will a circuit breaker designed for 230VAC work for 12VDC?

Use one designed for low voltage DC such as Eaton KD1 or this automotive type. A better option is also a Raychem Polyswitch which self resets when the current is removed. The problem with the 230V ones is the high trip current, have you really got more than 16 or 25 amps available from your 12V supply?

Edit: I didn't realise this related to an earlier question where you are talking about an automotive battery. In that case, yes, many amps are available (as opposed to a wall wart), and Andrew's comment applies - a breaker designed for AC should not be used on DC because it is a harder job to seperate the contacts on a DC current than on AC where the magnetic field falls to zero repeatedly due to the waveform.

MCBs (and fuses) have a characteristic curve showing time to trip against overcurrent. At the rated current, they should last indefinately, but as that is exceeded they trip faster. For a fast trip, you need the source to be capable of many times the rated trip current.

The key here is AC - alternating current - vs DC - direct current. They are different - AC oscillates while DC doesn't. That means, as the answer above notes, that AC breaks contact easier that DC and DC creates greater arcing across the contacts - AC doesn't to the same degree - use a DC circuit breaker - they are available.....

Same applies to standard switches. A 230v switch will fail very quickly if used in a DC circuit because of the arcing.