The first response from kleinstein is much better than the later response from Ozcar. Stepper motors usually have a driver IC to make the phase currents, and these work at the same time as a buck converter, and regulate the current through the motor coils.
Even if your motors power supply was 12V to begin with, if you just put 12Vdc on a motor winding, the motor is likely to fail. With a proper driver circuit however, the motor will run happily on an 24V power supply or even higher voltages (Depending on the driver IC used. Such IC's have a maximum input voltage).