Microalloying effects of Mo versus Cr in HSLA steels with ultrafine-grained ferrite microstructures.

Thermo-mechanical processing with warm-deformation in the ferrite is a promising pathway towards manufacturing of large volumes of steels with ultrafine grain sizes. We apply this processing to HSLA steels to invoke grain refinement coupled with strengthening arising from microalloying. The aim is to systematically unravel the strengthening via the addition of 'modern' microalloying elements Mo versus Cr besides Nb. These elements have been proposed to provide significant additional strengthening during such processing but the mechanisms yet remain unknown. Therefore, three model Fe-1.6Mn-0.04C-0.1Nb+0.5Mo/Cr steels were warm-rolled at ∼600 °C with an overall reduction of 50% followed by ageing. It was found that Mo leads to an increase in hardness of ∼20% compared to the base alloy, whereas Cr provides only a minor hardening increment. SEM, EBSD, TEM and atom probe were used to reveal the beneficial effect of Mo versus Cr. It was found that Mo is more effective delaying dislocation recovery. We also report that Mo partitions into nanoscale Nb–C solute clusters and precipitates of NbC and Fe 3 C during ageing, retarding the coarsening of these phases. However, Cr was found to partition into Fe 3 C only, and does not contribute to the nature of the dispersion of clusters and NbC. Image 1 • Systematic study of hardening through additions of Cr/Mo in a Nb-HSLA steel deformed in the ferrite. • Mo provides significant strengthening by precipitation, clustering, solute drag and interface segregation. • Mo partitions into NbC and cementite impeding their growth upon ageing at 600 °C. • Cr partitions into and impedes growth of cementite upon ageing at 600 °C only. • Nb–Mo is a more promising design approach than Nb–Cr for ultrafine-grained strong and tough HSLA steels. [ABSTRACT FROM AUTHOR]