Strengthening behaviour and failure analysis of hot-rolled Nb+V microalloyed steel processed at various coiling temperatures

The present work investigated the role of thermomechanical processing and coiling temperature on various strengthening mechanisms in a low carbon Nb + V microalloyed steel and its mechanical properties. The sample was subjected to 50 % thickness reduction by hot-rolling at 1100 °C and then air cooling directly to a coiling temperature maintained in the range of 650–500 °C. At the coiling temperature, an isothermal holding period was maintained for 1 h followed by water quenching to room temperature to preserve the sample microstructure. The microstructural characterization reveals a duplex/triplex phase mixture depending on the coiling temperature affecting its mechanical properties. For the coiling temperature range of 650–550 °C, the pro-eutectoid ferrite + pearlite phase mixture is evolved resulting an adequate ductility of ∼28 % with a tensile strength of ∼650 MPa. The strength-ductility combination was not altered much at the coiling temperature of 500 °C (i.e., near the bainitic transformation temperature domain), though a trace quantity (∼5 %) of Widmanstatten ferrite (WF) is nucleated along with the pro-eutectoid ferrite + pearlite microstructure. The isothermal holding further led to the nucleation and growth of niobium and vanadium-rich fine carbides with fcc crystal structure, which are mostly incoherent with the ferrite matrix phase. Though these precipitates were nucleated within the matrix grains due to induction of strain, the volume fraction was too low (∼1 %) to offer a substantial amount of precipitation strengthening (26–35 MPa) as compared to the grain refinement strengthening (126–156 MPa) and dislocation strengthening (115–150 MPa). The SEM analysis indicated that the segregated Mn and S atoms formed MnS inclusion at the phase interface of ferrite and pearlite, which operates as potential nucleation sites for crack initiation during tensile deformation.