rolling mill vs steel mill

rolling mill and steel mill both are nouns.

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

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

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

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