If you heat it to straiten it will, as Rog pointed out, lose it's 'temper'.

However, all is not lost. If you can in fact heat it to such a level (which to be honest is quite unlikey unless you use a forge - although an old fasioned parrafin blowlamp can give off a hell of a temp.) you can also re-temper it. I can't remember the exact colours but as the steel heats up it changes the structure slightly. It goes through being red to orange to yellow and finaly white. Somewhere around the 'straw' colour you can extract it and plunge it into a trough of cold water. It makes a very loud and satisfying hiss:-) Do this a couple of times and you will re-temper the steel to it's original hardness.

To do it properly of course you need a lifetime of experience in the skill of a blacksmith. Probably cheaper to but a new fork:-)

Cheers

Dave,