safe-deposit box vs strongbox

safe-deposit box and strongbox both are nouns.

Word	Noun	Adjective	Verb	Adverb
safe-deposit box	Yes	No	No	No
strongbox	Yes	No	No	No

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

safe-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 safe-deposit box include deedbox.

safe-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