Microstructural evolution of a niobium-microalloyed steel during hot shear deformation and subsequent cooling

Abstract For the purpose of obtaining a niobium-microalloyed steel with a preferable ultrafine-grained ferrite microstructure, a thermomechanical controlled processing (TMCP) strategy that included a hot shear deformation conducted near the local phase transformation temperature and a subsequent cooling process is proposed. As a large plastic strain was enforced by the simple hot shear deformation, severe plastic deformation (SPD) of the niobium-microalloyed steel was realized. The proposed TMCP strategy was simulated physically on a Thermecmastor-Z compression machine combined with a multi-type cooling system. Various cooling rates of different cooling methods resulted in reconstructive and displacive phase transformations from γ-Fe to α-Fe and led to different microstructural morphologies. In addition to phase transformations, the precipitation of carbides, grain growth, plastic deformation, discontinuous dynamic recrystallization (dDRX) of retained austenite grains, and continuous dynamic recrystallization (cDRX) of ferrite grains occurred during hot shear and subsequent cooling. The effects of the strain rate and forming temperature on the microstructural and textural evolution of niobium-microalloyed steel during hot shear and subsequent cooling were investigated and discussed. A niobium-microalloyed steel with a homogenous ultrafine-grained ferrite microstructure and intense γ-fiber texture was fabricated when the forming temperature, strain rate of hot shear deformation, and cooling rate of subsequent mist cooling were 1073 K, 20 s−1, and 10 K·s-1, respectively.