"Strength loss for steel is generally accepted to begin at about 300°C and increases rapidly after 400°C. By 550°C steel retains approximately 60% of its room temperature yield strength, and 45% of its stiffness. At high temperatures, steel is also subjected to significant thermal elongation, which may lead to adverse impacts, especially if it is restrained."

At 800 C ("dull cherry red"), the yield strength is down ~90%: https://www.steelconstruction.info/images/9/99/Steel_strengt...

This is talking about short term properties of steel, which would be important in building fires, but creep also increases with temperature.