Optimization of strength in V/N micro alloyed steel by varying vanadium content at fixed Nitrogen level

Abstract

The objective of present work was to study the precipitation behaviour of vanadium containing microalloyed steel with varying V/N ratio (2 to 6) keeping fixed nitrogen level (100-110 ppm) in the alloy and optimization of tensile strength w.r.t. V/N ratio. Five micro alloyed steel heats (~40 Kg each) with varying content of vanadium (0.02-0.06) wt.% & fixed nitrogen level of 100-110 ppm were prepared at CSIR-NML. To study the precipitation behaviour, as cast alloys were homogenised at ~1100ºC for 60 mins. and forged. Rolling was done after socking at 1100ºC for 60 minutes followed by normal air cooling (AC) to understand the effect of VC & V(CN) precipitates. Mechanical properties were evaluated and microstructural characterization was performed by using OM, SEM and TEM. Microstructural characterisation in OM and SEM revealed that the structure mainly constituted with pearlite and ferrite. However TEM, EELS and STEM-EDS study revealed very few VC & V(CN) precipitates in selected alloys in air cooled condition, indicating vanadium remained in solid solution. In such case, precipitation analysis confined on composition with V/N ratio 4.32 with different conditions decided jointly with TSL and suggestion from TSL-NML project evaluation committee. The conditions are 1. (Rolled without any HT); 2. (Forged +HT at 600 °C/1hr); 3. (Rolled +HT at 600 °C/1hr); 4. (Solutionized of Rolled sample at 1100°C for 2hr followed by WQ + HT at 600 °C/1hr). TEM study with aforesaid conditions had showed VC and V(CN) precipitates. Further, 'Thermocal' and 'JMatPro' software analysis were done to understand precipitation condition in reference of theoretical CCT diagram. The optimum combination of YS, UTS & ductility was found in the alloy with V/N ratio of 4.32.