A quantitative analysis of transformation, microstructure and properties of a low-carbon Nb-microalloyed steel for building after deformation

This study investigated the effects of deformation temperature and strain on phase transformation and mechanical properties in a low-carbon Nb-microalloyed building steel. Thermo-mechanical simulation tests were performed based on the continuous cooling transformation characteristics of the steel, and the microstructural evolution was analyzed using various microscopy techniques. Tensile testing was employed to evaluate the strengthening mechanisms. The results indicate that, at a strain of 30%, ferrite volume fraction initially increases and then decreases with rising deformation temperature, whereas at 50% strain, ferrite content consistently decreases. Higher deformation temperatures inhibit precipitation and increase solute content, which impedes the diffusion of carbon atoms and reduces ferrite-transformation dilatation. Deformation at 850 °C with 50% strain significantly promotes ferrite formation, resulting in the highest ferrite volume fraction and optimal mechanical properties, characterized by a low yield-to-tensile strength ratio of 0.66 and an excellent strength-ductility balance. These findings suggest that lower deformation temperature and higher strain are effective in optimizing the mechanical performance of the steel, providing valuable guidance for the industrial production of high-performance building steels.