It doesn't have to be hard in the sense of holding an edge (no cutting
involved). But it doesn't seem to me (by intuition ONLY) that mild
steel would be "strong" enough. It seems like it would bend too easily.

But not-bending is a matter of tensile strength, yes? And mild steel
and hard steel have the same tensile strength, right? Or, is it a
matter of yield verses ultimate strength?

Bottom line - what should I use use to make a pry bar? (It will have
very specific geometry, so store-bought is not an option.)

Thanks,
Bob