The deformation behavior of a vanadium-strengthened dual phase steel

A study has been made of the mechanical properties of dual phase (martensite plus ferrite) structures produced when a V containing HSLA steel is cooled in a controlled manner from either the austenite or austenite plus ferrite phase fields. Such a heat treatment results in the pearlite regions and carbide particles of the standard V steel being replaced by martensite; this leads to a decrease in the yield stress and an increase in ductility while the tensile strength is essentially unchanged. The fatigue of dual phase steels is slightly superior in the high strain life (ductility controlled) region and slightly inferior in the low strain life (yield dominated) region when compared to standard V steel. The replacement of the pearlite and cementite particles which can nucleate cracks, by more ductile martensite islands results in improved Charpy impact properties. The strength and the ductility of the dual phase materials is shown to be in agreement with a theory of composites with two ductile phases. This theory then allows one to understand the relative importance of various microstructural features in controlling strength and ductility. In this way it is found that the key to the superior elongation (at a constant tensile strength) is largely due to the high strength (fine grained), highly ductile ferrite matrix.