Behaviour of BS EN 10025 grade S355 steel

Grade S355 hot rolled structural steel also suffers losses in residual yield and tensile strength when subjected to temperature over 600ºC in fire. High strength steels, of which grade S355 is typical, obtain their characteristics as the result of the addition of strengthening elements, typically vanadium and niobium. At high temperatures these elements tend to precipitate out of the matrix creating a coarse distribution. As a result the reduction in yield strength at room temperature after the steel has been heated to temperatures above 600ºC, may be proportionately greater than for unalloyed mild steels.