Structure ? property relationship in a secondary steel

Microalloyed steels demonstrate superior mechanical properties through thermomechanically controlled rolling (TMCR). In this work, structure and properties of microalloyed steels of two different compositions (V-Al containing and V-Nb containing steels) are investigated. The investigated steels go through the same industrial forming process, but one of the two exhibits inefficient ductility. The work intends to understand the reasons for the lower ductility of the V-Nb containing steel.There is a strong relationship between the microstructure and mechanical properties of the steels, in which all of the strengthening mechanisms are in operation. The microstructure of the termomechanically controlled rolled steels primarily consisted of polygonal ferrite and perlite. When the steels austenitised at different temperatures and then air cooled, the microstructure of V-Al containing steel is consisted of polygonal ferrite and perlite while the other one contained bainitic(acicular) ferrite, perlite and upper bainite. Furthermore, the TEM study revealed that termomechanically controlled rolled V-Nb containing steel is consisted of degenerated perlite structure.In this study, the weldability property of these steels is also investigated. Microstructural study showed that one of the weld metal of the steels consisted of ferrite and perlite while the other one contained bainite. The variation in grain size of the HAZ and weld metal of the steels was also correlated with hardness apart from their impact properties. The experiments have shown that using the same processing parameters for these two steels produce different microstructures, and resultantly different mechanical properties.