steel mill vs steel plant

steel mill and steel plant both are nouns.

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

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

Other synonyms of steel mill include steel factory, steelworks.

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