"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."
"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.