Making an intuitive leap here: was cheap Russian steel used in the World Trade center buildings? Is that why they went "boom?"
The fire didn't "melt" the steel, but it likely changed the temper of the steel. Tempering steel affects its crystaline structure (going by memory here) and that's what gives it the ability to bear a load. If you change the temper of the steel, it loses its load-bearing capability, perhaps to the point of failure.