It is my understanding that kerosene, paper and plastic burn at low temperatures (relatively to the melting point of steel). Also, the smoke coming from the towers was black, indicating a sooty, oxygen-starved, low-temperature fire. No flames were visible from the outside (unlike in the case of Madrid's Windsor building). It is still possible that the fires were able to weaken the steel columns, as losing structural integrity is an entirely different business from melting. But you'd have to ask a metallurgist about that, the Handbook of Chemistry and Physics will tell you the melting point of steel, but not its compression modulus as a function of temperature. Also, the steel columns were coated in concrete, which is a good heat insulator, and the floors and ceilings were also slabs of reinforced concrete.

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