Strengthening Mechanism of Nb and V Microalloyed Low C Steel Rebars and Slabs-Phase-II

Abstract

Executive Summary: In the present study, precipitation behaviour/characteristic of Nb-containing steel rebar (32 mm) has been studied. Inconsistency in tensile properties for 32 mm steel rebarwas attributed because of heterogeneous distribution of NbC/Nb(CN) precipitate during the first phase of this project. Therefore, to find out the cause of such heterogeneous ‘Nb’ precipitations distribution – the said steel was exposed to various heat-treatment cycles followed by details microstructural characterization by using OM, SEM and TEM in the second phase of the project. The steel samples were solutionised at 1250oC for 30 minutes followed by (i) water quenching (WQ) and (ii) air cooling (AC). No precipitates/carbides were observed in the fully martensite (WQ) sample, whereas, AC sample shows a considerable amount of precipitates (mainly Cu precipitates and cementites and few Nb carbides), distributed in ferrite matrix. All the samples (WQ & AC) were then tempered at temperatures 550,600 & 650oC for 15, 30 & 60 minutes respectively. WQ sample tempered at 550oC shows only Cu precipitates & cementite and no Nb carbides were observed. Homogeneous ‘Nb’ carbides were observed in WQ sample only after tempering at 600 & 650oC. However, the sizes of the ‘Nb’ precipitates were found to be smaller than Cu precipitates & cementites. In AC tempered samples, only coarsening of precipitations/carbides and spherodising of cementites were observed at (550-650)oC. Therefore, readjustment of thermo-mechanical controlled processing (TMCP) parameters is required for homogeneous distribution of ‘Nb’ carbides in steel rebar production as the strain during processing is a controlling factor for forming of ‘Nb’ carbides. In vanadium micro-alloyed steel, better mechanical properties have been achieved by controlling the V/N ratio. Four numbers of heats of V-micro-alloying steels, 20kg each have been prepared with a pre-determined V/N ratio for rebar application. The V/N ratio was expected to have an impact on the mechanical properties with respect to the precipitation and coarsening of VN. It is found that in case of alloy 4 (higher amount of V and N) the YS and UTS values are superior compare to other alloys. It is expected that majority of V in this steel has been utilized. In alloy 4, the strengthening and work hardening is achieved by proper combination of V & N. The alloy 2 & 3 show distinct yield point phenomenon indicates that even after consuming all the V, some free N is still left in the respective steel that is undesirable.