear as the example shown in copper, but usable.
The punches showed no significant signs of damage from this use, so the hardening was effective.
The character size was a little smaller than I had envisioned. If I were to do it again for this purpose, I would get a 1/4 inch size. The 1/8 inch size is more appropriate for things like dog tags.
I found it hard to create a credible VIN when free-handing it. I had much better results by clamping a board to the metal and lining each punch up on the top edge of the board. I could get the horizontal spacing fairly well with that, and the board made sure that all characters were aligned vertically. Without the board the results looked like a wave on the ocean!