It's hard to keep track of the various temperatures involved in discussions around 9/11. So I made the above chart. It's a work in progress, so corrections and suggestions are much appreciated.
Melting points of Lead, Aluminum, and Steel: https://www.engineeringtoolbox.com/melting-temperature-metals-d_860.html
"Soft" temperature of steel: https://en.wikipedia.org/wiki/Forging_temperature
NatGeo Pit Fire: National Geographic Science & Conspiracy Part 3 (5:56)
Molten steel image: https://commons.wikimedia.org/wiki/File:Scunthorpe_Molten_Steel.jpg
Temperature colors (scale on left) https://en.wikipedia.org/wiki/Red_heat
I used the colors in the illustration there to create a photoshop scale between 550°C and 1300°C, then did white above, and a slight fade down to grey below.
Hydrocarbon fire range of 800°C-1200°C, various. Common temp cited for house fires is 1100°C, but range is more.
Strength reduction in steel - various, this is a topic in itself. There's a variety of graphs of this, and it varies between steel types and there's a different between yield strength and compressive strength.
(Diagrams from Brockenbrough and Merritt’s Structural Steel Designer’s Handbook.)
The "50% at 1200°F/650°C" actually seems a bit high a temperature, but it's a conservative round number.