Effect of deformation process on transformation and microstructure of low carbon containing Nb microalloy steel

Low-carbon-containing Nb microalloyed building structure steel was tested by Gleeble-3500 thermal simulation machine. Based on the thermal simulation data and microstructure analysis of optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM), the effect of different deformation temperatures (900 °C and 850 °C) and deformation amounts (30%, 40% and 50%) on the transformation and the final microstructure was clarified. When the strain amount is constant, with the decrease of deformation temperature from 900 °C to 850 °C, the initial temperature of ferrite transformation increases from 751 °C to 758 °C due to the increase in the amount of dislocation and deformation bands in original austenite grains. Meanwhile, the carbon content in the residual undercooled austenite after ferrite transformation increases, inhibiting the nucleation of medium-temperature bainite and decreasing the bainite transformation temperature from 558 °C to 544 °C. In addition, when the deformation temperature is constant, the grain boundary content of large angle (>15°) increases from 55.7% to 61.3%, and the starting temperature of ferrite transformation gradually increases from 740 °C to 758 °C with the increase of deformation amount from 30% to 50%. It is related to the amount of the second-phase precipitates. It indicates that the increase in deformation amount could promote the ferrite transformation and result in a smaller ferrite grain size.