Hmm.. Makes real sense. Let me re-frame my question.There is no clear limit. Elongation at break, fracture toughness, impact strength and other indicative and non-reproducible figures vary steadily with temperature.
Steel producers use to give figures or curves down to -50°C and these don't look damning. Steel won't break like glass. The need for toughness depends on you part's use: at a gas tank you want deformation without rupture, at a ball bearing any plastic deformation equals a failure.