Hi Jake,
creep is permanent deformation even under low loads. "Plastification" is the permanent deformation under very high loads (e.g. your steel example). I know that some people says that dyneema creeps only under high loads, but my expierence with (high quality) ropes of sk75 is a different one. Creep is a killer for the trim of your standing rigging. At least my feeling about it.
Cheers,
Klaus