Can working in a steelmill cause optical neuritis be?

Probably not: Unless you have contact with toxic solvents, or in contact with drugs which cause arteritis or vasculitis, it is most likely that your work has nothing to do with the eye issue, and other causes should be considered. Have a thorough evaluation with first an ophthalmologist and later a neurologist. If an MRI of brain shows additional lesions, this may be early ms.