No... definitely not by far the best, and probably not the best at all. 4x weaknesses are never good, especially with something as prominent as Fire, plus, it also takes away three resistances that Steel already had, besides Fire, and makes them neutral. I'd say its best dual-typing is maybe Flying. It only adds one weakness, Electric, which is a weakness Steel should obviously already have anyway, and it takes away Fighting as a weakness, and not only takes away ground, but makes it immune to it.