gregzo's comment is not silly, he's right. The difference between AddForce and AddRelativeForce is that AddRelativeForce applys relative to the object's rotation, while AddForce uses global directions. AddRelativeTorque is causing the object to spin, so the direction AddRelativeForce is applied in constantly changes along with it.

If you want to keep pushing it one direction while it spins, you'll want to use AddForce instead.

:edit: just realised gregzo and I were both making the assumption you applied this force repeatedly over time, which would result in it following a curving trajectory. It's also possible that it's curving downward because you did not disable gravity for the rigidbody!