The effect of molybdenum on clustering and precipitation behaviour of strip-cast steels containing niobium

Two high-strength low-alloy (HSLA) steels containing Nb-carbonitrides were produced, one contained Mo and the other was Mo-free. The alloys were produced by simulated direct strip casting, and were fully bainitic in the as-cast condition. Isothermal ageing treatments were carried out to precipitate harden the alloy, and the strength was measured using a shear punch test. The dislocation density was measured with X-ray diffraction (XRD), and was found to be larger in the alloy containing Mo in all ageing conditions. Atom probe tomography (APT) showed the presence of solute clusters in the as-cast condition, and the addition of Mo increased both size and volume fraction of these clusters. The solute clusters provided significant strengthening increments of up to 112 MPa, and cluster strengthening was larger in the Mo-containing alloy. Precipitation of Nb-carbonitrides was observed after longer ageing times, which were refined by the addition of Mo. This was attributed to the higher dislocation density that increased the number of nucleation sites. Precipitate chemistry was similar for both alloys, and contrary to some literature reports, minimal Mo was observed to segregate to the precipitates. A thermodynamic rationale is presented which describes the reasons that Mo segregates to the Nb-carbide in some alloys but not in others, despite the alloy chemistries being relatively similar.