steel factory vs steel mill

steel factory and steel mill both are nouns.

Word	Noun	Adjective	Verb	Adverb
steel factory	Yes	No	No	No
steel mill	Yes	No	No	No

As nouns, steel factory and steel mill are synonyms defined as:

Other synonyms of steel factory include steel plant, steelworks.

steel factory (noun)	steel mill (noun)
a factory where steel is made	a factory where steel is made