rolling mill vs steel factory

rolling mill and steel factory both are nouns.

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

As nouns, steel factory is a hypernym of rolling mill; that is, steel factory is a word with a broader meaning than rolling mill:

Other hypernyms of rolling mill include steel mill, steel plant, steelworks.

rolling mill (noun)	steel factory (noun)
steel mill where metal is rolled into sheets and bars	a factory where steel is made