If a relay has a coil voltage of 12vdc and a contact rating of 120vac @ 30 amps (example) would it still be possible to use the contacts with dc? A relay is a relay is a relay right??
Not only the amperage is lower, the max. DC voltage is lower by a lot too.
See attached image about relay rating from a well known relay manufacturer.
If you do the maths, your 120V AC @30A relay might be able to switch a DC load of 18A at 15V, but it is just speculation. The sure way to go is to look for DC rating printed on the relay casing, if there is any.