steel mill vs steel factory

steel mill and steel factory both are nouns.

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

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

Other synonyms of steel mill include steel plant, steelworks.

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