Properties of newly–developed hot–rolled high–strength sheet steels with phosphorus

Two types of hot–rolled high–strength sheet steels were developed. C–Mn–P steel with phosphorus content more than 0.04% and processed at a coiling temperature of less than 200°C exhibits very low yield ratio less than 70%, and superior properties in ductility, stretch flangeability, fatigue and bake–hardenability. It does not show 2nd cold work embrittlement due to P and brittleness of the heat affected zone of the welded joint. This steel is suitable for components which require high formability, such as wheel discs. The other type is an Nb–containing high–strength sheet steel with P addition instead of increasing C, that is, low carbon equivalent value. It also shows good ductility and superior stretch flangeability. Less hardenability of the welded zone of this steel results in a superior formability and touchness of the welded joint and makes it possible to apply this steel to a wheel rim.