safety deposit box vs strongbox

safety deposit box and strongbox both are nouns.

Word	Noun	Adjective	Verb	Adverb
safety deposit box	Yes	No	No	No
strongbox	Yes	No	No	No

As nouns, strongbox is a hypernym of safety deposit box; that is, strongbox is a word with a broader meaning than safety deposit box:

safety deposit box: a fireproof metal strongbox (usually in a bank) for storing valuables
strongbox: a strongly made box for holding money or valuables; can be locked

Other hypernyms of safety deposit box include deedbox.

safety deposit box (noun)	strongbox (noun)
a fireproof metal strongbox (usually in a bank) for storing valuables	a strongly made box for holding money or valuables; can be locked