Phosphorus-Added, Hot-Rolled, High-Strength Sheet Steel with Low Yield-to-Tensile Strength Ratio

A new hot rolled high strength sheet steel has been developed by utilizing controlled cooling technique after hot rolling to a 0.05wt%C-1.5wt%Mn-0.08wt%P steel. Phosphorus enhances ferrite transformation at higher temperature, which results in a fine dispersion of austenite phase during the cooling step after hot rolling. At lower temperature, P retards austenite decomposition and enhances martensite transformation when the strip is coiled at 250°C or below.

This new high strength steel exhibits low yield-to-tensile strength ratio of 60-70%, while in conventional high strength steels, for example, a 0.15wt%C-1.5wt%Mn steel hot rolled and coiled at 400°C or below after rapid cooling, the yield ratio is about 75%. Besides it exhibits higher ductility, good stretch-flangeability, good fatigue property, good weldability and extremely high bake-hardenability compared to the C-Mn steel. Embrittleness due to segregation of P to grain boundaries was not observed in sheets and welded joints.

This high strength steel has also good paintability and is now under commercial production for wheel discs or chassis components.