Effect of Vanadium Addition on the Strength of API X100 Linepipe Steel

Laboratory heats of high strength low alloy steels with yield strength of 100 ksi (API X100) were produced with and without vanadium addition. Upon mechanical testing of the rolled plates, it was determined that 0.06 wt.% V increased the yield and tensile strengths from 8 to 14% in all directions without adverse effects on toughness or ductility. Transmission Electron Microscopy (TEM) was used to study the distribution and structure and volume density of precipitation in the two steels. A higher number density of smaller carbonitride precipitates with more uniform distribution was observed in the V-added alloy compared to the steel without vanadium. Size and volume density of the carbonitride precipitates were measured based on EDX elemental maps of Nb, V and Ti, and measuring thickness of the TEM samples. It is shown that the volume density of carbonitride precipitates in the V-added sample is one order of magnitude higher than the base alloy. Ashby-Orowan analysis indicated that the additional precipitation density due to vanadium addition made partial contribution to the observed increase in strength. Other possible strengthening mechanisms caused by carbonitride precipitates such as their effect on dislocation recovery in bainite and refinement of bainite microstructure are also discussed.