At risk of repeating what has been said and in hopes of adding to it, carbon stiffens the steel in other ways by stiffening the crystaline structure. Brittleness implies that something is not as flexible and as temperatures drop, the molecules vibrate less and "stiffen" more. Flexibilty drops and the piece, or any metal for that matter becomes more rigid, thus being more prone to snap off. IN warmer temperatures, metal molecules vibrate more, become more "fluid" and thus a piece can bend more, including bending upon impact.
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