Strengthening mechanism of V and Nb microalloyed low C Steel rebars produced through TMT

Abstract

Present investigation aims details characterization of nano-sized carbides, nitrides, carbo-nitrides and various phases of V and Nb microalloyed low C Steel rebars produced through TMT. It includes size, volume fraction and distribution of the various precipitates. The investigated micro-alloyed steel in the form of re-bars contain Nb and V with varying amounts of N2 and the present work intends to get optimum combination of these for best possible mechanical properties by varying ratio of Nb/N2 or V/N2. More specifically the present investigation includes Nb-contained rebars of 32 mm, V- contained rebars of 20 mm & 16 mm diameter. All the re-bars were supplied by TATA STEEL. In case of 32 rebars, microstructure of rim consists of tempered martensite, precipitates and whereas core is entirely ferrite-pearlitic structure. The microstructure of rim of 16 mm consists of tempered martensite, precipitates and whereas core is consists of tempered bainite, ferrite & precipitates. In case of 20 mm rebar there is no rim portion and the structure at the edges consists of tempered bainite, ferrite, precipitates and core consists of ferrite-pearlite & precipitates. In addition, few heat treated condition of the same re-bars were included in the study. Besides, two heats of 40 kg each microalloyed steel were prepared at NML to find the role of nano size precipitation on the mechanical properties and contribution varying N2 microalloyed steel at constant vanadium. TEM investigation revealed non-uniform distribution of precipitations in Nb-containing 32 mm TMT rebar and have only carbo-nitrides with minor amount of ε-carbides in rim and there were few precipitates in core region , whereas in V-containing 16 mm TMT rebar - uniform distribution of precipitations were observed. But in air-cooled 20 mm rebar, there was no distinguished rim and uniform distribution of precipitation was observed from edge to core. In general, precipitates in the core region are smaller in size compare to rim and densities are also less in the core region. Similarly it was found that alloy-1 contains carbide and carbo-nitride along with ε-carbides but alloy-2 contains only carbo-nitride. For all the case it was found that some of the cementites are in broken condition. YS and UTS values were found satisfactory for alloy 1 but for alloy 2 it was below the target.