Steel does not burn, but it softens. The question in a fire is never whether the plate catches fire, but how much strength it keeps while the temperature climbs.
The numbers are simple. Up to about 300°C, structural steel barely changes. At 500°C, yield strength drops to roughly half of its room-temperature value. At 600°C, it falls to about a third – below the point where most designs assume failure. Building fires routinely reach 800 to 1000°C, which is why fire protection, not the steel grade, decides survival time.
What happens after matters too. Steel that has never melted usually keeps its properties once it cools; distortion is the real damage. A plate that warped in a fire can sometimes be straightened and reused after inspection, but that call belongs to an engineer, not the owner.
For buyers, three practical notes. First, fire protection design – spray coating, board or intumescent paint – is selected for a required fire rating time, and plate thickness plays into it. Second, grade does not buy fire resistance: A36, A572 and S355 all follow roughly the same strength loss curve. Third, keep the mill certificates after installation, because post-fire assessment starts from knowing what the structure is made of.
Steel gives you time, not immunity. Design the protection for the time you need.
