Effect of Mo on acicular ferrite transformation and interphase precipitation of Nb–V–N microalloyed steel during a continuous cooling process

The substantial influences of Mo contents varying from 0 to 0.26 and 0.50 wt.% on the microstructural evolution and MX (M = Nb, V and Mo; X = C and N) precipitation characteristics of Nb–V–N microalloyed steels processed by hot deformation and continuous cooling were studied using a Gleeble 3800 thermomechanical simulator. Metallographic analysis showed that the ferrite microstructure transformed from polygonal ferrite (PF) in 0Mo steel to both acicular ferrite (AF) and PF in 0.26Mo and 0.50Mo steels, and AF content first increased and then decreased. The thermodynamic calculations and the experimental results proved that the quantity of solid solution of Mo in austenite obviously increased, which reduced the austenite (γ) to ferrite (α) transformation temperature, consequently promoting AF formation in 0.26Mo steel and bainite transformation in 0.50Mo steel. Moreover, the submicron Nb-rich MX particles that precipitated at the temperature of the austenite region further induced AF heterogeneous nucleation with an orientation relationship of \((011)_{{{\text{MX}}}} //(100)_{{{\text{Ferrite}}}}\) and \([1\overline{1}1]_{{{\text{MX}}}} //[001]_{{{\text{Ferrite}}}}\). The interphase precipitation of the nanosized V-rich MX particles with Mo partitioning that precipitated during γ → α transformation exhibited a Baker–Nutting orientation relationship of \(\left( {100} \right)_{{{\text{MX}}}} //\left( {100} \right)_{{{\text{Ferrite}}}}\) and \(\left[ {001} \right]_{{{\text{MX}}}} //\left[ {01\overline{1}} \right]_{{{\text{Ferrite}}}}\) with respect to the ferrite matrix. With increasing Mo content from 0 to 0.26 and 0.50 wt.%, the sheet spacing decreased from 46.9–49.0 to 34.6–38.6 and 25.7–28.0 nm, respectively, which evidently hindered dislocation movement and greatly enhanced precipitation strengthening. Furthermore, facilitating AF formation and interphase precipitation was beneficial to improving steel properties, and the optimal Mo content was 0.26 wt.%.