Are you saying the motors are getting hot, or are the stepper modules on the RAMPS board getting hot?
In either case you probably have the current set too high for the motors. There's a tiny pot on each stepper driver on the RAMPS board that controls motor current and needs to be adjusted for the motors you're using.
Stepper motor voltage ratings are based on the resistance of the windings are are generally irrelevant. It is not unusual to power stepper motors from a source that is 10X their rated voltage. The driver module limits the average current to the motor according to (in this case) the pot setting on the driver module PCB.
