Influence of Cr and Cr+Nb on the interphase precipitation and mechanical properties of V–Mo microalloyed steels

There has been much research on steels strengthened by interphase precipitation, but the role of Cr remains unclear. Isothermal transformations have been performed at different temperatures to investigate the effect of Cr and Cr + Nb additions on the interphase precipitation resulting from the austenite-to-ferrite transformations in V–Mo based dual phase steels. Optical microscopy (OM) plus transmission electron microscopy (TEM) and tensile testing were used to characterize and correlate microstructural evolution to the tensile properties and the interphase precipitation formed during the austenite to ferrite (γ→α) transformation. It was found that an addition of 0.5 wt% Cr to V–Mo microalloyed steel accelerates the transformation rate of ferrite, producing a higher volume fraction of ferrite. Importantly, the addition of Cr reduces the diameter of the interphase precipitates giving an important contribution to the yield strength. A further addition of 0.03 wt% Nb to the Cr–V–Mo reduces the contribution made by the interphase precipitation and increased the grain refined contribution. A new method was used to calculate the volume fraction of interphase precipitation with TEM images from extraction replicas is presented and included in a root-mean-square model which includes the precipitation hardening made by IP on microalloyed DP steels to effectively predict their yield strength.